Crystalline Seeds Research Articles

Dealumination of MOR Zeolites with Different Crystal Morphologies

The process of dealumination of mordenites differing in morphology and crystal size obtained under identical conditions from reaction mixtures of similar composition has been studied. Parameters that have been chosen to change the morphology and crystal size are the dry matter concentration in the reaction mixture, the pH of the reaction mixture, the crystallization time, the presence of crystalline seeds, and agitation during the synthesis. Dealumination has been performed by heat treatment at 700°C followed by acid treatment. It has been shown that the stability of the mordenite crystal framework to dealumination depends on the thickness of the primary needle crystal, which forms the basis of large zeolite crystals, not on the size of the zeolite crystal as a whole. With a decrease in the average needle crystal thickness from 250 to 90 nm, the Si/Al molar ratio in the crystal lattice, determined from 29Si NMR data, increases from 6.8 to 9.5. The dealumination is accompanied by a change in the texture of large mordenite crystals associated with a decrease in the packing density of the primary needles.

Designing an ADME liquid formulation with matching exposures to an amorphous dosage form

Amorphous Solid Dispersion (ASD) based formulations have been frequently used to improve the bioavailability of poorly water soluble drugs, however, common processes to produce ASDs are not feasible for Absorption, Distribution, Metabolism and Excretion (ADME) studies with radio-labeled Active Pharmaceutical Ingredients (API) due to the complications associated with radioactive material handling. Liquid formulations are routinely used to support the ADME studies, though bridging the bioperformance between a liquid formulation to the amorphous dosage form for poorly soluble compounds has not been well studied, and can be challenging due to the potentially rapid in vitro and in vivo recrystallization and precipitation. Here we report the development of a fit for purpose liquid formulation that could accommodate the radioactive API and provide comparable bioavailability relative to the amorphous formulation without the need for dose adjustment. A number of formulation approaches were explored and the prototype formulations were evaluated by dissolution and preclinical pharmacokinetic studies. A PolyEthylene Glycol 400 (PEG 400) based solution formulation impregnated with a polymer, HydroxyPropyl MethylCellulose Acetate Succinate-L (HPMCAS-L), was identified as the lead formulation. It was found that the bioavailability of the formulation can be compromised by the presence of undissolved crystalline seeds, and the inclusion of HPMCAS-L can mitigate this effect, as well as potentially facilitate the nanoparticle formation. During the study, it is also noted that although dissolution test is instrumental in the formulation development, the in vitro study over predicted the extent of in vivo precipitation for PEG 400 formulation containing no crystalline seeds.

Simultaneous performance and stability improvement of perovskite solar cells by a sequential twice anti-solvent deposition process

Abstract In this paper, we report a sequential twice anti-solvent deposition (STAD) method for the preparation of high performance perovskite solar cells. In comparison with the traditional one-step anti-solvent processing method, this STAD method improves the perovskite thin film quality and consequently improves the device performance. For the CH3NH3PbI3-xClx cell, the highest power conversion efficiency (PCE) improved from 14.20% for the traditional cells to 15.70% for the STAD cells, corresponding to a 10% performance enhancement. PCE enhancement of around 5% was also found both for MAPbI3-xBrx and MAPbI3 cells, demonstrating the generality for the performance enhancement for the STAD process. In addition, operational stability of the perovskite cells fabricated by the STAD process was also found to be enhanced than the traditional one-step cells. Scanning electronic microscope (SEM) and atomic force microscope (AFM) analyses confirm that smaller perovskite crystal domains as well as smoother surface morphology for the perovskite films prepared by the STAD method. A mechanism was proposed as that perovskite nano crystals formed at the first deposition-annealing process step serves as the crystalline seeds for the crystal growth in the second deposition-annealing process, which leads to more homogeneous crystal growth and to a consequent smooth and condense perovskite film. The current work provides a feasible method to prepare high quality perovskite solar cells with higher device performance as well as device stability.

