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Effects of Ge and B substitution on thermoelectric properties of CoSi
Journal of Alloys and Compounds 392 (2005) 50–54
Effects of Ge and B substitution on thermoelectric properties of CoSi W.L. Rena,∗ , C.C. Lia , L.T. Zhanga , K. Itob , J.S. Wua a
b
Key Laboratory of the Ministry of Education for High Temperature Materials and Testing, School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200030, PR China Department of Materials Science and Engineering, Kyoto University, Sakyo-ku, Kyoto 606-8501, Japan Received 7 July 2004; received in revised form 14 September 2004; accepted 15 September 2004 Available online 18 November 2004
Abstract The effects of substitution of Ge and B for Si on the thermoelectric properties of CoSi were studied. The electrical resistivity, Seebeck coefficient, and thermal conductivity of CoSi, CoSi0.98 Ge0.02 and CoSi0.995 B0.005 were measured from room temperature to 973 K, and the power factor and figure of merit were evaluated. It was found that the substitution causes a decrease in the electrical resistivity and lattice thermal conductivity, as well as an increase in the absolute value of Seebeck coefficient and power factor. A theoretical analysis indicates the influence of substitution arises mainly from the increased carrier density and point-defect scattering of phonons. The thermoelectric figure of merit is enhanced 25–40% by substitution as compared with that of the nondoped CoSi. © 2004 Elsevier B.V. All rights reserved. Keywords: CoSi intermetallics; Thermoelectric properties; Ge and B substitution
1. Introduction
2. Experimental
Some transition-silicides with semiconducting or semimetallic properties have attracted considerable attention due to their practical applications in electronics and thermoelectrics [1]. CoSi has been reported to be one of the promising candidates for advanced thermoelectrics applications [2–4]. In order to improve the thermoelectric efficiency of CoSi, doping of Fe and Ni has been attempted during sample preparation. The doping elements Fe and Ni substituted for Co are known as good dopants for p-type and n-type CoSi materials, respectively. However, there is little information about the influence of the doping element substituting for Si on the thermoelectric properties of CoSi. Therefore, in this study, the preparation of CoSi with Ge and B substituted for Si was tried through a floating zone method, and the effects of Ge, B doping on the thermoelectric properties were investigated.
2.1. Crystal preparation
∗
Corresponding author. Tel.: +86 21 62932729; fax: +86 21 62932587. E-mail address: [email protected] (W.L. Ren).
0925-8388/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2004.09.036
The samples of CoSi, CoSi0.98 Ge0.02 and CoSi0.995 B0.005 were prepared by the floating zone technique. Firstly, feed and seed rods are prepared. High purity Co (99.98%), Si (99.999%), B (99.999%), Ge (99.999%) were used as starting materials. They were weighted in the desired ratio and melted together in an arc melting furnace on a round-shaped water-cooled copper hearth under a purified Ar atmosphere. Then, the buttons were remelted on a rod-shaped copper hearth in the same furnace to produce polycrystalline feed and seed rods with about 10 mm in diameter. Secondly, crystal growth was performed on an optical floating zone system type FZ-20035W apparatus at a rate of 5–7.5 mm h−1 under a high purity argon gas flow. Single crystal of CoSi, CoSi0.995 B0.005 and large grained CoSi0.98 Ge0.98 were obtained. Composition, microstructure and perfection of the crystals were investigated by X-ray diffraction (XRD), scanning electron microscope (SEM) including electron probe microanalysis (EPMA). The orientation of single
W.L. Ren et al. / Journal of Alloys and Compounds 392 (2005) 50–54
51
Fig. 1. Single crystal of CoSi grown by the floating zone technique. Table 1 Hall coefficient, carrier concentration, carrier mobility of the investigated compounds at room temperature Compounds
Hall coefficient (cm3 C−1 )
Carrier concentration (cm−3 )
Carrier mobility (cm2 V−1 s−1 )
CoSi CoSi0.995 B0.005 CoSi0.98 Ge0.02
−2.01 × 10−2 −1.38 × 10−2 −1.47 × 10−2
3.10 × 1020 4.53 × 1020 4.26 × 1020
114.9 103.0 88.7
crystals was determined by the X-ray Laue back scattering method. 2.2. Thermoelectric properties measurement Measurements of the electrical resistivity and the Hall coefficient RH were carried out in a temperature range from 300 to 973 K using the van der Pauw technique in an argon atmosphere. The Hall carrier concentration n was determined from the Hall coefficient RH using n = 1/RH q, where q is the carrier charge. The measurement of the Seebeck coefficient was performed by applying a small temperature difference (3 K) to the two ends of a specimen in a temperature range from 300 to 973 K in an argon atmosphere. The specific heat capacity was measured by DSC in Ar flow. The thermal dif-
fusivity was measured by the AC calorimetry method in a temperature range from 300 to 473 K. The thermal conductivity was calculated from the experimental values of thermal diffusivity, specific heat capacity and density.
