Ca3Co4O9 Phase Research Articles

Influence of ceramic particles additions on the properties of Ca3Co4O9

Ca3Co4O9 + x wt% B4C, AlN, TiC, TiB2, or TiN (x = 0.0, 0.25, 0.50, and 0.75) samples were prepared by the conventional solid-state route. In all samples, only the Ca3Co4O9 phase was identified by powder XRD. Nevertheless, microstructural studies have shown that most of the additives have reacted with air and Ca3Co4O9 phase on their surfaces, producing new phases. Moreover, it seemed that grain sizes were, at least, slightly reduced. On the other hand, while nearly no modification of the Seebeck coefficient has been observed, independently of the added compound and proportion, electrical resistivity decreased in all cases, when compared to the pristine sample. Consequently, the power factor of samples with additions was higher than the one determined for the pure sample. Linear thermal expansion also decreased with these additives, pointing out to the formation of relatively strong grain boundaries which can improve the carrier mobility and decrease the thermal expansion. The lowest thermal expansion value has been measured in 0.25 wt% B4C samples, being only around 20% higher than that of Al2O3, which can help to reduce the differential thermal expansion in thermoelectric modules working at high temperatures; these results may be very interesting for applications prospects.Article highlightsAddition of different ceramic additives to Ca3Co4O9 phase increases electrical conductivity, and power factor.All additive ceramics react with oxygen and the Ca3Co4O9 phase modifying the properties of pristine compound.The decrease of thermal expansion coefficient will allow reducing the differential thermal expansion in modules.

P-Ca3Co4O9/n-Zn0.98Al0.02O module for high temperature thermoelectric generator

The thermoelectric generators can generate electrical power from different temperatures of the top and bottom side thermoelectric modules or devices using n - and p - types thermoelectric materials. We synthesized the p-type Ca3Co4O9 and n-type Zn0.98Al0.02O thermoelectric materials by solid-state reaction (SSR) and hot-pressing (HP) methods then measured thermoelectric properties such as Seebeck coefficient, electrical resistivity, and thermal conductivity. The thermoelectric module of π-shape was fabricated 2 pair p-n junction then measured the electrical voltage, power, and efficiency. It was found that we obtained the single phase of Ca3Co4O9 and Zn0.98Al0.02O thermoelectric materials. The thermopower of Ca3Co4O9 and Zn0.98Al0.02O thermoelectric materials are measured by using steady-state method at hot side 976 K. The electricity of thermoelectric cells was increased with increasing temperature difference from 28–603 K. The thermoelectric cells p-n junction 2 pairs generated a maximum voltage about 93.04 mV, power 8 mW and efficiency of 0.23 % at temperature difference of 735 K (hot side 976 K). So, the thermoelectric module has been feasible to fabricate thermoelectric generators and developed a method for fabrication of high stability modules.

Growth and optical properties of CaxCoO2 thin films

The layered cobaltates AxCoO2 (A = Li, Na, Ca, Ba, Sr) are of interest for energy applications such as thermoelectrics and batteries. However, it is challenging to obtain these phases in pure from as thin films. Here, phase-pure CaxCoO2 (x ~ 0.5) thin films were obtained by annealing of Ca(OH)2/Co3O4 multilayers made by moisture treatment of sputter-deposited CaO/Co3O4 multilayer films. The pure CaxCoO2 thin films exhibit an average optical transmittance of approximately 36% in the visible region and greater than 70% in the near-infrared (NIR) region. In addition, the electrical conductivity can be increased by incorporating a secondary Ca3Co4O9 phase into the CaxCoO2 thin film without large changes in optical properties and Seebeck coefficient.

