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Ni nanocomposites
Materials Letters 58 (2003) 17 – 20 www.elsevier.com/locate/matlet
Gelcasting process of Al2O3/Ni nanocomposites B.-S. Kim *, T. Sekino, Y. Yamamoto, T. Nakayama, T. Kusunose, M. Wada, K. Niihara The Institute of Scientific and Industrial Research, Osaka University, 8-1 Mihogahoka, Ibaraki, Osaka 567-0047, Japan Received 14 March 2003; accepted 6 May 2003
Abstract Al2O3/Ni nanocomposites were fabricated by gelcasting an Al2O3/NiO mixture, followed by reduction – sintering process. Dispersant quantity was optimized at 1.75 wt.% with measurement of rheological behaviors to determine the best dispersion state of binary oxides colloidal suspension for casting. Hydrogen reduction of NiO in the shaped body was accomplished successfully through pore channels of debinded organic chemical volume. The reduced Al2O3/NiO green body comprised Al2O3 and Ni phases without a reacted phase. Onedimensional diametric shrinkage reached 21% after pressureless sintering treatment, while fairly high strength of 586 MPa was maintained. Dispersed Ni particles revealed typical ferromagnetic behavior in the Al2O3 matrix. Near-net shaping and sintering for the Al2O3/Ni nanocomposite were also achieved by present gelcasting, indicating applicability of this process for industrial use. D 2003 Elsevier B.V. All rights reserved. Keywords: Nanocomposites; Gelcasting; Colloidal; Ferromagnetism
1. Introduction High mechanical and multifunctional properties of ceramic/metal nanocomposites, in which nanosized metal particles are dispersed in a ceramic matrix, attract great interest [1,2]. For example, Al2O3/Ni nanocomposite, which is fabricated by H2 reduction and subsequent hot pressing of Al2O3/NiO mixture, has been found to exhibit peculiar multifunctional properties including ferromagnetism and high fracture strength [3,4]. However, these kinds of materials and various structured ceramics show increased difficulty in machining concomitant with the increase in their mechanical properties. This significant disadvantage leads to high machining costs for industrial applications [5]. Approaches to overcome these problems are development of a colloidal suspension technique of binary materials system and optimized consolidation process conditions. Gelcast processing is selected for the present study. In this relatively new shaping process, high solid loading is required in aqueous organic chemical suspensions [6,7]. A green body is fabricated by polymerization of organic monomers in cast molds. Well dried and debinded green bodies are
* Corresponding author. Tel.: +81-6-6879-8441; fax: +81-6-68798444. E-mail address: [email protected] (B.-S. Kim). 0167-577X/$ - see front matter D 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0167-577X(03)00397-5
densified by pressureless sintering process. However, mechanical properties of cast and pressureless sintered products are often relatively lower than those yielded by general sintering processes. From these points of view, Al2O3/Ni nanocomposite technique and gelcasting processes would constitute a complementary method to strengthening and shaping. Based upon these ideas, we investigated an optimum suspension condition and process route to fabricate complex-shaped Al2O3/Ni nanocomposite.
2. Experimental The solution chemical route was selected to obtain Al2O3/ NiO powder mixtures [3]. Weighed high purity Ni-nitrate (Ni(NO3)26H2O) was dissolved in ethanol, and then milled with a-Al2O3 (TM-DAR, 0.16 Am) powder, corresponding to 5 vol.% of Ni in the final Al2O3/Ni nanocomposite. The dried mixture was calcined to obtain the Al2O3/NiO mixture. It was again wet milled with ethanol and dry-ball milled for 24 h. As a monomer and cross-linker of gelcasting, methacrylamide and N,NV,-methylenebisacrylamide (total 8 wt.% of chemical, ratio 5:1) were dissolved in deionized water. Poly-acrylic acid (PAA) was added to this solution as a dispersant. A prepared Al2O3/NiO mixture (solid loading 50 vol.%) was then added to that solution and ball milled for 24 h. Rheological measurements were performed on a rotational
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B.-S. Kim et al. / Materials Letters 58 (2003) 17–20
stress-controlled rheometer to estimate the relationship between rheology behaviors and dispersant quantity. The reaction initiator and catalyst were added immediately before casting, where rectangular (64 43 12 mm) and complex nut-shaped molds were used for shaping. The drying process was performed in a relative humidity controlled chamber where RH was 90% and temperature was 30 jC. The debinding condition in air was determined by thermogravimetric and differential thermal analysis up to 500 jC. The fully dried and debinded green body was reduced by hydrogen flow at 700 jC for 1 h; then, it was sintered continuously at 1500 jC in an argon atmosphere. X-ray diffractometry (XRD) identified phase composition. A three-point bending test with 25 mm of span measured fracture strength. A SQUID magnetometer at 27 jC measured the magnetization loop. Scanning electron microscopy (SEM) showed sintered body fracture surfaces.
