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   1468篇
  免费   148篇
  国内免费   143篇
化学   1215篇
晶体学   14篇
力学   42篇
综合类   9篇
数学   99篇
物理学   380篇
  2023年   8篇
  2022年   18篇
  2021年   31篇
  2020年   51篇
  2019年   60篇
  2018年   39篇
  2017年   29篇
  2016年   60篇
  2015年   76篇
  2014年   74篇
  2013年   93篇
  2012年   102篇
  2011年   100篇
  2010年   73篇
  2009年   88篇
  2008年   82篇
  2007年   68篇
  2006年   64篇
  2005年   59篇
  2004年   48篇
  2003年   75篇
  2002年   69篇
  2001年   56篇
  2000年   38篇
  1999年   22篇
  1998年   11篇
  1997年   12篇
  1996年   20篇
  1995年   8篇
  1994年   15篇
  1993年   12篇
  1991年   13篇
  1990年   14篇
  1989年   6篇
  1988年   5篇
  1987年   7篇
  1986年   14篇
  1985年   20篇
  1984年   9篇
  1983年   8篇
  1982年   8篇
  1981年   15篇
  1980年   17篇
  1979年   9篇
  1978年   7篇
  1977年   7篇
  1976年   5篇
  1975年   5篇
  1974年   8篇
  1973年   8篇
排序方式: 共有1759条查询结果,搜索用时 47 毫秒
11.
Three New Diarylpropanes from Dioscorea composita   总被引:1,自引:0,他引:1  
Three new diarylpropanes were isolated from the dried rhizomes of Dioscorea composita Hemsl., and their structures were determined as 1, 3-bis-(2-hydroxy-4-methoxyphenyl) propane (1), 1, 3-bis-(2, 4-dihydroxyphenyl)propane (2), 1-(2′-hydroxy-4′-O-β-D- glucopymnosy-phenyl)-3-(2“, 4“-dihydroxyphenyl)propane (3), by spectroscopic analysis, respectively.  相似文献   
12.
An in‐line monitoring device using a quartz crystal resonator for thin film polymerization was proposed, and its performance has been evaluated by implementing in the UV polymerization of 2‐hydroxyethyl methacrylate with a photoinitiator of 1‐chloroanthraquinone. Because the variation of resonant resistance of the resonator is proportional to the square root of viscosity change that is closely related to the polymerization degree, the resistance can be used as a measure of the polymerization degree. The resistance measurements were compared with the outcome of instrumental analyses of polymerization degree using an FTIR spectrometer and a gel permeation chromatograph. The experimental results showed that the resistance measurements were consistent with the experimental outcome of the instrumental analyses, and this indicates the effectiveness of the proposed device. Owing to the simplicity and availability of the resonator system, its wide utilization in the monitoring of a variety of film polymerization processes, including photoresistor application, is expected. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 2428–2439, 2006  相似文献   
13.
Experimental observations for possible presence of 4f quadrupolar Kondo effect in PrCu2Si2 are critically discussed. A comparison of the low temperature properties of the alloys, Pr x Y1–x Cu2Si2 (x=0.05, 0.2 and 0.4), seems to signal the existence of 4f electric quadrupolar glass phenomenon around 4K forx=0.2.  相似文献   
14.
初级产品生产行业有两大特点:一是其初级投资有较大的不可逆转性,二是其产品价格具有较大的随机性(不确定性)。这两点的同时存在,给进入和退出初级产品行业造成壁垒。  相似文献   
15.
The regional cerebral blood flow (CBF) was determined on seventeen patients with brain tumors. Ring type single photon emission CT (SPECT) was used following intravenous injection of 133Xe. Case materials included eleven meningiomas and six malignant gliomas. Evaluation was performed with emphasis on the following points; Correlation of the flow data within tumors to the angiographic tumor stains, Influence of tumors on the cerebral blood flow of the normal brain tissue, Correlation between degree of peripheral edema and the flow data of the affected hemispheres. There was significant correlation between flow data within tumors and angiographic tumor stains in meningiomas. Influence of tumors on cerebral blood flow of the normal tissue was greater in meningiomas than in gliomas. There was negative correlation between the degree of peripheral edema and the flow data of the affected hemisphere. It has been concluded that the measurement of CBF in brain tumors is a valuable method in evaluation of brain tumors.  相似文献   
16.
17.
根据X射线衍射仪的结构特点,通过巧妙而简易的改装,增添了“同步照相”新功能。文中分析了开发新功能的可行性;列举了新功能在某些场合下,能使传统的照相法和现代的衍射仪法结合起来,取长补短的例子;指出新功能有助于提高分析测试水平,并可收到一机多用、节能高效的综合效果。  相似文献   
18.
提出了一个基于局部组成概念上的核磁共振模型. 利用该模型和以前别人提出的局部组成型粘度方程, 成功地同时关联属于传递性质的粘度数据和属于波谱性质的核磁共振化学位移数据,关联所得到化学位移的平均绝对偏差小于0.0072,粘度的平均绝对偏差小于0.0006 mPa•s,结果表明提出的局部组成模型是合理的.  相似文献   
19.
In order to clarify the mechanism of action for the antioxidative activity of fluvastatin sodium (FLV, (+/-)-sodium (3RS, 5RS, 6E)-7-[3-(4-fluorophenyl)-1-(1-methylethyl)-1H-indol-2-yl]-3, 5-dihydroxy-6-heptanoate) and its derivatives, reaction of the corresponding methyl ester of FLV with di-tert-butyl diperoxyoxalate was examined, and the corresponding keto derivative was isolated from the reaction mixture. On the basis of this result, it was concluded that the active site is the allylic carbon conjugated with the indole ring.  相似文献   
20.
A synthesis of 1,2,5,6-tetrahydro-4H-benzo[3,4]cyclohepta[1,2-e]imidazo[1,2-c]pyrimidine (XII) having a novel ring system is described. Antidepressive activity of XII and its precursors VII-X was screened by inhibitory action of reserpine-induced hypothermia.  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