Thermoelectric Properties of Highly Textured Ca-doped (ZnO) m In 2 O 3 Ceramics

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1 Speial Issue Proessing Design of Single Crystals and Textured Polyrystals for Advaned Eletroni Devies 37 Researh Report Thermoeletri Properties of Highly Textured Ca-doped (ZnO) m In O 3 Ceramis Hisashi Kaga, Ryoji Asahi, Toshihiko Tani Ca (ZnO) m In O 3 Abstrat Highly textured Ca-doped (ZnO) m In O 3 (m is an integer) eramis were fabriated by the reative templated grain growth (RTGG) method and their thermoeletri properties were examined. Platelike ZnSO 4 3Zn(OH) partiles were used as reative templates and mixed with In O 3 and CaCO 3 powders into a stak of tapes. In situ formation and subsequent sintering resulted in textured Ca-doped (ZnO) m In O 3 eramis. The eletrial ondutivity of the textured speimen along the ab-plane was almost two times larger than that of the textured speimen along the -axis and about 3% larger than that of a nontextured speimen. On the other hand, the Seebek oeffiients of the textured speimen exhibited a small anisotropy. The thermal ondutivity of the RTGG speimen along the ab-plane was higher than that of the RTGG speimen along the -axis. However, both speimens showed similar values at high temperatures. As a result, the Ca-doped speimen along the ab-plane with a omposite phase of (ZnO) 3 In O 3 and (ZnO) 4 In O 3 showed a ZT value of.31 (at 153 K), ompared with.3 (at 153 K) for the nontextured speimen. Keywords Thermoeletri, Layer-strutured, Textured eramis, Zin indium oxide, Ca doping (RTGG ) ab Ca (ZnO) m In O 3 ab ZnSO 4 3Zn(OH) In O 3 CaCO n (ZT)

2 38 1. Introdution Texture engineering is a key tehnology for improving the anisotropi properties of materials. Sine the introdution of the reative templated grain growth (RTGG) method, 1) it has been applied to layered oxide thermoeletri materials suh as Na CoO 4, ) [CaCoO 3 ].6 [CoO ], 3) and (ZnO) 5 (In.97 Y.3 ) O 3. 4) These textured eramis have been proved to show high thermoeletri performane along the layer than randomly oriented eramis mainly due to their high eletrial ondutivity, whih is one of the fators in the thermoeletri dimensionless figure of merit defined as ZT = σ S T/κ, (1) where σ, S, T, and κ are the eletrial ondutivity, Seebek oeffiient, absolute temperature, and thermal ondutivity, respetively. Layer-strutured homologous ompounds (ZnO) m In O 3 (m is an integer, Z m IO) have been extensively studied as andidates for n-type thermoeletri oxide materials. 4-9) Koumoto and oworkers first reported that Z 5 IO had a fairly high figure of merit, whih was then improved with Y substitution for an In site. 5-6) Further enhanement was ahieved by the RTGG method, realizing a ZT value of.33 at 173 K in highly textured eramis. 7) We reently optimized arrier onentrations and strutural fator m with various doping effets for nontextured Z m IO. The obtained ZT value was.3 at 153 K with nontextured Cadoped (ZnO) n In O 3 (n = ZnO/In O 3, 3< n <4), whih was relatively high among n-type oxides. 8) Ca doping effetively dereases thermal ondutivity at high temperatures. In this paper, we report on Ca-doped (ZnO) m In O 3 eramis fabriated by the RTGG method. The resulting textured speimens have shown signifiant improvement in their thermoeletri properties.. Experimental proedure Platelike powder of ZnSO 4 3Zn(OH) (Hakusui Teh Co., Osaka, Japan) with partile sizes of -1 μm and thiknesses of.1-.3 μm was used as the reative template. The ZnSO 4 3Zn(OH) platelets, In O 3, and CaCO 3 (Kojundo Chemial, Saitama, Japan) powders were weighed in speifi proportions to have the omposition of n = (ZnO)/(In.975 Ca.5 ) O 3 ( < n < 4) where the highest thermoeletri performane was realized in the nontextured eramis fabriated by the onventional solid-state reation proess. 