Bonds vitreous

Bond Type. Most bonded abrasive products are produced with either a vitreous (glass or ceramic) or a resinoid (usually phenoHc resin) bond. Bonding agents such as mbber, shellac, sodium siHcate, magnesium oxychloride, or metal are used for special appHcations. 

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.

The stmcture of vitreous siUca is a continuous random network of SiO tetrahedra, linked through the sharing of corners. It differs from crystalline sihca ia having a broader distribution of Si—O—Si bond angles and a more random distribution of one tetrahedron with respect to another (44). The density is 2.2 g/cm. ... 

The tetrahedral network can be considered the idealized stmcture of vitreous siUca. Disorder is present but the basic bonding scheme is still intact. An additional level of disorder occurs because the atomic arrangement can deviate from the hiUy bonded, stoichiometric form through the introduction of intrinsic (stmctural) defects and impurities. These perturbations in the stmcture have significant effects on many of the physical properties. A key concern is whether any of these defects breaks the Si—O bonds that hold the tetrahedral network together. Fracturing these links produces a less viscous stmcture which can respond more readily to thermal and mechanical changes. 

Vitreous sihca has many exceptional properties. Most are the expected result of vitreous sihca being an extremely pure and strongly bonded glass. Inert to most common chemical agents, it has a high softening point, low thermal expansion, exceUent thermal shock resistance, and an exceUent optical transmission over a wide spectmm. Compared to other technical glasses, vitreous sihca is one of the best thermal and electrical insulators and has one of the lowest indexes of refraction. 

Because all of these structures share the same short-range bonding scheme, the density differences iadicate that vitreous siUca has a substantial iaterstitial volume and can be compacted. 

A typical absorption curve obtained for a metal-free vitreous sihca after a large dose of y-rays is shown in Eigure 13. The main band is at 215 nm three smaller bands occur at 230, 260, and 280 nm. The 230-nm band may result from an electron trapped at a sihcon atom having an incomplete oxygen bond (205). 

Fig. 8. (a) Synthetic diamond grit for resinoid or vitreous bond (free-cutting) abrasive wheels, and (b) synthetic diamond grit for metal bond abrasive... 

Sedimentary rocks (like sandstone) have a microstructure rather like that of a vitreous ceramic. Sandstone is made of particles of silica, bonded together either by more silica or by calcium carbonate (CaCOj). Like pottery, it is porous. The difference lies in the way the bonding phase formed it is precipitated from solution in ground water, rather than formed by melting. 

Vitreous ceramics are different. Clay, when wet, is hydroplastie the water is drawn between the clay particles, lubricating their sliding, and allowing the clay to be formed by hand or with simple machinery. When the shaped clay is dried and fired, one component in it melts and spreads round the other components, bonding them together. 

The released hydroxyl ion interacts with the siloxane bond in the vitreous network... 

Dental silicate cement was also variously known in the past as a translucent, porcelain or vitreous cement. The present name is to some extent a misnomer, probably attached to the cement in the mistaken belief that it was a silicate cement, whereas we now know that it is a phosphate-bonded cement. It is formed by mixing an aluminosilicate glass with an aqueous solution of orthophosphoric acid. After preparation the cement paste sets within a few minutes in the mouth. It is, perhaps, the strongest of the purely inorganic cements when prepared by conventional methods, with a compressive strength that can reach 300 MPa after 24 hours (Wilson et al, 1972). 

The Raman spectra (0-1400 cm l) shown in Fig re 6 illustrate the structural changes which accompany the consolidation of silica gels. The 1100°C sample is fully dense, whereas the 50 and 600°C samples have high surface areas (1050 and 890 m2/g), respectively. The important features of the Raman spectra attributable to siloxane bond formation are the broad band at about 430 cm 1 and the sharp bands at 490 and 608 cm 1(which in the literature have been ascribed to defects denoted as D1 and D2, respectively). The D2 band is absent in the dried gel. It appears at about 200°C and becomes very intense at intermediate temperatures, 600-800°C. Its relative intensity in the fully consolidated gel is low and comparable to that in conventional vitreous silica. By comparison the intensities of the 430 and 490 cm 1 bands are much more constant. Both bands are present at each temperature, and the relative intensity of the 430 cm 1 band increases only slightly with respect to D1 as the temperature is increased. Figure 7 shows that in addition to elevated temperatures the relative intensity of D2 also decreases upon exposure to water vapor. 

Qualitatively, the dipole-dipole interactions between the macro-molecular chains and the halide salt compensate for the lattice energy of the halide crystal and tend to decrease the interactions existing in the glass between the oxide macroanions. This decrease is probably the reason for the significant drop in the glass transition temperature resulting from the addition of a halide salt (Reggiani et al, 1978). Furthermore this type of reaction is consistent with the fact that dissolution of a halide salt in a vitreous solvent requires the existence of ionic bonds provided by a network modifier. 

Radical intermediates are also trapped by intramolecular reaction with an alkene or alkyne bond. At a mercury cathode this process competes with formation of the dialkylmercury [51], At a reticulated vitreous carbon cathode, this intramolecular cyclization of radicals generated by reduction of iodo compounds is an important process. Reduction of l-iododec-5-yne 5 at vitreous carbon gives the cyclopentane... 

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