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Ion irradiation effects on thermal property changes are compared between aligned carbon nanotube (A-CNT) films and randomly entangled carbon nanotube (R-CNT) films. After H, C, and Fe ion irradiation, a focusing ion beam with sub-mm diameter is used as a heating source, and an infrared signal is recorded to extract thermal conductivity. Ion irradiation decreases thermal conductivity of A-CNT films, but increases that of R-CNT films. We explain the opposite trends by the fact that neighboring CNT bundles are loosely bonded in A-CNT films, which makes it difficult to create inter-tube linkage/bonding upon ion irradiation. In a comparison, in R-CNT films, which have dense tube networking, carbon displacements are easily trapped between touching tubes and act as intertube linkage to promote off-axial phonon transport. The enhancement overcomes the phonon transport loss due to phonon-defect scattering along the axial direction. A model is established to explain the dependence of thermal conductivity changes on ion irradiation parameters including ion species, energies, and current. [ABSTRACT FROM AUTHOR]

In recent years, the fundamental physics of spin-lattice (e.g., magnon-phonon) interaction has attracted significant experimental and theoretical interests given its potential paradigm-shifting impacts in areas like spin-thermoelectrics, spin-caloritronics, and spintronics. Modelling studies of the transport of magnons and phonons in magnetic crystals are very rare. In this paper, we use spin-lattice dynamics (SLD) simulations to model ferromagnetic crystalline iron, where the spin and lattice systems are coupled through the atomic position-dependent exchange function, and thus the interaction between magnons and phonons is naturally considered. We then present a method combining SLD simulations with spectral energy analysis to calculate the magnon and phonon harmonic (e.g., dispersion, specific heat, and group velocity) and anharmonic (e.g., scattering rate) properties, based on which their thermal conductivity values are calculated. This work represents an example of using SLD simulations to understand the transport properties involving coupled magnon and phonon dynamics. [ABSTRACT FROM AUTHOR]

We theoretically demonstrate that the junction between a phase-change material with a phase-invariant one can efficiently function as a conductive thermal diode. Analytical expressions for the heat flux and optimal rectification factor are derived and analyzed for junctions whose operations are driven by the thermal conductivity hysteresis of both VO2 and nitinol. It is shown that phase-change materials with higher thermal conductivity contrast, smaller thermal hysteresis, and faster phase transitions yield a conductive thermal diode with higher rectification of heat currents. Rectification factors of up to 19.7% and 18.8% are found for thermal diodes based on VO2 and nitinol, operating with a temperature difference between their terminals of 369.5 − 300 = 69.5 K and 388.2 − 273 = 115.2 K, respectively. These similar rectification factors could be enhanced by increasing the thermal conductivity variations of the diode terminals, and hence, the results obtained will be useful for guiding the development of phase-change materials capable of optimizing the rectification of conductive heat fluxes. [ABSTRACT FROM AUTHOR]

Understanding thermal transport in nanofiber networks is essential for their applications in thermal management, which are used extensively as mechanically sturdy thermal insulation or high thermal conductivity materials. In this study, using the statistical theory and Fourier's law of heat conduction while accounting for both the inter-fiber contact thermal resistance and the intrinsic thermal resistance of nanofibers, an analytical model is developed to predict the thermal conductivity of nanofiber networks as a function of their geometric and thermal properties. A scaling relation between the thermal conductivity and the geometric properties including volume fraction and nanofiber length of the network is revealed. This model agrees well with both numerical simulations and experimental measurements found in the literature. This model may prove useful in analyzing the experimental results and designing nanofiber networks for both high and low thermal conductivity applications. [ABSTRACT FROM AUTHOR]

Superlattices and nano phononic crystals have attracted significant attention due to their low thermal conductivities and their potential application as thermoelectric materials. A widely used expression to calculate thermal conductivity, presented by Klemens and expressed in terms of the relaxation time by Callaway and Holland, originates from the Boltzmann transport equation. In its most general form, this expression involves a direct summation of the heat current contributions from individual phonons of all wavevectors and polarizations in the first Brillouin zone. In common practice, the expression is simplified by making an isotropic assumption that converts the summation over wavevector to an integral over wavevector magnitude. The isotropic expression has been applied to superlattices and phononic crystals, but its validity for different supercell sizes has not been studied. In this work, the isotropic and direct summation methods are used to calculate the thermal conductivities of bulk Si, and Si/Ge quantum dot superlattices. The results show that the differences between the two methods increase substantially with the supercell size. These differences arise because the vibrational modes neglected in the isotropic assumption provide an increasingly important contribution to the thermal conductivity for larger supercells. To avoid the significant errors that can result from the isotropic assumption, direct summation is recommended for thermal conductivity calculations in superstructures. [ABSTRACT FROM AUTHOR]

