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.     

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.     

Synthesis, characterization and radionuclide (Cs) trapping properties of a carbon foam

Low-density carbon foam was synthesized from an aqueous acidic sucrose solution. The resin formed by heating this solution underwent foaming and set into a solid green foam which was sintered in the temperature range 573-1223 K. The green and the sintered foams were characterized by Fourier transform infrared spectroscopy, powder X-ray diffraction and scanning electron microscopy. A thermogravimetric analyzer coupled with a mass spectrometer was used to study the thermal stability of green foam under an argon atmosphere in the temperature range 303-1273 K. The suitability of this carbon foam and a commercially available reticulated vitreous carbon for trapping radioactive ¹³⁷Cs was studied by a tracer technique in static sodium at 473 K. A NaI(Tl) detector was used to measure ¹³⁷Cs activity trapped by these materials. Trapping efficiency for ¹³⁷Cs and distribution coefficient of ¹³⁷Cs between sodium and the carbon foam were found to be in the range 73-77% and 4.9-6.0 × 10², respectively.     

Carbon foam derived from various precursors

A series of carbon foams was developed by using low-cost precursors, such as coal, coal tar pitch and petroleum pitch. The properties of the resultant carbon foams cover a wide range, e.g., bulk density, 0.32-0.67 g/cm³, compressive strength, 2.5-18.7 MPa, isotropic and anisotropic microstructure, etc. The investigation of foaming mechanism and the relationship between properties and structure indicate that the fluidity and dilatation of the foaming precursors significantly affect the foaming performance and foam structure. Raw coal samples were foamed directly without pretreatment in this work. However, for the pitch based foaming precursor, a thermal pretreatment is necessary to adjust its thermoplastic properties to meet the foaming requirement. The mechanical strength of carbon foam is found to be related to not only the foam cell structure, but also the fluidity and anisotropic domain size of the foaming precursors. In addition, the micro and mesopore structure in carbon foam matrix was investigated by gas adsorption and it was found that it also affects the strength of carbon foam and is related to the fluidity of foaming precursor.     

Generation and characterization of carbon nano-fiber-poly(arylene ether sulfone) nanocomposite foams

In this study, carbon nano-fibers (CNFs) were used to increase the compressive properties of poly(arylene ether sulfone) (PAES) foams. The polymer composite pellets were produced by melt blending the PAES resin with CNFs in a single screw extruder. The pellets were saturated and foamed with water and CO₂ in a one-step batch process method. Dynamic mechanical thermal analysis (DMTA) was used to determine the reduced glass transition temperature (T_g) of the CNF-PAES as a result of plasticization with water and CO₂. Sharp transitions were observed as peaks in the tan δ leading to accurate quantitative values for the T_g. By accurately determining the reduced T_g, the foaming temperature could be chosen to control the foam morphology. Foams were produced which ranged in density from 290 to 1100 kg/m³. The foams had cell nucleation densities between 10⁹ and 10¹⁰ cells/cm³, two orders of magnitude higher than unreinforced PAES foam, suggesting that the CNFs acted as heterogeneous nucleating agents. The CNF-PAES foam exhibited improved compressive properties compared to unreinforced PAES foam produced from a similar method. Both the specific compressive modulus and strength increased by over 1.5 times that of unreinforced PAES foam. The specific compressive strength of 59 MPa for the CNF-PAES foam is similar to that of commonly used high performance structural foam, poly(methacrylimide foam).     

Tannin-based carbon foams

An easily-prepared self-blowing system based on low-cost precursors, mainly tannin and furfuryl alcohol, was pyrolysed at 900 °C. Extremely low-density glass-like carbon foam was obtained, with very high porosity (96.4%, of which 94% is open) and low surface area (below 1 m² g⁻¹). The material presents slightly anisotropic properties. The linear cell density is 100 and 190 ppi along two orthogonal directions, and the major physical properties were examined according to these same directions: electrical and thermal conductivities, mechanical strength and elastic modulus, coefficient of thermal expansion, permeability and fire resistance. Since most of these properties compare very correctly with available data from the literature, such a cheap material like the one reported here can compete with commercial vitreous carbon foams, which are all produced from more expensive, non-renewable, resources.     

