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SnO 2 , TiO 2 , In 2 O 3 , LaFeO 3 , and SmFeO 3 thick films have been prepared by screen-printing technology on alumina substrates with comb-type Au electrodes, starting either from commercial or laboratory-synthesized powders. An array of thick-film prototype sensors has been placed beside a conventional station for environmental monitoring. Field tests have been performed by measuring the change in conductivity of the thick films exposed to real atmosphere. Their electrical response has been compared with the results of the analytical instruments for environmental monitoring, approved by the international standards. The same trend was observed for both systems, with very promising results, when nanostructured oxides have been used for the preparation of the thick films. This allows us to consider as feasible the use of these sensors for cheap, innovative stations for air quality control.

Thick films of LaFeO 3 and SmFeO 3 were fabricated by screen-printing technology on alumina substrates with comb-type Au electrodes. The perovskite-type oxide powders used for the preparation of the thick films have been prepared by the thermal decomposition at 700°C of hexacyanocomplexes, Ln[Fe(CN) 6 ] · n H 2 O. These powders are ultrafine, homogeneous, and free of intragranular pores. The films have been fired at different temperatures in the 750–1000°C range, in N 2 and air atmospheres. The gas-sensitive electrical response of the thick films have been tested in laboratory, in environments with different gases (CO and NO 2 ) in dry and wet air. For field tests, the prototype sensors have been placed beside a conventional station for environmental monitoring. The electrical response of the thick films has been compared with the results of the analytical instruments for environmental monitoring. The same trend was observed for both systems, with very promising results.

This work gives further results about the properties of SnO2 nano-sized materials, prepared via a sol–gel route, pure or added with Mo at two different Mo loading (1.8 and 4.7 Mo atoms%). FT-IR spectroscopic and electrical measurements are employed on powders and films, respectively, to obtain information on the electronic effects due to the molybdenum addition. FT-IR spectra in air of the powders show that Mo lowers the concentration of the oxygen vacancies. Accordingly, electrical data show that molybdenum lowers the conductance of the films in air. Electrical measurements show that Mo in amount of 1.8 atoms% lowers the ability to sense NO2 of films fired at 650°C and leaves almost unaltered those of films fired at 850°C. At variance Mo in amount of 4.7 atoms% leaves almost unaltered the ability to sense NO2 of films fired at 650°C and enhances the ability to sense NO2 of films fired at 850°C. The sensing temperature of maximum response for all materials is in any case 150°C. FT-IR spectroscopy is not able to distinguish the different ability of the pure and Mo-added materials to sense NO2, although for some samples an electronic response is evident. At variance the spectroscopic technique has been employed to carefully investigate the nature of surface species formed by NO2/O2 interaction with the three materials and their stability at the working temperature.

Thick films of nanostructured TiO 2 and Niobium-doped TiO 2 have been fabricated by screen-printing technology starting from pure Titania and Niobium-doped Titania powders; the powders were prepared by laser pyrolysis method which provides nanosized particles. The laser powders are crystalline with anatase structure and an average specific surface area of approximately 100 m 2 g −1 ; their grain size ranges from 10 up to 25 nm, the particles are spherical monocristalline and without internal porosity. In this work we evidentiated that TiO 2 -based thick film sensors exhibit a suitable sensitivity to atmospheric environmental monitoring provided that the microstructural properties of the materials are suitably correlated to the required electrical features. Moreover nanostructured particles were obtained at high firing temperature by adding a proper metal ions to inhibit the grain sintering. Finally, prototype sensors based on pure titania sensing film have been prepared and tested in field for environmental monitoring application.