8) It should be noted that we will use the term "m" to refer to the homologous phase of Z m IO as distinguished from the n value. They were mixed with a solvent (6% toluene - 4% ethanol, v/v) and binder [poly(vinyl butyral)] in a ball mill for 5 h using zironia balls. A plastiizer (di-n-butyl phthalate) was then added to the slurry and the mixture was milled for another hour. The mixed slurry was tape-asted using a dotor blade to form a sheet with a thikness of about 15 μm, and the obtained sheet was dried in air at room temperature. The sheet was ut, staked, and pressed at 8 O C to form a billet with a thikness of about mm. The billet was ut into a retangular bar perpendiular and parallel to the original sheet plane for use in studying anisotropi transport properties. The speimens were heated at 173 K for 3 min in flowing air to remove organi substanes. After subjeting to old isostati pressing at 94 MPa, dewaxed speimens were synthesized at 115 O C for 1 h and then sintered at 13 O C for 4 h in flowing air (denoted as RTGG speimens). A referene speimen with a random grain orientation was also fabriated by the onventional solid-state sintering methods for omparison (denoted as an speimen). The detailed proedure and transport properties of the speimen are available elsewhere. 8) Crystalline phases were determined by X-ray diffration (XRD, Model RINT-TTR, Rigaku, Tokyo, Japan) analysis with CuKα radiation. The san was arried out in ranging from 5 to 8 O in - theta at a san rate of 4 O /min. For eah pattern, the peaks were indexed using the ard file patterns (JCPDF) of possible reation produts in omparison with the obtained patterns. The degree of orientation was evaluated in terms of the Lotgering fator, F L, using the equations desribed previously. 9) A refletion pole figure of (15) was also used to examine the detailed texture. In this measurement, the inident (θ) and diffration (θ) angles were maintained onstant, and azimuthal (β, O < β <36 O ) sans were arried out around the

3 39 normal diretion (ND), whih was perpendiular to the original sheet plane at various polar angles (α, O < α <75 O ). The degree of orientation for the ND, F ND, was alulated using 3 os α 1 F, () ND = with os α = I ( αβ, )os α sinα dβ dα, 36 I ( αβ, )sinα dβ dα (3) where I is the normalized diffration intensity with the bakground orretion. 1) For the RTGG speimens, thermoeletri properties (eletrial ondutivity, Seebek oeffiient, and thermal ondutivity) were examined on retangular bars parallel (//) and perpendiular ( ) to the original sheet plane, as ompared with the speimen. The relative densities of the speimens were determined by Arhimedes' method. 3. Results and disussion Figure 1 shows the XRD patterns of the Ca-doped RTGG [parallel (//) and perpendiular ( ) to the sheet plane] speimens with n = 3 ompared with the Ca-doped speimen with n = 3.5 that showed the highest figure of merit among the nontextured n-type oxides as previously reported. 8) The (//) speimen exhibited a high diffration intensity from { } planes but a low diffration intensity from other rystal planes, while the diffration lines of the { } planes were hardly observed for the ( ) speimen. The degree of { } orientation, F L, alulated by the Lotgering method for the (//) speimen reahed.94, showing that this speimen obtained by the RTGG method is highly textured. It should be noted that the XRD pattern of the Ca-doped RTGG speimen with n = 3 shows mixed phases of (ZnO) 3 In O 3 and (ZnO) 4 In O 3 and () 111 Intensity (Arb. Units) (b) RTGG( ) (a) RTGG(//) Fig theta XRD patterns of Ca-doped RTGG and speimens: (a) parallel and (b) perpendiular to sheet plane, ompared with () pattern for speimen. Peaks are indexed as (ZnO) 3 In O 3 ( ) and (ZnO) 4 In O 3 ( ) phases. Fig. (15) pole figure of Ca-doped Z 3 IO speimen sintered at 13 O C. Pole densities are normalized using the average density. The enter (α = O ) indiates the normal diretion, and β is measured from the diretion of tape-asting in the RTGG proess.

