Coalescence Of Nanocrystals Research Articles

Influence of purification methods on the extraction of nanocrystals from bio-hydroxyapatite under consideration of the coalescence phenomenon

The coalescence of nanocrystals that occurs during low-temperature thermal treatments during the purification of hydroxyapatite from pigs has not yet been studied in the context of low-temperature environmentally friendly purification. To preserve the nature of hydroxyapatite, two extraction methods were investigated: Route 1 (autoclave and Soxhlet) and Route 2 (calcination at low temperatures with different times). The focus is on observing the effectiveness of removing fats and proteins while maintaining crystallite size and morphology. DSC showed that coalescence of crystallites occurred at 452, 566, 648, and 704 °C. Raw has a higher IR FWHM (phosphate band) due to the organic material; R1 had a lower FWHM (partial removal of lipids and proteins). Increasing the calcination temperature decreases the FWHM, indicating a shift in vibrational states from surface to bulk, leading to an increase in apparent crystallite size due to coalescence. For 20–80 h at quasi-stable temperatures of 400 and 500 °C, no significant changes occur in the IR spectrum. X-ray diffraction indicates the removal of organic material during calcination, as the peaks become clearer with increasing temperature. The crystallite size is not affected and remains in similar ranges. Purification with hydroxide shows an increase in both lattice parameters, in addition to the fluctuations that occur due to the ionic substitutions that naturally occur in biogenic HAp. And the TEM study confirms that the changes in apparent size from 2D to 3D are the result of the phenomenon of interplanar coalescence at low temperatures that occurs in sintering processes.

Study of the coalescence phenomena in biogenic nano-hydroxyapatite produced by controlled calcination processes at low temperature

The coalescence of nanocrystals generated by thermal treatments is a phenomenon observed in many materials. In the case of hydroxyapatite (HAp) derived from a biological source such as bovine bone, this transition was observed at temperatures around 700 °C. However, whether this is also the case at lower temperatures has not yet been investigated. With this aim, this work examined HAp samples from cattle calcined at 400–720 °C in increments of 20 °C, to determine the temperature effect on HAp nanocrystals due to coalescence. Differential scanning calorimetry showed that the raw (as prepared) bovine hydroxyapatite exhibited four thermal transition events at which the coalescence of HAp nanocrystals occurred. Infrared analysis showed that the full width at half maximum (FWHM) of the phosphate band decreases with increasing calcination temperature. The FWHM of the X-ray diffraction peaks showed minima values just at the coalescence events, while the unit cell volumes exhibited an opposite trend. The crystallite size of the HAp increases with increasing calcination temperature during the coalescence events, and the Rietveld method shows that the unit cell volume follows the same behavior and shows an opposite behavior in terms of the micro-strain. Scanning electron microscopy (SEM) images of samples calcined at temperatures above 520 °C show the first signs of coalescence, as necks are formed by the union of adjacent HAp nanocrystals. N2 adsorption-desorption measurements have also shown that the average pore diameter is maintained during thermal processes. Finally, transmission electron microscopy (TEM) was used to observe the coalescence of the nanocrystals of the sample burned at 420 and 520 °C. These thermal events could be because the surface vibrational states of at least two adjacent HAp nanocrystals collapse and form new bulk vibrational states.

Grain growth competition during sintering of SrTiO3 nanocrystals: Ordered coalescence of nanocrystals versus conventional mechanism

Grain growth plays a crucial role in developing the microstructures of polycrystalline materials during heating treatment. In this work, SrTiO3 ceramics sintered at different heating rates (10°C/min and 100°C/min) display the distinctly different grain growth behaviors and microstructures between them. The onset temperature of rapid grain growth and mechanical-performance-related fracture mode are apparently different between these two sets of samples. Two grain growth mechanisms are demonstrated to dominate separately the sintering of SrTiO3 nanocrystals at different heating rates, i.e. conventional mechanism of atom-by-atom additions for grain growth at 10°C/min and ordered coalescence of nanocrystals for grain growth at 100°C/min. It is demonstrated that the switch of growth mechanisms is determined by the competition between the activation of grain motions and the formation of steady neck among grains.

