Interest in new thermal diodes, regulators, and switches has been rapidly growing because these components have the potential for rich transport phenomena that cannot be achieved using traditional thermal resistors and capacitors. Each of these thermal components has a signature functionality: Thermal diodes can rectify heat currents, thermal regulators can maintain a desired temperature, and thermal switches can actively control the heat transfer. Here, we review the fundamental physical mechanisms of switchable and nonlinear heat transfer which have been harnessed to make thermal diodes, switches, and regulators. The review focuses on experimental demonstrations, mainly near room temperature, and spans the fields of heat conduction, convection, and radiation. We emphasize the changes in thermal properties across phase transitions and thermal switching using electric and magnetic fields. After surveying fundamental mechanisms, we present various nonlinear and active thermal circuits that are based on analogies with well-known electrical circuits, and analyze potential applications in solid-state refrigeration and waste heat scavenging.

The thermal boundary conductance (TBC) of materials pairs in atomically intimate contact is reviewed as a practical guide for materials scientists. First, analytical and computational models of TBC are reviewed. Five measurement methods are then compared in terms of their sensitivity to TBC: the 3ω method, frequency-and time-domain thermoreflectance, the cut-bar method, and a composite effective thermal conductivity method. The heart of the review surveys 30 years of TBC measurements around room temperature, highlighting the materials science factors experimentally proven to influence TBC. These factors include the bulk dispersion relations, acoustic contrast, and interfacial chemistry and bonding. The measured TBCs are compared across a wide range of materials systems by using the maximum transmission limit, which with an attenuated transmission coefficient proves to be a good guideline for most clean, strongly bonded interfaces. Finally, opportunities for future research are discussed.

To study recombination at the amorphous/crystalline Si (aSi:H/c-Si) heterointerface, the amphoteric nature of silicon (Si) dangling bonds is taken into account. Modeling interface recombination measured on various test structures provides insight into the microscopic passivation mechanisms, yielding an excellent interface defect density reduction by intrinsic aSi:H and tunable field-effect passivation by doped layers. The potential of this model's applicability to recombination at other Si heterointerfaces is demonstrated. Solar cell properties of aSi:H/c-Si heterojunctions are in good accordance with the microscopic interface properties revealed by modeling, that are, e.g., slight asymmetries in the neutral capture cross-sections and band offsets. The importance of atomically abrupt interfaces and the difficulties to obtain them on pyramidally textured c-Si is studied in combination with transmission electron microscopy.

The effect of diamond surface treatment on the Thermal Boundary Conductance (TBC) between Al and diamond is investigated. The treatments consist in either of the following: exposition to a plasma of pure Ar, Ar:H and Ar:O, and HNO3:H2SO4 acid dip for various times. The surface of diamond after treatment is analyzed by X-ray Photoelectron Spectroscopy, revealing hydrogen termination for the as-received and Ar:H plasma treated samples, pure sp2 termination for Ar treated ones and oxygen (keton-like) termination for the other treatments. At ambient, all the specific treatments improve the TBC between Al and diamond from 23 ± 2 MW m–2 K–1 for the as-received to 65 ± 5, 125 ± 20, 150 ± 20, 180 ± 20 MW m–2 K–1 for the ones treated by Ar:H plasma, acid, pure Ar plasma, and Ar:O plasma with an evaporated Al layer on top, respectively. The effect of these treatments on temperature dependence are also observed and compared with the most common models available in the literature as well as experimental values in the same system. The results obtained show that the values measured for an Ar:O plasma treated diamond with Al sputtered on top stay consistently higher than the values existing in the literature over a temperature range from 78 to 290 K, probably due a lower sample surface roughness. Around ambient, the TBC values measured lay close to or even somewhat above the radiation limit, suggesting that inelastic or electronic processes may influence the transfer of heat at this metal/dielectric interface.

Understanding of polarization-heat interactions in pyroelectric and electrocaloric thin-film materials requires that the electrothermal response is reliably characterized. While most work, particularly in electrocalorics, has relied on indirect measurement protocols, here we report a direct technique for measuring both pyroelectric and electrocaloric effects in epitaxial ferroelectric thin films. We demonstrate an electrothermal test platform where localized high-frequency (∼ 1 kHz) periodic heating and highly-sensitive thin-film resistance thermometry allow direct measurement of pyrocurrents (< 10 pA) and electrocaloric temperature changes (< 2 mK) using the "2-omega" and an adapted "3-omega" technique, respectively. Frequency-domain, phase-sensitive detection permits extraction of pyrocurrent from the total current, which is often convoluted by thermally-stimulated currents. The wide frequency range measurements employed in this study further show the effect of secondary contributions to pyroelectricity due to the mechanical constraints of the substrate. Similarly, measurement of the electrocaloric effect on the same device in the frequency-domain (∼ 100 kHz) allows decoupling of Joule heating from the electrocaloric effect. Using one-dimensional, analytical heattransport models, the transient temperature profile of the heterostructure is characterized to extract pyroelectric and electrocaloric coefficients.

We report Time-Domain ThermoReflectance experiments measuring the Thermal Boundary Conductance (TBC) of interfaces between diamond and metal surfaces, based on samples consisting of [111] diamond substrates with hydrogen or with sp 2 carbon surface terminations created using plasma treatments. In a concurrent theoretical study, we calculate the work of adhesion between Ni, Cu and diamond interfaces with [111] surface orientation, with or without hydrogen termination of the diamond surface, using first-principles electronic structure calculations based on density functional theory (DFT).We find a positive correlation between the calculated work of adhesion and the mea-sured conductance of these interfaces, suggesting that DFT could be used as a screening tool to identify metal/dielectric systems with high TBC We also explain the negative effect of hydrogen on the thermal conductance of metal/diamond interfaces. a) Corresponding author,

Aluminum nitride (AlN) particle reinforced metal-matrix-composites produced by pressure infiltration are characterized in terms of their thermal conductivity. The composites are designed to cover a wide range of phase contrast between the dispersed particles and the matrix; this is achieved by changing the matrix conductivity using Cu, Al, Sn, and Pb as the matrix. The interface thermal conductance (hc) between AlN and the matrix metals is determined by varying the size of the AlN particles using the Hasselman–Johnson approach and the differential effective medium (DEM) model to calculate hc from measured composite conductivity values. In addition, hc is measured directly at the AlN/Al interface using the transient thermoreflectance (TTR) method on thin aluminum layers deposited on flat AlN substrates to find good agreement with the value derived directly from Al/AlN composites of variable particle size and thus confirm the approach used here to measure hc. Data from the study show that hc at AlN-metal interfaces increases with the metal/AlN Debye temperature ratio; however, the increase is much less than predicted by currently accepted models.

We demonstrate far-field optical thermometry using individual NaYF4 nanoparticles doped with 2% Er(3+) and 20% Yb(3+). Isolated 20 × 20 × 40 nm(3) particles were identified using only far-field optical imaging, confirmed by subsequent scanning electron microscopy. The luminescence thermometry response for five such single particles was characterized for temperatures from 300 K to 400 K. A standard Arrhenius model widely used for larger particles can still be accurately applied to these sub-50 nm particles, with good particle-to-particle uniformity (response coefficients exhibited standard deviations below 5%). With its spatial resolution on the order of 50 nm when imaging a single particle, far below the diffraction limit, this technique has potential applications for both fundamental thermal measurements and nanoscale metrology in industrial applications.