Microscopic Structure Of Defects Research Articles

Influence of cluster shaped defects on fracture process of fiber bundle

Defects that exist inside composites have an important effect on the tensile fracture properties of composites. The fiber bundle model is a theoretical model commonly used to study the tensile fracture properties of disorder materials. Existing work on fiber bundle models with single fiber defects shows that after single fiber defects are introduced into the fiber bundle model, the defects have a significant effect on the tensile fracture properties of the model. Since there are more complex microscopic defect structures in actual materials, such as voids, gaps, impurities, dislocations, micro-cracks, etc, it is necessary to build a multi-size defect model. In order to study the defects of different sizes and damage degrees existing in actual materials, the spatial size of the defect, the degree of defect and the distribution of fiber damage levels within the defect and other influencing factors are introduced to construct an extended fiber bundle model with cluster shaped defects. For the model, it is first assumed that the degree of defect of the fiber inside each cluster decays linearly from the center to the outside in two spatial attenuation forms: exponential decay and constant degree of defect. In the fiber bundle model of this cluster-shaped defect, the two most important factors are the number of defects <i>α</i> and the upper limit of defect size <i>β</i>. The numerical simulation method is used to analyze the influence of the number of defects, the upper limit of defect size, and spatial distribution of degree of defective fibers inside defect on the macroscopic mechanical properties and statistical properties of fracture when the model is subjected to quasi-static load borne under the nearest neighbor stress redistribution. Through the simulation analysis, it is found that owing to the overlapping competition mechanism of the defect spatial distribution, when the upper limit <i>β</i> of the defect size is large, the influence of the number of defects on the system load capacity trends to saturation. Since the defect degree of the defect center fiber is proportional to the defect size, with the upper limit <i>β</i> of the defect size increasing, its influence on the load capacity of the model becomes more and more significant. When large size defects exist, even if the number of defects is small, the load bearing performance of the material will be significantly reduced. The spatial distribution function of the damage degree of fiber inside the defect has no substantial influence on the above rules, and only changes the specific value of each fracture property. The simulation analysis results in this paper have certain theoretical significance in improving the mechanical properties of composite materials.

Resolving the positions of defects in superconducting quantum bits

Solid-state quantum coherent devices are quickly progressing. Superconducting circuits, for instance, have already been used to demonstrate prototype quantum processors comprising a few tens of quantum bits. This development also revealed that a major part of decoherence and energy loss in such devices originates from a bath of parasitic material defects. However, neither the microscopic structure of defects nor the mechanisms by which they emerge during sample fabrication are understood. Here, we present a technique to obtain information on locations of defects relative to the thin film edge of the qubit circuit. Resonance frequencies of defects are tuned by exposing the qubit sample to electric fields generated by electrodes surrounding the chip. By determining the defect’s coupling strength to each electrode and comparing it to a simulation of the field distribution, we obtain the probability at which location and at which interface the defect resides. This method is applicable to already existing samples of various qubit types, without further on-chip design changes. It provides a valuable tool for improving the material quality and nano-fabrication procedures towards more coherent quantum circuits.

Defect structure and holographic storage properties of LiNbO3:Zr:Fe:Cu crystals with various Li/Nb ratios

A series of LiNbO3:Zr:Fe:Cu single crystals with various Li/Nb ratios ranging from 0.946 to 1.38 were grown in air by the Czochralski method. Both X-ray powder diffraction and ultraviolet absorption spectra have been examined, and are discussed with regard to the defect structure of the as-grown crystals. In addition, the conventional two-wave coupling experiment has been conducted with 532 nm laser to reveal the holographic storage properties. The results have shown the macroscopic photorefractive properties to correspond well to the microscopic defect structure changing with the composition. More specifically, LiNbO3:Zr:Fe:Cu crystals possess at once relatively high sensitivity, fast response time and dynamic range comparable with that of LiNbO3:Zr:Fe crystals, and at the same time maintain high saturation diffraction efficiency (up to 68%) close to that of typical photorefractive LiNbO3:Fe crystals (70%), as such denoting their potential as prominent holographic storage media.

Research on failure of cementitious materials under rotary cavitation test

Utilizing the rotating cavitation erosion test device invented by Nanjing Hydraulic Research Institute (NHRI) to perform the persistent cavitation experiment on hardened cement paste and mortar, observing and analyzing the differences and similarities between cavitation and abrasion zone. Observing the cavitation erosion on the surface of cement-based materials and researching on the destruction of cement hydrations under cavitation process by adopting multiple modern material analysis apparatus, such as Scanning Electron Microscope (SEM), Energy dispersive X-Ray(EDX), X-Ray Diffraction(XRD) and thermal analysis(TG&DSC). Results implied that after continuous cavitation test for 2 hours, the microscopic structure of original defects becomes smooth, to be specific, the flake, rod shape-crystals are basically disappeared and the bottom of cavitation pits are shaped like some melted organizations. Hence the cavitation erosion on cement paste is not only a simple mechanical failure but also lead to the thermal decomposition on hydrations, which could cause the remarkable loss of hydration products on the surface of test blocks. The specific figures are as follows: the mass loss of cementitious substances is no less than 40% so as the loss of slaked lime is more than 25% during a 2 hours cavitation experiment. In that way, the considerable hydration mass loss is capable to cause a huge affection on deterioration of concrete structures.

