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   1917篇
  免费   427篇
  国内免费   344篇
化学   1532篇
晶体学   21篇
力学   70篇
综合类   7篇
数学   204篇
物理学   854篇
  2024年   4篇
  2023年   51篇
  2022年   82篇
  2021年   82篇
  2020年   93篇
  2019年   77篇
  2018年   65篇
  2017年   70篇
  2016年   93篇
  2015年   109篇
  2014年   119篇
  2013年   166篇
  2012年   210篇
  2011年   230篇
  2010年   151篇
  2009年   127篇
  2008年   137篇
  2007年   131篇
  2006年   119篇
  2005年   105篇
  2004年   94篇
  2003年   55篇
  2002年   49篇
  2001年   18篇
  2000年   34篇
  1999年   31篇
  1998年   34篇
  1997年   24篇
  1996年   28篇
  1995年   18篇
  1994年   16篇
  1993年   6篇
  1992年   8篇
  1991年   10篇
  1990年   12篇
  1989年   2篇
  1988年   2篇
  1987年   7篇
  1986年   9篇
  1985年   2篇
  1984年   1篇
  1983年   5篇
  1982年   1篇
  1981年   1篇
排序方式: 共有2688条查询结果,搜索用时 342 毫秒
991.
In an attempt to develop potent antitumor agents,new rhodacyanine analogues containing the pyridinium ring(5a-5h),the isoquinolinium ring(6a-6c) and the quinolinium ring(7a-7e) linked to the rhodanine ring via N-N covalent bond were designed, synthesized and evaluated for antitumor activity against human lung cancer cell line(H460) by MTT assay in vitro.Most of the tested compounds showed enhanced antitumor activity with IC50 values ranging from 0.006 to 9.2 u,mol/L as compared to the lead compound MKT-077.Among them,the most promising compound 7d(IC50 = 0.006μmol/L) was 216.7 times more active than MKT-077(IC50 = 1.3μmol/L).The preliminary structure-activity relationship of the target compounds was discussed.  相似文献   
992.
We have modified silica-coated Fe3O4 nanoparticles with 2,6-diaminopyridine and used these for selective magnetic solid-phase extraction of trace amounts of metal ions. The nanoparticles were characterized by transmission electron microscopy and Fourier transform infrared spectroscopy. Quantitative extraction of trace amounts of Cu(II) and Zn(II) from mixed-ion solutions was accomplished at an optimal pH value of 6 within less than 10?min. The metal ions were eluted from the sorbent with hydrochloric acid. Common electrolytes and chemically related metal ions do not interfere. The relative standard deviations of the method are <4?%. It was successfully applied to the separation and preconcentration of trace metal ions from the certified reference materials GBW 08301 (river sediment) and GBW 08607 (water solution), in natural water, and in samples of vegetable with satisfying results.  相似文献   
993.
以相应的氧化物粉末和盐为原料,通过甘氨酸-硝酸盐法合成出了中温固体氧化物燃料电池(IT-SOFC)Pr1.2Sr0.8NiO4(PSNO)阴极原料粉体,并制备出了烧结体试样.采用X射线衍射(XRD)分析对所合成粉体的相组成进行了分析,分别采用热膨胀仪和四端子法对PSNO烧结体试样的热膨胀系数和电导率进行了测定,同时对该阴极材料与Sm0.2Ce0.8O1.9(sco)电解质材料的电化学阻抗谱(EIS)进行了测试分析以SCO作电解质,分别以NiO/SCO和PSNO作阳极和阴极材料,制备出固体氧化物燃料单电池,并对其性能进行测试.实验结果表明,通过甘氨酸-硝酸盐法,在1050℃以上煅烧前驱体,可以获得具有K2NiF4结构的PSNO粉体.所制备的PSNO烧结体试样在200-800℃间的热膨胀系数约为12×10-6 K-1,在450℃下的电导率约为155 S· cm-1,在400-800℃,平均电导活化能为0.034 eV.电化学阻抗谱分析结果表明,在700 ℃下PSNO阴极和SCO电解质间的比表面阻抗(ASR)为0.37Ω·cm2,而Ni-SCO/SCO/PSNO单电池的比表面阻抗为0.61Ω·cm2;所制备的SOFC单电池在800℃下的输出功率为288 mW· cm-2,开路电压为0.75 V.本研究的初步结果表明PSNO 材料是一种综合性能较为优良的新型巾温固体氧化物燃料电池阴极材料.  相似文献   
994.
