全文获取类型
收费全文 | 14371篇 |
免费 | 730篇 |
国内免费 | 173篇 |
专业分类
化学 | 9045篇 |
晶体学 | 105篇 |
力学 | 596篇 |
综合类 | 4篇 |
数学 | 1628篇 |
物理学 | 2100篇 |
无线电 | 1796篇 |
出版年
2024年 | 45篇 |
2023年 | 167篇 |
2022年 | 747篇 |
2021年 | 698篇 |
2020年 | 584篇 |
2019年 | 656篇 |
2018年 | 650篇 |
2017年 | 526篇 |
2016年 | 832篇 |
2015年 | 543篇 |
2014年 | 758篇 |
2013年 | 1416篇 |
2012年 | 1031篇 |
2011年 | 1013篇 |
2010年 | 696篇 |
2009年 | 591篇 |
2008年 | 641篇 |
2007年 | 583篇 |
2006年 | 428篇 |
2005年 | 426篇 |
2004年 | 287篇 |
2003年 | 270篇 |
2002年 | 224篇 |
2001年 | 127篇 |
2000年 | 85篇 |
1999年 | 101篇 |
1998年 | 60篇 |
1997年 | 71篇 |
1996年 | 80篇 |
1995年 | 48篇 |
1994年 | 57篇 |
1993年 | 40篇 |
1992年 | 58篇 |
1991年 | 53篇 |
1990年 | 48篇 |
1989年 | 47篇 |
1988年 | 43篇 |
1987年 | 48篇 |
1986年 | 49篇 |
1985年 | 51篇 |
1984年 | 44篇 |
1983年 | 36篇 |
1982年 | 38篇 |
1981年 | 20篇 |
1980年 | 35篇 |
1979年 | 31篇 |
1978年 | 30篇 |
1976年 | 20篇 |
1975年 | 18篇 |
1974年 | 18篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
971.
Two novel bidesmosidic cycloartane-type glycosides, depressosides C and D were isolated from Corchorus depressus L. Their structures were elucidated as (22R)-16beta,22-epoxy-3beta,26-dihydroxy-9,19-cyclolanost-++ +24E-ene 3, 26-di-O-beta-D-glucopyranoside and (22R,24S)-22,25-epoxy-3beta,16beta,24-trihydroxy-9,19-cyclolano stane 3, 24-di-O-beta-glucopyranoside, respectively on the basis of chemical evidence and detailed spectroscopic studies. 相似文献
972.
[reaction: see text] Pyridinium hydrobromide perbromide (Py x HBr3) catalyzes effectively the aziridination of electron-deficient as well as electron-rich olefins using Chloramine-T (N-chloro-N-sodio-p-toluenesulfonamide) as a nitrogen source to afford the corresponding aziridines in moderate to good yields. 相似文献
973.
Mohammed Moussaoui Alberto Seeger 《Transactions of the American Mathematical Society》1999,351(9):3687-3711
The purpose of this work is twofold: on the one hand, we study the second-order behaviour of a nonsmooth convex function defined over a reflexive Banach space . We establish several equivalent characterizations of the set , known as the second-order subdifferential of at relative to . On the other hand, we examine the case in which is the functional integral associated to a normal convex integrand . We extend a result of Chi Ngoc Do from the space to a possible nonreflexive Banach space . We also establish a formula for computing the second-order subdifferential .
974.
Orbital stability of limit cycles is the result of the competing local tendencies of perturbations from the cycle to decay (during phases of local stability) and to grow (during phases of local instability), averaged over a cycle. We examine this coexistence of attractive and repulsive phases on limit cycles, including the local rates of expansion and contraction of phase space volumes. This is done in a frame of reference that moves along the orbit, to partially decouple motions tangential and perpendicular to the cycle. Dynamical systems used for illustration are the generalized Bonhoeffer-van-der-Pol and Rossler models, both far from and near to different types of bifurcations. Finally, it is shown that the nonuniformity of local stability in phase space affects the response of limit cycle oscillators to perturbations and gives rise to their phase-dependent response. (c) 1999 American Institute of Physics. 相似文献
975.
976.
In order to gain an insight into the mechanism of reduction of Mn(III) heteropoly ions, and also to establish the conditions for use of some of these ions as oxidizing agent, following measurements have been made. The pseudo-first order rate constants, kobs, have been determined and specific rate constants, k, were calculated from the plots of kobs against SO3 2? concentrations. A plot of ln(k2/T) against inverse temperature gives enthalpy of activation as 10.67 kJ mol?1 and entropy of activation as ?237.90 J K?1 mol?1. Effects of ionic strength and pH have also been studied over a limited range. 相似文献
977.
A. Mushtaq M. S. Mansur Ali Muhammad 《Journal of Radioanalytical and Nuclear Chemistry》1996,212(2):107-115
Hydrated antimony pentoxide (HAP) as an absorbent for column operation has been prepared by hydrolysis of SbCl5 with deionized water. Sorption behavior of Sn, Cd, Sb and In was studied on HAP in HCl medium. Radiochemical separation of no-carrier added113mIn from113Sn/125Sb and115mIn from115Cd wa achieved over a HAPO column. The separated products were of high radionuclidic purity. 相似文献
978.
Applied Biochemistry and Biotechnology - The effects of steam explosion pretreatment on the physical and chemical properties of rice straw and on the enzymatic saccharification of the straw were... 相似文献
979.
Basudeb Saha Sumana Gangopadhyay Mahammad Ali Pradyot Banerjee 《Journal of Chemical Sciences》1995,107(4):393-402
The kinetics of the reduction of [NiIII(L1)]2+ (where HL1 = 15-amino-3-methyl-4,7,10,13-tetraazapentadec-3-en-2-one oxime) by sulphur(IV) and selenium (IV) over the regions pH 2.50–8.02
and 2.01–4.00 respectively have been investigated at 30°C. Attempts were made to evaluate the reactivity of all the reacting
species, of sulphur(IV) and selenium(IV) by considering suitable pH ranges. The oxidation of SC2·H2O and HSO
3
−
is proposed to proceed through the formation of a hydrogen-bonded adduct. The reaction with SO
3
2−
seems to follow a direct outer-sphere route which is well supported by Marcus crossrelation calculation. The oxidation of
HSeO
l3
−
is ≈ 103 times slower than that of H2SeO3. The kinetic data indicate that the oxidation of sulphur(IV) by [NiIIIL1)]2+ is much more favourable as compared to the corresponding oxidation of selenium(IV). 相似文献
980.
Ali A. Moshfegh Effat Beladi Lida Radnia Afsaneh S. Hosseini Soosan Tofigh Gholam H. Hakimelahi 《Helvetica chimica acta》1982,65(4):1264-1270
The synthesis of the title compounds ( 1 and 3 ) is described. Some of the compounds prepared were found to be active against a number of pathogenic microorganisms in vitro. Structure-activity relationship is briefly discussed. 相似文献