Process for preparing YBCO superconducting films on a ceramic substrate

Abstract

A novel ceramic substrate useful for the preparation of superconducting films, said substrate having the formula REBa.sub.2 MO.sub.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.sub.2 Cu.sub.3 O.sub.7-.delta. thick films on new ceramic substrate.

Description

FIELD OF THE INVENTION

This invention relates to novel ceramic substrates for YBa.sub.2 Cu.sub.3 O.sub.7-.delta. films; a process for the preparation of such substrates and a process for the preparation of superconducting YBa.sub.2 Cu.sub.3 O.sub.7-.delta. thick films on the new ceramic substrates.

BACKGROUND OF THE INVENTION

In the preparation of YBa.sub.2 Cu.sub.3 O.sub.7-.delta. (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.sub.c(o) film. Besides, for microwave applications, the substrate should have very low dielectric constant (=10) and the loss factor (10.sup.-4 -10.sup.-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.degree. 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 (>700.degree. C.) chemical interdiffusion of the cation across the film boundary in YBCO--MgO is quite prominent (Hu et al., "Advances in Superconductivity", (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.degree. C., the Ba salt formation and cation interdiffusion reduce the superconducting transition t.sub.c(o) of YBCO thick film very appreciably (Bansal, Mater. Lett., 13, 7 (1992)). Other commercially available substrates such as Si, SiO.sub.2, Al.sub.2 O.sub.3, SrTiO.sub.3, GaAlO.sub.3, LaAlO.sub.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.sub.2 MO.sub.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.sub.2 Cu.sub.3 O.sub.7-.delta. films. These substrates are having a formula REBa.sub.2 MO.sub.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.sub.2 MO.sub.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.sub.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.sub.2 Cu.sub.3 O.sub.7-.delta. 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.sub.2 MO.sub.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 1000.degree. to 1200.degree. C., (iv) repeating the calcination process for 30 to 45 h, preferably 12 h for each calcination, at temperature in the range of 1000.degree. to 1200.degree. 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.sup.2, and (vi) sintering the resultant product at a temperature in the range of 1200.degree. to 1600.degree. 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 1000.degree. to 1200.degree. 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.sub.2 MO.sub.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.sub.2 Cu.sub.3 O.sub.7-.delta. thick films on new ceramic substrates of the formula REBa.sub.2 MO.sub.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.sub.2 MO.sub.6 substrates using a mesh size in the range of 325, (iv) drying the resulting films at a temperature in the range of 100.degree. to 150.degree. C., (v) heating the dried films at a rate of 150.degree. to 250.degree. C./h up to 960.degree. to 1000.degree. C. and soaking at this temperature for 1 to 5 minutes, (vi) cooling the film at a rate of 400.degree. to 600.degree. C./h to bring down the temperature to 800.degree. 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.degree. to 250.degree. C./h up to 600.degree. to 400.degree. 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.sub.2 NbO.sub.6, (B) NdBa.sub.2 SbO.sub.6 and (C) SmBa.sub.2 NbO.sub.6 ;

FIG. 2 is a graph showing X-ray diffraction patterns of (A) YBa.sub.2 Cu.sub.3 O.sub.7- (B) YBa.sub.2 Cu.sub.3 O.sub.7-.delta. --GdBa.sub.2 NbO.sub.6, (C) YBa.sub.2 Cu.sub.3 O.sub.7-.delta.- --NdBa.sub.2 SbO.sub.6 and (D) YBa.sub.2 Cu.sub.3 O.sub.7-.delta.- --SmBa.sub.2 NbO.sub.6 ;

FIG. 3 is a graph showing X-ray diffraction patterns of screen-printed YBa.sub.2 Cu.sub.3 O.sub.7-.delta. thick film on NdBa.sub.2 SbO.sub.6 (substrate peaks are marked by ".circle-solid.");

FIG. 4 is a graph showing X-ray diffraction patterns of screen-printed YBa.sub.2 Cu.sub.3 O.sub.7-.delta. thick film on SmBa.sub.2 NbO.sub.6 (substrate peaks are marked by ".circle-solid.");

FIG. 5 is a graph showing the temperature-resistance curve of screen-printed YBa.sub.2 Cu.sub.3 O.sub.7-.delta. thick film on NdBa.sub.2 SbO.sub.6 substrate; and

FIG. 6 is a graph showing the temperature-resistance curve of screen-printed YBa.sub.2 Cu.sub.3 O.sub.7-.delta. thick film on SmBa.sub.2 NbO.sub.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.sub.2 NbO.sub.6

GdBa.sub.2 NbO.sub.6 was prepared by solid state reaction method. Gd.sub.2 O.sub.3, BaCO.sub.3 and Nb.sub.2 O.sub.5 (purity 99%) were taken in stoichiometric ratio, mixed thoroughly in acetone medium and calcined in air at 1150.degree. C. for 12 h with two intermediate grindings. The resultant mixture was powdered and pressed at a pressure of 4 tons/cm.sup.2 in the form of circular pellets and sintered in air at 1450.degree. C. for 20 h.

