全文获取类型
收费全文 | 50524篇 |
免费 | 8589篇 |
国内免费 | 6672篇 |
专业分类
化学 | 36269篇 |
晶体学 | 653篇 |
力学 | 3084篇 |
综合类 | 509篇 |
数学 | 6263篇 |
物理学 | 19007篇 |
出版年
2024年 | 55篇 |
2023年 | 980篇 |
2022年 | 1224篇 |
2021年 | 1608篇 |
2020年 | 2041篇 |
2019年 | 2029篇 |
2018年 | 1778篇 |
2017年 | 1777篇 |
2016年 | 2433篇 |
2015年 | 2391篇 |
2014年 | 2972篇 |
2013年 | 3885篇 |
2012年 | 4622篇 |
2011年 | 4804篇 |
2010年 | 3483篇 |
2009年 | 3325篇 |
2008年 | 3576篇 |
2007年 | 3207篇 |
2006年 | 2969篇 |
2005年 | 2436篇 |
2004年 | 1799篇 |
2003年 | 1431篇 |
2002年 | 1270篇 |
2001年 | 1102篇 |
2000年 | 1017篇 |
1999年 | 1056篇 |
1998年 | 829篇 |
1997年 | 790篇 |
1996年 | 697篇 |
1995年 | 691篇 |
1994年 | 604篇 |
1993年 | 505篇 |
1992年 | 418篇 |
1991年 | 382篇 |
1990年 | 354篇 |
1989年 | 268篇 |
1988年 | 210篇 |
1987年 | 169篇 |
1986年 | 149篇 |
1985年 | 142篇 |
1984年 | 80篇 |
1983年 | 67篇 |
1982年 | 70篇 |
1981年 | 28篇 |
1980年 | 18篇 |
1979年 | 10篇 |
1971年 | 4篇 |
1959年 | 2篇 |
1957年 | 11篇 |
1936年 | 5篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
31.
An oriented tetrahedron defined on four vertices is a set of four cyclic triples with the property that any ordered pair of vertices is contained in exactly one of the cyclic triples. A tetrahedral quadruple system of order with index , denoted by , is a pair , where is an ‐set and is a set of oriented tetrahedra (blocks) such that every cyclic triple on is contained in exactly members of . A is pure if there do not exist two blocks with the same vertex set. When , the spectrum of a pure TQS has been completely determined by Ji. In this paper, we show that there exists a pure if and only if and . A corollary is that a simple also exists if and only if and . 相似文献
32.
The horizontal convection in a square enclosure driven by a linear temperature profile along the bottom boundary is investigated numerically by using a finite difference method. The Prandtl number is fixed at 4.38, and the Rayleigh number Ra ranges from107 to 1011. The convective flow is steady at a relatively low Rayleigh number, and no thermal plume is observed, whereas it transits to be unsteady when the Rayleigh number increases beyond the critical value. The scaling law for the Nusselt number Nu changes from Rossby's scaling Nu ~ Ra~(1/5) in a steady regime to Nu ~ Ra~(1/4) in an unsteady regime, which agrees well with the theoretically predicted results. Accordingly,the Reynolds number Re scaling varies from Re ~ Ra~(3/11) to Re ~ Ra~(2/5). The investigation on the mean flows shows that the thermal and kinetic boundary layer thickness and the mean temperature in the bulk zone decrease with the increasing Ra. The intensity of fluctuating velocity increases with the increasing Ra. 相似文献
33.
Liu Cungen Gao Chuang Liu Xiaoping Wang Huanqing Zhou Yucheng 《Nonlinear dynamics》2021,104(4):3655-3670
Nonlinear Dynamics - This paper is devoted to the adaptive finite-time prescribed performance control (FTPPC) for stochastic nonlinear systems with unknown virtual control coefficients (UVCCs),... 相似文献
34.
The satisfiability (SAT) problem is a core problem in computer science. Existing studies have shown that most industrial SAT instances can be effectively solved by modern SAT solvers while random SAT instances cannot. It is believed that the structural characteristics of different SAT formula classes are the reasons behind this difference. In this paper, we study the structural properties of propositional formulas in conjunctive normal form (CNF) by the principle of structural entropy of formulas. First, we used structural entropy to measure the complex structure of a formula and found that the difficulty solving the formula is related to the structural entropy of the formula. The smaller the compressing information of a formula, the more difficult it is to solve the formula. Secondly, we proposed a -approximation strategy to approximate the structural entropy of large formulas. The experimental results showed that the proposed strategy can effectively approximate the structural entropy of the original formula and that the approximation ratio is more than 92%. Finally, we analyzed the structural properties of a formula in the solution process and found that a local search solver tends to select variables in different communities to perform the next round of searches during a search and that the structural entropy of a variable affects the probability of the variable being flipped. By using these conclusions, we also proposed an initial candidate solution generation strategy for a local search for SAT, and the experimental results showed that this strategy effectively improves the performance of the solvers CCAsat and Sparrow2011 when incorporated into these two solvers. 相似文献
35.
36.
37.
Xu Yifan Zhang Hehe Ding Tangjing Tian Ruiqi Sun Dongmei Wang Ming-Sheng Zhou Xiaosi 《中国科学:化学(英文版)》2022,65(9):1807-1816
Science China Chemistry - Due to their enormous potential for large-scale energy storage, rechargeable potassium-ion batteries have been widely researched and developed. However, the drastic volume... 相似文献
38.
为探究加强型Bi-2212超导股线的力学性能以及不锈钢包带对超导股线力学性能的影响,运用Ansys软件建立加强型Bi-2212超导股线的三维模型,采用有限元分析方法,对其机械结构进行仿真,模拟高温高压热处理后轴向受拉形变量和应力应变分布情况。仿真结果显示,在同样施加100 N拉力的情况下,与超导原线(Bi-2212超导线)相比,密绕Ni80Cr20包带后的加强型Bi-2212超导股线轴向受拉形变程度减小,约10%;应力应变均降低,约30%。采用Ni80Cr20作为外包带制成的加强型Bi-2212超导股线力学性能显著,是未来超导线发展方向之一。 相似文献
39.
40.
Acta Mathematica Sinica, English Series - We are interested in the existence and asymptotic behavior for the least energy solutions of the following fractional eigenvalue problem $$\left({\rm{P}}... 相似文献