全文获取类型
收费全文 | 24835篇 |
免费 | 3810篇 |
国内免费 | 2792篇 |
专业分类
化学 | 13044篇 |
晶体学 | 178篇 |
力学 | 1330篇 |
综合类 | 179篇 |
数学 | 2309篇 |
物理学 | 7311篇 |
无线电 | 7086篇 |
出版年
2024年 | 106篇 |
2023年 | 651篇 |
2022年 | 686篇 |
2021年 | 1013篇 |
2020年 | 925篇 |
2019年 | 891篇 |
2018年 | 720篇 |
2017年 | 728篇 |
2016年 | 1111篇 |
2015年 | 1069篇 |
2014年 | 1259篇 |
2013年 | 1702篇 |
2012年 | 1995篇 |
2011年 | 1992篇 |
2010年 | 1478篇 |
2009年 | 1439篇 |
2008年 | 1515篇 |
2007年 | 1304篇 |
2006年 | 1349篇 |
2005年 | 1133篇 |
2004年 | 940篇 |
2003年 | 872篇 |
2002年 | 889篇 |
2001年 | 772篇 |
2000年 | 637篇 |
1999年 | 617篇 |
1998年 | 515篇 |
1997年 | 475篇 |
1996年 | 431篇 |
1995年 | 416篇 |
1994年 | 383篇 |
1993年 | 310篇 |
1992年 | 223篇 |
1991年 | 199篇 |
1990年 | 159篇 |
1989年 | 115篇 |
1988年 | 98篇 |
1987年 | 60篇 |
1986年 | 66篇 |
1985年 | 47篇 |
1984年 | 28篇 |
1983年 | 38篇 |
1982年 | 22篇 |
1981年 | 17篇 |
1980年 | 14篇 |
1979年 | 4篇 |
1978年 | 4篇 |
1975年 | 3篇 |
1974年 | 3篇 |
1957年 | 6篇 |
排序方式: 共有10000条查询结果,搜索用时 8 毫秒
11.
A route to the total synthesis of koumine has been suggested. Five 4-substituted 1, 2-d hydropyridine derivatives, which are supposed to be a conjugated diene of Diels-Alder reaction to produce the isoquinuclidine ring of koumine, have been synthesized. Only two of them were found to be able to undergo the Diels-Alder reaction. The stereochemistry of the reaction product has also been examined. 相似文献
12.
This paper proposes a new method to describe and identify a 3-D curved object for the purpose of validating a fabricated object to the design specification. Curved 3-D objects are, in general, difficult to represent and identify because they lack distinct properties such as edges, planes, or cylindrical surfaces which are the building blocks in representing objects. In this paper, the authors propose to use principal axes of a 3-D object to establish a reference for the representation. A method of obtaining an inertia matrix from a 3-D range image is developed. The unique set of principal axes is obtained from the inertia matrix of an object with an arbitrary 3-D position and orientation, and the object can be described uniquely on these principal axes. On the principal axes, an object is described by a set of features describing the shape of the object such as spine, section size, section orientation, and section contraction. The features are used for comparing two objects for the validation purpose. The authors also propose a direct measure of similarity between two objects as a mean-squared difference of radii. As an experiment, two 3-D object models are designed through a CAD package, and fabricated objects are compared with the designed models for validation purposes 相似文献
13.
In the reversible addition–fragmentation transfer (RAFT) copolymerization of two monomers, even with the simple terminal model, there are two kinds of macroradical and two kinds of polymeric RAFT agent with different R groups. Because the structure of the R group could exert a significant influence on the RAFT process, RAFT copolymerization may behave differently from RAFT homopolymerization. The RAFT copolymerization of methyl methacrylate (MMA) and styrene (St) in miniemulsion was investigated. The performance of the RAFT copolymerization of MMA/St in miniemulsion was found to be dependent on the feed monomer compositions. When St is dominant in the feed monomer composition, RAFT copolymerization is well controlled in the whole range of monomer conversion. However, when MMA is dominant, RAFT copolymerization may be, in some cases, out of control in the late stage of copolymerization, and characterized by a fast increase in the polydispersity index (PDI). The RAFT process was found to have little influence on composition evolution during copolymerization. The synthesis of the well‐defined gradient copolymers and poly[St‐b‐(St‐co‐MMA)] block copolymer by RAFT miniemulsion copolymerization was also demonstrated. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 6248–6258, 2004 相似文献
14.
15.
Yanbin Luo Jianhui Zhang Alexandrov P. Fursin L. Zhao J.H. Burke T. 《Electron Device Letters, IEEE》2003,24(11):695-697
This letter reports the design and fabrication of 4H-SiC bipolar junction transistors with both high voltage (>1kV) and high dc current gain (/spl beta/=32) at a collector current level of I/sub c/=3.83A (J/sub c/=319 A/cm/sup 2/). An Al-free base ohmic contact has been used which, when compared with BJTs fabricated with Al-based base contact, shows clearly improved blocking voltage. A specific on-resistance of 17 m/spl Omega//spl middot/cm/sup 2/ has been achieved for collector current densities up to 289 A/cm/sup 2/. 相似文献
16.
Retractable structures which alter their geometries according to practical requirements are widely used in roof structures of stadiums for their versatility. A retractable structure based on threefold-symmetric Bricard linkages and rotating rings of tetrahedra is proposed and developed in this paper. By replacing each link of a threefold-symmetric Bricard linkage with a tetrahedron, a retractable structure is obtained which can repeatedly open and close by rotating the tetrahedra. The geometric relationship between the retractable structure and the threefold-symmetric Bricard linkage is derived and ranges of main geometric parameters are determined to ensure a continuous and smooth movement in the deployment of the structure. Then the relative displacement of the support and the open rate of the structure, two main parameters concerned in practical applications, are investigated. Preferred ranges of main geometric parameters are proposed and a physical model is manufactured to verify them. A discussion on some possible improvements and modifications of the structure is also presented. 相似文献
17.
18.
19.
Qing Zhong Liu 《Microwave Theory and Techniques》1992,40(2):329-337
Unified analytical expressions have been derived for calculating the resonant frequencies, transimpedance and equivalent input noise current densities of the four most widely used tuned optical receiver front ends built with FETs and p-i-n diodes. A more accurate FET model has been used to improve the accuracy of the analysis. The Miller capacitance has been taken into account, and its impact on the performances of the tuned front ends has been demonstrated. The accuracy of the expressions has been verified by Touchstone simulations. The agreement between the calculated and simulated performances of the front ends is very good. The expressions can be used to investigate the performances of different tuned front ends in a very simple way and provide a good starting point for further computer optimizations of the front ends 相似文献
20.