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   2060篇
  免费   306篇
  国内免费   143篇
化学   1369篇
晶体学   16篇
力学   93篇
综合类   28篇
数学   228篇
物理学   775篇
  2024年   3篇
  2023年   30篇
  2022年   34篇
  2021年   42篇
  2020年   60篇
  2019年   72篇
  2018年   54篇
  2017年   38篇
  2016年   70篇
  2015年   65篇
  2014年   82篇
  2013年   121篇
  2012年   133篇
  2011年   151篇
  2010年   89篇
  2009年   100篇
  2008年   94篇
  2007年   101篇
  2006年   110篇
  2005年   90篇
  2004年   86篇
  2003年   74篇
  2002年   68篇
  2001年   39篇
  2000年   45篇
  1999年   59篇
  1998年   47篇
  1997年   39篇
  1996年   47篇
  1995年   50篇
  1994年   41篇
  1993年   33篇
  1992年   41篇
  1991年   30篇
  1990年   29篇
  1989年   47篇
  1988年   37篇
  1987年   24篇
  1986年   21篇
  1985年   19篇
  1984年   10篇
  1983年   12篇
  1982年   12篇
  1981年   8篇
  1980年   7篇
  1979年   9篇
  1978年   6篇
  1976年   6篇
  1974年   4篇
  1973年   4篇
排序方式: 共有2509条查询结果,搜索用时 15 毫秒
51.
Starting in 1989 an experiment was run at PSI to directly measure the final sticking probability in muon catalyzed dt fusion. This experiment was based on an active-target ionization chamber (IC) built at Gatchina, Russia, and an array of plastic neutron counters. In three runs approximately 5×106 isolated alpha signals were recorded with around one half of these occurring in the inner chamber region where we have more complete understanding of the systematic errors. Particularly from a long run in 1992 we were able to obtain a very clean sticking peak of some 5000 events. However, to reach an accurate value of sticking, all systematic effects and several major backgrounds had to be understood in detail. To this end a Monte Carlo code was written to simulate the full electrostatic environment of the IC and to recreate completely each signal type including the actual tritium decay noise from the live experiment. A slightly model dependent value of approx. 0.56±0.04% is obtained for final sticking.  相似文献   
52.
The triton energy of the muon capture reaction 3He t+v, where 3 He is the ground state of muonic3He, has been measured in order to investigate a possible heavy v admixture into the flavour with high sensitivity. 3 He has been formed via the pd fusion reaction by stopping in an ionization chamber (IC) filled with an H/D gas mixture of 3% D concentration at a pressure of 161 bar. In a first short experiment 650 triton events were observed yielding an upper limit for the -heavy v mixing strength of 2.3×10–3 atE 0v=60 MeV.  相似文献   
53.
Complementary to the investigations of the most efficient dt cycle, also the other muon-induced fusion cycles in mixtures of hydrogen isotopes have been studied. The results of these dedicated experiments provide rich information about muon-induced few-body reactions and contribute significantly to a better overall understanding of CF. A summary of the recent progress will be presented. Special emphasis will be put on two characteristic examples, namely a new experimental approach to study the muonic cascade in H-D mixtures and the systematic study of hyperfine effects in muon-induced reactions.  相似文献   
54.
For the first time the isomerization of 1,3,2-diazaphospholidine-2-sclenide dervatives in refluxing benzene in the presence of trace water was observed. The structures of isomerized products were determined by IR, ^1H NMR,^31P NMR, elemental analysis and X-ray crystallography. The mechanism of isomerization was also proposed.  相似文献   
55.
过取代苯氧乙酰氯和α-羟基烷基胰酸酯的缩合反应合成了8种标题化合物,用元素分析,红外,核磁及质谱作了表征,测定了它们的除草活性和植物生长调节活性。  相似文献   
56.
The dispersed fluorescence spectra of Pb_2 dimer were recorded from the excitation of Ar~+ laser in HPOR. By the detailed spectroscopic analysis for this well-resolved fluorescence spectra, the ground state of pb_2 molecule and one of its electronically excited state were assigned preliminarily to be X~1∑_g~+ and F′Ⅱ_u state respectively.The spectral constants of the X and F are ω_e″=108.5 cm~(-1),ω_e″X_e″ = 0.209 cm~(-1), ω_e′=152.4 cm~(-1), ω_e′X_e′=0.636 cm~(-1) and T_e=19818,1 cm~(-1)., respectively  相似文献   
57.
58.
Six chiral diamide stationary phases (CSPs), namely N-(3-carbobenzoxypropionyl)-L-Val-tert-butylamide (CSP-1), N-undecenoyl-L-Val-S-α-phenylethylamide (CSP-2), N-undecenoyl-L-Val-R-α-phenylethylamide (CSP-3), OV-225-L-Val-tert-butylamide (CSP-4), XE-60-L-Val-tert-butylamide (CSP-5) and polycyanoethyl vinyl siloxane-L-Val-tert-butylamide (CSP-6), were inves-tigated and CSP-6 was crosslinked within narrow bore (70 μm) fused silica capillary columns. Theseparation of amino acid enantiomers on this narrow bore column by gas chromatography (GC) isillustrated.  相似文献   
59.
Two new routes to hexafluorotropone have been developed, one from hexachlorotropone and a superior synthesis from hexafluorobenzene. Hexafluorotropone was found to be a very weak base, with a conjugate acid pK(a) of -6.2 +/- 0.5. The tropone adds in [6 + 4] fashion to cyclopentadiene and photocyclizes to hexafluorobicyclo[3.2.0]hepta-3,6-dien-2-one. Lithium hydroxide in benzene transforms the tropone into pentafluorotropolone, which functions as a bidentate ligand.  相似文献   
60.
Extended Gaussian orbital basis set calculations have been carried out on an assumed staggered and eclipsed classical geometrical configuration of C2H. The best total energies obtained for these geometries were ?78.170692 a.u. and ?78.170674 a.u. respectively, corresponding to a barrier to internal rotation of 1.8 × 10?5 a.u. or 11 kcal/mole. An analysis of the charge density matrix indicates that charge is distributed in these molecules in a manner consistent with the concept of hyperconjugation.  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