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   9486篇
  免费   728篇
  国内免费   1017篇
  2024年   20篇
  2023年   120篇
  2022年   305篇
  2021年   541篇
  2020年   403篇
  2019年   484篇
  2018年   396篇
  2017年   291篇
  2016年   421篇
  2015年   610篇
  2014年   728篇
  2013年   810篇
  2012年   903篇
  2011年   787篇
  2010年   499篇
  2009年   475篇
  2008年   540篇
  2007年   471篇
  2006年   390篇
  2005年   304篇
  2004年   296篇
  2003年   262篇
  2002年   212篇
  2001年   149篇
  2000年   128篇
  1999年   129篇
  1998年   83篇
  1997年   63篇
  1996年   52篇
  1995年   60篇
  1994年   64篇
  1993年   40篇
  1992年   36篇
  1991年   43篇
  1990年   31篇
  1989年   23篇
  1988年   11篇
  1987年   10篇
  1986年   12篇
  1985年   12篇
  1984年   4篇
  1983年   5篇
  1982年   3篇
  1981年   2篇
  1980年   3篇
排序方式: 共有10000条查询结果,搜索用时 265 毫秒
91.
Three dimeric cassane diterpenoids, caesalpanins A–C, were isolated from the seeds of Caesalpinia sappan L., as well as three known compounds. Their structures were determined via analysis of 1D‐, 2D‐NMR, and HR‐ESI‐MS data. Caesalpanins A and B were the second and third compounds that presented a nitrogen‐containing cassane diterpenoid dimer linked through one ether bond between C‐19 and C‐20′. Caesalpanin B exhibited moderate cytotoxic activity against MCF‐7 cell lines with IC50 value of 29.98 μm . Caesalpanins A and B had weak inhibitory effects against LPS‐induced nitric oxide (NO) production in RAW 264.7 macrophages at 50 μm with inhibitory rate of 36.01 % and 32.93 %, respectively.  相似文献   
92.
93.
Transition metal sulfides hold promising potentials as Li‐free conversion‐type cathode materials for high energy density lithium metal batteries. However, the practical deployment of these materials is hampered by their poor rate capability and short cycling life. In this work, the authors take the advantage of hollow structure of CuS nanoboxes to accommodate the volume expansion and facilitate the ion diffusion during discharge–charge processes. As a result, the hollow CuS nanoboxes achieve excellent rate performance (≈371 mAh g?1 at 20 C) and ultra‐long cycle life (>1000 cycles). The structure and valence evolution of the CuS nanobox cathode are identified by scanning electron microscopy, transmission electron microscopy, and X‐ray photoelectron spectroscopy. Furthermore, the lithium storage mechanism is revealed by galvanostatic intermittent titration technique and operando Raman spectroscopy for the initial charge–discharge process and the following reversible processes. These results suggest that the hollow CuS nanobox material is a promising candidate as a low‐cost Li‐free cathode material for high‐rate and long‐life lithium metal batteries.  相似文献   
94.
Compared to conjugated polymers, small‐molecule organic semiconductors present negligible batch‐to‐batch variations, but presently provide comparatively low power conversion efficiencies (PCEs) in small‐molecular organic solar cells (SM‐OSCs), mainly due to suboptimal nanomorphology. Achieving precise control of the nanomorphology remains challenging. Here, two new small‐molecular donors H13 and H14 , created by fluorine and chlorine substitution of the original donor molecule H11 , are presented that exhibit a similar or higher degree of crystallinity/aggregation and improved open‐circuit voltage with IDIC‐4F as acceptor. Due to kinetic and thermodynamic reasons, H13 ‐based blend films possess relatively unfavorable molecular packing and morphology. In contrast, annealed H14 ‐based blends exhibit favorable characteristics, i.e., the highest degree of aggregation with the smallest paracrystalline π–π distortions and a nanomorphology with relatively pure domains, all of which enable generating and collecting charges more efficiently. As a result, blends with H13 give a similar PCE (10.3%) as those made with H11 (10.4%), while annealed H14 ‐based SM‐OSCs have a significantly higher PCE (12.1%). Presently this represents the highest efficiency for SM‐OSCs using IDIC‐4F as acceptor. The results demonstrate that precise control of phase separation can be achieved by fine‐tuning the molecular structure and film formation conditions, improving PCE and providing guidance for morphology design.  相似文献   
95.
