MTL TR 90-51 AD

HOT ISOSTATIC PRESSING OFSUPERCONDUCTING CERAMICS

LflLfl

NC14 KERRY I. RICHARDS and CPT RICHARD H. BENFERI CERAMICS RESEARCH BRANCH

0

October 1990

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'JS ARMYLABORATORY COMMAND U.S. ARMY MATERIALS TECHNOLOG(Y LABORATORYMATeNKS TECHWCOY LADPTaAY Watertown, Massachusetts 02172-0001

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Final ReportHOT ISOSTATIC PRESSING OF SUPERCONDUCTINGCERAMICS -P- RAI-OF REPORT NUMBiER

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Kerry T. Richards and CPT Richard H. Benfer

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19 KEY WORDS Caots- -wnrtn Am ,ermarv and ' tfy a b bhlk numer)

Ceramic materials Oxvwen stoichiometrvHot isostatic pressing (HIP) Physical propertiesSuperconducting materials

20 ABSTRACT (CGrngini on -o,. n,& 1f nrm, v ad i rsfy by blm-k n.t lnI

(SEE REVERSE SIDE)

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Block No. 20

ABSTRACT

------ Hot isostatic pressing (HIP) was studied as a method for processing bulk sue on-ductors. Superconducting powder was derived from the calcination of nitrated Y 203,CuO and BaCO3 powder. The powder was HIPed using pressures of 70, 140, and210 MPa with temperatures of 820ec and 950°' for a hold time of 1 hour. The den-sity, hardness and Young's modulus of HIPed samples were higher than those ofsintered control samples. Superconducting transition temperatures >92"-K were achievedwithout post-HIP annealing of the samples. J-(/eXor5: Spercorndtcors/

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.....UNCLASSIEIED..SFCL~fITY M AS SIFICATION Or 1 HIS ~~ ~tx INVI IV,,

CONTENTS

Page

BACKGROUND........................................................1

EXPERIMFNTAL

Powder Preparation .. .. ... .. ... .. ... . .. .. .... .. .... .. ... ... ...... ... 2

Sam ple Preparation ... .......... .......... ....... ..... ....... ...... 3Characterization 5.................................................. 5

RESULTS AND DISCUSSION

E ncapsulation ....... .......... .......... ....... ...... ....... ..... 6Phase Identification . .. . .. .. ... .. ... .. .. . .. ... .. ... ... .. .. .. .... ... 6

Physical Properties ... .......... .......... ....... ...... ...... ...... 7T ransition Tem peratures . ........................................ .... 13

SU M M A R Y . ..... ....... .......... .......... ....... ...... ....... ..... 15

ACKNOW LEDG M ENTS . ................................................ 15

REFERENCES I.......................................................1b

Accession For

NTIS G1RA&IDTTC TAB 0Urrl"! r, :n I'c d El

j i. -,,ition

Av:! i.'tilty CodOS

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BACKGROUND

Much work has been devoted to high temperature ceramic superconductors since theirrecent discovery. 1,2 Most advances have come in the area of thin films where prototype appli-cations have been developed and tested. The processing of bulk shapes (e.g., bars, rods,rings) for use as components in larger systems (e.g., motors, rail guns) has progressed moreslowly.

Hot isostatic pressing (HIP) is a method of fabricating bulk ceramic shapes through theapplication of high pressures and temperatures. The HIP process can readily consolidate com-plex ceramic shapes at lower temperatures than conventional sintering. Currently, HIP wiltscan operate at pressures over 200 MPa and temperatures in excess of 20000 C. The pressureis applied by a high pressure gas which is commonly nitrogen or argon. More recently,oxygen/inert gas mixtures have been used.

A material must have closed porosity in order to be successfully HIPed without encapsula-tion. In other cases, the material must be sealed in a capsule which acts as a pressuretransfer membrane. The capsule material must be able to withstand the high temperaturesand pressures found in the HIP chamber. In addition, it must have the liexibility to conformto the sample material. Capsules are generally made of metals and glasses.

