Glass phases

Trapped gas in closed pores often limits densification when sintering witlr a liquid or viscous (glass) phase because rapid material transport tlirough tlie liquid often results in pore closure early in tlie sintering process. 

TRANSFORMED -8l3N4 GRAINS WITH RESIDUAL GLASS PHASE AT 20 C... 

For high temperatures, the spin-glass system behaves essentially the way conventional Ising-spin systems behave namely, a variety of different configurations are accessible, each with some finite probability. It is only at low enough tempera tures that a unique spin-glass phase - characterized chiefly by the appearance of a continuum of equilibrium states - first appears. 

The glassy state does not represent a true equilibrium phase. Below the transition into a glass phase, the material is regarded as being in a metastable state. If one holds the substances at temperatures somewhat below the glass transition temperature, heat evolution can often be observed over time as the molecules slowly orient themselves into the lower energy, stable crystalline phase. 

The glass phase furnished an active surface for the possible deposition of dispersed plutonium species. 

Sutton [1.15] studied the question of how quickly solutions with certain CPAs (GL, dimethylsulfoxide (DMSO) and others] have to be cooled in order to avoid crystallization. At 100 °C/min concentration of 42.1 % DMSO and 48.5 % for GL are necessary to achieve the glass phase. With a 32.5 % solution of (2R.3R)-(-)butan-2,3-dio, the same effect can be accomplished at = 50 °C/min. In Fig. 1.18 Sutton (Fig. 11 from [1.114]) showed, that polyethylene glycol with a molecular weight of 400 (PEG 400) reduced the critical cooling rate down to approx. 25 °C/min. The addition of PEG 8000 [1.115] improved the protection of lactate dehydrogenase (LDH) by maltodextrins, if maltodextrins with low dextrose equivalents are used. 

Since the definition of the glass phase is fuzzy as Reid et al. [1.17] note, and the methods of measurement can be chosen, the results for Tg- will have a range of variations. For freeze drying one can summarize ... 

The addition of certain carbohydrates increases, by varying degrees, the T. values and decreases the amount of UFW. These stabilize the glass phase to higher temperatures and permit higher drying temperatures. They can also bind volatile components. 

From DTA measurements phase diagrams can be constructed as shown for ethyleneglycol in Fig. 1.34. A solution of 40 % ethyleneglycol is only stabile in the glass phase below = -135 °C, at = -120 °C unfrozen water starts to crystallize, at = -65 °C a recrystallization is found, and at = —45 °C melting will start. As recrystallization is the growing of existing crystals, and not the nucleation of new ones, this event cannot be detected by DTA, but can be observed in a microscope when a transparent area becomes opaque. 

As shown in the following table (from [3.6]) the selection of the excipient also defines Tg- and the amount of unfreezable water (UFW) in the glass phase. 

Liposomes can, generally speaking, only be frozen without damage if the suspension is frozen in a glass phase of water. This requires the addition of CPAs e. g. mannitol, dextran or trehalose, and quick freezing (e. g. 10 °C/min by LN2) [3.37], (page 363). 

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... 

Figure 5 The upper panel shows the logarithm of the specific volume as a function of temperature for a cooling rate T = 52.083 10-6, with error bars determined from 55 independent cooling runs. The lines are fits with a constant expansion coefficient in the melt (continuous line) and glass phase (dashed line), respectively. The lower panel shows the common fit curve for all cooling rates in the melt and fit curves in the glass for four cooling rates given in the legend.

Cleavage and fracture fragments of the mineral and glass phases were reacted separately at 25°, 50°, and 100°C. In order to produce levels measurable by XrS, lxlO 1 molar alkali chloride solutions were required. After reaction, these samples were washed with deionized water and stored in a vacuum dessicator. 

The EPR spectra of various Tc complexes have been investigated in detail. The EPR of the six-coordinate, deep blue complex [AsPh4]2[Tc(NO)(NCS)5], prepared from [NH4][Tc04] in dmf and in the presence of KNCS and H2N0H HC1, has been studied in nonaqueous solution in the liquid and in frozen glass phases. With gx = (+)2.045 and gn = (-I-) 1.928, and A = 0.0236 and x = 0-0095 cm , the spectrum is characteristic of a low-spin 4quadrupole moment observed is solvent dependent. " When Hacac is... 

In this contribution we focus on the region that is sandwiched between the FE and glass phase states, i.e. the range of x-values 0.20 < x < 0.35. In [10] we have shown that the low temperature glass state (0.35 < x < 0.65) consists predominantly of short range ordered AFE clusters with a mean correlation length of about 1 nm. The fact that no FE clusters were found was explained by the unfavourable ratio of electric surface to volume energy, which makes... 

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