Growth Rate Anisotropy Research Articles

Fabrication of high-permeance SiC filters reinforced with in situ-grown mullite whiskers for gas/solid filtration

The pursuit of cost-efficiency, superior strength, and excellent gas permeance has driven the development of silicon carbide (SiC) membranes or filters for gas/solid filtration. In this study, SiC filters with high gas permeance reinforced with in situ-grown mullite whiskers were successfully prepared through reaction bonding. Low-cost kaolin and bauxite were used as sintering aids and raw materials for mullite whisker formation, and MoO3 was used as an additive to promote mullite whisker growth. The effects of different sintering temperatures and the addition of kaolin, bauxite, and MoO3 on filter performance were systematically investigated. When the sintering aid and MoO3 content additions were 6 and 1.5 wt%, respectively, and the sintering temperature was 1450 °C, the resulting ceramic filter exhibited a porosity of 45.5 %, a bending strength of 17.9 MPa, and an average pore size of 47.7 µm. Moreover, the filter exhibited a high Darcy’s permeability coefficient of 2.42 × 10−11 m2, satisfactory chemical stability, and thermal shock resistance. The addition of MoO3 facilitated the consumption of the molten phases and increased the anisotropic growth rate of the mullite whiskers, which contributed to the fabrication of the high-permeance filters. Finally, the ceramic filters demonstrated good performance for the filtration of dusty gases loaded with fly ash particles with a mean size of 15.3 µm at room temperature and 300 °C, and the removal rate reached > 99.9 %. The pressure drop was a little lower than that of our commercially available SiC filters. The SiC filters showed good prospects in the field of gas/solid filtration.

Impact of ITO layer on the spatial optical distribution of semipolar (20-21) InGaN/GaN multiple quantum wells with surface morphology.

Textured surface with micro-facets have been widely observed in semipolar and nonpolar III-nitride heterostructures, mainly resulted from the anisotropic growth rate in the growth plane. Polarization and the intensity distribution of surface emissions are both affected by the surface morphology. The indium tin oxide (ITO) layer, serving as the current spreading layer, are usually employed to enhance the current injection efficiency and light extraction efficiency in III-nitride emitters. For semipolar orientation, the introduction of an ITO layer could weaken the anisotropic optical property, especially for the spatial intensity distribution. This paper reports the influence of the ITO layer on the spatial intensity distribution of semipolar (20-21) InGaN/GaN multiple quantum wells. The intensity distribution could be shaped from a rectangular-like pattern to a circular-like pattern with the deposition of an ITO layer. The ITO layer allows more light along the [11-20] direction to emit out at a small angle with respect to the surface normal. By further increasing the ITO thickness, the influence of surface fluctuation of semipolar sample decreases, leading to an improvement in the proportion of the light at small angles and a slight decrease in the overall integrated intensity of whole far field. These results will help pave the way to high-performance semipolar emitters with great potential in general illumination and backlighting.

A Phase-Field Study of Spinodal Decomposition Impeded by Irradiation in U-Mo and U-Mo-Zr Alloys.

The phase-field method, coupled with the micro-elastic model and irradiation-induced cascade mixing model, has been employed to investigate the spinodal decomposition in U-Mo and U-Mo-Zr alloys. The microstructure evolution of U-Mo or U-Mo-Zr alloys under different initial conditions, such as the alloy composition, aging temperature and irradiation intensity, were simulated to study the effect of cascade mixing on the miscibility gap, morphology and volume fraction of the decomposed phases. The simulation results demonstrate that irradiation-induced cascade mixing impedes the process of spinodal decomposition, and that irradiation shrinks the composition range of the miscibility gap in the alloys. Irradiation-induced cascade mixing slows down the anisotropic growth rate of the spinodal decomposition, yet this phenomenon can be weakened with increasing aging temperature. Adding an appropriate amount of Zr to a U-Mo alloy can effectively prevent the contraction of the miscibility gap caused by irradiation.

