forquestions and additional information contact · a metallurgical approachtoward alloying in rare...

For questions and additional information contact:

George A. SamaraSandia National LaboratorieslNMPhone: (505) 844-6653Fax: (505) 844-4045E-mail: [email protected]

C The DOE Center ofExcellence for the it~~Zt~+~

S Synthesis and Processing '~~~~i

P f Ad d M t . 1 BASIC ENERGY SCIENCESo vance a erta s DIVISION OF MATERIALS SCIENCES

Research Briefs

January 1996

TABLE OF CONTENTS

PREFACE 4

THE CENTER'S MEMBER LABORATORIES 6

CENTER PROJECTS AND THEIR COORDINATORS 7

EXECUTIVE SUMMARy 8

Research Briefs

CONVENTIONAL AND SUPERPLASTIC METAL FORMING 14

New Scaling Relationships Have Been Discovered for the Dislocation Cell Structures inDeformed Metals 14

Ultrasonic Monitoring of Rolling and Recrystallization Textures in Aluminum 16

Formability of AI-Mg Alloys in the Solute Drag Regime 18

MATERIALS JOINING 20

Characterization of High-Speed AI-Cu Electron Beam Welds .20

Models Predicting Weld Solidification Behavior and Microstructure Are BeingDeveloped and Verified 22

Joining of SiC-Based Ceramics for Elevated Temperature Applications 24

Low-Temperature Joining of Ceramics and Ceramic Matrix Composites .26

NANOSCALE MATERIALS FOR ENERGY ApPLICATIONS 28

Synthesis and Properties of Semiconductor Nanocrystals 28

Applications of Semiconductor Nanocrystals 30

Early Stages of Sintering in Nanoparticle Systems 32

2

MICROSTRUCTURAL ENGINEERING WITH POLyMERS 34

Polyphosphazene Membranes for Separation and Speciation 34

Polymer-Based Materials With Controlled Porosity 36

Synthesis and Characterization of a New Family of Anion Complexing Agents 38

TAILORED MICROSTRUCTURES IN HARD MAGNETS .40

A Metallurgical Approach Toward Alloying in Rare Earth Permanent Magnet .40

Importance of Grain Boundary Composition and Structure to Magnetic ReversalProcesses in Advanced Permanent Magnets .42

Alloy Design ofNd2FeI4B .44

PROCESSING FOR SURFACE HARDNESS .46

Low-Stress, Ultra-Hard, Low-Wear Carbon Films .46

Synthesis of Hard Boron Suboxide Thin Films Using Electron CyclotronMicrowave Plasmas 48

Accurate Measurement of Film Stress Using Micromachined Cantilever Beams:Application to Cubic Boron Nitride 50

MECHANICALLY RELIABLE SURFACE OXIDES FOR HIGH-TEMPERATURE

CORROSION RESISTANCE 52

Oxidation Performance of Plasma-Synthesized Alumina Coatings 52

Impurity Segregation To Alumina-FeAICr Alloy Interfaces 54

Strain Development In Thin, Thermally Grown Alumina Layers 56

3

PREFACE

This publication, Research Briefs, is designed to inform present and potential customers and

partners of the DOE Center of Excellence for the Synthesis and Processing of Advanced Materials

about significant advances resulting from Center-coordinated research. The format for Research

Briefs is an easy-to-read, not highly technical, concise presentation of the accomplishments. Each

Brief provides a statement of the motivation for the research followed by a description of the

accomplishment and its significance.

The Center is a distributed center for promoting coordinated, cooperative research

partnerships related to the synthesis and processing of advanced materials. It was established by the

Department of Energy's Division of Materials Sciences, Office of Basic Energy Sciences and the

DOE Laboratories in recognition of the enabling role of materials synthesis and processing to

numerous materials fabrication- and manufacturing-intensive technologies. The participants include

investigators from 12 DOE national laboratories, universities and the private sector. The Center has

a technology perspective provided by a Technology Steering Group.

The overall objective ofthe Center is,

To enhance the science and engineering of materials synthesis and

processing in order to meet the programmatic needs of the

Department of Energy and to facilitate the technological exploitation

ofmaterials.

Synthesis and processing (S&P) are those essential elements of Materials Science and

Engineering (MS&E) that deal with (1) the assembly of atoms or molecules to form materials, (2) the

manipulation and control of the structure at all levels from the atomic to the macroscopic scale, and

(3) the development of processes to produce materials for specific applications. Clearly, S&P

represent a large area of MS&E that spans the range from fundamental research to technology. The

goal of basic research in this area ranges from the creation of new materials and the improvement of

the properties of known materials, to the understanding of such phenomena as diffusion, crystal

growth, sintering, phase transitions, etc., in relation to S&P. On the applied side, the goal of S&P is

to translate scientific results into useful materials by developing processes capable of producing high

quality cost-effective products.

4

The technical emphasis of the Center has been on a number of focused multilaboratory

projects which draw on the complementary strengths of the member institutions in their ongoing

research programs. These projects were selected on the basis of the following criteria:

• scientific excellence

• clear relationship to energy technologies

• involvement of several laboratories

• strong existing or potential partnerships with DOE Technologies-funded programs

• strong existing or potential in-kind partnerships with industry

Each Project is coordinated by a knowledgeable representative from one of the participating

laboratories. The Projects and their Coordinators are listed in the accompanying table. The first

seven Projects were initiated in the fall of 1994, whereas the last one on High Efficiency

Photovoltaics was recently developed for start in 1996. In this first issue of Research Briefs we have

selected a few accomplishments from each of the seven initial Center Projects. An Executive

Summary provides highlights of these accomplishments organized by Project. Readers are

encouraged to contact any of the Coordinators for information about the Center and its

accomplishments.

George A. SamaraJanuary 1996

5

THE CENTER'S MEMBER LABORATORIES

The member laboratories of the Center are:

• Ames Laboratory (Ames)

• Argonne National Laboratory (ANL)

• Brookhaven National Laboratory (BNL)

• Idaho National Engineering Laboratory (INEL)

• University of Illinois Frederick Seitz Materials Research Laboratory (UIIMRL)

• Lawrence Berkeley National Laboratory (LBNL)

• Lawrence Livermore National Laboratory (LLNL)

• Los Alamos National Laboratory (LANL)

• National Renewable Energy Laboratory (NREL)

• Oak Ridge National Laboratory (ORNL)

• Pacific Northwest National Laboratory (PNNL)

• Sandia National Laboratories (SNL)

6

CENTER PROJECTS AND THEIR COORDINATORS

PROJECT

Conventional andSuperplastic Metal Forming

Materials Joining

Nanoscale Materials forEnergy Applications

Microstructural Engineeringwith Polymers

Tailored Microstructures inHard Magnets

Processing for SurfaceHardness

Mechanically Reliable SurfaceOxides for High-TemperatureCorrosion Resistance

High Efficiency Photovoltaics

COORDINATOR(S)

W. (Bill) G. Wolfer (SNL/CA)Phone: (510) 294-2307FAX: (510) 294-3231E-mail: [email protected]

R. Bruce Thompson (Ames)Phone: (515) 294-9649FAX: (515) 294-4456E-mail: [email protected]

Daniel S. Chemla/Mark Alper (LBNL)Phone: (510) 486-6581FAX: (510) 486-4995E-mail: [email protected] & [email protected]

Gregory(Greg)J.Exarhos(PNNL)Phone: (509) 375-2440FAX: (509) 375-2186E-mail: si,[email protected]

Bob Dunlap (ANL)Phone: (708) 252-4925FAX: (708) 252-4798E-mail: [email protected]

James (Jim) B. Roberto (ORNL)Phone: (423) 576-0227FAX: (423) 574-4143E-mail: [email protected]

Linda L. Horton (ORNL)Phone: (423) 574-5081FAX: (423) 574-7659E-mail: [email protected]

Satyen Deb (NREL)Phone: (303) 384-6405FAX: (303) 384-6481E-mail: [email protected]

-6~~oc~~~~co~~~~~~-----------Goorn~A~S;~~;~NL~M)----------

Phone: (505) 844-6653FAX: (505) 844-4045E-mail: [email protected]

7

mailto:[email protected]









EXECUTIVE SUMMARY

The Research Briefs presented in this publication are intended to inform the Center's present

and potential customers and partners about significant advances resulting from Center-coordinated

research. Selected accomplishments from each of the Center's initial seven focused projects are

presented. This Executive Summary states the overall objective of each project followed by

highlights of the accomplishments presented later in more detail.

Conventional and Superplastic Metal Forming

Objective

Highlights

To concentrate on those aspects of deformation mechanisms whichimprove formability in conventional metal forming and whichcontribute to superplasticity in fine-grained light weight materials.

• Although the complexity of microstructures resulting from metal formingprocesses had appeared to defy any quantitative interpretation, Centerresearchers have recently discovered scaling relationships indicating thatuniversal laws govern the evolution of the dislocation structure andresulting grain refinement during large strain plastic deformation. Thisdiscovery has the potential of improving the formability of metals and theproperties of resulting products. (pp. 14-15)

• An ultrasonic technique sensitive to differences in bulk rolling andrecrystallization textures of Al has been developed. Key advantages ofthis technique over other approaches (e.g., X-ray measurements) are that itprovides a volume (not near surface) average of the texture and iscompatible with the manufacturing environment. Texture influences theformability and properties of sheet metal. (pp. 16-17)

• Results on AI-Mg alloys show that enhanced ductility through viscoussolute drag of dislocations in the warm forming regime may provide aneconomical way for improving the formability of these light weight alloys.(pp. 18-19)

8

Materials Joining

Objective

Highlights

To improve the reliability of the processes used to join materials intomore complex structures serving a variety ofenergy-relatedfunctions.

• Studies of the microstructure of AI-Cu alloys at low and high electronbeam welding speeds revealed fine equiaxed grains at low speeds andlarge columnar grains at high speeds, contrary to common microstructuraldevelopment in alloys. This indicates that grain growth and not nucleationis the controlling solidification mechanism in high-speed welding.(pp.20-21)

• Models are being developed to predict weld solidification behavior inorder to select weld processes and parameters for optimizing weldmicrostructure and performance. Model results on AI-Cu welds comparefavorably with experimental measurements. (pp.22-23)

• Two different techniques for joining silicon carbide (SiC)-to-SiC suitablefor high temperature applications have been developed. SiC-basedceramics and composites offer the potential for significant efficiencyimprovements in power generation and energy conversion. Bothtechniques can be adapted for in-field component fabrication and repair.(pp.24-27)

Nanoscale Materials For Energy Applications

Objective

Highlights

To significantly enhance the technological exploitation of novelnanoscale materials in energy and other commercial applications bydeveloping synthesis and processing approaches that allow control ofsize, surface passivation and the interconnection and assembly ofnanoparticles.

• A wide variety of high quality nanocrystalline semiconductors in a rangeof sizes have been successfully grown by colloidal, inverse micelle andion implantation techniques, and theoretical approaches have beendeveloped to understand their electronic/optical properties. Theseachievements represent important advances towards the ultimateutilization of these exciting materials. (pp.28-29)

9

• A prototype light emitting diode (LED) has been successfully fabricatedby sandwiching nanocrystalline CdSe layers between semiconductorpolymer contacts. The color of the emitted light can be controlled (red-toblue) by changing the size of the nanocrystals. (pp.30-31)

• A synthesis and characterization facility has been developed which iscapable of producing nanoparticles from various materials by magnetronsputtering, transporting these particles under ultra-high vacuum to a highresolution transmission electron microscope, and studying (in-situ) theirsintering and interactions with substrates and surrounding media. Thework is complemented by molecular dynamics simulation of the earlystages of the sintering process. (pp. 32-33)

Microstructural Engineering with Polymers

Objective

Highlights

To develop new multiphase materials that retain the processability oforganic polymers but share the properties normally associated withinorganic materials.

