Diffusion Of Gas Molecules Research Articles

Three-Dimensional Gas-Source Localization in Indoor Environment

Gas sensors are used to detect the presence of gaseous chemical species in air. Once the target gas is detected, it is often required to track the gas and locate its source. In principle, the gas source location can be determined by tracking the spatial gas concentration gradient, which can be measured using a spatially aligned array of gas sensors. However, it is not an easy task as it appears to be. Diffusion of gas molecules into air is generally extremely slow. Even in a closed room with no ventilation, there exists convective airflow, and its velocity dominates the molecular diffusion. Therefore, the gas molecules released from their source are carried by airflow and form an aerial trail. Since the gas plume thus formed has a shape elongated in the downwind direction, the gas concentration gradient along the airflow direction is extremely small. Moreover, the airflow in environments of practical interest is almost always turbulent. Since the airflow direction and velocity fluctuate randomly, the gas concentration measurements are unstable.In order to overcome these issues, we have proposed a directional gas-plume detector that consists of an anemometer in addition to an array of gas sensors. Here we report the experimental results on three-dimensional tracking of a gas plume under various conditions in an indoor environment. The main element of the gas-plume detector is an ultrasonic anemometer (Model 81000, Young). It allows accurate measurement of the three-dimensional direction of the airflow for a wide range of velocities ranging from 1 cm/s to 40 m/s. Six metal-oxide gas sensors (TGS2620, Figaro Engineering) are aligned around the anemometer, and are used to estimate the three-dimensional gas concentration gradient vector. The direction of the gas source is obtained by taking a linear combination of the airflow velocity vector and the gas concentration gradient vector. Owing to the elongated shape of the gas plume, the gas concentration gradient vector points not directly to the location of the gas source but rather toward the centerline of the plume. Meanwhile, if the presence of a gas plume is detected by the gas sensors, its source is estimated to be in the upwind direction. Therefore, the airflow velocity vector and the gas concentration gradient vector allow us to track the gas plume in the upwind direction while staying near the plume centerline. In the gas-source localization experiments, a smoke generator (SLN-200, Nissho Electric Works) was used as a gas source. White smoke of glycol mist was released from the smoke generator, and a smoke plume was formed in an experimental room. The smoke is detectable with the gas sensors, and at the same time, allows us to visually check the extension of the plume. The plume detector was fixed on a tripod, and was first placed in the downstream part of the plume. Then, the sensor readings were measured for 60 s, and the detector was manually moved by 20 cm in the estimated direction of the gas source. By repeating this process, the plume detector succeeded in tracking the gas plume three-dimensionally. As the detector approached the gas source, the plume becomes narrower. Therefore, a slight change in the airflow direction causes large fluctuations in the gas concentration at the locations of the gas sensors. As a result, the standard deviations of the gas sensor responses in the 60-s time period increased as the detector approached the gas source. These standard deviations can be used as indices showing the proximity of the gas-plume detector to the gas source.

Synthesis mechanism and gas-sensing application of nanosheet-assembled tungsten oxide microspheres

Nanosheet-assembled tungsten oxide microspheres have been synthesized using rapid sonochemistry followed by thermal treatment. Transient observation of controllable synthesis reveals that the morphological evolution of the product is highly dependent on the ultrasonication time. An assembly mechanism based on oriented attachment and reconstruction is proposed for the sonochemical formation of the nanosheet-assembled microspheres. The obtained samples possess intrinsic non-stoichiometry and a hierarchically porous nano/microstructure, which is beneficial for their utilization in sensing materials and for fast diffusion of gas molecules. The maximum response of the tungsten oxide hierarchical microspheres is 3 times higher than that of commercial nanoparticles for NO2 gas. The gas adsorption–desorption kinetics during the sensing process were mathematically simulated by a derivative method. The first-principles calculation reveals that the NO2 molecule is most likely adsorbed at the terminal O1c site of tungsten oxide, leading to the introduction of new surface states, which are responsible for the intrinsic NO2-sensing properties.