This paper reports the study of semiconducting oxides to develop gas sensors in thick-film form for use in atmospheric pollutant monitoring devices. The investigation was achieved with the following steps: selection of the suitable oxides and of their most appropriate processing method to obtain nano-sized powders, fabrication using screen-printing technology of thick-film sensors from these powders, and electrical measurements in laboratory and in the field. Chemical routes such as sol-gel techniques and thermal decomposition of heteronuclear complexes have been used to prepare nano-sized powders of n-type (TiO2) and p-type (LaFeO3 and SmFeO3) semiconducting oxides. Thick-film gas sensors have been produced by screen-printing technology. Pastes have been prepared and printed on laser precut 96% alumina substrates, each 2×2 mm element being provided with a heater, comb-type Au contacts and a Pt-100 resistor for controlling the operating temperature. The firing of the films has been performed in conditions able to keep grain size at nanometer level. Electrical responses to some major polluting gases (CO, NO, NO2 and O3) have been tested in laboratory and in the field, and compared with results of the analytical techniques approved by the international standards.

The aim of this work has been to obtain a better understanding of the influence of both morphology and palladium addition on the electrical and electronic properties of thick SnO2 films. Two different SnO2 powders, one commercial and one prepared in the laboratory, both pure and after Pd addition (0.36 Pd wt.%), have been studied. The morphology of the samples has been analyzed by transmission electron microscopy (TEM), the texture by volumetric measurements. Films made by commercial samples show particles with sharp borders and inhomogeneous in size (from 20 to 200 nm), while in the laboratory films particles with indented borders and very homogeneous in size (30 nm) are present. Fourier transform infrared (FT-IR) and UV–Vis spectroscopies together with impedance and resistivity measurements have been employed to provide information on the electronic and electrical properties of the four samples in wet air or in the presence of reducing gases. In particular, we have investigated the different responses to CH4 of the four films in the presence of wet air at 350 and 450°C. The morphological differences have been proposed to be at the origin of the different electronic phenomena showed by the commercial and laboratory powders. Palladium addition results in a resistivity increase on both commercial and laboratory samples, in wet air, but the effect is particularly enhanced for the laboratory sample. The response to 1000 ppm CH4 admission (measured by the resistivity decrease) becomes greater after palladium addition, but while commercial samples, both pure and with addition of Pd, show a higher response at 450°C, on the laboratory-prepared sample the Pd addition also lowers the temperatures of maximum sensibility from 450 down to 350°C.

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The influence of Rapid Thermal Annealing (RTA), on silicon electrical and structural characteristics is studied. A degradation directly connected to the technique is observed in electron beam annealed samples, while the lamp annealed ones do not change crystal properties for low temperature anneals.

Optochemical sensing properties of thick films of titanium dioxide (titania) in anatase phase have been studied and compared with tin dioxide cassiterite. Anatase titania exhibits a large photoluminescence response to nitrogen dioxide, which acts as a luminescence enhancer. Intrinsic surface phenomena rather than bulk defectivity are proposed to account for the behaviour and the experimental results are fitted with the Langmuir model. Good operational performances working at room temperature are achieved.

An impedance spectroscopic analysis has been performed on WO3 and MoO3−WO3 thick-film gas sensors, working under the flow through technique. The dielectric response of the metal oxides at typical operation temperature (250–300 °C) has been analyzed in the presence of a few parts per million of NO2, in dry air. The experimental data were processed to get the best equivalent circuit with known circuital elements such as resistance, capacitance, and constant phase element. The sensors have been modeled taking into account the contribution to the total impedance of the intergranular contacts and of the substrate. As a result, a bimodal distribution of grains was foreseen for the WO3 film, as confirmed by scanning electron microscope analysis. In the case of MoO3−WO3 it was also possible to determine the permittivity of the material and the depletion region width.

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We report developing a SnO2 thick-film gas sensor deposited by screen printing onto a micromachined dielectric stacked membrane equipped with an embedded polysilicon microheater and two resistors for temperature measurement. The microheaters were designed to enable an operating temperature of 400 °C at about 30 mW power consumption. A newly developed scheme for temperature measurement was adopted for on-line adjustment of the film temperature through a conventional low-power feedback circuit. The electrical response of the prototypes to CO and CH4 is discussed, and their performance is compared to traditional devices fabricated via thick-film methods.

