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Electrical and thermal properties of titanium hydrides
Journal of Alloys and Compounds 420 (2006) 25–28
Electrical and thermal properties of titanium hydrides Masato Ito ∗ , Daigo Setoyama, Junji Matsunaga, Hiroaki Muta, Ken Kurosaki, Masayoshi Uno, Shinsuke Yamanaka Division of Sustainable Energy and Environmental Engineering, Graduate School of Engineering, Osaka University, 2-1 Yamada-oka, Suita, Osaka 565-0871, Japan Received 27 September 2005; received in revised form 19 October 2005; accepted 20 October 2005 Available online 28 November 2005
Abstract Various shapes of flaw-free bulk titanium hydrides (TiHX : X = 1.53–1.75) were fabricated, and its electrical and thermal properties were studied. The electrical conductivity of titanium hydride was slightly lower than that of titanium metal, and it exhibited metal-like temperature dependence. The Seebeck coefficient of the Ti–H system changed from positive to negative due to hydrogenation, and its absolute value for titanium hydride remained at a low magnitude, which is typical for metallic materials. The thermal conductivity of the hydride was the same as that of the metal and increased slightly with increasing temperature. © 2005 Elsevier B.V. All rights reserved. Keywords: Metal hydrides; Electrical transport; Thermoelectric; Heat conduction
1. Introduction
2. Experimental procedure
Titanium and its alloys are useful materials in various kinds of plants because of their superior specific strength and resistance to corrosion. Therefore, they have been extensively utilized and expected for use in heat exchangers, steam condenser tubes, irradiation targets for transmuting radioactive wastes, and overpacks for geological disposal of high-level radioactive wastes [1,2]. On the other hand, titanium is known to absorb large quantities of hydrogen. This hydrogenation transforms titanium from metal (hcp A3, ␣-phase) to hydride (fcc C1, ␦-phase), and the precipitation of the hydride results in embrittlement. Therefore, the influence of the hydride has been keenly observed [3–5]; however, the information regarding the properties of titanium hydride is inadequate. Since a high reliability of the materials is required under various severe conditions, it is necessary to elucidate the properties of titanium hydrides with various hydrogen contents. Yamanaka et al. reported that the characteristics of zirconium changed considerably by hydrogenation [6–8]. Therefore, the properties of titanium hydrides are expected to be rather abnormal. In the present study, the electrical and thermal properties of titanium hydrides were studied.
In the present study, bulk titanium hydrides were fabricated directly from α titanium polycrystalline rods (Ø 10 mm × 5 mm t and Ø 5 mm × 15 mm t) with 99.9 wt.% purity. Hydrogenation was executed using a modified Sieverts’ UHV apparatus under a highly pure (7N) hydrogen gas atmosphere with an appropriate pressure. The authors have succeeded in fabricating flaw-free bulk titanium hydrides by regulating their hydrogen absorption rate. The details of this apparatus and the hydrogenation process are described in our previous papers [9,10]. X-ray diffraction measurement was performed at room temperature using Cu K␣ radiation (RINT-2000/PC, Rigaku Corp.). The crystal structure of the sample was analyzed and the lattice parameter was evaluated using this measurement. The hydrogen content was measured using a hydrogen analyzer (HORIBA, EMGA-621). The electrical conductivity was measured with a standard fourprobe DC analysis (ULVAC Inc., ZEM-1) in the temperature range from 323 to 773 K under He atmosphere. The thermal diffusivity was measured by a laser flash method (ULVAC Inc., TC-7000) at temperatures ranging from room temperature to 623 K in vacuum. The heat capacity was measured by a differential scanning calorimeter (ULVAC-RIKO Inc., Triple-cell DSC) in a temperature range from 323 to 623 K under a He flow. The thermal conductivity was estimated by employing the thermal diffusivity, heat capacity, and density.
∗
Corresponding author. Tel.: +81 6 6879 7905; fax: +81 6 6879 7889. E-mail address: [email protected] (M. Ito).
