Symmetric Twin Research Articles

Faceting of Σ3 Grain Boundaries in Al

The temperature dependence of the energy of various facets of twin GBs has been measured. For the investigation of GB faceting the Al bicrystals of 99.999% wt. purity were grown by the modified Bridgman technique. One grain in these bicrystals is semi-surrounded by another one. Bicrystals were coated with a layer of Sn–Al alloy and annealed at various temperatures. Contact angles at the junction of a GB and two solid/liquid interfaces have been measured. The ratios of GB energy to solid/liquid interface energy have been calculated. Using these data, the Wulff-Herring plots and GB phase diagrams were constructed. Three different crystallographic facets were observed for the coincidence GB. Two of them are stable at all studied temperatures, the third one becomes metastable below ~ 800K. In GBs with θ = 3° only one facet (symmetric twin GB) is stable.

Structural unit and faceting description of Σ3 asymmetric tilt grain boundaries

Atomistic simulations are employed to investigate the structure of Σ3 asymmetric tilt grain boundaries (ATGBs) with boundary planes rotated about the \(\langle 110 \rangle\) misorientation axis in Cu and Al. Results show that the structural units (SUs) and faceting of all 25 Σ3 ATGBs in Cu and Al intermediate to the coherent twin boundary and the symmetric incoherent twin boundary can be completely defined in terms of SUs for these two symmetric boundaries. A structural unit and faceting description for Σ3 asymmetric tilt grain boundaries is presented. Interestingly, this description is identical for both low stacking fault energy Cu and high stacking fault energy Al; only the dissociation width of the D structural unit on the incoherent twin facet differs in Cu and Al. A model based upon the coincidence plot and the structural units of the Σ3 coherent and incoherent twin boundaries is shown to accurately describe the structural units and faceting for all calculated Σ3 asymmetric tilt grain boundaries in this study. This model can also be extended to continuum descriptions of these boundaries to facilitate higher scale computational models.

Crack nucleation at the [formula omitted] symmetrical tilt grain boundary in tungsten

Crack nucleation mechanisms and nucleation conditions are still poorly understood. Experimental evidence suggests that crack nucleation mainly occurs when growing twins, slip or persistent slip bands collide with grain boundaries. We therefore started to investigate these processes by atomistic simulations. As a first step, we present here simulations of crack nucleation at the ∑ 9 ( 2 2 ¯ 1 ) grain boundary in tungsten resulting from the interaction with dislocations. Symmetric twin boundaries and most other high-angle grain boundaries are relatively stable and act as barriers to dislocation motion. When a dislocation is absorbed by such grain boundaries, a stress concentration develops at the absorbing site and a crack is nucleated there. Crack nucleation is investigated under different loading conditions. Our results show that crack nucleation is aided significantly by additional dislocation activity. The relevant mechanisms are significantly more complex than captured by simple pile-up models.

Four decades of conjoined twins at Red Cross Children's Hospital--lessons learned.

Conjoined twins represent a rare but fascinating congenital condition, the aetiology of which remains obscure. Over the past four decades, the paediatric surgeons at Red Cross Children's Hospital have been involved in the management of 46 pairs of conjoined twins, of which 33 have been symmetrical and 12 asymmetrical. Seventeen symmetrical twins have undergone separation with 22 children (65%) surviving; all of the live asymmetrical twins survived separation. We describe the important features of this unique cohort, outline our approach to management and present the results of this approach. We consider some of the ethical and moral dilemmas we have confronted, and discuss the prenatal diagnosis, obstetric implications and postnatal care of these children, including the relevant investigations and anaesthetic and surgical management. Specific aspects related to the cardiovascular system, hepatobiliary and gastrointestinal tracts, urogenital tract, central nervous system and musculoskeletal system are highlighted.

Liquid-Crystalline Dimers Composed of Bent-Core Mesogenic Units

Systematic studies on symmetric twin molecules composed of two bent-core mesogenic units are reported for the first time. It is shown that the formation of mesophases mainly depends on the chemical structure of the spacer. A sufficiently flexible spacer containing dimethylsiloxane units allows a decoupling of the mesogenic moieties; the related compounds exhibit a ferroelectric SmCP phase proved by electro-optical measurements. On the basis of X-ray data, a plausible structure model is presented. The connection of two bent-core mesogenic units by means of a tetraethylene glycol spacer results in dimers, which are able to form columnar phases. For twins containing a simple hydrocarbon chain as spacer, in most cases liquid-crystalline behavior could not be detected. These general tendencies remain valid if the bent-core unit is changed, e.g., by increasing the number of aromatic rings, by lateral substitution, and by inversion of the direction of one of the connecting groups between the aromatic rings, resp...

