Formation of Liquid Phases

Liquid phases are formed by the direct melting of batch components, by melting of decomposition products, and by melting of eutectic mixtures formed from the batch components. In soda-lime-silicate batches, we 

The time required to completely dissolve the original batch is known as the batch-free time. Although the definition of the batch-free time is straightforward, determination of the exact time at which the last remaining trace of batch remains in the melt is difficult. In general, determination of the batch-free time is subject to a variety of errors, including prejudice on the part of the researcher. 

The overall glass composition is by far the most important factor in controlling the batch-free time. Simple oxide mixtures, such as those used to produce calcium aluminate glasses, often form eutectic mixtures which melt directly with very short batch-free times. Many non-silicate melts are very fluid at any temperature above the melting point of their components and rapidly dissolve all batch particles. Borate, phosphate, and germanate melts can be formed at much lower temperatures than are typically required for silicate melts. As a result, it is usually easier to decrease their viscosity by increases in temperature, e.g., an increase in temperature from 1000 to 1200 C is more easily attained than an increase from 1400 to 1600 C. 

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The ore is ordinarily ground to pass through a ca 1.2-mm (14-mesh) screen, mixed with 8—10 wt % NaCl and other reactants that may be needed, and roasted under oxidizing conditions in a multiple-hearth furnace or rotary kiln at 800—850°C for 1—2 h. Temperature control is critical because conversion of vanadium to vanadates slows markedly at ca 800°C, and the formation of liquid phases at ca 850°C interferes with access of air to the mineral particles. During roasting, a reaction of sodium chloride with hydrous silicates, which often are present in the ore feed, yields HC1 gas. This is scmbbed from the roaster off-gas and neutralized for pollution control, or used in acid-leaching processes at the mill site. 

The endotherm that peaked at 651°C intensifies steadily with decreasing particle size, as is the case for the associated DTG peak. This endotherm represents increased formation of liquid phase with decreasing particle size. The enhanced weight loss, indicated by the DTG traces with decreasing particle size, does not coincide with the formation of detectable new crystalline phases in the coarse particle sizes, but does correspond to XRD detection of the formation of sodium disilicate in the fine particle size. Thus, decreasing particle size results in a significantly enhanced low temperature liquid phase attack on silica grains. 

Finally, a word of caution should be added. The successful transformation of the precursors to a homogeneous oxide or a finely interdispersed mixture of oxides demands that diffusion leading to segregation be made as difficult as possible. In particular, the formation of liquid phases during decomposition and calcination of the precursor should be completely avoided. What cannot be avoided is surface contamination. 

In fact, the hydrogenation enthalpies we cite were obtained by taking the difference of the archival enthalpies of formation of liquid phase unsaturated and saturated species, rather than by direct measurements (cf Reference 16). 

Formation of liquid phase melt at temperature higher than 1250°C. 

Preparation of the Bi-Sr-Ca-Cu-0 superconductors is essentially the same as in the Y-Ba-Cu-0 system, except the calcination temperature. The Bi (2223) phase is formed by firing at 850°C for 200 h in air atmosphere. To shorten the firing time, the conventional practice is to replace 10-20 atom% Bi by Pb. The replacement induces the formation of liquid-phase CaaPbOa, which accelerates the solid-state reaction. Lead atoms are incorporated in the Bi sites. The Bi (2212) phase is more stable than the Bi (2223) phase and is formed by calcin-... 

Erom the presented results, it follows that formation of liquid phase occurs in a small section of the throttle, in which the contraction changes from 0.3 to 0.24. The velocity in the latter section is equal to 5.3 Ui. 

The formation of liquid phase during the liquid-phase sintering is schematically shown in Fig. 5.28 [73]. It is started from a mixture of two powders, i.e., the major component and an additive. The additive is molten to liquid phase or reacts with the... 

KENNETH W. WHITE, FENG YU AND YI FANG (b) formation of liquid phase... 

The details of the core melt-down processes depend both on the particular composition of the respective reactor core and on the particular conditions of the accident sequence under consideration. However, as a comparative evaluation of the results from different experiments on core melt progression performed in test reactors and from examination of the damaged core of the TMI-2 reactor (Hob-bins et al., 1991) has shown, in spite of considerable differences in the conditions and physical scales, several important phenomena seem to be common to all such events. Eutectic interactions between core materials cause the formation of liquid phases and loss of original core geometry at comparatively low temperatures (about 1500 K). The first liquids to be formed are metallic in nature and consist... 

Sodium silicate forms one of the most effective dispersants for clays. As illustrated in Fig. 6.13, replacement of bivalent Ca " ions (or Mg " ions) more commonly present on the clay particle surfaces by monovalent Na ions produces less screening of the surface charge and, hence, to a greater repulsion between the clay particles. For advanced ceramics that must meet very specific property requirements, the use of inorganic dispersants may leave residual ions (e.g., sodium or phosphate), which even in very small concentrations can lead to the formation of liquid phases during sintering, thereby making microstructural control more difficult. 

Figure 3.1-4 Chemical interactions and formation of liquid phases in an LWR fuei rod bundle with increasing temperature.

The ability to remove ZrO layers on the boride particles and the formations of liquid phases have been advanced as a reason for the usefulness of many additives. PS studies of cold isostatically pressed ZrB with additions of MoSi (5-20vol%) powder at 1800-1850°C resulted in an increase in the RD from 86.3-99.7% (Sciti et al., 2005 Sciti et al., 2006). Similarly, ultrafine ZrB -SiC (20vol%)-Mo (4wt%)... 

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