Dielectric Thermal Conductivity Research Articles

Heat dissipation promotion of LED chips by c-BN film

The light-emitting diodes (LEDs) are now the most important lighting devices due to their excellent wall-plug efficiency (WPE). However, their unique structure means that heat dissipation from the chip can only be achieved by conduction. The insulating thermal conductivity layer becomes a bottleneck in the heat transfer of the chip, seriously affecting its lifetime and reliability. Cubic boron nitride (c-BN) has good thermal stability, electrical insulation and high thermal conductivity, making it a potential insulating and heat dissipating material. In this study, we investigate the application of c-BN films to promote heat dissipation in LEDs based on c-BN films grown by physical vapor deposition (PVD) sputtering. In our study, we highlighted the performance variations between c-BN and Si substrates under different driving powers, ranging from 9 W to 12 W. By incorporating the c-BN film onto the Si substrate, we observed a distinct improvement in the temperature distribution of the LED array. Specifically, with a driving power of 9 W, the temperature was reduced by approximately 4.1 °C, and with 12 W, the reduction reached 16.8 °C. This enhanced thermal management not only augments the stability and lifespan of the components but also boosts the optical output power of the LED chip array, with an increase ranging from 21.3% to 27.9%. We find that c-BN films can effectively enhance chip heat dissipation, significantly improve WPE, and suppress light decay, as demonstrated by temperature distribution and luminescence power measurements. The experimental and simulation results demonstrate that c-BN films can reduce the temperature of LED beads by approximately 5–17 °C compared to Si substrates. This c-BN thin film substrate shows promise for use in commercial device heat dissipation structures.

Constructing a highly vertically aligned network of h-BN/CF in silicone rubber composites: Achieving superior through-plane thermal conductivity and electrical insulation

Polymer composites with both high through-plane thermal conductivity (TC) and electrical insulation are desperately needed for the efficient thermal management of modern electronic devices. Herein, a highly vertically aligned network of hexagonal boron (h-BN) nitride/carbon fiber (CF) was constructed in silicone rubber composites via the combination of multistage stretching extrusion, layer-by-layer stacking and hot-pressing procedures. The composites exhibited superior through-plane TC and electrical insulation, simultaneously. The through-plane TC of the composites reached 9.5 W/(m·K) when the content of h-BN and CF was 120 phr and 5 phr, respectively. Meanwhile, the volume resistivity and breakdown voltage of the composites were 1.89 × 1013 Ω cm and 7.7 kV/mm, respectively, since the low content of CF was insufficient to form electrically conductive pathways. Furthermore, both simulation and experimental results indicated that the composites show outstanding heat dissipation performance. The composites with such hybrid networks are very promising for efficient heat dissipation in high-performance electronic devices.

Investigation of novel expandable polystyrene/alumina aerogel composite thermal insulation material

Expandable polystyrene (EPS) foam is one of the most commonly used building thermal insulation materials due to its low thermal conductivity, apparent density as well and low cost. However, its high flammability and the release of a large number of toxic gases during combustion hinder its application. In this paper, using EPS foam and alumina aerogel (Al2O3) as insulation components, epoxy resin (EP) as a binder, and expandable graphite (EG) and ammonium polyphosphate (APP) as composite flame-retardant, a novel organic-inorganic composite thermal insulation material (EPS-AEA) was designed and fabricated. Experiment results showed that adding Al2O3 to EPS could further reduce the thermal conductivity of the obtained composite, and improve its mechanical properties. The addition of EG and APP composite flame-retardant could markedly enhance flame-retardant performance. With the addition of 4.17% Al2O3, and 50% EG/APP (1/1 mass ratio), the obtained EPS-AEA-1/1 composite showed superior fire resistance (reaching UL-94 V-0 rating and limiting oxygen index (LOI of 40.6%), high thermal insulation (thermal conductivity of 0.0487 W/(m·K)and applicable mechanical properties. In addition, EPS-AEA had excellent anti-hygroscopic properties (moisture ab-desorption rate within 0.5%) and fairly low toxic gas emissions during combustion, indicating its potential in practical application.

