Peritectic freezing

Previous works reported on the study of the freezing process of aqueous solutions of ethanol in relation with the phase diagram. They were focused on the identification of different stable or metastable ethanol-hydrate structures using X-ray diffraction and differential scanning calorimetry (DSC). They also mentioned the formation of clathrate hydrates. In spite of the familiarity of alcohol-water systems and their importance in many fields of chemistry, cryobiology, astrophysics, there are still significant discrepancies in the literature with uncertainties concerning the composition and the structure of the stable or metastable eutectic or peritectic hydrates. ... 

Al—Cr. Although no commercial alloys are based on this system, chromium [7440-47-3], Cr, is an ingredient of several complex and commercially significant alloys (Fig. 16. The Cr is added for control of grain stmcture. The Al—Cr system has a peritectic portion at 661°C where soHd solubiHty is 0.7% Cr andHquid solubiHty is 0.4% Cr. Freezing of binary alloys containing >0.4% Cr produces coarse ptimary crystals (metalHc inclusions) of AlyCr [12005-37-7] which may adversely affect mechanical properties. In complex alloys, the Hquid solubiHty can be reduced such that formation of primary AlyCr crystals may occur at significantly lower Cr contents. 

Pure hydrates are crystalline ice-like solids usually prepared from water or ice and an appropriate guest species. Hydrates of water-soluble guests have well-defined phase diagrams characteristic of two-phase systems with compound formation."." If the hydrate melts congruently, it is simply a matter of freezing a solution of the correct composition to obtain the hydrate. For systems that melt incongruently, the aqueous solution must be quenched below the peritectic temperature and conditioned so that the crystalline hydrate can form on... 

Solidifying an off-eutectic composition from the melt will result in pro-eutectic grains with the first-to-freeze composition. The remaining melt then freezes with the eutectic composition. Attempts to solidify from the melt at the peritectic composition will result in highly cored grains because when the (3 first forms, it blocks the remaining a from reacting with the liquid to form new (3 phase. 

Liquid mixtures freeze at lower temperatures than the pure components of which they are comprised. The lowest possible temperature in which we only have liquid is called the eutectic point. The presence of solid compounds of stoichiometry aj y increases the complexity of their phase diagrams. A compound with a definite melting point is termed congruent, while a compound that is not stable all the way up to a well-defined melting point is said to have an incongru-ent melting point. The state at which the latter case dissociates is called the peritectic point. 

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