Isotropic Thermal Conductivity Research Articles

Preparations and Thermal Properties of PDMS-AlN-Al2O3 Composites through the Incorporation of Poly(Catechol-Amine)-Modified Boron Nitride Nanotubes

As one of the emerging nanomaterials, boron nitride nanotubes (BNNTs) provide promising opportunities for diverse applications due to their unique properties, such as high thermal conductivity, immense inertness, and high-temperature durability, while the instability of BNNTs due to their high surface induces agglomerates susceptible to the loss of their advantages. Therefore, the proper functionalization of BNNTs is crucial to highlight their fundamental characteristics. Herein, a simplistic low-cost approach of BNNT surface modification through catechol-polyamine (CAPA) interfacial polymerization is postulated to improve its dispersibility on the polymeric matrix. The modified BNNT was assimilated as a filler additive with AlN/Al2O3 filling materials in a PDMS polymeric matrix to prepare a thermal interface material (TIM). The resulting composite exhibits a heightened isotropic thermal conductivity of 8.10 W/mK, which is a ~47.27% increase compared to pristine composite 5.50 W/mK, and this can be ascribed to the improved BNNT dispersion forming interconnected phonon pathways and the thermal interface resistance reduction due to its augmented compatibility with the polymeric matrix. Moreover, the fabricated composite manifests a fire resistance improvement of ~10% in LOI relative to the neat composite sample, which can be correlated to the thermal stability shift in the TGA and DTA data. An enhancement in thermal permanence is stipulated due to a melting point (Tm) shift of ∼38.5 °C upon the integration of BNNT-CAPA. This improvement can be associated with the good distribution and adhesion of BNNT-CAPA in the polymeric matrix, integrated with its inherent thermal stability, good charring capability, and free radical scavenging effect due to the presence of CAPA on its surface. This study offers new insights into BNNT utilization and its corresponding incorporation into the polymeric matrix, which provides a prospective direction in the preparation of multifunctional materials for electric devices.

Anisotropic Fourier Heat Conduction and phonon Boltzmann transport equation based simulation of time domain thermo-reflectance experiments

The anisotropic Fourier Heat Conduction Equation (FHCE) and the multidimensional phonon Boltzmann Transport Equation (BTE) were solved numerically in cylindrical coordinates and in time domain to simulate a Time Domain Thermo-Reflectance (TDTR) experimental silicon/aluminum substrate/transducer setup. The out-of-phase response of the probe laser was predicted at various beam offset distances for a pump laser pulse frequency of 80 MHz and modulation frequency of 10 MHz and compared against experimental measurements for a silicon substrate. The isotropic FHCE was also solved for comparison. Results show that the isotropic FHCE with bulk thermal conductivity of 145 W/m/K significantly underpredicts the out-of-phase temperature difference, particularly at smaller beam offsets. With an isotropic thermal conductivity of 105 W/m/K, the computed results match experimental data at smaller beam offsets well, but overpredicts the experimental data at larger beam offsets. An almost-perfect match is obtained by using an anisotropic thermal conductivity wherein the radial (in-plane) thermal conductivity is set to 85 W/m/K and the axial (through-plane) conductivity is set to 130 W/m/K. The multidimensional frequency and polarization dependent phonon BTE is next solved. The BTE results for the out-of-phase temperature difference match experimental observations well at small and intermediate beam offsets, but overpredicts the experimental data at larger beam offsets. FHCE results are fitted to the BTE predictions, and the extracted (best fit) thermal conductivity is found to be 110 W/m/K.

Unleashing the Potential of Boron Nitride Spheres for High‐Performance Thermal Management

