Trace Amounts Of Water Vapor Research Articles

Experimental study of frosting characteristics of trace water vapor under cryogenic wind tunnel conditions

In cryogenic wind tunnels, the working fluid mixed with trace amounts of water vapor may form frost on the test model, which could affect the testing results. To investigate the frosting phenomenon under trace water condition, an experimental system is built for visualizing the frosting process on a cryogenic surface. The effects of variable factors on frost formation are analyzed focusing on the growth characteristics of frost layer. Under trace water and cryogenic surface conditions, the water vapor desublimates into crystals without obvious condensation. As the surface temperature increases from −183 ℃ to −123 ℃, the frost becomes thicker and more porous due to the thinner outer boundary layer. When the water vapor content rises from 30 ppm to 500 ppm, the frost morphology transforms from a flat layer to frost clumps with visible dendritic structure. Additionally, the decrease in nitrogen flow temperature and the increases in ambient pressure and surface roughness of the cryogenic surface are conducive to the development of the branches on the frost crystal, which accelerate the growth of the frost layer. Furthermore, a dimensionless correlation is proposed for predicting the frost thickness under cryogenic and trace water conditions, with an overall error within ±20 %.

Insights into internal oxidation of SiC/BN/SiC composites

AbstractThe article presents new observations of the physical manifestations of internal oxidation and volatilization in SiC/BN/SiC composites. The observations are made on both unbroken and broken minicomposite specimens before and after 12 h exposures at 1000°C in dry air with 10 ppm water vapor. The observations are enabled by a sample preparation method involving ion‐mill sectioning and polishing. Complementary analyses of volatilization and closure of resulting gaps are also presented. The observations show that BN is generally consumed in two stages: (i) through reaction with oxygen along the interfaces with both the fiber and the matrix, producing two concentric annular pockets of borosilicate glass and an intervening annulus of progressively thinning BN; and (ii) subsequent volatilization, through the reaction of boria with trace amounts of water vapor in the environment to form borohydroxide gases. The spatial extent to which these processes proceed is governed by a competition between the outward diffusion of reaction gases through both matrix cracks and interface gaps produced by boria volatilization, and the formation of oxides on the newly exposed surfaces of fibers, matrix, and coating.

In Situ TEM Studies of the Oxidation of Li Dendrites at High Temperatures

AbstractAlthough lithium (Li) is inert to dry oxygen at room temperature, it reacts with dry oxygen at elevated temperatures, causing thermal runaway and fire hazard in Li metal batteries (LMBs). However, the oxidation mechanism of Li at high temperatures is not explored. Here, real‐time transmission electron microscopy studies of the effect of temperature on the oxidation mechanisms of Li dendrites are reported. The oxidation is controlled by the outward diffusion of Li+ through the oxide layer (the Wagner oxidation mechanism), forming thin films of Li2O comprising nanograins at temperatures between 100 and 140 °C. When the temperature is between 160 and 200 °C, the oxidation product is plate‐like LiOH due to the presence of trace amount of water vapor in the atmosphere. When the temperature is above 300 °C, the oxidation product becomes single crystalline Li2O nanocubes. Density function theory calculations reveal the Li oxidation chemistry is controlled by the thermodynamics and kinetics of the interactions between O2 or H2O and Li2O at high temperatures. These results provide an important understanding of the microscopic oxidation mechanism of Li at elevated temperatures, which sheds lights on the thermal runaway of LMBs.

Ion-driven organic chemistry for Titan-like atmospheres: Implications for N-dominated super-Earth exoplanets

Context. Characterizing temperate (200–1000 K) super-Earth atmospheres is one of the future challenges in exoplanetary science. One of the major difficulties comes from the ubiquity of aerosols in these objects, which complicates the spectroscopic analyses. The knowledge gained on the Solar System is then crucial to better understand the chemical processes of exoplanet atmospheres. Aims. This work focuses on the impact of ion chemistry on molecular diversity in a specific Titan-like exoplanet atmosphere that would be dominated by molecular nitrogen. On the largest satellite of Saturn, Titan, ion chemistry is a major component of molecular growth that forms precursors for the observed photochemical organic hazes. Methods. Based on an experimental approach, we irradiated a gaseous mixture representative of a Titan-like atmosphere (N2-dominated with CH4) using an extreme-uv photon source (16.8 eV). Trace amounts of water vapor were added to the composition of the Titan-type gas mixture to simulate an exoplanet in the habitable zone. Results. A wide variety of molecules and ions have been detected and they cannot all be identified based on our current knowledge of the organic chemistry of planetary atmospheres (mostly N- and C-based chemistry). The presence of even trace amounts of H2O significantly broadens the product distribution, and H3O+ is found to be the most abundant ion. Conclusions. This work demonstrates the complexity of the chemistry within exoplanet atmospheres. Numerical models must consider oxygen chemistry and ion-molecule reactions in order to probe the habitability of a certain type of super-Earths. The abundance of H3O+ makes it a good candidate for future observations.

