High Supersaturation Research Articles

Hydration of salts as a two-step process: Water adsorption and hydrate formation

K2CO3 is a promising salt for thermochemical heat storage. For a high performance, the thermochemical reaction must take place as close as possible to the equilibrium, while ensuring sufficient reaction rates. In this work, we studied the near-equilibrium hydration kinetics of K2CO3 and other salts (CuCl2, MgCl2 and LiCl). We proposed a generic two-step mechanism for the hydration of salts, consisting of (1) adsorption of water vapour and dissolution of ions from the initial phase (a wetting film) and (2) formation of the hydrate crystal (crystallisation from the wetting film). The two steps are assumed to be in momentarious balance during the hydration process. As a result, nucleation is rate limiting at low supersaturations of water vapour (inside the metastable zone), and water diffusion to the wetting film is rate limiting at high supersaturations (outside the metastable zone). We have seen that the vapour pressure of the wetting film stabilises at the metastable zone boundary p*. The driving force for hydration outside the metastable zone (MZ) is therefore the pressure difference between the atmospheric vapour pressure and the vapour pressure of the wetting film, p − p*. Non-Parametric Kinetic analysis of the hydration of K2CO3 indicates that nucleation plays a central role inside the metastable zone (at low supersaturations) as expected. Outside the MZ, the analysis suggests a steady conversion rate, in agreement with a water vapour diffusion limitation. The diffusion limited process at high supersaturations hardly depends on the temperature, but mainly on the pressure difference, as expected. It is further shown that the diffusion limited process can be characterised with an apparent activation energy. However, this apparent activation energy is in fact the hydration enthalpy and does not refer to a real energy barrier.

Osteogenic and antibacterial surfaces on additively manufactured porous Ti-6Al-4V implants: Combining silver nanoparticles with hydrothermally synthesized HA nanocrystals

The recently developed additively manufacturing techniques have enabled the fabrication of porous biomaterials that mimic the characteristics of the native bone, thereby avoiding stress shielding and facilitating bony ingrowth. However, aseptic loosening and bacterial infection, as the leading causes of implant failure, need to be further addressed through surface biofunctionalization. Here, we used a combination of (1) plasma electrolytic oxidation (PEO) using Ca-, P-, and silver nanoparticle-rich electrolytes and (2) post-PEO hydrothermal treatments (HT) to furnish additively manufactured Ti-6Al-4V porous implants with a multi-functional surface. The applied HT led to the formation of hydroxyapatite (HA) nanocrystals throughout the oxide layer. This process was controlled by the supersaturation of Ca2+ and PO43− during the hydrothermal process. Initially, the high local supersaturation resulted in homogenous nucleation of spindle-like nanocrystals throughout the surface. As the process continued, the depletion of reactant ions in the outermost surface layer led to a remarkable decrease in the supersaturation degrees. High aspect-ratio nanorods and hexagonal nanopillars were, therefore, created. The unique hierarchical structure of the microporous PEO layer (pore size < 3 μm) and spindle-like HA nanocrystals (<150 nm) on the surface of macro-porous additively manufactured Ti-6Al-4V implants provided a favorable substrate for the anchorage of cytoplasmic extensions assisting cell attachment and migration on the surface. The results of our in vitro assays clearly showed the important benefits of the HT and the spindle-like HA nanocrystals including a significantly stronger and much more sustained antibacterial activity, significantly higher levels of pre-osteoblasts metabolic activity, and significantly higher levels of alkaline phosphatase activity as compared to similar PEO-treated implants lacking the HT.

Numerical Frameworks for Laser-Induced Cavitation: Is Interface Supersaturation a Plausible Primary Nucleation Mechanism?

