materials Flashcards
What mechanical behavior is the result of dislocations?
Ductility results from dislocations along the crystal lattice. This is what occurs within metals
Which mechanical behavior is improved by the presence of nanocrystals in a glass-ceramic?
The materials hardness is improved. This is the ability to diffuse stresses through diffuse crack propogation.
What is the effect of an impurity on the electrical conductivity of a metal?
An impurity lowers the electrical conductivity by increasing the electron scattering during the flow of electrons through the materials.
Which has greater electron mobility, glass or crystal?
A crystal has greater electron mobility because the crystal will have discrete patterned structures that lower the probability of electron scattering.
Would a polycrystalline ionic solid be transparent in the visible?
I would expect the material to be opaque because there would be a large amount of reflection due to the polycrystalline nature of the solid.
What are the two main contributions to the thermal conductivity of solids?
There is KL which is the lattice thermal conductivity and is the result of atomic vibrations throughout the crystal lattice transferred via phonons. Materials that do not have rigid bonds have a low lattice thermal conductivity which means they dissipate phonons. For example, diamond has a thermal conductivity of ~1000 W/mC but very poor electrical conductivity. Its strong bonds transfer phonons with ease though.
Ke is the electronic thermal conductivity which is a result of heat generation due to flowing electrons. In metals, most heat is generated from electron flow not lattice thermal conductivity (loose bonds = dissipation of phonons)
What is a phonon?
Phonons are vibrational particles/waves that move along a crystalline lattice and represent heat energy. They are considered a quasi-particle and are quantized as a standing wave through each atom within a lattice having some fixed 0 position.
Types of phonons and character
There are four phonon types:
Longitudinal optical phonons are phonons that move in the lattice vector direction but in opposite directions.
Transverse optical phonons move out of the lattice vector in opposite directions.
Longitudinal acoustic phonons move like sound waves along the crystalline vector
Transverse acoustic phonons move like ocean waves.
Graphite vs. Diamond: structure
Graphite and diamond are allotropes of one another. Graphite has a hexagonal 2d structure with 142 pm bonds in plane and 335 pm out of plane. Diomond is a network solid with .15 nm bond lengths.
Diamond k vs. metals
Diamonds have a thermal conductivity of ~1000 W/mK whereas a metal like copper have a k of ~ 500 W/mK
Graphite k isotropy
In plane graphite has an extremely high value of k (1950 W/mK) out of plane it is very poor (5.7 W/mK) This is because in plane graphite has extremely strong bonds that transfer phonons and electrons well but out of plane both dissapate.
What is the hardest materials?
Diamond is the hardest known material which is why it is used for indentation tests.
Diamond vs. Graphite: electrical conductivity
Diamond is a very poor conductor with sigma=10-16 and a band gap of 5.5 eV. Whereas graphite in plane has delocalized pi bonds which increase its conductivity to 105 and out of plane it is 10
Over what wavelengths is diamond transparent?
Diamond is transparent through the IR and into the UV ending at about .1 microns. This is because it is very rigid and isometric. The rigid means that it resonates with very short wavelengths and the isotropism means that it does not couple with incident IR.
What are the four main types of probes used to characterize materials?
There are visible light (IR-vis-UV), x-rays, ions, and electrons.