Rapid thermal annealing (RTA) techniques are increasingly used in many technological applications. To date, however, only a limited knowledge exists of the effects induced by these thermal treatments on the material characteristics. We have studied the defect equilibria in float zone (FZ) and Czochralski zone (CZ) silicon subjected to annealing at 400°C by employing two different heating techniques, namely electron beam and lamp system. By comparison of the variations induced in the minority carrier lifetime and the crystal quality by the two systems an insight into the defects due to the anneal technique itself is obtained.

The minority carrier diffusion coefficient in Si as a function of dopant density is evaluated by comparing the diffusion length, as obtained by a steady state method (using a combination of the photoelectromagnetic (PEM) effect and the steady state photoconductivity (PC) effect) and a life-time obtained by the photoconductivity decay (PCD) method. Within the limits of the experimental data dispersion, the electron minority diffusinn coefficients are found to be comparable to the majority ones in the doping range 3 × < NA < 3 × 1017 cm−3. The minority hole diffusion coefficients are about 36% higher than the majority hole coefficients in the doping range 6.1 × × 1013< ND < 7.1 × 1016cm−3. Die Diffusionskoeffizienten der Minoritatsladungstrager in Silizium werden aus dem Vergleich zwischen Diffusionslange, die mit einer stationaren Methode (der gleichzeitigen Anwendung des photoelektromagnetischen Effekts und des stationaren Photoleitungseffekts) gemessen wurde, und der Lebensdauer bestimmt, die aus Abklingen der Photoleitung ermittelt wurde. Innerhalb des experimentellen Mesfehlers sind im Dotierungsbereich 3 × 1014cm−3 < NA < 3 × 1017cm−3 die Diffusionskoeffizienten der Minoritatselektronen vergleichbar mit denen der Majoritatselek-tronen. Die Diffusionskoeffizienten der Minoritatslocher sind um 36% groser als die der Majoritats-locher im Dotierungsbereich 6,1 × 1013 < ND < 7,1 × 1016cm−3.

Nanosized titania thick and thin films were produced and shown to possess sensing capability. Correlation between electrical performance and structural features is inferred, with particular emphasis to the role played by the grain size. The nanostructured nature of the layers allows the films to be operated within a remarkably lower temperature range (350–800°C) with respect to previously existing titania bulk sensors. The films are sensitive to CO and NO2.

The extraordinary modification of bulk-material properties provided by nanosized materials, due to magnification of surface effects, has encouraged the scientific community to seek novel methodologies of synthesis. Among these, particular emphasis has been given to the techniques that envisage large-scale production and could improve the present-day technology. Nanostructured titania films possess an immense range of applications, e.g. in the field of optics, electrical insulation, photovoltaic solar cells, electrochromic displays, antibacterial coatings, photocatalytic reactors, high-performance anodes in ion batteries, and for gas sensing. Indeed, the production of nanostructured titania thin films has been recently carried out by several methods. It was also shown that some applications greatly benefited from a nanostructured phase for TiO2. [11]

An algorithm for compensating water vapor pressure in CO detection is proposed here and tested on SnO2 thick-film gas sensors. For each sensor working at a fixed temperature, the conductance, G, is fitted by an analytical surface, whose expression can be inverted to determine the CO concentration once the water partial pressure is measured. As soon as the rate of water-vapor pressure change is slower than about 300 Pa/min, G is a function of the temperature, water vapor and CO concentration. If quicker water vapor variations occur instead, the sensing film undergoes a non-negligible transitory phenomenon during which G assumes different values even at fixed water vapor pressure and temperature. This phenomenon prevents the compensation from working properly. An explanation of the behavior is offered by the interpretation of kinetics equations at surface.

Minority electron diffusion coefficients as a function of dopant density are evaluated from the comparison between a steady state diffusion length (photoelectromagnetic effect) and a transient lifetime (photoconductivity decay) measurement. The reliability of the method is discussed with reference to the limits inherent to the two techniques. Within the limits of the experimental data dispersion, minority values comparable to the majority ones for the doping range 3×1014