Role of Oxygen Vacancy Migration in Pyroelectric Currents of Nd₂Ti₂O₇ Ceramics as High Temperature Sensors

A high <tex-math notation="LaTeX">TC </tex-math> ferroelectric material <tex-math notation="LaTeX">Nd_2Ti_2O_7 </tex-math> (neodymium titanate) was studied using photocurrent measurement and impedance spectroscopy (IS) to investigate the intrinsic and extrinsic factors in the pyrocurrent generation. The alteration in the pyroelectric coefficient was observed while varying the heating and cooling rates, spanning from 5 °C/min to 30 °C/min, corresponding to values of 88– <tex-math notation="LaTeX">90~μ C / m^2K </tex-math> and 28– <tex-math notation="LaTeX">30~μ C / m^2K </tex-math> , respectively. At higher heating and cooling rates (from 30 °C/min to 80 °C/min), the pyroelectric coefficients remained relatively constant (21.8 <tex-math notation="LaTeX">± ~4.0~μ C / m^2K </tex-math> ). The dispersive nature of the pyrocurrent was verified through photocurrent studies followed by the IS analysis. The measurement outcomes reveal the presence of oxygen vacancies that dominate between 300 °C and 500 °C. Below 300 °C, extrinsic factors, such as shallow traps, consisting of space charge near electrode–material interface, could have a greater impact on the pyrocurrent at zero electric field. When an electric field is applied above 300 °C, oxygen vacancies migration can become the dominant conduction mechanism due to the collective effect of electric field and temperature. The estimated relaxation time ( <tex-math notation="LaTeX">τ _M\,) </tex-math> exhibits the Arrhenius behavior, with an activation energy of ~0.92 eV, which implies oxygen vacancy migration with the assistance of external electric field. However, the thermal cycling of samples provided a true estimate of intrinsic pyroelectric currents in NTO ceramics without the influence of the thermal migration of oxygen vacancies resulting into a temperature-independent pyroelectric coefficient of about 15 <tex-math notation="LaTeX">± ~0.5~μ C / m^2K </tex-math> in the temperature range from 300 °C to 500 °C. Thus, NTO ceramic can operate as a temperature sensor without influence of extrinsic factors for the temperature range from 300 °C to 500 °C.