全文获取类型
收费全文 | 393532篇 |
免费 | 4275篇 |
国内免费 | 1462篇 |
专业分类
化学 | 185263篇 |
晶体学 | 5307篇 |
力学 | 14311篇 |
综合类 | 10篇 |
数学 | 34683篇 |
物理学 | 101674篇 |
无线电 | 58021篇 |
出版年
2018年 | 2503篇 |
2016年 | 4747篇 |
2015年 | 3576篇 |
2014年 | 5023篇 |
2013年 | 15437篇 |
2012年 | 11157篇 |
2011年 | 13972篇 |
2010年 | 9440篇 |
2009年 | 10195篇 |
2008年 | 14066篇 |
2007年 | 14901篇 |
2006年 | 14318篇 |
2005年 | 13361篇 |
2004年 | 12184篇 |
2003年 | 10920篇 |
2002年 | 10606篇 |
2001年 | 12828篇 |
2000年 | 10192篇 |
1999年 | 8301篇 |
1998年 | 6829篇 |
1997年 | 6804篇 |
1996年 | 6630篇 |
1995年 | 6099篇 |
1994年 | 5926篇 |
1993年 | 5736篇 |
1992年 | 6400篇 |
1991年 | 6245篇 |
1990年 | 5773篇 |
1989年 | 5749篇 |
1988年 | 5498篇 |
1987年 | 4824篇 |
1986年 | 4576篇 |
1985年 | 6223篇 |
1984年 | 6202篇 |
1983年 | 5165篇 |
1982年 | 5440篇 |
1981年 | 5258篇 |
1980年 | 5092篇 |
1979年 | 5097篇 |
1978年 | 5379篇 |
1977年 | 5136篇 |
1976年 | 5066篇 |
1975年 | 4867篇 |
1974年 | 4674篇 |
1973年 | 4851篇 |
1972年 | 3068篇 |
1971年 | 2441篇 |
1968年 | 2716篇 |
1967年 | 2872篇 |
1966年 | 2544篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
61.
A homologous series of azomesogens, 2″-[4-(4′-n-alkoxybenzoyloxy)-2-chlorophenylazo] naphthalenes, with lateral chloro groups was synthesised. All the homologues synthesized
exhibit enantiotropic nematic mesophase. The mesomorphic properties of the present series are compared with other structurally
related series to evaluate the effect of lateral chloro group and its position on mesomorphism
This paper was presented at the 10th National Conference on Liquid Crystals held at Bangalore, India during 9–11 October 2003. 相似文献
62.
We extend the matrix version of Cochran's statistical theorem to outer inverses of a matrix. As applications, we investigate the Wishartness and independence of matrix quadratic forms for Kronecker product covariance structures. 相似文献
63.
64.
X Y Liu C J Li 《Science in China. Series B, Chemistry, life sciences & earth sciences》1989,32(10):1215-1223
To further evaluate the causality between endogenous pyrogen (EP)-induced fever and cyclic adenosine-3',5'-monophosphate (cyclic AMP) level, the effects of sodium salicylate (SS) on the febrile response and increased levels of cyclic AMP in both cerebrospinal fluid (c.s.f.) and plasma during EP-induced fever in rabbits were observed. The results suggest that cyclic AMP is probably involved in the central mediation of EP-induced fever and that increased concentration of cyclic AMP in c.s.f. associated with EP-induced fever is not the result of temperature elevation but appears to be caused by the increased synthesis in the central nervous system. In addition it is confirmed that blood is impossibly a contributory source of increased cyclic AMP in c.s.f. during EP fever, and that SS may act subsequent to the increase in cyclic AMP. 相似文献
65.
Summary Using a saturated non-boiling hydrocarbon mixture, the influence of two parameters on the results of field ionization mass spectrometry (FI-MS) measurements was studied: (a) the potential difference between the FI emitter and the counterelectrode; (b) the emitter temperature.Variation of the potential difference had only a minor effect on the average molecular mass measured and had no evident effect on the relative ring number distribution in the sample. In contrast, when the emitter temperature was increased, higher average molecular masses were recorded. Moreover, the average molecular masses shifted to higher ring numbers. In order to control the relationship between the described influences during mixture analysis, measuring instructions have been developed that enable the quantitative analysis of unknown saturated samples. However, average molecular mass of the mixture must be known.