INVESTIGATION OF THE INFLUENCE OF NANODISPERSED COMPOSITIONS OBTAINED BY PLASMOCHEMICAL SYNTHESIS ON THE CRYSTALLIZATION PROCESSES OF STRUCTURAL ALLOYS

The state of the problem of stabilizing the structure, improving the quality and properties of structural alloys is studied. To solve the problem, it is proposed to modify melts of low–alloyed alloys with nanodispersed compositions obtained by plasma–chemical synthesis. Process technological parameters are developed. Nanopowders of carbide and carbonitride class SiC and Ti (C, N) with a size of 50 ... 100 nm are obtained. The crystallographic parameters of the nanocompositions and the specific surface are determined, and the dependency curves are plotted. The macro– and microstructure of structural steels and alloys was studied before and after the modification. A significant (in 2 ... 3.5 times) grain refinement and stabilization of the alloy structure as a result of nanopowder modification of titanium carbonitride have been achieved. Thermodynamic calculations of the dimensions of crystalline seeds during the crystallization of steels and alloys are carried out. A complex criterial estimation of the efficiency of nanodispersed compositions in a steel melt is proposed. The features of crystallization and structure formation of modified structural steels are studied. The obtained results are of theoretical and practical importance for production of critical parts from structural steels and high–quality alloys.

Influence of annealing on crucible-free float zone melted LaB6-TiB2 composites

Polycrystalline and textured eutectics LaB6-TiB2 were obtained by the floating zone melting (FZM) method for powders using polycrystalline (PC) and (111) single crystalline (MC) seeds. The phase composition, microstructure, crystallographic orientation, residual stresses and mechanical characteristics in PC and MC composites after FZM process and their changes during annealing were studied. It is shown that the annealing temperature and the exposure time necessary to achieve the equilibrium eutectic phase and optimum mechanical properties in PC composites are lower than in the MC ones. Crack resistance and hardness in PC and MC composites after FZM process are smaller than the values in the equilibrium state and this difference is due in part to the residual stresses in both phases. It is established that under optimal annealing conditions, it is possible to substantially increase the mechanical characteristics of PС and MC eutectics. Fracture resistance rises in 1.5–1.6 times and hardness from 14 to 15 GPa to 25 and 27 GPa in MC and PC composites, respectively.

Effects of temperature on the ion-induced bending of germanium and silicon nanowires

Nanowires can be manipulated using an ion beam via a phenomenon known as ion-induced bending (IIB). While the mechanisms behind IIB are still the subject of debate, accumulation of point defects or amorphisation are often cited as possible driving mechanisms. Previous results in the literature on IIB of Ge and Si nanowires have shown that after irradiation the aligned nanowires are fully amorphous. Experiments were recently reported in which crystalline seeds were preserved in otherwise-amorphous ion-beam-bent Si nanowires which then facilitated solid-phase epitaxial growth (SPEG) during subsequent annealing. However, the ion-induced alignment of the nanowires was lost during the SPEG. In this work, in situ ion irradiations in a transmission electron microscope at 400 °C and 500 °C were performed on Ge and Si nanowires, respectively, to supress amorphisation and the build-up of point defects. Both the Ge and Si nanowires were found to bend during irradiation thus drawing into question the role of mechanisms based on damage accumulation under such conditions. These experiments demonstrate for the first time a simple way of realigning single-crystal Ge and Si nanowires via IIB whilst preserving their crystal structure.

Single crystals of crystalline block copolymers formed in n‐hexanol and methanol/DMF solutions: A comparative study

ABSTRACTLamellar single crystals of poly(ɛ‐caprolactone) (PCL)‐based crystalline/liquid‐crystalline block copolymer (BCP) were prepared from two methods: solution crystallization in n‐hexanol as well as addition of methanol dropwise to BCP/DMF solution. The crystalline morphologies of PCL single crystals prepared at different temperatures were investigated. It was observed that well‐developed single crystals were formed in n‐hexanol at high crystallization temperatures, while low crystallization temperatures favored to fabricate well‐developed nanosheets in methanol/DMF. Annealing at the melting temperature of single crystals yielded the crystalline seeds and unimers in n‐hexanol, while it formed spherical micelles in methanol/DMF system. As a result, the size of single crystals in n‐hexanol could be tuned by addition of different ratios of unimer/seed. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017, 134, 45089.

