Faceted Grains Research Articles

Cellular Automaton Modeling of the Transition of Multi-Crystalline Silicon from a Planar Faceted Front to Equiaxed Growth

A modeling approach combining the lattice Boltzmann (LB) method and the cellular automaton (CA) technique are developed to simulate the faceted front to equiaxed structure transition (FET) of directional solidification of multi-crystalline silicon. The LB method is used for the coupled calculation of velocity, temperature and solute content field, while the CA method is used to compute the nucleation at the silicon-crucible interface and on SiC particles, as well as the mechanism of growth and capturing. For silicon, the interface kinetic coefficient is rather low, which means that the kinetic undercooling can be large. A strong anisotropy in the surface tension and interfacial kinetics are considered in the model. A faceted front in conjunction with a sufficiently high carbon content can lead to equiaxed growth by nucleation on SiC. The temperature gradient in Si melt at the interface is negative, which leads to the occurrence of a faceted interface. The higher the absolute value of thermal gradients, the faster the growth velocity. Due to differences in the degree of undercooling, there will be the unification of facets in front of the solid-liquid interface. Transitions from faceted front to thermal equiaxed dendrites or faceted equiaxed grains are observed with smaller or larger impurity contents, respectively.

Influence of substrate material on spectral properties and thermal quenching of photoluminescence of silicon vacancy colour centres in diamond thin films

Abstract Nanocrystalline diamond films with bright photoluminescence of silicon-vacancy colour centres have been grown using a microwave plasma enhanced CVD technique. The influence of substrate material (quartz, Al2O3, Mo and Si) on a reproducible fabrication of diamond thin films with Si-V optical centres is presented. Film quality and morphology are characterized by Raman spectroscopy and SEM technique. SEM shows well faceted diamond grains with sizes from 170 to 300 nm. The diamond peak is confirmed in Raman spectra for all samples. In the case of the quartz substrate, a redshift of the diamond peak is observed (≈3.5 cm−1) due to tension in the diamond film. The steady-state photoluminescence intensity was measured in the temperature range from 11 K to 300 K. All spectra consist of a broad emission band with a maximum near 600 nm and of a sharp zero phonon line in the vicinity of 738 nm corresponding to Si-V centres that is accompanied with a phonon sideband peaking at 757 nm. Activation energies for the thermal quenching of Si-V centre photoluminescence were determined and the effect of the substrate on photoluminescence properties is discussed too.

Effect of holding time and interlayer's thickness on the crack initiation and propagation and the dissolving behavior of the heat-treated facet WC grains

Post welding heat treatment was performed on dissimilar weldments composed of WC-Co to 316L stainless steel with 1-mm-, 1.5-mm-, and 2-mm-thick Invar interlayers by heating the samples to 1250°C for 2h, 8h, and 16h respectively. Cracks initiation and propagation were observed in all samples because of the stress relieving after heat treatment. The increase of the interlayer's thickness tends to partially inhibit the initiation and propagation of cracks. When the holding time is 2h, the sample fractured in the heat affected zone (HAZ). With the increase of holding time (8h, 16h), the samples tended to fracture in the fusion zone. The dissolving behavior of prismatic facets estimated by means of high-angle annular dark field (HAADF) microscopy indicated that the shape of WC grains changed to faceted WC grains during the heat treatment, which exhibited abnormal grain growth in the matrix composing of α-Co and γ-(Fe, Ni). In the fusion zone adjacent to the WC-Co, W-M-C carbides skeleton were formed with a character of continuous rods and isolated islands. The islands had lower tungsten and more binders than the rods. By using transmission electron microscopy - energy dispersive spectroscopy (TEM-EDS), it was found that WC grains and WC/Co/WC interface showed strong plastic deformation with obvious elemental diffusion. Moiré fringes inside the WC and in the matrix were confirmed.

