Log-normal Size Distribution Research Articles

Analysis of terahertz wave attenuated by sand and dust storms with different modes

The research on space transmission characteristics of terahertz wave is of great significance for the application of terahertz wave in space. In order to study the transmission characteristics of terahertz wave in sand and dust storm weather, according to the lognormal distribution of dust particle sizes, Mie scattering theory and Monte Carlo method are used to analyze the attenuation characteristics of six dry sand modes of sand and dust storm in different regions of China in a frequency band of 1–10 THz, and the relationship of the extinction parameters and attenuation rate to the frequency is given. The results show that with the increase of frequency, the attenuation rate of 1–10 THz terahertz wave first increases and then decreases. Different mode of sand and dust storm leads to different frequency range of strong attenuation of terahertz wave. In order to analyze the influence of sand dust particle moisture content on terahertz wave propagation attenuation, the relationship of three efficiency factors to water content of sand dust particles with different sizes is calculated. The results show that the influence of water content on extinction is different from that of the particle size. Monte Carlo method is used to calculate the attenuation of terahertz wave by sand and dust storm in two kinds of wet sand modes, and the relationship of the attenuation rate and water content to the frequency is given, the results are compared with those from the dry sand mode, showing that the albedo of wet sand mode is obviously lower than that of dry sand mode with the same size distribution. The absorption of wet sand particles increases with water content increasing. The extinction of wet sand and dust storm results from scattering and absorption. With the increase of water content in sand particles, the frequency band with strong attenuation of terahertz wave by wet sand and dust storm moves toward low frequency. When the water content is less than 5%, the attenuation rate of terahertz wave increases significantly with the increase of water content. Sand and dust storms with higher humidity have a greater influence on the transmission attenuation of terahertz wave.

Antibody retention by virus filtration membranes: Polarization and sieving effects

Virus filtration is a key component of the overall virus clearance strategy in the production of monoclonal antibodies. These virus filtration membranes also provide one of the most selective membrane separations ever demonstrated, with more than 95% recovery of the antibody product in the filtrate with more than 99.99% retention of even small parvovirus, despite the less than 2-fold difference in size between the virus and antibody. However, there is currently no quantitative data on the intrinsic sieving characteristics of commercially available virus filters. Experiments were performed with the Viresolve® Pro and Pegasus™ SV4 virus filters both with and without stirring to control the effects of concentration polarization. The actual sieving coefficient of a highly purified monoclonal antibody was less than 0.05 for both membranes, demonstrating that the high antibody recovery during typical virus filtration processes is a direct result of the high degree of concentration polarization in these systems. The intrinsic selectivity of the virus filter was in good agreement with predictions of available hydrodynamic models accounting for a log-normal pore size distribution. The actual sieving coefficient also decreased with increasing antibody concentration, consistent with available models for proteins with attractive interactions (negative values of the interaction parameter). These results provide important insights into the transport characteristics of virus filters and their effect on the performance of these virus filters in bioprocessing.

Simulation Informed Design and Performance of In Vitro Bioequivalence Trials for Particle Size Distributions.

This study used statistical simulations to investigate the performance of the population bioequivalence test applied to image-based particle size measurements (such as morphologically directed Raman spectroscopy) and methods for designing in vitro bioequivalence trials using prior information. Simulations of in vitro population bioequivalence trials were conducted across a range of representative D50 (number-weighted median particle diameter from a log-normal particle size distribution) and span (which is defined as [Formula: see text] where D90 and D10 are the number-weighted 90th and 10th percentiles in particle diameters sampled from a log-normal particle size distribution) values respectively. The performance of the population bioequivalence test in the simulations was driven by an interplay between overall test variability and the widening or narrowing of the bioequivalence region due to variance terms in the test statistic definition. These findings were dependent upon differences in the variability of D50 and span and may generalise to a wider range of in vitro metrics. Trial design optimisation using power and assurance approaches followed patterns consistent with these findings. As more novel scientific methods are applied to the development of complex generic drug products, the procedures outlined in this study may be used at the inception stage of future in vitro bioequivalence trials to reduce the risk of conducting costly trials with low probabilities of success.

