全文获取类型
收费全文 | 16329篇 |
免费 | 1780篇 |
国内免费 | 1267篇 |
专业分类
化学 | 14882篇 |
晶体学 | 180篇 |
力学 | 210篇 |
综合类 | 28篇 |
数学 | 183篇 |
物理学 | 3893篇 |
出版年
2023年 | 151篇 |
2022年 | 341篇 |
2021年 | 439篇 |
2020年 | 592篇 |
2019年 | 502篇 |
2018年 | 454篇 |
2017年 | 406篇 |
2016年 | 622篇 |
2015年 | 583篇 |
2014年 | 764篇 |
2013年 | 1191篇 |
2012年 | 782篇 |
2011年 | 867篇 |
2010年 | 688篇 |
2009年 | 804篇 |
2008年 | 841篇 |
2007年 | 894篇 |
2006年 | 783篇 |
2005年 | 669篇 |
2004年 | 627篇 |
2003年 | 574篇 |
2002年 | 1651篇 |
2001年 | 359篇 |
2000年 | 296篇 |
1999年 | 281篇 |
1998年 | 273篇 |
1997年 | 175篇 |
1996年 | 190篇 |
1995年 | 191篇 |
1994年 | 130篇 |
1993年 | 107篇 |
1992年 | 92篇 |
1991年 | 83篇 |
1990年 | 54篇 |
1989年 | 37篇 |
1988年 | 38篇 |
1985年 | 108篇 |
1984年 | 118篇 |
1983年 | 109篇 |
1982年 | 153篇 |
1981年 | 117篇 |
1980年 | 103篇 |
1979年 | 102篇 |
1978年 | 108篇 |
1977年 | 150篇 |
1976年 | 119篇 |
1975年 | 130篇 |
1974年 | 169篇 |
1973年 | 136篇 |
1972年 | 83篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
61.
62.
The reaction pathway for the Ca3Al2O6 formation up to 1300°C, from mechanochemically treated mixtures of amorphous aluminum hydroxide and CaCO3, was studied in situ by differential thermal analysis, constant heating rate dilatometry and time-resolved neutron powder diffraction. The experiment was carried out, in an open system, on a sample with the nominal Ca3Al2O6 stoichiometry. The results obtained by neutron diffractometry and thermal analysis were in good agreement with the data obtained by scanning electron microscopy and X-ray diffraction on heat-treated and-quenched samples. The synthesis path implied the formation of cryptocrystalline Al2O3, crystalline CaO, CaAl2O4 and Ca12Al14O33 as transitory phases. Finally the nucleation and growth of the single phase Ca3Al2O6 took place at 1300°C and exhibited porous structure due to CO2 and H2O release. 相似文献
63.
The central zinc(II) atom in the title complex is tetrahedrally coordinated by four nitrogen atoms derived from 4‐methyl‐5‐imidazolecarboxyaldehyde ligands with Zn? N in the range 2.007(3) to 2.026(4) Å. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献
64.
A new hydrophilic copolymer having three kinds of hydroxyl groups, i.e. primary OH on -hydroxy allyl alcohol component, secondary OH on cyclic hemiacetal component and tertiary OH on -hydroxy acrylate component, was prepared by reducing a ‘polylactone’, poly(1-oxa-2-oxobutane-1,4:3,3-tetrayl), with NaBH4 in mixed solvents containing an alcohol. The structural analyses were performed by the solution 1H NMR and the solid state 13C NMR spectroscopies of the reduced samples. The composition of the three components turned out to be variable to the alcohol species used for the mixed solvent. 相似文献
65.
采用PM 3半经验方法对肾上腺素和二甲亚砜与肾上腺素形成的 1∶1复合物的结构进行了几何优化 .在此基础上用密度泛函理论 (DFT)的B3LYP方法 ,分别在 6 31G、6 31G 、6 31+G 基组水平上进行了精确计算 ,并通过单点能计算考察了它们之间在形成各种复合物前后的能量和分子结构参数变化特点 .在各基组水平上所有物种的能量均进行零点振动能 (ZPVE)校正 .计算结果表明 ,二甲亚砜与肾上腺素能形成 6种稳定的复合物 ,在这些复合物中都具有较强的氢键作用 .计算结果能够解释有关实验现象 相似文献
66.
Inés CarrancoJosé Luis D?&#x;az Oscar JiménezRodolfo Lavilla 《Tetrahedron letters》2003,44(46):8449-8452
The Lewis acid-catalyzed interaction of dihydropyridines with ethyl glyoxalate and primary aliphatic amines yields, depending on the reaction conditions, bicyclic aminals or rearranged tetrahydropyridines in one-pot multicomponent reactions. 相似文献
67.
《Tetrahedron letters》2003,44(40):7411-7415
The first total synthesis of two E type I phytoprostanes from furan, azelaic acid monomethyl ester and rac-1,2-epoxybutane is described. The key features of our synthetic strategy encompass an enzymatic kinetic resolution of a hydroxycyclopentenone, a Co-salen hydrolytic kinetic resolution of a terminal epoxide and a tandem conjugate addition/diastereoselective protonation sequence to construct the protected phytoprostanes. Mild cleavage of the silyl protective groups followed by enzymatic ester hydrolysis afforded the free E-type phytoprostanes. 相似文献
68.
69.
Taking both the heterogeneous catalytic processes, including the surface formation of particles with excited internal degrees of freedom, and the processes of multicomponent diffusion and heat transfer in the MESOX apparatus fully into account makes it possible to obtain a recombination coefficient and an accommodation coefficient of the oxygen-atoms-on-quartz recombination energy which are in good agreement with the experimental data. The heterogeneous catalysis model constructed can be used effectively for predicting the heat fluxes to the surface of reentry vehicles on their entry into the Earth’s atmosphere. 相似文献
70.
G. I. Nikishin L. L. Sokova V. D. Makhaev N. I. Kapustina 《Russian Chemical Bulletin》2008,57(1):118-123
The solid-phase oxidation of cinnamic, 4-methoxy- and 3,4-dimethoxycinnamic acids with Ce(NH4)2(NO3)6—MHal system leads to β-halostyrenes. Similar procedure in the absence of a metal halide results in a cleavage of the C=C
bond giving the corresponding benzaldehydes.
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 114–118, January, 2008. 相似文献