Abstract:
A novel ceramic substrate useful for the preparation of superconducting films, said substrate having the formula REBa 2  MO 6  where RE represents rare earth metals--Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu and M represents metals Nb, Sb, Sn, Hf, Zr; and a process for the preparation of superconducting YBa 2  Cu 3  O 7- δ  thick films on new ceramic substrate.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a division of application Ser. No. 08/924,662, filed Sep. 5, 1997, now U.S. Pat. No. 5,856,276, which is a continuation of application Ser. No. 08/506,885, filed Aug. 14, 1995, abandoned, which is a continuation of application Ser. No. 08/221,154, filed Mar. 31, 1994, abandoned. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to novel ceramic substrates for YBa 2  Cu 3  O 7- δ  films; a process for the preparation of such substrates and a process for the preparation of superconducting YBa 2  Cu 3  O 7- δ  thick films on the new ceramic substrates. 
     BACKGROUND OF THE INVENTION 
     In the preparation of YBa 2  Cu 3  O 7- δ  (YBCO) thick and thin films, choice of substrate is a key factor. Chemical non-reactivity between the substrate and YBCO is the most crucial for obtaining a superconducting high T c (o) film. Besides, for microwave applications, the substrate should have very low dielectric constant (=10) and the loss factor (10 -4  -10 -5 ) at GHz frequencies [Brandle and Fratello, J. Mater. Res., 5, 2160 (1990)]. For epitaxial growth of the film, lattice matching is also important. MgO is the most widely reported substrate material which satisfies the above substrate characteristics reasonably well for YBCO films. However, MgO do form an interlayer of Ba salt at the YBCO--MgO interface if the temperature of processing is above 700° C. [Koinuma et al., Jpn J. Appl. Phys. 27, L1216 (1988), Cheung and Ruckenstein, J. Mater. Res. 4, 1 (1989) and Preng et al., Supercond. Sci. Technol. 3, 233 (1990)]. At higher temperatures (&gt;700° C.) chemical interdiffusion of the cation across the film boundary in YBCO--MgO is quite prominent [Hu et al., &#34;Advances in Superconductivity&#34;, (Proc. 1st Inter. Symp. on Supercond. ISS 88. Aug. 28-31, 1988, Nagoya, Japan), Edited by K. Kitazawa and I. Ishiguro, Springer-Verlag, Tokyo, p. 647 (1989) and Li et al., J. Mater. Sci., 26, 1057 (1991)]. In particular, in the case of thick film preparation of YBCO using MgO substrate, where the processing temperature required is well above 900° C., the Ba salt formation and cation interdiffusion reduce the superconducting transition τ c (o) of YBCO thick film very appreciably [Bansal, Mater. Lett., 13, 7 (1992)]. Other commercially available substrates such as Si, SiO 2 , Al 2  O 3 , SrTiO 3 , GaAlO 3 , LaAlO 3  etc. either chemically react with YBCO or have a high dielectric constant and loss factor, which makes them unsuitable or less attractive as substrates for YBCO at microwave frequencies. The main object of the present invention is to produce a process for the preparation of ceramic substrates of REBa 2  MO 6  (where RE=Rare Earth metals--Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu and M=Nb, Sb, Sn, Hf, Zr) which are chemically compatible with YBCO and have very low dielectric constant and loss factor making them suitable for microwave applications. 
     SUMMARY OF THE INVENTION 
     We have now invented novel ceramic substrates useful for the preparation of superconducting YBa 2  Cu 3  O 7- δ  films. These substrates are having a formula REBa 2  MO 6  wherein RE represents metals--Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu and M represents metals Nb, Sb, Sn, Hf, Zr. 
     Another aspect of the present invention relates to a process for the preparation of new ceramic substrates of the formula REBa 2  MO 6  where RE represents (rare-earth metals Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu) and M represents metals Nb, Sb, Sn, Hf, Zr, useful for the preparation of superconducting films. 
     In addition, we have successfully screen-printed a thick film or YBCO with a zero transition temperature T c (o) =92 K on these substrates for the first time. 