• Two new routes for the synthesis of inorganic and molecularly mixedinorganic-organic polyphosphazene membranes and porous films havebeen developed. These polymers and polymer blends, which offersignificant advantages over other separation systems with respect todurability in harsh ambient environments, have many potentialapplications in the fuels industry and environmental cleanup. (pp. 34-35)

• Advanced routes have been developed for the synthesis and processing ofbulk and thin film low-density ceramics and polymer-based materialshaving continuous and controlled porosity at the nanometer scale. Thesematerials (aerogels) have exceptional insulating, acoustic and opticalproperties. (pp. 36-37)

• A new family of stable anion complexing agents based on either cyclic,linear or branched amines containing electron withdrawing side groups hasbeen developed. In addition to complexing anions in electrolyte batteries,these compounds increase the transport number of u' ions, a key toimproving the performance ofLi batteries. (pp.38-39)

10

Tailored Microstructures in Hard Magnets

Objective

Highlights

To improve hard magnets by understanding, in terms of themicrostructures achieved, the magnetic and mechanical properties ofmaterials produced by a number of synthesis and processingapproaches.

• The addition of transition metal carbides, especially TiC, has been shownto produce enhanced nucleation, and hence finer grain size and improvedmagnetic properties of the commercially important Nd2FeI4B-basedpermanent magnets. (pp. 40-41)

• High resolution electron microscopy and nanoscale chemical analysis haveled to the discovery of a previously unknown amorphous, iron-rich grainboundary phase in mechanically-processed rare-earth iron boron(RE2FeI4B) magnets. This result explains why the coercivity of thesealloys is below its theoretical limit and suggests metallurgicalmodifications to improve it. (pp.42-43)

• Motivated by the need to improve the poor fracture resistance ofNd2Fel4Bmagnets, a 3-point bend test was implemented and shown to be capable ofquantitatively distinguishing the fracture toughness among samplesprepared by different manufacturers and processing methods. Thisaccomplishment is a first step towards developing correlation betweenfracture toughness and microstructure. (pp.44-45)

Processing for Surface Hardness

Objective

Highlights

To use the integrated capabilities of the participating laboratories toaddress critical issues which limit synthesis and processing forimproved surface hardness.

• Alternating hard and soft multilayer carbon thin film structures have beensuccessfully grown by cathodic arc deposition. The hardness of thesestructures was found to be the weighted average of the constituent layers,whereas the observed growth-produced compressive stress in thestructures was ~50% lower (leading to better adhesion to the substrate)than the weighted average of the stresses in the constituent layers. Theaverage wear rate of the multilayer films was 10 - 1000X less than that forother hard coatings. (pp. 46-47)

11

• An electron cyclotron resonance (ECR) plasma reactor has been used tosuccessfully produce amorphous thin films of boron suboxide whosehardness (30 GPa) is comparable to, and hardness/modulus ratiosignificantly greater than that of sapphire. (pp. 48-49)

• A micromachined silicon cantilever beam technique has been developed toaccurately measure growth and processing stress in thin films. Thetechnique was used to show that film stress increased dramatically withincreasing cubic boron nitride (cBN) in ion-assisted BN deposition, butthe stress is not responsible for the increased cBN content, contrary toearlier belief. (pp. 50-51)

Mechanically Reliable Surface Oxides ForHigh-Temperature Corrosion Resistance

Objective

Highlights

To generate the knowledge required to establish a scientific basis forthe design and synthesis of improved (slow growing, adherent, sound)protective oxide coatings and scales on high temperature materialswithout compromising the requisite bulk material properties.

• It has been demonstrated that plasma-deposited, submicron thick aluminacoatings can provide excellent high temperature oxidation protection toFe-AI-Cr alloys. Adhesion of the alumina to the alloy is further enhancedby the presence of Zr in the alloy. Plasma synthesis has the potential ofdepositing dense, strongly adhering coatings at relatively low cost.(pp.52-53)

• Auger electron spectroscopy has revealed elevated levels of sulfur at theoxide-metal interface in thermally-grown alumina on a Fe-28AI-5Cr(at.%) alloy. The presence of sulfur at this interface correlated with poorscale adhesion. For an alloy with 0.1 at.% Zr added, no sulfur wasobserved at the interface and there is good scale adhesion -- results thatsuggest modification of the interface as a means for developing reliablesurface oxides. (pp. 54-55)

• The shift of the "ruby line" fluorescence has been used to provide aquantitative method of determining the compressive strain in thermallygrown alumina coatings on Fe-AI-Cr alloys. Lack of this information hasoften been a barrier to understanding and modeling how oxide coatingsand scales accommodate thermally-generated stresses and thus toassessing the mechanical reliability of surface oxides. (pp. 56-57)

12

RESEARCH BRIEFS

13


New Scaling Relationships Have Been Discoveredfor the Dislocation CellStructures in Deformed Metals

D. A. Hughes, Sandia National Laboratories, Livermore, CaliforniaD. C. Chrzan, Lawrence Berkeley National Laboratory

Q. Liu and N. Hansen, Riso National Laboratory, Roskilde, Denmark

Motivation -- Large strain deformations takeplace in many metal forming processes such assheet forming, forging, extrusion, and sheetrolling. In all these cases, the plasticdeformation creates a subdivision of the initialgrains into smaller crystallites separated bydislocation walls. This refinement of theoriginal grain structure by cold working is ofcourse an ancient technique for impartingsuperior mechanical properties to the finishedproduct. Modem applications of this techniqueinclude the angular equal-channel die pressingfor producing superplastic alloys to obtainsubmicron grain sizes. In spite of the fact thatdislocation cell formation and grain refinementhave been observed and studied extensively, thecomplexity of the deformed microstructureappeared to defy any quantitative interpretationin terms of universal models. The discovery ofthe scaling relationships described belowindicate that universal laws do exist whichcontrol the evolution of the dislocation structureas a function of the plastic deformation.

Accomplishment -- Samples of pure aluminumwith an initial grain size of 300 urn were coldrolled ranging from 5% to 50% reduction.Convergent beam Kikuchi analysis wasperformed on TEM specimens made from thedeformed samples and the misorientation anglesacross dislocation boundaries were determined.Two different types of dislocation boundarieswere identified. Long, continuous dislocation

boundaries which have been calledgeometrically necessary boundaries (GNBs),and smaller scale cell boundaries termedincidental dislocation boundaries (IDBs). Theaverage misorientation angles, Q, for IDBsincreases linearly with the square root of thestrain while that for the GNBs increases by ahigher power according to the results shown inFigure 1. The frequency distributions ofmisorientation angles, P(q, Q), were alsomeasured for each type and at four differentstrain levels, namely 0.06, 0.1, 0.4, and 0.8.Although each distribution is very different, itwas discovered that they can all be scaled

according to P(q,Q)=const. (q/Q)a-l exp(-a q/Q)where a was found to be 3. The scaleddistribution functions are shown in Fig. 2.

Significance The distributions ofmisorientation angles for different plastic strainscan all be characterized by a universal functionwith one characteristic parameter, the averagemisorientation angle. The latter in tum is relatedto the plastic strain. The existence of the scalingrelationship and the universal distributionfunction for the misorientations indicates thatthe deformation structure and its evolution aregoverned by relatively simple laws for thecollective properties of dislocationmicrostructures produced by plastic deformationof polycrystalline metals with medium to highstacking fault energies.

Contact: Darcy A. Hughes, Sandia National Laboratories/CaliforniaPhone: (510) 294-2686 Fax: (510) 294-3410 E-mail: [email protected]

14



10.......

y

A~e/ ..'GNB ....

> 1....

Cll •••·1~

....

~lOB..... 1/2

0.1E

vM

0.1 '---~-'--'- ...............~-----'--'--'--'- ...............0.01 1

Figure 1. Average misorientation as a function of the deformation (von Mises) strain.

1.0 !1.0 I

0.8i

0.8 GNBsIDBs

~

5%cr~ 0.6 •<r} 0.6 • 5% cr a:>e • 10% cra5 • 10% cr 30% cr0:. 30% cr

c, ...... ~~ ., 50% cr a:> 0.4

., 5O%cra:> 0.4 -fit-fit

0.2 0.2

0.0 0.00 1 2 3 4 5 0 1 2 3 4 5

S/Sav S/Sav

Figure 2. The scaled distributions of misorientation angles are shown to coincide for different levels ofdeformation. (cr = cold reduction)

15


Ultrasonic Monitoring ofRolling and Recrystallization Textures in Aluminum

R. B. Thompson, Ames LaboratoryR. Bolingbroke, Alcan International

Motivation -- The texture (preferred grainorientation) of aluminum alloys plays animportant role in determining the ability of sheetto be formed into complex parts such asbeverage cans or automotive components. Forexample, the processes of rolling andrecrystallization produce textures which havedissimilar influences on formability, andprocessmg is designed to balance theirinfluences such that optimum formingcharacteristics are realized. Quality controltools are desired which can be used to ensurethat this has been properly accomplished.

Accomplishment -- An ultrasonic technique hasbeen developed which is sensitive to thedifferences in rolling and recrystallizationtextures. The technique involves measuring theanisotropy of the velocity of guided modespropagating in the plane of the sheet. Inparticular, the velocities of longitudinal Lambmodes are measured for waves propagating at 0,45, and 90 with respect to the rolling direction.As shown in the top accompanying figure, theangular variation is dramatically different forrolling and recrystallization textures, with theformer having a minimum velocity near 45 andthe latter having a maximum. It is also possibleto extract from this information certainparameters which play a prominent role in thequantitative analysis of texture and formingproperties. For example, the parameter C4

13,

which is closely related to the degree ofrecrystallization of a sheet, is proportional to thequantity [v(O) + v(90) - v(45)], where v is themeasured velocity. The lower accompanyingfigure presents a validation of the ability ofultrasound to sense this parameter by plottingthe values of C4

13 inferred from ultrasonicsagainst the values inferred from x-rays for a setof six samples. An important aspect of thetechnique is that the waves are excited anddetected by electromagnetic-acoustictransducers (EMAT' s), which require nophysical couplant.

Significance -- The control of texture is animportant part of the strategy for processingsheet metal. Attractive features of the ultrasonictechnique include the facts that a volumeaverage is sensed and that there are no radiationhazards, both in contrast to x-ray measurements.The former is significant since it is the volumeaverage ofthe texture that controls the formingcharacteristics of the sheet, not just the textureof a very near surface skin as sensed by x-rays.The fact that the measurements can be madewith couplant-free EMAT's, as opposed to thetraditional fluid-coupled, piezoelectric probes,greatly increases the opportunity for makingmeasurements in the manufacturingenvironment, including the possibility of on-linemeasurements.

