High Thermal Conductivity Materials Research Articles

Phase change-related thermal property characterization and enhancement in carbon-based organic phase change composites

By utilizing the significant amount of energy absorbed and released during their phase transitions, phase change materials (PCMs) can capture and store thermal energy to fill gaps between supply and demand. Due to their many favorable properties, organic PCMs have gained attention in a wide range of applications. Nevertheless, their inherent low thermal conductivity has limited the direct use of organic PCMs in thermal energy storage (TES). Extensive research has been conducted on enhancing organic PCM thermal conductivity by incorporating high thermal conductivity materials. Owing to their high thermal conductivity and low density, carbon-based materials have been extensively used for thermal conductivity enhancement in phase change composites (PCCs). Carbon-based organic PCCs, which incorporate highly thermally conductive carbon allotropes and their direct chemical derivatives with organic PCMs, are a group of diverse PCCs with highly promising potential for TES applications. Adequate latent heat and shape stability performances are crucial to the success of the applicational performances of these PCCs. Much empirical research has pushed efforts to enhance these phase change properties, yet a logical understanding of these enhancement efforts based on the thermodynamics and intermolecular interactions of carbon-based organic PCCs has been elusive. In particular, the effect of characterization methods on the evaluation of phase change properties has been largely understudied. This review strives to provide novel physical and chemical insights into latent heat and shape stabilization evaluation processes and enhancement efforts in carbon-based organic PCCs through a detailed review and analysis of recent literature. The review provides an unprecedented comprehension of newly developed PCCs that challenge the traditional understanding that the latent heat of PCCs cannot exceed that of its base PCM. Efforts on phase change property enhancement driven by these new insights have the potential for carbon-based organic PCCs to succeed in a variety of TES applications, including solar-thermal harvesting, thermal management of batteries and electronics, thermoregulating textiles, and infrared stealth and infrared responsive materials.

Core-sheath smart polymer fiber composites with high elasticity and thermal conductivity

Thermal conductive polymer fibers have broad development prospects because of its excellent thermal and mechanical properties. However, currently reported polymer fibers cannot simultaneously possess high thermal conductivity and high resilience, which greatly limits the application of thermal conductive polymer fibers. Therefore, preparing fibers with high thermal conductivity and high resilience is extremely challenging. The thermoplastic polyurethane/boron nitride liquid metal (TPU/BN-LMs) fiber with core sheath structure is prepared by coaxial wet spinning technology, the boron nitride (BN) doped in thermoplastic polyurethane elastomer (TPU) as sheath and the liquid metal (LMs) as core. The maximum tensile strength of the fiber is 3.9 MPa, and the elongation can reach 400 %. After multiple cycles of stretching, it can return to its original state. In addition, the thermal conductivity of the fiber in the horizontal direction and the vertical direction are 10.0 W m−1 K−1 and 4.6 W m−1 K−1, respectively. This work solves the current problem of high thermal conductivity and high resilience of thermally conductive polymer fibers that are difficult to be compatible. And it provides a new research idea for the design and preparation of new high thermal conductivity and high resilience polymer fiber materials.

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

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.

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.

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.

Rigorous Reliability Testing of an Encapsulated Thermal Pyrolytic Graphite (TPG) Heat Spreader for Passive Thermal Management Applications

