全文获取类型
收费全文 | 28442篇 |
免费 | 2594篇 |
国内免费 | 1808篇 |
专业分类
化学 | 22969篇 |
晶体学 | 267篇 |
力学 | 277篇 |
综合类 | 137篇 |
数学 | 305篇 |
物理学 | 8889篇 |
出版年
2024年 | 45篇 |
2023年 | 327篇 |
2022年 | 631篇 |
2021年 | 744篇 |
2020年 | 864篇 |
2019年 | 839篇 |
2018年 | 810篇 |
2017年 | 1134篇 |
2016年 | 1315篇 |
2015年 | 1227篇 |
2014年 | 1362篇 |
2013年 | 2135篇 |
2012年 | 2040篇 |
2011年 | 1783篇 |
2010年 | 1439篇 |
2009年 | 1795篇 |
2008年 | 1571篇 |
2007年 | 1882篇 |
2006年 | 1518篇 |
2005年 | 1324篇 |
2004年 | 1205篇 |
2003年 | 1017篇 |
2002年 | 837篇 |
2001年 | 582篇 |
2000年 | 548篇 |
1999年 | 502篇 |
1998年 | 447篇 |
1997年 | 448篇 |
1996年 | 382篇 |
1995年 | 339篇 |
1994年 | 284篇 |
1993年 | 262篇 |
1992年 | 215篇 |
1991年 | 222篇 |
1990年 | 133篇 |
1989年 | 107篇 |
1988年 | 135篇 |
1987年 | 66篇 |
1986年 | 55篇 |
1985年 | 48篇 |
1984年 | 50篇 |
1983年 | 21篇 |
1982年 | 36篇 |
1981年 | 29篇 |
1980年 | 22篇 |
1979年 | 17篇 |
1978年 | 9篇 |
1977年 | 6篇 |
1975年 | 5篇 |
1973年 | 9篇 |
排序方式: 共有10000条查询结果,搜索用时 46 毫秒
141.
142.
Transmission fluctuation spectrometry (TFS) is a method for the analysis of particle size distributions based on the statistical fluctuations of a transmission signal. Complete information on the PSD and particle concentration can be retrieved by a special transformation of the transmission signal, whereby the expectancy of the transmission square (ETS) is determined after the signal has been subjected to a procedure of spatial and temporal averaging. By varying the averaging parameters over a wide range, a spectrum of ETSs is obtained and introduced into a linear equation system, which yields the PSD. In the experimental realization presented here, variable temporal averaging is realized in the frequency domain with a series of low pass filters at different cutoff frequencies while spatial averaging inevitably occurs as the particles pass through a focused Gaussian beam of finite cross section. Experimental results on spherical particles (glass beads) and non‐spherical particles (SiC) are presented. The PSDs are resolved in 30 intervals within a particle size range from 1–1000 μm, employing a modified Chahine inversion algorithm. So far, the measurements are limited to moderate particle concentrations. Some influences affecting the measurements, especially for higher particle concentrations, are discussed in detail. 相似文献
143.
J. J. Rehr 《Foundations of Physics》2003,33(12):1735-1742
The Golden rule expression for x-ray absorption spectra (XAS) is typically calculated within a one-particle (quasiparticle) approximation and generally leads to good agreement between theory and experiment. The fact that a quasiparticle approximation works fairly well is surprising, since it neglects satellite excitations and intrinsic losses due to a suddenly created core-hole. The resolution of this paradox requires physics beyond the independent particle approximation. This is discussed here using an effective Green's function formulation based on a quasi-boson model that takes interference between inelastic losses into account. This approach shows that inelastic excitations such as multi-electron excitations tend to be suppressed, and that the XAS is given by a broadened quasiparticle particle approximation, together with weak satellite structure and edge singularity effects. 相似文献
144.
