Showing papers on "Crystallization published in 1971"

Theory of Particulate Processes: Analysis and Techniques of Continuous Crystallization

Abstract: Theory of particulate processes; analysis and techniques of continuous crystallization , Theory of particulate processes; analysis and techniques of continuous crystallization , مرکز فناوری اطلاعات و اطلاع رسانی کشاورزی

Rapid Reversible Light-Induced Crystallization of Amorphous Semiconductors

Abstract: We have observed a high-speed crystallization of amorphous semiconductor films and the reversal of this crystallization back to the amorphous state using short pulses of laser light and evidenced by a sharp change in optical transmission and reflection. This optical switching behavior is analogous to the memory-type electrical switching effect in these materials which has received wide attention1 since the observation by S. R. Ovshinsky2 of both threshold and memory switching in amorphous semiconductors. In this letter, we propose a model which closely relates the optical and electrical switching behavior, and shows that the phase change from amorphous to crystalline state is not only a thermal phenomenon but is directly influenced by the creation of excess electron-hole carriers by either the light, or, for the electrical device, by the electric field. The reversibility of the phenomenon in this model is obtained through the large difference in crystallization rates with the light on or off.

The crystallization of calcium carbonate

Abstract: The kinetics of crystallization of calcium carbonate (calcite) at 25°C has been examined by following the changes in calcium and hydrogen ion concentration when stable supersaturated solutions are inoculated with calcite seed crystals. The calcite growth follows a rate equation second order with respect to concentration suggesting a surface-controlled process. Calcium ion concentration changes were followed using 45Ca radiotracer and atomic absorption spectroscopy. Heterogeneous exchange of 45Ca between a calcium carbonate solution and calcite seed crystals has also been examined (by a liquid scintillation counting technique) in order to analyze the crystal growth data.

Crystallization of abyssal tholeiites

Abstract: In some abyssal tholeiite magmas having a relatively high content of olivine component, olivine is the first mineral to crystallize. In others having a relatively high content of plagioclase component, plagioclase is the first mineral to crystallize. The rooks of these two groups are called OL- and PL-tholeiites respectively. Continued crystallization drives the residual liquids to the cotectic curve between the olivine and plagioclase fields. Subsequent cotectic crystallization results in an increase of pyroxene component in residual magmas, eventually leading to the crystallization of clinopyroxene. Marked enrichment of iron and TiO2 takes place in later stages. The total iron/MgO ratio may be used as an indicator for the degree of fractional crystallization. Average compositions of OL- and PL-tholeiites are calculated for a series of total iron/MgO ranges (Table 7). PL-tholeiites are usually but not always higher in Al2O3 than OL-tholeiites.

Grain refinement in aluminum alloyed with titanium and boron

Abstract: The aluminum corner of the ternary Al-B-Ti diagram was explored. A eutectic: Liq — Al + TiAl3 + (Al, Ti)B2 was found at approximately 0.05 wt pct Ti, 0.01 wt pct B; 659.5‡C. TiB2 and A1B2 form a continuous series of solid solutions, but no distinct ternary phase was found. The addition of boron to aluminum-titanium alloys expands the field of primary crystallization of TiAl3 toward lower titanium contents and steepens the liquidus. In equilibrium conditions, pronounced grain refinement is found only in alloys in which TiAl3 is primary and nucleates the aluminum solid solution before any other impurity can act. The peritectic reaction facilitates this priority but it is not necessary for grain refinement. Because of the low diffusivity of titanium and boron in aluminum, equilibrium is seldom attained and in commercial practice grain refinement by TiAl3 is found also outside its equilibrium field of primary crystallization.

The elastic and anelastic behavior of a metallic glass

Abstract: The temperature dependence of the internal friction and Young's modulus of the glassy and crystalline phases of a Pd 0.76 Au 0.75 Si 0.165 alloy have been determined by a resonance technique. The internal friction of the glassy alloy increases exponentially above 150°C at a frequency of 140 Hz. The apparent activation energy of the relaxation process increases upon annealing from 50 to 80 kcal/mole. It is suggested that relaxation processes possessing a broad spectrum of activation energies are active in the glassy alloy. Young's modulus of the glassy alloy decreases gradually with increasing temperature and increases approximately 28% upon crystallization.

Hydrodynamically induced crystallization of polymers from solution

Abstract: This paper deals with a study on the melting behaviour of polyethylene crystals showing a fibrillar morphology. These crystals were prepared by hydrodynamically induced crystallization from xylene solutions. Use was made of several techniques, such as differential scanning calorimetry, polarizing microscopy and infrared dichroism. The experimental observations indicate that the melting process of the fibrous crystals can only partly be related to morphological structures. The infrared dichroism and persistent birefringence up to temperatures of 200 °C are due mainly to the molecular orientation in the melt. The shrinkage on melting was found to depend on the thickness of and fibre orientation in the sample, as well as on the molecular weight of the polyethylene. A maximum shrinkage value of 93% was measured on fibrillar crystals of Marlex (M w =90 · 103). Calorimetric studies on the fusion of fibrillar crystals of high molecular weight polyethylene (M w =1.5 · 106) revealed that extendedchain backbones melt at 150 °C and even at 180 °C. The melt of these crystal structures remains partly oriented, as could be inferred from the constant temperature position of the melting peak on repeated crystallizing and melting.

