Microstructure Of Carbon Aerogels Research Articles

The effect of pulverization methods on the microstructure of stiff, ductile, and flexible carbon aerogels

Carbon aerogels find application in many fields. In most of the applications, they are used as powders and thus need to be pulverized. However, the pulverization could induce various changes in the microstructure of carbon aerogels. The extent of changes depends not only on the dominant forces of used technique, but also on the mechanical and structural properties of initial monolithic samples. In the present work, we discuss the influence of grinding, milling in shaker cryo-mill, and planetary ball mill on stiff, ductile and flexible carbon aerogels. Scanning electron microscopy and transmission electron microscopy images, gas sorption techniques, wide-angle X-ray scattering, and Raman spectroscopy show a strong dependency of the introduced energy amount while pulverization on the structure modification. Results show that stiff carbon aerogels do not undergo noticeable changes. In contrast, ductile carbon aerogels are very sensitive to friction forces. Soft and flexible carbon aerogels undergo drastic changes in the microstructure.

Starch-derived carbon aerogels with high-performance for sorption of cationic dyes

Environmentally green carbon aerogels have been prepared as adsorbents for dye-containing wastewater. The aerogels were prepared by carbonization of starch aerogels synthesized from soluble starch through a sol–gel process followed by drying at ambient pressure. The Brunauer-Emmett-Teller (BET) surface areas and pore size distribution were measured by N2 adsorption/desorption, and the surface zeta-potential and microstructure of carbon aerogels were characterized using a scanning electron microscope (SEM) and zeta-potential analyzer. SEM images indicate that the carbon aerogels consist of flakes with side length of 60–120 μm and thickness of 3–4 μm. The flakes are irregular in shape and composed of spherical carbon nanoparticles of 10–30 nm. The carbon aerogels have both microporous and mesoporous structures and exhibit high specific surface areas, the highest value is 1571 m2/g. The mean diameter of the micropores is 0.89 nm and that of the mesopores is 2–10 nm. At pH = 10, the carbon aerogels have a zeta-potential of −40 mV and exhibit high adsorption capacities for cationic dyes, such as crystal violet (CV), methyl violet (MV) and methylene blue (MB), from aqueous solution. The largest adsorption capacities for CV, MV and MB are 1515, 1423 and 1181 mg/g, respectively.

Preparation and microstructure control of carbon aerogels produced using m-cresol mediated sol-gel polymerization of phenol and furfural

Carbon aerogels rich in mesopores were prepared by the sol-gel polycondensation of phenol, m-cresol, and furfural by an acid catalyst in a 1-propanol solution, followed by supercritical 1-propanol drying and pyrolysis. The effect of the ratios of m-cresol to phenol (m-C/P) on the properties of aerogels was investigated. The aerogels were characterized by infrared spectroscopy, N2 adsorption, scanning and transmission electron microscopy. It was found that the microstructure of carbon aerogels could be adjusted by the m-C/P ratio. The average mesopore size of carbon aerogels decreased from 47 to 13 nm with increasing m-C/P ratio. The surface area calculated using the Brunauer-Emmett-Teller method, external surface area, and mesopore volume of carbon aerogels, all exhibited maxima at the m-C/P ratio of 0.33.

Preparation of cresol–formaldehyde carbon aerogels via drying aquagel at ambient pressure

Carbon aerogels were prepared from mixed cresol (Cm) and formaldehyde (F) via the sol–gel process followed by drying at ambient pressure and carbonization. The inexpensive feedstock of mixed cresol and the simple drying process could be as an alternative economical route to the classical resorcinol–formaldehyde synthesis process. In our process, organic precursor gels were synthesized via polycondensation of cresol with formaldehyde in an aqueous alkaline (NaOH) solution. After gelation the solvent was removed via drying at ambient pressure to obtain organic aerogels that exhibit a drying shrinkage below 5% (linear). Upon carbonization of the organic aerogels at 1173K, monolithic carbon aerogels (denoted as CmF carbon aerogels) can be produced. Nitrogen adsorption results showed that the CmF carbon aerogels have abundant mesopores and micropores with a dominant pore diameter of 25–40nm. An increase of the BET surface area and a modification of the pore size distribution of CmF can be realized by a CO2 activation. The images of scanning electron microscopy (SEM) indicated that the microstructure of carbon aerogels can be effectively controlled and tailored by varying the synthetic conditions during the initial sol–gel process.

Electrical Transport Properties of Carbon Aerogels

Carbon aerogels are prepared by sol-gel polymerization and supercriticle drying. By controlling the mass ratio of reactants and the molar ratio of resorcinol to catalyst (R/C), carbon aerogels with different microstructure can be developed. Hall effect is measured by Van der Bow method. The results show that the electrical conductivity increases with increasing density of carbon aerogel, and Hall mobility at high temperature is nearly constant, while at low temperature the absolute value of Hall mobility increases linearly with decreasing temperature. The abnormal sign of Hall mobility is probably related to the microstructure of carbon aerogels, which influence the transport path of carriers.

The investigation of the adsorption character of carbon aerogels

The influence of pyrolysis temperature and density on the microstructure of carbon aerogels is studied by nitrogen adsorption analysis. Resorcinol-formaldehyde (RF) aerogel and carbon aerogels all give type II isotherms and hysteresis loops of type A. Carbon aerogel pyrolized at 600°C has the largest specific surface area (SSA). The trend of pore size distribution of RF aerogel and its pyrolized derivatives is same. Carbon aerogels with low and high density have different pore size distribution, while carbon aerogel with low density has greater SSA.