24% Single‐Junction GaAs Solar Cell Grown Directly on Growth‐Planarized Facets Using Hydride Vapor Phase Epitaxy

AbstractA 24%‐efficient single‐junction GaAs solar cell grown directly on a faceted, spalled (100) GaAs substrate after in situ planarization growth by hydride vapor phase epitaxy (HVPE) is achieved. Controlled spalling, a promising low‐cost substrate reuse technique, produces large facets in (100)‐oriented GaAs substrates due to the orientation of the fracture planes used for lift‐off. Planarization by HVPE offers a path toward direct use of these spalled substrates without costly polishing steps. Here, the growth rate anisotropy enabling planarization arising from diffusion and differences in the adsorption of growth species on {n11}B‐type facets relative to (100) is determined. Consecutive planarization and device growth that results in a solar cell with a minimal performance difference relative to a control cell grown on an epitaxy‐ready substrate are demonstrated. These results show that controlled spalling coupled with HVPE planarization is a viable pathway for lowering the cost of III‐V photovoltaics.

Study of the Formation of Precursor Clusters in an Aqueous Solution of KH2PO4 by Small-Angle X-ray Scattering and Molecular Dynamics

The structure of an aqueous solution of potassium dihydrogen phosphate (KH2PO4, KDP) was studied by small-angle X-ray scattering (SAXS) and molecular dynamics (MD). According to SAXS data, the octameric species (KH2PO4)8 are formed in solution in addition to K+, (H2PO4)−, and KH2PO4, while the presence of other types of oligomers is not observed. When the temperature drops below the saturation temperature (~60 °C), the volume fraction of octamers increases sharply, reaching 50% at 4 °C. The results of MD calculations of the temporal stability relationships of dimers (KH2PO4)2, tetramers (KH2PO4)4, and octamers (KH2PO4)8 show that the dimers and tetramers disintegrate rapidly (50–100 ps), while the octamers remain stable. A comparative analysis of the bonds between the octamers and the KDP crystal lattice was carried out when the octamer was inserted during crystal growth in the directions [001] and [100]. The possible relationship of the obtained results with the changes in the anisotropy of growth rates (habitus) of KDP crystals at different degrees of supersaturation is discussed.

Modeling of the Atomic Structure in the Vicinity of Spherical Voids in Aluminum and the Calculation of Anisotropy of the Void Growth Rate

Modeling of the Atomic Structure in the Vicinity of Spherical Voids in Aluminum and the Calculation of Anisotropy of the Void Growth Rate

Simulation of the atomic structure near spherical void in aluminum and the calculation of the anisotropy of the void growth rate

We simulate structure in the vicinity of different size nanovoids using a new variant of the Molecular Statics, wherein atomic structure in the vicinity of nanovoids and the parameters that define the displacements of atoms placed in elastic continuum around main computation cell are determined in a self-consistent manner. Then, the previously obtained kinetic equations are applied to calculate the shifting rate of the elements of the void surface in certain crystallographic directions. These equations take into account the dependence of the vacancy flux on the deformation fields. The displacement rates in different crystallographic directions for bcc and fcc metals are significantly different. The results show that the effects studied by computer simulation can lead to a change in the shape of initially spherical nanopores and cause their transformation into cuboidal pores in metals with a cubic structure under irradiation.

Fatigue crack growth behavior in an aluminum alloy Al–Mg–0.3Sc produced by wire based directed energy deposition process

AbstractAdditive manufacturing (AM) of Al–Mg–Sc alloys has received considerable interest from the aerospace industry owing to their high specific strength and suitability for AM processes. This study has investigated the fatigue crack growth behavior in an Al–Mg–0.3Sc alloy made by wire and arc additive manufacturing. Tests were conducted with two different crack orientations at cyclic load ratios of 0.1 and 0.5. At the lower load ratio, the horizontal crack showed a faster growth rate owing to the smaller grains and coarser second‐phase particles that the crack tip had encountered when it propagated along the material build direction. The anisotropy in crack growth rate was mainly caused by the grain size effect. When the applied stress intensity factor range exceeded the value of 10 MPa m1/2, an isotropic crack growth rate between the two crack orientations was measured. This is due to the microstructural influence being overcome by the governing parameter of fracture mechanics. At the higher load ratio of 0.5, crack growth rate is isotropic, and the threshold stress intensity factor range was much lower than that tested under load ratio 0.1. Finally, the modified Hartman–Schijve equation has been successfully employed to represent the crack growth rates in all three regions.

