Glass borate-rich

Phase separated alkali borosilicate glasses tend to consist of a silica-rich phase and an alkali borate-rich phase. Several studies have shown that impurities, including transition metals and hydroxyl, are concentrated in the alkali borate-rich phase. Since the alkali borate-rich phase is much more readily dissolved in HCl than is the silica-rich... 

Fig. 1.15. Transmission electron micrograph of a sodium borosilicate glass [470], Secondary small droplets of a Si02-rich phase are embedded in a sodium borate rich matrix phase. The large droplets are also Si02-rich...

The HGMs produced were heat treated at 620 °C for 18 hours before they were leached in 4 molar HC1 solutions at 80 "C for 4 hours. Heat treatment produced phase separation which resulted in a silica-rich phase and a more soluble and interconnected, sodium borate phase. Leaching removed the soluble sodium borate phase of the glass and created porosity through the microsphere walls. The porous wall, hollow glass microspheres, sank to the bottom of the solution. These sinkers were collected, water washed and dried at 100 °C overnight. 

Under certain time and temperature conditions, the homogeneous glass separates into two phases. One of the phases consists substantially of silicon dioxide which is insoluble in mineral acid. The other phase represents a soluble coherent boric acid phase rich in alkali borate. If the boric acid phase is dissolved out of this heterogeneous glass structure with a mineral acid, a porous skeleton of substantially insoluble silicon dioxide is left. The phase separation region occurs between 500°C and 800 C. 

Controlled pore glass is prepared by heating certain types of borosilicate glass to 500-800 C until it separates into silicate-rich and boron-rich phases. (Figure 16-2). The borate phase is then dissolved with acid, producing a sponge like arrangement with pores from 2.5-7.0 nm. 

Boron has a great affinity for oxygen and occurs in nature only in boric acid or borates. Borates are composed from clusters of flat trigonal BO3 and tetrahedral BO4 groups. The structural chemistry of borates is as rich and complicated as those of silicates, borides, or boranes. Boron oxide is an essential part of borosilicate glasses such as Pyrex. Boron halides are volatile molecular compounds. They are Lewis acids and react violently with water. The subhalides consist of boron chains or clusters that have terminally bound halogen atoms. They are substitution derivatives of the lower boranes. 

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