Reduction in the Thermal Conductivity of Thermoelectric Titanium Oxide by Introduction of Planar Defects
- PDF / 454,956 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 18 Downloads / 347 Views
1218-Z01-04
Reduction in the Thermal Conductivity of Thermoelectric Titanium Oxide by Introduction of Planar Defects Shunta Harada, Katsushi Tanaka and Haruyuki Inui Department of Materials Science and Engineering, Kyoto University, Sakyo-ku Kyoto 606-8501, Japan ABSTRACT Thermoelectric properties of a homologous series of Magnéli phase titanium oxides TinO2n-1 (n = 2, 3..) have been investigated. Dense polycrystalline specimens with nominal composition of TiO2-x (x = 0.10, 0.20) have been prepared by conventional hot-pressing. X-ray diffraction analysis has revealed that prepared specimens are slightly reduced during hotpressing. Electrical conduction is of n-type for all prepared titanium oxides and electrical resistivity and absolute values of Seebeck coefficient decrease with increasing oxygen deficiency. The carrier concentration of Magnéli phase titanium oxide increases with increasing oxygen deficiency. Lattice thermal conductivity decreases with increasing oxygen deficiency by more than 60% at room temperature and 40% at 773K compared to TiO2, which can be due to the presence of dense planar defects. The largest thermoelectric figure of merit Z, 1.6×10-4 K-1 at 773K, was obtained in TiO1.90 hot pressed specimen. INTRODUCTION Thermoelectric materials have been attracting great interest due to their potential applications, such as electric power generation by waste heat and cooling system [1]. The efficiency of thermoelectric materials is evaluated by the dimensionless figure of merit ZT=α2T/ρλ where Z, α, ρ, λ and T correspond to the figure of merit, Seebeck coefficient, electrical resistivity, thermal conductivity and absolute temperature, respectively. Thermal conductivity is the sum of two contributions, one, κl of phonon, and the other, κe of conduction charge carriers. Since κe is related to electrical resistivity through Wiedemann-Franz law, materials with low lattice thermal conductivity, which is independent of other physical properties shown above, can possess high thermoelectric performance. Various approaches to reduce the lattice thermal conductivity have been reported including phonon scattering by mass fluctuation, grain boundary [2, 3], resonant scattering of phonons in clathrate compound [4] and super lattice structure [5]. However, many of them sometimes act as sources for carrier scattering as well, which leads to the reduction of thermoelectric performance. This indicates that some crystal lattice defects that act as sources for phonon scattering but not as sources for carrier scattering have to be introduced for thermoelectric materials to achieve the high performance. In the present study, we pay special attention to crystallographic shear structures introduced in some reduced TiO2 (rutile) of homologous series of the Magnèli phase. Homologous series of Magnèli phase titanium oxides are expressed with TinO2n-1 (n = 2, 3, ...) [6] and they form so-called crystallographic shear structures, in which dense planar defects are regularly introduced in the mother rutile structure with their spac
Data Loading...