Contact: R. Bruce Thompson, Ames LaboratoryPhone: (515) 294-9649 Fax: (514) 294-4456 E-mail: [email protected]

16



0.5650 ,..------------------,

0.5600

~'0 0.5550o~o"§ 0.5500lJ)

~5

••••••••••••••••••• •••••••••••••••••••••••••

........................................~Rolling Texture

Partial RecrystallizationTexture

0.5450

o.5400 ~....l....I.....l....I.....l...l-.l..I...I'-I....l....J...L...L..J..J...WL....L..J...J...l....w....L.L.l..I...IW-..L....l..J.J

o 10 20 30 40 50 60 70 80 90

Propagation Direction (Degrees)

3

2o

Strong RecrystallizationTexture

:Q1coC/)

~ 0....."5...........

C'?-.t

""0 -1

-2

-3-3 -2

•

o

-1 0 1

C~ (x-ray)

2 3

Ultrasonic velocity measurements can be used to monitor the degree ofrecrystallization in an aluminum sheet. The upper figure shows the change in thevelocity, as a function of angle in the plane of the sheet, as a rolling textureevolves into a strong recrystallization texture. The lower figure compares

numerical values of the texture coefficient c1 3, inferred from such velocity

measurements, to values determined by x-ray distraction. This coefficientprovides a measure of the degree of recrystallization.

17


Formability ofAI-Mg Alloys in the Solute Drag Regime

G. A. Henshall and D. R. Lesuer, Lawrence Livermore National Laboratory

Motivation -- Our objective is to examine"extended ductility" of Al-Mg alloys in thewarm forming regime (roughly 200-400°C). Inthis regime, solute drag of dislocations leads tostrain-rate sensitivities near 0.3 and, therefore,large tensile ductilities. Evaluation of alloychemistry, metallurgical variables, andprocessing conditions controlling ductility andwarm formability of these alloys will be used tobetter understand the deformation and failuremechanisms in solution-hardened Al-Mg alloys,and to define the critical processing variablesrequired to provide optimal formability.

Accomplishments Experiments wereperformed using two binary alloys: Al-2.8 wt.%Mg and Al-5.5 wt.% Mg. Within this range, theMg concentration only slightly affects strengthand has no significant effect on the failure strain(eA, which reached values as high as 325%.The tensile ductility was found to correlatestrongly with the strain-rate sensitivity (m) andfailure occurred by necking to a point. Thevalue of m was controlled by the diffusioncompensated strain rate during testing. Testsperformed under widely different temperaturesand strain rates resulted in a similar e/ if thediffusion-compensated strain rates were similar.This finding suggests that e/ is controlled by theviscous drag of solute atmospheres by movingdislocations during grain matrix flow.

A numerical model of neck growth properly

predicts e/ in these binary alloys, indicating

that tensile ductility is controlled by the value ofm when failure occurs by necking. Since thereis little strain hardening in the regime whereductilities are large, the strain-hardening ratedoes not need to be included in the analysis.

Further experiments using an extremely coarsegrained Al-2.8 wt.% Mg alloy showed thatenhanced ductility is not significantly degradedby an increase in grain size. This finding furthersupports the hypothesis that solute drag ofdislocations during grain matrix flow, not grainboundary sliding, accounts for the large tensileelongations observed in these materials.Finally, experiments were performed using highpurity Al-3Mg-0.25Mn and Al-3Mg-0.5Mnalloys. The compositions and processinghistories of these alloys were determinedfollowing consultation with 1. Vetrano at PNL.As shown in the figure, Mn decreases the strainrate sensitivity of Al-Mg during solute-dragdeformation, resulting in lower elongations of150% to 200%. Mn has a secondary effect ofcausing ductile fracture before necking to apoint when added to Al-Mg in a concentrationof 0.5 wt.%.

Significance -- These results indicate thatenhanced ductility through solute drag mayprovide an economical way to improve theformability of Al-Mg alloys.

Contact: G. A. Henshall, Lawrence Livermore National LaboratoryPhone: (510)423-4417 Fax: (510)423-7040 E-mail: [email protected]

18



400

- 350 1:1. AI-2.8 Mgtfl. 1:1. 0 AI-5.5 Mg-e 300 ~ AI-3.0 Mg-0.24 Mn::I 1:1. 0 1:1. • AI-3.0 Mg-0.50 Mn-"iii 250 0 8LL 0 00 200 ~ ~-C

~0 150 I •;; • ~ If>asC) 100 0e0 1:1.- Cbw 50 0 00 1:1.

0109 1011 1013 1015 1017

•em (m-2)

The tensile elongation to failure as a function of diffusioncompensated strain rate for two binary AI-Mg alloys and twoternary AI-Mg-Mn alloys. Ductilities are largest in the solute dragregime (diffusion-compensated strain rates below 7x1012 m"). Inthis regime, the ductility decreases as Mn concentration increases.

19

Materials Joining

Characterization ofHigh-Speed Al-Cu Electron Beam Welds

S. A. David and J. M. Vitek, Oak Ridge National LaboratoryR. Trivedi and M. Kramer, Ames Laboratory

Motivation -- The reliability and performanceof welds strongly depends on processingconditions, microstructure, and properties. Inaddition, the presence of gradients, which areintrinsic to the welding process, can often leadto the formation of defects and variations inproperties, and they may be the cause of poorperformance and lack of reliability. Themotivation for this Project is to address theproblem of gradients in weldments and to assessthe extent of the gradients as well as thepotential influence they may have on weldperformance. The specific objective of the OakRidge National Laboratory task is to investigatethe microstructural evolution and gradients inweldments as a function of welding process andprocess parameters. Grain structure formation,solidification substructure development, andnon-equilibrium phase formation are issues ofinterest. Analysis is carried out in conjunctionwith theoretical support from Ames.

Accomplishment -- The investigation of thealuminum-copper system was initiated. Twodifferent alloys were identified for testing, All %Cu and AI-4%Cu. In order to coordinateactivities with Ames Laboratory, the alloys werethe same as those being studied at Ames, andwere in fact supplied by Ames. Electron-beamwelds were made on these two alloys at weldingspeeds ranging from 30 to 1000 in/min. Theseextreme conditions were investigated in order toassess the tendency of these alloys to form

metastable phases during rapid solidificationconditions typical of electron beam and laserwelding. Fine, equiaxed microstructures werefound at low welding speeds while largecolumnar grains were found at high weldingspeeds. These results are contrary to typicalsolidification observations in which finermicrostructures are prevalent at the highercooling rates associated with high-speedwelding. An example of the typical weldmicrostructures is given in the figure. Inaddition, at very high welding speeds, bandingpatterns indicative of convective flow patternsin the weld pool were observed.

Significance -- The observation of coarsergrains at higher welding (solidification) speedsis contrary to common microstructuraldevelopment in that finer grains are normallyfound at higher solidification rates. The resultsindicate that there may be competition betweennucleation and growth such that at the higherwelding speeds, growth is the controllingsolidification mechanism and only limitednucleation takes place. Further analysis of theseresults is being carried out. The observation offlow patterns in the rapidly solidified materialmay be useful in that it may provide a means ofexperimentally studying flow behavior inweldments. Such information would be veryuseful in testing theoretical computationalmodels that predict flow behavior in welds.

Contact: R. Bruce Thompson, Ames LaboratoryPhone: (515) 294-9649 Fax: (514) 294-4456 E-mail: [email protected]

20


Materials Joining

a)

b)

500 ~m I I

Weld microstructures in electron-beam welded AI-4%Cu alloy. (a) 150 "/min weld showing finegrain structure in weld and (b) 500 "/min weld showing coarse grain structure and banding patternindicating convective flow in the weld pool.

21

Materials Joining

Models Predicting Weld Solidification Behavior and Microstructure Are Being*Developed and Verified

J. A. Brooks, N. Y. C. Yang, M. Li, Sandia National Laboratories, LivermoreA. W. Thompson, Lawrence Berkeley National Laboratory

Motivation -- During weld solidification,alloying elements partition between the solidand liquid, oftentimes leaving large compositional differences within different regions ofthe solidified dendritic structure. The nonuniform composition can result in weldproperties being considerably inferior to thoseof the finely-tailored base material. It isdesirable to be able to model the solidificationprocess so one can select weld processes andparameters to optimize weld microstructure andperformance.

Accomplishments --We have modeled thesolidification behavior of Al-Cu welds for arange of alloy compositions. The modelsincorporate a finite element (FEM) thermalanalysis which predicts the solidificationvelocity and cooling rates occurring alongdifferent regions of the weld pool interface.These are the two primary solidificationparameters which control the weld structure.The output from the thermal analysis isincorporated into a solidification model to

predict solidification temperature range, volumefraction of solid and liquid during differentstages of solidification, and the composition andvolume fraction of different phases of the finalweld structure. As the figure shows, results ofthe analysis compare favorably withexperimental measurements especially at lowCu concentrations.

Significance -- The weld solidification modelsare being incorporated into larger FEMthermomechanical codes to predict solidificationcracking behavior. These codes provide thewelding engineer with a powerful tool whichcan be used to develop a weld process in whichthese often catastrophic defects can beeliminated. Results from these codes are alsobeing used in conjunction with fracture studiesto better understand the mechanical behavior ofwelded structures. The ability to predictsolidification behavior and weld structure sothat one can optimize the weld process canresult in tremendous improvements to weldperformance and production costs.

Contact: John Brooks, Sandia National Laboratories, Livermore, CaliforniaPhone: (510) 294-2051 Fax: (510) 294-3410 E-mail: [email protected]

"Research sponsored by the Division of Materials Sciences and by Defense Programs, U.S. Department of Energy.

22


Materials Joining

Weld in Al-Cu alloy showing weld macrostructure, and, at higher magnification, theweld microstructure containing secondarysolidification product CuAlz atinterdendritic boundaries.

2.5c'"::Q,

2 0 ~.. ;-

oe.,'"1.5 oe::I"l

'"::-1.0 j;l-o'::

0.087

: ............- 0»> :

1-·····....·......·,,4-....·..·........·..

••- _- Ca lcu la ted C uAl,

- .-- Measured C uAl,

5 6(wt.%Cu)

! :

... .... .... : ...~ .. -j- ..

V..... .......:, .

3 4Bulk Composition

2

o ~

.. ....~+ .... ,. .. ..,....·c/

1

--{]-- Calculated Dendrite Co re Co ncentration

-0--- Meas ur ed Den drfte Co re Concentra tion

o

4

2

6

o

8

10

-e::U

Measured and calcu lated dendrite core concentration and volume fraction of eutectic solidificationconst ituent 8 (CuAlzl in Al-Cu welds made at 1.3 ern/sec.

23

Materials Joining

*Joining ofSiC-Based Ceramics for Elevated Temperature Applications

B. H. Rabin, Idaho National Engineering LaboratoryJ. K. Weddell, DuPont Lanxide Composites, Inc.

N. Q. Minh, AlliedSignal, Inc.

Motivation SiC-based ceramics andcomposites offer the potential for significantefficiency improvements in power generationand energy conversion systems. Practical andreliable ceramic-to-ceramic and ceramic-tometal joining techniques are required to fullyrealize the advantages of these materials inindustrial applications. In addition, numericalmodeling capabilities are needed to supportjoining development efforts and to assist in jointbehavior prediction and evaluation.