The thermal conductivity of three single crystal samples of n-type gallium nitride with electron densities of 4.0 × 1016, 2.6 × 1018, and 1.1 × 1020 cm-3 has been determined in the temperature range 4-320K. The measurements were carried out within the ab plane using the stationary method. The thermal conductivity depends strongly on the donor concentration. The analysis within the Callaway approach and the Debye model shows a significant influence of phonon-electron scattering on the thermal conductivity of the samples. In addition, some preliminary results obtained along the c axes of GaN layered samples are presented. The latter measurements have been carried out using the 3ω method. [ABSTRACT FROM AUTHOR]

The thermal conductivity, electrical conductivity, and Seebeck coefficient of a vaporgrown CdTe and two melt-grown Cd0.8Zn0.2Te crystals, including In-doped CZT-30 and un-doped CZT-34 samples, were measured from room temperature to 780#176;C. The measured thermal conductivity of CdTe was higher than the two CdZnTe samples in the low temperature range and the three sets of data merged together at temperature above 750#176;C. The measured electrical conductivities of CdTe and CZT-30 were close to be intrinsic from 190#176;C to 780#176;C. The CZT-34 sample showed higher electrical conductivity value at 290#176;C and merged with the data of CZT-30 at temperatures above 650#176;C. The measured Seebeck coefficients for all three samples showed high positive values, between 0.9 and 1.1mV/K, in the intermediate temperature range between 350 and 550#176;C and, as temperature increased, decreased slowly and converted to n-type. The Figure of Merit for the thermoelectric application, zT, calculated for the three samples were orders of magnitude lower than the state-of-the-art p-type thermoelectric materials mainly due to the low values of the electrical conductivity. A rough estimate was made on the hole concentration needed to improve the value of zT for CdTe to 1.0 at 500#176;C. A simple interpolation and extrapolation of the present data gives the value of zT for the n-type intrinsic CdTe to be 1.2±0.4 at 1050#176;C. [ABSTRACT FROM AUTHOR]

AgSbTe2 is a thermoelectric semiconductor with an intrinsically low thermal conductivity and a valence band structure that is favorable to obtaining a high thermoelectric figure of merit zT. It also has a very small energy gap Eg ~ 7.6 ± 3 meV. As this gap is less than the thermal excitation energy at room temperature, near-intrinsic AgSbTe2 is a two carrier system having both holes (concentration p) and electrons (n). Good thermoelectric performance requires heavy p-type doping (p >> n). This can be achieved with native defects or with extrinsic doping, e.g. with transition metal element. The use of defect doping is complicated by the fact that many of the ternary Ag-Sb-Te and pseudo-binary Sb2Te3-Ag2Te phase diagrams are contradictory. This paper determines the compositional region most favorable to creating a single phase material. Through a combination of intrinsic and extrinsic doping, values of zT > 1 are achieved, though not on single-phased material. Additionally, we show that thermal conductivity is not affected by defects, further demonstrating that the low lattice thermal conductivity of I-V-VI2 materials is due to an intrinsic mechanism, insensitive to changes in defect structure. [ABSTRACT FROM AUTHOR]

Since the thermal conduction equation has form invariance under coordinate transformation, one can design thermal metamaterials with novel functions by tailoring materials' thermal conductivities. In this work, we establish a different transformation theory, and propose a layered device with anisotropic thermal conductivities. The device is able to convert heat flux from parallel patterns into non-parallel patterns and vice versa. In the mean time, the heat flux pattern outside the device keeps undisturbed as if this device is absent. We perform finite-element simulations to confirm the converting behavior. This work paves a different way to manipulate the flow of heat at will. [ABSTRACT FROM AUTHOR]

Thermal transport in suspended graphene samples has been measured in prior works and this work with the use of a suspended electro-thermal micro-bridge method. These measurement results are analyzed here to evaluate and eliminate the errors caused by the extrinsic thermal contact resistance. It is noted that the roomtemperature thermal resistance measured in a recent work increases linearly with the suspended length of the single-layer graphene samples synthesized by chemical vapor deposition (CVD), and that such a feature does not reveal the failure of Fourier's law despite the increase in the reported apparent thermal conductivity with length. The re-analyzed apparent thermal conductivity of a single-layer CVD graphene sample reaches about 1680 ± 180 W m-1 K-1 at room temperature, which is close to the highest value reported for highly oriented pyrolytic graphite. In comparison, the apparent thermal conductivity values measured for two suspended exfoliated bi-layer graphene samples are about 880 ± 60 and 730 ± 60 Wm-1 K-1 at room temperature, and approach that of the natural graphite source above room temperature. However, the low-temperature thermal conductivities of these suspended graphene samples are still considerably lower than the graphite values, with the peak thermal conductivities shifted to much higher temperatures. Analysis of the thermal conductivity data reveals that the low temperature behavior is dominated by phonon scattering by polymer residue instead of by the lateral boundary. [ABSTRACT FROM AUTHOR]

