Characterization of partially sintered ceramic powder compacts using fluorinated gas NMR imaging.

We use nuclear magnetic resonance (NMR) imaging of C2F6 gas to characterize porosity, mean pore size, and permeability of partially sintered ceramic (Y-TZP Yttria-stabilized tetragonal-zirconia polycrystal) samples. Conventional measurements of these parameters gave porosity values from 0.18 to 0.4, mean pore sizes from 10 nm to 40 nm, and permeability from 4 nm(2) to 25 nm(2). The NMR methods are based on relaxation time measurements (T(1)) and the time dependent diffusion coefficient D(Delta). The relaxation time of C2F6 gas is longer in pores than in bulk gas and it increases as the pore sizes decrease. NMR yielded accurate porosity values after correcting for surface adsorption effects. A model for T(1) dependence on pore size that accounts for collisions between gas molecules and walls as well as surface adsorption effects is proposed. The model fits the experimental data well. Finally, the long time limit of D(Delta)/D(o), where D(o) is the bulk gas diffusion coefficient is useful for measuring tortuosity, while the short time limit was not achieved experimentally and could not be used for calculating surface-area to volume (S/V) ratios.     

Demixing transitions in a binary Gaussian-core fluid confined in narrow slit-like pores

Using a mean-field density functional approach we investigate phase separation transition in a binary mixture of Gaussian-core molecules confined in narrow slit-like pores. We consider pores with repulsive and attractive walls. In the case of fluid confinement in pores with repulsive and non-selective attracting walls, no phase separation in the confined fluid, prior to the bulk separation transition, was observed. However, in the case of pores with the walls selectively attracting fluid particles, we reveal that the separation transition may take place as a two-step process. During the first step the composition change occurs within a few layers adjacent to the pore walls, whereas in the second step, it takes place in the pore interior.     

Further observations on the polarized absorption spectrum of benzene crystals at 2000 Å

The structure of a hard sphere fluid confined by model slit and cylindrical pores is investigated. Results from grand canonical Monte Carlo (GCMC) simulations and from the hypernetted chain/mean spherical approximation (HNC/MSA) equation are reported. GCMC results are compared with those from the HNC/MSA equation, and agreement is good. The effect of confinement on liquids at the same chemical potentials is that the absorption of the hard sphere fluid into the pores decreases with increasing confinement, i.e., when going from planar to cylindrical geometry or by narrowing the pores. The adsorption on the pore walls has, in general, the opposite behaviour. For high bulk concentrations and certain values of cylindrical pore diameter the concentration profile is higher at the centre of the pore than next to the pore wall. A very strong, but continuous, transition occurs in the concentration profile, as a function of the cylinder's diameter. These results could be of some interest in catalysis studies.     

Molecular dynamics study of thermal diffusion in a binary mixture of alkanes trapped in a slit pore

We have used direct non-equilibrium molecular dynamics computer simulations to study the influence of an aluminosilicate slit pore on thermal diffusion in equimolar methane- n -decane. We have computed the Soret coefficient S T as a function of the pore width. The S T values deviate from those in a pore-free situation only for pores narrower than 35 Å. We have then investigated the possible causes for this deviation. We have noticed that the solid behaves as a thermal short circuit for the liquid but this has no consequence on the thermal and solutal profiles in the mixture. The main influence of the confinement of the liquid lies in the 'freezing' of the layer of molecules in contact with the pore walls. Outside this layer, the thermal diffusive behaviour of the mixture does not depart from that in the bulk fluid. This finding has enabled us to compute a 'corrected' Soret coefficient where the influence of the porous medium is eliminated.     

