全文获取类型
收费全文 | 184944篇 |
免费 | 2685篇 |
国内免费 | 845篇 |
专业分类
化学 | 101551篇 |
晶体学 | 2570篇 |
力学 | 8062篇 |
综合类 | 7篇 |
数学 | 20041篇 |
物理学 | 56243篇 |
出版年
2021年 | 1121篇 |
2020年 | 1303篇 |
2019年 | 1294篇 |
2018年 | 1313篇 |
2017年 | 1302篇 |
2016年 | 2609篇 |
2015年 | 2144篇 |
2014年 | 2765篇 |
2013年 | 8387篇 |
2012年 | 6901篇 |
2011年 | 8807篇 |
2010年 | 5302篇 |
2009年 | 5416篇 |
2008年 | 8020篇 |
2007年 | 8038篇 |
2006年 | 7792篇 |
2005年 | 7279篇 |
2004年 | 6547篇 |
2003年 | 5659篇 |
2002年 | 5549篇 |
2001年 | 6207篇 |
2000年 | 4735篇 |
1999年 | 3615篇 |
1998年 | 2779篇 |
1997年 | 2793篇 |
1996年 | 2772篇 |
1995年 | 2427篇 |
1994年 | 2315篇 |
1993年 | 2150篇 |
1992年 | 2471篇 |
1991年 | 2441篇 |
1990年 | 2140篇 |
1989年 | 2095篇 |
1988年 | 2155篇 |
1987年 | 2080篇 |
1986年 | 1959篇 |
1985年 | 2950篇 |
1984年 | 2952篇 |
1983年 | 2351篇 |
1982年 | 2604篇 |
1981年 | 2472篇 |
1980年 | 2426篇 |
1979年 | 2360篇 |
1978年 | 2422篇 |
1977年 | 2365篇 |
1976年 | 2293篇 |
1975年 | 2253篇 |
1974年 | 2169篇 |
1973年 | 2247篇 |
1972年 | 1263篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
901.
Summary Glass formation in nature and materials science is reviewed and the recent recognition of polymorphism within the glassy state,
polyamorphism, is discussed. The process by which the glassy state originates during the continuous cooling or viscous slowdown
process, is examined and the three canonical characteristics of relaxing liquids are correlated through the fragility. The
conversion of strong liquids to fragile liquids by pressure-induced coordination number increases is discussed, and then it
is shown that for the same type of system it is possible to have the same conversion accomplished via a first-order transition
within the liquid state. The systems in which this can happen are of the same type which exhibit polyamorphism, and the whole
phenomenology can be accounted for by a recent simple modification of the van der Waals model for tetrahedrally bonded liquids.
The concept of complex amorphous systems which can lose a significant number of degrees of freedom through weak first-order
transitions is then used to discuss the relation between native and denatured hydrated proteins, since the latter have much
in common with plasticized chain polymer systems. Finally, we close the circle by taking a short-time-scale phenomenon given
much attention by protein physicists,viz., the onset of an anomaly in the Debye-Waller factor with increasing temperature, and showing that for a wide variety of liquids,
including computer-simulated strong and fragile ionic liquids, this phenomenon is closely correlated with the experimental
glass transition temperature. This implies that the latter owes its origin to the onset of strong anharmonicity in certain
components of the vibrational density of states (evidently related to the boson peak) which then permits the system to gain
access to its configurational degrees of freedom. The more anharmonic these vibrational components, the closer to the Kauzmann
temperature will commence the exploration of configuration space and, for a given configurational microstate degeneracy, the
more fragile the liquid will be.
Paper presented at the I International Conference on Scaling Concepts and Complex Fluids, Copanello, Italy, July 4–8, 1994. 相似文献
902.
The processing of oil sands materials and upgrading of bitumens present a variety of problems for conventional technologies. This article describes a preliminary study of the concept of using microwave induced catalytic techniques to decompose the complex and viscous hydrocarbon compounds contained in these materials to allow efficient extraction of volatile and economically useful organic products such as C2 and C3 hydrocarbons. Addition of water to the tar sand material prior to reaction facilitated separation from the sand component as well as promoting the formation of economically very useful, liquid, oxygenated hydrocarbon products. 相似文献
903.
