全文获取类型
收费全文 | 2548篇 |
免费 | 40篇 |
国内免费 | 289篇 |
专业分类
化学 | 2244篇 |
晶体学 | 2篇 |
力学 | 107篇 |
数学 | 405篇 |
物理学 | 119篇 |
出版年
2024年 | 4篇 |
2023年 | 54篇 |
2022年 | 52篇 |
2021年 | 28篇 |
2020年 | 48篇 |
2019年 | 59篇 |
2018年 | 69篇 |
2017年 | 68篇 |
2016年 | 63篇 |
2015年 | 44篇 |
2014年 | 105篇 |
2013年 | 153篇 |
2012年 | 180篇 |
2011年 | 158篇 |
2010年 | 136篇 |
2009年 | 178篇 |
2008年 | 200篇 |
2007年 | 206篇 |
2006年 | 174篇 |
2005年 | 151篇 |
2004年 | 146篇 |
2003年 | 60篇 |
2002年 | 88篇 |
2001年 | 48篇 |
2000年 | 33篇 |
1999年 | 42篇 |
1998年 | 45篇 |
1997年 | 42篇 |
1996年 | 40篇 |
1995年 | 34篇 |
1994年 | 34篇 |
1993年 | 15篇 |
1992年 | 25篇 |
1991年 | 23篇 |
1990年 | 6篇 |
1989年 | 16篇 |
1988年 | 8篇 |
1987年 | 7篇 |
1986年 | 4篇 |
1985年 | 6篇 |
1984年 | 5篇 |
1983年 | 1篇 |
1982年 | 2篇 |
1981年 | 3篇 |
1980年 | 3篇 |
1979年 | 5篇 |
1977年 | 1篇 |
1976年 | 2篇 |
1974年 | 2篇 |
1973年 | 1篇 |
排序方式: 共有2877条查询结果,搜索用时 0 毫秒
951.
Dislocation events are seen as excursions or pop-in events in the load–displacement curve of nanoindentation experiments. Two nanoindenters have been used to examine the difference between quasi-static and dynamic loading during indentation. Yield excursions were present in the load–displacement curves of both the statically and dynamically loaded single crystal nickel samples. Only one major excursion occurred in each quasi-static indent, nominally loaded at 100 μN/s while staircase yielding was observed under dynamic loading indentation with a 45 Hz oscillation of 2 nm superimposed on a 60 μN/s loading rate. Thermal activation analysis is used to explain the arrest and reinitiation of the yielding with activation volumes being modeled. For nanoindentation experiments differences between quasi-static and dynamic loading are described by the models presented. It is proposed that insight into the plastic deformation mechanisms associated with such plastic instabilities will provide one of the keys to length scale effects necessary to understanding nanostructures. 相似文献
952.
Both nitrazepam and flunitrazepam have been determined by high-performance liquid chromatography dual electrode detection
(LC-DED) in the reductive–reductive mode, using a carbon fibre veil electrode (CFVE) in conjugation with a glassy carbon electrode.
Initial studies were made to optimise the chromatographic conditions. These were found to be 45% acetonitrile-55% acetate
buffer (50 mM, pH 4.1) at a flow rate of 1.0 ml/min, employing a Hypersil C18, 5 μm, 250 mm × 4.6 mm column. Cyclic voltammetric
studies performed to ascertain the redox behaviour of nitrazepam and flunitrazepam at a CFVE in the optimised mobile phase.
Studies showed that similar voltammetric behaviour was obtained to that report at Hg or glassy carbon based electrodes. Further
studies were then carried out to identify the optimum conditions required for the LC-DED determination of nitrazepam and flunitrazepam
in beverage samples. Hydrodynamic voltammetry was used to optimise the applied potential at the generator and detector cells;
these were identified to be −2.40 and −0.25 V, respectively. A linear range of 2.0 to 100 μg ml−1, with a detection limit of 20 ng ml−1 was obtained. A convenient and rapid method for the determination of both nitrazepam and flunitrazepam in beverage sample
was developed. Following a simple sample extraction procedure, extracts were examined using the optimised LC-DED procedure.
An average percentage recovery of 95.5% (%CV = 4.5%) for nitrazepam and 78.0% (%CV = 8.8%) was achieved for a sample of “Pepsi
Max” spiked at 1.0 μg ml−1 nitrazepam, 1.47 μg ml−1 flunitrazepam.
Presented at the 4th Annual Meeting of the Great Western Electrochemistry Group, 8th June 2005, University of the West of
England, Bristol, UK. 相似文献
953.
M. C. Bastos L. F. A. Proença M. M. M. Neto I. T. E. Fonseca 《Journal of Solid State Electrochemistry》2008,12(2):121-131
This paper reports an electrochemical study on the corrosion of brass in deoxygenated nonbuffered and buffered natural and
artificial seawater solutions under anaerobic conditions. Cyclic voltammograms of brass and copper in natural seawater (NSW)
and artificial seawater (ASW) were obtained in the passive and transpassive potential regions. The corrosion resistance of
brass in natural and artificial seawater was evaluated, and open-circuit potentials were recorded over exposure period of
1 week. Brass samples from 3-month exposures in deoxygenated nonbuffered ASW and NSW, under open-circuit potential, have been
imaged by scanning electron microscopy, and the elemental composition of the corrosion products was obtained by energy dispersive
spectrometry analysis. It has been concluded that, under anaerobic conditions, the aggressivity of NSW is higher, with brass
being less resistant to corrosion than copper, and that buffer contributes to reduce the aggressivity of both media. 相似文献
954.
