全文获取类型
收费全文 | 5648篇 |
免费 | 487篇 |
国内免费 | 422篇 |
专业分类
化学 | 1649篇 |
晶体学 | 21篇 |
力学 | 177篇 |
综合类 | 35篇 |
数学 | 1173篇 |
物理学 | 3502篇 |
出版年
2024年 | 5篇 |
2023年 | 58篇 |
2022年 | 115篇 |
2021年 | 151篇 |
2020年 | 120篇 |
2019年 | 184篇 |
2018年 | 172篇 |
2017年 | 155篇 |
2016年 | 190篇 |
2015年 | 169篇 |
2014年 | 340篇 |
2013年 | 423篇 |
2012年 | 327篇 |
2011年 | 334篇 |
2010年 | 342篇 |
2009年 | 424篇 |
2008年 | 373篇 |
2007年 | 392篇 |
2006年 | 310篇 |
2005年 | 269篇 |
2004年 | 250篇 |
2003年 | 213篇 |
2002年 | 174篇 |
2001年 | 105篇 |
2000年 | 147篇 |
1999年 | 98篇 |
1998年 | 112篇 |
1997年 | 88篇 |
1996年 | 71篇 |
1995年 | 65篇 |
1994年 | 61篇 |
1993年 | 49篇 |
1992年 | 50篇 |
1991年 | 52篇 |
1990年 | 25篇 |
1989年 | 32篇 |
1988年 | 21篇 |
1987年 | 14篇 |
1986年 | 10篇 |
1985年 | 16篇 |
1984年 | 14篇 |
1983年 | 3篇 |
1982年 | 4篇 |
1981年 | 6篇 |
1980年 | 3篇 |
1977年 | 2篇 |
1976年 | 3篇 |
1975年 | 4篇 |
1974年 | 4篇 |
1973年 | 3篇 |
排序方式: 共有6557条查询结果,搜索用时 62 毫秒
881.
Determination of titanium NMR parameters of ATiO3 compounds: correlations with structural distortion
Solid state 47,49Ti NMR spectra have been obtained for a number of perovskite and ilmenite ATiO3 compounds. The 49Ti quadrupole coupling constant varies from 2.75 MHz (CaTiO3) to 15.5 MHz (MgTiO3) and the electric field gradient at the titanium site was found to correlate well with the shear strain, independent of structure. The chemical shift in the perovskite structures varies by 160 ppm and correlates well with the mean Ti–O distance. The 25Mg and 113Cd NMR parameters are also reported for the relevant compounds. 相似文献
882.
J. Emery Y. Laligant J. Y. Buzar L. Smiri F. Hlel T. Ben-Chaabane 《Solid state nuclear magnetic resonance》2000,16(4)
To understand the surprising behavior between the variations of the P′–P–P″ angles and the correlated variations of the O′–P–O″ ones, two lithium cyclohexaphosphate compounds Li6P6O18·3H2O and Li6P6O18 are studied by solid state nuclear magnetic resonance (NMR) spectroscopy. The two compounds exhibit the same [P6O18]6− ring anions but with 3m or
internal symmetry, respectively. Such symmetries induce local distortions that are exhibited by NMR spectroscopy. One-dimensional (1D) NMR gives information on structural sites of 7Li and 31P ions and the crystallographic non-equivalencies are observed. Nevertheless, in the anhydrous compound, X-ray diffraction and NMR results do not completely agree and some discrepancy exists between the number of sites observed with the first technique and the number of lines exhibited in the NMR spectra either for 7Li or 31P nuclei. This problem is elucidated by using 2D double quantum NMR spectroscopy coupled with theoretical considerations. We find that the 31P chemical shift tensor is dependent on the deviations of the O–P–O angles from those in the regular tetrahedron. Within the same empirical model, we suggest that the surprising behavior between the variations of the P′–P–P″ and the ones of the O′–P–O″ is related to the overall charge on the PO4 group. We also find the positions of the isotropic lines for 7Li essentially depend on the site co-ordination of this nuclei. 相似文献
883.
