Large Thermal Conductivity Research Articles

Structural, elastic, anisotropic, electronic, thermal properties and tensile strength of AlTM2Ti (TM = Ni, Fe, Cu, Co, Au) studied by first-principles calculations

In the present work, we systematically studied the elastic properties, elastic anisotropy, electronic properties and thermal properties of ternary AlTM2Ti (TM = Ni, Fe, Cu, Co, Au) intermetallics by using first-principles calculations. The optimize the lattice parameters and phonon frequency confirm that the five intermetallics display dynamic stability. The elastic modulus of AlTM2Ti was calculated using the VRH approximation. On the one hand, according to the values of Poisson's ratio and G/B, AlTM2Ti intermetallics show the ductile behavior. In terms of density of states and electronic structure, AlTM2Ti (TM = Ni, Fe, Cu, Co, Au) show the metallic properties. Moreover, AlFe2Ti and AlCo2Ti show the magnetic properties. AlFe2Ti shows the strongest chemical bond, the largest hardness, the smallest anisotropy, the strongest deformation resistance and the largest thermal conductivity. The stress–strain curves are calculated and the ideal tensile strength follows the sequence: Al2FeTi > AlCo2Ti > AlNi2Ti > AlAu2Ti > AlCu2Ti.

Parameter study for oil spray cooling on endwindings of electric machines via Eulerian–Lagrangian simulation

The demand for larger power density and torque for the power traction motors used in electrified transportation puts forward a requirement for better thermal management methods. Spray cooling is a promising direct liquid cooling technique that has been proved to possess high heat removal capability in previous research. This paper investigates the heat transfer characteristics of spray cooling on endwindings of electric machines via numerical simulation through an Eulerian–Lagrangian approach. The utilized numerical models and calculated results are validated with experimentally measured data. The influence of different parameters and options involved in the simulation settings on the final results, like the stream numbers for the spray injector, the constant heat flux versus constant temperature thermal boundary condition, the influence of splashing, the effect of heat conduction in the endwindings and the Saffman lift force, only solving the energy equation for the air after its flow field reaches a steady-state, are evaluated. Parameter analyses are also conducted for operation conditions, configuration of the spray nozzles, and material properties of the coolant liquid. It is found that larger flow rate, smaller droplet size, lower spray height, more nozzle numbers, larger thermal conductivity and smaller viscosity of the coolant liquid tend to increase the overall heat transfer performance.

Compact Aluminium Foam Heat Exchangers

The aim of this study was to investigate the potential application of metal foams in shell-tube recuperators. A356 aluminium foam was cast around the internal and external surfaces of a thin-walled copper tube to enhance heat transfer between separated water streams at different temperatures. The results demonstrated that the aluminium foam drastically increased heat transfer efficiency due to its large volumetric surface area and high thermal conductivity. In the shell-tube foam recuperators, a maximum heat transfer efficiency of 48.1% was observed, compared to only 12.2% for a single copper tube without metal foam. The pressure drop across the external foam increased with the flow rate, from an average value of 1.19 kPa at 1.0 L/min to 7.36 kPa at 3.0 L/min. These findings suggest that metal foams have great potential for use in shell-tube recuperators, which could significantly improve the efficiency of heat transfer in various industrial and engineering applications.

Topology optimization of a thermal cloak in the frequency domain

Designing wideband thermal cloaks remains a challenge, especially at high frequencies. We propose an optimization approach for the design of a thermal cloak for an arbitrary object with large thermal conductivity (copper), in a given frequency band and for a specific diffusion direction. Cloaking performance is assessed as a function of frequency (in the optimal direction) and as a function of angle (at the optimal frequency). Near-perfect cloaking is achieved over a finite frequency band, and, moreover, the thermal cloak performs well in the time domain, including in the transient regime, irrespective of the initial temperature distribution. Interestingly, this specially optimized cloak also works fairly well for other objects with large thermal conductivity but breaks down for those of low thermal conductivity.

