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Electroless silver coating on fine copper powder and its effects on oxidation resistance
Materials Letters 57 (2003) 3987 – 3991 www.elsevier.com/locate/matlet
Electroless silver coating on fine copper powder and its effects on oxidation resistance Xinrui Xu *, Xiaojun Luo, Hanrui Zhuang, Wenlan Li, Baolin Zhang Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Dingxi Road, Shanghai 200050, China Received 10 September 2002; received in revised form 11 February 2003; accepted 19 February 2003
Abstract Electroless silver coating on fine copper powder (3.4 Am) and its effects on oxidation resistance were investigated by varying the silver contents. As-coated copper powders were characterized using X-ray diffraction (XRD) and scanning electron microscopy (SEM). It was found that the uniformity of silver coating was improved with silver content. When the silver content reached 20 wt.%, silver was homogeneously distributed around the copper particles and few free silver particles were detected. As a result, the sheet resistances of metal films reduced with the silver content (presenting the improvement of oxidation resistance), and at the level of 20 wt.% silver content, it had a minimum value and hardly increased with the increasing oxidation time. D 2003 Elsevier Science B.V. All rights reserved. Keywords: Silver coating; Electroless; Copper powder; Oxidation resistance
1. Introduction Metal particles used as electrically and thermally conductive fillers for polymeric materials typically comprise gold, silver, copper, nickel or aluminum. Gold and silver whilst possessing excellent conductivity properties are, however, expensive. Nickel and copper display good electrical conductivities but are deleteriously susceptible to oxidation, resulting in the concomitant diminution of desirable properties such as quality and stability. In order to overcome the problems associated with the oxidation of nickel and copper, or the high cost of using silver, the substitu-
* Corresponding author. Tel.: +86-21-52414213; fax: +86-2152413903. E-mail address: [email protected] (X. Xu).
tion by silver-coated copper particles for solid silver particles has been explored [1 – 3]. Generally, powder coating treatment can be achieved by electroplating [4], electroless plating [5,6], and vacuum process (evaporation, sputtering [7] etc.). It is well known that silver can be deposited on many substrates by electroplating [8]. However, the efficiency of electroplating and vacuum coating for powders is very low. Therefore, electroplating and vacuum processes are not feasible for commercial purpose. Electroless silver [2,3,9,10] coating, however, can be successfully achieved on copper powder and has much higher deposition rate than electroplating, although there are still difficulties in reducing process steps and coating on the fine copper powder which would have good sinterability. This paper described a simple electroless silver coating process on fine copper powder. The effects of
0167-577X/03/$ - see front matter D 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0167-577X(03)00252-0
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X. Xu et al. / Materials Letters 57 (2003) 3987–3991
silver coating on oxidation resistance of fine copper powder were also investigated by varying the silver contents. The coating structures at different silver content were studied in detail with special emphasis on their effects on the properties of oxidation resistance. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were employed as main techniques.
2. Experimental procedure 2.1. Silver coating Copper powder with an average particle size of 3.4 Am and chemical purity of greater than 99.9% was first dispersed in an ammonium sulphate (purity>99.0%) and ammonium hydroxide solution for 10 min to remove surface oxide film and activate the surface. After this pretreatment, a sodium potassium tartrate (purity>99.0%) solution was added to the slurry for reduction. An aqueous solution containing silver nitrate (purity>99.8%) and ammonium hydroxide was then added dropwise. Table 1 lists the typical silver coating process parameters. After termination of the silver coating process, the silver-coated copper powder was separated, then washed using deionised water and alcohol successively to remove the residual chemicals and impurities, and then baked at 100jC in a vacuum atmosphere.
2.3. Characterization The cross-sectional areas of the plated powders were investigated using scanning electron microscope (EPMA-8705Q). The phase composition of the prepared powder was determined by using XRD. The property of oxidation resistance was tested by measuring sheet resistances of metal films which changed with the increasing of oxidation time (oxidation time was the time when metal films were exposed in air at 150 jC). The sheet resistances of silver-coated copper thick films were measured by a semiconductor resistivity meter (BD-86A) on samples which have 1010 mm square metal film patterns.
