Brian McEnaney

Estimation of the dimensions of micropores in active carbons using the Dubinin-Radushkevich equation

Abstract Decreases in the characteristic energy of the Dubinin-Radushkevich equation, E 0 , are correlated with increases in micropore sizes determined by molecular probes, W m , using the empirical equation W m = 4.691 exp (−0.0666 E 0 ); a similar equation can also be used to correlate the more limited data relating E 0 to the Guinier radius of gyration, R g . A new, semiempirical method is proposed which relates E 0 to micropore sizes calculated from adsorption potentials in slit-shaped, model micropores. The two methods for estimating micropore size are applied to adsorption of Ar at 77 K on a steam-activated series of polymer carbons. Both methods show that micropore sizes increase progressively with activation, but for each carbon, the micropore size estimated from molecular probes is greater than that obtained using potentials in model micropores. The differences are attributed to different weightings of the pore size estimates in the micropore size distributions. Estimates based upon molecular probes are weighted towards the tail of the distribution, while those based on model pore potentials are weighted towards the smallest micropores. A method for estimating the size range of micropores is proposed where the lower limit is defined by the adsorption potential maximum in model pores and the upper limit is defined by the correlation between W m and E 0 .

Adsorption and structure in microporous carbons

Abstract The microporous structure of carbons consists of a tangled network of defective carbon layer planes in which micropores are the spaces between the layer planes. Adsorption of gases in micropores is characterized by (1) strong adsorption at low pressure due to overlap of force fields from opposite pore walls, (2) activated diffusion effects caused by constrictions in the microporous network, and (3) molecular size and shape selectivity (molecular sieving). The surface fractal dimension of activated carbons decreases from three to two with increasing activation, indicating that activation smoothens pore surfaces. Calculated adsorption potentials for slit-shaped model micropores show that adsorption potentials are enhanced by a factor of up to 2 and enable critical dimensions for diffusion of gases through micropores to be estimated. The Brunauer-Emmett-Teller equation is unsuitable for analyzing adsorption with a significant microporous contribution but may be used to estimate the nonmicroporous surface area, provided that the microporous contribution can be removed. The Dubinin-Radushkevich and Dubinin-Astakhov equations have been more successful when applied to microporous carbons because they reflect the influence of adsorbent heterogeneity, as they result from an approximation to the generalized adsorption isotherm (GAI). More exact solutions of the GAI enable adsorption energy distribution functions to be obtained. The possibility of extracting micropore size distributions from adsorption measurements is briefly considered.

Theoretical and experimental studies of methane adsorption on microporous carbons

Abstract GCEMC molecular simulations of methane adsorption in model slit-shaped carbon pores show that variations in the density of carbon atoms in the pore wall have a significant influence on adsorbed methane density, although variations in inter-layer spacing and the number of layer planes in the pore wall, n, (n > 2) have little effect. A model is proposed for calculating the influence of pore wall thickness on the stored volumetric methane capacities of a void-free microporous carbon monolith and model monoliths formed from close-packed powders and fibers. Volumetric methane capacities for the models were in reasonable agreement with measured values for carbon monoliths made from KOH-activated meso-carbon microbeads and for compacted activated carbon fibers.

Comparison of adsorption methods for characterizing the microporosity of activated carbons

Adsorption methods for characterizing the microporosity of activated carbons are discussed critically. Three methods—the αs-method and those based on the Dubinin-Radushkevich and Jaroniec-Choma isotherm equations—are compared with respect to the parameters that characterize the microporous structure of a solid. It is shown that the isotherm equations that account for the structural heterogeneity of activated carbons give values of the micropore volume similar to that obtained by the αs-method.

Structural studies of wear debris from carbon–carbon composite aircraft brakes

Abstract Microstructural studies using X-ray diffraction, SEM, and optical microscopy were made of wear debris from carbon–carbon composite aircraft brakes produced under different simulated braking operations. Under simulated cold taxiing conditions, particulate wear debris comprising a disordered carbon phase containing fibre fragments is formed. The disordered phase is produced mainly by shear deformation of the graphitic CVI matrix in the parent carbon–carbon composite. Under simulated landing conditions the wear debris is partly transformed by shear processes into a friction film with a similar disordered microstructure to the wear debris formed during taxiing. The friction film is formed as a result of the higher power density and interfacial temperatures during landing. The very high power densities and interface temperatures that apply during rejected take-off result in shear-stress assisted graphitisation of the friction film, but with a fine mosaic texture that is quite different from the texture of the CVI matrix in the parent composite.