Synthesis and Characterization of Ca3Co4O9 Ceramic by using Sol-Gel Method

We report the synthesis of Ca3Co4O9 ceramics by the Sol-Gel method with citric acid as the chelating agent. Calcium nitrate and cobalt nitrate as precursors were weighed at an atomic ratio of Ca: Co = 3: 4. The process starts with gel preparation, followed by drying, calcination, and compaction process. The pellets were sintered at 820°C, 860°C, and 900°C for 6 hours in a furnace without flowing gas. The effect of sintering temperature on the formation of the Ca3Co4O9 phase was analyzed using an X-ray diffractometer. The Ca3Co4O9 phase has formed in samples sintered at 820°C with the Co3O4 phase as an impurity. Based on the calculation, the crystallite size decreases with increasing sintering temperature

Exploring the High-Temperature Electrical Performance of Ca3−xLaxCo4O9 Thermoelectric Ceramics for Moderate and Low Substitution Levels

Aliovalent substitutions in Ca3Co4O9 often result in complex effects on the electrical properties and the solubility, and impact of the substituting cation also depends largely on the preparation and processing method. It is also well-known that the monoclinic symmetry of this material’s composite crystal structure allows for a significant hole transfer from the rock salt-type Ca2CoO3 buffer layers to the hexagonal CoO2 ones, increasing the concentration of holes and breaking the electron–hole symmetry from the latter layers. This work explored the relevant effects of relatively low La-for-Ca substitutions, for samples prepared and processed through a conventional ceramic route, chosen for its simplicity. The obtained results show that the actual substitution level does not exceed 0.03 (x < 0.03) in Ca3−xLaxCo4O9 samples with x = 0.01, 0.03, 0.05 and 0.07 and that further introduction of lanthanum results in simultaneous Ca3Co4O9 phase decomposition and secondary Ca3Co2O6 and (La,Ca)CoO3 phase formation. The microstructural effects promoted by this phase evolution have a moderate influence on the electronic transport. The electrical measurements and determined average oxidation state of cobalt at room temperature suggest that the present La substitutions might only have a minor effect on the concentration of charge carriers and/or their mobility. The electrical resistivity values of the Ca3−xLaxCo4O9 samples with x = 0.01, 0.03 and 0.05 were found to be ~1.3 times (or 24%) lower (considering mean values) than those measured for the pristine Ca3Co4O9 samples, while the changes in Seebeck coefficient values were only moderate. The highest power factor value calculated for Ca2.99La0.01Co4O9 (~0.28 mW/K2m at 800 °C) is among the best found in the literature for similar materials. The obtained results suggest that low rare-earth substitutions in the rock salt-type layers can be a promising pathway in designing and improving these p-type thermoelectric oxides, provided by the strong interplay between the mobility of charge carriers and their concentration, capable of breaking the electron–hole symmetry from the conductive layers.

Prospects for Electrical Performance Tuning in Ca3Co4O9 Materials by Metallic Fe and Ni Particles Additions.

This work further explores the possibilities for designing the high-temperature electrical performance of the thermoelectric Ca3Co4O9 phase, by a composite approach involving separate metallic iron and nickel particles additions, and by employing two different sintering schemes, capable to promote the controlled interactions between the components, encouraged by our recent promising results obtained for similar cobalt additions. Iron and nickel were chosen because of their similarities with cobalt. The maximum power factor value of around 200 μWm−1K−2 at 925 K was achieved for the composite with the nominal nickel content of 3% vol., processed via the two-step sintering cycle, which provides the highest densification from this work. The effectiveness of the proposed approach was shown to be strongly dependent on the processing conditions and added amounts of metallic particles. Although the conventional one-step approach results in Fe- and Ni-containing composites with the major content of the thermoelectric Ca3Co4O9 phase, their electrical performance was found to be significantly lower than for the Co-containing analogue, due to the presence of less-conducting phases and excessive porosity. In contrast, the relatively high performance of the composite with a nominal nickel content of 3% vol. processed via a two-step approach is related to the specific microstructural features from this sample, including minimal porosity and the presence of the Ca2Co2O5 phase, which partially compensate the complete decomposition of the Ca3Co4O9 matrix. The obtained results demonstrate different pathways to tailor the phase composition of Ca3Co4O9-based materials, with a corresponding impact on the thermoelectric performance, and highlight the necessity of more controllable approaches for the phase composition tuning, including lower amounts and different morphologies of the dispersed metallic phases.

