Enhanced thermal conductivity of epoxy–matrix composites with hybrid fillers

The results of thermal conductivity study of epoxy–matrix composites filled with different type of powders are reported. Boron nitride and aluminum nitride micro‐powders with different size distribution and surface modification were used. A representative set of samples has been prepared with different contents of the fillers. The microstructure was investigated by SEM observations. Thermal conductivity measurements have been performed at room temperature and for selected samples it was also measured as a function of temperature from 300 K down to liquid helium temperatures. The most spectacular enhancement of the thermal conductivity was obtained for composites filled with hybrid fillers of boron nitride–silica and aluminum nitride–silica. In the case of sample with 31 vol.% of boron nitride–silica hybrid filler it amounts to 114% and for the sample with 45 vol.% of hybrid filler by 65% as compared with the reference composite with silica filler. However, in the case of small aluminum nitride grains application, large interfacial areas were introduced, promoting creation of thermal resistance barriers and causing phonon scattering more effective. As a result, no thermal conductivity improvement was obtained. Different characters of temperature dependencies are observed for hybrid filler composites which allowed identifying the component filler of the dominant contribution to the thermal conductivity in each case. The data show a good agreement with predictions of Agari‐Uno model, indicating the importance of conductive paths forming effect already at low filler contents. Copyright © 2014 John Wiley & Sons, Ltd.     

Rice Husk‐derived Hierarchical Micro/Mesoporous Carbon–Silica Nanocomposite as Superior Filler for Green Electronic Packaging Material

In this study, a carbon‐controllable hierarchical micro/mesoporous carbon–silica material derived from agricultural waste rice husk was easily synthesized and utilized as filler in an epoxy matrix for electronic packaging applications. Scanning electron microscopy, thermogravimetric analysis, and N₂ adsorption/desorption isotherms were used to characterize the morphology, thermal stability, carbon content, and porous structural properties, respectively, of the as‐obtained carbon–silica material, namely rice husk char (RHC ). As a filler material, the uniformly dispersed RHC filler in the epoxy/RHC composite was easily prepared through hydrogen bonding of the silanol group of silica with the epoxy matrix. For electronic packaging applications, the thermal conductivity and thermomechanical properties (storage modulus and coefficient of thermal expansion) of the epoxy/RHC composites improved with increasing carbon content. Moreover, loading of the 40% RHC filler substantially enhanced the storage modulus of the epoxy/RHC composite (5735 MPa ) compared to the epoxy with 40% commercial silica filler (3681 MPa ). Considerable commercial potential is expected for the carbon–silica composite because of the simple synthesis process and outstanding performance of the prepared packaging material.     

Epoxy‐based composite adhesives: Effect of hybrid fillers on thermal conductivity,rheology, and lap shear strength

Thermal management is an important parameter in an electronic packaging application. In this work, three different types of fillers such as natural graphite powder (Gr) of 50‐μm particle size, boron nitride powder (h‐BN) of 1‐μm size, and silver flakes (Ag) of 10‐μm particle size were used for thermal conductivity enhancement of neat epoxy resin. The thermal properties, rheology, and lap shear strength of the neat epoxy and its composite were investigated. The analysis showed that the loading of different wt% of Gr‐based fillers can effectively increase the thermal conductivity of the epoxy resin. It has also been observed that the thermal conductivity of the hybrid filler (Gr/h‐BN/Ag) reinforced epoxy adhesive composite increased six times greater than that of neat epoxy resin composite. Further, the viscosity of hybrid filler reinforced epoxy resin was found to be increased as compared with its virgin counterpart. The adhesive composite with optimized filler content was then subsequently subjected to determine single lap shear strength. The degree of filler dispersion and alignment in the matrix were determined by scanning electron microscopy (SEM) analysis.     

