全文获取类型
收费全文 | 29338篇 |
免费 | 4790篇 |
国内免费 | 2126篇 |
专业分类
化学 | 6226篇 |
晶体学 | 96篇 |
力学 | 4204篇 |
综合类 | 714篇 |
数学 | 14047篇 |
物理学 | 10967篇 |
出版年
2024年 | 46篇 |
2023年 | 329篇 |
2022年 | 505篇 |
2021年 | 751篇 |
2020年 | 737篇 |
2019年 | 659篇 |
2018年 | 680篇 |
2017年 | 1104篇 |
2016年 | 1259篇 |
2015年 | 1043篇 |
2014年 | 1603篇 |
2013年 | 2110篇 |
2012年 | 1596篇 |
2011年 | 1766篇 |
2010年 | 1579篇 |
2009年 | 1810篇 |
2008年 | 1996篇 |
2007年 | 2081篇 |
2006年 | 1674篇 |
2005年 | 1482篇 |
2004年 | 1424篇 |
2003年 | 1277篇 |
2002年 | 1171篇 |
2001年 | 938篇 |
2000年 | 879篇 |
1999年 | 823篇 |
1998年 | 705篇 |
1997年 | 589篇 |
1996年 | 527篇 |
1995年 | 512篇 |
1994年 | 355篇 |
1993年 | 309篇 |
1992年 | 231篇 |
1991年 | 273篇 |
1990年 | 189篇 |
1989年 | 133篇 |
1988年 | 166篇 |
1987年 | 119篇 |
1986年 | 96篇 |
1985年 | 125篇 |
1984年 | 78篇 |
1983年 | 54篇 |
1982年 | 65篇 |
1981年 | 63篇 |
1980年 | 72篇 |
1979年 | 70篇 |
1978年 | 42篇 |
1977年 | 45篇 |
1976年 | 44篇 |
1974年 | 20篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
21.
Transformation hydrodynamics and the corresponding metamaterials have been proposed as a means to exclude the drag force acting on an object. Here, we report a strategy to deploy the hydrodynamic cloaks in a more practical manner by assembling different-shaped cloaking parts. Our strategy is to first model a square-shaped cloak and a carpet cloak and then combine them to conceal a more complex-shaped space in the three-dimensional hydrodynamic flow. With the derivation of transformation hydrodynamics, the coordinate transformations for each hydrodynamic cloaking are demonstrated with the calculated viscosity tensors. The pressure and velocity fields of the square, triangular (carpet), and exemplary three-dimensional house-shaped cloaks are numerically simulated, thus showing a cloaking effect and reduced drag. This study suggests an efficient way of cloaking complex architectures from fluid-dynamic forces. 相似文献
22.
《Tetrahedron letters》2019,60(24):1582-1586
Rh(II)-catalyzed decomposition of certain cyclic α-diazocarbonyl compounds in the presence of cyclic ethers has been shown to give bicyclic ring expansion products. These are thought to arise from a [1,4]-alkyl shift toward the carbonyl oxygen atom and are in contrast with the recently observed spirocyclic products of a Stevens-type [1,2]-alkyl shift within the postulated oxonium ylide intermediate. Quantum chemical calculations performed at the B3LYP/6-31G* level of theory showed that the former reaction pathway (toward fused bicycles) is kinetically preferred. 相似文献
23.
Abstract Gastrodigenin, also known as 4-hydroxybenzyl alcohol (HBA), is one of the main components of Gastrodia elata, which is a perfect lead compound of natural products. In order to get new active compounds, we modified the structure of HBA through esterification with carboxylic acid, and got a series of derivatives in which 4-hydroxybenzyl alcohol 2-naphthoate (NHBA) showed stronger antidepressant activity than HBA. In this paper, we firstly evaluated the antidepressant activity of NHBA by tail suspension test (TST) and forced swimming test (FST). Then, we carried out the biochemical assay and western blot to determine its mechanism. The results displayed that NHBA could increase the content of serotonin, dopamine, norepinephrine, γ-aminobutyric acid, brain-derived neurotrophic factor (BDNF) and tropomyosin receptor kinase B (TrkB) in mice brain. It suggested that NHBA exhibited an antidepressant-like effect through monoaminergic system, GABAergic system and BDNF/TrkB signaling pathways. 相似文献
24.
采用DIS数字信息化系统,对不同浓度的蓝墨水溶液在激光光源照射下的透射平均照度及照度分布图像进行实验研究,得到了平均照度值随溶液浓度变化的规律。对不同颜色塑料膜片对白炽灯光源透过照度值及照度分布图像进行实验研究得到及其相关之规律。 相似文献
25.
We investigate the problem of teleportation of unitary operations by unidirectional control-state telepor-tation and propose a scheme called unidirectional quantum remote control. The scheme is based on the isomorphismbetween operation and state. It allows us to store a unitary operation in a control state, thereby teleportation of theunitary operation can be implemented by unidirectional teleportation of the control-state. We find that the probabilityof success for implementing an arbitrary unitary operation on arbitrary M-qubit state by unidirectional control-stateteleportation is 4-M, and 2M ebits and 4M cbits are consumed in each teleportation. 相似文献
26.
An M/G/1 retrial queueing system with additional phase of service and possible preemptive resume service discipline is considered. For an arbitrarily distributed retrial time distribution, the necessary and sufficient condition for the system stability is obtained, assuming that only the customer at the head of the orbit has priority access to the server. The steady-state distributions of the server state and the number of customers in the orbit are obtained along with other performance measures. The effects of various parameters on the system performance are analysed numerically. A general decomposition law for this retrial queueing system is established. 相似文献
27.
28.
In a previous paper (Ref. 1), an exact solution of the optimal planar interception with fixed end conditions was derived in closed form. The optimal control was expressed as an explicit function of the state variables and two fixed parameters, obtained by solving a set of nonlinear algebraic equations involving elliptic integrals. In order to facilitate the optimal control implementation, the present paper derives a highly accurate simplified solution assuming that the ratio of the pursuer turning radius to the initial range is small. An asymptotic expansion further reduces the computational workload. Construction of a near-optimal open-loop control, based on the approximations, completes the present paper. 相似文献
30.
对由一类非线性抛物型变分不等方程所描述的无穷维动力系统,给出了存在全局吸引子及弱近似惯性流形的充分条件. 相似文献