Influence of the thermal conductivity of different CuCr0.8 substrate state on the formability of laser directed energy deposition Inconel718 single track

One effective method for fabricating liquid rocket engine thrust chambers involves using laser directed energy deposition (LDED) to produce Cu/Ni bimetals by depositing Ni-based superalloys onto a High Strength and High Conductivity (HSHC) copper alloy. However, it is difficult to fabricate strengthening layer to the surface of HSHC Cu alloy due to its extremely high thermal conductivity. This paper investigates the influence of states of the HSHC CuCuCr0.8 substrate e.g., as-built of LPBF, rolled and annealed, and direct age after LPBF at 480 °C for 4 h, on the formability of In718 single track by LDED. The results indicate that the thermal conductivity is significantly influenced by the state of the HSHC CuCuCr0.8 substrate. At room temperature and 300 °C, the order of the thermal conductivity from low to high is as-built of LPBF, rolled and annealed, and direct age after LPBF at 480 °C for 4 h, while at 500 °C, the as-built of LPBF substrate has the lowest thermal conductivity, while the rolled and annealed substrate has the largest thermal conductivity. Thermal conductivity has a significant effect on the formability of LDED. The LPBF-ed substrate has the best formability, with a minimum Ra value of 9.72 ± 0.12 μm, a maximum deposition depth of 193.00 ± 4.32 μm, and a minimum grain size of 5.2 ± 2.6 μm because of its smallest thermal conductivity. These findings demonstrate that using an as-built LPBF-ed substrate can improve the LDED formability of the In718 single track compared with the other two substrates. This paper presents a new approach for LDED on alloy surfaces with high thermal conductivity.

Morphological engineering of SiO2 interlayer towards meliorative dielectric and thermal properties in β‐SiCw@SiO2/PVDF composites

AbstractPolymers with high thermal conductivity (TC) and dielectric constant (ε) but low dielectric loss show enormously potential applications in electrical power systems. To enhance the ε and TC while significantly suppressing the loss of β‐silicon carbide whisker (β‐SiCw)/polyvinylidene fluoride (PVDF), in this study, SiO2 shells with different morphologies were constructed on the superficies of β‐SiCw via a facile oxidation method in air atmosphere. The SiO2 shell’ morphology on the TC and dielectric performances of the β‐SiCw@SiO2/PVDF are systematically explored as a function of the frequency and filler loading. The β‐SiCw@SiO2/PVDF demonstrate astonishingly restricted loss and meliorative TC when compared to the β‐SiCw/PVDF because the SiO2 shell not only suppresses the β‐SiCw from direct contact with one another and impedes the long‐distance charges migration thereby resulting in rather low loss and leakage conductivity, but also restrains the thermal interfacial resistance via increased interfacial compatibility and interactions. The concurrent enhancement effect in the ε and TC but low loss of the composites is more evident in the crystalline SiO2 interlayer with a high TC compared with amorphous one. These results provide insight into the exploration of composite nanodielectrics with large TC and ε but low loss for applications in electrical power systems.Highlights Core@shell structured β‐SiCw@SiO2/PVDF were obtained by a facile oxidation. The great reduction in both loss and conductivity is attributed to the SiO2 shell. Crystalline SiO2 shell can improve the composites' thermal conductivity. The SiO2 shell prevents long‐range carrier migration.

Droplet heating and evaporation: A new approach to the modeling of the processes

A new model for mono-component droplet heating/evaporation is developed, tested, and applied to the analysis of in-house experimental data. The new model links the previously developed liquid phase model, using the analytical solution to the heat transfer equation at each time step, and the gas phase model, using the solution to the equations of the conservation of mass, momentum, and energy leading to an explicit expression for the Nusselt number and implicit expression for evaporation rate of the droplet. The latter expressions are used as boundary conditions for the liquid phase model. The new model is verified using a comparison between its predictions of the droplet temperatures and radii for very large liquid thermal conductivity [1000 W/(m K)] and those of the model, using the assumption that the thermal conductivity of liquid is infinitely large. The closeness between the predictions of these models supports the reliability of both. The model is validated using the experimental data obtained at the Heat and Mass Transfer laboratory of Tomsk Polytechnical University with regard to the heating/evaporation of droplets. The deviations between the measured and predicted droplet radii and temperatures in most cases are shown to be within experimental error margins.

