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Thermoelectric properties of Fe-doped CoSb3 prepared by mechanical alloying and vacuum hot pressing
Journal of Alloys and Compounds 442 (2007) 358–361
Thermoelectric properties of Fe-doped CoSb3 prepared by mechanical alloying and vacuum hot pressing Soon-Chul Ur ∗ , Joon-Chul Kwon, Il-Ho Kim Department of MSE/ReSEM, Chungju National University, Chungju, Chungbuk 380-702, Republic of Korea Received 17 May 2006; received in revised form 10 July 2006; accepted 2 August 2006 Available online 30 January 2007
Abstract Fe doped skutterudite CoSb3 with a nominal composition of Fex Co4−x Sb12 (0 ≤ x ≤ 2.5), were synthesized by mechanical alloying of elemental powders, and consolidated by vacuum hot pressing. Nanostructured, single-phase skutterudites were successfully produced for the compositions of x ≤ 1.5, while second phase was found to form in case of x ≥ 2. Thermoelectric properties including thermal conductivity from 300 to 600 K were measured and discussed. Lattice thermal conductivity was greatly reduced by introducing Fe doping up to x ≤ 1.5 as well as by increasing phonon scattering in nanostructured skutterudite, leading to enhancement in the thermoelectric figure of merit. © 2007 Elsevier B.V. All rights reserved. PACS: 74.25.Fy; 84.60.Rb Keywords: Thermoelectric; Mechanical alloying; Skutteridite; Doping; Vacuum hot pressing
1. Introduction Recent search for novel thermoelectric materials revealed a new class of compounds with skutterudite structure as good candidates for higher performance thermoelectric conversion [1,2]. CoSb3 belongs to a skutterudite and it is expected to be a promising thermoelectric material having high figure of merit value, especially for power generation [1–3]. The figure of merit is defined as ZT = α2 T/ρκ, where α is the Seebeck coefficient, ρ is the electrical resistivity, and κ is the thermal conductivity. Undoped CoSb3 shows intrinsic p-type conduction in general [1,2]. Although high performance was anticipated in CoSb3 , binary compound itself could not provide high ZT due to relatively higher lattice thermal conductivity (κL ) [2–5]. In order to enhance their thermoelectric efficiency, doping and/or filling are proposed [2,3,5]. κL was shown to be greatly reduced by inserting small sized atoms (rattlers) into the residual voids in skutterudite structure [2,3], and some dopants such as Fe, Ni, Ir and Cr were also reported to reduce κL as well as to control carrier concentration [2,5,6].
∗
Corresponding author. Tel.: +82 43 841 5385; fax: +82 43 841 5380. E-mail address: [email protected] (S.-C. Ur).
0925-8388/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2006.08.369
The preparation of polycrystalline ␦-CoSb3 involves rather complicated processes and are frequently confronted with the formation of undesirable phases such as ␥-CoSb2 , phase decomposition and/or Sb evaporation [3,5]. Another difficulty in processing is a poor tendency to form CoSb3 due to inherently sluggish progress of peritectic transformation [5,6]. In order to address these drawbacks, mechanical alloying (MA) process was applied in this study [6,7]. MA is known as a unique technique for solid state alloying process [7], by which alloying proceeds with consecutive cold welding and fracturing, resulting in ultra fine grain size and phase homogenization. MAed materials having a fine grain size may improve thermoelectric conversion efficiency by the reduction in κL [8]. Fe doped skutterudite CoSb3 with a nominal composition of Fex Co4−x Sb12 (0 ≤ x ≤ 2.5) have been synthesized by MA of elemental powders, followed by vacuum hot pressing (VHP) in this study. Thermoelectric properties from 300 to 600 K were measured and their correlations to phase transformations were examined. 2. Experimental procedure Appropriate elemental powder mixtures of −325 mesh Co (99.9%), Sb (99.9%) and Fe (99.9%) for stoichiometric Fex Co4−x Sb12 (0 ≤ x ≤ 2.5) were prepared and MAed in a Szegavari type attrition mill for 100 h under an Ar atmosphere. As-milled powders were hot pressed in a cylindrical high strength
S.-C. Ur et al. / Journal of Alloys and Compounds 442 (2007) 358–361 graphite die at 823 K with a stress of 60 MPa for 2 h in vacuum. In order to investigate the phase transformation during synthesis, X-ray diffraction (XRD; Rigaku RINT2000) analyses using Cu K␣ radiation were carried out for the powders as well as hot pressed samples. Densities after hot consolidation were measured using a He pycnometer. In order to observe microstructures SEM (Hitachi S-2400) and TEM (FEI Tecnai F30 S-Twin model at 300 kV) were employed. Thermoelectric properties were measured from 300 to 600 K, and the resulting ZT was evaluated. α and ρ were measured by the differential and four-point probe methods (Ulvac-Riko ZEM2-M8). κ was evaluated from the measurements of thermal diffusivity, specific heat and density by the laser flash method (Ulvac-Riko TC7000).