Successful joining methods will permit thefabrication of large and complex shaped parts,and will allow integration of ceramiccomponents into existing structures. The goalof this task is to develop joining techniquesapplicable to SiC ceramics and fiber-reinforcedSiC matrix composites, with emphasis onproducing ceramic-to-ceramic joints for use inelevated temperature structural applications.

Accomplishment -- Research conducted at theIdaho National Engineering Laboratory (lNEL)has led to the development of a technique whichpermits the fabrication of SiC-to-SiC jointssuitable for elevated temperature applications.In this process, which employs methods similarto those used to manufacture commercialreaction bonded silicon carbide, liquid siliconcapillary infiltration of SiC+C tape castprecursors is carried out at ~1450°C. Jointshaving room temperature strengths exceeding250 MPa are obtained, and joint strengths aremaintained at temperatures up to ~1200°C.Joining can be accomplished without the needfor high pressure equipment, therefore the

technique can be modified to allow in-fieldcomponent fabrication and repair. Theaccompanying figure shows several examples ofpressureless sintered SiC parts that have beenjoined, along with a micrograph of a polishedjoint cross section. The joint material consistsprimarily of SiC and Si, so there is littleproperty mismatch and therefore low residualstress in the joints. Technology transferactivities are in progress to move the joiningmethod from the laboratory scale to industrialpractice.

The INEL research has also developed extensivemodeling capabilities for predicting residualstresses and thermomechanical behavior ofdissimilar material joints and graded materials,an important tool for further development ofuseful ceramic-to-metal joining methods.

Significance -- The ability to fabricate SiC-toSiC joints having excellent elevated temperatureproperties will allow the industrial utilization ofSiC in numerous applications where higheroperating temperatures result in improvedenergy efficiency. For example, collaborationwith AlliedSignal is focused on the developmentand construction of an industrial scale highpressure SiC heat exchanger using commerciallyavailable pressureless sintered SiC. Work withDuPont Lanxide Composites is examining theapplicability of the joining method to fiberreinforced SiC composites which offer thepossibility for improved reliability inapplications such as gas turbine components,radiant burner tubes, petrochemical processingsystems and heat exchangers.

24

B. H. Rabin, Idaho National Engineering LaboratoryPhone: (208) 526-0058 Fax: (208) 526-0690 E-mail: [email protected]

'Partially supported by the Continuous Fiber Ceramic Composites Program, Office of Industrial Technologies, Office of Energy Efficiencyand Renewable Energy.

Contact:


Materials Joining

Hexoloy ~' S A sin \efe d II_- S \ C

Examples ofjoined SiC parts.

Optical micrograph showing the microstructure of the ceramic-to-ceramic joint.

25

Materials Joining

*Low Temperature Joining ofCeramics and Ceramic Matrix Composites

I. E. Anderson, S. I-Maghsoodi, O. Unal, M. Nosrati and T. J. Barton, Ames LaboratoryB. C. Mutsuddy and A. Szweda, Dow Coming Corporation

W. E. Bustamante, Amercom, Inc.

Motivation -- U. S. industry that is involved inhigh temperature processing operations, e.g.,metal heat treating, petroleum refining, glassprocessing, and power generation, has a criticalneed for ceramic and ceramic fiber/ceramicmatrix composite (CFCC) materials, especiallySiC ceramics. The processing environment ofthese operations is typically characterized bysevere oxidation, corrosion, and erosionproblems that result in a short life for mostcommon materials. SiC ceramics and SiCbased CFCC materials should extend greatly thelifetime of such components. One criticalroadblock to implementation of a materialsubstitution is that, in both manufacturing andfield installation environments, there is a need toproduce bigger and more complex shapes byjoining simpler and smaller ceramic bodies.Unfortunately, a practical SiC ceramic joiningtechnology is currently nonexistent.

Accomplishment -- This Project has developeda novel approach for the joining of SiC ceramicsand CFCC components using "glue" which isconverted to a strong joint during simplethermal processing. Initial studies were focusedon SiC/SiC joining using preceramic polymersand metal alloy powders developed in the AmesLaboratory. The joining materials wereprepared by uniformly mixing liquid siliconacetylene polymers and fine spherical alloypowders (both prepared at Ames Lab) into aviscous paste-like material. The SiC surfaceswere cleaned and the joining paste was appliedmanually. Immediately after joining, the gluedassemblies were pressed together by hand in air

at room temperature, sandwiching the gluemixture so that the joining materials conformedto imperfections in the adhered surfaces givingthe green assemblies some strength. Finally, theassemblies were fired in an inert atmosphere byheating without pressure at lO°C per minute tol200°C, holding for 2 hours at l200°C, andcooling to room temperature. Initial studies ofthe microstructure revealed good adhesion withuniform loading. Initial ambient temperaturestrength measurements on SiC CFCC/CFCCbutt joints revealed a bend strength of 50 to70MPa. Qualitative testing of the effect ofprolonged exposure (100 hours) to air at hightemperature (1100°C) indicated that SiCCFCC/CFCC joints retained structural integrityfor mechanical handling at ambient temperature.More extensive testing in-house and at ourindustrial partners is in progress.

Significance -- An exciting aspect of thisjoining technique is the ease of field use. Whenglued green assemblies were fired in air with apropane flame, high quality joints wereobtained. The most important benefit of thistechnique is the application of heat only to thejoint area. In fact, we have repaired severalbroken SiC elements of a resistance heated tubefurnace by this method in less than 15 minutes.An example of this joining technique is shownin the following illustration. The repairedelements were used in the furnace for 3 monthsup to 1200°C without any problems. Recently,the propane flame firing approach has beendemonstrated on CFCC material as well.

Contact: Iver E. Anderson, Ames LaboratoryPhone: (515) 294-8252 Fax: (515) 294-8727 E-mail: [email protected]

'Partially supported by the Continuous Fiber Ceramic Composites Program, Office of Industrial Technologies, Office of Energy Efficiencyand Renewable Energy.

26


Materials Joining

Joining of SiC resistance heating elements with Ames Lab glue using a propanetorch flame (flame temperature of about 1200°C). The SiC element took less than 15minutes to repair and has lasted for more than 3 months of furnace service.

27

Nanoscale Materials for Energy Applications

Synthesis and Properties ofSemiconductor Nanocrystals

* *M. J. Heben, O. I. Mieie , A. J. Nozik and A. Zunger, National Renewable Energy LaboratoryC. W. White, J. D. Budai, J. G. Zhu and S. P. Withrow, Oak Ridge National Laboratory

J. P. Wilcoxon, D. E. Bliss, P. P. Newcomer and G. A. Samara Sandia National LaboratorieslNM

Motivation--Semiconductor nanocrystals orquantum dots (QDs) show very profoundchanges in their fundamental physicochemicalproperties when their size becomes comparableto the deBroglie wavelength of their chargecarriers. This size quantization effect producesunique carrier dynamics and has potentiallyimportant applications in novel solar photonconversion, optoelectronics and photocatalysis.In a coordinated effort, Center participants areemphasizing different material systems andapproaches to produce and understand theseQDs. (See also the following report).

Accomplishment -- Colloidal dispersions of theIII-V compounds GaP, InP and GaInP2 from 1.4to 4.0 nm in diameter were synthesized andstudied at NREL. TEM lattice fringe imagesand x-ray diffraction showed excellentcrystalline quality. The optical absorptionspectra were strongly blue shifted withdecreasing particle diameter, and for the firsttime the InP QDs showed strong excitonicfeatures at room temperature. The emissionspectra of GaP and InP QDs showed strong PLintensity and band edge emission. Thedependence of InP QD band gap on QD sizewas found to be much weaker than predicted bythe effective mass approximation. Newpseudopotential methods were developed thatcan treat 1000 atom nanostructures and thustranscend effective-mass based k-p methods.Optical properties of silicon QDs, wires, andfilms were calculated; and an indirect-to-directtransition in GaAs QDs as a function of theirradii was predicted.

At SNL the emphasis is on metalsulfides as potential photocatalysts. High quality

QDs of PbS, FeS2' and MoS2 were grown ininverse micelles and their structural and opticalproperties studied. Their bandgaps, which aretypically in the infrared in the bulk, can beshifted to the near ultraviolet by reducing size.Results on metal capping the QDs show thatsurface related states of one material can beexcited through states of a different interiormaterial. This effect has importantramifications for photocatalysts since thefunctions of energy absorption and energytransfer can be separated. Materials for theinterior of the QD can be chosen to be efficientabsorbers of light energy, while materials on thesurface can be chosen to be efficient attransferring energy to reactants.

At ORNL, Si and Ge QDs embedded inSi02 and Al203 were synthesized by ionimplantation followed by annealing. Thesecubic QDs are randomly oriented in amorphousSial> but are three-dimensionally aligned in thecrystalline a-A120 3 matrix. Strong visiblephotoluminescence near 1.7 eV is observed inSi02 samples containing Si nanocrystals. Byimplanting combinations of different ions, alloyand compound QDs (SiGe, GaAs, CdSe) havebeen successfully synthesized. SiGe and GaAspossess the expected diamond and zincblendestructures, respectively, but CdSe can beformed in either a hexagonal or zincblendestructure.

Significance --Our successful synthesis of awide variety of high quality QDs anddevelopment of novel theoretical approachesrepresent important advances towards theultimate understanding and utilization of theseexciting materials.

Contact: Mark Alper, Lawrence Berkeley National LaboratoryPhone: (510) 486-6581 Fax: (510) 486-4995 E-mail: [email protected]

Supported by BES/Division of Chemical Sciences. 28


A high resolution transmission electronmicrograph of a 3 nanometer (.0000003centimeter) diameter lead sulfide cluster on acarbon film. The image shows excellent latticefringes (parallel lines) revealing the highcrystalline quality of the nanocluster .


BulkInPhasbandgap1.35 eV (918 nm).

~c,e.!'!. 45Aaem-e0M

""

~--35A

26A

400 450 500 550 600 650 700 750 800

Wavele ngth. nm

Absorption spectra of In? quantum dots as afunction of particle diameter ranging from 45to 26A showing a large and increasing blueshift with decreasing size and structure in thespectra due to size quantization effects.

~~

v0 0

104 0 0~

~ ~

.c en en... « X-ray diffraction 8-28C'Cl N<II

~ Cl scans showing the103 0

>. 0 presence of GaAs~

.'!:: enIII -c nanocrystals in siliconc: <llQ)

102 CJ produced by ion-c: implantation.

10

10 20 30 40 50 60 7028(deg)

29


Applications ofSemiconductor Nanocrystals

Paul Alivisatos, Lawrence Berkeley National Laboratory

Motivation -- Researchers at LBNL havepioneered inexpensive, solution-phase, colloidalchemistry techniques that can prepare II-VIsemiconductor nanocrystals with highcrystallinity and narrow size distribution. Theelectronic properties of these nanocrystals (e.g.,the band gap) are a sensitive function of the sizeof the crystals. As one component of thisresearch effort, LBNL researchers arecollaborating with other personnel in the Centerfor Excellence to investigate ways in which suchnanocrystals may be applied to the developmentof new, low-cost electro-optic and photovoltaicdevices.

