全文获取类型
收费全文 | 173808篇 |
免费 | 1654篇 |
国内免费 | 451篇 |
专业分类
化学 | 80112篇 |
晶体学 | 2387篇 |
力学 | 6068篇 |
综合类 | 3篇 |
数学 | 13569篇 |
物理学 | 46489篇 |
无线电 | 27285篇 |
出版年
2021年 | 1140篇 |
2020年 | 1290篇 |
2019年 | 1372篇 |
2018年 | 1776篇 |
2017年 | 1804篇 |
2016年 | 2750篇 |
2015年 | 1641篇 |
2014年 | 2561篇 |
2013年 | 6424篇 |
2012年 | 5088篇 |
2011年 | 6259篇 |
2010年 | 4584篇 |
2009年 | 4926篇 |
2008年 | 6620篇 |
2007年 | 6684篇 |
2006年 | 6498篇 |
2005年 | 6132篇 |
2004年 | 5612篇 |
2003年 | 5229篇 |
2002年 | 5100篇 |
2001年 | 6623篇 |
2000年 | 5079篇 |
1999年 | 4047篇 |
1998年 | 3066篇 |
1997年 | 3060篇 |
1996年 | 2962篇 |
1995年 | 2816篇 |
1994年 | 2807篇 |
1993年 | 2594篇 |
1992年 | 3142篇 |
1991年 | 3184篇 |
1990年 | 2948篇 |
1989年 | 2851篇 |
1988年 | 2700篇 |
1987年 | 2411篇 |
1986年 | 2206篇 |
1985年 | 2712篇 |
1984年 | 2689篇 |
1983年 | 2171篇 |
1982年 | 2155篇 |
1981年 | 1976篇 |
1980年 | 1955篇 |
1979年 | 2197篇 |
1978年 | 2271篇 |
1977年 | 2184篇 |
1976年 | 2116篇 |
1975年 | 1973篇 |
1974年 | 1965篇 |
1973年 | 1995篇 |
1972年 | 1335篇 |
排序方式: 共有10000条查询结果,搜索用时 312 毫秒
171.
172.
Y Sonoda K Ichinose T Yoshimura Y Sato T Sasaki 《Chemical & pharmaceutical bulletin》1991,39(1):100-103
Lanosterol derivatives with a functional group at C-32 have been synthesized from 3 beta-acetoxylanostan-7 alpha-ol. The key reaction of the synthesis is the hypoiodite reaction of 3 beta-acetoxylanostan-7 alpha-ol. In vitro antitumor activity testing of the lanosterol derivatives revealed that 3 beta-hydroxylanost-7-en-32-oic acid has antineoplastic activity. 相似文献
173.
A comparison of steady-state evoked potentials to modulated tones in awake and sleeping humans. 总被引:12,自引:0,他引:12
L T Cohen F W Rickards G M Clark 《The Journal of the Acoustical Society of America》1991,90(5):2467-2479
Steady-state evoked potential responses were measured to binaural amplitude-modulated (AM) and combined amplitude- and frequency-modulated (AM/FM) tones. For awake subjects, AM/FM tones produced larger amplitude responses than did AM tones. Awake and sleeping responses to 30-dB HL AM/FM tones were compared. Response amplitudes were lower during sleep and the extent to which they differed from awake amplitudes was dependent on both carrier and modulation frequencies. Background EEG noise at the stimulus modulation frequency was also reduced during sleep and varied with modulation frequency. A detection efficiency function was used to indicate the modulation frequencies likely to be most suitable for electrical estimation of behavioral threshold. In awake subjects, for all carrier frequencies tested, detection efficiency was highest at a modulation frequency of 45 Hz. In sleeping subjects, the modulation frequency regions of highest efficiency varied with carrier frequency. For carrier frequencies of 250 Hz, 500 Hz, and 1 kHz, the highest efficiencies were found in two modulation frequency regions centered on 45 and 90 Hz. For 2 and 4 kHz, the highest efficiencies were at modulation frequencies above 70 Hz. Sleep stage affected both response amplitude and background EEG noise in a manner that depended on modulation frequency. The results of this study suggest that, for sleeping subjects, modulation frequencies above 70 Hz may be best when using steady-state potentials for hearing threshold estimation. 相似文献
174.
Partially supported by NSF Grants. 相似文献
175.
176.
177.
A narrow linewidth has been obtained in a new 1.5 ?m distributed feedback laser integrated monolithically with a tunable external cavity. The linewidth of 18 MHz has been achieved by tuning the current in the external cavity, and the FM response was flat from 100 kHz to 500 MHz. 相似文献
178.
A novel typed of hybrid-mode feed horn is presented. It comprises a hollow conical dielectric waveguide whose outer surface is metallized, and whose inner surface is coated with circumferentially oriented conducting strips. It may be designed to have mimimal cross polarization at two arbitrarily separated frequencies, as for a dual depth corrugated horn. Compared to the corrugated horn, it has the potential of lower weight and lower price. Small horns will have some cross polarization due to undesired radiation from the fields transmitted within the dielectric. 相似文献
179.
Let V be a hypersurface with an isolated singularity at the origin in ? n+1. It is a natural question to ask when V is defined by weighted homogeneous polynomial or homogeneous polynomial up to biholomorphic change of coordinates. In 1971, a beautiful theorem of Saito gives a necessary and sufficient condition for V to be defined by a weighted homogeneous polynomial. For a two-dimensional isolated hypersurface signularity V, Xu and Yau found a coordinate free characterization for V to be defined by a homogeneous polynomial. Recently Lin and Yau gave necessary and sufficient conditions for a 3-dimensional isolated hypersurface singularity with geometric genus bigger than zero to be defined by a homogeneous polynomial. The purpose of this paper is to prove that Lin-Yau’s theorem remains true for singularities with geometric genus equal to zero. 相似文献
180.
Summary The main purpose of this paper is to introduce the concepts of η-sets, ηζ-sets,η-continuity and ηζ-continuity and to obtain
a decomposition of continuity. 相似文献