Temperature crystallization

Determination of melting temperature (a) Maintain the apparatus at about 100 C below the melting temperature so that it is stable, (b) Heat the sample at a rate of lO Cmin up to 30 °C above the final temperature of melting, (c) When the melting temperature is determined using the sample conditioned by procedure (3), heat the sample at a rate of 10 °C min up to 30 C above the melting peak temperature, (d) Accumulate data in a computer or record the DTA or DSC curve. 

Determination of crystallization temperature (a) Heat the sample up to about 30 C above the final temperature of the melting peak by the above-mentioned procedure, (b) Maintain this temperature for 10 min, then cool to about 50 C below the final temperature of the melting peak at a rate of 5 or 10 Cmin. (c) Accumulate data or record the cooling curve, (d) If the crystallization rate is slow and it is difficult to determine the completion of the crystallization peak on the lower temperature side, the experiment should be stopped, (e) The sensitivity should 

The peak temperature of crystallization (Tp. ) is taken as the temperature at the top of the peak on the crystallization DTA or DSC curve the extrapolated onset temperature of crystallization (Tj. ) is defined as the intersection between the extrapolated baseline at the higher temperature and the tangent drawn at the point of greatest slope on the rising side of the crystallization peak. In the case of multiple peaks of crystallization, the corresponding characteristic temperature for each peak can be determined. In addition, if there are more than two overlapping peaks, Tj, several Tp. s and T s can be determined. Moreover, when the crystallization takes place continuously at a slower rate, it is difficult to determine the baseline on the lower temperature side and it is not necessary to determine T. .  

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Valov P M and Leiman V I 1997 Size effects in the melting and crystallization temperatures of copper chloride nanocrystals in glass JETP Lett. 66 510... 

The normal boiling point of 2-methylthiazole is 17 0= 128.488 0.005°C. The purity of various thiazoles was determined cryometrically by Handley et al. (292), who measured the precise melting point of thiazole and its monomethyl derivatives. Meyer et al. (293, 294) extended this study and, from the experimental diagrams of crystallization (temperature/degree of crystallization), obtained the true temperatures of crystallization and molar enthalpies of fusion of ideally pure thiazoles (Table 1-43). 

The lower the crystallization temperature, the lower the melting point. 

Melting occurs over a range of temperatures, as in Fig. 4.1. The range narrows as the crystallization temperature increases. This is probably due to a wider range of crystal dimensions and less perfect crystals under the lower temperatures of formation. 

The greater the undercooling, the more rapidly the polymer crystallizes. This is due to the increased probability of nucleation the more supercooled the liquid becomes. Although the data in Fig. 4.8 are not extensive enough to show it, this trend does not continue without limit. As the crystallization temperature is lowered still further, the rate passes through a maximum and then drops off as Tg is approached. This eventual decrease in rate is due to decreasing chain mobility which offsets the nucleation effect. 

Fractional crystallization may be accompHshed on a batch, continuous, or semicontkiuous basis. Oil is chilled continuously while passkig through the unit and is then passed over a continuous belt filter which separates soHd fat from the Hquid oil. The process gives poorer separation compared to solvent fractionation because oils are viscous at crystallization temperatures and are entrained to a significant extent ki the soHd fraction. The Hquid fraction, however, is relatively free of saturated material. 

A typical heat treatment cycle, as illustrated in Figure 1, comprises both nucleation and crystallization temperature holds, but some glass-ceramics are designed to nucleate and/or crystallize during the ramp itself, eliminating the need for multiple holds. 

Sodium Poly(4-styrene sulfonate). The sol—gel processing of TMOS in the presence of sodium poly-4-styrene sulfonate (NaPSS) has been used to synthesize inorganic—organic amorphous complexes (61). These sodium siUcate materials were then isotherm ally crystallized. The processing pH, with respect to the isoelectric point of amorphous siUca, was shown to influence the morphology of the initial gel stmctures. Using x-ray diffraction, the crystallization temperatures were monitored and were found to depend on these initial microstmctures. This was explained in terms of the electrostatic interaction between the evolving siUcate stmctures and the NaPSS prior to heat treatment at elevated temperatures. 

To erase information by the transition amorphous — crystalline, the amorphous phase of the selected area must be crystallized by annealing. This is effected by illumination with a low power laser beam (6—15 mW, compared to 15—50 mW for writing/melting), thus crystallizing the area. This crystallization temperature is above the glass-transition point, but below the melting point of the material concerned (Eig. 15, Erase). 

