S. Gherardi

Thick-film gas sensors based on vanadium–titanium oxide powders prepared by sol-gel synthesis

Two titania powders modified by 10 at.% of vanadium were prepared by two different sol-gel routes. The powders fired at 650 °C had the rutile structure. These powders were used to produce prototype thick-film sensors. Four series of thick-film samples were fabricated by screen-printing, fired for 1 h at 650 and 850 °C. The morphology and gas-sensing properties were examined and compared with those of pure and Ta-added titania films, previously studied by the authors. Ta addition inhibited the anatase-to-rutile phase transformation during heating and was also effective in keeping the TiO2 grain size in the nanometre range. On the contrary, V addition facilitated the anatase-to-rutile phase transformation. Thick films obtained from the two powders had similar conductance behaviour vs. temperature. The gas response of the films was affected by both the grain size and firing temperature.

Metal Sulfides as Sensing Materials for Chemoresistive Gas Sensors

This work aims at a broad overview of the results obtained with metal-sulfide materials in the field of chemoresistive gas sensing. Indeed, despite the well-known electrical, optical, structural and morphological features previously described in the literature, metal sulfides present lack of investigation for gas sensing applications, a field in which the metal oxides still maintain a leading role owing to their high sensitivity, low cost, small dimensions and simple integration, in spite of the wide assortment of sensing materials. However, despite their great advantages, metal oxides have shown significant drawbacks, which have led to the search for new materials for gas sensing devices. In this work, Cadmium Sulfide and Tin (IV) Sulfide were investigated as functional materials for thick-film chemoresistive gas-sensors fabrication and they were tested both in thermo- and in photo-activation modes. Furthermore, electrical characterization was carried out in order to verify their gas sensing properties and material stability, by comparing the results obtained with metal sulfides to those obtained by using their metal-oxides counterparts. The results highlighted the possibility to use metal sulfides as a novel class of sensing materials, owing to their selectivity to specific compounds, stability, and the possibility to operate at room temperature.

Nanostructured (Sn,Ti, Nb)O2 Solid Solution for Hydrogen Sensing

A novel co-precipitation route for preparing pure nanograined (Ti, Sn, Nb)O2 solid solution has been accomplished. The solid solution containing the three elements has been synthesized with the molar ratios for Sn:Ti:Nb 100:42:5. Electron microscopy and X-ray diffraction have been adopted to observe the morphology, the crystalline structure and the mean grain radius. Calcining at 550, 650, 850 or 1050 °C for 2h, showed rutile-like single-phase. The prepared powders have been deposited to produce gas sensors in form of thick films through screen-printing technology. SEM micrographs of both powders and films showed regularly-shaped particles with grain dimensions at nanometric level, the nanostructure being maintained up to 1050°C. The sensors have been tested with different reducing gases showing large responses to hydrogen and good selectivity.

Tin (IV) Sulfide chemoresistivity: A possible new gas sensing material

In the last years, the research in the gas sensor field had a significant upward thrust. Regarding the chemoresistive gas sensors, this has produced a remarkable study of metal oxides semiconductors which, however, have shown different limits. In particular their low selectivity and lack of stability take them to an unreliable responses over time. For this reason, in this work it was decided to study the chemoresistive behavior of a non metal oxide semiconductor as Tin (IV) Sulfide (SnS2). SnS2 nanoparticles was synthetized by precipitation reaction in aqueous solution. Then, structural chemical and morphological characterizations were carried out by means of X-Ray Diffraction and SEM techniques. Furthermore, the thermal stability of the powder was studied with thermogravimetric analysis. The sensitive films were obtained by preparing a screen-printing paste and then depositing it on alumina substrates by means of screen-printing technique. The sensing properties of the obtained devices were tested with several gases at different working temperatures. At the best working temperature, a high selectivity to ketones and aldehydes, with respect to different types of molecules, was observed.

ZnO as functional material for sub-ppm acetone detection

Zinc oxide nano-powders with different morphologies have been synthesized using a wet chemical route, starting from an aqueous solution of Zn(NO3)2·6H2O. The obtained powders shaped like nanoparticles, bipyramidal nano-aggregates, nanoneedles and flower-like nanostructures. The screen-printing technique has been adopted to deposit the functional materials in form of thick films onto miniaturized alumina substrates or onto silicon substrates for PL measurements. Acetone, acetaldehyde, formaldehyde, ethanol and ammonia have been used as testing gases, highlighting that the bipyramidal samples exhibited the best performances versus all gases, but especially toward acetone at sub-ppm level.

Detection of Tumor Markers and Cell Metabolites in Cell Cultures, Using Nanostructured Chemoresistive Sensors

Nowadays, tumor markers detection is one of the most dynamic field of research for medical technologies, as it seems a reliable source of screening technologies able to both detect neoplasms before their degeneration into malignant forms, and monitor possible relapses after the main cancer removal. On the other hand, studying neoplastic cell cultures behaviour, and their vitality in real time, places problems given to the high proliferation rate of the tumor cells. In this work, nanostructured chemoresistive sensors, sensing unit able to detect volatile chemicals in concentrations up to part per billions, have been used to detect neoplastic markers, with the idea to develop a technology able to follow in real time cell cultures and neoplasms growth, for both research and application in the medical field.

Sustainable Water Management: Sensors for Precision Farming

The application of Site Specific Crop Management (SSCM) consists of the knowledge of the variability of soil and yield. [...]

Silicon Carbide: A Gas Sensing Material for Selective Detection of SO2

Silicon carbide (SiC) is a long-time known material with exceptional mechanical properties. Ceramics obtained by sintering SiC grains are very hard and find application in car brakes, bulletproof vests and, in general, in high endurance applications. [...]

Mesoporous silicon gas sensors: design, fabrication and conduction model

Most chemoresistive gas sensors are supported by an insulating substrate, not integrable into silicon IC platforms, and need very high temperature to reach operating performance, this implies energy consumption and a risk factor in the presence of flammable gases. Therefore, porous silicon substrates represent a good choice, thanks to its chemical and physical properties. In this work we designed mesoporous silicon as substrate for gas sensors, and provided a theoretical investigation about the p-Si/PS/gas interface, by analysing the semiconductor band bending at the interface, the formation of a Schottky barrier and the consequent pinning of the Fermi level, due to the high density of surface states in porous silicon. The theoretical considerations have been verified through the experimental measurements with sensors based on p-Si substrate.