Materials For Thermal Management Applications Research Articles

Direct measurements of thermal transport in glass and ceramic microspheres embedded in an epoxy matrix

The time-domain thermoreflectance metrology is applied to evaluate the thermal conductivities of filler particles embedded in a composite matrix. Specifically, a system of glass and ceramic microspheres with a diameter of 100 to 150 μm embedded in an epoxy matrix was used as a representation of a typical composite thermal interface material (TIM) suitable for microelectronics applications. These measurements provide a direct characterization of the thermal properties of filler materials. The measured thermal conductivities of both borosilicate glass and yttria stabilized zirconia microspheres agree well with literature values for bulk materials, whereas the thermal conductivity of the alumina microspheres is nearly 50% lower than that of bulk crystals. The reduction in thermal conductivity of the alumina microspheres highlights how important this level of understanding is for TIM development and is attributed to enhanced phonon scattering due to structural heterogeneity, such as defects induced by phase mixing and microvoids. Combining sample preparation, structural characterization, and direct thermal measurements, our study reveals the structure–thermal property relationship for individual microspheres. The results of this work can facilitate the design and engineering of composite-based thermally conductive materials for thermal management applications.

Design of 3D printed bioinspired nacre-like structured materials with significantly enhanced thermal conductivity

In this Letter, we report a simple approach for the preparation of bioinspired nacre-like structured materials with achievable high in-plane or through-plane thermal conductivity via digital light processing 3D printing under optimized printing parameters. Based on the 3D layer-by-layer formation, a vertical force exerted on each printing layer during the 3D printing process makes 2D platelets well-ordered in ultraviolet curable resin (hereafter UV resin), which is proved by the images of the scanning electron microscope and spectra of x-ray diffraction. It is found that a lower printing layer thickness leads to a higher orientation of Al2O3 platelets in the UV resin and greater thermal conductivity of the composites. The thermal conductivity of the structured composites reaches up to 2.622 W m−1 K−1 along the oriented direction at the loading of 30 wt. % of 2D Al2O3 platelets under the designed 3D printing layer thickness of 15 μm, which is about 14 times greater than that of pure UV resin. The surface temperature variations of the composites with time during heating and cooling, observed from the infrared thermograph, indicate the great potential of the 3D-printed structured materials for thermal management applications in electronic devices and electric equipment. It is predicted that fillers with greater intrinsic thermal conductivity and a larger diameter than the 3D printing layer thickness will lead to composites with greater thermal dissipation capability.

High-contrast, reversible change of thermal conductivity in hexagonal nickel-iron sulfides

Materials showing abrupt change of thermal conductivity are strongly desired for rapidly developing thermal regulation applications. Here, we report a sharp change of thermal conductivity (Δκ/κ0) up to 200% within a temperature interval of ~ 40 K in hexagonal sulfides Ni1-xFexS bonded with silver. Such Δκ/κ0 values exceed those reported in other thermal regulation materials, e.g., VO2 series (Δκ/κ0<100%). The large jump of thermal conductivity roots from the dramatic modification of the electronic structure at the first-order phase transition. Moreover, the addition of silver overcomes the inherent brittleness of Ni1-xFexS and thus improves machinability and thermal cycling stability, which is beneficial to the practical applications. Our work presents a new promising material for thermal management applications.

Boron nitride–nanosheet enhanced cellulose nanofiber aerogel with excellent thermal management properties

Boron nitride nanosheets (BNNSs) are desirable materials for thermal management applications owing to their excellent thermal conductivity. Such property also related to the increased thermal transfer efficiency per unit mass, which can be improved by constructing three-dimensional (3D) structures. Cellulose nanofibers (CNFs) are often used to prepare 3D skeletons and stabilize the dispersion of inorganic fillers. Therefore, CNF/BNNS aerogels were prepared to disperse BNNS in a 3D network. CNF/BNNS aerogel exhibited a thermal conductivity of 0.57 W m−1 K−1, which is an enhancement of 443 % compared to 0.105 W m−1 K−1 of CNF aerogel. The surface temperature changes of the aerogels were confirmed a strong thermal management capacity. The BNNSs also resulted in the compressive strength increased from 0.25 MPa (CNF aerogel) to 0.37 MPa (CNF/BNNS aerogel). This study provides an effective strategy for the preparation of cellulose-based nanocomposite aerogels with excellent thermal management properties and mechanical properties.

