Defect-free Structures Research Articles

Crossed chiral band approximation for wide-band self-collimation of light

We propose a perspective to the evaluation of the wide bandwidth phenomenon for low-symmetric photonic structures, by introducing the band tailoring and chiral band approximation on the self-collimation effect. In the case of the crossing of the bands, we claim the excitation of the lower mode can provide the utilization of the entire bandwidth by suppressing the intersection regions where the frequencies tend to mix. Thereby, we design broadband self-collimation capable, defect-free photonic structures and examine their performances. A fractional bandwidth of 0.35 () and 0.37 () are achieved for radii of and , respectively. We explore the full-range collimation by using transmission and E-field intensity analyses in addition to band diagrams and group velocity dispersions. Moreover, we indicate all-angle collimation validity even for highly tilted sources up to an angle of .

GaAs nanoscale membranes: prospects for seamless integration of III-Vs on silicon.

The growth of compound semiconductors on silicon has been widely sought after for decades, but reliable methods for defect-free combination of these materials have remained elusive. Recently, interconnected GaAs nanoscale membranes have been used as templates for the scalable integration of nanowire networks on III-V substrates. Here, we demonstrate how GaAs nanoscale membranes can be seamlessly integrated on silicon by controlling the density of nuclei in the initial stages of growth. We also correlate the absence or presence of defects with the existence of a single or multiple nucleation regime for the single membranes. Certain defects exhibit well-differentiated spectroscopic features that we identify with cathodoluminescence and micro-photoluminescence techniques. Overall, this work presents a new approach for the seamless integration of compound semiconductors on silicon.

Effect of the Solvent System on the Morphology and Performance of Nylon 6 Nanofibre Membranes

Nylon 6 nanofibre membranes were prepared by electrospinning of nylon 6 solutions with various volume ratios of trifluoroethyl alcohol (TFE) and formic acid (FA). The effect of the solvent type on the morphology of nylon 6 nanofibre membranes was investigated. Results showed that all membranes studied showed uniform, defect-free structures with very thin nanofibre diameters. The addition of formic acid led to a significant decrease in average fibre diameters. The average fibre diameters were 660, 186, 87, 62 and 30 nm for nylon 6 nonofibre prepared using the binary solution system and trifluoroethyl alcohol/formic acid (100:0), (75:25), (50:50), (25:75) & (0:100) respectively. In addition, the nylon 6 nanofibre membranes prepared using formic acid showed the highest strength with the highest porosity and the lowest average fibre diameters.

Increasing Exposed Metal Site Accessibility in a Co-MOF-74 Material With Induced Structure-Defects

Metal-organic frameworks (MOFs) are promising nanoporous materials with many practical applications. This owes largely to their remarkable porosity and the presence of specific chemical functionalities, such as exposed metal sites (EMS). The MOF-74 structure is known for exhibiting one of the highest EMS densities among porous materials. Moreover, the inclusion of structural defects has been proposed to enhance activity further. This was previously achieved by mixing the original linker together with a second, having lower topology. The presence of structural defects was evidenced by the resulting crystalline properties and thermal stability. In this work, different mixtures of tetratopic 2,5 dihydroxyterephthalic acid with up to 60 % of the tritopic hydroxyterephtalic acid were used to synthesize crystalline Co-MOF-74-like materials. Materials synthesized from higher proportions than 30 % of hydroxyterephtalic acid in the synthesis media collapse upon partial removal of the solvent molecules. This indicates the presence of structural defects and the importance of the solvent molecules in stabilizing the crystalline structures. Electron microscope images show that crystal size reduces with inclusion of hydroxyterephtalic acid as the second linker. The presence of coordinated solvent molecules at the EMS was evaluated by FTIR spectroscopy, so that a higher degree of solvent-exchange was observed during washing for defective structures. Furthermore, TG analysis suggests defective structures exhibit lower desolvation temperatures than the defect-free structures. Finally, N2 adsorption-desorption analesis at -196 °C showed an enhanced accessibility of the gas to the inner porosity of the defective structures and therefore, the EMS of the material. All these finding make this pathway interesting to enhance the potential interest of these materials for an industrial application because of both a facilitated activation and a better access to the active sites.

Boron Nitride Nanotube Nucleation via Network Fusion during Catalytic Chemical Vapor Deposition.