Continued microelectronic miniaturization has resulted in higher power density, increasing the need for high performance thermal management. One possible solution for board-level thermal management is to fully encapsulate a high thermal conductivity material, such as Thermal Pyrolytic Graphite (TPG; ~1700 W/mK) in a metal (such as 6061 aluminum), producing an efficient passive heat spreader for thermal dissipation. The metal shell provides sufficient mechanical strength at a small detriment to the overall thermal conductivity. Reliability and consistent performance over time are critical for thermal management systems, especially in Defense and Aerospace applications. MIL-STD-883H provides test parameters for reliability involving exposure to mechanical shock (Method 2002.5), mechanical vibration (Method 2005.2), and thermal shock (Method 1011.9), with traditional evaluation criteria involving microelectronic device performance and/or a visual examination. For this work, the reliability of aluminum encapsulated TPG test coupons was evaluated with more demanding criteria: a visual examination, X-ray inspection, acoustic microscopy, and thermal conductivity performance testing, both before and after the previously discussed MIL-STD-883H exposures. The encapsulated coupons showed excellent robustness; the thermal conductivity properties remained constant with an average value around 965 W/mK, and the non-destructive imaging revealed excellent bond quality for the metal encapsulation, with no evidence of delamination. This more stringent evaluation criteria after MIL-STD-883H testing provides a better way to evaluate the reliability of passive thermal management systems. The study results also demonstrate the metal encapsulated TPG heat spreader as a superior thermal management solution with high performance and high reliability for demanding applications.

A New Type of Expanded Graphite-Based Heat-Conducting Glue and Its Application in Heat-Non-Burning Cigarette

With the popularity of heat non-combustible cigarettes, a new type of tobacco product, higher requirements have been put forward for the thermal conductivity of tobacco materials compared to traditional tobacco. In order to make the tobacco uniformly heated, adding high thermal conductivity materials to the tobacco to improve the thermal conductivity of the tobacco in the heat-non-burning smoke is a conventional practice. Therefore, this article introduces expanded graphite with high specific surface area and high thermal conductivity as the main material of thermal conductivity, and the polymer obtained by esterification reaction of biomass material polylactic acid, citric acid and diol are utilized as the adhesive glue. Research shows that this material enhances the heating performance of tobacco in heat non-combustible cigarettes. The thermal conductivity measured by adding 3 ml, 4 ml, and 6 ml of the expanded graphite viscous mixture to the filter paper spacer increased sequentially both at room temperature and at 100 degrees Celsius, and all of them were higher than that of the blank filter paper. This method effectively realized the fixation of the expanded graphite thermal conductive material on the surface of the tobacco, which greatly improved the thermal conductivity performance of the tobacco and made this new type of heat cigarettes more uniformly heated.

Understanding neural network tuned Langevin thermostat effect on predicting thermal conductivity of graphene-coated copper using nonequilibrium molecular dynamics simulations

Copper has always been used in thermoelectric applications due to its extensive properties among metals. However, it requires further improving its heat transport performance at the nanosized applications by supporting another high thermal conductivity material. Herein, copper was coated with graphene, and the neural network fitting was employed for the nonequilibrium molecular dynamics simulations of graphene-coated copper nanomaterials to predict thermal conductivity. The Langevin thermostat that was tuned with a neural network fitting (NNF), which makes up the backbone of deep learning, generated the temperature difference between the two ends of the models. The NNF calibrated the Langevin thermostat damping constants that helped to control the temperatures precisely. The buffer and thermostat lengths were also analyzed, and they have considerable effects on the thermostat temperatures and a significant impact on the thermal conductivity of the graphene-coated copper. Regarding thermal conductivity, the four different shapes of vacancy defect concentrations and their locations in the graphene sheets were further investigated. The vacancy between the thermostats significantly decreases the thermal conductivity; however, the vacancy defect in thermostats does not have a similar effect. When the graphene is placed between two copper blocks, the thermal conductivity decreases drastically, and it continues to drop when the sine wave amplitude on the graphene sheet increases.

Numerical thermal performance assessment of phase change process in a PCM/foam-fins enclosure under various thermal conditions

