Tin dioxide-based gas sensors for detection of hydrogen fluoride in air

This research concentrates on the sensitivity of semiconductor tin dioxide-based gas sensors to hydrogen fluoride in air. After evaluating the characteristic detection temperature, the sensor's signals were studied for different HF concentrations. Despite the corrosive effects of hydrogen fluoride, a reproducibility of the signal was found. Likewise, we did not observe any long-term degradation to the sensor. For the experiment, the sensor was exposed to a gas mixture formed by HF, O₂, N₂ with a constant flow rate of 150 ml min⁻¹. The semiconductor gas sensor reached maximum sensitivity near 380 °C, and a minimum concentration was detected approximately 50 ppb. Moreover, the detection phenomenon appears to be reversible when considering the electrical response under a constant air flow.     

Multimode detection of hydrogen gas using palladium-covered silicon micro-channels

Palladium was electrodeposited onto lithographically patterned Si(100) "micro-channels" with dimensions of 2 microm (width) x 100 microm (length). The properties of these Pd-covered Si micro-channels for detecting dihydrogen gas were then evaluated. Pd electrodeposition was carried out under conditions favoring an instantaneous nucleation and growth mechanism. This strategy produced size-similar Pd particles at a coverage of (4-6) x 10(9) cm(-2) within the confines of the Si micro-channel. When the mean particle radius, ro, was smaller than a critical value (ro < rc = 70-85 nm), each Pd particle was well separated on the surface from adjacent particles, on average, and no response to H2 gas attributable to the micro-channel was observed. As Pd particles were grown larger, to a mean radius of ro approximately equal to rc, adjacent particles on the surface touched and the electrical resistance of the micro-channel dropped by several orders of magnitude. These "type 2" H2 sensors exhibited a rapid (< 1 s), reversible decrease in their resistance in response to exposure to H2 above 0.5%, but a minimum resistance was observed at 1-2%, and a resistance increase was seen at higher H2 concentration. This complex behavior resulted from the existence of three mechanisms for charge transport across the micro-channel. If still larger quantities of Pd were deposited, the Pd particle ensemble coalesced into an electrically continuous film. These "type 3" sensors became more resistive in the presence of H2, not more conductive as seen for sensors of types 1 and 2, but the amplitude of this response was smaller than seen for type 2 sensors.     

Trace methane gas detection by wavelength modulated off-axis integrated cavity output spectroscopy

An instrument based on wavelength modulation off-axis integrated cavity output spectroscopy (WM-OA-ICOS) was developed for detection of trace methane (CH₄) gas. The wavelength modulation technique was employed to obtain the second harmonic of the CH₄ absorption signal. The modulation parameters were optimized to obtain a maximum second harmonic signal. A noise-equivalent absorption sensitivity of 9.4 × 10^?11 cm^?1 Hz^?1/2 (corresponding to a detection limit of 28.9 ppbv for CH₄ at this wavelength) was achieved using 2f detection. Compared with the traditional off-axis integrated cavity output spectroscopy (OA-ICOS) technique, WM-OA-ICOS provided an 8-fold improvement in detection sensitivity. CH4 concentration measurements were also achieved by normalization of second harmonic signal to first harmonic signal (2f/1f). The dynamic range and linearity of WM-OA-ICOS for both 2f method and 2f/1f method were investigated. The result showed that the 2f/1f method exhibited better linearity than 2f method.     

Nanocrystalline Pt-doped TiO2 thin films prepared by spray pyrolysis for hydrogen gas detection

Nanostructured pure and Pt-doped TiO₂ thin films were prepared by chemical spray pyrolysis technique. Aqueous solution of TiCl₃·6H₂O (0·01 M) was chosen as the starting solution for the preparation of pure TiO₂ thin film. Aqueous solutions of PtCl₆·6H₂O (0·01 M) and TiCl₃·6H₂O (0·01 M) were mixed in volume % of 1 : 99, 2·5 : 97·5 and 5 : 95 respectively to obtain Pt-doped TiO₂ thin films. The solutions were sprayed onto quartz substrate heated at 350 °C temperature to obtain the films. These thin films were fired for one hour at 550 °C. The sensing performance of these films was tested for various gases such as LPG, H₂, CO₂, ethanol, NH₃ and Cl₂ (1000 ppm). The Pt-doped TiO₂ (1 : 99) was observed to be most sensitive (572) to H₂ at 400 °C with high selectivity against other gases. Its response time was short (10 s) and recovery was also fast (14 s). To understand the reasons behind the gas-sensing performance of the films, their structural and microstructral properties were studied using X-ray diffraction and electron microscopy (FE–SEM and TEM), respectively. Thicknesses of all these samples were determined using Surface Profiler. The results are interpreted.     

