Polymer composites with enhanced thermal conductivity via oriented boron nitride and alumina hybrid fillers assisted by 3-D printing

Herein, oriented boron nitride (BN)/alumina (Al₂O₃)/polydimethylsiloxane (PDMS) composites were obtained by filler orientation due to the shear-inducing effect via 3-D printing. The oriented BN platelets acted as a rapid highway for heat transfer in the matrix and resulted in a significant increase in the thermal conductivity along the orientation direction. Extra addition of spherical Al₂O₃ enhanced the fillers networks and resulted in the dramatic growth of slurry viscosity. This, together with filler orientation induced the synergism and provided large increases in the thermal conductivity. A high orientation degree of 90.65% and in-plane thermal conductivity of 3.64 W/(m∙K) were realized in the composites with oriented 35 wt% BN and 30 wt% Al₂O₃ hybrid fillers. We attributed the influence of filler orientation and hybrid fillers on the thermal conductivity to the decrease of thermal interface resistance of composites and proposed possible theoretical models for the thermal conductivity enhancement mechanisms.     

Multi-contact hybrid thermal conductive filler Al2O3@AgNPs optimized three-dimensional thermal network for flexible thermal interface materials

In this work, a multi-contact Al₂O₃@AgNPs hybrid thermal conductive filler was synthesized by in-situ growth method to fill high thermal conductivity polydimethylsiloxane (PDMS)-based composites to prepare TIMs. And the thermal conductivity, electrical conductivity, and mechanical properties of the composite materials were studied. During the synthesis process of the multi-contact hybrid filler, different concentrations of silver ions were reduced to generate silver nanoparticles and attached to the surface of Al₂O₃. Al₂O₃@AgNPs/PDMS thermally conductive composites were prepared by changing the filler addition. Using SEM, XPS, and XRD is used to characterize the morphology and chemical composition of Al₂O₃@AgNPs hybrid filler. The thermal conductivity of PDMS-based composites with different AgNPs content under 70 wt% filler loading was studied. The results show that the thermal conductivity of PDMS-based composites filled with 7owt%Al₂O₃@3AgNPs/PDMS multi-contact hybrid filler is 0.67 W/m·K, which is 3.72 times that of pure PDMS, and is higher than that of unmodified Al₂O₃ with the same addition amount. /PDMS composite material has a high thermal conductivity of 24%. This work provides a new idea for the design and manufacture of high thermal conductivity hybrid fillers for TIMs.     

Spherical hybrid filler BN@Al2O3 via chemical adhesive for enhancing thermal conductivity and processability of silicon rubber

Hexagonal boron nitride (h-BN) is an ideal candidate material for electrical and electronic systems due to its excellent performance. However, the addition of platelet-like h-BN leads to a dramatic increase of viscosity of composites and anisotropic thermal conductivity of composites. Herein, modified h-BN (m-BN) was coated onto spherical α-Al₂O₃ via chemical adhesive, and core-shell structured hybrid spherical filler (m-BN@Al₂O₃) was prepared. Furthermore, the microstructure, rheology, mechanical properties, and thermal conductivity of hybrid filler/polydimethylsiloxane (PDMS) were studied. At 60 vol% filler loading, the thermal conductivity of m-BN@Al₂O₃/PDMS is up to 2.23 W·m⁻¹·K⁻¹, which is 86% higher than that of Al₂O₃/PDMS and the ratio of in-plane diffusivity to through-plane diffusivity decreases from 2.0 to 1.0. At meanwhile, the viscosity of m-BN@Al₂O₃/PDMS is about one fourth of the viscosity of m-BN/Al₂O₃/PDMS. This simple and versatile strategy opens a pavement for enhancing the thermal conductivity of polymer and has great potential in high-frequency communication.     