Inert fluorinated gas T1 calculator

The physics of spin-rotation interaction in roughly spherical perfluorinated gas molecules has been studied extensively. But, it is difficult to calculate a spin-lattice relaxation time constant T1 for any given temperature and pressure using the published literature. We give a unified parameterization that makes use of the Clausius equation of state, Lennard-Jones collision dynamics, and a formulaic temperature dependence for collision cross section for rotational change. The model fits T1s for SF6, CF4, C2F6, and c-C4F8 for temperatures from 180 to 360 K and pressures from 2 to 210 kPa and in mixtures with other common gases to within our limits of measurement. It also fits previous data tabulated according to known number densities. Given a pressure, temperature, and mixture composition, one can now calculate T1s for common laboratory conditions with a known accuracy, typically 0.5%. Given the success of the model's formulaic structure, it is likely to apply to even broader ranges of physical conditions and to other gases that relax by spin-rotation interaction.     

Effect of confinement in nano-porous materials on the solubility of a supercritical gas

By combining Gibbs Ensemble Monte Carlo simulations and density functional theory, we investigate the influence of confinement in a slit-shaped carbon pore on the solubility of a supercritical solute gas in a liquid solvent. In the cases studied here, competing adsorption of the solvent and solute determines whether the solubility is enhanced or suppressed for larger pores. We find that the solubility in the confined system is strongly dependent on pore width, and that molecular packing effects are important for small pore widths. In addition, the solubility decreases on increase in the temperature, as for the bulk mixture, but the rate of decrease is greater in the pore due to a decrease in the partial molar enthalpy of the solute in the pore; this effect becomes greater as pore width is decreased. The solubility is increased on increasing the bulk pressure of the gas in equilibrium with the pore, and obeys Henry's law at lower pressures. However, the Henry constant differs significantly from that for the bulk mixture, and the range of pressure over which Henry's law applies is reduced relative to that for the bulk mixture. The latter observation indicates that solute–solute interactions become more important in the pore than for the bulk at a given bulk pressure. Finally, we note that different authors use different definitions of the solubility in pores, leading to some confusion over the reported phenomenon of ‘oversolubility’. We recommend that solubility be defined as the overall mole fraction of solute in the pores, since it takes into account the increase in density of the solvent in the pores, and avoids ambiguity in the definition of the pore volume.     

Relaxation processes of water confined to AlMCM-41 molecular sieves. Influence of the hydroxyl groups of the pore surface

A series of AlMCM-41 molecular sieves was prepared with constant composition (Si/Al = 14.7) and presumably same pore structure but different pore diameters (from 2.3 to 4.6 nm). The pore size distribution is narrow for each sample. The rotational fluctuations of water molecules confined inside the pores were investigated applying broadband dielectric spectroscopy (10⁻²–10⁷ Hz) over a large temperature interval (213–333K). A relaxation process, slower than that expected for bulk water, was observed which is assigned to water molecules forming a surface layer on the pore walls. The estimated relaxation time has an unusual non-monotonic temperature dependence, which is rationalized and modeled assuming two competing processes: rotational fluctuations of constrained water molecules and defect formation (Ryabov model). This paper focuses on the defects and notably the influence of the hydroxyl groups of the pore walls. The Ryabov model is fitted to the data and characteristic parameters are obtained. Their dependence on pore diameter is considered for the first time. The found results are compared with those obtained for other types of molecular sieves and related materials.     

The phase behaviour of a fluid in a slitlike pore with permeable walls

The confinement of a lattice fluid in a set of slitlike pores separated by semipermeable walls with a finite width has been studied. The walls are modelled by a square-well repulsive potential with a finite height. The thermodynamic properties and the phase behaviour of the system are evaluated by means of Monte Carlo simulations. For some states theoretical calculations have been made using a mean-field-type theory. These investigations confirm previous findings for confined Lennard-Jones fluids, obtained from a density functional approach. For intermediate and low potential barriers that separate the pores, the isotherms exhibit two hysteresis loops and the liquid-vapour coexistence curve divides into two branches describing condensation inside the pore and inside the permeable wall. These two branches are separated by a triple point. At temperatures lower than the triple point temperature, the condensation takes place instantaneously in both the pore and inside the permeable wall. It was found that when the temperature is scaled by the bulk critical temperature, the phase diagram emerging from this simple mean-field treatment is close to the phase diagram obtained from simulation.     