Effect of the flow field of mechanical bileaflet mitral prostheses on valve closing

Antianatomically installed Jyros (JR) and ATS valves were compared with the St. Jude Medical (SJM) valve in the mitral position to study the effects of valve design on the downstream flow field and associated closing sounds using a particle image velocimetry (PIV) method utilizing a high-speed video flow visualization technique to map the velocity field and sound measurement to confirm claims by the manufacturer. Based on the experimental data, the following general conclusions can be made: in the velocity field directly below the mitral valve, where distinct characteristic differences in valve design can be seen, symmetrical twin circulations were observed because of the divergent nature of the flow generated by the two inclined half-disks installed in the antianatomical orientation; the SJM valve, which produced central downward circulation, is contrasted to the two other valves, which produced peripheral downward circulation. These differences may play an important role in the closing behavior of the valve leaflets, thus affecting the generation of the valve closing sound.

Time-resolved particle image velocimetry and laser doppler anemometry study of the turbulent flow field of bileaflet mechanical mitral prostheses

Dynamic particle image velocimetry (PIV) was applied to the study of the flow field associated with prosthetic heart valves. The results were compared with those of laser Doppler anemometry (LDA). Anatomically and antianatomically oriented Jyros (JR) and St. Jude Medical (SJM) valves were compared in the mitral position to study the effects of valve design on the downstream flow field. The experimental program used a dynamic PIV system utilizing high-speed, high-resolution video to map the true time-resolved velocity field inside the simulated ventricle. This system was complemented by a study using the more traditional LDA system for comparison. Based on the experimental data, the following general conclusions can be made. High-resolution dynamic PIV can capture true chronological changes in the velocity and turbulence fields. It also produces very detailed velocity and turbulence information comparable to the LDA results. In the vertical measuring plane that passes both the center of the aortic and mitral valves (A-A section), the two valves (the SJM and the JR) show distinct circulatory flow patterns when the valve is installed in the antianatomical orientation. Small differences in valve design can generate noticeable differences, particularly during the accelerating flow phase. The SJM valve maintains a relatively high velocity through the central orifice; the curved leaflets of the JR valve generate higher velocities with a divergent flow during the accelerating and peak flow phases. In the velocity field directly below the mitral valve and normal to the previous measuring plane (B-B section), where characteristic differences in valve design will be visible, symmetrical twin circulations were observed because of the divergent nature of the flow generated by the two inclined half-disks installed in the antianatomical orientation. The SJM valve, with a central downward flow near the valve, is contrasted with the JR valve, which has a peripheral downward circulation with higher, turbulent stresses.

Microstructure of top seeded melt-grown YBCO bulks with holes

Single-grain Y123/211 TSMG bulks with holes parallel to thec-axis were successfully grown. We show that the growth is more perfect when the seed is positionedin such a way that the growth sector boundary does not meet the holes. Collision of thea-growth sector with the hole leads to the formation of closeda-growth subsectors and does not disturb the growth front of a 123 crystal. Microstructureanalysis reveals significant reduction of sample porosity, formation of a 211 lowconcentration layer and a subgrain free region along the hole. Isothermal oxygenation at440 °C induces a circular stress field along the holes, whose tangential component is responsible for radialc-cracking starting at the surface of the hole. The stress field formationalong the holes during the oxygenation was confirmed by the observationof a symmetric twin complex pattern around the holes. The radialc-cracks degrade the trapped field of the sample, which is apparent in the measured trappedfield profile.

