The influence of long additional thermal treatment of controlled porous glasses on the structuralization of their silica network

Abstract

The materials used as sorbents in gas and liquid chromatography include controlled porosity glasses (CPG). The heating of CPG in the temperature range 400-800 ~ leads not only to dehydroxylation of their surface, but also to a diffusion of the boron atoms (and a smaller amount of sodium atoms) remaining in the silica network of the CPG towards the glass surface. Simultaneously, crystallization of the silica network of the CPG takes place.The present paper deals with the changes of the crystallographic structures in thermally treated glasses which differ in their mean pore diameters. The results show not only the existence of cristobalite in the siliceous lattice of the heated glasses, but also the presence of c~-quartz, the very symmetrical structure of SiO 2.Materials very often applied as sorbents, supports, catalysts or column packings for chromatography are controlled porous glasses (CPG). Their usefulness results from their chemical, mechanical and thermal resistance [1,2]. CPG are usually obtained by appropriate thermal treatment and the leaching of Vycor-type glass (R20--B203--SiO2, where R20 = Li20 or Na20 or K20 ) [l, 2].During thermal treatment, phase separation takes place in Vycor glass. In the continuous silica structure, the continuous alkali metal borate phase is formed. The latter can be removed from the thermally treated Vycor glass by means of a leaching process [1,2]. As a consequence of this procedure, porous materials (CPG) are obtained. The pore diameter in the CPG depends on the composition of the initial glass, the duration and temperature of thermal treatment (i.e. the prehistory of the liquification process), and the eaching procedure of the Vycor glass [1,2]. The porous networks of CPG are composed mainly of SiO 2 (ca. 94-98 %) and residual amounts of REO (ca. 0.1-0.5%) and B203 (1-6%). Additionally, their porous structure is completely different from that of silica gels [1].

Redistribution of o-Ps in Porous Vycor Glass. Crystallization of Silica at High Temperatures

“…This effect is well visible in lowering of the o-Ps intensity value which means vanishing of small voids. It is known from literature [16] that in fully crystallized silica one should not observe o-Ps formation, however this is not confirmed empirically [17][18][19]. In our case, the problem is much more complicated because of presence of big free spaces (pores) in the material.…”

Application of positron annihilation lifetime spectroscopy in studies of crystallization processes

“…As shown in the literature, the thermal treatment of sodiumboro-silicate glasses leads to crystallization of siliceous materials, and the degree of crystallization depends on the temperature and time of the treatment. 23 However, this process was not observed earlier using PALS method. For this reason a few series of material samples, thermally treated in different ways, were prepared in order to compare the results of both methods: PALS and X-ray diffraction.…”

“…It has already been stated 23 that the crystallization process in porous glasses is connected with migration of boron and sodium atoms in the siliceous structure of the studied material. In this experiment crystallization of silica only has also been achieved, but a part of sodium-borate still remains amorphous even at 975 K. A 87% crystallinity degree is however unexpected at first glance in the material investigated, because the Fig.…”