全文获取类型
收费全文 | 48063篇 |
免费 | 4160篇 |
国内免费 | 1496篇 |
学科分类
工业技术 | 53719篇 |
出版年
2024年 | 117篇 |
2023年 | 773篇 |
2022年 | 931篇 |
2021年 | 1774篇 |
2020年 | 1301篇 |
2019年 | 1296篇 |
2018年 | 1596篇 |
2017年 | 1534篇 |
2016年 | 1685篇 |
2015年 | 1694篇 |
2014年 | 2283篇 |
2013年 | 2942篇 |
2012年 | 3122篇 |
2011年 | 3664篇 |
2010年 | 2900篇 |
2009年 | 2850篇 |
2008年 | 2692篇 |
2007年 | 2404篇 |
2006年 | 2220篇 |
2005年 | 1922篇 |
2004年 | 1554篇 |
2003年 | 1576篇 |
2002年 | 1752篇 |
2001年 | 1558篇 |
2000年 | 1139篇 |
1999年 | 1055篇 |
1998年 | 1113篇 |
1997年 | 800篇 |
1996年 | 757篇 |
1995年 | 558篇 |
1994年 | 363篇 |
1993年 | 308篇 |
1992年 | 256篇 |
1991年 | 197篇 |
1990年 | 145篇 |
1989年 | 141篇 |
1988年 | 126篇 |
1987年 | 101篇 |
1986年 | 86篇 |
1985年 | 58篇 |
1984年 | 48篇 |
1983年 | 37篇 |
1982年 | 44篇 |
1981年 | 35篇 |
1980年 | 38篇 |
1979年 | 22篇 |
1977年 | 24篇 |
1976年 | 36篇 |
1975年 | 21篇 |
1974年 | 19篇 |
排序方式: 共有10000条查询结果,搜索用时 798 毫秒
71.
72.
Soo Jin Kim Wha Young Lee Yeungho Park Wansoo Huh Young Gwan Ko 《Polymer Engineering and Science》2003,43(5):1011-1017
Supported zirconcene catalysts on a new support, MgO, were prepared and tested in ethylene polymerization. Three types of impregnation methods were employed to find an optimum supporting method for MgO. The direct impregnation of Cp2ZrCl2 on MgO showed low metal loading and polymerization activity, while the catalyst had a higher metal loading and polymerization activity when MgO was treated with methylaluminoxane (MAO) before supporting. Treatment of MgO with MAO during the supporting step invoked two types of catalytic sites, which was evidenced by the bimodal molecular weight distribution of the polymer products. MgO is considered to have potential as a support for metallocenes. 相似文献
73.
Presents an obituary and information about the life and accomplishments of Josef Maria Bro?ek. Born August 14, 1913, in the ancient town of Melnik, in central Bohemia, today the Czech Republic, Josef spent his childhood in Poland (Warsaw, 1913- 1915) and in Siberia (1915-1920). His education in Czechoslovakia culminated with a thesis on "Memory, Its Measurement and Structure" and a PhD awarded in June 1937. In the history of psychology, his lifetime project bore the title "Historiography of Psychology Around the World," and it covered about 20 geographical areas. Extensive attention was devoted to institutional and organizational developments as journals, academic settings, archives, museums, research groups, conferences, and institutes. (PsycINFO Database Record (c) 2010 APA, all rights reserved) 相似文献
74.
75.
Dual command travel times and miniload system throughput with turnover-based storage 总被引:2,自引:0,他引:2
Byung Chun Park Robert D. Foley John A. White Edward H. Frazelle 《IIE Transactions》2003,35(4):343-355
We analyze travel times for automated storage/retrieval systems. In particular, we apply our travel time model to turnover-based storage systems and determine the mean and variance of dual command travel times. We present detailed numerical results for selected rack shape factors and ABC inventory profiles. We then investigate the effect of alternative rack configurations on travel time performance measures. We also show how to determine the throughput of miniload systems with turnover-based storage and exponentially distributed pick times. 相似文献
76.
This letter presents our investigation for the effect of symbol timing errors in orthogonal frequency-division multiple access (OFDMA) uplink systems. We express the symbol timing errors between users as the symbol timing misalignments with respect to the desired user. Then, we derive an explicit expression of the signal-to-noise ratio (SNR) as a function of the maximum value of the symbol timing misalignments. Analyses and simulation results show that, to achieve an SNR of 20 dB, the maximum value of the symbol timing misalignments must be less than the cyclic prefix duration plus 6.25% of the useful symbol duration. Based on the resulting SNR degradation, we evaluate the SNR gain with guard subcarriers in order to mitigate the effect of the symbol timing misalignments. 相似文献
77.
H.S. Seo Y.G. Choi B.J. Park D.H. Cho K.H. Kim 《Photonics Technology Letters, IEEE》2003,15(9):1198-1200
A flat signal gain over in the entire C- and L-bands by erbium (Er) ions' radiative transition and stimulated Raman scattering in an Er-doped germano-silica fiber can be obtained if proper values of the concentration of Er and background loss in a fiber core are obtained during the fiber fabrication process. The optimized conditions for the flat C- and L-band gain are analyzed as functions of Er concentrations. Even for a low-gain value provided by a germano-silica core fiber with a low Er concentration and an optimum fiber length, a relatively low pump is required to obtain the flat gain band. 相似文献
78.
79.
金属/Al2O3基纳米复合材料研究最新进展 总被引:2,自引:0,他引:2
金属/Al2O3纳米复合材料在保持原有金属的功能特性时,还可以获得 很好的力学性能,是有良好发展前景的一种纳米复合材料。本文回顾了近年来金属/Al2O3基纳米复合材料在制备工艺,微观结构和力学性能,增韧强化机理方面的最新进展,并指出了今后的研究方向。 相似文献
80.
Chel-Jong Choi Tae-Yeon Seong Key-Min Lee Joo-Hyoung Lee Young-Jin Park Hi-Deok Lee 《Electron Device Letters, IEEE》2002,23(4):188-190
The leakage mechanism in p+/n shallow junctions fabricated using Co silicidation and shallow trench isolation processes has been investigated using transmission electron microscopy (TEM) combined with selective chemical etching. TEM and TSUPREM-4 simulation results show that dopant profiles bend upward near the edge of the active region. The formation of the abnormal profile is attributed to transient enhanced diffusion induced by source/drain implantation. Based on the TEM and simulation results, it is suggested that the shallower junctions formed near the active edge can serve as a source for leakage current in the silicided p+ /n shallow junctions 相似文献