Synthesis of nanoporous carbide-derived carbon by chlorination of titanium silicon carbide

Synthesis of nanoporous carbide-derived carbon, CDC, by extraction of titanium and silicon from Ti₃SiC₂ by chlorine is discussed in this work. Thermodynamic simulations using a Gibbs free energy minimization program provided general guidelines to the experimental design. Raman spectroscopy, X-ray diffraction, and electron microscopy studies showed that the structure of CDC depends on the chlorination temperature. The low temperature synthesis resulted in an amorphous CDC structure. Noticeable graphite formation starts above 800 °C and well ordered graphite ribbons of 1-3 nm in thickness form at 1200 °C. The macroscopic volume and shape of Ti₃SiC₂ preform were preserved during the transformation. However, the chlorination resulted in the formation of cracks between the former grains of the polycrystalline Ti₃SiC₂ preform. These cracks are believed to be caused by a contraction in the direction perpendicular to the basal planes of Ti₃SiC₂. The synthesized nanoporous carbon demonstrated excellent sorption properties. Energy dispersive X-ray spectroscopy studies showed that Ti₃SiC₂ material chlorinated at 400 °C is capable of trapping over 40 wt.% of Cl₂.     

Synthesis and characterisation of nanoporous carbide-derived carbon by chlorination of vanadium carbide

Nanoporous carbide-derived carbon (CDC) was synthesised from vanadium carbide (VC) powder via gas phase chlorination in the temperature range from 500 to 1100 °C. The XRD analysis of nanoporous carbon powder samples was carried out to investigate the structural changes (graphitisation) of nanoporous carbons synthesised. The first-order Raman spectra showed the absorption peak at ∼1582 cm⁻¹ and the disorder-induced (D) peak at ∼1345 cm⁻¹. The low-temperature N₂ adsorption experiments were performed and a specific surface area up to 1305 m² g⁻¹ and total pore volume up to 0.66 cm³ g⁻¹ were obtained.     

Nanostructured carbide-derived carbon synthesized by chlorination of tungsten carbide

Nanostructured carbide-derived carbons were synthesized from α-tungsten carbide (WC-CDC) powder via gas phase chlorination within the temperature range from 700 to 1100 °C. Analysis of X-ray diffraction results showed that WC-CDC are mainly amorphous consisting of relatively small graphitic crystallites and the apparent crystallite size along the a- and c-directions of graphite structure L_a ≈ 4 nm and L_c ≈ 1.5 nm were calculated. The first-order Raman spectra showed the graphite-like absorption peak at ∼1590 cm⁻¹ and the disorder-induced peak at ∼1350 cm⁻¹. The low-temperature N₂ sorption experiments were performed and a specific micropore surface area up to 1550 m² g⁻¹ and total pore volume up to 0.89 cm³ g⁻¹ were obtained for WC-CDC synthesized at T = 1100 °C. High-resolution transmission electron microscopy and electron energy loss spectroscopy studies revealed that WC-CDC prepared at 800 °C correspond to the highly disordered carbon material but WC-CDC prepared at 1100 °C showed partial graphitization.     

Small-angle neutron scattering characterization of the structure of nanoporous carbons for energy-related applications