In situ biaxial stretching for the platelet formation of an ethylene–vinyl alcohol copolymer through multistage stretching extrusion and its effect on the gas‐barrier properties of polyethylene

ABSTRACTThe morphological evolution of ethylene–vinyl alcohol copolymer (EVOH) and its effect on the gas‐barrier properties of high‐density polyethylene (HDPE) were investigated. HDPE/EVOH blends were prepared through a multistage stretching extrusion, which combined an assembly of force‐assembling elements (FAEs) with an extruder. Scanning electron microscopy confirmed that with an increasing number of FAEs, the biaxial‐stretching field existing in each FAE transformed the dispersed EVOH phase into well‐defined platelets along the flowing plane. Dynamic rheological results further revealed that the formation of the platelets enlarged the interfaces between the dispersed barrier phase and the matrix; this not only led to the decline of the complex viscosity but also created more tortuous paths for the diffusion of gas molecules. Compared with that of the non‐FAE specimen blended with 25 wt % EVOH, the oxygen permeability coefficient decreased more than one order of magnitude when one FAE was applied. The structural model for permeability indicated that the enhanced barrier resulted from the increased tortuosity of the diffusion pathway, which was provided by the aligned high‐aspect‐ratio platelets. Compared with the previous biaxial‐stretching method, multistage stretching extrusion provided a simple and economical way to generate a laminar structure of the dispersed phase in the matrix phase without the application of an external stretching force. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 40221.

Effects of fullerene derivatives on the gas permeability of thermoplastic polyurethane elastomers

ABSTRACTIn an effort to improve the gas barrier properties of thermoplastic polyurethane (TPU) elastomers, fullerene derivatives were added as fillers, and the resulting O2 and CO2 permeabilities were analyzed. The addition of 5 wt % polyhydroxylated fullerene {fullerenol [C60(OH)n] mixture, where n = 6–12} decreased the gas permeability by approximately 10–20%. According to the hole volumes computed with the results from positron annihilation lifetime spectroscopy, the addition of fullerene derivatives did not produce any changes in the hole volumes of the TPUs. Thus, the reduction in the TPU gas permeability was not caused by changes in the hole volumes. Instead, an inhibited diffusion of gas molecules by fullerene particles was deduced as the cause of the decrease in gas permeability from changes in the diffusion coefficient with temperature. The addition of urethanized fullerene, prepared through the chemical modification of fullerenol, markedly affected the TPU gas barrier properties. As compared to fullerenol addition, the gas barrier properties improved approximately fourfold for O2 and approximately fivefold for CO2. These results suggest that the dispersability of urethanized fullerene in TPU was higher than that of fullerenol. We found that the gas barrier properties were independent of the structure of polyol. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2014, 131, 39986.

Multiphase transformation and hybrid nanostructure under non-equilibrium and equilibrium condition during high-energy ball milling of BaTiO3 powders

BaTiO3 is a well-known technologically important electroceramic material. In the present study high-energy ball-milling processing for producing distortion in the parent thermodynamically stable tetragonal BaTiO3 has been followed. This has produced nano-sized particles as well as a reactive surface. The intention of the present work is to study (i) structural changes on mechanical activation of perovskite BaTiO3 phase and (ii) possible formation of lattice defect as a result of lattice strain generated during milling that can serve peculiar channels of enhanced diffusion of gas molecules at ambient condition. Catalytic activity of nano-sized BaTiO3 has been explained on a result of high structural distortion brought into solid by milling.