Fig. 2. XRD profiles of each process steps; (a) dried body, (b) debinded body, (c) reduced body and (d) pressureless sintered Al2O3/Ni nanocomposites (.: Al2O3, j: Ni and z: NiO).
3. Results and discussion Suspension property conditions are consequence parameters to control optimized synthesizing processes. The complex shaped molding process requires good flow properties of suspension for appropriate forming; it also requires as high a solid loading as possible in the suspension for optimal green body density. To determine the appropriate amount of dispersant leading to the best dispersion state, 50 vol.% of solid containing Al2O3/NiO suspension was prepared with different dispersant amounts. Fig. 1 shows viscosity measurements. These results confirm that dispersant concentration strongly influenced rheological behavior of suspension. Below 1 wt.% dispersant, viscosity exceeded the available measurement range (dotted line). However, viscosity decreased drastically with dispersant additions up to 1.75 wt.%, pH 7.5. Further additions resulted in increased viscosity. Negative electrostatic surface charge of both oxides was achieved basically at intermediate pH ( f 7) of suspension because isoelectric points of a-Al2O3 and NiO were
Fig. 1. Rheological curves for 50 vol.% of solid loaded Al2O3/NiO suspension prepared with different amounts of dispersant. Measurement was carried out for the different share rate condition (1.74 and 17.4 s 1).
reported as pH of 8 –9 and pH of 10 –11, respectively [8]. This electrostatic force concept and addition of dispersant (PAA) for increasing steric repulsion prevented phase segregation during related processing. From these studies, 1.75 wt.% of dispersant was selected for further investigation to prepare Al2O3/NiO green bodies. Fig. 2 shows XRD profiles registered for all these process steps. The XRD patterns after drying and debinding contain characteristic peaks for a-Al2O3 and NiO (Fig. 2a,b). The debinded Al2O3/NiO green body involves internal voids similar to those of porous materials because the volume after burning out of polymerized organic chemicals remained as connected pores. This implies that the overall gas– solid reaction, reduction of NiO in the Al2O3/NiO mixture, can occur through these continuous pore channels [9]. The reduced green body comprised Al2O3 and Ni phases with no reaction phases (Fig. 2c). Since the Al2O3 phase is thermodynamically stable at NiO reduction temperatures, formation of Ni nanoparticles is unrelated to interaction between Al2O3 and NiO. Sintering was performed under an argon atmosphere; XRD analysis results in Fig. 2d show no reaction phase except matrix a-Al2O3 and the Ni dispersion phase. Fig. 3 shows a typical fracture surface of obtained Al2O3/ Ni nanocomposites. Fine nickel particles are dispersed mostly at the Al2O3 matrix grain surface. The fracture mode of this specimen dominated an inter-granular type during fracture test. This behavior seems to imply that dispersed Ni particles did not enhance grain boundary strength as much as hot-pressed nanocomposite systems [1,3,4], because the present densification method was pressureless sintering. However, the present Al2O3/5 vol.% Ni nanocomposite achieved 99.27% of relative density and 586 F 50 MPa of fracture strength, respectively. Although different methods and conditions used for mechanical testing make it difficult to establish an appropriate comparison, the strength result of
B.-S. Kim et al. / Materials Letters 58 (2003) 17–20
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Fig. 3. Typical SEM micrograph of gelcasting and reduction – pressureless sintered Al2O3/5 vol.% Ni nanocomposite at 1500 jC.
