Thermal Conduction Mechanism Research Articles

Elucidating thermal conductivity mechanism of Al-9Si based alloys with trace transition elements (Mn, Cr, V)

The effects of trace transition elements (Mn, Cr, V) on the solidification microstructure and the thermal conductivity of Al-9Si based alloys have been investigated. It was found that the effect of Mn, Cr, V on the solidification behavior and solidification microstructure is mainly related to α-Al grains via affecting heterogeneous nucleation and growth restriction, while no significant effect on the solidification behavior and microstructure of eutectic Si was observed. Compared with factors including grain size of α-Al and morphology characteristics (e.g. size, spacing, and shape factor) of eutectic Si, the solid solubility in Al matrix is confirmed to be one of the most important factors affecting thermal conductivity. Due to the larger partition coefficient of V compared with Cr and Mn, the solid solubility and the lattice distortion of α-Al in Al-9Si-xV alloy were increased significantly, thus the thermal conductivity of alloy was decreased steeply from 171.7 to 152.3 W/m·K with the content of V increasing from 15 to 1010 ppm. A linearly fitted curve reflecting the relationship between the content of the transition elements and the thermal conductivity of alloys has been proposed, which can be used to optimize the alloy composition in subsequent remelting or recycling process. Furthermore, two thermal conductivity models (H-S model and Maxwell model) also have been used to evaluate the effect of Mn, Cr, V on thermal conductivity, in which the thermal conductivity of α-Al was corrected based on the measured solid solubility. In this way, the data correlation between the experimental and fitted value based on H-S model and Maxwell model is increased from 0.18 to 0.96. These obtained results could be helpful to precisely control the composition and further improve the thermal conductivity of Al-Si based alloy during the primary production and / or recycling process.

Investigation on thermal conductivity property and hydration mechanism of graphene-composite cement for geothermal exploitation

In the process of geothermal exploitation, the thermal conductivity of casing cement affects the heat exchange efficiency of geothermal wells to a great extent. Because of this, combining with theoretical analysis and experimental study, this paper documented the development of a graphene-composite cement material for geothermal exploitation, based on the evaluation of thermal conductivity property and its hydration mechanism. Cement additives, including NS-600 and 1200 mesh silicon carbide (SiC) as basic thermal conductive fillers, graphene nanosheets (GNSs) aqueous solution for synergistic thermal conduction, and early strength agent (ZQ-4), structural stabilizer (FL-5), defoamer (MC-6) were utilized. The rheological properties, stability, compressive strength, and thermal conductivity of cement slurry were evaluated by laboratory instruments. X-ray diffraction (XRD) and environmental scanning electron microscope (ESEM) were used to analyze the phase composition and micro morphology of the optimized formulas, and the hydration and thermal conductivity mechanism of the graphene-composite cement was obtained. The thermal conductivity of the two optimized formulas obtained in the experiment were 2.141 W/(m · K) and 1.862 W/(m · K) respectively. The study results provide a new method to improve the properties of potential casing cement for enhancing the heat exchange efficiency of deep geothermal exploitation.

Experimental and theoretical investigation of an innovative composite nanofluid for solar energy photothermal conversion and storage

Molten salts play a key role in the heat transfer and thermal energy storage processes of concentrated solar power plants. A novel composite material was prepared in this work by adding micron-sized magnesium particles into Li2CO3-Na2CO3-K2CO3 molten salt, the heat transfer and thermal energy storage properties of the composites were studied experimentally. A stable composite nanofluid can be obtained, and a thermal conductivity of 0.728 W/(m·K) at 973 K with an enhancement of 31% is achieved for the Mg/molten carbonate nanofluid. And the strengthening mechanism of thermal conductivity was revealed by using ab-initio molecular dynamics method. It is found that the main bonding interactions exist between Mg and O atoms at the surface of Mg particles. A compressed ion layer with a more compact and ordered ionic structure is formed around Mg particles, and the Brownian motions of Mg particles lead to the micro-convections of carbonate ions around them. These factors are helpful to the enhancement of thermal conduction with the improved probability and frequency of ion collisions. This work can provide a guidance for further studies and applications on metal/molten salt composites with enhanced heat transfer and thermal energy storage capacity.

An investigation of internal heat flux in SiC/AlN/Epoxy composites by mesoscopic simulation

Polymer-matrix composites play an important role as packaging material in superconducting equipment. The research on thermal conductivity of composites by combining computer numerical simulation and reference data can not only save the expenses but also shorten the period of material development. Herein, the internal heat flow of silicon carbide/aluminum nitride/Epoxy composites was simulated by COMSOL finite element software. And the thermal conductivity calculated by the finite element software is compared with the existing thermal conductivity mechanism. Meanwhile, the influence of the fraction, gap distance, interfacial thermal resistance, etc. on the thermal conductivity of composites was analyzed. In short, it can provide a reference for the thermal conductivity design of polymer composites.

