Graphite foam from pitch and expandable graphite

Graphite foams were prepared from a coal tar pitch that was partially converted into mesophase. Expandable graphite was used instead of an inert gas to “foam” the pitch. The resulting foam was subjected to a series of heat treatments with the objective of first crosslinking the pitch, and thereafter carbonizing and graphitizing the resulting foam. XRD confirmed that the graphitization at 2600 °C resulted in a highly graphitic material. The porosity of this foam derives from the loose packing of the vermicular exfoliated graphite particles together with their internal porosity. During the foaming process the pitch tends to coat the outside surface of the expanding graphite flakes. It also bonds them together. The graphite foam prepared with 5 wt.% expandable graphite had a bulk density of 0.249 g cm⁻³, a compressive strength of 0.46 MPa and a thermal conductivity of 21 W m⁻¹ K⁻¹. The specific thermal conductivity (thermal conductivity divided by the bulk density) of this low-density carbon foam was 0.084 W m² kg⁻¹ K⁻¹ which is considerably higher than that of copper metal (0.045 W m² kg⁻¹ K⁻¹) traditionally used in thermal management applications.     

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.     

Microstructure and thermophysical properties of graphite foam/glass composites

Mesophase pitch based graphite foams with different thermal properties and cell structures were infiltrated with glass by pressureless infiltration to prepare potential alternative composites for cooling electronics. Microstructure, thermal diffusivity and coefficient of thermal expansion (CTE) of the obtained composites were investigated. It was demonstrated that there was excellent wettability of the graphite foam by molten glass, and the foam framework was retained well after infiltration, which could facilitate good heat transfer throughout the composites. The highest thermal diffusivity of the composites reached 202.80 mm²/s with a density of 3.81 g/cm³. And its CTE value was 4.53 ppm/K, much lower than the corresponding calculated result (7.46 ppm/K) based on a simple “rule of mixtures” without considering the space limitations of the graphite foams. Thus, the mechanical interlocking within the space limitations of the graphite network played a crucial role in limiting the thermal expansion of the glass. The CTEs of the graphite foam/glass composites varied from 4.53 to 7.40 ppm/K depending on the graphite foam density which varied from 0.82 to 0.48 g/cm³. The CTEs were a good match to those of semiconductor chips and packaging materials.     

Preparation of mesophase-pitch-based carbon foams at low pressures

A simple method for preparing the mesophase-pitch-based carbon foams at low pressures through prolonging the soaking time in the preparation process of the mesophase pitch was disclosed. The physical properties, morphologies and the crystal structure of the as-obtained foams were investigated. Bulk density of the resultant carbon foams cover a range 0.514-0.624 g/cm³, under the preparation pressure range 0.5-2 MPa. The SEM micrographs revealed that the thermal shrinkage of the graphitized foams derived from the higher softening point mesophase pitch was less than that of the foam from the lower softening point; Optical micrographs showed that higher softening point mesophase pitch derived carbon foams exhibited better orientation and less microcracks in both junctions and ligaments; The XRD results revealed that higher softening point pitch derived graphitized foams exhibited smaller interlayer spacing and larger crystallite size. The properties of the carbon foam can be severely affected by the properties of the precursor, so it is critical to tailor the properties of the pitch precursor so as to obtain high performance and low cost products.     

Modeling the principle physical parameters of graphite carbon foam

Graphite carbon foam, a mesophase, pitch-based material, portrays highly ordered topology structures which exhibit superior mechanical and thermal properties. Typical graphite carbon foam with dimensions 5 cm³, can have a surface area greater than 11 m², making it an excellent candidate for heat transfer applications. Accurate three dimensional modeling of carbon foams is necessary to study and predict their properties in simulation. This paper describes a computer algorithm for modeling POCO Foam® and similar carbon foams. The algorithm, written in MATLAB, captures the principle physical parameters of the carbon foam including bubble and pore diameter ranges and overall foam void percentage while retaining the random dispersal of spherical bubbles found in manufactured foams.     

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.     

