Investigation of the thermal expansion and heat capacity of the CaCu3Ti4O12 ceramics

The thermal expansion of the CaCu3Ti4O12 ceramics has been measured over a wide temperature range 120–1200 K. The high quality of the samples under study has been confirmed by good agreement of the results of measurements of the heat capacity in the range 2–300 K and in the vicinity of the phase transi tion of magnetic nature at 25 K with the data for the single crystal. No anomalies in the thermal expansion that can be associated with the phase transition at 726–732 K assumed by other investigators have been found. The influence exerted on the thermal expansion by the heat treatment of the sample in a helium atmosphere and in air has been investigated. DOI: 10.1134/S1063783412090120 2. SAMPLE PREPARATION AND EXPERIMENTAL TECHNIQUE Ceramic samples were prepared by the procedure described in [10]. The synthesis of CCTO was performed by the method of joint precipitation of precursor oxalates with the use of TiCl3, CaCl2, and CuCl2 · 2H2O. The starting compounds were dissolved in water in various ratios, while the precipitation was performed with the addition of the solution of the oxalic acid in ethanol. The TiCl3 compound was selected from reasons of operation simplicity compared with TiCl4. The com plete transformation of Ti3+ into Ti4+ in the solution was provided by air passage during the reaction and controlled by the variation of the color from white to blue. The water volume was selected much smaller than the volume of ethanol for decreasing the dielec tric constant of the precipitate. Since the nucleation is preferable compared with the growth, it is consider ably easier to obtain small particles homogeneous in size and composition. The solution was evaporated for 2 h, and the obtained precipitate was dried in a centri fuge. Precursors were then calcined in air at 900°C for 10 h to obtain oxides. Microphotographs obtained using a JEOL JSM 6700 scanning electron micro scope showed the homogeneity of the grains over the shape and size, which varied in a range of 0.2–1.0 μm. The obtained powder was pressed into the discs 6 mm in diameter and 5–6 mm thick at a pressure of 250 MPa. Green ceramics was obtained by sintering in air at 1050°C for 24 h. The X ray diffraction analysis showed that the sample contains only an insignificant amount of impurities (CaTiO3 and Cu2O), no larger than 3%. The unit cell parameter a0 = 7.391 Å deter mined at room temperature agrees well with the parameter known for the single crystal a0 = 7.3906 Å [11]. In order to characterize the sample by the physical properties, the heat capacity of the ceramics was investigated in a temperature range of 2–300 K by the relaxation method using the PPMS calorimeter (Quantum Design Inc., San Diego, CA). To form good thermal contact of the sample weighing ~150 mg with the measuring system, we used the vacuum low temperature grease Apiezon N. The heat capacity of the grease was measured in the separate experiment. The measurement error of the heat capacity was 1% below 60 K and ~0.25–0.50% above 100 K. The thermal expansion measurements were per formed using the DIL 402C induction dilatometer NETZSCH in a temperature range of 120–1200 K in the dynamic mode with heating and cooling rates of 3–5 K/min for ceramic samples with L ≈ 5 mm. To calibrate and take into account the thermal expansion of the measuring system, we used reference samples from the fused quartz and corundum. 3. EXPERIMENTAL RESULTS AND DISCUSSION