Improved properties of PTFE composites filled with glass fiber modified by sol–gel method

PTFE/GF(glass fiber) composites are widely applied in high-frequency printed circuit board (PCB) substrate materials due to the excellent dielectric properties of PTFE and the low thermal expansion coefficient of GF. However, the poor interface compatibility between PTFE and GF affects the performance of the composite substrates. In this study, tetraethyl orthosilicate (TEOS) was used as the silicon source, and polydimethylsiloxane (PDMS) was the organic precursor to modify the surface of GF through the sol–gel method to promote the interface compatibility of GF and PTFE. The modified GF noted T-GF was filled in PTFE to prepare PTFE/T-GF composites. SEM, FTIR, XPS, and contact angle confirmed that organic–inorganic hybrids were successfully loaded on GF's surface. Moreover, compared with PTFE/GF and the conventional coupling agent modified GF filled PTFE composites, the PTFE/T-GF exhibited improved dielectric constant (2.305), decreased dielectric loss (9.08E^?4), higher bending strength (21.45 MPa) and bending modulus (522 MPa), better thermal conductivity (0.268 W/m*K) and lower CTE (70 ppm/°C), making it has promising application as the substrate materials for high frequency PCB.

     

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.     

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.     

High conductive ethylene vinyl acetate composites filled with reduced graphene oxide and polyaniline

A conductive network composed of reduced graphene oxide (RGO) planes and polyaniline (PANI) chains was designed and fabricated by in situ polymerization of aniline monomer on the RGO planes. It was further used for fabrication of conductive composites with a polymer matrix–ethylene vinyl acetate (EVA). The composites achieve improved conductivity at a low filler loading although the host polymer–EVA–is of insulator. For instance, compared to the pure EVA polymer, the conductivity of the composite filled with 4.0 wt.% RGO and 8.0 wt.% PANI increases from 1.2 × 10^?14 S cm^?1 to 1.07 × 10^?1 S cm^?1. In addition, thermal stability of the composites is also enhanced by the filler loading.     

A novel fiber-reinforced polyethylene composite with added silicon nitride particles for enhanced thermal conductivity

A novel thermally conductive plastic composite was prepared from a mixture of silicon nitride (Si₃N₄) filler particles and an ultrahigh molecular weight polyethylene–linear low density polyethylene blend. The effects of Si₃N₄ particle sizes, concentration, and dispersion on the thermal conductivity and relevant dielectric properties were investigated. With proper fabrication the Si₃N₄ particles could form a continuously connected dispersion that acted as the dominant thermally conductive pathway through the plastic matrix. By adding 0–20% Si₃N₄ filler particles, the composite thermal conductivity was increased from 0.2 to ～1.0 W m^?1 K^?1. Also, the composite thermal conductivity was further enhanced to 1.8 W m^?1 K^?1 by decreasing the Si₃N₄ particle sizes from 35, 3 and 0.2 μm, and using coupling agent, for the composites with higher filler content. Alumina short fibers were then added to improve the overall composite toughness and strength. Optimum thermal, dielectric and mechanical properties were obtained for a fiber-reinforced polyethylene composite with 20% total alumina–Si₃N₄ (0.2 μm size) filler particles.     

Friction and wear behavior of polytetrafluoroethene composites filled with Ti3SiC2

In this work, polytetrafluoroethylene (PTFE) composites filled with Ti₃SiC₂ or graphite were prepared through powder metallurgy. The effects of different filling components, loads and sliding velocities on the friction performance of Ti₃SiC₂/PTFE composites were studied. Ti₃SiC₂/PTFE composites exhibit better wear resistance than graphite/PTFE composites due to the better mechanical properties of Ti₃SiC₂. The wear resistance was found to improve around 100× over unfilled PTFE with the addition of 1 wt.% Ti₃SiC₂. In addition, the 10 wt.% sample had the lowest wear rate of K = 2.1 × 10⁻⁶ mm³/Nm and the lowest steady friction coefficient with μ = 0.155 at the condition of 90 N–0.4 m/s. Ti₃SiC₂ was proved to promote the formation of a thin and uniform transfer film on counterpart surface and a protection oxide film on worn surface, which are the key roles for improving wear resistance.     

The desirable dielectric properties and high thermal conductivity of epoxy composites with the cobweb-structured SiCnw–SiO2–NH2 hybrids

We report the preparation of epoxy-based composites by intercalating low loading of core–shell silicon carbide nanowire-silica-amino (named as SiCnw–SiO₂–NH₂) hybrids, exhibiting simultaneously high permittivity and thermal conductivity (TC) and maintaining rather low dielectric loss. More interestingly, the epoxy composites with the cobweb-structured SiCnw–SiO₂–NH₂ hybrids exhibited high thermal conductivity at low filler loading due to space micro-structures and hydrogen bond interaction. Specifically, permittivity of the sample with 3.0 vol% SiCnw–SiO₂–NH₂ hybrids reaches 61.9 under 0.1 Hz, while its dielectric loss is only 0.012, and possessing a high TC of 1.59 W/m K, respectively.

