High Pressure Carbonization Research Articles

Three-dimensional porous nitrogen-doped carbon nanosheets with ultra-high surface area for high-performance oxygen reduction reaction electrocatalysts

A series of three-dimensional (3D) porous nitrogen-doped carbon nanosheets (PNCNs) have been successfully fabricated via a high-pressure carbonization followed by high-temperature thermal treatment. With the prolonging of thermal treatment time, the 3D PNCNs demonstrate several remarkable advantages for oxygen reduction reaction (ORR) electrocatalysts, including ultra-high specific surface area (more than 2100 m2/g−1), unique 3D porous structure, abundant defects, and suitable configuration of nitrogen. With these merits, the N-C-2 h based electrocatalyst exhibits outstanding ORR activities with a comparable onset potential of −40 mV (vs. Ag/AgCl), half-wave potential of −120 mV (vs. Ag/AgCl), satisfactory electron transfer number (4) and superior stability to those of commercial 20% Pt/C. The excellent electrochemical behaviors of 3D PNCNs are confirmed to be dominated by structural characteristics, which can be optimized by varying thermal treatment time. Our work offers a promising way for controllably constructing carbon-based materials with optimized ORR performance.

Gaseous formaldehyde adsorption by eco-friendly, porous bamboo carbon microfibers obtained by steam explosion, carbonization, and plasma activation

To develop an eco-friendly biomass adsorbent for indoor formaldehyde, porous bamboo carbon microfibers (BCMFs) with different types and contents of oxygen (O)-containing and nitrogen (N)-containing functional groups were firstly prepared by combining high-pressure steam explosion, carbonization, and plasma activation. The BCMF adsorbents were characterized, and their adsorption performance and mechanism were evaluated and theoretically calculated. BCMF carbonized at 800 °C (BCMF-800) showed the highest specific surface area of 526.862 m2 g−1. The surfaces of the BCMFs were etched, with many additional micro/nanoscale pores and cavities. Functional groups, including O–CO, CO, CO, quaternary N, oxidized N, pyridinic N, amine N, and pyrrolic N were successfully introduced by plasma activation at various powers and atmospheres. BCMF-800 samples activated by O2 plasma at 240 W (BCMF-800-O-240) and by N2 at 120 W (BCMF-800-N-120) exhibited the highest formaldehyde adsorption capacities (179.53 mg g−1 and 205.28 mg g−1), which were 85.90 % and 112.50 % higher than that of BCMF-800, respectively. The samples introduced by carboxyl and pyrrolic N showed the highest adsorption energies (−14.54 kcal mol−1 and −11.58 kcal mol−1) with an increase of 83.82 % and 46.40 %, respectively, compared with the control, which were proved to be the optimal O-containing and N-containing functional groups for enhancing formaldehyde adsorption. This work provides a new insight into the natural biomass-based adsorbents for gaseous formaldehyde with a clean and eco-friendly preparation and modification process.

Effect of graphitization temperature on microstructure, mechanical and ablative properties of C/C composites with pitch and pyrocarbon dual-matrix

Two graphitized C/C composites with a density of 1.83 g/cm3 were fabricated by CVI, high-pressure liquid-phase carbonization, and graphitization process. After graphitization at 2500 and 3000 °C, the graphitization degree of carbon phases in both composites shows a trend of pitch carbon > pyrocarbon > PAN carbon fiber. With increasing graphitization temperature, the orderly arrangement and growth of the graphite microcrystals cause the improved graphitization degree of the composites. Therefore, the thermal conductivity increases from 68.78 to 126.09 W/(m·K) in the transverse direction and 31.09 to 75.60 W/(m·K) in the thickness direction. Meanwhile, the increase in micro-cracks and pores formed at the interface induced the declined flexural and compressive strength, with the corresponding strength reducing to 104.17 and 113.92 MPa, respectively. During the first 22 s of ablation, the improved graphitization degree and higher thermal conductivity can transfer the heat rapidly, resulting in a lower surface temperature and a higher backside temperature. After 30 s ablation, the mass ablation rate decreases from 0.0030 to 0.0018 g/s, while the linear ablation rate increases from 0.0040 to 0.0093 mm/s due to the increased open porosity and defects under higher graphitization temperature.

