Effect Of Crystal Size Research Articles

Correlated Debye model for atomic motions in metal nanocrystals

The Correlated Debye model for the mean square relative displacement of atoms in near-neighbour coordination shells has been extended to include the effect of finite crystal size. This correctly explains the increase in Debye–Waller coefficient observed for metal nanocrystals. A good match with Molecular Dynamics simulations of Pd nanocrystals is obtained if, in addition to the phonon confinement effect of the finite domain size, proper consideration is also given to the static disorder component caused by the undercoordination of surface atoms. The new model, which addresses the analysis of the Pair Distribution Function and powder diffraction data collected at different temperatures, was preliminarily tested on recently published experimental data on nanocrystalline Pt powders.

High-performance thin-felt SS-fiber@HZSM-5 catalysts synthesized via seed-assisted vapor phase transport for methanol-to-propylene reaction: Effects of crystal size, mesoporosity and aluminum uniformity

A series of thin-felt SS-fiber@HZSM-5 catalysts are fabricated via the seed-assisted vapor-phase transport (VPT) method. The crystal size and mesoporosity of zeolite shell are controllably tuned toward enhanced diffusion by increasing the silicalite-1 seeding gel (SG) amount used. The microstructured catalyst obtained with a high SG amount of 50% contains a smaller crystal size of ∼70 nm and higher mesopore volume of 0.21 cm3 gzeolite−1, which markedly prolonged single-run lifetime with a remarkable decrease in the coking rate in the MTP process. The adverse effect of non-uniform aluminum in the zeolite shell on the MTP stability is revealed. The use of aluminum-containing SG to replace the pure-silica one makes the acid sites of as-obtained ZSM-5 mounted on SS-fiber distributed homogeneously thereby leading to a marked suppression of hydrogen transfer, and as a result, the catalyst single-run lifetime is further increased by ∼40%. This promising SS-fiber@HZSM-5 catalyst is stable for at least 1600 h (>94% conv.) with a high propylene selectivity of ∼36% at 450 °C using a WHSV of 1 h−1 for a feed of MeOH/H2O molar ratio of 1:1, by taking advantage of improved diffusion from mesoporosity development and uniform aluminum distribution as well as our distinctive microfibrous-engineered design.

Thermoelectric properties of nanocrystalline Sb2Te3 thin films: experimental evaluation and first-principles calculation, addressing effect of crystal grain size

The effect of crystal grain size on the thermoelectric properties of nanocrystalline antimony telluride (Sb2Te3) thin films was investigated by experiments and first-principles studies using a developed relaxation time approximation. The Sb2Te3 thin films were deposited on glass substrates using radio-frequency magnetron sputtering. To change the crystal grain size of the Sb2Te3 thin films, thermal annealing was performed at different temperatures. The crystal grain size, lattice parameter, and crystal orientation of the thin films were estimated using XRD patterns. The carrier concentration and in-plane thermoelectric properties of the thin films were measured at room temperature. A theoretical analysis was performed using a first-principles study based on density functional theory. The electronic band structures of Sb2Te3 were calculated using different lattice parameters, and the thermoelectric properties were predicted based on the semi-classical Boltzmann transport equation in the relaxation time approximation. In particular, we introduced the effect of carrier scattering at the grain boundaries into the relaxation time approximation by estimating the group velocities from the electronic band structures. Finally, the experimentally measured thermoelectric properties were compared with those obtained by calculation. As a result, the calculated thermoelectric properties were found to be in good agreement with the experimental results. Therefore, we can conclude that introducing the effect of carrier scattering at the grain boundaries into the relaxation time approximation contributes to enhance the accuracy of a first-principles calculation relating to nanocrystalline materials.

