Thermally conducting aluminum nitride polymer-matrix composites

Thermally conducting, but electrically insulating, polymer-matrix composites that exhibit low values of the dielectric constant and the coefficient of thermal expansion (CTE) are needed for electronic packaging. For developing such composites, this work used aluminum nitride whiskers (and/or particles) and/or silicon carbide whiskers as fillers(s) and polyvinylidene fluoride (PVDF) or epoxy as matrix. The highest thermal conductivity of 11.5 W/(m K) was attained by using PVDF, AlN whiskers and AlN particles (7 μm), such that the total filler volume fraction was 60% and the AlN whisker–particle ratio was 1:25.7. When AlN particles were used as the sole filler, the thermal conductivity was highest for the largest AlN particle size (115 μm), but the porosity increased with increasing AlN particle size. The thermal conductivity of AlN particle epoxy-matrix composite was increased by up to 97% by silane surface treatment of the particles prior to composite fabrication. The increase in thermal conductivity is due to decrease in the filler–matrix thermal contact resistance through the improvement of the interface between matrix and particles. At 60 vol.% silane-treated AlN particles only, the thermal conductivity of epoxy-matrix composite reached 11.0 W/(m K). The dielectric constant was quite high (up to 10 at 2 MHz) for the PVDF composites. The change of the filler from AlN to SiC greatly increased the dielectric constant. Combined use of whiskers and particles in an appropriate ratio gave composites with higher thermal conductivity and low CTE than the use of whiskers alone or particles alone. However, AlN addition caused the tensile strength, modulus and ductility to decrease from the values of the neat polymer, and caused degradation after water immersion.     

Zirconium tungstate/cyanate ester nanocomposites with tailored thermal expansivity

The dimensional stability of polymer matrix composites can be enhanced by reducing the mismatch in the coefficient of thermal expansion (CTE) between the high CTE polymer matrix and low CTE fiber reinforcements, which leads to development of residual stresses and matrix microcracking. A potential strategy to diminish these residual stresses involves development of polymer nanocomposites with well dispersed nanoparticles that reduce the extent of mismatch in CTE. In this work, we explore the potential for development of bulk polymer nanocomposites with tailored thermal expansivity through incorporation of zirconium tungstate nanoparticles that are characterized by a negative CTE in a unique low viscosity bisphenol E cyanate ester (BECy) thermosetting polymer matrix. Incorporation of up to 10 vol.% whisker-like nanoparticles, synthesized by a hydrothermal method, results in a 20% reduction in the CTE of the polymer matrix. However, the nanoparticles exert a dramatic catalytic effect on the cure reaction of BECy resin and subsequently decrease the onset temperature of the glass transition for the cured polymer network, at high filler loadings.     

Thermomechanical Behavior of Bulk Ni/MWNT Composites Produced via Powder Metallurgy

The thermal expansion behavior of Ni matrix composites reinforced with multiwalled carbon nanotubes (MWNT) fabricated by pressureless sintering and hot uniaxial pressing was studied in the range between 50 and 1050 °C and compared to that of pure Ni. The results show an active interaction between the MWNT and the Ni matrix by reducing the coefficient of thermal expansion (CTE) of pure Ni up to 76% between 50 and 400 °C. This reduction is due to the strong interfacial interaction between the matrix and the reinforcement and the low intrinsic CTE of the nanotubes. This outstanding behavior may be very useful in applications were low CTE is required as for example structural materials.     

Effect of copper content on the thermal conductivity and thermal expansion of Al–Cu/diamond composites

Al–Cu matrix composites reinforced with diamond particles (Al–Cu/diamond composites) have been produced by a squeeze casting method. Cu content added to Al matrix was varied from 0 to 3.0 wt.% to detect the effect on thermal conductivity and thermal expansion behavior of the resultant Al–Cu/diamond composites. The measured thermal conductivity for the Al–Cu/diamond composites increased from 210 to 330 W/m/K with increasing Cu content from 0 to 3.0 wt.%. Accordingly, the coefficient of thermal expansion (CTE) was tailored from 13 × 10⁻⁶ to 6 × 10⁻⁶/K, which is compatible with the CTE of semiconductors in electronic packaging applications. The enhanced thermal conductivity and reduced coefficient of thermal expansion were ascribed to strong interface bonding in the Al–Cu/diamond composites. Cu addition has lowered the melting point and resulted in the formation of Al₂Cu phase in Al matrix. This is the underlying mechanism responsible for the strengthening of Al–Cu/diamond interface. The results show that Cu alloying is an effective approach to promoting interface bonding between Al and diamond.     

