Appearance Of Facets Research Articles

Faceting mechanisms of GaN nanopillar under KOH wet etching

This study presents strategies for achieving GaN pillars with the desired m- (or even a-) oriented nonpolar facets through a top-down approach that combines plasma etching followed by room temperature KOH wet treatment processes. Indeed, GaN etching in KOH solution is an anisotropic process, meaning that it allows the appearance of stable facets at the macroscopic scale, while atomic processes, such as step-flow, drive the fundamental mechanisms of the wet etching at the microscopic scale. Our study highlights the key role played by both the shape (circular or hexagonal, aligned with either the m-planes or the a-planes) and the roughness of the hard mask in determining the resulting crystalline facet formation and their associated roughness. Furthermore, it underscores the importance of the GaN pillar profiles (reentrant, straight, tapered) after plasma patterning, as they strongly influence the subsequent wet etching mechanisms. Ultimately, the article demonstrates that smooth m-oriented facets can be achieved by using room temperature wet KOH (44 wt%) on slightly sloped GaN profiles after plasma etching, in conjunction with the use of hexagonal m-oriented hard masks.

Structurally colored silica foams using colloidal templating

We produce solid silica dry foam structures that exhibit structural color. For this purpose, we use densely packed colloidal microgels decorated with silica nanoparticles that are much smaller than the microgels. By concentrating and overpacking the microgel particles, we can control their morphology so that the silica nanoparticles spontaneously form honeycomb-like structures or networks. We obtain pure silica dry foam structures by burning the organic polymer, leaving behind silica networks. The resulting foams exhibit vivid structural colors. We can control the color by changing the microgel particles’ size and degree of compression. Furthermore, thanks to the softness of the particles, compression can also lead to the appearance of facets. This faceting behavior distinguishes microgels from hard spherical particles. We present details about the synthesis and fabrication strategies and report on a comprehensive characterization of optical and structural properties. Our study provides insight into the fabrication of innovative self-assembled materials with structural colors by colloidal templating.

Friction surfacing assisted refilled friction stir spot welding of AA6061 alloy and Q235 steel

The AA6061 alloy and Q235 steel were well joined by refilled friction stir spot welding without stirring steel, as a 1 mm thick coating was deposited on the steel by friction surfacing before welding. The coating was composed of highly refined grains. The materials experienced shear deformation in the stir zones and the extrusion in hook and thermo-mechanical affected zones, resulting different predominant textures in the weld. A Fe-rich layer was depicted from the interface due to the enhanced atomic diffusion, and the layer's thickness increased as the sleeve plunged deeper during welding. Al-Fe intermetallics, including Al13Fe4, Al5Fe2, formed in the Fe-rich layer, since the solubility of Fe in Al was far lower than the measured content. The optimal shear strength reached 7.31 kN in this study. The welds fractured along the weld boundary and the Al/steel interface. The fracture initiated at the edge of the gap between AA6061 plate and coating, and propagated along the weld boundaries, resulting in the shear dimples and the faceted appearance. However, the spot weld with 2.9 mm penetration was fractured along the Al/steel interface mainly, which was attributed to the over formed intermetallics at the interface.

Epitaxial growth of β -Ga2O3 on (110) substrate by plasma-assisted molecular beam epitaxy

Epitaxial growth of β-Ga2O3 films on (110) substrates has been performed via plasma-assisted molecular beam epitaxy (PAMBE). The atomic force microscopy scan shows a very low root mean square roughness of 0.08 nm for the surface of the as-received (110) substrates. High-resolution x-ray diffraction measurements reveal a 2.5 nm/min growth rate of β-Ga2O3 films on (110) substrates for conventional PAMBE growth conditions (∼700 °C), which is comparable to that on (010) substrates. The surface morphology of β-Ga2O3 epitaxial films is smooth and has a similar dependence on Ga flux to (010) growth. However, the (110) plane does not have a tendency to show a well-defined step–terrace structure in spite of the appearance of (110) facets in the growth of (010) β-Ga2O3. Indium catalyzed growth was also demonstrated to improve the growth rate up to 4.5 nm/min and increase the maximum growth temperature up to 900 °C of (110) β-Ga2O3.

