Organic and carbon aerogels derived from poly(vinyl chloride)

Organic aerogels were derived from dimethylformamide solution of poly(vinyl chloride) (PVC) via dehydrochlorination using a strong base, 1,8-diazabicyclo[5,4,0]undec-7-ene, and supercritical drying using carbon dioxide. From these organic aerogels, carbon aerogels were yielded via stabilization and carbonization. Changes in the porous structure of the aerogels during the preparation process and influences of the preparation conditions on the porous structure were investigated. The framework of the aerogels composed the walls of the meso- and macropores. The volume and the size of these pores were reduced during stabilization and carbonization due to the shrinkage of the framework caused by the release of decomposition gases and densification of the material. Simultaneously, the release of decomposition gases produced additional micropores. The extent of dehydrochlorination, the concentration of PVC in the starting solution and the molecular weight of PVC were the factors with which the porous structure of the aerogels could be controlled over a wide range. In addition, the stabilization conditions notably influenced the carbonization behavior of the organic aerogels and the porous structure of the carbon aerogels. The optimum stabilization conditions that minimized the loss of mass and maximized the pore volume of the carbon aerogels were determined.     

Morphology of heat-treated tunsgten doped monolithic carbon aerogels

The morphology of a tungsten-doped monolithic organic aerogel, prepared by the sol-gel method from the polymerisation of a resorcinol, formaldehyde and ammonium tungstate mixture, and of its carbonized derivatives at 500 and 1000 °C was studied by scanning and high-resolution electron microscopy. Tungsten influenced the surface morphology of the carbon aerogels. The tungsten-containing phase was homogeneously distributed in the organic aerogel and its heat treatment produced changes in the metal phase distribution throughout the pellets. Tungsten oxide particles of needle-like structure similar to hollow tubules were detected after heat treatments at different temperatures. In addition, particles of dendritic appearance, formed by tungsten carbide and an intermediate Magnelli phase, appeared when the heat treatment was carried out at 1000 °C.     

Preparation of mesoporous carbon by freeze drying

Resorcinol–formaldehyde (RF) cryogels were synthesized by sol-gel polycondensation of resorcinol with formaldehyde and freeze drying with t-butanol. The cryogels were characterized by nitrogen adsorption and density measurements. Their porous properties were compared with those of RF aerogels prepared by supercritical drying with carbon dioxide. RF cryogels were mesoporous materials with large mesopore volumes >0.58 cm³/g. Although surface areas and mesopore volumes of the cryogels were smaller than those of the aerogels, the cryogels were useful precursors of mesoporous carbons. Carbon cryogels were obtained by pyrolyzing RF cryogels in an inert atmosphere. Carbon cryogels were mesoporous materials with high surface areas >800 m²/g and large mesopore volumes >0.55 cm³/g. When pyrolyzed, micropores are formed inside the cryogels more easily than inside the aerogels.     

Interaction of hydrogen with carbon coils at low temperature

Hydrogen was adsorbed on carbon coils under 10 MPa hydrogen gas at liquid nitrogen temperature. The equilibrium pressure of hydrogen desorbed from the as-grown carbon coils with an amorphous structure was three to four times greater than those of multiwall carbon nanotubes (MWNT) and active carbons (AC). The heat treatment of the carbon coils at 850 °C contributed to an increase in hydrogen adsorption by 20% compared to the as-grown carbon coils. On the other hand, the adsorption of hydrogen gas on the carbon coils decreased significantly after heat treatment at a temperature higher than 1000 °C due to formation of capsule-like carbon composed of 10-20 layers on the surface of the carbon coils.     

Preparation and pore control of highly mesoporous carbon from defluorinated PTFE

Porous carbon having more than 2000 m²/g of BET specific surface area was synthesized by defluorination of polytetrafluoroethylene (PTFE) at 473 K using sodium metal. The porous carbon as-prepared had a large amount of narrow mesopores 2-3 nm in pore width, together with micropores. Control of the pore structure was attempted by simple heat-treatment of the carbon in nitrogen, and change of the porous structures was characterized by nitrogen adsorption techniques. As a result, it was found that the ratio between micro- and mesopores was easily varied. Electric double layer capacitance was measured as one of the applications for the mesoporous carbon with specific porosity, and the effect of pore control on capacitance was investigated.     

High purity multiwalled carbon nanotubes under high pressure and high temperature

We have studied the structural modifications that multi-walled carbon nanotubes (MWNTs) undergo when treated under high pressure and high temperature (HP-HT). Uniaxial pressures of 5.5-6.5 GPa were applied to samples of purified MWNTs between 850 and 950 °C for 30 min. The resulting material was then analysed by X-ray diffractometry, Raman spectroscopy and transmission electron microscopy. Comparison of the data obtained by such analyses before and after treatment does show changes in the Raman spectra, a slight linear decrease of the d₀₀₂ spacing, and some structural modifications that could be due to MWNT segmentation and interlinking.     