Synergistic effect of hybrid graphene and boron nitride on the cure kinetics and thermal conductivity of epoxy adhesives

In the present study, the synergistic effect of hybrid boron nitride (BN) with graphene on the thermal conductivity of epoxy adhesives has been reported. Graphene was prepared by chemical reduction of graphite oxide (GO) in a mixture of concentrated H₂SO₄/H₃PO₄ acid. The particle size distribution of GO was found to be ~10 μm and a low contact angle of 54° with water indicated a hydrophilic surface. The structure of prepared graphene was characterized by Fourier transform infrared (FTIR), X‐ray diffraction (XRD), Raman spectroscopy and atomic force microscopy (AFM). The thermal conductivity of adhesives was measured using guarded hot plate technique. Test results indicated an improvement in the thermal conductivity up to 1.65 W/mK, which was about ninefold increase over pristine epoxy. Mechanical properties of different epoxy formulations were also measured employing lap shear test. The surface characterization of different epoxy adhesive systems was characterized through XRD, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) studies. Fourier transform infrared also served to determine the nature of interactions between filler particles and epoxy resin. Non‐isothermal differential scanning calorimetric (DSC) technique was used to investigate the effects of graphene and BN particles on the cure kinetics and cross‐linking reaction of epoxy cured with amine curing agent. The Kissinger equation, the model‐free isoconversional Flynn–Wall–Ozawa method and the Ozawa model were used to analyze the kinetic parameter. Copyright © 2017 John Wiley & Sons, Ltd.     

Thermal properties and moisture absorption of nanofillers‐filled epoxy composite thin film for electronic application

This current study aimed to enhance the thermal conductivity of thin film composites without compromising other polymer qualities. The effect of adding high thermal conductivity nanoparticles on the thermal properties and moisture absorption of thin film epoxy composites was investigated. Three types of fillers in nanosize with high thermal conductivity properties, boron nitride (BN), synthetic diamond (SD), and silicon nitride (Si₃N₄) were studied. SN was later used as an abbreviation for Si₃N₄. The contents of fillers varied between 0 and 2 vol.%. An epoxy nanocomposite solution filled with high thermal conductivity fillers was spun at 1500–2000 rpm to produce thin film 40–60 µm thick. The effects of the fillers on thermal properties and moisture absorption were studied. The addition of 2 vol.% SD produced the largest improvement with 78% increment in thermal conductivity compared with the unfilled epoxy. SD‐filled epoxy thin film also showed good thermal stability with the lowest coefficients of thermal expansion, 19 and 124 ppm, before and after T_g, respectively, which are much lower compared with SN‐filled and BN‐filled epoxy thin film composites. However the SD‐filled epoxy film has its drawback as it absorbs more moisture compared with BN‐filled and SN‐filled epoxy film. Copyright © 2012 John Wiley & Sons, Ltd.     

Novel polyphenylene oxide microcapsules filled with epoxy resins

Novel polyphenylene oxide (PPO) microcapsules filled with epoxy resins (PPOMCs) were synthesized by in situ polymerization technology with 2, 6‐dimethy phenol as shell materials and diglycidyl ether of bisphenol A epoxy resins as core materials. The structures and morphologies of PPOMCs were characterized using Fourier‐transform infrared spectroscopy, micro‐confocal Raman microscope, laser scanning confocal microscopy, scanning electron microscopy and optical microscopy, respectively. The thermal properties of PPOMCs were investigated using differential scanning calorimetry and thermogravimetric analysis. The influences of different processing parameters such as the weight ratio of shell material to core material, kind of surfactant and reaction temperature on the morphologies and sizes of PPOMCs were investigated. Preliminary investigation on application of PPOMCs to thermosetting resins 4,4′‐bismaleimidodiphenylmethane/O,O′‐diallylbisphenol A (BMI/BA) system was conducted. Results indicate that PPOMCs can be synthesized successfully. The sizes and surface morphologies of PPOMCs may be significantly affected by different processing parameters. PPOMCs can be well prepared at about 30°C, and they depend strongly on the kind of surfactant and the weight ratio of shell material to core material. PPOMCs basically exhibit high thermal stability when the temperature is below 258°C. The addition of PPOMCs can improve the mechanical properties and maintain the thermal properties of BMI/BA system. The released core materials from PPOMCs may repair the matrix cracks through the polymerization of epoxy resins initiated by curing agent. Copyright © 2012 John Wiley & Sons, Ltd.     