Enhanced etch rate of deep-UV laser induced etching of diamond in low pressure conditions

Ultraviolet laser-induced etching is a method of machining and nanostructuring diamond surfaces in which carbon is removed from the surface via a photochemical process involving oxygen. We show here that using a dry source of oxygen at pressures in the range of 0.01–1 Torr leads to a 10-fold increase in the etch rate compared to etching in atmospheric air. The enhanced etch rate is also found to be accompanied by a marked change in the nanopatterned surface morphology. We developed a rate equation model for the etch rate that provides good agreement with measurements for pressures up to approximately 0.1 Torr. For higher pressures, the reduced etch rate and departure from the model are attributed to the contamination of the diamond surface by trace amounts of water vapor, introduced as an impurity from the gas sources. The results provide a method for markedly increasing the etch rate, as well as a better understanding of the role of gas impurities on the etch mechanism and emergent nanopattern formation.

Multi-factor study of the effects of a trace amount of water vapor on low concentration CO2 capture by 5A zeolite particles.

Water vapor is ubiquitous and affects the performance of an adsorbent. In this work, a grand-canonical Monte Carlo method (GCMC) combining with dispersion-corrected density functional theory (DC-DFT) calculation is adopted to investigate the effect of a trace amount of water vapor on low concentration CO2 capture in 5A zeolite particles. The force field parameters for the interactions among CO2, water, and 5A zeolite are obtained via DC-DFT calculations. The effects of the charges of water molecules on the CO2 and N2 adsorption amounts and the selectivity of the CO2/(N2 + O2) gas mixture under different trace amounts of water vapor ranging from 0.05 ppm to 5 ppm are studied. The results show that the presence of the water vapor in 5A zeolite particles increases or decreases the CO2 adsorption amount, which is strongly determined by the trace amount of water. Specifically, when the water vapor concentration is less than 0.1 ppm, the CO2 adsorption amount is increased by 0.7–53.4%, whereas when the water vapor concentration is greater than 0.3 ppm, the amount of adsorbed CO2 decreases, with the reduction proportional to the amount of trace water. However, the N2 adsorption amount and the selectivity of the CO2/(N2 + O2) gas mixture decrease with an increasing amount of trace water. This indicates that the electrostatic interactions induced by the water molecules are the dominant factor influencing the CO2 and N2 adsorption amount and the selectivity of the CO2/(N2 + O2) gas mixture. Therefore, to achieve the desired adsorption performance, a trace amount of water vapor (<0.1 ppm) is recommended for CO2 adsorption, whereas low trace amounts of water vapor (<0.1 ppm) are also recommended for the selectivity of the CO2/(N2 + O2) gas mixture in the 5A zeolite particle.

Force-Field Development from Electronic Structure Calculations with Periodic Boundary Conditions: Applications to Gaseous Adsorption and Transport in Metal-Organic Frameworks.

We present a systematic and efficient methodology to derive accurate (nonpolarizable) force fields from periodic density functional theory (DFT) calculations for use in classical molecular simulations. The methodology requires reduced computation cost compared with other conventional ways. Moreover, the whole process is performed self-consistently in a fully periodic system. The force fields derived by using this methodology nicely predict the CO2 and H2O adsorption isotherms inside Mg-MOF-74, and is transferable to Zn-MOF-74; by replacing the Mg-CO2 interactions with the corresponding Zn-CO2 interactions, we obtain an accurate prediction of the corresponding isotherm. We have applied this methodology to address the effect of water on the separation of flue gases in these materials. In general, the mixture isotherms of CO2 and H2O calculated with these derived force fields show a significant reduction in CO2 uptake with the existence of trace amounts of water vapor. The effect of water, however, is found to be quantitatively different between Mg- and Zn-MOF-74.

A review of contamination-resistant antireflective sol–gel coatings

Sol–gel derived silica antireflective (AR) coatings have been widely used as the optical components for high peak power laser systems because of their excellent optical properties and high laser-induced damage thresholds. However, the sol–gel derived coatings have a high surface area that is more susceptible to be contaminated by absorption of trace amounts of water vapor and other volatile organic compounds from the environment. In this paper, the major approaches to fabricate contamination resistant sol–gel derived silica AR coatings have been extensively reviewed. Different approaches, including solution-phase and vapor-phase silanization, ammonia–water vapor treatment and fluorine modification have been discussed. The optical properties and laser-induced damage thresholds of modified coatings have also been evaluated. The improved sol–gel AR coatings have been shown to possess superior contamination resistance to work in vacuum systems compare to the traditional sol–gel AR coatings.

