全文获取类型
收费全文 | 181篇 |
免费 | 17篇 |
国内免费 | 30篇 |
专业分类
化学 | 205篇 |
综合类 | 2篇 |
物理学 | 21篇 |
出版年
2024年 | 1篇 |
2023年 | 5篇 |
2022年 | 6篇 |
2021年 | 6篇 |
2020年 | 7篇 |
2019年 | 11篇 |
2018年 | 13篇 |
2017年 | 12篇 |
2016年 | 13篇 |
2015年 | 4篇 |
2014年 | 17篇 |
2013年 | 30篇 |
2012年 | 13篇 |
2011年 | 10篇 |
2010年 | 13篇 |
2009年 | 19篇 |
2008年 | 18篇 |
2007年 | 3篇 |
2006年 | 3篇 |
2005年 | 4篇 |
2004年 | 3篇 |
2003年 | 3篇 |
2002年 | 4篇 |
2001年 | 3篇 |
1999年 | 1篇 |
1995年 | 2篇 |
1993年 | 1篇 |
1992年 | 1篇 |
1991年 | 1篇 |
1983年 | 1篇 |
排序方式: 共有228条查询结果,搜索用时 15 毫秒
71.
Mansour H. Almatarneh Abd Al‐Aziz A. Abu‐Saleh Kabir M. Uddin Raymond A. Poirier Peter L. Warburton 《International journal of quantum chemistry》2017,117(3):180-189
A detailed computational study of the deamination reaction of melamine by OH–, n H2O/OH–, n H2O (where n = 1, 2, 3), and protonated melamine with H2O, has been carried out using density functional theory and ab initio calculations. All structures were optimized at M06/6‐31G(d) level of theory, as well as with the B3LYP functional with each of the basis sets: 6‐31G(d), 6‐31 + G(d), 6‐31G(2df,p), and 6‐311++G(3df,3pd). B3LYP, M06, and ω B97XD calculations with 6‐31 + G(d,p) have also been performed. All structures were optimized at B3LYP/6‐31 + G(d,p) level of theory for deamination simulations in an aqueous medium, using both the polarizable continuum solvation model and the solvation model based on solute electron density. Composite method calculations have been conducted at G4MP2 and CBS‐QB3. Fifteen different mechanistic pathways were explored. Most pathways consisted of two key steps: formation of a tetrahedral intermediate and in the final step, an intermediate that dissociates to products via a 1,3‐proton shift. The lowest overall activation energy, 111 kJ mol?1 at G4MP2, was obtained for the deamination of melamine with 3H2O/OH?. 相似文献
72.
Samantha J Cloake Her Shuang Toh Patricia T Lee Chris Salter Colin Johnston Richard G Compton 《ChemistryOpen》2015,4(1):22-26
The influence of nanoparticle aggregation on anodic stripping voltammetry is reported. Dopamine-capped silver nanoparticles were chosen as a model system, and melamine was used to induce aggregation in the nanoparticles. Through the anodic stripping of the silver nanoparticles that were aggregated to different extents, it was found that the peak area of the oxidative signal corresponding to the stripping of silver to silver(I) ions decreases with increasing aggregation. Aggregation causes incomplete stripping of the silver nanoparticles. Two possible mechanisms of ‘partial oxidation’ and ‘inactivation’ of the nanoparticles are proposed to account for this finding. Aggregation effects must be considered when anodic stripping voltammetry is used for nanoparticle detection and quantification. Hence, drop casting, which is known to lead to aggregation, is not encouraged for preparing electrodes for analytical purposes. 相似文献
73.