     

Characterization of mechanochemically synthesized MHSO4–H4SiW12O40 composites (M = K,NH4, Cs)

Anhydrous proton conductive MHSO₄–H₄SiW₁₂O₄₀ (MHS–STA) composites were successfully synthesized using mechanochemical treatment. 80MHS·20STA (mol%) composite, for example, showed very high anhydrous proton conductivity above 10^?3 S cm^?1 in a temperature range from 160 to 60 °C under ambient pressure. From the X-ray diffraction study, it was confirmed that the mechanochemical treatment induced chemical interactions via ion-exchange between M⁺ ion in MHS and H⁺ ion in STA. Furthermore, phase-transition of raw substances, such as melting, dehydration and superprotic phase-transition, was suppressed in mechanochemically synthesized MHS–STA composites, indicating that improvement of anhydrous proton conductivity for MHS–STA composites is caused by the changes in protic conduction behavior.     

Microstructure and properties of the Si3N4/silica aerogel composites fabricated by the sol–gel method via ambient pressure drying

Si₃N₄ particle reinforced silica aerogel composites have been fabricated by the sol–gel method via ambient pressure drying. The microstructure and mechanical, thermal insulation and dielectric properties of the composites were investigated. The effect of the Si₃N₄ content on the microstructure and properties were also clarified. The results indicate that the obtained mesoporous composites exhibit low thermal conductivity (0.024–0.072 Wm^− 1 K^− 1), low dielectric constant (1.55–1.85) and low loss tangent (0.005–0.007). As the Si₃N₄ content increased from 5 to 20 vol.%, the compressive strength and the flexural strength of the composites increased from 3.21 to 12.05 MPa and from 0.36 to 2.45 MPa, respectively. The obtained composites exhibit considerable promise in wave transparency and thermal insulation functional integration applications.     

Sandwiched epoxy–alumina composites with synergistically enhanced thermal conductivity and breakdown strength

Development of polymer-based composites with simultaneously high thermal conductivity and breakdown strength has attracted considerable attentions owing to their important applications in both electronic and electric industries. In this study, we successfully design novel epoxy-based composites with nano-Al₂O₃/epoxy composite layer sandwiched between micro-Al₂O₃/epoxy composite layers, which show synergistically and significantly enhanced thermal conductivity and breakdown strength. Compared with the traditional composites, the bottleneck that both thermal conductivity and breakdown strength cannot be simultaneously enhanced can be overcome successfully. An optimized sandwiched alumina–epoxy composite with 70 wt% micro-Al₂O₃ fillers in the outer layers and 3 wt% nano-Al₂O₃ in the middle layer simultaneously displays a high thermal conductivity of 0.447 W m^?1 K^?1 (2.4 times of that of epoxy) and a high breakdown strength of 68.50 kV mm^?1, which is 6.3 % higher than that of neat epoxy (64.45 kV mm^?1). The experimental results on the thermal conductivity of multi-layered alumina–epoxy composites were in well accordance with the theoretical values predicted from the series conduction model. This novel technique simultaneously improves thermal conductivity and breakdown strength, which is of critical importance for design of perspective composites for electronic and electric equipments.     

Enhanced thermal conductivity and dielectric properties of Al/β-SiCw/PVDF composites

Polymeric composites with high thermal conductivity, high dielectric permittivity but low dissipation factor have wide important applications in electronic and electrical industry. In this study, three phases composites consisting of poly(vinylidene fluoride) (PVDF), Al nanoparticles and β-silicon carbide whiskers (β-SiC_w) were prepared. The thermal conductivity, morphological and dielectric properties of the composites were investigated. The results indicate that the addition of 12 vol% β-SiC_w not only improves the thermal conductivity of Al/PVDF from 1.57 to 2.1 W/m K, but also remarkably increases the dielectric constant from 46 to 330 at 100 Hz, whereas the dielectric loss of the composites still remain at relatively low levels similar to that of Al/PVDF at a wider frequency range from 10⁻¹ Hz to 10⁷ Hz. With further increasing the β-SiC_w loading to 20 vol%, the thermal conductivity and dielectric constant of the composites continue to increase, whereas both the dielectric loss and conductivity also rise rapidly.     

