Sharma, Raghubir Saran (1956)
Optical studies on the etching of glasses.
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Optical studies on the etching of quartz crystals (Both natural and synthetic) with the solution and fusion of Sodium Hydroxide, employing sensitive Optical and Interferometric techniques are described in this work. Similar studies have also been made on the etching of optical glasses with HP Acid Vapour and of fused silica with both NaOH and HP acid vapour. The thesis comprises of three parts. It begins with a historical review of the etch-figures obtained on crystal faces and their theoretical interpretation. The views of a large number of investigators in connection with the mechanism of etching have also been discussed. All the experimental and microscopic techniques are described in Chapter 2. In Part II is given a general introduction regarding the studies of the natural and etched faces of Quartz crystals, with special reference to the artificially etched quartz with HP acid and alkaline solutions, used to infer its probable symmetry and also to determine its electric axes. However, in the present investigations an attempt is made not only to interpret its symmetry from the etch-figures on the different faces of the two types of crystals, but to unfold some of the mysteries of the growth mechanism. A quick method of determining its electric axes has also been suggested. Almost invariably the initial chemical attack takes place at particular sites whereas in the advanced stages of etching the whole surface is filled with etch-pits. The crystallography of such particular regions of high chemical potential is studied by etching the crystals under different conditions. It seems that the initiation of an etch-pit and its subsequent development is governed by the physical character of the surface. Thus it has been amply shown in chapters 5, 6 and 7 that etching takes place preferentially on the weak spots such as strained parts, stacking faults, slip bands and grain boundaries etc. Not all twin boundaries resist etching as previously supposed. In addition, the growth centres, the edges of the growth sheets, surface irregularities and imperfections are the most probable sites for the first attack of the etchant. It is suggested that the deepest pits are situated at places of imperfections and shallow ones on the regions free from such flaws. Etch channels are formed by the intergrowth of a number of pits. The surface along such directions seems to be a misfit area due to the presence of vicinal planes. In chapter 8 the character of the striations on the prism faces of quartz is discussed. The etching of such prism faces has helped considerably in understanding the real nature of striations. The origin of these striations has been suggested on the basis of the proposed growth mechanism that quartz grows by accretion of layers parallel to the rhombo-hedral planes. The rectilinear pattern on the R-faces of one crystal and slip bands on two of the r-faces of the same crystal revealed by the preferential etching along them, have been discussed in Chapter 5 in the light of the proposed growth mechanism. In Part III, the etching of ten optical glasses of known composition, fire-polished and mechanically polished plate-glass and fused Silica is described# The chemical action of HP acid on Silica which is an important constituent of Optical glass determines the etching. The etching experiments on glass described in the Chapters 10 and 11, have been carried out under three headings: 1. To make a comparative study of the depth of etching. 2. To reveal the existence of a surface layer in fire-polished and mechanically polished glass. 3. To reveal the surface cracks associated with the so-called Griffith cracks which are ordinarily invisible. In addition evidence is produced to show the existence of a polish layer by the hardness tests carried out on fire-polished and mechanically polished glass surfaces. This study has also proved that the optical glass is subject to plastic deformation under static indentations. In Chapter 12 the etching of fused Silica with NaOH and HP acid is discussed. Fused Silica with weak cohesive forces etches readily in comparison to crystalline silica. The hemispherical pits thus abundantly reveal the isotropy of the material.
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Institution: University of London, Royal Holloway College (United Kingdom).