2-Chemical Structure of Materials

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Chemical structure of

materials

1. Types of chemical bond2. Chemical structures of materials

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Types of chemical bond

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Atomic structure

• Bohr model

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Atomic structure

• Orbital

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Valence electrons

&

periodic table5

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Atomic bond

• Ftotal

 –  Attractive force(FA)

 –  Repulsive force(FR)

• Bond length

• Bond energy

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Primary bond

• Ionic bond

• Covalent bond

• Metallic bond

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Ionic bond

• Electronegativity(전기음성도) 차이가 큰 원소간 결합

 –  Alkali metal-halogen

• Electrostatic attraction, Coulombic force

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Ionic bond

• Potential energy versus internuclear distance

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Ionic bond

• Isotropic

• Ionic crystal

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Covalent bond

• Electronegativity(전기음성도) 차이가 적은 비금속 원소간 결합

• Valence electrons 서로 공유

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Covalent bond

• 원자중심과 공유전자 간에 존재하게 되는 힘

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Covalent bond

• Potential energy versus internuclear distance

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Covalent bond

• Three different ways to form an electron-pair bond

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Covalent bond

• Hybridization of s and p orbitals

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Covalent bond

• Hybridization of s and p orbitals

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Covalent bond

• σ bond, π bond

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Covalent bond

• σ bond, π bond

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Metallic bond

• 금속 원자간 결합

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Metallic bond

• Electron-sea model

• Isotropic

• Ion core와 전자구름 간의 정전기적 인력

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Secondary bond

• 전자의 이동이나 공유를 수반하지 않는 분자간 인력

• Primary bond에 비해 약한 결합력

• 분자적 쌍극자(dipole) : 일시적, 영구적(극성분자)

• Van der Waals bond

• Hydrogen bond

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Secondary bond

• Hydrogen bond

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Bonding energy25

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Chemical structure of materials

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Solid materials

• Rigid structure

 –  Component atoms, ions, or molecules are usually locked into place

• Extended three-dimensional patterns

• Allotrope

 –  Diamond

 –  Graphite –  C60

 –  Carbon nanotubes

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Crystalline & amorphous

• Crystalline solid

 –  Crystal lattice : regular repeating three-dimensional structure

• Amorphous solid : “shapeless”

 –  No particular order

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Metallic structure

• Metallic bond

 –  Free electron : high conductivity (electricity, heat)

 –  Good mechanical properties

 –  Crystal lattice : high ductility, tensile strength

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Unit cell

• Seven basic unit cells

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Defect

• Real crystals

 –  Large numbers of defects (typically more than 104 per milligram)

• Point defect

• Line defect

• Plane defect

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Ceramic structures

• Two or more elements (compound) : ex) Al2O3

• Covalent and ionic bond, predominately ionic bonding –  Stronger than metallic bond

 –  High hardness, high compressive strength, and chemical inertness

 –  Low ductility and low tensile strength

• No free electron : low conductivity (electricity, heat)• Variable structures

• Charge balance

• Glassy or crystalline

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Ceramic structures

• Ceramic glass : ex) pure

SiO2 (tetrahedrons) –  Network of chain or ring

structures

 –  Entirely glassy structure

 –  No sharp melting or freezingpoint

 –  High T : easily slide past eachother, viscous

 –  Low T : rigid, hard and brittle

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Ceramic structures

• Ceramic crystalline or partially crystalline material

 –  Most ceramics• Metallic and nonmetallic elements with ionic or covalent bonds

 –  Silicate Ceramics (SiO2)

• Pyramid (tetrahedral or four-sided) units

 –  Cement (19~25% SiO2, 5~9% Al

2O

3, 60~64% CaO, 2~4% FeO)

• One of the main ingredients of concrete

• Solution, recrystallization, and precipitation of a silicate structure

 –  Nitride Ceramics

• High thermal and mechanical stability

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Cutting tools, wear-resistant parts• TiN : cubic structure

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Defect

• Electroneutrality

 –  Point defects• Frenkel defect

• Schottky defect

• Impurity – both substitutional and interstitial

 –  Linear Defects

• Dislocations

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Polymeric structure

• Monomer, repeating unit

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Polymeric structure

• Addition & condensation

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Polymeric structure

• Types by monomers

 –  Homopolymer –  Copolymer

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Polymeric structure

• Types by structures

 –  Linear polymer –  Branched polymer

 –  Network(crosslinked) polymer

 –  Dendrimer

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Chemical bonds –  Covalent + secondary bond

• Anisotropic

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Polymeric crystal

• Crystalline, semi-crystalline, amorphous

• Crystallinity

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Others

• Carbon materials

 –  Crystalline :diamond, graphite, carbon nanotubes –  Glassy carbon

 –  Pyrolytic carbon

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