Juan C. Rendón-Angeles

Preparation of Selected Ceramic Compounds by Controlled Crystallization Under Hydrothermal Conditions

The main aim of this chapter is to review some particular aspects related with the hydrothermal crystallization process for preparing some selected oxide and non-oxide powders, namely functional compound with electric, piezoelectric, ionic conducting and catalytic properties. The literature on the hydrothermal crystallization of oxide and nonoxide powders is vast; the references cited here are those more appropriated to illustrate this review. A particular attempt is made to broaden the traditional concepts of processing perovskite powders with controlled chemical composition and particle morphology, those aspects will be discussed based on the chemical reactivity of the precursor reactants (gels). Furthermore, an additional approach that takes into account the solubility of the solid species (mineral reagents) that are employed as a precursor in the hydrothermal systems on controlling the crystallization process of oxide particles was further investigated for preparing titanates oxide ceramic. The specific reaction pathways and kinetic aspects are discussed and illustrated by experimental setups for the solution of selected problems in hydrothermal crystallization. Also the chapter includes recent work on the formation of inorganic salts of Sr or Ba, under ordinary hydrothermal treatments via the anionic replacement in sulphate minerals, because these particular reactions have promoted peculiar microstructure on the crystallized material that preserves the bulk geometrical features of particular mineral specie. Therefore, this method could be attractive for preparing net-shaped materials with controlled porosity, with optimized functional properties, because can be used as gas sensor, substrates for porous catalytic materials, filters, among other potential applications.

High Temperature Chemical Interaction Between SSiC Substrates and Ag-Cu Based Liquid Alloys in Vacuo

The interfaces formed at 850 °C under vacuum between polished or oxidized substrates of pressureless sintered α-SiC (SSiC) and Cusil, Cusil-ABA and Incusil-ABA brazing alloys have been characterized by X-ray diffraction, scanning electron microscopy and energy dispersive spectroscopy. No chemical interaction is observed for Cusil on both SSiC substrates. In contrast, the Ti contained in the Cusil-ABA and Incusil-ABA alloys promotes the occurrence of chemical reactions at the metal/ceramic interface with both SSiC substrates. The formation of TiC and Ti5Si3 is observed for Cusil-ABA and Incusil-ABA on untreated SSiC, following the sequence SiC → TiC → Ti5Si3 at the metal/ceramic interface. The formation of Ti5Si3 and Cu3Ti3O, following the sequence SiC → Ti5Si3 → Cu3Ti3O, is observed for both Ti-containing alloys on pre-oxidized SSiC. During the wetting experiments, Ti5Si3 and Cu3Ti3O detach from the ceramic surface, floating away from the ceramic/metal interface into the liquid alloy, where the latter phase partially dissolves. It is concluded that for both Ti-containing alloys in contact with pre-oxidized SSiC, the Ti remaining after the reaction with the silica film is insufficient to decrease the contact angle to the values observed for untreated SSiC or to produce a strong metal/ceramic joint.

Synthesis of Perovskite Oxides by Hydrothermal Processing – From Thermodynamic Modelling to Practical Processing Approaches

The present chapter aims to provide a conscious review of the principles associated with the chemical reactions and the control of the parameters related to the hydrothermal processing of perovskite-structured compounds. Highlights on fundamental principles of the thermodynamic modelling coupled with the relevant technical expertise gained during the past two decades are discussed. Achievements conducted in the early 1990s on thermodynamic modelling of hydro‐ thermal reactions, leading to the estimation of the chemical reaction equilibrium occurring under specific conditions, i.e. above 100°C and 0.1 MPa, are discussed. Additional efforts resulted in dif‐ ferent thermodynamic models that predict crystal growth kinetics and the stability for particle nu‐ cleation; the models based on chemical population balance approaches are also considered. However, these models do not apply for perovskite compounds containing rare earth elements that crystallize under hydrothermal conditions above 250°C, i.e. orthorhombic lanthanum chro‐ mite perovskite. Hence, the final part comprises a literature survey for the experimental research work conducted on various perovskite species produced via hydrothermal treatments, emphasiz‐ ing the relevant conditions that led to the stoichiometric single-phase crystallization.

Titanium Coatings on AISI 316L Stainless Steel Formed by Thermal Decomposition of TiH 2 in Vacuum

Ti-coated AISI 316L stainless steel, for potential biomedical applications, is obtained by thermal decomposition of TiH 2 under vacuum. The presence of hydrogen in the coating material facilitates the sintering process of Ti particles, with simultaneous formation of several inter-diffusion layers at the substrate/coating interface, whose thickness and chemical composition depend mainly on the treatment temperature. Coatings prepared at 1100°C exhibit formation of a wide zone at the substrate/coating interface, which is associated with the appearance of cracks, and which consists of a mixture of λ + χ + α-Fe phases. Formation of abundant microporosity is also observed in this region, which is attributed to the Kinkerdall effect.