Low Supersaturation Conditions Research Articles

Nanosynthesis Without a Twist

The synthesis of many nanoscale materials occurs under conditions of changing saturation because generation of product decreases the concentration of reactants. Morin et al. (p. [476][1]) used a flow reactor to maintain conditions of low supersaturation during the growth of zinc oxide nanotubes and

Spiral Growth on Nanoporous Silicoaluminophosphate STA-7 as Observed by Atomic Force Microscopy

Atomic force microscopy was used to study the surface of STA-7 crystals. STA-7 is a silicoaluminophosphate, nanoporous solid formed by interlinked double six ring units (D6R). Observations showed the formation of three distinct types of spirals at low supersaturation conditions. The {001} face shows spirals with isotropic shapes and a Burgers vector of 0.9 nm, which corresponds to one D6R or one unit cell along the ⟨001⟩ direction. The {100} face contains two distinct types of spirals. The first has a Burgers vector of 0.9 nm, or half a unit cell along ⟨100⟩. This dislocation produces a change in the “stacking” sequence of the D6Rs generating an overgrowth with the AEI structure. The second type is an interlaced spiral and is generated by a dislocation with a Burgers vector of 1.8 nm or one unit cell, leading to the formation of two substeps each with a different growth anisotropy. This anisotropy is directed by the shape of the substep and the energetics of template attachment. The preponderance of a surface coating of a secondary phase will have significant consequences on applications reliant on intracrystalline diffusion, such as catalysis, where, owing to diffusion limitations, the outermost structure dominates the functional properties.

Magnetic effects on crystal growth rate of calcite in a constant-composition environment

The crystal growth rates of calcite in a constant-composition environment were measured in the presence of a magnetic field. The magnetic effect to be influenced by any one of the solution variables, including pH, degree of relative supersaturation ( σ), ionic strength ( I), Ca 2+ to CO 3 2− ionic activity ratio ( R), and superficial velocity ( v), was investigated while keeping the other variables constant. The growth rate was suppressed completely in the presence of the magnetic field under low pH and supersaturation conditions. By contrast, the growth rate seemed to increase at high pH and relative supersaturation. A more surprising result is that the magnetic suppression effect on CaCO 3 growth was observed except at unity activity ratio. The information gathered in this experiment might be used as a reference for understanding the performance of the anti-scale magnetic treatment device.

Growth, structure and electrical conduction of WO 3 nanorods

We present a very simple method to obtain tungsten trioxide nanorods. The nanorods are epitaxially grown on a mica substrate in low supersaturation conditions. Investigations of morphology, crystallographic structure and chemical composition of the nanorods allow us to propose a growth model in which the potassium ions of the substrate play a major role inducing the one-dimensional structure. The nanorod growth is initiated by the formation of a hexagonal tungsten bronze (HTB) epitaxially oriented on the mica. By using a conductive atomic force microscopy technique, we characterise the electrical conduction of WO 3 networks.

Rare-earth elements modified hydrotalcites and corresponding mesoporous mixed oxides as basic solid catalysts

The inspection of a series of hydrotalcites (HTs) samples prepared under different supersaturation conditions, has allowed the selection of the low supersaturation method conducting to a more disordered HT structure as appropriate to accommodate rare-earth elements (REE) species in HTs structures. The goal is to prepare mesoporous mixed oxides to be used as catalysts for basic catalyzed organic reactions. A series of REE (Y 3+, Dy 3+, Gd 3+, Sm 3+, La 3+) modified HTs samples are prepared under low supersaturation conditions. Their derived mixed oxides were obtained by the thermal decomposition of HTs at 460 °C for 18 h under argon flow. The properties of HTs modified REE and of their corresponding mixed oxides are analyzed from XRD structural data, textural measurements and basicity data. The HTs modified REE in an amount of approximately 10% (w/w), from 6.78% (w/w) for Mg 0.75Al 0.23Y 0.02 to 11.74% (w/w) for Mg 0.75Al 0.23Dy 0.02 proved to be effective precursors for the manufacturing of mixed oxides catalysts very active and selective in the reaction of cyanoethylation of ethanol with acrylonitrile. A relation between their conversions and their properties, in particular, the number of weak and medium-strength basic sites could be proposed.