HIPing has been used to fabricate new high temperature superconductors. 3-6 Tien et al. 3

HIPed YBa2Cu 30 7. x using copper cans at 100 MPa and temperatures of 750')C and 9000C.The resulting material was 99.3% dense and after heat treatment in air the material exhibitedMeissner exclusion. Meissner exclusion indicated the presence of a superconducting phase.They note, however, that it is preferable to produce the final oxygen stoichiometry of theceramic in the as-HIPed state.

Other researchers have also noted the requirement of an oxygen anneal after HIPing inorder to regain superconducting properties. Snow et al.4 HIPcd the YBa 2Cu 3O7T, material instainless steel cans at 103 MPa and 850)C to over 95% of theoretical density. An oxygenanneal at 850 0 C for 2 hours was required for superconducting properties to appear. Meissnerexclusion at low temperatures approached 100%.

Sadananda et al.5 studied HIPing superconductors in both glass (Pyrex) and metal (stain-less steel) tubes. HIPing at 8500 C and 207 MPa for 1 hour in the Pyrex tube resulted in amaterial that became superconducting at 650 K without anneal. This group also noted a grainsize refinement where the average grain size in the compact decreased after HIPing. Theypropose that the PdV work done during HIPing caused the fracture of brittle ceramic oxideparticles. The low transition temperature (Tc) of 650 K is attributed to oxygen loss occurringduring vacuum encapsulation or during pressing.

I. BEDNORZ, j. G. and MULLER, K. A. Possible tligh-T Superconductivity in the Ba-La-Cu-O System. Z. Phys. B., v, 64. 1986. p. 189-193.2. WU, M. K. ci al. Sulpeconductivity at 93.K in a New Mired Phase Y-Ia-Cu-O Compound System at Ambient Pressure. Phys. Rev. ILct..

v. 58. 1987, p. 908.3. TIEN. J. K.. I IENI)RIX, B. C., 13OROFKA, J. C., and ABE. T. h ot Isostatic lessing (1111) for the Densifcticjon of Oid, Soperconihictors.

Matf. Rcs. Soc. Spring Mccing, 1988.4. SNOW. 1). II., WI-INIFRGI-, It . R., Pi,'FEITSON. G. G., 1,YNDS, L., EASTON, I1., BURIIA. C. 1'.. I'OTRI'EPKA. I). M.. and

KUWA IARA. M. Processing Microstnicntre and Properties of Illa.,cu3O7.dAg Sulerconductors. Proceedings o1 I'MS, March 1989.5. SADANANI)A. K., SINGh i. ,%. K.. IMAN. M. A.. OSO[:SKY, M., I,TOUIRNEA3. V.. and RICI IARIDS, I. F. Effect of 'lot lsostth

Presig on Rllah 't.O7 St'erconthictorv. Advanced Ceramic Malcrials. v. 3. no. 5. 19,8, p. 524.526

Niska et al.6 have noted that HIPing of the YBa 2Cu3OT7 - can lead to oxygen loss and aresulting orthorhombic-to-tetragonal phase transformation. This reversible structural translorma-tion occurs with changing oxygen stoichiometry, going from orthorhombic at x = 0 to tetrago-

7nal at x = 1.0. They also noted that glass encapsulated material retained the super-conducting orthorhombic phase while the stainless steel encapsulated material became tctrago-nal. They explain that oxygen is evolved by the superconducting material while it decomposesduring heating. This oxygen may more readily diffuse out through the metal, and more impor-tantly, the oxygen can react with the metal canister to form an oxide. A glass encapsulatedsystem, therefore, results in a higher equilibrium partial pressure of oxygen which favors theretention of the orthorhombic phase. However, this higher oxygen pressure can also opposethe densification process. Therefore, processing conditions must be carefully chosen in orderto maximize both T, and density.