Preparation and characterisation of carbon-coated ceramic foams for organic vapour adsorption

Ceramic foam substrates of various porosities were coated with novolak resin, which was then subsequently carbonised and activated to develop a pore structure. The carbon forming the layer was characterised by thermal analysis, TPD and N₂, CO₂ and hexane vapour adsorption. It was found to be microporous with a high surface area (up to 1400 m² g⁻¹), which made it a good adsorbent for hexane vapour at ambient temperature. The carbon-ceramic interface was examined using SEM. The coated foams displayed a reduced pressure drop compared to carbon granules and had good dynamic hexane vapour adsorption characteristics. Depending on the foam architecture, hexane breakthrough was delayed for up to 78 min.     

Mechanical properties of copper-coated carbon foams

The mechanical properties of copper-coated carbon foam were investigated. Reticulated vitreous carbon cell type foams, with 97% porosity and 10 ppi pore size, were electroplated with copper for different periods of time to obtain coatings with different thicknesses and foams with different porosities. Compression tests were performed to determine the Young’s modulus and the plateau stress. The copper electroplating technique improved these two properties, with the modulus increasing from 4.5 to 8.6 MPa for the sample electroplated for 40 min and the plateau stress increasing from 54 to 171 kPa for the foam coated for 80 min. The relationships between the measured properties and the copper weight ratio were determined.     

Anisotropy of mesophase pitch-derived carbon foams

Carbon foams with uniform pore size were produced from mesophase pitch in a high-pressure chamber. The morphologies, crystal structure and mechanical properties of as-grown carbon foams, carbonized and graphitic foams were investigated. The microwave absorbing properties of carbon foam sandwich composites at 2-18 GHz were also examined. It was found that the extension of pores in the xz-plane direction (parallel to the direction of gravitational force) was larger than that in the xy-plane direction (perpendicular to the direction of gravitational force). The compressive strength and modulus of resultant foams in the xz-plane direction were higher than those in the xy-plane direction. Further, the reflection loss of the sandwich composites composed of xy-plane carbon foam slabs was lower than that of xz-plane composites. The results of this work showed that carbon foam was a kind of anisotropic material.     

Rheology of foamed cement

Foams are being used in a number of petroleum industry applications that exploit their high viscosity and low density. Foamed cement slurries can have superior displacement properties relative to non-foamed cement slurries. This article presents results of an experimental study of foamed cement rheology. Viscosity curves of foamed cements were obtained using a flow-through rotational viscometer. Foamed cements with different foam qualities were generated under different pressures using a foam generator/viscometer apparatus. The foam qualities during the tests ranged from 0% to 30%, and the shear rate varied between 5 s^− 1 and 600 s^− 1. Experimental results indicate that: i) unlike conventional aqueous foams, low-quality cement foams have a lower viscosity than the base fluid; ii) as the cement foam quality (gas volumetric fraction) increases from 10% to 30%, the viscosity also increases; and iii) the viscosity of low-quality cement foam slightly increases after depressurization or expansion.     

Preparation and properties of multi-walled carbon nanotube/carbon/polystyrene composites

Multi-walled carbon nanotube (MWCNT)/C/polystyrene (PS) composite materials were prepared by in situ polymerization of monomer in preformed MWCNT/C foams. MWCNT/C foams were preformed using polyurethane foam as template. The preformed MWCNT/C foams had a more continuous conductive structure than the carbon nanotube networks formed by free assembly in composites. The structure of the MWCNT/C foam network was characterized with scanning electron microscopy. The MWCNT/C/PS composites have an electric conductivity higher than 0.01 S/cm for a filler loading of 1 wt.%. Enhancement of thermal conductivity and mechanical properties by the preformed MWCNT/C foam were also observed.     

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.     

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.     

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.     

Hydrodynamic and mass transfer efficiency of ceramic foam packing applied to distillation

In addition to a high void volume and specific area, solid foams possess other properties (low density, good thermal, mechanical, electrical, and acoustical behaviour) that make them attractive for applications such as heat exchangers and reformers. Applications using foams as catalysts or structured catalyst supports have demonstrated higher performance than classical catalysts. Several studies have explored the hydrodynamic behaviour of foams in monophasic and countercurrent systems and have reported very low pressure drops. This paper describes the application of ceramic foam to distillation. The β-SiC foam contains 5 pores per inch (PPI) with a 91% void volume and a surface area of 640 m²/m³. Performance parameters including pressure drop for the dry and wet packing, flooding behaviour, and dynamic liquid hold-up were measured in a column of 150 mm internal diameter. The mass transfer efficiency in terms of the height equivalent to theoretical plate (HETP) was determined by total reflux experiments using a mixture of n-heptane and cyclohexane at atmospheric pressure. The experimental results were used to develop a set of correlations describing pressure drop and liquid hold-up in terms of a dimensionless number. The hydrodynamic performance and mass transfer efficiency were compared with classical packing materials used in distillation.     

