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   125037篇
  免费   1339篇
  国内免费   461篇
化学   68484篇
晶体学   2037篇
力学   4957篇
综合类   5篇
数学   12106篇
物理学   39248篇
  2016年   1275篇
  2015年   983篇
  2014年   1319篇
  2013年   4777篇
  2012年   3379篇
  2011年   4385篇
  2010年   2733篇
  2009年   2513篇
  2008年   3923篇
  2007年   3981篇
  2006年   4038篇
  2005年   3962篇
  2004年   3453篇
  2003年   3151篇
  2002年   3057篇
  2001年   3574篇
  2000年   2674篇
  1999年   2195篇
  1998年   1886篇
  1997年   1872篇
  1996年   1802篇
  1995年   1772篇
  1994年   1545篇
  1993年   1534篇
  1992年   1753篇
  1991年   1745篇
  1990年   1677篇
  1989年   1683篇
  1988年   1685篇
  1987年   1674篇
  1986年   1587篇
  1985年   2135篇
  1984年   2262篇
  1983年   1887篇
  1982年   2212篇
  1981年   2027篇
  1980年   2073篇
  1979年   2058篇
  1978年   2186篇
  1977年   2091篇
  1976年   2115篇
  1975年   2053篇
  1974年   1895篇
  1973年   2103篇
  1972年   1282篇
  1971年   975篇
  1970年   912篇
  1969年   923篇
  1968年   1050篇
  1967年   1101篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
991.
The surface enhanced Raman spectrum of benzotriazole (BTAH) adsorbed on a copper electrode has been studied as a function of the potential applied to the electrode. The effect of pH and of the type of halide in the electrolyte solution has also been investigated. The presence of some complexes involving Cu(I), benzotriazole or benzotriazolate (BTA-) and the halide has been characterized. The protective film formed on copper surface, in the presence of benzotriazole, has been identified as cuprous benzotriazolate [Cu(I)BTA].  相似文献   
992.
Methods for the investigation of pore and molecular structure of synthetic membranes are reviewed. Membranes are classified as coarse-porous, fine-porous, and solution-diffusion membranes, on one hand; and homogeneous, asymmetric, and composite on the other, Pore structure of synthetic membranes can be elucidated in detail only by electron and raster electron microscopic investigations. Inspection of molecular structure requires diversely specific test probes such as low-energy neutron scattering and/or diffraction, and gas sorption and permeability measurements, as well as thermodynamic and thermomechanical analysis. Other methods used to elucidate pore and molecular structure of synthetic membranes are discussed and, concurrently, membrane structure is correlated with membrane permeability.  相似文献   
993.
The Sulfenic Fluorides CF3SF and CF2CISF and their Dimers The reactions of RfSCl (Rf = CF3, CF2Cl) with HgF2 and AgF give RfSF and the dimer product RfSF2SRf in high yield and various ratios, in contrast, activated KF leads only to RfSF in low yield. A complex of transition metal and sulfenic halide as an intermediate step is discussed for the dimerisation. As liquid CF3SF2SCF3 disproportionates into CF3SF3 and CF3SSCF3 and the hydrolysis of CF3SF2SCF3 gives the stable compound CF3S(O)SCF3 · PF3 reacts with RfSF as well as with RfSF2SRf to RfSPF4. The products of the spontaneous decomposition of CF2ClSF were investigated. I.r., n.m.r., and mass spectra are reported and discussed. It was possible to carry out vapour pressure measurements of CF2ClSF.  相似文献   
994.
Metabolites from Microorganisms, 6-Chlorogenistein and 6,3′-Dichlorogenistein A strain of Streptomyces griseus produced chlorinated metabolites in media containing soybean meal. Two of these metabolites were isolated together with a chlorine free compound which could be identified as the isoflavone genistein (I) by its spectral properties and by comparison with an authentic sample. By NMR. and mass spectroscopy the chlorinated compounds were shown to be 6-chlorogenistein (II) and 6,3′-dichlorogenistein (III). The 7-glucoside of genistein is a constituent of soybean meal; it is hydrolysed and chlorinated by Streptomyces griseus.  相似文献   
995.
