Sealing hydrothermal

The hydrothermal sealing (90 to 100°C) process consists of immersing the anodized parts in a solution of boiling water or other solution such as nickel acetate, wherein the aluminum oxide is hydrated. The hydrated form of the oxide has greater volume than the unhydrated form and thus the pores of the coating are filled or... 

Sulphuric acid electrolytes are widely used in concentrations between 5% and 25% wt. and temperatures between 15 °C and 38 °C. The films are suitable for hydrothermal sealing (Yaffe 1990 Moutarlier et al., 2005). They are less protective than those formed by CAA. Baths with additions of boric or tartaric acids have been developed as replacements for CAA. 

Phosphoric acid-based electrolytes are used mainly to generate oxides that provide enhanced adhesion with paints and adhesives (Anowsmith and Clifford 1985 Arrowsmith et al., 1992 Johnsen et al, 2004 Zhang et al, 2008). The baths generally contain between 10% and 30% wt. phosphoric acid, with a temperature between 15 °C and 30 °C. The films produced are generally unsuited to hydrothermal sealing (Konno et al, 1982). 

Equipment. A typical commercial quartz-growing autoclave is Hlustrated in Figure 1. The material of constmction for use at 17 MPa (25,000 psi) and 400°C can be a low carbon steel, such as 4140, or various types of low aHoy steel. The closure, a modified Bridgeman closure, is based on the unsupported area principle (12). That is, the pressure in the vessel is transmitted through the plunger to the steel surfaces which initially are nearly line contacts. Thus, the pressure in the seal surface gready exceeds the pressure in the vessel because most of the area of the plunger is unsupported. Hydrothermal equipment has been further discussed (10). 

The solvothermal reaction between metal halides and polysulfide anions is also a useful method for the synthesis of metal-polysulfide clusters. Hydrothermal reaction of K2PtCl4 with K2S4 (5 eq) at 130 °C in a sealed tube... 

Fig. 13. Digital photograph showing various stages of the hydrothermal reduction of reaction of CU2O in a sealed tube.

This solution of lOO ml was transferred to a 250 ml Teflon container held in a stainless-steel vessel. After the vessel was tightly sealed, it was heated at 120 200°C for 5 h. After hydrothermal treatment, the TiOi particles were separated in a centrifuge at 10,000 rpm for 10 min and were then washed in distilled water. The particles were dried at 105 C for 12 h and were then calcined at 300 700 C for 3 h. 

Hydrothermal stability (HTS). The stability of the chlorinated resins was determined by a test procedure described in the experimental. The resin according to the standard test is treated with water in a sealed flask at 200 °C for 24 hours to determine the loss in acid functionality and additionally, the level of chlorine released into the aqueous phase. 

Bermin J, Vance D, Archer C, Statham PJ (2006) The determination of the isotopic composition of Cu and Zn in seawater. Chem Geol 226 280-297 Berndt ME, Seal RR, Shanks WC, Seyfried WE (1996) Hydrogen isotope systematics of phase separation in submarine hydrothermal systems experimental calibration and theoretical models. 

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. 

Samples were studied after drying (100°C), after calcination in air to 540°C, and after hydrothermal treatment with 95% steam/5% Np mixtures at 730°C. Calcination and steaming were performed in quartz reactors. For electron paramagnetic resonance studies samples were sealed off in 2 mm quartz tubes luminescence studies were performed with the same sealed off tubes. Samples sealed in Teflon cells were used for Mossbauer studies. 

Hydrothermal synthesis does not require the water to be above its critical point. Huan, et al. published a synthesis of VOC6H5PO3XH2O prepared from phenylphosphonic acid, CeH5PO(OH)2 and vanadium(III) oxide, V2O3 (Huan et al., 1990). The two reagents were added to water, sealed in a Teflon acid digestion bomb, and heated to 200°C. Pure water has a vapor pressure of 225 PSI at 200°C, well within the bursting pressure of the bomb (1800 PSI). Unlike the quartz example, in this case, the solvent became incorporated into the final product. 

Some interesting devices, both on classical (46) and miniamrized scale (47, 48) have been developed for the combinatorial hydrothermal synthesis of zeolites (49,50). Solutions of suitable precursors were dispensed into the wells of the multireactor autoclaves, which were then sealed and heated following appropriate protocols. X-ray characterization of the individuals allowed the determination of the properties of the library individuals. Both protocols and instruments allowed the synthesis of hundreds of composites and their automated (47) or semiautomated (46, 48) work-up and characterization. Structural information was derived from both reports, proving the applicability of combinatorial technologies to hydrothermal synthesis procedures. 

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. 

The cerium hydroxyl carbonate could be obtained through hydrothermal treatments, for example the hydrothermal treatment of cerium oxalate (Li et al., 1996). The most feasible way is to utilize the sealed reaction of Ce with urea CO(NH2)2 in aqueous solution. The hydrolysis of urea in water leads to (NH4)2C03 and provides the base to change the... 

---