Thermal Conduction Mechanism Research Articles

The liquid-exfoliation of graphene assisted with hyperbranched polyethylene-g-polyhedral oligomeric silsesquioxane copolymer and its thermal property in polydimethylsiloxane nanocomposite

Thermal interface materials with high thermal conductivity are essential to transfer the redundant heat and improve the reliability of integrated circuits. Here we reported high thermal conductivity in polydimethylsiloxane (PDMS) nanocomposite incorporated with few-layer graphene, which was exfoliated in chloroform with assistance of hyperbranched polyethylene-g-polyhedral oligomeric silsesquioxane copolymer (HBPE@POSS) as the stabilizer. In order to improve the compatibility and enhance the thermal property, the HBPE@POSS copolymer was synthesized via the unique chain walking polymerization mechanism, which subsequently was applied to exfoliate natural graphite into few-layer graphene in low-boiling-point solvents. The majority of resultant nanosheets with low defects was verified with lateral dimension of ~400 nm and the thickness of ~1.6 nm, which is attributed to the presence of CH-π noncovalent interaction between graphene and HBPE@POSS copolymer. The graphene nanoplates (GNPs)/polydimethylsiloxane (PDMS) nanocomposites were prepared by solution casting, in which graphene nanofillers were dispersed uniformly in the matrix due to good compatibility between PDMS and oligomeric silsesquioxane segments adsorbed on the nanosheets. The thermal conductivity of 4.0 wt% GNPs/PDMS nanocomposite reaches 0.93 W m−1 K−1, which is 400% higher than that of pure PDMS. The PDMS nanocomposite incorporated with few-layer graphene exhibits a promising prospect in thermal interface for thermal management of electronic devices, and sheds a light on the interfacial improvement mechanism of thermal conductivity for polymer composite.

Механизм электропроводности и теплопроводности в AgSbSe2

Механизм электропроводности и теплопроводности в AgSbSe2

Improved interfacial properties for largely enhanced thermal conductivity of poly(vinylidene fluoride)-based nanocomposites via functionalized multi-wall carbon nanotubes

Interfacial properties between fillers and polymer matrix are crucial for the enhanced thermal conductivity of composites. Considering that vinyl-containing groups can be compatible with poly(vinylidene fluoride) (PVDF) matrix, triethoxyvinylsilane (YDH-151) functionalized multi-wall carbon nanotubes (s-MWCNTs) were prepared and blended into PVDF to achieve high thermal conductive s-MWCNTs/PVDF nanocomposites. Functionalized s-MWCNTs not only showed better dispersion, but also significantly increased the interfacial compatibility with PVDF, contributing to the largely enhanced thermal conductivity. A thermal conductivity of 1.552 W/(m·K) was achieved in s-MWCNTs/PVDF composite with 10 wt% s-MWCNTs loading, about 9 times in comparison to that of pure PVDF matrix, which was much higher than that of the non-functionalized MWCNTs/PVDF composite (0.478 W/(m·K)) under the same loading weight. The enhanced thermal conductivity was verified by both theorical and experimental results. A classic Effective Medium Theory model proved that YDH-151 functionalization on MWCNTs significantly improved their dispersibility in PVDF matrix, and more importantly reduced the interfacial thermal resistance of composite, which was 68% lower than that of original MWCNTs/PVDF composite. Experimental rheological measurement confirmed the improved interfacial properties greatly promoted the formation of denser MWCNTs network structure in nanocomposites. This work highlights an effective strategy for realizing excellent thermal conductive polymeric composites and provides useful information to further reveal the mechanism of thermal conductivity.

