'J!

NSWC TR 90-48

AD-A221 616

A SILVER-BEARING, HIGH-TEMPERATURE,SUPERCONDUCTING (HTS) PAINT

BY WILLIAM A. FERRANDO

RESEARCH AND TECHNOLOGY DEPARTMENT

15 FEBRUARY 1990

DTICft ELECTE11MAY111MOIU

Approved for public release; distribution is unlimited. S D

I NAVAL SURFACE WARFARE CENTERDahigren, Virginia 22448-5000 e Silver Spring, Maryland 20903-5000

, J >,

NSWC TR 90-48

A SILVER-BEARING, HIGH-TEMPERATURE,SUPERCONDUCTING (HTS) PAINT

BY WILLIAM A. FERRANDORESEARCH AND TECHNOLOGY DEPARTMENT

15 FEBRUARY 1990

Approved for public release; distribution is unlimited.

NAVAL SURFACE WARFARE CENTERDahlgren, Virginia 22448-5000 * Silver Spring, Maryland 20903-5000

NSWC TR 90-48

FOREWORD

While the desirability of producing the high-temperature superconducting(HTS) materials in wire form is undeniable, there is a subgroup of devices whichmight be improved significantly if the superconductor were available as a thick filmor paint with appropriate properties. Such a paint could be applied by brush or sprayand heat treated to produce the superconducting properties. Paint coatings underdiscussion here fall within the range of about 10-100 microns. Potential applicationsof thick film coatings include infrared detectors, magnetic field sensors, magneticshielding, and microwave cavities.

The HTS paint system described here is capable of providing a fullysuperconductive coating at 77K with the additional benefits of good bonding to thesubstrate, resistance to microcracking, and environmental protection.

The author wishes to acknowledge Mr. Dave Divecha of the Naval SurfaceWarfare Center (NSWC) for his encouragement in this investigation. The work wassupported by NSWC Independent Exploratory Development (IED) funds.

Approved by:

CARL E. MUELLER, HeadMaterials Division

Acoesslon For

NTIS GRA&IDTIC TAB 0Unannounced 01Justificat1o

ByDiqtributiou/

Availability Codes[vail and/or

Dist Speoial

i/ii

NSWC TR 90-48

CONTENTS

Chapter Page

1 INTRODUCTION .......................................... 1

2 BACKGROUND AND APPLICATIONS ...................... 3INFRARED DETECTORS ............................... 3GRADIOMETER AND MAGNETOMETER SENSORS ..... 3MAGNETIC SHIELDING AND WAVEGUIDE

CAVITY COATINGS ................................. 3

3 FORMULATION, APPLICATION, AND PROCESSINGOF THE COATING ..................................... 5SUBSTRATE REQUIREMENTS ......................... 7

4 EXPERIMENTAL PROCEDURE AND RESULTS ............ 9

5 SUM M ARY ............................................... 15

REFERENCES ............................................ 17

DISTRIBUTION ........................................... (1)

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NSWC TR 90-48

ILLUSTRATIONS

Figure Page

1 FLOW CHART FOR PRODUCTION AND APPLICATIONOF GLYCOL-BASED 123/AgNO3 PAINT .................. 6

2 SEM PHOTOGRAPH OF 123/Ag PAINTED SURFACE(ALUM[NA FLAT) WITH Ag DISTRIBUTION MAP ....... 10

3 PHOTOGRAPH OF PAINTED THORIA TUBE TESTSAMPLE MOUNTED ON LOW-TEMPERATUREPROBE PLUG .......................................... 11

4 ABSOLUTE TEMPERATURE VERSUS RESISTANCEOF THORIA TUBE COATED WITH 123/AgN03PAINT ATER THERMAL PROCESSING ................ 12

iv

NSWC TR 90-48

CHAPTER 1

INTRODUCTION

For the initial promise of the high-temperature superconducting (HTS) mate-rials to be sustained, it is imperative that some early application be made. It is likelythat this will take the form of component modification of an already existing systemor device to accommodate the superconductor. It seems obvious, also, that earlyapplications should not rely on extreme material properties such as very high criticaltemperature (Tc ) or current (Jc). Likewise, an early application involving a long,ductile wire of these materials may be unrealistic.