Winter snow conditions on Arctic sea ice north of Svalbard during the Norwegian young sea ICE (N‐ICE2015) expedition

AbstractSnow is a crucial component of the Arctic sea ice system. Its thickness and thermal properties control heat conduction and radiative fluxes across the ocean, ice, and atmosphere interfaces. Hence, observations of the evolution of snow depth, density, thermal conductivity, and stratigraphy are crucial for the development of detailed snow numerical models predicting energy transfer through the snow pack. Snow depth is also a major uncertainty in predicting ice thickness using remote sensing algorithms. Here we examine the winter spatial and temporal evolution of snow physical properties on first‐year (FYI) and second‐year ice (SYI) in the Atlantic sector of the Arctic Ocean, during the Norwegian young sea ICE (N‐ICE2015) expedition (January to March 2015). During N‐ICE2015, the snow pack consisted of faceted grains (47%), depth hoar (28%), and wind slab (13%), indicating very different snow stratigraphy compared to what was observed in the Pacific sector of the Arctic Ocean during the SHEBA campaign (1997–1998). Average snow bulk density was 345 kg m−3 and it varied with ice type. Snow depth was 41 ± 19 cm in January and 56 ± 17 cm in February, which is significantly greater than earlier suggestions for this region. The snow water equivalent was 14.5 ± 5.3 cm over first‐year ice and 19 ± 5.4 cm over second‐year ice.

Citrate gel-combustion synthesis and sintering of nanocrystalline ThO2 powders

A systematic study of the influence of citric acid to nitrate mole (R) ratio (R = 0 to 0.50) on the citrate gel-combustion synthesis of nanocrystalline (nc) ThO2 in bulk quantities (30 g) by using citrate gel-combustion was carried out. The nc-ThO2 powders were characterized for their bulk density, size distribution of particles, specific surface area, carbon residue and X-ray crystallite size. All these powders were compacted at pressures varying from 60 to 353 MPa and sintered by using the “two-step sintering” method. Powders prepared from a mixture with an “R” value of 0.125 compacted at 243 MPa yielded a maximum sintered density of 98.8 ± 0.3% T.D. For nc-ThO2, this is the highest sintered density reported so far. The microstructural investigations on nc-ThO2 powders were carried out by using both scanning electron microscopy (SEM) and transmission electron microscopy (TEM). SEM images of the sintered thoria monoliths revealed faceted grains with well defined grain boundaries. Shrinkage anisotropy factor (α) revealed that the compacts prepared from the powders obtained from starting mixtures with R values of 0.125–0.50 had undergone uniform sintering (near isotropic shrinkage).

{1 1 1} facet growth laws and grain competition during silicon crystallization

Directional solidification from mono-crystalline Si seeds having different orientations along the growth direction is studied. Due to the frequent twinning phenomenon, new grains soon nucleate during growth. The grain competition is then characterized in situ by imaging the dynamic evolution of the grain boundaries and of the corresponding grain boundary grooves that are formed at the solid-liquid interface. To perform this study, an experimental investigation based on Bridgman solidification technique coupled with in situ X-ray imaging is conducted in an original device: GaTSBI (Growth at high Temperature observed by X-ray Synchrotron Beam Imaging). Imaging characterisation techniques using X-ray synchrotron radiation at ESRF (European Synchrotron Radiation Facility, Grenoble, France) are applied during the solidification to study the growth dynamics. Facetted/facetted grain boundary grooves only are studied due to their importance in the grain competition because of their implication in the twinning mechanism. The maximum undercooling inside the groove is calculated from the groove depth knowing the local temperature gradient. Additionally, thanks to dynamic X-ray images, the global solid-liquid interface growth rate and the normal growth rate of the {111} facets existing at the grooves and at the edges are measured. From these measurements, experimental growth laws that correlate the normal velocity of the {111} facets with the maximum undercooling of the groove are extracted and compared to existing theoretical models. Finally, the experimental laws found for the contribution to the undercooling of the {111} facets are in good agreement with the theoretical model implying nucleation and growth eased by the presence of dislocations. Moreover, it is shown that, for the same growth parameters, the undercooling at the level of the facets (always lower than 1K) is higher at the edges so that there is a higher probability of twin nucleation at the edges which is in agreement with the grain structure development characterised in the present experiments as well as in the literature.

