Si-SiC based materials obtained by infiltration of silicon: study and applications

Many efforts and investments have been made in the last decades in order to obtain materials able to withstand extreme conditions in several technological fields such as future military, industrial and space based projects. The present PhD thesis addresses this problem dealing Ultra High Temperature Ceramics (UHTCs) and in particular focusing on Si-SiC-ZrB2 composites obtained with a technique named silicon reactive infiltration (SRI). Each major chapter of the present thesis relates to an article either published or in press on the journals indicated at the end in the seventh chapter (List of author's publications). The sequence of these chapters is obviously not random, but trying to follow the streamline that leads from the physico-chemical and technological properties of the material to its applications, in my case this is meant its use in industrial burners within an european project: CEREXPRO, that was financially supported by the European Commission in the FP7 (7th Framework Programme). The reader is introduced to the current state of the art of UHTC (Chapter 1) having a historical review of UHTC research carried out thus far. Since the most comprehensive research has been focused on compositions containing HfB2 or ZrB2, they are the primary materials discussed. The chapter concludes by illustrating a particular type of UHTC: the siliconized silicon carbide (Si-SiC) and the infiltration method used to obtain it. In the second chapter a methodology to produce Si-SiC-ZrB2 bulk ceramics, taking advantage of the reactive infiltration technique, is proposed. Preforms with different contents of SiC and ZrB2 were compacted with phenolic resin, pyrolysed and finally infiltrated with molten silicon at temperature above its melting point. A detailed study of the mutual interactions between the Si-SiC-ZrB2 constituent materials during their processing and of their chemical activity with oxygen at high temperatures in the prospect of using this material as a matrix of a continuous fiber reinforced composite is addressed in Chapter 3. The study of the early stage of oxidation is the first part of this chapter, while the second part is focused on the long-term oxidation of some selected samples at a high temperature. The aim of chapter four is to give an overview of the processing routes currently available for the preparation of macro-porous ceramics, with particular emphasis on the processing-microstructure-property relations inherent to each process. At the end, the chapter presents several high temperature applications in which Si-SiC foams are currently applied because of their superior thermo-mechanical properties. The Chapter 5, in fact, aims to track the changes of Si-SiC foam properties during the first part of a long time aging condition, typical of industrial burner applications. The project "CEREXPRO" (CERamic heat EXchangers with enhanced materials PROperties), discussed in the Chapter 6, was aimed to develop a new generation of ceramic heat exchangers for high temperature heat recovery with the target of significantly reducing the size and weight as well as also the price of such components by simplifying the manufacturing process and allowing a higher flexibility in the heat exchanger geometry. The use of precursors/template materials taken from the textile industries and a subsequent ceramic conversion, via liquid silicon infiltration, is proposed as the main technological path for reaching the above objectives. The main role of the Polytechnic within this project was the characterization process of material