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排序方式: 共有333条查询结果,搜索用时 15 毫秒
31.
Alexander B. Weberg Subhajyoti Chaudhuri Thibault Cheisson Christian Uruburo Ekaterina Lapsheva Pragati Pandey Michael R. Gau Patrick J. Carroll George C. Schatz Eric J. Schelter 《Chemical science》2022,13(23):6796
The separation and purification of niobium and tantalum, which co-occur in natural sources, is difficult due to their similar physical and chemical properties. The current industrial method for separating Ta/Nb mixtures uses an energy-intensive process with caustic and toxic conditions. It is of interest to develop alternative, fundamental methodologies for the purification of these technologically important metals that improve upon their environmental impact. Herein, we introduce new Ta/Nb imido compounds: M(tBuN)(TriNOx) (1-M) bound by the TriNOx3− ligand and demonstrate a fundamental, proof-of-concept Ta/Nb separation based on differences in the imido reactivities. Despite the nearly identical structures of 1-M, density functional theory (DFT)-computed electronic structures of 1-M indicate enhanced basic character of the imido group in 1-Ta as compared to 1-Nb. Accordingly, the rate of CO2 insertion into the M Nimido bond of 1-Ta to form a carbamate complex (2-Ta) was selective compared to the analogous, unobserved reaction with 1-Nb. Differences in solubility between the imido and carbamate complexes allowed for separation of the carbamate complex, and led to an efficient Ta/Nb separation (STa/Nb = 404 ± 150) dependent on the kinetic differences in nucleophilicities between the imido moieties in 1-Ta and 1-Nb.A selective separation of the critical metals tantalum and niobium was accomplished from π-bonding-based reactivity differences of imido complexes. New insights into Ta/Nb separations were gained through detailed kinetic and computational studies. 相似文献
32.
M.A. Goetschalckx R.L. Mowery E.R. Krausz W.C. Yeakel P.N. Schatz B.S. Ault L. Andrews 《Chemical physics letters》1977,47(1):23-27
MCD measurements on XeF, XeCl, XeBr and XeI matrix isolated in Ar at low temperature are reported. Some relationships among the various spectroscopic parameters are established by combining these data existing absorption and ESR measurements. 相似文献
33.
E. Morenzoni M. Birke H. Glückler A. Hofer J. Litterst M. Meyberg C. Niedermayer Th. Prokscha G. Schatz Th. Wutzke 《Hyperfine Interactions》1997,106(1-4):229-235
At the Paul Scherrer Institute very slow, nearly 100% polarized, positive muons with an energy of \sim\mbox10 eV are produced
by moderating a secondary beam of surface muons in a thin film of an appropriate condensed gases. These epithermal muons can
be used as a source of a tertiary beam of tunable energy between \sim\mbox10 eV and \sim\mbox20 keV. Such a beam allows the
μSR technique to be extended to the study of thin films and surfaces.
In order to be able to perform time differential μSR experiments we have developed an ultra‐thin detector that registers the
passage of keV muons and permits to trigger the experiment. The results achieved so far demonstrate that first investigations
of thin film samples can be performed with the present set‐up.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
34.
Louguet S Kumar AC Sigaud G Duguet E Lecommandoux S Schatz C 《Journal of colloid and interface science》2011,359(2):413-422
The adsorption behavior of poly(ethylene oxide)-b-poly(L-lysine) (PEO(113)-b-PLL(10)) copolymer onto silica nanoparticles was investigated in phosphate buffer at pH 7.4 by means of dynamic light scattering, zeta potential, adsorption isotherms and microcalorimetry measurements. Both blocks have an affinity for the silica surface through hydrogen bonding (PEO and PLL) or electrostatic interactions (PLL). Competitive adsorption experiments from a mixture of PEO and PLL homopolymers evidenced greater interactions of PLL with silica while displacement experiments even revealed that free PLL chains could desorb PEO chains from the particle surface. This allowed us to better understand the adsorption mechanism of PEO-b-PLL copolymer at the silica surface. At low surface coverage, both blocks adsorbed in flat conformation leading to the flocculation of the particles as neither steric nor electrostatic forces could take place at the silica surface. The addition of a large excess of copolymer favoured the dispersion of flocs according to a presumed mechanism where PLL blocks of incoming copolymer chains preferentially adsorbed to the surface by displacing already adsorbed PEO blocks. The gradual addition of silica particles to an excess of PEO-b-PLL copolymer solution was the preferred method for particle coating as it favoured equilibrium conditions where the copolymer formed an anchor-buoy (PLL-PEO) structure with stabilizing properties at the silica-water interface. 相似文献
35.
36.
