全文获取类型
收费全文 | 921篇 |
免费 | 175篇 |
国内免费 | 116篇 |
专业分类
化学 | 637篇 |
晶体学 | 13篇 |
力学 | 73篇 |
综合类 | 2篇 |
数学 | 91篇 |
物理学 | 396篇 |
出版年
2024年 | 7篇 |
2023年 | 15篇 |
2022年 | 39篇 |
2021年 | 44篇 |
2020年 | 42篇 |
2019年 | 33篇 |
2018年 | 27篇 |
2017年 | 44篇 |
2016年 | 56篇 |
2015年 | 52篇 |
2014年 | 73篇 |
2013年 | 63篇 |
2012年 | 94篇 |
2011年 | 100篇 |
2010年 | 51篇 |
2009年 | 54篇 |
2008年 | 34篇 |
2007年 | 65篇 |
2006年 | 49篇 |
2005年 | 39篇 |
2004年 | 25篇 |
2003年 | 24篇 |
2002年 | 26篇 |
2001年 | 21篇 |
2000年 | 20篇 |
1999年 | 8篇 |
1998年 | 5篇 |
1997年 | 19篇 |
1996年 | 12篇 |
1995年 | 3篇 |
1994年 | 8篇 |
1993年 | 7篇 |
1992年 | 5篇 |
1991年 | 3篇 |
1990年 | 16篇 |
1989年 | 10篇 |
1987年 | 4篇 |
1986年 | 1篇 |
1985年 | 2篇 |
1982年 | 2篇 |
1981年 | 4篇 |
1979年 | 1篇 |
1977年 | 2篇 |
1976年 | 1篇 |
1975年 | 2篇 |
排序方式: 共有1212条查询结果,搜索用时 16 毫秒
961.
Ruonan Wang Jingcai Wang Qingxuan Tang Xin Zhao Junfeng Wang Yuting Leng Yangjie Wu Junbiao Chang Yusheng Wu Zhaoda Zhang Shiwei Wang 《Tetrahedron letters》2019,60(8):586-590
A mild and efficient trifluoromethylation of imidazopyridines with Togni’s reagent in the presence of inexpensive base has been realized at room temperature. This methodology has several advantages: (1) using efficient copper(I) as catalyst and inexpensive K3PO4 as base, (2) no additional oxidants required for this reaction. (3) good functional group tolerance. An array of trifluoromethylated imidazopyridines with broad functionalities were obtained in acceptable yields. 相似文献
962.
Ruonan Wang Shiwei Wang Dayong Li Feiyang Ye Yuting Leng Yangjie Wu Junbiao Chang Yusheng Wu 《Tetrahedron》2019,75(15):2298-2305
A mild, metal-free and efficient synthesis of 2,3-disubstituted acrylonitriles and α-iminonitriles through radical ring opening reaction of pyridine fused heterocycles has been developed. The tetra-n-butylammonium iodide catalyst acts as a formal one-electron donor. Compared to the previous methods, which require excessive amounts of highly explosive azide sources and the addition of oxidants, this is a safe and convenient transformation. 相似文献
963.
Zolkeflee Nur Khaleeda Zulaikha Maulidiani M. Khoo Leng Wei Shaari Khozirah Abas Faridah 《Chemistry of Natural Compounds》2022,58(1):167-171
Chemistry of Natural Compounds - 相似文献
964.
965.
966.