3. Results and discussion 3.1. Orientation of crystal A typical as-grown crystal is shown in Fig. 1. The Xray and metallographic investigations permit the conclusion that the samples CoSi and CoSi0.995 B0.005 are single crystalline and CoSi0.98 Ge0.02 is polycrystalline with very large grains. Fig. 2(a) and (b) are the photo and simulation of Laue diffraction spots for the cross-section of CoSi, respectively. The simulation is in good agreement with the Laue pictures, so the growth direction is determined to be [2 1¯ 1]. The microstructure of each compound is of single phase. 3.2. Electrical resistivity The electrical properties of these compounds were studied. Table 1 indicates the Hall coefficient, carrier concentra-
Fig. 2. (a) Photo and (b) simulation of Laue diffraction spots for the cross-section of the floating zone grown CoSi crystal.
52
W.L. Ren et al. / Journal of Alloys and Compounds 392 (2005) 50–54
Fig. 3. Temperature dependence of electrical resistivity of CoSi, CoSi0.98 Ge0.02 and CoSi0.995 B0.005 .
tion and carrier mobility at room temperature. As Table 1 shows, the Hall coefficient of all studied specimens is negative, meaning that they are of n-type conductivity. The room temperature carrier concentration of the doped materials is higher than that of the undoped one. Fig. 3 shows the temperature dependence of the electrical resistivity of CoSi, CoSi0.98 Ge0.02 and CoSi0.995 B0.005 . The electrical resistivity of all the samples increases steadly with increasing temperature over the whole range studied, because of the increased scattering due to the thermal vibrations of the lattice. B and Ge substitution reduces the electrical resistivity in the investigated temperature range. The reduction for CoSi0.995 B0.005 is larger than that for CoSi0.98 Ge0.02 . The decrease in the electrical resistivity is considered to be caused by the increase in the carrier density due to doping. It has been reported that Fe-doping can increase the electrical resistivity of CoSi, while Ni-doping decreases it [2,3].
Fig. 4. Temperature dependence of Seebeck coefficient of CoSi, CoSi0.98 Ge0.02 and CoSi0.995 B0.005 .
for CoSi and CoSi0.98 Ge0.02 may be explained by the fact that Ge and Si are in the same group in the periodic system. One B atom substituting for one Si atom produces approximately one hole, so the sign of the Seebeck coefficient for CoSi0.995 B0.005 should be opposite to that of CoSi. When CoSi is slightly doped with Fe and Al [2,5], the S values remains negative. Upon further substitution, the hole concentration increases and consequently reverses the sign of S from negative to positive. Such a phenomenon can be ascribed to the compensation of p- and n-typed carriers involved in the heat transport processes by substitution. Therefore, the unchanged sign of S for B-doped alloy may be understood by low concentration of B. The power factor P is calculated from Seebeck coefficient S and the electrical resistivity ρ in the equation P = S2 /ρ. Fig. 5 shows the temperature dependence of the power factor of all the samples. Ge and B doping enhances the power factor compared to that of the nondoped sample over the entire temper-
3.3. Seebeck coefficient and power factor Fig. 4 shows the temperature dependence of the Seebeck coefficient, S, of CoSi, CoSi0.98 Ge0.02 and CoSi0.995 B0.005 . All the samples show a p-type Seebeck coefficient over the entire temperature range, indicating that electron-type carriers dominate the transport properties. The absolute value of S for CoSi0.98 Ge0.02 and CoSi0.995 B0.005 is bigger than that of CoSi. The negative S values for the stoichiometric compound of CoSi is consistent with the sign of the Hall coefficient and with previous results [2,3]. The sign of the Seebeck coefficient can be explained within the framework of two-carrier electrical conduction. Accordingly, the total S can be expressed as [4] S = (σp SP + σn Sn )/(σp + σn ), where Sp , Sn and σ p , σ n represent the Seebeck coefficients and electrical conductivities for the p- and n-type carriers, respectively. Since the signs of Sp and Sn are opposite, tuning these quantities could result in a sign change in S. In the present study, the consistency of the sign of the Seebeck coefficient
Fig. 5. Temperature dependence of the power factor of CoSi, CoSi0.98 Ge0.02 and CoSi0.995 B0.005 .
