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The starting point of our research is the following problem: given a doubling metric ?=(V,d), can one (efficiently) find an unweighted graph G′=(V′,E′) with V?V′ whose shortest-path metric d′ is still doubling, and which agrees with d on V×V? While it is simple to show that the answer to the above question is negative if distances must be preserved exactly. However, allowing a (1+ε) distortion between d and d′ enables us bypass this hurdle, and obtain an unweighted graph G′ with doubling dimension at most a factor O(log?ε ?1) times the doubling dimension of G.More generally, this paper gives algorithms that construct graphs G′ whose convex (or geodesic) closure has doubling dimension close to that of ?, and the shortest-path distances in G′ closely approximate those of ? when restricted to V×V. Similar results are shown when the metric ? is an additive (tree) metric and the graph G′ is restricted to be a tree. 相似文献
4.
In the framework of a rigid-ion model and using realistic Greens function theory, we have studied systematically the influence of doping on the lattice dynamics of III–V-nitrides. Our theoretical analysis is based on a sound physical appeal for the perturbation caused by different dopants in compound semiconductors. After analyzing the dynamical behavior for two sets of impurities with closest masses occupying Ga- and As-sites in GaAs, we have proposed simple empirical relationships providing corrections to the impurity–host interactions for isoelectronic defects and for dopants carrying static charges. Despite the simplicity of perturbation models, our Greens function theory provided results in good agreement with the recent Fourier-transform infrared (FTIR) absorption data for the magnitude and splitting of N-isotopic shifts of localized vibrational modes in GaAs. With high-resolution FTIR spectroscopy, we strongly believe that the 14NAs-related broad local mode (471 cm−1) in GaAs can be resolved into fine structures due to Ga-host isotopes. Theoretical results for the in-band modes of AsN in cubic gallium nitride are compared and discussed with the existing Raman data on molecular beam epitaxy (MBE) grown GaN layers on GaAs substrates, as well as on GaN samples intentionally doped with As. 相似文献
5.
G.J. Talwar C.P. JoshiS.V. Moharil S.M. DhopteP.L. Muthal V.K. Kondawar 《Journal of Alloys and Compounds》2011,509(35):8742-8747
Metasilicates are important class of materials. They find applications in many diverse areas. Silicates are widely used as adsorbents, sensors, in production of ceramics, as bioactive materials, as microwave dielectric etc. A large number of phosphors based on metasilicate hosts are known. The conventional routes for synthesis of silicates are the solid state reaction and sol-gel. The synthesis temperature for metasilicates is usually in excess of 1100 °C. Here, we report for the first time the wet-chemical preparation of alkaline earth metasilicates. Though the as-prepared compounds were poorly crystallized, annealing at 700 °C was sufficient for the formation of well defined crystalline phases. Photoluminescence was also observed in these compounds after activating with suitable dopants. 相似文献
6.
Jini connection technology forms a network of devices on the fly, without manual connection or configuration. It can also complement other technologies that strive for "anytime, anywhere" connectivity. The article looks at lookup and discovery services; reliability and scalability; Jini architectural requirements; service for non-Jini devices; Jini mobile edition for wireless devices; Jini implementations; and competing and related technologies. 相似文献
7.
ZnO and Zn1−xCdxO nanocrystallites were prepared by oxidation of zinc arachidate-arachidic acid and zinc arachidate-cadmium arachidate-arachidic acid LB multilayers, respectively. The metal content of the multilayers was controlled by manipulation of subphase composition and pH. Precursor multilayers were oxidized in the temperature range of 400 °C-700 °C. The formation of ZnO and Zn1−xCdxO was confirmed by UV-Visible spectroscopy. Uniformly distributed, isolated and nearly mono-dispersed nanocrystallites of ZnO (11 ± 3 nm) and Zn1−xCdxO (18 ± 6 nm) were obtained. 相似文献
8.
W. Wayne Chen Q. Jane Wang Yuchuan Liu Wei Chen Jiao Cao Cedric Xia Raj Talwar Rick Lederich 《Tribology Letters》2007,28(1):27-38
Interaction of nominally flat engineering surfaces that leads to a large contact area exists in many mechanical systems. Considering
periodic similarity of surface geometry, a numerical three-dimensional elasto-plastic contact model can be used to simulate
the contact behaviors of two nominally flat surfaces with the assistance of the continuous convolution and Fourier transform
(CC-FT) algorithm. This model utilizes the analytical frequency response functions (FRF) of elastic/plastic responses of materials
and provides contact performance results, including the average surface gap, the contact area ratio, and the volume of plastically
deformed material, which may be defined as performance variables. Following the digital filtration technology, rough surfaces
can be numerically generated with specified autocorrelation length and the first four orders of statistical moments. A group
of contact simulations are conducted with various working conditions. The effects of topographic and material properties on
the contact behaviors are discussed. With a multi-variables regression method, empirical formulas are developed for the performance
variables as functions of surface statistical characteristics, material properties, a hardening parameter, and the applied
load in terms of pressure. 相似文献
9.
Tiesheng Li Shuji Okada Hirohito Umezawa Hitoshi Kasai Hachiro Nakanishi Satya S. Talwar Tatsumi Kimura Hiro Matsuda 《Polymer Bulletin》2006,57(5):737-746
Summary 1,4-Bis(3-quinolyl)-1,3-butadiyne (DQ) is known to be polymerized in solid state to give the corresponding polydiacetylene.
However, the polymer yield of DQ bulk crystals is low. Thus, we prepared several types of DQ crystals by different procedures
to find the reason for the low polymer yield. We found three modifications of DQ crystals and they were evaluated by spectroscopic
measurements and X-ray diffraction. DQ bulk crystals (Crystal I) and thermally grown DQ crystals on Crystal I (Crystal II)
have the same structure classified to Type A, which gives regular polydiacetylene structure in low polymer yields. DQ crystals
grown on glass plates by sublimation (Crystal III) belong to the second modification of Type B. DQ nanocrystals prepared by
the reprecipitation method (Crystal IV) are Type B, and thermally grown DQ crystals on nanocrystals (Crystal V) are Type C.
Crystals of Types B and C could be polymerized in low yields without showing excitonic absorption of polydiacetylene indicating
irregular polymerization other than 1,4-addition. For crystals of Type A, we found that the 1,4-addition polymerization proceeded
only in near-surface portions of the crystals. It can be plausibly explained that mobile monomers in the near-surface portion
are only able to take part in their polymerization, resulting in low polymer conversion. 相似文献
10.
The photoelectret properties of pure and acrylic acid (AA) doped polystyrene (PS) films formed by an isothermal immersion technique have been studied. The photoelectrets were formed by simultaneous application of u.v. radiation for a short time period (2–10 min) and d.c. electric field (3–18 V). The depolarization current was obtained by reilluminating the photoelectret with the same radiation. It is found that the photo depolarization current increases with polarizing voltage and is greater for the doped samples than for the undoped samples, prepared under identical conditions. The current increases with the dopant concentration and with duration of polarization. The total charge, found by integrating the depolarization current-time curve of doped photoelectrets, tends to saturation at a certain voltage value. Doped PS gives better photoelectrets. 相似文献