Thermal Conductivity Of Materials Research Articles

Thermal transport in thermoelectric materials of SnSSe and SnS[formula omitted]: A non-equilibrium Monte-Carlo simulation of Boltzmann transport equation

In the present work, thermal transport and the energy conversation in two thermoelectrically efficient candidates of Janus SnSSe and SnS2 are investigated within the non-equilibrium Monte Carlo simulation of the phonon Boltzmann equation. The phonon analysis has been performed to determine the contributed phonon in heat transport. The results present that the dominant participating phonons are the ones from the longitudinal acoustic (LA) branch while the least belongs to the transverse acoustic (TA) modes. Both materials reached the very high maximum temperature in response to the implied wasted heat. This is attributed to the low presence of the critical TA phonons which are on one hand fast enough to leave the hotspot but on the other hand have frequencies commensurable to ZA phonons’ frequency. Also, the temperature profile achieved during the heating and cooling of the materials is studied. It is obtained that the heat propagation through the SnS2 is, at first, swifter, which results in a temperature gradient through the whole material which is less than that of the SnSSe. As the time passes, the heat transfer that is directly related to the material thermal conductivity, slows down. So, the behavior of the SnS2 and SnSSe, in case of the heat propagation status, becomes similar. Moreover, the behavior of the thermoelectric figure of merit (zT), the efficiency (η), and the generated voltage have been figured out. It is stated that the higher zT and η do not guarantee a larger generated Seebeck voltage. This is true, while the generated Seebeck voltage is related to the temperature difference between the heated and the cold junction. Accordingly, how far the temperature of matter rises in response to the implied wasted heat is directly related to the obtained voltage. Mainly, it is presented that the maximum temperature that a material achieves, alongside the temperature gradient and the material property Seebeck coefficient, are essential in introducing thermoelectrically efficient materials with reasonable thermal to electrical energy conversion, useful for thermal engineering.

Thermal Management System with Intrinsically High Thermal Conductive and Antileakage Composite Phase-Change Materials for Battery Module

Thermal Management System with Intrinsically High Thermal Conductive and Antileakage Composite Phase-Change Materials for Battery Module

Anisotropy Reversal of Thermal Conductivity in Silicon Nanowire Networks Driven by Quasi-Ballistic Phonon Transport.

Nanostructured semiconductors promise functional thermal management for microelectronics and thermoelectrics through a rich design capability. However, experimental studies on anisotropic in-plane thermal conduction remain limited, despite the demand for directional heat dissipation. Here, inspired by an oriental wave pattern, a periodic network of bent wires, we investigate anisotropic in-plane thermal conduction in nanoscale silicon phononic crystals with the thermally dead volume. We observed the anisotropy reversal of the material thermal conductivity from 1.2 at 300 K to 0.8 at 4 K, with the reversal temperature of 80 K mediated by the transition from a diffusive to a quasi-ballistic regime. Our Monte Carlo simulations revealed that the backflow of the directional phonons induces the anisotropy reversal, showing that the quasi-ballistic phonon transport introduces preferential thermal conduction channels with anomalous temperature dependence. Accordingly, the anisotropy of the effective thermal conductivity varied from 2.7 to 5.0 in the range of 4-300 K, indicating an anisotropic heat manipulation capability. Our findings demonstrate that the design of nanowire networks enables the directional thermal management of electronic devices.

Preparation of high thermal conductivity vat photopolymerization used UV-curable resin synergistically enhanced by silicon nitride and boron nitride

Additive manufacturing (3D printing) technologies, especially vat photopolymerization (VPP), provide a cost-effective and efficient approach for the fabrication of heat dissipation devices with complex geometric designs. The incorporation of inorganic thermal conductive fillers into resin matrix is widely recognized as an effective way to enhance thermal conductivity. Notably, the synergistic effect of hybrid fillers can further enhance thermal conductivity. However, the consequent drastic increase in viscosity of the UV-curable resin may decrease the success rate and accuracy of printing on commercial VPP 3D printers. In this work, hexagonal boron nitride (h-BN) platelets and spherical h-BN together with β-Si3N4 are introduced into the UV-curable resin system. Corresponding results proved that h-BN together with β-Si3N4 provide a synergistic effect on the thermal conductivity enhancement compared to the filler in single morphology only. Moreover, owing to the spherical structure, low-viscosity UV-curable resin with high solid content can be prepared. In specific, a thermal conductive polymer composite material with a solid content of 60 wt% is prepared, which exhibits high thermal conductivity (1.42 W/mK), representing an enhancement of up to 610 % compared to neat resin. In subsequent application demonstrations, 3D printed heat dissipation devices with as-prepared resin composite material have been shown to effectively enhance the heat dissipation rate for the heating device.

