Initial sintering of magnesium oxide in carbon dioxide

Abstract

Studies have been made of the initial sintering (surface area diminution and crystallite growth) of MgO freely dispersed in CO₂. The sintering rate at 1123 K was directly proportional to the CO₂ pressure in the range 0.67–93.1 kPa, and the apparent activation energy for sintering was 153 ± 11 kJ mol^–1. Increases in the cation vacancy concentration by doping with Mn ions resulted in virtually no variation in the sintering rate. From sintering measurements in ¹⁸O-enriched CO₂, the bulk ¹⁸O distribution in crystallites grown by sintering was determined and it was shown that most of the anions migrate only on the surface during sintering. Relative sintering rate constants at 1123 K were (3.4 ± 0.3)× 10⁶, 38 ± 8, 1.3 ± 0.1 and 1 in H₂O, CO₂, O₂ and Ar, respectively.

It is concluded, on the basis of these facts, that sintering is enhanced by increased surface migration of O^2– ions (the slower moving ions), caused by repetition of the adsorption–desorption cycle of CO₂ molecules (anion-exchange mechanism).