Hydrogen Gas Concentration Research Articles

Changes in Work Function and Electrical Resistance of Pt Thin Films in the Presence of Hydrogen Gas

The changes in the electrical properties, such as work function and resistance, of Pt thin films in the presence of hydrogen gas were studied. They were simultaneously measured with a flow-through cell at different concentrations of hydrogen gas in atmosphere containing gaseous nitrogen and that containing air. The resistance was measured by a four-terminal sensing method and the relative work function changes were measured using a field effect transistor. In both atmospheres, the resistance decreased as the concentration of hydrogen gas increased. This result was repeatable only in air because of the differences in the dynamic mechanism of increased density of electrical carriers inside the Pt film as a result of diffused H atoms. In the nitrogen atmosphere, the diffused H atoms were not easily released because of the surface barrier. On the other hand, oxygen gas reacted with H atoms at the surface and this reaction accelerated atom release into air. The work function showed repeatable responses in both atmospheres, but the response characteristics were different. The equilibrium reaction between the adsorption and desorption of hydrogen occurred at the surface in the nitrogen atmosphere, whereas the equilibrium reaction of hydrogen and oxygen to form water molecules occurred in air. The changes in work function and resistance in the presence of hydrogen were due to changes in dissociated hydrogen intensity in the bulk, as well as to the surface reactions.

Growth Kinetics of Vertically Aligned Carbon Nanotube Arrays in Clean Oxygen-free Conditions

Vertically aligned carbon nanotubes (CNTs) are an important technological system, as well as a fascinating system for studying basic principles of nanomaterials synthesis; yet despite continuing efforts for the past decade many important questions about this process remain largely unexplained. We present a series of parametric ethylene chemical vapor deposition growth studies in a "hot-wall" reactor using ultrapure process gases that reveal the fundamental kinetics of the CNT growth. Our data show that the growth rate is proportional to the concentration of the carbon feedstock and monotonically decreases with the concentration of hydrogen gas and that the most important parameter determining the rate of the CNT growth is the production rate of active carbon precursor in the gas phase reaction. The growth termination times obtained with the purified gas mixtures were strikingly insensitive to variations in both hydrogen and ethylene pressures ruling out the carbon encapsulation of the catalyst as the main process termination cause.

Synthesis and oxidation of luminescent silicon nanocrystals from silicon tetrachloride byvery high frequency nonthermal plasma

Silicon nanocrystals have recently attracted significant attention for applications inelectronics, optoelectronics, and biological imaging due to their size-dependent optical andelectronic properties. Here a method for synthesizing luminescent silicon nanocrystals fromsilicon tetrachloride with a nonthermal plasma is described. Silicon nanocrystals with meandiameters of 3–15 nm are synthesized and have a narrow size distribution with thestandard deviation being less than 20% of the mean size. Control over crystallinity isachieved for plasma pressures of 1–12 Torr and hydrogen gas concentrations of5–70% through adjustment of the plasma power. The size of nanocrystals, andresulting optical properties, is mainly dependent on the gas residence time in theplasma region. Additionally the surface of the nanocrystals is covered by bothhydrogen and chlorine. Oxidation of the nanocrystals, which is found to follow theCabrera–Mott mechanism under ambient conditions, is significantly faster than hydrogenterminated silicon due to partial termination of the nanocrystal surface by chlorine.

Silicon epitaxial growth process using trichlorosilane gas in a single-wafer high-speed substrate rotation reactor

A silicon epitaxial growth process in a trichlorosilane–hydrogen system using a single-wafer high-speed substrate rotation reactor was studied by means of experiments and numerical calculations taking into account the transport phenomena and surface chemical reactions. Hydrogen chloride gas produced at the substrate surface is effectively replaced with the source gases of trichlorosilane and hydrogen, due to the effective mass transport provided by high-speed substrate rotation. Because the intermediate species produced due to chemisorption of trichlorosilane fully occupies the silicon surface, the rate of its decomposition by hydrogen gas governs the entire silicon epitaxial growth rate. Thus, the epitaxial growth rate in this reactor simply depends on the hydrogen gas concentration at the silicon substrate surface.

