全文获取类型
收费全文 | 197420篇 |
免费 | 3451篇 |
国内免费 | 445篇 |
专业分类
化学 | 112863篇 |
晶体学 | 2672篇 |
力学 | 7649篇 |
综合类 | 3篇 |
数学 | 21808篇 |
物理学 | 56321篇 |
出版年
2021年 | 1574篇 |
2020年 | 1886篇 |
2019年 | 1918篇 |
2018年 | 2689篇 |
2017年 | 2555篇 |
2016年 | 4126篇 |
2015年 | 3018篇 |
2014年 | 3834篇 |
2013年 | 9045篇 |
2012年 | 8309篇 |
2011年 | 9496篇 |
2010年 | 6362篇 |
2009年 | 5988篇 |
2008年 | 8768篇 |
2007年 | 8559篇 |
2006年 | 8074篇 |
2005年 | 7549篇 |
2004年 | 6695篇 |
2003年 | 5897篇 |
2002年 | 5576篇 |
2001年 | 6420篇 |
2000年 | 4740篇 |
1999年 | 3619篇 |
1998年 | 2551篇 |
1997年 | 2590篇 |
1996年 | 2575篇 |
1995年 | 2228篇 |
1994年 | 2292篇 |
1993年 | 2101篇 |
1992年 | 2621篇 |
1991年 | 2643篇 |
1990年 | 2451篇 |
1989年 | 2432篇 |
1988年 | 2339篇 |
1987年 | 2352篇 |
1986年 | 2141篇 |
1985年 | 2753篇 |
1984年 | 2750篇 |
1983年 | 2140篇 |
1982年 | 2254篇 |
1981年 | 2087篇 |
1980年 | 2010篇 |
1979年 | 2308篇 |
1978年 | 2362篇 |
1977年 | 2257篇 |
1976年 | 2244篇 |
1975年 | 2100篇 |
1974年 | 2089篇 |
1973年 | 2115篇 |
1972年 | 1341篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Saraph G.P. Antonsen T.M. Jr. Levush B. Lin G.I. 《IEEE transactions on plasma science. IEEE Nuclear and Plasma Sciences Society》1992,20(3):115-125
The problem of mode competition in a high-power gyrotron oscillator is considered. The regions of parameter space in which a preexisting large-amplitude mode is able to suppress competing satellites are determined for cases in which the coupling coefficients and cavity quality factors for the operating and satellite modes are different. In addition, the effect of beam quality on the regimes of stable single-mode operation is investigated. Generally speaking, the requirement of stable operation favors devices whose interaction length measured by the parameter μ is not too large. It is found that for μ near 10 the operation is relatively stable and μ near 17 is not 相似文献
992.
993.
Sulfide cluster-derived ensembles are promising models of the active sites in commercial hydrotreatment catalysts. A series of sulfide clusters were adsorbed intact onto high-surface-area γ-alumina, magnesium oxide and activated carbon supports, then pretreated to produce highly dispersed catalytic ensembles with sizes similar to those of their precursor clusters. The activities of the bimetallic cluster-derived catalysts were significantly higher than those of the monometallic catalysts. We took this as evidence that direct interactions between molybdenum and the promoter element cause the promotional effect observed in commercial hydrotreatment catalysts. The hydrodesulfurization and hydrodenitrogenation activities correlated with the extent of molybdenum reduction. Our results suggested that the active sites in promoted hydrotreatment catalysts are centered on molecular-scale ensembles containing molybdenum, sulfur and the promoter element. 相似文献
994.
995.
996.
The gamma subunit of enolase (gamma-enolase) was purified from the brain tissues of cow, dog, goat, pig, rabbit, and rat. The purification was achieved in only three steps: ammonium sulfate-precipitation, DE 53 cellulose ion-exchange chromatography, and polyacrylamide gel electrophoresis (PAGE) in a preparative mode. The purification procedure was comparatively more simple than previously reported methods, and the yield of gamma-enolase was sufficient for subsequent structural and immunological analyses. In all mammals, the purified gamma-enolase migrated in sodium dodecyl sulfate-PAGE (SDS-PAGE) with a molecular mass of 46 kilodaltons (kDa), and the immunological cross-reactivity between those gamma-enolases was very strong. The structural homology of these gamma-enolases was examined by peptide mapping using cyanogen bromide cleavage and subsequent two-dimensional electrophoresis. The resulting peptide patterns were highly similar and in cow, dog, and goat, the patterns were almost identical. These results indicate that structural homology, that is, the species non-specificity of gamma-enolase, appears to be very high. 相似文献
997.
T. Butz Ch. Meinecke M. Morawski T. Reinert M. Schwertner D. Spemann J. Vogt 《Applied Surface Science》2005,252(1):43-48
This contribution deals with the morphological and elemental characterisation with high-energy (MeV) focused ion beams (in particular protons) with special emphasis on high spatial resolution in the sub-micrometer regime and very low minimum detection limits (sub-ppm) in trace element analysis. The most important methods like particle induced X-ray emission (PIXE), Rutherford backscattering spectrometry (RBS), as well as scanning transmission ion microscopy (STIM) and STIM-tomography will be illustrated by examples from material and life sciences. 相似文献
998.
The macroscopic approach for two-particle transfer reactions is applied in normal spherical nuclei and pairing deformation parameters βp are estimated. For Ni isotopes the macroscopic approach works reasonably well while for the lead region strong energy dependence of theβ p values has been observed. It is shown that alpha-transfer reactions can also be treated within this formalism as couplings of the two-neutron and two-proton channels. 相似文献
999.
1000.
V. F. Tret’yakov A. G. Zakirova V. A. Matyshak T. N. Burdeinaya V. N. Korchak L. S. Glebov V. V. Lunin 《Kinetics and Catalysis》2006,47(6):873-880
According to X-ray diffraction analysis data, the test catalyst was a Ni-Cr spinel with an impurity of NiO. With the use of in situ IR spectroscopy, it was found that nitrite, nitrate, and acetate surface complexes occurred under the reaction conditions of the selective catalytic reduction of nitrogen oxides by propane in the presence of oxygen on the nickel-chromium catalyst. As the temperature was increased, the nitrite complexes were converted into nitrate species. The molar absorption coefficient of surface nitrate complexes was determined. According to IR-spectroscopic and TPD data, the nitrate complexes were bound relatively weakly to the surface. The temperature region of their existence was 50–200°C. The temperature region of existence of the surface acetate complexes was 200–400°C. The individual adsorption of oxygen was not observed; however, oxygen-containing surface sites (Cr5+=O) participated in the formation of the surface complexes of reactants. 相似文献