High-performance flexible thermoelectric generator based on silicone rubber and cover with graphite sheet

Harvesting thermal energy through a flexible thermoelectric generator (FTEG) offers an excellent micro-power solution for energizing node sensors in the realm of Internet of Things (IoT) and wearable electronics. Nonetheless, current FTEG suffer from drawbacks including low efficiency, significant thermal resistance, and complex manufacturing procedures. In this study, a high-performance FTEG using silicone rubber was designed and fabricated using a straightforward process. The finite-element method was used to optimize the copper electrode thickness, and the bendable substrate layers with various thermal conductivity were studied for the first time. The copper electrode thickness of 0.1 mm was selected because it offers high flexibility and bendability while still providing a relatively high-power output. The 5 × 5 cm2 FTEG device was fabricated and covered with a bendable substrate. Silicon rubber (0.08 Wm−1K−1), silicon rubber added 5% graphene (0.14 Wm−1K−1), and graphite sheets (15 Wm−1K−1) were used as bendable substrates. The FTEG cover with graphite sheets has a maximum output voltage of 1.1 V under a temperature difference (ΔT) at 65 °C. Its maximum output power is 162.4 mW, corresponding to a power density of 6499.1 µW/cm2 under the same above ΔT. The experimental findings indicated that integrating a bendable substrate with high thermal conductivity and electrical insulation properties enhances the performance of the FTEG.

Constructing Hierarchical Polymer Nanocomposites with Strongly Enhanced Thermal Conductivity.

The rapid advancement of communication technology has substantially increased the demand for advanced electronic packaging materials with high thermal conductivity and outstanding electrical insulation properties. In this study, we design polyvinyl alcohol/polydopamine-modified boron nitride nanosheet (PVA/BNNS@PDA) nanocomposites with hierarchical structures by combining electrospinning, vacuum filtration deposition, and hot pressing. The modified BNNS@PDA improves the interaction between the filler and the polymer matrix while reducing the interfacial thermal resistance, resulting in superior thermal conductivity, excellent insulation, and perfect flexibility. The PVA/BNNS@PDA nanocomposites possess an ultrahigh in-plane thermal conductivity of 16.6 W/(m·K) at 35.54 wt % BNNS@PDA content. Even after 2000 folds, the nanocomposites do not undergo any crack, showing their ultrahigh thermal conductivity behavior. Furthermore, the nanocomposites exhibit a volume resistivity above 1014 Ω·cm, which is well above the standard for insulating materials. Based on these results, this work provides a novel method to produce nanocomposites with high thermal conductivity, offering a new perspective to design advanced thermal management materials.

Heat Transfer Model and Thermal Insulation Characteristics of Surrounding Rock of Thermal Insulation Roadway in a High-Temperature Mine

The thermal insulation method is one of the effective methods for controlling the thermal environment of a high-temperature mine. In order to explore the thermal insulation mechanism and characteristics of thermal insulation roadways in high-temperature mines, a heat transfer model for the surrounding rock of the thermal insulation roadway was established based on the steady heat transfer theory. The temperature field of the surrounding rock of the thermal insulation roadway was studied, and the effects and sensitivities of thermal insulation layer thickness and thermal conductivity, convective heat transfer coefficient between roadway wall and airflow, and roadway radius on the thermal insulation performance of thermal insulation roadway were discussed. The results suggest the following: (1) The temperature gradient inside the thermal insulation layer is greater than that inside the surrounding rock. The thermal insulation roadway reduces the temperature difference between the original rock and the outside surface of the thermal insulation layer, thereby reducing the heat dissipation of the surrounding rock. (2) As the thermal insulation layer thickness increases, the thermal insulation capacity gradually increases, but its enhancement rate gradually weakens; as the thermal conductivity of the thermal insulation layer or the roadway radius increases, the thermal insulation capacity gradually decreases and its decline rate gradually weakens; and the convective heat transfer coefficient between the roadway wall and airflow has almost no effect on the thermal insulation capacity. (3) The thermal insulation performance of the thermal insulation roadway is highly sensitive or above the thickness and thermal conductivity of the thermal insulation layer, as well as the roadway radius. The sensitivity of thickness and thermal conductivity of the thermal insulation layer is greater than that of roadway radius. Therefore, the research results have guiding significance for the application of thermal insulation methods in the prevention and control of thermal hazards in mines.