Accomplishment -- Semiconductor nanocrystalscan be deposited readily as monolayers or inmultilayer structures, allowing theirincorporation into electronic devices. Centerresearchers have fabricated prototype lightemitting diodes (LEDs) by sandwichingnanocrystalline CdSe layers betweensemiconductor polymer contacts (see figure).The researchers demonstrated that the lightemitted by the device could be controlled bychanging the size of the nanocrystals - smallercrystals emitted blue light, larger ones red. It ispossible that by using multilayers ofnanocrystalline materials with different sizes,LEDs with a variable color output may befabricated. This is not possible in LEDs

fabricated from bulk semiconducting materials.Because of their large surface to volume ratio,nanocrystals melt at substantially lowertemperatures than the corresponding bulk solid.It is possible that thin semiconductor films canbe formed by depositing nanocrystals via wetchemistry techniques, followed by a heating stepthat fuses the crystals together. The lowermelting point of the nanocrystals will allowdeposition of semiconductor layers on heatsensitive substrates (e.g. glass), due to the lowerprocessing temperatures required. In addition,the cost of such a fabrication technology ispotentially very low compared to currentchemical vapor deposition methods becauseexpensive vacuum processing steps are notrequired. As a first step in demonstrating theviability of this technology, a reduction inmelting point of over 1000°C was demonstratedin CdTe nanocrystals (see figure). Acollaboration has been established with NREL toinvestigate the feasibility of this technique forfabricating large-area photovoltaics at low cost.

Significance -- The Center researchers havedemonstrated that device fabrication usingsemiconducting nanocrystals may represent alow-cost and versatile alternative to traditional,vacuum-based semiconductor processing. Bothlight emitting and photovoltaic devices can befabricated with wet-chemistry techniques.

Contact: A. Paul Alivisatos, Lawrence Berkeley National LaboratoryPhone: (510) 643-7371 Fax: (510) 642-6911 E-mail: [email protected]

30


Voltage-dependent color in a lightemitting CdSe nanocrystal diode.

1800

-bulk1600

~

~~

1400CIl...::::J-ctI...

1200CIlCoECIl;, 1000l:-CIl 800:2

600

40010 20 30 40 50

Radius (Al

31

Nanoscale Materials for Ener& Applications

A dramatic decrease in the melting point ofnanocrystals is observed as their size decreases.The melting point of 15A CdTe nanocrystals ismore than 1000°C less than the value for bulkmaterials.


Early Stages ofSintering in Nanoparticle Systems

D. Olynick, M. Ghaly, H. Zhu, J.M. Gibson, and R.S. Averback, University ofIllinois/Urbana

MOTIVATION The incorporation ofnanoparticles in the synthesis of new materialsfor catalysis, magnetics, electro-optics, andcoatings has been undergoing enormousadvances. In several of these applications, theinteractions of the nanoparticles with themselves(for sintering), with substrates (for catalysis),and with a surrounding matrix (for magneticsand electro-optics) significantly influences theproperties of the material. For continued rapidprogress in this field, an understanding of thedifferent nanoparticle interactions is essential.

ACCOMPLISHMENT -- The difficulty ofexperimentally investigating nanoparticleinteractions lies in the need to synthesize,process, and characterize these materials, allwithout ever exposing them to contaminatinginfluences. Over the past two years we havedeveloped, with support of the Center, a systemin which nanoparticles of nearly any materialstype, can be synthesized and transported througha UHV line to a high resolution, UHV electronmicroscope (UHV-HRTEM). We furthershowed, that owing to the ultra small size of thenanoparticles, molecular dynamics (MD) is apractical means to investigate their interactions,and that the combination of TEM and MD isparticularly illuminating. Figure aschematically illustrates the nanoparticlecharacterization facility (NCF). Magnetronsputtering vaporizes materials from any target ina high pressure background of inert or reactivegases to form nanoparticles. The particles arethen transported to the UHV-HRTEM for

observation. In situ thermal treatment or gasexposure is also possible. Figure b, e.g., showsthe effect of oxygen exposure on the sinteringand morphology of Cu nanoparticles. Noticethat the clean nanoparticles are sintered, even atambient temperature. The MD simulations,illustrated in Figure c, show that sinteringoccurs by plastic deformation of the particles,owing to their large surface stresses. Asurprising feature is that even at roomtemperature the particles rotate to form a lowenergy (twin) boundary.

SIGNIFICANCE -- The development of theNCF now makes it possible to investigate a widerange of problems involving nanoparticlesystems. Most promising is the capability for insitu studies of nanoparticle interactions withsubstrates and the effects of gas-dosing onnanoparticle structure, in the context of catalysis.The versatility of the NCF makes studies ofvarious nanoparticle systems possible. Our firstinvestigations of sintering illustrate that the hugestresses developed at the nanoparticle contactslead to sintering and other structural changes inthe particles. This leads to a wealth ofopportunities for building nanoparticle-substrateheterostructures, where the structure is tuned forspecific applications.

The NCF facility is expected to soon become afocus for collaborations in the Center especiallywith the work on magnetic nanoparticles andcatalysis.

CONTACT: Robert Averback, University of Illinois/UrbanaPhone: (217) 333-4302 Fax: (217) 244-2278 E-mail: [email protected]

32


Nanoscalc Materials for Energy Applications

-- c

Figure a: Experimental Apparatus A) Production chamber.B) Transport Assembly. C) UHV-HRTEM chamber.

Figure b: Oxygen effects on copper nano-particle morphology - oxygenexposure partial pressure increases from 0 to I x 10-3 torr from left to right.

Figure c: Molecular dynamics simulation of twocopper particles sintering at time = 5, 20, and 40 ps.

33


Polyphosphazene Membranes for Separation and Speciation

E.S. Peterson, Idaho National Engineering LaboratoryW.D. Samuels and M. A. Josowicz, Pacific Northwest National Laboratory

Motivation -- Phosphazene polymers andpolymer blends offer enhanced radiation,thermal and chemical stability over conventionalorganic polymer systems rendering them usefulfor the development of molecularly specificsensors or for applications as separationmembranes in hostile environments. Suchmaterials can be custom tailored to a specificpurpose while maintaining both chemical andphysical stability in harsh environments. Inaddition, these polymer systems have beenfound to exhibit unrivaled versatility in threekey separation areas: pervaporation, vaportransport, and nanofiltration. New routes to thepreparation of these materials in the form oflayers or thin films which support enhanceddiffusion have recently been developed.

Accomplishment Inorganic polymermembranes and molecularly mixed inorganicorganic porous polymer films were prepared bymeans of a thermal ring-opening polymerizationroute or a recently patented anodicelectropolymerization process. Molecularblends of polyphosphazene separationmembranes, -[N=PRR']x' have been prepared,where the substituent groups R and R' representboth hydrophilic and hydrophobic moieties.The hydrophobic groups impart good filmforming characteristics to the blend, while thehydrophilic groups enhance water diffusionthrough the film thereby improving separationefficiency. Figure 1 illustrates a solventswelling mechanism whereby an increase in

flow through the membrane is achieved. Analternate method for enhancing flow requiresdevelopment of a homogeneous pore structurein the membrane. This has been achievedrecently using anodic electropolymerization toassemble a molecularly mixed quinonephosphazene film at an electrode surface ascurrent is passed through a solution containingone of several quinone type aromatics and acyclic phosphazene trimer. By varyingprecursor concentrations, electrode potential,and current, a porous microstructure can beengineered into the film as shown in Figure 2.Both synthetic routes produce membranes whichexhibit high diffusivity.

Significance -- Phosphazene polymers andpolymer blends offer significant advantagesover other separation systems with respect todurability in harsh ambient environments. Forexample, fuel gases such as methane can beseparated readily from acid gases (C02, H2S,S02) at high temperatures with little measurabledegradation to the membrane or separationefficiency over time. Possible applications inthe fuels industry as well as in activitiesrequiring clean-up of combustion emissions areevident. In addition, such membranes areeffective at removing chlorinated hydrocarbonsfrom both air and water waste streams.Appropriately tailored inorganic polymermembrane materials also may find applicationsas exchange media for clean-up of coolant waterin electric power generating plants.

Contact: Greg Exarhos, Pacific Northwest National LaboratoryPhone: (509) 375-2440 Fax: (509) 375-2186 E-mail: [email protected]

34



Physical Description ofDynamic Membrane Mechanism

(1)Dry dense polymerfilm on a support

(2)Solvent swollenpolymer film on asupport

swoUenfilm

(3)Solvent swollenpolymer film on asupport with avacuum applied

thin active layer

veccern

Figure l. Vacuum filtration enhances fluid transport through the membrane

Figure 2. Atomic Force Microscope image showing the porous microstructure of amolecular polymer blend (quinone + hexachlorophosphazene trimer) formed bythe anodic e1ectropolymerization process.

35


Polymer-Based Materials With Controlled Porosity

R.W. Pekala, Lawrence Livermore National LaboratoryA. J. Hurd, Sandia National Laboratories

W.D. Samuels, Pacific Northwest National Laboratory

Motivation -- Advanced processing routes forthe synthesis of low density ceramic andpolymer based materials having continuous andcontrolled porosity at the nanometer level havestimulated industrial interest as a result of theatypical thermal, acoustic, optical, and electricalproperties which these tailored materials exhibit.From a thermal conductivity standpoint, thepresence of a homogeneous distribution ofnano-pores in a support matrix, which can bederived through a super-critical fluid processingroute, affords a significant decrease in heat flow,rendering such materials superior for insulationapplications. To improve the mechanicalproperties of these otherwise fragile monolithsfor building applications, polymer blend andpolymer-reinforced ceramic composites havebeen developed.

Accomplishment -- Different compositions ofresorcinol-formaldehyde (RlF) polymer gelshave been prepared by a polycondensation routeusing selected additions of a polymerizationcatalyst (C). The RIC ratio was found to alterthe surface area, pore size, and attendantmechanical properties of the derived aerogelmaterials. Competing effects of particleinterconnectivity and pore size on measuredthermal conductivity are illustrated in the topfigure. A broad minimum is observed since thematrix conductivity increases monotonicallywith density while the gaseous and radiativecontributions to the conductivity decrease withdensity. Similar results are observed in porousmaterials prepared by means of supercriticalfluid processing of silica aerogels in which the

precursor alkoxide containing solutions alsocontain fluorinated phosphazene polymers. Theadded polymer coats the porous matriximparting enhanced mechanical stability. In thedried material, egress of water through theporous matrix is hampered as a result of thehydrophobicity of the polymer coating.

Once thought impossible to produce, aerogelfilms became a reality via an ambient-pressureaerogel process developed by Center researchersthat does not require autoclave conditions (seeFig.). The measurement of aerogel film thermalproperties is proving very difficult though thestructure of the films appears to mimic bulk.

Significance -- Some 40% of the total energyused in the United States can be apportioned tothe building industry. Heat loss from or heategress into a dwelling is mitigated by thermalinsulation which is integrated into the walls,roofs, and windows. In addition appliances,

... pipes, and both heating and cooling systemsrequire insulation in order to curb energy usage.Improvement to existing thermal insulationmaterial can result in a marked increase inenergy efficiency and associated reduction intotal energy cost. Materials having controlledpore nanostructures have been shown tosignificantly reduce thermal conductivity andare candidates for next generation insulationmaterials. Continuing activities in this Projectare focused on the development of advancedprocessing methods to implement batchprocessing of these materials.

