全文获取类型
收费全文 | 4743篇 |
免费 | 638篇 |
国内免费 | 372篇 |
专业分类
化学 | 1173篇 |
晶体学 | 39篇 |
力学 | 576篇 |
综合类 | 119篇 |
数学 | 1750篇 |
物理学 | 2096篇 |
出版年
2024年 | 9篇 |
2023年 | 34篇 |
2022年 | 85篇 |
2021年 | 102篇 |
2020年 | 152篇 |
2019年 | 151篇 |
2018年 | 145篇 |
2017年 | 145篇 |
2016年 | 176篇 |
2015年 | 143篇 |
2014年 | 220篇 |
2013年 | 420篇 |
2012年 | 266篇 |
2011年 | 279篇 |
2010年 | 232篇 |
2009年 | 303篇 |
2008年 | 286篇 |
2007年 | 287篇 |
2006年 | 265篇 |
2005年 | 272篇 |
2004年 | 229篇 |
2003年 | 215篇 |
2002年 | 201篇 |
2001年 | 143篇 |
2000年 | 141篇 |
1999年 | 116篇 |
1998年 | 115篇 |
1997年 | 114篇 |
1996年 | 62篇 |
1995年 | 77篇 |
1994年 | 68篇 |
1993年 | 46篇 |
1992年 | 42篇 |
1991年 | 41篇 |
1990年 | 29篇 |
1989年 | 18篇 |
1988年 | 23篇 |
1987年 | 12篇 |
1986年 | 16篇 |
1985年 | 14篇 |
1984年 | 10篇 |
1983年 | 3篇 |
1982年 | 15篇 |
1981年 | 5篇 |
1980年 | 3篇 |
1979年 | 6篇 |
1978年 | 2篇 |
1977年 | 6篇 |
1976年 | 2篇 |
1959年 | 4篇 |
排序方式: 共有5753条查询结果,搜索用时 62 毫秒
121.
邢凤鸣 《中国科学B辑(英文版)》1991,(11)
It is considered that the early Jinning period granitoids from southern Anhui Province belong to the continental crust transformation (S) type and have their own characteristics which are different from those for general continental crust transformation type granitoids. They were formed at the early stage of the late Proterozoic era when that region was at an island-arc stage of the continental margin environment. By that time the continental crust was just formed and had a nature of juvenile crust. It is this juvenile crust that determined a series of characteristics of that type granitoids and made them a relatively independent rock type. They are the products formed by the continental crust in a certain stage of development. 相似文献
122.
123.
An innovative route for the synthesis of substituted dibenzofurans has been delineated through a ring transformation reaction of suitably functionalized 2H-pyran-2-ones by reaction with 7-methoxybenzofuran-3-one, in high yield. The novelty of the procedure lies in the creation of an aromatic ring from a 2H-pyran-2-one involving the -COCH2-moiety of the substrate. 相似文献
124.
On the plastic deformation of bulk syndiotactic polypropylene 总被引:1,自引:0,他引:1
The plastic deformation of syndiotactic polypropylene (sPP) bulk samples has been investigated. A structural comparison of
the initial states before and after plastic deformation by necking is carried out by X-ray diffraction observations. Independent
of the initial states (amorphous, semi-crystalline with different crystal phases), only the planar all-trans crystal form of sPP is present in the deformed samples after necking. Form these results, molecular mechanisms of the plastic
deformation in the neck zone of semi-crystalline polymers will be discussed.
Received: 11 February 1997 Accepted: 15 August 1997 相似文献
125.
