The compound bis(ethylenediamine-N,N′)-(oxalato-O,O′)-cobalt biphenyl-4,4′-disulfonate hydrate, refcode TIQHOR, crystallizes with four Co(III) cations in the asymmetric unit. Another pair of compounds, rac-bis(ethylenediamine-N,N′)-(oxalato-O,O′)-cobalt(III) l-hydrogenaspartate, refcode VAGBOU, and tetra-ammine-(N-benzylethanediamine)-cobalt(III) trinitrate sesquihydrate, refcode ZIFHEB crystallize both as conglomerates and kryptoracemates. Their stereochemical characteristics are described, and the fact that they display more than one mode of crystallization in a single lattice, is illustrated numerically and visually. 相似文献
The crystallization kinetics of Cu55Zr45 (at%) glassy alloy is studied under isothermal condition using differential scanning calorimetry (DSC). The plot of correlation between the crystallized volume fraction α and annealing time t shows a sigmoid-type curve, which is steeper with higher annealing temperature. Furthermore, in isothermal crystallization condition, local activation energy Eα values, determined using the Arrhenius equation, range from 181.1 to 187.8 kJ/mol, which is nearly a constant. The local Avrami exponent n(α) values, obtained by the Johnson-Mehl-Avrami equation, which range from 2.2 to 4.0 at different annealing temeperatures, which indicates that the crystallization mechanism is diffusion-controlled transformation. Moreover, n(α) becomes greater with increasing annealing temperature, which indicates that annealing temperature can affect nucleation rate and growth type. 相似文献
A kinetic study of the crystallization processes was performed for some decorative ceramic glazes in the PbO-SiO2-Na2O-K2O-CaO(BaO)-Al2O3-B2O3 system with addition of 10% TiO2 and ZnO. The crystallization kinetics has been studied in non-isothermal conditions using DTA technique. The apparent activation
energies of the crystallization processes were calculated using the Kissinger method. The main crystalline phase, which provides
the decorative effect, is rutile. This has been identified by X-ray diffraction and it is clearly visible in the optical microscopy
images taken in transmitted light, as needle-like or even prismatic crystals arranged in radial-fibrous aggregates. 相似文献
The enthalpies of crystallization of NaCl, KCl, LiCl·H2O, MgCl2·6H2O, CaCl2·6H2O and BaCl2·2H2O from aqueous solution were determined by means of different calculation methods on the basis of the earlier-measured differential and integral enthalpies of solution of the above salts. The obtained crystallization enthalpies are discussed and compared with the appropriate literature data.
Zusammenfassung Es wurden die Kristallisationenthalpien des NaCl, KCl, LiCl·H2O, MgCl2·6H2O, CaCl2·6H2O und BaCl2·2H2O aus wässerigen Lösungen bestirnt. Es wurden vorher gemessene differenzierte und integrale Enthalpien von oben genannten gelösten Salze ausgenutzt, bei denen verschiedene Berechnungsmethoden angewandt wurden. Die gewonenen Kristallisationenthalpien wurden mit entsprechenden Literaturangaben erörtert und vergliechen.
Using a layer‐multiplying coextrusion process to fabricate films with thousands of alternating polymer nanolayers, we report here a new crystalline morphology in confined polymer nanolayers and an abrupt transition in the crystallization habit. At higher temperatures, poly(ethylene oxide) crystallizes as large, in‐plane lamellae. A 5 °C change in the crystallization temperature produces an on‐edge lamellar orientation. The results point to a transition from heterogeneous nucleation to substrate‐assisted nucleation. This may be a general phenomenon that accounts for previously unexplained differences in the preferred chain alignment of confined polymer crystals.
