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Sillimanite‐mullite transformation observed in synchrotron X‐ray diffraction experiments
Author(s) -
Igami Yohei,
Ohi Shugo,
Miyake Akira
Publication year - 2017
Publication title -
journal of the american ceramic society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.9
H-Index - 196
eISSN - 1551-2916
pISSN - 0002-7820
DOI - 10.1111/jace.15020
Subject(s) - mullite , activation energy , arrhenius equation , sillimanite , materials science , nucleation , diffusion , crystallography , analytical chemistry (journal) , mineralogy , chemistry , thermodynamics , physics , metallurgy , ceramic , quartz , chromatography , biotite
High‐resolution synchrotron powder X‐ray diffraction ( XRD ) experiments were conducted to clarify the transformation of sillimanite to mullite (mullitization) and determine the mullitization temperature ( T c ). We were able to distinguish sillimanite and mullite in the XRD patterns, despite their very similar crystallographic parameters, and to detect the appearance of small mullite peaks among sillimanite peaks. Analysis of the J ohnson‐ M ehl‐ A vrami ( JMA ) equation for mullitization ratio (ζ) revealed that at temperatures T ≥1240°C the mullitization had the same kinetics. The activation energy E at T ≥1240°C obtained from the Arrhenius plot was 679.8 kJ mol −1 . In analysis using a time‐temperature‐transformation diagram for mullitization, a mullitization curve of ζ=1% can be described asln t = 1 n ln ln 1 1 − ζ− lnk 0 + A / [ R T T − T c 2 ] + E A / R Twhere t is time, n is a reaction‐mechanism‐dependent parameter determined as 0.324 by JMA ‐analysis, k 0 is the frequency factor, E A is the activation energy for atomic diffusion, and A /T c − T 2represents the activation energy for nucleation. The results of fitting the data to this equation were T c =1199°C, A =3.9×10 6 kJ mol −1 K −2 , E A =605 kJ mol −1 , and k 0 =3.65×10 15 . We conclude that the boundary between sillimanite and mullite+ SiO 2 in the phase diagram is ~1200°C.