Simultaneously improving the thermal conductive and flame retardant performance for epoxy resins thermosets by constructing core‐shell‐brush structure and distributing of MWCNTs in brush intervals

Fire safety and thermal dissipation performance of epoxy resins thermosets were critical for its application in key fields such as electronic devices. The simultaneous improvement of flame retardant and thermal conductivity properties were still a challenge. Herein, ammonium polyphosphate (APP) was firstly encapsulated with 5‐wt% epoxy resins based on APP and then surface grafted with polyurethane polymer chain, and the resulting APP with core‐shell‐brush structure was constructed. Finally, the multiwalled carbon nanotube (MWCNT) was assembled in the intervals of polymer brush on APP surface, and the prepared filler was defined as MF‐APP. Its chemical structure and morphologies were characterized and confirmed. The wettability of MF‐APP was evaluated by water contact angles tests (WCA) and MF‐APP exhibited hydrophobic property with the WCA of 138°. When 9‐wt% MF‐APP was incorporated into EP thermosets, the thermal conductive value of EP/MF‐APP achieved 1.02 Wm^?1 K^?1, and the MWCNTs concentration was only 1.8 wt% in thermosets. Compared with the previous work, the prepared EP/MF‐APP thermosets exhibited outstanding thermal conductive efficiency because of the homogeneously distribution of MWCNTs. Moreover, the samples fulfilled UL‐94 V‐0 grade during vertical burning tests with the limiting oxygen index of 30.8%. As a result, the thermal conductivity and flame retardancy of EP thermosets were simultaneously enhanced with a relatively low addition amount of MF‐APP, which would bring more chance for wider application of EP thermosets in key fields.     

Enhanced Thermal Stability in SiO2/Carbon Filler Derived from Rice Husk via Microwave Treatment for Electronic Packaging Application

Here we report the effect of microwave treatment on a silica–carbon (SiO₂ /C) filler derived from rice husk and the function of the microwave‐treated filler in an epoxy matrix for electronic packaging applications. Thermogravimetric analysis revealed improved thermal stability of the SiO₂ /C filler upon microwave treatment. X‐ray diffraction analysis indicated partial SiC formation after the microwave treatment. For packaging applications, compared to that of the pure epoxy polymer, the thermal conductivity of the epoxy–SiO₂ /C composite was improved by 178% at 40 wt % content of the microwave‐treated SiO₂ /C filler. Furthermore, an improvement of 149% in storage modulus and 17.6°C in glass transition temperature of the epoxy–SiO₂ /C composites was realized. The improvement in thermal stability of SiO₂ /C filler could be achieved via a simple microwave treatment, which in turn enhanced the thermal stability, thermal conduction, and thermomechanical strength of the electronic packaging materials.     

Silane surface modification of boron nitride for high thermal conductivity with polyphenylene sulfide via melt mixing method

Polyphenylene sulfide (PPS) is a promising engineering polymer, which is used for various industrial applications. In this study, we developed a highly thermally conductive PPS composite containing boron nitride (BN) as a thermally conductive ceramic filler. (3‐Aminopropyl) triethoxysilane was doped onto the surface of hydroxyl‐functionalized BN using a simple sol–gel process. The modified BN particles were embedded in a PPS matrix via a melt mixing process using a twin extruder to form BN‐Si composites. The maximum thermal conductivity 3.09 W/m·K was exhibited by the surface‐modified BN‐Si containing 60 wt%. This value was 116% higher than the thermal conductivities of the pristine BN and PPS matrix, respectively. The surface‐treated composites also showed an improved storage modulus because of an improvement in the interfacial adhesion and interaction between the BN filler and the PPS matrix. Copyright © 2017 John Wiley & Sons, Ltd.     