纳米ZnO气敏元件对H2的测定研究

以采用物理热蒸发法制备的纯ZnO纳米线以及Ag掺杂ZnO纳米线为气敏基料制备成旁热式气敏元件,用静态配气法对不同浓度的H2进行气敏性能测试。利用测试结果,绘制元件灵敏度与所测气体浓度的关系曲线,并对此曲线进行了线性拟合。结果表明,Ag掺杂纳米ZnO元件与纯纳米ZnO元件相比会明显提高对H2的灵敏度,两类元件的气敏性能与所测气体浓度呈现相同的变化规律。用拟合方程计算出的气体浓度值与实际检测值间吻合较好,误差小于10%。因此,可以利用这两类元件及其拟合直线对H2气体浓度进行测定。     

A single nanotrench in a palladium microwire for hydrogen detection

The hydrogen sensing characteristics of a single nanotrench fabricated by focused ion beam milling (FIB) in an evaporated palladium microwire are presented. In situ atomic force microscopy (AFM) measurements proved that, in the presence of H(2), the trench closes and electrically connects the initially separated parts of the wire due to the increase in volume of the material. Therewith, an electrical current can be switched through the wire. With experiments under various H(2) concentrations and a mathematical model, we describe the closing mechanism of the trench with respect to various parameters, including the substrate material, film thickness, trench size and wire dimensions. Results have been compared with those from equivalent continuous wires. Thin SiO(2) and polyimide (PI) layers on silicon were used to study the effect of substrate elasticity. Sufficient lateral expansion of Pd to close trenches of up to 70?nm in width has only been observed on PI, which we attribute to its advantageous elastic properties. The scale of the response times allowed the observation of two superposing effects: the chemical conversion of Pd to PdH(x) and the mechanical closing of the trench.     

Fiber-optic cavity sensing of hydrogen diffusion

A novel type of fiber-optic cavity sensor for hydrogen diffusion into and out of fibers is presented. The sensor is an implementation of a cavity ringdown scheme in a silica-based single-mode fiber that has been exposed to gaseous hydrogen at normal pressure. The measured ringdown times during the H2 diffusion show good agreement with a theoretical diffusion model. This model allows the determination of the diffusion coefficient of hydrogen in silica, resulting in D = (3.02 +/- 0.07) x 10(-15) m2/s at 30 degrees C.     

Ultra-sensitive bolometers for atomic hydrogen detection

Two different types of bolometers have been developed to be used for spin-polarized atomic hydrogen detection. One of them uses a thin Cu shell as the absorber and a doped Ge chip as the thermometer element. The other uses a silver film as the absorber and a superconductor-insulator-normal metal junction to measure the temperature rise of the electrons in the absorber. These bolometers have a calculated noise equivalent power of 10^–15 WHz^–1/2 and 10^–17 WHz^–1/2, respectively, and can detect down to 6 × 10³ and 800 particles, respectively, at an operating temperature of 100 mK. If these detectors perform as predicted, the minimum number of detectable hydrogen atoms will be improved by several orders of magnitude.     

Spin-polarized hydrogen: A weakly interacting boson gas

These lectures on a gas of spin-polarized hydrogen discuss the developments in this field up to the latest results. First, hydrogen is treated as a stable non-recombining gas and many fundamental properties such as Bose-Einstein condensation (BEC), the two-dimensional Kosterlitz-Thouless transition to superfluidity, and interactions with helium are discussed. In the second section the experimental production and handling of spin-polarized hydrogen is treated. This includes theoretical and experimental discussions of two- and three-body recombination as well as relaxation, both in the normal and condensate phase. These processes have so far prevented the experimental observation of degenerate quantum effects, as they either limit the density or the temperatures of samples. In the final section several experimental ideas for BEC, currently under development, are discussed. These include the magnetostatic trap, the microwave trap, compression of hydrogen as bubbles in helium, compression with magnetic field gradients, and the use of the Lyman- laser source in the study of atomic hydrogen.Presented by Isaac F. Silvera.     

