全文获取类型
收费全文 | 802篇 |
免费 | 70篇 |
国内免费 | 1篇 |
专业分类
化学 | 695篇 |
晶体学 | 4篇 |
力学 | 4篇 |
数学 | 55篇 |
物理学 | 69篇 |
无线电 | 46篇 |
出版年
2023年 | 13篇 |
2022年 | 12篇 |
2021年 | 15篇 |
2020年 | 26篇 |
2019年 | 24篇 |
2018年 | 11篇 |
2017年 | 14篇 |
2016年 | 31篇 |
2015年 | 39篇 |
2014年 | 42篇 |
2013年 | 41篇 |
2012年 | 56篇 |
2011年 | 74篇 |
2010年 | 50篇 |
2009年 | 38篇 |
2008年 | 69篇 |
2007年 | 68篇 |
2006年 | 41篇 |
2005年 | 24篇 |
2004年 | 37篇 |
2003年 | 29篇 |
2002年 | 34篇 |
2001年 | 11篇 |
2000年 | 12篇 |
1999年 | 4篇 |
1998年 | 7篇 |
1997年 | 2篇 |
1996年 | 3篇 |
1995年 | 8篇 |
1993年 | 4篇 |
1992年 | 2篇 |
1990年 | 1篇 |
1989年 | 2篇 |
1988年 | 2篇 |
1986年 | 1篇 |
1985年 | 6篇 |
1983年 | 2篇 |
1982年 | 3篇 |
1981年 | 1篇 |
1980年 | 1篇 |
1979年 | 2篇 |
1978年 | 1篇 |
1977年 | 1篇 |
1976年 | 1篇 |
1975年 | 1篇 |
1974年 | 1篇 |
1973年 | 2篇 |
1972年 | 1篇 |
1968年 | 1篇 |
1925年 | 1篇 |
排序方式: 共有873条查询结果,搜索用时 10 毫秒
71.
The new binaphthyl-based amino acid 6-amino-6′-carboxyethyl-2-methoxy-2′-hydroxy-1,1′-binaphthyl (Bna) is presented, which combines the axially chiral binaphthyl core, a phenolic OH-group as well as terminating amino and carboxyl groups in one structure. The large aromatic rings of the compound provide molecular spacing and π-surface attraction in assembled Bna oligoamides. The synthesis of Bna derivatives is reported, both with the (R)- and with the (S)-binaphthyl skeleton. Several dipeptides of (R)- or (S)-Bna units combined with natural amino acids, were prepared as ‘building blocks’ for the synthesis of extended Bna peptides. The tetrapeptide Boc-(S)-Val-(S)-Bna(OH)-(S)-Val-(S)-Bna(OPiv)-O-n-But (12) and the pentapeptide Boc-(S)-Val-(S)-Bna(OH)-(S)-Val-(S)-Bna(OH)-Gly-OH (13) were prepared via conventional solution phase synthesis and solid phase synthetic techniques, respectively. Compound 12 shows an interesting dynamic 1H NMR spectrum suggesting compact and aggregated forms in dichloromethane. Compound 13 accelerates the enolisation of acetone. The use of more complex Bna peptides as organo catalysts is proposed. 相似文献
72.
Müllner M Yuan J Weiss S Walther A Förtsch M Drechsler M Müller AH 《Journal of the American Chemical Society》2010,132(46):16587-16592
We report the preparation of water-soluble organo-silica hybrid nanotubes templated by core-shell-corona structured triblock terpolymer cylindrical polymer brushes (CPBs). The CPBs consist of a polymethacrylate backbone, a poly(tert-butyl acrylate) (PtBA) core, a poly(3-(trimethoxysilyl)propyl acrylate) (PAPTS) shell, and a poly(oligo(ethylene glycol) methacrylate) (POEGMA) corona. They were prepared via the "grafting from" strategy by the combination of two living/controlled polymerization techniques: anionic polymerization for the backbone and atom transfer radical polymerization (ATRP) for the triblock terpolymer side chains. The monomers tBA, APTS, and OEGMA were consecutively grown from the pendant ATRP initiating groups along the backbone to spatially organize the silica precursor, the trimethoxysilyl groups, into a tubular manner. The synthesized core-shell-corona structured CPBs then served as a unimolecular cylindrical template for the in situ fabrication of water-soluble organo-silica hybrid nanotubes via base-catalyzed condensation of the PAPTS shell block. The formed tubular nanostructures were characterized by transmission electron microscopy (TEM), cryogenic TEM, and atomic force microscopy. 相似文献
73.
