全文获取类型
收费全文 | 782篇 |
免费 | 17篇 |
专业分类
化学 | 389篇 |
晶体学 | 6篇 |
力学 | 6篇 |
数学 | 51篇 |
物理学 | 347篇 |
出版年
2021年 | 9篇 |
2019年 | 6篇 |
2016年 | 11篇 |
2015年 | 13篇 |
2014年 | 6篇 |
2013年 | 38篇 |
2012年 | 27篇 |
2011年 | 39篇 |
2010年 | 8篇 |
2009年 | 8篇 |
2008年 | 24篇 |
2007年 | 25篇 |
2006年 | 20篇 |
2005年 | 38篇 |
2004年 | 32篇 |
2003年 | 36篇 |
2002年 | 16篇 |
2001年 | 19篇 |
2000年 | 25篇 |
1999年 | 16篇 |
1998年 | 11篇 |
1997年 | 6篇 |
1996年 | 11篇 |
1995年 | 17篇 |
1994年 | 12篇 |
1993年 | 22篇 |
1992年 | 15篇 |
1991年 | 9篇 |
1990年 | 11篇 |
1989年 | 7篇 |
1988年 | 8篇 |
1987年 | 6篇 |
1986年 | 10篇 |
1985年 | 19篇 |
1984年 | 6篇 |
1983年 | 15篇 |
1982年 | 15篇 |
1981年 | 6篇 |
1978年 | 8篇 |
1977年 | 9篇 |
1976年 | 9篇 |
1975年 | 10篇 |
1974年 | 18篇 |
1973年 | 9篇 |
1972年 | 13篇 |
1971年 | 6篇 |
1970年 | 6篇 |
1969年 | 11篇 |
1968年 | 7篇 |
1966年 | 8篇 |
排序方式: 共有799条查询结果,搜索用时 15 毫秒
771.
772.
The cross sections for the reactions N14(n, α)B11 and N14(n, t)C12 have been measured in the neutron energy range 4.0 to 6.4 MeV and at 2.5 MeV. Mono-energetic neutrons were produced in the D(d, n) He3 reaction using a gas target. The (n, α) and (n, t) disintegrations were detected in a gridded ionization chamber filled with an argonnitrogen mixture. The response of the chamber under different operation conditions is described. The excitation functions, measured with a neutron energy resolution of 40 to 50 keV, are given for theα 0 group from the N14(n,α)B11 reaction over the entire neutron energy range and for theα 1 group and the t0 group from N14(n, t) C12 for neutron energies above 4.3 and 5.6 MeV, respectively. 相似文献
773.
Walter Fink 《Helvetica chimica acta》1967,50(4):1144-1153
Linear polymers are built up by polycondensation of aminodimethylsilyl- or aminodiphenylsilyl-terminated monomeric or oligomeric 1,3-diaza-2-sila-cyclo-pentanes with bisphenols or disilanols. The thermal properties of the polymers are determined. 相似文献
774.
Thermodynamic and Structural Investigations on Compounds of the Systems KCl/Mcl2 (M=Ca, Cd, Co, Ni) . The formation of ternary chlorides according to the equation nKCl + MCl2 = KnMCln+2 with M = Ca, Cd, Co, Ni was investigated with a galvanic cell for solid electrolytes. Additionally to the Gibbs energies for the solid reactions, ΔGR, the entropies, ΔSR, and enthalpies, ΔHR were determined by the dependence of the e.m.f. on temperature. But the stability of the double chlorides is given by the free enthalpy of synproportionation from the neighboured compounds. In case of RbSrCl3 and RbPbCl3 (stable only at higher temperature) the synproportionation is endothermic; the loss of enthalpy is compensated by a gain of entropy. The same is valid for K2CoCl4. Lattice parameters were measured for the modifications of KCaCl3; the structure of KCdCl3 was refined by single crystal measurements. 相似文献
775.
A graph israndomly matchable if every matching of the graph is contained in a perfect matching. We generalize this notion and say that a graphG israndomly H-coverable if every set of independent subgraphs, each isomorphic toH, that does not cover the vertices ofG can be extended to a larger set of independent copies ofH.
Various problems are considered for the situation whereH is a path. In particular, we characterize the graphs that are randomlyP
3
-coverable. 相似文献
776.
Gerhard Talsky und Gerhard Fink 《Fresenius' Journal of Analytical Chemistry》1967,228(6):419-422
Zusammenfassung Es wird über eine modifizierte Thunberg-Methode berichtet, die es gestattet, die Kinetik von Reaktionen unter anaeroben Bedingungen mit hoher Genauigkeit (Fehler ± 1%) zu verfolgen. Als Anwendungsbeispiel wird die AktivitÄtsbestimmung von Xanthinoxydase mit Xanthin und Methylenblau als Acceptor beschrieben.
Summary A modified Thunberg method is proposed allowing the observation of the kinetics of reactions under anaerobic conditions with high accuracy (error ±1%). As an example of application the estimation of the activity of xanthine oxidase acting on xanthine and methylene blue as an acceptor is described.相似文献
777.
The solution-phase, parallel synthesis and evaluation of a library of 132 (+)-1,2,9,9a-tetrahydrocyclopropa[c]benz[e]indol-4-one (CBI) analogues of CC-1065 and the duocarmycins containing dimeric monocyclic, bicyclic, and tricyclic heteroaromatic replacements for the DNA-binding domain are described. This systematic study revealed clear trends in the structural requirements for observation of potent cytotoxic activity and DNA alkylation efficiency, the range of which spans a magnitude of > or =10 000-fold. Combined with related studies, these results highlight that the role of the DNA-binding domain goes beyond simply providing DNA-binding selectivity and affinity (10-100-fold enhancement in properties), consistent with the proposal that it contributes significantly to catalysis of the DNA alkylation reaction accounting for as much as an additional 1000-fold enhancement in properties. 相似文献
778.
779.
Experimental results of time-reversal focusing in a high-order multiple scattering medium are presented and compared to theoretical predictions based on a statistical model. The medium consists of a random collection of parallel steel rods. An ultrasonic source (3.2 MHz) transmits a pulse that undergoes multiple scattering and is recorded on an array. The time-reversed waves are sent by the array back to the source through the scattering medium. The quality of temporal focusing is very well predicted by a simple statistical model. However, for thicker samples, persistent temporal side-lobes appear. We interpret these side-lobes as a consequence of the growing number of crossing paths in the sample due to high-order multiple scattering. As to spatial focusing, the resolution is practically independent from the array's aperture. With a 16-element array, the resolution was found to be 30 times finer than in a homogeneous medium. Resolutions of the order of the wavelength (0.5 mm) were attained. These results are discussed in relation with the statistical properties of time-reversal mirrors in a random medium. 相似文献
780.
Elastic waves propagating inside a solid chaotic cavity create a diffusive random field that contains both longitudinal and shear waves. In the current paper, we are interested in the field radiated in a fluid in contact with such cavity. The goal of this paper is to predict the spatial focusing properties that can be obtained in the fluid using a time-reversal piezoelectric transducer in contact with the cavity. We present a statistical approach that supposes a fully diffused wavefield inside the cavity with an equipartition of energy between longitudinal and shear waves. We show that the critical angles of transmission in the solid-fluid interface generate a cut-off of the spatial frequencies and then a degradation in the spatial focusing. This limitation can be overcome using a rough surface. A set of experiments conducted in the MHz range confirm the theoretical model. 相似文献