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   23092篇
  免费   2146篇
  国内免费   2012篇
化学   8775篇
晶体学   142篇
力学   2710篇
综合类   262篇
数学   10625篇
物理学   4736篇
  2024年   72篇
  2023年   325篇
  2022年   615篇
  2021年   700篇
  2020年   870篇
  2019年   756篇
  2018年   749篇
  2017年   864篇
  2016年   917篇
  2015年   691篇
  2014年   1150篇
  2013年   2359篇
  2012年   1278篇
  2011年   1280篇
  2010年   1069篇
  2009年   1467篇
  2008年   1503篇
  2007年   1397篇
  2006年   1137篇
  2005年   1024篇
  2004年   895篇
  2003年   722篇
  2002年   689篇
  2001年   544篇
  2000年   546篇
  1999年   548篇
  1998年   434篇
  1997年   386篇
  1996年   345篇
  1995年   300篇
  1994年   243篇
  1993年   211篇
  1992年   157篇
  1991年   144篇
  1990年   98篇
  1989年   86篇
  1988年   73篇
  1987年   68篇
  1986年   56篇
  1985年   84篇
  1984年   88篇
  1983年   41篇
  1982年   55篇
  1981年   52篇
  1980年   40篇
  1979年   36篇
  1978年   19篇
  1977年   23篇
  1976年   15篇
  1974年   7篇
排序方式: 共有10000条查询结果,搜索用时 10 毫秒
41.
An efficient, more sensitive and accurate spectrophotometric process is possible and can be used to obtain qualitative or quantitative results for analytes at limiting concentrations lower than usual. Transmission measurements (incident light power, P0, or transmitted, P) are performed with a fluorescence spectrometer. One cell only is used to measure standards or unknown solutions, placed between the standard holder cuvette and the emission monochromator of the instrument. The source of the radiant power (P0) is not the xenon lamp but the fluorescence or scattered radiation from the holder cuvette filled with an appropriate solution. The analyte concentration is found from a calibration graph, based upon Beer’s law or the approximate formula P0P=2.303P0εbc, which is valid for dilute solutions. Determination of iron in a reference material, using 1,10-phenanthroline as the chromogenic reagent, was chosen as an example to demonstrate the suitability of the proposed method and clarify several important statistical questions. Also discussed is why and to what extent molecular fluorescence methods are more sensitive than molecular absorption methods.  相似文献   
42.
The structures and energies of the 1:1 acetylene/cyanoacetylene, acetylene/dicyanoacetylene and cyanoacetylene/dicyanoacetylene complexes in solid argon matrices have been investigated using FT-IR spectroscopy and ab initio calculations, at the B3LYP/6-31G** level of theory. For the three complexes, predicted frequency shifts for the L shaped structures, characterized by a hydrogen bond between the nitrogen of the cyano group and the acetylenic proton, were found to be in good agreement with those experimental. Only in the case of acetylene/cyanoacetylene complex, we obtained a second minimum with a T shaped structure characterized by an interaction between the proton of cyanoacetylene and the Π system of acetylene. It appears clearly that HC3N acts as an electrophile or as a nucleophile in these complexes.  相似文献   
43.
In the UV-Vis spectra of pure light-scattering systems, there is an exponential relationship between absorbance and wavelength (A = Kλ^-n). Here, the exponent n is named as flocculation-coagulation parameter. In the present paper, the effects of different additives on the stability of poly(N,N'-methylenebisacrylamide-co-4-vinylpyridine) (poly(Bis-co-4-VP)) microgel dispersion were studied in detail via this parameter. The results showed that the stability of the dispersion mainly comes from the ionization of pyridine groups, making the microgel positively charged on its surface. This was confirmed by the measurement of Zeta potential and the result of conductometric titration. The result of fluorescence analysis indicated that the hydrophobicity in the microgels is enhanced with the increase in total 4-VP unit content.  相似文献   
44.