Thermal lagging effect on heat transport within living biological tissues

Thermal lagging effect on heat transport within living biological tissues is explored in the context of different relaxation mechanisms. The governing equations of temperature rooted the delay effect between heat flux and temperature gradient, as well as non-equilibrium effect between blood vessel and peripheral tissues, are constructed separately via the classic dual-phase lag model (DPL) and generalized dual-phase lag model (GDPL). An analytical procedure based on the Laplace transform technique and its limit theorem is then proposed. The expressions and effective values of phase lags rooted different effect have been derived and further estimated. On this basis, the temperature response of a biological tissue with its boundary exposed to a pulse heat flux is analyzed. The detailed parametric study has been conducted to explore the dependence of these phase lags on tissue structures and their effect on heat transport properties. The results highlight the contributions of the delay effect and non-equilibrium effect in temperature prediction and state that the delay effect is dominated for a larger tissue size. Meanwhile a significant difference has also been observed with the boundary conditions that obey different thermal conduction mechanism.

Quantum phonon transport through channels and molecules—A Perspective

Phonon transport is a dominant mechanism of thermal conduction in solids that has been studied for decades. A good understanding of many transport regimes in micro- and nanostructures has been established, including ballistic and diffusive transport, mode softening, or band structure engineering in phononic crystals. However, the limit of quantized transport and the engineering of single transport channels is much less explored. In this Perspective, we discuss concepts and theoretical and experimental progress in the field of quantized phonon transport in channels, such as molecular systems. We particularly highlight open questions and research opportunities that should be within experimental reach. Challenges in experimental sensitivity and control hinder fast experimental progress. Recently, however, heat transport measurements through quantum channels and single molecules have become available at room temperature using break junction techniques. These techniques are well established in the molecular electronics community and have recently been expanded to the measurement of heat transport on the single-molecule level. Given the new experimental capabilities, it is now inviting to address the rather unexplored area of molecular phonon-engineering. Several interesting theoretical predictions concern the realization of the phonon quantum interference effect, suppression of phonon current via the introduction of side groups to molecules, and the construction of a phonon diode device based on molecular anharmonicity and asymmetry. This Perspective should serve the experimental and theory community by listing key challenges, thus a roadmap for making progress in the field of quantized phonon transport.

Mechanisms for thermal conduction in molten salt-based nanofluid

The addition of nanoparticles to molten salt can enhance its utility as a thermal energy storage material for concentrating solar power plants. The heat transfer property of solar salt loaded with Al2O3 nanoparticle is investigated by molecular dynamics simulations. The results show that the introduction of Al2O3 nanoparticles improves the thermal conductivity of solar salt, but its enhancement effect is not as significant as SiO2 nanoparticles. The increased thermal conductivity cannot be explained by the previously proposed hypothesis, such as the Brownian motion of nanoparticles, microconvection of base liquid, ordered layer of base liquid and coulombic potential energy. New insights into heat conduction performance of molten salt based nanofluid are given from the perspectives of material components and heat flux fluctuation modes. It is found that the nanofluid thermal conductivity is mainly contributed by the atom motions and interactions between atoms in Al2O3 nanoparticle, rather than in base fluid. The heat flux is concentrated in nanoparticle, which promotes the heat transfer. Furthermore, the collision involving the work done by the interatomic forces or the virial interaction dominates the heat conduction in nanofluid with lower mass fraction (< 6%), while the potential energy and collision both contribute to the nanofluid with higher mass fraction (> 6%). These new findings make a step forward in understanding the heat conduction mechanisms of molten salt based nanofluid.

Thermal conduction mechanism of ferroelastic Zr‐Y‐Yb‐Ta‐Nb‐O high‐entropy oxides with glass‐like thermal conductivity

AbstractFerroelastic high‐entropy oxides (HEOs) with high fracture toughness and low thermal conductivity are promising topcoat materials for next‐generation thermal barrier coatings (TBCs). However, phonon transmission characteristics in the ferroelastic HEOs are less well understood due to their complicated and highly‐disordered structure. Here we prepared two types of ferroelastic Zr‐Y‐Yb‐Ta‐Nb‐O HEOs and investigated their thermal conduction behaviors at 100–900°C. Hybrid Monte Carlo and molecular dynamic simulations were employed to understand the thermal conduction mechanism in this high‐entropy system. Results show the HEOs present glass‐like, low thermal conductivity (&lt; 1.30 W m–1 K–1 at 100°C), which is attributed to the diffused lattice vibration mode with random polarization distribution. Dominant delocalized phonon modes in the HEOs are beneficial to strengthening phonon scattering and consequently decrease the thermal conductivity significantly. Lower thermal conductivity can be achieved by tailoring the composition and crystal structure of the HEOs to increase the contribution from diffused lattice vibration and “diffusion”. This work provides a fundamental insight into the thermal conduction mechanisms of HEOs, which may facilitate materials design of low thermal conductivity materials for TBCs applications.