There is, however, a class of potential applications for HTS materials in theform of a paint coating on suitable substrates and in appropriate geometries whichmay not rely on the extremes of HTS material properties. Those of potential near-term interest to the Navy are described below. The paint method proposed is aresponse to these reported needs.

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NSWC TR 90-48

CHAPTER2

BACKGROUND AND APPLICATIONS

INFRARED DETECTORS

This class of detector depends upon the photoconductive properties of the film.Current infrared (IR) detector technology includes Hg-Cd-Te cooled to 77K and dopedGe cooled to 4.2K. These materials are expensive and difficult to configure in arrays.The photoconductive carrier lifetimes in these materials must be reduced by dopingto obtain the proper frequency response. This doping, however, tends to reduce sensi-tivity and to increase noise and stray capacitance of the film.

HTS films have a natural network of grain boundaries which can cause prob-lems in some applications, but which may be exploited to advantage in this case.Films of HTS materials having prominent grain boundary configuration show alarge reduction of voltage (resistance) upon illumination with light.1 Thus, sincethe required type of photo response has been obtained with HTS materials for thisapplication, materials optimization and fast detector development remain.

GRADIOMETER AND MAGNETOMETER SENSORS

Superconducting Quantum Interference Device (SQUID) magnetometers arethe most sensitive measuring devices of magnetic fields known. SQUIDs operating at4.2K have a theoretical sensitivity of about 2X10-7 gauss. HTS SQUIDs have beendeveloped with laboratory demonstrated performance at 77K equal to that of com-mercial SQUIDs at 4.2K utilizing conventional niobium (Nb) technology. 2 The high-est sensitivity, after material and design optimization, always will occur at liquidhelium temperature. This is due to the reduction of Johnson noise and thermallyinduced magnetic noise in the normal metals present and the more desirable mag-netic properties of helium itself. Microscopically uniform addition of silver mayimpart a higher density, degree of coupling, and uniformity to the HTS particulatecoatings. This added to an oriented grain structure may provide the coating mediumrequired for improved performance in such devices.

MAGNETIC SHIELDING AND WAVEGUIDE CAVITY COATINGS

Although they are substantially different applications, both magnetic shieldingand waveguide cavities depend upon the "skin" or "penetration depth" to relate to theother coating properties. This is a measure of the depth a magnetic field of givenmagnitude and frequency will penetrate a superconducting surface. This depth isalso a function of the "reduced' temperature of the superconductor, T/To. Forshielding applications, the film thickness should be greater than the penetration

3

NSWC TR 90-48

depth. For waveguide applications, the penetration depth is a measure of losses asthe signal is transmitted along the tube. The depth must be minimized for highestefficiency or "Q". The presence of finely deposited and highly bonding silver shouldhelp in bridging microcracking, relieving stresses, and minimizing chemical reactionwith the substrate which have been encountered in previous efforts. 3 Finally, silveraddition was found to significantly reduce environmental degradation of HTS filmsdue to incursion and reaction of moisture along grain boundaries. 4

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NSWC TR 90-48

iJ

CHAPTER 3

FORMULATION, APPLICATION, AND PROCESSINGOF THE COATING

The chemical reactions and phase changes of AgNO 3 which allow it to be usedin the context of micro-coating of ceramic superconductor particles were discussedpreviously.5 The thermal behavior of AgNO 3 is:

AgNO3 > AgN03 (liquid) @ 222°C

(brown fumes)2AgNO 3 - --- > 2AgO + 2NO 2 ' @ 4440C

2AgO ----- > 2Ag + 02

or perhaps more correctly:

2AgNO 3 - --- > 2Ag + 02 + 2NO 2 @ 444-C

Results on bulk superconductors indicate the decomposition of AgNO3 toprovide a superior method of Ag addition.