Effect of filler geometry on coefficient of thermal expansion in carbon nanofiber reinforced epoxy composites

This study involves the investigation of the geometry effect of nano-fillers on thermally induced dimensional stability of epoxy composites by experimentally evaluating the linear coefficient of thermal expansion (CTE). Carbon nanofibers (CNF) were chosen as the filler in epoxy matrix to investigate the effect of an aspect ratio on the CTE of the nanocomposites at three different volume fractions of 0.5, 1, and 2% of the nano-filler. The composites were fabricated using a mechanical mixing method. The CTE values were evaluated by measuring thermal strains of the composites and also compared with a micromechanics model. It was observed that the composites with short CNF (average L/d = 10) show better thermal stability than one of the composites with long CNF (average L/d = 70), and the thermal stability of the composites was proportional to the volume fraction of the filler in each composite. In addition, the CTE of mutliwalled carbon nanotubes (MWNT) reinforced epoxy composites was evaluated and compared with the CTE of the CNF reinforced composites. Interestingly, the MWNT reinforced composites show the greatest thermal stability with an 11.5% reduction in the CTE over the pure epoxy. The experimental data was compared with micromechanics model.     

Thermal expansion behaviour of aluminum matrix composites with densely packed SiC particles

The coefficient of thermal expansion (CTE) of Al-based metal matrix composites containing 70 vol.% SiC particles (AlSiC) has been measured based on the length change from room temperature (RT) to 500 °C. In the present work, the instantaneous CTE(T) of AlSiC is studied by thermo-elastic models and micromechanical simulation using finite element analysis in order to explain abnormalities observed experimentally. The CTE(T) is predicted according to analytical thermo-elastic models of Kerner, Schapery and Turner. The CTE(T) is modelled for heating and cooling cycles from 20 °C to 500 °C considering the effects of microscopic voids and phase connectivity. The finite element analysis is based on a two-dimensional unit cell model comparing between generalized plane strain and plane stress formulations. The thermal expansion behaviour is strongly influenced by the presence of voids and confirms qualitatively that they cause the experimentally observed decrease of the CTE(T) above 250 °C.     

Thermomechanical properties of copper–carbon nanofibre composites prepared by spark plasma sintering and hot pressing

Several types of carbon nanofibres (CNF) were coated with a uniform and dense copper layer by electroless copper deposition. The coated fibres were then sintered by two different methods, spark plasma sintering (SPS) and hot pressing (HP). The Cu coating thickness was varied so that different volume fraction of fibres was achieved in the produced composites. In some cases, the CNF were pre-coated with Cr for the improvement the Cu adhesion on CNF. The results show that the dispersion of the CNF into the Cu matrix is independent of the sintering method used. On the contrary, the dispersion is directly related to the efficiency of the Cu coating, which is tightly connected to the CNF type. Overall, strong variations of the thermal conductivity (TC) of the composites were observed (20–200 W/mK) as a function of CNF type, CNF volume fraction and Cr content, while the coefficient of thermal expansion (CTE) in all cases was found to be considerably lower than Cu (9.9–11.3 ppm/K). The results show a good potential for SPS to be used to process this type of materials, since the SPS samples show better properties than HP samples even though they have a higher porosity, in applications where moderate TC and low CTE are required.     

Comparison of the thermal properties between composites reinforced by raw and amino-functionalized carbon materials

Carbon materials, such as graphite oxides, carbon nanotubes and graphenes, have exceptional thermal conductivity, which render them excellent candidates as fillers in advanced thermal interface materials for high density electronics. In this paper, these carbon materials were functionalized with 4,4′-diaminodiphenyl sulphone (DDS), to enhance the bonding between the carbon materials and the resin matrix. Their visibly different properties were investigated. It seems that DDS-functionalization can obviously improve the interfacial heat transfer between the carbon materials and the epoxy matrix. The thermal conductivity enhancement of D-Graphene composites (0.493 W/m K) was about 30% higher than that of D-MWNTs composites (0.387 W/m K) at 0.5 vol.% loading. The different effects among EGO, D-EGO, MWNTs, D-MWNTs and D-Graphene in polymer composites were also discussed. It was demonstrated that DDS-functionalized carbon materials had an obvious effect on the thermal performances of composite materials and were more effective in thermal conductivity enhancement.     

