Non-isothermal crystallization in amorphous selenium films

The usual determination of kinetic parameters of crystallization of amorphous products is based on isothermal measurements. In general the crystallization of amorphous selenium thin films is studied by non-isothermal experiments (DTA). The adaptation to non-isothermal crystallization of the Avrami transformation rate equation allows us to determine different types of crystallization. These different regions enable one to determine the variations of the growth rate and of the nucleation rate versus temperature. The influence of the wavelength of illumination during the crystallization time on these parameters is also investigated.

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Zusammenfassung Die übliche Bestimmung kinetischer Parameter der Kristallisation amorpher Produkte basiert auf isothermen Messungen. Die Kristallisation von amorphem Selen in dünnen Schichten wird im allgemeinen in nicht-isothermen Experimenten (DTA) untersucht. Die Adaptation der Transformationsgeschwindigkeitsgleichung nach Avrami an die nicht-isotherme Kristallisation ermöglicht die Bestimmung verschiedener Kristallisationtypen. Dadurch wird es möglich, die Veränderungen der Wachstumsgeschwindigkeit und der Geschwindigkeit der Nukleation in Abhängigkeit von der Temperatur zu bestimmen. Der Einfluß der Wellenlänge bei der Bestrahlung während der Kristallisation auf diese Parameter wird ebenfalls untersucht.

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Structural development of mercaptophenol self-assembled monolayers and the overlying mineral phase during templated CaCO3 crystallization from a transient amorphous film

Formation of biomineral structures is increasingly attributed to directed growth of a mineral phase from an amorphous precursor on an organic matrix. While many in vitro studies have used calcite formation on organothiol self-assembled monolayers (SAMs) as a model system to investigate this process, they have generally focused on the stability of amorphous calcium carbonate (ACC) or maximizing control over the order of the final mineral phase. Little is known about the early stages of mineral formation, particularly the structural evolution of the SAM and mineral. Here we use near-edge X-ray absorption spectroscopy (NEXAFS), photoemission spectroscopy (PES), X-ray diffraction (XRD), and scanning electron microscopy (SEM) to address this gap in knowledge by examining the changes in order and bonding of mercaptophenol (MP) SAMs on Au(111) during the initial stages of mineral formation as well as the mechanism of ACC to calcite transformation during template-directed crystallization. We demonstrate that formation of ACC on the MP SAMs brings about a profound change in the morphology of the monolayers: although the as-prepared MP SAMs are composed of monomers with well-defined orientations, precipitation of the amorphous mineral phase results in substantial structural disorder within the monolayers. Significantly, a preferential face of nucleation is observed for crystallization of calcite from ACC on the SAM surfaces despite this static disorder.     

A shape-persistent polymeric crystallization template for CaCO3

Alanyl-alanine-derived poly(isocyanide)s possess a helical structure that persists even in aqueous media. These rigid macromolecules possess a regular distribution of carboxylic acid-terminated side chains which for the first time allows the study of the templating process involved in biomineralization using a shape-persistent polymeric template. It is demonstrated that in the case where the polymer derived from l-Ala-d-Ala is used the formation of apple core-type calcite crystals is controlled both by nucleation of the (01.1) face and subsequent adsorption of the polymer to the {hk.0} faces of the growing crystal. A small change in the secondary structure of the template that is introduced by using the polymer derived from l-Ala-l-Ala is directly reflected in the lower degree of control over the crystallization process.     

Controlling the growth of zirconia needles precursor from a liquid crystal template

This work reports the directional growth of macroscopic ceramic needles by combining a thermo-responsive sulfated hydroxy zirconyl (SHZ) hydrosol with a swollen hexagonal liquid crystal (SHLC). The effect of the molar ratio R_s = [Zr⁴⁺]/[SO₄^2?] on the thermo-stability of both the SHZ hydrosol and the SHLC, and also on the nucleation and 1D-growth of ceramic particles was investigated. The dynamics of the thermo-induced aggregation growth process was monitored in situ by rheological property measurements. The results show that by increasing the R_s, the kinetic stability of the system in the sol state increases, leading to the formation of a less branched framework during the thermo-induced aggregation process. Furthermore, the small-angle X-ray diffraction (XRD) results show that the thermo-stability of the SHLC employed as soft template is improved by increasing the R_s. Both effects allow us to induce the particle's nucleation by heating the SHLC template to a moderate temperature (≈50 °C), while the kinetics of the needle's growth is enhanced by increasing the [Zr⁴⁺]/[SO₄^2?] molar ratio.     

