Small-angle grain boundaries in quasicrystals

The Read-Shockley treatment of small-angle grain boundaries in crystals is generalized to the case of quasicrystals. The dependence of the grain-boundary energy on the angle of mismatch between abutting quasicrystalline grains is calculated. It is found that, even for a symmetric tilt boundary in a quasicrystal, dislocations with at least two types of Burgers vectors are required; these dislocations have to be arranged quasiperiodically along the boundary. The possible clumping of these dislocations to form composites is discussed. Explicit calculations are presented for a pentagonal quasicrystal

Small-Angle Grain Boundaries in Quasicrystals

The Read-Shockley treatment of small-angle grain boundaries in crystals is generalized to the case of quasicrystals. The dependence of the grain-boundary energy on the angle of mismatch between abutting quasicrystalline grains is calculated. It is found that, even for a symmetric tilt boundary in a quasicrystal, dislocations with at least two types of Burgers vectors are required; these dislocations have to be arranged quasiperiodically along the boundary. The possible clumping of these dislocations to form composites is discussed. Explicit calculations are presented for a pentagonal quasicrystal. Translationally ordered solids, either crystals or quasicrystals, can have grain boundaries, which are interfaces between regions with different orientations. Such boundaries are well known in periodic crystals,' and they have been reported recently in quasicrystals. 2 Frank 3 and Read and Shockley 4 have shown how small-angle grain boundaries in crystals can be described as arrays of dislocations [planar (linear) arrays in three (two) dimensions]. Read and Shockley use this description to calculate the dependence of the (zero-temperature) grainboundary energy on the angle of mismatch between abutting crystalline grains. In this paper we generalize the Read-Shockley treatment of small-angle grain boundaries to quasicrystals. Our principal results are as follows: (l) Whereas a symmetric tilt boundary (the simplest) in a crystal can be obtained by using an array of only one type of dislocation, 4 in a quasicrystal it can be obtained only by using an array of at least two types of dislocations, alternating quasiperiodically along the boundary. The dislocations in the array must be chosen and arranged in such a way that the sum of their Burgers vectors has a phonon part that scales as L and a phason part that vanishes as L ~[, where L is the linear size of the array. This was shown first for an incommensurate smectic liquid crystal. 5 (2) An array of dislocations that does not satisfy the above conditions leads to strains in the quasicrystals that do not vanish infinitely far from the array. Thus, the energy -T; \UiiUn /j= y /« y -+ fly«,-) is the strain tensor, w// = di\Vj, and we sum over repeated i and j indices. Note that E e \ has rotational invariance built into it: Fields with constant, finite Vxu do not cost any energy. However, w fields with constant, finite Vxw do cost energy, because they lead to relative rotations of the density waves (see below) of which the quasicrystal is constituted. It is easy to see now why in a quasicrystal an array of only one type of dislocation yields an energy proportional to L d : As in a crystal, such an array leads to a relative rotation of the regions of quasicrystal on either side of cost of such an array scales as L d for a ^-dimensional quasicrystal and not as L d~' as required for a true grain boundary. (3) As in a crystal, 4 the intensive energy per unit area (length if d = 2) of a grain boundary is E =C\6 -C2#ln0, where 9 is the tilt angle, and Ci and Ci depend on the orientation of the grain boundary and on the elastic constants of the quasicrystal. We calculate these for a symmetric tilt boundary in a pentagonal quasicrystal, 7 In the remaining part of this paper we give the arguments that lead to the results summarized above. These arguments hold for all quasicrystals with dimensions d>2. We substantiate these arguments with explicit calculations for two-dimensional, pentagonal quasicrystals. The elastic energy density of a quasicrystal depends on derivatives of two fields: u (the phonon part) and w (the phason part). [Spatially uniform u (i.e., uniform translations) and spatially uniform w (this shifts density waves relative to one another in a density-wave description) do not cost any energy.] For example, for pentagonal quasicrystals, the elastic energy density is 7 ' I the array; i.e., far from the array there is a u field with uniform nonzero Vxu, which costs no energy. In a quasicrystal, however, energy dislocation has both u and w parts, and so the same argument yields a constant, nonzero Vx w far from the array. Such a field has a nonzero energy density at infinity. Thus the total energy of an array of only one type of dislocation scales as L d in a quasicrystal. Such an array is clearly not a grain boundary. We now show how, using more than one type of dislocation, it is possible to construct an array which leads t

