Production of UHTC Complex Shapes and Architectures

Production of UHTC Complex Shapes and Architectures

Valentina Medri (National Research Council of Italy - Institute of Science and Technology for Ceramics, Italy), Diletta Sciti (National Research Council of Italy - Institute of Science and Technology for Ceramics, Italy) and Elena Landi (National Research Council of Italy - Institute of Science and Technology for Ceramics, Italy)
DOI: 10.4018/978-1-4666-4066-5.ch008
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Abstract

In spite of the difficult sinterability of Zr and Hf borides and carbides, recent results highlight that these ceramics can be produced with full density, fine microstructure, and controlled mechanical and thermal properties, through different procedures: pressureless sintering and hot pressing with proper sintering aids, reactive synthesis/sintering procedures starting from precursors, and field assisted technologies like spark plasma sintering. More recently, the use of near net shaping techniques and the development of UHTC porous components open the way to further and innovative applications, where the performances, fixed the material, are linked to 2D or 3D architectures and the high ratio of specific surface area to volume of the component and to the features of the porosity itself. Structural lightweight parts, insulator panels, filters, radiant burners, and solar absorbers are some of the possible applications.
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State Of The Art On The Production Of Dense Uhtcs

The performance of a structural ceramic is closely linked with the parameters involved in the production process: starting powder, compositions, forming and densification process, microstructure, intergranular secondary phases of the sintered materials and so on. SiC, Si3N4, oxide ceramics, and composites of these materials are typical structural materials for use in high-temperature oxidizing environments. The limit temperature of using the silicon-based ceramics in an oxidizing atmosphere is 1600 °C. Above this temperature, the protective SiO2 surface film, responsible for the thermal stability, begins to soften dramatically and develops substantial vapour pressure (Berton et al., 1992; Guo, 2009; Upadhya et al., 1997). Among the refractory oxides that are stable in an oxidizing environment at T ≥ 2000 °C, zirconia (ZrO2) and hafnia (HfO2) typically have the highest melting points, ~2700 °C and ~2800 °C, respectively (Scott, 1975; Upadhya et al., 1997). However, they are susceptible to thermal shock, and exhibit high creep rates and phase transition at higher temperatures. The need of structural materials for using in oxidizing environments at temperature over 1600 °C has moved the research interest toward the development of the so called ultra-high temperature ceramics (UHTCs). As for common structural ceramics, the understanding of the relation of performance with all the parameters involved in the production process is fundamental for exploiting the potentialities of these ceramics.

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