Electrosurface transfer in the CaWO₄-WO₃ system

A novel solid-phase phenomenon is studied, namely, transfer of WO₃ onto inner surface of the CaWO₄ ceramics induced by an electric field. The phenomenon is shown to be intrinsic to any CaWO₄ ceramics notwithstanding the technique of its production. The mass of WO₃ transferred and the front penetration depth are found to be proportional to the charge passed. The process rate depends linearly on the porosity of the ceramics, which determines the magnitude of cross-section for the surface charge and mass transfer. The transfer is shown to be an activation process with an activation energy of 110 ± 4 kJ mol^-1. In order to characterize the activity of the substrates and the current efficiency of the process, a carry-away coefficient is introduced, which is defined as the number of equivalents of WO₃ transported by a faraday of electricity. The surface compositions of the initial ceramics and the CaWO₄-WO₃ composite produced are shown to be close to [W]/[Ca] = 2.