Shapes of drums with lowest base frequency under non-isotropic perimeter constraints

We study the minimizers of the sum of the principal Dirichlet eigenvalue of the negative Laplacian and the perimeter with respect to a general norm in the class of Jordan domains in the plane. This is equivalent (modulo scaling) to minimizing the said eigenvalue (or the base frequency of a drum of this shape) subject to a hard constraint on the perimeter. We show that, for all norms, a minimizer exists, is unique up to spatial translations and is convex but not necessarily smooth. We give conditions on the norm that characterize the appearance of facets and corners. We also demonstrate that near minimizers have to be close to the optimal ones in the Hausdorff distance. Our motivation for considering this class of variational problems comes from a study of random walks in random environment interacting through the boundary of their support.

Microstructural influence on fatigue crack propagation during high cycle fatigue testing of additively manufactured Alloy 718

A study of the microstructure of additively manufactured Alloy 718 was performed in order to better understand the parameters that have an influence on the fatigue properties of the material. The specimens were manufactured using two powder bed fusion techniques – Electron Beam Melting (EBM) and Selective Laser Melting (SLM). Four point bending fatigue tests were performed at room temperature with a stress ratio of R = 0.1 and 20 Hz frequency, on material that was either in hot isostatically pressed (HIP) and solution treated and aged (STA) condition or in STA condition without a prior HIP treatment. The grains in the SLM material in the HIP + STA condition have grown considerably both in the hatch and the contour regions; EBM material, in contrast, shows grain growth only in the contour region. Fractographic analysis of the specimens in HIP + STA condition showed a faceted appearance while the specimens in STA condition showed a more planar crack appearance. The crack propagation occurred in a transgranular mode and it was found that precipitates such as NbC, TiN or δ-phase, when present, did not affect the crack path. The areas with larger grains corresponded to the faceted appearance of the fracture surface. This could be attributed to the plastic zone ahead of the crack tip being confined within one grain, in case of the larger grains, which promotes single shear crack growth mode.

Implications of dynamic strain aging under LCF-HCF interactions in a type 316LN stainless steel

Influence of dynamic strain aging (DSA) under sequential low cycle fatigue (LCF) and high cycle fatigue (HCF) loading was investigated by conducting HCF tests on specimens subjected to prior LCF cycling over a wide range of temperature from 573 to 973K. DSA was found to be pronounced at 823–873K depending on the magnitude of the stress employed under HCF cycling. DSA was seen to have contrasting implications under LCF and HCF deformation resulting in an anomalous fatigue behavior in terms of remnant HCF life under LCF-HCF interaction. LCF-HCF interaction was found to be pronounced at intermediate levels of prior LCF exposure, where the remnant HCF life is dictated by competitive damage mechanism resulting from the influence of DSA under LCF as well as HCF. Detailed fracture surface examination revealed that extensive hardening associated with DSA leads to an extended zone of faceted appearance with river markings (Stage-I crack) under HCF cycling (with or without LCF exposure). This reduces the crack growth rate, delaying the transition of crack from Stage-I to Stage-II, thereby leading to an extension of life in such cases. On the other hand, a highly striated fracture surface indicating a quick transition in crack from Stage-I to Stage-II, was observed for loading conditions with minimal or no influence of DSA, thus leading to lower life compared to the previous case.

Synergistic effect of Al and Gd on enhancement of mechanical properties of magnesium alloys

The effect of Gd/Al ratio on the properties of as-cast Mg-Gd-Al-Zn alloys was investigated by changing the chemical composition from that of AZ61 to GZ61. At the ratio of 1, the Al2Gd phase becomes predominant and Mg17Al12 is hardly seen in the microstructure. As a potent inoculant, the Al2Gd phase resulted in intense grain refinement and enhancement of strength, ductility and toughness. For instance, the tensile strength and elongation to failure of Mg-3Gd-3Al-1Zn alloy were enhanced by ~4% and 180% compared with those of AZ61 alloy, respectively. However, at high Gd/Al ratios, the Al2Gd phase was replaced by (Mg,Al)3Gd and Mg5Gd phases and very large grain sizes were achieved, which led to poor tensile properties and the appearance of cleavage facets on the fracture surfaces. Therefore, it can be deduced that the presence of Gd and Al, in appropriate amounts to reach Gd/Al ratio of ~ 1, is required for the achievement of grain refinement, good ductility, high strength, and the appearance of ductile fracture surfaces in the Mg-Gd-Al-Zn system. Conclusively, the Mg-Gd-Al-Zn alloys can be considered as a new class of structural magnesium alloy and it is superior to both AZ (Mg-Al-Zn) and GZ (Mg-Gd-Zn) series of alloys.

