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Author(s): Dames, C; Urban, JJ; Kang, H | Abstract: Sugar alcohols (SAs) have attractive characteristics as phase-change materials, but their relatively high melting temperature limits their application in the real world. Nanoconfinement can be a useful parameter to reduce the melting temperature to pragmatic ranges. Using molecular dynamics simulations, we investigate the phases and behaviors of encapsulated SA in ZIF-8 and ZIF-11, which cannot be experimentally observed. Based on reliable partial charges for the zeolitic imidazolate framework (ZIF) structures calculated by a density functional theory, structural analysis shows that the SA's attractive interaction with the ZIF structure frustrates the SA crystallization and also elucidates the second-order phase transition between amorphous phases. A methodology is suggested to determine the phase transition temperature of confined materials and used to quantify the melting temperature depression of the ZIF-confined SAs. We also explored the thermal conductivity of SA-in-ZIF composites. Phonon frequency analysis verifies that the presence of SA molecules enhances the heat transfer by adding heat pathways between the nanoporous structure of ZIFs.

In an isothermal process, applying an electric field E to a ferroelectric material can change its entropy S through two distinct mechanisms: namely, through changing the dipole alignment or configurational entropy (ΔSconf), and the vibrational entropy due to the intrinsic structure response (ΔSvib). Previous numerical investigations yield only the total entropy change ΔS but cannot separate these two contributions. Here we develop a full first-principles method to extract ΔSvib and the corresponding induced adiabatic temperature change ΔTvib under E, i.e., the electrocaloric effect (ECE), and compare them to the total ΔS and ΔT from molecular dynamics simulation based on a first-principles effective Hamiltonian model. For both single crystal PbTiO3 (PTO) and PbZr0.5Ti0.5O3 (50/50 PZT), the calculation results show that for T far from the phase transition temperature Tpt, ΔSvib plays an important and even dominant role in the ECE. On the other hand, for T close to Tpt, ΔSconf dominates the ECE. Moreover, for PTO, we find that positive E can cause positive ECE, while negative E cause negative ECE. Therefore, by combining both positive and negative ECE in a “bipolar” Ericsson cycle, the cooling energy density can significantly increase as compared to that of a unipolar cycle.

The electron beam (e-beam) in the scanning electron microscopy (SEM) provides an appealing mobile heating source for thermal metrology with spatial resolution of ∼1 nm, but the lack of systematic quantification of the e-beam heating power limits such application development. Here, we systemically study e-beam heating in LPCVD silicon nitride (SiNx) thin-films with thickness ranging from 200 to 500 nm from both experiments and complementary Monte Carlo simulations using the CASINO software package. There is good agreement about the thickness-dependent e-beam energy absorption of thin-film between modeling predictions and experiments. Using the absorption results, we then demonstrate adapting the e-beam as a quantitative heating source by measuring the thickness-dependent thermal conductivity of SiNx thin-films, with the results validated to within 7% by a separate Joule heating experiment. The results described here will open a new avenue for using SEM e-beams as a mobile heating source for advanced nanoscale thermal metrology development.

Applications such as solid-state waste-heat energy conversion, infrared sensing, and thermally-driven electron emission rely on pyroelectric materials (a subclass of dielectric piezoelectrics) which exhibit temperature-dependent changes in polarization. Although enhanced dielectric and piezoelectric responses are typically found at polarization instabilities such as temperature- and chemically induced phase boundaries, large pyroelectric effects have been primarily limited in study to temperature-induced phase boundaries. Here, we directly identify the magnitude and sign of the intrinsic, extrinsic, dielectric, and secondary pyroelectric contributions to the total pyroelectric response as a function of chemistry in thin films of the canonical ferroelectric PbZr1-xTixO3 (x = 0.40, 0.48, 0.60, and 0.80) across the morphotropic phase boundary. Using phase-sensitive frequency and applied dc-bias methods, the various pyroelectric contributions were measured. It is found that the total pyroelectric response decreases systematically as one moves from higher to lower titanium contents. This arises from a combination of decreasing intrinsic response (-232 to -97 μC m-2 K-1) and a sign inversion (+33 to -17 μC m-2 K-1) of the extrinsic contribution upon crossing the morphotropic phase boundary. Additionally, the measured secondary and dielectric contributions span between -70 and -29 and 10-115 μC m-2 K-1 under applied fields, respectively, following closely trends in the piezoelectric and dielectric susceptibility. These findings and methodologies provide novel insights into the understudied realm of pyroelectric response.

