Exam review
al_awa
361_07_defects_diffusion_c_am.ppt- Crystal Growth YouTube videos:
- Growing minerals in a fracture (Structural Modeling, analogue vein fill)
Timelapse photography - Crystal Growth - Experiment #1 - Slide-lok
Lysozyme—An enzyme occurring naturally in egg white, human tears, saliva, and other body fluids, capable of destroying the cell walls of certain bacteria and thereby acting as a mild antiseptic. Also called muramidase.
KDP Crystal Growth (single xl growth)
Timelapse of Crystals Growing (3:58 interaction)
YouTube: Diffusion posted by smcblackburn
Diffusion video
Materials Moments:
Mauricio G and Rudy G—Particle Board
*
Sections 4.1-4.3
Imperfections in Solids:
Point defects
*
http://www.ndt-ed.org/EducationResources/CommunityCollege/Materials/Graphics/EdgeDislocation1.jpg
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 the 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: No composition change
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) Self-interstitial
Fig. 4.1
Rare—too much lattice strain
requires too much energy
II. Extrinsic Point Defects: Composition changes
1) Substitutional
STM: Manganese substituted into GaAs (makes semiconductor magnetic)
Fig 4.2
STM image: http://www.mse.engin.umich.edu/research/highlights/189/the_image_pop
*
When can an impurity atom
be substitutional?
Fig. 4.2
II. Extrinsic Point Defects:
2) Interstitial
When can an impurity atom
be
interstitial?
Most common interstitial elements:
- Nitrogen
- Oxygen
- Carbon
- Hydrogen
Example: Brass = Cu + Zn
- Zn atoms replace some Cu atoms
FCC structure
Example: Steel
- Small C atom added to Fe
- How much C?
- How does C strengthen Fe?
Steel = C + Fe
Interstitial C imposes
lattice strains
in Femakes it difficult for atoms to move to accommodate strain
strengthens
Sections 4.5-4.6
Imperfections in Solids:
Linear defects (Dislocations)
Interfacial defects
*
Line Defects:
Edge Dislocations
Extra ½ plane of atoms in xl structure
STM Image
Line Defects:
Screw Dislocations
1) Screw dislocation—formed by shear stress
STM Image
Screw dislocation:
Image shows only atomic planes involved
Crystal growth on screw dislocation
Screw dislocation wire growth
- Nanowire growth in PbS
- Driven by screw dislocations w/o aid of catalysts
Line Defects: Mixed Dislocations
Reality: Most line defects are combinations of edge and screw disclocations
Interfacial Defects
External Surfaces
Grain Boundaries
Twinning
Important Source for Zinc & Piezoelectrics
Metal Sulfide–Sphalerite–ZnS
- External Surfaces
Interfacial Defects
Why a defect?
Unsatisfied bonds!
Interfacial Defects
Grain Boundaries
(polycrystalline materials)
Fig. 4.7
Fig. 3.18
f17_03_pg65
Polycrystalline grain growth
Formation of Grain Boundaries
Misalignment of crystals
Unsatisfied bonds!
*
f17_03_pg65.jpg
Polycrystalline copper (SEM)
Grain boundaries in Polycrystalline Copper
Interfacial Defects
3) Twinning
Twinning in Al
Whiting School of Engr., Johns Hopkins
Section 5.1-5.3
Diffusion:
Introduction
*
YouTube: Diffusion
0:16-1:10
View the rest of this video if you have trouble understanding the concept of diffusion.
Diffusion
- Gases
- Liquids
- Solids
Diffusion
Connect 2 gas tanks: Ar & He
- Assume equal T & P
- Gasses diffusereach uniform comp.
- 2 gases: Ar & He
- 2 liquids: water & alcohol
- Diffuse until uniform composition
Diffusion
- 2 gases: Ar & He
- 2 liquids: water & alcohol
- 2 solids: Blocks of Cu & Ni
- Heat at elevated T (below Tm)
- Leave for days
- Eventually reaches uniform composition
Diffusion
Rate
- 2 gases: Ar & He Fastest
- 2 liquids: water & alcohol
- 2 solids: Blocks of Cu & Ni Slowest
Diffusion
In all cases, diffusion rate increases as T increases.
Fig. 5.3
f03_05_pg112
Vacancy
Diffusion
Interstitial
Diffusion
Diffusion Mechanisms
*
f03_05_pg112.jpg
Fig. 5.3
f03_05_pg112
Vacancy
Diffusion
Interstitial
Diffusion
Diffusion Mechanisms
*
f03_05_pg112.jpg
f03_05_pg112
Interdiffusion
Example: Cu, Ni
Vacancy diffusion or interstitial diffusion?
*
*
f03_05_pg112.jpg
Metallic xl Structures
Face-Centered Cubic (FCC)
Cu, Al, Ag, Au, Pb, Ni, Pt
Body-Centered Cubic (BCC)
Na, Fe, Cr, Mo, W
Hexagonal Close-Packed (HCP)
Ti, Zn, Cd, Co, Mg
Example: Cu, Ni
*
Thermocouples and resistors whose resistance is stable across changes in temperature contain the 55% copper-45% nickel alloy (constantan).
Monel metal is a nickel-copper alloy, containing a minimum of 63% nickel.
Cupronickel or copper-nickel or "cupernickel" is an alloy of copper that contains nickel and strengthening elements, such as iron and manganese.
f03_05_pg112
Interdiffusion
Vacancy diffusion
Example: Cu, Ni
*
*
f03_05_pg112.jpg
Initially
See Figs. 5.1 and 5.2
Interdiffusion
After some time
*
*
http://www.yale.edu/yaleche/chemeng/eia/eia.htm
Eric I. Altman
“Vacancy diffusion in a layer of adsorbed Br atoms on Cu(100)”
What’s Cu(100)?
f03_05_pg112
Interdiffusion
- Vacancy diffusion
- Interstitial diffusion
- Other paths
- Dislocations
- Grain boundaries
- Free surfaces
*
*
f03_05_pg112.jpg
f03_05_pg112
Two conditions for diffusion
- Empty site near the diffusing atom
- Diffusing atom needs enough energy to break existing bonds.
*
*
f03_05_pg112.jpg
QuickTime™ and a
decompressor
are needed to see this picture.
