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   2031篇
  免费   242篇
  国内免费   212篇
化学   1228篇
晶体学   33篇
力学   92篇
综合类   43篇
数学   198篇
物理学   891篇
  2024年   8篇
  2023年   38篇
  2022年   70篇
  2021年   61篇
  2020年   58篇
  2019年   83篇
  2018年   77篇
  2017年   49篇
  2016年   77篇
  2015年   66篇
  2014年   79篇
  2013年   112篇
  2012年   154篇
  2011年   146篇
  2010年   101篇
  2009年   114篇
  2008年   108篇
  2007年   93篇
  2006年   94篇
  2005年   109篇
  2004年   93篇
  2003年   74篇
  2002年   66篇
  2001年   93篇
  2000年   48篇
  1999年   53篇
  1998年   51篇
  1997年   40篇
  1996年   33篇
  1995年   36篇
  1994年   25篇
  1993年   23篇
  1992年   16篇
  1991年   16篇
  1990年   33篇
  1989年   17篇
  1988年   10篇
  1987年   8篇
  1986年   19篇
  1985年   8篇
  1984年   3篇
  1983年   3篇
  1981年   2篇
  1979年   4篇
  1978年   4篇
  1977年   2篇
  1976年   1篇
  1975年   1篇
  1974年   3篇
  1973年   1篇
排序方式: 共有2485条查询结果,搜索用时 15 毫秒
991.
甘峰  龚范 《分析化学》1998,26(1):51-54
将序贯为九论优化法,结合采用约束背影双线性比模型用于多组分“灰色”体系同时测定,且对SNTO的收缩比和收敛判据进行了改进。结果表明,SNTO法是一种实用的全局优化方法,收缩比和收敛判据的改进可大大缩短计算时间。  相似文献   
992.
超细碳化钨及其复合粉末的制备   总被引:9,自引:0,他引:9  
徐志花  马淳安  甘永平 《化学通报》2003,66(8):544-548,543
超细碳化钨(WC)是近年发展起来的硬质合金材料,本文根据超细碳化钨粉末在还原碳化过程中是否连续,将其制备方法分成一步法和两步法进行综述。同时,归纳了超细WC-Co复合粉末的主要制备方法,其中喷雾干燥-流化床法具有操作过程连续、复合粉与反应气体接触充分、复合粉末的活性高、反应温度低和最终粉末的晶粒细小均匀等优点,可望作为制备超细碳化钨及其复合粉末的重要方法。  相似文献   
993.
生物降解ε—己内酯/d,l—丙交酯共聚物的合成与表征   总被引:6,自引:0,他引:6  
采用一种新型的稀土配位化合物Y(CF3COO)3/Al(i-Bu)3为催化剂,制备了不同组成的ε-己内酯/d,l-丙交酯共聚物,并用GPC、NMR和DSC表征了共聚物的结构.结果表明通过改变初始投料中两种单体的比例,可以调节共聚酯的化学结构,而共聚物的形态则受结构影响很大.  相似文献   
994.
Sensitized photolysis of iminodiacetic acid methyl ester and maleimides follows a [2 + 3] cycloaddition pathway yielding pyrrolidine derivatives. This is similar to the photochemical reaction between C(60) and amines. A series of pyrrolidine derivatives are prepared by the method including multipyrrolidines from bis- and tris-maleimide starting materials. The yields range from 13% to 85%. The reaction is highly stereoselective. All the isolated products have the 1,3-dimethoxycarbonyl groups in the cis configuration. Various sensitizers may be used with slightly different yields. A plausible mechanism is proposed that involves the singlet oxygen abstraction of two alpha hydrogen atoms from the iminodiacetate and formation of a 1,3-dipole with a structure similar to the classical thermally generated 1,3-dipole.  相似文献   
995.
本文报道CAEM过程配位吸附溶出可逆波理论,用铟(Ⅲ)-铜铁试剂-醋酸和醋酸钠缓冲体系进行验证,实验结果与理论相符。  相似文献   
996.
997.
A new fullerene self-assembled monolayer (SAM) which has the property of photoelectric conversion is reported here. The SAM was fabricated on hydrophilic substrates by an esterification reaction. The SAM is characterized by contact angle, AFM, UV spectrum, and cyclic voltammetry. A cathodic photocurrent of 226 nA/cm(2) was obtained. Copyright 2000 Academic Press.  相似文献   
998.
Self-assemblies of octakis(octyloxy) phthalocyanine (PcOC8) and its sandwich lanthanide complex, substituted praseodymium bis(phthalocyanine) (Pr(PcOC8)(2)), with oligo(phenylene-ethynylene) (OPE) have been investigated by scanning tunneling microscopy (STM) on a highly oriented pyrolytic graphite (HOPG) surface. The assemblies were prepared by dissolving the molecules in phenyloctane solution. It was found that both PcOC8 and Pr(PcOC8)(2) can form 4-fold or 6-fold symmetrical adlayers on HOPG. The intramolecular structure of Pr(PcOC8)(2) molecule was revealed by a high-resolution STM image. When OPE molecules are added into phenyloctane solvent, Pr(PcOC8)(2) molecules prefer to form an ordered adlayer at the top of the OPE adlayer, while PcOC8 molecules adsorb on HOPG surface directly and form separated domains with OPE. These results may be helpful to construct surface assemblies and develop molecular electronic devices in the future.  相似文献   
999.
[reaction: see text] tert-Butylperoxy radicals add to [70]fullerene to form a mixture of adducts C(70)(OO(t)()Bu)(n)() (n = 2, 4, 6, 8, 10). Four isomers were isolated for the bis-adduct with the two tert-butylperoxo groups attached at 1,2-, 5,6-, 7,23-, and 2,5-positions, respectively. Two isomers were isolated for the tetrakis-adduct with the tert-butylperoxo groups located along the equator in C(s)() symmetry and on the side in C(1) symmetry, respectively. Similarly, two isomers were isolated for the hexakis-adducts with a structure related to the tetrakis-adducts, one of which has the cyclopentadienyl substructure. No isomer was detected for the octakis- and decakis-adducts. The C(s)()-symmetric octakis- and C(2)-symmetric decakis-adducts have all the tert-butylperoxo groups located along the equator. The decakis-adduct is the major product under optimized conditions. The compounds were characterized by their spectroscopic data. Chemical correlation through further addition of tert-butylperoxy radicals to isolated pure derivatives confirmed the structure assignment. Mechanisms of the tert-butylperoxy radical addition to C(70) follow two pathways: equatorial addition along the belt and cyclopentadienyl addition on the side.  相似文献   
1000.
Gan F  Xu QS  Liang YZ 《The Analyst》2001,126(2):161-168
In order to deal with the problem of simultaneous qualitative and quantitative analysis of the complicated samples in analytical chemistry, two novel procedures for automatic resolution of two-way data from coupled chromatography were developed in this work. The first can be used to determine automatically the number of components involved in a certain peak cluster. The second method is an iterative resolution procedure named the stepwise key spectrum selection which has also been developed to resolve automatically the spectra and chromatographic profiles of every component in the peak cluster investigated. The results obtained from simulated and real data show that the proposed methods perform fairly well.  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