Development of Na0.5CoO2 Thick Film Prepared by Screen-Printing Process.

The Na0.5Co0.9Cu0.1O2 thick film with the same thermoelectric performance as a Na0.5CoO2 bulk was formed on an alumina substrate by the screen-printing process. The power factor exceeded 0.3 mW/K2m, with the resistivity of 3.8 mΩcm and the thermopower of 108 µV/K. The thick film without any cracks strongly adhered to the substrate. The high-quality thick film had been realized through the carefully designed and improved process, mixing NaCl to promote the anisotropic sintering of Na0.5Co0.9Cu0.1O2, inserting a CuO interlayer to adhere the film and substrate, and Co-Cu substituting Cu for Co to control the sintering temperature.

Co-Substitution Effect in Room-Temperature Ferromagnetic Oxide Sr3.1Y0.9Co4O10.5.

We investigated the Co substitution effect for the magnetic properties in room-temperature ferromagnetic oxide Sr3.1Y0.9Co4O10.5. The substituted element (Al and Ga) and low-spin state Co3+, which was changed from a high-spin or intermediate-spin state by Al or Ga substitution, reduced the Curie temperature to even 1.5 times lower than the temperature estimated from a simple dilution effect. Al3+ preferentially substituted for intermediate-spin-state Co3+ in the ferrimagnetic CoO6 layer and deteriorated the saturation magnetization of Sr3.1Y0.9Co4O10.5. By contrast, Ga3+ substituted for high-spin-state Co3+ in the CoO6 layer and/or the antiferromagnetic CoO4.25 layer and enhanced the saturation magnetization per Co ion. These results indicate that the magnetic properties of Sr3.1Y0.9Co4O10.5 can be controlled by selectively substituting for Co3+ with different spin states.

Unusually Small Thermal Expansion of Ordered Perovskite Oxide CaCu3Ru4O12 with High Conductivity.

We measured the coefficient of thermal expansion (CTE) of conducting composite ceramics 30 vol.% CuO-mixed CaCu3Ru4O12 together with CaCu3Ru4O12 and CuO. Although conducting ceramics tend to show higher CTE values than insulators, and its CTE value does not match with other ceramic materials, the CTE of CaCu3Ru4O12 (7⁻9 × 10-6/K) was as small as those of insulators such as CuO (9 × 10-6/K), alumina (8 × 10-6/K), and other insulating perovskite oxides. We propose that the thermal expansion of CaCu3Ru4O12 was suppressed by the Cu-O bond at the A-site due to the Jahn⁻Teller effect. This unusually small CTE of CaCu3Ru4O12 compared to other conducting oxides plays a vital role enabling successful coating of 30 vol.% CuO-mixed CaCu3Ru4O12 thick films on alumina substrates, as demonstrated in our previous study.

Trial of an All-Ceramic SnO2 Gas Sensor Equipped with CaCu3Ru4O12 Heater and Electrode.

We have constructed a gas sensor of SnO2 equipped with ceramic electrodes and a heater made of CaCu3Ru4O12, which demonstrated good device performance at high temperature. The CaCu3Ru4O12-based electrodes and heater were formed on Al2O3 substrates using a screen-printing process, which is cost-effective and suitable for mass-production. This all-ceramic device reached 600 °C at the lowest, and remained intact after one week of operation at 500 °C and rapid thermal cycling of 500 °C temperature changes within 10 s. We propose CaCu3Ru4O12 as a robust and reliable conducting material that can be a substitute for Pt in various devices.

Study of entropic characteristics of strongly correlated systems using VO2 as a model case.

To explain the huge caloric effects often observed in the first-order electronic phase transition in the strongly correlated oxides, the entropic characteristics are investigated in VO2. By evaluating the spin and charge fluctuations based on the local moment model and the Sommerfeld coefficient in the high-temperature rutile phase, it is found that these fluctuations of the high-temperature phase are the main source of the entropic change during the transition. This mode of entropic change is realized by the quenching of these fluctuations owing to the formation of a singlet bonding state in the low-temperature monoclinic phase. By introducing oxygen deficiency, a vagueness in the gap at the Fermi level is confirmed by the transport data, the X-ray photoelectron spectra and also the electronic structure calculated by the first-principles calculations. In this case, the entropic feature at the transition is weakened. Consequently, the large caloric phenomena of the strongly correlated oxides are a result of the conversion of the internal energy gain owing to the orbital selection at the ground state into the free energy gain owing to the spin and charge fluctuations at finite temperature.