Fused silica preform

Prior to extrusion the fused silica preform is usually treated with dilute hydrofluoric acid to remove any imperfections and deformations present on the inner and outer surfaces and then rinsed with distilled water and followed by annealing (55). In a cleanroom atmosphere, the preform is vertically drawn through a furnace maintained at approximately 2000°C. Guidance and careful control of the drawing process is achieved by focusing an infrared laser beam down the middle of the capillary in conjunction with feedback control electronic circuitry in order to maintain nniformity in the specifications of the inner and outer diameters in the final prodnct. 

Three steps are involved in the preparation of a fused sihca column (1) the high-temperature extrusion of the blank capillary tubing from a preform, where the capillary receives a protective outer coating in the same process (2) the deactivation of the inner surface of the column and (3) the uniform deposition of a stationary phase of a desired film thickness on the deactivated inner surface. In this section the extrusion of fused silica will be described the procedures employed for the deactivation and coating of fused-silica capillaries are presented in the following section. 

Tensile strength versus cross-sectional area for SiOj fibers prepared from viscous sols and heated at 500, 800, and 1000"C compared to conventional fused silica fibers. From Sakka [205], in Sol-Gel Technology for Thin Films, Preforms, Electronics, and Speciality Shapes, ed. L.C. Klein (Noyes Publications, Park Ridge, New Jersey, 1988). 

Fusing the preformed compounds or the mixtures of the elements in evacuated silica capsules at 900- -1160°C. 

Modern Manufacturing Techniques. Manufacturing techniques for making bulk vitreous silica are for the most part improved variations of the historical processes. The main exception is the sol—gel process (see Sol-gel technology). All processes involve the fusion or viscous sintering of silica particles. The particles can be in the form of a loose powder or a porous preform. The powders can be made from natural quartz or from the decomposition of chemical precursors, such as silicon tetrachloride, and tetraethylorthosilicate (1 EOS). In some approaches, such as flame hydrolysis, the powder is produced and fused in a single step. The improvements made to these techniques deal mainly with the procedures used to prepare the powders, that is, to control purity and particle size, and the specific conditions under which the powders are consolidated. 

A capillary is essentially a tube with a large ratio of outer to inner diameters. It is drawn to the correct dimensions starting from a larger preform of fused synthetic silica which is heated in an induction furnace to its softening temperature and pulled... 

---