Eric B. Brouwer Robin Challoner Robin K. Harris 《Solid state nuclear magnetic resonance》2000,18(1-4)
The supramolecular 1 : 1 host–guest inclusion compound, p-tert-butylcalix[4]arene ·α,α,α-trifluorotoluene, 1, is characterized by 19F and 13C solid-state NMR spectroscopy. Whereas the 13C NMR spectra are easily interpreted in the context of earlier work on similar host–guest compounds, the 19F NMR spectra of solid 1 are, initially, more difficult to understand. The 19F{1H} NMR spectrum obtained under cross-polarization and magic-angle spinning conditions shows a single isotropic resonance with a significant spinning sideband manifold. The static 19F{1H} CP NMR spectrum consists of a powder pattern dominated by the contributions of the anisotropic chemical shift and the homonuclear dipolar interactions. The 19F MREV-8 experiment, which minimizes the 19F–19F dipolar contribution, helps to identify the chemical shift contribution as an axial lineshape. The full static 19F{1H} CP NMR spectrum is analysed using subspectral analysis and subsequently simulated as a function of the 19F–19F internuclear distance (DFF = 2.25 ± 0.01 Å) of the rapidly rotating CF3 group without including contributions from additional libration motions and the anisotropy in the scalar tensor. The shielding span is found to be 56 ppm. The width of the centerband in the 19F{1H} sample-spinning CP NMR spectrum is very sensitive to the angle between the rotor and the magnetic field. Compound 1 is thus an attractive standard for setting the magic angle for NMR probes containing a fluorine channel with a proton-decoupling facility. 相似文献
884.
Carole T. Ferrand 《Journal of voice》2000,14(1):17-21
This study examined harmonics-to-noise ratios (HNR) in 4 groups of normally speaking children. HNRs were calculated for the vowels /ai/ and /inverted v/, selected from conversational speech samples of 80 children aged 4, 6, 8, and 10 years (10 boys and 10 girls at each age level). HNR values for /inverted v/ were significantly higher than those for /ai/. Significant age differences emerged for /ai/ between ages 4 and 8, and ages 8 and 10. Girls obtained significantly higher HNRs than boys for the /ai/ vowel. Overall, the HNR values for these normally speaking children were lower than those reported for normally speaking adults. These findings suggest that acoustic values for children cannot be validly compared to those for adults, and that the child's gender and age should be taken into account when applying spectral analyses to research and/or clinical situations. 相似文献
885.
886.
有机染料TMQ掺杂的SiO2薄膜的光谱特性 总被引:5,自引:1,他引:4
采用溶胶 -凝胶法合成了紫外波段有机染料 TMQ掺杂的 Si O2 薄膜和块体材料。薄膜中掺杂的染料浓度高达 1.2 4× 10 -2 mol/ L,块体材料中染料浓度可掺至 1.5× 10 -3 mol/ L。由于Si O2 “笼”的束缚作用 ,在吸收光谱中未观察到二聚体的特征谱带 ,在荧光光谱中未观察到荧光猝灭现象 ;同时由于 Si O2 “笼”的极化作用 ,其吸收峰和发射峰的位置相对于其在环己烷溶液中的吸收峰和发射峰位置发生了 10 nm左右的红移。 相似文献
887.
铅测定中的二条谱线对比 总被引:2,自引:0,他引:2
本文介绍了对铅的二条灵敏谱线进行的实验对比,结果证明217.0nm谱线的灵敏度增加一倍以上,其他指标也接近283.3nm谱线,可以用于卫生检测。 相似文献
888.
This paper reports a hidden chaotic system without equilibrium point. The proposed system is studied by the software of MATLAB R2018 through several numerical methods, including Largest Lyapunov exponent, bifurcation diagram, phase diagram, Poincaré map, time-domain waveform, attractive basin and Spectral Entropy. Seven types of attractors are found through altering the system parameters and some interesting characteristics such as coexistence attractors, controllability of chaotic attractor, hyperchaotic behavior and transition behavior are observed. Particularly, the Spectral Entropy algorithm is used to analyze the system and based on the normalized values of Spectral Entropy, the state of the studied system can be identified. Furthermore, the system has been implemented physically to verify the realizability. 相似文献
889.
Shuhei Fukawa Masahiko Hada Ryoichi Fukuda Shinji Tanaka Hiroshi Nakatsuji 《Journal of computational chemistry》2001,22(5):528-536
Linear and nonlinear halogen dependencies of the 13C magnetic shielding constants of CH4−nIn, CH4−nBrn, CCl4−nIn, and CBr4−nIn were fairly reproduced by the ab initio generalized unrestricted Hartree–Fock (GUHF)/finite perturbation (FP) method including spin‐orbit (SO) interaction and spin‐free relativistic (SFR) terms. As seen from the experimental trends, the calculated 13C chemical shifts in CCl4−nIn and CBr4−nIn depend linearly on n=0–4, while those in CH4−nIn and CH4−nBrn depend nonlinearly. We found that both the linear and nonlinear dependencies are due to the relativistic effects, and especially due to the Fermi–Contact (FC) term originating from the SO interaction. © 2001 John Wiley & Sons, Inc. J Comput Chem 22: 528–536, 2001 相似文献
890.