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   5898篇
  免费   241篇
  国内免费   28篇
化学   3971篇
晶体学   71篇
力学   163篇
数学   439篇
物理学   1523篇
  2024年   4篇
  2023年   30篇
  2022年   87篇
  2021年   161篇
  2020年   110篇
  2019年   108篇
  2018年   87篇
  2017年   94篇
  2016年   151篇
  2015年   172篇
  2014年   221篇
  2013年   383篇
  2012年   432篇
  2011年   464篇
  2010年   297篇
  2009年   249篇
  2008年   431篇
  2007年   366篇
  2006年   378篇
  2005年   307篇
  2004年   289篇
  2003年   239篇
  2002年   234篇
  2001年   168篇
  2000年   111篇
  1999年   95篇
  1998年   54篇
  1997年   29篇
  1996年   45篇
  1995年   56篇
  1994年   34篇
  1993年   38篇
  1992年   29篇
  1991年   15篇
  1990年   31篇
  1989年   29篇
  1988年   14篇
  1987年   15篇
  1985年   22篇
  1984年   10篇
  1983年   11篇
  1982年   14篇
  1981年   13篇
  1980年   10篇
  1979年   3篇
  1978年   4篇
  1977年   5篇
  1975年   3篇
  1974年   3篇
  1969年   3篇
排序方式: 共有6167条查询结果,搜索用时 171 毫秒
201.
202.
Cultivation is the most expensive step in the production of biodiesel from microalgae, and substantial research has been devoted to developing more cost-effective cultivation methods. Plant hormones (phytohormones) are chemical messengers that regulate various aspects of growth and development and are typically active at very low concentrations. In this study, we investigated the effect of different phytohormones on microalgal growth and biodiesel production in Chlamydomonas reinhardtii and their potential to lower the overall cost of commercial biofuel production. The results indicated that all five of the tested phytohormones (indole-3-acetic acid, gibberellic acid, kinetin, 1-triacontanol, and abscisic acid) promoted microalgal growth. In particular, hormone treatment increased biomass production by 54 to 69 % relative to the control growth medium (Tris–acetate–phosphate, TAP). Phytohormone treatments also affected microalgal cell morphology but had no effect on the yields of fatty acid methyl esters (FAMEs) as a percent of biomass. We also tested the effect of these phytohormones on microalgal growth in nitrogen-limited media by supplementation in the early stationary phase. Maximum cell densities after addition of phytohormones were higher than in TAP medium, even when the nitrogen source was reduced to 40 % of that in TAP medium. Taken together, our results indicate that phytohormones significantly increased microalgal growth, particularly in nitrogen-limited media, and have potential for use in the development of efficient microalgal cultivation for biofuel production.  相似文献   
203.
A total synthesis of the three anti-coccidial agents WS-5995 A, B, and C by a Diels-Alder route is described. An intramolecular lactonization involving the displacement of a chlorine atom on a naphthoquinone was the key step in the synthesis of A.  相似文献   
204.
We establish a change of path formula for generalized Wiener integrals and develop a rotation theorem for Wiener measure with respect to Gaussian paths.  相似文献   
205.
In this study, we developed a method for the determination of PF‐04620110 (2‐{(1r,4r)‐4‐[4‐(4‐amino‐5‐oxo‐7,8‐dihydropyrimido[5,4‐f][1,4]oxazepin‐6(5H)‐yl)phenyl]cyclohexyl}acetic acid), a novel diacylglycerol acyltransferase 1 (DGAT‐1) inhibitor, in rat plasma and validated it using liquid chromatography–tandem mass spectrometry (LC‐MS/MS). Rat plasma samples were processed following a protein precipitation method by using acetonitrile and were then injected into an LC‐MS/MS system for quantification. PF‐04620110 and imipramine (internal standard) were separated using a Hypersil Gold C18 column, with a mixture of acetonitrile and 10 mm ammonium formate (90:10, v/v) as the mobile phase. The ion transitions monitored in positive‐ion mode [M + H]+ of multiple‐reaction monitoring were m/z 397.0 → 260.2 for PF‐04620110 and m/z 280.8 → 86.0 for imipramine. The detector response was specific and linear for PF‐04620110 at concentrations within the range 0.05–50 µg/mL and the signal‐to‐noise ratios for the samples were ≥10. The intra‐ and inter‐day precision and accuracy of the method matched the acceptance criteria for assay validation. PF‐04620110 was stable under various processing and/or handling conditions. PF‐04620110 concentrations in the rat plasma samples could be measured up to 24 h after intravenous or oral administration of PF‐04620110, suggesting that the assay is useful for pharmacokinetic studies in rats. Copyright © 2013 John Wiley & Sons, Ltd.  相似文献   
206.
We propose a method for fast switching of vertically aligned (VA) negative liquid crystals (LCs) by hiding the relaxation process of LCs. During the turn-off process, a strong in-plane electric field is applied for a short duration of time instead of relying solely on the slow relaxation of LCs. The LC molecules are rotated to the transmission axis of one of the polarisers by the applied in-plane electric field, resulting in turn-off switching that is 5.8 times faster than that of a conventional VA cell. By applying an overdriving scheme, we experimentally obtained a total response time of 3.3 ms.  相似文献   
207.
208.
We investigate the role of the t-channel meson exchange in various photoproduction processes to discuss features of the respective production mechanism. For the less model-dependent analysis we work with the t-channel meson pole reggeized in the Born approximation amplitude. With the meson–baryon coupling constants chosen consistently with symmetry prediction we show that the Reggeized pole model could reproduce the experimental data to a good degree in the lower energy region. Numerical consequences show the significance of the tensor meson exchange in the γ pK +Λ, the dominance of the pseudoscalar meson exchange in the πΔ (and KΣ*) processes, and the sizable role of the vector-meson magnetic moment in the charged ρ (and K*) photoproductions, respectively. These new features from the present analyses could provide a useful guide for future study of the N* resonances in the low energy region.  相似文献   
209.
210.
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