Design and Fabrication of MOSFET Type Hydrogen Gas Sensor Using MEMS Process

In this study, MOSFET type micro hydrogen gas sensors with platinum catalytic metal gates were designed, fabricated, and their electrical characteristics were analyzed. The devised MOSFET Hydrogen Sensors, called MHS-1 and -2, were designed with a platinum gate for hydrogen gas adsorption, and an additional sensing part for higher gas sensitivity and with a micro heater for operation temperature control. In the electrical characterization of the fabricated Pt-gate MOSFET (MHS-1), the saturated drain current was 3.07 mA at 3.0 V of gate voltage, which value in calculation was most similar to measurement data. The amount of threshold voltage shift and saturated drain current increase to variation of hydrogen gas concentration were calculated and the hydrogen gas sensing properties were anticipated and analyzed. (Received October 14, 2010)

Hydrogen Gas Sensor Based on Highly Ordered Polyaniline/Multiwall Carbon Nanotubes Composite

In this work, the structural and gas sensing properties of an electropolymerized, polyaniline (PANI)/multiwall carbon nanotube (MWNT) composite based surface acoustic wave (SAW) sensor are reported. Thin films made of PANI nanofibers were deposited onto 36 lithium tantalate (LiTaO3) SAW transducers using electropolymerization and were subsequently dedoped. Scanning electron microscopy (SEM) revealed the compact growth of the composites which is much denser than that of PANI nanofibers. The PANI/MWNT composite based SAW sensor was then exposed to different concentrations of hydrogen (H2) gas at room temperature with a demonstrated electrical response.

Voltammetric Determination of Both Concentration and Diffusion Coefficient by Combinational Use of Regular and Microelectrodes

AbstractTwo Pt disks with 1.6 and 0.1 mm diameter, whose sizes were accurately evaluated, were applied to voltammetric diffusion‐controlled currents to determine the concentrations of redox species. Since voltammetric peak currents at a regular electrode, Ip, and limiting currents at a microelectrode, IL, are proportional to D1/2 and D, respectively, the ratio Ip2/IL is independent of the diffusion coefficient but dependents on the concentration. We used this fact to determine the concentrations of ferrocenyl derivatives, hemin, hexacyanoferrate(II), dioxygen gas and hydrogen gas. New insights are not only the absolute determination of concentrations but also the sensitive detection of adsorption of electrochemical products which blocked the voltammetric currents.

Simulating the impact of suppression of methanogenesis in continuous flow biohydrogen reactors

A calibrated model of the patent-pending integrated biohydrogen reactor clarifier system (IBRCS) using BioWin was used to evaluate the impact of sludge and/or feedstock pre-treatment for methanogens inhibition in a dynamic simulation for 90 days with and without methanogens suppression. Dynamic simulations at four different OLRs ranging from 6.5 to 103 gCOD/L-d have shown that without any pre-treatment, the system was capable of washing out methanogens and enriching hydrogen producers. The average methane gas content in the reactor’s headspace was 4% after 7 days of continuous operation, decreasing sharply to less than 0.5%, while the maximum reduction in hydrogen gas was <10%. The hydrogen gas content in the headspace ranged from 65% to 72%. Simulating the impact of extended SRT ranging from 3 days to 20 days on the performance of the IBRCS revealed that up to an SRT of 5 days hydrogen production was predominant with a reasonable deterioration in the production rate by 20%. Biomass distribution showed that at SRTs up to 20 days, acetoclastic methanogens were naturally eliminated. However, hydrogenotrophic methanogens had a significant impact on the overall hydrogen production rate where most of the hydrogen gas produced was consumed at SRTs of 10 days and 20 days.

A Room Temperature Conductometric Hydrogen Sensor with Polyaniline/WO3 Nanocomposite

Polyaniline/W03 nanocomposite with high volume of W03 compared to polyaniline has been synthesized through a template-free synthetic route. Characterization of the developed nanomaterial was conducted through Scanning Electron Microscopy (SEM), X-Ray Diffraction (XRD), and Fourier Transform Infrared Spectroscopy (FT-IR). A conductometric transducer was fabricated and used to measure the resistance of polyaniline/W03 nanofiber films during interaction with different concentrations of hydrogen gas at room temperature. The concentration of aniline monomer to W03 was 1 to 8 on the solution. It was observed that the sensor has a stable baseline of 33.2 KO under synthetic air. A decrease in the sensitive film resistance of i .4 K!l was noticed during the Interaction with 1% H2 with 90% response time of 60 seconds. Due to the high operating temperature of ali metal oxides based gas sensors, this sensor offers low power consumption because of its room temperature operation.

Polyaniline/MWCNT Nanocomposite Based Hydrogen Sensor Operating at Room Temperature

A nanocornposite of polyaniline (PANI) and multiwall carbon nanotubes (MWCNT) has been synthesized and investigated as a sensing material for hydrogen gas at room temperature. The PANI/MWCNT nanocomposite was prepared by in situ chemical polymerization. The nanomaterial characterization was conducted through Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM). From these data it was concluded that the PANI formed conformal layer with thickness of approximately 7 nm aligned to the outer layer of the MWCNT. A hydrogen sensor was developed by airbrushing the solution over the surface of heated conductometric transducer. The sensor was tested towards different concentrations of hydrogen gas at room temperature. At 1% of H(2) an increase in the sensing film resistance of 112 Omega was observed. The sensor maintained its stable baseline and repeatability during the testing period which makes it useful for commercialization purposes.