Pediaditakis A Schroeder M Sagawe V Ludwig T Hillebrecht H 《Inorganic chemistry》2010,49(23):10882-10893
Single crystals of dark-red MgB(7) were grown from the elements in a Cu-melt. The crystal structure (Pearson symbol oI64; space group Imma; a = 10.478(2) ?, b = 5.977(1) ?, c = 8.125(2) ?, 2842 reflns, 48 params, R(1)(F) = 0.018, R(2)(I) = 0.034) consists of a hexagonal-primitive packing of B(12)-icosahedra and B(2)-units in trigonal-prismatic voids. According to the UV-vis spectra and band structure calculations MgB(7) is semiconducting with an optical gap of 1.9 eV. The long B-B distance of 2.278 ? within the B(2)-unit can be seen as a weak bonding interaction. The new Mg(~5)B(44) occurs beside the well-known MgB(12) as a byproduct. Small fragments of the black crystals are dark-yellow and transparent. The crystal structure (Pearson symbol tP196, space group P4(1)2(1)2, a = 10.380(2) ?, c = 14.391(3) ?, 4080 reflns, 251 params, R(1)(F) = 0.025, R(2)(I) = 0.037) is closely related to tetragonal boron-II (t-B(192)). It consists of B(12)-icosahedra and B(19+1)-units. With a charge of -6 for the B(19+1)-units and a Mg-content of ~20 Mg-atoms per unit cell the observed Mg content in Mg(~5)B(44) is quite close to the expected value derived from simple electron counting rules. All compositions were confirmed by EDXS. The microhardness was measured on single crystals for MgB(7) (H(V) = 2125, H(K) = 2004) and MgB(12) (H(V) = 2360, H(K) = 2459). 相似文献
74.
Thorsten Perl Bertram Bödeker Melanie Jünger Jürgen Nolte Wolfgang Vautz 《Analytical and bioanalytical chemistry》2010,397(6):2385-2394
Multicapillary column (MCC) ion mobility spectrometers (IMS) are increasingly in demand for medical diagnosis, biological
applications and process control. In a MCC-IMS, volatile compounds are differentiated by specific retention time and ion mobility
when rapid preseparation techniques are applied, e.g. for the analysis of complex and humid samples. Therefore, high accuracy
in the determination of both parameters is required for reliable identification of the signals. The retention time in the
MCC is the subject of the present investigation because, for such columns, small deviations in temperature and flow velocity
may cause significant changes in retention time. Therefore, a universal correction procedure would be a helpful tool to increase
the accuracy of the data obtained from a gas-chromatographic preseparation. Although the effect of the carrier gas flow velocity
and temperature on retention time is not linear, it could be demonstrated that a linear alignment can compensate for the changes
in retention time due to common minor deviations of both the carrier gas flow velocity and the column temperature around the
MCC-IMS standard operation conditions. Therefore, an effective linear alignment procedure for the correction of those deviations
has been developed from the analyses of defined gas mixtures under various experimental conditions. This procedure was then
applied to data sets generated from real breath analyses obtained in clinical studies using different instruments at different
measuring sites for validation. The variation in the retention time of known signals, especially for compounds with higher
retention times, was significantly improved. The alignment of the retention time—an indispensable procedure to achieve a more
precise identification of analytes—using the proposed method reduces the random error caused by small accidental deviations
in column temperature and flow velocity significantly. 相似文献
75.
Melanie Zimmermann Jeroen Volbeda Karl W. Törnroos Reiner Anwander 《Comptes Rendus Chimie》2010,13(6-7):651-660
The protonolysis reaction of [Ln(AlMe4)3] with H(Cp′) (Cp′ = C5Me4H) gives access to half-sandwich complexes [(Cp′)Ln(AlMe4)2]. X-ray structure analyses of the samarium, neodymium, and lanthanum derivatives reveal a distinct [AlMe4] coordination (one η2, one bent η2) for the two smaller rare-earth metals. The lanthanum complex displays an unprecedented dimeric structure with two μ2-η1:η2 coordinating [AlMe4] ligands in the solid state. Treatment of complexes [(Cp′)Ln(AlMe4)2] with perfluorinated organoborates and -boranes produces discrete contact ion-pairs, which are characterized by 1H, 13C, 27Al, 19F, and 11B NMR spectroscopy and act as efficient initiators for the fabrication of trans-1,4 polyisoprene. The polymerization performance is hereby affected by the rare-earth metal cation size, the type of boron cocatalyst, and the polymerization temperature. 相似文献
76.