Influence of template on crystallization of ZSM-5 zeolites

Pentasil zeolites of ZSM-5 type are synthesised hydrothermally using triethyl-n-proplyammonium bromide (TEPA-Br) and triethyl-n-butylammonium bromide (TEBA-Br). The crystallization kinetics, followed by XRD, SEM and thermal analysis, clearly demonstrate the influence of size and molecular weight of the templating quaternary ammonium cation (QAC) species on the rates of nucleation and crystallization. The values of the apparent activation energies for nucleation and crystal growth indicate that both nucleation and crystal growth are faster when TEPA-Br rather than TEBA-Br is used as a template. The quantitative identification of intergrown phases characterizes both the phases to be ZSM-5 zeolite. Thermoanalytical curves for both these zeolites in as-synthesised forms exhibit two-step oxidative decomposition of the occluded organic species. This suggests that the quaternary ammonium cation may be located at two energetically different sites within the zeolite channels. The equilibrium sorption capacity, however, is found to increase in the order of size and molecular weight of the templating species in both the zeolites. The nature of acid site distribution, obtained from the temperature programmed desorption of ammonia is found to be independent of the templating species used during the synthesis.     

Room temperature crystallization of amorphous polysiloxane using photodimerization

Bulk crystallization in flexible polymeric systems is difficult to control due to the random orientation of the chains. Here we report a photo cross-linking strategy that results in simultaneous cross-linking and crystallization of polysiloxane chains into millimeter sized leaf-like polycrystalline structures. Polymers containing pendant anthracene groups are prepared and undergo [4+4] photocycloaddition under 365 nm irradiation at room temperature. The growth and morphology of the crystalline structures is studied using polarized optical microscopy (POM) and atomic force microscopy and is found to progress through three unique stages of nucleation, growth, and constriction. The mobility of the individual chains is probed using pulsed-field gradient (PFG) NMR to provide insights into the diffusion processes that may govern chain transport to the growing crystal fronts. The room temperature crystallization of this conventionally amorphous polymer system may allow for a new level of morphological control for silicone materials.

Millimeter sized polymer crystallites grow rapidly when anthracene-containing siloxanes are photocrosslinked using UV light.     

Inhibiting surface crystallization of amorphous indomethacin by nanocoating

An amorphous solid (glass) may crystallize faster at the surface than through the bulk, making surface crystallization a mechanism of failure for amorphous pharmaceuticals and other materials. An ultrathin coating of gold or polyelectrolytes inhibited the surface crystallization of amorphous indomethacin (IMC), an anti-inflammatory drug and model organic glass. The gold coating (10 nm) was deposited by sputtering, and the polyelectrolyte coating (3-20 nm) was deposited by an electrostatic layer-by-layer assembly of cationic poly(dimethyldiallyl ammonium chloride) (PDDA) and anionic sodium poly(styrenesulfonate) (PSS) in aqueous solution. The coating also inhibited the growth of existing crystals. The inhibition was strong even with one layer of PDDA. The polyelectrolyte coating still permitted fast dissolution of amorphous IMC and improved its wetting and flow. The finding supports the view that the surface crystallization of amorphous IMC is enabled by the mobility of a thin layer of surface molecules, and this mobility can be suppressed by a coating of only a few nanometers. This technique may be used to stabilize amorphous drugs prone to surface crystallization, with the aqueous coating process especially suitable for drugs of low aqueous solubility.     

Glass-forming ability and crystallization kinetics of amorphous palladium-boron alloys

Palladium-boron alloys have been prepared to check their ability to produce metallic glasses when spun from the melt.An amorphous alloy with 31.5 at.% boron obtained in the form of ribbon has been submitted to both isothermal and non-isothermal DSC tests. The isothermal crystallization kinetics have been analyzed according to the Avrami law for phase transformations in solids.The n exponent of the law has been determined to obtain information on the geometrical features of the growing crystals. A confirmation of the calculated n value has been sought through an analysis of non-isothermal DSC peaks     

A new method for studying crystallization of amorphous alloys

Experiments on the crystallization of amorphous Fe?Si?B alloys were carried out by thermogravimetric analysis (TG). This new method gives us some important information about the magnetic phase transformation of amorphous alloys, especially the magnetic volume change in crystallization beside the energy change obtained by the traditional DSC and DTA methods. Crystallization activation energies of Fe?Si?B amorphous alloy are calculated from both TG and DTA curves. The experiment also showed that the addition of Nb, Cu and Mo would influence the crystallization transition temperature of amorphous Fe?Si?B alloys greatly.     