Kinetics and dynamics of planar abnormal grain growth in nanocrystalline nickel

Growth of abnormally large grains has been observed previously in nanocrystalline Nickel both in-directly by electron imaging after heat treatment and by in-situ resistivity measurement. However the form and rate of the growth has been derived indirectly. This work makes use of in-situ heat treatment combined with back scattered electron imaging illustrates that the abnormal micro-planar faceted grains grow with a dependence not only on the surrounding but also as the orientation of the growing facet. This observation has shown that the grains grow in a semi-constrained manner such that high index faces such as {211} grow faster than stable {111} faces.

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.

Self-Terminating Confinement Approach for Large-Area Uniform Monolayer Graphene Directly over Si/SiOx by Chemical Vapor Deposition.

To synthesize graphene by chemical vapor deposition (CVD) both in large area and with uniform layer number directly over Si/SiOx has proven challenging. The use of catalytically active metal substrates, in particular Cu, has shown far greater success and therefore is popular. That said, for electronics applications it requires a transfer procedure, which tends to damage and contaminate the graphene. Thus, the direct fabrication of uniform graphene on Si/SiOx remains attractive. Here we show a facile confinement CVD approach in which we simply "sandwich" two Si wafers with their oxide faces in contact to form uniform monolayer graphene. A thorough examination of the material reveals it comprises faceted grains despite initially nucleating as round islands. Upon clustering, they facet to minimize their energy. This behavior leads to faceting in polygons, as the system aims to ideally form hexagons, the lowest energy form, much like the hexagonal cells in a beehive, which requires the minimum wax. This process also leads to a near minimal total grain boundary length per unit area. This fact, along with the high graphene quality, is reflected in its electrical performance, which is highly comparable with graphene formed over other substrates, including Cu. In addition, the graphene growth is self-terminating. Our CVD approach is easily scalable and will make graphene formation directly on Si wafers competitive against that from metal substrates, which suffer from transfer. Moreover, this CVD route should be applicable for the direct synthesis of other 2D materials and their van der Waals heterostructures.

Kinetics and high-temperature oxidation mechanism of ceramic materials in ZrB2–SiC–MoSi2 system

The paper focuses on the production of compact ceramics ZrB2–SiC–(MoSi2) using the hybrid SHS + HP technology, as well as on its phase composition, structure and high-temperature oxidation kinetics. Reaction mixtures were obtained according to the following scheme: mechanical activation of Si + C powders; wet admixing of Zr, B and MA powders of Si + C mixture; drying of mixtures in a drying cabinet. The composite SHS powder ZrB2–SiC was obtained in the SHS-reactor in combustion mode by element synthesis. Compact samples were produced using the hot pressing method by SHS powder consolidation. Resulting samples characterized by a homogeneous structure and low residual porosity not exceeding 1,3 %. In total, two compositions were chosen for tests: the first one rated for ZrB2 + 25 % SiC formation, the second one similar to the first one, but with the addition of 5 % commercial MoSi2 powder. The microstructure of samples is represented by dispersed dark gray rounded SiC grains distributed among the light faceted ZrB2 grains. The sample with the MoSi2 additive has a more finely dispersed structure. High-temperature oxidation of samples at 1200 °С forms complex oxide films SiO2–ZrO2–(B2O3) about 20–30 μm in thickness on their surface, which serve as an effective diffusion barrier and reduce oxidation rate. The complex ZrSiO4 oxide is also present in the oxide film structure after long holding times (more than 10 hours). In addition, after 10 hours of testing, a slight decrease in the mass of the samples is observed, which is due to the volatilization of B2O2, CO/CO2, MoO3 gaseous oxidation products. The sample with MoSi2 added demonstrates better resistance to oxidation.