High-quality MgB2 nanocrystals synthesized by using modified amorphous nano-boron powders: Study of defect structures and superconductivity properties

Nano sized magnesium diboride (MgB2) samples were synthesized using various high-quality nano-B precursor powders. The microscopic defect structures of MgB2 samples were systematically investigated using X-ray powder diffraction, Raman, resistivity measurements and electron paramagnetic resonance spectroscopy. A significant deviation in the critical temperature Tc was observed due to defects and crystal distortion. The symmetry effect of the latter is also reflected on the vibrational modes in the Raman spectra. Scanning electron microscopy analysis demonstrate uniform and ultrafine morphology for the modified MgB2. Defect center in particular Mg vacancies influence the connectivity and the conductivity properties which are crucial for the superconductivity applications.

Defect-relevant piezoelectric and ferroelectric properties in LiCuTa3O9-doped K0.5Na0.5NbO3 lead-free piezoceramics

Lead-free K0.5Na0.5NbO3 + x mol LiCuTa3O9 (abbreviated to KNN-xLCT) piezoceramics are synthesized via a conventional sintering technique. All ceramics exhibit perovskite structure and their densification is improved after the addition of LCT. The doping of small quantities of LCT (x ≤ 0.015) results in the generation of two sorts of defect complexes [i.e., $${\left({\text{Cu}}_{\text{Nb}}^{{\prime }{\prime }{\prime }}-{\text{V}}_{\text{o}}^{\bullet \bullet }\right)}^{{\prime }}$$ and $${\left({\text{V}}_{\text{o}}^{\bullet \bullet} - {\text{Cu}}_{\text{Nb}}^{\prime \prime \prime}-{\text{V}}_{\text{o}}^{\bullet \bullet}\right)}^{\bullet}$$ ], inducing greatly hardening electrical behaviors with high mechanical quality factor Qm of ~ 780 at x = 0.015. However, excess LCT (x ≥ 0.015) contributes to the substantial reduction of defect complexes, and thus the ceramics are softened, presenting a relatively low Qm of ~ 480 at x = 0.03. It is noted that the ceramics with x = 0.03 remain comparatively great piezoelectric performances: d33 = 96 pC/N and kp = 37%. Our study indicates that the electrical properties of KNN-based ceramics doped with LCT are closely related with microscopic defect structure in the materials.

Defect structure‐electrical property relationship in Mn‐doped calcium strontium titanate dielectric ceramics

AbstractCa0.6Sr0.4TiO3 (CST) ceramics with different amounts of Mn dopant (0‐2.0 mol%) were prepared by solid‐state reaction method. The electric field and temperature stability of energy storage performance was found to be greatly enhanced with moderate doped level of 0.5 mol%. The dielectric loss‐frequency spectra revealed the existence and evolution of defect dipoles at elevated temperature, which was confirmed directly by electron paramagnetic resonance (EPR) spectra. The response of defect dipoles was characterized by thermally stimulated depolarization current (TSDC), where the activation energy and the concentration evolution of defect dipoles were calculated, with the highest values observed for 0.5% doped samples. The dissociation of defect dipoles and the movement of free were analyzed by high‐temperature impedance spectra analysis, with the activation energy of 1.04‐1.60 eV, and 0.5% doped samples also demonstrated the highest Ea. The relationship between microscopic defect structure and macroscopic electrical behavior was established in this work.

The heliconical nematic twist-bend phase from “classic” bent-core benzylideneanilines with oligomethylene cores

ABSTRACTWe report the synthesis and properties of several new heliconical twist-bend nematic (NTB) bimesogens possessing “classic” bent-core benzylideneaniline wings combined with oligomethylene spacers of odd parity. While short alkoxy tails combined with a resorcinol diester core in the classic system appear to give SmA phases (not B2 phases), in the present system, methoxy or ethoxy tails exhibit the NTB phase as evidenced by textural analysis. Several of these compounds also exhibit microscopic helical defect structures in the NTB phase, which have been reported in other lamellar and pseudo-lamellar liquid crystal phases, but not in the NTB phase.