Probing length-dependent thermal conductivity of a given material has been considered as an important experimental method to determine the length of ballistic thermal conduction, or equivalently, the averaged phonon mean free path (l). However, many previous thermal transport measurements have focused on varying the lateral dimensions of samples, rendering the experimental interpretation indirect. Moreover, deducing l is model-dependent in many optical measurement techniques. In addition, finite contact thermal resistances and variations of sample qualities are very likely to obscure the effect in practice, leading to an overestimation of l. We point out that directly investigating one-dimensional length-dependent (normalized) thermal resistance is a better experimental method to determine l. In this regard, we find that no clear experimental data strongly support ballistic thermal conduction of Si or Ge at room temperature. On the other hand, data of both homogeneously-alloyed SiGe nanowires and heterogeneously-interfaced Si-Ge core-shell nanowires provide undisputed evidence for ballistic thermal conduction over several micrometers at room temperature. [ABSTRACT FROM AUTHOR]

Thermal phenomena in many biological systems offer an alternative detection opportunity for quantifying relevant sample properties. While there is substantial prior work on thermal characterization methods for fluids, the push in the biology and biomedical research communities towards analysis of reduced sample volumes drives a need to extend and scale these techniques to these volumes of interest, which can be below 100 pl. This work applies the 3ω technique to measure the temperature-dependent thermal conductivity and heat capacity of de-ionized water, silicone oil, and salt buffer solution droplets from 24 to 80 °C. Heater geometries range in length from 200 to 700 µm and in width from 2 to 5 µm to accommodate the size restrictions imposed by small volume droplets. We use these devices to measure droplet volumes of 2 µl and demonstrate the potential to extend this technique down to pl droplet volumes based on an analysis of the thermally probed volume. Sensitivity and uncertainty analyses provide guidance for relevant design variables for characterizing properties of interest by investigating the tradeoffs between measurement frequency regime, device geometry, and substrate material. Experimental results show that we can extract thermal conductivity and heat capacity with these sample volumes to within less than 1% of thermal properties reported in the literature. [ABSTRACT FROM AUTHOR]

Molecular dynamics simulations are performed to evaluate the effect of van der Waals forces among single-wall carbon nanotubes (SWNTs) on the interfacial thermal conductance between a SWNT array and silicon substrate. First, samples of SWNTs vertically aligned on silicon substrate are simulated, where both the number and arrangement of SWNTs are varied. Results reveal that the interfacial thermal conductance of a SWNT array/Si with van der Waals forces present is higher than when they are absent. To better understand how van der Waals forces affect heat transfer through the interface between SWNTs and silicon, further constructs of one SWNT surrounded by different numbers of other ones are studied, and the results show that the interfacial thermal conductance of the central SWNT increases with increasing van der Waals forces. Through analysis of the covalent bonds and vibrational density of states at the interface, we find that heat transfer across the interface is enhanced with a greater number of chemical bonds and that improved vibrational coupling of the two sides of the interface results in higher interfacial thermal conductance. Van der Waals forces stimulate heat transfer at the interface. [ABSTRACT FROM AUTHOR]

We discuss the textbook presentation of the concept of umklapp vs normal phonon-phonon scattering processes in the context of lattice thermal conductivity. A simplistic picture, in which the "momentum conservation" in a normal process leads to the conservation of the heat flux, is only valid within the single-velocity Debye model of phonon dispersion. Outside this model, the simple "momentum conservation" argument is demonstrably inaccurate and leads to conceptual confusion. Whether or not an individual scattering event changes the direction of the energy flow is determined by the phonon group velocity, which, unlike the quasimomentum, is a uniquely defined quantity independent of the choice of the primitive cell in reciprocal space. Furthermore, the statement that normal processes do not lead to a finite thermal conductivity when umklapp processes are absent is a statistical statement that applies to a phonon distribution rather than to individual scattering events. It is also important to understand that once umklapp processes are present, both normal and umklapp processes contribute to thermal resistance. A nuanced explanation of the subject would help avoid confusion of the student and establish a connection with cutting edge research. [ABSTRACT FROM AUTHOR]

