全文获取类型
收费全文 | 680篇 |
免费 | 16篇 |
国内免费 | 2篇 |
专业分类
化学 | 521篇 |
晶体学 | 1篇 |
力学 | 25篇 |
数学 | 114篇 |
物理学 | 37篇 |
出版年
2024年 | 1篇 |
2023年 | 1篇 |
2022年 | 13篇 |
2021年 | 22篇 |
2020年 | 11篇 |
2019年 | 13篇 |
2018年 | 19篇 |
2017年 | 5篇 |
2016年 | 21篇 |
2015年 | 18篇 |
2014年 | 18篇 |
2013年 | 42篇 |
2012年 | 39篇 |
2011年 | 48篇 |
2010年 | 39篇 |
2009年 | 31篇 |
2008年 | 46篇 |
2007年 | 38篇 |
2006年 | 33篇 |
2005年 | 34篇 |
2004年 | 34篇 |
2003年 | 26篇 |
2002年 | 17篇 |
2001年 | 5篇 |
2000年 | 14篇 |
1999年 | 12篇 |
1998年 | 9篇 |
1997年 | 11篇 |
1996年 | 9篇 |
1995年 | 8篇 |
1994年 | 3篇 |
1993年 | 8篇 |
1992年 | 5篇 |
1991年 | 4篇 |
1990年 | 3篇 |
1989年 | 4篇 |
1988年 | 4篇 |
1987年 | 1篇 |
1986年 | 3篇 |
1985年 | 4篇 |
1984年 | 5篇 |
1983年 | 4篇 |
1982年 | 4篇 |
1981年 | 2篇 |
1980年 | 2篇 |
1979年 | 1篇 |
1978年 | 1篇 |
1977年 | 1篇 |
1975年 | 1篇 |
1973年 | 1篇 |
排序方式: 共有698条查询结果,搜索用时 406 毫秒
51.
The rationale for the extension of the rH-metric standardization and measurements to aqueous-organic solvent mixtures is here introduced, and examples of establishment of the ranges of conventional rH-metric scales as well as primary standards rHs in some typical aqueous-organic solvent mixtures are described. 相似文献
52.
Kolbitsch C Lorenz IH Hörmann C Hinteregger M Löckinger A Moser PL Kremser C Schocke M Felber S Pfeiffer KP Benzer A 《Magnetic resonance imaging》2002,20(7):535-541
Conflicting results reported on the effects of hyperoxia on cerebral hemodynamics have been attributed mainly to methodical and species differences. In the present study contrast-enhanced magnetic resonance imaging (MRI) perfusion measurement was used to analyze the influence of hyperoxia (fraction of inspired oxygen (FiO2) = 1.0) on regional cerebral blood flow (rCBF) and regional cerebral blood volume (rCBV) in awake, normoventilating volunteers (n = 19). Furthermore, the experiment was repeated in 20 volunteers for transcranial Doppler sonography (TCD) measurement of cerebral blood flow velocity in the middle cerebral artery (CBFV(MCA)). When compared to normoxia (FiO2 = 0.21), hyperoxia heterogeneously influenced rCBV (4.95 +/- 0.02 to 12.87 +/- 0.08 mL/100g (FiO2 = 0.21) vs. 4.50 +/- 0.02 to 13.09 +/- 0.09 mL/100g (FiO2 = 1.0). In contrast, hyperoxia diminished rCBF in all regions (68.08 +/- 0.38 to 199.58 +/- 1.58 mL/100g/min (FiO2 = 0.21) vs. 58.63 +/- 0.32 to 175.16 +/- 1.51 mL/100g/min (FiO2 = 1.0)) except in parietal and left frontal gray matter. CBFV(MCA) remained unchanged regardless of the inspired oxygen fraction (62 +/- 9 cm/s (FiO2 = 0.21) vs. 64 +/- 8 cm/s (FiO2 = 1.0)). Finding CBFV(MCA) unchanged during hyperoxia is consistent with the present study's unchanged rCBF in parietal and left frontal gray matter. In these fronto-parietal regions predominantly fed by the middle cerebral artery, the vasoconstrictor effect of oxygen was probably counteracted by increased perfusion of foci of neuronal activity controlling general behavior and arousal. 相似文献
53.
54.
