全文获取类型
收费全文 | 86862篇 |
免费 | 2186篇 |
国内免费 | 1404篇 |
专业分类
化学 | 30502篇 |
晶体学 | 866篇 |
力学 | 7245篇 |
综合类 | 52篇 |
数学 | 32900篇 |
物理学 | 18887篇 |
出版年
2024年 | 47篇 |
2023年 | 234篇 |
2022年 | 357篇 |
2021年 | 412篇 |
2020年 | 403篇 |
2019年 | 382篇 |
2018年 | 10705篇 |
2017年 | 10482篇 |
2016年 | 6515篇 |
2015年 | 1288篇 |
2014年 | 872篇 |
2013年 | 1074篇 |
2012年 | 4663篇 |
2011年 | 11304篇 |
2010年 | 6196篇 |
2009年 | 6594篇 |
2008年 | 7152篇 |
2007年 | 9218篇 |
2006年 | 692篇 |
2005年 | 1682篇 |
2004年 | 1813篇 |
2003年 | 2179篇 |
2002年 | 1189篇 |
2001年 | 411篇 |
2000年 | 482篇 |
1999年 | 368篇 |
1998年 | 359篇 |
1997年 | 303篇 |
1996年 | 377篇 |
1995年 | 257篇 |
1994年 | 193篇 |
1993年 | 182篇 |
1992年 | 151篇 |
1991年 | 118篇 |
1990年 | 123篇 |
1989年 | 112篇 |
1988年 | 84篇 |
1987年 | 85篇 |
1986年 | 94篇 |
1985年 | 84篇 |
1984年 | 55篇 |
1983年 | 53篇 |
1982年 | 49篇 |
1981年 | 42篇 |
1980年 | 52篇 |
1979年 | 46篇 |
1914年 | 45篇 |
1912年 | 40篇 |
1909年 | 41篇 |
1908年 | 40篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
121.
122.
Xiao Xiaohua Cao Yuguan Liu Xia Jiang Shengxiang 《Journal of inclusion phenomena and macrocyclic chemistry》2004,48(3-4):111-115
A PM3 calculation in vacuum of the inclusion complexation ofo-, m-, p-nitro-phenol with calix[n]arenes is performedsuccessfully. The pathways for inclusion process are describedand the most probable structures of the 1:1 complex are soughtthrough a potential energy scan. The energy differences betweenthe inclusion complexes and the hosts, by calculation, show thatthe most stable complexation is calix[4]-p-nitro phenol andcalix[6]-m-nitro phenol. 相似文献
123.
The gas‐transport properties of poly[2,6‐toluene‐2,2‐bis(3,4‐dicarboxylphenyl)hexafluoropropane diimide] (6FDA‐2,6‐DAT) have been investigated. The sorption behavior of dense 6FDA‐2,6‐DAT membranes is well described by the dual‐mode sorption model and has certain relationships with the critical temperatures of the penetrants. The solubility coefficient decreases with an increase in either the pressure or temperature. The temperature dependence of the diffusivity coefficient increases with an increase in the penetrant size, as the order of the activation energy for the diffusion jump is CH4 > N2 > O2 > CO2. Also, the average diffusion coefficient increases with increasing pressure for all the gases tested. As a combined contribution from sorption and diffusion, permeability decreases with increases in the pressure and the kinetic diameter of the penetrant molecules. Even up to 32.7 atm, no plasticization phenomenon can be observed on flat dense 6FDA‐2,6‐DAT membranes from their permeability–pressure curves. However, just as for other gases, the absolute value of the heat of sorption of CO2 decreases with increasing pressure at a low‐pressure range, but the trend changes when the feed pressure is greater than 10 atm. This implies that CO2‐induced plasticization may occur and reduce the positive enthalpy required to create a site into which a penetrant can be sorbed. Therefore, a better diagnosis of the inherent threshold pressure for the plasticization of a glassy polymer membrane may involve examining the absolute value of the heat of sorption as a function of pressure and identifying the turning point at which the gradient of the absolute value of the heat of sorption against pressure turns from a negative value to a positive one. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 354–364, 2004 相似文献
124.