Constructing 3D cross-connected networks in spongy MgO/Ni/MWCNT composites to synchronously boost microwave absorption and thermal conductivity

To address the issues of electromagnetic (EM) radiation and waste heat engendered in modern electrics, we pioneer the utilization of 3D cross-connected MgO/Ni/MWCNT networks as a magnetic-dielectric-dual-loss filler with electron-phonon dual thermal carriers to synchronously boost EMW absorption and thermal conductivity. The carbothermic reduction of magnesium oxalate/nickel oxalate core-shell hexahedra produced from a cryogenic precipitation/precipitate transfer route causes the in-situ self-catalytic growth of MWCNTs on the surfaces of porous MgO/Ni composites. The component, microstructure, and texture of the composites and their resulting performance can be tuned by some dynamic factors (i.e., [Ni2+], Ts, etc.). The spongy MgO/Ni/MWCNT composites formed under [Ni2+] = 1.6 M and Ts = 600 ℃ exhibit the optimal EMW absorption and thermal conductivity. Compared with the most previously reported materials, they have a larger thermal conductivity (3.61 W/(m⋅K)), a larger EAB/d value (∼4.61 GHz/mm), stronger absorption (−51.33 dB), and a smaller thickness (1.7 mm). The synchronously enhanced properties are attributed to magnetic/dielectric dual losses, electron/phonons dual thermal carriers, and 3D interconnected networks in MgO/Ni/MWCNT composites, which generate the strong attenuation, good low-frequency matching performance, and continuous heat transfer path. Noteworthy, spongy MgO/Ni/MWCNT composites are promising as a multifunctional filler to address the issues of EMI and thermal dissipation.

Significance of Joule heating for radiative peristaltic flow of couple stress magnetic nanofluid

Nanofluids are gaining more importance in heat transfer applications because of their excellent thermal features, such as larger thermal conductivity, enhanced convective heat transfer coefficients, and better thermal stability. In a range of technical applications, including cooling structures, exchangers for heat, and thermal energy storage systems, they have the potential to increase the efficiency of heat transfer. Main aim is to address the peristalsis of couple-stress nanomaterial in a channel subject to partial slip conditions. Channel walls are compliant. Buongiorno model for thermophoresis and Brownian motion is accounted. Energy equation is modeled with considering the features of Ohmic heating and thermal radiation. The model is initially developed as a set of non-linear partial differential equations (PDEs). Using th appropriate similarity transformations, these are turned into a set of dimensionless ordinary differential equations (ODEs). The governing system of ODEs is solved by using numerical technique NDsolve. Detailed analysis for concentration, temperature, velocity, heat transfer rate, pressure gradient, skin friction coefficient, Sherwood and Nusselt numbers are analyzed. Reduction of problems with small Reynolds number and large wavelength is made. Concentration and velocity have reverse trends for higher couple stress fluid parameter. In addition, the radiation and Hartman number behave in an opposite sense for temperature. Rate of heat transfer is significantly accelerated by nanoparticles. Further, Nusselt number increases enhances via Brinkman number.

Thermo-optic coefficient of chemical vapor deposited graphene multilayers

Observation of strong inelastic light-matter interaction and large thermal conductivity of novel two-dimensional graphene has invigorated the present research. Herein, the thermo-optic coefficient of graphene multilayers is obtained from their nonlinear refraction coefficients calculated from Z-scan experiment in closed aperture geometry. The refractive optical nonlinearity was obtained employing a continuous wave He-Ne laser and occurs due to thermal lensing effect. The graphene multilayers behave as the diverging (concave) lens giving rise to self-defocusing effect leading to negative sign of refractive nonlinearity. Accordingly, a large negative thermo-optic coefficient (-2.43×10-3 K-1) inferred for the graphene multilayers suggests its utility for thermo-optic device applications.
 Keywords: Graphene, Optical Nonlinearity, Thermal lens, Self-defocusing, Thermo-optic coefficients

Electron cooling in graphene enhanced by plasmon-hydron resonance.