0925-8388/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2005.10.032
3. Results and discussion The hydrogen content of the prepared sample is in the range from 1.53 to 1.75 in atomic ratio [H/Ti]. Fig. 1 shows the X-ray diffraction patterns of titanium hydride employing the literature data [11,12]. From these patterns, it is found that all the samples
26
M. Ito et al. / Journal of Alloys and Compounds 420 (2006) 25–28 Table 1 Sample characteristics and physical properties of Ti and TiH1.75
Crystal system Lattice parameters at room temperature (nm) Sample bulk density [g cm−3 ] [%T.D.] Electrical conductivity at 323 K (−1 m−1 ) Seebeck coefficient at 323 K (V K−1 ) Thermal conductivity at 323 K (W m−1 K−1 )
Ti
TiH1.75
a
hcp A3 (␣) 0.2950
fcc C1 (␦) 0.4437
c
0.4684
–
σ
4.50 99.9 1.59 × 106
3.82 99.9 1.16 × 106
S
7.19
−0.07
κ
21.8
19.7
observed that the conductivity σ of the hydride is independent of the hydrogen content, σ of the titanium hydride is expressed as follows: 292.2 σ[×106 −1 m−1 ] = 0.378 + (1) T
Fig. 1. X-ray diffraction patterns of Ti, TiH1.75 , and TiH1.53 .
obtained in the present study have a fluorite-type structured fcc C1(␦-TiH2−X ) single phase or a ␣ + ␦ biphasic material. Fig. 2 shows the crystal structure of ␦-phase titanium hydride. The sample characteristics are summarized in Table 1. It is also found that the geometrical density of titanium hydride, which is determined from its weight and dimensional measurements, is approximately equal to the theoretical density, which is determined from the lattice parameter. Therefore, voids are assumed to be scarcely present in the sample. Fig. 3(a) shows the temperature dependence of electrical conductivity σ of titanium metal and its hydrides. For titanium hydride, σ is slightly lower than that of the metal and titanium hydride exhibits metal-like temperature dependence. Since it is
Fig. 2. Crystal structure of ␦-phase titanium hydride TiH2 .
Fig. 3. Electrical properties of Ti, TiH1.53 , TiH1.66 , and TiH1.75 : (a) electrical conductivity, (b) Seebeck coefficient, and (c) power factor.
M. Ito et al. / Journal of Alloys and Compounds 420 (2006) 25–28
One of the causes for these differences between the metal and its hydride is the decrease in the number of the electrons as carriers because some free electrons are utilized in the metalhydrogen bond. Since the hydrogen effect on electrical conductivity of the Ti–H system is similar to that of the Zr–H system [7], it is assumed that group IV metal hydrides have unique electrical properties. Fig. 3(b) shows the temperature dependence of the Seebeck coefficient S of titanium metal and its hydrides. For the Ti–H system, S changes from positive to negative due to hydrogenation and its absolute value for titanium hydride remains at a low magnitude; this is typical for metallic materials. However, TiH1.53 shows a relatively higher S, which might be due to the influence of ␣ + ␦ two-phase mixture or lesser hydrogen content. Fig. 3(c) shows the temperature dependence of the power factor of titanium metal and its hydrides. It is found that the power factor of the hydrides increases with increasing temperature, although that of the metal decreases with increasing temperature. Further, it is found that the power factor of TiH1.53 , which has an ␣ + ␦ biphasic phase, is five times as large as that of the metal, whereas the power factor of the hydrides, which have a ␦-single phase, are comparable in magnitude to that of the metal. Since the electrical properties are also closely related to the carrier state, experimental and theoretical evidence of the mobility of carriers is required for a detailed explanation of the electrical properties of titanium hydride. The thermal conductivity κ was derived from the product of thermal diffusivity α, heat capacity CP , and density ρ: κ = αCP ρ
(2)
Fig. 4(a) shows the temperature dependence of the estimated thermal conductivity of titanium metal and its hydrides. The thermal conductivity κ of the hydride is the same as that of the metal and it slightly increases with increasing temperature. The conductivity κ of TiH1.53 is slightly lower than those of the other hydrides due to the scattering of the heat carriers by the phase boundary. The total thermal conductivity κtotal can be approximately represented as the sum of the electron (κel ) and phonon (κph ) components as follows: κtotal = κel + κph
Fig. 4. (a) Temperature dependence of the thermal conductivities of Ti, TiH1.53 , TiH1.66 , and TiH1.75 ; (b) contributions of electrons and phonons to thermal conductivity κ in TiH1.75 .
