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   5343篇
  免费   666篇
  国内免费   536篇
化学   3826篇
晶体学   49篇
力学   260篇
综合类   42篇
数学   820篇
物理学   1548篇
  2024年   11篇
  2023年   75篇
  2022年   131篇
  2021年   181篇
  2020年   209篇
  2019年   215篇
  2018年   182篇
  2017年   160篇
  2016年   240篇
  2015年   227篇
  2014年   260篇
  2013年   357篇
  2012年   419篇
  2011年   453篇
  2010年   288篇
  2009年   259篇
  2008年   336篇
  2007年   248篇
  2006年   234篇
  2005年   230篇
  2004年   200篇
  2003年   186篇
  2002年   231篇
  2001年   175篇
  2000年   131篇
  1999年   98篇
  1998年   90篇
  1997年   51篇
  1996年   106篇
  1995年   72篇
  1994年   56篇
  1993年   42篇
  1992年   42篇
  1991年   35篇
  1990年   39篇
  1989年   26篇
  1988年   25篇
  1987年   27篇
  1986年   16篇
  1985年   25篇
  1984年   19篇
  1983年   8篇
  1982年   27篇
  1981年   21篇
  1980年   17篇
  1979年   19篇
  1977年   9篇
  1976年   9篇
  1975年   6篇
  1974年   5篇
排序方式: 共有6545条查询结果,搜索用时 15 毫秒
991.
A new method for biochemical oxygen demand (BOD) determination, which combines the series piezoelectric quartz crystal (SPQC) technique with the growth of a microorganism is presented in this paper. This method needs no immobilization of bacteria and is simple and convenient. When a calibration technique was used for BOD analysis, the detection time was 2.5 h at 37°C. There was a good linear relationship between the frequency shift and BOD value in the range 2.2–11 mg/L and the regression equation was ΔF = 64.10 + 11.23[BOD]. The proposed method was compared with the conventional BOD5 method. This method was more rapid than the BOD5 method and the results obtained by the former were in good agreement with those obtained by the latter. The experimental conditions are also discussed in detail.  相似文献   
992.
N,N,N',N'-Tetrabutyladipicamide (TBAA) was used for the extraction of nitric acid and uranyl(II) ion from nitric acid media into toluene. The effects of nitric acid, uranyl(II) ion, and extractant concentration, temperature and back extraction on the distribution coefficient of uranyl(II) ion have been studied. The main adduct of TBAA and HNO3 is TBAA·HNO3 in 1.0 mol/l nitric acid solution. The 1:2:2 complex of uranyl(II) ion, nitrate ion and TBAA as extracted species is further confirmed by IR spectra of the extraction of uranyl(II) ion with TBAA. The values of the thermodynamic parameters have also been calculated.  相似文献   
993.
In order to obtain the exact information of atomic number density in the ternary system of HNO3−UO2(NO3)2−H2O, the densities were measured with an Anton-Paar DMA60/602 digital density meter thermostated at 298.15±0.01 K. The apparent molal volumes for the systems were calculated from the experimental data. The present measured apparent molar volumes have been fitted to the Pitzer ion-interaction model, which provides an adequate representation of the experimental data for mixed aqueous electrolyte solutions up to 6.2 mol/kg ionic strength. This fit yields θ V , and ψ V , which are the first derivatives with respect to pressure of the mixing interaction parameters for the excess free energy. With the mixing parameters θ V , and ψ V , the densities and apparent molar volumes of the ternary system studied in this work can be calculated with good accuracy, as shown by the standard deviations.  相似文献   
994.
铝-荧光镓极谱络合吸附波的研究   总被引:2,自引:0,他引:2  
包晓玉  罗红霞  李南强 《分析化学》1999,27(11):1329-1332
在pH5.72的缓冲溶液中,用单扫示波极谱法可获得灵敏的铝-荧光镓络合吸附波。检测限达1.0×10^-8mol/L。测定了自来水中铝的含量,并测定了络合物组成和表面电极反应速度常数。  相似文献   
995.
Based on the phase-space generating functionalof the Green function, the canonical Ward identities(CWI) under local, nonlocal, and global transformationsin phase space for a system with a regular and singular Lagrangian have been derived. Therelation of global canonical symmetries to conservationlaws at the quantum level is presented. The advantage ofthis formulation is that one does not need to carry out the integration over canonicalmomenta in a phase-space path (functional) integral asin the traditional treatment in configuration space. Ingeneral, the connection between global canonicalsymmetries and conservation laws in classical theories isno longer preserved in quantum theories. Applications ofour formulation to the non-Abelian Chern-Simons (CS)theory are given, and new forms for CS gauge-ghost field proper vertices and the quantal conservedangular momentum of this system are obtained; thisangular momentum differs from the classical one in thatone needs to take into account the contribution of angular momenta of ghost fields.  相似文献   
996.
Reactions of trialkyltin oxides with 2‐, 3‐, and 4‐pyridinecarboxylic acids in 1:1 stoichiometry yield 10 corressponding trialkyltin esters R3SnO2CC5H4N. All compounds are characterized by elemental analysis, IR, 1H, 113C, and 119Sn NMR. The crystal structure of tributyltin ester of 2‐pyridinecarboxylic acid is determined by single crystal X‐ray diffraction. In this compound, the tin atom is rendered five‐coordinate in a trigonal bipyramidal structure by coordinating with 2‐pyridinecarboxylate group. The resulting structure is a linear polymer containing Sn O bonds (0.2278(5) and 0.2308(6) nm). © 2004 Wiley Periodicals, Inc. Heteroatom Chem 15:524–529, 2004; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/hc.20057  相似文献   
997.
采用扫描电子显微镜、X-光衍射分析和高真空苯吸附量的测定等方法,研究了在一定的化学环境下沸石分子筛的介稳状态;探讨了不同转晶条件对沸石重新晶化的影响;分别研究了在Y型、AG4型或Ω型沸石的合成条件下,加入其它型号的沸石转变为Y型、AG4或Ω型沸石的过程;并研完了在Y型沸石合成的化学环境下,A型沸石向Y型沸石转变的过程,并探讨其机理。  相似文献   
998.
999.
张宝峰  廖惕生 《大学物理》2000,19(8):33-35,37
利用微波分光仪测量介质板的微波吸收系数,考虑了接收器和介抽板对微波的反射影响,沿波传播方向移动介质板的位置来测量同波强度的极大值和极小值,以它们的平均值作为透射波的强度。还考虑了介质板两个界面的反射对入射波强度的影响。  相似文献   
1000.
依据基模(TEM00)高斯光束本身的横向光强分布,结合一类光强分布转换器件的转换特性,提出一种衫型低能耗光学超分辨的新方案,高斯光中央峰入射这类器件后,将从器件的边缘出射,而入射到边缘的光则从中央出射,由此,在成像系统中就可实现光学超分辨,与原有的滤波器比较,大大降低了能量的损耗。  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