Effect of nano/micro‐mixed ceramic fillers on the dielectric and thermal properties of epoxy polymer composites

Nano/micro ceramic‐filled epoxy composite materials have been processed with various percentage additions of SiO₂, Al₂O₃ ceramic fillers as reinforcements selected from the nano and micro origin sources. Different types of filler combinations, viz. only nano, only micro, nano/micro, and micro/micro particles, were designed to investigate their influence on the thermal expansion, thermal conductivity, and dielectric properties of epoxy polymers. Thermal expansion studies were conducted using thermomechanical analysis that revealed a two‐step expansion pattern consecutively before and after vitreous transition temperatures. The presence of micro fillers have shown vitreous transition temperature in the range 70–80°C compared with that of nano structured composites in which the same was observed as ~90°C. Similarly, the bulk thermal conductivity is found to increase with increasing percentage of micron‐size Al₂O₃. It was established that the addition of micro fillers lead to epoxy composite materials that exhibited lower thermal expansion and higher thermal conductivity compared with nano fillers. Moreover, nano fillers have a significantly decisive role in having low bulk dielectric permittivity. In this study, epoxy composites with a thermal expansion coefficient of 2.5 × 10^?5/K, thermal conductivity of 1.18 W/m · K and dielectric permittivity in the range 4–5 at 1 kHz have been obtained. The study confirms that although the micro fillers seem to exhibit good thermal conductivity and low expansion coefficient, the nano‐size ceramic fillers are candidate as cofillers for low dielectric permittivity. However, a suitable proportion of nano/micro‐mixed fillers is necessary for achieving epoxy composites with promising thermal conductivity, controlled coefficient of thermal expansion and dielectric permittivity. Copyright © 2013 John Wiley & Sons, Ltd.     

Transition‐Metal Nitride Core@Noble‐Metal Shell Nanoparticles as Highly CO Tolerant Catalysts

Core–shell architectures offer an effective way to tune and enhance the properties of noble‐metal catalysts. Herein, we demonstrate the synthesis of Pt shell on titanium tungsten nitride core nanoparticles (Pt/TiWN) by high temperature ammonia nitridation of a parent core–shell carbide material (Pt/TiWC). X‐ray photoelectron spectroscopy revealed significant core‐level shifts for Pt shells supported on TiWN cores, corresponding to increased stabilization of the Pt valence d‐states. The modulation of the electronic structure of the Pt shell by the nitride core translated into enhanced CO tolerance during hydrogen electrooxidation in the presence of CO. The ability to control shell coverage and vary the heterometallic composition of the shell and nitride core opens up attractive opportunities to synthesize a broad range of new materials with tunable catalytic properties.     

Enhanced conductivity composites for aircraft applications: carbon nanotube inclusion both in epoxy matrix and in carbonized electrospun nanofibers

The potential of carbonized electrospun nanofiber mats to render epoxy resin composites for aircraft applications electrically and thermally more conductive was investigated. The effect of carbon nanotube inclusion both inside the carbon nanofiber and in the epoxy resin matrix material was studied, in order to reveal any synergistic effects of multilevel presence of nanosized reinforcements on the conductivity and mechanical properties. The carbon nanotube inclusion into the carbonized nanofibers increased the electrical conductivity of the samples by 20–50% and the thermal conductivity by approximately three times leading to a higher value than that of the conventional composites. The preparation of layered composites with a conductive upper layer containing nonwoven carbon nanofabric and a load bearing lower layer with conventional unidirectional carbon fiber reinforcement can offer a cost‐effective and weight‐saving solution for the replacement of metal meshes in structural aircraft composites. Copyright © 2014 John Wiley & Sons, Ltd.     

Polymer/boron nitride nanosheet composite with high thermal conductivity and sufficient dielectric strength

An efficient method was reported to fabricate boron nitride (BN) nanosheets using a sonication–centrifugation technique in DMF solvent. Then non‐covalent functionalization and covalent functionalization of BN nanosheets were performed by octadecylamine (ODA) and hyperbranched aromatic polyamide (HBP), respectively. Then, three different types of epoxy composites were fabricated by incorporation of BN nanosheets, BN‐ODA, and BN‐HBP. Among all three epoxy composites, the thermal conductivity and dielectric strength of epoxy composites using BN‐HBP nanosheets display the highest value, efficiently enhancing to 9.81 W/m K at 50 vol% and 34.8 kV/mm at 2.7 vol% (increase by 4057% and 9.4% compared with the neat epoxy), respectively. The significantly improved thermal conductivity and dielectric strength are attributed to the large surface area, which increases the contact area between nanosheets and nanosheets, as well as enhancement of the interfacial interaction between nanosheets and epoxy matrix. Copyright © 2015 John Wiley & Sons, Ltd.     

