High Supersaturation Research Articles

A contemporary look at the accuracy of the ‘double pulse technique’ for measuring rates of crystal nucleation; molecular-kinetic and non-classical mechanisms of initial growth of the just-born crystals

Microscopically visible crystals are indispensable for measuring nucleation rates by means of the ‘double pulse technique’ (DPT). Therefore, the accuracy of the DPT depends on the survivability of the just-born crystals during their initial growth and (possible) dissolution. First, because the critical nuclei stay in a labile equilibrium with the mother phase (meaning equal growth and dissolution probabilities), half of the critical nuclei must be dissolved. Second, because of interphase fluctuations and crystal coalescence, the number densities of super-critically sized nuclei can also be decreased. To estimate the survivability of super-critical crystals, the probabilities for attachment/detachment of molecules to/from such crystals are considered in full detail from a molecular-kinetic point of view. It is shown that the probability of decomposition down to subcritical dimension, which is of importance for the accuracy of the DPT, is negligibly small already for crystals by only three molecules larger than the critical size. Besides, except for extremely high supersaturations, also the oriented attachment of nanocrystals is of little consequence for the accuracy of the DPT. The conclusion is that, because the nucleation of crystals is a stochastic process, the nucleation rates measured by means of the DPT are dependable.

Nucleation experiments on a titanium-carbon system imply nonclassical formation of presolar grains.

Just as the shapes of snowflakes provide us with information on the temperature and humidity of the upper atmosphere, the characteristics of presolar grains in meteorites place limits on their formation environment in a stellar outflow. However, even in the case of well-characterized presolar grains consisting of a titanium carbide core and a graphitic carbon mantle, it is not possible to delimit their formation environment. Here, we have demonstrated the formation of core-mantle grains in gravitational and microgravity environments and have found that core-mantle grains are formed by a nonclassical nucleation pathway involving the three steps: (i) primary nucleation of carbon at a substantially high supersaturation, (ii) heterogeneous condensation of titanium carbide on the carbon, and (iii) fusion of nuclei. We argue that the characteristics of not only core-mantle grains but also other presolar and solar grains might be accurately explained by considering a nonclassical nucleation pathway.

Induced-charge electroosmosis for rapid mixing of reactive precipitation systems to obtain small and uniform particles

The control of product quality in precipitation systems is often difficult due to high supersaturation levels and fast kinetics. Rapid and uniform mixing of fluids is required to minimize supersaturation gradients before the onset of nucleation. This study proposes an active mixing platform utilizing induced-charge electroosmosis (ICEO) to better control crystal quality attributes such as the size distribution. The mixing performance of the ICEO-based micromixer is characterized through experiments as a function of the solution properties and the electric field parameters such as phase angle, frequency, and voltage. Optimal mixing is achieved at lower conductivities when the phase difference is 180°. Furthermore, computational fluid dynamics simulations demonstrate that uniform supersaturation is achieved at shorter mixing lengths of the device along with a narrower residence time distribution in the presence of ICEO. Additionally, the performance of the ICEO-based crystallizer is demonstrated for the reactive precipitation of silver chloride particles and carbamazepine-succinic acid cocrystals. The application of ICEO leads to the production of smaller silver chloride particles with a narrower size distribution and an improved yield for cocrystallization compared to cases without ICEO. The ICEO-based micromixer characterized in this work is generally attractive for the continuous production of particles through reactive precipitation with stringent requirements on product uniformity and yield.

