The influence of light on the gas sensitive properties of microstructured metal oxide thin films

Metal oxide gas sensors are widely used for different applications and operate normally at high temperatures between 300°C and 600°C. Such high temperatures are mainly needed to speed up the desorption of molecules from the gas sensor surface. Goal of the reported investigations is the reduction of the operating temperatures of such devices by the influence of radiation on the gas adsorption/desorption process. Therefore, the influences of radiation on the gas sensing mechanisms at surfaces of different metal oxides (SnO 2, ZnO, WO 3 and Cr 2-x Ti x O 3+z) have been studied for different wavelengths. The experiments were carried out at an operating temperature of 130°C as well as at room temperature. As radiation sources LEDs emitting at different wavelength were used. The sensor response to NO 2, CO, NH 3 and H 2 has been measured with and without illumination. The investigations have shown that light mainly influences the photo-activation of electron-hole pairs, which results in an increasing of the electrical conductivity of the illuminated metal oxide. The observed influences of photoadsorption and photocatalytic effects are small compared to the photoelectric effect. Only a weak increase of the NO 2 sensitivity during illumination has been measured

Biological water contamination in some cattle production fields of Argentina subjected to runoff and erosion

Grain production has displaced livestock to marginal lands in most of the productive regions in Argentina since 1990. In the fertile Rolling Pampa region, extensive cattle production has been concentrated in lowlands subjected to flooding, salt excess, erosion and sedimentation processes but also in some feedlots recently located in sloping arable lands prone to soil erosion. We studied the concentration of microbiological contamination indicators in runoff water and sediments accumulated in depressions along the tributary network from these lands devoted to cattle production. The aims of this work were: (i) to gather a reliable set of data from different monitoring periods and scales, (ii) to search for simple and sensible variables to be used as indicators for surface water quality advising purposes and (iii) to corroborate previous biological contamination conceptual models for this region. Concentration of pollution indicators in these ponds was related to mean stocking rates from nearby fields and proved to depend significantly on the accumulated water and sediments. Viable mesophiles and total coliforms were found mainly attached to large sediments rather than in the runoff water phase. Seasonal sampling showed that the time period between the last significant runoff event and each sampling date regarding enterococci proved to be a sensible variable for predicting contamination. Enterococci concentration tended to increase gradually until the next extraordinary runoff event washed away contaminants. The mentioned relationship may be useful for designing early warning surface water contamination programs regarding enterococci dynamics and other related microbial pollutants as well

Thermal and mechanical design optimisation of a micro machined mid-infrared emitter for optical gas sensing systems

The numerical pre-design of a novel micro-machined thermal infrared emitter, using a spider type hotplate concept is presented. The spider concept introduced allows the fast transient operation of the emitter and thus a direct modulation of the radiation without additional mechanical tools like choppers. The thermal radiation source is excited by a pulsed electrical voltage and is thermally separated from the carrier substrate. Miniaturisation, cost reduction and economy of scale can be realized by applying the silicon on insulator (SOI) technology in combination with KOH-etching. In order to reach the maximum performance in the operation-relevant wavelength range over 8 mum, a coupled field simulation of the electro-thermal heating and the transient thermal behaviour considering thermal c onduction, convection and radiation was performed. With respect to the required long term reliability of the emitter, the mechanical stability of the component was investigated and improved by additional structure-mechanical modelling and calculations of the electric current density of the heating structure to avoid electro migration effects. The advantageous reliability properties of the new designs were validated by experimental tests performed on prototype samples

Micromachined multicavity grey body emitter for the use in MIR spectroscopic systems

A grey body emitter based on a microcavity array with Pt-heater on the backside is presented. The microcavity array is made by electro-chemical etching of silic on. It has been shown in a previous work, that this emitter has especially in th e spectral region >8 µm significantly higher emissivity than commercial availabl e emitters. Due to the thin-film technology of MEMS-based emitters, these types can be typically operated with a maximum temperature of 700°C to 800°C. Higher t emperature causes degradation of the heater. But higher temperatures also mean a n enhancement in radiation power and thus open a wider area of application. The presented work shows a temperature enhanced thermal emitter with a ceramic heate r passivation. Short time tests show the possibility of a maximum temperature of 1000°C. The part of light emitted by the microcavities in comparison to the who le device as well as the influence of the pore size concerning the emitted spect ral range is investigated. The results are the basis for a redesign of the micro cavity array for an enhancement of the geometry tuned emissivity

Conduction model of SnO2 thin films based on conductance and Hall effect measurements

Thin and porous SnO2 films (70 nm thick with grain size between 10 and 30 nm) have been prepared by e-beam evaporation onto alumina substrate provided with platinum electrodes. The Ohmic character of the contacts was preserved in all measurement conditions utilized for investigations. The dependence of electrical conduction on the composition of the ambient atmosphere has been studied by means of Hall and four point conductance measurements. The experiments were performed in different gas atmospheres containing N2, O2, and CO and at different operation temperatures (between room temperature and 420 °C). A relatively low effective mobility (530 cm2 V1 s1) and a high charge carrier effective concentration (10181019 cm3) were deduced when using the single crystals recipe, as required by the established models for granular materials. The analysis of these experimental data showed the inadequacy of the geometrical models and effective medium theories to correctly extract the electrokinetic parameters from conductance and Hall measurements in the case of gas sensitive layers and to predict their temperature and gas composition dependences. The conventional approach fails because it considers the samples at different temperatures as one physical system while, in fact, the surface chemistry in oxygen atmosphere leads to new trap generation, which is equivalent to the doping level modification. The use of a nonconventional approach, taking into account the film interaction with the ambient through quasichemical equations, and associated mass action laws together with the surface scattering influence on the carrier mobility allowed for the understanding of the involved mechanisms and good fits for the experimental data