Evidence is accumulating for the crucial role of a solid's free electrons in the dynamics of solid-liquid interfaces. Liquids induce electronic polarization and drive electric currents as they flow; electronic excitations, in turn, participate in hydrodynamic friction. Yet, the underlying solid-liquid interactions have been lacking a direct experimental probe. Here we study the energy transfer across liquid-graphene interfaces using ultrafast spectroscopy. The graphene electrons are heated up quasi-instantaneously by a visible excitation pulse, and the time evolution of the electronic temperature is then monitored with a terahertz pulse. We observe that water accelerates the cooling of the graphene electrons, whereas other polar liquids leave the cooling dynamics largely unaffected. A quantum theory of solid-liquid heat transfer accounts for the water-specific cooling enhancement through a resonance between the graphene surface plasmon mode and the so-called hydrons-water charge fluctuations-particularly the water libration modes, which allows for efficient energy transfer. Our results provide direct experimental evidence of a solid-liquid interaction mediated by collective modes and support the theoretically proposed mechanism for quantum friction. They further reveal a particularly large thermal boundary conductance for the water-graphene interface and suggest strategies for enhancing the thermal conductivity in graphene-based nanostructures.

Thermoelectric hotspot cooling using thermally conductive fillers

The commercial applications of thermoelectric coolers (TECs) are limited to niche markets owing to their low cooling efficiency. For hotspot cooling, in which the Fourier heat is added to the Peltier heat, thermoelectric (TE) materials should possess both large power factors and high thermal conductivities. However, the interdependence among TE material properties renders the development of such materials challenging. Herein, we provide a design strategy of TEC that addresses these material challenges at the device level. As proof of concept, we demonstrate that the cooling capability of filler-embedded TECs (F-TECs), which consist of TE legs with large power factors and a thermally conductive filler material, can be significantly improved compared to that of conventional TEC, owing to the 42.5% enhanced effective thermal conductance. Additionally, infrared imaging shows that the filler can decrease the temperature gradient and maximum temperature of the TE legs, which may improve the thermal stability of the TECs. Our proposed F-TEC concept can facilitate the development of effective TEC devices that can be widely used in hotspot cooling applications, such as in microprocessors, batteries and photovoltaic cells.

One‐Particle and Excitonic Band Structure in Cubic Boron Arsenide

Cubic BAs has received recent attention for its large electron and hole mobilities and large thermal conductivity. This is a rare and much desired combination in semiconductor industry: commercial semiconductors typically have high electron mobilities, or hole mobilities, or large thermal conductivities, but not all of them together. Herein, predictions from an advanced self‐consistent many‐body perturbative theory are reported and it is shown that with respect to one‐particle properties, BAs is strikingly similar to Si. There are some important differences, notably there is an unusually small variation in the valence band masses. With respect to two‐particle properties, significant differences with Si appear. The excitonic spectrum for both q = 0 and finite q is reported, and it is shown that while the direct gap in cubic BAs is about 4 eV, dark excitons can be observed down to about ≈1.5 eV, which may play a crucial role in application of BAs in optoelectronics.

Highly thermally conductive flexible insulated PI/BNNS@rGO nanocomposite paper with a three-dimensional network bridge structure

The sharp increases in power consumption and heating capacity caused by the emergence of intelligent electronic devices necessitate the development of highly thermally conductive thermal interface materials (TIMs) with good heat dissipation properties. Boron nitride nanosheets (BNNS) are ideal materials with high thermal conductivity. Hence, it should be possible to produce flexible high thermal conductivity nanocomposites with a three-dimensional network containing ultrathin, large, and uniformly thick BNNS. In this study, large-scale (1–1.5 µm) fewer-layered (2 nm) BNNS with a high yield of 80% were prepared through the separation of a NaOH–LiCl aqueous solution by a hydrothermal method and in ball milling. Highly thermally conductive insulating nanocomposite paper with a three-dimensional bridging structure of two-component nanosheets filled with nanofibers was fabricated by a simple electrospinning–electrospraying technique. The mechanical properties of the polyimide (PI)/BNNS@reduced graphene oxide nancomposite paper were improved by 168% as compared with those of the PI/BNNS composite. With an increase in the BNNS content, a layered microstructure similar to that of natural nacre was produced, which resulted in a large in-plane thermal conductivity of 16.92 W/m·K. The described method can facilitate the design of TIMs with good electrical insulation properties, thermal stability, and flexibility.

