The reactivity in the system CuCl-Si related to the activation of silicon in the Direct Synthesis

The reaction between cuprous chloride CuCl and silicon is the classical model reaction for activation of the contact mass in Direct Synthesis. It had been found by several authors that the reactivity of CuCl strongly depends on its quality and the route of synthesis. In the present study the reactivities of two CuCl charges, prepared from the elements at high temperatures and by reduction in aqueous solution, are compared by means of calorimetric studies. The aqueous route CuCl was found to have a higher reactivity, indicated by lower reaction start temperatures and 100% CuCl conversion with respect to the other charge. It was found that the reactivity depends neither on intrinsic properties (defect concentration) nor on properties that can be derived from microscopical or thermoanalytical characterizations or by X-ray powder diffraction. The initial step of reaction between cuprous chloride and silicon was identified as a solid state reaction. Therefore, the role of the CuCl surface is discussed and a model for the initial step is suggested.