全文获取类型
收费全文 | 43090篇 |
免费 | 7618篇 |
国内免费 | 5712篇 |
专业分类
化学 | 31791篇 |
晶体学 | 654篇 |
力学 | 2392篇 |
综合类 | 365篇 |
数学 | 4326篇 |
物理学 | 16892篇 |
出版年
2024年 | 174篇 |
2023年 | 883篇 |
2022年 | 1622篇 |
2021年 | 1770篇 |
2020年 | 1860篇 |
2019年 | 1859篇 |
2018年 | 1633篇 |
2017年 | 1549篇 |
2016年 | 2139篇 |
2015年 | 2183篇 |
2014年 | 2702篇 |
2013年 | 3385篇 |
2012年 | 3926篇 |
2011年 | 4018篇 |
2010年 | 2832篇 |
2009年 | 2671篇 |
2008年 | 2972篇 |
2007年 | 2643篇 |
2006年 | 2366篇 |
2005年 | 1932篇 |
2004年 | 1481篇 |
2003年 | 1190篇 |
2002年 | 1238篇 |
2001年 | 1014篇 |
2000年 | 789篇 |
1999年 | 810篇 |
1998年 | 659篇 |
1997年 | 604篇 |
1996年 | 611篇 |
1995年 | 513篇 |
1994年 | 429篇 |
1993年 | 328篇 |
1992年 | 323篇 |
1991年 | 267篇 |
1990年 | 230篇 |
1989年 | 175篇 |
1988年 | 110篇 |
1987年 | 102篇 |
1986年 | 117篇 |
1985年 | 91篇 |
1984年 | 46篇 |
1983年 | 53篇 |
1982年 | 38篇 |
1981年 | 25篇 |
1980年 | 10篇 |
1979年 | 11篇 |
1978年 | 5篇 |
1976年 | 6篇 |
1975年 | 4篇 |
1957年 | 5篇 |
排序方式: 共有10000条查询结果,搜索用时 46 毫秒
81.
Mingyang Zhao Warren T. Ford Stefan H. J. Idziak Nicholas C. Maliszewskyj Paul A. Heiney 《Liquid crystals》1994,16(4):583-599
The hexa-4-dodecyloxybenzoyl derivative 1 of azacrown [18]-N6 was originally reported to have a 'tubular' mesophase on the basis of its large central ring and 6-fold symmetry. Starting in the mesophase, annealing of 1 under a cover slip results in formation of a new crystalline phase that melts directly to an isotropic liquid at the temperature previously observed for the mesophase to isotropic transition. Thus the phase behaviour of 1 is kinetically controlled. The analogous hexa-3,4-bisdodecyloxybenzoyl derivative 2 of azacrown [18]-N6 has no kinetic limitations to its phase changes and has an enantiotropic columnar liquid crystalline phase. We have synthesized side chain copolysiloxanes with a (CH2)11 spacer and 75-84 per cent by weight of the same 4-dodecyloxybenzoyl-[18]-N6 mesogen. The polysiloxanes also display a liquid crystalline phase. 相似文献
82.
c轴定向氮化铝薄膜的制备 总被引:3,自引:0,他引:3
利用电子回旋共振 (ECR)微波增强化学气相沉积法 (PECVD)并使用氮气 (N2 ) ,氩气 (Ar)和AlCl3蒸气作为气源在直径为 6 .35cm的 (10 0 )单晶硅片表面制备了c轴定向氮化铝 (AlN)薄膜 ,并使用X射线衍射仪及其X射线特征能谱和扫描电镜 (SEM)分析了薄膜特征 ,研究了微波功率、基板温度和N2 流量对薄膜c轴定向的影响 ,得到了c轴偏差角小于 5°的高质量大面积AlN薄膜。 相似文献
83.
YBa2Cu3O7-δ and Tl2Ba2CaCu2O8 thin films for microwave filters were synthesized by pulsed laser deposition and the two-step thalliation process. Substrate
quality requirements and the relation of thin film morphology, microstructure with microwave surface resistance were discussed. 相似文献
84.
记Ore2=min{d(y) d(x)|x,y∈V(G),d(x,y)=2},本得到:若n阶图G的Ore2≥n 1,则G是[5;n]泛连通图。此是比Faudree等人的定理进一步的结果。 相似文献
85.
The study of bed-load transport is of great significance both in theory and in practice. This paper discusses the saltation
of bed-load solid grains in flowing water. Experiments and theoretic analysis have been made by means of high-speed photographing
and advanced data processing technique with a combined method based on mechanical and statistical theories. It indicates that
the saltation is the main form of the bed-load transport under ordinary flowing conditions. In the meantime, taking successive
saltation as the model of bed-load transport, systematic analysis has been made with regard to the kinematic properties and
mechanism of saltation. The statistical analysis shows that the probability density functions of the relative height and length
of saltation are in conformity with Γ-type distribution, while the probability density functions of the relative velocities
of saltation are in conformity with the Gaussian distribution.
The project supported by National Natural Science Foundation of China 相似文献
86.
Let φ be a normal function on [0,1) and A'(φ)(1
相似文献
87.
Applied Biochemistry and Biotechnology - A novel, anaerobic fermentation process is developed to produce calcium magnesium acetate (CMA) from cheese whey. CMA can be used as a noncorrosive road... 相似文献
88.
用600keV的Kr~ 离子轰击Al/Cr双层薄膜样品进行界面原子反应及相互混合的研究。实验样品是在单晶硅上蒸镀约500nm厚的铝膜,相继再蒸上所需厚度的铬膜而制成的。轰击剂量为2.0×10~(15)-2.5×10~(16)Kr~ /cm~2。用2.0MeVa粒子对轰击前后的样品进行了卢瑟福背散射(RBS)分析,发现界面处有明显的原子混合存在;当轰击剂量≥1.0×10~(16)Kr~ /cm~2时,RBS谱出现有明显的坪台,经拟合计算和x射线衍射(XRD)测量证实确有化合物Al_(13)Cr_2存在;还分别得到了原子混合量及混合效率与轰击剂量的关系;最后对界面处的原子混合机制进行了讨论。 相似文献
89.
We describe one convenient synthesis route to boron nitride (BN) nanotube by the reaction of boron powder, iron oxide, and ammonium chloride at 600 °C for 12 h. Characterized by XRD, FTIR, XPS, TEM and SAED, the composition and morphology of the products are confirmed. The possible reaction mechanism is also discussed. 相似文献
90.
HAO Wei-Fang Tang Hui ZHU Xiao-Qing CHENG Jin-Pei 《有机化学》2003,23(Z1):172-172
Great interests have been accumulated in recent years in the chemistry and biochemistry of nitric oxide (NO) since the remarkable discoveries of its key roles in a wide range of human physiological processes. To elucidate the mechanistic details of NO migration from its donor to its acceptor, it is necessary to determine the Y-NO bind energy that registers the thermodynamic driving force for NO release and capture. In this paper the heterolytic and homolytic N-NO bond dissociation energies [ i. e., △Hhet(N-NO) and △Hhomo(N-NO)] for ten N-nitroso-p-substituted-benzensulfonyl methylamines in acetonitrile are offered, which were obtained from titration calorimetry and thermodynamic cycles, respectively (Scheme 1). 相似文献