Elastic, electronic, optical and thermoelectric properties of Ca5Si2N6 and Sr5Ge2N6 ternary nitrides

In order to expand the scope of application of ternary nitrides and explore more thoroughly their potential as novel materials, ab initio calculations founded on density functional theory are executed to examine the mechanical, electronic, optical and transport properties of Ca5Si2N6 and Sr5Ge2N6 nitrides. These compounds are thermodynamically stable established on coherent energy and enthalpy of formation, with Ca5Si2N6 having the best stability. The estimated values of elastic constants and their derived properties suggest that the items under examination are mechanically stable, ductile and elastically anisotropic. Electronic properties have been evaluated using the GGA and TB-mBJ exchange-correlation potentials. The band structure calculated via TB-mBJ reveals that these nitrides are semiconductors, where Ca5Si2N6 has a direct energy gap of 3.55 eV and Sr5Ge2N6 an indirect band-gap of 3.15 eV. By combining Debye temperature and band-gap values, it was concluded that the nitridosilicate compound can be used as phosphor material. According to the optical response, which was examined in terms of dielectric function, complex refractive index, absorption, reflectivity and energy damage function, the potential applications of Ca5Si2N6 and Sr5Ge2N6 have been discussed. Further, thermoelectric properties are computed utilizing the semi-classical Boltzmann transport notion. The obtained results show a large strength operator and depressed thermal conductivity driving to rising figure of merit values, particularly at 300 K (0.978–0.986), signifying that the studied ternary nitrides are favorable nominees for thermoelectric employments at both low and room temperatures.

Simultaneous Enhancement of the Power Factor and Phonon Blocking in Nb-Doped WSe2

Transition-metal dichalcogenide WSe2 is a potentially good thermoelectric (TE) material due to its high thermopower (S). However, the low electrical conductivity (σ), power factor (PF), and relatively large lattice thermal conductivity (κL) of pristine WSe2 degenerate its TE performance. Here, we show that through proper substitution of Nb for W in WSe2, its PF can be increased by ∼10 times, reaching 5.44 μW cm-1 K-2 (at 850 K); simultaneously, κL lowers from 1.70 to 0.80 W m-1 K-1. Experiments reveal that the increase of PF originates from both increased hole concentration due to the replacement of W4+ by Nb3+ and elevated thermopower (S) caused by the enhanced density of states effective mass, while the reduced κL comes mainly from phonon scattering at point defects NbW. As a result, a record high figure of merit ZTmax ∼0.42 is achieved at 850 K for the doped sample W0.95Nb0.05Se2, which is ∼13 times larger than that of pristine WSe2, demonstrating that Nb doping at the W site is an effective approach to improve the TE performance of WSe2.

Characteristics of thermophysical parameters in the Wugongshan area of South China and their insights for geothermal genesis

The Wugongshan area is rich in medium–low temperature convective geothermal resources, among which there are more than 10 geothermal fields in Wentang, Wanlongshan, Wenjia, Hongjiang, etc. There are few basic geothermal geological studies in the geothermal fields and their peripheral areas; thus far, no systematic research work into the thermophysical parameters has been carried out. In this paper, 85 rock samples were collected from the surface and boreholes covering the strata and magmatic rocks in the study area. The results show that the average radioheat generation rate, the average thermal conductivity, and the average specific heat are 0.24–5.49 (μW/m3), 1.995–4.390 (W/mK), and 1.318–4.829 (MJ/m3K), respectively. The average thermal diffusivity ranges from 1.115 to 1.611 × 10-6 m2/s. The highest radioheat generation rate is Jurassic granite, and the lowest is quartz vein. The largest thermal conductivity and specific heat is the siliceous quartzite, and the smallest is the quartz vein. The highest thermal diffusivity is Cambrian metamorphic mica schist, and the lowest is siliceous quartzite. The radioactive heat generation rate, thermal conductivity, specific heat, and thermal diffusivity are closely related to the chemical composition, mineral composition, rock fabric, porosity, water content, and temperature and pressure conditions of rocks in the whole area. There is a linear relationship between thermal conductivity (K) and thermal diffusivity (κ), and the correlation equation is K = −0.3144k + 3.2172. Combined with the characteristics of thermophysical parameters, the genetic theory of deep crust heat generation + structural heat accumulation + siliceous quartzite heat conduction + granite heat preservation is preliminarily proposed.

