H-2 sensing properties and mechanism of Nb2O5-Bi2O3 varistor-type gas sensors

Hydrogen sensing properties and mechanism of Nb2O5 varistors mixed with Bi2 O3 (0-16.7 mol %) were investigated in the H-2 concentration range of 0.2-2 .0 % at 400-700 degrees C. Pure Nb2O5 showed higher breakdown voltage and h igher sensitivity of 1,200 V mm(-1) to 2.0 % H-2 at 400 degrees C than the ZnO- and SnO2-based varistors reported before. The H-2 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 se nsitivity at 400 degrees C among the varistors tested. However, further add ition of Bi2O3 resulted in significant deterioration of the sensitivity. Th e addition of Bi2O3 led to a slight decrease in the grain size, a change in shape of Nb2O5 particles and formation of Bi2Nb10O28 at the surface of Nb2 O5 particles. A.c. impedance measurement was performed to investigate the e lectric and electrochemical properties of the varistors. Resistances of the Nb2O5-Bi2O3 varistors were decomposed into four components, (i) bulk resis tance (R-0), (ii) grain boundary resistance (R-1), (iii) resistance of oxid e ion conduction (R-2) and (iv) electrode-oxide interface resistance (R-3) The R-1, R-2 and R-3 decreased drastically with increasing H-2 concentratio n, while R-0 remained almost unchanged at 400 degrees C. Further studies ha ve confirmed that R-1 mainly dominated the breakdown voltage of the Nb2O5-B i2O3 varistors, and then the change in the potential barrier height per gra inboundary determined the magnitude of the H-2-induced shift in the breakdo wn voltage.