全文获取类型
收费全文 | 211976篇 |
免费 | 2889篇 |
国内免费 | 863篇 |
专业分类
化学 | 96804篇 |
晶体学 | 2531篇 |
力学 | 7861篇 |
综合类 | 7篇 |
数学 | 19294篇 |
物理学 | 55274篇 |
无线电 | 33957篇 |
出版年
2021年 | 1203篇 |
2020年 | 1321篇 |
2019年 | 1296篇 |
2018年 | 1372篇 |
2017年 | 1356篇 |
2016年 | 2735篇 |
2015年 | 2184篇 |
2014年 | 2914篇 |
2013年 | 8765篇 |
2012年 | 7007篇 |
2011年 | 8836篇 |
2010年 | 5708篇 |
2009年 | 6192篇 |
2008年 | 8904篇 |
2007年 | 9287篇 |
2006年 | 9035篇 |
2005年 | 8493篇 |
2004年 | 7585篇 |
2003年 | 6759篇 |
2002年 | 6543篇 |
2001年 | 7526篇 |
2000年 | 5903篇 |
1999年 | 4724篇 |
1998年 | 3849篇 |
1997年 | 3809篇 |
1996年 | 3708篇 |
1995年 | 3328篇 |
1994年 | 3262篇 |
1993年 | 3178篇 |
1992年 | 3403篇 |
1991年 | 3363篇 |
1990年 | 3005篇 |
1989年 | 2896篇 |
1988年 | 2787篇 |
1987年 | 2367篇 |
1986年 | 2266篇 |
1985年 | 3244篇 |
1984年 | 3186篇 |
1983年 | 2603篇 |
1982年 | 2787篇 |
1981年 | 2676篇 |
1980年 | 2621篇 |
1979年 | 2488篇 |
1978年 | 2580篇 |
1977年 | 2494篇 |
1976年 | 2428篇 |
1975年 | 2397篇 |
1974年 | 2277篇 |
1973年 | 2387篇 |
1972年 | 1427篇 |
排序方式: 共有10000条查询结果,搜索用时 45 毫秒
991.
Within the framework of a simple electrostatic model, as compared to recent experimental results, we here discuss the stability
of very weakly bound molecular negative ions. In contrast with the case of conventional valence anions, the excess electron
is then located in a very diffuse orbital and is mainly bound by electrostatic dipolar, quadrupolar, and polarization forces,
at large distances from the neutral molecular core. By fitting a single repulsion parameter of the model to the available
experimental data, it is possible to make quantitative predictions of the excess-electron binding energies in these species.
Critical values of the dipole moment, quadrupole moment or polarizability required for the observation of stable multipole-bound
negative ions are predicted and compared to available experimental data and ab initio calculations.
Received October 24, 2001; accepted for publication November 16, 2001 相似文献
992.
A system has been developed for tracking the motion of objects in two dimensions in real-time. The system consists of a conventional CCD camera linked to a transputer-based frame grabber and an array of nine transputers. A parallel moments algorithm is used to extract the co-ordinates of the object's centre of gravity and orientation at field rate, i.e. 60 Hz. Since the position data are made available in real-time—with a small time delay—the system has the potential for inclusion in a feedback loop. Results are presented for tracking the trajectory of a chocolate bar diverted by an air jet. The potential of the system for higher sampling rates—up to 200 Hz—is discussed. 相似文献
993.
Several extraction procedures are described allowing arsenic speciation in sediments. The extraction of organometallic compounds
such as dimethylarsinic acid or monomethylarsonic acid is quite simple since these compounds are stable in the different extraction
media (HCl/ HNO3, H3PO4, ammonium oxalate) and are easily released independent of the extraction mode (magnetic stirring or microwave solubilization).
Extraction yields are higher than 96% for these two arsenic forms. An HCl/HNO3 microwave solubilization procedure allows the quantitative solubilization of mineral arsenic, but the differentiation between
the two oxidation states is not possible owing to the oxidation of As(III) to As(V). Extractions with orthophosphoric acid
or ammonium oxalate allow the solubilization of mineral arsenic with extraction yields ranging from 90 to 95% and the differentiation
between As(III) and As(V). Nevertheless, the amount of As(III) is underestimated owing to its partial oxidation. The usefulness
and advantages of microwave solubilization compared with conventional extraction procedures are discussed.
