全文获取类型
收费全文 | 73159篇 |
免费 | 12811篇 |
国内免费 | 3995篇 |
专业分类
化学 | 63908篇 |
晶体学 | 690篇 |
力学 | 3179篇 |
综合类 | 248篇 |
数学 | 6741篇 |
物理学 | 15199篇 |
出版年
2023年 | 438篇 |
2022年 | 607篇 |
2021年 | 1041篇 |
2020年 | 2187篇 |
2019年 | 3522篇 |
2018年 | 2000篇 |
2017年 | 1664篇 |
2016年 | 4644篇 |
2015年 | 4566篇 |
2014年 | 4771篇 |
2013年 | 5948篇 |
2012年 | 5101篇 |
2011年 | 4478篇 |
2010年 | 4508篇 |
2009年 | 4422篇 |
2008年 | 4129篇 |
2007年 | 3483篇 |
2006年 | 2936篇 |
2005年 | 2806篇 |
2004年 | 2448篇 |
2003年 | 2312篇 |
2002年 | 3091篇 |
2001年 | 2290篇 |
2000年 | 2023篇 |
1999年 | 1129篇 |
1998年 | 684篇 |
1997年 | 673篇 |
1996年 | 685篇 |
1995年 | 574篇 |
1994年 | 572篇 |
1993年 | 508篇 |
1992年 | 493篇 |
1991年 | 478篇 |
1990年 | 431篇 |
1989年 | 388篇 |
1988年 | 323篇 |
1987年 | 333篇 |
1986年 | 321篇 |
1985年 | 355篇 |
1984年 | 337篇 |
1979年 | 316篇 |
1978年 | 339篇 |
1977年 | 328篇 |
1976年 | 408篇 |
1975年 | 364篇 |
1974年 | 394篇 |
1973年 | 401篇 |
1972年 | 372篇 |
1971年 | 331篇 |
1970年 | 315篇 |
排序方式: 共有10000条查询结果,搜索用时 62 毫秒
81.
V. N. Plugar' Ya. V. Rashkes A. Ovezdurdyev I. D. Sham'yanov 《Chemistry of Natural Compounds》1994,29(4):462-466
The main directions of fragmentation and the EI of six pseudoguaianolides related to carpesiolin have been studied. It has been established that acylation of the OH group at C-6 suppresses the selective breakdown of the guaiane skeleton, while the simultaneous presence of AcO groups at C-2 and C-4 favors the formation of a number of characteristic ions. The hydrogenation of the exomethylene group has no appreciable influence on direction of fragmentation.Institute of the Chemistry of Plant Substances, Academy of Sciences of the Republic of Uzbekistan, Tashkent. Translated from Khimiya Prirodnykh Soedinenii, No. 4, pp. 530–535, July–August, 1993. 相似文献
82.
M. S. Tashmukhamedov O. K Zakirova Sh. Ya. Mirzaakhmedov Sh. I. Salikhov I. T. Yakubov M. M. Rakhimov 《Chemistry of Natural Compounds》1994,30(1):97-100
Phospholipase A2 has been isolated fromthe venom of he Central Asian green toad Bufo viridis in the electroophoretically homogeneous state for the first time by biospecific absorption chromatography. The affinity sorbent used was Affi-Gel 10 with covalently bound phosphatidylserine (PDS). The chemical inertness of the bound PDS to the hydrolytic action of phospholipase A2 and its high tendency to form biospecific complexes have been shown. Some biochemical characteristics of the enzyme isolated have been studied: influence of the pH and of the Ca2+ concentration and dependence of activity on substrate concentration.A. S. Sadykov Institute of Bioorganic Chemistry, Academy of Sciences of the Republic of Uzbekistan, Tashkent, fax 627071. Tashkent State University, fax 462472. Translated from Khimiya Prirodnykh Soedinenii, No. 1, pp. 105–109, January–February, 1994. 相似文献
83.
84.
85.
固体力学有限元体系的结构拓扑变化理论 总被引:2,自引:1,他引:1
本文是文[1]的继续.文[1]提出了杆件系统的结构拓扑变化理论和拓扑变化法本文将这一理论和方法推进到连续体有限元体系;且在此基础上揭示出有限元体系的一个新性质,称为基本位移之梯度的正交性定理,从而给出一套设计敏度的显式表达式,可直接用于计算. 相似文献
86.
Ma Jiangle Yang Hong-shen Zhang Zhongxi Lu Zhong-zuo 《International Journal of Infrared and Millimeter Waves》1993,14(5):1069-1076
A simple method is introduced to improve the analysing accuracy of circular groove guide. With this method, the characteristic equation of circular groove guide is derived and its solution is given and discussed.The Project Supported by National Natural Science Foundation of P.R.China 相似文献
87.
The stress–strain state of rectangular plates with a variable thickness and constant weight is analyzed. The laws of variation in the thickness are specified to depend on parameters so that the plate weight remains constant for any of their values. To solve the problem, discrete Fourier series are used, which makes it possible to reduce a two-dimensional problem to one-dimensional. Calculation results are presented. 相似文献
88.
采用基于第一性原理的密度泛函理论研究了四角晶相二氧化铪(t-HfO2)体相及 其(001)表面的原子几何与电子结构.理论计算结果表明,t-HfO2(001)表面不会 产生重构现象.与体相电子结构相比, t-HfO2(001)表面态密度明显高于体相态 密度.其次,表面原子的态密度更靠近费米能级(EF),价带往低能量处移动,并 有表面态产生.计算结果表明了t-HfO2表面禁带宽度明显低于体相的禁带宽度. t-HfO2(001)的表面态产生以及表面禁带宽度减小是由于Hf原子与O原子的配位 数减少,表面原子周围的环境发生变化而引起的.
关键词:
密度泛函理论
2(001)')" href="#">t-HfO2(001)
表面电子结构 相似文献
89.
90.
Jian Shen Huizhong Zeng Zhihong Wang Shengbo Lu Huidong Huang Jingsong Liu 《Applied Surface Science》2006,252(22):8018-8021
Polycrystalline Pb(Zr0.55Ti0.45)O3 thin film was deposited on Pt/Ti/SiO2/Si(1 0 0) by radio-frequency-magnetron sputtering method, and the writing of charge bits on the surface of PZT thin film was studied by Kelvin probe force microscopy. It is found that the surface potential of the negative charge bits are higher than those of the corresponding positive ones. When ferroelectric polarization switching occurs, the potential difference becomes even more remarkable. A qualitative model was proposed to explain the origin of the asymmetric charge writing. It is demonstrated that the internal field in the interface layer, which is near the ferroelectric/electrode interface in ferroelectric film, is likely to be the cause for the occurrence of this phenomenon. 相似文献