Thermophysical properties of layered rare earth copper oxides

Thermophysical properties of layered rare earth copper oxides

Journal of Alloys and Compounds 349 (2003) 269–272 L www.elsevier.com / locate / jallcom Thermophysical properties of layered rare earth copper oxi...

290KB Sizes 0 Downloads 20 Views

Journal of Alloys and Compounds 349 (2003) 269–272

L

www.elsevier.com / locate / jallcom

Thermophysical properties of layered rare earth copper oxides Shinsuke Yamanaka, Hirokazu Kobayashi, Ken Kurosaki* Department of Nuclear Engineering, Graduate School of Engineering, Osaka University, Yamadaoka 2 -1, Suita, Osaka 565 -0871, Japan Received 28 May 2002; received in revised form 17 June 2002; accepted 17 June 2002

Abstract The series of layered rare earth copper oxides with RE 2 CuO 4 type formula, where RE is neodymium, samarium, or gadolinium, has been prepared and various thermophysical properties, such as elastic moduli, Debye temperature, and thermal conductivity, have been evaluated. The relationships between several properties of RE 2 CuO 4 have also been studied. It was found that RE 2 CuO 4 have relatively high thermal conductivity at around room temperature, which decreases with increasing temperature. Gd 2 CuO 4 has the highest thermal conductivity compared with those of Nd 2 CuO 4 and Sm 2 CuO 4 in the whole temperature range, and the value at 330 K of Gd 2 CuO 4 is 30.0 W m 21 K 21 .  2002 Elsevier Science B.V. All rights reserved. Keywords: Superconductors; Powder metallurgy; Electrical transport; Heat conduction; Elasticity

1. Introduction Layered rare earth copper oxides with RE 2 CuO 4 type formula have been known as high temperature superconductors [1,2]. The compounds have one or more twodimensional (2-D) CuO 2 planes in a unit cell, which affects high temperature superconductivity. Additionally, due to the layered structure, the anisotropy of the conduction is very strong, for example, the electrical conductivity of c-axis direction is much larger than that in the ab-plane direction [3]. Because of their anisotropic electrical properties, the compounds may have a potential as thermoelectric materials. The electrical properties of the compounds below room temperature have been studied in much detail because of their superconductivity, while the thermophysical properties, especially above room temperature, have been scarcely studied. In the present study, therefore, the 2-1-4 type layered rare earth copper oxides, RE 2 CuO 4 where RE is neodymium, samarium, or gadolinium, are selected and the thermophysical properties such as elastic moduli, Debye temperature, and thermal conductivity are evalu-

*Corresponding author. Tel.: 181-6-6879-7905; fax: 181-6-68797889. E-mail address: [email protected] (K. Kurosaki).

ated. Relationships between several properties are also studied.

2. Experimental Sintered samples of RE 2 CuO 4 (RE: Nd, Sm, or Gd) were prepared by mixing stoichiometric amounts of RE 2 O 3 and CuO powders followed by pressing to pellets and heating at 1373 K for 30 h in air. The crystal structure of the samples was analyzed by the powder X-ray diffraction method at room temperature using Cu Ka radiation. For thermophysical properties measurements, appropriate shapes of the samples were cut from the pellets. The density of the samples was calculated from the measured weight and dimensions. The longitudinal and shear sound velocities were measured by an ultrasonic pulse-echo method at room temperature to evaluate the elastic moduli and Debye temperature. Hardness measurements were also performed, at room temperature, using a micro-Vickers hardness tester. In the temperature range from room temperature to about 1000 K, the thermal conductivity k was calculated from the measured thermal diffusivity D, specific heat capacity CP , and measured density d using the following relationship:

k 5 DCP d.

0925-8388 / 02 / $ – see front matter  2002 Elsevier Science B.V. All rights reserved. PII: S0925-8388( 02 )00875-7

S. Yamanaka et al. / Journal of Alloys and Compounds 349 (2003) 269–272

270

were measured at room temperature, and the elastic moduli were evaluated. For isotropic media, the shear modulus G, Young’s modulus E, and bulk modulus K can be written in terms of the longitudinal sound velocity VL and shear sound velocities VS [9] by: G 5 rV S2 , 2

2

Gs3V L 2 4V Sd E 5 ]]]] , sV 2L 2V 2Sd

S

D

4 K 5 r V 2L 2 ]V 2S , 3

Fig. 1. Relationship between lattice parameter and lanthanide ionic radius.

The thermal diffusivity was measured by a laser flash method using ULVAC TC-7000 in vacuum. The heat capacity of RE 2 CuO 4 was evaluated from the Neumann– Kopp law using literature data [4,5] of RE 2 O 3 and CuO.