Bilayer lattice structure integrated with phase change material for innovative thermal protection system design

In recent years, ceramic matrix composite (CMC) lattice structure has become a promising candidate for a new generation of integrated thermal protection systems (ITPSs) used in the ultrahigh-temperature environment. However, the thermal short circuit effect caused by lattice cores limits their application. In this work, we propose a bilayer CMC lattice structure filled with phase change material (PCM) and thermal insulation material for integrated thermal protection systems (ITPSs) to mitigate the thermal short circuit effect. The numerical simulation method is used to verify the effectiveness of the bilayer lattice structure in mitigating the thermal short circuit effect compared with one-layer ITPS. By inserting the middle face sheet, the thermal insulation effect enhances about 16%. Furthermore, the effects of thermal insulation materials, thermal conductivity and thickness of paraffin/expanded graphite composite PCM, and the diameter of the lattice core strut on the thermal insulation performance are discussed. The results show that the thickness of PCM has an optimal solution. Optimizing results show that when the aerogel insulation fiber is used as the thermal insulation material, the thermal conductivity k=0.82 W/(m⋅K) and the thickness t3=10 mm of PCM, the diameter of the core strut d=1.5 mm, the bilayer PCM-ITPS has the best thermal insulation performance. When the ultrahigh-temperature load is continuously loaded, the highest temperature of Top Face Sheet is more than 2300°C, but the highest temperature of Bottom Face Sheet is only 30.12°C, which thermal insulation effect is far better than the traditional ITPS. This work gives a novel scheme to solve the thermal short circuit effect, improving the thermal protection performance and accelerate the application of lattice structure in ITPS.

Ultra-high thermal conductivity FGN/PVA/MXene composite films with good electrical insulation

The polyvinyl alcohol (PVA) -based composite films with the high thermal conductivity and excellent electrical insulation have attracted more and more attention due to their advantages such as light weight and easy processing. Herein, a series of three-component fluorographene nanosheets (FGN)/PVA/MXene(FPM) films were prepared by vacuum assisted filtration technology with the single layer FGN (1.5–2.5 μm) and PVA as main raw materials and a small amount of Ti3C2Tx MXene nanosheets (200–600 nm) as the third phase. The FPM films had ultra-high in-plane thermal conductivity of 122.5 W/(m·K) when the addition of Ti3C2Tx MXene was 0.8 wt% of the composite film. The reason is that the Ti3C2Tx MXene nanosheets replace the high thermal resistance PVA molecules and connect the adjacent FGN powders horizontally (via hydrogen bonding), thus greatly reducing phonon scattering at the gap. The FPM films achieve extremely high in-plane thermal conductivity while maintaining excellent insulation and can be used for heat dissipation of small electrical appliances (such as mobile phone CPU). Meanwhile, it provides a new idea for the preparation of composite films with the low cost and high thermal conductivity.

Chitosan/silica hybrid aerogels with synergistic capability for superior hydrophobicity and mechanical robustness

Chitosan aerogels could be applied potentially in thermal insulation for energy-saving buildings, separation/adsorption, and catalysis. However, disadvantages of chitosan aerogels include their hydrophilicity and low insufficient mechanical strength. Here we propose a silica-phase hybriding route to create chitosan/silica hybrid aerogels with a synergistic capability for favourable hydrophobicity and superior mechanical strength, demonstrating an emergent finding (hydrophobicity optimised with the improved mechanical strength). The aerogels exhibit low drying shrinkage (as low as 13.41 %), lightweight (lowest to 0.149 g cm−1), high-efficient thermal insulation (thermal conductivity as low as to 0.024 W m−1 K−1 at room temperature and normal pressure) either under cryogenic (−196 °C) or high-temperature conditions, exceptional fire-retardancy (self-extinguishing in 1.8 s) and environmentally friendly characteristic (initial mineralisation after 10 d). High hydrophobic property (water contact angle up to 142°) of the aerogels were achieved depending upon 1H, 1H, 2H, 2H-perfluorodecyltriethoxysilane of vapor deposition, presenting a discovery concerning substantial improvement of mechanical properties (up to 0.188 MPa at 5 % strain, increased by 25 %). Furthermore, we demonstrate that a plausible mechanism for simultaneous hydrophobic and mechanical enhancement is depending upon the modulation of networking skeletons at the nanoscale.

