Polycrystalline Particles Research Articles

Effect of polyethylene glycol on crystal growth and photocatalytic activity of anatase TiO2 single crystals.

In order to evaluate the effect of polyethylene glycol (PEG) on the growth of TiO2 crystals, anatase TiO2 crystals with different morphologies and structures were synthesized by controlling the content and type of PEG in a solvothermal system. Then, their morphology and structure were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Characterization results show that hydrofluoric acid can promote the formation of high activity (001) facets. Experiments on the effect of PEG on crystal growth show that the low molecular weight PEG (PEG400) can accelerate crystal differentiation and relieve the agglomeration of crystals in the presence of hydrofluoric acid. Besides, according to the experimental results, we found that PEG400 can reduce the agglomeration size and number of TiO2 polycrystalline particles. Research on the photocatalytic activity proposed that the independence of single crystal and the integrity of (001) facets are the critical factors in advanced oxidation reaction. The resultant anatase TiO2 single crystals could produce more hydroxyl radicals (˙OH) in the photocatalytic system, which exhibited remarkable photocatalytic performance for the degradation of Acid Red B.

Magnetic and magnetocaloric properties of crystalline DyFe1−xMnxO3

DyFe[Formula: see text]Mn[Formula: see text]O3(x[Formula: see text]=[Formula: see text]0, 0.025, 0.075, 0.15) polycrystalline samples were prepared using a traditional solid-state reaction route. The structural, magnetic and magnetocaloric properties of these samples were investigated. X-ray diffraction patterns showed that the samples exist as single-phase crystallines without peaks. The results of the Scanning Electron Microscopy (SEM) revealed that the average size of the polycrystalline particles decreased from 4.34 to 3.00 [Formula: see text]m as the Mn doping amount increased from 0.00 to 0.15. The magnetization versus temperature (M[Formula: see text]−[Formula: see text]T[Formula: see text]) plots showed that the order temperature of dysprosium (Dy) ions gradually decreased and the Morin-like transition temperature moved to the high-temperature region as x increased. The T[Formula: see text] gradually decreased from 13 to 10 K and the isotropic interaction of Fe[Formula: see text]-Fe[Formula: see text] was weakened as x increased from 0.00 to 0.15. The polycrystalline samples appeared as pre-formed clusters. The magnetization (M[Formula: see text]−[Formula: see text]H[Formula: see text]) plots revealed that all the samples underwent a first-order magnetic phase transition. The maximum magnetic entropy change, occurring near the Curie temperature, obtained at a magnetic-field span of 7 T, was 18.2 J/kg K. The maximum cooling capacity of the polycrystalline DyFe[Formula: see text]Mn[Formula: see text]O3(x[Formula: see text]=[Formula: see text]0, 0.025, 0.075, 0.15) samples was 441 J/kg.

Design of Preferentially Faceted Zinc to Improve the Rechargeability of Alkaline Batteries

Rechargeable Zinc-based batteries have attracted a growing interest due to their intrinsic high safety as well as the low environmental impact and low cost of Zinc. However, there still exists some major obstacles in achieving these low cost secondary cells, including shape change during repeated charge/discharge (dendrite formation) and limited reversibility of the Zinc electrodes. Traditional Zinc anodes are formed from large, polycrystalline particles with limited control over the morphology. Many studies have looked to improve morphology control of Zinc through the design of Zinc nanostructures 1, 2, but the future implementation of these nanomaterials is hindered by the high rate of corrosion for Zinc nanostructures 3, 4 as well as the expense and scalability of manufacturing such nanostructures. Therefore, there is a need to develop a scalable synthesis method that allows for the creation of large monocrystalline particles with controlled shape. In this work, we report a facile electrolytic method to synthesize micron-sized preferentially faceted, hexagonal-shaped Zinc particles. Using a three-electrode electrochemical cell, the charge and discharge behavior of these Zinc particles were studied in alkaline (6M KOH) and mildly acidic (1M ZnSO4) aqueous electrolytes. It was confirmed that the dendrites form during cycling the Zinc particles in KOH electrolytes, however, using ZnSO4 electrolytes no dendrite formation was observed, and the particles were able to retain their hexagonal structures ever after cycling for than 100 times at 1C. References M. Chamoun, B. J. Hertzberg, T. Gupta, D. Davies, S. Bhadra, B. Van Tassell, C. Erdonmez, and D. A. Steingart, NPG Asia Materials, 7 (4), e178 (2015).T.-H. Wu, Y. Zhang, Z. D. Althouse, and N. Liu, Materials Today Nano, 100032 (2019).E. Faegh, T. Omasta, M. Hull, S. Ferrin, S. Shrestha, J. Lechman, D. Bolintineanu, M. Zuraw, and W. E. Mustain, Journal of The Electrochemical Society, 165 (11), A2528-A2535 (2018).Y.-D. Cho and G. T.-K. Fey, Journal of Power Sources, 184 (2), 610-616 (2008).

