Hydrogen Concentration In Air Research Articles

Investigations on the resistance of the metal-free phthalocyanine and palladium bilayer sensor structure influenced by hydrogen

Results concerning resistance measurements of two metal-free phthalocyanine (Pc) and palladium (Pd) bilayer sensor structures under the influence of medium hydrogen concentrations (0.5–4%) in nitrogen and air are presented. These two bilayer structures of different thicknesses of metal-free phthalocyanine (55 nm and 80 nm) and the same thickness of palladium (∼10 nm) have been realized in an electric system. For the interaction in nitrogen the bigger response has been observed in a sample with a thinner phthalocyanine layer. For interaction in air a change in the response character (increase–decrease of resistance) it is clearly visible in a sample with a thinner Pc film. Between 2% and 2.5% hydrogen concentration in air the structure of the palladium phase changes from α to β. The resistance of the α-Pd structure increases with the increasing hydrogen concentration whereas in the case of the β-Pd structure an opposite behavior is observed. Such a change is observed only concerning the interaction in air and for a thinner phthalocyanine (Pc) layer. In a structure with a thicker phthalocyanine film this phenomenon occurs at a higher hydrogen concentration.

MOS-Junction-Based Nanostructures by Thermal Oxidation of Silicon Wires for Hydrogen Detection

Heavily p-doped monocrystalline silicon wires have been fabricated by employing isotropic Si wet etch and thermal oxidation to achieve a nanometric cross section-a gate-oxide growth and a final palladium evaporation made up the MOS junction able to detect hydrogen concentration in air. Several types of wire dimensions have been designed and fabricated: length ranges from 5 to 70 /spl mu/m; the smallest widths obtained are around 250-300 nm, while the biggest are up to 7 /spl mu/m. Preliminary experimental results show a high signal/noise ratio sensor response to 100 ppm concentration of H/sub 2/ at room temperature, 1-atm air.

Comparative hydrogen sensing performances of Pd– and Pt–InGaP metal–oxide–semiconductor Schottky diodes

The hydrogen sensing performances of Pd– and Pt–InGaP metal–oxide–semiconductor (MOS) Schottky diodes are systematically studied and compared under different operating temperature. It is known that, from experimental results, the Pd–InGaP MOS Schottky diode exhibits higher hydrogen detecting capability at lower hydrogen concentration regimes. On the contrary, the MOS Schottky diode with Pt catalytic material shows better hydrogen detecting capability and higher operating temperature. It is believed that the refractory Pt metal is responsible for detecting high hydrogen concentration in air, especially at high operating temperature. Furthermore, the temperature dependence of equilibrium constants is investigated and reported. In order to study the steady-state reaction kinetics, the van’t Hoff equation is used to estimate the values of initial heat of adsorption for both devices. Based on the Temkin isotherm model, the experimental results and theoretical simulation of hydrogen coverage are also studied and compared.

Critical tube measurements at elevated initial mixture temperatures

Experimental results obtained on the transmission of a planar detonation wave from a cylindrical tube into an unconfined volume are reported. Experiments are performed using a 9.1 mm long tube connected to an 88.9 cm inner diameter receiver vessel. Both the detonation tube and receiver vessel can be heated. Tests are performed using 10, 20, and 27.3 cm tubes. The minimum, or critical, hydrogen concentration in air that results in detonation transmission into the receiver vessel is measured at an initial pressure of 1 atm and initial temperatures of 300, 500, and 650 K. The value of the ratio of the tube diameter, d, and the critical mixture detonation cell size, u c , is found to be between 18 and 24. The data show no correlation between the value of d/ u c and the initial mixture temperature and there is no observable effect of temperature on the regularity of the soot foil line spacing. The uncertainty in the average cell size measurement of - 25% is not sufficient to explain the significant departure from the classical empirical correlation d/ u c =13. It is proposed that the d/ u c =13 is neither a unique nor adequate correlation for describing detonation diffraction in mixtures considered to have a regular or irregular detonation cellular structure.

Difference in hydrogen sensitivity between Pt and Pd field-effect devices

An explanation is given for the large differences in the hydrogen sensitivity in air observed for gas sensitive field-effect devices with palladium and platinum gates, respectively. It is demonstrated that this difference is mainly due to a difference in the hydroxyl formation rate between the two metals. The water production rates are, however, almost the same for the two metals. The considerably smaller sensitivity of platinum devices in air is then due to the much lower steady-state hydrogen atom concentration on the platinum surface compared with the palladium surface. This leads to a smaller coverage of hydrogen atoms at the metal–oxide interface and thus a smaller response of the device at a given hydrogen concentration in air.

