Reactivity of Ceramic Superconductors with Palladium Alloys

zyxw zyx J. Am. C e m . Soc.,73 161 1760-62 (1990) z zyxwvu zyxwvu zyx zyxwvu Reactivity of Ceramic Superconductors with Palladium Alloys Jennifer L. Porter,* Thirukumar K. Vethanayagam,* Robert L. Snyder,* and Jenifer A. T. Taylor* New York State College of Ceramics at Alfred University, Alfred, New York 14802 Palladium alloy compositions were investigated for suitability as a nonreactive material for the processing of ceramic superconductors. Barium-based superconductors were tested on Pd-Au and Pd-Ag alloys for reactivity. Bismuth-based superconductors were tested on a Pd-Ag alloy. The least reactive was found to be 70% Pd-30% Ag for barium-based high-temperature superconductors (HTSC), whereas 30% Pd-70% Ag was found to be least reactive for bismuth-based HTSC. [Key words: superconductors, barium, bismuth, electrical properties, palladium.] I. Introduction P alloys were investigated as a possible barrier layer between high-temperature superconductors (HTSC) and substrates. The processing of superconducting materials greatly influences their final properties. According to studies by Roth,' Frase: and others, BazYCu307-x(BYC213) material is essentially a point component. Hence, it is difficult to produce pure BYC213, especially because of its reactivity with other compounds. One frequently observes crack formation, crystal orientation, phase separation, and chemical reactions, all of which involve interactions between the superconductor and the substrate.' ALLADIUM R . S. Roth-contributing editor Manuscript No. 198220. Received July 15, 1989; approved January 26, 1990. Supported by the Center for Advanced Ceramic Technology. 'Member, American Ceramic Society. 11. Experimental Procedure The specimens were prepared with the raw materials and firing profiles described in Tables I and 11. The mixed powders (50 g) were milled for 24 h in a small polypropylene container with distilled water and zirconia grinding media. The suspension was dried overnight, and the powder was calcined, ground, and recalcined. Bars 25 mm x 6 mm x 3 mm were formed from 3 g of powder; they were cut in half and placed on foil on an alumina setter. After being cleaned with acetone, the foil was flattened to make sure there was consistent contact between the bar and the foil. Note that flattening can induce dislocations and possibly increase the foil reactivity. The bars were fired in a furnace with heating and cooling rates of approximately 10"C/min. 111. Discussion of Results ( I ) BYC213 Fired on 95% Au-5% Pd The 95% Au-5% Pd alloy in contact with BYC213 during firing at 980°C showed evidence of reaction on the foil (Fig. 1). A scanning electron microscopy (SEM) photograph (Fig. 2) taken at 3 0 0 ~depicts a reaction site on the foil. Through energy dispersive spectroscopy (EDS) analysis, done on the backside of the foil, it was found that the center of the reaction site (labeled 1 on Fig. 2) contains all three original raw materials (Cu, Ba, and Y)in addition to the alloy (Fig. 3). The outer edge of the reaction site (labeled 2 on Fig. 2) is higher in Cu and Ba. The Au-Pd matrix directly around the reaction site (labeled 3 on Fig. 2) also contains Cu which possibly suggests that Cu has the highest diffusion coefficient of the three (Fig. 4). Similar results were found upon examining the bar and foil fired at 1050°C. Table I. Summary of Experimental Parameters for BYC213 Study Values Parameters Setter Y2O3,CuO, BaC03 900°C for 12 h in oxygen 900°C for 12 h in oxygen Alumina with