Preparation of nano‐reinforced thermal conductive natural rubber composites

Natural rubber (NR) composites highly filled with nano‐α‐alumina (nano‐α‐Al₂O₃) modified in situ by the silane coupling agent bis‐(3‐triethoxysilylpropyl)‐tetrasulfide (Si69) were prepared. The effects of various modification conditions and filler loading on the properties of the nano‐α‐Al₂O₃/NR composites were investigated. The results indicated that the preparation conditions for optimum mechanical (both static and dynamic) properties and thermal conductivity were as follows: 100 phr of nano‐α‐Al₂O₃, 6 phr of Si69, heat‐treatment time of 5 min at 150°C. Furthermore, two other types of fillers were also investigated as thermally conductive reinforcing fillers for the NR systems: (1) hybrid fillers composed of 100 phr of nano‐α‐Al₂O₃ and various amounts of the carbon black (CB) N330 and (2) nano‐γ‐Al₂O₃, the particles of which are smaller than those of nano‐α‐Al₂O₃. The hybrid fillers had better mechanical properties and dynamic performance with higher thermal conductivity, which means that it can be expected to endow the rubber products serving under dynamic conditions with much longer service life. The smaller sized nano‐γ‐Al₂O₃ particles performed better than the larger‐sized nano‐α‐Al₂O₃ particles in reinforcing NR. However, the composites filled with nano‐γ‐Al₂O₃ had lower thermal conductivity than those filled with nano‐α‐Al₂O₃ and badly deteriorated dynamic properties at loadings higher than 50 phr, both indicating that nano‐γ‐Al₂O₃ is not a good candidate for novel thermally conductive reinforcing filler. POLYM. COMPOS., 37:771–781, 2016. © 2014 Society of Plastics Engineers     

Novel heat‐conductive composite silicone rubber

The silicone rubber with good thermal conductivity and electrical insulation was obtained by taking vinyl endblocked polymethylsiloxane as basic gum and thermally conductive, but electrically insulating hybrid Al₂O₃ powder as fillers. The effects of the amount of Al₂O₃ on the thermal conductivity, coefficient of thermal expansion (CTE), heat stability, and mechanical properties of the silicone rubber were investigated, and it was found that the thermal conductivity and heat stability increased, but the CTE decreased with increasing Al₂O₃ fillers content. The silicone rubber filled with hybrid Al₂O₃ fillers exhibited higher thermal conductivity compared with that filled with single particle size. Furthermore, a new type of thermally conductive silicone rubber composites, possessing thermal conductivity of 0.92 W/mK, good electrical insulation, and mechanical properties, was developed using electrical glass cloth as reinforcement. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 104: 2478–2483, 2007     

Synthesis of a novel biphenyl epoxy resin and its hybrid composite with high thermal conductivity

A novel biphenyl epoxy monomer of p-methyl phenylhydroquinone epoxy resin (p-MEP) was synthesized and characterized. We researched its potential in the area of thermal conduction application and prepared a series of hybrid composites based on it with different mass ratios of sphere Al₂O₃ filler. From the good mobility and low viscosity of p-MEP, it allowed mixing with more Al₂O₃ fillers. The hybrid epoxy resins owned the advantages of traditional epoxy resins as well as quite considerable thermal conductivity. Therefore, the hybrid composite at the maximum mass fraction of 70% possess the highest thermal conductivity of 5.6 W mK⁻¹, which is 5.6 times higher than that of pristine p-MEP (0.1 W mK⁻¹). © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47078.     

Development of a ceramic-based composite for direct bonded copper substrate

In the present paper, a computational approach is presented to design alumina-based composite with tailored properties that could replace commercial alumina used in Direct Bonded Copper (DBC) substrates for applications in power electronic modules. A mean-field homogenization and effective medium approximation (EMA) using an in-house code is used for predicting potential optimum thermal and structural properties for DBC substrates by considering the effect of filler type, volume, and size in the alumina matrix. The primary goal for designing such alumina-based composites is to have enhanced thermal conductivity for effective heat dissipation and spreading capabilities together with a coefficient of thermal expansion (CTE) value that is close to the silicon chips in electronic circuits in order to avoid interface layers. At the same time, other functional properties like elastic modulus and electrical conductivity have to be maintained. Our strategy incorporates thermal and structural properties of composites as a constraint on the design process. Among various metallic and carbon-based fillers, chromium, silicon carbide and diamond fillers were found suitable candidates that could enhance the thermal and structural performance of the alumina-based substrates. As a validation, we developed alumina-silicon carbide (Al₂O₃-SiC) composites in line with the designed range of filler size and volume fraction using Spark Plasma Sintering (SPS) process. Thermal and structural properties including thermal conductivity, CTE, and elastic modulus are measured to complement the computational design. It is found that the developed computational design tool is accurate enough in predicting the desired properties of composite materials for DBC substrate applications.     

