Silica based glasses

In the near-IR, sensors almost exclusively rely on silica fibres (standard or low-OH) as they are accepted as industrially fully applicable32, 33 Silica-based glass fibres are chemically and mechanically robust, easy to handle, inexpensive, available with various core and outer diameters, a core-clad transfer fibres or bare sensing fibres, and have successfully been optimised to their theoretical attenuation limit.34. The spectral window allows application up to 2,5 pm. 

Silica analysis of water, 26 39 Silica-based glass fibers fabrication of, 77 136-137 transparency of, 77 132 Silica brick, ASTM classifications and specifications for, 27 509 Silica components, production of, 23 56 Silica fibers... 

Fig. 4.4 (a) Comparison on a logarithmic scale of the conductivity ratio and the thermodynamic activity ratio of alkali oxide in several silica based glasses. Activity ratios are deduced from potentiometric measurements, (b) On the same scale, conductivity vs activity of AgX (X = Cl, Br, I) in phosphate glasses. Activity values are deduced from calorimetric measurements. 

Over the past 40 years a great deal of research has been done on the nucleation and growth of fibers for industrial uses. The predominant industrial fiber is a silica-based glass. Whiskers, with a high degree of internal structural perfection, have been produced under a variety of special conditions from an extraordinarily wide range of compounds. 

Figure 4.2 Dependence of viscosity on temperature for several silica-based glasses. Reprinted, by permission, from W. Callister, Materials Science and Engineering An Introduction, 5th ed., p. 425. Copyright 2000 by John Wiley Sons, Inc.

High-purity silica-based glasses are used as the fiber material, with fiber diameters ranging between about 5 and 100 ptm. The fibers are carefully fabricated to be virtually free of flaws and, as a result, are extremely strong and flexible. We will examine this unique fabrication process in more detail in the next chapter. 

Silica-based glasses Si02 with 10% GeOj Optical fibres for communication... 

Oxide Al203/ (AI2O3 + Si02), Zr02 Silica based glasses, etc. 

One of the main spectroscopic properties that differentiate fluoride glasses from silica-based glasses is the low multiphonon emission rate. These non-radiative relaxations that may strongly compete with radiative processes in rare-earth ions are nearly three orders of magnitude lower in ZBLAN glass than in silicate, as shown in Fig. 2. This property is directly related to the fundamental vibration modes of the host and, therefore, varies basically in the same manner as the infrared absorption edge. 

Physical dimensions or size of a fiber determine whether or not it is respirable. For example, for silica-based glass fiber, fibers less than 3 jxm in diameter and... 

Spinning fibers from a melt is perhaps the most common technique used to make fibers from polymeric and silica-based glasses. Such a technique, however, is not very suitable for metallic fibers because molten metals have very low viscosity. 

Ceramics are primarily compounds. Ceramics other than glasses generally have a crystalline structure, while silica-based glasses, a subclass of ceramic materials, are noncrystalline. In crystalline ceramic compounds, stoichiometry dictates the ratio of one element to another. Nonstoichiometric ceramic compounds, however, occur frequently. Some important ceramic materials are listed in Table... 

The last quarter of the twentieth century saw tremendous advances in the processing of continuous, fine diameter ceramic fibers. Figure 6.4 provides a summary of some of the important synthetic ceramic fibers that are available commercially. We have included in Fig. 6.4 two elemental fibers, carbon and boron, while we have excluded the amorphous, silica-based glasses. Two main categories of synthetic ceramic fibers are oxide and nonoxides. A prime example of oxide fibers is alumina while that of nonoxide fibers is silicon carbide. An important subclass of oxide fibers are silica-based glass fibers and we devote a separate chapter to them because of their commercial importance (see chapter 7). There are also some borderline ceramic fibers such as the elemental boron and carbon fibers. Boron fiber is described in this chapter while carbon fiber is described separately, because of its commercial importance, in Chapter 8. 

Silica-based glass fiber has been around for a long time. Coirunon glass fiber is readily available commercially in a variety of different chemical compositions. 

A great advantage of any silica-based glass is its ease of fabrication, which allows processes such as melt infiltration and compression molding to be used. 

About 8.5% of glass containers used in 1986 wa recycled [11], Since only about 20% of all silica-based glass goes into other products (flat glass, fiba glass, etc.), the 8.5% figure is rather impressive. Most recyclers request (and most municipalities that recycle require) that glass be separated into clear and colored portions. 

F. Lahoz, I. R. Martin, J. Mendez-Ramos, P. Nuflez, Dopant distribution in a Tm -Yb codoped silica based glass ceramic An infrared-laser induced upconversion study, J. Chem. Phys. 120, 6180-90 (2004)... 

Silicon alkoxides have been widely used for the sol-gel synthesis of silica-based glasses and ceramics. Silicon remains fourfold coordinated N = 4) in the precursor as well as in the oxide. All silicon alkoxides Si(OR)4 are therefore monomeric and tetrahedral. Their reactivity decreases when the size of the alkoxy group increases this is mainly caused by steric hindrance factors, which play a major role during the formation of hypervalent silicon intermediates [1]. 

The Kozuka and Sakka chapter demonstrates how nucleation and polymerization play a role in the formation of silica gels and subsequently silica-based glasses. The section concludes with Healy s chapter, which presents a model of the variations in sihca sol coagulation effected by pH changes and addition of electrolytes. As a final note, a list of references [1-15] is included. 

Since it is silica-based, glass fiber for polymer reinforcement could be thought of as a cousin to mineral fillers. But glass fiber is more carefully produced in controlled, uniform, and symmetrical shapes with extremely high aspea ratios, with particle dimensions that are (usually) visible to the human eye. Glass-fiber reinforcement is probably the most cost-effective and most proven way of reinforcing polymers to inaease tensile and flexural modulus and strength. 

We will see that the silica-based glasses are somewhere between elemental semiconductors and ionic materials, but introduce other challenges. 

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