Vapor axial deposition

Fig. 21. Schematic illustration of the four primary vapor-phase deposition processes used in optical-fiber fabrication outside vapor deposition (OVD), modified chemical vapor deposition (MCVD), plasma vapor deposition (PVD), and vapor axial deposition (VAD) (115).

Blanks can also be made using Vapor Axial Deposition (VAD) (Fig. 21). The process involves simultaneous flame deposition of both core- and cladding-glass soots onto the end (ie, axially) of a rotating fused-siUca target rod. The finished perform is then consoHdated in a process similar to the OVD process. 

There are many potential variations in the processes used to manufacture optical fiber, but the basic premise remains essentially the same for each. This section will introduce the basics of fiber manufacturing and briefly discuss the three primary methods currently employed to support the economical production of optical fiber on a large scale. They are outside vapor deposition (OVD), vapor axial deposition (VAD), and inside vapor deposition (IVD). 

The vapor axial deposition (VAD) process is basically similar to the OVD process, except that the soot is deposited by fixed burners located near the end of a vertically mounted seed rod. This type of process allows for several fixed burners to be used in laydown and the cylindrical soot rod created in this way is drawn upward as the burner assembly deposits additional layers (see Figure 9.18). Again, as in the OVD process, the seed rod is removed before the next step. 

Heraeus, Shin-Etsu, and Nippon Silica Glass employ variations of this process called vapor axial deposition (VAD), whereby the silica soot is deposited on the end of a continuously withdrawing rotating mandrel and is consohdated to full density in a separately heated zone of the processing apparatus. This process is illustrated in Fig. 6.7. 

FIGURE 6.7 Vapor axial deposition (VAD) process for synthetic fused silica. (From Optical Fiber Waveguides by R. M. Klein in Glass Science and Technology, Vol. 2, Processing, edited by D. R. Uhlmann and N. J. Kreidl, copyright 1983 by Academic Press, reproduced by permission of the publisher.)... 

In a variation of the outside process, called the axial process, the soot is deposited at the end of the rotating preform as it is gradually pulled away (withdrawn) from the burners, as illustrated in Fig. 6.45c. The preform is thus created from the end, the core and cladding materials being deposited essentially simultaneously. This process is sometimes referred to as vapor axial deposition (VAD), axial vapor deposition (AVD) or even axial flame hydrolysis (AFH). 

There are four principal processes that may be used to manufacture the glass body that is drawn into today s optical fiber. "Outside" processes—outside vapor-phase oxidation and vertical axial deposition— produce layered deposits, of doped silica by varying the concentration of SiCl4 and dopants passing through a torch. The resulting "soot" of doped silica is deposited and partially sintered to form a porous silica boule. Next, the boule is sintered to a pore-free glass rod of exquisite purity and transparency. 

Positronium formation is also sensitive to ion-implanted amorphous Si02. Figure 9.7 shows the intensity of the long-lived component, IL, as a function of the positron incident energy for Xe ion-implanted amorphous Si02 ([22]). The sample was obtained by a vapor-phase axial deposition (VAD) method. Xe ions of 400 keV were implanted into the sample to doses of 1 x 1014 and 5 x 1015 ions/cm2 at room temperature. While there is a small difference between IL of 1 x 1014 and II of 1 x 1015, both have a minimum at around 4-5 keV, corresponding to the mean positron implantation depth of -200 nm at which the ions are implanted. 

Finally, Vapor Phase Axial Deposition (VAD) is used for forming optical fibers. This... 

Manufacturing economics require that many devices be fabricated simultaneously in large reactors. Uniformity of treatment from point to point is extremely important, and the possibility of concentration gradients in the gas phase must be considered. For some reactor designs, standard models such as axial dispersion may be suitable for describing mixing in the gas phase. More typically, many vapor deposition reactors have such low L/R ratios that two-dimensional dispersion must be considered. A pseudo-steady model is... 

For many applications, like chemical-vapor-deposition reactors, the semi-infinite outer flow is not an appropriate model. Reactors are often designed so that the incoming flow issues through a physical manifold that is parallel to the stagnation surface and separated by a fixed distance. Typically the manifolds (also called showerheads) are designed so that the axial velocity u is uniform, that is, independent of the radial position. Moreover, since the manifold is a solid material, the radial velocity at the manifold face is zero, due to the no-slip condition. One way to fabricate a showerhead manifold is to drill many small holes in a plate, thus causing a large pressure drop across the manifold relative to the pressure variations in the plenum upstream of the manifold and the reactor downstream of the manifold. A porous metal or ceramic plate would provide another way to fabricate the manifold. 

This model has been applied to vacuum coalers where the material being vapor deposited is evaporated from one or more point sources. Note that and Dr are empirical parameters that account for both convection and diffusion. Rotary vacuum coalers avoid any dependence in the 6 direction by rotating the substrate as it is coated. The false-transient method works here as in Section 16.2.4, but the axial and radial diffusivities are now separate and empirical. 

Significant hydrothermal sites are known from a number of on- and off-axis seamounts. These include the Axial Volcano site on the JFR, a large sulfide deposit on a near-axis volcano at 13°N EPR, Loihi seamount in the Hawaiian-Emperor chain, the Lucky Strike hot-spot-related seamount site on the MAR, and a number of other localities. Axial Volcano and Lucky Strike have been studied most thoroughly, and have high-temperature hydrothermal systems. The Ashes vent field on the summit of Axial Volcano was the first to show effects of boiling at the reduced pressures encountered on the seamount relative to a normal ridge crest (Massoth et al. 1989). Many ridge-crest vent fields have been discovered in the last decade that show the effects of phase-separation into low-salinity vapor and more saline fluid (Butterfield 2000). 

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