Mechanical Characterization of Additive Manufacturing Processes

Objectives: To compare two of the most popular rapid prototyping processes of Stereolithography (SLA) and Selective Laser Sintering (SLS) from a product-based perspective, to help customers analyze them and make a choice between the two. Methods: This paper includes the qualitative testing of identical specimens created by SLA and SLS. The specimens are evaluated on the parameters of dimensional accuracy, tensile strength, water absorption, surface roughness, density, Vickers hardness and microscopic defect structure. The outcome of this study aims at helping people to understand SLS and SLA better in terms of the products they create so that it becomes easier for users to make a choice between the two. It also aims at highlighting the above mentioned statistical information about SLA and SLS so that they may be improvised and enhanced in the future. Findings: Based on the tests conducted, it was confirmed that the SLA specimens were better than the SLS specimens in the tensile strength, water absorption, surface roughness and density tests. The SLS specimens outperformed the SLA specimens in the dimensional accuracy and Vickers hardness tests. Thus it was concluded that the SLA specimens exhibited better mechanical and physical characteristics than the SLS specimens.

Al-doped MgB2 materials studied using electron paramagnetic resonance and Raman spectroscopy

Undoped and aluminum (Al) doped magnesium diboride (MgB2) samples were synthesized using a high-temperature solid-state synthesis method. The microscopic defect structures of Al-doped MgB2 samples were systematically investigated using X-ray powder diffraction, Raman spectroscopy, and electron paramagnetic resonance. It was found that Mg-vacancies are responsible for defect-induced peculiarities in MgB2. Above a certain level of Al doping, enhanced conductive properties of MgB2 disappear due to filling of vacancies or trapping of Al in Mg-related vacancy sites.

Electron paramagnetic resonance and Raman spectroscopy studies on carbon-doped MgB2 superconductor nanomaterials

Undoped and carbon-doped magnesium diboride (MgB2) samples were synthesized using two sets of mixtures prepared from the precursors, amorphous nanoboron, and as-received amorphous carbon-doped nanoboron. The microscopic defect structures of carbon-doped MgB2 samples were systematically investigated using X-ray powder diffraction, Raman and electron paramagnetic resonance spectroscopy. Mg vacancies and C-related dangling-bond active centers could be distinguished, and sp3-hybridized carbon radicals were detected. A strong reduction in the critical temperature Tc was observed due to defects and crystal distortion. The symmetry effect of the latter is also reflected on the vibrational modes in the Raman spectra.

Regularities of the Rehbinder effect in polymers. Review

Data are analyzed that are related to particular details of manifestations of the Rehbinder effect in the case of polymer deformation in adsorption-active media (AAMs). It is shown that, as a result of such a deformation, a vitreous polymer is dispersed, forming a system of fibrillar aggregates of oriented macromolecules with a diameter of 1–20 nm separated in space by microcavities of similar size (the crazing phenomenon). It is shown that polymer crazing in AAMs is a complex multistage process. Its first stage is initiation of crazes related to the microscopic surface defect structure of the actual polymer and is controlled by the Griffith criterion. Craze growth and broadening are typical thermally activated processes of plastic deformation of the polymer. The main areas for practical application of the Rehbinder effect in polymers are pointed out.

Bond angles for O-H defects in SnO2from polarization properties of their vibrational modes

Infrared absorption experiments made with polarized light yield significant insights into the possible structures of one- and two-O-H defects in SnO${}_{2}$ that are produced by thermal annealing treatments. These polarized absorption results reveal that a two-O-H defect must involve symmetry-equivalent O-H sites and that the axes of both one- and two-O-H defects are 63${}^{\ensuremath{\circ}}$--68${}^{\ensuremath{\circ}}$ from the $c$ axis of the rutile structure. These O-H bond angles found by experiment restrict the microscopic defect structures that are possible and suggest structures associated with either a metal atom substituting for Sn or an interstitial metal atom (such as Sn).

EXAFS investigations on PbMoO4 single crystals grown under different conditions

Extended X-ray absorption fine structure (EXAFS) measurements on PbMoO\(_{\boldsymbol{4}}\) (LMO) crystals have been performed at the recently-commissioned dispersive EXAFS beamline (BL-8) of INDUS-2 Synchrotron facility at Indore, India. The LMO samples were prepared under three different conditions viz. (i) grown from a stoichiometric starting charge in air ambient, (ii) grown from a stoichiometric starting charge in argon ambient and (iii) grown from PbO-rich starting charge in air ambient. The EXAFS data obtained at both Pb \({\boldsymbol{L}}_{\boldsymbol{3}}\) and Mo K edges of LMO have been analysed to determine Pb–O, Pb–Mo and Mo–O bond lengths in the crystals. The information thus obtained has been used to examine the microscopic defect structures in crystals grown under different conditions.