3. Results and discussion XRD analysis revealed that single phase of ␦-(Co,Fe)Sb3 powders could not be obtained after MA for 100 h, but those existed in a meta-stable state similar to other MA studies [9]. As-milled powder size was typically less than 10 m. Complete alloying to a single phase can be accomplished by either powder annealing or hot consolidation [9]. In order to accomplish the phase transition and the consolidation concurrently, MAed Fex Co4−x Sb12 (0 ≤ x ≤ 2.5) powders were VHPed at 823 K with a pressure of 3 ton for 2 h, and the resulting XRD patterns were presented in Fig. 1(A). All specimens showed over 96% of theoretical density. SEM observation revealed no microcracks, but some voids were unavoidable. Mocrostructure with an average grain size of 40 nm was obtained by this process, as shown in Fig. 1(B). All compositions of x ≤ 1.5 resulted in single phase ␦-(Fe,Co)Sb3 , while second phase in the form of marcasite structure FeSb2 was found to exist in case of x ≥ 2. According to an analogue study, the solubility limit of Fe with Co was reported to be adjacent to x = 1.5 [10]. However, the solubility exceeds the limit in this study, presumably due to the formation of supersaturated solid solution which frequently occurs in MA process [7,9]. Thermoelectric properties as function of temperature with Fe contents were evaluated. Seebeck coefficients in all the specimens at test range showed positive values, representing p-type conductivity, except an undoped specimen which showed ntype conductivity at room temperature, as presented in Fig. 2(a). Undoped CoSb3 sometimes shows n-type conduction at room
359
temperature depending on processes, which might cause variation in carrier concentration [1,2,4], because Sb sublimation is possible during hot pressing so that the conductivity can be changed [3]. α increased with increasing temperature as in similar studies [3,4]. Those also show a decreasing trend as per Fe addition, possibly due to the increase in hole concentration caused by Fe substitution for Co [11]. α in undoped specimen reached to the maximum at 550 K while those in Fe doped specimens did not, indicating that application temperature would be elevated per Fe doping. ρ decreases with increasing temperature in undoped specimen, representing a non-degenerate semiconducting material, while Fe doped specimens represent highly degenerate semiconductors, as shown in Fig. 2(b). Interestingly, Fe doped specimens showed abrupt decrease in ρ with an order of magnitude. It is also attributed to the increase in hole concentration caused by Fe substitution for Co [11]. However, no specific trends in ρ variations as a function of Fe content were found in this case. Thermal conductivities as a function of temperature up to 600 K were presented in Fig. 2(c). κ was shown to decrease drastically as per Fe addition. This would be attributed to the induced phonon scattering by lattice distortion which would be caused by Fe substitution [3,11]. It is worth noting that the lowest κ is shown in the composition of x = 1.5, and the κ markedly increases at the composition of x ≥ 2, at which FeSb2 begin to appear. It is well known that κ consists of lattice component (κL ) and electron component (κe ), and the two components can be sorted by simple calculation using the Wiedemann–Frantz law (κe = LT/ρ). Here, L is Lorenz number (2.44 × 10−8 V2 /K2 ). For the composition of Fe1.5 Co2.5 Sb12 which shows the optimum thermoelectric property in this study, each component was possible to obtain as presented in Fig. 2(d). Lattice component prevails at test temperature range, and decreases with increasing temperature. This is attributed to the increasing phonon scattering upon increasing temperature, leading to lowering the κL . Electron contribution in κ gradually increases with increasing temperature, and becomes nearly the same as lattice contribution at near 600 K. From the values of ρ, α and κ obtained above, ZT values were possible to calculate, as presented in Fig. 3. Fe doping up to x = 1.5 in Fex Co4−x Sb12 appeared to increase ZT and the
Fig. 1. (A) XRD patterns of Fex Co4−x Sb12 processed by vacuum hot pressing at 823 K for 2 h: (a) x = 0, (b) x = 0.5, (c) x = 1, (d) x = 1.5, (e) x = 2, and (f) x = 2.5, (B) TEM micrograph of hot pressed MA Fex Co4−x Sb12 (x = 0), showing average grain size of 40 nm.