Despite boron nitride nanotubes (BNNTs) first being synthesized in the 1990s, their nucleation mechanism remains unknown. Here we report nonequilibrium molecular dynamics simulations showing how BNNT cap structures form during Ni-catalyzed chemical vapor deposition (CVD) of ammonia borane. BN hexagonal ring networks are produced following the catalytic evolution of H2 from the CVD feedstock, the formation and polymerization of B-N chain structures, and the repeated cleavage of homoelemental B-B/N-N bonds by the catalyst surface. Defect-free BNNT cap structures then form perpendicular to the catalyst surface via direct fusion of adjacent BN networks. This BNNT network fusion mechanism is a marked deviation from the established mechanism for carbon nanotube nucleation during CVD and potentially explains why CVD-synthesized BNNTs are frequently observed having sharper tips and wider diameters compared to CVD-synthesized carbon nanotubes.

3D-customized thin-walled ceramic mouldings produced by deep drawing of green tapes

Thin-walled 3D-customized ceramic components can be used, e.g. for inert and thermal resistant housings as well as special surface structured kiln furniture. For manufacturing such components, deep drawing of ceramic green tapes has been used. The technology has been almost not applied for processing of ceramic green tapes. A new approach was developed to realize a homogenous ceramic particle packing and hence uniform green density within a deep drawn green tape. After debinding and sintering, defect-free model structures with a dense microstructure (density > 99%) were achieved. Within this study, the deep drawing of ceramic green tapes by using a new approach was investigated. The developed approach brings several beneficial properties together, e.g. a reproducible form deviation and more importantly a homogeneous particle distribution allowing homogeneous and dense structures. With this results, the transformation of non-complex and cheap ceramic green tapes to 3D-customized and near-net-shape thin-walled mouldings becomes possible.

Capabilities and Limits to Form High Aspect-Ratio Microstructures by Molding of Borosilicate Glass

This paper presents an experimental study of capabilities to fabricate high aspect-ratio microstructures in glass. The fabrication was done by molding borosilicate glass in deep silicon trenches created by deep reactive ion etching in silicon and releasing the structures by potassium hydroxide (KOH) and Xenon difluoride (XeF2) etching of the Si mold. Subsequent over-etching of the glass can also be used to increase the aspect ratio of the glass structures. By using this molding method, glass structures have been successfully fabricated achieving an aspect ratio of 42:1, a height of 212 $\mu \text{m}$ , and vertical sidewall angle up to 89.7°. We also investigated the effect of molding parameters (heat treatment time, temperature, and pressure) on the process capabilities and limitations for defect free structures. This work demonstrates the capabilities to fabricate high aspect-ratio glass structures that cannot be achieved by conventional etching techniques to date, opening new perspectives for the use of glass in MEMS and microfluidics.

Microstructure and thermal properties of unalloyed tungsten deposited by Wire + Arc Additive Manufacture

Tungsten is considered as one of the most promising materials for nuclear fusion reactor chamber applications. Wire + Arc Additive Manufacture has already demonstrated the ability to deposit defect-free large-scale tungsten structures, with considerable deposition rates. In this study, the microstructure of the as-deposited and heat-treated material has been characterized; it featured mainly large elongated grains for both conditions. The heat treatment at 1273 K for 6 h had a negligible effect on microstructure and on thermal diffusivity. Furthermore, the linear coefficient of thermal expansion was in the range of 4.5 × 10−6 μm m−1 K−1 to 6.8 × 10−6 μm m−1 K−1; the density of the deposit was as high as 99.4% of the theoretical tungsten density; the thermal diffusivity and the thermal conductivity were measured and calculated, respectively, and seen to decrease considerably in the temperature range between 300 K and 1300 K, for both testing conditions. These results showed that Wire + Arc Additive Manufacture can be considered as a suitable technology for the production of tungsten components for the nuclear sector.

A comprehensive study on the fabrication and characterization of Ti–48Al–2Cr–2Nb preforms manufactured using electron beam melting

This feasibility study examines the additive manufacturing of Ti–48Al–2Cr–2Nb preforms in a quest to make more complex engine components from this promising but hard to machine, brittle material. The preforms are built using the electron beam melting technique; hot isostatic pressing (HIP) was performed as the only post-process. Microstructural characterizations were carried with scanning electron microscopy techniques, X-ray diffraction and X-ray computed tomography while mechanical properties were obtained via tensile testing. We exploited computational thermodynamic modeling (widely known as CALPHAD) approach to investigate the phase stability pertinent to Nb and Cr modified TiAl. Defect-free structures were obtained in the HIPed condition with mechanical behavior comparable to/better than traditionally manufactured counterparts. Equiaxed duplex microstructures were achieved in the as-built condition that were sustained post-HIP. The effect of the build conditions and phase transformation kinetics on the observed microstructures is discussed. The microstructure-mechanical behavior relationship is also discussed.

