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Lanthanum cobaltate, LaCoO3, with a perovskite type of structure has been recognized as attractive thermoelectric material due to its high Seebeck coefficient (S>500 V/K at room temperature) and high thermal stability. However, the electrical resistivity of LaCoO3 is relatively гigh (about 10 Ωcm at 300 K), which lowers the thermoelectric activity (ZT

Ca3Co4O9 ceramic is an attractive lead-free alternative to conventional semiconductor materials, based on bismuth and tellurium with a practical application in converting waste heat to electricity, solar thermoelectric generators and refrigeration devices [1]. Ca3Co4O9 is a thermoelectric material representing the family of the so-called misfit layered oxides. It consists of a Ca2CoO3 layer which is an insulating subsystem that maintains the charge supply for the conducting CoO2 layer, where both Co3+ and Co4+ ions coexist [2]. In addition to the research carried out on the thermoelectric properties of Ca3Co4O9, several studies have reported results elucidating crystal information and electronic structure [3,4]. Recently, a giant dielectric permittivity was measured in Ca3Co4O9 ceramic at room temperature [5]. In this work, we investigated and compared the structural and electrical properties of Ca3Co4O9 (CCO) ceramics and thin films. The CCO thin films were produced from a stoichiometric, in-house made CCO ceramic by pulsed laser deposition (PLD) onto (100) oriented lanthanum aluminate (LaAlO3, LAO) substrates. Both the CCO ceramics and thin films were subsequently characterised by X-ray diffraction (XRD). To evaluate the samples’ dielectric properties, capacitor structures were formed. Their capacitances were measured and the corresponding permittivities were evaluated in a temperature range RT – 450 K. Both CCO ceramics and thin films were pure misfit phases. The thin film exhibited epitaxial growth with preferred (00l) orientation. For the CCO ceramics, the measured dielectric permittivity was increasing moronically with no maximum observed at temperatures up to 450 K (Figure 1a). The estimated value of the dielectric permittivity ~ 5,300 measured at room temperature corresponds well with the values reported elsewhere [5]. Similar behaviour of the capacitance as a function of the temperature with a maximum at 430K was measured for the thin film sample (Figure 1b). Due to the small thickness of the layer, the error associated with the dielectric permittivity evaluation procedure is very high. Nevertheless, one could speculate that it is in the same order of magnitude as those of the CCO ceramic. The observed reduction of the phase-transition temperature is associated with the residual strain in the thin film sample introduced by the substrate.

Thermoelectric oxides are considered as perspective materials for energy conversion due to their high thermal and chemical stability, low cost and environmental compatibility in comparison with the conventional semiconductors based on bismuth and tellurium [1]. Herein we report firstly the thermoelectric properties of composites between perovskite and layered oxides. The proof-of-concept studies conducted on LaCo0.8Ni0.1Fe0.1O3 perovskite and layered Ca3Co4O9 oxides demonstrate the improved thermoelectric properties of composites through the enhancing the boundaries phonon scattering without affecting the electronic transport. The oxide composites are formed by mechanical milling followed by annealing at 900 oC. This procedure allows to achieve a good contact between particles without changing the structure and composition of two components. The thermoelectric efficiency of the composites is determined by the dimensionless figure of merit (ZT) calculated from the independently measured Seebeck coefficient, electrical resistivity and thermal conductivity. The observed changes in the electrical resistivity of the composites are explained by the particle size effects and the contact between particles. It is found that the thermoelectric activity of the composites is higher than that of the individual components.

The present research is devoted to studying the influence of high oxygen pressure on the thermoelectric properties of cobalt-based perovskites. Two model compositions were selected as the subject of research: mono-substituted with Ni perovskites LaCo0.9Ni0.10O3 and doubly substituted with Ni and Fe perovskites LaCo0.8Ni0.1Fe0.1O3. It is characteristic of them that they show better thermoelectric properties than unsubstituted lanthanum cobaltite. Oxygen pressure was 200 bar and it has a direct effect on the superoxygen content of the perovskite. The structural characteristics of the perovskites were investigated by X-ray structural analysis using the Rietveld method. The thermal properties of the punched calcined under high oxygen pressure were also studied by DTA and TG analyses. Electrical properties were measured on an MMR apparatus (K2500-5SLP-SP) using the Van der Pauw method in a temperature range of 300 to 600 K and a magnetic field of 5 T. All measurements were performed on tablets with minimal porosity. Настоящето изследване е посветено на изучаване на влиянието на високото кислородно налягане върху термоелектричните свойства на перовскити на основата на кобалт. Като обект на изследване са подбрани два моделни състава: еднозаместени с Ni перовскити LaCo0.9Ni0.10O3 и двойнозаместени с Ni и Fe перовскити LaCo0.8Ni0.1Fe0.1O3. Характерно за тях е, че те показват по-добри термоелектрични свойства от незаместения лантанов кобалтит [2]. Кислородно налягане бе 200 bar и то оказва пряко въздействие върху свръхкислородното съдържание на перовскита. Структурните характеристики на перовскитите са изследвани чрез рентгеноструктурен анализ по метода на Ритвелд. Термичните свойства на пробите накалени под високо кислородно налягане бяха проучени и чрез ДТА И ТГ анализи. Електричните свойства са измерени на апаратура MMR (K2500-5SLP-SP) по метода на Van der Pauw в температурния интервал от 300 до 600 K и магнитно поле от 5 T. Всички измервания са извършени на таблетки с минимална порьозност.

In this report, the electrical properties of cobalt oxides with layered and perovskite structures, as well as composites between them, will be compared. The purpose of the research is to study the conditions of formation of composites between perovskites and layered oxides and how they affect their electrical properties. The layered and perovskite oxides were obtained by the methods of soft chemistry by using the Pechini reaction, and the composites by mechanical grinding and annealing. Structural and morphological characteristics were determined by X-ray diffraction and SEM analyses. В настоящия доклад ще бъдат сравнени електричните свойства на кобалтови оксиди със слоеста и перовскитова структури, както и на композити между тях. Целта на изследването е да се изучат условията на формиране на композити между перовскити и слоести оксиди и как те влияят върху електричните им свойства. Слоестите и перовскитни оксиди са получени по методите на меката химия чрез използване на реакцията на Печини, а композитите чрез механично смилане и отгряване. Структурните и морфологични характеристики са определени чрез рентгенова дифракция и СЕМ анализи.