Ultra high thermal conductivity polymer composites

Epoxy composites based on vapor grown carbon fiber (VGCF) were fabricated and analyzed for room temperature thermophysical properties. An unprecedented high thermal conductivity of 695 W/m K for polymer matrix composites was obtained. The densities of all the composites are lower than 1.5 g/cc. In addition the high value of coefficient of thermal expansion (CTE) of the polymer material was largely reduced by the incorporation of VGCF. Also, unlike metal matrix composite (MMC), the epoxy composite has an electrically insulating surface. Based on the composite thermal conductivities, the room temperature thermal conductivity of VGCF, heat-treated at 2600°C, was estimated to be 1260 W/m K. Furthermore, the longitudinal CTE of the heat-treated VGCF was determined, for the first time, to be −1.5 ppm/K.     

Oxidative stabilization of PAN/SWNT composite fiber

PAN/SWNT composite fibers have been spun with 0, 5, and 10 wt% single wall carbon nanotubes (SWNTs). Tensile fracture surfaces of polyacrylonitrile (PAN) fibers exhibited extensive fibrillation, while for PAN/SWNT composite fibers, tendency to fibrillate decreased with increasing SWNT content. The reinforcing effect of SWNTs on the oxidized polyacrylonitrile (PAN) fiber has been studied. At 10 wt% SWNTs, breaking strength, modulus, and strain to failure of the oxidized composite fiber increased by 100%, 160%, and 115%, respectively. Tensile fracture surfaces of thermally stabilized PAN and the PAN/SWNT fibers exhibited brittle behavior and well distributed SWNT ropes covered with the oxidized matrix can be observed in the tensile fracture surfaces of the fibers. No de-bonding has been observed between unoxidized or the oxidized PAN matrix and the nanotube ropes. Higher strain to failure of the oxidized composite fiber as compared to that of the oxidized control PAN fiber also suggests good adhesion/interaction between SWNT and the oxidized matrix. Thermal stresses generated on the composite fiber during the oxidation process were lower than those for the control fiber. The potential of PAN/SWNT composite fiber as the precursor material for the carbon fiber has been discussed.     

Polypyrrole/carbon composite electrode for high-power electrochemical capacitors

Nano-thin polypyrrole (PPy) layers with thickness from ∼5 nm to several 10s nm were deposited on vapor grown carbon fibers (VGCF) by an in situ chemical polymerization. Using different concentrations of the pyrrole could control the thicknesses of deposited PPy layers. Surface morphology and thickness of the deposited PPy layers were confirmed by means of scanning electron microscopy and scanning transmission emission microscopy. Pseudo-capacitive behavior of the deposited PPy layers on VGCF investigated by means of cyclic voltammetry. Then, the PPy/VGCF composites were mixed with activated carbons (AC) at various mixing ratios. For the PPy/VGCF/AC composite electrodes, characteristics of specific capacitance and power capability were examined by half-cell tests. As results of this study, it was investigated that nano-thin PPy layer below ∼10 nm deposited on VGCF had high pseudo-capacitance and fast reversibility. Its specific capacitance per averaged weight of active material (PPy) was obtained as ∼588 F g⁻¹ at 30 mV s⁻¹ and maintained as ∼550 F g⁻¹ at 200 mV s⁻¹ of scan rate. Also, from the mixing 60 wt.% of the PPy/VGCF with 25 wt.% of AC, the PPy/VGCF/AC composite electrode exhibited higher power capability maintaining the specific capacitance per active materials of PPy and AC as ∼300 F g⁻¹ at 200 mV s⁻¹ in 6 M KOH.     

Evolution of the structure and mechanical behaviour of a carbon foam at very high temperatures

Mechanical behaviour of a low density carbon/carbon composite at very high temperature is studied in relation with its microstructure. This composite is a syntactic foam made of carbon microbeads with a binder and voids. The resulting geometrical density is 0.3 g cm⁻³. Compressive tests from room temperature up to 3100 °C with a very high heating rate (180 °C s⁻¹) have been conducted. Intermediate temperature tests have also been performed and show an obvious modification of mechanical behaviour from around 2000 °C. This result is related to a sudden modification of structure and texture of the carbonaceous matter during the high temperature mechanical test. A strong plastic deformation occurs when the mechanical experiment is performed at 3100 °C whereas the material elastically deforms at room temperature.     

