全文获取类型
收费全文 | 228115篇 |
免费 | 1695篇 |
国内免费 | 551篇 |
专业分类
化学 | 107245篇 |
晶体学 | 3419篇 |
力学 | 10324篇 |
综合类 | 10篇 |
数学 | 22488篇 |
物理学 | 65295篇 |
无线电 | 21580篇 |
出版年
2021年 | 2235篇 |
2020年 | 2317篇 |
2019年 | 2956篇 |
2018年 | 4048篇 |
2017年 | 4057篇 |
2016年 | 5391篇 |
2015年 | 2629篇 |
2014年 | 4846篇 |
2013年 | 9484篇 |
2012年 | 7504篇 |
2011年 | 8862篇 |
2010年 | 7160篇 |
2009年 | 7552篇 |
2008年 | 8706篇 |
2007年 | 8725篇 |
2006年 | 7778篇 |
2005年 | 6962篇 |
2004年 | 6589篇 |
2003年 | 6056篇 |
2002年 | 5979篇 |
2001年 | 6422篇 |
2000年 | 4938篇 |
1999年 | 3900篇 |
1998年 | 3364篇 |
1997年 | 3420篇 |
1996年 | 3094篇 |
1995年 | 2762篇 |
1994年 | 2839篇 |
1993年 | 2744篇 |
1992年 | 3024篇 |
1991年 | 3138篇 |
1990年 | 3063篇 |
1989年 | 3084篇 |
1988年 | 2940篇 |
1987年 | 2793篇 |
1986年 | 2634篇 |
1985年 | 3333篇 |
1984年 | 3349篇 |
1983年 | 2813篇 |
1982年 | 2728篇 |
1981年 | 2677篇 |
1980年 | 2489篇 |
1979年 | 2824篇 |
1978年 | 2790篇 |
1977年 | 2962篇 |
1976年 | 2970篇 |
1975年 | 2753篇 |
1974年 | 2682篇 |
1973年 | 2809篇 |
1972年 | 2145篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
881.
A. K. Ghosh A. D. Williams J. M. Zucker J. L. Mathews N. Spinhirne 《Experimental Mechanics》2008,48(2):139-152
In nature, shape and structure evolve from the struggle for better performance. Often, biological structures combine multiple
beneficial properties, making research into mimicking them very complex. Presented here is a summary of observations from
a series of experiments performed on a material that closely resembles the human skull bone’s cancellous structure under acoustic
loads. Transmission loss through flat and curved open-cell polyurethane foam samples is observed using air and water as the
two interstitial fluids. Reduction in strength and stiffness caused by porosity can be recovered partially by filling the
interstitial pores with a fluid. The test findings demonstrate the influence of the interstitial fluid on the mechanical characteristics
of a porous structure in a quantitative manner. It is also demonstrated that the transmission loss does not depend only on
the mass per unit area of the structure as predicted by acoustic mass law. Current tests also demonstrate that the transmission
loss is more sensitive to the interstitial fluid than the shape and support conditions of the structures. Test observations
thus support the concepts of “moisture-sensitivity of biological design” and the “law of hierarchy in natural design”. 相似文献
882.
G. N. Kozhemyakin D. V. Lutskiy M. A. Rom P. V. Mateychenko 《Crystallography Reports》2008,53(7):1267-1271
The growth conditions and structural quality of Sb-Bi gradient single crystals with Bi content from 2 to 18 at %, grown by
the Czochralski method with solid phase feed, are investigated. Bi distribution in the crystals along their pulling direction
are studied by electron probe microanalysis and the change in the interplanar spacing is analyzed by double-crystal X-ray
diffraction. It is established that the pulling rate and feed mass affect the Bi distribution in Sb-Bi single crystals. 相似文献
883.
Several sophisticated methods to solution of symmetry specified enumeration problems are available in the modern literature.
In this paper we propose a simple technique that allows one to manually compute the exact numbers of fixed-symmetry derivatives
for a given structure either with inclusion or ignoring the substitution patterns. The basic idea of the method suggested
consists in the derivation of Pólya-like cycle indices for the automorphism groups of specially constructed orbit partition
graphs; the expansion of these indices and subsequent simple calculations result in the desired numbers of substituted derivatives
with achiral substituents. Limitations of the new technique (and a method suggested earlier) depend on the relevance of the
orbit partitions for particular subgroups of the point symmetry group. For illustration purposes, the results obtained for
the prismane (D
3h
) and adamantane (T
d
) structures are discussed. In the former case the numbers of substituted derivatives can be found for all subgroups of the
D
3h
group, whereas in the latter case these numbers can be determined for eight out of eleven subgroups of the T
d
point symmetry group.