Predicting the role of seed morphology in the evolution of anisotropic nanocatalysts.

Controlling the structure of nanocrystals is an effective way to tune their properties and improve performance in a wide variety of applications. However, the atomic pathways for achieving this goal are difficult to identify and exercise, due to competing kinetic and thermodynamic influences during formation. In particular, an understanding of how symmetry, and symmetry breaking, determine nanocrystal morphology would significantly advance our ability to produce nanomaterials with prescribed functions. In this study we present results of a detailed computational study into the atomic structure of platinum nanoparticles at early growth stages of formation, as a function of temperature and atomic deposition rates. We investigate the impact of different types of crystalline seeds and characterize the emergent structures via simulated High Resolution Transmission Electron Microscopy (HRTEM) images. We find that the choice of initial seed is an important driver for symmetry breaking, due to a combination of atomic deposition and etching on different seed facets. A mix of low index facets causes the formation of important surface defects, in addition to the absorption/adsorption of single atoms, which can be correlated with different catalytic reactions as the process perpetuates. These findings provide new insights into nanocrystal shape-control mechanisms and suggest new opportunities for future design of this important class of nanomaterials.

Contrasting transparent conductive properties of ZnO films on amorphous and crystalline substrates in view of thickness dependence

The authors conducted comparative studies on ZnO films deposited on various substrates to elucidate how the different nucleation and crystallization processes affect their transparent conductive properties. The resistivity versus thickness curves of Ga-doped ZnO films deposited on a-SiNx:H films and glass substrates coincided within the experimental error. This result means that as long as the amorphous substrate is neither reactive with the deposited film nor providing crystalline seeds, resistivity is determined only by self-crystallization. In contrast, the resistivity of undoped ZnO films on sapphire c-planes was about half that on glass substrate even when the films were deposited at room temperature, indicating that the crystal template of sapphire stimulates local crystallization of ZnO films, though they are not epitaxial. With regard to the dependence on deposition temperature, a sudden drop in carrier concentration of undoped ZnO films was commonly observed between 200 and 300 °C for both glass and sapphire substrates, as a result of eliminating crystal disorder that facilitates holding donors. A significant difference was manifested between 300 and 500 °C; ZnO films on glass were nearly insulating, whereas those on sapphire were conductive, reflecting higher mobility and more reduced state. On sapphire substrates, the resistivity and sheet resistance versus thickness curves exhibited a monotonic decrease below 200 °C, whereas a plateau region appeared between 20 and 100 nm at 300 and 400 °C. This corresponds to the existence of electrically dead or inactive region near the interface probably because of depletion of carriers in the lattice-matched epitaxial layer. ZnO films became well conductive only when they were sufficiently thick.

Heteroepitaxial Writing of Silicon-on-Sapphire Nanowires.

The heteroepitaxial growth of crystal silicon thin films on sapphire, usually referred to as SoS, has been a key technology for high-speed mixed-signal integrated circuits and processors. Here, we report a novel nanoscale SoS heteroepitaxial growth that resembles the in-plane writing of self-aligned silicon nanowires (SiNWs) on R-plane sapphire. During a low-temperature growth at <350 °C, compared to that required for conventional SoS fabrication at >900 °C, the bottom heterointerface cultivates crystalline Si pyramid seeds within the catalyst droplet, while the vertical SiNW/catalyst interface subsequently threads the seeds into continuous nanowires, producing self-oriented in-plane SiNWs that follow a set of crystallographic directions of the sapphire substrate. Despite the low-temperature fabrication process, the field effect transistors built on the SoS-SiNWs demonstrate a high on/off ratio of >5 × 104 and a peak hole mobility of >50 cm2/V·s. These results indicate the novel potential of deploying in-plane SoS nanowire channels in places that require high-performance nanoelectronics and optoelectronics with a drastically reduced thermal budget and a simplified manufacturing procedure.

