Physical properties whiskers

Aluminate cement, 2 415-416 Aluminate ions, silica sols and, 22 394s Aluminates, 2 273-279 analysis, 2 275-276 chemical reactions, 2 273-274 dispersants, 8 710t economic aspects, 2 275 health and safety factors, 2 276 manufacture, 2 274-275 physical properties of, 2 273-274 uses of, 2 276-277 Alumina trihydrate (ATH), 2 274 in synthetic fillers, 11 314-315 Alumina whisker reinforcement, 5 574t Alumina xerogels, X-ray diffraction of, 23 78... 

Whipple s rules, 20 138 Whisker reinforcement, 5 554, 555, 654 performance in ceramic—matrix composites, 5 572-575 physical properties, 5 557t synthesis, 5 642-643 and toughening, 5 622 Whiskers, silicon carbide, 22 533-534 White... 

Small single crystals, such as those of potassium titanatc, are being used at an annual rate of over 10,000 tons for the reinforcement of nylon and other thermoplastics. These composites are replacing die-cast metals in many applications. Another microfiber, sodium hydroxycarbonate (Dawsonite), also improves the physical properties and flame resistance of many polymers. Many other single crystals, called whiskers, such as alumina, chromia, and boron carbide, have been used for making high-performance composites. 

The bulk analysis of /3-SiC whiskers shows the least variation in chemistry. In some whiskers, the residual metals content can vary, most likely, as a result of additives that used as catalysts during synthesis. These include iron, cobalt, and chromium. Studies by Karasek et al. [56] have shown that the physical properties of silicon carbide whisker-reinforced composites do not correlate to the bulk properties of the whiskers significantly. This lack of significant correlation is mainly due to the fact that the important phase chemistry of the whisker-matrix interface is controlled by the matrix chemistry and the surface chemistry of the whiskers. There seems to be little impact of the diffusion of materials into or out of the bulk whisker material. 

Whenever silicon nitride is synthesized in the presence of aluminum-containing compounds (frequently used as a flux material in process of growing whiskers), there is a high probability of the formation of /3 -SiA10Ns. Up to two-thirds of the silicon in /3-Si3N4 can be substituted by Al without a change of structure. The /3 -SiAl()N has mechanical and physical properties similar to y3-Si3N4. It is, however, thermodynamically more stable than silicon nitride. 

Because SiC and aliunina have significantly different properties, the composites have physical properties, which are a combination of both [19]. The addition of SiC whiskers to an alumina matrix increases the hardness and thermal conductivity as shown in Figs. 10 and 11, respectively. Conversely, the electrical resistivity and thermal expansion are observed to decrease with SiC whisker additions as shown in Figs. 12 and 13, respectively. Other detailed property summaries on SiC whisker reinforced alumina can be found in Reference 20. 

Polymer/chitin whiskers nanocomposites produced by the two techniques usually showed different physical properties due to the different morphology of the composite and the fact that different interactions between whiskers can be established. 

A hybrid composite [121,122] can contain more than one type of reinforcement and/or more than one type of matrix, with the objective of improving or lowering the cost of the basic composite [123]. The second reinforcement may be a fiber (continuous or chopped), particles or whiskers. The fiber reinforcement can be in the same laminae and interspersed using any textile process such as weaving, or in different laminae, interspersing plies to obtain the desired mechanical/ physical properties. A sandwich composite is a special case which has an interlayer of a material such as A1 foil or a honeycomb. The matrix may be different for each type of reinforcement, or added to infiltrate the reinforced matrix (e.g. a thermoplastic resin such as PSU) to confer controlled viscosity in the matrix, or an elastomer (e.g. CTBN) for increased toughness. 

In this section, limits were imposed on the improvement of toughness in ceramics by the incorporation of britde, discontinuous reinforcing phases. Observations indicate that such additions bridge cracks in the regions behind the crack tips. Whiskers act in the same manner as discontinuous reinforcing phases. Bridging models may be used to optimize toughening effects and to allow for the choice and modification of pertinent material characteristics, such as physical properties and microstructure. 

A composite material or simply a composite is a duplex and multifunctional material composed of at least two elements working together to produce a structural material with mechanical and physical properties that are greatly enhanced compared to the properties of the components taken separately. In practice, most composites consist of a bulk material called the matrix and a reinforcement material or filler, added primarily to increase the mechanical strength and stiffness of the matrix but also sometimes to modify its thermal conductivity and electrical resistivity. This reinforcement is usually made of fibers (e.g., monofilaments, whiskers) but can also be particulates (i.e., dispersion strengthened and particle reinforced) or even material having a more complex shape (e.g., mesh, ribbon, laminates, etc.). Composites are first classified according to their matrix phase into three major classes ... 

Incorporation of nanomaterials having different size and shapes such as spheres, fibers, whiskers, or plates into polymer matrices enhances the mechanical properties like tensile strength, modulus, stifhiess, and impact strength significantly. Also other physical properties like barrier, optical, thermal resistance, nonflammability, etc., can be improved by the introduction of nanomaterials. Nanomaterials exhibit some unique properties, which are completely different from their corresponding bulk materials. There are mainly three reasons for the improved performance of polymer nanocomposites. The first reason is the increased relative surface area (aspect ratios) and its associated quantum effects exhibited by nanoparticles. As the size of a particle decreases, the proportion of the number of atoms present on the surface will be more as compared to the atoms present in the bulk. The smface atoms exhibit very different properties compared to that of bulk atoms and hence, the properties of nanomaterials are determined by the properties of surface atoms, rather than that of bulk atoms. Thus the nanoparticles that possess a large surface area per unit mass exhibits, totally different quantum mechanical effects. As the size of the material reaches to nanometer size, most of the properties like mechanical, catalytic, electrical, optical properties, etc., can change. 

Silicon carbide (SiC) whiskers are extremely anisotropic, short fibrous crystals and grow from SiC particles along the [111] plane through catalysis. At present, SiC whiskers are usually prepared by vapor reaction and solid methods, and the latter is more economical and more suitable for industrial production. Figure 3.1 shows the microstructures of SiC whiskers. Table 3.1 shows the chemical and physical properties of SiC whiskers. 

Zinc oxide whiskers possess the following attractive physical properties ... 

In addition to the aforementioned whiskers, there are many others whiskers, such as graphite (C) whiskers, barium titanate (BaTiOj) whiskers, titanium boride (TiB2) whiskers, titanium oxide (Ti02) whiskers, Si02-Mg0-Ca0 (SMC) whiskers, and so forth. Table 3.12 shows the chemical and physical properties of different whiskers. 

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