Hydrothermal technique

Beryl. Beryl [1302-52-9] Be Al Si O g, is called aquamarine [1327-51 -1] when pale green or blue from inclusion of Fe emerald [12415-33-7] when dark green from Cr or at times V, and morganite or red beryl when pink or red, respectively, from Mn. Only the synthetic emerald is in commercial production, although the other colors can also be grown. Both the flux and hydrothermal techniques are used to grow this luxury synthetic. 

A thin layer of dark green beryl had been grown by a hydrothermal technique over the surface of a pale beryl to imitate emerald. It has been suggested that such stones should be called synthetic emerald-beryl doublets (16). The abiHty to grow thin, but not thick, single-crystal diamond on the surface of natural diamond (17) leads to the possibiHty of growing such a thin film colored blue with boron this has been done experimentally (18). 

The recycling of PVC by hydrothermal techniques is described, in which PVC is thermally converted in a steam atmosphere into hydrogen chloride, hydrogen, carbon dioxide, carbon monoxide and some gaseous and liquid hydrocarbons. Whilst gasification with only steam is an endothermic reaction, partial combustion of PVC by the addition of small amounts of air, enables autothermic operation of the process to take place. This work deals... 

A specific feature of reactions occurring in the autoclave is that the least soluble compounds are always precipitated from the homogeneous phase of the reaction. As a result, the equilibrium of the reaction is always shifted to the formation of these very insoluble compounds. Thus, it becomes clear that by varying the composition of the reaction mixture (mainly due to the introduction of new cations and anions) practically all types of the cluster forms being generated in the given system can be obtained in the solution. This is a clear advantage of the hydrothermal technique for cluster synthesis in the autoclave. 

In the present study low temperature (373 K) hydrothermal technique has been employed for the synthesis of Na8[GaSi04]6(C104)2, containing perchlorate as a guest anion. The product obtained was characterized by x-ray powder diffraction, IR and Raman spectroscopy. The product crystallizes with the cubic sodalite in the space... 

A hydrothermal technique operating at 450°C has been shown to yield samples with narrow transition temperature widths (24)(25). Growth times on the order of 3 days were found to yield crystals 1mm in diameter. 

Table HI. Typical Impurity Content of KTi0P04 and KTi0As04 Crystals Grown by the Flux and Hydrothermal Techniques (Parts Per Million by Weight)...

Yanagisawa K., Ovenstone J. (1999) Crystallization of Anatase from Amorphous Titania Using the Hydrothermal Technique Effects of Starting Material and Temperature, J. Phys. Chem B. 103(37), 7781-7787. 

Hydrothermal Techniques. Hydrothermal reactions typically produce nanometer-sized particles that can be quenched to form a nanoparticle powder, or cross finked to produce nanocrystaUine structures (Feng and Xu, 2001). Hydrothermal conditions allow for reduction in solubihties of ionic materials and thus more rapid nucleation and increased ion mobihty, resulting in faster growth. Via judicious choice of the hydrothermal conditions, a measure of control can be exerted over the size and morphology of the materials. As mentioned earher, the viscosity and ionic strength of solvents is a function of the temperature and pressure at which the reaction is carried out. Other experimental parameters, such as the precursor material and the pH, have an impact on the phase purity of the nanoparticle. The two principal routes for the formation of nanoparticles are hydrolysis/oxidation, and the neutrahzation of hydroxides. There have been limited successes with solvents other than water. 

Hydrothermal-growth is usually defined as the use of an aqueous solvent at elevated temperature and pressure to dissolve a solute which would be insoluble under ordinary conditions. The advantages of the hydrothermal technique are a low growth temperature, a AT close to 0 at liquid/solid interface, an easily scalable technique, the reduction of most of the impurities from source. The disadvantages are the presence of intermediate products, the lithium or sodium or potassium incorporation when such solvents as LiOH, NaOH or KOH are used, the slow growth rates ( 10 mils per day), the inert liner needed, the occasional incorporation of OH and H2O. 

Despite of applications in geochemistry and in crystal growth long before, the hydrothermal technique has only recently been exploited in the routine preparation of inorganic materials. Nevertheless, hydrothermal and related syntheses have evolved into an area of rapidly increasing importance for the preparation of new and structurally complex solid-state compounds [15-18]. It was only during the last two decades that borates synthesized by utilizing hydrothermal and related techniques have occasionally been reported [19,20]. 

Employing a hydrothermal technique, 2 and 3 were also synthesized in the presence of ethylenediamine and methylamine. The role of ethylenediamine and methylamine... 

Aqueous methods and conversion methods In aqueous solutions, the Ce " ions would precipitate even in quite acidic solutions while the Ce " " ions would precipitate as Ce(OH)3 at much higher pH values (7-9) Ce(OH)3 precipitate could be oxidized into ceria in air. In nonaqueous solutions, ceria forms with decomposition of appropriate precursor, with or without the oxidation by air or other oxidative species. If doping is a target, the doping ions would be introduced during the synthesis, and the coprecipitation, sol-gel, combustion/spray pyrolysis, or hydrothermal techniques are usually employed for such a task. 

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