ABSTRACT The low ability of phase change material (PCM) to transfer thermal energy has created a serious challenge for the development of latent heat storage (LHTES) units. Embedding foam with high thermal conductivity material like copper has attracted significant attention. Besides, high porosity, lightweight, and easy usage are other important benefits of copper. However, its higher cost and decreasing effective volume of PCM led to the creation of amendments in the foam design. Employment of the foam in the form of fin or foam fin has emerged as an alternative method to benefit from the advantages of porous material and overcome its defects. In this study, various LHTES units with different numbers of foam fins have been simulated numerically and compared with pure PCM and fully foam PCM enclosures. Two practical boundary conditions including constant temperature and constant heat flux were examined for the designed systems. The obtained results demonstrated that foam fins have a remarkable effect on the thermal performance enhancement of LHTES under constant temperature boundary condition. For LHTES with six foam fins, 42% and 30% reductions in melting time were achieved for both types of boundary conditions including constant temperature and heat flux, respectively. At the beginning of charging process, a significant enhancement of input heat rate was proved for LHTES with 6 foam fins and fully foam up to 112% and 155%, respectively. In addition, the foam fins method provided some free volume of PCM to permit the falling out of natural convection of molten PCM.

Investigation on the improvement method of ampacity for cable in duct bank: Using the mixed filler with high thermal conductivity material and phase change material

Duct laying is a common cable laying way for the case of not meeting excavation conditions. However, due to the slow air flow rate in the duct, the cable section laying the duct becomes one ampacity bottleneck. Backfilling the duct with the mixed filler of high thermal conductivity material and phase change material is an effective method to solve the problem. In this paper, the simulation models of cable in ducts with backfilling the mixed filler were used to analysis the effect of relative position of cable and duct on temperature distribution and guide the thermal modeling of external environment. Then, the thermal model of cable in duct was conducted and the layered method was adopted to optimize the model. The real cable experiment was designed to verify the accuracy of proposed model. The improvement effect of mixed filler with different thermal conductivity and phase transition region on the cable ampacity were discussed by using the proposed model. The investigations in this paper show that the use of mixed filler material can have a great contribution to ensure the safe operation of cable in duct.

Nickel catalyst supported on SiC incorporated SiO2 for CO2 methanation: Positive effects of dysprosium promoter and microwaves heating method

In this work, Dy-promoted Ni/SiC-SiO2 catalysts with varied Dy loadings (0–1 %) were developed using the wet impregnation method and evaluated for their effectiveness in CO2 methanation. The characterization results reveal that Dy addition can reduce the Ni crystallite size, enhance the dispersion of active phase and increase the catalyst’s reducibility. Among the prepared catalysts, the 0.5 %Dy-10 %Ni/SiC-SiO2 demonstrated the highest CH4 selectivity (100 %) and CO2 conversion (73.9 %) at 350 °C owing to its highest basic sites and H2 chemisorption capacity. The performance of this Dy-promoted catalyst diluted with different amounts of SiC, which was employed as a microwave susceptor and high thermal conductive dilution material, was also investigated in both microwave and electric reactor. Microwave heating leads to higher CO2 conversion compared to electric heating, regardless of reaction temperature and dilution factor due to selective heating and the random occurrence of hot spots, which are usually considered as micro-plasmas in the catalyst bed. However, further increase quantity of SiC decreased both CO2 conversion and CH4 selectivity regardless of heating method. This may be attributed to the appearance of extreme overheating and the absence of gentle overheating with increased amount of SiC in the microwave and electric reactors, respectively. The temperature-programmed oxidation results demonstrated that carbon deposition on the 0.5 %Dy-10 %Ni/SiC-SiO2 catalyst during 30 h of reaction can be significantly suppressed by microwave.

Experimental and Computational Evidence of Damped Axial Conduction with Reciprocating Flow

Abstract Axial conduction is a crucial performance deteriorating factor in miniaturized heat transfer devices, primarily due to the low fluid flow rates, high solid cross-sectional to free-flow area ratio, and use of high thermal conductivity materials. These causative factors, inherent to micro-scale systems, should be such that the axial conduction is minimum. The reciprocating flow of the convective fluid (instead of steady unidirectional flow) is proposed per se as an alternative, which directly alters the solid temperature profile, the root cause of axial conduction. An experimental setup is built as a proof of the concept. In the test rig, a double-acting reciprocating pump generates a fully reversing periodic flow of air through a flow channel carved into a steel block embedded with a heater. The experimental temperature profile in the solid at the cyclic steady state is bell-shaped, indicating a virtual adiabatic plane capable of restricting axial heat transfer. The experimental results are verified taking the help of an independent finite-element-based numerical analysis. Similarly, the non-dimensional interfacial flux ratio (?_0 ), integrally related to axial conduction, for unidirectional and reciprocating flow is significantly different. This ratio in the vicinity of the inlet is ~53% less with the reciprocating compared to the equivalent unidirectional flow. The optimal thermal performance with the reciprocating flow is correlated through a critical Strouhal number expression, Sr ≤ (pDh)/L. In thermal management applications employing reciprocating flow, the limiting relation can be used to determine flow parameters and optimum geometry design.