Simultaneous analysis of 70 pesticides using HPlc/MS/MS: a comparison of the multiresidue method of Klein and Alder and the QuEChERS method 总被引:1,自引:0,他引:1
Since 2003, two new multipesticide residue methods for screening crops for a large number of pesticides, developed by Klein and Alder and Anastassiades et al. (Quick, Easy, Cheap, Effective, Rugged, and Safe; QuEChERS), have been published. Our intention was to compare these two important methods on the basis of their extraction efficiency, reproducibility, ruggedness, ease of use, and speed. In total, 70 pesticides belonging to numerous different substance classes were analyzed at two concentration levels by applying both methods, using five different representative matrixes. In the case of the QuEChERS method, the results of the three sample preparation steps (crude extract, extract after SPE, and extract after SPE and acidification) were compared with each other and with the results obtained with the Klein and Alder method. The extraction efficiencies of the QuEChERS method were far higher, and the sample preparation was much quicker when the last two steps were omitted. In most cases, the extraction efficiencies after the first step were approximately 100%. With extraction efficiencies of mostly less than 70%, the Klein and Alder method did not compare favorably. Some analytes caused problems during evaluation, mostly due to matrix influences. 相似文献
77.
Electron-electron coincidence spectra of Ar-Kr clusters after photoionization have been measured. An electron with the kinetic energy range from 0 to approximately 1 eV is found in coincidence with the Ar 3s cluster photoelectron. The low kinetic energy electron can be attributed to an Ar + Kr+ + Kr+ final state which forms after electron transfer mediated decay. This autoionization mechanism results from a concerted transition involving three different atoms in a van der Waals cluster; it was predicted theoretically, but hitherto not observed. 相似文献
78.
This paper presents a time integrator, which is based on a time discrete spatially weak finite element formulation, but fulfills the same balance laws as the underlying (five) differential equations. Namely, in addition to the balances of linear and angular momentum as well as entropy, also the balances of total energy and LYAPUNOV function are fulfilled. The spatially weak formulation is obtained by integration by parts. Where the resulting virtual stress power term is well-known, the virtual entropy production by conduction of heat is less known. The time discretisation is based on the midpoint rule and non-standard time discrete differential operators. This time integrator is a further development of the TC integrator of I. ROMERO. (© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
79.
Steven J. Granger Roland Bol Wolfram Meier‐Augenstein Melanie J. Leng Helen F. Kemp Tim H. E. Heaton Sue M. White 《Rapid communications in mass spectrometry : RCM》2010,24(5):475-482
Stable isotopes of water have been previously used in catchment studies to separate rain‐event water from pre‐event groundwater. However, there are a lack of studies at the smaller scale looking at the separation of event water from pre‐event water. This is particularly relevant for heavy clay soil systems through which the movement of water is uncertain but is thought to be rainwater‐dominated. The data presented here were collected at a rural site in the south‐west of England. The historic rainfall at the site was isotopically varied but similar to the global meteoric water line, with annual weighted means of ?37‰ for δ2H and ?5.7‰ for δ18O and with no seasonal variation. Drainage was sampled from the inter‐flow (surface runoff + lateral through‐flow) and drain‐flow (55 cm deep mole drains) pathways of two 1 ha lysimeters during two rainfall events, which had δ2H values of ?68‰ and ?92‰, respectively. The δ2H values of the lysimeter drainage water suggest that there was no contribution of event water during the first, small discharge (Q) event; however, the second larger event did show isotopic variation in δ2H values negatively related to Q indicating that rainwater was contributing to Q. A hydrograph separation indicated that only 49–58% of the inter‐flow and 18–25% of the drain‐flow consisted of event water. This was surprising given that these soil types are considered retentive of soil water. More work is needed on heavy clay soils to understand better the nature of water movement from these systems. Copyright © 2010 John Wiley & Sons, Ltd. 相似文献
80.