Nanocrystalline‐silicon hole contact layers enabling efficiency improvement of silicon heterojunction solar cells: Impact of nanostructure evolution on solar cell performance

AbstractHydrogenated amorphous silicon (a‐Si:H) is a key enabler in high‐efficiency crystalline silicon solar cells known as the silicon heterojunction technology. Although efforts have been devoted to replacing doped a‐Si:H contact layer by hydrogenated nanocrystalline silicon (nc‐Si:H) to take advantage of its superior optoelectrical properties, it is still unclear whether the nc‐Si:H outperforms the a‐Si:H at the high efficiency level. Here, we show that boron‐doped (p)nc‐Si:H prepared by plasma‐enhanced chemical vapor deposition (PECVD) acts as an efficient hole contact layer, providing not only a mitigation of the parasitic absorption loss but also improvements in passivation and electrical contact properties. This results in an efficiency increase by 0.3%–0.6% absolute compared to the reference cell with the (p)a‐Si:H, and a best cell efficiency of 23.54%. We find that the critical thickness of the (p)nc‐Si:H layers required for gaining high efficiency (tc ~ 15–30 nm) is a factor of 3–6 greater than that of the (p)a‐Si:H. UV Raman spectroscopy and electrical conductivity measurements reveal that the tc of the (p)nc‐Si:H is associated with the layer growth needing for the surface coalescence of nanocrystals, determining the hole selectivity and the contact resistivity at the electrode/(p)nc‐Si:H interface. Our results suggest that such nanostructure evolution can be hastened by using a very‐high‐frequency PECVD process.

Study of Crystallization and Coalescence of Nanocrystals in Amorphous Glass at High Temperature.

Amorphous fluoride glass is used as the reaction chamber (i.e., solid cell) to grow Ba2LaF7 (BLF) nanocrystals at elevation temperatures (i.e., 300-500 °C) so that in situ real-time crystallization and coalescence of BLF nanocrystals can be observed. Due to the inherent advantages of the liquid-like solid medium, high temporal and spatial resolution transmission electron microscopy images can be obtained. Hence, we reveal that the twinned and quadruplet BLF nanocrystals are formed at low temperature (≤430 °C) and the unification of two nanocrystals via the two pathways (i.e., migration with and without rotation) to a single defect-free BLF nanocrystal is favored at high temperature (≥470 °C).

Doping and Photon Induced Defect Healing of Hybrid Perovskite Thin Films: An Approach Towards Efficient Light Emitting Diodes

AbstractHere, we demonstrate a simple method to improve photoluminescence and electroluminescent properties of MAPbBr3 perovskite thin films by monovalent Cs cation doping and treatment with UV light. It is known that the intermediate species of lead halides, which limits the photoluminescence quantum yield (PLQY) of perovskite thin films, are formed due to the presence of an organic polar solvent. In such cases, coalescence of nanocrystals is unavoidable when the thin‐films are thermally annealed to remove the intermediate phases and solvent residues. We find that PbBr2(DMSO)x complexes can effectively tune the morphology, grain boundary, PLQY, and non‐radiative loss of perovskite thin films under UV light irradiation. Upon UV exposure, highly uniform MA0.87Cs0.13PbBr3 ultra‐thin (45 nm) films with a full surface coverage of neatly packed nanocrystallites are obtained. When these films are integrated into electroluminescent devices, a current efficiency (CE) of 18.71 Cd/A and an external quantum efficiency (EQE) of 5.63% are observed for green perovskite‐based light emitting diodes (PeLEDs). These values of CE and EQE are respectively thirty‐four and forty‐three times higher than those achieved in pure MAPbBr3 based devices.