Crystallization has been observed in laser-induced cavities in saturated solutions, but the mechanisms behind nucleation of crystals are not entirely clear. A hypothesis is that high solution supersaturation during the bubble growth period triggers the nucleation. Because of small spatiotemporal scales of the cavitation event, the supersaturation is very difficult to measure experimentally. To test the nucleation hypothesis, we perform a two-dimensional axisymmetric direct numerical simulation of an experimentally observed laser-induced cavitation event with crystallization. We demonstrate a significant degree of supersaturation and argue that the nucleation hypothesis is indeed plausible. To analyze factors that lead to a high supersaturation, we develop a comprehensive one-dimensional model for spherical laser-induced cavities. We conduct an extensive investigation on how the solute solubility, solute diffusivity, laser pulse energy, and superheated liquid volume affect the supersaturation. We show that high supersaturation is possible under a range of relevant conditions but not readily obtained for all solutions and laser setups. Guidelines are provided to identify if a specific solution or laser setup may attain high supersaturation. The insights obtained and the numerical methods formulated in this work can be applied to assess and design new laser-induced cavitation setups that allow for precise control of the duration and degree of the supersaturation

Implementing Superhydrophobic Surfaces within Various Condensation Environments: A Review

AbstractSteam condensation is omnipresent, and inevitable in many industrial processes. A classic example is seen within condensers that are used to liquify various gasses. In consequence, water‐vapor is eventually deposited upon the exterior of heat pipes that passively cool the gas to form a thin film of liquid. This macroscopic film is responsible for the degradation of heat transfer efficiencies. Liquid repellent surfaces can microscopically manipulate the hydrodynamics of formulating condensate to transition toward dropwise/jumping‐droplet condensation. This effect will save operating and maintenance costs of steam condensers within power‐plants, subsequently reducing emissions, operating power, and fuel consumption. However, the challenges associated with the stabilization of dropwise/jumping‐droplet condensation, especially at large subcooling temperatures and high supersaturation ratios, have prohibited the use of liquid repellent surfaces. This review aims to discuss recent improvements and understanding of dropwise/jumping‐droplet condensation surfaces on superhydrophobic surfaces. Numerous examples shall be included, showing experimental and numerical studies of various innovative properties of superhydrophobic structures. In addition, various fabrication techniques of superhydrophobic surfaces that are applicable within industrial systems are discussed. These topics are consolidated to provide guidelines for future research into dropwise/jumping‐droplet condensation on liquid repellent surfaces.

Activation properties of aerosol particles as cloud condensation nuclei at urban and high-altitude remote sites in southern Europe

Understanding the activation properties of aerosol particles as cloud condensation nuclei (CCN) is important for the climate and hydrological cycle, but their properties are not fully understood. In this study, the CCN activation properties of aerosols are investigated at two different sites in southern Spain: an urban background station in Granada and a high altitude mountain station in the Sierra Nevada National Park, with a horizontal separation of 21 km and vertical separation of 1820 m.CCN activity at the urban environment is driven by primary sources, mainly road traffic. Maximum CCN concentrations occurred during traffic rush hours, although this is also when the activation fraction is lowest. This is due to the characteristics of the rush hour aerosol consisting of ultrafine and less hygroscopic particles. In contrast, the mountain site exhibited larger and more hygroscopic particles, with CCN activity driven by the joint effect of new particle formation (NPF) and vertical transport of anthropogenic particles from Granada urban area by orographic buoyant upward flow. This led to the maximum concentrations of CCN and aerosol particles occurring at midday at the mountain site. Clear differences in the diurnal evolution of CCN between NPF events and non-event days were observed at the Sierra Nevada station, demonstrating the large contribution of NPF to CCN concentrations, especially at high supersaturations. The isolated contribution of NPF to CCN concentration has been estimated to be 175% higher at SS = 0.5% relative to what it would be without NPF. We conclude that NPF could be the major source of CCN at this mountain site.Finally, two empirical models were used to parameterize CCN concentration in terms of aerosol optical or physical parameters. The models can explain measurements satisfactorily at the urban station. At the mountain site both models cannot reproduce satisfactorily the observations at low SS.