     Thus, the present invention provides novel ceramic substrates, a novel process for preparing these substrates and a process for the preparation of superconducting YBa 2  Cu 3  O 7- δ  thick films on these new ceramic substrates. 
     Accordingly, one embodiment of the present invention relates to a process for the preparation of new ceramic substrates of the formula REBa 2  MO 6  where RE represents rare earth metals and M represents metals Nb, Sb, Sn, Hf, Zr useful for the preparation of superconducting films which comprises, 
     (i) reacting, salts of the rare earth metals, barium and Nb, Sb, Sn, Hf, Zr in an organic medium, 
     (ii) pressing the resultant mixture in the form of pellets, 
     (iii) calcining the pellets by heating at a temperature in the range of 10000 to 1200° C., 
     (iv) repeating the calcination process for 30 to 45 h, preferably 12 h for each calcination, at temperature in the range of 10000 to 1200° C. till a highly homogeneous mixture is formed, 
     (v) grinding the calcined material and pelletizing at a pressure in the range of 3 to 6 tons/cm 2 , and 
     (vi) sintering the resultant product at a temperature in the range of 12000 to 1600° C. for a period of 10 to 30 h preferably 20 h and then furnace cooled to room temperature. 
     The salts of the rare earths, barium and other metals used may be selected from oxides, carbonates or nitrates. The purity of the salts may be of 99%. The organic medium used may be selected from organic solvents such as acetone, ethyl alcohol, isopropyl alcohol. Three multiple calcinations of the pellets may be conducted at temperature 10000 to 1200° C. for a period ranging from 10 to 15 h preferably 12 h for each calcination. The sintering of the final product may effected for a period of 10 to 30 h, preferably for 20 h. 
     In view of the suitability of REBa 2  MO 6  substrates, we have successfully screen-printed a thick film of YBCO with a zero superconducting transition up to 92 K on these substrates. 
     Thus, yet another aspect of the present invention relates to a process for the preparation of superconducting YBa 2  Cu 3  O 7  -δ thick films on new ceramic substrates of the formula REBa 2  MO 6  where RE represents rare earth metals and M represents metals Nb, Sb, Sn, Hf, Zr, useful for the preparation of superconducting films which comprises, 
     (i) mechanically polishing the ceramic substrate of the above said formula to get highly smooth and shining surfaces, 
     (ii) preparing thick film of YBCO by known methods, 
     (iii) screen printing YBCO on said polished REBa 2  MO 6  substrates using a mesh size in the range of 325, 
     (iv) drying the resulting films at a temperature in the range of 1000 to 150° C., 
     (v) heating the dried films at a rate of 1500 to 250° C./h up to 960° to 1000° C. and soaking at this temperature for 1 to 5 minutes, 
     (vi) cooling the film at a rate of 400° to 600° C./h to bring down the temperature to 800° C. and keeping the films at this temperature for a period of 20 to 40 minutes, and 
     (vii) cooling the film at a rate of 150° to 250° C./h up to 600° to 400° C. and keeping the film at this temperature for a period of 1 to 2 h. Finally the film is furnace cooled to room temperature. 
     All the above steps are carried out in the presence of air or flowing oxygen. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a graph showing X-ray diffraction patterns of sintered (A) GdBa 2  NbO 6 , (B) NdBa 2  SbO 6  and (C) SmBa 2  NbO 6  ; 
     FIG. 2 is a graph showing X-ray diffraction patterns of (A) YBa 2  Cu 3  O 7-  (B) YBa 2  Cu 3  O 7- δ  --GdBa 2  NbO 6 , (C) YBa 2  Cu 3  O 7- δ- --NdBa 2  SbO 6  and (D) YBa 2  Cu 3  O 7- δ- --SmBa 2  NbO 6  ; 
     FIG. 3 is a graph showing X-ray diffraction patterns of screen-printed YBa 2  Cu 3  O 7- δ  thick film on NdBa 2  SbO 6  (substrate peaks are marked by &#34;•&#34;); 
     FIG. 4 is a graph showing X-ray diffraction patterns of screen-printed YBa 2  Cu 3  O 7- δ  thick film on SmBa 2  NbO 6  (substrate peaks are marked by &#34;•&#34;); 
     FIG. 5 is a graph showing the temperature-resistance curve of screen-printed YBa 2  Cu 3  O 7- δ  thick film on NdBa 2  SbO 6  substrate; and 
     FIG. 6 is a graph showing the temperature-resistance curve of screen-printed YBa 2  Cu 3  O 7- δ  thick film on SmBa 2  NbO 6  substrate. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The details of the invention are described in the Examples given below which are provided by way of illustration only and should not be construed to limit the scope of the invention. 