A prediction model using response surface methodology based on cell size and foam density to predict thermal conductivity of polystyrene foams

Polymeric foams are one of the most important insulating materials due to their low thermal conductivity. The comprehensive investigation of the thermal behavior of these materials is experimentally difficult, costly, and time-consuming. In this study, a step-wise approach is used in order to predict thermal conductivity of the polymeric foams. The results demonstrate that two different models estimate the thermal conductivity of the polystyrene foams with cell sizes smaller than 100 μm and larger than 100 μm with an error almost smaller than 10%. According to these theoretical models, a comprehensive investigation is performed on the thermal-insulation performance of polystyrene foams. The results indicate that the thermal conductivity reduces by decreasing the cell size but there is an optimum foam density for achieving the smallest thermal conductivity. The effects of foam density and cell size are studied on the different mechanisms of thermal conductivity. In the following, a new approach is offered based on the cell size and the foam density using response surface method (RSM). The reliability of the proposed regression model is checked not only using analysis of variance (ANOVA) tool of RSM but also in comparison to empirical results.

Tailored interphase and thermal interface resistance of self‐assembled thermally reduced graphene oxide–polyamide hybrid/epoxy composites with enhanced thermal conductivity

ABSTRACTThermally reduced graphene oxide–polyamide (TrGO‐PA) hybrids were fabricated by self‐assembly between TrGO nanosheets and PA microparticles, and the dispersibility, interphase extension, and thermal conduction mechanism of TrGO‐PA/epoxy (EP) composites were investigated. Most of the oxygen‐containing functional groups of TrGO were removed, and a conjugated structure of graphene was recovered. TrGO was distributed evenly on the PA surface via electrostatic adsorption between TrGO and PA, which resulted in the inhibition of TrGO aggregation in the epoxy matrix. Compared with that of TrGO/EP and PA/EP composites, the thermal interface resistance (RTIM) of TrGO‐PA/EP composites was greatly decreased to 38.3 mm2 kW−1 and the thermal conductivity was improved to 0.268 W/(m K), which was attributed to the enhanced dispersibility of TrGO‐PA and the enlarged interphase in TrGO‐PA/EP composites. A schematic model of thermal conduction mechanisms was proposed based on the formation of contiguous thermal transfer pathways by bridged TrGO adsorbed on well‐dispersed PA microparticles in TrGO‐PA/EP composites. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019, 136, 47826.

Predicting the effective thermal conductivity of silica/clay mineral nanocomposite aerogels

The crucial effect of the morphology of nanocomposite aerogel on its effective thermal conductivity caused the presence of comprehensive theoretical investigation. Classic unit cell model provided an analytical relationship between microstructure and thermal conductivity of pristine silica aerogels, but the predicted results showed more than 250% deviation from the experimental results. In this work, the classic unit cell model was modified for nanocomposite aerogel by considering the effect of phonon scattering, secondary porosity, and clay mineral presence. The modified classic unit cell model (MCUM) acted as a powerful tool in the analysis of thermal conductivity mechanism of silica/clay mineral nanocomposite aerogels. Clay mineral was utilized to improve thermal radiation insulation properties of nanocomposite aerogels. Experimental results revealed that addition of 7 wt% clay mineral to silica aerogel led to a 55% reduction in thermal radiation heat transport at 700 °C. MCUM showed good agreement with experimental results with a less than 10% deviation for neat and nanocomposite aerogel.

Diffuson-driven ultralow thermal conductivity in amorphous Nb2O5 thin films

Niobium pentoxide (Nb2O5) has been extensively reported for applications of electrochemical energy storage, memristors, solar cells, light emitting diodes (LEDs), and electrochromic devices. The thermal properties of Nb2O5 play a critical role in device performance of these applications. However, very few studies on the thermal properties of Nb2O5 have been reported and a fundamental understanding of heat transport in Nb2O5 is still lacking. The present work closes this gap and provides the first study of thermal conductivity of amorphous Nb2O5 thin films. Ultralow thermal conductivity is observed without any size effect in films as thin as 48 nm, which indicates that propagons contribute negligibly to the thermal conductivity and that the thermal transport is dominated by diffusons. Density-function-theory (DFT) simulations combined with a diffuson-mediated minimum-thermal-conductivity model confirms this finding. Additionally, the measured thermal conductivity is lower than the amorphous limit (Cahill model), which proves that the diffuson model works better than the Cahill model to describe the thermal conduction mechanism in the amorphous Nb2O5 thin films. Additionally, the thermal conductivity does not change significantly with oxygen vacancy concentration. This stable and low thermal conductivity facilitates excellent performance for applications such as memristors.

