全文获取类型
收费全文 | 74633篇 |
免费 | 16382篇 |
国内免费 | 6938篇 |
专业分类
化学 | 68262篇 |
晶体学 | 826篇 |
力学 | 3187篇 |
综合类 | 372篇 |
数学 | 6432篇 |
物理学 | 18874篇 |
出版年
2024年 | 151篇 |
2023年 | 800篇 |
2022年 | 1481篇 |
2021年 | 1656篇 |
2020年 | 2789篇 |
2019年 | 3938篇 |
2018年 | 2378篇 |
2017年 | 1940篇 |
2016年 | 5114篇 |
2015年 | 5253篇 |
2014年 | 5630篇 |
2013年 | 6846篇 |
2012年 | 6469篇 |
2011年 | 5855篇 |
2010年 | 5308篇 |
2009年 | 5257篇 |
2008年 | 5126篇 |
2007年 | 4225篇 |
2006年 | 3824篇 |
2005年 | 3515篇 |
2004年 | 2911篇 |
2003年 | 2525篇 |
2002年 | 3299篇 |
2001年 | 2405篇 |
2000年 | 2151篇 |
1999年 | 1326篇 |
1998年 | 772篇 |
1997年 | 686篇 |
1996年 | 725篇 |
1995年 | 611篇 |
1994年 | 512篇 |
1993年 | 406篇 |
1992年 | 378篇 |
1991年 | 334篇 |
1990年 | 265篇 |
1989年 | 226篇 |
1988年 | 182篇 |
1987年 | 133篇 |
1986年 | 128篇 |
1985年 | 101篇 |
1984年 | 70篇 |
1983年 | 64篇 |
1982年 | 40篇 |
1981年 | 44篇 |
1980年 | 21篇 |
1979年 | 8篇 |
1974年 | 10篇 |
1971年 | 7篇 |
1957年 | 10篇 |
1936年 | 9篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
钙钛矿型复合氧化物对许多反应具有较高的催化活性,它们被广泛地用作CO氧化的催化剂.陈水华等对SmMeO_3(Me:V、Cr、Mn、Fe、Co)和Tascon等对LaMeO_3(Me:V、Cr、Mn、Fe、Co、Ni)上CO催化活性评价说明,LnMnO_3和LnCoO_3(Ln=La、Sm)活性最好,而LaNiO_3则表现和LaCoO_3相似活性,由此可推断这些氧化物具有相似的催化性能.本文用TPR-GC和XRD考察了在还原气氛下,LaNiO_3的还原机理,这对研究其电、磁特性具有一定的指导意义. 相似文献
992.
Zhanglin Liu Dong Tian Fei Shen Lulu Long Yanzong Zhang Gang Yang Yongmei Zeng Jing Zhang Jinsong He Ying Zhu Shihuai Deng 《中国化学快报》1990,30(12):2221-2224
Four distinct biochars were employed to remove three typical pollutants, meanwhile, path analysis, a multi-statistical regression method, was performed to elucidate the dominant factors of biochar adsorption. This work can provide a new insight to prepare a targeted biochar as adsorbents. 相似文献
993.
Jo‐Nan Chen Fang‐Ming Hsu Hui‐Chun Wang Chung‐Wen Wu Pao‐Swu Cheng Wen‐Liang Tsai 《中国化学会会志》2006,53(4):931-938
Four series of compounds 11?50 containing terminal alicyclic rings such as cyclohexylmethyl, cyclopentylmethyl, cyclobutylmethyl, and cyclopropylmethyl rings were synthesized and their liquid crystal behavior studied. The ring size and the length of flexible alkoxy chain influence the phase formation in different ways. While the smaller ring and the shorter alkoxy chain tend to favor the formation of the N phase, the larger ring and the longer alkoxy chain tend to favor the formation of the SmC phase. All the compounds except 11 and 21 exhibit SmA phases. The widest temperature range of the N, SmA, and SmC phases are found in the compounds 41 , 46 , and 20 , respectively, which are 75 °C for 41 , 115 °C for 46 , and 100 °C for 20 . 相似文献
994.
