2002

The work is intended to focus the attention on different aspects of sinterhardening, starting from the main reason to use such a process to the final characterisation of produced samples. Sintehardening allows the production of high apparent hardness parts; the process is rather attractive for components difficult to be quenched because of their dimensions. Admixed alloys, diffusion alloyed mixes, prealloyed ones or hybrid alloys can be used as starting powders, each having positive and negative aspects.

Sol-gel processed NASICON-type with new compositions in the Na3Zr2-(x/4)Si2-xP1+xO12 system showed an improved sinterability with an increase in the x value. This is attributed to liquid phase sintering. This dense electrolyte system is suitable for the application as gas sensors. The CO2 gas sensors using highly dense x = 0.667 (sample B) and x = 1.333 (sample C) samples show a stable EMF response in dry atmosphere which is very close to the theoretical value.

A β-alumina-based gas sensor for automotive exhaust application (hydrocarbon, CO, NO2 detection in 10-1000 ppm concentration range) has been developed by thick film technology (screen-printing) in the frame of a European project. The sensing device consists of a solid electrolyte (β alumina) and of two metallic electrodes having different catalytic properties, the whole system being in contact with the surrounding atmosphere to be analyzed.

As a part of our research on new gas sensors, an original potentiometric gas sensor was developed. The gas sensor consists of a solid electrolyte associated with two different electrodes located in the same gas mixture. Owing to the interesting behavior of this sensor to the action of CO and NOx, it was decided to develop this device within the framework of a European contract and to produce it industrially by screen-printing. The sensor is designed for use in automotive applications. We propose to expose the problems encountered during the development phase as well as the chosen solutions.

A case history of a severe degradation of concrete foundation plinths and piers of an about-35-year-old building located in Northern Italy is described. Significant amounts of gypsum, near ettringite and/or thaumasite were detected by X-ray diffraction analyses performed on ground concrete samples. Large gypsum crystals were mainly located at the interface between the cement paste and aggregates, as observed by scanning electron microscopy coupled to energy-dispersive spectroscopy microanalysis.

The oxidation behaviour of LnSiAlON (Ln=Y, La) glasses was studied at different temperatures (990–1150 °C) and under different water vapour pressures (360–2690 Pa). These results were also compared with those obtained under O2, N2/H2O or O2/H2O mixtures. When glasses are treated under a N2/H2O mixture, optical and SEM observations show porous scales. Transformations of the reaction rate data and a kinetic model show that there is only one limiting process occurring during oxidation. This rate limiting step is the progress of the chemical reaction at the internal interface.