COMPUTATIONAL INVESTIGATION OF THE EFFECTS OF SPHERICAL FILLER MORPHOLOGY AND LOADING ON DIFFUSION TORTUOSITY AND RUBBER PERMEABILITY

ABSTRACT Inflation pressure loss in tires degrades performance, raises rolling resistance, and reduces fuel economy. The incorporation of solid fillers, such as carbon black, at relatively high loadings in tire innerliners helps minimize these pressure losses by reducing innerliner permeability due to increases in average gas molecule diffusion path lengths (tortuosity), as well as reductions in diffusion pathway density (capacity). The effects of filler morphology and loading on diffusion path tortuosity can be explored by modeling biased random-walk diffusion through impermeable sphere-filled matrices. Modeled diffusion rate was found to decrease with increased filler loading, reduced filler sphere sizes, increased random-walk step sizes, and the aggregation of filler spheres. Initial correlations with limited empirical permeability measurements are used to validate the model approach.

Enhanced acetone sensing characteristics by decorating Au nanoparticles on ZnO flower-like structures

A well-organized hierarchical structure of ZnO was developed by chemical bath deposition and used as templates for making gold-coated ZnO (Au/ZnO) hybrid nanostructures. The coverage of Au nanoparticles (Au NPs) on ZnO was controlled by changing the amount of the Au precursor. The Au/ZnO hybrids were applied as gas sensing materials to detect acetone. The improved sensor response, selectivity and short response, and recovery time to acetone vapor due to Au NPs on ZnO nanostructures has been observed and explained by considering the formation of Au/ZnO heterostructures, which are favorable for the diffusion of gas molecules. In addition, the dependence of Au amount on gas sensor properties was systematically investigated. ZnO decorated by 6 wt% Au NPs displayed a 9.05-fold enhancement in gas response to 100 ppm of acetone at 280 °C compared to pristine ZnO.

ZnO@ZnS hollow dumbbells–graphene composites as high-performance photocatalysts and alcohol sensors

Novel ZnO@ZnS hollow dumbbells–graphene composites have been successfully fabricated through a polymer-assisted hydrothermal reaction and sulfurization treatment. Because of the formation of heterostructures and a new transfer pathway of electrons from ZnS to graphene, the composites show significantly superior photocatalytic activities than ZnO dumbbells and ZnO@ZnS hollow dumbbells. The improved electric conductivity and the improved adsorption and diffusion of gas molecules also endow them with excellent sensitivity and selectivity toward alcohol gas. This paper presents a promising method to fabricate semiconductor heterostructure–graphene composites for high-efficiency photocatalysts and gas sensors.

Diffusive counter dispersion of mass in bubbly media

We consider a liquid bearing gas bubbles in a porous medium. When gas bubbles are immovably trapped in a porous matrix by surface-tension forces, the dominant mechanism of transfer of gas mass becomes the diffusion of gas molecules through the liquid. Essentially, the gas solution is in local thermodynamic equilibrium with vapor phase all over the system, i.e., the solute concentration equals the solubility. When temperature and/or pressure gradients are applied, diffusion fluxes appear and these fluxes are faithfully determined by the temperature and pressure fields, not by the local solute concentration, which is enslaved by the former. We derive the equations governing such systems, accounting for thermodiffusion and gravitational segregation effects, which are shown not to be neglected for geological systems-marine sediments, terrestrial aquifers, etc. The results are applied for the treatment of non-high-pressure systems and real geological systems bearing methane or carbon dioxide, where we find a potential possibility of the formation of gaseous horizons deep below a porous medium surface. The reported effects are of particular importance for natural methane hydrate deposits and the problem of burial of industrial production of carbon dioxide in deep aquifers.

Preparation of Flower-like SnO2 Nanostructures and Their Applications in Gas-Sensing and Lithium Storage

