KINETICS AND MECHANISM OF DISSOLUTION OF MGO CRYSTALS

The kinetics of dissolution of {1 0 0}, {1 1 0} and{1 1 1} surfaces of MgO crystals have been investigated in H2SO4 and H3PO4. It is observed that in concentrated H2SO4 and in concentrated and dilute H3PO4, the etch rates change in the sequence {1 1 1}>{1 1 0}> {1 0 0}; and in dilute H2SO4 in the sequence {1 0 0}>{1 1 1}>{1 1 0}. From the etch rates determined for 4N H2SO4, 36N H2SO4 and 4N H2SO4-glycerine solutions at different temperature, the values of activation parameters are calculated. It is found that the value of the activation energy in a particular acid is practically constant for different planes, but that of the pre-exponential factor changes. Increase in the viscosity of 4N H2SO4 due to the addition of glycerine lowers the etch rate but the value of activation energy remains unchanged. However, in highly viscous 4N H2SO4 etchant, the value of the activation energy of selective etching decreases somewhat above 60° C, and the etch pits formed at dislocations are deeper and larger than those at screw dislocations. The results are discussed and the important conclusions are enumerated. © 1979 Chapman and Hall Ltd.