Received: 17 May 1996 / Revised: 19 September 1996 / Accepted: 25 September 1996 相似文献
994.
This paper develops a method for simultaneously designing the power stage and controller for a switching power supply. The method utilizes a numerical optimization procedure, which facilitates computer-aided design. It is found that better performance can be achieved than with a traditional two-step design process, where the power stage and controller are designed sequentially. Optimization and simulation results for a buck power converter are presented to illustrate the design process and benefits 相似文献
995.
A. Kussmaul S. Vernon P. C. Colter R. Sudharsanan A. Mastrovito K. J. Linden N. H. Karam N. H. Karam S. C. Warnick M. A. Dahleh 《Journal of Electronic Materials》1997,26(10):1145-1153
We have used spectroscopic ellipsometry to perform real-time monitoring during metalorganic chemical vapor deposition growth
of AlGaAs (on GaAs) and InGaAs (on GaAs and InP). Optical constants for these materials were obtained up to growth temperatures
of 600 to 700°C. This information permits real-time extraction of composition and layer thickness from the raw ellipsometric
data at sample rates on the order of 0.5 Hz. We describe closed-loop control of composition and total layer thickness on AlGaAs-based
structures, including Bragg reflectors. In-situ data obtained on double-heterostructure quantum-well laser structures demonstrate that spectroscopic ellipsometry is an extremely
powerful monitoring and quality-control tool, giving important real-time information on complex structures that would be difficult
and time-consuming to obtain after growth. 相似文献
996.
The authors consider the problem of detecting visual evoked potentials (VEP's). A matched subspace filter is applied to the detection of the VEP and is demonstrated to perform better than a number of other evoked potential detectors. Unlike single-harmonic detectors, the matched subspace filter (MSF) detector is suitable for detecting multiharmonic VEP's. Moreover, the MSF is optimal in the uniformly most powerful sense for multiharmonic signals with unknown noise variance 相似文献
997.
Summary Hydroxypropyl methyl cellulose (HPMC) and polyethylene glycol 400 (PEG 400) content in HypoTears™ Daily Dose ophthalmic solution
are determined simultaneously by size exclusion chromatography. The retention times of HPMC and PEG 400 are 10.6 and 15.4
minutes, respectively. The method requires minimal sample pretreatment and is accurate and reproducible. The peak area response
from a refractive index detector versus HPMC and PEG 400 concentration is linear over the range of 50–150 % of their label
claims of 2.5 mg/mL and 10 mg/mL, respectively. The mean absolute recoveries of HPMC and PEG 400 at their label claim using
the described method are 98.9±1.3 % and 100.4±1.2 % (mean±SD, n=12), respectively. 相似文献
998.
This paper presents a few novel results, and collects together what is known and conjectured about the branching graph of a polyhex. 相似文献
999.
G.Biju Kumar Hetal.V Patel Amrish C Shah Markus Trenkle Christine J Cardin 《Tetrahedron: Asymmetry》1996,7(12):3391-3396
N-Arylsulfonamides of (R)- and (S)-2-amino-1-butanol, on condensation with aromatic aldehydes produced diastereomerically pure 2-aryl-3-arenesulfonyl 4-ethyl-1,3-oxazolidines. The absolute configurations of one enantiomeric pair have been determined from two fully refined X-ray structures, supplemented by nmr data. 相似文献
1000.
S. A. J. Wiegers B. Tieke U. Zeitler R. Fletcher A. K. Geim J. C. Maan M. Henini 《Czechoslovak Journal of Physics》1996,46(5):2461-2462
We have investigated the interaction of phonons with a 2DEG in the FQH regime with phonon drag thermoelectric power (TEP). We find that the TEP at filling factors with the same even denominator is identical and at other even denominator filling factors they differ only by a constant. Assuming these states to be Composite Fermions (CF), we can explain our observations by extending a zero magnetic field theory for phonon drag to the CF-phonon interaction. This analysis is further corroborated by the observed T4 dependence of the CF TEP. 相似文献