全文获取类型
收费全文 | 3740篇 |
免费 | 423篇 |
国内免费 | 249篇 |
专业分类
化学 | 2516篇 |
晶体学 | 14篇 |
力学 | 163篇 |
综合类 | 22篇 |
数学 | 512篇 |
物理学 | 1185篇 |
出版年
2024年 | 13篇 |
2023年 | 66篇 |
2022年 | 88篇 |
2021年 | 101篇 |
2020年 | 103篇 |
2019年 | 92篇 |
2018年 | 76篇 |
2017年 | 62篇 |
2016年 | 144篇 |
2015年 | 117篇 |
2014年 | 169篇 |
2013年 | 253篇 |
2012年 | 323篇 |
2011年 | 308篇 |
2010年 | 221篇 |
2009年 | 185篇 |
2008年 | 253篇 |
2007年 | 202篇 |
2006年 | 187篇 |
2005年 | 175篇 |
2004年 | 146篇 |
2003年 | 150篇 |
2002年 | 126篇 |
2001年 | 100篇 |
2000年 | 73篇 |
1999年 | 67篇 |
1998年 | 48篇 |
1997年 | 61篇 |
1996年 | 63篇 |
1995年 | 48篇 |
1994年 | 43篇 |
1993年 | 39篇 |
1992年 | 30篇 |
1991年 | 20篇 |
1990年 | 29篇 |
1989年 | 18篇 |
1988年 | 16篇 |
1987年 | 15篇 |
1986年 | 15篇 |
1985年 | 21篇 |
1984年 | 23篇 |
1983年 | 13篇 |
1982年 | 15篇 |
1981年 | 8篇 |
1980年 | 14篇 |
1979年 | 14篇 |
1978年 | 7篇 |
1976年 | 9篇 |
1975年 | 6篇 |
1974年 | 7篇 |
排序方式: 共有4412条查询结果,搜索用时 20 毫秒
61.
高效液相色谱法测定柳树皮提取物中的水杨甙 总被引:2,自引:0,他引:2
建立了一种测定柳树皮提取物中水杨甙的反相高效液相色谱法。该法采用KromasilC18(4 6mmi.d .× 2 5 0mm ,5 μm)色谱柱 ,以甲醇 0 0 1mol/LKH2 PO4 缓冲液 (pH 4 0 1) (体积比为 15∶85 )的混合溶液作流动相 ,流速为1 0mL/min ,紫外检测波长为 2 6 5nm ,灵敏度为 0 0 4AUFS。在 8 89mg/L~ 2 84 40mg/L的范围内 ,水杨甙的峰面积Y与其质量浓度X的线性关系良好 ,回归方程为Y =- 2 91 5 2 6 0 +5 0 0 0 0 34X(r =0 9998)。水杨甙的平均回收率为 96 1%~ 10 1 2 % (n =5 ) ,相对标准偏差 (RSD)为 1 43%。该法操作简便、快速、准确。 相似文献
62.
63.
64.
Xiudi Lai Junni Li Xue Gong Xiaojing Lin Gengqiu Tang Rong Li 《Analytical letters》2018,51(17):2745-2760
The potential of near-infrared spectroscopy (NIRS) for the quality control of traditional Chinese medicine has been evaluated. Seven quantitative parameters, andrographolide, deoxyandrographolide, dehydroandrographolide, neoandrographolide, moisture, ash content, and alcohol-soluble extract of Andrographis paniculata, were evaluated by NIRS. The reference values of andrographolides were determined by high-performance liquid chromatography, and the others were obtained using the standard methods of the 2015 Chinese Pharmacopoeia. The predicted values were determined by a quantitative model using NIRS based on partial least square regression. Different spectral preprocessing methods, spectral ranges, and optimum number of factors were selected to optimize the models. All models were estimated by the combination of various parameters, including the correlation coefficient of calibration for andrographolide, deoxyandrographolide, dehydroandrographolide, neoandrographolide, moisture, ash content, alcohol-soluble extract (values of 0.980, 0.984, 0.989, 0.983, 0.987, 0.988, 0.979, respectively), root mean square error of calibration (values of 0.156, 0.038, 0.050, 0.029, 0.604, 0.431, 0.135, respectively), root mean square error of prediction (values of 0.169, 0.041, 0.050, 0.033, 0.280, 0.493, 0.140, respectively), root mean square error of cross-validation (values of 0.626, 0.114, 0.158, 0.046, 1.145, 0.774, 0.508, respectively), and ratio of standard deviation to standard error of prediction (values of 4.583, 4.690, 4.796, 4.899, 4.899, 4.690, 5.099, respectively). The results show that the calibration models by NIRS are reliable and can be applied for the quantification for seven parameters from A. paniculata for quality control in traditional Chinese medicine production and processing. 相似文献
65.
