Temperature Dependent Piezoelectric and Pyroelectric Response of Nb Doped (Na0.41 K0.09 Bi0.5) TiO3 Ceramics

In the present study, temperature dependence of the piezoelectric and pyroelectric coefficients of niobium (Nb) doped ($\left.\mathrm{Na} 0.41 \mathrm{~K}_{0.09} \mathrm{Bi}_{0.5}\right) \mathrm{TiO}_3$ [NKBT] ceramics has been investigated. The piezoelectric charge coefficient $\left(d_{31}\right)$ and electromechanical coupling coefficient $\left(k_{\mathrm{p}}\right)$ increase gradually with temperature till close to the depolarization temperature and thereafter fall sharply. The highest piezoelectric coefficients were obtained for $\mathrm{x}=0.005(0.5$ at. %Nb) which are $d_{33} \sim 185 \mathrm{pC} / \mathrm{N}, d_{31} \sim 51 \mathrm{pC} / \mathrm{N}$ and $\mathrm{k}_{\mathrm{p}} \sim 0.35$ at room temperature. Like the piezoelectric response, the pyroelectric coefficient also shows an anomaly in the vicinity of T
d
. At room temperature the pyroelectric coefficient $\left(p_3\right)$ is in the range of 2-10 $\mathrm{nC} / \mathrm{cm}^2 \mathrm{~K}$ for all the compositions. The maximum value of p
3
observed near T
d
is $4.6 \mu \mathrm{C} / \mathrm{cm}^2 \mathrm{~K}$. The measurements reveal the lowering of T
d
with increase in Nb content which can be explained on the basis of donor type behavior of niobium $\left(\mathrm{Nb}^{5+}\right)$ ion. For energy harvesting, the generated output voltage shows maximum value for a periodic temperature oscillation of $\Delta \mathrm{T} \sim 15 \mathrm{~K}$ at $\mathrm{dT} / \mathrm{dt} \sim 0.44 \mathrm{~K} / \mathrm{s}$ for sample with $\mathrm{x}=0.005$. The study suggests that by using a proper rectifier circuit a pulsating DC output with maximum voltage of $\sim 3 \mathrm{~V}$ can be obtained.