Adsorption of Hydrogen in Graphitic Slit Pores

Molecular simulations of adsorption isotherms for hydrogen on graphite and graphitic slit pores are presented. The simulations employ the path integral isomorphism of Feynman to rigorously account for the quantum nature of the adsorbate. The isosteric heat of adsorption of para-hydrogen on planar graphite is computed from several different solid–fluid potential models and compared with experiment. The adsorption isotherm for hydrogen on the graphite basal plane was computed at 20 K and compared with experiment. Agreement with experiment is very good. Adsorption isotherms for hydrogen in slit pores of two different pore widths are computed at 20 K and compared with results from classical simulations. Quantum and classical isotherms exhibit qualitatively different behavior.     

Expansion or Contraction of Slit Pores Due to Gas Uptake

Adsorption inside a slit pore of flexible width w is explored, with a focus on how w varies as a function of the external pressure P of a gas in equilibrium with the adsorbate within the pore. The analysis is first carried out in general, using a minimization of the thermodynamic grand potential energy of the system. This leads to an equation predicting both the gas uptake N as a function of chemical potential ?? and the expansion (or contraction) of the pore in response to the adsorbate??s pressure. The resulting equilibrium behavior depends on the elastic parameters of the host material. Explicit results are derived for three adsorption systems: a low density fluid, Ar (a classical fluid at finite temperature T) in a graphite pore and ⁴He within a Au pore at T=0. The resulting behaviors include some situations where the pore expands and others for which it contracts. The difference arises from the sign of the thermodynamic response of the fluid as a function of slit width.     

Effects of Substrate Relaxation on Adsorption in Pores

Fluids in porous media are commonly studied with analytical or simulation methods, usually assuming that the host medium is rigid. By evaluating the substrate’s response (relaxation) to the presence of the fluid we assess the error inherent in that assumption. One application is a determination of the ground state of ³He in slit and cylindrical pores. With the relaxation, there results a much stronger cohesion than would be found for a rigid host. Similar increased binding effects of relaxation are found for classical fluids confined within slit pores or nanotube bundles.     

Fluid Helium in a Narrow Pore at Zero Temperature

The zero temperature adsorption isotherm of ⁴He in a rigid cylindrical pore with alkali metal walls is computed within finite-range density functional theory, with the radius of the tube as a parameter. It is shown that starting from narrow pores and increasing the radius, the adsorbed helium first becomes bound in a quasi-onedimensional phase, and finally condenses into a quasi-twodimensional configuration by going through an almost filling situation. The results are in agreement with recent calculations for carbon nanotubes based on thermodynamical approaches.     

Helium Adsorption on Lithium Substrates

We have developed a cryogenic pulsed laser deposition (PLD) system to deposit lithium films onto a quartz crystal microbalance (QCM) at 4 K. Adsorption isotherms of ⁴He on lithium were measured in the temperature range between 1.42 K and 2.5 K. The isotherms are qualitatively different from isotherms on strong substrates such as gold and weak substrates such as cesium. There is no evidence of the formation of solid-like layers of helium, and the helium coverage is approximately linear in the pressure over a wide range. By measuring the low coverage slope of the isotherms, the binding energy of helium to lithium was found to be approximately −13.6 K. For lithium substrates less than approximately 100 layers thick, the chemical potential at which the superfluid transition was observed was surprisingly sensitive to the details of lithium deposition. This work was supported by NSF grant DMR 0509685.     

Adsorption of Helium in a Deformed Pore

The adsorption potential in the interior of an infinite nanopore along the z-axis, with varying radius R(z), is derived as a function of a deformation parameter. Detailed calculations for a carbon nanotube indicate that according to the magnitude of the deformation the potential landscape can be substantially modified, giving rise to regions of weakened confining strength, and even changing the nature of the minima. This modified field may induce thermodynamic instabilities in the adsorbed fluid. A similar situation takes place for other materials, such as alkaline pores with variable cross-section. A general argument can be provided that explains the origin of such instabilities.     

Adsorption of Helium on Au Nanowires

The growth of helium films on Au nanowires is investigated within finite range density functional theory. It is shown that as in previous studies of helium adsorption on spherical surfaces, submonolayer condensation and stable layering transitions take place, and that a growth instability appears above a finite film thickness. The competition between unstable film growth on a single tube and stable capillary condensation in an array of nanowires is examined in a simplified mean field-like model. Results are displayed to illustrate adsorption and its stable and unstable patterns outside solid nanowires, cavities in bulk gold and in the gap between a cavity and an inner concentric cylinder.     

The Production of Narrow Slit Images by a Scanning Technique

Transparent slits 5 cms. long, approximately 5μ wide and straight to within l line width were required. Lenses having the requisite resolving power do not have the necessary covering power. This paper describes a scanning microphotographic technique for greatly extending the length of line covered whilst retaining the edge delineation characteristics of microscope objectives. The design and construction of the apparatus is described, problems involved are discussed, and results shown.     

