全文获取类型
收费全文 | 3311篇 |
免费 | 117篇 |
国内免费 | 23篇 |
专业分类
化学 | 2127篇 |
晶体学 | 27篇 |
力学 | 63篇 |
数学 | 628篇 |
物理学 | 606篇 |
出版年
2023年 | 18篇 |
2022年 | 37篇 |
2021年 | 75篇 |
2020年 | 71篇 |
2019年 | 85篇 |
2018年 | 68篇 |
2017年 | 62篇 |
2016年 | 131篇 |
2015年 | 117篇 |
2014年 | 133篇 |
2013年 | 243篇 |
2012年 | 218篇 |
2011年 | 225篇 |
2010年 | 150篇 |
2009年 | 158篇 |
2008年 | 200篇 |
2007年 | 200篇 |
2006年 | 162篇 |
2005年 | 125篇 |
2004年 | 110篇 |
2003年 | 85篇 |
2002年 | 91篇 |
2001年 | 38篇 |
2000年 | 46篇 |
1999年 | 31篇 |
1998年 | 23篇 |
1997年 | 26篇 |
1996年 | 36篇 |
1995年 | 32篇 |
1994年 | 29篇 |
1993年 | 20篇 |
1992年 | 29篇 |
1991年 | 16篇 |
1990年 | 24篇 |
1989年 | 16篇 |
1988年 | 18篇 |
1987年 | 16篇 |
1986年 | 14篇 |
1985年 | 21篇 |
1984年 | 32篇 |
1983年 | 15篇 |
1982年 | 21篇 |
1981年 | 12篇 |
1980年 | 19篇 |
1979年 | 15篇 |
1978年 | 30篇 |
1977年 | 22篇 |
1976年 | 13篇 |
1970年 | 9篇 |
1968年 | 8篇 |
排序方式: 共有3451条查询结果,搜索用时 15 毫秒
81.
Lucjan Chmielarz Piotr Kustrowski Marek Michalik Barbara Dudek Magdalena Czajka Roman Dziembaj 《Reaction Kinetics and Catalysis Letters》2007,91(2):369-378
Natural phlogophite, pre-treated with acids and intercalated with alumina pillars, was used as catalytic support. Transition
metals (Fe, Cu) were deposited on the surface of the modified clay materials by an ion-exchange method. The obtained samples
were characterized with respect to structure (XRD), texture (BET), composition (EPMA) and chemical nature of the deposited
transition metals species (UV-vis-DRS). The phlogophite based materials have been found to be active and selective catalysts
of the DeNOx process. The Fe-containing samples were catalytically active at lower temperatures than the clays modified with
copper. A competitive ammonia oxidation by oxygen decreased the effectiveness of the DeNOx process in the high temperature
range. 相似文献
82.
Pascual JI Barth JV Ceballos G Trimarchi G De Vita A Kern K Rust HP 《The Journal of chemical physics》2004,120(24):11367-11370
We report scanning tunneling microscopy observations on the restructuring of a Ag(110) surface induced by the molecule 4-[trans-2-(pyrid-4-yl-vinyl)]benzoic acid (PVBA). Our data reveal that the surface undergoes a mesoscopic step faceting following exposure to submonolayer coverages and thermal activation. A sawtooth arrangement evolves implying long-range mass transport of substrate atoms and forming a regular arrangement of kink sites. Its formation is associated with the molecules' functional headgroups forming carboxylates with [100] Ag microfacets at step edges, and eventually operating to reshape the surface morphology. Interestingly, the resulting microfacets act as chiral templates for the growth of supramolecular PVBA structures. Theoretical modeling based on ab initio results indicates that chiral recognition processes discriminating between the two enantiomers of adsorbed PVBA molecules occur in this process. 相似文献
83.
The occurrence of a series of new constituents which can be considered as Diels-Alder adducts of methyl vinyl ketone and ocimene (→1–4), myrcene (→ 9 , 10 ) or β-far-nesene ( → 11 , 12 ), respectively, was reported. Furthermore, the structures of four isomeric cyclohexene derivatives could be established as adducts 21–24 of (E, Z)- and (E, E)-1,3,5-undecatrience and methyl vinyl ketone. Another series of constituents having the norbornane skeleton represents adducts 25–32 , and 33–40 of methyl cyclopentadiene and 1-octen-3-one or methyl vinyl ketone, respectively. In accordance with Alder's endo-rule the endo-isomers are preponderant in the natural as well as in the synthetic mixtures. Most of these constituents could also be identified in a lavender absolute as well as in a freshly prepared hexane extract of lavender flowers (Lavandula officinalis CHAIX ). 相似文献
84.
E. Roman M. Sánchez-Avedillo J. L. de Segovia 《Applied Physics A: Materials Science & Processing》1984,35(1):35-40
In the present work, results of the interaction of O2 with polycrystalline titanium using AES, and ELS techniques, are presented. Changes in the shapes of Ti(LMV) and Ti(LMM) transitions and in the Ti(LM)O(V)/Ti(LMV) and Ti(LMV) Ti(LMM) amplitude ratios as well as a shift of the 34 eV loss peak [Ti(3p level], are studied as function of the oxygen exposure.At O2 pressures equal or less than 10–6 Pa and exposure up to 2000 L(O2) (weak oxidation), the Ti(LM)O(V)/Ti(LMV) and Ti(LMV)/Ti(LMM) ratios show three clearly distinct regions: (i) up to 20L there is a chemisorbed phase with the Ti(LMV)/Ti(LMM) ratio remaining constant and a fast linear increase in the Ti(LM)O(V)/Ti(LMM) ratio. (ii) At exposures higher than 20L the oxidation begins being characterized by a linear variation of the Ti(LM)O(V)/Ti(LMV) and Ti(LMV)/Ti(LMV) ratios, with positive and negative slopes, respectively. (iii) At 125L there is a change in both slopes but no stable value is achieved, despite the O(KLL)/Ti(LMM) ratio remains constant. Hence, it can be concluded that TiO is the final oxide obtained after that treatment, from features like the shape of the transitions and a maximum shift of 3eV of the Ti(3p) level, complemented with ESD experiments.On the other hand, experiments at pressures higher than 10–4Pa with or without the surfaces at 900K (strong oxidation) indicate that the most likely final oxide is TiO2.Work presented to the IXIVC-VICSS. Madrid (1983) 相似文献
85.
