Thermal properties of copper-coated carbon foams

Carbon foams, with 97% porosity, were electroplated with copper for different periods of time to achieve desired copper thicknesses and foam porosity. A light flash diffusivity instrument was used to measure the thermal conductivity of the coated samples. An analytical model was developed to calculate the effective thermal conductivity of the coated foams. It was observed that the copper-coated carbon foam with 50% porosity can attain a thermal conductivity of 180 W/m K. The results from the analytical model were compared to the experimental results and they were in a very good agreement. The above analyses demonstrated the significance of copper coating in tailoring carbon foam thermal properties. The developed analytical model was adopted to predict the thermal conductivity of the copper-coated carbon foams.     

A graphite foam reinforced by graphite particles

Graphite foam was obtained after carbonization and graphitization of a pitch foam formed by the pyrolysis of coal tar based mesophase pitch mixed with graphite particles in a high pressure and temperature chamber. The graphite foam possessed high mechanical strength and exceptional thermal conductivity after adding the graphite particles. Experimental results showed that the thermal conductivity of modified graphite foam reached 110 W/m K, and its compressive strength increased from 3.7 MPa to 12.5 MPa with the addition of 5 wt% graphite particles. Through the microscopic observation, it was also found that fewer micro-cracks were formed in the cell wall of the modified foam as compared with pure graphite foam. The graphitization degree of modified foam reached 84.9% and the ligament of graphite foam exhibited high alignment after carbonization at 1200 °C for 3 h and graphitization at 3000 °C for 10 min.     

Fabrication and thermal conductivity of AlN/BN ceramics by spark plasma sintering

Aluminum nitride/boron nitride (AlN/BN) ceramics with 15–30 vol.% BN as secondary phase were fabricated by spark plasma sintering (SPS), using Yttrium oxide (Y₂O₃) as sintering aid. Effects of Y₂O₃ content and the SPS temperature on the density, phase composition, microstructure and thermal conductivity of the ceramics were investigated. The results revealed that with increasing the amount of starting Y₂O₃ in AlN/BN, Yttrium-contained compounds were significantly removed after SPS process, which caused decreasing of the residual grain boundary phase in the sintered samples. As a result, thermal conductivity of AlN/BN ceramics was remarkably improved. By addition of Y₂O₃ content from 3 wt.% to 8 wt.% into AlN/15 vol.% BN ceramics, the thermal conductivity increased from 110 W/m K to 141 W/m K.     

Carbon foams with high compressive strength derived from mixtures of mesocarbon microbeads and mesophase pitch

Carbon foam with relatively high compressive strength and suitable thermal conductivity was prepared from mixtures of mesocarbon microbeads (MCMBs) and mesophase pitch, followed by foaming, carbonization and graphitization. The influence of addition amount of MCMB on the properties of as-prepared carbon foams was investigated in detail. Results showed that addition of MCMBs into mesophase pitch could significantly reduce the amount and length of cracks in carbon foams, which results in increase of compressive strength of carbon foams. Carbon foam with high compressive strength of 23.7 MPa and suitable thermal conductivity of 43.7 W/mK, was obtained by adding 50% MCMBs into mesophase pitch, followed by foaming, carbonization and graphitization.     

Application of a high thermal conductivity C/C composite in a heat-redistribution thermal protection system

A carbon/carbon (C/C) composite with a thermal conductivity greater than 500 W/m K was used as a heat-redistribution material in a new thermal protection system. Using this material, the temperature at the hot head of a model hypersonic aircraft re-entry cone was sharply decreased, which effectively reduced the thermal burden and protected the hot side from being destroyed.     

Preparation of phenolic-based carbon foam with controllable pore structure and high compressive strength

Phenolic-based carbon foams with controllable pore structure and high compressive strength were prepared by foaming of resin solution under the pressure of 4 MPa and then carbonizing. Results showed that the average pore size of carbon foam ranging from 20 to 180 nm can be controlled by changing the resin concentration. The nanometer pore structure resulted in significant improvement of compressive strength and thermal insulation properties of the carbon foams. Carbon foam with bulk density of 0.73 g/cm³, average pore size of 20 nm, compressive strength of 98.3 MPa and thermal conductivity of 0.24 W/mK was obtained.     