Contact: Greg Exarhos, Pacific Northwest National LaboratoryPhone: (509) 375-2440 Fax: (509) 375-2186 E-mail: [email protected]

36


Microstructural Engineering with Polvmers

~

:.::'7e 0.020

.!,.,...0.015>;::

U::l"0<:0 0.010uiOe~

'" 0.005s:-0

0 100 200

density (kg m-3)

0= RIC 50d- RIC 200O. RIC 300

300

Varia tion in thermal conductivity as a function of density for nanoporous resorcinol-formaldehyde polymers preparedusing several resorcinol/catalyst ratios.

SS film ~

~ 2 2~

~ ~

"~

;.§ "~.. eo.9 .9.a o 0 .a~ ~Q.

"Q.

" "~ 8~

-2e::

is0 2

Film thickness (um)

e 0.1

" 3';;' 5 5;§ ".. ;.§.9 ...a .9.a~ 0 0

~Q.

"Q.

u "3 -38

~ -3 e::DS film

is

Ellipsometric images of a zone near the drying line during processing of an aerogel film. The standard sol (SS)displays normal drying whi le the deriva tized sol (DS) that makes aerogel films exhibits "springback" in which thethickness rebounds duri ng drying.

37


Synthesis and Characterization ofa New Family ofAnion Complexing Agents

H. S. Lee, X. Q. Yang and J. McBreen, Brookhaven National Laboratory

Motivation -- Improved battery performance inlithium based systems can be realized if anelectrolyte can be developed which increasesboth the extent of ion dissociation and the Li+transport number. What is needed are anioncomplexing agents that are stable in the batteryenvironment. Although many cation-selectivecrown ethers and cryptands are known, only afew host compounds bind anions selectively.The field of anion receptors is a very active fieldof research with most of the work aimed atmolecular recognition to mimic how ionbinding protons control ion transport inbiological membranes. Present anion receptorsare based either on positively charged sites,hydrogen bonding, or Lewis acid metal centers.None of these are suitable for use in batteries orelectrochemical sensors.

Accomplishment -- A new family of stableanion complexing agents has been synthesized.These are either cyclic, linear or branched azacompounds in which the H on the N is replacedby an electron withdrawing group such asCF3S02, CF3CO, CN or S02CN. They functionwith anions in much the same way as crownethers act with cations. This has been confirmedin extensive x-ray absorption (XAS) studies inLiBr, LiCI at the respective Br and CI Kedgesand in LiI at the I L, edge. These agents greatlyincrease the conductivity of non-aqueous

electrolytes. For instance, the conductivity of0.1 M LiBr in tetrahydrofuran (THF) isincreased by two orders of magnitude on theaddition of 0.1 M of a cyclic aza compound.

Significance -- The key to improving batteryperformance is to increase both ion dissociationand the u' transport number in the electrolyte.In the past cation complexing agents such ascrown ethers and cryptands have been used asadditives in polymer electrolytes. The new azacompounds are preferable since they complexanions and will increase the transport number

+for Li ions. Unlike most known anioncomplexing agents, which are based onhydrogen bonding, these complexing agents arestable in the presence of lithium. Hence theycan be considered for use in lithium batteries.The XAS results indicate that the ability of thelinear aza compounds to complex anionsdepends on the anion size and the chain lengthof the aza compound. Thus it may be possibleto tailor complexes for specific anions. Thiscould have application in molecular recognitionin the biomedical field and in sensors. Also itmay be possible to devise polymer electrolytesbased on anion complexation. These wouldhave very high transference numbers for thecation.

Contact: James McBreen, Brookhaven National LaboratoryPhone: (516) 282-4513 Fax: (516) 282-4071 E-mail: [email protected]

38

http://bnl.gov


uo in THF

......... (a)tI)+.0.-t::::J

.0 R rI R\ /g uo + eN N)

N N

t::(b) IU\

.9-<-J

f:- R, rr»; /R0tI)

LiCl + c: N).0<J:: N

(c) R. R.

N«()

LiCI +N N NR. R. R.

(d)

2790 2810 2830 2850 2870Energy (eV)

Figure showing near edge x-ray absorption (NEXAFS) at the CI K edge for LiCI solutions in THF;(a) 0.2 M LiCI and in the same solution with additions of cyclic (b), linear (c) and branched (d) azacomplexing agents (R = CF3SOr ), aza structures are shown in the figure. The absorption edge at2830 eV is due to x-rays having sufficient energy to excite C1 Is core electrons. The featuresbetween 2830 and 2840 eV are due to transitions of the Is electrons into empty 3p states. In asymmetrical environment ((a) and (b)) the 3p states are degenerate and there is a single peak. WhenC1 is complexed with the linear or branched aza compounds ((c) and (d)) the degeneracy is lifted andthe peaks are split.

39


A Metallurgical Approach Toward Alloying in Rare Earth Permanent MagnetSystems

R. W. McCallum, M. J. Kramer, K. W. Dennis, Ames LaboratoryD. J. Branagan, Idaho Engineering National Laboratory

Motivation Bulk magnetic materialscomprise a large economically important classof materials. For a ferromagnetic material, theresponse of the bulk magnetization to anexternal field is governed by the creation andmotion of domain walls which separate thematerial into regions with different directions ofmagnetization. If the domain walls are hard tonucleate or are strongly pinned to defects orgrain boundaries in the material, large magneticfields are required to change the netmagnetization and the material is consideredmagnetically hard. The dependence of domainwall formation and motion on extrinsic variablesmeans that microstructure plays a crucial role indetermining the bulk magnetic properties ofthese materials. The microstructure of amaterial may be controlled through the use ofmetallurgical additions. We have studied therole of a number of additions in thecommercially important Nd2Fe14B basedpermanent magnets.

Accomplishments Starting withstochiometric Nd2Fe14B, the role of transitionmetal carbide additions in both the directsolidification of the hard magnetic phase fromthe melt and the crystallization of the hard phasefrom an amorphous precursor has been studied.Compound additions of group IVA, VA, and

VIA transition metals along with carbon wereadded to the Nd2Fe14B system. Transition metalcarbide (TMC) formation was found for thegroup IVA and VA transition metals. All theTMC precipitates observed formed with a one toone stoichiometry. The most promisingaddition was found to be TiC which influencedthe microstructure in a number of ways. First,the solidification rates necessary to obtain anamorphous precursor were drastically reduced.In addition, the crystallization temperature ofthe glass was increased, resulting in a finer,more uniform microstructure withcorrespondingly enhanced magnetic properties.Finally the TiC precipitates which formedduring crystallization served to pin the grainboundaries during high temperature annealingso that good magnetic properties are maintainedover a higher processing temperature range.

Significance -- The ability to produce materialswith good permanent magnetic properties at lowsolidification rates opens up a variety ofpossible processing routes for producingpowders for bonded magnets. Not only maysuch processes as gas atomization be used toproduce powders, but the process parameters arerelaxed so that high temperatures can be used toform more compact and durable parts with anoptimum microstructure.

Contact: R. William McCallum, Ames LaboratoryPhone: (515) 294-4736 FAX: (515) 294-4291 E-mail: [email protected]

40



+ base 10

.X=2 TiC

~X=6 TiC

.X=2C8

e X=6 C

.X=2 Ti 6

i~tsX=6 Tiffi ~

4 ~~---

2

0-12 -10 -8 -6 -4 -2 0

Thousands

Magnetic Field (H)

18

16

X

~14

ctlE <.9 12I ~ 10co ...... TiC

8

60 2 4 6

X=

Demagnetization curves and maximum energy product for (Nd2/17FeI41l7Bl/]7)lOO-2x + Ti.C, ,(Nd2/17FeI4/17Bl/]7)lOO-x + Ti, , and (Nd2/17FeI4/17B1/17)lOO-x + Cx. The alloys were melt spun at40m/sec to obtain an amorphous precursor and then annealed at 650°C for 1 hour to crystallize thematerial. For small concentrations, all additives produced enhanced nucleation, and hence finergrain size and better magnetic properties. For higher concentrations, the individual addition of Ti orC resulted in a loss of the hard magnetic phase while the TiC addition maintains the phase.

41


Importance ofGrain Boundary Composition and Structure to Magnetic ReversalProcesses in Advanced Permanent Magnets

L. H. Lewis, Yimei Zhu and D. O. Welch, Brookhaven National Laboratory

Motivation -- While magnets based on therare-earth (RE)/transition metal compounds withthe composition RE2Fe14B (~ RE12FeS2B6) offerexcellent room-temperature properties, atelevated temperatures they are plagued by a lowresistance to demagnetization (coercivity),among other difficulties. For bonded magnetsmade from RE2Fe14B this means that operatingconditions are restricted to temperatures in therange 140a C - 180a C, which limits their use inmany motor applications. To improve themagnetic properties at elevated temperatures itis necessary to obtain a detailed understandingof the interactions, both exchange andmagnetostatic, that result from the magnet'smicrostructure, which controls the coercivityand its temperature dependence, and to use suchunderstanding to engineer improved magnets.

Accomplishment -- High-resolution electronmicroscopy and nanoscale chemical analysisdemonstrated that the nature of the intergranular phases depends on the character of theboundary between the grains and detected thepresence of a previously unknown amorphous,iron-rich grain boundary phase in two thermomechanically-processed rare-earth magnets withbulk compositions Nd13.75Fes025B6 andPr13.75FesO.25B6' The intergranular phase occursprimarily on grain boundaries parallel to thetetragonal RE2Fe14B phase c-axis and has an

average width of 8 - 12 A in the Nd-basedmagnet and 15 - 20 A in the Pr-based magnet.The microstructural and nanocompositionalevidence, combined with initial magnetizationcurves and the character of coercivitydevelopment at elevated temperatures suggestthat the dominant reversal mechanism in thesemagnets is nucleation of reversed domains in theiron-rich intergranular region which likelypossesses reduced anisotropy. The presence ofthis inter-granular phase is also likely topromote exchange coupling between adjacentgrains, which can reduce the coercivity belowthat for uncoupled grains.

Significance -- This research helps to clarifysome of the reasons that the coercivity of polycrystalline RE2Fe14B permanent magnets isreduced below its theoretical limit and why it issensitive to temperature. The conclusionsreached suggest methods to improve thetemperature coefficient of coercivity in dieupset RE2Fe14B magnets. An idealmicrostructure for a nucleation-type magnetconsists of grains of a magnetically hard phasethat are isolated from interacting via exchangeinteractions with one another. Therefore,segregation of non-magnetic atoms to the grainboundary phase via metallurgical manipulationwould produce higher-coercivity magnets.

Contact: Laura H. Lewis, Brookhaven National LaboratoryPhone: (516) 344-2861 Fax: (516) 344-4071 E-mail: [email protected]

42


High-resolution images of large-ang le grain boundaries in (a) Nd 13."Feso."B6 and (b), (c) Prl3 ." Feso."B6. Anamorphized intergranular phase is visible in (a) and (b) but not in (c) . The boundary planes are as follows: (a)(lJOlA//(OOI)B; (b) (109), //(00 1)B; (e) (OO l lAi/(OOI)B. GB indica tes the position of the grain boundaries.dl l2 , dll o, dOO I ' l and d fX)] indicate the spacings of the corresponding latt ice planes.

