ICT2014 Abstracts

Thermoelectric (TE) property of Na_xCoO₂ (NCO, P-type) gives many impacts to develop oxide TE materials. Low thermal conductivity (κ) is found in oxide TE materials, and it favorable to increase dimension less figure of merit (ZT=S²σT/κ, S: Seebeck coefficient, σ:electrical conductivity). Although partly reduced Na (κ_Na>100 W/mK) layer involves in NCO, κ is possible to be decreased until 20 W/mK with conductive CoO₂ layered crystal structure. Nb-doped SrTiO₃ (STO, N-type) also shows excellent TE properties, on the other hand k of STO is slightly high vale (10 W/mK) with cubic structure. Tungsten bronze type oxide (TB) such as Sr_1-xBa_xNb₂O₆ (SBN) and Ba₂NaNb₅O₁₅ (BNN) are well known for electro-optic crystal. N-type TE property of reduced SBN single crystal is firstly reported about typical S and σ for c-axis. As a function of oxygen reduction, total κ^tot (κ^tot=κ_ph+κ_e) of reduced BNN is affected by decreasing κ_ph and increasing κ_e. We evaluate κ of reduced BNN from theoretical evaluation and experimental study. In experimental, BNN was prepared from BaCO₃, NaCl and Nb₂O₅ powders at 1100^oC-1300^oC for 12h by solid state reaction. Needle-like (Ba, Na)Nb₂O₆ (N-BNN) particles were synthesized at 900^oC-1100^oC for 4h in closed crucible. Mixture of BNN and N-BNN was made by tape casting for in-plane orientation, and was sintered at 1340^oC for 12h. Orient BNN was partly reduced at 1050^oC-1100^oC for 2-5h in CO atmosphere. Temperature dependence of S and that of σ for reduced BNN were measured from 20^oC to 300^oC by conventional two probe method and four-terminal method, respectively. Curve fitting (follows Gasumyants’ eq.) was carried out for temperature dependence of S data. ZT and κ of BNN were measured by using Harman method. In presentation, we will report about κ of non-stoichiometric BNN utilizing Harman method for ZT. κ and comparison with theoretical evaluation (WIEN2K, BoltzTrap).