Jack Graham Firth

The effect of the physical form of the oxide on the conductivity changes produced by CH4, CO and H2O on ZnO

Abstract A study has been made of the effects of carbon monoxide, methane and water vapour on the electrical conductivity of zinc oxide at atmospheric pressure. Three physical forms of zinc oxide were investigated; polycrystalline masses, single crystals and compressed discs. Results are presented which show major differences in behaviour between the different physical forms. Methane and water vapour have much greater effects on the conductances of polycrystalline oxide than on those of single crystals. Single crystals of zinc oxide are shown to be potentially useful as sensors for low concentrations (ppm) of carbon monoxide in the presence of high concentrations (per cent) of methane.

The principles of the detection of flammable atmospheres by catalytic devices

Abstract Most instruments used for measuring the explosibility of fuel/air atmospheres use catalytic oxidation as a method of measurement. The detailed mechanism of this method has been examined and equation describing the output from these devices have been derived. The output V(LEL) at the lower explosive limit of a fuel has the general form V (LEL) =K D 12 δH[LEL], where D12, ΔH, and [LEL] are respectively the diffusion coefficient, heat of oxidation, and the lower explosive limit of the fuel in air, and K is a constant. Calculations have been made which enable the responses to explosive gas/air mixtures to be predicted and correction factors to be derived for practical devices. A new method for the measurement of explosiveness is discussed based on the empirical correlation between the heat of oxidation of the fuel and its lower explosive limit.

The effect of oxygen on the electrical conductivity of some metal oxides in inert and reducing atmospheres at high temperature

Abstract An investigation has been carried out on the elctrical behaviour at 900 °C of some ceramic oxides as a function of oxygen pressure. Measurements have been made in an inert (N 2 ) atmosphere and in an inert atmosphere containing reducing gases (CO, H 2 and CH 4 ). The oxides for which data are presented are Cr 2 O 3 , Nb 2 O 5 , CeO 2 , ThO 2 and Ga 2 O 3 . The factors affecting the electrical conductivities of these materials are briefly reviewed. The conductivities of Cr 2 O 3 , Nb 2 O 5 and CeO 2 showed a switch-like behaviour at the stoichiometric mixture of oxygen and reducing gas. Ga 2 O 3 and ThO 2 and do not exhibit this type of behaviour, the conductivities changing monotonically with increasing oxygen pressure. The possible reasons for this difference in behaviour are discussed. Data are also presented showing the behaviour of a sensor based on Ga 2 O 3 in a vehicle exhaust. This shows its potential as a means of monitoring exhaust emissions with a view to controlling the air/fuel ratios at around the stoichiometric mixture.

Catalytic oxidation of hydrogen over platinum

The oxidation of hydrogen on platinum has been studied using a microcalorimetric technique. The reaction takes place by two mechanisms depending on whether the surface is in an oxidised or reduced state. On the reduced surface, hydrogen is dissociatively adsorbed and competes with adsorbed molecular oxygen for adsorption sites. The reaction rate is higher on the oxidised surface where hydrogen molecules are weakly adsorbed and react with strongly adsorbed oxygen.