AbstractHighly integrated and miniaturized electronic devices require advanced thermal management techniques to improve reliability and performance. Thanks to their high thermal conductivity and electrical insulation, boron nitride nanosheets (BNNSs) are commomly used as fillers to construct thermally conductive polymer composites for heat dissipation. However, the BNNS reinforced composites exhibit anisotropic thermal conductivity due to the anisotropic structure of BNNSs. Micro‐sized boron nitride spheres (BNSs) with isotropic thermal conductivity are considered one of the best solutions. Nevertheless, precisely measuring the thermal conductivity of BNSs remains a challenge, limiting the understanding of the thermal transport mechanism. Herein, we have successfully estimated the thermal conductivity of BNSs using the laser flash method. Factors influencing BNSs’ thermal conductivity, including precursor, polymer binder and sintering temperature, are also investigated. Under optimized conditions, BNSs exhibit high, isotropic thermal conductivity of 37.2±2.5 W/(m ⋅ K), and the BNS pellet outperforms its h‐BN counterpart in heat dissipation for an LED light. This superiority is attributed to outstanding heat transfer performance in the cross‐plane direction, in addition to high in‐plane thermal conductivity. This study provides a feasible method to estimate the thermal conductivity of spherical materials and highlights promising boron nitride materials with isotropic thermal conductivity for heat dissipation in advanced electronics.

Studies on 8.4 W/m·K thermally conductive silicone rubber with high compressibility, high electrical insulation, high thermal reliability, and low cost

Thermally conductive silicone rubber (TCSR) has been widely used to enhance heat dissipation in electronics and energy storage devices. Currently, it is a challenge to produce TCSR which combines high thermal conductivity, high compressibility, high thermal reliability, high electrical insulation, and low cost. Here, we report a state-of-the-art TCSR which achieves such a combination. Owing to the optimized alumina gradation, the loading level of alumina in the TCSR reaches 96 wt%. The TCSR reaches a high and isotropic thermal conductivity of 8.4 W/m·K, a high compression ratio of 48 % (under a pressure of 6 psi or 42.4 kPa), and a high dielectric strength (>6 kV/mm). After heating at 180 °C for 100 h, little oil bleeding is observed. More importantly, these properties remain stable (variation < 10 %) after 2000 h of thermal aging at 135 °C, indicating high thermal reliability.

Application-Driven High-Thermal-Conductivity Polymer Nanocomposites.

Polymer nanocomposites combine the merits of polymer matrices and the unusual effects of nanoscale reinforcements and have been recognized as important members of the material family. Being a fundamental material property, thermal conductivity directly affects the molding and processing of materials as well as the design and performance of devices and systems. Polymer nanocomposites have been used in numerous industrial fields; thus, high demands are placed on the thermal conductivity feature of polymer nanocomposites. In this Perspective, we first provide roadmaps for the development of polymer nanocomposites with isotropic, in-plane, and through-plane high thermal conductivities, demonstrating the great effect of nanoscale reinforcements on thermal conductivity enhancement of polymer nanocomposites. Then the significance of the thermal conductivity of polymer nanocomposites in different application fields, including wearable electronics, thermal interface materials, battery thermal management, dielectric capacitors, electrical equipment, solar thermal energy storage, biomedical applications, carbon dioxide capture, and radiative cooling, are highlighted. In future research, we should continue to focus on methods that can further improve the thermal conductivity of polymer nanocomposites. On the other hand, we should pay more attention to the synergistic improvement of the thermal conductivity and other properties of polymer nanocomposites. Emerging polymer nanocomposites with high thermal conductivity should be based on application-oriented research.

Ultrafast sintering of boron nitride nanosheet assembled microspheres with strong processability for high-performance thermal management materials

A novel strategy produces BNNS microspheres for isotropic thermal conductivity, utilizing high-temperature ultrafast sintering and surface engineering to enhance processability for producing thermal management materials.

Magnetically assisted construction of Al2O3 platelets dual network and its excellent thermal conductivity in epoxy resin composites

Constructing three-dimensional thermally conductive networks in polymer matrix is a key strategy to improve the thermal management performance of electronic packaging materials. However, optimizing the thermal conductivity of the composite while retaining its electrical insulation properties still requires further structural design. Herein, the heat-transferring Al2O3 platelets dual network was constructed by using a simple compression moulding method combined with a magnetic alignment strategy. Al2O3@Fe3O4 was used for the magnetic orientation arrangement in the vertical direction of the laminated Al2O3 framework. And the obtained Al2O3/Al2O3@Fe3O4/Epoxy composites possessed satisfactory thermal conductivity. The thermal conductivity of the composites containing 34.63 vol% filler was as high as 2.30 W/mK (K‖) and 1.83 W/mK (K⊥), which were significantly higher by 945 % and 732 % compared to epoxy resin. In addition, the introduction of Al2O3@Fe3O4 in the composite promoted isotropic thermal conductivity and provided electromagnetic shielding properties.

