PhD Theses - Nicholas Ashley
Defect Properties of Binary Non-Oxide Ceramics
This thesis considers the structures associated with nonstoichiometry in nitride materials, namely TiN, ZrN and HfN. Although these materials exhibit the simple rock-salt structure, they exist over an exceptionally wide range of nitrogen deficient compositions, MN1-x, were 0 < x < 0.3, without significant changes in lattice parameter. Predictions of lattice parameters and properties are used to identify the mechanism responsible for accommodating this broad range of compositions and the properties associated with these materials. Clustering of defects for highly defective materials is also considered, significant improvement in the predictive model. For each of the nitride materials examined, a clear improvement in the range of compositions predicted to form is demonstrated. Binary and ternary solid solutions of the mono-nitrides are predicted, and compared with Vegards Law, with both the structure and properties of ternary solid solutions showing a close correlation with this relationship.
The same method is applied to the construction of a model to predict the structure and properties of UN. Similar behaviour to the other nitride materials is observed, however, when predicted values are validated against experimental observations discrepancies are seen. The results are useful in helping to understand the deficiencies of the model.
Nonstoichiometric compounds of TiC, ZrC and HfC behave very similarly to their equivalent nitride materials. None of the carbide materials examined is seen to exhibit metal deficient nonstoichiometry, however, variations in carbon deficient nonstoichiometry are again associated with a remarkably small deviation in lattice parameter, and local relaxation. Predictions of binary and ternary solid solutions of mono-carbides are also presented, and deviations from Vegards law are discussed.