Supercritical hydrothermal crystallization

In this section, we describe selected examples of inorganic reactions that are carried out in supercritical water (SCH2O) and ammonia (SCNH3), critical temperatures and pressures of which are listed in Table 8.11. An important application of SCH2O is in the hydrothermal generation of metal oxides from metal salts (or supercritical hydrothermal crystallization). Equations 8.84 and 8.85 summarize the proposed steps for conversion of metal nitrates to oxides where, for example, M = Fe(III), Co(II) or Ni(II). 

In this chapter, first the ionic reaction equilibrium, phase behavior, and solubility of metal oxides in supercritical water are discussed. Next, the specific features of hydrothermal synthesis under supercritical conditions are discussed based on the experimental results. The supercritical hydrothermal crystallization method was applied to the production of functional materials, barium hexaferrite (BaFei20i9), metal-doped oxide [Al5(Y- -Tb)30i2, YAG Tb], and Li ion battery cathode material (LiC02O4). The importance of understanding the chemical reaction equilibrium and phase behavior is discussed. 

Hydrothermal crystallisation processes occur widely in nature and are responsible for the formation of many crystalline minerals. The most widely used commercial appHcation of hydrothermal crystallization is for the production of synthetic quartz (see Silica, synthetic quartz crystals). Piezoelectric quartz crystals weighing up to several pounds can be produced for use in electronic equipment. Hydrothermal crystallization takes place in near- or supercritical water solutions (see Supercritical fluids). Near and above the critical point of water, the viscosity (300-1400 mPa s(=cP) at 374°C) decreases significantly, allowing for relatively rapid diffusion and growth processes to occur. 

Separation of Metals from Simulated Mixed Waste Streams through Hydrothermal Crystallization in Supercritical Water... 

T Adschiri, K Kanazawa, K Arai. Rapid and continuous hydrothermal crystallization of metal oxide particles in supercritical water. J Am Ceram Soc 75 1019-1023,... 

Depolymerization, e.g., polyethylene terephthalate and cellulose hydrolysis Hydrothermal oxidation of organic wastes in water Crystallization, particle formation, and coatings Antisolvent crystallization, rapid expansion from supercritical fluid solution (RESS)... 

A solvothermal process is one in which a material is either recrystallized or chemically synthesized from solution in a sealed container above ambient temperature and pressure. The recrystallization process was discussed in Section 1.5.1. In the present chapter we consider synthesis. The first solvothermal syntheses were carried out by Robert Wilhelm Bunsen (1811-1899) in 1839 at the University of Marburg. Bunsen grew barium carbonate and strontium carbonate at temperatures above 200°C and pressures above 100 bar (Laudise, 1987). In 1845, C. E. Shafhautl observed tiny quartz crystals upon transformation of freshly precipitated silicic acid in a Papin s digester or pressure cooker (Rabenau, 1985). Often, the name solvothermal is replaced with a term to more closely refer to the solvent used. For example, solvothermal becomes hydrothermal if an aqueous solution is used as the solvent, or ammothermal if ammonia is used. In extreme cases, solvothermal synthesis takes place at or over the supercritical point of the solvent. But in most cases, the pressures and temperatures are in the subcritical realm, where the physical properties of the solvent (e.g., density, viscosity, dielectric constant) can be controlled as a function of temperature and pressure. By far, most syntheses have taken place in the subcritical realm of water. Therefore, we focus our discussion of the materials synthesis on the hydrothermal process. 

The hydrothermal method for producing single crystals of quartz works by taking advantage of the slight solubility of SiOa) ) in supercritical water ... 

Solvothermal process is now becoming a powerful technique for preparing nanomaterials. It is analogous to hydrothermal synthesis, except that non-aqueous solvents replace water as reaction medium. From the chemical reaction point of view, solvents in supercritical conditions play a significant role in reaction and crystallization. New materials, especially those having metastable phases and special nanostructures, can be obtained under mild conditions. By sealing the reaction system in an autoclave, the reactants and products prevent effectively from oxidation, hydrolysis and volatilization, and the reaction and crystallization can be realized synchronously. 

According to the reaction temperature, hydrothermal and solvothermal synthesis can be classified into subcritical and supercritical synthesis reactions. In subcritical synthesis, the temperature is in the range of 100 to 240 °C, while in supercritical synthesis, the temperature could reach 1000 °C and the pressure could reach 0.3 GPa. By using the special properties of solvent water and other reactants under supercritical high temperature and pressure, various syntheses with specific features could be conducted, resulting in the formation of numerous crystal materials with simple to very complex structures. In addition, it should be pointed out that some crystal materials cannot be obtained by using other preparation approaches except for using hydrothermal or solvothermal synthesis routes. 

Like the oxides, several metal sulfides have also drawn the attention of crystal growth experts. Thus important compounds like ZnS can be grown in electronic-grade quality in supercritical water [101,102]. Hydrolysis is occasionally observed under certain conditons, but is not generally a problem. A variety of other related compounds, such as 61283, Ag3AsSe3, and CdTe, have been prepared hydrothermally, but their growth chemistry has not been studied in great detail [103]. 

Investigations of hydrothermal processes and development of hydrothermal equipment have their origin in the attempts to understand and mimic nature which produces thousands of well-crystallized minerals at very modest temperatures [19]. The hydrothermal processes in this paper deal with water or aqueous fluids in the supercritical range of water, above its critical point = 374°C, Pc = 22.14 MPa). This is opposed to just boiling in water , and supercritical processes dealing merely with hydrocarbons or halogenated hydrocarbons [35]. 

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