1. Crystal Systems

2. Crystal System Terms Unit Cell
smallest repeating unit of a crystal structure
Slip Planes - surface along which layers of atoms can slide past one another
plane of closely packed atoms
Use the dowel set up to demonstrate slip planes. If the atoms are close together, other atoms can easily slip past, if the atoms are further apart, it is more difficult to slip past.

5. Crystal System Terms
Void or Interstice-
empty space in a crystal

6. Crystal Packing – loosely packed
Is there another way these atoms could be arranged?
Show next slide

7. Crystal Packing – More Densely Packed
Most metals are close packed - that is, they fit as many atoms as possible into the available volume
Alternating the atoms has made more room – the 4 halves could be joined and added to the bottom still leaving space.

8. FCC – Face Centered Cubic BCC- Body Centered Cubic
simple cubic
FCC – Face Centered Cubic
BCC- Body Centered Cubic
HCP – Hexagonal
Close Packed

9. Slip Planes of FCC Morton Schaffer
Let’s take a look at a FCC cubic structure and count the slip planes in this slip system. Even though there is a convention to name the planes, that level of detail is not within the scope of this course. Therefore, I’m just going to show pictorially where the planes are.

10. Slip Planes Morton Schaffer1

11. Slip Planes Morton Schaffer2

12. Slip Planes Morton Schaffer3

13. Slip Planes Morton Schaffer4

14. Slip Planes Morton Schaffer5

15. Slip Planes Morton Schaffer6

16. Slip Planes Morton Schaffer7

17. Slip Planes Morton Schaffer
Up to 48 Slip Planes
12 Slip Planes
center atom
As noted earlier, the more slip planes there are, the more ductile the material trends to be. Look at the examples of metals that have the indicated structure. Compare the properties of these metals and get a “feel” for why thes metals tend to behave the way they do. Keep in mind that slip planes do not necessarily determine how strong the metal is, but rather how easy it is to bend, form, etc. without tearing or breaking.
Body-centered cubic (BCC)
Face-centered cubic (FCC)
Hexagonal close-packed (HCP)
3 Slip Planes

18. Each group makes all of them but every group is making them at the same time---so all make b together and then all make c together and they talk about it
And then stack the layers
Have the kids make them but have the kids figure out what name goes with which structure
Then talk about gappiness and slip planes for each one---kids would write it down
For FCC—diagnol slip planes are harder to see—show the tennis ball model from Tom who is a retired NASA scientist who does taxes for a hobby for elderly…

19. STOP – Crystal Models Lab

20. HEXAGONAL CLOSE PACKED
SIMPLE CUBIC
FACE CENTERED CUBIC
(FCC)
BODY CENTERED CUBIC
(BCC)
Each group makes all of them but every group is making them at the same time---so all make b together and then all make c together and they talk about it
And then stack the layers
Have the kids make them but have the kids figure out what name goes with which structure
Then talk about gappiness and slip planes for each one---kids would write it down
For FCC—diagnol slip planes are harder to see—show the tennis ball model from Tom who is a retired NASA scientist who does taxes for a hobby for elderly…
HEXAGONAL CLOSE PACKED
(HCP)

22. Why do Crystal Systems Matter?
Workability
changing the shape of a solid without breaking or cracking
Malleability
ability of being hammered into thin sheets
Ductility
ability of being drawn into wires

23. Workability Which crystal structure is more workable?
Many slip planes or few slip planes?
Tightly packed or loosely packed?
Malleability and ductility is a type of workability
How easy you can manipulate a solid
Many slip planes determines good workability
Common sense
Common sense more gappiness has more room to move around—but not true—do ice tray demo
Less gappiness is more workable
Based on what we did and showed which crystal is more workable? FCC most packed and most slip planes

24. Type of crystal structure
Models of Crystals Lab
*more tightly packed = more workable
*more slip planes = more workable
Type of crystal structure
Closely packed?
Many slip planes?
Workability
FCC
BCC
HCP
This slide68 – S
Chalk Demo 62 - S

25. Type of crystal structure
Models of Crystals Lab
*more tightly packed = more workable
*more slip planes = more workable
Type of crystal structure
Closely packed?
Many slip planes?
Workability
FCC
Yes
Highest
BCC No
Medium
HCP
Lowest
This slide68 – S
Chalk Demo 62 - S

26. Crystal Structures & Metals
BCC
FCC
HCP
Other

27. Crystal Structures & Metals
BCC
FCC
HCP
Other
Chromium
Aluminum
Cobalt
Manganese
Iron (<910°C)
Calcium
Magnesium
tin
Molybdenum
Copper
Titanium
Sodium
Gold
zinc
tungsten
Iron (>910°C)
Lead
Nickel
Platinum
silver

28. Sargent Welch Periodic Table
Crystal structures on the back.

29. Crystalline vs. Amorphous?
Orderly arrangement
Repeating pattern
Predictable
Opaque (not see through)
Random arrangement
No repeating pattern
Not predictable
Clear

30. Allotropes (review)
Different forms of the same element in the same physical state
Difference is in how the atoms are arranged
Also called polymorphism
Examples:
Carbon – diamond, graphite, buckyballs
Oxygen – O2 (atmospheric) and O3 (ozone)
Sulfur – rhombic, monoclinic, amorphous

31. Allotropes of Carbon buckyball
Great web page about crystals

32. Allotropes of Sulfur
rhombic
amorphous
monoclinic

33. Solid State Phase Change
Change in crystal structure while remaining a solid.
Example:
Amorphous sulfur changing to crystalline sulfur