Pickering emulsions stabilized by hemp protein nanoparticles: Tuning the emulsion characteristics by adjusting anti-solvent precipitation

Hemp protein is an important dietary source rich in cannabinoid (CBD) and comprehensive amino acids, but its application in food is limited due to its poor emulsibility and solubility. In this study, hemp seed proteins were used to produce hemp protein nanoparticles (HPNs) by anti-solvent precipitation for the first time using formic acid as solvent. During anti-solvent precipitation, the supersaturation can control the number and size of the nanoparticles produced. Three important factors, including the solvent-antisolvent ratio, the dropping rate and the stirring rate were used to adjust the supersaturation. At moderate supersaturation, such as 1:10 (v/v) solvent-antisolvent ratio, a dropping rate of 15 mL min−1 or a stirring rate of 1000 rpm, HPNs exhibited smaller particle size (∼130 nm), greater ζ-potential (∼50 mV) and stronger wettability to water compared to HPNs prepared under high or low supersaturation. HPNs prepared under moderate supersaturation exhibited more random coil (24.83%) and β-sheet (61.75%), exposing more hydrophobic amino acid residues. All O/W Pickering emulsions were able to remain stable during 3 weeks storage at 20 °C. In the presence of HPNs prepared under moderate supersaturation, the emulsions exhibited larger droplet size than HPNs prepared under low or high supersaturation, increasing from ∼300 nm to ∼450 nm. Because many HPNs were separated from the interface, Pickering emulsions prepared with these HPNs exhibited higher apparent viscosity than HPNs prepared under low or high supersaturation. This research will expand the way of hemp proteins utilization, and also broaden the preparation method and regulation means of protein-based Pickering emulsions.

Calcium Carbonate Hexahydrate (Ikaite): History of Mineral Formation as Recorded by Stable Isotopes

Calcium carbonate hexahydrate (ikaite) is a rare mineral that forms as metastable species in the organic-carbon-rich sediments of the King George Basin, Bransfield Strait, Antarctica, as a consequence of early diagenetic decomposition of organic matter under cold water (−1.4 °C) and high pressure (200 bar) conditions. Large crystals grow in the sediment immediately below the diagenetic transition between microbial sulfate reduction and methanogenesis at ~320 cm below sea floor (bsf). This process is reflected in the dissolved sulfate, total carbon dioxide, and methane concentrations, as well as in the carbon, hydrogen, and oxygen isotope chemistries of the interstitial fluids and dissolved gases of the host sediment. The ikaite crystal faithfully records in its zonal structure the changing carbon isotope ratio of the total dissolved carbon dioxide pool as it gradually diminishes during methanogenesis (δ13Cikaite = −17.5 to −21.4‰). These changes in the crystal’s host environment follow general Rayleigh carbon isotope fractionation. The oxygen isotopes of the ikaite carbonate (δ18Oikaite = 1.46 to 4.45‰) also show a strong zonal distribution, unrelated to temperature of formation, but perhaps controlled by the degree of recrystallization of ikaite to calcite. The crystal water of the ikaite is depleted 11‰ in 2H/1H (VSMOW) relative to the coexisting interstitial water, which is in excellent agreement with the isotope fractionation of other hydrated minerals. In addition to the in situ temperature and pressure, nucleation of the ikaite crystals in the Bransfield Basin sediments may be induced by the high alkalinity, high phosphate concentrations, and dissolved organic compounds. Intense microbial metabolism generates such compounds; of these, aspartic acid and glutamic acid may play an important role, as they do in biological and extracellular carbonate mineral precipitation. All indications are that low temperatures (such as of polar environments), high calcium carbonate supersaturation caused by interstitial methanogenesis, and a sufficiently large supply of dissolved phosphate and amino acids favor metastable ikaite formation. These conditions, modified by recrystallization, may be preserved in calcite glendonites, thinolites, and other calcitic pseudomorphs derived from ikaite and found throughout the ancient sedimentary record.

Crystallization by selective evaporation using membrane pervaporation: Application to l-glutamic acid to control polymorphism

Crystallization is an important unit operation in process industries, driven by supersaturation, i.e. the solute concentration difference between solution and equilibrium. Hence, to finely control the product qualities, controlling mass and/or heat transfer is crucial. Membrane pervaporation, based on a selective evaporation technique, appears as a process able to limit phase transition and control polymorphism. In this work, a semi-continuous system is developed for the crystallization of l-glutamic acid, which has two polymorphic forms. Hydrophilic HybSi® membranes are chosen. The influence of different operating parameters on process performance in terms of solvent separation, solute crystallization and control of polymorphism is investigated. Results validate the proof of concept and highlight the preferential crystallization of a polymorphic form over the other. Hence, the α-form is usually favoured except when the β-form is seeded, or at high temperature. Temperature appears as the parameter influencing mostly the membrane selectivity and the polymorph generated. However, concentration polarization and fouling, due to the deposit of l-glutamic acid crystals on the membrane surface, are reported and decrease the process performance. Both phenomena are caused by a high local supersaturation close to the membrane surface in addition to a local cooling due to the water evaporation during pervaporation.

