Influence of pressure assisted sintering and reaction sintering on microstructure and thermoelectric properties of bi-doped and undoped calcium cobaltite

Abstract

Calcium cobaltite (Ca3Co4O9) is considered as one of the most promising thermoelectric p-type oxides for energy harvesting applications at temperatures above 500 °C. It is challenging to sinter this material as its stability is limited to 920 °C. To facilitate a practicable and scalable production of Ca3Co4O9 for multilayer generators, a systematic study of the influence of powder calcination, Bi doping, reaction sintering, and pressure-assisted sintering (PAS) on microstructure and thermoelectric properties is presented. Batches of doped, undoped, calcined, and not calcined powders were prepared, tape-cast, and sintered with and without uniaxial pressure at 900 °C. The resulting phase compositions, microstructures, and thermoelectric properties were analyzed. It is shown that the beneficial effect of Bi doping observed on pressureless sintered samples cannot be transferred to PAS. Liquid phase formation induces distortions and abnormal grain growth. Although the Seebeck coefficient is increased to 139 μV/K by Bi doping, the power factor is low due to poor electrical conductivity. The best results were achieved by PAS of calcined powder. The dense and textured microstructure exhibits a high power factor of 326 μW/m K2 at 800 °C but adversely high thermal conductivity in the relevant direction. The figure of merit is higher than 0.08 at 700 °C.

Redox-Promoted Tailoring of the High-Temperature Electrical Performance in Ca3Co4O9 Thermoelectric Materials by Metallic Cobalt Addition

“…Some of the best-known individual or combined approaches for enhancing the TE performances of Ca 3 Co 4 O 9 include cation substitutions in both calcium and cobalt sites, microstructural engineering techniques and some composite approaches [37][38][39][40][41][42][43][44][45]. The density and grain connectivity may be improved by using specific processing methods, like SPS and LFZ [46][47][48], and high-quality, ultrafine precursor powders [49][50][51].…”

Influence of pressure and dwell time on pressure‐assisted sintering of calcium cobaltite

“…The heat capacity c p of the material was measured by differential scanning calorimetry (STA449F3 Jupiter, Netzsch, Selb, Germany) in N 2 with a heating rate of 20 K/min up to 1000 K. The thermal conductivity of the textured samples in ab‐direction κ ab was measured with hot‐disc method with a slab module (Hot Disk TPS 2500S, C3 Prozess‐ und Analysetechnik GmbH, Haar, Germany) at room temperature. For the detailed measurement setup, the reader is referred to reference 12.…”

“…3,11 The anisotropy of the particle can be assigned to the polycrystalline material by texturing the microstructure. A detailed literature review about the texturing of Co349 can be found in our previous work 12 or by Prasad et al 13 Here we only focus on some aspects of texturing Co349. Dry-pressing with high pressure up to 1000 MPa is a simple way to increase texture, 7,14 but the sample size is limited.…”

“…With a pressure of 30 MPa, Kenfaui et al 9 reached σ (900K) = 8 ∙ σ ref . Due to the enhanced charge carrier mobility in ab‐direction, the thermal conductivities of hot‐pressed specimens measured in ab‐direction are enhanced by the factor of 10 12 or 4.7 10 compared to the reference. Liu et al 20 and Delorme et al 21 used spark plasma sintering (SPS) with a pressure of 50 MPa 20 (70 MPa 21 ).…”

“…Based on the work of Schulz et al, 16 we recently reported on the combination of tape casting and pressure‐assisted sintering to texture Co349 12 . In this work, 12 we focused on the influence of powder composition on the thermoelectric properties when using pressure‐assisted sintering. We found that Bi‐doping (Ca 2.7 Bi 0.3 Co 4 O 9) ) is not beneficial as it leads to distortions and abnormal grain growth due to a liquid phase when a pressure of 10 MPa is applied during sintering.…”