Hierarchical flower-like SnO2 nanospheres have been prepared by one step hydrothermal reaction with surfactant-free. A possible growth mechanism is proposed. The application of this nanostructure in gas sensor for detecting ethanol reveals that the as-prepared SnO2 nanospheres exhibit a high sensitivity and short response/recovery time. The flower-like SnO2 nanostructures were also used as anode materials for Li-ion battery, whose cyclic voltammetry, charge–discharge galvanstatic behavior and cycle performance were examined. For the sample with 6 h hydrothermal time, a highly stable capacity about 570 mA g–1 after 100 cycles is achieved. The gas sensing properties are mainly determined by the specific surface area, and the 3D nanostructured films composed of flower-like SnO2 nanospheres are favorable for the diffusion of gas molecules. At the same time, the high specific surface area of the flower-like SnO2 nanospheres can provide more electrochemical active sites for Li storage, and the 3D nanostructured films composed of this nanostructure facilitate the diffusion of electrolyte. Furthermore, the short rods on the nanospheres may provide a buffer to alleviate volume change during the alloying and dealloying reactions between Sn and Li. The superior properties achieved here suggest that the as-prepared hierarchical flower-like SnO2 nanostructures have potential application in Li-ion battery and gas sensor.

The Enhancement of Response Speed by Loading the Noble Metal into the Sensing Layer for FET-Type NO2 Sensors

In order to enhance the response speed, noble metal (Ru) loading into the sensing layer for the field effect transistor (FET)-based sensors was investigated. The sensing layer was prepared by heat-treating the mixture powder of and noble metal (Ru)-loaded at . The effects of loading amounts of the noble metal on sensing performances as well as the microstructure of the sensing layer have been examined. It was found that the optimization of the noble metal loading amount into the sensing layer was important for the improvement of 90% response- and recovery-times. Field emission scanning electron microscopy observations revealed that the sensing layer became porous with increasing noble metal loading amounts. However, extensive addition of Ru (above to ) into the sensing layer resulted in the degradation of sensing characteristics, although the sensing layer showed a highly porous structure. According to theoretical analysis based on Knudsen gas diffusion reported by Matsunaga et al. , the diffusion of gas molecules into such a porous sensing layer is very fast. Therefore the obtained results suggested that the improvement of the response- and recovery-speeds is owing to the large contribution of noble metal nanoparticle to the electrochemical promotion rather than the diffusion of gas into the sensing layer.

Analysis of permeability; solubility and diffusivity of carbon dioxide; oxygen; and nitrogen in crystalline and liquid crystalline polymers

In general, permeability and diffusivity in crystalline and liquid crystalline polymers are thought to decrease with increasing crystallinity. However, according to our statistical literature data analysis with 300 of crystalline and liquid crystalline polymers, permeability and diffusivity of carbon dioxide, oxygen, and nitrogen were not significantly affected by crystallinity at the lower crystallinity ranges. Some of them were observed with almost constant or slightly increased values of the permeability and diffusivity at the lower crystallinity range. In contrast, finally, at a higher crystallinity, gas permeability and diffusivity decreased. No increase in the solubility is observed for crystalline and liquid crystalline polymers, regardless of the crystallinity. The crystalline structure could form discontinuously in crystalline and liquid crystalline polymers at a lower crystallinity. The crystal structure gradually grew like a phase transition from the discontinuous domains with increasing crystallinity. At a higher crystallinity, the crystal structure might effectively prevent the diffusion of gas molecules because the crystal structure could be changed from discontinuous to continuous crystal domains after phase transitions. Therefore, gas permeability and diffusivity began to diminish after the phase transition of the crystal structures.

Capillary Filling in Closed End Nanochannels

We investigated the interactions between liquid, gas, and solid phases in the capillary filling process of closed-end nanochannels. This paper presents theoretical models without and with absorption and diffusion of gas molecules in the liquid. Capillary filling experiments were carried out in closed-end silicon nanochannels with different lengths. The theoretical and measured characteristics of filling length versus time are compared. The results show that the filling process consists of two stages. The first stage resembles the capillary filling process in an open-end nanochannel. However, a remarkable discrepancy between the experimental results and the theory without gas absorption is observed in the second stage. A closer investigation of the second stage reveals that the dissolution of gas in the liquid is important and can be explained by the model with gas absorption and diffusion.

