Chiaki Ishii

Origin of superhigh surface area and microcrystalline graphitic structures of activated carbons

Abstract The microcrystalline graphitic structures of microporous carbon fibers determined by x-ray diffraction, high-resolution transmission electron microscopy, and magnetic susceptibility measurements were determined. The relationship between the microcrystalline graphitic structures and specific surface area was geometrically derived on the basis of the structural data. A simple new method for determining specific surface area for microporous carbons, which is designated the subtracting pore effect (SPE) method, was proposed. The surface area by the SPE method was compared with that by the BET method for high surface area-activated carbon fibers (ACFs); the BET surface area of more than 2000 m2g−1, which is determined by the data in the relative pressure of 0.1-0.3, was overestimated by 40%–50%. The valid relative pressure region of the BET plot for microporous carbons was suggested. The micrographitic structural changes of ACFs with water adsorption were examined by in situ x-ray diffraction and in situ small angle x-ray scattering. Water adsorption led to marked compression of the micrographitic pore walls, accompanying the clear swelling and development of the micropore.

Characterization of porous carbons with high resolution αs-analysis and low temperature magnetic susceptibility

Abstract The effectiveness of integrated characterization of porous carbons with different approaches such as the high resolution α s -method for N 2 adsorption, X-ray diffraction, and magnetic susceptibility measurement is shown. The theoretical basis for the high resolution α s -analysis for N 2 adsorption using GCMC simulation and the background of magnetic susceptibility of carbon are given. The applicability of the integrated characterization to changes of ACF with heating at 1473–3173 K in Ar is shown. The activated mesocarbon microbead (a-MCMB) and activated carbon aerogel (a-CA) were examined by the integrated characterization method. a-MCMB showed an unusual magnetic relaxation below 10 K due to random magnetism. The random magnetism was associated with the mutually isolated network structure of nanographitic units. a-CA also showed random magnetism at low temperature, which was associated with the spin isolated nanographitic structure similar to a-MCMB.

GAS ADSORPTION EFFECTS ON STRUCTURAL AND ELECTRICAL PROPERTIES OF ACTIVATED CARBON FIBERS

Activated carbon fibers (ACF) are microporous carbon consisting of a three-dimensional network of micrographites having the size of ∼3 nm. We investigate structural and electronic properties of ACFs in relation to the adsorption of helium, nitrogen, and oxygen gases, by means of adsorption isotherm, ESR (electron spin resonance), magnetic susceptibility, and electrical conductivity measurements. The linewidth of ESR associated with dangling bond spins on micrographites decreases with gas uptake at low pressures below 0.1 kPa regardless of gas species. The similar behavior of oxygen to that of nonmagnetic helium and nitrogen demonstrates that adsorbed oxygen molecules having chemisorption feature are stabilized in the singlet ground state in the low-pressure range. Taking into account that the linewidth is governed by dipole–dipole interaction between dangling bond spins, the reduction in the ESR linewidth proves the swelling of micropores induced by gas uptake. The conductivity decreases with gas uptake i...

Anomalous helium-gas-induced spin-lattice relaxation and the evidence for ultra micropores in microporous carbon

The spin-lattice relaxation mechanism of dangling bond spins in activated carbon fibers (ACF) with huge specific surface areas (∼3000m2/g) was investigated by ESR measurements in the presence of the gases, He, Ne, Ar, H2, N2, and O2. The introduction of helium gas remarkably enhances the spin-lattice relaxation rate, suggesting the participation of the collisional process of helium atoms in the spin relaxation from the dangling bond spins to the lattice. Taking into account an exceptionally large condensation of helium gas, this proves that ACF has ultra micropores which can accommodate only the small diameter helium atoms, resulting in a novel molecular sieve effect.

Size evaluation of graphitic crystallites in activated carbon fibers from diamagnetic susceptibility measurements

Abstract Diamagnetic susceptibilities of various kinds of activated carbon fibers (ACFs), nonporous carbon black, and Grafoil were measured under an N 2 atmosphere in the temperature range of 273 to 423 K. An empirical relationship between the diamagnetic susceptibility and molecular structure was derived from the literature for polynuclear aromatic compounds without hydrogen atoms. The unit size of graphitic crystallites in ACFs was evaluated from the derived relationship.

