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Defense Advanced Research Project Agency's (DARPA's) thermal ground plane (TGP) effort was aimed at combining the advantages of vapor chambers or two-dimensional (2D) heat pipes and solid conductors by building thin, high effective thermal conductivity, flat heat pipes out of materials with thermal expansion coefficients that match current electronic devices. In addition to the temperature uniformity and minimal load-driven temperature variations associated with such two phase systems, in their defined parametric space, flat heat pipes are particularly attractive for Department of Defense and commercial systems because they offer a passive, reliable, light-weight, and low-cost path for transferring heat away from high power dissipative components. However, the difference in thermal expansion coefficients between silicon or ceramic microelectronic components and metallic vapor chambers, as well as the need for a planar, chip-size attachment surface for these devices, has limited the use of commercial of the shelf flat heat pipes in this role. The primary TGP goal was to achieve extreme lateral thermal conductivity, in the range of 10 kW/mK–20 kW/mK or approximately 25–50 times higher than copper and 10 times higher than synthetic diamond, with a thickness of 1 mm or less.

This paper presents the fabrication and application of a micro-scale hybrid wicking structure in a flat polymer-based heat pipe heat spreader, which improves the heat transfer performance under high adverse acceleration. The hybrid wicking structure which enhances evaporation and condensation heat transfer under adverse acceleration consists of 100 μm high, 200 μm wide square electroplated copper micro-pillars with 31 μm wide grooves for liquid flow and a woven copper mesh with 51 μm diameter wires and 76 μm spacing. The interior vapor chamber of the heat pipe heat spreader was 30 × 30 × 1.0 mm 3 . The casing of the heat spreader is a 100 μm thick liquid crystal polymer which contains a two-dimensional array of copper-filled vias to reduce the overall thermal resistance. The device performance was assessed under 0-10 g acceleration with 20, 30 and 40 W power input on an evaporator area of 8 × 8 mm 2 . The effective thermal conductivity of the device was determined to range from 1653 W (m K) −1 at 0 g to 541 W (m K) −1 at 10 g using finite element analysis in conjunction with a copper reference sample. In all cases, the effective thermal conductivity remained higher than that of the copper reference sample. This work illustrates the possibility of fabricating flexible, polymer-based heat pipe heat spreaders compatible with standardized printed circuit board technologies that are capable of efficiently extracting heat at relatively high dynamic acceleration levels.

The thermal performance of a miniature, three-dimensional flat-plate oscillating heat pipe (3D FP-OHP) was experimentally investigated during high-gravity loading with nonfavorable evaporator positioning. The heat pipe had dimensions of 3.0 × 3.0 × 0.254 cm3 and utilized a novel design concept incorporating a two-layer channel arrangement. The device was charged with acetone and tested at a heat input of 95 W within a spin-table centrifuge. It was found that the heat pipe operated and performed near-independent of the investigated hypergravity loading up to 10 g. Results show that at ten times the acceleration due to gravity (10 g), the effective thermal conductivity was almost constant and even slightly increased which is very different from a conventional heat pipe. The gravity-independent heat transfer performance provides a unique feature of OHPs.

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This report describes an experimental investigation into the effect of electric current in reducing the supercooling of erythritol. Previous studies have identified erythritol as a prime material candidate for moderate temperature thermal energy storage (TES) systems due to its high latent heat of fusion and melting temperature (118°C), but it has also shown excessive supercooling, sometimes exceeding 65°C [1]. Various methods for controlling or reducing supercooling are reviewed, including work by Shichiri and Hozumi showing that a small electric current passed through supercooled water is highly effective in initiating nucleation [2,3]. In the present study, the authors demonstrate a similar effect with erythritol by subjecting a sample to repeated thermal cycles with and without the application of a direct electric current. The control cases without electric current showed a highly variable recrystallization temperature ranging from 67°C to 109°C (or supercooling magnitudes from 9 to 51°C). Passing a direct current through the sample using silver wire electrodes significantly shifted the material’s nucleation behavior. The local nucleation temperature only varied from 108°C to 112°C (or 6–10°C of supercooling), and nucleation always occurred on the positive electrode surface. Control cases both before and after the electrical trials indicated no noticeable change in sample crystallization behavior.

The paper highlighted one of the benefits of thin film pyroelectrics versus bulk systems, namely, the ability to withstand large electric fields. When using considering a PEC technique that uses cyclic charging and discharging, this benefit becomes apparent upon inspection of the cycle (Figure 1a) and work (W) equationFor maximum power density each PEC cycle should experience as large of a ΔD as possible, enabled by large pyroelectric coefficients, and be subjected to as wide a ΔE as possible. The electrocaloric community has exploited this benefit, showing a number of material systems with excellent energy conversion potential under high applied fields. [2][3][4][5][6] Thin film pyroelectrics also have the benefit of low thermal mass, which enables faster energy conversion at higher cycle frequencies thus producing higher power. [7] Equation (2) relates the work to power simplywhere P and f are the cycle power and frequency, respectively. Bhatia et al. showed that the strongest correlation to PEC power density came from cycle frequency and not material properties.Using an electrically driven resistive heater, Bhatia et al. demonstrated energy conversion cycles at 1 kHz obtaining record high power densities of 3 W cm −3 . [8] This is in contrast to a majority of studies using bulk pyroelectrics where power densities were limited to <200 mW cm −3 due to systems relying on fluid flow over pyroelectric structures for transient temperature variations. [9,10] Radiatively stimulated PEC has been under consideration since the 1960s. A thermo-dielectric energy system utilizing concentrated solar energy was patented in 1960. [11] In 1982, NASA commissioned a study on using PEC on rotating satellites, where periodic solar incident energy would provide the time variant temperature. [12] Targeting applications where PEC was optically driven, Zabek, et al., showed that using micropatterned top electrodes on a free-standing polymer pyroelectric improved PEC performance by 380% by enabling greater optical absorption by the material while only minimally affecting the capacitance. [13] Thermal energy harvesting was demonstrated using a heat lamp and off-the-shelf pyroelectric material at a frequency of <0.5 Hz, resulting in harvested energies in the milliwatt range. [14] Thin film pyroelectrics, which have the greatest potential for high power and efficiency conversion, have infrequently been used for real conversion cycles.Distributed, small sensor systems show promise for creating a more connected human experience through the internet of things, yet their energy needs are not being satisfied by current on-sensor storage mechanisms, such as batteries, or energy harvesting approaches. One alternative is to wirelessly transmit power to sensor nodes. Over short distances (<10 cm) this is accomplished via inductive or capacitive coupling. Long distance transmission (>10 m) requires a low-divergence signal source, such as a laser, and a receiver that can convert either light or heat into electricity. Pyroelectric thin films receivers...