Studies of methods to restrict the grain growth of nanocrystalline metal oxides

There is considerable interest in nanocrystalline materials. This thesis is concerned with nanocrystalline oxides and the development of methods to prevent their grain growth on heating. This growth, which is evident at temperatures as low as 400°C, presents a serious problem in the study and applications of nanocrystalline oxides. The systems that were studied were nanocrystalline magnesium oxide, zirconium oxide, cerium oxide and tin oxide. The methods of preventing grain growth included the encapsulation of the oxide in the pores of porous silica, mixing with nanocrystals of alumina and treating the surface with a silanising agent, hexamethyldisilazane. All the methods employed showed some effect on reducing the grain growth. Encapsulation in the pores of silica was effective, however it proved difficult to get large amounts of the oxides into the pores. A more efficient method of preparing large samples was the incorporation of alumina, which was achieved by a sol-gel process. An alkoxide of the target oxide and an aluminium alkoxide were mixed and then hydrolsed and calcined. This proved very effective for magnesium oxide, zirconium oxide and tin oxide. For example, heating zirconium oxide at 1000°C for 60 minutes causes the nanocrystals to grow to about 50nm. Treatment with alumina restricts the growth to 12nm. Similar effects were found for the other oxides, although magnesium oxide showed a reaction with alumina at the highest temperatures. Silanising the surface was only studied for tin oxide and it restricted growth at 1000°C to 27nm, compared to 88nm for an untreated sample. A full description is given of the preparative methods and structural studies of the systems using X-ray diffraction and X-ray absorption spectroscopy.