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Preparation of YBaCuOAg composites by control of the sintering atmosphere
~Solid State Communications, Vol. 74, No. 12, PP. 1291-1294,1990. Printed in Great Britain.
0038-1098/9053.00+.00 Pergamon Press plc
PREPARATION OF YBaCuO-Ag COMPOSITES BY CONTROL OF THE SINTERING ATMOSPHERE J.S. Moya and E. Saiz Instituto de Cer~rtica y Vidrio, C.S.I.C. 28500 Arganda del Rey, Madrid, Spain. Received Feb. 8, 1990 by M. Cardona Close to theoretical density YBaCuO/25 wt % Ag composites have been obtained at 930 gC in Pch--0.21 atm. This composite shows -10 times higher oxygen gain rate at 350 ~C and -35 times lower degradation kinetics by water than the YBaCuO compacts. The YBaCuO-Ag compact reaches saturation of 02 after 8 h annealing in air at 350 -°C and a sharp transition at 92 K.
INTRODUCTION Since the discovery of the new ceramic superconductors a tremendous effort has been made to obtain materials with the optimal properties susceptible of technological applications. However at present manufacturing of the new ceramic superconductors is prevented due to the following problems: i) Poor mechanical properties [ 1]. ii) Chemical instability [2]. iii) Difficulty in obtaining materials with density close to theoretical, maximum oxygen content and sharp superconducting transitions at Tc>90K [3,4]. iv) Lack of reliability of the processing flow chart [5]. iv) Low critical currents [1]. In the present work we show the possibility of drastically improving these properties in the YBaCuO superconductors by: i) Metallic silver addition. ii) Control of the sintering atmosphere.
EXPERIMENTAL Powders of YBa2Cu+OT~ were obtained by appropriate mixing of C u t (99.9%, Merck, FRG), Y203 (99.99%, Atomergic Chemetals Corp., USA) and BaCO3 (99.7%, Pro-Bys, Spain) by attrition milling in isopropyl alcohol media with zirconia balls,subsequently dried at 60 ~C for 20 h, and calcined twice at 910 QC 20 h in air. The powders were ground to a grain size <35 I.tm, after each calcination. After the synthesis the powders were attrition milled for 1 h. to reach ~5 ~tm average particle size. XRD study of the obtained powder showed pure 1:2:3 with orthorhombic structure. These powders were homogeneously mixed with metallic silver of -0.5 ~tm average particle size (99.9%, Western Gold and Platinum, USA) in isopropyl media in different proportions (0/25 wt % of
silver), and then uniaxilly pressed at 190 MPa. into pellets of ~10 mm diameter, - 3 mm thickness. The influence of the sintering atmosphere was followed by hot stage optical microscopy up to 1010 ~C in air and pure 02 using unfired samples with 25 wt % silver content. The samples were sintered in air at 930/950 ~C-18 h with subsequent annealing in air at 350 gC and then analyzed by XRD, RLOM SEM and EDX. Resistivity measurements of the samples in the range 30 to 300 K were carried out using the four probe technique with silver paint contacts. The dependance of the oxidation rate upon silver content was analyzed by isothermal TG in air at 350 °C. Degradation tests were conducted in air by immersing a sintered sample (-0.5 g) in 20 ml of deionized water following the variation of pH with time. The tests were performed using a plastic container to avoid the presence of leachates from the glass. Chemical analyses of the water after the test were performed by ICP technique. Four probe critical current measurements of pure YBaCuO and 25 % Ag sintered samples at 75.5 K were performed using 1 ms current pulses.