Mechanisms of thermal conduction at grain boundaries and in nanocrystalline materials by computational and machine learning techniques

Mechanisms of thermal conduction at grain boundaries and in nanocrystalline materials by computational and machine learning techniques

Thermal conduction mechanism of graphene-like carbon nitride structure (C&lt;sub&gt;3&lt;/sub&gt;N)

As a new graphene-based two-dimensional semiconductor material, C&lt;sub&gt;3&lt;/sub&gt;N has received extensive attention from researchers due to its excellent mechanical and electronic properties. Whether there is any difference in the phonon transport mechanism among different C&lt;sub&gt;3&lt;/sub&gt;N structures remains to be further investigated. Therefore, four kinds of C&lt;sub&gt;3&lt;/sub&gt;N structures with different patterns are constructed in this paper, and their thermal conduction mechanisms are studied by the non-equilibrium molecular dynamics (NEMD) method. The research results are shown as follows. 1) Among these four patterns, the C&lt;sub&gt;3&lt;/sub&gt;N (M3) with the perfect structure has the highest thermal conductivity, followed by M1, and M4 has the lowest thermal conductivity. 2) Moreover, the thermal conductivities of C&lt;sub&gt;3&lt;/sub&gt;N with different patterns have obviously different size and temperature effects. When the sample length is short, the phonon transport is mainly ballistic transport, while diffusion transport dominates the heat transport when the sample length further increases. As the temperature increases, Umklapp scattering dominates the heat transport, making the thermal conductivity and temperature show a 1/&lt;i&gt;T&lt;/i&gt; trend. 3) Comparing with M3 , the patterns of M1 and M4 have large phonon band gaps, and their dispersion curves are further softened. At the same time, regardless of low-frequency or high-frequency phonons, localized features appear in the M1 and M4 (especially the M4), which has a significant inhibitory effect on thermal conductivity. This paper provides an idea for the better design of thermal management materials.

Research progress of high thermal conductivity polyimide dielectric films

In the era of highly thin, multi-functional and integrated electronic devices, it will inevitably lead to the heat accumulation inside the composite material, thereby seriously affecting the operation stability and service life of the equipment. How to realize the rapid and efficient heat conduction and heat dissipation of dielectric materials has become a bottleneck problem restricting the further development of electronic devices. The intrinsic thermal conductivity of traditional polyimide is low, which limits its application in electrical equipment, smart grid and other fields. The development of new high thermal conductivity polyimide dielectric film materials has become the focus of research. This paper introduces the thermal conduction mechanism of composite materials, summarizes the research progress and development status of thermally conductive polyimide films in recent years, and focuses on the effects of thermally conductive fillers, interface compatibility, and molding process of the thermal conductivity of materials. Finally, some key scientific and technical issues in the research are summarized and prospected in combination with the future development needs of thermally conductive polyimide composite dielectric materials.

Thermal Conductivity Mechanism Analysis and Neural Network Model of Clay Backfill Mixed with Different Amounts of Steel Slag

Thermal Conductivity Mechanism Analysis and Neural Network Model of Clay Backfill Mixed with Different Amounts of Steel Slag

Strong phonon coupling induces low thermal conductivity of one-dimensional carbon boron nanotube

Recently, one-dimensional materials have drawn much attention due to their excellent performance in the field of energy storage and thermal management. As a newly synthesized representative of 1D carbon boron compounds, BC3 nanotube (BC3NT), its thermal conductivity and phonon transport mechanism are urgently required for designing electron-related devices and have been still lacking. Herein, the thermal transport properties of BC3NT are investigated using non-equilibrium molecular dynamic simulations. Compared with the carbon nanotube, the lattice analysis and differential charge density revealed that the much lower thermal conductivity of BC3NT stems from the strong phonon coupling induced by the disorders from atomic mass discrepancy and the polarized covalent bond. Furthermore, the temperature and strain effect of thermal conductivity of BC3NT are explored. Interestingly, it is found that for both CNT and BC3NT, the small tensile strain can enhance the thermal conductivity while the large tensile and compressive strain would impede the phonon transport, indicating that the strain strategy can effectively modulate the thermal conductivity of 1D systems. Our studies not only provide a newly perspective to understand phonon transport of 1D systems and could be helpful to extend phonon coupling mechanism to low-dimensional systems.

