2000 年 68 巻 1 号 p. 24-31
Hydrogen sensing properties and mechanism of Nb2O5 varistors mixed with Bi2O3 (0-16.7 mol %) were investigated in the H2 concentration range of 0.2-2.0 % at 400-700 °C. Pure Nb2O5 showed higher breakdown voltage and higher sensitivity of 1,200 V mm−1 to 2.0 % H2 at 400 °C than the ZnO- and SnO2-based varistors reported before. The H2 sensitive properties of a Nb2O5 varistor were improved by the addition of Bi2O3 up to 5.0 mol % and the Nb2O5 varistor mixed with 1.0 mol % Bi2O3 exhibited the highest sensitivity at 400 °C among the varistors tested. However, further addition of Bi2O3 resulted in significant deterioration of the sensitivity. The addition of Bi2O3 1ed to a slight decrease in the grain size, a change in shape of Nb2O5 particles and formation of Bi2Nb10-O28 the surface of Nb2O5 particles. A.c. impedance measurement was performed to investigate the electric and electrochemical properties of the varistols. Resistances of the Nb2O5-Bi2O3 varistors were decomposed into four components, (i) bulk resistance (R0), (ii) grain boundary resistance (R1), (iii) resistance of oxide ion conduction (R2) and (iv) electrode-oxide interface resistance (R3). The R1, R2 and R3 decreased drastically with increasing H2 concentration, while R0 remained almost unchanged at 400 °C. Further studies have confirmed that R1 mainly dominated the breakdown voltage of the Nb2O5-Bi2O3 varistors, and then the change in the potential barrier height per grainboundary determined the magnitude of the H2-induced shift in the breakdown voltage.