Structural Characterization of Heat-Treated Activated Carbon Fibers

Pitch (PIT) and cellulose (CEL)-based activated carbon fibers (ACFs) were heated at 1473–3173 K under an Ar atmosphere. The N2 adsorption, X-ray diffraction (XRD), and magnetic susceptibility of the heat-treated ACFs were measured. The specific surface area of ACF samples was determined by the subtracting pore effect (SPE) analysis using the N2 adsorption isotherm.Both stacking height, Lc and stacking width, La of ACFs began to increase remarkably above 2000 K. The amounts of N2 adsorption on PIT and CEL became nil by heating above 1773 K and 2073 K, respectively. The relationships between the heating temperature and the magnetic susceptibility of ACFs near room temperature were divided into three regions: below 1773 K, from 1773 K to 2473 K, and above 2473 K.The results from gas adsorption, X-ray diffraction and magnetic susceptibilities have been explained with respect to the changes taking place in these three regions.

Surface and physical properties of microporous carbon spheres

The surface area of activated mesocarbon microbeads (activated MCMBs) was determined by N2 adsorption using the subtracting pore effect (SPE) method. The microcrystalline graphitic structure of activated MCMBs was determined by X-ray diffraction. Electron spin resonance (ESR) spectra and magnetic susceptibility were measured for analysis of structure and magnetic properties. Spin concentration and spin-lattice relaxation time (T1) were determined from ESR data. The spin concentration of activated MCMB increased with the increase of surface area. The higher the surface area of activated MCMB, the longer the T1. The longer T1 indicates that micrographitic units of activated MCMB of higher surface area should be more isolated. A single electron spin was presumed to be situated on each micrographitic unit. The isolated spin system gave a random magnetism of a long magnetic relaxation (>3000 s).

Low temperature magnetic properties of an O2 and H2O mixed molecular assembly, confined in a graphitic nanospace

Abstract The magnetic susceptibility (χ) of O2, in a slit-shaped graphitic nanospace of pore width w=0.9 nm, was measured as a function of the fractional filling (φ) by O2 and the ratio Rw of coexistent H2O to O2 in the temperature range 2 to 100 K. The effect of coexistent H2O on the χ of the bulk O2 was examined for comparison. The H2O destroyed the antiferromagnetic ordering of the α-phase of the bulk solid O2 inducing paramagnetism. O2 in the nanospace at low φ showed the paramagnetism and coexistent H2O of Rw≤0.35 accelerated the spin cluster formation. The presence of excess H2O enhanced the paramagnetism of the confined O2 due to the perfect isolation of an O2 spin by clathrate formation.

Random magnetism of superhigh surface area carbon having minute graphitic structures

Abstract The magnetic properties of micrographitic structure-oriented carbons with different surface areas were examined over the temperature range of 1.7 to 300 K. Superhigh surface area carbon of specific surface area = 3110 m 2 g −1 , whose surface atom ratio is 0.94, showed a remarkable magnetic hysteresis and ξ − T relation with maximum below 4.2 K. The magnetic hysteresis depended on the measuring temperature; the residual magnetization was 0.016 emu G g −1 at 1.7 K and 0.015 emu G g −1 at 300 K. The ferromagnetic behavior was dependent on the measuring time, and thereby the maximum of the ξ − T relation shifted to lower temperature with a longer measuring time. On the contrary, low surface area carbon of surface area = 570 m 2 g −1 showed no ferromagnetism.

Novel Structure of Microporous Activated Carbon Fibers and Their Gas Adsorption

Activated carbon fibers are a kind of microporous carbon. Using dangling bond spins attached to the peripheries of the micropores, we investigated the microporous structures in relation to the heat-treatment and gas adsorption effects. Functional groups weakly bonded to the graphitic backbone are removed by the heat-treatment at moderate temperatures 200-400°C, resulting in the generation of a variety of dangling bond spins. The heattreatment above 500°C brings about homogenization of the dangling bond spins. For gas adsorption, the introduction of helium gas strongly enhances the spin-lattice relaxation rate for the dangling bond spins. In addition to a remarkably large condensation of helium gas in the microporous region, the enhancement proves the presence of ultra-micropores which can accommodate only the smallest diameter helium atoms.