Studio del processo CVI/CVD per lo sviluppo di compositi ceramici rinforzati a fibra lunga

The most relevant thermo-mechanical properties of SiC or C based CFCCs are high strength, high toughness, low weight, high reliability, thermal shock and fatigue resistance. Thanks to these special characteristics, the CFCCs are the best candidates to substitute metals and monolithic ceramics, traditionally employed to realize components in energy, aeronautic and nuclear fields. Among the commonly techniques for the CFCCs production, CVI still represents the most significant one. Its main advantages are the versatility, the high quality deposits and the fact that it is conducted under mild temperature conditions. On the other hand, this technique is quite complex, therefore the set up of all process parameters needs long development time. The main purpose of the present study was to analyze the parameters controlling the CVD and CVI processes. Specifically, deposition and infiltration of SiC and Py-C tests were conducted on non-porous and porous substrates. The experiments were performed with a pilot size Isothermal/Isobaric CVI plant, designed and developed by ENEA. To guarantee the control of the process parameters, a previously optimization of the plant was needed. Changing temperature, pressure, flow rates and methane/hydrogen ratio, the Py-C deposition rate value, for an optimal fibre/matrix interphase thickness, was determined. It was also underlined the hydrogen inhibiting effect over the Py-C deposition rate. Regarding SiC morphologies, a difference between the inner and outer substrate surfaces was observed, as a consequence of a flow rate non-uniformity. In the case of the Cf/C composites development, the key parameter of the CVI process was the gas residence time. In fact, the hydrogen inhibiting effect was evident only with high value of residence time. Furthermore, lower the residence time more homogeneous the Py-C deposition rate was obtained along the reaction chamber axis. Finally, a CVD and CVI theoretical modelling was performed.

Aging of nuclear power plant cables: in search of non-destructive diagnostic quantities

The safety systems of nuclear power plants rely on low-voltage power, instrumentation and control cables. Inside the containment area, cables operate in harsh environments, characterized by relatively high temperature and gamma-irradiation. As these cables are related to fundamental safety systems, they must be able to withstand unexpected accident conditions and, therefore, their condition assessment is of utmost importance as plants age and lifetime extensions are required. Nowadays, the integrity and functionality of these cables are monitored mainly through destructive test which requires specific laboratory. The investigation of electrical aging markers which can provide information about the state of the cable by non-destructive testing methods would improve significantly the present diagnostic techniques. This work has been made within the framework of the ADVANCE (Aging Diagnostic and Prognostics of Low-Voltage I\&C Cables) project, a FP7 European program. This Ph.D. thesis aims at studying the impact of aging on cable electrical parameters, in order to understand the evolution of the electrical properties associated with cable degradation. The identification of suitable aging markers requires the comparison of the electrical property variation with the physical/chemical degradation mechanisms of polymers for different insulating materials and compositions. The feasibility of non-destructive electrical condition monitoring techniques as potential substitutes for destructive methods will be finally discussed studying the correlation between electrical and mechanical properties. In this work, the electrical properties of cable insulators are monitored and characterized mainly by dielectric spectroscopy, polarization/depolarization current analysis and space charge distribution. Among these techniques, dielectric spectroscopy showed the most promising results; by means of dielectric spectroscopy it is possible to identify the frequency range where the properties are more sensitive to aging. In particular, the imaginary part of permittivity at high frequency, which is related to oxidation, has been identified as the most suitable aging marker based on electrical quantities.