Creep Deformation and Dynamic Grain Growth in an Interstitial-Free Steel

Dynamic grain growth is demonstrated to be much faster than static grain growth in a body-centered-cubic, interstitial-free steel sheet material at 850 $$\,^\circ {\rm{C}}$$ . Dynamic grain growth occurs during concurrent plastic deformation at elevated temperature, whereas static grain growth occurs during static annealing. Grain growth during steady-state plastic flow in tension at 850 $$\,^\circ {\rm{C}}$$ to a true strain of 0.2 at a true-strain rate of $$10^{-4}$$ $${\rm{s}}^{-1}$$ doubled grain size, while static annealing for the same time produced no increase in grain size. This is described as dynamic normal grain growth (DNGG) because no abnormally large grains were observed. The recrystallized microstructure of the steel demonstrated a log-normal distribution of grain sizes. DNGG produced bimodal grain size distributions that deviate from the theoretical expectation of a simple shift to larger sizes during normal growth. The bimodal distributions contained a remnant of small grains that were not consumed during grain growth. DNGG produced a crystallographic texture that is unique from both the recrystallized material and that produced by lattice rotation alone. DNGG strengthened the $$\{111\} \langle 110 \rangle $$ and $$\{111\} \langle 112 \rangle $$ components of the strong $$\gamma $$ -fiber component in the original recrystallization texture. Lattice rotation from tensile deformation, by contrast, strengthened the $$\alpha $$ -fiber components that intersect the original $$\gamma $$ -fiber.

Citrate-silver nanoparticles and their impact on some environmental beneficial fungi.

Colloidal suspensions of silver nanoparticles (AgNPs) with surface modified by capping with citrate ions were synthesized by chemical reduction method. Transmission and Scanning Electron Microscopy as well as darkfield Optical Microscopy provided information on the nanoparticle morphology, with fine symmetrical grains and log-normal fitted size distribution. Small Angle X-ray Scattering method allowed theoretical confirmation of colloidal silver nanoparticle fine granularity, based on measurements in the native fluid sample. UV–Vis spectrophotometry allowed studying the Localized Surface Plasmon Resonance band versus the stability of the citrate-AgNP sample after storage and after UV-C exposure. The colloidal AgNP impact on Phanerochaete chrysosporium environmental microorganisms was studied by specific biochemical investigations. Silver released from the colloidal suspension of AgNPs was supposed to induce changes in some antioxidant enzymes and in some enzymes of Krebs’ cycle. Catalase activity was moderately changed (an increase with over 50%) as well as superoxide dismutase activity, while the diminution of the activities of four dehydrogenases synthesized in the fungus mycelium was emphasized also: a decrease with about 60% for malate dehydrogenase, with over 50% for isocitrate dehydrogenase and succinate dehydrogenase and with about 40% for alpha-ketoglutarate dehydrogenase. These findings suggested the nano-toxicological issues of citrate-AgNPs impact on the environmental beneficial microorganisms.

Effect of lognormal particle size distributions of non-spherical particles on hopper discharge characteristics

This study focusses on the hopper discharge characteristics of lognormal particle size distributions (PSDs) of non-spherical particles (namely, ellipsoids, cylinders and cuboids). Discrete element method is employed to understand the coupled effects of sphericity and lognormal PSD width. The results reveal that: (i) the ellipsoids have the most similar discharge behaviors with the spheres, but the differences increase as the PSD width increases; (ii) the packing density in the hopper increases as PSD width increases and particle sphericity decreases; (iii) regardless of sphericity, the discharge rate decreases and the jamming probability increases as the PSD width increases; (iv) for the same PSD width, the jamming probability is the greatest for cuboids and least for the ellipsoids; and (v) the lower-sphericity cylinder and cuboid have some residual particles left in the hopper upon complete discharge, and the number of residues is greater and increases with PSD width for the latter. The results provide new insights regarding the behavior of wide PSDs of non-spherical particles in the hopper, highlighting the non-negligible impact of the coupled effect of PSD and particle non-sphericity.

PARAMETERIZATION OF MAGNETIC NANOPARTICLES MATHEMATICAL MODEL USING EVOLUTIONARY ALGORITHMS

In this paper, the evolutionary algorithms approach is applied to the parameterization of a mathematical model describing the Mössbauer spectra of nanogranular (or nanoparticle) magnetic systems. These systems exhibit physical properties very different from bulk specimens being of great interest for material science and its use as biosensors, magneto sensors, data storage, and magnetic fluids. The purpose of this work is to compare the performance between the Differential Evolution and the Evolutionary Strategies algorithms to optimize the model parameters which best fit the experimental Mössbauer spectra of nanoscale magnetic particles. Spectra of two samples (??iron foil and NiFe2O4 nanoparticles) were recorded, at room temperature, by a conventional Mössbauer spectrometer using a scintillation detector in transmission geometry with a 57Co/Rh source. Fits to Mössbauer spectra were done using spin hamiltonians to describe both the electronic and nuclear interactions; a model of superparamagnetic relaxation of two levels (spin ½) and stochastic theory; a lognormal particle size distribution function as well as a dependency of the magnetic transition temperature and the anisotropy constant on particle diameter. The evolutionary algorithms have been implemented using Python programming language. For comparison, the two algorithms obey the termination criterion of 6,000 evaluations of the objective function. The results presented show the efficiency of these algorithms in the optimization of the parameters and on the fits of the spectra.

