Chemoresistive Gas Sensors for Sub-ppm Acetone Detection☆

New sensors for detecting acetone directly in the exhaled breath have been developed. Several metal oxide materials, ZnO in different nano-morphologies, WO3 pure and Zr-loaded and TixSn1-xO2 solid solutions, have been considered for comparing the functional properties of the corresponding thick films with respect to the target gas. ZnO nano-sheets and bipyramidal nano-aggregates exhibited the large responses towards acetone, however ZnO aggregates of nanocrystals, Zr-WO3 and TixSn1-xO2 showed better performances as regards the influence of humidity on the acetone response

Thick film gas sensors

Usually a thick-film gas sensor means a metal-oxide semiconductor sensor obtained by thick-film screen-printing technology. The chemically sensitive layer consists of a paste prepared from metal-oxide powder, inorganic additives and organic binders. The paste is printed over an alumina substrate containing metal film electrodes and a back heating resistor; the paste is then sintered in a thermal or IR belt furnace. Since stannic oxide is the most frequently used material, we shall restrict our discussion to SnO2-based sensors, which are currently prepared and characterized in our laboratory. SnO2 is extremely sensitive to trace concentration of reactive gases in air, while it presents a lack of intrinsic selectivity; however, the selectivity can be improved by the addition of catalysts and promoters. The conductivity of a high-porosity n-type thick film changes in the presence of a reducing gas due to a combustion process that lowers the surface coverage of oxygen ions, causing the return of electrons to the conduction band and lowering the Schottky-barrier heights at the intergranular contacts. On both pure and Pd-doped samples measurements of conductance and capacitance have been made by varying the temperature at fixed gas concentration or by varying the concentration at fixed temperature. The conductance variations on pure and Pd-doped samples by switching periodically between dry and wet air are outlined. Experimental correlations between the energy barrier in gas and in air and the parameter, which relates the conductance G to the partial pressure PR Of the reducing agent (G&unknown;PR), are discussed too

Moisture effects on pure and Pd-doped SnOz thick films analysed by FTIR spectroscopy and conductance measurements

Pure SnO, and Pd/SnO, (0.4 wt.%) pastes have been prepared starting from SnO* powder obtained with the procedures generally used for gas-sensor materials. The pastes are printed on 96% alumina substrates for electrical measurements and the layers detached from the support are used for Fourier-transform infrared (FTIR) investigations. Conductance measurements in dry and wet air are presented together with the FTIR spectra for temperatures ranging from 100 to 450 “C. By alternating wet and dry air, it has been observed that the conductance of pure SnO, samples changes in a reversible way only for temperatures over approximately 200 “C, while the samples catalytically treated with Pd do not present any significant irreversible component. The IR analysis on pure SnOZ samples treated in wet air at temperatures above 200 “C shows the formation of a very broad absorption of electronic nature almost completely destroyed by a subsequent dry-air treatment. The absorption may be due to two families of donor levels at 01.5-0.18 and 0.45-0.50 eV, respectively, from the bottom of the conduction band. Changes of the electronic absorption shape, intensity and reversibility to dry-air contact are observed on the same samples treated in wet air below 200 “C. These results are compared with those obtained for Pd catalytically modified materials. The conductance and impedance measurements in the presence of methane are presented for both the wet- and dry-air treated pure and Pd-doped thick film

Influence of room temperature on a thick-film gas sensor

Nanostructured semiconductor gas sensors are notoriously very sensitive to a huge number of environmental parameters, such as humidity and ambient temperature. It can be noticed that even if we keep the film temperature constant through an electronic feedback, variations of the ambient temperature lead to conductance variations. In this work we try to understand the nature of this dependence. In order to study the correlation between the response and the temperature of the film, of the air near the film and of the ambient temperature a thermal exchange mathematical model has been developed. A simple experimental configuration has been taken into account, with the heated film placed inside the protection cap, the whole sensor inside the test box and the test box subjected to environmental temperature, and the model was solved numerically and compared with the experimental data

Automotive pollution monitoring by an array of thick film gas sensors from nanosized semiconducting oxides