We shall answer several questions concerning additive or multiplicative commutators in a division ring which are algebraic of bounded degree over its center. 相似文献
125.
Panos M. Pardalos Wanpracha Chaovalitwongse Leonidas D. Iasemidis J. Chris Sackellares Deng-Shan Shiau Paul R. Carney Oleg A. Prokopyev Vitaliy A. Yatsenko 《Mathematical Programming》2004,101(2):365-385
There is growing evidence that temporal lobe seizures are preceded by a preictal transition, characterized by a gradual dynamical change from asymptomatic interictal state to seizure. We herein report the first prospective analysis of the online automated algorithm for detecting the preictal transition in ongoing EEG signals. Such, the algorithm constitutes a seizure warning system. The algorithm estimates STLmax, a measure of the order or disorder of the signal, of EEG signals recorded from individual electrode sites. The optimization techniques were employed to select critical brain electrode sites that exhibit the preictal transition for the warning of epileptic seizures. Specifically, a quadratically constrained quadratic 0-1 programming problem is formulated to identify critical electrode sites. The automated seizure warning algorithm was tested in continuous, long-term EEG recordings obtained from 5 patients with temporal lobe epilepsy. For individual patient, we use the first half of seizures to train the parameter settings, which is evaluated by ROC (Receiver Operating Characteristic) curve analysis. With the best parameter setting, the algorithm applied to all cases predicted an average of 91.7% of seizures with an average false prediction rate of 0.196 per hour. These results indicate that it may be possible to develop automated seizure warning devices for diagnostic and therapeutic purposes.Mathematics Subject Classification (1991):20E28, 20G40, 20C20 相似文献
126.
Harish Seshadri 《Mathematische Zeitschrift》2004,247(3):487-503
We study closed Einstein 4-manifolds which admit S1 actions of a certain type, i.e., warped products. In particular, we classify them up to isometry when the fixed point of the S1 action satisfies certain natural geometric conditions. The proof uses the Bochner-Weitzenböck formula for 1-forms and the theory of minimal surfaces in 3-manifolds.in final form: 22 January 2003 相似文献
127.
128.
Fabio A. C. C. Chalub Peter A. Markowich Benoît Perthame Christian Schmeiser 《Monatshefte für Mathematik》2004,142(1-2):123-141
Kinetic models for chemotaxis, nonlinearly coupled to a Poisson equation for the chemo-attractant density, are considered. Under suitable assumptions on the turning kernel (including models introduced by Othmer, Dunbar and Alt), convergence in the macroscopic limit to a drift-diffusion model is proven. The drift-diffusion models derived in this way include the classical Keller-Segel model. Furthermore, sufficient conditions for kinetic models are given such that finite-time-blow-up does not occur. Examples are given satisfying these conditions, whereas the macroscopic limit problem is known to exhibit finite-time-blow-up. The main analytical tools are entropy techniques for the macroscopic limit as well as results from potential theory for the control of the chemo-attractant density.Present address: Centro de Matemática e Aplicações Fundamentais, Universidade de Lisboa, Av. Prof. Gama Pinto 2, 1649-003, Lisboa, Portugal 相似文献
129.
Zhaoyang Yin 《Monatshefte für Mathematik》2004,141(4):337-344
We prove under quite general assumptions the existence of a bounded positive solution to the semilinear Schrödinger equation in a two-dimensional exterior domain. 相似文献
130.
On a Definitizable Analog of the Trigonometric Moment Problem Generating an Indefinite Toeplitz Form
We prove the existence of an integro-polynomial representation for a sequence of numbers such that there exists a difference operator mapping this sequence to a sequence that generates the solvable trigonometric moment problem. A similar result related to the power moment problem was given in [12]. 相似文献