 In April 1998 Raimond Castaing left the world of electrons, of ions and others particles, his wife and his family, his numerous students, for the world of stars. Raimond Castaing (Fig. 1) had a very strong personality. No one will forget their first meeting with him and all his students remember how brilliant he was as a teacher. A lot of anecdotes about his famous hot temper are still circulating among his friends and his former students. But in this paper, we would like to evoke Castaing’s memory through his achievements in Instrumental Physics, from the time of his doctoral thesis to later developments with his students, which were all centred on the imaging of the microstructures of materials and their quantitative chemical analysis.  相似文献   
45.
Time- and temperature-dependent SAXS-experiments were used to determine the effect of octene and butene co-units on the lamellar structure and the melting properties of polyethylene. As expected, melting points decrease with increasing co-unit content, but crystal thicknesses are not affected and depend on the crystallization temperature only. Results can be cast into some simple equations which describe the dependence:
1. Of the melting point Tf on the crystal thickness dc and the co-unit content xB
2. Of the equilibrium melting point on xB
3. Of dc on the crystallization temperature Tc
4. Of the long spacing L on Tc, xB and the molar mass
5. Of Tf on Tc.
  相似文献   
46.
    
An attempt has been made to apply Green’s function and partitioning technique to the case of XY2Z (ClO2F) pyramidal type molecule belonging toC, point group. The isotopic rules have been formulated. The force constants, rotation distortion constants and mean amplitude of vibration have been computed and compared with other observed and calculated values whichever is available.  相似文献   
47.
A new dicarboxylic acid, 2,4-bis(N-trimellitoyl)triphenylamine, bearing two preformed imide rings was synthesized from the condensation of 2,4-diaminotriphenylamine and trimellitic anhydride at 1:2 molar ratio. A series of poly(amide-imide)s (PAIs) with inherent viscosities of 0.38-0.66 dl/g were prepared by triphenyl phosphite-activated polycondensation from the diimide-dicarboxylic acid with various aromatic diamines. All the resulting PAIs were readily soluble in a variety of organic solvents and formed strong and tough films via solution casting. These PAIs have useful levels of thermal stability associated with moderately high glass-transition temperatures (259-314 °C) and 10% weight loss temperatures in excess of 530 °C in nitrogen or in air.  相似文献   
48.
The beta-delayed two-proton decay of theT z =–5/2 nuclide39Ti has been observed. The39Sc isobaric analog state has been calculated to lie at 8.82 MeV using the measured two-proton sum energy of 4750±40 keV for its decay to the37K ground state. Combining this excitation energy with a Coulomb displacement energy calculation has lowered the energy available for ground state two-proton emission of39Ti from 760 to 530 keV.This work was supported by Director, Office of Energy Research, Division of Nuclear Physics of the Office of High Energy and Nuclear Physics of the U.S. Depeartment of Energy under Contracts DE-AC03-76SF00098 with Lawrence Berkeley Laboratory, DE-AC02-76CH00016 with Brookhaven National Laboratory and DE-AC06-76RL0 1830 with Pacific Northwest Laboratories.  相似文献   
49.
Silyleneisthesimilarityofsilicontocarbene,and,likecarbeneinorganicreactions,itplaysaveryimportantroleinorganosiliconreactions.Inrecentyears,therehavebeenmanyreportsonsilylenesinquantumchemicalresearches[1—3].Investigationonitsorganicreactionshowsthatreali…  相似文献   
50.
The influence of the vapors ofn-amyl orn-decyl alcohol on the stability of single thin liquid films, single bubbles, and foam columns was determined. It was found that the presence of surfactant vapors lowered the stability of foams and single foam films. The mechanism of the destabilizing action of the surfactant vapors on wet, dynamic foams under dynamic conditions is discussed. It is shown that the destabilizing action of the surfactant vapors is a further indication that surface elasticity forces are the main factor determining stability of wet, dynamic foams.  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