全文获取类型
收费全文 | 62591篇 |
免费 | 10474篇 |
国内免费 | 1962篇 |
专业分类
化学 | 58552篇 |
晶体学 | 599篇 |
力学 | 1573篇 |
综合类 | 41篇 |
数学 | 4553篇 |
物理学 | 9709篇 |
出版年
2023年 | 208篇 |
2022年 | 517篇 |
2021年 | 762篇 |
2020年 | 1722篇 |
2019年 | 3029篇 |
2018年 | 1407篇 |
2017年 | 1030篇 |
2016年 | 4036篇 |
2015年 | 4062篇 |
2014年 | 4164篇 |
2013年 | 5519篇 |
2012年 | 4908篇 |
2011年 | 4359篇 |
2010年 | 4113篇 |
2009年 | 3937篇 |
2008年 | 4109篇 |
2007年 | 3386篇 |
2006年 | 3000篇 |
2005年 | 3034篇 |
2004年 | 2674篇 |
2003年 | 2300篇 |
2002年 | 2936篇 |
2001年 | 2066篇 |
2000年 | 1848篇 |
1999年 | 767篇 |
1998年 | 373篇 |
1997年 | 397篇 |
1996年 | 387篇 |
1995年 | 327篇 |
1994年 | 337篇 |
1993年 | 331篇 |
1992年 | 315篇 |
1991年 | 238篇 |
1990年 | 189篇 |
1989年 | 155篇 |
1988年 | 161篇 |
1987年 | 132篇 |
1986年 | 125篇 |
1985年 | 194篇 |
1984年 | 129篇 |
1983年 | 113篇 |
1982年 | 134篇 |
1981年 | 101篇 |
1980年 | 85篇 |
1978年 | 84篇 |
1977年 | 91篇 |
1976年 | 96篇 |
1975年 | 104篇 |
1974年 | 87篇 |
1973年 | 110篇 |
排序方式: 共有10000条查询结果,搜索用时 109 毫秒
51.
52.
Science China Mathematics - We prove that if a compact Riemannian 4-manifold with positive sectional curvature satisfies a strengthened Kato type inequality, then it is definite. We also discuss... 相似文献
53.
54.
55.
56.
Uhm H.S. Lee P.H. Kim Y.I. Kim J.H. Chang H.Y. 《IEEE transactions on plasma science. IEEE Nuclear and Plasma Sciences Society》1995,23(4):628-635
A theory is developed for the density profile of low temperature plasmas confined by applied magnetic field and an experiment of the electron-cyclotron-resonance (ECR) plasma is conducted to compare the theoretical prediction and experimental measurements. Due to a large electron mobility along the magnetic field, electrons move quickly out of the system, leaving ions behind and building a space charge potential, which leads to the ambipolar diffusion of ions. In a steady-state condition, the plasma generation by ionization of neutral molecules is in balance with plasma loss due to the diffusion, leading to the electron temperature equation, which is expressed in terms of the plasma size, chamber pressure, and the ionization energy and cross section of neutrals. The power balance condition leads to the plasma density equation, which is also expressed in terms of the electron temperature, the input microwave power and the chamber pressure. It is shown that the plasma density increases, reaches its peak and decreases, as the chamber pressure increases from a small value (0.1 mTorr). These simple expressions of electron temperature and density provide a scaling law of ECR plasma in terms of system parameters. After carrying out an experimental observation, it is concluded that the theoretical predictions of the electron temperature and plasma density agree remarkably well with experimental data 相似文献
57.
Using the continuous shape space formalism, we develop an immune system model involving both B lymphocytes and antibody molecules. The binding and cross-linking of receptors on B cells stimulates the cells to divide and, with a lag, to secrete antibody. Using the method of multiple scales, we show how to correctly formulate long-time-scale equations for the population dynamics of B cells, the total antibody concentration, and rate of antibody secretion. We compare our model with previous phenomenological formulations. 相似文献
58.
59.
5‐Amino‐4‐methyl‐2‐phenyl‐6‐substitutedfuro[2,3‐d]pyrimidines ( 2a‐c ) were reacted with 2,5‐dimethoxytetrahydrfuran to afford the pyrrolyl derivatives 3a‐c . Compound 3a was chosen as intermediate for the synthesis of poly fused heterocycles incorporated furopyrimidines moiety 4–11 . Some of the synthesized compounds were screened for their antibacterial and antifungal activities. 相似文献
60.
Efficient measurement of the performance index (the distance of a loading parameter from the voltage collapse point) is one of the key problems in power system operations and planning and such an index indicates the severity of a power system with regard to voltage collapse. There exist many interesting methods and ideas to compute this index. However, some successful methods are not yet mathematically justified while other mathematically sound methods are often proposed directly based on the bifurcation theory and they require the initial stationary state to be too close to the unknown turning point to make the underlying methods practical.This paper first gives a survey of several popular methods for estimating the fold bifurcation point including the continuation methods, bifurcation methods and the test function methods (Seydel's direct solution methods, the tangent vector methods and the reduced Jacobian method) and discuss their relative advantages and problems. Test functions are usually based on scaling of the determinant of the Jacobian matrix and it is generally not clear how to determine the behaviour of such functions. As the underlying nonlinear equations are of a particular type, this allows us to do a new analysis of the determinants of the Jacobian and its submatrices in this paper. Following the analysis, we demonstrate how to construct a class of test functions with a predictable analytical behaviour so that a suitable index can be produced. Finally, examples of two test functions from this class are proposed. For several standard IEEE test systems, promising numerical results have been achieved. 相似文献