Effects of Grinding on the Photoconductive Properties of CdS Powder

Abstract

The effects of ball milling on the photoconductivity process in ground CdS powders were investigated. A sample powder was prepared from aqueous solution of cadmium sulphate. The powder was found to have a hexagonal structure, and the mean particle size was about 10μm. The powder was ground in an alumina ball mill for different periods from 30min to 30hr, In the early stage of the grinding process, the mean particle size decreased rapidly to about 2μm and it did not decrease any more even if grinding time was increased.The X-ray analysis showed the following results: (1) The transformation of CdS from hexagonal to cubic does not take place during grinding by using the alumina ball mill. (2) The degree of lattice distortion increases with increasing grinding time.Reflectance of the ground CdS powders decreased slightly with grinding time in the wavelength region of 550∼700nm. This reduction could be explained by an increase in lattice distortion.Using an interdigital electrode, the spectral response of photoconductivity, the photocurrent-light intensity characteristic and the decay of photocurrent were measured for the ground CdS powders. The photocurrent decreased considerably in the early stage of the grinding process, and then decreased gradually with increasing grinding time. This change was similar to the change of the mean particle size with grinding time. As the result of this similarity, it became evident that the considerable decrease in photocurrent by grinding is atributable to an increase in interparticle contact resistance of pressed CdS powders.The photoconductivity spectral response of the ground CdS powders showed a larger decrease in the short wavelength region than in the long wavelength region. This could be explained by structural imperfections generated near the CdS surface. The imperfections generated by grinding acted as the recombination centers and lowered the photosensitivity, especially in the short wavelength region. It was found also that the recombination centers facilitated the decay of photoconductivity. Moreover imperfections could also act as the trapping centers and lower the speed of response.