Nonisothermal crystallization kinetics of potassium chloride produced by stirred crystallization

The elucidation of the crystal nucleation and growth process can help control crystal size and quality. Here,the classical theory of primary nucleation was first used to study the homogeneous and heterogeneous nucleation mechanism of potassium chloride (KCl) crystallization. The growth kinetics in the presence of seed in KCl was then investigated. To understand the nucleation mechanism of KCl, the induction period of KCl was analyzed using classical nucleation theory to calculate the interfacial tension (γ) and various nucleation parameters, including the radius of the critical nucleus (r*), the critical free energy of the nucleus (ΔG*), and the molecular number of the critical nucleus (i*). The results showed a homogeneous nucleation mechanism at high supersaturation ratios (≥1.10) and a heterogeneous nucleation mechanism at low supersaturation ratios (<1.08). Meanwhile, the KCl crystal exhibited a continuous growth mechanism as inferred by the surface entropy factor (f) value. The SEM data demonstrated that the increase in crystallization temperature and supersaturation ratio could increase the average crystal size. The particle size-independent growth model is suitable for the growth process according to the relationship between crystal particle number density and particle size. In addition, the crystal nucleation and growth rates in stirred crystallization were calculated using the population balance equation, and then various kinetic parameters were estimated. The estimated parameters suggested an increase in growth with an increase in supersaturation ratio and stirring rate. Meanwhile, the impact of each parameter in the nucleation rate equation showed that the degree of influence is suspension density, supersaturation ratio, and finally, stirring rate.

Effect of Solvent Composition on Solubility, Thermodynamics, Metastable Zone Width (MSZW) and Crystal Habit of L-Ascorbic Acid

This work investigates the effect of different solvent systems on solubility, thermodynamics, metastable zone width (MSZW), and crystal habit of ascorbic acid, in order to help optimize its crystallization process. The solubility curves and metastable zone (MSZ) limits were determined experimentally using the polythermal method in pure solvents: water and alcohols (methanol/ethanol/isopropanol), as well as water-alcohol binary solvent systems. The solubility decreases with increasing alcohol composition for all solvent systems. The solubility data were well correlated using the Jouyban–Acree model as a function of two variables: temperature and solvent composition. The dissolution enthalpy (ΔHdiss), dissolution entropy (ΔSdiss), and Gibbs free energy (ΔGdiss) were determined using Van’t Hoff and Jouyban–Acree models. The thermodynamic properties increase with increasing alcohol composition. The lowest and highest values of enthalpy were obtained for water (20.52 kJ mol −1) and isopropanol (35.33 kJ mol−1), respectively. Pure alcohols as solvents widen the metastable zone width, indicating high supersaturation required for the nucleation. Crystal images captured during cooling crystallization in water confirm the cubic crystal habit formation, whereas increasing alcohol composition in the solvent system promotes preferential growth along one crystallographic axis, leading to elongated prism-shaped crystals in methanol and ethanol and needle-shaped crystals in isopropanol.