The formulation of an HTS paint using AgNO3 is rather simple. Figure 1outlines the process. AgNO3 is ground and added to the HTS powder. (YBa 2 Cu307-..x,designated 123, was used in this work, but another HTS compound could be substi-tuted.) Uniform fine particle (small compared with the coating thickness) HTSpowder should be used. Generally, a quantity of AgNO3 is added toproduce about15wt % after decomposition. Sufficient pure ethylene glycol is added to provide agood consistency for brushing (or spraying). The glycol is inert to the 123 compoundand dissolves the AgNO3 , spreading it uniformly throughout the entire mass. Sol-ubilit of AgNO 3 in glycol was found to be at least 7.5 g/10 cc, a fraction greater thanwould ever be needed in an HTS paint.

The suspension mixture is applied to the substrate by brushing or spraying.The choice of substrate is critical for successful HTS coating. The substrate must beable to tolerate the sinter temperature without cracking or chemically reacting withthe HTS compound. Its thermal expansion must match that of the superconductor, sothat the film remains under compression during thermal cycling. This retards theformation of microcracks which impair Je and broaden the superconducting tran-sition. The substrate must not produce too great a compression on cooling,however,or separation of the film could occur.

5

NSWC TR 90-48

mix IADD ETHYLENE GLYCOL TOAgNO3 AgNO3

++

123 123MIXTURE

BRUSH OR SPRAYONTO SUBSTRATE

DRY( -50o C)

SINTER ( - 900oC. 3% 02)

OXYGENATE(700-4000C. 100% 02)

TO YIELDAg COATED 123

PAINT

FIGURE 1. FLOW CHART FOR PRODUCTION AND APPLICATION OF

GLYCOL-BASED 123/AgNO3 PAINT

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NSWC TR 90-48

SUBSTRATF REQUIREMENTS

Various substrates have been tested in the course of several HTS thin filmefforts. 3,4 These include Ba-Ti, Ni-Al-Ti, A12 0 3, spinel, MgO, SiO 2 , SrTiO 3, andYSZ (yttria stabilized zirconia). Those containing nickel (Ni) or titanium (Ti)experienced some degree of chemical reaction. The most successful substrates werethose of spinel and YSZ. These yielded complete superconducting transitions with T,of about 81K and transition widths of less than 10K.

7/8

NSWC TR 90-48

CHAPTER4

EXPERIMENTAL PROCEDURE AND RESULTS

A 2-inch diameter tube of A120 3 was brush painted with the 123/AgNO3/glycolmixture and dried at about 50°C (Figure 1). The tube was subsequently fired at

460°C to decompose the AgNO3. Care was taken to completely evaporate the glycolprior to firing; otherwise, the resulting combustion will alter the final coating prop-erties. This operation was carried out several times to produce a coating thickness ofseveral mils. The tube was sintered overnight at 900°C in 02. Four contacts wereattached using silver paint. Measurement of T, was made using the apparatus andtechrdque described in Reference 5.

A sharp drop in resistance was observed at about 85K; however, a smallresidual resistance remained at 77K. The curve remained substantially unchangedwith further annealing of the tube in 02. This indicated the possibility of unhealedmicrocracks in the coating due to mismatch in thermal expansion with the tube. Thecurved tube presented difficulties for microscopic observation. Thus an alumina flatwas identically coated and heat treated for microscopic study. Figure 2 shows thepainted surface (top) at high magnification and the Ag elemental map (bottom). Thisshows the degree of Ag dispersion achievable by the AgN0 3 decomposition method.The distribution is remarkable for its homogeneity and microparticulate character.

Available thoria (ThO2 ) tube sections were tested as an alternative substratematerial. To help obviate the problem of electrical contact, four silver metal stripswere bonded along the inside of the thoria tube using small amounts of AgNO 3 priorto painting. Then the tube inside was painted with the 123-AgNO3-ethylene glycolmixture. The ethylene glycol was allowed to evaporate at -50°C. The tube wastransferred to a furnace, sintered in 02 for 16 hours at 900'C, and allowed to cool inthe furnace to ambient.

