Giant power output in lead-free ferroelectrics by shock-induced phase transition

The force-electric effect in ferroelectrics is characterized by the release of bound charge during pressure/shock-induced depolarization. In contrast to other electrical energy storage systems, the charge-storage/release by the force-electric effect of ferroelectrics is determined by polarization switching or polar-nonpolar phase transition. This offers a further set of options for materials design in the realm of energy conversion, especially for the high power density applications. Here, we report that a ferroelectric ceramic, $\mathrm{N}{\mathrm{a}}_{0.5}\mathrm{B}{\mathrm{i}}_{0.5}\mathrm{Ti}{\mathrm{O}}_3$ (NBT), can generate a high power output $(3.04\times {10}^8\mathrm{W}/\mathrm{kg})$ under shock compression, which is one of the highest values achieved by the force-electric effect. The in situ synchrotron x-ray diffraction studies reveal that this power output mainly arises from a polar-nonpolar phase transition (rhombohedral-orthorhombic). First-principles calculations show that this is a first-order phase transition that undergoes two-step structure changes. These results extend the application of the force-electric effect and are a key step in understanding the phase transition behaviors of NBT under high pressure.

Figure 1

Schematic of the dynamic compression experiments.

Figure 2

Force-electric effect (a): The shock compression induces the depolarization with all bound charge released. Piezoelectric effect (b): The shock compression does not change the $P_r$ value and the current in the external circuit is generated by the piezoelectric effect. The typical current wave forms generated by force-electric effect (red) and piezoelectric effect (blue) in the external circuit (c). The power (d) and voltage (e) could achieve to 453 kW, 17.16 kV, based on a small power source with a volume of $0.256\mathrm{c}{\mathrm{m}}^3$, and the current (f) is 26.4 A, under a shock pressure of 4.93 GPa. The plot (g) of specific power against specific energy for various electrical energy storage devices shows that the force-electric power source based on NBT has a higher power density than other power sources. Compared to PZT and ${\mathrm{BaTiO}}_3$ ferroelectrics, the power density and energy density of NBT power source are both improved. The data error is ±10%.

Figure 3

Current and released charge of NBT ceramics under different shock pressures (a)–(f); with the particle speed–shock wave speed curve ($u_s\text{−}{u}_p$) and pressure-volume curve ($P\text{−}V$) (g). The released charge is calculated from the current data. The $r$ value is the ratio of released charge over the bound charge calculated from $P_r$.

Figure 4

Pressure dependence of the phase transition in NBT has been studied by the in situ synchrotron x-ray diffraction. The x-ray diffraction spectra of NBT ferroelectric materials at selected pressures (a). The XRD peaks of the phase are marked by the red spades. The NBT is rhombohedral ($R3c$) structure at low pressure, and it changes into orthorhombic structure ($Pnma$) at high pressure. The normalized $P\text{−}V$ curve of NBT according to the $Z=6$ (b), and the schematic diagram of the structure change during the phase transition is shown in (c)–(e). The red cuboid in (d) is a prototype of the orthorhombic unit cell ($Pnma$) before phase transition in rhombohedral ($R3c$) lattice, which changes into orthorhombic at high pressure (e).

Figure 5

First-principles calculations of the $R3c$ and $Pnma$ phases as a function of pressure. The enthalpy ($H$) calculated by first-principles simulation for $R3c$ and $Pnma$ phases at different pressures, respectively (a). The enthalpy change of $R3c$ phase could be divided into two regions (A and B). When the pressure is below 1.9 GPa (region A), the enthalpy of $R3c$ increases sharply due to the volume decreasing, which is shown in (b). When the pressure is above 1.9 GPa (region B), the enthalpy of $R3c$ phase increases gently, which is mainly due to the ${\mathrm{O}}^{2-}$ ions displacing following the red arrows in (c).