No abstract

Phase change materials (PCMs) provide a high energy density for thermal storage systems but often suffer from limited power densities due to the low PCM thermal conductivity. Much like their electrochemical analogs, an ideal thermal energy storage medium combines the energy density of a thermal battery with the power density of a thermal capacitor. Here, we define the design rules and identify the performance limits for rationally-designed composites that combine an energy dense PCM with a thermally conductive material. Beginning with the Stefan-Neumann model, we establish the material design space using a Ragone framework and identify regimes where hybrid conductive-capacitive composites have thermal power densities exceeding that of copper and other high conductivity materials. We invoke the mathematical bounds on isotropic conductivity to optimize and define the theoretical limits for transient cooling using PCM composites. We then demonstrate the impact of power density on thermal transients using copper inverse opals infiltrated with paraffin wax to suppress the temperature rise in kW cm−2 hotspots by ∼10% compared to equivalent copper thin film heat spreaders. These design rules and performance limits illuminate a path toward the rational design of composite phase change materials capable of buffering extreme transient thermal loads.

No abstract

The increasing complexity and power density of electronic systems have necessitated the development of thermal circuits that can not only remove but actively redirect the flow of heat. Passive thermal regulators are promising as heat routing components that can mitigate large temperature spikes by transitioning between high and low resistance states without external actuation. Existing regulators, however, are often either limited to fixed temperature regulation ranges due to solid-state material property limitations or are difficult to package in a compact form factor. Here, we present a passive, compact (1 × 1 cm2 active area), and tunable thermal regulator that functions based on the dynamics of vapor transport through a noncondensable gas cavity. The device demonstrates a switching resistance ratio of 4 in response to variations in the input power ranging from approximately 0.6 W to 14 W. The device is also able to set the temperature difference across the hot and cold sides to a fixed, “clamped” value that is reasonably independent of heat flow. Both the overall resistance and the clamped temperature difference can be easily tuned by presetting the pressure of the noncondensable gas. We present a brief analysis of the physical operating principles of the device and lay the groundwork for the development of future passive and tunable thermal circuitry components.