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排序方式: 共有493条查询结果,搜索用时 15 毫秒
41.
42.
Arsenic compounds including arsenous acid (As(III)), arsenic acid (As(V)), dimethylarsinic acid (DMA) and monomethylarsonic acid (MMA) were separated by high-performance liquid chromatography (HPLC) and detected by inductively coupled plasma mass spectrometry (ICP-MS). A Hamilton PRX-100 anionic-exchange column and a pH 8.5 K2HPO4/KH2PO4 5.0 × 10−3 mol L−1 mobile phase were used to achieve arsenic speciation. The separation of arsenic species provided peaks of As(III) at 2.75 min, DMA at 3.33 min, MMA at 5.17 min and As(V) at 12.5 min. The detection limits, defined as three times the standard deviation of the lowest standard measurements, were found to be 0.2, 0.2, 0.3 and 0.5 ng mL−1 for As(III), DMA, MMA and As(V), respectively. The relative standard deviation values for a solution containing 5.0 μg L−1 of As(III), DMA, MMA and As(V) were 1.2, 2.1, 2.5 and 3.0%, respectively. This analytical procedure was applied to the speciation of arsenic compounds in drinking (soft drink, beer, juice) samples. The validation of the procedure was achieved through the analysis of arsenic compounds in water and sediment certified reference materials. 相似文献
43.
Alessandro Gandini Dora Coelho Armando J.D. Silvestre 《European Polymer Journal》2008,44(12):4029-4036
The model Diels-Alder reaction of furfuryl acetate with N-methylmaleimide and the linear polymerization of the corresponding difunctional monomers were followed by both UV and 1H NMR spectroscopy. The results obtained from this study provided clear-cut indications about the most appropriate conditions to be applied in the preparation of novel macromolecular materials based in part on renewable resources and possessing thermoreversible, mendable and recycling features, among other properties. 相似文献
44.
We discuss the existence of tilting modules which are direct limits of finitely generated tilting modules over tilted algebras.
相似文献
45.
Kinga Lasek Jingfeng Li Sadhu Kolekar Paula Mariel Coelho Lu'an Guo Min Zhang Zhiming Wang Matthias Batzill 《Surface Science Reports》2021,76(2):100523
Layered transition metal dichalcogenides (TMDs) are a diverse group of materials whose properties vary from semiconducting to metallic with a variety of many body phenomena, ranging from charge density wave (CDW), superconductivity, to Mott-insulators. Recent interest in topologically protected states revealed also that some TMDs host bulk Dirac- or Wyle-semimetallic states and their corresponding surface states. In this review, we focus on the synthesis of TMDs by vacuum processes, such as molecular beam epitaxy (MBE). After an introduction of these preparation methods and categorize the basic electronic properties of TMDs, we address the characterization of vacuum synthesized materials in their ultrathin limit-mainly as a single monolayer material. Scanning tunneling microscopy and angle resolved photoemission spectroscopy has revealed detailed information on how monolayers differ in their properties from multi-layer and bulk materials. The status of monolayer properties is given for the TMDs, where data are available. Distinct modifications of monolayer properties compared to their bulk counterparts are highlighted. This includes the well-known transition from indirect to direct band gap in semiconducting group VI-B TMDs as the material-thickness is reduced to a single molecular layer. In addition, we discuss the new or modified CDW states in monolayer VSe2 and TiTe2, a Mott-insulating state in monolayer 1T-TaSe2, and the monolayer specific 2D topological insulator 1T′-WTe2, which gives rise to a quantum spin Hall insulator. New structural phases, that do not exist in the bulk, may be synthesized in the monolayer by MBE. These phases have special properties, including the Mott insulator 1T-NbSe2, the 2D topological insulators of 1T′-MoTe2, and the CDW material 1T-VTe2. After discussing the pure TMDs, we report the properties of nanostructured or modified TMDs. Edges and mirror twin grain boundaries (MTBs) in 2D materials are 1D structures. In group VI-B semiconductors, these 1D structures may be metallic and their properties obey Tomonaga Luttinger quantum liquid behavior. Formation of Mo-rich MTBs in Mo-dichalcogenides and self-intercalation in between TMD-layers are discussed as potential compositional variants that may occur during MBE synthesis of TMDs or may be induced intentionally during post-growth modifications. In addition to compositional modifications, phase switching and control, in particular between the 1H and 1T (or 1T′) phases, is a recurring theme in TMDs. Methods of phase control by tuning growth conditions or by post-growth modifications, e.g. by electron doping, are discussed. The properties of heterostructures of TMD monolayers are also introduced, with a focus on lateral electronic modifications in the moiré-structures of group VI-B TMDs. The lateral potential induced in the moiré structures forms the basis of the currently debated moiré-excitons. Finally, we review a few cases of molecular adsorption on nanostructured monolayer TMDs. This review is intended to present a comprehensive overview of vacuum studies of fundamental materials' properties of TMDs and should complement the investigations on TMDs prepared by exfoliation or chemical vapor deposition and their applications. 相似文献
46.