Mechanical qualification and microstructural analysis of alumina produced by material extrusion

Ceramics with their advantageous properties such as high thermal conductivity and electrical insulation are attractive to circuit carrier materials in electronics production. Using material extrusion (MEX-AM) for reasons of flexible manufacturing, this work produces green bodies with low surface roughness, which is necessary for subsequent metalizing to create 3D circuit carriers. Systematic variation of post-treatment profiles (debinding, sintering) leads to different microstructures and thus influences mechanical properties. A comprehensive mechanical qualification of ceramic samples produced with MEX-AM and post-treated differently has been carried out, in which tensile, compressive and flexural strength are analyzed. Using the Anderson Darling Test, it is shown that statistical methods standardized for conventional ceramics are applicable to additively manufactured specimens. A mean bending fracture strength comparable to conventional ceramics could be achieved combining a low heating rate profile for debinding with a two-step sintering (TSS) approach, which resulted in reduction of grain size, as SEM analysis confirmed.

Carbonized CMPs Hollow Microspheres-Based Hydrogel Composite Membranes with a Hierarchical Architecture for Efficient Solar-Driven Interfacial Evaporation.

Solar-driven interfacial evaporation (SDIE) with excellent photothermal conversion efficiency is emerging as one of the frontier technologies for freshwater production. In this work, novel carbonized conjugate microporous polymers (CCMPs) hollow microspheres-based composite hydrogel membranes (CCMPsHM-CHM) for efficient SDIE are reported. The precursor, CMPs hollow microspheres (CMPsHM), is synthesized by an in situ Sonogashira-Hagihara cross-coupling reaction using a hard template method. The as-synthesized CCMPsHM-CHM exhibit significantly excellent properties, i.e., 3D hierarchical architecture (from micropore to macropore), superior solar light absorption (more than 89%), better thermal insulation (thermal conductivity as low as 0.32-0.42 W m-1K-1 in the wet state), superhydrophilic wettability with a water contact angle (WCA) of 0°, superior solar efficiency (up to 89-91%), a high evaporation rate of 1.48-1.51 kg m-2 h-1 under 1 sun irradiation, and excellent stability which maintains an evaporation rate of more than 80% after 10 cycles and over 83% evaporation efficiency in highly concentrated brine. In this case, the removal rate of metal ions in seawater is more than 99%, which is much lower than the ion concentration standard for drinking water set by the World Health Organization (WHO) and the United States Environmental Protection Agency (USEPA). Taking advantage of its simple and scalable manufacture, our CCMPsHM-CHM may have great potential as advanced membranes for various applications for efficient SDIE in different environments.

Thermally Insulating SiO2/PSA Composite Aerogels with Heat Resistance and Mechanical Properties via the Thiol-Yne Click Reaction.

Polymer-reinforced silica aerogels are thermally insulating materials employed to enhance mechanical properties; however, they exhibit low heat stability and require a complex production process. The main body of this work concerns the synthesis of silicon-containing polyarylacetylene (PSA) resin, which has exceptional thermal properties and is used to strengthen the gel skeleton and significantly improve the heat resistance of the polymer reinforcement phase. The honeycomb-like porous SiO2/PSA aerogels derived from directional freezing were obtained via click reaction, gel aging, freeze-drying, and curing without the requirement for time-consuming solvent replacement. The prepared SiO2/PSA aerogel is low density (∼0.3 g/cm3) and high porosity (∼80%), which provides the material with low levels of thermal conductivity (∼0.06 W/m·K) and excellent thermal insulation performance. When compared to the majority of polymer aerogels and aerogel-like materials, the prepared SiO2/PSA aerogels have high Td5 (∼460 °C) and Yr800 (∼80%) and compressive strength (compression strength > 1.5 MPa). SiO2/PSA composite aerogel has numerous functions in areas where materials must withstand extremely elevated temperatures, such as the aerospace industry.

