The need for efficient energy utilization is driving research into ways to harvest ubiquitous waste heat. Here, we explore pyroelectric energy conversion from low-grade thermal sources that exploits strong field- and temperature-induced polarization susceptibilities in the relaxor ferroelectric 0.68Pb(MgNb)O-0.32PbTiO. Electric-field-driven enhancement of the pyroelectric response (as large as -550 μC m K) and suppression of the dielectric response (by 72%) yield substantial figures of merit for pyroelectric energy conversion. Field- and temperature-dependent pyroelectric measurements highlight the role of polarization rotation and field-induced polarization in mediating these effects. Solid-state, thin-film devices that convert low-grade heat into electrical energy are demonstrated using pyroelectric Ericsson cycles, and optimized to yield maximum energy density, power density and efficiency of 1.06 J cm, 526 W cm and 19% of Carnot, respectively; the highest values reported to date and equivalent to the performance of a thermoelectric with an effective ZT ≈ 1.16 for a temperature change of 10 K. Our findings suggest that pyroelectric devices may be competitive with thermoelectric devices for low-grade thermal harvesting.

Harvesting waste heat for useful purposes is an essential component of improving the efficiency of primary energy utilization. Today, approaches such as pyroelectric energy conversion are receiving renewed interest for their ability to turn wasted energy back into useful energy. From this perspective, the need for these approaches, the basic mechanisms and processes underlying their operation, and the material and device requirements behind pyroelectric energy conversion are reviewed, and the potential for advances in this area is also discussed.

Understanding of polarization-heat interactions in pyroelectric and electrocaloric thin-film materials requires that the electrothermal response is reliably characterized. While most work, particularly in electrocalorics, has relied on indirect measurement protocols, here we report a direct technique for measuring both pyroelectric and electrocaloric effects in epitaxial ferroelectric thin films. We demonstrate an electrothermal test platform where localized high-frequency (∼ 1 kHz) periodic heating and highly-sensitive thin-film resistance thermometry allow direct measurement of pyrocurrents (< 10 pA) and electrocaloric temperature changes (< 2 mK) using the "2-omega" and an adapted "3-omega" technique, respectively. Frequency-domain, phase-sensitive detection permits extraction of pyrocurrent from the total current, which is often convoluted by thermally-stimulated currents. The wide frequency range measurements employed in this study further show the effect of secondary contributions to pyroelectricity due to the mechanical constraints of the substrate. Similarly, measurement of the electrocaloric effect on the same device in the frequency-domain (∼ 100 kHz) allows decoupling of Joule heating from the electrocaloric effect. Using one-dimensional, analytical heattransport models, the transient temperature profile of the heterostructure is characterized to extract pyroelectric and electrocaloric coefficients.

We demonstrate capillary fed porous copper structures capable of dissipating over 1200 W cm À2 in boiling with water as the working fluid. Demonstrated superheats for this structure are dramatically lower than those previously reported at these high heat fluxes and are extremely insensitive to heat input. We show superheats of less than 10 K at maximum dissipation and varying less than 5 K over input heat flux ranges of 1000 W cm À2. Fabrication of the porous copper layers using electrodeposition around a sacrificial template allows fine control of both microstructure and bulk geometry, producing structures less than 40 lm thick with active region lateral dimensions of 2 mm  0.3 mm. The active region is volumetrically Joule heated by passing an electric current through the porous copper bulk material. We analyze the heat transfer performance of the structures and suggest a strong influence of pore size on superheat. We compare performance of the current structure to existing wick structures. V

Temperature-dependent changes in spontaneous polarization (i.e., the pyroelectric effect or PEE) [1] have the potential to impact applications in waste-heat energy conversion [2,3] and thermal imaging. [4] Similarly, the inverse thermodynamic effect (i.e., the electrocaloric effect or ECE) [1] where an electric field perturbs the dipolar order and therefore the entropy of the system can enable solid-state cooling devices. [5,6] Key to such applications is the ability to manipulate and control the temperatureand field-dependence of polarization and entropic changes in ferroic materials. In turn, research has focused on finding pathways to enhance the pyroelectric π = ∂ Ferroelectric Thin FilmsThe complex interplay of polarization (P), temperature (T), entropy (S), and electric field (E) in ferroic materials enables electrothermal susceptibilities useful for a range of applications.

This paper reports the first integration of laser-etched polycrystalline diamond microchannels with template-fabricated microporous copper for extreme convective boiling in a composite heat sink for power electronics and energy conversion. Diamond offers the highest thermal conductivity near room temperature, and enables aggressive heat spreading along triangular channel walls with 1:1 aspect ratio. Conformally coated porous copper with thickness 25 µm and 5 µm pore size optimizes fluid and heat transport for convective boiling within the diamond channels. Data reported here include 1280 W cm −2 of heat removal from 0.7 cm 2 surface area with temperature rise beyond fluid saturation less than 21 K, corresponding to 6.3 × 10 5 W m −2 K −1 . This heat sink has the potential to dissipate much larger localized heat loads with small temperature nonuniformity (5 kW cm −2 over 200 µm × 200 µm with <3 K temperature difference). A microfluidic manifold assures uniform distribution of liquid over the heat sink surface with negligible pumping power requirements (e.g., <1.4 × 10 −4 of the thermal power dissipated). This breakthrough integration of functional materials and the resulting experimental data set a very high bar for microfluidic heat removal.

Applications such as solid-state waste-heat energy conversion, infrared sensing, and thermally-driven electron emission rely on pyroelectric materials (a subclass of dielectric piezoelectrics) which exhibit temperaturedependent changes in polarization. Although enhanced dielectric and piezoelectric responses are typically found at polarization instabilities such as temperature-and chemically induced phase boundaries, large pyroelectric effects have been primarily limited in study to temperature-induced phase boundaries. Here, we directly identify the magnitude and sign of the intrinsic, extrinsic, dielectric, and secondary pyroelectric contributions to the total pyroelectric response as a function of chemistry in thin films of the canonical ferroelectric PbZr 1−x Ti x O 3 (x = 0.40, 0.48, 0.60, and 0.80) across the morphotropic phase boundary. Using phase-sensitive frequency and applied dc-bias methods, the various pyroelectric contributions were measured. It is found that the total pyroelectric response decreases systematically as one moves from higher to lower titanium contents. This arises from a combination of decreasing intrinsic response (−232 to −97 μC m −2 K −1) and a sign inversion (+33 to −17 μC m −2 K −1) of the extrinsic contribution upon crossing the morphotropic phase boundary. Additionally, the measured secondary and dielectric contributions span between −70 and −29 and 10−115 μC m −2 K −1 under applied fields, respectively, following closely trends in the piezoelectric and dielectric susceptibility. These findings and methodologies provide novel insights into the understudied realm of pyroelectric response.

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