Analysis of Crystalline Defects Caused by Growth on Partially Planarized Spalled (100) GaAs Substrates

We analyze the effect of growth on non-(100) surfaces resulting from incomplete planarization of spalled GaAs wafers on the defect structure of GaAs solar cell layers grown by hydride vapor phase epitaxy (HVPE). Controlled spalling of (100)-oriented GaAs has the potential to reduce substrate costs for III-V epitaxy; however, it creates regularly faceted surfaces that may complicate the growth of high-quality III-V optoelectronic devices. We leverage the anisotropic growth rate of HVPE to planarize these faceted GaAs substrates, reducing the surface roughness and degree of faceting. We observe degraded solar cell performance and material quality in sample areas where facets are not completely removed. We used dark lock-in thermography and photoluminescence to identify recombination in areas that were not fully planarized. We used cathodoluminescence to identify the presence of extended defects in these regions, which are correlated with bandgap fluctuations in the material. We hypothesize that these defects were created by strain from compositional fluctuations in ternary alloys grown on the faceted surfaces. This work elucidates the potential issues of solar cells grown on faceted surfaces and builds understanding toward realizing high performance III-V photovoltaics with the cost-reduction potential of controlled spalling.

Highly Uniform Array of Hexagonally Symmetric Micro‐Pyramid Structures for Scalable and Single Quantum Dot Emitters (Adv. Mater. Interfaces 8/2023)

Uniform Single Quantum Dot Emitters Controlling the position, size and shape of group III-nitride quantum dots is critical for the development of scalable single quantum emitters operable at room temperature. In article number 2202085, Yong-Hoon Cho and colleagues present a methodology for highly uniform, site-controlled GaN micro-pyramidal structures with a high degree of hexagonal symmetry by adjusting growth conditions under the self-limited growth mechanism. By successively growing a thin InGaN layer on the uniform pyramidal arrays with growth rate anisotropy, high-performance single photon emission from the apex of each pyramid can be achieved with suppressed background emission from side quantum wells of the pyramids.

Experimental investigation on anisotropic fatigue crack growth characteristics of Q420C steel

The hot-rolled steel sheets often exhibit anisotropy in their mechanical characteristics due to the grain’s elongation along the rolling orientation. The anisotropy characteristic also appears in the fatigue crack growth behaviour. However, it has not been thoroughly studied for some types of steel materials although they have been widely used for wind turbine towers. In this study, the longitudinal fatigue crack growth behaviour of the hot-rolled steel sheet (Q420C) is investigated by an experimental programme. A total of six specimens were tested under constant amplitude loading cases with different load ratios. It was found that longitudinal cracks are approximately 15% faster than transverse cracks at the load ratio of 0.1; at the load ratio of 0.3, they are approximately 22% faster. The anisotropy of crack growth rates is attributed to differences in the microstructure of the material. A comparison of the crack growth rate is carried out for Q420C steel and some widely used American-made steel materials. Furthermore, the applicability of design curves in the British Standard BS 7910 is explored, the crack closure phenomenon is analysed and the fatigue crack growth mechanism is discussed.

