Multicomponent supercritical adsorption in microporous activated carbon materials

In this paper, a new technique for predicting multicomponent adsorption equilibria of supercritical fluids in microporous carbons is presented. In difference from adsorption on a surface, which is a function of the fluid-solid interaction only, adsorption in porous media is influenced by the proximity of the pore walls, resulting in the enhancement in adsorption affinity. The degree of this enhancement is different for different adsorbates, and it increases with a decrease in pore size. The theory is applied to a number of carbonaceous systems with good success.

Comparative study of hydrogen storage in Li- and K-doped carbon materials - theoretically revisited

Hydrogen adsorption in alkali-doped carbon materials is investigated theoretically. Our calculations show that hydrogen molecules can be physically adsorbed on alkali-doped graphite at 0 K but such an adsorption is thermodynamically unfavourable. The binding energy of hydrogen adsorption decreases significantly with the increase in temperature and becomes nearly zero at ambient temperature. We suggest that it may be unlikely to observe any hydrogen uptake in alkali-doped carbon materials at or above ambient temperature in the TGA (thermogravimetric) system, the previously reported hydrogen uptake in alkali-doped carbon materials was caused by either uncyclable chemisorbed hydrogen on the defects of carbon (defects were produced by repeated heat treatment) and/or moisture adsorption. (C) 2004 Elsevier Ltd. All rights reserved.

Insights into hydrogen atom adsorption on and the electrochemical properties of nitrogen-substituted carbon materials

The nitrogen substitution in carbon materials is investigated theoretically using the density functional theory method. Our calculations show that nitrogen substitution decreases the hydrogen adsorption energy if hydrogen atoms are adsorbed on both nitrogen atoms and the neighboring carbon atoms. On the contrary, the hydrogen adsorption energy can be increased if hydrogen atoms are adsorbed only on the neighboring carbon atoms. The reason can be explained by the electronic structures analysis of N-substituted graphene sheets. Nitrogen substitution reduces the pi electron conjugation and increases the HOMO energy of a graphene sheet, and the nitrogen atom is not stable due to its 3-valent character. This raises an interesting research topic on the optimization of the N-substitution degree, and is important to many applications such as hydrogen storage and the tokamaks device. The electronic structure studies also explain well why nitrogen substitution increases the capacitance but decreases the electron conductivity of carbon electrodes as was experimentally observed in our experiments on the supercapacitor.

Drying of porous building materials: hydraulic diffusivity and front propagation

One-dimensional drying of a porous building material is modelled as a nonlinear diffusion process. The most difficult case of strong surface drying when an internal drying front is created is treated in particular. Simple analytical formulae for the drying front and moisture profiles during second stage drying are obtained when the hydraulic diffusivity is known. The analysis demonstrates the origin of the constant drying front speed observed elsewhere experimentally. Application of the formulae is illustrated for an exponential diffusivity and applied to the drying of a fired clay brick.

Effect of carbon/noncarbon addition on hydrogen storage behaviors of magnesium hydride

Various Mg/carbon and Mg/noncarbon composite systems were prepared by mechanical milling and their hydrogen storage behaviors were investigated. It was found that all the carbon additives exhibited prominent advantage over the noncarbon additives, such as BN nanotubes (BNNTs) or asbestos in improving the hydrogen capacity and dehydriding/hydriding kinetics of Mg. And among the various carbon additives, purified single-walled carbon nanotubes (SWNTs) exhibited the most prominent catalytic effect on the hydrogen storage properties of Mg. The hydrogen capacities of all Mg/C composites at 573 K reached more than 6.2 wt.% within 10 min, about 1.5 wt.% higher than that of pure MgH2 at the identical operation conditions. Under certain operation temperatures, H-absorption/desorption rates of Mg/carbon systems were over one order of magnitude higher than that of pure Mg. Furthermore, the starting temperature of the desorption reaction of MgH2 has been lowered to 60 K by adding SWNTs. On the basis of the hydrogen storage behavior and structure/phase investigations, the possible mechanism involved in the property improvement of Mg upon adding carbon materials was discussed. (c) 2005 Elsevier B.V. All rights reserved.

New insights into the interaction of hydrogen atoms with boron-substituted carbon

Boron substitution in carbon materials has been comprehensively investigated using the density functional theory method. It was found that there is a correlation between the stability of the graphene sheet, the distribution of T electrons, the electrostatic potential, and the capability for hydrogen-atom adsorption. Boron substitution destabilizes the graphene structure, increases the density of the electron wave around the substitutional boron atoms, and lowers the electrostatic potential, thus improving the hydrogen adsorption energy on carbon. However, this improvement is only ca. 10-20% instead of a factor of 4 or 5. Our calculations also show that two substitutional boron atoms provide consistent and reliable results, but one substitutional boron results in contradictory conclusions. This is a warning to other computational chemists who work on boron substitution that the conclusion from one substitutional boron might not be reliable.