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S.-C. Ur et al. / Journal of Alloys and Compounds 442 (2007) 358–361
Fig. 2. Thermoelectric properties as a function of temperature in Fex Co4−x Sb12 processed by VHP at 823 K for 2 h: (a) Seebeck coefficients, (b) electrical resistivity, (c) thermal conductivity, and (d) thermal conductivity component variations in VHPed Fe1.5 Co2.5 Sb12 .
maximum value was found to be 0.32 at 600 K in the composition of Fe1.5 Co2.5 Sb12 . It can be noticed that Fe in CoSb3 system appeared to act effectively as a dopant and to reduce thermal conductivity, leading to enhancing ZT. It is also worth noting that previous study revealed that κ was reduced due to increasing phonon scattering in nanostructured CoSb3 , resulting in enhancement in ZT [12]. Thus, appropriate doping into the
CoSb3 along with producing ultra fine microstructure can be a promising route for this system. 4. Conclusion Fe doped skutterudite CoSb3 with a nominal composition of Fex Co4−x Sb12 (0 ≤ x ≤ 2.5) were synthesized by mechanical alloying of elemental powders. Single phase skutterudites were successfully produced by vacuum hot pressing using asmilled powders without subsequent annealing. However, second phase in the form of FeSb2 was found to form in case of x ≥ 2, suggesting the solubility limit of Fe with Co in this system. Fe in CoSb3 showed to effectively act as a dopant and to reduce thermal conductivity, resulting in enhanced ZT. Fe doping up to x = 1.5 with Co in Fex Co4−x Sb12 appeared to increase ZT and the maximum value was found to be 0.32 at 600 K in the composition of Fe1.5 Co2.5 Sb12 . Acknowledgements This research was supported by the Regional Innovation Center (RIC) Program which was conducted by the Ministry of Commerce, Industry and Energy of the Korea Government. References
Fig. 3. The dimensionless figure of merit (ZT) temperature as a function of temperature in Fex Co4−x Sb12 processed by VHP at 823 K for 2 h: (a) x = 0, (b) x = 0.5, (c) x = 1, (d) x = 1.5, (e) x = 2, and (f) x = 2.5.
[1] G.S. Nolas, D.T. Morelli, T.M. Tritt, Annu. Rev. Mater. Sci. 29 (1999) 89–116.
S.-C. Ur et al. / Journal of Alloys and Compounds 442 (2007) 358–361 [2] T. Caillat, A. Borschchevsky, J.-P. Fleurial, J. Appl. Phys. 80 (8) (1996) 4442–4449. [3] J.W. Sharp, E.C. Jones, R.K. Williams, P.M. Martin, B.C. Sales, J. Appl. Phys. 78 (2) (1995) 1013–1018. [4] Y. Kawaharada, K. Kurosaki, M. Uno, S. Yamanaka, J. Alloys Compd. 315 (2001) 193–197. [5] J.X. Jang, Q.M. Lu, K.G. Liu, L. Zhang, M.L. Zou, Mater. Lett. 58 (2004) 1981–1984. [6] J. Yang, Y.C. Chen, J. Peng, X. Song, W. Zhu, J. Su, R. Chen, J. Alloys Compd. 375 (2004) 229–232.
361
[7] C.C. Koch, Annu. Rev. Mater. Sci. (1989) 121–143. [8] D.M. Rowe, V.S. Schukla, J. Appl. Phys. 52 (12) (1981) 7421– 7426. [9] S.-C. Ur, P. Nash, I.-H. Kim, J. Alloys Compd. 361 (2003) 84–91. [10] S. Katsuyama, Y. Shichijo, M. Ito, K. Majima, H. Nagai, J. Appl. Phys. 84 (1998) 6708–6712. [11] J. Nagao, M. Ferhat, H. Anno, K. Matsubara, E. Hatta, K. Mukasa, Appl. Phys. Lett. 76 (23) (2000) 3436–3438. [12] S.-C. Ur, J.-C. Kwon, M.-K. Choi, S.-Y. Kweon, T.-W. Hong, I.-H. Kim, Y.-G. Lee, J. Key Eng. Mater., in press.
