Mullitization from a Multicomponent Oxide System in the Temperature Range 1200°–1500°C

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Abstract
Mullitization from a multicomponent oxide system (alumina–kaolin–quartz–feldspar–talc) was analyzed as a function of firing temperature from 1200° to 1500°C based on quantitative XRD and SEM. In the present study, whisker forms of mullite grew in three characteristic stages. In the first stage (1255°–1295°C), mullitization (nucleation) took place from glass via alumina dissolution into glass under the condition of no apparent change in glass content. The reaction in this stage was rate‐limited by alumina dissolution into glass. Extensive mullitization occurred in the 1295°–1335°C range (second stage) directly from glass. Unlike in the sol–gel‐based binary system, alumina dissolution into glass was not shown to be the rate‐controlling mechanism during this extensive mullitization stage. Finally (>1335°C, third stage), the reaction was saturated, accompanied by an apparent decrease in glass consumption rate. The impingement of mullite whiskers by other whiskers and crystals was speculated to cause mullite to grow in the transverse direction, yielding a diminished reaction rate in the final stage. Mullitization stages in this work were compared with those of the alumina–silica binary system shown in the literature.