Treatment of TNT Red Water Using Ice Crystallization

AbstractCrystallization of ice from TNT red water was studied using a scraped unit. Growth rate, crystal size distribution, and median particle size of ice crystals were measured. The effect of subcooling, temperature, growth rate, and crystal size on the treatment of red water was investigated. The relationship between ice crystal growth rate and effective distribution coefficient was correlated. From the plot of ice growth rate against subcooling degree, the order of the growth kinetic was in the range of 1.2 to 2.0 for various concentrations. The purity of ice crystals increased with increasing amounts of partial melt. Above 99 % of dinitrotoluene sulfonates in red water were removed after crystallization at a cooling rate of 1.0 K/min, a final crystallization temperature of −10 °C, and a yield of 50 %. The purity of ice crystal was significantly affected by the subcooling degree. Finally, it was shown that TNT red water of 9800 mg/L COD can be treated to provide water of 54 mg/L COD by scraped ice crystallization at a yield of 50 %.

Effect of crystal grain size on stress and strain fields of polycrystalline pure copper under macroscopic heterogeneous

Effect of crystal grain size on stress and strain fields of polycrystalline pure copper under macroscopic heterogeneous

Effect of particle size of HZSM-5 zeolite on the catalytic depolymerization of organosolv lignin to phenols

The effect of particle on the catalytic activity of HZSM-5 zeolite in organosolv lignin to phenols was investigated in the supercritical ethanol-water under the operating conditions of 350 °C and 4 h. Two samples comprising a micro and a nano size HZSM-5 were prepared by reflux and hydrothermal crystallization method, respectively. The catalyst samples were characterized with XRD, SEM, BET and NH3-TPD analytical techniques. The effect of crystal size on the lignin conversion to phenols and the structure changes of lignin residue were studied. The nano-HZSM-5 showed higher yield of phenols and lower yield of char as compared to the micro-HZSM-5. The enhanced content of aromatic C-C structures and extenuative content of aromatic C-O and C-H showed that intramolecular dehydration and deoxygenation were occurred with nano- and micro-HZSM-5, and the nano-HZSM-5 activity significantly stronger than micro-HZSM-5.

Effect of crystal size on the tribological behavior of manganese phosphate coatings under lubricated sliding

In this study, the wear and friction behavior of manganese phosphate coatings with different crystal sizes were investigated. Crystal size was controlled modifying the chemical composition of the phosphating bath, particularly the concentration of the activator which modifies the number of nuclei for crystal growth. Activator concentration range used for this work varied from 0 to 0.7 g/L, and crystal size was measured using image analysis software on scanning electron microscopy photographs. Available volume for lubricant retention was determined measuring the phosphated surface with a 3D optical profilometer. At the same time, lubricated wear tests were carried out using a ring-on-block configuration at low speeds (23 mm/s) and high loads (14,500 N). Wear behavior was determined as the sliding distance to failure, which was noticed through signs of removal of the phosphate along with the increase of coefficient of friction. It was found that there is a competition between the availability of volume to hold the lubricant, which increases with the crystal size, and the surface coverage, which diminishes as the crystal size grows. Optimal results were obtained for an activator concentration of 0.3 g/L, which meant an average crystal size of 16 µm.

Effect of Pr inoculation and crystal size on the Hall-Petch relationship for Al-30 wt%Mg2Si composites

The effect of varying levels of Pr inoculation on the microstructure, granularity, and mechanical properties of an Al-30%Mg2Si composite was studied. When the Pr content was increased to 0.8 wt%, the refinement was ideal. Similarly, the mechanical properties such as Ultimate Tensile Stress (UTS), Breaking Elongation (BE), and Vickers Microhardness (VM) values were improved. The relationship between crystal size and UTS or VM was described using the Hall-Petch relationship.

Mixed Membrane Matrices Based on Nafion/UiO-66/SO3H-UiO-66 Nano-MOFs: Revealing the Effect of Crystal Size, Sulfonation, and Filler Loading on the Mechanical and Conductivity Properties.