We study the effect of grain size on thermal conductivity of thin film barium titanate over temperatures ranging from 200 to 500 K. We show that the thermal conductivity of Barium Titanate (BaTiO3) decreases with decreasing grain size as a result of increased phonon scattering from grain boundaries. We analyze our results with a model for thermal conductivity that incorporates a spectrum of mean free paths in BaTiO3. In contrast to the common gray mean free path assumption, our findings suggest that the thermal conductivity of complex oxide perovskites is driven by a spectrum of phonons with varying mean free paths. [ABSTRACT FROM AUTHOR]

Recently, a pump beam size dependence of thermal conductivity was observed in Si at cryogenic temperatures using time-domain thermal reflectance (TDTR). These observations were attributed to quasiballistic phonon transport, but the interpretation of the measurements has been semiempirical. Here, we present a numerical study of the heat conduction that occurs in the full 3D geometry of a TDTR experiment, including an interface, using the Boltzmann transport equation. We identify the radial suppression function that describes the suppression in heat flux, compared to Fourier's law, that occurs due to quasiballistic transport and demonstrate good agreement with experimental data. We also discuss unresolved discrepancies that are important topics for future study. [ABSTRACT FROM AUTHOR]

We demonstrate the impact on thermal conductivity of varying the concentration of oxygen vacancies and reduced cations in Pr0.1Ce0.9O2-δ thin films prepared by pulsed laser deposition. The oxygen vacancy concentration is controlled by varying the oxygen partial pressure between 1×10-4 and 1 atm at 650 °C. Corresponding changes in the oxygen non-stoichiometry (δ) are monitored by detecting the lattice parameters of the films with high-resolution X-ray diffraction, while the thermal properties are characterized by time-domain thermoreflectance measurements. The films are shown to exhibit a variation in oxygen vacancy content, and in the Pr3+/Pr4+ ratio, corresponding to changes in δ from 0.0027 to 0.0364, leading to a reduction in the thermal conductivity from k=6.62±0.61 to 3.82±0.51 W/m-K, respectively. These values agree well with those predicted by the Callaway and von Baeyer model for thermal conductivity in the presence of point imperfections. These results demonstrate the capability of controlling thermal conductivity via control of anion and cation defect concentrations in a given reducible oxide. [ABSTRACT FROM AUTHOR]

Ab initio predictions indicating that alloying RuO2 with La, Eu, or Lu can increase the Seebeck coefficient α manifold due to quantum confinement effects are validated in sputter-deposited La-alloyed RuO2 films showing fourfold α increase. Combinatorial screening reveals that α enhancement correlates with La-induced lattice distortion, which also decreases the thermal conductivity twentyfold, conducive for high thermoelectric figures of merit. These insights should facilitate the rational design of high efficiency oxide-based thermoelectrics through quantum-guided alloying. [ABSTRACT FROM AUTHOR]

We theoretically predict some thermal properties versus temperature dependence of one dimensional (1D) material nanowire systems. A known method is used to provide an efficient and reliable analytical procedure for wide temperature range. Predicted formulas are expressed in terms of Bloch-Grüneisen functions and Debye functions. Computing results has proved that the expressions are in excellent agreement with the results reported in the literature even if it is in very low dimension limits of nanowire systems. Therefore the calculation method is a fully predictive approach to calculate thermal conductivity and heat capacities of nanowire material systems. [ABSTRACT FROM AUTHOR]

Using ab initio calculations, we have investigated the phonon linewidths and the thermal conductivity (κ) of monolayer MoS2. κ for a typical sample size of 1 μm is 83 W/m K at room temperature in the completely rough edge limit, suggesting κ is not a limiting factor for the electronic application of monolayer MoS2. κ can be further increased by 30% in 10 μm sized samples. Due to strong anharmonicity, isotope enhancement of room temperature κ is only 10% for 1 μm sized samples. However, linewidths can be significantly reduced, for instance, for Raman active modes A1g and E2g1, in isotopically pure samples. [ABSTRACT FROM AUTHOR]

Most previous analytical theories for microscale heat transfer are limited to low temperatures (≤100 K). We present simple yet general analytical formulae which reveal the essential features of the thermal properties of two-dimensional nanomaterials in a wide range of temperatures by full coverage of guided wave and bulk wave modes. In particular, we are able to handle the intermediate ballistic-diffusive regime. As an illustration, the formulae are applied on graphene to obtain its specific heat, thermal conductance, and thermal conductivity. The predictions are remarkably consistent with existing theories and experiments. [ABSTRACT FROM AUTHOR]

