Peculiarities of gallium crystallization in confined geometry

The freezing and melting phase transitions for gallium embedded into a porous glass with a pore size of about 8 nm were studied using acoustic, NMR, and x-ray techniques. It was shown that the broadened solidification and melting transitions upon deep cooling up to complete freezing at 165 K were due to the formation of β-Ga within pores. The offset of confined β-Ga melting was lowered by about 21 K compared to the bulk β-Ga melting point. Both melting and freezing in pores were irreversible. The fulfillment of some special thermal conditions led to gallium crystallization into other modifications. The role of heterogeneous crystallization in freezing of confined gallium is discussed.     

Monodisperse spherical meso–macroporous silica particles: Synthesis and adsorption of biological macromolecules

Monodispersed spherical silica particles, including large mesopores (over 10 nm) and macropores (up to 100 nm) were obtained by chemical etching in an autoclave. A method for introducing globular protein myoglobin molecules into the pores is developed. The method of filling is based on a high adsorption capacity of the developed internal pore structure of the particles. The structure and adsorption properties of the materials are studied.     

Melting of Naphthalene Confined in Mesoporous Silica MCM-41

The ²H nuclear magnetic resonance (NMR) solid-echo spectra of naphthalene molecules as guests in the mesopores of neat MCM-41 with a pore width of 3.3 nm were measured in the temperature regime from 180 to 250 K. A strong reduction of the melting point of the naphthalene molecules by 152 K is observed. The line shape changes in the melting region were simulated with two different models, namely, the model of a narrow distribution of activation energies, which is typical for a crystal-like phase, and a two-phase model. Both models indicate a relatively narrow distribution of melting points of the naphthalene molecules inside the pores, indicative of a rather well-defined structure of the naphthalene molecules inside the pores. This finding supports the proposal of a plastic crystalline phase previously proposed by other groups.     

Effects of CO2 activation on porous structures of coconut shell-based activated carbons

In this paper, textural characterization of an activated carbon derived from carbonized coconut shell char obtained at carbonization temperature of 600 °C for 2 h by CO₂ activation was investigated. The effects of activation temperature, activation time and flow rate of CO₂ on the BET surface area, total volume, micropore volume and yield of activated carbons prepared were evaluated systematically. The results showed that: (i) enhancing activation temperature was favorable to the formation of pores, widening of pores and an increase in mesopores; (ii) increasing activation time was favorable to the formation of micropores and mesopores, and longer activation time would result in collapsing of pores; (iii) increasing flow rate of CO₂ was favorable to the reactions of all active sites and formation of pores, further increasing flow rate of CO₂ would lead carbon to burn out and was unfavorable to the formation of pores. The degree of surface roughness of activated carbon prepared was measured by the fractal dimension which was calculated by FHH (Frenkel-Halsey-Hill) theory. The fractal dimensions of activated carbons prepared were greater than 2.6, indicating the activated carbon samples prepared had very irregular structures, and agreed well with those of average micropore size.     

Effect of activation on the porous structure and the strain and strength properties of beech wood biocarbon

The effect of activation on the size, specific volume, and surface area of pores in a monolithic biomorphic material obtained by carbonization of beech wood is studied. It is shown that under optimal activation mode with a steam heated to 970°C, the total pore volume and surface, determined by adsorption curves, increased by 20 and 18 times, respectively. With the use of high-precision interferometric procedure, strain curves are obtained under uniaxial compression with a stepwise loading, and the strain rate is measured with a step of moving of 325 nm for activated and nonactivated samples. Despite an increase in porosity, the strength and maximum deformation of the samples do not decrease. The behavior of the strain rate jumps is analyzed in the micro- and nanometer range. It is shown that the maximum size of the micrometer jumps (4 μm) correlates well with the average size of the possible strain area in the samples (the average distance between the pores of small size), and the minimum dimensions of the strain jumps are close to the size of mesopores. Assessment of the strain change and its rate upon activation indicates that the effect of activation on the strain and strength characteristics is defined by nanometer defects, the most likely of which are microand mesopores.     