the present gelcast Al2O3/Ni nanocomposites was much higher than that of reported conventional pressureless sintered pure Al2O3 results (294 f 393 MPa) [10]; moreover, this value closely approximated other reported pressureassisted sintering of pure Al2O3 value (605 MPa) [11]. Fig. 4 shows a model-made complex nut shaped Al2O3/ NiO green body (left) and its sintered body (Al2O3/Ni nanocomposite, right). One-dimensional percentage diametric shrinkage of a nut-shaped specimen reached about 21% during pressureless sintering with no visible cracks. Such shrinkage achievements originated from homogeneous density distribution of the gelcast Al2O3/NiO green body. Fig. 5 shows dependence of magnetization (I) on the applied magnetic field (H) for sintered Al2O3/5 vol.% Ni nanocomposites. A magnetic hysteresis loop clearly indicated typical ferromagnetic behavior. Magnetization saturated near 250 kA/m of the magnetic field; the saturated magnetization value was 50.9 emu/g of Ni dispersion. Coercive force, 3.3 kA/m, is approximately two orders of magnitude larger than that of pure Ni metal of 70 A/m. Fine ferromagnetic Ni particles are known to exhibit high magnetic coercive force due to their characteristic magnetic domain
Fig. 5. The magnetization versus applied magnetic field loop for gelcasting and reduction – pressureless sintered Al2O3/5 vol.% Ni nanocomposite.
structure [12,13]. When particle size of a magnetic material decreases, its magnetic structure changes from a multidomain to a single domain state to reduce total energy of the system. The coercive force of Al2O3/5 vol.% Ni nanocomposites by hot-press sintering was reported to be 4.4 kA/ m [3]. The present gelcasted nanocomposite showed a slightly lower value. It is considered due to the different stress state of Ni dispersion in the composite, because coercive force is affected by not only particle size but also the residual stress and/or dislocations in a magnetic matter. Nevertheless, a reasonable inference is that such coerciveforce enhancement in the present nanocomposite is caused both by the size effect of ferromagnetic Ni particles and by formation of a magnetic single-domain structure of some small, dispersed nickel particles.
4. Conclusions Magnetically functionalized Al2O3/Ni nanocomposites were fabricated successfully by gelcasting technique of chemically prepared Al2O3/NiO binary oxide and reduction – sintering process. Rheological behavior was optimized by compositional control of organic species for gelcastable suspension with higher solid loading of 50 vol.%. A complexly shaped green body was obtained by casting and followed polymerization of used organic monomers; its shape was maintained successfully after burning out polymers, reduction and sintering. The gelcasting process for the ceramic/metal nanocomposite material system presents great advantages for industrial fabrication applications to available shaped components as a complementary method.
Acknowledgements Fig. 4. The model near-net shaping of complex nut-shaped green body (left) and sintered final component (right).
This work is partly supported by the Japan Society for the Promotion of Science (JSPS) Joint Research Projects under
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B.-S. Kim et al. / Materials Letters 58 (2003) 17–20
the Bilateral Programs between Japan and Korea (Core University Program).
References [1] K. Niihara, J. Ceram. Soc. Jpn. 99 (1991) 974. [2] R. Roy, R.A. Roy, D.M. Roy, Mater. Lett. 4 (1986) 323. [3] T. Sekino, T. Nakajima, S. Ueda, K. Niihara, J. Am. Ceram. Soc. 80 (1997) 1139. [4] W.H. Tuan, R.J. Brook, J. Eur. Ceram. Soc. 10 (1992) 95. [5] H. Kawaoka, Y.-H. Kim, T. Sekino, Y.-H. Choa, T. Kusunose, T. Nakayama, K. Niihara, J. Ceram. Process. Res. 2 (2001) 1.
[6] A.C. Young, O.O. Omatete, M.A. Janney, P.A. Menchhofer, J. Am. Ceram. Soc. 74 (1991) 612. [7] R. Gilissen, J.P. Erauw, A. Smolders, E. Vanswijgenhoven, J. Luyten, Mater. Des. 21 (2000) 251. [8] J.A. Lewis, J. Am. Ceram. Soc. 83 (2000) 2341. [9] J. Szekely, J.W. Evans, H.Y. Shon, Gas – solid Reaction, Academic Press, New York, 1976. [10] G.V. Samsonov, The Oxide Handbook, IFI/Plenum Data Company, New York, 1982. [11] R.M. German, Sintering Theory and Practice, Wiley, New York, 1996. [12] A.YE. Yermakov, O.A. Ivanov, YA.S. Shur, R.M. Grechishkin, G.V. Ivanova, Phys. Met. Metallogr. 33 (1972) 99. [13] R. Kamel, A. Reffat, Solid State Commun. 8 (1970) 821.
