TlxBa2Ca3Cu4Oy superconductors from coprecipitated oxalate precursors

PHYSICA

Physica C 196 (1992) 259-263 North-Holland

TlxBa2Ca3Cu4Oy superconductors from coprecipitated oxalate precursors Klaus Bernhard and G e r h a r d Gritzner InstitutJ~r Chemische TechnologieAnorganischer Stoffe, Johannes Kepler Universitdt Linz, A-4040 Linz, Austria

Received 4 April 1992

Precursor material for TI-Ba-Ca-Cu-O superconductorswas prepared by coprecipitation of the cations as oxalates. Following calcination at 600°C, the powders were attrited, then uniaxially compacted at 7000 bar and converted into superconducting ceramicsby sintering in the temperature range of 880°C to 915 °C. The thallium content and the Tc(0) depended stronglyon the sintering conditions. Best results were obtained for samples with the compositionT1L4Ba2Ca3Cu40~t.4yieldingsuperconductors with a T¢(0) of 118 K and a critical current density of 19.4A cm- 2at 77 K.

I. Introduction The T I - B a - C a - C u - O superconductors show transition temperatures up to 125 K [ 1-4]. At least two superconducting phases, commonly called "222Y' and "2212" phases, were found, although neither one of the two superconducting phases corresponded exactly to the formulas T12Ba2Ca2Cu3Ox and T12Ba2CalCu2Oy. Rather compositions such as Tll.64Ba2Cal.s7Cu3.11 and Tll.7oBa2Cal.o6Cu2.3 were published [5]. Tl-based superconductors were generally made by grinding together the respective oxides [3,6], or oxides and carbonates [7]. In this study we investigated the possibility to prepare such superconductors from coprecipitated oxalate precursor material. Coprecipitation leads to an intimate mixing of the components of the superconducting material, which cannot be obtained by grinding. Such intimate mixing is especially necessary for the T1based superconductors, since sintering times must be short to avoid sublimation of T120. Coprecipitation from aqueous solutions is hindered by the considerable solubility of thallium oxalate in water; we therefore studied other solvent systems and found that almost complete coprecipitation of all cations can be achieved in glacial acetic acid, a solvent which was already successfully applied for the preparation of B i - C a - S r - C u - O superconductors [ 8 ]. Literature reports favour a composition of ap-

proximately T12Ba2Ca3Cu4Oy over T12Ba2CazCu3Ox as starting material for the formation of the high-To "2223" superconducting phase [4]. Preliminary studies at our laboratory confirmed this observation. Thus we adjusted the concentrations of the cations to obtain oxalates with molar ratios ofTl: Ba: Ca: Cu of 2 : 2 : 3 : 4 . Producing Tl-based superconducting material is complicated by the high volatility of thallium oxide at the sintering temperatures. For this reason we studied three methods of converting the calcined powders into superconducting material, namely sintering without gold foil, with gold foil [ 3 ], and sintering first with, then without gold foil, and investigated the effects of sintering on the remaining Tl-concentration and its influence on phase compositions and the electrical properties.

2. Experimental 2.1. Apparatus

Atomic absorption analysis (AAS) was carried out on a Unicam SP 1900 atomic absorption spectrophotometer. X-ray diffraction spectra were performed on a Geigerflex D-max II a (Rigaku, Japan) employing Ni filtered Cu Kct radiation. A TG-770 thermobalance (Stanton Redcraft, UK) was used for thermogravimetric measurements. The specific sur-

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K. Bernhard, G. Gritzner/ TIxBa2CajCu4Q, superconductors

face of calcined powders was determined on a Quantasorb (Quantachrome, USA) by the BET method. Particle size distributions of coprecipitated and calcined powders were measured on a Coulter LS 130 laser particle sizer (Coulter Electronics, Inc., USA). The change in the resistance with temperature and current density was measured by the standard fourpoint DC-technique. The specific resistivity at room temperature was determined by the van der Pauw method. 2.2. Analysis The cation concentration in the supernatant solutions after coprecipitation were determined by AAS at the following wave lengths: T1 +, 276.78 nm; Ca 2+, 422.67 nm; Ba 2+, 553.56 nm, Cu 2+, 324.75 rim. The amount of Cu 3+ was analysed by a method published in ref. [ 9 ]. The concentration of T1203 in sintered samples was derived from thermogravimetric measurements [6] and by polarography in 5 vol.% perchloric acid as supporting electrolyte. The data obtained from these two techniques agreed within 1.5%. 2.3. Coprecipitation of the oxalates The precursor compounds for superconducting materials were coprecipitated as oxalates in the following manner: 5.268 g (0.02 mol) TI(CH3COO) (Johnson Matthey, 99.99%), 3.003 g (0.03 tool) CaCO3 (Merck, volumetric standard), 3.947 g (0.02 mol) BaCO3 (Merck, > 9 9 % ) and 7.986 g (0.04 mol ) Cu (CH3COO) 2.H20 were dissolved in 200 ml of 25 vol.% acetic acid and the solution was evaporated to dryness to convert all compounds into acetates. The residue was then dissolved in 350 ml of conc. acetic acid. Clear blue solutions were obtained in all cases. The solution containing the respective cations was slowly added into a stirred solution of 0.11 tool (13.86 g) of oxalic acid dissolved in 200 ml conc. acetic acid. The resulting suspension was stirred for another hour and kept overnight. Upon filtration the precipitate was washed twice with 50 ml of conc. acetic acid and then dried at 80°C for 20 h. The filtrate was analysed by atomic absorption spectroscopy. The analysis showed that less than 0.35% of the original amount of Tl+, less than 0.10%