Photonic crystals and glasses from monodisperse spherical mesoporous silica particles filled with nickel

Films of photonic crystals and photonic glasses have been grown by the methods of vertical deposition and sedimentation from a suspension of monodisperse spherical mesoporous silica particles (MSMSP). A method of filling MSMSP pores with nickel, which is based on the adsorption and capillary phenomena in mesopores, has been developed. The composition and structure of the obtained materials have been studied.     

Peculiarities of melting and crystallization of n-decane in a porous glass

Melting and crystallization of n-decane embedded into porous glass with the mean pore size of about 6.4 nm were studied using acoustic and DSC methods. Smearing of the phase transitions, decrease of melting and freezing temperatures, pronounced hysteresis between melting and crystallization were revealed by both methods. In DSC measurements for the pore filling factors 70% and higher double peaks were observed upon cooling while only single peaks were present upon heating. Also a high reduction of the corresponding phase transition heats was revealed. Melting and freezing intervals determined by acoustic and DSC methods strongly differed from each other. A model which qualitatively explains the observed anomalies is proposed. It supposes the formation of liquid layers on the surface of the pores.     

High-pressure differential thermal analysis measurements of the melting curve of lithium

The pressure effect on the melting behavior of lithium has been measured by observing the latent heat signatures of melting and freezing using differential thermal analysis (DTA). Samples were repeatedly melted and recrystallized at selected pressures up to 15 GPa in a multianvil press. Despite the weak DTA signals due to small sample sizes at high pressure, the melting and freezing temperatures were clearly determined from the derivatives of the DTA traces. We measured a drop in the melting temperature between 9 and 12 GPa, yielding a maximum at 10(2) GPa and 245(2) ^°C. This work highlights both the successes and failures of recent theoretical models for the melting behavior of this and related systems.     

Multidimensionally resolved pore size distributions

A novel method of determining median pore size and pore size distributions as a function of spatial position inside a porous sample is described. Pore sizes have been measured with 1-, 2- and 3-dimensional spatial resolution, using NMR cryoporometry in conjunction with magnetic resonance imaging techniques. The method is suitable for pore diameters in the range of 30 Å to over 2000 Å pore diameter, and is based on the technique of freezing a liquid in the pores and measuring the melting temperature by nuclear magnetic resonance. Since the melting point is depressed for crystals of small size, the melting point depression gives a measurement of pore size.     

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.     

The structure of n-alkane binary mixtures adsorbed on graphite

The thermodynamics and structure of the surface adsorbed phase in binary C15-C16 and C15-C17 n-alkane mixtures confined in graphite pores have been studied by differential scanning calorimetry and small-angle X-ray scattering. The previously observed selective adsorption of the longer alkane for chain length differences greater than five carbon atoms is verified but reduced for chain length differences less than or equal to two. With a difference in chain length of one carbon atom, Vegard's law is followed for the melting points of the adsorbed mixture and the (0 2) d-spacing is a continuous function of the mole fraction x. With a two-carbon atom difference, samples aged for 1 week have a lamellar structure for which the entities A_1−xB_x try to be commensurate with the substrate. The same samples aged for 1 month show a continuous parabolic x-dependence for both the melting points and the d-spacings. An explanation in terms of selective probability of adsorption is proposed based on crystallographic considerations.     

Molecular dynamics simulation of thermodynamical properties of copper clusters

The melting and freezing processes of Cu_N (N=180, 256, 360, 408, 500, 628 and 736) nanoclusters are simulated by using micro-canonical molecular dynamics simulation technique. The potential energies and the heat capacities as a function of temperature are obtained. The results reveal that the melting and freezing points increase almost linearly with the atom number in the cluster increasing. All copper nanoclusters have negative heat capacity around the melting and freezing points, and hysteresis effect in the melting/freezing transition is derived in Cu_N nanoclusters for the first time.     

Gas adsorption in mcm-41 porous silicas dynamic measurements using SANS

The isothermal gas adsorption of two hexane isomers (n-hexane and cyclohexane) in the mesopores of MCM-41 silica have been investigated by small angle neutron scattering (SANS). This has been achieved by performing SANS measurements under contrast matching conditions for the silica matrix and condensed hydrocarbon confined in the mesopores. Insight into the kinetics of adsorption have been derived from changes in the intensity of the (100) diffraction peak associated with the ordered hexagonal mesoporous structure of the MCM-41 silica.     

