全文获取类型
收费全文 | 1225篇 |
免费 | 142篇 |
国内免费 | 121篇 |
专业分类
化学 | 958篇 |
晶体学 | 41篇 |
力学 | 51篇 |
综合类 | 4篇 |
数学 | 161篇 |
物理学 | 273篇 |
出版年
2023年 | 12篇 |
2022年 | 14篇 |
2021年 | 28篇 |
2020年 | 18篇 |
2019年 | 34篇 |
2018年 | 23篇 |
2017年 | 26篇 |
2016年 | 50篇 |
2015年 | 57篇 |
2014年 | 63篇 |
2013年 | 89篇 |
2012年 | 86篇 |
2011年 | 96篇 |
2010年 | 53篇 |
2009年 | 41篇 |
2008年 | 78篇 |
2007年 | 63篇 |
2006年 | 61篇 |
2005年 | 50篇 |
2004年 | 58篇 |
2003年 | 45篇 |
2002年 | 61篇 |
2001年 | 48篇 |
2000年 | 32篇 |
1999年 | 28篇 |
1998年 | 22篇 |
1997年 | 13篇 |
1996年 | 15篇 |
1995年 | 11篇 |
1994年 | 17篇 |
1993年 | 17篇 |
1992年 | 12篇 |
1991年 | 8篇 |
1990年 | 14篇 |
1989年 | 7篇 |
1988年 | 5篇 |
1987年 | 8篇 |
1986年 | 5篇 |
1985年 | 8篇 |
1984年 | 12篇 |
1982年 | 15篇 |
1981年 | 11篇 |
1979年 | 8篇 |
1978年 | 9篇 |
1976年 | 6篇 |
1974年 | 8篇 |
1971年 | 6篇 |
1970年 | 4篇 |
1968年 | 4篇 |
1967年 | 4篇 |
排序方式: 共有1488条查询结果,搜索用时 15 毫秒
991.
We synthesized the Mn-doped Mg(In2−xMnx)O4 oxides with 0.03?x?0.55 using a solid-state reaction method. The X-ray diffraction patterns of the samples were in a good agreement with that of a distorted orthorhombic spinel phase. Their lattice parameters and unit-cell volumes decrease with x due to the substitution of the smaller Mn3+ ions to the larger In3+ ions. The undoped MgIn2O4 oxide presents diamagnetic signals for 5 K?T?300 K. The M(H) at T=300 K reveals a fairly negative-sloped linear relationship. Neither magnetic hysteresis nor saturation behavior was observed in this parent sample. For the Mn-doped samples, however, positive magnetization were observed between 5 and 300 K even if the x value is as low as 0.03. The mass susceptibility enhances with Mn content and it reaches the highest value of 1.4×10−3 emu/g Oe (at T=300 K) at x=0.45. Furthermore, the Mn-doped oxides with x=0.06 and 0.2, respectively, exhibit nonlinear magnetization curves and small hysteretic loops in low magnetic fields. Susceptibilities of the Mn-doped samples are much higher than those of MnO2, Mn2O3 oxides, and Mn metals. These results show that the oxides have potential to be magnetic semiconductors. 相似文献
992.
993.
994.
995.
996.
Lutetium oxyorthosilicate (LSO) thin films with a cerium thickness gradient were sputter deposited to investigate the optimum cerium concentration for emission intensity. Thin film cerium concentration ranged from 0.06 to 0.88 at%. To compare the thin film samples to single crystal LSO, a set of single crystal LSO samples were investigated with cerium concentrations of 0.0015, 0.0095 and 0.078 at%. The thin film samples showed peak photoluminescence emission intensity at a cerium concentration of 0.35 at%; however, the single crystal samples exhibited peak photoluminescence emission intensity at a lower cerium concentration of 0.0095 at%. The photoluminescence excitation and emission spectra as a function of concentration demonstrate the concentration quenching behavior and the mechanisms are speculated to be due to radiative (self-absorption) and non-radiative energy transfer, which may be phonon assisted. 相似文献
997.
Evaluation of liquid chromatography–ion trap–time of flight hybrid mass spectrometry on the quantitative analysis for ginsenosides
下载免费PDF全文
![点击此处可从《Biomedical chromatography : BMC》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Yan Liang Rong Xing Tai Rao Lijun Zhou Qian Wang Hanxu Fu Wei Ye Guangji Wang 《Biomedical chromatography : BMC》2014,28(7):1003-1010
It is ideal and desirable for a single instrument to meet the requirement of both qualitative and quantitative analysis of complicated components in pharmacokinetic research for herbal medicine. Liquid chromatography combined with hybrid ion trap and time‐of‐flight mass spectrometry (LCMS‐IT‐TOF) was recently confirmed as a very powerful tool for the qualitative analysis of both target and nontarget components in herbal medicines. The present study was designed to investigate the feasibility of LCMS‐IT‐TOF on quantitative analysis of ginsenosides in biological matrices. A simple liquid–liquid extraction procedure was followed by injection of the extracts onto a C18 column with gradient elution and detection based on LCMS‐IT‐TOF system in negative scan mode. The developed method was validated with respect to the limit of quantification, linear dynamic range, precision, accuracy, matrix effects and stabilities. All the results suggested that the presently developed method was sufficiently sensitive and robust enough to simultaneously monitor 15 ginsenosides with diverse properties and a large range of concentration differences. Therefore, this method would be expected to be highly useful for comprehensive studies of ginsenosides in complicated matrix. Copyright © 2014 John Wiley & Sons, Ltd. 相似文献
998.
Chun‐Wei Lin Pi‐Tai Chou Prof. Yong‐Hong Liao Ying‐Chih Lin Prof. Ching‐Ting Chen Yu‐Chun Chen Chin‐Hung Lai Dr. Bo‐So Chen Yi‐Hung Liu Dr. Chih‐Chieh Wang Prof. Mei‐Lin Ho Prof. 《Chemistry (Weinheim an der Bergstrasse, Germany)》2010,16(12):3770-3782
Strategically designed salen ligand 2,3‐bis[4‐(di‐p‐tolylamino)‐2‐hydroxybenzylideneamino]maleonitrile ( 1 ), which has pronounced excited‐state charge‐transfer properties, shows a previously unrecognized form of photoisomerization. On electronic excitation (denoted by an asterisk), 1Z *→ 1E isomerization takes place by rotation about the C2? C3 bond, which takes on single‐bond character due to the charge‐transfer reaction. The isomerization takes place nonadiabatically from the excited‐state ( 1Z ) to the ground‐state ( 1E ) potential‐energy surface in the singlet manifold; 1Z and 1E are neither thermally inconvertible at ambient temperature (25–30 °C), nor does photoinduced reverse 1E *→ 1Z (or 1Z *) isomerization occur. Isomers 1Z and 1E show very different coordination chemistry towards a ZnII precursor. More prominent coordination chemistry is evidenced by a derivative of 1 bearing a carboxyl group, namely, N,N′‐dicyanoethenebis(salicylideneimine)dicarboxylic acid ( 2 ). Applying 2Z and its photoinduced isomer 2E as building blocks, we then demonstrate remarkable differences in morphology (sphere‐ and needlelike nanostructure, respectively) of their infinite coordination polymers with ZnII. 相似文献
999.
1000.
Three new chiral heterocalix [4] crowns containing aza thio atoms bearing two chiral sites provided by (R)‐cysteine ester were synthesized. All new compounds were characterized by 1H NMR, MS and elemental analysis. 相似文献