Vitrimeric phase change material facilitating 3D-structured boron nitride network for reliable thermal conductive interface materials

Phase change materials (PCMs) shows great promise as thermal interface materials (TIMs) due to their compliance and thermal management properties from solid-liquid phase transition characteristics. However, the significantly reduced thermal conductivity resulting from that randomly dispersed thermal conductive fillers susceptible to localized volume change by solid-liquid phase transition deteriorates the reliable heat transfer of TIM but is often overlooked. In this work, 3D-structured network of boron nitride (BN) fillers is created in vitrimeric PCM (vPCM) consisting of dynamic covalent crosslinked polyolefin elastomer (POE)/paraffin wax (PW) blends. The resulted composites with interconnected BN network show a through-plane thermal conductivity (κ⊥) of 3.08 W m−1 K−1 at 29.8 vol% BN. Significantly, the composites can keep the BN network continuous when heated, and remain κ⊥ of about 1.4 W m−1 K−1 after the solid-liquid phase transition of PW, which ensures reliable heat transfer when achieving thermal management by melting endotherm of PW. Moreover, the composites conform to the surface topography of sandwiching surfaces through solid plasticity under low stress when the crosslinked network rearranges via the exchange of dynamic covalent bonds activated by the melting of POE, reducing the thermal contact resistance. When the composites are used as TIM through thermally triggered reconfigurable package procedures at 80 °C and 50 kPa, the heat dissipation is better than a commercial thermal pad with κ⊥ of 3.86 W m−1 K−1. Vitrimeric PCM facilitates reliable 3D-structured BN network and thermally triggered conformability, paving a promising way for high-performance TIM with excellent heat dissipation.

Research on the heating effect evaluation of the electromagnetic induction heating system at low temperature based on the electrochemical-thermal coupling model

The extremely fast electromagnetic induction heating system (EIHS) was recently introduced to improve the poor charge and discharge performance of lithium-ion batteries (LiBs) at low temperature. In this work, aimed to investigate the heating effect of EIHS, the accurate electrochemical-thermal coupling model, validated against the charging capacity calibration with various C-rates and HPPC tests at different environments, was developed in depth to reflect the electrochemical and heat transfer behaviors of the LiB exposed to external cool air during the electromagnetic induction heating process. When the copper coil was subjected to a sinusoidal alternating current (SAC) excitation at 10 A, 3 kHz, the LiB can be warmed up to 293.15 K from 243.15 K within 742 s with an even heat generation inside the LiB. The impact of the SAC parameters on the needed heating time was illustrated with time maps, a series of contours suggested that the heating velocity increase with the increase of both the amplitude and frequency. As compared with the low-temperature state, the internal resistance had a more than 3-fold decrease, which can improve the discharging voltage platform and terminal voltage output, thus increasing the pulse power and usable capacity. Neither noticeable capacity retention difference before and after the heating cycles nor obvious capacity loss discrepancy between the normal charge-discharge and heating cycle tests was found at the end of the 200 heating cycles, so the EIHS can achieve a good tradeoff among temperature rising effect, working behaviors and LiB cycle lifespan, which had a considerable potential to enhance the applicability of EVs (electric vehicles) in frigid weather. Moreover, a better heat transfer condition, higher material thermal conductivity and lower specific heat capacity play an important positive role on the improvement of temperature rising performance.

A study on aramid nano-aerogel fiber and its fabric thermal conductivity

Aramid nano-aerogel represents an ideal structure for the fabrication of next-generation high-performance thermal insulation fibers and textiles. Single aramid nano-aerogel fiber can be achieved via wet spinning. The thermal conductivity of these fibers, a crucial determinant of textile insulation performance, remains under-explored in terms of numerical modeling. This research aims to establish a numerical inversion model to determine the thermal conductivity of single aramid nano-aerogel fiber. By simulating the three-dimensional geometry of plain fabric, the volume fraction of gas and solid within the fabrics are computed. Drawing from the law of energy conservation and Fourier's law of heat conduction, a relationship model is proposed between the thermal conductivity of single aramid nano-aerogel fiber and the overall thermal conductivity of the fabric. The finite difference method is employed to ascertain the thermal conductivity of single aramid nano-aerogel fiber. Furthermore, the influence of the geometric structure and the fiber diameter on the thermal insulation performance of the single-layer fabric is analyzed, which provides a feasible approach for the subsequent design of fabric structures with excellent thermal insulation properties.

