Solid-state Gas Research Articles

Solid-state mixed potential gas sensors: theory, experiments and challenges

Solid-state mixed potential electrochemical sensors sense gases using differential electrocatalysis on dissimilar electrode materials. The response theory is typically expressed in terms of models invoking Butler–Volmer kinetics at high overpotentials (Tafel behavior). This model is not adequate for describing all types of mixed potential sensor responses. For low concentrations of analyte gas, mass transport limitations must also be considered. Experiments with sensors with air reference electrodes also demonstrate the importance of low overpotential oxygen reduction kinetics in establishing the device response. A sensor response model that predicts a linear relationship between response voltage and analyte gas concentration is derived. The development of oxide electrode based devices offers improved long-term response stability over metal electrode based devices.

Structural and dimensional control in micromachined integrated solid state gas sensors

The application of electron microscopy for quality control in the fabrication of solid state integrated tin oxide gas sensors is described. Among the parameters that influence the performance and mechanical stability of these devices are the thickness, grain size, porosity and point defect (oxygen vacancy) distribution of the active SnO 2 layer and the intrinsic stresses and interfacial adhesion in the support layers. The sputtered SnO 2 films were highly crystalline with a columnar growth structure and equiaxed grains of mean diameter ∼10 nm. HREM studies revealed evidence of CSP defects, which act as traps for free carriers and can therefore be expected to influence the conductivity of the oxide layer. Increasing the thickness of the film from 300 to 600 nm lead to an increase in the sensitivity to low concentrations of NO 2 (∼1 ppm) by up to a factor of two. Sensitivity is also affected by surface roughness and film porosity, which increase the effective area on which gas molecules can be adsorbed.

An amperometric solid-state NO 2 sensor with a solid polymer electrolyte and a reticulated vitreous carbon indicator electrode

Reticulated vitreous carbon (RVC) was tested as a material for the preparation of the indicator electrode in solid-state gas sensors. The test planar sensor contained a RVC indicator, a platinum auxiliary and a Pt/air reference electrode, with a solid polymer electrolyte (SPE) of 10% PVC, 3% tetrabutylammonium hexafluorophosphate (TBAHFP) and 87% of 2-nitrophenyloctyl ether (NPOE). The analyte, gaseous nitrogen dioxide in air, was monitored by reduction at −500 mV vs. the Pt/air electrode. The reduction current was linearly dependent on the analyte concentration over the whole test range, from 0.2 to 2.2 ppm (v/v). The sensitivity of determination amounted to 277 nA/ppm, the limit of detection was 80 ppb (at a relative humidity, RH, of 54%) and 90% of the steady-state signal was attained within 19 s. The measuring sensitivity depended on the temperature, the gas flow rate and the RH with slopes of 1.04% per 1°C, 32% per 1 ml/s and 0.16% per 1% RH, respectively. It has been demonstrated that RVC can successfully replace noble metals in gas solid-state sensors.

Memory effect and its switching by electric field in solid-state gas sensors

The Au/n-SnO 2/SiO 2/p-Si/Al heterostructures were grown using aerosol-gel and spray pyrolysis methods. The effect of capacitance memory was discovered in the presence of polar gas molecules: C 2H 5OH, NH 3, H 2O. The mechanism of gas sensitivity, capacitance memory and its switching was associated with the process taking place on the SnO 2/SiO 2 interface.

Materials for solid-state gas sensors

The major problems in the development of inorganic gas-sensor materials are discussed. The general principle of semiconductor gas sensors is considered, and the band-structure parameters sensitive to the gas-phase composition are determined. Ways of improving sensor selectivity are examined. The composition, microstructure, and gas sensitivity of nanocrystalline SnO2 and ZnO are investigated. The dopant content and grain size of Ni-doped SnO2 are optimized for H2S detection. The prospects of employing systems of two or more nanocrystalline oxides (nanocomposites) for gas detection are discussed

SO2 gas sensor utilizing stabilized zirconia and sulfate salts with a new working mechanism

A solid-state gas sensor for sulfur dioxide with a new working mechanism was fabricated and its sensing properties investigated. The sensor was constructed using yttria-stabilized zirconia as the oxygen ion conductor, silver sulfate based salts as the auxiliary phase, a silver sensing electrode and a platinum reference electrode. When sulfur dioxide was detected, the potential of the sensing electrode changed to negative with the new working mechanism. The 90% response time of the sensor to 100 ppm SO2 was ca. 40 s at 600°C, and the electromotive force (EMF) value obtained was perfectly linear to the logarithm of SO2 concentration over a wide range (1 to 1000 ppm). The Nernst’s slope corresponded exactly to the theoretical value based on a two-electron electrochemical reaction.