Shift of triple point in confined systems with curved interfaces

The unified thermodynamic approach to the analysis of melting/freezing phenomena in confined systems is proposed. The approach relates the shift of the triple point in a confined system in comparison with the bulk system with the physico-chemical parameters of the bulk system and the boundary layer. The application of general equations to particular types of confinement is illustrated for small particles and substance embedded inside porous matrices. The analysis of equations derived indicates that for the systems considered the shift of triple point in the boundary layers and in the uniform core part of the system in the general case differ from each other. The shift of triple point may be either positive or negative and may be controlled for paricles by variation of the interfacial energy, and for the pores by variation of interfacial energy and/or pore walls wettability.     

Dynamics of plastic and liquid cyclohexane in bulk and in porous glasses studied by NMR methods

Cyclohexane was investigated both in bulk and in porous glasses with pore diameters between 4 and 208 nm in the temperature range 136 K≤T≤300 K. The methods involved were field-cycling NMR relaxometry, field-gradient NMR diffusometry, transverse-relaxation spectroscopy, and differential scanning calorimetry (DSC). The field-cycling data for the bulk material can best be described assuming translational modulation of intermolecular dipole-dipole coupling. This interpretation is confirmed by experiments with different degrees of deuteration, and is in accordance with diffusion coefficients determined with the aid of field-gradient diffusometry. The confinement in pores produces substantial changes in the phase behaviour and in molecular dynamics. For pore diameters of 30 nm and above, a non-frozen two monolayers thick film on the surface retains a diffusivity about one order of magnitude lower than in the bulk liquid, but two orders of magnitude larger than in the bulk plastic phase. Experiments indicate an exchange mechanism between this layer and the crystallite inside the pore. In glass with a pore diameter of 4 nm, all applied methods corroborate DSC results of the virtual absence of a phase transition and reveal a continuously decreasing translational mobility down to temperatures more than 100 K below the bulk liquid/cubic phase transition temperature.     

Phase behaviour of a Lennard-Jones fluid in a pore with permeable walls of a finite thickness: a density functional approach

A confinement of a Lennard-Jones fluid in a system of slitlike pores separated by semipermeable walls of a finite width is studied. The walls are modelled by square-well repulsive potential wells. The structure of the confined fluid is investigated by means of a density functional method. For high potential barriers separating the pores, the phase behaviour of the system is similar to that for a single slitlike pore with impenetrable walls. For intermediate and low potential barriers the system shows different phase behaviour. Within some temperature range the isotherms exhibit two hysteresis loops, which characterize the condensation of the fluid in different parts of the system, namely in the pore and inside the semipermeable walls. The systems characterized by low and intermediate potential barriers exhibit the triple point, such that at temperatures below that triple point the condensation instantaneously takes place in both the pore and inside the permeable wall.     

Nanopore wall effect on surface tension of methane

Local pressure is known to be anisotropic across the interfaces separating fluids in equilibrium. Tangential pressure profiles show characteristic negative peaks as a result of surface tension forces parallel to the interface. Nearby attractive forces parallel to the interface are larger than the repulsive forces and, hence, constitute the surface tension. In this work, using molecular dynamics simulations of methane inside nano-scale pores, we show this surface tension behaviour could be significantly influenced by confinement effects. The layering structure, characterised by damped oscillations in local liquid density and tangential pressures, extends deep into the pore and can be a few nanometers thick. The surface tension is measured numerically using local pressures across the interface. Results show that the tension is smaller under confinement and becomes a variable in small pores, mainly controlled by the thickness of the liquid density layering (or liquid saturation) and the pore width. If the liquid saturation inside the pore is high enough, the vapour–liquid interface is not interfered by the pore wall and the surface tension remains the same as the bulk values. The results are important for understanding phase change and multi-phase transport phenomena in nanoporous materials.     

受限导致硬椭球液体的取向有序

Monte Carlo模拟研究了两平行硬墙受限下各向异相的硬椭球液体形成的有序结构. 墙壁对椭球粒子的过量吸附和椭球的长短径之比有关;对于高密度的椭球液体,它在一个临界的长短径之比 a/b=2.9处达到最大值,表明在墙表面附近发生了取向有序. 墙表面附近的密度分布和取向序参量进一步证实这结论. 相同密度条件下体相中的取向序参量计算表明体相结构均为各向同性,这是由于体系的密度依然低于各向同性相向相列向转变的临界密度. 该受限椭球液体中墙表面的取向有序是由墙面的几何受限所导致的.     

