全文获取类型
收费全文 | 993篇 |
免费 | 21篇 |
国内免费 | 6篇 |
专业分类
化学 | 601篇 |
晶体学 | 7篇 |
力学 | 19篇 |
数学 | 107篇 |
物理学 | 286篇 |
出版年
2020年 | 13篇 |
2019年 | 9篇 |
2016年 | 18篇 |
2015年 | 20篇 |
2014年 | 13篇 |
2013年 | 19篇 |
2012年 | 32篇 |
2011年 | 42篇 |
2010年 | 31篇 |
2009年 | 32篇 |
2008年 | 26篇 |
2007年 | 38篇 |
2006年 | 41篇 |
2005年 | 46篇 |
2004年 | 36篇 |
2003年 | 22篇 |
2002年 | 18篇 |
2001年 | 24篇 |
2000年 | 11篇 |
1999年 | 18篇 |
1998年 | 16篇 |
1997年 | 16篇 |
1996年 | 12篇 |
1995年 | 11篇 |
1994年 | 14篇 |
1993年 | 11篇 |
1992年 | 11篇 |
1991年 | 8篇 |
1989年 | 11篇 |
1987年 | 9篇 |
1985年 | 9篇 |
1984年 | 8篇 |
1983年 | 14篇 |
1982年 | 15篇 |
1981年 | 18篇 |
1980年 | 13篇 |
1978年 | 10篇 |
1976年 | 8篇 |
1975年 | 9篇 |
1974年 | 9篇 |
1973年 | 8篇 |
1943年 | 12篇 |
1933年 | 11篇 |
1931年 | 8篇 |
1930年 | 9篇 |
1926年 | 7篇 |
1924年 | 8篇 |
1923年 | 10篇 |
1902年 | 7篇 |
1881年 | 7篇 |
排序方式: 共有1020条查询结果,搜索用时 10 毫秒
21.
H. J. Walther H. Eyer U. P. Schlunegger C. J. Porter E. A. Larka J. H. Beynon 《Journal of mass spectrometry : JMS》1982,17(2):81-85
Phenol and five acyclic isomeric compounds have been investigated using electron impact and field ionization techniques, mass analysed ion kinetic energy spectrometry and collision induced dissociation mass analysed ion kinetic energy spectrometry. The kinetic energy release data corresponding to the elimination of carbon monoxide from the molecular ions show that at least two structures of the reactant ion are involved. The electron impact and field ionization collision induced dissociation mass analysed ion kinetic energy spectra indicate that under electron impact conditions, the phenol ion partially isomerizes to another ion structure. An isomerization of about 40% to the structure of cis-2-hexen-4-yn-1-al is in good accordance with the spectral data. 相似文献
22.
A. Zschunke H. Meyer I. Heidlas B. Messbauer B. Walther H.-D. Schdler B. Thomas 《无机化学与普通化学杂志》1983,504(9):117-127
31P and 195Pt N.M.R. Investigations on Diplatinum (I) Complexes of the Type [Pt2(μ-SPR2)2L2] (L = PR3, PhP(OPh)2, P(OPh)3, CNR) 31P-, 195Pt-chemical shifts and 195Pt–31P- resp. 31P–31P-coupling constants of a series of doubly bridged diplatinum(I) complexes are reported. 31P-coordination chemical shifts of the terminal ligands of complexes of type [Pt2(μ-SPR2)2(P′R3′)2] and some of the various coupling constants are strongly influenced by the π-acceptor strength of these ligands. J(195Pt–195Pt) is found to change the sign among the series of complexes investigated. Thermal singlett triplet exitation giving rise to the paramagnetism of these complexes observed by preliminary EPR-measurements and confirmed by EHT-calculations is deduced from the large values of 2J(P–P′) and 3J(P′P′) as well as the unusually high temperature dependence of some coupling constants and other NMR features. The chemical stability of the doubly bridged core, the coordination shifts of the bridging phosphorus atoms and EHT-calculations suggest a view of aromaticity of the [Pt2(μ-SPR2)2](M–M) unit of these complexes. 相似文献
23.
24.
Richard Behnke Franka Stahl Kathrin Duske Mareike Warkentin Margit Schwartz Burkhard Hinz Udo Walther 《Molecules (Basel, Switzerland)》2022,27(12)
Depending on their composition, plastics have a cytotoxic potential that needs to be evaluated before they are used in dentistry, e.g., as orthodontic removable appliances. Relevant guidelines set out requirements that a potential new resin in the medical field must meet, with a wide scope for experimental design. In the present study, test specimens of different geometries consisting of varying polymers (Orthocryl®, Orthocryl® LC, Loctite® EA 9483, Polypropylene) were soaked for different periods of time, then transferred to cell culture medium for 24 h, which was subsequently used for 24-h cultivation of A549 cells, followed by cytotoxicity assays (WST-1, Annexin V-FITC-propidium iodide (PI) flow cytometry). In this context, a reduction in the cytotoxic effect of the eluates of test specimens prepared from Orthocryl® LC and Loctite® EA 9483 was particularly evident in the Annexin V-FITC-PI assay when the soaking time was extended to 48 h and 168 h, respectively. Consistent with this, a reduced release of potentially toxic monomers into the cell culture medium, as measured by gas chromatography-mass spectrometry, was observed when the prior soaking time of test specimens of all geometries was extended. Remarkably, a significant increase in cytotoxic effect was observed in the WST-1 assay, which was accompanied by a higher release of monomers when the thickness of the test sample was increased from 0.5 to 1.0 mm, although an elution volume adapted to the surface area was used. However, further increasing the thickness to 3.0 mm did not lead to an increase in the observed cytotoxicity or monomer release. Test specimens made of polypropylene showed no toxicity under all test specimen sizes and soaking time conditions. Overall, it is recommended to perform toxicity studies of test specimens using different geometries and soaking times. Thereby, the influence of the different specimen thicknesses should also be considered. Finally, an extension of the test protocols proposed in ISO 10993-5:2009 should be considered, e.g., by flow cytometry or monomer analysis as well as fixed soaking times. 相似文献
25.