A combined Na and Cl treatment to promote grain growth in MOCVD grown CdTe thin films

The role of sodium (Na) in cadmium telluride (CdTe) thin film photovoltaic absorbers deposited on a metal-coated substrate is investigated. Introducing Na during the growth of the structure influences the morphological and crystallographic properties of the CdTe layer grown by metal-organic chemical vapour deposition (MOCVD). It is observed that the introduction of Na between the metal and CdTe layers induces a slight randomisation via promotion of (400) and (220) orientations. It is shown that the inclusion of Na between the substrate and the Mo back metal contact enlarges the CdTe grains following a CdCl2 treatment by 50% but weakens the adhesion to the substrate. The introduction of Na between the Mo back contact and CdTe layer promotes the formation of large faceted grains for the as-grown films with an average grain size ten times larger than in the case of Na free deposition while maintaining good adhesion to the substrate. There is no further grain growth following CdCl2 treatment however the CdTe grain size is still double that of the Na-free samples.

Internal fatigue crack initiation in drawn Ti–6Al–4V wires

Fatigue cracks in titanium alloys are often found to initiate at faceted alpha grains. In the very high cycle fatigue regime, crack initiation tends to shift from the surface towards the interior of the material, and more initiation facets can be found on the fracture surface. In this study, fatigue tests were performed on drawn and heat-treated Ti–6Al–4V wires. Only a few samples fractured due to interior initiation. The facets at the initiation sites of these samples were not flat, but had markings on the nano-scale, and were highly inclined. A possible explanation for these aspects is the crystallographic texture of the wire, and a reflection is made on the suggested mechanisms of facet formation.This paper is part of a Themed Issue on Euromech 570: Interface-dominated materials.

Microwave dielectric properties of potassium sodium niobate ceramics with different K/Na ratios

Potassium sodium niobate (KxNa1−xNbO3) ceramics with different K/Na ratios were prepared by traditional solid reaction method and the complex permittivity at 8.2–12.4GHz was measured in this study. Then the complex permittivity at GHz was compared with that at low frequency (KHz and MHz). And the influences of phase structure and microstructure on the dielectric properties were investigated. A lattice parameter discontinuity at x=0.5 was found and the ceramic with x=0.5 has well faceted and homogeneous grain size distribution compared with others. In addition, the reflection loss of the ceramics was calculated. All the ceramics possess better absorption property at the frequency around 11GHz, while the effective absorption bandwidth is narrow. The ceramic with x=0 shows the best absorption property because of its relatively low dielectric permittivity.

Coarsening of boron carbide grains during the infiltration of porous boron carbide preforms by molten silicon

This work investigates the coarsening of boron carbide grains during the infiltration of porous boron carbide preforms by molten silicon with respect to fabrication of reaction-bonded boron carbide ceramics. Experimental results reveal that the shape of boron carbide grains evolve from the irregular shape to faceted shape due to dissolution-precipitation during infiltration. For infiltration temperatures below 1750°C, the boron carbide grains are irregular and exhibit an unimodal size distribution, which can be ascribed to the normal grain growth. The growth of the irregular grains follow a cubic law of diffusion control. In contrast, for infiltration temperatures above 1750°C, the boron carbide grains become faceted and exhibit a bimodal size distribution, indicative of the typical abnormal grain growth. The abnormal growth of faceted grains is proposed to be controlled by coalescence-enhanced two-dimensional nucleation.

Microstructural influence on piezoresponse and leakage current behavior of Na0.5Bi0.5TiO3 Thin Films

AbstractSodium bismuth titanate (Na0.5Bi0.5TiO3: NBT) a lead free piezoelectric; exhibits promising features such that it could be an alternate to lead based piezoelectrics. In this work, we report the microstructural influence on piezoelectric and leakage current behavior of NBT thin films grown by pulsed laser ablation (PLD). Various microstructural features like coarse faceted grains and fine spherical grains was achieved by effective optimization of substrate temperature and oxygen partial pressures. The studies reveals that, leakage current of NBT thin films were dominated by interface limited modified Schottky emission type of conduction. The piezoelectric domain studies reveal that for NBT thin films with fine spherical grain the domain pattern was highly dominated by the morphology and in the case of coarse faceted grains the domains were relatively large and the domains were extending beyond the grain boundaries.

