CaMn_1−x Nb_xO₃ (x ≤ 0.08) Perovskite-Type Phases As Promising New High-Temperature n-Type Thermoelectric Materials

Abstract

Perovskite-type CaMn(1-x)Nb(x)O(3+/-delta) (x = 0.02, 0.05, and 0.08) compounds were synthesized by applying both a "chimie douce" (SC) synthesis and a classical solid state reaction (SSR) method. The crystallographic parameters of the resulting phases were determined from X-ray, electron, and neutron diffraction data. The manganese oxidations states (Mn(4+)/Mn(3+)) were investigated by X-ray photoemission spectroscopy. The orthorhombic CaMn(1-x)Nb(x)O(3+/-delta) (x = 0.02, 0.05, and 0.08) phases were studied in terms of their high-temperature thermoelectric properties (Seebeck coefficient, electrical resistivity, and thermal conductivity). Differences in electrical transport and thermal properties can be correlated with different microstructures obtained by the two synthesis methods. In the high-temperature range, the electron-doped manganate phases exhibit large absolute Seebeck coefficient and low electrical resistivity values, resulting in a high power factor, PF (e.g., for x = 0.05, S(1000K) = -180 microV K(-1), rho(1000K) = 16.8 mohms cm, and PF > 1.90 x 10(-4) W m(-1) K(-2) for 450 K < T < 1070 K). Furthermore, lower thermal conductivity values are achieved for the SC-derived phases (kappa < 1 W m(-1) K(-1)) compared to the SSR compounds. High power factors combined with low thermal conductivity (leading to ZT values > 0.3) make these phases the best perovskitic candidates as n-type polycrystalline thermoelectric materials operating in air at high temperatures.

Review—Towards the Next Generation of Thermoelectric Materials: Tailoring Electronic and Phononic Properties of Nanomaterials

“…55 To date the most promising results are obtained in a thin film of 2%-Nb-doped STO grown on LSAT, with a carrier concentration of 8.37 × 10 20 cm −3 , carrier mobility of 5.4 cm 2 /Vs, electrical conductivity of 723.5 S/cm, Seebeck coefficient of −320 ± 15 μV/K, and a thermal conductivity of 2.70 ± 0.01 W/mK, yielding a ZT of 0.8 ± 0.1, where all values at room temperature. This value compares very well to ZT at 1000 K of 0.37 obtained in single crystal and polycrystalline ceramics 56,57 and the largest ZT obtained in a superlattice of SrTiO 3 /Nb:SrTiO 3 , namely, 2.4 at 300 K.…”

Enhancement of thermoelectric performance in strontium titanate by praseodymium substitution

“…The latter emphasizes a strong necessity in seeking highly performing n-type thermoelectric oxides. Among oxides, manganates 10,11 and strontium titanate SrTiO 3 , 12,13 having perovskite-type lattice are considered promising thermoelectric materials. However, the bulk thermoelectric performance of pure SrTiO 3 is low, with wide band-gap and relatively high lattice thermal conductivity.…”

Nanostructured Nb-substituted CaMnO3 n-type thermoelectric material prepared in a continuous process by ultrasonic spray combustion

“…11. The disorder caused by this Ruddlesden-Popper phase is another mechanism limiting the thermal conductivity in our compound similar to the twinned domains observed by Bocher et al 8,9 For the determination of the thermoelectric properties, a bulk sample was produced from the USC powder as described above. These additional heat treatments are clearly contradictory to the advantages of the USC powder but necessary for the low-cost production of thermoelectric legs for high-temperature converters.…”