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相关概念视频

Ionic Crystal Structures02:42

Ionic Crystal Structures

18.0K
Ionic crystals consist of two or more different kinds of ions that usually have different sizes. The packing of these ions into a crystal structure is more complex than the packing of metal atoms that are the same size.
Most monatomic ions behave as charged spheres, and their attraction for ions of opposite charge is the same in every direction. Consequently, stable structures for ionic compounds result (1) when ions of one charge are surrounded by as many ions as possible of the opposite...
18.0K
Metallic Solids02:37

Metallic Solids

16.4K
Metallic solids such as crystals of copper, aluminum, and iron are formed by metal atoms. The structure of metallic crystals is often described as a uniform distribution of atomic nuclei within a “sea” of delocalized electrons. The atoms within such a metallic solid are held together by a unique force known as metallic bonding that gives rise to many useful and varied bulk properties.
All metallic solids exhibit high thermal and electrical conductivity, metallic luster, and...
16.4K
Valence Bond Theory02:42

Valence Bond Theory

8.9K
Coordination compounds and complexes exhibit different colors, geometries, and magnetic behavior, depending on the metal atom/ion and ligands from which they are composed. In an attempt to explain the bonding and structure of coordination complexes, Linus Pauling proposed the valence bond theory, or VBT, using the concepts of hybridization and the overlapping of the atomic orbitals. According to VBT, the central metal atom or ion (Lewis acid) hybridizes to provide empty orbitals of suitable...
8.9K
Radicals: Electronic Structure and Geometry01:07

Radicals: Electronic Structure and Geometry

4.1K
This lesson delves into the geometry of a radical, which is influenced by the electronic structure of the molecule. The principle is similar to that of a lone pair, where the unpaired electron influences the geometry at the radical center.
Accordingly, the structure of a trivalent radical lies between the geometries of carbocations and carbanions. An sp2-hybridized carbocation is trigonal planar, while an sp3-hybridized carbanion is trigonal pyramidal. Here, the difference in geometry is...
4.1K
Polymers: Molecular Weight Distribution01:10

Polymers: Molecular Weight Distribution

4.0K
For any given polymer, the weight average molecular weight (Mw) is higher than, if not equal to, the number average molecular weight (Mn). The only situation in which the weight average molecular weight and the number average molecular weight are equal is when a polymer consists only of chains with equal molecular weight. However, this never happens in a synthetic polymer, since it is difficult to control the polymerization process up to a molecular level with accuracy to a hundred percent.
4.0K
Types of Semiconductors01:20

Types of Semiconductors

1.8K
Intrinsic semiconductors are highly pure materials with no impurities. At absolute zero, these semiconductors behave as perfect insulators because all the valence electrons are bound, and the conduction band is empty, disallowing electrical conduction. The Fermi level is a concept used to describe the probability of occupancy of energy levels by electrons at thermal equilibrium. In intrinsic semiconductors, the Fermi level is positioned at the midpoint of the energy gap at absolute zero. When...
1.8K

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相关实验视频

Updated: May 3, 2026

Characterization of Nanocrystal Size Distribution using Raman Spectroscopy with a Multi-particle Phonon Confinement Model
06:54

Characterization of Nanocrystal Size Distribution using Raman Spectroscopy with a Multi-particle Phonon Confinement Model

Published on: August 22, 2015

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在环恒星碳化中多种类型的分布

T L Daulton1, T J Bernatowicz, R S Lewis

  • 1Materials Science Division, Argonne National Laboratory, Argonne IL, 60439-4838, USA. tdaulton@nrlssc.navy.mil

Science (New York, N.Y.)
|June 8, 2002
PubMed
概括

碳化 (SiC) 的天文红外光谱存在争议. 从石中直接分析 presolar SiC 发现只有两个常见的多类型,简化了以前的模型.

科学领域:

  • * 天体化学和材料科学.
  • *对前太阳颗粒的表征.
  • * 恒星进化和核合成.

背景情况:

  • *环恒星碳化 (SiC) 的晶体结构受到争论,天文红外光谱表明了多种类型的复杂混合物.
  • *以前的研究依赖于间接的光谱分析,导致对恒星外流中形成的实际SiC结构的不确定性.

研究的目的:

  • *直接确定环恒星碳化 (SiC) 的多类型分布.
  • *为了解决围绕SiC的争议,从天文数据推断出SiC的结晶学类.
  • * 了解SiC在恒星环境中的形成条件.

主要方法:

  • *使用传输电子显微镜 (TEM) 进行直接结构分析.
  • *对从默奇森碳化石中提取的前太阳碳化 (SiC) 颗粒的检查.
  • * 在样本中存在的SiC多种类型的识别和量化.

主要成果:

  • *只有两种SiC多种类型被确定:立方3C和六角2H.
  • * 这两种多种类型的交叉生长也被观察到.
  • *观察到的结构简单性与光谱分析表明的多种多样性形成鲜明对比.

结论:

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Fabrication and Optimization of Type II Silicon Clathrate Films
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Characterization of Nanocrystal Size Distribution using Raman Spectroscopy with a Multi-particle Phonon Confinement Model

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Fabrication and Optimization of Type II Silicon Clathrate Films

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  • * 限定的多类型分布归因于环恒星外流中的低压.
  • * 在这些环境中,碳化的低凝结温度有利于仅形成3C和2H多种类型.
  • * 这一发现简化了我们对进化恒星中SiC形成的理解.