Enhancing the thermal conductivity of polyacrylonitrile- and pitch-based carbon fibers by grafting carbon nanotubes on them

The thermal conductivities of ultrahigh tensile strength polyacrylonitrile (PAN)-based (T1000GB) and ultrahigh modulus pitch-based (K13D) carbon fibers with carbon nanotubes (CNTs) grown on them using chemical vapor deposition were measured using a thermal diffusivity meter. The thermal conductivities of the resulting hybrid materials were calculated to be 18.6 ± 1.7 and 965.6 ± 30.0 W/m K for T1000GB and K13D, respectively, while the respective original conductivities were 12.6 ± 1.0 and 745.5 ± 16.0 W/m K. The results clearly show that the CNTs grafting improves the thermal conductivities of both types of fiber.     

Measurement of through-plane effective thermal conductivity and contact resistance in PEM fuel cell diffusion media

An experimental study was performed to determine the through-plane thermal conductivity of various gas diffusion layer materials and thermal contact resistance between the gas diffusion layer (GDL) materials and an electrolytic iron surface as a function of compression load and PTFE content at 70 °C. The effective thermal conductivity of commercially available SpectraCarb untreated GDL was found to vary from 0.26 to 0.7 W/(m °C) as the compression load was increased from 0.7 to 13.8 bar. The contact resistance was reduced from 2.4×10⁻⁴ m²°C/W at 0.7 bar to 0.6×10⁻⁴ m²°C/W at 13.8 bar. The PTFE coating seemed to enhance the effective thermal conductivity at low compression loads and degrade effective thermal conductivity at higher compression loads. The presence of microporous layer and PTFE on SolviCore diffusion material reduced the effective thermal conductivity and increased thermal contact resistance as compared with the pure carbon fibers. The effective thermal conductivity was measured to be 0.25 W/(m °C) and 0.52 W/(m °C) at 70 °C, respectively at 0.7 and 13.8 bar for 30%-coated SolviCore GDL with microporous layer. The corresponding thermal contact resistance reduced from 3.6×10⁻⁴ m²°C/W at 0.7 bar to 0.9×10⁻⁴ m²°C/W at 13.8 bar. All GDL materials studied showed non-linear deformation under compression loads. The thermal properties characterized should be useful to help modelers accurately predict the temperature distribution in a fuel cell.     

Processing and characterization of syntactic carbon foams containing hollow carbon microspheres

The effects of heat-treatment on the properties of carbon foams were studied. The carbon foam was first prepared by adding hollow carbon microspheres to phenolic resin, followed by post-curing, pre-carbonization and carbonization. The mechanisms of failure behaviour and the increase of electrical and thermal conductivities showed that the properties of the foams were influenced by the heat-treatment temperature. Results showed that the introduction of more interval voids during carbonization resulting in a reduction of the mechanical properties. Carbon foams with electrical conductivity of 1.20 S/cm and thermal conductivity of 12.85 W/mK were obtained.     

Effects of pitch fluoride on the thermal conductivity of carbon foam derived from mesophase pitch

Carbon foams with high thermal conductivity were obtained from mixtures of mesophase pitch and pitch fluoride. The addition of pitch fluoride in mesophase pitch could significantly increase the specific thermal conductivity of as-prepared carbon foams. After graphitization at 2873 K, the specific thermal conductivity of carbon foams increased from 82 up to 155.4 (W/mK)/(g/cm³) when the content of pitch fluoride was 3% in the raw material.     

Improved thermal conductivity for chemically functionalized exfoliated graphite/epoxy composites

Chemically functionalized exfoliated graphite-filled epoxy composites were prepared with load levels from 2% to 20% by weight. The viscosities of the composites having load levels >4% by weight were over the processing window for the vacuum-assisted resin transfer molding process. Wide-angle X-ray diffraction revealed a rhombohedral carbon structure in the filler. Enhanced interaction between the epoxy and the graphite filler was evidenced by an improvement in the rubber modulus for the chemically functionalized graphite/epoxy composites. The thermal and electrical properties of the nanoparticle-filled epoxy composites were measured. The electrical property of the chemically functionalized graphite/epoxy composite deteriorated. Thermal conductivity of the chemically functionalized graphite/epoxy composite, however, increased by 28-fold over the pure epoxy resin at the 20% by-weight load level, increasing from 0.2 to 5.8 W/m K.     

