Process for preparing a single crystal of oxide superconducting material

Abstract: Single crystal of superconducting compound oxide of Tl, Bi, Sr, Ca and Cu is prepared by mixing powders of oxides of Tl, Bi, Sr, Ca and Cu, melting the oxide powder mixture at 950-1050°C and then cooling down the resulting melt.

Description: Background of the Invention Field of the Invention The present invention relates to a process for preparing single crystals of superconducting oxide compounds. More particularly, it relates to superconducting oxide materials containing Tl and/or Bi having high superconducting critical temperature (Tc) and high superconducting critical current density (Jc), processes for preparing the same and their applications as wires or magnets. Description of the related art Discovery of new oxide type superconductor by Bednorz and Müller revealed the possibility of high temperature superconductors (Z. Phys. B64, 1986 p 189) and study to find much higher Tc superconductors have been continued. C. W. Chu et al. reported another superconducting material of so-called YBCO type represented by YBa 2 Cu 3 O 7-x having the critical temperature of about 90 K (Physical Review letters, Vol. 58, No. 9, p 908). Maeda et al reported the other type new superconducting compound oxide of Bi-Sr-Ca-Cu-O system which show the critical temperature of more than 100 k (Japanese Journal of Applied Physics. Vol. 27, No. 2, p 1209 to 1210). The Tl-Ba-Ca-Cu-O system compound oxide reported by Hermann et al. in Appl. Phys. Lett. 52 (20) p 1738 is also a high Tc superconductor of about 120 K. The Tl-Ba-Ca-Cu-O system and Bi-Sr-Ca-Cu-O system oxide superconductors show very high critical temperatures which is enough higher than boiling point of liquid nitrogen of 77 K and hence are expected to be applicable to actual uses. However, the critical current density (Jc) of these systems at 77 K drops sharply under higher magnetic field of more than 1 T. In fact, the Bi-Sr-Ca-Cu-O system oxide is a mixture of a high Tc phase with Tc of 100 K and a low Tc phase with tc of 80 K and it is difficult to separate them. The Tl-Ba-Ca-Cu-O system compound oxide also is composed of a plurality of different phases such as Tl 2 Ca 2 Ba 2 Cu 3 O x and Tl 2 Ca 1 Ba 2 Cu 3 O y and is extremely difficult to isolate the high Tc phase alone. In the case of Bi-Sr-Ca-Cu-O system oxide, it was proposed to add Pb to this system in order to obtain nearly 90 % single phase of Bi-Pb-Sr-Ca-Cu-O system having a critical temperature of higher than 100 K. However, in order to obtain this Bi-Pb-Sr-Ca-Cu-O system, the material powder must be subjected to a very long sintering operation. Still more, the resulting sintered oxide shows anisotropy, in crystal structure and exhibits a very low critical current density under a strong magnetic field. The Tl-Sr-Ca-Cu-O system compound is also reported. This system shows the relatively lower critical temperature of 75 K. Tl-Pb-Sr-Ca-Cu-O system compound is reported in Science, 242 n°4876, 14/10/88, page 249-252. Single crystals of this compound can be grown from oxide mixture. An object of the present invention is to overcome the problems of the prior art and to provide improved superconducting materials which exhibit higher critical current density under strong external magnetic field as well as high critical temperature, processes for preparing the same and their applications as superconducting wire and magnets. The object of the present invention is to provide a process for producing single crystals of these superconducting compound oxides. Summary of the Invention The present invention provides a method for producing a single crystal of a superconducting oxide material compound oxide having a composition represented by the formula: [(Tl 1-x Bi x ) 1-p Pb p ] q Sr y Ca z Cu v O w in which "x","y","z","p", "q", "v" and "w" are numbers each satisfying