全文获取类型
收费全文 | 29396篇 |
免费 | 5053篇 |
国内免费 | 3636篇 |
专业分类
化学 | 21138篇 |
晶体学 | 463篇 |
力学 | 1792篇 |
综合类 | 306篇 |
数学 | 3358篇 |
物理学 | 11028篇 |
出版年
2024年 | 56篇 |
2023年 | 680篇 |
2022年 | 781篇 |
2021年 | 998篇 |
2020年 | 1317篇 |
2019年 | 1225篇 |
2018年 | 1008篇 |
2017年 | 953篇 |
2016年 | 1381篇 |
2015年 | 1348篇 |
2014年 | 1609篇 |
2013年 | 2084篇 |
2012年 | 2620篇 |
2011年 | 2614篇 |
2010年 | 1778篇 |
2009年 | 1716篇 |
2008年 | 1937篇 |
2007年 | 1762篇 |
2006年 | 1651篇 |
2005年 | 1363篇 |
2004年 | 1054篇 |
2003年 | 898篇 |
2002年 | 796篇 |
2001年 | 673篇 |
2000年 | 654篇 |
1999年 | 709篇 |
1998年 | 565篇 |
1997年 | 500篇 |
1996年 | 523篇 |
1995年 | 450篇 |
1994年 | 409篇 |
1993年 | 370篇 |
1992年 | 304篇 |
1991年 | 251篇 |
1990年 | 262篇 |
1989年 | 176篇 |
1988年 | 133篇 |
1987年 | 93篇 |
1986年 | 108篇 |
1985年 | 76篇 |
1984年 | 47篇 |
1983年 | 53篇 |
1982年 | 42篇 |
1981年 | 22篇 |
1980年 | 9篇 |
1979年 | 8篇 |
1978年 | 2篇 |
1976年 | 2篇 |
1973年 | 3篇 |
1957年 | 6篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
992.
Tinghua Wu Mingqiao Zhu Zhenjiang Niu Yijun Zhong Yan Guan Ya Liu Qiangu Yan Zelin Li Huilin WanZhejiang Normal University Institute of Physical Chemistry Jinhua ChinaZhejiang University Department of Chemical Engineering Hangzhou ChinaXiamen University Department of Chemistry Xiamen China 《天然气化学杂志》2002,(Z2)
The catalytic properties of several supported metal catalysts on different carriers were studied in the partial oxidation of methane (POM) to syngas. In our experiment, supported noble metal catalysts exhibited better performance than the other supported transition metal catalysts. The catalyst performances were significantly influenced by the d-electron configuration of the active metal components and the dispersion of active metal components on the support. A catalyst with a moderate number of unpaired electrons in the d-orbital of the active metal support without obvious acidity or redox activity (e.g. MgO) was suitable for POM performance. The Rh/SiO2 catalyst was the best in the POM reaction, among those investigated. Reaction conditions apparently also affected the POM performance of the catalyst. The conversion of methane and the selectivity for CO increased with the reaction temperature, and a high CH4/O2 ratio was not beneficial for POM performance. 相似文献
993.
<正> 聚对苯二甲酸乙二酯(PET)的重复单元中有乙撑基。它的碳碳键内旋转产生反式构象(T)和左右式构象(G)。用其不同的构象来表征PET的聚集态结构,对研究结构与性能的关系是十分重要的。许多工作都已指出,乙撑基反式构象的谱带热处理时随结晶度的增加而增加。X-射线衍射的结果证实 相似文献
994.
995.
Dan Zhu Yi-xiang Cheng Xiao-wei Zou Ling-wu Chen Jin-feng Song Zhi-liu Wang College of Chemistry Chemical Engineering Nanjing University Nanjing China 《高分子科学》2006,(6)
Chiral polymer was synthesized by the polymerization of (R)-6,6'-bistributylstannyl-2,2'-binaphtho-20-crown-6 (M-1) with 1,4-dibromo-2,3-bisbutoxy-naphthyl (M-2) by Pd(PPh3)4 catalyzed Stille coupling reaction. Both monomer and polymer were analyzed by NMR, MS, FT-IR, UV, polarimetry, DSC-TGA, CD, fluorescent spectroscopy and GPC. The major difference between monomer and polymer is that a long wavelength Cotton Effect was observed for the polymer due to its more extended conjugation in the repeating unit and a highly rigid backbone in the polymer chain. Polymer has strong blue fluorescence due to the efficient energy migration from the extended n-electronic structure of the repeating unit of the polymer to the chiral binaphthyl core and is expected to have potential application in the materials of fluorescent sensors and chiral chromatographic packing for resolution of racemic amino acid. 相似文献
996.
