Trend Of Thermal Conductivity Research Articles

Anharmonic properties inMg2X(X=C,Si,Ge,Sn,Pb)from first-principles calculations

Thermal conductivity reduction is one of the potential routes to improve the performance of thermoelectric materials. However, detailed understanding of the thermal transport of many promising materials is still missing. In this paper, we employ electronic-structure calculations at the level of density functional theory to elucidate thermal transport properties of the $\mathrm{M}{\mathrm{g}}_{2}X$ ($X=\mathrm{C}$, Si, Ge, Sn, and Pb) family of compounds, which includes $\mathrm{M}{\mathrm{g}}_{2}\mathrm{Si}$, a material already identified as a potential thermoelectric. All these materials crystallize into the same antifluorite structure. Systematic trends in the anharmonic properties of these materials are presented and examined. Our calculations indicate that the reduction in the group velocity is the main driver of the thermal conductivity trend in these materials, as the phonon lifetimes in these compounds are very similar. We also examine the limits of the applicability of perturbation theory to study the effect of point defects on thermal transport and find that it is in good agreement with experiment in a wide range of scattering parameter values. The thermal conductivity of the recently synthesized $\mathrm{M}{\mathrm{g}}_{2}\mathrm{C}$ is computed and predicted to be 34 W/mK at 300 \ifmmode^\circ\else\textdegree\fi{}C.

Temperature dependence of electrical and thermal conduction in single silver nanowire.

In this work, the thermal and electrical transport in an individual silver nanowire is characterized down to 35 K for in-depth understanding of the strong structural defect induced electron scattering. The results indicate that, at room temperature, the electrical resistivity increases by around 4 folds from that of bulk silver. The Debye temperature (151 K) of the silver nanowire is found 36% lower than that (235 K) of bulk silver, confirming strong phonon softening. At room temperature, the thermal conductivity is reduced by 55% from that of bulk silver. This reduction becomes larger as the temperature goes down. To explain the opposite trends of thermal conductivity (κ) ~ temperature (T) of silver nanowire and bulk silver, a unified thermal resistivity () is used to elucidate the electron scattering mechanism. A large residual Θ is observed for silver nanowire while that of the bulk silver is almost zero. The same ~T trend proposes that the silver nanowire and bulk silver share the similar phonon-electron scattering mechanism for thermal transport. Due to phonon-assisted electron energy transfer across grain boundaries, the Lorenz number of the silver nanowire is found much larger than that of bulk silver and decreases with decreasing temperature.

Effect of long- and short-range order on SiGe alloy thermal conductivity: Molecular dynamics simulation

We report the role of long- and short-range order on the thermal conductivity and mode relaxation times of a model ${\mathrm{Si}}_{0.5}{\mathrm{Ge}}_{0.5}$ alloy using molecular dynamics simulation. All interactions used the Stillinger-Weber potential and the Si and Ge atoms differed only by their mass. The simulated alloys were generated using a Monte Carlo approach to decouple the short-range order from the long-range order. The thermal conductivity is almost entirely determined by the alloy's nearest-neighbor short-range order. Changes to the mode relaxation times between $\ensuremath{\sim}3$ and $\ensuremath{\sim}6\phantom{\rule{0.28em}{0ex}}\mathrm{THz}$ upon short-range ordering, and the observed ${f}^{\ensuremath{-}2}$ power law trend, suggest that short-range ordering reduces the anharmonic scattering rate of low frequency modes. The trend of thermal conductivity with short-range order may be transferred to real ${\mathrm{Si}}_{0.5}{\mathrm{Ge}}_{0.5}$ and other semiconductor alloys to the extent that scattering from mass disorder dominates their thermal conductivities.