3. Results and discussion 3.1. Silver coating on copper particles from silver ion containing ammonia solutions In an ammonia solution, it is estimated that the silver reduction process was controlled by the following reactions [10]: 1. Dissolution of surface oxides and hydroxides. Cu2 O þ 2NH4 OH þ ðNH4 Þ2 SO4 ¼ ½CuðNH3 Þ2 2 SO4 þ 3H2 O
ð1Þ
CuO þ 2NH4 OH þ ðNH4 Þ2 SO4
2.2. Metal paste preparation To investigate the property of oxidation resistance, the prepared silver-coated copper powder was mixed with appropriate amount of ethyl cellulose –terpineol solution in a three-roll mill. The silver-coated copper paste was ground to the required consistency, and screened on to the ceramic substrates, and then dried at 100 jC for 20 min in vacuum.
¼ CuðNH3 Þ4 SO4 þ 3H2 O
ð2Þ
CuðOHÞ2 þ 2NH4 OH þ ðNH4 Þ2 SO4 ¼ CuðNH3 Þ4 SO4 þ 4H2 O
ð3Þ
2. Dissolution of silver nitrate in ammonium hydroxide. 2AgNO3 þ 2NH4 OH
Table 1 Composition of the electroless plating bath and operating parameters Bath composition
Operating parameters
(NH4)2SO4 NH3 H2O AgNO3 C4H4O6KNa
Temperature pH value Reduction time Powder load
0.4 – 0.8 mol/l 0.8 – 1.3 mol/l 25 – 120 g/l 300 – 600 g/l
25 jC 8 – 12 5 – 20 min 80 – 150 g/l
¼ Ag2 O # þ2NH4 NO3 þ H2 O
ð4Þ
Ag2 O þ 4NH4 OH ¼ 2½AgðNH3 Þ2 OH þ 3H2 O ð5Þ ½AgðNH3 Þ2 OH þ NH4 NO3 ¼ ½AgðNH3 Þ2 NO3 þ NH4 OH
ð6Þ
X. Xu et al. / Materials Letters 57 (2003) 3987–3991
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3. Displacement reaction
ð7Þ
During electroless plating, silver metal was deposited by reduction – oxidation reactions involving the Ag-amine as described above in Eq. (7). A comparison of the standard reduction –oxidation potentials (DEo) shown in Eq. (7) indicates that the deposition of silver is favored. A similar observation was also reported by Tan et al. [11]. In addition, there were also side reactions:
2AgðNH3 Þþ 2 þ 2OH ¼ Ag2 O # þ4NH3 þ H2 O ð8Þ
3.3. Morphologies and phase composition
3Ag2 O þ C4 H4 O2
6 þ 2OH
¼ 6Ag # þ2C2 O2
4 þ 3H2 O
that the ratio of ammonia to ammonium sulphate should be less than 4:1. The silver ion solution used in this study was a freshly prepared aqueous solution of silver nitrate and ammonium hydroxide. The amount of ammonium hydroxide required in the silver ion solution was dependent on when the Ag2O precipitation was dissolved completely. It was found that the suitable molar ratio of silver nitrate and ammonium hydroxide was 1:3.
ð9Þ
The Ag2O produced from solution in Eq. (8) could be reduced by successive reaction as shown in Eq. (9). Sodium potassium tartrate used in this study functioned as both a mild reducing agent and as a complex agent for the Cu2+ ions to suppress precipitation of copper hydroxides. 3.2. Effects of coating parameters on silver reduction As is well known, heterogeneous electroless metal depositions are required to minimize the precipitation of free metal powder from the solution (this may also decompose the electroless solution) and the surface activation is therefore, important. However, since the electroless silver plating demonstrated an autocatalytic process [12], the reduction –oxidation reactions occurred preferentially around the silver seeds in the absence of surface activation. They were initiated by the reaction of copper with ammonia (Eq. (7)), with simultaneous release of electrons, which were then used for the decomposition of Ag-amine complexes. Subsequently, the silver was deposited onto the nuclei resulting in growth. The quantity of ammonia controlled the rate of copper dissolution, and it was found
The homogeneous distribution of silver coating over copper surface is required for ideal silver coating. This might be difficult to achieve in practice, particularly at low silver content and on fine copper powder which always attempt to aggregate. In this study, the ultrasonic and stirring methods were employed to disperse copper powder throughout the process, and copper powders coated with 5 to 20 wt.% silver were achieved by the above electroless plating. Fig. 1 shows the cross-section images of silver-coated copper powders. It can be seen that the uniformity of silver coating was improved with the increase of silver content. At the level of 5 wt.% silver content, no evident silver layer is observed indicating insufficient silver content for the fine copper particles (3.4 Am). Compared with the 5 wt.% silver-coated copper powder, the cross-section image of 10 wt.% silver-coated copper powders presents thin, but not continuous silver layer (see Fig. 1b). With the increasing silver content, a thicker and partially continuous silver layer is formed on copper particles at the level of 15 wt.% silver content (see Fig. 1c). When the silver content increases to 20 wt.%, silver is homogeneously distributed around the copper particles and few free silver particles are detected.
