S. S. Yakimov

Propane:butane semiconductor gas sensor with low power consumption

Abstract The semiconductor gas sensors which have a reduced power consumption as low as 100 mW were developed for the application to portable gas detection devices. Sensors with a base material of tin dioxide and 4 wt.% of Pd dopant were fabricated by means of thick film technology. Both the composition parameters of the sensing layer and the heater were optimized in order to sense propane/butane in air with the threshold level of 100 ppm. The original method of tin dioxide preparation for the sensing layer and for the thick film heater being stable at temperatures up to 600°C was developed.

Mössbauer spectra of hematite and magnetite nanoparticles in polymer composites

It is experimentally demonstrated that the shape of Mossbauer spectra of polymer composites based on single-domain magnetic particles depends more on an inter-particle interaction than on superparamagnetic properties of a separate particle. The characteristic features of the spectra of such systems with strong inter-particle interactions are separated, such as an asymmetric shape of lines with sharp outer and smeared inward sides and presence of central anomalously splitted doublet. It is shown that the three-level relaxation model, which takes into account the precession and diffusion of magnetization of single-domain particles between two local energy minima and one absolute energy maximum, allows to describe all characteristic features of the spectra of such systems.

Hydrocarbon and Fluorocarbon Monitoring by MIS Sensors Using an Ni Catalytic Thermodestructor

An increase in the number of gases detectable by sensors based on Pd-SiO₂-Si (MIS) and Pt-LaF₃-Si₃N₄-SiO₂-Si (MEIS) structures was achieved by the application of an external catalyst element (CE). It was shown that as a result of the decomposition of hydrocarbon and fluorocarbon molecules on a Ni coil (CE), the products detectable by metal-insulator-semiconductor (MIS) and metal-electrolyte-insulator-semiconductor (MEIS) sensors are formed. The simultaneous catalytic oxidation of hydrocarbons and their thermal decomposition result in an optimum CE temperature of about 1050 K for propane. The kinetics of the thermal decomposition of gases on Ni were investigated. The activation energy of the reaction for C₃H₈ and the enthalpy in the case of CF₃-CCl were estimated

Electrode structure effect on the selectivity of gas sensors

The effect of the catalytic metal gate structure on the selectivity of MOS gas sensors has been described. It has been shown that hydrogen cyclic treatment (HCT) modifies the structure of the palladium film evaporated on the silicon dioxide. Pore formation in the gate gives rise to the ammonia and carbon monoxide sensitivity of the field-effect sensor. Ammonia adsorption and desorption processes on the sensor surface have been investigated. These processes are of a unimolecular nature with an adsorption activation energy of 0.08±0.02 eV/molecule in an inert atmosphere.

Phonon softening near superconducting transition temperatures in high-Tc ceramics

Abstract Anomalous behavior of the Mossbauer factor ƒ for iron and tin impurity atoms in the ceramic of the 1-2-3 composition, which indicates a phonon softening near Tc and Tt≈190 K, was found by Mossbauer spectroscopy.

MOS structure (Pd-SiO2-Si) based gas sensor with an external catalyst element

Abstract Physicochemical properties of a gas sensor with the Pd-SiO2-Si-based sensitive element with an external catalyst element (CE) have been investigated. It is shown that as a result of the homolytical splitting of carbon-carbon bonds in alkane molecules, MOS sensors showed sensitivity to propane and butane both in inert and oxygen containing atmospheres. The dependencies of the sensors signals on the temperature of the CE was obtained for different gas mixtures containing H2, CH4, C3H8 and C4H10. The temperature of the CE (platinum coil) located above the sensor could be varied within a wide range (up to 1100 K in the air and 700 K in helium), while the temperature of the sensitive element was kept constant (463 K). A semi-empiric model describing the temperature dependence of the sensors signal was suggested. The magnitudes of the activation energy of the dissociation methane (3.5 eV) and propane (1.1 eV) on platinum was determined. When the initial rate of change of the flat-band voltage of the MOS capacitor ∂ΔU|∂t∣t=0t = 0 was taken as the sensors signal, a linear dependence of the signal versus the propane and butane concentration in air from 100 to 2000 ppm was obtained. A possibility of using pulse heating of the CE for the formation of the sensors signal was demonstrated.

Detection of phosphine and arsine in air by sensors based on SnO2 and ZnO

Abstract The conductivity of sensors based on SnO 2 and ZnO has been investigated in air mixed with PH 3 (0.02–0.42 ppm) and AsH 3 (0.015–0.3 ppm). The sensors are made of SnO 2 or ZnO thin films (0.1 μm) sputtered on sapphire substrates, an RuO 2 heater being pasted on the other side of the substrate. The resistance of the oxide film is 10 5 –10 6 Ω in pure air at room temperature; the heater resistance is 10–30 Ω. The changes of conductivity of the SnO 2 and ZnO films have been measured under impulses of gaseous hydrides of different concentrations in air. It has been shown that the maximum sensitivity of the sensors is achieved at a film temperature of 350 °C (heater power = 0.35 W). The typical response time is within 1–2 min. As a result, the sensitivity (relative change of conductivity Δ G / G 0 ) versus hydride concentration in air has been obtained.

An investigation of the hydrogenated Si-SiO2 interface by X-ray diffraction

Abstract An asymptotic Bragg diffraction method is applied for the first time to the investigation of a hydrogenated Si-SiO 2 interface. It appears that the method may be useful for physical and chemical characterization of Pd-SiO 2 -Si type sensor systems.

System to detect hydrogen in water

Abstract In this paper we present a system to control hydrogen in the heat-transport medium (water) designed to operate in nuclear power stations. Its main elements are a MOS sensor with a Pd-electrode and hydrogen-permeable membrane, which is produced from Pd-Ru alloy. The hydrogen is given off from the analysed medium by means of this membrane and is transferred to the sensor by helium. We report results of investigations using this system. An effect of water on the interaction between hydrogen and the membrane surface is found.

Oxygen effect on the operation of the MOS-structure-based hydrogen sensor

Abstract The influence of oxygen on the operation of hydrogen sensors based on MOS devices (capacitor) with palladium and platinum electrodes has been investigated. An effect of oxygen on the effective hydrogen sensitivity energy and the flat-band voltage-change kinetics Δ U(t) of the sensor with a Pt gate has been detected. The dependence of the initial rate of change of the flat-band voltage upon introduction of a gas mixture versus oxygen concentration has been obtained. A difference between them for sensors based on PtSiO 2 Si and PdSiO 2 Si is observed at a working temperature above 130 °C.