Ceramic powder production

These developments have only been possible because of a continuing improvement in understanding of the basic solid state science of the ceramic dielectrics, of the electrode metals and of the interaction between the two, and particularly of the technologies of ceramic powder production and the MLCC fabrication processes. 

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 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. 

Binders in Ceramics, Powder Metallurgy, and Water-Based Coatings of Fluorescent Lamps. In coatings and ceramics appHcations, the suspension rheology needs to be modified to obtain a uniform dispersion of fine particles in the finished product. When PEO is used as a binder in aqueous suspensions, it is possible to remove PEO completely in less than 5 min by baking at temperatures of 400°C. This property has been successfully commercialized in several ceramic appHcations, in powder metallurgy, and in water-based coatings of fluorescent lamps (164—168). 

Specialty Aluminas. Process control (qv) teclmiques permit production of calcined specialty aluminas ha nng controlled median particle sizes differentiated by about 0.5 ]lm. Tliis broad selection enables closer shrinkage control of high tech ceramic parts. Production of pure 99.99% -AI2O2 powder from alkoxide precursors (see Alkoxides, metal), apparently in spherical form, offers the potential of satisfying the most advanced appUcations for calcined aluminas requiring tolerances of 0.1% shrinkage. 

Vapor—vapor reactions (14,16,17) are responsible for the majority of ceramic powders produced by vapor-phase synthesis. This process iavolves heating two or more vapor species which react to form the desired product powder. Reactant gases can be heated ia a resistance furnace, ia a glow discharge plasma at reduced pressure, or by a laser beam. Titania [13463-67-7] Ti02, siUca, siUcon carbide, and siUcon nitride, Si N, are among some of the technologically important ceramic powders produced by vapor—vapor reactions. 

HTS materials, because of their ceramic nature, are quite brittle. This has introduced problems relative to the winding of superconducting magnets. One solution is to first wind the magnet with the powder-in-tube wire before the ceramic powder has been bonded and then heat treat the desired configuration to form the final product. Another solution is to form the superconductor into such fine fila-... 

Applications. CVD ceramic powders such as SiC and Si3N4 are used to produce ceramic bodies for a wide variety of applications, either experimentally or in production. These include structural applications in high temperature or corrosive environments where metals are not suitable, in such areas as reciprocating engines, gas turbines, turbochargers, bearings, machinery, and process equipment. 

Firstly it can be used for obtaining layers with a thickness of several mono-layers to introduce and to distribute uniformly very low amounts of admixtures. This may be important for the surface of sorption and catalytic, polymeric, metal, composition and other materials. Secondly, the production of relatively thick layers, on the order of tens of nm. In this case a thickness of nanolayers is controlled with an accuracy of one monolayer. This can be important in the optimization of layer composition and thickness (for example when kernel pigments and fillers are produced). Thirdly the ML method can be used to influence the matrix surface and nanolayer phase transformation in core-shell systems. It can be used for example for intensification of chemical solid reactions, and in sintering of ceramic powders. Fourthly, the ML method can be used for the formation of multicomponent mono- and nanolayers to create surface nanostructures with uniformly varied thicknesses (for example optical applications), or with synergistic properties (for example flame retardants), or with a combination of various functions (polyfunctional coatings). Nanoelectronics can also utilize multicomponent mono- and nanolayers. 

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