Thermoelectric properties of Fe-doped CoSb3 prepared by mechanical alloying and vacuum hot pressing Soon-Chul Ur ∗ , Joon-Chul Kwon, Il-Ho Kim Department of MSE/ReSEM, Chungju National University, Chungju, Chungbuk 380-702, Republic of Korea Received 17 May 2006; received in revised form 10 July 2006; accepted 2 August 2006 Available online 30 January 2007
Abstract Fe doped skutterudite CoSb3 with a nominal composition of Fex Co4−x Sb12 (0 ≤ x ≤ 2.5), were synthesized by mechanical alloying of elemental powders, and consolidated by vacuum hot pressing. Nanostructured, single-phase skutterudites were successfully produced for the compositions of x ≤ 1.5, while second phase was found to form in case of x ≥ 2. Thermoelectric properties including thermal conductivity from 300 to 600 K were measured and discussed. Lattice thermal conductivity was greatly reduced by introducing Fe doping up to x ≤ 1.5 as well as by increasing phonon scattering in nanostructured skutterudite, leading to enhancement in the thermoelectric figure of merit. © 2007 Elsevier B.V. All rights reserved. PACS: 74.25.Fy; 84.60.Rb Keywords: Thermoelectric; Mechanical alloying; Skutteridite; Doping; Vacuum hot pressing
1. Introduction Recent search for novel thermoelectric materials revealed a new class of compounds with skutterudite structure as good candidates for higher performance thermoelectric conversion [1,2]. CoSb3 belongs to a skutterudite and it is expected to be a promising thermoelectric material having high figure of merit value, especially for power generation [1–3]. The figure of merit is defined as ZT = α2 T/ρκ, where α is the Seebeck coefficient, ρ is the electrical resistivity, and κ is the thermal conductivity. Undoped CoSb3 shows intrinsic p-type conduction in general [1,2]. Although high performance was anticipated in CoSb3 , binary compound itself could not provide high ZT due to relatively higher lattice thermal conductivity (κL ) [2–5]. In order to enhance their thermoelectric efficiency, doping and/or filling are proposed [2,3,5]. κL was shown to be greatly reduced by inserting small sized atoms (rattlers) into the residual voids in skutterudite structure [2,3], and some dopants such as Fe, Ni, Ir and Cr were also reported to reduce κL as well as to control carrier concentration [2,5,6].
∗
Corresponding author. Tel.: +82 43 841 5385; fax: +82 43 841 5380. E-mail address: [email protected] (S.-C. Ur).
0925-8388/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2006.08.369
The preparation of polycrystalline ␦-CoSb3 involves rather complicated processes and are frequently confronted with the formation of undesirable phases such as ␥-CoSb2 , phase decomposition and/or Sb evaporation [3,5]. Another difficulty in processing is a poor tendency to form CoSb3 due to inherently sluggish progress of peritectic transformation [5,6]. In order to address these drawbacks, mechanical alloying (MA) process was applied in this study [6,7]. MA is known as a unique technique for solid state alloying process [7], by which alloying proceeds with consecutive cold welding and fracturing, resulting in ultra fine grain size and phase homogenization. MAed materials having a fine grain size may improve thermoelectric conversion efficiency by the reduction in κL [8]. Fe doped skutterudite CoSb3 with a nominal composition of Fex Co4−x Sb12 (0 ≤ x ≤ 2.5) have been synthesized by MA of elemental powders, followed by vacuum hot pressing (VHP) in this study. Thermoelectric properties from 300 to 600 K were measured and their correlations to phase transformations were examined. 2. Experimental procedure Appropriate elemental powder mixtures of −325 mesh Co (99.9%), Sb (99.9%) and Fe (99.9%) for stoichiometric Fex Co4−x Sb12 (0 ≤ x ≤ 2.5) were prepared and MAed in a Szegavari type attrition mill for 100 h under an Ar atmosphere. As-milled powders were hot pressed in a cylindrical high strength
S.-C. Ur et al. / Journal of Alloys and Compounds 442 (2007) 358–361 graphite die at 823 K with a stress of 60 MPa for 2 h in vacuum. In order to investigate the phase transformation during synthesis, X-ray diffraction (XRD; Rigaku RINT2000) analyses using Cu K␣ radiation were carried out for the powders as well as hot pressed samples. Densities after hot consolidation were measured using a He pycnometer. In order to observe microstructures SEM (Hitachi S-2400) and TEM (FEI Tecnai F30 S-Twin model at 300 kV) were employed. Thermoelectric properties were measured from 300 to 600 K, and the resulting ZT was evaluated. α and ρ were measured by the differential and four-point probe methods (Ulvac-Riko ZEM2-M8). κ was evaluated from the measurements of thermal diffusivity, specific heat and density by the laser flash method (Ulvac-Riko TC7000).