In this study, the synthesis, structural characterisation and mesomorphic and optical properties of seven new bent-shaped and polycatenar bent-shaped compounds derived from chalcone and cyanopyridine are reported. The mesomorphic behaviour was investigated by differential scanning calorimetry (DSC), polarised optical microscopy (POM) and X-ray diffraction (XRD) and correlated with the molecular structure. Two bent-core hexacatenars molecules (Ic and IIc) presented liquid crystalline properties, showing a hexagonal columnar (Colh) phases at room temperature, being each disc constituted by two mesogens. Optical studies were also performed for the final molecules, being conducted by ultraviolet-visible and fluorescence spectrometry. The cyanopyridine derivatives show moderate luminescence quantum yields, ranging between 18% and 27%, with emission maxima around 371 nm. It is also shown that while the chalcone central unit favours a calamitic liquid crystalline behaviour in molecules with lower number of aliphatic chains, a polycatenar structure with cyanopyridine as the central unit favours a Colh arrangement, also providing luminescence properties to the molecule. 相似文献
47.
Adamson P Auty DJ Ayres DS Backhouse C Barr G Bishai M Blake A Bock GJ Boehnlein DJ Bogert D Cavanaugh S Cherdack D Childress S Coelho JA Coleman SJ Corwin L Cronin-Hennessy D Danko IZ de Jong JK Devenish NE Diwan MV Dorman M Escobar CO Evans JJ Falk E Feldman GJ Frohne MV Gallagher HR Gomes RA Goodman MC Gouffon P Graf N Gran R Grant N Grzelak K Habig A Harris D Hartnell J Hatcher R Himmel A Holin A Huang X Hylen J Ilic J Irwin GM Isvan Z Jaffe DE James C Jensen D Kafka T Kasahara SM Koizumi G 《Physical review letters》2011,107(1):011802
Results are reported from a search for active to sterile neutrino oscillations in the MINOS long-baseline experiment, based on the observation of neutral-current neutrino interactions, from an exposure to the NuMI neutrino beam of 7.07×10(20) protons on target. A total of 802 neutral-current event candidates is observed in the Far Detector, compared to an expected number of 754 ± 28(stat) ± 37(syst) for oscillations among three active flavors. The fraction f(s) of disappearing ν(μ) that may transition to ν(s) is found to be less than 22% at the 90% C.L. 相似文献
48.
Adamson P Andreopoulos C Armstrong R Auty DJ Ayres DS Backhouse C Barr G Bishai M Blake A Bock GJ Boehnlein DJ Bogert D Cavanaugh S Cherdack D Childress S Choudhary BC Coelho JA Coleman SJ Corwin L Cronin-Hennessy D Danko IZ de Jong JK Devenish NE Diwan MV Dorman M Escobar CO Evans JJ Falk E Feldman GJ Frohne MV Gallagher HR Gomes RA Goodman MC Gouffon P Graf N Gran R Grant N Grzelak K Habig A Harris D Hartnell J Hatcher R Himmel A Holin A Huang X Hylen J Ilic J Irwin GM Isvan Z Jaffe DE 《Physical review letters》2011,106(18):181801
Measurements of neutrino oscillations using the disappearance of muon neutrinos from the Fermilab NuMI neutrino beam as observed by the two MINOS detectors are reported. New analysis methods have been applied to an enlarged data sample from an exposure of 7.25×10(20) protons on target. A fit to neutrino oscillations yields values of |Δm(2)|=(2.32(-0.08)(+0.12))×10(-3) eV(2) for the atmospheric mass splitting and sin(2)(2θ)>0.90 (90% C.L.) for the mixing angle. Pure neutrino decay and quantum decoherence hypotheses are excluded at 7 and 9 standard deviations, respectively. 相似文献
49.
I.P. Coelho M.S. VasconcelosC.G. Bezerra 《Journal of magnetism and magnetic materials》2011,323(23):3162-3167
In this paper we investigated the influence of mirror symmetry on the transmission spectra of quasiperiodic magnonic multilayers arranged according to Fibonacci, Thue-Morse and double period quasiperiodic sequences. We consider that the multilayers composed of two simple cubic Heisenberg ferromagnets with bulk exchange constants JA and JB and spin quantum numbers SA and SB, respectively. The multilayer structure is surrounded by two semi-infinite slabs of a third Heisenberg ferromagnetic material with exchange constant JC and spin quantum number SC. For simplicity, the lattice constant has the same value a in each material, corresponding to epitaxial growth at the interfaces. The transfer matrix treatment was used for the exchange-dominated regime, taking into account the random phase approximation (RPA). Our numerical results illustrate the effects of mirror symmetry on (i) transmission spectra and (ii) transmission fingerprints. 相似文献
50.