Response of 45S5 Bioglass foams to tensile loading

The tensile strength test of highly porous ceramic foams has been developed and first results have been obtained on bioactive glass foams. The tested material was a 45S5 Bioglass^® derived foam-like scaffold intended for use in bone tissue engineering which was manufactured by Bioglass^® slurry coating of polyurethane foam and subsequent sintering. The Bioglass^® foam structure was investigated in two states: uncoated (as fabricated) and with a PDLLA polymer coating. The tensile testing procedure is based on fixation of the foam into aluminium pots by a suitable adhesive. Tensile test samples having cross-section of 10×10 mm² and a length of 30 mm were used for the experiments. Basic fractographic analysis was applied to get relevant information about specimens' behaviour during tensile loading. In Bioglass^® based scaffolds, the presence of PDLLA coating led to a significant increase of the fracture strength, which is attributed to the interaction of the polymer phase with propagating cracks, e.g. enabling a crack bridging mechanism to take place.     

Extrusion-cooking of starch protective loose-fill foams

One of the most interesting alternatives to EPS is extrusion of starch-based materials. TPS-based biocomposites can be processed, in a one-step process, via an extrusion-cooking. Wide program of the experimental works with application of extrusion-cooking for production of starchy loose-fill foams has been started in the Department of Food Process Eng., Lublin University of Life Sciences in 2012. The object of the study is to achieve commercially acceptable biodegradable products based on locally produced potato, corn and wheat starch, which can replace popular EPS. Results of the first phase of the study are presented in the paper. The measurements of glass transition temperature of TPS samples showed that with glycerol content growth in the blend, the Tg of the obtained material decreases almost linearly. In the case of potato TPS, the highest observed Tg was 187.7 ?C for 7.0% glycerol and the lowest was at 18.1 ?C for 30% glycerol. Properties of the loose-fill foams highly depend on raw materials and process parameters used in production. In addition of plasticiser or other additives, different temperature of processing is causing changes in product's properties. All starch-based foams had high open-cell content and the expansion was attributable to the escape of water as steam during processing, resulting more than 80% open cells. The foam density of starch-based products ranged between 18.7 and 30.5 kg/m³. The products were at least 2.5 times more dense than EPS-based foams. The best products achieved by us up till now are the corn TPS-based foams containing 3% plasticizer and 1% poly-vinyl alcohol. The energy consumption during extrusion-cooking depended on the material composition of the blends, temperature and the screw rotation speed used during processing. Average value of SME was about 2.52 × 10⁵ J/kg, which is equivalent to 0.07 kW h/kg.     

Mechanical performance of highly compressible multi-walled carbon nanotube columns with hyperboloid geometries

Multi-walled carbon nanotube (MWCNT) columns are formed from the frit compression of a random distribution of MWCNTs in a casting solvent; its drying led to the formation of hyperboloid geometry. Uniaxial loading of MWNT columns mimics an open-cell foam behaviour and possesses an expansion rate in excess of 250 mm min⁻¹ and an elastic modulus of 10-12 MPa, thus superior to conventional low-density flexible foams. Successive compression-expansion cycling within the Hookean region reveals a hysteresis loop in the stress-strain curve that stabilises at a final value of ε_F = 18%, but on contact with its casting solvent and subsequent drying, the sample can be regenerated to within ε_R = 6% according to a memory effect and is repeatable in successive stress cycles and solvent regeneration. The system was modelled for the macroscopic stress-strain behaviour of the MWCNT column to reveal the contributions of linear dependence, elasticity-plasticity and elasticity-plasticity with hardening, revealing good agreement with the stress-strain data. MWCNT columns should prove useful as an energy adsorbing device.     

Electrochemical impedance study of polyaniline electrocoated porous carbon foam

Electropolymerization of aniline on mesophase pitch based carbon foam has been studied in order to evaluate the influence of conductive polymer coating on the properties of carbon foam. The surface morphology of the coating was determined by scanning electron microscopy (SEM). Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were used to investigate the electrochemical properties of resulting modified carbon foam samples. Polyaniline (PANI) electrocoated-mesophase pitch based carbon foam showed good capacitor behavior in 0.5 M H₂SO₄. Better capacitive behavior is obtained for 100 and 150 mV/s compared to other scan rates, under these faster scan rates thinner films of PANI coatings were combined with more porous structure of carbon foam. Conductivity of the carbon foam was increased from 9.23 to 13.73 S/cm by electrocoating of PANI.