43


Alloy Design ofNd2Fe14B Permanent Magnets

J. A. Horton and D. S. Easton, Oak Ridge National LaboratoryJ. W. Herchenroeder, Magnequench International, Inc.

Motivation -- Permanent magnet motorsusing Nd2Fe14B magnets are generally lighterand more energy efficient than the motors theyreplace. New applications include appliancemotors, automotive accessory motors such asan on-demand electric power steering pumpand electric vehicle wheel motors. Poorfracture resistance is a major concern forindustrial production, handling, and use. Thedemanding electric vehicle wheel motorapplication requires an improvement in thefracture stress before commercial productionis feasible. The hard magnetic phase,Nd2Fe14B, has a large unit cell, low symmetry,and no deformation modes. In order toimprove the toughness, techniques derivedfrom toughening of ceramics and fromalleviating the brittleness of other intermetallicalloys are being applied to the Nd2Fe14Bmagnets.

Accomplishment -- Measurements of thefracture toughnesses of commercial magnetsranged from 2.5 to 5.5 MPa-Vm (see table).The measurements are sensitive enough todistinguish between manufacturer andprocessing method. A chevron notch, 3-pointbend test that is used to test ceramics andother brittle intermetallics was utilized.Because of the large deliberate notch, this testminimizes the effect of flaws and surfacecracks and yields a toughness value

representative of the innate properties of thematerial. In addition, fewer tests are neededto result in reproducible values as compared toa strength test in which surface flaws andcracks are present. Toughness itself isessentially a measure of the absorbed energyduring crack propagation. For the testsreported here, with the specimen geometryused, and with toughnesses greater than 3MPa-Vm, controlled crack propagationoccurred. For the specimens with the lowervalues, catastrophic failure occurred and adifferent method was used to calculate thetoughness. (For comparison purposes,alumina has a toughness of 3 to 4 MPa-Vm.)

Significance Demonstrating that aquantitative measurement can yieldreproducible results and can distinguishbetween manufacturers, and between the twomain processing methods is an important firststep to improve the mechanical properties ofthe Nd2Fe14B magnets. Through this Center'sactivities a common site will be used to makethese measurements for magnets produced byother laboratories using different methods ofsynthesizing and producing magnets. Acorrelation of toughness with microstructureshould lead to improvements of mechanicalproperties without degrading the magneticproperties.

Contact: Joe Horton, Oak Ridge National LaboratoryPhone: (423)574-5575 Fax: (423)574-7659 E-mail: [email protected]

44



Fracture Toughness, K IO of Commercial Nd2Fe\4B Permanent Magnets*

Process Manufacturer Grade or K\c. MPa"m # of testsenergy product

Spin Cast & Hot Magnequench El 2.5 ± 0.2f 8Pressed MQ2

Spin Cast & Die Magnequench E l 2.7 ± 0.5f 6Upset MQ3

Spin Cast & Die Magnequench H 4.4 ± 0.3 8Upset MQ3**

Crushed & ShinE tsu 45 3.9 ± 0.2 7Sintered

Cr ushed & Sumitomo 40 4.4 ± 0.2 6Sintered

Crushed & Vacuumschmelze 42 5.5 ± 0.2 8Sinte red

• All specimens, 40 mm span. "DIe upset by Daido Steel Co.' Calcu lated by peak load method, others calculated by area method for controlled crack growth .

Intergranular fracture surface in a crushed andsintered Nd2Fet4B magnet showing thebehavio r of the second phase particles.

10 ~m

A chevron notch, 3-point bend test is able to distingui sh the fracture toughness level betweenmanufacturer and production method for Nd2Fet4B permanent magnets . Baseline propertymeasurements correlated with microstructure and the abili ty to reliably track small changes in thetoughness is essential to improving the mechanical properties of the Nd2Fel4B magnets. Indemanding applications such as electric vehicle wheel motors, an improvement in the mechanicalproperties is needed before commercial production is feasible.

45


Low-Stress, Ultra-Hard, Low-Wear Carbon Films

J. W. Ager III, S. Anders, A. Anders, I. G. Brown, Lawrence Berkeley National LaboratoryK. Walter and M. Nastassi, Los Alamos National Laboratory

Motivation -- Researchers at LBNL andelsewhere have discovered that thin, ultrahardfilms of carbon can be synthesized by depositingcarbon ions from the gas phase under conditionsin which the ion energy is controlled precisely.Film with a hardness of up to 60% of that ofdiamond (the hardest known material) and 85%diamond-like Sp3 bonding can be made at anoptimal ion energy of 100 - 200 eV. However,substantial residual stresses in excess of 10 GPaappear to be an intrinsic part of the growthprocess. These large compressive growthstresses adversely affect the adhesion of thefilms such that they often fail by delamination inwear tests. Furthermore, the intrinsic film stresslimits the thickness to which the ultrahard filmcan be grown; above a critical thickness that isonly 50 nm for the hardest films, the stresscauses the film to delaminate from the substrate.

Accomplishment -- LBNL researchers have adeveloped a method to independently controlhardness and stress in ultrahard amorphouscarbon films.

Aided by modeling provided by researchers atLos Alamos National Laboratory, the LBNLresearchers designed multilayer structures andgrew them by cathodic arc deposition. Thestructures consist of an initial thin layer of lowstress material grown with a 2000 V substratebias which mixes carbon ions into the substratefor enhanced adhesion, followed by alternatingharder (high stress) and softer (low stress) layers

(see figure). The relative thicknesses of the hardand soft layers can be controlled and optimizedfor best performance. It was found that thehardness of the multilayer structures is theweighted average of the constituent layers (i.e.alternating layers of the same thickness of ahard and a soft film would give a hardnesshalfway between the layer hardnesses).However, the observed stress was found to beapproximately 50% lower than the weightedaverage of the stresses. Films with lower stressare expected to have superior adhesion. Thewear rates of the multilayer films have beenmeasured by Center researchers at LANL andare exceedingly low (see figure). The averagewear rate of the multilayer films was 10-1 OOOxless than that of other hard coatings. Themultilayer film that consists of equal layers ofhard and soft material had an average wear ratethat was 5-10x less than that of the monolithichard and soft films.

Significance -- To date, only very simplemultilayer structures have been explored. It ispossible that more sophisticated structures willproduce even lower stress films, while retainingthe film hardness and low wear rate. Notably,multilayer structures are no more costly toproduce in a cathodic arc deposition chamberthan are single layer films. Applications for themultilayer carbon films includes coating forcomputer hard disk and read-write heads,tooling, and wear components in automobiles.

Contact: Joel W. Ager III, Lawrence Berkeley National LaboratoryPhone: (510) 486-6715 Fax: (510) 486-4114 E-mail: [email protected]

46



T200 nm hard, high stress layer

soft, low stress layer

Wear rates of Protective films

10-3 r----------- - --,--- ----------,

IOther hard coatings

IMonolithic

LBNL monolithic andmultilayer films

Monolithic

10""

10""

0% 10% 50% 90% 100% a-c :H a-c TIC PVC TI (B,N)% of soft phase Cr·N

in multilayer carbon film

Alternating layers of softer and harder amorphous carbon films are deposited using a cathodic arcprocess to make multilayer films. The multilayer film has 50% less stress than expected from themonolithic hard and soft films; this leads directly to increased adhesion in the multilayer films. Thewear rates of the multilayer films were measured under a variety of conditions and compared to otherhard films. The average wear rates of the multilayer films are exceedingly low -- e.g., more than100x lower than titanium carbide . The optimal multi layer film has equal layers of hard and softmaterial - its average wear rate is 5-1Ox lower than of the monolithic hard and soft films.

47


Synthesis ofHard Boron Suboxide Thin Films UsingElectron Cyclotron Microwave Plasmas

s. M. Gorbatkin, R. L. Rhoades, C. Doughty, K. B. Alexander, T. Y. Tsui, and W. C. OliverOak Ridge National Laboratory

D. L. Medlin, Sandia National Laboratories, LivermoreA. Jankowski, Lawrence Livermore National Laboratory

Motivation Boron suboxides (B/O atomratios > 1) are an interesting class of hardmaterials that, similar to the cases of diamondand cubic-BN, have been synthesized in variousforms for over 25 years using high temperaturesand pressures. They are hard and resistant tothermal and chemical degradation, and somephases have been found to scratch the (111)diamond face and easily wear down a diamondtip. Furthermore, boron oxide also exists in aform known to have excellent lubricationproperties, and thus it may be possible tosynthesize a thin film materials system which iscomprised of only two elements and providesboth a hard underlying coating with alubricating overlayer. Despite such encouragingresults, little research has been directed towardsboron suboxide thin films.

Accomplishment Unique thin films ofboron suboxide have been fabricated which havefriction coefficients of 0.13, hardnesses of 30GPa, near that of sapphire, and significantlyhigher hardness/modulus ratios of 0.10compared to 0.068 for sapphire. The hardness/modulus ratio indicates how much strain thematerial can accommodate elastically, and ahigh value is desirable for protective materials.The technique used to fabricate the thin films atOak Ridge National Laboratory couples eitheran effusion cell or sputter target, which serves asa source of pure boron, to an argon/oxygen

plasma formed using electron cyclotronresonance. Typical ion energies are less than 50eV, with approximately 100 eV incident on thegrowing film surface per deposited atom.

Transmission electron microscopy (TEM)studies at Sandia National Laboratories revealan amorphous structure for the hard films, withthe nucleation of nanometer size crystallineregions present in softer films with higheroxygen levels. Comparisons using TEM tosputtered pure boron films prepared at LawrenceLivermore National Laboratory, combined withelectron energy loss spectroscopy, reveal similarlocal structural environments for the boronatoms in all films.

Significance Any application requmngresistance to friction and wear is a potential enduse of these films, including power generation,machining, magnetic data storage, and optics.The development of amorphous, boron suboxidethin films represents the discovery of analternative to amorphous hard carbon, alreadywidely used in the microelectronics industry forhard disk coatings. Development of a detailedunderstanding of the formation mechanisms ofhard, amorphous boron suboxide hard coatingmay also provide some clues on how to obtaineven harder phases, comparable to those whichscratched diamond.

Contact: Steven M. Gorbatkin, Oak Ridge National LaboratoryPhone: (423) 576-6502 Fax: (423) 576-8135 E-mail: [email protected]

48



The figure shows a high resolution transmission electron microscopy (HRTEM) image of a boronsuboxide hard coating near the interface with a silicon substrate. The inset shows a selected areaelectron diffraction pattern obtained from the film. The results show that the film is amorphous anddevoid of long range structural order. Our current electron microscopy efforts are focused onapplying diffraction and spectroscopic methods to determi ne the short-range order in theseamorphous films with the goal of determining the influence of oxygen on the film structure andproperties.

49

Mechanically Reliable Surface Oxides for High-Temperature Corrosion Resistance

Oxidation Performance ofPlasma-Synthesized Alumina Coatings *

I. G. Brown, P. Y. Hou, Lawrence Berkeley National LaboratoryP. F. Tortorelli, K. B. Alexander, Oak Ridge National Laboratory

Motivation - Minimizing the deleterious effectsof elevated-temperature oxidizing environments isa key aspect of the development of hightemperature materials. The resistance to suchreactions is best afforded by the formation ofstable surface oxides that are slow growing,sound, and adherent to the substrate and/or by thedeposition of coatings that contain or developoxides with similar characteristics. This latterapproach can be used as an alternative to relianceon a protective surface oxide grown duringexposure of the substrate material to a hightemperature oxidizing environment. The use ofplasma synthesis to deposit thin alumina coatingson selected alloys is being examined as a methodof tailoring the characteristics of surface oxides toestablish exceptional high-temperature corrosionresistance. This technique was chosen because ithas the potential to deposit dense films withstrong adhesion to the substrate.