respective range of 0 ≤ x ≤ 1.0, 0.5 ≤ y ≤ 4.0, 0.5 ≤ z ≤ 4.5, 0 ≤ p ≤ 0.6, 0.5 ≤ q ≤ 3.0, and 1.0 ≤ v ≤ 5.5 and w is an atomic ratio of oxygen which satisfies chemical valence theory. The superconducting oxide material according to the present invention preferably has a value of "x" which lies in the range of 0.1 ≤ x ≤ 0.5. The superconducting oxide material according to the present invention preferably is a crystalline compound oxide having a layered structure of a tetragonal system and having a composition represented by the general formula in which "y" = 2 ±0.5, "z" = n±0.5 (n = 1, 2, 3 or 4), "q" = m ± 0.5 (m = 1 or 2) and "v" = (n+1) ±0.5. A preferable example of the superconducting oxide material according to the present invention is a Tl-Bi-Sr-Ca-Cu system compound oxide which is expressed by the general formula when "p" = 0. In this Tl-Bi-Sr-Ca-Cu system, the "x", "y", "z", "q", "v" and "w" are numbers each satisfying following respective range: 0 ≤ x ≤ 1.0, 0.5 ≤ y ≤ 3.0, more preferably 1.5 ≤ y ≤ 2.5, 0.5 ≤ z ≤ 3.0, more preferably 1.5 ≤ z ≤ 2.5, 0.5 ≤ q ≤ 3.0, more preferably 0.5 ≤ q ≤ 2.5, 1.0 ≤ v ≤ 4.0, more preferably 2.5 ≤ v ≤ 3.5, and w is an atomic ratio of oxygen which satisfies chemical valence theory. Another preferable example of the superconducting oxide material according to the present invention is a Tl-Bi-Pb-Sr-Ca-Cu system compound oxide which is expressed by the general formula when p > 0. In this Tl-Bi-Pb-Sr-Ca-Cu system, the "x", "y", "z" "p", "q "v" and "w" are numbers each satisfying following respective range: 0 ≤ x ≤ 1.0, 0.1 ≤ p ≤ 0.6, 0.5 ≤ y ≤ 3.0, more preferably 1.5 ≤ y ≤ 2.5, 0.5 ≤ z ≤ 3.0, more preferably 1.5 ≤ z ≤ 2.5, 0.5 ≤ q ≤ 3.0, more preferably 0.5 ≤ q ≤ 2.5, 1.0 ≤ v ≤ 4.0, more preferably 2.5 ≤ v ≤ 3.5, and w is an atomic ratio of oxygen which satisfies chemical valence theory. These Tl-Bi-Sr-Ca-Cu and Tl-Bi-Pb-Sr-Ca-Cu systems have preferably the range of "x" of 0.≤ x ≤ 0.5 in the general formula and are composed preferably of a layered crystal structure of a tetragonal system. One of the most important properties of the superconducting oxide material according to the present invention is that the superconducting oxide material exhibits and maintains high critical current density even in strong magnetic field. In fact, the superconducting oxide material according to the present invention exhibits such critical current density Jc (B) that satisfies the following equation: 0.5 ≤ Jc B Jc 0.2T ≤ 1.5 in which Jc (0.2T) is the critical current density determined when the intensity of magnetic field is 0.2 T and Jc (B) is the critical current density determined at any intensity (B) of magnetic field between 0.2 T ≤ B ≤ 2T. The present invention provides a process for producing a single crystal of the superconducting material by using a crystal growth method. When the single crystal of Tl-Bi-Sr-Ca-Cu-O system compound oxide is grown by the method according to the present invention, powders of oxides containing Tl, Bi, Sr, Ca and Cu are mixed in atomic ratios of (Tl+Bi): Sr: Ca : Cu = 2 : 2 : (n-1) : n, in which 1 < Tl/Bi ≤ 3 and "n" is a number between 3 and 7 and are melted at 950 to 1,050 °C at first. Then the resulting melt is cooled down at a cooling rate of 1 to 20 °C/hour to ambient temperature. The product of this method is a single crystal growth represented by (Tl, Bi) 1 Sr 2 Ca 2 Cu 3 O w (w is a number of about 9). Another superconducting compound oxide of the Tl-Bi-Pb-Sr-Ca-Cu-O system also can be obtained by this single crystal growth method. In this system, powders of oxides containing Tl, Bi, Pb, Sr, Ca and Cu are mixed in atomic ratios of (Tl+Bi+Pb) : Sr : Ca : Cu = 2 : 2 : (n-1) : n, in which "n" is a number between 4 and 7 when Tl/(Bi+Pb) > 1 and "n" is a number between 5 and 7 when Tl/(Bi+Pb) = 1, and Bi/(Tl+Bi+Pb) ≥ 0.125 and are melted at 950 to 1,050 °C. After then, the resulting melt