Sintered materials polycrystalline

The single crystals thus far prepared are too small for many physical property measurements. Also, the two techniques described for preparation of powders result in fine grained (and not sintered) materials which are also not appropriate for many measurements. A different technique, however, was recently developed which produces near theoretical density polycrystalline pellets (56). Stoichiometric mixtures of BaO, K02 and Bi2Os are mixed and melted in N2 gas and quickly quenched onto a copper block under the N2 atmosphere. The process must be performed... 

Polycrystalline ceramics have a structure consisting of many grains. The size, shape, and orientation of the grains play a key role in many of the macroscopic properties of these materials, for example, mechanical strength. In most ceramics, more than one phase is present, with each phase having its own structure, composition, and properties. Control of the type, size, distribution, and amount of these phases within the material provides a means to control properties. The microstructure of a ceramic is often a result of the way it was processed. For example, hot-pressed ceramics often have very few pores. This may not be the case in sintered materials. 

Klimke J, Krell A (2005) Polycrystalline Zr02-transparent ceramics with high refractive index. Fraunhofer Institute for Ceramic Technologies and Sintered Materials (IKTS), p 23... 

Reviews on the synthesis of sinter materials consisting of p-BN and/or y-BN and their use as machining tools are numerous and have been cited with the respective applications. Variable intergrain bonding additives are contained in polycrystalline hard boron nitride sinters, and it is difficult to decide if a material should be classified as a polycrystalline hard boron nitride sinter, as a sintered boron nitride base ceramic, or as hard metal-bonded dense boron nitride. 

The term ceramics refers to a broad range of materials including not only polycrystalline materials, but also powdered materials, thin films and single crystals, and glassy inorganic materials. In this section, we will use a slightly narrower definition of ceramics as polycrystalline sintered materials. 

The mechanical properties and the failure mechanisms of transition-metal carbides are reviewed in detail by Toth.t l Generally, large spreads in the reported values found in the literature are common. This is particularly true in older reports which were mostly performed on sintered materials. More recently, testing has been switched to single crystals or polycrystalline materials obtained from the melt or by thin-film deposition. These are believed to yield more accurate and consistent information. Yet, any test must be carefully characterized in order to be meaningful. The following factors influence mechanical testing.l H ... 

For the final stage sintering of polycrystalline materials, the powder system is idealized in terms of an array of equal-sized tetrakaidecahedra with spherical pores of the same size at the corners (Fig. 8.8d). The tetrakaidecahedron has 24 pores (one at each corner), and each pore is shared by four tetrakaidecahedra, so the pore volume associated with a single tetrakaidecahedron is Vp = (24/4)(4/ 3) TTr, where r is the radius of a pore. Making use of Eq. (8.36), the porosity in a single tetrakaidecahedron is... 

Diffusion of Be through polycrystalline unirradiated specimens of BeO was studied. The isotope was deposited on one end of cylindrical specimens by exchange and adsorption from aqueous solution. The depth of penetration perpendicular to the deposition surfaces was determined after annealing. A simple lapping method of high precision was used to determine the penetration depths. For poly crystalline cold-pressed and sintered material, the diffusion behaviour at 1150 to 1800C were described by ... 

Too great a load chosen for deep penetration to overcome or minimize the sample surface factors can produce erratic hardness results as brittle ceramics crack locally around the indent and energy is expended on crack propagation. Deliberate overloading has now been shown to be a nondestructive microscopic way of determining important properties of ceramic systems, and as such occupies the whole of Chapter 5. However, when a polycrystalline ceramic is not obviously cracked and the indented area extends over many grains, then other microstructural features become dominant. For example, equation (1.2) is found to relate hardness to porosity in sintered materials. ... 

The results of this investigation indicate that material transport during sintering of polycrystalline calcium fluoride takes place by slip due to dislocation motion when the stress exerted by surface tension forces or by a combination of surface tension forces and an external force is high. When the stress becomes smaller than a critical stress, which is estimated to be in the neighborhood of 10 dyn/cm (1.4 psi) for calcium fluoride with an average grain size of 40 //, material transport takes place by diffusional flow. 

A wide range of cutting-tool materials is available. Properties, performance capabilities, and cost vary widely (2,7). Various steels (see Steel) cast cobalt alloys (see Cobalt and cobalt alloys) cemented, cast, and coated carbides (qv) ceramics (qv), sintered polycrystalline cubic boron nitride (cBN) (see Boron compounds) and sintered polycrystalline diamond tbin diamond coatings on cemented carbides and ceramics and single-crystal natural diamond (see Carbon) are all used as tool materials. Most tool materials used in the 1990s were developed during the twentieth century. The tool materials of the 1990s... 

The properties of siHcon carbide (4—6) depend on purity, polytype, and method of formation. The measurements made on commercial, polycrystalline products should not be interpreted as being representative of single-crystal siHcon carbide. The pressureless-sintered siHcon carbides, being essentially single-phase, fine-grained, and polycrystalline, have properties distinct from both single crystals and direct-bonded siHcon carbide refractories. Table 1 Hsts the properties of the hiUy compacted, high purity material. 

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 ... 

It has been illustrated that polycrystalline materials can be operated in regenerative electrolytic solar cells yielding substantial fractions of the respectable energy conversion efficiency obtained by using single crystals. Pressure-sintered electrodes of CdSe subsequently doped with Cd vapor have presented solar conversion efficiencies approaching 3/4 of those exhibited by single-crystal CdSe electrodes in alkaline polysulfide PEC [84]. 

In order to interpret correctly the results of electrophysical measurements conducted on vacuum - sintered ZnO semiconductor films one should answer the question concerning the origin of contacts between specific crystallites controlling the electric conductivity of the material. This was accomplished in paper [37] using the method of prerelaxation VAC to run a comparative analysis of mechanisms of charge transfer in thin sintered (in vacuum) films and pressed polycrystalline ZnO samples. 

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