全文获取类型
收费全文 | 250650篇 |
免费 | 18032篇 |
国内免费 | 9334篇 |
专业分类
电工技术 | 13382篇 |
技术理论 | 13篇 |
综合类 | 14211篇 |
化学工业 | 41737篇 |
金属工艺 | 14983篇 |
机械仪表 | 15228篇 |
建筑科学 | 17349篇 |
矿业工程 | 6399篇 |
能源动力 | 5871篇 |
轻工业 | 14355篇 |
水利工程 | 4654篇 |
石油天然气 | 14530篇 |
武器工业 | 1706篇 |
无线电 | 26140篇 |
一般工业技术 | 36450篇 |
冶金工业 | 15343篇 |
原子能技术 | 5833篇 |
自动化技术 | 29832篇 |
出版年
2024年 | 1016篇 |
2023年 | 3610篇 |
2022年 | 6793篇 |
2021年 | 9233篇 |
2020年 | 7054篇 |
2019年 | 5954篇 |
2018年 | 7393篇 |
2017年 | 8216篇 |
2016年 | 7370篇 |
2015年 | 8679篇 |
2014年 | 11099篇 |
2013年 | 13617篇 |
2012年 | 14716篇 |
2011年 | 15810篇 |
2010年 | 13441篇 |
2009年 | 13198篇 |
2008年 | 12934篇 |
2007年 | 12256篇 |
2006年 | 11972篇 |
2005年 | 10407篇 |
2004年 | 7545篇 |
2003年 | 7071篇 |
2002年 | 6917篇 |
2001年 | 6242篇 |
2000年 | 5937篇 |
1999年 | 5849篇 |
1998年 | 4726篇 |
1997年 | 3898篇 |
1996年 | 3719篇 |
1995年 | 3152篇 |
1994年 | 2633篇 |
1993年 | 2072篇 |
1992年 | 1887篇 |
1991年 | 1603篇 |
1990年 | 1473篇 |
1989年 | 1334篇 |
1988年 | 1159篇 |
1987年 | 1019篇 |
1986年 | 883篇 |
1985年 | 800篇 |
1984年 | 757篇 |
1982年 | 704篇 |
1979年 | 742篇 |
1978年 | 779篇 |
1977年 | 739篇 |
1976年 | 755篇 |
1975年 | 713篇 |
1974年 | 719篇 |
1973年 | 723篇 |
1972年 | 705篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
71.
This study addresses the influence the 7-substituent on the cytotoxicity of pyrrolo[1,2-alpha]-benzimidazole quinones possessing a 6-aziridinyl group (PBIs) and a 6-acetamido group (APBIs). Reduction of a PBI to the aziridinyl hydroquinone results in both nucleophile trapping (alkylation) and 1,5-sigmatropic shift reactions. The latter process is essentially an internal redox reaction wherein the hydroquinone causes reductive opening of the aziridinyl ring. The 7-substituent controls the fate of the aziridinyl ring by means of steric and electronic effects. An electron-rich 7-substituent favors the 1,5-sigmatropic shift reaction. If the 7-substituent distorts the 6-aziridinyl group from the conformation required for the 1,5-sigmatropic shift, then nucleophile trapping occurs. The 7-methyl substituent results in significant nucleophilic trapping, and the 7-unsubstituted and 7-methoxy substituents favor the 1,5-sigmatropic reaction. Thus, the 7-methyl PBIs show the most cytotoxicity of the analogues studied. The APBIs are cytotoxic only as quinones, and reduction to the hydroquinone results in loss of activity. Consistent with this observation, the change from 7-methyl to the more electron-rich 7-methoxy results in a substantial loss of APBI cytotoxicity as well as decreased topoisomerase II inhibition. The mechanism of inhibition is thought to involve the interacalation of only electron deficient APBIs into DNA. 相似文献
72.
The magnetization of a melt-texture growth (MTG) HTSC ring has been studied. It is shown that the magnetic field inside the ring is larger than the external field under a certain range of external magnetic fields. We have also investigated the magnetic field dependence of the response of a detective coil near a rotating superconducting ring. The responses of the MTG sample are different for different cooling methods. 相似文献
73.
Comparative analysis of the characteristics of amplitude-dependent internal friction (IF) was carried out to explain reasons leading to an increase of the creep limit of the butt welded joint in PT-3V titanium alloy (PT-2V filler material) after cyclic loading. IF was measured by the method of the direct torsional pendulum on cylindrical specimens representing the gage part of the specimens subjected to preliminary cyclic loading. The data were processed using the Granato-Lucke theory. The results show that the reduction of IF after cyclic loading took place as a result of an increase of the number and redistribution of pinning points of the dislocations. Consequently, the density of mobile dislocations decreases by 8% and the length of dislocation segments by 18%. These processes of stabilization of the structure were completed within 5·10
4
cycles of preliminary loading. Stabilization of the dislocation structure of the material of the welded joint increases its creep limit.Translated from Problemy Prochnosti, No. 4, pp. 35–40, April, 1996. 相似文献
74.
75.
In this paper, symbolic code matrix ,constant matrix and count matrix are defined .The first twomatrices are used to describe the elemental expression of augmented matrix and the nede admittance equa-tion is thus obtained. The third matrix is used to obtain the incoming degree matrix, and according to thematrix all the 1- factors of the Coates graph are given. By using the data code, the determinant is expandedand the same items in the expansion are merged. Thus the symbolic network function in which no term can-cellation occurs is generated. 相似文献
76.
Yu. I. Berdyshev 《Journal of Computer and Systems Sciences International》2006,45(4):526-531
An algorithm for construction of the reachability domain by the given time instant ? for a nonlinear third order system under the condition of preliminary approach to a predefined point is proposed. All possible cases of location of this point in the motion plane of the system for different values of the time instant ? are studied. 相似文献
77.
78.
本文介绍了秸杆毡的成型工艺 ,通过植物所需的水、肥料和空气三种因素对秸杆毡的化学和物理性能进行了研究 ,该材料可被用作草皮栽培基质 相似文献
79.
80.
根据托海拱坝5号坝段、7号坝段例垂线实测资料分析,说明了新疆寒冷地区坝体采取聚胺脂喷涂施工措施后对坝体变形的影响,以及右坝肩渗漏对右坝肩岩体的影响,分析结果为托海水电站通过国家电力公司大坝安全监察中心大坝安全首次定检提供了科学依据。 相似文献