Adsorption in slit-like pores of activated carbons: Improvement of the Horvath and Kawazoe method

Anew thermodynamic approach has been developed in this paper to analyze adsorption in slitlike pores. The equilibrium is described by two thermodynamic conditions: the Helmholtz free energy must be minimal, and the grand potential functional at that minimum must be negative. This approach has led to local isotherms that describe adsorption in the form of a single layer or two layers near the pore walls. In narrow pores local isotherms have one step that could be either very sharp but continuous or discontinuous benchlike for a definite range of pore width. The latter reflects a so-called 0 --> 1 monolayer transition. In relatively wide pores, local isotherms have two steps, of which the first step corresponds to the appearance of two layers near the pore walls, while the second step corresponds to the filling of the space between these layers. All features of local isotherms are in agreement with the results obtained from the density functional theory and Monte Carlo simulations. The approach is used for determining pore size distributions of carbon materials. We illustrate this with the benzene adsorption data on activated carbon at 20, 50, and 80 degreesC, argon adsorption on activated carbon Norit ROX at 87.3 K, and nitrogen adsorption on activated carbon Norit R1 at 77.3 K.

Microstructural evidence for the mechanism of the alpha-beta phase transformation in ytterbium SiAlON ceramics

Detailed microstructural evidence for the mechanism of the alpha-beta phase transformation in ytterbium SiAlON ceramics is presented. Grains, which show partial transformation, have been examined using transmission electron microscopy. We suggest that the transformation proceeds as a discernable reaction front and the accompanying lattice mismatch is accommodated be a series of complex dislocations. The stabilizing cation is ejected from the transformed alpha- phase and diffuse along the dislocation to accumulate as isolated pockets in a way similar to that observed in metal systems and termed pipe diffusion. High-resolution electron microscopy reveals the details of each of these features.

Inorganic nanoparticles as carriers for efficient cellular delivery

Cellular delivery involving the transfer of various drugs and bio-active molecules (peptides, proteins and DNAs, etc.) through the cell membrane into cells has attracted increasing attention because of its importance in medicine and drug delivery. This topic has been extensively reviewed. The direct delivery of drugs and biomolecules, however, is generally inefficient and suffering from problems such as enzymic degradation of DNAs. Therefore, searching for efficient and safe transport vehicles (carriers) to delivery genes or drugs into cells has been challenging yet exciting area of research. In past decades, many carriers have been developed and investigated extensively which can be generally classified into four major groups: viral carriers, organic cationic compounds, recombinant protiens and inorganic nanoparticles. Many inorganic materials, such as calcium phosphate, gold, carbon materials, silicon oxide, iron oxide and layered double hydroxide (LDH), have been studied. Inorganic nanoparticles show low toxicity and promise for controlled delivery properties, thus presenting a new alternative to viral carriers and cationic carriers. Inorganic nanoparticles generally possess versatile properties suitable for cellular delivery, including wide availability, rich functionality, good biocompatibility, potential capability of targeted delivery (e.g. selectively destroying cancer cells but sparing normal tissues) and controlled release of carried drugs. This paper reviews the latest advances in inorganic nanoparticle applications as cellular delivery carriers and highlights some key issues in efficient cellular delivery using inorganic nanoparticles. Critical proper-ties of inorganic nanoparticles, surface functionalisation (modification), uptake of biomolecules, the driving forces for delivery, and release of biomolecules will be reviewed systematically. Selected examples of promising inorganic nanoparticle delivery systems, including gold, fullerences and carbon nanotubes, LDH and various oxide nanoparticles in particular their applications for gene delivery will be discussed. The fundamental understanding of properties of inorganic nanoparticles in relation to cellular delivery efficiency as the most paramount issue will be highlighted. (c) 2005 Elsevier Ltd. All rights reserved.