Statistical Analysis of Photopyroelectric Signals using Histogram and Kernel Density Estimation for differentiation of Maize Seeds

Considering the necessity of photothermal alternative approaches for characterizing nonhomogeneous materials like maize seeds, the objective of this research work was to analyze statistically the amplitude variations of photopyroelectric signals, by means of nonparametric techniques such as the histogram and the kernel density estimator, and the probability density function of the amplitude variations of two genotypes of maize seeds with different pigmentations and structural components: crystalline and floury. To determine if the probability density function had a known parametric form, the histogram was determined which did not present a known parametric form, so the kernel density estimator using the Gaussian kernel, with an efficiency of 95 % in density estimation, was used to obtain the probability density function. The results obtained indicated that maize seeds could be differentiated in terms of the statistical values for floury and crystalline seeds such as the mean (93.11, 159.21), variance \((1.64\times 10^{3}, 1.48\times 10^{3})\), and standard deviation (40.54, 38.47) obtained from the amplitude variations of photopyroelectric signals in the case of the histogram approach. For the case of the kernel density estimator, seeds can be differentiated in terms of kernel bandwidth or smoothing constant h of 9.85 and 6.09 for floury and crystalline seeds, respectively.

Amorphous, ultra-nano- and nano-crystalline tungsten-based coatings grown by Pulsed Laser Deposition: mechanical characterization by Surface Brillouin Spectroscopy

Pulsed Laser Deposition allows to obtain W and W-Ta alloy coatings with different nanostructures, monitored by X-ray diffraction. The correlation between such structures and the elastic properties is investigated for amorphous-like, ultra-nano- and nano-crystalline coatings obtained by tuning the gas pressure during deposition, annealing temperature and Ta concentration. The full elastic characterization is achieved by surface Brillouin spectroscopy, interpreted by isotropic and anisotropic film models. Amorphous like coatings are obtained with He pressures of tens of Pa. In comparison with bulk W, they have lower stiffness, by about 60%, closely correlated to the mass density (lower by about 40%). In the nanocrystalline regime the stiffness is more correlated to the average grain size, approaching the bulk values for increasing crystallite size. Vacuum annealing of amorphous like coatings leads to the nucleation of ultra-nano crystalline seeds, embedded in an amorphous matrix with intermediate values for mass density and stiffness. Here, the stiffness results from an interplay between the crystal size and the density. Alloying with Ta leads to properties which are consistent with the lever rule in the nanocrystalline regime, and deviate from it when the higher Ta concentration, interfering with crystal growth, induces an ultra-nano crystalline structure.

Grain growth in weak electric fields in strontium titanate: Grain growth acceleration by defect redistribution

The impact of DC electric fields on grain growth in strontium titanate is investigated between 1350°C and 1550°C for fields of up to 50V/mm. To prevent joule heating by electrical currents, insulating Al2O3 plates separate electrodes from samples. The seeded polycrystal technique is used, which allows evaluating gradients induced by electric fields. The growth direction of the single crystalline seeds is perpendicular to the electric field; hence electrostatic forces do not influence its growth. Below 1425°C, the influence of electric fields is very weak. Above 1425°C the field results in an increase of the grain boundary mobility at the negative electrode.The enhancement of the boundary mobility at the negative electrode is attributed to electric field induced defect redistribution. Oxygen vacancies migrate towards the negative electrode, while strontium vacancies accumulate at the positive electrode. This defect redistribution is connected to the defect chemistry dependent grain growth in strontium titanate.