Microstructure fibers for gas detection

The paper deals with experimental investigation of capabilities of microstructure fibers (MSFs) for evanescent-wave chemical sensing. It presents results on the sensitivity of MSFs of a “grapefruit” type to gaseous toluene in mixtures with nitrogen which flow through the fiber holes. The sensitivity is determined on the basis of changes of the output optical power from the fiber caused by refractive-index changes of the fiber cladding induced by toluene and determined at 670 nm. Specific changes of spectral absorbance due to C–H overtones of toluene in a range of 1600–1800 nm are also utilized. Results obtained both with unmodified fibers and with fibers modified by the application of methyltriethoxysilane-based xerogel layers onto the hole walls are presented. The results show that fibers modified by the xerogel layers are sensitive to gaseous toluene in the cladding holes. Changes of the output power can also be determined with fibers unmodified by the xerogel layers, namely for saturated toluene mixtures. The presence of toluene in the detection layers was proved by spectral measurements.     

Trace detection of dissolved hydrogen gas in oil using a palladium nanowire array

The electrical resistance, R, of an array of 30 palladium nanowires is used to detect the concentration of dissolved hydrogen gas (H(2)) in transformer oil over the temperature range from 21 to 70 °C. The palladium nanowire array (PdNWA), consisting of Pd nanowires ～100 nm (width), ～20 nm (height), and 100 μm (length), was prepared using the lithographically patterned nanowire electrodeposition (LPNE) method. The R of the PdNWA increased by up to 8% upon exposure to dissolved H(2) at concentrations above 1.0 ppm and up to 2940 ppm at 21 °C. The measured limit-of-detection for dissolved H(2) was 1.0 ppm at 21 °C and 1.6 ppm at 70 °C. The increase in resistance induced by exposure to H(2) was linear with [H(2)](oil)(1/2) across this concentration range. A PdNWA sensor operating in flowing transformer oil has functioned continuously for 150 days.     

Tribotronic transistor sensor for enhanced hydrogen detection

Hydrogen detection with a high sensitivity is necessary for preventing potential explosions and fire.In this study,a novel ZnO tribotronic transistor is developed by coupling a ZnO field effect transistor (FET) and triboelectric nanogenerator in free-standing mode and is used as a sensor for hydrogen detection at room temperature.Tribotronic modulated performances of the hydrogen sensor are demonstrated by investigating its output characteristics at different sliding distances and hydrogen concentrations.By applying an external mechanical force to the device for sliding electrification,the detection sensitivity of the ZnO tribotronic transistor sensor is improved,with a significant enhancement achieved in output current by 62 times at 500 ppm hydrogen and 1 V bias voltage.This study demonstrates an extension of the applications of emerging tribotronics for gas detection and a prospective approach to improve the performance of the hydrogen sensor via human-interfacing.     

Morphology tuned ZnS nanostructures for hydrogen gas sensing

In this paper we present the effect of dimensionality of ZnS nanostructures on hydrogen gas sensing characteristics. Vapor Liquid Solid growth mode was employed to synthesize ZnS nanostructures with different dimensions by controlling the growth parameters, i.e., variation in the substrate temperature and the carrier gas flow rate. The growth was explained by using the chemical tension model and the saturation conditions were determined for each growth. X-ray diffraction and scanning electron microscopy were used to determine the phase, shape, size and density of the nanostructures. Optical properties of nanostructures also confirmed the presence of different phases of ZnS. A variety of these nanostructures were tested for hydrogen gas sensing. The rapid response time was obtained for nanowires in few hundred millisecond’s range with a sensitivity of 8, which was due to its high aspect ratio as compared to the other nanostructures.     

