Materials science has made progress in maximizing or minimizing the thermal conductivity of materials; however, the thermal effusivity—related to the product of conductivity and capacity—has received limited attention, despite its importance in the coupling of thermal energy to the environment. Herein, we design materials that maximize the thermal effusivity by impregnating copper and nickel foams with conformal, chemical-vapor-deposited graphene and octadecane as a phase change material. These materials are ideal for ambient energy harvesting in the form of what we call thermal resonators to generate persistent electrical power from thermal fluctuations over large ranges of frequencies. Theory and experiment demonstrate that the harvestable power for these devices is proportional to the thermal effusivity of the dominant thermal mass. To illustrate, we measure persistent energy harvesting from diurnal frequencies, extracting as high as 350 mV and 1.3 mW from approximately 10 °C diurnal temperature differences.
Thermal diodes are materials that allow for the preferential directional transport of heat and are highly promising devices for energy conservation, energy harvesting, and information processing
Next generation off-the-grid electronic systems call for alternative modes of energy harvesting. The past two decades have witnessed the evolution of a wide spectrum of low dimensional carbon materials with extraordinary physical and chemical properties, ideal for micro-scale electrical energy storage and generation. Tremendous progress has been made in harnessing the energy associated with the interactions between these nano-structured carbon substrates and the surrounding molecular phases, subsequently converting them into useful electricity. This review summarizes the important theoretical and experimental milestones the field has reached to date, and further classifies these energy harvesting processes based on underlying physics, into five mechanistically distinct classesphonon coupling, Coulombic scattering, electrokinetic streaming, asymmetric doping, and capacitive discharging. With a special mechanistic focus, the authors hope to resolve the fundamental attributes shared by this diverse array of molecular scale energy harvesting schemes, offer perspectives on key challenges, and ultimately establish design principles that guide further device optimization.
Materials designed to undergo a phase transition at a prescribed temperature have been advanced as elements for controlling thermal flux. Such phase change materials can be used as components of reversible thermal diodes, or materials that favor heat flux in a preferred direction; however, a thorough mathematical analysis of such diodes is thus far absent from the literature. Herein, it is shown mathematically that the interface of a phase change material with a phase invariant one can function as a simple thermal diode. Design equations are derived for such phase change diodes, solving for the limits where the transition temperature falls within or outside of the temperature gradient across the device. Criteria are derived analytically for the choice of thermal conductivity of the invariant phase to maximize the rectification ratio. Finally, the model is applied to several experimental systems in the literature, providing bounds on observed performance. This model should aid in the development of materials capable of controlling heat flux.
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