Large Increase In Thermal Conductivity Research Articles

The Effect of Expanded Graphite Content on the Thermal Properties of Fatty Acid Composite Materials for Thermal Energy Storage.

The mass content of expanded graphite (EG) in fatty acid/expanded graphite composite phase-change materials (CPCMs) affects their thermal properties. In this study, a series of capric-myristic acid/expanded graphite CPCMs with different EG mass content (1%, 3%, 5%, 8%, 12%, 16%, and 20%) were prepared. The adsorption performance effect of EG on the PCMs was observed and analyzed. The structure and thermal properties of the prepared CPCMs were characterized via scanning electron microscopy, differential scanning calorimetry, thermal conductivity measurements, and heat energy storage/release experiments. The results show that the minimum mass content of EG in the CPCMs is 7.6%. The phase-change temperature of the CPCMs is close to that of the PCMs, at around 19 °C. The latent heat of phase change is equivalent to that of the PCM at the corresponding mass content, and that of phase change with an EG mass content of 8% is 138.0 J/g. The CPCMs exhibit a large increase in thermal conductivity and a significant decrease in storage/release time as the expanded graphite mass content increases. The thermal conductivity of the CPCM with a mass content of 20% is 418.5% higher than that with a mass content of 5%. With an increase in the EG mass content in CPCMs, the heat transfer mainly transitions from phase-change heat transfer to thermal conductivity.

Enhancing the Conductivity and Thermoelectric Performance of Semicrystalline Conducting Polymers through Controlled Tie Chain Incorporation.

Conjugated polymers are promising materials for thermoelectric applications, however, at present few effective and well-understood strategies exist to further advance their thermoelectric performance. Here a new model system is reported for a better understanding of the key factors governing their thermoelectric properties: aligned, ribbon-phase poly[2,5-bis(3-dodecylthiophen-2-yl)thieno[3,2-b]thiophene] (PBTTT) doped by ion-exchange doping. Using a range of microstructural and spectroscopic methods, the effect of controlled incorporation of tie-chains between the crystalline domains is studied through blending of high and low molecular weight chains. The tie chains provide efficient transport pathways between crystalline domains and lead to significantly enhanced electrical conductivity of 4810 S cm-1, which is not accompanied by a reduction in Seebeck coefficient or a large increase in thermal conductivity. Respectable power factors of 173 µW m-1 K-2 are demonstrated in this model system. The approach is generally applicable to a wide range of semicrystalline conjugated polymers and could provide an effective pathway for further enhancing their thermoelectric properties and overcome traditional trade-offs in optimization of thermoelectric performance.

Numerical experiments on thermal convection of highly compressible fluids with variable viscosity and thermal conductivity in 2-D cylindrical geometry: implications for mantle convection of super-Earths

SUMMARY We conduct a series of numerical experiments of thermal convection of highly compressible fluids in 2-D cylindrical annulus, in order to study the mantle convection on super-Earths. The variations in thermodynamic properties (thermal expansivity and reference density) with depth are taken to be relevant for the super-Earths with 10 times the Earth’s mass, while those in transport properties (viscosity and thermal conductivity) are modelled by an exponential dependence on temperature and/or depth. From our experiments we identified a distinct regime of convecting flow patterns induced by the interplay between the adiabatic temperature change and the spatial variations in viscosity and thermal conductivity. That is, for the cases with strong temperature-dependent viscosity and large increase in thermal conductivity with depth, a ‘deep stratosphere’ of stable thermal stratification is formed at the base of the mantle, in addition to thick stagnant lids at their top surfaces. In the ‘deep stratosphere’, the fluid motion is insignificant particularly in the vertical direction in spite of smallest viscosity owing to its strong dependence on temperature. From the comparison with the experiments with the Cartesian geometry, we also found that the occurrence of ‘deep stratosphere’ tends to be suppressed for the cases with cylindrical geometry, owing to the reduction of the surface area with depth which helps increase the temperature gradient in the lowermost mantle. Our finding may further imply that both the effects of adiabatic compression and those of spherical (or cylindrical) geometry of mantle are of crucial importance in understanding the mantle dynamics of massive super-Earths in the presence of spatial variations in physical properties.