Emilie S. Vosegaard Dr. Maja K. Thomsen Dr. Lennard Krause Thomas B. E. Grønbech Dr. Aref Mamakhel Seiya Takahashi Prof. Dr. Eiji Nishibori Prof. Dr. Bo B. Iversen 《Chemistry (Weinheim an der Bergstrasse, Germany)》2022,28(54):e202201295
Melamine is a precursor and building block for graphitic carbon nitride (g-CN) materials, a group of layered materials showing great promise for catalytic applications. The synthetic pathway to g-CN includes a polycondensation reaction of melamine by evaporation of ammonia. Melamine molecules in the crystal organize into wave-like planes with an interlayer distance of 3.3 Å similar to that of g-CN. Here we present an extensive investigation of the experimental electron density of melamine obtained from modelling of synchrotron radiation X-ray single-crystal diffraction data measured at 25 K with special focus on the molecular geometry and intermolecular interactions. Both intra- and interlayer structures are dominated by hydrogen bonding and π-interactions. Theoretical gas-phase optimizations of the experimental molecular geometry show that bond lengths and angles for atoms in the same chemical environment (C−N bonds in the ring, amine groups) differ significantly more for the experimental geometry than for the gas-phase-optimized geometries, indicating that intermolecular interactions in the crystal affects the molecular geometry. In the experimental crystal geometry, one amine group has significantly more sp3-like character than the others, hinting at a possible formation mechanism of g-CN. Topological analysis and energy frameworks show that the nitrogen atom in this amine group participates in weak intralayer hydrogen bonding. We hypothesize that melamine condenses to g-CN within the layers and that the unique amine group plays a key role in the condensation process. 相似文献
74.
以三聚氰胺和氰尿酸为原料,以水为溶剂,考察了高压法合成阻燃剂MCA的工艺。在反应压力为120 KPa、三聚氰胺与氰尿酸的摩尔比约为1∶1.02下,采用改变单一因素的方法探讨了在不同温度、压力、反应时间及不同p H值等情况下对反应结果的影响,得出较优的工艺条件为反应时间为1.5 h、反应的最佳温度为105℃、p H值保持在7左右最佳,在此条件下,产品纯度可达98%以上,产品质量稳定。 相似文献
75.
A. I. Konovalov I. S. Ryzhkina L. I. Murtazina A. P. Timosheva R. R. Shagidullin A. V. Chernova L. V. Avvakumova S. G. Fattakhov 《Russian Chemical Bulletin》2008,57(6):1231-1238
Bis(hydroxymethyl)phosphinic acid and melamine were shown by FTIR and UV spectroscopy to form the salt (melafen), whose cations
and anions involving water molecules can be joined into supramolecular structures due to electrostatic interactions and hydrogen
bonds. The conductometry, dielcometric titration, and dynamic light scattering methods showed that melafen in water and chloroform
in a concentration range of 10−10–10−4 mol L−1 involving the solvent structures exist as supramolecular polymeric nanostructures, whose size and properties change nonlinearly
depending on the melafen concentration.
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 1207–1214, June, 2008. 相似文献
76.
77.
三聚氰胺焦磷酸盐阻燃剂的合成及性能表征 总被引:2,自引:1,他引:2
三聚氰胺焦磷酸盐自膨胀阻燃剂[1]无卤、低烟、低毒,其热解温度与被阻燃材料的降解温度的匹配,效果很好。本文合成了三聚氰胺焦磷酸盐(MPP)[2],用元素分析,XRD、TG、DSC、IR等表征,并对其阻燃性能进行了测定,讨论了阻燃机理。1 实验部分1.1 仪器与试剂三聚氰胺(化学纯,含量不少于98.5%,北京化工厂);焦磷酸钠(Na4P2O7·10H2O,分析纯,含量不少于99.0%,上海科昌精细化学品公司);浓硫酸(分析纯,成都新都金牛王化工厂);硝酸(分析纯,成都有机化学试剂厂)。岛津TG 50H热重分析仪和DSC 50H差热分析仪;PerKin Eimer240C元素分析仪;… 相似文献
78.
An anionic poly(urethane–urea) dispersion (PUD) was cross-linked with different amount of partially methylated melamine formaldehyde
(PMMF). The isothermal curing behavior was observed by a rigid-body pendulum rheometer. The test results showed that cure
response of PUD cross-linked with PMMF was a function of the concentration of PMMF. Also, PMMF self-condensation could take
place during the curing process. In this experiment, the anionic poly(urethane–urea) dispersion has a large number of >N–H
cross-linking or branching sites in urethane and urea groups per molecule that allow a large number of PMMF to couple into
elastic PUD backbone to form branched structure with partial cross-linking. The dynamic mechanical properties of PUD cross-linked
with PMMF were affected by the concentration of PMMF. It was further shown that the tensile properties were strongly influenced
by the concentration of PMMF and curing temperature. 相似文献
79.
80.