Isothermal study of crystallization of Pd-Ni-Si amorphous metallic materials

The isothermal crystallization of amorphous alloys of the type (Pd_100?x Ni _x )₈₃Si₁₇ has been investigated by DSC. The character of the transformation depends upon the content of Ni. Ifx>30, material crystallizes probably without nucleation in a very narrow temperature range. Ifx?30, the crystallization process is controlled by a nucleationgrowth mechanism. The initial stages of transformation are characterized by a relatively short incubation period with an Arrhenius temperature-dependence. Afterwards, the time-dependence of the degree of conversion is sigmoidal. Ifα<0.1, thenp～1 characterizes a homogeneous nucleation. If 0.1<α<0.9 than the Avrami exponent isp(α)～3–4. Atα>0.9 so-called tails are observed. Two kinds of unseparable non-stationarities of the nucleation-growth process at the beginning of crystallization of the amorphous material are discussed.     

Structure of hard-sphere fluid and precursor structures to crystallization

The structural origin of the commonly observed split second peak of the radial distribution function of a supercooled or glassy liquid is examined in this work using the hard-sphere fluid as an example. A novel approach to the analysis of the microscopic structure of a fluid is described, which permits the decomposition of both the radial distribution function and bond-angle distribution function of a system of particles into contributions from a small number of ring structures. The method uses a modified shortest-path definition of rings appropriate to the analysis of the medium-range structure of dense systems. It is shown that the split peak is an indicator of the emergence of precursor structures to crystal formation. The origin of the split peak provides a structural link between fluid and crystalline phases and our results suggest that it is neither a structural feature peculiar to glassy phases nor a smooth structural continuation of the stable-fluid phase. This structural feature of simple glassy systems is more appropriately described as a signifier of the frustration of emerging crystalline order in a fluid.     

Formation of gas hydrate during crystallization of ethane-saturated amorphous ice

Layers of ethane-saturated amorphous ice were prepared by depositing molecular beams of water and gas on a substrate cooled with liquid nitrogen. The heating of the layers was accompanied by vitrification (softening) followed by spontaneous crystallization. Crystallization of condensates under the conditions of deep metastability proceeded with gas hydrate formation. The vitrification and crystallization temperatures of the condensates were determined from the changes in their dielectric properties on heating. The thermal effects of transformations were recorded by differential thermal analysis. The crystallization of the amorphous water layers was studied by electron diffraction. Formation of a metastable packing with elements of a cubic diamond-like structure was noted.     

Development of one-dimensional nanostructures through the crystallization of amorphous colloids

In this paper we have demonstrated that the crystallization method of amorphous colloids is convenient and feasible in the large-scale production of one-dimensional (1D) nanostructures. For the crystals with highly anisotropic structures, such as orthorhombic, trigonal, and hexagonal crystals, the crystallization generally tends to occur along the (001) axis. The preparation of orthorhombic bismuth sulfide (Bi2S3) nanorods and trigonal selenium ( t-Se) nanowires by the crystallization route was used as typical examples to illustrate the process and mechanism of crystallization. The as-prepared products were characterized with transmission electron microscopy, field-emission scanning electron microscopy, X-ray diffraction, and selected area electron diffraction. Additionally, the detailed crystal growth processes involved in the crystallization of amorphous Bi2S3 colloid were investigated by studying the morphology and structure of intermediates. It demonstrates that the growth of the nanorods is through two key steps: (1) the formation of multiple activated sites on the surface of spherical Bi2S3 colloid and (2) the subsequent preferential growth along these sites.     

Growth of ultrathin films of amorphous ruthenium-phosphorus alloys using a single source CVD precursor

Thin films ( approximately 30 nm) of amorphous RuP alloys (P approximately 15-20%) can be grown by CVD from the single source precursor cis-H2Ru(PMe3)4 at 250-300 degrees C and 200 mTorr pressure on native SiO2.     

Properties of amorphous calcium carbonate and the template action of vaterite spheres

The fast mixing of aqueous solutions of calcium chloride and sodium carbonate could immediately result in amorphous calcium carbonate (ACC). Under vigorous stirring, the formed ACC in the precipitation system will dissolve first and, then, transform within minutes to produce crystalline forms of vaterite and calcite. After that, the solution-mediated mechanism dominates the transformation of the thermodynamically unstable vaterite into the thermodynamically stable calcite. Although ACC is the least stable form of the six anhydrous phases of calcium carbonate (CaCO(3)), it could be, however, produced and stabilized by a variety of organisms. To better understand the formation-transformation mechanism of ACC and vaterite into calcite, ex-situ methods (i.e., scanning electron microscopy, Fourier transform infrared spectroscopy, and X-ray diffraction spectroscopy) were used to characterize the formation-transformation process of ACC and vaterite in aqueous systems without organic additives, showing that ACC sampled at different conditions has different properties (i.e., lifetime, morphology, and spectrum characterization). It is also very interesting to capture the obviously polycrystalline particles of CaCO(3) during the transformation process from vaterite to calcite, which suggests the formation mechanism for the calcite superstructure with multidimensional morphology.     

