Thermal treatment of glass

B) Thermal treatment of glass obtained in stage A. The glass (in pieces or as crushed and sieved powder) is reheated for several hours to several days [16,17] to achieve phase separation and to increase the microheterogeneities which are the alkali-borate phase. The thermal history determines the later size of the CPG pores. The separation process for alkali-borate glasses occurs in the temperature range from about 500 to 700°C. 

Example 2. An example of thermal treatment of glass and the consecutive change in the Raman scattering is illustrated in Fig. 27. 

Post-heat treatment of glass induces phase separation and crystalliza tion A very precise secondary heat treatment is necessary to develop and control photochromic properties. Glass-ceramics receive a thermal ceramming process that induces crystallization... 

Selenium purification by zone refining is not feasible. At practical zone-refining speeds, crystallization does not occur and impurities do not segregate. However, a controlled differential thermal treatment of selenium in a long vertical glass tube has been described (45). The treatment time is several weeks to several months. 

The glass transition temperatures ofthe polyimides are 195-250 °C their 10% weight loss temperamres (dynamic thermogravimetric analysis, air, AT = 4.5 °C/min) are 390-422 °C. Of particular interest are the dielectric constants of these polyimides. At a relative humidity of 50% these constants are 2.70-2.90 and are comparable with constants of the best fluorinated polyimides [21, 50-55]. The lowest dielectric constant (2.70) was observed for polyimide based on 6F dianhydride, containing the highest amount of fluorine. Thermal treatment of this polymer film at 280-290 °C for 1 hour led to a decrease (2.45) of dielectric constant due to the possible formation of nanofoams [56]. 

Yanes et al. (2004) observed a very interesting size selective spectroscopy in 0.4 mol% Eu3+ SnC>2 nanocrystals ( 4 nm) embedded in SiC>2 glass prepared by thermal treatment of sol-gel glasses. The mean size of SnC>2 nanocrystals is comparable to the bulk exciton Bohr radius (4.8 nm). Thus the band-gap excitation energy depends on the nanocrystal size. 

PbS nanoparticles with features of quantum dots (QDs) have been fabricated in boron-silicate glass matrix. Their mean diameter was found to be in the range of 3.4-8.2 nm from the optical spectroscopy data due to their explicit quantum confinement effect. The particle size and position of the absorption bands can be controlled through the regimes of thermal treatment of the glasses. SAXS technique showed near to monodispetse size-distribution of QDs and possible ordering within the glass matrix. 

Aluminium titanate (tielite, Al2Ti05) has excellent thermal shock resistance but poor mechanical strength which can, however, be improved by reinforcing with whiskers of a related phase such as potassium hollandite (K2Al2Ti60i6). Such composites can be formed by thermal decomposition of sol-gel precursors, reaction sintering of the two phases or by thermal treatment of an appropriate glass-ceramic material. Al MS NMR has been used to study the co-formation of these two phases during thermal treatment, and indicates that hollandite crystallises as whiskers within the tielite matrix (Kohn and Jansen 1998). 

Phosphosilicate glasses. The formation of phosphosilicate glasses by thermal treatment of gel precursors has been studied by P MAS NMR (Clayden et al. 2001). The dried gels were found to consist of siloxane frameworks containing trapped... 

Investigations of surface free energy (SFE) of controlled porosity glasses and silica gels carried out more recently showed certain similarity in the properties of bare materials and important differences caused by thermal treatment [49-56]. Dispersive interactions expressed as dispersive component of SFE (7 ) and polar interactions expressed as polar component of SFE (7 ) measured by means of hexane and toluene respectively are similar for both materials. The average value of 7 for silica gel equals 35.6 mj/m and for CPG 35.0 mJ/m. The mean values of 7P for silica gel and CPG are 159.8 mj/m and 159.2 mJ/m, respectively. The thermal treatment of both materials leads to a small increase of dispersive interactions and simultaneously causes a significant drop of polar interactions. 

Dawidowicz, A. et al.. The effect of thermal treatment of controlled porosity glasses (CPGs) on the properties of the electrical double layer at the CPG/electrolyte interface, J. Colloid Interf. Sci., 115, 555, 1987. 

Zhdanov (Institute of Silicate Chemistry, the U.S.S.R. Academy of Sciences, Leningrad) (154) showed (1949) that the adsorption of water vapor by SiC>2 (porous glasses, silica gels) strongly depends on the temperature of the preliminary thermal treatment of the adsorbent. Calcination of 300-500 °C resulted in a sharp decrease in the adsorption of H2O at low values of pressure over initial pressure p/p0 (<0.3), and the adsorption isotherms were found to be irreversible. On the other hand, the adsorption isotherms of water on silica subjected to calcination in vacuo at <500 °C (but after the sample was kept in contact with water vapor or liquid water at room temperature) again became reversible that is, the adsorption activity of Si02 was restored. 

Surface silylation of solid supports, glass columns, inserts, or even glass-wool spacers and glassware for the sake of surface deactivation remains highly recommended in biochemical GC. An alternative approach to surface deactivation is the method of Aue et al. [93], in which thermal treatment of polymer-coated supports results in a partial linkage of the macromolecule to the surface. This approach has been successfully employed with both packed and capillary columns. 

The thermal treatment of the glasses results in growth of the particles only a little (Fig. 1), and their concentration remains practically unchanged. Therefore, the nucleation process is finished completely at the step of glass preparation. The subsequent heating produces no new particles. Perhaps, a creation of the prime particles prevented supersaturation in glass. 

Thermal treatment of photochromic glasses is generally possible, but the tempering process must be very closely controlled. The thermal stability depends on the type of the glass. If photochromic glasses are thermally overheated they may become distorted and have a degraded photochromic performance. It is therefore recommended to keep the temperature below about 200°C and to use only short durations of tempering. 

Commercial powders of reagent grade were mixed and melted in a platinum crucible for 1 h in an electric furnace at 1400°C. The melt was poured between two iron plates and the thickness of the obtained sample was 1.6 mm. The glass ceramic was obtained by thermal treatment of the precursor glass at 620°C for 2 hours. It was used to compare with measurements in the locally damage zone by laser action. 

The relationship between observed enthalpy-volume relaxations and thermal treatment of slightly oriented industrial PVC films was investigated. Differential scanning calorimetry at 20 -C per minute and specific volume analysis (density gradient column) were used to study the effects of annealing near and below Tg. Nonlinear effects in the volume relaxation at relatively long times and temperatures close to the glass transition produce deviations in the specific heat curves at temperatures far above Tg in addition to the normal overshoot effects. 

The thermal treatment of inorganic substmices has a great synthetic potential because it can turn simple compoimds into advanced materials such as ceramics, catalysts, glasses, etc. 

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