Pore formation in in situ processed MgB2 superconductors

MgB₂ bulks were prepared by an in situ process which utilizes the reaction between boron and magnesium powder. The reaction time was fixed at 0.5 h and the temperature was changed from 600 °C to 1000 °C. The density decrease due to pore formation and mass (mainly magnesium) loss during the formation reaction of MgB₂ was observed in all samples. In addition to the pore formation, a pellet expansion which can be explained by the outgrowth of MgB₂ grains was also observed. Two different mechanisms were adopted to explain the pore formation; Kirkendall pores formed at a temperature below the melting point (m.p.) of magnesium by a difference in the diffusivity between magnesium and boron, and the pores formed at a temperature above the m.p. by melting of magnesium and a capillary movement. The density, T_c and J_c results suggest that the current carrying capacity can be improved by a careful control of the process parameters regarding a pore evolution.     

Optical properties of Mg<Subscript>0.05</Subscript>Zn<Subscript>0.95</Subscript>O/SiO<Subscript>2</Subscript> nanocomposite films prepared by sol–gel technique

Mg_0.05Zn_0.95O/SiO₂ nanocomposite films in the molar ratio 25:75 consisting of Mg_0.05Zn_0.95O nanoparticles embedded in a dielectric matrix were prepared by sol–gel technique (spin coating). Optical transmittance, Raman effect and photoluminescence measurements of the composites indicated effective capping of the Mg_0.05Zn_0.95O nanoparticles (radii 1. 61–1.68 nm) in the host showing practically no variation of particle size with the post deposition annealing treatments. The blue shift of the band gap (4.29–4.23 eV) from that of bulk Mg_0.05Zn_0.95O indicated strong carrier confinement for samples annealed at T ≤ 873 K. Highly intense UV emission ( ∼ 4.14 eV) compared to that of defect related emission (2.59 eV) at room temperature was obtained by incorporating 5% Mg in ZnO.This revised version was published online in August 2005 with a corrected issue number.     

Conductance studies in a double-bend quantum structure

Transport phenomena in a double-bend quantum structure fabricated in the two-dimensional electron gas of a modulation doped GaAs/AlGaAs structure, are studied experimentally. The structure consists of an electrostatically defined quantum dot with two one-dimensional wires connected on opposite corners of the dot. The current–voltage characteristics of such devices exhibit quantized conductance breakdown (non-linear behavior), conductance variation with confinement, and non-linear and asymmetric behavior at high bias condition. Low temperature conductance of this structure shows evidence of resonant tunneling, while the peaks of the conductance vary with temperature.     

MoO3/Al2O3催化剂中Mo分散的正电子研究

用浸渍法制备了一系列不同Mo含量的MoO₃/Al₂O₃催化剂.测量了这些样品的正电子湮没寿命谱(PALS)与符合多普勒展宽(CDB)谱,以研究其孔洞结构以及Mo分散.正电子寿命测量结果表明,Al₂O₃载体中存在两种不同尺寸的孔洞.掺入MoO₃之后,Mo原子主要进入Al₂O₃的大孔中,使孔洞体积减小.符合多普勒展宽谱结果表明,当MoO 关键词： 3/Al₂O₃催化剂')" href="#">MoO₃/Al₂O₃催化剂 正电子湮没寿命谱 符合多普勒展宽 Mo 分散     

Proton Diffusion andT1Relaxation in Polyacrylamide Gels: A Unified Approach Using Volume Averaging