In Situ Atomic-Scale Study of Particle-Mediated Nucleation and Growth in Amorphous Bismuth to Nanocrystal Phase Transformation.

Understanding classical and nonclassical mechanisms of crystal nucleation and growth at the atomic scale is of great interest to scientists in many disciplines. However, fulfilling direct atomic‐scale observation still poses a significant challenge. Here, by taking a thin amorphous bismuth (Bi) metal nanosheet as a model system, direct atomic resolution of the crystal nucleation and growth initiated from an amorphous state of Bi metal under electron beam inside an aberration‐corrected transmission electron microscope is provided. It is shown that the crystal nucleation and growth in the phase transformation of Bi metal from amorphous to crystalline structure takes place via the particle‐mediated nonclassical mechanism instead of the classical atom‐mediated mechanism. The dimension of the smaller particles in two contacted nanoparticles and their mutual orientation relationship are critical to governing several coalescence pathways: total rearrangement pathway, grain boundary migration‐dominated pathway, and surface migration‐dominated pathway. Sequential strain analyses imply that migration of the grain boundary is driven by the strain difference in two Bi nanocrystals and the coalescence of nanocrystals is a defect reduction process. The findings may provide useful information to clarify the nanocrystal growth mechanisms of other materials on the atomic scale.

Ordered coalescence of nano-crystals in alkaline niobate ceramics with high remanent polarization

Lead-free alkali niobates Na0.5K0.5NbO3 (NKN) ceramics, with significantly enhanced ferroelectric remanent polarization (Pr), were prepared using Spark Plasma Sintering (SPS). Three types of boundaries were observed in the ceramics, being grain boundaries between faceted grains, domain boundaries that separate ferroelectric domains inside individual grains, and nanoscale sub-grain boundaries that reveal the nano-scale mosaicity of individual grains. Part of the sub-grain boundaries were from initial powder particles. The other sub-grain boundaries were built by ordered coalescence of nano-crystals during rapid SPS process. It was worthwhile to emphasize that the ordered coalescence of nano-crystals in bulk ceramics during sintering takes place and completes within minutes. These sub-grain features would disappear at higher temperature by long time sintering. Rapid Spark Plasma Sintering allowed us to capture this transient microstructure. The significantly enhanced ferroelectric Pr of NKN was attributed to nanoscale sub-boundaries, which stimulated the dynamics of ferroelectric domain formation and switching.

Direct observation of shear–induced nanocrystal attachment and coalescence in CuZr-based metallic glasses: TEM investigation

In-situ tensile straining tests were performed in a transmission electron microscope (TEM) to analyse the deformation processes in CuZr-based metallic glasses and to directly observe the phase transformation occurrence. We report evidence of shear induced coalescence of nanocrystals in the vicinity of deformed regions. Nanocrystals grow in shear bands, come into contact, being attached and progressively coalesce under applied shear stress.

The crystal structures of sintered copper nanoparticles: A molecular dynamics study

The coalescence of nano-crystals during sintering is often found to result in interesting crystalline structures such as multi-fold twins, and yet the plasticity mechanism accompanying their formation is unclear. In this work, the sintering behavior of two unsupported copper nanoparticles initially at room temperature is investigated by molecular dynamics simulations under the constant-energy ensemble. The results reveal that once the two nanoparticles are brought into contact, they often go through drastic structural changes with the inter-particle grain boundary quickly eliminated, and single- and multi-fold twinning occurs frequently in the coalesced product. Whereas the formation of single twins is found to be via the more usual mechanism of emission of Shockley partials on {111} planes, the formation of fivefold twins, however, takes place via a novel dislocation-free mechanism involving a series of shear and rigid-body rotation processes caused by elastic waves with amplitudes not corresponding to any allowable Burgers vector in the fcc lattice. Such a lattice-wave, dislocation-free twinning mechanism has never been reported before.