4 4 orresponds to the XRD pattern of the nontextured Ca-doped speimen with n = 3.5. Figure shows the (15) pole figure obtained from the RTGG (//) speimen. The azimuthal angle β was obtained from the diretion of the tape asting in the RTGG proess. A large peak at the enter in the pole figure indiates that the (15) plane is preferentially aligned along the ND. The degree of orientation, F ND, reahed.84. On the other hand, the irularly distributed (15) plane implies that there is no distint texture on the ab-plane. These results indiate the feasibility of the present RTGG method by whih grain growth follows the morphology of the platelike ZnSO 4 3Zn(OH) templates via Ostwald ripening, and the tape-asting does not indue any uniaxial distortion. Figure 3 shows the temperature dependene of the eletrial ondutivity, σ, for the Ca-doped RTGG [(//) and ( )] and speimens with n = 3 and n = 3.5, respetively. The eletrial ondutivities of the textured speimen exhibited a strong anisotropy: that is, σ (//) was almost twie as large as σ ( ) in a high temperature region. In addition, σ (//) was about 3 % larger than σ of the speimen over the measured temperatures. Note that σ () was not between σ (//) and σ ( ) in a low temperature region as usually expeted for anisotropi rystal systems. The reason for this is that large grains, whih are well aligned as a result of the topotati reation in the RTGG method, enhane the ondutivities in both // and diretions. For the high-temperature region, σ ( ) was dereased by the ontribution of the interfaial eletron phonon sattering in the layered struture. Figure 4 shows the Seebek oeffiient, S, as a funtion of temperature. The Seebek oeffiient exhibited a small anisotropy, i.e., S(//) was ~1.1 S( ). This result suggests that the Seebek oeffiient is rather insensitive to the type of mirostruture. The thermal ondutivities, κ, of three speimens as funtions of temperature are shown in Fig. 5. The thermal ondutivities of all speimens dereased as temperature inreased. The speifi heat apaity (C p ) inreased but the thermal diffusivity (D T ) dereased further, leading to a derease in thermal ondutivity (κ = ρ C P D T, ρ being the density) with an inrease in temperature. The eletrial ontribution was alulated using the Wiedeman-Franz law. The eletrial part was only about 1 % of the total thermal ondutivity and slightly inreased at high temperatures. κ ( ) was lower than κ (//) over the measured temperature, indiating interfaial phonon-phonon sattering that makes the phonon mean free path in κ ( ) shorter than that in κ (//). The thermoeletri dimensionless figure of merit was alulated and is shown in Fig. 6. The RTGG (//) speimen, in whih eletrial and thermal fluxes are presumed to be parallel to the preferential.6-6 [log(s/m)] Eletrial ondutivity, σ RTGG(//) RTGG( ) Seebek oeffiient, S ( V/K) RTGG(//) RTGG( ) Fig. 3 Temperature dependene of eletrial ondutivity of Ca-doped textured [(// ) and ( )] and speimens. Fig. 4 Temperature dependene of Seebek oeffiient of Ca-doped textured [(//) and ( )] and speimens.