Rebuilding phonon mean free path (MFP) spectra from experimental data is integral to phonon MFP spectroscopy. However, being based on effective thermal conductivity, the current integral eq...

Considerable advances in manipulating heat flow in solids have been made through the innovation of artificial thermal structures such as thermal diodes, camouflages, and cloaks. Such thermal devices can be readily constructed only at the macroscale by mechanically assembling different materials with distinct values of thermal conductivity. Here, we extend these concepts to the microscale by demonstrating a monolithic material structure on which nearly arbitrary microscale thermal metamaterial patterns can be written and programmed. It is based on a single, suspended silicon membrane whose thermal conductivity is locally, continuously, and reversibly engineered over a wide range (between 2 and 65 W/m·K) and with fine spatial resolution (10-100 nm) by focused ion irradiation. Our thermal cloak demonstration shows how ion-write microthermotics can be used as a lithography-free platform to create thermal metamaterials that control heat flow at the microscale.

The poor performance of lithium-ion batteries in extreme temperatures is hindering their wider adoption in the energy sector. A fundamental challenge in battery thermal management systems (BTMSs) is that hot and cold environments pose opposite requirements: thermal transmission at high temperature for battery cooling, and thermal isolation at low temperature to retain the batteries’ internally generated heat, leading to an inevitable compromise of either hot or cold performances. Here, we demonstrate a thermal regulator that adjusts its thermal conductance as a function of the temperature, just as desired for the BTMS. Without any external logic control, this thermal regulator increases battery capacity by a factor of 3 at an ambient temperature (Tambient) of −20 °C in comparison to a baseline BTMS that is always thermally conducting, while also limiting the battery temperature rise to 5 °C in a very hot environment (Tambient = 45 °C) to ensure safety. The result expands the usability of lithium-ion batteries in extreme environments and opens up new applications of thermally functional devices.

The need for efficient energy utilization is driving research into ways to harvest ubiquitous waste heat. Here, we explore pyroelectric energy conversion from low-grade thermal sources that exploits strong field- and temperature-induced polarization susceptibilities in the relaxor ferroelectric 0.68Pb(Mg1/3Nb2/3)O3–0.32PbTiO3. Electric-field-driven enhancement of the pyroelectric response (as large as −550 μC m−2 K−1) and suppression of the dielectric response (by 72%) yield substantial figures of merit for pyroelectric energy conversion. Field- and temperature-dependent pyroelectric measurements highlight the role of polarization rotation and field-induced polarization in mediating these effects. Solid-state, thin-film devices that convert low-grade heat into electrical energy are demonstrated using pyroelectric Ericsson cycles, and optimized to yield maximum energy density, power density and efficiency of 1.06 J cm−3, 526 W cm−3 and 19% of Carnot, respectively; the highest values reported to date and equivalent to the performance of a thermoelectric with an effective ZT ≈ 1.16 for a temperature change of 10 K. Our findings suggest that pyroelectric devices may be competitive with thermoelectric devices for low-grade thermal harvesting. Pyroelectric energy conversion in a thin-film relaxor ferroelectric is studied under an electric field, resulting in high energy and power densities. Performance is equivalent to a ZT = 1.16 thermoelectric, competitive for low-grade thermal harvesting.

