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We show that the effects of microstructure on the response of gas-sensitive resistors based on semiconducting oxides can be understood in a pragmatic and practically useful way using a simple three-element resistance network, in which only one of the elements is gas-sensitive. This model, with the gas-sensitive resistance showing a simple form of response consistent with surface reaction models, displays the power-law response to variation of gas concentration (P g ) shown by practical devices: G = A g P β g , where G is (R − R 0 )/R 0 for resistance increase or (σ − σ 0 )/σ 0 for conductance increase. The observations that β varies widely between preparations, is different for different gases on the same sensor, and changes with change of the relative humidity of the gas, are simply explained as being due to changes in the relative values of the resistors in the network, related to the microstructure. The model predicts that, for a range of sensor preparations responding to a given gas, A g and β should be correlated. The predictions are confirmed by measurements of the response of a wide range of microstructures of sensors of both tin dioxide and chromium titanium oxide to toluene, ethanol and carbon monoxide in atmospheres of varying relative humidity. We show that the correlation of A g and β is a powerful tool for discovering subtle effects on the sensor response. These include: effects due to gas concentration gradients within the sensing layer, effects of variation in microstructure throughout the sensing layer, the extent of sintering of the material in the finished sensor, and whether water vapour acts on the sensor surface synergistically or independently of the reactive gas being measured.

Chromium titanium oxide (CTO) Cr 22x Ti x O 3 (x~0.05 to 0.4) is the first gas-sensitive resistor material successfully to be commercialised since tin dioxide in the 1960s. Microspheres were synthesised by the solemulsion-gel method and characterised by X-ray diffraction, scanning electron microscopy (SEM/EDAX) and X-ray photoelectron spectroscopy (XPS). Three differently sized microspheres of size 10, 2 and 0.7 mm were synthesised. The microspheres showed minimal variation in size and good compositional homogeneity. The CTO powders fired at 1000 ‡C crystallised as single phases for Cr 1.95 Ti 0.05 O 3 , Cr 1.9 Ti 0.1 O 3 and Cr 1.8 Ti 0.2 O 3 . For xw0.2 the formation of a secondary CrTiO 3 phase was noted. EDAX and XPS measurements revealed the absence of impurities and a surface segregation of the Ti atoms.

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The evolution of conduction and polarization phenomena is studied as a function of dehydration temperature (TT varying from 100 up to 400 °C) on a faujasite Y zeolite. The measurements are carried out as a function of frequency at different temperatures. In all cases, the conductivity is ascribed to the migration of cations in large cages involving energies varying from 0.61 to 0.85 eV. Two relaxation domains are observed. It is assumed that they are due to the movement of cations of sites III‘ and II. The existence of these two domains is confirmed by the technique of the thermally stimulated currents. The spreading domain of relaxation depends on the dehydration state of the zeolite.