Characterization of carbon aerogels by transport measurements

Abstract

Carbon aerogels are a special class of low-density microcellular foams. These materials are composed of interconnected carbon particles with diameters of approximately 10 nm. The temperature dependence of the dc electrical resistivity and magnetic susceptibility (χ) from 4 K to room temperature, magnetoresistance (MR) in a magnetic field up to 15 T, and Raman scattering were measured as a function of aerogel density. While Raman scattering measurements are not sensitive to variations in density, the χ data show that there are more free carriers in samples of higher density. Aerogel samples with different densities all show a negative temperature coefficient of resistivity and a positive MR. The less dense samples exhibit a stronger temperature dependence of resistivity and a stronger field dependence of the MR, indicating that with decreasing density and increasing porosity, charge carriers are more localized. Data analysis precludes variable-range hopping in favor of nearest-neighbor hopping and fluctuation-induced tunneling as the most likely conduction mechanisms for carbon aerogels.

Unconventional magnetism in all-carbon nanofoam

“…The nanofoam has the highest spin density reported for a three-dimensional pure carbon solid, although we note that similarly high spin densities have been reported for sp 3 -rich amorphous carbon as thin films. 40,42 C. Nanostructure Low-resolution TEM images clearly show the presence of small randomly interconnected clusters with the diameters in the range from 4 to 9 nm, with the average size of 6.1 nm (Fig. 1).…”

“…This behavior implied at once that the dielectric properties of the carbon nanofoam are different from those of carbon aerogels and other carbon nanometer-size porous structures. [39][40][41] This difference can be attributed to the formation conditions: the foam clusters are formed in a very high temperature environment of partially ionized carbon plasma. 38 The carbon nanofoam has a significant sp 3 / ͑sp 2 + sp 3 ͒ bonding ratio of 35% on average, and up to 60% in some regions.…”

Photoconductivity of carbon aerogels

“…13 Fung et al 12 Table II. fit the electrical conductivity with various conduction models, including nearest-neighbor hopping, variable range hopping, and charge-energy-limited-tunneling, and found a good fit for their data to the FIT model. The FIT model was proposed by Sheng et al 13 to explain the conduction mechanism for carbon-PVC composites, consisting of conductive carbon particles of 140-350 A in diameter, embedded in an insulating PVC matrix.…”

“…6 " 12 The aerogels are cluster-assembled, low-density porous materials, consisting of interconnected carbon particles with diameters of -120 A. Within each particle, a glassycarbon-like nanostructure is observed, consisting of an a) Present address: Toshiba R&D Center, 1, Komukai Toshiba-cho, Saiwaiku, Kawasaki 210, Japan.…”

Carbon aerogel evolution: Allotrope, graphene-inspired, and 3D-printed aerogels

“…However, in the past 30 years, there has been a collective push to not only develop new methods 5 to produce traditional metal oxides but also to expand the variety of materials that aerogels cover. Some notable examples of these new aerogels include reports of organic aerogels, 6 carbons, [7][8][9][10] conducting oxides, 11 chalcogenides, 12 metals, [13][14][15] and various two-dimensional (2D) materials. [16][17][18] A major driver for realizing aerogels from a wider materials set is the potential to unlock novel properties that these materials only exhibit as aerogels.…”