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91.
Raheleh Nikonam M. Martin D. Pugh Robin A.L. Drew 《Journal of the European Ceramic Society》2019,39(4):915-927
In porous reaction bonded silicon nitride, whiskers normally grow in globular clusters as the dominant morphology and deteriorate the pore interconnectivity. However, the ceramic microstructure was significantly transformed with the addition of MgO; specifically, the morphology was modified to a combination of matte and hexagonal grains. Microstructural observation along with thermodynamic studies suggest that MgO interfered with the presence and nitridation of SiO(g). Consequently, rather than being involved in the whiskers’ formation, surface silica instead reacted with volatile MgO to form intermediate products. Through these reactions, whisker formation was blocked, and a porous interconnected structure formed which was confirmed by 3D tomography. After heat-treatment at 1700 °C, β-Si3N4 crystallized in a glassy matrix containing magnesium. Resulting samples had an open-pore structure with porosity of 74–84 vol. %, and density of 0.48-0.75 g.cm?3. Combination of high porosity and pore size of <40 μm led to compressive strengths of 1.1–1.6 MPa. 相似文献
92.
Optimal designs depend upon a prespecified model form. A popular and effective model-robust alternative is to design with respect to a set of models instead of just one. However, model spaces associated with experiments of interest are often prohibitively large and so algorithmically generated designs are infeasible. Here, we present a simple method that largely eliminates this problem by choosing a small set of models that approximates the full set and finding designs that are explicitly robust for this small set. We build our procedure on a restricted columnwise-pairwise algorithm, and explore its effectiveness for two model spaces in the literature. For smaller full model spaces, we find that the designs constructed with the new method compare favorably with robust designs that use the full model space, with construction times reduced by orders of magnitude. We also construct designs that heretofore have been unobtainable due to the size of their model spaces. Supplementary material (available online) includes code, designs, and additional results. 相似文献
93.
Enrico Ianni M. Veronica Sofianos Drew A. Sheppard Matthew R. Rowles Terry D. Humphries Shaomin Liu Craig E. Buckley 《Journal of Materials Science》2018,53(2):1076-1087
A simple and cost-effective method for the synthesis of a porous Al scaffold has been optimised using only NaAlH4 and TiCl3. The starting materials were compacted into a pellet and sintered under dynamic vacuum to remove the Na and H2. The sintering conditions, such as vacuum level, temperature, and time, were the key factors that influenced both the extraction of Na and H2 from the pellet and the overall porosity. Quantitative phase analysis by X-ray diffraction revealed that after the sintering process, the as-prepared porous Al scaffold consisted primarily of Al. Morphological observations conducted by scanning electron microscopy showed that the scaffold exhibited an open network of pores with a small number of mesopores and no formation of micropores. The specific surface area of the scaffold was determined to be 7.9 ± 0.1 and 6.0 ± 0.5 m2/g by the Brunauer–Emmet–Teller method and from small-angle X-ray scattering measurements, respectively. The total porosity of the Al scaffold was 44.6%. 相似文献
94.
The presence of magnesium, calcium and oxygen has been detected in the intergranular fracture surface of an Si3N4-7% MgO hot-pressed material by Auger electron spectroscopy, together with a chemical shift in the silicon Auger peak. The composition of this grain-boundary phase has been estimated as (0.40 ± 0.03) CaO. (0.75 ± 0.10) MgO. 2SiO2 by comparison with spectra obtained from a bulk glass specimen of similar composition. The reduction in strength of the hot-pressed material at temperatures above 1000° C has been attributed to the decrease in viscosity of this phase. 相似文献
95.
Mariusz Mital Kosma Szutkowski Karolina Bossak-Ahmad Piotr Skrobecki Simon C. Drew Jarosaw Poznaski Igor Zhukov Tomasz Frczyk Wojciech Bal 《International journal of molecular sciences》2020,21(23)
The A peptide is a major beta-amyloid species in the human brain, forming toxic aggregates related to Alzheimer’s Disease. It also strongly chelates Cu(II) at the N-terminal Phe-Arg-His ATCUN motif, as demonstrated in A and A model peptides. The resulting complex resists ROS generation and exchange processes and may help protect synapses from copper-related oxidative damage. Structural characterization of Cu(II)A complexes by NMR would help elucidate their biological function, but is precluded by Cu(II) paramagneticism. Instead we used an isostructural diamagnetic Pd(II)-A complex as a model. To avoid a kinetic trapping of Pd(II) in an inappropriate transient structure, we designed an appropriate pH-dependent synthetic procedure for ATCUN Pd(II)A, controlled by CD, fluorescence and ESI-MS. Its assignments and structure at pH 6.5 were obtained by TOCSY, NOESY, ROESY, H-C HSQC and H-N HSQC NMR experiments, for natural abundance C and N isotopes, aided by corresponding experiments for Pd(II)-Phe-Arg-His. The square-planar Pd(II)-ATCUN coordination was confirmed, with the rest of the peptide mostly unstructured. The diffusion rates of A, Pd(II)-A and their mixture determined using PGSE-NMR experiment suggested that the Pd(II) complex forms a supramolecular assembly with the apopeptide. These results confirm that Pd(II) substitution enables NMR studies of structural aspects of Cu(II)-A complexes. 相似文献
96.