Metal-Free CVD Graphene Synthesis on 200 mm Ge/Si(001) Substrates.

Good quality, complementary-metal-oxide-semiconductor (CMOS) technology compatible, 200 mm graphene was obtained on Ge(001)/Si(001) wafers in this work. Chemical vapor depositions were carried out at the deposition temperatures of 885 °C using CH4 as carbon source on epitaxial Ge(100) layers, which were grown on Si(100), prior to the graphene synthesis. Graphene layer with the 2D/G ratio ∼3 and low D mode (i.e., low concentration of defects) was measured over the entire 200 mm wafer by Raman spectroscopy. A typical full-width-at-half-maximum value of 39 cm-1 was extracted for the 2D mode, further indicating that graphene of good structural quality was produced. The study also revealed that the lack of interfacial oxide correlates with superior properties of graphene. In order to evaluate electrical properties of graphene, its 2 × 2 cm2 pieces were transferred onto SiO2/Si substrates from Ge/Si wafers. The extracted sheet resistance and mobility values of transferred graphene layers were ∼1500 ± 100 Ω/sq and μ ≈ 400 ± 20 cm2/V s, respectively. The transferred graphene was free of metallic contaminations or mechanical damage. On the basis of results of DFT calculations, we attribute the high structural quality of graphene grown by CVD on Ge to hydrogen-induced reduction of nucleation probability, explain the appearance of graphene-induced facets on Ge(001) as a kinetic effect caused by surface step pinning at linear graphene nuclei, and clarify the orientation of graphene domains on Ge(001) as resulting from good lattice matching between Ge(001) and graphene nucleated on such nuclei.

Seeing Molecular Configuration in Twisted Crystal Form

AbstractStructural chemistry began with Louis Pasteur, who first saw the configuration of molecules manifest in the shapes of crystals. Here is reviewed forgotten, century‐old efforts to make molecular configuration vivid in polycrystalline ensembles. Resorcinol, and a variety of other simple molecular crystals, when grown from the melt in the presence of resolved chiral additives, such as tartaric acids, form spectacular bull's‐eye patterns evident in the petrographic microscope. Concentric, rhythmic optical bands in so‐called banded spherulites are tell‐tale signs of the twisting of helicoidal crystalline lamellae. The sense of twisting was assayed by correlating displacements of extinction bands on rotation with additive configurations. Thus, the arrangements of atoms in space were made manifest in crystal form – not in the appearance of hemihedral facets, but rather, in the chirality of nonclassical morphologies with Gaussian curvature. The far reaching consequences of these experiments for molecular crystal growth and form, long ago foreseen, are evaluated with the hindsight of an extra century of experience and sophistication. The narrative aims to draw a continuous thread through the work of Pasteur, Grigorii Vyroubov, Frédéric Wallerant, John Monteath Robertson, Hélène Metzger, and Jack Dunitz.

Stratigraphy of a diamond epitaxial three-dimensional overgrowth using doping superlattices

The selective doped overgrowth of 3D mesa patterns and trenches has become an essential fabrication step of advanced monolithic diamond-based power devices. The methodology here proposed combines the overgrowth of plasma-etched cylindrical mesa structures with the sequential growth of doping superlattices. The latter involve thin heavily boron doped epilayers separating thicker undoped epilayers in a periodic fashion. Besides the classical shape analysis under the scanning electron microscope relying on the appearance of facets corresponding to the main crystallographic directions and their evolution toward slow growing facets, the doping superlattices were used as markers in oriented cross-sectional lamellas prepared by focused ion beam and observed by transmission electron microscopy. This stratigraphic approach is shown here to be applicable to overgrown structures where faceting was not detectable. Intermediate growth directions were detected at different times of the growth process and the periodicity of the superlattice allowed to calculate the growth rates and parameters, providing an original insight into the planarization mechanism. Different configurations of the growth front were obtained for different sample orientations, illustrating the anisotropy of the 3D growth. Dislocations were also observed along the lateral growth fronts with two types of Burger vector: b011¯=12[011¯] and b112=16[112]. Moreover, the clustering of these extended defects in specific regions of the overgrowth prompted a proposal of two different dislocation generation mechanisms.