Magnetic graphite suspensions with reversible thermal conductivity

Magnetic graphite nanoflake (GN) suspensions with reversible thermal conductivity (TC) are reported. A process of sulfuric acid intercalation, microwave expansion magnetization, and ultrasonic dispersion is followed to make magnetic GN Poly-Alpha-Olefin (PAO) suspensions. Magnetic field was observed to increase the TC of the suspensions dramatically, up to 325% with 0.8% (w/w) of magnetic GNs. After removing the magnetic field, the TC trends to decrease reversibly. The maximal TC contrast ratio reaches 3 times before and after the effects of magnetic field. Such materials with magnetically reversible TC have great potential in next generation “smart” cooling devices.

Carrier interactions and porosity initiated reversal of temperature dependence of thermal conduction in nanoscale tin films

Recently, tin has been identified as an attractive electrode material for energy storage/conversion technologies. Tin thin films have also been utilized as an important constituent of thermal interface materials in thermal management applications. In this regards, in the present paper, we investigate thermal conductivity of two nanoscale tin films, (i) with thickness 500 ± 50 nm and 0.45% porosity and (ii) with thickness 100 ± 20 nm and 12.21% porosity. Thermal transport in these films is characterized over the temperature range from 40 K–310 K, using a three-omega method for multilayer configurations. The experimental results are compared with analytical predictions obtained by considering both phonon and electron contributions to heat conduction as described by existing frequency-dependent phenomenological models and BvK dispersion for phonons. The thermal conductivity of the thicker tin film (500 nm) is measured to be 46.2 W/m-K at 300 K and is observed to increase with reduced temperatures; the mechanisms for thermal transport are understood to be governed by strong phonon-electron interactions in addition to the normal phonon-phonon interactions within the temperature range 160 K–300 K. In the case of the tin thin film with 100 nm thickness, porosity and electron-boundary scattering supersede carrier interactions, and a reversal in the thermal conductivity trend with reduced temperatures is observed; the thermal conductivity falls to 1.83 W/m-K at 40 K from its room temperature value of 36.1 W/m-K. In order to interpret the experimental results, we utilize the existing analytical models that account for contributions of electron-boundary scattering using the Mayadas-Shatzkes and Fuchs-Sondheimer models for the thin and thick films, respectively. Moreover, the effects of porosity on carrier transport are included using a previous treatment based on phonon radiative transport involving frequency-dependent mean free paths and the morphology of the nanoporous channels. The systematic modeling approach presented in here can, in general, also be utilized to understand thermal transport in semi-metals and semiconductor nano-porous thin films and/or phononic nanocrystals.

Experimental and Theoretical Analysis on Thermal Conductivity of Fired Clay Bricks Incorporated with Cigarette Butts

Currently higher requirements in thermal performance is needed for the quality of building materials especially fired clay bricks. Thermal conductivity is an important criterion as it will influences the heat losses from building. The objective of this study is to validate the estimation value of thermal conductivity by using theoretical value with the experimental work conducted in the laboratory. The experiment data was collected in order to compare with a theoretical model that obtained the thermal conductivity value based on it relationship with dry density of fired clay bricks. Different percentages of CBs (0%, 2.5% and 5.0%) were incorporated into fired clay bricks. Different heating rates were applied during firing stage, which are 1°C/min, 3°C/min and 5°C/min respectively. All samples were fired up to 1050°C. The experimental work for thermal conductivity was carried out using the Hot Guarded Plate Method. Meanwhile, the theoretical result was obtained from the previous study using model developed. Throughout statistical analysis, some trend of thermal conductivity and dry density were revealed. The analysis results show that as the dry density decreased, thermal conductivity also decreased.

Tailoring of thermal and dielectric properties of LDPE-matrix composites by the volume fraction, density, and surface modification of hollow glass microsphere filler

