Defect Loops Research Articles

Atomistic simulation study of clustering and evolution of irradiation-induced defects in zirconium

Zirconium (Zr) alloys have been widely used as structural materials for in-core components in water-cooled nuclear reactors. During normal operation, these materials are exposed to a neutron flux in the core of the reactor, resulting in material degradation such as irradiation-induced anisotropic growth, thus affecting their performance in the long-term. Experimental and theoretical studies have shown that irradiation-induced defects in zirconium lead to the formation of defect clusters and loops. The anisotropy in the migration of defects has been suggested to play an important role in irradiation growth in pure Zr and its alloys. However, the mechanisms that govern the microstructural evolution that lead to the observed anisotropic growth of Zr is still unclear. In the present work, we perform a molecular dynamics simulation study of irradiation-induced lattice defects in Zr to investigate the formation of clusters and loops. Irradiation-induced damage is modeled by constrained stochastic formation of vacancies and self-interstitial atoms in bulk Zr. Using this approach, the formation and evolution of defect clusters and loops were determined. The dynamic properties of lattice defect structures were investigated through the evaluation of their migration and diffusivity. We found that the diffusivity of vacancy and interstitial clusters is anisotropic and slow, while the diffusivity of large loops is relatively high and confined to the ⟨a⟩ plane.

Topology of Three-Dimensional Active Nematic Turbulence Confined to Droplets

Active nematics contain topological defects which under sufficient activity move, create and annihilate in a chaotic quasi-steady state, called active turbulence. However, understanding active defects under confinement is an open challenge, especially in three-dimensions. Here, we demonstrate the topology of three-dimensional active nematic turbulence under the spherical confinement, using numerical modelling. In such spherical droplets, we show the three-dimensional structure of the topological defects, which due to closed confinement emerge in the form of closed loops or surface-to-surface spanning line segments. In the turbulent regime, the defects are shown to be strongly spatially and time varying, with ongoing transformations between positive winding, negative winding and twisted profiles, and with defect loops of zero and non-zero topological charge. The timeline of the active turbulence is characterised by four types of bulk topology-linked events --- breakup, annihilation, coalescence and cross-over of the defects --- which we discuss could be used for the analysis of the active turbulence in different three-dimensional geometries. The turbulent regime is separated by a first order structural transition from a low activity regime of a steady-state vortex structure and an offset single point defect. We also demonstrate coupling of surface and bulk topological defect dynamics by changing from strong perpendicular to inplane surface alignment. More generally, this work is aimed to provide insight into three-dimensional active turbulence, distinctly from the perspective of the topology of the emergent three-dimensional topological defects.

Topological defects in tubular network block copolymers

Materials based on the double gyroid multicontinuous tubular structure can exhibit exotic properties due to the unique 3D network structure. The nature, frequency and potential influence of topological defects present in the interpenetrating gyroid networks on various physical properties is an open question. We examined a polystyrene – polydimethylsiloxane diblock copolymer possessing the double gyroid morphology in order to identify and classify the types of topological defects present. The large volume reconstructions were made by sequentially ion beam milling and low voltage electron beam imaging. Different types of local point-like topological defects were found including node functionality defects, network breaks and network-network joints (bridges) as well as two types of intra-network loop defects. Such topological defects will influence properties ranging from charge and mass transport, to wave propagation and band gaps, and battery performance in materials with the gyroid structure.

Fission product release in thoria and urania defective fuel experiments FDO-680 and FDO-681

FDO-680 (thoria as ThO2-4%UO2) and FDO-681 (urania) were circa 1975 intentionally defected fuel experiments conducted in the National Research Experimental (NRX) reactor X-2 defect loop in Chalk River, Canada. The intent of these experiments was to determine the release rates of fission gases and radioiodines from a defected ThO2-4%UO2 element and to determine the extent to which a moderately powered ThO2-4%UO2 element would deteriorate when defected. Detailed analysis of the FDO-680 and FDO-681 fission product release data has only recently been undertaken. The objective of analyzing this data is to address a gap in defected thoria fuel behaviour knowledge enroute to commercialisation of thoria fuels. The analysis suggests: 1) thoria fuel has superior stability when defected relative to urania, 2) thoria results in a lower fission product burden in the Primary Heat Transport System (PHTS) than urania under steady-state conditions, and 3) the fission product transport models developed for urania cannot be directly applied to thoria.