Crystal Engineering in Antisolvent Crystallization of Rare Earth Elements (REEs)

Antisolvent crystallization is a separation technology that separates the solute from the solvent by the addition of another solvent, in which the solute is sparingly soluble. High yields are achieved by using higher antisolvent-to-aqueous ratios, but this generates higher supersaturation, which causes excessive nucleation. This results in the production of smaller particles, which are difficult to handle in downstream processes. In this work, the effect of varying the organic (antisolvent)-to-aqueous (O/A) ratio and seed loading on the yield, particle size distribution, and morphology of neodymium sulphate product, during its recovery from an aqueous leach solution using antisolvent crystallization, was investigated. A batch crystallizer was used for the experiments, while ethanol was used as an antisolvent. Neodymium sulphate octahydrate [Nd2(SO4)3.8H2O] seeds were used to investigate the effect of seed loading. It was found that particle sizes increased as the O/A ratio increased. This was attributed to the agglomeration of smaller particles that formed at high supersaturation. An O/A ratio of 1.4 resulted in higher yields and particles with a plate-like morphology. The increase in yield was attributed to the increased interaction of ethanol molecules with the solvent, which reduced the solubility of neodymium sulphate. Increasing the seed loading resulted in smaller particle sizes with narrow particle size distribution and improved filtration performance. This was attributed to the promotion of crystal growth and suppression of agglomeration in the presence of seeds.

Microstructural characteristics and deformation behaviors of an Al–Mg–Si alloy with improved strength and conductivity processed by continuous casting and expansion extrusion

The industrial applications of Al–Mg–Si alloys conductors still meet the big challenges of strength and conductivity trade-off. In the present work, a special continuous expansion extrusion forming (CEEF) with violent shearing deformation was performed on continuous cast 6101 Al alloy rods to process the sheets with improved mechanical and electrical properties by combining various artificial aging treatments. The results showed that refined dendrites with few intergranular phases and high supersaturation solid solubility could be obtained in the continuous casted rods with fast cooling rate. In the CEEF sheet, the gradient distribution of grain size from the core to the surface of the sheet was observed, where the grain size of processed sheet was refined remarkably by dynamic recovery (DRV) and dynamic recrystallization (DRX). When artificial aging was conducted on the as-extruded, solid-solution and pre-deformed alloy specimens respectively, different precipitation responses were found. The high-density dislocations in the pre-deformed alloy acted as pipes for rapid diffusion of solid solution atoms to accelerate their precipitation, reducing the matrix solid solution atom content and mitigating electron scattering phenomena. As a result, simultaneous improvements of strength and conductivity could be obtained by combining the above optimized strategies.

Kinetic modeling of interfacial abruptness in axial nanowire heterostructures

Kinetic modeling of the formation of axial III–V nanowire heterostructures grown by the Au-catalyzed vapor–liquid–solid method is presented. The method is based on a combination of kinetic growth theory for different binaries at the liquid–solid interface and thermodynamics of ternary liquid and solid alloys. Non-stationary treatment of the compositional change obtained by swapping material fluxes allows us to compute the interfacial abruptness across nanowire heterostructures and leads to the following results. At high enough supersaturation in liquid, there is no segregation of dissimilar binaries in solid even for materials with strong interactions between III and V pairs, such as InGaAs. This leads to the suppression of the miscibility gaps by kinetic factors. Increasing the Au concentration widens the heterointerface at low Au content and narrows it at high Au content in a catalyst droplet. The model fits quite well the data on the compositional profiles across nanowire heterostructures based on both group III and group V interchange. Very sharp heterointerfaces in double of InAs/InP/InAs nanowire heterostructures is explained by a reduced reservoir effect due to low solubility of group V elements in liquid.

Hot-Wall MOCVD for High-Quality Homoepitaxy of GaN: Understanding Nucleation and Design of Growth Strategies

Thick GaN layers with a low concentration of defects are the key to enable next-generation vertical power electronic devices. Here, we explore hot-wall metalorganic chemical vapor deposition (MOCVD) for the development of GaN homoepitaxy. We propose a new approach to grow high-quality homoepitaxial GaN in N2-rich carrier gas and at a higher supersaturation as compared to heteroepitaxy. We develop a low-temperature GaN as an optimum nucleation scheme based on the evolution and thermal stability of the GaN surface under different gas compositions and temperatures. Analysis in the framework of nucleation theory of homoepitaxial layers simultaneously grown on GaN templates on SiC and on hydride vapor phase epitaxy GaN substrates is presented. We show that residual strain and screw dislocation densities affect GaN nucleation and subsequent growth leading to distinctively different morphologies of GaN homoepitaxial layers grown on GaN templates and native substrates, respectively. The established comprehensive picture provides a guidance for designing strategies for growth conditions optimization in GaN homoepitaxy. GaN with atomically flat and smooth epilayer surfaces with a root-mean-square roughness value as low as 0.049 nm and low background carbon concentration of 5.3 × 1015 cm–3 has been achieved. It is also shown that there is no generation of additional dislocations during homoepitaxial growth. Thus, our results demonstrate the potential of the hot-wall MOCVD technique to deliver high-quality GaN material for vertical power devices.