All-Metallic Phase Change Thermal Management Systems for Transient Spacecraft Loads

In this work, we explore the thermal properties of gallium as an effective phase change material for thermal management applications. Thermal storage and dissipation of gallium manufactured heat sinks were compared to conventional phase change heat sinks. The comparison revealed a 50-fold (80 K versus 1.5 K) potential reduction in temperature during the phase change process due to the high density, thermal conductivity, and latent heat of fusion. The gallium creates shallow thermal gradients when transiently heated, producing a nearly isothermal process. Computational estimates using lumped sum parameters were able to provide simple modeling to predict the results. Gallium based phase change devices offer a combination of low volume, small temperature drops across the device, simplicity of manufacture and design, and high energy storage applications.

Anisotropic Thermal Transport in Spray-Coated Single-Phase Two-Dimensional Materials: Synthetic Clay Versus Graphene Oxide.

Directional control on material properties such as mechanical moduli or thermal conductivity are of paramount importance for the development of nanostructured next-generation devices. Two-dimensional materials are particularly interesting in this context owing to their inherent structural anisotropy. Here, we compare graphene oxide (GO) and synthetic clay sodium fluorohectorite (Hec) with respect to their thermal transport properties. The unique sheet structure of both allows preparation of highly ordered Bragg stacks of these pure materials. The thermal conductivity parallel to the platelets strongly exceeds that perpendicular to them. We find a significant difference in the performance between GO and synthetic clay. Our analysis of the textured structure, size of the platelets, and chemical composition shows that Hec is a superior two-dimensional component to GO. Consequently, synthetic clay is a promising material for thermal management applications in electronic devices where electrically insulating materials are prerequisites.

Coefficient of thermal expansion (CTE) study in metal matrix composite of CuSiC vs AlSiC

Metal matrix composites are widely used as advanced materials for thermal management applications. In view of the demands in higher temperature products application, materials with low coefficient of thermal expansion (CTE) and high thermal conductivity are required. Copper Silicon Carbide (CuSiC) is highly rated as the best materials selected for this application because it has good thermal characteristics especially in CTE, but its density is almost high for high temperature electronic device application. An aluminium silicon carbide (AlSiC) composite, on the other hand, is a new candidate for electronic device application since it has low density and good CTE characteristics. CuSiC and AlSiC composites were fabricated via powder metallurgy method through variety volume fractions such as 5%, 10%, 15% and 20%. In the present study, the comparison of CTE between CuSiC and AlSiC composites with different volume fraction of SiC particles has been carried out. The CTE of CuSiC and AlSiC composites were investigated and their responses between 25°C to 400°C were studied using horizontal dilatometer. Based on the experiment results, the density of CuSiC composites are decrease while the density of AlSiC composites is increase with increasing in SiC particles content. However, the porosity for both composites is increases with increasing in volume fraction of SiC particles. As of CTE, both composites showed decreased with increasing in SiC particles content that dispersed in their matrix. The experimental CTE values for both composites seem to be much lower than theoretical CTE value, which means that there has been excellent bonding between each particle after sintering process. The particles distribution has been observed to understand more on the aspect of SiC particles homogeneity in copper and aluminium matrix.

Ragone Relations for Thermal Energy Storage Technologies

The Ragone relation is a facile approach to assess and compare electro-chemical battery performance in terms of two critical performance parameters: power density and energy density. This power and energy nexus is equally relevant for thermal energy storage materials for thermal management applications that require a balance between energy storage capacity and on-demand cooling or heating rates. Here, thermal energy storage is evaluated for sensible heating and for phase-change materials (PCMs). We propose an analytic expression using a lumped mass model for thermal storage through an analogy with heat diffusion that allows for intuitive mapping of materials and components in power-energy space. In addition, a previously proposed figure-of-merit, $\eta_q$, describing the intrinsic capability of PCMs to rapidly absorb or discharge heat is placed in the context of the thermal Ragone (power-energy) relation. This figure of merit serves as a proxy for the cooling power of PCMs and single-phase materials to store thermal energy. Thus, $\eta_q$ plotted against energy density can serve graphically to illustrate performance tradeoffs between different thermal storage materials, as well as composites composed of different materials.