A novel high-sensitive Pd/InP hydrogen sensor fabricated by electroless plating

In this work, a novel electroless plating technique was proposed to fabricate high-sensitive Pd/InP Schottky diode hydrogen sensors. The Schottky current–voltage ( I– V) characteristics were investigated at hydrogen concentrations in air ranging from 15–10,000 ppm. Experimental results show that the Schottky diode characteristics for the sensor devices obtained by electroless plating are superior to those obtained by the conventional thermal evaporation. Furthermore, due to the low-energy fabrication, the Fermi-level pinning effect can be avoided and therefore, the sensor device exhibits high sensitivity on hydrogen. Even at very low hydrogen concentration of 15 ppm, the saturation sensitivity reaches about 2.07. Further analyzing the I– V data, it shows that the experimental results are in a good agreement with the proposed hydrogen adsorption model.

Mechanism for electric breakdown in a chemically nonequilibrium system and the influence of the chain oxidation reaction in an H2-air mixture on the breakdown threshold

A mathematical model is developed and a numerical analysis is performed for an electric breakdown in a hydrogen-air mixture with a low concentration of H2. It is shown that, at sufficiently low pressures p<10−2 atm, a small molecular-hydrogen additive (η=5×10−5–5×10−3) decreases the reduced field of an electric breakdown in air by a factor of more than 2 because of the appearance of an additional detachment process associated with the chain hydrogen-oxidation reaction. Detailed calculations are performed for the mean number density of negative oxygen ions [O2−]=103 cm−3 and the hydrogen concentration in air [H2]=0.5, 0.05, and 0.005%. It is found that, for [H2]=0.005%, the breakdown can develop under the action of a geoelectric field of 1.3 V/cm at p≃10−4 atm.

Interaction of hydrogen with a metal-oxide-semiconductor structure containing an additional solid-electrolyte layer

The effect of hydrogen on the flat-band voltage of the structure Pt/LaF3/SiO2/SiC, which is the sensitive element of a high-temperature fluorochlorocarbons sensor, is investigated. It is shown that at 441 K temperature and 10−2 hydrogen concentration in air, the capacitance-voltage characteristic of the structure shifts in the direction of negative voltages by about 160 mV. Increasing the temperature decreases this amount, apparently because of the catalytic oxidation of oxygen on the platinum electrode of the structure. The hydrogen sensitivity of the structure investigated can affect the kinetics of the response of a sensor interacting with hydrogen-containing fluorochlorocarbons.

Modified heterojunction based on zinc oxide thin film for hydrogen gas-sensor application

Device-quality ZnO thin film is deposited by using an indigenously developed modified CVD method and a Pd/ZnO/p-Si heterojuncion is fabricated. A study of the current-voltage characteristics of the heterojunction in pure air and in air with different concentrations (2000−20 000 ppm) of hydrogen reveals that the device can be used as a room-temperature hydrogen sensor down to a level of hydrogen below its explosion mixture with air. The saturation sensitivity and time response of the device with respect to the different hydrogen concentrations in air has been studied. The device operating at 1 V forward bias shows a maximum saturation sensitivity in different hydrogen concentrations. At 20 000 ppm. H 2 in air, the sensitivity is found to be 5×10 2, while the time response is 162 s. The increase of surface conductivity of ZnO at the Pd/ZnO interface due to the adsorption and chemical interaction of hydrogen is found to play a key role in sensing. This mechanism of hydrogen sensing is verified by a double-metal-gate heterojunction.

Fiber Optic Sensor for Hydrogen Concentrations near the Explosive Limit

A new hydrogen sensor has been demonstrated that provides a large signal for concentrations of hydrogen in air near the explosive limit. This optical fiber sensor is intrinsically safe for use in potentially explosive environments. The sensor responds to changes in the optical properties of a thin palladium film exposed to hydrogen.

The effects of gas explosions in free and partially confined fuel/air mixtures

AbstractTo obtain a further insight into the complex flame acceleration process of deflagrating fuel/air mixtures experimental investigations were carried out in free and partially confined homogeneous clouds with hydrogen as a fuel. The results of the experiments in an unconfined hemispherical configuration show a distinct dependence of the flame velocity on the cloud size which does not exceed, however, an upper limiting value. The experiments under partially confined conditions without additional turbulence indicate that the flame velocity and consequently the pressure field is governed by the hemispherical flame front initially formed; i.e. the flame velocity is not affected by the partial confinement. To increase the turbulence effects further experiments were carried out with a fan and with jet ignition. In the case of fan induced turbulence the flame velocity was strongly influenced by the fan speed. At a certain fan capacity the transition of a deflagration into a detonation was observed. With jet ignition produced by venting an explosion chamber into the partially confined cloud the transition occured even with hydrogen concentrations in air as low as 22 vol%. The results which are discussed on the basis of turbulence models show that high damaging overpressures or the transition from a deflagration into a detonation are only generated by strong turbulences as a consequence of flow and/or obstacles.