ON SPIN HAMILTONIAN FITS TO MÖSSBAUER SPECTRA OF NiFe2O4 NANOPARTICLES SYNTHESIZED BY CO-PRECIPITATION

Nanocrystalline NiFe2O4 particles prepared by chemical co-precipitation method were studied using magnetic measurements, 57Fe Mössbauer spectroscopy, X-ray diffraction, and transmission electron microscopy. Fits to Mössbauer spectra, in the range of 4.2 K – 300 K, were done using spin hamiltonians to describe both the electronic and nuclear interactions, a model of superparamagnetic relaxation of two levels (spin ½) and stochastic theory, a log-normal particle size distribution function as well as a dependency of the magnetic transition temperature and the anisotropy constant on particle diameter. We have used evolutionary strategies to fit the more complex Mössbauer spectra line shapes. The nanoparticles have an average size of 7 nm and exhibit superparamagnetism at room temperature. The saturation magnetization (Ms) at 4.2 K was determined from M vs. 1/H plots by extrapolating the value of magnetizations to infinite fields, to 24.21 emu/g and coercivity to 3.15 kOe. A magnetic anisotropy energy constant (K) 1.9´105 J/m3, at 4.2 K, were calculated from magnetization measurements. The synthesis, characterization, and functionalization of magnetic nanoparticles is a highly active area of current research located at the interface between materials science, biotechnology, and medicine. Superparamagnetic iron oxides nanoparticles have unique physical properties and have emerged as a new class of diagnostic probes for multimodal tracking and as contrast agents for magnetic resonance imaging (MRI).

The importance of representative aerosol diameter and bubble size distribution in pool scrubbing

Mitigation methods that reduce the amount of radioactive materials leaking into the environment have emerged as a critical issue. Many researchers have found that pool scrubbing can play a crucial role in this process. Pool scrubbing is a practice that removes radioactive aerosols using pool water when a gas containing aerosols rises in a pool. Several codes have been developed to evaluate how much aerosol is removed during the pool scrubbing process. This study aims to improve the pool scrubbing code in a bubbly flow to make it more realistic. The importance of selecting a representative diameter of aerosol particles with a lognormal distribution is discussed. Moreover, unlike the BUSCA (BUbble SCrubbing Algorithm) code and the SPARC (Suppression Pool Aerosol Removal Code) code, which do not account for the condensation effect of the steam in the injection region or for thermophoresis, respectively, the presented code, called POSCAR (POol SCrubbing Aerosol Removal code), takes these two phenomena into account. Additionally, the calculation is performed considering the aerosol removal effect with the lognormal bubble size distribution, which had not been considered in the two aforementioned codes. Finally, the performance of this code is evaluated based on experimental results to further improve existing codes.

Frequency Dependence of Initial Heat Generation in Granular Magnetite Nanoparticles

The frequency dependence of heat generation in granular magnetite nanoparticles was studied using vibrating sample magnetometry and the nanoTherics Magnetherm™ hyperthermia testing system. First, the particle size and its distribution were obtained by fitting the M-H curve to the classical Langevin function weight-averaged with a modified log-normal size distribution function. Next, the ac power dissipation model for monodispersed nanoparticles was extended to the polydispersed case, and the volumetric heating rate was predicted as a function of the frequency of the applied field under the assumption of the Neel relaxation mechanism. Finally, the temperature increase caused by the application of an ac magnetic field was measured experimentally at frequencies of 50, 112, 261, 335, 474 and 523 kHz, with the root-mean-squared field fixed to 250 Oe, and the results were compared with those from theoretical calculation. The frequency dependence of the initial heating rates was found to be in good agreement with the results predicted by using the polydispersed power dissipation model.

