topik 3 - crystal structure - ilmu bahan
TRANSCRIPT
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8/11/2019 Topik 3 - Crystal Structure - Ilmu Bahan
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Dr.Ir.Asep Handaya Saputra, MEng.
Lecture Note: Material Science (source: virginia univ)
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Crystal Structures
3rd Topics
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Dr.Ir.Asep Handaya Saputra, MEng.
Lecture Note: Material Science (source: virginia univ)
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Fundamental Concepts
Atoms self-organize in crystals, most of the time. Thecrystalline lattice, is a periodic array of the atoms. Whenthe solid is not crystalline, it is called amorphous.Examples of crystalline solids are metals, diamond andother precious stones, ice, graphite. Examples ofamorphous solids are glass, amorphous carbon (a-C),amorphous Si, most plastics
To discuss crystalline structures it is useful to consideratoms as being hard spheres, with well-defined radii. In
this scheme, the shortest distance between two likeatoms is one diameter.
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Dr.Ir.Asep Handaya Saputra, MEng.
Lecture Note: Material Science (source: virginia univ)
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Unit Cells
The unit cell is the smallest structure thatrepeats itself by translation through the crystal.
We construct these symmetrical units with thehard spheres.
The most common types of unit cells are the
faced-centered cubic (FCC), the body-centeredcubic (BCC) and the hexagonal close-packed(HCP).
Other types exist, particularly among minerals.The simple cube (SC) is often used for didacticalpurpose, no material has this structure.
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Dr.Ir.Asep Handaya Saputra, MEng.
Lecture Note: Material Science (source: virginia univ)
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Metallic Crystal Structures
Important properties of the unit cells areThe type of atoms and their radii R.cell dimensions (side a in cubic cells, side of base a and height cin HCP) in terms of R.
n, number of atoms per unit cell. For an atom that is shared withm adjacent unit cells, we only count a fraction of the atom, 1/m.
CN, the coordination number, which is the number of closestneighbors to which an atom is bonded.
APF, the atomic packing factor, which is the fraction of the
volume of the cell actually occupied by the hard spheres.APF =Sum of atomic volumes/Volume of cell.
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Dr.Ir.Asep Handaya Saputra, MEng.
Lecture Note: Material Science (source: virginia univ)
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Metallic Crystal StructuresUnit Cell n CN a/R APF
SC 1 6 2 0.52
BCC 2 8 4 3 0.68
FCC 4 12 2 2 0.74
HCP 6 12 0.74
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Dr.Ir.Asep Handaya Saputra, MEng.
Lecture Note: Material Science (source: virginia univ)
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Simple Cubic CrystalIn the Simple Cubic (SC) unit
cell there is one lattice point ateach of the eight corners of a
cube.
Unit cells in which there arelattice points only at the eight
corners are called primitive.
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Dr.Ir.Asep Handaya Saputra, MEng.
Lecture Note: Material Science (source: virginia univ)
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Simple Cubic Crystal
An example of a simple cubic crystal is CsCl.
Each ion resides on a separate,
interpenetrating Simple Cubic lattice.The cation is in the center of the anion unit
cell and visa versa, so the two lattices have
the same unit cell dimensions.
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Dr.Ir.Asep Handaya Saputra, MEng.
Lecture Note: Material Science (source: virginia univ)
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Face Centered CubicIn the Face Centered Cubic(FCC) unit cell there is one hostatom at each corner and onehost atom in each face.
Since each corner atom
contributes one eighth of itsvolume to the cell interior, andeach face atom contributes onehalf of its volume to the cell
interior (and there are six faces),then n = 1/8.8 + 1/2.6 = 4
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Dr.Ir.Asep Handaya Saputra, MEng.
Lecture Note: Material Science (source: virginia univ)
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Face Centered CubicIn a NaCl crystal, each ion resides on a
separate, interpenetrating FCC lattice, andthe two lattices have the same unit cell
dimensions.
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Dr.Ir.Asep Handaya Saputra, MEng.
Lecture Note: Material Science (source: virginia univ)
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Hexagonal Closest Packing
(HCP)
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Dr.Ir.Asep Handaya Saputra, MEng.
Lecture Note: Material Science (source: virginia univ)
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Density Computations
The density of a solid is that of the unit
cell, obtained by dividing the mass of theatoms (n atoms x Matom) and dividing by Vcthe volume of the cell (a3 in the case of a
cube). If the mass of the atom is given in
amu (A), then we have to divide it by the
Avogadro number to get Matom. Thus, theformula for the density is:
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Dr.Ir.Asep Handaya Saputra, MEng.
Lecture Note: Material Science (source: virginia univ)
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Polymorphism and AllotropySome materials may exist in more than
one crystal structure, this is calledpolymorphism.
If the material is an elemental solid, it iscalled allotropy. An example of allotropy is
carbon, which can exist as diamond,
graphite, and amorphous carbon.
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Dr.Ir.Asep Handaya Saputra, MEng.
Lecture Note: Material Science (source: virginia univ)
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Crystalline MaterialsSingle Crystals
Crystals can be single crystals where the wholesolid is one crystal. Then it has a regulargeometric structure with flat faces.
Polycrystalline Materials
A solid can be composed of many crystallinegrains, not aligned with each other. It is calledpolycrystalline. The grains can be more or less
aligned with respect to each other. Where theymeet is called a grain boundary.
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Dr.Ir.Asep Handaya Saputra, MEng.
Lecture Note: Material Science (source: virginia univ)
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Dr.Ir.Asep Handaya Saputra, MEng.
Lecture Note: Material Science (source: virginia univ)
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Crystals Growth