Studies of intrawall porosity in the hexagonally ordered mesostructures of SBA-15 by small angle X-ray scattering and nitrogen adsorption

Ordered mesoporous silicas (OMSs) such as SBA-15 (p6mm symmetry group) synthesized in the presence of block copolymers containing poly(ethylene oxide) blocks possess irregular complementary pores in the walls of ordered mesopores. The X-ray scattering caused by this complementary porosity contributes to the background of the SAXS patterns. This work shows the possibility of using the SAXS data for the study of intrawall channels interconnecting ordered cylinders in SBA-15. The proposed SAXS analysis was tested by using a series of SBA-15 samples obtained at different temperatures of hydrothermal treatment (from 60 to 180 °C). The structural modelling of the SAXS patterns recorded for a series of SBA-15 samples was performed by using the continuous density function (CDF) technique in combination with the derivative difference minimization (DDM) method of full-profile refinement. This method is well suited for extraction of the background curves from the SAXS patterns. The resulting smooth background curves were analyzed by the well-known method in the SAXS theory used for evaluation of heterogeneity distributions, which in this case characterize the intrawall complementary porosity. A relatively good agreement has been observed between the data obtained by SAXS and nitrogen adsorption analysis. The SAXS analysis is sufficiently sensitive for examination of heterogeneous microporosity in SBA-15 materials. The average diameter of intrawall pores for the SBA-15 sample obtained at 60 °C was only about 1.4 nm. However, this diameter increased with the increasing temperature of hydrothermal treatment; namely, it was 1.5, 1.8, 2.6, 2.6, 3.5 and 5.2 nm for the SBA-15 samples hydrothermally treated at 80, 100, 120, 140, 160 and 180 °C, respectively.     

铜原子纳米团簇热力学性质的分子动力学模拟研究

利用分子动力学模拟方法,研究了CuN(N=80,140,216,312,408,500,628和736)纳米团簇在热化和冷凝过程中结构和热力学性质的变化,模型采用的是Johnson的EAM作用势.模拟结果表明:铜团簇的熔点和凝固点随其尺寸线性增加,并逐渐向大块晶体靠拢;所有团簇的凝固点都低于熔点,出现凝固过程中的滞后现象;在熔点和凝固点附近,团簇都具有负热容特性,负热容是由相变前后团簇内部结构突变引起的.     

Phase transitions of fluids confined in porous silicon: A differential calorimetry investigation

The phase transitions of non-polar organic fluids and of water, confined in the pores of porous silicon samples, were investigated by Differential Scanning Calorimetry (DSC). Two types of PS samples (p^- and p⁺ type) with different pore size and morphology were used (with spherical pores with a radius of about 1.5 nm and cylindrical shape with a radius of about 4 nm respectively). The DSC results clearly show that the smaller the pores are, the larger is the decrease in the transition temperature. Moreover, a larger hysteresis between melting and freezing is observed for p⁺ type than for p^- type samples. A critical review of the thermodynamical properties of small particles and confined fluids is presented and used to interpret and discuss our DSC results. The effects of the chemical dissolution as well as the influence of anodization time are presented, showing that thick p⁺ type porous silicon layers are non-homogeneous. The DSC technique which was used for the first time to investigate fluids confined in porous silicon, enables us to deduce original information, such as the pore size distribution, the decrease in the freezing temperature of confined water, and the thickness of non-freezing liquid layer at the pore wall surface. Received: 11 May 1998 / Revised and Accepted: 29 July 1998     

Clathrate formation and dissociation in vapor/water/ice/hydrate systems in SBA-15, sol-gel and CPG porous media, as probed by NMR relaxation, novel protocol NMR cryoporometry, neutron scattering and ab initio quantum-mechanical molecular dynamics simulation

The Gibbs-Thomson effect modifies the pressure and temperature at which clathrates occur, hence altering the depth at which they occur in the seabed. Nuclear magnetic resonance (NMR) measurements as a function of temperature are being conducted for water/ice/hydrate systems in a range of pore geometries, including templated SBA-15 silicas, controlled pore glasses and sol-gel silicas. Rotator-phase plastic ice is shown to be present in confined geometry, and bulk tetrahydrofuran hydrate is also shown to probably have a rotator phase. A novel NMR cryoporometry protocol, which probes both melting and freezing events while avoiding the usual problem of supercooling for the freezing event, has been developed. This enables a detailed probing of the system for a given pore size and geometry and the exploration of differences between hydrate formation and dissociation processes inside pores. These process differences have an important effect on the environment, as they impact on the ability of a marine hydrate system to re-form once warmed above a critical temperature. Ab initio quantum-mechanical molecular dynamics calculations are also being employed to probe the dynamics of liquids in pores at nanometric dimensions.     