Ultrasound as a tool for polymorph control and high yield in flow crystallization

Ortho-aminobenzoic acid (o-ABA) has three polymorphs from which form-I is the most stable and forms-II and III are meta–stable ones at 25 °C. In flow crystallization, the meta-stable form-II is quite persistent and prevents nucleation of the stable form-I. This is contrary to batch crystallization and possibly a result of high local supersaturation. In this work, seeded, anti-solvent crystallization of o-ABA was conducted to overcome nucleation of the meta-stable form and obtain a high yield of form-I. Experiments were conducted in a tubular flow crystallizer under silent and sonicated conditions. Seeding was effective in producing pure form-I only when ultrasound was applied. Under silent conditions, poor dispersion of seeds and heterogeneous wall nucleation led to the growth of form - II. Ultrasound improved dispersion and reduced nucleation on walls and thus a pure form-I product was obtained. Particle sizes and SEM images showed evidence of secondary nucleation as the mechanism for form-I occurring in seeded, sonicated experiments rather than seed growth. A high yield, 48% by mass and 85% of maximum crystallizable mass, could be achieved. Ultrasound increased the potential of the flow crystallization system by combining high yield with polymorph control.

Improving the reproducibility of size distribution of protein crystals produced in continuous slug flow crystallizer operated at short residence time

Continuous crystallization of proteins at short residence time and high supersaturation level is attractive in terms of the space-time yield and production efficiency. Nevertheless, it is rarely pursued due to its less-than-desirable crystal size distribution (CSD) characterized by the abundance of small crystals due to high nucleation rate. The small crystals were prone to random agglomeration, resulting in poor crystals’ residence time distribution, hence low CSD’s reproducibility. Herein we developed a segmented slug flow crystallizer (SFC) design operated at short residence time (<30 min) comprising a short nucleation segment and a growth segment operated at different temperature and fluid velocity. The SFC design improved the CSD’s reproducibility by limiting small crystals and large-sized agglomerates formations as evidenced by the small coefficient-of-variations between replicates (<10%). Lysozyme crystals having size of roughly 13–14 µm with well-preserved bioactivity were produced at yield and space-time yield of approximately 67% (w/w) and 93 g/L·h, respectively.

α or β?-hemihydrates transformed from dihydrate calcium sulfate in a salt-mediated glycerol–water solution

α-and β-hemihydrate calcium sulfate (HH), the most important calcium sulfate phases, are usually achieved by autoclave hydrothermal reaction and calcination in air, respectively. Herein, we report an investigation on preparation of HH from dihydrate calcium sulfate(DH) in a salt-mediated glycerol-water solution. We found that both α-HH and β-HH in forms of apparently single crystals could be obtained at 110 ℃ only by tuning the concentrations of glycerol, Na2EDTA, and NaCl. TG-DSC, XRD, SEM, and Raman spectroscopy were used to identify and characterize the product crystals. The results indicate that the high relative supersaturation of HH could be the key factor determining the generation of β-HH crystal in solution. Glycerol and NaCl generally decrease the water activity, enhance the crystallization kinetics, and therefore allow the presence of a metastable HH phase (especially for compact α-HH bulk crystal). But too high concentration of glycerol (i.e., ≥70%) and NaCl (i.e., ≥0.2 M) would be favorable for β-HH crystallization with obviously defective crystal morphology due to the dramatically increased local relative supersaturation. Na2EDTA could retard the recrystallization but well regulate the morphology of HH bulk crystals.