of Ca 2+, less than 0.15% of Ba 2+ and less than 0.08% of Cu 2+ remained in solution. The X-ray diffraction spectroscopy revealed the amorphous nature of the precipitated powders. 2.4. Formation and characterization of the superconducting oxides The thermal decomposition of the coprecipitated oxalates was analysed via thermogravimetry in an argon atmosphere (fig. 1). The results were interpreted by comparison with the thermogravimetric behaviour of the individual oxalates. The weight loss around 200 °C is due to the conversion of T12(C204) tO T120 and of Cu (C204) to CuO. Between 600 and 800°C Ca(C204) decomposes to CaCO3 followed by the conversion to CaO and Ba(C204) converts to BaCO3. Sublimation of T120 occurs in the range between 800 and 1000°C, the formation of BaO from BaCO3 around 1050 ° C. Based on these observations a calcination temperature of 600°C and a duration of 20 h in air was chosen to avoid losses on thallium oxide and to allow partial reaction of the BaCO3 with the oxidic compounds. The heating and cooling rates were 1 K m i n - ' . X-ray diffraction of the calcined powders indicated the presence of BaCO3, BaCuO2 and Ca2CuO3. T1203formed by the calcination in air was found at the beginning of the calcination process, but apparently reacted to form thallium cuprates. X-ray diffraction data for T1-Cu-O systems are not available at present, preventing an identification of the reaction product. The calcined powders contained agglomerates and were therefore sub10

s 6

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o 200

I

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400

600

800

I

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1000 1200 1400 1600 Temperature["C]

Fig. 1. Thermogravimetryof coprecipitated thallium-calciumbarium-copper oxalates.

261

K. Bernhard, G. Gritzner / TlxBa2Ca3Cu+Or superconductors

temperature range, between 900°C and 910°C, lead to samples, which consisted predominantly of the "2223" phase with small amounts of BaCO3 (fig. 3). X-ray diffraction spectra of samples sintered in goldfoil at temperatures below 905°C following method (2) or (3) showed the lines for the "2122" phase and BaCO3. Sintering at temperatures above 905°C yielded mainly BaCuO2 and CaCuO3 besides small amounts of "2212" and "2223" phase. Only sintering at 905 °C yielded mixtures of the "2122" phase and the "2223" phase, besides small amounts of BaCO3. The Tc (0) values of the different samples are summarized in table 1. Samples sintered without goldfoil between 880 ° C-915 °C showed superconductivity above 100 K. The highest To(0), l l 8 K, could be measured for samples sintered at 905°C and 910°C. Sintering temperatures higher than 910°C and lower than 905°C lead to a decrease of the Tc(0)-values. Samples sintered with goldfoil showed only at high sintering temperatures (910°C-915°C) To(0) values above 100 K. Disks sintered at temperatures between 880 °C and 905 °C had critical temperatures of about 80 K. Samples first sintered with and then without goldfoil showed lower critical temperatures than those sintered without goldfoil. Detailed measurements were carried out on a sam-

jected to attrition milling in 2-propanol. The particle size distribution of the calcined and the attrited powders is given in fig. 2. Surface area measurements yielded 0.80 m 2 g-~. The calcined, attrited powders were uniaxially compressed into disks of 13 m m diameter at 7000 bar. Due to the high volatility of thallium oxide three different ways of sintering were studied: ( 1 ) sintering without goldfoil, (2) sintering of the compacted powders wrapped in a goldfoil with additional 100 mg T1203 and ( 3 ) sintering of the specimen as described in (2) followed by sintering without goldfoil. All sintering procedures were carried out in oxygen atmosphere at temperatures between 880 °C and 915 °C for a period of 6 min with heating and cooling rates of 2 K m i n - ~. The results of the thallium analysis of the sintered samples are shown in table 1. Samples sintered without goldfoil at temperatures between 880 and 900°C for 6 min showed high amounts of the "2122" phase and of BaCO3 and low amounts of the "2223" phase. Samples sintered at temperatures between 910 ° C and 915 ° C consisted mainly of BaCuO2 and Ca2CuO3 and small amounts of the "2223" phase, since most of the T1203 had converted and sublimed as T120. Only a very limited

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Fig. 2. Particle size distribution of calcined (...) and attrited (

) powders.