A mathematical model of pulmonary gas exchange under inflammatory stress

During a severe local or systemic inflammatory response, immune mediators target lung tissue. This process may lead to acute lung injury and impaired diffusion of gas molecules. Although several mathematical models of gas exchange have been described, none simulate acute lung injury following inflammatory stress. In view of recent laboratory and clinical progress in the understanding of the pathophysiology of acute lung injury, such a mathematical model would be useful. We first derived a partial differential equations model of gas exchange on a small physiological unit of the lung (≈25 alveoli), which we refer to as a respiratory unit (RU). We next developed a simple model of the acute inflammatory response and implemented its effects within a RU, creating a single RU model. Linking multiple RUs with various ventilation/perfusion ratios and taking into account pulmonary venous blood remixing yielded our lung-scale model. Using the lung-scale model, we explored the predicted effects of inflammation on ventilation/perfusion distribution and the resulting pulmonary venous partial pressure oxygen level during systemic inflammatory stresses. This model represents a first step towards the development of anatomically faithful models of gas exchange and ventilation under a broad range of local and systemic inflammatory stimuli resulting in acute lung injury, such as infection and mechanical strain of lung tissue.

Enhancement corrosion resistance of (3-methacryloxypropyl)silsesquioxane hybrid films and its validation by gas-molecule diffusion coefficients using MD simulation

In the first part of this work, based on silsesquioxanes (SSO) derived from hydrolytic condensation of (3-methacryloxypropyl)trimethoxysilane (MPMS) and tetraethoxysilane (TEOS), two hybrid films, f-MPMS-SSO (f-MS) and f-MPMS-TEOS-SSO (f-MTS) modified with 15 wt% TEOS, were prepared. The anticorrosion properties (corrosion potential, E corr, and corrosion current density, I corr) of the bare aluminum alloy (AA) and the two films on AA were tested by electrochemical measurements with typical potentiodynamic polarization curves. The I corr values of three samples are significantly different with the order of f-MTS < f-MS < AA which implies that the TEOS addition in the f-MTS coating indeed enhances the electrochemical corrosion resistance. Correlations between the structures of two films and anticorrosion properties were discussed. In the second part of this work, two different 3D-amorphous cubic unit cells, cell(f-MS) and cell(f-MTS), as models were employed to investigate self-diffusion coefficients by molecular dynamics (MD) simulation for the NO2, SO2 and H2O molecules. All three self-diffusion coefficients of NO2, SO2 and H2O diffusing in cell(f-MTS) were less than the coefficients in cell(f-MS), which validates the corresponding anticorrosion-experiment results. Two reasons why the addition of TEOS in the system of f-MTS leads to a lowering of the gas self-diffusion coefficient compared to the f-MS system, were discussed.

Enhancement Corrosion Resistance of Two (Vinyl)Silsesquioxane Hybrid Films and their Validation by Gas-Molecule Diffusion Coefficients Using Molecular Dynamics Simulation

Based on silsesquioxanes (SSO) derived from hydrolytic condensation of (vinyl)trimethoxysilane (VMS) and tetraethoxysilane (TEOS), two hybrid films, f-VMS-SSO (f-VS) and f-VMS-TEOS-SSO (f-VTS) modified with 15 wt-% TEOS, were prepared. The anticorrosion properties (corrosion potential, Ecorr, and corrosion current density, Icorr) of the bare aluminum alloy (AA) and the two films on AA were tested by electrochemical measurements with typical potentiodynamic polarization curves. The Icorr values of three samples are significantly different in the order of f-VTS&lt;f-VS&lt;AA which implies that the TEOS addition in the f-VTS coating indeed enhances the electrochemical corrosion resistance. Correlations between the structures of two films and anticorrosion properties were discussed. Four different 3D-amorphous cubic unit cells, two cell(f-VS)s and two cell(f-VTS)s, as models were employed to investigate self-diffusion coefficients by molecular dynamics (MD) simulation for the NO2, SO2 and H2O molecules. All three self-diffusion coefficients of NO2, SO2 and H2O in cell(f-VTS) were less than the corresponding values in cell(f-VS), which validates the anticorrosion experiment results. The reasons why the addition of TEOS in the system of f-VTS leads to a lower gas self-diffusion coefficient compared to the f-VS system, were discussed.

