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71.
72.
Tracie L. Williams James L. Stephenson Richard A. Yost 《Journal of the American Society for Mass Spectrometry》1997,8(5):532-538
The quadrupole ion trap is commonly operated with a constant background pressure of an inert, low molecular weight buffer gas. This inclusion of a buffer gas has been shown to increase the sensitivity and mass resolution of the instrument. Research to gain an understanding of these effects, both experimental and through simulations, has typically assumed that it is optimal to maintain a constant buffer gas pressure throughout the entire experiment This article describes the effects of the pulsed introduction of buffer gas at strategic points within the analytical scan and evaluates those events during which the presence of buffer gas is critical. By incorporating a pulsed valve within the ion trap manifold, both the presence and pressure of the buffer gas can be controlled and varied during the individual steps of the scan. The presence of helium buffer gas just before the ion ejection and detection event showed a greater increase in intensity of the ion signal than at any other time in the analytical scan. In addition, this increase in intensity upon pulsed introduction of buffer gas prior to detection is constant over a wide range of pulsed valve open times (i.e., pressures), whereas the signal enhancement upon pulsed introduction of the buffer gas before ionization is observed only over a narrow range of pulsed valve open times. 相似文献
73.
Stephenson BC Rangel-Yagui CO Pessoa Junior A Tavares LC Beers K Blankschtein D 《Langmuir : the ACS journal of surfaces and colloids》2006,22(4):1514-1525
Surfactants can be used to increase the solubility of poorly soluble drugs in water and to increase drug bioavailability. In this article, the aqueous solubilization of the nonsteroidal, antiinflammatory drug ibuprofen is studied experimentally and theoretically in micellar solutions of anionic (sodium dodecyl sulfate, SDS), cationic (dodecyltrimethylammonium bromide, DTAB), and nonionic (dodecyl octa(ethylene oxide), C12E8) surfactants possessing the same hydrocarbon "tail" length but differing in their hydrophilic headgroups. We find that, for these three surfactants, the aqueous solubility of ibuprofen increases linearly with increasing surfactant concentration. In particular, we observed a 16-fold increase in the solubility of ibuprofen relative to that in the aqueous buffer upon the addition of 80 mM DTAB and 80 mM C12E8 but only a 5.5-fold solubility increase upon the addition of 80 mM SDS. The highest value of the molar solubilization capacity (chi) was obtained for DTAB (chi = 0.97), followed by C12E8 (chi = 0.72) and finally by SDS (chi = 0.23). A recently developed computer simulation/molecular-thermodynamic modeling approach was extended to predict theoretically the solubilization behavior of the three ibuprofen/surfactant mixtures considered. In this modeling approach, molecular-dynamics (MD) simulations were used to identify which portions of ibuprofen are exposed to water (hydrated) in a micellar environment by simulating a single ibuprofen molecule at an oil/water interface (modeling the micelle core/water interface). On the basis of this input, molecular-thermodynamic modeling was then implemented to predict (i) the micellar composition as a function of surfactant concentration, (ii) the aqueous solubility of ibuprofen as a function of surfactant concentration, and (iii) the molar solubilization capacity (chi). Our theoretical results on the solubility of ibuprofen in aqueous SDS and C12E8 surfactant solutions are in good agreement with the experimental data. The ibuprofen solubility in aqueous DTAB solutions was somewhat overpredicted because of challenges associated with accurately modeling the strong electrostatic interactions between the anionic ibuprofen and the cationic DTAB. Our results indicate that computer simulations of ibuprofen at a flat oil/water interface can be used to obtain accurate information about the hydrated and the unhydrated portions of ibuprofen in a micellar environment. This information can then be used as input to a molecular-thermodynamic model of self-assembly to successfully predict the aqueous solubilization behavior of ibuprofen in the three surfactant systems studied. 相似文献
74.
XiGuang Cao Xun Chen YunHua Chen XiangYi Cui DeQing Fang ChangBo Fu Karl L. Giboni HaoWei Gong GuoDong Guo Ming He Jie Hu XingTao Huang XiangDong Ji YongLin Ju ShaoLi Li Qing Lin HuaXuan Liu JiangLai Liu Xiang Liu Wolfgang Lorenzon YuGang Ma YaJun Mao KaiXuan Ni Kirill Pushkin XiangXiang Ren Michael Schubnell ManBing Shen YuJie Shi Scott Stephenson AnDi Tan Greg Tarlé HongWei Wang JiMing Wang Meng Wang XuMing Wang Zhou Wang YueHuan Wei ShiYong Wu MengJiao Xiao Xiang Xiao PengWei Xie Tao Ye YingHui You XiongHui Zen Hua Zhang Tao Zhang HaiYing Zhao Li Zhao XiaoPeng Zhou ZhongHua Zhu 《中国科学:物理学 力学 天文学(英文版)》2014,57(8):1476-1494
75.
