Sintering organic particle

Figure C2.11.3. A scanning electron micrograph of tire spherical alumina granules produced by spray drying a ceramic slurry. The granules are comprised of individual alumina particles, sintering additives, and an organic binder.

Low-temperature synthesis. Due to the homogeneity of the dried powder/gel product and the smaller particle size when compared to grind-and-fire methods, nucleation and growth of crystalline phases can occur at lower temperatures. This also allows the synthesis of metastable phases and the inclusion of organic or other compounds with low thermal stability into the final product. Volatile materials are no longer lost and amorphous phases can be formed. Lower temperatures avoid the effects of particle sintering. 

Just as in condensation polymerization between organic molecules, a small molecule (water, in this case) is produced by the combination of atoms from the two monomers. Repeated condensation produces an insoluble chain of metal atoms connected by oxygen atoms. The polymer chmn may contain branches (i.e., it may be three dimensional) because all three of the hydroxide groups attached to the central metal ion are identical (and are thus equally likely to react). The suspension of the metal oxide-hydroxide polymer is called a gel. The gel is earefully heated to remove the liquid, and what remains is a collection of tiny, remarkably unifomdy sized particles. Sintering of material produced by the sol-gel process produces a cCTamic with relatively few imperfections. 

Extra-Fine Precipitated Hydroxide. Very fine (< 1 /im-diameter) particle size hydroxide is produced by precipitation under carefully controlled conditions using specially prepared hydroxide seed. Precipitation is usually carried out at low (30 —40°C) temperatures causing massive nucleation of fine, uniform hydroxide particles (Fig. 5). Tray or tumiel Ape dry ers are used to dry the thorouglily washed filter cake to a granular product wliich is easily pulverized to obtain the fine hydroxide. Alternatively, the washed product is spray dried. Precipitation from an organic-free aluniinate Hquor, such as that obtained from the soda—sinter process, fields a very wliite product. Tlie fine precipitated hydroxide is used by the paper and plastic industries as fillers. 

Example. The Pechini method for fuel cell electrode preparation. La, Ba, Mn niU ates - - CgHgO — citrate complex - - C2FI6O2 — gel. Metal nitrates are complexed with citric acid, and then heated with ethylene glycol to form a transparent gel. This is then heated to 600 K to decompose the organic content and then to temperatures between 1000 and 1300K to produce tire oxide powder. The oxide materials prepared from the liquid metal-organic procedures usually have a more uniform particle size, and under the best circumstances, this can be less than one micron. Hence these particles are much more easily sintered at lower temperatures than for the powders produced by tire other methods. 

Zorbax PSM particles are made from small (80-2000 A), extremely uniform colloidal silica sol beads. In a patented polymerization process, these beads are agglutinated to form spherical particles. The size of the Zorbax PSM particles is controlled by the polymerization process, and the pore size is determined by the size of the silica sol beads. After polymerization, the silica is heated to remove the organic polymer and sinter the particles. The result is a spherical, porous, mechanically stable, pure silica particle that provides excellent chromatographic performance (Pig. 3.1). 

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