W.L. Ren et al. / Journal of Alloys and Compounds 392 (2005) 50–54
Fig. 6. Temperature dependence of the thermal conductivity of CoSi, CoSi0.98 Ge0.02 and CoSi0.995 B0.005 .
ature range, because the electrical resistivity is reduced and the absolute value of the Seebeck coefficient is increased by doping. 3.4. Thermal conductivity Fig. 6 shows the temperature dependence of the thermal conductivity of CoSi, CoSi0.98 Ge0.02 and CoSi0.995 B0.005 . The thermal conductivity of all the samples decreases monotonously with increasing temperature in the investigated range. This is considered to be caused by enhanced the lattice vibrations due to a temperature increase. Compared to the undoped sample, the thermal conductivity is lowered by Ge substitution and slightly increased by B substitution in the investigated temperature range. Substitution of Fe for Co also reduced the thermal conductivity in CoSi [2,3]. Generally, the thermal conductivity κ for semimetals is the sum of carrier κcar and lattice κl terms. The carrier thermal conductivity can be evaluated using the Wiedemann–Franz relationship [5] κcar = TL/ρ, where L is the Lorentz number (L = 2.45 × 10−8 W /K2 ), ρ is the electrical resistivity, and T is the absolute temperature. The lattice thermal conductivity is obtained by subtracting κcar from the observed total thermal conductivity. Fig. 7 shows the carrier thermal conductivity κcar and lattice thermal conductivity κl versus temperature for the samples. As seen from Fig. 7, κl is predominant in the total thermal conductivity of all the samples. The lattice thermal conductivity of all the samples decreases with increasing temperature. κl is lowered by substitution of B and Ge for Si. The reduction in magnitude for Ge substitution is much larger. The reason may be the appearance of point defects by Ge and B doping. These point defects originate from the atom size and mass fluctuations between Si and Ge, B, since their atom size and mass differences are much more than 4%. Moreover, lattice imperfections, such as vacancies, are introduced with Ge and B substitution, which in turn give rise to a considerable amount of point defects in the studied
53
Fig. 7. Temperature dependence of carrier thermal conductivity (κcar ) and lattice thermal conductivity (κl ) of CoSi, CoSi0.98 Ge0.02 and CoSi0.995 B0.005 .
samples. The point defects enhance the phonon scattering. On the other hand, the carrier thermal conductivity increases with increasing temperature. κcar is increased by substitution of B and Ge for Si. The increase for B substitution is larger. This can be ascribed to the increasing carrier density by the doping. Therefore, the reason that Ge substitution decreases the total thermal conductivity is that the decrease in κl is larger than the increase in κcar . The reason that B substitution increases the total thermal conductivity is that the decrease in κl is lower than the increase in κcar . 3.5. Figure of merit From the application viewpoint, the efficiency of a thermoelectric material is characterized by the dimensionless ZT values. This is given by, ZT = (S2 σ/κ)T, where S, thermoelectric power, σ, electrical conductivity, and κ, thermal conduc-
Fig. 8. Temperature dependence of ZT for the CoSi, CoSi0.98 Ge0.02 and CoSi0.995 B0.005 .