Pore structure characterization and effective thermal conductivity modeling of fibrous building thermal insulation materials based on X-ray tomography

The existing structural characterization methods of building materials as well as thermal conductivity calculation models can not be directly applied when considering fibrous building thermal insulation materials with intricate skeletons. In this study, rock wool is considered the research object, and the X-ray tomography method is used to characterize the pore structure and extract structural parameters. Based on the actual pore morphology of the material, the pores in the representative elementary volume (REV) are reasonably assumed to be cubic. Additionally, considering the contact thermal resistance of fibers and the randomness of pore distribution, the curved pattern of heat conduction pathway with the ever-changing diameter is reasonably simplified into a series-parallel form of the curved heat chain with the same diameter. On this basis, the fractal models of effective thermal conductivity are established in horizontal and vertical directions. Finally, a modified formula for the effective thermal conductivity calculation of fiber thermal insulation materials is proposed, taking into account the influence of fiber orientation on the material thermal conductivity. It was shown that the relative deviation between the calculated correction value and the experimental value of the effective thermal conductivity is all less than 16.13%.

Phase change composites with ultra-high through-plane thermal conductivity achieved by vertically-aligned graphite film and double-shelled microcapsules

The heat dissipation in electronic devices has become bottleneck problem. One of the best solutions is to develop high performance materials which can not only dissipate the heat away, but also absorb heat energy. This kind of double function materials can be obtained by simultaneously using high thermal conductivity (TC) materials and phase change materials. The formation of continuous network of thermally conductive fillers is demonstrated as one of the most effective methods for fabricating highly thermally conductive composites. Here, graphite films (GF) are stacked and vertically cut to endow the composites with high through-plane TC. The obtained composites exhibit a through-plane TC of 51.55 W m−1 K−1 and a low thermal resistance of 0.398 °C cm2 W−1. Besides, double-shelled PDA@SiO2@paraffin (Pa) microcapsules are embedded into the composite to give its good heat-absorption capacity. Via a CPU thermal stress test, the composites demonstrate satisfactory temperature controlling performance.

Structural and thermoelectric properties of doped Bi2Te3 crystalline alloys

Bulk alloys of Bi2Te3 doped with various amounts of Se, Sb and Na were synthesized by simple melting. Structural and morphological properties have been checked using x-ray diffraction (XRD) and scanning electron microscope (SEM) observations. XRD and SEM investigations revealed that the obtained samples are of high crystallinity with the structural characteristics of the Bi2Te3 phase. Electrical and thermoelectric properties of the prepared samples were investigated over the temperature range 283–473 K. Seebeck coefficient and electrical and thermal conductivities were measured and studied as functions of temperature between 283 K and 473 K. The power factor (PF) was calculated and examined against the temperature increasing, as well. The thermoelectric figure of merit (ZT) was estimated from the materials power factor and thermal conductivity. The Sb-doped alloy exhibited the largest power factor, recorded at 285.23 µW/m.K2 at 430 K. On the other side, the Sb-doped and the Na-doped Bi2Te3 showed increased ZT than that of the pristine alloy. The maximum ZT was recorded at 1.97 for the highest Sb-doped Bi1.94Sb0.06Te3 sample, observed at 430 K. It is believed that the increased number of grain boundaries besides the decreased grain size are main reasons behind the high observed ZT in this work, especially because the phonon scattering on grain boundaries results in reducing the lattice part of the thermal conductivity.

Quasi-Hyperbolic Framework Graphite Foam-Based Composites with High Thermal Conductivity and Electromagnetic Shielding Properties Fabricated by an Electrochemical Expansion Method.

Nowadays, the rapid development of electronic devices requires composites with high thermal conductivity and good electromagnetic shielding properties. The key challenge lies in the construction of high-performance conductive networks. Herein, an electrochemical expansion graphite foam (EEG) with a quasi-hyperbolic framework was prepared by an electrochemical expansion method, and then the epoxy resin (EP) was filled to fabricate the composites. The graphite plate was first electrochemically intercalated and then foamed, in which plasticization was caused by weak oxidation in intercalation and the quasi-hyperbolic framework was induced by foaming during expansion. These processes were characterized by Fourier transform infrared (FTIR), micro-Raman, X-ray photoelectron spectroscopy (XPS), and so on. Based on the highly efficient quasi-hyperbolic framework and high-quality graphite structure, the thermal conductivity of the composite reached 43.523 W/(m·K), and total electromagnetic interference (EMI) shielding (SET) reached 105 dB. The heat transfer behavior was simulated by finite element analysis (FEA) in detail. This method of preparing high thermal conductivity and electromagnetic shielding materials has a good application prospect.

