Ultrafine oxide dispersions

I. A Novel Processing Method to Achieve Ultrafine Oxide Dispersions... 

Mavoori, H. Jin, S. New creep-resistant, low melting point solders with ultrafine oxide dispersions. J. Electron. Mater. 1998, 27 (11), 1216-1222. 

Traditionally, diemiites are prepared by mixing fine conqwnent powders, such as ferric oxide and aluminum. Mixing fine metal powders by conventional means can be an extreme fire hazard sol-gel mediods reduce that hazard while achievii ultrafine particle dispersions that are not possible with normal processing methods. In conventional mixing, domains rich in either fiiel or oxidizer exist, vdiich limit die mass transport and dierefore decrease the efficiency of die bum. However, sol-gel derived nanoconqiosites should be more uniformly mixed, thus reducing die magnitude of this problem. 

Inorganics can also be synthesized and used as templates. Thus, controlled siloxane networks were formed when dispersions of alkoxysilanes (such as (MeO)3SiMe) are mixed with the suitable template matrixes. Ultrafine particles of metal oxides can be used as starting materials for the formation of metal oxide films. For instance, a mixture of a double-chained ammonium amphiphile and an aqueous solution of aluminum oxide particles (diameter about 10 100 nm) gives a multilayered aluminum oxide film when calcinated at over 300°C. 

One important class of particulate composites is dispersion-hardened alloys. These composites consist of a hard particle constituent in a softer metal matrix. The particle constituent seldom exceeds 3% by volume, and the particles are very small, below micrometer sizes. The characteristics of the particles largely control the property of the alloy, and a spacing of 0.2-0.3 tim between particles usually helps optimize properties. As particle size increases, less material is required to achieve the desired interparticle spacing. Refractory oxide particles are often used, although intermetallics such as AlFes also find use. Dispersion-hardened composites are formed in several ways, including surface oxidation of ultrafine metal powders, resulting in trapped metal oxide particles within the metal matrix. Metals of commercial interest for dispersion-hardened alloys include aluminum, nickel, and tungsten. 

In preparing fine particles of inorganic metal oxides, the hydrothermal method consists of three types of processes hydrothermal synthesis, hydrothermal oxidation, and hydrothermal crystallization. Hydrothermal synthesis is used to synthesize mixed oxides from their component oxides or hydroxides. The particles obtained are small, uniform crystallites of 0.3-200 jim in size and dispersed each other. Pressures, temperatures, and mineralizer concentrations control the size and morphology of the particles. In the hydrothermal oxidation method, fme oxide particles can be prepared from metals, alloys, and intermciallic compounds by oxidation with high temperature and pressure solvent, that is, the starting metals are changed into fine oxide powders directly. For example, the solvothermal oxidation of cerium metal in 2-mcthoxycthanol at 473-523 K yields ultrafine ceria particles (ca 2 nm). 

Dispersed ultrafine colloidal inorganic oxide particles... 

A sol-gel process is an important method to synthesize many materials in a variety of shapes and forms such as particles, films, and bulks. This method is especially suited for the synthesis and preparation of ultrafine rare earths oxide particles at relatively low temperatures. A sol is a stable colloidal dispersion of small particles suspended in a liquid. The particles are amorphous or crystalline and particle aggregation is prevented by electrostatic repulsion. The particles in some sols interact to form a continuous network of connected particles called a gel, instead of aggregating to form larger particles (precipitates). 

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