全文获取类型
收费全文 | 295篇 |
免费 | 136篇 |
国内免费 | 181篇 |
专业分类
化学 | 331篇 |
晶体学 | 8篇 |
力学 | 13篇 |
综合类 | 3篇 |
数学 | 30篇 |
物理学 | 227篇 |
出版年
2023年 | 2篇 |
2022年 | 9篇 |
2021年 | 12篇 |
2020年 | 5篇 |
2019年 | 13篇 |
2018年 | 11篇 |
2017年 | 20篇 |
2016年 | 20篇 |
2015年 | 12篇 |
2014年 | 26篇 |
2013年 | 41篇 |
2012年 | 37篇 |
2011年 | 62篇 |
2010年 | 51篇 |
2009年 | 43篇 |
2008年 | 45篇 |
2007年 | 35篇 |
2006年 | 27篇 |
2005年 | 18篇 |
2004年 | 21篇 |
2003年 | 10篇 |
2002年 | 14篇 |
2001年 | 9篇 |
2000年 | 9篇 |
1999年 | 11篇 |
1998年 | 6篇 |
1997年 | 6篇 |
1996年 | 4篇 |
1995年 | 3篇 |
1994年 | 1篇 |
1993年 | 4篇 |
1992年 | 5篇 |
1991年 | 2篇 |
1990年 | 3篇 |
1989年 | 1篇 |
1988年 | 4篇 |
1987年 | 5篇 |
1986年 | 1篇 |
1985年 | 1篇 |
1984年 | 1篇 |
1983年 | 1篇 |
1964年 | 1篇 |
排序方式: 共有612条查询结果,搜索用时 375 毫秒
51.
Lu YH Lu YW Wu CL Shao Q Chen XL Bimbong RN 《Spectrochimica acta. Part A, Molecular and biomolecular spectroscopy》2006,65(3-4):695-701
Salicyladehyde benzoylhydrazone (SBH) has three groups suitable for forming coordination bond with transition metal. The UV-vis absorption spectra of SBH and its Co(II) complexes in various media were studied by using the deconvolution method. It is found that the structure of complex in solution is different from those in solid crystals. The nature of complexes in solution depends on acidity of the phenolic proton of SBH and on the medium. In neutral or slightly acid medium, the SBH is a non-charged bidentate ligand. And the "free" hydroxyl group on the SBH molecule makes it possible to form hydrogen bonds in solution. In basic medium, the SBH is a mono, negatively charged tridentates ligand. 相似文献
52.
联苯二胺分子器件电输运特性的取代效应 总被引:1,自引:1,他引:0
在第一性原理基础上,理论研究了苯环上含有不同取代物的四个联苯分子的电输运特性.计算结果表明,当苯环上的氢被取代时,联苯分子的两个苯环之间的扭转角增大.分子结的电导随苯环间扭转角的增加而减少,且电导值与扭转角余弦平方成线性关系.在低偏压下,分子结的电输运机制是电子通过π轨道的隧穿过程.计算结果与实验结果符合的较好. 相似文献
53.
Bang-Bang Jiang Ming Pan Chen Wang Hua-Fei Li Ning Xie Hai-Yan Hu Fan Wu Xiao-Ling Yan Marvin H. Wu K. Vinodgopal Gui-Ping Dai 《Journal of Saudi Chemical Society》2019,23(2):162-170
Controlled growth of single-crystal high-quality ‘track-and-field ground’ shaped graphene domains and the morphological evolution from hexagonal to hexagram graphene domain even square and circular graphene domain has been achieved by low-pressure CVD on solid copper substrate, thereby demonstrating that the shape of the graphene grains can potentially be precisely tuned by optimizing growth parameters. The etching reaction of graphene has also been studied, and results show that a low flow rate of hydrogen (99.999%) is favorable to form hexagonal structure for the etching reaction of graphene due to the exist of oxygen or oxidizing impurities in hydrogen gas commonly used. Controlled growth and etching reaction of graphene determine the final shape of graphene domains and all these efforts contribute to the study of size and morphology and the growth mechanism of graphene domains. 相似文献
54.
