全文获取类型
收费全文 | 65712篇 |
免费 | 19789篇 |
国内免费 | 2912篇 |
专业分类
化学 | 68980篇 |
晶体学 | 285篇 |
力学 | 3003篇 |
综合类 | 217篇 |
数学 | 4503篇 |
物理学 | 11425篇 |
出版年
2024年 | 497篇 |
2023年 | 4533篇 |
2022年 | 2417篇 |
2021年 | 3411篇 |
2020年 | 5568篇 |
2019年 | 3190篇 |
2018年 | 3218篇 |
2017年 | 1492篇 |
2016年 | 6679篇 |
2015年 | 6622篇 |
2014年 | 6293篇 |
2013年 | 6938篇 |
2012年 | 5339篇 |
2011年 | 3354篇 |
2010年 | 4870篇 |
2009年 | 4761篇 |
2008年 | 2684篇 |
2007年 | 2132篇 |
2006年 | 1449篇 |
2005年 | 1204篇 |
2004年 | 896篇 |
2003年 | 725篇 |
2002年 | 631篇 |
2001年 | 541篇 |
2000年 | 531篇 |
1999年 | 643篇 |
1998年 | 539篇 |
1997年 | 565篇 |
1996年 | 534篇 |
1995年 | 571篇 |
1994年 | 451篇 |
1993年 | 530篇 |
1992年 | 350篇 |
1991年 | 307篇 |
1990年 | 273篇 |
1989年 | 230篇 |
1988年 | 220篇 |
1987年 | 184篇 |
1986年 | 135篇 |
1985年 | 140篇 |
1982年 | 123篇 |
1977年 | 162篇 |
1976年 | 182篇 |
1975年 | 185篇 |
1974年 | 194篇 |
1972年 | 153篇 |
1971年 | 122篇 |
1970年 | 207篇 |
1969年 | 125篇 |
1968年 | 133篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
11.
Mo2Ni3Si/NiSi metal silicide composite coatings with a fine microstructure consisting of Mo2Ni3Si primary dendrites and the interdendritic Mo2Ni3Si/NiSi eutectics were fabricated on austenitic stainless steel AISI 321 by laser cladding process. Small amplitude reciprocating sliding wear resistance of the coatings is evaluated as functions of normal load and slip amplitude and the wear mechanisms were discussed based on worn surface morphology observations. Results showed that the Mo2Ni3Si/NiSi coatings have excellent small amplitude reciprocating sliding wear resistance. 相似文献
12.
Stabilized hybrid finite element methods based on the combination of saddle point principles of elasticity problems 总被引:2,自引:0,他引:2
Tianxiao Zhou. 《Mathematics of Computation》2003,72(244):1655-1673
How, in a discretized model, to utilize the duality and complementarity of two saddle point variational principles is considered in the paper. A homology family of optimality conditions, different from the conventional saddle point conditions of the domain-decomposed Hellinger-Reissner principle, is derived to enhance stability of hybrid finite element schemes. Based on this, a stabilized hybrid method is presented by associating element-interior displacement with an element-boundary one in a nonconforming manner. In addition, energy compatibility of strain-enriched displacements with respect to stress terms is introduced to circumvent Poisson-locking.
13.
Guangchang Zhou Jibao He Issifu I. Harruna 《Journal of polymer science. Part A, Polymer chemistry》2007,45(18):4204-4210
Amphiphilic tris(2,2′‐bipyridine)ruthenium‐cored star‐shaped polymers consisting of one polystyrene block and two poly(N‐isopropylacrylamide) blocks were prepared by the “arm‐first” method in which RAFT polymerization and nonconvalent ligand–metal complexation were employed. The prepared amphiphilic star‐shaped metallopolymers are able to form micelles in water. The size and distribution of the micelles were studied by dynamic light scattering and transmission electron microscopy techniques. Preliminary studies indicate that the polymer concentration and the hydrophilic poly(N‐isopropylacrylamide) block length can affect the morphologies of the formed metal‐interfaced core–shell micelles in water. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 4204–4210, 2007 相似文献
14.
