Exam review

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361_06_packingxlinity_pointdefects_c_am2.ppt
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Materials Moments:

Rich W—Volleyballs

Elijah W—Cooking surfaces

Crystallographic Directions and Planes
Solved Examples posted on Canvas:
Files>Solved Problems, Assignments, Extra Credit

Packing
and
Close-packed Planes

Close-packed Xl Structures

  • FCC & HCP—APF = .74
  • Most efficient packing

for equal-sized spheres.

  • BCC—APF = .68
  • Not as efficient

BCC: Which is the closest-packed plane?

z

z

A)

B)

C)

D)

z

x

y

y

x

z

y

x

y

x

Figs. 3.11, 3.12

f10_03_pg58

(110) plane packing

  • FCC—Not a close-packed plane

  • BCC—close-packed plane

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f10_03_pg58.jpg

(111)

(110)

(0001)

Close-Packed Planes

Different Packing…
So What?

Packing along planes:

  • Strongly affects deformation
  • Atoms can slip past each other on tightly packed planes (plastic deformation).

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f11_03_pg59.jpg

SEM {100} planes

SEM single cadmium crystal deforming by dislocation slip on {100} planes.

SEM study of slip in deformed cadmium single crystal

Sections 3.13-3.15

Single Crystals
Polycrystalline Materials
Anisotropy

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Terms

  • Crystalline–Regular repeating order over long distances
  • Crystal structure–shape of atomic arrangement
  • Crystal lattice
  • Single crystal
  • Polycrystalline

Crystal lattice:

http://chemed.chem.wisc.edu/chempaths/GenChem-Textbook/Lattices-and-Unit-Cells-837.html

Unit Cell

Crystal Lattice

Long-range 3-D representation of a crystal

Specific location for each atom

Single Crystals

Crystal structure repeats perfectly over large atomic distance

Galena (Lead ore)

Polycrystalline materials

Many small crystals grow together

Polycrystalline materials

Extremely small crystals grow together.

Fig. 3.18

f17_03_pg65

Polycrystalline grain growth

Nucleation sites have random orientations  xls have random orientations

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f17_03_pg65.jpg

Polycrystalline metal

Micrograph of a polycrystalline metal; grain boundaries evidenced by acid etching.

http://en.wikipedia.org/wiki/Grain_boundary

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Polycrystalline copper (SEM)

Anisotropy
Properties dependent on
crystallographic direction

Isotropy

Properties independent of crystallographic direction.

Random orientations of anisotropic material
yield isotropic behavior

Polycrystalline Calcite

Metal Thread Anisotropy

Section 3.17

Non-Crystalline Solids

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Non-Crystalline Solids

  • Amorphous—without ordered form.

  • Formed by rapid cooling from a melt

Non-crystalline solids

www.msm.cam.ac.uk

Hand sample of fractured glass

SEM of fractured glass

Glass: SiO2

(Glassy texture)

Fig. 3.23

f22_03_pg71

Crystalline SiO2

Non-Crystalline SiO2

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f22_03_pg71.jpg

Sections 4.1-4.3
Imperfections in Solids:
Point defects

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Perfect Crystals

Crystal structure

Crystal structure

Crystal lattice

Real World of Crystals

The perfect crystal doesn’t exist.

  • All materials have defects & impurities
  • 99.9999% pure metals have

1022 – 1023 impurity atoms/m3

Crystal defects

  • Defect—place where perfect periodicity of unit cell is interrupted.
Dimension Defect Type Examples
0 Point Vacancy, Substitutional
1 Line Dislocations
2 Interfacial Free surface, Grain boundary
3 Volume Pores, cracks, other phases

Point defects

  • I) Intrinsic—flaws in xl lattice geometry
    (no impurities)

  • II) Extrinsic—impurities

I. Intrinsic Point Defects:

1) Vacancy

STM Germanium (55x70 Å2)

I. Intrinsic Point Defects:

1) Vacancy

Fig. 4.1

An STM image of a self-assembled Au cluster array. The hexagonal lines illustrate the unit cell properties of the cluster array. A defect vacancy is clearly evident. The image was taken under  ultra-high vacuum conditions. Image by T. Lee.

http://www.physics.purdue.edu/nanophys/newpage10-03/gallery/index.htm

I. Intrinsic Point Defects:

2) Interstitial

Fig. 4.1

II. Extrinsic Point Defects:

1) Substitutional

STM: Manganese substituted into GaAs (makes semiconductor magnetic)

Fig 4.2

http://www.mse.engin.umich.edu/research/highlights/189/the_image_pop

When can an impurity atom
be substitutional?

Substitutional Atoms:

  • Atomic radii of host and impurity must be ± 15%

Fig. 4.2

II. Extrinsic Point Defects:

2) Interstitial

When can an impurity atom
be interstitial?

Most common interstitial elements:

  • Nitrogen
  • Oxygen
  • Carbon
  • Hydrogen

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