[25] Protein crystallization: Micro techniques involving vapor diffusion

Abstract: Publisher Summary Microtechniques have received widespread use in the crystallization of tRNA's, where only extremely small amounts of material are available. The protein to be crystallized is brought to the desired concentration in dilute buffer and a 10 μl droplet is placed in the well of a siliconed microculture slide. At any pH a preliminary test should be made to establish the concentration of salt, for example ammonium sulfate, at which the protein precipitates. This can be accomplished by adding strongly buffered saturated salt solutions to the 10 μl droplet with a 10 μl Hamilton syringe until the precipitation point is reached. Large crystals may sometimes be encouraged to grow by the use of a seeding solution. Such a solution is prepared by crushing a single crystal or group of crystals in a “stabilizing” solvent in which the crystal fragments will not redissolve. The suspension may then be centrifuged with a low speed clinical centrifuge to remove large fragments and diluted in the stabilizing solvent to minimize the concentration of nucleation sites. The sample to be crystallized is brought to a precipitant concentration slightly lower than that required to yield crystals without seeding and is then touched with a glass fiber which has been wetted with the seeding solution. This procedure frequently leads to the production of a few large crystals.

Dielectric and optical properties of transparent ferroelectric glass-ceramic systems

Abstract: The properties and behavior of optically transparent materials prepared by controlled crystallization of a ferroelectric phase from a glass are discussed. The dielectric and optical properties are different from those of the bulk materials and are shown to be a function of the crystallite size. The control of the crystallite size to values below 0.2 μm allows one to prepare transparent specimens which exhibit a large electrooptic effect. The dielectric and electrooptic properties are shown for a representative number of glass-ceramic systems.

Dilute Solution Theory of Polymer Crystal Growth: A Kinetic Theory of Chain Folding

Abstract: A kinetic theory of polymer crystallization from dilute solution is formulated for linear chain molecules of finite molecular weight (monodisperse). Two models of crystal growth are considered; both are essentially “regular” chain folding type models. Formulas for the crystal growth rates are derived as a function of the fundamental rate constants associated with the various states of molecular crystallization. These rate constants are evaluated as a function of polymer concentration, molecular weight, crystallization temperature, and crystal thickness. Consideration of finite molecular weight molecules requires an understanding of how these molecules are incorporated into the crystal and what happens to chain ends. Attention is focused on these problems and a description of how “cilia” are formed in polymer crystals is given. A remarkable aspect of cilia formation is that the uncrystallized portion of a chain molecule which dangles in the solution can participate in nucleating a new growth strip (fold pl...

Structure of crystalline polymers with unbranched long side chains

Abstract: The structure and thermodynamic properties of atactic and isotactic acrylic and methacrylic polymers containing 16–18 carbon atoms in the n-aliphatic side chains, and of copolymers of hexadecyl acrylate with isopropyl acrylate were studied by means of x-ray and differential thermal analysis. The crystallization of branched acrylic and methacrylic polymers and of acrylic copolymers proceeds in the form of a hexagonal crystal, regardless of the configuration of the backbone chain. Methods of ordering branched macromolecules are proposed, and the melting points, heats and entropies of fusion determined. The role of flexibility of the backbone chains in ordering and the crystallization processes was determined. In the case of poly(n-alkyl acrylates) the backbone chain is involved in the crystalline lattice; this is not the case in methacrylates and copolymers of hexadecyl acrylate with isopropyl acrylate. Some similarity was assumed between the structure of biopolymers and synthetic branched polymers.

Crystallization and Preliminary Structural Analysis of Dihydrolipoyl Transsuccinylase, the Core of the 2-Oxoglutarate Dehydrogenase Complex

Abstract: Dihydrolipoyl transsuccinylase, one of the three enzymes comprising the Escherichia coli 2-oxoglutarate dehydrogenase (EC 1.2.4.2) complex, has been crystallized. Studies by x-ray diffraction and electron microscopy establish that the transsuccinylase has octahedral (432) symmetry, i.e., it consists of 24 subunits that are structurally identical.