A thermal property microscopy technique based on frequency domain thermoreflectance (FDTR) is presented. In FDTR, a periodically modulated laser locally heats a sample while a second probe beam monitors the surface reflectivity, which is related to the thermal properties of the sample with an analytical model. Here, we extend FDTR into an imaging technique capable of producing micrometer-scale maps of several thermophysical properties simultaneously. Thermal phase images are recorded at multiple frequencies chosen for maximum sensitivity to thermal properties of interest according to a thermal model of the sample. The phase versus frequency curves are then fit point-by-point to obtain quantitative thermal property images of various combinations of thermal properties in multilayer samples, including the in-plane and cross-plane thermal conductivities, heat capacity, thermal interface conductance, and film thickness. An FDTR microscope based on two continuous-wave lasers is described, and a sensitivity analysis of the technique to different thermal properties is carried out. As a demonstration, we image ∼3 nm of patterned titanium under 100 nm of gold on a silicon substrate, and simultaneously create maps of the thermal interface conductance and substrate thermal conductivity. Results confirm the potential of our technique for imaging and quantifying thermal properties of buried layers, indicating its utility for mapping thermal properties in integrated circuits. [ABSTRACT FROM AUTHOR]

Monte Carlo simulations were employed to investigate the effective thermal conductivity (Keff) of multi-walled carbon nanotube-epoxy (MWNT-epoxy) nanocomposites with and without coating the MWNTs with silica. The numerical approach was validated with experimental data and values of the Kapitza resistance for the silica-coated MWNT-epoxy composite were calculated for realistic configurations of the MWNTs. While the Kapitza resistance was found to be 40% smaller than for the case of pristine MWNTs, it was also observed that the effect of persistence length of the MWNT on Keff is as important as the effect of the Kapitza resistance. [ABSTRACT FROM AUTHOR]

Thin diamond films of thickness near 1 μm can have highly nonuniform thermal conductivities owing to spatially varying disorder associated with nucleation and grain coalescence. Here, we examine the nonuniformity for nanocrystalline chemical vapor deposited diamond films of thickness 0.5, 1.0, and 5.6 μm using picosecond thermoreflectance from both the top and bottom diamond surfaces, enabled by etching a window in the silicon substrate. The extracted local thermal conductivities vary from less than 100 W m-1 K-1 to more than 1300 W m-1 K-1 and suggest that the most defective material is confined to within 1 μm of the growth surface. [ABSTRACT FROM AUTHOR]

We report the thermal properties of carbon nanowall layers produced by expanding beam radio-frequency plasma. The thermal properties of carbon nanowalls, grown at 600 °C on aluminium nitride thin-film sputtered on fused silica, were measured with a pulsed photo-thermal technique. The apparent thermal conductivity of the carbon at room temperature was found to increase from 20 to 80 Wm-1 K-1 while the thickness varied from 700 to 4300 nm, respectively. The intrinsic thermal conductivity of the carbon nanowalls attained 300 Wm-1 K-1 while the boundary thermal resistance with the aluminium nitride was 3.6 × 10-8 Km2 W-1. These results identify carbon nanowalls as promising material for thermal management applications. [ABSTRACT FROM AUTHOR]

The authors report on synthesis and thermal properties of the electrically conductive thermal interface materials with the hybrid graphene-metal particle fillers. The thermal conductivity of resulting composites was increased by ∼500% in a temperature range from 300 K to 400 K at a small graphene loading fraction of 5-vol.-%. The unusually strong enhancement of thermal properties was attributed to the high intrinsic thermal conductivity of graphene, strong graphene coupling to matrix materials, and the large range of the length-scale-from nanometers to micrometers-of the graphene and silver particle fillers. The obtained results are important for the thermal management of advanced electronics and optoelectronics. [ABSTRACT FROM AUTHOR]

Thermal characterization of materials, especially civil engineering materials, in the way of non-destructive methods, are more and more widespread. In this article, we show an original point of view to describe the used method, the thermal waves, to obtain the thermal impedance of the studied system, using a specific sensor - a fluxmeter. The identification technique, based on a frequential approach, is optimized by applying a random input to the system. This kind of random heating is shown to provide a frequency range where the thermal effusivity is able to be identified and not correlated to another parameter. The strength of the method is also the determination of the contact resistance of the system, that allows to validate the identification process. Experimental results obtained from a sample with well-known thermal properties (polyvinyl chloride) are used to validate the proposed method. [ABSTRACT FROM AUTHOR]

