全文获取类型
收费全文 | 3398篇 |
免费 | 492篇 |
国内免费 | 458篇 |
专业分类
化学 | 2686篇 |
晶体学 | 35篇 |
力学 | 198篇 |
综合类 | 43篇 |
数学 | 375篇 |
物理学 | 1011篇 |
出版年
2024年 | 12篇 |
2023年 | 71篇 |
2022年 | 114篇 |
2021年 | 136篇 |
2020年 | 153篇 |
2019年 | 140篇 |
2018年 | 103篇 |
2017年 | 115篇 |
2016年 | 151篇 |
2015年 | 149篇 |
2014年 | 199篇 |
2013年 | 245篇 |
2012年 | 261篇 |
2011年 | 304篇 |
2010年 | 206篇 |
2009年 | 180篇 |
2008年 | 189篇 |
2007年 | 176篇 |
2006年 | 158篇 |
2005年 | 152篇 |
2004年 | 125篇 |
2003年 | 100篇 |
2002年 | 116篇 |
2001年 | 100篇 |
2000年 | 86篇 |
1999年 | 92篇 |
1998年 | 81篇 |
1997年 | 60篇 |
1996年 | 63篇 |
1995年 | 49篇 |
1994年 | 51篇 |
1993年 | 34篇 |
1992年 | 43篇 |
1991年 | 24篇 |
1990年 | 30篇 |
1989年 | 14篇 |
1988年 | 9篇 |
1987年 | 6篇 |
1986年 | 10篇 |
1985年 | 10篇 |
1984年 | 14篇 |
1983年 | 8篇 |
1982年 | 3篇 |
1981年 | 3篇 |
1980年 | 1篇 |
1978年 | 1篇 |
1957年 | 1篇 |
排序方式: 共有4348条查询结果,搜索用时 500 毫秒
41.
众所周知,铑催化剂是醇类羰基化的有效催化剂,但是铑价格昂贵且对设备有腐蚀性,因此,寻找出对醇类羰基化催化活性好的非贵金属催化剂,对醇类羰化的工业化有十分重要的意义。 本文在温和条件下,合成了四种含有相同配体(Ph_2PPy)不同金属的有机配合物(金属基分别为Ni、Co、Pd、Fe),并探索了它们对乙醇羰基化反应的催化活性。 相似文献
42.
Solvent-free organic reactions have been attracting great interest of chemists due to the elimination of the usage of harmful organic solvents,low costs,and simplicity in the procedure1.Solvent-free mechanochemical reactions of fullerenes were explored and are significant for the reactions of fullerenes because the low solubility of fullerenes in common organic solvents requires large quantity of organic solvents and some novel fullerene reactions could only occur in the solid-state reaction2.… 相似文献
43.
把无相移滤波技术引入了分析化学信号处理领域。其方法为:先将输入序列按顺序滤波,然后将所得结果逆转后反向通过滤波器,再将所得结果逆转后输出。通过对色谱信号的实验研究表明,与普通的数字滤波方法相比较,无相移滤波不但具有普通数字滤波的优点,而且不会产生滤波前后的相位偏移,具有良好的应用前景,尤其对于需要准确计算保留时间的场合,用此预处理方法非常适合。 相似文献
44.
Heat capacities of Pr(NCS)3·7H2O and Nd(NCS)3· 7H2O haw been measured from 13 to 300 K by using a fully automated adiabatic calorimeter. Schottky anomaly was observed for Pr(NCS)3·7H2O below 50 K. The polynomial equations for calculating the heat capacity values of the two compounds in the range of 13-300 K were obtained by the least-squares fitting based on the experimental Cp data. The Cp values below 13 K were estimated by using the Debye-Einstein and Schottky heat capacity functions. The standard molar thermodynamic functions were computed from 0 to 300 K. The standard entropies and Gibbs energies of formation of the two compounds were also calculated. 相似文献
45.
Guan X Lin X Kwok WM Du Y Li YL Zhao C Wang D Phillips DL 《The journal of physical chemistry. A》2005,109(6):1247-1256
Ultraviolet photolysis of low concentrations of CH2I2 in methanol solution found that CH2I2 is converted into dimethoxymethane and some H+ and I- products. Picosecond time-resolved resonance Raman (ps-TR3) experiments observed that the isodiiodomethane (CH2I-I) photoproduct decayed faster as the concentration of methanol increases, suggesting that isodiiodomethane is reacting with methanol. Ab initio calculations indicate isodiiodomethane is able to react with methanol via an O-H insertion/HI elimination to form an iodoether (ICH2-O-CH3) and HI products. The iodoether can then further react via another O-H insertion/HI elimination reaction to form the dimethoxymethane (CH3-O-CH2-O-CH3) observed in the photochemistry experiments. A reaction mechanism consistent with these experimental and theoretical observations is proposed. 相似文献
46.
The ultraviolet photolysis of low concentrations of CH(2)XI (X = Cl, Br, I) were investigated in water and saltwater solutions by photochemistry and picosecond time-resolved resonance Raman spectroscopy. Photolysis in both kinds of solutions formed mostly CH(2)(OH)(2) and HI and HX products. However, photolysis of the CH(2)XI molecules in saltwater resulted in production of some CH(2)XCl products not observed in aqueous solutions without salt present. The appearance of these new products in saltwater solutions is accompanied by a decrease in the amount of CH(2)(OH)(2), HI, and HX products compared to photolysis in aqueous solutions without salt present. The possible implications for photolysis of CH(2)XI and other polyhalomethanes in seawater and other salt aqueous environments compared to nonsaltwater solvated environments is briefly discussed. 相似文献
47.
Silver halide (AgX) microcrystal was used as template to synthesize hollow polyelectrolyte capsules. These hollow capsules were characterized by laser light scattering (LLS) used to measure the size of the capsules in solution. The ratio of hydrodynamic radius (Rh) from dynamic LLS to the radius of gyration (Rg) from static LLS is almost unity, revealing that the entities are hollow in solution. The results suggest that the LLS method can be regarded as a good complement to the confocal laser scanning microscopy (CLSM) method for the characterization of small hollow capsules, and it possesses the advantage of not needing fluorescence labeling. 相似文献
48.
49.
50.