D.R. Demers J. Lei U. Shah P.M. Schoch K.A. Connor T.P. Crowley J.G. Schatz J.K. Anderson J.S. Sarff 《Czechoslovak Journal of Physics》2001,51(10):1065-1077
Measurement of the plasma potential in the core of MST marks both the first interior potential measurements in an RFP, as well as the first measurements by a Heavy Ion Beam Probe (HIBP) in an RFP. The HIBP has operated with (20-110) keV sodium beams in plasmas with toroidal currents of (200-480) kA over a wide range of densities and magnetic equilibrium conditions. A positive plasma potential is measured in the core, consistent with the expectation of rapid electron transport by magnetic fluctuations and the formation of an outwardly directed ambipolar radial electric field. Comparison between the radial electric field and plasma flow is underway to determine the extent to which equilibrium flow is governed by E×B. Measurements of potential and density fluctuations are also in progress.Unlike HIBP applications in tokamak plasmas, the beam trajectories in MST (RFP) are both three-dimensional and temporally dynamic with magnetic equilibrium changes associated with sawteeth. This complication offers new opportunity for magnetic measurements via the Heavy Ion Beam Probe (HIBP). The ion orbit trajectories are included in a Grad-Shafranov toroidal equilibrium reconstruction, helping to measure the internal magnetic field and current profiles. Such reconstructions are essential to identifying the beam sample volume locations, and they are vital in MST's mission to suppress MHD tearing modes using current profile control techniques. Measurement of the electric field may be accomplished by combining single point measurements from multiple discharges, or by varying the injection angle of the beam during single discharges.The application of an HIBP on MST has posed challenges resulting in additional diagnostic advances. The requirement to keep ports small to avoid introducing magnetic field perturbations has led to the design and successful implementation of cross-over sweep systems. High levels of ultraviolet radiation are driving alternative methods of sweep plate operation. While, substantial levels of plasma flux into the HIBP diagnostic chambers has led to the use of magnetic plasma suppression. 相似文献
37.
Banholzer MJ Li S Ketter JB Rozkiewicz DI Schatz GC Mirkin CA 《The journal of physical chemistry. C, Nanomaterials and interfaces》2008,112(40):15729-15734
We have developed a method to smooth the end sections of nanowires and nanograps generated via the On-Wire Lithography process and studied these rods with optical spectroscopies and theoretical modeling (Discrete Dipole Approximation). The first step of the smoothing process is a reductive one aimed at controlling the diffusion and migration of metal ions to the growing nanorod surface by adjusting the applied potential and concentration of the metal ions in the growth solution. A second oxidative smoothing step, based in part on the energetic differences between topologically rough and smooth surfaces, is used to further smooth the nanorod. The RMS roughness can be reduced over five fold to approximately 5 nm. The properties of these smoothed rods were investigated by empirical and theoretical methods, where it was found the smoothed rods have sharper plasmon resonances and decreased SERS intensity. 相似文献
38.
Tu XL Xu HS Wang M Zhang YH Litvinov YA Sun Y Schatz H Zhou XH Yuan YJ Xia JW Audi G Blaum K Du CM Geng P Hu ZG Huang WX Jin SL Liu LX Liu Y Ma X Mao RS Mei B Shuai P Sun ZY Suzuki H Tang SW Wang JS Wang ST Xiao GQ Xu X Yamaguchi T Yamaguchi Y Yan XL Yang JC Ye RP Zang YD Zhao HW Zhao TC Zhang XY Zhan WL 《Physical review letters》2011,106(11):112501
Mass excesses of short-lived A=2Z-1 nuclei (63)Ge, (65)As, (67)Se, and (71)Kr have been directly measured to be -46,921(37), -46,937(85), -46,580(67), and -46,320(141) keV, respectively. The deduced proton separation energy of -90(85) keV for (65)As shows that this nucleus is only slightly proton unbound. X-ray burst model calculations with the new mass excess of (65)As suggest that the majority of the reaction flow passes through (64)Ge via proton capture, indicating that (64)Ge is not a significant rp-process waiting point. 相似文献
39.
Hannah DC Dunn NJ Ithurria S Talapin DV Chen LX Pelton M Schatz GC Schaller RD 《Physical review letters》2011,107(17):177403
We report heat dissipation times in semiconductor nanocrystals of CdSe. Specifically, a previously unresolved, subnanosecond decay component in the low-temperature photoluminescence decay dynamics exhibits longer decay lifetimes (tens to hundreds of picoseconds) for larger nanocrystals as well as a size-independent, ~25-meV spectral shift. We attribute the fast relaxation to transient phonon-mediated relaxation arising from nonequilibrium acoustic phonons. Following acoustic phonon dissipation, the dark exciton state recombines more slowly via LO-phonon assistance resulting in the observed spectral shift. The measured relaxation time scales agree with classical calculations of thermal diffusion, indicating that interfacial thermal conductivity does not limit thermal transport in these semiconductor nanocrystal dispersions. 相似文献
40.
M. Hausmann J. Stadlmann F. Attallah K. Beckert P. Beller F. Bosch H. Eickhoff M. Falch B. Franczak B. Franzke H. Geissel Th. Kerscher O. Klepper H.-J. Kluge C. Kozhuharov Yu.A. Litvinov K.E.G. Löbner G. Münzenberg N. Nankov F. Nolden Yu.N. Novikov T. Ohtsubo T. Radon H. Schatz C. Scheidenberger M. Steck Z. Sun H. Weick H. Wollnik 《Hyperfine Interactions》2001,132(1-4):289-295
A novel method for mass measurements of short-lived exotic nuclides is presented. Exotic nuclides were produced and separated
in flight at relativistic energies with the fragment separator (FRS) and were injected into the experimental storage ring
(ESR). Operating the ESR in the isochronous mode we performed mass measurements of neutron deficient fragments of 84Kr with half-lives larger than 50 ms. However, this experimental technique is applicable in a half-life range down to a few
μs. A mass resolving power of 110000 (FWHM) has been achieved. Results are presented for the masses of 68As, 70,71Se and 73Br.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献