Thermal Conductivity Contrast Research Articles

Room temperature thermal rectification in suspended asymmetric graphene ribbon

Thermal rectifiers are essential in optimizing heat dissipation in solid-state devices to enhance energy efficiency, reliability, and overall performance. In this study, we experimentally investigate the thermal rectification phenomenon in suspended asymmetric graphene ribbons (GRs). The asymmetry within the graphene is introduced by incorporating periodic parallel nanoribbons on one side of the GR while maintaining the other side in a pristine form. Our findings reveal a substantial thermal rectification effect in these asymmetric graphene devices, reaching up to 45% at room temperature and increasing further at lower environmental temperatures. This effect is attributed to a significant thermal conductivity contrast between pristine graphene and nanoribbon graphene within the asymmetric structure. We observe that the incorporation of nanoribbons leads to a notable reduction in thermal conductivity, primarily due to phonon scattering and bottleneck effects near the nanoribbon edges. These findings suggest that graphene structures exhibiting asymmetry, facilitated by parallel nanoribbons, hold promise for effective heat management at the nanoscale level and the development of practical phononic devices.

Modeling and Analysis of a Radiative Thermal Memristor

This study presents a theoretical framework for a radiative thermal memristor (RTM), utilizing Tungsten-doped vanadium dioxide (WVO) as the phase-change material (PCM) and silicon carbide (SiC) in the far-field regime. The behavior of the RTM is depicted through a Lissajous curve, illustrating the relationship between net flux (Q) and a periodically modulated temperature difference ΔT(t). It is established that temperature variations in the memristance (M) of the RTM form a closed loop, governed by PCM hysteresis. The analysis explores the impact of thermal conductivity contrast (r) and periodic thermal input amplitude (θ) on the Q–ΔT curve and the M–ΔT curve and negative differential thermal resistance (NDTR), revealing notable effects on the curve shapes and the emergence of NDTR. An increasing r leads to changes in the Lissajous curve’s shape and enhances the NDTR influence, while variations in both r and (θ) significantly affect the Q values and Lissajous curve amplitudes. In the M–ΔT curve, the height is linked to thermal conductivity contrast (r), with increasing r resulting in higher curve heights.

Exploration of Thermal Bridging Through Shrub Branches in Alpine Snow

AbstractIn the high Arctic, thermal bridging through frozen shrub branches has been demonstrated to cool the ground by up to 4°C during cold spells, affecting snow metamorphism and soil carbon and nutrients. In alpine conditions, the thermal conductivity contrast between shrub branches and snow is much less than in the Arctic, so that the importance of thermal bridging is uncertain. We explore this effect by monitoring ground temperature and liquid water content under green alders and under nearby alpine tundra in the Alps. During a January 2022 cold spell, the ground temperature at 5 cm depth under alders is 1.3°C colder than under alpine tundra. Ground water freezing under alders is complete, while water remains liquid under tundra. Finite element simulations reproduce the observed temperature difference between both sites, showing that thermal bridging does affect ground temperature also under Alpine conditions.

Thermal conductivity switching for a Y–Mg alloy hydride thin film due to hydrogenation/dehydrogenation reactions using dilute H2 gas

Thermal switching requires a significant contrast in thermal conductivity between the on and off states. We focus on thermal conductivity switching performance and mechanism for switchable mirror materials, which changes reversible metallic and semiconductor states due to hydrogenation and dehydrogenation. A thin film of yttrium–magnesium (Y–Mg) alloy hydride covered with a Pd catalyst layer was fabricated on quartz glass substrates by dc magnetron sputtering using a 60 at. % Y and 40 at. % Mg alloy target and a mixture of 50% Ar and 50% H2 gases. The crystal structure, electrical conductivity, and thermal conductivity in each state were measured using in situ x-ray diffraction analysis, Hall effect measurement, and thermoreflectance apparatus, respectively. The Y–Mg alloy hydride film was hydrogenated and dehydrogenated on exposure to a mixture of 3% H2 in N2 gas and air, respectively. The structural change in Y hydrides due to hydrogenation and dehydrogenation was clarified, whereas Mg or Mg hydride in the film showed no apparent crystallization. The thermal conductivity of the on-state was 4.5 times larger than that of the off-state. The thermal conductivity change from hydrogenated to dehydrogenated state was ∼5.4 W m−1 K−1, and ∼2.5 W m−1 K−1 of thermal conductivity change could be attributed to electron contribution based on the estimation using Wiedemann–Franz law. The thermal conductivity changes of Y–Mg alloy hydrides due to hydrogenation/dehydrogenation resulted from both electrons and phonons.