Mixed membrane matrices (MMMs) made up with Nafion and nanocrystals of zirconium metal-organic framework (MOF) UiO-66 or the analogous sulfonated SO3H-UiO-66 were prepared by varying the filler loading and the size of the crystals. The combined effects of size and loading, together with the presence of sulfonic groups covalently linked to the MOFs, were studied with regard to the conductivity and mechanical properties of the obtained composite matrices. A large screening of membranes was preliminarily made and, on the most promising samples, an accurate conductivity study at different relative humidities and temperatures was also carried out. The results showed that membranes containing large crystals (200 nm average size) in low amounts (around 2%) displayed the best results in terms of proton conductivity values, reaching values by 30% higher than those of pure Nafion, while leaving the mechanical properties substantially unchanged. On the contrary, MMMs containing MOFs of small size (20 nm average size) did not show any conductivity improvements if compared to pure Nafion membranes. The effect of MOF sulfonation was negligible at low filler loading whereas it became important at loading values around 10%. Finally, membranes with a high filler loading (up to 60 wt %) of sulfonated UiO-66 showed a slight reduction of conductivity in comparison with membranes loaded at 20% of nonsulfonated ones.

Analysis of thickness-dependent optical parameters of a-Si:H/nc-Si:H multilayer thin films

Multilayer thin films with alternate hydrogenated amorphous (a-Si:H) and nanocrystalline silicon (nc-Si:H) layers are deposited in hot wire chemical vapor deposition (HWCVD) chamber using hydrogen (H2) dilution of silane (SiH4). Two sets (two samples for each set) have different nc-Si:H layer thickness with the same a-Si:H thickness. The transmittance spectra, refractive index and absorption coefficient are obtained using UV–Vis spectroscopy. An analysis of Tauc plot suggests thickness-dependent optical band gap shift. The films with thick nc-Si:H layers show a decrease in band gap from 1.78 to 1.68 eV, whereas the films with lower nc-Si:H layer thickness have an increased band gap from 1.64 to 1.72 eV. Further, the slope of the Tauc plot (B) and disorder indicator (Urbach energy, Eu) show that the films with thick nc-Si:H layers are poorly ordered as hydrogen dilution increases. These results illustrate that band gap shift can be attributed to the crystal size effect and film disorder. This shifting of optical band gap with the nc-Si:H layer thickness is useful in tuning of the absorber layer band gap which finds an application in photovoltaics.

Anomalous hardening in magnesium driven by a size-dependent transition in deformation modes

Here, we report a comprehensive study that combines in situ scanning electron microscopy experiments and atomistic simulations to quantify the effect of crystal size on the transformation in deformation modes in a-axis oriented Mg single crystals at room temperature. The experimental results indicate that the deformation is dominated by the nucleation and propagation of tensile twins. The stress required for twin propagation was found to increase with decreasing sample size, showing a typical “smaller is stronger” behavior. Furthermore, an anomalous increase in strain hardening is first reported for microcrystals having diameters larger than ∼18 μm, which is induced by twin-twin and dislocation-twin interactions. The hardening rate gradually decreases toward the bulk response as the microcrystal size increases. Below 18 μm, deformation is dominated by the nucleation and propagation of a single tensile twin followed by basal slip activity in the twinned crystal, leading to no apparent hardening. In addition, molecular dynamics simulations indicate a transition from twinning mediated plasticity to dislocation mediated plasticity for crystal sizes below a few hundred nanometers in size. A deformation mechanism map for twin oriented Mg single crystals, ranging from the nano-scale to bulk scale is proposed based on the current simulations and experiments. The current predicted size-affected deformation mechanism of twin oriented Mg single crystals can lead to better understanding of the competition between dislocations plasticity and twinning plasticity.