Iodine-mediated high-pressure high-temperature carbonization of hydrocarbons and synthesis of nanodiamonds

Carbonization behavior of iodinated adamantane, adamantane and naphthalene in mixtures with germanium iodide or iodine was studied at pressures of 3.5 and 8 GPa to elucidate diamond-forming catalytic activity of iodine. Using XRD, Raman spectroscopy and TEM as characterization methods, we have found that pyrolysis of iodinated adamantane at 8 GPa is accompanied with formation of nanodiamonds at temperatures exceeding 800 °C. Post-pyrolysis growth of nanodiamonds points out the diamond-forming catalytic activity of iodine. In the samples obtained from precursor mixtures, only graphite was detected as a pyrolysis product, which means that hydrocarbon iodination is a prerequisite for iodine-mediated synthesis of nanodiamonds.

Sublimation and oxidation zone ablation behavior of carbon/carbon composites

Three-dimensional carbon/carbon (C/C) composites comprising four reinforcement directions (4D) were fabricated using intermediate modulus carbon fibers and densified using a hybrid process. This consists of a pre-densification step using a thermal-gradient chemical vapor infiltration process followed by a high-pressure pitch impregnation and carbonization process. The specimens machined along Z-axis of the preform architecture were tested in an arc plasma heater for studying its ablation behavior at different temperatures. Regimes from ultra-high temperature (4750K) sublimation to high-temperature (2467K) oxidation zones were created by varying the mass flow rate of secondary air in the heater. The ablation rate showed a progressive increase as the environment changed from oxygen-lean sublimation to oxygen-rich oxidation conditions while the back-face temperature showed a similar temperature profile during the plasma exposure period. The thermal diffusivity value decreased with the rise in temperature till 1173K and later on became fairly flat till 1523K and onwards. In the compression test, 4750K exposed specimen showed toughening in the plasma-affected zone and crushed with shear mode from the opposite face while the 2467K exposed specimen showed end brushing in the plasma heat-affected face with a lower residual strength and Young’s modulus.

Preparation of a high performance carbon/carbon composite throat insert woven with axial carbon rods

A new kind of C/C composite was made to meet the requirements of the severe conditions suffered by a high performance throat-insert of a solid rocket motor using a method that includes four steps: preform preparation, pressure impregnation and carbonization, high pressure impregnation and carbonization, and high temperature treatment. The preform was made of soft carbon fiber yarns and rigid carbon fiber/resin rods, in which the carbon rods were distributed in an hexagonal arrangement parallel to the axial direction and carbon fiber yarns were laid in a plane perpendicular to the axial carbon rods with a nearest interyarn angle of 60°. The influence of the main processing conditions on the key properties of these composites was investigated. It is found that the composites have excellent mechanical and thermal properties, and their ablation rates are equivalent to those of C/C composites reinforced with an integral felt preform and 60% lower than those of the C/C composites woven with radial carbon rods.

Preparation of a high performance carbon/ carbon composite throat insert woven with axial carbon rods

A new kind of C/C composite was made to meet the requirements of the severe conditions suffered by a high performance throat-insert of a solid rocket motor using a method that includes four steps: preform preparation, pressure impregnation and carbonization, high pressure impregnation and carbonization, and high temperature treatment. The preform was made of soft carbon fiber yarns and rigid carbon fiber/resin rods, in which the carbon rods were distributed in an hexagonal arrangement parallel to the axial direction and carbon fiber yarns were laid in a plane perpendicular to the axial carbon rods with a nearest inter yarn angle of 60°. The influence of the main processing conditions on the key properties of these composites was investigated. It is found that the composites have excellent mechanical and thermal properties, and their ablation rates are equivalent to those of C/C composites reinforced with an integral felt preform and 60% lower than those of the C/C composites woven with radial carbon rods.