     EXAMPLE 1 
     Preparation of Ceramic Substrate of the Formula GdBa 2  NbO 6   
     GdBa 2  NbO 6  was prepared by solid state reaction method. Gd 2  O 3 , BaCO 3  and Nb 2  O 5  (purity 99%) were taken in stoichiometric ratio, mixed thoroughly in acetone medium and calcined in air at 1150° C. for 12 h with two intermediate grindings. The resultant mixture was powdered and pressed at a pressure of 4 tons/cm 2  in the form of circular pellets and sintered in air at 1450° C. for 20 h. 
     EXAMPLE 2 
     Preparation of Ceramic Substrate of the Formula NdBa 2  SbO 6   
     NdBa 2  SbO 6  was prepared by solid state reaction method. Nd 2  O 3 , BaCO 3  and Sb 2  O 3  (purity 99%) were taken in stoichiometric ratio, mixed thoroughly in acetone medium and calcined in air at 1100° C. for 10 h with two intermediate grindings. The resultant mixture was powdered and pressed at a pressure of 5 tons/cm 2 , in the form of circular pellets and sintered in air at 1400° C. for 16 h. 
     EXAMPLE 3 
     Preparation of Ceramic Substrate of the Formula SmBa 2  NbO 6   
     SmBa 2  NbO 6  was prepared by solid state reaction method. Sm 2  O 3 , BaCO 3  and Nb 2  O 5  were taken in stoichiometric ratio, wet mixed in acetone medium and calcined in air at 1200° C. for 15 h with two intermediate grindings. Calcined powder was finally, ground and pelletized at a pressure of 5 tons/cm 2  in the form of circular pellets and sintered at 1350° C. for 15 h in air. 
     Structure Determination 
     The structure of the sintered materials was examined by X-ray diffraction (XRD) method and it is found that all these materials are isostructural having a cubic perovskite structure as shown in the XRD patterns in FIG. 1 (Sheet No. 1) of the drawings accompanying this specification for three typical samples GdBa 2  NbO 6  (Example 1), NdBa 2  SbO 6  (Example 2) and SmBa 2  NbO 6  (Example 3). In tables 1a to c we have given the computerized XRD data for these materials. 
     
                       TABLE 1(a)______________________________________Computerized XRD data GdBa.sub.2 NbO.sub.6No.    2          Intensity                      Width   d    I/Io______________________________________1      29.780     7988     0.459   2.998                                   1002      36.730      594     0.480   2.445                                    73      42.660     2358     0.525   2.118                                   304      52.890     2882     0.555   1.730                                   36______________________________________ 
    
     
                       TABLE 1(b)______________________________________Computerized XRD data NdBa.sub.2 SbO.sub.6No.    2          Intensity                      Width   d    I/Io______________________________________1      29.740     9506     0.375   3.002                                   1002      36.690      250     0.210   2.447                                    33      42.510     2776     0.420   2.125                                   294      52.680     3004     0.555   1.736                                   32______________________________________ 
    
     
                       TABLE 1(c)______________________________________Computerized XRD data SmBa.sub.2 NbO.sub.6No.    2          Intensity                      Width   d    I/Io______________________________________1      29.730     7988     0.420   3.003                                   1002      36.630      476     0.270   2.451                                    63      42.570     2569     0.435   2.122                                   324      52.780     2750     0.495   1.733                                   34______________________________________ 
    
     Dielectric Properties 
     The dielectric properties of the substrate materials were measured at 30 Hz to 13 MHz frequencies and values of the dielectric constant (ε&#39;) and loss factor (tan δ) at 13 MHz frequency are 24 and 10 -4  respectively at room temperature. At GHz frequencies the values of ε&#39; and tan δ are found to be -6 and 10 -5  respectively, which are ideally suitable for microwave applications. 
