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Low temperature aerosol synthesis (LTAS) of nanostructured alumina particles

M.E. Rabanal1*, M.I. Martn

1, L.S. Gmez

1, J.M. Torralba

1, O. Milosevic

2

1University Carlos III of Madrid, Avda. de la Universidad, 30, 28911 Legans, Madrid, Spain

2Institute of Technical Sciences of Serbian Academy of Sciences and Arts, K. Mihajlova 35/IV,

11000 Belgrade, Serbia

Abstract

Metal matrix composites (MMCs) having fine-scale anduniformly dispersed phases, are of great technologicalinterest because of improved mechanical properties, particularly the hardness, wear resistance, elasticmodulus and yield strength. The addition of ceramicalumina nanoparticles into metal matrix compositesmight have a huge effect for their implementation intoautomotive, defense and aerospace application. This

paper will present the preparation of nanostructuredspherical alumina particles (< 500 nm sized) by lowtemperature aerosol synthesis (LTAS) for theapplication in MMCs reinforcement. Synthesis procedure includes aerosol formation ultrasonicallyfrom alumina nitrate water solution and itsdecomposition into a tubular flow reactor at 400C.Consequently, as-obtained particles are spherical,smooth, amorphous and in non-agglomerated state.The phase crystallization, either to or -Al2O3 is promoted by additional thermal treatment in the rangefrom 900 to 1300C. Detailed phase and structuralanalysis were proceeded in accordance to X-ray powder

diffraction, and electron microscopy (SEM/EDS andTEM).

Keywords: Alumina;Nanoparticles; Spray Pyrolysis;Ceramic Materials.

Introduction

Small aluminium oxide nanoparticles have importantapplications in the ceramic industry[1,2] and can be usedas an abrasive material, in heterogeneous catalysis, as anabsorbent, a biomaterial and as reinforcements of metal-

matrix composites (MMCs)

[3-5]

. In order to be used foreffective discontinuous reinforcements in a continuousmetal matrix, several structural and morphologicalaspects for Al2O3 particles have to be fulfilled: smallparticle size and narrow size distribution, large surfacearea, spherical morphology and the absence ofagglomerates. When the hot wall aerosol synthesismethod (spray pyrolysis), as a basically chemical routefor obtaining a versatility of advanced materials isconsidered, several advantages over conventionalsynthesis have been reported for the preparation of welldefined oxide powders[6].

The technique is based on the generation ofmicrometric-sized aerosol droplets by ultrasonic wavesand their decomposition at intermediate temperatures(400-800C). Due to the precipitation, decomposition

and chemical reaction occur in a dispersed phase and ina single step, there is a possibility to control theimportant particle properties (size, morphology,chemical composition, etc.) by simple controlling the process parameters (residence time, decomposition

temperature)[7-10].

This paper describes the low temperature aerosolsynthesis (LTAS) of nanostructured alumina particles

for the applications in nanoreinforcements of metal-matrix composites. For that purpose, the conditions forthe production of high-purity nano-particles, sphericaland free of aggregates will be optimized by assessingthe influence of processing parameters (ultrasoundfrequency, decomposition temperature, residence time,solution properties) on the morphology and structuralproperties of the nanopowders.

Experimental

Particle preparation

The spray pyrolysis experimental set-up consists of anultrasonic nebulizer, a quartz tube located inside acylindrical furnace and a particle collector (Fig. 1). Thefine drops of precursor solutions were carried out by theairflow regulated with a flow controller. Al2O3 fine particles were synthesized by pyrolysis of an aerosolgenerated by ultrahigh frequency of aluminium nitrateaqueous solution: Al(NO3)39H2O (0.1 M) according tothe following experimental conditions: furnacetemperatures: 400 C, gas flow rate: 1.5 l/min,frequency of the ultrasonic atomizer (RBI, France): 2.1MHz, carrier gas: pure air.

Fig. 1. Schematic representation of the processingequipment.

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After synthesis, the powders were annealed isothermallyat 700-1300C for 12 h in a chamber furnace in air(CHESA).

Particle characterization

The crystal structure of the as-prepared and thermally

treated powders was analysed by X-ray diffraction(XRD) collected in an automatic X Pert Philipsdiffractometer, and using a CuK source. Data werecollected in the 2 ranges from 10 to 70 in step-scanning mode with a step size of 0.04 and a countingtime of 2.70 s per step. Crystalline phases wereidentified and indexed using the software X-RayDiffraction Philips Analytical[11] and the Pcpdfwindatabase - JCPDS-ICDD[12].

