Vitrification glasses

Figure 3.1 Various molecular structures of shape memory polymers (SMPs). A stable network and a reversible switching transition are the prerequisites for the polymers to show the shape memory effect (SME). The stable network can be molecule entanglement, chemical cross-hnking, crystaUization, and IPN the reversible switching transition can be the crystalUzation/melting transition, vitrification/glass transition, anisotropic/isotropic transition, reversible chemical cross-linking, and association/disassociation of supramolecular structures. Source [22] Reproduced with permission from Elsevier...

Vitrification, glass formation and crystallization of glasses of rare earth borates have been studied by Mukhin and Shmatok (1968) and Gutkina et al. (1975). The rare earth borate glasses are of optical grade and their chemical resistance is good (Ohora, 1981). 

Vitrification processes are being developed in several countries in which the dried waste is calcined and heated with ground glass frit to produce a borosilicate glass which can be stored or disposed of more permanently if there is agreement on suitable sites. 

The slow rate of dissolution of, or leaching from, durable glasses has led to proposals for the vitrification of nuclear waste. Glasses based on the sodium borosilicate system appear to be favoured because of their ability to dissolve the waste, combined with good chemical durability. Intensive development has taken place over recent ycars and a regular journal is devoted to this topic . 

We thus see that the RFOT theory provides a rather complete picture of vitrification and the microscopies of the molecular motions in glasses. The possibility of having a complete chart of allowed degrees of freedom is veiy important, because it puts strict limitations on the range of a priori scenarios of structural excitations that can take place in amorphous lattices. This will be of great help in the assessment of the family of strong interaction hypotheses mentioned in the introduction. To summarize, the present theory should apply to all amorphous materials produced by routine quenching, with quantitative deviations expected when the sample is partially crystalline. The presence and amount of crystallinity can be checked independently by X-ray. It is also likely that other classes of disordered materials, such as disordered crystals, will exhibit many similar traits, but of less universal character. 

In this section we continue to explore the consequences of the existence of the low temperature excitations in amorphous substances, which, as argued in Section III, are really resonances that arise from residual molecular motions otherwise representative of the molecular rearrangements in the material at the temperature of vitrification. We were able to see why these degrees of freedom should exist in glasses and explain their number density and the nearly flat energy spectrum, as well as the universal nature of phonon scattering off these excitations at low T < 1 K). 

The next stage, baking, begins at about 600°C, when small amounts of flux within the clay mixture melt and induce incipient melting of the surrounding clay, which is therefore converted to glass (see Chapter 3). The melting, or vitrification process, starts at isolated sites within the clay mixture... 

Example Optimization of an Eleven Component Glass Formulation. Piepel (6) discussed the generation and analysis of a mixture design consisting of eleven oxides used to prepare glasses for waste vitrification. Although many responses must be considered for the end use of this composition, the intent of Piepel s study was to minimize the response of leachability subject to the compositional constraints of ... 

AVM [Atelier de Vitrification de Marcoule] A continuous process for immobilizing radioactive waste by incorporation in a borosilicate glass. Developed at Marcoule, France, in 1972, based on the earlier PIVER process. In 1988, two larger vitrification plants were... 

HARVEST [Highly Active Residue Vitrification Experimental Studies] A process for immobilizing nuclear waste by incorporation in a borosilicate glass. Developed from FIN-GAL. Piloted by the UK Atomic Energy Authority at Sellafield, in the late 1970s, but abandoned in 1981 in favor of AVM, the French vitrification process. 

Crowe and Crowe [3.39] proved that it is sufficient for certain liposomes, e. g. egg phosphatidyl-choline (DPPC), to be vitrified by trehalose or dextran during freezing and freeze drying. In trehalose the retention rate was almost 100 %, and in dextran more than 80 %. This did not apply to egg PC-liposomes Dextran as CPA alone led to an almost total loss of the CF-indicator, but addition of dextran into a trehalose solution (Fig. 3.20) also reduced the retention rate of CF substantially, e. g. from 90 % in a pure trehalose to approx. 45 % if trehalose and dextran were in equal amounts in the solution. Since T of dextran is approx. -10 °C and Tg- of trehalose is -30 to -32 °C, dextran should form a glass phase at much higher temperatures than trehalose. Therefore the stabilization of egg- PC with trehalose cannot be related with the vitrification. Crowe showd with IR spectroscopy that egg-PC freeze dried with 2 g trehalose/g lipid had almost the identical spectrographic characteristics as the hydrous lipid Trehalose molecules replaced the water molecules, and hydrogen... 

---