Nanoscale Water Condensation on Click-Functionalized Self-Assembled Monolayers

We have examined the nanoscale adsorption of molecular water under ambient conditions onto a series of well-characterized functionalized surfaces produced by Cu(I)-catalyzed alkyne-azide cycloaddition (CuAAC or "click") reactions on alkyne-terminated self-assembled monolayers on silicon. Water contact angle (CA) measurements reveal a range of macroscopic hydrophilicity that does not correlate with the tendency of these surfaces to adsorb water at the molecular level. X-ray reflectometry has been used to follow the kinetics of water adsorption on these "click"-functionalized surfaces, and also shows that dense continuous molecular water layers are formed over 30 h. For example, a highly hydrophilic surface, functionalized by an oligo(ethylene glycol) moiety (with a CA = 34°) showed 2.9 Å of adsorbed water after 30 h, while the almost hydrophobic underlying alkyne-terminated monolayer (CA = 84°) showed 5.6 Å of adsorbed water over the same period. While this study highlights the capacity of X-ray reflectometry to study the structure of adsorbed water on these surfaces, it should also serve as a warning for those intending to characterize self-assembled monolayers and functionalized surfaces to avoid contamination by even trace amounts of water vapor. Moreover, contact angle measurements alone cannot be relied upon to predict the likely degree of moisture uptake on such surfaces.

Nanoscale condensation of water on self-assembled monolayers

We demonstrate that water is almost universally present on apparently dry self-assembled monolayers, even on those considered almost hydrophobic by conventional methods such as water contact goniometry. The structure and kinetics of nanoscale water adsorption onto these surfaces were investigated using X-ray and neutron reflectometry, as well as atomic force microscopy. Condensation of water on hydrophilic surfaces under ambient conditions formed a dense sub-nanometre surface layer; the thickness of which increased with exponentially limiting kinetics. Tapping mode AFM measurements show the presence of nanosized droplets that covered a small percentage (∼2%) of the total surface area, and which became fewer in number and larger in size with time. While low vacuum pressures (∼10−8 bar) at room temperature did nothing to remove the adsorbed water from these monolayers, heating to temperatures above 65 °C under atmospheric conditions did lead to evaporation from the surface. We demonstrate that water contact angle measurements are not necessarily sensitive to the presence of nanoscale adsorbed water and do not vary with time. For the most part they are a poor indicator of the kinetics and the amount of water condensation onto these surfaces at the molecular level. In summary, this study reveals the need to exclude air containing even trace amounts of water vapor from such surfaces when characterizing using techniques such as X-ray reflectometry.

The promotional effect of NO on N 2O decomposition over the bi-nuclear Fe sites in Fe/ZSM-5

Combining transient response method, steady-state kinetics, in situ IR, and in situ Raman spectroscopic techniques, the effect of NO on N 2O decomposition over Fe/ZSM-5 catalyst was investigated in detail, which reveals that the active sites are bi-nuclear Fe sites. The presence of NO increases the desorption rate of O 2 and lowers the apparent activation energy of N 2O decomposition. Trace amount of water vapor can lead to the deactivation of the Fe/ZSM-5 catalyst due to the oxidation of the active Fe 2+ sites during N 2O decomposition. On the other hand, NO treatment causes the regeneration of the inactive Fe sites by releasing O 2 at relatively low temperature. For the first time, it was found that a Raman band at 743 cm −1, assigned to the bi-nuclear Fe sites, changes accordingly with the deactivation and regeneration of the Fe/ZSM-5 catalyst. The in situ IR spectroscopic study reveals that NO treatment of the deactivated catalyst can remove the hydroxyl groups bound to the inactive Fe 3+ species. These results demonstrate that H 2O and NO play important roles in the structural transformation between the hydroxylated bi-nuclear Fe 3+ sites and the dehydroxylated bi-nuclear Fe 2+ sites in Fe/ZSM-5. Our conclusion is that the positive effect of NO on N 2O decomposition is due to the dehydroxylation of the hydroxylated bi-nuclear Fe sites facilitated by NO, as well as the enhancement of the O 2 desorption rate regulated by NO.