Electrochemical-Shock Resistant Single-Crystal Ni-Rich Layered Cathode Materials

Ni-rich layered oxides LiNi x M1 -x O2 (x ≥ 0.6, M = combinations of Co, Mn and/or Al) are technologically important cathode materials for lithium-ion batteries due to their high energy density, good power capability, and lower material cost than LiCoO2. These materials are usually prepared by a co-precipitation and calcination method that produces micron-sized polycrystalline (PC) particles consisting of densely packed nanoscale grains. The unique microstructure and anisotropic Vegard coefficient of these materials make them vulnerable to grain boundary fracture (“electrochemical shock”), a major cause for battery impedance growth and performance degradation. We have recently developed a scalable method to prepare a variety of single-crystal (SC) Ni-rich cathode materials, such as LiNi0.6Mn0.2Co0.2O2 (NMC622), LiNi0.6Mn0.2Co0.2O2, and LiNi0.8Co0.15Al0.05O2. SC-NMC622 outperforms the commercially available PC-NMC622 in terms of cycling stability, rate performance, and thermal stability. Post-cycling structural characterizations reveal that the SC-NMC622 do not crack even when charged to 4.9 V vs Li+/Li and its surface undergoes less phase transformation than PC-NMC622 during cycling in a normal voltage range (4.3-2.8 V). Single-crystal Ni-rich cathode materials offer hope for fabricating mechanically reliable solid-state batteries and provide a unique platform for studying doping and surface modification methods to further improve the durability of Ni-rich cathode materials.

Synthesis of (sub)microcrystals of dinuclear terbium(III) carboxylate (Hpiv)6Tb2(piv)6 and polymeric terbium(III) carboxylate {Tb(piv)3}n in supercritical carbon dioxide. Photoluminescence and magnetic properties

Supercritical fluid micronization by rapid expansion of supercritical solutions setup using supercritical carbon dioxide as a fluid and the dinuclear complex (Hpiv)6Tb2(piv)6 (1) as the starting compound allows the preparation of either the single-phase dinuclear complex (Hpiv)6Tb2(piv)6 (1a) or the coordination polymer {Tb(piv)3}n (2a, 2b, 2c) as (sub)microcrystals with a controlled size depending on the processing parameters. It was shown that {Tb(piv)3}n (2) can in principle be synthesized by supercritical carbon dioxide processing as polycrystalline particles with different morphologies. A decrease in the size of polycrystalline particles (aggregates) of 1 and 2 and an increase in the fraction of defects in the crystal structure, respectively, lead to an increase in the degree of photoluminescence quenching at these defects and a change in magnetic properties of 1.

Eddy currents in fine polycrystalline metal particle

The magnetic dipole absorption in fine metal polycrystalline particle is considered. The kinetic equation with a collision integral that takes into account the influence of electron scattering at the grain boundaries on the properties of the metal has been solved. We consider the case of diffuse reflection of electrons from the sample surface Analysis of the dependence of the absorption cross section of particles from the parameter describing the degree of influence of the scattering on the grain boundaries of polycrystalline particles on the kinetic processes has been considered.