Graphene Interlayer for Much Enhanced Interface Thermal Conductance in Metal Matrix Composites: An Approach Beyond Surface Metallization and Matrix Alloying

Effective heat dissipation in highly integrated electronic (nano-)devices is being a technical bottleneck in the microelectronic industry, where diamond/copper matrix (Dia/Cu) composite is considered as a promising heat sink material for thermal management applications. However, poor wettability and acoustic mismatch between diamond and Cu lead to low interfacial thermal conductance (ITC), producing much lower thermal conductivity (TC) of such a composite than that of pure Cu. Surface metallization and matrix alloying have widely been utilized for interface engineering by introducing carbide interlayer, while an uniform and nano-sized (< 100 nm) interlayer cannot be obtained and alloying decreases overall TC of matrix. Here, a novel and highly thermal conductive Dia/Cu composite component, using in-situ grown graphene as the highly effective interlayer, was prepared by vacuum hot pressing. The graphene interlayer improves interfacial wetting and mitigates acoustic mismatch between diamond and Cu, thereby improving ITC of composite. As results, the TC of diamond/graphene/copper (Dia/Gr/Cu) composite is 61% higher than that of the Dia/Cu counterpart. Differential effective medium (DEM) calculation further indicates that the Dia/Gr/Cu interface exhibits an ITC ~3.7 times higher than the Dia/Cu counterpart. This study provides a new approach for interface modification by 2D materials for high TC Dia/Cu composite beyond surface metallization and matrix alloying.

Enzymatic degradation, electronic, and thermal properties of graphite‐ and graphene oxide‐filled biodegradable polylactide/poly(ε‐caprolactone) blend composites

ABSTRACTCommodity polymers are the most widely used materials for electronic packaging applications. However, they are nondegradable and causing serious environmental damage. Addressing this challenge, the relative effects of graphite (G) and graphene oxide (GO) dispersion on the enzymatic degradation, electronic properties, thermal degradation, and crystallization behavior of enzyme degradable polylactide/poly(ε‐caprolactone) blend composites is investigated. Owing to the oxygenated surface functionalities and excellent thermal conductivity arising from the carbon structure, the randomly dispersed GO particles do not provide electrical pathways and facilitate large enhancements in the electrical resistivity (126%) and thermal conductivity (72%) of the blend composites. However, while the G particles enhanced the thermal conductivity of the composites, they had little effect on enzymatic degradation. Furthermore, they reduced the electrical resistivity, particularly at high concentration (0.25 wt % G), as a result of the conducting delocalized electrons in the G structure and due to network formation. We also find that the energy required to initiate and propagate the thermal degradation process for GO‐filled blend composites is relatively lower than that of G‐filled blend composite. However, the former composites show higher crystallization rate coefficients value than that of G‐filled composites and the neat blend, thereby providing better crystallization ability and miscibility with the matrix. In summary, the GO‐filled blend composites are observed to show potential for use in sustainable materials for thermal management applications. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019, 136, 47387.

Filler free technology for enhanced thermally conductive optically transparent polymeric materials using low thermally conductive organic linkers

This work presents for the first time a non-conventional technology for higher thermally conductive optically transparent polymeric material by employing lower thermally conductive organic molecules. In last couple of decades, practically all work on thermally conductive polymer based materials has been mainly confined to conventional polymer-filler composites which comes with various challenges like high loading, expensive fillers, etc., in addition to pronounced phonon scattering at filler-polymer interface. In this work, we have thrown light on alternating yet promising strategy for thermally conductive polymer based materials without incorporating any traditional metallic, ceramic or carbon fillers. One of the fascinating fact is the achievement of effective higher thermal conductivity in spite of using constituent components with lower thermal conductivity. This was achieved by rationally designed polymer-organic molecules system where thermal conduction was induced by engineering intermolecular interaction within the polymer chain. Additionally, these materials were optically transparent. Apart from presenting these new classes of materials, this study generates some of the prominent fundamental understanding on the heat transport of polymers. Short chains of diethylene glycol (DEG) were incorporated into base polymer matrix of polyvinyl alcohol (PVA) which lead to remarkable thermal conductivity enhancement of 260% and 175% than DEG and neat PVA respectively. Phonon transport was driven by the “thermal bridges” designed through intermolecular hydrogen bonding between PVA and DEG. A further impact of the length of “thermal bridges” and its functional groups (hydroxyl, carboxyl, carbonyl) at terminal position were investigated. It was found that shorter organic molecules are more efficient in driving thermal conduction owing to less inter-chain resistance. Interestingly, these materials were found to have inverse thermal conductivity–crystallinity relationship which is usually an opposite trend to the present belief. Overall, it was found that size, molecular geometry, terminal functional group of the thermal bridging chain in host polymer are very crucial factors in determining thermal conductance. Overall, this work demonstrates an intriguing tale of engineering intermolecular interaction in polymer based material for the development of thermally conductive and optically transparent materials for thermal management applications.