Effect of alumina on the structure and properties of polyimide matrix films

To improve the properties of polyimide (PI), different mass fractions of alumina (Al₂O₃) nanoparticles, unmodified or modified by KH550, were incorporated into PI matrix to form PI/Al₂O₃ hybrid films by in situ polymerisation. The effects of Al₂O₃ additives on the structure, dielectric and mechanical properties of the films were studied. Fourier transform infrared spectroscopy confirmed the successful preparation of PI/Al₂O₃ hybrid films, and the microstructures of the samples showed a more uniform dispersion of the modified Al₂O₃ nanoparticles than the unmodified ones in the matrix. The dielectric constant of the films increased with increasing filler content, and the maximum electrical breakdown strength of 311 MV m^?1 was obtained with a filler content of 8.0 wt-% modified Al₂O₃ in the matrix. Both unmodified and modified Al₂O₃-reinforced PI hybrids demonstrated improved mechanical properties compared with the PI matrix. Moreover, the properties of films with Al₂O₃ modified by KH550 were better.     

Properties of thermally conductive silicone rubbers filled with admicellar polymerized polypyrrole-coated Al2O3 particles

Polypyrrole (PPy) nanolayers were introduced on the surface of alumina (Al₂O₃) particles via admicellar polymerization. The properties of silicone rubbers (SRs) filled with PPy-coated Al₂O₃ and pristine Al₂O₃ as thermally conductive fillers were studied and compared. The results demonstrate that the addition of PPy-coated Al₂O₃ leads to a better interfacial compatibility but lower cross-linking density of the composites than pristine Al₂O₃. The improvement in the compatibility and the decrease in the cross-linking density are paradoxes in affecting mechanical properties. The improvement in the compatibility shows a slight predominance on the strength at low-filler contents. Lower cross-linking density of modified-Al₂O₃/SR composites led to a better processing performance and a higher maximum filler loading amount than the pristine Al₂O₃/SR composites, which is beneficial to increasing the thermal conductivity and maintaining a relatively good strength. The PPy-coated Al₂O₃/SR composite with 83 wt% filler content has a thermal conductivity of 1.98 W/(m K) and a tensile strength of 2.9 MPa, and the elongation at break was 63%. Functionalized fillers by admicellar polymerization used in the fabrication of filler/SR composites not only improve the interfacial compatibility but also optimize and expand the functions of the composites, which has great significance for the production and application of thermally conductive SR in some branches of industry (automotive, electrical engineering, etc.) in the future.     

Study on thermally conductive ESBR vulcanizates

The thermal conductivities of emulsion polymerized styrene-butadiene rubber (ESBR) vulcanizates filled with alumina (Al₂O₃), zinc oxide (ZnO), carbon nanotubes (CNTs), silicon carbide (SiC), are measured by steady-state method. The effects of types and loadings of the fillers and their mixture on thermal conductivities of the ESBR vulcanizates are investigated. The results show that the thermal conductivity of ESBR vulcanizates filled with alumina or zinc oxide, increases nearly linearly with increasing loading when the filler loading exceeded 20 phr; the ESBR vulcanizates filled with CNTs have the highest thermal conductivity at a given filler loading in comparison with other composite vulcanizates. At a given loading of 100 phr, the ESBR vulcanizate filled with two different particle sizes SiC of 1–3 and 5–11 μm at the mass ratio of 1:1 has the highest thermal conductivity and relatively good mechanical properties. The experimental results are analyzed using Geometric mean model and Agari’s equation to explain the effect of filler types and particle sizes on the formation of thermal conductive networks. The thermal conductivity of the ESBR vulcanizates filled with Al₂O₃ or ZnO or CNTs could be well predicted by optimized parameters using Agari’s equation for a polymer composite filled with mixtures of particles.     