J. C. S. Moraes E. M. Telles F. C. Cruz A. Scalabrin D. Pereira C. Garelli N. Ioli A. Moretti F. Strumia 《International Journal of Infrared and Millimeter Waves》1991,12(12):1475-1486
We report the discovery of 57 new fir laser lines from13CD3OH molecule optically pumped by a waveguide CO2 laser of 300 MHz tunability. For all lines, precise frequency offset measurements between the CO2 line center and the center of the absorbing13CD3OH line were performed using the transferred Lamb-Dip technique. We have also measured directly the frequency of seven FIR laser lines by heterodyning with already known laser lines. We present a complete list of all known laser lines (134) and frequency measurements (24) for this molecule.Work supported by CNPq, FAPESP, FAEP-Brasil, and CNR-Italia 相似文献
904.
P. C. Roy 《等离子体物理论文集》1992,32(2):101-107
By using the two-point space correlation function an equation for the power spectral density for a random Langmuir field has been derived. The dispersion relation for a monochromatic wave is regained for a delta spectrum. For a Gaussian spectrum, the maximum growth rate is less than that for a monochromatic wave. For a “meander spectrum”, the growth rate is increased with the width of the spectrum in the first stage then decreased for further increase of the width. 相似文献
905.
An analysis of the effects of combined evaporation and quench periods on the formation of asymmetric membranes by the phase inversion technique is presented. The model for the evaporation period assumes binary diffusion and includes composition dependence in the mutual diffusion coefficient and free convection mass transfer in the gas phase. The quench period model includes the use of 4 composition-dependent diffusion coefficients and incorporates a modified from of the interface jump mass balance which enables efficient numerical analysis of composition gradients in the film at the beginning of the quench period. Results for the effects of evaporation-period variable, including: evaporation time, initial cast film thickness, casting surface dimension, and vapor-phase composition, clearly indicate that significant effects of the evaporation period on membrane structure formation and its reproducibility occur within 20 seconds. 相似文献
906.
A. Ross M. Czisch G.C. King 《Journal of magnetic resonance (San Diego, Calif. : 1997)》1997,124(2):355-365
A theoretical approach to calculate the time evolution of magnetization during a CPMG pulse sequence of arbitrary parameter settings is developed and verified by experiment. The analysis reveals that off-resonance effects can cause systematic reductions in measured peak amplitudes that commonly lie in the range 5–25%, reaching 50% in unfavorable circumstances. These errors, which are finely dependent upon frequency offset and CPMG parameter settings, are subsequently transferred into erroneousT2values obtained by curve fitting, where they are reduced or amplified depending upon the magnitude of the relaxation time. Subsequent transfer to Lipari–Szabo model analysis can produce significant errors in derived motional parameters, with τeinternal correlation times being affected somewhat more thanS2order parameters. A hazard of this off-resonance phenomenon is its oscillatory nature, so that strongly affected and unaffected signals can be found at various frequencies within a CPMG spectrum. Methods for the reduction of the systematic error are discussed. Relaxation studies on biomolecules, especially at high field strengths, should take account of potential off-resonance contributions. 相似文献
907.
908.
I. Ortalli G. Pedrazzi K. Jiang X. Zhang C. Carlo-Stella V. Rizzoli 《Il Nuovo Cimento D》1992,14(4):351-358
Summary A new approach to cancer treatment using low-energy gamma-rays is discussed. Cultures of bone marrow mononuclear cells collected
from normal donors and patients with chronic myelogenous leukaemia have been studied after the addition of hematin, at different
concentrations, and irradiation by a M?ssbauer gamma-ray source. Growth stimulation has been observed when hematin is added
to normal bone marrow cultures, while growth inhibition is observed when hematin is added to leukaemic cultures. The effect
becomes then more pronounced when hematin is used in combination with M?ssbauer gamma-rays. 相似文献
909.
We show that it is a NP-complete problem to decide whether a finite poset arises as the (Birkhoff) dual of the Frattini sublattice of some finite distributive lattice.This work was supported in part by Swiss NSF grant 20-32644.91. 相似文献
910.
C. A. Beattie M. B. Boisen Jr. L. W. Johnson 《Journal of Optimization Theory and Applications》1991,68(1):203-203
Corrections to the last two equations of Ref. 1 are given. 相似文献