全文获取类型
收费全文 | 6277篇 |
免费 | 1123篇 |
国内免费 | 1119篇 |
专业分类
化学 | 5404篇 |
晶体学 | 61篇 |
力学 | 270篇 |
综合类 | 83篇 |
数学 | 774篇 |
物理学 | 1927篇 |
出版年
2024年 | 10篇 |
2023年 | 104篇 |
2022年 | 93篇 |
2021年 | 143篇 |
2020年 | 234篇 |
2019年 | 274篇 |
2018年 | 233篇 |
2017年 | 173篇 |
2016年 | 287篇 |
2015年 | 320篇 |
2014年 | 355篇 |
2013年 | 485篇 |
2012年 | 541篇 |
2011年 | 516篇 |
2010年 | 426篇 |
2009年 | 420篇 |
2008年 | 439篇 |
2007年 | 403篇 |
2006年 | 386篇 |
2005年 | 343篇 |
2004年 | 324篇 |
2003年 | 302篇 |
2002年 | 363篇 |
2001年 | 233篇 |
2000年 | 193篇 |
1999年 | 197篇 |
1998年 | 131篇 |
1997年 | 106篇 |
1996年 | 72篇 |
1995年 | 59篇 |
1994年 | 62篇 |
1993年 | 34篇 |
1992年 | 37篇 |
1991年 | 46篇 |
1990年 | 33篇 |
1989年 | 23篇 |
1988年 | 17篇 |
1987年 | 14篇 |
1986年 | 19篇 |
1985年 | 20篇 |
1984年 | 9篇 |
1983年 | 8篇 |
1982年 | 7篇 |
1981年 | 4篇 |
1979年 | 4篇 |
1977年 | 2篇 |
1975年 | 2篇 |
1974年 | 2篇 |
1968年 | 3篇 |
1936年 | 2篇 |
排序方式: 共有8519条查询结果,搜索用时 312 毫秒
991.
992.
The possible geometrical and the electronic structures of small MgnNi(n = 1-7) clusters are optimised by the density functional theory with a LANL2DZ basis set.The binding energy,the energy gap,the electron affinity,the dissociation energy and the second difference in energy are calculated and discussed.The properties of Mg n Ni clusters are also discussed when the number of Mg atom increases. 相似文献
993.
Baohuai Sheng 《分析论及其应用》2006,22(4):319-328
In the present paper,we provide a way of constructing translation network operators by Bernstein-Durrmeyer operators. 相似文献
994.
The effect of glottal angle on intraglottal pressure 总被引:1,自引:0,他引:1
Li S Scherer RC Wan M Wang S Wu H 《The Journal of the Acoustical Society of America》2006,119(1):539-548
Intraglottal pressure distributions depend upon glottal shape, size, and diameter. This study reports the effects of varying glottal angle on intraglottal and transglottal pressures using a three-dimensional Plexiglas model with a glottis having nine symmetric glottal angles and a constant minimal glottal diameter of 0.06 cm. The empirical data were supported by computational results using FLUENT. The results suggested that (1) the greater the convergent glottal angle, the greater outward driving forces (higher intraglottal pressures) on the vocal folds; (2) flow resistance was greatest for the uniform glottis, and least for the 10 degrees divergent glottis; (3) the greatest negative pressure in the glottis and therefore the greatest pressure recovery for diverging glottal shapes occurred for an angle of 10 degrees; (4) the smaller the convergent angle, the greater the flow resistance; (5) FLUENT was highly accurate in predicting the empirical pressures of this model; (6) flow separation locations (given by FLUENT) for the divergent glottis moved upstream for larger flows and larger glottal angles. The results suggest that phonatory efficiency related to aerodynamics may be enhanced with vocal fold oscillations that include large convergent angles during glottal opening and small (5 degrees - 10 degrees) divergent angles during glottal closing. 相似文献
995.
Three experiments were conducted to study relative contributions of speaking rate, temporal envelope, and temporal fine structure to clear speech perception. Experiment I used uniform time scaling to match the speaking rate between clear and conversational speech. Experiment II decreased the speaking rate in conversational speech without processing artifacts by increasing silent gaps between phonetic segments. Experiment III created "auditory chimeras" by mixing the temporal envelope of clear speech with the fine structure of conversational speech, and vice versa. Speech intelligibility in normal-hearing listeners was measured over a wide range of signal-to-noise ratios to derive speech reception thresholds (SRT). The results showed that processing artifacts in uniform time scaling, particularly time compression, reduced speech intelligibility. Inserting gaps in conversational speech improved the SRT by 1.3 dB, but this improvement might be a result of increased short-term signal-to-noise ratios during level normalization. Data from auditory chimeras indicated that the temporal envelope cue contributed more to the clear speech advantage at high signal-to-noise ratios, whereas the temporal fine structure cue contributed more at low signal-to-noise ratios. Taken together, these results suggest that acoustic cues for the clear speech advantage are multiple and distributed. 相似文献
996.
Structural model of eumelanin 总被引:1,自引:0,他引:1
Melanin is a ubiquitous pigment in living organisms with multiple important functions, yet its structure is not well understood. We propose a structural model for eumelanin protomolecules, consisting of 4 or 5 of the basic molecular units (hydroquinone, indolequinone, and its tautomers), in arrangements that contain an inner porphyrin ring. We use time-dependent density functional theory to calculate the optical absorption spectrum of the structural model, which reproduces convincingly the main features of the experimental spectrum of eumelanin. Our model also reproduces accurately other important properties of eumelanin, including x-ray scattering data, its ability to capture and release metal ions, and the characteristic size of the protomolecules. 相似文献
997.
A new guaiane-type diterpenoid, (1alpha,5beta,7alpha)-3,10(18),11-dictytriene-19-acid, was obtained from the roots of Euphorbia wallichii. This is the first isolation of guaiane diterpene from this genus of Euphorbia. The structure was elucidated by spectral methods. And the compound was tested for the cytotoxicities on the cancer cell line P-388 and A-549 in vitro. 相似文献
998.
alpha-Helix formation is known to be opposed by the entropy loss due to the folding and favored by the energy of molecular interactions. However, the underlying mechanism of these factors is still being discussed. Here we have used the experimental and calculation data for short alanine-based peptides embedded in water to model the mechanism of helix folding and unfolding and to calculate microscopically the free energy factors of alanine in the frame of helix coil conformational integrals. Classical helix-coil transition theories take into account the interactions in a peptide chain only if the i, i + 3 peptide bond participates in hydrogen bonding. But quantum mechanical calculations showed that interactions of the i, i + 2 peptide bond play an important role in helix folding too. We also included the short-range repulsive interactions due to molecular steric clashes and the end effects due to polar/hydrogen-bonding interactions at the N and C termini. The helix and coil regions of peptide conformational space were defined using an experimental steric criterion for hydrogen bonding. Arginine helix propensity was discussed and estimated. Monte Carlo numerical simulations of thermodynamics and kinetics for the 21 amino acid alpha-helical polypeptide Ac-A5(AAARA)3A-NMe were carried out and found to be in an agreement with the experimental results. 相似文献
999.
1000.