Predicting lattice thermal conductivity of semiconductors from atomic-information-enhanced CGCNN combined with transfer learning

Rapid identification of lattice thermal conductivity of semiconductors from their crystal structure is required in the discovery of functional materials. A promising strategy is using a machine learning method based on a first-principles dataset, which, however, suffers from the dilemma of too little data available. In this work, the crystal graph convolutional neural networks (CGCNN) model was improved by enhancing the information of atomic descriptors (for short CGCNN-D), and the transfer learning (TL) method was combined to overcome the problem of small datasets. It is found that the CGCNN-D has improved predicting performance for both electronic bandgap with large data volume and thermal conductivity with small data volume, with the mean absolute error reducing 7% and 10%, respectively, indicating the importance of the improved atomic description. Applying TL with electronic bandgap as a proxy into the CGCNN-D further upgrades the prediction accuracy for thermal conductivity that has only 95 pieces of data, yielding 19% decrease in the mean absolute error as compared to the original CGCNN. The trained CGCNN-D-TL model was used to quickly estimate the thermal conductivities of thousands of semiconductors, and the materials identified with potentially high thermal conductivity were further screened by the optimized Slack model. Finally, the most promising BC2N was discovered and then confirmed by the first-principles calculations, which shows room-temperature thermal conductivities of 731, 594, and 500 W m−1 K–1 along the three principal axes of its lattice structure.

Constructing “sesame‐biscuit” structured hybrids with SiC and Ag particles for enhancing thermal conductivity and dielectric constant of elastomer composites

AbstractWith the development of multifunctional and miniaturized integrated circuits, polymeric materials with large dielectric constant and high thermal conductivity have major significance in modern electronic industry. In this work, “sesame‐biscuit” structured thermally conductive hybrids were prepared by polydopamine (PDA) modification and in‐situ electroless silver (Ag) plating on the surface of silicon carbide (SiC), referred as SiC‐P‐Ag. Then, the SiC‐P‐Ag was incorporated into epoxidized natural rubber (ENR) matrix to yield polymeric composites (ENR/SiC‐P‐Ag) with excellent thermal and dielectric properties. It was found that PDA improved the interfacial compatibility and reduced the interface thermal resistance between SiC and ENR matrix. In addition, the formation of heat conduction channels and conductive Ag nanoparticles caused an increase in thermal conductivity of the 50 vol% ENR/SiC‐P‐Ag composite to 0.5655 W/(mK), which was 540% that of the pure ENR (0.1048 W/(mK)). Meanwhile, the 50 vol% ENR/SiC‐P‐Ag composite showed a low AC conductivity of 2.98 × 10−11 S/cm and a high dielectric constant of 35 at 10 Hz. It is noteworthy that this simple and efficient route for preparing ENR/SiC‐P‐Ag composites can also be applied in other fillers for simultaneously enhancing dielectric constant and thermal conductivity of polymeric composites.

First-principles study on the lattice thermal conductivity of layered Dirac semimetal BeN4

A novel two-dimensional Dirac semimetal BeN 4 was successfully synthesized in a recent experiment (Bykov et al. Phys. Rev. Lett. 126, 175501 (2021)). Here, we have studied the lattice thermal conductivities of bulk BeN 4 by the first-principles calculations. Due to the anisotropic crystal structure, the lattice thermal conductivities of BeN 4 show large anisotropy and the values at 300 K are 91.5, 42.1, and 5.0 W/m ⋅ K along the armchair, zigzag, and cross-plane directions respectively based on the three-phonon scattering. We also find that four-phonon scattering only reduces the lattice thermal conductivities of BeN 4 by about 9%. Due to the relatively weak Be-N bonds and complex crystal structure, BeN 4 has small phonon group velocities and large phonon anharmonicity, resulting in its much lower lattice thermal conductivity than that of hexagonal boron nitride and graphene. Our work would be helpful for future experimental and theoretical studies on the Dirac semimetal BeN 4 . • The lattice thermal conductivity of Dirac semi-metal BeN 4 was studied by the first-principles calculations. • BeN 4 has large anisotropic thermal conductivity due to its anisotropic crystal structure. • Despite the light elements, BeN 4 has a relatively low lattice thermal conductivity due to its large anharmonicity.