RESULTS AND DISCUSSION Figure 1 shows the effect of sintering atmospheres on the shrinkage rate of h2:3/25 wt % Ag composites. As observed the effect of the oxygen partial pressure, Po2, on densification process is drastic. In the case of Po2--0.21, 50 vol % shrinkage vs. 8 vol % shrinkage in pure oxygen (Po2=l) at 910 QC was obtained. Taking into account this result the pellets were sintered in air at 930 gC-18 h obtaining 99 % th. density for sample containing 25 wt % Ag. In samples without Ag the maximum density was <92% th. The microstructure of the 1:2:3/ 25 wt % Ag composite is shown in Fig. 2. The SEM micrograph
1291
1292
VOI. 74, NO. 12
CONTROL OF THE SINTERING ATMOSPHERE
dearly shows that the Ag completely wets the 1:2:3 superconducting grains. This microstructure was not observed in samples sintered in Po2=l atm; in this particular case silver is dispersed as individual grains [6,7].This result is in complete agreement with the wetting behaviour of Ag on 1:2:3 compact as a function of 1'o2 observed by Tomsia et al. [8] by using a sessile drop technique. Figure 3 shows the oxygen gain rate for 1:2:3/25% Ag sintered composite. In spite of the substantial difference in density between both compacts (-20% th), the oxygen rate value corresponding to silver containing composite is -10 times higher than the one corresponding to 1:2:3 compound. This behaviour can be explained considering the particular microstructure of this composite (fig. 2) where -250 nm continuous silver layer between grains allows a fast transport of the 02 into the bulk of the samples. The effect of the annealing time on the resisdv~y measurements in dense (99% th.) 1:2:3/25% Ag composite and in 85% th 1:2:3 compacts is shown in fig. 4. The sample without silver has T(p=0)=42 K after 8h annealing; after reannealing for 48 h the same sample reaches T(9=0)=75 K with a A=6 K. Conversely the silver containing sample shows maximum oxygen content after 8 h annealing and a T( 9--0)---92 K and A = I K. These results clearly confirm the beneficious effect of the mentioned microstructure in the oxidation rate of dense superconducting compacts.
Fig. 1. Hot stage optical micrographs corresponding to 1:2:3/25 wt. % Ag composition at 20~C and at 910 -°C in two different fwing atmospheres.
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Fig.2. SEM micrographs corresponding to the fracture surface of 1:2:3/25 wt. % Ag compact sintered in air ( Po2--0.21 atm. ) at 930 ~C-18 h.
Fig. 3. Resistivity vs absolut temperature curves corresponding to: (A) 1:2:3 compact 8 h, annealing in air, (B) the same sample after additiaonal 48 h. annealing and (C) 1:2:3/25 wt. % Ag compact annealed in air for 8 h.
Vol. 74, No. 12
CONTROL OF THE SINTERING ATMOSPHERE
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Fig. 4. Weight gain vs. time at 350 °C in air for sintered compact containing different amounts of silver. Degradation kinetics is another important parameter that can be affected by the addition of silver. Fig 5 shows the variation of pH vs. time in the water containing the superconducting sample; this variation is mainly due to selective barium leaching from the superconducting policrystal. As higher is the pH of the liquid, higher is the amount of barium present as has been determined by chemical analysis. The slope of the pH variation vs. time is about 35 times higher in the case of 1:2:3 compact. It seems that silver can act in two ways: i) Filling voids difficulting the access of water to the interior of the sample but allowing the introduction of oxygen due to the large solubility of oxygen in silver. ii) Cleaning grain boundaries of phases that can be susceptible to water attack (BaCO3, BaCuO2 ...). As can be seen in figure 6 the addition of a 25 wt. % Ag enhances the critical current from 37 to 80 A/cm2. Probably optimization of the thickness and composition in the intergranular layer can lead to higher critical current densities. The reliability of the results obtained in the present investigation was confirmed in at least 10 different batches and the electrical measurements were very similar in three different laboratories (Dpt. Materials Science Univ. of Cantabria, Sandia Nat. Lab. and Inst. Ceram. y Vidrio). Considering that silver is a very plastic material the presence of this metal enhances the thoughness of YBaCuO/Ag composites as has been reported by Nishio et al. [7]. Up to now, the obtention of high dense YBaCuO compact (>95% th.) requires non conventional processing such as hot pressing, or the presence of a liquid phase during sintering wich degradates the grain boundaries and consequently the Jc and Tc. In the case of pure YBaCuO dense compacts, a serious problem is the oxidation rate during annealing. Clarke et al.[4] have studied this fact and concluded that several hundred hours annealings are required to completely saturate the structure of oxygen.
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Fig. 5. pH vs. times plots for (A) 1:2:3 sintered compacts; (B) 1:2:3/25 wt. % Ag compact.
