全文获取类型
收费全文 | 689篇 |
免费 | 205篇 |
国内免费 | 232篇 |
专业分类
化学 | 591篇 |
晶体学 | 59篇 |
力学 | 5篇 |
综合类 | 7篇 |
数学 | 28篇 |
物理学 | 436篇 |
出版年
2023年 | 7篇 |
2022年 | 22篇 |
2021年 | 33篇 |
2020年 | 23篇 |
2019年 | 16篇 |
2018年 | 14篇 |
2017年 | 35篇 |
2016年 | 34篇 |
2015年 | 24篇 |
2014年 | 67篇 |
2013年 | 72篇 |
2012年 | 58篇 |
2011年 | 74篇 |
2010年 | 61篇 |
2009年 | 78篇 |
2008年 | 73篇 |
2007年 | 68篇 |
2006年 | 52篇 |
2005年 | 36篇 |
2004年 | 41篇 |
2003年 | 23篇 |
2002年 | 35篇 |
2001年 | 26篇 |
2000年 | 20篇 |
1999年 | 14篇 |
1998年 | 22篇 |
1997年 | 10篇 |
1996年 | 6篇 |
1995年 | 10篇 |
1994年 | 22篇 |
1993年 | 13篇 |
1992年 | 10篇 |
1991年 | 6篇 |
1990年 | 1篇 |
1989年 | 3篇 |
1988年 | 4篇 |
1987年 | 2篇 |
1986年 | 2篇 |
1985年 | 1篇 |
1984年 | 1篇 |
1983年 | 2篇 |
1982年 | 1篇 |
1980年 | 2篇 |
1977年 | 1篇 |
1973年 | 1篇 |
排序方式: 共有1126条查询结果,搜索用时 203 毫秒
101.
使用草酸盐共沉淀法合成了LiNi0.5Mn0.5O2, 并研究了共沉淀时的pH条件对终产物的结构、形貌及电化学性能的影响. 采用X射线衍射(XRD)和扫描电镜(SEM)表征了在pH值为4.0、5.5、7.0和8.5时得到的共沉淀和终产物LiNi0.5Mn0.5O2的结构和形貌. 使用充放电实验研究了不同pH条件下得到的LiNi0.5Mn0.5O2的电化学性能. 结果表明, pH为7.0时, 合成的材料颗粒更小、分布最均匀, 材料具有良好的层状特征, 且材料中锂镍的混排程度最小. 电化学测试结果印证了pH为7.0时合成的材料具有更好的电化学性能, 在0.1C的倍率下, 材料的首次放电比容量达到了185 mAh·g-1, 在循环20周后, 放电比容量仍然保持在160 mAh·g-1. X射线光电子能谱(XPS)测试结果表明, pH为7.0时合成的LiNi0.5Mn0.5O2中Ni为+2价, Mn为+4价. 相似文献
102.
Jian Wang Guang-bo Che Qing-wei Wang Wan-xi Zhang Li-min Wang 《Frontiers of Physics》2011,6(2):209-213
The Ti0.9Zr0.1V0.2Ni1.5La0.5 alloy samples were synthesized by melt-spinning technique at the different wheel velocity (cooling rate), and the structure and electrochemical hydrogen storage properties were investigated. The result indicated that the structure of the melt-spun ribbons mainly contains C14 Laves phase and V-based solid solution phase. The discharge capacity, cyclic stability, high-rate discharge ability and electrochemical kinetic of the alloy electrodes are correlated with the cooling rate (wheel velocity), and the maximum discharge capacity is over 200 mA·h/g at the wheel velocity of 20 m/s. 相似文献
103.
Sm3+ doped CaO-MgO-Al2O3-SiO2 glass and glass ceramics have been prepared. The diopside crystal (CaMgSi2O6) was identified in the glass ceramics by X-ray diffraction analysis. X-ray photoelectron spectra of the glass and glass ceramics were measured by a monochromatised Al-Kα XPS instrument. Sm 3d core level spectra for the Sm doped samples showed that Sm ions are predominantly in the Sm (III) state in glass and glass ceramics. The O 1s core spectra could be fitted by summing the contributions from bridging oxygen (BO) and non bridging oxygen (NBO) for samarium undoped glass, BO, NBO and Si-O-Sm for the doped glass. The O 1s XPS spectrum of undoped glass ceramics was curve fitted with BO and NBO in glass phase, as well as SiOSi, SiOMg and SiOCa in diopside. In addition to the five components above mentioned, SiOSm in diopside also appeared in O 1s XPS spectra of samarium doped glass ceramics. According to the fitting results, we demonstrate that the Sm2O3 exist in glass network as a glass modifier. After heat treatment, nearly all the Sm3+ existed in diopside phase as the substitution for Ca2+. 相似文献
104.