Techno-economic Feasibility of Modified Pulse Arc Deposition on Thick Section of Quenched and Tempered Steel

Quenched and Tempered (Q&T) steels welded structures that have numerous applications, particularly in the defence industry. However these steels are particularly prone to Hydrogen Assisted Cold Cracking (HACC) and require a highly-skilled welder to fabricate defect-free structures. This is due to the selection of the manual metal arc welding process of shielded metal arc welding (SMAW). The introduction of Modified Pulsed arc mode of depositions; a variation to Pulsed Arc deposition, has advanced deposition rates and can be employed by welders with a greater variation in skill. In this body of work, full strength butt welds are fabricated on 20mm, sections of Q&T AS/NZS 3597 Grade 700 steel under a high level of restraint using Modified Pulse Gas Metal Welding (GMAW-P) and conventional Shielded Metal Arc Welding (SMAW). The study investigates the economic feasibly of the two modes of deposition and the propensity for cracking when welded under high restraint. The study concluded that modified GMAW-P achieved reduction of 63% in the ‘Arc-On' time and an 88% reduction in the total normalised fabrication time. However, due to the increased propensity to lack of fusion type defects, strict controls must be employed in optimising the welding procedure to mediate for such defects if GMAW-P is to provide a techno-economically beneficial alternative to conventional SMAW when welding Q&T steels.

Self-healing ability assessment of irradiated multilayered composites: A continuum approach

Atomistic simulations have revealed an unconventional behavior of point defects at interfaces found in multilayer composites synthesized by physical vapor deposition but the observed mechanisms that involve point-defect annihilation are subject to time-scale limitations. So, a mathematical model that describes long-term evolution of point defects in such materials under irradiation is presented in this work. Firstly, the effect of interface point-defect trapping and recombination mechanisms on point-defect concentrations has been studied. In addition, the effect of interface self-interstitial atoms loading, which has been seen during collision cascades, and constitutional vacancies has been studied too. Two interface configurations have been considered between metals in a β-α-β three-layer system (α = Cu and β = Nb, or V), KSmin and KS1. These interfaces correspond to ground-state and defect-free KS structures respectively. The respond to irradiation of the systems investigated here, Cu/Nb and Cu/V, depends on both, interface characteristics and bulk properties. Nonetheless, the influence of the properties of one metal in the point-defect evolution of the other metal is only effective if there are constitutional vacancies at the interface, i.e., for KSmin. Especial attention has been paid to the behavior of the same metal (Cu) when it is surrounded by diverse metals (Nb, or V) with the aim of comparing quantitatively our model predictions with experimental results reported elsewhere. The lower concentration of vacancies in Cu layer of Cu/Nb system at steady state is due to the low mobility of vacancies in niobium.

Removal Pathways of Out-of-Plane Defects in Thin Films of Lamellar Forming Block Copolymers

Understanding the defect removal process is crucial for fabricating defect-free self-assembled structures in block copolymer thin films. Most previous studies mainly focused on the removal of in-plane dislocation and disclination defects, while out-of-plane defects receive less attention. In this study, the removal of two types of out-of-plane defects of lamellar forming block copolymer thin films, the tilted domain defect and the cross-sectional edge dislocation defect, are studied in detail using the string method coupled with the numerical self-consistent field theory (SCFT). It is found that the removal of the tilted domain defect can be regarded as an order–order transition process controlled by the nucleation and growth mechanism. On the other hand, the cross-sectional edge dislocation can be eliminated by either evaporating or growing its core (a partial domain). For both cases, multiple removal pathways have been identified by varying the height of the partial domains and the segregation strength ...

Carbon nanotube bundles with tensile strength over 80 GPa.

Carbon nanotubes (CNTs) are one of the strongest known materials. When assembled into fibres, however, their strength becomes impaired by defects, impurities, random orientations and discontinuous lengths. Fabricating CNT fibres with strength reaching that of a single CNT has been an enduring challenge. Here, we demonstrate the fabrication of CNT bundles (CNTBs) that are centimetres long with tensile strength over 80 GPa using ultralong defect-free CNTs. The tensile strength of CNTBs is controlled by the Daniels effect owing to the non-uniformity of the initial strains in the components. We propose a synchronous tightening and relaxing strategy to release these non-uniform initial strains. The fabricated CNTBs, consisting of a large number of components with parallel alignment, defect-free structures, continuous lengths and uniform initial strains, exhibit a tensile strength of 80 GPa (corresponding to an engineering tensile strength of 43 GPa), which is far higher than that of any other strong fibre.