Defect structure and evolution in silicon carbide irradiated to 1 dpa-SiC at 1100 °C

Transmission electron microscopy (TEM), swelling measurements, isochronal annealing, and thermal diffusivity testing were used to characterize the effects of radiation damage in SiC. Together, these techniques provided a comprehensive set of tools for observing and characterizing the structure and evolution of radiation-induced defects in SiC as a function of irradiation temperature and dose. In this study, two types of dense, crystalline, monolithic SiC were subjected to irradiation doses up to 1 dpa-SiC at a temperature of 1100 °C, as well as post-irradiation annealing up to 1500 °C. The microscopic defect structures observed by TEM were correlated to changes in the macroscopic dimensions, thermal diffusivity and thermal conductivity. The results demonstrated the value of using ultrapure β-SiC as an effective reference material to characterize the nature of expected radiation damage in other, more complex, SiC-based materials such as SiC/SiC composites.

Proton-implantation-induced defects inn-type6H- and4H−SiC:An electron paramagnetic resonance study

The microscopic structure and introduction rate of point defects in n-type 6H- and 4H-SiC generated by room-temperature proton implantation have been studied by the electron paramagnetic resonance technique. In order to selectively study the effects of defect introduction in the trace region, 12-MeV implantation in 300-\ensuremath{\mu}m-thick samples was employed, for which the protons completely cross the sample. In both polytypes we observe three dominant paramagnetic defects attributed to the Si monovacancy in the negative charge state and the neutral Si monovacancy in the hexagonal and quasicubic lattice sites, respectively. The concentration of all three defects increases linearly with proton dose. Their total introduction rate is \ensuremath{\sim}19 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$, which amounts only to 4% of the concentration expected from SRIM simulations. No carbon-vacancy-related defect is observed. Thermal annealing at 1100 \ifmmode^\circ\else\textdegree\fi{}C is sufficient to anneal out the ${V}_{\mathrm{Si}}$ defects and to restore n-type conductivity. The observation of the neutral Si vacancy at hexagonal and quasicubic sites under thermal equilibrium conditions at 4 K does not support their previous assignment to an excited state.

Microscopic structure of defects in microcrystalline silicon

Electron spin resonance (ESR) on paramagnetic defects in hydrogenated microcrystalline silicon (μc-Si:H) has been studied to investigate its microscopic structure. From the anisotropy of preferentially oriented sample along 〈1 1 0〉 we suggest that the fine structure and its angular dependence arise from the grain boundary having 〈1 1 1〉 axial symmetry with g-tensors of g ‖ ∼2.0022 and g ⊥ ∼2.0078 such as the Pb center. A simulation study indicates the presence of isotropic components arising from grain boundary defects on randomly oriented crystallites and dangling bonds in the amorphous structure. Thermal desorption of hydrogen from μc-Si:H was correlated with the isochronal annealing of the ESR spectra.

μm-resolved high resolution X-ray diffraction imaging for semiconductor quality control

We present a method and an equipment for performing μm-resolved X-ray diffraction area maps, and apply it to wafer defect analysis and industrial wafer quality inspection. The method determines simultaneously the macroscopic warpage, the mesoscopic curvature and the microscopic defect structure of semiconductor wagers. It is based on X-ray diffraction rocking curve imaging of the whole wafer with down to 1 μm 2 resolution. The new wafer testing equipment determines the maximal and integral peak intensities, the peak position and the half width of the rocking curves with a microscopic resolution, thus imaging simultaneously the macroscopic quality parameters and the microscopic defect structure. This permits to establish a direct one-to-one correlation between the microscopic defects and the resulting macroscopic effects. As an example, wafers of different materials, fabrication technology and resulting perfection are studied. When investigating layered samples, the technique allows furthermore to determine the influence of the wafer quality on the layer properties. The method can be applied generally to characterize non-destructively the quality of all kinds of crystalline structures, like, e.g. microelectronic and optoelectronic devices.

Microscopic structure of the substitutional Al defect in α quartz

First-principles pseudopotential calculations are reported for the lattice distortion and electronic properties of the Al substitutional defect in $\ensuremath{\alpha}$ quartz. We determine microscopical properties of the center such as Al coordination, symmetry of the distorted lattice and defect-induced electronic states. The localization properties of the electronic spin density of this paramagnetic color center are investigated. Our results show that the spin density is evenly distributed on the four oxygen nearest neighbors to Al in contrast to phenomenological model results.

Microscopic structure of oxygen defects in gallium arsenide

Accurate total-energy pseudopotential methods are used to study the structures, binding energies, and local vibrational modes of various models for the Ga-O-Ga defect in GaAs. We find that the previously proposed models, OAs ~an off-centered substitutional oxygen in arsenic vacancy! and OI ~an oxygen atom occupying a tetrahedral interstitial site!, are inconsistent with experimental data. We introduce a model, (AsGa)2-OAs ~two arsenic antisites and one off-centered substitutional oxygen in arsenic vacancy!, the properties of which are in excellent agreement with experimental characterizations. @S0163-1829~99!51648-2#