Local Thermal Conductivity Research Articles

Thermal conductivity and annealing effect on structure of lignin-based microscale carbon fibers

This work reports on systematic investigation of the structure and thermal conductivity of lignin-based carbon fibers (CF) at the microscale. The lignin-based CF is produced by melt-spinning pyrolytic lignin derived from red oak. The 0 K-limit phonon scattering mean free path uncovers a characteristic structure size of ∼1.2 nm, which agrees well with the crystallite size by X-ray scattering (0.9 and 1.3 nm) and the cluster size by Raman spectroscopy (2.31 nm). The thermal conductivity of as-prepared CFs is determined at ∼1.83 W/m·K at room temperature. The thermal reffusivity of CFs shows little change from room temperature down to 10 K, uncovering the existence of extensive defects and grain boundaries which dominate phonon scattering. The localized thermal conductivity of CFs is increased by more than ten-fold after being annealed at ∼2800 K, to a level of 24 W/m·K. Our microscale Raman scanning from less annealed to highly annealed regions shows one-fold increase of the cluster size: from 1.83 nm to 4 nm. This directly confirms structure improvement by annealing. The inverse of the thermal conductivity is found linearly proportional to the annealing temperature in the range of 1000–2800 K.

Self-Generated Magnetic Fields in the Stagnation Phase of Indirect-Drive Implosions on the National Ignition Facility.

Three-dimensional extended-magnetohydrodynamic simulations of the stagnation phase of inertial confinement fusion implosion experiments at the National Ignition Facility are presented, showing self-generated magnetic fields over 10^{4} T. Angular high mode-number perturbations develop large magnetic fields, but are localized to the cold, dense hot-spot surface, which is hard to magnetize. When low-mode perturbations are also present, the magnetic fields are injected into the hot core, reaching significant magnetizations, with peak local thermal conductivity reductions greater than 90%. However, Righi-Leduc heat transport effectively cools the hot spot and lowers the neutron spectra-inferred ion temperatures compared to the unmagnetized case. The Nernst effect qualitatively changes the results by demagnetizing the hot-spot core, while increasing magnetizations at the edge and near regions of large heat loss.

Thermal conductivity of free-standing CVD diamond films by growing on both nuclear and growth sides

Polycrystalline CVD diamond typically has inhomogeneous structure in cross section, with grain size monotonically increasing from bottom (nucleation) side to top (growth) side, this resulting in a significant difference in local thermal conductivity in the layers adjacent to the two surfaces. The polycrystalline diamond films with both sides of similar structure with coarse-grains are of interest for thermal management applications. Here, we produced a double-side coarse-grain polycrystalline diamond film by microwave plasma assisted chemical vapor deposition (MPCVD), by a repeated diamond growth on nucleation side of a primary free-standing film. The grain size, texture, morphology and phase purity of the samples were characterized with SEM, XRD and Raman spectroscopy, while the thermal conductivity (TC) perpendicularly to the film plane was measured by a laser flash technique in the temperature range of 250–400K. The thermal conductivity of a single layer and bi-layered films are found to be almost identical, while the local TC values for latter on both sides are expected to become similar. A simple model discretizing the film in two regions, “poor” and “good” diamond layers, is considered to explain the observed TC temperature dependences.

Influence of container material on the heat transfer characteristics of nanofluids

ABSTRACTThe average and local thermal conductivity measurements of water-based Ag-nanofluid held in polypropylene and metallic containers using transient hot-wire method revealed a new phenomenon. The local thermal conductivity of water-based Ag nanofluid measured at different locations of containers was found to depend strongly upon the metallic container, but not on the polypropylene container. Similar observations have been found in water-based NiAl nanofluid, but not in water-based Al2O3 nanofluid. In contrast, this phenomenon was not observed for ethylene glycol-based Ag nanofluid, possibly due to the insignificant charge on the container wall, which partly explains the diversity in thermal conductivity by different researchers.

