全文获取类型
收费全文 | 3978篇 |
免费 | 452篇 |
国内免费 | 316篇 |
专业分类
化学 | 2599篇 |
晶体学 | 27篇 |
力学 | 198篇 |
综合类 | 20篇 |
数学 | 488篇 |
物理学 | 1414篇 |
出版年
2023年 | 51篇 |
2022年 | 121篇 |
2021年 | 105篇 |
2020年 | 133篇 |
2019年 | 98篇 |
2018年 | 103篇 |
2017年 | 95篇 |
2016年 | 130篇 |
2015年 | 138篇 |
2014年 | 167篇 |
2013年 | 238篇 |
2012年 | 322篇 |
2011年 | 294篇 |
2010年 | 220篇 |
2009年 | 193篇 |
2008年 | 259篇 |
2007年 | 216篇 |
2006年 | 198篇 |
2005年 | 162篇 |
2004年 | 128篇 |
2003年 | 113篇 |
2002年 | 102篇 |
2001年 | 89篇 |
2000年 | 102篇 |
1999年 | 73篇 |
1998年 | 66篇 |
1997年 | 55篇 |
1996年 | 75篇 |
1995年 | 56篇 |
1994年 | 60篇 |
1993年 | 53篇 |
1992年 | 54篇 |
1991年 | 36篇 |
1990年 | 28篇 |
1989年 | 41篇 |
1988年 | 37篇 |
1987年 | 31篇 |
1986年 | 41篇 |
1985年 | 30篇 |
1984年 | 35篇 |
1983年 | 26篇 |
1982年 | 20篇 |
1981年 | 14篇 |
1980年 | 22篇 |
1979年 | 12篇 |
1978年 | 11篇 |
1977年 | 15篇 |
1976年 | 17篇 |
1975年 | 17篇 |
1973年 | 9篇 |
排序方式: 共有4746条查询结果,搜索用时 15 毫秒
981.
A new synthetically useful decarboxylative reduction of γ-carbamoyloxy-α,β-unsaturated esters with lithium dialkylcuprate under mild condition was described. 相似文献
982.
In the context of manpower planning, goal programming (GP) is extremely useful for generating shift duties of fixed length. A fixed-length duty consists of a fixed number of contiguous hours of work in a day, with a meal/rest break somewhere preferably around the middle of these working hours. It is such properties that enable the straightforward, yet flexible GP modeling. We propose GP models for an integrated problem of crew duties assignment, for baggage services section staff at the Hong Kong International Airport. The problem is solved via decomposition into its duties generating phase—a GP planner, followed by its GP scheduling and rostering phase. The results can be adopted as a good crew schedule in the sense that it is both feasible, satisfying various work conditions, and “optimal” in minimizing idle shifts. 相似文献
983.
984.
985.
986.
987.
§1 引言 限量分配问题是古典概率论,组合论的重要内容。本文将文献[1]、[2]中的一类广泛的限量分配问题给以统一的处理并加以推广,归结为如下问题: 问题Ⅰ 内无序分配问题。给定m类盒和n类球,假定第i类球和第j类盒的个数分别为r_i、s_j(1≤i≤n,1≤j≤m),即所谓球的规格为(?)=(r_1,r_2,…,r_n)和盒的规格为(?)=(s_1,s_2,…,s_m)。已知第i类盒对于球的限量集为A_i(这里A_i∈N_0~t,其中每个元素表示该类盒所能容纳之球的规格,1≤i≤m),记A=(A_1,A_2,…,A_m)。则分配规格为(?)的球至规 相似文献
988.
In Hwan Jung Young Kwan Jung Jonghee Lee Jong‐Hwa Park Han Young Woo Jeong‐Ik Lee Hye Yong Chu Hong‐Ku Shim 《Journal of polymer science. Part A, Polymer chemistry》2008,46(21):7148-7161
We synthesized two fluorene‐based copolymers poly[(2,5‐bis(4‐hexylthiophen‐2‐yl)thiazolo[5,4‐day]thiazole‐5,5′‐diyl)‐alt‐(9,9′‐dioctylfluorene‐2,7‐diyl)] ( PF‐TTZT), and poly[(5,5′‐bis(4‐hexylthiophen‐2‐yl)‐2,2′‐bithiazole‐5,5′‐diyl)‐alt‐(9,9′‐dioctylfluorene‐2,7‐diyl)] (PF‐TBTT), which contain the electron‐withdrawing moieties, thiazolothiazole, and bithiazole, respectively. Through electrochemical studies, we found that these two polymers exhibit stable reversible oxidation and reduction behaviors. Moreover, the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of PF‐TBTT are lower than those of PF‐TTZT, and the bandgap of PF‐TBTT is smaller than that of PF‐TTZT. Thus the bithiazole moiety in PF‐TBTT is more electron‐withdrawing than the thiazolothiazole moiety in PF‐TTZT. Light‐emitting devices with indium tin oxide (ITO)/poly(3,4‐ethylene dioxythiophene):poly(styrenesulfonate)(PEDOT)/polymer/bis(2‐methyl‐8‐quinolinato)‐4‐phenylphenolate aluminum (BAlq)/LiF/Al configurations were fabricated. The performance of the PF‐TBTT device was found to be almost three times better than that of the PF‐TTZT device, which is because electron injection from the cathode to PF‐TBTT is much easier than for PF‐TTZT. We also investigated the planarity and frontier orbitals of the electron donor‐acceptor (D‐A) moieties with computational calculations using ab initio Hartree–Fock with the split‐valence 6‐31G* basis set. These calculations show that TBTT has a more nonplanar structure than TTZT and that the bithiazole moiety is more electron‐withdrawing than thiazolothiazole. These calculations are in good agreement with the experimental results. © Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 7148–7161, 2008 相似文献
989.
Mu-hua Chen Wei Chu Jun-jie Zhu Lin Dong 《Journal of Sol-Gel Science and Technology》2008,47(3):354-359
Novel cobalt catalysts were prepared by sol–gel method, and enhanced by plasma treatment, for methane catalytic combustion.
These samples were characterized using X-ray diffraction, X-ray photoelactron spectroscopy, UV-vis spectroscopy, Fourier transform
infrared spectroscopy, thermal gravimetrical analysis, N2 Adsorption–desorption, temperature-programmed reduction and hydrogen–oxygen titration technologies. The XPS characterizations
suggested that plasma treatment was favorable for the enrichment of surface cobalt, with a value of surface cobalt from 2.2%
to 8.5% in mole. The specific surface area of the glow plasma assised sample (Co-Plas-Solgel-2) increased to 320 m2/g comparing with 305 m2/g of the conventional sample (Co-Solgel-1). The ignition temperature (T10%) of Co-Plas-Solgel-2 catalyst was about 50 °C lower than that of Co-Solgel-1, and its CH4 conversion was two times higher than that of Co-Solgel-1 during the whole range of catalytic combustion activity test (340–520 °C).
With a better dispersion and more active sites, the plasma assisted sample exhibited significant enhancement in catalytic
performances. 相似文献