Microstructures and Formation of Tundish Clogging Deposits in Ti-Alloyed Al-Killed Steel

The continuous casting of Ti-alloyed Al-killed steels traditionally experiences clogging in the tundish, impeding stable operation. We studied the microstructures and mode of formation of such clogging deposits in tundish skulls recovered after long cast series. We sampled the steel skull along the whole tundish bottom from under the ladle shroud (impact zone) up to the tundish well and nozzle. The microstructures imply that the steel solidifies to the refractory (gunning mass) and remains partially solid through the entire cast. Thus, a “mushy zone” of steel loaded with solid delta ferrites is present along the margins of the steel during casting. In this mushy zone, the widely observed clusters of variably sintered alumina (NMI) accumulate, along with a substantial amount of gas bubbles acting as inclusion scavengers. The clustered alumina particles show a lognormal particle size distribution (PSD) in all locations, which becomes more pronounced if correcting for the observable sintering. Sintering of alumina size distributions linearizes a PSD, and further evolution, once the deposit is locked in place, occurs by Ostwald ripening, introducing skewness to the distributions. The observed PSDs contrast with the established log-linearity of PSDs of individual floating alumina NMI in the bulk steel, and suggest that the source of the alumina in the clogging deposits is not the passing bulk steel. However, ubiquitous sedimentary features show that the alumina particles are also not grown by reoxidation in situ. An upstream source separate from the bulk stream must be inferred, which is also required by the consistent sporadic presence of oxidic materials which cannot be derived from local sidewall reaction. In summary, the development of clogging deposits in this type of steel is an example of “slurry casting” of partially solidifying steel over thermal loss surfaces of refractories in the nozzle area.

A Method for Retrieving Stratospheric Aerosol Extinction and Particle Size from Ground-Based Rayleigh-Mie-Raman Lidar Observations

We report on the retrieval of stratospheric aerosol particle size and extinction coefficient profiles from multi-color backscatter measurements with the Rayleigh–Mie–Raman lidar operated at the Arctic Lidar Observatory for Middle Atmosphere Research (ALOMAR) in northern Norway. The retrievals are based on a two-step approach. In a first step, the median radius of an assumed monomodal log-normal particle size distribution with fixed width is retrieved based on a color index formed from the measured backscatter ratios at the wavelengths of 1064 nm and 532 nm. An intrinsic ambiguity of the retrieved aerosol size information is discussed. In a second step, this particle size information is used to convert the measured lidar backscatter ratio to aerosol extinction coefficients. The retrieval is currently based on monthly-averaged lidar measurements and the results for March 2013 are discussed. A sensitivity study is presented that allows for establishing an error budget for the aerosol retrievals. Assuming a monomodal log-normal aerosol particle size distribution with a geometric width of S = 1.5, median radii on the order of below 100 nm are retrieved. The median radii are found to generally decrease with increasing altitude. The retrieved aerosol extinction profiles are compared to observations with the OSIRIS (Optical Spectrograph and InfraRed Imager System) and the OMPS-LP (Ozone Mapping Profiling Suite Limb Profiler) satellite instruments in the 60∘ N to 80∘ N latitude band. The extinction profiles that were retrieved from the lidar measurements show good agreement with the observations of the two satellite instruments when taking the different wavelengths of the instruments into account.

Revealing Nanoscale Domains in Cu2ZnSnS4 Thin Films by Catalyzed Chemical Etching

The presence of ZnS precipitates is common in Cu2ZnSnS4 (CZTS) films due to the preferred Cu‐poor and Zn‐rich composition for high‐efficiency CZTS devices. It is important to identify their distribution in the film as it can serve as a fingerprint for the CZTS forming process. However, limited methods are viable to quantify their distribution as they are small in size and few in number. Herein, a fast and cost‐effective chemical treatment is presented that can reveal the nanoscale ZnS domains within CZTS films to be captured under a common scanning electron microscope (SEM). This method exploits a hole‐injection process driven by gold catalysis and the band alignment between CZTS and ZnS. ZnS precipitates as small as 10 nm in diameter can be revealed clearly. Using this approach, a characteristic lognormal size distribution of the ZnS precipitates close to the CZTS film surface with a median diameter of 80 nm and a density of around 2 in a 1 μm2 surface area is demonstrated. Energy‐dispersive X‐ray spectroscopy (EDS) mapping and scanning transmission electron microscopy (STEM) imaging further confirm the existence of ZnS precipitates in the film. This method provides a fast means for capturing fingerprints of ZnS precipitations in CZTS process.