Electrical properties of polyethylene and carbon black particle blends prepared by gelation/crystallization from solution

Composite materials based on low molecular weight polyethylene (LMWPE), ultra-high molecular weight polyethylene (UHMWPE) and carbon black (CB) particles were prepared by gelation/crystallization from solution. The positive temperature coefficient (PTC) intensity for the 90/10 (LMWPE/UHMWPE) composition exceeded five orders of magnitude for the specimens heat-treated at a suitable temperature, which was almost equal to that observed with LMWPE-CB blends prepared by a kneading method. In comparison with LMWPE-CB blends, much promoted reproducibility of PTC effect and inhibition of the negative temperature coefficient (NTC) effect were achieved.     

Effect of heat treatment on the tribological behavior of 2D carbon/carbon composites

The effect of high temperature heat treatment on the tribological behavior of carbon/carbon (C/C) composites has been investigated. C/C composite preforms were made from 1K PAN plain carbon cloth, and densified using rapid directional diffusion (RDD) CVI processes. Four specimens treated at 1800, 1800+2000, 2000, and 2300 °C, respectively, were prepared. A ring-on-ring specimen configuration was used to simulate aircraft brakes. The brake initial angular velocity ranged from 1800 to 7500 rpm (6.2-26.0 m s⁻¹ average linear sliding velocity). The specific pressure and moment of inertia were 392-784 kPa and 0.25-0.31 kg m², respectively (1.9-42.3 MJ m⁻² kinetic energy loading per unit friction surface area). The results showed that the stability of the brake moment-time curves increased with increasing heat treatment temperature (HTT) for the four composites, and those treated at 2300 °C possessed the lowest initial brake moment peak ratio values (from 1.1 to 1.3). The high degree of graphitization and low shear forces of the matrix carbon resulting from the high HTT could allow friction films to develop and reduce those values under the present brake conditions. The friction coefficients of four RDD CVI C/C composites decreased with an increase in specific pressure. The resulting changes in the friction coefficient of the four composites due to the specific pressure changes have basically nothing to do with the interface temperature under those conditions. According to the practical brake conditions, the friction properties of RDD CVD C/C composites could be improved by regulating the structure of the brake discs, changing the specific pressure exerted on the discs and the heat treatment. The linear wear rates of the four materials increased with increasing HTT. The composites treated at 2000 °C had both high enough friction coefficients and the lower linear wear rates. The different heat treatment methods at 2000 °C had no obvious effect on the friction and wear properties of RDD CVI C/C composites.     

Thermal and ablative properties of low temperature carbon fiber-phenol formaldehyde resin composites

The thermal and ablative properties of phenol formaldehyde resin (PF) composites reinforced with carbon fibers heat-treated at low temperature have been investigated. Low temperature carbon fibers (LTCF) were obtained by a continuous carbonization process from stabilized PAN fibers at 1100 °C. The properties of LTCF reinforced PF (LTCF-PF) composites are compared with those of high temperature carbon fiber (HTCF) reinforced PF (HTCF-PF) composites. The thermal conductivity of the LTCF-PF composite is lower than that of HTCF-PF composite by about 35 and 10% along the directions parallel and perpendicular to the laminar plane, respectively. It was found from the ablation test using an arc plasma touch flame that the erosion rate is higher by about 30% in comparison with HTCF-PF composite. The result suggests that use of LTCFs as reinforcement in a composite may improve the thermal insulation of the composite but decrease the ablative resistance.     