The ammonium perchlorate solution is spray-dried to the desired crystal size at air temperatures below 150°C and crystal temperatures of about 110°C. This procedure provides a pure product having a controlled grain size. Prior mechanical and thermal treatment affects the isothermal... 

Other crystallization parameters have been determined for some of the polymers. The dependence of the melting temperature on the crystallization temperature for the orthorhombic form of POX (T = 323K) and both monoclinic (T = 348K) and orthorhombic (T = 329K) modifications of PDMOX has been determined (284). The enthalpy of fusion, Aff, for the same polymers has been determined by the polymer diluent method and by calorimetry at different levels of crystallinity (284). for POX was found to be 150.9 J/g (36.1 cal/g) for the dimethyl derivative, it ranged from 85.6 to 107.0 J/g (20.5—25.6 cal/g). Numerous crystal stmcture studies have been made (285—292). Isothermal crystallization rates of POX from the melt have been determined from 19 to —50 C (293,294). Similar studies have been made for PDMOX from 22 to 44°C (295,296). 

Pa-s (45 P) flash point, 11°C crystallization temperature, 7°C miscible in alcohols inmiscible in hydrocarbons. 

Composition, % AI2O2 PH Specific gravity at 15.6°C Crystallization temperature, °C... 

Fig. 4. Relation between crystallization temperature and melting temperature for natural mbber.

Barium acetate [543-80-6] Ba(C2H202)2, crystallines from an aqueous solution of acetic acid and barium carbonate or barium hydroxide. The level of hydration depends on crystallization temperature. At <24.7°C the trihydrate, density 2.02 g/mL is formed from 24.7 to 41 °C barium acetate monohydrate [5908-64-5] density 2.19 g/mL precipitates and above 41 °C the anhydrous salt, density 2.47 g/mL results. The monohydrate becomes anhydrous at 110°C. At 20°C, 76 g of the monohydrate dissolves in 100 g of water. Barium acetate is used in printing fabrics, lubricating grease, and as a catalyst for organic reactions. 

Table 7. Alcohol content as a function of Fractional Crystallization Temperature.

Anhydrous caustic soda (NaOH) is available but its use is generally not considered practical in water and wastewater treatment applications. Consequently, only liquid caustic soda is discussed here. Liquid caustic soda is generally shipped at two concentrations, 50 percent and 73 percent NaOH. The densities of the solutions as shipped are 12.76 Ib/gal for the 50 percent solution and 14.18 Ib/gal for the 73 percent solution. These solutions contain 6.38 Ib/gal NaOH and 10.34 Ib/gal NaOH, respectively. The crystallization temperature is 53 F for the 50 percent solution and 165 F for the 73 percent solution. The molecular weight of NaOH is 40. The pH of a 1 percent solution of caustic soda is 12.9. 

It is known that elevating the crystallization temperature [42,49] or annealing above the crystallization temperature [50] of PE results in a thicker folded-chain lamella of up to —200 nm. In addition to the higher temperature, if high pressure is applied, crystals can grow as thick as several micrometers in the chain axis direction [2-4,51]. 

Balabanov et al. [499] found an endothermic effect in the thermographic pattern of the decomposition of niobium hydroxide at 435°C that corresponds to complete removal of water. At the above temperature, amorphous niobium hydroxide also converts into amorphous niobium oxide. Ciystallization of the amorphous oxide occurs at a higher temperature with the release of energy [28]. Researchers [499] reported on another exothermal effect at 549°C that was attributed to the crystallization temperature of amorphous niobium oxide. Decomposition of tantalum hydroxide and its conversion into crystalline tantalum oxide occurs at about 710°C [502] or at 670-700°C according to another source [132]. 

Crystallization The formation of crystallites or groups of plastic molecules in an ordered structure within the plastic as the plastic is cooled from its amorphous state to a temperature below its crystallization temperature. 

Supercooling The rapid cooling of a normally crystalline plastic through its crystallization temperature, so it does not get a chance to crystallize and it remains in the amorphous state. 

The change in Gibbs free energy, AG, in the formation of FCC and ECC, depending on the drawing ratio of the melt ft and the crystallization temperature, is given by14 ... 

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