Inverse design method of thermal devices with thermal Hall effect

Efficient thermal management is important for both performance and efficiency improvements of thermal devices. For designing reasonable materials and structures of such devices, various design methods were proposed where the material thermal conductivity tensors were positive definite and symmetric based on the physical requirements. Here, we propose an inverse design method for thermal devices considering the thermal Hall effect, which makes the material thermal conductivity tensor asymmetric. Enlarging the design space consisting of the symmetric constitutive tensors to that of the asymmetric ones, there is a possibility of improving the theoretical performance limit of thermal devices. We formulate an inverse problem based on the free material optimization formalism, parameterizing the design space so that the physically available property could be naturally satisfied. Several numerical experiments are provided to show the validity and the utility of the proposed method.

Anisotropic Rotunda-Shaped Carboxymethylcellulose/Carbon Nanotube Aerogels Supported Phase Change Materials for Efficient Solar-Thermal Energy Conversion.

For the drawbacks of phase change materials such as poor shape stability and weak solar-thermal conversion ability, a rotunda-shaped carboxymethylcellulose/carbon nanotube aerogel (CA) with three-dimensional network was constructed by freeze casting with a special mold, and then impregnated with polyethylene glycol (PEG) in this work. The PEG/CA had an enthalpy of 183.21 J/g, and a thermal conductivity of 0.324 W m-1 K-1, which was 57 % higher than the pure PEG. The ability of PEG/CA to convert solar energy to thermal energy was a positive correlation between the inclusion of CNTs and the composite material's thermal conductivity. Under simulated sunlight, its solar-thermal conversion efficiency reaches 94.41 %, and after 10 min of irradiation, the surface temperature can reach 65 °C and the internal temperature can reach 44.67 °C. This rotunda-shaped PEG/CA is promising for the efficient use of renewable solar energy due to its strong solar-thermal conversion and thermal storage capabilities.

Searching for low thermal conductivity materials for thermal barrier coatings: A theoretical approach

Searching for low thermal conductivity materials for thermal barrier coatings: A theoretical approach

Numerical modeling and optimization of tube flattening impacts on the cooling performance of the photovoltaic thermal system integrated with phase change material

In this research, a computational fluid dynamics-based numerical simulation is performed to evaluate the influence of tube flattening on the cooling performance of a photovoltaic thermal system integrated with phase change material. Such a study is executed to obtain the four evaluation parameters, including average photovoltaic panel temperature, phase change material melting percentage, tube pressure drops, and the difference between the highest and lowest temperature points on the photovoltaic panel. To obtain the optimum percentage of tube flattening to optimize the cooling performance of the systems, multi-objective optimization by utilizing response surface methodology is done by considering some design requirements, including minimizing average photovoltaic panel temperature, maximizing phase change material melting percentage, minimizing tube pressure drops, and minimizing the difference between the highest and lowest temperature points on the photovoltaic panel. Finally, a parametric analysis is done to explore how certain critical factors (solar radiation, fluid mass flow rate, phase change material thermal conductivity, phase change material enthalpy of fusion, and phase change material melting point) affect the efficiency of the systems. The findings indicate that as tube flattening rises from 0 % to 75 %, the average photovoltaic panel temperature and the difference between the highest and lowest temperature points on the photovoltaic panel decrease which is a positive effect. Conversely, increasing the tube flattening results in an increase in the tube pressure drops and a reduction in the phase change material melting percentage. These changes adversely affect the cooling performance. Considering these impacts and conducting an optimization process, the ideal tube flattening value is 54.78 %. In this optimized state, the average photovoltaic panel temperature reaches 55.21 ˚C, phase change material melting percentage comprises 42.54 %, tube pressure drops is 184.8 Pa, and the difference between the highest and lowest temperature points on the photovoltaic panel is 33.42 ˚C.

Study on thermal conductivity of micron BN-nano SiO2 combined modified epoxy resin composites

Epoxy resin has attracted widespread attention in the field of power electronic packaging due to its favorable processing technology, high adhesion, and excellent dielectric properties. The enhancement of thermal conductivity in epoxy resin through the incorporation of diverse particles is a prominent area of interest in the realm of modified insulation materials. In this study, hexagonal boron nitride (h-BN) and silicon dioxide (SiO2) with good thermal conductivity were used as doped fillers to modify epoxy resin (EP). The analysis of experimental results indicated that the thermal conductivity of materials with a total doping amount of 20 wt% and a mass ratio of BN to SiO2 of 70:30 to 60:40 could increase the thermal conductivity by 118.1~126.0%. In contrast to pure EP, the thermal conductivity of composite epoxy resin materials has been significantly enhanced.