Hollow glass microsphere (HGM) filled low-density polyethylene (LDPE) composites were prepared, and the effects of density, content, and surface modification of HGM on the thermal and dielectric properties of the composites were investigated. It is found that the thermal conductivity of the composites decreases with increasing HGM content or decreasing HGM density. At the same HGM content and density, the composites filled with suitable amount of silane coupling agent (KH570) modified HGM exhibit higher thermal conductivity than unmodified-HGM filled composites. The dielectric constant at 1MHz of the composites also decreases with increasing HGM content or decreasing HGM density, but their dielectric loss increases with increasing HGM content or increasing HGM density. By modifying the surface of HGM with suitable amount of KH570, the dielectric constant and loss at 1MHz of the composites can be decreased at the same time. The results of microwave dielectric properties of the composites indicate that the dielectric constant decreases with increasing HGM content or decreasing HGM density, the quality factor (Q×f) decreases with increasing HGM content or increasing HGM density, but both dielectric constant and quality factor are slightly affected by the surface modification of HGM. Due to lower intrinsic thermal conductivity and dielectric constant but higher dielectric loss of HGM than LDPE, the thermal conductivity and dielectric properties of the composites can be controlled with adding HGM and varying its volume fraction. The surface modification of HGM improves the interface contact between HGM and LDPE in the composites, which is confirmed by the SEM observation, and thus the heat conduction and dielectric properties at low frequency are improved. Based on calculated thermal conductivity and dielectric constant of HGM, the experimental trends of thermal conductivity and dielectric constant at 1MHz of the composites are analyzed by using different models, including typical models for particles-filled composites and special models developed for hollow microsphere filled composites. The results from suitable models show close correlation with the experimental values.

Thermal and mechanical response of [0001]-oriented GaN nanowires during tensile loading and unloading

Molecular dynamics simulations are carried out to investigate the thermal and mechanical responses of GaN nanowires with the [0001] orientation and hexagonal cross sections to tensile loading and unloading. The thermal conductivity of the nanowires at each deformed state is calculated using the Green-Kubo approach with quantum correction. The thermal conductivity is found to be dependent on the strain induced by tensile loading and unloading. Phase transformations are observed in both the loading and unloading processes. Specifically, the initially wurtzite-structured (WZ) nanowires transform into a tetragonal structure (TS) under tensile loading and revert to the WZ structure in the unloading process. In this reverse transformation from TS to WZ, transitional states are observed. In the intermediate states, the nanowires consist of both TS regions and WZ regions. For particular sizes, the nanowires are divided into two WZ domains by an inversion domain boundary (IDB). The thermal conductivity in the intermediate states is approximately 30% lower than those in the WZ structure because of the lower phonon group velocity in the intermediate states. Significant effects of size and crystal structure on mechanical and thermal behaviors are also observed. Specifically, as the diameter increases from 2.26 to 4.85 nm, the thermal conductivity increases by 30%, 10%, and 50%, respectively, for the WZ, WZ-TS, and WZ-IDB structured wires. However, change in conductivity is negligible for TS-structured wires as the diameter changes. The different trends in thermal conductivity appear to result from changes in the group velocity which is related to the stiffness of the wires and surface scattering of phonons.

A molecular dynamics study of the thermal conductivity of graphene nanoribbons containing dispersed Stone–Thrower–Wales defects

Classical molecular dynamics with the AIREBO potential is used to investigate the thermal conductivity of both zigzag and armchair graphene nanoribbons possessing different densities of Stone–Thrower–Wales (STW) defects. Our results indicate that the presence of the defects can decrease thermal conductivity by more than 50%. The larger the defect density, the lower the conductivity, with the decrease significantly higher in zigzag than in armchair nanoribbons for all defect densities. The effect of STW defects in the temperature range 100–600K was also determined. Our results showed the same trends in thermal conductivity decreases at all temperatures. However, for higher defect densities there was less variation in thermal conductivity at different temperatures.