The development of ultrahigh-temperature thermoelectric materials could enable thermoelectric topping of combustion power cycles as well as extending the range of direct thermoelectric power generation in concentrated solar power. However, thermoelectric operation temperatures have been restricted to under 1,500 K due to the lack of suitable materials. Here, we demonstrate a thermoelectric conversion material based on high-temperature reduced graphene oxide nanosheets that can perform reliably up to 3,000 K. After a reduction treatment at 3,300 K, the nanosheet film exhibits an increased conductivity to ~4,000 S cm−1 at 3,000 K and a high power factor S2σ = 54.5 µW cm−1 K−2. We report measurements characterizing the film’s thermoelectric properties up to 3,000 K. The reduced graphene oxide film also exhibits a high broadband radiation absorbance and can act as both a radiative receiver and a thermoelectric generator. The printable, lightweight and flexible film is attractive for system integration and scalable manufacturing. The Carnot efficiency and the power output of thermoelectric power generation increase with temperature but current thermoelectrics are characterized up to 1,500 K. Here, Li et al. develop reduced graphene oxide films that can convert heat up to 3,000 K with high power factors, opening the door for novel applications.

Thermal radiation from a black body increases with the fourth power of absolute temperature (T4 ), an effect known as the Stefan-Boltzmann law. Typical materials radiate heat at a portion of this limit, where the portion, called integrated emissivity (eint ), is insensitive to temperature (|deint /dT| ≈ 10-4 °C-1 ). The resultant radiance bound by the T4 law limits the ability to regulate radiative heat. Here, an unusual material platform is shown in which eint can be engineered to decrease in an arbitrary manner near room temperature (|deint /dT| ≈ 8 × 10-3 °C-1 ), enabling unprecedented manipulation of infrared radiation. As an example, eint is programmed to vary with temperature as the inverse of T4 , precisely counteracting the T4 dependence; hence, thermal radiance from the surface becomes temperature-independent, allowing the fabrication of flexible and power-free infrared camouflage with unique advantage in performance stability. The structure is based on thin films of tungsten-doped vanadium dioxide where the tungsten fraction is judiciously graded across a thickness less than the skin depth of electromagnetic screening.

Ferroelectric materials attract attention as potential solid-state thermal switches, due to various phenomena near their solid-state phase transition. In principle these phenomena could also be triggered by an external electric field, though recent experiments on thin films have shown only weak effects. The authors explore this issue by solving the linearized Boltzmann equation with first-principles force constants, to study the thermal conductivity of PbTiO${}_{3}$ as a function of electric field. The results emphasize the need for large-domain samples and lower temperatures, to have the best chance at large voltage-controlled thermal switching in this class of materials.

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.

Highly efficient broadband thermal radiation from reduced graphene oxide (RGO) paper mixed with single-walled carbon nanotubes (CNTs) is reported. These RGO-CNT paper ribbons routinely reach 3000 K before failure, with some samples exceeding 3300 K, higher than any other carbon nanomaterial. Excellent performance is achieved, with ≈90% radiation efficiency, 200 000 on/off cycles, and stable operation for more than 50 hours.

Many materials have anisotropic thermal conductivity, with diverse applications such as transistors, thermoelectrics, and laser gain media. Yet measuring the thermal conductivity tensor of such materials remains a challenge, particularly for materials lacking in-plane symmetry (i.e., transversely anisotropic materials). This paper demonstrates thermal conductivity tensor measurements for transversely anisotropic materials, by extending beam-offset frequency-domain thermoreflectance (BO-FDTR) methods which had previously been limited to transversely isotropic materials. Extensive sensitivity analysis is used to determine an appropriate range of heating frequencies and beam offsets to extract various tensor elements. This technique is demonstrated on a model transversely anisotropic material, x-cut quartz ( α-SiO2), by combining beam offset measurements from different sample orientations to reconstruct the full in-plane thermal conductivity tensor. The technique is also validated by measurements on two transversely isotropic materials, sapphire and highly oriented pyrolytic graphite (HOPG). The anisotropic measurements demonstrated very good self-consistency in correctly identifying isotropic directions when present, with residual anisotropy errors below 4% for sapphire and 2% for HOPG and quartz. Finally, a computational case study (simulated experiment) shows how the arbitrary in-plane thermal conductivity tensor of a fictitious material with high in-plane anisotropy can in principle be obtained from only a single sample orientation, rather than multiple orientations like the experiments on x-cut quartz.