surface coating of BYC213 Superconducting phase confirmation X-ray diffraction. Forming pressure Sintering 95% Au-~%Pd 20 MPa Raw materials Calcined Recalcined zyxw zyxwv W, looo", lOW, 1060",and 1125"C, each for 2 h followed by 550°C for 2 h, at 1 atm oxygen 980°C for 12 h, 1100°C for 6 h, followed b 550°C for 12 h; 30% Ag-78% Pd (mp = 1140°C)' 40% Ag-60% Pd (mp = 1200°C) 50% &SO% Pd (mp = 1260°C) 60% 440% Pd (mp = 1315°C) 70% Ag-30% Pd (mp = 1375°C) Ag-Pd *X-ray diffraction on powder taken from 20" to 60" 28; count time 2 s per 0.05" step. 'Melting point of Pd-Ag binary. 1760 zyxwvutsrqp zyxwvutsrq zyxwvuts 1761 Communications of the American Ceramic Society June 1990 Table 11. Summary of Experimental Parameters for Bismuth HTSC Study* Parameters Values Raw materials Bi203, CuO, CaCO3, SrC03 Product Calcined B~ZS~Z,SC~O.~C~ZO, 650°C for 12 h in air Recalcined 825°C for 16 h in air Setter Alumina Forming pressure 27 MPa Sinterin Ag-Pf zyxwvutsr zyxwvuts 865"C, 980°C for 12 h 30% Ae-70% Pd 40% A & O % Pd 50% Ag-50% Pd Pd 60% 440% 70% &30% Pd zyxwvut zyxwvu *Note: superconducting phase was not identified. The Meissner effect was a qualitative technique used to evaluate the effect of temperature on BYC213. Bars fired at 980" and 1OOO"C showed visible Meissner effect; but the bars fired at 1050" and 1060°C did not. Melt-processed BYC213 showed enhanced superconducting properties despite incongruent melting which usually causes an increase in the volume percent of BYC121.4 However, reaction of the liquid phase with the foil caused a change in stoichiometry, leading to significant decrease in the volume percent of BYC123, as indicated by the visible Meissner effect. During the 1125°C firing, the Au-Pd foil melted into the porous alumina setter showing that the reference value of 1160°C as the melting point of 95% Au-5% Pd is not valid when firing BYC213 in oxygen. (The furnace had been calibrated with a National Bureau of Standards standard thermocouple and was controlled to within 2°C of the programmed temperature). Cu and Au form a eutectoid at 889°C: Apparently, at this temperature, Cu is more stable in Au than in BYC213, causing the diffusion of Cu out of the BYC213 into the Au-Pd foil. (2) BYC213 Fired on Pd-Ag EDS analysis on the various Pd-Ag compositions fired at 980°C (Fig. 5) showed that Ba and Cu are both present in the 30% Pd-70% Ag, but reactivity decreased as the percentage of the Pd concentration increased. Therefore, it is better to use the 70% Pd-30% Ag foil. Similar results were obtained upon analyzing a cross section of the bars in contact with the foil. Even at 11OO"C, the bar fired on the 70% Pd-30% Ag Fig. 2. Microgra h of the back side of the 95% Au-5% Pd foil in Fig. 1: area found to contain Ba, Y, and Cu; (2) outer ed e of the reaction site contains Ba and Cu; and (3) Au-!d matrix directly surrounding the reaction site contains Cu (bar = 40 pm). (5 revealed a distinct gap between the BYC213 and the foil with no reaction apparent (Fig. 6), whereas, as seen in Fig. 7, there was a reaction interface between the bar and 30% Pd-70% Ag. As the percentage of Ag decreased, the reactivity with BYC213 also decreased. This observation is in agreement 213 R l SNC - WPO FOlL zyxwv EDCE OF RERRED ST01 C E N l E R OF REAClEO SPOl Fig. 3. EDS of the reaction site in area 1 in Fig. 2 shows the presence of Ba, Y, and Cu as well as Au and Pd. el3 RT *--RI-PO FOlL WTRIX REGION 8ssx COUWTS cu Fig. 1. 