Kapitza conductance of symmetric tilt grain boundaries in graphene

Non-equilibrium molecular dynamics simulations were employed to study the Kapitza conductance of symmetric tilt grain boundaries in the monolayer graphene sheet. Both armchair and zig-zag oriented bicrystal graphene were investigated. The Kapitza conductance of the interface shows length dependence up to 300 nm, which arises from the fact that long-wavelength phonons allowed in large-size graphene are able to transmit through the interface contributing to the Kapitza conductance. The Kapitza conductance exhibits monotonic increase with temperature, opposite to the trend of thermal conductivity of bulk graphene above room temperature. We found that the Kapitza conductance is inversely proportional to the number of dislocations per length of grain boundaries. The facts that the phonon density of states (DOS) shows no difference between the two crystals separated by the grain boundary and the vibrational DOS of grain boundary region atoms deviates from that of bulk atoms reveal that the interfacial thermal resistance arises from the structure defects, causing additional phonon scattering for the mismatched phonon spectrum of defects. The predicted length-independent Kapitza conductance ranges from 19 to 47 GW/Km2, which is larger than that of any other interfaces reported in the literature. Finally, theoretical analysis was carried out to explain why the thermal resistance scales with the number of defects per unit length.

Thermal conductivity of high-temperature Si–B–C–N thin films

In this study, thermal transport was investigated for ceramic films with different silicon, boron, carbon, and nitrogen (Si–B–C–N) compositions. In order to investigate the effect of morphology on thermal barrier properties, the microstructure of these materials was varied from amorphous to nanocrystalline. Thermal conductivity trends of several ceramic thin films were characterized with a time-domain thermoreflectance (TDTR) technique. Samples containing two different Si–B–C–N chemical compositions were created by reactive magnetron sputtering and then subjected to annealing at temperatures up to 1400°C. The room temperature thermal conductivity of the samples prepared via a 50% Ar/50% N2 gas mixture remained constant near 1.3Wm−1K−1, while samples prepared via a 75% Ar/25% N2 gas mixture exhibited an increase in the thermal conductivity of 2.2Wm−1K−1 (or higher). X-ray diffraction data demonstrated that the former samples were amorphous, while the latter samples formed silicon nitride (Si3N4) crystals. The experiments reveal which Si–B–C–N film composition remains stable in the amorphous state at high temperatures, thereby retaining lower thermal transport properties. These material aspects are ideal for thermal barrier applications such as non-oxide based ceramic coatings for high-temperature protective systems of aircrafts, as well as surfaces of cutting tools and optical devices.

Atomistic Simulations of Heat Transport in Carbon Nanotubes Effected by Temperature and Stretch Strain

Carbon nanotubes (CNTs) is a well thermal transport nano materials, however, the thermal conductivity of CNTs has not been well established, only a few groups had reported experimental data and the existed simulation results ranged widely. Specially, the conclusions in low temperature section and dynamic structures were not very clearly. In this paper, the methods based on phonon scattering theory were applied to explore the thermal transport properties CNTs. The investigation was carried out under the conditions of temperature and axial strain. In the consideration of quantum effect, the thermal conductivity increased linearly with the growth of temperature in low-temperature section, and began to decrease gradually when the temperature exceeded a definite value. If an axial strain was concerned, there was an increasing trend of thermal conductivity as the stretch strain increases. However, after the strain exceeded a particular value the thermal conductivity decreased significantly. In addition, the high frequency phonon peak in PDOS was found to be an important parameter in describing thermal transport properties of dynamic structures.

Cross-plane thermal conductivity of superlattices with rough interfaces using equilibrium and non-equilibrium molecular dynamics

This paper describes a heat transfer study of binary Lennard Jones superlattices, focusing on the influence of interface topology on cross-plane thermal conductivity, by using both non-equilibrium and equilibrium molecular dynamics methods. Both methods reveal the same trends of thermal conductivity. In particular, interfacial roughness is shown to slightly increase cross-plane thermal conductivity in comparison to smooth interfaces. Our results highlight paths for optimizing superlattices for thermoelectric conversion applications and for thermal management solutions in micro- and nano-systems.