Study of heat transfer by using DEM–CFD method in a randomly packed pebble-bed reactor

The pebble-bed reactor is one of the most promising designs for the nuclear energy industry. In this paper, a discrete element method–computational fluid dynamics (DEM–CFD) approach that includes thermal conduction, radiation, and natural convection mechanisms was proposed to simulate the thermal-fluid phenomena after the failure of forced circulation cooling system in a pebble-bed core. The whole large-scale packed bed was created using the DEM technique, and the calculated radial porosity of the bed was validated with empirical correlations reported by researchers. To reduce computational costs, a segment of the bed was extracted, which served as a good representative of the large-scale packed bed for CFD calculation. The temperature distributions simulated with two different fluids in this DEM–CFD approach were in good agreement with SANA experimental data. The influence of the natural convection mechanism on heat transfer must be taken into account for coolants with strong convective capacity. The proposed DEM–CFD methodology offers a computationally efficient and widely applied method for understanding the heat transfer process in a pebble-bed core. The method can also be easily extended to assess the passive safety features of newly designed fluoride-salt-cooled pebble-bed reactors.

Effect of Grain Size and Film Thickness on the Thermoelectric Properties of Flexible Sb2Te3 Thin Films

In this work, stoichiometric Sb2Te3 thin films with various thicknesses were deposited on a flexible substrate using RF magnetron sputtering. The grain size and thickness effects on the thermoelectric properties, such as the Seebeck coefficient (S), electrical conductivity (σ), power factor (PF), and thermal conductivity (k), were investigated. The results show that the grain size was directly related to film thickness. As the film thickness increased, the grain size also increased. The Seebeck coefficient and electrical conductivity corresponded to the grain size of the films. The mean free path of carriers increases as the grain size increases, resulting in a decrease in the Seebeck coefficient and increase in electrical conductivity. Electrical conductivity strongly affects the temperature dependence of PF which results in the highest value of 7.5 × 10−4 W/m·K2 at 250°C for film thickness thicker than 1 µm. In the thermal conductivity mechanism, film thickness affects the dominance of phonons or carriers. For film thicknesses less than 1 µm, the behaviour of the phonons is dominant, while both are dominant for film thicknesses greater than 1 µm. Control of the grain size and film thickness is thus critical for controlling the performance of Sb2Te3 thin films.

Constructing bioinspired hierarchical structure in polymer based energetic composites with superior thermal conductivity

Inspired by the microlayered and highly oriented structure of naturally nacre, we reported a thermally conductive polymer based energetic composite constructed with an alternating microlayered structure, in which the one-dimensional (1D) CNTs filled layer acted as bridge to link the adjacent two-dimensional (2D) graphene filled layer. This ordered arrangement of microlayers was similar to the hierarchical ‘brick-and-mortar’ structure of nacre. As a result, the thermal conductivity (K) of composites with an artificial nacre-like structure was obviously higher than those with other structure designs, such as filled by pure graphene/CNTs, or hybrid graphene/CNTs. Moreover, the K enhancing effect could become more pronounced with the layer number due to the synergetic effect of microlayered structure. In addition, the nonlinear dependence of K on the layer number was well fitted by an analytical model which incorporated the synergetic effect. The thermal conduction mechanism could change from series thermal structure to parallel thermal structure with the layer number increasing. Finally, the calculated results of thermal shock resistance and thermal stress distribution confirmed that the enhanced thermal environment adaptability by the construction of alternating microlayered micro/nano-scale structure. This thought based on bioinspired engineering provides a creative opportunity for design and fabrication of novel thermally conductive materials.