Computational simulation study of the impact of isotropic GDL thermal conductivity on PEMFC characteristics

Computational simulation study of the impact of isotropic GDL thermal conductivity on PEMFC characteristics

Effects of high-temperature thermal reduction on thermal conductivity of reduced graphene oxide polymer composites

The graphitic crystalline structure of reduced graphene oxide (rGO) can be improved by high-temperature thermal reduction at various heat-treatment temperatures ranging from 1000 to 2500 °C. The crystallinity significantly increased with increasing heat-treatment temperature. The electrical conductivities of the rGOs heat-treated at 2000 and 2500 °C (h-rGO-2000 and h-rGO-2500, respectively) were similar to those of commercial graphite. The isotropic thermal conductivity of rGO/epoxy composite with 10 wt% h-rGO-2500 (2.56 W/mK) was 11.6 times higher than that of pristine rGO (p-rGO; 0.22 W/mK) and significantly superior to those of epoxy composites with commercial graphite (0.82 W/mK) and mesophase pitch-based carbon fibers (MPCFs; 1.29 W/mK). Moreover, owing to the synergistic effect operating in the MPCF–h-rGO hybrid filler in epoxy composites, this combination of fillers increased the thermal conductivity to a greater extent than the MPCF–p-rGO hybrid filler. Optimum synergistic effects on the isotropic and in-plane thermal conductivities were achieved with an MPCF:h-rGO-2000 weight ratio of 49:1 (11.90 and 17.93 W/mK, 1.48 and 1.85 times higher than 8.02 and 9.69 W/mK for MPCF–p-rGO, respectively). Finally, a machine learning method that could predict and optimize the properties of rGOs based on their heat-treatment temperatures and material compositions was developed.

Three-sensor 2ω method with multi-directional layout: A general methodology for measuring thermal conductivity of solid materials

Anisotropic thermal transport plays a key role in both theoretical study and engineering practice of heat transfer, but accurately measuring anisotropic thermal conductivity remains a significant challenge. To address this issue, we propose the three-sensor 2ω method in this study, which is capable of accurately measuring the isotropic or anisotropic thermal conductivity of solid materials. In this method, several three-sensor groups following the design guidelines are fabricated upon the sample along different characteristic directions, and each group consists of three parallel metal sensors with unequal widths and distances optimally designed based on sensitivity analysis. Among the three sensors, the outer two serve as AC heaters and the middle one as a DC detector. The 2ω voltage signals across the detector in each three-sensor group are measured, and then the data are processed by the proposed Intersection Method to derive thermal conductivities along directions of interest. The application of the detector's 2ω instead of the heater's 3ω voltage signals eliminates errors introduced by the uncertainties of thermal resistance in superficial structures (metal layer, insulation layer, interface, etc.). Meanwhile, by replacing the fitting algorithm with the Intersection Method, the local optimum trap of multivariate fitting is avoided. To verify the accuracy and reliability, four typical monocrystalline semiconductors, i.e., Si, GaN, AlN, and β-Ga2O3, are measured, and the results are consistent with the literature. This method will provide a comprehensive and versatile solution for the thermal conductivity measurements of solid materials.

Construction of three-dimensional interconnected boron nitride/sliver networks within epoxy composites for enhanced thermal conductivity

With the advent of the 5G era, the conspicuous increases in operating power and thermogenic capacity of integrated electronic components necessitated the development of enhanced thermal conductive polymeric composites with excellent heat dissipation properties. Herein, the polydopamine (PDA) and silver (Ag) modified BN (BN-PDA-Ag) was synthesized and a three-dimensional (3D) interconnected BN-PDA-Ag porous network was successfully fabricated by the salt template-assisted method to improve the thermal conductivity property of epoxy based composites. The high thermal conductive epoxy resin composites (3D BN-PDA-Ag/EP) were obtained by infiltrating epoxy into the 3D BN-PDA-Ag porous network. On one hand, the 3D BN-PDA-Ag porous skeleton provided continuous transfer paths for phonons, which was conducive to reducing phonon scattering. On the other hand, the Ag nanoparticles loaded on the BN surface bridged the adjacent BN sheets, significantly decreasing the thermal contact resistance between adjacent fillers. The isotropic thermal conductivity (TC) of 3D BN-PDA-Ag/EP composite with a BN-PDA-Ag content of 20.3 wt% reached 1.37 W/(m·K), which exhibited a evident enhancement of 661% compared with pure epoxy. The thermal management simulation and application results showed the composite had enormous application potential for electronic devices and components in heat dissipation.

