全文获取类型
收费全文 | 1864篇 |
免费 | 52篇 |
国内免费 | 7篇 |
专业分类
化学 | 1492篇 |
晶体学 | 9篇 |
力学 | 20篇 |
数学 | 161篇 |
物理学 | 241篇 |
出版年
2024年 | 1篇 |
2023年 | 9篇 |
2022年 | 41篇 |
2021年 | 105篇 |
2020年 | 41篇 |
2019年 | 61篇 |
2018年 | 75篇 |
2017年 | 51篇 |
2016年 | 72篇 |
2015年 | 80篇 |
2014年 | 64篇 |
2013年 | 165篇 |
2012年 | 86篇 |
2011年 | 115篇 |
2010年 | 95篇 |
2009年 | 84篇 |
2008年 | 111篇 |
2007年 | 129篇 |
2006年 | 88篇 |
2005年 | 93篇 |
2004年 | 62篇 |
2003年 | 65篇 |
2002年 | 54篇 |
2001年 | 17篇 |
2000年 | 18篇 |
1999年 | 23篇 |
1998年 | 16篇 |
1997年 | 17篇 |
1996年 | 23篇 |
1995年 | 5篇 |
1994年 | 6篇 |
1993年 | 7篇 |
1992年 | 12篇 |
1991年 | 7篇 |
1989年 | 2篇 |
1988年 | 2篇 |
1987年 | 1篇 |
1986年 | 1篇 |
1985年 | 2篇 |
1984年 | 1篇 |
1982年 | 1篇 |
1981年 | 1篇 |
1980年 | 1篇 |
1979年 | 4篇 |
1978年 | 3篇 |
1977年 | 2篇 |
1975年 | 1篇 |
1973年 | 1篇 |
1972年 | 1篇 |
1968年 | 1篇 |
排序方式: 共有1923条查询结果,搜索用时 15 毫秒
931.
Pidko EA Xu J Mojet BL Lefferts L Subbotina IR Kazansky VB van Santen RA 《The journal of physical chemistry. B》2006,110(45):22618-22627
A FT-IR spectroscopic study of methane, ethane, and propane adsorption on magnesium and calcium forms of zeolite Y reveals different vibrational properties of the adsorbed molecules depending on the exchanged cation. This is attributed to different adsorption conformations of the hydrocarbons. Two-fold eta(2) coordination of light alkanes is realized for MgY, whereas in case of CaY zeolite quite different adsorption modes are found, involving more C-H bonds in the interaction with the cation. The topological analysis of the electron density distribution function of the adsorption complexes shows that when a hydrocarbon coordinates to the exchanged Mg(2+) ions, van der Waals bonds between H atoms of the alkane and basic zeolitic oxygens significantly contribute to the overall adsorption energy, whereas in case of CaY zeolite such interactions play only an indirect role. It is found that, due to the much smaller ionic radius of the Mg(2+) ion as compared to that of Ca(2+), the former ions are significantly shielded with the surrounding oxygens of the zeolitic cation site. This results in a small electrostatic contribution to the stabilization of the adsorbed molecules. In contrast, for CaY zeolite the stabilization of alkanes in the electrostatic field of the partially shielded Ca(2+) cation significantly contributes to the adsorption energy. This is in agreement with the experimentally observed lower overall absorption of C-H stretching vibrations of alkanes loaded to MgY as compared to those for CaY zeolite. The preferred conformation of the adsorbed alkanes is controlled by the bonding within the adsorption complexes that, in turn, strongly depends on the size and location of the cations in the zeolite cavity. 相似文献
932.
Min W Gopich IV English BP Kou SC Xie XS Szabo A 《The journal of physical chemistry. B》2006,110(41):20093-20097
Enzymes are dynamic entities: both their conformation and catalytic activity fluctuate over time. When such fluctuations are relatively fast, it is not surprising that the classical Michaelis-Menten (MM) relationship between the steady-state enzymatic velocity and the substrate concentration still holds. However, recent single-molecule experiments have shown that this is the case even for an enzyme whose catalytic activity fluctuates on the 10(-4)-10 s range. The purpose of this paper is to examine various scenarios in which slowly fluctuating enzymes would still obey the MM relationship. Specifically, we consider (1) the quasi-static condition (e.g., the conformational fluctuation of the enzyme-substrate complex is much slower than binding, catalysis, and the conformational fluctuations of the free enzyme), (2) the quasi-equilibrium condition (when the substrate dissociation is much faster than catalysis, irrespective of the time scales or amplitudes of conformational fluctuations), and (3) the conformational-equilibrium condition (when the dissociation and catalytic rates depend on the conformational coordinate in the same way). For each of these scenarios, the physical meaning of the apparent Michaelis constant and catalytic rate constant is provided. Finally, as an example, the theoretical analysis of a recent single-molecule enzyme assay is considered in light of the perspectives presented in this paper. 相似文献
933.