A defective flexible loop contributes to the processing and gating defects of the predominant cystic fibrosis-causing mutation.

People with the genetic disease cystic fibrosis (CF) often carry a deletion mutation ΔF508 on the gene encoding the CF transmembrane conductance regulator (CFTR) Cl- channel. This mutation greatly reduces the CFTR maturation process and slows the channel opening rate. Here, we investigate whether residues near F508 contribute to these defects in ΔF508-CFTR. Most deletion mutations, but not alanine substitutions, of individual residues from positions 503 to 513 impaired CFTR maturation. Interestingly, only protein processing of ΔY512-CFTR, like that of ΔF508-CFTR, was greatly improved by low-temperature culture at 27°C or small-molecule corrector C18. The 2 mutant Cl- channels were equally slow to open, suggesting that they may share common structural flaws. Studies on the H3-H4 loop that links residues F508 and Y512 demonstrate that G509A/V510G mutations, moving G509 1 position backward in the loop, markedly enhanced ΔF508-CFTR maturation and opening rate while promoting protein stability and persistence of the H3 helix in ΔF508 nucleotide-binding domain 1. Moreover, V510A/S511A mutations noticeably increased ΔY512-CFTR maturation at 27°C and its opening rate. Thus, loop abnormalities may contribute to ΔF508- and ΔY512-CFTR defects. Importantly, correcting defects from G509 displacement in ΔF508-CFTR may offer a new avenue for drug discovery and CF treatments.-Chen, X., Zhu, S., Zhenin, M., Xu, W., Bose, S. J., Wong, M. P.-F., Leung, G. P. H., Senderowitz, H., Chen, J.-H. A defective flexible loop contributes to the processing and gating defects of the predominant cystic fibrosis-causing mutation.

Creation and topological charge switching of defect loops on a long fibre in the nematic liquid crystal

ABSTRACTWe demonstrate new type of topological defects on a homeotropic fibre aligned perpendicular to the nematic director in a planar nematic cell. Contrary to expectations we can create defect loops which are encircling the fibre along its short axis and are strongly tilted with respect to the fibre. Such loops are always accompanied by two topological solitons, which emanate from the loop and propagate to the left- and right-hand side of the fibre. Unlike previously reported closed loops of either positive and negative charge, encircling the fibre parallel to the nematic director, these loops can carry either positive or negative charge, or can be charge neutral and very stable. We show how to switch the charge of individual loops from positive to neutral and negative charge by adding unit monopoles of appropriate topological charge. We demonstrate new type of interaction of dipolar colloids with these new topological entities on a fibre.

Optimalisasi Proses Produksi Celana Panjang Melalui Pendekatan Six Sigma

PT. Tainan Enterprises Indonesia branch of Cianjur produce various kinds of garment products. One of the most widely product is trousers. The product are still many defects. This is showed from the number of defective products during February 2015 until mid- March 2015 which reached 509 products unit of 10.138 trousers produced. Therefore, appropriate quality control is required to optimize the quality of trousers products. This research method uses Six Sigma approach with DMAIC cycle (Define, Measure, Analyze, Improvement, Control). Based on the results of the research is known the most dominant type of defect is oblique belt loop and skid jump defects. Factors causing of oblique belt loop defects are sourced from human factors (operator does not follow signs and operators are fatigue), engine factor (old machine age and engine setting less than perfect), material factor (needle material less reliable and materials need to be heating which is high), and the method factor (information on working methods is still lacking). While the factors causing of skid jump defects are sourced from human factors (operator is less careful in choosing to sewing needles and operators experience fatigue), machine factor (old machine age and no regular machine maintenance), material factor (needle less reliable sewing), and method factors (production supervision and quality control are still weak). Quality Increasing could be can increase the value of Six sigma from 3.82 to 4.27.