The goal of this research was to develop a HIP process for densifying YBa 2Cu 3OT7 . whilesimultaneously retaining high temperature Te's. This would eliminate the need for a post-HIPoxygen anneal step. The encapsulation method was designed to adjust the oxygen equilibriumin the samples through the addition of an oxygen donor material. This material would decom-pose and evolve oxygen in order to increase the oxygen partial pressure. BaO 2 was chosenas the oxygen donor for this work because of its compatibility with the YBa 2Cu 3OT7 x system.With the appropriate choice of process conditions, it was expected that fully denseYBa 2Cu 307.x could be HIPed while still retaining its superconducting properties.

An alternative method for controlling oxygen stoichiometry is accomplished by the recentlydeveloped oxygen HIP. Kobe Steel of Japan, for instance, has developed a HIP unit thatuses an oxygen/inert gas mixture and plans to use it for their superconductor processingresearch.8 However, due to the problems associated with high pressure oxygen at high temper-atures, the oxygen HIP unit is more complex and expensive than a standard HIP unit. Themethod outlined in this report is inexpensive and readily adopted.

EXPERIMENTAL

Powder Preparation

The starting superconducting powder was prepared by the solid-state reaction of Y20 3,BaCO 3 and CuO powders. Figure 1 illustrates a flowchart of the powder processingsequence. The powders were mixed in stoichiometric ratio, reacted with nitric acid and driedat 1500C. The nitrated powder was reacted at 6000 C in air for 20 hours and then groundusing a mortar and pestle. The powder was then calcined in air twice, with an intermediategrinding, at 9400 C for 6 hours. The resulting YBa 2Cu 30 7.x powder was then heated to500 0C and cooled to 275 0C over 12 hours in order to maximize the oxygen content by thecompound. The powder was examined by an X-ray diffractometer (Diano Model 8535) usingCu-Ka radiation.

h. NISKA.J., IOE3 Ii, B.. and IFASTERLING, K. Effect of O.n'n Loss on kciisification Wlcn lieo Isostatic Prcssing )71a'Ci1407.%J. Am. Ccram. Soc.. v. 72. no. 8. 1989. p. 1508-1510.

7. (iAI .A i IER. P. K., O11RYAN. I I. M., SUNSI IINi. S. A.. and MURPI IY. 1). W. O gcgn .teijcionctnr in la2Y('C.O,. Mat. R is. Bull..v. 22. 1987, p. (Y)5-1000.

8. ()n cn iIP for Superconducting Ceramics Research Fabrication. Ncbca Kceia Shimbunsha, January 24, 19., p. 4.

2

Mix Y2 0 3 , BaCO 3. Cuc)

[I

SRatw HIFire mixture at 600"C

Fu Fire Powder at p40rCesinto form din phase

S e aRepeat

P Grind to a fine oow der

from 500 275hC

Figure 1. Flowchart of the powder processingsteps used for producing single phaseYBa2Cu307., material.

Sample Preparation

Powder samples were cold isostatically pressed (CIPd) in flexible molds at 160 MPa.The resulting sample size was approximately 13 mm long x 9 mm diameter and weighed 4 g.

Both sintered and HIPed samples were produced from the same batch of YBa2Cu3T-x pow-der. The sintered samples were used for comparison purposes. Table I outlines the process-ing conditions used in both sintering and HIPing.

Table 1. PROCESSING CONDITIONS

Tmerature Pressure TimeC) (MPa) (Hrs.)

Sinter 960 Atmospheric 6

HIP 820 69 1

138 1

207 1

950 69 1

138 1

207 1

3

Sintercd samples were fired in air at 9600 C for 6 heirs. A slow anneal in air from5000 C to 2000C over 6 hours was used to maximize the ox)ger content of the samples.

Two types of HIP sample configurations w're used. Both types were vacuum-scaled inPyrex capsules. The type A configuration consisted of a pressed superconducting powder sam-pie surrounded by silica c!oth. Type B consisted of a pressed powder samplc and I g ofBaO 2 surrounded by silica cloth.

HIPing was performed in an Autoclave Engineers 30M hot isostatic press using N2 gasand a graghite furnace. Six temperature/pressure conditions for HIPing were chosen(T = 820 C and 9500 C; P = 69, 138 and 207 MPa). Four encapsulated samples were HIPedduring each run; two of type A and two of type B. Samples were held at temperature andpressure for 1 hour, after which the furnace was shut off and the samples allowed to cool.Figure 2 illustrates typical pressure and temperature schedule for a 138 MPa and 8200 C HIPrun.

