全文获取类型
收费全文 | 319篇 |
免费 | 60篇 |
国内免费 | 69篇 |
专业分类
化学 | 251篇 |
晶体学 | 17篇 |
综合类 | 2篇 |
物理学 | 178篇 |
出版年
2023年 | 2篇 |
2022年 | 5篇 |
2021年 | 10篇 |
2020年 | 5篇 |
2019年 | 6篇 |
2018年 | 10篇 |
2017年 | 11篇 |
2016年 | 11篇 |
2015年 | 11篇 |
2014年 | 21篇 |
2013年 | 37篇 |
2012年 | 13篇 |
2011年 | 37篇 |
2010年 | 23篇 |
2009年 | 32篇 |
2008年 | 18篇 |
2007年 | 22篇 |
2006年 | 26篇 |
2005年 | 25篇 |
2004年 | 23篇 |
2003年 | 15篇 |
2002年 | 11篇 |
2001年 | 7篇 |
2000年 | 11篇 |
1999年 | 8篇 |
1998年 | 7篇 |
1997年 | 5篇 |
1996年 | 6篇 |
1995年 | 9篇 |
1994年 | 6篇 |
1993年 | 2篇 |
1992年 | 5篇 |
1991年 | 3篇 |
1988年 | 1篇 |
1986年 | 1篇 |
1985年 | 2篇 |
1980年 | 1篇 |
排序方式: 共有448条查询结果,搜索用时 15 毫秒
51.
分子筛改性对一步法合成二甲醚的影响 总被引:5,自引:2,他引:3
采用浸渍法制备了MgO、CaO、ZnO改性的HZSM 5分子筛,并以改性HZSM 5为脱水剂与JC207甲醇合成催化剂组成双功能催化剂,在固定床反应器上考察了其对一步法合成二甲醚影响。结果表明,适量碱性氧化物的引入,引起分子筛表面的B酸中心(强酸中心)向L酸中心(弱酸中心)转变,而弱酸和中强酸中心是甲醇脱水生成二甲醚的活性中心,强酸中心会造成二甲醚进一步脱水生成烃类副产品,所以改性后产物中二氧化碳和烃类的选择性下降,二甲醚选择性升高。这种趋势在CaO/HZSM 5脱水剂上表现的更为明显。 相似文献
52.
Catalytic direct dehydrogenation of methanol to formaldehyde was carried out over Ag‐SiO2‐MgO‐Al2O3 catalysts prepared by sol‐gel method. The optimal preparation mass fractions were determined as 8.3% MgO, 16.5% Al2O3 and 20% silver loading. Using this optimum catalyst, excellent activity and selectivity were obtained. The conversion of methanol and the selectivity to formaldehyde both reached 100%, which were much higher than other previously reported silver supported catalysts. Based on combined characterizations, such as X‐ray diffraction (XRD), scanning electronic microscopy (SEM), diffuse reflectance ultraviolet‐visible spectroscopy (UV‐Vis, DRS), nitrogen adsorption at low temperature, temperature programmed desorption of ammonia (NH3‐TPD), desorption of CO2 (CO2‐TPD), etc., the correlation of the catalytic performance to the structural properties of the Ag‐SiO2‐ MgO‐Al2O3 catalyst was discussed in detail. This perfect catalytic performance in the direct dehydrogenation of methanol to formaldehyde without any side‐products is attributed to its unique flower‐like structure with a surface area less than 1 m2/g, and the strong interactions between neutralized support and the nano‐sized Ag particles as active centers. 相似文献
53.
In response to recent helium atom scattering (HAS) and neutron scattering results, Monte Carlo simulations and perturbation theory calculations have been performed for D2 on MgO(0 0 1). Monte Carlo simulations predict that D2 molecules form a series of interesting structures, p(2 × 2) → p(4 × 2) → p(6 × 2), with coverages Θ = 0.5, 0.75, 0.83 respectively, and followed by a formation of a top layer of p(6 × 2) unit cell symmetry. The three types of mono-layers are stable up to 13 K, whereas the top layer still exists up to 10 K. This is in partial agreement with the neutron scattering and HAS results that report c(2 × 2), c(4 × 2) and c(6 × 2); they agree in terms of coverage and stability, but disagree in terms of symmetry. A quantum mechanical examination of the D2 molecules’ rotational motion shows the molecular axes are azimuthally delocalized and hence the simulated structures are c-type rather than p-type. These calculations also indicate that ortho-D2 and helicoptering para-D2 prefer cationic sites, while cartwheeling para-D2 prefers anionic sites. 相似文献
54.