Tuning the Morphology of Te 1D Nanostructures By Template-Free Electrochemical Deposition in an Ionic Liquid

Tellurium and its alloys are used in many applications such as photoconductors, piezoelectric devices, thermoelectric generators and coolers [1]. For thermoelectric applications, the transport properties (thermal conductivity, electrical resistivity, Seebeck coefficient) of tellurium and its compounds can be improved by nanostructuring [2]. One-dimensional nanostructures like nanowires [3] and core-shell nanowires [4] are particularly promising due to the high phonon interface scattering and the blocking of the phonon conduction along the wire axis which lead to an increase of the thermoelectric conversion efficiency. Te nanowires can represent a base to create core-shell structures, being the core that could be further capped with a shell to get arrays of Te-based thermoelectric nanostructures like BiTe [5], AgTe [6] or LaTe [7]. Template assisted electrodeposition is a low cost technique widely used in literature for the synthesis of nanowires but implies several preparation steps and the removal of the template after the growth of the nanowires. The aim of this work is to develop a template free synthesis route that would be more convenient. For this purpose, Room-Temperature Ionic Liquids (RTILs) appear as promising solvents. Indeed, it has been highlighted that they can act as stabilizing agents for nanoparticles and nanostructured films synthesis [8]. In a previous work ([9]), our laboratory showed that Te nanowires can be obtained in an ionic liquid binary mixture: 1-ethyl-1-octyl-piperidinium bis(trifluoromethylsulfonyl)imide:1-ethyl-1-octyl-piperidinium bromide (EOPipTFSI:EOPipBr). In the present work, several morphologies were obtained by varying the synthesis parameters (applied fixed potential, Te(IV) concentration, electrolyte composition). Under charge transfer control, micrometer faceted grains are formed. Under diffusion control, the deposits are composed of nanowires, which are single crystalline as proven by SAED (Selected Area Electronic Diffraction) and TEM (Transmission Electron Microscopy) analyses. In highest tested overpotential conditions, nanowires turn progressively into crystallized hollow nanostructures (HNs) when the coulometric charge increases. This phenomenon is more accentuated at low concentration and disappears when using a soluble anode, showing that the formation of HNs is due to a limitation by species supply. The geometry of HNs, with a high surface to volume ratio, is particularly interesting to create high performances thermoelectric materials. Indeed, this geometry could further decrease the lattice part of the thermal diffusion coefficient by phonon confinement. By decreasing the bromide content of the electrolyte, hair-like single crystalline nanowires with a high aspect ratio can be obtained. HRTEM (High Resolution Transmission Electron Spectroscopy) analyses reveal a growth along the c-axis, with a smooth interface. No surface contaminants were observed by STEM (Scanning Transmission Electron Microscopy), proving that halogenide ions do not play the role of capping agents, unlike observed in other works [10].

Characteristics of Reactive Ni3Sn4 Formation and Growth in Ni-Sn Interlayer Systems

The near-isothermal growth and formation of Ni3Sn4 intermetallic compounds (IMC) in Ni-Sn interlayer systems was studied in the solid state at 473 K (200 °C) and under solid–liquid conditions at 523 and 573 K (250 °C and 300 °C) from an initial state of a few seconds. Scalloped solid-state IMC formation was mainly driven by grain boundary diffusion of Ni through the IMC layer combined with the grain coarsening of the IMC layer. Under solid–liquid conditions, the formation of faceted and needle-shaped Ni3Sn4 grains as well as an atypical IMC growth behavior with similar parabolic growth constants for 523 K and 573 K (250 °C and 300 °C) was observed within the first 180 seconds of the holding time, and IMC growth occurred as an isothermal solidification from the Ni-saturated Sn melt. Due to the progressive densification of the IMC layer and the diffusion-controlled growth, the kinetics slowed down by approximately one order of magnitude after 180 seconds of annealing. The final stage was characterized by the formation of IMC islands ahead of the interfacial Ni3Sn4 layer. Needle-like IMC growth was effectively suppressed under combined solid-state and solid–liquid conditions. Textured Ni3Sn4 IMC formation at the Ni-Sn interface was approved with pole figure measurements. The activation energy Q for solid–liquid IMC formation was calculated as 43.3 kJ/mol, and processing maps for IMC growth and Sn consumption were derived as functions of temperature and time, respectively.