Conversion of ethanol to higher alcohols on Ni/MxOy-Al2O3 (M=La, Ce, Zr, Mg and Ti) catalysts: Influence of support characteristics

9-22A new series of alumina supported nickel (8% w/w) catalysts, modified with promoters, La2O3, CeO2, ZrO2, MgO and TiO2, highly active for the conversion of ethanol to butanol and higher alcohols, at 200°C-220°C, in batch mode, under autogenous pressure, has been investigated. XRD and XPS results indicate the presence of metallic Ni and Ni aluminate as the active phases. H2-TPR studies reveal that the introduction of promoters improves nickel dispersion, reducibility and moderates the metal-support interactions.TPD of ammonia and CO2 studies establish the strong influence of the promoter oxides on the strength and population of acidic and basic sites. Ethanol conversion at 200°C varies in a narrow range, 36-42%. CeO2 and MgO modified catalysts display maximum selectivity towards butanol (48%) and higher alcohols, (81% and 75%) in comparison with the catalyst based on pristine alumina (28.9% and 40.5%). While the selectivity for butanol and higher alcohols is governed by the basicity of the catalysts, both metal function and basicity are required to drive ethanol conversion. Moderation of acidity helps in minimizing the formation of ethylene and other gaseous products. Analysis of used catalyst indicates that the structural and active phase characteristics are retained during use

Particle-resolved lattice Boltzmann simulations of 3-dimensional active turbulence

Collective behaviour in suspensions of microswimmers is often dominated by the impact of long-ranged hydrodynamic interactions. These phenomena include active turbulence, where suspensions of pusher bacteria at sufficient densities exhibit large-scale, chaotic flows. To study this collective phenomenon, we use large-scale (up to $N=3\times 10^6$) particle-resolved lattice Boltzmann simulations of model microswimmers described by extended stresslets. Such system sizes enable us to obtain quantitative information about both the transition to active turbulence and characteristic features of the turbulent state itself. In the dilute limit, we test analytical predictions for a number of static and dynamic properties against our simulation results. For higher swimmer densities, where swimmer-swimmer interactions become significant, we numerically show that the length- and timescales of the turbulent flows increase steeply near the predicted finite-system transition density

Giant breast tumors: Surgical management of phyllodes tumors, potential for reconstructive surgery and a review of literature

<p>Abstract</p> <p>Background</p> <p>Phyllodes tumors are biphasic fibroepithelial neoplasms of the breast. While the surgical management of these relatively uncommon tumors has been addressed in the literature, few reports have commented on the surgical approach to tumors greater than ten centimeters in diameter – the giant phyllodes tumor.</p> <p>Case presentation</p> <p>We report two cases of giant breast tumors and discuss the techniques utilized for pre-operative diagnosis, tumor removal, and breast reconstruction. A review of the literature on the surgical management of phyllodes tumors was performed.</p> <p>Conclusion</p> <p>Management of the giant phyllodes tumor presents the surgeon with unique challenges. The majority of these tumors can be managed by simple mastectomy. Axillary lymph node metastasis is rare, and dissection should be limited to patients with pathologic evidence of tumor in the lymph nodes.</p

Retrospective evaluation of whole exome and genome mutation calls in 746 cancer samples