Effect of nano‐modified SiO2/Al2O3 mixed‐matrix micro‐composite fillers on thermal,mechanical, and tribological properties of epoxy polymers

Thermo‐mechanically durable industrial polymer nanocomposites have great demand as structural components. In this work, highly competent filler design is processed via nano‐modified of micronic SiO₂/Al₂O₃ particulate ceramics and studied its influence on the rheology, glass transition temperature, composite microstructure, thermal conductivity, mechanical strength, micro hardness, and tribology properties. Composites were fabricated with different proportions of nano‐modified micro‐composite fillers in epoxy matrix at as much possible filler loadings. Results revealed that nano‐modified SiO₂/Al₂O₃ micro‐composite fillers enhanced inter‐particle network and offer benefits like homogeneous microstructures and increased thermal conductivity. Epoxy composites attained thermal conductivity of 0.8 W/mK at 46% filler loading. Mechanical strength and bulk hardness were reached to higher values on the incorporation of nano‐modified fillers. Tribology study revealed an increased specific wear rate and decreased friction coefficient in such fillers. The study is significant in a way that the design of nano‐modified mixed‐matrix micro‐composite fillers are effective where a high loading is much easier, which is critical for achieving desired thermal and mechanical properties for any engineering applications. Copyright © 2016 John Wiley & Sons, Ltd.     

An overview of boron nitride based polymer nanocomposites

Recently, boron nitride (BN) based materials have received significant attention in both academic and industrial sectors due to its interesting properties like large energy band gap, good resistance to oxidation, excellent thermal conductivity, thermal stability, chemical inertness, significant mechanical property and widespread applications. This review article deals with the preparation and properties of boron nitride and its nanocomposites with various polymers. Diverse polymers have been explored for the preparation of boron nitride filled polymer nanocomposites by adopting different mixing methods. Properties of the resulting polymer nanocomposites mainly depend up on filler size and dispersion, mixing conditions and type of interaction between polymer matrix and the filler. Herein, the structure, preparation and properties of various boron nitride based polymer nanocomposites are reviewed in detail along with a brief overview of different classes of BN nanomaterials.     

Highly thermal conductive benzoxazine‐epoxy interpenetrating polymer networks containing liquid crystalline structures

A diamine‐based benzoxazine monomer (Bz) and a liquid crystalline epoxy monomer (LCE) are synthesized, respectively. Subsequently, a benzoxazine‐epoxy interpenetrating polymer network (PBEI) containing liquid crystalline structures is obtained by sequential curing of the LCE and the Bz in the presence of imidazole. The results show that the preferential curing of LCE plays a key role in the formation mechanism of liquid crystalline phase. Due to the introduction of liquid crystalline structures, the thermal conductivity of PBEI increases with increasing content of LCE. When the content of LCE is 80 wt %, the thermal conductivity reaches 0.32 W m^?1 K^?1. Additionally, the heat‐resistance of PBEI is superior to liquid crystalline epoxy resin. Among them, PBEI55 containing equal weight of Bz and LCE has better comprehensive performance. Its thermal conductivity, glass transition temperature, and the 5 % weight loss temperature are 0.28 W m^?1 K^?1, 160 °C, and 339 °C, respectively. By introducing boron nitride (BN) fillers into PBEI55, a composite of PBEI/BN with the highest thermal conductivity of 3.00 W m^?1 K^?1 is obtained. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017 , 55 , 1813–1821     

Preparation,Characterization and Thermal Energy Storage Properties of Micro/Nano Encapsulated Phase Change Material with Acrylic-Based Polymer

