Use this URL to cite or link to this record in EThOS:
Title: Microstructure and thermal expansion behaviour of magnesia-magnesium aluminate composites
Author: Buggakupta, Wantanee
ISNI:       0000 0001 3508 277X
Awarding Body: University of Surrey
Current Institution: University of Surrey
Date of Award: 2008
Availability of Full Text:
Access from EThOS:
Access from Institution:
Knowledge of the coefficient of thermal expansion (CTE) of a ceramic material is important in many application areas. Whilst the CTE can be measured, it would be useful to be able to predict the expansion behaviour of multiphase materials. There are several models for the CTE, however, most require a knowledge of the elastic properties of the constituent phases and do not take account of the microstructural features ·of the material. If the CTE could be predicted on the basis ofmicrostructural information, this would then lead to the ability to engineer the microstructure of multiphase ceramic materials to produce acceptable thermal expansion behaviour. To investigate this possibility, magnesia-magnesium aluminate spinel (MMAS) composites, consisting of a magnesia matrix and magnesium aluminate spinel (MAS) particles, were studied. Having determined a procedure to produce MAS from alumina and magnesia, via solid state sintering, magnesia-rich compositions with various magnesia contents were prepared to make the MMAS composites. Further, the MMAS composites prepared from different powders (i.e. from an alumina-magnesia mixture and from a magnesia-spinel powder) were compared. Com starch was added into the powder mixtures before sintering to make porous microstructures. Microstructural development and thermal expansion behaviour of the MMAS composites were investigated. Microstructures of the MAS and the MMAS composites as well as their porous bodies were quantified from backscattered electron micrographs in terms of the connectivity of solids i.e. solid contiguity by means of linear intercept counting. Solid contiguity decreased with increasing pore content and varied with pore size, pore shape and pore distribution whereas the phase contiguity depended strongly on the chemical composition and was less influenced by porosity. The thermal expansion behaviour of the MAS and the MMAS composites between 100 and 1000 °C was determined experimentally. Variation in the CTE of the MAS relates to the degree of spinel formation while the thermal expansion of the MMAS composites depends strongly on phase content. However, the MMAS composites with similar phase compositions but made from different manufacturing processes showed differences in microstructural features and thermal expansion behaviour. Predictions of the CTE values for composites based on a simple rule-of-mixtures (ROM) using volume fraction were compared with the measured data. A conventional ROM accurately predicted the effective CTE of a range of dense alumina-silicon carbide particulate composites but was not very accurate for porous multiphase structures. It provided an upper bound prediction as all experimental values were lower. Hence, the conventional ROM was modified to take account of quantitative microstructural parameters obtained from solid contiguity. The modified ROM predicted lower values and gave a good agreement with the experimental data. Thus, it has been shown that quantitative microstructural information can be used to predict the CTE of multiphase ceramic materials with complex microstructures.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available