Graphics cards based topography artefacts simulations in Scanning Thermal Microscopy

We present an approach for simulation of topography related artefacts in local thermal conductivity measurements using Scanning Thermal Microscopy (SThM). Due to variations of the local probe-sample geometry while the SThM probe is scanning across the surface the probe-sample thermal resistance changes significantly which leads to distortions in the measured data. This effect causes large uncertainty in the local thermal conductivity measurements and belongs between most critical issues in the SThM when we want to make the technique quantitative. For a known probe and sample geometry the topography artefacts can be computed by solving the heat transfer in the SThM for different probe positions across the surface, which is however very slow and limited to single profiles only, if we use standard tools (like commercially available Finite Element Method solvers). Our approach is based on an assumption of diffusive heat transfer between the probe and the sample surface (and within them) and on the use of a Finite Difference solver that is optimized for the needs of a simulated SThM images computing. Using a graphics card we can achieve computation speed that is sufficient for a virtual SThM image generation on the order of few hours, which is already sufficient for practical use. We can therefore use the measured sample topography and convert it to a virtual SThM image – which can be then e.g. compared to real measurement or used for artefacts compensation. The possibility of performing fast simulations of topography artefacts is also useful when uncertainties of the SThM measurements are evaluated.

Anderson Localization of Thermal Phonons Leads to a Thermal Conductivity Maximum.

Our elastic model of ErAs disordered GaAs/AlAs superlattices exhibits a local thermal conductivity maximum as a function of length due to exponentially suppressed Anderson-localized phonons. By analyzing the sample-to-sample fluctuations in the dimensionless conductance, g, the transition from diffusive to localized transport is identified as the crossover from the multichannel to single-channel transport regime g ≈ 1. Single parameter scaling is shown to hold in this crossover regime through the universality of the probability distribution of g that is independent of system size and disorder strength.

Relation of vibrational excitations and thermal conductivity to elastic heterogeneities in disordered solids

In crystals, molecules thermally vibrate around the periodic lattice sites. Vibrational motions are well understood in terms of phonons, which carry heat and control heat transport. The situation is notably different in disordered solids, where vibrational excitations are not phonons and can be even localized. Recent numerical work has established the concept of elastic heterogeneity: disordered solids show inhomogeneous local mechanical response. Clearly, the heterogeneous nature of elastic properties strongly influences vibrational and thermal properties, and it is expected to be the origin of anomalous features, including boson peak, vibrational localization, and temperature dependence of thermal conductivity. These are all crucial long-standing problems in materials physics, which we address in the present work. We have considered a toy model able to stabilize different states of matter, by introducing an increasing amount of size disorder. The phase diagram generated by molecular dynamics simulations encompasses the perfect crystalline state with a spatially homogeneous elastic moduli distribution, multiple defective phases with increasing moduli heterogeneities, and eventually a series of amorphous states. We have established clear correlations among the heterogeneous local mechanical response, vibrational states, and thermal conductivity. We provide evidence that elastic heterogeneity controls both vibrational and thermal properties, and is a key concept to understand the anomalous puzzling features of disordered solids.

Interannual perturbations of the Martian surface heat flow by atmospheric dust opacity variations

AbstractThe InSight (Interior Exploration using Seismic Investigations, Geodesy and Heat Transport) mission will perform the first Martian in situ heat flow measurement by deploying the Heat Flow and Physical Properties Package (HP3) onto the Martian surface. In order to estimate the heat flow coming from the planetary interior, HP3 will measure the local subsurface thermal gradient as well as the local thermal conductivity to a depth of up to 5 m. From these measurements, local heat flow can be determined, but this will in general differ from the heat flow emanating from the planetary interior due to atmosphere‐induced perturbations. Here we quantify heat flow perturbation induced by dust loading of the Martian atmosphere using dust opacity data obtained by the Mars Exploration Rover Opportunity. Dust opacity data span the time period between Mars year (MY) 27 and MY 32, thus incorporating the global dust storm event of MY 28 as a signal. We consider two end‐member cases for the regolith thermal conductivity and find that the background planetary heat flow is superposed by atmosphere‐induced perturbations of less than 1.5 mW m−2 at depths below 2 m if regolith thermal conductivity is low and around 0.025 W m−1 K−1 on average. If thermal conductivity is high and around 0.05 W m−1 K−1 on average, perturbations are less than 2.5 mW m−2 at depths below 3 m. Overall, the influence of interannual variability on subsurface heat flow is found to be moderate following a global dust storm. Considerably smaller perturbations are introduced by regional dust storms, which are of shorter duration and smaller magnitude.