This work presents a method to measure the thermal conductivity and heat capacity of electrically conducting small-volume liquid samples using the 3ω technique. A mathematical model of heat transfer is derived to determine the thermal properties from the 3ω signal considering the device geometry. In order to validate the model, an experimental apparatus has been designed and set up to measure the thermal properties (thermal conductivity and heat capacity) of seven different liquid samples. The results show good agreement with other literature values, demonstrating that the suggested method is effective for measuring the thermal properties of electrically conducting liquids. More importantly, the result with a sample volume of 1 μl demonstrates the resolution of the thermal conductivity as precise as 0.01% which corresponds to a thermal-conductivity change of 10-4 W/m K in the case of water-based solutions. [ABSTRACT FROM AUTHOR]

The 3-omega method is widely used to measure thermal properties of thin films and interfaces. Generally, one-dimensional heat conduction across the film is assumed and the film capacitance is neglected. The change in the in-phase (real part) temperature response for the film-on-substrate case relative to the substrate-only case is, therefore, attributed to the sum of the bulk thermal resistance of the film and the thermal boundary resistance between the film and the substrate. Based on a rigorous and intuitive mathematical derivation, it is shown that this approach represents a limiting case, and that its use can cause significant errors in rather realistic situations when the underlying assumptions are not met. This article quantifies the error by introducing a new parameter called the ratio function R, which modifies the film thermal resistance and mathematically shows that it depends only on three dimensionless parameters that combine thermal properties and geometries of the film and the heated linewidth. A new data reduction scheme is suggested accordingly to determine the film thermal conductivity (cross-plane), anisotropic thermal conductivity ratio between the in-plane direction and the cross-plane direction, and the interface thermal conductance. [ABSTRACT FROM AUTHOR]

A novel calorimeter, termed as the Peltier tip calorimeter, is developed. The calorimeter consists of a single thermocouple junction, called the Peltier tip, which is used as a heater and a sensor simultaneously. We demonstrate that the Peltier tip calorimeter is capable of measuring the heat capacity of a small solid sample with submilligrams of mass and also the thermophysical properties of a liquid such as heat capacity and thermal conductivity. It is also proposed that a Peltier tip be used as a local calorimeter for scanning thermal microscopy. [ABSTRACT FROM AUTHOR]

Although various useful techniques exist for nondestructively measuring homogeneous thermal properties, a few useful type techniques for measuring inhomogeneous thermal properties are reported such that there are many structures, materials, substances, and living things whose thermal properties cannot be measured. The temperature distribution of a target, however, can be nondestructively measured with a very high accuracy using infrared, pyroelectric, ultrasound, or magnetic resonance sensors. In this report, we describe an inverse problem technique for determining in situ the thermal conductivity distribution of a target having an arbitrary geometry using only internal steady temperature distribution measurements. The target distribution is directly determined by linearly solving heat transfer equations as the first-order partial differential equations in which temperature distributions and reference thermal conductivities of the region of interest are used as inhomogeneous coefficients. Under proper configurations of reference regions and thermal sources/sinks, this technique enables the determination of the conductivity distribution without disturbing the temperature distribution. A stable numerical solution that considers measurement noise and improper configurations of reference regions and thermal sources/sinks is also described. Technique feasibility is confirmed using two-dimensional problems through simulations and experiments using conventional infrared thermography. [ABSTRACT FROM AUTHOR]

Thermal diffusivity and specific heat can be estimated from thermal conductivity measurements made using a standard needle probe and a suitably high data acquisition rate. Thermal properties are calculated from the measured temperature change in a sample subjected to heating by a needle probe. Accurate thermal conductivity measurements are obtained from a linear fit to many tens or hundreds of temperature change data points. In contrast, thermal diffusivity calculations require a nonlinear fit to the measured temperature change occurring in the first few tenths of a second of the measurement, resulting in a lower accuracy than that obtained for thermal conductivity. Specific heat is calculated from the ratio of thermal conductivity to diffusivity, and thus can have an uncertainty no better than that of the diffusivity estimate. Our thermal conductivity measurements of ice Ih and of tetrahydrofuran (THF) hydrate, made using a 1.6 mm outer diameter needle probe and a data acquisition rate of 18.2 points/s, agree with published results. Our thermal diffusivity and specific heat results reproduce published results within 25% for ice Ih and 3% for THF hydrate. [ABSTRACT FROM AUTHOR]

