V. Lantto

Semiconductor gas sensors based on nanostructured tungsten oxide

Semiconductor gas sensors based on nanocrystalline WO3 films were produced by two different methods. Advanced reactive gas evaporation was used in both cases either for a direct deposition of films ...

Electrical studies on the reactions of CO with different oxygen species on SnO2 surfaces

Electrical conductance measurements and a quick cooling of porous SnO 2 samples from initial temperatures above about 250°C to final temperatures below 150°C give a possibility to study the amounts of different oxygen species (O 2 − , O − ) ionosorbed on the surfaces at different initial temperatures and also the catalytic reactions of reducing agents like CO with these oxygen species at final temperatures. At low temperatures the ionosorbed oxygen species stay frozen for a long time after such a quick cooling. Commercial TGS gas sensors and some thick film samples containing 10 wt% of A1 2 O 3 mixed with SnO 2 were used in the present study. The primary aim was to get information about the 0 − /0 2 − balance on SnO 2 surfaces at different temperatures between 250 and 400°C, and about the reaction rates of CO with the O − and O 2 − species at low temperatures below 150°C. It was found that O 2 − is the major ionosorbed oxygen species up to the temperature of 400°C and the reaction rate of CO with O − was found to be at 125°C about two decades higher than that with O 2 − . At 125°C the rate constants for both of these reactions were found to be independent of the oxygen partial pressure in nitrogen. The rate constant for the reaction of CO with O − was almost of the first order in CO partial pressure in the range of 10 to 100 ppm whereas the rate constant for the reaction with O 2 − had a much weaker dependence on CO partial pressure in the range of 10 to 1000 ppm. This kind of results may be of value, for instance, in work where sensitivity and selectivity problems of semiconductor gas sensors are attempted to be solved using temperature cycling.

A study of the temperature dependence of the barrier energy in porous tin dioxide

Abstract The dependence of conductance on temperature of some commercial and thick-film SnO2 semiconductor gas sensors is measured in the temperature range 350 K to 875 K at different partial pressures of oxygen in nitrogen. The measurements are carried out using different heating and cooling rates. The values of the barrier enrgy between particles in the ceramic material are also calculated from these results on the basis of the single barrier model for the conductance. The value of the barrier energy in pure oxygen at a fixed temperature is first determined using the temperature-stimulated change in the conductance. The results are discussed in the light of some other experiments and theoretical models given to describe the barrier energy. A discussion on the effect of different barrier heights between different particles is also given on the basis of a random barrier network model. By using quick cooling from 875 K to 350 K, it is possible to get high barrier values also at this low temperature. It is beleived that this may be the modus operandi of temperature-pulsed sensors.

Nanocrystalline tungsten oxide thick-films with high sensitivity to H2S at room temperature

Ultra-fine powder of tungsten oxide was made by evaporation of tungsten metal by an electric are discharge in a reactive atmosphere. The obtained WO3 powder displayed a mixture of monoclinic and te ...

TiO2 thick-film gas sensors and their suitability for NOx monitoring

The aim of our laboratory study was to test the possibility to use TiO2-based thick-film gas sensors for exhaust NOx monitoring. The lack of selectivity, being one of the main troubles with the semiconductor gas sensors, has been controlled and improved by doping the base material with trivalent additives (Al3+ and In3+). The other way to affect the selectivity and also the sensitivity of semiconductor gas sensors is to change the operation temperature of the sensor. The response measurements are performed in a temperature range of 400 to 550 °C. The conductance response of the sensors is tested using both NOx (NO and NO2) and interfering gases CO, SO2 and O2. The concentration of NO, NO2, CO and SO2 cover the range from tens to hundreds of ppm.

The effect of microstructure on the height of potential energy barriers in porous tin dioxide gas sensors

A theoretical study is made on the height of potential energy barriers formed at intergranular contacts in porous tin dioxide. Expressions are derived for the height of barriers in a one‐dimensional case when the donor concentration is constant and also when there is a concentration gradient in the surface. Expressions are also derived for the case when the intergranular contact is in the shape of a cylinder. The treatment in this case, together with the application of Fermi–Dirac statistics, leads to a conclusion of the existence of a distribution of the height of barriers. The potential barrier on the center line of the cylinder vanishes above a critical radius. The effect of the distribution of the height of barriers on the activation energy of the conductance of porous tin dioxide is discussed on the basis of a random barrier network model.

Simultaneous response of work function and resistivity of some SnO2-based samples to H2 and H2S

Abstract The simultaneous response of the work function and resistivity to H 2 and H 2 S of some pure and doped SnO 2 samples in the form of thick films is measured in humid laboratory air. The gas-sensitive layer of thick-film samples is screen printed on alumina substrates containing gold electrodes on a finger-like structure. Palladium and silver are used as catalytic activators in the SnO 2 . The work function change is measured with a vibrating capacitor (Kelvin probe) automatically compensated by a feedback system and with graphite as the reference electrode. Silver doping has a large effect on the response to H 2 S at temperatures around 150°C, but no effect is found in the case of exposure to H 2 at these temperatures. Palladium doping has an effect only in the case of H 2 exposure, the effect being strongest at lower temperatures around 150°C. The resistance changes give much higher values for the calculated work function changes as compared to the measured ones in the case of H 2 S exposure at 420 and 620 K, whereas in the case of H 2 exposure, the measured one are higher and a linear dependence between the calculated and measured changes of work function is obtained.

Morphological rank of nano-scale tin dioxide films deposited by spray pyrolysis from SnCl4·5H2O water solution

The morphology and some principal details of the crystallographic grain structure of tin dioxide thin (20–300 nm) films obtained from SnCl4·5H2O water solution by spray pyrolysis deposition were studied using scanning electron microscopy (SEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) methods. Direct correlation between the pyrolysis temperature and several fundamental nanoscale grain shapes and crystallographic features successively replacing each other with Tpyr was shown. These were: (I) separate crystallites with a sphere-like shape (spherulites), existing at 300–350 °C; (II) agglomerated spherulites at T=350–410 °C; and (III) nanocrystals with regular crystal faceting at T=410–530 °C. In turn, the nanocrystals can be subdivided into three types of crystal habit: needle or long prismatic, prismatic and pyramidal habits, each of which corresponds to its own temperature range in increasing order. XRD study and crystallographic form analysis of the cassiterite phase allowed us to suggest identification of the grain facets observed in our experiments. An explanation is also suggested for the gas-sensing properties of such films, which are strongly dependent on the crystallographic grain habit {hkl} and may be attributed to surface orientation effects.

Neutron Diffraction Studies of Pb(ZrxTi1-x)O3 Ceramics

Neutron diffraction studies of lead zirconate titanate Pb(ZrxT1-x)O-3 (PZT) powders, 0.20 less than or equal to x less than or equal to 0.54, were carried out at 10 K and 297 K, The phase transitio ...

Surface relaxation of the (110) face of rutile SnO2

Surface relaxation of the stoichiometric and reduced SnO2 (110) surfaces is studied with first-principles calculations. Calculations are carried out with two different self-consistent ab initio LDA methods, which lead to similar results. The most prominent feature in the relaxation is that the surface layer oxygens of the reduced surface move outwards about 0.4 A with respect to the surface tin atoms. The stoichiometric (oxidized) surface is stabilized by the “bridging” oxygen atoms, and therefore, relaxes less. The valence band density-of-states is similar at both surfaces, except that removing bridging oxygens leaves behind electrons that occupy gap states formed at the reduced tin atoms.