High-temperature thermoelectric performance of (W1−xTix)18O49. Issue 4 (April 2022)

• Record Type: Journal Article
• Authors: Tran, Nhat Quang Minh; Ohtaki, Michitaka; Suekuni, Koichiro
• Is Part Of: Journal of the European Ceramic Society. Volume 42:Issue 4(2022)
• Page Start: 1486
• Page End: 1492
• Publication Date: 2022-04
• Subjects: Thermal conductivity -- Electrical conductivity -- Transitional metal oxides -- Spark plasma sintering -- Thermoelectricity; Ceramic materials -- Periodicals; Composite materials -- Periodicals; Matériaux céramiques -- Périodiques; Composites -- Périodiques; Ceramic materials; Composite materials; Periodicals; Electronic journals; 666.05
• DOI: 10.1016/j.jeurceramsoc.2021.12.014
• Languages: English
• ISSNs: 0955-2219

Abstract

Highlights: Samples of (W1− x Ti x )18 O49 ( x = 0, 0.03, 0.05, 0.1, 0.15, 0.2, 0.25) were synthesized by direct reaction of single metal oxides as starting materials (WO3, WO2, and TiO2 rutile form) in vacuum at 1073 K followed by densification via spark plasma sintering at 1423 K. Whereas the undoped W18 O49 sample expressed metallic behavior with large σ and relatively small | S |, t he Ti-substitution at the W site effectively reduced the carrier concentration and improved | S | and hence σ S 2 at high temperature region until x = 0.2. The void tunnels in the crystal structure, expected to act as the naturally formed phonon scattering centers together with the bulky penta-columns, would lead to an intrinsically low κ lat . Moreover, the κ lat values decreased as the amount of the Ti-substitution increased, reaching the lowest value of 0.45 ± 0.40 WK −1 m −1 ( x = 0.1) and 0.47 ± 0.27 WK −1 m −1 ( x = 0.15) at 1073 K. The highest ZT = 0.50 ± 0.07 at 1073 K was attained for the sample with x = 2, and ZT = 0.20 ± 0.03 at 1073 K ( x = 0.1) for the samples without the secondary phase. The solubility limit for the Ti dopant falls between x = 0.1 and x = 0.15, which limits the performance enhancement by the carrier concentration tuning via the Ti-doping for the intrinsically over-doped W18 O49 -based oxide. However, the Jonker-type analysis predicts better thermoelectric performance with the ZT value as high as 0.9 under an assumption that the carrier concentration will be optimized within the single phase region. More suitable dopants or a method to expand the solubility limit of Ti in the oxide is strongly anticipated. Abstract: High-temperature thermoelectric properties of tungsten-based Magnéli phase oxide (W1- x Ti x )18 O49 (0 ≤ x ≤ 0.25) prepared by solid state reaction followed by densification via spark plasma sintering (SPS) were studied. The Ti substitution increased the Seebeck coefficient, the power factor, and decreased both the electronic and lattice thermal conductivity. The synergistic substitution effect on the electrical and thermal properties and inherently low total thermal conductivity of 1.46 ± 0.08 WK −1 m −1 originating from the tunnel-like crystal structure of the oxide led to a significantly high ZT of 0.50 ± 0.07 at 1073 K for the sample with x = 0.2.