J-H. Lee et al.: Observation of intergranular films in BaB2O4-added BaTiO3 ceramics
perimental results signify that the composition of film
need not be identical to that of the bulk liquid located at
triple junctions.
diffusivity in a viscous melt, shows that the diffusion
coefficient is inversely proportional to viscosity. Thus,
the nucleation rate is inversely proportional to viscosity
because high viscosity reduces the jump frequency of
atoms, i.e., a lower viscosity of liquid with the dissolu-
tion of certain components into liquid phases increases
the nucleation rate.32
In addition to the segregation, there is also a necessity
to have a viewpoint of compositional difference with
respect to the microstructural difference between two-
grain and triple-junction regions. The radius of the cur-
vature of solid–liquid interface may be an important
geometrical factor, which determines the chemical
potential of atoms at the interface. According to the
Thompson–Freundlich equation,27 the solubility of solid
particles in the liquid phase is inversely proportional to
the radius of solid particles. Because the curvatures of
solid particle at triple junction regions are small com-
pared with two-grain junction that have flat boundary
morphology, the solubility of solid particles in the liquid
phase at triple junction regions are also higher than those
at the two-grain junction as shown in Fig. 5. Concerning
the concentration of solute at triple junction, Green-
wood27 and others28,29 have demonstrated the concentra-
tion gradient profile of a solute with distance. The
approximate formula derived to predict the solute con-
centration gradient around each particle showed that the
effective influential distance of concentration to be the
radius of the solute particle from the surface. Under this
condition, the concentration profiles of each particle at
triple-junction overlap with each other, and a higher con-
centration due to higher solubility is expected.
During the sintering process, in our system, the
BaB2O4 phase will melt at the eutectic temperature and
some of the Ti and Ba ions will be concurrently dissolved
into BaB2O4 melts from BaTiO3 grains, and then form
Ba–Ti–B–O glass. In Ba–Ti–B–O glass, B and Ba com-
ponents act as glass network former and modifier, re-
spectively. Thus, the dissolving of Ba and Ti components
may lead to the lowering of the viscosity of the liquid and
supersaturation of glass with Ba and Ti components con-
currently. Since the radius of curvature in solid–liquid
interface is small at the liquid pocket, the amount of
dissolved Ba and Ti components is also large compared
with the liquid film. When the specimen was cooling
down after sintering, the crystallization of BaB2O4 was
thought to occur at the bulk-liquid pocket located at triple
junctions, which had rather low viscosity and high su-
persaturation, compared with the liquid film at two-grain
junction. Therefore, such a high solubility of solid par-
ticles at triple junctions into the liquid phase may accel-
erate the crystallization of the liquid phase during the
cooling process.
When a new phase is formed by a nucleation and
growth process in a liquid, we can consider factors such
as supercooling, degree of supersaturation, and viscosity.
If we consider the effect of increasing supersaturation at
a constant temperature, the rate of nucleation is very
sensitive to the degree of supersaturation especially when
the supersaturation approaches some critical value.30 On
the other hand, the viscosity of a liquid should also be
considered as an important factor. The Stokes–Einstein
relation,31 which is frequently used to estimate the atom
IV. CONCLUSION
Two kinds of distribution characteristics of boundary
phases in BaB2O4 added BaTiO3 ceramics were observed
by HRTEM: (i) triple junction filled with a mixture of
liquid phase and crystallized BaB2O4 phase, and (ii) two-
grain junction with a flat amorphous thin film. This kind
of boundary phase distribution characteristic was ex-
plained with a viewpoint of compositional difference
with respect to the microstructure difference between
two-grain and triple-junction regions. Because the solu-
bility of solid particles in the liquid phase are inversely
proportional to the radius of solid particles, the solid
particle at the triple-junction region has smaller curvature
and are more soluble than the two-grain junction. Such a
high solubility of a solid particle to the liquid phase
concurrently leads to supersaturation of the glass phase
and lowers the viscosity of the liquid, which eventually
accelerates crystallization of the liquid phase during the
cooling process.
ACKNOWLEDGMENT
The authors greatly appreciate the helpful advice of
Prof. Yet-Ming Chiang at the Department of Materials
Science and Engineering, MIT, Cambridge, MA.
FIG. 5. Schematic representation of a triple junction with liquid films.
J. Mater. Res., Vol. 15, No. 7, Jul 2000
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