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   1640篇
  免费   187篇
  国内免费   291篇
  2024年   8篇
  2023年   55篇
  2022年   66篇
  2021年   89篇
  2020年   97篇
  2019年   114篇
  2018年   92篇
  2017年   75篇
  2016年   64篇
  2015年   64篇
  2014年   75篇
  2013年   126篇
  2012年   68篇
  2011年   71篇
  2010年   58篇
  2009年   70篇
  2008年   72篇
  2007年   96篇
  2006年   74篇
  2005年   67篇
  2004年   53篇
  2003年   61篇
  2002年   68篇
  2001年   54篇
  2000年   45篇
  1999年   35篇
  1998年   35篇
  1997年   40篇
  1996年   21篇
  1995年   19篇
  1994年   27篇
  1993年   20篇
  1992年   13篇
  1991年   18篇
  1990年   9篇
  1989年   16篇
  1988年   10篇
  1987年   11篇
  1986年   6篇
  1985年   11篇
  1984年   9篇
  1982年   9篇
  1981年   3篇
  1980年   4篇
  1979年   4篇
  1978年   5篇
  1976年   3篇
  1975年   2篇
  1974年   3篇
  1973年   1篇
排序方式: 共有2118条查询结果,搜索用时 46 毫秒
91.
92.
The derivatives of 1,4‐dimethoxybenzene are thus far the best performing redox shuttle additives for overcharge protection of Li‐ion batteries. The most durable molecules of this kind typically possess two in‐plane methoxy groups that are equivalent by inversion symmetry. However, such geometry leads to a vanishing average dipole moment that causes poor solubility of these molecules in carbonate‐based electrolytes. In this study, a novel redox shuttle additive, 1,2,3,4‐tetrahydro‐6,7‐dimethoxy‐1,1,4,4‐tetramethyl‐naphthalene (TDTN), is introduced. It has been demonstrated that reversible oxidation at 4.05 V versus Li+/Li, high polarity, high solubility (around 0.4 m ), and excellent electrochemical stability (150 overcharge cycles at C/2 rate with 100% overcharge) can all be achieved simultaneously by the imposition of axial symmetry in the corresponding radical cation that is generated by electrochemical oxidation of TDTN in the battery. The intricate interplay between the symmetry and the chemical stability of the radical cation is scrutinized using magnetic resonance spectroscopy and electron structure modeling.  相似文献   
93.
A major hallmark of the autoimmune demyelinating disease multiple sclerosis (MS) is immune cell infiltration into the brain and spinal cord resulting in myelin destruction, which not only slows conduction of nerve impulses, but causes axonal injury resulting in motor and cognitive decline. Current treatments for MS focus on attenuating immune cell infiltration into the central nervous system (CNS). These treatments decrease the number of relapses, improving quality of life, but do not completely eliminate relapses so long-term disability is not improved. Therefore, therapeutic agents that protect the CNS are warranted. In both animal models as well as human patients with MS, T cell entry into the CNS is generally considered the initiating inflammatory event. In order to assess if a drug protects the CNS, any potential effects on immune cell infiltration or proliferation in the periphery must be ruled out. This protocol describes how to determine whether CNS protection observed after drug intervention is a consequence of attenuating CNS-infiltrating immune cells or blocking death of CNS cells during inflammatory insults. The ability to examine MS treatments that are protective to the CNS during inflammatory insults is highly critical for the advancement of therapeutic strategies since current treatments reduce, but do not completely eliminate, relapses (i.e., immune cell infiltration), leaving the CNS vulnerable to degeneration.  相似文献   
94.
【背景】瓜实蝇是一种重要的世界性检疫害虫,广泛分布在我国的热带、亚热带地区。在室内和田间测定了液体植物保护膜对瓜实蝇的控制效能,并初步探索了该保护膜的作用方式,以期为瓜实蝇的绿色防控提供一定的技术支撑。【方法】测定了应用保护膜后瓜实蝇的产卵、存活以及对苦瓜的为害率。【结果】保护膜显著影响了瓜实蝇产卵、存活以及选择性。随着保护膜浓度的升高,瓜实蝇的存活率和在供试苦瓜段上的产卵量逐渐下降,但300和400倍保护膜之间的差异不显著。当浓度达到400倍时产卵量最低,为0.33粒,死亡率最高,约为50%;室内研究发现保护膜可持续影响瓜实蝇产卵5 d;此外,通过将保护膜应用在损伤的苦瓜上,发现保护膜并没有抑制瓜实蝇产卵。在田间,研究发现苦瓜的果长影响了保护膜的应用效能,2种果长的苦瓜应用保护膜后12~14 cm苦瓜的被害率显著低于6~8 cm苦瓜的被害率。【结论与意义】400倍的植物液体保护膜是防治瓜实蝇的最佳浓度,应用时间建议每4 d一次。保护膜最合适的应用时期为果长发育至12~14 cm时,即生长后期。施用保护膜时一定均匀,且在寄主果实受到损伤时不宜喷施保护膜。  相似文献   
95.
