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Superconducting properties and microstructure of YBCO HTSC materials with K2CO3 addition
Physica C 235-240 (1994) 805-806
PHY$1(A
North-Holland
SUPERCONDUCTING PROPERTIES AND MICROSTRUCTURE OF YBCO HTSC MATERIALS WITH K2CO 3 ADDITION A. Venevaa, I. Nedkova, V. Lovchinovb alnstitute of Electronics, Bulgarian Academy of Sciences, Blvd. Tzarigradsko Chaussee 72, Sofia 1784, Bulgaria blnstitute of Solid State Physics, Bulgarian Academy of Sciences, Blvd. Tzarigradsko Chaussee 72, Sofia 1784,
Bulgaria The influence of K2CO s additive on the superconducting properties and microstructure of samples with nominal compositions Yl-o. 2,,Ba2.o. 2,,K,, Cus 07.),,I"1Ba2 Cus. s.,,Kx 07.y , ~) Ba2.,,K,, Cus 07.y , ( x = 0 to 1.50), were investigated. Only traces of K (lxl0 "2 wt.%) were observed in all samples after the final baking and no proof could be found for alkali metal cations participating in the Perovsldte crystal cell. Potassium carbonate eliminated the CuO second phase from the system, because of a KCu-compound formation which evaporated above 900 ° C. The influence of the K2COs was positive for x = 0.40 to 0.75 where we observed homogenous grain-size ceramic structure and ItTSC Y1B%Cus@ samples with values of T c = 93.4 K and AT = 1.5 K.
INTRODUCTION The effect of adding carbonates of some alkali metals, particularly K2COs, in synthesizing YlBa2Cu30y high-temperature superconductor (HTSC) is an open question periodically arising in the literature without being unambiguously answered. Some authors have determined it as "catalytic" (1). Beating in mind the large ionic radius of potassium (K+, r i = 1.51/k) (2) it has been assumed that it can be introduced at barium's (Ba2÷, r i = 1.42 /k) place (3, 4). There have been reports that no trace of potassium could be found in the samples after the final thermal-treatment (5). In this work, we report investigations on the adding of I~2C0 s in a wide concentration range in the initial batch of YBCO system and the influence of the addition on the superconducting properties dll~d,
llllt~l
t J ~ t . lt L I ~ I . U l
~,..
EXPERIMENTAL Three series of materials were prepared with nominal compositions Yl-o. 2xBa2-o. :x K~ Cu s 07_y, 111Ba2Cu3. s-xKx 0T-y, YI Ba2-xKx Cus 0T-y, where x = 0 to 1.50. When sintering the HTSC, special attention was paid to the purity of the initial components: the basic 7(, 0 s was 99.99 %; BaCO s, 99.99 %; CuO, 99.99 %; and K2COsxl.5H20 was
99.90 %. The samples were obtained following the classic ceramic technology consisting in mixing of oxides and carbonales in the appropriate ratios and calcining the mixture in air at 900 ° C for 24 hours. The calcined powders with l-gin grain size were pressed into pellets which were then sintered once more. The samples formation was momtored by means of X-ray diffraction analysis (XRD); atomic absorption spectroscopy (AAS), scanning electron microscopy, energy dispersive X-ray spectra (SEM EDS) were used to study the microstructure and morphology of the crystalline grains in the samples prepared. The transition temperature dependence of HTSC samples was determined using the foux-probe "resistive" technique. RESULTS AND DISCUSSIONS We found that adding K2COs to the stardng batch lowered the sintering temperature of the highTc orthorhombic phase down to 950 ° C compaxed to that of pure giB%Cus@ (6). After the first heat° treatment (900 ° C/24 h) the YlBa2Cus@ phase was about 75 wt. % and the ~ data showed (SEM and EDS confirmed it) that BaCuO: , CuO and C u # phases existed in samples with higher K2CO3 content. Peaks of K-additive related phase were also