Thermodynamic analysis and purifying an amorphous phase of frozen crystallization centers

The possibility of dissolving frozen crystallization centers in amorphous alloys of the Fe–B system is considered by means of thermodynamic calculations. This can in turn improve the thermal stability of an amorphous alloy. The effect isothermal annealing has on the thermal stability of multicomponent amorphous alloys based on iron is investigated via the highly sensitive dilatometric technique, measurements of microsolidity, and electron microscopic investigations. The annealing temperature is determined empirically on the basis of the theses of the thermodynamic theory of the high temperature stability of multicomponent amorphous alloys, according to which there exists a range of temperatures that is characterized by a negative difference between the chemical potentials of phases in a heterogeneous amorphous matrix–frozen crystallization centers system. The thermodynamic condition of the possible dissolution of frozen crystallization centers is thus met. It is shown that introducing regimes of thermal processing allows us to expand the ranges of the thermal stability of iron-based amorphous alloys by 20–40 K through purifying an amorphous matrix of frozen crystallization centers. This conclusion is proved via electron microscopic investigations.     

SAXS study on the structure and crystallization of amorphous metallic alloys

The change in structure of several amorphous alloys as Fe-P-C, Fe-B and Pd-Au-Si alloys during isothermal ageing was examined using small angle X-ray scattering technique as well as transmission electron microscopy. The SAXS intensity was related to two different types of scattering sources depending on ageing period and temperature. As-splatted amorphous alloys and the specimens aged for short period at relatively low temperature, where specimens remained in amorphous state, showed a weak and spreaded SAXS intensity. This indicates an existence of electron density fluctuation in amorphous phase. The dimension of their scattering regions was estimated to be 1.8 to 2.4 nm for Fe-P-C alloys, and 1.2 to 0.8 rim. for Fe-B alloys, respectively, by analysing the electron density-density correlation function derived from observed SAXS intensity. The origin has been discussed based on one dimensional model. A compositional fluctuation was made clear to occur in fine scale of about 1 nm in amorphous Fe-B and Fe-P-C alloys. When the alloys were aged for longer period below their crystallization temperatures, fine crystalline particles precipitate; those are a two phase lamella structure of Fe₃P and-Fe for a Fe₈₃P₁₀C₇ alloy, an Ferich single phase for Fe₈₃B₁₇ alloy, and a Pd-rich single phase for Pd₇₆Au₆Si₁₈8 alloy, respectively. Their size and structure were determined from the analysis of SAXS data and TEM observation.     

Orientation of the remaining amorphous chains during crystallization: Isotactic polystyrene

The orientation of statistical chain segments in the amorphous regions of a semicrystalline polymer can be characterized quantitatively by x-ray diffraction. A calculated background (which is orientation independent) must be subtracted, which requires that the intensity profile of the amorphous halo be measured on an absolute scale. Application is illustrated to samples of isotactic polystyrene which were isothermally crystallized at fixed elongation. These samples differ from those of the previous study due to the unintentional introduction of a small amount of crystallinity (0.25% to 1%) which serves as tie points in the drawing operation, giving a stronger amorphous orientation. Preferred orientation of the amorphous chain segments is rapidly depleted during the first few percent of crystallization. The crystallite orientation distribution is quite sharp, and broadens only slowly with crystallization. These results imply that, for the crystallization conditions employed, the crystallite orientation must be determined by nuclei which are formed at the earliest stages of the crystallization process.     

Effect of amorphous seeds on the crystallization kinetics of Na-A zeolite

The formation of crystal nuclei occurs when alkaline aluminosilicate gels are treated at temperatures below the crystallization temperature or during crystallization of gels rather than during precipitation of the gels. The linear growth rate of the crystals of Na-A zeolites yielded by alkaline aluminosilicate gels in the presence of amorphous seeds is a function of the product of the concentrations of the silicate and aluminate ions in the intermicellar liquid.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 799–804, May, 1994.     

Formation of single-crystalline aragonite tablets/films via an amorphous precursor

Thin tablets and films of calcium carbonate have been grown at the air-water interface via an amorphous precursor route using soluble process-directing agents and a Langmuir monolayer based on resorcarene. By using appropriate concentrations of poly(acrylic acid-sodium salt) in combination with Mg2+ ion, an initially amorphous film is deposited on the monolayer template, which subsequently crystallizes into a mosaic film composed of a mixture of single-crystalline and spherulitic patches of calcite and aragonite. Of particular importance is the synthesis of single-crystalline "tablets" of aragonite (approximately 600 nm thick), because this phase generally forms needle-like polycrystalline aggregates when grown in vitro. To our knowledge, a tabular single-crystalline morphology of aragonite has only been observed in the nacreous layer of mollusk shells. Therefore, this in vitro system may serve as a useful model for examining mechanistic issues pertinent to biomineralization, such as the influence of organic templates on nucleation from an amorphous phase.