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Depending on their structure and order (individual, films, bundled, buckypapers, etc.), carbon nanotubes (CNTs) demonstrate different values of thermal conductivity, from the level of thermal insulation with the thermal conductivity of 0.1 W/mK to such high values as 6600 W/mK. This review article concentrates on analyzing the articles on thermal conductivity of CNT networks. It describes various measurement methods, such as the 3-ω method, bolometric, steady-state method and their variations, hot-disk method, laser flash analysis, thermoreflectance method and Raman spectroscopy, and summarizes the results obtained using those techniques. The article provides the main factors affecting the value of thermal conductivity, such as CNT density, number of defects in their structure, CNT ordering within arrays, direction of measurement in relation to their length, temperature of measurement and type of CNTs. The practical methods of using CNT networks and the potential directions of future research in that scope were also described. [ABSTRACT FROM AUTHOR]

To pursue the advanced properties of 2D materials, we explore the single-layer WSe2, one of the most studied transitional metal chalcogenide-based magnetic structures. Based on the first-principle calculations, we predict the excellent ferromagnetic coupling between the doped Cu and its nearby Se and W atoms in the single-layer WSe2 (SL-WSe2). Our calculations point out that the Cu-d electrons and the interactions between the dopant and the nearby atoms are the major cause for the induced magnetism. The emerging electronic states around the Fermi level, arising from the doped Cu and its nearby W and Se atoms, not only introduce magnetism into SL-WSe2, but also low its energy gap largely. We also demonstrate a semiconducting-metallic transition in the Cu-doped SL-WSe2 caused by the applied compressive strain and a semiconducting-half metal transition under the applied tensile strain. Moreover, we show the feasibility of doping concentrations and external strain on manipulating the conductivity and magnetism of the Cu-doped SL-WSe2. [ABSTRACT FROM AUTHOR]

To pursue the advanced properties of 2D materials, we explore the single-layer WSe2, one of the most studied transitional metal chalcogenide-based magnetic structures. Based on the first-principle calculations, we predict the excellent ferromagnetic coupling between the doped Cu and its nearby Se and W atoms in the single-layer WSe2 (SL-WSe2). Our calculations point out that the Cu-d electrons and the interactions between the dopant and the nearby atoms are the major cause for the induced magnetism. The emerging electronic states around the Fermi level, arising from the doped Cu and its nearby W and Se atoms, not only introduce magnetism into SL-WSe2, but also low its energy gap largely. We also demonstrate a semiconducting-metallic transition in the Cu-doped SL-WSe2 caused by the applied compressive strain and a semiconducting-half metal transition under the applied tensile strain. Moreover, we show the feasibility of doping concentrations and external strain on manipulating the conductivity and magnetism of the Cu-doped SL-WSe2. [ABSTRACT FROM AUTHOR]

Diamond/aluminum (Dia/Al) composites were fabricated by powder metallurgy using starting Al powders of different size. Effect of matrix-to-reinforcement particle size ratio (PSR) on diamond particle dispersion was revealed, and higher thermal conductivity of Dia/Al composites with a certain volume fraction was achieved by changing PSR. The results indicated that, with PSR increasing from 0.225 to 0.9, diamond particles tended to show a connecting dispersion and the thermal conductivity of 40 and 50 vol.% Dia/Al composites increased by 21% (from 389 to 472 W/mK) and by 42% (from 442 to 628 W/ mK), respectively. The underlying cause was discussed from the point of the coordination number of Al powders around diamond particles. This study supplies a new idea to improve thermal conductivity of Dia/Al composites by tuning particle dispersion for the first time, which is also applicable to other metal matrix composites. [ABSTRACT FROM AUTHOR]

It is a great challenge for facile fabrication of thermal materials with both high thermal conductivity and excellent mechanical behaviors. Here, using tannic acid (TA) as a surface modifying agent, few-layer boron nitride nanosheets (BNNS) were effectively obtained by exfoliation of h-BN microplatelets through wet ball-milling. The existence of TA on the surface of BNNS also increased the dispersibility and stability in aqueous solution. By means of vacuum filtration of the dispersion of graphene oxide (GO) and BNNS, layered composite films of GO/BNNS were fabricated easily and at low cost. Because of the orderly orientation of GO and BNNS along the horizontal direction and high inherent thermal conductivity of the nanosheets used, a high in-plane thermal conductivity of 11.9 W/mK was reached with 80 wt% of BNNS. We also found that the thermal conductivity of the film could be further enhanced through adding 1.6 wt% PVA in the system to fill the voids between the nanosheets. The resulting composite film also exhibited excellent tensile strength of 41.0 MPa, flexibility potential for utilization on a curved surface, and electrical insulation up to 6.8 × 10 Ω cm. [ABSTRACT FROM AUTHOR]

