首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   7215篇
  免费   464篇
  国内免费   824篇
化学   7016篇
晶体学   40篇
力学   334篇
综合类   85篇
数学   141篇
物理学   887篇
  2024年   13篇
  2023年   81篇
  2022年   361篇
  2021年   333篇
  2020年   265篇
  2019年   271篇
  2018年   233篇
  2017年   263篇
  2016年   389篇
  2015年   324篇
  2014年   338篇
  2013年   692篇
  2012年   417篇
  2011年   345篇
  2010年   295篇
  2009年   329篇
  2008年   303篇
  2007年   385篇
  2006年   339篇
  2005年   301篇
  2004年   295篇
  2003年   242篇
  2002年   192篇
  2001年   176篇
  2000年   128篇
  1999年   145篇
  1998年   112篇
  1997年   111篇
  1996年   123篇
  1995年   89篇
  1994年   96篇
  1993年   83篇
  1992年   73篇
  1991年   47篇
  1990年   54篇
  1989年   37篇
  1988年   40篇
  1987年   29篇
  1986年   21篇
  1985年   22篇
  1984年   17篇
  1983年   12篇
  1982年   23篇
  1981年   6篇
  1980年   11篇
  1979年   13篇
  1978年   7篇
  1977年   6篇
  1976年   6篇
  1973年   3篇
排序方式: 共有8503条查询结果,搜索用时 420 毫秒
1.
《Mendeleev Communications》2022,32(3):395-396
A new method for assembling 1,3-selenazolines by the iodine- mediated reaction of the simplest building blocks such as elemental selenium, alkenes and acetonitrile has been discovered. A proposed mechanism includes the addition of the intermediate selenium iodides to alkene with subsequent solvent interception by the formed seleniranium ion.  相似文献   
2.
《印度化学会志》2022,99(11):100719
The essential intention of the existing article is to illustrate the effect of wall properties on flow and thermal behavior, through a sinusoidal inclined wavy pipe. To succeed this phenomena we assume a sinusoidal pipe consist of wavy surface, whose walls traveling down to its borders and located at an inclined position, moreover titanium dioxide (TiO2), ferrosoferric oxide (Fe3O4) and nichrome (80% Ni and 20% Cr) are considered as nano-particles and unused engine oil is assumed as a base fluid. The arising mathematical equations for thermal and flow ratio with wall slip impact are solved by mathematica. Also, evaluate graphically the thermal and flow behavior of nanofluids for multi values of solid volume fractions (η), rigidity parameter (M1), stiffness parameter (M2), viscous damping parameter (M3), Grashof number (Gr), slip parameter (β) and heat generation parameter (H) also discuss the streamlines for different values of solid volume fraction.  相似文献   
3.
4.
We explore the influence of two solvents, namely water and the ionic liquid 1‐ethyl‐3‐methylimidazolium acetate (EmimAc), on the conformations of two cellulose models (cellobiose and a chain of 40 glucose units) and the solvent impact on glycosidic bond cleavage by acid hydrolysis by using molecular dynamics and metadynamics simulations. We investigate the rotation around the glycosidic bond and ring puckering, as well as the anomeric effect and hydrogen bonds, in order to gauge the effect on the hydrolysis mechanism. We find that EmimAc eases hydrolysis through stronger solvent–cellulose interactions, which break structural and electronic barriers to hydrolysis. Our results indicate that hydrolysis in cellulose chains should start from the ends and not in the centre of the chain, which is less accessible to solvent.  相似文献   
5.
The synthesis of vinyl alcohol copolymers is limited due to the poor radical reactivity of vinyl acetate (VAc), the traditional precursor to polyvinyl alcohol (PVA). Main group monomers such as BN 2-vinylnaphthalene (BN2VN) have attracted attention as alternatives to VAc to form side chain hydroxyls via oxidation, but outstanding questions of molecular weight control remain. Herein we report systematic investigation of solvent, temperature, and initiator concentration as factors influencing BN2VN degree of polymerization. We find increased chain transfer to toluene, hypothesized to arise from differences in radical stabilization and reactivity by aromatic and BN aromatic rings. As a result of these combined efforts, high molecular weight (Mw ~ 105 g mol−1) BN2VN homopolymers and BN2VN-styrene copolymers were obtained.  相似文献   
6.
