Relative crystallinity composite materials

Oxidizer. The major component, by weight and volume, of composite solid propellants is the oxidizer. By far, the most important oxidizer used is AP, a crystalline solid material ground to exacting particle size distributions. This chemical possesses the desirable properties of high density, good thermal stability, and oxygen availability, and relatively low reactivity and cost. Properties of AP and several other materials that are used as oxidizers are summarized in Table 37.5. 

Minerals are inorganic compounds that are found in nature and have both a well-defined composition and crystalline arrangement of atoms. Coal and petroleum hydrocarbons are organic and thus not minerals. Obsidian is not a mineral because it has neither crystalline structure nor a specific composition. Stones such as chert and flint, which are mainly silica, SiO, have a relatively precise composition but lack crystalline structure, so are not minerals. While copper is a mineral, brass and bronze do not occur in nature and do not have a fixed elemental composition, so they are not minerals. A synthetic material can be a mineral, however, as long as it is also found in nature. Hematite can be produced artificially by firing ceramics in an oxidizing environment, but it is still considered mineral because hematite can be found in nature. A synthetic ruby is likewise a mineral because rubies do occur in nature, but modem cubic zirconia is not. 

Zeolites, which form one great family of crystalline porous materials, are broadly used in catalysis (petrochemicals cracking). However, postmodification ofmicrop-orous zeolites is limited to cation exchange or silanation. In addition, zeolites also suffer a drastic limitation in their small pore sizes. Among other porous materials, MS materials, such as MCM-41 and SBA-15 (SBA, Santa Barbara amorphous) [66, 67], are widely used as adsorbents or catalyst supports however, unlike the highly ordered MOFs, their walls are amorphous and thus exhibit relatively disordered surface hydroxyl group distribution [68]. In addition, the diversity of MS materials is limited in terms of composition and porous structure. 

WAXS measurements are based on the ratio of the intensity of the crystalline peaks and the amorphous background. Fitting the amorphous background is best done with the help of a computer. Various corrections can be applied to obtain more accurate absolute crystallinities. However, most frequently we are interested in approximate absolute but accurate relative crystallinities and the corrections are not always necessary. The peak width can give information on crystal perfection - sharper peaks indicate more perfect crystals. Two-dimensional diffraction patterns of isotropic materials show rings corresponding to the diffraction peaks. Intensity variations within the rings can be used to assess crystalline orientation. WAXS only looks at material which can be penetrated by X-rays and so the depth of the analysis is limited. The patterns for filled systems can be hard to interpret but various methods have been developed - for example for use in continuous carbon-fibre composites. 

Polymer dispersed liquid crystals (PDLCs) are a relatively new class of composite materials consisting of micrometric liquid crystalline droplets dispersed into a polymer matrix (Bronnikov et al. 2013). Thus, they combine the unique optical properties of liquid crystals (LCs) with the film-forming ability and mechanical properties of a polymer matrix—resulting in appropriate materials for a large variety of fiexible opto-electronic applications (Bouteiller et al. 1996 Bronnikov et al. 2013 Dierking 2000 Drzaic 2006 Kitzerow 1994 Smith 1993). As evidenced by... 

Let us look at the mechanical properties of polycrystaUine ceramics and composites in detail. Let c = AZ/Zq denote relative strain of material (where Zq stands for the initial length, AI—for length increase of the material subjected to load), and o— for load applied to the material. The load evokes in the material a stress, i.e. force acting on a unit of area of a given volume of material. Except for the immediate vicinity of defects of the crystalline stmcture, the stress has an identical value with the load, o. 

The C-C linkage in tire polymeric [60]fullerene composite is highly unstable and, in turn, tire reversible [2+2] phototransfonnation leads to an almost quantitative recovery of tire crystalline fullerene. In contrast tire similarly conducted illumination of [70]fullerene films results in an irreversible and randomly occurring photodimerization. The important aspect which underlines tire markedly different reactivity of tire [60]fullerene polymer material relative to, for example, tire analogous [36]fullerene composites, is tire reversible transfomration of tire fomrer back to the initial fee phase. 

Solid state NMR is a relatively recent spectroscopic technique that can be used to uniquely identify and quantitate crystalline phases in bulk materials and at surfaces and interfaces. While NMR resembles X-ray diffraction in this capacity, it has the additional advantage of being element-selective and inherently quantitative. Since the signal observed is a direct reflection of the local environment of the element under smdy, NMR can also provide structural insights on a molecularlevel. Thus, information about coordination numbers, local symmetry, and internuclear bond distances is readily available. This feature is particularly usefrd in the structural analysis of highly disordered, amorphous, and compositionally complex systems, where diffraction techniques and other spectroscopies (IR, Raman, EXAFS) often fail. 

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