Study On Transparent Ceramics MgAl₂O₄ By Positron Annihilation Lifetime Technique

In this thesis positron annihilation technique and irradiation effect are outlined , and the defects of transparent ceramics MgAl2O4 are studied by positron annihilation lifetime spectra.Transparent ceramic MgAl2O4 is a kind of poly-functional material . Because of its transparent and anti-irradiation it can be used as windows for infrared laser , tips of infrared missile and lens of camera working in high temperature and irradiation environment. As a novel material it can be used in wide range .The quality of MgAl2O4 can be depended on its transparent and anti-irradation capability while it is used as window , and thus depend on the defects in it. Positron annihilation lifetime spectra technique is a special technique useful to research the defects in material . It is so sensitive to defects that we use it to study the transparent ceramics MgAl2O4.We find that there exist some grain boundaries in transparent ceramic MgAl2O4. So the lifetimes of positron of grain boundaries and irradiation defects are mixed up . This make T2 have different changing character for ceramics MgAl2O4 and crystal MgAl2O4.After irradiated by electron , proton and Y -ray there have a lot of cationvacancies in the material . With the dose of Y -ray increasing the defects changing in three stages , this correspond to defects' creation ,decomposition and coalescence . The samples are annealed after irradiated . At first the defects in samples are decomposed when annealing temperature is 250℃ . With the annealing temperature increasing the defects are recoalesced , and then disappear at 500℃ . This means that after irradiated by electron , proton and y -ray the type of defects created in samples are all similar . So they have similar annealing character .The samples doped with La are studied too . We find that some La atoms are doped into vacancies when Al2O3/MgO is 2.3 and quantity of La lower than 50ppm. But this effect is not fount when Al2O3/MgO is 2 .Comparing the positron lifetime data . We can conclude that the stoichoimetry samples have more ability of resisting against irradiation than the nonstoichoimetry samples and the samples doped with La.