ms115a principles of materials science fall 2012
DESCRIPTION
MS115a Principles of Materials Science Fall 2012. Instructor: Prof. Sossina M. Haile 307 Steele Laboratories, x2958, [email protected] http://addis.caltech.edu/teaching/MS115a/MS115a.html Class Meetings: MWF 11am-noon; 080 Moore; to 12:30pm?? Teaching Assistant: - PowerPoint PPT PresentationTRANSCRIPT
MS115a Principles of Materials ScienceFall 2012
• Instructor: – Prof. Sossina M. Haile
– 307 Steele Laboratories, x2958, [email protected]
• http://addis.caltech.edu/teaching/MS115a/MS115a.html• Class Meetings: MWF 11am-noon; 080 Moore; to 12:30pm??• Teaching Assistant:
– Alex Zevalkink, 317 Steele, x4804, [email protected]
• TA Office Hours: TBA (likely Tuesdays)• All recommended and reference texts on reserve in SFL• Recommended:
– “Understanding Solids,” Tilley; “Intro to Mat Sci for Engineers,” Shackelford
• Additional references:– “The Principles of Engineering Materials,” Barrett, Nix & Tetelman– “Phase Transformations in Metals and Alloys,” Porter & Easterling– “Quantum Chemistry,” Levine
Chemistry / Composition
What is Materials Science?
Processing+
Structure
Properties / Performance
??
MS 115a (MS 115b)
thermodynamics kinetics
Course Content
• Introduction to Materials Science– Chemistry + Processing Structure Properties
• Structure– Review: Structure of the Atom & Chemical Bonding– Crystalline Structure– Structural Characterization (X-ray diffraction)– Amorphous Structure– Microstructure
• Defects in Crystalline Solids, Connections to Properties– Point Defects (0-D) and Diffusion & Ionic Conductivity– Dislocations (1-D) and Mechanical Deformation– Surfaces and interfaces (2-D)– Volume Defects (3-D) and Fracture
Course Content
• Electrons in Solids– Chemical Bonding, Revisited– Band Structure– Electronic Conductivity: Metals vs. Insulators
• Thermodynamics– 1st and 2nd Laws– Gibb’s Free Energy– Phase Diagrams
• Some Other Properties Along the Way– Thermal: Thermal Expansion, Heat Capacity, Thermal
Conductivity– Optical: Refraction, Reflection; Absorption, Transmission,
Scattering, Color
• Conceptual vs. Highly Mathematical
Course Structure
• Homework: weekly 50%– Assigned Wednesdays– Due following Wednesday, 5pm– Place in course mailbox, 3rd floor Steele
• Midterm HW: Oct 31 - Nov 6 15%– Solo homework
• Final: Dec 12 - 14 35% – Take home
HW Collaboration Policy• Students are encouraged to discuss and work on
problems together. – During discussion, you may make/take notes
– However, do not bring and/or exchange written solutions or attempted solutions you generated prior to the discussion.
– If you’ve worked the problem out and you plan to help a friend, you should know the solution cold.
• Do not consult old problem sets, exams or their solutions.
• Solutions will be handed out on Friday, or possibly Monday. Assignments turned in late, but before solutions are available, will receive 2/3 credit. Assignments will not be accepted after solutions are handed out.
Midterm Homework• In lieu of a midterm exam there will be homework to be
performed on an individual basis. This homework must be completed without collaborative discussion.
• The problem set will focus primarily on recent lectures, but material from early topics may also be included.
• Similar to other homeworks, you will have one week to complete the assignment.
• You are permitted to utilize all available resources, with the exception of previous solutions; this exception includes solutions from earlier in the year.