New insights into thermal conductivity of uniaxially stretched high density polyethylene films

Thermal conductivity of drawn polyethylene (PE) fibers has been extensively studied, while thermal conductivity for the stretched PE sheets still remains unknown. In addition, no consensus conclusions have been reached on the thermal conduction mechanisms for drawn PE. In this work, melt processed PE sheets are subjected to uniaxial stretching under different stretch ratios. The in-plane and out-of-plane thermal conductivity of stretched PE has been measured and the crystalline structure has been analyzed through 2-dimensional wide-angle X-ray diffraction (2D-WAXD) and scanning electron microscopy (SEM). Results show that both in-plane and out-of-plane thermal conductivities of stretched PE increase with increasing stretch ratios. The enhancement of thermal conductivity has been ascribed to the formation of interlocking shish-kebab superstructures induced by the stretching process. A new model is proposed to demonstrate the thermal conductivity improvement seen for the uniaxial-stretched PE. To the best of our knowledge, it is the first time that the mechanisms of thermal conductive enhancement were revealed for uniaxially stretched semi-crystalline PE sheet along with the understanding of the associated process-structure-property relationship. This work may provide key information towards design and manufacturing of polymer-based materials for thermal management applications.

Thermal, Electrical, and Mechanical Properties of Various Thermal Conductive Powder Filled Polyamide 6 Composite Materials for Thermal Management Applications

Thermal, Electrical, and Mechanical Properties of Various Thermal Conductive Powder Filled Polyamide 6 Composite Materials for Thermal Management Applications

Thermal conductivity of pillared graphene-epoxy nanocomposites using molecular dynamics

Thermal conductivity in a pillared graphene-epoxy nanocomposite (PGEN) is studied using equilibrium molecular dynamics simulations. PGEN is a proposed material for advanced thermal management applications because it combines high in-plane conductivity of graphene with high axial conductivity of a nanotube to significantly enhance the overall conductivity of the epoxy matrix material. Anisotropic conductivity of PGEN has been compared with that of pristine and functionalized carbon nanotube-epoxy nanocomposites, showcasing the advantages of the unique hierarchical structure of PGEN. Compared to pure carbon allotropes, embedding the epoxy matrix also promotes a weaker dependence of conductivity on thermal variations. These features make this an attractive material for thermal management applications.

VO2-dispersed glass: A new class of phase change material

Energy storage technology is crucial for a sustainable society, and its realisation strongly depends on the development of materials. Oxide glass exhibits high durability. Moreover, the amorphous structure of the glass without periodic ordering demonstrates excellent formability and controllability, thus enabling a large-scale production. These factors provide impetus for the development of new materials for thermal management applications. As vanadium dioxide (VO2) with a strongly correlated electron system exhibits a structural phase transition, leading to a large heat of transition. Therefore, VO2 demonstrates immense potential as a phase change material (PCM). This study reports the fabrication of VO2-dispersed glass and examines its potential as a new latent heat storage material, which can be applied for massive PCM heat storage applications.

Experimental and numerical investigation on the performance of carbon-based nanoenhanced phase change materials for thermal management applications

Phase change material (PCM)-based thermal management is one of the most promising approaches for the thermal management of energy conversion devices such as electric vehicle batteries and photovoltaic cells. However, most of the PCM present inherently low thermal conductivities, which result in deficient temperature control capacity in these applications. Several types of carbon-based nanoparticle have been explored to enhance the thermal conductivity of pure PCM. Despite the significant improvements in their thermal conductivity, the heat transfer in the liquid phase nano-enhanced PCM is expected to decrease due to the presence of the nanoparticles matrices. In this study, three types of carbon-based nanostructures are embedded in a paraffin to investigate their heat transfer performance for thermal management applications. The experimental results indicate that the addition of nanoadditives can improve the heat conduction of solid phase paraffin with slight latent heat degradation. But it also drastically increases the dynamic viscosity of composites which suppresses the natural convection heat transfer in the melted PCM. The thermal behavior of nanocomposites during the melting process is experimentally and numerically examined. Results reveal that adding carbon nanofibers and graphene platelets deteriorate the thermal performance of the pure paraffin. However, an enhanced thermal response observed when graphite-based nanocomposites at 7.5 and 10wt% are used due to their 620% and 1100% solid phase thermal conductivity enhancement, respectively. The results of the current work suggest that there is a trade-off between the thermal conductivity enhancement and natural convection suppression of nanocomposites that can be used in the optimal design of PCM-based thermal management systems.