Thermal conductivity of biomorphic SiC/Si, a silicon carbide + silicon containing two phase material, was evaluated using the laser steady-state heat flux method. These materials were processed via silicon melt infiltration of wood-derived carbon scaffolds. In this approach, heat flux was measured through the thickness when one side of the specimen was heated with a 10.6-µm CO laser. A thin mullite layer was applied to the heated surface to ensure absorption and minimize reflection losses, as well as to ensure a consistent emissivity to facilitate radiative loss corrections. The influence of the mullite layer was accounted for in the thermal conductivity calculations. The effect of microstructure and composition (inherited from the wood carbonaceous performs) on measured conductivity was evaluated. To establish a baseline for comparison, a dense, commercially available sintered SiC ceramic was also evaluated. It was observed that at a given temperature, thermal conductivity falls between that of single-crystal silicon and fine-grained polycrystalline SiC and can be rationalized in terms of the SiC volume fraction in biomorphic SiC/Si material. [ABSTRACT FROM AUTHOR]

In this paper, we report an ultralow thermal conductivity and a high-temperature phase stability of the (NdCe)ZrO system over the temperature range from room temperature to 1600 °C and over a wide composition range (0.2 ≤ x ≤ 0.8), and the (NdCe)ZrO system is therefore considered a strong candidate material for the fabrication of next-generation high-temperature thermal barrier coatings. The observed thermal conductivities (0.65-1.0 W/mK) are about 60-40% lower than those of undoped NdZrO over the same temperature range (100-700 °C) and indicate a glass-like behavior. For comparison, the variation in the thermal conductivity with the temperature of the (GdCe)ZrO system with similar point defects was also measured, and the observed behavior was almost the same as that of undoped GdZrO and was mostly determined by phonon-phonon scattering ( λ ∝ 1/ T). The effect of point defect scattering and strong phonon scattering sources (rattlers) on the thermal conductivity is also discussed in this paper. The results of this study suggest that the ultralow thermal conductivity of (NdCe)ZrO can be attributed to the presence of rattlers because of the large difference between the ionic radii of the Nd and Ce ions. [ABSTRACT FROM AUTHOR]

Utilizing a combination of atomic layer deposition and dip-coating techniques, we have incorporated natural nanocellulose fibers into an inorganic matrix in order to create a layered hybrid inorganic-organic thin-film structure. Such layer-engineered hybrid materials with an unorthodox combination of components are highly potential candidates for exciting new properties. Here, we show a more than an order of magnitude reduction in the cross-plane thermal conductivity for ZnO thin films achieved with the regular inclusion of the cellulose nanofiber layers. We foresee that a similar approach as presented here for ZnO could also be applied to other inorganic materials based on earth-abundant elements to influence their thermal transport properties. [ABSTRACT FROM AUTHOR]

Development of polymer-based composites with simultaneously high thermal conductivity and breakdown strength has attracted considerable attentions owing to their important applications in both electronic and electric industries. In this study, we successfully design novel epoxy-based composites with nano-AlO/epoxy composite layer sandwiched between micro-AlO/epoxy composite layers, which show synergistically and significantly enhanced thermal conductivity and breakdown strength. Compared with the traditional composites, the bottleneck that both thermal conductivity and breakdown strength cannot be simultaneously enhanced can be overcome successfully. An optimized sandwiched alumina-epoxy composite with 70 wt% micro-AlO fillers in the outer layers and 3 wt% nano-AlO in the middle layer simultaneously displays a high thermal conductivity of 0.447 W m K (2.4 times of that of epoxy) and a high breakdown strength of 68.50 kV mm, which is 6.3 % higher than that of neat epoxy (64.45 kV mm). The experimental results on the thermal conductivity of multi-layered alumina-epoxy composites were in well accordance with the theoretical values predicted from the series conduction model. This novel technique simultaneously improves thermal conductivity and breakdown strength, which is of critical importance for design of perspective composites for electronic and electric equipments. [ABSTRACT FROM AUTHOR]

The thermal conductivity of heat-treatable steels is highly dependent on their thermo-mechanical history and the alloying degree. Besides phase transformations like the martensitic $$\upgamma \rightarrow \upalpha ^{\prime }$$ or the degree of deformation, the precipitation of carbides exerts a strong influence on the thermal conductivity of these steels. In the current work, thermal and electrical conductivity of a 0.45 mass% C steel is investigated during an isothermal heat treatment at 700 $$^{\circ }$$ C and correlated with the precipitation kinetics of cementite. To include processes in the short-term as well as in the long-term range, annealing times from 1 s to 200 h are applied. This investigation includes microstructural characterization, diffusion simulations, and electrical and thermal conductivity measurements. The precipitation of carbides is connected with various microstructural processes which separately influence the thermophysical properties of the steel from the solution state to the short-term and long-term annealing states. In the early stages of cementite growth, an interstitial-dominated diffusion reaction takes place (carbon diffusion in the metastable condition of local equilibrium non-partitioning). Afterwards, substitutional-dominated diffusion controls the kinetics of the reaction. The electrical and thermal conductivity increase differently during the two stages of the carbide precipitation. The increment is associated to the binding of alloying elements into the carbides and to the reduction of the distortion of the martensitic matrix. Both factors increase the electron density and reduce the electron and phonon scattering. The correlation of the precipitation kinetics and the thermophysical properties are of general interest for the design of heat-treatable steels. [ABSTRACT FROM AUTHOR]

