3.3. Dielect ric and piezoel ectric propert ies
With the increase of sintering temperature, the bulk density
of the samples increased approaching that of dense materials
which could be inferred from Fig. 3. Therefore, the relative
dielectric constant of the samples, measured at 1 kHz as shown
in Fig. 5, increased due to the increase in proportion of active
piezoelectric ceramic phase and the dominating effects of it
over passive phase (air). The result was easily understood and
consistent with former research [7,13].
Fig. 6 shows the change of piezoelectric constant d33 and
HFOM with sintering temperature. According to the study of
Okazaki and Nagata [9], there are certain space-charge sites