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Thermal Expansion Discontinuities of Mullite
Author(s) -
Schneider Hartmut,
Rodewald Katia,
Eberhard Emil
Publication year - 1993
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/j.1151-2916.1993.tb04033.x
Subject(s) - thermal expansion , mullite , classification of discontinuities , discontinuity (linguistics) , materials science , tetrahedron , mineralogy , condensed matter physics , thermodynamics , composite material , chemistry , crystallography , mathematical analysis , physics , mathematics , ceramic
Thermal expansion data of mullite were collected between 25° and 800°C with high accuracy by means of an X‐ray high‐temperature Guinier powder camera, and high‐resolution single‐crystal Bond techniques. 3/2‐type mullites, synthesized from chemically coprecipitated precursors and 2/1‐type fused‐mullite single crystals, were used for the measurements. Mullite displays low and nonlinear thermal expansions along the crystallographic a, b , and c axes below about 200°C, and linear expansions between 200° and 400°C. Between about 400° and 500°C, expansion curves are discontinuous. Above 500°C, a, b , and c expansions are linear again. The temperature‐induced expansion discontinuities are relatively strong parallel to a , but are much weaker parallel to b and c . The slopes of the a and volume expansion curves are higher above the expansion discontinuity than below, while there is no significant tendency for those of b and c . Thermal expansion coefficients and discontinuity effects are reversible without any hysteresis. The increase of the a and volume expansion curves above the expansion discontinuities may be due to possible deformations, rotations, and tiltings of the [AlO 6 ] octahedra. Another factor responsible for the higher a and volume expansion coefficients at high temperatures may be a geometrical deformation of the [Al ⋆ O 4 ] tetrahedra. At present, no conclusive model exists to explain the discontinuous change of thermal expansion coefficients between 400° and 500°C.