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At the atomic level, heat is viewed as energy for lattice vibrational waves, i.e., a mechanical wave. Correspondingly, the strain as atomic displacement can have a profound impact on the thermal transport. Despite numerous atomistic simulations, fewer experimental efforts can be found for strain-dependent thermal properties of individual nanostructures and thin films. In this work, suspended 2 μm-thick Si films were stretched to reveal the influence of the uniaxial tensile strain on in-plane thermal conductivity along the stretching direction. In a high vacuum, the room-temperature thermal conductivity of a 2 μm-thick Si film decreased from 135.5 ± 6.9 to 127.2 ± 6.5 W/m K under a ∼0.44% tensile strain. This thermal conductivity decrease followed the predicted trend for Si films. In addition, the heat transfer coefficient of representative thin films in the air was also measured to reveal the impact of the heat loss along the sample sidewall on previous in-air thermal measurements. [ABSTRACT FROM AUTHOR]

Localization is a well-known wave phenomenon that significantly impedes transport, as uncovered by a pioneering work of Anderson. The localization of thermal phonons based on the phonon wave nature is widely represented in disordered atomic systems. Compared with electron and photon localization, the observations of phonon localization are much rarer, owning to the broadband nature of phonon thermal transport. In this Perspective, we summarize the experimental and theoretical evidences of phonon Anderson localization in disordered atomic systems from the aspects of vibrational spectroscopy, thermal conductivity measurement, phonon transmission, phonon wave packet, phonon participation ratio, and energy distribution. [ABSTRACT FROM AUTHOR]

We show that the centroid molecular dynamics (CMD) method provides a realistic way to calculate the thermal diffusivity a = λ/ρcV of a quantum mechanical liquid such as para-hydrogen. Once a has been calculated, the thermal conductivity can be obtained from λ = ρcVa, where ρ is the density of the liquid and cV is the constant-volume heat capacity. The use of this formula requires an accurate quantum mechanical heat capacity cV, which can be obtained from a path integral molecular dynamics simulation. The thermal diffusivity can be calculated either from the decay of the equilibrium density fluctuations in the liquid or by using the Green–Kubo relation to calculate the CMD approximation to λ and then dividing this by the corresponding approximation to ρcV. We show that both approaches give the same results for liquid para-hydrogen and that these results are in good agreement with the experimental measurements of the thermal conductivity over a wide temperature range. In particular, they correctly predict a decrease in the thermal conductivity at low temperatures—an effect that stems from the decrease in the quantum mechanical heat capacity and has eluded previous para-hydrogen simulations. We also show that the method gives equally good agreement with the experimental measurements for the thermal conductivity of normal liquid helium. [ABSTRACT FROM AUTHOR]

The 3ω method is a well-established thermal technique used to measure the thermal conductivity of materials and the thermal resistance of interfaces. It has significant advantages over other steady state and transient thermal techniques in its ability to provide spatially resolved thermal property measurements over a wide range of thermal conductivity. Despite its advantages, it has been restricted to lab-scale use because of the difficulty involved in sample preparation and sensor fabrication and is limited to non-metallic substrates. High-throughput 3ω measurements with reusable sensors have not been realized yet. In this work, we demonstrate a method of applying reusable 3ω sensors fabricated on flexible polyimide films to measure bulk and spatially resolved thermal properties. We establish the limits of thermal conductivity measurement with the method to be 1 to 200 W/mK, and within the measurement limit, we verify the method by comparing the measured thermal conductivities of standard samples with established values. From the 3ω measurements, we also determine the thermal resistance of an interlayer of thermal grease as a function of pressure and compare it against the resistance calculated from direct thickness measurements to demonstrate the ability of this method to provide spatially resolved subsurface information. The technique presented is general and applicable to both metallic and non-metallic substrates, providing a method for high-throughput 3ω measurements with reusable sensors and without considerable sample preparation. [ABSTRACT FROM AUTHOR]

Photothermal deflection (PTD) has been frequently utilized to measure the thermal properties of thin solid films on a substrate. In the models commonly used to interpret PTD data, the substrate is assumed to be an ideal thermal insulator. This assumption poses important restrictions on the reliability of these thermal measurements and limits the possibility to use PTD for also measuring the specific heat of the samples. Simultaneous knowledge of specific heat and thermal diffusivity is necessary to determine the thermal conductivity of thin solid films. In this work, we calculated the phase and amplitude of the PTD signal at the two opposites sides (film-side and substrate-side) of a thin-film substrate system. We find that, on both sides, the phases of the PTD signal primarily depend on the thermal diffusivity of the thin film, while the amplitudes primarily depend on the specific heat. By using the phases and amplitudes at the two sides, we show that the accuracy of thermal conductivity measurements by PTD can be dramatically improved. We validate our theoretical model by measuring, in a scanning PTD apparatus, the thermal properties of gold thin films, which are in excellent agreement with, and improve on, existing data from the literature. [ABSTRACT FROM AUTHOR]