The emergent behavior of three-dimensional and three-phase thermal network model for the heterogeneous materials

The study of the heat transfer properties of heterogeneous materials has been a world-wide research focus, for example in sectors related to thermal management in aerospace, architecture, and geology. In this paper, the emergent behavior and thermal conduction characteristics of three-dimensional and three-phase heterogeneous materials thermal networks are studied using the finite element method. The results show that the existence of percolation paths of each phase has a strong impact on the effective properties of the network when the contrast in thermal conductivities of each phase is high, and percolation also affects the effective thermal conductivity of the whole thermal network system. However, when the contrast in thermal conductivity between the two phases is low, the thermal networks exhibit a more consistent and “emergent” behavior, and the effective thermal conductivity of thermal networks at the same volume fraction changes to a lower extent from network to network. This paper also demonstrates that a logarithmic mixing rule can predict the effective thermal conductivity in the low contrast emergent region in three-dimensional networks, and the modeling method provides new approaches for the design of multi-phase composites and prediction of their thermal conduction properties.

Surface entrapment of micromotors by a background temperature field

The fabrication of self-propelling micromotors and the study of their propulsion strategies have gained attention due to their wide range of applications in the medical, engineering, and environmental fields. The role of a background temperature field in the precise navigation of a self-thermophoretic micromotor near an insulated wall has been investigated by employing exact solutions to the energy equation and creeping flow. We report bound states for half-coated micromotors appearing as steady-state sliding, damped, and periodic oscillations when the dimensionless external temperature gradient (S) is in the range of 0.15≤S<0.26. The sliding height is lower with S but remains insensitive to the thermal conductivity contrast. Moreover, the stationary states for the self-propelled, asymmetrically coated micromotors transform into scattering trajectories. We highlight the combinations of S and coating coverage needed for guided swimming up or against the field along with a broad spectrum of counter-intuitive temporal variations of its navigating locations. These unique observations have been ascribed to a confinement-mediated dynamic coupling between the passive and active propulsion mechanisms.

Geothermal Heat Flow Anomalies Caused by Thermal Conductivity Contrast

Heat flow measurements are usually made in boreholes considering that the heat flow field is homogeneous and the flux vector is vertical. This is not true in regions where thermal conductivity anomalies exist. In this paper we study configurations which may disturb heat flow density distribution. A numerical method is used to investigate the two-dimensional character of a heat flow field which could be associated with three situations: a layer almost vertical, an inclined layer and a layer situated between two media of different conductivities. Vertical and horizontal heat flow components are calculated along the surface of the earth and through the region of study. The results indicate that the heat flow is greatly disturbed by thermal conductivity anomalies.As medidas de fluxo de calor são usualmente feitas em furos considerando-se que o campo de fluxo térmico é homogêneo e o vetor fluxo é vertical. Isto não é verdade em regiões onde anomalias de fluxo térmico existem. Neste trabalho estudamos as configurações que podem perturbar a distribuição de densidade de fluxo térmico. Um método numérico é usado para investigar o caráter bidimensional do campo de fluxo térmico que pode estar associado com três situações: uma camada quase vertical, uma camada inclinada e uma camada situada entre dois meios com diferentes condutividades. As componentes de fluxo térmico vertical e horizontal são calculadas sobre a superfície da Terra e através da região em estudo. Os resultados indicam que o fluxo térmico é fortemente perturbado por anomalias de condutividade térmica.