The eutectic mixture of octacosane (C28)–heptadecane (C17) as core material was successfully encapsulated with an acrylic-based polymer (polymethylmethacrylate; PMMA) as shell material through emulsion polymerization. The polymeric reaction occurred around the core material was characterized by FTIR spectroscopy analysis. The polarized optical microscopy, scanning electron microscopy and particle size distribution investigations showed that the most of the prepared capsules had almost spherical shape with non-unimodal size distribution in micro-nano range. The differential scanning calorimetry analysis results exhibited that the capsules including highest amount of the eutectic PCM had a melting temperature of about 21°C and a latent heat capacity of about 98 J/g. The high thermal durability of the prepared capsules was confirmed by thermogravimetric analysis. The thermal cycling test designated that the synthesized capsules had good long-term usage latent heat thermal energy storage (LHTES) performance and chemical stability. Furthermore; the fabricated capsules with (1: 2) shell/core ratio had a reasonable thermal conductivity. It can be drawn a conclusion from all results that the prepared capsules especially PMMA/(C28–C17) (1: 2) product is a hopeful PCM that can be evaluated for low-temperature LHTES applications.     

Manufacturing Graphene-Encapsulated Copper Particles by Chemical Vapor Deposition in a Cold Wall Reactor

Functional fillers, such as Ag, are commonly employed for effectively improving the thermal or electrical conductivity in polymer composites. However, a disadvantage of such a strategy is that the cost and performance cannot be balanced simultaneously. Therefore, the drive to find a material with both a cost efficient fabrication process and excellent performance attracts intense research interest. In this work, inspired by the core–shell structure, we developed a facile manufacturing method to prepare graphene-encapsulated Cu nanoparticles (GCPs) through utilizing an improved chemical vapor deposition (CVD) system with a cold wall reactor. The obtained GCPs could retain their spherical shape and exhibited an outstanding thermal stability up to 179 °C. Owing to the superior thermal conductivity of graphene and excellent oxidation resistance of GCPs, the produced GCPs are practically used in a thermally conductive adhesive (TCA), which commonly consists of Ag as the functional filler. Measurement shows a substantial 74.6 % improvement by partial replacement of Ag with GCPs.     

Epoxy nanocomposites with high thermal conductivity and low loss factor: Realize 3D thermal conductivity network at low content through core-shell structure and micro-nano technology

Dielectric polymers with high thermal conductivity are very promising in the fields of aerospace and electronic device packaging. However, composites with excellent dielectric properties usually have low thermal conductivity. It is usually to fill the polymer with thermal conductivity particles to improve the thermal conductivity, but the high content of filler often reduces the mechanical properties of the polymer. In this paper, the traditional insulating polymer epoxy resin was used as the matrix, by covering the surface of silicon carbide with graphene to form a core-shell structure and co-filled with nano diamonds to achieve the preparation of high-performance epoxy resin at low content. The results showed that at the filling content of 30 wt%, the thermal conductivity of epoxy nanocomposites showed a dramatic thermal conductivity enhancement of 1263%, the energy storage modulus increased by 1.1 GPa, and the dielectric loss remained unchanged at 50 Hz. The advantages of the composite are the structural design and surface modification of the filler, which not only take advantage of its inherent advantages, but also improve the interface area with the epoxy matrix. The composite materials with excellent properties are expected to provide theoretical guidance for the application of high thermal conductivity dielectric materials.     

Moisture–absorption,dielectric relaxation,and thermal conductivity studies of polymer composites

In the search for new packaging materials for the electrical/electronics industry, three types of polymer composites have been studied. Silicone/boron nitride powders, polyurethane/alumina powders, and polyurethane/carbon fibers have all been synthesized to study the moisture–absorption kinetics, thermal conductivities, and the dielectric loss spectra under various levels of humidity. The water uptake data indicate that water molecules are absorbed not only by the polymer matrix, but also by the interfaces introduced by the fillers. For all materials, the dielectric relaxation spectroscopy shows the presence of a peak in the 175–200 K range, which is largely due to absorbed water. The silicone/boron nitride samples absorbed the least amount of moisture. Incorporating this result with the thermal conductivity data of the three types of polymer composites, it is concluded that silicone polymers embedded with boron nitride can best serve as the coating for the electronic devices that require heat dissipation and moisture resistance, in addition to electrical insulation. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 2259–2265, 1998