Radiative thermal conduction of molten tin sulfide estimated from its optical emission spectrum

Molten semi-conductors have potential utility in thermoelectrics or heat-management at high temperature (900 °C and above), though their development requires further analysis of their thermal conduction mechanisms, in particular radiative heat-transfer. Using a container-less method based on the floating zone furnace, the optical emission properties of a pendant droplet of molten tin sulfide (SnS) are investigated in the UV–visible (200–850 nm) and near IR (900–2050 nm) ranges. The emissivity results suggest a low emissivity for molten SnS at the peak of radiation for the temperature range of 890–950 °C. Corresponding estimates of radiative thermal conductivity suggest its minor contribution to the overall thermal conduction of molten SnS.

Melt Crystallization Behavior of Injection-Molded High-Density Polyethylene Based Upon a Solidification Kinetic Analysis

Rheological characterization and an in-situ temperature measurement were carried out to investigate the solidification behavior and crystallization kinetics of injection-molded (IM) high-density polyethylene (HDPE) samples. The temperature profiles obtained via the enthalpy transformation method (ETM) were also used to disclose the effect of cooling rate on the solidification kinetics during the IM process. A four parameter model (FPM) was developed in the present work, based on a three-parameter model (TPM) we proposed previously, with the FPM shown to have an obviously better fitting effect. It was seen that heat transfer at locations χ ≤ 0.5 was primarily dominated by a thermal conduction mechanism, which was unlike the situation when χ > 0.5, with χ being the fractional distance from the mold surface to the center. The results of the present study are suggested to be of significance to further research on the correlation between microstructures and properties of IM products.

On plane thermoelastic waves in hemitropic micropolar continua

Рассматриваются связанные термические и динамические уравнения гемитропной термоупругой микрополярной среды относительно подлежащих определению полей перемещений, микровращений и температурного инкремента. Механизм теплопроводности предполагается термодиффузионным. Определяющие постоянные гемитропного термоупругого тела редуцированы к минимальному набору, обеспечивающему его термоупругую полуизотропность. Изучаются решения связанных уравнений в форме распространяющихся плоских волн. Определены их пространственные поляризации. Получено алгебраическое бикубическое уравнение для определения волновых чисел и установлено, что для связанной волны в действительности существуют ровно три нормальных комплексных волновых числа. Исследуется также холодная атермическая волна. Пространственные поляризации в этом случае образуют (вместе с волновым вектором) триэдр взаимно ортогональных направлений. Для атермической волны находятся (в зависимости от случая) либо два вещественных нормальных волновых числа, либо одно.

Specific heat measurements of CNT nanofluids

There have been numerous research studies regarding nanofluid for last two decades. The major topic was mostly focused on the thermal conductivity increase with small particle concentration. However, a conclusion regarding thermal conductivity increase and heat transfer mechanism was not achieved yet due to large discrepancies among the experimental results. In parallel with the thermal conductivity, the specific heat is a crucial thermal property in the application of the nanofluid but wide research has not been conducted. In this study, the specific heat of carbon nanotube (CNT) nanofluids were experimentally measured and the experimental results were compared with possible models to explain the results. The CNT nanofluids were produced with DI water and ethylene glycol (EG) as base fluids. Since CNTs are not mixed with DI water, gum Arabic was used as a surfactant. On the other hand, CNTs were mixed with EG without any surfactant. The specific heat was measured with Calvet calorimeter. Temperature and heat flow sensors of Calvet calorimeter were calibrated with reference materials. The experiment method was validated by comparing the base fluids with reference values showing very good agreement. The measured specific heat of CNT nanofluids decreased with particle concentration due to the lower specific heat of CNT than that of base fluids. It is found that the thermal equilibrium model of nanofluid specific heat successfully predicts the experimental results. This result can be very useful in generating desired working fluids in the diverse thermal systems because the control of the specific heat of nanofluid can be achieved.