Gelcasting process of Al2O3/Ni nanocomposites B.-S. Kim *, T. Sekino, Y. Yamamoto, T. Nakayama, T. Kusunose, M. Wada, K. Niihara The Institute of Scientific and Industrial Research, Osaka University, 8-1 Mihogahoka, Ibaraki, Osaka 567-0047, Japan Received 14 March 2003; accepted 6 May 2003
Abstract Al2O3/Ni nanocomposites were fabricated by gelcasting an Al2O3/NiO mixture, followed by reduction – sintering process. Dispersant quantity was optimized at 1.75 wt.% with measurement of rheological behaviors to determine the best dispersion state of binary oxides colloidal suspension for casting. Hydrogen reduction of NiO in the shaped body was accomplished successfully through pore channels of debinded organic chemical volume. The reduced Al2O3/NiO green body comprised Al2O3 and Ni phases without a reacted phase. Onedimensional diametric shrinkage reached 21% after pressureless sintering treatment, while fairly high strength of 586 MPa was maintained. Dispersed Ni particles revealed typical ferromagnetic behavior in the Al2O3 matrix. Near-net shaping and sintering for the Al2O3/Ni nanocomposite were also achieved by present gelcasting, indicating applicability of this process for industrial use. D 2003 Elsevier B.V. All rights reserved. Keywords: Nanocomposites; Gelcasting; Colloidal; Ferromagnetism
1. Introduction High mechanical and multifunctional properties of ceramic/metal nanocomposites, in which nanosized metal particles are dispersed in a ceramic matrix, attract great interest [1,2]. For example, Al2O3/Ni nanocomposite, which is fabricated by H2 reduction and subsequent hot pressing of Al2O3/NiO mixture, has been found to exhibit peculiar multifunctional properties including ferromagnetism and high fracture strength [3,4]. However, these kinds of materials and various structured ceramics show increased difficulty in machining concomitant with the increase in their mechanical properties. This significant disadvantage leads to high machining costs for industrial applications [5]. Approaches to overcome these problems are development of a colloidal suspension technique of binary materials system and optimized consolidation process conditions. Gelcast processing is selected for the present study. In this relatively new shaping process, high solid loading is required in aqueous organic chemical suspensions [6,7]. A green body is fabricated by polymerization of organic monomers in cast molds. Well dried and debinded green bodies are
* Corresponding author. Tel.: +81-6-6879-8441; fax: +81-6-68798444. E-mail address: [email protected] (B.-S. Kim). 0167-577X/$ - see front matter D 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0167-577X(03)00397-5
densified by pressureless sintering process. However, mechanical properties of cast and pressureless sintered products are often relatively lower than those yielded by general sintering processes. From these points of view, Al2O3/Ni nanocomposite technique and gelcasting processes would constitute a complementary method to strengthening and shaping. Based upon these ideas, we investigated an optimum suspension condition and process route to fabricate complex-shaped Al2O3/Ni nanocomposite.
2. Experimental The solution chemical route was selected to obtain Al2O3/ NiO powder mixtures [3]. Weighed high purity Ni-nitrate (Ni(NO3)26H2O) was dissolved in ethanol, and then milled with a-Al2O3 (TM-DAR, 0.16 Am) powder, corresponding to 5 vol.% of Ni in the final Al2O3/Ni nanocomposite. The dried mixture was calcined to obtain the Al2O3/NiO mixture. It was again wet milled with ethanol and dry-ball milled for 24 h. As a monomer and cross-linker of gelcasting, methacrylamide and N,NV,-methylenebisacrylamide (total 8 wt.% of chemical, ratio 5:1) were dissolved in deionized water. Poly-acrylic acid (PAA) was added to this solution as a dispersant. A prepared Al2O3/NiO mixture (solid loading 50 vol.%) was then added to that solution and ball milled for 24 h. Rheological measurements were performed on a rotational
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B.-S. Kim et al. / Materials Letters 58 (2003) 17–20
stress-controlled rheometer to estimate the relationship between rheology behaviors and dispersant quantity. The reaction initiator and catalyst were added immediately before casting, where rectangular (64 43 12 mm) and complex nut-shaped molds were used for shaping. The drying process was performed in a relative humidity controlled chamber where RH was 90% and temperature was 30 jC. The debinding condition in air was determined by thermogravimetric and differential thermal analysis up to 500 jC. The fully dried and debinded green body was reduced by hydrogen flow at 700 jC for 1 h; then, it was sintered continuously at 1500 jC in an argon atmosphere. X-ray diffractometry (XRD) identified phase composition. A three-point bending test with 25 mm of span measured fracture strength. A SQUID magnetometer at 27 jC measured the magnetization loop. Scanning electron microscopy (SEM) showed sintered body fracture surfaces.
