全文获取类型
收费全文 | 56413篇 |
免费 | 5418篇 |
国内免费 | 11789篇 |
专业分类
化学 | 52126篇 |
晶体学 | 1977篇 |
力学 | 766篇 |
综合类 | 480篇 |
数学 | 4146篇 |
物理学 | 14125篇 |
出版年
2024年 | 170篇 |
2023年 | 1030篇 |
2022年 | 1966篇 |
2021年 | 1944篇 |
2020年 | 1973篇 |
2019年 | 1819篇 |
2018年 | 1517篇 |
2017年 | 1927篇 |
2016年 | 2022篇 |
2015年 | 1745篇 |
2014年 | 2308篇 |
2013年 | 5097篇 |
2012年 | 3545篇 |
2011年 | 3849篇 |
2010年 | 3313篇 |
2009年 | 3965篇 |
2008年 | 3822篇 |
2007年 | 3897篇 |
2006年 | 3680篇 |
2005年 | 3224篇 |
2004年 | 3095篇 |
2003年 | 2524篇 |
2002年 | 2141篇 |
2001年 | 1718篇 |
2000年 | 1639篇 |
1999年 | 1316篇 |
1998年 | 1123篇 |
1997年 | 1000篇 |
1996年 | 879篇 |
1995年 | 893篇 |
1994年 | 799篇 |
1993年 | 635篇 |
1992年 | 593篇 |
1991年 | 447篇 |
1990年 | 313篇 |
1989年 | 286篇 |
1988年 | 233篇 |
1987年 | 152篇 |
1986年 | 131篇 |
1985年 | 133篇 |
1984年 | 115篇 |
1983年 | 44篇 |
1982年 | 79篇 |
1981年 | 110篇 |
1980年 | 83篇 |
1979年 | 84篇 |
1978年 | 57篇 |
1977年 | 55篇 |
1976年 | 38篇 |
1973年 | 32篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
992.
Tsuyoshi Nakajima 《Journal of fluorine chemistry》2007,128(4):277-284
Recent results on the surface modification of petroleum cokes and their electrochemical properties as anodes of secondary lithium batteries are summarized. The surface of petroleum coke and those heat-treated at 1860-2800 °C were fluorinated by elemental fluorine (F2), chlorine trifluoride (ClF3) and nitrogen trifluoride (NF3). No surface fluorine was found except only one sample when ClF3 and NF3 were used as fluorinating agents while surface region of petroleum coke was fluorinated when F2 was used. Transmission electron microscopic (TEM) observation revealed that closed edge of graphitized petroleum coke was destroyed and opened by surface fluorination. Raman spectra showed that surface fluorination increased the surface disorder of petroleum cokes. Main effect of surface fluorination with F2 is the increase in the first coulombic efficiencies of petroleum cokes graphitized at 2300-2800 °C by 12.1-18.2% at 60 mA/g and by 13.3-25.8% at 150 mA/g in 1 mol/dm3 LiClO4-ethylene carbonate (EC)/diethyl carbonate (DEC) (1:1, v/v). On the other hand, main effect of the fluorination with ClF3 and NF3 is the increase in the first discharge capacities of graphitized petroleum cokes by ∼63 mAh/g (∼29.5%) at 150 mA/g in 1 mol/dm3 LiClO4-EC/DEC. 相似文献
993.
994.
M.L. Foo Q. Huang Wei-Li Lee I.S. Hagemann N.P. Ong 《Journal of solid state chemistry》2006,179(2):563-572
The synthesis, structure, and physical properties of five R-type Ru ferrites with chemical formula BaMRu5O11 (M=Li and Cu) and BaM′2Ru4O11 (M′=Mn, Fe and Co) are reported. All the ferrites crystallize in space group P63/mmc and consist of layers of edge sharing octahedra interconnected by pairs of face sharing octahedra and isolated trigonal bipyramids. For M=Li and Cu, the ferrites are paramagnetic metals with the M atoms found on the trigonal bipyramid sites exclusively. For M′=Mn, Fe and Co, the ferrites are soft ferromagnetic metals. For M′=Mn, the Mn atoms are mixed randomly with Ru atoms on different sites. The magnetic structure for BaMn2Ru4O11 is reported. 相似文献
995.
