Materials Final Flashcards
Even though Silicon is an intrinsic semiconductor, how can it be made into an extrinsic semiconductor?
By intentionally adding a small number of impurities (doping)
-Doping an element of different valance into the Si
If a material is amorphous, it can be characterized as having:
An atomic structure without long-range order
A thermoplastic polymer is best described as being compromised of:
Covalently bonded, long-chain molecules held together by van der Waal’s forces with or without long range order
What is the most important physical characteristic that influences the elastic properties of a material?
Bond strength
As it refers to polymers, the term cross linking means:
The formation of covalent bonds between polymer chains
Although the density can vary widely within any class of materials, the density of a metal is generally greater than the density of a ceramic because:
-atoms are more closely packed in metals than in ceramics
-ceramics are usually a mixture of metal atoms and lighter, non-metallic elements
The bulk modulus of a material is used to characterize:
The resistance of a material to volume change during hydrostatic loading
As it refers to the atomic structure of a crystalline material, what is a “line” defect?
An edge dislocation
The Burgers vector of a dislocation:
Has a magnitude equal to the shear displacement caused by the motion of the dislocation
Plastic deformation increases the yield strength of a metal because:
The number of entangled dislocations increases
Brass (a Cu-Zn alloy) is stronger than pure copper. This is because:
The zinc atoms distort the copper lattice
Is the following statement true? The structure of a thermoset is a 3-D network of covalently bonded molecules
True
Is the following statement true? The Young’s modulus of a thermostat is generally higher than a thermoplastic polymer
True
Crystalline ceramics do not experience significant plastic deformation because:
-the stress required to move dislocations is greater than the fracture stress
-the ionic bonds are too strong
-Small flaws in the material act as stress concentrators, causing fracture before plastic deformation can occur
The vulcanizing process is used to:
Create cross-links in elastomers
The specific heat Cp of a material is equal to:
The energy required to raise the temperature of a unit mass of the material by one degree
Metals typically have a higher value of fracture toughness than ceramics because:
Metals plastically deform
The basic result obtained from a creep test is a plot with which two parameters:
Strain and time
Creep deformation becomes an important consideration at which homologous temperature?
0.4
How many atoms are in a simple cubic?
1 atom
How many atoms are in a face centred cubic unit cell?
4 atoms
How many atoms are in a body-centred cubic unit cell?
2 atoms
The number of independent close-packed directions in a close-packed plane of atoms is:
3
What is the definition of a nanoparticle?
A structure with at least 1 dimension less than 1 micrometer
What are examples of nanoparticles?
-Sphere-like particles
-Rod-like particles
-Tube-like particles
List the first unique property of nanoparticles
-Large surface to volume ratio
List the second unique property of nanoparticles
-High percentage of atoms/molecules on the surface
List the third unique property of nanoparticles
-Surface forces are very important, while bulk forces are not as important
List the fourth unique property of nanoparticles
-Metal nanoparticles have unique light scattering properties and exhibit plasmon resonance
List the fifth unique property of nanoparticles
-Semiconductor nanoparticles may exhibit confined energy states in their electronic band structure
List the sixth unique property of nanoparticles
-Can have unique chemical and physical properties
List the seventh unique property of nanoparticles
-Same size scale as many biological structures
What range of atoms do molecules have?
1-10
What range of atoms do nanoparticles have?
100-100,000
What range of atoms do bulk materials have?
greater than 1 000 000
Which one has lower a melting point? Nanoparticles or bulk materials
Nanoparticles
What is a Carbon Nanotube?
a tubular form of carbon with diameter as small as 1 nm
What are the types of Carbon Nanotubes (CNT)?
-Single Wall CNT
-Multiple Wall CNT
True or False? CNT have very high current carrying capacity
-true
What is a graphene?
Can be described as a one atom thick layer of carbon (graphite)
Which is stronger? Graphene or structure steel? By how much?
Graphene is 200 times stronger than the structure steel
What is the Pauli exclusion principle?
Two electrons of opposite spin per orbital
How is the “sea of electrons” in metallic bonding formed?
In metals, the valance band and the conduction band overlap
Define Band Gap Energy (EG)
The energy that must be applied to a non-metal to force a valance electron to break its inter atomic bonds and begin to move to conduct electricity
What is Fermi energy (EF)?
The energy of the most energetic electron when only the lowest energy levels are filled
What happens when voltage is less than the Band Gap Energy?
Electricity isn’t conducted
What happens when voltage is greater than the Band Gap Energy?
Electricity is conducted
What are examples of materials that are intrinsic semiconductors?
-Sillicon
-Germanium
What are intrinsic semiconductors?
-Their valance bands and conduction bands do not overlap, but their energy gaps are small
-So even at room temperature, some electrons have enough thermal energy to enter the conduction band
-When electrons move onto the conduction band, they leave a hole behind in the valance band that moves in the opposite direction
What is the major difference between intrinsic and extrinsic semiconductors?
-Intrinsic semi conductors are sensitive to temperature and its more difficult to accurately control their conductivity
-Extrinsic semi conductors have constant conductivity over a wide temperature range
How do we create extrinsic semiconductors?
-By intentionally adding a small number of impurities (doping)
What are most electrical devices made of?
Extrinsic semiconductors
What happens if you add a n-type into a semiconductor?
Introduces extra electrons into the crystal lattice
What happens if you add a p-type into a semiconductor?
Creates “holes” in the crystal lattice
What is extrinsic semi conduction?
When donor electron in an n-type semi conductor dominate conductivity
What is donor exhaustion?
When all donor electrons are in the conduction band as the temperature increases
-conductivity is constant
How do p-type semiconductors work?
-The holes that are created are filled by other valance electrons in the valance band
-This movement requires less energy than the Band Gap energy
How can we control the conductivity in a material?
-By changing the number of carriers
-Influencing the mobility
What is drift velocity?
The average electron velocity in the direction of the force imposed by the applied field
What is mobility of an electron
Indication of the frequency of scattering events
What is corrosion?
The deterioration of a materials by chemical reactions with its environment
What is an oxidation reaction?
A reaction where a chemical species produces free electrons