Canopy carbon and oxygen isotope composition of 9-year-old hoop pine families in relation to seedling carbon isotope composition, growth, field growth performance, and canopy nitrogen concentration

Carbon isotope composition (delta C-13), oxygen isotope composition (delta O-18), and nitrogen concentration (N-mass) of branchlet tissue at two canopy positions were assessed for glasshouse seedlings and 9-year-old hoop pine (Araucaria cunninghamii Ait. ex D. Don) trees from 22 open-pollinated families grown in 5 blocks of a progeny test at a water-limited and nitrogen-deficient site in southeastern Queensland, Australia. Significant variations in canopy delta C-13, delta O-18, and N-mass existed among the 9-year-old hoop pine families, with a heritability estimate of 0.72 for branchlet delta C-13 from the upper inner canopy position. There was significant variation in canopy delta C-13 of glasshouse seedlings between canopy positions and among the families, with a heritability estimate of 0.66. The canopy delta C-13 was positively related to canopy N-mass only for the upper outer crown in the field (R = 0.62, p < 0.001). Phenotypic correlations existed between tree height and canopy delta C-13 (R = 0.37-0.41, p < 0.001). Strong correlations were found between family canopy delta C-13 at this site and those at a wetter site and between field canopy delta C-13 and glasshouse seedling delta C-13. The mechanisms of the variation in canopy delta C-13 are discussed in relation to canopy photosynthetic capacity as reflected in the N-mass and stomatal conductance as indexed by canopy delta O-18.

Use of liquid phase adsorption for characterizing pore network connectivity in activated carbon

A simple percolation theory-based method for determination of the pore network connectivity using liquid phase adsorption isotherm data combined with a density functional theory (DFT)-based pore size distribution is presented in this article. The liquid phase adsorption experiments have been performed using eight different esters as adsorbates and microporous-mesoporous activated carbons Filtrasorb-400, Norit ROW 0.8 and Norit ROX 0.8 as adsorbents. The density functional theory (DFT)-based pore size distributions of the carbons were obtained using DFT analysis of argon adsorption data. The mean micropore network coordination numbers, Z, of the carbons were determined based on DR characteristic plots and fitted saturation capacities using percolation theory. Based on this method, the critical molecular sizes of the model compounds used in this study were also obtained. The incorporation of percolation concepts in the prediction of multicomponent adsorption equilibria is also investigated, and found to improve the performance of the ideal adsorbed solution theory (IAST) model for the large molecules utilized in this study. (C) 2002 Elsevier Science B.V. All rights reserved.

Service performance of engineering materials

Corrosion research by Atrens and co-workers has made significant contributions to the understanding of the service performance of engineering materials. This includes: (1) elucidated corrosion mechanisms of Mg alloys, stainless steels and Cu alloys, (2) developed an improved understanding of passivity in stainless steels and binary alloys such as Fe-Cr, Ni-Cr, Co-Cr, Fe-Ti, and Fe-Si, (3) developed an improved understanding of the melt spinning of Cu alloys, and (4) elucidated mechanisms of environment assisted fracture (EAF) of steels and Zr alloys. This paper summarises contributions in the following: (1) intergranular stress corrosion cracking of pipeline steels, (2) atmospheric corrosion and patination of Cu, (3) corrosion of Mg alloys, and (4) transgranular stress corrosion cracking of rock bolts.

Electrochemical characteristics of ordered mesoporous carbon used as electrode material in Li-ion battery

Ordered mesoporous carbon CMK-5 was comprehensively tested for the first time as electrode materials in lithium ion battery. The surface morphology, pore structure and crystal structure were investigated by Scanning Electronic Microscopy (SEM), N-2 adsorption technique and X-ray diffraction (XRD) respectively. Electrochemical properties of CMK-5 were studied by galvanostatic cycling and cyclic voltammetry, and compared with conventional anode material graphite. Results showed that the reversible capacity of CMK-5 was 525 mAh/g at the third charge-discharge cycle and that CMK-5 was more compatible for quick charge-discharge cycling because of its special mesoporous structure. Of special interest was that the CMK-5 gave no peak on its positive sweep of the cyclic voltammetry, which was different from all the other known anode materials. Besides, X-ray photoelectron spectroscopy (XPS) and XRD were also applied to investigate the charge-discharge characteristics of CMK-5.

Synthesis of ordered nanoporous carbon and its application in Li-ion battery

Ordered nanoporous carbon (ONC) was comprehensively tested for the first time as electrode material in lithium-ion battery. Structure characterization shows the order nanoporous structure and tiny crystallite structure of as-synthesized ONC. The electrochemical properties of this carbon were studied by galvanostatic cycling and cyclic voltammetry. Of special interest is that ONC gave no peak on its positive sweep of the cyclic voltammetry, which was different from other known anode materials. Besides, X-ray photoelectron spectroscopy (XPS) and XRD were also used to investigate the electrochemical characteristics of ONC. (c) 2006 Elsevier Ltd. All rights reserved.