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   1723篇
  免费   124篇
  国内免费   395篇
化学   1847篇
晶体学   81篇
力学   17篇
综合类   7篇
数学   2篇
物理学   288篇
  2024年   4篇
  2023年   17篇
  2022年   42篇
  2021年   41篇
  2020年   52篇
  2019年   49篇
  2018年   40篇
  2017年   59篇
  2016年   71篇
  2015年   87篇
  2014年   103篇
  2013年   191篇
  2012年   123篇
  2011年   104篇
  2010年   93篇
  2009年   100篇
  2008年   105篇
  2007年   99篇
  2006年   104篇
  2005年   99篇
  2004年   96篇
  2003年   88篇
  2002年   68篇
  2001年   46篇
  2000年   50篇
  1999年   54篇
  1998年   41篇
  1997年   24篇
  1996年   26篇
  1995年   31篇
  1994年   28篇
  1993年   11篇
  1992年   18篇
  1991年   16篇
  1990年   11篇
  1989年   8篇
  1988年   7篇
  1987年   4篇
  1986年   9篇
  1985年   6篇
  1984年   3篇
  1983年   2篇
  1982年   1篇
  1981年   3篇
  1980年   4篇
  1979年   1篇
  1978年   1篇
  1977年   1篇
  1975年   1篇
排序方式: 共有2242条查询结果,搜索用时 15 毫秒
31.
蚊蝇杀灭器及其发射光谱分析   总被引:1,自引:0,他引:1  
文章首先介绍了蚊蝇杀灭器在佛山市两个单位的职工饭堂厨房和会议室或值班室的试验情况。然后围绕文中四个图说明了下面四个问题:一,展示了进行光谱分析的测量装置。二,将蚊蝇杀灭器黑光灯的发射光谱和日用荧光灯发射光谱作分析比较。三,指出黑光灯也是一种荧光灯,说明了它的发光机理。四,展示了国外一种高效黑光灯的发射光谱,它是用活性铈钙磷酸脂做荧光涂料,我们进一步深入探讨了这种黑光灯的发光机制,并介绍了它的其他用  相似文献   
32.
掺钕磷酸盐激光玻璃的光谱特性   总被引:2,自引:1,他引:2  
施旗  程红  吕景文  孙彧 《发光学报》2005,26(3):359-364
掺钕激光玻璃广泛应用于核聚变、高功率激光放大器和光纤激光器等领域。磷酸盐玻璃热膨胀系数高、热稳定性和化学稳定性差、热机械强度低。通过改变玻璃组分,即添加Al2O3和F2,并改进制备工艺来降低热膨胀系数,除去铂和分子水。测量了磷酸盐玻璃中Nd3+离子的荧光光谱、吸收光谱及玻璃的热膨胀系数。根据吸收光谱计算掺钕磷酸盐激光玻璃的光谱参数。通过对掺钕磷酸盐玻璃的热膨胀系数的实际测量和计算,分析了玻璃的热稳定性。结果表明,在基质玻璃中引入Al2O3使激光玻璃的热膨胀系数可降低到α=38.75×10-7/℃,引入F2既达到了除水的目的又降低了玻璃的声子能量,提高了荧光发射的量子效率,并优化了光谱性能,拓展了掺钕磷酸盐激光玻璃的应用范围。  相似文献   
33.
Abstract

1-Oxo-4-chlorocarbony1-1-phospha-2,6,7-trioxabicyclo[2.2.2]octane (5) was obtained from phosphorus oxychloride. Benzyl chloroformate was synthesized by the reaction of benzyl alcohol and triphosgene in good yield for the first time. N-r-Butyl-N-benzoylhydrazine(II) was prepared in a new and convenivent procedure with good yield. The reaction of 5 and II proceeded smoothly in the presence of sodium carbonate and afforded the desired compound 13 in good yield, while in the presence of triethylamine, the elimination of butyl was observed and afforded the compound 12.  相似文献   
34.
In this work, the calcium phosphate nanoparticles have been produced by new reverse micro emulsion method containing β‐cyclodextrin, poly(oxyethylene)5 nonyl phenol ether and cyclohexane. Scanning electron microscope, transmission electron microscope, fourier transform infrared spectroscope and X‐ray diffraction were used to characterize the particles. The sizes of the nanoparticles were identified between 70‐80 nm. In conclusion, these results suggested that the developed reverse micro emulsion system based nanoparticles seem to be a promising formulation for calcium phosphate nanoparticles synthesis and it has immense potential in delivery of drugs and vaccines.  相似文献   
35.
A new and efficient method to synthesize a 3,3,6,6,9‐aryl‐1,8‐dioxo‐octahydroxanthene derivative using diammonium hydrogen phosphate as catalyst was performed in water at room temperature in a short periods.  相似文献   
36.
