Yumi Masuoka

Nitrogen oxides sensing characteristics of Zn2SnO4 thin film

Abstract A thin film of Sb-doped Zn2SnO4 with spinel-type structure was deposited on an alumina substrate by sputtering from multi-targets of constituent metals, followed by annealing at 1000°C in air for 1 h. The film showed fairly good sensing characteristics to nitrogen dioxide over the range from 0 to 300 ppm at 600°C, though the interference by nitrogen monoxide and iso-butane was rather large. The durability of the film in a reductive atmosphere at 600°C was proved. The electrical resistance of the film was found to respond well to the total concentration of nitrogen oxides in an engine exhaust.

NO2 sensing characteristics of Nb doped TiO2 thin films and their electronic properties

Thin films of Ti–Nb alloy were deposited on an Al2O3 substrate by sputtering from multi-targets of constituent metals, followed by annealing at 1000°C in 1% O2/N2. The films consisted of rutile-type TiO2 structure and showed fairly good sensing characteristics to NO2 over the range from 100 to 300 ppm at 600°C. According to the Arrhenius plot between the conductivity and the reciprocal temperature, the conductivity of the fairly Nb doped films were high and showed a saturated range among 300–600°C, on the other hand, that of undoped or slightly doped films were low and no saturation range was observed. After the temperature programmed desorption (TPD) measurement, no differences were observed on the desorption characteristics of NO2 between different amount of Nb-doped powder samples. A numerical analysis taking account of carrier concentration and mobility indicated that the majority electronic carrier has been alternated from p-type to n-type according to the amount of doped Nb, and also reproduced some complicated behavior of slightly doped film. As a result, NO2 sensing characteristics of the Nb doped TiO2 films seem to mainly depend on the electronic properties of the film.

Field-modulated thermopower in SrTiO3-based field-effect transistors with amorphous 12CaO・7Al2O3 glass gate insulator

We report transistor characteristics and field-modulated thermopower (S) for single crystal SrTiO3-based field-effect transistors (FETs). We use 150-nm-thick amorphous 12CaO⋅7Al2O3 glass as the gate insulator of the SrTiO3-FET. The resulting SrTiO3-FET exhibits excellent transistor characteristics at room temperature: on-to-off current ratio greater than 106, threshold gate voltage of +1.1 V, subthreshold swing of approximately 0.3 V decade−1, and effective mobility of 2 cm2 V−1 s−1. The field-modulated S-value of the SrTiO3-FET varied from −900 to −580 μV K−1 with electric fields of up to 2 MV cm−1, demonstrating the effectiveness of the FET structure for the exploration of thermoelectric materials.

A thick-film NO2 sensor fabricated using Zn-Sn-Sb-O composite material

Thick film NO2 sensors comprised of various Zn–Sn–Sb–O composite materials have been studied. The gas-sensing materials were prepared by sintering a mixture of ZnO, SnO2 and Sb2O3 powders at 800, 1000 and 1200°C. Thick films were fabricated on an Al2O3 substrate using a conventional screen-printing method. The atomic ratios of the fabricated films were confirmed by the results obtained from EPMA (Electron probe micro analysis). The dominant compositions of the sintered gas-sensing materials were characterized by XRD (X-ray diffraction). It was found that the composition of the gas-sensing material depended on the initial Sb/Zn atomic ratio, where the gas-sensing material shows almost single-phase ZnSb2O6 when Sb/Zn = 2.0, a mixture of ZnSb2O6 and SnO2 when Sb/Zn = 1.0 and a mixture of SnO2 and an unknown crystal when Sb/Zn = 0.5. The sensing film shows high sensitivity and excellent selectivity for ppm levels of NO2 gas because the film shows no sensitivity for i-C4H10 and CO gases. The capability for NO2 detection in a practical application was examined by using the sensor to monitor the gas which was emitted from a kerosene heater.