全文获取类型
收费全文 | 46541篇 |
免费 | 1531篇 |
国内免费 | 19篇 |
专业分类
化学 | 28314篇 |
晶体学 | 242篇 |
力学 | 769篇 |
综合类 | 1篇 |
数学 | 8830篇 |
物理学 | 9935篇 |
出版年
2023年 | 332篇 |
2022年 | 303篇 |
2021年 | 483篇 |
2020年 | 673篇 |
2019年 | 613篇 |
2018年 | 951篇 |
2017年 | 868篇 |
2016年 | 1686篇 |
2015年 | 1390篇 |
2014年 | 1351篇 |
2013年 | 3041篇 |
2012年 | 2889篇 |
2011年 | 2695篇 |
2010年 | 1804篇 |
2009年 | 1519篇 |
2008年 | 2380篇 |
2007年 | 2160篇 |
2006年 | 1932篇 |
2005年 | 1993篇 |
2004年 | 1721篇 |
2003年 | 1436篇 |
2002年 | 1250篇 |
2001年 | 958篇 |
2000年 | 963篇 |
1999年 | 667篇 |
1998年 | 516篇 |
1997年 | 437篇 |
1996年 | 580篇 |
1995年 | 417篇 |
1994年 | 491篇 |
1993年 | 446篇 |
1992年 | 477篇 |
1991年 | 399篇 |
1990年 | 444篇 |
1989年 | 365篇 |
1988年 | 370篇 |
1987年 | 331篇 |
1986年 | 321篇 |
1985年 | 446篇 |
1984年 | 413篇 |
1983年 | 328篇 |
1982年 | 327篇 |
1981年 | 341篇 |
1980年 | 279篇 |
1979年 | 274篇 |
1978年 | 270篇 |
1977年 | 261篇 |
1976年 | 277篇 |
1974年 | 250篇 |
1973年 | 264篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
91.
We consider toy cosmological models in which a classical, homogeneous, spinor field provides a dominant or sub-dominant contribution to the energy-momentum tensor of a flat Friedmann-Robertson-Walker universe. We find that, if such a field were to exist, appropriate choices of the spinor self-interaction would generate a rich variety of behaviors, quite different from their widely studied scalar field counterparts. We first discuss solutions that incorporate a stage of cosmic inflation and estimate the primordial spectrum of density perturbations seeded during such a stage. Inflation driven by a spinor field turns out to be unappealing as it leads to a blue spectrum of perturbations and requires considerable fine-tuning of parameters. We next find that, for simple, quartic spinor self-interactions, non-singular cyclic cosmologies exist with reasonable parameter choices. These solutions might eventually be incorporated into a successful past- and future-eternal cosmological model free of singularities. In an Appendix, we discuss the classical treatment of spinors and argue that certain quantum systems might be approximated in terms of such fields. 相似文献
92.
Lubomír Kubáček 《Mathematica Slovaca》2007,57(6):571-588
The multivariate model, where not only parameters of the mean value of the observation matrix, but also some other parameters
occur in constraints, is considered in the paper. Some basic inference is presented under the condition that the covariance
matrix is either unknown, or partially unknown, or known.
Supported by the grant of the Council of Czech Republic MSM 6 198 959 214. 相似文献
93.
We prove that directly reducible lattices and selfdual subdirectly irreducible lattices of locally finite length are determined by their sublattice-lattices. As a corollary we obtain that splitting varieties are closed under the isomorphism of sublattice-lattices iff they are selfdual. A class of selfdual non-closed varieties is given too. 相似文献
94.
V. D. Fedotov N. P. Obuchov R. A. Zadikhanov J. Spěváček J. Straka 《Applied magnetic resonance》1993,4(4):491-511
The combined analysis of1H and13C NMR relaxation data in solid lysozyme and some typical homopolypeptides was carried out by using “model-free” approach. Three types of relaxation transitions (γ’, γ and β) were revealed in the temperature range investigated. The microdynamical parameters of these motions were determined. From the comparison of these parameters with those of selected synthetic polymers it follows that the molecular motions in proteins and synthetic polymers are of the same nature. All these motions show pronounced anisotropic character. In the investigated temperature range no molecular motions corresponding to α-relaxation (liquid-like) transition were revealed. The hydration effects on parameters of the motions in proteins were considered. The most pronounced effect takes place for β-transition. The effect of Brownian rotation of protein molecule in solution on measured correlation function of local motions was also discussed. 相似文献
95.