Ivan?GutmanEmail author Ned?ad?Turkovi? Jelena?Jovi?i? 《Monatshefte für Chemie / Chemical Monthly》2004,135(11):1389-1394
Summary. Cyclic conjugation in benzo-annelated perylenes is studied by means of the energy-effects of their six-membered rings. Classical theoretical approaches (based on Kekulé structures, Clar formulae, or conjugated circuits) predict that the central ring in benzo-annelated perylenes is empty and thus negligibly contributes to cyclic conjugation. Our calculations show that this is true only to a limited degree. In particular, rings angularly annelated relative to the central ring significantly increase the extent of its cyclic conjugation. 相似文献
955.
Heinrich Hahn Wolfgang Meindl Kurt Utvary 《Monatshefte für Chemie / Chemical Monthly》1985,116(2):157-166
Photochemical chlorination of (CH3NPF3)2 yields ClCH2(NPF3)2CH3(I) and ClCH2(NPF3)2CH2Cl (II). By reaction with organometallics unsymmetric N-substituted hexafluorodiazadiphosphetidines are synthesized. Depending on the nucleophilic strength of the used organometallic the chlorine atom of the N-CH2Cl group can be selectively substituted versus an organic substituent without side-reaction at the phosphorus atom. 相似文献
956.
Reaction of (CH3NPF3)2 with lithiummethylat yields the 2,4-dimethoxysubstituted diazadiphosphetidine which shows a good insecticide effect as proven by screening tests. In order to investigate the influence of the substituent of the nitrogen atom, a number of different 2,4-dimethoxy-substituted diazadiphosphetidines were synthesized. 相似文献
957.
A new method for the analysis of cyclic peptides (Pseudostellarins) in traditional Chinese medicine, Pseudostellaria heterophylla (Miq.) Pax, was established by high performance liquid chromatography–atmospheric pressure chemical ionization–mass spectrometry
(HPLC–APCI–MS). The real samples were separated by a reversed-phase C18 column using a binary eluent under gradient conditions. Six cyclic peptides were isolated and detected with a DAD detector.
The Pseudostellarins components were identified by APCI–MS in a negative mode. In the negative ion APCI mode, the best sensitivity
and the lowest detection limit for Pseudostellarins were obtained by using a mobile phase consisting of acetonitrile and pure
water. The APCI spectra were characterized by [M-H]− peak. The parent ions [M-H]− with highest intensity was used for quantification of one cyclic peptide-Pseudostellarin B. 相似文献
958.
Johan F. Billing 《Tetrahedron》2005,61(4):863-874
An Fmoc-protected δ-sugar amino acid, prepared by oxidation of a glucosamine derivative, was coupled to three different tripeptide tert-butyl esters (H-Tyr-Tyr-Tyr-OtBu, H-Tyr-Glu(OBzl)-Tyr-OtBu and H-Tyr-Arg(Mtr)-Tyr-OtBu) and the resulting sugar amino acid/amino acid hybrids were transformed into dimers that were subsequently cyclized to give three C2-symmetric macrocycles. The macrocycles were deprotected and their binding properties towards p-nitrophenyl glycosides, nucleotides, and purines were examined. Of the ligands screened, only some of the purines showed weak, but significant, binding. 相似文献
959.
Ljiljana S. Jovanović Nikola J. Ević Luka J. Bjelica Biljana F. Abramović Ferenc F. Gaál Nándor L. Rehák 《Monatshefte für Chemie / Chemical Monthly》1992,123(12):1071-1079
Summary The acid-base properties of the -oxo bridged dimeric iron complexes [FeL]2O with the ligands based on S-alkyl-1,4-bis(substituted salicylidene)isothiosemicarbazide and a tetramer with S-methyl-1,4-bis(salicylidene)isothiosemicarbazide, {[FeL]2O}2I3·I2, were investigated by cyclic voltammetry at glassy carbon electrode inDMF. Studies were carried out in the presence of either a weak (phenol) or a strong (HClO4 aq.) acid. The stoichimoetry of the reaction, changes in the general voltammetric pattern and the electrode reaction mechanism were discussed. These studies served as the basis for three-electrode amperometric titrations to determine the content of several of these complexes.
Voltammetrische und titrimetrische Untersuchung der Säure-Base-Eigenschaften einiger -oxo dimerer Eisen(III)-Komplexe inDMF-Lösungen
Zusammenfassung Die Säure-Base-Eigenschaften einiger dimerer -oxo-Eisen(III)-Komplexe [FeL]2O mit Liganden auf Basis von 1,4-bis(substituierten Salicyliden)S-(alkyl)isothiosemicarbaziden sowie eines tetrameren Komplexes mit 1,4-Bis(salicyliden)S-methylisothiosemicarbazid, {[FeL]2O}2I3·I2, wurden mittels zyklischer Voltammetrie an der Glaskohlenstoffelektrode inDMF-Lösungen untersucht. Die Messungen wurden in Gegenwart entweder einer schwachen (Phenol) oder einer starken (HClO4 aq.) Säure ausgeführt. Die Stöchiometrie der Reaktion, die Änderungen der voltammetrischen Kurven und der Elektroden-Reaktionsmechanismus wurden diskutiert. Diese Untersuchungen stellten eine Basis für Drei-Elektroden amperometrische Titrationen dar, die für die Bestimmung des Gehalts einiger dieser Komplexe angewendet wurden.相似文献
960.
The first simple cyclic silyl peroxide, namely 1,1,4,4,-tetramethyl-1,4-disila-2,3-dioxane (1), was prepared by classical synthetic methodology from its corresponding cyclic disilazane and the urea complex of hydrogen peroxide. 相似文献