Modelling the Age-Hardening Precipitation by a Revised Langer and Schwartz Approach with Log-Normal Size Distribution

A new numerical modelling approach integrating the Langer and Schwartz approach and log-normal particle size distribution has been developed to depict the precipitation kinetics of age-hardening precipitates in Al alloys. The modelling framework has been implemented to predict the precipitation behavior of the key secondary phases in 6xxx and 7xxx Al alloys subjected to artificial aging. The simulation results are in good agreement with the available experimental data in terms of precipitate number density, radius, and volume fraction. The initial shape parameter of the log-normal size distribution entering the modeling framework turns to play an important role in affecting the later-stage evolution of precipitation. It is revealed that the evolution of size distribution is not significant when a small shape parameter is adopted in the modelling, while an initial large shape parameter will cause substantial broadening of the particle size distribution during aging. Regardless of the magnitude of shape parameter, a broadening of the particle size distribution as predicted by the present model is in agreement with experimental observations. It is also shown that large shape parameter will accelerate the coarsening rate at later aging stage, which induces fast decreasing of number density and increased growth rate of mean/critical radius. A comparison to the Euler-like multi-class approach demonstrates that the integration of more realistic log-normal distribution and Langer and Schwartz model make the present modelling faster and equivalently accurate in precipitation prediction.

Efficient Method of Moments for Simulating Atmospheric Aerosol Growth: Model Description, Verification, and Application

AbstractThe atmospheric aerosol dynamics model (AADM) has been widely used in both comprehensive air quality model systems and chemical transport modeling globally. The AADM consists of Smoluchowski's coagulation equation (SCE), whose solution undergoing Brownian coagulation in the free molecular regime is a challenge because it is inconsistent with aerosols whose size distribution cannot exactly follow the lognormal size distribution. Thus, a new method for solving the SCE without assuming lognormal size distribution is proposed and developed. The underlying principle of this method is that the hybridization of the well‐established method of moments with the assumed lognormal size distribution (log MOM) and Taylor‐series expansion method of moments (TEMOM) is implemented. This method shows excellent agreement with the sectional method (SM) which is used as reference. The accuracy of these two specific models closely approaches that of the TEMOM, but overcomes the limitation of the classical log MOM. The computational time of this scheme is considerably lower than that of the SM. The new method was successfully implemented to reveal the formation and growth of secondary particles emitted from a vehicle exhaust tailpipe. It was found that the formation of new particles only occurs in the interface region of the turbulent exhaust jet (which is very close to the tailpipe exit), whereas no new particles are formed in the mixture of the exhaust jet plume and the surrounding cold air downstream. The new method is verified as an efficient and reliable numerical scheme for studying atmospheric aerosol dynamics.

Mathematical Modelling of Evaporating Droplets Dynamics in a Vortex Ring Using Moment Method

The quadrature method of moments (QMOM) and method of moments with a lognormal particle size distribution (PSD) are applied to simulate a cloud of evaporating droplets in a vortex ring flow. Results predicted by both methods are compared for the cases with a sinusoidal and lognormal initial particle size distribution, using Lagrangian particle tracking as a reference solution. Spatial distributions of droplet number density obtained by different methods are shown to be in a qualitative agreement. The method of moments with a lognormal PSD overestimates disappearance of particles leading to the underestimation of droplet mean size and higher values of the variance of PSD. QMOM showed better agreement with the Lagrangian approach than the method of moments with a lognormal PSD.

Effect of size distribution of monomers on the radiometric properties of micro/nano aggregates for different materials

The radiometric properties of micro/nano aggregates formed by nano monomers are essential in many scientific disciplines. The aggregate model of single-sized monomers is often used to calculate the radiometric properties of micro/nano aggregates. However, the size distribution of monomers commonly exists in most of the actual aggregates and critically influences the radiometric properties. However, the influencing mechanism remains unclear. This paper discusses the influence of size distribution of monomers on the radiometric properties of micro/nano aggregates formed by nano monomers. The monomers of the aggregates have lognormal size distributions. Diffusion-limited aggregation method is used to generate the structure of aggregate, and multi-sphere T-matrix method is used to calculate the extinction efficiency, absorption efficiency, scattering efficiency, and phase function of aggregate for three representative materials and different size distributions of monomers. The influence of the geometric standard deviation of lognormal distribution and monomer number on the radiometric properties of aggregate for the three representative materials is investigated, and the mechanism is explained. Results show that the extinction and absorption efficiencies of gold aggregate decrease as the geometric standard deviation increases with the same average monomer size due to the coupling particle–plasma mode oscillation alternating between super-radiation and sub-radiation. Meanwhile, the extinction efficiency of ice and black carbon aggregates increases due to the increase in large monomers as the geometric standard deviation increases from 0 to 0.5. The size distribution of monomers mainly affects the distribution of back-scattering in comparison with forward-scattering. For extinction and scattering efficiencies, the aggregate model with size distribution of monomers and the aggregate model with single-sized monomers are close for gold aggregate; the former model is larger than the latter model for black carbon and ice aggregates. For absorption efficiency, the former model is lower for gold aggregate and higher for black carbon aggregate.

