全文获取类型
收费全文 | 88540篇 |
免费 | 4772篇 |
国内免费 | 11424篇 |
专业分类
化学 | 69632篇 |
晶体学 | 1111篇 |
力学 | 2362篇 |
综合类 | 821篇 |
数学 | 12018篇 |
物理学 | 18792篇 |
出版年
2024年 | 113篇 |
2023年 | 657篇 |
2022年 | 1846篇 |
2021年 | 1762篇 |
2020年 | 2161篇 |
2019年 | 2102篇 |
2018年 | 1801篇 |
2017年 | 2640篇 |
2016年 | 2756篇 |
2015年 | 2317篇 |
2014年 | 3218篇 |
2013年 | 6358篇 |
2012年 | 6091篇 |
2011年 | 4854篇 |
2010年 | 4122篇 |
2009年 | 5646篇 |
2008年 | 5836篇 |
2007年 | 6073篇 |
2006年 | 5503篇 |
2005年 | 4643篇 |
2004年 | 4291篇 |
2003年 | 3562篇 |
2002年 | 4797篇 |
2001年 | 2727篇 |
2000年 | 2538篇 |
1999年 | 2376篇 |
1998年 | 2064篇 |
1997年 | 1612篇 |
1996年 | 1352篇 |
1995年 | 1290篇 |
1994年 | 1171篇 |
1993年 | 978篇 |
1992年 | 936篇 |
1991年 | 615篇 |
1990年 | 528篇 |
1989年 | 506篇 |
1988年 | 365篇 |
1987年 | 294篇 |
1986年 | 273篇 |
1985年 | 223篇 |
1984年 | 234篇 |
1983年 | 135篇 |
1982年 | 208篇 |
1981年 | 172篇 |
1980年 | 189篇 |
1979年 | 177篇 |
1978年 | 164篇 |
1977年 | 121篇 |
1976年 | 105篇 |
1973年 | 66篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Xiaoxia WangYongmin Zhang 《Tetrahedron》2003,59(23):4201-4207
N-Acylbenzotriazoles, when treated with samarium diiodide in THF, undergo self-coupling reaction to afford 1,2-diketones in good to excellent yields; while when treated with samarium diiodide in CH3CN, they undergo ring-opening reaction to afford 1-acylamido-2-alkyl (or aryl) benzimidazoles in reasonable to good yields. A plausible mechanism was suggested. 相似文献
992.
在自制的硅基纤维素-三(3,5-二甲基苯基氨基甲酸酯)高效液相色谱手性固定相上(HPLC-CSP),优化了1-(6'-甲氧基萘)乙醇氢酯基化反应产物-萘普生甲酯手性分离的条件,测定了相应的一系列不对称氢酯基化反应产物的对映体过剩值(e.e.值)。结果表明,在CDMPC-CSP手性柱上用HPLC测定此类不对称催化反应的光学产率,评价催化剂体系的手性选择是一种非常理想的方法。 相似文献
993.
994.
酞侧基聚芳醚酮的热学性能谢红卫,李滨耀(中国科学院长春应用化学研究所长春130022)关键词酞侧基聚芳醚酮,pvT行为,导热系数,定压比热容酞侧基聚芳醚酮(PEK-C)具有较高的玻璃化转变温度,其流变加工性能和力学性能已被广泛研究[1~3],被证明是... 相似文献
995.
Two new esters, methyl 4‐(prenyloxy)dihydrocinnamate and methyl 4‐(geranyloxy)dihydrocinnamate, together with fourteen known compounds have been isolated from the stem bark of Zanthoxylum pistaciiflorum. The structures of two new compounds were determined through spectral analyses. Among the isolates, four compounds exhibited effective cytotoxicities against P‐388 and HT‐29 cell lines in vitro. 相似文献
996.