《Magnetic resonance in chemistry : MRC》2003,41(2):139-142
Bioassay‐guided fractionation of the methanol extract of Odontadenia macrantha afforded a new limonoid, odontadenin A (1) and two known triterpenoids, lupeol (2) and α‐amyrin (3). The structure of 1 was established on the basis of 1D and 2D NMR and high‐resolution fast atom bombardment mass spectrometric data. The new compound was found to possess moderate cytotoxicity against A2780, the ovarian cancer cell line. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献
145.
Phenethylammonium-based perovskites, which can be regarded as a semiconductor/insulator multiple quantum well consisting of lead halide semiconductor layers sandwiched between phenethylammonium insulator layers were prepared. To investigate the effects of the electronic state and the orientation of organic insulator layers on the optical properties of layered perovskites, fluorine substituted analogues were also prepared. The structure and optical properties were investigated by the XRD, UV–Vis absorption, and fluorescence measurements. The exciton absorption peak was shifted by the substitution of fluorine atoms in organic ammonium compounds. It became clear that the optical properties of two-dimensional perovskite compounds were controlled by the substitution of fluorine atoms. 相似文献
146.
147.
J. L. Gardea-Torresdey K. J. Tiemann J. R. Peralta-Videa J. G. Parsons M. Delgado 《Microchemical Journal》2004,76(1-2):65
Traditional treatment methods used to clean-up heavy metal contamination of soils and waters are cost intensive whereas more cost effective methods need to be developed. The use of plant materials to remediate heavy contamination has been studied for the past two decades. This technique has shown much promise for many of the common heavy metal contaminants, but few studies have focused on the lanthanide series elements. By investigating the binding and interactions of the lanthanide elements to alfalfa biomass, a more complete understanding of the binding mechanisms and the interactions of heavy metals with biomaterials can be obtained. Different chemical functional groups on the alfalfa biomass, carboxyl, amino, sulfur, and ester groups, were modified to investigate the binding mechanisms of erbium(III) and holmium(III). Batch experiments were performed with native and chemically modified alfalfa biomass suggesting that the carboxyl groups play a major role in the binding of erbium(III) and holmium(III) to the alfalfa biomass. In addition, X-ray absorption spectroscopy (XAS) studies corroborated the data obtained from the batch experiments. 相似文献
148.
G. Palfinger B. Bitnar H. Sigg E. Müller S. Stutz D. Grützmacher 《Physica E: Low-dimensional Systems and Nanostructures》2003,16(3-4):481
In order to obtain a low band gap photocell based on the widely spread silicon technology, e.g. for thermophotovoltaics, SiGe nanostructures can be introduced into a monocrystalline silicon photocell. Beforehand, it is necessary to know the absorption coefficient of the SiGe quantum wells. On a silicon (1 0 0) substrate multiple Si/SiGe quantum well structures were grown by UHV-CVD. The Ge concentration and the well width were used as growth parameters. To obtain significant absorption, the experiment was set up to allow for 200 internal reflections.The total reflection of the light results in a standing electromagnetic wave. The absorption coefficient was obtained from the experimental data taking the geometry and the electric field distribution in the absorbing layer into account. The influence of well width and germanium content on the absorption was investigated with the goal of maximizing the absorption for photons with energies below the band gap energy of silicon. The measurement results are compared with a theoretical model, which takes the band structure of strained SiGe including confinement effects into account. 相似文献
149.
150.
Summary The potential of liquid chromatography-mass spectrometry (LC-MS) has been studied for the simultaneous determination of sixteen carbamate and organophosphorus pesticides in honeybees using a traditional sample preparation protocol based on acetone extraction and dichloromethane partitioning. The performances of both atmospheric pressure chemical ionization (APCI) and electrospray (ES) interfaces were compared. APCI offered better sensitivity and specificity for a higher range of pesticides. Limits of quantification were from 0.01 to 0.17 mg kg–1, at which recoveries obtained were between 64 and 93%, except for pirimicarb that was at 13%, with relative standard deviations ranging from 7 to 20%. Fenitrothion, fenoxycarb, methiocarb and phoxim were found in bees from Valencian Community beehives at concentrations between 0.03 and 3.75 mg kg–1. 相似文献