Comparison of direct numerical simulation of lean premixed methane–air flames with strained laminar flame calculations

Direct numerical simulation (DNS) with complex chemistry was used to study statistics of displacement and consumption speeds in turbulent lean premixed methane–air flames. The main focus of the study is an evaluation of the extent to which a turbulent flame in the thin reaction zones regime can be described by an ensemble of strained laminar flames. Conditional averages with respect to strain for displacement and consumption speeds are presented over a wide range of strain typically encountered in a turbulent flame, compared with previous studies that either made local pointwise comparisons or conditioned the data on small strain and curvature. The conditional averages for positive strains are compared with calculated data from two different canonical strained laminar configurations to determine which is the optimal representation of a laminar flame structure embedded in a turbulent flame: the reactant-to-product (R-to-P) configuration or the symmetric twin flame configuration. Displacement speed statistics are compared for the progress-variable isosurface of maximum reaction rate and an isosurface toward the fresh gases, which are relevant for both modeling and interpretation of experiment results. Displacement speeds in the inner reaction layer are found to agree very well with the laminar R-to-P calculations over a wide range of strain for higher Damköhler number conditions, well beyond the regime in which agreement was expected. For lower Damköhler numbers, a reduced response to strain is observed, consistent with previous studies and theoretical expectations. Compared with the inner layer, broader and shifted probability density functions (PDFs) of displacement speed were observed in the fresh gases, and the agreement with the R-to-P calculations deteriorated. Consumption speeds show a poorer agreement with strained laminar calculations, which is attributed to multidimensional effects and a more attenuated unsteady response to strain fluctuations; however, they also show less departure from the unstrained laminar value, suggesting that detailed modeling of this quantity may not be critical for the conditions considered. For all quantities investigated, including CO production, the R-to-P laminar configuration provides an improved description relative to the twin flame configuration, which predicts qualitatively incorrect trends and overestimates extinction.

Congenital Duplication of the Caudal Region (Monocephalus Dipygus) in a Kid Goat

A case of congenital duplication (monocephalus dipygus) in a goat is described. Two pelvis and four pelvic limbs were observed in the kid. Conjoined or fused symmetric twins were diagnosed. Associated abnormalities were cleft palate and anal atresia. Most of the classically recognized teratogens were ruled out by history and serology. However, progenitors were related in the second degree. Thus, genetic factors could be suspected in this case. To the authors' best knowledge, this is the first report of monocephalus dipygus in a goat.

A study on the effect of carboxylate ester and thioester linking groups in ferroelectric liquid crystal dimers

Two series of symmetrical liquid crystal twins possessing a chiral centre at the inner side of the molecules were synthesized and characterized. Structural effects on the mesomorphic and physical properties were investigated in terms of (a) variation of carboxylate and thioester groups linking the spacer and core, (b) variation in achiral chain length between two mesogenic units. The mesomorphic investigation reveals that these compounds exhibit a BP, N*, SmA* and SmC* mesophase sequence for the BDPNP series, and a SmA and SmC* sequence for the ABPNTP-n series. The P s values were measured and a largest of 38.5 nC cm−2 was observed for ABPNTP-12.

Effect of introducing thioether linkages on the molecular organization of chiral twin liquid crystals

Novel symmetric and non-symmetric chiral twin compounds possessing one or two thioether groups in a central spacer were prepared, and the effect of substituting oxygen for sulphur on the liquid crystalline properties investigated. Chiral twin compounds possessing an alkylsulphanyl spacer showed an antiferroelectric phase exclusively. However, replacing the alkyloxy chain of the analogous monomer by the alkylsulphanyl chain has no significant effect on the phase transition behaviour, i.e. both of the monomers showed the ferro- and ferri-electric phases as well as the antiferroelectric phase. Thus, different effects of introducing the thioether linkage were for the first time observed between twin and monomeric systems. The introduction of oxygen or sulphur atoms into the central alkyl spacer of the chiral twin was also investigated, and these modifications were found to stabilize the SmA phase. Furthermore, the twin compound possessing a thiaalkyl spacer showed two different molecular assemblies in the smectic A phase.