4. Conclusion The electrical conductivity of titanium hydride was slightly lower than that of titanium metal and almost independent of the hydrogen content, exhibiting metal-like temperature dependence. There was no marked change in the electrical conductivities of the hydrides with a change in the hydrogen content. The Seebeck coefficient for the Ti–H system changed from positive to negative due to hydrogenation, and its absolute value for titanium hydride remained at low magnitude, which is typical for metallic materials. The thermal conductivity of the hydride was the same as that of the metal and it slightly increased with increasing temperature. Both electron and phonon contributions played an important role in the thermal conductivity of titanium hydride.
(3)
The electron contribution was estimated with the use of the Wiedemann–Franz relationship expressed as κel = σLT
27
(4)
where L, σ and T are the Lorentz number, electrical conductivity, and absolute temperature, respectively. The phonon contribution κph was estimated by subtracting the electron contribution κel from the measured thermal conductivity κtotal . Fig. 4(b) shows the contributions of electrons and phonons to the thermal conductivity κ in TiH1.75 . It is found that both κel and κph , which have the same values, play an important role in the thermal conductivity of titanium hydride.
Acknowledgements We would like to express our special thanks to Research Associate Dr. B. Tsuchiya for advice on suggesting the development of the fabrication of flaw-free bulk titanium hydride. This study is carried out within the framework of the proposed public research by the Japan Nuclear Energy Safety Organization (JNES)—an incorporated administrative agency. References [1] M. Yamawaki, H. Suwarno, T. Yamamoto, T. Sanda, K. Fujimura, K. Kawashima, K. Konashi, J. Alloys Compd. 271–273 (1998) 530.
28
M. Ito et al. / Journal of Alloys and Compounds 420 (2006) 25–28
[2] K. Ishiguro, K. Kawamura, H. Sonobe, M. Nodaka, T. Arai, Y. Yusa, N. Tsunoda, Nucl. Eng. Des. 116 (1) (1989) 61. [3] D. Hardie, S. Ouyang, Corros. Sci. 41 (1999) 155. [4] H.G. Nelson, in: A.W. Thompson, N.R. Moody (Eds.), Hydrogen Effects in Materials, TMS, Warrendale, PA, 1996, p. 699. [5] H.L. Yakel Jr., Acta Cryst. 11 (1958) 46. [6] S. Yamanaka, K. Yamada, K. Kurosaki, M. Uno, K. Takeda, H. Anada, T. Matsuda, S. Kobayashi, J. Nucl. Mater. 294 (2001) 94. [7] S. Yamanaka, K. Yamada, K. Kurosaki, M. Uno, K. Takeda, H. Anada, T. Matsuda, S. Kobayashi, J. Alloys Compd. 330–332 (2002) 99.
[8] S. Yamanaka, M. Kuroda, D. Setoyama, Trans. Atom. Energ. Soc. Jpn. 1 (4) (2002) 323 (in Japanese). [9] S. Yamanaka, T. Tanaka, M. Miyake, J. Nucl. Mater. 167 (1989) 231. [10] D. Setoyama, J. Matsunaga, H. Muta, M. Uno, S. Yamanaka, J. Alloys Compd. 381 (2004) 215. [11] JCPDS card, 25-982 (TiH1.92 ). [12] JCPDS card, 44-1294 (Ti).
