全文获取类型
收费全文 | 1105篇 |
免费 | 39篇 |
国内免费 | 71篇 |
专业分类
化学 | 240篇 |
力学 | 74篇 |
数学 | 714篇 |
物理学 | 187篇 |
出版年
2023年 | 13篇 |
2022年 | 12篇 |
2021年 | 18篇 |
2020年 | 22篇 |
2019年 | 18篇 |
2018年 | 21篇 |
2017年 | 17篇 |
2016年 | 18篇 |
2015年 | 22篇 |
2014年 | 54篇 |
2013年 | 124篇 |
2012年 | 47篇 |
2011年 | 59篇 |
2010年 | 48篇 |
2009年 | 74篇 |
2008年 | 88篇 |
2007年 | 80篇 |
2006年 | 45篇 |
2005年 | 50篇 |
2004年 | 33篇 |
2003年 | 33篇 |
2002年 | 28篇 |
2001年 | 32篇 |
2000年 | 20篇 |
1999年 | 23篇 |
1998年 | 46篇 |
1997年 | 27篇 |
1996年 | 26篇 |
1995年 | 14篇 |
1994年 | 18篇 |
1993年 | 16篇 |
1992年 | 9篇 |
1991年 | 6篇 |
1990年 | 6篇 |
1989年 | 7篇 |
1988年 | 5篇 |
1987年 | 2篇 |
1986年 | 3篇 |
1985年 | 3篇 |
1984年 | 6篇 |
1983年 | 2篇 |
1982年 | 4篇 |
1981年 | 4篇 |
1980年 | 4篇 |
1978年 | 2篇 |
1977年 | 2篇 |
1976年 | 1篇 |
1974年 | 1篇 |
1973年 | 1篇 |
1970年 | 1篇 |
排序方式: 共有1215条查询结果,搜索用时 15 毫秒
21.
22.
Su-Hong Ge Xin-Lu Cheng Xin-Xing Wang Guang-xing Dong Gui-hua Sun 《Structural chemistry》2007,18(6):985-991
Some explosives are stable molecules with large energy barriers to chemical reaction, and in shock or impact initiation, a
sizable amount of phonon energy must be converted to the molecular internal higher vibrations by multiphonon up pumping. To
investigate the relationship between impact sensitivities and energy transfer rates, the number of doorway modes of explosive
molecules is estimated by a simple theory in which the rate is proportional to the number of normal mode vibrations. We evaluated
frequencies of normal mode vibrations of 13 explosive molecules which are CHNO nitramine-contained and have not been analyzed
previously. The number of doorway modes in the regions of 200–700 cm−1 was evaluated by the direct counting method. For more clear investigation of the relationship we have classified these 13
nitramine explosive molecules, by the number of nitramine group they contained, into two groups. There are eight molecules
that contained one nitramine group and five molecules that contained poly-nitramine groups. It is found that the number of
doorway modes shows a linearly correlation to the impact sensitivities derived from drop hammer tests. This result is in agreement
with that of several previous works. Besides, it is also noted in our study that in those nitramine explosives molecules with
similar molecular structure (similar number nitramine group they contained) and similar molecular weight, the correlation
between the sensitivity and the number of doorway modes is higher. We found that the vibrational frequency of ω corresponds
to nitro group motions of every molecule is contributed to the number of doorway modes in the regions of 200–700 cm−1. 相似文献
23.
Synthesis, Crystal Structure and Spectroscopic Properties of the Cluster Anions [(Mo6Br )X ]2? with Xa = F, Cl, Br, I The tetrabutylammonium (TBA), tetraphenylphosphonium (TPP) and tetraphenylarsonium (TPAs) salts of the octa-μ3-bromo-hexahalogeno-octahedro-hexamolybdate(2?) anions [(Mo6Br)X]2? (Xa = F, Cl, Br, I) are synthesized from solutions of the free acids H2[(Mo6Br)X] · 8 H2O with Xa = Cl, Br, I. The crystal structures show systematic stretchings in the Mo? Mo bond length and a slight compression of the Bri8 cube in the Fa to Ia series. The cations do not change much. The i.r. and Raman spectra show at 10 K almost constant frequencies of the (Mo6Bri8) cluster vibrations, whereas all modes with Xa ligand contribution are characteristically shifted. The most important bands are assigned by polarization measurements and the force constants are derived from normal coordinate analysis. The 95Mo nmr signals are shifted to lower field with increasing electronegativity of the Xa ligands. The fluorine compound shows a sharp 19F nmr singlet at ?184.5 ppm. 相似文献
24.