Heterogeneously structured conductive carbon fiber composites by using multi-scale silver particles

This paper reports a new approach to enhance the through-thickness thermal conductivity of laminated carbon fabric reinforced composites by using nanoscale and microscale silver particles in combination to create heterogeneously structured continuous through-thickness thermal conducting paths. High conductivity of 6.62 W/(m K) with a 5.1 v% silver volume fraction can be achieved by incorporating these nanoscale and microscale silver particles in EWC-300X/Epon862 composite. Silver flakes were distributed within the inter-tow area, while nanoscale silver particles penetrated into the fiber tows. The combination of different sizes of silver fillers is able to effectively form continuous through-thickness conduction paths penetrating fiber tows and bridging the large inter-tow resin rich areas. Positive hybrid effects to thermal conductivity were found in IM7/EWC300X/sliver particle hybrid composites. In addition, microscale fillers in resin rich areas showed less impact on tensile performance than nanoscale particles applied directly on fiber surface.     

Fabrication and thermal conductivity of copper matrix composites reinforced with Mo2C or TiC coated graphite fibers

Graphite fiber reinforced Cu-based composites have good thermal conductivity, low coefficient of thermal expansion for heat sink applications. In these composites, the quality of interfacial bonding between the copper matrix and the graphite fibers has significant influence on the thermal properties of composites. In this study, two different carbide coatings (Mo₂C or TiC) were synthesized on graphite fiber to promote the interfacial bonding in composites. Fibers/Cu composites had been produced by spark plasma sintering process. The results showed that the densification, interfacial bonding and thermal conductivity of coated composites were improved distinctly compared to that of uncoated ones. The enhanced composites present 16–44% increase of thermal conductivity in X–Y plane. An original theoretical model was proposed to estimate the interface thermal resistance. The result showed that the interfacial thermal resistance was largely reduced by one order of magnitude with the introduction of carbide interlayer.     

Thermal conductivity and dielectric properties of Al/PVDF composites

Polymeric composites with relatively high thermal conductivity, high dielectric permittivity, and a low dissipation factor are obtained in the present study. Three types of core-shell-structured aluminum (Al) particles are incorporated in poly(vinylidene fluoride) (PVDF) by melt-mixing and hot-pressing processes. The morphological, thermal, and dielectric properties of the composites are characterized using thermal analysis, a scanning electron microscope, and a dielectric analyzer. The results indicate that the Al particles decrease the degree of crystallinity of PVDF, and that the particle size and shape of the filler affect the thermal conductivity and dielectric properties of Al/PVDF. No variation in the dissipation factor is observed up to 60 wt.% Al. Thermal conductivity and dielectric permittivity values as high as 1.65 W/m K and 230, respectively, as well as a low dissipation factor of 0.25 at 0.1 Hz, are realized for the composites with 80 wt.% spherical Al.     

Thermo-physical Properties of Continuous Carbon Fiber Reinforced Copper Matrix Composites

Continuous carbon fiber reinforced copper matrix composites with 70％(volume fraction)of carbon fibers prepared by squeeze casting technique have been used for investigation of the coefficient of thermal expansion(CTE)and thermal conductivity．Thermo-physical properties have been measured in both, longitudinal and transversal directions to the fiber orientation．The results showed that Cf／Cu composites may be a suitable candidate for heat sinks because of its good thermo-physical properties e．g．the low CTE(4.18×10-6／K)in longitudinal orientation and(14．98×10-6／K)in transversal orientation at the range of 20-50℃，a good thermal conductivity(87．2 W／m·K)in longitudinal orientation and(58．2 W／m·K)in transversal orientation．Measured CTE and thermal conductivity values are compared with those predicted by several well-known models．Eshelby model gave better results for prediction of the CTE and thermal conductivity of the unidirectional composites．     