Electrical and thermal properties of titanium hydrides Masato Ito ∗ , Daigo Setoyama, Junji Matsunaga, Hiroaki Muta, Ken Kurosaki, Masayoshi Uno, Shinsuke Yamanaka Division of Sustainable Energy and Environmental Engineering, Graduate School of Engineering, Osaka University, 2-1 Yamada-oka, Suita, Osaka 565-0871, Japan Received 27 September 2005; received in revised form 19 October 2005; accepted 20 October 2005 Available online 28 November 2005
Abstract Various shapes of flaw-free bulk titanium hydrides (TiHX : X = 1.53–1.75) were fabricated, and its electrical and thermal properties were studied. The electrical conductivity of titanium hydride was slightly lower than that of titanium metal, and it exhibited metal-like temperature dependence. The Seebeck coefficient of the Ti–H system changed from positive to negative due to hydrogenation, and its absolute value for titanium hydride remained at a low magnitude, which is typical for metallic materials. The thermal conductivity of the hydride was the same as that of the metal and increased slightly with increasing temperature. © 2005 Elsevier B.V. All rights reserved. Keywords: Metal hydrides; Electrical transport; Thermoelectric; Heat conduction
1. Introduction
2. Experimental procedure
Titanium and its alloys are useful materials in various kinds of plants because of their superior specific strength and resistance to corrosion. Therefore, they have been extensively utilized and expected for use in heat exchangers, steam condenser tubes, irradiation targets for transmuting radioactive wastes, and overpacks for geological disposal of high-level radioactive wastes [1,2]. On the other hand, titanium is known to absorb large quantities of hydrogen. This hydrogenation transforms titanium from metal (hcp A3, ␣-phase) to hydride (fcc C1, ␦-phase), and the precipitation of the hydride results in embrittlement. Therefore, the influence of the hydride has been keenly observed [3–5]; however, the information regarding the properties of titanium hydride is inadequate. Since a high reliability of the materials is required under various severe conditions, it is necessary to elucidate the properties of titanium hydrides with various hydrogen contents. Yamanaka et al. reported that the characteristics of zirconium changed considerably by hydrogenation [6–8]. Therefore, the properties of titanium hydrides are expected to be rather abnormal. In the present study, the electrical and thermal properties of titanium hydrides were studied.
In the present study, bulk titanium hydrides were fabricated directly from α titanium polycrystalline rods (Ø 10 mm × 5 mm t and Ø 5 mm × 15 mm t) with 99.9 wt.% purity. Hydrogenation was executed using a modified Sieverts’ UHV apparatus under a highly pure (7N) hydrogen gas atmosphere with an appropriate pressure. The authors have succeeded in fabricating flaw-free bulk titanium hydrides by regulating their hydrogen absorption rate. The details of this apparatus and the hydrogenation process are described in our previous papers [9,10]. X-ray diffraction measurement was performed at room temperature using Cu K␣ radiation (RINT-2000/PC, Rigaku Corp.). The crystal structure of the sample was analyzed and the lattice parameter was evaluated using this measurement. The hydrogen content was measured using a hydrogen analyzer (HORIBA, EMGA-621). The electrical conductivity was measured with a standard fourprobe DC analysis (ULVAC Inc., ZEM-1) in the temperature range from 323 to 773 K under He atmosphere. The thermal diffusivity was measured by a laser flash method (ULVAC Inc., TC-7000) at temperatures ranging from room temperature to 623 K in vacuum. The heat capacity was measured by a differential scanning calorimeter (ULVAC-RIKO Inc., Triple-cell DSC) in a temperature range from 323 to 623 K under a He flow. The thermal conductivity was estimated by employing the thermal diffusivity, heat capacity, and density.
∗
Corresponding author. Tel.: +81 6 6879 7905; fax: +81 6 6879 7889. E-mail address: [email protected] (M. Ito).