Improved Heat Transfer in Guar Gel Composites Reinforced with Randomly Distributed Glass Microspheres

Improved heat transfer in composites consisting of guar gel matrix and randomly distributed glass microspheres is extensively studied to predict the effective thermal conductivity of composites using the finite element method. In the study, the proper and probabilistic three-dimensional random distribution of microspheres in the continuous matrix is automatically generated by a simple and efficient random sequential adsorption algorithm which is developed by considering the correlation of three factors including particle size, number of particles, and particle volume fraction controlling the geometric configuration of random packing. Then the dependences of the effective thermal conductivity of composite materials on some important factors are investigated numerically, including the particle volume fraction, the particle spatial distribution, the number of particles, the nonuniformity of particle size, the particle dispersion morphology and the thermal conductivity contrast between particle and matrix. The related numerical results are compared with theoretical predictions and available experimental results to assess the validity of the numerical model. These results can provide good guidance for the design of advanced microsphere reinforced composite materials.

Thermal Percolation in Well-Defined Nanocomposite Thin Films.

Thermal percolation in polymer nanocomposites─the rapid increase in thermal transport due to the formation of networks among fillers─is the subject of great interest in thermal management ranging from general utility in multifunctional nanocomposites to high-conductivity applications such as thermal interface materials. However, It remains a challenging subject encompassing both experimental and modeling hurdles. Successful reports of thermal percolation are exclusively found in high-aspect-ratio, conductive fillers such as graphene, albeit at filler loadings significantly higher than the electrical percolation threshold. This anomaly was attributed to the lower filler-matrix thermal conductivity contrast ratio kf/km ∼104 compared to electrical conductivity ∼1012-1016. In a randomly dispersed composite, the effect of a low contrast ratio is further accentuated by uncertainties in the morphology of the percolating network and presence of other phases such as disconnected aggregates and colloidal dispersions. Thus, the general properties of percolating networks are convoluted as they lack a defined structure. In contrast, a prototypical system with controllable nanofiller placement enables the elucidation of structure-property relations such as filler size, loading, and assembly. Using self-assembled nanocomposites with a controlled 1,2,3-dimension nanoparticle (NP) arrangement, we demonstrate that thermal percolation can be achieved in spite of using spherical, nonconductive fillers (kf/km ∼60) at a low volume fraction (9 vol %). We observe that the effects of volume fraction, interfacial thermal resistance, and filler conductivity on thermal conductivity depart from effective medium approximations. Most notably, contrast ratio plays a minor role in thermal percolation above kf/km ∼60─a common range for semiconducting nanoparticles/polymer ratios. Our findings bring new perspectives and insights to thermal percolation in nanocomposites, where the limits in contrast ratio, interfacial thermal conductance, and filler size are established.

A paradigm shift in liquid cooling by multitextured surface design

A paradigm shift in liquid cooling by multitextured surface design

Thermal conductivity contrast effect of organic soils and its environmental implications

Thermal conduction control is important for retarding permafrost degradation and mitigating of frost geohazards. Similar to a thermodiode, high thermal conductivity contrast (HTCC) materials can serve as good thermal insulators. A preferred HTCC material for ground cooling is larger in thermal resistance in summer and smaller in winter. Because of contrasting thermal conductivity under frozen and thawed states, organic soil is blessed with such a property. This study quantified and reported the HTCC effects on a range of soil organic matter concentrations (SOMC) and soil moisture saturation degree (SMSD). Using the COMSOL, influences of different SOMC and SMSD on ground temperatures were simulated and compared with laboratory-measured properties. Simulation results demonstrated that with constant SMSD at 20% throughout the year, the thermal insulation effect was strengthened with increasing SOMC. A better insulating effect was judged by lower annual amplitudes and smaller depths of zero annual amplitude of ground temperatures. In case of low SMSD in summer (20%) and high SMSD in winter (60–80%), the HTCC effect of soil is enhanced with increasing SOMC. This enhancement was evidenced by increased thermal offsets and decreased maximum summer and average near-surface soil temperatures. With constant SOMC and increasing SMSD, the rising HTCC effect gradually cools the ground. An integral analysis indicates that the higher the SOMC and SMSD in winter, the larger the thermal offset and the lower the ground temperature, i.e., the greater the HTCC effect of organic soil. This study may provide geocryological bases for engineering and environmental applications in cold regions.

