Journal of Electronic Materials, Vol. 24, No. 12, 1995 Special Issue Paper

Powder Processing of High Temperature Ceramic Superconductor TI2Ba2Ca2Cu30 o

Y. XIN,* B.R. XU,* W.S. HE,* G.F. SUN,* R. FARR,* D.F. LU,* K.W. WONG,* and D. KNAPP*

*Midwest Superconductivity, Inc., 1315 Wakarusa Dr., Lawrence, KS 66049 *Department of Physics and Astronomy, University of Kansas, Lawrence, KS 66045 *Department of Geology, University of Kansas, Lawrence, KS 66045

We have developed a technique to produce high quality T12Ba2Ca2Cu3010 pow- ders used for making superconducting wire, tape, lead, shield, and other large scale bulk applications. Starting with T1203, BaO~, CaO, and CuO, we mix and grind these chemicals with a machine ball mill and then press the ground mixture into pellets. The pellets are sintered at about 895~ for at least 30 h in an oxygen atmosphere. The sintered material is mainly the T12Ba2Ca2Cu30~0 compound. To get more homogeneous superconductor powders, we pulverize the sintered material and use a magnetic superconducting material selector to separate and grade the material. Finally, the top grade material has a phase purity of >98% and a T c (r = 0) of 123-126K.

Keywords : Magnetic separation, phase purity, synthesis, superconductor, T12Ba2Ca2Cu3010

INTRODUCTION

Superconductivity in thallium cuprates was dis- covered by Sheng et al. in the winter of 1987-1988.1 T1-Ba-Cu-O, the first non-rare earth superconductor with a T c above liquid nitrogen temperature, was later identified stoichiometrically as T12Ba2CuO6 .2 The ad- dition of Ca to the new non-rare earth T1 based cuprate superconductor resulted in a transition tem- perature of above 100K2 The chemical composition and crystal structures of these T1-Ba-Ca-Cu-O com- pounds were then worked out 2 as Tl~Ba2CuO6, T12Ba2CaCu20 s, and T12Ba2C%Cu3Olo. This is the first family of thallium cuprate superconductors, which is featured by two T1 atoms in each chemical formula.

Genera l ly , we can wri te the fo rmula as T12Ba2Can_~CunO2n§ , where n = 1, 2, 3,... Structurally, this family is featured by the two neighboring T1-O layers sliding a displacement of 1/2 (a+ b), which is known as T1 double layer family. Soon after the

(ReCeived February 14, 1995; revised July 10, 1995)

discovery, a series of T1 containing superconductors with different chemical and structural features were recognized. 4,5 This time the general chemical formula is T1Ba~Ca 1CunO2,.3 , where n = 1, 2, 3,... Since there is no T1-O layers neighboring each other in the crystal structure of the new compounds, these compounds were then called the T1 single-layer family. This was not the end of the story. It was realized lateP that the Ba cation in the T1 single-layer family can be replaced by Sr. Then chemically, we can divide the T1 cuprates into two categories, i.e. T1-Ba-Ca-Cu-O (double layer and single layer) and T1-Sr-Ca-Cu-O (single-layer only). The chemical difference, as well as the struc- tural difference, makes a significant contrast in physi- cal and mechanical properties between the T1-Ba-Ca- Cu-O and the T1-Sr-Ca-Cu-O compounds. Among the T1-Ba-Ca-Cu-O compounds, T12Ba2Ca2CuaOlo (T12223) has the highest T c of about 125K. It is chemically more stable than rare earth and Bi based cuprates, which have Tcs of 90 and 85-110K, respectively. Generally speaking, we believe that T12Ba2Ca2Cu3Olo is the most promising material to date for large scale bulk super-

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1822 Xin, Xu, He, Sun, Farr, Lu, Wong, and Knapp

Fig. 1. Furnace setup diagram for sintering the Tt-containing super- conductors.