Assessment of particle size magnifier inversion methods to obtain the particle size distribution from atmospheric measurements

Abstract. Accurate measurements of the size distribution of atmospheric aerosol nanoparticles are essential to build an understanding of new particle formation and growth. This is particularly crucial at the sub-3 nm range due to the growth of newly formed nanoparticles. The challenge in recovering the size distribution is due its complexity and the fact that not many instruments currently measure at this size range. In this study, we used the particle size magnifier (PSM) to measure atmospheric aerosols. Each day was classified into one of the following three event types: a new particle formation (NPF) event, a non-event or a haze event. We then compared four inversion methods (stepwise, kernel, Hagen–Alofs and expectation–maximization) to determine their feasibility to recover the particle size distribution. In addition, we proposed a method to pretreat the measured data, and we introduced a simple test to estimate the efficacy of the inversion itself. Results showed that all four methods inverted NPF events well; however, the stepwise and kernel methods fared poorly when inverting non-events or haze events. This was due to their algorithm and the fact that, when encountering noisy data (e.g. air mass fluctuations or low sub-3 nm particle concentrations) and under the influence of larger particles, these methods overestimated the size distribution and reported artificial particles during inversion. Therefore, using a statistical hypothesis test to discard noisy scans prior to inversion is an important first step toward achieving a good size distribution. After inversion, it is ideal to compare the integrated concentration to the raw estimate (i.e. the concentration difference at the lowest supersaturation and the highest supersaturation) to ascertain whether the inversion itself is sound. Finally, based on the analysis of the inversion methods, we provide procedures and codes related to the PSM data inversion.

Tailoring the nanostructure of laser powder bed fusion additively manufactured maraging steel

Abstract Grade 300 maraging steel specimens were synthesized using the additive manufacturing technique laser powder bed fusion (LPBF). The influence of heterostructures, i.e. cellular/dendritic regions with micro-segregations, formed during LPBF processing on the microstructure evolution, particularly the formation of nano-precipitates were investigated. We applied aging and solution- aging post heat-treatments to allow the precipitation in presence and after elimination of the heterostructures, respectively. A model describing the precipitation mechanisms and sequence for both heat-treatment routes is proposed based on the three-dimensional elemental distribution at near-atomic scale and the quantification of nano-scale segregated regions performed by atom probe tomography (APT). We observed that the heterostructures favored the austenite retention in the as-built condition, while they promoted the martensite-to-austenite reversion in the aged condition. On the contrary, the retention and reversion processes are suppressed in the solution-aged condition. The formation of nanosized (Ni,Fe,Co)3(Ti,Mo,Al) and (Fe,Ni,Co)7(Mo)6 precipitates was confirmed for both post heat-treatment routes. However, the nano-precipitates were smaller in size and higher in number density in the solution-aged condition, as compared to the aged state. The current work demonstrates that the elimination of heterostructures in the solution-aged condition, especially Ni micro-segregations, led to a high supersaturation and a concomitant increase in number density of (Ni,Fe,Co)3(Ti,Mo,Al) precipitates. The corresponding Ni-depleted matrix inhibited the martensite-to-austenite reversion. In contrast, the partial retention of heterostructures and the presence of retained austenite during aging without prior solution treatment resulted in a reduced number density of (Ni,Fe,Co)3(Ti,Mo,Al) precipitates, which can potentially trigger the reversion transformation supported by the rejection of Ni in front of (Fe,Ni,Co)7(Mo)6 precipitates.

CCN activation of carbonaceous aerosols from different combustion emissions sources: A laboratory study

A laboratory-based experiment has been carried out to understand the cloud condensation nuclei (CCN) activation of carbonaceous aerosols from various sources of emissions such as combustion of wood, cow dung cake, wood-coal burning and diesel generator (DG). The aerosol particles from DG exhaust found to activate completely to cloud condensation nuclei (CCN) at a higher supersaturation (SS) of 0.9% (100.9% RH). However, the CCN activation was faster in carbonaceous aerosol emission from combustion of cow dung cake, where complete CCN activation of particles was observed at 0.3% SS. Wood burning also showed a faster activation at 0.4% SS, whereas for wood-coal burning, the CCN activation has occurred at 0.7% SS. Organic carbon (OC) was found to be the dominant carbon component in all four emission sources compared to elemental carbon (EC) and black carbon (BC). Among all four combustion emissions, wood-burning had the highest OC/EC ratio (8.35) and the lowest was for DG emissions (2.03). BC mass concentration was higher in DG emissions compared to other emissions for the same number of aerosol particles. The non-refractory aerosol chemical composition analysis showed that organic species dominate inorganic species in all the emissions. The study indicates that the carbonaceous aerosols originating from direct emission sources have the potential to get activated as CCN at high supersaturations.