Analysis of heat flow in modified transient plane source (MTPS) measurements of the thermal effusivity and thermal conductivity of materials.

An iterative algorithm for the diffusion of heat in layered structures is solved in cylindrical coordinates for the geometry used in measurements of thermophysical properties of materials by the modified transient plane source (MTPS) method. This solution for the frequency-domain temperature response is then used to model the transient temperature excursion and evaluate the accuracy of the measurements. We evaluate when the MTPS method is capable of separately determining the thermal conductivity and heat capacity per unit volume of a material. For a typical sensor design, data acquisition, and data analysis, the MTPS measurement has a small sensitivity to the thermal diffusivity of the sample when the thermal diffusivity is <5 mm2 s-1. We analyze the propagation of errors from uncertainties in the thermal contact between the sensor and the sample and evaluate the limitations of the MTPS method in accurately measuring samples with extremely low thermal effusivity, e.g., low density foam insulation. We find that uncertainties in the thickness of the contact region limit the accuracy of MTPS measurements when the data are analyzed in a conventional manner based on a single parameter, m-1, the inverse of the slope of the temperature excursion as a function of the square root of time.

Vacuum filtration-assisted design of hierarchical structure in polyimide-BN composite films: Toward enhanced thermal conductivity, low dielectric and electrical insulation

With the advent of the 5G era, the demand for heat dissipation using high thermal conductivity (TC) materials is becoming increasingly urgent. The electrical insulation and thermal stability of dielectric polymer composites are also notable in electrical and electronic applications. Nevertheless, the improvement in TC of dielectric polymer composites is typically accomplished at the expense of electrical insulating capabilities. Herein, we fabricated polyimide (PI) composites with ordered structure by combining electrostatic spinning, vacuum filtration deposition and blade-coating. The modified boron nitride nanosheets (mBN) not only enhanced the interaction between filler and PI matrix, but also reduced the interface thermal resistance. Benefiting from the hierarchical structure, the in-plane TC of prepared composite was 17.44 W/mK at 18.79 vol% mBN content, which is 9.6 times higher than that of pure PI. Moreover, the composites were electrically insulating, with the volume resistivity of about 1013 Ω cm, and exhibited great flexibility, exceptional thermal stability (THRI ＞ 269 °C), excellent thermal dissipation capability and low dielectric characteristics. In summary, this work offers a novel perspective to design highly thermally conductive composites and these composites have vast application potential in flexible electronic devices or circuits.

Enhancing the melting performance in latent heat storage systems with fin-foam combination at high temperatures: Configurational optimization and economic assessment

High-temperature latent heat thermal energy storage is restricted by the low thermal conductivity materials. To achieve rapid heat storage, a fin-foam synergistic heat transfer enhancement is proposed to break the marginal improvement of unilateral enhancement. The effect of different enhancement methods is explored by constructing 3D numerical models. The numerical result indicates the optimal fin-foam synergistic enhancement can effectively reduce the full melting time by 59.7%, compared with the smooth-foam unilateral enhancement. Further, the relationship between temperature uniformity and thermal performance has been refined. Internal fins achieve more efficient heat transfer from the tube center to the tube edge, accompanied by a 29.1% improvement in temperature uniformity of heat transfer fluid. A comprehensive index including the pump cost is established for economic assessment. The fin-foam synergistic enhancement has been proved to be the most cost-effective, achieving relatively high performance and avoiding excessive pressure drop.

Preparation of functionalized boron nitride nanosheets by one-step method for improving the thermal conductivity of polymers

Boron nitride nanosheets (BNNSs) with high thermal conductivity (TC) have promising applications in the thermal conduction. However, the preparation of BNNSs is cumbersome and BNNSs are prone to aggregation in the polymer matrix, which reduces the efficient enhancement for the TC of BNNSs/polymer composite. Herein, we propose a one-step method for the simultaneous exfoliation and functionalization of BNNSs with polymer latex as an agent. The obtained WPU-BNNSs displayed as few-layers with sizes of 100-500 nm and were mixed with WPU matrix as filler. The results demonstrated that the BNNSs/WPU composites have higher TC than the h-BN/WPU with the same filler content. When the filler content reached 50%, its TC was 251.4% higher than that of pure WPU. The excellent TC arose from the outstanding dispersion uniformity of WPU-BNNSs in the matrix. This study provides an economical and efficient method of fabricating BNNSs-based thermal conductive composite materials.