全文获取类型
收费全文 | 1267篇 |
免费 | 120篇 |
国内免费 | 219篇 |
专业分类
化学 | 1301篇 |
晶体学 | 25篇 |
力学 | 3篇 |
综合类 | 9篇 |
数学 | 2篇 |
物理学 | 266篇 |
出版年
2024年 | 2篇 |
2023年 | 13篇 |
2022年 | 22篇 |
2021年 | 26篇 |
2020年 | 35篇 |
2019年 | 45篇 |
2018年 | 36篇 |
2017年 | 46篇 |
2016年 | 60篇 |
2015年 | 49篇 |
2014年 | 74篇 |
2013年 | 121篇 |
2012年 | 69篇 |
2011年 | 67篇 |
2010年 | 76篇 |
2009年 | 63篇 |
2008年 | 70篇 |
2007年 | 81篇 |
2006年 | 63篇 |
2005年 | 69篇 |
2004年 | 72篇 |
2003年 | 50篇 |
2002年 | 53篇 |
2001年 | 40篇 |
2000年 | 28篇 |
1999年 | 34篇 |
1998年 | 39篇 |
1997年 | 18篇 |
1996年 | 21篇 |
1995年 | 29篇 |
1994年 | 37篇 |
1993年 | 23篇 |
1992年 | 14篇 |
1991年 | 10篇 |
1990年 | 8篇 |
1989年 | 5篇 |
1988年 | 7篇 |
1987年 | 7篇 |
1985年 | 3篇 |
1984年 | 4篇 |
1983年 | 4篇 |
1982年 | 5篇 |
1981年 | 3篇 |
1980年 | 1篇 |
1979年 | 2篇 |
1975年 | 1篇 |
1969年 | 1篇 |
排序方式: 共有1606条查询结果,搜索用时 15 毫秒
991.
采用水热法合成了一种非稀土上转换发光材料,由钠铝氟化物和BaF2组成。采用X射线衍射(XRD)、傅里叶红外光谱(FTIR)和扫描电镜(SEM)对其进行了表征,分析表明,钠铝氟化物主要以NaF和Na3AlF6两种晶态存在于BaF2晶体表面,形成化学复合,呈现包覆结构。荧光光谱测试表明,样品在583nm可见光和863nm近红外光室温激发下,均能产生304和324nm的宽波段紫外光发射。时间分辨光谱表明,发光均有一个上升和衰减过程,衰减大约自15ms开始,磷光寿命大于18.4ms。借助上转换发光能量转换过程对发光机理进行了研究,分析表明,发光源于量子限制效应的带间光吸收和界面缺陷的光吸收,发光机制属于量子限制效应-界面发光中心复合发光。 相似文献
992.
Poly(vinylidene fluoride) (PVDF) membranes were hydrophilic modified with hydroxyl group terminated hyperbranched poly(amine‐ester) (HPAE). Fourier transform infrared spectroscopy (FT‐IR) was used to study the chemical change of PVDF membranes. X‐ray photoelectron spectroscopy (XPS) indicated that some HPAE molecules were retained in PVDF membrane through polymer chain coiling. The presence of HPAE would improve the hydrophilicity of PVDF membrane. Scanning electron microscopy (SEM) was employed to characterize the morphology of different membranes. The thermodynamic stability for PVDF/DMAc/HPAE/Water system was characterized by the determination of the gelation values. Precipitation kinetics for PVDF/DMAc/HPAE/Water system was studied by precipitation time measurement. The water contact angle indicated that the hydrophilicity and the biocompatibility corresponding to protein adsorption of PVDF membrane were improved significantly after blending with hydrophilic HPAE molecules. Copyright © 2011 John Wiley & Sons, Ltd. 相似文献
993.
I.A. Godunov N.N. Yakovlev D.V. Maslov S.I. Bokarev 《Journal of Molecular Spectroscopy》2010,260(2):124-129
The gas phase absorption spectrum of oxalyl fluoride in the region of 37 000–29 300 cm−1 has been examined at high resolution. Singlet–singlet and singlet–triplet electronic transitions of the trans-conformer were found in the spectrum. The fundamental frequencies of trans-oxalyl fluoride in the and electronic states were determined.In the low resolution ultraviolet absorption spectrum of oxalyl fluoride in the gas phase the transition of the cis-conformer (νmax) was found to be shifted to the blue by about 6000 cm−1 relative to the transition of the trans-conformer. 相似文献
994.
James Cederberg Christopher Conklin Emily Berger 《Journal of Molecular Spectroscopy》2010,263(2):142-144
The molecular beam electric resonance technique has been used to conduct a high precision examination of the hyperfine spectrum of 23Na19F. Coupling constants for the nuclear electric quadrupole interactions, the spin-rotation interactions, the tensor and scalar spin-spin interactions, and their dependence on vibrational and rotational state have been determined. 相似文献
995.