Crop protection is an integral part of establishing food security, by protecting the yield potential of crops. Cereal aphids cause yield losses by direct damage and transmission of viruses. Some wild relatives of wheat show resistance to aphids but the mechanisms remain unresolved. In order to elucidate the location of the partial resistance to the bird cherry–oat aphid, Rhopalosiphum padi, in diploid wheat lines of Triticum monococcum, we conducted aphid performance studies using developmental bioassays and electrical penetration graphs, as well as metabolic profiling of partially resistant and susceptible lines. This demonstrated that the partial resistance is related to a delayed effect on the reproduction and development of R. padi. The observed partial resistance is phloem based and is shown by an increase in number of probes before the first phloem ingestion, a higher number and duration of salivation events without subsequent phloem feeding and a shorter time spent phloem feeding on plants with reduced susceptibility. Clear metabolic phenotypes separate partially resistant and susceptible lines, with the former having lower levels of the majority of primary metabolites, including total carbohydrates. A number of compounds were identified as being at different levels in the susceptible and partially resistant lines, with asparagine, octopamine and glycine betaine elevated in less susceptible lines without aphid infestation. In addition, two of those, asparagine and octopamine, as well as threonine, glutamine, succinate, trehalose, glycerol, guanosine and choline increased in response to infestation, accumulating in plant tissue localised close to aphid feeding after 24 h. There was no clear evidence of systemic plant response to aphid infestation.  相似文献   
96.
97.
在国际趋势的压力下,兼受跨太平洋伙伴关系协定(TPP)影响,我国实施药品数据保护制度已成大势所趋。鉴于药品数据保护与药品注册审批体系紧密相关,我国化学药注册分类改革后,药品数据保护对医药产业发展必然会带来不同影响。结合我国分类注册改革新标准,在理论上对数据保护制度实施效果进行研究,进而探讨新注册分类下数据保护的实施对1至4类药品及医药产业的影响。  相似文献   
98.
Novel quinazolin- and benzothiazol-6-yloxyacetamides show excellent in vivo activity against the three economically most important Oomycete pathogens Phytophthora infestans, Plasmopara viticola and Pythium ultimum. They are polar analogs of known quinolin-6-yloxyacetamides, which are not active against the soil-borne damping-off disease caused by Pythium ultimum. The Bogert quinazoline synthesis, an almost forgotten heterocyclization technique, proved to be highly useful for the concise construction of required quinazolin-6-ol building blocks.  相似文献   
99.
游径系统一直是广东城乡规划建设的亮点,也是推动高质量发展与高水平保护的重要手段。以广东省绿道建设和古驿道建设为例,通过厘清两类游径的缘起和作用,总结风景园林专业力量在绿道建设实施过程中形成的风景园林遗产保护、大地景观规划、园林与景观设计、园林工程与技术、园林植物应用、景观生态学等专业设计之“道”,提出在南粤古驿道建设的大背景下,风景园林人应充分发挥风景园林专业优势,以“道”传“道”,将绿道专业设计之“道”传承到南粤古驿道建设中去,推动南粤古驿道建设高质量发展,同时为风景园林专业设计人员以南粤古驿道为载体进行景观创作提供路径与方法。  相似文献   
100.
植物作为自然界生态系统的重要组成部分,其多样性的保护与发展是人类可持续发展的物质基础之一,对维护生态环境的平衡与稳定具有重要意义,也是当今中国生态园林城市建设的重要内容。以上海滨江森林公园一期工程为例,通过总结建设过程中自然生境的恢复与重建、现状植物保护和开发的经验,探索郊野公园设计建造中植物多样性保护的有效措施,为今后城市郊野公园的建设和原有绿地的绿化改造等提供参考借鉴。  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