The effect of crystal size on tunneling phenomena in luminescent nanodosimetric materials

The study of luminescence signals from nanodosimetric materials is an active research area, due to the many possible practical applications of such materials. In several of these materials it has been shown that quantum tunneling is a dominant mechanism for recombination processes associated with luminescence phenomena. This paper examines the effect of crystal size on quantum tunneling phenomena in nanocrystals, based on the assumption of a random distribution of electrons and positive ions. The behavior of such random distributions is determined by three characteristic lengths: the radius of the crystal R, the tunneling length a, and the initial average distance 〈d〉 between electrons and positive ions (which is directly related to the density of charges in the material). Two different cases are examined, depending on the relative concentrations of electrons and ions. In the first case the concentration of electrons is assumed to be much smaller than the concentration of positive ions. Examination of a previously derived analytical equation demonstrates two different types of crystal size effects. When the tunneling length a is much smaller than both R and 〈d〉, the analytical equations show that smaller crystals exhibit a faster tunneling recombination rate. However, when the tunneling length a is of the same order of magnitude as both R and 〈d〉, the opposite effect is observed, with smaller crystals exhibiting a slower tunneling recombination rate. As the crystal size increases, the rate of tunneling in both cases reaches the limit expected for bulk materials. In the second case we examine the situation where the concentrations of electrons and positive ions are equal at all times. In this situation there is no analytical equation available to describe the process, and the crystal size effects are simulated by using Monte Carlo (MC) techniques. The two opposite behaviors as a function of the crystal size are also observed in these MC simulations. The effect of sample temperature is also studied by extending the MC simulations to include thermal characteristics of the defects. The relevance of the simulated results for luminescence dosimetry is discussed.

Numerical simulations with the P-Hydroslag model to predict phosphorus removal by steel slag filters

The first version of the P-Hydroslag model for numerical simulations of steel slag filters is presented. This model main original feature is the implementation of slag exhaustion behavior, crystal growth and crystal size effect on crystal solubility, and crystal accumulation effect on slag dissolution. The model includes four mineral phases: calcite, monetite, homogeneous hydroxyapatite (constant size and solubility) and heterogeneous hydroxyapatite (increasing size and decreasing solubility). In the proposed model, slag behavior is represented by CaO dissolution kinetic rate and exhaustion equations; while slag dissolution is limited by a diffusion rate through a crystal layer. An experimental test for measurement of exhaustion equations is provided. The model was calibrated with an experimental program made of three phases. Firstly, batch tests with 300 g slag sample in synthetic solutions were conducted for the determination of exhaustion equation. Secondly, a slag filter column test fed with synthetic solution was run for 623 days, divided into 9 cells and sampled at the end of the experiment. Finally, the column was dismantled, sampled and analyzed with XRD, TEM and SEM. Experimental column curves for pH, oPO4, Ca and inorganic carbon were well predicted by the model. Crystal sizes measured by XRD and TEM validated the hypothesis for homogeneous precipitation while SEM observations validated the thin crystal layer hypothesis. A preliminary validation of the model resulted in successful predictions of a steel slag filter longevity fed with real wastewater.

Effect of Crystal Quality and Particle Size of HMX on the Creep Resistance for TATB/HMX Composites

AbstractTwo kinds of reduced sensitivity high explosive 1,3,5,7‐tetranitro‐1,3,5,7‐tetrazocane (RS‐HMX) with different particle sizes were selected to enhance the energy output and the mechanical properties of insensitive high explosive 1,3,5‐triamino‐2,4,6‐trinitrobenzene (TATB). Mechanical sensitivities, dynamic mechanical analysis, and non‐linear time dependent creep behaviors of TATB/HMX composites were investigated and discussed in relation to the structural characteristics. Compared with TATB/conventional HMX (C‐HMX) sample, both the impact and friction sensitivities of TATB/RS‐HMX were reduced. It revealed that TATB/fine grains RS‐HMX composites had the highest storage modulus and minimum steady‐state creep strain rate due to the increased coherence strength and the inhibited slide of the single layer of TATB crystal. The creep resistance also showed clear dependence on the particle size of RS‐HMX. The overall results indicated that RS‐HMX had good potential in high energetic, safe, and load‐bearing material applications.

Crystal Face Distributions and Surface Site Densities of Two Synthetic Goethites: Implications for Adsorption Capacities as a Function of Particle Size.