We used small-angle neutron scattering (SANS) and neutron contrast variation to study the structure of four nanoporous carbons prepared by thermo-chemical etching of titanium carbide TiC in chlorine at 300, 400, 600, and 800 °C with pore diameters ranging between ∼4 and ∼11 Å. SANS patterns were obtained from dry samples and samples saturated with deuterium oxide (D₂O) in order to delineate origin of the power law scattering in the low Q domain as well as to evaluate pore accessibility for D₂O molecules. SANS cross section of all samples was fitted to Debye-Anderson-Brumberger (DAB), DAB-Kirste-Porod models as well as to the Guinier and modified Guinier formulae for cylindrical objects, which allowed for evaluating the radii of gyration as well as the radii and lengths of the pores under cylindrical shape approximation. SANS data from D₂O-saturated samples indicate that strong upturn in the low Q limit usually observed in the scattering patterns from microporous carbon powders is due to the scattering from outer surface of the powder particles. Micropores are only partially filled with D₂O molecules due to geometrical constraints and or partial hydrophobicity of the carbon matrix. Structural parameters of the dry carbons obtained using SANS are compared with the results of the gas sorption measurements and the values agree for carbide-derived carbons (CDCs) obtained at high chlorination temperatures (>600 °C). For lower chlorination temperatures, pore radii obtained from gas sorption overestimate the actual pore size as calculated from SANS for two reasons: inaccessible small pores are present and the model-dependent fitting based on density functional theory models assumes non-spherical pores, whereas SANS clearly indicates that the pore shape in microporous CDC obtained at low chlorination temperatures is nearly spherical.     

Comparative study of carbide-derived carbons obtained from biomorphic TiC and SiC structures

Biomorphic carbide ceramics, TiC and SiC, derived from paper performs by chemical vapor infiltration were converted into high porous carbon by carbide-derived carbon (CDC) approach using selective etching in chlorine or hydrogen/chlorine gas mixture in a temperature range of 400-1200 °C. A comparative study of both carbide precursors was performed regarding reaction kinetics, influence of hydrogen as well as microstructure of the resulting carbon. SiC showed lower reactivity than TiC. Temperatures below 650 °C are not sufficient to remove Si from SiC. Addition of hydrogen to the reactive gas inhibits the chlorination reaction. A linear decrease of etching rate with increasing hydrogen/chlorine ratio was observed for both carbide precursors. A critical ratio, where no etching takes place, was estimated to be 0.72 for TiC-CDC and 0.66 for SiC-CDC. The etching rate of TiC is independent from the temperature. In the case of SiC activation energy of the chlorination reaction of about 50 kJ/mol was estimated in the temperature range 650-800 °C. The structural ordering of CDC with increasing synthesis temperature affects also its oxidation resistance as shown by thermo gravimetric analysis.     

Synthesis and characterization of boron carbide films by plasma-enhanced chemical vapor deposition

The plasma-enhanced chemical vapor deposition of boron carbide was investigated on quartz glass and alumina substrates from a gas mixture of BCl 3 , H 2 , and CH 4 in an inductively coupled plasma (ICP) medium produced by a radio frequency (RF) discharged onto the gases passing through a tubular reactor under atmospheric pressure. A thin solid boron carbide coating with a gray color was deposited on both substrates. The results of XRD revealed that the major solid phase formed in the coating material was β-rhombohedral B 4 C. The SEM analysis showed that the surface homogeneity increased with an increase in the exposure time, and different boron carbide structures were formed at different RF powers and exposure times.     

Mechanism of formation and electrochemical performance of carbide-derived carbons obtained from different carbides

Porous carbide-derived carbons (CDCs) are synthesized from different carbide (VC, TiC, NbC) as electrode materials for electrochemical capacitors. The process of carbide–carbon transformation is investigated by observations at different carbide/CDC interfaces. It is found that the restructuring process has much influence on formation of microstructure as well as the resultant electrochemical performance. The carbon structure in the produced CDC is well in accordance with that formed at the carbide/CDC interface, indicating that the microstructure in the produced CDC is decided by re-bonding of the residual carbon atoms. It is further found that the internal stress during carbide–carbon transformation has much influence on the CDC microstructure. In addition, the microstructure in CDCs is dependent on the volumetric concentration of carbon atoms in carbide precursor. Lower volumetric concentration of carbon atoms facilitates the formation of CDC with short and curved graphene structure, which owns easily accessible pores and large specific surface area, and thus high electrochemical performance for ultracapacitor. A novel strategy that controlling microstructure of CDC through controlling the volumetric concentration of carbon atoms in carbide precursor is presented. This strategy is very effective to form designed microstructure of CDC for electrochemical applications.     