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Fig. 6. Four probe critical current measurement for (A) 1:2:3 sintered compact; (B) 1:2:3/25 wt. % Ag compact.
These facts are serious inconvenients to produce reliable results and justify the large variety of values obtained in the literature. In summary the results obtained in the present work offer the possibility of manufacturing real superconducting materials such as wires, magnetic shielding, tores etc, with almost theoretical density, maximum oxygen content, high chemical resistance and improved mechanical properties following a simple ceramic processing route. Acknowledgements- The authors thank S. de Aza (I.C.V.) and A.P. Tomsia (U.C. Berkeley) for helpful discussions and J.C. Gomez Sal and J. Rodriguez (U.Cantabria) and R. Loehman (Sandia Nat. Lab.) for experimental assistance.
1294
CONTROL OF THE SINTERING ATMOSPHERE
Vol. 74, NO. 12
REFERENCES 1. L. Gervin, P. Campbell, Nature, 330, 611 (1987). 2. K.G. Frase, E.C. Liniger, D.R. Clarke, Adv. Ceram. Mater., 2 NO 3B, 698(1987). 3. R.E. L o e h m a n , W.F. Hammeter, E.L. Venturini, R.H. Moore, F.P. Gersfle Jr, J. Am. Ceram. Soc., 70 [4], 669(1989). 4. D.R. Clarke, T.M. Shaw, D. Dimos, J. Am. Ceram. Soc., 72 [7], 1103(1989). 5. K.G. Frase, D.R. Clarke, Adv. Ceram. Mater., 2 NO 3B, 295(1987).
6. C.S. Pande, H.A. Hoff, T.C. Francavilla, L.E. Richards, H.R. Khan, High Temperature Superconducting Compounds Processing & Related Properties, Edited by S.H. Whang and A. DasGupta, 155(1989). 7. T. Nishio, Y. Itoh, F. Ogasawasa, M. Suganum, Y. Yamada, O. Mizutani, J. Mater. Sci., 24, 3228 (1989). 8. A.P. Tomsia, J.A. Pask, R.L. Loehman, 41st Pacific Coast Regional Meeting, American Ceramic Society, 1988.
0038-1098/9053.00+.00 Pergamon Press plc
PREPARATION OF YBaCuO-Ag COMPOSITES BY CONTROL OF THE SINTERING ATMOSPHERE J.S. Moya and E. Saiz Instituto de Cer~rtica y Vidrio, C.S.I.C. 28500 Arganda del Rey, Madrid, Spain. Received Feb. 8, 1990 by M. Cardona Close to theoretical density YBaCuO/25 wt % Ag composites have been obtained at 930 gC in Pch--0.21 atm. This composite shows -10 times higher oxygen gain rate at 350 ~C and -35 times lower degradation kinetics by water than the YBaCuO compacts. The YBaCuO-Ag compact reaches saturation of 02 after 8 h annealing in air at 350 -°C and a sharp transition at 92 K.
INTRODUCTION Since the discovery of the new ceramic superconductors a tremendous effort has been made to obtain materials with the optimal properties susceptible of technological applications. However at present manufacturing of the new ceramic superconductors is prevented due to the following problems: i) Poor mechanical properties [ 1]. ii) Chemical instability [2]. iii) Difficulty in obtaining materials with density close to theoretical, maximum oxygen content and sharp superconducting transitions at Tc>90K [3,4]. iv) Lack of reliability of the processing flow chart [5]. iv) Low critical currents [1]. In the present work we show the possibility of drastically improving these properties in the YBaCuO superconductors by: i) Metallic silver addition. ii) Control of the sintering atmosphere.