Surface heat flow in Western Anatolia (Turkey) and implications to the thermal structure of the Gediz Graben

Knowledge of heat flow density on the Earth's surface and subsurface temperature distribution is essential for the interpretation of several processes in the crust such as for the evaluation of the geothermal potential of a region. With this study, we investigate the conductive heat flow distribution in western Anatolia to understand the thermal state and its relationship to regional tectonics in the region. The new heat flow data are collected and combined with previously published data to obtain the new heat flow map of western Anatolia. Analysis of data sets after appropriate corrections yields a better picture of the regional distribution of heat flow within the region. Generally, high values are observed around the grabens of Menderes Massif due to the intense tectonic activity. We also present the 2D steady-state thermal model of Gediz. The modeled temperatures are validated by temperature measurements from two deep wells. Numerical simulation results show that the dominant heat transfer mechanism in Gediz graben can be explained by conduction. Temperature distribution in the deep subsurface of the graben is controlled by both thickness distribution and thermal properties of the different stratigraphic sections. Thermal conductivity contrast between different stratigraphic sections causes anomalously elevated heat flow values at the edges of the graben. The comprehensive results of this study will bring a new perspective to geothermal studies in particular Enhanced Geothermal Systems (EGS) resource estimations in Gediz graben.

Thermal refraction: implications for subglacial heat flux

AbstractIn this study, we explore small-scale (~1 to 20 km) thermal-refractive effects on basal geothermal heat flux (BGHF) at subglacial boundaries resulting from lateral thermal conductivity contrasts associated with subglacial topography and geologic contacts. We construct a series of two-dimensional, conductive, steady-state models that exclude many of the complexities of ice sheets in order to demonstrate the effect of thermal refraction. We show that heat can preferentially flow into or around a subglacial valley depending on the thermal conductivity contrast with underlying bedrock, with anomalies of local BGHF at the ice–bedrock interface between 80 and 120% of regional BGHF and temperature anomalies on the order of ±15% for the typical range of bedrock conductivities. In the absence of bed topography, subglacial contacts can produce significant heat flux and temperature anomalies that are locally extensive (>10 km). Thermal refraction can result in either an increase or decrease in the likelihood of melting and ice-sheet stability depending on the conductivity contrast and bed topography. While our models exclude many of the physical complexities of ice behavior, they illustrate the need to include refractive effects created by realistic geology into future glacial models to improve the prediction of subglacial melting and ice viscosity.

Pool boiling heat transfer enhancement by bi-conductive surfaces

The use of the bi-conductive surface provides a promising prospect for the enhancement of the boiling phase change heat transfer. In this study, a theoretical model of two-phase flow and boiling heat transfer on bi-conductive surfaces is developed. With the consideration of thermal conductivity contrast and conjugate heat transfer, the boiling behavior is numerically investigated based on a modified lattice Boltzmann method. The bubble dynamics during the boiling process on the bi-conductive surface are reproduced and the effect of the structural size of these insets on the boiling performance is explored. The results indicate that boiling performance is enhanced on the bi-conductive surface when compared to homogeneous ones. In-plane temperature variation on the surface contributes to the pinning effect on bubble contact lines, and thus promotes boiling heat transfer. Boiling enhancement gets optimized once the embedding pitch space equal to the capillary length of the boiling fluid. Increasing the width of the insets also improves the boiling heat transfer coefficient on the bi-conductive surface under low superheat conditions, but also leads to the advance of CHF. However, changing the depth of the insets shows almost no effect on the boiling heat transfer performance.