3. Results and discussion XRD analysis revealed that single phase of ␦-(Co,Fe)Sb3 powders could not be obtained after MA for 100 h, but those existed in a meta-stable state similar to other MA studies [9]. As-milled powder size was typically less than 10 m. Complete alloying to a single phase can be accomplished by either powder annealing or hot consolidation [9]. In order to accomplish the phase transition and the consolidation concurrently, MAed Fex Co4−x Sb12 (0 ≤ x ≤ 2.5) powders were VHPed at 823 K with a pressure of 3 ton for 2 h, and the resulting XRD patterns were presented in Fig. 1(A). All specimens showed over 96% of theoretical density. SEM observation revealed no microcracks, but some voids were unavoidable. Mocrostructure with an average grain size of 40 nm was obtained by this process, as shown in Fig. 1(B). All compositions of x ≤ 1.5 resulted in single phase ␦-(Fe,Co)Sb3 , while second phase in the form of marcasite structure FeSb2 was found to exist in case of x ≥ 2. According to an analogue study, the solubility limit of Fe with Co was reported to be adjacent to x = 1.5 [10]. However, the solubility exceeds the limit in this study, presumably due to the formation of supersaturated solid solution which frequently occurs in MA process [7,9]. Thermoelectric properties as function of temperature with Fe contents were evaluated. Seebeck coefficients in all the specimens at test range showed positive values, representing p-type conductivity, except an undoped specimen which showed ntype conductivity at room temperature, as presented in Fig. 2(a). Undoped CoSb3 sometimes shows n-type conduction at room
359
temperature depending on processes, which might cause variation in carrier concentration [1,2,4], because Sb sublimation is possible during hot pressing so that the conductivity can be changed [3]. α increased with increasing temperature as in similar studies [3,4]. Those also show a decreasing trend as per Fe addition, possibly due to the increase in hole concentration caused by Fe substitution for Co [11]. α in undoped specimen reached to the maximum at 550 K while those in Fe doped specimens did not, indicating that application temperature would be elevated per Fe doping. ρ decreases with increasing temperature in undoped specimen, representing a non-degenerate semiconducting material, while Fe doped specimens represent highly degenerate semiconductors, as shown in Fig. 2(b). Interestingly, Fe doped specimens showed abrupt decrease in ρ with an order of magnitude. It is also attributed to the increase in hole concentration caused by Fe substitution for Co [11]. However, no specific trends in ρ variations as a function of Fe content were found in this case. Thermal conductivities as a function of temperature up to 600 K were presented in Fig. 2(c). κ was shown to decrease drastically as per Fe addition. This would be attributed to the induced phonon scattering by lattice distortion which would be caused by Fe substitution [3,11]. It is worth noting that the lowest κ is shown in the composition of x = 1.5, and the κ markedly increases at the composition of x ≥ 2, at which FeSb2 begin to appear. It is well known that κ consists of lattice component (κL ) and electron component (κe ), and the two components can be sorted by simple calculation using the Wiedemann–Frantz law (κe = LT/ρ). Here, L is Lorenz number (2.44 × 10−8 V2 /K2 ). For the composition of Fe1.5 Co2.5 Sb12 which shows the optimum thermoelectric property in this study, each component was possible to obtain as presented in Fig. 2(d). Lattice component prevails at test temperature range, and decreases with increasing temperature. This is attributed to the increasing phonon scattering upon increasing temperature, leading to lowering the κL . Electron contribution in κ gradually increases with increasing temperature, and becomes nearly the same as lattice contribution at near 600 K. From the values of ρ, α and κ obtained above, ZT values were possible to calculate, as presented in Fig. 3. Fe doping up to x = 1.5 in Fex Co4−x Sb12 appeared to increase ZT and the
Fig. 1. (A) XRD patterns of Fex Co4−x Sb12 processed by vacuum hot pressing at 823 K for 2 h: (a) x = 0, (b) x = 0.5, (c) x = 1, (d) x = 1.5, (e) x = 2, and (f) x = 2.5, (B) TEM micrograph of hot pressed MA Fex Co4−x Sb12 (x = 0), showing average grain size of 40 nm.