Adsorption of CO2 and SO2 on activated carbons with a wide range of micropore size distribution

The adsorption isotherms of N2 at 77K, CO2 at 251, 273 and 298K, and SO2 at 262 and 273K have been determined on a series of physically activated carbons with a wide range of micropore size distributions. Since the series includes carbons with very high burn-off, it shows the problems involved in the characterization of microporsity in superactivated carbons. On the other hand, the results show that the carbon surface-adsorbate interactions for SO2 at low relative pressures are weaker than for N2 and CO2, as a result of the strong adsorptive-adsorptive interactions in the bulk gas phase.

Fracture processes in graphite and the effects of oxidation

Measurements were made of 3-point bend strength, σb, critical stress intensity factor. KIc, dynamic elastic modulus, E, and specific fracture surface energy, γ, for two graphites before and after oxidation up to 7% weight loss in CO25% CO gas at 1173 K. The fracture path cleaves grains parallel to basal planes and traverses pores in the binder phase. Calculated critical flaw sizes were larger than any single microstructural feature, implying that they develop by sub-critical cracking. The decrements in σb, E and KIc, with increases in fractional porosity P following oxidation are described by exponential equations; the decrements in γ are better described by linear equations; each of the properties is reduced by 40–50% for up to 7% wieght loss. There is no fractographic evidence for changes in fracture mechanism upon oxidation and the calculated flaw size does not change significantly; the decrements in KIc are entirely accounted for by decreases in E and γ and there is evidence for nonideal brittle behaviour in both graphites. The most significant change in the microstructure after oxidation is the development of narrow, slit-shaped shrinkage cracks in the binder phase which are mainly oriented parallel to basal planes. The elongation of these cracks is attributed to the reaction anisotropy of graphite. The severe decrement in E after oxidation is attributed to the development of these slit-shaped pores of high aspect ratio.

Oxidation and fracture of a woven 2D carbon-carbon composite☆

Abstract Measurements were made of flexural strength and dynamic elastic modulus of a woven, 2D carbon-carbon composite before and after oxidation in flowing, dry air. The microstructure and fracture surfaces were examined using optical and scanning electron microscopy. Oxidation preferentially attacks the fibre/matrix interfaces and weakens fibre bundles. The unoxidised material fails catastrophically by delamination cracking between plies and at bundle/bundle interfaces within plies. As oxidation progresses, failure becomes a multistep process with less delamination cracking and more cross-bundle cracking. This change of mode of failure with oxidation is attributed to more severe attack within bundles than at bundle/bundle interfaces. For a weight loss on oxidation of 10%, the reduction in elastic modulus and flexural strength was 30% and 50% respectively.

The development of structure and microporosity in cellulose carbon

Abstract Changes in structure and microporosity of a non-graphitizing cellulose carbon upon heattreatment to 1870 K were followed by electron microscopy, X-Ray measurements, evolved hydrogen analysis and adsorption of carbon dioxide. The 1170 K carbon has a disordered structure containing distorted, defective layer planes which occur singly or in small stacks. Rapid lateral growth of layer planes and improvement in planarity occur on heat treatment to 1670 K and are associated with expulsion of hydrogen. Microporosity is an interconnected network of slit-like pores between layer planes. Layer plane growth above 1670 K causes a collapse of some ultra-micropores to turbostratic spacings resulting in growth of layer stacks and conversion of open micropores to closed micropores. The loss of some ultra-micropores by conversion to turbostratic spacings causes a progressive increase in mean micropore sizes with heat treatment.

The influence of dissolved oxygen concentration on the corrosion of grey cast iron in water at 50°C

Abstract The formation of corrosion scales has been studied on grey cast iron in flowing water at 50°C as a function of O 2 concentrations from 0.1 to 3.95 ppm O 2 . Below 1.0 ppm O 2 , nodular scales form containing Fe 3 O 4 and a green rust, GR. At higher O 2 concentrations, a continuous scale eventually forms, consisting of a porous subscale of Fe 2 O 4 + GR overlaid with a compact crust of Fe 3 O 4 + GR and a thin surface layer of γ-FeOOH. ‘Chimneys’ oriented in the water flow direction grow out of the crust. γ-FeOOH is reduced to Fe 3 O 4 which becomes the principal constituent of the scale. Scales on cast iron components from central heating systems closely resemble those found in the present work.