Entwicklung einer quantitativen FI-MS-Methode zur Charakterisierung von gesättigten hoch- und nichtsiedenden Kohlenwasserstoffgemischen相似文献
66.
67.
Sarto M.S. Voti R.L. Sarto F. Larciprete M.C. 《Electromagnetic Compatibility, IEEE Transactions on》2005,47(3):602-611
New nanolayered coatings are designed and deposited on flexible plastic substrate having the thickness of 100 /spl mu/m, in order to realize lightweight ultrathin transparent shielding foils. The structure of the coating is optimized considering three figures of merit: the average transmittance in the visible range for normal incidence, the normalized average transmittance for oblique incidence at 550 nm, and the transmittance quality factor. The nanotechnology exploited for the deposition of the transparent metals is the dual ion beam sputtering. Tests of durability, optical transmission, and shielding effectiveness demonstrate that the film has a high adhesion under mechanical solicitation, high resistance against aging, peak transmittance in the visible range higher than 70%, omnidirectional properties in the range 0/spl deg/-60/spl deg/, and shielding effectiveness of 40 dB up to 6 GHz. 相似文献
68.
H E Cline W E Lorensen R J Herfkens G A Johnson G H Glover 《Magnetic resonance imaging》1989,7(1):45-54
A three-dimensional examination of blood vessels is provided using MR data from seven cases. The vascular surfaces are constructed with an algorithm that automatically follows the selected artery or vein and generates a projected three-dimensional gradient shaded image. Fast 3DFT pulse sequences were optimized to enhance the time-of-flight contrast of the intravascular region. By increasing the surface threshold value in a three-dimensional head study, the flesh of a patient's face was peeled away to demonstrate the superfacial temporal artery. Gated cardiac images show the great vessels and cardiac chambers. A three-dimensional view of the aorta shows an irregular surface in the vicinity of an adrenal tumor. 3D MR exams provide a non-invasive technique for assessing vascular morphology in a clinical setting. 相似文献
69.
K. C. Kumara Swamy E. Balaraman M. Phani Pavan N. N. Bhuvan Kumar K. Praveen Kumar N. Satish Kumar 《Journal of Chemical Sciences》2006,118(6):495-501
The diversity of products in the reaction of diethyl azodicarboxylate (DEAD)/diisopropyl azodicarboxylate (DIAD) and activated
acetylenes with PIII compounds bearing oxygen or nitrogen substituents is discussed. New findings that are useful in understanding the nature
of intermediates involved in the Mitsunobu reaction are highlighted. X-ray structures of two new compounds (2-t-Bu-4-MeC6H3O)P (μ-N-t-Bu)2P+[(NH-t-Bu)N[(CO2]-i-Pr)(HNCO2-i-Pr)]](Cl-)(2-t-Bu-4-MeC6H3OH)(23)and [CH2(6-t-Bu-4-Me-C6H2O)2P(O)C(CO2Me)C-(CO2Me)CClNC(O)Cl] (33) are also reported. The structure of23 is close to one of the intermediates proposed in the Mitsunobu reaction. 相似文献
70.
Seo K. Heiblum M. Knoedler C.M. Oh J.E. Pamulapati J. Bhattacharya P. 《Electron Device Letters, IEEE》1989,10(2):73-75
A high-gain ballistic hot-electron device is described. The GaAs-AlGaAs heterostructure device, with a 21-mm-thick pseudomorphic In 0.12Ga0.88As base, had a current gain of 27 at 77 K and 41 at 4.2 K. As characteristically seen in ballistic devices, transfer into the L valley limited the maximum gain. The Γ-L valley separation in the strained In0.12Ga0.88As was estimated to be about 380 meV 相似文献