26.
Virtual prototyping plays an ever increasing role in the engineering disciplines. Nowadays, engineers can rely on powerful tools like object oriented modeling languages, e.g., Modelica. Models written in this language can be simulated by open source software as well as commercial tools. The advantage of this approach is that the engineers can concentrate themselves on modeling, whereas the numerical intricacies of the simulation are handled by the software. On the other hand the simulations are usually slower than implementations which are parallelized and optimized manually. This can lead to computation times which are infeasible in practice, e.g., when a real time simulation is necessary for a hardware-in-the-loop simulation. In this contribution we are concerned with speeding up such automated simulations by parallelization (on desktop hardware as well as HPC systems). We examine the parallelism across the system approaches. Additionally, the influence of the problem formulation on the simulation time is discussed. The implemented methods are demonstrated on engineering examples. (© 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
27.
Simon Ludwanowski Dr. Meral Ari Karsten Parison Somar Kalthoum Paula Straub Nils Pompe Prof. Dr. Stefan Weber Priv.-Doz. Dr. Michael Walter Prof. Dr. Andreas Walther 《Chemistry (Weinheim an der Bergstrasse, Germany)》2020,26(58):13203-13212
Arylazopyrazoles are an emerging class of photoswitches with redshifted switching wavelength, high photostationary states, long thermal half-lives and facile synthetic access. Understanding pathways for a simple modulation of the thermal half-lives, while keeping other parameters of interest constant, is an important aspect for out-of-equilibrium systems design and applications. Here, it is demonstrated that the thermal half-life of a water-soluble PEG-tethered arylazo-bis(o-methylated)pyrazole (AAP) can be tuned by more than five orders of magnitude using simple pH adjustment, which is beyond the tunability of azobenzenes. The mechanism of thermal relaxation is investigated by thorough spectroscopic analyses and density functional theory (DFT) calculations. Finally, the concepts of a tunable half-life are transferred from the molecular scale to the material scale. Based on the photochromic characteristics of E- and Z-AAP, transient information storage is showcased in form of light-written patterns inside films cast from different pH, which in turn leads to different times of storage. With respect to prospective precisely tunable materials and time-programmed out-of-equilibrium systems, an externally tunable half-life is likely advantageous over changing the entire system by the replacement of the photoswitch. 相似文献
28.
Walther Burchard 《Macromolecular Symposia》1996,101(1):103-113
The main features of dynamic light scattering are briefly outlined. The capability of this method to characterizing molar mass distributions and to distinguishing different molecular architectures is demonstrated with some examples. Special attention is given to the question of internal mobility and the spectrum of relaxation times. Recent results on asymptotic behavior are presented and discussed. Scaling behavior of the first cumulant with respect of space and of the time correlation function with respect of time (shape function) is found. All examples studied so far demonstrate the dynamic scaling of Zimm-Rouse chains, but the prefactors strongly depend on branching and crosslinking. 相似文献
29.
30.
Walther C Rothe J Fuss M Büchner S Koltsov S Bergmann T 《Analytical and bioanalytical chemistry》2007,388(2):409-431
Polynuclear species of zirconium in acidic aqueous solution are investigated by combining X-ray absorption spectroscopy (XAFS)
and nanoelectrospray mass spectrometry (ESI-MS). Species distributions are measured between pHC 0 and pHC 3 for [Zr] = 1.5–10 mM. While the monomer remains a minor species, with increasing pH the degree of polymerization increases
and the formation of tetramers, pentamers, octamers, and larger polymers is observed. The high resolution of the mass spectrometer
permits the unambiguous determination of polynuclear zirconium hydroxide complexes by means of their isotopic patterns. The
relative abundances of mononuclear and polynuclear species present simultaneously in solution are measured, even if one of
the species contributes only 0.1% of the Zr concentration. For the first time it has been directly observed that the hydrolysis
of polynuclear Zr species is a continuous process which leads to charge compensation through the sequential substitution of
water molecules by hydroxide ligands until doubly charged polymers dominate at conditions (H+ and Zr concentrations) close to the solubility of Zr(OH)4(am). The invasiveness of the electrospray process was minimized by using very mild declustering conditions, leaving the polynuclear
species within a solvent shell of approximately 20 water molecules.
Figure Schematic Diagram of Multiplexed Measurement of 9 Anti-Nuclear Antibodies Using the AtheNa Multilyte Assay 相似文献