Synergistic thermal conductivity enhancement of PC/ABS composites containing alumina/magnesia/graphene nanoplatelets

Graphene nanoplatelets (GNPs) have attracted considerable attention in the field of thermal management materials due to their unique structure and exceptional thermal conductive properties. In this work, we demonstrate a significant synergistic effect of GNPs, alumina (Al₂O₃), and magnesia (MgO) in improving the thermal conductivity of polycarbonate/acrylonitrile‐butadiene‐styrene polymer alloy (PC/ABS) composites. The thermal conductivity of the composites prepared through partial replacement of Al₂O₃ and MgO with GNPs could increase dramatically compared with that without GNPs. The maximum thermal conductivity of the composite is 3.11 W mK⁻¹ at total mass fraction of 70% with 0.5 wt% GNPs loading. It increases 60% compared with that without GNPs (1.95 W mK⁻¹). The synergistic effect results from the compact packing structure formed by Al₂O₃/MgO and the bridging of GNPs with Al₂O₃/MgO, thus promoting the formation of effective thermal conduction pathways within PC/ABS matrix. More importantly, together with the intrinsically high thermal conductivity of GNPs, boosted and effective pathways for phonon transport can be created, thus decrease the thermal resistance at the interface between fillers and PC/ABS matrix and increase the thermal conductivity of composites. POLYM. COMPOS., 38:2221–2227, 2017. © 2015 Society of Plastics Engineers     

Cure Characteristics,Mechanical and Thermal Properties of Al2O3 and ZnO Reinforced Silicone Rubber

The effects of alumina (Al₂O₃) and zinc oxide (ZnO) fillers on the curing characteristics, thermal and mechanical properties of silicone rubber were studied. Rheometer results indicate that the incorporation of ZnO fillers retards the curing process, whereas an enhancement in cure rate was observed for Al₂O₃. Higher maximum torque (M_H) and minimum torque (M_L) values was also observed for ZnO silicone rubber compounds compared to Al₂O₃. Thermogravimetric analysis (TGA) showed that ZnO silicone rubber compounds are thermally more stable than Al₂O₃; however, the coefficient of thermal expansion of the Al₂O₃ silicone rubber compounds are lower than that of ZnO. Comparison in mechanical strength between the two silicone rubber hybrids indicates that ZnO is a better reinforcement filler, as evidenced in the tensile strength, elongation at break, and modulus at 300% elongation.     

Rapid preparation of Palygorskite/Al2O3 composite aerogels with superhydrophobicity,high-temperature thermal insulation and improved mechanical properties

Al₂O₃ aerogels are widely employed in heat insulation and flame retardancy because of their unique combination of low thermal conductivity and exceptional high-temperature stability. However, the mechanical properties of Al₂O₃ aerogel are poor, and the preparation time is considerably long. In this study, we present a simple and scalable approach to construct monolithic Pal/Al₂O₃ composite aerogels using solvothermal treatment instead of traditional solvent replacement, which remarkably shortened the preparation time. Subsequently, to obtain stable superhydrophobicity (θ > 152°), the Pal/Al₂O₃ aerogel was modified by gas-phase modification method. The obtained Pal/Al₂O₃ composite aerogels demonstrate the integrated properties of low density (0.078–0.106 g/cm³), low thermal conductivity (1000 °C, 0.143 W/(m·K)), good mechanical properties (Young's modulus, 1.6 MPa), and good heat resistance. The monolithic Pal/Al₂O₃ composite aerogels with improved mechanical performance and improved thermal stability can show great potential in the field of thermal insulation.     

Simultaneously enhanced mechanical properties and thermal properties of ultrahigh‐molecular‐weight polyethylene with polydopamine‐coated α‐alumina platelets

Polydopamine (PDA) was employed to modify micrometric Al₂O₃ platelets to improve the interfacial compatibility between α‐Al₂O₃ powder and ultrahigh‐molecular‐weight polyethylene (UHMWPE). The structure of PDA‐coated Al₂O₃ and UHMWPE composites was investigated via Fourier transform infrared spectroscopy, scanning electron microscopy and X‐ray photoelectron spectroscopy. The thermal stability and mechanical performance of the samples were also evaluated. It is clear that UHMWPE/PDA‐Al₂O₃ composites exhibit better mechanical properties, higher thermal stability and higher thermal conductivity than UHMWPE/Al₂O₃ composites, owing to the good dispersion of Al₂O₃ powder in the UHMWPE matrix and the strong interfacial force between the macromolecules and the inorganic filler caused by the presence of PDA. The tensile strength and the tensile elongation at break of UHMWPE/PDA‐Al₂O₃ composite with 1 wt% PDA‐Al₂O₃ are 62.508 MPa and 462%, which are 1.96 and 1.98 times higher than those of pure UHMWPE, respectively. The thermal conductivity of UHMWPE/PDA‐Al₂O₃ composite increases from 0.38 to 0.52 W m^?1 K^?1 with an increase in the dosage of PDA‐Al₂O₃ to 20 wt%. The results show that the prepared PDA‐coated Al₂O₃ powder can simultaneously enhance the mechanical properties and thermal conductivity of UHMWPE. © 2018 Society of Chemical Industry     