As device length scales trend downward, small feature sizes and steep temperature gradients require thermometers with increasingly fine spatial resolution in order to capture the true peak temperature. Here, we develop analytical expressions for the true and measured temperature rises as a function of thermometer size for Gaussian, disk-shaped, and rectangular surface heat sources. We find that even a thermometer the same size as the hotspot can underestimate the true peak temperature rise by more than 15%, and this error frequently exceeds 75% and can approach 90% for certain geometries when the thermometer is ten times larger than the measured hotspot. We show that a thermometer with resolution approximately two times smaller than the hotspot size is required to measure the peak temperature rise with less than 5% error for several common hotspot geometries. We also experimentally demonstrate that a 50 × 50 × 50 nm3 individual upconverting NaYF4:Yb3+,Er3+ nanoparticle thermometer captures the peak temperature rise due to laser heating more accurately than conventional diffraction limited optical techniques that our modeling results show would underestimate this value. In contrast to apparent self-heating effects that spuriously increase the nanoparticle thermometry signal at high excitation intensities, we measure true laser heating, as confirmed by comparing measurements on glass and diamond substrates.

The hot disk transient plane source (TPS) method is a widely used standard technique (ISO 22007–2) for characterizing the thermal properties of materials, especially the thermal conductivity, k. Despite its well-established reliability for a wide variety of common materials, the hot disk TPS method is also known to suffer from substantial systematic errors when applied to low-k thermal insulation materials, because of the discrepancies between the idealized model used for data analysis and the actual heat transfer process. Here, we present a combined numerical and experimental study of the influence of the geometry of the hot disk sensor on the measured k value of low-k materials. We demonstrate that the error is strongly affected by the finite thickness and thermal mass of the sensor's insulation layer as well as the corresponding increase of the effective heater size beyond the radius of the embedded metal heater itself. We further numerically investigate the dependence of the error on the sample thermal properties, confirming that the errors are worse in low-k samples. A simple polynomial correction function is provided based on the numerical error analysis, which converts the apparent (erroneous) result from a standard hot disk TPS measurement to a more accurate value of k. To experimentally validate the conclusions from numerical simulations, standard polyimide (Kapton) sensors are systematically optimized (thinned) by etching and used to measure low-k materials, including a standard polystyrene foam, a commercial Airloy® x56 aerogel, and a commercial Hydrophobic Silica Disk aerogel from Aerogel Technologies, LLC. The experimental results clearly demonstrate the strong influence of the sensor thickness. The k results of these samples obtained using either the optimized sensor or the pristine sensor then corrected with the corresponding polynomial correction functions are in good agreement with the values measured independently using a steady-state heat flowmeter (HFM) method: whereas the raw values measured with the pristine sensor are in error by 35% and 40% compared to the HFM reference values for the Airloy® x56 and the hydrophobic aerogel, respectively, the correction function greatly reduces those errors to

Individual luminescent nanoparticles enable thermometry with sub-diffraction limited spatial resolution, but potential self-heating effects from high single-particle excitation intensities remain largely uninvestigated because thermal models predict negligible self-heating. Here, we report that the common “ratiometric” thermometry signal of individual NaYF4:Yb3+,Er3+ nanoparticles unexpectedly increases with excitation intensity, implying a temperature rise over 50 K if interpreted as thermal. Luminescence lifetime thermometry, which we demonstrate for the first time using individual NaYF4:Yb3+,Er3+ nanoparticles, indicates a similar temperature rise. To resolve this apparent contradiction between model and experiment, we systematically vary the nanoparticle’s thermal environment: the substrate thermal conductivity, nanoparticle-substrate contact resistance, and nanoparticle size. The apparent self-heating remains unchanged, demonstrating that this effect is an artifact, not a real temperature rise. Using rate equation modeling, we show that this artifact results from increased radiative and non-radiative relaxation from higher-lying Er3+ energy levels. This study has important implications for single-particle thermometry., Nanoparticles are often used as nanothermometers by measuring their luminescence from upconverted energy under illumination. The authors uncover the artificial appearance of a temperature rise at high excitation intensities due to effects involving higher energy states.