54
W.L. Ren et al. / Journal of Alloys and Compounds 392 (2005) 50–54
tivity. Fig. 8 shows the ZT value as a function of temperature for all the samples. The ZT value increased 25–40% by Ge and B substitution. But the improvement reason is not same. The ZT improvement by B is achieved by the increase of the power factor, while that by Ge is due to both the increase of the power factor and the decrease of thermal conductivity.
4. Conclusion The effects of substitution of Ge and B for Si on the thermoelectric properties of CoSi were investigated in the temperature range from 300 to 973 K. Compared with that of the nondoped CoSi, the n-type conduction of CoSi0.98 Ge0.02 and CoSi0.995 B0.005 remains unchanged. The electrical resistivity is reduced by the Ge and B substitution in the investigated temperature range, which is related to an increased carrier density. The substitution causes an increase in the absolute value of the Seebeck coefficient and power factor. The thermal conductivity of CoSi0.98 Ge0.02 is smaller than that of the undoped sample, while that of CoSi0.995 B0.005 is slightly higher than that of CoSi. This depends on the relative changes in the value of the carrier thermal conductivity and lattice
thermal conductivity by substitution. The figure of merit was enhanced 25–40% by substitution as compared with that of the nondoped CoSi.
Acknowledgments This work was supported by the Shanghai-Applied Materials Research Development Foundation (contract No. 0317) and the National Natural Science Foundation of China (contract No. 50131030). W.L. Ren acknowledges the China and Shanghai Postdoctoral Science Foundation for their financial support.
References [1] D.M. Rowe, CRC Handbook of Thermoelectrics, CRC Press, Boca Raton, FL, 1995. [2] S. Asanabe, D. Shinoda, Y. Saski, Phys. Rev. A 134 (1964) 774. [3] G.T. Alekseeve, V.K. Zailsev, A.V. Petrov, V.I. Tarasov, M.I. Fedorov, Sov. Phys. Solid State 23 (1981) 1685. [4] S.W. Kim, Y. Mishima, D.C. Choi, Intermetallics 10 (2002) 177. [5] C.S. Lue, Y.K. Kuo, C.L. Huang, W.J. Lai, Phys. Rev. B 69 (2004) 125111-1.
Effects of Ge and B substitution on thermoelectric properties of CoSi W.L. Rena,∗ , C.C. Lia , L.T. Zhanga , K. Itob , J.S. Wua a
b
Key Laboratory of the Ministry of Education for High Temperature Materials and Testing, School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200030, PR China Department of Materials Science and Engineering, Kyoto University, Sakyo-ku, Kyoto 606-8501, Japan Received 7 July 2004; received in revised form 14 September 2004; accepted 15 September 2004 Available online 18 November 2004
Abstract The effects of substitution of Ge and B for Si on the thermoelectric properties of CoSi were studied. The electrical resistivity, Seebeck coefficient, and thermal conductivity of CoSi, CoSi0.98 Ge0.02 and CoSi0.995 B0.005 were measured from room temperature to 973 K, and the power factor and figure of merit were evaluated. It was found that the substitution causes a decrease in the electrical resistivity and lattice thermal conductivity, as well as an increase in the absolute value of Seebeck coefficient and power factor. A theoretical analysis indicates the influence of substitution arises mainly from the increased carrier density and point-defect scattering of phonons. The thermoelectric figure of merit is enhanced 25–40% by substitution as compared with that of the nondoped CoSi. © 2004 Elsevier B.V. All rights reserved. Keywords: CoSi intermetallics; Thermoelectric properties; Ge and B substitution
1. Introduction
2. Experimental
Some transition-silicides with semiconducting or semimetallic properties have attracted considerable attention due to their practical applications in electronics and thermoelectrics [1]. CoSi has been reported to be one of the promising candidates for advanced thermoelectrics applications [2–4]. In order to improve the thermoelectric efficiency of CoSi, doping of Fe and Ni has been attempted during sample preparation. The doping elements Fe and Ni substituted for Co are known as good dopants for p-type and n-type CoSi materials, respectively. However, there is little information about the influence of the doping element substituting for Si on the thermoelectric properties of CoSi. Therefore, in this study, the preparation of CoSi with Ge and B substituted for Si was tried through a floating zone method, and the effects of Ge, B doping on the thermoelectric properties were investigated.