Abstract

The effects of two metal complexes of 2,2′‐dipyridylamine (bpya) ligand, [(bpya)Cu]Cl2 and [(bpya)Zn]Cl2, in promoting the hydrolysis of bis(4‐nitrophenyl) phosphate (BNPP) have been kinetically investigated in Brij35 micellar solution and at 298 K, pH ranging from 6.41 to 8.6. In neutral micellar solution at 298 K, pH 7.02, the rate constants for the catalytic hydrolysis of BNPP by [(bpya)Cu]Cl2 and [(bpya)Zn]Cl2 are 1.2 × 106 times and 1.5 × 105 times higher than those for the spontaneous hydrolysis, respectively. Kinetic studies show that the active species in the catalytic hydrolysis of BNPP is the aquo‐hydroxy form, and the relative kinetic and thermodynamic parameters indicate that the mechanism of the reaction involves intramolecular nucleophilic attack on the metal center‐bound diester.  相似文献   
37.
《Current Applied Physics》2015,15(4):541-546
Porous LiFePO4 is synthesized and coated with amorphous carbon by using high energy nano-mill (HENM) processed solid-state reaction method. FeCl3 (38%) containing water solution which is originated from pickling of steel scrap (waste liquid) is used as a source material in this study. The result indicates that LiFePO4 powders are well coated with the amorphous carbon. HENM process successfully produces the porous LiFePO4 with homogeneously distributed pores and a well networked carbon web, which delivers an enhanced electrochemical rate capability. HENM process is incorporated as an effective route for reducing particle size, distributing particle homogeneously and averting agglomeration of particles of precursor in this study. X-ray diffraction, scanning electron microscopy with elemental mapping, transmission electron microscopy with selected area (electron) diffraction, Raman spectroscopy, cyclic voltammetry, and galvanostatic charge/discharge are employed to characterize the final product. Electrochemical measurement shows that the synthesized LiFePO4/C composite cathode delivers an initial discharge capacity of 161 mAhg−1 at 0.1C-rate between 4.2 and 2.5 V. Remarkably, the cathode delivers 101.9 mAhg−1 at high charge/discharge rate (10 C).  相似文献   
38.
Amorphous carbon and graphene co-modified LiFePO_4 nanocomposite has been synthesized via a facile polyol process in connection with a following thermal treatment.Various characterization techniques,including XRD.Mossbauer spectra,Raman spectra,SEM,TEM,BET,O_2-TPO,galvano charge-discharge,CV and EIS were applied to investigate the phase composition,carbon content,morphological structure and electrochemical performance of the synthesized samples.The effect of introducing way of carbon sources on the properties and performance of LiFePO_4/C/graphene composite was paid special attention.Under optimized synthetic conditions,highly crystalized olivine-type LiFePO_4was successfully obtained with electron conductive Fe_2P and FeP as the main impurity phases.SEM and TEM analyses demonstrated the graphene sheets were randomly distributed inside the sample to create an open structured LiFePO_4 with respect to graphene,while the glucosederived carbon mainly coated over LiFeP04 particles which effectively connected the graphene sheets and LiFePO_4 particles to result in a more efficient charge transfer process.As a result,favorable electrochemical performance was achieved.The performance of the amorphous carbon-graphene co-modified LiFePO_4 was further progressively improved upon cycling in the first 200 cycles to reach a reversible specificcapacity as high as 97 mAh·g~(-1) at 10 C rate.  相似文献   
39.
One novel organically templated zincophophate(C5N2H14)·[Zn3(OH2)(PO4)2(HPO4)] has been synthesized under hydrothermal conditions and characterized by single-crystal X-ray diffraction together with elemental analysis, infrared spectroscopy, thermogravimetric analysis, and powder X-ray diffraction. The title compound crystallizes in the monoclinic space group P21/n, with a = 9.7904(11), b = 14.0287(14), c = 11.8651(13) , β = 104.690(3), V = 1576.4(3) 3, Z = 4, T = 296(2) K, Mr = 601.31 and Dc = 2.533 g/cm3. The compound consists of a macroanionic [Zn3(OH2)(PO4)2(HPO4)]2- framework and(C5N2H14)2+ cations, and its structure is built up from ZnO3(OH2), ZnO4, HPO4 and PO4 tetrahedral units that result in 4, 8 and 10-ring channels.  相似文献   
40.
The polymerization of isodecyl acrylate (ISODAC) in vesicles made from an anionic surfactant—sodium di-2-ethylhexyl phosphate (SEHP)—and from water is studied by 1H-NMR, transmission electron microscopy, and quasielastic light scattering. High polymerization rates and high conversion rates are achieved with both water-soluble initiator, K2S2O8 (potassium persulfate), and oil-soluble initiator, AIBN (azoisobisbutyronitrile). ISODAC is probably located inside the vesicle bilayer(s) because of its high hydrophobicity. Particles stable at room temperature with a mean diameter of about 50 nm are obtained. Kinetic orders of ISODAC polymerization are determined and the characterization of the resulting particles during and after polymerization are studied. © 1996 John Wiley & Sons, Inc.  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