Structure and Energetics of Helium Adsorption on Nanosurfaces

The ground and excited state properties of small helium clusters, ⁴He _N , containing nanoscale (～3–10 Å) planar aromatic molecules have been studied with quantum Monte Carlo methods. Ground state structures and energies are obtained from importance-sampled, rigid-body diffusion Monte Carlo. Excited state energies due to helium vibrational motion are evaluated using the projection operator, imaginary time spectral evolution technique. We examine the adsorption of N helium atoms (N≤24) on a series of planar aromatic molecules (benzene, naphthalene, anthracene, tetracene, phthalocyanine). The first layer of helium atoms is well-localized on the molecule surface, and we find well-defined localized excitations due to in-plane vibrational motion of helium on the molecule surface. We discuss the implications of these confined excitations for the molecule spectroscopy.     

X-ray Scattering Measurements of Helium Adsorption into Silica Aerogel

We have performed simultaneous adsorption isotherm and small angle x-ray scattering (SAXS) measurements of ⁴ He adsorption into 98% porous aerogel at 3.5K. Measurements indicate a two phase coexistence at vapor pressures slightly below the bulk liquid-vapor equilibrium point. SAXS data are interpreted according to a model which consists of the coexistence of a film phase where the aerogel strands are coated with a liquid film and a filled-pore phase were the pores defined by neighboring strands are filled with liquid.     

Finite Size Effects in Adsorption of Helium Mixtures by Alkali Substrates

We investigate the behavior of mixed ³He–⁴He droplets on alkali surfaces at zero temperature, within the frame of Finite Range Density Functional theory. The properties of one single ³He atom on $^4 He_{N_4 }$ droplets on different alkali surfaces are addressed, and the energetics and structure of $^4 He_{N_4 } + ^3 He_{N_3 }$ systems on Cs surfaces, for nanoscopic ⁴He drops, are analyzed through the solutions of the mean field equations for varying number N ₃ of ³He atoms. We discuss the size effects on the single particle spectrum of ³He atoms and on the shapes of both helium distributions.     

A Temperature-Controlled Device for Volumetric Measurements of Helium Adsorption in Porous Media

We describe a set-up for studying adsorption of helium in silica aerogels, where the adsorbed amount is easily and precisely controlled by varying the temperature of a gas reservoir between 80 K and 180 K. We present validation experiments and a first application to aerogels. This device is well adapted to study hysteresis, relaxation, and metastable states in the adsorption and desorption of fluids in porous media.     

Adsorption of Helium and Other Gases to Carbon Nanotubes and Nanotube Bundles

There is considerable interest in the adsorption of helium and other gases to unconventional substrates. Carbon nanotubes and bundles of such tubes provide a relatively unique and interesting substrate opportunity. We describe recent experimental and theoretical work from a number of groups that sheds light on the behavior of various gases adsorbed to these unusual substrates.     

Adsorption of Thin Liquid Helium Films on Cs Measured via Photoelectron Tunneling

The alkali metals Cs and Rb are the only surfaces which are not wetted by superfluid ⁴ He below a certain temperature. In our experiments, using the photoelectron tunneling method, we can highly resolve the growth of the non-wetting thin-film state of ⁴ He on a quench-condensed Cs surface. It turns out that far from coexistence there is little adsorption of helium. In contrast, close to coexistence a rapid growth up to two monolayers of helium is observed, but the surface is still non-wet under the usual convention.     

氦制冷机和液化器中的纯化系统

主要介绍了氦制冷机和氦液化器中应用的几种典型的氦纯化器的工作原理，主要工作程序和优缺点，并介绍了讨论了几种氦纯化器的设计方案，吸附量和计算以及可用于氦纯化器的工程设计。     

Low Temperature Adsorption of Helium on Quench-Condensed Solid Neon Probed by Electron Emission from the Solid

Cyclotron resonance of free electrons (ECR) emitted from the solid Ne under VUV irradiation is observed. A flow of He gas supplied to the substrate avoiding a gas discharge zone is found to decrease the ECR amplitude. The decrease has been attributed to different mechanisms at the sample temperatures of 4.2 K and 1.6 K. He monolayer formation on the Ne surface is suggested in the low temperature region.      

Get 210-2019: State Primary Standard of Units of Specific Adsorption of Gases,Specific Surface Area of Pores,Specific Volume of Pores,Dimension of Pores,Open Porosity,and Coefficient of Gas Permeability of Solid Substances and Fabricated Materials

Measurement Techniques - Methods and means of measurement of the characteristics of the porosity and permeability of solid substances and fabricated materials that are incorporated into GET...