Fig. 2. XRD profiles of each process steps; (a) dried body, (b) debinded body, (c) reduced body and (d) pressureless sintered Al2O3/Ni nanocomposites (.: Al2O3, j: Ni and z: NiO).
3. Results and discussion Suspension property conditions are consequence parameters to control optimized synthesizing processes. The complex shaped molding process requires good flow properties of suspension for appropriate forming; it also requires as high a solid loading as possible in the suspension for optimal green body density. To determine the appropriate amount of dispersant leading to the best dispersion state, 50 vol.% of solid containing Al2O3/NiO suspension was prepared with different dispersant amounts. Fig. 1 shows viscosity measurements. These results confirm that dispersant concentration strongly influenced rheological behavior of suspension. Below 1 wt.% dispersant, viscosity exceeded the available measurement range (dotted line). However, viscosity decreased drastically with dispersant additions up to 1.75 wt.%, pH 7.5. Further additions resulted in increased viscosity. Negative electrostatic surface charge of both oxides was achieved basically at intermediate pH ( f 7) of suspension because isoelectric points of a-Al2O3 and NiO were
Fig. 1. Rheological curves for 50 vol.% of solid loaded Al2O3/NiO suspension prepared with different amounts of dispersant. Measurement was carried out for the different share rate condition (1.74 and 17.4 s 1).
reported as pH of 8 –9 and pH of 10 –11, respectively [8]. This electrostatic force concept and addition of dispersant (PAA) for increasing steric repulsion prevented phase segregation during related processing. From these studies, 1.75 wt.% of dispersant was selected for further investigation to prepare Al2O3/NiO green bodies. Fig. 2 shows XRD profiles registered for all these process steps. The XRD patterns after drying and debinding contain characteristic peaks for a-Al2O3 and NiO (Fig. 2a,b). The debinded Al2O3/NiO green body involves internal voids similar to those of porous materials because the volume after burning out of polymerized organic chemicals remained as connected pores. This implies that the overall gas– solid reaction, reduction of NiO in the Al2O3/NiO mixture, can occur through these continuous pore channels [9]. The reduced green body comprised Al2O3 and Ni phases with no reaction phases (Fig. 2c). Since the Al2O3 phase is thermodynamically stable at NiO reduction temperatures, formation of Ni nanoparticles is unrelated to interaction between Al2O3 and NiO. Sintering was performed under an argon atmosphere; XRD analysis results in Fig. 2d show no reaction phase except matrix a-Al2O3 and the Ni dispersion phase. Fig. 3 shows a typical fracture surface of obtained Al2O3/ Ni nanocomposites. Fine nickel particles are dispersed mostly at the Al2O3 matrix grain surface. The fracture mode of this specimen dominated an inter-granular type during fracture test. This behavior seems to imply that dispersed Ni particles did not enhance grain boundary strength as much as hot-pressed nanocomposite systems [1,3,4], because the present densification method was pressureless sintering. However, the present Al2O3/5 vol.% Ni nanocomposite achieved 99.27% of relative density and 586 F 50 MPa of fracture strength, respectively. Although different methods and conditions used for mechanical testing make it difficult to establish an appropriate comparison, the strength result of
B.-S. Kim et al. / Materials Letters 58 (2003) 17–20
19
Fig. 3. Typical SEM micrograph of gelcasting and reduction – pressureless sintered Al2O3/5 vol.% Ni nanocomposite at 1500 jC.
