全文获取类型
收费全文 | 4894篇 |
免费 | 151篇 |
国内免费 | 14篇 |
专业分类
化学 | 2957篇 |
晶体学 | 89篇 |
力学 | 153篇 |
数学 | 350篇 |
物理学 | 1510篇 |
出版年
2023年 | 46篇 |
2022年 | 94篇 |
2021年 | 85篇 |
2020年 | 108篇 |
2019年 | 104篇 |
2018年 | 119篇 |
2017年 | 88篇 |
2016年 | 144篇 |
2015年 | 114篇 |
2014年 | 174篇 |
2013年 | 280篇 |
2012年 | 365篇 |
2011年 | 355篇 |
2010年 | 202篇 |
2009年 | 180篇 |
2008年 | 214篇 |
2007年 | 213篇 |
2006年 | 201篇 |
2005年 | 169篇 |
2004年 | 130篇 |
2003年 | 118篇 |
2002年 | 97篇 |
2001年 | 75篇 |
2000年 | 62篇 |
1999年 | 54篇 |
1998年 | 36篇 |
1997年 | 56篇 |
1996年 | 64篇 |
1995年 | 45篇 |
1994年 | 75篇 |
1993年 | 82篇 |
1992年 | 80篇 |
1991年 | 50篇 |
1990年 | 51篇 |
1989年 | 54篇 |
1988年 | 35篇 |
1987年 | 40篇 |
1986年 | 37篇 |
1985年 | 36篇 |
1984年 | 33篇 |
1983年 | 37篇 |
1982年 | 43篇 |
1981年 | 51篇 |
1980年 | 42篇 |
1979年 | 32篇 |
1978年 | 27篇 |
1977年 | 34篇 |
1975年 | 24篇 |
1974年 | 27篇 |
1973年 | 33篇 |
排序方式: 共有5059条查询结果,搜索用时 15 毫秒
61.
This article describes a nutrient‐phytoplankton‐zooplankton system with nutrient recycling in the presence of toxicity. We have studied the dynamical behavior of the system with delayed nutrient recycling in the first part of the article. Uniform persistent of the system is examined. In the second part of the article, we have incorporated diffusion of the plankton population to the system and dynamical behavior of the system is analyzed with instantaneous nutrient recycling. The condition of the diffusion driven instability is obtained. The conditions for the occurrence of Hopf and Turing bifurcation critical line in a spatial domain are derived. Variation of the system with small periodicity of diffusive coefficient has been studied. Stability condition of the plankton system subject to the periodic diffusion coefficient of the zooplankton is derived. It is observed that nutrient‐phytoplankton‐zooplankton interactions are very complex and situation specific. Moreover, we have obtained different exciting results, ranging from stable situation to cyclic oscillatory behavior may occur under different favorable conditions, which may give some insights for predictive management. © 2014 Wiley Periodicals, Inc. Complexity 21: 229–241, 2015 相似文献
62.
Effect of Hall current on the unsteady free convection flow of a viscous incompressible and electrically conducting fluid past a fluctuating porous flat plate with internal heat absorption/generation in the presence of foreign gasses (such as H2, CO2, H2O, NH3) was investigated. The results are discussed with the effect of the parameters m, the Hall current, Mt, the hydromagnetic parameter, G r the Grashoff number for heat transfer, G c , the Grashoff number for mass transfer, S, the internal heat absorption/generation parameter, α, the transpiration parameter, S c , the Schmidt parameter, and K c the chemical reaction parameter for Prandtl number P r = 0.71, which represents air. Further, the present study accounts for the 1st order chemical reaction affecting the flow characteristics. The governing equations are solved in closed form applying Hh n (x) function. The effects of pertinent parameters characterizing the flow field are discussed with the help of graphs and tables. The important observation of the present study is that heat generation/absorption modifies the flow of current simultaneously to a magnetic force and thermal bouncy force. Heat generation combined with blowing leads to a sharp fall of temperature. 相似文献
63.
Carrier transport in gated 2D graphene monolayers is considered in the presence of scattering by random charged impurity centers with density n(i). Excellent quantitative agreement is obtained (for carrier density n>10(12) cm(-2)) with existing experimental data. The conductivity scales linearly with n/n(i) in the theory. We explain the experimentally observed asymmetry between electron and hole conductivities, and the high-density saturation of conductivity for the highest mobility samples. We argue that the experimentally observed saturation of conductivity at low density arises from the charged impurity induced inhomogeneity in the graphene carrier density which becomes severe for n less, similarn(i) approximately 10(12) cm(-2). 相似文献
64.
We investigate the doping dependence of the penetration depth versus temperature in electron-doped Pr(2-x)Ce(x)CuO(4-delta) using a model which assumes the uniform coexistence of (mean-field) antiferromagnetism and superconductivity. Despite the presence of a d(x2-y2) pairing gap in the underlying spectrum, we find nodeless behavior of the low-T penetration depth in the underdoped case, in accord with experimental results. As doping increases, a linear-in-T behavior of the penetration depth, characteristic of d-wave pairing, emerges as the lower magnetic band crosses the Fermi level and creates a nodal Fermi surface pocket. 相似文献
65.