Contradictory Evidence for the Role of Temperature and Particle Size in Nanofluid Thermal Conductivity

ABSTRACTThe prospects for increased cooling capacity from the use of nanofluid coolants has created a tremendous amount of interest. However, in the years since the initial thermal conductivity measurements of nanoparticle suspensions were reported, there has been much inconsistency in data published in the literature. The International Nanofluids Benchmarking Exercise was a significant step towards creating a reliable set of data on the thermal conductivity enhancement of stable nanofluids, however there remain many unanswered questions. Most significant, perhaps, is the contradictory results on the effects of particle size and temperature. In the past year alone it is possible to find published reports on nominally identical samples claiming precisely opposing trends in thermal conductivity with decreasing particle size at room temperature. Some studies also claim an increasing enhancement at higher temperatures, sometimes linking this to small particle sizes. In this work we review the literature claims for particle size and temperature results, the theories used to support those claims, as well as presenting new data with the aim of resolving the dispute and identifying the origins of the evidence for contradictory claims.

Yttria-stabilized zirconia-based composites with adaptive thermal conductivity

Thermal conductivity trends in a “chameleon coating” thin film were characterized with a time-domain thermoreflectance (TDTR) technique. A yttria-stabilized zirconia (YSZ)-based nanocomposite material containing ∼21 vol.% silver (Ag) was employed for this study. The thermal conductivity ( k) of the as-deposited composite film was measured with TDTR and found to have a value of 7.4 ± 1.4 W m −1 K −1. The film was then annealed at 500 °C for 1 h to stimulate Ag flow from within the composite to the surface via diffusion. The Ag that coalesced on the surface during annealing was removed to expose the underlying porous YSZ matrix, and the sample was reexamined with the TDTR technique. The thermal conductivity of the porous nanocomposite YSZ material was then measured to be 1.6 ± 0.2 W m −1 K −1, which is significantly lower than a fully dense control sample of pure nanocrystalline YSZ (2.0 ± 0.1 W m −1 K −1). The annealed film displayed a 20% reduction in thermal conductivity as compared to the control sample and a 4–5-fold reduction in thermal conductivity as compared to the as-deposited material. The experiments demonstrate temperature triggering of a composite material, resulting in self-modifying thermal conductivity and diffusion-controlled porosity. These aspects can be used to enhance or restrict thermal transport (i.e., a thermal switch). The applicability of the TDTR technique to measurements of thin, nanoporous film materials is also demonstrated.

Thermal conductivity measurements of particulate materials: 5. Effect of bulk density and particle shape

Thermal conductivities were measured with a line‐heat source for three particulate materials with different particle shapes under low pressures of a carbon dioxide atmosphere and various bulk densities. Less than 2 μm kaolinite exhibited a general decrease in thermal conductivity with increasing bulk density. For the range of atmospheric pressures appropriate for Mars, a reduction in porosity of 24% decreased the thermal conductivity by 24%. Kaolinite manifests considerable anisotropy with respect to thermal conductivity. As the particles align the bulk thermal conductivity measured increasingly reflects the thermal conductivity of the short axis. When kyanite is crushed, it forms blady particles that will also tend to align with increasing bulk density. Without any intrinsic anisotropy, however, kyanite particles, like other particulates exhibit an increase in thermal conductivity with increasing bulk density. Under Martian atmospheric pressures, a reduction in porosity of 30% produces a 30% increase in thermal conductivity. Diatomaceous earth maintains a very low bulk density due to the highly irregular shape of the individual particles. A decrease in porosity of 17% produces an increase in thermal conductivity of 27%. The trends in thermal conductivity with bulk density, whether increasing or decreasing, are often not smooth. Whether oscillations in the trends presented in this paper and elsewhere have any physical significance or whether they are merely artifacts of the precision error is unclear. Clarification of this question may not be possible without higher‐precision measurements from future laboratories and further development of theoretical modeling.