Funder: NCI U24CA211006Abstract: The Cancer Genome Atlas (TCGA) and International Cancer Genome Consortium (ICGC) curated consensus somatic mutation calls using whole exome sequencing (WES) and whole genome sequencing (WGS), respectively. Here, as part of the ICGC/TCGA Pan-Cancer Analysis of Whole Genomes (PCAWG) Consortium, which aggregated whole genome sequencing data from 2,658 cancers across 38 tumour types, we compare WES and WGS side-by-side from 746 TCGA samples, finding that ~80% of mutations overlap in covered exonic regions. We estimate that low variant allele fraction (VAF < 15%) and clonal heterogeneity contribute up to 68% of private WGS mutations and 71% of private WES mutations. We observe that ~30% of private WGS mutations trace to mutations identified by a single variant caller in WES consensus efforts. WGS captures both ~50% more variation in exonic regions and un-observed mutations in loci with variable GC-content. Together, our analysis highlights technological divergences between two reproducible somatic variant detection efforts

Reactions of 1,3,5-benzenetricarboxylic acid with Zn(II) ion in the presence of added amines: Isolation and structure determination of zinc coordination polymers with tetrahedral and octahedral zinc centres^§,†

2267-2276The formation of two metal-organic coordination polymers and an organic supramolecular network based on 1,3,5-benzenetricarboxylic ac id (H3BTC) is reported. While [Zn3(BTC)2(DMP)6 2.5H2O] (1) is formed by the addition of zinc acetate and 3,5-dimethylpyrazole (DMP) to H3BTC, the addition of zinc acetate to 4-aminopyridine and H3BTC results in previously reported [Zn3(BTC)2.12H2O] (2). On the other hand, the interaction of H3BTC with ZnSO4 in the presence of pi-perazine (PIP) leads to the formation of [H2BTC]2[H2PIP] (3) as soluble single crystals in addition to un insoluble zinc-BTC- piperazine complex with unknown composition. The solid -state structures as determined by single crystal X-ray diffraction studies reveal that compounds 1-3 form 3-D polymeric networks with the aid of extensive hydrogen bonding. In 1, the adjacent dimeric units of [(DMP)4Zn2(BTC)2] are bridged by [Zn(DMP)2] units to form a polymeric chain structure. These chains are in turn linked to each other through hydrogen bonding to form a 2-D sheet network. The 2-D sheets are held together by weak π-π interactions to form a 3-D network. Compound 2 is made up of a zig-zag chain of alternating BTC3- ion and Zn(H2O)4 octahedral units. These chains are held together by numerous hydrogen bonding interactions resulting in a porous solid. The supramolecular assembly in 3 can be described as a host-guest complex in which the hydrogen bonded [H2BTC-]n network hosts the piperazinium cations

Reactions of 1,3,5-benzenetricarboxylic acid with Zn(II) ion in the presence of added amines: Isolation; structure determination of zinc coordination polymers with tetrahedral; octahedral zinc centres

The formation of two metal-organic coordination polymers and an organic supramolecular network based on 1,3,5-benzenetricarboxylic acid (H3BTC) is reported. While [Zn3(BTC)2(DMP)6.2.5H2O] (1) is formed by the addition of zinc acetate and 3,5-dimethylpyrazole (DMP) to H3BTC, the addition of zinc acetate to 4-aminopyridine and H3BTC results in previously reported [Zn3(BTC)2.12H2O] (2). On the other hand, the interaction of H3BTC with ZnSO4 in the presence of piperazine (PIP) leads to the formation of [H2BTC]2[H2PIP] (3) as soluble single crystals in addition to an insoluble zinc-BTC-piperazine complex with unknown composition. The solid-state structures as determined by single crystal X-ray diffraction studies reveal that compounds 1-3 form 3-D polymeric networks with the aid of extensive hydrogen bonding. In 1, the adjacent dimeric units of [(DMP)4Zn2(BTC)2] are bridged by [Zn(DMP)2] units to form a polymeric chain structure. These chains are in turn linked to each other through hydrogen bonding to form a 2-D sheet network. The 2-D sheets are held together by weak π-π interactions to form a 3-D network. Compound 2 is made up of a zig-zag chain of alternating BTC3- ion and Zn(H2O)4 octahedral units. These chains are held together by numerous hydrogen bonding interactions resulting in a porous solid. The supramolecular assembly in 3 can be described as a host-guest complex in which the hydrogen bonded [H2BTC]n network hosts the piperazinium cations