Preparation of γ-LaSy (1.33<y<1.50) alloys by the pressure-assisted reaction sintering method and their thermoelectric properties

Abstract

The pressure-assisted reaction sintering (PARS) method was used to prepare thermoelectric elements of the γ-phase lanthanum sesquisulfide solid solution alloys which exist over a wide homogeneity range: LaSy (1.33<y<1.5). A mixture of powders of stoichiometric lanthanum sesquisulfide (La2S3) and lanthanum trihydride (LaH3) was reaction sintered in a hot-press to make the desired composition of γ-LaSy. This method produced black dense single-phase γ-LaSy pellets with a theoretical density of 97% or better. The thermoelectric properties (electrical resistivity and Seebeck coefficient) of the samples prepared by the PARS technique were measured. The room temperature electrical resistivity and Seebeck coefficient of γ-LaSy varied in a systematic manner with the electron concentration indicating that γ-LaSy alloys behave like a simple metal. Although the current carriers appear to be quite heavy (drift mobility ∼3 cm2/V s), the temperature dependence (up to 1000 °C) of these properties of LaS1.42 were found to be linear functions, which is in accord with the above observations. The thermoelectric figure of merit based on the measured resistivity and Seebeck coefficient and an estimated thermal conductivity for LaS1.42 is 0.75×10−3 °C−1 at 1000 °C, which is comparable to those of the state-of-the-art materials such as silicon–germanium alloys at 1000 °C.