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Volume phase gratings have been fabricated by controlled generation of periodic striations during the growth of copper doped potassium lithium tantalate niobate crystals. Gratings with periods ranging from below I to 5 mu m were f...
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Volume phase gratings have been fabricated by controlled generation of periodic striations during the growth of copper doped potassium lithium tantalate niobate crystals. Gratings with periods ranging from below I to 5 mu m were fabricated. It is shown that the fabricated composition grating induces a refractive index grating which is a superposition of a fixed grating and an electrically controlled (electrooptic) grating. The electrooptic grating is produced due to the generation of a spatial modulation of the Curie temperature which is manifested as a correlated modulation of the static dielectric constant. It was also observed that when operated at the immediate vicinity of the phase transition temperature the diffraction efficiency from these gratings was bi-stable at a specific electric field due to an induced shift of the Curie temperature. (c) 2007 Elsevier B.V. All rights reserved.
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We present a systematic modelling methodology using the spiral growth mechanism of Burton, Cabrera and Frank to predict the steady-state shape of organic molecular crystals grown from solution. This methodology has been developed ...
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We present a systematic modelling methodology using the spiral growth mechanism of Burton, Cabrera and Frank to predict the steady-state shape of organic molecular crystals grown from solution. This methodology has been developed to eliminate the need for special modifications for each new crystal system studied. Therefore, the mechanisms and choices for spiral shapes, edges and evolution are mathematically determined as governed by the underlying solid-state chemistry and physics. The power of the approach is demonstrated for several crystal systems: naphthalene grown from both ethanol and cyclohexane; anthracene grown from 2-propanol; and glycine grown from water. The predicted crystal shapes are in good agreement with experiment.
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We report measurements of the growth rates of the basal (0001) and prism (1010) facets of ice as a function of surface supersaturation, in a regime where the growth dynamics are dominated by the presence of surface melting. Our me...
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We report measurements of the growth rates of the basal (0001) and prism (1010) facets of ice as a function of surface supersaturation, in a regime where the growth dynamics are dominated by the presence of surface melting. Our measurements were made using freely falling columnar ice crystals, growing a solvent gas of air at a pressure of one atmosphere, with ambient supersaturation levels 1＜σ∞<11/100. Condensation coefficients for the two facets were inferred by modeling the effects of both particle and heat diffusion, which also yielded surface supersaturation levels 0. 75<σ_surf<2.2/100. Our results show a strong supersaturation dependence in the crystal growth, as for both facets the condensation coefficient exhibited a precipitous drop below σ_surf≈1/100. In addition, the condensation coefficient for the basal facet was found to be nearly a factor of 10 larger than for the prism facet. These results support a model in which the ice growth rates are governed by 2D nucleation at the interface between the crystalline solid and a quasi-liquid surface layer.
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Generally, diffusion through film thickness is used for correlating data in the industrial crystallization processes. Furthermore, mass transfer is an important phenomenon in most chemical processes and studies involving determina...
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Generally, diffusion through film thickness is used for correlating data in the industrial crystallization processes. Furthermore, mass transfer is an important phenomenon in most chemical processes and studies involving determination of mass transfer coefficients are necessary for a better estimation of equipment performance. Solid-liquid mass transfer coefficients in stirred systems have received substantial attention in the past due to their practical applications. In contrast, little information is available on solid-liquid mass transfer in crystallization systems, despite the importance of crystallization. In this work, an expression for the mass transfer coefficients in solution crystallization has been developed based on the Stefan problem formulation. The model is able to predict a finite mass transfer coefficient when the layer thickness vanishes. The obtained mass transfer coefficients agree with previously reported experimental data. Furthermore, it is found that the stagnant film for mass transfer has oscillatory behavior, and this behavior is a function of residence time.
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The generic heterogeneous effect of foreign particles on 3D nucleation was examined in terms of the relative size and the interfacial interaction and structural match with the crystallizing phase in terms of a so-called interfacia...
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The generic heterogeneous effect of foreign particles on 3D nucleation was examined in terms of the relative size and the interfacial interaction and structural match with the crystallizing phase in terms of a so-called interfacial correlation function f(m,x). At low supersaturation, nucleation is controlled by foreign bodies with a relatively small curvature and/or an optimal interfacial interaction and structural match with the crystallizing phase. This is characterized by a small interfacial correlation function f(m,x) f(m,x) --> 0). At high supersaturation, nucleation on foreign particles having a weak interaction and poor structural match with the nucleating phase (f(m,x) --> 1) will govern the kinetics. It follows that if a wide range of supersaturations are applied in a crystallization system, a sequence of progressive heterogeneous nucleation processes, characterized by different interfacial correlation function f(m,x)s, can be identified, where genuine homogeneous nucleation can be regarded as the up-limit of heterogeneous nucleation. To check the above results, experiments on the nucleation of organic crystals and CaCO3 from aqueous solutions were carried out under gravity and microgravity. The results are in excellent agreement with our prediction. [References: 27]
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Sectored vesuvianite showing optically triclinic propertieswas studied by X-ray and P-FTIR analyses, and the origins ofthe internal optical texture are discussed. A monoclinic refinement(space group P2/n) suggests that site occ...