Runx1-Stat3 signaling regulates the epithelial stem cells in continuously growing incisors

Rodent incisors grow permanently and the homeostasis of enamel production is maintained by a continuous supply of epithelial progenitors from putative stem cells in the cervical loop. We herein report that Runx1 regulates the Lgr5-expressing epithelial stem cells and their subsequent continuous differentiation into ameloblasts. Mice deficient in epithelial Runx1 demonstrate remarkable shortening of the incisors with underdevelopment of the cervical loop and enamel defects. In this mutant cervical loop, the proliferation of the dental epithelium was significantly disturbed and the expression of Lgr5 and enamel matrix proteins was remarkably downregulated. Interestingly, the expression of Socs3, an inhibitor of Stat3 signaling, was upregulated and Stat3 phosphorylation was suppressed specifically in the mutant cervical loop. The expression of Lgr5 and the enamel matrix protein in the wild-type incisor germs is disturbed by pharmaceutical Stat3 inhibition in vitro., of. Conversely, pharmaceutical activation of Stat3 rescues the defective phenotypes of the Runx1 mutant with upregulated Lgr5 and enamel matrix protein genes. The present results provide the first evidence of the role of Runx1 regulates the Lgr5-expressing epithelial stem cells and differentiation of ameloblast progenitors in the developing incisors. Our study also demonstrates that Stat3 modulates the Runx1-Lgr5 axis in the cervical loop.

Softening due to Grain Boundary Cavity Formation and its Competition with Hardening in Helium Implanted Nanocrystalline Tungsten

The unique ability of grain boundaries to act as effective sinks for radiation damage plays a significant role in nanocrystalline materials due to their large interfacial area per unit volume. Leveraging this mechanism in the design of tungsten as a plasma-facing material provides a potential pathway for enhancing its radiation tolerance under fusion-relevant conditions. In this study, we explore the impact of defect microstructures on the mechanical behavior of helium ion implanted nanocrystalline tungsten through nanoindentation. Softening was apparent across all implantation temperatures and attributed to bubble/cavity loaded grain boundaries suppressing the activation barrier for the onset of plasticity via grain boundary mediated dislocation nucleation. An increase in fluence placed cavity induced grain boundary softening in competition with hardening from intragranular defect loop damage, thus signaling a new transition in the mechanical behavior of helium implanted nanocrystalline tungsten.

Object kinetic Monte Carlo model for neutron and ion irradiation in tungsten: Impact of transmutation and carbon impurities

We upgrade our object kinetic Monte Carlo (OKMC) model, aimed at describing the microstructural evolution in tungsten (W) under neutron and ion irradiation. Two main improvements are proposed based on recently published atomistic data: (a) interstitial carbon impurities, and their interaction with radiation-induced defects (point defect clusters and loops), are more accurately parameterized thanks to ab initio findings; (b) W transmutation to rhenium (Re) upon neutron irradiation, impacting the diffusivity of radiation defects, is included, also relying on recent atomistic data. These essential amendments highly improve the portability of our OKMC model, providing a description for the formation of SIA-type loops under different irradiation conditions. The model is applied to simulate neutron and ion irradiation in pure W samples, in a wide range of fluxes and temperatures. We demonstrate that it performs a realistic prediction of the population of TEM-visible voids and loops, as compared to experimental evidence. The impact of the transmutation of W to Re, and of carbon trapping, is assessed.

Chiral symmetry-breaking dynamics in the phase transformation of nematic droplets

ABSTRACTDynamic simulations of the isotropic–nematic phase transformation of liquid crystal droplets with homeotropic surface anchoring are found to predict chiral symmetry-breaking dynamics. These observations occur when using material parameters for pentyl-cyanobiphenyl (5CB) but not with the single elastic constant approximation for this material, which is frequently used in simulations. The twisting dynamic process occurs during the relaxation of the domain from an unstable radial texture to a stable uniform texture and involves simultaneous defect loop motion and twisting of the bulk nematic texture.

Mechanism of damage buildup in ion bombarded ZnO

Ion beam induced defect processes in wide bandgap compound semiconductors are rather complex because of the formation of different types of defects and their transformations. In this paper, we report results of the study on the mechanism of defect formation, migration and agglomeration in 300 keV Ar-ion bombarded ZnO single crystals. RBS/c analysis performed with the use of the unique McChasy code allowed determination of depth distributions for different defect types separately. Complementary HRXRD analysis was applied to study lattice deformation due to ion bombardment. It has been observed that migration of simple defects and their agglomeration lead to the formation of two types of dislocation loops: basal loops located at the depth corresponding to the range of incident Ar ions and prismatic loops located beyond the ion range. The stress induced by defects in the bombarded layer has been identified as the driving force for defect migration and loop formation. As soon as the critical stress is attained bombarded layer undergoes plastic deformation resulting in the formation of a dense dislocation tangle.

