3. Results and discussion
3.1. Sintering behavior and piezoelectric properties of
CuO-doped BNKT ceramics
Fig. 1 shows the surface SEM micrographs of undoped and
CuO-doped BNKT ceramics sintered at 975 8C for 2 h in air.
Undoped samples exhibit little grain growth with poor
densification at such low Ts, however, both the average grain
size and relative density increase up to 1.14 mm and 96%
respectively, by adding excess CuO even as low as 1 mol%.
Further increase in the amount of CuO aid only results in slight
enhancement in the average grain size. It should be noted that
the secondary phases are observed at grain boundaries and
corners as indicated by arrows on the figures, which strongly
suggests the presence of a liquid phase during sintering
similarly to CuO added KNN ceramics [11]. However, XRD
analysis detected little secondary phases, implying that the
observed unwanted phase between grains is amorphous.
The d33 and kp were determined for CuO added specimens
and the results are shown in Fig. 2. The piezoelectric
parameters for high temperature (HT) fired BNKT ceramics
are also given for comparison because the piezoelectric
properties of low temperature (LT) fired BNKT ceramics
without CuO sintering aid cannot be measured due to their poor
density. The d33 and kp of HT fired specimen with x = 0 are
146 pC/N and 34%, respectively, which are higher than those of
LT fired specimens that moreover show degradations in the
piezoelectric properties with increasing CuO content. These
results seem to be arisen from the experimental evidence that
Cu ions generally act as acceptors in ABO3 type perovskite
structures [14], which is also supported by the fact that the ionic
radius of a Cu2+ is closer to that of Ti4+ than those of Bi3+, Na+,
or K+ ions [15].