全文获取类型
收费全文 | 79346篇 |
免费 | 16772篇 |
国内免费 | 7266篇 |
专业分类
化学 | 71331篇 |
晶体学 | 1048篇 |
力学 | 3240篇 |
综合类 | 432篇 |
数学 | 7391篇 |
物理学 | 19942篇 |
出版年
2024年 | 105篇 |
2023年 | 890篇 |
2022年 | 1136篇 |
2021年 | 1646篇 |
2020年 | 2902篇 |
2019年 | 4120篇 |
2018年 | 2455篇 |
2017年 | 2110篇 |
2016年 | 5253篇 |
2015年 | 5454篇 |
2014年 | 5856篇 |
2013年 | 7143篇 |
2012年 | 6849篇 |
2011年 | 6337篇 |
2010年 | 5715篇 |
2009年 | 5648篇 |
2008年 | 5514篇 |
2007年 | 4508篇 |
2006年 | 4102篇 |
2005年 | 3683篇 |
2004年 | 3078篇 |
2003年 | 2672篇 |
2002年 | 3326篇 |
2001年 | 2404篇 |
2000年 | 2302篇 |
1999年 | 1337篇 |
1998年 | 900篇 |
1997年 | 791篇 |
1996年 | 789篇 |
1995年 | 687篇 |
1994年 | 598篇 |
1993年 | 488篇 |
1992年 | 471篇 |
1991年 | 392篇 |
1990年 | 329篇 |
1989年 | 277篇 |
1988年 | 217篇 |
1987年 | 195篇 |
1986年 | 169篇 |
1985年 | 173篇 |
1984年 | 115篇 |
1983年 | 78篇 |
1982年 | 65篇 |
1981年 | 42篇 |
1980年 | 21篇 |
1979年 | 14篇 |
1978年 | 5篇 |
1977年 | 6篇 |
1976年 | 5篇 |
1957年 | 4篇 |
排序方式: 共有10000条查询结果,搜索用时 62 毫秒
991.
Z. P. Hu Z. X. Shen L. Qin S. H. Tang M. H. Kuok G. Q. Xu K. F. Mok H. H. Huang 《Journal of Molecular Structure》1995,356(3):163-168
The high-pressure Raman studies of 7,7,8,8-tetracyanoquinodimethane (TCNQ) single crystals and polycrystalline CuTCNQ are presented in this paper. TCNQ shows a phase transition at 22 kbar, a pressure higher than reported earlier. CuTCNQ undergoes a first order phase transition at 30 kbar, which is characterized by the abrupt disappearance of all the Raman bands. 相似文献
992.
Multivariate classification methods were used to evaluate data on the concentrations of eight metals in human senile lenses measured by atomic absorption spectrometry. Principal components analysis and hierarchical clustering separated senile cataract lenses, nuclei from cataract lenses, and normal lenses into three classes on the basis of the eight elements. Stepwise discriminant analysis was applied to give discriminant functions with five selected variables. Results provided by the linear learning machine method were also satisfactory; the k-nearest neighbour method was less useful. 相似文献
993.
994.
The bridged tri-imidazoliums 3.3X^--5.3X^-(X^-=PF6^-,Br^-,I^-)and bis-imidazoliums 6.2PF6^- were synthesized by N-quaternization of imidazole derivative 1 in acetonitrile under reflux.UV spectroscopic titration experiments showed that the halide salts and hexafluorophosphate salts of these imidazoliums exhibited good recognition toward anions in water and in acetonitrile,respectively. 相似文献
995.
Four lanthanide coordination polymers with benzophenone‐4,4′‐dicarboxylic acid (H2bpndc) and 1,10‐phenanthroline (phen), [Ln2(bpndc)3(phen)] (Ln=La (1), Pr (2) and Tb (3)), [Yb(bpndc)15(phen)].05H2O (4) were obtained through solvothermal synthesis. The crystallographic data show that 1, 2, and 3 are isostructural, the Ln(III) ions in 1, 2 and 3 are all eight‐ and ten‐coordinated, respectively, and thus the Ln(III) ions are connected by bpndc ligands, resulting in an interpenetrating 3D structure. While in 4, the Yb(III) ions are eight‐coordinated and connected by bpndc ligands into a 3D structure with 1D rhombic channels, which result from the effect of lanthanide contraction from La(III) to Yb(III) ions, and the bpndc ligands in 1, 2, 3, and 4 display three types of coordination modes. 相似文献
996.
