全文获取类型
收费全文 | 16022篇 |
免费 | 2922篇 |
国内免费 | 2207篇 |
专业分类
化学 | 16008篇 |
晶体学 | 209篇 |
力学 | 321篇 |
综合类 | 81篇 |
数学 | 132篇 |
物理学 | 4400篇 |
出版年
2024年 | 50篇 |
2023年 | 182篇 |
2022年 | 932篇 |
2021年 | 819篇 |
2020年 | 875篇 |
2019年 | 650篇 |
2018年 | 528篇 |
2017年 | 575篇 |
2016年 | 827篇 |
2015年 | 746篇 |
2014年 | 808篇 |
2013年 | 1493篇 |
2012年 | 1001篇 |
2011年 | 884篇 |
2010年 | 781篇 |
2009年 | 870篇 |
2008年 | 927篇 |
2007年 | 976篇 |
2006年 | 892篇 |
2005年 | 817篇 |
2004年 | 797篇 |
2003年 | 733篇 |
2002年 | 533篇 |
2001年 | 464篇 |
2000年 | 457篇 |
1999年 | 420篇 |
1998年 | 380篇 |
1997年 | 282篇 |
1996年 | 232篇 |
1995年 | 239篇 |
1994年 | 202篇 |
1993年 | 133篇 |
1992年 | 130篇 |
1991年 | 73篇 |
1990年 | 53篇 |
1989年 | 63篇 |
1988年 | 48篇 |
1987年 | 34篇 |
1986年 | 38篇 |
1985年 | 46篇 |
1984年 | 29篇 |
1983年 | 10篇 |
1982年 | 14篇 |
1981年 | 23篇 |
1980年 | 22篇 |
1979年 | 13篇 |
1978年 | 10篇 |
1977年 | 8篇 |
1975年 | 10篇 |
1973年 | 7篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
91.
HL-2A tokamak, the first tokamak with divertor in China, has been constructed and put into operation in 2002. The main parameters are R=1.65 m, a=0.4 m, BT=2.8 T, Ip = 0. 48 MA. The divertor of HL-2A is unique, because it is characterized with a large closed divertor chamber. The device has double divertor chamber, but now it is operating with lower single null configuration to study the physics of divertor for the next step design of a divertor. Supersonic molecular beam injection (SMBI) system with LN2 cooling trap was first installed and demonstrated on the HL-2A tokamak in 2004. The first results of SMBI into HL-2A plasma are to demonstrate the function of the HL-2A divertor and to observe the cold pulse propagation during multi-pulse SMBI on HL-2A Tokamak. 相似文献
92.
93.
Akira Shimazu Tukasa Miyazaki Shigeru Katayama Yasuo Ito 《Journal of Polymer Science.Polymer Physics》2003,41(3):308-318
The ortho‐positronium (o‐Ps) lifetime τ3 and its intensity I3 in various fluorinated polyimides were determined by the positron annihilation technique and were studied with the spin–lattice relaxation time T1 and the propylene permeability, solubility, diffusivity, and permselectivity for propylene/propane in them. τ3, I3, and the distribution of τ3 changed when the bulky moieties in the polyimides were changed. The polyimides, having both large τ3 and I3 values, exhibited a short T1 and a high permeability with a low permselectivity. The propylene permeability and diffusivity were exponentially correlated with the product of I3 and the average free‐volume hole size estimated from τ3. In highly plasticized states induced by the sorption of propylene, the permeability increased with the propylene pressure in excellent agreement with the change in the free‐volume hole properties probed by o‐Ps. The large and broad distribution of the free‐volume holes and increased local chain mobility for the 2,2‐bis(3,4‐decarboxyphenyl) hexafluoropropane dianhydride‐based polyimides are thought to be important physical properties for promoting penetrant‐induced plasticization. These results suggest that o‐Ps is a powerful probe of not only the free‐volume holes but also the corresponding permeation mechanism and penetrant‐induced plasticization phenomenon. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 308–318, 2003 相似文献
94.
本文报导了聚4-氨基联苯的电化学合成,测定了它的ESR、IR及紫外可见光谱。聚合物在THF、DMF和DMSO中能全部溶解。界面移动法测得聚4-氨基联苯的DMF饱和溶液中正离子的迁移率为1.48×10~(-8)m~2·S~(-1)·V~(-1)。 相似文献
95.
96.
Novel oscillatory flow results of phenoxy/organoclay nanocomposites are analysed considering the blocking effect of nanostructure on polymer chain mobility. The modification provoked by this hindering effect on loss tangent spectrum is investigated. The study of three different systems, involving a pristine montmorillonite and two montmorillonites modified with one alkyl tail and two alkyl tails, respectively, leads to conclude that polymer-alkyl repulsive interactions play the most important role in the chain mobility obstruction process. Our results suggest that polymer-alkyl interactions increase with temperature. 相似文献
97.
98.
S.F. Lienin R. Brüschweiler R.R. Ernst 《Journal of magnetic resonance (San Diego, Calif. : 1997)》1998,131(2):184-190
The viscosity-dependent retarding effect of a polymeric solvent on the rotation of small solute molecules is investigated by13C NMR relaxation measurements. It is found that the relaxation data of 1,3-dibromoadamantane in highly viscous polymeric chlorotrifluoroethene can be explained neither by isotropic nor by realistic anisotropic tumbling in a single environment. The experimental data are rationalized in terms of fast exchange between at least two environments with correlation times differing by up to two orders of magnitude. The study shows that a uniform retardation of molecular tumbling by a polymeric solvent, desirable for shifting the NMR observation window in studies of intramolecular mobility, is not always feasible. 相似文献
99.
100.
Sridevi Kaiti Eric Fossum 《Journal of polymer science. Part A, Polymer chemistry》2006,44(6):2099-2106
A thorough study of the polymerization behavior of 4‐fluoro‐4′‐hydroxytriphenyl‐phosphine oxide, 2 , under nucleophilic aromatic substitution reactions has been carried out. The synthesis of 2 was achieved in excellent yields by the reaction of bis(4‐fluorophenyl)phenylphosphine oxide, 1 , with one equivalent of potassium hydroxide in DMSO/water. The structure and purity of 2 were confirmed via 1H, 13C, and 31P NMR spectroscopy along with elemental analysis. Polymerization reactions of 2 in NMP or DMSO at 180 °C provided the corresponding linear poly(arylene ether phosphine oxide)s, PAEPOs, with number average molecular weights, Mn, ranging from 11,700 to 36,500 Da. All of the polymer samples were completely soluble in chloroform, tetrahydrofuran, DMSO, NMP, and DMAc. The polymerization reactions were accompanied by a competing intramolecular process that resulted in the formation of cyclic oligomeric species that were removed via a final precipitation from methanol. Analysis using 31P NMR spectroscopy and size exclusion chromatography (SEC) confirmed that the majority of the lower molecular weight cyclic species were removed via this process. The polymer samples formed tough films when chloroform solutions were slowly evaporated on a glass slide. The PAEPO samples prepared in this study exhibited excellent thermal stability with Td (5%) values between 503 and 542 in air while the glass transition temperatures ranged from 223 to 237 °C. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 2099–2106, 2006 相似文献