Semibatch monomer addition as a general method to tune and enhance the mechanics of polymer networks via loop-defect control

Controlling the molecular structure of amorphous cross-linked polymeric materials is a longstanding challenge. Herein, we disclose a general strategy for precise tuning of loop defects in covalent polymer gel networks. This "loop control" is achieved through a simple semibatch monomer addition protocol that can be applied to a broad range of network-forming reactions. By controlling loop defects, we demonstrate that with the same set of material precursors it is possible to tune and in several cases substantially improve network connectivity and mechanical properties (e.g., ∼600% increase in shear storage modulus). We believe that the concept of loop control via continuous reagent addition could find broad application in the synthesis of academically and industrially important cross-linked polymeric materials, such as resins and gels.

Electrophoretic assembly and topological weaving of crumpled two-dimensional sheets with entangled defect loops

Transport, relocation, and self-assembly of nano and microparticles in colloidal systems are highly demanded in nanotechnology, photonics, microfluidics, and biotechnology; topological charges can provide an effective means for these purposes. We report that crumpled two-dimensional (2D) graphene oxide (GO) particle sheets in nematic fields can serve as a nest for complicated topological defect loops, which, in turn, provide mobility and inter-adhesiveness to the GO particles. The application of electric fields actuated the GO particles orthogonally, inducing their coalescence into large radial clusters upon absorption of other GO particles. In contrast, in the isotropic phase, where no topological defects existed, the GO particles electrostatically repelled each other owing to the presence of surface charges with equal sign. We also demonstrate that predesigned shallow surface trenches on a substrate can anchor seed GO particles, which attract other GO particles to create a macroscopic structure along the trench.

Quantifying the impact of molecular defects on polymer network elasticity.

Elasticity, one of the most important properties of a soft material, is difficult to quantify in polymer networks because of the presence of topological molecular defects in these materials. Furthermore, the impact of these defects on bulk elasticity is unknown. We used rheology, disassembly spectrometry, and simulations to measure the shear elastic modulus and count the numbers of topological "loop" defects of various order in a series of polymer hydrogels, and then used these data to evaluate the classical phantom and affine network theories of elasticity. The results led to a real elastic network theory (RENT) that describes how loop defects affect bulk elasticity. Given knowledge of the loop fractions, RENT provides predictions of the shear elastic modulus that are consistent with experimental observations.

Stretching the connections

Polymer Science Measuring the elastic behavior of polymers is complicated by their long chain architecture, which can lead to transient entanglements and time-dependent behavior. A further complication arises in the case of covalently linked polymers, such as hydrogels, that contain fixed loop defects. By controlling defect structure during polymer synthesis, Zhong et al. can count the defects in a polymer gel and correlate them to elastic and rheological behavior. Current models are not able to capture their observations, but a real elastic network theory, based on a modified phantom network theory, can account for the impact of topological molecular defects. Science , this issue p. [1264][1] [1]: /lookup/doi/10.1126/science.aag0184

Spatial prediction of crystalline defects observed in molecular dynamic simulations of plastic damage

ABSTRACTMolecular dynamic computer simulation is an essential tool in materials science to study atomic properties of materials in extreme environments and guide development of new materials. We propose a statistical analysis to emulate simulation output with the ultimate goal of efficiently approximating the computationally intensive simulation. We compare several spatial regression approaches including conditional autoregression (CAR), discrete wavelets transform (DWT), and principle components analysis (PCA). The methods are applied to simulation of copper atoms with twin wall and dislocation loop defects, under varying tilt tension angles. We find that CAR and DWT yield accurate results but fail to capture extreme defects, yet PCA better captures defect structure.

Talented 12 class of 2015: Where are they now?