Sensoring hydrogen gas concentration using electrolyte made of proton conductive manganese dioxide

Hydrogen gas promises to be a major clean fuel in the near future. Thus, sensors that can measure the concentrations of hydrogen gas over a wide dynamic range (e.g., 1–99.9%) are in demand for the production, storage, and utilization of hydrogen gas. However, it is difficult to directly measure hydrogen gas concentrations greater than 10% using conventional sensor [1–11]. We report a simple sensor using an electrolyte made of proton conductive manganese dioxide that enables in situ measurements of hydrogen gas concentration over a wide range of 0.1–99.9% at room temperature.

Bi-layer nanostructures of CuPc and Pd in SAW and resistance hydrogen sensor

Bi-layer nanostructures of copper phthalocyanine (CuPc ∼ 80nm, ∼100nm and ∼200nm) with a very thin film of palladium (Pd ∼18nm) on the top, have been studied for hydrogen gas-sensing application at ∼30-38°C, and medium (0,5 – 4%), and low (0,01-0,08%) hydrogen concentrations in synthetic dry air. The structures were obtained by vacuum deposition (first the CuPc and then the Pd film) onto a LiNbO3 (Y- cut Z-propagating) and glass substrates and were tested by means of the Surface Acoustic Wave (in a dual channel delay line configuration) and resistance methods. Very repeatable results have been observed for these three nanostructures with changes in frequency on the level of 100 to 600Hz, for hydrogen concentration from 0,5 to 4% in air. The absolute response value depends on the CuPc film thickness, the interaction temperature and hydrogen gas concentration and is greater for the bi-layer nanostructure with a medium (100nm) CuPc film.

Relationship between intestinal gas and the development of right colonic diverticula.

Backgrounds/AimsHigh intraluminal pressure has been reported to cause left colonic diverticula. However, the pathophysiology of right colonic diverticula is still unclear. Methane gas has been reported to delay small intestinal transit and to increase intraluminal pressure. The aim of this study was to evaluate the relationship between right colonic diverticula and intestinal gas produced by enteric bacteria.MethodsLactulose breath tests were performed in 30 patients who were diagnosed with right colonic diverticula via colonoscopy. The control group consisted of 30 healthy adults with no specific symptoms or medical histories. A hydrogen or methane producer was defined in 2 ways: either one that exhibited a breath hydrogen level ≥ 20 ppm (methane ≥ 10 ppm) baseline or one that exhibited an increase in breath hydrogen ≥ 20 ppm (methane ≥ 10 ppm) above baseline within the first 90 minutes of the test.ResultsThe lactulose breath test (LBT) positivity in the diverticular group and the control group were 40.0% and 33.3%, respectively, without a statistically significant difference. The concentrations of methane and hydrogen gas measured by LBT increased over time, but there was no significant difference between the control and the diverticular groups.ConclusionsThere was no significant relationship between right colonic diverticula and intestinal gases produced by enteric bacteria. However, time-dependent formation of diverticula should be taken into consideration, therefore long-term, large-scale follow-up studies may reveal further pathogenesis of right colonic diverticulosis.

Suppression of phase transitions in Pd thin films by insertion of a Ti buffer layer

We investigated the effects of a thin titanium (Ti) buffer layer on structural changes and electrical responses of palladium (Pd) thin films. A Ti buffer layer was inserted between a Pd film and the substrate, with varying thickness from 0.5 to 80 nm. Unlike pure Pd films, Ti-buffered Pd films showed no structural deformations after cyclic exposure to hydrogen gas, leading to a linear relationship between sensitivity and hydrogen gas concentration over the measured concentration range of 0 to 2%. This was attributed to the suppression of phase transitions from the α to the β phase in Pd films, due to the reinforced film adhesion by the inserted Ti layer. Our results highlight the practical usability of Pd thin films as reliable and sensible hydrogen sensors, enabled simply by the insertion of a thin Ti buffer layer.

Enhancement of photohydrogen production using phbC deficient mutant Rhodopseudomonas palustris strain M23

This study used a DNA recombination method to knock out the poly-β-hydroxybutyrate (PHB) synthesis gene phbC in the photosynthetic bacterium Rhodopseudomonas palustris WP3-5. The experimental results indicated that the mutant strain Rps. palustris M23 could be successfully screened. Fluorescent observation with Nile blue staining showed no significant PHB granule accumulation in the mutant cells. Batch mode experiments using acetic acid as a carbon source revealed a 29.1% and 25.9% hydrogen gas content from M23 and WP3-5, respectively. However, this trend did not appear when using propionic acid as carbon source. Under continuous operation, the hydrogen gas content from M23 could be maintained above 72%. The average hydrogen production rates of the WP3-5 and M23 strains were 264 mL-H 2/L/day and 457 mL-H 2/L/day, respectively. The total biogas volume collected from M23 was 1.7 times higher than that from the wild type.