Stabilization and carbonization of gel spun polyacrylonitrile/single wall carbon nanotube composite fibers

Gel spun polyacrylonitrile (PAN) and PAN/single wall carbon nanotube (SWNT) composite fibers have been stabilized in air and subsequently carbonized in argon at 1100 °C. Differential scanning calorimetry (DSC) and infrared spectroscopy suggests that the presence of single wall carbon nanotube affects PAN stabilization. Carbonized PAN/SWNT fibers exhibited 10-30 nm diameter fibrils embedded in brittle carbon matrix, while the control PAN carbonized under the same conditions exhibited brittle fracture with no fibrils. High resolution transmission electron microscopy and Raman spectroscopy suggest the existence of well developed graphitic regions in carbonized PAN/SWNT and mostly disordered carbon in carbonized PAN. Tensile modulus and strength of the carbonized fibers were as high as 250 N/tex and 1.8 N/tex for the composite fibers and 168 N/tex and 1.1 N/tex for the control PAN based carbon fibers, respectively. The addition of 1 wt% carbon nanotubes enhanced the carbon fiber modulus by 49% and strength by 64%.     

Production and characterisation of vapour grown carbon fiber/polypropylene composites

Polymeric composites are widely used in the aircraft and automotive industries. Their high strength-to-weight ratio makes significant weight reduction possible. Beside this advantage, the polymer materials also offer a good corrosion resistance but the mechanical and electrical properties are not satisfactory. In order to increase these properties, vapour grown carbon fibers (VGCF) with high strength and metal-like electrical conductivity can be embedded in the polymeric matrix. To ensure a good adhesion between the fibers and the polymer matrix a functionalization of the chemically inert surface of the fibers is necessary.In the present research work oxygen-containing functional groups were introduced on the fiber surface through cold plasma treatment. Measurements of the fiber surface energy after plasma functionalization showed an enhancement of at least 50% of the initial value. The VGCF/PP composites with different amounts of VGCF were made through extrusion and injection molding. The results show that the degree of fiber surface functionalization and the fiber distribution and orientation in the polypropylene (PP) matrix may strongly influence the mechanical properties of the composite.     

Characterizations of expanded graphite/polymer composites prepared by in situ polymerization

Poly(styrene-co-acrylonitrile)/expanded graphite composite sheets with very low in-plane (8.5 × 10⁻³ Ω cm) and through-thickness (1.2 × 10⁻² Ω cm) electrical resistivities have been prepared. The expanded graphite was made by oxidation of natural graphite flakes, followed by thermal expansion at 600 °C. Microscopic results disclosed that the expanded graphite has a legume-like structure, and each “legume” has a honeycomb sub-structure with many diamond-shaped pores. After soaking the expanded graphite with styrene and acrylonitrile monomers, the polymer/expanded graphite composite granules were obtained by in situ polymerization of the monomers inside the pores at 80 °C. The functional groups and microstructures of the oxidized graphite, expanded graphite and composites in the forms of particles or sheets were carefully characterized using various techniques, including X-ray powder diffraction, thermogravimetry, optical and electron microscopy. It was found that the honeycomb sub-structure survived after hot-pressing, resulting in a graphite network penetrating through the entire composite body, which produces a composite with excellent electrical conductivity.     

Enhanced microwave-absorption properties of polymer-derived SiC/SiOC composite ceramics modified by carbon nanowires

Novel microwave-absorbing SiOC composite ceramics with dual nanowires (carbon nanowires (CNWs) and SiC nanowires) with high performances were fabricated by using the polymer-derivation method and heat treatment in Ar atmosphere. The introduction of CNWs in the amorphous SiOC ceramics promotes the ceramic crystallization into SiC nanoparticles and SiC nanowires at lower annealing temperatures, which leads to multi-phases and multiple nano heterogeneous interfaces. The distinctive architectures largely increase the interfacial and dipole polarizations of the composite ceramics. The CNWs/SiC/SiOC composite ceramics exhibit excellent microwave-absorption properties in the Ku band (12.4–18 GHz). The minimum reflection coefficient (RC) is -24.5 dB at a thickness of 1.8 mm, while the maximum effective absorption bandwidth (EAB, the corresponding frequency band in which RC is smaller than -10 dB) is 4.8 GHz at a thickness of 1.9 mm, which make the CNWs/SiC/SiOC composite ceramics promising electromagnetic-wave-absorbing materials.     

