Dielectric properties
Fig. 4 exhibits the temperature dependent dielectric properties
of BST ceramics derived from different routes at 10 kHz within a
temperature range of 120 to 100 C. For the traditional sample,
a sharp cubic-tetragonal phase transition TTC is observed at about
2 C, and a tetragonal-orthorhombic phase transition TOT is
around 64 C, as ‘c/t’ and ‘t/o’ marked in Fig. 4(a), respectively.
For the HEBM specimens, as shown in Fig. 4(b), the transition peak
TTC is broadened and shifted to lower temperature range of 66 to
27 C. It is also observed from this figure that the lower the sintering
temperature is, the lower the TTC will be. The shifted and
broadened phase transition can be explained by the ‘‘internalstress
model’’—which is related to the grain size refinement
[9,18]. In addition, as discussed above, the formation of BW phase
leads to a reduction of Ba/Sr ratio less than 0.6/0.4, which in turn
contributes to the shifted TTC. This is mainly because the lower
Ba/Sr ratio always corresponds to the lower TTC [19]. Meanwhile,
as Fig. 4(b) shows, the TOT peaks cannot be observed in the samples
sintered at lower temperatures (1100 and 1200 C), but reappear
in the samples sintered at 1250 C, which also corroborates
the viewpoint that the internal-stress can help fine-grained materials
overcome the energy barrier of the phase transition [7,20].
Fig. 4(c and d) shows the temperature dependent dielectric loss
of the specimens obtained differently. Unlike the sharp peak observed
in the traditional one [Fig. 4(c)], the dielectric loss curves
of the HEBM specimens are smeared, as shown in Fig. 4(d), and
keep at low values (