Nathan J Reh R Ankoudinova I Ankoudinova G Chang B Heckenlively J Hurley JB 《Photochemistry and photobiology》2006,82(6):1489-1494
Methods that allow specific manipulations of the mouse genome have made it possible to alter specific aspects of photoreceptor function within the mouse retina. Mice with photoreceptors that have altered photosensitivities and altered photoresponse kinetics are now available. Methods are needed that can show how those perturbations in photoreceptor response characteristics translate into perturbations in visual sensitivity and perception. We have adapted a previously described method to evaluate visual threshold, spatial discrimination and temporal discrimination in mice swimming in a water maze. In this report we describe the sensitivities of rod-mediated and cone-mediated vision using GNAT1-/-and GNAT2-/- mice. Cone-mediated vision is approximately 10,000 times less sensitive than rod-mediated vision in mice. We also demonstrate that mice can distinguish striped from solid objects in the water maze and that they can distinguish flickering from continuous illumination. 相似文献
934.
Wafaa Zaaraoui Audrey Rico Bertrand Audoin Françoise Reuter Irina Malikova Elisabeth Soulier Patrick Viout Yann Le Fur Sylviane Confort-Gouny Patrick J. Cozzone Jean Pelletier Jean-Philippe Ranjeva 《Magnetic resonance imaging》2010
Background
Acute symptomatic inflammation is a main feature of multiple sclerosis but pathophysiological processes underlying total or partial recovery are poorly understood.Objective
To characterize in vivo these processes at molecular, structural and functional levels using multimodal MR methods.Methods
A neuroimaging 3-year follow-up (Weeks 0, 3, 11, 29, 59 and 169) was conducted on a 41-year-old woman presenting at baseline with a large acute demyelinating lesion of multiple sclerosis. Conventional magnetic resonance imaging (MRI), magnetization transfer imaging, diffusion-weighted imaging, functional MRI and magnetic resonance spectroscopy were conducted at 1.5 T.Results
Patient presenting with subacute left hemiplegia recovered progressively (expended disability status scale 7 to 5.5). The MR exploration demonstrated structural functional and metabolic impairments at baseline. Despite restoration of the blood brain barrier integrity, high lactate levels persisted for several weeks concomitant with glial activation. Slow and progressive structural and metabolic restorations occurred from baseline to W169 (lesion volume −64%; apparent diffusion coefficient −14.7%, magnetization transfer ratio +14%, choline −51%, lipids −78%, N-acetylaspartate +77%) while functionality of the motor system recovered.Conclusions
Multimodal MRI/MRS evidenced long-term dynamics recovery processes involving tissue repair, glial activation, recovery of neuronal function and functional systems. This may impact on customized rehabilitation strategies generally focused on the first months following the onset of symptoms. 相似文献935.
Art van der Est Sara Chirico Irina Karyagina Rachel Cohen Gaozhong Shen John H. Golbeck 《Applied magnetic resonance》2010,37(1-4):103-121
The ambient temperature and low-temperature electron transfer properties of Photosystem I (PS I) from the M688NPsaA and M668NPsaB mutant strains of the cyanobacterium Synechocystis sp. PCC 6803 are studied using transient electron paramagnetic resonance (EPR) and continuous-wave (CW) EPR. The two mutations are expected to alter the midpoint potentials of, and the reorganization energies around, the primary electron chlorophyll acceptors A0A and A0B, which should lead to a change in the yield and/or rate of electron transfer to the phylloquinone acceptors A1A and A1B, respectively. At ambient temperature it is known that both quinone acceptors are active in electron transfer. At low temperature there are at least two fractions that undergo either reversible or irreversible electron transfer. The EPR data of the two PS I variants are used to investigate the relationship between these low-temperature fractions and the ambient temperature electron transfer pathway. The results show that mutation in the PsaA-branch increases the rate of $ {\text{A}}_{{1{\text{A}}}}^{. - } $ to FX electron transfer at ambient temperature, while the corresponding mutation in the PsaB-branch has no effect on the electron transfer rate observable by transient EPR. An analysis of the complete time/field datasets from both variants suggests that the yield of electron transfer in the branch carrying the mutation is reduced. The mutations have no effect on the low-temperature CW EPR spectra of the iron–sulfur clusters if the samples are frozen under illumination but they both cause a decrease in the yield of reduced FA and FB if the samples are frozen in the dark and then illuminated. The PsaA-branch mutation greatly reduces the intensity and changes the polarization pattern of the radical pair $ {\text{P}}_{700}^{ + } {\text{A}}_{1}^{. - } $ . Possible causes of the changes in the polarization pattern are discussed and it is suggested that the mutations introduce structural heterogeneity in the vicinity of the A0 binding site. No clear correlation between the yield of electron transfer in a particular branch and the yield of stable charge separation is found. 相似文献
936.