Spherulitic growth and morphology control of lithium carbonate: the stepwise evolution of core-shell structures

In this paper, we demonstrate morphologies, controlling factors (temperature, supersaturation and additives) and formation processes of lithium carbonate (Li2CO3) spherulites. Li2CO3 crystals were synthesized via a reaction crystallization and the effect of supersaturation was studied by changing temperature, adding antisolvent and altering Li+ ions concentration. Star-like and flower-like polycrystalline particles were produced without additives. By addition of sodium hexametaphosphate (SHMP), spherical particles could be obtained composed of radially arranged crystallites and core-shell structure was evolved at lower temperature. Time-resolved experiments were performed to detect the nucleation time and morphological changes. It was found that SHMP strongly inhibited nucleation and induced noncrystallographic branching. Two formation mechanisms were proposed to illustrate the stepwise evolution of core-shell spheres under different conditions. In addition, sodium tripolyphosphate exhibits similar effects with SHMP, while polymaleic acid modulates a different spherulitic growth mode. These findings provide new insights into spherulitic growth and fabrication of spherical particles.

Photoluminescence and thermoluminescence properties of the Sr3Al2O6:Eu3+/Eu2+,Tb3+ persistent phosphor

Eu3+,Tb3+-doped Sr3Al2O6 powder phosphor was synthesized via a precursor route and subjected to a subsequent thermal treatment in reducing atmosphere. Photoluminescence and thermoluminescence properties of Sr3Al2O6:Eu3+/Eu2+,Tb3+ were investigated. The structure and morphology of oxides were investigated by X-ray diffraction (XRD) and scanning electron microscopy (SEM). X-ray photoelectron spectroscopy (XPS) was used for the nanocrystals surface composition analysis. X-ray diffraction patterns confirmed the formation of the cubic structure specific to Sr3Al2O6 with space group Pa3 and lattice parameter a = 15.8322 Å, while SEM investigations revealed equiaxial, polycrystalline particles, with sizes in the submicronic range, for both Sr3Al2O6:Eu3+,Tb3+ and Sr3Al2O6:Eu3+/Eu2+,Tb3+ samples. The photoluminescence spectra showed the typical f-f luminescence lines of the Tb3+ and Eu3+ - ions, accompanied by a broad Eu2+ luminescence band at 510 nm (after calcination in reducing atmosphere). The “after-glow” luminescence signal and the thermoluminescence were assigned to the recombination of close neighbor partners (electron and Eu2+ - hole centers) within the same complex of defects.

Direct ultrasound synthesis of vanadyl pyrophosphate catalyst for partial oxidation of n-butane to maleic anhydride

Three VPO catalyst were prepared via direct ultrasound technique using organic, sesquihydrate and dihydrate routes, denoted as VPOuo, VPOus and VPOud respectively. These catalysts were synthesised solely on direct ultrasound technique and calcined in n-butane/air mixture. All catalysts exhibited well-crystallised (VO)2P2O7 phase. VPOus and VPOud showed αII-VOPO4, which led to an increase in average oxidation state of vanadium. All catalysts were showing O1s approx. 530 eV, similar P2p value and V2p3/2 at approx. 517 eV, giving vanadium oxidation state of approx. 4.0 – 4.2. FE-SEM micrographs showed the secondary structure consisting of thin plate-like crystals in different sizes agglomerate to each other due to cavitation effect. HR-TEM demonstrated the existence of polycrystalline phase. The nature of the oxidants was investigated by TPR in H2. VPOus showed highest amount of total removal of oxygen species suggesting that it had highest activity compared to VPOuo and VPOud. The XANES measurement of these catalysts showed the occurrence of vanadium oxide reductions in flowing hydrogen gas, which indicates the presence of V4+ and V5+ species. Catalytic tests demonstrated that the activity and selectivity to maleic anhydride increased with direct ultrasound technique. This study showed that catalyst synthesis time can be reduced to only 2 hours and giving polycrystalline particles compared to conventional method.

X‐Ray Phase Contrast Imaging of the Impact of a Single HMX Particle in a Polymeric Matrix

AbstractA complete understanding of the mechanisms by which high explosives (HEs) are shock initiated, especially at the particle scale, is still in demand. One approach to explain shock initiation phenomenon is hot spot theory, which suggests that distributed energy in energetic material is localized due to shock or impact to generate the high temperatures for ignition. This study focuses on the impact response of a HE polycrystalline particle, specifically HMX, in a polymer matrix. This represents a simplified analog of a traditional polymer‐bonded explosive (PBX) formulation. A light gas gun, together with high‐speed x‐ray phase contrast imaging (PCI), was used to study the impact response of a single particle of production‐grade HMX in a Sylgard‐184® matrix. The high‐speed x‐ray PCI allows for real‐time visualization of HE particle behavior. The experiments revealed that, at impact velocities of ∼200 m s−1, the energetic particle was cracked and crushed. When the impact velocity was increased to 445 m s−1, a significant volume expansion of the particle was observed. This volume expansion is considered to be the result of chemical reaction within the HE particle.