Grain growth by multiple ordered coalescence of nanocrystals during spark plasma sintering of SrTiO3 nanopowders

Sintering is the most widely applied densification process for manufacturing polycrystalline materials in powder metallurgy and ceramic industries. Grain growth behavior during sintering has a crucial influence on the final microstructure and thus the achieved performance. So far, it has been accepted that grain growth, based on classic crystal growth theory, takes place via atomic diffusion driven by excess interfacial energy. This paper presents a novel grain growth mechanism resulting from multiple ordered coalescence of nanocrystals via the activation of rapid grain motions. In rapid solid-state sintering of a strontium titanate (SrTiO3) nanopowder, individual SrTiO3 nanocrystals can act as the building blocks and self-assemble to form larger grains. A quasi-liquid interfacial film achieved by surface melting of the nanocrystals plays an essential role in this new process by facilitating the grain motion and ordered coalescence of nanocrystals. The imperfect ordered coalescence of nanocrystals introduce deep structural heterogeneities characterized by the unique quasi-interfaces inside grown grains of single crystal signature. The quasi-interfaces consist of the vacancy arrays and/or aggregated line defects.

Ferroelectric ceramics with enhanced remnant polarization by ordered coalescence of nano-crystals

An exceptional high ferroelectric remnant polarization (Pr) was observed in BaTiO3 ceramics owing to the formation of micron-sized grains possessing nano-scale mosaicity. Such a structural hierarchy was developed via a novel crystal-growth mechanism, namely ordered coalescence of nano-crystals achieved by synergetic atomic epitaxial growth and self-assembly of nano-crystals. The accommodating lattice defects in sub-grain boundaries due to the imperfect assembly of nano-crystals significantly contribute to the Pr enhancement by stimulating the dynamics of ferroelectric domain formation and switching. This finding defines a new approach to nanopowder sintering leading to enhanced properties sensitive to lattice defects.

Effect of Non-equilibrium Pulsed Ejection of Si Species into Background Gas on the Formation of Si Nanocrystallite and Nanocrystal-film

ABSTRACTThe Si nanocrystal-films are prepared by pulsed laser ablation of Si target in a mixture of helium and hydrogen gas. The total gas pressure and hydrogen partial gas pressure were varied to control structure of nanocrystal-film. The surface of Si nanocrystallite was hydrogenated and degree of hydrogenation increased with increasing hydrogen partial gas pressure. The aggregate structure of nanocrystal-film depended on both the total gas pressure and the hydrogen partial gas pressure. The former and the latter alter spatial confinement of Si species during deposition and the surface hydrogenation of individual nanocrystal, respectively. Spatial confinement increases probability of collision between nanocrystals in the plume. While, surface hydrogenation prevents coalescence of nanocrystals. The individual or aggregated nanocrystals formed in the plume reach the substrate and the nanocrystal-film is deposited on the substrate. The non-equilibrium growth processes during pulsed laser ablation are essential for the formation of the surface structure and the subsequent nanocrystal-film growth. Our results indicate that the structure of nanocrystal-film depends on the probabilities of collision and coalescence between nanocrystals in the plume. These probabilities can be varied by controlling the total gas pressure and the hydrogen partial gas pressure.

Ordered coalescence of nano-crystals during rapid solidification of ceramic melts

Rapid cooling of an aluminosilicate-zirconia melt after laser sintering results in the formation of zirconia nano-crystals and dendritic zirconia crystals embedded in a glass matrix. The nano-cryst ...

Ordered coalescence of nanocrystals: a path to strong macroporous nanoceramics

A versatile approach for integrating two apparently conflicting physical properties, highporosity and high mechanical strength, in polycrystalline bulks is established anddemonstrated for the case of alumina ceramics. Macroporous alumina nanoceramics aresynthesized by stimulating coalescence-mediated necking, which enables the formation ofstrong crystallographically coherent necks between adjacent grains. The workplaces a general emphasis on manipulating crystal growth on the nanoscale and onpreparing highly porous polycrystalline bulk ceramics with improved mechanicalrigidity.