Wallentin CJ Nguyen JD Finkbeiner P Stephenson CR 《Journal of the American Chemical Society》2012,134(21):8875-8884
Herein, the development of visible light-mediated atom transfer radical addition (ATRA) of haloalkanes onto alkenes and alkynes using the reductive and oxidative quenching of [Ir{dF(CF(3))ppy}(2)(dtbbpy)]PF(6) and [Ru(bpy)(3)]Cl(2) is presented. Initial investigations indicated that the oxidative quenching of photocatalysts could effectively be utilized for ATRA, and since that report, the protocol has been expanded by broadening the scope of the reaction in terms of the photocatalysts, substrates, and solvents. In addition, further modifications of the reaction conditions allowed for the efficient ATRA of perfluoroalkyl iodides onto alkenes and alkynes utilizing the reductive quenching cycle of [Ru(bpy)(3)]Cl(2) with sodium ascorbate as the sacrificial electron donor. These results signify the complementary nature of the oxidative and reductive quenching pathways of photocatalysts and the ability to predictably direct reaction outcome through modification of the reaction conditions. 相似文献
76.
Gutt C Wochner P Fischer B Conrad H Castro-Colin M Lee S Lehmkühler F Steinke I Sprung M Roseker W Zhu D Lemke H Bogle S Fuoss PH Stephenson GB Cammarata M Fritz DM Robert A Grübel G 《Physical review letters》2012,108(2):024801
We measured the transverse and longitudinal coherence properties of the Linac Coherent Light Source (LCLS) at SLAC in the hard x-ray regime at 9 keV photon energy on a single shot basis. Speckle patterns recorded in the forward direction from colloidal nanoparticles yielded the transverse coherence properties of the focused LCLS beam. Speckle patterns from a gold nanopowder recorded with atomic resolution allowed us to measure the shot-to-shot variations of the spectral properties of the x-ray beam. The focused beam is in the transverse direction fully coherent with a mode number close to 1. The average number of longitudinal modes behind the Si(111) monochromator is about 14.5 and the average coherence time τ(c)=(2.0±1.0) fc. The data suggest a mean x-ray pulse duration of (29±14) fs behind the monochromator for (100±14) fc electron pulses. 相似文献
77.
Dr. Joohyun Lim Se-Ho Kim Raquel Aymerich Armengol Dr. Olga Kasian Prof. Dr. Pyuck-Pa Choi Dr. Leigh T. Stephenson Dr. Baptiste Gault Prof. Dr. Christina Scheu 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2020,132(14):5700-5704
The incorporation of impurities during the chemical synthesis of nanomaterials is usually uncontrolled and rarely reported because of the formidable challenge in measuring trace amounts of often light elements with sub-nanometer spatial resolution. And yet, these foreign elements (introduced by doping, for example) influence functional properties. We demonstrate how the hydrothermal growth and a partial reduction reaction on hollow TiO2 nanowires leads to the introduction of parts per millions of boron, sodium, and nitrogen. This doping explains the presence of oxygen vacancies and reduced Ti states at the surface, which enhance the functional properties of TiO2. Our results were obtained on model metal oxide nanomaterials and they shed light on a general process that leads to the uncontrolled incorporation of trace impurities in TiO2, thereby, having a strong effect on applications in energy-harvesting. 相似文献
78.
George A. Olah Maurice Stephenson Joe G. Shih V. V. Krishnamurthy G. K. Surya Prakash 《Journal of fluorine chemistry》1988,40(2-3)
One bond 13C-19F coupling constant is used as a criterion to measure the extent of fluorine non-bonded electron pair back-donation to a carbenium center in a series of α-fluorodiarylcarbenium ions
. The electron demand at the cationic center was probed by systematically changing the nature of substituents on the phenyl rings, applying the tool of increasing electron demand to the study. 相似文献
79.
The 14N NQR frequencies of selected aromatic C-nitroso compounds were measured using cross relaxation double resonance. The monomers
have high 14N NQR frequencies with Χ and η values of 5700 to 6100 kHz and 0.5 ± 0.1, respectively. The dimers and polymer show lower frequencies with values of 2300
to 2600 kHz and 0.8 ± 0.05. Benzofuroxan has values typical of both the monomer and dimer. 相似文献
80.
S Kailas A Saxena P P Singh Q Chen D L Friesel P Schwandt E J Stephenson 《Pramana》1989,33(3):365-379
The giant resonance region in90Zr and116Sn excited by 270 MeV helions has been measured up to about 35 MeV excitation energy. The low and the high energy octupole
resonances are seen prominently in addition to the quadrupole and the monopole resonances. The angular distribution data for
the various multipoles are satisfactorily explained by the collective model calculations. The percentatge energy weighted
sum rule strengths have been determined for all the prominent resonances. 相似文献