Short residence time graphitization of mesophase pitch-based carbon fibers

The effects of graphitization time and temperature on the properties of three mesophase pitch-based carbon fibers have been characterized. Graphitization temperatures studied were 2400, 2700, and 3000 °C and residence times ranged from 0.7 to 3600 s. Helium pycnometry, measurements of fiber tow resistance, and X-ray diffraction were employed to study fiber properties. As anticipated, substantial variations in fiber properties were noted for the range of graphitization conditions studied and among the three fiber types. Significant structural evolution and property development occurred even at the shortest furnace residence times. For example, for one of the fibers, a furnace residence time of 0.7 s at 3000 °C resulted in a degree of graphitization value of ∼50%, a density of 1.98 g/cm³, and an electrical resistivity of 6.3 μΩ m (corresponding thermal conductivity ∼200 W m⁻¹ K⁻¹). A simple energy consumption analysis suggests that short residence time graphitization at high temperature may result in both lower costs and substantially higher production rates for fibers prepared from mesophase pitch.     

Oxidative stabilization of PAN/VGCF composite

An oxidative stabilization process to prepare carbon films was carried out for a new kind of precursor using the composite of polyacrylonitrile (PAN) and vapor‐grown carbon fiber (VGCF) by a process of gelation/crystallization from dilute solutions. It was found that the new precursor has special features in the stabilization process different from those of the homopolymer in regard to thermal and morphological aspects. In the stabilization process under heat treatment at 180–350°C in an oxidative atmosphere, it was inferred that, although the introduction of VGCF hinders the initiation and propagation step of the cyclization and dehydrogenation reactions, the precursor helps the oxidation and the additional aromatization and intermolecular crosslinking reactions in the stabilization process, thus promoting the formation of the later carbon product in film's shape with good performance. From characteristic works by wide‐angle X‐ray diffraction (WAXD), FTIR, Raman, and DSC, the changes of the precursors in structure, morphology, and mechanical property in terms of different heat‐treat temperatures and tensions were studied. Through a series of experimental results, the effect of the VGCF's introduction on those changes was discussed. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 87: 2063–2073, 2003     

Effect of carbonization rate on the properties of a PAN/phenolic-based carbon/carbon composite

The effect of carbonization rate in a wide range (1, 100 and 1000 °C/min) on the properties of a PAN/phenolic-based carbon/carbon (C/C) composite was studied. The results indicated that the composite processed at a higher carbonization rate had a higher porosity level, more large pores and a more graphitic structure than that processed at a lower carbonization rate. After second graphitization the bending properties of composites carbonized at 1 °C/min and 1000 °C/min were comparable. The composite carbonized at 1000 °C/min had the highest fracture energy. The composite carbonized at 100 °C/min showed the worst mechanical performance among three. The large increase in carbonization rate can be beneficial to the industry from an economic point of view.     

Preparation and properties of 2D C/SiC-ZrB2-TaC composites

Two-dimensional (2D) C/SiC-ZrB₂-TaC composites were fabricated by chemical vapor infiltration (CVI) combined with slurry paste (SP) method. 2D laminate was prepared by stacking carbon cloth that was pasted with a mixture of polycarbosilane-ZrB₂-TaC slurry. A small amount of carbon fiber tows were introduced into the preform in the vertical direction. After heat-treated at 1800 °C, the 2D laminate was densified with SiC by CVI to obtain 2D C/SiC-ZrB₂-TaC composites. Properties including flexural strength, interlaminar shear strength, and thermal expansion of the composites were investigated. The ablation test was carried out under an oxyacetylene torch flame. The morphologies of the ablated specimens were analyzed. The results indicate that the adding vertical fiber tows and heat-treatment at 1800 °C can greatly improve the mechanical properties of the composites. The co-addition of TaC and ZrB₂ powders into C/SiC composite effectively enhance its ablation resistance.     