Temperature (C) Pressure (MPa)1000

140

800 ,. ,d120

/ - 100

600 -

I/ -80

~-60400 - 60

' -' 40

200

-"20

01-10 50 100 150 200

Time (minutes)

Figure 2. Pressure and temperature measurements for a 138 MPa and 8200C HIP run.Note that heat is applied first in order to soften the encapsulating glass. Hold time attemperature and pressure is 60 minutes.

Samples were removed from the glass capsule and then sliced into sections with a dia-mond saw. Measurements were performed on these sliced sections. Sample sections used forporosit, measurements and hardness testing were mounted and polished in kerosene. Allmaterials were stored in a vacuum desiccator.

4

Characterization

Samples were characterized for (1) density, (2) porosity, (3) Young's Modulus, (4) Knoophardness, (5) crystalline phases and (6) Tc.

Bulk density was measured by the Archimedes method using distilled water. Porosity mea-surements were made in accordance with ASTM E 562-83 (Standard Practice for DeterminingVolume Fraction by Systematic Manual Point Count). The point count was made using ascanning electron microscope at 500X magnification and an 81 point grid. One hundredfields %%ere counted for each sample.

Young's modulus was determined by ultrasound longitudinal and shear wave measurementsusing 6.4 mm diameter, 5 MHz longitudinal and shear wave transducers. Hardness testingwas performed with the Knoop hardness tester using a 100 g load.

Sliced sections were ground for powder X-ray diffraction. X-ray patterns were generatedwith an X-ray diffractometer (Diano Model 8535) using Cu-Ka radiation. Phase identificationwas determined by comparison with JCPDS cards and published patterns.

Transition temperatures were determined through the alternate current (a.c.) magnetiza-tion tests. Samples cut to approximately 2 mm x 2 mm x 5 mm were used. Measurementswere made by placing the samples within concentric coils driven by a 20 kHz, I volt rmssource (HP651B Test Oscillator). A lock-in amplifier (EG&G Model 128A) was used to filterand amplify the pick-up signal. Temperature was measured through a Type K thermocouplereferenced to 0°C. Critical temperatures were determined by noting the abrupt change in theoutput voltage which corresponds to the onset of diamagnetism in the sample. Measurementswere made as the sample was warmed through the Tc. Figure 3 shows a schematic diagramof the measuring system.

Cryogenic Samole

Off cAlna

i look r

Ty 0 K Thermocou pleL

DIglIa IsAgl3

Figure 3. Diagram of the transition temperature measurement system.Cryogenic chamber uses liquid nitrogen as a coolant.

'Barium (opper Yttrium Oxide, Iil'(7u1YO7. Powder I)ilfraction. v. 2, 1997, p. 192.tl;arium Copper Yltnum Oxide. Ih;AVuI'O2 . Powder D)ilTraction, v. 2, 1997, p. 192.

5

RESULTS AND DISCUSSION

Encapsulation

The encapsulation method proved to be relatively simple and reliable. The silica glasscloth separated the sample from the BaO 2 and from the capsule. It also helped prevent damn-age to the green body while sealing the capsule. Cooling the samples quickly from theHIPing tern perature normally caused the en~capsulating glass to craick, making removal of' thesample from the capsule at simple matter of peeling away the cracked glass. Samples HiPedat 9500 C were normally more difficult to remove from the capsule than samples HIPed at850)0C.

Phase Identification

The X-ray, diffraction pattern of starting superconducting powders was compared to pub-lished patterns tand identified as a single phase composition of orthcrhombic YBa2CU.1O 7 ..

3The theoretical density of this phase is 6.383 glcm

The decomposition of barium carbonate presents the greatest problems during the calcina-tion Process. 9 Significant amounts of BaCO 3 can remain up to 12000C. 10 Therefore. nitratcdprecursors were used in this work. No r, .idual BaCO3 was detected in X-ray diffraction pat-terns of the starting powder. These results may be explained by the difference in decomposi-tion temperatures for barium carbonate and barium nitrate (1450('C versus 6000C). 11 Thenitrate yields a more complete reaction at lower temperatures.