Thermal Cloak: Theory, Experiment and Application.

In the past two decades, owing to the development of metamaterials and the theoretical tools of transformation optics and the scattering cancellation method, a plethora of unprecedented functional devices, especially invisibility cloaks, have been experimentally demonstrated in various fields, e.g., electromagnetics, acoustics, and thermodynamics. Since the first thermal cloak was theoretically reported in 2008 and experimentally demonstrated in 2012, great progress has been made in both theory and experiment. In this review, we report the recent advances in thermal cloaks, including the theoretical designs, experimental realizations, and potential applications. The three areas are classified according to the different mechanisms of heat transfer, namely, thermal conduction, thermal convection, and thermal radiation. We also provide an outlook toward the challenges and future directions in this fascinating area.

Photoinduced anisotropic lattice dynamic response and domain formation in thermoelectric SnSe

Identifying and understanding the mechanisms behind strong phonon–phonon scattering in condensed matter systems is critical to maximizing the efficiency of thermoelectric devices. To date, the leading method to address this has been to meticulously survey the full phonon dispersion of the material in order to isolate modes with anomalously large linewidth and temperature-dependence. Here we combine quantitative MeV ultrafast electron diffraction (UED) analysis with Monte Carlo based dynamic diffraction simulation and first-principles calculations to directly unveil the soft, anharmonic lattice distortions of model thermoelectric material SnSe. A small single-crystal sample is photoexcited with ultrafast optical pulses and the soft, anharmonic lattice distortions are isolated using MeV-UED as those associated with long relaxation time and large displacements. We reveal that these modes have interlayer shear strain character, induced mainly by c-axis atomic displacements, resulting in domain formation in the transient state. These findings provide an innovative approach to identify mechanisms for ultralow and anisotropic thermal conductivity and a promising route to optimizing thermoelectric devices.

Mechanism of low thermal conductivity for Fe76Si13B8Nb2Cu1 amorphous and nanocrystalline alloys at room temperature

Fe-based amorphous and nanocrystalline alloys offer application prospects in the field of coating materials, and their thermal transport property is crucial for their application in thermal insulation coating. However, the mechanism of thermal conductivity in the process of isothermal annealing preparation of Fe-based nanocrystalline alloys are still unclear. In this work, the thermal conductivity of Fe76Si13B8Nb2Cu1 amorphous alloy isochronal annealed at different temperature was examined. It was found that the thermal conductivity of the amorphous alloy is as low as 7.06 W/mK, whereas significantly increases to 10.38 W/mK after annealing at 693 K for 1800s. The result of the limited increase in thermal conductivity at annealing temperatures below the 693 K can be ascribed to the conflicting variations of electronic and phonon contributions with increasing temperature. At annealing temperature above 693 K, the two contributors start to cooperate, leading to abrupt enhancement of thermal conductivity. This work provides an insight into the thermal transport mechanisms for amorphous alloys.

Ablation behavior and thermal conduction mechanism of 3D ZrC–SiC-modified carbon/carbon composite having high thermal conductivity using mesophase-pitch-based carbon fibers and pyrocarbon as heat transfer channels

A three-dimensional ZrC–SiC-modified Carbon/Carbon composite having high thermal conductivity is prepared to significantly reduce the ablation rate and improve dimensional stability of composites used in thermal protection systems. The mesophase-pitch-based carbon fibers/pyrocarbon skeleton is designed as the thermal diffusion channel using the intermediate state mesophase-pitch-based carbon fibers and chemical vapor infiltration method . After successive graphitization and polymer precursor impregnation and pyrolysis, the obtained ZrC–SiC-modified carbon/carbon composite exhibits a high thermal conductivity of 206.5 W m −1 K −1 in the X(Y) direction. The ablation results indicate that the composite exhibits a surface temperature of 241 °C, which is lower than that of the composite without mesophase-pitch-based carbon fibers, thus verifying the improved ablation resistance. The linear and mass ablation rates of the composite are found to be 0.33 μm s −1 and 0.56 mg cm −2 s −1 , respectively. The thermal conduction mechanism of the ZrC–SiC-modified carbon/carbon composite near the ablation center is investigated to show the combined phonon heat conduction and thermal radiation . This study provides a protocol for the design and construction of modified carbon/carbon composites having high thermal conductivity for application in hypersonic vehicles . • A C/C–ZrC–SiC composite with a thermal conductivity of 206.5 W m −1 K −1 in the ablation direction was fabricated. • The CF MP /C skeleton acted as the thermal diffusion channels during ablation. • The composite showed improved ablation resistance due to a reduction in surface temperature.