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Fig. 1. Cross-section micrographs of silver-coated copper powder with (a) 5, (b) 10, (c) 15, and (d) 20 wt.% silver contents.
A series of performance tests were carried out for various content of silver-coated copper powders. Fig.
3 summarizes the effects of silver content on oxidation resistance. The sheet resistances of metal film prepared from 5 wt.% silver-coated copper powder, increased dramatically with increasing oxidation time, which was consistent with the microstructure depicted in Fig. 1a. At this level of silver content, the little silver coating on copper powder was not sufficient to prevent copper powder from oxidation, so the increasing rate of sheet resistance with oxidation time was
Fig. 2. XRD pattern of the typical silver-coated copper powder.
Fig. 3. Dependence of sheet resistance of metal films prepared from 5, 10, 15, and 20 wt.% silver-coated copper powder on oxidation time (the oxidation time is the time when metal films were exposed in air at 150 jC).
Fig. 2 shows an XRD spectrum of the 15 wt.% silver-coated copper powder. In addition to copper, a pure silver phase was clearly identified and it possessed a crystalline rather than an amorphous structure. 3.4. Performance comparison of oxidation resistance
X. Xu et al. / Materials Letters 57 (2003) 3987–3991
steep. With the increase of silver content, the uniformity of silver coating increased, implicating that the properties of oxidation resistance were improved. As a result, the sheet resistance increment slowed down with the oxidation time. For the 20 wt.% silver, a fairly continuous silver layer was formed on copper particle surface and few free silver particles were detected (Fig. 1d), thus the sheet resistance of this metal film had a minimum value compared with the other three samples and hardly increased with increasing oxidation time. It should also be noted that the sheet resistance values after 24 h oxidation are lower than initial values for the 10 to 20 wt.% silver, as shown in Fig. 3. It may due to the volatilization of the residual organic solvent in metal film that would isolate the metal particles.
4. Conclusion Successful silver coatings have been achieved on fine copper powder particles by a simple electroless plating technique. As-coated copper powders were characterized using X-ray diffraction and scanning electron microscopy. It was found that the uniformity of silver coating and properties of oxidation resistance were improved with increasing silver content. A fairly continuous silver layer was formed on copper particle surface and few free silver particles were detected when the silver content was increased to 20 wt.%. As a result, the sheet resistances of metal films reduced
3991
with the increasing silver content, and at the level of 20 wt.% silver, it had a minimum value and hardly increased with increasing oxidation time.
Acknowledgements This work was supported by National Natural Science Foundation of China (69836030). We also thank Dr. Yunzhen Cao for sheet resistance tests of silver-coated copper films.
References [1] D. Russev, D. Raclev, S. Kava, J. Met. Finish. 81 (1983) 27 – 30. [2] Koto, et al., US Patent No. 4,652,465 (1987). [3] T. Hayashi, US Patent No. 5,178,909 (1993). [4] E. Takeshima, K. Takatsu, Y. Kojima, T. Fujji, US Patent No. 4,954,235 (1990). [5] N. Feldstein, US Patent No. 4,305,997 (1981). [6] J. Kondo, K. Murakawa, K. Nomoto, F. Ishikawa, N. Ishida, J. Ishikawa, US Patent No. 4,956,014 (1990). [7] B. Wang, Z. Ji, F.T. Zimone, G.M. Janowski, J.M. Rigsbee, Surf. Coat. Technol. 91 (1997) 64 – 68. [8] A. Vaskelis, E. Matulionis, E. Norkus, A. Jagminiene, R. Juskenas, Surf. Coat. Technol. 82 (1996) 165 – 168. [9] J.N. Keuler, L. Lorenzen, R.D. Sanderson, V. Prozesky, W.J. Przybylowicz, Thin Solid Films 347 (1999) 91 – 98. [10] D. Djokic, et al., US Patent No. 5,945,158 (1999). [11] F. Tan, Y. Cai, L. Zhao, F. Huang, Precious Metals 21 (2000) 8 – 11 (in Chinese). [12] K.L. Yeung, S. Christiansen, A. Varma, J. Membr. Sci. 159 (1998) 107 – 122.