LPE of Silicon Carbide Using Diluted Si-Ge Flux

Epitaxial growth of 4H-SiC has been carried out at temperatures up to 1650 oC on 4HSiC substrates dipped in strongly diluted Si-based solutions. Liquid Phase Epitaxy (LPE) in conditions of low supersaturation was shown to be an effective technique to overgrow micropipe defects (MPs) in SiC wafers prepared by the Physical Vapour Transport (PVT) technique. The aim of this work was to investigate the SiC growth morphology and the dependence of MP elimination efficiency on Si-Ge flux composition. Macroscopically flat, single crystalline SiC layers of a thickness up to 10 µm were grown with a growth rate of about 0.5 µm/h. Stepped growth morphology was observed independent of the melt composition. Micropipes with the diameter below 5 µm were closed with an efficiency of about 80%. SEM investigations as well as inspection under reflected/transmitted light did not show any specific distortion of the growth morphology at the micropipe healing place.

Structural promotion and stabilizing effect of Mg in the catalytic decomposition of nitrous oxide over calcined hydrotalcite-like compounds

Catalytic decomposition of nitrous oxide to nitrogen and oxygen has been carried out on in-situ generated calcined hydrotalcite-like compounds containing different combinations of bivalent (Mg, Co, Ni) and trivalent (Al, La, Rh) cations with carbonate as interlayer anion. The precursors were prepared by co-precipitation under low supersaturation conditions and characterized by XRD and TG/DSC. The mixed oxides derived after calcination at 723 K were characterized by XRD, N 2 adsorption at 77 K, and XRF. These catalysts show high activity on a mass basis in comparison with the active catalyst Co-ZSM-5 under similar experimental conditions. The presence of Mg in the catalysts exerts an important influence on the catalytic behaviour, increasing the surface area of the calcined materials, acting as a stabilizing agent against SO 2 inhibition at low temperature and eliminating the initial deactivation, but decreasing the specific activity. Addition of Rh or La to the cobalt-based catalyst improves the catalytic activity considerably. Ex-Co-Rh,Al-HTlc (Co/Al = 3/1 and 0.7 wt.% Rh), with and without Mg, proved to be the most active catalyst, followed by ex-Co-La,Al-HTlc (Co/La/Al = 3/1/1).

Equilibrium crystal shapes for 6H AND 4H SiC grown on non-planar substrates

Growth of 6H and 4H SiC has been performed on Si-face substrates with etched stripe mesas under low supersaturation conditions (which is obtained by keeping the C:Si ratio in the gas phase low) at the two different growth temperatures of 1480 and 1620°C. The growth around the mesas was investigated by scanning electron microscopy and the facets which appear around the mesas could be identified as low index lattice planes. The difference in growth rate between these lattice planes was found to be less than ±25%. Modified polar Wulff plots are used to illustrate the growth rate in the different lattice directions, and could also be used to predict the growth habit for different geometries. As expected, the average growth rate is larger on the {11 2 n} than on the {1 1 0n} planes, but slow and fast growing planes exist for both sets of planes. Generally the growth rate is larger for planes with high n, and for the {1 1 0n} planes it is found that a slow growth rate could be correlated with a larger percentage of C ad-atoms to the plane. At the lower growth temperature a larger difference in growth rate between the fast and slow growing planes could be observed, probably due to a larger influence of the surface energies under these conditions.