Drop Size Distribution Broadening Mechanisms in a Bin Microphysics Eulerian Model

Abstract In this study, processes that broaden drop size distributions (DSDs) in Eulerian models with two-moment bin microphysics are analyzed. Numerous tests are performed to isolate the effects of different physical mechanisms that broaden DSDs in two- and three-dimensional Weather Research and Forecasting Model simulations of an idealized ice-free cumulus cloud. Sensitivity of these effects to modifying horizontal and vertical model grid spacings is also examined. As expected, collision–coalescence is a key process broadening the modeled DSDs. In-cloud droplet activation also contributes substantially to DSD broadening, whereas evaporation has only a minor effect and sedimentation has little effect. Cloud dilution (mixing of cloud-free and cloudy air) also broadens the DSDs considerably, whether or not it is accompanied by evaporation. This mechanism involves the reduction of droplet concentration from dilution along the cloud’s lateral edges, leading to locally high supersaturation and enhanced drop growth when this air is subsequently lifted in the updraft. DSD broadening ensues when the DSDs are mixed with those from the cloud core. Decreasing the horizontal and vertical model grid spacings from 100 to 30 m has limited impact on the DSDs. However, when these physical broadening mechanisms (in-cloud activation, collision–coalescence, dilution, etc.) are turned off, there is a reduction of DSD width by up to ~20%–50% when the vertical grid spacing is decreased from 100 to 30 m, consistent with effects of artificial broadening from vertical numerical diffusion. Nonetheless, this artificial numerical broadening appears to be relatively unimportant overall for DSD broadening when physically based broadening mechanisms in the model are included for this cumulus case.

Prediction of Crystal–Solvent Interactions in a Supercritical Medium: A Possible Way to Control Crystal Habit at High Supersaturations with Molecular Modeling

The purpose of this work is to contribute to a better control of the crystallization process which occurs in a supercritical medium, especially during the Supercritical AntiSolvent (SAS) process. I...

Distribution Characteristics of Aerosol Size and CCN during the Summer on Mt. Tian and Their Influencing Factors

The aerosol size distribution and cloud condensation nuclei (CCN) number concentration were measured using a wide-range particle spectrometer (WPS) and a cloud condensation nuclei counter (CCNC) on Mt. Tian from 31 July to 9 September, 2019. Combined with meteorological data, distribution characteristics of aerosol size and CCN and their influencing factors were analyzed. The results indicated that the mean aerosol number concentration was 5475.6 ± 5636.5 cm−3. The mean CCN concentrations were 183.7 ± 114.5 cm−3, 729.8 ± 376.1 cm−3, 1630.5 ± 980.5 cm−3, 2162.5 ± 1345.3 cm−3, and 2575.7 ± 1632.9 cm−3 at supersaturation levels of 0.1%, 0.2%, 0.4%, 0.6%, and 0.8%, respectively. The aerosol number size distribution is unimodal, and the dominant particle size is 30–60 nm. Affected by the height of the boundary layer and the valley wind, the diurnal variation in aerosol number concentration shows a unimodal distribution with a peak at 17:00, and the CCN number concentration showed a bimodal distribution with peaks at 18:00 and 21:00. The particle size distribution and supersaturation have a major impact on the activation of the aerosol into CCN. At 0.1% supersaturation (S), the 300–500 nm particles are most likely to activate to CCN. Particles of 100–300 nm are most easily activated at 0.2% (S), while particles of 60–80 nm are most likely activated at high supersaturation (≥0.4%). The concentrations of aerosol and CCN are higher in the northerly wind. Ambient relative humidity (RH) has little relationship with the aerosol activation under high supersaturation. According to N = CSk fitting the CCN spectrum, C = 3297 and k = 0.90 on Mt. Tian, characteristic of the clean continental type.