Structure and thermal conductivity of powder injection molded AlN ceramic

AlN powders doped with Y₂O₃ (5 wt.%) were compacted by employing powder injection molding (PIM) technique. The binder consisted of paraffin wax (PW, 60 wt.%), polypropylene (PP, 35 wt.%) and stearic acid (SA, 5 wt.%). The feedstock was prepared with a solid loading of 62 vol.%. The binder was removed through debinding process in two steps, solvent debinding followed by thermal debinding. At last, the debound samples were sintered in flowing nitrogen gas at atmospheric pressure. The result reveals that thermal debinding atmosphere has significant effect on the thermal conductivity and structure of AlN ceramics. The thermal conductivity of injection molded AlN ceramics thermal debound in flowing nitrogen gas is 231 W m^?1 K^?1.     

Dielectric spectroscopy of polypyrrole–γ–Fe2O3 composites

In situ polymerization of pyrrole was carried out in the presence of γ–Fe₂O₃ (FE) to synthesize polypyrrole–γ–Fe₂O₃ composites (PPy/FE) by chemical oxidation method. The PPy/FE composites have been synthesized with various compositions viz., 10, 20, 30, 40 and 50 wt.% of γ–Fe₂O₃ in pyrrole. The AC conductivity was studied in the frequency range 10²–10⁷ Hz. The dielectric behaviour was also investigated in the frequency range 10²–10⁷ Hz. The dimensions of γ–Fe₂O₃ particles in the matrix have a greater influence on the conductivity values and the observed dielectric values.     

Electrochemical characterization of LaBaCuCoO5+δ–Sm0.2Ce0.8O1.9 composite cathode for intermediate-temperature solid oxide fuel cells

In order to improve the electrochemical performance of the LaBaCuCoO_5+δ (LBCC) electrode, LaBaCuCoO_5+δ–Ce_0.8Sm_0.2O_1.9 (LBCC–SDC) are prepared and characterized for potential application as a cathode material for intermediate-temperature solid oxide fuel cells (IT-SOFCs) based on an SDC electrolyte. Electrical conductivity, thermal expansion and electrochemical properties are investigated by four probing DC technique, dilatometry, AC impedance and polarization techniques, respectively. It is found that the thermal expansion coefficient (TEC) and electrical conductivity decrease with the increase of SDC content in LBCC–SDC composites. AC impedance spectra based on SDC electrolyte measured at intermediate temperatures show that the addition of SDC to LBCC improves the electrochemical performance of a LBCC cathode, and that a LBCC–SDC20 cathode exhibits superior electrochemical performance in the LBCC–SDCx composite cathodes. Moreover, even when the content of SDC is up to 40 wt%, the area specific resistance of the LBCC–SDC40 composite cathode on SDC electrolyte is lower than the corresponding interfacial resistance for pure LBCC at 650–800 °C. The power density of the Ni–SDC/SDC/LBCC–SDC20 cell is 615 mW cm^?2 at 800 °C. These results indicate that LBCC–SDCx is a potential cathode material for application in IT-SOFCs.     

Effect of processing technique on the transport and mechanical properties of graphite nanoplatelet/rubbery epoxy composites for thermal interface applications

Graphite nanoplatelet (GNP)/rubbery epoxy composites were fabricated by mechanical mixer (MM) and dual asymmetric centrifuge speed mixer (SM). The properties of the GNP/rubbery epoxy were compared with GNP/glassy epoxy composites. The thermal conductivity of GNP/rubbery epoxy composite (25 wt.% GNP, particle size 15 μm) reached 2.35 W m⁻¹ K⁻¹ compared to 0.1795 W m⁻¹ K⁻¹ for rubbery epoxy. Compared with GNP/rubbery epoxy composite, at 20 wt.%, GNP/glassy epoxy composite has a slightly lower thermal conductivity but an electrical conductivity that is 3 orders of magnitude higher. The viscosity of rubbery epoxy is 4 times lower than that of glassy epoxy and thus allows higher loading. The thermal and electrical conductivities of composites produced by MM are slightly higher than those produced by SM due to greater shearing of GNPs in MM, which results in better dispersed GNPs. Compression and hardness testing showed that GNPs increase the compressive strength of rubbery epoxy ∼2 times without significantly affecting the compressive strain and hardness. The GNP/glassy epoxy composites are 40 times stiffer than the GNP/rubbery epoxy composites. GNP/rubbery epoxy composites with their high thermal conductivity, low electrical conductivity, low viscosity before curing and high conformability are promising thermal interface materials.     

