Effects of Pr and Yb Dual Doping on the Thermoelectric Properties of CaMnO3

Abstract

There has been research on CaMnO3 with natural abundance, low toxicity, and low cost as promising candidates for n-type thermoelectric (TE) materials. In this paper, Ca1−2xPrxYbxMnO3 with different Pr and Yb contents (x = 0, 0.01, 0.02, 0.03, 0.04 and 0.05) were synthesized by means of coprecipitation. With X-ray diffraction (XRD), scanning electron microscopy (SEM), and high-resolution transmission electron microscopy (HRTEM), researchers characterized the phase structure and morphology of all the samples. The oxidation states of manganese were determined by X-ray photoemission spectroscopy (XPS). The role of Ca-site dual doping in the TE properties was also investigated. Increasing the Pr and Yb contents leads to decreases in the electrical resistivity and Seebeck coefficient, leading to a power factor of 3.48 × 10−4 W·m−1·K−2 for x = 0.04 at 773 K, which is its maximum. Furthermore, the thermal conductivity (κ) decreases with increasing x, and κ = 1.26 W·m−1·K−1 is obtained for x = 0.04 at 973 K. Ca0.92Pr0.04Yb0.04MnO3 exhibit a ZT (thermoelectric figure of merit) value of 0.24 at 973 K, approximately 3 times more than that of the pristine CaMnO3. Thus, the reported method is a new strategy to enhance the TE performance of CaMnO3.

Microstructural Influence on Thermoelectric Properties of CaMnO3 Ceramics

“…where k B is the Boltzmann constant and n and m* refer to the carrier concentration and the effective mass of the carrier, respectively. Snyder and Toberer 1 and Li et al 66 concluded that materials with low charge carrier concentration present as consequence high Seebeck coefficients values and low electrical conductivities, as also observed in this study. Srivastava et al 67 , observed in SrTiO 3 ceramics the reduction of the Seebeck coefficient and the increase in electrical conductivity with the increase of the sintering time.…”

Key properties of inorganic thermoelectric materials—tables (version 1)

“…There is also a metal-insulator transition at high temperature due to a change in the spin state of Mn ions. Substituting Ca with heavy rare earths has been particularly successful in reducing thermal transport [613]; dual doping on both Ca and Mn sites can also enhance charge transport [614,616]. Theoretical work suggests that zT max of greater than 1.0 can be achieved in CMO [670], but to date the use of soft chemistry processing to develop sub-micron grains with nanosized twinned domains, has produced the best performing CMO with a zT max of 0.32 at 1060 K [613].…”

High Power Factor vs. High zT—A Review of Thermoelectric Materials for High-Temperature Application

“… ( a ) p -Type; ( b ) n -type type-I Ioffe plots; ( c ) p -type; ( d ) n -type type-II Ioffe plots and ( e ) p -type and ( f ) n -type zT -electrical conductivity plots for several doped oxidic thermoelectric materials and BiCuSeO [ 10 , 11 , 39 , 69 , 74 , 80 , 96 , 102 ]. Dashed lines show the corresponding absolute values of the Seebeck coefficient given in V K −1 .…”