the present gelcast Al2O3/Ni nanocomposites was much higher than that of reported conventional pressureless sintered pure Al2O3 results (294 f 393 MPa) [10]; moreover, this value closely approximated other reported pressureassisted sintering of pure Al2O3 value (605 MPa) [11]. Fig. 4 shows a model-made complex nut shaped Al2O3/ NiO green body (left) and its sintered body (Al2O3/Ni nanocomposite, right). One-dimensional percentage diametric shrinkage of a nut-shaped specimen reached about 21% during pressureless sintering with no visible cracks. Such shrinkage achievements originated from homogeneous density distribution of the gelcast Al2O3/NiO green body. Fig. 5 shows dependence of magnetization (I) on the applied magnetic field (H) for sintered Al2O3/5 vol.% Ni nanocomposites. A magnetic hysteresis loop clearly indicated typical ferromagnetic behavior. Magnetization saturated near 250 kA/m of the magnetic field; the saturated magnetization value was 50.9 emu/g of Ni dispersion. Coercive force, 3.3 kA/m, is approximately two orders of magnitude larger than that of pure Ni metal of 70 A/m. Fine ferromagnetic Ni particles are known to exhibit high magnetic coercive force due to their characteristic magnetic domain
Fig. 5. The magnetization versus applied magnetic field loop for gelcasting and reduction – pressureless sintered Al2O3/5 vol.% Ni nanocomposite.
structure [12,13]. When particle size of a magnetic material decreases, its magnetic structure changes from a multidomain to a single domain state to reduce total energy of the system. The coercive force of Al2O3/5 vol.% Ni nanocomposites by hot-press sintering was reported to be 4.4 kA/ m [3]. The present gelcasted nanocomposite showed a slightly lower value. It is considered due to the different stress state of Ni dispersion in the composite, because coercive force is affected by not only particle size but also the residual stress and/or dislocations in a magnetic matter. Nevertheless, a reasonable inference is that such coerciveforce enhancement in the present nanocomposite is caused both by the size effect of ferromagnetic Ni particles and by formation of a magnetic single-domain structure of some small, dispersed nickel particles.
4. Conclusions Magnetically functionalized Al2O3/Ni nanocomposites were fabricated successfully by gelcasting technique of chemically prepared Al2O3/NiO binary oxide and reduction – sintering process. Rheological behavior was optimized by compositional control of organic species for gelcastable suspension with higher solid loading of 50 vol.%. A complexly shaped green body was obtained by casting and followed polymerization of used organic monomers; its shape was maintained successfully after burning out polymers, reduction and sintering. The gelcasting process for the ceramic/metal nanocomposite material system presents great advantages for industrial fabrication applications to available shaped components as a complementary method.
Acknowledgements Fig. 4. The model near-net shaping of complex nut-shaped green body (left) and sintered final component (right).
This work is partly supported by the Japan Society for the Promotion of Science (JSPS) Joint Research Projects under
20
B.-S. Kim et al. / Materials Letters 58 (2003) 17–20
the Bilateral Programs between Japan and Korea (Core University Program).
References [1] K. Niihara, J. Ceram. Soc. Jpn. 99 (1991) 974. [2] R. Roy, R.A. Roy, D.M. Roy, Mater. Lett. 4 (1986) 323. [3] T. Sekino, T. Nakajima, S. Ueda, K. Niihara, J. Am. Ceram. Soc. 80 (1997) 1139. [4] W.H. Tuan, R.J. Brook, J. Eur. Ceram. Soc. 10 (1992) 95. [5] H. Kawaoka, Y.-H. Kim, T. Sekino, Y.-H. Choa, T. Kusunose, T. Nakayama, K. Niihara, J. Ceram. Process. Res. 2 (2001) 1.
[6] A.C. Young, O.O. Omatete, M.A. Janney, P.A. Menchhofer, J. Am. Ceram. Soc. 74 (1991) 612. [7] R. Gilissen, J.P. Erauw, A. Smolders, E. Vanswijgenhoven, J. Luyten, Mater. Des. 21 (2000) 251. [8] J.A. Lewis, J. Am. Ceram. Soc. 83 (2000) 2341. [9] J. Szekely, J.W. Evans, H.Y. Shon, Gas – solid Reaction, Academic Press, New York, 1976. [10] G.V. Samsonov, The Oxide Handbook, IFI/Plenum Data Company, New York, 1982. [11] R.M. German, Sintering Theory and Practice, Wiley, New York, 1996. [12] A.YE. Yermakov, O.A. Ivanov, YA.S. Shur, R.M. Grechishkin, G.V. Ivanova, Phys. Met. Metallogr. 33 (1972) 99. [13] R. Kamel, A. Reffat, Solid State Commun. 8 (1970) 821.