Damage mechanism and morphology characteristics of chromium film in femtosecond laser rear-side ablation 总被引:1,自引:0,他引:1
Wenjun Wang Xuesong Mei Kedian Wang Jinyou Shao Chengjuan Yang 《Applied Surface Science》2010,256(11):3612-3617
In this paper, damage mechanism and morphology characteristics of chromium film in femtosecond laser rear-side ablation are investigated. The film removing process includes two key sub-processes: the laser ablation dynamic process and subsequent breaking and ejecting dynamic process. Film morphology in rear-side ablation is determined by the interrelation between the laser energy and the film strength. When lower laser energy is used, residual out-layer film is relative thick and tends to break into some large fragments, which results in an irregular ablation shape. While when higher pulse energy is used, thinner residual film with weaker break strength breaks into small fragments, the ablation quality improves correspondingly. Besides laser energy and film property, energy distribution of laser beam also affects the ablation quality. In experiments, this kind of effect is researched by changing the focal position. It is found that ripples, which are familiar nano-structures in front-side ablation, also exist in rear-side ablation. These ripples are formed initially at the interface between quartz substrate and film, and their coverage varies with the energy distribution. Additionally, increasing number of scans is an effective method to shorten the period of ripples. 相似文献
967.
Jian Leng Yuqiong Li Dongpu Zhang Xiaoyi Liao Wei Xue 《Applied Surface Science》2010,256(20):5832-5836
Y2O3 thin films were deposited by ion beam assisted deposition (IBAD) and the effects of fabrication parameters such as substrate temperature and ion energy on the structure, optical and electrical properties of the films were investigated. The results show that the deposited Y2O3 films had less optical absorption, larger refractive index, and better film crystallinity with the increase of substrate temperature or ion energy. The as-deposited Y2O3 films without ion-beam bombardment had larger relative dielectric constant (?r) and the ?r decreased with time even over by 40%, while the ?r of films prepared with high ion energy had less changes, only less than 3%. Also, with the increase of ion energy, the electrical breakdown strength and the figure of merit increased. 相似文献
968.
Jianjun Li Guodong Li Jinsong Leng 《Journal of magnetism and magnetic materials》2010,322(21):3396-3400
MnFe2O4 nanoparticles have been synthesized with a sol-gel method. Both differential thermal and thermo-gravimetric analyses indicate that MnFe2O4 nanoparticles form at 400 °C. Samples treated at 450 and 500 °C exhibit superparamagnetism at room temperature as implied from vibrating sample magnetometry. Mössbauer results indicate that as Mn2+ ions enter into the octahedral sites, Fe3+ ions transfer from octahedral to tetrahedral sites. When the calcination temperature increases from 450 to 700 °C, the occupation ratio of Fe3+ ions at the octahedral sites decreases from 43% to 39%. Susceptibility measurements versus magnetic field are reported for various temperatures (from 450 to 700 °C) and interpreted within the Stoner-Wohlfarth model. 相似文献
969.
应用动态光散射、透射电镜和粘度-剪切模型3种方法互相配合,研究了果胶与乳清分离蛋白的混合体系在热力学不相容条件下的微观结构。在90℃及pH7.4条件下,向5%蛋白浓度的乳清蛋白溶液中加入带有负电的果胶分子,可导致热变性乳清蛋白分子之间发生损耗聚集,并引发混合体系内的相分离现象。确切地说,当果胶与乳清蛋白的混合重量比小于0.08时,溶液内可观察到粒径小于300nm的聚集颗粒,体系呈现牛顿流体特征;而当混合重量比大于0.08时,体系粘度上升,切稀特征逐渐明显,聚集集团粒径接近700nm。 相似文献
970.
Natural systems are typically nonlinear and complex, and it is of great interest to be able to reconstruct a system in order to understand its mechanism, which cannot only recover nonlinear behaviors but also predict future dynamics. Due to the advances of modern technology, big data becomes increasingly accessible and consequently the problem of reconstructing systems from measured data or time series plays a central role in many scientic disciplines. In recent decades, nonlinear methods rooted in state space reconstruction have been developed, and they do not assume any model equations but can recover the dynamics purely from the measured time series data. In this review, the development of state space reconstruction techniques will be introduced and the recent advances in systems prediction and causality inference using state space reconstruction will be presented. Particularly, the cutting-edge method to deal with short-term time series data will be focused on. Finally, the advantages as well as the remaining problems in this field are discussed. 相似文献