全文获取类型
收费全文 | 2202篇 |
免费 | 45篇 |
国内免费 | 76篇 |
专业分类
化学 | 288篇 |
晶体学 | 10篇 |
力学 | 15篇 |
数学 | 1788篇 |
物理学 | 222篇 |
出版年
2023年 | 5篇 |
2022年 | 5篇 |
2021年 | 8篇 |
2020年 | 12篇 |
2019年 | 56篇 |
2018年 | 51篇 |
2017年 | 36篇 |
2016年 | 28篇 |
2015年 | 28篇 |
2014年 | 66篇 |
2013年 | 132篇 |
2012年 | 51篇 |
2011年 | 132篇 |
2010年 | 103篇 |
2009年 | 141篇 |
2008年 | 197篇 |
2007年 | 160篇 |
2006年 | 144篇 |
2005年 | 124篇 |
2004年 | 95篇 |
2003年 | 73篇 |
2002年 | 56篇 |
2001年 | 67篇 |
2000年 | 46篇 |
1999年 | 54篇 |
1998年 | 41篇 |
1997年 | 39篇 |
1996年 | 75篇 |
1995年 | 52篇 |
1994年 | 42篇 |
1993年 | 43篇 |
1992年 | 29篇 |
1991年 | 24篇 |
1990年 | 26篇 |
1989年 | 9篇 |
1988年 | 4篇 |
1987年 | 16篇 |
1986年 | 7篇 |
1985年 | 7篇 |
1984年 | 4篇 |
1983年 | 4篇 |
1982年 | 8篇 |
1981年 | 3篇 |
1980年 | 3篇 |
1979年 | 1篇 |
1978年 | 2篇 |
1977年 | 5篇 |
1976年 | 5篇 |
1975年 | 1篇 |
1973年 | 3篇 |
排序方式: 共有2323条查询结果,搜索用时 15 毫秒
991.
ShengAnTANG ZhiWeiDENG JunLI HongZhengFU YueHuPEI SiZHANG WenHanLIN 《中国化学快报》2005,16(3):353-355
From the marine sponge Jaspis sp., a new isomalabaricane triterpenoid 22, 23-dihydrostellettin D (1) was isolated, and its structure was established on the basis of IR, MS and extensive 2D NMR spectroscopic analysis. It is a unique skeleton compound rarely obtained from Chinese marine organisms. 相似文献
992.
Ion exchange was made on MCM-22 and MCM-49 zeolites with different Si/Al molar ratios, with Li+, Na+, K+, and Cs+ ions and the study of the influence of alkali metal cations on CO2 adsorption properties was performed. The degree of ion-exchange decreased for larger cations (Cs+) apparently due to steric hindrances. The exchange with different cations led to a decrease in the surface area and the micropore
volume. Our study shows that the adsorption capacity of the tested zeolites depends significantly on the nature and the concentration
of the charge-compensating cations. The highest CO2 adsorption capacity was obtained on the MWW zeolites with the lowest Si/Al molar ratio and the Li+ or K+ cations. 相似文献
993.
Homobrassinolide (2) and (22S, 23S)-22,23-epi-homobrassinolide (4) were synthesized fromstigmasterol in seven steps respectively in 4.7% and 24.1% overall yields. The ratio of 2 to 4is 1:5. The key step is the highly regioselective formation of the B--homo-7-oxa-lactone ringby oxidation of an enol silyl ether with 3--chloro-peroxybenzoic acid. 相似文献
994.
H. M. Ortner und A. Janku 《Fresenius' Journal of Analytical Chemistry》1973,264(3):185-192
Zusammenfassung Die Methode wurde für ThO2-Gehalte von 1–5% in stabförmigen Wolfram-Schweiß-elektroden ausgearbeitet. Die gammaspektrometrische Aktivitätsmessung kann unter Verwendung eines einfachen Einkanal-Gammaspektrometers mit NaJ (Tl)-Bohrlochkristall erfolgen. Der günstigste Meßbereich umfaßt224xp22/xxlarge947.gif" alt="gamma" align="MIDDLE" BORDER="0">-Energien von 30–500 keV. Eine Zählzeit von 5–40 sec ist je nach ThO2-Gehalt und Elektrodendurchmesser ausreichend. Chemische Eichanalysen für aus verschiedenen ThO2-Chargen hergestellte Elektroden sind unerläßlich. Brauchbare Messungen werden erst 1 Woche nach der Sinterung der Elektroden erhalten. Dies erscheint jedoch für die Endkontrolle auszuliefernder Elektroden unwesentlich. Untersuchungen betreffend die radiale und axiale ThO2-Verteilung an verschiedenen Elektroden wurden durchgeführt.