Fernando Mendizabal Dana Reyes Claudio Olea‐Azar 《International journal of quantum chemistry》2006,106(4):906-912
Hydrogen bonding and metallophilic attractions are studied in the model systems: [(AuNH3Cl)2], [(AuNH(CH3)2Cl)2], [{Au2(μ‐SH)(PH2O)(PH2OH)}2], [(CuNH3Cl)2], and [{Cu(NH3)Cl}4] at the Hartree–Fock (HF) and second‐order Møller–Plesset (MP2) levels. The two interactions are found to be comparable and prevailing in the final structure. It is determined that the aurophilic contact has a same magnitude that the hydrogen bonding, and is stronger than the cuprophilic interaction. The presence of hydrogen bond directs the growth of the crystal. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2006 相似文献
995.
Crystal structures of new nitronyl nitroxide derivatives 1, 2 and 3 were determined with X‐ray diffraction analysis: 1, monoclinic, C2/c, a = 1.2404(5) nm, b = 0.9730(5) um, c = 2.7049(10) ran, β = 98.189(15)°, V = 3.2315(24) nm3, Z = 8; 2, or‐thorhombic, Pbca, a = 0.61262(2) nm, b = 1.11426(6) nm, c = 2.30543(13) nm, V = 1.57373(13) nm3, Z = 4; 3, monoclinic, P2(1)/n, a=0.64253(4) nm, b=2.55003(17) nm, c = 1.15497(6) nm, β = 95.000(3)°, V = 1.8852(2) nm3, Z = 4. Their magnetic properties were measured with SQUID and analyzed based on their crystal structures with simple singlet‐triplet, modified one dimensional antiferromagnetic chain and modified singlet‐triplet models respectively: 1, J/kb= ?2.5 K; 2, J/kb = 7.8 K, θ = 2.8 K; 3, J/kb = ?0.96 K, θ = 0.21 K. 相似文献
996.
Crystal Structure of non‐aqueous Melaminium Chloride Melaminium chloride was obtained as colorless, needle‐shaped, single crystalline material from solid state reactions between melamin and ammonium chloride. The structure of [C3N6H7]Cl was refined by single crystal X‐ray diffraction: I2/m, Z = 8, a = 852.87(4), b = 1704.4(1) c = 918.44(4) pm and β = 92.165(6)°. The crystal structure contains melaminium ions stacked to columns along [100] and linked via N‐H···N bridges to form bands along [001]. Both of the two distinct chloride ions are stabilized by six hydrogen bonds through distorted trigonal prismatic arrangements of hydrogen atoms to yield a supramolecular structure. 相似文献
997.
998.
J. M. Hernndez Ana M. Herrera J. García‐Serrano J. F. Rivas‐Silva 《International journal of quantum chemistry》2002,88(3):342-346
Conformations of p‐methacryloylaminophenylarsonic acid (p‐MAPHA) are determined through molecular mechanics and DFT/B3LYP calculations. Solvation effects are studied within the self‐consistent isodensity continuum model (SCI‐PCM). The stationary points were found to correspond to minima as verified by the analysis of vibrational frequencies in the molecule. The molecular optical absorption was obtained by using different solvent environments. The present results are in good agreement with the available experimental data. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2002 相似文献
999.
Sixteen compounds containing the bicyclic moiety (3,8,10-trisubstituted 2,9-dioxo-5-thia-1,8-diazabicyclo[4.4.0]decane) were produced via solid-phase synthesis. Differing substitution at the 3- and 10-positions was used. These were analyzed using 2-D NMR techniques (ROESY) to determine the stereoselectivity of ring formation in the core heterocycle. Conformational analysis of the proposed transition state structure using Sybyl 6.8® was used to rationalize the stereochemical outcome of ring formation. 相似文献
1000.
The interaction between gatifloxacin mesylate (GM) and salmon sperm DNA was studied by fluorescence spectrometry and ultra‐violet (UV) spectrometry. Additions of salmon sperm DNA to GM solution resulted in its strong fluorescence quenching and UV absorbance decrease due to the strong interaction between GM and salmon sperm DNA. Both the maximum fluorescence emission wavelength and the maximum UV absorbance wavelength of GM did not change with the increasing concentration of salmon sperm DNA, indicating no intercalative binding existed between them. The Stern‐Volmer plot indicated that the fluorescence‐quenching constant at different temperatures or different salmon sperm DNA concentration ranges was different. Effects of ionic strength and I? on the fluorescence quenching of GM by salmon sperm DNA indicated that electrostatic interaction and groove binding coexisted between them. 相似文献