Effects of Different Particle Size Distributions on Aluminum Particle–Air Detonation

Due to the involvement of powder materials in dust explosion hazards and detonation experiments, it is imperative to analyze the heterogeneous detonation in a polydisperse suspension with a continuous particle size distribution. Notably, most current studies are limited to monodisperse suspensions with only one particle size. In this study, the rich aluminum particle–air detonation with two particle size distributions (namely, the monodisperse and the polydisperse with log-normal particle size distribution) is numerically studied by using the Eulerian–Lagrangian method along with a new hybrid aluminum combustion model. Significant discrepancies of the one-dimensional detonation front structures are observed between the monodisperse detonation and the polydisperse counterpart. And, the physical mechanisms of these discrepancies have been revealed by decoupling the gas–particles interactions with the one-dimensional flow theory. It is mainly caused by the different timings of the particle phase transition processes and the consequently different heat transfer characteristics, which are the effects of multiple timescales and length scales in the polydisperse detonation. Furthermore, owing to the wider reaction zones of polydisperse detonations than that of the monodisperse counterpart, discrepancies of two-dimensional detonation cell sizes are observed as well. This study reveals the great importance of considering particle size distribution in heterogeneous detonation simulations.

Synthesis of multiphase binary eutectic Al–Mg alloy-nanoparticles by electrical wire explosion technique for high-energy applications, its characterisation and size-dependent thermodynamic and kinetic study

Abstract In the present study, Al–Mg binary eutectic alloy nanoparticles were synthesized through the electrical wire explosion technique (EWET) in an inert ambience. High-speed imaging was carried out to visualize sublimation process of alloy wire and subsequent formation of nanoparticle through the condensation process. The fundamental gas-phase kinetics is investigated by the embedded atom method (EAM) and reactive hard-sphere model to elucidate the mechanisms governing the formation of finer particles of eutectic Al-Mg alloy. The condensed phase transformations are analysed through size-dependent thermodynamics and growth kinetics modelling, which has shown that the adsorption controlled growth is responsible for the generation of polydispersed particles. The formation of Al20Mg23 nanoparticles and its morphological features were characterised through XRD and SEM/EDS analysis. High-resolution transmission electron microscopy (HRTEM) along with the selected area electron diffraction (SAED) confirmed the spherical morphology, crystallinity, lognormal particle size distribution and the multiphase microstructure of the eutectic AlMg nanoparticles. The formation of multiphase alloy nanoparticles is attributed to the insignificant difference in melting temperatures of Al and Mg and phase boundary modifications due to size-effect. Scanning transmission electron microscopy (STEM) revealed the spatial distribution of Al and Mg within the alloy nanoparticle. Differential scanning calorimetry (DSC) confirmed the reduction in melting temperature and fusion enthalpy of eutectic Al–Mg alloy nanoparticles. It is proposed as a potential substitute for Al nanoparticles to reduce the ignition temperature, agglomeration and two-phase flow losses for realising improved combustion performance in solid rocket propellants.

The Nature of Native MgO in Mg and Its Alloys

Native MgO particles in Mg-alloy melts have been recently exploited as potential substrates for heterogeneous nucleation during solidification, leading to significant grain refinement of various Mg-alloys. However, our current knowledge of the nature of the native MgO particles is still limited. In this work, we study both the physical and chemical nature of the native MgO in commercial purity Mg and Mg-9Al alloy by means of advanced electron microscopy. We found that as oxidation products MgO aggregates exist in the alloy melt in three different forms: dominantly young oxide film, occasionally old oxide film and ingot skin, all consisting of discrete nano-sized MgO particles. Detailed analysis shows that the native MgO particles have an octahedral or cubic morphology, a nano-scale particle size and a log-normal size distribution. The mechanisms underlying the formation of the two types of MgO were investigated, and we found that octahedral MgO is formed by oxidation of Mg melt and cubic MgO by oxidation of Mg vapor. With a large lattice misfit with α-Mg, the native MgO particles are impotent for heterogeneous nucleation, but can be made effective for grain refinement.