X-ray diffraction patterns of the samples HiPed at 8200 C were ideintified as an orthorhom-bic YBa12CU-1O7-x phase. Residual amounts of orthorhombic Y2BaCuO 5 phase ("211"), CuO,and Ba(OH) 2 , were also detected and are highlighted on the X-ray pattern in Figure 4. 1These phases have been shown to be a product of the following decomposition react.on 1with water:

2YBa2Cu3O7 + 3H 20 -~ Y2BaCuO 5 + 5CuO + 3Ba(OH)2 + 1/202(I

This reaction takes place slowly with humid air at room temperature and rapidly in air at850 C and 85% relative humidity.'13 Therefore, the "211" phase may have developed duringcutting, storage, or during preparation of the X-ray samples.

Fllanr Copper Yttnum Oxide, I3a2CU3YO7. Powder Diffraction, v. 2. 1987. p. 192.tBarium Copper Yttrium Oxide. BaCuT.:Oi. Powder Diffraction, v. 2. 1987, p. 192.9. YAN. M. F., LING. 11. C., OJ3RYAN. If. M., GALLAGhEFR. P. K.. and RHODES, W. W. Plrocss-redated Pro!-kc~ns ofY )'7?a&U30,

Superconductors. Mar. Sc,. and FEng.. v. DIl 1988, p. 119-129.10. NAVEII. J and PFIAN.. 1. On the Preparation oif 171a:Cm.O7., Ceramnic I1iikh-Temnpcranire Superconductor Mar. Res. [luli.. v. 24. 1999.

p. 12227.H. TARASCON. J. M., IIARIIOUX. P. BI., iAGITY. 13. Gi.. GREE'NF, . I Ii., and I Gii.. , W On Svntlrcsis of Hih T* Supt-rccohcuin

I'erovskitcs- Mat. Soi. and Eng.. %,. Ill. i8. p. z;)-_-;.12. YAN. M. F. BARNS, R. L ., OIIRYAN. Jr., ii. M.. GAL iAGIi IE;R. 1'. K.. Si N:RWOOD. R. C., and JIN. S. warer Interaction with thec

superconducting YIia2CuiOi Phbase. Appi. i'hys. Il.. v. 51, 1987. p. 532.13. 1DARNS. R. I_ and L AUISF 11Z2 N.A Stability of .Yuperconrdchitnx Y~hj'i0 ,~Inl thre Prcsc'wc iif 1iatr. .*ppl. i'lw'. L ewr. '.. 51. no. 17.

1987, p. 1373-1375.

01

Relative Intensity140 _________

120 F(101)

00

60

(200)40

2C '21 V Cuo 105013)10 2

B3(OH131)(000)

25 '(120) AJ k25 27 29 31 33 35 37 39 41 43 45 47 49

Two-Theta AngleW

Figure 4. X-ray diffraction pattern for a sample HIPed at 69 MPa and 8200C with Ba0 2.The (hkl) planes corresponding to the major peaks for YBa 2Cu3O7 are identified.Note also the peaks attribted to the "211" phase, CuO -nnd Ba(OH) 2.

Physical Properties

Thc density data is plotted in Figures 5a and 5b. Samples show an increase in the den-sity with an increase in HIP pressure. Samples HIPed at 8200 C have higher dens;ities thanthose HIPed at 950 0C. For each temperature, type B samples had lower densities than typeA samples.

Density (9/Cm3)6.5

6

5.5-4-

5

4.5

Type A

4 - TypesB

C3 Sintered

3.5 1 I

0 50 100 150 200Proce ing Pressure WMa

Figure 5a. Density measurement results for samples HIPed at 820 0C.

7

Density (g/cm)6.5

6 ,

5.5

5

4.5

* Type A4 + Type 8

O Sintered

3.50 50 100 150 200

Processing Pressure (MPa)

Figure 5b. Density measurement results for samples HIPed at 9500 C.