Structural and surface property changes of macadamia nut-shell char upon activation and high temperature treatment

Structural and surface property changes of macadamia nut-shell (MNS) char upon activation and high temperature treatment (HTT) were studied by high-resolution nitrogen adsorption, diffuse reflectance infra-red Fourier transform spectroscopy, X-ray photoelectron spectroscopy, and temperature-programmed desorption. It is found that activation of MNS char can be divided into the low extent activation which may involve the reactions of internal oxygen-containing groups and leads to the formation of comparatively uniform micropores, and the high extent activation which induces reactions between carbon and activating gas and produces a large amount of micropores. The surface functional groups (SFGs) basically increase with the increase of activation extent, but high extent activation preferentially increases the amount of -C-O and -CO. HTT in air for a short time at a high temperature (1173 K) greatly increases the micropore volume and the amounts of SFGs. By appropriately choosing the activation and HTT conditions, it is possible to control both the textural structure and the type and amounts of SFG.     

A treatment method to give separated multi-walled carbon nanotubes with high purity, high crystallization and a large aspect ratio

CNT agglomerates, prepared by catalytic chemical vapor deposition in a nano-agglomerate fluidized-bed reactor are separated and dispersed. The effects of shearing, ball milling, and ultrasonic and chemical treatments on the dispersing of the carbon nanotubes were studied using SEM, TEM/HRTEM and a Malvern particle size analyser. The resulting microstructures of the agglomerates and the efficiency of the different dispersion methods are discussed. Representative results of annealed CNTs are highlighted. The as-prepared CNT product exists as loose multi-agglomerates, which can be separated by physical methods. Although a concentrated H₂SO₄/HNO₃ (v/v=3:1) treatment is efficient in severing entangled nanotubes to enable their dispersion as individuals, damage to the tube-wall layers is serious and unavoidable. A high temperature annealing (2000 °C, 5 h) before the acid treatment (140 °C, 0.5 h) is recommended and can give well separated nanotubes with a high aspect ratio and 99.9% purity. These highly dispersed CNTs contain few impurities and minimal defects in their tube-bodies and will be of use in further research and applications.     

Carbon aerogels derived from cresol–resorcinol–formaldehyde for supercapacitors

The objective of the present paper is to demonstrate the possibility to synthesize mixed carbon aerogels (denoted C_mRF) from cresol (C_m), resorcinol (R) and formaldehyde (F), as an alternative economic route to the classical RF synthesis. These porous carbon aerogels can be used as electrode materials for supercapacitors with a high volume-specific capacitance. Organic precursor gels were synthesized via polycondensation of a mixture of resorcinol and cresol with formaldehyde in an aqueous alkaline (NaOH) solution. After gelation and aging the solvent was removed via drying at ambient pressure to produce organic aerogels. Upon pyrolysis of the organic aerogels at 1173 K, monolithic carbon aerogels can be obtained. By controlling the catalyst (Cat) molar ratio (C_m+R/Cat) in the range 200–500, up to 70% of the resorcinol can be replaced with the cheap cresol. The resulting homogeneous organic aerogels exhibit a drying shrinkage below 15% (linear). The shrinkage and mass loss upon pyrolysis of the mixed aerogels increase with increasing cresol content. Nitrogen adsorption at 77 K was employed to characterize the microstructure of the carbon aerogels. The data show that the porous structure of mixed carbon aerogels is similar to that of RF carbon aerogels. Cyclic voltammetry measurements show that the as-prepared C_mRF carbon aerogels exhibit a high volume-specific capacitance of up to 77 F/cm³.     

Microporosity in carbon blacks

Microporous carbon blacks can be characterized by the same techniques as activated carbons, using the classical DR equation and comparison plots based on non-porous materials. The CO₂ adsorption isotherm at 273 K, combined with computer modelling, also leads to an assessment of microporosity. The results agree with independent techniques such as immersion calorimetry into liquids of variable molecular dimensions and a modified Dubinin equation. The study also confirms that the comparison plots based on N₂ (77 K), CO₂ (273 K) and C₆H₆ (293 K) do not necessarily lead to overlapping results for the total micropore volume and the external surface area of the carbons.     

Porosity and surface area of monolithic carbon aerogels prepared using alkaline carbonates and organic acids as polymerization catalysts

Carbon aerogels were prepared by polymerization of a resorcinol-formaldehyde mixture using different polymerization catalysts such as: sodium or potassium carbonates, oxalic acid or para-toluenesulfonic acid. The carbon aerogel obtained with this last acid was further CO₂-activated to 8.5% and 22% burn-off. All samples were characterized by N₂ and CO₂ adsorption at −196 and 0 °C, respectively, and by mercury porosimetry, scanning electron microscopy, and thermogravimetric analysis. Samples prepared using Na₂CO₃ were denser than those prepared using K₂CO₃. In addition, the density of samples prepared under acidic conditions was greater than that of samples prepared using alkaline carbonates as catalysts. Most of the carbon aerogels prepared were mesoporous with narrow pore size distributions. Results obtained showed that the nature of the acid used in the preparation of these aerogels only affected the gelation process. Finally, it is noteworthy that CO₂ activation of the carbon aerogel prepared with para-toluenesulfonic acid as catalyst only increased and widened the microporosity and had virtually no effect on the mesoporosity.     