3ω methods are commonly used to measure the thermal conductivity of a substrate adjacent to a strip heater or the thermal conductivity and specific heat of a suspended wire. Here we consider the general case of a line heater that is also used to sense temperature. Analysis of all harmonics is presented in terms of generic thermal and electrical transfer functions and is readily adapted to other experimental configurations. We identify voltage signals at 2ω and 1ω that contain the same information about the thermal properties as the 3ω signal. The 2ω voltage requires a dc offset at the current source. The 1ω voltage requires a very stable current source, but eliminates the need for higher-harmonic detection, and is advantageous for studying the dynamics of systems with very fast thermal response times. The 1ω, 2ω, and 3ω methods compare favorably with experiments using a suspended platinum wire and a line heater on a Pyrex substrate. With a modern lock-in amplifier, no common-mode voltage subtraction is necessary, which simplifies the experiment compared to the common practice of balancing a bridge or using a multiplying digital-to-analog converter. We also show that the widespread practice of using a voltage source to approximate a current source is only valid when the sample resistance is small compared to the total electrical resistance of the circuit, and derive and experimentally verify a correction factor to be used otherwise. [ABSTRACT FROM AUTHOR]

Transient hot probe methods are widely used for the measurement of thermal properties in semi-infinite media. In this article, the three-dimensional temperature impulse response of a rectangular probe is analyzed using an analytical solution based on the Green’s functions, such that two characteristic times relative to the asymptotic behaviors of the probe can be calculated. A strategy for the estimation of two uncorrelated properties is deduced from this approach. The identification technique, based on a fractional model approach, is optimized by applying a random input heat flux to the probe. This kind of random heating is shown to provide a frequency content in the range where both the thermal conductivity and thermal effusivity are uncorrelated. Some experimental results obtained with samples with well-known thermal properties are used to validate the proposed method. [ABSTRACT FROM AUTHOR]

The thermal conductivity of individual ZnTe nanowires (NWs) was measured using a suspended micro-bridge device with built-in resistance thermometers. A collection of NWs with different diameters were measured, and strong size-dependent thermal conductivity was observed in these NWs. Compared to bulk ZnTe, NWs with diameters of 280 and 107 nm showed approximately three and ten times reduction in thermal conductivity, respectively. Such a reduction can be attributed to phonon-surface scattering. The contact thermal resistance and the intrinsic thermal conductivities of the nanowires were obtained through a combination of experiments and molecular dynamic simulations. The obtained thermal conductivities agree well with theoretical predictions. [ABSTRACT FROM AUTHOR]

The influence of guest atoms on thermal conduction in filled skutterudites was studied using ultrafast reflectance spectroscopy. Different filling species cause coupled vibrational modes between the guest atoms and the host lattice at different frequencies, which scatter phonons in different spectral spans. Using a Debye model for the measured lattice thermal conductivity together with the measured vibration frequencies and scattering rates, it is shown that scattering due to the coupled vibrational modes has a considerable contribution to the suppression of lattice thermal conduction. This demonstrates that filling with multiple species can efficiently reduce the lattice thermal conductivity in skutterudites. [ABSTRACT FROM AUTHOR]

In the light of recently reported pressure-volume-temperature relationships in diamond, we show that the self-diffusion coefficient of diamond as a function of temperature and pressure can be satisfactory reproduced in terms of the bulk elastic and expansivity data by means of a thermodynamical model that interconnects the parameters of point defects to the bulk properties. Our calculated self-diffusion coefficients are in good agreement with the experimental ones when the uncertainties are taken into account. [ABSTRACT FROM AUTHOR]

Tantalum Nitride (TaN) films carry high heat fluxes in a variety of applications including diffusion barriers in magnetoresistive random access memory and buffer/absorbers in extreme ultraviolet masks. The thicknesses of these films are usually of the same order as the thermal energy carrier mean free path, which complicates the study of heat conduction. This paper presents thermal (cross-plane) and electrical (in-plane) conductivity measurements on TaN films with thicknesses of 50, 75, and 100 nm. Picosecond thermoreflectance is used to extract the thermal boundary resistance between TaN and Al and the intrinsic thermal conductivity of TaN for temperatures of 300-700 K. The data and the relative importance of boundary resistances, electron-boundary scattering, and electron-defect scattering are interpreted using the electrical and thermal transport data. These data facilitate comparison of the phonon and electron contributions to thermal conduction in TaN. [ABSTRACT FROM AUTHOR]