Effect of Variation in Temperature on Thermal Conductivity of Compressed Earth Block (CEBs) in Dry and Ambient States

In our days, the hydrothermal behavior of building materials raises a deep interest. The search for energy savings, by using techniques of optimization of thermo-physiques characteristics of materials of the building walls, is grown. It is important to measure the local thermal conductivity in such materials when they are facing changes in temperature gradients and humidity. Our primary objective is to determine thermal conductivity (λ) of ambient and dry material of compressed earth blocks stabilized by cement (CEBs). Usually this characteristic is measured experimentally and numerically by using the transfer heat into compressed earth block (CEBs). The numerical model used to solve the 2-D is based on the finite volume method. The obtained algebraic systems are solved by the double scanning mesh method. The measurement technique used to determine the thermal conductivity is called 'hot rings method'.

Pore Network Simulations of Heat and Mass Transfer inside an Unsaturated Capillary Porous Wick in the Dry-out Regime

In this work, a two-dimensional pore network model is developed to study the heat and mass transfer inside a capillary porous wick with opposite replenishment in the dry-out regime. The mass flow rate in each throat of the pore network is computed according to the Hagen–Poiseuille law, and the heat flux is calculated based on Fourier’s law with an effective local thermal conductivity. By coupling the heat and the mass transfer, a numerical method is devised to determine the evolution of the liquid–vapor interface. The model is verified by comparing the effective heat transfer coefficient versus heat load with experimental observations. For increasing heat load, an inflation/deflation of the vapor pocket is observed. The influences of microstructural properties on the vapor pocket pattern and on the effective heat transfer coefficient are discussed: A porous wick with a non-uniform or bimodal pore size distribution results in a larger heat transfer coefficient compared to a porous wick with a uniform pore size distribution. The heat and mass transfer efficiency of a porous wick comprised of two connected regions of small and large pores is also examined. The simulation results indicate that the introduction of a coarse layer with a suitable thickness strongly enhances the heat transfer coefficient.

Thermal shock resistance of alumina ceramics enhanced by nanostructured conformal coatings using metal–organic frameworks

In this article we provide a large-scale and low-cost method, based on metal–organic frameworks, to fabricate nanostructured conformal coatings with the same material as the substrate. Such design can reduce the thermal mismatch between coatings and substrate and massively change the local thermal conductivity and interfacial heat transfer coefficient. Therefore, the thermal shock resistance can be enhanced to about 75%. We describe all aspects of such nanostructured conformal coatings from fabrication to characterization. Moreover, the mechanism of this enhancement due to high porosity and low pore size of nanoporous coating is discussed.

Effect of magnetic field on laminar convective heat transfer characteristics of ferrofluid flowing through a circular stainless steel tube

Ferrofluids have promising potential for heat transfer applications, as they can be easily controlled and manipulated by applied magnetic field produced by either permanent magnets or electromagnets. The present study reports the effect of magnetic field on the convective heat transfer characteristics of ferrofluid flowing through a circular stainless steel (SS) tube (2mm ID×2.6mm OD) under constant heat flux conditions. The convective heat transfer coefficient for various ferrofluid flow rates and applied magnetic field gradients is reported using infrared thermography (IRT) technique. COMSOL simulation has been carried out to calculate the magnetic field and magnetic force distribution inside the SS tube. Bright field visualization of clusters of nanoparticles and aggregation of nanoparticles at the inner wall of the SS tube in the presence of magnetic field have also been carried out in order to explain the possible mechanism for heat transfer enhancement for ferrofluid in the presence of magnetic field. The convective heat transfer coefficient for ferrofluid flow in the presence of magnetic field may increase or decrease depending upon several factors such as, ratio of magnetic force to inertia force acting on the ferrofluid, interaction of ferrofluid flow with the aggregate of nanoparticles formed at the wall of the tube near the vicinity of the magnets, and enhancement in local thermal conductivity of ferrofluid flow resulting from the chain like clusters of nanoparticles within the ferrofluid in the presence of magnetic field.