Effects of aluminium nitride inclusions on thermal and electrical properties of epoxy and polypropylene: An experimental investigation

Thermal and dielectric properties of polymers reinforced with micro-sized aluminium nitride (AlN) particles have been studied. A set of epoxy–AlN composites, with filler content ranging from 0 to 25 vol% is prepared by hand lay-up technique. With similar filler loading, polypropylene -AlN composites are fabricated by compression molding technique. Density (ρ_c), effective thermal conductivity (k_eff), glass transition temperature (T_g), coefficient of thermal expansion (CTE) and dielectric constant (ε_c) of these composites are measured experimentally. The various experimental data were interpreted using appropriate theoretical models. Incorporation of AlN in both the resin increases the k_eff and T_g whereas CTE of composite decreases favourably. The dielectric constant of the composite also found to get modified with filler content. With improved thermal and modified dielectric characteristics, these AlN filled polymer composites can possibly be used for microelectronics applications.     

Internal stresses and voids in SiC particle reinforced aluminum composites for heat sink applications

Metal-matrix composites (MMC) are being developed for power electronic IGBT modules, where the heat generated by the high power densities has to be dissipated from the chips into a heat sink. As a means of increasing long term stability a base plate material is needed with a good thermal conductivity (TC) combined with a low coefficient of thermal expansion (CTE) matching the ceramic insulator. SiC particle reinforced aluminum (AlSiC) offers the high TC of a metal with the low CTE of a ceramic. Internal stresses are generated at the matrix-particle interfaces due to the CTE mismatch between the constituents of the MMC during changing temperatures. Neutron and synchrotron diffraction was performed to evaluate the micro stresses during thermal cycling. The changes in void volume fraction, caused by plastic matrix deformation, are visualized by synchrotron tomography. The silicon content in the matrix connecting the particles to a network of hybrid reinforcement contributes essentially to the long term stability by an interpenetrating composite architecture.     

Thermal conductivity of Cu–Zr/diamond composites produced by high temperature–high pressure method

Copper matrix composites reinforced with about 90 vol.% of diamond particles, with the addition of zirconium to copper matrix, were prepared by a high temperature–high pressure method. The Zr content was varied from 0 to 2.0 wt.% to investigate the effect on interfacial microstructure and thermal conductivity of the Cu–Zr/diamond composites. The highest thermal conductivity of 677 W m⁻¹ K⁻¹ was achieved for the composite with 1.0 wt.% Zr addition, which is 64% higher than that of the composite without Zr addition. This improvement is attributed to the formation of ZrC at the interface between copper and diamond. The variation of thermal conductivity of the composites was correlated to the evolution of interfacial microstructure with increasing Zr content.     

Interfacial microstructure and its effect on thermal conductivity of SiCp/Cu composites

Thermal conductivity of SiCp/Cu composites was usually far below the expectation, which is usually attributed to the low real thermal conductivity of matrix. In the present work, highly pure Cu matrix composites reinforced with acid washed SiC particles were prepared by the pressure infiltration method. The interfacial microstructure of SiCp/Cu composites was characterized by layered interfacial products, including un-reacted SiC particles, a Cu–Si layer, a polycrystalline C layer and Cu–Si matrix. However, no Cu₃Si was found in the present work, which is evidence for the hypothesis that the formation of Cu₃Si phase in SiC/Cu system might be related to the alloying elements in Cu matrix and residual Si in SiC particles. The thermal conductivity of SiCp/Cu composites was slightly increased with the particle size from 69.9 to 78.6 W/(m K). Due to high density defects, the real thermal conductivity of Cu matrix calculated by H–J model was only about 70 W/(m K). The significant decrease in thermal conductivity of Cu matrix is an important factor for the low thermal conductivity of SiCp/Cu composites. However, even considered the significant decrease of thermal conductivity of Cu matrix, theoretical values of SiCp/Cu composites calculated by H–J model were still higher than the experimental results. Therefore, an ideal particle was introduced in the present work to evaluate the effect of interfacial thermal resistance. The reverse-deduced effective thermal conductivities of ideal particles according to H–J model was about 80 W/(m K). Therefore, severe interfacial reaction in SiCp/Cu composites also leads to the low thermal conductivity of SiCp/Cu composites.     