Morphology and Activity of Electrolytic Silver Catalyst for Partial Oxidation of Methanol to Formaldehyde Under Different Exposures and Oxidation Reactions

Electrolytic silver particles were studied in relation to its morphology changes under different reactive and non-reactive atmospheres, and its catalytic activity in oxidation of methanol to formaldehyde (MTF), carbon monoxide to carbon dioxide, and hydrogen to water. Scanning electron microscopy and X-ray diffraction (XRD) were applied to analyze structural changes in the silver catalyst after exposure or interaction with nitrogen, oxygen, methanol/water, carbon monoxide and hydrogen, applied either individually or in selected combinations, at temperatures approaching 700 °C. The as-received Ag catalyst consists of agglomerated, faceted, polycrystalline particles. These undergo massive morphological changes during MTF reaction conditions. It was found that Ag catalysts exposed to oxygen-free atmospheres (N2, H2/N2 and CH3OH/H2O/N2) at 650 °C exhibit minimal changes in surface morphology compared to the fresh catalyst, while severe restructuring occurs on the mesoscopic scale under oxygen containing atmospheres (O2/N2, H2/O2/N2 and CO/O2/N2) at elevated temperature. This restructuring renders a smoothened surface with refacetted areas and many pinholes, while a small primary crystallite size (~ 40 nm, XRD) is maintained. Such pinholes are commonly described as a result of sub-surface oxygen/hydrogen/hydroxyl interactions. Here, they are present in all samples exposed to oxygen, indicating that presence of hydrogen is not prerequisite. For the CO and H2 oxidation sub-systems, the initial activity was comparable. But, while the conversion of H2 is preserved during 70 h time on stream, the CO conversion gradually reduces from 70 to 10%. This suggests that the restructuring associated with dissolution of O at high temperature inhibits the CO to CO2 pathway.

Flame‐assisted spray pyrolysis of lithium and manganese precursors to polycrystalline LiMn2O4

AbstractLithium manganese oxide cathodes have a smaller environmental footprint compared to cathodes with cobalt and nickel. Flame assisted spray pyrolysis (FASP) is an emerging technique that may improve the economics of the process at the commercial scale while controlling purity. reacts with to form . Carbon from the flame coats the particles and, together with the Mn oxides, they reduce electrical capacity. The reaction temperature and droplet residence time are the main parameters that determine manganese oxide content and therefore the product purity. LECO carbon analysis confirms that the fuel and precursor type affect carbon content. It increases from % with nitrate precursors to 2 % with carbonate and acetate precursors. The temperature profile as well as the solute type and concentration change the product morphology. The particle surface is wrinkled at a mild temperature and blowholes form at a high temperature. Primary nano‐crystals (5–8 nm) agglomerate to form 1 polycrystalline particles from a 0.5 nitrate precursors solution, while a 5 solution produces 10 powders. We modelled the reaction kinetics based on thermogravimetric analyses to identify the principle reaction steps. Oxide formation increases below 400C and above 800C, while fast heating rates and short residence time reduce purity.

Reversed Crystal Growth

In the last decade, a reversed growth route has been found in many crystal growth processes. In these systems, a single crystal does not develop from a single nucleus. The precursor molecules/ions or nanocrystallites aggregate into some large amorphous or polycrystalline particles. Multiple-nucleation on the surface of the amorphous particles or surface re-crystallization of the polycrystalline particles then takes place, forming a single crystal shell with a regular morphology. Finally, the crystallization extends from the surface to the core to form single crystals. This non-classical crystal growth route often results in some special morphologies, such as core-shell structures, hollow single crystals, sandwich structures, etc. This article gives a brief review of the research into reversed crystal growth and demonstrates that investigation of detailed mechanisms of crystal growth enables us to better understand the formation of many novel morphologies of the crystals. Some unsolved problems are also discussed.

