Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

2 THE SOLID-GAS INTERFACE 213L,.iFig. 19. Alternating current bridge circuit for the measwement of electrical conductivity.lo-’ ohms the Kelvin double bridge can be employed’46. Alternating currentmethods can be used to overcome difficulties associated with direct currentmeasurements. Amongst these are barriers at internal surfaces and contact resistances,which occur particularly in powders and evaporated films. The bridge usedrequires two balances, one for the resistive and one for the capacitative elementsof the impedance being studied (Fig. 19). Current is supplied to the bridge froman ac oscillator with a frequency range of 20 cycles per sec to 10 Mc per sec.The detector D is normally a pair of earphones connected directly, or in the caseof radiofrequencies, through a radio receiver. Since most adsorption studies havebeen conducted on powders or evaporated films in order to obtain a sufficientlyhigh surface-to-volume ratio, the ac bridge method has a particular advantage.Other high frequency techniques which have been used are the Q-meteri4’ (whichmeasures the breadth of a resonance curve) and microwave ad~orption’~~.(b) SemiconductorsThe term “semiconductor” covers a wide range of materials, including theelements silicon and germanium, and compounds such as oxides, sulphides,selenides, etc. A survey of the physics and chemistry of semiconductors is givenin the book edited by Hannay14’ and particular reference to semiconductivityand adsorption on oxide surfaces is to be found in an article by GraylS0.In semiconductors, the surface electrical double layer penetrates the crystal toa distance of about lov4 cm, and the mobility, as well as the number, of currentcamers may be altered by the adsorption process. For this reason, conductivitymeasurements alone are insufficient to quantitatively determine the extent ofcharge transfer across the surface. For a simple n-type semiconductor, adsorptionReferences pp. 270-278