全文获取类型
收费全文 | 2719篇 |
免费 | 188篇 |
国内免费 | 1263篇 |
专业分类
化学 | 3797篇 |
晶体学 | 23篇 |
力学 | 86篇 |
综合类 | 60篇 |
数学 | 33篇 |
物理学 | 171篇 |
出版年
2024年 | 9篇 |
2023年 | 30篇 |
2022年 | 71篇 |
2021年 | 93篇 |
2020年 | 131篇 |
2019年 | 117篇 |
2018年 | 95篇 |
2017年 | 120篇 |
2016年 | 148篇 |
2015年 | 135篇 |
2014年 | 165篇 |
2013年 | 222篇 |
2012年 | 191篇 |
2011年 | 159篇 |
2010年 | 142篇 |
2009年 | 147篇 |
2008年 | 150篇 |
2007年 | 194篇 |
2006年 | 179篇 |
2005年 | 190篇 |
2004年 | 211篇 |
2003年 | 198篇 |
2002年 | 152篇 |
2001年 | 143篇 |
2000年 | 135篇 |
1999年 | 105篇 |
1998年 | 87篇 |
1997年 | 78篇 |
1996年 | 73篇 |
1995年 | 64篇 |
1994年 | 66篇 |
1993年 | 45篇 |
1992年 | 27篇 |
1991年 | 22篇 |
1990年 | 24篇 |
1989年 | 16篇 |
1988年 | 13篇 |
1987年 | 8篇 |
1986年 | 4篇 |
1985年 | 5篇 |
1984年 | 2篇 |
1981年 | 2篇 |
1980年 | 1篇 |
1979年 | 1篇 |
排序方式: 共有4170条查询结果,搜索用时 156 毫秒
61.
相转移催化法合成糖苷化合物 总被引:1,自引:0,他引:1
糖苷化合物广泛存在于生物体内,中草药和天然药物中的许多抗病活性化合物也是糖苷化合物。根据糖苷化合物分子结构中的配糖体与糖环碳原子相连的原子类型可把糖苷化合物分为氧苷、氮苷、硫苷和碳苷等。未作特殊说明的糖苷化合物均指氧苷,且配糖体为羧酸的糖苷化合物亦称为糖酯。有关糖苷(酯)化合物和其合成方法的研究很多,其中应用相转移催化法较为常见。我们曾用相转移催化法合成过许多糖酯化合物并对其生理活性进行研究报道。本文在原合成糖酯的基础上通过相转移催化法,选用不同于文献报道催化剂及溶剂体系, 相似文献
62.
本文继采用含硫配体(-SH,-SR)对高分子保护金属胶体的配位俘获之后,采用锚联在SiO_2表面的较含硫配体配位能力弱的P,N配体分别与聚乙烯吡咯烷酮(PVP)保护的胶体钯在室温下反应,在P/Pd大于3.5,N/Pd大于7时,可以实施对保护金属胶体的配位俘获,证实了配位俘获法是一种具有普遍意义的方法,经原子吸收光谱分析,溶液中残留的金属钯的量小于0.36 ppm(P/Pd=3.5)。经电子能谱证实,锚联膦胺配体与金属钯之间存在配位作用,这是实施保护胶体负载化的关键。透射电镜分析表明,在整个配位俘获的过程中,无论是在载体上或是在溶液中,胶体钯的颗粒大小与分布均保持不变。因此,配位俘获法是一种能控制多相催化剂中金属颗粒大小与分布的有效方法。由此合成的钯催化剂具有很高的催化活性,良好的选择性和稳定性,在对Cis,Cis-1,3-环辛二烯的选择性加氢中,其活性仅比均相的PVP-Pd胶体催化剂稍低,选择性达100%。在周转数达12,000次/个钯原子后,其催化活性仍保持不变,随着P/Pd比的增大,催化活性迅速下降。 相似文献
63.
64.
1INTRODUCTIONTherehasbeenconsiderableattentionfocusedonthestudyofpalladiumcomplexinorganicsynthesis,catalyticprocesses,chemicalreactivity,spectroscopyandstructuralanalysisandfunctionalmaterials[1].Manypalladacycleshavebeenappliedinthepreparationofliquid-crystallinecomplexes[2],andsomeofthemshowantitumoractivity.Theincreasingimportanceofsuchcyclo-organometalliccomplexleadsustoinvestigatethecyclicPdcomplexeswithenhancedchemicalandthermalstability,fromwhichdetailedspectroscopicandstructuralc… 相似文献
65.
