engleski

Calcium manganite coatings for energy conversion

Calcium manganite, CaMnO3, is a ceramic material of perovskite structure with the attractive property of transformation of thermal into electric energy. But perovskite materials also have excellent properties as the absorption layer in photovoltaic cells. Most commonly researched are lead halides, which have poor stability when exposed to higher temperatures or humidity, and the synthesis of which involves harmful and toxic compounds. Calcium manganite is both chemically and thermally stable, as well as easily synthesised from widely accessible raw materials (i.e. calcium carbonate, CaCO3 and manganese dioxide, MnO2). Electron structure of CaMnO3 makes it suitable for the absorption layer in photovoltaic cells, and its optical properties largely match those of a commonly used lead halide, which makes it an interesting alternative to those compounds. The goal of our research is to synthesise CaMnO3 in a form suitable for thin film deposition (e.g. by doctor blade or spin- coating). We have prepared crystalline CaMnO3 powder by auto-combustion method (CMc) and by powder homogenisation (CMh). The first method consists in preparation of aqueous solution of nitrate salts of Ca2+ and Mn2+ with citric acid as a complexing agent and “fuel”. Upon boiling, the solution self-ignites due to decomposition of nitrates and rapid oxidation of the citric acid. The resulting ashes were found to contain CaCO3 and probably poorly crystallized MnO2. The second method consists in repeated homogenisation of mixture of pure CaCO3 and MnO2 powders in an agate mortar. Both the ashes and the homogenised mixture were calcined at 900 °C for 2 h in order to obtain fully crystallized perovskite structure. Resulting crystalline powders were CaMnO3 with traces of CaMn2O4, with following mean particle diameter and specific surface area: 38 μm and 4.1 m2/g for CMc and 226 μm and 4, 3 m2/g for CMh. Scanning electron microscopy (SEM) has shown that diameters correspond to sintered branched agglomerates of particles whose primary size is under 1 μm. Both powders were suspended in ethanol using ultrasonic agitation in order to obtain a stable suspension, and suspension of CMh has proven more stable. Suspension of CMh was deposited by drop casting and the resulting coatings examined under SEM: quality was poor, and it will be necessary to obtain finer powders for this manner of CaMnO3 deposition. Calcined CMh powder was compacted into a tablet and sintered at 1200 °C to obtain a target for pulsed laser deposition (PLD). There was no change in phase composition during sintering. PLD was carried out in a vacuum chamber using Nd:YAG laser with 1064 nm, 10 J/cm2 and 5 Hz repetition rate. Thin film was deposited on a silicon substrate with 2000 laser pulses while distance between target and substrate was 3 cm. The film was characterised by grazing incidence X-ray diffraction. The results show that CaMnO3 during the deposition process dissociates into CaO and most likely Mn2O3 with a bixbyite structure (space group Ia3). To avoid dissociation of deposited films we plan to further optimize PLD parameters such as laser wavelength, fluence and deposition atmosphere (addition of inert or reactive gas, deposition within plasma medium and deposition in with colliding laser plasmas).

calcium manganite ; synthesis ; morphology ; coatings

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