全文获取类型
收费全文 | 7923篇 |
免费 | 272篇 |
国内免费 | 35篇 |
专业分类
化学 | 6449篇 |
晶体学 | 54篇 |
力学 | 99篇 |
数学 | 926篇 |
物理学 | 702篇 |
出版年
2024年 | 10篇 |
2023年 | 96篇 |
2022年 | 410篇 |
2021年 | 430篇 |
2020年 | 237篇 |
2019年 | 236篇 |
2018年 | 198篇 |
2017年 | 164篇 |
2016年 | 316篇 |
2015年 | 230篇 |
2014年 | 318篇 |
2013年 | 488篇 |
2012年 | 533篇 |
2011年 | 581篇 |
2010年 | 392篇 |
2009年 | 298篇 |
2008年 | 481篇 |
2007年 | 455篇 |
2006年 | 421篇 |
2005年 | 323篇 |
2004年 | 279篇 |
2003年 | 204篇 |
2002年 | 183篇 |
2001年 | 98篇 |
2000年 | 67篇 |
1999年 | 48篇 |
1998年 | 55篇 |
1997年 | 43篇 |
1996年 | 55篇 |
1995年 | 39篇 |
1994年 | 34篇 |
1993年 | 38篇 |
1992年 | 32篇 |
1991年 | 33篇 |
1990年 | 25篇 |
1989年 | 32篇 |
1988年 | 21篇 |
1987年 | 22篇 |
1986年 | 28篇 |
1985年 | 45篇 |
1984年 | 24篇 |
1983年 | 17篇 |
1982年 | 29篇 |
1981年 | 21篇 |
1980年 | 22篇 |
1979年 | 12篇 |
1978年 | 6篇 |
1977年 | 9篇 |
1976年 | 9篇 |
1932年 | 5篇 |
排序方式: 共有8230条查询结果,搜索用时 12 毫秒
71.
Ultrasound assisted enhancement in natural dye extraction from beetroot for industrial applications and natural dyeing of leather 总被引:1,自引:0,他引:1
Venkatasubramanian Sivakumar J. Lakshmi Anna J. Vijayeeswarri G. Swaminathan 《Ultrasonics sonochemistry》2009,16(6):782-789
There is a growing demand for eco-friendly/non-toxic colorants, specifically for health sensitive applications such as coloration of food and dyeing of child textile/leather garments. Recently, dyes derived from natural sources for these applications have emerged as an important alternative to potentially harmful synthetic dyes and pose need for suitable effective extraction methodologies. The present paper focus on the influence of process parameters for ultrasound assisted leaching of coloring matter from plant materials. In the present work, extraction of natural dye from beetroot using ultrasound has been studied and compared with static/magnetic stirring as a control process at 45 °C. The influence of process parameters on the extraction efficiency such as ultrasonic output power, time, pulse mode, effect of solvent system and amount of beetroot has been studied. The use of ultrasound is found to have significant improvement in the extraction efficiency of colorant obtained from beetroot. Based on the experiments it has been found that a mixture of 1:1 ethanol–water with 80 W ultrasonic power for 3 h contact time provided better yield and extraction efficiency. Pulse mode operation may be useful in reducing electrical energy consumption in the extraction process. The effect of the amount of beetroot used in relation to extraction efficiency has also been studied. Two-stage extraction has been studied and found to be beneficial for improving the yield for higher amounts of beetroot. Significant 8% enhancement in % yield of colorant has been achieved with ultrasound, 80 W as compared to MS process both using 1:1 ethanol–water. The coloring ability of extracted beet dye has been tested on substrates such as leather and paper and found to be suitable for dyeing. Ultrasound is also found to be beneficial in natural dyeing of leather with improved rate of exhaustion. Both the dyed substrates have better color values for ultrasonic beet extract as inferred from reflectance measurement. Therefore, the present study clearly offers efficient extraction methodology from natural dye resources such as beetroot with ultrasound even dispensing with external heating. Thereby, also making eco-friendly non-toxic dyeing of fibrous substances a potential viable option. 相似文献
72.
Macrì MA Garreffa G Giove F Guardati M Ambrosini A Colonnese C Maraviglia B 《Magnetic resonance imaging》2004,22(10):1385-1393
A quantitative analysis of cerebellar metabolites in normal subjects has been performed by proton MR spectroscopy (MRS) with relaxation time correction. Quantitation was carried out in seven healthy human subjects with the well-established LCModel program. The prior knowledge utilized for quantitation was obtained from solutions containing the major brain metabolites and MRS investigated under the same experimental conditions. The tissue water signal was used as an internal standard for the in vivo studies. Both in vitro (for the prior knowledge template) and in vivo data were acquired separately at 1.5 T by PRESS sequence (TR, 1500 ms; TE, 30 ms). The absolute concentration of main cerebellar metabolites was corrected for relaxation time effects. Different noise and line broadening conditions were considered and simulated in the spectral processing in order to evaluate the effect of spectral quality on the concentration estimates. 相似文献
73.