Porous graphite matrix for chemical heat pumps

Expanded graphite powders were prepared by rapid heating of expandable graphite powders intercalated with sulfuric acid at different heat treatment temperatures (HTT). Porous graphite matrices with 100–400 kg m⁻³ of bulk density were fabricated by pressing expanded graphite powders in order to use as heat conductive media. They were characterized using an C/S analyzer, inductively coupled plasma spectroscopy, X-ray diffraction, scanning electron microscopy, Fourier transform infrared (FTIR), nitrogen adsorption, optical microscopy and helium pycnometer before and after heat treatment. Gas permeability and thermal conductivity were measured for porous graphite matrices with different HTT and bulk densities. Chemical analysis and FTIR showed that as the HTT of expandable graphite powders increase, the residual sulfur content decreased remarkably. Nitrogen adsorption experiments for expanded graphite powders showed that specific surface area and total pore volume increased with HTT. Helium penetration results showed that porous graphite matrices with different HTT have noticeably different open porosities which were attributed to the different degrees of expansion of graphite layers. The gas permeability of porous graphite matrices was in the range of 10⁻¹²–10⁻¹⁵ m² and exhibited higher values with low HTT. Thermal conductivity values in the axial and the radial directions were in the range of 4.1–20.0 and 4.6–42.3 W mK⁻¹, respectively. A semi-empirical model was developed that can be used to correlate with the thermal conductivity of graphite matrix on the basis of solid conductivity, bulk density and porosity.     

Near net-shape fabrication and properties of graphitic carbon foam with a continuous network of graphite nanosheets

A near net-shape graphitic carbon foam (GCF) with a continuous network of graphite nanosheets was prepared by direct carbonization of epoxy resin filled with nano-Al₂O₃. The effects of carbonization temperature on the properties of the resulting carbon foams were investigated by SEM, TEM, XRD, Raman, thermal conductivity and compression strength test. The results show that the as-prepared GCF can maintain well dimensional stability upon carbonization. The carbothermal reaction between the nano-Al₂O₃ and carbon foam matrix greatly influences the microstructure of carbon foam and promotes its growth of the continuous network of graphite nanosheets. In addition, the GCF prepared at 1700 °C possesses a compressive strength of 2.34 MPa with a bulk density of 0.19 g cm^-3, and meanwhile presents a high graphitization degree of 65.12% and a thermal conductivity of 2.02 W/mK. The continuous network of graphite nanosheets favors the enhancement of thermal conductivity of carbon foam and simultaneously prevents the decline of compressive strength further.     

Thermal conductivity and permeability of consolidated expanded natural graphite treated with sulphuric acid

The thermal conductivity and permeability of consolidated expanded natural graphite treated with sulphuric acid (ENG-TSA) were measured both parallel and perpendicular to the direction of compression used to produce the samples. Results showed that the thermal conductivity and permeability were highly anisotropic. The thermal conductivity perpendicular to the direction of compression was 50 times higher than that parallel to the direction of compression and the permeability was 200 times higher. The maximum thermal conductivity measured was 337 W m⁻¹K⁻¹ at a bulk density of 831 kg m⁻³. The permeability perpendicular to the direction of compression varied in the range of 10⁻¹¹ to 10⁻¹⁶ m² as the density increased from 111 to 539 kg m⁻³. The specific heat was measured, and the average value is 0.89 kJ kg⁻¹K⁻¹ in the temperature range 30–150 °C. As a type of heat transfer matrix the thermal diffusivity was about five times higher than that of, for example, pure aluminium due to the combination of improved thermal conductivity with comparatively low density and reasonable specific heat.     