The intention of this work presented is to introduce new processing principles for the crystallization of watery or fatty phases in food systems, and in particular to quantify the process — microstructure — product quality relationships for such food systems. The crystallization processes demonstrated in detail are high shear crystallization (i), spray crystallization (ii) and spray powder based seed crystallization (iii). Crystalline, semi-crystalline and/or amorphous structures were analysed by calorimetric, mechanical/rheological and microscopical methods. Quality aspects of the final food products, which are related to the structure of the crystalline and/or amorphous components, were investigated additionally.This revised version was published online in November 2005 with corrections to the Cover Date. 相似文献
Poly(3,3-dimethyl oxetane) fractions ranging in number average molecular weights from 18500 to 130000 have been isothermally crystallized from the relaxed melt state in the temperature range from 12 to 44 °C, where only the monoclinic modification is formed. The influence of molecular weight and undercooling in crystallization kinetics has been analyzed. The level of crystallinity is very slightly dependent on molecular weight but the influence of this parameter on the time scale of the crystallization is relatively pronounced. The crystallization temperature coefficient was determined and it was found a constant value of the product of the interfacial energies in the range of molecular weights which has been analyzed. Growth rate measurements were carried out for fraction ¯Mn=130000 and it was found that the temperature coefficients for overall and growth rates are equal. Finally, the comparison of the experimental results for this polymer with those reported for poly(oxetane) shows two main differences: first, the crystallization rate is slower for poly(3,3-dimethyl oxetane) and second, the temperature coefficient is smaller for this polymer. 相似文献
Poly(oxetane) fractions ranging in number-average molecular weights from 7800 to 157000 have been isothermally crystallized in the temperature range from –50 to 19 C, using dilatometric and calorimetric techniques. In both cases, reproducible isotherms were obtained with an Avrami exponent equal to three. The crystallization rate against crystallization temperature presents a maximum at –30 C. The level of crystallinity changes with molecular weight and the influence of this parameter on the rate of crystallization is pronounced. The crystallization temperature coefficient was studied using nucleation theory and it was found an slight increase in the basal interfacial free energy for the lowest molecular weight fraction. For the analysis of the temperature coefficient at the higher undercoolings, different approximations for the free energy of fusion and the transport term have been considered. The conclusion of this analysis is that, independently of these approximations, the obtained temperature coefficients are the same. 相似文献
Summary Summarizing a number of experiments concerning crystallization of polymers in oriented state, and attempting to compare them with predictions of various theories one comes to the conclusion, that in many areas of the field the experimental results remain in qualitative agreement with theory. It is much difficult to find such agreement concerning quantitative aspects. The reason for that comes from both sides; experiments are usually made at conditions, which do not correspond to theoretical assumptions, and theories usually discuss particular models, which are not able to describe full variety of morphologies and situations occurring in reality.It is believed that measurements performed at constant stress will provide data, that could be used to verify some theories. Basing on these data, it should be also possible to predict behaviour in other, more complex conditions. This is the matter of practical importance, and finally it could give better insight into phase transition occurring in such processes like fiber spinning, film blowing or injection molding, at each of them the molecular orientation takes place.Also explanation of morphological peculiarities appearing in oriented crystallization, as well as deeper insight into crystallization mechanisms can be expected from this type of measurements.
Flow Induced Crystallization (FIC) is the common term to indicate the acceleration in polymer crystallization kinetics due to the action of flow. When modeling FIC, two major challenges are encountered. On the one hand, the model must be able to produce quantitative reliable results, while correctly describing the coupling between the intrinsic (quiescent) crystallization kinetics and the rheological response of the polymer. On the other hand, the model must be able to describe the complex kinematics taking place in real industrial processes. In this paper, we present the predictions of a recently proposed model for FIC in the case of a mixed flow, where both shear and extensional components are present at the same time. In particular, the effects of the overall flow intensity and of relative weight between shear and extension on the enhancement in nucleation rate are presented and discussed. Some guidelines for future development are also proposed. 相似文献
Herein, we demonstrate the potential of droplet‐based microfluidics for controlling protein crystallization and generating single‐protein crystals. We estimated the critical droplet size for obtaining a single crystal within a microdroplet and investigated the crystallization of four model proteins to confirm the effect of protein molecular diffusion on crystallization. A single crystal was obtained in microdroplets smaller than the critical size by using droplet‐based microfluidics. In the case of thaumatin crystallization, a single thaumatin crystal was obtained in a 200 μm droplet even with high supersaturation. In the case of ferritin crystallization, the nucleation profile of ferritin crystals had a wider distribution than the nucleation profiles of lysozyme, thaumatin, and glucose isomerase crystallization. We found that the droplet‐based microfluidic approach was able to control the nucleation of a protein by providing control over the crystallization conditions and the droplet size, and that the diffusion of protein molecules is a significant factor in controlling the nucleation of protein crystals in droplet‐based microfluidics. 相似文献