Characterization And Properties Of The Nonstoichiometric Magnesia-alumina Spinel

Spinel,which is a derivative structure of the basic structure rock salt,is known as its superior mechanical properties and radiation resistance or lower chemical leaching.It has important application prospects in immobilization for high-level radioactive waste,a host of inert matrix fuels in light water reactors?LWRs?and minor actinide transmutation targets.The application of a material in the corresponding nuclear industry is dependent on its composition,mechanical properties and radiation resistance.The studies described in this project examine the nonstoichiometric magnesia-alumina spinel of varying composition.The investigations discussed in this thesis divided three objectives:?1?to study composition and structure of nonstoichiometric magnesia-alumina spinel?MgO·nAl₂O₃?;?2?to study the mechanical properties of single-phase spinel,and?3?to study the radiation resistance of single-phase spinel.A series of samples with different compositions?MgO·n Al₂O₃?was prepared by high temperature solid-state reaction at 1600?to study the ranges of single-phase spinel.Composition and microstructure were examined by various techniques including X-ray diffraction?XRD?,scanning electron microscopy?SEM?and elemental analysis.The results indicate that the single-phase magnesia-alumina spinels have been prepared successfully for the first time ranging from n=0.667 to n=1.5,which is beyond the previous reported ranges of n?0.91.This result can modify the previous T-C synthesis phase diagram and provide a basis for further study of the composition of n<1.The degree of disorder decreases first and then increases when we introduce excess MgO or excess Al₂O₃ to MgO·Al₂O₃.For the magnesia rich or alumina rich compositions,most of Mg²⁺or Al³⁺tend to occupy the tetrahedral or octahedral sites considering the ionic radius and valence,which will lead to a decrease in the ratio of cation inverse.The excess Mg²⁺or Al³⁺would result in an increased inverse when the tetrahedral sites of Mg²⁺or the octahedral sites of Al³⁺reaches a saturated amount.On the other hand,the mechanical properties and radiation resistance of single-phase compositions,MgO·nAl₂O₃?n=0.667-1.5?,have been systematically studied.Mechanical properties are characterized by hardness and elastic modulus,which are measured by nanoindentation.These spinel compositions synthesized in this study exhibit a hardness of 18-21 GPa and a modulus of 300-320 GPa.It means that our samples possess superior resistance to elastic and plastic deformations.In addition,the hardness and modulus show a tendency to decrease with increasing n,which is contrary to the trend of grain size.In the irradiation experiment with 400 keV Ne²⁺,it can be found that the structure of the irradiated samples did not change significantly.It shows that the spinel has extremely high radiation resistance.According to the relation of lattice swelling rate of sample,it can be found that the lattice swelling rate of all component samples is increasing first and then decreasing.The lattice swelling rate of aluminum excess spinel?n>1?was significantly lower than that of other components?n?1?,which indicated that Al excess spinel had better anti-lattice-swelling ability.The conclusions in this paper provide an important basis for the study of non-stoichiometric spinel.