Hierarchical evaluation of effective thermal conductivities of needled composites

Thermal conductivity (TC) is essential to characterize for thin-walled parts made of needled carbon/carbon (C/C) composites in the aviation sector. This paper thus proposes a hierarchical multiscale model to predict the TCs of needled C/C composites. This model can comprehensively consider the effects of the anisotropy of pyrolytic carbon matrices and the orientation and elongation of pores. At the microscale, representative volume element models of the needled fibers and weftless plies are established, respectively, and their TCs are estimated using the finite element homogenization method. Meanwhile, a combination of the finite element and orientation average method is developed for short-cut fiber plies. At the macroscale, a generated needled laminated model consisting of the weftless plies, short-cut fiber plies, and needled region, is established, where input properties are from homogenization results of microscale models. By comparing with experimental results, the proposed multiscale model is validated. The results show that effective TCs in the directions perpendicular and parallel to needled fibers drop by 13.18% and 13.06% respectively as the porosity increases from 6% to 14%. Additionally, it is found by microstructural parameter studies that it is possible to realize needled C/C composites with isotropic TCs.

ANSYS investigation of solar photovoltaic temperature distribution for improved efficiency

&lt;p&gt;A computational analysis of the influence of varying solar module material properties on operating temperature is presented and related to electrical conversion efficiency through the devised method. By varying the properties of density, specific heat capacity, and isotropic thermal conductivity for each material that comprises a solar module, density, and specific heat capacity were found to have the greatest influence on decreasing the operating temperature when increased by a factor of 50% for the glass layer, resulting in a decrease in temperature of 5.33 °C. Utilizing the devised method, which is based on the work of Palumbo, this temperature decrease was correlated to an electrical efficiency increase of 3.08%.&lt;/p&gt;

Clouds accreting from the IGM are not able to feed the star formation of low-redshift disc galaxies

ABSTRACT Galactic halos accrete material from the intergalactic medium (IGM) and part of this accretion is expected to be in the form of cool (T ∼ 104 K) gas. A signature of this process could reside in the detection of numerous clouds in the circumgalactic medium (CGM). However, whether this material is able to accrete onto the galaxies and feed their star formation or, instead, evaporates into the CGM hot phase (corona, T ∼ 106 K), is not yet understood. Here, we investigate the evolution of cool CGM clouds accreted from the IGM and falling through the hot corona of low-redshift disc galaxies, using 3D high-resolution hydrodynamical simulations. We include the effects of gravity due to the dark matter halo, isotropic thermal conduction, radiative cooling, and an ionizing UV background. We explored different values of parameters such as the halo mass, coronal mass, initial cloud velocity and strength of the thermal conduction. We find that the clouds lose the vast majority of their mass at distances larger than half of the galaxy virial radius and are completely dissolved in the corona before reaching the central galaxy. Resolving the Field length with at least 5–7 cells is crucial to correctly capture the evolution of the infalling cool gas. Our results indicate that cool IGM accretion can not feed star formation in z ∼ 0 star-forming galaxies in halos with masses of 1011.9 M⊙ or above. This suggests that present-day massive star-forming galaxies can sustain their star formation only via the spontaneous or induced cooling of their hot corona.

Enhancement of Isotropic Heat Dissipation of Polymer Composites by Using Ternary Filler Systems Consisting of Boron Nitride Nanotubes, h-BN, and Al2O3.