Highly Uniform Array of Hexagonally Symmetric Micro‐Pyramid Structures for Scalable and Single Quantum Dot Emitters

AbstractControlling the site, size, and shape of group III‐nitride quantum dots (QDs) is critical for the development of mass‐producible single‐photon sources for scalable quantum technologies operable at room temperature. Herein, a methodology is proposed for fabricating high‐purity single QD emitters by controlling site‐controlled GaN micro‐pyramid structures with a high degree of uniformity and symmetry. To achieve a uniformly grown, hexagonally symmetric micro‐pyramid array, the H2/N2 carrier gas ratio, growth temperature, and V/III ratio are controlled to attain self‐limited growth regime and self‐limited width at the GaN pyramid apex. A thin InGaN layer is consecutively grown on a pyramid array under the growth condition for enhancing the growth rate anisotropy to hinder the growth of InGaN quantum wells (QWs) at semi‐polar facets. As a result, single‐photon emission is observed from apex QD with suppressed background side QW emission while maintaining more than 90% high hexagonal QD symmetry over the large area of the wafer.

Tuning the microstructure and mechanical properties in nanocrystalline Cr coatings by sputtering power

A significant sputtering power effect on the microstructure and mechanical properties was investigated in the nanocrystalline Cr coatings that were prepared by employing magnetron sputtering. Experimental results demonstrated that all the Cr coatings exhibited a columnar grain structure with preferred (110) out-of-plane orientation. However, with raising the sputtering power from 50 to 250 W, the surface morphology changed from the faceted pyramid to vermiform-like shape due to the anisotropy of growth rate along different crystallographic plane. Meanwhile, raising sputtering power also increased the grain size and coating thickness, between of which followed a power-law relationship, indicative of an evolutionary overgrowth mechanism. In addition, both the nanoindentation hardness and elastic modulus of Cr coatings exhibited a monotonic increment with reducing the grain size. This size-dependent strengthening behavior was rationally interpreted by the classical Hall-Petch theory, and a grain coalescence model for columnar grain morphology was developed to quantitatively describe the size effect on elastic modulus of nanocrystalline Cr coatings.

Kinematics of Crystal Growth in Single‐Seal Syntaxial Veins in Limestone ‐ A Phase‐Field Study

AbstractBuilding on recent developments in phase‐field modeling of structural diagenesis, we present an analysis of single‐seal syntaxial calcite vein microstructure in a variety of limestones. We focus on the effects of fracture aperture, intergranular versus transgranular fracturing, crystal habit and the presence of second phases in the host rock, to systematically investigate a simplified set of models covering the main classes of limestone in 2D. We incorporate the kinematic process of growth competition between differently oriented crystals, growth rate anisotropy between rough and faceted crystal surfaces and different growth rates on intergranular to transgranular fractures. Results show that within the considered parameter space we can reproduce a wide range of vein microstructures in limestone known in nature, such as stretched crystals, wide‐blocky veins, and elongated crystals. We identify five archetypes of vein microstructures in limestones, which are diagnostic for different kinematics and evolution of transport processes and illustrate the effect of key parameters in microstructure maps. We show how syntaxial veins with median line form after intergranular fracturing, while stretched crystals indicate transgranular fracturing. Intergranular fracturing leads to stronger growth competition and more prominent CPO in syntaxial veins. Our results can be extended to 3D to include multiple crack‐seal events, pore‐space cementation and simulation of fluid flow, providing a generic platform for modeling structural diagenesis in limestones.

Lateral Growth of Uniformly Thin Gold Nanosheets Facilitated by Two-Dimensional Precursor Supply.

The nanosheets of highly symmetric materials with a face-centered cubic lattice such as gold have been synthesized by adsorbing the precursors on a flat surface, whose chemical specificity induces the anisotropy of growth rates. We have succeeded in the fabrication of gold nanosheets in a hydrophilic space inside highly separated bilayers, which work as two-dimensional hydrophilic reactors, in a hyperswollen lamellar liquid crystalline phase of an amphiphile solution. One of the physical properties, amphiphilicity, confines the ingredients therein. The nanosheets can only grow in the in-plane direction due to the inhibition of the out-of-plane growth rather than the anisotropy of growth rates probably. Thus, the synthesis can be accelerated; the particles can be completed within 15 min. As not relying on chemical specificity, silver nanosheets could also be synthesized in the same way. The suspension of gold and silver nanosheets without any amphiphiles could be obtained, and the solvent is replaceable. We found that the width of the obtained gold nanosheets is proportional to the Reynolds number of the solution because the area of the bilayer in the hyperswollen lamellar phase depends on shear stress. This implies that the areas of gold nanosheets depend on the areas of the bilayers, and it can be controlled by changing the Reynolds number. This method could be widely used to continuously obtain large-area nanosheets of various materials in a roll-to-roll manufacturing process.