Step Coalescence by Collective Motion at an Incommensurate Grain Boundary.

Using extended time series scanning transmission electron microscopy, we investigate structural fluctuations at an incommensurate grain boundary in Au. Atomic-resolution imaging reveals the coalescence of two interfacial steps, or disconnections, of different height via coordinated motion of atoms along close-packed directions. Numerical simulations uncover a transition pathway that involves constriction and expansion of a characteristic stacking fault often associated with grain boundaries in face-centered cubic materials. It is found that local atomic fluctuations by enhanced point defect diffusion may play a critical role in initiating this transition. Our results offer new insights into the collective motion of atoms underlying the lateral advance of steps that control the migration of faceted grain boundaries.

Synthesis of sodium beta alumina films by heat treatment of sodium aluminum oxides

Sodium beta alumina (Na-β-alumina) films were synthesized by heat treatment of NaAl6O9.5 and γ-NaAlO2 films at temperatures of 1 373-1 573 K. Single-phase γ-NaAlO2 and NaAl6O9.5 films were prepared by laser chemical vapor deposition at the deposition temperatures of 976 and 1 100 K, respectively. Subsequent heat treatment of the films resulted in the formation of Na-β-alumina with α-Al2O3 at temperatures above 1 373 K for NaAl6O9.5 and 1 473 K for γ-NaAlO2. On heat treatment at temperatures of 1 473-1 573 K, the faceted morphology with terraces of the as-deposited (110)-oriented γ-NaAlO2 films transformed to a porous morphology with platelet grains comprising Na-β-alumina and α-Al2O3. On heat treatment at temperatures of 1 373-1 473 K, the pyramidal, faceted grains of as-deposited NaAl6O9.5 films transformed to planer, shapeanisotropic morphology in the film of mixed Na-β-alumina and α-Al2O3. A dense morphology was observed in both the as-deposited and heat-treated NaAl6O9.5 films.

Solvent-molecule-mediated manipulation of crystalline grains for efficient planar binary lead and tin triiodide perovskite solar cells.

Binary lead and tin perovskites offer the benefits of narrower band gaps for broader adsorption of solar spectrum and better charge transport for higher photocurrent density. Here, we report the growth of large, smooth crystalline grains of bianry lead and tin triiodide perovskite films via a two-step solution process with thermal plus solvent vapor-assisted thermal annealing. The crystalline SnxPb1-xI2 films formed in the first step served as the templates for the formation of crystalline CH3NH3SnxPb1-xI3 films during the second step interdiffusion of methylammonium iodide (MAI). Followed by dimethylsulfoxide (DMSO) vapor-assisted thermal annealing, small, faceted perovskite grains grew into large, smooth grains via the possible mechanism involving bond breaking and reforming mediated by DMSO solvent molecules. The absorption onset was extended to 950 and 1010 nm for the CH3NH3SnxPb1-xI3 perovskites with x = 0.1 and 0.25, respectively. The highest PCE of 10.25% was achieved from the planar perovskite solar cell with the CH3NH3Sn0.1Pb0.9I3 layer prepared via the thermal plus DMSO vapor-assisted thermal annealing. This research provides a way to control and manipulate film morphology, grain size, and especially the distribution of metal cations in binary metal perovskite layers, which opens an avenue to grow perovskite materials with desired properties to enhance device performance.