Controlled alloying of Pt-Rh nanoparticles by the polyol approach

We report on a facile synthesis strategy for the reaction matrix platinum(II)-acetylacetonate − rhodium(III)-acetylacetonate − polyvinylpyrrolidone − 1,4-butanediol and show that phase pure Pt-Rh alloyed nanoparticles can be achieved with compositional control based on simultaneous decomposition of Pt- and Rh-metal precursors. By combining data from powder X-ray diffraction, (high-resolution) transmission electron microscopy [(HR)TEM], energy dispersive X-ray spectroscopy (EDS) and kinetic experiments, we describe key parameters for compositional control. Powder X-ray diffraction suggests a crystallite size of around 4 nm, less than the size of polycrystalline particles, of around 7 nm in average and with different morphologies, as imaged by TEM. High-Angle Annular Dark Field Scanning TEM and EDS show some compositional fluctuations between and within in the received Pt-Rh nanoparticles. Based on insight to the reaction kinetics of the individual Pt and Rh metal precursors, recipes are fine-tuned to provide bimetallic nanoparticles with controlled average composition, including achieving the identical Pt/Rh molar ratio for the resulting nanoparticles as defined by the precursor mixture prior to the simultaneous reduction step. The adopted approach adds value beyond the Pt-Rh system and shows that control of precursor reactivity is essential to provide exact metal compositions of bimetallic nanoparticles for use in fundamental studies and applications.

Production of alumo-matrix composite material modified by nanostructured cubic boron nitride

The structure and microhardness of an aluminum alloy with additives of nanostructured cubic boron nitride (cBN) after treatment under high pressure and temperature are investigated. А nanostructured powder of cBN with primary particles within 50–200 nm is used as a filler. A preliminary chemical-thermal modifying of the nanostructured cBN, which consists in its high-temperature annealing in the temperature range of 750–950 °C in a medium of aluminum-contai ning compounds, is carried out to increase the chemical affinity of the nanostructured cBN to the aluminum matrix. It is shown that the modifying of nanostructured cBN with aluminum increases the strength of the additives retention in the aluminum matrix. At the same time the increase in the concentration of BN additives from 1.5 to 5 wt.% as well as the increase in the treatment temperature at a fixed pressure promotes the increase in the microhardness of the material by a factor of 1.5 to 2 as compared with the base aluminum alloy without the addition of a modifier. An increase in the cBN concentration to 5 % by weight results in an increase in the fraction of smaller particle conglomerates (1–5 μm) in the material and in a decrease in the size of large inclusions to 10–20 μm. In this case, the distribution of BN particles in the aluminum matrix is more uniform in comparison with a material with a cBN content of 1.5 wt.%. In the material with the growth of temperature up to 1000 °С, cBN in aggregates is recrystallized with the formation of single-crystal (polycrystalline) particles with the size of 1–10 μm with faceting specific for cBN micron particles.

Study of dynamic behaviors for nano Fe-doped MgB2 superconductor via ac-susceptibility measurements

Nano-sized superconducting polycrystalline particles of Mg1-xFexB2 (0.00 ≤ x ≤ 0.04; space group P6/mmm) were synthesized through the solid state reaction route. The effect of nano-Fe substitution on ac-susceptibility at varying frequency range from 1 to 3 kHz in addition with ac-amplitude (Hac) and dc-applied field (Hdc) were explored. The dynamical behavior of nano-particles shows the strong magnetic interaction between the nano-clusters with the increase of doping content. This is due to the successful inclusion of Fe3+ ions in the matrix of MgB2. X-ray powder diffraction (XRD) reveals the reduction of average grain size with increasing the doping level. Neel-Arrhenius (N-A) and Vogel-Fulcher (V-F) models were discussed to distinguish the dynamical behaviors of the nano-particles. Remarkably, the V-F model elucidates the experimental results much better than N-A model with the support of some parameters like jump attempt frequency, activation energy, magnetic anisotropy constant and V-F temperature.