Thermal Conductivity of Standard Sands. Part I. Dry-State Conditions

A comprehensive thermal conductivity (λ) database of three dry standard sands (Ottawa C-109, Ottawa C-190, and Toyoura) was developed using a transient line heat source technique. The database contains λ data representing a variety of soil compactions and temperatures (T) ranging from 25 °C to 70 °C. The tested standard sands, due to their repeatable physical characteristics, can be used as reference materials for validation of thermal probes applied to similar dry granular materials. The measured data show an increasing trend of thermal conductivity at dryness (λdry) against T in spite of declining quartz λ with T. The air content (porosity) controls the λ of dry sands by acting as a very effective thermal insulator around solid soil particles. As a result, a diminutive increase of λdry with T is driven by increasing λ of air. The experimental λ data of dry sands were exceptionally well predicted by de Vries and Woodside–Messmer models, and also by a thermal conductance model, a product of λ of solids and the thermal conductance factor.

Thermal conductivity effects on steady state propagation speed during self-propagating high-temperature synthesis of Ti + C green compacts

Thermal conductivities of mixtures of titanium (Ti) and graphite (C) powder compacts are experimentally investigated and the results are presented. These compacts are used in self-propagating high-temperature synthesis (SHS) of refractory, ceramic, and composite materials. Thermal conductivity values of these compacts are important for accurate modeling of the SHS process. Results show that, thermal conductivities of the compacts increase for an increase in initial density, but decrease with an increase of Ti/C mixture ratios. The experimental results also show the dependency trend of thermal conductivity on particle size of the constituents. A numerical model was also developed to predict the steady state propagation speed along a vertical cylindrical compact employing Kanury kinetic model. An important dimensionless parameter, the Damkohler number was introduced. The effects of the variation of thermal conductivity on propagation speed due to the changes in initial density and composition mixture ratio are also presented.

Thermal conductivity of zirconia based inert matrix fuel: use and abuse of the formal models for testing new experimental data

An inert matrix fuel material based on yttria-stabilized cubic zirconia: Er x Y y Pu z Zr 1− x− y− z O 2−( x + y)/2 ( x+ y=0.15, z: [0.05–0.15]) was proposed for burning excess plutonium in light water reactors. The studied inert matrix fuel is made of cubic stabilized zirconia. The limited number of experimental thermal conductivity data justifies this formal and intensive study. Approaches derived from Klemens theory were revisited and the derived conductivity model applied for zirconia, accounting the effects of phononic scattering centers. The hyperbolic thermal conductivity trend with temperature known for pure zirconia, is reduced by isotopes, impurities, dopants and oxygen vacancies, which act as scattering centers and contribute to conductivity reduction to a flat plot with temperature for stabilized zirconia. It is experimentally observed that the thermal conductivity derived from laser flash measurements for Er x Y y M z Zr 1− x− y− z O 2−( x + y)/2 (with M=Ce or Pu, z=0 or ∼0.1 and x+ y=0.15) is rather constant as a function of temperature in the range 300–1000 K. The thermal conductivity was observed to depend on the concentration of dopants such as YO 1.5 and/or ErO 1.5, CeO 2 (analogous of PuO 2) or PuO 2. The bulk material conductivity of Er 0.05Y 0.10Pu 0.10Zr 0.75O 1.925 is about 2 W m −1 K −1. In this study, the thermal conductivity data of both monoclinic and stabilized cubic zirconia based IMF are tested with the model approach in order to understand the experimental data in a semi-quantitative way.

Experimental Measurements of the Thermal Conductivity of Ash Deposits: Part 2. Effects of Sintering and Deposit Microstructure

This paper describes an experimental study that examines the influence of sintering and microstructure on ash deposit thermal conductivity. The measurements are made using a technique developed to make in situ, time-resolved measurements of the effective thermal conductivity of ash deposits formed under conditions that closely replicate those found in the convective pass of a commercial boiler. The technique is designed to minimize the disturbance of the natural deposit microstructure. The initial stages of sintering and densification are accompanied by an increase in deposit thermal conductivity. Subsequent sintering continues to densify the deposit, but has little effect on deposit thermal conductivity. SEM analyses indicate that sintering creates a layered deposit structure with a relatively unsintered innermost layer. We hypothesize that this unsintered layer largely determines the overall deposit thermal conductivity. A theoretical model that treats a deposit as a two-layered material predicts the observed trends in thermal conductivity.