TALENTED 12 Talented 12 class of 2015: Where are they now?Changing the world takes a while. We asked our inaugural class what they’ve been up to this past year. ShareShare onFacebookTwitterWechatLinked InRedditEmail C&EN, 2016, 94 (33), p 50August 22, 2016Cite this:C&EN 94, 33, 50Here, Erb (left) celebrates the feat with grad student Thomas Schwander. “Put your helmets on, we are going to cycle CO2!” (Credit: Erb Lab )Figure1of2Balskus, a microbial metabolism expert, poses with Hitomi Nakamura (left) and Smaranda Bodea (right). (Credit: Courtesy of Emily Balskus)Figure1of2 Brad OlsenMIT Olsen reports that his group published its first paper on loop defects in polymer networks in Physical Review Letters (DOI: 10.1103/physrevlett.116.188302). Tobias ErbMax Planck Institute for Terrestrial Microbiology Erb’s team successfully engineered enzymes to pull CO2 from the atmosphere and efficiently convert it into useful carbon-based compounds. Luke LavisHoward Hughes Medical Institute’s Janelia Research Campus “After a year of heavy traveling and writing papers, I finally got back into the lab and made some dyes with my own hands.” Matt MacDonaldAir Products & Chemicals MacDonald tells us he’s been whipping up new silicon-based compounds to build microelectronic devices with three-dimensional memory technology. Troy ListerSpero Therapeutics Lister disclosed his company’s lead drug candidate, SPR741, for the first time at a meeting in Boston. SPR741 is a molecule that disrupts the outer membrane of resistant bacteria so that antibiotics can sneak in and attack. Denis MalyshevTop secret start-up “I got a job at a biotech company that I’m super stoked about.” The firm is still in stealth mode, though, so Malyshev wouldn’t name names. Kami HullUniversity of Illinois, Urbana-Champaign Hull reports that her lab has “successfully done both directed and undirected anti-Markovnikov-selective amination” this year. (Translation for nonorganickers: They’ve achieved both tight and loose control over a reaction that adds amines to the most crowded carbon in a carbon-carbon double bond.) Hosea NelsonUCLA “My lab ran its first experiment. It worked!” Nelson says. The organic chemist adds that he also taught his first classes ever. “The second class was 100 times better than the first.” Karen HavenstriteTangible Science Havenstrite says her firm was issued two patents and received U.S. Food & Drug Administration clearance for its contact lens technology, which helps treat dry eye. Emily BalskusHarvard “I graduated my first class of Ph.D. students this year!” Jacob HookerHarvard Medical School Hooker celebrated his first graduate student getting her Ph.D. And the radiochemist will soon celebrate the birth of his second child, in mid-September. Matt KananStanford Kanan pointed us to a paper his team published in Nature on a reaction that uses simple molten salts to convert greenhouse gas CO2 into building blocks for making a type of polyester plastic (DOI: 10.1038/nature17185).

P-148: 3D Modelling of Twist Wall at the Electrode Edge of Liquid Crystal Cells

Q-tensor simulation of the liquid crystal structure at the edge of electrodes has been carried out. The modeling shows a twist wall, which reverses direction to form a zig-zag structure. The results are compared with experiment. Also a defect loop is found in micro-lenses formed using a hole electrode structure.

Universe and Matter Conjectured as a 3-Dimensional Lattice with Topological Singularities

One fundamental problem of modern physics is the search for a theory of everything able to explain the nature of space-time, what matter is and how matter interacts. There are various propositions, as Grand Unified Theory, Quantum Gravity, Supersymmetry, String and Superstring Theories, and M-Theory. However, none of them is able to consistently explain at the present and same time electromagnetism, relativity, gravitation, quantum physics and observed elementary particles. In this paper, one summarizes the content of a new book, published in English [2] and in French [3], in which it is suggested that Universe could be a massive elastic 3D-lattice, and that fundamental building blocks of Ordinary Matter could consist of topological singularities of this lattice, namely diverse dislocation loops and disclination loops. For an isotropic elastic lattice obeying Newton’s law, with specific assumptions on its elastic properties, one obtains the result that the behaviours of this lattice and of its topological defects display “all” known physics, unifying electromagnetism, relativity, gravitation and quantum physics, and resolving some longstanding questions of modern cosmology. Moreover, studying lattices with axial symmetries, represented by “colored” cubic 3D-lattices, one has identified a lattice structure whose topological defect loops coincide with the complex zoology of elementary particles, which could open a very promising field of research. Here, only main steps and principal results of the new theory are presented and discussed, without showing the mathematical concepts and developments contained in the book.