Fast production of monolithic carbide-derived carbons with secondary porosity produced by chlorination of carbides containing a free metal phase

Hierarchical structured carbide-derived carbons (CDC) are produced by high temperature chlorination of silicon carbides containing free silicon (Si/SiC). The influence of free silicon in the precursor carbide on the resulting pore and carbon structure and production rate is studied. The two phases – free silicon and silicon carbide – of Si/SiC gives the possibility to synthesize a monolithic carbon with the typical microporous character and narrow pore size distribution combined with larger voids in the micrometer range, while the carbon structure itself stays unchanged. The study revealed that using Si/SiC material increases the production rate for carbide-derived carbons dramatically, due to higher reactive surface area and lower mass transfer limitations, which allows for the time effective production of larger monoliths.     

Synthesis of boron carbide by calciothermic reduction process

Boron carbide (B₄C) powders were synthesized by calciothermic reduction process from the mixtures of borax (or boron trioxide), petroleum coke, and calcium. The synthesized materials were characterized by XRD, SEM, and chemical analysis. The article is published in the original.     

碳化物衍生碳的制备及其应用研究进展

碳化物衍生碳（CDC）是除去碳化物中非碳元素后得到的产物,本文综述了卤素刻蚀法、超临界水热法、酸浸泡法、碳化钙反应法、高温热解法、高温熔盐电化学刻蚀法六种制备方法,其中,氯气刻蚀法效率最高。文中指出：CDC因具有轻质、高孔隙率、孔径可调、高比表面积、碳形态多样、生物相容性好等优势,可用于电化学储能（如超级电容器、锂离子电池、燃料电池）、吸附、生物医药和摩擦学领域中;而影响CDC孔结构的因素有很多,如前体类型、反应温度、反应气氛、反应时间、刻蚀方式以及活化方式。通过选择不同的合成参数,可以制备出满足不同应用场景需求的CDC材料。本文还展望了CDC在未来实现商业化的可能性及需要满足的五点要求。     

Fabrication and characterization of hot-pressed B-rich boron carbide

Boron carbide has a wide solubility range owing to the substitution of B and C atoms in the crystal. In this study, boron carbides with different stoichiometric ratios were prepared using a hot-pressing sintering method, and the influences of the B/C atomic ratio on the microstructures and properties were explored in detail. X-ray diffraction analysis showed that excessive B atoms caused lattice expansion. Raman spectroscopy analysis showed disordered substitution of B atoms in the chains and icosahedra. Analysis of the densification process and microstructure evolution revealed that the addition of B promoted densification, and more stacking faults and twins occurred in B-rich boron carbide, and result in the densification mechanism gradually changes from atomic diffusion mechanism driven by thermal energy to plastic deformation mechanism dominated by the proliferation of dislocation and substructures. The introduction of chemical composition changes by dissolving excessive B into boron carbide further affected the microstructure and consequently the mechanical properties. The Vickers hardness, modulus, and sound velocity all decreased with the increase in B content. Moreover, the fracture toughness improved with increased B content. The flexural strength of the samples was optimised at the B/C stoichiometric ratio of 6.1.     