Jian Xiao Li-Bin Lin Jia-Yao Hu Dong-Zhu Duan Wei Shi Qiang Zhang Wen-Bo Han Le Wang Xiao-Ling Wang 《Tetrahedron letters》2018,59(18):1772-1775
Pestalustaines A (1) and B (2), one unique sesquiterpene possessing an unusual 5/6/7-fused tricyclic ring system and one unprecedented coumarin derivative bearing 6/6/5/5-fused tetracyclic ring system, were isolated from the plant-derived Pestalotiopsis adusta. Their structures with absolute configurations were established by extensive NMR analysis, X-ray crystallography, and CD spectra associated with TD-DFT calculation. Hypothetical biosynthetic pathways for compounds 1 and 2 are proposed. Compounds 1 and 2 showed weak to moderate cytotoxic activities against three human tumor cell lines HeLa, HCT116, and A549, whose IC50 values were ranged from 21.01 to 55.43?μM. 相似文献
55.
Xiao-Ling Zhao Hong-Qi Zhang Bin Zhao Hong-Fei Guo Ji-Lin Cao 《Journal of solution chemistry》2018,47(9):1419-1437
The solubilities in the quaternary system K+, \( {\text{NH}}{_4^{+}} \)//Cl?, \( {\text{SO}}{_4^{2-}} \)–H2O and its two ternary subsystems NH4Cl–KCl–H2O, (NH4)2SO4–K2SO4–H2O at 80.0 °C were measured using the isothermal dissolution equilibrium method under atmospheric pressure, and the corresponding phase diagrams were plotted. In the phase diagram of the NH4Cl–KCl–H2O system, there are three crystalline zones, which correspond to (K1?m,(NH4)m)Cl, ((NH4)n,K1?n)Cl and the co-existence zone of (K1?m,(NH4)m)Cl and ((NH4)n,K1?n)Cl, respectively. In the phase diagram of the (NH4)2SO4–K2SO4–H2O system, there is only one crystalline zone for (K1?t,(NH4)t)2SO4. In the phase diagram of the K+, \( {\text{NH}}{_4^{+}} \)//Cl?, \( {\text{SO}}{_4^{2-}} \)–H2O system, there are three crystal zones, which correspond to (K1?t,(NH4)t)2SO4, (K1?m,(NH4)m)Cl and ((NH4)n,K1?n)Cl, respectively. According to the analysis and the calculations for the phase diagrams of the K+, \( {\text{NH}}{_4^{+}} \)//Cl?, \( {\text{SO}}{_4^{2 -}} \)–H2O system at 80.0 °C and 50.0 °C, this paper proposes a technological process. In the process, the (K1?t,(NH4)t)2SO4 can be prepared at 80.0 °C and the ((NH4)n,K1?n)Cl can crystallize out at 50.0 °C. The mass fraction of K2SO4 in product L1 (K1?t,(NH4)t)2SO4 (t?=?0.1465) is 88.48%. The composition of solid solutions in the K+, \( {\text{NH}}{_4^{+}} \)//Cl?, \( {\text{SO}}{_4^{2 -}} \)–H2O system was experimentally determined and then theoretical calculations about the process can be carried out. 相似文献
56.
Xiu-Qi Huang Feng-Qi Liu Xiao-Ling Che Jun-Qi Liu Wen Lei Zhan-Guo Wang 《Journal of Crystal Growth》2004,270(3-4):364-369
Self-assembled InAs quantum dots (QDs) in an InAlGaAs matrix, lattice-matched to InP substrate, have been grown by molecular beam epitaxy (MBE). Transmission electron microscopy (TEM), double-crystal X-ray diffraction (DCXRD) and photoluminescence (PL) are used to study their structural and optical properties. In InAs/InAlGaAs/InP system, we propose that when the thickness of InAs layer deposited is small, the random strain distribution of the matrix layer results in the formation of tadpole-shaped QDs with tails towards random directions, while the QDs begin to turn into dome-shaped and then coalesce to form islands with larger size and lower density to release the increasing misfit strain with the continuous deposition of InAs. XRD rocking curves showing the reduced strain with increasing thickness of InAs layer may also support our notion. The results of PL measurements are in well agreement with that of TEM images. 相似文献
57.