Nicholas R. Glavin Christopher Muratore Michael L. Jespersen Jianjun Hu Phillip T. Hagerty Al M. Hilton Austin T. Blake Christopher A. Grabowski Michael F. Durstock Michael E. McConney Drew M. Hilgefort Timothy S. Fisher Andrey A. Voevodin 《Advanced functional materials》2016,26(16):2640-2647
Next‐generation nanoelectronics based on 2D materials ideally will require reliable, flexible, transparent, and versatile dielectrics for transistor gate barriers, environmental passivation layers, capacitor spacers, and other device elements. Ultrathin amorphous boron nitride of thicknesses from 2 to 17 nm, described in this work, may offer these attributes, as the material is demonstrated to be universal in structure and stoichiometric chemistry on numerous substrates including flexible polydimethylsiloxane, amorphous silicon dioxide, crystalline Al2O3, other 2D materials including graphene, 2D MoS2, and conducting metals and metal foils. The versatile, large area pulsed laser deposition growth technique is performed at temperatures less than 200 °C and without modifying processing conditions, allowing for seamless integration into 2D device architectures. A device‐scale dielectric constant of 5.9 ± 0.65 at 1 kHz, breakdown voltage of 9.8 ± 1.0 MV cm?1, and bandgap of 4.5 eV were measured for various thicknesses of the ultrathin a‐BN material, representing values higher than previously reported chemical vapor deposited h‐BN and nearing single crystal h‐BN. 相似文献
97.
Broadband antihole photon sieve telescope 总被引:1,自引:0,他引:1
A broadband-corrected optical telescope has been constructed from a photon sieve with five million holes. Through careful optimization of hole size, this "antihole" sieve has holes centered on the dark underlying Fresnel zones. The diffraction-limited performance of a 1 m focal-length, f/10 element is demonstrated with a view toward constructing large lightweight telescopes for space applications. 相似文献
98.
Drew MS Finlayson GD 《Journal of the Optical Society of America. A, Optics, image science, and vision》2007,24(2):294-303
The measured light spectrum is the result of an illuminant interacting with a surface. The illuminant spectral power distribution multiplies the surface spectral reflectance function to form a color signal--the light spectrum that gives rise to our perception. Disambiguation of the two factors, illuminant and surface, is difficult without prior knowledge. Previously [IEEE Trans. Pattern Anal. Mach. Intell.12, 966 (1990); J. Opt. Soc. Am. A21, 1825 (2004)], one approach to this problem applied a finite-dimensional basis function model to recover the separate illuminant and surface reflectance components that make up the color signal, using principal component bases for lights and for reflectances. We introduce the idea of making use of finite-dimensional models of logarithms of spectra for this problem. Recognizing that multiplications turn into additions in such a formulation, we can replace the original iterative method with a direct, analytic algorithm with no iteration, resulting in a speedup of several orders of magnitude. Moreover, in the new, logarithm-based approach, it is straightforward to further design new basis functions, for both illuminant and reflectance simultaneously, such that the initial basis function coefficients derived from the input color signal are optimally mapped onto separate coefficients that produce spectra that more closely approximate the illuminant and the surface reflectance for any given dimensionality. This is accomplished by using an extra bias correction step that maps the analytically determined basis function coefficients onto the optimal coefficient set, separately for lights and surfaces, for the training set. The analytic equation plus the bias correction is then used for unknown input color signals. 相似文献
99.
Shindell D 《Philosophical transactions. Series A, Mathematical, physical, and engineering sciences》2007,365(1860):2675-2694
I investigate the potential for sudden climate change during the current century. This investigation takes into account evidence from the Earth's history, from climate models and our understanding of the physical processes governing climate shifts. Sudden alterations to climate forcing seem to be improbable, with sudden changes instead most likely to arise from climate feedbacks. Based on projections from models validated against historical events, dramatic changes in ocean circulation appear unlikely. Ecosystem-climate feedbacks clearly have the potential to induce sudden change, but are relatively poorly understood at present. More probable sudden changes are large increases in the frequency of summer heatwaves and changes resulting from feedbacks involving hydrology. These include ice sheet decay, which may be set in motion this century. The most devastating consequences are likely to occur further in the future, however. Reductions in subtropical precipitation are likely to be the most severe hydrologic effects this century, with rapid changes due to the feedbacks of relatively well-understood large-scale circulation patterns. Water stress may become particularly acute in the Southwest US and Mexico, and in the Mediterranean and Middle East, where rainfall decreases of 10-25% (regionally) and up to 40% (locally) are projected. 相似文献
100.
Drew Rutherford Jerrick J.J. Juliette Christian Rocaboy István T. Horváth J.A. Gladysz 《Catalysis Today》1998,42(4):443-388
Biphase systems were generated by combining toluene solutions of alkenes (2-cyclohexen-1-one, 1-dodecene, cyclododecene, 4-bromostyrene) and CF3C6F11 solutions of the pre-catalyst ClRh[P(CH2CH2(CF2)5CF3)3]3 ((1); 1.1–0.8 mol%) and placed under 1 atm of H2. The perfluoroalkyl segments in (1) confer high affinities for fluorocarbons. After 8–26 h at 45°C, the CF3C6F11 phases were separated, and extracted with an equal volume of toluene. GLC analyses of the toluene solutions showed 98–87% yields of the hydrogenation products cyclohexanone, dodecane, cyclododecane, and 4-bromoethylbenzene (turnover numbers 120–87). The CF3C6F11 phases were charged with new toluene solutions of 2-cyclohexen-1-one or 1-dodecene, and similarly treated with H2. There was no significant loss of catalyst activity over three cycles. A preparative reaction gave cyclohexanone in 89% yield. Some C=C isomerization could be detected during the course of 1-dodecene hydrogenation, and (1) gave a dihydride complex in the absence of alkene. 相似文献