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Material science summary
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Summary of 32 pages for the course Material Science at RuG (Material science)
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Material science lecture 1To repair human function we need biomaterials.
3 classes of materials are available
• Metals
• Plastics
• Ceramics
Composite materials have more materials than one
Nano materials their structure is controlled on the nano level
Requirements for materials
Decision based on pro’s and con’s for materials.
Processing-> Structure -> Properties -> Performance
Performance depends on the properties, which depends on structure
Processing van change structure, for example how fast you cool a material which result in
different structures.
Chemical processing can change chemical composition which results in different properties.
Thoughness.
Transmittance
Metals:
• Strong, ductile
• High thermal & electrical conductivity
• Opaque, reflective
Polymers/plastics: Covalent bonding
• Soft, ductile, low strength, low density
• Thermal & electrical insulators
• Optically translucent or transparent
Ceramics: ionic bonding(refractory) – compounds of metallic & non-metallic elements
(oxides, carbides, nitrides, sulphides)
• Brittle, glassy, elastic
• Non-conducting (insulators)
• High melting temperatures
• Wear resistant
Atom -> electrons, protons and neutrons
Atomic number (Z) – number of protons in nucleus of atom
Atomic mass – number of protons (Z) + number of neutrons (N)
A = atomic mass unit = amu = 1/12 atomic mass of ^12C
Atomic wt = wt of 6.023 x 10^23 molecules or atoms
1amu/atom = 1 g/mol
Electron energy states
Valence electrons (outer) shell not filled completely -> not stable
In the periodic table columns have similar valence structure
Electronegativity, large values: tendency to acquire electrons, lower values: no tendency to
aquire electrons
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,Ionic bond between metal (donates electrons) and non-metal (accepts electrons)
Requires electron transfer
Large difference in electronegativity required.
Electrons repel electrons and nucleus repel nucleus.
Balance of attractive and repulsive terms with ionic bonds
En = Ea + Er = -(A:r) +(B: r^8)
Covalent bonding:
Similar electronegativity, shared electrons
Bonds determined by valence
Binding energies are intermediate
Breaking covalent bonds require lots of energy
Different types of covalent binding by orbital overlap
The number of possible covalent bonds depends on the valance electrons (N’) = 8 - N’
For carbon, N’ = 4 therefor number of covalent bonds = 4
Ionic-covalent mixed bonding
% ionic character = (1-e(- (Xa-Xb)^2):4) x 100%)
Where Xa & Xb are Pauling electronegativities
Primary bonding
Metallic bonding:
• Materials have 1, 2 or at most 3 valance electrons
• Valence electrons are not bound to nucleus but freely float all over the material (free
electrons)
• This make the metals good thermal and electrical conductors
• Low binding energies
Secondary bonding
Van der Waals interaction arises from interaction between dipoles
Fluctuating dipoles & Permanent dipoles-molecule induced
Type Bond energy Comments
Ionic Large! Nondirectional (ceramics)
Covalent Variable large-diamond Directional (semicondutors,
small-bismuth ceramics polymer chains)
Metallic Variable large-tungsten Nondirectional (metals)
small-mercury
Secondary Smallest Directional inter-chain
(polymer) inter-molecular
Properties from bonding: Melting point Tm
Bond energy
Tm is larger if Eo is larger
Relation between melting temp and bond energy.
Coefficient of thermal expansion(a)(alpha)
A(alpha) is larger if Eo is smaller.
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,Properties from bonding: Stiffness (G)
G is larger if Eo is larger
Ceramics (ionic & covalent bonding):
• Large bond energy
• Large Tm
• Large G
• Small a
Metals (Metallic bonding):
• Variable bond energy
• Moderate Tm
• Moderate G
• Moderate a
Polymers (Covalent & secondary):
• Directional properties
• Small Tm
• Small G
• Large a
Binding Metallic Covalent Ionic v.d. Waals +
covalent
Binding energy Low Intermediate High Very low
Material Metal Ceramic/ Ceramic Polymer
semiconductors
Electrical High Low Low Low
conductivity
Thermal High Low Low Low
conductivity
Ductility High Low Low High
Hardness Low High High Low
Melting point High High High Low
• There are three types of primary bonds (covalent, ionic and metallic)
• Position occupied by the participating atoms in the periodic table indicates the type
of bonds
• In general binding influence many material properties.
Crystalline solids
Energy and packing:
• Dense, ordered packing
• Non-dense, random packing
Dense, ordered packed structures tend to have lower energies and thus stronger
Noncrystalline materials = ‘’amporphous’’
• Atoms have no periodic packing
• Occurs for: - complex structures & rapid cooling
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, Crystalline materials
• Atoms pack in periodic, 3D arrays
• Typical of metals, many ceramics and some polymers
Crystal = entity x grid
Unit cell: smallest repetitive volume which contains the complete lattice pattern of a crystal.
Metals have the simplest crystal structures which are densely packed.
Metallic crystal structures
1. Simple cubic structure (SC) cube formed, rare due to low packaging density
Coordination number = 6 (number of nearest neighbours)
Atomic packing factor (APF) = Volume of atoms in unit cell: Volume of unit cell
2. Body centred cubic structure (BCC) – atoms touch each other along cube diagonals
Coordination number = 8
3. Face centred cubic structure (FCC) coordination number = 12
Material science lecture 2
Anstroms
Hexagonal Close-packed structure (HCP)
Theoretical density, p
Density = p = Mass of atoms in unit cell: Total volume of unit cell
P= nAw:VcNa
Densities of material classes in general:
P metals > P ceramics> P polymers
Single crystals are required in SOME engineering applications
Properties of crystalline materials are often related to crystal structure
Isotropic and anisotropic (wood) (not directional)
Polycrystalline materials (many single crystals)
Most engineering materials are polycrystals
SINGLE CRYSTALS
• Properties vary with direction: anisotropic
POLYCRYSTALS
• Properties may/may not vary with direction
• If grains are randomly oriented: isotropic
• If grains are textured, anisotropic
Polymorphism – two or more distinct crystal structures for the same material
(allotropy/polymorphism)
Point coordinates X, Y, Z
Crystallographic directions
Algorithm
1. Vector repositioned (if necessary) to pass through origin
2. Read off projections in terms of unit cell dimensions a, b and c
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