全文获取类型
收费全文 | 78310篇 |
免费 | 13389篇 |
国内免费 | 8705篇 |
专业分类
化学 | 55571篇 |
晶体学 | 747篇 |
力学 | 5057篇 |
综合类 | 456篇 |
数学 | 9060篇 |
物理学 | 29513篇 |
出版年
2024年 | 184篇 |
2023年 | 1639篇 |
2022年 | 2856篇 |
2021年 | 2850篇 |
2020年 | 3298篇 |
2019年 | 3096篇 |
2018年 | 2667篇 |
2017年 | 2484篇 |
2016年 | 3740篇 |
2015年 | 3600篇 |
2014年 | 4327篇 |
2013年 | 5606篇 |
2012年 | 7061篇 |
2011年 | 7269篇 |
2010年 | 4856篇 |
2009年 | 4653篇 |
2008年 | 5011篇 |
2007年 | 4458篇 |
2006年 | 4096篇 |
2005年 | 3468篇 |
2004年 | 2707篇 |
2003年 | 2208篇 |
2002年 | 2114篇 |
2001年 | 1790篇 |
2000年 | 1541篇 |
1999年 | 1688篇 |
1998年 | 1392篇 |
1997年 | 1338篇 |
1996年 | 1364篇 |
1995年 | 1113篇 |
1994年 | 984篇 |
1993年 | 806篇 |
1992年 | 708篇 |
1991年 | 606篇 |
1990年 | 536篇 |
1989年 | 404篇 |
1988年 | 347篇 |
1987年 | 290篇 |
1986年 | 270篇 |
1985年 | 216篇 |
1984年 | 147篇 |
1983年 | 111篇 |
1982年 | 86篇 |
1981年 | 71篇 |
1980年 | 66篇 |
1979年 | 34篇 |
1978年 | 30篇 |
1977年 | 40篇 |
1976年 | 31篇 |
1973年 | 29篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
992.
流动注射在线阴离子树脂预富集火焰原子吸收测定痕量铜 总被引:10,自引:0,他引:10
研究了以阴离子交换树脂作吸附剂的流动注射在线微柱预富集火焰原子吸收测定铜的新方法。在最佳条件下富集150s,富集23倍,检出限0.2μg/L,RSD为1.6%,分析速度20次/h。天然水样中可能存在的离子不干扰铜的测定。所建立的方法已用于水样中痕量铜的测定。 相似文献
993.
A nickel catalyst was nodeled with ligand L^2,[NH=CH-CH=CH-0]^-,which should have potential use as a syndiotactic plyolefin catalyst,and the reaction mechanism was studied by theoretical calculations using the density functinal method at the B3LYP/LANL2MB level.The mechanism involves the formation of the intermediate [NiL^2Me]^ ,in which the metal occupies a T-shaped geometry.This intermediate has two possible structures with the methyl group trans either to the oxygen or to the nitrogen atom of L^2.The results show that both structures can lead to the desired product via similar reaction paths,A and B.Thus,the polymerization could be considered as taking place either with the alkyl group occupying the position trans to the Ni-0 or trans to the Ni-N bond in the catalyst.The polymerization process thus favors the catalysis of syndiotactic polyolefins.The syndiotactic synthesis effects could also be enhanced by varations in the ligand substituents.From energy considerations,we can conclude that it is more favorable for the methyl ttrans-O position to form a complex than to occupy the trans-N position.From bond length considerations,it is also more favoured for ethene to occupy the trans-O position than to occupy the trans-N position. 相似文献
994.
详细分析并解答了第33届中国化学奥林匹克(初赛)试题。本届初赛试题对知识储备的要求有所降低,但对思维能力与解题准确度的要求略有提高。每一道题目都附有详尽的分析、讨论与解答过程,引导读者综合运用所学的化学知识,通过推理、演算、论证等方法顺利解题。对于部分题目,还给出了科学背景介绍、知识拓展与思考题,鼓励读者了解题目背后的科学思想,举一反三。 相似文献
995.
996.
季铵盐用于示波极谱滴定法测定培氟沙星和环丙沙星 总被引:4,自引:0,他引:4
介绍采用季铵盐 十六烷基三甲基溴化铵作滴定剂的示波极谱滴定法测定培氟沙星和环丙沙星含量的方法。在pH 4 .8HAc NaAc缓冲溶液中 ,培氟沙星和环丙沙星均能与四苯硼钠 (Na TPB)作用生成沉淀 ,干过滤后 ,用十六烷基三甲基溴化铵标准溶液代替在示波极谱滴定法中常用的有毒硫酸亚铊作为滴定剂 ,回滴滤液中过量的Na TPB ,由示波极谱图 [dE dt=f(E)曲线 ]上TPB的切口消失指示终点。本法终点直观、灵敏 ,操作简便、快速 ,应用于多批原料药样品的测定 ,均获得满意的结果 ,其回收率在 99.9%~ 10 0 .3%之间 ,最大相对误差 <± 0 .3% ,与非水滴定法测得结果基本吻合。 相似文献
997.
Liu GD Zhong TS Huang SS Shen GL Yu RQ 《Fresenius' Journal of Analytical Chemistry》2001,370(8):1029-1034
A renewable amperometric immunosensor based on the sol-gel technique has been constructed by dispersing graphite, complement 3 (C3) antiserum, and sol-gel at low temperature. The prepared immunosensor is rigid, porous, and has a renewable external surface. A competitive binding assay has been used to determine C3 in human serum with the aid of C3 labeled with horseradish peroxidase. The enzyme-labeled antigen can readily diffuse toward the encapsulated antibody, which retains its binding properties. The experimental conditions for the assay with the biocomposite, including the loading of C3 antiserum in the biocomposite, the amount of labeled C3 in incubation solution, incubation time, and temperature, have been optimized. Using C3 labeled with horseradish peroxidase, and o-AP as the substrate, amperometric detection at -150 mV (relative to the SCE) results in a linear detection range of 1.17-35.1 microg mL(-1), with a detection limit of 0.56 microg mL(-1). Serum samples have been assayed and the results demonstrate the feasibility of the proposed immunosensor for clinical analysis. The surface of the immunosensor can be renewed simply by polishing to obtain a fresh immunocomposite ready to use in a new competitive assay. 相似文献
998.
Summary Direct separations of the enantiomers of cetirizine and related compounds have been achieved by reversed-phase HPLC on the
Chiralcel OD-R, a polysaccharide-derived chiral stationary phase; the mobile phase was usually perchlorate solution supplemented
with acetonitrile. Resolution of the enantiomers of cetirizine and related compounds was good. The effect of the acetonitrile
content of the mobile phase was investigated, and the effect of the structure of the chiral compounds on their behavior on
the Chiralcel OD-R column is discussed. 相似文献
999.
Hongyu Liu Xinhui Guo Juncheng Chen Aobo Song Gu Jin Qizhi Yao Jiao Li Lingling Li 《大学化学》1986,35(12):221-226
This paper introduces an experiment about waste liquid recovery and effect evaluation in laboratory. Through this experiment, the basic operations of crystal preparation, chemical analysis and instrumental analysis are further consolidated. At the same time, it allows students to experience a series of possible problems of condition selection and control in the process of waste liquid treatment. We hope to improve the students' ability to analyze and solve problems, enhance their interest in learning, and strengthen their humanistic education. 相似文献
1000.
Twelve compounds each having a six membered Si-contained heterocycle, have been successfully synthesized. Of these eleven compounds have never been reported in the literature. The structures were confirmed by Spectral analysis and elementary analysis. The stability and properties of these compounds is also discussed. 相似文献