Elastic ceramics Aerogels hold promise as lightweight replacements for thermal insulation. However, poor mechanical stability has hampered progress in moving toward commercialization. Xu et al. designed a mechanical metamaterial that pinches in a small amount when you compress it (see the Perspective by Chhowalla and Jariwala). This is characteristic of materials with a negative Poisson's ratio and dramatically improves mechanical stability. The trick was using three-dimensional graphene structures to template the ceramic aerogels, thus producing a superinsulating material endowed with excellent mechanical properties. Science , this issue p. 723 ; see also p. 694

Temperature- and electric-field-induced structural transitions in a polydomain ferroelectric can have profound effects on its electrothermal susceptibilities. Here, the role of such ferroelastic domains on the pyroelectric and electrocaloric response is experimentally investigated in thin films of the tetragonal ferroelectric PbZr0.2 Ti0.8 O3 . By utilizing epitaxial strain, a rich set of ferroelastic polydomain states spanning a broad thermodynamic phase space are stabilized. Using temperature-dependent scanning-probe microscopy, X-ray diffraction, and high-frequency phase-sensitive pyroelectric measurements, the propensity of domains to reconfigure under a temperature perturbation is quantitatively studied. In turn, the "extrinsic" contributions to pyroelectricity exclusively due to changes between the ferroelastic domain population is elucidated as a function of epitaxial strain. Further, using highly sensitive thin-film resistive thermometry, direct electrocaloric temperature changes are measured on these polydomain thin films for the first time. The results demonstrate that temperature- and electric-field-driven domain interconversion under compressive strain diminish both the pyroelectric and the electrocaloric effects, while both these susceptibilities are enhanced due to the exact-opposite effect from the extrinsic contributions under tensile strain.

We quantify the tradeoff between ZT and materials cost for thermoelectric energy scavengers, considering 31 real materials. It is nearly always practical to aggressively miniaturize the legs until the overall system cost is limited by the heat exchangers and ceramic spreader plates. For thermoelectric materials costing less than C crit . ‴ ∼ 1 $/cm3 (∼100 $/kg), the material might as well be free, and even costs as high as ∼10 $/cm3 (∼1000 $/kg) may still be acceptable. Thus, the materials goal is maximizing ZT at (almost) any cost.

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.

Author(s): Pandya, S; Wilbur, JD; Bhatia, B; Damodaran, AR; Monachon, C; Dasgupta, A; King, WP; Dames, C; Martin, LW | Abstract: An 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 (approximately 1 kHz) periodic heating and highly sensitive thin-film resistance thermometry allow the direct measurement of pyrocurrents (l10 pA) and electrocaloric temperature changes (l2 mK) using the "2-omega" and an adapted "3-omega" technique, respectively. Frequency-domain, phase-sensitive detection permits the extraction of the 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 (at approximately 100 kHz) allows for the decoupling of Joule heating from the electrocaloric effect. Using one-dimensional, analytical heat-transport models, the transient temperature profile of the heterostructure is characterized to extract pyroelectric and electrocaloric coefficients.

High-quality crystals minimize conductivity losses caused by phonon scattering at defects

Phonons can display both wave-like and particle-like behaviour during thermal transport. While thermal transport in silicon nanomeshes has been previously interpreted by phonon wave effects due to interference with periodic structures, as well as phonon particle effects including backscattering, the dominant mechanism responsible for thermal conductivity reductions below classical predictions still remains unclear. Here we isolate the wave-related coherence effects by comparing periodic and aperiodic nanomeshes, and quantify the backscattering effect by comparing variable-pitch nanomeshes. We measure identical (within 6% uncertainty) thermal conductivities for periodic and aperiodic nanomeshes of the same average pitch, and reduced thermal conductivities for nanomeshes with smaller pitches. Ray tracing simulations support the measurement results. We conclude phonon coherence is unimportant for thermal transport in silicon nanomeshes with periodicities of 100 nm and higher and temperatures above 14 K, and phonon backscattering, as manifested in the classical size effect, is responsible for the thermal conductivity reduction., Low thermal conductivities in nanomeshes have been attributed to both wave-like and particle-like behaviour of phonons. Here, the authors use periodicity-controlled silicon nanomeshes to show that the particle backscattering effect dominates for periodicities above 100 nm and temperatures above 14 K.