2.1. Crystal preparation
∗
Corresponding author. Tel.: +86 21 62932729; fax: +86 21 62932587. E-mail address: [email protected] (W.L. Ren).
0925-8388/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2004.09.036
The samples of CoSi, CoSi0.98 Ge0.02 and CoSi0.995 B0.005 were prepared by the floating zone technique. Firstly, feed and seed rods are prepared. High purity Co (99.98%), Si (99.999%), B (99.999%), Ge (99.999%) were used as starting materials. They were weighted in the desired ratio and melted together in an arc melting furnace on a round-shaped water-cooled copper hearth under a purified Ar atmosphere. Then, the buttons were remelted on a rod-shaped copper hearth in the same furnace to produce polycrystalline feed and seed rods with about 10 mm in diameter. Secondly, crystal growth was performed on an optical floating zone system type FZ-20035W apparatus at a rate of 5–7.5 mm h−1 under a high purity argon gas flow. Single crystal of CoSi, CoSi0.995 B0.005 and large grained CoSi0.98 Ge0.98 were obtained. Composition, microstructure and perfection of the crystals were investigated by X-ray diffraction (XRD), scanning electron microscope (SEM) including electron probe microanalysis (EPMA). The orientation of single
W.L. Ren et al. / Journal of Alloys and Compounds 392 (2005) 50–54
51
Fig. 1. Single crystal of CoSi grown by the floating zone technique. Table 1 Hall coefficient, carrier concentration, carrier mobility of the investigated compounds at room temperature Compounds
Hall coefficient (cm3 C−1 )
Carrier concentration (cm−3 )
Carrier mobility (cm2 V−1 s−1 )
CoSi CoSi0.995 B0.005 CoSi0.98 Ge0.02
−2.01 × 10−2 −1.38 × 10−2 −1.47 × 10−2
3.10 × 1020 4.53 × 1020 4.26 × 1020
114.9 103.0 88.7
crystals was determined by the X-ray Laue back scattering method. 2.2. Thermoelectric properties measurement Measurements of the electrical resistivity and the Hall coefficient RH were carried out in a temperature range from 300 to 973 K using the van der Pauw technique in an argon atmosphere. The Hall carrier concentration n was determined from the Hall coefficient RH using n = 1/RH q, where q is the carrier charge. The measurement of the Seebeck coefficient was performed by applying a small temperature difference (3 K) to the two ends of a specimen in a temperature range from 300 to 973 K in an argon atmosphere. The specific heat capacity was measured by DSC in Ar flow. The thermal dif-
fusivity was measured by the AC calorimetry method in a temperature range from 300 to 473 K. The thermal conductivity was calculated from the experimental values of thermal diffusivity, specific heat capacity and density.
3. Results and discussion 3.1. Orientation of crystal A typical as-grown crystal is shown in Fig. 1. The Xray and metallographic investigations permit the conclusion that the samples CoSi and CoSi0.995 B0.005 are single crystalline and CoSi0.98 Ge0.02 is polycrystalline with very large grains. Fig. 2(a) and (b) are the photo and simulation of Laue diffraction spots for the cross-section of CoSi, respectively. The simulation is in good agreement with the Laue pictures, so the growth direction is determined to be [2 1¯ 1]. The microstructure of each compound is of single phase. 3.2. Electrical resistivity The electrical properties of these compounds were studied. Table 1 indicates the Hall coefficient, carrier concentra-
Fig. 2. (a) Photo and (b) simulation of Laue diffraction spots for the cross-section of the floating zone grown CoSi crystal.