Thermal conductivity of high porosity alumina refractory bricks made by a slurry gelation and foaming method

Alumina has high heat resistance and corrosion resistance compared to other ceramics such as silica or mullite. However, for its application to refractory bricks, its high thermal conductivity must be reduced. To reduce this thermal conductivity by increasing the porosity, a GS (gelation of slurry) method that can produce high porosity solid foam was applied here to produce the alumina refractory brick. This method was successfully applied to produce alumina foam with high porosity and thermal conductivity of the foam is evaluated. At room temperature, the thermal conductivity was about 0.12 W/mK when the foam density was 0.1 g/cm³. At elevated temperature above 783 K, thermal conductivity of the foam was strongly affected by heat radiation and increased with increasing temperature, in contrast to the thermal conductivity of alumina itself, which decreased with increasing temperature. The alumina foams developed here achieved sufficient thermal insulating properties for use in refractory bricks.     

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.     

Preparation of erythritol–graphite foam phase change composite with enhanced thermal conductivity for thermal energy storage applications

Three-dimensional interconnected graphite composite foam as a heat conductive matrix was fabricated by using low cost polymeric precursors and polyurethane (PU) foam as carbon source and sacrificial macroporous template, respectively. Erythritol–graphite foam as a stable composite phase change material (PCM) was obtained by incipient wetness impregnation method. The thermophysical properties such as thermal diffusivity, specific heat, thermal conductivity and latent heat of the erythritol–graphite composite foam were measured. From the results, it was found that the thermal conductivity of the erythritol–graphite composite foam (3.77 W/mK) was enhanced 5 times as compared with that of pristine erythritol (0.72 W/mK). This enhancement can significantly reduce the charging and discharging times of the PCM storage system. There is no chemical reaction between erythritol and graphite as confirmed by X-ray diffractometer (XRD). The PCM/foam composite has a melting point of 118 °C and latent heat of 251 J/g which corresponds to the mass percentage (75 wt.%) of the erythritol within the composite foam. The obtained results confirmed the feasibility of using erythritol–graphite foam as a new phase change composite for thermal energy storage (TES) applications, thus it can contribute to the efficient utilization and recovery of solar heat or industrial waste heat.     

Preparation of cellular SiBOC foams by thermo-foaming of polymethylvinylborosiloxane–wheat flour dough

A novel method for the preparation of SiBOC foams from a polymethylvinylborosiloxane (MVBS) solution in ethanol using wheat flour as a foam stabilizer and pore template has been reported. A dough prepared by mixing the MVBS solution and wheat flour undergoes foaming at 180°C due to the stabilization of bubbles created by ethanol vapor by the adsorption of wheat flour particles on their surface. The ceramization of the foamed body at 1500°C followed by burnout of carbon produced from the wheat flour results in SiBOC foams of hierarchical pore structure. The foam density (.68–.44 g cm⁻³), average cell size (1.81–.58 mm), compressive strength (3.9–1.7 MPa), and thermal conductivity (.33–.21 W m⁻¹ K⁻¹) decrease with an increase in wheat flour to MVBS weight ratio from .25 to 1. The population of smaller pores (∼5–50 μm) created by wheat flour particles on cell walls and struts increases with an increase in wheat flour to MVBS weight ratio. The presence of β-SiC and turbostratic graphite nanodomains in amorphous SiBOC is evidenced by X-ray diffraction and transmission electron microscopy analysis.     

Characterization of polyurethane foams prepared from liquefied sawdust by crude glycerol and polyethylene glycol

The aim of this study was to investigate polyurethane foams (PUF) properties obtained from crude glycerol (CG) and polyethylene glycol (PEG) based liquefaction of sawdust. The four types of foam were prepared by producing polyols from different weight loadings of PEG to CG as the liquefaction solvent. The produced polyurethane foams showed densities from 0.042 to 0.08 g/cm³ and compressive strengths from 200 to 311 kPa. The foams obtained from CG/PEG based liquefaction, had more uniform and regular cell structure than foams derived from liquefaction by CG. Also with the increasing percentage of PEG to CG in liquefaction, closed cell content of the synthesized foams increased and the size of cells decreased. The thermal conductivity of the produced foams was between 0.031 and 0.040 W/m K. Foams produced from liquefaction by binary solvent had lower thermal conductivity. However all foams showed approximately similar thermal degradation curves; maximum thermal decomposition temperature was seen for the foam produced from higher weight ratio of PEG to CG in liquefaction. PU foams produced from PEG/CG based liquefaction process had improved properties over from foams derived from sawdust liquefaction by CG.     