A Novel Application of High Thermal Conductivity Material on Superconducting RF Cavities: The Inner-wall Thermal Conducting Film (ITCF)

Superconducting Radiofrequency (SRF) technology has developed rapidly for the requirements of high-energy accelerators in fundamental physics research and nuclear industry applications. Thus, the SRF community endeavored to improve the performance from every aspect for niobium and thin-film cavities and made significant breakthroughs in the past decades. However, the positive feedback effect from the RF loss seriously limits the cavity’s maximum acceleration gradient and causes the high field’s unstable operation. Mitigating this effect requires increasing the cavity thermal conductivity to reduce the temperature at the heating point. Conventional technology conducts the RF-loss heat from the surface to the liquid helium outside the cavity while the inner cavity remains vacuum. This paper introduces a new strategy which coats an inner-wall thermal-conducting film (ITCF) in the cavity for the first time globally. The COMSOL simulation results show that ITCF absorbs heat from the RF surface and transmits the heat to the distance on the inner wall, generating a different heat transfer route, thereby reducing the heating-point temperature more efficiently.

Towards Controlled Transfer of (001) β-Ga2O3 to (0001) 4H-SiC Substrates

We demonstrate successful blistering of He-implanted (001) β-Ga2O3, bonding to (0001) 4H-SiC, and initial results towards large-area transfer of (001) β-Ga2O3 to 4H-SiC. Compatible with large-scale processing, exfoliation is an important step in controlled-thickness transfer of films for heterogeneous integration. Furthermore, integration of β-Ga2O3 to high thermal conductivity materials will be crucial for thermal management of β-Ga2O3-based power devices. Two-inch (001) β-Ga2O3 wafers were implanted with He+ at an energy of 160 keV and a dose of 5×1016 cm-2, which are the same implant parameters used previously to successfully exfoliate (010) β-Ga2O3.1 Strain fringes were observed after the implant which corresponded to a maximum strain of ~1.7%, which is higher than the ~1% strain when implanting (010) β-Ga2O31 likely due to the larger Poisson’s ratio of (001) β-Ga2O3.2 The implanted substrates were then bonded to (0001) 4H-SiC at room temperature using a ~5 nm thin Ti interlayer to assist with the bond. Both unbonded implanted substrates and the bonded structures were first annealed at 200 °C for 10 hours to simultaneously initiate He bubble nucleation at the implanted projected range (~0.68 μm) and strengthen the bond. Then, even after annealing at 500 °C for 10 hours to initiate He bubble growth, the (001) β-Ga2O3 did not transfer to the (0001) 4H-SiC. Unlike what was observed for (010) β-Ga2O3,1 blistering did not even occur at this temperature. Annealing at 800 °C for up to 12 hours resulted in ~10 μm blisters for the unbonded substrates, which is typically an indication that large wafer-area transfer can be achieved if bonding was done prior to annealing.3 However, the β-Ga2O3 substrate did not wafer split from the bonded structure, and instead only small area transfers up to ~200 μm were achieved. Only ~7% of the total bonded area transferred while the entire structure remained bonded. Strategies to improve transfer will be presented, including initiating a cold split prior to exfoliation and refined annealing strategies to improve He bubble nucleation and larger bubble growth at the projected range. These are promising results towards achieving large wafer-scale (001) β-Ga2O3 composite wafers, and when combined with subsurface damage-free chemical mechanical polishing,4 these composite wafers would be suitable for subsequent devices and/or epitaxial growth.The authors would like to acknowledge the support from the Office of Naval Research through a MURI program, grant No. N00014-18-1-2429. This research was performed while M.E.L. and J.S.L. held an NRC Research Associateship award at the U.S. Naval Research Laboratory.References M.E. Liao, et al., ECS J. Solid State Sci. Technol., 8(11), P673 (2019).K. Adachi, et al., J. Appl. Phys., 124, 085102 (2018).M. Bruel, et al., Jpn. J. Appl. Phys., 36, 1636 (1997).M.E. Liao, et al., J. Vac. Sci. Technol. A, 41, 013205 (2023). Figure 1