Moisture driven thermal conduction in polymer and polymer blends

Though lots of work is available in thermally conductive filler-polymer composite, clear understanding of the heat conduction mechanism in neat polymers or polymer blends is a subject of great investigation especially for the promotion and development of new thermal management technologies. This study shows an interesting tale of intermolecular interaction in governing thermal conduction in polymers and how the quality of “thermal connections” through intermolecular interactions like hydrogen bonding can effectively drive heat propagation in polymers. Here by engineering intermolecular interaction through hydrogen bonding in polymer chains, a significant enhancement in thermal conductivity was achieved and corresponding decrement was observed by breaking those interactions. Thermal conductivity of free standing PVA films with different molecular weights and its blends with PEG and PVP was investigated. Effect of the absorption of vapor molecules of water, ethanol, methanol and ethylene diamine on polymer film's thermal conductivity was studied. Penetration of water molecule in films was found to drastically alter inter-molecular interaction contributing to remarkable 200% increment in thermal conductivity. Thermal conduction of polymer blends with varied intermolecular interactions was further explored. Neat PVA films with lower molecular weight had higher thermal conductivity due to less dense and hindered polymer chains and ease of interaction to form continuous thermal network. This study unveils some of the important yet fascinating drivers of thermal conductivity in polymers & polymer blends which can pave the way for development and fabrication of polymer based materials for thermal management applications.

Microstructure and Thermal Conductivity of Carbon Nanotube Reinforced Cu Composites.

Microstructure and thermal conductivity (TC) of carbon nanotubes reinforced Cu (CNT-Cu) composites have been studied. When CNTs were coated with nano Cu by electroless plating, the TC of CNT-Cu composites showed a noticeable improvement and increased with CNT contents. When 1.0 vol% CNTs was added, the TC of CNT-Cu composites increased to 420.4 W/(m · K), 30% higher than that of monolithic Cu (323.1 W/(m · K)). According to the measured TC of CNT-Cu composites, the interfacial thermal resistance of CNT-Cu composites was calculated as 3.0 × 10⁻⁹ m² K/W which was lower than the reported values of CNTs reinforced polymer matrix composites and ceramic matrix composites. Microstructures showed that CNTs modified with nano Cu were homogeneously dispersed and embedded in the Cu matrix, indicating that there was strong bonding between CNTs and Cu. The homogeneously dispersed CNTs and reduction of interfacial thermal resistance resulted in the improvement of thermal conductivity of CNT-Cu composites. Therefore, the prepared CNT-Cu composites are promising materials for thermal management applications.

Fabrication of TiCx-TiB₂/Al Composites for Application as a Heat Sink.

Metal matrix composites reinforced with ceramic particles have become the most attractive material in the research and development of new materials for thermal management applications. In this work, 40–60 vol. % TiCx-TiB2/Al composites were successfully fabricated by the method of combustion synthesis and hot press consolidation in an Al-Ti-B4C system. The effect of the TiCx-TiB2 content on the microstructure and compression properties of the composites was investigated. Moreover, the abrasive wear behavior and thermo-physics properties of the TiCx-TiB2/Al composite were studied and compared with the TiCx/Al composite. The compression properties, abrasive wear behavior and thermo-physics properties of the TiCx-TiB2/Al composite are all better than those of the TiCx/Al composite, which confirms that the TiCx-TiB2/Al composite is more appropriate for application as a heat sink.

Thermal transport across carbon nanotube-graphene covalent and van der Waals junctions

Carbon nanotubes and graphene are promising materials for thermal management applications due to their high thermal conductivities. However, their thermal properties are anisotropic, and the radial or cross-plane direction thermal conductivity is low. A 3D Carbon nanotube (CNT)-graphene structure has previously been proposed to address this limitation, and direct molecular dynamics simulations have been used to predict the associated thermal conductivity. In this work, by recognizing that thermal resistance primarily comes from CNT-graphene junctions, a simple network model of thermal transport in pillared graphene structure is developed. Using non-equilibrium molecular dynamics, the resistance across an individual CNT-graphene junction with sp2 covalent bonds is found to be around 6×10−11 m2K/W, which is significantly lower than typical values reported for planar interfaces between dissimilar materials. In contrast, the resistance across a van der Waals junction is about 4×10−8 m2K/W. Interestingly, when the CNT pillar length is small, the interfacial resistance of the sp2 covalent junction is found to decrease as the CNT pillar length decreases, suggesting the presence of coherence effects. To explain this intriguing trend, the junction thermal resistance is decomposed into interfacial region and boundary components, and it is found that while the boundary resistance has little dependence on the pillar length, the interfacial region resistance decreases as the pillar length decreases. This is explained by calculating the local phonon density of states (LDOS) of different regions near the boundary. The LDOS overlap between the interfacial region and the center region of CNT increases as the pillar length decreases, leading to the decrease of interfacial region resistance. The junction resistance Rj is eventually used in the network model to estimate the effective thermal conductivity, and the results agree well with direct MD simulation data, demonstrating the effectiveness of our model.