Experimental Phonon Dispersion and Lifetimes of Tetragonal CH3NH3PbI3 Perovskite Crystals.

Hybrid organic-inorganic perovskites were reported to have ultralow thermal conductivity in recent studies. In this Letter, we report the first experimental phonon dispersion and lifetimes of tetragonal CH3NH3PbI3 single crystals at both 200 and 300 K by high-energy resolution inelastic X-ray scattering, which enables a thorough understanding of the underlying mechanisms for the ultralow thermal conductivity. Notably, we observed unusual and significant phonon dips along the [100] and [110] directions at both temperatures. The ultralow thermal conductivity can be attributed to small group velocities due to ultrasoft acoustic modes and short phonon lifetimes originating from the strong acoustic-optical coupling. We further provided the structural origins for these peculiar phonon features. Moreover, our results and interpretation are consistent with the reported temperature-dependent trend for thermal conductivity of CH3NH3PbI3. Our work offers critical guidelines for accelerating the design and discovery of novel hybrid materials for energy applications including photovoltaics and thermoelectrics.

EFFICIENCY IMPROVEMENT OF DUAL COMPLETION BY ROD PUMP AND ELECTRIC CENTRIFUGAL PUMP UNITS

Background The introduction by the oil companies of programs energy efficiency and increasing the profitability of oil field development in modern market conditions causes the wide spread of dual completion technology using pumping units. Oil production by rod units, including rod and electric centrifugal pumps, is one of the most common ways of developing oil fields with a significant differentiation and technical specifications of the reservoir fluids. The specific features of the development of the upper productive reservoir by a rod pump in this dual completion scheme are associated with low reservoir productivity due to the low mobility of the reservoir oil under natural temperature and pressure conditions. Aims and Objectives To research the possibility of natural heating of the upper productive layer due to the thermal energy of the lower layer and the heat produced by the rod and electric centrifugal pumps. Results A mathematical model has been developed for calculating the temperature field of the «well - productive formations - rod pump - electrical centrifugal pump» system taking into account the mechanisms of convective heat transfer, thermal conductivity, thermodynamic effects, and heat generation in the electrocentrifugal and rod pumps. A method for calculating the temperature of the fluid at the wellhead has been developed. By numerical solution of the heat conduction equation, a detailed study of heat distribution process in the upper productive formation interval was carried out. It is shown that due to the relatively small thermal conductivity of the fluid in the annular space of the well there is a significant temperature gradient, causing a decrease in temperature in the radial direction. To improve the heating of the upper productive formation efficiency with heated fluid in tubing, an improved design of dual completion unit using a heat exchanger to increase the heat transfer rate in the well has been proposed. The efficiency of the implementation of the periodic pumping mode by a rod pump using the proposed dual completion unit is shown.

Pressure Effect of the Vibrational and Thermodynamic Properties of Chalcopyrite-Type Compound AgGaS₂: A First-Principles Investigation.

To explore the structural, vibrational, and thermodynamic properties of the chalcopyrite-type compound AgGaS2 under pressure, we applied hydrostatic pressure to the relaxed compound based on the first principles calculation and quasi-harmonic approximation. The structural parameters, including lattice constants and bond lengths decrease monotonically with the increasing pressure. The phonon dispersion curves under various pressures reveal the structural phase transition of chalcopyrite-type compound AgGaS2 at about 4 GPa. The intrinsic mechanism of thermal conductivity for the chalcopyrite-type compound AgGaS2 has been shown with phonon anharmonicity. The frequencies of the optical phonons at the center point Γ of the first Brillouin zone were calculated with the longitudinal optical–transverse optical (LO–TO) splitting mode. The dependence of the frequencies of the optical phonons on the pressure provides the information for the Raman spectroscopic study under high pressure. The pressure dependence of the Grüneisen parameters indicates that the instability of chalcopyrite-type compound AgGaS2 is associated with the softening of the acoustic phonon modes at around the center point Γ. The thermal conductivity for chalcopyrite-type compound AgGaS2 could be reduced by applying external pressure. The various thermodynamic properties, such as the Helmholtz free energy, entropy, and heat capacity, at different temperatures and pressures were discussed and analyzed based on the phonon properties.