96.
97.
98.
Developing a titanium dioxide (TiO2)‐based anode with superior high‐rate capability and long‐term cycling stability is important for efficient energy storage. Herein, a simple one‐step approach for fabricating blue TiO2 nanoparticles with oxygen vacancies is reported. Oxygen vacancies can enlarge lattice spaces, lower charge transfer resistance, and provide more active sites in TiO2 lattices. As a result, this blue TiO2 electrode exhibits a highly reversible capacity of 50 mAh g?1 at 100 C (16 800 mA g?1) even after 10 000 cycles, which is attributable to the combination of surface capacitive process and remarkable diffusion‐controlled insertion revealed by the kinetic analysis. The strategy of employing oxygen‐deficient nanoparticles may be extended to the design of other robust semiconductor materials as electrodes for energy storage.  相似文献   
99.
The development of Pt‐free catalysts for the alkaline hydrogen evolution reaction (HER), which is widely used in industrial scale water‐alkali electrolyzers, remains a contemporary and pressing challenge. Ruthenium (Ru) has excellent water‐dissociation abilities and could be an alternative water splitting catalyst. However, its large hydrogen binding energy limits HER activity. Here, a new approach is proposed to boost the HER activity of Ru through uniform loading of Ru nanoparticles on triazine‐ring (C3N3)‐doped carbon (triNC). The composite (Ru/triNC) exhibits outstanding HER activity with an ultralow overpotential of ≈2 mV at 10 mA cm?2; thereby making it the best performing electrocatalyst hitherto reported for alkaline HER. The calculated metal mass activity of Ru/triNC is >10 and 15 times higher than that of Pt/C and Pt/triNC. Both theoretical and experimental studies reveal that the triazine‐ring is a good match for Ru to weaken the hydrogen binding on Ru through interfacial charge transfer via increased contact electrification. Therefore, Ru/triNC can provide the optimal hydrogen adsorption free energy (approaching zero), while maintaining the strong water‐dissociation activity. This study provides a new avenue for designing highly efficient and stable electrocatalysts for water splitting.  相似文献   
100.
目的:探讨沐舒坦联合布地奈德雾化治疗新生儿胎粪吸入综合征的临床效果及安全性。方法:选择2017年1月~2018年2月我院新生儿科收治的76例新生儿胎粪吸入综合征患儿,按照随机数字表法将其分成两组,每组38例。对照组患者采用布地奈德雾化治疗,观察组在对照组的治疗基础上加用沐舒坦治疗,分析和比较两组的治疗效果,患儿治疗前后动脉血气分析指标变化以及预后情况。结果:治疗后,观察组临床总有效率明显高于对照组,观察组呼吸困难缓解时间、肺部湿罗音消失时间、发绀消失时间、血氧饱和度恢复时间均显著较对照组短(P0.05)。两组患儿治疗后PaCO_2、FIO_2、OI均较治疗前降低,PaO_2均较治疗前上升,其中观察组PaCO_2、FIO_2、OI明显低于对照组,PaO_2高于对照组,上述差异均具有统计学意义(P0.05)。观察组总并发症发生率显著低于对照组(P0.05),患儿治愈率显著高于对照组(P0.05),两组死亡率比较差异无统计学意义(P0.05)。结论:与布地奈德雾化治疗相比,沐舒坦联合布地奈德雾化治疗新生儿胎粪吸入综合征患儿可以更有效缩短临床症状改善时间,改善患儿肺功能及预后,且安全性更高。  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