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   3127篇
  免费   88篇
  国内免费   14篇
化学   2495篇
晶体学   32篇
力学   51篇
数学   131篇
物理学   520篇
  2022年   21篇
  2021年   21篇
  2020年   34篇
  2019年   35篇
  2018年   31篇
  2017年   17篇
  2016年   51篇
  2015年   44篇
  2014年   70篇
  2013年   130篇
  2012年   139篇
  2011年   144篇
  2010年   103篇
  2009年   99篇
  2008年   175篇
  2007年   132篇
  2006年   186篇
  2005年   179篇
  2004年   143篇
  2003年   139篇
  2002年   117篇
  2001年   68篇
  2000年   75篇
  1999年   52篇
  1998年   34篇
  1997年   47篇
  1996年   46篇
  1995年   33篇
  1994年   47篇
  1993年   30篇
  1992年   45篇
  1991年   29篇
  1990年   34篇
  1989年   33篇
  1988年   30篇
  1987年   49篇
  1986年   30篇
  1985年   44篇
  1984年   41篇
  1983年   30篇
  1982年   48篇
  1981年   42篇
  1980年   46篇
  1979年   42篇
  1978年   37篇
  1977年   41篇
  1976年   31篇
  1975年   20篇
  1974年   26篇
  1973年   22篇
排序方式: 共有3229条查询结果,搜索用时 31 毫秒
981.
We prepared a phenobarbital (PB) solid dispersion (SD) with phosphatidylcholine (PC). PB was present in an amorphous state in SD if its mole fraction was under 0.75. An infrared (IR) spectra study suggested a hydrogen bond between NH in PB and phosphate in PC, with a ratio of about 1:1. When the mole fraction of PB was less than 0.50, differential scanning calorimetry (DSC) curves showed endothermic peaks at 57, 90 and 145 degrees C, and an exothermic peak at 60 degrees C. The IR spectrum and X-ray diffraction pattern changed after holding at 70 degrees C, so at this point it is considered that the metastable state of SD changed into a stable state, and extra energy was released. When the mole fraction of PB was high, PB also arranged near hydrophobic group because an endothermic peak was observed at 46-52 degrees C, which was lower than fully hydrated PC. PB is similar to indomethacin (IM) in molecular shape and to phenytoin (PHT) in chemical structure. Its DSC curve and IR spectra are similar to PHT, and the limit ratio of its amorphous state is the same as IM. It is considered that the chemical structure is an important factor in its interaction to PC, and also, the molecular shape is important to arrange into PC lattice.  相似文献   
982.
Properties of proton diffractive dissociation have been investigated for four-prong final states in proton-proton reactions at 360 GeV/c, in particular forppppπ +π? 0(m=0,1,2). Mass distributions and decay angular distributions are given. The decay of the diffractive system is seen to be very anisotropic, and largep T is suppressed at higher masses. It is found that the “pomeron” couples with a single valence quark of the incident proton, but indications for a diquark-pomeron coupling are also found. Similarities with fragmentation in lepton-hadron deep inelastic scattering are underlined.  相似文献   
983.
In the presence of an optical active amyloxyaluminum or menthoxyaluminum chloride the asymmetric-induction alternating copolymerization of benzofuran or 1,3-cyclooctadiene and acrylonitrile or methacrylonitrile was investigated. The sign of optical rotation of copolymers was positive or negative, depending on the kind of catalyst used. The optical activity of copolymers of benzofuran and acrylonitrile attains a maximum value at an equimolar composition. The highest specific rotation [α]D of copolymers of benzofuran and acrylonitrile prepared in the presence of stoichiometric amount of (—)-menthoxyaluminum dichloride with respect to acrylonitrile was about ?8°. The stereo-structure of acrylonitrile unit may be controlled by the optical active aluminum compound onto which a strong coordination of acrylonitrile may occur.  相似文献   
984.
Dynamic theory developed previously by the authors is applied to photoionization from delocalized molecular orbitals. EXAFS-like oscillation is observed in any case, but the origin, period and magnitude differ from those of EXAFS usually observed in core photoionization. Numerical calcu- lations are performed in the energy range k = 3–6 a.u. for three molecules: H2O, CO and CO2. Analysis of such oscillatory behavior aids in the characterization of photoionized orbitals.  相似文献   
985.
The crystal and molecular structure of [N,N′-(3,3′-dipropylamine)bis(salicyclideneaminato)-monoacetate]cobalt(III) complex has been determined by a three-dimensional X-ray diffraction study. In the complex, the cobalt ion has an octahedral coordination environment with cis-geometry for the two salicyclideneaminato moieties. An oxygen atom of the acetate ion is coordinated to cobalt ion and another oxygen atom is hydrogen-bonded to the secondary amine nitrogen atom.  相似文献   
986.
Summary A numerical method for determining the distribution function of relaxation times from the decay curve of electric birefringence in macromolecular solutions has been described. The distribution function is expanded in a series of appropriate functions, and the best values of the coefficients are determined by the method of least squares. An application of this method to poly-Lglutamic acid in methanol is presented. It is shown that the distribution of molecular lengths can be obtained from the relaxation spectrum in the case of helical polypeptides in solution.
Zusammenfassung Es wird eine numerische Methode beschrieben, die die Relaxationszeit-Verteilungsfunktion aus der Abklingkurve der elektrischen Doppelbrechung bei makromolekularen Lösungen zu bestimmen erlaubt. Die Verteilungsfunktion wird in eine Reihe geeigneter Funktionen entwickelt und die besten Werte der Koeffizienten werden nach der Methode der kleinsten Quadrate bestimmt. Eine Anwendung dieser Methode zur Poly-L-glutaminsäure in Methanol-Lösung wird exemplifiziert. Es zeigt sich, daß die Verteilung von Moleküllängen im Falle helicaler Polypeptide aus dem Relaxationsspektrum gewonnen wird.
  相似文献   
987.
988.
989.
990.
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