Significant enhancement of the power factor in Ca3Co4O9 by incorporating Cu2Se

Composites of calcium cobalt oxide and copper selenide (Ca3Co4O9/xCu2Se, where x = 0, 5, and 10 wt%) were prepared using the polymerized complex method followed by hot-press sintering. X-ray diffraction analysis (XRD) results revealed the crystalline phase of Ca3Co4O9 with low diffraction peaks of Cu2Se phases, indicating that Ca3Co4O9/xCu2Se composites were successfully obtained. Field emission scanning electron microscopy (FESEM) images showed randomly oriented plate-like grains of pristine Ca3Co4O9, while fine grains were observed after incorporation of Cu2Se. Temperature dependence of the electrical resistivity and Seebeck coefficient was investigated using a four-probe method with temperature ranging from 300 K to 862 K. Measurement of the Hall effect showed higher carrier concentration of Ca3Co4O9/xCu2Se composite materials, with increasing Cu2Se contents resulting in decreased electrical resistivity, while the power factor increased with increasing Cu2Se. Lowest electrical resistivity of 115 μΩ m at 862 K was achieved for Ca3Co4O9/10 wt%Cu2Se, while highest Seebeck coefficient of 228 μV K−1 was obtained at 410 K due to phase transition of the Cu2Se matrix. Maximum power factor of Ca3Co4O9/10 wt%Cu2Se was 270 W m−1 K−2, higher than pristine Ca3Co4O9 (104 μW m−1 K−2) at 862 K. The enhanced power factor of Ca3Co4O9/xCu2Se bulk materials occurred as a result of significantly higher hole concentration of Cu2Se, while enhancement of grain connectivity had a strong influence on decreased electrical resistivity.

Drastic enhancement of mechanical properties of Ca3Co4O9 by B4C addition

Ca3Co4O9 with B4C additions in different proportions (0, 0.10, 0.25, 0.5, and 0.75 wt.%) have been fabricated using the classical solid-state method. Powder XRD patterns have displayed that Ca3Co4O9 phase is the major one in all samples. Microstructural observations showed that B4C has been superficially oxidized, producing liquid B2O3 during sintering, and reacting with the Ca3Co4O9 grains to produce bridges between them. In spite of the increase of porosity, these bridges led to an important raise (more than two times) of mechanical properties when compared to the pristine materials. On the other hand, while B4C addition has not influenced S values, it has decreased electrical resistivity, thermal conductivity, and thermal expansion. Consequently, ZT values have been also increased, reaching 0.24 at 800 °C in 0.25 wt.% B4C containing samples, which is very close to the best values reported in the literature, and two times higher than the obtained in pure materials in this work.

Tuning Ca3Co4O9 thermal and transport properties by TiC nanoparticles addition

Ca3Co4O9+xwt.% TiC (x=0, 0.25, 0.5, 0.75, and 1.0) polycrystalline thermoelectric ceramics have been prepared through the classical ceramic route. XRD characterization has demonstrated that all samples are mainly composed by the Ca3Co4O9 phase, while microstructural observation has shown that no reaction between both components has been produced. Moreover, TiC particles are well distributed for small additions, and start to agglomerate from 0.75wt.% content. Density measurements showed that nearly no changes have been produced by TiC addition and nearly the whole porosity appears as open one. Furthermore, electrical resistivity decreases up to 0.75wt.% addition, increasing for higher content. On the other hand, Seebeck coefficient has been maintained unchanged in all samples. In spite of an irregular behaviour of thermal conductivity with temperature, it tends to decrease when the temperature is raised, displaying the lowest values for the 0.25wt.% samples. These data led to an increase of about 40% in ZT values at 800°C for samples with 0.25wt.% addition, when compared with the pure ones. Finally, linear thermal expansion coefficient is decreased when TiC content is increased, which can be exploited to fit the thermal expansion coefficients of all the components used to build a power generation thermoelectric module.