Effect of precipitation kinetics on microstructure and properties of novel Al-Mn-Cr-Zr based alloys developed for powder bed fusion – laser beam process

This study investigates the precipitation kinetics and resulting effect on microstructure and property of a family of novel high strength Al-Mn-Cr-Zr alloys designed for powder bed fusion – laser beam process. The alloys have been shown to be printable without solidification cracking along with high supersaturation of solutes in as-printed state. Upon direct ageing, two families of precipitates namely Al-Mn and Al-Zr are observed. Al-Mn containing precipitates, which are observed in as-printed condition as nanometric precipitates decorating special regions are seen to grow preferentially at grain boundaries, which is followed by growth in bulk of the sample. A possible explanation is suggested to be a higher diffusivity at grain boundaries leading to faster growth while depleting solutes around grain boundary region quickly. The Al-Zr precipitation, which normally follows bulk precipitation is observed to co-precipitate with Al-Mn precipitates. Optimised heat treatments are seen to achieve peak hardness of 143 HV at 623 K for 24 h and 142 HV at 648 K for 14 h as compared to 102 HV in as-printed condition for one of the alloys. This overall hardening effect is attributed majorly to Al 3 Zr nanoprecipitates along with semi-coherent Al 12 Mn precipitates. • Alloy design tailored for PBF-LB process created Al-Mn-Cr-Zr family of Al alloys. • Alloying elements are supersaturated in solid solution after printing. • On direct ageing, two novel precipitation phenomenon observed. • Feature analysis enabled quantitative precipitation analysis with chemical data.

Biological Mineralization of Hydrophilic Intraocular Lenses

Biomaterials calcify upon implantation in contact with biological fluids, which are supersaturated with respect to more than one crystalline phase of calcium phosphate. The implantation of intraocular lenses (IOLs) for cataract treatment has been hailed as a major advance. Hydrophilic acrylic IOLs, made of Poly(2-hydroxyethyl methacrylate) (PHEMA), upon contact with aqueous humor, exhibit significant incidence of opacification, due to the formation of calcium phosphate crystals, mainly hydroxyapatite (Ca5(PO4)3OH, HAP) on the surface or in their interior. The aqueous humor is supersaturated with respect to HAP. Clinical findings were duplicated by laboratory experiments through the development of appropriate experimental models which included batch reactors, well stirred operating at constant supersaturation (CCR) and reactors simulating anterior eye chamber (ECSR). In both CCR and ECSR, simulated aqueous humor was used. In ECSR the flow rate was the same as in the eye chamber (2.5 mL per 24 h). HAP formed both on the surface and inside the IOLs tested. Induction times preceding the crystallization of HAP on the surface of the IOLs and crystal growth rates were measured. Surface hydroxyl ionized groups favored the development of locally high supersaturation by surface complexation. In the interior of the IOLs, HAP formed by the diffusion of the calcium and phosphate ions inside the polymeric matrix.