Three-dimensional aspects of sidebranch formation during the growth of snow crystals

The nature of sidebranching in the facetted regions of snow crystals is investigated. It is shown that during the growth of a snow crystal, an initial sidebranch-pair forms after step bunching occurs on the leading prism planes, followed by step bunching on the adjacent flat basal face, producing a macrostep between the main branch and sidebranch feature. A Michelson interferometer–reflecting microscope setup was used to in-situ observe the three-dimensional morphology of a growing snow-crystal branch in a diffusion chamber. The macrostep on the basal plane was found to occur in the cases of simple facet growth, repetitive facet growth, and rounded tip growth, with the rounded tips forming at higher supersaturations. Thus, snow-crystal sidebranching involves a separation of the supply route of diffusing molecules on basal plane from the tip of the main-branch. Moreover, the sidebranches would fully sprout when the supersaturation increased, at which time the facetted branch tip abruptly narrowed and became rounded. These results indicate important roles of both the surface diffusion of water molecules and step migration for dendritic morphological instability on snow crystals.

Rapid synthesis of sodium-rich Prussian white for Sodium-ion battery via a bottom-up approach

In this work, combining homogeneous rapid precipitation in microreactor and following thermal aging treatment, we proposed a bottom-up approach to afford rapid and productive preparation of NaxMn[Fe(CN)6]y with the low defect and high sodium content. It was revealed that the key to a successful bottom-up synthesis was to enhance the aggregation-mediated growth of building blocks. Homogeneous rapid precipitation in microreactor decouples the nucleation and growth under high supersaturation and thus overwhelmingly obtain uniform building blocks. Thus, the Ostwald ripening growth is seriously weakened due to narrow size distribution and these building blocks are more prone to fusion growth due to similar crystal properties and lower aggregation barrier. The as-prepared MnHCF with high crystallinity delivers a high capacity (162.4 mAhg−1 at 15 mAg−1), an excellent rate performance (144.8 mAhg−1 at 150 mAg−1, 109.2 mAhg−1 at 750 mAg−1) and satisfied cyclic retention (with a capacity retention of 76% for 100 cycles at the current density of 150 mAg−1).

Prospects of Bulk Growth of 3C-SiC Using Sublimation Growth

Free standing 3C-SiC wafers with a dimeter of 50 mm and a thickness of ca. 0.8 mm have been grown on a regular base using 3C-SiC CVD seed transfer from Si wafers to a poly-SiC-carrier and a sublimation epitaxy configuration. Up to the thickness of almost 1 mm, stable growth conditions of the cubic polytype have been achieved. The high supersaturation was kept stable by the proper design of the hot zone that enables a high axial temperature gradient at the growth interface. The Sirich gas phase was realized by the application of a Tantalum getter that was integrated into the graphitebased growth cell. Furthermore, an adaption of the growth setup allowed the growth of 3C material with a diameter of 95 mm and bulk material up to 3 mm on 25 mm diameter. Computer simulations were used to determine the supersaturation of the growth setup for different source-to-seed distances. The minimum supersaturation necessary for stable growth of cubic SiC was found to be higher 0.1 for seed already containing the required 3C polytype.

Estimation of cloud condensation nuclei number concentrations and comparison to in situ and lidar observations during the HOPE experiments