     Chemical Compatibility with YBCO 
     The most important characteristics of REBa 2  MO 6 , which makes these materials ideal as substrates for YBCO, is their chemical non-reactivity with YBCO even at extreme processing conditions. The chemical reactivity between YBCO and REBa 2  MO 6  was studied by mixing 1:1 molar mixture of YBCO and REBa 2  MO 6  and heating the pressed samples at 950° C. for 15 h. In FIG. 2 (sheet No. 2), the XRD pattern of a pure YBa 2  Cu 3  O 7- δ  superconductor (FIG. 2a) and three representative XRD patterns of annealed YBa 2  Cu 3  O 7- δ --GdBa 2  NbO 6  (FIG. 2b). YBa 2  Cu 3  O 7  --NdBa 2  SbO 6  (FIG. 2c) and YBa 2  Cu 3  O 7- δ --SmBa 2  NbO 6  (FIG. 2d) samples mixed in 1:1 molar ratio and heated at 950° C. for 15 h have been given. The XRD patterns clearly show that there is no additional phase formed besides YBCO and REBa 2  MO 6  in the composite. This indicates that there is no reaction taking place between YBCO and REBa 2  MO 6  even under severe heat treatment. REBa 2  MO 6  are insulating oxides with room temperature resistivities of the order of 10 10  Ωcm. Detailed percolation studies carried out on YBCO--REBa 2  MO 6  composites also showed that the presence of REBa 2  MO 6  up to 60 vol % in the composite did not show any detrimental effect on the super-conducting transition temperature of YBCO. 
     Because of the excellent dielectric properties and chemical compatibility of the novel substrates with YBCO as described above, we also invented a process for the preparation of superconducting YBa 2  Cu 3  O 7- δ  thick films on the newly developed ceramic substrates of REBa 2  MO 6  as described above. In other words, in view of the suitability of REBa 2  MO 6  substrates, we have successfully screen printed a thick film of YBCO with a zero superconducting transition up to 92 K on these substrates. 
     We quote a few prior art references regarding the formation of superconducting films here below: 
     Superconducting thick films have wide applications in microwave integrated circuits transmission lines and other microelectronic devices [Alford et al., Supercond. Sci. Technol., 4, 433 (1991), Khare et al., Supercond. Sci. Technol, 4, 107 (1991) and Alford et al., Nature, 349, 680 (1991)]. Screen printing is a straight forward and highly economical process for the deposition of thick films of YBa 2  Cu 3  O 7- δ (YBCO) superconductors [Budhani et al., Appl. Phys. Lett., 51, 1277 (1987), Shih and Riu, Appl. Phys. Lett., 52, 748 (1988), Koinuma et al., Jpn. J. Appl. Phys., 26, L399 (1987), Bansal and Simon, Appl. Phys. Lett., 53, 603 (1988) and Bansal, Mater. Lett., 13, 7 (1992)]. In the preparation of superconducting thick films, the choice of substrate is a key factor. Chemical non-reactivity between the substrates and YBCO is most crucial for obtaining superconducting films with high transition temperature (≈90 K). 