This work is based on the text of the lecture presented by the authors at the 5th All-Russia Conference on Molecular Modeling
(Moscow, April 2007). The paper deals with the methodology and detailed treatment of applied aspects related to solution of
enumeration problems for substituted derivatives with prescribed symmetry groups. Unlike the known methods of symmetry specified
enumeration, the technique suggested is simple enough and may be regarded as generalization of the Pólya methodology, which
is widely used by chemists.
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 227–245, February, 2008. 相似文献
884.
The reaction of 3,5-dibromo-4-hydroxybenzoic acid, its sodium salt, and also sodium 3,5-dibromo-2-hydroxybenzoate with NaNO2 in a glacial acetic acid at room temperature led to the formation of a mixture of dibromonitrophenol resulting from nitrosodecarboxylation accompanied by a rearrangement processes and followed by oxidation of the arising nitrosophenols. 相似文献
885.
The types of quartz textures found in a large collection of multiphase rocks from different regions of the earth are analyzed. Crystallographic textures of granulite, amphibolite, slate, and gneiss samples are measured, classified, and compared with the similar textures of monomineral rocks. 相似文献
886.
S. E. Dizhur I. N. Kotel’nikov E. M. Dizhur 《Journal of Communications Technology and Electronics》2006,51(5):588-595
Al-δ-GaAs structures are studied where tunneling to one or more subbands of the 2D electron system of a near-surface delta-doped layer is observed. Reflection of electrons at the threshold of the emission of an LO-phonon is observed. This phenomenon occurs when a new subband is involved in the tunneling process and when intersubband transitions of the electrons in a 2D-system with the emission of an LO-phonon are added to the inelastic tunneling within a single subband. It is shown that, under the conditions of an intersubband polaron resonance, the reflection processes dominate during tunneling into the delta-layer. If, however, tunneling from the delta-layer occurs, the reflection processes are observed when two subbands are occupied. 相似文献
887.
888.
T. M. Prokop’eva V. A. Mikhailov M. K. Turovskaya E. A. Karpichev N. I. Burakov V. A. Savelova I. V. Kapitanov A. F. Popov 《Russian Journal of Organic Chemistry》2008,44(5):637-646
Bis(dialkylamide)hydrogen dibromobromates were synthesized and their reactivity was investigated in decomposition of diethylphosphonic, diethylphosphoric, and 4-toluenesulfonic acids 4-nitrophenyl esters. The nucleophilic reactivity of a typical α-nucleophile, hypobromite ion, is independent of the source of the active bromine. The cetyltrimethylammonium dibromobromate is a unique reagent for destruction of ecotoxicants. In weakly alkaline media the half-life of 4-nitrophenyl diethylphosphonate did not exceed 6 s at [BrO?] 0.02 mol l?1, and the apparent reaction rate compared to water increased ~40-fold. The main factor governing the micellar effects of surfactants is concentrating the substrate in the micellar pseudophase. 相似文献
889.
M. A. Katkova V. A. Ilichev A. N. Konev M. N. Bochkarev 《Russian Chemical Bulletin》2008,57(11):2281-2284
Rare-earth metal 8-hydroxyquinolinates (q) were studied as emission layers for organic light-emitting diodes. Compounds Dyq3, Tbq3, and Smq3 possess weak electroluminescence due to the emission of the ligand, and the electroluminescence of Hoq3, Tmq3, and Ybq3 is due to electronic transitions of the lanthanide ion. The best electroluminescence characteristics were demonstrated by
Scq3 and Yq3. The power efficiency of the organic light emitter based on ITO/TPD/Scq3/Yb (ITO is tin-alloyed indium oxide, TPD is N,N′-bis(3-methylphenyl)-N,N′-diphenylbenzidine) is 2.6 lm W−1 at a luminance of the device of 300 cd m−2. 相似文献
890.
E. V. Sennikova A. S. Antsyshkina G. G. Sadikov A. V. Bicherov O. Yu. Korshunov G. S. Borodkin M. S. Korobov V. S. Sergienko N. N. Kharabaev A. D. Garnovskii 《Russian Journal of Coordination Chemistry》2008,34(5):315-321
The chelate and molecular complexes of 2-(2-benzazolyl, perimidyl)-indandiones-1,3 were synthesized for the first time. The structures of the ligands (tautomeric forms) and complex compounds were determined from the X-ray diffraction, 1H, 13C NMR, and IR data and the quantum-chemical calculations (RHF SCF in 6-31 G** basis). 相似文献