Silica glass, elastic properties

Irradiation by fast neutrons causes a densification of vitreous silica that reaches a maximum value of 2.26 g/cm3, ie, an increase of approximately 3%, after a dose of 1 x 1020 neutrons per square centimeter. Doses of up to 2 x 1020 n/cm2 do not further affect this density value (190). Quartz, tridymite, and cristobalite attain the same density after heavy neutron irradiation, which means a density decrease of 14.7% for quartz and 0.26% for cristobalite (191). The resulting glass-like material is the same in each case, and shows no x-ray diffraction pattern but has identical density, thermal expansion (192), and elastic properties (193). Other properties are also affected, ie, the heat capacity is lower than that of vitreous silica (194), the thermal conductivity increases by a factor of two (195), and the refractive index, increases to 1.4690 (196). The new phase is called amorphous silica M, after metamict, a word used to designate mineral disordered by radiation in the geological past (197). 

Studies of the viscous-elastic behavior of OMC with various water-glass silica moduli and MGF-9 contents have revealed the considerable influence of these factors on composite viscous-elastic properties. Evidently, study of the viscous-elastic properties of OMC is an effective method for prediction of physical, mechanical, and operating characteristics of the composites investigated. 

Vitreous Silica Vitreous sdica is a glass form of sdica composed of Si02. It may be transparent, translucent, or opaque. It has a number of abnormal and anomalous properties in thermal expansion, viscosity, bulk density, compressibility, and elasticity. These properties depend on thermal history and preparation method. Vitreous sdica exhibits high resistance to chemical attack. At ambient temperature, it is not attacked by any chemical except hydrofluoric acid. 

Although Si02 is a typical inorganic glass, it is also atypical in many ways. Several properties of vitreous silica are known to vary anomalously at low temperatures. Anomaly in the low temperature specific heat is the most notable and well investigated. This is reflected in serious disagreement between Debye temperatures calculated from thermal and acoustic measurements. (thermal) and(elastic) are respectively given by (Anderson and Dienes, 1960),... 

Because of their large pore volume, the mechanical properties of dried gels are several orders of magnitude lower than those of the dense silica, but the mechanical behavior seems identical to that of brittle materials like glasses. The bulk density is the main parameter which defines the elastic and brittle features, but two others parameters (pore size distribution and OH content) are also significant to describe and imderstand the whole mechanical behavior. Toughness measurements have shown that the flaw size responsible... 

Regarding PEN nanocomposites, Si02-based systems have been developed since 2004 by Ahn, Kim, and Lee (2004), who surface-modified silica nanoparticles to improve their dispersion in the polymer during melt blending in an internal mixer. They reported on the increase of the elastic modulus and elongation at break at low nanoparticle content (0.4%wt). On the same nanocomposite system, Kim et al. showed an increase of several chemical-physical properties (thermal stability, dynamic mechanical behavior, and crystallization kinetics) after nanoparticle addition, and also a fourfold gain in the elastic modulus above the glass... 

---