Ice fog observed at cirrus temperatures at Dome C, Antarctic Plateau

Abstract. As the near-surface atmosphere over the Antarctic Plateau is cold and pristine, its physico-chemical conditions resemble to a certain extent those of the high troposphere where cirrus clouds form. In this paper, we carry out an observational analysis of two shallow fog clouds forming in situ at cirrus temperatures – that is, temperatures lower than 235 K – at Dome C, inner Antarctic Plateau. The combination of lidar profiles with temperature and humidity measurements from advanced thermo-hygrometers along a 45 m mast makes it possible to characterise the formation and development of the fog. High supersaturations with respect to ice are observed before the initiation of fog, and the values attained suggest that the nucleation process at play is the homogeneous freezing of solution aerosol droplets. This is the first time that in situ observations show that this nucleation pathway can be at the origin of an ice fog. Once nucleation occurs, the relative humidity gradually decreases down to subsaturated values with respect to ice in a few hours, owing to vapour deposition onto ice crystals and turbulent mixing. The development of fog is tightly coupled with the dynamics of the boundary layer which, in the first study case, experiences a weak diurnal cycle, while in the second case, it transits from a very stable to a weakly stable dynamical regime. Overall, this paper highlights the potential of the site of Dome C for carrying out observational studies of very cold cloud microphysical processes in natural conditions and using in situ ground-based instruments.

Number‐Size Distribution and CCN Activity of Atmospheric Aerosols in the Western North Pacific During Spring Pre‐Bloom Period: Influences of Terrestrial and Marine Sources

AbstractA cruise observation of atmospheric aerosols and gaseous air/seawater compounds was performed in the western North Pacific during the spring pre‐bloom period. The aerosol size distribution and cloud condensation nuclei (CCN) activity were characterized according to the source type: terrestrial/pollution, marine, and mixed. The CCN activity was represented by the aerosol hygroscopicity parameter (ĸCCNC) from the activation diameter (dact) in water‐vapor supersaturation (SS), which is controlled by the composition of particles with sizes around dact. For marine type, whereas a high hygroscopicity parameter value of 0.86 for accumulation mode (ĸaccum) suggests a strong influence of sea salt and sulfate, a lower value of 0.49 in Aitken mode (ĸAit) suggests the contribution of biogenic organic matter (OM). Lower ĸAit and ĸaccum for terrestrial/pollution type (0.12 and 0.62, respectively) suggests greater contributions of OM. Under the influence of marine sources, ĸCCNC at a dact of ∼110 nm (low SS) was not responsive to the chlorophyll a (Chl‐a) concentration, whereas ĸCCNC at ∼50 nm (high SS) correlated negatively with Chl‐a. The hygroscopicity parameter of OM was estimated as 0.13–0.29 considering the aerosol composition. During a new particle formation event, the ĸCCNC at 0.60% SS increased; nucleated particles grew in size to become CCN. Sensitivity analysis indicated that changes in the size distribution and chemical composition of the aerosol contributed to the increase in CCN concentration. This study highlights the high variability in size distribution and CCN activity of off‐coast aerosols from different sources, including marine biological activities.

Fluid Pressure Changes Recorded by Trace Elements in Quartz

AbstractFluid pressure is a key parameter in earthquake mechanics, controlling seismic failure and plate coupling in convergent zones. Yet fluid pressure is also extremely difficult to quantify at seismogenic depth, which limits our knowledge of the stress state in accretionary prisms. Here, we show that the geochemical record of exhumed hydrothermal quartz veins may be used to place quantitative bounds on fluid pressure variations in subduction zones. The studied veins come from sediments accreted and exhumed by plate convergence in southwestern Japan. Quartz in veins displays growth rims of contrasted bright blue/dark brown cathodoluminescence (CL) colors, high/low Al concentrations, and low/high fluid inclusion abundance. Because Si‐Al substitution (and charge compensation by Li) strongly depends on the rate of quartz precipitation and Si solubility, Al‐Li concentrations must be sensitive to fluid pressure. This is confirmed by fluid inclusions, the density of which, converted into trapping pressures, record fluid pressure drops by up to ∼70 MPa from CL‐brown, Al‐Li‐poor rims to CL‐blue, Al‐Li‐rich quartz rims. CL‐blue rims grow at a fast rate, high Si supersaturation and low fluid pressure whereas CL‐brown rims grow at a slower pace, lower Si supersaturation, and higher fluid pressure. Quartz trace element chemistry thus offers a promising tool to quantify deep fluid pressure variations and their relationships to earthquakes.