Fabrication and evaluation of structural, thermal, mechanical and optical behavior of epoxy–TEOS/MWCNTs composites for solar cell covering

In the present study, hybrid organic–inorganic composites were fabricated from epoxy–TEOS (tetraethyl orthosilicate, Si(OC2H5)4) with various ratios (0–10 wt%) of multiwall carbon nanotubes (MWCNTs) as reinforcing nanofillers by the sol–gel method. The effect of the MWCNTs ratios on the structural, optical and mechanical characteristics and the thermal conductivity of the epoxy–TEOS/MWCNTs composites is investigated. The X-ray diffraction (XRD) analysis reveals that the pure epoxy-TEOS is amorphous, while epoxy–TEOS/MWCNTs composites are crystalline with an orthorhombic crystal structure that has an average crystallite size of 3.9 ± 0.15 nm. In addition, thermal stability and thermal conductivity were improved by adding TEOS and MWCNTs, whereas the exothermic peak temperature decreases compared with pure epoxy-TEOS. Similarly, the hardness Shore-D and tensile strength reach the optimum value at 4 wt% MWCNTs content. The significant improvement in the mechanical and thermal properties of the prepared composites could be attributed to the synergistic effect of MWCNTs and epoxy–TEOS which was emphasized by Fourier transform infrared (FTIR) spectroscopy. Moreover, epoxy-TEOS sample has high optical transmittance (T) within the visible region, but the composites samples are transparent at λ < 800 nm and have a lower value of T. The indirect optical band gap decreases from 3.59 to 2.91 eV with an increase in MWCNTs fractions from 0 to 10 wt%, respectively. However, the glass transition reflects the onset of decomposition temperatures was also considerably increased. The acquired outcomes such as a large increase in thermal conductivity and tensile stress coupled with reduced T make the composites readily applicable for a variety of applications.

Thermally conductive thin films derived from defect free graphene-natural rubber latex nanocomposite: Preparation and properties

Commercially useful rubber products viz. gloves, condoms, tyres, and rubber hoses used in high temperature environments, etc., require efficient thermal conductivity, which increases the lifetime of these products. Graphene can fetch this property, if it is effectively incorporated into the rubber matrix. The great challenge in preparing graphene-rubber nanocomposites is formulating a scalable method to produce defect free graphene and its homogeneous dispersion into polymer matrices through an aqueous medium. Here, we used a simple method to produce defect free few layer (2–5) graphene, which can be easily dispersed into natural rubber (NR) latex without adversely affecting its colloidal stability. The resulting new composite showed large increase in thermal conductivity (480–980%) along with 40% increase in tensile properties and 60% improvement in electrical conductivity. This study provides a novel and generalized approach for the preparation of graphene based thermally conductive rubber nanocomposites.

The impact of hydrogenation on the thermal transport of silicene

Silicene, the silicon counterpart of graphene, has been identified as a promising 2D material for electronics applications. The reported very low thermal conductivity of silicene can potentially pose challenges on the thermal management of such nanoelectronics, which can in turn influence the device performance and reliability. Although the thermal conductivity of silicene has been studied, the impact of hydrogenation of silicene, which can happen spontaneously due to the resultant lower energy state, on its thermal transport ability is not clear. In this paper, we use first-principles calculations and iterative solution of phonon Boltzmann transport equation (BTE) to investigate and compare the thermal transport property of silicene and hydrogenated silicene. Surprisingly, we predict that the hydrogenation can lead to a large increase in thermal conductivity (from 22.5 W m−1 K−1 for silicene to 78.0 W m−1 K−1 for hydrogenated silicene at 300 K). We also find that the main contributor for such an improvement is the transverse acoustic phonon modes, and the reasons are the reduced anharmonicity as well as the reduced three-phonon scattering phase space after hydrogenation. This research may help better understand thermal transport in functionalized 2D materials and inspire new strategies to manipulate their thermal properties, which is of critical importance for designing high performance and reliable nanoelectronic devices.

Thermally induced chain orientation for improved thermal conductivity of P(VDF-TrFE) thin films

A large increase in thermal conductivity κ was observed in a P(VDF-TrFE) thin film annealed above melting temperature due to extensive ordering of polymer backbone chains perpendicular to the substrate after recrystallization from the melt. This finding may lay out a straightforward method to improve the thin film κ of semicrystalline polymers whose chain orientation is sensitive to thermal annealing.

Atomistic Modeling of Thermal Conductivity of Epoxy Nanotube Composites

The Green–Kubo method was used to investigate the thermal conductivity as a function of temperature for epoxy/single wall carbon nanotube (SWNT) nanocomposites. An epoxy network of DGEBA-DDS was built using the ‘dendrimer’ growth approach, and conductivity was computed by taking into account long-range Coulombic forces via a k-space approach. Thermal conductivity was calculated in the direction perpendicular to, and along the SWNT axis for functionalized and pristine SWNT/epoxy nanocomposites. Inefficient phonon transport at the ends of nanotubes is an important factor in the thermal conductivity of the nanocomposites, and for this reason discontinuous nanotubes were modeled in addition to long nanotubes. The thermal conductivity of the long, pristine SWNT/epoxy system is equivalent to that of an isolated SWNT along its axis, but there was a 27% reduction perpendicular to the nanotube axis. The functionalized, long SWNT/epoxy system had a very large increase in thermal conductivity along the nanotube axis (~700%), as well as the directions perpendicular to the nanotube (64%). The discontinuous nanotubes displayed an increased thermal conductivity along the SWNT axis compared to neat epoxy (103–115% for the pristine SWNT/epoxy, and 91–103% for functionalized SWNT/epoxy system). The functionalized system also showed a 42% improvement perpendicular to the nanotube, while the pristine SWNT/epoxy system had no improvement over epoxy. The thermal conductivity tensor is averaged over all possible orientations to see the effects of randomly orientated nanotubes, and allow for experimental comparison. Excellent agreement is seen for the discontinuous, pristine SWNT/epoxy nanocomposite. These simulations demonstrate there exists a threshold of the SWNT length where the best improvement for a composite system with randomly oriented nanotubes would transition from pristine SWNTs to functionalized SWNTs.