Electrical connections (visible in the picture) were made to the ends of the silvertabs now embedded in the coating. Tc measurement was made by slowly lowering thetube into a liquid nitrogen dewar while monitoring the coating resistance with aKeithley Model 503 milliohmmeter. Temperature and resistance were recordedusing a Houston Instruments dual pen recorder. Figure 3 shows the tube section withattached four contact leads and mounted on a four-pin plug assembly. The data isshown plotted in Figure 4. A complete superconducting transition of several degreeswidth and Tc of about 81K was observed (circle points). After thermal cycling andseveral months exposure to ambient conditions, during which no particularprecautions to protect the tube were taken, a second measurement was made. Thisset of data (square points) indicates essentially the same superconducting behavior.Hence, a reasonably stable paint film, fully superconducting at 77K, has beenapplied. An optimally stable HTS paint coating will depend strongly on the phaseCurity of the 123 starting material and the high degree of sealing of the HTS grain

undaries against moisture by the Ag (see Reference 4).

9

N SWC 'FR 90 -4 8

FIGURE 2. SEM III OTOGRAIPI I OF 123/Ag PAINTEI) SU RFACE (ALU MINA FLAT)WvITHI Ag D)ISTRIBUTI'IION MA I)

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4

FIGURE 3. PHOTOGRAPH OF PAINTED THORIA TUBE TEST SAMPLE MOUNTEDON LOW-TEMPERATURE PROBE PLUG

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100

0 INITIAL TEST80 -E RETESTED AFTER 2 MONTHS

60-RESISTANCE

(MILLI-OHMS) _

40-

20-

0 t , _ I50 75 100 125 150

ABSOLUTE TEMPERATURE (K)

FIGURE 4. ABSOLUTE TEMPERATURE VERSUS RESISTANCE OF THORIA TUBECOATED WITH 123/AgNO3 PAINT AFTER THERMAL PROCESSING

12

NSWC TR 90-48

A crude measurement of Jc for this film was made. The estimated paintthickness is approximately equal to that of the embedded Ag contact strips whichwere .005 inch (.013 cm). The tube length was -1 inch (2.5 cm). The tube carriedabout 0.01 amps in the superconducting state. Thus, the Jc of the paint cross sectionmay be computed as Jc = .01/(.013 x 2.54) = .30 A/cm 2. While some applications ofHTSC coatings might not need a substantial Jc, most involving microwave handlingwill require at least moderately high Jc.

Proper choice of substrate and sintering/oxygenation heat treatment schedule 6

should produce the improvement required for the HTS paint applications notedabove. Other HTS compounds also can be used with the AgNO 3 decompositionprocess to produce paint films. The presence of the thoroughly dispersed andintimately bonded Ag should reduce the incidence of microcracking, leading togood thermal cycling performance.

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NSWC TR 90-48

CHAPTER 5

SUMMARY

A well-bonded, environmentally stable superconducting paint film containingAg has been produced on the inside surface of a thoria tube. A method of AgNO 3decomposition was used for Ag dispersion in the 123 superconductor material and forbonding with an ethylene-glycol vehicle for application. This film displays fullsuperconductivity at 77K and retains its properties for an extended time underambient unprotected conditions. Improvement upon these results certainly is likelyusing other substrates and more careful heat treatment.

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NSWC TR 90-48

REFERENCES

1. Aponick, A., Carey, C., and Avarbock, G., Evaluation of High T. Super-conductors as Infrared Detectors--Final Report, Contract #F08635-88-C-0310,Dept. of the Air Force, Armament Division, Eglin AFB, FL 32542-5000,Aug 1989.

2. Clem, T. R., Gershenson, M., and Purpura, J. W., High-Temerature Super-conductors for an Advanced Magnetic Gradiometry, NCSC LR 211-89-001,Apr 1989, Naval Coastal Systems Center, Panama City, FL 32407-5000.

