Hydrothermal process

The process temperatures in hydrothermal synthesis (up to 400°C) are generally lower than the temperatures in flux methods. Sometimes the reaction temperature is even kept below room temperature, where the driving force for formation of low-entropy and low-symmetry compounds is comparatively high. 

An autoclave for a hydrothermal reactor is a thick-walled steel reactor able to contain high pressures at elevated temperatures. Powders dissolve in the lower and hotter part of the reactor and crystallize in the form of single crystals in the upper part. Crystals can also be grown by gradually lowering the temperature of the entire reactor vessel large PbO and Na2CoGe04 crystals have been made this way. 

Sodalite, NagAl6Si6024(0H)2 nH20, is a piezoelectric low-density solid that can be easily made photochromic with suitable doping from solution. Sodalite crystals are grown hydrothermally from a concentrated NaOH solution at 100 bars and a temperature of 360-400°C. It has been shown that crystals can be grown epitaxially even if their facets have been covered with a gold coating. 

The hydrothermal (hot water) cycling of seawater at mid-ocean ridges has a profound effect on the chemistry and budgets of some major and trace elements in seawater. To understand the chemical changes it is necessary to know a little about geological processes at mid-ocean ridges. 

The deep waters of the oceans are cool (around 2-4°C) and dense relative to overlying seawater. This dense water percolates into fissures in the basaltic crust. Slowly it penetrates deep into the crust, gradually heating up, particularly as it approaches the heat source of the underlying magma chamber. This massive heat 

Axial hydrothermal convection Off-axis convection Recycling through 

The high temperatures encountered in hydrothermal circulation cells at midocean ridges increase substantially the rate and extent of chemical reaction 

In the following section we describe the effects of hydrothermal activity on major ions in the oceans the effects on minor components in the oceans are discussed in Sections 6.5.2 and 6.5.5. 

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Powder Preparation. The goal in powder preparation is to achieve a ceramic powder which yields a product satisfying specified performance standards. Examples of the most important powder preparation methods for electronic ceramics include mixing/calcination, coprecipitation from solvents, hydrothermal processing, and metal organic decomposition. The trend in powder synthesis is toward powders having particle sizes less than 1 p.m and Httie or no hard agglomerates for enhanced reactivity and uniformity. Examples of the four basic methods are presented in Table 2 for the preparation of BaTiO powder. Reviews of these synthesis techniques can be found in the Hterature (2,5). 

Greater dimensional control and thinner tapes in multilayer ceramics are the driving forces for techniques to prepare finer particles. Metal organic decomposition and hydrothermal processing are two synthesis methods that have the potential to produce submicrometer powders having low levels of agglomeration to meet the demand for more precise tape fabrication. 

A commercial process which uses hydrothermal leaching on a large scale is the Bayer process for production of aluminum oxide (see Aluminum compounds). This process is used to extract and precipitate high grade alurninum hydroxide (gibbsite [14762-49-3]) from bauxite [1318-16-7] ore. The hydrothermal process step is the extraction step in which concentrated sodium hydroxide is used to form a soluble sodium aluminate complex ... 

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