Electroless silver coating on fine copper powder and its effects on oxidation resistance Xinrui Xu *, Xiaojun Luo, Hanrui Zhuang, Wenlan Li, Baolin Zhang Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Dingxi Road, Shanghai 200050, China Received 10 September 2002; received in revised form 11 February 2003; accepted 19 February 2003
Abstract Electroless silver coating on fine copper powder (3.4 Am) and its effects on oxidation resistance were investigated by varying the silver contents. As-coated copper powders were characterized using X-ray diffraction (XRD) and scanning electron microscopy (SEM). It was found that the uniformity of silver coating was improved with silver content. When the silver content reached 20 wt.%, silver was homogeneously distributed around the copper particles and few free silver particles were detected. As a result, the sheet resistances of metal films reduced with the silver content (presenting the improvement of oxidation resistance), and at the level of 20 wt.% silver content, it had a minimum value and hardly increased with the increasing oxidation time. D 2003 Elsevier Science B.V. All rights reserved. Keywords: Silver coating; Electroless; Copper powder; Oxidation resistance
1. Introduction Metal particles used as electrically and thermally conductive fillers for polymeric materials typically comprise gold, silver, copper, nickel or aluminum. Gold and silver whilst possessing excellent conductivity properties are, however, expensive. Nickel and copper display good electrical conductivities but are deleteriously susceptible to oxidation, resulting in the concomitant diminution of desirable properties such as quality and stability. In order to overcome the problems associated with the oxidation of nickel and copper, or the high cost of using silver, the substitu-
* Corresponding author. Tel.: +86-21-52414213; fax: +86-2152413903. E-mail address: [email protected] (X. Xu).
tion by silver-coated copper particles for solid silver particles has been explored [1 – 3]. Generally, powder coating treatment can be achieved by electroplating [4], electroless plating [5,6], and vacuum process (evaporation, sputtering [7] etc.). It is well known that silver can be deposited on many substrates by electroplating [8]. However, the efficiency of electroplating and vacuum coating for powders is very low. Therefore, electroplating and vacuum processes are not feasible for commercial purpose. Electroless silver [2,3,9,10] coating, however, can be successfully achieved on copper powder and has much higher deposition rate than electroplating, although there are still difficulties in reducing process steps and coating on the fine copper powder which would have good sinterability. This paper described a simple electroless silver coating process on fine copper powder. The effects of
0167-577X/03/$ - see front matter D 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0167-577X(03)00252-0
3988
X. Xu et al. / Materials Letters 57 (2003) 3987–3991
silver coating on oxidation resistance of fine copper powder were also investigated by varying the silver contents. The coating structures at different silver content were studied in detail with special emphasis on their effects on the properties of oxidation resistance. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were employed as main techniques.
2. Experimental procedure 2.1. Silver coating Copper powder with an average particle size of 3.4 Am and chemical purity of greater than 99.9% was first dispersed in an ammonium sulphate (purity>99.0%) and ammonium hydroxide solution for 10 min to remove surface oxide film and activate the surface. After this pretreatment, a sodium potassium tartrate (purity>99.0%) solution was added to the slurry for reduction. An aqueous solution containing silver nitrate (purity>99.8%) and ammonium hydroxide was then added dropwise. Table 1 lists the typical silver coating process parameters. After termination of the silver coating process, the silver-coated copper powder was separated, then washed using deionised water and alcohol successively to remove the residual chemicals and impurities, and then baked at 100jC in a vacuum atmosphere.
2.3. Characterization The cross-sectional areas of the plated powders were investigated using scanning electron microscope (EPMA-8705Q). The phase composition of the prepared powder was determined by using XRD. The property of oxidation resistance was tested by measuring sheet resistances of metal films which changed with the increasing of oxidation time (oxidation time was the time when metal films were exposed in air at 150 jC). The sheet resistances of silver-coated copper thick films were measured by a semiconductor resistivity meter (BD-86A) on samples which have 1010 mm square metal film patterns.
3. Results and discussion 3.1. Silver coating on copper particles from silver ion containing ammonia solutions In an ammonia solution, it is estimated that the silver reduction process was controlled by the following reactions [10]: 1. Dissolution of surface oxides and hydroxides. Cu2 O þ 2NH4 OH þ ðNH4 Þ2 SO4 ¼ ½CuðNH3 Þ2 2 SO4 þ 3H2 O
ð1Þ
CuO þ 2NH4 OH þ ðNH4 Þ2 SO4
2.2. Metal paste preparation To investigate the property of oxidation resistance, the prepared silver-coated copper powder was mixed with appropriate amount of ethyl cellulose –terpineol solution in a three-roll mill. The silver-coated copper paste was ground to the required consistency, and screened on to the ceramic substrates, and then dried at 100 jC for 20 min in vacuum.