Rigid Polyurethane Foam Production from Palm Oil-Based Epoxidized Diethanolamides

A new type of rigid polyurethane foam was produced by incorporating oxazolidone heterocyclic rings on to polyurethane backbones. Epoxidized diethanolamides were synthesized by reacting palm oil blends of epoxidized palm olein and refined bleached deodorized palm kernel olein with diethanolamine to produce rigid polyurethane foams. Epoxides, retained in the diethanolamides, reacted with isocyanate during foam production in the presence of AlCl₃–THF complex catalyst to form oxazolidone linkages in the polyurethane network. The carbonyl stretch of oxazolidone was identified at 1,750 cm⁻¹ through Fourier Transform Infra Red analysis. Chemical modifications of the polyurethane network also improved the thermal and mechanical properties of the foams. In addition, isocyanate index 1.4 was determined to be the most suitable in the production of foams from this newly synthesized epoxidized diethanolamides.     

Carbon foams prepared from polyimide using urethane foam template

Polyimide and carbon foams were successfully prepared using polyurethane foams as a template. Impregnation of polyimide precursor, poly(amide acid), followed by imidization at 200 °C gave polyurethane/polyimide (PU/PI) composite foams, which resulted in PI foams by heating above 400 °C and then carbon foams above 800 °C. Foams carbonized at 1000 °C were graphitized by the heat treatment at 3000 °C, keeping foam characteristics. Two applications of these carbon foams, i.e., an adsorbent of ambient water vapor and a substrate of photocatalyst anatase TiO₂, were experimentally confirmed. For the former application, the present foam could be characterized by prompt adsorption of ambient water vapor. Some of carbon foams prepared were floating on water, even after loading photocatalyst anatase, which might be advantageous for photodecomposition of pollutants in water in respect to the UV rays efficiency.     

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.     

Conductive elastomeric foams prepared by in situ vapor phase polymerization of pyrrole and copolymerization of pyrrole and N-methylpyrrole

Electrically conductive elastomer foams were prepared by in situ vapor phase copolymerization of pyrrole and N-methylpyrrole within preformed polyurethane foams. The conductive polymer is formed within the dense polymer phase of the foam, and the conductivity of the composite foam ranged from 10^?1 to 10^?7 S/cm. The conductvity depended on the density of the foam, the amount of conducting polymer incorporated, and the copolymer composition. Replacing with N-methylpyrrole in the copolymer lowered the conductivity.     

Properties of a novel inherently flame-retardant rigid polyurethane foam composite bearing imide and oxazolidinone

A novel inherently flame-retardant rigid polyurethane (PU) foam with imide and oxazolidinone was prepared by using 3,3′,4,4′-biphenyltetracarboxylic dianhydride (PTDA) and 9,10-dihydro-9-oxa-(10-glycidoxypropylene)-10-phosphap-henanthrene-10-oxide (e-DOPO) as reactive flame retardants. The physical and mechanical properties of the prepared PU foams were investigated. The compressive strength was improved to 0.22 MPa, the thermal conductivity decreased, and the density hardly changed. Thermogravimetric (TG) analysis and TG analysis coupled with Fourier transform infrared spectroscopy indicated that the PTDA and e-DOPO showed a small improvement in thermal properties. The fire behaviors were evaluated based on the limited oxygen index (LOI), cone-calorimetry experiment, and smoke-density test. The LOI of the PU foam with PTDA and e-DOPO reached 22.4%. The peak of heat release rate and total heat release decreased to 227.50 kW m⁻² and 11.27 MJ m⁻² from 281.28 kW m⁻² and 14.05 MJ m⁻², respectively. The morphologies of the PU foam and residues after the cone-calorimetry test were characterized by scanning electron microscopy. X-ray photoelectron spectroscopy analysis indicated that PTDA and e-DOPO lead to an increase in graphite in the residue and the formation of a better barrier to prevent burning by the condensed-phase mechanism. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47943.     

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.