USING OF THE DIELECTRIC BARRIER DISCHARGE FOR DETECTION OF THE SURFACE DEFECTS OF MATERIALS

Modern requirements to the quality of materials entail the necessity of refining methods for their control and developing various procedures for analysis of the state of the surface with the goal of having defect-free structures for high-precision instruments and devices. Gas-discharge visualization of the defects at atmospheric pressure consists in obtaining bright plasma formations in the vicinity of the surface with a defect against the background of a weak glow of the air plasma in electromagnetic fields of high strength. The potentialities of plasma technologies regarding testing the surfaces of solid materials for various kinds of defects using a self-sustained dielectric barrier discharge (DBD) at atmospheric pressure are considered. The developed rapid technique provides quick rejection of defective materials or samples prior to their technological use. A set of diagnostic tools and auxiliary equipment has been developed to visualize the mechanical defects of different origin and evaluate the quality of raw materials. A number of important issues are considered when controlling the quality of high-grade surfaces, transparent and opaque materials, including metal surfaces. The special attention is paid to the search for defects in the structural elements of industrial equipment and determination of the degree of moisture of fibrous structures, for example, woven materials.

Effect of Surface and Defect Chemistry on the Photocatalytic Properties of Intentionally Defect-Rich ZnO Nanorod Arrays.

Due to the abundance of intrinsic defects in zinc oxide (ZnO), the material properties are often governed by same. Knowledge of the defect chemistry has proven to be highly important, especially in terms of the photocatalytic degradation of pollutants. Given the fact that defect-free materials or structures exhibiting only one type of defect are extremely difficult to produce, it is necessary to evaluate what influence various defects may have when present together in the material. In this study, intentionally defect-rich ZnO nanorod (NR) arrays are grown using a simple low-temperature solution-based growth technique. Upon changing the defect chemistry using rapid thermal annealing (RTA) the material properties are carefully assessed and correlated to the resulting photocatalytic properties. Special focus is put on the investigation of these properties for samples showing strong orange photoluminescence (PL). It is shown that intense orange emitting NR arrays exhibit improved dye-degradation rates under UV-light irradiation. Furthermore, strong dye-adsorption has been observed for some samples. This behavior is found to stem from a graphitic surface structure (e.g., shell) formed during RTA in vacuum. Since orange-luminescent samples also exhibit an enhancement of the dye adsorption a possible interplay and synergy of these two defects is elucidated. Additionally, evidence is presented suggesting that in annealed ZnO NRs structural defects may be responsible for the often observed PL emission at 3.31 eV. However, a clear correlation with the photocatalytic properties could not be established for these defects. Building on the specific findings presented here, this study also presents some more general guidelines which, it is suggested, should be employed when assessing the photocatalytic properties of defect-rich ZnO.

Creation of film-free freestanding 3D microstructures by aligned mask micromolding

There are established macroscale fabrication methods to create parts of virtually any shape or size; but for components in the microscale range (thousands to tens of microns) there is a dearth of options for making 3D parts at high enough throughput to make them practical. Here we use traditional photolithography with an aligned mask micromolding method to fabricate freestanding 3D microstructures that can be used as components of microrobotics and microfluidics devices for biomedical applications. The overall goal of this research is to create novel micromolding processes that are capable of creating defect-free microscale structures with controllable 3D geometry, a variety of material properties, and high fabrication throughput. While some methods are available that are capable of meeting a few of these goals, nothing exists in the current microfabrication repertoire that can accomplish all simultaneously.

Growth mechanism of one-step self-masking reactive-ion-etching (RIE) broadband antireflective and superhydrophilic structures induced by metal nanodots on fused silica.

This work presents a low-cost, simple, convenient, advanced technology to prepare large-area defect-free subwavelength structures (SWSs). SWSs were obtained by a metal-induced one-step self-masking RIE process on a fused-silica surface, in which metal-fluoride (mainly ferrous-fluoride) nanodots were used to induce and gather stable fluorocarbon polymer etching inhibitors in the RIE polymers as masks. The SWS growth processes are visible with an increase in etching time and some exhibit prominent broadband antireflective properties from the visible to the near-infrared wavelength range. Transmission in the 600-900-nm range increased from approximately 93% for the polished fused silica to above 99% for the double-side SWSs on fused silica. A theoretical simulation by a finite-difference time-domain method agreed well with the experiments. Moreover, the surface of the SWSs exhibits excellent superhydrophilic properties.