Microwave heating synthesis and thermoelectric property characterization of highly dense Ca3Co4O9 bulk

The present study is focused on synthesis and sintering of calcium cobaltite (Ca3Co4O9) by solid-state reaction of calcium carbonate (CaCO3) and cobalt oxide (Co3O4) using microwave heating at 850, 900, and 950 °C. Thermogravimetric Analysis (TGA) and Differential Thermal Analysis (DTA) results show a weight loss of ~26% due to the decomposition of CaCO3 in the temperature range of 600–820 °C, resulting into single phase of Ca3Co4O9 which remains stable up to 920 °C. The Ca3Co4O9 phase is observed in samples heated at 850 °C and 900 °C, whereas at 950 °C, Ca3Co4O9 is transformed to Ca3Co2O6 as identified by X-Ray Diffractometry. 98.76% of relative density is obtained by microwave sintered sample at 950 °C with 1hr holding time. Changes in synthesis temperature did not show any effect on the Seebeck coefficient (119–155 μV/K). The specimens sintered at 900 °C exhibit a low resistivity (150-160 μΩ-m) and better power factor (94–145 μW/mK2) in the 35–500 °C temperature range.

Enhanced thermoelectric properties of Ca3Co4O9+δ ceramics by Sr substitution

Ca3-xSrxCo4O9+δ (CCO-Srx) with varying strontium (Sr) content (x = 0.4, 0.5, 0.6, and 0.7) were synthesized by solid-state method. The crystal structures and morphologies of Ca3-xSrxCo4O9+δ thermoelectric ceramics were analyzed by XRD and SEM. Micro structural characterizations have shown that all samples are composed by the same oriented plate-like grains with similar sizes about 1–3 μm diameters and 0.2–0.5 μm thicknesses. The results of XRD have shown that no secondary phases have been produced, which indicated that Sr has been incorporated into the Ca3Co4O9 phase. The diffraction peak (00l) shifted to a lower angle and the offset is increasing with the increase of Sr content, which indicated that the Sr cations can enter the Ca-sites of the Ca3-xSrxCo4O9+δ, and cause increase of the length of the C axis. Slight improvements in the Seebeck coefficient (S) and electrical resistivity (ρ) were observed across all Sr-substitution samples when the mol% of Sr was adjusted. The improvement in both electrical resistivity and Seebeck coefficient leads to higher power factor (PF) values for the ceramics. The measured results show that the highest power factor value around 0.21 mW m−1K−2 can be obtained in Ca2.4Sr0.6Co4O9+δ at 650 °C, which is 16.7% higher than the undoped sample, The ZT value of 0.37 at 650°Cfor the CCO–Sr0.6 sample is significantly higher than that of the pure CCO sample. The results show that Sr can be as an effective doping element to reduce the thermal conductivity and enhance the Seebeck coefficient for the CCO system. It can be concluded that x = 0.6 is the optimal Sr for Ca substitution in this material system.

The effect of calcium impurities of β″-alumina on the degradation of NaxCoO2 cathodes in all solid state sodium-ion batteries

β″-alumina is a promising solid electrolyte candidate for room-temperature all-solid-state sodium-ion batteries due to its high sodium ionic conductivity of up to 10−2 S cm−1 at room temperature. However, calcium impurities are known to be present in the Al2O3 precursors used to produce commercial β″-alumina. Here we present a study on the impact of the calcium impurities which are diffusing in a temperature controlled process from the β″-alumina substrates into the NaxCoO2 layered oxide cathode. The increasing calcium content in the NaxCoO2 cathode leads to a degradation due to the formation of the calcium-rich Ca3Co4O9 phase. The electrochemical performance of sodium-ion batteries is investigated with NaxCoO2/β″-alumina/Na solid-state model batteries. The thin-film cathodes were grown onto commercial β″-alumina ceramic substrates by pulsed laser deposition and cycled between 2.0 and 3.8 V with metallic sodium anodes.

