We present detailed data on the performance of microstructured geometries for use in the evaporator section of a vapor chamber heat pipe. The central innovation of the geometries is their hierarchical structure, involving the use of large microchannels in order to reduce liquid flow drag while fabricating microscale pin fin arrays whose small pores increase capillary suction. The overall conductance in such a geometry is dependent on the extent of thin liquid film (thickness ∼few microns), which is manipulated by increasing the surface area-to-volume ratio through the use of microstructuring. Experiments were conducted for a heater area of 1cm 2 , with the wick in a vertical orientation. Results are presented for fixed microchannel widths of 30–60 microns, with pin fin diameters ranging from 4 to 32 microns, and pin fin array widths of 150 to 300 microns. The competing effects of increase in surface area due to microstructuring, and the suppression of evaporation due to reduction in pore scale are explored. In the evaporative regime, conductances of the order of 6 W/cm 2 -K are attained at heat fluxes of up to 140 W/cm 2 , until the capillary limit is reached and the wick dries out.

The loop heat pipe (LHP) was invented in Russia in the early 1980’s. It is a two-phase heat transfer device that utilizes the evaporation and condensation of a working fluid to transfer heat, and the capillary force developed in fine porous wicks to circulate the fluid. The temperature of LHP evaporator as functions of the heat load, sink temperature, ambient temperature is an important parameter which can reflect the performance of an LHP. Many factors can affect the LHP operating temperature and which can be divided into two parts: external and internal. The external factors including heat sink temperature, ambient temperature, fluid forces, the position between heat source and heat sink and the heat exchange between LHP and ambient. The internal factors related to the design and structure of the LHP, for example, the charging amount of the working fluid and the distribution status of the liquid phase during the LHP operating. Based on Sinda/Fluint software an ammonia-stainless steel steady state model of loop heat pipe was established, the impacts on the LHP operating temperature induced by alterable heat loads under 3 operating cases (the different position between evaporator and condenser, the changing of ambient temperature and the changing of heat sink temperature) were analyzed and conclusions were made. Changing the position between evaporator and condenser has a significant influence on the LHP operating temperature. Anti-gravity operation will reduce the performance of the LHP, this phenomenon is obviously in low heat load range. Further more, increasing of fluid pressure drop in the loop will induce decreasing of the LHP performance. The temperature difference between ambient and heat sink will influence the transition heat load (from variable conductance mode to fixed conductance mode), the bigger the temperature difference the higher the transition heat load.

Thermal transport at the interface between Lennard-Jones crystals is explored via non-equilibrium molecular dynamics simulations. The vibrational properties of each crystal are varied by changing the atomic mass of the crystal. By applying a constant thermal flux across the two-crystal composite system, a steady-state temperature gradient is established and thermal boundary conductance at the interface between the crystals is calculated via Fourier’s law. With the material properties of the two crystals fixed, thermal boundary conductance can be affected by an intermediate layer inserted between the two crystals. It is found that when the interstitial layer atomic mass is between those values of the crystals comprising the interface, interfacial transport is enhanced. This layer helps bridge the gap between the different vibrational spectra of the two materials, thus enhancing thermal transport, which is maximized when the interstitial layer atomic mass approaches the average mass of the two fixed crystals. The degree of enhancement depends on the vibrational mismatch between the interstitial layer and the crystals comprising the interface, and we report an increase in thermal boundary conductance of up to 50%.

Thermal boundary resistance dominates the thermal resistance in nanosystems since material length scales are comparable to material mean free paths. The primary scattering mechanism in nanosystems is interface scattering, and the structure and composition around these interfaces can affect scattering rates and, therefore, device thermal resistances. In this work, the thermal boundary conductance (the inverse of the thermal boundary resistance) is measured using a pump-probe thermoreflectance technique on aluminum films grown on silicon substrates that are subjected to various pre-Al-deposition surface treatments. The Si surfaces are characterized with Atomic Force Microscopy (AFM) to determine mean surface roughness. The measured thermal boundary conductance decreases as Si surface roughness increases. In addition, stripping the native oxide layer on the surface of the Si substrate immediately prior to Al film deposition causes the thermal boundary conductance to increase. The measured data are then compared to an extension of the diffuse mismatch model that accounts for interfacial mixing and structure around the interface.

This paper considers phonon transport behavior in graphene nanoribbons (GNRs) that bridge semi-infinite graphene contacts. The work employs an atomistic Green’s function method to investigate phonon wave effects in zigzag and armchair edge ribbons. Phonon transmission functions and thermal conductances are found to be sensitive to the edge shape of structures. The thermal conductances of GNRs with different widths are normalized by the quantum of thermal conductance to reveal the relation between number of phonon modes and conductance as a function of temperature. In addition, the phonon transmission functions of nano ribbons with defects are evaluated by artificially creating mismatches at interfaces. By comparing the transmission function of different defect patterns and the corresponding thermal conductances, the reduction of phonon transport is quantified. The length of defects is found to be important to phonon transport. Constriction effects are also studied at abrupt mismatched interfaces, and the reduction of thermal conductance is found to be moderately high.

Synthetic diamond has potential as a heat spreading material due to its uniquely high thermal conductivity. In small-scale devices, interfaces can dominate the resistance to heat transport, and thus play an important role in determining device performance. Here we use transient thermoreflectance techniques to measure the thermal interface conductance at metal-diamond interfaces. We study single crystal diamond samples with various surface terminations. We measure thermal interface conductance values over a range of temperatures from 88 K to 300 K, and find roughly 60 percent higher thermal interface conductance between Al and oxygenated diamond samples as compared to hydrogen terminated samples. The results reported here will be useful for device design and for advancing models of interfacial heat transport.

Electronic transport in molecular junctions has been studied through measurements of junction thermopower to evaluate the feasibility of thermoelectric (TE) energy generation using organic-inorganic hybrid materials. Energy transport and conversion in these junctions are heavily influenced by transport interactions at the metal-molecule interface. At this interface the discrete molecular orbitals overlap with continuum electronic states in the inorganic electrodes to create unique energy landscapes that cannot be realized in the organic or inorganic components alone. Over the past decade, scanning probe microscopes have been used to study the electronic conductance of single-molecule junctions[1–5]. Recently, we conducted measurements of junction thermopower using a modified scanning tunneling microscope (STM)[6]. Through our investigations, we have determined: (i) how the addition of molecular substituent groups can be used to predictably tune the TE properties of phenylenedithiol (PDT) junctions[7], (ii) how the length, molecular backbone, and end groups affect junction thermopower[8], and (iii) where electronic transport variations originate[9]. Furthermore, we have recently found that large (10 fold) TE enhancement can be achieved by effectively altering a (noble) metal junction using fullerenes (i.e., C 60 , PCBM, and C 70 ). We associate the enhancement with the alignment of the frontier orbitals of the fullerene to the chemical potential of the inorganic electrodes. We further found that the thermopower can be predictably tuned by varying the work function of the contacts. This yields considerable promise for altering the surface states at interfaces for enhanced electronic and thermal transport. This paper highlights our work using thermopower as a probe for electronic transport, and reports preliminary results of TE conversion in fullerene-metal junctions.