全文获取类型
收费全文 | 6293篇 |
免费 | 1352篇 |
国内免费 | 1943篇 |
专业分类
化学 | 4309篇 |
晶体学 | 285篇 |
力学 | 379篇 |
综合类 | 190篇 |
数学 | 876篇 |
物理学 | 3549篇 |
出版年
2024年 | 17篇 |
2023年 | 67篇 |
2022年 | 230篇 |
2021年 | 212篇 |
2020年 | 228篇 |
2019年 | 221篇 |
2018年 | 199篇 |
2017年 | 289篇 |
2016年 | 225篇 |
2015年 | 315篇 |
2014年 | 377篇 |
2013年 | 531篇 |
2012年 | 494篇 |
2011年 | 510篇 |
2010年 | 547篇 |
2009年 | 507篇 |
2008年 | 603篇 |
2007年 | 516篇 |
2006年 | 530篇 |
2005年 | 449篇 |
2004年 | 366篇 |
2003年 | 275篇 |
2002年 | 314篇 |
2001年 | 264篇 |
2000年 | 254篇 |
1999年 | 150篇 |
1998年 | 87篇 |
1997年 | 85篇 |
1996年 | 65篇 |
1995年 | 62篇 |
1994年 | 67篇 |
1993年 | 54篇 |
1992年 | 81篇 |
1991年 | 58篇 |
1990年 | 49篇 |
1989年 | 60篇 |
1988年 | 37篇 |
1987年 | 35篇 |
1986年 | 29篇 |
1985年 | 12篇 |
1984年 | 15篇 |
1983年 | 15篇 |
1982年 | 14篇 |
1981年 | 21篇 |
1980年 | 16篇 |
1979年 | 11篇 |
1978年 | 9篇 |
1977年 | 4篇 |
1965年 | 6篇 |
1963年 | 2篇 |
排序方式: 共有9588条查询结果,搜索用时 15 毫秒
51.
本文首先运用考虑非期望产出的SBM-SupSBM模型估计我国各省市绿色技术创新效率并进行分类:稳定上升类、平稳类、大幅度波动类以及下降类。之后从企业内部和外部两方面对绿色技术创新效率提升的影响因素提出假设,着重考虑大幅度波动类别省份的效率提升。运用面板数据模型各影响因素进行验证,结论显示:企业规模对绿色技术创新效率波动较大的省份起到显著的抑制作用,环境规制和技术进步对绿色技术创新效率波动较大的省份作用不明显。针对分析结果,提出结论:在提升绿色技术创新效率的过程中,应注重企业规模和技术进步的协同作用,以及技术的开放程度。 相似文献
52.
Hang GONG Nian Fa YANG? Wei Jun TANG Qing Hua FAN Li Wen YANG The College of Chemistry Xiangtan University Xiangtan Laboratory of Chemical Biology Center for Molecular Science Institute of Chemistry Chinese Academy of Sciences Beijing 《中国化学快报》2005,16(8)
Among the reactions in which C-C bonds are formed, the Baylis-Hillman coupling of aldehydes with α, β-unsaturated carbonylic compounds is currently attracting much interest due to the atom economy, the mild conditions and the generation of functional groups1,2. Furthermore, compared to the Heck, Suzuki and other palladium catalyzed C-C bond forming reaction3, the Baylis-Hillman reaction can be promoted by using organic bases in the complete absence of any metal4. However, almost all the … 相似文献
53.
A pyrryl substituted photocbromic fulgide, (1-p-methoxyphenyl-2-methyl-5-phenyl)-3-pyrryl-ethylidene (isopropylidene) succinic anhydride, was synthesized. Its chemical structure was identified by IR, 1H NMR MS and elemental analysis. The title compound was used to prepare the optical disks by the spin coating method and the vacuum evaporation method. The results of the static state examination show that the optical disks prepared with this kind of molecules possess excellent thermal stability and fatigue resistance, i.e. over 450 writing-erasing recycles, the relative contrast between writing state and erasing state remains at 40%. 相似文献
54.
The glassy transition of the polyethylene terephthalate (PET) sampleswhich have been subjected to solvent induced crystallization (SINC) was investigated bymodulated differential scanning calorimetry (MDSC) and density measurement. The dif-ferential of heat capacity signal, d C_p/dT from MDSC, was used to monitor the SINCprocess. It reveals that the T_g temperature shifts to higher value with the advancement ofSINC. When the toluene-immersing time was longer (168h), the detection of T_g becomemore difficult, because some smaller peaks emerged at the lower temperatures and theseare explained as the movement of small segments in the amorphous region. These observedresults are due to the morphology and structure introduced by the SINC process. 相似文献
55.
Photochromism of two bispyrryl-substituted ethenes, 2, 3-bis-(1-p-methoxyphenyl-5-phenyl-2-methyl-3-pyrryl)-2-butene (BPE1) and 2,3-bis(1-p-bromophenyl-4-phenyl-2-methyl-3-pyrryl)-2-butene (BPE2), was studied by laser flash photolysis technique. The results indicate that photocyclization of these compounds proceeds mainly via the excited triplet state, and the cis-trans isomerization proceeds mainly via the excited singlet state. After UV laser pulse irradiation, both photocylization and cis-trans isomerization of BPEl occur, but photocydization is the main reaction. On the other hand, laser photolysis of BPE2 leads mainly to photocydization. The effects of the substituents on the photochromic mechanism are also discussed. 相似文献
56.
Seven polystyrene-type adsorbents (Ⅰ—Ⅶ) were synthesized by the modification of styrene-divinylbenzene copolymer respectively with hydroxymethyl, aminomethyl, methoxymethyl, phenoxymethyl, (4-hydroxy phenoxy)methyl, (4-acetylphenoxy)methyl, and acetyl groups. Their structures were characterized with chemical methods and physical techniques. The adsorption capacities of the adsorbents to stevioside and gypenosides were determined via batch test in which the HPLC method was used for the analysis of Saponin concentration of solution. The results showed that all the adsorbents could adsorb these saponins to a certain extent, but Ⅵ and Ⅶ had large capacities owing to their moderate polarities. 相似文献
58.
The atom transfer radical polymerization(ARP)of (-)-menthyl methacrylatc((-)-MnMA) mediated by CuCl/bipyridine and ethy1 2-bromopropionate or 1-phenylethyl bromide in THF system has been studied.The dependence of the specific rotation on molecular weight and the CD of Poly((-)-MnMA) thus obatined was investigated. 相似文献
59.
60.