High activity and stability of the Rh‐free Co‐based ex‐hydrotalcite containing Pd in the catalytic decomposition of N2O

The catalytic decomposition of nitrous oxide to nitrogen and oxygen has been studied over calcined hydrotalcite‐like compounds containing different combinations of bivalent (Co, Pd, Mg) and trivalent (Al, La, Rh) cations with carbonate as interlayer anion. The precursors were prepared by co‐precipitation under low supersaturation conditions and characterized by XRD and TG/DSC. The mixed oxides derived after calcination at 723 K were characterized by XRD, N2 adsorption at 77 K, and XRF. The presence of Rh, La, or Pd in the Co‐based HTlc's improves considerably the catalytic activity. Co–Rh,Al‐HTlc (Co/Rh/Al==3/0.02/1) proved to be a very active catalyst, although the presence of the noble metal Pd in this catalyst ex‐Co,Pd–La,Al‐HT (Co/Pd/La/Al=3/1/1/1) produces a similar catalytic activity to that of Rh‐containing catalyst, both in a N2O‐containing stream and in one containing also SO2 and O2, but with a better performance in stability tests. PdO phase has been identified by XRD as being responsible for the considerable improvement in the activity. The presence of Mg as spinel structure exerts a stabilizing effect in the more active catalysts when mixtures of SO2 and O2 are considered.

Inherently Stable High-Aspect-Ratio Silver Chloride Tabular Emulsions

A wide variety of silver bromide crystal morphologies can be produced in a straightforward manner by changing the excess bromide level during emulsion precipitation. A number of these conditions produce morphologies bounded by {111} faces, most notably the twinned tabular form. Silver chloride does not inherently lend itself to such a variety in part because the {111} crystal face is not stable without strongly adsorbed growth modifiers. A new type of inherently stable high-aspect-ratio tabular silver chloride has been precipitated by adding small amounts of iodide (less than 0.1 mole% overall) early in the process to induce anisotropic grain growth under low supersaturation conditions. Unlike the traditional twinned tabular morphology, these new crystals possess stable {100} major faces, contain no twin planes, and do not require organic growth modifiers for formation or stability. Photographic performance consistent with the high surface-to-volume ratio, such as high light absorption and covering power, is obtained using conventional methods of spectrochemical sensitization.

The growth and perfection of organic non-linear optical crystals: N-methyl urea (NMU) from methanol solution. III. The growth of large single crystals for optical examination

Using growth conditions defined from previous studies of the kinetics of growth of this material, large crystals of N-methyl urea (NMU) have been prepared by seeded solution growth using the temperature lowering technique. The principal constraints to good growth are that crystals should be prepared using seeds cut from good quality crystals previously prepared by this technique and that they should be developed under conditions of low supersaturation (σ < 0.5−3%). Efficient stirring conditions should be used with flow reversal to minimise the effect of volume diffusion and macrostep formation which in turn leads to the inclusion of solvent in some sectors. The role of seed orientation in the definition of the development process is discussed. Under well-defined conditions of seed orientation and development crystals of dimensions 5 × 4 × 4 cm3 have been produced of high optical and structural perfection and with efficient non-linear optical performance. This material shows a figure of merit performance which improves on urea. For the doubling of light of wavelength 871.6 nm; d14 ≈ 2pm V−1, cf. urea d14 = 1.7pm V−1.

Scaling of single-bubble growth in a porous medium.

Mass-transfer driven growth of a single gas cluster in a porous medium under the application of a supersaturation in the far field is examined. We discuss the growth pattern and its growth rate. Contrary to compact (spherical) growth in the bulk, growth patterns in porous media are disordered and vary from percolation to diffusion-limited aggregation (DLA) as the cluster size increases. At conditions of low supersaturation, scaling laws for the boundaries that delineate these patterns and of the corresponding growth rates are derived. In three dimensions (3D), it is found that the cluster grows as ${\mathit{R}}_{\mathit{g}}$\ensuremath{\sim}${\mathit{t}}_{\mathit{f}}^{1/(\mathit{D}}$-1), where ${\mathit{D}}_{\mathit{f}}$ is the pattern fractal dimension (\ensuremath{\approxeq}2.50 for percolation or DLA). A similar result involving logarithmic corrections is found for 2D. These results generalize the classical scaling ${\mathit{R}}_{\mathit{g}}$\ensuremath{\sim}${\mathit{t}}^{1/2}$ to fractal clusters.