¼ CuðNH3 Þ4 SO4 þ 3H2 O
ð2Þ
CuðOHÞ2 þ 2NH4 OH þ ðNH4 Þ2 SO4 ¼ CuðNH3 Þ4 SO4 þ 4H2 O
ð3Þ
2. Dissolution of silver nitrate in ammonium hydroxide. 2AgNO3 þ 2NH4 OH
Table 1 Composition of the electroless plating bath and operating parameters Bath composition
Operating parameters
(NH4)2SO4 NH3 H2O AgNO3 C4H4O6KNa
Temperature pH value Reduction time Powder load
0.4 – 0.8 mol/l 0.8 – 1.3 mol/l 25 – 120 g/l 300 – 600 g/l
25 jC 8 – 12 5 – 20 min 80 – 150 g/l
¼ Ag2 O # þ2NH4 NO3 þ H2 O
ð4Þ
Ag2 O þ 4NH4 OH ¼ 2½AgðNH3 Þ2 OH þ 3H2 O ð5Þ ½AgðNH3 Þ2 OH þ NH4 NO3 ¼ ½AgðNH3 Þ2 NO3 þ NH4 OH
ð6Þ
X. Xu et al. / Materials Letters 57 (2003) 3987–3991
3989
3. Displacement reaction
ð7Þ
During electroless plating, silver metal was deposited by reduction – oxidation reactions involving the Ag-amine as described above in Eq. (7). A comparison of the standard reduction –oxidation potentials (DEo) shown in Eq. (7) indicates that the deposition of silver is favored. A similar observation was also reported by Tan et al. [11]. In addition, there were also side reactions:
2AgðNH3 Þþ 2 þ 2OH ¼ Ag2 O # þ4NH3 þ H2 O ð8Þ
3.3. Morphologies and phase composition
3Ag2 O þ C4 H4 O2
6 þ 2OH
¼ 6Ag # þ2C2 O2
4 þ 3H2 O
that the ratio of ammonia to ammonium sulphate should be less than 4:1. The silver ion solution used in this study was a freshly prepared aqueous solution of silver nitrate and ammonium hydroxide. The amount of ammonium hydroxide required in the silver ion solution was dependent on when the Ag2O precipitation was dissolved completely. It was found that the suitable molar ratio of silver nitrate and ammonium hydroxide was 1:3.
ð9Þ
The Ag2O produced from solution in Eq. (8) could be reduced by successive reaction as shown in Eq. (9). Sodium potassium tartrate used in this study functioned as both a mild reducing agent and as a complex agent for the Cu2+ ions to suppress precipitation of copper hydroxides. 3.2. Effects of coating parameters on silver reduction As is well known, heterogeneous electroless metal depositions are required to minimize the precipitation of free metal powder from the solution (this may also decompose the electroless solution) and the surface activation is therefore, important. However, since the electroless silver plating demonstrated an autocatalytic process [12], the reduction –oxidation reactions occurred preferentially around the silver seeds in the absence of surface activation. They were initiated by the reaction of copper with ammonia (Eq. (7)), with simultaneous release of electrons, which were then used for the decomposition of Ag-amine complexes. Subsequently, the silver was deposited onto the nuclei resulting in growth. The quantity of ammonia controlled the rate of copper dissolution, and it was found
The homogeneous distribution of silver coating over copper surface is required for ideal silver coating. This might be difficult to achieve in practice, particularly at low silver content and on fine copper powder which always attempt to aggregate. In this study, the ultrasonic and stirring methods were employed to disperse copper powder throughout the process, and copper powders coated with 5 to 20 wt.% silver were achieved by the above electroless plating. Fig. 1 shows the cross-section images of silver-coated copper powders. It can be seen that the uniformity of silver coating was improved with the increase of silver content. At the level of 5 wt.% silver content, no evident silver layer is observed indicating insufficient silver content for the fine copper particles (3.4 Am). Compared with the 5 wt.% silver-coated copper powder, the cross-section image of 10 wt.% silver-coated copper powders presents thin, but not continuous silver layer (see Fig. 1b). With the increasing silver content, a thicker and partially continuous silver layer is formed on copper particles at the level of 15 wt.% silver content (see Fig. 1c). When the silver content increases to 20 wt.%, silver is homogeneously distributed around the copper particles and few free silver particles are detected.