3. Results and discussion The powder X-ray diffraction pattern at room temperature of the sample shows that single phase RE 2 CuO 4 (RE: Nd, Sm, or Gd) is obtained in the present study. The lattice parameter and X-ray density were obtained from the X-ray diffraction analysis. The lattice parameters agree well with literature data [6,7]. The bulk densities of the samples were 80–85% of the theoretical densities. The lattice parameters are plotted in Fig. 1, as a function of the ionic radius for RE 31 in eight-coordination reported by Shannon [8]. The lattice parameters increase smoothly with increasing lanthanide ionic radius, as might be expected. The sample characterization of RE 2 CuO 4 is shown in Table 1. The longitudinal and shear sound velocities of RE 2 CuO 4

where r is the sample density. In the present study, however, VS could not be measured, probably due to appreciable effects of the internal friction. Therefore, the following relationship is assumed, and Young’s and shear moduli were evaluated: 1 VS 5 ] Œ]3 VL . The values of the elastic moduli calculated from the sound velocities are shown in Table 1. For some substances, Young’s modulus is proportional to q(RT m /Vm ) [10], where q is the number of atoms in the chemical formula, T m is the melting temperature in K, Vm is the molar volume, and R is the gas constant. For pure metals, we have obtained the following relationship [11]:

S D

RT m E 5 97.9 ? q ]] , Vm which is in good agreement with the results of Frost and Ashby [10]. In the present study, the melting temperature of RE 2 CuO 4 is referred to literature data [12–14]. A comparison of Young’s modulus is made between RE 2 CuO 4 and other substances in Fig. 2, indicating that the proportionality between E and q(RT m /Vm ) of RE 2 CuO 4

Table 1 Sample characterization and thermophysical properties of RE 2 CuO 4

Lanthanide ionic radius (nm) [8] Lattice parameters (nm) Bulk density (%T.D.) Melting temperature (K) [20–22] Shear modulus (GPa) Young’s modulus (GPa) Bulk modulus (GPa) Vickers hardness (GPa) Debye temperature (K) Thermal conductivity at 330 K (W m 21 K 21 )

Nd 2 CuO 4

Sm 2 CuO 4

Gd 2 CuO 4

0.111

0.108

0.105

Tm

0.394 1.217 79.6 1543

0.392 1.197 86.4 1493

0.389 1.189 85.0 1458

G E K HV uD k330

30.8 77.1 51.4 1.49 324.7 23.1

42.0 105.0 70.0 3.18 354.0 20.4

38.8 97.0 64.7 2.74 334.9 30.0

a c

Fig. 2. Relationship between Young’s modulus E and q(RT m /Vm ).

S. Yamanaka et al. / Journal of Alloys and Compounds 349 (2003) 269–272

271

is almost identical with that of other oxides such as UO 2 and ThO 2 . The relatively low value of Young’s modulus of Nd 2 CuO 4 compared with those of Sm 2 CuO 4 and Gd 2 CuO 4 is probably due to its low bulk density. Vickers microhardness measurements were repeated 12 times for a sample, and the applied load and loading time were chosen to be 1.96 N and 30 s. The hardness values obtained for RE 2 CuO 4 are shown in Table 1. For some oxide and carbide ceramics, the Vickers hardness HV was proportional to Young’s modulus E with the values of [15]: HV ] ¯ 0.05. E We have evaluated the HV /E ratio for pure metals using literature data [16] and obtained the values of 0.006, 0.003 and 0.004 for bcc, fcc and hcp metals, respectively [11]. The value of HV of RE 2 CuO 4 is plotted in Fig. 3 as a function of Young’s modulus E together with data of other substances [17]. As shown in this figure, the value of HV /E ratio for RE 2 CuO 4 is around 0.03, which is equal to that of UO 2 . The Debye temperature uD of RE 2 CuO 4 can be calculated from the sound velocities and lattice parameters. The Debye temperature uD is related to the longitudinal and shear sound velocities [18] as follows:

S DF

h 9N uD 5 ] ? ]]]]]] 23 kB 4pV ?sV L 1 2V S23d

G

1 ] 3

,

where h is the Plank constant, k B is the Boltzmann constant, N is the number of atoms in a unit cell, and V is the unit cell volume. The values of uD of RE 2 CuO 4 evaluated in the present study are shown in Table 1. It is known that the Debye temperature uD can be related to the melting temperature T m in K, the molar mass M and

Fig. 3. Relationship between Vickers hardness HV and Young’s modulus E.

Fig. 4. Relationship between Debye temperature uD (MV 2m/ 3 ))1 / 2 .

and q(T m /

the molar volume Vm by the Lindemann relationship [19]. The relationships were reexamined for perovskite type 2/3 1/2 oxides, and the ratio of uD to q 5 / 6 (T m /(MV m )) was evaluated to be 1.60 [20], where q is the number of atoms in the chemical formulas. Fig. 4 shows this relationship for RE 2 CuO 4 , together with other substances data [20–22]. The proportionality constant of RE 2 CuO 4 differs from those of both perovskite type oxides and SiO 2 glass, and is evaluated to be 1.35. The thermal conductivities at 330 K k330 of RE 2 CuO 4 are shown in Table 1, in which the thermal conductivities were corrected to 100% of the theoretical density by using the Loeb equation [23]. The temperature dependence of the thermal conductivity k of RE 2 CuO 4 is shown in Fig. 5. As can be seen in the figure, the thermal conductivities decrease gradually with increasing temperature, showing phonon conduction. It was found that RE 2 CuO 4 have relatively high thermal conductivity at around room tem-

Fig. 5. Temperature dependence of thermal conductivity k.

