chap2_9

chap2_9 - 材材材材材材 Fundamental of Materials Prof:...

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Unformatted text preview: 材材材材材材 Fundamental of Materials Prof: Tian Min Bo Prof: Tian Tel: 62795426 材 62772851 E-mail: tmb@mail.tsinghua.edu.cn tmb@mail.tsinghua.edu.cn Department of Material Science and Engineering Tsinghua University. Beijing 100084 Lesson seven §2.9 Some Additional Terminology in Crystallography Crystallography Ⅰ. Single crystal versus polycrystal 1.. For single crystals there is only one type of structure with 1 same orientation. same Single crystals is characterized by anisotropy. Single 2.. Polycrystal consists of a lot of grain, small single crystals, 2 with same structure but different orientations. These small single crystals are called grains. The boundaries between them are called grain boundaries. grain grain boundary single crystal polycrystal Ⅱ. Allotropy Same crystal has different crystallographic structure at different condition. structure Examples: C , graphite , diamond , CNT Examples: α − Fe BCC 910°C γ − Fe FCC 1400°C δ − Fe BCC 1540°C Liquid Graphite C60 molecule Carbon Nanotubes Ⅲ. Atomic radii Definition: S AB r= 2 Examples and Discussions Examples Exercise Exercise Thank you ! Thank you ! 7 ...
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