Enhancement Corrosion Resistance of (3–Glycidoxylpropyl)- Silsesquioxane Hybrid Films and its Validation by Gas-Molecule Diffusion Coefficients Using MD Simulation

Based on silsesquioxanes (SSO) derived from hydrolytic condensation of (–glycidoxylpropyl) trimethoxysilane (GPMS) and tetraethoxysilane (TEOS), two hybrid films, f-GPMS-TEOS-SSO (f-GTS) modified with 10 wt-% TEOS and f-GTS modified with 50 wt-% DGEBA (f-GTSD), were prepared. The anticorrosion properties (corrosion potential, Ecorr, and corrosion current density, Icorr) of the bare aluminum alloy (AA) and the two films on AA were tested by electrochemical measurements with typical potentiodynamic polarization curves. The Icorr values of three samples were significantly different with the order of f-GTSD&lt;f-GTS&lt;AA， which implies that the DGEBA addition in the f-GTSD coating indeed enhances the electrochemical corrosion resistance. Two different 3D-amorphous cubic unit cells, cell(f-GTS) and cell(f-GTSD), as models were employed to investigate self-diffusion coefficients by MD simulation for the NO2, SO2 and H2O molecules. All the three self-diffusion coefficients of NO2, SO2 and H2O diffusing in cell(f-GTSD) were less than the coefficients in cell(f-GTS), which validates the corresponding anticorrosion-experiment results. Two reasons that the addition of DGEBA in the system of f-GTS lead to the decrease of the gas self-diffusion coefficient compared to the f-GTS system were discussed.

Advantages of Slow High-Vacuum Pumping for Suppressing Excessive Gas Load in Dynamic Evacuation Systems

In dynamic diffusion pump systems, such as vacuum evaporators and sputter coating systems, where the chamber is frequently evacuated from atmospheric pressure to high vacuum, the oil-vapor backstreaming problem sometimes occurs owing to the excessive gas load flowing into the high-vacuum pump just after switching the evacuation mode from low-speed roughing to high-speed, high-vacuum pumping. Two kinds of excessive gas loads exist just after crossover: (1) gas molecules in the vacuum chamber space and (2) temporarily increased outgassing from the chamber wall surface. The outgassing rate from the chamber wall surface becomes very large with the rapid reduction of pressure owing to high-speed, high-vacuum pumping, because the time constant of diffusion of gas molecules in the wall surface is much larger compared with the time constants of pumping down and the resultant reduction of sorption rate of impinging gas molecules. Slow high-vacuum pumping, followed by high-speed pumping, is very effective to suppress the temporarily increased outgassing load and the adverse effect of the space gas load, and to meet the maximum throughput capacity of the diffusion pump. Providing with a low-conductance bypass valve makes it possible to use a small-volume buffer tank and a low-speed rotary pump as a backing pump, leading to a reduction in the cost of high-vacuum evacuation systems.

Molecular Dynamics Simulation of Gas Molecules Dissolved in Lubricant under Shear

Molecular dynamics simulations were conducted in order to investigate the behaviors of dissolved gas molecules in lubricating film under shear. The lubricants were n-hexane and perfluorohexane, and the dissolved components were hydrogen. When the lubricant film was sheared, the dissolved gas molecules moved, and the number of molecules that adsorb on the solid surfaces increased. Shear induced the diffusion of gas molecules in lubricants. The behaviors of the dissolved molecules were affected by a sliding velocity.

Enhancement corrosion resistance of (3-methacryloxypropyl)silsesquioxane hybrid films and its validation by gas-molecule diffusion coefficients using MD simulation

Enhancement corrosion resistance of (3-methacryloxypropyl)silsesquioxane hybrid films and its validation by gas-molecule diffusion coefficients using MD simulation