Density functional theory has been used to study the mechanism of the decomposition of peroxyacetyl nitrate (CH3C(O)OONO2) in hydrogen fluoride clusters containing one to three hydrogen fluoride molecules at the B3LYP/6‐311++G(d,p) and B3LYP/6‐311+G(3df,3pd) levels. The calculations clarify some of the uncertainties in the mechanism of PAN decomposition in the gas phase. The energy barrier decreases from 30.5 kcal mol?1 (single hydrogen fluoride) to essentially 18.5 kcal mol?1 when catalyzed by three hydrogen fluoride molecules. As the size of the hydrogen fluoride cluster is increased, PAN shows increasing ionization along the O? N bond, consistent with the proposed predissociation in which the electrophilicity of the nitrogen atom is enhanced. This reaction is found to proceed through an attack of a fluorine to the PAN nitrogen in concert with a proton transfer to a PAN oxygen. On the basis of our calculations, an alternative reaction mechanism for the decomposition of PAN is proposed. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2010 相似文献
996.
Phase transformations in LnF3-(Ln, Zr)-ZrF4 systems (where Ln = Sm, Eu, Tm, Yb) in the temperature range 700-900 °C have been investigated by chemical and X-ray phase analyses, UV and IR spectroscopy. Synthesis conditions of nonstoichiometric samarium, europium, ytterbium and thulium fluorides, which are products of interaction between LnF3 and Ln and original substances for the preparation of compounds such as LnZrF6 are given. The crystal lattice of the fluorozirconates obtained is similar to the Yb27F64 superstructure. Thulium fluorozirconate (TmZrF6), whose structure is indexed as the Tm13F32−δ superstructure of hexagonal system, is an exception. For all compounds obtained, the lattice parameters have been calculated and the absorption bands of IR spectra have been interpreted. 相似文献
997.
L.Q. Lobo I.M.A. Fonseca A.G.M. Ferreira A.M.P. Senra 《The Journal of chemical thermodynamics》2006,38(12):1532-1538
The equilibrium pressure of ternary mixtures of {x1CH3F + x2HCl + x3N2O} covering the entire composition range has been measured at temperature of 182.33 K by the static method. The system exhibits a minimum pressure for the binary {x1CH3F + x2HCl}. The molar excess Gibbs free energy has been calculated from the experimental equilibrium pressure. For the equimolar mixture . The (p, x, y) surface for the ternary system and the corresponding curves for the three constituent binary mixtures obtained from the Peng-Robinson equation of state are in agreement with the experimental data. 相似文献
998.
Autoxidation/Aldol Tandem Reaction of 2‐Oxindoles with Ketones: A Green Approach for the Synthesis of 3‐Hydroxy‐2‐Oxindoles 下载免费PDF全文
Qing‐Bao Zhang Wen‐Liang Jia Yong‐Liang Ban Yong Zheng Prof. Dr. Qiang Liu Prof. Dr. Li‐Zhu Wu 《Chemistry (Weinheim an der Bergstrasse, Germany)》2016,22(8):2595-2598
In the presence of tetrabutylammonium fluoride and molecular sieves (MS) 4 Å in DMF, an efficient autoxidation reaction of 2‐oxindoles with ketones under air at room temperature has been developed. This approach may provide a green, practical, and metal‐free protocol for a wide range of biologically important 3‐hydroxy‐3‐(2‐oxo‐alkyl)‐2‐oxindoles. 相似文献
999.
Enrique J. Baran 《光谱学快报》2013,46(8):599-603
Abstract Pertechnetyl fluoride, TcO3F, had been prepared some years ago1, but its vibrational spectrum was only recently recorded and interpreted2. The spectroscopic study reveals that, as expected, TcO3F posseses C3v symmetry, and the values found for the fundamental vibrations are in good agreement with those previously estimated by Müller et al.3 in a paper concerning with the vibrational behaviour of MO3Xn? species, and some systematic relations among them (cf. also4). 相似文献
1000.
Stanislav Průša Pavel Bábík Tomáš Šikola Hidde H. Brongersma 《Surface and interface analysis : SIA》2020,52(12):1000-1003
Low-energy ion scattering (LEIS) probes the atomic composition of the outer surface. Well-defined reference samples are used for the quantitation. For elements like fluorine and calcium, it is not easy to find suitable, clean, and homogeneous references, since fluorine is a gas and calcium is a very reactive metal. In contrast to surface analytic techniques such as XPS, the extreme surface sensitivity of LEIS makes it difficult to use stable compounds like CaF2 as reference, since these compounds are not homogeneous at the atomic scale. With LEIS, CaF2 is not expected to show an atomic ratio F/Ca = 2.0. Thus, before CaF2 can be used as reference, its atomic surface concentrations have to be determined. Here, 3-keV He+ scattering by a LiF(001) single crystal, an evaporated layer of Ca, and a Cu foil are used as basic references. High-purity CaF2 is available in two forms: a single crystal and a powder. For a practical reference, powders are preferred, since if bulk impurities segregate to the surface, they will be dispersed over a large surface area. It is found that both CaF2 (111) and powder have similar F/Ca atomic ratios. This confirms the F termination for both samples. For the powder, the F and Ca signals are reduced by 0.77 ± 0.03 in comparison with those for the single crystal. The atomic sensitivity factors and relative sensitivity factors have been determined for F, Ca, and Cu. 相似文献