EXPERIMENTAL Powders of YBa2Cu+OT~ were obtained by appropriate mixing of C u t (99.9%, Merck, FRG), Y203 (99.99%, Atomergic Chemetals Corp., USA) and BaCO3 (99.7%, Pro-Bys, Spain) by attrition milling in isopropyl alcohol media with zirconia balls,subsequently dried at 60 ~C for 20 h, and calcined twice at 910 QC 20 h in air. The powders were ground to a grain size <35 I.tm, after each calcination. After the synthesis the powders were attrition milled for 1 h. to reach ~5 ~tm average particle size. XRD study of the obtained powder showed pure 1:2:3 with orthorhombic structure. These powders were homogeneously mixed with metallic silver of -0.5 ~tm average particle size (99.9%, Western Gold and Platinum, USA) in isopropyl media in different proportions (0/25 wt % of
silver), and then uniaxilly pressed at 190 MPa. into pellets of ~10 mm diameter, - 3 mm thickness. The influence of the sintering atmosphere was followed by hot stage optical microscopy up to 1010 ~C in air and pure 02 using unfired samples with 25 wt % silver content. The samples were sintered in air at 930/950 ~C-18 h with subsequent annealing in air at 350 gC and then analyzed by XRD, RLOM SEM and EDX. Resistivity measurements of the samples in the range 30 to 300 K were carried out using the four probe technique with silver paint contacts. The dependance of the oxidation rate upon silver content was analyzed by isothermal TG in air at 350 °C. Degradation tests were conducted in air by immersing a sintered sample (-0.5 g) in 20 ml of deionized water following the variation of pH with time. The tests were performed using a plastic container to avoid the presence of leachates from the glass. Chemical analyses of the water after the test were performed by ICP technique. Four probe critical current measurements of pure YBaCuO and 25 % Ag sintered samples at 75.5 K were performed using 1 ms current pulses.
RESULTS AND DISCUSSION Figure 1 shows the effect of sintering atmospheres on the shrinkage rate of h2:3/25 wt % Ag composites. As observed the effect of the oxygen partial pressure, Po2, on densification process is drastic. In the case of Po2--0.21, 50 vol % shrinkage vs. 8 vol % shrinkage in pure oxygen (Po2=l) at 910 QC was obtained. Taking into account this result the pellets were sintered in air at 930 gC-18 h obtaining 99 % th. density for sample containing 25 wt % Ag. In samples without Ag the maximum density was <92% th. The microstructure of the 1:2:3/ 25 wt % Ag composite is shown in Fig. 2. The SEM micrograph
1291
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VOI. 74, NO. 12
CONTROL OF THE SINTERING ATMOSPHERE
dearly shows that the Ag completely wets the 1:2:3 superconducting grains. This microstructure was not observed in samples sintered in Po2=l atm; in this particular case silver is dispersed as individual grains [6,7].This result is in complete agreement with the wetting behaviour of Ag on 1:2:3 compact as a function of 1'o2 observed by Tomsia et al. [8] by using a sessile drop technique. Figure 3 shows the oxygen gain rate for 1:2:3/25% Ag sintered composite. In spite of the substantial difference in density between both compacts (-20% th), the oxygen rate value corresponding to silver containing composite is -10 times higher than the one corresponding to 1:2:3 compound. This behaviour can be explained considering the particular microstructure of this composite (fig. 2) where -250 nm continuous silver layer between grains allows a fast transport of the 02 into the bulk of the samples. The effect of the annealing time on the resisdv~y measurements in dense (99% th.) 1:2:3/25% Ag composite and in 85% th 1:2:3 compacts is shown in fig. 4. The sample without silver has T(p=0)=42 K after 8h annealing; after reannealing for 48 h the same sample reaches T(9=0)=75 K with a A=6 K. Conversely the silver containing sample shows maximum oxygen content after 8 h annealing and a T( 9--0)---92 K and A = I K. These results clearly confirm the beneficious effect of the mentioned microstructure in the oxidation rate of dense superconducting compacts.
Fig. 1. Hot stage optical micrographs corresponding to 1:2:3/25 wt. % Ag composition at 20~C and at 910 -°C in two different fwing atmospheres.
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0.05
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.......
Fig.2. SEM micrographs corresponding to the fracture surface of 1:2:3/25 wt. % Ag compact sintered in air ( Po2--0.21 atm. ) at 930 ~C-18 h.
Fig. 3. Resistivity vs absolut temperature curves corresponding to: (A) 1:2:3 compact 8 h, annealing in air, (B) the same sample after additiaonal 48 h. annealing and (C) 1:2:3/25 wt. % Ag compact annealed in air for 8 h.