52
W.L. Ren et al. / Journal of Alloys and Compounds 392 (2005) 50–54
Fig. 3. Temperature dependence of electrical resistivity of CoSi, CoSi0.98 Ge0.02 and CoSi0.995 B0.005 .
tion and carrier mobility at room temperature. As Table 1 shows, the Hall coefficient of all studied specimens is negative, meaning that they are of n-type conductivity. The room temperature carrier concentration of the doped materials is higher than that of the undoped one. Fig. 3 shows the temperature dependence of the electrical resistivity of CoSi, CoSi0.98 Ge0.02 and CoSi0.995 B0.005 . The electrical resistivity of all the samples increases steadly with increasing temperature over the whole range studied, because of the increased scattering due to the thermal vibrations of the lattice. B and Ge substitution reduces the electrical resistivity in the investigated temperature range. The reduction for CoSi0.995 B0.005 is larger than that for CoSi0.98 Ge0.02 . The decrease in the electrical resistivity is considered to be caused by the increase in the carrier density due to doping. It has been reported that Fe-doping can increase the electrical resistivity of CoSi, while Ni-doping decreases it [2,3].
Fig. 4. Temperature dependence of Seebeck coefficient of CoSi, CoSi0.98 Ge0.02 and CoSi0.995 B0.005 .
for CoSi and CoSi0.98 Ge0.02 may be explained by the fact that Ge and Si are in the same group in the periodic system. One B atom substituting for one Si atom produces approximately one hole, so the sign of the Seebeck coefficient for CoSi0.995 B0.005 should be opposite to that of CoSi. When CoSi is slightly doped with Fe and Al [2,5], the S values remains negative. Upon further substitution, the hole concentration increases and consequently reverses the sign of S from negative to positive. Such a phenomenon can be ascribed to the compensation of p- and n-typed carriers involved in the heat transport processes by substitution. Therefore, the unchanged sign of S for B-doped alloy may be understood by low concentration of B. The power factor P is calculated from Seebeck coefficient S and the electrical resistivity ρ in the equation P = S2 /ρ. Fig. 5 shows the temperature dependence of the power factor of all the samples. Ge and B doping enhances the power factor compared to that of the nondoped sample over the entire temper-
3.3. Seebeck coefficient and power factor Fig. 4 shows the temperature dependence of the Seebeck coefficient, S, of CoSi, CoSi0.98 Ge0.02 and CoSi0.995 B0.005 . All the samples show a p-type Seebeck coefficient over the entire temperature range, indicating that electron-type carriers dominate the transport properties. The absolute value of S for CoSi0.98 Ge0.02 and CoSi0.995 B0.005 is bigger than that of CoSi. The negative S values for the stoichiometric compound of CoSi is consistent with the sign of the Hall coefficient and with previous results [2,3]. The sign of the Seebeck coefficient can be explained within the framework of two-carrier electrical conduction. Accordingly, the total S can be expressed as [4] S = (σp SP + σn Sn )/(σp + σn ), where Sp , Sn and σ p , σ n represent the Seebeck coefficients and electrical conductivities for the p- and n-type carriers, respectively. Since the signs of Sp and Sn are opposite, tuning these quantities could result in a sign change in S. In the present study, the consistency of the sign of the Seebeck coefficient
Fig. 5. Temperature dependence of the power factor of CoSi, CoSi0.98 Ge0.02 and CoSi0.995 B0.005 .
W.L. Ren et al. / Journal of Alloys and Compounds 392 (2005) 50–54
Fig. 6. Temperature dependence of the thermal conductivity of CoSi, CoSi0.98 Ge0.02 and CoSi0.995 B0.005 .
