Thermoelectric Performance Of Ca3Co4O9 Research Articles

Effect of Y doping at Co site on thermoelectric properties of Ca3Co4O9

In this work, our effort was to improve the thermoelectric properties of Ca3Co4O9 thermoelectric material. Ca3Co[Formula: see text]YxO9 polycrystalline thermoelectric ceramics have been fabricated by sol–gel method plus spontaneous combustion and cold isostatic pressing process. The structure, surface topography and thermoelectric properties of Ca3Co[Formula: see text]YxO9 polycrystalline thermoelectric were investigated by X-ray diffraction (XRD), SEM and ZEM3, respectively. The results showed that [Formula: see text] doping affected the microstructure and thermoelectric performance remarkable. With the increasing of [Formula: see text] doping, the electrical conductivity had a maximum value, and thermal conductivity has a minimum value. Seebeck coefficient gradually increased with [Formula: see text] doping content increasing always. The figure of merit was 0.07 at 673[Formula: see text]K for the Ca3Co[Formula: see text]Y[Formula: see text]O9 sample. The results showed that the thermoelectric performance of Ca3Co4O9 could be improved 39% with the substitution of [Formula: see text] than the un-doped.

A new approach to obtain calcium cobalt oxide by microwave-assisted hydrothermal synthesis

This study describes the effect of microwave-assisted hydrothermal synthesis on the crystalline phase-formation, microstructural characteristics, and thermoelectric performance of Ca3Co4O9. Synthesis parameters, such as time, temperature, KOH concentration, and oxidation states of Co ions, to produce this compound, were studied. X-ray diffraction and X-ray photoelectron spectroscopy showed that the most critical parameters for the phase formation are the oxidation states of Co ions. Ca3Co4O9 can be obtained by microwave-assisted hydrothermal synthesis at 503 K for 30 min and thermal treatment at 1173 K for 120 min. Scanning electron microscopy images showed that the sample exhibits randomly oriented particles with plate-like morphology and range size between 0.4 and 1.2 μm. From measurements of electrical resistivity, thermal conductivity, and Seebeck coefficient, we obtained a figure of merit of ~0.008 at 300 K.

Significant enhancement in Seebeck coefficient and power factor of Ca3Co4O9 thermoelectric ceramics by SiC addition

The ceramics of Ca3Co4O9/xwt.% SiC (x = 0, 2, 3, 5, 7 and 10) have been prepared by using a solid state reaction method. The effects of SiC on the structure and thermoelectric performance of Ca3Co4O9 have been systematically investigated. The results show that major phase is the Ca3Co4O9, and the grain-oriented structure (00l) appears in the Ca3Co4O9/xwt.% SiC. Compared with a pure Ca3Co4O9 sample, the electrical resistivity of the SiC adding samples decreases with the SiC content at room temperature. Seebeck coefficient (S) increases from 52.52 μV/K (pure Ca3Co4O9) to 232.45 μV/K (Ca3Co4O9/10 wt% SiC) at 800 °C. The power factor (P) of Ca3Co4O9/10 wt% SiC samples shows the maximum values 247.926μW/K2m at 51.73 °C, which is 28.8 times larger than the power factor of pure Ca3Co4O9.

Preparation and Thermoelectric Properties of Bi Doped Ca<sub>3</sub>Co<sub>4</sub>O<sub>9</sub> Based Thermoelectric Materials

Polycrystalline Ca3-xBixCo4O9 samples have been prepared by solid-phase reaction followed by spark plasma sintering process. The thermoelectric properties have been systematically investigated from room temperature to near 1000K. It is found that the change of the carrier concentration leads to the change of resistivity, which is mainly associated with doping induced point defect phonon scattering. The change of the thermal potential mainly comes from the spin entropy. In addition, polycrystalline Ca3-xBixCo4O9 had a maximum figure of merit of 0.30 at 973 K, which was about 50% higher than Ca3Co4O9. It indicated that doping approach can effectively improve the thermoelectric performance of Ca3Co4O9+δ-based material.

