全文获取类型
收费全文 | 2187篇 |
免费 | 37篇 |
国内免费 | 9篇 |
专业分类
化学 | 1432篇 |
晶体学 | 10篇 |
力学 | 37篇 |
数学 | 392篇 |
物理学 | 362篇 |
出版年
2022年 | 16篇 |
2021年 | 30篇 |
2020年 | 21篇 |
2019年 | 22篇 |
2018年 | 20篇 |
2017年 | 19篇 |
2016年 | 38篇 |
2015年 | 30篇 |
2014年 | 33篇 |
2013年 | 120篇 |
2012年 | 91篇 |
2011年 | 110篇 |
2010年 | 67篇 |
2009年 | 54篇 |
2008年 | 96篇 |
2007年 | 95篇 |
2006年 | 108篇 |
2005年 | 118篇 |
2004年 | 88篇 |
2003年 | 75篇 |
2002年 | 66篇 |
2001年 | 29篇 |
2000年 | 27篇 |
1999年 | 21篇 |
1998年 | 22篇 |
1997年 | 26篇 |
1996年 | 29篇 |
1995年 | 30篇 |
1994年 | 23篇 |
1993年 | 27篇 |
1992年 | 30篇 |
1991年 | 27篇 |
1990年 | 17篇 |
1989年 | 18篇 |
1988年 | 26篇 |
1986年 | 17篇 |
1985年 | 45篇 |
1984年 | 42篇 |
1983年 | 31篇 |
1982年 | 45篇 |
1981年 | 41篇 |
1980年 | 47篇 |
1979年 | 29篇 |
1978年 | 37篇 |
1977年 | 32篇 |
1976年 | 27篇 |
1975年 | 29篇 |
1974年 | 24篇 |
1973年 | 19篇 |
1972年 | 15篇 |
排序方式: 共有2233条查询结果,搜索用时 7 毫秒
21.
A comparative study has been made of the reactions of metal ions with dithizone in the two media, chloroform and molten naphthalene. The absorption spectra of several metal dithizonates, prepared in molten naphthalene, were recorded and were found to be the same as those of the dithizonates obtained by extraction of metal ions from aqueous solution with chloroform dithizone. 相似文献
22.
Abstract— We have devised a method of obtaining simultaneous action spectra for photoreactions I and II by analysis of direct and indirect effects involved in enhancement. The method requires previous determination of the neutral wavelength which gives maximum quantum yield by virtue of equal fractions of open reaction centers ( p and q ) for each photoreaction. A sufficient intensity of the neutral wavelength is used as a constant background. Upon addition of a weak modulated measuring light of intensity Im and wavelength λ m two amperometric signals are obtained for rate of oxygen evolution. A modulated signal (AC¯) isolates the direct effect of Im and gives action of photoreaction II as AC/ Im . An increment in total rate (ΔDC) also includes an indirect effect of Im in perturbing reaction center conditions ( p and q ). From analysis of interaction of the two photoreactions, action for photoreaction I can be estimated as (2 ΔDC-AC)/ Im . The method is applicable to whole cells, properly scales the two action spectra to each other, and removes contribution of the State 1-State 2 phenomena. Action spectra were obtained for Chlorella. 相似文献
23.
24.
Treatment of 2-phenyl-2,3-dihydro-1,4-benzodiazepin-5(4H)-one and a series of 1,4-benzoxazepinone analogues with tert-butyl hypochlorite affords products shown, by NMR and MS spectroscopy, to be chlorinated exclusively in the A-ring at positions 7 and/or 9. 相似文献
25.
Kristjan S. Gudmundsson Jack M. Hinkley Michael S. Brieger John C. Drach Leroy B. Townsend 《合成通讯》2013,43(5):861-870
An efficient large scale synthesis of 2-amino-4-chloropyridine (3) has been achieved through a modification of existing literature procedures. Compound 3 was used to prepare the previously unreported 2-amino-4,5-dichloropyridine (4). The known 2-amino-3,4-dichloropyridine (5) and 2-amino-3,4,5-trichloropyridine (6) were prepared from 3 by new routes and in higher yields than previously reported. 相似文献
26.
27.
28.
Jack Brimberg William J Hurley 《The Journal of the Operational Research Society》2015,66(6):1035-1043
Public sector managers, particularly those at the highest level of government, tend to view lapsed (or unused) funds at the end of a fiscal year as a consequence of poor management and/or inadequate financial controls. The aim of this paper is to challenge this view. We show that the planning environment in the public sector is in essence the classical Newsvendor Problem. This simple model argues that lapsed funds are a direct consequence of a manager doing his job properly; that is, lapsed funds occur from time to time when a manager is maximizing value for the organization. An extension of the model shows that allowing low-value year-end spending has an undesirable effect on the value of spending during the year and this suggests a role for a strong audit function for year-end spending. 相似文献
29.
Simon R. Drake Brian F. G. Johnson Jack Lewis 《Journal of organometallic chemistry》1988,340(3):C31-C36
The heteronuclear clusters [Os10C(CO)24(MPR3nm− (n = 1, m = 1; 2: M = Au; 3: M = Ag; 4: M = Cu; 5 n = 2, m = 0, M = Ag) have been prepared. These clusters undergo molecular rearrangements in solution, and two isomeric forms of 2, 3, 4, 5 and 6 have been identified. This interconversion is thought to involve a cap [lrarr2] edge bridge [lrarr2] cap pathway. 相似文献
30.
In this paper, we apply an iterative parameter adaption scheme based on chaos synchronization to estimate system parameters of the asymmetric vocal folds from glottal area time series. The original asymmetric vocal-fold system associated with recurrent laryngeal paralysis shows chaotic vibrations with positive Lyapunov exponents. Aperiodic glottal area time series from the original system will be applied as the feedback variable coupling the simulative and the original vocal-fold systems. The parameter adaption technique based on chaos synchronization is employed to manipulate the simulative system parameters. The chaotic vibrations, system parameters, and the bifurcation diagram of the original vocal-fold system can be exactly reproduced in the simulative system, and the two chaotic systems can be synchronized. Furthermore, the effects of noise, sampling rate, and equation difference due to nonlinear spring terms on vocal-fold parameter estimations are investigated. Despite large noise perturbations, large equation differences, and low sampling rate, the parameter adaption scheme can effectively estimate the original vocal-fold system parameters. This study provides a theoretical base to apply chaos synchronization to estimate the vocal-fold system parameters from the glottal area data and show its potential application in laryngeal physiology. 相似文献