A titania nanotube-array room-temperature sensor for selective detection of hydrogen at low concentrations

A tremendous variation in electrical resistance, from the semiconductor to metallic range, has been observed in titania nanotube arrays at room temperature, approximately 25 degrees C, in the presence of < or = 1000 ppm hydrogen gas. The nanotube arrays are fabricated by anodizing titanium foil in an aqueous electrolyte solution containing hydrofluoric acid and acetic acid. Subsequently, the arrays are coated with a 10 nm layer of palladium by evaporation. Electrical contacts are made by sputtering a 2 mm diameter platinum disk atop the Pd-coated nanotube array. These sensors exhibit a resistance variation of the order of 10(4) in the presence of 100 ppm hydrogen at 25 degrees C. The sensors demonstrate complete reversibility, repeatability, high selectivity, negligible drift and wide dynamic range. The nanoscale geometry of the nanotubes, in particular the points of tube-to-tube contact, is believed to be responsible for the outstanding hydrogen gas sensitivities.     

Nonstationary gas discharge from a semiclosed cavity

A mechanism of formation of pressure fluctuations in a closed cylindrical cavity filled with compressed gas is studied. Pressure fluctuations occur with sudden rupture of the cavity over the entire section. Boundary conditions imposed on an open boundary in the case of quick interchange of discharge and inflow through it are suggested. A comparative analysis of the nonstationary process of intense gas discharge from cylindrical and conical cavities closed from one side is made. Scientific-Research Institute of Applied Mathematics and Mechanics, Tomsk. Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 70, No. 5, pp. 820–823, September-October, 1997.     

Evaluation of small hydrogen sulfide concentrations in calibrating commercial gas analyzers for hydrogen sulfide

Conclusions Small concentrations (10^–5–10^–2 by volume) of hydrogen sulfide in gas mixtures can be analyzed by means of the photocolorimetric method based on the formation of molybdenum blue by the interaction of hydrogen sulfide with ammonium molybdate in an acidic medium.The optimum conditions for reducing molybdate by hydrogen chloride are provided by a content of 50 mg/ml of ammonium molybdate in the absorbing solution, a sulfuric solution with an acidity of 0.6 N, and a coloration ripening time of 15–20 min.     

Static Fourier-transform ultraviolet spectrometer for gas detection

We report the design, construction, and evaluation of a static Fourier-transform ultraviolet spectrometer. The spectrometer is based on Wollaston prisms that form an interferogram in the spatial domain, which is recorded with a detector array. We demonstrate the application of the spectrometer to gas detection. Using a deuterium light source, we measured a detection limit, with a 1-s integration time, for hydrogen sulfide and sulfur dioxide, corresponding to 0.2 ppm over a 5-m path length.     

Multiline absorption spectroscopy for methane gas detection

A multiline absorption spectroscopy technique was investigated based on the single-line absorption spectroscopy technique. An open-path methane-detecting system was designed. An LED was used as a broadband source, and a Fabry-Perot interferometer whose transmission peaks matched the methane R-branch absorption lines was used to enhance the detectable sensitivity. We demonstrate a minimum-detectable concentration of 7600 +/- 10% ppm (parts per million) with a multiline differential absorption spectroscopy technique and a concentration of 1000 +/- 10% ppm with a multiline wavelength modulation spectroscopy technique.     

A driftless gas proportional scintillation counter for muonic hydrogen X-ray spectroscopy under strong magnetic fields

An experiment involving muonic hydrogen requires an X-ray detector having large area and working under strong magnetic fields (5 T) with good energy and timing resolution. A compact, driftless gas proportional scintillation counter (GPSC) capable of operating under such magnetic fields is investigated. This GPSC uses a CsI photocathode deposited onto a microstrip plate as the UV scintillation readout photosensor. This photocathode has the advantage of operating in direct contact with the scintillation gas. The detector is filled with pure xenon and is designed to have a high detection efficiency for 2 keV X-rays. Energy resolutions of 23% and 22% were obtained for 1.74 and 2.3 keV X-rays, respectively. The low-energy detector limit due to the electronic noise is 300 eV. Its performance in the presence of strong magnetic fields was tested. At magnetic field of 5 T the detector pulse amplitudes are reduced by less than 25%, while the detector energy resolution and pulse rise time present a relative increase of less than 10%.