Influence of topographically patterned angled guidelines on directed self-assembly of block copolymers.

Single chain in mean-field Monte Carlo simulations were employed to study the self-assembly of block copolymers (BCP) in thin films that use trapezoidal guidelines to direct the orientation and alignment of lamellar patterns. The present study explored the influence of sidewall interactions and geometry of the trapezoidal guidelines on the self-assembly of perpendicularly oriented lamellar morphologies. When both the sidewall and the top surface exhibit preferential interactions to the same block of the BCP, trapezoidal guidelines with intermediate taper angles were found to result in less defective perpendicularly orientated morphologies. Similarly, when the sidewall and top surface are preferential to distinct blocks of the BCP, intermediate tapering angles were found to be optimal in promoting defect free structures. Such results are rationalized based on the energetics arising in the formation of perpendicularly oriented lamella on patterned substrates.

Preparation of Regular Cement Hydration Crystals and Ordered Microstructures by Doping GON and an Investigation into Its Compressive and Flexural Strengths

In this work, we found that by controlling the size of the graphene oxide nanosheets (GON) at 5–140 nm, 5–260 nm and 5–410 nm, respectively, we could prepare regular shaped cement hydration crystals with the shape of nano-needle-like, flower-like, and polyhedron-like crystals, respectively. Together, these crystals formed an ordered structure of cement composites on both the micro and macro levels, and the compressive and flexural strengths of the cement composites obviously increased when compared to the control samples. Our results indicated that the smallest structural unit of regular crystals was the nano-polyhedron-like crystals, which consisted of AFt, AFm, CH, and crystallization C–S–H, and could assemble into regular needle-like crystals, flower-like crystals, and polyhedron-like crystals, as well as an ordered structure on the micro and macro levels. Most of the C-S-H was transferred into a monoclinic system crystals and the remainder played the role of an adhesive in the forming process of regular crystals and structure. The cracks and holes in the cement composites disappeared by the self-repairing effect of the growing hydration crystal. The results indicate that ordered structural cement composites with defect-free structures and high strength can be prepared using GON with a suitable size range.

Controlled Synthesis of Ultralong Carbon Nanotubes with Perfect Structures and Extraordinary Properties.

Carbon nanotubes (CNTs) have drawn intensive research interest in the past 25 years due to their excellent properties and wide applications. Ultralong CNTs refers to the horizontally aligned CNT arrays which are usually grown on flat substrates, parallel with each other with large intertube distances. They usually have perfect structures, excellent properties, and lengths up to centimeters, even decimeters. Ultralong CNTs are promising candidates as building blocks for transparent displays, nanoelectronics, superstrong tethers, aeronautics and aerospace materials, etc. The controlled synthesis of ultralong CNTs with perfect structures is the key to fully exploit the extraordinary properties of CNTs. CNTs are typical one-dimensional single-crystal nanomaterials. It has always been a great challenge how to grow macroscale single-crystals with no defects. Thus, the synthesis of ultralong CNTs with no defect is of significant importance from both fundamental and industrial aspects. In this Account, we focus on our progress on the controlled synthesis of ultralong CNTs with perfect structures and excellent properties. A deep understanding of the CNT growth mechanism is the first step for the controlled synthesis of ultralong CNTs with high quality. We first introduce the growth mechanism for ultralong CNTs and the main factor affecting their structures. We then discuss the strategies to control the defects in the as-grown ultralong CNTs. With these approaches, ultralong high-quality CNTs with different structures can be obtained. By completely eliminating the factors which may induce defects in the CNT walls, ultralong CNTs with perfect structures can be obtained. Their chiral indices keep unchanged for several centimeters long along the axial direction of the CNTs. The defect-free structures render the ultralong CNTs with excellent electrical, mechanical and thermal properties. The as-grown ultralong CNTs exhibit superhigh mechanical strength (>100 GPa) and their breaking strain (>17.5%) reach the theoretical limits. They also show excellent electrical and thermal properties. In addition, centimeters long CNTs showed macroscale interwall superlubricious properties due to their defect-free structures. Ultralong, defect-free CNTs with controlled structures are highly desirable for many high-end applications. We hope that this Account will shed light on the controlled synthesis of ultralong CNTs with perfect structures and excellent properties. Moreover, the growth mechanism and controlled synthesis of ultralong CNTs with perfect structures also offers a good model for other one-dimensional nanomaterials.