5 41 ab-plane, exhibits higher ZT values than the RTGG( ) speimen beause the anisotropy observed in σ exeeds that of κ. The dimensionless figure of merit of the RTGG(//) speimen inreased by about 3% ompared with that between the speimens of the speimen, mainly due to the differene in eletrial ondutivity. As a result, the Ca-doped textured speimen with n = 3 shows a ZT value of.31 at 153 K. (W/mK) Thermal ondutivity, κ Fig. 5 ZT Fig κ e, RTGG(//) κ, RTGG( ) e κ e, RTGG (//) κ, RTGG(//) κ, RTGG( ) κ, Temperature dependene of total thermal ondutivity (κ) of Ca-doped textured [(//) and ( )] and speimens. The eletroni ontribution (κ e ) was alulated using the Weidemann-Franz law Temperature dependene of dimensionless figure of merit of Ca-doped textured and speimens. 4. Conlusions Highly textured Ca-doped (ZnO) m In O 3 eramis were fabriated by the RTGG method using platelike ZnSO 4 3Zn(OH) partiles as reative templates. The XRD results showed a highly textured omposite phase of Z 3 IO and Z 4 IO for a designed molar ratio of (ZnO)/(In.975 Ca.5 )O 3 = 3. The textured speimen exhibited a higher in-plane eletrial ondutivity than the nontextured speimen, leading to a higher figure of merit. The textured Ca-doped speimen showed a ZT value of.31 (at 153 K), whih is one of the best values in n-type oxides. Aknowledgments The work was partly arried out as a joint researh and development projet with the International Center for Environment Tehnology Transfer in -3, ommissioned by the Ministry of Eonomy, Trade and Industry. Referenes 1) Tani, T. : J. Korean Phys. So., 3(1998), S117 ) Tajima, S., Tani, T., Isobe, S. and Koumoto, K. : Mater. Si. Eng. B, 86(1), 3) Itahara, H., Xia, C., Seno, Y., Sugiyama, J., Tani, T. and Koumoto, K. : Pro. nd Int. Conf. Thermoeletris, La Grande-Motte, Frane, (3), 188 4) Hiramatsu, H., Ohta, H., Seo, W. -S. and Koumoto, K. : J. Jpn. So. Powder Powder Metallurg., 44(1997), 44 5) Ohta, H., Seo, W.-S. and Koumoto, K. : J. Am. Ceram. So., 79(1996), 193 6) Kazeoka, M., Hiramatsu, H., Seo, W. -S. and Koumoto, K. : J. Mater. Res., 13(1998), 53 7) Isobe, S., Tani, T., Masuda, Y., Seo, W. -S. and Koumoto, K. : Jpn. J. Appl. Phys., 41(), 731 8) Kaga, H., Asahi, R. and Tani, T. : Jpn. J. Appl. Phys., 43(4), 354 9) Tani, T., Isobe, S., Seo, W. -S. and Koumoto, K. : J. Mater. Chem., 11(1), 34 1 Hermans, P. H. and Platzek, P. : Kolloid-Z, 88(1939) 68 11) Grimvall, G. : Thermophysial Properties of Materials, (1999), 59, Enlarged and revised ed., North-Holland, Amsterdam Reprinted with permission from The Japan Soiety of Applied Physis (Jpn. J. Appl. Phys., 43-1(4), ).

6 4 Hisashi Kaga Researh fields : Thermoeletri materials Aademi degree : Ph. D. Aademi soiety : Ceram. So. Jpn., Am. Ceram. So. Awards : World Young Fellow Meeting Best Presentation Award, 4 Present affiliation : Advaned Sintering Tehnology Group National Institute of Advaned Industrial Siene and Tehnology (AIST) Ryoji Asahi Researh field : Computational physis and materials design of funtional materials Aademi degree : Ph. D. Aademi soiety : Jpn. So. Appl. Phys., Phys. So. Jpn., Am. Phys. So. Awards : Tehnial Award of Jpn. Fine Ceram. Asso., 3 Toshihiko Tani Researh fields : Synthesis and texture engineering of funtional eramis Aademi degree : Ph. D. Aademi soiety : Ceram. So. Jpn., Am. Ceram. So., Jpn. So. Appl. Phys., Jpn. So. Powder Powder Metallurg., Mater. Res. So. Jpn. Awards : Jpn. So. Powder Powder Metallurg. Award for Innovatory Researh, Ceram. So. Jpn. Award for Aademi Ahievements, 5 Dr. Tani onurrently serves as a professor of Toyota Tehnologial Institute.