Thermophysical properties of carbon/carbon composites and physical mechanism of thermal expansion and thermal conductivity

Five different carbon/carbon composites (C/C) have been prepared and their thermophysical properties studied. These were three needled carbon felts impregnated with pyrocarbons (PyC) of different microstructures, chopped fibers/resin carbon + PyC, and carbon cloth/PyC. The results show that the X-Y direction thermal expansion coefficient (CTE) is negative in the range 0-100 °C with values ranging from −0.29 to −0.85 × 10⁻⁶/K. In the range 0-900 °C, their CTE is also very low, and the CTE vs. T curves have almost the same slope. In the same temperature range composites prepared using chopped fibers show the smallest CTE values and those using the felts show the highest. The microstructure of the PyC has no obvious effect on the CTE for composites with the same preform architecture. Their expansion is mainly caused by atomic vibration, pore shrinkage and volatilization of water. However, the PyC structure has a large effect on thermal conductivity (TC) with rough laminar PyC giving the highest value and isotropic PyC giving the lowest. All five composites have a high TC, and values in the X-Y direction (25.6-174 W/m K) are much larger than in the Z direction (3.5-50 W/m K). Heat transmission in these composites is by phonon interaction and is related to the preform and PyC structures.     

Thermal and mechanical properties of graphite foam/Wood’s alloy composite for thermal energy storage

Thermal and mechanical properties of graphite foam/Wood’s alloy (50Bi/27Pb/13Sn/10Cd) composites for thermal energy storage were investigated. As compared with the alloy and graphite foam, thermal conductivity of the composites (193.74 W/mK) increased 2 times. Significant reduction in coefficient of thermal expansion of the composite compared with that of the alloy (7.82 vs. 24.81 ppm/K) was obtained. The latent heat of the composite remained very close to the value of the alloy alone. Moreover, alloy filled into graphite foam enhanced the mechanical properties of the graphite foam both in the solid phase and melting phase.     

Effect of functionalized carbon nanotubes on the thermal conductivity of epoxy composites

Direct functionalized carbon nanotubes (CNTs) were utilized to form the heat flow network for epoxy composites through covalent integration. A method of preparing a fully heat flow network between benzenetricarboxylic acid grafted multi-walled carbon nanotubes (BTC-MWCNTs) and epoxy matrix is described. A Friedel-Crafts modification was used to functionalize MWCNTs effectively and without damaging the MWCNT surface. Raman spectra, X-ray photoelectron spectra and thermogravimetric analysis reveal the characteristics of functionalized MWCNTs. The scanning electron microscope images of the fracture surfaces of the epoxy matrix showed BTC-MWCNTs exhibited higher solubility and compatibility than pristine-MWCNTs. The MWCNTs/epoxy composites were prepared by mixing BTC-MWCNTs and epoxy resin in tetrahydrofuran, followed by a cross-linking reaction with a curing agent. The BTC was grafted onto the MWCNTs, creating a rigid covalent bond between MWCNTs and epoxy resin and forming an effective network for heat flow. The effect of functionalized MWCNTs on the formation of the heat flow network and thermal conductivity was also investigated. The thermal conductivity of composites exhibits a significant improvement from 0.13 to 0.96 W/m K (an increase of 684%) with the addition of a small quantity (1-5 vol%) of BTC-MWCNTs.     

Thermal properties of diamond/Ag composites fabricated by eletroless silver plating

Thermal management in microelectronic technology has become an important issue due to the increase of device power and integration levels. Diamond and silver were selected for the fabrication of composites with high thermal conductivity and low coefficient of thermal expansion (CTE). Diamond reinforcement powders with varied types, shapes and sizes were electroless plated by silver. Then these powders were hot-pressed in air at 600 °C, 500 MPa for 30 min to produce bulk silver matrix composites. The thermal conductivity and the CTEs of the composite at 20 vol.% are 420 W/m K and 12 ppm/K, respectively. These diamond/Ag composites have potential applications for the high integration electronic devices.     

Nano- and microstructural effects on thermal properties of poly (l-lactide)/multi-wall carbon nanotube composites

In this work the thermal properties of poly (l-lactide)/multi-wall carbon nanotube (PLLA/MWCNT) composites have been investigated. Thermal conductivity was determined after measuring specific heat capacity (C_p), thermal diffusivity (D) and bulk density (ρ) of composites. Thermal conductivity rises up to 0.345 W/m K at 5 wt.% after reaching a minimum value of about 0.12 W/m K at 0.75 wt.%. In order to understand the heat-conduction process, experimentally obtained thermal conductivities were fitted to an existing theoretical model. The much lower thermal conductivity of composites compared with the value estimated from the intrinsic thermal conductivity of the nanotubes and their volume fraction could be explained in terms of the obtained large thermal resistance (R_k) of 1.8 ± 0.3 × 10^?8 m² K/W at nanotube–matrix interface. The CNT dispersion in the composites was analyzed by atomic force microscopy (AFM) and transmission electron microscopy (TEM). Although the thermal resistance dramatically reduces the estimated bulk thermal conductivity of composites, the existence of an interconnected conductive nanotube network for thermal diffusion in PLLA/MWCNT composites demonstrates that the addition of carbon nanotubes represents an efficient strategy in order to successfully enhance the thermal conductivity of insulator polymers.     