WANG Xiao-Ling② 《结构化学》2008,27(10):1250-1254
A new hasubanane-type alkaloid, 8-demethoxyrunanine, was isolated from Sino- menium acutum and characterized by melting point, HREIMS, 1H NMR, and X-ray diffraction analysis. X-ray diffraction reveals that the title compound crystallizes in the orthorhombic system, space group P212121 with a = 7.308(1), b = 21.742(5), c = 22.893(4)A, V = 3637.5(11) A^3, Z = 8, Dx = 1.254 g/cm^3, F(000) = 1472, μ(MoKα) = 0.087 mm^-1, the final R = 0.0438 and wR = 0.0575 for 4497 independent reflections with Rint = 0.0192 and 2091 observed reflections with I 〉 2σ(I). Four rings (ring A: one benzene ring, ring B: one hexagon carbon ring in a half-chair conformation, ring C: one hexagon carbon ring with α,β-unsaturated ketone segment (-CR2=CR1-C=O) in a screw-boat conformation, and ring D: one nonplanar tetrahydropyrrole) form a hasubanane-type alkaloid. 相似文献
58.
Liao GH Luo L Xu HX Wu XL Lei L Tung CH Wu LZ 《The Journal of organic chemistry》2008,73(18):7345-7348
Irradiation of 2-naphthalenecarbonitrile (2-NpCN) in solution with a light lambda > 280 nm results in the formation of three rigid cubane-like photodimers, anti-head-to-head 1, anti-head-to-tail 2, and syn-head-to-tail 3, which are not in line with the previously recognized regioselectivity. These cubane-like photodimers have been well characterized by spectroscopic investigation and/or X-ray crystal structural analysis in this work. Moreover, the separation of the optically pure enantiomers of 1, 2, and 3 has been achieved by HPLC resolution. 相似文献
59.
60.
By optimizing the extraction, separation, and analytical conditions, a reliable and accurate high-performance liquid chromatography method coupled with evaporative light scattering detection (ELSD) was developed for simultaneous determination of five terpenoids, i.e., triptolide, tripchlorolide, demethylzelastral, wilforlide B, and wilforlide A, in root, stem, leaves, root bark, twig, and root without bark of Tripterygium wilfordii Hook. f and six of its herbal preparations. This approach would thus provide a more accurate and general method for evaluating the quality of the herb and its preparations. Separation of these five terpenoids was achieved on a ZORBAX Eclipse XDB-C8 column with gradient elution using water and acetonitrile as solvents, both containing 0.05% formic acid, at a temperature of 30 degrees C and a flow rate of 0.8 mL/min. The drift tube temperature of ELSD was set at 100 degrees C, and the nitrogen flow rate at 1.5 L/min. Good linear relationships were obtained with correlation coefficients for the analytes exceeding 0.992, and the LOD and LOQ were less than 0.149 microg and 0.297 microg on column, respectively. Intra-day and inter-day precision of the analytes were less than 1.25% and 5.97%, respectively, and the average recovery rates obtained were in the range of 95.9 +/- 3.7% to 100.4 +/- 5.0% for all terpenoids with RSDs below 4.99%. Quantitative analysis of the five terpenoids in different parts of Tripterygium wilfordii and its six preparations showed that the contents of the terpenoids varied significantly. The tender root contained higher concentrations of triptolide, tripchlorolide, demethylzelastral, and wilforlide B than any other part of the herb. Correspondingly, the root bark contained the greatest concentration of wilforlide A, and the stem and twig came in second and third. This suggested that we could infer whether the medicinal materials were absolute roots without bark or not from the comparative contents of these terpenoids in the tablets in view of the fact that only the roots without bark are the valid officinal part of the plant. This method and the quantitation results obtained can provide a scientific and general as well as simple and convenient approach for the product manufacturers to set up quality control standards and for informing the public about the quality and safety of the preparations. 相似文献