全文获取类型
收费全文 | 192279篇 |
免费 | 2760篇 |
国内免费 | 801篇 |
专业分类
化学 | 104516篇 |
晶体学 | 2580篇 |
力学 | 8269篇 |
综合类 | 16篇 |
数学 | 20920篇 |
物理学 | 59539篇 |
出版年
2021年 | 1315篇 |
2020年 | 1428篇 |
2019年 | 1386篇 |
2018年 | 1489篇 |
2017年 | 1482篇 |
2016年 | 2858篇 |
2015年 | 2347篇 |
2014年 | 2984篇 |
2013年 | 8517篇 |
2012年 | 7397篇 |
2011年 | 9321篇 |
2010年 | 5573篇 |
2009年 | 5645篇 |
2008年 | 8403篇 |
2007年 | 8376篇 |
2006年 | 8007篇 |
2005年 | 7964篇 |
2004年 | 7242篇 |
2003年 | 6005篇 |
2002年 | 5601篇 |
2001年 | 6502篇 |
2000年 | 5049篇 |
1999年 | 3744篇 |
1998年 | 2837篇 |
1997年 | 2845篇 |
1996年 | 2888篇 |
1995年 | 2535篇 |
1994年 | 2429篇 |
1993年 | 2320篇 |
1992年 | 2615篇 |
1991年 | 2519篇 |
1990年 | 2195篇 |
1989年 | 2174篇 |
1988年 | 2181篇 |
1987年 | 2100篇 |
1986年 | 2012篇 |
1985年 | 2952篇 |
1984年 | 2913篇 |
1983年 | 2349篇 |
1982年 | 2536篇 |
1981年 | 2418篇 |
1980年 | 2366篇 |
1979年 | 2299篇 |
1978年 | 2382篇 |
1977年 | 2316篇 |
1976年 | 2233篇 |
1975年 | 2214篇 |
1974年 | 2145篇 |
1973年 | 2211篇 |
1972年 | 1239篇 |
排序方式: 共有10000条查询结果,搜索用时 328 毫秒
961.
In this paper we define the vertex-cover polynomial Ψ(G,τ) for a graph G. The coefficient of τr in this polynomial is the number of vertex covers V′ of G with |V′|=r. We develop a method to calculate Ψ(G,τ). Motivated by a problem in biological systematics, we also consider the mappings f from {1, 2,…,m} into the vertex set V(G) of a graph G, subject to f−1(x)f−1(y)≠ for every edge xy in G. Let F(G,m) be the number of such mappings f. We show that F(G,m) can be determined from Ψ(G,τ). 相似文献
962.
963.
K. C. Nicolaou Dionisios Vourloumis Tianhu Li Joaquin Pastor Nicolas Winssinger Yun He Sacha Ninkovic Francisco Sarabia Hans Vallberg Frank Roschangar N. Paul King M. Ray V. Finlay Pareskevi Giannakakou Pascal Verdier-Pinard Ernest Hamel 《Angewandte Chemie (International ed. in English)》1997,36(19):2097-2103
964.
Il Nuovo Cimento D - The applicability of the Kosterlitz-Thouless theory to HTSC is examined. The model initially presented in its general lines for bidimensional systems is then applied to... 相似文献
965.
Cornelis Van de Panne 《The Journal of the Operational Research Society》1992,43(12):1159-1171
The paper deals with the appropriate form of interaction between two refineries with different demand patterns. This problem can be formulated as finding a decentralized solution of linear programming problems linked by buying and selling activities. The complete problem is first solved for central values of product demands and costs and revenues. The structure of the basis then determines the organization of the interaction in terms of which unit sets quantities and which prices, or whether centralized decisions should be made. If, for expected values of product demand and costs, the structure of the basis is the same then the related organization of trading can be used for day-to-day transactions. For a well-known oil refinery model it is found that, for fairly large demand variations, decentralized interaction is effective, but that the structure of the basis changes easily with crude price variations, and that simulations did not converge for these variations. 相似文献
966.
S. Coffa D. C. Jacobson J. M. Poate F. Priolo 《Applied Physics A: Materials Science & Processing》1992,54(6):481-484
We have investigated ion-beam-enhanced diffusion of Au in undoped and B doped amorphous Si. The diffusion coefficients depend linearly on ion flux and exibit an Arrhenius-like temperature dependence with an activation energy of 0.37 eV in the temperature range 200–350° C. Moreover the diffusivity is enhanced by a factor of 5 by B-doping at a concentration of 1×1020 atoms/cm3. A similar enhancement is observed in thermal diffusion of Au which has an activation energy of 1.5 eV. On the basis of these results a model for the ion-beam-enhanced diffusion of Au is proposed where the high density of defects present in amorphous Si act as traps for the fast moving interstitial Au atoms. The effectiveness of this trapping process can be changed by the high concentration of mobile defects generated by the beam and also by a change in the charge state of the traps induced by the presence of B. 相似文献
967.
M. A. Macrì C. Del Gratta L. Di Donato S. Di Luzio G. L. Romani S. Della Penna A. Pasquarelli 《Il Nuovo Cimento D》1994,16(5):425-432
Summary A preliminary study is here reported on a new potential marker for biomagnetic measurements. The marker consists of superparamagnetic
polymer microspheres which were detected in the presence of external steady magnetic fields by means of an r.f.-SQUID magnetometer.
The particles were prepared in samples differing in the concentration value and immersed in a homogeneous magnetic field of
variable intensity. A simple model was taken into account for the distribution of the microspheres in the samples, so that
the theoretical values were compared to the marker field values measured by the biomagnetic sensor. The overall sensitivity
of the experimental apparatus and the minimum concentration value of the marker were then estimated. 相似文献
968.
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
969.
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
970.
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