High-quality fabrication of diamond straight microchannels heat sink with large aspect ratio microchannels using UV nanosecond laser based on multi-feed method

The high heat flux density of modern chips presents a major challenge to their development. Single-phase or two-phase flow heat dissipation techniques using high thermal conductivity materials are seen as a potential solution. However, creating microfluidic channels from ultra-high thermal conductivity diamond materials is a significant challenge. In this paper, we present an experimental study on the fabrication of diamond heat sinks with high aspect ratio microchannels using a UV-nanosecond laser processing system, along with two multi-feed processing techniques for the collecting grooves and microchannels. We used a 1 μm spacing grid path combined with a polished multi-feed process to fabricate a 1600 μm deep collecting groove with a smooth bottom morphology. We also proposed a multi-area multi-feed fabrication technique for microchannels with an aspect ratio of 8:1 and a depth of 1600 μm. Finally, we have successfully fabricated a diamond straight microchannels heat sink, featuring a high aspect ratio and low phase change damage.

An evaluation of the use of air cooling to enhance photovoltaic performance

The rapid rise in global energy consumption and its consequences on climate change has made incorporating renewable energy sources like solar photovoltaics into the building envelope easier. However, in spite of extensive uses and significant technological advances, the lower solar panel efficiencies caused by high temperatures remain a significant barrier to the viability of deploying photovoltaic technology in regions with hot climates utilizing computational fluid dynamics (CFD). This research examines the cooling effectiveness of air-cooled photovoltaic (PV) under the climate of Nablus - Palestine. This study presents a numerical model designed to cool solar panels using various air-cooled channel configurations. Rectangular fins made of high thermal conductivity materials such as copper were used in this study. The parametric study was based on the changing baseplate thickness, fin spacing, height, and thickness through a stepwise optimization process to enhance the heat transfer mechanism. The results show that the optimum design of average volume temperatures for the PV cell models in air-cooled channel configurations with and without fins were 40.28 °C and 42.58 °C, respectively. The optimum design was obtained at 3, 110, 60, and 4 mm for baseplate thickness, fin spacing, height, and thickness, respectively. This optimum design was responsible for the average PV panel temperature drop by 1.6 %, 1.3 %, 5.9 %, and 6.2 % for baseplate thickness, fin spacing, height, and thickness, respectively. The optimum design of an air-cooled cooling channel for PV is an important insight provided by this work, and it may help in the future development of more effective and affordable cooling methods.

Snow-melting performance of the thermally conductive concrete pavement – experimental evaluation in field application