On model for three-dimensional Carreau fluid flow with Cattaneo–Christov double diffusion and variable conductivity: a numerical approach

The present article investigates the steady 3D flow of a Carreau liquid influenced by bidirectional stretching surface. In the presence of Cattaneo–Christov double diffusion with temperature-dependent thermal conductivity, the heat and mass transfer mechanisms have been scrutinized. The alteration of nonlinear PDEs to nonlinear ODEs is equipped via apposite conversions and then resolved numerically by means of bvp4c scheme. The graphical assessment is exposed to portray the essential features of somatic parameters on Carreau liquid temperature and concentration distributions. This study indicates that the variable conductivity parameter enhances the liquid temperature, while the thermal relaxation time parameter and Prandtl number are diminishing functions of temperature field. Furthermore, our results illustrate that the concentration relaxation time parameter and Schmidt number diminish the concentration field. The assertion of present exploration is asserted by emergent assessment with former outcomes vacant in prose, which sets a benchmark for execution of computational methodology. Moreover, graphical assessment is also vacant for two altered techniques, namely analytical (HAM) and numerical (bvp4c).

Preparation and thermophysical properties of Ti4+ doped zirconia matrix thermal barrier coatings

There is an upper limited temperature of about 1200 °C for yttrium oxide partially stabilized zirconia coatings. With the increasing temperature of inlet gas (>1200 °C), new thermal barrier coatings are urgently needed to replace traditional yttrium oxide partially stabilized zirconia. Ti4+ ion doping has been recognized as an effective method to achieve this. In this work, a novel ceramic coating with low thermal conductivity at 1600 °C is synthesized, and the microstructure, chemical components, phase structures are analyzed. The thermal diffusivity and thermal conductivity of the as-sprayed coatings initially exhibited low values up to 1600 °C. The mechanism of low thermal conductivity is analyzed. The oxygen vacancies effectively reduce thermal conductivity, which can be proved by the ablation experiment (at 1600 °C). The effect of lattice defects caused by differences in the masses and radii of Ti4+ and Zr4+ ions on the phonon scattering coefficient is analyzed quantitatively. These results indicate that this coating is a promising candidate for thermal barrier coatings at high temperatures.

Efficiently Controlling the 3D Thermal Conductivity of a Polymer Nanocomposite via a Hyperelastic Double‐Continuous Network of Graphene and Sponge

AbstractGraphene‐reinforced polymer composites with high thermal conductivity show attractive prospects as thermal transfer materials in many applications such as intelligent robotic skin. However, for the most reported composites, precise control of the thermal conductivity is not easily achieved, and the improvement efficiency is usually low. To effectively control the 3D thermal conductivity of graphene‐reinforced polymer nanocomposites, a hyperelastic double‐continuous network of graphene and sponge is developed. The structure (orientation, density) and thermal conductivity (in‐plane, cross‐plane) of the resulting composites can be effectively controlled by adjusting the preparation and deformation parameters (unidirectional, multidirectional) of the network. Based on the experimental and theoretical simulation results, the thermal conduction mechanism is summarized as a two‐stage transmission of phonons. The in‐plane thermal conductivity increases from 0.175 to 1.68 W m−1 K−1 when the directional compression ratio increases from 0% to 95%, and the corresponding enhancement efficiency exceeds 300. The 3D thermal conductivity reaches a maximum of 2.19 W m−1 K−1 when the compression ratio is 70% in three directions, and the graphene content is 4.82 wt%. Moreover, the thermal conduction network can be largely prepared by power‐driven roller equipment, making the composite an ideal candidate for sensitive robotic skin for temperature detection.