Steering a thermally activated micromotor with a nearby isothermal wall

Selective heating of a microparticle surface had been observed to cause its autonomous movement in a fluid medium due to self-generated temperature gradients. Here, we theoretically investigate the response of such an auto-thermophoretic particle near an isothermal planar wall. We derive an exact solution of the energy equation and employ the Lorentz reciprocal theorem to obtain the translational and rotational swimming velocities in the creeping-flow limit. We report fixed points for vertical movement of the micromotor for its specific orientations relative to the wall. The critical wall gap for fixed points shows unique non-monotonic dependence on the metallic coating coverage on the particle. Also, the micromotor trajectories can be switched either from wall-bound sliding or stationary state to escape from the near-wall zone by tuning the particle and the surrounding fluid pair thermal conductivity contrast. The scenario holds several exclusive distinguishing features from the otherwise extensively studied self-diffusiophoresis phenomenon near an inert wall, despite obvious analogies in the respective constitutive laws relating the fluxes with the gradients of the concerned forcing parameters. The most contrasting locomotion is the ability of a self-thermophoretic micromotor with a large heated cap to migrate towards the wall even if it is initially directed away from the wall. During the stationary states of swimming, the cold portion on the micromotor surface faces away from the wall under all conditions. Such unique aspects hold the potential of being harnessed in practice towards achieving intricate control over the autonomous motion of microparticles in thermally regulated fluidic environments.

Nanoslot Patterns for Enhanced Thermal Anisotropy of Si Thin Films

Materials with a large thermal anisotropy can be widely used to guide the heat flow for various energy-related applications. Other than the well-known thermal conductivity contrast along the in-plane and cross-plane directions of a thin film, the in-plane thermal anisotropy within a thin film is less emphasized. However, such a thermal anisotropy can be easily adopted for the thermal management of thin-film-based electronic devices in practice. In this work, two-dimensional Si thin films patterned with through-film nanoslots are studied for their enhanced in-plane thermal anisotropy. Specifically, the anisotropy of the nanoslot pattern can be further increased if rows of nanoslot patterns have an offset. Within the limitation of nanofabrication techniques, offset nanoslot patterns can provide more scopes for tailoring the transport properties. These nanoslot-patterned thin films can be widely used for applications such as heat guide, thermal insulation, and thermoelectrics.

Paraffin/polyacrylonitrile hybrid nanofibers for thermal hysteresis enhancement of paraffin actuators

Fast and facile one-step preparation of paraffin/polyacrylonitrile hybrid nanofibers via single needle (uniaxial) electrospinning system was studied. As-spun paraffin/polyacrylonitrile nanofibers were used for thermal hysteresis enhancement of paraffin actuators. Solid paraffin with the melting point of 32, 58, 89 and 114°C were employed for the preparation of the paraffin/polyacrylonitrile nanofibers. Differential scanning calorimetry measurements revealed that the melting point of the paraffin in paraffin/polyacrylonitrile hybrid nanofiber was clearly detectable and the melting entalpy coming from the paraffin part gradually increased from 9.6 to 101.5 J/g with the increase in the melting points of the added same amount of paraffins in paraffin/polyacrylonitrile nanofibers. When both calorimetric and weight loss measurements were considered, the paraffin which has the melting point of 32°C was found to be suitable to produce hybrid nanofibers paraffin actuator. Therefore, this hybrid nanofiber was selected for the application in paraffin actuators for e-vehicle battery cooling systems where the battery temperature must be kept between 15 and 35°C. Paraffin compound of the paraffin actuators was prepared with a mixture of pure paraffin and paraffin/polyacrylonitrile nanofiber with the wt.% of 2.5, 5, 7.5 and 10. In the hysteresis measurements, the hysteresis value at 3 mm stroke was successfully enhanced as 1.7, 3.4, 11.9 and 15.3% sequentially for the samples produced with the above ratios. Beyond hysteresis enhancement, the phenomena of thermal percolation threshold effect and thermal conductivity contrast ratio effect in nano scale were emprically exposed on opening and closing behavior of the paraffin actuator.