We report experiments and models of self-heating in piezoresistive microcantilevers that show how cantilever measurement resolution depends on the thermal properties of the surrounding fluid. The predicted cantilever temperature rise from a finite difference model is compared with detailed temperature measurements on fabricated devices. Increasing the fluid thermal conductivity allows for lower temperature operation for a given power dissipation, leading to lower force and displacement noise. The force noise in air is 76% greater than in water for the same increase in piezoresistor temperature. [ABSTRACT FROM AUTHOR]

Without requiring regression for parameter determination, one-dimensional (1D) analytical models are used by many research groups to extract the thermal properties in frequency-domain thermoreflectance measurements. Experimentally, this approach involves heating the sample with a pump laser and probing the temperature response with spatially coincident probe laser. Micron order lateral resolution can be obtained by tightly focusing the pump and probe lasers. However, small laser beam spot sizes necessarily bring into question the assumptions associated with 1D analytical models. In this study, we analyzed the applicability of 1D analytical models by comparing to 2D analytical and fully numerical models. Specifically, we considered a generic n-layer two-dimensional (2D), axisymmetric analytical model including effects of volumetric heat absorption, contact resistance, and anisotropic properties. In addition, a finite element numerical model was employed to consider nonlinear effects caused by temperature dependent thermal conductivity. Nonlinearity is of germane importance to frequency domain approaches because the experimental geometry is such that the probe is always sensing the maximum temperature fluctuation. To quantify the applicability of the 1D model, parametric studies were performed considering the effects of: film thickness, heating laser size, probe laser size, substrate-to-film effusivity ratio, interfacial thermal resistance between layers, volumetric heating, substrate thermal conductivity, nonlinear boundary conditions, and anisotropic and temperature dependent thermal conductivity. [ABSTRACT FROM AUTHOR]

Thermal properties in multiwall carbon nanotubes carpets and micro-devices are investigated using a nanosecond photothermal method. Gradually, the identification model and experimental protocol are performed to increase the method accuracy for the thermal conductivity determination. In the experimental protocol, a nanosecond UV monopulse laser beam is used to heat the surface of a multilayer (600 nm of Ti/20 μm of carbon nanotube carpet) sample. In the 1D identification model with two layers and a thermal contact resistance, the effect of the laser excitation temporal shape is taken into account. In this study, this first approach allows to improve the accuracy of apparent thermal conductivity measurements of multiwall carbon nanotubes carpet. The carbon nanotubes carpet apparent thermal conductivity value went from being to 180 ± 5 W×m-1×K-1. In the second approach, two laser beams are coupled in order to increase the interaction time duration from 27 ns to 60 ns. It becomes possible to probe different depths in the carpet. The obtained value (180 W×m-1×K-1) confirms the pulsed photothermal method consistency for porous samples. Finally, assuming that the carbon nanotubes are parallel and without any defects, the equivalent intrinsic thermal conductivity of a single carbon nanotube is estimated to be around 3600 W×m-1×K-1. [ABSTRACT FROM AUTHOR]

The effects on thermal and electrical properties of adding small amounts of TiO2 nanoinclusions to bulk Ba8Ga16Ge30 clathrate have been investigated. The thermal properties were analysed using the transient plane source technique and the analysis showed a significant decrease in thermal conductivity as the volume fraction of TiO2 increased from 0 vol. % to 1.2 vol. %. The introduction of TiO2 nanoparticles caused a shift in the peak value of the Seebeck coefficient towards lower temperatures. The maximum value of the Seebeck coefficient was, however, only little affected. The introduction of TiO2 nanoparticles into the bulk Ba8Ga16Ge30 resulted in an increased electrical resistivity of the sample, thus simultaneously reducing the charge carrier contribution to the thermal conductivity, partly explaining the decrease in total thermal conductivity. Due to the large increase in resistivity of the samples, ZT was only somewhat improved for the material with 0.4 vol. % TiO2 while the ZT values of the other materials were lower than for the reference Ba8Ga16Ge30 material without TiO2 nanoparticles. The combined results are consistent with a scenario where the nanoparticle introduction causes a light doping of the semiconductor matrix and an increased concentration of phonon scattering centres. [ABSTRACT FROM AUTHOR]

Refrigerators for space and other applications working around 100 mK require lightweight components with good thermal properties. We have measured the thermal properties of high-purity (99.95%) magnesium, which is five times lighter than copper, over the 60–150 mK range and found that it is well-behaved down to these temperatures. Both conductivity and heat capacity are in good agreement with extrapolations from measurements at higher temperatures. The heat capacity per unit volume is about the same as copper and the thermal conductivity about 2.7 times lower than copper of similar residual resistivity ratio, as expected from magnesium’s higher room-temperature resistivity. [ABSTRACT FROM AUTHOR]