利用分子筛择形特点,对煤直接液化油中的混合酚实施高效分离。本研究选取间甲酚和对甲酚作为分离煤直接液化油馏分段混合酚的模型化合物,采用化学液相沉积法对HZSM-5吸附剂的孔口结构进行改变,分析分子筛硅铝比及颗粒粒径对模型化合物间甲酚和对甲酚吸附分离性能的影响,以获得高性能固相吸附剂,并将其应用于180-190℃馏分段混合酚分离。结果表明,当分子筛硅铝比为25、粒径为3-5 μm时,分子筛的孔口结构调节效果最优;当正硅酸乙酯的最小用量为0.2 mL/g时,固相吸附剂的吸附量为0.03 g/g,对甲酚选择性高于95%。由于外表面沉积物对吸附剂的孔口结构变化,导致对甲酚选择性的提高。进一步采用HZSM-5(1)吸附剂对真实煤直接液化油混合酚的分离中发现,苯酚和对甲酚的选择性均达到100%。  相似文献   
7.
In this research work, selenium dioxide (SeO2) nanorods have been prepared by a solvothermal method in which a strong reducing agent (NaBH4) was used to reduce precursor salt into SeO2 nanorods. X-ray diffraction (XRD) technique was applied to observe the crystal structure which confirmed its tetragonal geometry. Moreover, morphology and particle size were studied by scanning electron microscopy (SEM). SEM fully described the 1-dimensional morphology of SeO2 nanorods which then arranged themselves to create a 3-dimensional flower-like structure with an average particle size of 50 nm. Also, the catalytic activity of SeO2 nanorods as diesel-additive was studied by defining different parameters such as fire and flash points, calorific value, cloud and pour points, specific gravity, and kinematic viscosity. Subsequently, SeO2 nanorods proved to be an excellent diesel additive due to higher total heat content and lower value of kinematic viscosity which enhances the better performance of the diesel engine.  相似文献   
8.
Thin films (monolayer and bilayer) of cylinder forming polystyrene‐block‐polydimethylsiloxane (PS‐b‐PDMS) were shear aligned by the swelling and deswelling of a crosslinked PDMS pad that was physically adhered to the film during solvent vapor annealing. The nanostructures formed by self‐assembly were exposed to ultraviolet‐ozone to partially oxidize the PDMS, followed by calcination in air at 500 °C. In this process, the PS segments were fully decomposed, while the PDMS yielded silica nanostructures. The highly aligned PDMS cylinders were thus deposited as silica nanolines on the silicon substrate. Using a bilayer film, the center‐to‐center distance of these features were effectively halved from 38 to 19 nm. Similarly, by sequential shear‐alignment of two distinct layers, a rhombic array of silica nanolines was fabricated. This methodology provides a facile route to fabricating complex topographically patterned nanostructures. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015 , 53, 1058–1064  相似文献   
9.
10.
新疆野生黑加仑与马林果实的营养成分分析   总被引:2,自引:0,他引:2  
通过对新疆野生黑加仑和野生马林的脂肪油,8种微量元素,17种氨基酸以及维生素C的含量进行测定得出:黑加仑脂肪油含量为17.9%,马林的脂肪油含量为22.3%,其中,黑加仑脂肪油中,r-亚麻酸的含量为19.02%,而马林中,其含量为9.40%,黑加仑中K,Na,Ca,Cu,Zn,Fe的含量比马林中的含量高,而Mn,Mg元素的含量却比马林含量低;马林的维生素C含量为16.22mg/g,黑加仑的含量为108mg/g,黑加仑的氨基酸的总量为1.63%,马林为1.14%。  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号