Rosen MJ Wang H Shen P Zhu Y 《Langmuir : the ACS journal of surfaces and colloids》2005,21(9):3749-3756
The interfacial tension (IFT) between alkanes and several individual surfactants and their mixtures has been investigated, using three kinds of alkyl hydrocarbons: decane, dodecane, and tetradecane. For individual and mixed surfactant systems, critical micelle concentrations and areas per molecule at the hydrocarbon-aqueous solution interface were calculated; for the mixed surfactant systems, betasigma(LL), the molecular interaction parameter at the hydrocarbon-aqueous solution interface, and beta(M), the molecular interaction parameter in mixed micelle formation, were calculated. It was found that IFT in the 10(-3) mN/m (ultralow) range can be obtained at surfactant concentrations below 0.05 wt % and even at concentrations below 0.01 wt %, when mixtures of certain surfactants are used at the proper ratio. Surfactants with branched-chain alkyl groups show a much better IFT reduction effectiveness than those with straight-chain alkyl groups. Contrary to what has been observed at the air-aqueous solution surface, mixtures of two homologues with two hydrophobic groups show significant molecular interactions, with both betasigma(LL) and beta(M) having negative values in the 4-5 range in some cases, with the betasigma(LL) value more negative than beta(M). The relationship between micellar shape and ultralow IFT was investigated by calculating the critical packing parameter of the surfactants. It was found that ultralow IFT between the surfactant mixtures and the three hydrocarbons investigated could reach ultralow (<10(-2) mN/m) values when the critical packing parameter is very close to 1. 相似文献
997.
The adsorption of (R)- and (S)-2-phenylpropionamide (PPA, C(9)H(11)ON) molecules on a Cu(111) surface has been investigated using the density functional method with supercell models. The adsorption orientations of both (R)- and (S)-PPA molecules on the surface are the same: the phenyl rings are approximately parallel to the Cu(111) surface and positioned in the hollow sites, the amino and methyl groups occupy two-bridge sites, and the carbonyl occupies the top site. After the adsorption, the bond lengths in the two enantiomers are almost unchanged, but the changes for two dihedral angles show differences, especially for (R)-PPA molecule. The first angles between the (N,C9,C7) plane and the (C9,C7,C6) plane are 19.4 and 0.7 degrees for (R)- and (S)-PPA molecules, respectively, and the second angles between the (C8,C7,C6) plane and the (C7,C6,C5) plane are 74.8 and 0.4 degrees for (R)- and (S)-PPA molecules, respectively. The adsorption energies of (R)- and (S)-PPA molecules are calculated to be -34 and -26 kJ mol(-1), respectively. The simulated scanning tunneling microscopy (STM) images of (R)- and (S)-PPA molecules on the Cu(111) surface display different features and are coincident with the experimental ones. The interaction between the adsorption molecule and the metal surface is found to be responsible for the discrimination of (R)- and (S)-PPA molecules on the surface. 相似文献
998.
采用双股并流共沉淀方法制备了SnO2含量从10%至90%的锡锆体系DeNOx催化剂,用XRD、微区电子衍射、FT-Raman及FT-IR等技术深入研究了锡锆体系氧化物的结构及其随组成的变化规律.结果表明,由于Sn4+与Zr4+离子半径接近,SnO2与ZrO2易于形成固溶体,并随组成变化表现出不同的结构特征.纯ZrO2为单斜相,当少量Sn4+(SnO2 ≤ 20%)进入ZrO2晶格时形成四方相富锆固溶体,Sn4+起到稳定ZrO2四方相的作用;随着SnO2含量的增大,结构从无定形或微晶态的富锆固溶体(含SnO2 30~50%)经富锆固溶体与金红石结构的富锡固溶体在55% SnO2含量的共存状态变化到具有金红石结构的富锡固溶体(SnO2 ≥ 60%).FT-Raman和FT-IR光谱测试证明,Zr进入SnO2晶格使得Sn-O键的结合减弱,Sn离子上的有效正电荷减小,降低了SnO2对丙烯的燃烧能力,从而提高了对NO的还原活性. 相似文献
999.
The enthalpies of mixing of glycine, l-α-alanine, l-γ-aminobutyric acid, l-α-valine, l-α-serine and l-α-threonine with cyclohexanone in aqueous solutions and their respective enthalpies of dilution have been measured by calorimetry at 298.15 K. Experimental enthalpies of dilution and mixing have been correlated with the virial expansion equation that was obtained with the McMillan-Mayer theory. The enthalpic interaction parameters hxy, hxxy and hxyy of the amino acids studied with cyclohexanone in aqueous solutions have been evaluated, and the heterotactic enthalpic pair interaction coefficients (hxy) are discussed in terms of solute-solute interactions. 相似文献
1000.
Xueliang Zhu Xiangyang Qi Jing Wang Jingnian Yue Zhida Sun Wenjun Lei 《Chromatographia》2007,65(9-10):625-628
A rapid method based on matrix solid-phase dispersion (MSPD) was developed for the determination of procymidone, pentachloroaniline and methyl-pentachloro-phenylsulfide in wine. After the optimisation of different parameters such as the type of adsorbent, the extraction solvent, and the extraction assistance by sonication. The analysis of samples was accomplished using gas chromatography with electron-capture detector (GC-ECD). Spiked blank samples were used as standards to counteract the matrix effect observed in the chromatographic determination. The recovery of the method was in the range 82.4–93.7% and a good linear relationship (R 2 ≥ 0.9992) with relative standard deviations lower than 8%. The limits of detection (LOD) ranged from 0.1 to 0.4 μg L?1. The proposed method was applied to the analysis of these compounds in commercial wine samples. 相似文献