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   4163篇
  免费   606篇
  国内免费   467篇
化学   2994篇
晶体学   40篇
力学   238篇
综合类   51篇
数学   503篇
物理学   1410篇
  2024年   10篇
  2023年   101篇
  2022年   108篇
  2021年   141篇
  2020年   190篇
  2019年   166篇
  2018年   142篇
  2017年   112篇
  2016年   177篇
  2015年   191篇
  2014年   176篇
  2013年   290篇
  2012年   297篇
  2011年   305篇
  2010年   223篇
  2009年   229篇
  2008年   249篇
  2007年   212篇
  2006年   205篇
  2005年   204篇
  2004年   184篇
  2003年   126篇
  2002年   123篇
  2001年   117篇
  2000年   117篇
  1999年   101篇
  1998年   87篇
  1997年   81篇
  1996年   84篇
  1995年   62篇
  1994年   72篇
  1993年   52篇
  1992年   46篇
  1991年   36篇
  1990年   41篇
  1989年   28篇
  1988年   24篇
  1987年   25篇
  1986年   18篇
  1985年   22篇
  1984年   13篇
  1983年   3篇
  1982年   7篇
  1981年   12篇
  1980年   7篇
  1979年   4篇
  1973年   3篇
  1972年   2篇
  1969年   2篇
  1967年   2篇
排序方式: 共有5236条查询结果,搜索用时 109 毫秒
991.
In the present work the feasibility of β-cyclodextrin in complexation was explored, as a tool for improving the solubility and biological ability of daidzein derivatives. A series of phosphorylated daidzein derivatives featuring different chain lengths were synthesized through a modified Atherton-Todd reaction and their inclusion complexes with βCD were prepared by coprecipitation method. The inclusion complexation behavior was studied by fluorescence, UV, FT-IR, MS and (1)H NMR. The results showed that only phosphorylated daidzein derivative carrying small substituent group ((C(2)H(5)O)(2)PO) entered the cavity of βCD and formed 1:1 inclusion complex. The formation constant was 175(mol/L)(-1).  相似文献   
992.
PET/silica nanocomposite fibers of high quality were fabricated from electrospinning by choosing appropriate surface modification of inorganic fillers, solution properties, and processing conditions. The existence of an immobilized layer around silane-modified silica particles in PET fibers was verified by Fourier transform infrared spectroscopy, the results of which confirm previous thermal analysis studies. The influence of silica particles on the crystal growth during isothermal crystallization as well as the phase structure of the crystallized nanocomposite fibers were examined using differential scanning calorimetry. The PET crystallization rate increases significantly with increasing silica content, which indicates that the silica nanoparticles act as an efficient nucleating agent to facilitate PET crystallization. Using Avrami analysis, for the first time, preferred 1-D crystal growth was confirmed for geometrically confined nanocomposite fibers. Addition of silica particles makes the crystal growth more likely to occur in a 1-D manner.  相似文献   
993.
The Bi5FeTi3O15 (BFTO) films of layered structure have been fabricated on Pt/Ti/SiO2/Si substrates by the sol–gel method. The thermal decomposition behaviors of precursor powder were examined using thermo-gravimetric and differential scanning calorimeters analysis. The optimal heat treatment process for BFTO films were determined to be low-temperature drying at 200 °C for 4 min and high-temperature drying at 350 °C for 5 min followed by annealing at 740 °C for 60 min, which led to the formation of compact films with uniform grains of ~300 nm. The structural, surface topography, ferroelectric and magnetic properties of the films were investigated. The remnant polarization (2P r) of BFTO thin films under an applied electric field of ~550 kV/cm are determined to be 67.5 μC/cm2 . Meanwhile, the weak ferromagnetic properties of the BFTO films were observed at room temperature.  相似文献   
994.
Diverse fused thiophenes with electron-rich and electron-deficient blocks have been synthesized and employed as the π-conjugated spacers of organic dyes for the dye-sensitized solar cells (DSSCs). The effects of these fused thiophenes were investigated by their absorption spectra, electrochemical and photovoltaic properties. For a typical device a maximum power conversion efficiency of 6.11% was obtained under simulated AM 1.5 irradiation (100 mW cm?2): a short-circuit current (JSC) of 14.47 mA cm?2, an open-circuit voltage (VOC) of 670 mV, and a fill factor (FF) of 0.63.  相似文献   
995.