Improving thermoelectric properties of Ca3Co4O9+δ through both Na doping and K addition at optimal values

Ca2.93Na0.07Co4Oy/x wt% K2CO3 (x = 0.00, 0.01, and 0.03) polycrystalline ceramics were prepared by conventional solid-state method. XRD results have shown that all samples predominantly include Ca3Co4O9 phase together with small amounts of secondary phases. SEM images show that all samples have randomly oriented plate-like grains in different sizes. The electrical resistivity measurement showed that electrical properties of Ca3Co4O9 ceramics can be improved significantly by both, Na doping and K addition at their optimal values. The effect of dopants on thermoelectric properties of Ca3Co4O9 was examined by both, Seebeck coefficient and power factor, being higher in K-added than in the pure samples, indicating that thermoelectric properties of samples are positively affected when alkaline elements enter into their structure.

Nanoporous Ca3Co4O9 Thin Films for Transferable Thermoelectrics.

The development of high-performance and transferable thin-film thermoelectric materials is important for low-power applications, e.g., to power wearable electronics, and for on-chip cooling. Nanoporous films offer an opportunity to improve thermoelectric performance by selectively scattering phonons without affecting electronic transport. Here, we report the growth of nanoporous Ca3Co4O9 thin films by a sequential sputtering-annealing method. Ca3Co4O9 is promising for its high Seebeck coefficient and good electrical conductivity and important for its nontoxicity, low cost, and abundance of its constituent raw materials. To grow nanoporous films, multilayered CaO/CoO films were deposited on sapphire and mica substrates by rf-magnetron reactive sputtering from elemental Ca and Co targets, followed by annealing at 700 °C to form the final phase of Ca3Co4O9. This phase transformation is accompanied by a volume contraction causing formation of nanopores in the film. The thermoelectric propoperties of the nanoporous Ca3Co4O9 films can be altered by controlling the porosity. The lowest electrical resistivity is ∼7 mΩ cm, yielding a power factor of 2.32 × 10–4 Wm–1K–2 near room temperature. Furthermore, the films are transferable from the primary mica substrates to other arbitrary polymer platforms by simple dry transfer, which opens an opportunity of low-temperature use these materials.

High-temperature thermoelectric properties of Sm3+-doped Ca3Co4O9+δ fabricated by spark plasma sintering

Ca3-xSmxCo4O9+δ (0 ≤ x ≤ 0.3) samples were fabricated by the sol-gel method followed by spark plasma sintering in vacuum. The high-temperature thermoelectric properties of the Ca3-xSmxCo4O9+δ were also studied, with an emphasis placed on the partial substitution of Sm3+ for Ca2+. The sintered Ca3-xSmxCo4O9+δ formed a monoclinic Ca3Co4O9 phase and exhibited fine lamellar grains and dense morphology. With increased Sm3+ content, the electrical and thermal conductivities decreased, whereas the Seebeck coefficient significantly increased. Of the prepared samples, Ca2.7Sm0.3Co4O9+δ had the largest dimensionless figure-of-merit (0.175) at 800°C. The results showed that the partial substitution of Sm3+ for Ca2+ in Ca3Co4O9+δ is effective for enhancing its thermoelectric properties.

Nanostructural Tailoring to Induce Flexibility in Thermoelectric Ca3Co4O9 Thin Films.

Because of their inherent rigidity and brittleness, inorganic materials have seen limited use in flexible thermoelectric applications. On the other hand, for high output power density and stability, the use of inorganic materials is required. Here, we demonstrate a concept of fully inorganic flexible thermoelectric thin films with Ca3Co4O9-on-mica. Ca3Co4O9 is promising not only because of its high Seebeck coefficient and good electrical conductivity but also because of the abundance, low cost, and nontoxicity of its constituent raw materials. We show a promising nanostructural tailoring approach to induce flexibility in inorganic thin-film materials, achieving flexibility in nanostructured Ca3Co4O9 thin films. The films were grown by thermally induced phase transformation from CaO–CoO thin films deposited by reactive rf-magnetron cosputtering from metallic targets of Ca and Co to the final phase of Ca3Co4O9 on a mica substrate. The pattern of nanostructural evolution during the solid-state phase transformation is determined by the surface energy and strain energy contributions, whereas different distributions of CaO and CoO phases in the as-deposited films promote different nanostructuring during the phase transformation. Another interesting fact is that the Ca3Co4O9 film is transferable onto an arbitrary flexible platform from the parent mica substrate by etch-free dry transfer. The highest thermoelectric power factor obtained is above 1 × 10–4 W m–1 K–2 in a wide temperature range, thus showing low-temperature applicability of this class of materials.