This paper evaluated the snow-melting performance of thermally conductive concrete incorporating silicon carbide (SiC) and copper powder (CP) in the field experiment. Before conducting the in-situ test, a mock-up test was previously performed. In the investigated properties of the mock-up test, the thermal conductivity test, normal concrete was 1.71 W/mK, the concrete with SiC 50% and CP 10% was 6.82 W/mK, and the concrete with SiC 100% and CP 10% was 9.22 W/mK. In addition, the ice layer melting test followed the thermal conductivity results as expected. The possibility of the snow-melting performance improvement was found in the mock-up test, therefore, the in-situ test was conducted. In the in-situ experiment, the thermal conductivity result of normal concrete (OPC-F) was 1.62 W/mK and the thermally conductive concrete (TC-F) was 3.23 W/mK. In addition, the compressive strength of OPC-F was 41 MPa and TC-F was 45 MPa. This phenomenon is related to the filler effect of SiC and CP according to the existing studies, it could be found in the field test as well. The key point was the snow-melting performance of TC-F, TC-F showed faster melting speed than OPC-F the 2 h after the starting time of the snow-melting test. Consequently, substituting a portion of aggregate with SiC and adding CP significantly enhances thermal transfer performance and snow-melting efficiency in cement composites. This study highlighted the importance of high thermal conductivity materials for cold-region road pavements and demonstrated practical application potential.

Facile synthesis of functionalized boron nitride nanosheets for photo‐cured 3D printing of high thermal conductive composites

AbstractAddressing the concern of heat accumulation in electronic devices, considerable research efforts have been dedicated to develop composites that are thermally conductive yet electrically insulative. Incorporating hexagonal boron nitride (h‐BN) as fillers shows promise in enhancing the thermal conductivity of polymer composites. However, notable challenges arise from difficulties in achieving well dispersion and establishing efficient three‐dimensional thermal conductive networks. This work demonstrated an efficient photocurable direct ink writing (DIW) printing method to fabricate functional h‐BN nanosheets (A‐BNNS)/epoxy acrylate (EA) resin‐based thermal conductive composites (A‐BNNS/EA). Apple polyphenol‐assisted ball milling was employed to fabricate functionalized BN nanosheets (A‐BNNS). Subsequently, A‐BNNS/EA inks were 3D printed using DIW to create thermally conductive composites, which were then UV‐cured. The composite containing 55 wt% A‐BNNS demonstrated an impressive thermal conductivity (TC) of 3.44 W/(m K). The utilization of 3D printing technology enables an onion‐like dispersion of A‐BNNS within the polymer matrix, effectively constructing three‐dimensional thermal conductive networks. This contribution significantly enhances the overall thermal conductivity.Highlights The functional BNNS (A‐BNNS) was achieved by apple polyphenol‐assisted ball milling. The utilization of photocurable 3D printing technology for producing high thermal conductivity materials are efficient. A‐BNNS exhibited an onion‐like distribution and effectively constructed a 3D thermal conductive network within the polymer matrix The composites demonstrate excellent thermal conductivity and favorable heat dissipation property.

Extreme sensitivity of higher-order interatomic force constants and thermal conductivity to the energy surface roughness of exchange-correlation functionals

In this Letter, we report that the fourth-order interatomic force constants (4th-IFCs) are significantly sensitive to the energy surface roughness of exchange-correlation (XC) functionals in density functional theory calculations. This sensitivity, which is insignificant for the second- (2nd-) and third-order (3rd-) IFCs, varies for different functionals in different materials and can cause misprediction of thermal conductivity by several times of magnitude. As a result, when calculating the 4th-IFCs using the finite difference method, the atomic displacement needs to be taken large enough to overcome the energy surface roughness, in order to accurately predict phonon lifetime and thermal conductivity. We demonstrate this phenomenon on a benchmark material (Si), a high-thermal conductivity material (BAs), and a low thermal conductivity material (NaCl). For Si, we find that the LDA, PBE, and PBEsol XC functionals are all smooth to the 2nd- and 3rd-IFCs but all rough to the 4th-IFCs. This roughness can lead to a prediction of nearly one order of magnitude lower thermal conductivity. For BAs, all three functionals are smooth to the 2nd- and 3rd-IFCs, and only the PBEsol XC functional is rough for the 4th-IFCs, which leads to a 40% underestimation of thermal conductivity. For NaCl, all functionals are smooth to the 2nd- and 3rd-IFCs but rough to the 4th-IFCs, leading to a 70% underprediction of thermal conductivity at room temperature. With these observations, we provide general guidance on the calculation of 4th-IFCs for an accurate thermal conductivity prediction.