Layered Calcium Cobaltite Based Ceramics for Thermoelectric Applications

  • Jincheng Yu

Student thesis: Phd

Abstract

Layer-structured calcium cobaltite is considered a promising p-type thermoelectric material due to its intrinsically low thermal conductivity and good high-temperature stability. In this study, the effects of composition, heat-treatment and fabrication route on the thermoelectric performance of calcium cobaltite-based materials were investigated in detail. Ca2.7Bi0.3CoyO9+δ (y = 3.92-4.0) ceramics were prepared by solid state reaction. By introducing 2.0 at.% cobalt deficiency and 10.0 at.% Bi3+ substitution for Ca2+ in calcium cobaltite, the largest lattice expansion was achieved and poorly conducting secondary phases were efficiently reduced. However, the electrical conductivity showed strong dependence on porosity; the lowest porosity in Ca2.7Bi0.3Co3.92O9+δ ceramic resulted in much reduced carrier scattering, thereby leading to the highest power factor of 98.0 µWm-1K-2 at 823 K. The optimal composition (Ca2.7Bi0.3Co3.92O9+δ) was selected for fabrication of thick films by screen printing. When thick films were sintered at 1203 K for 8 h, the single-phase products exhibited the maximum power factor of 55.5 µWm-1K-2 at 673 K. The enhanced electrical conductivity resulted mainly from the increased carrier concentration and increased mobility through control of the microstructure. An order-disorder transition in the structural arrangements of grains near the interface between the film and substrate is believed to result from constrained sintering caused by tensile stresses. Highly dense Ca3-xBixCoyO9+δ (x = 0.1-0.3; y = 3.92-4.0) ceramics were successfully prepared by liquid-phase-assisted spark plasma sintering. The electrical conductivity of the original SPS-processed samples was severely limited by poorly conducting secondary phases. By annealing at 1023 K in air, the reaction forming calcium cobaltite was promoted, the texture was improved and more oxygen diffused into the crystal lattice, thereby leading to increased carrier concentration and mobility. Doping with ‘heavy’ Bi ions helped to increase lattice parameters and phonon scattering, while cobalt deficiency provided opportunities to generate crystalline defects. As a result, the maximum ZT value (//ab) of 0.16 at 823 K was achieved in Ca2.7Bi0.3Co3.92O9+δ ceramic annealed at 1203 K for 12 h. Both single-fired and double-fired Ca2.7Bi0.3Co3.92O9+δ powders were densified by cold sintering. A double calcination process helped to reduce secondary phases in the resulting powders, thereby modifying phase composition and texture development in the cold sintered ceramics. The electrical conductivity was significantly enhanced by extending the annealing time, due to reduction of secondary phases and decreased grain boundary density. The maximum power factor of 0.28 mWm-1K-1 at 823 K was obtained for cold sintered samples prepared from the double-calcined powders annealed at 1203 K for 24 h. Ca2.63Bi0.3M0.07Co3.92O9+δ (M = Sr and Ba) ceramics were prepared by solid state reaction and spark plasma sintering. Bi and Sr dopants were confirmed to enter the crystal lattice, whilst the Ba dopant participated in forming Ba-rich phases. The electrical conductivity of the solid state synthesized samples was limited by the high porosity, although thermal conductivity was significantly reduced. The annealed Bi/Sr co-doped, SPS-processed samples showed enhanced electrical conductivity mainly due to the combined effects of severe lattice expansion, higher densification and improved texture. Furthermore, phonon transport was inhibited because of the higher porosity, atomic mass variations and strain field effects. The maximum ZT value of 0.14 at 800 K was achieved in both the solid state synthesized Bi/Ba co-doped and the annealed SPS-processed Bi/Sr co-doped samples.
Date of Award31 Dec 2021
Original languageEnglish
Awarding Institution
  • The University of Manchester
SupervisorRobert Freer (Supervisor) & David Lewis (Supervisor)

Keywords

  • Calcium cobaltite
  • Thermoelectric mateirals
  • Layered structure

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