     Our process for fabrication of YBCO screen-printed thick film on REBa 2  MO 6  substrates is given below: 
     Before screen-printing YBCO, REBa 2  MO 6  substrates were polished mechanically to get highly smooth and shining surfaces. Thick film paste of YBCO, which we used for screen printing was made by mixing YBCO powder with an organic vehicle and well mixed paste was achieved by hand milling. Film thickness was controlled via viscosity control of YBCO paste. Thick film of this YBCO paste was then screen-printing on REBa 2  MO 6  substrate using a mesh size of 325. Printed films were dried at 100° to 150° C. for 2 to 3 h. Dried films were heated in a programmable furnace at a rate of 150° to 250° C./h up to 950-1000° C. and soaked at this temperature for 1-5 min. The films were cooled down at a rate of 400°-600° C./h and brought down to 800°-900° C. and kept at this temperature for 20-40 min and films were then cooled down at a rate of 150° C. to 250° C./h up to 600°-400° C. and kept at this temperature for 1 to 2 h and after that cooled down to room temperature. All the above processing were done either in air or in flowing oxygen. The structure of the film was examined by X-ray diffraction method and XRD patterns of two typical screen-printed YBCO films on NdBa 2  SbO 6  and SmBa 2  NbO 6  substrates are shown in FIGS. 3 and 4 (given in sheet Nos. 3 and 4) respectively. The XRD patterns of these YBCO thick films showed that except for the characteristic peaks of NdBa 2  SbO 6  and SmBa 2  NbO 6  substrates, all other peaks could be assigned to a phase pure orthorhombic superconducting YBCO structure. 
     In the following examples illustrate the preparation of superconducting films employing the novel substrates: 
     EXAMPLE 4 
     Preparation of Superconducting Film Using Ceramic Substrate of the Formula NdBa 2  SbO 6   
     A YBa 2  Cu 3  O 7- δ  paste was made by mixing fine YBa 2  Cu 3  O 7- δ  powder with organic vehicle and its viscosity was controlled by adding fish oil. This paste was screen-printed on a highly polished NdBa 2  SbO 6  substrate using a screen of 325 mesh size. The film, after drying at 100° C. for 90 minutes was heated in a programmable furnace at a rate of 3° C./minute up to 980° C. and soaked at this temperature for 5 minutes. It was then cooled at a rate of 10° C./minute to bring down the temperature to 900° C. and kept at 900° C. for 30 minutes. It was then cooled to 600° C. at a rate of 4° C./minute and again kept at this temperature for 60 minutes. The film was finally furnace cooled to room temperature. Entire processing was carried out in air. 
     EXAMPLE 5 
     Preparation of Superconducting Film Using Ceramic Substrate of the Formula SmBa 2  NbO 6   
     A YBa 2  Cu 3  O 7- δ  paste was made by mixing fine YBa 2  Cu 3  O 7- δ  powder with organic vehicle and its viscosity was controlled by adding fish oil. The paste was screen-printed on highly polished SmBa 2  NbO 6  substrate using a screen of 325 mesh size. The film, after drying at 120° C. for 60 minutes was heated in a programmable furnace at a rate of 3° C./minute up to 980° C., soaked at this temperature for 5 minutes. It was then cooled at a rate of 8° C./minute to bring down the temperature to 900° C. for 30 minutes. It was then cooled to 550° C. at a rate of 3° C./minute and again kept at this temperature for 60 minutes. The film was finally furnace cooled to room temperature. The entire processing was done in air. 
     The structure of the film was examined by X-ray diffraction method. The XRD patterns of two typical screen-printed YBCO films on NdBa 2  SbO 6  (Example 4) and SmBa 2  NbO 6  (Example 5) substrates are shown in FIGS. 3 and 4 of the drawings (Sheet Nos. 3 and 4) accompanying this specification. The XRD pattern of these YBCO thick films showed that except for the characteristic peaks of NdBa 2  SbO 6  and SmBa 2  NbO 6  substrates, all other peaks could be assigned to a phase pure orthorhombic superconducting YBCO structure. 
     Superconductivity in these YBCO screen-printed thick films on REBa 2  MO 6  substrates were studied by temperature-resistance measurements. The films show a metallic behavior in the normal state and give superconducting transition temperature of 88-94 K (T c  onset) and 85-92 K (T c (o) as shown in the two representative examples given in FIGS. 5 and 6 (sheet Nos. 5 and 6). These films give a current density of the order of 10 5  A/cm 2 .