A stationary phase based on poly(1-vinylimidazole)-grafted silica has been prepared by the surface radical chain-transfer reaction. The stationary phase was characterized by infrared spectra, X-ray photoelectron spectroscopy and elemental analysis. Chromatographic characteristics of the stationary phase were investigated in normal-phase HPLC. The results showed that both weak polar compounds (polycyclic aromatic hydrocarbons, dialkyl phthalates) and polar compounds (anilines, phenols) could be successfully separated on this stationary phase, implying better separation performance than blank silica and conventional aminopropyl-bonded silica under the same conditions. The excellent performance can be attributed to multiple interactions between surface modifier and the analytes that might include dipole, hydrogen bonding, H-π, electrostatic and inductive interactions. 相似文献
25.
Summary A set of hydrophilicity parameters in a normal-phase liquid chromatography of peptides is presented in order to clarify the
contribution of individual amino acid residues to peptide retention and to predict retention times. The retention of 100 peptides
was studied using normal-phase liquid chromatography on amide, diol and silica columns. An acetonitrile-water mixed solution
containing 0.2% trifluoroacetic acid +0.2% triethylamine was used as the mobile phase in a linear gradient elution system.
The contribution of each residue upon retention was calculated by linear multiple regression analysis. This paper described
the contribution values as “hydrophilicity retention coefficients”. Using these hydrophilicity retention coefficients, retention
times could be predicted for peptides of known amino acid content and sequence. A set of hydrophilicity retention coefficients
on each column was successfully explained by contributions to the degree of retention. 相似文献
26.
27.
文章简介1939年,马大猷[1]在美国声学学会会刊(JASA)上发表了一篇在室内声学中具有里程碑性的文章.文章论述了房间尺寸在声波长范围内,准确计算矩形房间内简正模态数量(简正模数)的公式:N=4πVf3u/3c3(1+3Sc/16V 1/fu+3Lc2/8πV 1/f2u式中,V为房间的体积;S为房间内表面的总面积... 相似文献
28.
以培养的原发性肝癌细胞HepG2和正常肝细胞L02为研究对象,采用细胞裂解液提取总蛋白,用PNGase F酶解释放N-糖链,以微晶纤维素柱结合石墨碳柱纯化分离N-糖链,通过电喷雾电离质谱(ESI-MS)和串联质谱(MS/MS)对N-糖链进行序列鉴定,以β-环糊精为内标对2种细胞系的N-糖链进行了定量比较分析.结果表明,在肝癌细胞系HepG2和正常细胞系L02中共检测到26种N-糖链,与L02相比,HepG2的大多数高甘露糖型糖链、唾液酸化糖链和岩藻糖基化糖链的数量都明显升高,其中有15种糖链在数量上具有极显著性差异(p0.01),1种糖链具有显著性差异(p0.05).本研究为进一步探索肝癌中各类N-糖链的表达特点及发现早期肝癌糖链标志物提供了参考. 相似文献
29.
Emmanuel Urandu Mapesa Ludwig Popp Wycliffe Kiprop Kipnusu Martin Tress Friedrich Kremer 《Soft Materials》2014,12(4):S22-S30
Broadband Dielectric Spectroscopy (BDS) is used to probe the molecular dynamics of Type A polymer, poly(cis-1,4-isoprene), when confined in the 1-dimensional (1D) exploring space of thin layers and the 2-dimensional (2D) constraining geometry of unidirectional anodic aluminum oxide (AAO) nanopores. For both cases, it was observed that the structural relaxation remains bulk-like in its mean relaxation rate, although the distribution of its relaxation times is broadened in 2D confinement. Furthermore, the fluctuation of the end-to-end vector is interrupted, with the 1D case being relatively less pronounced. By this clear-cut comparison, it is demonstrated that the effects of confinement on molecular dynamics depend, inter alia, on the dimensionality of the restricting space. 相似文献
30.
We consider a normal–superconducting junction in order to investigate the effect of new physical ingredients on waiting times. First, we study the interplay between Andreev and specular scattering at the interface on the distribution of waiting times of electrons or holes separately. In that case the distribution is not altered dramatically compared to the case of a single quantum channel with a quantum point contact since the interface acts as an Andreev mirror for holes. We then consider a fully entangled state originating from splitting of Cooper pairs at the interface and demonstrate a significant enhancement of the probability to detect two consecutive electrons in a short time interval. Finally, we discuss the electronic waiting time distribution in the more realistic situation of partial entanglement. 相似文献