In this research article, a poly(dimethylsiloxane) (PDMS)-based composite was postulated adapting an interactive ternary filler system consisting of Al2O3, hexagonal boron nitride (h-BN), and boron nitride nanotubes (BNNT) to construct a continuous three-dimensional (3D) structure for thermal attenuation. Al2O3 was imposed as a main filler, while h-BN and BNNT were assimilated to form interconnected heat conduction pathways for effective thermal dissipation. The structured framework articulates a profound improvement in isotropic thermal conductivity considering both axial and radial heat dissipation. The presence of h-BN entails uniform heat distribution in a planar mode, eliminating the occurrence of hotspots, while BNNT constructed a connecting phonon pathway in various directions, which insinuates effective overall thermal transport. The generated ternary filler composites attained an isotropic ratio of 1.35 and a thermal conductivity of 7.50 W/mK, which is a 36-fold (∼3650%) increase compared to neat PDMS resin and almost 3-fold (∼297%) that of the Al2O3 unary filler composite and ∼53% that of its binary counterpart, partaking interfacial thermal gaps of ∼36.15 and ∼62.24% on practical heating performance relative to its counterparts. Moreover, the incorporation of BNNT on a traditional spherical and planar filler offers an advantage not only in thermal conductivity but also in thermal and structural stability. Improvement in thermal stability is stipulated due to a melting point (Tm) shift of ∼11 °C upon the assimilation of BNNT. Mechanical permeance reinforcement was also observed with the presence of BNNT, showcasing a 31.5% increase in tensile strength and a 53% increase in Young's modulus relative to the singular filler composite. This exploration administers a new insight into heat dissipation phenomena in polymeric composites and proposes a simple approach to their design and assembly.

Near-Theoretical Thermal Conductivity Silver Nanoflakes as Reinforcements in Gap-Filling Adhesives.

The rapid development of highly integrated microelectronic devices causes urgent demands for advanced thermally conductive adhesives (TCAs) to solve the interfacial heat-transfer issue. Due to their natural 2D structure and isotropic thermal conductivity, metal nanoflakes are promising fillers blended with polymer to develop high-performance TCAs. However, achieving corresponding TCAs with thermal conductivity over 10W m-1 K-1 at filler content below 30 vol% remains challenging so far. This longstanding bottleneck is mainly attributed to the fact that most current metal nanoflakes are prepared by "bottom-up" processes (e.g., solution-based chemical synthesis) and inevitably contain lattice defects or impurities, resulting in lower intrinsic thermal conductivities, only 20-65% of the theoretical value. Here, a "top-down" strategy by splitting highly purified Ag foil with nanoscale thickness is adopted to prepare 2D Ag nanoflakes with an intrinsic thermal conductivity of 398.2W m-1 K-1 , reaching 93% of the theoretical value. After directly blending with epoxy, the resultant Ag/epoxy exhibits a thermal conductivity of 15.1W m-1 K-1 at low filler content of 18.6 vol%. Additionally, in practical microelectronic cooling performance evaluations, the interfacial heat-transfer efficiency of the Ag/epoxy achieves ≈1.4 times that of the state-of-the-art commercial TCA.

Molecular Regulation of Flexible Composite Solid-Solid Phase Change Materials with Controllable Isotropic Thermal Conductivity for Thermal Energy Storage.

In recent years, graphene has been introduced into phase change materials (PCMs) to improve thermal conductivity to enhance the heat transfer efficiency in thermal energy storage. However, graphenes tend to aggregate in PCMs, leading to the low thermal conductivity efficient enhancement (TCEE), anisotropic thermal conductivity, and deterioration of mechanical performance of PCMs. In this work, we fabricated biomimetic thermally conductive solid-solid PCMs (SSPCMs) by facile blending of the graphene into well-designed polyurethane SSPCMs, in which the graphene established a controllable and highly efficient isotropic thermally conductive pathway based on the π-π stacking between the graphene and the polymer aromatic ring segment. The as-fabricated SSPCMs showed high TCEE (156.78%), excellent flexibility (328% elongation at break), high enthalpy value (>101 J/g), and solid-solid phase transition properties, under 2% loading of graphene. The proportion of in-plane to through-plane thermal conductivity can be adjusted by an elaborate design of the aromatic ring segment in polyurethane SSPCMs. We further demonstrated mechanical flexibility and photothermal property of the composites to reveal their potential in practical applications.

Reprocessable and Chemically Recyclable Hard Vitrimers Based on Liquid-Crystalline Epoxides.