Thermal Conductivity and Mechanical Properties of Magnesium Oxide Reinforced Polyamide-66 Composites

Magnesium oxide (MgO) reinforced polyamide-66 (PA66) composites were prepared through melt-compounding method in order to determine the possibility of using MgO particle as conductive filler in the polymer-based composite. The effects of MgO filler content on the thermal conductivity and mechanical properties for the PA66/MgO composites were investigated. The results showed that the addition of MgO filler to the PA66 matrix led to a large increase in thermal conductivity of the PA66/MgO composites. Tensile strengths of the PA66/MgO composites were slightly decreased as MgO filler loading increased. However, flexural strength and flexural modulus were improved with increasing filler loading. Notched Izod impact strengths were dramatically lowered by the addition of MgO filler.

Plasma nanocoated carbon nanotubes for heat transfer nanofluids

Multi-wall carbon nanotubes (CNTs) were plasma-treated using glow discharges of argon, oxygen andmethane/oxygen mixtures and then dispersed into a base fluid of water. It was found that proper plasmatreatments of CNTs using nanoscale plasma coatings significantly improve the dispersionand stabilize the suspension of CNTs in the base fluid. With 0.01 vol% addition ofplasma-treated CNTs, a 25% initial increase in thermal conductivity was achievedand a stabilized 20% increase was observed with the resulting nanofluids after 5days’ settling. It should particularly be pointed out that such a large increasein thermal conductivity was achieved when plasma-treated CNTs were stablydispersed in water without adding any dispersing agents or surfactants. Surfacemodification of the CNTs was confirmed by Raman spectroscopy and ultra-thin (∼2 nm) plasma nanocoatings were noted on the treated CNT surfaces by transmission electronmicroscopy (TEM).

Thermal conductivity of polymer composites with close-packed structure of nano and micro fillers

Thermal conductivity of polymer composites with nano and micro fillers has been investigated numerically and experimentally. The nano fillers used were multi-walled carbon nanotubes (MWNTs) and alumina nanoparticles, and the spherical alumina particles were selected as the micro fillers. A periodic unit cell with a random close-packed structure was created using a packing algorithm that treat the micro filler as spheres. Finite element analyses were also performed to predict the potential of nano fillers to enhance thermal conductivity of the composites and to analyze the effect of microstructure of micro fillers. Additionally, the polymer composites with nano and micro fillers were made and the thermal conductivity of the composites were measured. The results showed that the addition of MWNTs to the matrix lead to a large increase in thermal conductivity of the composites. The proposed thermal model predicted a thermal conductivity in good agreement with experiment.

Microwave sintering of high-density, high thermal conductivity AlN

Microwave energy was used to sinter high thermal conductivity AlN ceramics (160–225 W/mK). The effects of sintering time, temperature, and amount of additive on phase composition, phase distribution, densification behavior, grain growth, and thermal conductivity were studied. The thermal conductivity of AlN was greatly improved by the addition of Y2O3, extended sintering time, and higher sintering temperatures. Thermal conductivity development in Y2O3-doped AlN showed two distinctive time regimes: (i) densification, where full densification, secondary phase formation, concentration and segregation, and rapid purification of AlN grains occur, accompanied by a large increase in thermal conductivity; (ii) postdensification, where grain growth and secondary phase sublimation/evaporation occur, yielding a further increase in thermal conductivity. Our results indicate that microwave sintering is a promising approach for synthesis of high thermal conductivity AlN ceramics.

Piezo-Thermal Conductivity Effect in Germanium

The effect of static elastic strain on the thermal conductivity of germanium has been measured in the temperature interval 1.5\ifmmode^\circ\else\textdegree\fi{} to 4.2\ifmmode^\circ\else\textdegree\fi{}K. A large increase in thermal conductivity was produced by [111] or [110] tension in antimony-doped samples in which the scattering of phonons is primarily due to donors. The theory of scattering of phonons by donors predicts the sign and magnitude of the effect. Samples without strong donor scattering show small piezo-thermal conductivity effects.