The design and development status of Sm2Co17 magnet blocks for two in-vacuum undulators (IVU20) at the SSRF with the same hybrid design has been described. By the technological improvement of some processes and comparison with the experimental Sm2Co17 magnet blocks for the IVU25A, magnetic properties such as the intrinsic coercive force Hci and the average magnetic moment M are increased, the bend point magnetic field Hk value and pass rate are significantly increased, and the magnetic field uniformity of the magnet blocks are significantly improved. The basic developmental method of high uniformity Sm2Co17 magnet blocks for IVU20 is presented. The magnetic field qualities of the magnet blocks, including the magnetic property, the magnetic moment distribution, the magnetization deviation angle and the magnetic field uniformity, basically satisfy the specifications of the two IVU20 in-vacuum undulators. 相似文献
105.
Jijing XuGuijuan Ji Haifeng ZouYanhua Song Shucai Gan 《Journal of magnetism and magnetic materials》2011,323(1):156-161
Sm-substituted barium hexaferrites, Ba3−xSmxCo2Fe24O41 (x=0-0.25), were prepared by a conventional ceramic sintering method. The microstructure, complex permittivity, complex permeability and static magnetic properties of the samples were studied using powder X-ray diffraction, field emission scanning electron microscopy, vector network analyzer and vibrating sample magnetometry. The results reveal that by introducing a relatively small amount of Sm3+ instead of Ba2+ an important modification of both structure and high-frequency electromagnetic properties can be obtained. Doping of Sm3+ suppressed the grain growth and gave rise to a decrease of the grain size. As the Sm content increases, the static magnetic properties continuously increase. The real part and imaginary part of complex permittivity initially increase with Sm content, and then decreases when x>0.10. The imaginary part of complex permeability decreases after Sm3+ is doped. There is no obvious change in the real part of the complex permeability for different Sm contents. The reasons are discussed using electromagnetic theory. 相似文献
106.
Sagar E. Shirsath R.H. KadamAnil S. Gaikwad Ali GhasemiAkimitsu Morisako 《Journal of magnetism and magnetic materials》2011,323(23):3104-3108
Sintering temperature and particle size dependent structural and magnetic properties of lithium ferrite (Li0.5Fe2.5O4) were synthesized and sintered at four different temperatures ranging from 875 to 1475 K in the step of 200 K. The sample sintered at 875 K was also treated for four different sintering times ranging from 4 to 16 h. Samples sintered at 1475 K have the cubic spinel structure with a small amount of α-Fe2O3 (hematite) and γ-Fe2O3 (maghemite). The samples sintered at≤1275 K do not show hematite and maghemite phases and the crystals form the single phase spinel structure with the cation ordering on octahedral sites. Particle size of lithium ferrite is in the range of 13-45 nm, and is depend on the sintering temperature and sintering time. The saturation magnetization increased from 45 to 76 emu/g and coercivity decreases from 151 to 139 Oe with an increase in particle size. Magnetization temperature curve recorded in ZFC and FC modes in an external magnetic field of 100 Oe. Typical blocking effects are observed below about 244 K. The dielectric constant increases with an increase in sintering temperature and particle size. 相似文献
107.
LI Ming DAI ChangJian & XIE Jun Key Laboratory of Display Materials Photoelectric Devices Ministry of Education Tianjin China School of Science Tianjin University of Technology Tianjin 《中国科学:物理学 力学 天文学(英文版)》2011,(6)
The odd-parity high-lying states of the Sm atom are investigated systematically by a three-color resonant excitation scheme with two different excitation paths.The two intermediate states,4f 6 6s7s 7 F 0 and 4f 6 6s7s 7 F 1 are employed for paths I and II,respectively.Fifty-seven bound states are detected with paths I and II in the energy range between 44188 and 45515 cm 1,while 64 autoionizing states are found in the energy range from 45528 to 45761 cm 1.Not only the level energies of all observed states,b... 相似文献
108.
109.
Two novel chiral Br?nsted acids 3b and 3c were prepared without involving the complexity of Suzuki coupling. Catalyst 3c bearing two additional hydroxyl groups at 3 and 3′ positions of axially chiral 1,1‐binaphthalene‐2,2′‐diol phosphoric acid was applied in a model Mannich reaction to afford β‐amino ester in high yield (92%) and enantiomeric excess (91%) at low reacting temperature of ?40°C. In addition, those β‐amino ester derivatives with high yields and excellent enatioselectivities were obtained in the presence of catalyst 3c under the above condition. 相似文献
110.
Ola Basa’ar 《液相色谱法及相关技术杂志》2017,40(17):879-886
Supercritical fluid extraction (SFE) achieves a wide range of applications since the past decade as a sustainable green technology. The present study investigates the process for producing high yield by supercritical carbon dioxide (Sc-CO2) extraction form Gymnema sylvestre R.Br. roots. The effect of temperature and pressure on the percentage of accumulative yield is demonstrated. It is found that the highest yield is obtained at the temperature of 60°C and the pressure of 10?MPa. A proper review showed that there is a lack in the study of Sc-CO2 extraction of this plant especially for optimization of SFE process which makes this study useful and valuable. For more benefit, the extract which achieves highest percentage is subjected to gas chromatography–mass spectrometry to study its chemical composition and detect the active principle compounds which present with high concentration and expected to be responsible of the pharmaceutical properties of the extract. 相似文献