Vol. 74, No. 12
CONTROL OF THE SINTERING ATMOSPHERE
1293
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Fig. 4. Weight gain vs. time at 350 °C in air for sintered compact containing different amounts of silver. Degradation kinetics is another important parameter that can be affected by the addition of silver. Fig 5 shows the variation of pH vs. time in the water containing the superconducting sample; this variation is mainly due to selective barium leaching from the superconducting policrystal. As higher is the pH of the liquid, higher is the amount of barium present as has been determined by chemical analysis. The slope of the pH variation vs. time is about 35 times higher in the case of 1:2:3 compact. It seems that silver can act in two ways: i) Filling voids difficulting the access of water to the interior of the sample but allowing the introduction of oxygen due to the large solubility of oxygen in silver. ii) Cleaning grain boundaries of phases that can be susceptible to water attack (BaCO3, BaCuO2 ...). As can be seen in figure 6 the addition of a 25 wt. % Ag enhances the critical current from 37 to 80 A/cm2. Probably optimization of the thickness and composition in the intergranular layer can lead to higher critical current densities. The reliability of the results obtained in the present investigation was confirmed in at least 10 different batches and the electrical measurements were very similar in three different laboratories (Dpt. Materials Science Univ. of Cantabria, Sandia Nat. Lab. and Inst. Ceram. y Vidrio). Considering that silver is a very plastic material the presence of this metal enhances the thoughness of YBaCuO/Ag composites as has been reported by Nishio et al. [7]. Up to now, the obtention of high dense YBaCuO compact (>95% th.) requires non conventional processing such as hot pressing, or the presence of a liquid phase during sintering wich degradates the grain boundaries and consequently the Jc and Tc. In the case of pure YBaCuO dense compacts, a serious problem is the oxidation rate during annealing. Clarke et al.[4] have studied this fact and concluded that several hundred hours annealings are required to completely saturate the structure of oxygen.
I
Fig. 5. pH vs. times plots for (A) 1:2:3 sintered compacts; (B) 1:2:3/25 wt. % Ag compact.
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Fig. 6. Four probe critical current measurement for (A) 1:2:3 sintered compact; (B) 1:2:3/25 wt. % Ag compact.
These facts are serious inconvenients to produce reliable results and justify the large variety of values obtained in the literature. In summary the results obtained in the present work offer the possibility of manufacturing real superconducting materials such as wires, magnetic shielding, tores etc, with almost theoretical density, maximum oxygen content, high chemical resistance and improved mechanical properties following a simple ceramic processing route. Acknowledgements- The authors thank S. de Aza (I.C.V.) and A.P. Tomsia (U.C. Berkeley) for helpful discussions and J.C. Gomez Sal and J. Rodriguez (U.Cantabria) and R. Loehman (Sandia Nat. Lab.) for experimental assistance.
1294
CONTROL OF THE SINTERING ATMOSPHERE
Vol. 74, NO. 12
REFERENCES 1. L. Gervin, P. Campbell, Nature, 330, 611 (1987). 2. K.G. Frase, E.C. Liniger, D.R. Clarke, Adv. Ceram. Mater., 2 NO 3B, 698(1987). 3. R.E. L o e h m a n , W.F. Hammeter, E.L. Venturini, R.H. Moore, F.P. Gersfle Jr, J. Am. Ceram. Soc., 70 [4], 669(1989). 4. D.R. Clarke, T.M. Shaw, D. Dimos, J. Am. Ceram. Soc., 72 [7], 1103(1989). 5. K.G. Frase, D.R. Clarke, Adv. Ceram. Mater., 2 NO 3B, 295(1987).
6. C.S. Pande, H.A. Hoff, T.C. Francavilla, L.E. Richards, H.R. Khan, High Temperature Superconducting Compounds Processing & Related Properties, Edited by S.H. Whang and A. DasGupta, 155(1989). 7. T. Nishio, Y. Itoh, F. Ogasawasa, M. Suganum, Y. Yamada, O. Mizutani, J. Mater. Sci., 24, 3228 (1989). 8. A.P. Tomsia, J.A. Pask, R.L. Loehman, 41st Pacific Coast Regional Meeting, American Ceramic Society, 1988.