Impurity and vacancy clustering at the Σ3(111)[1−10] grain boundary in strontium titanate

The interaction of selected point defects and the Σ3(1 1 1)[1 −1 0] symmetrical twin boundary in SrTiO3 is investigated by density-functional band-structure calculations. The pristine boundary is SrO3-terminated and exhibits electronic properties which are comparable with the pure bulk phase. Both the mirror-symmetric twin and some laterally shifted structures have low grain boundary energies, thus they may coexist in a real crystal if external stress is applied. With varying chemical potential of the electrons, the TiO3 sublattice of the pristine boundary is destabilised by both electron enrichment and electron depletion. These results may explain, why different translation states can be observed by electron microscopy within the same bicrystal. Substitutional doping of the Ti columns next to the boundary plane with formally 3+ cations yields a contraction compared with the pure boundary. The same result, accompanied by more pronounced local geometry changes, is obtained for the translation state in the presence of O vacancies. Neutral O vacancies in the boundary plane are the most stable species followed by positively charged O vacancies in the neighbouring Sr/O plane parallel to the boundary. The Fermi level is shifted either upward to metal-derived states by substitutional doping or downward into the O-2p manifold for O vacancies. These findings may rationalise both the observed low, mainly electronic, grain boundary conductivity of SrTiO3 boundaries and the changes in local potential found at the grain boundary by tunnelling and force microscopy experiments.

Numerical Experiment of Flow around a Self-Oscillating Circular Cylinder by a Vortex Method Combined with Boundary Layer Calculation. (The Flow Features and the Motion of Circular Cylinder in the Initial Stage of Self-Oscillation)

In this study, the flow features of vortex shedding from a self-oscillating circular cylinder and the characteristics of fluid force were examined using a vortex method simulation combined with boundary layer calculation in a uniform flow at high Reynolds number in order to investigate validity and applicability to the analysis of the flow-induced vibration problems. The calculations were performed for the parameters of the mass ratio M (=1.78, 5.15), the damping factor C (=0.0, 0.0153) and the reduced velocity Vr (=1.38-4.13) at the Reynolds number Re=1×105 based on the uniform flow velocity U and the cylinder diameter D. As a result of the calculations, two typical patterns of vortex shedding were observed, one of which is symmetric twin vortex shedding and the other is antisymmetric twin vortex shedding. It was confirmed that the calculated flow patterns and the free-oscillation motions of the cylinder, were in reasonable agreement with experimental results. The relationship between the fluid force and the resultant cylinder's displacement was investigated.

A symmetric chiral liquid-crystalline twin exhibiting stable ferrielectric and antiferroelectric phases and a chirality-induced isotropic–isotropic liquid transition

In this article we report the first example of a pair of symmetrically substituted chiral twin enantiomers that exhibit stable wide temperature range ferrielectric and antiferroelectric phases, and which have large tilt angles of over 45°. In addition, both of the enantiomers exhibited an isotropic–isotropic liquid transition just above the transition to the liquid crystalline state.

Directional Effects of the Carbonyl Group on the Mesogenic Properties of Twin Troponoids

Transition temperatures of four twin types of troponoid liquid crystals, in which the direction of the tropone carbonyl groups and a kind of the spacer were different from each other, were discussed. Symmetrical twin dimers 1, in which two tropone carbonyl groups direct inside, had monotropic smectic C phases whereas another symmetrical twin dimers 3 were less mesogenic than 1. Unsymmetrical twin dimers 4 had smectic A phases. Compared their thermal stabilities, unsymmetrical 4 had the highest clearing point when the number of atoms of the inner spacer was identical.

STUDY OF M82 USING SPECTRA FROM THE INFRARED SPACE OBSERVATORY

We have studied the central parts of M82, which is a well-known infrared luminous, starburst galaxy, by analyzing archival data from the Infrared Space Observatory (ISO). M82 was observed at 11 positions covering 45 from the center along the major axis. We analyzed 4 emission lines, [ArIII] 8.99 , 17.034 , [FeII] 25,98 , and [SiII] 34,815 from data. The integrated flux distributions of these lines are quite different. The line shows symmetric twin peaks at 18 from the center, which is a general characteristic of molecular lines in starburst or barred galaxies. This line appears to be associated with the rotating molecular ring at around 200 pc just outside the inner spiral arm. The relative depletion of the line at the center may be due to the active star formation activity which dissociates the molecules. The other lines have peaks at the center and the distributions are nearly symmetric. The line profiles are deconvolved assuming that both intrinsic and instrumental profiles are Gaussian. The velocity dispersion outside the core is found to be . The central velocity dispersion is much higher than , and different lines give different values. The large central velocity dispersion () is mostly due to the rotation, but there is also evidence for a high for [ArIII] line. We also generated position-velocity maps for these four lines. We found very diverse features from these maps.