Aluminum oxide particles/silicon carbide whiskers’ synergistic effect on thermal conductivity of high-density polyethylene composites

Aluminum oxide (Al₂O₃) particles and silicon carbide (SiC) whiskers improved the thermal conductivity of high-density polyethylene (HDPE). To improve the dispersion of inorganic fillers in the matrix, 5 wt% of maleic anhydride-modified polyethylene was added into HDPE as a compatibilizer, and the hybrid matrix was denoted as mHDPE. The thermal conductivity, heat resistance, and tensile properties of resulting HDPE composites were characterized. The results showed that the thermal conductivity reached its maximum value of 0.8876 W/(m K) at 1/4 weight ratio of Al₂O₃/SiC, which was 110.3, 54.8, and 8.8% higher than that of pure HDPE, mHDPE/Al₂O₃, and mHDPE/SiC composites, in the order given, indicating that hybrid fillers have synergistic effect on the thermal conductivity of HDPE composites. Moreover, they also have a synergistic effect on the heat resistance and Young’s modulus. As the SiC content increases, the heat resistance of the composites increases at first and then falls, and the maximum VST is reached at an Al₂O₃/SiC weight ratio of 3/2, which is 5.4 °C higher than that of HDPE. The maximum Young’s modulus of the composites (1160 MPa) is obtained at an Al₂O₃/SiC weight ratio of 1/4, and the yield strength increases gradually as the SiC whiskers’ content increases.     

Heat transport enhancement of thermal energy storage material using graphene/ceramic composites

Graphene/ceramic composites are proposed by directly depositing graphene on the insulating Al₂O₃ particles by chemical vapor deposition without any metal catalysts. Carbothermic reduction occurring at the Al₂O₃ surface is vital during the initial stage of graphene nucleation and the graphene sheet can connect with neighboring sheets to completely cover Al₂O₃ particles. The quality and layer number of graphene on Al₂O₃ can be finely tailored by changing the growth temperature and gas ratio. Graphene coated Al₂O₃ (G-Al₂O₃) composites are used as effective fillers of stearic acid (SA) to increase the thermal transport property. By the optimization of the layer number of graphene, size of Al₂O₃ particles and ratio of G-Al₂O₃/SA in a quantitative, their thermal conductivities significantly increase up to 11 folds from 0.15 to 1.65 W m⁻¹ K⁻¹. The great improvement is attributed to the high thermal transfer performance of graphene and excellent wettability between graphene and SA. When the G-Al₂O₃/SA composites with the graphene coated porous Al₂O₃ foam, the thermal conductivity further reaches to 2.39 W m⁻¹ K⁻¹, and the corresponding latent heat is 38 J g⁻¹. It demonstrates the potential applications of graphene in thermal transport and thermal energy storage devices.     

Effects of the surface treatment of the Al2O3 filler on the lithium electrode/solid polymer electrolyte interface properties

The lithium deposition-dissolution process in solid polymer electrolytes containing Al₂O₃ filler treated under different conditions has been investigated comparing with the ionic conduction behavior of the electrolyte. The composite electrolytes were prepared from poly(ethylene oxide) (PEO), LiBF₄ and α-Al₂O₃ filler by using a dry process, where the surface of α-Al₂O₃ was beforehand modified by a wet process. The exchange current densities, i₀, of the lithium electrode process in P(EO)₂₀LiBF₄ with and without Al₂O₃ filler were determined by a micro-polarization method. The temperature dependence of i₀ provided similar values for activation energy, ca. 25 and 70 kJ mol⁻¹ in both temperature regions above and below 60 °C, respectively. The effect of the surface treatment of the filler on the lithium electrode process gave a different tendency from that on the ionic conductivity. The Al₂O₃ surface treated by alkali solution enhanced the electrode process to the largest extent among the fillers used here, while it led to rather poor cycling stability in voltammetry. The enhanced reaction rate at the lithium electrode/solid polymer electrolyte interface has probably resulted in the improved ion dissociation by the surface groups of the Al₂O₃ filler.     