conductor applications. Like other high T c cuprates, the T1 based super-

conductors are most commonly prepared by solid state reaction. The early day synthesis of the T1 based compounds was featured by a two-step preparation and a very short time for sintering because of the concern ofT1203 evaporation. It is obvious under such procedure that the solid state reaction of the compo- nent chemicals is far from sufficient. Another com- monly held practice in early day synthesis ofT1 based compounds was the off-stoichiometric starting com- position. An off-stoichiometric starting composition (most of them using excess Ca or Cu or both), in some cases, would facilitate the formation of the 2223 phase. However, the excessive chemicals may form some parasitic nonsuperconducting crystalline or become precipitate particles. From the current carry- ing capability point of view, the presence of impurity phases among superconductive grains impedes the transport of the electrical current, resulting in low critical current density (J). Someone may argue that impurities can work as flux pinning centers and will increase J in the case when a magnetic field is present. Therefore, it would follow that the impurity issue may not be the major issue in achieving high Jr for the high T cuprate superconductors. However, we believe this argument is not quite correct. As a matter of fact, the impurities (impurity phases and voids) have been over dosed in the cuprate superconductors so that under a magnetic field the flux cores are heavily overlapping and nearly free creeping in these poor purity materials, which destroy the supercurrent. This is why the J is low for poor purity high T c superconductors. Effective flux pinning centers cer- tainly need to be finely dispersed in a well-controlled manner. In any case, it is necessary to first synthesize high purity superconductive material as starting material and subsequently follow a well defined pro- cedure to generate the desired secondary impurity phase as pinning centers. Nonetheless, the ability to prepare large quantity powders with high phase pu- rity is essential to practical applications of high T c

Fig. 2. The photograph of the prototype magnetic superconducting material separator.

superconductors. We have established a synthesis procedure for

fabricating high phase purity T12Ba2Ca2Cu3010. The production rate is 200-400 grams per batch per fur- nace. In average, for the sintered material, 30% is with a 2223 phase purity >95%, 40% is with the phase purity ranged from 85 to 95%, and 30% has the phase purity <85%. To increase the production rate of high purity material, we built a magnetic superconducting material selector, which gives us enhanced capability of further up-grading the material.

EXPERIMENTAL

T1203(>99.99%) , Ba02(>95%) , Ca0(>99.9%), and

Powder Processing of High Temperature Ceramic Superconductor Tl~BazCa~Cu~O~o

CUO(>99.5%) at the molar ratio of cations of 2:2:2:3 were mixed and ground with a machine ball mill for about 5 h. Twenty to 30 grams of the ground powders were pelletized with a density of about 5g/cm 3 in a circular die with a hydraulic press. Fifteen to 20 pellets in each batch were placed in an open-ends alumina cylindrical crucible. The crucible with the contents was positioned in a three-zone-heating tube furnace (Blue M Model 55347 or 55647). Figure 1 illustrates the furnace setup. Oxygen was slowly flowing through the furnace tube during the sinter- ing. To filter out the toxic particles in the exhaust fume, we let the fume go through a 10% nitrogen acid solution and then released it to the fume hood drain- out fan. The temperature of the furnace was raised at a rate of 20~ to about 895~ The temperature was kept at 895~ for 30 h or longer, then was reduced at a rate of l~ to about 650~ The material was annealed at 650~ for 10 h. Finally, the power of furnace was cut off and the furnace slowly cooled down to room temperature. The resulting pellets were then pulverized into powders for making wire, tape, rod, hollow cylinder, plate, and other bulk supercon- ductor parts. In the material analysis, the resistance of the material was measured. Ac susceptibility was measured using a mutual inductance method at the frequency of 400 Hz. The resistance and ac suscepti- bility measurements were carried out in the tempera- ture range from room temperature to 70K. Powder x- ray diffraction was performed by Cu-K~ radiation using a DIANO DTM 1057 diffractometer. The phase purity of the materials was determined by comparing the intensity of peaks of the 2223 phase with the intensity of impurity peaks. A Quantum Design model MPMS-5S SQUID Magnetometer was used for the magnetization measurement in the temperature range from room temperature to 4.2K.