0921-4534/94/$07.00 © 1994 - Elsevier Science B.V. All rights reserved. SSDI 0921-4534(94)00966-X
806
,4. Veneva et al./Physica C 235-240 (1994) 805-806
found. Some aut',ors (4) claim to have observed KCuO peaks. The analysis of our X-ray patterns revealed the existence of KCu-compounds peaks up to 900 ° C, determined to be K:CuO 2 : (204, 302), d = 2.632 A,; (220), d = 3.040 A; (213), d = 2.906 A, (7). The lack of K~CO3 phase in the X-ray patterns at 900 ° C allows the supposition that CuO reacts with K2COs forming one or two types of potassiumcepper compounds. This interaction is possible as a result of the reduction of CuO to Cup at temperature above 800 ° C (8). After a second heattreatment at 950 ° C for 48 h the XRD patterns show that the system with x = 0 to 0.75 is YiBaFusOy single-phase ; for x > 0.95 the system ~as multiphase. Using AAS, (the samples were dissolved in HCI), we studied the K content in the samples. The AAS data of a series of samples baked at 950 ° C for 48 h with x varying from 0 to 0.75 show that potassium remains in all samples in amounts within lx 10z wt.%. Fig. 1 illustrates the characteristic stratification of a sample where the K-additive in the starting batch was x = 0.95.
Fig. 1. SEM micrograph of Yo.s~Ba:.sl Ko.p5Cu.~07.,, sample, where (A) is YiBa2Cu3Oz.y phase; (B) is KsCuO2 phase. The SEM EDS data confirm potassiumcopper compound (B phase). Fig. 2 shows typical results of electrical resistance measurements on samples with d~fferent K content in the starting batch. For values of x up to approximately 0.75, the R (T) data demonstrated that the materials regularly exhibits T c = 93.4 K and AT = 1.5 to 1.7 K. T~ explain the peculiarities observed resulting from
adding K we turned to the following changes in the polycrystalline microstructure. . . . . . . . . . . .
(d)
f
i,---
(c)
-)
{B)
--
78 s2 g6 90
9'8 . . . .
I
30o
TEMPERATURE, K Fig. 2. Temperature dependence of the resistance for: (a) Yi Ba2 Cus O7_y; (b) Yt-o.2xBa2-o.2xKxCupT-v (c) Yt Ba2.xK:,Cu3 07-y ; (d) Yi Ba2 Cus. s-~K~O7.y , where x = 0.75. When K2CO3 was increasing in the initial batch we observed a highly homogenous sma!l-grain ceramic structure. This gave us grounds to assume that the potassium-copper compound observed participates actively in the polycrystalline structure formation, most probably as a liquid film on the grains boundaries preventing ~eir growth; in addition, this second phase does not degrade the system's superconducting properties in the concentration range x = 0.40 to 0.75. REFEI~NCES ('i) R.J. Cava, J. J. Kraje.wski, W. F. Peck, B. Bat|ogg, I,. M. Rupp Jr., Letters to Nature, 338 (1989) 328. (2) R.D. Shannon, Acta Crystallogr., A32 (1976) 751. (3) Y. Matsumoto, Mat. Res. Bull, 23 (1988) 1242. (4) Y. Saito, T. Noji, A Endo, N. Higuchi, K. Fugimoto, T. © ' k ~ ,-"h3-~i.-,,C, !48B (!987) 336. (5) P.N. Miheenko, G. E. Chatalova, Superconductivity, Physics,. Chemisa.ryand rechnics. (iv,P,u~i~q), 4 ~r~ao~ . . . . ,/1564. (6) I Nedkov, A, Veneva, S. ~Aiteva, ~oc. of the European Conf. on Appl. Sup~cond., GottingeL Germany, Oct. 4-8, 1993. (7) JCPDSpowder diffraction file, no 38-0971. (8) B.T. Ahn, T. M. Gut, R A_ }-Iiggins, Physica C, 153-155 (1988) 590.