Two synthetic goethites of varying crystal size distributions were analyzed by BET, conventional TEM, cryo-TEM, atomic resolution STEM and HRTEM, and electron tomography in order to determine the effects of crystal size, shape, and atomic scale surface roughness on their adsorption capacities. The two samples were determined by BET to have very different site densities based on CrVI adsorption experiments. Model specific surface areas generated from TEM observations showed that, based on size and shape, there should be little difference in their adsorption capacities. Electron tomography revealed that both samples crystallized with an asymmetric {101} tablet habit. STEM and HRTEM images showed a significant increase in atomic-scale surface roughness of the larger goethite. This difference in roughness was quantified based on measurements of relative abundances of crystal faces {101} and {201} for the two goethites, and a reactive surface site density was calculated for each goethite. Singly coordinated sites on face {210} are 2.5 more dense than on face {101}, and the larger goethite showed an average total of 36% {210} as compared to 14% for the smaller goethite. This difference explains the considerably larger adsorption capacitiy of the larger goethite vs the smaller sample and points toward the necessity of knowing the atomic scale surface structure in predicting mineral adsorption processes.

Development of Pharmaceutically Acceptable Crystalline Forms of Drug Substances via Solid-State Solvent Exchange

In the pharmaceutical industry, solid form screening plays an important role to discover forms that exhibit desired physicochemical properties for drug product development. This work describes an approach to meet this objective by the transformation of undesirable solvates to hydrates or cosolvates with water via solid-state solvent exchange. Case studies of two drug substances, imatinib mesylate and linagliptin, are discussed, where linaglipitin methanol/ethanol solvate was converted to an iso-structural hydrate and, similarly, imatinib mesylate methanol–water cosolvate was converted to a predominantly water-containing cosolvate. Through quality by design based optimization, temperature and relative humidity were identified as critical process parameters that impacted the rate of solvent exchange during humidification. In addition, crystallization parameters that impacted the crystal size were found to play a key role in determining the extent of solvent exchange. This unexpected effect of crystal size w...

Product shape selectivity of MFI-type, MEL-type, and BEA-type zeolites in the catalytic hydroconversion of heptane

The influence of product shape selectivity on the bifunctional conversion of n-C7 by zeolite catalysts is investigated. Three different zeolite catalysts with different pore sizes (MFI-type, MEL-type, and BEA-type zeolites) have been investigated experimentally. For all three catalysts, n-C7 is isomerized to monobranched isomers which are further isomerized into dibranched isomers, and these dibranched molecules are converted into cracking products. More dibranched isomers and less cracking products are produced by BEA-type zeolite compared to MFI-type and MEL-type zeolites and clear differences are observed in the distribution of dibranched isomers produced by different catalysts. Molecular simulation is used to compute the adsorption isotherms and free energy barriers for diffusion of dibranched isomers in MFI-type, MEL-type, and BEA-type zeolites. Combining simulation results and experimental observations, it is shown that product shape selectivity can explain the distribution of dibranched molecules while transition state shape selectivity fails to do so. For the medium-pore zeolites (MFI-type and MEL-type zeolites), free energy barriers for diffusion of dibranched molecules are significant. For MFI-type and MEL-type zeolites, the dibranched molecule that has to overcome lower diffusion barrier is produced with a higher yield and the distribution of dimethylpentane molecules is determined by their diffusion rate. It is shown that there is almost no free energy barrier for the diffusion of any of these molecules in BEA-type zeolite. As BEA-type zeolite imposes no free energy barrier for diffusion of any of dibranched isomers, the distribution of dibranched isomers is very close to the equilibrium distribution in the gas phase. Due to the limited mobility of dimethylpentanes within the pores of MFI-type and MEL-type zeolites, most of the dimethylpentane molecules are trapped inside the zeolite and undergo consecutive cracking. Dimethylpentane molecules diffuse sufficiently fast in the large pores of BEA-type zeolite and transfer to the gas phase, before consecutive reaction converts these molecules into cracking products. Moreover, the effect of the MFI-type crystal size on the production of dibranched isomers is investigated. The yield of dibranched isomers reduces by increasing the size of the crystal and larger part of dibranched isomers are cracked as the crystal size of MFI-type is increased.

