全文获取类型
收费全文 | 3580篇 |
免费 | 42篇 |
国内免费 | 20篇 |
专业分类
化学 | 2275篇 |
晶体学 | 17篇 |
力学 | 94篇 |
数学 | 738篇 |
物理学 | 518篇 |
出版年
2020年 | 30篇 |
2019年 | 24篇 |
2018年 | 29篇 |
2017年 | 27篇 |
2016年 | 31篇 |
2015年 | 37篇 |
2014年 | 58篇 |
2013年 | 196篇 |
2012年 | 144篇 |
2011年 | 160篇 |
2010年 | 91篇 |
2009年 | 94篇 |
2008年 | 174篇 |
2007年 | 172篇 |
2006年 | 189篇 |
2005年 | 172篇 |
2004年 | 157篇 |
2003年 | 130篇 |
2002年 | 122篇 |
2001年 | 54篇 |
2000年 | 50篇 |
1999年 | 39篇 |
1998年 | 43篇 |
1997年 | 54篇 |
1996年 | 58篇 |
1995年 | 41篇 |
1994年 | 43篇 |
1993年 | 47篇 |
1992年 | 56篇 |
1991年 | 45篇 |
1990年 | 34篇 |
1989年 | 35篇 |
1988年 | 29篇 |
1987年 | 38篇 |
1986年 | 25篇 |
1985年 | 68篇 |
1984年 | 67篇 |
1983年 | 47篇 |
1982年 | 62篇 |
1981年 | 50篇 |
1980年 | 60篇 |
1979年 | 64篇 |
1978年 | 67篇 |
1977年 | 61篇 |
1976年 | 49篇 |
1975年 | 54篇 |
1974年 | 52篇 |
1973年 | 57篇 |
1971年 | 18篇 |
1969年 | 17篇 |
排序方式: 共有3642条查询结果,搜索用时 15 毫秒
61.
M. D. Cohler D. L. Watson R. Wadsworth S. M. Lane M. J. Smithson Ronald E. Brown J. -C. Peng N. Stein J. W. Sunier D. M. Drake 《Zeitschrift für Physik A Hadrons and Nuclei》1984,319(1):107-109
A measurement of the mass excess of110Pd and114Pd has been made by detecting emergent16O ions in aQ3D magnetic spectrometer using the112Cd(14C,16O)110Pd and116Cd(14C,16O)114Pd reactions at 60 MeV. The results are compared with theoretical predictions. 相似文献
62.
The authors introduce a new concept of measure-valued solution for the 3-D incompressible Euler equations in order to incorporate the complex phenomena present in limits of approximate solutions of these equations. One application of the concepts developed here is the following important result: a sequence of Leray-Hopf weak solutions of the Navier-Stokes equations converges in the high Reynolds number limit to a measure-valued solution of 3-D Euler defined for all positive times. The authors present several explicit examples of solution sequences for 3-D incompressible Euler with uniformly bounded local kinetic energy which exhibit complex phenomena involving both persistence of oscillations and development of concentrations. An extensions of the concept of Young measure is developed to incorporate these complex phenomena in the measure-valued solutions constructed here.Partially supported by N.S.F. GrantPartially supported by N.S.F. Grant 84-0223 and 86-11110 相似文献
63.
Sharon Peled Ching-Hua Tseng Aaron A. Sodickson Ross W. Mair Ronald L. Walsworth David G. Cory 《Journal of magnetic resonance (San Diego, Calif. : 1997)》1999,140(2):320-324
A single-shot pulsed gradient stimulated echo sequence is introduced to address the challenges of diffusion measurements of laser polarized 3He and 129Xe gas. Laser polarization enhances the NMR sensitivity of these noble gases by >10(3), but creates an unstable, nonthermal polarization that is not readily renewable. A new method is presented which permits parallel acquisition of the several measurements required to determine a diffusive attenuation curve. The NMR characterization of a sample's diffusion behavior can be accomplished in a single measurement, using only a single polarization step. As a demonstration, the diffusion coefficient of a sample of laser-polarized 129Xe gas is measured via this method. 相似文献
64.
The assumption that the same universal scale of time applies for different physical laws depends uponthe hypothesis that there is a unique universal scale of time in the universe. Intrinsic to that hypothesis is the assumption that if extraneous factors that affect physical clocks and processes that run on different physical principles could be removed, all clocks are synchronizable with one another. The consequences of that hypothesis not being true are considered. It was found thatclock asynchrony implies that: (1) a mass variation theory is probable; (2) there is a need for multiple clocks; and (3) t in the equations of physics is referring to clocks rather than time such that the properties of time are reduced to its direction (as characterized by thermodynamics and cosmology). 相似文献
65.
During phonation, air pressures act upon the vocal folds to help maintain their oscillation. The air pressures vary dynamically along the medial surface of the vocal folds, although no live human or excised studies have shown how those pressure profiles vary in time. The purpose of this study was to examine time-dependent glottal pressure profiles using a canine hemilarynx approach. The larynx tissue was cut in the midsaggital plane from the top to about 5 mm below the vocal folds. The right half was replaced with a Plexiglas pane with imbedded pressure taps. Simultaneous recordings were made of glottal pressure signals, subglottal pressure, particle velocity, and average airflow at various levels of adduction. The data indicate that the pressures in the glottis (on the Plexiglas) vary both vertically and longitudinally throughout the phonatory cycle. Pressures vary most widely near the location of maximum vibratory amplitude, and can include negative pressures during a portion of the cycle. Pressures anterior and posterior to the maximum amplitude location may have less variation and may remain positive throughout the cycle, giving rise to a new concept called dynamic bidirectional pressure gradients in the glottis. This is an important concept that may relate strongly to tissue health as well as basic oscillatory mechanics. 相似文献
66.