Nonlinear stimulation of the vorticity mode caused by losses in the momentum of sound in a chemically reacting gas is considered.
The instantaneous dynamic equation for the vorticity mode is derived. It includes a quadratic nonlinear acoustic source, which
reflects the fact that the reason for the interaction between sound and the vorticity mode is nonlinear. Both periodic and
aperiodic sound may be considered as the origin of the vorticity flow. The equation governing the mean flow (the acoustic
streaming) in the field of periodic sound is also derived. In the non-equilibrium regime of a chemical reaction, there may
exist streaming vortices whose direction of rotation is opposite to that of the vortices in the standard thermoviscous flows.
For periodic sound, this is illustrated by an example. The theory and the example describe both equilibrium and non-equilibrium
chemical reactions. 相似文献
74.
Chen J De Angelis AA Mandelshtam VA Shaka AJ 《Journal of magnetic resonance (San Diego, Calif. : 1997)》2003,162(1):74-89
An efficient way to treat two-dimensional (2D) constant-time (CT) NMR data using the filter diagonalization method (FDM) is presented. In this scheme a pair of N- and P-type data sets from a 2D CT NMR experiment are processed jointly by FDM as a single data set, twice as large, in which the signal effectively evolves in time for twice as long. This scheme is related to "mirror-image" linear prediction, but with the distinction that the data are directly used, without any preprocessing such as Fourier transformation along one dimension, or point-by-point reflection. As the signal has nearly perfect Lorentzian line shape in the CT dimension, it can be efficiently handled by the FDM approach. Applied to model and experimental signals, the scheme shows significant resolution improvement, and appears to tolerate noise reasonably well. Other complex aspects of multidimensional FDM are discussed and illustrated. 相似文献
75.
We examine the contribution of pairing fluctuations to the superfluid order parameter for harmonically trapped atomic Fermi gases in the BCS regime. In the limit of small systems we consider, both analytically and numerically, their space and temperature dependence. We predict a parity effect, i.e., that pairing fluctuations show a maximum or a minimum at the center of the trap, depending on the value of the last occupied shell being even or odd. We propose to detect pairing fluctuations by measuring the density-density correlation function after a ballistic expansion of the gas. 相似文献
76.
Anna Jenčová 《International Journal of Theoretical Physics》2004,43(7-8):1635-1649
We use a class of generalized relative entropies on density matrices to obtain one-parameter families of torsion-free affine connections. 相似文献
77.
A.?N.?WilsonEmail author G.?D.?Dracoulis A.?P.?Byrne P.?M.?Davidson G.?J.?Lane R.?M.?Clark P.?Fallon A.?G?rgen A.?O.?Macchiavelli D.?Ward 《The European Physical Journal A - Hadrons and Nuclei》2005,24(2):179-183
A superdeformed band has been observed in the N = 108 isotope 190Pb. This is the most neutron-deficient Pb isotope in which superdeformed states have been observed. Several theoretical approaches have predicted that N = 108 will mark the limit of observable superdeformation in the Pb isotopes. The band, which consists of five (possibly six) transitions, is observed to feed at least one isomeric level in its decay to the ground state. This decay pattern supports a spin assignment of 10 for the lowest observed level. 相似文献
78.
Pons M Ahufinger V Wunderlich C Sanpera A Braungardt S Sen De A Sen U Lewenstein M 《Physical review letters》2007,98(2):023003
We demonstrate the possibility of realizing a neural network in a chain of trapped ions with induced long range interactions. Such models permit one to store information distributed over the whole system. The storage capacity of such a network, which depends on the phonon spectrum of the system, can be controlled by changing the external trapping potential. We analyze the implementation of error resistant universal quantum information processing in such systems. 相似文献
79.
Anna Wypych-Stasiewicz Adam Szejgis Agnieszka Chmielewska Adam Bald 《Journal of Molecular Liquids》2007,130(1-3):34-37
The electrical conductivities of NaBPh4, NBu4I, NaI, NaCl, NaBr and NaClO4 have been studied in the mixtures of propan-1-ol with water. The obtained results were analysed using the Fuoss–Justice equation. The individual limiting ionic conductivities of Na+, NBu4+, BPh4−, I−, Cl−, Br−, ClO4− ions have been determined using the Fuoss–Hirsch assumption. The dependencies of the limiting molar conductances Λo and Walden products Λoη versus mixed solvent composition have been discussed. 相似文献
80.
Adam Rudziński Anna Tyszka-Zawadzka Paweł Szczepański 《Optical and Quantum Electronics》2007,39(4-6):501-510
The aim of this paper is to present the analysis of influence of defects in 1D photonic crystal (PC) on the density of states
and simultaneously spontaneous emission, in both spatial and frequency domains. In our investigations we use an analytic model
of 1D PC with defects. Our analysis reveals how presence of a defect causes a defect mode to appear. We show that a defect
in 1D PC has local character, being negligible in regions of PC situated far from the defected elementary cell. We also analyze
the effect of multiple defects, which lead to photonic band gap splitting. 相似文献