全文获取类型
收费全文 | 3307篇 |
免费 | 397篇 |
国内免费 | 397篇 |
专业分类
化学 | 2251篇 |
晶体学 | 34篇 |
力学 | 194篇 |
综合类 | 35篇 |
数学 | 490篇 |
物理学 | 1097篇 |
出版年
2024年 | 7篇 |
2023年 | 65篇 |
2022年 | 105篇 |
2021年 | 133篇 |
2020年 | 171篇 |
2019年 | 127篇 |
2018年 | 103篇 |
2017年 | 115篇 |
2016年 | 158篇 |
2015年 | 149篇 |
2014年 | 168篇 |
2013年 | 214篇 |
2012年 | 263篇 |
2011年 | 280篇 |
2010年 | 175篇 |
2009年 | 187篇 |
2008年 | 193篇 |
2007年 | 182篇 |
2006年 | 172篇 |
2005年 | 167篇 |
2004年 | 95篇 |
2003年 | 70篇 |
2002年 | 70篇 |
2001年 | 59篇 |
2000年 | 68篇 |
1999年 | 71篇 |
1998年 | 57篇 |
1997年 | 50篇 |
1996年 | 75篇 |
1995年 | 45篇 |
1994年 | 43篇 |
1993年 | 40篇 |
1992年 | 33篇 |
1991年 | 42篇 |
1990年 | 29篇 |
1989年 | 18篇 |
1988年 | 17篇 |
1987年 | 16篇 |
1986年 | 12篇 |
1985年 | 11篇 |
1984年 | 11篇 |
1983年 | 5篇 |
1982年 | 8篇 |
1981年 | 5篇 |
1980年 | 4篇 |
1979年 | 5篇 |
1977年 | 2篇 |
1975年 | 2篇 |
1959年 | 1篇 |
1957年 | 1篇 |
排序方式: 共有4101条查询结果,搜索用时 15 毫秒
991.
Xiaofeng Chen Rahmi Ozisik Sanat K. Kumar Phillip Choi 《Journal of polymer science. Part A, Polymer chemistry》2007,45(24):3349-3360
Coarse-grained, on-lattice Monte Carlo simulations are performed to investigate the role of stereotacticity defects along an isotactic polypropylene chain on the formation of 31-helices, which form the basic crystalline order within the chain. For this reason, systems with various stereoerror configurations are studied and are compared to neat isotactic polypropylene. All systems are equilibrated above the melting temperature and are cooled to lower temperatures in a stepwise manner, making sure each system is equilibrated at every temperature. Results indicate that chain ends have the lowest probability of being found in helices. Addition of a single stereoerror (mrm) decreases the probability of five repeat units' participation in helices (the repeat unit that contains the stereoerror and two nearest repeat units on both sides). The probability profile becomes more complicated when the number of stereoerrors increases, however, the results indicate that the effects of many stereoerrors can be explained by a simple addition of the effect of each stereoerror considered individually. The results also indicate that the presence of even a single stereoerror eliminates (within the temperature range studied) the transition to longer, more stable helices observed in neat isotactic polypropylene. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 3349–3360, 2007 相似文献
992.
993.
Jian‐Min Zhang Qing Pang Ke‐Wei Xu Vincent Ji 《Surface and interface analysis : SIA》2008,40(10):1382-1387
We present first‐principles calculations on the (001) surfaces of cubic PbTiO3 with PbO and TiO2 terminations. The cleavage energy, surface energy, surface grand potential, surface relaxation and surface electronic structure have been investigated by using the projector‐augmented wave method under generalized gradient approximation (GGA). The results show that surface energy of a TiO2‐terminated surface is a little lower than that of a PbO‐terminated one, thus allowing both terminations to coexist. The PbO‐termination is stable in O‐ and Pb‐rich environments, while on the contrary, the TiO2‐termination is stable in O‐ and Pb‐poor conditions. In addition, the surface rumpling S of a PbO‐terminated surface is slightly larger than that of a TiO2‐terminated one. The relaxations dominantly take place on the outermost three layers, and an oscillatory (? + ?) damping (|Δd12 | > | Δd23 | > | Δd34|) relaxation phenomenon appears for both terminations. The band gaps of both PbO‐ and TiO2‐terminations are slightly lower than that of the bulk. Moreover, the DOS curves of each layer show that for the TiO2‐termination, the top of the valence band of the first and third TiO2 layers moves toward Fermi level. Copyright © 2008 John Wiley & Sons, Ltd. 相似文献
994.
Xuan Ding Ye Chen Shenghui Chen Jun Li Xiaofeng Wang Huaping Wang 《Journal of Macromolecular Science: Physics》2013,52(6):1199-1207
A series of aromatic polysulfonamides with various monomer ratios of 4,4′-diaminodiphenylsulfone (p-DDS) and 3,3′-diaminodiphenylsulfone (m-DDS) were synthesized to investigate the relationship between copolymer composition and their properties. The copolymer composition was determined from nuclear magnetic resonance (NMR). It was found that the copolymer composition of the polysulfonamides varied little from the monomer ratio; thus, similar reactivity of the two diaminodiphenylsulfones was shown by the results. From the analysis of the thermal properties of the polysulfonamides, high thermal stability of the copolymers was shown both in N2 and air and the decomposition temperature exhibited little dependence on the copolymer composition. However, the glass transition temperatures (Tg ) decreased obviously with increasing m-DDS segments, which is helpful to improve the solubility and spinnability of these polysulfonamides. 相似文献
995.
<正>The method of fitting damage probability curves of laser-induced damage is introduced to investigate the laser-conditioning mechanism of ZrO_2/SiO_2 high reflection(HR) films.The laser-induced damage thresholds(LIDTs) of the sample are tested before and after the laser-conditioning scanning process.The parameters of the defects are obtained through the fitting process of the damage probability curve.It can be concluded that the roles of laser conditioning include two aspects:removing defects with lower threshold and producing new defects with higher threshold.The effect of laser conditioning is dependent on the competition of these two aspects. 相似文献
996.
997.
998.
999.
[reaction: see text] In connection with the known diyne-ene [2 + 2 + 2] cycloaddition reactions mediated by titanium aryloxides, the ability of titanium alkoxides to promote coupling of a titanacyclopentadiene with an alkene has been assessed for the isomerization-free preparation of 1,3-cyclohexadienes. The successful cycloaddition by titanium alkoxides is predicated on the use of homoallylic alcohols as the olefin component. With secondary homoallylic alcohols, high 1,3-diastereoselectivity is observed, which lends itself to enantioselective preparation of functionalized 1,3-cyclohexadienes. 相似文献
1000.
Yu Chuan Li Si Ping Pang Yong Zhong Yu 《中国化学快报》2007,18(5):516-518
The cyclization reaction of D-aspartic acid was studied, the carboxyl groups of D-aspartic acid were protected by benzyl alcohol to give compound D-dibenzyl aspartate. Then (4R)-benzyl azetidine-2-one-4-carboxylate and meso-3,6-disubstituted piperazine- 2,5-diones were synthesized via intramolecular cyclization and intermolecular cyclization of D-dibenzyl aspartate, respectively, and their structures were confirmed by 1H NMR and MS. Both cyclization reaction conditions were also investigated in detail. 相似文献