Preparation of mesoporous spherical magnesium hydroxide particles via the static self-assembled method

As the original first time, the novel mesoporous spherical magnesium hydroxide (MH) particles were synthesized via static self-assembled method using magnesium sulfate, ammonia solution, ethanol and sodium hydroxide as raw materials. To control the structure of the mesoporous spherical material, the effects of reaction time, MgSO4 concentration, reaction temperature, contacting area and volume ratio were comprehensively investigated. Structural characterizations of these products were analyzed by SEM, XRD, TEM, EDX, SAED, FT-IR, TG-DTG and BET. The optimal parameters of mesoporous spherical MH preparation were obtained as follows: the reaction time was 25 h, the MgSO4 concentration was 1–2 mol/L, the reaction temperature was 25–45 °C, the gas-liquid two-phase contacting area was 100–150 m2/m3. The structural characterizations analysis indicated that the mesoporous spherical materials prepared under optimal conditions were polycrystalline particles with many properties, such as high specific surface area, slit porous channels and uniform particle size distribution.

Hydrochlorination of acetylene catalyzed by activated carbon supported highly dispersed gold nanoparticles

A series of Au catalysts were prepared with several representative solvents and evaluated for acetylene hydrochlorination. The results revealed that the activity of the catalyst is closely linked with the intrinsic properties of solvents. The catalytic performance of the catalysts increased with decreasing polarity of the solvents, and superior performance was achieved over the Au-isopropanol/AC catalyst, with a 84% stable conversion under reaction conditions of 180 °C and a gas hourly space velocity (GHSV) of 1200 h−1; relative increases of 700.0% and 483.3%, respectively, in acetylene conversion were achieved compared with that achieved with traditional catalysts prepared with water and aqua regia. The substitution of the highly polar water and aqua regia with weakly polar and volatile alcohols altered the crystallization process of Au nanoparticles (NPs) during their formation. The various edges or defects in the formed multiple-twinned or polycrystalline particles provided new active sites for reactants. The altered solvents may enhance the interaction between the support and the Au species, highly dispersing and anchoring the active species and inhibiting their agglomeration and loss during the reaction. Moreover, the interaction also strengthens the adsorption capacity for reactants of the catalysts, enhancing their catalytic performance. This approach may provide an effective reference for exploring environmentally benign mercury-free catalysts for acetylene hydrochlorination.

Highly efficient hydrogen generation of BiI3 nanoplates decorated with Ag nanoparticles

This work presents the photocatalytic hydrogen generation of BiI3 films when they are exposed under simulated solar irradiation. Those films were fabricated by a simple bath chemical deposition method. The images of Scanning electron microscopy show that the BiI3 films are formed by polycrystalline particles with hexagonal shape and a closer inspection shows that nanoplates grown into them. The performance for the hydrogen generation of BiI3 films synthesized with different times (from 1 to 3 h), BiI3 films decorated with silver nanoparticles (BiI3-Ag) and BiI3 powders were compared. We found that the presence of Ag nanoparticles in the BiI3 films increased the hydrogen generation from 9 mmol/g·h to 14 mmol/g·h because these nanoparticles improved electron conductivity, reduced the amount of defects in the BiI3 films, raised the optical absorption, and produced higher photocurrents (with respect to the BiI3 films without nanoparticles) as confirmed by electrochemical experiments. On the contrast, the BiI3 powders had a poor hydrogen generation rate of only 0.5 mmol/g·h, this was due to the fact that they are formed by nanoplates 10 times bigger than these ones grown on the BiI3 films, consequently, they had lower surface area than the nanoplates in the BiI3 films. A possible mechanism for the hydrogen generation by the BiI3 and BiI3-Ag films is also discussed.

Automatic characterization of iron ore by digital microscopy and image analysis

This paper presents an automatic system for mineralogical and textural characterization of iron ores based on digital microscopy and image analysis. It employs a motorized and computer-controlled reflected light microscope in a correlative approach that combines bright field and circular polarization modes. Mosaic images covering large areas of polished sections are acquired to image thousands of particles. Different classifiers discriminate compact and non-compact hematite, polycrystalline and monocrystalline particles, and identify particles as granular, lamellar, and lobular. The entire process is automatic and produces a full pdf report containing typical images and the quantification of mineral and textural phases.