ature range, because the electrical resistivity is reduced and the absolute value of the Seebeck coefficient is increased by doping. 3.4. Thermal conductivity Fig. 6 shows the temperature dependence of the thermal conductivity of CoSi, CoSi0.98 Ge0.02 and CoSi0.995 B0.005 . The thermal conductivity of all the samples decreases monotonously with increasing temperature in the investigated range. This is considered to be caused by enhanced the lattice vibrations due to a temperature increase. Compared to the undoped sample, the thermal conductivity is lowered by Ge substitution and slightly increased by B substitution in the investigated temperature range. Substitution of Fe for Co also reduced the thermal conductivity in CoSi [2,3]. Generally, the thermal conductivity κ for semimetals is the sum of carrier κcar and lattice κl terms. The carrier thermal conductivity can be evaluated using the Wiedemann–Franz relationship [5] κcar = TL/ρ, where L is the Lorentz number (L = 2.45 × 10−8 W /K2 ), ρ is the electrical resistivity, and T is the absolute temperature. The lattice thermal conductivity is obtained by subtracting κcar from the observed total thermal conductivity. Fig. 7 shows the carrier thermal conductivity κcar and lattice thermal conductivity κl versus temperature for the samples. As seen from Fig. 7, κl is predominant in the total thermal conductivity of all the samples. The lattice thermal conductivity of all the samples decreases with increasing temperature. κl is lowered by substitution of B and Ge for Si. The reduction in magnitude for Ge substitution is much larger. The reason may be the appearance of point defects by Ge and B doping. These point defects originate from the atom size and mass fluctuations between Si and Ge, B, since their atom size and mass differences are much more than 4%. Moreover, lattice imperfections, such as vacancies, are introduced with Ge and B substitution, which in turn give rise to a considerable amount of point defects in the studied
53
Fig. 7. Temperature dependence of carrier thermal conductivity (κcar ) and lattice thermal conductivity (κl ) of CoSi, CoSi0.98 Ge0.02 and CoSi0.995 B0.005 .
samples. The point defects enhance the phonon scattering. On the other hand, the carrier thermal conductivity increases with increasing temperature. κcar is increased by substitution of B and Ge for Si. The increase for B substitution is larger. This can be ascribed to the increasing carrier density by the doping. Therefore, the reason that Ge substitution decreases the total thermal conductivity is that the decrease in κl is larger than the increase in κcar . The reason that B substitution increases the total thermal conductivity is that the decrease in κl is lower than the increase in κcar . 3.5. Figure of merit From the application viewpoint, the efficiency of a thermoelectric material is characterized by the dimensionless ZT values. This is given by, ZT = (S2 σ/κ)T, where S, thermoelectric power, σ, electrical conductivity, and κ, thermal conduc-
Fig. 8. Temperature dependence of ZT for the CoSi, CoSi0.98 Ge0.02 and CoSi0.995 B0.005 .
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W.L. Ren et al. / Journal of Alloys and Compounds 392 (2005) 50–54
tivity. Fig. 8 shows the ZT value as a function of temperature for all the samples. The ZT value increased 25–40% by Ge and B substitution. But the improvement reason is not same. The ZT improvement by B is achieved by the increase of the power factor, while that by Ge is due to both the increase of the power factor and the decrease of thermal conductivity.
4. Conclusion The effects of substitution of Ge and B for Si on the thermoelectric properties of CoSi were investigated in the temperature range from 300 to 973 K. Compared with that of the nondoped CoSi, the n-type conduction of CoSi0.98 Ge0.02 and CoSi0.995 B0.005 remains unchanged. The electrical resistivity is reduced by the Ge and B substitution in the investigated temperature range, which is related to an increased carrier density. The substitution causes an increase in the absolute value of the Seebeck coefficient and power factor. The thermal conductivity of CoSi0.98 Ge0.02 is smaller than that of the undoped sample, while that of CoSi0.995 B0.005 is slightly higher than that of CoSi. This depends on the relative changes in the value of the carrier thermal conductivity and lattice
thermal conductivity by substitution. The figure of merit was enhanced 25–40% by substitution as compared with that of the nondoped CoSi.
Acknowledgments This work was supported by the Shanghai-Applied Materials Research Development Foundation (contract No. 0317) and the National Natural Science Foundation of China (contract No. 50131030). W.L. Ren acknowledges the China and Shanghai Postdoctoral Science Foundation for their financial support.
References [1] D.M. Rowe, CRC Handbook of Thermoelectrics, CRC Press, Boca Raton, FL, 1995. [2] S. Asanabe, D. Shinoda, Y. Saski, Phys. Rev. A 134 (1964) 774. [3] G.T. Alekseeve, V.K. Zailsev, A.V. Petrov, V.I. Tarasov, M.I. Fedorov, Sov. Phys. Solid State 23 (1981) 1685. [4] S.W. Kim, Y. Mishima, D.C. Choi, Intermetallics 10 (2002) 177. [5] C.S. Lue, Y.K. Kuo, C.L. Huang, W.J. Lai, Phys. Rev. B 69 (2004) 125111-1.