报道从中国南海网胰藻Hydroclathrus clathratus中分离得到3个单体化合物,经过MS,IR,1HNMR,13CNMR(DEPT),HMQC和HMBC等波谱技术鉴定为:(6R,7aS)-6-羟基-4,4,7a-三甲基-2,4,5,6,7,7a-六氢苯并呋喃-2-酮(1),1-(2-脱氧-β-D-呋喃核糖)-5-甲基-1,2,3,4-四氢嘧啶-2,4-二酮(2),尿嘧啶(3),并通过X射线单晶衍射实验确定化合物1的立体结构,其中化合物1是首次从自然界中分离得到. 相似文献
996.
Piotr Michorczyk Piotr Kutrowski Lucjan Chmielarz Jan Ogonowski 《Reaction Kinetics and Catalysis Letters》2004,82(1):121-130
Different Fe-containing catalysts (pure Fe2O3, Fe2O3 supported on active carbon or g-Al2O3, and hydrotalcite derived Mg-Fe oxides) were examined in the dehydrogenation of propane performed in an Ar or CO2 atmosphere at 873 K. A promoting effect of carbon dioxide was found for the Fe2O3 and Fe2O3/AC samples. The catalytic results are discussed in terms of redox properties of the catalysts determined by temperature-programmed
reduction (TPR).
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
997.
用密度泛函理论研究了CF3SO3CF2CF3 F^-的碳氧键断裂反应的机理。首先,用DFT方法优化了反应物、中间体、过渡态、产物的平衡构型,分析了碳氧键断裂反应的势能面变化,发现在SN2反应机理中,除了S-O断裂SN2反应外,引起C-O键断裂的同面进攻也是一个可能的反应途径。理论计算表明,最终反应的产物是受热力学控制的,S-O键的断裂绝对地优于C-O的断裂。因此,C-O断裂的同面机理虽然是可能的,但却难以被实验观察到,本文还讨论了端基-CF3在同面SN2反应中的邻位效应,以及基组对这个效应的影响。 相似文献
998.
In this work, Ba(NO3)2 crystals with single crystal face were induced by using the the method of bio-mimetic mineralization and double LB films of behenic acid (BA) as the template. The crystals were characterized by Scanning Electron Microscope (SEM) and X-Ray Diffraction (XRD). The crystals were observed in regular square shape with uniform size about 5~8 μm by SEM, and they were found by XRD to grow along the (111) plane. From these experiments, we can conclude that the good selection of the (111) crystal face of Ba(NO3)2 is due to the electrostatic interactions , the match between this crystal face and the definite lattice structure of the LB films. 相似文献
999.
Push-pull 2-alkylidene-4-oxothiazolidine vinyl bromides undergo efficient C(5) functionalization through DMSO-assisted carbon-bromine cleavage, followed by a bromine transfer-substitution (or elimination) sequence. A mechanism for this novel transformation is proposed. 相似文献
1000.
P. M. Kochergin I. A. Mazur G. K. Rogul'chenko E. V. Aleksandrova B. E. Mandrichenko 《Chemistry of Heterocyclic Compounds》2001,37(2):224-226
Two novel preparative methods have been developed for the synthesis of 2,3-dihydroimidazo[1,2-a]pyrimidine from 2-aminopyrimidine. These are the reaction with 1,2-dibromoethane and also the reaction with ethylene chloro(bromo)hydrin, subsequent treatment of 1--hydroxyethyl-2-imino-1,2-dihydropyrimidine with thionyl chloride, and cyclization of the intermediate 1--chloroethyl-2-imino-1,2-dihydropyrimidine in the presence of sodium hydroxide. 相似文献