MANGANESE SEGREGATION ON THE SURFACE OF SNO2 BASED POWDERS

SnO2 based powders, containing different manganese amounts (0 less tha n or equal to Mn/(Mn+Sn)less than or equal to<0.15 cationic ratio) pre pared by a chemical route and calcined in the same time and temperatur e conditions have a specific surface area which increases with the man ganese concentration. Such a behaviour has been related to a surface p henomenon, namely segregation of manganese at the gain surfaces and re lated defects, i.e. oxygen vacancies. A model, based on the variation of the total number of oxygen vacancies in the grain peripheral layers , is proposed to explain the observed variations of the powder charact eristics. This model is in agreement with the densification behaviour of these powders. Shrinkage starts after grain growth and is effective for a constant manganese concentration on the SnO2 surface equal to 5 .10(-6) mol/m(2).