全文获取类型
收费全文 | 7406篇 |
免费 | 1336篇 |
国内免费 | 1374篇 |
专业分类
化学 | 6223篇 |
晶体学 | 139篇 |
力学 | 349篇 |
综合类 | 129篇 |
数学 | 810篇 |
物理学 | 2466篇 |
出版年
2024年 | 42篇 |
2023年 | 166篇 |
2022年 | 334篇 |
2021年 | 335篇 |
2020年 | 408篇 |
2019年 | 441篇 |
2018年 | 361篇 |
2017年 | 362篇 |
2016年 | 440篇 |
2015年 | 480篇 |
2014年 | 506篇 |
2013年 | 655篇 |
2012年 | 633篇 |
2011年 | 684篇 |
2010年 | 544篇 |
2009年 | 489篇 |
2008年 | 523篇 |
2007年 | 423篇 |
2006年 | 372篇 |
2005年 | 304篇 |
2004年 | 211篇 |
2003年 | 202篇 |
2002年 | 215篇 |
2001年 | 195篇 |
2000年 | 138篇 |
1999年 | 107篇 |
1998年 | 72篇 |
1997年 | 51篇 |
1996年 | 40篇 |
1995年 | 46篇 |
1994年 | 35篇 |
1993年 | 30篇 |
1992年 | 22篇 |
1991年 | 21篇 |
1990年 | 30篇 |
1989年 | 23篇 |
1988年 | 20篇 |
1987年 | 16篇 |
1986年 | 9篇 |
1985年 | 12篇 |
1984年 | 9篇 |
1983年 | 14篇 |
1982年 | 8篇 |
1979年 | 6篇 |
1977年 | 7篇 |
1975年 | 9篇 |
1974年 | 8篇 |
1972年 | 11篇 |
1971年 | 6篇 |
1969年 | 5篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
171.
172.
Preparation and Characterization of Microencapsulated Hexadecane Used for Thermal Energy Storage 总被引:3,自引:0,他引:3
GuangLongZOU ZhiChengTAN XiaoZhengLAN LiXianSUN TaoZHANG 《中国化学快报》2004,15(6):729-732
Polyurea microcapsules about 2.5μm in diameter containing phase change material for thermal energy storage application were synthesized and characterized by interfacial polycondensation method with toluene-2,4-diisocyanate and ethylenediamine as monomers in an emulsion system. Hexadecane was used as a phase change material and OP, which is nonionic surfactant, and used as an emulsifier. The chemical structure and thermal behavior of the microcapsules were investigated by FTIR and thermal analysis respectively. The results show encapsulated hexadecane has a good potential as a solar energy storage material. 相似文献
173.
Getahun Z Huang CY Wang T De León B DeGrado WF Gai F 《Journal of the American Chemical Society》2003,125(2):405-411
It is well-known that the C=N stretching vibration in acetonitrile is sensitive to solvent. Therefore, we proposed in this contribution to use this vibrational mode to report local environment of a particular amino acid in proteins or local environmental changes upon binding or folding. We have studied the solvent-induced frequency shift of two nitrile-derivatized amino acids, which are, AlaCN and PheCN, in H(2)O and tetrahydrofuran (THF), respectively. Here, THF was used to approximate a protein's hydrophobic interior because of its low dielectric constant. As expected, the C=N stretching vibrations of both AlaCN and PheCN shift as much as approximately 10 cm(-1) toward higher frequency when THF was replaced with H2O, indicative of the sensitivity of this vibration to solvation. To further test the utility of nitrile-derivatized amino acids as probes of the environment within a peptide, we have studied the binding between calmodulin (CaM) and a peptide from the CaM binding domain of skeletal muscle myosin light chain kinase (MLCK(579-595)), which contains a single PheCN. MLCK(579-595) binds to CaM in a helical conformation. When the PheCN was substituted on the polar side of the helix, which was partially exposed to water, the C=N stretching vibration is similar to that of PheCN in water. In constrast, when PheCN is introduced at a site that becomes buried in the interior of the protein, the C=N stretch is similar to that of PheCN in THF. Together, these results suggest that the C=N stretching vibration of nitrile-derivatized amino acids can indeed be used as local internal environmental markers, especially for protein conformational studies. 相似文献
174.
175.