Faceting and Roughening of the Asymmetric Twin Grain Boundaries in Zinc

Faceting of grain boundaries (GBs) or surfaces can be considered as a phase transition when the original surface or GB dissociates onto flat segments whose energy is less than that of original surface or GB. Zn [1120] flat single crystals were grown using the modified Bridgman technique from Zn of 99.999 wt% purity. Individual elongated twin plates having very uniform thickness were produced with the aid of slight deformation of single crystals. Parallel elongated sides of the twin plate are formed by the coherent symmetric twin (1102)1‖(1102)2 grain boundary (STGB) facets. Due to its optical anisotropy, zinc allows one to study the shape of the GB with the aid of polarised light. The stationary shape of the slowly migrating tip of the twin plate has been studied in situ. The hot stage of optical microscope was used. The temperature interval from 592 to 692 K was investigated. Below 632 K the twin tip contains only one plane facet 1 which is nearly parallel to the (1102)2 plane and has the angle of 84° with the coherent STGB. Above 632 K the second facet 2 appears at the tip of the twin plate. This facet is nearly parallel to the (1100)1 plane and has the angle of 46° with the coherent STGB. Between 632 and 682 K both 84° and 46° facets coexist, and 84° facet gradually disappear with increasing temperature. Above 682 K only 46° facet is present in the twin tip. The indications of the GB roughening phase transition were also observed, namely the edges of the facets become smoother with increasing temperature. The GB phase diagram for the twin GBs in zinc containing the lines of two GB faceting phase transitions has been constructed. Schematic Wulff-Herring diagrams explaining these transitions are presented.

Impact of head-to-abdominal circumference asymmetry on outcomes in growth-discordant twins

Objective: Our aim was to evaluate head-to-abdominal circumference asymmetry as a marker for adverse outcomes in growth-discordant twins. Study Design: We conducted a retrospective cohort study of asymmetric and symmetric twins with ≥25% growth discordance, comparing their outcomes with those in concordant symmetric twins. Growth was termed asymmetric on the basis of a head circumference/abdominal circumference ratio at ≥95th percentile on ultrasonography performed ≤4 weeks before delivery. Results: We evaluated 572 twin pairs. Asymmetric discordant twins were more likely than symmetric concordant twins to be delivered at ≤34 weeks’ gestation (57% vs 27%), to require intubation (36% vs 7%), to remain in intensive care >1 week (36% vs 3%), and to have an outcome composite that included respiratory morbidity, intraventricular hemorrhage, sepsis, or neonatal death (29% vs 6%), all P <.05. Symmetric discordant and symmetric concordant twins had similar outcomes. Conclusions: Discordant twins with head-to-abdominal circumference asymmetry have an increased risk of morbidity. Moreover, in the absence of asymmetry, outcomes are comparable among discordant and concordant twins. (Am J Obstet Gynecol 2000;183:1082-7.)

Grain-boundary structures in polycrystalline metals at the nanoscale

We present a detailed analysis of grain-boundary structures in computer-generated Cu and Ni three-dimensional nanocrystalline samples. The study includes both totally random and textured microstructures with grain sizes in the range of 5\char21{}12 nm. A detailed direct visualization technique is used at the atomic scale for studying the grain-boundary structural features. The study focuses on determining the presence of regions in the boundary exhibiting order and structural units normally expected for high-angle boundaries. For low-angle boundaries we investigate the presence of dislocation networks accommodating the misfit between the grains. A significant degree of crystalline order is found for all the boundaries studied. The highest degree of structural order was identified for boundaries with misfits within about 10\ifmmode^\circ\else\textdegree\fi{} deviation from the perfect twin. These grain boundaries contain a repeated building structure consisting of structural units typical of a $\ensuremath{\Sigma}=3$ symmetrical tilt twin boundary and highly disordered steps between those structural units. For all other types of misfit, we also observe some degree of structural coherence, and misfit accommodation occurs in a regular pattern. The cases studied include grain boundaries with a high-index common axis and show structural coherency that is independent of the grain size. Similar results are obtained for textured samples containing only low-angle grain boundaries, where regular dislocation arrays that are typical of larger grain materials are observed. These results provide evidence against the view of grain boundaries in nanocrystals as highly disordered, amorphous, or liquidlike interfaces. The results suggest that the grain-boundary structure in nanocrystalline materials is actually similar to that found in larger grain polycrystals.