The structure of polyacrylamide gels was studied using proton spin–lattice relaxation and PFG diffusion methods. Polyacrylamide gels, with total polymer concentrations ranging from 0.25 to 0.35 g/ml and crosslinker concentrations from 0 to 10% by weight, were studied. The data showed no effect of the crosslinker concentration on the diffusion of water molecules. The Ogston–Morris and Mackie–Meares models fit the general trends observed for water diffusion in gels. The diffusion coefficients from the volume averaging method also fit the data, and this theory was able to account for the effects of water-gel interactions that are not accounted for in the other two theories. The averaging theory also did not require the physically unrealistic assumption, required in the other two theories, that the acrylamide fibers are of similar size to water molecules. Contrary to the diffusion data,T₁relaxation measurements showed a significant effect of crosslinker concentration on the relaxation of water in gels. The model developed using the Bloch equations and the volume averaging method described the effects of water adsorption on the gel medium on both the diffusion coefficients and the relaxation measurements. In the proposed model the gel medium was assumed to consist of three phases (i.e., bulk water, uncrosslinked acrylamide fibers, and a bisacrylamide crosslinker phase). The effects of the crosslinker concentration were accounted for by introducing the proton partition coefficient,K^eq, between the bulk water and crosslinker phase. The derived relaxation equations were successful in fitting the experimental data. The partition coefficient,K^eq, decreased significantly as the crosslinker concentration increased from 5 to 10% by weight. This trend is consistent with the idea that bisacrylamide tends to form hydrophobic regions with increasing crosslinker concentration.     

Heat of adsorption and density distribution in slit pores with defective walls: GCMC simulation studies and comparison with experimental data

The adsorption behavior (capacity, density distribution and packing density) and the isosteric heat versus loading in a slit pore whose walls contain defective graphene layers are investigated in this paper. The defective wall is characterized by the extent and size of the defect. Simulation results obtained with the Grand Canonical Monte Carlo method reveal complex patterns of isosteric heat, and this complex behavior is a result of the interplay between three factors: (i) the surface heterogeneity (solid-fluid interaction, sites with varying degree of affinity), (ii) fluid-fluid interaction and (iii) the overlapping of potentials exerted by the two defective walls. We illustrate this with argon adsorption in pores of various sizes, and results obtained from the simulation agree qualitatively with the experimental data at 77 K on Saran microporous S600H and micro-mesoporous S84 charcoals of Beebe et al. [R.A. Beebe, B. Millard, J. Cynarski, J. Am. Chem. Soc. 75 (1953) 839]. The S600H was found to contain pores predominantly in the neighborhood of 7 Å with 30% of defect and a defective size of 2.84 Å. This is consistent with the argument made by Beebe et al. that this sample is a microporous solid and most pores can accommodate only one layer. The other sample, S84, has larger pores than S600H, and it is found that it has a wider pore size distribution and the pore width is centered at about 12 Å.     

NMR investigations of hexamethyldisilane confined in controlled pore glasses: Pore size distribution and molecular dynamics studies

In this work, the melting-point depression and molecular dynamics of hexamethyldisilane confined within five controlled pore glasses, with mean diameters ranging from 7.9 to 23.9 nm, are studied by high-field (9.4 T) nuclear magnetic resonance (NMR), and the results are discussed with reference to the bulk substance. The melting-point depression in pores with radiusR follows the simplified Gibbs-Thompson equation ΔT=k _p/(R−s) with ak _p value of 74 K · nm and ans value of 1 nm. To our knowledge, this is the first time thek _p value of hexamethyldisilane is reported. Proton spin-lattice relaxation times (T ₁), spin-spin relaxation times (T ₂), and diffusivities (D) are reported as a function of temperature. The confinement in the pores gives rise to substantial changes in the molecular dynamics and the phase behavior. The line-shape measurements reveal a two-phase system assigned to a relatively mobile component at the pore walls and a crystalline solid at the center of the pores. However, theT ₂ measurements show that the mobile phase also embraces a minor component attributed to nonfrozen liquid in pockets or micropores. The diffusivity of the major narrow-line component is approximately three orders of magnitude larger than that in the plastic bulk phase, reflecting fast diffusion of mobile molecules. Below the melting region,T ₁ of the narrow line is significantly shorter thanT ₁ of the broad line, suggesting that the molecular reorientation is more hindered close to the surface than at the center of the pore.