全文获取类型
收费全文 | 60篇 |
免费 | 24篇 |
国内免费 | 19篇 |
专业分类
化学 | 35篇 |
晶体学 | 13篇 |
力学 | 5篇 |
综合类 | 2篇 |
数学 | 9篇 |
物理学 | 39篇 |
出版年
2023年 | 1篇 |
2022年 | 2篇 |
2021年 | 3篇 |
2020年 | 6篇 |
2018年 | 4篇 |
2017年 | 5篇 |
2016年 | 3篇 |
2015年 | 5篇 |
2014年 | 4篇 |
2013年 | 3篇 |
2012年 | 4篇 |
2011年 | 6篇 |
2010年 | 3篇 |
2009年 | 4篇 |
2008年 | 6篇 |
2007年 | 10篇 |
2006年 | 11篇 |
2005年 | 4篇 |
2003年 | 1篇 |
2002年 | 4篇 |
2001年 | 1篇 |
2000年 | 5篇 |
1996年 | 4篇 |
1994年 | 2篇 |
1992年 | 1篇 |
1990年 | 1篇 |
排序方式: 共有103条查询结果,搜索用时 375 毫秒
11.
Wu GP Wei SH Ren WM Lu XB Xu TQ Darensbourg DJ 《Journal of the American Chemical Society》2011,133(38):15191-15199
Selective transformations of carbon dioxide and epoxides into biodegradable polycarbonates by the alternating copolymerization of the two monomers represent some of the most well-studied and innovative technologies for potential large-scale utilization of carbon dioxide in chemical synthesis. For the most part, previous studies of these processes have focused on the use of aliphatic terminal epoxides or cyclohexene oxide derivatives, with only rare reports concerning the synthesis of CO(2) copolymers from epoxides containing electron-withdrawing groups such as styrene oxide. Herein we report the production of the CO(2) copolymer with more than 99% carbonate linkages from the coupling of CO(2) with epichlorohydrin, employing binary and bifunctional (salen)cobalt(III)-based catalyst systems. Comparative kinetic studies were performed via in situ infrared measurements as a function of temperature to assess the activation barriers for the production of cyclic carbonate versus copolymer involving two electronically different epoxides: epichlorohydrin and propylene oxide. The relative small activation energy difference between copolymer versus cyclic carbonate formation for the epichlorohydrin/CO(2) process (45.4 kJ/mol) accounts in part for the selective synthesis of copolymer to be more difficult in comparison with the propylene oxide/CO(2) case (53.5 kJ/mol). Direct observation of the propagating polymer-chain species from the binary (salen)CoX/MTBD (X = 2,4-dinitrophenoxide and MTBD = 7-methyl-1,5,7-triazabicyclo[4.4.0]dec-5-ene) catalyst system by means of electrospray ionization mass spectrometry confirmed the perfectly alternating nature of the copolymerization process. This observation in combination with control experiments suggests possible intermediates involving MTBD in the CO(2)/epichlorohydrin copolymerization process. 相似文献
12.
磨西河流域泥石流流域形态的非线性特征 总被引:1,自引:0,他引:1
泥石流是磨西河流域地质环境演化的一个重要地表过程,研究该流域内泥石流的地貌发育及演化特征可为磨西河流域第四纪以来地质环境的演变提供重要信息。通过对流域51条泥石流沟地貌形态的统计分析发现,磨西河流域内泥石流的分布在空间上表现出明显的不均性,流域内泥石流沟的数量与对应的主沟长度、主沟纵比降及流域面积之间存在着明显的分形现象,具有较高的自相似性。结果表明,泥石流沟谷形态的非线性演化特征及现象是对磨西河流域冰川环境和地质构造环境响应的直接结果,主要体现在现代冰川对流域的强烈侵蚀和贡嘎山隆升而导致流域的强烈下切。研究结果可为流域内泥石流灾害的形成机理、防灾减灾及流域环境演化提供理论依据。 相似文献
13.