is cooled down at a cooling rate of 1 to 20 °C/hour. The product of this method is a single crystal growth represented by (Tl, Bi, Pb) 1 Sr 2 Ca 2 Cu 3 O w (w is a number of about 9). One of the important properties of the superconducting oxide material according to the present invention is that the superconducting oxide material contain a layered crystal structure of the tetragonal system having one or two layers of Tl-O, (Tl, Pb)-O, (Tl, Bi)-O or (Tl, Bi, Pb)-O. The superconducting oxide material according to the present invention has a perovskite type crystal structure. Another important property of the superconducting oxide material according to the present invention is that the superconducting oxide material exhibits very high critical current density (Jc) under a strong magnetic field and furthermore such high critical current density (Jc) does not change even if the strength of the external magnetic field varies. This property may come from the fact that the the superconducting oxide material according to the present invention consists mainly of the high Tc phase. Owing to this important property, the the superconducting oxide material according to the present invention is suitable for producing superconducting wires. Still another important property of the superconducting oxide material according to the present invention is that the superconducting oxide material can be obtained easily in a form of a single crystal having a composition of (Tl, Bi)Sr 2 Ca 2 Cu 3 O w (w is a number of about 9) or (Tl, Bi, Pb)Sr 2 Ca 2 Cu 3 O w (w is a number of about 9) by using the melt-grow method which promotes a much more homogeneous reaction than solid reaction or gas-phase reaction. As is described hereinabove, the oxide superconducting material according to the present invention exhibits very much higher critical current density than the known oxide superconductors such as the Bi-Sr-Ca-Cu-O system and the Tl-Sr-Ca-Cu-O system while they exhibit the same or even higher critical temperature with respect to such known oxide superconductors. It is thought that such an advantage might be obtained from such facts that the superconducting oxide materials according to the present invention have a novel layered crystal structure of the tetragonal system having one or two layers of Tl-O, (Tl, Pb)-O, (Tl, Bi)-O, or (Tl,Pb,Bi)-O and that the superconducting oxide materials according to the present invention are obtainable in the form of a single phase very easily. One of the very important properties or characteristics of the superconducting oxide materials according to the present invention resides in that the critical current density can be maintained at a relatively constant value even if the external magnetic field is changed and is less sensitive to the direction of the magnetic field. This feature is very advantageous in the application as magnetic coils. Brief Description of the Drawing Fig. 1 shows a temperature curve which was programmed to obtain single crystals of oxides represented by (Tl,Bi)Sr 2 Ca 2 Cu 3 O w and (Tl,Bi, Pb)Sr 2 Ca 2 Cu 3 O w prepared in Example 1 and 2. Fig. 2 shows a relation between the critical temperature (Tc) and a value "n" in a composition of (Tl, Bi)SrCaCuO system oxides. Fig. 3 is a X-ray diffraction chart of a high Tc phase of (Tl,Bi)Sr 2 Ca 2 Cu 3 O w in the single crystal. Fig. 4 is a chart obtained by an energy dispersive X-ray spectrometer and showing a composition of the high Tc phase of (Tl,Bi)Sr 2 Ca 2 Cu 3 O w in the single crystal. Fig. 5 is a X-ray diffraction chart of a low Tc phase of (Tl,Bi)Sr 2 Ca 2 Cu 3 O w in the single crystal. Fig. 6 is a chart obtained by an energy dispersive X-ray spectrometer and showing a composition of the low Tc phase of (Tl,Bi)Sr 2 Ca 2 Cu 3 O w in the single crystal. Fig. 7 is a X-ray diffraction chart of a high Tc phase of (Tl,Bi,Pb)Sr 2 Ca 2 Cu 3 O w in the single