95% Au-5% Pd foil shows the site of a reaction between BYC213 and alloy (sintered at 980°C for 2 h and annealed at 550°C for 6 h in oxygen) (bar = 600 pm). nu Fig. 4. EDS of area 3 in Fig. 2 shows the presence of Cu that has diffused from the primary reaction site. 1762 zy z zyxwvutsrqpo Communications of the American Ceramic Society Vol. 73, No. 6 U W - M C SUBS1RnlE Bfi-RBRsED S I C 38PD-78RE SUBSlRRlE 3.08 PO CDu)(lS 0.0 Fig. 5. and Cu. WUICT IKEVI zyxwvutsr zyx t8. EDS of 30% Pd-70% Ag foil shows the Presence of Ba Fig. 7. Reaction zone develops at t h e interface between BYC213 and 30% Pd-70% Ag after sintering at 1100°C (bar = 20 pm). with work done at the National Institute for Standards and Technology which showed that 30% Pd-70% Ag reacts with BYC213.6 (3) Bismuth Compound Fired on Pd-Ag SEM and EDS were performed on the foils and cross section of the bars for the various Pd-Ag compositions fired at 865°C. These techniques showed that Cu was present in the 70% Pd-30% Ag foil, but decreased in concentration as the percentage of Pd in the foil decreased. SEM analysis of the various foils supports the conclusion that as the Pd content of the foil increased so does the reactivity. The most reacted foil, containing 70% Pd, is shown in Fig. 8. Upon analysis of a cross section of the bars in contact with the foil, the bar that was fired on the high-Pd-content foil was most reacted. zyx IV. Conclusions It was determined that 95% Au-5% Pd reacts with BYC213 superconductor at 980°C. At this temperature, in the presence of Au-Pd in oxygen, Cu appears to be more stable as a eutectoid with Au. Under these conditions it is apparent that the 95% Au-5% Pd alloy is not a satisfactory substrate for BYC213, because of the high reactivity between Au and Cu. The 70% Pd-30% Ag, foil is a suitable substrate for the BYC213 superconductor both at 980" and 1100°C. The results Fig. 8. Micrograph of 70% Pd foil that has reacted with t h e bismuth compound (bar = 200 rm). of this study indicate potential application of the 70% Fkl30% Ag foil as a conductive barrier layer between substrate and the BYC213 superconductor. In the case of the bismuth superconductor fired at 88OoC,the W o Pd-70% Ag foil is the least reactive of the compositions investigated. Future studies on the bismuth superconductor will need to be made to determine if 30% Pd-70% Ag is acceptable as a conductive barrier. Acknowledgment: The authors thank Williams Precious Metals for pmviding the alloys for thls study. References Fig. 6. Micrograph reveals a distinct gap between the BYC213 and t h e 70% Pd-30% A g foil after sintering at 1100°C (bar = 40 pm). 'R. S. Roth, K. L. Davis, and J. R. Dennis, "Phase Equilibria and Crystal Chemistry in the System Ba-Y-Cu-0," A d m Cemm. hfafer..,2 [3B] 303-12 (1987). 2K. G. Frase and D. R. Clarke, "Phase Compatibilities in the System Y2O,-BaO-CuO," A d m Ccmm. Mater., 2 [3B] 295-302 (1987). X.T. Cheung and E. Ruckenstein, "Superconductor-Substrate Interactions of the Y-Ba-Cu Oxide,"l Mafcr. Rcs., 4 [l] 1-15 (1989). '(a) K. Salama, V. Selvamanickam, L. Gao, and K. Sun, "High Current Density in Bulk BYC213 Superconductor,"A&. h y s . Lctr., 54 [23] 2352-54 (1989). (b) H.Kumakura, K. Takahashi, M.Uehara, and K. Togano, "Large Magnetization in BYC213 Prepared by Sintering at High-Temperature,"Ipn. 1 Appl. Phy~.Left., 27 [2] L188-L191 (1988). 'R. M. Brick, A.W. Pense, and R. B. Gordon, Structure and Properties of Engineering Materials, 4th ed.; p. 469. McCraw-Hill. New York, 1977. 6J. R. Verkouteren, "SEM Analysis of Interactions between Pt, Au, and AgPd Capsules and Barium-Yttrium-Copper Superconductors," Mafcr. Lett., 8 [1,2] 59-63 (1989). 0 zyxwv