采用光化学反应法在稀酸条件下制备出薄片状溴氧化铋(BiOBr),将其分散于含有过硫酸铵和十六烷基三甲基溴化铵的水溶液中,通过吡咯的一步聚合反应原位制备出聚吡咯(PPy)修饰的BiOBr复合材料(BiOBr/PPy)。通过扫描电子显微镜、透射电子显微镜、X射线衍射、拉曼光谱、X射线光电子能谱、紫外可见光谱及荧光光谱等综合表征技术对样品的晶体结构、形貌特征和光电特性等进行测试。结果显示,PPy成功修饰到BiOBr薄片上,BiOBr与PPy接触紧密且相互作用强。与纯BiOBr相比,BiOBr/PPy复合材料具有更强的可见光吸收效率和增强的光催化降解甲基橙(MO)染料活性。通过优化PPy和BiOBr的组合比例,当BiOBr质量分数约为7%时,BiOBr/PPy-2在50 min内对MO (30 mg·L-1)的降解率为87.3%;另外,循环光催化活性虽有降低但仍高于纯BiOBr和纯PPy (10.4%)。这表明BiOBr与PPy之间较强的相互作用和良好的界面结合可以有效地促进光生电子与空穴的分离效率。反应体系中分离的光生空穴、衍生自由基在染料氧化降解中发挥了重要作用。 相似文献
14.
15.
An electrostatic deflector for separating the fusion evaporation residues from the beam-like products in heavy ion reactions was installed. The evaporation residue separation and identification with the electrostatic deflector setup was tested with the reaction ^32S+^96Zr at several energies. The fusion evaporation residues and the beam-like particles were well separated after the electrical separation and the experimental fusion cross section obtained from the angular distribution is in good agreement with the calculated value well above the Coulomb barrier. This confirms the reliability of the setup. 相似文献
16.
以芒硝(Na2SO4· 10H2O)为主相变材料,采用物理共混法,筛选出合适添加辅助材料组成多元混盐体系,制备出相变温度在25℃左右的Na2SO4· 10H2O基复合相变材料,研究了原料配比对相变材料相变温度和相变潜热的影响.通过热重分析、T-history曲线、差示扫描量热法等表征了制备的芒硝基相变储能材料性质.结果表明:二元相变材料相变温度随着Na2CO3·10H2O增加而先降低后增大;相变材料相变潜热密度基本不变.三元相变材料相变温度随着NaCl、KCl、NH4Cl量增加相变温度降低;相变材料相变潜热密度相比于二元体系有所减小,且在一定范围内随着成核剂硼砂含量的增加该体系相变潜热密度基本保持不变,过冷度有明显下降.当Na2SO4· 10H2O量为86.4;、Na2CO3 · 10H2O量为9.6;、NaCl量为4;,对该配方改性添加3;的硼砂和0.1;的羧甲基纤维素钠时,此相变材料的相变温度为24.6℃,过冷度为0.3℃,相变潜热密度为179.6 J/g. 相似文献
17.
18.
19.
Chang-Hong Yang Xiu-Feng Cheng Xiu-Jie Yi Guang-Peng Ma Fu-Yi Jiang Jian-Ru Han 《Solid State Communications》2005,134(7):461-464
K0.5Bi0.5TiO3 thin films were deposited on fused quartz, n-type Si(100) and Pt/TiO2/SiO2/Si substrates by repeated coating/dying cycles. X-ray diffraction analysis shows that the films annealed at 700 °C for 10 min present perovskite phase. Atomic force microscopy reveals that the surface morphology is smooth, the grain sizes of the films on Si(100) are quite larger than on fused quartz. The capacitance-voltage hysteresis loops at various sweeping speed are collected as are polarization types. The films in the ON and OFF states are relatively stable. The films also exhibit a hysteresis loop at an applied voltage of 4 V, with a remanent polarization of 9.3 μC/cm2 and a coercive voltage of 2 V. The insulating property of negative bias voltage is better than that of positive bias voltage. The transmittance of the films is between 74 and 82% in the wavelength range of 200-2000 nm. 相似文献
20.
A regioselective, solvent-free and catalyst-free synthesis of isoxazoles has been successfully developed under ball milling conditions. Milling the mixtures of N-hydroxybenzimidoyl chlorides and enamino carbonyl compounds in a ball mill at a frequency of 14.6?Hz for 20–60?min afforded isoxazoles in up to 86% yields. A possible reaction mechanism leading to the formation of the observed isoxazoles is proposed. 相似文献