Hydrothermal growth system

Porous materials can also be coated with zeolite films by direct synthesis. For example, microcellular SiOC ceramic foams in the form of monoliths were coated on their cell walls with thin films of silicalite-1 and ZSM-5 using a concentrated precursor solution for in situ hydrothermal growth (Fig. 9).[62] The zeolite-coated monoliths show a bimodal pore system and are thermally stable to at least 600 °C. A related strategy is based on the conversion of macroporous Vycor borosilicate glass beads, having pores of about 100 nm, to MFI-type zeolite-containing beads retaining the same macroscopic shape.[63] This conversion was achieved by hydrothermal treatment with an aluminium source and a template such as TPABr. 

In some cases the subsequent hydrothermal growth of the target zeolite results in preferential crystal alignment in the films. For example, oriented UTD-1 membranes were obtained via laser ablation from UTD-1 zeolite and deposition on silicon or porous stainless steel, followed by hydrothermal treatment (Fig. 10).[76] The resulting films showed densely packed plank-like crystals of UTD-1 with the large one-dimensional pore system oriented away from the substrate. 

X-Z Zhao, R Roy, KA Cherian, A Badzian. Hydrothermal growth of diamond in metal-C-H20 Systems. Nature 385 513, 1997. 

Substantial differences between solid-phase reactions and hydrothermal synthesis reactions have been stated in numerous investigations. In solid-phase processes, the sequence of intermediate products formation does not depend on reagents ratio in the initial mixture, and the excess product appears to be a compound with the highest crystallization temperature. On contrary, for the formation of a definite product by hydrothermal synthesis, the initial mixture should contain reagents at an exact stoichiometric ratio [19,20]. In solid-phase reactions, the interaction rate is determined by the rate of diffusion processes, while in hydrothermal processes the determining factor is the rate of dissolution of the initial products in the water. Water simplifies diffusion transport of particles in the system the formation of nuclei and crystal growth occur faster than in solids. 

Main factors which affect a hydrothermal reaction are the initial eomposition, reaction temperature and time. In mild hydrothermal synthesis, reaction temperatures lower than 240 °C are respected for both safety of high pressure in normal autoclaves and protection of softness of Teflon line. In our specific synthesis system, high temperature favorites the reaction and the most important factor was the base concentration in the initial reaction mixtures. The reaction time associated with reaction temperature affected the reaction. Crystallization kinetic experiment for a typical reaction showed that a reaction time more than lOh gave well-crystallized product and the further crystal growth needed additional time. Table 1 lists the starting reaction compositions and phase identification of products obtained at 240 °C for lOh. 

Hydrothermal and related processes are relatively complex. Many factors, such as initial reactants, starting concentration, pH value, crystallization temperature and pressure may affect the formation and crystal growth of products. The presence of ethylenediamine in the crystallization system for 1 seems to be critical for the growth... 

Water is the main natural explosive agent on the Earth. This fact is well demonstrated by all forms of volcanic and hydrothermal explosive manifestations, characterized by a sudden and brutal vaporization of water and other dissolved volatiles from a condensed state, either from aqueous solutions or from supersaturated magmas. This paper is mainly devoted to the first case, i.e. the explosivity of aqueous solutions. Explosions can be defined as violent reactions of systems, which have been perturbed up to transient and unstable states by physico-chemical processes. As such, the traditional approach to such problems is to rely on kinetic theories of bubble nucleations and growths, and this topic has been already the subject of an abundant literature (see references therein ). We apply here an alternative and complementary method by... 

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