A method for the isotopic determination of selenium in biological matrices is described. The method is based on hydride generation inductively coupled plasma mass spectrometry (ICP-MS). The development is specifically related to the requirements of stable isotope tracer studies in human subjects. The method is based on isotope dilution using 82Se as the in vitro spike and can quantify the 74Se and 77Se contents of samples. It involves wet oxidation (HNO3 - H2O2 or HNO3 - HClO4) of the 82Se-spiked matrix, reduction to selenite by boiling with HCl followed by measurement of the isotope ratios (82Se/77Se and 74Se/77Se) in the gas stream (H2Se) generated from on-line reduction of the sample selenite with NaBH4. Compared with the isotopic signal resulting from a selenite solution containing 5 ng ml-1 of Se, the total sample blank contributions at m/z = 74, 77 and 82 were less than 5% of the respective isotope signal. Worst-case absolute detection limits were 0.2-0.9 ng of Se, depending on the isotope used. Ion beam intensity ratios were measured with an over-all precision [relative standard deviation (RSD)] of 1% for both isotope pairs. Measured ratios (MRa/b) were stable during a given day's operation within the expected precision of the measurements but varied for different days. The magnitude of MRa/b was generally independent of the nature of the matrix. Highly linear relationships were found between ion beam intensity ratios (MRa/b) and the corresponding true isotope ratios for calibration solutions whose isotope ratios had been altered by as much as one order of magnitude.(ABSTRACT TRUNCATED AT 250 WORDS) 相似文献
176.
Zong Y Tawa K Menges B Rühe J Knoll W 《Langmuir : the ACS journal of surfaces and colloids》2005,21(15):7036-7043
To generate command surfaces for all-optical switching, highly ordered polymeric Langmuir-Blodgett-Kuhn (LBK) multilayers were fabricated onto silicon substrates and gold-coated optical glass slides from novel azobenzene-bearing polyamic acid systems. Pronounced Bragg peaks and well-defined Kiessig fringes observed in the X-ray reflectivity measurement for samples on silicon substrates indicate that these films possess a regularly repeated Y-type LBK multilayer structure and ultrasmooth surfaces. Fourier transform infrared (FT-IR) spectra taken by grazing incidence reflection suggest specific orientations of the functional groups in the layers. The excellent film-forming properties of the polyamic acid allow for a smooth buildup of several hundreds of layers of the LBK films onto gold-coated glass slides, which in turn allows for determining the geometrical thickness and the anisotropic refractive indices of the films by using optical waveguide spectroscopy. Interestingly, the probe laser beam induced a distinct fluorescence signal from the films, which remained even after the film underwent a thermal imidization process at 160 degrees C for 8 h in vacuo. LBK films fabricated from these compounds can be successfully applied for all-optically switching the alignment of liquid crystals by irradiation with light of different wavelengths. 相似文献
177.
178.
Lewis acid/base addition between Ln(NO3)3 · 6H2O (Ln = Pr, Nd, Sm, Eu, Tb and Lu) and H2salen [H2salen = N,N′-ethylenebis(salicylideneimine)] gives rise to an array of coordination polymeric structures. Crystal structural analysis reveals that Salen effectively functions as a bridging ligand in these compounds. The size of the lanthanide ions controls the structures of these Salen lanthanide complexes. Two representative structures with one dimensional and two dimensional topologies, viz. [Pr(H2salen)(NO3)3(CH3OH)2]n (1) and [Ln(H2salen)1.5(NO3)3]n [Ln = Pr (2), Nd (3), Sm (4), Eu (5), Tb (6) and Lu (7)] are reported. Luminescent spectra of complexes 4 and 5 exhibit characteristic metal-centered emission lines. However, the characteristic luminescence of the terbium(III) ion is not observed either in solution or in the solid state of complex 6. 相似文献
179.
采用密度泛函理论的b3lyp方法在6-311++G**基组上对15种分子式为N6H6的氮氢化合物进行了理论计算, 并且应用了自然键轨道理论(Nature Bond Orbital, NBO)和分子中的原子理论(Atoms In Molecules, AIM)分析了这些化合物的成键特征和相对稳定性. NBO分析表明N原子孤对电子到相邻的氮氮键的超共轭作用是影响氮氮键长变化的主要因素, AIM计算的氮氮键的键临界点电荷密度与键长呈反比关系. 而且, NBO的立体和超共轭分析表明立体交换排斥能和超共轭作用对这些分子的相对稳定性起了重要作用. G3MP2计算结果表明氮氢化合物的生成热均为正, 并且环状分子的能量和生成热都高于链状分子. 相似文献
180.
The crystal structure of [Cd(BDA)(phen)2(H2O)](H2O)2 (1) (BDA=6,6′-dibromo-2,2′-dimethoxy-1,1′-binaphthylene-4,4′-disulfonate, phen=1,10-phenanthroline)consists of a cadmium center whose coordination environment can be best described as a slightly distorted octahedron defined four nitrogen atoms from two phen ligands and two oxygen atoms differently from BDA ligand and water. There are strong hydrogen-bonding interactions between water and sulfonate group of BDA ligands to construct the 3D network. CCDC: 277921. 相似文献