Polyurethane-templated 3D BN network for enhanced thermally conductive property of epoxy composites

Utilizing three-dimensional filler has been deem as a promising approach to improve the thermal conductivity of polymer composites. In this research, we developed a polyurethane sacrificial template technique to fabricate a novel three-dimensional porous carbon bonded BN foam (3D-CBNF), aiming to improve the thermal performance of polymers (epoxy). The as-prepared 3D-CBNF/epoxy composite exhibits a high thermal conductivity (TC) of 2.11 Wm−1K−1 at a relatively low BN loading of 24.7 wt%, corresponding to a TC enhancement (TCE) of 1072% compared with the pure epoxy resin. The analysis of the thermal conductivity mechanism based on EMT and Foygel models and finite element simulation indicated that the high thermal conductivity of composites was due to the efficient heat transfer pathways provided by the 3D interconnected BN network and lower interface thermal resistance in internal of it. In addition, the as-obtained composite possessed a relatively low dielectric constant (below 4.5 at 1 MHz) and a high volume electrical resistivity (beyond 1015 Ω·cm). Overall, a novel and promising method of preparation for 3D construction with thermally conductive filler, which is designed to attain composites with high thermal conductivity, has been proposed in this work.

Optical phonon mode assisted thermal conductivity in p-type ZrIrSb half-Heusler alloy: a combined experimental and computational study

Half-Heusler (HH) alloys with an 18 valence electron count have attracted significant interest in the area of research related to thermoelectrics (TEs). Understanding the novel transport properties exhibited by these systems with semiconducting ground state is an important focus area in this field. The large thermal conductivity shown by most of the HH alloys poses a major hurdle to improving the figure of merit (ZT). In addition, understanding the mechanism of thermal conduction in heavy-constituent HH alloys is an interesting aspect. Here, we have investigated the high-temperature TE properties of ZrIrSb through experimental studies, phonon dispersion and electronic band structure calculations. ZrIrSb is found to exhibit a substantially lower magnitude of resistivity and Seebeck coefficient near room temperature due to the existence of anti-site disorder between Ir/Sb and vacant sites. Interestingly, in ZrIrSb, lattice thermal conductivity is governed by a coupling between the acoustic and low-frequency optical phonon modes, which originates due to heavier Ir/Sb atoms. This coupling leads to an enhancement in the Umklapp processes due to the optical phonon excitations near the zone boundary, resulting in a lower magnitude of κ L. Our studies point to the fact that the simultaneous existence of two heavy mass elements within a simple unit cell can substantially decrease the lattice degrees of freedom.

Mechanical behavior, durability, thermal performances and microstructure of GGBFS – Modified MPC solidified dredged sludge

Solidification is an effective way to solve the poor mechanical properties of sludge. In order to improve strength and reduce the cost, the influence of the ground granulated blast furnace slag (GGBFS) content on the properties of solidified sludge (SS) with magnesium phosphate cement (MPC) was investigated. The mechanical behavior, durability, capillary water absorption, microstructure, thermal conductivity (λ) and interaction mechanism between GGBFS-MPC and sludge were analyzed. Finally, microscopic analysis methods (TG-DTG/DSC, XRD, SEM) were used to examine the SS. Results showed that GGBFS-MPC (MgO:GGBFS = 1:1, H3) has excellent mechanical properties (unconfined compressive strength (UCS) splitting tensile strength (STS)) and low shrinkage rate. H3 has the highest UCS and the highest STS with the largest failure displacement, showing good tensile properties. After adding GGBFS, the water resistance increased by about 21.03%-24.05%. XRD detects new hydration products in GGBFS-MgO-ADP, such as Ca(H2PO2)2, CaP2O6, Ca2P2O7·2H2O, CaH2P2O7, Ca4H(PO4)3·2.5H2O, NH4MgPO4·6H2O, NH4MgPO4·H2O, Mg(OH)2, AlPO4, which cemented sludge and synergistically improved the mechanical properties and water resistance. The water sorptivity coefficient of H3 and H5 was small, which attributes to chemical effect and filling effect by hydration products. As the increase of apparent density, the λ increased. The λ does not change much with temperature under dry state. As the water content increased, the λ increased non-linearly. TG-DTA/DSC showed that the weight loss was caused by the loss of free water and bound water. SEM revealed that a small amount of Mg(OH)2 was formed, and struvite cemented the sludge to form agglomerates.