Magnetic properties of Mn0.1Mg0.2TM0.7Fe2O4 (TM = Zn, Co, or Ni) prepared by hydrothermal processes: The effects of crystal size and chemical composition

Nano-crystalline Zn-, Co-, and Ni-substituted Mn-Mg ferrites were prepared by hydrothermal process and annealed at 1100°C. Annealing conditions are critical on the crystalline phase. TEM and XRD data reveal particle sizes between 8nm and 15nm for the as-prepared fine powders, which increase to about 73nm after sintering at 1100°C. Mӧssbauer spectra show well resolved magnetic splitting in bulk samples. The as-prepared fine powders show weak hyperfine splitting and broad central doublets associated with fine particles. Magnetization data reveal a high coercive field at about 300K of about 945 Oe in the Co-based nanosized oxide, which reduces to about 360 Oe after thermal annealing at 1100°C. The magnetization curves of Zn- and Ni-based samples show much lower coercive fields indicative of superparamagnetic nanoparticles. The crystallite size and chemical composition have significant effects on the properties of Mn0.1Mg0.2(Zn,Co,Ni)0.7Fe2O4 investigated.

Finite versus small strain discrete dislocation analysis of cantilever bending of single crystals

Plastic size effects in single crystals are investigated by using finite strain and small strain discrete dislocation plasticity to analyse the response of cantilever beam specimens. Crystals with both one and two active slip systems are analysed, as well as specimens with different beam aspect ratios. Over the range of specimen sizes analysed here, the bending stress versus applied tip displacement response has a strong hardening plastic component. This hardening rate increases with decreasing specimen size. The hardening rates are slightly lower when the finite strain discrete dislocation plasticity (DDP) formulation is employed as curving of the slip planes is accounted for in the finite strain formulation. This relaxes the back-stresses in the dislocation pile-ups and thereby reduces the hardening rate. Our calculations show that in line with the pure bending case, the bending stress in cantilever bending displays a plastic size dependence. However, unlike pure bending, the bending flow strength of the larger aspect ratio cantilever beams is appreciably smaller. This is attributed to the fact that for the same applied bending stress, longer beams have lower shear forces acting upon them and this results in a lower density of statistically stored dislocations.

Structural and Mechanistic Differences in Mixed-Linker Zeolitic Imidazolate Framework Synthesis by Solvent Assisted Linker Exchange and de Novo Routes.

Mixed-linker zeolitic imidazolate frameworks (ZIFs) are a subclass of metal-organic frameworks (MOFs) amenable to significant property tuning by altering the functional groups on the imidazolate linkers. Solvent assisted linker exchange (SALE) and de novo synthesis of mixed-linker ZIFs have been demonstrated, but the differences in structural properties-most importantly the linker distributions-and synthesis mechanisms of these two different types of hybrid ZIFs are unknown. In this work, a combination of 1H NMR combined rotation and multiple pulse spectroscopy (CRAMPS), water adsorption, and nitrogen measurements reveal distinct differences in linker mixing between SALE and de novo ZIF-8-90 hybrids. Native-fluorescence confocal microscopy is shown to provide a direct means to visualize these differences. The effects of crystal size, temperature, and SALE duration were studied in detail, and a generalizable mechanism for SALE processes in ZIFs is proposed. The SALE process is found to follow a diffusion-limited behavior leading to core-shell morphologies. Under harsher SALE conditions, deviations from diffusion-limited behavior are found due to etching and partial dissolution of the initial ZIF-8 crystals. With the selection of appropriate reaction conditions, SALE processes appear to be capable of generating controlled core-shell ZIF structures of good morphological quality that complement the well-mixed structures obtained by de novo methods.