A highly efficient catalyst system assembled from enantiomerically pure diaminocyclohexane and Ni(OAc)(2) is, for the first time, used to catalyze the cascade Michael-Henry reaction of various diones and substituted nitroalkenes. A series of polyfunctionalized bicyclo[3.2.1]octane derivatives containing four stereogenic centers are prepared with excellent enantioselectivities (up to >99% ee) and diastereoselectivities (up to 50 : 1 dr) with high yields. In addition, via this chiral diamine-Ni(OAc)(2) catalyst system, the base-induced epimerization leading to the decrease of stereoselectivity can be prevented.  相似文献   
996.
We prove two identities related to overpartition pairs. One of them gives a generalization of an identity due to Lovejoy, which was used in a joint work by Bringmann and Lovejoy to derive congruences for overpartition pairs. We apply our two identities on pairs of partitions where each partition has no repeated odd parts. We also present three partition statistics that give combinatorial explanations to a congruence modulo 3 satisfied by these partition pairs.  相似文献   
997.
We proposed a geometrical form factor (GFF) calculation using Monte Carlo integration (GFF–MC) for lidar that is practical and can be applied to any laser intensity distribution. Theoretical results have been calculated with our method based on the functions of measured, uniform and Gaussian laser intensity distribution. Two experimental GFF traces on clear days are obtained to verify the validity of the theoretical results. The results indicated that the measured distribution function outperformed the Gaussian and uniform functions. That means that the deviation of the measured laser intensity distribution from an ideal one can be too large to neglect. In addition, the theoretical GFF of the uniform distribution had a larger error than that of the Gaussian distribution. Furthermore, the effects of the inclination angle of the laser beam and the central obstruction of the support structure of the second mirror of the telescope are discussed in this study.  相似文献   
998.
A series of novel (Z)‐1‐tert‐butyl (or phenyl)‐2‐(1H‐1,2,4‐triazol‐1‐yl)‐ethanone O‐[2,4‐dimethylthiazole (or 4‐methyl‐1,2,3‐thiadiazole) ?5‐carbonyl] oximes 5a – 5c and (1Z, 3Z)‐4,4‐dimethyl‐1‐substitutedphenyl‐2‐(1H‐1,2,4‐triazol‐1‐yl)‐pent‐1‐en‐3‐one O‐[2,4‐dimethylthiazole (or 4‐methyl‐1,2,3‐thiadiazole)‐5‐carbonyl] oximes 6a – 6e were synthesized by the condensations of (Z)‐1‐tert‐butyl (or phenyl)‐2‐(1H‐1,2,4‐triazol‐1‐yl)‐ethanone oximes 3 or (1Z, 3Z)‐4,4‐dimethyl‐1‐substitutedphenyl‐2‐(1H‐1,2,4‐triazol‐1‐yl)‐pent‐1‐en‐3‐one oximes 4 with 2,4‐dimethylthiazole‐5‐carbonyl chloride or 4‐methyl‐1,2,3‐thiadiazole‐5‐carbonyl chloride in the basic condition. Their structures were confirmed by IR, 1H NMR, mass spectroscopy, and elemental analyses. The results of preliminary bioassays showed the title compounds 5 and 6 exhibited moderate to good fungicidal activities. For example, compound 6c possessed 86.4% inhibition against Fusarium oxysporum, and compound 6b exhibited 86.4 and 100% inhibition against Fusarium oxysporum and Cercospora arachidicola Hori at the concentration of 50 mg/L, respectively.  相似文献   
999.
The partial molar volumes (V 2) for hexaamminecobalt(III) nitrate in proteated and deuterated water were determined at 0.00 (for H2O only), 5.00, 10.00, 15.00, and 20.00?°C. The increase of the solute??s partial molar volume with increasing concentration and the negative second derivative of the solute??s partial molar volume at infinite dilution with respect to temperature were interpreted in terms of the solvent structure breaking property of the solute. In addition, the difference at each temperature between the solute??s partial molar volume at infinite dilution for proteated and deuterated water was used to estimate the solvent coordination number at each temperature.  相似文献   
1000.
Responsive polyelectrolyte multilayers (PEMs) of poly(diallyl dimethyl ammonium chloride) (PDADMAC) and poly(styrene sodium sulfonate) (PSS) with thicknesses between 350 and 400 nm for 11 deposited polyelectrolyte layers were fabricated assembling the polyelectrolytes at 3 M NaCl. When the 3 M NaCl bulk solution is replaced by water, the PEMs release water, approximately a 46% of the total mass, and experience a thickness reduction of more than 200 nm. Changes in thickness and water content are fully reversible. The film recovers its original thickness and water content when it is exposed again to a 3 M NaCl solution. A responsive polymer film is achieved with the capability of swelling at high ionic strength and collapsing in water with variations in thickness of hundred of nanometers.  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