Si-doped diamond films prepared by chemical vapour deposition

The effects of Si doping on morphology, components and structure characteristics of CVD diamond films were studied. Si-doped CVD diamond films were deposited on Si substrate by adding tetraethoxysilane (TEOS) into acetone as source of reactant gas. The morphology and microstructure of diamond films were characterized by scanning electron microcopy (SEM). The crystalline quality of diamond films was studied by Raman spectroscopy and X-ray diffractometry (XRD). The surface roughness of the films was evaluated with surface profilometer. The results suggest that Si doping tends to reduce the crystallite size, enhance grain refinement and inhibit the appearance of (111) facets. Raman spectra indicate that Si doping can enhance the formation of sp2 phase in diamond films. Moreover, Raman signal of SiC was detected, which suggests the existence of Si in the diamond films. Smooth fine-grained diamond (SFGD) film was synthesized at Si to C ratio of 1%.

Preparation of Aluminum Metal Matrix Composite with Novel In situ Ceramic Composite Particulates, Developed from Waste Colliery Shale Material

A novel method is adapted to prepare an in situ ceramic composite from waste colliery shale (CS) material. Heat treatment of the shale material, in a plasma reactor and/or in a high temperature furnace at 1673 K (1400 °C) under high vacuum (10−6 Torr), has enabled in situ conversion of SiO2 to SiC in the vicinity of carbon and Al2O3 present in the shale material. The composite has the chemical constituents, SiC-Al2O3-C, as established by XRD/EDX analysis. Particle sizes of the composite range between 50 nm and 200 μm. The shape of the particles vary, presumably rod to spherical shape, distributed preferably in the region of grain boundaries. The CS composite so produced is added to aluminum melt to produce Al-CS composite (12 vol. pct). For comparison of properties, the aluminum metal matrix composite (AMCs) is made with Al2O3 particulates (15 vol. pct) with size <200 μm. The heat-treated Al-CS composite has shown better mechanical properties compared to the Al-Al2O3 composite. The ductility and toughness of the Al-CS composite are greater than that of the Al-Al2O3 composite. Fractographs revealed fine sheared dimples in the Al-CS composite, whereas the same of the Al-Al2O3 composite showed an appearance of cleavage-type facets. Abrasion and frictional behavior of both the composites have been compared. The findings lead to the conclusion that the in situ composite developed from the colliery shale waste material has a good future for its use in AMCs.

Fracture behavior of Alloy 625 with different precipitate microstructures

To study the effect of various ordered phases and their growth on room temperature tensile properties and fracture behavior of Alloy 625, specimens of this alloy were held isothermally at 813K, 973K and 1123K for 10h, 100h and 1200h, respectively. Specimens held at the aforementioned temperatures were subjected to transmission electron microscopic investigation to characterize Ni2(Cr,Mo), γ″ and δ ordered phases. Tensile testing of the heat-treated samples revealed the influence of these ordered phases on the tensile properties and fractures. Primary carbides, which were identified semi-qualitatively under electron-probe micro-analyzer (EPMA) have been found responsible for void nucleation resulting in large dimples in Alloy 625 with solid-solution matrix. Early stages of Ni2(Cr,Mo) and γ″ and their subsequent growth at 813K, was responsible for increase in strength without reducing ductility considerably. This behavior has been correlated with the reduction in size and depth of the dimples in the fractographs. Transmission electron micrographs showing three variants of γ″ in three orthogonal directions in the alloy matrix have revealed that the morphology of the ordered phase is lens shaped and the precipitate can grow up to ~150nm. With the growth of γ″ the alloy is seen to fracture in transgranular cleavage manner following ~23% of uniform strain. Influence of plate shaped, long and thick δ phase on the strength and ductility appears similar to that of γ″. δ—precipitation after 1200h of isothermal holding at 1123K is also seen to cause faceted appearance of the fracture surface, which is similar to that of γ″ after 1200h of isothermal holding at 973K. However, the facets are seen to be consisting of dimples, which are small and uniformly distributed over the surface.

Evolution of Bottom c-Plane on Wet-Etched Patterned Sapphire Substrate

Wet-etched pattern sapphire substrate (PSS) has been employed to improve the epitaxy of GaN-based LEDs. It was found that the crystal quality and performance of LEDs improved with decrease in c-plane areas of PSS. However, further decrease in bottom c-plane areas makes epitaxy of GaN film very difficult. In this research, the evolution of bottom c-plane was investigated through a systematic study. It was found that epitaxy difficulty might be due to the appearance of 6C facets and the disappearance of bottom c-plane.