采用激光闪光光解-瞬态吸收光谱技术研究了355nm激光作用下芘四磺酸钠(PyTS)水溶液的光化学反应机理及其产生水合电子的动力学行为.研究首次发现PyTS水溶液激发单线态(PyTS1*)在260nm、激发三线态(PyTS3*)在300nm及阴离子自由基(PyTS-?)在330nm处的特征吸收峰;分析了生成的水合电子(e-aq)的主要反应途径包括自猝灭反应及与PyTS的反应,得到水合电子与PyTS反应的准一级速率常数为2.7′105s-1;并计算得到在此实验条件下,PyTS水溶液经双光子吸收产生的水合电子量子产率为3.2′10-2.  相似文献   
995.
A wet-chemical etching method is reported for the scale-up preparation of carbon-coated hierarchical micro/nano porous silicon powders as a Li-ion battery anode, which shows no capacity fading at 1500 mA h g(-1) in 50 charge/discharge cycles.  相似文献   
996.
Being promising candidates for anodes in lithium-ion batteries (LIBs), transition metal oxide anodes utilizing the so-called conversion mechanism principle typically suffer from the severe capacity fading during the 1st cycle of lithiation-delithiation. Herein, we directly investigated these processes using an individual CuO nanowire anode and constructed a lithium ion battery (LIB) prototype within a transmission electron microscope.  相似文献   
997.
We demonstrated a facile and green approach to synthesize bifunctional fluorescent carbon nanodots via soy milk, which not only showed favorable photoluminescent properties, but also exhibited good electrocatalytic activity towards oxygen reduction reaction.  相似文献   
998.
The present study was undertaken to investigate the anti-inflammatory and analgesic activity of total flavone of branches and leaves of Cunninghamia lanceolata (TFC) to provide a scientific basis for its clinical use and resource development. TFC was evaluated for anti-inflammatory and analgesic activity in mice or rats using chemical and thermal models of nociception, including acetic acid-induced writhing test, hot plate latency test, formalin test and carrageenan induced paw oedema test. Results showed that TFC given orally can significantly attenuate acetic acid-induced writhing in mice in a dose-dependent manner. In the hot plate latency test, TFC showed common activity in prolonging duration time only at the highest dose (400 mg/kg). Each dose of TFC could not significantly inhibit the first phase but was active in the later phase of formalin-induced pain, whereas morphine showed notable activity in the two phases. In the carrageenan-induced paw oedema model, TFC could significantly and dose-dependently reduce the carrageenan-induced paw edema at the third and fifth hour, and decrease the content of PEG2 in paw edema tissue and that of COX-2 in blood serum. It may be concluded that TFC showed both anti-inflammatory and analgesic effects, showing that it can be of importance in drug development, especially in the field of pain and inflammation.  相似文献   
999.
Pure-phase CoO octahedral nanocages were successfully fabricated by a novel simple method. The coordination etching agents play key roles in the formation of these non-spherical hollow structures. When tested as anode materials in lithium ion batteries (LIBs), these nanocages showed excellent cycling performance, good rate capability and enhanced lithium storage capacity.  相似文献   
1000.
Addition of alkali metal ions significantly promotes the activity of the Pt/TiO(2) catalyst for the HCHO oxidation reaction by stabilizing an atomically dispersed Pt-O(OH)(x) alkali metal species and opening a new low-temperature reaction pathway. The atomically dispersed Na-Pt-O(OH)(x) species can effectively activate H(2) O and catalyze the facile reaction between surface OH and formate species to total oxidation products.  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