Sintering ceramic powders

A. Caprihan, C. F. M. Clewett, D. O. Kuethe, E. Fukushima, S. J. Glass 2001, (Characterization of partially sintered ceramic powder compacts using fluori-nated gas NMR imaging), Magn. Reson. Imag. 19, 311-317. 

Cannon, M.R., Danforth, S.C., Flint, J.H., Haggerty, J.S., and Marra, R.A., Sinterable Ceramic Powders from Laser-Driven Reactions I, Process Description and Modeling. J. Amer. Ceram. Soc. 65 [7] 324-3% (1982). 

Ultimately, the surface energy is used to produce a cohesive body during sintering. As such, surface energy, which is also referred to as surface tension, y, is obviously very important in ceramic powder processing. Surface tension causes liquids to fonn spherical drops, and allows solids to preferentially adsorb atoms to lower tire free energy of tire system. Also, surface tension creates pressure differences and chemical potential differences across curved surfaces tlrat cause matter to move. 

Conventional Sintering. Ceramic sintering is usually accompHshed by heating a powder compact to ca two-thirds of its melting temperature at ambient pressure and hoi ding for a given time. Densification can occur by soHd-state, Hquid-phase, or viscous sintering mechanisms. 

Dielectric, piezoelectric and pyroelectric properties of LiTa03 derived ceramics containing additives of LiF and MgF2 were investigated and reported on in [407]. The materials were prepared at 900°C by means of two methods Reaction sintering, yielding powdered polycrystalline material ... 

If the temperature and supersaturation are sufficiently high in a CVD reaction, the product is primarily powder precipitated from the gas phase (see Ch. 2). Such powders have few impurities provided that the CVD precursors are carefully purified. Their small diameter and great uniformity are important factors in the production of high quality hot-pressed or sintered ceramic bodies with good mechanical and electrical properties. In addition, the sintering temperatures required for CVD powders are lower than those for conventional powders. 

Other potential applications are ceramic powders coated with their sintering aids, zirconia coated withyttria stabilizer, tungsten carbide coated with cobalt, or nickel, alumina abrasive powders coated with a relatively brittle second phase such as MgAl204 and plasma spray powders without the segregation of alloying elements. 

Ceramic matrix composites are produced by one of several methods. Short fibers and whiskers can be mixed with a ceramic powder before the body is sintered. Long fibers and yams can be impregiated with a slurry of ceramic particles and, after drying, be sintered. Metals (e.g., aluminum, magnesium, and titanium) are frequently used as matrixes for ceramic composites as well. Ceramic metal-matrix composites are fabricated by infiltrating arrays of fibers with molten metal so that a chemical reaction between the fiber and the metal can take place in a thin layer surrounding the fiber. 

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. 

Fig. 3.5.8 Schematic and NMR image of C4F8 gas at 80 kPa in a hybrid phantom containing Vycor glass, a nanoparticulate AI2O3 powder, a nanoparticulate ZnO powder and sintered ceramics made from each of these powders. Dashed boxes indicate the regions of interest (ROIs) from which the isotherms in Figure 3.5.9 were extracted. Adapted from Ref. [21].

Most modern materials are formed empirically by solid-state methods. These methods generally involve more processing activity than chemical synthesis (for example, sintering of ceramic powders, modifying concrete by polymers, thermomechanical processing of alloys, layering polymeric membranes for... 

Examples of computer reconstructed DPF porous media are given in Fig. 5 and encompass all currently available filtration media extruded ceramic filters (including reaction formed media as cordierite and grain-sintered media as SiC), fibrous filters, foams and sintered metal powder/wiremesh. 

Sintering of powdered metals such as aluminum, beryllium, tungsten, and zinc as well as ceramics under pressure is widely practiced as a shaping process, but that is different from the sintering process described here. 

Describe what happens when a ceramic powder is sintered. 

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