Unusual freezing and melting of gallium encapsulated in carbon nanotubes

The freezing and melting behavior of gallium (Ga) encapsulated in carbon nanotubes was investigated through in situ observation in a transmission electron microscope. It is shown that Ga remains liquid up to -80 degrees C when encapsulated in carbon nanotubes. Results of detailed electron diffraction analysis show that the encapsulated Ga can crystallize in either beta phase or gamma phase rather than the common alpha phase upon freezing. Both beta-Ga and gamma-Ga melt at around -20 degrees C. While this is very close to the melting point of bulk beta-Ga (-16 degrees C), it is considerably higher than that of bulk gamma-Ga (-35.6 degrees C). It was observed that upon solidification, Ga has its unique crystallographic orientation relative to the host carbon nanotube.     

Entropic effects in diffusion-adsorption processes in micropores

We analyze the kinetics of diffusion-adsorption processes of one component systems in micropores. In realistic situations, the pores exhibit irregular forms which give rise to inhomogeneous adsorption with preferred sites for the adsorbing particles. By modeling the tortuosity of the pores by means of entropic barriers, we obtain a kinetic equation for the averaged concentration of particles along the pore and on its surface. The analysis performed yields expressions for the adsorption rate, the effective diffusion coeffcient, the adsorption isotherms and the concentration of the adsorbed particles. It is shown that this last quantity strongly depends on the form of the pore. This feature opens the possibility to design micropores with an optimal adsorption rate at selected sites. Our results show that to consider the geometry of the pore in the reaction-diffusion scheme is crucial to reproduce experimental observations of the concentration of adsorbed particles in micropores.     

Surface studies of novel hydrophobic active carbons

The efficient adsorption of toxic organic species from humid airstreams by active carbons is impeded by the competitive adsorption of water vapour which, at low values of p/p^s, occurs at specific (polar) adsorption sites located at the edges of the carbon layer-planes and at in-plane defects. At higher pressures, adsorption in micropores and mesopores also occurs. The concentration of polar adsorption sites therefore determines the hydrophilicity of the carbon structure and their accelerated formation, by exposure to air and water vapour, is also responsible for the ‘ageing’ of active carbons. Overall, the adsorption of water reduces the volume of porosity available for the adsorption of target species and the hydrophilic nature of active carbons is recognized as a major barrier to their effective use in many applications.We present here results for the adsorption of nitrogen, organic and water vapours by a hydrophobic respirator granular active carbon produced by the thermal treatment of a base carbon, to desorb polar oxygen groups, followed by use of a plasma enhanced chemical vapour deposition (PECVD) treatment to apply a hydrophobic, fluorine containing, surface nanolayer. We show that at equivalent %RH values the treated carbon adsorbs significantly less water compared to an untreated (control) carbon and that the treatment does not reduce the levels of open porosity or impede the adsorption of a range of organic vapours at ambient temperatures. Preliminary evidence for the presence, after treatment, of constrictions at pore entrances which act as molecular gates is also presented. The treated carbon (after ageing for 6 weeks at 80%RH) is shown to have greater adsorptivity than an untreated base carbon toward hexane present in a humid (80%RH) airstream. This results in a 39% increase in break-through time. These hydrophobic properties persist one year after manufacture. The mechanism leading to the modified water adsorption properties is the partial desorption of polar oxygen sites followed by deposition at the external carbon surfaces of hydrophobic plasma polymer species. This reduces the polar surface free energy of the carbon and hence the amount of water adsorption occurring by the primary mechanism. This in turn retards the diffusion of water molecules into the micropores and leads to lower adsorption volumes at higher pressures.