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   3525篇
  免费   422篇
  国内免费   505篇
化学   1719篇
晶体学   37篇
力学   451篇
综合类   63篇
数学   1545篇
物理学   637篇
  2024年   8篇
  2023年   30篇
  2022年   61篇
  2021年   71篇
  2020年   91篇
  2019年   88篇
  2018年   87篇
  2017年   135篇
  2016年   117篇
  2015年   119篇
  2014年   197篇
  2013年   296篇
  2012年   199篇
  2011年   209篇
  2010年   164篇
  2009年   192篇
  2008年   182篇
  2007年   209篇
  2006年   176篇
  2005年   182篇
  2004年   191篇
  2003年   171篇
  2002年   153篇
  2001年   110篇
  2000年   124篇
  1999年   135篇
  1998年   106篇
  1997年   110篇
  1996年   63篇
  1995年   64篇
  1994年   63篇
  1993年   41篇
  1992年   40篇
  1991年   40篇
  1990年   30篇
  1989年   17篇
  1988年   22篇
  1987年   21篇
  1986年   26篇
  1985年   15篇
  1984年   14篇
  1982年   13篇
  1981年   7篇
  1980年   8篇
  1979年   12篇
  1978年   12篇
  1977年   9篇
  1976年   5篇
  1975年   3篇
  1972年   4篇
排序方式: 共有4452条查询结果,搜索用时 8 毫秒
51.
Mixed strategy -equilibrium points are given forN-person games with cost functions consisting of quadratic, bilinear, and linear terms and strategy spaces consisting of closed balls in Hilbert spaces. The results are applied to linear-quadratic differential games with no information and quadratic integral constraints on the control functions.This work was supported by a Commonwealth of Australia, Postgraduate Research Award.  相似文献   
52.
High-temperature superconductivity exists in layered, square-planar cuprates, but is almost absent in most other Cu(II) compounds and in most Ag(II) and Au(II) compounds. Valence state II is quite unusual in silver and gold and often disproportionates to valence states I and III ("negative-U compounds"). The two-electron difference in oxidation state is suggestive of electron pairing, a prerequisite for superconductivity. In the present paper the connection between disproportionation and geometrical structure on one hand and superconductivity on the other is discussed by using the accepted theory for mixed valence complexes. It is concluded that absence of superconductivity in gold and silver compounds can be connected to the instability of oxidation state II and the large difference in equilibrium geometry between oxidation states I and III.  相似文献   
53.
研究了具有简单结构的bola型阴离子表面活性剂二十酸二钠(C_(20)Na_2)与 阳离子型普通表面活性剂溴化十二烷基三乙铵(C_(12)Et_3)混合体系的表面性质 ,发现混合体系的cmc和γ_(cmc)比C_(12)Et_3单一体系未有显著降低。以负染色 ,FF-TEM,动态光散射(DLS)及粘度方法研究了混合体系的聚集行为,发现混合 体系中同时形成球形囊泡和管状聚集体,提出了产生这种聚集行为的机制。  相似文献   
54.
刘耀虎  刘鸣华 《中国化学》2002,20(6):601-605
IntroductionReversiblecolorchangesuponexternalorinternalstimulationshavebeenattractingmuchattentionduetotheirutilityasfunctionalmaterials .Forexample ,pho tochromismdescribesthecolorchangesinducedbypho toirradiationandcanbeusedasphoto recordingmateri als…  相似文献   
55.
Applying different surface and spreading techniques to form binary monolayers in a different mixing state, the mixing behavior of the three binary systems cholesteryl formiate/stearic acid, cholesteryl acetate/stearic acid, and cholesteryl-n-propionate/stearic acid were investigated and compared.Analyzing the force ()/area (a) isotherms and the equilibrium spreading pressures (e of the binary monolayers, it can be concluded that the components of the three binary systems do not mix within the whole concentration range. The lipids in the binary monolayers are completely immiscible.  相似文献   
56.
分光光度法同时测定中草药中的钼和铜   总被引:2,自引:2,他引:2  
本文建立了在混合显色剂(PF+PAN)的胶束体系中,利用分光光度法同时测定中草药中的钼和铜。实验表明:在混合体系中,钼量在0~10μg/25mL,铜量在0~50μg/25mL的范围内符合比尔定律,ε′Mo=9.2×104L·mol-1·cm-1;ε′Cn=1.5×104L·mol-1·cm-1。该法用于中草药样品的分析,结果满意,回收率可达92~111%。  相似文献   
57.
Substitution of Pb for Bi in the recently characterized mixed-valence lead-platinum oxide PbPt2O4 was attempted and a Pb1−xBixPt2O4 solid solution was obtained for 0≤x≤0.3. Powder X-ray diffraction study showed that all substituted compounds crystallize with similar triclinic unit cell and PbPt2O4 lattice parameters. The structural model of Pb0.7Bi0.3Pt2O4 was refined from powder X-ray diffraction data using the Rietveld method and the results indicate the same crystal structure than PbPt2O4 with one mixed Pb/Bi atomic site. Neutron diffraction realized on the two limit compositions of the solid solution (x=0 and 0.3) allowed to confirm the PbPt2O4 and Pb0.7Bi0.3Pt2O4 stoichiometries. Mean oxidation degree of Pt atoms in the [PtO4] infinite chains decreases from +3 for PbPt2O4 to +2.7 for Pb0.7Bi0.3Pt2O4. Conductivity measurements show a metallic behavior for all the compositions except the limit composition x=0.3 for which a semiconducting behavior appears.  相似文献   
58.
Transition metal molybdates of the formulaAMoO4 whereA=Fe, Co or Ni exhibit a first-order phase transition between 670K–970K. An investigation of the lowtemperature (lt) and high-temperature (ht) phases by x-ray photoelectron spectroscopy, x-ray absorption spectroscopy, magnetic susceptibility and other physical methods shows that the phase transition is associated with a valence change of the typeA 2++Mo6+αA 3++Mo5+ in the cases of iron and cobalt molybdates. Contribution No. 311 from the Solid State and Structural Chemistry Unit.  相似文献   
59.
Both single crystals of VCl3(THF)3 as well as isotypic cocrystals of the composition MCl3(THF)3, M=Ti/V 1/3, undergo a topotactic reversible phase transition to a hitherto unknown low-temperature modification. The close relationship between this new structure and the room-temperature phase determined by Cottonet al. is discussed from the molecular and the intermolecular point of view: Both modifications are built up by conformationally very similar molecules which change their arrangement during the phase transition. Lattice energy calculations confirm that these two alternative arrangements correspond to minima of almost the same packing energy.Dedicated to Professor Dr. G. E. Herberich on the occasion of his 60th birthday.  相似文献   
60.
The components of synthesis gas, CO, H2, and CO2, may be converted into CH4 biologically through either acetate or H2/CO2 as intermediates. Of these two routes, conversion through H2/CO2 is preferred. This paper presents results of mixed-culture studies employing the photosynthetic bacteriumR. rubrum for converting CO to CO2 and H2 by the water gas shift reaction and two methanogens,M. formicicum andM. barkeri, for converting CO2 and H2 into CH4. Results are presented for triculture operation in two types of reactors, the packed bubble column and the trickle-bed reactor.  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