Substitution of the end-member oxides in the ternary sodium borosilicate system diagram was studied. Replacing SiO, in this ternary system with a combination of CeOz and NbzOs was found to produce glasses which, after heat treatment, decomposed into two immiscible microphases of which one is water-soluble. The mode of phase decomposition is sensitive to impurities. The importance of crucible selection, e.g. ,4120, or Pt/Rh, is demonstrated by differences in pore size, surface area, heat resistance, and structure of the insoluble phase for a series of Ce203-Nb205 and Ce203-Nb205-A1203 glassceramics. These new materials have a specific surface area of up to 217 m'/g and a pore radius of between 1.1 and 54.5 nm.

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A porous glass that has a high BET surface area and an alkali resistance superior to that of porous silica glass was prepared by leaching a glass in which silica had been totally substituted by a combination of Y203 and ZrO 2. This porous glass has an improved alkali resistance and a larger surface area than the materials which could be produced from the systems where silica was substituted by a combination of La203 and TiO2 [1] or La203 and Ta205 [2].The batch was melted without difficulty in a platinum-rhenium crucible at 1400°C for 4h in a normal atmosphere. The melt was cast in iron moulds preheated to 550°C and immediately annealed, after which the samples were heat treated at 550 ° C for 3 h. The samples were clear glass before and after the heat treatment, showing no visual signs of phase separation or crystallization. The heat-treated samples were cut into 2 mm thick slices and leached in distilled water at 95 ° C for 96 h, using a reflux system. The leached glass, now porous, was dried at 140 ° C for at least 2 h before the properties were measured. Pieces of the leached glass were subsequently sintered at temperatures ranging from 600 to 1200°C, with a soak time of 5 min.The BET surface area and void volumes were determined according to the techniques described previously [1,2]. The alkali resistance of the leached glass was established by a method based on ISO 695 [3], taking into account the measured BET surface area.Inductively coupled plasma atomic emission spectroscopy was used to analyse the leached glass while bulk densities were determined using the Archimedes method or, for samples having open porosity, by direct mass and volume measurements.The properties of the annealed and the leached glass are given in Table I. The leaching process removed much of the soluble sodium borate leaving a porous material composed predominantly of B 2 0 3 , Y203 and ZrO2. The leached samples were mechanically weak and crumbled easily during drying and handling, and Fig. 1 is an electron micrograph of a fracture surface. The extremely fine texture of the morphology is characteristic of a porous glass. This is supported by X-ray diffraction results which confirm the noncrystalline state for the leached sample. Fig. 2 shows the change in bulk density of the leached sample after the sintering procedure. Samples sintered below about 850°C were transparent and