Mechanical characterization of boron carbide single crystals

Out-of-plane anisotropy in the mechanical response of boron carbide was studied by performing nanoindentation experiments on four specific crystallographic orientations of single crystals, that is, , , , and . For each orientation of the single crystals, in-plane variations of indentation modulus and hardness were also studied by monitoring the relative rotation between the crystal surface and a Berkovich indenter tip. A significant out-of-plane anisotropy in indentation modulus was observed with ~80 GPa difference between the highest and lowest values. A smaller but measurable out-of-plane anisotropy in indentation hardness was also observed. In-plane anisotropy, on the other hand, was found to be significantly influenced by the scatter in the data and geometrical imperfections of the indenter tip. Investigations of indentation pop-in events suggested that deformation is entirely elastic prior to the first pop-in. Furthermore, quasi-plastic flow along the orientation of the single crystals was found to be more homogeneous than the other tested orientations. For select indents, cross-sectional transmission electron microscopy (TEM) of the indented regions showed formation of a quasi-plastic zone in the form of lattice rotation and various microstructural defects. The quasi-plastic zone grew in size with increasing the indentation depth. The TEM observations also suggested the crystal slip to be a potential mechanism of quasi-plasticity and a precursor for formation of amorphous bands that could eventually lead to cracking and fragmentation. The proposed failure mechanism provides valuable insights for calibrating constitutive computational models of failure in boron carbide.     

Toughness control of boron carbide obtained by spark plasma sintering in nitrogen atmosphere

Boron carbide ceramic was prepared by reactive Spark Plasma Sintering under N₂-atmosphere and for different heating times and maximum pressure regimes. Split-Hopkinson Pressure Bar (SHPB), indentation, XRD and microscopy measurements were performed for samples characterization. It is shown that SHPB toughness control depending on SPS regime is possible and the main reason is introduction of nitrogen into B₄C ceramic. Complex relationships between processing conditions, sintering mechanism, material's specifics, static and dynamic mechanical properties are discussed. Improvement of dynamic toughness is through mechanisms resembling those working for static load conditions such as cracks deflection and pull out, but there are also significant differences.     

Densification and characterization of rapid carbothermal synthesized boron carbide

Submicrometer boron carbide powders were synthesized using rapid carbothermal reduction (RCR) method. Synthesized boron carbide powders had smaller particle size, lower free carbon, and high density of twins compared to commercial samples. Powders were sintered using spark plasma sintering at different temperatures and dwell times to compare sintering behavior. Synthesized boron carbide powders reached >99% TD at lower temperature and shorter dwell times compared to commercial powders. Improved microhardness observed in the densified RCR samples was likely caused by the combination of higher purity, better stoichiometry control, finer grain size, and a higher density of twin boundaries.     

Preparation and characterization of pillared carbons obtained by pyrolysis of silylated graphite oxides

Pillared carbons were prepared by pyrolyzing various graphite oxides silylated by 3-aminopropylmethyldiethoxysilane. They were formed when silylated graphite oxides with silicon contents of 12.6% or higher were pyrolyzed in vacuo at 500-600 °C. Their interlayer spacings were 1.23-1.31 nm. When silylated graphite oxide was prepared at 90 °C, the reductive decomposition of graphite oxide by amino groups of 3-aminopropylmethyldiethoxysilane was suppressed and pillared carbon with higher crystallinity was obtained. At higher temperatures of pyrolysis, silylated graphite oxide decomposed to residual carbon without pillars. The pillars between the carbon layers contained methyl groups originating from the 3-aminopropylmethyldiethoxysilane. Based on the interlayer spacing and elemental analysis data, a structure model for the pillar is proposed. Pillared carbons showed type IV nitrogen adsorption isotherms and they contained both mesopores and a small volume of micropores. The BET surface area of the pillared carbon reached a maximum value of 236 m²/g, when it was prepared from graphite oxide silylated at 105 °C for 20 days.     

Spherical carbons: Synthesis,characterization and activation processes

Spherical carbons have been prepared through hydrothermal treatment of three carbohydrates (glucose, saccharose and cellulose). Preparation variables such as treatment time, treatment temperature and concentration of carbohydrate have been analyzed to obtain spherical carbons. These spherical carbons can be prepared with particle sizes larger than 10 μm, especially from saccharose, and have subsequently been activated using different activation processes (H₃PO₄, NaOH, KOH or physical activation with CO₂) to develop their textural properties. All these spherical carbons maintained their spherical morphology after the activation process, except when KOH/carbon ratios higher than 4/1 were used, which caused partial destruction of the spheres. The spherical activated carbons develop interesting textural properties with the four activating agents employed, reaching surface areas up to 3100 m²/g. Comparison of spherical activated carbons obtained with the different activating agents, taking into account the yields obtained after the activation process, shows that phosphoric acid activation produces spherical activated carbons with higher developed surface areas. Also, the spherical activated carbons present different oxygen groups’ content depending on the activating agent employed (higher surface oxygen groups content for chemical activation than for physical activation).     