EXAMPLE 2

Preparation of Ceramic Substrate of the Formula NdBa.sub.2 SbO.sub.6

NdBa.sub.2 SbO.sub.6 was prepared by solid state reaction method. Nd.sub.2 O.sub.3, BaCO.sub.3 and Sb.sub.2 O.sub.3 (purity 99%) were taken in stoichiometric ratio, mixed thoroughly in acetone medium and calcined in air at 1100.degree. C. for 10 h with two intermediate grindings. The resultant mixture was powdered and pressed at a pressure of 5 tons/cm.sup.2.sub.1, in the form of circular pellets and sintered in air at 1400.degree. C. for 16 h.

EXAMPLE 3

Preparation of Ceramic Substrate of the Formula SmBa.sub.2 NbO.sub.6

SmBa.sub.2 NbO.sub.6 was prepared by solid state reaction method. Sm.sub.2 O.sub.3, BaCO.sub.3 and Nb.sub.2 O.sub.5 were taken in stoichiometric ratio, wet mixed in acetone medium and calcined in air at 1200.degree. C. for 15 h with two intermediate grindings. Calcined powder was finally, ground and pelletized at a pressure of 5 tons/cm.sup.2 in the form of circular pellets and sintered at 1350.degree. 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.sub.2 NbO.sub.6 (Example 1), NdBa.sub.2 SbO.sub.6 (Example 2) and SmBa.sub.2 NbO.sub.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 (.di-elect cons.') and loss factor (tan .delta.) at 13 MHz frequency are 24 and 10.sup.-4 respectively at room temperature. At GHz frequencies the values of .di-elect cons.' and tan .delta. are found to be -6 and 10.sup.-5 respectively, which are ideally suitable for microwave applications.

Chemical Compatibility with YBCO

The most important characteristics of REBa.sub.2 MO.sub.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.sub.2 MO.sub.6 was studied by mixing 1:1 molar mixture of YBCO and REBa.sub.2 MO.sub.6 and heating the pressed samples at 950.degree. C. for 15 h. In FIG. 2 (sheet No. 2), the XRD pattern of a pure YBa.sub.2 Cu.sub.3 O.sub.7-.delta. superconductor (FIG. 2a) and three representative XRD patterns of annealed YBa.sub.2 Cu.sub.3 O.sub.7-.delta. --GdBa.sub.2 NbO.sub.6 (FIG. 2b). YBa.sub.2 CU.sub.3 O.sub.7 --NdBa.sub.2 SbO.sub.6 (FIG. 2c) and YBa.sub.2 Cu.sub.3 O.sub.7-.delta. --SmBa.sub.2 NbO.sub.6 (FIG. 2d) samples mixed in 1:1 molar ratio and heated at 950.degree. C. for 15 h have been given. The XRD patterns clearly show that there is no additional phase formed besides YBCO and REBa.sub.2 MO.sub.6 in the composite. This indicates that there is no reaction taking place between YBCO and REBa.sub.2 MO.sub.6 even under severe heat treatment. REBa.sub.2 MO.sub.6 are insulating oxides with room temperature resistivities of the order of 10.sup.10 .OMEGA.cm. Detailed percolation studies carried out on YBCO--REBa.sub.2 MO.sub.6 composites also showed that the presence of REBa.sub.2 MO.sub.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.sub.2 Cu.sub.3 O.sub.7-.delta. thick films on the newly developed ceramic substrates of REBa.sub.2 MO.sub.6 as described above. In other words, in view of the suitability of REBa.sub.2 MO.sub.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.sub.2 Cu.sub.3 O.sub.7-.delta. (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 (.about.90 K).

Our process for fabrication of YBCO screen-printed thick film on REBa.sub.2 MO.sub.6 substrates is given below:

Before screen-printing YBCO, REBa.sub.2 MO.sub.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.sub.2 MO.sub.6 substrate using a mesh size of 325. Printed films were dried at 100.degree. to 150.degree. C. for 2 to 3 h. Dried films were heated in a programmable furnace at a rate of 150.degree. to 250.degree. C./h up to 950-1000.degree. C. and soaked at this temperature for 1-5 min. The films were cooled down at a rate of 400.degree.-600.degree. C./h and brought down to 800.degree.-900.degree. C. and kept at this temperature for 20-40 min and films were then cooled down at a rate of 150.degree. C. to 250.degree. C./h up to 600.degree.-400.degree. 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.sub.2 SbO.sub.6 and SmBa.sub.2 NbO.sub.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.sub.2 SbO.sub.6 and SmBa.sub.2 NbO.sub.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.sub.2 SbO.sub.6