Structure of the Atom• “Electron in a box” – use quantum mechanics to solve
electron wave functions– Electron quantum numbers: describe orbitals– Electrical properties
• Qualitative description of chemical bonding– Electrons ‘orbit’ atomic nucleus
K
ML
Q.N.
n K, L, M “shell” n = 1, 2, 3 radius
l s, p, d “orbital” l = 0, 1 …. n-1
m px, py, pz “orientation” m = -l, -l+1, … 0, ... l-1, l
s spin s = ± ½
K-shell: n = 1 l = 0
1
32
1s m = 0 s = ± ½
L-shell: n = 2 l = {0, 1} 2s, 2p
m = 0 m = {-1, 0, 1} px. py. pz
Chemical notation
Structure of the Atom
• Electrons occupy these orbitals
• Pauli exclusion principle– Only one electron with a given set of QNs– For a multi-electron atom, fill up orbitals
beginning with lowest energy & go up
• Order: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s,..
Chemical Bonding
• Atoms Molecules Solids
• Bonds form so as to produce filled outer shells
• Some atoms are a few electrons short– Electronegative: readily pick up a few electrons from other
atoms, become negatively charged
• Some atoms have a few electrons too many– Electropositive: readily give up a few electrons to other atoms,
become positively charged
• Noble gases: filled outer shell, limited chemistry
electronegativity
• Primary– Ionic
• Electronegative/Electropositive
– Metallic• Electropositive – give up electrons
– Colavent• Electronegative – want electrons
• Shared electrons along bond direction
• Secondary– Fluctuating/instantaneous dipoles
– Permanent dipoles (H-bonds)
Types of Chemical Bonds
Isotropic, filled outer shells
+ - +
- + -
+ - +
+ + +
+ + +
+ + +
e-
e-
e-
Chemical Bonding• Covalent – between electronegative elements• Metallic – between electropositive elements• Ionic – between different elements with differing
electronegativities • Clear distinction between metallic & non-metallic• Ionic & covalent – somewhat qualitative
boundary
• ‘% ionic chararacter”: 1 – exp( -¼ (xA – xB)2)
– xA, xB = electronegativities
• Some properties from “bond-energy” curve
Some Properties
E
R (interatomic distance)long range attraction
1~
R
short range repulsion1
~nR E = ER + EA
E0
R0
E0 : decrease in energy due to bond formation
this much energy is required to break the bond define as bond energy sets the melting temperature
R0 : interatomic distance that minimizes E
is the equilibrium bond distance
The bond energy curve
E
R (interatomic distance)
E = ER + EA
Heat the material
More Properties
Ethermal = kbTT
R0 as T
Asymmetry in E(R) sets thermal expansion coefficient
ER (interatomic distance)
F = dE/dR
Some Mechanical Properties
E0
R0
Fattr
activ
ere
puls
ive
R (interatomic distance)
The bond force curve
at R0 no net force (equilibrium bond distance)
R0
Elastic constants relate stress to strain Stress – related to force Strain – related to displacement
F = k x
stress*area strain*length
stress k strain
k
Elastic constants given by slope of B.F. curve at R0
given by curvature of B.E. curve at R0
Covalent Bonds• Locally well-defined orbitals• Elements with electrons up to 2p or 3p states
– Filled outer shell octet rule (s + p 8 states)– Rule: 8 – N bonding electrons = n bonds
• Example: carbon (C)– 6 electrons total: 1s22s22p2
– 2s22p2 N = 4 n = 4
s orbital p orbitals
– solution: sp2 or sp3 hybrization
http://www.emc.maricopa.edu/faculty/farabee/BIOBK/orbitals.gif
how can carbon atoms fill px, py and pz orbitals if the other element is also electronegative?bonding electrons bonds
Hybridized Bonds
one s + three p orbitals 4 (x 2) electron states(resulting orbital is a combination)
sp3 hybridization
diamond
also methane: CH4
• Elemental carbon (no other elements)
Summary• Nature of the bonds formed depends on the
chemical nature of the elements (as given by placement on the periodic table)
• Bond energy / bond force curve gives– Equilibrium bond distance
– Melt temperature
– Thermal expansion coefficient
– Elastic constants
• In general, there is not a direct correlation between type of bond and value of properties