Preparation of isotropic submicron spherical boron nitride particles by restricted template method

AbstractHexagonal boron nitride (h‐BN) has received considerable attention, due to its high thermal conductivity and electrical insulation. However, the intrinsic platelike structure with the strong anisotropic property restricts its applications, and it is necessary to synthesize isotropic spherical h‐BN particles (SP‐BNs) with submicron size. Till now, methods to prepare (SP‐BN) still exist problems, such as high oxygen impurities and pollution, generated by the ammonia and pyrolysis of precursors. Here, a relatively green reaction between the restricted template of carbon nanospheres and boron trioxide (B2O3) under elevated temperature is conducted, and the SP‐BNs with an average diameter of 200–300 nm (named Nano‐BN‐s) have been successfully synthesized. Comprehensive scanning electron microscopy, transmission electron microscopy, and X‐ray diffraction characterizations confirm the obtained products are spherical boron nitride. With the analysis of X‐ray photoelectron spectroscopy and Fourier transform infrared, the reaction mechanism is briefly discussed. These results indicate the reaction occurs on the restricted template of carbon nanospheres, and the C atoms are substituted by B and N atoms as the reaction progress, forming the Nano‐BN‐s. What is more, the restricted template method plays a key role in the design and improves h‐BN‐based materials in the future and may also be extended to form other novel materials.

Preparation of h‐BN microspheres for nanocomposites with high through‐plane thermal conductivity

AbstractIn recent times, electronics have been increasingly minimized, and hence, heat dissipation has become essential. Owing to its high thermal conductivity and superior electrical insulation, hexagonal‐boron nitride (h‐BN) has been regarded as an appropriate ceramic material to increase the thermal conductivity of polymer nanocomposites for effective heat dissipation. However, the poor through‐plane thermal conductivity of h‐BN severely restricts its practical uses, and it is favorable for heat to radiates in the in‐plane direction. In this study, densified spherical h‐BN (sph‐BN) microspheres, composed of as‐synthesized nano‐sized h‐BN (nano‐BN), were manufactured by a spray‐drying process with variations in organic and/or inorganic binders followed by sintering. After incorporating various sph‐BN particles as fillers into polydimethylsiloxane (PDMS), the through‐plane thermal conductivity of composites embedded with sph‐BN, assisted by a sodium silicate binder, enhanced the highest through‐plane and in‐plane thermal conductivities. The composites exhibited high thermal isotropy (through‐plane thermal conductivity/in‐plane thermal conductivity: ) of 0.77. The out‐of‐plane thermal conductivity of the composites was remarkably enhanced by over 2000% compared with pristine PDMS, which can be attributed to the synergy combined with the synthesis of the densified spherical BN initiated by nano‐BN, sintering, and the application of inorganic binders. This study proposes a simple method to prepare polymer composites with h‐BN that exhibit high through‐plane thermal conductivity and are promising materials for heat removal in electronics.

First-principles calculations of the AlN/Ti interface properties

AlN substrates are widely used in power modules and have high thermal conductivity and good insulation performance. In order to study the thermal stability of AlN substrates coated with the active metal, the AlN/Ti interface structure and the temperature dependence of the AlN layer orientation were investigated using the first principles calculations. The calculated work of separation and distance at elevated temperatures demonstrated good thermal stability of the interface. Elastic and shear modulus were calculated for the interface elastic anisotropy analysis and a certain orientation dependence was found. This study is helpful for the development of novel high-power microelectronic devices.

Design of Silicon Rubber/BN Film with High Through-plane Thermal Conductivity and Ultra-low Contact Resistance

Polymer-hexagonal boron nitride (BN) composite has become an ideal thermal interface material (TIM) for electronic devices because of its high thermal conductivity and superior electronic insulation. However, owing to the 2D shape and chemical inertness of BN filler, the vertical alignment of BN and the huge thermal resistance are current challenges, which hinder the efficient heat transfer of polymer/BN composites. Herein, by a novel non-solvent induced phase separation process combined “in-situ welding” strategy, we present the fabrication of silicone rubber film with finger-like continuous BN-welded filler skeleton, which reveals a high through-plane thermal conductivity of 15.4 W m−1K−1 at only ∼ 15 wt% BN. Finite element simulation and nonlinear model analyses theoretically confirm that the filler-to-filler interfacial thermal resistance (ITR) is halved after in-situ welding process. In addition, thanks to the excellent compressibility and conformability of silicon rubber matrix, the contact thermal resistance (<70 Kmm2W−1) of this composite film is much lower than that of the commercial thermal pad under different pressure. The proposed strategy opens up a novel and high-throughput preparation strategy for the high-performance TIM for modern electronic devices.