Manfra MJ Hwang EH Das Sarma S Pfeiffer LN West KW Sergent AM 《Physical review letters》2007,99(23):236402
We present a study of the temperature and density dependence of the resistivity of an extremely high quality two-dimensional hole system grown on the (100) surface of GaAs. For high densities in the metallic regime (p > or approximately4x10;{9} cm;{-2}), the nonmonotonic temperature dependence ( approximately 50-300 mK) of the resistivity is consistent with temperature dependent screening of residual impurities. At a fixed temperature of T=50 mK, the conductivity versus density data indicate an inhomogeneity driven percolation-type transition to an insulating state at a critical density of 3.8x10;{9} cm;{-2}. 相似文献
66.
We describe how the spin coherence time of a localized electron spin in solids, i.e., a solid state spin qubit, can be prolonged by applying designed electron spin resonance pulse sequences. In particular, the spin echo decay due to the spectral diffusion of the electron spin resonance frequency induced by the non-Markovian temporal fluctuations of the nuclear spin flip-flop dynamics can be strongly suppressed using multiple-pulse sequences akin to the Carr-Purcell-Meiboom-Gill pulse sequence in nuclear magnetic resonance. Spin coherence time can be enhanced by factors of 4-10 in GaAs quantum-dot and Si:P quantum computer architectures using composite sequences with an even number of pulses. 相似文献
67.
Significant progress has been made in the last few years toward synthesizing highly dispersible inorganic catalysts for application in the electrodes of direct methanol fuel cells. In addition, research toward achieving an efficient catalyst supporting matrix has also attracted much attention in recent years. Carbon black- (Vulcan XC-72) supported Platinum and Platinum-Ruthenium catalysts have for long served as the conventional choice as the cathode and the anode catalyst materials, respectively. Oxygen reduction reaction at the cathode and methanol oxidation reaction at the anode occur simultaneously during the operation of a direct methanol fuel cell. However, inefficiencies in these reactions result in a generation of mixed potential. This, in turn, gives rise to reduced cell voltage, increased oxygen stoichiometric ratio, and generation of additional water that is responsible for water flooding in the cathode chamber. In addition, the lack of long-term stability of Pt-Ru anode catalyst, coupled with the tendency of Ru to cross through the polymer electrolyte membrane and eventually get deposited on the cathode, is also a serious drawback. Another source of potential concern is the fact that the natural resource of Pt and the rare earth metal Ru is very limited, and has been predicted to become exhausted very soon. To overcome these problems, new catalyst systems with high methanol tolerance and higher catalytic activity than Pt need to be developed. In addition, the catalyst-supporting matrix is also witnessing a change from traditionally used carbon powder to transition metal carbides and other high-performance materials. This article surveys the recent literature based on the advancements made in the field of highly dispersible inorganic catalysts for application in direct methanol fuel cells, as well as the progress made in the area of catalyst-supporting matrices. 相似文献
68.
There exist model calculations showing the modification of the hadronic parameters of ρ meson in the nuclear environment. From these parameters, we extract the ρ-meson-nucleus optical potential and show the medium effect due to this potential on the ρ-meson mass distribution spectra in the photonuclear reactions. The calculated results reproduced reasonably the measured e + e − invariant mass, i.e., ρ-meson mass, distribution spectra in γ, ρ 0 → e + e − reactions on nuclei.
相似文献69.
D. Thao Nguyen Matthias Freitag Christian Gutheil Dr. Kai Sotthewes Dr. Bonnie J. Tyler Dr. Marcus Böckmann Mowpriya Das Friederike Schlüter Prof. Dr. Nikos L. Doltsinis Prof. Dr. Heinrich F. Arlinghaus Prof. Dr. Bart Jan Ravoo Prof. Dr. Frank Glorius 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2020,132(32):13754-13759
70.
AbstractEffective extraction of phyto-biomolecules insures retaining maximum functionality along with higher recovery. In this study, ultrasound-solvent assisted extraction (USAE) was employed for optimal extraction of phyto-biomolecules from Sesamum indicum (sesame) leaves using the approach of Response Surface Methodology (RSM). The optimized condition of 200?W power, 59% methanol concentration with 1:14?g/mL solid–liquid ratio and 15?min of extraction time yielded 367.39?±?1.85?mg GAE/100?g of total phenolic content, 96.72?±?3.27% of free radical scavenging activity and 81.20?±?2.87% of iron chelating activity respectively. The extract consist of essential phytocomponents like gallic acid, chlorogenic acid, and quercetin with lipid peroxidation activities of >50% over incubation time of 48?h. Also, showed antimicrobial activity against various Gram’s negative and positive food borne pathogens. The results of this study implied the importance of USAE for effective and optimum recovery of phyto-biomolecules from Sesame leaves with retained functional properties. 相似文献