Hydrothermal powder synthesis

However, one should note that in the field of gas sensor design, the technology of hydrothermal powder synthesis is the most usable. One can find in reviews prepared by Byrappa and co-workers... 

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). 

The basic approach to classify powder production methods is based on whether a method is top-down or bottom-up. In a top-down method, micro- and nano-particles are produced due to the stracture and size refinement through the breakdown of the larger particles in a bottom-up method, the mechanism of particle formation is usually by means of nucleatimi, growth and aggregation of atoms and molecules. In a more practical approach, one may divide the powder synthesis methods as follows (1) wet chemistry, such as the chemical precipitation, sol-gel, microemulsion, sonochemistry, and hydrothermal synthesis methods (2) mechanical attrition, grinding and milling (3) gas phase methods, such as the chemical and physical vapor deposition (4) liquid phase spray methods, such as the molten metal spray atomization, spray pyrolysis, and spray drying, and (5) liquid/gas phase methods. 

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. 

The most significant commercial product is barium titanate, BaTiO, used to produce the ceramic capacitors found in almost all electronic products. As electronic circuitry has been rniniaturized, demand has increased for capacitors that can store a high amount of charge in a relatively small volume. This demand led to the development of highly efficient multilayer ceramic capacitors. In these devices, several layers of ceramic, from 25—50 ]lni in thickness, are separated by even thinner layers of electrode metal. Each layer must be dense, free of pin-holes and flaws, and ideally consist of several uniform grains of fired ceramic. Manufacturers are trying to reduce the layer thickness to 10—12 ]lni. Conventionally prepared ceramic powders cannot meet the rigorous demands of these appHcations, therefore an emphasis has been placed on production of advanced powders by hydrothermal synthesis and other methods. 

The presence of triethylenetetramine in the hydrothermal synthesis of open-framework zinc phosphates results in a number of frameworks with one- to three-dimensional structures. The structures include one-dimensional ladders, two-dimensional layer structures, and one structure where the tetramine is bound to the zinc center. The structural type was highly sensitive to the relative concentration of the amine and phosphoric acid.411 Piperazine and 2-methylpiperazine can be used as templating molecules in solvothermal syntheses of zinc phosphates. The crystallization processes of the zinc compounds were investigated by real time in situ measurements of synchrotron X-ray powder diffraction patterns.412... 

Lu, C. and Shen, B. (2010) Electrochemical characteristics of LiNijy 3Coi/3Mni/302 powders prepared from microwave-hydrothermally derived precursors. Synthesis and Reactivity in Inorganic and Metal-Organic Chemistry, 497, 159-165. 

Moreira, M.L., Andres, J., Varela, J.A. and Longo, E. (2009) Synthesis of fine microsized BaZr03 powders based on a decaoctahedron shape by the microwave-assisted hydrothermal method. Crystal Growth and Design, 9, 833-839. 

Shen, B., Ma, J., Wu, H. and Lu, C. (2008) Microwave-mediated hydrothermal synthesis and electrochemical properties ofLiNi1/3Co1/3Mn1/302 powders. Materials Letters, 62, 4075—4077. 

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... 

ZnO particle morphologies are very complex and diversiform in comparison with Ti02. Thus, monodispersed ZnO particles with well-defined morphological characteristics, such as spherical, ellipsoidal, needle, prismatic, and rod-like shapes, have been obtained. Aggregates composed of these basic shape particles have also been achieved. The methods used for synthesis of these ZnO powders include alkali precipitation [214-216], thermal decomposition [217], hydrothermal synthesis [218], organo-zinc hydrolysis [219], spray pyrolysis [220], and other routes. 

Hydrothermal Synthesis of Brookite. Typically, NaOH solution (2M) was added to 5 mL titanium tetrachloride with stirring to adjust the pH value of the solution to 10, producing a basic colloidal solution. The solution obtained was transferred into an autoclave and heated at 200°C for 24 h. The white precipitates were separated by centrifugation and washed three times with deionized water and once with ethanol. The product was dried at 70°C in air overnight and was ground to a fine powder before the deposition of gold precursor on its surface. 

Hydrothermal synthesis of ZnSe and CdSe, using powdered Se with either Zn or Cd heated to 180°C in a water filled autoclave, yields 70-100 nm particles without the need of any stabilizer or capping agent. While most hydrothermal synthesis routes of chalcogenides involve the preparation of binary systems, more complicated ternary compounds can also be prepared. For example, Znln2S4, a ternary chalogenide photocatalyst has been hydrothermally prepared and found to photocatalytically reduce water under visible... 

Single crystals of K NbeOn can be obtained by slow cooling of the melt near 1200°C.4,34,355 Powder samples can be prepared by conventional ceramic methods at about 1100°C. X-ray powder patterns have been reported in the literature.4 The BET surface area of the ceramic preparation85 is typically of the order of 1 m2g-1. Hydrothermal synthesis can be used to prepare the single-nhase oxide at temperatures as low as 280°C with four times the surface area, as reported in the literature.365 Catalytic properties of such preparations have not oeen examined. 

Ti-Mordenite has been obtained by reaction of TiCI4 with dealuminated mordenite and by hydrothermal synthesis. The reaction of TiCl4 with dealuminated mordenite was carried out at 723 K following the method of Kraushaar and van Hooff (1988, 1989). In the hydrothermal synthesis, Si02 powder, NaOH, Al isopropoxide, and TBOT react at 448 K for 3 days (Kim et al., 1993). 

Barium ferrite adequate for certain permanent magnet applications can be prepared from crude mineral iron oxide. A past interest in the material for perpendicular magnetic recording stimulated studies of a variety of chemical routes, including hydrothermal synthesis, to produce powders [12]. 

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