272

S. Yamanaka et al. / Journal of Alloys and Compounds 349 (2003) 269–272

perature and the thermal conductivity of Gd 2 CuO 4 is higher than those of Nd 2 CuO 4 and Sm 2 CuO 4 in the whole temperature range. As mentioned above, RE 2 CuO 4 -type oxides are considered to have a potential as thermoelectric materials because of its anisotropic electrical properties. Now we are studying the thermoelectric properties, viz. electrical resistivity and Seebeck coefficient, and evaluating the potential for thermoelectric applications of the RE 2 CuO 4 type oxides. These results will be reported in near future.

4. Conclusion In the present study, the thermophysical properties of RE 2 CuO 4 (RE: Nd, Gd, or Sm) are measured and the relationships between several properties are studied. The relationship between the hardness and Young’s modulus of RE 2 CuO 4 show ceramic characteristics. The proportionality between Young’s modulus and q(RT m /Vm ) of RE 2 CuO 4 is almost identical with that of other oxides such as UO 2 and ThO 2 . The Lindemann relationship of RE 2 CuO 4 differs from those of both perovskite type oxides and SiO 2 glass, and the ratio of uD to q 5 / 6 (T m / (MV 2m/ 3 ))1 / 2 is evaluated to be 1.35. It was found that RE 2 CuO 4 have relatively high thermal conductivity at around room temperature. The thermal conductivity of RE 2 CuO 4 decreases gradually with increasing temperature, showing that the phonon conduction is predominant.

References [1] Y. Tokura, H. Takagi, S. Uchida, Nature 337 (1989) 345. [2] H. Takagi, S. Uchida, Y. Tokura, Phys. Rev. Lett. 62 (1989) 1197.

[3] M. Sera, S. Shamoto, M. Sato, Solid State Commun. 68 (1988) 649. [4] Japan Thermal Measurement Society, Thermodynamics database for personal computer MALT2. [5] The SGTE Pure Substance and Solution databases, GTT-DATA SERVICES. [6] P. Adelmann, R. Ahrens, G. Czjzek, G. Roth, H. Schmidt, C. Steinleitner, Phys. Rev. B 46 (1992) 3619. [7] V.A. Polyakov, I.A. Zobkalo, D. Petitgrand, P. Bourges, L. Boudarene, S.N. Barilo, D.N. Zhigunov, A.G. Gukasov, Solid State Commun. 95 (1995) 533. [8] R.D. Shannon, Acta Crystallogr. Sect. A 32 (1976) 751. [9] Thermophysical Properties Handbook, Nippon Netu Bussei Gakkai, Youkendou, Tokyo, 1990. [10] H.J. Frost, M.F. Ashby, in: Deformation-Mechanism Maps, Pergamon Press, Oxford, 1982. [11] S. Yamanaka, K. Yamada, T. Tsuzuki, T. Iguchi, M. Katsura, Y. Hoshino, W. Saiki, J. Alloys Comp. 271–273 (1998) 549. [12] H. Kojima, T. Watanabe, N. Komai, I. Tanaka, Mol. Cryst. Liq. Cryst. 184 (1990) 69. [13] H. Takeda, M. Okuno, M. Ohgaki, K. Yamashita, T. Matsumoto, J. Mater. Res. 15 (2000) 1905. [14] W. Zhang, K. Osamura, Metal. Trans. B 22B (1991) 705. [15] K. Tanaka, H. Koguchi, T. Mura, Int. J. Eng. Sci. 27 (1989) 11. [16] S. Yamamoto, T. Tanabe, in: Atarashii Zairyoukagaku, Showadou, Kyoto, 1990. [17] K. Yamada, S. Yamanaka, M. Katsura, J. Alloys Comp. 275–277 (1998) 725. [18] H. Inaba, T. Yamamoto, Netsu Sokutei 10 (1983) 132. [19] F.A. Lindemann, Phys. Z. 14 (1910) 609. [20] The Chemical Society of Japan (Eds.), Kikan Kagaku Sousetsu, Perovskite-Related Comp. 32 (1997) 37–51. [21] K. Yamada, S. Yamanaka, T. Nakagawa, M. Uno, M. Katsura, J. Nucl. Mater. 247 (1997) 289. [22] A. Bartolotta, G. Carini, G. D’Angelo, A. Fontana, F. Rossi, G. Tripodo, J. Non-Cryst. Solids 245 (1999) 9. [23] A.L. Loeb, J. Am. Ceram. Soc. 37 (1954) 96.