Fatigue crack growth behavior of Ti-6Al-4V alloy fabricated via laser metal deposition: Effects of building orientation and heat treatment

This work studied the effect of building orientations and post-process heat treatments on the fatigue crack growth behavior of Ti-6Al-4V manufactured by laser metal deposition. The fatigue crack growth rate experiments were carried out in three sampling directions (YZ, ZY, XY) in order to evaluate the anisotropy. In addition, the heat-treated samples (YZ, ZY, XY) were used for comparison, which were heat-treated under the vacuum at 900 ℃ for 2 h followed by air cooling and 550 ℃ for 4 h followed by air cooling. The results showed that the ∆KT values before and after heat treatment were 13.6 and 15 MPa∙m1/2, respectively. In the hypo-transitional region, the effect of columnar grains orientation and uniaxial residual stress led to anisotropy in fatigue crack growth rate for as-built specimens. This effect is significantly attenuated after the post-process heat treatment. The coarsened α laths effectively changed the crack propagation behavior and improve the fatigue crack growth resistance of the material. In the hyper-transitional region, the fatigue crack growth was controlled by the inherent fatigue crack growth resistance rather than the microstructure, resulting in negligible differences in all fatigue crack growth rate curves.

The growth of a large GdPO4 crystal guided by theoretical simulation and the study of its phonon properties.

It has remained a big challenge to grow large crystals of compounds at very anisotropic crystal growth rates. GdPO4 is such an example, which usually grows into a needle-like shape. This work solved this problem by using Li+ ions, predicted by first-principles calculations, to adjust the crystal morphology of GdPO4. Based on the calculated surface energies and the polar properties of the surfaces, we identified two crystal planes, i.e. (-102) and (-111), which can be used to adjust the morphology of the GdPO4 crystal by using extrinsic cations in the flux system. With a predicted Li containing flux system, Li2O-MoO3-B2O3, a large crystal with a record size, 10.0 × 9.0 × 18.2 mm3, was grown by using the top-seeded solution growth (TSSG) method. Based on the grown large crystal and the polarized Raman measurements, four new Raman bands missing in early studies and other bands were unambiguously assigned. By introducing a new formula, the phonon-phonon interactions were shown to be weak in the temperature range from 83 to 803 K. Our study indicates that the crystal of GdPO4 can be used as a promising laser host material working under relatively high temperature conditions.

Modeling the atomic structure in the vicinity of the spherical voids and calculation of void growth rate anisotropy in bcc iron and tungsten

We study the influence of the atomic structure in the vicinity of voids on their growth rate anisotropy. In the first part, we model the atomic structure in the vicinity of nanovoids in α-Fe and W using the advanced Molecular Statics method. In the second part, we use the earlier obtained equations that taking the influence of elastic fields into account to calculate the shifting rate of the void surface elements, and to evaluate the components of the strain tensor we use the atomic structure modeling results from the first part. The calculations are performed for voids of several sizes at certain oversaturation’s in a wide temperature range. The simulation results for the mentioned metals with a bcc structure show that displacements of atoms located along the crystallographic directions of the <100>, <110>, <111> types in the vicinity of the voids are significantly different, and this anisotropy of atom displacements leads to a reduction of spherical symmetry for the shifting rate of the surface elements. As the result, the initially spherical void shape becomes faceted.