GS0609 接点温度法による絶縁物の局所熱伝導率計測

GS0609 接点温度法による絶縁物の局所熱伝導率計測

接点温度法による局所熱伝導率計測の適用条件

The purpose of this study is a development of the local thermal conductivity measurement method by contact temperature method. In this paper, the applicable conditions of this method were examined. First, by the numerical simulation of heat conduction and the measurement test for non-conductive materials coated with gold film, it was found that the proposed method with the platinum wire probe is useful for thermal conductivity of 1 to 100 W/mK. Spatial resolution of this system was also examined. It was noted the resolution for low conductivity sample is approximately 100 μm.

Imaging Spatial Variations in the Dissipation and Transport of Thermal Energy within Individual Silicon Nanowires Using Ultrafast Microscopy.

Thermal management is an important consideration for most nanoelectronic devices, and an understanding of the thermal conductivity of individual device components is critical for the design of thermally efficient systems. However, it can be difficult to directly probe local changes in thermal conductivity within a nanoscale system. Here, we utilize the time-resolved and diffraction-limited imaging capabilities of ultrafast pump-probe microscopy to determine, in a contact-free configuration, the local thermal conductivity in individual Si nanowires (NWs). By suspending single NWs across microfabricated trenches in a quartz substrate, the properties of the same NW both on and off the substrate are directly compared. We find the substrate has no effect on the recombination lifetime or diffusion length of photogenerated charge carriers; however, it significantly impacts the thermal relaxation properties of the NW. In substrate-supported regions, thermal energy deposited into the lattice by the ultrafast laser pulse dissipates within ∼10 ns through thermal diffusion and coupling to the substrate. In suspended regions, the thermal energy persists for over 100 ns, and we directly image the time-resolved spatial motion of the thermal signal. Quantitative analysis of the transient images permits direct determination of the NW's local thermal conductivity, which we find to be a factor of ∼4 smaller than in bulk Si. Our results point to the strong potential of pump-probe microscopy to be used as an all-optical method to quantify the effects of localized environment and morphology on the thermal transport characteristics of individual nanostructured components.

Local measurement of thermal conductivity and diffusivity.

Simultaneous measurement of local thermal diffusivity and conductivity is demonstrated on a range of ceramic samples. This was accomplished by measuring the temperature field spatial profile of samples excited by an amplitude modulated continuous wave laser beam. A thin gold film is applied to the samples to ensure strong optical absorption and to establish a second boundary condition that introduces an expression containing the substrate thermal conductivity. The diffusivity and conductivity are obtained by comparing the measured phase profile of the temperature field to a continuum based model. A sensitivity analysis is used to identify the optimal film thickness for extracting the both substrate conductivity and diffusivity. Proof of principle studies were conducted on a range of samples having thermal properties that are representatives of current and advanced accident tolerant nuclear fuels. It is shown that by including the Kapitza resistance as an additional fitting parameter, the measured conductivity and diffusivity of all the samples considered agreed closely with the literature values. A distinguishing feature of this technique is that it does not require a priori knowledge of the optical spot size which greatly increases measurement reliability and reproducibility.

The Effect of Particle Size on Thermal Conduction in Granular Mixtures.

Shredded rubber tire is a geomaterial that is potentially useful in environmental and engineering projects. Here, we study the effect of particle size ratio on the thermal conductivity of granular mixtures containing rubber tire particles. Glass beads were mixed at various volume fractions with rubber particles of varying size. The 3D network model analysis using synthetic packed assemblies was used to determine the dominant factors influencing the thermal conduction of the mixtures. Results present that mixtures with varying size ratios exhibit different nonlinear evolutions of thermal conductivity values with mixture fractions. In particular, mixtures with large insulating materials (e.g., rubber particles) have higher thermal conduction that those with small ones. This is because the larger insulating particles allow better interconnectivity among the conductive particles, thereby avoiding the interruption of the thermal conduction of the conductive particles. Similar tests conducted with natural sand corroborate the significant effect of the relative size of the insulating particles. The 3D network model identifies the heterogeneity of local and effective thermal conductivity and the influence of connectivity among conductive particles. A supplementary examination of electrical conductivity highlights the significance of local and long-range connectivity on conduction paths in granular mixtures.

