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   142366篇
  免费   33710篇
  国内免费   10122篇
化学   119764篇
晶体学   1269篇
力学   8867篇
综合类   549篇
数学   14551篇
物理学   41198篇
  2024年   364篇
  2023年   5304篇
  2022年   4146篇
  2021年   5884篇
  2020年   8144篇
  2019年   6162篇
  2018年   5338篇
  2017年   3670篇
  2016年   9984篇
  2015年   9675篇
  2014年   10244篇
  2013年   12180篇
  2012年   12130篇
  2011年   10707篇
  2010年   9577篇
  2009年   9314篇
  2008年   7501篇
  2007年   6723篇
  2006年   5578篇
  2005年   4674篇
  2004年   3456篇
  2003年   2715篇
  2002年   2451篇
  2001年   2017篇
  2000年   1787篇
  1999年   2098篇
  1998年   1872篇
  1997年   1803篇
  1996年   1968篇
  1995年   1636篇
  1994年   1544篇
  1993年   1377篇
  1992年   1166篇
  1991年   1050篇
  1990年   847篇
  1989年   638篇
  1988年   577篇
  1987年   480篇
  1986年   454篇
  1985年   414篇
  1984年   346篇
  1977年   337篇
  1976年   397篇
  1975年   488篇
  1974年   516篇
  1972年   387篇
  1971年   377篇
  1970年   554篇
  1969年   437篇
  1968年   466篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
951.
设T是m-增生算子,I是恒等映射.本文通过一个反例说明(T+λI)-1(λ>0)不一定是非扩张映射.  相似文献   
952.
A laser micro-chopper has been designed to respond fast to a molecular beam. A method of spectral structure measurement is established to analyse the responding signals. In experiments, the minimum detectable signals correspond to an air molecular-beam intensity of 5 × 108/s  相似文献   
953.
在pH5.3的HAc-NaAc、2-(3,5-二溴-2-吡啶偶氮)-5-二乙氨基酚(3,5-Br2-PADAP)、溴代十六烷基三甲铵(CTMAB)溶液中,用线性扫描示波极谱法可得到锑(Ⅲ)的络合物吸附波。峰电位为-0.70V(vs.SCE)。二阶导数档峰高与Sb(Ⅲ)浓度在3.3X10-8~2.5X10-6mol/L范围内呈线性关系。检出限为8.2X10-9mol/L。用多种电化学方法研究了极谱波的性质及电极反应机理。络合物组成为Sb(Ⅲ):3,5-Br2-PADAP=1∶2。加入CTMAB可消除试剂峰且提高灵敏度。试验了30多种离子对峰电流的影响,用流基棉分离干扰离子。方法已用于矿样中锑的测定。  相似文献   
954.
关于结合方案和PBIB设计的定义及所用的有关符号见文献[1].设F_q为特征为2的有限域,q=2~r.熟知F_q上的对称矩阵合同于以下三种形式的矩阵(见[1]p.24).  相似文献   
955.
The black carbon aerosols are now recognized for their potential impact on climate and the halogenated compounds are thought to be very important in atmospheric chemistry. However, there are very few studies on the interactions between soot aerosols and halogenated compounds. We report the measurement of the uptake of CH3I on soot. The measured uptake coefficient is 0.0103 ± 0.0015 at a total pressure of 2–4 Torr and 298 K. The results suggest that the uptake of CH3I occurs efficiently on soot particles. The present work suggests that heterogeneous loss of CH3I on soot may influence the concentration of CH3I throughout the atmosphere.  相似文献   
956.
957.
Qiang Wang  Qing Dai 《Journal of Non》2007,353(4):354-365
A room temperature method for the encapsulation of pyrene in SiO2 nanoparticles is described. The relation between alkoxysilane surfactant chain length, reactant molar ratios and the uptake of dye, sample morphology, photophysical properties, and the ability of the silicate matrix to protect the encapsulated dye was examined. The synthesis can easily be adapted for the encapsulation of other hydrophobic and thermolabile substances, and used in the development of nanostructured optically active coatings, films and monoliths.  相似文献   
958.
959.
Usually, Sm2+ ions could be reduced by heating the materials in reducing atmospheres. Exposure to ionizing radiations is also known to cause Sm3+→Sm2+ conversion. In this work, BaBPO5 doped with the samarium ion was prepared by high temperature solid-state reaction. Sm2+ ions were obtained by two different reduction methods, i.e., heating in H2 reduced atmosphere and X-ray irradiation. The measurements of X-ray diffraction (XRD), and scanning electron microscope (SEM) were investigated. It is found that the conversion of Sm3+→Sm2+ is very efficient in BaBPO5 hosts after X-ray irradiation. Sm2+ ions under these two reduction methods exhibit different characteristics that were studied by measurements of luminescence and decay. The results showed that the luminescence properties of Sm2+ ions in BaBPO5 were highly dependent on the sample preparation conditions.  相似文献   
960.
Tao Wang  Jinfa Cai 《ZDM》2007,39(4):287-300
This study investigates Chinese teachers’ cultural beliefs concerning effective mathematics teaching through semi-structured interview with nine experienced teachers. For the Chinese teachers, an effective teacher should always be passionate and committed to the teaching profession. She should not only understand the knowledge in the textbook thoroughly but also be able to carefully craft the knowledge from the textbook for teaching by predicting possible students’ difficulties. Although Chinese teachers emphasize student participation and flexible teaching, they tend to see the teacher’s ability to design and lead coherent lessons as the key for facilitating students’ understanding. The result of this study helps researchers and educators understand the teacher-designed and content-oriented teaching model in Chinese classrooms.  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