J. M. Pico C. P. Menaut J. Fernández J. L. Legido M. I. Paz Andrade 《Journal of solution chemistry》1991,20(1):115-124
Excess molar enthalpies hE at 25 and 35° C and atmospheric pressure, are reported for the binary mixtures formed by a 2-butanone and 2-pentanone with 1-chlorobutane, 1-chloropentane, 1-chlorohexane, or 1-chlorooctane. The hE values for all the mixtures are positive, increasing as the 1-chloroalkane length increases and as the ketone length decreases. Excess molar enthalpies depend slightly on the temperature. The experimental values together with those from the literature were used to calculate the interaction parameters for the Dang-Tassios version of the UNIFAC model.Communicated at the Festsymposium celebrating Dr. Henry V. Kehiaian's 60th birthday, Clermont-Ferrand, France, 17–18 May 1990. 相似文献
96.
Jesús M. Velásquez Bermúdez 《Annals of Operations Research》2002,117(1-4):21-31
This document presents theoretical considerations about the solution of dynamic optimization problems integrating the Benders Theory, the Dynamic Programming approach and the concepts of Control Theory. The so called Generalized Dual Dynamic Programming Theory (GDDP) can be considered as an extension of two previous approaches known as Dual Dynamic Programming (DDP): The first is the work developed by Pereira and Pinto [3–5], which was revised by Velásquez and others [8,9]. The second is the work developed by Read and others [2,6,7]. 相似文献
97.
Arthur Greenberg Oleg P. Charkin Ralph H. Bartram István Hargittai Gyula Beke 《Structural chemistry》1992,3(3):221-224
Unsigned book reviews are by the Book Review Editor. 相似文献
98.
P. I. H. Bastiaens S. G. Mayhew E. M. O'Naulláin A. van Hoek A. J. W. G. Visser 《Journal of fluorescence》1991,1(2):95-103
Both a mode-locked argon-ion laser and synchrotron radiation were used as excitation sources to obtain time-resolved polarized fluorescence of the two FAD cofactors in electron transferring flavoprotein fromMegasphaera elsdenii. Red-edge excited and blue-edge detected fluorescence anisotropy decay curves did not contain a fast relaxation process which was observed upon mainband excitation and detection. This relaxation was assigned to homo-energy transfer between the two FAD cofactors. Failure of energy transfer as observed with edge spectroscopy on this protein excludes restricted reorientational motion of the flavins as a possible mechanism of depolarization. From the global analysis of the fluorescence anisotropy decay surface obtained at multiple excitation and detection wavelengths, the distance between and the relative orientation of the flavins could be estimated. The methodology described has general applicability in other multichromophoric biopolymers and has the potential to acquire accurate geometrical parameters in these systems. 相似文献
99.
Robert Andreu Gerard Lligadas Joan Carles Ronda Marina Galià Virginia Cádiz 《Journal of polymer science. Part A, Polymer chemistry》2007,45(21):4965-4973
A new oxazolidine derivative was obtained from phenol, 2‐amino‐2‐methylpropane‐1,3‐diol and paraformaldehyde. The reaction of this novel oxazolidine diol with phenylisocyanate lead to a urethane model compound which can be polymerized thermally by oxazolidine ring opening to give a Mannich bridge structure. Linear segmented polyurethanes were prepared by reaction of different ratios of oxazolidine diol and commercial polyethylenglycol (Mw ~ 400) with 4,4′‐methylenbis (cyclohexylisocyanate) (HMDI, 90% isomers mixture). The polyurethanes were thermally characterized and crosslinked by oxazolidine ring opening to obtain materials which showed improved thermal stability. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 4965–4973, 2007 相似文献
100.