Graphitization and amorphization of textured carbon using high-energy nanosecond laser pulses

Laser pulses can effectively induce local structural changes and modify the physical properties of carbon allotropes. So far, only graphitization has been demonstrated using low laser energies (≤1 J/cm2). The novelty of this paper is a result of laser-induced amorphization of a highly anisotropic carbon allotrope by using high energy (1.5–15.4 J/cm2) 5 ns, 532 nm Nd-YAG laser pulses. Moreover, cycling phase change, between an amorphous and a crystalline phase, is also obtained by adjusting the pulse energy. However, cycling ability is restricted to a few cycles as a consequence of laser-induced surface damages caused by both high temperatures during and high thermal gradients during and after laser exposure. The occurrence of graphitization or amorphization depends on the amount of solid crystalline seeds during solidification from the melt, which is controlled by the post-pulse temperature of the carbon surface. This study uncovers new applications of carbon allotropes, such as optically-controlled reversible phase-change memories.

Enhanced hydrothermal stability of ZSM-5 formed from nanocrystalline seeds for naphtha catalytic cracking

We successfully synthesized hydrothermally stable ZSM-5 with crystalline nano seeds. We employed a template-free method using ZSM-5 crystalline nano seeds and sodium silicate as a silica source. The prepared ZSM-5 exhibited uniform crystal size and relative crystallinity greater than 100 %. The size of the crystalline nano seed in the scale of 100 nm was found to be the optimum size for obtaining uniform, highly crystalline ZSM-5 with structural stability. After P-modification, the synthesized ZSM-5 with the optimally sized seed showed high hydrothermal stability and improved catalytic naphtha cracking activity compared to a commercial ZSM-5 catalyst. In order to find the elements for the increased hydrothermal stability, the samples were evaluated by studying crystallinity, aluminum spectrum, and acidity using XRD, solid-state NMR, and NH3-TPD, respectively after steaming at 800 °C for 24 h. It is speculated that the increased hydrothermal stability of the ZSM-5 resulted mainly from the increased aluminum structural stability.

Effect of Different Mediated Agents on Morphology and Crystallinity of Synthesized Silver Nanowires Prepared by Polyol Process

Synthesis and characterization of multiple crystalline silver nanowires (NWs) with uniform diameters were carried out by using 1,2-propandiol and ethylene glycol (EG) as comediated solvents and FeCl3as mediated agent in the presence of poly(vinyl pyrrolidone) (PVP). Experimental data and structural characterizations revealed that AgNWs have evolved from the multiple crystalline seeds initially generated by reduction of AgNO3with EG and 1,2-propandiol followed by reducing Fe(III) to Fe(II) which in turn reacts with and removes adsorbed atomic oxygen from the surfaces of silver seeds. In addition, uniform silver nanowires were obtained by using FeCl2and AlCl3as mediated agents in EG solution. Field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) showed uniform nanowires in both diameter and length. UV-Vis spectra showed adsorption peaks confirming the formation of nanowires. X-ray diffraction (XRD) patterns displayed the final product with high crystallinity and purity. In this study, a growth mechanism for forming AgNWs was proposed and a comparison between different mediated agents was carried out.

Photocatalytic behavior of TiO2 films synthesized by microwave irradiation

Titanium dioxide was synthesized on glass substrates from titanium (IV)isopropoxide and hydrochloride acid aqueous solutions through microwave irradiation using as seed layer either fluorine-doped crystalline tin oxide (SnO2:F) or amorphous tin oxide (a-SnOx). Three routes have been followed with distinct outcome: (i) equimolar hydrochloride acid/water proportions (1HCl:1water) resulted in nanorod arrays for both seed layers; (ii) higher water proportion (1HCl:3water) originated denser films with growth yield dependent on the seed layer employed; while (iii) higher acid proportion (3HCl:1water) hindered the formation of TiO2. X-ray diffraction (XRD) showed that the materials crystallized with the rutile structure, possibly with minute fractions of brookite and/or anatase. XRD peak inversions observed for the materials synthesized on crystalline seeds pointed to preferred crystallographic orientation. Electron diffraction showed that the especially strong XRD peak inversions observed for TiO2 grown from the 1HCl:3water solution on SnO2:F originated from a [001] fiber texture. Transmittance spectrophotometry showed that the materials with finer structure exhibited significantly higher optical band gaps. Photocatalytic activity was assessed from methylene blue degradation, with the 1HCl:3water SnO2:F material showing remarkable degradability performance, attributed to a higher exposure of (001) facets, together with stability and reusability.