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Sectored vesuvianite showing optically triclinic propertieswas studied by X-ray and P-FTIR analyses, and the origins ofthe internal optical texture are discussed. A monoclinic refinement(space group P2/n) suggests that site occupancies are slightlydifferent among the Al(2) series, though the OH– dipoleis randomly oriented in all sectors. A relationship betweenthe surface and internal texture suggests that these sectoralstructures were produced during crystal growth, not by phasetransitions.收起
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Sectored vesuvianite showing optically triclinic properties was studied by X-ray and P-FTIR analyses, and the origins of the internal optical texture are discussed. A monoclinic refinement (space group P2/n) suggests that site occ...
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Sectored vesuvianite showing optically triclinic properties was studied by X-ray and P-FTIR analyses, and the origins of the internal optical texture are discussed. A monoclinic refinement (space group P2/n) suggests that site occupancies are slightly different among the Al(2) series, though the OH~- dipole is randomly oriented in all sectors. A relationship between the surface and internal texture suggests that these sectoral structures were produced during crystal growth, not by phase transitions.
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In the search for a greater understanding of polymer crystallization,numerous experimental observations with regards to microscopic structures and macroscopic properties have been reported in the past half-century.There are genera...
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In the search for a greater understanding of polymer crystallization,numerous experimental observations with regards to microscopic structures and macroscopic properties have been reported in the past half-century.There are generally two types of experimental results to provide information about the mechanisms of polymer crystal growth,i.e.molecular dynamic/scattering and structural/morphological.Since we cannot follow the trajectory of individual chain molecules when they undergo the transition from liquid to solid state during the crystallization process,structural/morphological analysis of polymer crystals reveal information recorded during this process.Namely,the final structure and morphology of polymer crystals have atomic,stem and global chain conformation information embedded in them during crystallization which provides evidence which can be used to deduce molecular aspects of the polymer crystallization process.It is commonly understood that polymer crystallization,from the thermodynamic perspective,is a first-order transition involving the relaxation of a metastable undercooled melt towards the equilibrium state which is rarely reached in polymer crystals.This process is controlled by a free energy barrier.A molecular model is required to describe the landscape of the free energy barriers and to provide an analytical explanation concerning and predictions about polymer crystallization.The Hoffman-Lauritzen (HL) theory,which was put forward more than 40 years ago,was one of the first analytical theories to illustrate how polymers crystallize.Since then,modifications to the HL theory and suggestions for new approaches have been reported,but the core physical picture of the HL theory has largely remained intact.This article consists of four major parts:(1) we will compare the crystallization of small molecules and long chain molecules,and the relationship between crystallization and crystal habits.The diversity of crystalline structures and morphologies of semi-crystalline polymers must be taken into account when studying the crystallization mechanism of polymers (2) this article also serves as a brief review of the HL theory and its importance in our understanding of polymer crystallization (3) we have tried to answer the question:what is the nucleation barrier?Specifically,we will illustrate that the nucleation barrier in polymer crystallization consists of both enthalpic and entropic components as deduced from experimental results.This barrier,from our perspective,consists of selection processes taking place in different length- and time-scales (4) finally,there is a brief discussion on what issues remain,in particular,in the areas of undercooled liquid structures and the initial stages of crystallization.
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Inorganic perovskites, particularly Cs4PbBr6, possess great potential applications for radiation detection, pho-tonic and optoelectronic devices due to their peculiar yet robust photoluminescence properties and spectral color puri...
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Inorganic perovskites, particularly Cs4PbBr6, possess great potential applications for radiation detection, pho-tonic and optoelectronic devices due to their peculiar yet robust photoluminescence properties and spectral color purity. Herein, a facile and industrial route to synthesize centimeter-size (1.2 x 1.1 x 0.8 cm3) Cs4PbBr6 single crystals was developed by modified seed crystal induced the precision-controlled temperature-gradient crystal-lization method. The effects of the temperature and supersaturation on growth, nucleation kinetics of large Cs4PbBr6 single crystals have been investigated. Our findings will provide a new strategy for the fabrication of large perovskite-related single crystals and could further strengthen their competitiveness for the field of lighting or display.
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Needle-shaped GaN nanowires have been synthesized on Si (111) substrate through ammoniating Ga_2O_3/MgO films under flowing ammonia atmosphere at the temperature of 950℃. The as-synthesized GaN nanowires were characterized by X-r...
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Needle-shaped GaN nanowires have been synthesized on Si (111) substrate through ammoniating Ga_2O_3/MgO films under flowing ammonia atmosphere at the temperature of 950℃. The as-synthesized GaN nanowires were characterized by X-ray diffraction (XRD), Fourier transformed infrared (FTIR) spectroscopy, scanning electron microscope (SEM) and high-resolution transmission electron microscopy (HRTEM). The results demonstrate that these nanowires are hexagonal GaN and possess a smooth surface with an average diameter about 200 nm and a length ranging from 5 μm to 15 μm. In addition, the diameters of these nanowires diminish gradually. The growth mechanism of crystalline GaN nanowires is discussed briefly.
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