Integrated solid-state gas sensors based on SnO 2(Pd) for CO detection

Thin films of polycrystalline Pd-doped tin dioxide SnO 2(Pd), widely used as sensitive element in gas sensors devices for atmospheric pollutant as CO, have been deposited on Si substrates through the ultrasonic aerosol pyrolysis technique. It is firstly shown that the sensitivity S=( G− G 0)/ G 0 to CO at low temperature (80–120°C), measured on steady state regime by conductance–temperature characteristics, depends on the concentration x of Pd incorporated into the film and on the SnO 2 grain size D. The highest sensitivity S corresponds to a very sharp ( x, D) domain. Secondly, the films synthesised under these optimal conditions have been integrated to a silicon-based prototype of micro-sensor. This device provides stable response in dynamic regime ( S=17) at low temperature (∼100°C) to 50 ppm CO by using a short-term annealing ( T annealing=350°C, t annealing=5 min) before each measurement.

5841021 Solid state gas sensor and filter assembly : De Castro Emory S; Genders J Davi; Weinberg Norman L Nahant, MA, United States

5841021 Solid state gas sensor and filter assembly : De Castro Emory S; Genders J Davi; Weinberg Norman L Nahant, MA, United States

Study of the interferences of NO 2 and CO in solid state commercial sensors

Commercial solid state gas sensors have been used to measure NO 2 and CO concentrations in the range of the first alarm thresholds for urban environments. Their mutual interference has been studied at three operating temperature ranges. The results showed that different sensitivities and selectivities can be obtained with the same type of sensors. This fact is used to make a multisensorial system based on commercial solid state gas sensors. Using artificial neural networks analysis, we detect the presence of NO 2 and CO gases, and even measure the concentration of both gases with a sufficient accuracy.

SO x solid state gas sensors: A review

The performance of any solid state electrochemical gas sensor is always rated on its response time, thermodynamic stability, operating temperature, gas sensing ability, sensitivity and gas concentration range which is sensed. Here, we have reviewed the factors contributing towards a gradual development of electrochemical solid state SO x sensor in terms of a continuous tailoring of its two basic components, i.e. solid electrolyte and reference electrode with high ionic and mixed (ionic + electronic) conductivities, respectively.

Nafion-based solid-state gas sensors: Pt / Nafion electrodes prepared by an impregnation-reduction method in sensing oxygen

The influences of the reductant concentration of NaBH4 and the quantity of Pt loading on the active surface area and the sensitivity of the Pt/Nafion electrodes prepared by an impregnation-reduction method in detecting oxygen were investigated in this study. The Pt/Nafion electrodes with a Pt loading of 4.99 mg/cm2, obtained at 0.0107 M Pt(NH3)4Cl2 and 0.06 M NaBH4, show maximum sensitivities of 0.0528 A/ppm and 0.0538 A/ppm obtained in O2 concentration regions of 0–5000 and 5000–50 000 ppm, respectively. A sensing model was also proposed to illustrate the sensing phenomenon.

Synthesis and Spectroscopic Studies of Endohedral Metallofullerenes Tb@C2N

Endohedral metallofullerenes Tb@C2n were synthesized and extracted with high-yield by K-H carbon-are evaporation and an effective pyridine extraction technique at high-temperature high-pressure. Laser-desorption-ionization time-of-flight (LD-TOF) mass spectrometry, X-ray photoelectron spectroscopy (XPS), solid-state fluorescent emission spectroscopy and gas phase derivation reaction with the self-chemical ionization mass spectrometric ion system of vinyl acetate were employed for studying the electronic structures, fluorescent properties and gas phase reactivities of metallofullerenes Tb@C2n. The experimental results suggest that endohedral metallofullerenes Tb@C2n would have the approximate structures of Tb3+@C2n 3+ similar to other metallofullerenes, good fluorescent emission properties and active reactivities in gas phase ion-molecular reactions.

Stability and oxidation of the sandwich type gas sensors based on thin metal films

Abstract The stability of the properties is investigated in the new family of the solid-state gas sensors based on various thin metal films (Pt, Au, Ni, Mo) with the modifying top layer of tin oxide. The resistance response to exposure to gas is demonstrated being stable within certain interval of temperatures. The limits of the interval depend on the metal selected for the basic layer. The variation of the chemical composition is also studied in the sensors after overheating in the clean or contaminated air. The drift of the electric resistance and the mechanism of gas sensitivity are discussed on the basis of the oxidation of the metal film.

A Broadband X‐Ray Study of the Young Neutron Star PSR B1706−44

The young Vela-like neutron star PSR B1706-44 was imaged by both the HRI aboard ROSAT and the solid-state imaging spectrometers and gas imaging spectrometers aboard ASCA during a 6 month period in late 1994 and early 1995. The broadband data set, extending over two decades in energy, allowed a detailed spatial, spectral, and temporal study to be conducted. PSR B1706-44 is found embedded in a compact synchrotron nebula of physical size ~0.32 pc and displays the morphology characteristic of the other young neutron stars in the 104-105 yr age range. No pulsations at the radio period were detected in either the ROSAT or the ASCA data. The spectrum is well described by a power-law distribution, and the data are consistent with there being no spectral break from the radio through the soft X-ray band. The unpulsed TeV emission that has been observed from PSR B1706-44 can be reconciled with these observations if the TeV emission results from the up-scattering of background IR photons by the population of high-energy particles that produce the soft X-ray photons.