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X. Xu et al. / Materials Letters 57 (2003) 3987–3991
Fig. 1. Cross-section micrographs of silver-coated copper powder with (a) 5, (b) 10, (c) 15, and (d) 20 wt.% silver contents.
A series of performance tests were carried out for various content of silver-coated copper powders. Fig.
3 summarizes the effects of silver content on oxidation resistance. The sheet resistances of metal film prepared from 5 wt.% silver-coated copper powder, increased dramatically with increasing oxidation time, which was consistent with the microstructure depicted in Fig. 1a. At this level of silver content, the little silver coating on copper powder was not sufficient to prevent copper powder from oxidation, so the increasing rate of sheet resistance with oxidation time was
Fig. 2. XRD pattern of the typical silver-coated copper powder.
Fig. 3. Dependence of sheet resistance of metal films prepared from 5, 10, 15, and 20 wt.% silver-coated copper powder on oxidation time (the oxidation time is the time when metal films were exposed in air at 150 jC).
Fig. 2 shows an XRD spectrum of the 15 wt.% silver-coated copper powder. In addition to copper, a pure silver phase was clearly identified and it possessed a crystalline rather than an amorphous structure. 3.4. Performance comparison of oxidation resistance
X. Xu et al. / Materials Letters 57 (2003) 3987–3991
steep. With the increase of silver content, the uniformity of silver coating increased, implicating that the properties of oxidation resistance were improved. As a result, the sheet resistance increment slowed down with the oxidation time. For the 20 wt.% silver, a fairly continuous silver layer was formed on copper particle surface and few free silver particles were detected (Fig. 1d), thus the sheet resistance of this metal film had a minimum value compared with the other three samples and hardly increased with increasing oxidation time. It should also be noted that the sheet resistance values after 24 h oxidation are lower than initial values for the 10 to 20 wt.% silver, as shown in Fig. 3. It may due to the volatilization of the residual organic solvent in metal film that would isolate the metal particles.
4. Conclusion Successful silver coatings have been achieved on fine copper powder particles by a simple electroless plating technique. As-coated copper powders were characterized using X-ray diffraction and scanning electron microscopy. It was found that the uniformity of silver coating and properties of oxidation resistance were improved with increasing silver content. A fairly continuous silver layer was formed on copper particle surface and few free silver particles were detected when the silver content was increased to 20 wt.%. As a result, the sheet resistances of metal films reduced
3991
with the increasing silver content, and at the level of 20 wt.% silver, it had a minimum value and hardly increased with increasing oxidation time.
Acknowledgements This work was supported by National Natural Science Foundation of China (69836030). We also thank Dr. Yunzhen Cao for sheet resistance tests of silver-coated copper films.
References [1] D. Russev, D. Raclev, S. Kava, J. Met. Finish. 81 (1983) 27 – 30. [2] Koto, et al., US Patent No. 4,652,465 (1987). [3] T. Hayashi, US Patent No. 5,178,909 (1993). [4] E. Takeshima, K. Takatsu, Y. Kojima, T. Fujji, US Patent No. 4,954,235 (1990). [5] N. Feldstein, US Patent No. 4,305,997 (1981). [6] J. Kondo, K. Murakawa, K. Nomoto, F. Ishikawa, N. Ishida, J. Ishikawa, US Patent No. 4,956,014 (1990). [7] B. Wang, Z. Ji, F.T. Zimone, G.M. Janowski, J.M. Rigsbee, Surf. Coat. Technol. 91 (1997) 64 – 68. [8] A. Vaskelis, E. Matulionis, E. Norkus, A. Jagminiene, R. Juskenas, Surf. Coat. Technol. 82 (1996) 165 – 168. [9] J.N. Keuler, L. Lorenzen, R.D. Sanderson, V. Prozesky, W.J. Przybylowicz, Thin Solid Films 347 (1999) 91 – 98. [10] D. Djokic, et al., US Patent No. 5,945,158 (1999). [11] F. Tan, Y. Cai, L. Zhao, F. Huang, Precious Metals 21 (2000) 8 – 11 (in Chinese). [12] K.L. Yeung, S. Christiansen, A. Varma, J. Membr. Sci. 159 (1998) 107 – 122.