Kai‐lu Yu Chang‐jun Liu Yue‐ping Zhang Fei He Xin‐li Zhu Baldur Eliasson 《Plasma Chemistry and Plasma Processing》2004,24(3):393-403
PdO/Al2O3 catalysts prepared by glow discharge plasma treatment followed by thermal calcination show a much higher dispersion and a better catalytic activity for methane combustion at relatively low temperatures. The dispersion of palladium active species by such plasma prepared catalysts is 29.7%, 5.4 times higher than that of conventional catalysts. XPS analysis indicates that a surface enrichment of Pd active species (PdO) has been achieved after plasma treatment. The surface atomic composition of PdO of plasma prepared catalysts reaches 10.5%. XRD characterization also confirms a wellcrystallized PdO phase present on the plasma prepared catalyst. The lightoff temperature of the plasma prepared catalyst is 370°C, 50°C lower than that obtained from the conventional catalyst. 相似文献
66.
Changes in binding affinity to catalytic antibody 6D9 of chloramphenicol phosphonate derivatives (CPDs) containing H or F were investigated by performing free energy calculations based on molecular dynamics simulations. We calculated the binding free energy, enthalpy, and entropy changes (DeltaDeltaG, DeltaDeltaH, and -TDeltaDeltaS) attributable to H-->F substitution by comparing results for CPDs containing a trifluoroacetylamino group (CPD-F) or an acetylamino group (CPD-H). The calculated DeltaDeltaG, DeltaDeltaH, and -TDeltaDeltaS values were -2.9, -6.3, and 3.5 kcal mol(-1) and close to experimental values observed for a series of similar ligands, chloramphenicol phosphonates with F and H (-1.4, -3.5, and 2.1 kcal mol(-1)). Therefore, CPD-F binds more strongly to 6D9 than does CPD-H. To clarify the origin of the large difference in DeltaDeltaG, we apportioned the calculated values of DeltaDeltaG and DeltaG for the associated and dissociated states into contributions from various atomic interactions. We found that the H-->F substitution increased the binding affinity mainly by decreasing the hydration free energy and not by increasing favorable interactions with the antibody. The decreased hydration free energy of the ligand was mainly due to unfavorable coulombic interactions between the trifluoroacetylamino group and solvent waters, which increased the free energy of the dissociated state (by about 3.7 kcal mol(-1)). Also, the trifluoroacetylamino group slightly increased the free energy level of the associated state (about 0.8 kcal mol(-1)) because favorable van der Waals interactions compensated for unfavorable coulombic interactions with antibody atoms. In addition, the enthalpy and entropy changes, DeltaDeltaH and -TDeltaDeltaS (computationally -6.3 and 3.5 kcal mol(-1)), originated mainly from a decrease in hydration free energy in the dissociated state. The CPD-F and CPD-H ligands had substantially different structures in the dissociated and complexed states. 相似文献
67.
Byung-Sik Jang Kyo-Hyun Cho Kyung-Hoon Kim Dae-Won Park 《Reaction Kinetics and Catalysis Letters》2005,86(1):75-82
Summary The performance of acid-treated montmorillonite catalysts in the degradation of polystyrene (PS) was investigated in this study. The degradation was carried out in a semi-batch reactor with a mixture of PS and catalyst at 400-450oC. The commercial Süd Chemie acid-treated montmorillonite clays (K-series) showed good catalytic activity for the degradation of PS. The styrene monomer and ethylbenzene were major liquid products. The increase of surface acidity enhanced further cracking of styrene dimer and trimer to produce styrene monomer. Higher production of ethylbenzene for K30 may be related to its bigger pore volume and surface area compared to those of K5. High degradation temperature favored styrene monomer production. 相似文献
68.
69.
甲烷直接氧化制甲醇Ⅲ.反应气吹扫催化膜反应器(RSCMR) 总被引:2,自引:0,他引:2
用溶胶-凝胶法制备了“SiO_2/陶瓷”非对称无机膜,并用该膜制备了反应气吹扫催化膜反应器(RSCMR)装置。在RSCMR上考察了甲烷一步催化氧化制甲醇反应。结果表明,在研究的范围内,增加氧气或甲烷的浓度和吹扫气的流速(即总的反应气流量)有利于提高甲醇的收率;甲醇在反应条件下的热不稳定性是影响目标反应选择性的重要原因。RSCMR较膜反应器(CMR)系统能更有效地抑制甲醇的热分解,因而可得到较CMR更高的甲醇收率。当反应温度为700℃时,甲醇的收率在CMR中为0.5g/m ̄2h,在RSCMR中可达0.9g/m ̄2h。 相似文献
70.