VGCF填充PVDF/PMMA复合材料导电性能的研究

以气相生长碳纤维（VGCF）为导电填料，聚偏氟乙烯（PVDF）、聚甲基丙烯酸甲酯（PMMA）为基体制备复合型导电高分子材料。考察了填料用量、基体种类、配比以及PVDF结晶行为对复合材料导电性能的影响。结果表明，VGCF填充PMMA、PVDF、PVDF／PMMA（50／50）体系的渗滤阔值分别为5、4、3phr的填料用量。VGCF的加入会导致PVDF／PMMA体系发生微观相分离，而且VGCF会选择性富集在PVDF的非晶相中，所以PVDF／PMMA／VGCF体系的导电性呈现双重渗滤现象，该体系的体积电阻率不仅取决于富集相中VGCF的含量，而且还与PVDF相的连续性及其结晶行为密切相关。     

Small angle X-ray scattering from voids within fibers during the stabilization and carbonization stages

Five kinds of commercial carbon fibers heat-treated at various temperatures were adopted to investigate the scattering from voids within the fibers by small angle X-ray scattering (SAXS), since the voids produced during stabilization and carbonized processes provide unfavorable influence on mechanical properties of the carbon fibers. The theoretical calculation was carried out for SAXS by assuming ellipsoidal voids with different shapes. In this model system, the long axis of all ellipsoidal voids on the two-dimensional plane was assumed to be oriented predominantly with respect to the machine direction but the center of gravity of each void are assumed to be arranged in the transverse direction assuring one-dimensional lattice. To analyze inter-particle interference effect from ellipsoidal voids, the function H₁(y) proposed by Hoseman was introduced to represent the probability of finding the nearest-neighbor void at a displacement vector. In comparison with observed and calculated SAXS patterns, the inter-particle interference effect was observed for the fibers carbonized at 1200-1700 °C but the effect disappeared with further heat-treatment at 3000 °C. Namely, the scattering from the fiber heat-treated at 3000 °C, which is so-called graphite fiber, showed no inter-particle interference effect. Namely, void distribution of high modulus carbon fiber with graphite crystallites was different from that of high strength carbon fiber with turbostratic structures.     

Development of thermally conductive packing for gas separation

A new highly thermally conductive composite material made of activated carbon in-situ activated within a consolidated expanded natural graphite (CENG) matrix is developed in the specific case of gas separation processes. According to the particular chosen testing case of CO₂/N₂ mixture, the microporous characteristics of the adsorbent fraction have been selected and the various properties of the whole material studied. Activated carbon effective densities, overall thermal conductivities and external heat transfer coefficients up to 650 kg×m⁻³, 32 W×m⁻¹×K⁻¹ and 3000 W×m⁻²×K⁻¹ were obtained, respectively. Two macroscopic external shapes were realized, namely a corrugated-sheets static mixer and a multi-axial-diffuser cylindrical monolith which will be further tested in a pilot equipment for comparison with conventional adsorbents.     

A Si-SnSb/pyrolytic PAN composite anode for lithium-ion batteries

Polyvinyl chloride (PVC) powders are introduced as carbon sources to prepare Si-SnSb alloy based on carbothermal method. Then the as-prepared alloy was mixed with pyrolytic polyacrylonitrile (PAN) to synthesize a composite anode of Si-SnSb/ pyrolytic polyacrylonitrile (PAN) with the pyrolysis of PAN at 400 °C and 600 °C. The material exhibits a high-specific capacity and a good-cycling stability due to its multiphase characteristics. The Si-SnSb blended with PAN pyrolyzed at 600 °C shows better electrochemical performance, its initial Coulombic efficiency is 62.6%, and the specific capacity is 658.4 mAh g⁻¹ after the 10th cycle. The pyrolytic PAN improves the performance of the Si-SnSb alloy by means of dispersing the alloy efficiently, and the pyrolytic PAN enhances the performance of the pure alloy simultaneously.     

Oxidation and defect control of CVD SiC coating on three-dimensional C/SiC composites

Two kinds of multi-layer CVD SiC coatings were prepared on a three-dimensional C/SiC composite. Oxidation behavior of the coating and the composite were studied and the effect of defects in the coating on its oxidation protection property were investigated. Above 1200 °C, thickness of the oxide film formed on the coating was related to oxidation time by the Fick’s first law X(t)²=Bt, the diffusion rate constant increased with oxidation temperature according to the Arrhenius’ relation ln B=−32?483/T+1.4048. Morphology of the interface between the CVD SiC and its oxide film was different after oxidation at temperatures from 1200 to 1500 °C. It was interpreted by consideration of the interfacial stress produced by thermal expansion mismatch and the CO gas pressure produced by interfacial reaction.