全文获取类型
收费全文 | 51930篇 |
免费 | 12369篇 |
国内免费 | 4269篇 |
专业分类
化学 | 53122篇 |
晶体学 | 530篇 |
力学 | 1698篇 |
综合类 | 249篇 |
数学 | 4118篇 |
物理学 | 8851篇 |
出版年
2024年 | 40篇 |
2023年 | 361篇 |
2022年 | 665篇 |
2021年 | 809篇 |
2020年 | 1946篇 |
2019年 | 3261篇 |
2018年 | 1573篇 |
2017年 | 1257篇 |
2016年 | 4178篇 |
2015年 | 4297篇 |
2014年 | 4330篇 |
2013年 | 5273篇 |
2012年 | 4478篇 |
2011年 | 3605篇 |
2010年 | 3927篇 |
2009年 | 3791篇 |
2008年 | 3446篇 |
2007年 | 2694篇 |
2006年 | 2349篇 |
2005年 | 2413篇 |
2004年 | 2111篇 |
2003年 | 1927篇 |
2002年 | 2734篇 |
2001年 | 1997篇 |
2000年 | 1731篇 |
1999年 | 809篇 |
1998年 | 404篇 |
1997年 | 317篇 |
1996年 | 308篇 |
1995年 | 240篇 |
1994年 | 224篇 |
1993年 | 185篇 |
1992年 | 188篇 |
1991年 | 153篇 |
1990年 | 110篇 |
1989年 | 70篇 |
1988年 | 68篇 |
1987年 | 55篇 |
1986年 | 64篇 |
1985年 | 46篇 |
1984年 | 25篇 |
1983年 | 28篇 |
1982年 | 16篇 |
1981年 | 15篇 |
1980年 | 7篇 |
1979年 | 6篇 |
1974年 | 5篇 |
1957年 | 5篇 |
1936年 | 5篇 |
1935年 | 2篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
151.
Hans‐Peter Brack Denise Ruegg Heinrich Bührer Michal Slaski Selmiye Alkan Günther G. Scherer 《Journal of Polymer Science.Polymer Physics》2004,42(13):2612-2624
The influence of irradiation and grafting on the crystallinity of three base polymers has been investigated with differential scanning calorimetry. Grafting has the largest effect on the base polymer crystallinity and results in a reduction of the crystallinity. The thermal degradation of the base polymers and grafted films has been investigated with thermogravimetric analysis. The extent of the fluorination of the base polymer, the irradiation method, and the graft level all influence the thermal degradation and its activation energy. It is proposed that the variation of the chain lengths of the grafted polystyrene chains is actually a primary underlying factor responsible for the influence of these various parameters on the degradation process. The first results of a comparative thermal analysis of some fuel‐cell membranes are also presented, and the promise and shortcomings of this method are discussed. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 2612–2624, 2004 相似文献
152.
对于超细长弹性杆静力学的Kirchhoff方程,用动力学的概念和方法研究其常值特解 和稳定性问题.计算了Kirchhoff方程相对固定坐标系、截面主轴坐标系以及中心线Frenet 坐标系的常值特解,进行了Kirchhoff动力学比拟,用一次近似理论分别讨论了它们的Lyapu nov稳定性,导出了若干稳定性判据,并在参数平面上绘出了稳定域.
关键词:
超细长弹性杆
Kirchhoff方程
常值特解
Lyapunov稳定性 相似文献
153.
154.
Qing An WU Zhen Yuan XU Mei ZHENG Dan Qian XU Jun XU Yin Chu SHENCollege of Chemical Engineering Materials Zhejiang University of Technology Hangzhou 《中国化学快报》2004,(7)
Six new 13-O-acylavermectin Bl aglycones(3-8) were synthesized from avermectin B1 aglycone and their bioactivities were evaluated against Spodoptera exigua, Spodoptera eridania, Tetranychus urticae and Aphis fabae. 相似文献
155.
Ri Cheng XUAN* Wei Xiao HU Zhong Yu YANG Ya Ping LU College of Pharmaceutical Sciences Zhejiang University of Technology Hangzhou 《中国化学快报》2004,(2)
2-Chloro-5-methylpyridine is an important intermediate for the preparation of biological active compounds, especially insecticides1, e.g. imidacloprid2, and is usually manufac- tured from 3-methylpyridine N-oxide. However, the manufacturing process also forms the by-products: 3-methylpyridine and its isomer 2-chloro-3-methylpyridine3. The pro- perties of the isomers are similar, it is difficult to separate by ordinary methods, such as distillation. Scheme 1 79.4%13.6%5.2%NClCH3NCH3ClNC… 相似文献
156.
Baker‘s yeast mediated reduction of optically active diketone is described. The two keto groups are efficiently differentiated and the ee value of the recovered material is considerably raised. It affords highly optically active key intermediates efficiently for the synthesis of natural polyhydroxylated agarofuran products. 相似文献
157.
XiangBaoMENG HuiLI QingLI ZhongJunLI 《中国化学快报》2004,15(7):777-778
An unusual reductive ring-opening reaction of phthalimide with sodium hydride in anhydrous DMF was observed for the first time. The presumed mechanism was described in detail. 相似文献
158.
Zhong Hong YAN Chun Hao YANG Xi Han WU Yu Yuan XIEDivision of Chemistry Shanghai Second Medical University Shanghai Sate Key Laboratory of Drug Research Shanghai Institute of Materia Medica Shanghai Institutesfor Biological Sciences Shanghai 《中国化学快报》2004,(4)
The first and facile synthesis of (±)syringaresinol was described. 相似文献
159.
Anna Dbiska‐Nagrska Andrzej Just Zdzisaw Stempie 《Mathematical Methods in the Applied Sciences》2004,27(2):221-230
This paper presents the Galerkin approximation of the optimization problem of a system governed by non‐linear second‐order evolution equation where a non‐linear operator depends on derivative of the state of the system. The control is acting on a non‐linear equation. After giving some results on the existence of optimal control we shall prove the existence of the weak and strong condensation points of a set of solutions of the approximate optimization problems. Each of these points is a solution of the initial optimization problem. Finally we shall give a simple example using the obtained results. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
160.
A. Bourgeat O. Gipouloux E. Marui‐Paloka 《Mathematical Methods in the Applied Sciences》2004,27(4):381-403
In this paper, we study the global behaviour of an underground waste disposal in order to have an accurate upscaled model suitable for the computations involved in safety assessment processes. We start from a detailed model describing the transport of pollutant leaking from a high number of units. Using the method of homogenization, going to the limit, we obtain first a macroscopic model where the sources are now appearing globally. Then we compute a first‐order matched asymptotic expansion and we give the error estimates for this approximation. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献