crystal. Fig. 8 is a chart obtained by an energy dispersive X-ray spectrometer and showing a composition of the high Tc phase of (Tl,Bi,Pb)Sr 2 Ca 2 Cu 3 O w in the single crystal. Fig. 9 is a chart obtained by an energy dispersive X-ray spectrometer and showing a composition of a low Tc phase of (Tl,Bi,Pb)Sr 2 Ca 2 Cu 3 O w in the single crystal. Fig. 10 shows the relation between the critical temperature (Tc) and a value "n" in a composition of (Tl, Bi, Pb)SrCaCuO system oxides. Fig. 11 is a X-ray diffraction chart obtained from Sample No 3 in Example 1. Fig. 12 and 13 show the temperature dependency of magnetic susceptibility and a temperature dependency of electric resistance of Sample No. 3 in Example 1 respectively. Fig. 14 (a) is a photo of of the low Tc phase of the single crystal of superconducting oxide of (Tl, Bi)Sr 2 Ca 2 Cu 3 O w by scanning electron microscope. Fig. 14 (b) is a photo of of the high Tc phase of the single crystal of superconducting oxide of (Tl, Bi)Sr 2 Ca 2 Cu 3 O w by scanning electron microscope. Fig. 14 (c) is a photo of of the low Tc phase of the single crystal of superconducting oxide of (Tl, Bi, Pb)Sr 2 Ca 2 Cu 3 O w by scanning electron microscope. Fig. 14 (d) is a photo of of the high Tc phase of the single crystal of superconducting oxide of (Tl, Bi, Pb)Sr 2 Ca 2 Cu 3 O w by scanning electron microscope. Description of the Preferred Embodiments Now, the present invention will be described in more details by examples, but the scope of the present invention should not limited to the following special examples. Example 1 Single crystal growth of (Tl,Bi)Sr 2 Ca 2 Cu 3 O w Powders of Tl 2 O 3 , Bi 2 O 3 , SrO, CaO and CuO were mixed in atomic ratios of (Tl+Bi) : Sr : Ca : Cu = 2 : 2 : (n-1) : n, in which Tl/Bi = 3, "n" was selected as n = 3, 4, 5, 6 and 7. The resulting powder mixture was packed in a tube made of gold. After the gold tube was closed, the gold tube was placed in a furnace and was heated according to a heating program shown in Fig. 1. Namely, the powder mixture was heated up to 1,000 °C to obtain a melt and then the melt was cooled at a cooling rate of 5 °C/hr down to 800 °C. Then the temperature of 800 °C was maintained for 3 hours to anneal the product. After this annealing, the product was left in the furnace de-energized until the product was cooled down to ambient temperature The critical temperature was determined by DC magnetic susceptibility measurement. Table 1 shows the result. n 3 4 5 6 7 critical temperature (K) 107 108 110 106 105 Fig. 2 shows the relation between the critical temperatures (K) with respect to a variation of "n". The resulting crystal was analyzed by X-ray diffraction to find that the the resulting crystal was a single crystal represented by (Tl, Bi) 1 Sr 2 Ca 2 Cu 3 O w . Fig. 3 is a X-ray diffraction pattern of a typical sample. The composition of the resulting crystal was analyzed by an energy dispersive X-ray spectrometer. Table 2 shows a composition of the typical sample determined by the energy dispersive X-ray spectrometer and Fig. 4 is a powder X-ray diffraction analysis of the sample. Element Tl Bi Sr Ca Cu atomic ratio 0.8 0.3 1.8 2.0 3.0 As comparative examples, the same procedure as above was repeated but the atomic ratios were changed as following: Tl/Bi 1 and "n" = 2, 3, 4, 5 and 6 and Tl/Bi = 3 and "n" = 2. The resulting critical temperatures (K) are summarized in the Fig. 2. As is shown in Fig.2, the critical temperatures are relatively lower (75 to 85 K). A composition of a sample (Tl/Bi = 3 and "n" = 2) in these crystals determined by an energy dispersion type X-ray analyzer is shown in Table 3. Element Tl Bi Sr Ca Cu atomic ratio 0.8 0.3 1.7 1.0 2.0 Fig. 5 is an X-ray diffraction pattern of this sample (Tl/Bi = 3 and "n" = 2) and Fig. 6 as Fig. 4 is a pattern of the energy dispersion type X-ray diffraction analysis of this sample. Fig. 14 (a) is a