Precursor powders

After preparing a homogeneous solution of the precursors, powder precipitation is accompHshed through the addition of at least one complexing ion. For PLZT, frequently OH in the form of ammonium hydroxide is added as the complexing anion, which results in the formation of an amorphous, insoluble PLZT-hydroxide. Other complexing species that are commonly used are carbonate and oxalate anions. CO2 gas is used to form carbonates. Irrespective of the complexing anion, the precipitated powders are eventually converted to the desired crystalline oxide phase by low temperature heat treatment. 

Bulk ceramics are produced conventionally by the sintering of powders. The strength, toughness, thermal stability, and dielectric properties of the fired ceramic depend strongly on the size and uniformity of the precursor powder and on the chemical properties of the powder smface. 

Figure 6.13. Experimental arrangement of the hot-explosive compaction method for the preparation of consolidated Ni-Al alloys (after Kecskes etal. 2004). (a) Precursor powder sample inside a steel-tube container placed in, (b) an asbestos thermal insulation sheet (c) a concentric card-box filled with the powdered explosive (80% NH4NO3 + 20% TNT) (d) threaded steel plugs serving as contacts for the preliminary heating and to be lifted off just before detonating the explosive (e) detonating cords.

Generally, the preparation of washcoated structured catalysts is governed by several parameters, such as the nature and particle size of the precursor powder, loading of powder, nature and concentration of dispersants, temperature of the slurry, use of binders in the slurry and deposition of a primer layer on the monolith. 

E. Bermejo, T. Becue, C. Lacour, M. Quarton, Synthesis of nanoscaled iron particles from freeze-dried precursors, Powder Technol. 94 (Issues 1) (1997) 29-34. 

Hirano, T. and Niihara, K. (1996), Thermal shock resistance of Si3N4/SiC nanocomposites fabricated from amorphous Si-C-N precursor powders , Mater. Lett., 26, 285-289. 

An aqueous solution of salt monomer 9PME was mixed with tetramethoxysi-lane, yielding a gel, which was then vacuum-dried forming a precursor powder composed of silica gel and the salt monomer. This was subjected to high-pressure polycondensation under 235 MPa at 230 °C for 5 h, producing polyimide-silica hybrid molding. By varying the ratio of tetramethoxysilane to the salt... 

Cables via the powder-in-tube (PIT) method The approach is especially suited to processing Bi-2223 into leads and cables for power applications. A silver or silver alloy tube, filled with the partially reacted precursor powders formulated to yield Bi-2223, is drawn down to a wire 1-2mm diameter. The wire is rolled into a tape if that is the required form, usually with a width-to-thickness ratio of approximately 10 1. The composite is then heated to 800-900 °C when the powder partially melts. The recrystallization process is controlled and the pure Bi-2223 phase develops with large grains oriented so that the Cu-O planes lie parallel to the silver surface to optimize Jc. 

The segregation observed in these precursor powders is often lost by calcining the powders where solid state interdiffiision and reaction of the different metals takes place. If the segregation is large scale, then it will take a long time for this interdiffiision to take place. For this reason an understanding of the segregation processes in coprecipitation is important. 

For the deposition of active phase(s), impregnation, adsorption and ion exchange, (co)precipitation, deposition precipitation, and in situ crystallization methods can be used. Moreover, it is possible to mix the active phase in the mixture for extrusion or to deposit the active phase by using a mixed sol containing both the precursors of the oxidic species and the active phase or a slurry with the precursor powder of the active phase. Other coating techniques, e.g., CVD or CVI techniques, can also be used. The dispersion of the active phase depends strongly on the method and conditions used, its precursors form as well as on the history of the active phase. 

Zeta-process and conventionally calcined powders can be made flowable for rapid mold filling in automated isostatic pressing of thin-walled cylindrical tubes by the technique of slurry spray drying (S D). Here the calcined and milled precursor powder is dispersed in an aqueous slurry (i.e., a stable, high solids content, low viscosity suspension) prior to spray-drying into spherical agglomerates. 

The foregoing powder synthesis techniques, along with others such as polymerization and decomposition of complex double alkoxides and the melt and spray decomposition of mixed nitrates, produce precursor powders with a high degree of chemical homogeneity. However upon subsequent calcination of the products of these reactions, the crystallization path invariably passes through the following sequence of reactions ... 

Historically, stabilized (and partially stabilized) zirconia ceramics were prepared from powders in which the component oxides are mechanically blended prior to forming and sintering. Because solid state diffusion is sluggish, firing temperatures in excess of 1800°C are normally required. Furthermore, the dopant was nonuniformly distributed, leading to inferior electrical properties. Trace impurities in the raw materials can also lead to enhancement of electronic conductivity in certain temperature ranges, which is also undesirable. To overcome these problems, several procedures have been developed to prepare reactive (small particle size) and chemically pure and homogeneous precursor powders for both fully stabilized and partially stabilized material. Two of these are alkoxide synthesis and hydroxide coprecipitation. 

---