Zn2+-Pb2+-doped calcite shrub fabrics: Abiotic morphogenesis of travertine-like dripstone encrustation at the Jersey Zinc Mine, southeastern British Columbia

A cm-thick calcite dripstone on the floor of the abandoned Jersey Zinc Mine in southeastern British Columbia, western Canada, consists of a mixture of shrub and radial fibrous elongated columnar crystal fabrics that resulted from Zn2+ and Pb2+ doping of the calcite crystallographic lattice structure. These heavy metal elements were sourced from sulfide mineralized veins in the overlying limestone beds. Zn2+ and Pb2+ substitution for Ca2+ reduced the size of the calcite crystal structure, impacting configuration of the calcite crystal fabrics. Lower concentration levels of dopants resulted in a radial fibrous elongated columnar crystal fabric with high inter-crystalline porosity. Increased levels of Zn2+ and Pb2+ dopants resulted in a crystal splitting process forming the shrub fabric. Growth of normally flat rhombohedral face on the external surface of the dripstone encrustation was disrupted by mixtures of isotropic and anisotropic growth rates. Inhibited growth rates resulted in early stage crystal splitting processes, forming parallel arrays of precursor crystallites aligned as step-down micro-terraces separated by dam-like micro-barrage partitions with normal growth rates. Shrub fabrics developed below the encrustation surface as ion-laden dripwater films infiltrated the 20% to 25% inter-crystalline porosity. This early stage of crystal splitting transitioned into a shrub fabric substrate, each consisting of fan-shaped crystal arrays distributed along a main stem, all in optical continuity. Extreme levels of Zn2+ dopant resulted in split crystal formation of micro-spherulites on the external surface of the dripstone. Spherulite neomorphism occurred, resulting in replacement of the crystallites by an individual calcite spar during envelopment by the columnar crystal domain fabric. The spheroid forms were preserved within the columnar fabric as spheroid zones of Zn-calcite and outlined by microcrystalline ferroan calcite. The shrub fabrics have morphological similarity to dendritic branching shrubs commonly associated with biotic and abiotic travertine and some biotic tufa deposits, which form under very different depositional conditions. Abiotic Zn2+-Pb2+ dopants absorbed into the calcite crystal structure of this mine floor encrustation precluded the necessity for carbonate precipitation from supersaturated carbonate water or precursor microbial induced nucleation sites, unlike similar travertine and tufa shrub fabrics elsewhere.

Controlling the Morphology and Titanium Coordination States of TS-1 Zeolites by Crystal Growth Modifier.

Developing an effective strategy to synthesize perfect titanosilicate TS-1 zeolite crystals with desirable morphologies, enriched isolated framework Ti species, and thus enhanced catalytic oxidation properties is a pervasive challenge in zeolite crystal engineering. We here used an amino acid l-carnitine as a crystal growth modifier and ethanol as a cosolvent to regulate the morphologies and the Ti coordination states of TS-1 zeolites. During the hydrothermal crystallization process, the introduced l-carnitine can not only tailor the anisotropic growth rates of zeolite crystals but also induce the formation of uniformly distributed framework Ti species through building a suitable chemical interaction with the Ti precursor species. Condition optimizations could afford the generation of perfect hexagonal plate TS-1 crystals and elongated platelet TS-1 crystals enriched in tetrahedral framework Ti sites (TiO4) or mononuclear octahedrally coordinated Ti species (TiO6). Both samples showed significant improvement in catalytic activity for the H2O2-mediated epoxidation of alkenes. In particular, the elongated platelet TS-1 enriched in "TiO6" species afforded the highest activity in 1-hexene epoxidation, with a turnover frequency (TOF) of up to 131 h-1, which is approximately twice as high as that of the conventional TS-1 zeolite (TOF: 65 h-1) and even higher than those of the literature-reported TiO6-containting TS-1 catalysts derived from the hydrothermal post-treatment of TS-1 zeolites. This work demonstrates that the morphologies and the titanium coordination states of TS-1 zeolites can be effectively tuned by directly introducing suitable crystal growth modifiers, thus providing new opportunities for developing highly efficient titanosilicate zeolite catalysts for important catalytic applications.