Continuous glass fiber polymer-matrix composites are electrically insulating and used for printed wiring boards, but their thermal conductivity needs to be increased without sacrificing the electrical insulation ability. The through-thickness thermal conductivity of these epoxy-matrix composite laminates with in-plane fibers is found to be effectively modeled using the Rule of Mixtures with fibers and matrix mainly in parallel in the through-thickness direction, in contrast to the series model that is effective for previously studied carbon fiber composites. For the glass fiber composites, the through-thickness conductivity is similar to the in-plane conductivity. The conductivity for woven fiber composites is increased by up to 80 % by curing pressure increase (from 0.69 to 4.0 MPa), up to 50 % by solvent (toluene or ethanol) treatment of the prepreg for partial surface resin removal, and up to 90 % by boron nitride nanotube (BNNT) incorporation along with solvent treatment. The highest through-thickness thermal conductivity reached is 1.2 W/(m K), which is higher than those of all prior reports on glass fiber composites. The interlaminar interfaces are negligible in through-thickness thermal resistance compared to the laminae, as for previously studied carbon fiber composites. The fiber contribution dominates the lamina resistance. The fiber-fiber interface contribution to the lamina resistance decreases significantly with curing pressure increase or composite modification involving BNNT incorporation or solvent treatment of the prepreg. [ABSTRACT FROM AUTHOR]

This research study investigates the influence of heat treatment on the thermal conductivities of three different tool steels at room temperature. The results reveal not only that tempering plays a decisive role in their thermophysical properties, but also that the changes in thermal conductivity due to heat treatment are dependent on the degree of alloying. Isobaric heat capacities c are found to be less dependent on heat treatment than thermal diffusivities a. The results are discussed with respect to the resulting microstructures and, for high-tempered conditions, under consideration of Calphad calculations. The results are relevant for the thermal design of tools, in particular, for tailored tempering of tools used for press hardening. [ABSTRACT FROM AUTHOR]

The thermal conductivities of nanoporous gold (NPG) microwires annealed at different temperatures have been measured in the temperature range from 100 to 320 K. Considering the electron-surface scattering, the thermal conductivity is expected to increase with the increase of ligament diameter. However, the thermal conductivity of NPG microwire is found to decrease after thermal coarsening, and has a maximum value at around 250 K for the as-dealloyed sample. We suggest that the defects accumulating at a relatively high temperature and the reduction in defect spacing may cause these temperature behaviors of thermal conductivity. Taking into account the electron scattering on ligament surfaces and defects, a modified theoretical model for the thermal conductivity of nanoporous metal is proposed to agree with our experimental results. [ABSTRACT FROM AUTHOR]

The density gradients present in several aluminium foams, produced by the powder metallurgical route, have been analysed by using computed tomography and by measuring the effective thermal conductivity (λ). The method used to measure λ, Transient Plane Source (TPS) technique, allows obtaining values of the local thermal conductivity, i.e. conductivity of a localised zone within the sample. These values have been related to the density of the measured zone, which was obtained from the computed tomography experiments. A power law relationship between local effective thermal conductivity and local density has been obtained. [ABSTRACT FROM AUTHOR]

The joining of tungsten to copper and the ongoing search for commercially viable production techniques is one of the challenging issues in the field of composite materials. The reason why this material combination is of essential importance is its ability to withstand erosion and high temperatures on the tungsten side and to remove big quantities of heat on the copper side. Due to the mismatch of thermal expansion and Young’s moduli, the direct joining of these two materials results in high residual and thermal stresses at the interface, ultimately reducing component lifetime. One potential answer to this problem is functionally graded structures of W and Cu, which smoothen the transition of material properties. The present study focuses on vacuum plasma spraying (120 mbar, Ar) of W/Cu-gradients and composites with defined mixing ratios. The influence of the fabrication process and the W:Cu ratio on the microstructure has been investigated and results from thermo-mechanical and thermo-physical results analyses are presented. Finite element modeling has been used to demonstrate the positive effect of gradients on the elastic and elastic–plastic response within two different model-geometries. Partial gradients, ranging from pure tungsten to 75 vol.% tungsten, exhibit the best results and improve the expected life-time performance significantly by reducing the stresses at both interfaces, W/FGM and FGM/Cu, compared to a reference interface between W and Cu. [ABSTRACT FROM AUTHOR]