A novel hyperbranched polyester acrylate (HPEA) was synthesized based upon ethylenediamine tetraacetic acid as a “core” molecule, 5‐hydroxyisophthalic acid as an AB2 monomer, and 2‐hydroxyethyl acrylate as an endcapping reagent. The obtained oligomer has an unsaturation concentration of 4.10 mmolC?C g?1 measured by nuclear magnetic resonance and a wide molecular weight distribution of 1.64 measured by gel permeation chromatography. The two‐photon absorption (TPA) photopolymerization of HPEA under the exposure of a Ti : sapphire femtosecond laser with a wavelength of 800 nm was investigated through laser exposure dose‐dependant spatial resolutions of its resins. The TPA photopolymerization thresholds at the range 1.6–4.3 × 107 mJ cm?2, and exposure dose windows at the range 3.4–4.3 for three formulations were determined. A spatial resolution of 0.85 μm was obtained through the TPA photopolymerization of the formulation containing 1 wt% photoinitiator and 0.3 wt% photoinhibitor. A diffraction grating and real three‐dimensional coupled gear wheel created by TPA photopolymerization were described to demonstrate the unique capability of HPEA in microfabrications. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
997.
A superoxochromium(III) ion, CraqOO2+, acts as a catalyst for the co-oxidation of alcohols and nitrous acid with molecular oxygen according to the stoichiometry: CH3OH+HNO2+O2→CH2O + NO3− + H2O+H+. The kinetics are second order in [HNO2] and independent of the concentrations of the superoxochromium catalyst, substrate, and O2. The proposed mechanism features the disproportionation of HNO2 to NO and NO2, both of which react rapidly with CraqOO2+. The CraqOO2+/NO reaction generates another equivalent of NO2 and a mole of CraqO2+, the active oxidant. The two-electron oxidation of the alcohol by CraqO2+ produces Craq2+, which reacts rapidly with O2 to regenerate the catalyst, CraqOO2+. The NO2/CraqOO2+ reaction yields the peroxynitrato complex, CraqOONO22+, in a dead-end equilibrium process that has no effect on the catalytic reaction. The disproportionation of NO2 yields the final nitrogen-containing product, NO3−, and regenerates an equivalent of HNO2. Under a fixed set of conditions, the relative catalytic efficiency (CE) of CraqOO2+ decreases as its concentration increases owing to the competition between O2 and CraqOO2+ for the intermediate Craq2+. 相似文献
998.
V. A. Dorokhov L. S. Vasil'ev N. Kyu Khoa N. Kong Hao V. S. Bogdanov 《Russian Chemical Bulletin》1997,46(11):1967-1969
A method for the synthesis of 2,2′-bipyridin-4-one from 4-amino-4-(2-pyridyl)-but-3-en-2-onevia its diphenylboron, chelates was proposed.
Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 11, pp. 2071–2073, November, 1997. 相似文献
999.
The stability of hexacyanoferrate(II)-amine(methylamine, ethylenediamine, diethylenetriamine and tetraethylenepentamine) was determined potentiometrically. Species Fe(CN)6(A)H
j
(j–4)
(A=amine) are formed in all the systems investigated, with j=1...n+2 (n=number of aminogroups). Some other complexes Fe(CN)6(A)iHj (with i>1) were also found. The stability of these complexes is fairly high: the full protonated amine species, show for the reaction Fe(CN)6
4- + HnAn+ = Fe(CN)6(A)Hn
(n-4) an equilibrium constant given by logK=0.686+2.10n. Factors affecting the stability are discussed in comparison with similar systems, together with the importance of interferences. 相似文献
1000.
以交联聚乙烯醇为载体的离子交换剂对蛋白质的分离性能 总被引:2,自引:0,他引:2
本文对阴离子交换剂DEA—PVT常压液相离子交换色谱分离蛋白质的性能、分离条件进行了探讨。结果表明其对蛋白质的分离性能良好,容易洗脱。与载体交联聚乙烯醇相比,DEA—PVT对蛋白质的非特异性吸附明显降低。 相似文献