Crystallisation time-temperature-transformation

An analysis of crystallisation rates is conveniently performed in terms of the so-called time-temperature-transformation (TTT) curves, which relate the time taken to crystallise a given fraction of the undercooled liquid or the supersaturated solution to the temperature. Experimentally, the crystallisation rates are measured by quenching the liquid phase to some predetermined temperature T and measuring the time taken for the solid to crystallise at that temperature, either by monitoring the latent heat of crystallisation or by microscopic observation. The volume fraction 4>(T) that crystallises out in time 1 is given by one form of the Avrami equation ... 

Polymorphism. Many crystalline polyolefins, particularly polymers of a-olefins with linear alkyl groups, can exist in several polymorphic modifications. The type of polymorph depends on crystallisa tion conditions. Isotactic PB can exist in five crystal forms form I (twinned hexagonal), form II (tetragonal), form III (orthorhombic), form P (untwinned hexagonal), and form IP (37—39). The crystal stmctures and thermal parameters of the first three forms are given in Table 3. Form II is formed when a PB resin crystallises from the melt. Over time, it is spontaneously transformed into the thermodynamically stable form I at room temperature, the transition takes about one week to complete. Forms P, IP, and III of PB are rare they can be formed when the polymer crystallises from solution at low temperature or under pressure (38). Syndiotactic PB exists in two crystalline forms, I and II (35). Form I comes into shape during crystallisation from the melt (very slow process) and form II is produced by stretching form-1 crystalline specimens (35). 

Law of Successive Reactions.— When sulphur vapour is cooled at the ordinary temperature, it first of all condenses to drops of liquid, which solidify in an amorphous form, and only after some time undergo crystallisation or when phosphorus vapour is condensed, white phosphorus is first formed, and not the more stable form, violet phosphorus. It has also been observed that even at the ordinary temperature (therefore much below the transition point) sulphur may crystallise out from solution in benzene, alcohol, carbon disulphide, and other solvents, in the monoclinic form, the less stable crystals then undergoing transformation into the rhombic form a similar behaviour... 

To demonstrate the "overlapping" conditions between chemical reactions and surface processes controlled by "specific" relaxation times we can compare processes in both areas. Such areas are diffusion of components, formation of new phases, e.g. crystallisation, evaporation, freezing, the transformation from an aggregate to a monomer state or vice versa. For example the component A produces the component B in dependence of temperature, pressure or other forces, such as outer electric fields. 

Investigations on the hydrothermal crystallisation of nitrate enclathrated cancrinite were performed using the alkaline transformation of zeolites A and X at a temperature of 353 K in 2-molar and 16-molar NaOH-solutions. The conversion of the zeolites was followed in the early stage of the reactions for times up to 48 hours by XRD and IR- spectroscopy. A fast and total transformation of zeolite X into cancrinite could be stated in most of the experiments, independent of the alkalinity. In contrast the conversion of zeolite A under low alkaline conditions was slower and accompanied by a sodalite-cancrinite cocrystallisation as well as the formation of an intermediate phase between both structure types. The results indicate a more kinetically controlled reaction mechanism for zeolite A transformation. 

From these somewhat discordant investigations, it would seem that both the detailed chemist and kinetics involved are very dependent on both precipitation and ageing conditions. At the temperatures used in most of the studies (60-80 C) the time for malachite to crystallise from the initial amorphous precipitate is less than 9 minutes [ 17 ]. Most of the samples examined were consequently well "aged" only Pollard and Shen sampling early in a precipitation of rapid addition have detected amorphous phases which transform or react to produce crystalline copper/zinc minerals. 

There exist three solid forms of SO3 called A, B and C. Only the A-form is stable, while B and C are not stable. The melting point of the A-form is 62°C, at which temperature the vapour pressure of liquid SO3 is 2.5 atmospheres. However, it appears that the A-form never crystallises from the liquid phase, that only forms if B crystals are present initially. These B-form crystals will in time (which might be very long) transform into A, therefore it is essential to prevent the formation of B crystals. 

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