Effect of Industrial Processing on the Morphology of Crystalline Polymers

Abstract: Our understanding of crystailinity in polymers was greatly advanced by the discovery of the folded-chain lamella as the unit of structure in crystallization both from solution and the melt. Subsequently, it was found that the organization of lamellae into larger units is dependent on the nature of the nuclei initiating crystallization. Under quiescent conditions, nuclei are randomly distributed throughout the crystallizing volume and growth proceeds independent of neighboring structures. In the presence of shearing stress, either in a melt or in solution, nuclei are not randomly distributed but appear along “rows”. However, what seems to be a row of nuclei actually is a special kind of nucleus, a fibril of oriented molecular chains, about 300 A in diameter. The presence of these fibril nuclei determines whether the polymer will crystallize spherulitically or in a “row structure” morphology. The type of morphology, in turn, influences many of the physical properties, particularly the stress-strain...

Differences between the Conformation of Arsanilazotyrosine 248 of Carboxypeptidase A in the Crystalline State and in Solution

Abstract: Coupling of carboxypeptidase A crystals with diazotized arsanilic acid specifically labels tyrosine 248, an active-site residue of the enzyme. Many azophenols are yellow and their zinc complexes are red; the “yellow” absorption spectrum of zinc arsanilazocarboxypeptidase crystals is characteristic of the arsanilazotyrosyl group, not of the zinc complex. This is consistent with the interpretation of x-ray data on native crystals of carboxypeptidase A, indicating that tyrosine 248 and the zinc atom are too far apart to form a complex. However, the enzyme in solution is red, denoting the formation of a complex between zinc and arsanilazotyrosine 248. The most likely interpretation of the data is that the orientation of arsanilazotyrosine 248 in solution and in the crystal is different. If the unlabeled tyrosine 248 of native carboxypeptidase undergoes similar changes, these data may bear upon the low activity of the enzyme in the crystalline state and on the catalytic mechanism of the enzyme based on the crystal structure. The opportunities for analogous spectrochemical studies of other, similar systems are pointed out.

Crystallographic evidence for the tetrameric subunit structure of L-asparaginase from Escherichia coli.

Abstract: The analysis of a monoclinic crystal modification of l-asparaginase from Escherichia coli supports the chemical and physio-chemical evidence for a tetrameric subunit structure. The pseudo-symmetries occurring in the crystal structure provide evidence that the molecules lie on positions with approximate point symmetry 222.

[23] Crystallization as a purification technique

Abstract: Publisher Summary Crystallization of an enzyme intends to achieve one of two aims: purification of the protein or preparation of a product suitable for structural studies. The purification method depends on the decrease in solubility of proteins in ammonium sulfate solution attendant to an increase in temperature. Protein, precipitated with salt, is extracted with ammonium sulfate solutions of decreasing concentration at or near 0°. Thus, contaminating proteins, soluble at higher concentrations of ammonium sulfate, are extracted first and thereby removed. Extracts obtained in this manner are allowed to warm to the temperature of the room during which period crystallization results. Crystallization can take place within minutes of transfer of the extract to room temperature but may require an hour or overnight incubation. Crystallization is signaled by the appearance of turbidity. The silky sheen, often termed “schlieren,” which is observed with several crystalline enzymes, is usually not realized and is never evident in the first few hours of crystallization.

Crystallization of polyethylene at elevated pressures

Abstract: The crystallization from the melt of three sharp polyethylene fractions has been studied at 5 kbar. It has been shown that the thickness of so-called extended-chain lamellae is a function of time, temperature, and molecular weight. There is by no means just the fully extended molecular configuration present. Crystallization is qualitatively similar to that of chain-folded crystals at 1 bar, giving an optimum lamellar thickness which increases with time and decreasing supercooling. Fractional crystallization is widespread and is a major cause of disparate lamellar thickness. Isothermal thickening of lamellae during crystallization has been established directly. Morphological detail suggests further that layers can increase their thickness tenfold over their initial size.

Theory of deformation and strain‐induced crystallization of an elastomeric network polymer

Abstract: A new theory of deformation and strain induced crystallization of network polymers has been developed. The effects of lattice vacancies, variation in distribution of trans and gauche bond conformations in stretched amorphous polymers, and the crystallite orientation in the partially crystalline stretched vulcanizate were considered in the evaluation of their partition functions. Stress-extension ratio relationships were evaluated for the amorphous and semicrystalline polymers. The rise in melting temperature due to strain induced crystallization is discussed. The new theory seems to be in closer agreement with the actual strain-induced crystallization process than earlier research.

Effect of composition and gelation conditions on structural changes accompanying the gelation of PAN, PVA and gelatin solutions

Abstract: For three types of polymer-solvent systems: polyacrylonitrile in dimethylformamide, polyvinyl alcohol in water and gelatin in water the isothermal gelation process was followed with viscometry, light scattering and X-ray diffraction techniques. Effects of thermal history (quenching, melting) and solvent composition were investigated. It was found that the variation of structure as revealed by these methods (molecular aggregation, crystallization etc.) is not specific for the gelation itself but associated with some side-processes which result from retarded phase separation. The molecular mechanism of “pure gelation” proceeding at unchanged supramolecular structure is discussed.