photo by scanning electron microscope of the single crystal of superconducting oxide shown in Table 3 and Fig. 14 (b) is a photo by scanning electron microscope of the single crystal of superconducting oxide of (Tl, Bi)Sr 2 Ca 2 Cu 3 O w according to the present invention. Example 2 Single crystal growth of (Tl, Bi,Pb)Sr 2 Ca 2 Cu 3 O w Powders of Tl 2 O 3 , Bi 2 O 3 , PbO, SrO, CaO and CuO were mixed in the following atomic ratios of (Tl+Bi+Pb) : Sr : Ca : Cu=2 : 2 : (n-1) n : n = 4, 5, 6 and 7 when Tl/(Bi+Pb) > 1 and n = 5, 6 and 7 when Tl/(Bi+Pb) = 1. The resulting powder mixture was packed in a tube made of gold. After the gold tube was closed, the gold tube was placed in a furnace and was heated according to a heating program shown in the Fig. 1. The procedure was the same as in Example 1. The critical temperature was determined by DC magnetic susceptibility measurements. Table 4 shows the result. Tl/(Bi+Pb) > 1 n 4 5 6 7 critical temperature (K) 109 110 108 107 Tl/(Bi+Pb) = 1 n -- 5 6 7 critical temperature (K) --- 109 110 108 The resulting crystal was analyzed by a powder X-ray diffraction to find that the the resulting crystal was a single crystal represented by (Tl, Bi, Pb) 1 Sr 2 Ca 2 Cu 3 O w . Fig. 7 is a powder X-ray diffraction pattern obtained from a crystal of a typical sample. The composition of the resulting crystal was analyzed by an energy dispersive X-ray spectrometer. Table 5 shows a composition of the typical sample determined by the energy dispersive X-ray spectrometer and Fig. 8 is a pattern of the energy dispersive X-ray spectrometer of the sample. Element Tl Bi Pb Sr Ca Cu atomic ratio 0.7 0.2 0.3 1.8 2.0 3.0 As comparative examples, the same procedure as above was repeated but the atomic ratios was changed as following: Tl/(Bi+Pb) = 1 and "n" = 2, 3 and 4 and Tl/(Bi+Pb) > 1 and "n" = 3. The resulting critical temperatures (K) were relatively lower (75 to 85 K). The composition of the crystal of a typical sample determined by an energy dispersive X-ray spectrometer is shown in Table 6. Element Tl Bi Pb Sr Ca Cu atomic ratio 0.7 0.2 0.3 1.8 1.0 2.0 Fig. 10 shows the relation between the critical temperatures (K) with respect to a variation of the atomic ratios of Tl:Bi:Pb when the number of "n" is changed. Fig. 14 (c) is a photo of Scanning Electron Microscope of the single crystal of superconducting oxide shown in Table 6 and Fig. 14 (d) is a photo of Scanning Electron Microscope of the single crystal of superconducting oxide of (Tl, Bi, Pb)Sr 2 Ca 2 Cu 3 O w according to the present invention.

Claims: A process for producing a single crystal of superconducting compound oxide of Tl, Bi, Sr, Ca and Cu, comprising mixing powders of oxides containing Tl Bi, Sr, Ca and Cu in atomic ratios of (Tl+Bi) : Sr: Ca : Cu = 2 : 2 : (n-1) : n, in which 1 < Tl/Bi ≤ 3 and "n" is a number between 3 and 7, melting the oxide powder mixture at 950 to 1,050 °C, and then cooling down the resulting melt at a cooling rate of 1 to 20 °C/hour. A process for producing a single crystal of superconducting compound oxide of Tl, Bi, Pb, Sr, Ca and Cu, comprising mixing powders of oxides containing Tl, Bi, Pb, Sr, Ca and Cu in atomic ratios of (Tl+Bi+Pb) : Sr: Ca : Cu = 2 : 2 : (n-1) : n, in which "n" is a number between 4 and 7 when Tl/(Bi+Pb) > 1 and "n" is a number between 5 and 7 when Tl/(Bi+Pb) = 1, and Bi/(Tl+Bi+Pb) ≥ 0.125 melting the oxide powder mixture at 950 to 1,050 °C, and then cooling down the resulting melt at a cooling rate of 1 to 20 °C/hour.

Inventors: KOTANI TOSHIHIRO, OHKURA KENGO, TORII YASUKO, KOTANI, TOSHIHIRO, OHKURA, KENGO, TORII, YASUKO, Kotani, Toshihiro, c/o Osaka Works of Sumitomo, Ohkura, Kengo, c/o Osaka Works of Sumitomo, Torii, Yasuko, c/o Osaka Works Of Sumitomo
Applicants: SUMITOMO ELECTRIC INDUSTRIES, SUMITOMO ELECTRIC INDUSTRIES, LTD.
Application year: 1989
Main classification: H01L39
IPC codes: C04B35, H01L39