Reduction of the electrocaloric entropy change of ferroelectric ${\mathrm{PbZr}}_{1-x}{\mathrm{Ti}}_x{\mathrm{O}}_3$ epitaxial layers due to an elastocaloric effect

We report laser-based direct measurements of the pyroelectric and electrocaloric effects of ${\mathrm{PbZr}}_{1-x}{\mathrm{Ti}}_x{\mathrm{O}}_3$ epitaxial layers as a function of temperature and electric field. By evaluating the differences between the total pyroelectric and electrocaloric coefficients, we determine the secondary contribution to the electrocaloric effect that is created by a combination of piezoelectric and elastocaloric effects. The secondary contribution to the electrocaloric coefficient has the opposite sign as the primary effect and therefore reduces the overall entropy change of ${\mathrm{PbZr}}_{1-x}{\mathrm{Ti}}_x{\mathrm{O}}_3$ in an electric field. The absolute magnitude of the secondary effect is comparable to the primary effect at elevated temperatures of ≈200 °C.

Figure 1

(a) Frequency dependence of the pyroelectric current and (b) frequency dependence of the temperature change of the V film for a PZT/GSO capacitor at room temperature. Measured data are shown as open circles, and the solid lines are the model calculations used to determine (a) the total pyroelectric coefficient $\Pi =-170\mu \mathrm{C}{\mathrm{m}}^{-2}{\mathrm{K}}^{-1}$ and (b) the total electrocaloric coefficient $\Sigma =-130\mu \mathrm{C}{\mathrm{m}}^{-2}{\mathrm{K}}^{-1}$. The laser power used in the pyroelectric measurement is 1 mW. The dc bias voltage and ac voltage used in the electrocaloric measurement are 1.4 V and 1 V, respectively.

Figure 2

Hysteresis measurement of the total pyroelectric, electrocaloric coefficient, and polarization of the PZT layers at room temperature. For the electrocaloric and pyroelectric measurements, a dc bias voltage is applied to the capacitors, and the time between changes in the bias voltage is ∼30 s with a increment of 0.1 V. Polarization is measured at a frequency of 1 kHz.

Figure 3

Dependence on sample temperature of the total electrocaloric (triangle symbols) and pyroelectric (square symbols) coefficients of PZT epitaxial layers on ${\mathrm{DyScO}}_3$ and ${\mathrm{GdScO}}_3$ substrates.

Figure 4

Temperature dependence of the difference between the total electrocaloric coefficient and pyroelectric coefficient of PZT epitaxial layers due to the secondary contribution. The converse piezeoelectric coefficient ${\mathrm{d}}_{33}$ calculated based on Eq. (6).The error bar is calculated with uncertainties of the pyroelectric and electrocaloric measurement to be 5%.

Figure 5

(a) The schematic of the electrocaloric measurement. (b) Thermal model for bidirectional heat conduction into the V film and substrate.

Figure 6

A schematic of the 2-omega method to measure the thermoreflectance coefficient.

Figure 7

(a) The schematic of the pyroelectric measurement. (b) The heat transport model for the pyroelectric measurement

Figure 8

Sensitivity of the electrocaloric measurement (a) and the pyroelectric measurement (b) to the thermophysical properties of PZT, DSO, and V as a function of frequencies. Sensitivity to the laser power is 1. Sensitivity to the thickness of the PZT films in the electrocaloric measurement is 0.