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   4654篇
  免费   381篇
  国内免费   835篇
化学   5156篇
晶体学   144篇
力学   21篇
综合类   42篇
数学   6篇
物理学   501篇
  2024年   9篇
  2023年   43篇
  2022年   115篇
  2021年   104篇
  2020年   161篇
  2019年   145篇
  2018年   121篇
  2017年   160篇
  2016年   178篇
  2015年   189篇
  2014年   237篇
  2013年   464篇
  2012年   345篇
  2011年   266篇
  2010年   226篇
  2009年   262篇
  2008年   296篇
  2007年   272篇
  2006年   277篇
  2005年   288篇
  2004年   231篇
  2003年   249篇
  2002年   173篇
  2001年   135篇
  2000年   147篇
  1999年   100篇
  1998年   85篇
  1997年   105篇
  1996年   86篇
  1995年   85篇
  1994年   70篇
  1993年   43篇
  1992年   56篇
  1991年   32篇
  1990年   25篇
  1989年   16篇
  1988年   11篇
  1987年   9篇
  1986年   11篇
  1985年   7篇
  1984年   8篇
  1983年   2篇
  1982年   5篇
  1981年   2篇
  1980年   3篇
  1979年   8篇
  1978年   3篇
  1976年   1篇
  1975年   1篇
  1972年   2篇
排序方式: 共有5870条查询结果,搜索用时 0 毫秒
31.
Several new lightweight polymer concretes derived from industrial, agricultural, and naturally occurring waste materials were investigated. Various physicomechanical properties of these materials were studied. Such inexpensive and lightweight polymer composites, which have properties superior to ordinary reinforced cement concrete, may find wide applications in building technology and related areas and may serve as better substitutes for cement. Furthermore, recycling of such wastes from various sources may help to solve the present environmental pollution problems.  相似文献   
32.

A new anti‐static agent was synthesized from zinc oxide‐adipic acid‐polyethylene glycol and caprolactam by three‐step reactions. The antistatic agent (called poly(ether ester amide zinc oxide) or PEEAZ) was analyzed by IR and DSC. The results showed that zinc oxide existed in the main chain of PEEAZ. The glass temperature and melt temperature of poly(ether ester amide zinc oxide) (referred to as PEEAZ in the following) decreased with increasing poly(ether ester zinc oxide) increasing in PEEAZ. Antistatic PA6 fiber was obtained by adding PEEAZ 2–8% (wt/wt) to PA6 during blend spinning. The specific resistance and the static half‐value period of PA6 fiber was less than 109Ω · cm and 60 sec, respectively. Excellent antistatic property remained after being washed 30 times.  相似文献   
33.
Abstract

Zinc dust serves as a reducing agent in the presence of ammonium formate and NaOH, and is highly effective for the hydrodebromination of decabromodiphenyl ether to give diphenyl ether and the less brominated diphenyl ethers.  相似文献   
34.
《合成通讯》2013,43(22):3449-3454
ABSTRACT

A series of substituted 5-oxo-1,2,3,4,5,6,7,8-octahydroquinoline derivatives have been synthesized from 5,5-dimethyl-1,3-cyclohexane-dione (dimedone) and 1,3-diaryl-2-propen-1-one in DMF at 80°C in the presence of ammonium acetate with high yields (64–98%), the structure of the product was confirmed by X-ray analysis.  相似文献   
35.
A new cyclization route, triggered by epoxide opening, has been performed to provide the key intermediates for isoindolobenzapine alkaloids, lennoxamine and chilenine. The epoxide was prepared by the Stille reaction using vinyltributylstannane and the following dioxirane treatment. Cyclization under the treatment of BF3 · OEt2 provided an azepine moiety, and the oxidative cyclization toward the known precursor for the alkaloids has been achieved by reaction with a stoichiometric amount of Pd(OAc)2. This formal synthesis suggests a new route to the alkaloids.  相似文献   
36.
The first example of the stereoselective synthesis of (Z)‐ and (E)‐allyl aryl sulfides and selenides from Baylis? Hillman acetates under neutral conditions in H2O by supramolecular catalysis involving β‐cyclodextrin is reported. β‐Cyclodextrin can be recovered and reused. The reaction is very efficient in providing allyl aryl sulfides and selenides in good‐to‐excellent yields with clean reaction profiles under mild reaction conditions.  相似文献   
37.
Zinc-mediated facile and efficient chemoselective S-alkylation of 5-aryl 1,3,4-oxadiazole-2-thiols in the presence of a catalytic amount of tetra butyl ammonium iodide was described. The reaction was performed under neutral conditions. The chemoselectivity of the alkylation was confirmed by NMR spectroscopy and x-ray crystallography.  相似文献   
38.
《合成通讯》2013,43(9):1189-1195
Abstract

An efficient, cost‐effective, and mild method for the N‐acetylation of anilines and secondary amines with ammonium acetate in acetic acid media at reflux temperature in good yield is described.  相似文献   
39.
《合成通讯》2013,43(24):4477-4482
Abstract

Regioselective synthesis of α‐iodoacetates from alkenes, ammonium acetate, and iodine in acetic acid is reported. The reaction is facile, fast, environmentally, friendly, and cost effective. α‐Iodoacetates are obtained from both acyclic and cyclic alkenes in high yields ranging from 80–95% within 10–20 min.  相似文献   
40.
In this article, a mononuclear Mn(Niten)2H2O complex was prepared to catalyze hydrolysis of p-nitrophenyl acetate (PNPA) in presence of Cetyltrimethylammonium bromide (CTAB) micellar system in different pH range from 6.5–10 at 25°C. These results obtained indicate that the complex exhibits good catalytic function. It also appears the complex accelerates the hydrolytic cleavage of PNPA in cationic CTAB micellar solution which may be due to coordinating ability of substrate to complex, electrostatic interaction between micelles and complex and due to electrostatic interaction between micelles with reactant.  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