Akira Yanagisawa Prof. Dr. Takahiro Suzuki Takanori Koide Shogo Okitsu Takayoshi Arai Prof. Dr. 《化学:亚洲杂志》2008,3(10):1793-1800
A Barbier‐type regioselective propargylation of aldehydes and ketones with (3‐bromobut‐1‐ynyl)trimethylsilane has been achieved using reactive barium as a low‐valent metal in THF. Especially in the case of ketones, the corresponding homopropargylic alcohols form almost exclusively. In the reaction of α,β‐unsaturated carbonyl compounds, only 1,2‐adducts have been observed. This method is also applicable to propargylation of imines, and the corresponding homopropargylic amines are obtained regiospecifically in good yields with diastereomeric ratios of up to 87:13. 相似文献
15.
16.
J.J. Pan Xiangdong Qiu Kejian Guan Liang Dong FengQing Zhou Wei Wang Mingjie Zhang James Guo Jun Yang Chunmeng Wu 《光学学报》2003,23(Z1)
A low cost, coolerless 980nm diode pumped, gain flattened L band EDFA with fast transient control, high pump efficiency and gain clamping effect was realized by using FBGs as C band seed generators. 相似文献
17.
In this article, we present results of an empirical study with 500 German students of grades 7 and 8. The study focussed on students' mathematics achievement and their interest in mathematics as well as on the relation between these two constructs. In particular, the results show that the development of an individual student's achievement between grade 7 and grade 8 depends on the achievement level of the specific classroom and therefore on the specific mathematics instruction Interest in mathematics could be regarded a predictor for mathematics achievement Moreover, our findings suggest that the students show hardly any fear of mathematics independent of their achievement level. 相似文献
18.
Behavior of cellulose in NaOH/Urea aqueous solution characterized by light scattering and viscometry
Cellulose was dissolved in 6 wt % NaOH/4 wt % urea aqueous solution, which was proven by a 13C NMR spectrum to be a direct solvent of cellulose rather than a derivative aqueous solution system. Dilute solution behavior of cellulose in a NaOH/urea aqueous solution system was examined by laser light scattering and viscometry. The Mark–Houwink equation for cellulose in 6 wt % NaOH/4 wt % urea aqueous solution at 25 °C was [η] = 2.45 × 10?2 weight‐average molecular weight (Mw)0.815 (mL g?1) in the Mw region from 3.2 × 104 to 12.9 × 104. The persistence length (q), molar mass per unit contour length (ML), and characteristic ratio (C∞) of cellulose in the dilute solution were 6.0 nm, 350 nm?1, and 20.9, respectively, which agreed with the Yamakawa–Fujii theory of the wormlike chain. The results indicated that the cellulose molecules exist as semiflexible chains in the aqueous solution and were more extended than in cadoxen. This work provided a novel, simple, and nonpollution solvent system that can be used to investigate the dilute solution properties and molecular weight of cellulose. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 347–353, 2004 相似文献
19.
20.
LiAlO2 single crvstals doped with Ti at concentration 0.2at.% are grown by the Czochralskl technique with dimensions φ42×55mm. Ti ions in the crystal are quadrivalence proven by comparing the absorption and fluorescence spectra of pure LiAlO2 and Ti: LiAlO2. After air and Li-rich atmosphere annealing, the absorption peaks in the range of 600-800nm disappear. We conclude that 682 and 756nm absorption peaks are attributed to the VLi and Vo absorptions, respectively: The peaks at 716nm and 798nm may stem from the VLi^+ and absorptions. The colour-centre model can be applied to explain the experimental phenomena. Ti^4+-doping produces more lithium vacancies in the LiAlO2 crystal. The intensities of [LiO4] and the associated bonds remain unchanged, which improves the anti-hydrolyzation and thermal stability of LiAlO2 crystals. 相似文献