Synthesis and physicochemical properties of cobalt aluminium hydrotalcites

Cobalt aluminium hydrotalcites with different compositions were prepared by a coprecipitation method under low supersaturation conditions. The compounds were characterized by X-ray diffraction (XRD), infrared absorption (IR), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC) and BET surface area measurements. XRD and IR studies revealed that all the compounds are single-phase crystallized under a hydrotalcite-like network. Hydrothermal treatments given to the aged sample increased the crystallinity of the samples. TG studies showed two stages of weight loss, the first due to the removal of interlayer water and the second ascribed to the removal of water molecules from the brucite sheet and CO2 from the interlayer carbonate anion, whose transition temperature depends on the Co/Al atomic ratio. Thermal calcination of these materials results in the formation of high surface area non-stoichiometric spinel phase whose crystallinity increases with increase in the calcination temperature attributed to the sintering of the particles.

Light-scattering investigations of nucleation processes and kinetics of crystallization in macromolecular systems.

Quasi-elastic light scattering (QELS) was used to investigate quantitatively the mechanisms of nucleation, postnucleation growth, and dissolution in ensembles of both crystalline and amorphous aggregates of satellite tobacco mosaic virus (STMV), ferritin, apoferritin and pumpkin seed globulin. At low supersaturation conditions, as described previously for small molecule crystallization, the metastable region was obtained. Under these conditions aggregation took place, but crystallization did not proceed and critical nuclei did not form over a long period of time. The critical solution supersaturation necessary to obtain crystals, sigma = ln(c/s) where c and s are concentration and solubility of protein, varied from approximately 0.1 for pumpkin seed globulin to approximately 0.9 for STMV. For higher supersaturation conditions when aggregation processes leading to formation of crystals are not established immediately but after a certain induction period, the supersaturation-dependent critical nuclear size, R(c), for different macromolecular systems was estimated from time-dependent size-distribution analyses to be in the range of approximately 10(3) for proteins such as pumpkin globulin to approximately 10 for virus particles. From the same data, the molar interfacial free energy was deduced to be 3.3-9.2 kJ mol(-1). These are believed to be among the first estimates for macromolecular crystals. Under conditions of moderate supersaturation where induction periods preceded the appearance of critical nuclei, the potential barriers for formation were estimated to be in the range 8.3-50 kJ mol(-1). Growth and dissolution kinetics for pumpkin seed globulin were investigated. These experiments allowed determination of protein solubility versus solution temperature, protein and precipitant concentrations. Aggregation patterns which lead to crystal formation are distinctly different to those which produce an amorphous precipitate. The results provide additional evidence that QELS can be used to find general criteria that allow one to discriminate between conditions for a given protein system leading to crystalline or amorphous states at early stages of the aggregation process.

Dislocation characterization in crystals of potash alum grown by seeded solution growth under conditions of low supersaturation

An analysis has been carried out of the genesis and character of growth dislocations present in all growth sectors of single crystals of potash alum. The crystals, grown from seeded solutions by the temperature lowering method under conditions of low supersaturation, presented the well-developed forms: {111} dominant, {100} and {110}. Growth dislocations formed predominately during refacetting of the edges and corners of the seed, rounded during preparation and insertion into the supersaturated solution. From here they become refracted into the {111} sectors which proved to be the most defective. Smaller numbers of dislocations form at the {111}, {100} and {110} seed interfaces and propagate in these sectors. In crystals of inferior quality, a number of inclusions were found predominantly in the fast growing {100} sectors which become the source of additional dislocations. Dislocations present in the original seed did not propagate across the interface into the developing crystal. Dislocations of all characters were observed. The principal Burgers vectors were found to be 〈100〉, 〈110〉 and 〈111〉.

Epitaxial Lateral Overgrowth of Silicon on SiO2 Investigated by X-Ray Topography

ABSTRACTSilicon lamellae grown laterally over silicon dioxide by liquid phase epitaxy are investigated using X-ray double crystal topography. The lamellae are grown from Indium solution. All of the lamellae show growth striations with indium concentration differences of about 1016...4x1017 cm−3 Overgrowth widths up to 320μm are obtained on sides of the seeding window by growth under conditions of low supersaturation. Facetting of the lamella edges then becomes less pronounced and, therefore,striations are detected farther away from the windows.About 60% of the lamellae are free of crystallographic defects. In the defective lamellae single dislocations parallel to the sample surface are detected. They extend between seeding window edges and re—entrant corners at the lamella edges. Dislocations probably form during the cooling process due to strains near the edges of the seeding windows, and as a result of indium incorporation in high concentration.

