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Cai Y Liu X Li J Chen W Wang W Lin L Feng X 《Chemistry (Weinheim an der Bergstrasse, Germany)》2011,17(52):14916-14921
Highly diastereo‐ and enantioselective iodoamination of chalcones, 4‐aryl‐4‐oxobutenoates, and a trifluoro‐substituted enone has been accomplished in the presence of a chiral N,N′‐dioxide/[Sc(OTf)3] complex (0.5–2 mol %), delivering the desired vicinal anti‐α‐iodo‐β‐amino carbonyl compounds regioselectively in high yields (up to 97 %) and with excellent diastereoselectivities (>99:1 d.r.) and enantioselectivities (up to 99 % ee). Enantiopure syn‐α‐iodo‐β‐amino products could also be obtained from the isomerization of particular iodo compounds. TsNHX species (X=Cl, Br, I), generated from the reactions between the halo sources and TsNH2, were further confirmed as the active species in the haloamination reactions involved in the formation of the key halonium ion intermediates. A typical haloamination dependency was observed, with reactivity decreasing in the order NBS>NIS?NCS. 相似文献
43.
Wenshuai Chen Haipeng Yu Yixing Liu Yunfei Hai Mingxin Zhang Peng Chen 《Cellulose (London, England)》2011,18(2):433-442
Cellulose nanofibers (CNFs) were isolated from four kinds of plant cellulose fibers by a chemical-ultrasonic treatment. The
chemical composition, morphology, crystalline behavior, and thermal properties of the nanofibers and their intermediate products
were characterized and compared. The CNFs extracted from wood, bamboo, and wheat straw fibers had uniform diameters of 10–40 nm, whereas the flax fibers were not uniformly nanofibrillated because of their initially high cellulose content. The chemical
composition of each kind of nanofibers was mainly cellulose because hemicelluloses and lignin were significantly removed during
chemical process. The crystallinity of the nanofibers increased as the chemical treatments were applied. The degradation temperature
of each kind of nanofiber reached beyond 330 °C. Based on the properties of the CNFs, we expect that they will be suitable
for use in green nanocomposites, filtration media and optically transparent films. 相似文献
44.
Single photon pulse ranging system with extremely high sensitivity has been widely used in distance measurement and 3D imaging. To analyze the factors that affect the measurement precision and accuracy will help to improve system performance. According to system structure and principle, we mainly discussed the following factors: laser intensity, pulse width, detection efficiency and time jitter. A simulation model based on Monte Carlo stochastic method was constructed in this paper, and we get the specific influence of factors on measurement precision and accuracy by simulation. Finally, we set up laboratory experiment system and took effective experiments on ranging precision and accuracy. 相似文献
45.
The formation of a manganese (II) coordination supramolecular polymer was studied by resonance light scattering spectra for manganese (II) detection. Bis-sulfosalophen (a ditopic tetradentate Schiff base ligand) and bis-phenanthroline-glutaraldehyde (a ditopic bidentate ligand) were prepared for the construction of the supramolecular polymer. In a procedure of manganese (II) detection, manganese (II) ion reacts with bis-sulfosalophen to form a binuclear manganese (II) complex. The binuclear complex then self-assembles with bis-phenanthroline-glutaraldehyde to form the supramolecular polymer, resulting in the production of strong resonance light scattering signal. The amount of manganese was detected by measuring the resonance light scattering intensity. Under optimal conditions, a linear range was found to be 0.5–50.0 ng/mL, with a detection limit of 0.1 ng/mL. The method has been successfully applied to determine manganese in vegetable and tea samples with relative standard deviations of less than 5% and recoveries of 95.8–105.7%. 相似文献
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Yunfei Lv Yongzhen Pei Shujing Gao Changguo Li 《Nonlinear Analysis: Real World Applications》2010,11(5):3608-3619
Considering that some phytoplankton and zooplankton are harvested for food, a phytoplankton–zooplankton model with harvesting is proposed and investigated. First, stability conditions of equilibria and existence conditions of a Hopf-bifurcation are established. Our results indicate that over exploitation would result in the extinction of the population and an appropriate harvesting strategy should ensure the sustainability of the population which is in line with reality. Furthermore, the existence of bionomic equilibria and the optimal harvesting policy are discussed. The present value of revenues is maximized by using Pontryagin’s maximum principle subject to the state equations and the control constraints. We discussed the case of optimal equilibrium solution. It is found that the shadow prices remain constant over time in optimal equilibrium when they satisfy the transversality condition. It is established that the zero discounting leads to the maximization of economic revenue and that an infinite discount rate leads to complete dissipation of economic rent. Finally, some numerical simulations are given to illustrate our results. 相似文献
48.
We have used vibrational spectroscopy to study the formula and molecular structure of the mineral penkvilksite Na 2TiSi 4O 11·2H 2O. Penkvilksite is a mineral which may be used in the uptake of radioactive elements. Both Raman and infrared spectroscopies identify a band at ~3638 cm?1 attributed to an OH-stretching vibration of hydroxyl units. The inference is that OH units are involved in the structure of penkvilksite. The formula may be well written as Na 2TiSi 4O 10(OH)2·H 2O. The mineral is characterised by a very intense Raman band at 1085 cm?1 and a broad infrared band at 1080 cm?1 assigned to SiO-stretching vibrations. Raman bands at 620, 667 and 711 cm?1 are attributed to SiO and TiO chain bonds. Water-stretching vibrations are observed as Raman bands at 3197, 3265, 3425 and 3565 cm?1. Vibrational spectroscopy enables aspects of the molecular structure of the mineral penkvilksite to be ascertained. Penkvilksite is a mineral which can incorporate actinides and lanthanides from radioactive waste. 相似文献
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