Effect of sol-gel and solid-state synthesis techniques on structural, morphological and thermoelectric performance of Ca3Co4O9

Two methods of obtaining calcium cobalt oxide (Ca3Co4O9) thermoelectric materials were studied: (I) solid state synthesis (SS) followed by high-energy ball-milling (HEBM) and (II) modified sol-gel method (SG). The obtained powders were heated at 900 °C for 12 h. They were subsequently compacted using spark plasma sintering (SPS). The crystal structure and morphology were characterized by X-ray diffraction and scanning electron microscopy. The calculated average crystallite sizes, before and after SPS, alter from 43 to 61 nm and from 38 to 41 nm, for the SS and SG powders, respectively. XRD studies of the obtained powders revealed presence of Co3O4 secondary phases, which turned into metallic cobalt after SPS. Thermoelectric measurements of sintered samples were performed in the range, 50–500 °C. The maximum Seebeck coefficient is 120 µV/K and 110 µV/K, for the SS and SG synthesized samples, respectively. The highest ZT was found to be 0.02 and 0.04 at 500 °C for the SS and SG samples, respectively.

The effect of Fe substitution on the electrical and thermal conductivity and thermopower of Ca3(FexCo1−x)4O9 synthesised by a sol–gel process

The effect of Fe substitution for Co on direct current (DC) electrical and thermal conductivity and thermopower of Ca3(Co1−xFex)4O9 (x = 0, 0.05, 0.08), prepared by a sol–gel process, was investigated in the temperature range from 380 down to 5K. The results indicate that the substitution of Fe for Co results in an increase in thermopower and DC electrical resistivity and substantial (14.9–20.4% at 300K) decrease in lattice thermal conductivity. Experiments also indicated that the temperature dependence of electrical resistivity ρ for heavily substituted compounds Ca3(Co1−xFex)4O9 (x = 0.08) obeyed the relation lnρ ∝ T−1/3 at low temperatures, T < ~55K, in agreement with Mott’s two-dimensional (2D) variable range hopping model. The enhancement of thermopower and electrical resistivity was mainly ascribed to a decrease in hole carrier concentration caused by Fe substitution, while the decrease of thermal conductivity can be explained as phonon scattering caused by the impurity. The thermoelectric performance of Ca3Co4O9 was not improved in the temperature range investigated by Fe substitution largely due to great increase in electrical resistivity after Fe substitution.

Enhanced high temperature thermoelectric characteristics of transition metals doped Ca3Co4O9+δ by cold high-pressure fabrication

A series of Fe, Mn, and Cu doped Ca3Co4O9+δ samples, Ca3(Co,M)4O9+δ (M=Fe, Mn, and Cu), were fabricated by cold high-pressure compacting technique. Their thermoelectric properties were investigated from room temperature up to 1000 K. The cold high-pressure compacting method is advantageous to increasing density and texture, in favor of the improvement of thermoelectric performance. The electrical transport measurements indicate that Fe/Mn substitutes for Co mainly in [CoO2] layers whereas the substitution of Cu for Co takes place in [Ca2CoO3] layers. The thermoelectric properties as well as electronic correlations depend not only on the substitution ion but also the Co site that is replaced. Thermopower can be well calculated by the carrier effective mass according to Boltzmann transport model, indicating that the electronic correlation plays a crucial role in the unusual thermoelectric characteristics of this system. From the changes in thermopower, resistivity, and thermal conductivity, thermoelectric performance of Ca3Co4O9+δ is efficiently improved by these transition metals doping. Fe doped samples possess the highest ZT values. Combining cold high-pressure technique, ZT of Ca3Co3.9Fe0.1O9+δ can reach ∼0.4 at 1000 K, which is quite large among ceramic oxides, suggesting that Fe doped Ca3Co4O9+δ could be a promising candidate for thermoelectric applications at elevated temperatures.