Hydrogen production from lignite via supercritical water in flow-type reactor

A supercritical water reactor with throughput of 10 kg/h was set up, which was operated with continuous feeding of coal water slurry. The effects of reaction temperature (500–650 °C), pressure (20.0–30.0 MPa), Ca/C molar ratio (0–0.45) and O/C molar ratio (0–0.35) on the hydrogen generation characteristics were investigated. It is found that there is a notable increase in the hydrogen content and yield with the increase of reaction temperature. The hydrogen yield increases from 24.67 ml/g to 135.73 ml/g when the temperature increases from 500 °C to 650 °C. The contents of CO 2 in gas product decrease, while that of hydrogen increases with the increase of Ca/C molar ratio. At Ca/C molar ratio of 0.45, nearly all CO 2 is fixed. Correspondingly, the content of hydrogen in gas is 73.29%, and the yield of hydrogen is 348.30 ml/g compared to 135.42 ml/g in the absent of CaO. Moreover, both of CaO and KOH catalyze gasification and water-shift reaction. The formation of hydrogen and the carbon gasification efficiency are improved by the added H 2O 2 when O/C ratio is less than 0.3.

Remote Sensing of Hydrogen Concentration by Low Power Laser

An eye-safe Raman lidar for remote measurement of hydrogen gas concentration was constructed. The lidar used a low power diode-pumped solid state laser of wavelength 349nm and pulse energy 120μJ. Detection was possible for H2 gas concentration in the gas cell of more than 4% at a distance of 5m from the lidar. The concentration of hydrogen gas leaked into the open air at a distance of 10m from the lidar was successfully measured in outdoor conditions. From these experimental results, the detection sensitivity of the receiver was evaluated by calculated using a biaxial lidar model. As a result, it was confirmed that H2 gas concentration of 1% up to a distance of 10m was able to detect using a Fresnel lens of aperture 305mm. This result showed that the eye-safe Raman lidar could be used for hydrogen gas leakage detection.

Differential Platinum Thin Film Hydrogen Gas Sensor Fabricated by MEMS Techniques

In this paper, thermo-resistive platinum (Pt) thin film sensor, which was produced using Micro Electro Mechanical Systems (MEMS) fabrication techniques, is proposed. The sensor design incorporated resistor elements that would facilitate the temperature-resistance characteristics and mechanisms of Pt based thin film thermo-resistors. Especially, this paper reports the characteristics of a new differential Pt thin film hydrogen gas sensor. The fabricated hydrogen sensor detected the concentration of hydrogen gas in the air from 2% to 10%. It was possible to achieve the correct the heat transfer with this new differential Pt thin film hydrogen gas sensor. The characteristics of sensor's output are higher than conventional ones that we developed.

Hydrogen gas diffusion imaging using multiple FET type gas sensors

Imaging of hydrogen gas concentrations were demonstrated using arrayed sensors system in the cylindrical diffusion tank-shaped simulator. The field effect transistor type hydrogen sensors with platinum gate electrode were used. The sensitivity of the sensors was around -34.1 mV/dec. and the response time was 1.2 sec. in average. By applying a sensor drive circuit to each sensor, the diffusion processes of the hydrogen gas were successfully obtained in time domain and two different processes were demonstrated using 100% hydrogen gas. These results indicate that real-time monitoring of the hydrogen gas distribution can be possible. Additionally, the calibration method of the acquired data was proposed.

Enhancement of Carbon Nanofilaments Formation Density and the Surface Electrical Conductivity by the Gas Phase Composition Cycling

For the enhancement of carbon nanofilaments (CNFs) formation density and the surface electrical properties, the source gas flow was intentionally manipulated as the cyclic on/off modulation of C2H2/H2 flow in a thermal chemical vapor deposition system. CNFs formation density on the substrate and surface electrical conductivity of as-deposited substrate were investigated as a function of the cyclic modulation interval time for one cycle, namely the time for H2 + C2H2 flow (C2H2 flow on) plus the time for H2 flow (C2H2 flow off). The optimal interval time for the enhancement of both CNFs formation density and the surface electrical conductivity was around 3.5 minutes. The cause for the variation in the characteristics of CNFs according to the interval time of the cyclic modulation process was discussed in association with the variation of the remaining time of pure hydrogen gas concentration.