Experimental assessment of thermal break strips performance in load-bearing and non-load-bearing LSF walls

A reliable evaluation of thermal behaviour and energy efficiency of buildings depends on the accurate thermal characterization of the envelope components. One of the most reliable methodologies to perform this thermal characterization is the measurements under laboratory-controlled conditions. The thermal performance assessment of lightweight steel-framed (LSF) building components exhibits particular additional challenges related to the strong thermal conductivity contrast between cavity insulation and steel frame materials, which may originate unwanted significant thermal bridge effects. The use of thermal break (TB) strips is one of the most currently used thermal bridge mitigation strategies. It was not found in the literature any experimental campaign for TB strips thermal performance evaluation in LSF elements. In this paper the thermal performance of twenty load-bearing (LB) and non-load-bearing (NLB) LSF walls configurations are measured, using the heat flow meter (HFM) method under controlled laboratory conditions. Three thermal break (TB) strip materials and three TB strip locations in the steel stud flanges are assessed. It was found that the inner and outer TB strips show very similar thermal performances, while double TB strips have a relative significant thermal performance increase. Aerogel was the best performance TB material, exhibiting a substantial improvement relatively to recycled rubber and cork/rubber composite TB strips. Furthermore, the TB strips performance was identical for the evaluated structural (LB) and non-structural (NLB) LSF walls.

New exact multi-coated ellipsoidal inclusion model for anisotropic thermal conductivity of composite materials

The present study deals with a new micromechanical modeling of the thermal conductivity of multi-coated inclusion-reinforced composites. The proposed approach has been developed in the general frame of anisotropic thermal behavior per phase and arbitrary ellipsoidal inclusions. Based on the Green's function technique, a new formulation of the problem of multi-coated inclusion is proposed. This formulation consists in constructing a system of integral equations, each associated to the thermal conductivity of each coating and the reference medium. Thanks to the concept of interior- and exterior-point Eshelby's conduction tensors, the exact solution of the problem of multicoated inclusion is obtained. Analytical expressions of the intensity in each phase and the effective thermal conductivity of the composite, through homogenizations schemes such as Generalized self-consistent and Mori-Tanaka models are provided. Results of the present model are successfully compared with those issued from both analytical models and finite elements methods for composites with doubly coated inclusions. Moreover, the developed micromechanical model has been applied to a three phase composite materials in order to analyze combined effects of the aspect ratio and the volume fraction of the ellipsoidal inclusions, the anisotropy of the thermal conductivity of interphase, the thermal conductivity contrast between local phases on the predicted effective thermal conductivity.

The Emergent Behaviour of Thermal Networks and Its Impact on the Thermal Conductivity of Heterogeneous Materials and Systems

The properties of thermal networks are examined to understand the effective thermal conductivity of heterogeneous two-phase composite materials and systems. At conditions of high contrast in thermal conductivity of the individual phases (k1 and k2), where k1 << k2 or k1 >> k2, the effective thermal conductivity of individual networks of the same composition was seen to be highly sensitive to the distribution of the phases and the presence of percolation paths across the network. However, when the contrast in thermal conductivities of the two phases was modest (k1/k2 ~ 10−2 to 102), the thermal networks were observed to exhibit an emergent response with a low variability in the effective thermal conductivity of mixtures of the same composition. A logarithmic mixing rule is presented to predict the network response in the low variability region. Excellent agreement between the model, mixing rule and experimental data is observed for a range two-phase porous and granular media. The modelling approach provides new insights into the design of multi-phase composites for thermal management applications and the interpretation or prediction of their heat transfer properties.