Porous Ca3Co4O9 with enhanced thermoelectric properties derived from Sol–Gel synthesis

Highly porous Ca3Co4O9 thermoelectric oxide ceramics for high-temperature application were fabricated by sol–gel synthesis and subsequent conventional sintering. Growth mechanism of misfit-layered Ca3Co4O9 phase, from sol–gel synthesis educts and upcoming intermediates, was characterized by in-situ X-ray diffraction, scanning electron microscopy and transmission electron microscopy investigations. The Ca3Co4O9 ceramic exhibits a relative density of 67.7%. Thermoelectric properties were measured from 373K to 1073K. At 1073K a power factor of 2.46μWcm−1K−2, a very low heat conductivity of 0.63Wm−1K−1 and entropy conductivity of 0.61mWm−1K−2 were achieved. The maintained figure of merit ZT of 0.4 from sol–gel synthesized Ca3Co4O9 is the highest obtained from conventional, non-doped Ca3Co4O9. The high porosity and consequently reduced thermal conductivity leads to a high ZT value.

Experimental Determination of the Formation Enthalpy of Calcium Cobaltate from Sol–Gel Precursors

Calcium cobaltate (Ca3Co4O9) remains one of the most promising p-type oxide materials for high-temperature thermoelectric energy conversion. While much progress has been made in refining our understanding of the unique structure of the material, as well as optimization of the transport properties for thermoelectric efficiency, there remains a gap in the knowledge, both experimental and theoretical, of the thermodynamics of the system. Presented herein is an analysis of the heat of formation of the Ca3Co4O9 phase from sol–gel precursors using a highly sensitive differential scanning calorimeter, as well as observations of its decomposition into the Ca3Co2O6 phase. The reaction enthalpy of forming Ca3Co4O9 from CaCO3 and Co3O4 sol–gel precursors was determined to be +284 (±2%) kJ/mol, leading to a standard enthalpy of Ca3Co4O9 of −3307 (±3.5%) kJ/mol.

Phase evolution and thermoelectric performance of calcium cobaltite upon high temperature aging

Crystal phase transformation and their nanostructure evolutions were studied for baseline thermoelectric ceramic Ca3Co4O9, and for a sample aged at 1283K, which is significantly higher than the Ca3Co4O9 decomposition temperature of 1199K. X-ray diffraction from both samples only reveals peaks from Ca3Co4O9 phase. TEM examination reveals that in the aged sample, CaO nano-phase exists between neighboring Ca3Co4O9 nano-lamella grains. Moreover, a small amount of Co3O4 and Ca2Co2O5 phase were also observed in the aged sample. The Ca2Co2O5 phase does not exhibit the same lamella nanostructure as Ca3Co4O9 and displayed few crystal defects. Except for Ca3Co4O9, Ca2Co2O5, and Co3O4, no other Ca–Co–O cobaltite phase, such as Ca3Co2O6, was found in the aged sample. Electrical and thermal properties of both baseline and aged sample were measured over the temperature range of 320–1033K.

Effect of Na doping on the Ca3Co4O9 thermoelectric performance

Ca3−xNaxCo4O9 polycrystalline thermoelectric ceramics with small amounts of Na (x=0, 0.01, 0.03, 0.05, 0.07, and 0.10) have been prepared using the classical solid state method. Microstructural characterization has shown that Na has been incorporated into the Ca3Co4O9 phase and that no Na-based secondary phases have been produced. It has also been found that Na addition promotes grain growth and favours sintering due to the formation of a small amount of liquid phase. Electrical resistivity decreases when Na content increases until 0.07Na addition while Seebeck coefficient is maintained practically unchanged. The improvement in electrical resistivity leads to higher power factor values than the usually obtained in samples prepared by more complex and expensive techniques.