采用直流磁控溅射并通过优化工艺参数,在(100)Si衬底上成功制备了高度(100)择优的MgO薄膜和MgO/TiN双层膜结构.对 (100)MgO择优取向温度影响机理做了详细讨论,并利用XRD,AFM,FESEM等手段研究了在(100)Si和(100)TiN/Si两种衬底上,不同工艺条件下MgO薄膜的表面和断面微观结构,表征了MgO薄膜的柱状生长结构和与TiN薄膜的良好外延关系.在对薄膜光学特性的研究中,利用Sellmeier模型获得了Si上MgO薄膜在可见光波段的折射率参数(550 nm处折射率为1.6
关键词:
MgO薄膜
择优取向
直流溅射
折射率拟合 相似文献
55.
对基于周期极化掺镬铌酸锂晶体的信号光单谐振光学参量振荡器的输出特性进行了实验研究.讨论了光学参量振荡器谐振腔的腔长、周期极化铌酸锂晶体的通光长度、输出镜的透过率以及抽运光的脉冲宽度对光学参量振荡器谐振阈值的影响.光学参量振荡器的抽运源采用输出波长为1 064 nm的声光调Q Nd:YVO_4激光器,在重复频率为2 kHz、周期极化掺镁铌酸锂晶体的温度为30℃的条件下,光学参量振荡器的振荡阈值仅为48 mW.当抽运功率为94 mW时获得了25 mW的信号光输出,其光-光转换效率为26.6%. 相似文献
56.
María M. Branda Angel Horacio Rodríguez Norberto J. Castellani 《Surface science》2009,603(8):1093-1098
In this work, the ethanol adsorption on a perfect MgO(1 0 0) surface, and also on topologic surface defects of MgO, is studied. Terrace, edge and corner sites were analyzed, whose O and Mg ions are five, four and three fold coordinated, respectively. All the calculations were performed using a cluster approach and the DFT based method. The ethanol molecule chemisorbs non-dissociatively on terrace sites of MgO by means of a two fold interaction while strong dissociative chemisorptions are produced on edge and corner sites. The weakened alcohol OH group is always oriented so that its oxygen (Oa) atom is linked to a Mg cation and the H atom to a surface O anion. The MgOa distance for edge and corner sites is smaller than that for the terrace site. This indicates that lowering the coordination number of ions in the adsorption site yields an increase of molecule-surface bond strength in agreement with a greater basicity of low coordinate sites. The behavior of the adsorption energy and the charge transfers are in accord with the idea of a strong basic character for the MgO substrate, which is more pronounced as the coordination number of ions decrease. 相似文献
57.
58.
以MgO为缓冲层的硅基铌酸锶钡薄膜取向特性与其波导结构的设计研究 总被引:1,自引:0,他引:1
采用溶胶-凝胶法在Si(100)基片上制备出择优取向的MgO薄膜,随后在其上生长出具有择优取向的铌酸锶钡铁电薄膜.实验发现,MgO缓冲层的应用可以大大提高SBN薄膜的择优取向性能.同时,用五层对称理想波导耦合模理论,以SBN为波导层,分析了波导损耗与厚度的关系. 通过对计算出的理想结果与实际相结合,以及对SBN在生长过程工艺与损耗关系的研究,制备出高质量、低损耗的SBN薄膜,为其在电光波导调制器等微系统中的应用打下良好的基础. 相似文献
59.
Ilya V. Mishakov Ekaterina V. Ilyina Alexander F. Bedilo Aleksey A. Vedyagin 《Reaction Kinetics and Catalysis Letters》2009,97(2):355-361
Nanocrystalline aerogel VOx/MgO catalysts for the oxidative dehydrogenation of propane with high surface area and uniform vanadium distribution were
synthesized by co-gelation followed by supercritical drying. The catalysts were shown to have superior performance compared
to nanocrystalline VOx/MgO catalysts prepared by impregnation. 相似文献
60.
Changhyun Jin Jungkeun Lee Sunghoon Park Chongmu Lee 《Crystal Research and Technology》2011,46(3):315-320
MgO nanorods were grown by the thermal evaporation of Mg3N2 powders on the Si (100) substrate coated with a gold thin film. The MgO nanorods grown on the Si (100) substrate were a few tens of nanometers in diameter and up to a few hundreds of micrometers in length. MgO/SiO2 core‐shell nanorods were also fabricated by the sputter‐deposition of SiO2onto the MgO nanorods. Transmission electron microscopy (TEM) and X–ray diffraction (XRD) analysis results indicated that the cores and shells of the annealed core‐shell nanorods were a face‐centered cubic‐type single crystal MgO and amorphous SiO2, respectively. The photoluminescence (PL) spectroscopy analysis results showed that SiO2 coating slightly decreased the PL emission intensity of the MgO nanorods. The PL emission of the MgO/SiO2 core‐shell nanorods was, however, found to be considerably enhanced by thermal annealing and strongly depends on the annealing atmosphere. The PL emission of the MgO/SiO2 core‐shell nanorods was substantially enhanced in intensity by annealing in a reducing atmosphere, whereas it was slightly enhanced by annealing in an oxidative atmosphere. The origin of the PL enhancement by annealing in a reducing atmosphere is discussed with the aid of energy‐dispersive X‐ray spectroscopy analyses. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献