A challenge in free energy calculation for complex molecular systems by computer simulation is to obtain a reliable estimate within feasible computational time. In this study, we suggest an answer to this challenge by exploring a simple method, overlap sampling (OS), for producing reliable free-energy results in an efficient way. The formalism of the OS method is based on ensuring sampling of important overlapping phase space during perturbation calculations. This technique samples both forward and reverse free energy perturbation (FEP) to improve the free-energy calculation. It considers the asymmetry of the FEP calculation and features an ability to optimize both the precision and the accuracy of the measurement without affecting the simulation process itself. The OS method is tested at two optimization levels: no optimization (simple OS), and full optimization (equivalent to Bennett's method), and compared to conventional FEP techniques, including the widely used direct FEP averaging method, on three alchemical mutation systems: (a) an anion transformation in water solution, (b) mutation between methanol and ethane, and (c) alchemical change of an adenosine molecule. It is consistently shown that the reliability of free-energy estimates can be greatly improved using the OS techniques at both optimization levels, while the performance of Bennett's method is particularly striking. In addition, the efficiency of a calculation can be significantly improved because the method is able to (a) converge to the right answer quickly, and (b) work for large perturbations. The basic two-stage OS method can be extended to admit additional stages, if needed. We suggest that the OS method can be used as a general perturbation technique for computing free energy differences in molecular simulations. 相似文献
126.
127.
X. Bokhimi A. Morales E. Ortíz T. López R. Gómez J. Navarrete 《Journal of Sol-Gel Science and Technology》2004,29(1):31-40
Sol-gel titania was sulfated by using sulfuric acid as hydrolysis catalyst, or by impregnating with ammonium sulfate fresh samples prepared with nitric acid or ammonium hydroxide as hydrolysis catalyst. Samples were characterized with X-ray powder diffraction, infrared spectroscopy, thermogravimetry and atomic absorption spectroscopy. Sulfate ions were found anchored to brookite and anatase phases, because they have short O—O atomic bond lengths slightly larger than the largest O—O bond length of sulfate ion. Since the shortest O—O atomic bond in anatase is smaller than the one in brookite, the sulfate ions are then less deformed when they are anchored to anatase, and consequently more stable. Therefore when the sample temperature is raised, the brookite with sulfate ions was transformed mainly to anatase and not into rutile, which is the most probably transformation when these ions are not involved. Sulfate ions also hindered anatase and brookite crystallite growing and stabilized the crystalline structure of anatase. When the sulfate ions are lost the crystalline anatase phase is transformed into rutile, leaving a large number of vacancies that favored atom diffusion and consequently the growing of rutile crystallites. The crystalline evolution of the samples as a function of the annealing temperature is almost independent of the sulfating method. 相似文献
128.
129.
Yuichi Michiue 《Journal of solid state chemistry》2006,179(8):2578-2583
Phase relations of rutile, freudenbergite, and hollandite structures were examined in the pseudobinary system NaCrO2-TiO2 (i.e., NaxCrxTi8−xO16) at 1350 °C. The hollandite structure was obtained in the composition range 1.7?x?2.0. The symmetry of the samples at room temperature was tetragonal for x=1.7 and 1.75, and monoclinic for x=1.8 and above. Single crystals of monoclinic hollandite Na2Cr2Ti6O16 were grown and the structure refinement has been carried out using an X-ray diffraction technique. The space group was I2/m and cell parameters were a=10.2385(11), b=2.9559(9), c=9.9097(11)Å, and β=90.545(9)° with Z=1. The Na ion distribution in the tunnel was markedly deformed from that in the tetragonal form. It was suggested that Cr/Ti ratios were different between the two framework metal sites. 相似文献
130.
Jungui DaiShujun Zhang Jun-ichi SakaiJiao Bai Yoshiki OkuMasayoshi Ando 《Tetrahedron letters》2003,44(5):1091-1094
Three C-14 oxygenated taxanes, 2α,5α,10β,14β-tetraacetoxytaxa-4(20),11-diene (1), 2α,5α,10β-triacetoxy-14β-(2-methylbutyryloxy)taxa-4(20),11-diene (2), and yunanaxane (3), major products of callus cultures of Taxus spp., were regio- and stereoselectively hydroxylated at the 7β position by a fungus, Absidia coerulea IFO 4011. Intriguingly, when 1 was co-administered with β-cyclodextrin and incubated with the fungus cell cultures, three other compounds 5α,9α,10β,13α-tetraacetoxytaxa-4(20),11-dien-14β-ol (7), 5α,9α,10β,13α-tetraacetoxytaxa-4(20),11-dien-1β-ol (8) and 5α,9α,10β,13α-tetraacetoxy-11(15→1) abeotaxa-4(20),11-dien-15-ol (9) were obtained. 相似文献