Optimal Stability of Advection-Diffusion Lattice Boltzmann Models with Two Relaxation Times for Positive/Negative Equilibrium 总被引:1,自引:0,他引:1
Irina Ginzburg Dominique d’Humières Alexander Kuzmin 《Journal of statistical physics》2010,139(6):1090-1143
Despite the growing popularity of Lattice Boltzmann schemes for describing multi-dimensional flow and transport governed by
non-linear (anisotropic) advection-diffusion equations, there are very few analytical results on their stability, even for
the isotropic linear equation. In this paper, the optimal two-relaxation-time (OTRT) model is defined, along with necessary and sufficient (easy to use) von Neumann stability conditions for a very general
anisotropic advection-diffusion equilibrium, in one to three dimensions, with or without numerical diffusion. Quite remarkably,
the OTRT stability bounds are the same for any Peclet number and they are defined by the adjustable equilibrium parameters.
Such optimal stability is reached owing to the free (“kinetic”) relaxation parameter. Furthermore, the sufficient stability bounds tolerate negative
equilibrium functions (the distribution divided by the local mass), often labeled as “unphysical”. We prove that the non-negativity
condition is (i) a sufficient stability condition of the TRT model with any eigenvalues for the pure diffusion equation, (ii) a sufficient stability condition of its OTRT and BGK/SRT sub-classes, for any linear anisotropic advection-diffusion equation, and (iii) unnecessarily more restrictive for any Peclet number than the optimal sufficient conditions. Adequate choices
of the two relaxation rates and the free-tunable equilibrium parameters make the OTRT sub-class more efficient than the BGK
one, at least in the advection-dominant regime, and allow larger time steps than known criteria of the forward time central
finite-difference schemes (FTCS/MFTCS) for both, advection and diffusion dominant regimes. 相似文献
937.
Vyacheslav Vasilevich Slepushkin Boris Mikhailovich Stifatov Sergei Borisovich Stifatov Irina Alexandrovna Tikhonova 《Journal of Solid State Electrochemistry》1999,3(4):234-238
Local electrochemical analysis with a pressed-on electrochemical cell is suggested for the estimation of the quality of coatings
protecting the surface of materials, for the detection of different defects and for the control of the thickness and composition
of coatings. The suggested local electrochemical analysis allows the determination of the throwing power of the electrolytes
used in electrodepositions.
Received: 8 December 1997 / Accepted: 21 August 1998 相似文献
938.
Victor N. Khrustalev Mikhail Yu. Antipin Nikolay N. Zemlyansky Irina V. Borisova Yuri A. Ustynyuk Valery V. Lunin Keith Izod 《应用有机金属化学》2005,19(3):360-362
An X‐ray diffraction study reveals an unusual structure of the new thermally stable germanium(II) ate complex [Ph3PiPr][Ge(OAc)3] (4) containing a discrete [Ge(OAc)3](?) anion containing monodentate acetate ligands with a trigonal pyramidal germanium centre. Copyright © 2005 John Wiley & Sons, Ltd. 相似文献
939.
Alexei D. Averin Anton V. Shukhaev Alexei K. Buryak Franck Denat Roger Guilard Irina P. Beletskaya 《Tetrahedron letters》2008,49(24):3950-3954
New bicyclic cryptand type compounds are synthesized by reacting 1,7-di(3-bromobenzyl)cyclen with 1 equiv of linear polyamines under dilute conditions using Pd-catalyzed amination. Bis(cyclen) and tris(cyclen) compounds containing linear polyamine linkers between benzylated cyclens are obtained by a similar procedure using different reaction conditions. Cyclization of these species via intramolecular catalytic diamination led to tri- and tetracyclic polyaza compounds. 相似文献
940.
Cornelia Fischer Evgeni Sorokin Irina T. Sorokina Markus W. Sigrist 《Optics and Lasers in Engineering》2005,43(3-5):573
We present the first spectroscopic measurements using a tunable solid state Cr2+:ZnSe laser emitting at wavelengths between 2.2 μm and 2.8 μm. Photoacoustic measurements on various gases such as methane, carbon monoxide, carbon dioxide, water vapour, nitrous oxide, and ambient air were carried out. In this paper, we present measurements on methane, nitrous oxide, and ambient air. The deduced detection limits are in the low ppm or sub-ppm range, e.g., 0.2 ppm for carbon dioxide, 0.8 ppm for methane and 2.7 ppm for carbon monoxide. 相似文献