On the Control of Diamond Deposition Structure and Morphology

Various methods have been employed to deposit diamond crystals and films from the vapor phase. It is clear that similar deposits were obtained by the different techniques, which seems to indicate that a single general mechanism independent of the deposition method is responsible for diamond deposition and deposit structure. A global approach to the deposition process is proposed here which is consistent with all the important experimental observations. No mechanistic hypothesis are made on the nature of the hydrocarbon species important for growth, as they can vary together with their relative concentrations as functions of the deposition conditions and methods. The interaction of the active species with the surface is characterized as a deposition from some neutrals and etching by hydrogen atoms, and it is shown how the relative concentration of these species controls the supersaturation at the gas‐solid interface. From the sole hypothesis that the reaction order for graphite etching by hydrogen atoms is higher than for diamond, it can be shown how the supersaturation, and therefore the nucleation and growth steps, and finally the deposit structure and morphology, can be controlled either by varying the experimental parameters in one deposition method, or by utilizing different deposition techniques. All the experimental deposition conditions, from different techniques, are correlated on a single diagram with the different morphologies, their degree of structural order and the growth rate. The different methods appear as a function of their ability to produce low or high partial pressures of hydrogen atoms. Finally, it is suggested that high‐pressure methods could be used in conditions of low supersaturation to grow single‐crystal films.

Crystallization in bile.

The formation of crystalline components in gallstones is governed by the physical-chemical factors controlling the crystallization of minerals in aqueous systems. The elucidation of the mechanism of these reactions, especially at the low supersaturations of interest in vivo, is facilitated by the use of a constant composition method in which the reactions are studied at supersaturation levels automatically sustained, potentiometrically, during the experiments. Not only is it possible to distinguish between the various theoretical models of crystal growth, but the influence of in vivo modifiers of mineralization can be investigated at the same conditions of low supersaturation. The method is illustrated for the calcium phosphate system in which it is shown that precursor phases may be stabilized kinetically for long periods of time despite the fact that the driving forces for the formation of the thermodynamically most stable phase are appreciable. These factors will have important consequences during the precipitation of mineral phases in the formation of gallstones.

Characterization of healed fracture surfaces in fluorite by etching and proton irradiation

Healed fracture surfaces in fluorite, which are not visible by conventional optical microscopy, can be revealed by etching or proton irradiation. Due to the semi-brittle nature of the mineral under conditions appropriate to the late stages of development of many ore deposits, healed fractures are often associated with some evidence of plastic deformation. Simple healed fractures consist of a single sub-boundary of very low angle, typically <0.02 degree, often decorated by <50 μm diameter fluid inclusions. Complex healed fractures consist of a zone with at least two sub-boundaries enclosing a thin layer of fluorite, which is almost in the same orientation as the surrounding crystal. This type of healed fracture is accompanied by small numbers of large fluid inclusions, typically >100 μm diameter. Simple healed fractures form by auto-epitaxial growth under conditions of high supersaturation in tight cracks. Complex healed fractures originate from more open cracks, the surfaces being initially etched before being healed by growth of a layer of fluorite under conditions of low supersaturation, probably by the screw dislocation mechanism.

Observations on the morphology and crystallography of proeutectoid ferrite

Proeutectoid ferrite precipitates in Fe-0.47% C austenite partially transformed at 730°C have been studied using Kossel X-ray diffraction and detailed two-surface serial sectioning. Previous observations of habit planes near {111} in this system have been confirmed. However, there are many morphological irregularities and a minority of precipitates possess major interfaces which clearly deviate from {111} by 10–15°, which from measurements of the orientation relationship and maximum growth direction appear essentially to be of the approximate {223} type observed recently. The results are compared with various previous studies and it is concluded that the near {111} habit is not simply a pseudo-habit-plane characteristic only of longer reaction times and furthermore may only occur when a precipitate is formed under conditions of low supersaturation.