Research on thermal conductivity and electrical insulation properties of epoxy composites with fluorescent grafted zero-dimensional Al2O3

Research on thermal conductivity and electrical insulation properties of epoxy composites with fluorescent grafted zero-dimensional Al2O3

Lightweight, electrical insulating, and high thermally conductive all-polymer composites with reinforced interfaces

High thermally conductive materials with good electrical insulating and mechanical properties are indispensable for electronics. Aiming for all-polymer composites with light weight, electrical insulation and high thermal conductivity (TC), ultrahigh molecular weight polyethylene (UHMWPE) fibers/epoxy composites with two kinds of fiber orientation structures (0°/90°, ±45°) were fabricated by hot-pressing. It is found that the in-plane TC of ±45° composite is approximately 1.3–1.4 times that of 0/90° one, and the corresponding mechanism is verified by finite element simulation and model calculation. Thermally conductive pathways formed by UHMWPE fibers endow 0/90° and ±45° composites with the in-plane TC of 9.94 and 13.61 Wm−1K−1, respectively. Through depositing polydopamine (PDA) and grafting polyether amine (PEA) onto the surface of fibers, the poor fiber/epoxy interface was improved, resulting in 40.7% and 52.3% improvement in interlaminar shear strength (ILSS) and shear modulus, respectively. Simultaneously, a higher in-plane TC as high as 15.76 Wm−1K−1 is achieved. The heat dissipation evaluation indicates that the composite can reduce the temperature of the resistance wire by 14 °C. This work reveals the great potential of UHMWPE fibers in heat dissipation and provides a strong candidate for electrical insulating support materials with high TC for heat management of electronics.

Graphene nanosheet stabilized Pickering emulsion and its polystyrene nanocomposites

AbstractHigh thermal conductivity and electrical insulation materials are necessary for packaging materials of miniaturized electronic equipment at present. Graphene nanosheets (GNs) have great potential in polymer composites due to its excellent thermal conductivity. However, the ultra‐high conductivity of GNs will reduce the insulation of polymer composites, so that the GN composites have good thermal conductivity and electrical insulation has become the focus of research. Herein, the closed cell polystyrene nanocomposites are polymerized by using a Pickering emulsion route. Based on the exfoliating effect of pristine graphite at the oil–water interface and the amphiphilic properties of GNs during the preparation of emulsion, the resulting graphite can achieve good dispersion in the polystyrene resin. Furthermore, the high heat transfer rate of the resulting graphite makes the temperature of polystyrene resin increase by 60.2°C at room temperature within 1 min. Meanwhile, the high volume electrical resistivity (3.63 × 109 Ω·m) and breakdown strength (33.8 kV·mm−1) confirms the PNGL0.610%/PS composite keep excellent electrical insulation at 10.0 wt% filler loading.

Coating dense silicon carbide layer on artificial graphites to achieve synergistically enhanced thermal conductivity and electronic insulation of polymer composites

AbstractAs a potential thermal conductive filler, spherical graphite (SG) has high thermal conductivity, low density, and a good spherical shape structure. However, the poor electrical insulation of SG will seriously limit its application in highly integrated electronic device packaging. Herein, through a simple liquid–solid chemical reaction, the surface of the SG particle is firmly covered by a uniform silicon carbide (SiC) ceramic coating. Subsequently, a polycarbosilane precursor is immersed into the pore channels of the SiC layer and pyrolyzed at a high temperature to densify the coating. As compared to the original SG thermal pad, the SiC‐coated SG‐derived pad exhibits significantly enhanced electrical insulation and even higher thermal conductivity. As a result, the SiC‐coated SG‐based composite displays a high thermal conductivity of 3.36 W m−1 K−1, a satisfactory resistivity of 7.10 × 1010 Ω cm accompanied with a low density of 1.65 g/cm3. This work could provide the instructional designing protocol of the highly thermal conductive carbon‐based particles by coating ceramic layers for potential application in electronic packaging.