The thermal conductivity and thermal diffusivity of both metallic and non-metallic microwires are simultaneously measured by a cross-wire geometry. In this method, the heating wire serves both as a thermometer and a heater. The deflection of the heating wire is in situ modified by using the Ampère force to contact and separate the test wire. By using the quasi-steady-state measurement, the thermal contact resistances under different contact conditions are obtained so that the effect on thermal conductivity can be eliminated. This method is verified by both the metallic wires and carbon fiber to clarify the effect of the surface radiation heat loss of the test wire. The obtained thermal properties are repeatable though the magnitude of the thermal contact resistance under different contact conditions changes significantly. The cross-wire geometry overcomes the obstacle introduced by different thermal interfacial materials, which provides an accurate and convenient way to measure the thermal properties of microwires. [ABSTRACT FROM AUTHOR]

Scanning Thermal Microscopy (SThM) provides efficient thermal property measurement with micro- or nanoscale spatial resolution. However, the sensitivity and accuracy of state-of-the-art thermal probes have been limited by excessive thermal contact resistance between the probe and sample. Introduced herein is a robust thermal microprobe that can increase the probe-sample contact force by more than two orders of magnitude, thereby reducing the probe-sample thermal contact resistance by as much as 96% and increasing measurement sensitivity by more than 240% compared to a commercial thermal probe with the same dimensions and measurement principle. The relationship between the probe-sample thermal contact resistance, thermal exchange radius, and sample thermal conductivity is determined experimentally. Simultaneous measurement of thermal conductivity and Seebeck coefficient with unprecedented sensitivity is demonstrated using the enhanced scanning thermal microprobe on samples of an extended range of thermal conductivity up to 18 W/m K, increasing the range of samples applicable to SThM when compared to the conventional commercial probe with diminished measurement sensitivity above ∼10 W/m K. The probe is further demonstrated by simultaneously mapping thermal conductivity and Seebeck coefficient as a function of depth from the irradiated surface of an ion-irradiated bulk nanostructured thermoelectric material. In addition to enabling microscale thermal conductivity and Seebeck coefficient measurement of materials previously not applicable to SThM, the probe can also facilitate high-throughput characterization of combinatorial materials to aid the rapid discovery of compositions and processing conditions that yield highly desired thermal and thermoelectric properties. [ABSTRACT FROM AUTHOR]

Measuring the thermal conductivity of sub-surface buried substrates is of significant practical interests. However, this remains challenging with traditional pump–probe spectroscopies due to their limited thermal penetration depths. Here, we experimentally and numerically investigate the TPD of the recently developed optical pump–probe technique steady-state thermoreflectance (SSTR) and explore its capability for measuring the thermal properties of buried substrates. The conventional definition of the TPD (i.e., the depth at which temperature drops to 1/e value of the maximum surface temperature) does not truly represent the upper limit of how far beneath the surface SSTR can probe. For estimating the uncertainty of SSTR measurements of a buried substrate a priori, sensitivity calculations provide the best means. Thus, detailed sensitivity calculations are provided to guide future measurements. Due to the steady-state nature of SSTR, it can measure the thermal conductivity of buried substrates that are traditionally challenging by transient pump–probe techniques, exemplified by measuring three control samples. We also discuss the required criteria for SSTR to isolate the thermal properties of a buried film. Our study establishes SSTR as a suitable technique for thermal characterizations of sub-surface buried substrates in typical device geometries. [ABSTRACT FROM AUTHOR]

The development of high thermal conductivity thin film materials for the thermal management of electronics requires accurate and precise methods for characterizing heat spreading capability, namely, in-plane thermal conductivity. However, due to the complex nature of thin film thermal property measurements, resolving the in-plane thermal conductivity of high thermal conductivity anisotropic thin films with high accuracy is particularly challenging. Capable transient techniques exist; however, they usually measure thermal diffusivity and require heat capacity and density to deduce thermal conductivity. Here, we present an explicit uncertainty analysis framework for accurately resolving in-plane thermal conductivity via two independent steady-state thermometry techniques: particle-assisted Raman thermometry and electrical resistance thermometry. Additionally, we establish error-based criteria to determine the limiting experimental conditions that permit the simplifying assumption of one-dimensional thermal conduction to further reduce thermal analysis. We demonstrate the accuracy and precision (<5% uncertainty) of both steady-state techniques through in-plane thermal conductivity measurements of anisotropic nanocrystalline diamond thin films. [ABSTRACT FROM AUTHOR]

A method for calculating the effective thermal conductivity and diffusivity for two-layer structures in photoacoustic experiments is proposed. Unlike previous methods, the proposed procedure provides effective thermal parameters preserving their original physical meaning and without artificial resonances as a function of the modulation frequency of the incident laser beam. [ABSTRACT FROM AUTHOR]