Evidence for the Role of Ions in Aerosol Nucleation

Aerosol nucleation has been studied experimentally in purified, atmospheric air, containing trace amounts of water vapor, ozone, and sulfur dioxide. The results are compared with model calculations. It is found that an increase in ionization by a factor of 10 increases the production rate of stable clusters by a factor of approximately 3, probably due to ion-induced nucleation.

Vertically-aligned carbon nanotubes prepared by water-assisted chemical vapor deposition

Vertically-aligned single walled carbon nanotubes (V-SWNTs) were prepared by water-assisted and ethylene-pyrolyzing chemical vapor deposition. Water vapor was introduced into the growth environment by passing a small fraction of Ar carrier gas through a water bubbler. The effects of processing gas flow rate, growth time, and water vapor content on the growth of SWNTs using Mo/Fe as the catalyst were systematically studied. For the catalyst of SWNTs growth, multi-metal layers consisting of an Al underlayer with 10 nm in thickness and Mo/Fe layers on top were deposited onto the silica film. Evidences of V-SWNTs with high-purity were found by Raman spectroscopy and TEM morphology. The trace amount of water vapor enhanced the activity of metal catalysts to further assist the growth of SWNTs. For the Mo (0.5 nm)/Fe (1 nm) structure, V-SWNTs grew from surface when the growth time lasted 10 min shown in SEM diagram meanwhile the portion of SWNTs increased until 30 min and then a layer of SWNTs developed shown in the RBM peak in the Raman spectra.

Influence of Water Vapor on Photochemical Ozone Generation with Efficient 172 nm Xenon Excimer Lamps

The influence of water vapor on photochemical ozone generation has been investigated. Tests of a coaxial ozone generator driven by an efficient, tubular, 172 nm xenon excimer lamp revealed that ozone saturation concentration strongly depends on moisture concentration in the process gas. In order to adequately model the data, catalytic ozone destruction by OH and HO2 radicals formed by reactions with trace amounts of water vapor in the process gas had to be included in the photochemical ozone production rate equation system. Based on the model, optimized ozone photoreactor designs for ambient air, dry air and dry oxygen are described.

Reducing the Uncertainty of Industrial Trace Humidity Generators through NIST Permeation-Tube Calibration

Permeation-tube moisture generators (PTGs) are commonly used by the semiconductor industry as transfer standards for the calibration of hygrometer systems measuring trace amounts of water vapor in gases (water vapor mole fractions typically below 1 × 10−6). They are relatively simple devices that generate a steady stream of humidified gas by diluting water vapor delivered at a constant rate from a permeable capsule with precisely metered purified gas, usually nitrogen. Here a new calibration service enabling the measurement of PTG permeation rates directly in terms of NIST primary standards of trace humidity generation is described. Rather than using commonly employed gravimetric methods for permeation-tube calibration, the method applied here links the permeation rate of the permeation tube to the thermodynamic properties of ice. Using a hygrometer based on cavity ringdown spectroscopy, we compare the water vapor concentrations produced by the NIST low frost-point generator (LFPG) and a specially constructed PTG containing the permeation tube undergoing calibration. A least squares fit of the data determines the permeation rate of the tube under test. We describe the calibration system, experimental procedure and present sample calibration data. The expanded relative uncertainty of NIST permeation-tube calibrations is 1.8% with a coverage factor k = 2, dominated by the Type A uncertainties.

Direct observation of PFPE lubricant molecules by cryogenic AFM under ultra-high vacuum

Surface lubrication is one of the essential technologies in modern magnetic disk systems and improvement of the surface lubrication is very important in the development of next generation systems. In this study, we used AFM for the direct observation of perfluoropolyether (PFPE) lubricant molecules on atomically flat surfaces. We used a cryogenic non-contact AFM to observe the molecules in a frozen state of micro-Brownian motion of PFPE segments, because the glass transition temperature of PFPE is very low. To avoid freezing a trace amount of water vapor on the sample surface at liquid nitrogen temperatures, the AFM observation was performed under ultra-high vacuum. We observed that on a gold surface the size of the molecules increases with repeated AFM scans. This is because the mechanical stimulus causes the fusion of PFPE lubricant molecules to form reversed micelles at the non-polar surface. At a hydrophilic silicon wafer surface, however, we succeeded in observing single lubricant molecules. This is because almost all PFPE lubricant molecules are fixed to the hydrophilic solid surface by polar–polar bond formation and they cannot move around on the surface and thus they cannot fuse to each other. As formation of the reversed micelle structure is a rather general phenomenon in the PFPE lubricant thin layer at non-polar surfaces, we also will discuss briefly the expected molecular structures of PFPE lubricants at the surface of the carbon overcoat of magnetic disks.

