Pore-free Sintering

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. 

When the densified component is pore-free, Vflnai = VsoUds and Dr = 1.0. Anything less than this value indicates the presence of porosity. Control of Dr requires control of both the microstructure of the as-formed part and the firing, because both affect the sintering shrinkage. 

Fig. 4. A 0.09 La203 0.91 Y20, pore-free microstructure achieved by sintering in the two-phase (2150°C) and annealing in the single-phase (1900°C) region of the phase diagram.

At about 1300-1340 °C (binary eutectic temperature WC-Co 1310 °C ternary eutectic temperature W-Co-C 1275/80 °C) partial melting occurs, and more WC is dissolved until the eutectic concentration (54% Co and 46% WC) is reached [9.8]. A further increase in temperature results in additional dissolution of WC and complete melting of the binder phase. In this stage, rapid final densification occurs and the sintered body is practically pore-free. 

So-called self-flowing Cr-Ni-boron (B)-silicium (Si) alloys, which, after the actual spraying process without any adhesion layer, are submitted in a second operation to subsequent treatment at about 1100°C, liquid phase sintering. This process produces virtually pore-free layers with good adhesion characteristics. 

Green samples or net shape articles should have the optimum initial porosity (0o)- On the one hand, 0q should be as small as possible, which makes it easy to produce pore-free material after combustion sintering. On the other hand, as shown above, Bq is one of the main parameters that defines the combustion regime and the degree of final conversion for example, too small Bq could make impossible to accomplish the steady-state combustion regime. 

After the initial demonstration of feasibility, fundamental investigations established the knowledge required to create a commercial process, i.e., to better understand the chemistry of MCVD required to control the Incorporation of Ge02 and limit hydroxyl impurities. To increase deposition effidency, it was necessary to understand the mechanism by which particles deposit on the substrate tube and sinter to pore-free glass. Although process development preceded quantitative understanding, this knowledge formed the basis for optimization of the commercial process. 

Fig. 5.20 Pores in SiC pipe and a disk joined to it by spark plasma sintering (SPS) a pores in the pipe and the interface to pore free the disk is seen, b indent by nano indentation and cracks emanation from the corners. Stern et al. (unpublished). Courtesy of Prof A. Stern...

Three major quantities are used in P/M the bulk density of the loose iron powder (e.g., 3000 kg.m for water-atomized powder) which is lower than the apparent density due to air space. After compression, the compressed density doubles to about 6000 kg.m Finally, after sintering, the fusion that occurs between particles increases the steel s density to a density approaching the theoretical density or pore-free density. Three types of iron powders are available commercially. 

However, this now appears unlikely. Ion-free silica was prepared by Goodman. and Gregg (348) for a careful study of sintering silica free from impurities. The gel was made by hydrolyzing tetraethyl silicate (tetraethoxysilane). The. micropore diameter remained constant at 6.0 0.3 A from 200 to 800°C, whereas the surface area and pore volume decreased in a similar manner, their ratio remaining constant. The surface arM dropped from about 700 to 450 m g as the gel shrank in three dimensions. 

Selective laser sintering Solvent free good mechanical properties high porosity, interconnectivity, and surface area Laser beam limits the resolution high temperature limited pore size 108,114,118... 

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