Dreos R Nardin G Randaccio L Siega P Tauzher G Vrdoljak V 《Inorganic chemistry》2003,42(21):6805-6811
The reduction of [Co(III)(tmsalen)py(2)](+)ClO(4)(-), where tmsalen = 4,4',7,7'-tetramethylsalen, with NaBH(4)/PdCl(2) in alkaline methanolic solution, followed by the oxidative addition of CH(2)ClI, leads to the expected trans organometallic dimeric species 1, [CH(2)ClCo(tmsalen)](2), provided that the product is recovered from the reaction mixture immediately after the completion of the reaction. If 1 is left for longer time in contact with the reaction mixture, the intramolecular reaction of the axial chloromethyl group with the equatorial chelate leads to the formation of the monocationic complex 2, containing a seven-membered ring. In this complex the novel tetradentate ligand coordinates Co in a cis fashion, the other two positions being occupied by one py and one water molecule. The resulting complex is chiral, even if the reaction product is a racemic compound. The unidentate ligands of 2 have been exchanged quantitatively for N-MeIm, and the resulting complex 3 still maintains the beta cis geometry. Therefore, 2 may be considered the precursor of a new class of organocobalt derivatives with a folded tetradentate ligand and two adjacent exchangeable sites. On the basis of the geometry of the tetradentate Schiff bases in complexes, where they adopt a planar geometry, it was suggested that there is a significant electron density delocalization involving the metal center over the two chemically equivalent moieties of Co(chel). Comparison of the geometry of the planar salicylaldiminate (sal) moiety with that of the cyclized methoxy-iminate one (imi) in 2 and 3 strongly supports that the delocalization, still present in sal, is essentially either lost or strongly reduced in imi. 相似文献
55.
Patrizia Longobardi Mercede Maj Akbar Rhemtulla 《Proceedings of the American Mathematical Society》2000,128(3):637-641
If a group has an ascending series of subgroups such that for each ordinal , and has no non-abelian free subsemigroup, then is right orderable if and only if it is locally indicable. In particular if is a radical-by-periodic group, then it is right orderable if and only if it is locally indicable.
56.
Let be a vector lattice of real functions on a set with , and let be a linear positive functional on . Conditions are given which imply the representation , , for some bounded charge . As an application, for any bounded charge on a field , the dual of is shown to be isometrically isomorphic to a suitable space of bounded charges on . In addition, it is proved that, under one more assumption on , is the integral with respect to a -additive bounded charge.
57.
Sulphur-headgroup organic molecules have been chemisorbed on Cu(1 0 0) as self-assembled monolayers (SAMs) in highly-ordered two-fold symmetry structures, and the electronic states induced at the interface have been measured by photoemission: a close similarity of the main interface states for methane-thiolate and mercaptobenzoxazole on Cu(1 0 0) in the same p(2 × 2)-phase is observed. The bonding states for methane-thiolate/Cu(1 0 0) in the p(2 × 2) and c(2 × 2) structures have been compared to ab-initio calculation of the total density of states (DOS) for the S/Cu(1 0 0) system in the same phases. The major role of the S-Cu bonding to determine the density of state evolution at the interface is brought to light. The observed differences in the two phases depend mainly on the charge distribution associated to the different molecular packing, with a minor role of the radical group. 相似文献
58.
Paolo Matteini Giovanni Agati Patrizia Pinelli Andrea Goti 《Monatshefte für Chemie / Chemical Monthly》2011,45(2):885-893
Abstract
The interaction of the model flavonoid glycoside rutin with the flavonoid reagent diphenylborinic acid 2-aminoethyl ester (DPBA) was investigated using a combination of HPLC–DAD–ESI–MS analysis, UV–visible spectroscopy, and semiempirical calculations. Mass spectra and spectroscopic data made it possible to describe the complexation pathway as addition of diphenylboron groups to the two available 3′,4′-o-diphenolic and the 5-hydroxy-4-keto coordinating sites of rutin. Semiempirical calculations were carried out to obtain the conformation of the most stable DPBA/rutin adducts. The results showed that a number of complexation dynamics can occur as a function of the characteristics of the medium (type and pH of the solvent) and of the amount of DPBA. This work suggests the possibility of substantially improving existing procedures for recognition of flavonoid compounds by choice of suitable experimental conditions. 相似文献59.
Patrizia Trovalusci Danilo Capecchi Giuseppe Ruta 《Archive of Applied Mechanics (Ingenieur Archiv)》2009,79(11):981-997
This paper presents an overview of the origin of multiscale approaches in mechanics. While the pioneer molecular models of
linear elastic bodies by Navier, Cauchy and Poisson were contradicted by experiments, the phenomenological energetic approach
by Green still seems suitable for simple materials only. Voigt’s molecular model, here reinterpreted in the light of contemporary
mechanics, reconciled the two approaches providing a conceptual guideline for developing constitutive models based on a direct
link between continuum and discrete solid mechanics. Such a theoretical background proves to be especially suitable for new
complex materials. An example referred to masonry-like materials is given. 相似文献
60.