Roman Mazurkiewicz 《Monatshefte für Chemie / Chemical Monthly》1989,120(11):973-980
Summary N-Acylanthranilamides react with dibromotriphenylphosphorane in the presence of triethylamine as HBr captor to give 4-imino-4H-3,1-benzoxazines in good yields. If the reaction is carried out without acid acceptor, N-acetylanthranilamides yield 2-methyl-4-quinazolones, whereas N-benzoylanthranilamides give 2-phenyl-4-imino-4H-3,1-benzoxazines. It has also been found that 2-methyl-4-imino-4H-3,1-benzoxazines rearrange under the influence of HCl or HBr into the respective 2-methyl-4-quinazolones; 2-phenyl-4-imino-4H-3,1-benzoxazines, however, do not undergo such a rearrangement.
Synthese und Umlagerung von 4-Imino-4H-3,1-benzoxazinen
Zusammenfassung Die Umsetzung von N-Acyl-anthranilsäure-amiden mit Triphenyldibromphosphoran in Gegenwart von Triethylamin als HBr-Akzeptor führt mit guten Ausbeuten zu 4-Imino-4H-3,1-benzoxazinen. Wird die Reaktion ohne säurebindendes Mittel durchgeführt, dann entstehen aus N-Acetyl-anthranilsäure-amiden 2-Methylchinazolone-4, jedoch erhält man aus N-Benzoylanthranilsäure-amiden 2-Phenyl-4-imino-4H-3,1-benzoxazine. 2-Methyl-4-imino-4H-3,1-benzoxazine erleiden unter dem Einfluß von HBr oder HCl eine Umlagerung in entsprechende 2-Methylchinazolone-4, während 2-Phenyl-4-imino-4H-3,1-benzoxazine zu einer solchen Umlagerung nicht befähigt sind.相似文献
86.
The Callan-Symanzik- and-functions are calculated analytically for Q.E.D. in the limit of a large number of leptons (N
F) up to terms of order 1/N
F inclusive. We give closed analytic expressions for the coefficients of these terms in their series expansion in powers ofK N
F/. We have been able to sum these series and to obtain some striking results. 相似文献
87.
Roman Pogreb Oleg Popov Vlad Lirtsman Oleg Pyshkin Alexander Kazachkov Albina Musin Gene Whyman Binyamin Finkelshtein Yuri Shmukler Dan Davidov Alexander Gladkikh Edward Bormashenko 《先进技术聚合物》2006,17(1):20-25
The work is devoted to luminescent properties of trivalent lanthanide complexes dispersed in thermoplastic host matrices. Polyethylene films and polypropylene‐rods, both doped with these complexes, were manufactured using an extrusion technique. Two kinds of dopants were used: Eu(III)‐thenoyltrifluoroacetone‐1,10‐phenanthroline complex (1) and Eu(III)‐La(III)‐1,10‐phenanthroline complex (2). Absorption, excitation, emission spectra and lifetime of luminescence were studied. The impact of the polymer matrix on the emission spectra was investigated. Emission spectra of the films were studied at room and helium temperatures. Time‐of‐flight secondary ion mass spectrometry (TOF‐SIMS) surface mapping showed that in the Eu(III)‐La(III) complex europium forms islands (clusters) with a dimension of 1 µm, whereas lanthanum was dispersed more uniformly in the polymer matrix. Dependence of emission intensity on the excitation was determined. Copyright © 2006 John Wiley & Sons, Ltd. 相似文献
88.
Roman Boča 《Theoretical chemistry accounts》1982,61(2):179-192
The perturbative configuration interaction using localized orbitals (the PCILO method) was extended in the way that current limitations to the two-centre bond approach were overcome. The localized molecular orbitals contain an arbitrary number of the basis set components; this follows from the a priori stated localized bonding model of a molecule. The extended PCILO method was formulated for the CNDO, INDO and NDDO Hamiltonian approximations. The configuration interaction was performed using the Rayleigh-Schrödinger many-body perturbation theory with the Møller-Plesset type of Hamiltonian partitioning, similar to that used in the so-called modified PCILO method. Applications to molecules with semi-localized and/or semi-delocalized bonds, as benzene or diborane, are presented. 相似文献
89.
90.
Herbert Schumann Kerstin Zietzke Frank Erbstein Roman Weimann 《Journal of organometallic chemistry》1996,520(1-2):265-268
LnCl3 reacts with Na(C5Me41Bu) in THF to form the organolanthanoid chlorides [(C5Me41Bu)2LnCl(THF)] (Ln = La (1a), Lu (1b)). Compounds 1a and 1b yield in reaction with NaO2CCH3 the monomeric organolanthanoid acetates [(C5Me41Bu)2LnO2CCH3] (Ln = La (2a), Lu (2b)). The single crystal X-ray structure analysis of 2b as well as the cryoscopic molecular weight investigation of 2a verify the monomeric structure of these complexes. 相似文献