Solid-state NMR study of methanol conversion over ZSM-23, SAPO-11 and SAPO-5 molecular sieves. Part 2

The chemistry of methanol-to-hydrocarbon conversion over H-ZSM-23, H-SAPO-11 and H-SAPO-5 was studied using solid-state NMR and gas chromatography. The first hydrocarbons observed in the very early stages of the reaction were either C1 and C2 or C1–C4, depending upon loading. The various selectivities of SAPO-5 for hydrocarbon products are considered to be related to the stability of isobutyl cation intermediate and the loadings of the catalyst. The formation of higher carbon number products for SAPO-11 is caused by the lower reactivity of the active centres for cracking reactions. To gain a better understanding of the intermediates involved in the conversion reaction, premethylated samples were treated with hydrogen and dimethyl sulfide.

A micromachined solid state integrated gas sensor for the detection of aromatic hydrocarbons

Preliminary studies to obtain an integrated solid state gas microsensor are presented in this work. FEM thermal simulations have been carried out to optimize the membrane and heater layouts with the aim of increasing temperature uniformity on the sensitive area and decreasing power consumption. Heater materials and full process flow have been defined in order to achieve compatibility with a CMOS technology with minor changes. On the other hand, first tests of sensitivity with pure tin oxide and Pt-doped tin oxide deposited by a sputtering process on alumina substrates, for benzene and toluene, have given good results for low concentrations (100 ppm) at 400 and 300°C respectively.

Comparison of light emission from room temperature light emitting diodes with InAs active regions grown by LPE

InAs and InAsSbP are promising materials for the fabrication of mid-infrared light sources for use in solid state gas sensors. In this paper, we report work on n-i-p and p-i-n. InAs light emitting diode structures grown using liquid phase epitaxy (LPE) on InAs substrates. These homoepitaxial diodes were compared to double heterojunction InAsSbP/InAs LEDs with InAs active regions. Room temperature emission at wavelengths between 3.3 µm and 3.7 µm (dependent on structure) was readily obtained. Electroluminescence spectra of these devices are presented, and a comparison with photoluminescence spectra of the epitaxial material is used to determine the region of the diode giving rise to the light emission. At room temperature, in the double heterojunction LEDs, the light emission was observed to originate from the InAs active region. However, in the homoepitaxial LEDs, the light was generated both in the undoped and p+ regions of the device, depending on the precise structure.

Vitality protection solid-state gas sensors based on macrocyclic cobalt(III) complexes

The electrochemical low-cost sensors for the quantitative analysis of toxic and explosive gases in workplace air are being reported. The sensors are of voltammetric (galvanic) type based on proton conductive solid-state electrolyte with high conductivity at an ambient temperature and are fabricated as pellets with the dimensions 12 × 5 mm. The implementation of macrocyclic cobalt(III) complexes with different ligands as the counter electrode materials allows the development of the sensors with good output linearity, stability and high selectivity towards the gases measured, and long operating lifetime, up to several years.

Hydrogen sensor based on antimonium pentoxide-phosphoric acid solid electrolyte

A solid composite electrolyte with high proton conductivity based on antimonium pentoxide with additives of phosphoric acid has been obtained. A potentiometric solid-state gas sensor using this electrolyte has been developed for detecting small amounts of hydrogen (10–2000 ppm) in gas mixtures at ambient temperature. The sensor consists of the reference electrode: Ag or Ag/(Ag + Ag 2SO 4), the solid composite electrolyte and H 2-sensitive electrode: Pt or Pd. The electromotive force (e.m.f.) of the sensor varies logarithmically with H 2 concentration for hydrogen partial pressures in the range 100–2000 ppm and depends on the oxygen partial pressure. The slope of e.m.f.-log( pH 2) dependence is 170 and 200 mV for Pt and Pd, respectively, which exceeds the Nernst value, presumably due to the formation of a mixed potential. The sensor can operate at a wide range (20–95%) of a relative humidity.

Precise measurement of methane in air using diode-pumped 3.4-μm difference-frequency generation in PPLN

volume multi-pass absorption cell with an 18-m path length. The methane mixing ratio was determined by comparing the direct optical absorption measured in the sample with that measured in a reference gas at 100 torr and room temperature. Relative accuracy of better than 1 ppb (parts in 109, by mole fraction) was achieved in measurements of natural air that contained 1700–1900 ppb methane. The typical measurement time for each sample was 60 seconds. The accuracy was limited by residual interference fringes in the multi-pass cell that resulted from scattering. Without the use of reference samples, the relative accuracy was 20 ppb; it was limited by the long-term reproducibility of the spectroscopic baseline, which was affected by drift in the optical alignment coupled to changes in the ambient temperature. This work demonstrates the use of diode-pumped difference-frequency generation (DFG) in PPLN in a high-precision infrared spectrometer. Compact, room-temperature solid-state gas sensors can be built based on this technology, for accurate real-time measurements of trace gases in the 3–5 μm spectroscopic region.