Ni–ZrO2 Composite Coating by Electro-Co-Deposition

In the present investigation Ni–ZrO2 metal matrix composite coatings were prepared on steel substrate using watt’s type solution through electro-co-deposition process with different weight percentages of zirconia powder dispersed in the bath. In the coating, nickel is present with faceted appearance along with ZrO2. The microhardness and wear resistance of the coatings increase with increasing weight percentage of particles content in the coating. The hardness of the resultant coatings was found to be 325 VHN for pure Ni coating whereas 401VHN for Ni–ZrO2 (15 g/l ZrO2) coating depending on the particle volume in the Ni matrix. The results also showed that the wear resistance of the composite coatings was improved as compared to unreinforced Ni deposited material. Strengthening of the coating was attributed to the ZrO2 dispersion and partially favorable texture.

Growth and strain characterization of high quality GaN crystal by HVPE

Freestanding GaN crystals were fabricated by hydride vapor phase epitaxy using a random-islands facet-initiated epitaxial overgrowth technique. In this method, small micrometer sized GaN islands were firstly deposited on a TiC buffer layer on a sapphire substrate. Successive three-dimensional growth of GaN was controlled to a thickness of a few hundred micrometers on the buffer layer. Finally, a thick GaN layer was grown and high quality freestanding GaN crystals (dislocation density: <3×106cm−2, radius of curvature: >5m) were obtained by self-separation from the sapphire substrate. It was found that the dislocation density was drastically reduced in the initial growth stage of this method by the appearance of sidewall facets. Depth profiles of the residual strain in the freestanding GaN substrates have been successfully measured by a novel method employing cross-sectional micro-reflectance spectroscopy. It was found that the intrinsic strain, the driving force of wafer bending, can be greatly reduced by the introduction of three-dimensional growth in the initial growth stage.

Developments in Gemstone Analysis Techniques and Instrumentation During the 2000s

The first decade of the 2000s continued the trend of using more powerful analytical instruments to solve gem identification problems. Advances in gem treatment and synthesis technology, and the discovery of new gem sources, led to urgent needs in gem identification. These, in turn, led to the adaptation of newer scientific instruments to gemology. The past decade witnessed the widespread use of chemical microanalysis techniques such as LA-ICP-MS and LIBS, luminescence spectroscopy (particularly photoluminescence), real-time fluorescence and X-ray imaging, and portable spectrometers, as well as the introduction of nanoscale analysis. Innovations in laser mapping and computer modeling of diamond rough and faceted stone appearance changed the way gemstones are cut and the manner in which they are graded by gem laboratories.

Effects of hydrogen on the fracture toughness of a X70 pipeline steel

Effects of hydrogen on the fracture toughness of a X70 pipeline steel were investigated in the cases of hydrogen pre-charging and dynamic hydrogen charging in 0.5 mol/L H 2SO 4 solution under slow strain rate tensile testing. Under the hydrogen pre-charging, the fracture toughness decreased in a linear relationship with the hydrogen concentration as the hydrogen concentration was more than 1 ppm in weight. The fracture surfaces were characteristic of dimples. Under the dynamic hydrogen charging, the fracture toughness for hydrogen-induced cracking decreased linearly with logarithm of the hydrogen concentration without stress. The hydrogen-induced fracture had the appearance of cleavage facets.

Selective etching of Si 1 − xGe x versus Si with gaseous HCl for the formation of advanced CMOS devices

In this paper, we have studied the selective etching of Si 1 − x Ge x layers against Si in a Chemical Vapour Deposition (CVD) epitaxy reactor. Indeed, HCl gas, available in such tools, can be used to remove Si 1 − x Ge x selectively compared to Si which can be useful in numerous advanced CMOS applications. Firstly, the etching morphologies obtained at different HCl partial pressures are presented. These experiments permitted us to point out the appearance of facets at the edges of the formed tunnels under certain conditions. Secondly, SEM cross-section observations allowed us to measure the lateral etch kinetics of Si 1 − x Ge x layers with 15% < x < 30% on rotated substrates, along the <100> directions. And, the apparent activations energies and their dependency on both HCl partial pressure and Ge composition are also reported. Finally, ellipsometry measurements permitted us to measure selectivities as high as 50 for SiGe(25%) and 90 for SiGe(30%) between lateral SiGe and direct vertical Si etches. Finally, this technique appears to be very attractive for the formation of thin-Si film transistors such as SON devices with perfectly uniform Si channels and good interfaces quality.