200

K. Bernhard, G. Gritzner / TlxBa2Ca3Cu40y superconductors

262

Table 1 T1203 content of sintered TlxBa2Ca3Cu4Oy samples in wt.%, the ratio x and the transition temperature To(0), obtained at different sintering temperatures Temp. (°C)

880 885 890 895 900 905 910 915

Sintered without gold foil

Sintered with gold foil

Sintered first with, then without gold foil

%

x

Tc (K)

%

x

T¢ (K)

%

x

Tc (K)

33.5 33.2 28.1 28.4 28.4 28.3 27.0 20.2

1.75 1.73 1.35 1.38 1.38 1.37 1.28 0.88

< 77 105 107 113 115 118 118 114

50.6 44.5 38.1 42.0 40.7 39.7 40.3 30.7

3.55 2.78 2.13 2.51 2.38 2.29 2.34 1.53

< 77 78 78 78 80 80 100 114

33.9 33.3 27.9 28.1 28.0 27.5 27.0 19.7

1.78 1.73 1.34 1.36 1.35 1.32 1.28 0.85

113 108 108

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1

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40

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60

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Fig. 3. X-ray diffraction of Tli,37Ca3Ba2Cu40,,.42, sintered at 905 °C for 6 rain without gold foil; BaCO3 is indicated as ( A ) , the low-T~ phase as ( [] ), the high-Tc phase as ( o ).

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0.8

0.6

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200

250

300

T[K]

Fig. 4. Temperature dependence of the electrical resistance of Tll.37Ca3Ba2Cu4011.42, sintered at 905 °C for 6 min without goldfoil.

K. Bernhard, G. Gritzner / TlxBa2Ca3Cu40y superconductors

263

ues [6 ]. W r a p p i n g in goldfoil to obtain stoichiometric c o m p o s i t i o n s did not result in the best superconducting material. We found that a thallium content above the stoichiometric number, above 1.4, led to a general decrease in Tc values with increasing Tl-content. Variations in the heat t r e a t m e n t had only m o d e s t effects on this general trend. Tl-contents below the stoichiometric n u m b e r o f 1 also resulted in a decrease o f the T~ values. F u r t h e r loss o f T1 led to the d e c o m p o s i t i o n o f the superconducting phases. Thus the sintering conditions to adjust the p r o p e r Tl-content r e m a i n the critical step in preparing T1based superconductors. Fig. 5. Scanning electron micrograph of Zll.37Ca3Ba2Cu4Oll.42, sintered at 905 ° C for 6 min without goldfoil.

pie

with

the

overall composition Tlt.37Ba2Ca3Cu4Oll.42 sintered for 6 m i n at 905°C without goldfoil, which h a d the highest Tc a n d which consisted p r e d o m i n a n t l y o f the " 2 2 2 3 " phase. The specific resistance at r o o m t e m p e r a t u r e was 0.414 f~m, the To(0) 118 K (fig. 4). The density o f a sintered disk was 4.74 g cm -3. The p a r a m e t e r s o f the tetragonal cell were a = 0 . 3 9 0 6 nm, c = 3 . 4 2 2 9 nm. These values are very similar to the values reported in the literature ( a = 0 . 3 9 1 4 nm, c = 3.3644 nm, [7], and a = 0 . 3 8 6 nm, c = 3.575 n m [3] ). The m a x i m u m current density at 77 K was 19.4 A c m -2. The scanning electron micrograph clearly shows that partial melting had occurred during the sintering step (fig. 5).

3. Conclusions The use o f acetic acid as solvent lead to an almost complete coprecipitation o f the cations as oxalates. This precursor material could be converted into superconducting material. D u e to the volatility o f T120 the quality o f the superconductors strongly dep e n d e d on the sintering conditions. As observed by other groups we also noted that a thallium deficit in the " 2 2 2 3 " phase considerably i m p r o v e d the Tc val-

Acknowledgements

The authors thank D.1. Ratajski for the scanning electron micrographs and Mr. Kellner for the X-ray diffraction spectra. Financial support by the Jubil~iumsfond der 0sterreichischen N a t i o n a l b a n k is gratefully acknowledged.

References

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