Active thermal materials like thermal diodes, regulators, and switches have the potential to revolutionize thermal management, creating an opportunity for significant energy savings. We present results on a proposed thermal switching composite that changes its thermal conductivity based on applied strain. The composite is constructed of highly crystalline, high aspect ratio cellulose nanocrystal (CNC) nanorods embedded in a shape-memory polymer matrix. The properties of the matrix allow for changes to the mechanical state to be indefinitely retained and also for the state to be reversed; this work is the first step in demonstrating that the thermal state exhibits similar reversibility. Measurements of the neat matrix polymer show a factor of three increase in thermal conductivity with applied strain up 100% and abrupt decrease beyond this strain level. A twofold increase in the thermal conductivity is achieved for the proof-of-concept composite at 100% strain. By comparing the measured results to a Maxwell mixing model, the primary drivers of the thermal conductivity change are traced to changes in crystallinity of the matrix and CNC alignment.

This work demonstrates an innovative microfabricated air-cooling technology that employs an electrohydrodynamic (EHD) corona discharge (i.e., ionic wind pump) for electronics cooling applications. A single, microfabricated ionic wind pump element consists of two parallel collecting electrodes between which a single emitting tip is positioned. A grid structure on the collector electrodes can enhance the overall heat-transfer coefficient and facilitate an IC compatible batch process. The optimized devices studied exhibit an overall device area of 5.4 mm × 3.6 mm, an emitter-to-collector gap of ∼0.5 mm, and an emitter curvature radius of ∼12.5 μm. The manufacturing process developed for the device uses glass wafers, a single mask-based photolithography process, and a low-cost copper-based electroplating process. Various design configurations were explored and modeled computationally to investigate their influence on the cooling phenomenon. The single devices provide a high heat-transfer coefficient of up to ∼3200 W/m2 K and a coefficient of performance (COP) of up to ∼47. The COP was obtained by dividing the heat removal enhancement, ΔQ by the power consumed by the ionic wind pump device. A maximum applied voltage of 1.9 kV, which is equivalent to approximately 38 mW of power input, is required for operation, which is significantly lower than the power required for the previously reported devices. Furthermore, the microfabricated single device exhibits a flexible and small form factor, no noise generation, high efficiency, large heat removal over a small dimension and at low power, and high reliability (no moving parts); these are characteristics required by the semiconductor industry for next generation thermal management solutions.

A steady state thermal conductivity measuring setup based on the comparative radial heat flow method is presented. The setup consists of a pair of coaxial cylinders as its main components, with test fluid placed in the annular space between these cylinders with water tight cover plates at the top and bottom of the cylinders. Experiment involves heating the coil at the concentric-center of the inner cylinder; steady state data are acquired for the calculation of the thermal conductivity. Thermal conductivity is calculated by comparing the radial heat flow between the cylinders and the test fluid (comparative method). Thermal conductivity of water, glycerol, and ethylene glycol was measured for varying temperatures and is in good agreement with the published thermal conductivity values in literature.

Low-dimensional nanostructured materials are promising candidates for high efficiency solid-state cooling devices based on the Peltier effect. Thermal transport in these low-dimensional materials is a key factor for device performance since the thermoelectric figure of merit is inversely proportional to thermal conductivity. Therefore, understanding thermal transport in nanostructured materials is crucial for engineering high performance devices. Thermal transport in semiconductors is dominated by lattice vibrations called phonons, and phonon transport is often markedly different in nanostructures than it is in bulk materials for a number of reasons. First, as the size of a structure decreases, its surface area to volume ratio increases, thereby increasing the importance of boundaries and interfaces. Additionally, at the nanoscale the characteristic length of the structure approaches the phonon wavelength, and other interesting phenomena such as dispersion relation modification and quantum confinement may arise and further alter the thermal transport. In this paper we discuss phonon transport in semiconductor superlattices and nanowires with regards to applications in solid-state cooling devices. Systematic studies on periodic multilayers called superlattices disclose the relative importance of acoustic impedance mismatch, alloy scattering, and crystalline imperfections at the interfaces. Thermal conductivity measurements of mono-crystalline silicon nanowires of different diameters reveal the strong effects of phonon-boundary scattering. Experimental results for Si/SiGe superlattice nanowires indicate that different phonon scattering mechanisms may disrupt phonon transport at different frequencies. These experimental studies provide insight regarding the dominant mechanisms for phonon transport in nanostructures. Finally, we also briefly discuss Peltier coolers made from nanostructured materials that have shown promising cooling performance.

Molecular dynamics simulations are used to examine how thermal transport is affected by the presence of one or more interfaces. Parameters such as film thickness, the ratio of respective material composition, the number of interfaces per unit length, and lattice strain are considered. Results indicate that for simple nanoscale strained heterostructures containing a single interface, the effective thermal conductivity may be less than half the value of an average of the thermal conductivities of the respective unstrained thin films. Increasing the number of interfaces per unit length, however, does not necessarily result in a corresponding decrease in the effective thermal conductivity of the superlattice.