Crystalline quartz

In 1921, a discontinuous index of refraction of vitreous Si02 near the (X—(3 transition of quartz (crystalline Si02) was noted (17). These data and subsequent x-ray investigations of vitreous silica led to the suggestion (18) that crystallites on the order of 1.5 nm were present. It was demonstrated, however, that the crystal size would be less than 0.8 nm, and it was suggested that the term crystal loses meaning for these dimensions (19,20). 

There are two types of quartz crystalline and cryptocrystalline or microcrystalline. Crystalline species often occur as terminated hexagonal prismatic crystals, while cryptocrystalline varieties never form crystals, but occur in homogeneous masses. All quartzes have the same crystalline structure at die molecular level. 

Figure 8.4 shows the thermal conductivity for quartz (crystalline Si02) and amorphous silica (a-Si02) [7]. The quartz data follows the T3 behavior at low temperature, peaks at about 10 K, and then drops with increasing temperature. As discussed before, this is the expected trend for a crystalline solid. However, amorphous silica behaves very differently. The value of the thermal conductivity is much lower than that of the crystalline sample for all values of temperature. In addition, the temperature dependence of the conductivity is also vastly different. Hence, the model proposed for crystalline solids cannot be applied for such a case. Note that the relation k = Cvtl3 is still valid although the heat capacity and the mean free path cannot be determined by relations used for crystalline solids. 

Ceramic materials play an important role in the electronics industry. Semiconductor integrated circuits are typically mounted on a ceramic substrate, usually alumina. Some ceramics, notably quartz (crystalline Si02), are piezoelectric, which means that they generate an electrical potential when subjected to mechanical stress. This property enables us to use piezoelectric materials to control frequencies in electronic circuits, as in quartz watches and ultrasonic generators. 

The decrease in the values of can be explained by the increase in the work of nucleus formation due to an increase in the interfacial free energy at the boundary between the nucleus and another particle. Indeed, particles of amorphous silica are chemically most similar to the nuclei of contact. The values of for the contact between silica-modified threads and quartz glass (a highly condensed polysUicic acid) or quartz (crystalline silica) were higher than those for contacts between silica-modified threads and other materials because of a higher degree of chemical similarity. This further emphasizes the role of chemical composition and structure of the substance in the process. 

There are two classes of solids that are not crystalline, that is, p(r) is not periodic. The more familiar one is a glass, for which there are again two models, which may be called the random network and tlie random packing of hard spheres. An example of the first is silica glass or fiised quartz. It consists of tetrahedral SiO groups that are linked at their vertices by Si-O-Si bonds, but, unlike the various crystalline phases of Si02, there is no systematic relation between... 

Fig. 2. The distribution of silicon—oxygen—silicon bond angles in vitreous siUca (22,25). The function V(a) is the fraction of bonds with angles normalized to the most probable angle, 144°. This distribution gives quite a regular stmcture on the short range, with gradual distorting over a distance of 3 or 4 rings (2—3 nm). Crystalline siUca such as quartz or cristobaUte would have a narrower distribution around specific bond angles.

Hydrothermal crystallisation processes occur widely in nature and are responsible for the formation of many crystalline minerals. The most widely used commercial appHcation of hydrothermal crystallization is for the production of synthetic quartz (see Silica, synthetic quartz crystals). Piezoelectric quartz crystals weighing up to several pounds can be produced for use in electronic equipment. Hydrothermal crystallization takes place in near- or supercritical water solutions (see Supercritical fluids). Near and above the critical point of water, the viscosity (300-1400 mPa s(=cP) at 374°C) decreases significantly, allowing for relatively rapid diffusion and growth processes to occur. 

Transmission electron microscopy (tern) is used to analyze the stmcture of crystals, such as distinguishing between amorphous siUcon dioxide and crystalline quartz. The technique is based on the phenomenon that crystalline materials are ordered arrays that scatter waves coherently. A crystalline material diffracts a beam in such a way that discrete spots can be detected on a photographic plate, whereas an amorphous substrate produces diffuse rings. Tern is also used in an imaging mode to produce images of substrate grain stmctures. Tern requires samples that are very thin (10—50 nm) sections, and is a destmctive as well as time-consuming method of analysis. 

Results obtained at high temperatures indicate that the solubihties of the crystalline modifications of sihca are in the order tridymite > cristobahte > quartz, an order that parallels to some extent the chemical reactivity of these forms. Lower values for solubihty of crystalline as compared to amorphous sihca are consistent with the free-energy differences between them. 

The density of vitreous sihca is lower than those of the low pressure crystalline phases of sihcon dioxide (121), especially that of quartz. 

Silicon dioxide [7631-86-9] Si02, exists in both crystalline and glassy forms. In the former, the most common polymorph is a-quartz (low quartz). All commercial appHcations of crystalline quartz use a-quartz, which is stable only below ca 573°C at atmospheric pressure. Some of the properties of a-quartz are Hsted in Table 1. 

The principal health hazard that may be associated with shicon and shicon alloys is caused by the crystalline form of the oxide, ie, quartz, used as a raw material. Shica ia its crystalline form is the chief cause of disabling pulmonary fibrosis, such as shicosis. Over a period of years, the breathing of air containing excessive amounts of crystalline shica can cause shortness of breath (30). 

Most igneous and metamorphic rocks are composed predominantly of alurninosiHcate minerals, including feldspar such as albite (NaAlSi Og) or anorthite (CaAl2Si20g) and crystalline forms of siHca such as quartz (Si02). Various mixed metal-plus-siHcon oxides such as oHvine [(Mg,Fe)2(SiO ] and... 

Of greater interest in recent years have been the peculiar piezolectric properties"" of polyfvinylidene fluoride). In 1969 it was observed" that stretched film of the polymer heated to 90°C and subsequently cooled to room temperature in a direct current electric field was 3-5 times more piezoelectric than crystalline quartz. It was observed that the piezolectric strain coefficients were higher in the drawn film and in the normal directions than in the direction transverse to the film drawing. 

Respiratory fibrogens Fibrogenic dusts e.g. Free crystalline silica, (quartz, tridymite, cristobalite), asbestos (chrysotile, amosite, crocidolite etc.), talc. 

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