Reaction-induced grain growth impediment during sintering of strontium hexaferrite with silica addition
The present paper, referrinf to our previous studies on the action of silica during sintering of SrO excess strontium hexaferrite (SrFe12O19) is focused on the mechanism of the grain growth impediment. It is concluded that none of the three “classical” mechanisms - pore drag, precipitate drag and impurity drag - can explain satisfactorily the observed grain growth behaviour. Suggested by the observed correlation between dissolving of silica in hexaferrite and stopped grain growth, a new mechanism is put forward : “Reaction-induced grain growth impediment”. 
 
In this mechanism, a chemical force (Fc) is introduced, which counteracts the driving force for grain growth (Fs). This chemical force results from the fact that the concentration gradients of the solute at both sides of the curved grain boundary are different during the (dissolving) reaction of a reactive secondary phase. The ratio Fc/Fs determines what happens : impeded grain growth, stopped grain growth or even growth of the smaller grains resulting in a decreased spread in grain size.