Electrical conductivity and thermal properties of spark plasma sintered Al2O3-SiC-CNT hybrid nanocomposites

In this study, we report the electrical conductivity and thermal properties of Al₂O₃-SiC-CNT hybrid nanocomposites processed via ball milling (BM) and spark plasma sintering (SPS). The initial powders and consolidated samples were characterized using transmission electron microscopy (TEM) and field emission scanning electron microscopy (FE-SEM), respectively. A multifunction calibrator and a high-resolution digital multimeter were used to measure the electrical conductivity. The thermal properties were measured using a thermal constants analyser. The SiC and CNT-reinforced alumina hybrid nanocomposites exhibited a significant increase in their room-temperature electrical conductivity, which made them suitable for electrical discharge machining. The Al₂O₃-5SiC-2CNTs had a high electrical conductivity value of 8.85 S/m compared to a low value of 6.87×10⁻¹⁰ S/m for the monolithic alumina. The addition of SiC and CNTs to alumina decreased its room-temperature thermal properties. The increase in temperature resulted in a decrease in the thermal conductivity and thermal diffusivity but an increase in the specific heat of the monolithic alumina and the hybrid nanocomposites. These properties were correlated with the microstructure, and possible transport mechanisms were discussed.     

Joining of alumina using magnesium- or calcium-aluminosilicate glass–ceramic fillers

Magnesium- and calcium-aluminosilicate (MAS and CAS) glass–ceramics were used to join alumina with six different compositions. The fillers were applied onto the alumina by screen-printing, and then joining was performed slightly below and above the filler melting temperature (T_m). The evolution of various intermediate compounds upon heat treatment between the filler itself and at the joining interface was compared. MgAl₂O₄ and CaO·6Al₂O₃ was the main crystalline phase presented at the joining interface for the MAS and CAS system, respectively, while more intermediate compounds were observed when only filler was heat-treated. The formation of MgAl₂O₄ and CaO·6Al₂O₃ was attributed to the diffusion of Al ions from the alumina base, which is desirable for obtaining a sound joint due to the similar coefficient of thermal expansion to the base alumina. The maximum joint strength of 250 ± 41 and 301 ± 48 MPa was obtained for MAS and CAS filler system, respectively, after joining at T ≥ T_m due to complete interfacial wetting.     

Al2O3/h-BN/epoxy based electronic packaging material with high thermal conductivity and flame retardancy

The long-term and stable operation of integrated circuits and microelectronics requires packaging epoxy resin (EP) exhibit high thermal conductivity for efficient heat dissipation, and excellent flame retardancy in case of thermal runaway. We achieved such EP composite via filling poly-dopamine (PDA) modified nanoscale Al₂O₃ spheres and microscale h-BN sheets. The PDA modification increases the compatibility between fillers and EP and largely reduces the viscosity, improving the dispersion of fillers in EP thus the thermal conductivity of EP composites. In addition, NH₃, H₂O, and N₂ generated during the combustion of phenolic hydroxyls and aminos in PDA combined with the physical barrier effect of Al₂O₃ and h-BN can improve the flame retardancy of EP composites. As a consequence, the EP composite filled with PDA modified Al₂O₃ (26.67 wt%) and h-BN (13.33 wt%) (i.e., PDA-BNAO/EP) shows a thermal conductivity of 1.192 W/mK (654.9% of EP), a peak heat release rate of 194.9 W/g (33.8% of EP), and total heat release of 15.2 kJ/g (54.5% of EP), respectively. What's more, the viscosity of PDA-BNAO/EP is 20,443 mPa s, which is only 20% of BNAO/EP (whose viscosity is 102,281 mPa s). More importantly, the PDA-BNAO/EP has good dynamic mechanical properties with the storage modulus of 14.69 Gpa, glass transition temperature of 91.9°C and good electrical insulation, which is desired for packaging of microelectronics. PDA-BNAO/EP composite should be a promising candidate for widespread packaging materials of microelectronics.