Production of anorthite refractory insulating firebrick from mixtures of clay and recycled paper waste with sawdust addition

Production of porous anorthite refractory insulating firebricks from mixtures of two different clays (K244 clay and fireclay), recycled paper processing waste and sawdust addition are investigated. Suitability of alkali-containing-clay, low-alkali fireclay, pore-making paper waste and sawdust in the products was evaluated. Prepared slurry mixtures were shaped, dried and fired. Highly porous anorthite ceramics from the mixtures with up to 30% sawdust addition were successfully produced. Physical properties such as bulk density, apparent porosity, percent linear change were investigated as well as the mechanical strengths and thermal conductivity values of the samples. Thermal conductivities of the samples produced from fireclay and recycled paper waste decreased from 0.25 W/mK (1.12 g/cm³) to 0.13 W/mK (0.64 g/cm³) with decreasing density. Samples were stable at high temperatures up to 1100 °C, and their cold strength was sufficiently high. The porous anorthite ceramics produced in this study can be used for insulation in high temperature applications.     

Use of exfoliated graphite filler to enhance polymer physical properties

Three sets of exfoliated graphite filled polymers, having three different particle sizes, were prepared with load levels from 0.1-20% by weight. After sieving, the particles were reduced to nanometer size through exfoliation, shear mixing, and ultrasonication, which further breaks and delaminates them. The electrical, thermal and mechanical properties of the nanoparticle filled polymers were measured. In addition, light, scanning electron, and atomic force microscope characterization were performed. The exfoliated graphite filled polymer material could be tailored to be high modulus and vary from insulating to highly conducting. Compared with the pure polymer, the polymers filled with 20% wt. exfoliated graphite has seen an significant reduction in electrical resistivity from 1.5⁸ to 0.5 Ω cm. The thermal conductivity for the polymers containing 20% wt exfoliated graphite has also been drastically improved, increasing from 0.2 to 5 W/m K. The flexural modulus achieved a maximum increase of 3.8 GPa which is a 60% above the value for the pure polymer (2.4 GPa).     

Graphite blocks with high thermal conductivity derived from natural graphite flake

In this work, natural graphite flake (NG) and mesophase pitch were used as precursor carbons to prepare the graphite blocks, which were doped with Si and Ti powders. After hot-pressed at 2700 °C, we investigated the effect of mean size of NG on properties and microstructure of the graphite blocks. Results showed that both thermal conductivity and flexural strength of the graphite blocks were improved as mean size of NG in raw material increased from 50 to 246 μm. However, a decrease of thermal conductivity was observed when mean size of NG was higher than 246 μm. The density and open porosity were respectively 2.26 g/cm³ and 5.82% when mean size of NG in raw material was 246 μm. The thermal conductivity was enhanced, however, the flexural strength was reduced as hot-pressing temperature increased from 2300 to 3000 °C. The thermal conductivity and flexural strength of the graphite block were respectively 704 W/m K and 21.1 MPa when hot-pressing temperature was 3000 °C. Phase analysis demonstrated there were diffraction peaks of graphite, TiC and α-SiC crystals in the graphite block as the hot-pressing temperature was less than 2500 °C. No SiC crystals were evident when the hot-pressing temperature was 2700 °C or above.     

Preparation of Si–diamond–SiC composites by in-situ reactive sintering and their thermal properties

This article described a novel method of preparation of Si–diamond–SiC composites by in-situ reactive spark plasma sintering (SPS) process. The relative packing density of Si–diamond–SiC composite was 98.5% or higher in a volume fraction range of diamond between 20% and 60%. Si–diamond–SiC composites containing 60 vol% diamond particles yielded a thermal conductivity of 392 W/m K, higher than 95% the theoretical thermal conductivity calculated by Maxwell–Eucken's equation. Coefficients of thermal expansion (CTEs) of the composites are lower than the values of theoretical models, indicating strong bonding between the diamond particle and the Si matrix in the composite. The microstructures of these materials were studied by field emission scanning electron microscope (FE-SEM) and X-ray diffraction (XRD). As a result of reaction between diamond and silicon, SiC phase formed.