Sulfur-doped porous carbons: Synthesis and applications

Heteroatom doping of carbon materials may become the “Next Big Thing” in materials science further enhancing research concerning carbon nanostructures. In particular, the S-doped porous carbons have gained a great deal of attention in the last few years. They are already proven to be versatile functional materials with a wide range of potential applications, including heterogeneous catalysis, sorption, as well as in the areas of energy conversion and storage. To date, a few approaches have been developed to intrinsically blend sulfur into the carbon matrix. Yet there is still a need to design new porous structures with controllable porosity and well defined chemical status of sulfur doped into the carbon matrix. In this review, we summarize recent reports on the preparation of S-doped carbons, with special emphasis on porous carbons with intrinsically doped sulfur. The effect of S-doping on the properties determining applications is delineated. Special attention is paid to differentiate between elemental sulfur impregnation, intercalation, surface functionalization and S bulk doping of porous carbons. To this end, synthesis and applications of S-impregnated, S-functionalized and S-intercalated carbons are shortly discussed before the intrinsically S-doped carbons are presented in detail. The importance of the sulfide –C–S–C– system for the properties of S-doped carbon is stressed. At the very end, Se-doped carbons are shortly presented as a promising next generation of chalcogen-doped carbon.     

Effective control of the microstructure of carbide-derived carbon by ball-milling the carbide precursor

A microstructure control strategy for carbide-derived carbon (CDC) by ball-milling the metal carbide precursor prior to CDC synthesis is investigated. This work explores the effect of chlorination temperature and ball-milling time on the microstructure, specific surface area (SSA) and the pore size distribution. It is found that the degree of order of CDC obtained from the milled titanium carbide (TiC) is obviously high and can be well tuned by controlling the ball-milling time at a lower chlorination temperature (400–800 °C). As the chlorination temperature rises to 1000 °C, an obvious decrease in the degree of order is observed and many cubic diamond-like carbon nanoparticles with larger d-spacing are formed. In addition, the produced CDC has a high SSA with both micro- and meso-pores. The effect of ball-milling TiC precursor on the microstructure of CDC can be attributed to the iron (Fe) in the TiC from the milling balls and jar to a great extent. The Fe promotes the formation of the better-organised carbon at lower chlorination temperature and the formation of the nano-diamond at higher temperature.     

EDLC performance of carbide-derived carbons in aprotic and acidic electrolytes

This study shows that carbide-derived carbons (CDCs) with average pore size distributions around 0.9-1 nm and effective surface areas of 1300-1400 m² g⁻¹ provide electrochemical double-layer capacitors with high performances in both aqueous (2M H₂SO₄) and aprotic (1M (C₂H₅)₄NBF₄ in acetonitrile) electrolytes.In the acidic electrolytic solution, the gravimetric capacitance at low current density (1 mA cm⁻²) can exceed 200 F g⁻¹, whereas the volumetric capacitance reaches 90 F cm⁻³. In the aprotic electrolyte they reach 150 F g⁻¹ and 60 F cm⁻³.A detailed comparison of the capacitive behaviour of CDCs at high current density (up to 100 mA cm⁻²) with other microporous and mesoporous carbons indicates better rate capabilities for the present materials in both electrolytes. This is due to the high surface area, the accessible porosity and the relatively low oxygen content.It also appears that the surface-related capacitances of the present CDCs in the aprotic electrolyte are in line with other carbons and show no anomalous behaviour.