Enhancing the Thermoelectric Performance of Calcium Cobaltite Ceramics by Tuning Composition and Processing

Abstract

Calcium cobaltite (Ca3Co4O9) is a promising p-type thermoelectric oxide material. Here, we present an approach to optimize the thermoelectric performance of Ca3Co4O9 by controlling the chemical composition and fabrication process. Ca3–x Bi x Co3.92O9+δ (0.1 ≤ x ≤ 0.3) and Ca2.7Bi0.3Co y O9+δ (3.92 ≤ y ≤ 4.0) ceramics were prepared by Spark Plasma Sintering (SPS). Stoichiometric mixtures of raw materials were combined and calcined at 1203 K for 12 h, followed by SPS at 1023 K for 5 min at 50 MPa. The samples were subsequently annealed at 1023 or 1203 K for 12 h in air. XRD and HRTEM analyses confirmed the formation of the cobaltite misfit phase with minor amounts of secondary phases; SEM-EDS showed the presence of Bi-rich and Co-rich secondary phases. After annealing at 1203 K, the secondary phases were significantly reduced. By controlling the cobalt deficiency and level of bismuth substitution, the electrical conductivity was enhanced without degrading Seebeck coefficients, promoting a high power factor of 0.34 mW m–1 K–2 at 823 K (parallel to the ab planes, //ab). Due to enhanced phonon scattering, the thermal conductivity was reduced by 20%. As a result, a highly competitive ZT­(//ab) of 0.16 was achieved for Ca2.7Bi0.3Co3.92O9+δ ceramics at 823 K.

Controlling the Thermoelectric Properties of Nb-Doped TiO2 Ceramics through Engineering Defect Structures

“…Oxide materials based on earth-abundant elements exhibit exceptional chemical and thermal stability at high temperatures under oxidizing environments; they show considerable potential as candidates for high-temperature TE applications. − A wide range of oxide thermoelectrics have been investigated; cobaltites (including NaCo 2 O 4 , Ca 3 Co 4 O 9 , Ca 2 Co 2 O 5 , and Bi 2 Sr 2 Co 2 O y ) are the most popular p-type TE materials. Among the corresponding n-type TE materials, SrTiO 3 , CaMnO 3 , donor-doped TiO 2 , , , Magnéli phases (Ti n O 2 n –1 , and Nb 12 O 29 ), In 2 O 3 , , and tungsten bronze structured oxides (Ba 5.19 Nd 8.54 Ti 18 O 54 and Ba 6 Ti 2 Nb 8 O 30 ) show much promise.…”

Calcium cobaltite, a promising oxide for energy harvesting: effective strategies toward enhanced thermoelectric performance

“…More generally, the presence of poorly conducting secondary phases in calcium cobaltite based materials can be efficiently suppressed during sintering by promoting reactions that form the primary phase by optimizing the sintering conditions [67,90]; this also helps to increase electrical conductivity. For instance, Yu et al [67] prepared single-phase Ca 2.7 Bi 0.3 Co 3.92 O 9+δ thick films by sintering at 1203 K and adjusting the dwell time; in this way a maximum power factor of 55.5 µWm −1 K −2 was achieved at 673 K. Furthermore, an additional annealing step in air or oxygen atmosphere for as-sintered calcium cobaltite based ceramics can be beneficial, enhancing grain growth, texture development, the fraction of the main phase, and the concentration of Co 4+ by reducing oxygen vacancies in the lattice [18,29], thereby enhancing both the carrier concentration and mobility.…”

“…Indeed, for a given chemical composition, elemental segregation is sensitive to the processing conditions. For example, Yu et al [18] and Boyle et al [66] examined the effects of bismuth substitution on microstructural evolution in calcium cobaltite ceramics. The former reported the formation of Bi-rich secondary phases for 10.0 at.% Bi substituted calcium cobaltite prepared by SPS, whilst the latter only observed Bi segregation at grain boundaries without any impurity phase in high-pressure synthesized calcium cobaltite with the Bi substitution up to 13.3 at.%.…”

“…Whilst the performance of most oxides is limited by their high thermal conductivity and modest electrical conductivity [16,17], several show outstanding TE properties. Among oxide-based TE materials, the layer-structured, p-type calcium cobaltite has long been of interest because of its intrinsically low thermal conductivity and good high temperature stability [18]. For single crystal Ca 3 Co 4 O 9 , a high ZT value of 0.87 at 973 K, was reported by Shikano et al [19] In contrast, polycrystalline ceramics of similar compositions exhibit lower TE performance because of modest densification caused by the layered structure [20].…”

Enhanced Thermoelectric Performance of Na0.55CoO2 Ceramics Doped by Transition and Heavy Metal Oxides

“…Waste heat, evolving into the environment by industrial enterprises, cars, and households, can be directly and effectively converted into electrical energy using thermoelectrogenerators (TEGs). The concept of TEGs can be realized with the use of thermoelectric materials, which should possess at the same time both low thermal conductivity (λ) and electrical resistivity (ρ) and high values of the Seebeck coefficient (S) [1][2][3][4][5]. Due to the strong interconnection of the above mentioned properties, the number of thermoelectric materials is strongly restricted.…”