Accomplishment - Plasma synthesis has beenused to deposit thin (0.5 - 1 urn) alumina on FeAI-Cr(-Zr) alloys. Preliminary experiments haveshown that at 1000°C, the overall oxidation rate islower than that of the matching uncoated alloy.During exposure in the oxidizing environment,oxide was formed between the plasmasynthesized coating and the substrate on which itwas deposited, thereby indicating transport ofoxygen ions through the original alumina, whichwas amorphous as deposited but slowlytransformed to a-AI20 3. The combined thicknessof the thermally formed and deposited surfaceoxides was less than that of the alumina grown onthe exposed, but uncoated Fe-AI-Cr for the sameamount of time. It appeared that the presence of

the alumina deposit effectively eliminated therapid reaction rate associated with the transientoxidation stage that occurs when alumina isgrown on an uncoated alloy surface at hightemperature. Because the thermally grownalumina forms under the deposit, the adherence ofthe coating is controlled by the strength of theinterface between the alloy and the oxide thatdevelops during exposure to the oxidizingenvironment. Adhesion at this interface has beenshown to be improved when Zr is present in thesubstrate.

Significance - The demonstration that a plasmasynthesized Al20 3 coating can provide hightemperature oxidation protection that isequivalent, or even superior, to that proffered bythermally grown alumina is an important first stepin synthesizing improved surface oxides for hightemperature corrosion resistance. Further work isrequired to establish whether the observedimprovement in oxidation resistance is maintainedat extended exposure times and to determine howto best control deposition parameters to achieveoptimal microstructures and properties. Progressin this latter area is linked to other Projectactivities aimed at establishing the relationshipsamong composition and properties of the substrateand scale/coating adherence, damage tolerance,and micromechanical properties. In this respect,the finding that coating adherence is controlled bythe interfacial bonding between the alloy andthermally grown oxide allows exploitation of theknowledge base that exists for controlling nativeoxide adhesion by manipulation of substratecomposition.

Contact: Peggy Hou, Lawrence Berkeley National LaboratoryPhone: (510) 486-5560 FAX: (510) 486-4995 E-mail: [email protected]

'Research sponsored by Division of Materials Science and Office of Fossil Energy, U. S. Department of Energy, and the Electric Power Research Institute.

52



a

/ \

b 2~

Cross-sections of uncoated (a) and coated (b) Fe-28 al.% Al - 5% Cr - 0.1 % Zr exposed for 96 h at1000°C in air. The surface in (b) was first coated with 0.6 urn of alumina deposited by plasmasynthesis yet showed a thinner overall oxide layer after exposure; new oxide formed at the interfacebetween the coating and substrate. The bright features indicated by arrows are zirconia-richparticles. The indentations shown are from micromechanical characterization of the surface oxidesand substrate.

53


*Impurity Segregation To Alumina-FeAICr Alloy Interfaces

P. Y. Hou, R. M. Cannon, Lawrence Berkeley LaboratoryK. Natesan, B. W. Veal,Argonne National Laboratory

Motivation - The protective surface oxides thatprovide environmental protection of materials athigh temperatures must not only be chemicallystable but also adherent to the substrate onwhich they are grown or deposited. Theeffective adhesion at the oxide-substrateinterface is determined not only by theatomistics of bonding between the principalelements, but also by the interfacial structure,geometric factors, and the nature and extent ofdefects. The presence of impurities at theinterface can influence bonding and defectdevelopment as well as reduce the interfacialfracture energy at the lower temperatures wherespallation of oxide layers occurs. The chemicalcharacterization of the interface is therefore animportant issue in understanding therelationships between oxidation resistance,microstructure, and the mechanical behavior andadhesion of protective surface oxides. Theselinks serve as the foundation of the effort todevelop mechanically reliable scales andcoatings for high-temperature corrosionresistance. One component of the interfacialcharacterization effort is the use of Augerelectron spectroscopy (AES) to probe thechemistry at oxide-metal interface producedduring the high-temperature oxidation ofFeAICr alloys to form surface alumina. Theissue of segregation to alumina-alloy interfacesis not only of great current interest for oxidationprotection but also for the durability of thermalbarrier coatings, which often are designed withan underlying alumina-forming bond coat layer.

Accomplishment - For Al20 3 thermally grownon certain Fe-28AI-5Cr (at.%) alloys, elevated

levels of sulfur were detected at the oxide-metalinterface by AES. Such observations weremade by in-situ monitoring of a slowly growingscale as well as by ambient-temperature analysisof as-grown oxides. In the latter technique, anoxidized specimen is inserted into a scanningAES system after which the oxide is scratchedto expose the oxide-metal interfacial region foranalysis. No other impurities were consistentlyobserved at these interfaces, nor was asignificant amount of sulfur found in the bulk ofthe surface oxide. The presence of sulfur at thisinterface can be correlated with poor scaleadhesion: in each case where sulfur wasdetected, extensive spallation of the thermallygrown oxide occurred. On the other hand, thescratch method revealed the absence of sulfurfor the case of an alloy (Fe-28AI-5Cr withO.1Zr) that has been observed to have goodadhesion of its thermally grown alumina scale.

Significance - Despite a significant amount ofstudy of Fe-AI-Cr alloys over recent years, thepresent observations related to sulfur at theoxide-metal interface and the correlation of itspresence with scale adhesion were the first suchresults for this system. These findings are alsorelevant to other Project work on aluminacoatings on FeAICr alloys: Al20 3 formsunderneath the deposit during high-temperatureoxidation and its ability to bond to the substratedetermines whether the coating is adherent.This work is the initial step in defining the roleof substrate compositional manipulations (suchas the addition of Zr) on impurity segregationand the modification of interface structure indeveloping reliable surface oxides.

Contact: Peggy Hou, Lawrence Berkeley LaboratoryPhone: (510) 486-5560 FAX: (510) 486-4995 E-mail: [email protected]

'Research sponsored by Division of Materials Science and Office of Fossil Energy, U. S. Department of Energy, and the Electric Power Research Institute.

54



10

9

8

:!::7

"0 6W'" 5-W- '"Z

3

2

1

0

e

AI

Fe

200

10

9

8

7:t:::c 6

W 5'"- Cw- 4Z

3 AI2 Fe

0200

4110 600 800

Kinetic Energy, eV

(a)

Fe

400 600 800

Kinetic Energy, eV

(b)

1000

1000

1200

1200

1400

1400

Auger electron spectra from specimens of Fe - 28 at.% Al - 5% Cr (a) and Fe - 28 at.% Al - 5% Cr 0.1% Zr (b) that were first oxidized at 1000 and 1200°C, respectively, then scratched to expose thealumina-alloy interfaces for analysis. The alloy with sulfur at the oxide-metal interface showed atendency for extensive spallation of the thermally grown scale during cooling from the oxidationtemperature. The alloy containing Zr did not show sulfur segregation and displayed good scaleadhesion during thermal cycling.

55


*Strain Development In Thin, Thermally Grown Alumina Layers

B. W. Veal, M. Grimsditch, K. Natesan, Argonne National LaboratoryR. L. Williamson, Idaho National Engineering Laboratory

R. M. Cannon, P. Y. Hou, Lawrence Berkeley National Laboratory

Motivation - Protection from environmentalcorrosion and degradation is required to fullyexploit the potential of advanced high-temperaturematerials designed to improve energy efficiencyand minimize environmental impact. Theresistance to such reactions is best afforded by theformation or deposition of a stable, slowlygrowing surface oxide that retains its mechanicalintegrity in the presence of stresses imposed athigh temperature and during thermal transients. Amajor barrier to understanding and modeling howoxide coatings and scales accommodate suchstresses is the lack of accurate and sensitivemethods to characterize the strain in these thin(0.5 - 3 urn) surface layers. To this end,fluorescence spectroscopy is being used tomeasure strains in alumina layers on Fe-Al-Cralloys. For comparison to the measured values,finite element techniques are being used tocalculate strains for Al20 3 layers on thesesubstrates based on differences in thermalexpansion coefficients.

Accomplishment - Compressive strains in thinthermally grown Al20 3 were measured at roomtemperature by observing the peak shift of the Crfluorescence line as the growth temperature (and,consequently, scale thickness) was varied. This"ruby line" fluorescence is caused by theexcitation of a very small amount of substitutedchromium in the alumina. Based on the knownpiezospectroscopic coefficients and elasticconstants for Al20 3, the observed fluorescenceshifts are translated into measurements of strain inthe scales. The magnitude of the measured strainswere in general agreement with the predicted

values calculated from the differences in therespective coefficients of thermal expansion(CTEs) of the oxide layer and alloy substrate. Theamount of strain (peak shift) increased withincreasing reaction temperature until crackingand/or spallation relaxed the stress; at that point,the ruby lines were again located at wavelengthscharacteristic of an unstrained lattice. Themaximum strain measured in this way was foundto depend on the composition of the alloysubstrate. It was greater for the oxide grown onFe-28Al-5Cr-0.1Zr (at %) than for the one on Fe28Al-5Cr. This finding correlates with resultsfrom work showing that the presence of Zrsubstantially improves scale adherence for the Fe28Al-5Cr system.

Significance - This use of ruby fluorescence is animportant development as it forms a keycomponent of determining the overall mechanicalreliability of protective surface oxides that offercorrosion resistance at high temperatures. Theagreement between measured strains andcalculated values based on differences in theCTEs of the alumina and substrate indicate that amajor (but possibly not only) contribution tostress generation in these layers comes fromthermal cycling; it also validates the use of finiteelement techniques to model strain developmentunder a variety of geometric and structuralconditions. The demonstration that the maximumstrains correlate with the onset of observed scalefailure indicates that this technique will bevaluable in determining whether compositionalmodification of the substrate or the particularsynthesis route can affect strain accommodation.

Contact: Boyd Veal, Argonne National LaboratoryPhone: (708) 252-4957 FAX: (708) 252-4798 E-mail: [email protected]

"Research sponsored by Division of Materials Science and Office of Fossil Energy, U. S. Department of Energy, and the Electric Power Research Institute.

56



3.0

2.5 AI203 Grown OnFe-28%AI-5%Cr-O.1 %Zr

• •/

."",-../.' ../.' "\.

./ 0' ../ ....( •

~Calculated Strain

From CTE Mismatch

// .

•,·0

/

0'.~.

o

•

.'0

2.0

-~0- 1.5E:CIS~-fI)

1.0

0.5 AI203 Grown OnFe-28%AI-5%Cr

0'·

0.0700 800 900

.Q ··· 80

1000 1100 1200

Temperature (OC)

Compressive strain, as measured with the ruby fluorescence technique, inalumina scales thermally grown on Fe-Cr-Al alloys as a function of exposuretemperature. Strain in the scales increased with reaction temperature (andscale thickness) until strain relief occurred due to scale cracking and failure.Note that scales on Fe-28 at.% Al-5% Cr-O.l% Zr sustained substantiallygreater strains than those on Fe-28% Al-5% Cr. The solid line shows thecalculated scale compressive strain based on differences in the respectivecoefficients of thermal expansion for the oxide and for an Fe-Al-Cr alloywhen cooled from the oxidation temperature.

57

forquestions and additional information contact · a metallurgical approachtoward alloying in rare...

Documents