全文获取类型
收费全文 | 10966篇 |
免费 | 1569篇 |
国内免费 | 918篇 |
专业分类
化学 | 5401篇 |
晶体学 | 113篇 |
力学 | 907篇 |
综合类 | 83篇 |
数学 | 806篇 |
物理学 | 6143篇 |
出版年
2024年 | 18篇 |
2023年 | 93篇 |
2022年 | 274篇 |
2021年 | 282篇 |
2020年 | 307篇 |
2019年 | 303篇 |
2018年 | 294篇 |
2017年 | 326篇 |
2016年 | 431篇 |
2015年 | 361篇 |
2014年 | 546篇 |
2013年 | 929篇 |
2012年 | 657篇 |
2011年 | 786篇 |
2010年 | 574篇 |
2009年 | 689篇 |
2008年 | 747篇 |
2007年 | 783篇 |
2006年 | 637篇 |
2005年 | 540篇 |
2004年 | 468篇 |
2003年 | 438篇 |
2002年 | 395篇 |
2001年 | 321篇 |
2000年 | 297篇 |
1999年 | 268篇 |
1998年 | 218篇 |
1997年 | 177篇 |
1996年 | 130篇 |
1995年 | 164篇 |
1994年 | 118篇 |
1993年 | 148篇 |
1992年 | 111篇 |
1991年 | 86篇 |
1990年 | 69篇 |
1989年 | 52篇 |
1988年 | 60篇 |
1987年 | 39篇 |
1986年 | 39篇 |
1985年 | 51篇 |
1984年 | 44篇 |
1983年 | 21篇 |
1982年 | 32篇 |
1981年 | 26篇 |
1980年 | 25篇 |
1979年 | 12篇 |
1978年 | 15篇 |
1977年 | 17篇 |
1976年 | 11篇 |
1973年 | 7篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
961.
在众多可产生磁共振现象的原子核中,1H核凭借其在生物体中含量高、磁共振信号强的优势,成为磁共振成像的主要研究对象.但其它杂核在生命科学相关研究中同样具有不可替代的独特性,如31P核广泛参与了生物体内的能量代谢过程,是非质子成像研究领域的重要内容.MRI向更高场强的发展使得杂核成像逐渐普及,其核心部件是高质量的1H/31P双调谐射频线圈.本文总结了与1H/31P双调谐射频线圈相关的研究与应用,展示了9.4 T下小鼠脑的质子磁共振成像及磁共振磷谱,并讨论了高场1H/31P双调谐射频线圈的潜在应用价值. 相似文献
962.
超疏水表面液滴的振动特性与接触线的移动、液滴体积、基底振幅等因素密切相关.本文在基底振幅较小且恒定的条件下,研究了超疏水表面液滴的共振振幅、模式区间、共振频率等振动特性及其与液滴体积(20—500μL)的关系.此外,将基于一般性疏水表面建立的Noblin共振频率计算模型应用于超疏水表面,并提出“虚驻点”的概念,借此对模型进行了误差分析和修正.研究表明:1)共振时,液滴高度变化率即比振幅随体积增大而增大,随阶数增大而减小;2)各模式区间的起止频率首尾相接,其范围随体积增大而减小;3)液滴体积越大,共振频率越小,随着阶数增大,共振频率f与体积V的关系趋于f-V–0.4,不同于一般性疏水表面上的f-V–0.5;4)直接应用Noblin模型计算共振频率会产生较大误差,主要原因在于液滴表面波波段数量统计存在较大偏差,而修正后的模型可以准确计算超疏水表面大体积液滴的共振频率. 相似文献
963.
C-2位单取代降冰片烯衍生物中降冰片烯环存在各向异性,且多数为外型和内型异构体的混合物,导致其结构解析困难.针对这一问题,本文在制备了单一构型的C-2位羧基和羟甲基取代的降冰片烯衍生物的基础上,利用1H NMR、DEPT135、1H-1H COSY、1H-13C HMQC、1H-1H NOESY谱和相应的耦合裂分信息,对外型-5-降冰片烯-2-羧酸、内型-5-降冰片烯-2-羧酸、外型-5-降冰片烯-2-甲醇和内型-5-降冰片烯-2-甲醇的1H和13C NMR信号进行归属,并探讨了降冰片烯衍生物的取代基种类及空间构型对1H NMR化学位移的影响. 相似文献
964.
965.
966.
967.
