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   1171篇
  免费   168篇
  国内免费   157篇
化学   593篇
晶体学   11篇
力学   240篇
综合类   19篇
数学   109篇
物理学   524篇
  2024年   4篇
  2023年   18篇
  2022年   41篇
  2021年   37篇
  2020年   44篇
  2019年   29篇
  2018年   39篇
  2017年   38篇
  2016年   54篇
  2015年   45篇
  2014年   57篇
  2013年   87篇
  2012年   65篇
  2011年   73篇
  2010年   53篇
  2009年   60篇
  2008年   74篇
  2007年   68篇
  2006年   70篇
  2005年   65篇
  2004年   60篇
  2003年   48篇
  2002年   54篇
  2001年   36篇
  2000年   30篇
  1999年   29篇
  1998年   29篇
  1997年   44篇
  1996年   27篇
  1995年   24篇
  1994年   19篇
  1993年   14篇
  1992年   7篇
  1991年   7篇
  1990年   12篇
  1989年   3篇
  1988年   6篇
  1987年   1篇
  1986年   8篇
  1985年   3篇
  1984年   3篇
  1983年   2篇
  1982年   3篇
  1981年   4篇
  1979年   1篇
  1978年   1篇
排序方式: 共有1496条查询结果,搜索用时 0 毫秒
961.
The production of radicals in the ozonolysis of ethene in air in a flow tube was monitored directly by a chemical amplification instrument at room temperature (298±2) K and 1 × 10^5 Pa. The radical yield is 0.50± 0.08 (σ) and found to be independent of CO. The result shows that the indirectly measured radical yields for the ozonolysis of ethene may be underestimated by a factor of 2.  相似文献   
962.
A functionality of the novel black silicon based nanostructured surfaces (BS 2) with different metal surface modifications was tested by time-of-flight secondary ion mass spectrometry (TOF SIMS). Mainly two surface functions were studied: analytical signal enhancement and analyte pre-ionization effect in SIMS due to nanostructure type and the assistance of the noble metal surface coating (Ag or Au) for secondary ion formation. As a testing analyte a Rhodamine 6G was applied. Bi+ has been used as SIMS primary ions. It was found out that SIMS signal enhancement of the analyte significantly depends on Ag layer thickness and measured ion mode (negative, positive). The best SIMS signal enhancement was obtained at BS2 surface coated with 400 nm of Ag layer. SIMS fragmentation schemes were developed for a model analyte deposited onto a silver and gold surface. Significant differences in pre-ionization effects can play an important role in the SIMS analysis due to identification and spectra interpretation.  相似文献   
963.
ABSTRACT. Given a paucity of empirical data, policymakers are forced to rely on modeling to assess potential impacts of creating marine reserves to manage fisheries. Many modeling studies of reserves conclude that fishing yield will increase (or decrease only modestly) after creating a reserve in a heavily exploited fishery. However, much of the marine reserves modeling ignores the spatial heterogeneity of fishing behavior. Contrary to empirical findings in fisheries science and economics, most models assume explicitly or implicitly that fishing effort is distributed uniformly over space. This paper demonstrates that by ignoring this heterogeneity, yield‐per‐recruit models systematically overstate the yield gains (or understate the losses) from creating a reserve in a heavily exploited fishery. Conversely, at very low levels of exploitation, models that ignore heterogeneous fishing effort overstate the fishing yield losses from creating a reserve. Starting with a standard yield‐per‐recruit model, the paper derives a yield surface that maps spatially differentiated fishing effort into total long‐run fishing yield. It is the curvature of this surface that accounts for why the spatial distribution of fishing effort so greatly affects predicted changes from forming a reserve. The results apply generally to any model in which the long‐run fishing yield has similar curvature to a two‐patch Beverton‐Holt model. A simulation of marine reserve formation in the California red sea urchin fishery with Beverton‐Holt recruitment, eleven patches, and common larval pool dispersal dynamics reinforces these results.  相似文献   
964.
田间喷施植物保健素或健壮素均能提高同源四倍体大麦叶绿素的含量,延缓功能叶的衰老,有利于可溶性糖从茎秆等营养器官向穗部生殖器官运转,使单株穗数从2. 1提高到2. 8 ,结实率从68. 7%提高到77.。%,千粒重从56. 60g提高到59. 80g.  相似文献   
965.
Improved intranuclear cascade model for nucleon-nucleus interactions   总被引:1,自引:0,他引:1  
  相似文献   
966.
作物产量构成因素通径分析方法的再商榷   总被引:3,自引:0,他引:3  
本文讨论了作物产量构成因素通径分析的方法。提出按因果关系将各因素对产量的效应剖分为多个路段,并进而计算与比较其效应总和及总决定程度。在各因素的变异较大时,对数转换后再进行通径分析才合理。  相似文献   
967.
葡萄籽中原花青素提取方法的进一步研究   总被引:10,自引:0,他引:10  
本次实验是在前人对葡萄籽原花青素提取方法研究的基础上,采用大孔树脂和聚酰氨柱层析的双重纯化,省略了许多复杂烦琐的工艺过程,提取出的葡萄籽原花青素纯度可达到99.37%,并能够获得3.82%的得率,取得了理想的效果。  相似文献   
968.
 分析了放电管稳定试验前的烘烤排气对等离子体焦点装置试验结果的影响,介绍了双真空烘烤排气台的原理和结构设计,并利用试验数据说明了该排气台在等离子体焦点装置试验中的重要性,在实际应用中具有潜在的实用价值。  相似文献   
969.
The synthesis of conjugated polymers 1 – 5 functionalized with 4,9‐dihydro‐s‐indaceno[1,2‐b:5,6‐b′]dithiophene‐4,9‐dione in the backbone is reported and their use in the construction of organic solar cells is demonstrated. Increasing the molar ratio of 2,7‐dibromo‐3,8‐dihexyl‐4,9‐dihydro‐s‐indaceno[1,2‐b:5,6‐b′]dithiophene‐4,9‐dione, relative to 4,4′‐dihexyl‐5,5′‐dibromo‐2,2′‐bithiophene, in the copolymer synthesis significantly lowers the solubility of these polymers. The incorporation of highly conjugated 3,8‐dihexyl‐4,9‐dihydro‐s‐indaceno[1,2‐b:5,6‐b′]dithiophene‐4,9‐dione unit into the polymer backbone has been confirmed by UV–vis absorption. The observation of decreasing quantum yield for the emission in the order of 1 , 2 , 3 is consistent with copolymers with different comonomer content. The power conversion efficiencies of solar cells using blends of these polymers with PCBM ([6,6]‐phenyl C61‐butyric acid methyl ester) were determined to be 0.11% for polymer 1 , 0.33% for 2 , and 0.26% for 3 , respectively. Under identical white light illumination, the power conversion efficiency of the device based on polymer 2 /PCBM as the active layer was three times higher compared to that of device based on polymer 1 /PCBM. Owing to the limited solubility and poor film‐forming ability of polymer 3 , the power conversion efficiency of solar cell based on 3 /PCBM blend is lower than that of 2 /PCBM blend, but is still larger than that of 1 /PCBM blend. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 2680–2688, 2008  相似文献   
970.
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