Degradation behavior of 4D carbon/carbon composites under supersonic oxidative air plasma

Abstract Four-directional (4D) carbon/carbon (C/C) composites were prepared using an intermediate modulus carbon fiber as the reinforcement and two kinds of coal tar pitches as the source for carbon matrix. After repeated high-pressure impregnation, carbonization and graphitization at 2250 °C, a high density of 1.86 g/cm3 was achieved. A plasma arc jet was applied to the composite for studying its degradation behavior. The oxidative supersonic jet was generated by an air plasma torch. For studying various stages of degradation during a long exposure time of 90-s, the intermittent ablation and erosion rates were measured for the C/C composite at time intervals of 70, 50 and 30 s. The mass ablation rate was increased rapidly with the time, while the erosion showed a bilinear response with a decreasing trend. The results were discussed based on the machined and eroded surfaces of the test samples. For studying the residual ablative performance, the samples of 30–70 s were then re-ablated and re-eroded in steps of 20-s so that all the samples are equal in total exposure time. The samples showed decreasing trends in the re-ablation and re-erosion rates and ultimately became zero for the last 20-s exposure time. There was a progressive increase in the char layer on the samples with the re-ablation. The porous char layer protected the samples against chemical and mechanical actions of the plasma stream. Compressive tests were carried out, and the decrease in the modulus was due to the presence of microcracks resulting from the mismatch in thermal expansion of the fiber and the matrix. The SEM micrographs reveal that inter- and intra-bundle cracks tend to grow through the weaker matrix pockets.

Converting Poly(ethylene terephthalate) Waste into Carbon Microspheres in a Supercritical CO2 System

Well-shaped carbon spheres in micrometer dimensions were prepared by pyrolyzing postconsumer poly(ethylene terephthalate) (PET) in supercritical carbon dioxide at 500-650 °C for 3 h. It was also found that the yield of carbon microspheres increased as the reaction temperature increased and the reaction time was prolonged. Carbon microspheres were obtained in 47.5% yield as the reaction occurred at 650 °C for 9 h. A high-pressure carbonization mechanism of aromatic hydrocarbons decomposed from PET waste was proposed according to gas chromatography combined with mass spectrometry analysis. One possible application of the carbon microspheres as a negative electrode material for lithium ion batteries was evaluated.

Ablative Properties of 4D Carbon/Carbon Composites by Combustion Test

The factors that influence ablation resistance in fiber composites are properties of the reinforced fiber and matrix, plugging quantity of fiber, geometrical arrangement, crack, pore size, and their distributions. To examine ablation resistance according to distribution of crack and pore size that exist in carbon/carbon composites, this study produced various sizes of unit cells of preforms. They were densified using high pressure impregnation and carbonization process. Reinforced fiber is PAN based carbon fiber and composites were heat-treated up to <TEX>$2800^{\circ}C$</TEX>. The finally acquired density of carbon/carbon composites reached more than <TEX>$1.932\;g/cm^3$</TEX>. The ablation test was performed by a solid propellant rocket engine. The erosion rate of samples is below 0.0286 mm/s. In conclusion, in terms of ablation properties, the higher degree of graphitization is, the more fibers that are arranged vertically to the direction of combustion flame are, and the less interface between reinforced fiber bundle and matrix is, the better ablation resistance is shown.

Flexural Behaviors of 4D Carbon/carbon Composites with the Preform Architectures

Multidirectional reinforcement is aimed primarily at overcoming interlaminar weakness, hence a major interest lies in the mechanical properties of multidirectional carbon/carbon composites. Mechanical properties depend on the type of carbon fiber, the size of the fiber bundle, the spacing of the bundles, the angles of the bundles relative to the axes of the block, and matrix formation. In the present studies, PAN based carbon fiber preforms manufactured different size of unit cell have been prepared. Densification of these used high pressure infiltration and carbonization technique with coal tar pitch as matrix precursor was carried out. Scanning electron microscopy has been used to study the fracture behavior of composites. The size of unit cell of the preforms has considerably affected on the flexural properties as well as microstructure of the carbon/carbon composites.