Probing technique for localized thermal conductivity measurement

A low-cost and non-destructive measurement technique based on the combination of a temperature sensor and a heater integrated in a very sharp tip is proposed for the determination of thermal conductivity of planar materials. The thermal sensor is fabricated by means of microtechnology technique. Associated to the system, an analytical thermal model is developed to express the measured Seebeck voltage as a function of the material thermal resistance. A numerical analysis based on COMSOL MultiphysicsⓇ is then developed to extract the thermal conductivity from the thermal resistance. To validate the approach proposed, experiments on planar dielectric materials and metals are conducted. Precision around 0.1 W m−1 K−1 for thermal conductivities lower than 2 W m−1 K−1 is obtained.

Local thermal transport of liquid alkanes in the vicinity of α-quartz solid surfaces and thermal resistance over the interfaces: A molecular dynamics study

Thermal transport in liquid n-alkanes in the vicinity of α-quartz substrates and thermal boundary resistance between the liquid n-alkanes and the α-quartz substrates have been investigated using nonequilibrium molecular dynamics simulations. The study considers two liquid alkanes, methane and decane, and three crystal orientations of α-quartz substrate terminated with -H or -OH groups. The local thermal conductivity (LTC), defined in the same manner as with macroscopic thermal conductivity, is used to measure the efficiency of thermal energy transport of the liquids in the vicinity of the solid surface. The variations in the LTC of the liquid alkanes in the layered region next to the surface of the substrate were examined. The modeled LTC values of the alkanes were found to oscillate in the solid-liquid interface region. These fluctuations were typically proportional to the oscillations in the local density profile. The correlation between the thermal conductivity and density was linear in the bulk liquid region. The correlation between LTC and local density in the first adsorption layer is not a straightforward extension of that of the bulk liquid, which is mostly due to the specific molecular-scale ordering structure that occurs in the liquids formed by the proximity of the solid substrate. Thermal boundary resistance between the liquid alkanes and the quartz substrate was also evaluated. It was observed that thermal boundary resistance is relatively large when the in-plane molecular-scale structure in the first adsorption layer is sparse, and is lower when the liquid structure is dense in the adsorption layer.

High accuracy thermal conductivity measurement of aqueous cryoprotective agents and semi-rigid biological tissues using a microfabricated thermal sensor.

An improved thermal-needle approach for accurate and fast measurement of thermal conductivity of aqueous and soft biomaterials was developed using microfabricated thermal conductivity sensors. This microscopic measuring device was comprehensively characterized at temperatures from 0 °C to 40 °C. Despite the previous belief, system calibration constant was observed to be highly temperature-dependent. Dynamic thermal conductivity response during cooling (40 °C to –40 °C) was observed using the miniaturized single tip sensor for various concentrations of CPAs, i.e., glycerol, ethylene glycol and dimethyl sulfoxide. Chicken breast, chicken skin, porcine limb, and bovine liver were assayed to investigate the effect of anatomical heterogeneity on thermal conductivity using the arrayed multi-tip sensor at 20 °C. Experimental results revealed distinctive differences in localized thermal conductivity, which suggests the use of approximated or constant property values is expected to bring about results with largely inflated uncertainties when investigating bio-heat transfer mechanisms and/or performing sophisticated thermal modeling with complex biological tissues. Overall, the presented micro thermal sensor with automated data analysis algorithm is a promising approach for direct thermal conductivity measurement of aqueous solutions and soft biomaterials and is of great value to cryopreservation of tissues, hyperthermia or cryogenic, and other thermal-based clinical diagnostics and treatments.