Ceramics, Ceramic Processing

Abstract

Processing is key to the reproducible manufacture of ceramics. The tolerance of a finished ceramic to defects determines the raw materials selected, and the control that must be exercised during processing. More expensive advanced ceramics require higher quality, more expensive raw materials coupled with more carefully controlled manufacturing processes.Ceramic processing is complicated both by the number of steps required in manufacture (Fig. 1), and by requirements to optimize the processing in the different steps. These factors are often opposed. For example, a fine particle size provides improved plasticity for forming and a higher thermodynamic driving force for sintering; however, electrostatic attraction and van der Waals forces promote agglomeration, ie, the formation of weakly bound particle clusters, and caking, making mixing and packing difficult. Submicrometer particles can be more easily mixed and packed in liquids, but the finer interparticle pore structure results in higher forming and drying stresses, as well as longer forming and drying times.Ceramics are basically flaw intolerant materials. Consequently chemical and physical defects can severely degrade properties. Additionally, mistakes are cumulative in ceramic processing and these generally cannot be corrected during sintering and post-sintering processing (as they can be in metals processing, for example). The quality of the product is only as good as the quality of the raw materials used, and the control exercised in each of the process steps. Raw MaterialsRaw materials for ceramic processing range from relatively impure clay materials (see CLAYS) mined from natural mineral deposits, to ultrahigh purity powders 638 CERAMICS, PROCESSING Vol. 5Kirk-Othmer Encyclopedia of Chemical Technology. Fig. 1. Flow diagram of the steps and processes involved in manufacturing a ceramic.Vol. 5 CERAMICS, PROCESSING CERAMICS, PROCESSING Vol. 5 by chemical synthesis pose some unresolved processing problems in the areas of handling and mixing. Solid State. Ceramic compounds can be formed by reacting constituent oxides and/or thermally decomposed salts at an elevated temperature in a solid-state process described as calcination (2,13-17). For example, spinel [1302-67-6], MgAl 2 O 4 , can be formed by reacting magnesia [1309-48-4], MgO, and alumina [1344-28-1], Al 2 O 3 .Because solid-state diffusion is inherently slow, fine, well-mixed powders are required to ensure that reactions go to completion and that chemical and structural homogeneity are achieved during calcination. Often multiple calcination, grinding, and mixing steps are performed to ensure homogeneity. Purity of the product is limited by the purity of the constituent raw materials, and by the impurities introduced during grinding. Ceramic powders formed by calcination are typically less expensive (<$1=kg) than those formed by liquid or vapor techniques ($$25 À 50=kg), but are also typically less pure and larger in size. Solution Chemistry. Submicrometer particle size, high purity ceramic powders ca...

Comparative Characterization of Different Cutting Strategies for the Sintered ZnO Electroceramics

“…This suggests that the variation in green compaction pressures and processing conditions had a significant influence on the microstructural integrity and resulting mechanical properties. Typically, with lower green density, the sintering can cause particle coalescence only to an extent such that internal porosity still remains widely distributed [31,32]. In this case, it is more apparent from Figure 13 that larger size and regular grain shaped pores had formed, which suggests that ZnO grains were pulled out of the matrix instead of only the smaller secondary phases in Figure 12 within a denser matrix.…”

3-D Structures via Tape Casting and Lamination

“…To co-fire different ceramic materials at one temperature requires the use of glass-ceramic composites with suitable glasses of low softening temperature like LTCC, in which the ceramic powder act as a filler [19]. LTCC is composed of glass-ceramic composites, which can be densified around 900 °C.…”

Residual Stress Distribution and Characterization on Multi‐Curved Armor Tiles

“…Both SiC and B 4 C are produced from fine powders that can be consolidated into multi‐shaped components through either a hot pressing or pressureless sintering operation . However, to develop the desired microstructure and properties, the green body must be consolidated in a furnace at temperatures in excess of 2000°C.…”