Proton-ligand association constants of 1-benzoyl(1,2,4-triazol-3-yl)thiourea (BTTU) and its complex formation constants with some bivalent metal ions Ni(II), Co(II), Mn(II), Zn(II), and Cu(II), have been determined potentiometrically in 50% EtOH–H2O and 0.1 M NaNO3. The complexes formed in solution have a stoichiometry of 1:1 and 1:2 [M:L], where M represents the metal ion and L the BTTU ligand. The corresponding thermodynamic parameters are derived and discussed. The complexes are stabilized by enthalpy changes and the results suggest that complexation is an enthalpy-driven process. The effects of metal ion, ionic radius, electronegativity, and nature of ligand on the formation constants are discussed. The formation constants of the complexes with 3d transition metals follow the order Mn2+ < Co2+ < Ni2+ < Cu2+ > Zn2+. The metal complexes were synthesized and characterized by elemental analyses, conductance, IR, 1H NMR, and magnetic measurements. The low magnetic moment of 0.11 BM for the Cu(II) complex is suggestive of dimerization through Cu–Cu interaction. The concentration distribution diagrams of the complexes were evaluated. The ligands and their metal complexes have been screened in vitro against some bacteria and fungi. 相似文献
968.
Carla F. C. Ruschel Marcelo C. A. Marcelo Francisco P. dos Santos Marco F. Ferrão 《Analytical letters》2017,50(11):1767-1777
Automotive fuel adulteration is an old and significant problem. One common type of fuel adulteration is the addition of diesel to gasoline. Unsupervised models were developed through hierarchical cluster and principal component analysis models. Supervised models through partial least square discriminant analysis using 1H nuclear magnetic resonance spectra as the input were used to classify samples as adulterated or unadulterated. Quantitative models were developed using partial least squares to determine the gasoline and diesel concentrations in the samples. This set contained samples composed of pure gasoline and anhydrous ethanol reproducing commercial gasoline and other samples treated with diesel. Hierarchical cluster and principal component analysis did not distinguish between adulterated and unadulterated samples except for the most adulterated materials. However, partial least square discriminant analysis classified 100% of the samples correctly. The partial least square algorithm provided excellent regression models for the gasoline and diesel content. The determination coefficient was 0.9920 for both models, whereas the root mean square error of cross-validation and root mean square error of prediction for the diesel model were 2.32 and 1.42%, respectively, and 2.40 and 1.38% for the gasoline model. 相似文献
969.
Here, the synthesis, characterization, and volatile organic compound (VOCs) sensing of a 1,3-dimethyl polyphenylene vinylene polymer is reported. The synthesis was performed by a Witting condensation through the reaction of 1,4-terphthaldehyde with the phosphonium chloride of meta-xylene. The material was characterized by infrared spectroscopy, elemental analysis, and thermogravimetric analyses. Thin films of the polymer were prepared by spin coating at speeds from 1000 to 5000?rpm. Ultraviolet–visible spectroscopy and surface plasmon resonance were used to characterize the spin-coated films. The thicknesses of the films were estimated by fitting the curves and were between 4.5 and 24.5?nm depending on the speed. The refractive index of the new polymer was 1.72. The polymer spin-coated films were exposed to volatile organic vapors to characterize their sensing properties by surface plasmon resonance as a function of time. The results showed that the new material responded rapidly, sensitively, and reversibly to VOCs. 相似文献
970.
《中国化学会会志》2017,64(5):522-530
In this study, we report the substituent effect on the structures, frontier orbital analysis, and spectroscopic properties (IR , 13C , 29Si NMR ) in the molybdenum silylidyne complexes CpMo (CO )2(≡Si‐para ‐C6H4X ) (X = H, F, Cl, CN , NO2 , Me, OMe , NH2 , NHMe ) using MPW1PW91 quantum chemical calculations. The calculated structural parameters and spectral parameters are compatible with the experimental values in similar complexes. The nature of the chemical bond between the [Cp(OC ) 2Mo ]− and [Si‐para ‐C6H4X ]+ fragments was explored with energy decomposition analysis (EDA ). The percentage composition in terms of the defined groups of frontier orbitals for CpMo (CO )2(≡Si‐para ‐C6H4X ) complexes was investigated to explore the character of the metal–ligand bonds. The linear correlations between the properties and Hammett constants (σ p) were illustrated. Natural bond orbital analysis (NBO ) was used to illustrate the electronic structure of the complexes. 相似文献