Road traffic and utilities fuel combustion strongly influence the air quality in the urban atmosphere; Carbon Monoxide and Nitrogen Oxides polluting the atmosphere are mainly caused by the exhaust fumes released from motor vehicles. Atmospheric pollution control is today performed, in the required range, by means of traditional analytical techniques such as chromatography, infrared spectroscopy and so on, according to standardized procedures. The draw-back consists in the limited number of air quality monitoring locations due to the high cost of the equipment and to the necessity of skilled personnel. A spread of monitoring sites is greatly important for achieving a better control of the air quality and also for studying the distribution of the pollutants. Such a result could be reached through the development and exploitation of cheap, reliable and selective solid-state sensors. Recently it has been shown that gas sensor performances are improved when the particle sizes decrease to nanometric level, due to the strongly increased specific surface area. Nevertheless to achieve stable, selective and reliable ceramic sensors, the nanostructured materials need a careful control of their structural, physico-chemical and electrical properties. Correct powder preparation is a crucial point and many factors have been considered, including grain shape and size, size distribution, intragranular porosity, and surface conditions. Nanosized powders of n-type and p-type semiconducting materials obtained by chemical routes such as sol-gel techniques, laser-assisted pyrolysis and thermal decomposition of heteronuclear complexes have been used to prepare thick film gas sensors . The screen-printing technology has been adapted for the fabrication of layers composed of nanosized particles; the sensors were printed on miniaturized laser-precut 96% alumina substrates, each 2x2 mm element being provided with a heater, Au interdigitated contacts and a Pt-100 resistor for controlling the operating temperature. Despite the high firing temperature necessary to guarantee long term stability at the working temperature, control and inhibition in grain coalescence have been reached by adding appropriate transition metal ions to the nanostructured ceramic materials. In fact the usage of homogeneous nanometric powder particles, joined together with high temperature (800-1000C) treatments, leads to a great improvement in the detection properties and long-term stability. It must be highlighted once again that repetitive results can be reached only by a very careful technological control of the sensor manufacturing. Electrical, structural and optical characterization of the thick film sensors obtained by different semiconducting oxides will be presented. The correlation between nanoparticle engineering (grain size, doping, thermal treatment) and the final properties of the sensing films will be emphasized. The gas-sensitive electrical properties of the films were studied in laboratory under different gases and in the field. We have recently demonstrated the possibility of performing an environmental monitoring of CO, NOx and ozone by an array of nanostructured thick film sensors which detect the pollutant concentrations with a limited error. This result has been obtained by calibrating different sensor arrays through the pollutant concentrations obtained by internationally recognized analytical techniques

Costruzione degli integrali formali del moto per un sistema di particelle con interazione Lennard-Jones

Per una catena lineare di molecole interagenti con il potenziale di Lennard-Jones tra primi vicini si costruiscono gli integrali formali del sistema che riducono la funzione di Hamilton in forma normale

Modelling of the inter-granular energy-barrier height in very-fine nanograins through a semi-classical approach

Modeling of the electron charge distribution for very fine nanograins ( ≈1 nm) has been addressed via a semi-classical method such as the Thomas–Fermi approximation. This method takes into account quantum effects of electron confinement within a nanograin and allows one to calculate the height of the inter-grain energy barrier, which is a physical quantity that determines the conductive properties of semi- conductor metal-oxides. Comparison with non-quantum models and with experimental measurements is given

Comportamento degli integrali formali per un sistema hamiltoniano vicino a un punto di equilibrio

Si analizza con metodi numerici l'evoluzione temporale delle costanti formali di un sistema hamiltoniano nell'intorno di un punto di equilibrio. Valori positivi dell'esponente di Layapunov rivelano l'esistenza di regioni stocastiche invarianti

Characterization of SnO2 gas sensors. A spectroscopic and electrical study of thick films from commercial and lab prepared samples

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 450oC. 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 450oC, on the laboratory-prepared sample the Pd addition also lowers the temperatures of maximum sensibility from 450 down to 350oC

New phenomenon in the stochastic transition of coupled oscillators

A numerical investigation around the region of transition from quasiperiodic to stochastic motions for a chain of particles with Lennard-Jones interaction is performed. The curve for the maximal Lyapunov number presents sucessive bifurcations