Dank. Für wertvolle Diskussionen sowie für die Aufnahme von224xp22/xxlarge947.gif" alt="gamma" align="MIDDLE" BORDER="0">-Spektren mit hoher Auflösung sind wir Herrn Dr. Danneberg der Osram Studiengesellschaft in München zu aufrichtigem Dank verpflichtet, ebenso Herrn Dipl.-Ing. Christ vom Metallwerk Plansee für seine Rechenhilfe. 相似文献
995.
Representations of groups of loops in U(N), SO(N) and various subgroups are studied. The representations are defined on fermion Fock spaces, and may be regarded as local gauge groups in the context of the two-dimensional many-particle Dirac theory for charged or neutral particles with rest mass m0. For m=0, the representations are shown to give rise to type I factors, while for m>0 hyperfinite, type III1 factors arise. A key point in the structure analysis is a convergence result: We prove that suitably rescaled representers of certain nonzero winding number loops converge to the free Dirac fields. We also present applications to cyclicity and irreducibility questions concerning the Dirac currents, and to the representation theory of a class of Kac-Moody Lie algebras. 相似文献
996.
This work is devoted to the study of variational problems encountered in the theory of locking materials introduced by Prager. In a first step, we are interested in the dual principles governing the equilibrium state of an elastic-locking material. We establish the inf-sup equality by means of a penalty method. In a second step, we introduce and discuss the locking limit analysis problem. This problem allows one to determine which displacements can be imposed on a locking body before complete locking. We show the equivalence of the statical and the kinematical methods. Two appendices close the paper. The first one focuses the attention on stress concentrations in a problem of torsion, and the second is devoted to materials with limited compressibility which lead to simpler mathematical problems.
Résumé Ce travail est consacré à l'étude des problèmes variationnels posés par les matériaux à blocage introduits par Prager. Dans un premier temps nous nous intéressons aux principes duaux qui gouvernent l'équilibre d'un milieu élastique à blocage. Nous établissons l'égalité Inf-Sup par une méthode de pénalisation. Dans un deuxième temps nous introduisons et discutons l'analyse limite de blocage qui permet de déterminer les déplacements susceptibles d'être imposés au corps avant son blocage complet. On montre que les approches statiques et cinématiques conduisent à la même valeur de la charge limite de blocage. Le travail se termine par deux annexes: l'une met l'accent sur la possibilité de concentration de contraintes (qui sont alors des mesures) dans un problème de torsion; l'autre étudie les matériaux à compressibilité limitée pour lesquels les problèmes mathémqtiques se simplifient notablement.相似文献
997.
Hassib Tounsi Samir Djemel Abdelhamid Ghorbel Gérard Delahay Louis Charles de Menorval Bernard Coq 《Reaction Kinetics and Catalysis Letters》2004,81(1):33-40
The steam treatment of Cu-ZSM-5 prepared by solid ion exchange involves an important decrease of the NO conversion. Characterization of the solids by NO-TPD, H2-TPR, 27Al MAS NMR shows that the catalyst deactivation is mainly due to change in copper species rather than the collapse of the zeolite framework.This revised version was published online in December 2005 with corrections to the Cover Date. 相似文献
998.
999.
In this work we introduce a class of Sikorski differential spaces (M, D) called pre-Fr¨olicher spaces, on which the process of yielding a Fr¨olicher structure on the underlying set M is D preserving, their category we denote by preFrl. We investigate some algebraic properties on these spaces whose subsequent geometric properties are mostly similar to those of smooth manifolds, except for the invariance of dimension, and also that preFrl naturally induces a Cartesian closed subcategory of the category Frl in which there is no discrete object. Using this Cartesian property, it is shown that the Gelfand representation is a smooth map, that the tangent as well as cotangent bundles are made smooth spaces in an unusual but more natural way via smooth curves. 相似文献
1000.
Precise Modulation of the Breathing Behavior and Pore Surface in Zr‐MOFs by Reversible Post‐Synthetic Variable‐Spacer Installation to Fine‐Tune the Expansion Magnitude and Sorption Properties 下载免费PDF全文
Cheng‐Xia Chen Zhangwen Wei Ji‐Jun Jiang Yan‐Zhong Fan Shao‐Ping Zheng Chen‐Chen Cao Yu‐Hao Li Prof. Dieter Fenske Prof. Cheng‐Yong Su 《Angewandte Chemie (International ed. in English)》2016,55(34):9932-9936
To combine flexibility and modifiability towards a more controllable complexity of MOFs, a post‐synthetic variable‐spacer installation (PVSI) strategy is used to implement kinetic installation/ uninstallation of secondary ligands into/from a robust yet flexible proto‐Zr‐MOF. This PVSI process features precise positioning of spacers with different length, size, number, and functionality, enabling accurate fixation of successive breathing stages and fine‐tuning of pore surface. It shows unprecedented synthetic tailorability to create complicated MOFs in a predictable way for property modification, for example, CO2 and R22 adsorption/separation, thermal/chemical stability, and extended breathing behavior. 相似文献