| Research Abstract |
We have focused on preparing misfit-layered sulfides [Ln_2S_2]_pTS_2 (Ln:lanthanide ions, T:transition metal ions), in which a large thermoelectric power factor and a low lattice thermal conductivity can be simultaneously realized, to design novel Peltier materials. First of all, a T=Nb system has been explored to prepare new p-type materials. Except for Ln=Eu and Lu, all the lanthanides ions including Y are found to form [Ln_2S_2]_pNbS_2 phases. Three-types of structure with different superspace group are discovered depending on their ionic radii. Larger ions with Ln=La,Ce and Pr form orthorhombic Fm2m(p00)00s, while the samples with Ln=Nd and Sm form Fm2m(p00)000. For smaller ions with Ln=Gd,Tb,Dy,Ho,Er,Tm,Yb and Y, monoclinic F2(p0q) phases are obtained. The sample with Ln=Yb exhibits the highest thermoelectric figure-of-merit Z=3.2 x 10^<-4> K^<-1>, with the Seebeck coefficient S=60μVK^<-1>, resistivity ρ=1.4 mΩcm and κ=0.80 WK^<-1>m^<-1>, at around 300 K, yielding the dimensionles
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s figure-of-merit ZT〜0.1. The Z value still increases with increasing temperature, but the maximum value of Z as well as the highest limit temperature to be utilized are not determined in detail. Next, we have fabricated a thermoelectric pn junction (1 pair) using a Yb-Nb-S compound as a p-type leg and a Sn-Ti-S compound as an n-type leg. The pn uncouple was attached to an Al sheet on its top with silver paste and then was subjected to the Peltier property measurements. At the beginning, a small temperature difference with several kelvin was in fact observed but the difference became small and reached zero with further applying an electric current. This behavior can be ascribed to the fact that the Seebeck coefficient of the both legs was not enough (〜70μVK^<-1>) to keep generating temperature difference because the generated Joule heat easily overcame the temperature difference. Hence, further study is necessary to design sulfide thermoelectric materials which have larger figure-of-merit. Less
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