A new steady-state apparatus is designed and constructed for the measurement of thermal conductivity (up to 25 W/mK) on a square specimen (300 mm side) with a heating temperature range from 30 °C to 900 °C. A vacuum container, of which the pressure can reach to 1 Pa, is also built for materials which can be easily oxidized. The structure of the facility is different from that of traditional steady-state devices, especially for the design of heating plate and heat sink. To verify the temperature uniformity of the heating plate, a simulation analysis is carried out in this paper. Besides, the heating system, the heat sink, the measuring system, and the vacuum system are presented in detail. In addition, the thermal conductivities of a heat insulation tile, 304L stainless steel, n-docosane, and erythritol are measured by this apparatus. Finally, an uncertainty analysis is discussed depending on different temperatures and materials. [ABSTRACT FROM AUTHOR]

Simplified heat-transfer models are widely employed by heated probe scanning thermal microscopy techniques for determining thermal conductivity of test samples. These parameters have generally been assumed to be independent of sample properties; however, there has been little investigation of this assumption in non-contact mode, and the impact calibration procedures have on sample thermal conductivity results has not been explored. However, there has been little investigation of the commonly used assumption that thermal exchange parameters are sample independent in non-contact mode, or of the impact calibration procedures have on sample thermal conductivity results. This article establishes conditions under which quantitative, localized, non-contact measurements using scanning thermal microscopy with heated microprobes may be most accurately performed. The work employs a three-dimensional finite element (3DFE) model validated using experimental results and no fitting parameters, to determine the dependence of a heated microprobe thermal resistance as a function of sample thermal conductivity at several values of probe-to-sample clearance. The two unknown thermal exchange parameters were determined by fitting the 3DFE simulated probe thermal resistance with the predictions of a simplified probe heat transfer model, for two samples with different thermal conductivities. This calibration procedure known in experiments as the intersection method was simulated for sample thermal conductivities in the range of 0.1-50 W m -1 K-1 and clearance values in the 260-1010 nm range. For a typical Wollaston wire microprobe geometry as simulated here, both the thermal exchange radius and thermal contact resistance were found to increase with the sample thermal conductivity in the low thermal conductivity range while they remained approximately constant for thermal conductivities >1 W m -1 K -1, with similar trends reported for all clearance values investigated. It is shown that versatile sets of calibration samples for the intersection method should employ either medium range (1 W m -1 K -1) and (2 W m -1 K -1) thermal conductivities, or wide range (0.5 W m -1 K-1) and (50 W m -1 K -1). The medium range yielded results within 1.5% –20.4% of the expected values of thermal conductivity for specimens with thermal conductivity within 0.1-10 W m -1 K-1, while the wide range yielded values within 0.5%-19.4% in the same range. [ABSTRACT FROM AUTHOR]

Germanane (GeH), a hydrogen-terminated layered germanium structure, has recently been synthesized. Here, we employed a four-probe thermal transport measurement method to obtain the basalplane thermal conductivity of thin exfoliated GeH flakes and correlated the measurement results with the crystal structure. The obtained thermal conductivity increases with increasing temperature, suggesting that extrinsic grain boundary and defect scattering dominate over intrinsic phononphonon scattering. Annealing a polycrystalline GeH sample at 195 °C caused it to become amorphous, reducing the room-temperature thermal conductivity from 0.53±0.09W m-1 K-1, which is close to the value calculated for 16 nm grain size, to 0.29±0.05W m-1 K-1, which approaches the calculated amorphous limit in the basal plane thermal conductivity. [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]

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 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]

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]

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]

Ga3PO7 crystal with large size and good optical quality has been grown by the top-seeded solution growth slow-cooling method from a Li2O–3MoO3 flux system. Thermal properties of the as-grown Ga3PO7 crystal have been carefully studied by measuring the anisotropic thermal expansion, specific heat, thermal diffusion, and thermal conductivity. The results show that Ga3PO7 crystal possesses low thermal expansion coefficients and good thermal conductivity. The calculated average thermal expansion coefficients along a and c axes are αa=1.66×10-6 and αc=3.53×10-6 K-1 from 293.15 to 773.15 K. The thermal conductivities along a and c axes are up to 6.34 and 6.18 W m-1 K-1 at the temperature of 303.15 K. [ABSTRACT FROM AUTHOR]

We have measured the temperature-dependent thermal conductivity κ(T) of boron–carbon–nitride (B–C–N) and boron nitride (BN) nanotube mats between room temperature and 20 K. For both materials, κ(T) increases with increasing temperature, with no sign of saturation. We employ an analysis method to estimate the intrinsic κ(T) of BN nanotubes converted from B–C–N nanotubes, and find that at room temperature κ(T) of a multiwalled BN nanotube can be comparable to that of a multiwalled carbon nanotube and may exceed it if made isotopically pure. At low temperature, the functional form of κ(T) reflects dimensional confinement. [ABSTRACT FROM AUTHOR]

We investigate the thermal conductivity of four types of deformed carbon nanotubes by using the nonequilibrium molecular dynamics method. It is reported that various deformations have different influences on the thermal properties of carbon nanotubes. For bending carbon nanotubes, the thermal conductivity is independent of the bending angle. However, the thermal conductivity increases lightly with xy-distortion and decreases rapidly with z-distortion. The thermal conductivity does not change with the screw ratio before the breaking of carbon nanotubes, but it decreases sharply after the critical screw ratio. [ABSTRACT FROM AUTHOR]