Abstract. Atmospheric aerosol particles are the precondition for the formation of cloud droplets and therefore have large influence on the microphysical and radiative properties of clouds. In this work, four different methods to derive or measure number concentrations of cloud condensation nuclei (CCN) were analyzed and compared for present-day aerosol conditions: (i) a model parameterization based on simulated particle concentrations, (ii) the same parameterization based on gravimetrical particle measurements, (iii) direct CCN measurements with a CCN counter, and (iv) lidar-derived and in situ measured vertical CCN profiles. In order to allow for sensitivity studies of the anthropogenic impact, a scenario to estimate the maximum CCN concentration under peak aerosol conditions of the mid-1980s in Europe was developed as well. In general, the simulations are in good agreement with the observations. At ground level, average values between 0.7 and 1.5×109 CCN m−3 at a supersaturation of 0.2 % were found with the different methods under present-day conditions. The discrimination of the chemical species revealed an almost equal contribution of ammonium sulfate and ammonium nitrate to the total number of CCN for present-day conditions. This was not the case for the peak aerosol scenario, in which it was assumed that no ammonium nitrate was formed while large amounts of sulfate were present, consuming all available ammonia during ammonium sulfate formation. The CCN number concentration at five different supersaturation values has been compared to the measurements. The discrepancies between model and in situ observations were lowest for the lowest (0.1 %) and highest supersaturations (0.7 %). For supersaturations between 0.3 % and 0.5 %, the model overestimated the potentially activated particle fraction by around 30 %. By comparing the simulation with observed profiles, the vertical distribution of the CCN concentration was found to be overestimated by up to a factor of 2 in the boundary layer. The analysis of the modern (year 2013) and the peak aerosol scenario (expected to be representative of the mid-1980s over Europe) resulted in a scaling factor, which was defined as the quotient of the average vertical profile of the peak aerosol and present-day CCN concentration. This factor was found to be around 2 close to the ground, increasing to around 3.5 between 2 and 5 km and approaching 1 (i.e., no difference between present-day and peak aerosol conditions) with further increasing height.

Hybrid neutralization and membrane process for fluoride removal from an industrial effluent

This study aims to investigate at a laboratory scale fluorides removal from an industrial wastewater having excessive F- concentration through a hybrid process combining neutralization and membrane separation. For the membrane separation operation, both Reverse Osmosis (RO) and Nanofiltration (NF) were investigated and confronted. The optimized neutralization step with hydrated lime allowed reaching fluoride removal rates of 99.1 ± 0.4 %. To simulate continuous process, consecutive batch treatments with full recirculation of membrane process brines were conducted. Despite the relatively high super saturations with respect to CaF2, no membrane cloaking was observed. The RO polishing treatment allowed decreasing the permeate fluoride concentration to 0.9 &#xb1 0.3 mg/L with a fluoride rejection rate of 93&#xb1 2% at the optimal transmembrane pressure of around 100 psi. When NF membrane was used to treat neutralization filtrate, the permeate fluoride concentration dropped to 1.1&#xb1 0.4 mg/L with a fluoride rejection rate of 88&#xb1 5% at the optimal pressure of around 80 psi. Thus, with respect to RO, NF allowed roughly 20% decrease of the driving pressure at the expense of only 5% drop of rejection rate. Both NF and RO permeates at optimal operating transmembrane pressures respect environmental regulations for reject streams discharge into the environment.

Effect of Radius on Crystal Structure Selection in III–V Nanowire Growth

The radius of III-V nanowires is known to have an effect on the resulting crystal structure during particle-assisted growth; however, the causes behind this effect remain under debate. In this work, we use stochastic simulations of nanowire growth to evaluate how the radius (R) affects the growth dynamics and how this in turn affects the crystal structure selection. This is due to the geometry of the growing nanowire: The number of atoms in the seed particle scales with R3, and the number of III-V pairs per layer scales with R2. The influx of growth species to the seed can, for instance, scale with the surface area of the seed particle for direct impingement (?R2) or the perimeter of the nanowire for sidewall diffusion (?R1) or be radius-independent for substrate diffusion in some cases (?R0). These differences in radius dependencies cause the particle composition to change more rapidly for thinner nanowires, which in turn leads to nucleation at higher supersaturations and promotion of the wurtzite structure. In addition, the geometry can also make the influx V/III ratio dependent on the nanowire radius, which can influence the selection of crystal structure in different ways depending on the materials system and growth regime. (Less)