Biomedical Engineering Reference
In-Depth Information
13.18
Observations of the grain growth in BaTiO
3
powder at different
temperatures from 940
C (d to o). Grains
grow through reduction of smaller grains and enlargement of larger
ones. The distance between the particle centers decreases
simultaneously.
8
C (a), 950
8
C (b, c) to 960
8
Considering that nano particles are usually not at equilibrium states and
are likely to contain excess amounts of various defects that are created
during the production of nano particles, there will be a relaxation period for
migration, redistribution and annihilation of the defects.
96,97
Owing to the
non-equilibrium structure of nano particles, diffusivity is dramatically
enhanced during the relaxation process,
98-101
which may contribute to
dynamic grain growth at the beginning of sintering. Dynamic grain growth
usually dominates during the heat-up stage and for the first few minutes
after reaching a preset isothermal holding temperature. Therefore, rapid
dynamic grain growth accounts for the experimental observation that the
first data point during isothermal holding is several times that of the initial
grain size. The relaxation time depends on materials, nano particle
production methods and temperature.
The role of grain boundary migration should also be considered in