The rapid increase in demand for recyclable and reusable thermosets has necessitated the development of materials with chemical structures that exhibit these features. Thus, functional mesogenic epoxide monomers bearing both ester and imine groups that can be vitrimerized and recycled are reported herein. The compounds show mesophase characteristics at 100-200°C and can be converted into hard epoxides by a common curing reaction. The obtained hard epoxides have high isotropic thermal conductivity (≈0.64 W m-1 K-1 ), which is derived from their highly ordered microstructures. The cured products can be easily reprocessed through imine metathesis and transesterification, and decomposed products can be obtained through imine hydrolysis under acidic or basic conditions and subsequently be re-cured. Surprisingly, recycled materials can be repeatedly reprocessed or chemically decomposed. The reprocessed materials retain the properties of their pristine counterparts, and the recycled products preserve the advantages of the hard thermosets without alteration to any of their unique properties. A dehydration reaction occurs between the residual hydroxyl groups during the re-hardening, which dramatically increases the glass transition temperature by ≈60°C. These reprocessable and recyclable vitrimers demonstrate the effectiveness and environmental friendliness of the molecular design strategy reported herein.

Joint‐Inspired Liquid and Thermal Conductive Interface for Designing Thermal Interface Materials with High Solid Filling yet Excellent Thixotropy

AbstractFor advanced thermal interface materials (TIMs), massive inorganic addition for high isotropic thermal conductivities conflicts with suitable rheological viscosity for low contact thermal resistance. Traditional strategies rarely resolve such a contradiction, and it remains an academic and industrial challenge. Herein, inspired by the structure and function of the bone joint, a best‐of‐both‐worlds approach is reported that endows a standard polydimethylsiloxane/alumina (PDMS/Al2O3) TIM with simultaneously enhanced rheological mobility and thermal conductivity. It is conducted by employing morphology‐controllable gallium‐based liquid metal (LM) to the surface of Al2O3 by a scalable mechanochemical process. At the typical polymer‐LM‐Al2O3 interface, LM droplets with low cohesive energy can release the freedom for macromolecular chain relaxation and reduce the viscosity, successfully allowing the high‐loading TIMs (79 vol.%) to keep the thixotropic state and effectively reducing its contact thermal resistance with a copper substrate by 65%. At the same time, adjacent LMs merge to thermally bridge separate Al2O3 particles, which facilitates the interfacial thermal conduction and enhances the thermal conductivity from 5.9 to 6.7 W m−1 K−1. Along with additional electrical insulation, this filler modification strategy is believed to inspire others to develop high‐performance polymer‐based TIMs for future advanced electronics.

Thermal analysis of surface micromachined porous silicon membranes using the 3ω method: Implications for thermal sensing

Investigations into porous silicon thermo-resistive type thermal sensors and their advantages in speed-sensitivity trade-off relative to other comparable thermal materials are presented in this work. Porous silicon films were suspended above a silicon substrate using successive patterning and micromachining steps. The 3ω method was used in both supported and suspended configurations, allowing the analysis of both cross-plane and in-plane thermal properties of the micromachined films. By utilising a low noise, broad-frequency measurement of the in-phase and out-of-phase temperature components, thermal conductivity and thermal diffusivity were simultaneously determined. The measurements showed isotropic thermal conductivity of 0.38 ± 0.02 W/m K and 0.12 ± 0.01 W/m K, and thermal diffusivity of 0.39 ± 0.04 mm2/s and 0.23 ± 0.02 mm2/s, for the surface micromachined released porous silicon films at 50 % and 77 % porosity, respectively. The implemented suspended 3ω method leveraged a thermal model that accounted for the finite length of the heater(180 µm) and membranes (400 µm × 400 µm), eliminating the millimetre scale dimensional constraints in previous works. Use of a thermal passivation technique rendered the thermal properties of the films robust against successive photolithography and micromachining. The obtained thermal properties were utilised in finite element modelling of a thermo-resistive thermal sensor with 50 µm × 50 µm × 100 nm dimensions. The modelling results suggest that a thermal time constant of 5 ms could be achieved, comparable to that of amorphous silicon based thermal sensors but with 2–4 times larger temperature sensitivity due to smaller intrinsic thermal conductivity and heat capacity in the porous silicon films, thus achieving a significant improvement in overall speed-sensitivity trade-off.