Compositional homogeneity and particle morphologywere analysed by scanning electron microscopy(SEM/EDS) on a Philips XL Series XL 30 and

transmission electron microscopy (TEM, JEOL-JEM,400kV.)

Results and discussion

Fig. 2 shows X-ray diffraction patterns for as-preparedand thermally treatment powder samples. Two phasesare formed as a result of the annealing process. In bothas prepared and thermally treated powder samples(800C) the amorphous character is typically reflectedin the shape of the diffractograms. However, after

thermal treatment at 900C/12h begins to appear thechange in the amorphous behaviour coming out with thelow intensities reflections corresponding to the -Al2O3 phase (JCPDS 10-0425, S.G. 227 a = 7.924), wherethe (400) and the (440) planes maximums at 2 45.8and 67, respectively, can be easily identified.Increasing the temperature begins to resolve othermaximums related to the phase, although the shape ofthe diffractograms is still broad. At 1100C the main peaks of phase are clearly defined. After thistemperature the peaks begin to broaden probably causedby the appearance of the second phase that is obviouslycoexistent with the phase. Based on the low intensity

peaks at approximately 2 43, 57.4 and a peak over35, corresponding to the (113), (116) and (104)reflections, respectively, the maximums identifiedcorrespond to the -Al2O 3 phase (JCPDS-42-1468, S.G. 167, a = 4.758 , c = 12.99). After 1300 /12hours annealing, only well defined peaks of -Al2O3 phase are presented. Besides the main phasesencountered in the sample, the small peakscorresponding to the SiO2 residues of the reactor quartztube, are also detected.

Fig. 2. Experimental X-ray diffraction patterns for as-prepared and thermally treatment powder samples.

Fig. 3a shows SEM micrographs of as prepared Al2O3 particles. It can be seen that as-prepared particlesderived through aerosol decomposition are highlyspherical, smooth, non-aggregated and relativelyuniform in size (below 500 nm). EDS analysis (Fig. 3b) prove high compositional uniformity and the singlepresence of the constitutive elements.

Fig. 3: SEM micrograph of as preparedAl2O3particles(a) and the corresponding EDS analysis (b).

Fig. 4 corresponds to SEM images taken with secondaryelectron mode detector for the thermally treated powdersamples at 700-1200 C during 12 h. It is obvious that

particle morphology does not change significantly withannealing. Powders persist in their un-agglomeratedform although high temperature regime provokes furthercrystallisation and growth of the primary particles[6].

20 30 40 50 60 700

100

200

300

400

500

600

700

800

900

1000

(220)

(220)

(220)

(104)

(311)

(110)

(113)

(116)

(400)

(311)

(311)

(311)

(222)

(222)

(222)

(400)

(4

00)

(400)

(440)

(440)

(440

)

(440)

(111)

(300)

(214)

(116)

(024)

(113)

(110)

(104)

(012)

: -Al2O

3

: -Al2O

3

*: SiO2

1300 C

1200 C

1100 C

1000 C

900 C

800 C

700 C

as prepared

*

*

*

*

*

*

Intensity

(a.u.

)

2

(a)

(b)

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Fig. 4: Scanning electron micrographs of the thermallytreated powder samples: (a) 700C/12 h, (b) 800C/12 h,(c) 1100C/12 h, (d) 1200C/12 h.

Particle morphology does not change significantly withthermal treatment at 1300C/12 h as being implied atFig. 5 showing spherical and non-aggregated particles.

Fig. 5: SEM micrograph of sample annealed at1300C/12 h: (a) secondary electrons (SE), (b)

backscattered electrons (BSE).

Fig. 6 shows a low magnification in bright field mode ofthe sample annealed at 1100C/ 12 h confirming thespherical character in the secondary nanoparticle with adiameter of 331 nm. Individual primary particles can beresolved with a diameter mean of 17.822 6.5 nm.

Fig. 6: Low magnification TEM micrograph in brightfield mode of the sample annealed at 1100C/12 h.

Conclusions

The low temperature aerosol synthesis (LTAS) of fineAl2O3particles (

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11. Software of Philips Analytical X-Ray. PhilipsElectronics N.V. 1996-1999.

12. Pcpdfwin program, version 1.1 - September (1995).