Elevated temperature tensile properties and thermal expansion of CNT/2009Al composites

1.5 vol.% and 4.5 vol.% carbon nanotubes reinforced 2009Al (CNT/2009Al) composites with homogeneously dispersed CNTs and refined matrix grains, were fabricated using powder metallurgy (PM) followed by 4-pass friction stir processing (FSP). Tensile properties of the composites between 293 and 573 K and the coefficient of thermal expansion (CTE) from 293 to 473 K were tested. It was indicated that load transfer mechanism still takes effect at temperatures elevated up to 573 K, thus the yield strength of the 1.5 vol.% CNT/2009Al composite at 423–573 K, was enhanced compared with the 2009Al matrix. However, for the 4.5 vol.% CNT/2009Al composite, the yield strength at 573 K was even lower than that for the matrix, due to the quicker softening of ultrafine-grained matrix. Compared with the 2009Al matrix, the CTEs of the composites were greatly reduced for the zero thermal expansion and high modulus of the CNTs and could be well predicted by the Schapery’s model.     

Mechanical, thermal and electrical properties of aluminum nitride/polyetherimide composites

The mechanical, thermal and electrical properties of modified AlN/polyetherimide (PEI) composites were investigated. It revealed that the surface of AlN modified by silane could effectively increase the adhesion with matrix, which was beneficial for AlN to reinforce the polyetherimide matrix. After silane modification, the AlN showed good dispersion and wetibility in the polyetherimide matrix and imparted excellent mechanical, electrical and thermal properties. The tensile strength, modulus, electrical and thermal stability were improved with the increasing of AlN content. The tensile strength of AlN/PEI composites increased by 27% when 12.6 vol.% AlN was added to neat polyetherimide. The thermal conductivity of the 57.4 vol.% AlN/PEI composites increased three times compared with neat polyetherimide. Test results indicate that the silane grafted AlN incorporated into the polyetehetimide matrix effectively enhance the thermal stability, thermal conductivity and mechanical properties of the polyetherimide composites.     

热压烧结镀Cr碳纤维/Cu复合材料的制备及热性能

采用热压法将拥有超高导热率和负热膨胀系数（CTE）的中间相沥青基短碳纤维（CF_s）与Cu复合,并利用化学气相沉积技术对CF_s镀Cr以改善其与Cu的结合状况,研究了所制备的镀Cr CF_s/Cu复合材料的显微结构与热性能。结果表明：在制备中Cr层的大部分与CF_s表层的C反应形成连续、均匀的界面薄层Cr₇C₃,少量的扩散于Cu基体中,使CF_s与Cu之间的界面由结合极差的机械结合转化成良好的冶金结合,有效提升了复合材料的热性能。CF_s含量为40vol%~55vol%时,镀Cr CF_s/Cu复合材料致密度高于97.5%,平面方向上的热导率达393~419 W（mK）^-1,平面方向的CTE在5.1×10^-6~8.4×10^-6 K^-1之间。高的热导率、低的CTE以及优良的可加工性能使其成为极有潜力的电子封装材料。     

Multi-walled carbon nanotube/silver nanoparticles used for thermal transportation

A green method was applied to prepare composites of multi-walled carbon nanotubes (MWNTs) decorated with silver nanoparticles (Ag-NPs). MWNTs were functionalized using ball milling technology in the presence of ammonium bicarbonate, and the traditional method of silver mirror was used to decorate MWNTs to obtain Ag/MWNT composite. The obtained Ag/MWNT composites were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier transmission infrared spectroscopy, and Brunauer–Emmet–Teller (BET) surface area analysis. SEM characterization showed that Ag-NPs distributed uniformly on the walls of MWNTs. The content and size of Ag-NPs could be controlled by adjusting the redox time. XRD patterns demonstrated that the Ag-NPs are composed of pure Ag and crystallized well. BET analysis indicated that the specific surface areas of Ag/MWNT decrease with increasing the content of Ag-NPs, and this result is similar to that of the literature. The measurement results of the thermal property showed that the thermal conductivity of the nanofluid containing Ag/MWNT composites was higher than that of nanofluid containing pristine or functionalized MWNTs.