全文获取类型
收费全文 | 26089篇 |
免费 | 2941篇 |
国内免费 | 6850篇 |
专业分类
化学 | 32641篇 |
晶体学 | 381篇 |
力学 | 209篇 |
综合类 | 191篇 |
数学 | 267篇 |
物理学 | 2191篇 |
出版年
2024年 | 24篇 |
2023年 | 238篇 |
2022年 | 508篇 |
2021年 | 733篇 |
2020年 | 1026篇 |
2019年 | 781篇 |
2018年 | 852篇 |
2017年 | 876篇 |
2016年 | 1290篇 |
2015年 | 1300篇 |
2014年 | 1596篇 |
2013年 | 2585篇 |
2012年 | 1851篇 |
2011年 | 1885篇 |
2010年 | 1603篇 |
2009年 | 1830篇 |
2008年 | 1922篇 |
2007年 | 1947篇 |
2006年 | 1930篇 |
2005年 | 1842篇 |
2004年 | 1774篇 |
2003年 | 1537篇 |
2002年 | 937篇 |
2001年 | 668篇 |
2000年 | 566篇 |
1999年 | 561篇 |
1998年 | 503篇 |
1997年 | 463篇 |
1996年 | 394篇 |
1995年 | 377篇 |
1994年 | 304篇 |
1993年 | 289篇 |
1992年 | 279篇 |
1991年 | 141篇 |
1990年 | 99篇 |
1989年 | 94篇 |
1988年 | 76篇 |
1987年 | 44篇 |
1986年 | 30篇 |
1985年 | 38篇 |
1984年 | 28篇 |
1983年 | 12篇 |
1982年 | 10篇 |
1981年 | 9篇 |
1980年 | 9篇 |
1978年 | 2篇 |
1976年 | 3篇 |
1975年 | 4篇 |
1974年 | 2篇 |
1968年 | 2篇 |
排序方式: 共有10000条查询结果,搜索用时 375 毫秒
231.
232.
立方介孔相含钇氧化硅的合成与表征 总被引:4,自引:0,他引:4
以低浓度的十六烷基三甲基溴化铵为结构导向剂,在碱性条件下通过水热法合成了具有立方结构的含钇Y MCM 48介孔分子筛材料。XRD测试表明当Y/Si<0.05时可以获得典型的长程有序介孔立方结构相,随Y/Si比的增加,晶胞参数的增大和红外吸收光谱(FTIR)的变化为Y进入介孔分子筛骨架中提供了有力证据。N2吸附-脱附实验给出这种立方介孔材料的BET表面积为1180m2·g-1,BJH平均孔径为3.4nm。紫外 可见漫反射光谱(UV vis)证明钇以一种六配位的形式存在。X射线光电子能谱(XPS)进一步证明钇以三价形式存在于立方介孔分子筛骨架中。 相似文献
233.
234.
Substantial progress has been made recently in extending the supramolecular assembly of biomimetic structures to vesicle-based sophisticated nanocomposites and mesostructures. We report herein the successful preparation of unilamellar surfactant vesicles coated with a monolayer of ring-shaped [Mo(154)] polyoxometalate (POM) nanoclusters, (NH(4))(28)[Mo(154)(NO)(14)O(448)H(14)(H(2)O)(70)]. approximately 350 H(2)O, by coulomb attractions using preformed didodecyldimethylammonium bromide (DDAB) surfactant vesicles as templates. The resultant vesicle-templated supramolecular assemblies are robust (they do not disintegrate upon dehydration) both at room-temperature ambient and vacuum conditions, as characterized by conventional transmission electron microscopy (TEM) and atomic force microscopy (AFM). The flexibility of the complex soft assemblies was also revealed by AFM measurements. The effect of POM-vesicle coulomb attractions on the dimensions of the templating vesicles was also investigated by using dynamic light scattering (DLS). Although origins of the structure stability of the as-prepared supramolecular assemblies are not clear yet, the nanometer scale cavities and the related properties of macroions of the POM clusters may play an important role in it. 相似文献
235.
236.
Hydrothermal Synthesis and Structure of a Copper(II) Complex, [Cu(4,4''''bpy)(H_2O)_3]SO_42H_2O 总被引:1,自引:0,他引:1
1 INTRODUCTION Recently, there has been increasing interest in crystal engineering of supramolecular architectures by means of covalent bonding, hydrogen bonding or other weak intermolecular interactions[1, 2], which reflect their potential applications in host-guest che- mistry, catalysis and function materials[3~5]. The construction of such materials is mainly provided by geometry of transition metal extending through rigid ligands. The oxidation state and coordination predisposition … 相似文献
237.
A novel linear polymer (PFABT) containing bithiazole rings was synthesized by polycondensation of 2,2′‐diamino‐4,4′‐bithiazole (DABT) and formaldehyde. The complexes of PFABT with two transition metal ions (Fe2+, Cu2+) were prepared for the first time. The polymer was determined through FT‐IR, 1H‐NMR and elemental analysis (EA), and the complexes were characterized by FT‐IR. The magnetic behaviors of these complexes were measured as a function of magnetic field strength (0–50 kOe) at 4 K and as a function of temperature (4–300 K) under an applied magnetic field of 30 kOe. The results show that PFABT‐Cu2+ is a ferromagnet while PFABT‐Fe2+ is an anti‐ferromagnet. Copyright © 2007 John Wiley & Sons, Ltd. 相似文献
238.
2-杂萘满并喹啉衍生物的合成及其抑菌活性刘永军,刘英(曲阜师范大学化学系曲阜273165)关键词杂萘满酮,合成,抑菌活性在2-杂萘满酮的化学结构,最常见的杂原子为O、S、N,分别俗称异色满酮、硫代异色满酮、异喹啉酮.自然界中的许多天然或代谢产物如卡拉... 相似文献
239.
6-Acetoxy-4-alkenyl arenes undergo regio- and stereoselective intramolecular Friedel-Crafts reaction affording benzocycles in moderate to excellent yields in TFA/HOAc (3:1). It was observed that introduction of alkyls or phenyl group to the allylic acetate moiety facilitates the cyclization reaction. The optically active tricyclic (4bR,8aS)-4b,7,8,8a,9,10-hexahydrophenanthrene skeleton could also be easily obtained in excellent yields. 相似文献
240.