Many researchers have made great efforts to produce bulk materials having smaller grains because of their excellent mechanical, optical, magnetic and electronic properties. But, the prospects for practical production of nanocrystalline materials have been seriously hampered by difficulties in controlling the rapid grain growth of nanoparticles during a sintering process. The rapid grain growth on the final stage sintering has been considered as almost inevitable, therefore, many researchers have resorted to high pressure consolidation at elevated temperatures, which has limitations in practical applications.

Since unagglomerated and smallest possible nanoparticles are usually needed to satisfy a necessary condition suppressing rapid grain growth, the controlled synthesis of nanoparticles should be very important. Internal defects such as oxygen vacancy are known to accelerate the rate of grain growth, therefore, synthesizing defect-free pure nanocrystals should be also important. Since we could produced much smaller unagglomerate spherical anatase nanoparticles by reducing the characteristic time of coalescence of nanoparticles at high concentrations and also the particles are nearly oxygen-vacancy defect free, our control method might give a solution of the above problems.

Producing a full dense titania bulk ceramic with 60 nm grains  

Nanoparticles synthesized from our proposed method exhibited not only excellent thermal stability, but also unusually small grain growth during a simple pressureless sintering, which ultimately lead to a full dense titania bulk ceramic with 60 nm grains. Such small grain size in full dense ceramics has not been achieved before without employing special sintering techniques. We are now trying to produce other bulk nanocrystalline materials with smaller grains using the same principle.

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