The adsorption of molecular oxygen at defective edge sites of zigzag and armchair graphite surfaces has been investigated by adopting cluster models in conjunction with density functional theory. Several different types of chemisorbed O2 species are identified. It was found that the defect edge sites exhibit the significant catalytic role toward the adsorption and activation of molecular oxygen. The O2 molecule is not only able to strongly bind to these edge sites, but the O–O bond strength is obviously weakened. Moreover, the calculated adsorption energy for O2 adsorbed on the clean graphite basal surface is fairly consistent with the weak interaction nature of O2 with the surface observed in the experiment, indicating one-layer cluster model is an effective way to study O2 adsorption on graphite surface in terms of accuracy and computational cost, which is in agreement with previous experience. Whereas, we note that the local detailed arrangement of edge carbon atoms can play an important effect on the adsorption of O2 on defect surfaces. 相似文献
997.
Ion-exchanger phase Spectrophotometry withp-nitrochlorophosphonazo (CPApN) has been developed for the determination of scandium. The linear range is 1–8 g of scandium in 50 ml of solution, using 0.8 g of resin, with an apparent molar absorptivity of 2.76 × 105 1 mol–1 cm–1. Aluminum and rare earth elements in reasonable amounts do not interfere. The method has been applied successfully to the determination of scandium in alloys, with relative standard deviations of 2–4%. 相似文献
998.
热动力学的研究 X. 一级反应的热谱峰高法 总被引:1,自引:0,他引:1
本文根据一级反应的热谱曲线方程、建立了速度常数的一种新算法。这种新方法,只需要热谱曲线在各个特征时刻的峰高,而不需要任何峰面积,即可算得反应体系的无量纲参数和速度常数,称为一级反应的热谱峰高法。 相似文献
999.
利用水热合成技术成功制备出一种新型多钒硼氧化合物, 用X射线单晶衍射分析技术对其晶体结构和分子结构进行了确定。结果表明在该化合物中多钒硼氧阴离子具有一个新颖的三明治结构。上下两个结构单元都是由六个VO5四角锥交替地通过顺式和反式共边的方式连接起来构成的一个钒氧三角形结构。中间的结构单元是由BO3平面三角形和BO4四面体以共角的方式相互连接形成的一个折叠型的B18O36(OH)6环。三明治结构中层与层之间通过桥氧相连。一个水分子处于它的核心位置上,与每个VO5四角锥中的钒原子都保持几乎相等的距离。该化合物及其晶体中存在着丰富的化学结构和成键信息,同时也有作为氧化还原反应催化剂的潜能。 相似文献
1000.
Two new isostructural tellurites, Pb(4)Te(6)M(10)O(41) (M = Nb(5+) or Ta(5+)), have been synthesized by standard solid-state techniques using PbO, Nb(2)O(5) (or Ta(2)O(5)), and TeO(2) as reagents. The structures of Pb(4)Te(6)Nb(10)O(41) and Pb(4)Te(6)Ta(10)O(41) were determined by single-crystal and powder X-ray diffraction. The materials exhibit a three-dimensional framework consisting of layers of corner-shared NbO(6) octahedra connected by TeO(3) and PbO(6) polyhedra. The Nb(5+), Te(4+), and Pb(2+) cations are in asymmetric coordination environments attributable to second-order Jahn-Teller effects. The Nb(5+) cations undergo an intraoctahedral distortion either toward a face or a corner, whereas the Te(4+) and Pb(2+) cations are in distorted environments attributable to their lone pair. In addition, the TeO(3) polyhedra strongly influence the direction of the Nb(5+) intraoctahedral distortion. Infrared and Raman spectroscopy, thermogravimetric analysis, and dielectric measurements are also presented. Crystal data: Pb(4)Te(6)Nb(10)O(41), monoclinic, space group C2/m (No. 12), with a = 23.412(3) A, b = 20.114(3) A, c = 7.5008(10) A, beta = 99.630(4) degrees, V = 3482.4(8) A(3), and Z = 4; Pb(4)Te(6)Ta(10)O(41), monoclinic, space group C2/m (No. 12), with a = 23.340(8) A, b = 20.068(5) A, c = 7.472(2) A, beta = 99.27(3) degrees, V = 3453.8(2) A(3), and Z = 4. 相似文献