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S.-C. Ur et al. / Journal of Alloys and Compounds 442 (2007) 358–361
Fig. 2. Thermoelectric properties as a function of temperature in Fex Co4−x Sb12 processed by VHP at 823 K for 2 h: (a) Seebeck coefficients, (b) electrical resistivity, (c) thermal conductivity, and (d) thermal conductivity component variations in VHPed Fe1.5 Co2.5 Sb12 .
maximum value was found to be 0.32 at 600 K in the composition of Fe1.5 Co2.5 Sb12 . It can be noticed that Fe in CoSb3 system appeared to act effectively as a dopant and to reduce thermal conductivity, leading to enhancing ZT. It is also worth noting that previous study revealed that κ was reduced due to increasing phonon scattering in nanostructured CoSb3 , resulting in enhancement in ZT [12]. Thus, appropriate doping into the
CoSb3 along with producing ultra fine microstructure can be a promising route for this system. 4. Conclusion Fe doped skutterudite CoSb3 with a nominal composition of Fex Co4−x Sb12 (0 ≤ x ≤ 2.5) were synthesized by mechanical alloying of elemental powders. Single phase skutterudites were successfully produced by vacuum hot pressing using asmilled powders without subsequent annealing. However, second phase in the form of FeSb2 was found to form in case of x ≥ 2, suggesting the solubility limit of Fe with Co in this system. Fe in CoSb3 showed to effectively act as a dopant and to reduce thermal conductivity, resulting in enhanced ZT. Fe doping up to x = 1.5 with Co in Fex Co4−x Sb12 appeared to increase ZT and the maximum value was found to be 0.32 at 600 K in the composition of Fe1.5 Co2.5 Sb12 . Acknowledgements This research was supported by the Regional Innovation Center (RIC) Program which was conducted by the Ministry of Commerce, Industry and Energy of the Korea Government. References
Fig. 3. The dimensionless figure of merit (ZT) temperature as a function of temperature in Fex Co4−x Sb12 processed by VHP at 823 K for 2 h: (a) x = 0, (b) x = 0.5, (c) x = 1, (d) x = 1.5, (e) x = 2, and (f) x = 2.5.
[1] G.S. Nolas, D.T. Morelli, T.M. Tritt, Annu. Rev. Mater. Sci. 29 (1999) 89–116.
S.-C. Ur et al. / Journal of Alloys and Compounds 442 (2007) 358–361 [2] T. Caillat, A. Borschchevsky, J.-P. Fleurial, J. Appl. Phys. 80 (8) (1996) 4442–4449. [3] J.W. Sharp, E.C. Jones, R.K. Williams, P.M. Martin, B.C. Sales, J. Appl. Phys. 78 (2) (1995) 1013–1018. [4] Y. Kawaharada, K. Kurosaki, M. Uno, S. Yamanaka, J. Alloys Compd. 315 (2001) 193–197. [5] J.X. Jang, Q.M. Lu, K.G. Liu, L. Zhang, M.L. Zou, Mater. Lett. 58 (2004) 1981–1984. [6] J. Yang, Y.C. Chen, J. Peng, X. Song, W. Zhu, J. Su, R. Chen, J. Alloys Compd. 375 (2004) 229–232.
361
[7] C.C. Koch, Annu. Rev. Mater. Sci. (1989) 121–143. [8] D.M. Rowe, V.S. Schukla, J. Appl. Phys. 52 (12) (1981) 7421– 7426. [9] S.-C. Ur, P. Nash, I.-H. Kim, J. Alloys Compd. 361 (2003) 84–91. [10] S. Katsuyama, Y. Shichijo, M. Ito, K. Majima, H. Nagai, J. Appl. Phys. 84 (1998) 6708–6712. [11] J. Nagao, M. Ferhat, H. Anno, K. Matsubara, E. Hatta, K. Mukasa, Appl. Phys. Lett. 76 (23) (2000) 3436–3438. [12] S.-C. Ur, J.-C. Kwon, M.-K. Choi, S.-Y. Kweon, T.-W. Hong, I.-H. Kim, Y.-G. Lee, J. Key Eng. Mater., in press.