A YBa.sub.2 Cu.sub.3 O.sub.7-.delta. paste was made by mixing fine YBa.sub.2 Cu.sub.3 O.sub.7-.delta. powder with organic vehicle and its viscosity was controlled by adding fish oil. This paste was screen-printed on a highly polished NdBa.sub.2 SbO.sub.6 substrate using a screen of 325 mesh size. The film, after drying at 100.degree. C. for 90 minutes was heated in a programmable furnace at a rate of 3.degree. C./minute up to 980.degree. C. and soaked at this temperature for 5 minutes. It was then cooled at a rate of 10.degree. C./minute to bring down the temperature to 900.degree. C. and kept at 900.degree. C. for 30 minutes. It was then cooled to 600.degree. C. at a rate of 4.degree. 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.sub.2 NbO.sub.6

A YBa.sub.2 Cu.sub.3 O.sub.7-.delta. paste was made by mixing fine YBa.sub.2 Cu.sub.3 O.sub.7-.delta. powder with organic vehicle and its viscosity was controlled by adding fish oil. The paste was screen-printed on highly polished SmBa.sub.2 NbO.sub.6 substrate using a screen of 325 mesh size. The film, after drying at 120.degree. C. for 60 minutes was heated in a programmable furnace at a rate of 3.degree. C./minute up to 980.degree. C., soaked at this temperature for 5 minutes. It was then cooled at a rate of 8.degree. C./minute to bring down the temperature to 900.degree. C. for 30 minutes. It was then cooled to 550.degree. C. at a rate of 3.degree. 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.sub.2 SbO.sub.6 (Example 4) and SmBa.sub.2 NbO.sub.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.sub.2 SbO.sub.6 and SmBa.sub.2 NbO.sub.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.sub.2 MO.sub.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.sub.c onset) and 85-92 K (T.sub.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.sup.5 A/cm.sup.2.

Claims

1. A process for the preparation of a superconducting YBa.sub.2 Cu.sub.3 O.sub.7-.delta. film on a ceramic substrate, said process comprising:

(i) providing the ceramic substrate having the formula REBa.sub.2 MO.sub.6, wherein RE represents a rare earth metal selected from the group consisting of Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu, and wherein M represents a metal selected from the group consisting of Nb, Sb, Sn, Hf, and Zr;

(ii) mechanically polishing the ceramic substrate to provide a smooth, shining surface thereon;

(iii) preparing a film of YBa.sub.2 Cu.sub.3 O.sub.7-.delta. by mixing YBa.sub.2 Cu.sub.3 O.sub.7-.delta. with an organic vehicle;

(iv) printing the YBa.sub.2 Cu.sub.3 O.sub.7-.delta. film on the polished substrate; and

(v) drying the printed film to obtain the superconducting YBa.sub.2 Cu.sub.3 O.sub.7-.delta. film on a ceramic substrate,

wherein all the above steps are carried out in the presence of air or of flowing oxygen.

2. The process as claimed in claim 1 where the printing of the film is conducted by screen printing using a mesh size in the range of 325.

3. The process as claimed in claim 1 where the film is dried at a temperature in the range of 100.degree. C. to 150.degree. C.

4. The process as claimed in claim 1 where the dried film is heated at a rate of 150.degree. C. to 250.degree. C./h up to a maximum temperature of 950.degree. C. to 1000.degree. C. and then is soaked at this temperature for 1 to 5 minutes.

5. The process as claimed in claim 4 which further comprises cooling the heated film at a rate of 400.degree. to 600.degree. C./h to reduce the temperature to 800.degree. to 900.degree. C. and maintaining the film at this temperature for a period of 20 to 40 minutes.

6. The process as claimed in claim 4 where the film is further cooled the film at a rate of 150.degree. C. to 250.degree. C./h down to 600.degree. to 400.degree. C. and is maintained at this temperature for a period of 1 to 2 h.

7. A process for the preparation of superconducting YBa.sub.2 Cu.sub.3 O .sub.7-.delta. thick films on a ceramic substrate, having the formula REBa.sub.2 MO.sub.6, wherein RE represents a rare earth metal selected from the group consisting of Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu, and wherein M represents a metal selected from the group consisting of Nb, Sb, Sn, Hf, and Zr, said process comprising:

(i) mechanically polishing the ceramic substrate of the above said formula to get smooth and shining surfaces;

(ii) preparing a thick film of YBa.sub.2 Cu.sub.3 O.sub.7-.delta. by mixing YBa.sub.2 Cu.sub.3 O.sub.7-.delta. with an organic vehicle;

(iii) screen-printing the thick film of YBa.sub.2 Cu.sub.3 O.sub.7-.delta. on REBa.sub.2 MO.sub.6 substrate using a mesh size in the range of 325;

(iv) drying the resulting film at a temperature in the range of 100.degree. C. to 150.degree. C.;

(v) heating the dried film at a rate of 150.degree. C. to 250.degree. C./h up to a maximum temperature of 950.degree. C. to 1000.degree. C. and soaking at this temperature for 1 to 5 minutes;

(vi) cooling the film at a rate of 400.degree. to 600.degree. C./h to bring down the temperature to 800.degree. to 900.degree. 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.degree. C. to 250.degree. C./h down to 600.degree. to 400.degree. C. and keeping the film at this temperature for a period of 1 to 2 h,

wherein all the above steps are carried out in the presence of air or of flowing oxygen.