Superoxide Stabilization and a Universal KO2 Growth Mechanism in Potassium–Oxygen Batteries 下载免费PDF全文
Wanwan Wang Dr. Nien‐Chu Lai Zhuojian Liang Yu Wang Prof. Yi‐Chun Lu 《Angewandte Chemie (International ed. in English)》2018,57(18):5042-5046
Rechargeable potassium–oxygen (K‐O2) batteries promise to provide higher round‐trip efficiency and cycle life than other alkali–oxygen batteries with satisfactory gravimetric energy density (935 Wh kg?1). Exploiting a strong electron‐donating solvent, for example, dimethyl sulfoxide (DMSO) strongly stabilizes the discharge product (KO2), resulting in significant improvement in electrode kinetics and chemical/electrochemical reversibility. The first DMSO‐based K‐O2 battery demonstrates a much higher energy efficiency and stability than the glyme‐based electrolyte. A universal KO2 growth model is developed and it is demonstrated that the ideal solvent for K‐O2 batteries should strongly stabilize superoxide (strong donor ability) to obtain high electrode kinetics and reversibility while providing fast oxygen diffusion to achieve high discharge capacity. This work elucidates key electrolyte properties that control the efficiency and reversibility of K‐O2 batteries. 相似文献
66.
We have synthesized a new series of chromium-group 15 dihydride and hydride complexes [H(2)As(Cr(CO)(5))(2)](-) (1) and [HE(Cr(CO)(5))(3)](2)(-) (E = As, 2a; E = Sb, 2b), which represent the first examples of group 6 complexes containing E-H fragments. The contrasting chemical reactivity of 2a and 2b with organic halogen derivatives is demonstrated. The reaction of 2a with RBr (R = PhCH(2), HC triple bond CCH(2)) produces the RX addition products [(R)(Br)As(Cr(CO)(5))(2)](-) (R = PhCH(2), 3; R = C(3)H(3), 4), while the treatment of 2b with RX (RX = PhCH(2)Br or HC triple bond CCH(2)Br, CH(3)(CH(2))(5)C(O)Cl) forms the halo-substituted complexes [XSb(Cr(CO)(5))(3)](2-) (X = Br, 5; X = Cl, 6). Moreover, the dihaloantimony complexes [XX'Sb(Cr(CO)(5))(2)](-) can be obtained from the reaction of 2b with the appropriate organic halides. In this study, a series of organoarsenic and antimony chromium carbonyl complexes have been synthesized and structurally characterized and the role of the main group on the formation of the resultant complexes is also discussed. 相似文献
67.
Copper complexes of chiral quinolinyl‐oxazoline have been studied as the catalysts for enantioselective allylic oxidation of cycloalkenes with tert‐butyl perbenzoate. Using 5 mol% of these chiral catalysts, optical active allylic benzoates were obtained in moderate enantiomeric excesses. CuOTf prepared in situ, CuClO4 and CuPF6 were found to be good precatalysts in acetone. 相似文献
68.
The potential energy surface for the decomposition of HXSiS (X = H, F, and Cl) on the singlet state has been explored by B3LYP and CCSD(T) calculations. Five different types of reaction are proposed: (A) 1,1‐HX elimination, (B) 1,2‐H shift, (C) 1,2‐X shift, (D) H · and XSiS · radical formation, and (E) X · and HSiS · radical formation. These results show interesting trends for the HXSiS isomers. Our theoretical investigations suggest that the doubly bonded species HXSiS should be the lowest energy structure among the isomers from both kinetic and thermodynamic viewpoints. We also report theoretical predictions of molecular parameters and vibrational infrared (IR) spectra of the monohalogen‐substituted silanethione, which should be useful for future experimental observations. © 2001 John Wiley & Sons, Inc. Int J Quant Chem 82: 14–25, 2001 相似文献
69.
The polarographic behaviour of dithiodiacetic acid and that of U(VI) in a solution containing dithiodiacetic acid as complexing agent have been investigated. For the dithiodiacetic acid system, two waves appear over the pH range studied. The prewave is kinetic in nature and the mainwave is diffusion-controlled. However, as U(VI) is added into the dithiodiacetic acid system, the polarogram changes due to the existence of a complex. The current-potential curve of the first wave is not the normal S shape. This is due to the superposition of the first wave of the ligand and the wave due to the reduction of the U(VI) in the complex to U(V). The second wave is due to the reduction of the complex The first wave is an adsorption-controlled current and the second wave is partly diffusion-controlled and partly adsorption-controlled. We propose an electrode reaction mechanism for both systems and the complex species. The dissociation constant of the complex HASSAUO+2 is found to be of the order of 10?4. 相似文献
70.
urokinase(简称UK)属于絲氨酸蛋白酶,其一级结构已经测定。去掉前135个残基后得到的低分子量UK(LUK)氨基酸残基的顺序与糜胰蛋白酶、胰蛋白酶等絲氨酸蛋白酶十分类似。从二硫桥的配置来看LUK与糜胰蛋白酶比较接近,而UK对Lys侧链处肽键的专一性则和胰蛋白酶相近。在这一类酶中,起到活性中心作用的氨基酸为Ser195,His57及Asp102.在UK中此三个氨基酸都保留了下来。蛋白水解酶中起专一性作用的残基主要是 相似文献