Spg Membrane

2.1. Preparation of SPG Membrane

In recent years, porous glass became attractive as a material for formation of unique structures. They are expected to find application in various industries (16). In the earlier stage, Na20-B203-SiC>2 type glass, which was made by utilizing phase separation of Na20, B2O3 and SiC>2, was used as a primary glass for producing porous materials (17). Recently, Nakashima and Shimizu found a new stable phase separation region in Ca0-Al203-B203-Si02 type glass by utilizing "Shirasu" and made a unique new type of porous glass (10).

Figure 1 shows the process for manufacturing SPG membrane using the CaO-Al203-B203-Si02 type glass as a primary glass.

Figure 1 shows the process for manufacturing SPG membrane using the CaO-Al203-B203-Si02 type glass as a primary glass.

Figure 1. Production procedure for SPG

The basic process of separating the phase by heat treatment and dissolving the separated phase in an acidic solution is identical with that of Corning Process, so-called Porous Silica Glass (PSG), except for the addition of CaO and AI2O3 as major components. "Shirasu", used as a raw material, is volcanic ash and has a composition listed in Table 1.

After primary glass was formed into a tube, plate, powder, beads or fiber depending on the application of SPG membrane, it was subjected to heat treatment at a temperature of 600 to 800°C for several hours. This heat treatment causes the homogeneous primary glass to produce phase separation of Si02-Al203-rich and Ca0-B203-rich glasses. Since the latter phase easily dissolves in acidic solution, the desired porous SPG can be obtained by immersing the phase-separated glass into a hydrochloric acid solution. Table 2 summarizes the chemical composition of the primary glass and SPG along with that of PSG made by the Corning process. It can be seen clearly from this table that the chemical components of SPG differ from PSG because it contains less Si02 and more AI2O3.

Table 1

Chemical Composition of "Shirasu"! 1

Table 1

Chemical Composition of "Shirasu"! 1

Components

wt.%

Si02

0 0

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