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   201372篇
  免费   2250篇
  国内免费   495篇
化学   110270篇
晶体学   3357篇
力学   7993篇
综合类   11篇
数学   19194篇
物理学   63292篇
  2020年   1726篇
  2019年   1920篇
  2018年   2237篇
  2017年   2356篇
  2016年   3519篇
  2015年   2196篇
  2014年   3549篇
  2013年   9083篇
  2012年   6506篇
  2011年   8016篇
  2010年   5699篇
  2009年   5657篇
  2008年   7161篇
  2007年   7074篇
  2006年   6700篇
  2005年   6054篇
  2004年   5524篇
  2003年   5099篇
  2002年   4868篇
  2001年   6142篇
  2000年   4627篇
  1999年   3518篇
  1998年   2745篇
  1997年   2703篇
  1996年   2593篇
  1995年   2441篇
  1994年   2333篇
  1993年   2165篇
  1992年   2828篇
  1991年   2733篇
  1990年   2673篇
  1989年   2691篇
  1988年   2714篇
  1987年   2721篇
  1986年   2591篇
  1985年   3304篇
  1984年   3313篇
  1983年   2599篇
  1982年   2712篇
  1981年   2700篇
  1980年   2475篇
  1979年   2821篇
  1978年   2821篇
  1977年   2946篇
  1976年   2784篇
  1975年   2529篇
  1974年   2473篇
  1973年   2439篇
  1972年   1676篇
  1968年   1705篇
排序方式: 共有10000条查询结果,搜索用时 546 毫秒
921.
Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 50, No. 3, pp. 385–391, March, 1989.  相似文献   
922.
Summary The influence of internal degrees of freedom on the behaviour of one-dimensional systems is discussed. For systems with half-filled bands the coupling to internalviz. lattice coordinates decides whether Peierls distortion is caused by intramonomer coordinates or by a lattice coordinate. Thereby the various intramonomer degrees of freedom act cooperatively. We show that there is a small regime of parameters where both kinds of distortion exist simultaneously. For increasing temperature we find that distortions can also move from the lattice coordinate to the intramonomer coordinate.  相似文献   
923.
High-average-power and high-energy lasers with good beam quality are fundamental tools for pumping high-peak and high-average-power ultrafast Ti:sapphire laser systems. Laser-diode (LD) pumped solid slate lasers with phase conjugation offer a unique combination of efficiency, reliability, and compactness. We present the design details and performance characteristics of a 360-W 1-kHz LD pumped Nd:YAG phase-conjugated laser. We also discuss the basic design aspects and present our preliminary experimental investigations on ultrashort-pulse laser systems.  相似文献   
924.
925.
The dissolution behavior of the aspirin enteric granule prepared using acylglycerols, glyceryl monostearate (GMS) and glyceryl trilaurate (GTL), was investigated in vitro and in human subjects in a fasting or non-fasting state. Aspirin was slowly released from the granule in vitro at pH 1.2. No acceleration of the aspirin dissolution rate in the medium without lipase and cholic acid was observed when the pH level of the medium increased to a neutral region (pH 6.4). However, the dissolution of aspirin was significantly increased by increasing the concentrations of lipase and cholic acid in the medium. Lipase appears to play an essential role in the dissolution process of aspirin granules. In human subjects, the average levels of the cumulative amount of total salicylate excreted in a urine-time curve, and the mean residence time (MRT) obtained after oral administration of a granule in the fasting state were markedly delayed in comparison with the results observed using an aqueous solution and a crystalline form of aspirin. In comparing the fasting condition with the non-fasting condition (after food ingestion), no significant difference was recognized in the total amount of salicylate excreted in urine to an infinite time (Ae(infinity)), whether the MRT was obtained by granule, crystalline form or aqueous solution. It can be concluded that aspirin granule prepared by GMS and GTL has a property of pancreatic lipase-sensitive dissolution, and its bioavailability is unaffected by food intake.  相似文献   
926.
We found a strong paraoxon-hydrolyzing activity (23.4 +/- 8.50 nmol/h/individual and 137 +/- 86.2 nmol/h/mg protein) in the crude extract from larvae of Culex tritaeniorhynchus Toyama 89, which is markedly resistant to organophosphorous insecticides. The activity was higher than those from Cx. tritaeniorhynchus re-e-ae (0.175 +/- 0.0336 and 1.83 +/- 0.651), Anopheles omorii (0.112 +/- 0.0301 and 1.86 +/- 0.746) and An. stephensi (0.0651 +/- 0.0713 and 0.789 +/- 0.910), which are susceptible to organophosphorous insecticides. These facts suggest that the high paraoxon-hydrolyzing activity plays a role in the development of organophosphorous resistance in Cx. tritaeniorhynchus. The enzyme preparation obtained from Toyama 89 showed higher activity in the alkaline pH range and its Km values to paraoxon were 0.67 mM in larvae and 0.50 mM in adults. A calcium ion was strictly required for the hydrolysis of paraoxon. Fenitroxon was also hydrolyzed, in addition to paraoxon. However, it did not degradate parathion and fenitrothion at all. Dichlorvos and phenyl acetate competitively inhibited the enzyme. The phenyl acetate-hydrolyzing activity in the preparation of Toyama 89 was significantly (p less than 0.01) lower than those in susceptible strains, and was irreversibly inhibited by paraoxon. Therefore, the paraoxon-hydrolyzing activity belongs to the class of organophosphate compound hydrolases; it must be thus distinguished from bacterial phosphotriesterase.  相似文献   
927.
928.
The optical properties of CdSe/ZnS semiconductor nanocrystals with the core-shell structure are studied upon visible-laser excitation in a wide range of flux densities. It is demonstrated that the dimensional quantization effect is preserved in the films with a limiting high concentration of nanocrystals. A strong bathochromic shift of the absorption and luminescence peaks relative to the peak positions in the corresponding spectra of nanocrystals in films with a relatively low concentration of nanocrystals and solutions is caused by a high concentration of nanocrystals and the dipole moment related to the asymmetry of the nanoparticles. The shift is varied from 35 to 50 nm depending on the film thickness. The luminescence spectra of the films remain unchanged upon an increase in the laser intensity to 1 × 106 W/cm2. The laser action on the nanoparticle films is studied at intensities (5 × 106?1 × 109 W/cm2) higher than the damage threshold.  相似文献   
929.
A binary alloy Schottky barrier diode on zinc oxide (ZnO) was developed using the combinatorial ion beam-assisted deposition system. The compositional fraction of the binary alloy was continuously varied using the composition-spread technique, to control the Schottky barrier height. After metal deposition, patterned Schottky diodes were fabricated on a ZnO single-crystal substrate. Pt-Ru alloy was selected from the work function viewpoint. Our experiments showed that the compositional fraction of the Schottky binary alloys changed continuously as designed and the Schottky barrier heights measured by current-voltage (I-V) measurements increased with increasing Pt content. Maximum barrier height difference for ZnO was 137 meV. Using ion beam deposition in parallel with the combinatorial system showed that the Schottky barrier heights for ZnO can be controlled by binary metal alloying.  相似文献   
930.
We give a concise review and extension of S-procedure that is an instrumental tool in control theory and robust optimization analysis. We also discuss the approximate S-Lemma as well as its applications in robust optimization.The many suggestions and detailed corrections of an anonymous referee are gratefully acknowledged.  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