We show that the evolution equations for a perfect fluid coupled to general relativity in a general lapse and shift, are Hamiltonian relative to a certain Poisson structure. For the fluid variables, a Lie-Poisson structure associated to the dual of a semi-direct product Lie algebra is used, while the bracket for the gravitational variables has the usual canonical symplectic structure. The evolution is governed by a Hamiltonian which is equivalent to that obtained from a canonical analysis. The relationship of our Hamiltonian structure with other approaches in the literature, such as Clebsch potentials, Lagrangian to Eulerian transformations, and its use in clarifying linearization stability, are discussed.Research supported in part by NSF grant MCS 81-08814(A02)Research supported in part by NSF grant MCS 81-07086 相似文献
67.
Adams GJ Greene J Vick GW Harrist R Kimball KT Karmonik C Ballantyne CM Insull W Morrisett JD 《Magnetic resonance imaging》2004,22(9):1249-1258
BACKGROUND AND PURPOSE: Magnetic resonance imaging (MRI) can accurately and reproducibly measure the volume of atherosclerotic plaque in human carotid arteries. Atherosclerotic plaques may either progress or regress over time, depending on individual risk factors and treatment regimens. This study was designed to determine if regression or progression of human carotid atherosclerosis in patients receiving statin therapy over 24 months can be detected by high-resolution MRI. METHODS: In 11 subjects who had undergone unilateral carotid endarterectomy and were on statin therapy, volumes for total carotid artery, concentric wall (normal wall), eccentric wall (plaque), and lumen were quantified at 0, 16 and 24 months using a 1.5-T human imager equipped with 6-cm phased array coils. RESULTS: The interobserver mean coefficient of variation (CV) was lowest for the lumen volume (3.1%) and highest for the plaque volume (9.8%). The interscan mean CV was lowest for the total artery volume (3.2%) and highest for the plaque volume (9.9%). As much as 26% regression and 35% progression were observed in individual subject's carotid artery eccentric wall (plaque) volumes over time. Mean eccentric wall volume increased 5% by 16 months and 8% by 24 months. Mean total wall volume increased slightly at both 16 and 24 months (+1.2% and +1.8%). CONCLUSIONS: High-resolution MRI provides a noninvasive reproducible method of tracking changes in carotid atherosclerosis. This pilot study detected changes in individual subjects at both 16 and 24 months. MRI tracking of changes in atherosclerotic plaques should prove useful in assessing vascular disease risk and monitoring the efficacy of interventions designed to induce regression or retard progression. 相似文献
68.
It has been known for over six decades that the dissolution of minute amounts of high molecular weight polymers in wall-bounded turbulent flows results in a dramatic reduction in turbulent skin friction by up to 70%. First principles simulations of turbulent flow of model polymer solutions can predict the drag reduction (DR) phenomenon. However, the essential dynamical interactions between the coherent structures present in turbulent flows and polymer conformation field that lead to DR are poorly understood. We examine this connection via dynamical simulations that track the evolution of hairpin vortices, i.e., counter-rotating pairs of quasistreamwise vortices whose nonlinear autogeneration and growth, decay and breakup are centrally important to turbulence stress production. The results show that the autogeneration of new vortices is suppressed by the polymer stresses, thereby decreasing the turbulent drag. 相似文献
69.
Jiankun Shao Rishav Choudhary David F. Davidson Ronald K. Hanson Samuel Barak Subith Vasu 《Proceedings of the Combustion Institute》2019,37(4):4555-4562
The need for more efficient power cycles has attracted interest in super-critical CO2 (sCO2) cycles. However, the effects of high CO2 dilution on auto-ignition at extremely high pressures has not been studied in depth. As part of the effort to understand oxy-fuel combustion with massive CO2 dilution, we have measured shock tube ignition delay times (IDT) for methane/O2/CO2 mixtures and hydrogen/O2/CO2 mixtures using sidewall pressure and OH* emission near 306?nm. Ignition delay time was measured in two different facilities behind reflected shock waves over a range of temperatures, 1045–1578?K, in different pressures and mixture regimes, i.e., CH4/O2/CO2 mixtures at 27–286 atm and H2/O2/CO2 mixtures at 37–311 atm. The measured data were compared with the predictions of two recent kinetics models. Fair agreement was found between model and experiment over most of the operating conditions studied. For those conditions where kinetic models fail, the current ignition delay time measurements provide useful target data for development and validation of the mechanisms. 相似文献
70.
G. Manasa Arvind K. Bhakta Zineb Mekhalif Ronald J. Mascarenhas 《Electroanalysis》2019,31(7):1363-1372
Increased concern over the risk resorcinol (RS) pose to ecology and humans, led to its position in European Union Category 1 list of endocrine disruptors. Legal measures restricted RS utilization and hence crucial to monitor its levels in the environment. Herein we report development of highly efficient and economically viable electrochemical sensor for quantitative determination of RS based on 77Maghemite/MultiWall Carbon Nanotube (M/MWCNT) modified carbon paste electrode. M/MWCNT was synthesized via strategic IR irradiation for the first time, a promising approach to overcome other complicated chemical routes. Powder X‐ray diffraction (PXRD), Transmission electron microscopy (TEM), Field emission scanning electron microscopy (FESEM) and Energy dispersive X‐ray (EDX) were used for characterization. Using Differential Pulse Voltammetry (DPV), we report the lowest detection limit at 0.02 μM. The potential application of the sensor was accomplished as a result of excellent recoveries made from real samples fortified with RS. Results indicated the proficiency of the sensor reliable for rapid, onsite monitoring of RS water contamination and in biological matrices. 相似文献