Results of porosity measurements are shown in Figures 6a and 6b. Porosity decreasedwith increasing HIP pressure. Samples HIPed at 8200 C have less porosity than those HIPcdat 950"C. For a given HIP temperature, type B samples had greater porosity than type Asamples. As expected, decreasing porosity was found to correlate well with increasing density(see Figure 6c).

Porosity (%)14

12

10

8 0

6

4[ - Type

A

2 -- Type BCJ [ Sintered

0 " I0 0 100 150 200

Pressure (MPa)

Figure 6a. Porosity measurements for samples HIPed at 8200C.

8

Porn-!ty .

12

10

8

6

4

- Type A

2 Type B

C] Sintered

0 I 1 1 1

0 50 100 150 200

Processing Pressure (MPa)

Figure 6b. Porosity measurements for samples HIPed at 9500C.

Porosity (%)14

12

100 o

x8

0

6 A 950

+ A 8204

0 B 950 0

0] B 820 +,

2X sintered _

03.5 4 4.5 5 5.5 6 6.5

Density (g/cm')

Figure 6c. Porosity versus density correlation for all samples.

Young's modulus results are presented in Figures 7a and 7b. The modulus increased with

increasing pressure. Samples HIPcd at 820()C had higher modulus values than those HIPcd at

950(C. For a given HIP temperature, type B samples had lower modulus values than type A

samples. Figure 7c shows the trend of increasing Young's modulus with higher density.

9

E (GPa)140,

120 L

100 /

80

60 -

40O4 Type A

20 - Type B

C Sintered0 ! I I I I

0 50 100 150 200

Processing Pressure (MPa)

Figure 7a. Young's modulus results for samples HIPed at 8200C.

E (GPa)140

120 L100 -

So B60

40 -Type A

20 + Type 8 +

C Sintered

0 1 1 1 1 1

0 50 100 150 200

Processing Pressure (MPa)

Figure 7b. Young's modulus results for samples HIPed at 9500C.

10

E (GPa)140

A g50120 + A 820 +

0 8 950 %100 * +

1 0" B 820

X sintered80

Do60 o

40 0

0

20

0 I 1 I

3.5 4 4.5 5 5.5 6 6.5

Density (g/cm')

Figure 7c. Young's modulus versus density correlation for all samples.

Figures 8a and 8b show the Knoop hardness data. The hardness increased with increas-ing HIP pressure. Samples HIPed at 8200 C had a higher hardness than those HIPed at950 0C. For each temperature, type B samples had lower hardness values than type A sam-ples. Figure 8c indicates that hardness rises with increasing density of the sample.

Knoop hardness (GPa)5

4

0

3

2

1 TypeA

+ Type B

o Sintered

00 50 100 150 200

Processing Pressure (MPa)

Figure 8a. Knoop hardness results for samples HIPed at 8200C.

11

Knoop Hardness (GPa)5 -

. Type A

+ Type B4 C Sintered

3

2 0+

+0 +

0 I I

0 50 100 150 200

Processing Pressure (MPa)

Figure 8b. Knoop hardness results for samples HIPed at 9500 C.

Knoop Hardness (GPa)5

A 950+ A 820

O B 950

0 B 820 X0

3 X sintered ++

• 02~ 0

0

1i 0

0 L I f

3.5 4 4.5 5 5.5 6 6.5Density (/cm )

Figure 8c. Knoop hardness versus density correlation for all samples.

The following explanation of the above trends is proposed. All samples were sealedunder vacuum in glass tubes. During the initial sequence of the HIP cycle, the samples wereheated to the soak temperature (820')C and 950 0C) with the HIP chamber under vacuum orvery low pressure. Figure 2 illustrates these conditions, which allowed oxygen from the

12

samples in the tube to dissociate. The evolved gas became entrained in the sample, inhibitingpore closure during HIPing. Samples heated to 950 0C generated more oxygen gas, contribut-ing to their higher porosity and lower density. The lower densities of these samples thenresulted in poorer physical properties (hardness, modulus).