Effects of pressure on the pore formation of carbon/carbon composites during carbonization

In this study, porosity and graphitizability of coal tar pitch with the treatment pressure were investigated. 4-directional carbon/carbon composites (4D C/C) were made from the matrix precursor of coal tar pitch through the process of impregnation and carbonization. Then the effects of applied pressure during the densification on the composites were observed. The matrix pitch which had 600 bar applied during the carbonization process had one and a half times less pore area ratio than that treated at 1 bar. When the pitch was heat treated up to 2300°C after the high pressure carbonization, the degree of graphitization was improved on a small scale and the crystal size tended to reduce. As the applied pressures to 4D C/C composites increased from 1 to 600 bar, the densification ratio was greatly improved. In the pore size distribution of the 4D C/C composites, the macropore portion was decreased while the mesopore portion increased, when high pressures were applied.

Effect of shock heating on the strength and structure of coke produced by high-pressure carbonization of low-rank bituminous coals

In the BGL slagging gasifier, graded coal is gasified in a fixed reactor. A study has been made of the carbonization of large particles (∼25–30 mm) of three high-volatile UK bituminous coals at pressures in the range 0.5–8 MPa in an autoclave using a shock beating technique. The cokes were characterized in detail and compared with the corresponding high-pressure cokes produced under slow heating conditions. The coke yields increased significantly with increase in carbonization pressure. On the other hand, cokes produced under shock heating conditions resulted in a lower yield than the corresponding cokes prepared under slow heating conditions. These shock-heated cokes gave higher relative apparent densities and exhibited greater strength, such as higher abrasion resistance.

Effect of high pressure carbonization on the strength and structure of cokes from low rank bituminous coals

Commercial gasification processes are usually operated at high pressure for economic considerations. However, there is very little information available on the influence of pressure on the carbonization of coal under conditions relevant to high pressure fixed bed gasifiers. This investigation involved high pressure carbonization studies (0.5–8 MPa) simulating the conditions found in the British Gas-Lurgi gasifier. Two low rank bituminous coals typical of likely gasifier feedstocks were used in the study. The physical properties of the resultant cokes, such as porosity, tensile strength and abrasion resistance, were measured and several interrelationships between the coke structural parameters, mechanical properties and experimental conditions have been established. In general, for the two low rank coals studied the main effect of pressure is to initially enhance macropore development during carbonization at slow heating rates resulting in highly macroporous cokes, which consequently have relatively low tensile strength and low resistance to abrasive forces compared with metallurgical cokes. The results are discussed in terms of the relationships between coal thermoplasticity, coke structural characteristics and mechanical properties.

The release of nitrogen oxides from chars derived from high-pressure carbonization of carbazole and naphthol during oxidative gasification

The release of nitrogen species from model carbons derived from high pressure co-carbonization of carbazole and naphthol in argon was investigated using a thermogravimetric analyser coupled to a mass spectrometer. The NO CO 2 and NO CO ratios increased substantially with increasing burnoff over the range of gasification temperatures 873–1273 K. The CO CO 2 ratio for the gasification of the carbon of HTT 1273 K varied little with burnoff; it did not vary with gasification temperature up to 1023 K but decreased thereafter. The amount of HCN detected was small. Comparison with the results for carbons derived from acridine and naphthol shows that 1. (1) the carbazole-naphthol carbon was more reactive at a given temperature in the chemical-control region, 2. (2) the CO CO 2 ratio was usually higher for the carbazole-naphthol carbon, and 3. (3) the NO CO 2 ratio increased with increasing burnoff for both carbons. The results suggest that during gasification of the nitrogen in the char the concentrations of nitrogen-containing species relative to the carbon species vary with reaction temperature, heat-treatment temperature and burnoff. The results also clearly suggest that the functionality in the precursor material is important in determining char reactivity and nitrogen release.

