W.M. Sears

Algorithms to improve the selectivity of thermally-cycled tin oxide gas sensors

The conductance of a commercial tin oxide gas sensor (TGS#812) is measured as a function of concentration for a number of different gases under conditions of thermal cycling. This information is used to examine the criteria that could be used to improve the selectivity of detection. Different conductance-time curves (signatures) are seen for the various gases tested, which show gas-specific concentration dependences. Algorithms are presented that can distinguish the responses to propane, carbon monoxide or hydrogen from each other and from other gases (alcohols) over wide ranges of concentration. Irreversible poisoning effects occur under long-term exposure of the sensor to strong reducing gases, such as high concentrations of CO or H2. However, the conditions causing poisoning do not apply in most practical applications of gas sensors.

The gas-sensing properties of bismuth molybdate evaporated films

Abstract A gas sensor that operates by the detection of changes in the bulk conductivity of a bismuth molybdate semiconductor catalyst is presented. The reduction of the catalyst by organic vapours produces highly mobile oxygen vacancies, which have a direct effect on the carrier density throughout the sample. This reaction is completely reversible in air. The sensor can be prepared as a mixture of the alpha and gamma phases of bismuth molybdate evaporated onto a quartz substrate. The prototype sensors studied show long-term stability of response and insensitivity to water vapour. A degree of selectivity to alcohols and ketones and some alkenes over other reducing agents such as alkanes, hydrogen and carbon monoxide is shown. At an operating temperature of 330 °C, increases in conductivity of a factor of 30 after exposure to 200 ppm of ethanol were commonly observed, with sensitivities down to 5 ppm. Eventually this class of sensors may find use in breathalyser devices.

Selective thermally cycled gas sensing using fast Fourier-transform techniques

Abstract The conductance versus time signatures of three types of thermally cycled gas sensors have been measured for a number of different reducing gases over a wide range of concentrations. A fast Fourier-transform data analysis technique is used to produce first-harmonic polar plots of phase angle versus magnitude of both single gases and mixtures as a function of concentration. It is found that this produces a good separation among many of the gases tested and leads to a clear selective criterion for the detection of single unknown gases. Gas mixtures can sometimes be distinguished as well, but with less reliability.

The gas-sensing properties of sintered bismuth iron molybdate catalyst

Abstract Sintered pellets of bismuth iron molybdate catalysts act as sensors of reducing vapors by bulk conductivity changes mediated by oxygen vacancy donors. These donors, originating from the direct reduction of the semi-conductor catalyst, are highly mobile and can quickly diffuse through the sample, thereby having a direct effect on the electronic carrier density throughout. In air the vacancies are reoxidized into lattice oxygen. The sensitivity and selectivity of pellets embedded with gold wire contacts are examined for a wide number of gases and vapors and the effect of high-temperature sintering (calcining) as well as the effects of platinum doping is examined. Mechanisms are proposed to explain the power law dependencies of the sensor conductivity versus concentration and the response time versus concentration. Direct measurements are made on the diffusion of oxygen vacancies under voltage and thermal-induced drift. Bismuth iron molybdate is shown to be almost completely immune to the effects of water vapor. The pellets show great long-term stability of sensor response under most operating conditions, as well as great sensitivity to very small amounts of organic vapor (less than 1 ppm).

Positive ion emission from a platinum hot wire gas sensor

Abstract It was found that hot (500 °C to 1000 °C) metal wires in contact with organic vapors or contaminated by surface carbon will emit positive ions in air. With appropriate bias and collection geometry, currents up to 100 nA can be detected. As the carbon burns on a contaminated wire and the wire becomes cleaner, the current decays to zero. A clean platinum wire that is a good oxidation catalyst produces a steady positive ionic current in the presence of organic vapors. A number of different vapors were tested and it was concluded that higher responses were obtained for vapors with higher numbers of carbon atoms per molecule and greater ease of oxidation by the wire. Oxidizable gases with little or no carbon produced little or no ionic response. The Saha-Langmuir equation is used to calculate the ionization energies required to emit positive ions from the surface of the hot metal wire. This gave ionization potentials of about 6 eV, which are too low to represent ionization potentials for carbon itself or an oxide of carbon and therefore must represent some, as yet unknown, intermediate of the oxidation reaction. It is concluded that both the clean and carbon-contaminated wire responses can be used to design selective gas sensors. At 800 °C, for example, a clean platinum wire works as a highly reproducible gas sensor, giving a linear response from about 10 ppm to 1% vapor concentration of acetone.

Photocapacitance spectroscopy of thin film cadmium selenide electrodes

Abstract Photocapacitance spectroscopic measurements were made on thin film CdSe that was prepared by electrodeposition from a solution of selenosulfite and cadmium ions. The spectra were very similar for five spots on three films that were prepared under identical conditions. In 0.5 M KOH solution, the film appeared stable for a number of measuremens at various applied potentials. At pH 10 the film was relatively stable for applied potentials negative of 0.0 V (SCE). At more positive potentials in this solution, surface oxidation appears to occur, resulting in the formation of a surface layer that alters the spectrum. In both solutions, the photocapacitance spectrum appears independent of applied potential over the range employed. The continuous rise without any abrupt steps in the spectrum as the photon energy is increased indicated that there are probably a series of bandgap states of comparable density. The energy levels range from E c − 1.1 eV to E c − 1.6 eV. In contrast with thin film CdSe prepared by vacuum coevaporation, the electrodeposited material did not require low frequency (7 to 70 Hz) for the measurement.

The influence of residual gas on the properties of evaporated selenium films

The photoconductivity of vacuum-evaporated vitreous selenium films has been found to depend strongly on residual gases which exist in the vacuum chamber. Specific effects depend on whether these gases are the result of increased system outgassing or purge gas leaked into the evaporation chamber. Films that were evaporated in the presence of 6*10-3 Torr of argon, oxygen, air or water-vapour-saturated air had greatly enhanced photoconductivity for superband gap radiation under positive bias. In contrast, increasing the vacuum pressure by partially closing a valve to reduce the pumping speed resulted in a decreased response. The use of a strong reducing agent, such as hydrogen, also caused a reduction in photoconductivity. Films produced from selenium containing 7% tellurium were less sensitive to residual gas. The photoconductivity measured under negative front-surface bias showed a more complicated dependence on evaporation conditions. The enhanced photoconductivity seen in the selenium films was usually correlated with an increased crystallinity, as determined by X-ray diffraction. The data are interpreted in terms of density changes in midgap traps resulting from changes in the concentration of chemically active gaseous impurities.