0925-8388/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2005.10.032
3. Results and discussion The hydrogen content of the prepared sample is in the range from 1.53 to 1.75 in atomic ratio [H/Ti]. Fig. 1 shows the X-ray diffraction patterns of titanium hydride employing the literature data [11,12]. From these patterns, it is found that all the samples
26
M. Ito et al. / Journal of Alloys and Compounds 420 (2006) 25–28 Table 1 Sample characteristics and physical properties of Ti and TiH1.75
Crystal system Lattice parameters at room temperature (nm) Sample bulk density [g cm−3 ] [%T.D.] Electrical conductivity at 323 K (−1 m−1 ) Seebeck coefficient at 323 K (V K−1 ) Thermal conductivity at 323 K (W m−1 K−1 )
Ti
TiH1.75
a
hcp A3 (␣) 0.2950
fcc C1 (␦) 0.4437
c
0.4684
–
σ
4.50 99.9 1.59 × 106
3.82 99.9 1.16 × 106
S
7.19
−0.07
κ
21.8
19.7
observed that the conductivity σ of the hydride is independent of the hydrogen content, σ of the titanium hydride is expressed as follows: 292.2 σ[×106 −1 m−1 ] = 0.378 + (1) T
Fig. 1. X-ray diffraction patterns of Ti, TiH1.75 , and TiH1.53 .
obtained in the present study have a fluorite-type structured fcc C1(␦-TiH2−X ) single phase or a ␣ + ␦ biphasic material. Fig. 2 shows the crystal structure of ␦-phase titanium hydride. The sample characteristics are summarized in Table 1. It is also found that the geometrical density of titanium hydride, which is determined from its weight and dimensional measurements, is approximately equal to the theoretical density, which is determined from the lattice parameter. Therefore, voids are assumed to be scarcely present in the sample. Fig. 3(a) shows the temperature dependence of electrical conductivity σ of titanium metal and its hydrides. For titanium hydride, σ is slightly lower than that of the metal and titanium hydride exhibits metal-like temperature dependence. Since it is
Fig. 2. Crystal structure of ␦-phase titanium hydride TiH2 .
Fig. 3. Electrical properties of Ti, TiH1.53 , TiH1.66 , and TiH1.75 : (a) electrical conductivity, (b) Seebeck coefficient, and (c) power factor.
M. Ito et al. / Journal of Alloys and Compounds 420 (2006) 25–28
One of the causes for these differences between the metal and its hydride is the decrease in the number of the electrons as carriers because some free electrons are utilized in the metalhydrogen bond. Since the hydrogen effect on electrical conductivity of the Ti–H system is similar to that of the Zr–H system [7], it is assumed that group IV metal hydrides have unique electrical properties. Fig. 3(b) shows the temperature dependence of the Seebeck coefficient S of titanium metal and its hydrides. For the Ti–H system, S changes from positive to negative due to hydrogenation and its absolute value for titanium hydride remains at a low magnitude; this is typical for metallic materials. However, TiH1.53 shows a relatively higher S, which might be due to the influence of ␣ + ␦ two-phase mixture or lesser hydrogen content. Fig. 3(c) shows the temperature dependence of the power factor of titanium metal and its hydrides. It is found that the power factor of the hydrides increases with increasing temperature, although that of the metal decreases with increasing temperature. Further, it is found that the power factor of TiH1.53 , which has an ␣ + ␦ biphasic phase, is five times as large as that of the metal, whereas the power factor of the hydrides, which have a ␦-single phase, are comparable in magnitude to that of the metal. Since the electrical properties are also closely related to the carrier state, experimental and theoretical evidence of the mobility of carriers is required for a detailed explanation of the electrical properties of titanium hydride. The thermal conductivity κ was derived from the product of thermal diffusivity α, heat capacity CP , and density ρ: κ = αCP ρ
(2)
Fig. 4(a) shows the temperature dependence of the estimated thermal conductivity of titanium metal and its hydrides. The thermal conductivity κ of the hydride is the same as that of the metal and it slightly increases with increasing temperature. The conductivity κ of TiH1.53 is slightly lower than those of the other hydrides due to the scattering of the heat carriers by the phase boundary. The total thermal conductivity κtotal can be approximately represented as the sum of the electron (κel ) and phonon (κph ) components as follows: κtotal = κel + κph
Fig. 4. (a) Temperature dependence of the thermal conductivities of Ti, TiH1.53 , TiH1.66 , and TiH1.75 ; (b) contributions of electrons and phonons to thermal conductivity κ in TiH1.75 .