The influence of matrix microstructure on the mechanical properties of CFRC composites

A suite of carbon fibre reinforced carbon (CFRC) composites and carbon matrices with a range of matrix microstructures have been fabricated using a multiple liquid vacuum impregnation process and high-pressure carbonization. The carbon matrices were derived from a range of petroleum and coal tar pitches in addition to pitch/phenolic resin mixtures. The efficiency with which the pitches densified their respective CFRC composites was dependent initially on their carbon yield, but after repeated densification on their rheological properties. The densification efficiency of the pitch/phenolic resin matrix precursors was dependent solely on their carbon yields. The variation of phenolic resin/ pitch composition of the precursor for the matrix carbon had a profound influence on the structural and mechanical properties of the resultant carbon. The intrinsic mechanical strength (microstrength) of a matrix carbon cannot always be translated to its corresponding CFRC composite because of fibre matrix interfacial phenomena. Studies of the systematic variation of matrix microstructure have shown that in the absence of transverse shrinkage phenomena, maximum interlaminar shear strength (ILSS) corresponds to a matrix of mosaic optical texture. Maximum compressive strength corresponds to a matrix with an axial preferred anisotropic optical texture. Consequently, it is possible to fabricate CFRC composites with specific properties by modifying their matrix microstructures.

Thermoplastic behaviour of coal and its relationship to coke structure during high pressure carbonization

The thermoplastic properties of three coals of rank 301a to 602 in the British Coal Classification Scheme were investigated using a high pressure dilatometer and a high pressure constant shear rate plastometer. The results show that some measurements of thermoplastic behaviour, for example, dilatation and coking intensity, are very dependent on heating rate and pressure. In contrast, the plastometry caking intensity only varies slightly with heating rate and pressure. The optical anisotropy of the carbonized residues from the high pressure thermoplasticity experiments increased markedly with increasing heating rate. The relationships between dilatometry, plastometry and optical anisotropy under a range of experimental conditions are complex and indicate that the measurements are most relevant when they are carried out under simulated process conditions.

Pitch-based processing of carbon-carbon composites

A microstructural approach was used to study the process mechanisms limiting the efficiency of pitch-based composite fabrication, which should have advantages of low cost and high carbon yield with a graphitizable matrix. The microstructural framework is established by mesophase mechanisms active in the final stages of liquid phase pyrolysis. The formation of mesophase in fiber bundles and 3D preforms, for various fibers and pitches, and as affected by pressure, displayed common patterns of wetting, mesophase alignment on fiber substrates, and disclination structures. Mesophase bloating by pyrolysis gases can expel matrix from the preform, and the need was seen for a means to fix the mesophase in place before carbonization. An oxidation process, similar to that used in the stabilization of mesophase fiber, was found to be applicable in depth to mesophase-impregnated preforms because a network of access porosity in the matrix results from the mismatch of thermal expansivities of fiber and mesophase. The successful application of oxidation processing to 2D preforms, which are particularly vulnerable to bloating in the direction lacking reinforcement, showed that high-pressure carbonization is not required for pitch-based processing. With a method for matrix stabilization in hand, process development may turn to use of the flow capability of mesophase pitch to impregnate the fiber preform.

Catalytic graphitization of model compound chars by aluminum and beryllium

The use of metal additives for the catalytic graphitization of chars prepared by high temperature, high pressure carbonization of anthracene, phenanthrene or biphenyl has been investigated. In particular, the extent and type of catalysis initiated by aluminum or beryllium has been examined. Quantitative X-ray intensity measurements have been performed to show the existence of at least two concurrent mechanisms of graphitization catalysis. Evidence for the preferential attack on the disorganized carbon by the graphitization catalyst has been found.