Comparison of the HIPed samples with the sintered control samples shows that animprovement in the physical properties was attained using HIPing. Properties are mostenhanced for samples HIPed at 8200C and 207 MPa. Under those conditions, HIPed sampleshad significantly better properties than the sintcrcd samples. An overall evaluation showsthat HIPing samples at 8200C and 207 MPa with BaO 2 produced samples with good physicaland superconducting properties.

Transition Temperatures

A typical example of the trace derived from a.c. magnetization measurements is shown inFigure 9. The ordinate is shown in arbitrary units while the abscissa is in degrees Kelvin.The sharp bend in the curve is indicative of diamagnetic shielding and defines the T c of thematerial. Use of this magnetic transition, instead of the resistive transition, offers two dis-tinct advantages. First, no leads or connections must be made, making the measurement non-intrusive. Second, this method avoids the spurious results of resistive measuieimt;nts caused bylead connections and grain boundary effects.

Output (V)0.78

92 K

0.77

0.76

0.75

0.74

0.73 ,,

0.72 1I75 80 85 90 95 100 105 110 115

Temperature (K)

Figure 9. A.C. magnetization trace for a sintered sample.The sharp bend in the curve indicates the Tc.

Figure 10a ;hows the Tc of samples HIPed at 820')C. Type B samples recorded higherTc's than type A samples. The difference in Tc varied between 6 and 12 degrees Kelvin.

The Tc's of the type B samples compared favorably with those of the sintercd control specimens.

13

Transition Temperature (K)100

9 5

90 F

85

80 - * Type A

+ Type B

0 Sintered

75 I I

0 50 100 150 200

Processing Pressure (MPa)

Figure lOa. Tc for samples HIPed at 8200C.

The release of oxygen from the BaO2 at elevated temperatures establishes a higher oxy-gen partial pressure within the capsule. The higher Tc's for the type B samples suggests thatthis environment promotes a higher final oxygen content in the HIPed sample. This hypothe-sis agrees with reported literature which finds that the Tc increases with increasing oxygen con-tent of the ceramic. 14

An overall evaluation shows that samples HIPed with BaO 2 at 8200 C and 207 MPa pos-sessed good physical properties and high superconducting Tc's. These characteristics are equalto or surpassing those of sintered samples. In addition, these properties are obtained at aprocessing temperature 140 0 C lower than that used for sintering.

Figure l0b shows the Tc's of samples HIPed at 950 0 C. These results are even lowerthan those for samples HIPed at 820C. Some T.'s were below 77 0 K and could not be mea-sured by the test system used in this work.

In YBa 2Cu 3OT. x, TC is maximized at x = 0 and it decreases with the decreasing oxygencontent of the material. 1 Therefore, any processing step which leads to decreased oxygencontent in the material will lower that material's Tc. At 950 0 C, both a greater dissociation ofYBa 2Cu 307.x and an increased loss of oxygen through the encapsulant material occur. It isproposed that these processes lead to a decreased oxygen content in the HIPed samples andresults in lower Tc's for these samples.

14. MI-,PII IY. 1). W,, JOI INSON. Jr.. 1). W., JIN. S.. and I IOWARD, It. i. i'rtwing Tchnolost'es for the 9.r"K Supercondcior BO2l'(YCHO7.

Science, v. 241, August 19. 1988. p. 923.

14

Transition temperature (K)100

95

9] --

90

85

+ +

80 * Type A

+ Type B±

C Sintered

7 5 ' p I

0 50 100 150 200Processing Pressure (MPa)

Figure lOb. Tc for samples HIPed at 9500C.

SUMMARY

HIPing of the ceramic superconductor YBa 2Cu 307-. was found to be a simple and reli-able method for the consolidation of bulk shapes. The following observations are made:

" HIPing can produce samples with better physical properties at lower processingtemperatures than conventional sintering. These improved physical properties areshown to correlate well with higher densities.

* The addition of the BaO2 oxidant allows for HIPed samples to retain high Tc's.Samples HIPcd at 8200 C with BaO2 had Tc's greater than 920 K.