The naphthalene absorption in the spectral region of 3500–4100 Å consists of two perpendicularly polarized band systems. This can be shown by polarization measurements. The arguments given do not allow a decision as to whether or not the short axis polarized band is the 3Ag3Biu+ transition or an induced vibronic band of the 3B3g3Biu+ transition generated by a b3g vibration mode. The T-T polarization of phenanthrene as a function of the excitation wavenumber is quantitatively compared with the equivalent S1 ← S0 fluorescence polarization. There is a good agreement between the calculated and the measured curve.  相似文献   
996.
Zusammenfassung Ein Gemisch von-Naphthylisocyanat mit etwa 6 1,4-Diazabicyklo-[2,2,2]-oktan (10–15 ml) reagiert quantitativ und sofort mit mg-Mengen Wasser unter Bildung von Kohlendioxid, das anschließend automatisch titriert wird. Wassermengen von etwa 0,3–3 mg lassen sich in etwa 8 min mit einer Standardabweichung von 0,8% rel. bestimmen.
The automatic determination of carbon and hydrogen. IV
Summary A mixture of about 10–15 ml of-naphthylisocyanate with about 6%. 1,4-diazabicyclo [2,2,2]-octane reacts instantaneously and quantitatively with water in mg quantities. The resulting carbon dioxide is automatically titrated. Amounts of water of about 0.3–3 mg were determined within 8 min. with a standard deviation of 0.8% rel.
  相似文献   
997.
Zusammenfassung Einige Änderungen an der Apparatur zur Stickstoffschnellbestimmung nachMerz wurden vorgenommen. Dadurch wird erreicht, daß zwischen den einzelnen Verbrennungen Sauerstoff das Verbrennungsrohr durchspült und vor dem Reduktionsrohr nach außen tritt. Der Verbrennungsraum ist daher bereits zu Beginn jeder Analyse mit Sauerstoff gefüllt, wodurch günstigere Bedingungen für die schlagartige Verbrennung erzielt werden. Der Umbau der von der Firma Heraeus gelieferten Apparatur und die Herstellung eines Oxydationskatalysators (Kobaltoxid-Wolframoxid) werden beschrieben.
Contribution to the rapid nitrogen determination according to merz
Summary The apparatus for rapid nitrogen determination according toMerz has been modified. Between each two combustions, oxygen is flushed through the combustion tube and is vented before the reduction tube. Therefore, at the beginning of each analysis, the combustion chamber is filled with oxygen, providing a more favorable atmosphere for the rapid ignition. The alterations in the Hereaus apparatus and the preparation of an oxidation catalyst (cobalt oxide and tungsten oxide) are described.
  相似文献   
998.
A series of phenylated dihydrophthalimides has been synthesized by the Diels-Alder reaction of tetraphenylcyclopentadienone with maleimide, N-phenylmaleimide, and N,N'-o-, -m-, and -p-phenylenedimaleimide. Subsequent dehydrogenation of these compounds yielded the corresponding phenylated phthalimides. These phthalimides were also obtained in one step when the initial reactions were carried out in refluxing nitrobenzene.  相似文献   
999.
Spectra of fluorobenzene (FB), pentafluorobenzene (PFB) and hexafluorobenzene (HFB) from 50000 to 93000 cm−1 are presented. Rydberg series converging to the first and to a higher ionization limit are found in each molecule. The orbital natures of the excited states and ions are discussed.  相似文献   
1000.
Zusammenfassung Es werden die Eigenschaften und Reaktionen von 1,3-Dichloruretidindion, einem neuen Dimeren des Chlorisocyanats, beschrieben. Durch thermische Isomerisierung entsteht der bereits bekannte N,N-Dichlor-N-carbonylharnstoff. Die IR-Spektren beider Verbindungen werden mitgeteilt.
Chemistry of chloro-isocyanate, III: 1,3-Dichlorouretidinedione, a new chloroisocyanate dimer
The properties and reactions of 1,3-dichloro-uretidinedione, a new dimer of chloro-isocyanate, are described. Its thermic isomerization gives the known N,N-dichloro-N-carbonyl-urea. The IR-spectra of both compounds are published.


Mit 2 Abbildungen  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