Molecular dynamics simulations are performed to investigate the thermal transport properties of a three-dimensional (3D) anisotropic phononic crystal consisting of silicon nanowires and films. The calculation shows that the in-plane thermal conductivity is negatively correlated with the out-of-plane thermal conductivity upon making geometric changes, whether varying the nanowire diameter or the film thickness. This enables the anisotropy ratio of thermal conductivity to be tailored over a wide range, in some cases by more than a factor of 20. Similar trends in thermal conductivity are also observed from an independent phonon ray tracing simulation considering only diffuse boundary scattering effects, though the range of anisotropy ratios is smaller than that obtained in MD simulation. By analyzing the phonon dispersion relation with varied geometric parameters, it is found that increasing the nanowire diameter increases the out-of-plane acoustic phonon group velocities, but reduces the in-plane longitudinal and fast transverse acoustic phonon group velocities. The calculated phonon irradiation further verified the negative correlation between the in-plane and the out-of-plane thermal conductivity. The proposed 3D phononic crystal may find potential application in thermoelectrics, energy storage, catalysis and sensing applications owing to its widely tailorable thermal conductivity.

Decoupling charge and heat transport In metals, electrons carry both charge and heat. As a consequence, electrical conductivity and the electronic contribution to the thermal conductivity are typically proportional to each other. Lee et al. found a large violation of this so-called Wiedemann-Franz law near the insulator-metal transition in VO 2 nanobeams. In the metallic phase, the electronic contribution to thermal conductivity was much smaller than what would be expected from the Wiedemann-Franz law. The results can be explained in terms of independent propagation of charge and heat in a strongly correlated system. Science , this issue p. 371

Scanning transmission electron microscopy (STEM) thermometry techniques offer the potential for mapping temperature (T) with high spatial resolution. Existing STEM thermometry methods based on thermally induced strains must contend with small thermal expansion coefficients [ 1000 ppm/K) for almost all materials at room temperature. This T-dependent TDS has not been leveraged for STEM thermometry, however, due to experimental difficulties in quantifying the relatively small thermal signals. Here, we demonstrate quantitative TDS measurements using STEM by measuring diffuse scattering in energy-filtered scanning electron nanodiffraction patterns. Applying virtual apertures to these diffraction patterns during post-processing allows us to quantify the T-dependent TDS between the Bragg spots. We measure a position-averaged temperature coefficient of 2400±400 ppm/K for a single-crystal gold film averaged between T=100 K and T=300 K and compare this result with the predictions of Debye-Waller theory. This TDS-based STEM thermometry technique demonstration provides a step towards the goal of non-contact nanoscale temperature mapping of thin nanostructures.

Scanning electron microscopy (SEM) is ubiquitous for imaging but is not generally regarded as a tool for thermal measurements. Here, the temperature dependence of secondary electron (SE) emission from a sample's surface is investigated. Spatially uniform SEM images and the net charge flowing through a sample were recorded at different temperatures to quantify the temperature dependence of SE emission and electron absorption. The measurements also demonstrated charge conservation during thermal cycling by placing the sample inside a Faraday cup to capture the emitted SEs and back-scattered electrons from the sample. The temperature dependence of SE emission was measured for four semiconducting materials (Si, GaP, InP, and GaAs) with response coefficients found to be of magnitudes ∼100−1000 ppm/K. The detection limits for temperature changes were no more than ±8 °C for 60 s acquisition time.

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 ana...

On page 4684, C. Dames, L. Hu and co-workers report highly efficient, broadband lighting from printed hybrid nanocarbon structures with carbon nanotubes and reduced graphene oxides. The fast response and excellent stability of the flexible lighting can find applications in a range of emerging applications where the shape and format, as well as being lightweight, are important.

The effects of in-plane (IP) and cross-plane (CP) interatomic bonding strengths on the IP and CP thermal conductivities of a model layered material are investigated using molecular dynamics and lattice dynamics. Increasing the IP bonding strength while holding the CP bonding constant increases the IP thermal conductivity, but reduces the CP thermal conductivity. Analogous but weaker trends are seen when increasing the CP bonding strength while holding the IP bonding constant. These results show how both low- and high-symmetry directions must be considered to understand the effects of phonon focusing on the thermal conductivity tensor of highly anisotropic materials.