CFD simulation and experimental study of antisolvent precipitation through impinging jets for synthesis of nanodrug particles

The nucleation and growth kinetics of loratadine, one of the primary antihistamines among drugs, are determined experimentally in confined liquid impinging jet channels, followed by simulating the anti-solvent precipitation process to obtain ultrafine loratadine nanoparticles. The effect of supersaturation is assessed as the essential factor on the nucleation and growth rates. Based on classical nucleation theory, induction time and nucleation rate showed homogeneous and heterogeneous nucleation regions in high and low supersaturations, respectively. The interfacial free energy measured from the induction time is obtained at about 18.99 mJ/m2. The size-dependent particle growth and growth dispersion effects are assessed through the experimental data. The nonlinear curve fitting with the experimental data in log population density plot at residence time of 7.69 s reveals that the four-parameter model has a better fit, with no growth dispersion. The microchannel mixing performance is assessed by analyzing the Damkohler number under different Reynolds numbers. The loratadine particle size distribution (CSD) is obtained by combining the population balance equation with the computational fluid dynamics in following the discretization method with and without turbulence aggregation kernel. The simulated CSD with turbulence aggregation kernel are 7 % more consistent with experimental results. The effects of collision angle, outlet nozzle length, Reynolds number, and solution concentration on the average particle size are assessed. An increase in the collision angle from 60 to 180°decreases the average particle size. The ultrafine loratadine nanoparticles of 49 nm are obtained with a narrow distribution, related to the angle of 180⁰, outlet nozzle length of 0.5 cm, Reynolds number of 4875, and the solution concentration of 35 mg/ml.

Effect of crucible location on heat transfer in GaN crystal growth using Na flux method

A global model of heat transfer, considering convection, conduction, radiation as well as mass transfer, is established for GaN single crystal growth system employing the Na flux method. Global simulations are carried out to investigate the effects of crucible locations on the heat and mass transfer during growing. It was found that the crucible location has a significant effect on the heat transfer in the melt. When the crucible moves down, the flow in the upper region of melt is enhanced, while that near the crucible bottom is weaken. This flow structure leads to high supersaturation at the crucible bottom, which is favorable to accelerate crystallization. Therefore, it can be concluded that the crucible location should be lowered properly to achieve a better condition for crystallization. The research provides directions to the design of GaN growing systems.

Chemical characterization, absolute configuration and optical purity of (1S)-(+)- and (1R)-(-)-10-camphorsulfonic acid.

Comprehensive chemical characterization for two isomers of camphorsulfonic acid (CSA), occasionally used in the manufacture of active pharmaceutical ingredients (APIs), was performed by nuclear magnetic resonance (NMR) spectroscopy, high-resolution mass spectroscopy in negative electrospray ionization mode and gas chromatography/mass spectrometry (GC/MS) in electron ionization mode. Electronic circular dichroism (ECD) spectra together with quantum chemical calculations using time-dependent density functional theory (TD-DFT) were used to assign the stereochemistry for CSA for the first time and these assignments were then confirmed by single-crystal X-ray diffraction. As crystals were grown under the same conditions of high supersaturation using a mixed solvent without water removal, the crystal structures of the two enantiomers contained one ordered molecule of water in the asymmetric unit. The crystals of the (+)-enantiomer have a 1S,4R configuration and the H atom of the sulfonic acid group combines with the water molecule to form a hydronium ion, namely, hydronium (1S,4R)-(7,7-dimethyl-2-oxobicyclo[2.2.1]heptan-1-yl)methanesulfonate, H3O+·C10H15O4S-. The crystals of the (-)-enantiomer have a 1R,4S configuration. The determination of the optical purity of CSA using NMR spectroscopy with a chiral solvating agent, (1R,2R)-1,2-diphenylethane-1,2-diamine, and GC/MS with a chiral column has been well explored. The results showed that the examined samples of these two isomers of CSA proved to be enantiomerically pure. In particular, for (1R)-(-)-10-camphorsulfonic acid, this is, to our knowledge, the first description on its spectral characterization in a scientific context.