Dielectric relaxation and ac conductivity of polyaniline–zinc ferrite composite

In situ polymerization of aniline is carried out in the presence of zinc ferrite to synthesize polyaniline/ZnFe₂O₄ composites (PANI/ZnFe₂O₄) by chemical oxidation method. The composite has been synthesized with various compositions (10, 20, 30, 40 and 50 wt.%) of zinc ferrite in PANI. From the infrared spectroscopy (FTIR) studies on polyaniline/ZnFe₂O₄ composites, the peak at 1140 cm^?1 is considered to be measure of the degree of electron delocalization. The surface morphology of these composites is studied with scanning electron micrograph (SEM). The ac conductivity and dielectric properties are studied in the frequency range from 10² to 10⁶ Hz. The results obtained for these composites are of scientific and technological interest.     

Microstructural and thermochemical properties of Al/AlN/CuAl2 composite prepared by a combination of combustion synthesis and spark plasma sintering

A combination of combustion synthesis (CS) and spark plasma sintering (SPS) technology was employed in the fabrication of Al/AlN/CuAl₂ dense composites. Al/AlN/CuAl₂ composite powders in which a portion of the AlN was present in macro- and nanofiber forms were prepared by combustion of Al–Cu–5 wt.% (C₂F₄)_n, under a nitrogen atmosphere. The resulting composite powders were then subjected to consolidation by SPS at a dwell temperature level of 1500 °C, mechanical pressure of 60 MPa, and a non-isothermal heating time of 10 min. It is found that the actual thermal conductivity of Al/AlN/CuAl₂ composites fabricated with 5 wt.% (C₂F₄)_n is much higher than that of materials prepared in the absence (C₂F₄)_n. Maximum thermal conductivity (320 W/m K) was recorded for the samples prepared from an 0.8Al–0.2Cu–5 wt.% (C₂F₄)_n mixture. The influence of (C₂F₄)_n on the growth mechanism of AlN fibers and thermal conductivity of composite samples is discussed in light of the experimental data.     

Thermal conductivity of Cu/diamond composites prepared by a new pretreatment of diamond powder

Cu/diamond composites were fabricated by spark plasma sintering (SPS) after the surface pretreatment of the diamond powders, in which the diamond particles were mixed with copper powder and tungsten powder (carbide forming element W). The effects of the pretreatment temperature and the diamond particle size on the thermal conductivity of diamond/copper composites were investigated. It was found that when 300 μm diamond particles and Cu–5 wt.% W were mixed and preheated at 1313 K, the composites has a relatively higher density and its thermal conductivity approaches 672 W (m K)⁻¹.     

Lightweight multi-walled carbon nanotube buckypaper/glass fiber–epoxy composites for strong electromagnetic interference shielding and efficient microwave absorption

Multi-walled carbon nanotube buckypaper (BP) reinforced glass fiber–epoxy (GF/EP) composites were selected to fabricate electromagnetic interference (EMI) shielding and microwave absorbing materials. Six different composite configurations with 3.0 mm thick have been conceived and tested over the X-band (8.2–12.4 GHz). Flexible and low-density (0.29 g/cm³) BP provided a high specific EMI SE of 76 dB with controlled electrical conductivity. GF/EP/BP₁₁₁ and GF/EP/BP₁₀₁ composites possess EMI SE as high as of 50–60 dB, which can be attributed to the number of BP inserted and variation in the wave-transmitting layer of the laminates. Furthermore, the shielding mechanism was discussed and suggested that the absorption was the dominant contribution to EMI SE. GF/EP/BP₁₁₀ laminate demonstrated suitable EMI performance (~?20 dB), whereas GF/EP/BP₀₁₁ composite revealed excellent microwave performance, achieving an effective ? 10 dB bandwidth of 3.04 GHz and minimum reflection loss (RL) value of ? 21.16 dB at 10.37 GHz. On the basis of these results, GF/EP/BP composites prepared in this work have potential applications as both EMI shielding and microwave absorber materials given their facile preparation and lightweight use.

     

Thermal conductivity of diamond/Ag composites with chromium carbide coated diamonds for the building materials of high power modules

Abstract

This paper reports on a study of the preparation and characterisation of diamond/Ag composites for the building materials of high power modules. The Cr₇C₃ coated diamond particles are utilised to improve the interfacial bonding between the Ag and diamond and composites are prepared by hot pressing technique. The characteristics of Cr₇C₃ coating layers were investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The results show that the Cr₇C₃ coatings on the diamonds result in a strong interfacial bonding and a greatly enhanced thermal conductivity of the composites. A largely enhanced thermal conductivity of 768 W m^?1 K^?1 is obtained in Cr₇C₃ coated composites, which increases 168% relative to that of uncoated composites at 65% diamond volume fraction. The measured thermal conductivity agrees reasonably well with the predictions by a differential effective-medium (DEM) model.