Sorbents for air prepurification in air separation

Air fed to air separation units such as cryogenic distillation columns needs to be prepurified; that is, the concentration levels of air-borne impurities such as water vapor, CO 2, and light hydrocarbons need to be brought down below the tolerable limits. This process is commonly carried out by using adsorptive methods such as pressure swing adsorption (PSA) or temperature swing adsorption (TSA). This work deals with the study of adsorption characteristics of two conventional microporous adsorbents, namely 13X zeolite molecular sieves, and activated γ-Al 2O 3, and three non-conventional adsorbents, namely a natural zeolite (clinoptilolite), and its K +- and Ca 2+-ion exchanged forms. A noteworthy feature of this work is the measurement of adsorption isotherms at very low partial pressures of the adsorbate gas (to a few ppm). The relative merits of these adsorbents for the removal of trace amounts of water vapor, CO 2, and hydrocarbons such as CH 4, C 2H 4, and C 2H 6 are discussed. The isotherm data for 13X zeolite and γ-Al 2O 3 has been fit to the Langmuir–Freundlich, Tòth, and Dubinin–Astakhov (DA, or potential theory) isotherm models. It has been found that the potential theory model is the most suitable one for description of low pressure or concentration data. The origin of the better fit by potential theory is that its corresponding energy distribution function follows a quasi-Gaussian distribution with a broadening at high adsorption energies, and the high-energy sites are important for adsorption at low pressures or concentrations. Finally, the possibility of using H 2O adsorption isotherm to evaluate the pore size distribution by the Horvàth–Kawazoe approach is discussed.

A Numerical Study of Low Temperature Silane Combustion

A mechanism of low temperature silane combustion has been proposed in the present work based on the assumption that a trace amount of water vapor helps the occurrence of spontaneous ignition at room temperature. This assumption has been made based upon the fact that the combustion product of silane influences positively the occurrence of spontaneous ignition. Energetic calculation of the reaction path way for low temperature silane combustion also supports this assumption [Kondo et al., 1999]. A numerical model has been constructed which can interpret the spontaneous ignition limit at room temperature, the ignition delay times, and the second explosion limit of silane mixtures simultaneously.

Gas-Phase Oxidation of 1-Butene Using Nanoscale TiO2 Photocatalysts

The effects of preparation methods, humidity, and calcination temperatures on the behavior of nanoscale TiO2 photocatalysts were investigated in this study. Application of these photocatalysts in the gas-phase decomposition of 1-butene demonstrated that the nanoscale catalysts, prepared by the acid-assisted sol–gel technique, showed higher photocatalytic reactivity than commercially available Degussa P-25 TiO2, while lower photoactivity was obtained on TiO2 catalysts prepared without adding acid to titanium isopropoxide. Our experimental results revealed that the oxidation rates of 1-butene decreased exponentially with increasing water concentrations in the flowing stream. However, a trace amount of water vapor was indispensable in maintaining the stability of the catalysts. Proper calcination temperature (300°C) could promote the resistance of catalysts against the poisoning effects of humidity. Higher calcination temperature (400°C) unfavorably lowered the photoactivity due to phase transformations occurring at such a temperature. Amorphous and rutile-typed TiO2 showed less photocatalytic reactivity. XRD patterns and BET measurements indicated that moderate surface areas (from 100 to 160 m2/g), appropriate crystallite sizes (5∼6 nm), and crystallinity of anatase were beneficial to the photoactivity of TiO2 catalysts. In situ FTIR studies indicated that catalyst surfaces contained large amounts of chemisorbed water and hydroxyl groups, which are considered to be active sites in photocatalytic reactions. The accumulation of carbonate species on active sites resulted in the deactivation of TiO2 catalysts under dry conditions. Quantum size effects were thought to be responsible for the high photoactivity achieved on the nanoscale TiO2 catalysts prepared by sol–gel methods.

Growth of InP by infinite solution LPE

Epitaxial layers of InP were grown by the infinite solution technique. The layers were characterized by Hall-effect measurements, SIMS, photoluminescence measurements, and Auger spectroscopy. Si and S were identified as major background donors. Si predominates at growth temperatures above about 700°C. The concentration was reduced by adding trace amounts of water vapor to the hydrogen ambient and by reducing the growth temperature. Silicon's predominance is consistent with the thermodynamic constraints of Weiner's model describing Si contamination controlled by the quartz-hydrogen reaction. However, the fast response of the solution to variations in the partial pressure of water vapor indicates the importance of a complementary reaction involving the direct conversion of Si in the solution to SiO 2. A study of the electrical, optical, and chemical properties of the material strongly suggests the amphoteric nature of Si in InP.