全文获取类型
收费全文 | 14899篇 |
免费 | 2968篇 |
国内免费 | 4372篇 |
专业分类
化学 | 11077篇 |
晶体学 | 512篇 |
力学 | 969篇 |
综合类 | 540篇 |
数学 | 2119篇 |
物理学 | 7022篇 |
出版年
2024年 | 50篇 |
2023年 | 266篇 |
2022年 | 650篇 |
2021年 | 661篇 |
2020年 | 652篇 |
2019年 | 642篇 |
2018年 | 574篇 |
2017年 | 755篇 |
2016年 | 651篇 |
2015年 | 838篇 |
2014年 | 1000篇 |
2013年 | 1246篇 |
2012年 | 1263篇 |
2011年 | 1269篇 |
2010年 | 1188篇 |
2009年 | 1163篇 |
2008年 | 1320篇 |
2007年 | 1186篇 |
2006年 | 1141篇 |
2005年 | 824篇 |
2004年 | 714篇 |
2003年 | 537篇 |
2002年 | 579篇 |
2001年 | 594篇 |
2000年 | 601篇 |
1999年 | 356篇 |
1998年 | 198篇 |
1997年 | 145篇 |
1996年 | 139篇 |
1995年 | 90篇 |
1994年 | 126篇 |
1993年 | 126篇 |
1992年 | 100篇 |
1991年 | 81篇 |
1990年 | 78篇 |
1989年 | 62篇 |
1988年 | 49篇 |
1987年 | 65篇 |
1986年 | 42篇 |
1985年 | 39篇 |
1984年 | 26篇 |
1983年 | 27篇 |
1982年 | 22篇 |
1981年 | 17篇 |
1980年 | 19篇 |
1979年 | 20篇 |
1978年 | 7篇 |
1977年 | 10篇 |
1976年 | 5篇 |
1974年 | 9篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
101.
102.
激光光散射研究丙烯酰胺-二甲基二烯丙基氯化铵共聚物的溶液行为 总被引:6,自引:0,他引:6
用激光光散射技术表征了阳离子含量受控的丙烯酰胺 二甲基二烯丙基氯化铵共聚物 (简称P(AM DMDAAC) )在 0 1mol LNaCl溶液中的基本参数和溶液行为 .表观基本参数 : Mw ,app =5 0 4× 10 5g·mol- 1 、app =39 9nm和A2 ,APP =1 5× 10 - 4cm3·mol·g- 2 .动态光散射研究结果表明 ,扩散系数Dt 与角度、浓度皆呈非线性依赖关系 ,当C >C 或在较大的散射角域 ,Dt 偏离线性关系所预示的理论值而向减小的方向发展 ,表明存在缔合物 ;流体力学半径Rh 分布有大、小两个范围 ,并随温度升高向小尺寸方向发展 ,证明了体系中存在缔合行为以及升温的解缔合作用 .在一定条件下 ,“亚稳定性”的存在有力地旁证了体系的缔合行为 相似文献
103.
This paper compares the kinetics of exchanges of phenylethanethiolate ligands (PhC2S-) of the monolayer-protected clusters (MPCs) Au(38)(SC2Ph)(24) and Au(140)(SC2Ph)(53) with p-substituted arylthiols (p-X-PhSH), where X = NO(2), Br, CH(3), OCH(3), and OH. First-order rate constants at 293 K for exchange of the first ca. 25% of the ligands on the molecule-like Au(38)(SC2Ph)(24) MPC, measured using (1)H NMR, vary linearly with the in-coming arythiol concentration; ligand exchange is an overall second-order reaction. Remarkably, the second-order rate constants for ligand exchange on Au(38)(SC2Ph)(24) are very close to those of corresponding exchange reactions on the larger nanoparticle Au(140)(SC2Ph)(53) MPCs. These are the first results that quantitatively show that the chemical reactivity of different sized nanocrystals is almost independent of size; presumably, this is because the locus of the initial ligand exchanges is a common kind of site, thought to be the nanocrystal vertexes. The rates of later stages of exchange (beyond ca. 25%) differ for Au(38) and Au(140) cores, the latter being much slower presumably due to its larger terrace-like surface atom content. The reverse exchange reaction was studied for Au(38)(p-X-arylthiolate)(24) MPCs (X = NO(2), Br, and CH(3)), where the in-coming ligand is now phenylethanethiol. Remarkably, the rate constants of both forward and reverse exchanges display identical substituent effects, which implies a concurrent bonding of both in-coming and leaving ligands to the Au core in the rate-determining step, as in an associative mechanism. X = NO(2) gives the fastest rates, and the ratio of forward and reverse rate constants gives an equilibrium constant of K(EQ,PE) = 4.0 that is independent of X. 相似文献
104.
105.
Rui Qi SONG * Jian Qing ZENG Bing ZHONG Guangzhou Institute of Chemistry Chinese Academy of Sciences Guangzhou State Key Laboratory of Coal Chemistry Institute of Coal Chemistry Chinese Academy of Sciences Taiyuan School of 《中国化学快报》2002,13(10)
We have studied the palladium-catalyzed alkoxycarbonylation of allyl bromide in supercritical (sc) CO2 and found the reaction rate in sc CO2 was lower than those in some organic solvents1. The possible reason was the lower solubility of sc CO2 for the palladium catalyst. As an effective approach to improve the solubility of varieties of solutes, cosolvent such as ethanol has been widely used in the sc CO2 extraction2,3. Based on the above consideration, we chose several organic reagents… 相似文献
106.
Two new pyrrolizidines named lankongensisine A (1), B (2) were isolated from the roots of Ligularia lankongensis collected in Lijiang, Yunnan, and their structures were established by spectroscopic analysis. 相似文献
107.
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
108.
Two fluorescent "off-on" probes YYH1 and YYH2 were used for bioimaging mitochondrial polarity and viscosity. 相似文献
109.
Borba A Gómez-Zavaglia A Simões PN Fausto R 《Spectrochimica acta. Part A, Molecular and biomolecular spectroscopy》2005,61(7):1461-1470
The preferred conformations of dimethyl sulfate and their vibrational spectra were studied by matrix-isolation FT-IR spectroscopy and theoretical methods (DFT and MP2, with basis sets of different sizes, including the quadruple-zeta, aug-cc-pVQZ basis). Conformer GG (of C2 symmetry and exhibiting OSOC dihedral angles of 74.3 degrees ) was found to be the most stable conformer in both the gaseous phase and isolated in argon. Upon annealing of the matrix, the less stable observed conformer (GT; with C1 symmetry) quickly converts to the GG conformer, with the resulting species being embedded in a matrix-cage which corresponds to the most stable matrix-site for GG form. The highest energy TT conformer, which was assumed to be the most stable conformer in previous studies, is predicted by the calculations to have a relative energy of ca. 10 kJ mol-1 and was not observed in the spectra of the matrix-isolated compound. 相似文献
110.