34. Milk Jug Demo
What is happening when you heat the plastic to the crystal structure?
Before heat  Crystalline b/c it was opaque
During heating  Changing to amorphous, became clear
After heat  atoms were able to go back to close to original states & went back to crystalline

35. Growing Crystals
Sulfur Lab

36. Sulfur MSDS

37. Sulfur MSDS

38. Sulfur MSDS

39. Sulfur MSDS

40. Review lab on Friday!
Monday, 9/19/16

41. Part A – Rhombic Sulfur Forming crystals from a solution
What did we do?
Heated in mineral oil to dissolve
What did we see?
Crystals formed in solution

43. Part B: Monoclinic Sulfur
Forming crystals from a melted substance
What did we do?
1. Fill a test tube approximately 1/2 full with sulfur. Keep the sulfur powder off the sides of the test tube.
2. Make a cone out of filter paper and place it in a funnel.
3. Heat the test tube of sulfur very slowly - passing it back and forth above the flame. Totally melt to a liquid. Use Bunsen burner and test tube clamp. Keep the sulfur yellow.
4. Pour liquid sulfur into filter paper cone. As soon as a crust forms, open the filter paper to original shape.

46. What did we see? Forming crystals from a melted substance
Long skinny pointy crystals

49. Part C: Amorphous Sulfur
What did we do?
Heat sulfur slowly. It will pass through stages.
Pour hot sulfur into beaker of cold water. (quench)
What did we see?
melt to yellow liquid
individual rings of 8
red liquid
short chains of 8 – 16 sulfur atoms
dark reddish-brown thick syrup
longer chains of sulfur atoms that entangle
dark runny liquid
longer chains of sulfur atoms that have enough energy to flow

50. Ring of 8 sulfur atoms
Chain of sulfur atoms

51. Amorphous Sulfur

52. Crystalline balls
of sulfur

53. Crystal Defects What does crystalline mean?
How easy is it to grow a perfect crystal?
What types of defects are possible?

54. CRYSTAL TERMS Grains individual crystals
They grow in different directions and meet up with each other
Grain Boundary
boundaries where grains meet

56. “Crystal grains” are regions of regularity
“Crystal grains” are regions of regularity. At the grain boundaries, atoms have become misaligned.
Heating a metal tends to shake the atoms into a more regular arrangement - decreasing the number of grain boundaries, and so making the metal softer. Banging the metal around when it is cold tends to produce lots of small grains. Cold working therefore makes a metal harder. To restore its workability, you would need to reheat it.
You can also break up the regular arrangement of the atoms by inserting atoms of a slightly different size into the structure. Alloys such as brass (a mixture of copper and zinc) are harder than the original metals because the irregularity in the structure helps to stop rows of atoms from slipping over each other.
Crystal Grains

57. Crystal Defects and Imperfections
Impurities and disruptions in the pattern of atoms
Affect many physical properties
Three main types
Ask the students what comes to mind when you hear the word “defect”. They usually say something “bad” or “unwanted” or “negative”. When discussing that point defects can be used to make alloys point out that we create “defects” on purpose b/c they result in useful changes in properties.
Reinforce the idea that defects can lead to positive changes when discussing that line defects can increase the hardness or strength of metals.
Ask students to predict how crystals may “mess up”.

58. 1. Interfacial Defects grain boundaries affected by size of grains
Use BB board to show

60. By cooling more quickly, a smaller grain size results and this has the effect of making the resulting solid stronger.
Figure 2 and Figure 3 are examples of metals with different grain size – the one exhibited in Figure 3 will be significantly stronger than its counterpart in Figure 2.
Figure 2
Figure 3

61. Application in Industry
Creep
High temperatures with less grain boundaries
(30-50% of melting temperature)
Stronger
Stretching
Low temperatures with more grain boundaries
Increases tensile strength – more stretch
Read Case Study on “Creep of Turbine Blades”

62. Phenyl Salicylate Demo
freezing a liquid to grow crystals
nucleation site – location where a crystal starts growing
watch for grains and grain boundaries

63. 2. Dislocation – Line Defects
regions in crystals where atoms are not perfectly aligned – an extra partial plane
can move
a small number make a metal more workable
a large number make a metal harder to work
dislocations can get “jammed” or “pinned”
makes the metal harder = work-hardening
Becomes so strong it becomes brittle and breaks
-use overheads to illustrate
Deck of cards with one partial card inserted

68. Dislocation Movement

69. 3. Point Defects
single atom defects

70. Substitutional replace some of the original atoms with different atoms
used in making alloys
examples – brass and bronze

71. Interstitial
extra atoms inserted in the gaps between the regular atoms
used in making alloys
example - steel

72. Vacancy atom is missing (void) move around in crystal by diffusion
-use BB board to illustrate
Have students draw an example in their journals

73. Silver Nitrate on Copper Wire
Growing Crystals
Silver Nitrate on
Copper Wire

74. Growing Silver Crystals
Place a flattened 1” piece of Cu wire on a microscope slide.
Focus one end of wire under stereoscope.
Have the teacher drop AgNO3 (silver nitrate) on the wire.
Make observations:
Low and high power
Light from above and below

75. Crystal Definitions Dendrites crystal branches
crystal growth pattern – directional
grow until they eventually become large enough to impinge upon (interfere with) each other
spaces between the dendrite arms
crystallize to make a more regular
crystal

76. Once nucleated, the dendrites spread sideways and the secondary arms generate further tertiary arms and so on. When solidification is complete, all the dendrites that have formed knit together to form grains (or crystals).

78. Snow - ice
Snow - Ice

79. Dendritic copper crystals