Crystals with Relatively Isotropic Structures

Examples of materials falling into this class include salts such as sodium chloride, diamond, sapphire and other similar minerals, and solid nonmetallic elements such as krypton. The softer members of the class are generally found to obey Amonton s laws with frictional coefficients falling in the range of 0.5-1.0. The harder, more brittle substances such as sodium chloride tend to suffer extensive surface damage due to cracking but still hold more or less to normal behavior. 

Diamond and sapphire differ in that they have lower than normal ju.f values in the area of 0.1, and its value depends on the load, as might be expected for materials that deform elastically rather than plastically. Such materials also begin to show surface damage beyond a certain load. Under very clean conditions, m for diamond has been found to rise to 0.6, suggesting that some mechanism such as the adsorption of a monomolecular water layer or slight surface oxide formation may act to lubricate the diamond surface naturally. 

Many commonly encountered solid lubricants are in fact highly anisotropic crystalline materials that can form layered structures. These include graphite, molybdenum disulfide, and talc, all of which are known as good lubricants under certain conditions. What might one expect to be the primary mechanism of lubricating action is such systems  

FIGURE 18.4. Certain crystalline structures such as graphite and molybdinum sulfide have flat, diskhke structures. Such materials make good lubricants because they can orient themselves and facilitate the sliding of one surface past the other. 

Plastic material under pressure rolls up to lubricate interface 

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Whereas in many metals with relatively simple and isotropic crystal structures the parameter / has values between 0.5 and 1, it can have much more extreme values in materials in which the mobile species move through much less isotropic structures with 1-D or two-dimensional (2-D) channels, as is often the case with insertion reaction electrode materials. As a result, radiotracer experiments can provide misleading information about self-diffusion kinetics in such cases. 

As their name implies, liquid crystals are materials whose structures and properties are intermediate between those of isotropic liquids and crystalline solids (2). They can be of two primary types. Thermotropic liquid crystalline phases are formed at temperatures intermediate between those at which the crystalline and isotropic liquid phases of a mesogenic compound exist. Substances which exhibit thermotropic phases are generally rod- or disc-like in shape, and contain flexible substituents attached to a relatively rigid molecular core. Lyotropic liquid crystalline phases are formed by amphiphilic molecules (e.g. surfactants) in the presence of small amounts of water or other polar solvent. In general, the constituent molecules in a liquid crystal possess orientational order reminiscent of that found in the crystalline phase, yet retain some degree of the fluidity associated with the isotropic liquid phase. 

In the (crystalline) solid phase, the relative locations of all atoms in the material are fixed, defining the crystal structure that can adopt a variety of symmetries commensurate with our three-dimensional (3D) Euclidean space. The crystal is optically isotropic if it crystallizes in a cubic space group, and anisotropic otherwise, with distinct physical features along different directions in the crystal. There is some uncertainty in atomic... 

When a chiral material is added to a nematic liquid crystal at low concentrations, the pitch p appears to vary linearly with the concentration. A mixture of two compounds of opposite chirality can produce a nematic phase at a certain composition. At this compensation point, the pitch becomes infinite. Unwinding of helical structures can be achieved by external fields. Finally, it is mentioned that, for chiral nematics with relatively short pitch, there exist several intermediate phases known as the blue phases between the isotropic and the chiral nematic phases. These blue phases are... 

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