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排序方式: 共有80条查询结果,搜索用时 3 毫秒
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Filip Botha Jan Budka Václav Eigner Oldřich Hudeček Lukáš Vrzal Ivana Císařová Pavel Lhoták 《Tetrahedron》2014
A new type of calixarene-based receptor designed for the recognition of chiral anions was prepared by the introduction of (S)-2-methylbutan-1-ol moieties into the lower rim of calixarene. The immobilization of calixarene skeleton in the 1,3-alternate conformation enabled the construction of a cavity consisting of preorganised ureido functions and chiral substituents in close proximity. This cavity is capable of chiral discrimination of selected anions as demonstrated on d- and l-phenylalaninates. 相似文献
3.
M.A. Lephoto O.M. Ntwaeaborwa Shreyas S. Pitale H.C. Swart J.R. Botha B.M. Mothudi 《Physica B: Condensed Matter》2012,407(10):1603-1606
Combustion method was used in this study to prepare BaAl2O4:Eu2+ phosphors co-doped with different trivalent rare-earths (Re3+=Dy3+, Nd3+, Gd3+, Sm3+, Ce3+, Er3+, Pr3+ and Tb3+) ions at an initiating temperature of 600 °C. The phosphors were annealed at 1000 °C for 3 h. As confirmed from the X-ray diffraction (XRD) data, both as prepared and post annealed samples crystallized in the well known hexagonal structure of BaAl2O4. All samples exhibited bluish-green emission associated with the 4f65d1→4f7 transitions of Eu2+ at ∼500 nm. Although the highest intensity was observed from Er3+ co-doping, the longest afterglow (due to trapping and detrapping of charge carriers) was observed from Nd3+ followed by Dy3+ co-doping. The traps responsible for the long afterglow were studied using thermoluminescence (TL) spectroscopy. 相似文献
4.
The rehydration characteristics of a commercially produced hydromagnesite and two basic magnesium carbonates synthetically
produced from Mg(OH)2, are presented. The products were dehydrated and dehydroxylated at 325°C before rehydration was attempted. DTA and FT-IR
were used to follow the structural changes that occurred during the rehydration processes. The results obtained for the commercially
and synthetically produced hydromagnesite products indicated that the original symmetry of the groups was reclaimed during
rehydration. This was not observed for the synthetically produced unidentified basic magnesium carbonate product. This investigation
provides insight into the rehydration characteristics of a select group of basic magnesium carbonates.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
5.
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Theresa Hague Andrea Petroczi Paul LR Andrews James Barker Declan P Naughton 《Chemistry Central journal》2010,4(1):2
This is a correction to the following paper: Hague T, Petroczi A, Andrews PR, Barker J, Naughton DP: Determination of metal
ion content of beverages and estimation of target hazard quotients: a comparative study. Chem Central J 2008, 2:13. 相似文献
7.
Peter Reischig Jorik Blaas Charl Botha Alberto Bravin Liisa Porra Christian Nemoz Arie Wallert Joris Dik 《Journal of synchrotron radiation》2009,16(2):310-313
One of the most fascinating objects in the Rijksmuseum (Amsterdam, The Netherlands) is an early 16th century prayer nut. This spherical wooden object measures 4 cm in diameter and consists of two hemispheres connected with a small hinge so that it can be opened. The interior of the nut holds wood carvings with scenes from the life of Christ. These miniature reliefs show an incredible degree of finish with carving details well beyond the millimetre scale. In the present paper it is shown how synchrotron‐based computer X‐ray tomography revealed the structure and fabrication method of the bead. The central part of the relief was cut from a single piece of wood, rather than assembled from multiple components, underlining the extraordinary manual dexterity of its maker. In addition, a piece of fibrous material contained in the inner structure of the bead is revealed. This may have served as a carrier for an odorous compound, which would be in line with the religious function of the prayer nut. 相似文献
8.
The optical properties of ZnO grown on (1 0 0) GaAs substrate using metalorganic chemical vapor deposition are investigated by photoluminescence (PL) spectroscopy. Postgrowth annealing in nitrogen and oxygen was performed for different times and temperatures in order to incorporate As from the substrate into the ZnO thin films. The PL spectra of the samples annealed in different ambients reveal that the effect of As diffusion into the ZnO thin films is more pronounced when the annealing is performed in oxygen at 550 °C. The 11 K PL spectra show the appearance of a transition at ∼3.35 eV after annealing in oxygen at 550 °C for 1 h. A further increase in the annealing temperature leads to the disappearance of this line, while for annealing times longer than 2 h at 550 °C, it is no longer prominent. The increase in intensity of this new transition is also accompanied by the enhancement of radiative centers related to structural defects, such as the stacking fault-related transition at 3.31 eV and the Y-line. Temperature dependent PL illustrates the excitonic nature of the new transition at ∼3.35 eV, which is therefore assigned to (A0, X) transition, where the acceptor is possibly the 2VZn-AsZn complex, with an activation energy EA in the range of 160-240 meV. Furthermore, the enhancement of the radiative centers related to structural defects is regarded as evidence that As atoms tend to segregate in the vicinity of structural defects to relieve local strain. 相似文献
9.
Accreditation and Quality Assurance - The production of reference materials (RMs) is a key activity for the improvement and maintenance of a worldwide coherent measurement system. General... 相似文献
10.
A unification of some factorization results regarding products of positive-definite matrices, commutators, and products of involutions is presented. This topic was first presented by Sourour for fields with, in the latter two classes, sufficiently many elements in terms of the order of the matrix being factored. The current presentation is valid for matrices over any field with at least four elements, and is independent of the order of the matrix being factored. 相似文献