Fabrication and thermoelectric properties of fine-grained TiNiSn compounds
Graphical abstract
Nearly single-phased TiNiSn-based half-Heusler compound polycrystalline materials with fine grains were fabricated by combining mechanical alloying (MA) and spark plasma sintering (SPS). A high ZT value for undoped TiNiSn was obtained because of the reduced thermal conductivity.
Introduction
Thermoelectric materials have received great attention in recent years because of their promising applications of electronic refrigeration and power generation. The energy conversion efficiency of a thermoelectric device is mainly determined by the dimensionless figure of merit (ZT) of its corresponding thermoelectric materials. ZT is expressed as ZT=α2T/ρκ, where T is the absolute temperature, α is the Seebeck coefficient or thermopower, ρ is the electrical resistivity, and κ is the thermal conductivity. Good thermoelectric materials should have large ZT values, which require high α but low ρ and κ.
Ternary half-Heusler compounds MNiSn (M=Ti, Zr, Hf) have a cubic MgAgAs-type structure with a space group of . These compounds show promising thermoelectric properties because of their narrow band gap [1], [2], [3], [4], [5], about 0.1–0.2 eV at the Fermi level. These alloys exhibit high Seebeck coefficient and low electrical resistivity. For example, Bhattacharya et al. obtained a high power factor (PF=α2/ρ) up to 6150 μW m−1 K−2 at 650 K in a sample of TiNiSn0.95Sb0.05 [6], showing that the TiNiSn-based half-Heusler alloys have a great potential as a mediate-temperature thermoelectric material. It is well known that undoped MNiSn compounds usually have low ZT values about 0.005–0.01 at room temperature [7]. Through doping, ZT values reach 0.7 and 0.78 for the composition Zr0.5Hf0.5Ni0.8Pd0.2Sn0.99Sb0.01 at 800 K and Ti0.95Hf0.05NiSn0.99Sb0.01 at 770 K as reported by Shen et al. [8] and Kim et al. [9], respectively. Even an extremely high ZT value of 1.5 at 700 K was reported for the composition (Zr0.5Hf0.5)0.5Ti0.5NiSn0.998Sb0.002 [10].
However, a dominant drawback of this material is their high lattice thermal conductivity (κL), which is as high as 10 W/m K at room temperature [7]. Researchers have done lots of work to reduce the thermal conductivity by isoelectronic alloying and atomic substitution. Shen et al. [8] reported the values of κ=2 W/m K at 800 K in a sample of Zr0.5Hf0.5Ni0.5Pd0.5Sn0.99Sb0.01 and κ=3 W/m K in the sample Zr0.5Hf0.5Ni0.8Pd0.2Sn0.99Sb0.01. It is obvious that their compositions are complex. In addition, these materials are usually fabricated by arc melting which often causes compositional segregation. Although the compositional segregation can be reduced by long time annealing, the microstructure usually has coarse grains characteristic of the melting process [11], [12], [13]. Goldsmid et al. [14] presented a concept of reducing room temperature thermal conductivity by grain boundary scattering in the half-Heusler system, in other words, reducing grain size to decrease κL. Bhattacharya et al. [11], [12] also reported that boundary scattering can affect the thermal conductivity of half-Heusler alloys. Some researchers tried a process to grind arc-melted bulks to fine powders then use hot pressing to produce fine-grained materials, but such a route complicated the fabrication process [9], [10].
Recently, the combination of MA and SPS was widely used to synthesize thermoelectric materials, such as Bi2Te3 [15], [16], CoSb3 [17] and AgPbmSbTe2+m systems [18], [19]. This method is useful to avoid compositional segregation and coarse grains. The fine-grained materials obtained in this way are favorable for thermoelectric applications because of the reduced thermal conductivity caused by grain boundary scattering. In addition, MA is also an effective method to fabricate the compound that is composed by elements with significantly different melting points [20]. For TiNiSn compound, the great difference of melting points between Sn and the other two elements increases the processing complexity for the traditional melting techniques. In this work, the combination of MA and SPS was applied to synthesize TiNiSn half-Heusler compounds. The thermoelectric properties were investigated in the temperature range from 300 to 900 K.
Section snippets
Experimental procedure
Starting materials were highly pure powders of titanium (99.99%), nickel (99.99%) and tin (99.99%). The powder mixtures at the determined ratios were milled in a planetary mill (Pulveristte 6, Germany) at 300 rpm under the protection of Ar atmosphere, using zirconia-lined vial and zirconia ball. Obtained powders were sintered by SPS in a graphite die at a heating rate of 100 K/min and with a soaking duration of 10 min.
The phase structures of the MA-treated powders and the SPSed bulk materials were
Results and discussion
Although TiNiSn compound powders were not synthesized directly by MA, it was found that the MA treatment enhanced the reaction of forming TiNiSn compounds during SPS. Fig. 1 shows the XRD patterns of the raw materials, the powder mixture after MA (2.5 h) and the corresponding bulk sample after SPS (1073 K), respectively. In comparison with the powder mixture before MA, the component phases of the powder changed dramatically after MA for 2.5 h. A compound identified as Ni3Sn4 (PDF#65-4310) was
Conclusions
The present study confirmed that the combined process of MA and SPS is applicable to the synthesis of the TiNiSn-based half-Heusler thermoelectric materials. Almost single-phased TiNiSn-based compounds were prepared by taking measures to compensate for the Ti loss during the MA and SPS processes. The resultant materials show dense and uniform microstructure with small grains of about 200–400 nm in diameter. Because of the refined grain sizes compared with the previous arc melting process, the
Acknowledgments
The authors acknowledge financial support from the National Basic Research Program of China (Grant no. 2007CB607500) and Tsinghua-Toyota Collaborative Research Project (no. 0307J36) as well as National Nature Science Foundation (Grants no. 50820145203).
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