PHY$1(A
North-Holland
SUPERCONDUCTING PROPERTIES AND MICROSTRUCTURE OF YBCO HTSC MATERIALS WITH K2CO 3 ADDITION A. Venevaa, I. Nedkova, V. Lovchinovb alnstitute of Electronics, Bulgarian Academy of Sciences, Blvd. Tzarigradsko Chaussee 72, Sofia 1784, Bulgaria blnstitute of Solid State Physics, Bulgarian Academy of Sciences, Blvd. Tzarigradsko Chaussee 72, Sofia 1784,
Bulgaria The influence of K2CO s additive on the superconducting properties and microstructure of samples with nominal compositions Yl-o. 2,,Ba2.o. 2,,K,, Cus 07.),,I"1Ba2 Cus. s.,,Kx 07.y , ~) Ba2.,,K,, Cus 07.y , ( x = 0 to 1.50), were investigated. Only traces of K (lxl0 "2 wt.%) were observed in all samples after the final baking and no proof could be found for alkali metal cations participating in the Perovsldte crystal cell. Potassium carbonate eliminated the CuO second phase from the system, because of a KCu-compound formation which evaporated above 900 ° C. The influence of the K2COs was positive for x = 0.40 to 0.75 where we observed homogenous grain-size ceramic structure and ItTSC Y1B%Cus@ samples with values of T c = 93.4 K and AT = 1.5 K.
INTRODUCTION The effect of adding carbonates of some alkali metals, particularly K2COs, in synthesizing YlBa2Cu30y high-temperature superconductor (HTSC) is an open question periodically arising in the literature without being unambiguously answered. Some authors have determined it as "catalytic" (1). Beating in mind the large ionic radius of potassium (K+, r i = 1.51/k) (2) it has been assumed that it can be introduced at barium's (Ba2÷, r i = 1.42 /k) place (3, 4). There have been reports that no trace of potassium could be found in the samples after the final thermal-treatment (5). In this work, we report investigations on the adding of I~2C0 s in a wide concentration range in the initial batch of YBCO system and the influence of the addition on the superconducting properties dll~d,
llllt~l
t J ~ t . lt L I ~ I . U l
~,..
EXPERIMENTAL Three series of materials were prepared with nominal compositions Yl-o. 2xBa2-o. :x K~ Cu s 07_y, 111Ba2Cu3. s-xKx 0T-y, YI Ba2-xKx Cus 0T-y, where x = 0 to 1.50. When sintering the HTSC, special attention was paid to the purity of the initial components: the basic 7(, 0 s was 99.99 %; BaCO s, 99.99 %; CuO, 99.99 %; and K2COsxl.5H20 was
99.90 %. The samples were obtained following the classic ceramic technology consisting in mixing of oxides and carbonales in the appropriate ratios and calcining the mixture in air at 900 ° C for 24 hours. The calcined powders with l-gin grain size were pressed into pellets which were then sintered once more. The samples formation was momtored by means of X-ray diffraction analysis (XRD); atomic absorption spectroscopy (AAS), scanning electron microscopy, energy dispersive X-ray spectra (SEM EDS) were used to study the microstructure and morphology of the crystalline grains in the samples prepared. The transition temperature dependence of HTSC samples was determined using the foux-probe "resistive" technique. RESULTS AND DISCUSSIONS We found that adding K2COs to the stardng batch lowered the sintering temperature of the highTc orthorhombic phase down to 950 ° C compaxed to that of pure giB%Cus@ (6). After the first heat° treatment (900 ° C/24 h) the YlBa2Cus@ phase was about 75 wt. % and the ~ data showed (SEM and EDS confirmed it) that BaCuO: , CuO and C u # phases existed in samples with higher K2CO3 content. Peaks of K-additive related phase were also