Growth of single crystalline seeds into polycrystalline strontium titanate: Anisotropy of the mobility, intrinsic drag effects and kinetic shape of grain boundaries

We present a suite of measurements and combined analyses of grain growth in SrTiO3 for oriented single crystals into polycrystals. The growth distance and standard deviation and the microstructure evolution along the single crystal–matrix interface are used to locally characterize the change in migration behavior as a function of temperature, time and interface orientation. The relative grain boundary mobility was determined between 1250°C and 1600°C for four crystallographic orientations {100}, {110}, {111} and {310}. An absolute mobility of these orientations is estimated. Under fast growth conditions the morphology of single crystals shows macroscopic stepping with parts of the interface rotating to low mobility orientations. This effect represents a kinetic influence on the grain boundary morphology. The results also indicate dragging effects on microstructure coarsening, which indicate the existence of a critical driving force for grain growth. This critical driving force seems to be related to an ‘intrinsic’ interface drag similar to the solute drag, but based on intrinsic defects. At 1460°C the growth of single crystals was significantly faster than expected from the mobility of the polycrystal and was identified as exaggerated grain growth. The findings give new insights into the recently published grain growth anomaly of strontium titanate, leading to a hypothesis based on the temperature dependent relative mobility of {100} oriented grain boundaries.

Thermal Effects of Laser Irradiation on Maize Seeds

Abstract It is important to know the temperature changes in seeds that have been irradiated with laser light because this could have substantial practical and theoretical importance. Thus, the thermal effects of low intensity laser irradiation on seeds was studied, showing variation of temperature produced by laser light applied during 60 s on two maize seed genotypes, ‘Toluqueño’ and ‘Cacahuazintle’: crystalline and floury, respectively, under two different conditions: natural colour and dyed black, evaluating the temperature changes by a thermal camera. The optical absorption spectra and the non-radiative relaxation time of the seeds were obtained using photoacoustic spectroscopy. The results indicate that it is possible to produce temperature changes, detected by an infrared camera, in crystalline and floury seeds when they are irradiated with a laser beam at a 650 nm wavelength and 27.4 mW power. The highest variation of temperature in the seeds was obtained for the black-dyed condition, these variations being 5.56 and 9.28°C for crystalline and floury seeds, respectively. Among the seeds, in the dyed condition, the floury seed had the lower non-radiative relaxation time, the higher optical absorption coefficient and a lower optical penetration length at the laser wavelength (650 nm).

A hydrothermal peroxo method for preparation of highly crystalline silica–titania photocatalysts

A new completely inorganic method of preparation of silica–titania photocatalyst has been described. It has been established that the addition of silica promotes crystallinity of TiO2 anatase phase. Relative crystallinity and TiO2 crystal size in the silica–titania particles increase with the silica content until SiO2/TiO2 molar ratio of 0.9, but at higher molar ratios they start to decrease. The single-source precursor containing peroxo titanic (PTA) and silicic acids has been proved to be responsible for high crystallinity of TiO2 encapsulated into amorphous silica. It has been proposed that peroxo groups enhance rapid formation of crystalline titania seeds, while silica controls their growth. It has been concluded from the TEM that the most morphologically uniform anatase crystallites covered with SiO2 particles are prepared at SiO2/TiO2 molar ratio of 0.4. This sample, according to 29Si NMR, also shows the high content of hydroxylated silica Q3 and Q2 groups, and it is the most photocatalytically active in UV-assisted decomposition of methylene blue among the tested materials. It has been determined that the increase in the amount of the condensed Q4 silica in the mixed oxides leads to the decrease in photocatalytic performance of the material, despite its better crystallinity. High crystallinity, low degree of incorporation of Ti atoms in SiO2 in the mixed oxide and adsorption of methylene blue in the vicinity of photoactive sites on the hydroxylated silica have been considered as the main factors determining the high degradation degree of methylene blue in the presence of silica–titania.