3. Chang, C., "Reduced Moisture-Induced Degradiation of YBaCuO Super-conductivity Films by Silver and High Deposition Temperature,"Appl. Phys. Lett., Vol. 53(12), 19 Sep 1988, p. 1114.

4. Bansal, N. P., Simons, R. N., and Farrell, D. E., "High T: Screen-PrintedYBa 2 Cu3O7.x Films: Effect of the Substrate Material," Appl. Phys. Lett.,Vol. 53(7), 15 Aug 1988, p. 604.

5. Ferrando, W. A., Fabrication of High Temperature Superconductor Wires byAgNOg Decomposition, NSWC TR 89-338, 15 Dec 1989 (to be published).

6. Goretta, K. C., Poeppel, R. B., Shi, D., Chen, N., Rothman, S., J.,Routbort, J. L., and Stoessel, J. P., "Diffusion and Processing of YBa2Cu 3Ox,"submitted to the First International Ceramic Science and Technology Congress,31 Oct-3 Nov 1989, Anaheim, CA, sponsored by the American Ceramics Society.

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4. TITLE AND SUBTITLE S. FUNDING NUMBERS

A Silver-Bearing, High-Temperature Superconducting (HTS) Paint

6. AUTHOR(S)

William A.Ferrando

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) 8. PERFORMING ORGANIZATIONREPORT NUMBER

Naval Surface Warfare CenterWhite Oak Laboratory NSWC TR 90-4810901 New Hampshire AvenueSilver Spring, MD 20903-5000

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13. ABSTRACT (Maximum 200 words)

A substantial set of device applications awaits development of a workable, durable, high-temperature superconducting (HTS) paint. Such a paint should be truly superconducting with its criticaltemperature T,>77K. For most of these applications, a high critical current (J) is not required, althoughprobably desirable.

A process is described which can be used to produce silver-bearing HTS paint coatings on manyengineering materials. Preliminary tests have shown good adherence to several ceramics and the abilityto meet the superconducting criteria. Moreover, the coatings withstand multiple thermal cycling andstability under laboratory ambient storage conditions for periods of at least several months.

14. SUBJECT TERMS 15. NUMBER OF PAGES

23Superconductivity > Superconducting Paint, 6. PRICE CODEHigh+:Temperature Superconducting (HTS)

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-LOCK 6. Authorts). Name(s) of person(s)esponsible for writing the report, performing the Block 14. Subiect Terms. Keywords or phrasesesearch, or credited with the content of the report. identifying major subjects in the report.'editor or compiler, this should follow the name(s).

Block 15. Number of Pages. Enter the totallock 7. Performing Organization Name(s) and number of pages.

Address(es). Self-explanatory.Block 16. Price Code. Enter appropriate price code

Block 8. Performing Organization Report Number. (NTIS only)Enter the unique alphanumeric report number(s)assigned by the organization performing the report. Blocks 17.-19. Security Classifications. Self-

Block 9. Sponsoring/Monitoring Agency Name(s) explanatory. Enter U.S. Security Classification in

and Address(es). Self-explanatory. accordance with U.S. Security Regulations (i.e.,UNCLASSIFIED). If form contains classified

Block 10. Sponsoring/Monitoring Agency Report information, stamp classification on the top andNumber. (If Known) bottom of the page.

i3lock 11. Supplementary Notes. Enter information Block 20. Limitation of Abstract. This block must",ot included elsewhere such as: Prepared in coop- be completed to assign a limitation to the abstract.S,:'ation with...; Trans. of...; To be published in.... Enter either UL (unlimited) or SAR (same as report).v,; nen a report is revised, include a statement An entry in this block is necessary if the abstract is,.hether the new report supersedes or supplements to be limited. If blank, the abstract is assumed tom~e older report. be unlimited.

Standard Form 298 Back (Rev 2-89)