Energy Conversion from Heat to Electricity by Highly Reversible Phase-Transforming Ferroelectrics

The search for performant multiferroic materials has attracted general research interest in energy science as they have been increasingly exploited as the conversion media among thermal, electric, magnetic, and mechanical energies by using their temperature-dependent ferroic properties. Here we report a material development strategy that guides us to discover a reversible phase-transforming ferroelectric material exhibiting enduring energy harvesting from small temperature differences. The material satisfies the crystallographic compatibility condition between polar and nonpolar phases, which shows only $2.5{}^{\circ }\mathrm{C}$ thermal hysteresis and a high figure of merit. It stably generates $15\mu \mathrm{A}$ of electricity in consecutive thermodynamic cycles in the absence of any bias fields. We demonstrate our device to consistently generate $6\mu \mathrm{A}/{\mathrm{cm}}^2$ current density near $100{}^{\circ }\mathrm{C}$ over 540 complete phase transformation cycles without any electric and functional degradation. The energy-conversion device can light up an LED directly without attaching an external power source. This promising material candidate brings the low-grade waste-heat harvesting closer to a practical realization, e.g., small temperature fluctuations around the water boiling point can be considered as a clean energy source.

Figure 1

Ferroelectric, thermal, and structural characterizations of first-order phase transformation in polycrystalline tridoped barium titanate crystals. (a),(b) Temperature-dependent polarization and absolute value of pyroelectric coefficient $dP/dT$. (c) Thermal analysis of reversible phase transformations by DSC. (d) Temperature-dependent $c/a$ ratio by neutron diffraction experiments. The inset of (a) is the polarization versus electric field of $\mathrm{Zr}$0.005 $\mathrm{Ce}$0.005 at $40{}^{\circ }\mathrm{C}$.

Figure 2

Neutron diffraction pattern of $\mathrm{Zr}$0.005 $\mathrm{Ce}$0.005 polycrystalline bulk sample at elevated temperatures from $40$ to $140{}^{\circ }\mathrm{C}$ indexed by $Pm\overline{3}m$ (cubic) and $P4mm$ (tetragonal) symmetries.

Figure 3

Contours of (a) FOM, (b) thermal hysteresis, and (c) compatibility condition in two-dimensional composition space interpreted from experimental data by linear interpolation.

Figure 4

The grain morphology of (a) polycrystalline ${\mathrm{Ba}}_{0.95}{\mathrm{Ca}}_{0.05}{\mathrm{Ti}\mathrm{Zr}}_x{\mathrm{Ce}}_y{\mathrm{O}}_3$. Orientation maps by synchrotron Laue microdiffraction for (b) $\mathrm{Zr}$0.005 $\mathrm{Ce}$0.005 and (c) $\mathrm{Zr}$0.01 $\mathrm{Ce}$0.001 grown by floating-zone method. The color map indicates the orientation of normal direction (ND) of the surface.

Figure 5

Ferroic properties of $\mathrm{Zr}$0.005 $\mathrm{Ce}$0.005 single crystal and $\mathrm{Zr}$0.01 $\mathrm{Ce}$0.001 coarse-grain crystal. (a) Temperature-dependent polarization and (b) absolute value of $dP/dT$ in a zoomed temperature range between 100 and $130{}^{\circ }\mathrm{C}$.

Figure 6

(a) Schematic polarization-field curves of tetragonal ferroelectric phase (blue) and cubic paraelectric phase (red). The yellow area demonstrates the thermodynamic cycle of the process without application of external bias electric field. (b) Battery detached setup of the transforming capacitor with polarization jump between ferroelectric and paraelectric phases.

Figure 7

Energy conversion by phase-transforming ferroelectric crystals. (a) Pyro-current density generated by the $\mathrm{Zr}$0.005 $\mathrm{Ce}$0.005 single-crystal capacitor (red) and the $\mathrm{Zr}$0.01 $\mathrm{Ce}$0.001 coarse-grained capacitor (blue) over hundreds of transformation cycles. (b),(c) Pyroelectric current density of 47 to 52 cycles corresponding to the applied temperature profiles.

Figure 8

Differential scanning calorimetry measurement of (a) $\mathrm{Zr}$0.005 $\mathrm{Ce}$0.005 single-crystal plate (red) and (b) $\mathrm{Zr}$0.01 $\mathrm{Ce}$0.001 coarse-grained plate (blue) at initial and final thermal cycles.

Figure 9

(a) Generated current by a grid of phase-transforming $\mathrm{Zr}$0.005 $\mathrm{Ce}$0.005 capacitors connected in parallel under $\pm 20{}^{\circ }\mathrm{C}$ temperature differences around $110{}^{\circ }\mathrm{C}$. (b) The demonstration to light up an LED light during heating.