Novel carbon molecular sieve honeycomb membrane module: configuration and membrane characterization

A new production concept of carbon molecular sieve membranes modules is described. The supported membrane synthesis process allows for industrial production with uniform and reproducible properties, using flat membranes as a starting precursor. The corresponding membrane modules are made in a honeycomb configuration, leading to high packing density and thermochemical stability. Some of the carbon membranes produced for use in these modules are characterized. Species H₂, O₂, N₂, CO₂ and SF₆ were used in monocomponent adsorption and permeation experiments to obtain adsorption equilibrium and permeation data. High permeabilities were attained in combination with high selectivities. The pore size distribution of the membrane was determined. A number of complementary morphological and structural characterizations were also performed.     

Raman spectroscopy and nitrogen vapour adsorption for the study of structural changes during purification of single-wall carbon nanotubes

Raman spectroscopy and nitrogen adsorption measurements were combined to study the surface features of semi-conducting and metallic single-wall nanotubes (SWNTs). The nanotubes were treated chemically and with heat under moderate conditions that more than doubled the mesopore volume of the tested samples, which consistently led to a significant rise in the total surface area of up to 1550 m²/g. The large increase in the number of micropores of less than 1 nm in diameter was associated with the loosening of nanotube bundles as well as the creation of structural flaws on the surface of individual SWNTs due to chemical treatment. Micropores in the 1.0-1.8 nm range were associated with the holes created on the surface of individual tubes. Heating at 1000 °C was shown to restore nanotube diameter to their initial pre-chemical treatment levels with the change in the chirality of SWNTs and diminish the porosity by closing small holes. It was assumed that the intermediate frequency range (500-1100 cm⁻¹) was associated with the degree of imperfection of HiPco SWNTs crystalline structures, and therefore provided information about the degree of tube surface damage due to the presence of functional groups. A hypothesis explaining the transformation of SWNT porous structure during heat treatment is proposed.     

Preparation of low-density carbon aerogels by ambient pressure drying

With the addition of hexamethylenetetramine (HMTA) and alcohol as solvent, an ambient pressure drying technique was developed for the fabrication of low-density organic aerogels and related carbon aerogels. When a suitable ratio of resorcinol to HMTA (R/H ratio) and ratio of resorcinol to solvents (R/S ratio) are selected, the low-density alco-gels obtained can be dried under ambient pressure conditions without observable shrinkage. The addition of HMTA increases the size of carbon nano-particles and the pore size of the aerogels that are produced. The carbon aerogels prepared in this work have similar nano-particle structures typical of the aerogels prepared with CO₂ or by the isopropanol supercritical drying technique.     

Characterization of activated carbon fibers using argon adsorption

In this paper, we present results of the internal structure (pore size and pore wall thickness distributions) of a series of activated carbon fibers with different degrees of burn-off, determined from interpretation of argon adsorption data at 87 K using infinite and finite wall thickness models. The latter approach has recently been developed in our laboratory. The results show that while the low bun-off samples have nearly uniform pore size (<0.6 nm), the pore size distribution of the high burn-off samples becomes broader, with a significant increase in proportion of larger pores. The results of pore wall thickness distribution are generally consistent with development of porosity with increasing degree of burn-off. Further they show good correspondence with X-ray diffraction.     

Preparation and characterization of antibacterial silver-dispersed activated carbon aerogels

Silver-dispersed carbon aerogels (CAs) were obtained by direct immersion of organic aerogels prepared by ambient pressure drying technique in AgNO₃ aqueous solution and then carbonization. The effect of preparation conditions such as the resorcinol/catalyst ratio, the feed AgNO₃ concentration, the ratio of aerogel mass/solution volume, immersion time and carbonization temperature on the bulk density and silver content as well as the BET surface area of the dispersed CAs was studied. The dispersion and structure of silver nanoparticles in obtained materials were investigated by means of scanning electron microscopy, transmission electron microscopy and X-ray diffraction. The Ag-dispersed CAs prepared exhibit strong and long-term antibacterial activity.     

Structure of coke powder heat-treated with boron

Coal tar pitch-based coke powder with a fine mosaic texture was heat-treated with various concentrations of boron powder at 2900°C. Increasing the boron amount led to smaller d₀₀₂ and larger d₁₁₀, and made the original fine texture coarser. Some small particles showed specific structures of polyhedrons, of which surfaces are 002 planes of graphite lattice, after heat treatment with boron. The size of the polyhedron increased with boron content. Boron concentration was lower at the surface than at the inner portions of particles for a powder heat-treated with a higher amount of boron, while it depended less on the depth for that heat-treated with a lower amount of boron. The formation mechanism of the polyhedron particle is discussed.