排序方式: 共有86条查询结果,搜索用时 15 毫秒
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用磁控溅射方法制各纯Fe薄膜,并硫化合成FeS2.采用同步辐射X射线近边吸收谱与X射线光电子能谱研究了薄膜的电子结构.结果表明,合成的FeS2薄膜,在费米能级附近,有较强的Fe 3d态密度存在,同时,在价带谱中2-10eV处有强度较大的S 3p态密度存在;Fe的3d轨道在八面体配位场作用下分别为t2g和eg轨道,实验中由Fe的吸收谱计算得到两分裂能级之差为2.1eV;实验测得FeS2价带结构中导带宽度约为2.4eV,导带上方仍存在第二能隙,其宽度约为2.8eV. 相似文献
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以硝酸亚铈(Ce(NO3)3·6H2O)和正硅酸四乙酯(C8H20O4Si)为前驱体,采用溶胶-凝胶法合成了系列具有大比表面积的xCeO2-(1-x)SiO2(x = 0,0.25,0.50,0.75,1)复合氧化物载体,然后浸渍活性组分Ni制得用于甲烷部分氧化制合成气的Ni催化剂。运用N2物理吸附-脱附、X射线粉末衍射、扫描电镜、紫外-可见漫反射光谱、氢程序升温还原、氨程序升温脱附和热重等手段对所得催化剂的组织结构、还原性、表面酸性和积炭行为等进行了表征;同时考察了催化剂的组成、焙烧温度和反应时间等对催化剂在甲烷部分氧化制合成气中催化性能的影响。表征结果表明,该系列Ni/CeO2-SiO2催化剂具有大比表面积,CeO2晶粒较小,NiO的分散性好且易被还原,表面酸性弱,不容易积炭。当Ce/Si摩尔比为1:1,活性组分Ni的质量分数为10%,焙烧温度为700℃时,所制备的Ni/CeO2-SiO2催化剂表现出较好的稳定性、最高的CH4转化率(~84%)和对产物CO及H2的选择性(>87%)。 相似文献
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针对惯例复制过程中与外部情景难以匹配的复制困境,分析了常规惯例复制和柔性惯例复制对知识转移的单独以及联合作用的关系悖论,对比分析作用机制在网络闭合性和环境不确定性约束下的权变性影响。以研发密集型企业为对象,运用多元回归分析进行实证检验。结果表明:网络闭合性和环境不确定性是惯例复制对知识转移的单独以及联合作用的权变条件;网络闭合性强化了常规惯例复制机制的促进作用,但限制了柔性惯例复制机制的效能发挥;环境不确定性削弱了常规惯例复制机制的促进作用,对柔性惯例复制机制的作用效果不显著;网络闭合性低或环境不确定性低时,两种惯例复制机制的联合作用相互补充,而网络闭合性高或环境不确定性高时,两种惯例复制机制相互替代。研究结论有助于揭示惯例复制的权变作用,对提升技术创新网络的知识转移效率具有重要意义。 相似文献
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Accurate attenuation correction is required in dedicated breast PET imaging systems for image artifact removal and quantitative studies. In this study, a method using only emission data based on consistency conditions is proposed for attenuation correction in breast PET imaging systems. The consistency conditions are exploited to evaluate the accuracy of the estimated attenuation distribution and find the appropriate parameters that yield the most consistent attenuation distribution with the measured emission data. We have proved the validity of the method with experimental investigations and single-patient studies using a dedicated breast PET. The results show that the method is capable of accurately estimating the attenuation distribution of a uniform attenuator from the experimental data with various relatively low activities. The results also show that in single-patient studies, the method is robust for the irregular boundary of breast tissue and provides a distinct improvement in image quality. 相似文献
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以石墨粉为原料,采用Hummers氧化法合成氧化石墨烯(GO).然后在超声作用下,将不同含量的Ag3PO4沉积在GO上,制备了一系列4% (w,质量分数) GO/Ag3PO4、8% GO/Ag3PO4、16% GO/Ag3PO4和32% GO/Ag3PO4复合光催化剂.对所制备的光催化剂运用N2物理吸附、X射线粉末衍射(XRD)、扫描电子显微镜(SEM)、透射电子显微镜(TEM)、拉曼光谱、傅里叶变换红外(FT-IR)光谱、紫外-可见漫反射吸收光谱(UV-Vis DRS)等手段进行了表征,并在可见光下考察了GO含量对Ag3PO4光催化降解甲基橙(MO)的性能.结果表明, GO能够和Ag3PO4实现均匀复合.复合GO提高了催化剂的比表面积,改善了催化剂的吸附性能.复合16% GO使Ag3PO4光催化活性提高最显著, 120 min内对MO的降解率达到83%,是纯Ag3PO4光催化活性的7.5倍. GO能提高催化剂的比表面积,促进光生电子-空穴(e-/h+)的分离,产生更多活性自由基,从而提高Ag3PO4光催化的活性和稳定性. 相似文献
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简要介绍了基于飞行时间(Time of Flight, 简称TOF)技术的正电子发射断层扫描成像(Positron Emission Tomography, 简称PET)的基本原理, 回顾了TOF-PET成像设备的发展历史, 讨论了影响TOF PET时间分辨率的主要因素, 并对TOF-PET给图像重建技术带来的挑战进行了分析。 最后, 重点介绍了TOF-PET技术所带来的优势, 使用TOF技术可以有效地改善图像质量, 并且为临床诊断和临床前研究带来便利。 The principle of time of flight (TOF) positron emission tomography (PET) and a brief review of the history of TOF-PET are introduced. The factors influencing the time resolution of a TOF-PET scanner are presented, especially focus on the intrinsic properties of scintillators and front end electronics. Challenges and achievements of the structure of data organization and image reconstruction are reviewed. Finally, the benefits of TOF-PET on image quality improvement and tumor detection are emphasized. 相似文献
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A dedicated breast CT system (DBCT) is a new method for breast cancer detection proposed in recent years. In this paper, the glandular dose in the DBCT is simulated using the Monte Carlo method. The phantom shape is half ellipsoid, and a series of phantoms with different sizes, shapes and compositions were constructed. In order to optimize the spectra, monoenergy X-ray beams of 5-80 keV were used in simulation. The dose distribution of a breast phantom was studied: a higher energy beam generated more uniform distribution, and the outer parts got more dose than the inner parts. For polyenergtic spectra, four spectra of Al filters with different thicknesses were simulated, and the polyenergtic glandular dose was calculated as a spectral weighted combination of the monoenergetic dose. 相似文献