" Unlike results reported previously in the literature, these Tc's were achieved in thefinal shape without a post-HIP anneal.

" The careful tailoring of HIP processing conditions (temperature, pressure, cncapsulantand oxidant) can result in samples produced with both enhanced physical propertiesand retained superconducting properties.

ACKNOWLEDGMENTS

The authors thank L. Tardiff for ultrasonic measurements and Young's Modulus determina-tion, R. Lancto for Knoop hardness measurements and R. Hinxman for X-ray diffraction work.

15

REFERENCES

1. BEDNORZ, J. G. and MULLER. K. A. Possible lligh-T, Superconductivi.ty in the Ba-La-Cu-O System. Z. Phys. B.. v. (, 1986, p. 189-193-2. WU. M. K. et al. Superconductivity at 93 K in a New Mired Phase Y-Ba-Cu-O Compound system at Ambient Pressure. Phys. Rev. I-elt.

v. 5s, 1987. p. 908.3. TIEN. J. K., HENDRIX. 13. C., BOROFKA, J. C., and ABE, T. Hot Isostatic Pressing (l11P) for the Densification of Oxide Sunercondactiors.

Mat. Res. Soc. Spring Meeting, 1988.4. SNOW, D. B., WEINBERGER, B. R., PETERSON. G. G., LYNDS. L., EASTON, H., BURILA. C. T., POTREPKA. D. M.. and

KUWABARA, M. Processing Microstructure and Properties of YBa,('u3O7.,lg Superconductors. Proceedings of TMS, March 1989.5. SADANANDA. K., SINGII. A. K., IMAN, M. A.. OSOFSKY, M., LETOU RNE-AU, V., and RICHARDS, L. E-. Effect of Hot Isostatic

Pressing on Rla.,(u O7 Sttuercotductors. Advanced Ceramic Materials. v. 3, no. 5, 1988. p. 524-526.6. NISK.A. J., LOBERG, B., and EASTERLING. K. Effect of OMgen Loss on Densiication Whlen Hot Isostatic Pressing Yla.'Cu.0 .,

J. Am. Ceram. Soc., v. 72, no. 8. 1989, p. 1508-1510.7. GALLAGHER, P. K. O'BRYAN, H. M., SUNSHINE, S. A., and MURPHY, D. W. (Mgen Stoichiomevy in Ba2YCu3O. Mat. Res. Bull..

v. 22, 1987, p. 995-1006.8. O gen HIP for Superconducting Ceramics Research Fabrication. Nebea Keizai Shimbunsha. January 24, 1988, p. 4.9. YAN, M. F., LING, H. C., O'BRYAN H M. GALLAGHER, P. K., and RHODES, W. W. Process-related Problems of YBa2Cu30,

Superconductor. Mat. Sci. and Eng., v. BI, 1488, p. 119-129.10. NAVEH, J. and PELLY, I. On the Preparation of YBa2Cu3O7. Ceramic Iligh-Temperatre Superconductor. Mat. Res. Bull., v. 24, 1989,

p. 282-287.11. TARASCON, J. M., BARBOUX, P. B.. BAGLEY B. G., GREENE, L. H., and HULL, G. W. On Srnthesis of High Tc Superconducting

Perotskites. Mat. Sci. and Eng., v. Bi, 1988, p. 29-6.12. YAN, M. F., BARNS, R. L., O'BRYAN, Jr.. H. M., GALLAGHER, P. K., SHERWOOD, R. C., and JIN. S. Water Interaction with the

Superconducting YBa2Cu307 Phase. Appl. Phys. Lett., v. 51, 1987, p. 532.13. BARNS, R. L. and LAUDISE. R. A. Stabihir. of Superconducting Ya2CuO7 in the Presence of Water. Appi. Phys. Lett., v. 51. no. 17,

1987, p. 1373-1375.14. MURPHY. D. W., JOHNSON. Jr., D. W., JIN, S.. and HOWARD, R. E. Processing Technologies for the 93K Superconductor Ba'YCuO7.

Science, v. 241, August 19, 1988, p. 923.

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