A prototype magnetic superconducting material separator was designed and constructed for selecting and grading the sintered materials. Figure 2 is the photograph of the prototype. The superconductor powders are cooled to about 77K in the cooling tunnel which is surrounded by liquid nitrogen. Then the auger is rotated to propel the powder dropping through the vertical pass pipe. When the powders enter the magnet wheel region, the interaction between super- conducting particles and magnets makes the separa- tion possible. Details about the apparatus will be published elsewhere#

RESULTS AND DISCUSSIONS

As mentioned in the introduction part, people are inclined to use two-step methods to prepare T1-Ba-Ca- Cu-O superconductors. After mixing the Ba-Ca-Cu-O precursor and T1203, the final sintering time is usu- ally very short, such as a few minutes to a few hours, to avoid the excessive evaporation of T1203. That concern is understandable because the melting point ofT1203 is 717~ and beyond this point the evapora- tion of the thallium oxide is rather fast if it stands alone. However, the short sintering time results in an

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inadequate solid state reaction which can rarely pro- duce high phase purity materials. The synthesis tech- nique we developed for preparing T12Ba2Ca2Cu3Olo compound employs a long time high temperature one- step sintering. We found that the vaporization of T1203 becomes significantly slower as soon as it is participating in the formation of the superconducting compounds. Therefore, we use a rather rapid heating up rate to raise the furnace temperature to about 895~ in less than 50 min. The weight loss typically is less than 5% during the whole sintering process, which is attributed to the evaporation of T1203, O 2 in BaOe, as well as the moisture. We sort the prepared material into three grades, A, B, and C. Figure 3 is the powder x-ray diffraction patterns for the three grades of materials. Grade A material has a TlzBa2Ca2Cu~O lo phase purity >95%. Grade B has the 2223 phase purity of 85%--95% while Grade C material is of less phase purity. Some researchers 8 have claimed that the longer sintering time than several hours at above 890~ results in a majority of the lower T 2212 phase. Our experiences do not support the conclusion. Sin- tering up to 100 h basically produces the same mate- rial yield. In contrast, if the sintering time is less than 20 h, the volume of the 2212 phase will significantly increase. The T of Grade A material is about 125K in both resistance and ac susceptibility measurements. The grain J~ of Grade A material is evaluated by

- l - -

r

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Fig. 3. Powder x-ray diffraction patterns of Grade A (a), Grade B (b), and Grade C (c) materials.

1824 Xin, Xu, He, Sun, Farr, Lu, Wong, and Knapp

_S 123

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z i , i

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(d)

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20(degree) Fig. 4. Comparison of x-ray diffraction patterns for the material before separation (a), in collector 2 (b), in collector 3 (c), and in collector 4 (d).

magnetization measurement according to the Bean Model, which is 1.7 • 107 A/cm 2, 1.2 • 107 A/cm 2, 2.2 • 106 A/cm 2, and 6.3 • 104 A/cm 2 at 4.5, 10, 30, and 80K, respectively.

To increase the production rate of Grade A ma- terial, we used our magnetic separation machine to further separate our Grade B and Grade C materials. The difference in superconducting volume and the difference in superconducting phase among the par- ticles can be distinguished by the horizonal moving distance under the superconductor and magnet inter- action. Nonsuperconducting particles are dropped directly to the collector right below the vertical pass pipe since they have no interaction with the magnets. Figure 4 shows the results of the separation by the powder x-ray diffractions. Four peaks, 017(2223), 017(2212), 019(2223), 110(2223 and 2212 overlap), in the 20 range from 28 to 34 ~ are used for the analysis. The 017,019, and 110 peaks represent the three most intensive reflections in the 2223 structure. On the other hand, the 107 and 110 reflections are the two most intensive ones in the 2212 phase. Figure 4a shows the diffraction pattern of the unselected mate- rial which contains a significant amount of the 2212 phase material. Figure 4b shows the pattern of the material in the second collector. It is obvious that the portion of 2212 phase material is greatly reduced. The higher purity material is obtained in the third and fourth collectors (see Fig. 4c and 4d). The material in the fourth collector is pure 2223 phase material. It should be mentioned that the method of comparison among these prominent reflection peaks to determine the phase ratio of the T1 2223 phase to the T1 2212 phase is a simple and valid tool for the purity analysis.

CONCLUSION

Quantity production of the T12Ba2Ca2Cu3010 com- pound is achieved with a one-step method. Long time sintering at about 895~ results in high quality su- perconducting material. Further separation of the sintered material can enhance the production rate of the pure phase 2223 material. It becomes feasible to produce high grade T12Ba2Ca2Cu3010 powders for large scale applications.

ACKNOWLEDGMENT

This work was supported by Midwest Superconduc- tivity, Inc.

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