0921-4534/94/$07.00 © 1994 - Elsevier Science B.V. All rights reserved. SSDI 0921-4534(94)00966-X
806
,4. Veneva et al./Physica C 235-240 (1994) 805-806
found. Some aut',ors (4) claim to have observed KCuO peaks. The analysis of our X-ray patterns revealed the existence of KCu-compounds peaks up to 900 ° C, determined to be K:CuO 2 : (204, 302), d = 2.632 A,; (220), d = 3.040 A; (213), d = 2.906 A, (7). The lack of K~CO3 phase in the X-ray patterns at 900 ° C allows the supposition that CuO reacts with K2COs forming one or two types of potassiumcepper compounds. This interaction is possible as a result of the reduction of CuO to Cup at temperature above 800 ° C (8). After a second heattreatment at 950 ° C for 48 h the XRD patterns show that the system with x = 0 to 0.75 is YiBaFusOy single-phase ; for x > 0.95 the system ~as multiphase. Using AAS, (the samples were dissolved in HCI), we studied the K content in the samples. The AAS data of a series of samples baked at 950 ° C for 48 h with x varying from 0 to 0.75 show that potassium remains in all samples in amounts within lx 10z wt.%. Fig. 1 illustrates the characteristic stratification of a sample where the K-additive in the starting batch was x = 0.95.
Fig. 1. SEM micrograph of Yo.s~Ba:.sl Ko.p5Cu.~07.,, sample, where (A) is YiBa2Cu3Oz.y phase; (B) is KsCuO2 phase. The SEM EDS data confirm potassiumcopper compound (B phase). Fig. 2 shows typical results of electrical resistance measurements on samples with d~fferent K content in the starting batch. For values of x up to approximately 0.75, the R (T) data demonstrated that the materials regularly exhibits T c = 93.4 K and AT = 1.5 to 1.7 K. T~ explain the peculiarities observed resulting from
adding K we turned to the following changes in the polycrystalline microstructure. . . . . . . . . . . .
(d)
f
i,---
(c)
-)
{B)
--
78 s2 g6 90
9'8 . . . .
I
30o
TEMPERATURE, K Fig. 2. Temperature dependence of the resistance for: (a) Yi Ba2 Cus O7_y; (b) Yt-o.2xBa2-o.2xKxCupT-v (c) Yt Ba2.xK:,Cu3 07-y ; (d) Yi Ba2 Cus. s-~K~O7.y , where x = 0.75. When K2CO3 was increasing in the initial batch we observed a highly homogenous sma!l-grain ceramic structure. This gave us grounds to assume that the potassium-copper compound observed participates actively in the polycrystalline structure formation, most probably as a liquid film on the grains boundaries preventing ~eir growth; in addition, this second phase does not degrade the system's superconducting properties in the concentration range x = 0.40 to 0.75. REFEI~NCES ('i) R.J. Cava, J. J. Kraje.wski, W. F. Peck, B. Bat|ogg, I,. M. Rupp Jr., Letters to Nature, 338 (1989) 328. (2) R.D. Shannon, Acta Crystallogr., A32 (1976) 751. (3) Y. Matsumoto, Mat. Res. Bull, 23 (1988) 1242. (4) Y. Saito, T. Noji, A Endo, N. Higuchi, K. Fugimoto, T. © ' k ~ ,-"h3-~i.-,,C, !48B (!987) 336. (5) P.N. Miheenko, G. E. Chatalova, Superconductivity, Physics,. Chemisa.ryand rechnics. (iv,P,u~i~q), 4 ~r~ao~ . . . . ,/1564. (6) I Nedkov, A, Veneva, S. ~Aiteva, ~oc. of the European Conf. on Appl. Sup~cond., GottingeL Germany, Oct. 4-8, 1993. (7) JCPDSpowder diffraction file, no 38-0971. (8) B.T. Ahn, T. M. Gut, R A_ }-Iiggins, Physica C, 153-155 (1988) 590.