4. Conclusion The electrical conductivity of titanium hydride was slightly lower than that of titanium metal and almost independent of the hydrogen content, exhibiting metal-like temperature dependence. There was no marked change in the electrical conductivities of the hydrides with a change in the hydrogen content. The Seebeck coefficient for the Ti–H system changed from positive to negative due to hydrogenation, and its absolute value for titanium hydride remained at low magnitude, which is typical for metallic materials. The thermal conductivity of the hydride was the same as that of the metal and it slightly increased with increasing temperature. Both electron and phonon contributions played an important role in the thermal conductivity of titanium hydride.
(3)
The electron contribution was estimated with the use of the Wiedemann–Franz relationship expressed as κel = σLT
27
(4)
where L, σ and T are the Lorentz number, electrical conductivity, and absolute temperature, respectively. The phonon contribution κph was estimated by subtracting the electron contribution κel from the measured thermal conductivity κtotal . Fig. 4(b) shows the contributions of electrons and phonons to the thermal conductivity κ in TiH1.75 . It is found that both κel and κph , which have the same values, play an important role in the thermal conductivity of titanium hydride.
Acknowledgements We would like to express our special thanks to Research Associate Dr. B. Tsuchiya for advice on suggesting the development of the fabrication of flaw-free bulk titanium hydride. This study is carried out within the framework of the proposed public research by the Japan Nuclear Energy Safety Organization (JNES)—an incorporated administrative agency. References [1] M. Yamawaki, H. Suwarno, T. Yamamoto, T. Sanda, K. Fujimura, K. Kawashima, K. Konashi, J. Alloys Compd. 271–273 (1998) 530.
28
M. Ito et al. / Journal of Alloys and Compounds 420 (2006) 25–28
[2] K. Ishiguro, K. Kawamura, H. Sonobe, M. Nodaka, T. Arai, Y. Yusa, N. Tsunoda, Nucl. Eng. Des. 116 (1) (1989) 61. [3] D. Hardie, S. Ouyang, Corros. Sci. 41 (1999) 155. [4] H.G. Nelson, in: A.W. Thompson, N.R. Moody (Eds.), Hydrogen Effects in Materials, TMS, Warrendale, PA, 1996, p. 699. [5] H.L. Yakel Jr., Acta Cryst. 11 (1958) 46. [6] S. Yamanaka, K. Yamada, K. Kurosaki, M. Uno, K. Takeda, H. Anada, T. Matsuda, S. Kobayashi, J. Nucl. Mater. 294 (2001) 94. [7] S. Yamanaka, K. Yamada, K. Kurosaki, M. Uno, K. Takeda, H. Anada, T. Matsuda, S. Kobayashi, J. Alloys Compd. 330–332 (2002) 99.
[8] S. Yamanaka, M. Kuroda, D. Setoyama, Trans. Atom. Energ. Soc. Jpn. 1 (4) (2002) 323 (in Japanese). [9] S. Yamanaka, T. Tanaka, M. Miyake, J. Nucl. Mater. 167 (1989) 231. [10] D. Setoyama, J. Matsunaga, H. Muta, M. Uno, S. Yamanaka, J. Alloys Compd. 381 (2004) 215. [11] JCPDS card, 25-982 (TiH1.92 ). [12] JCPDS card, 44-1294 (Ti).