Midterm/last set of slides info Flashcards
(35 cards)
What is an x-ray
high energy (1-100keV) electromagnetic radiations that are between 0.01nm to 10 nm in wavelength
How are x-rays generated?
When electrically charged particles are decelerated?
Why are x-rays interesting for the purpose of biomedical instrumentation?
Because they have a wavelength that is similar spacing of atoms in matter
What is a crystal?
any solid material characterized by a near-perfect repeating arrangement of molecules (unit cell)that forms a lattice
When does diffraction occur?
When a wave hits an object and the path is changed of the wave but the wavelength stays constant
How is crystallography done and what does it use?
It is done using x-ray diffraction, it uses this to find the arrangement of atoms
What is braggs law and what is it used for?
ngamma=2dsintheta, it is used to find interference in the measurement
What can diffraction study?
Can study all states of matter as long as they have a wavelength similar to the distance between the atomic or molecular structures of interest
What are the applications of XRD?
Measure the average spacing between layers or rows of atoms
Determine the orientation of a grain or crystal
Find the crystal structure of an unknown material
Measure size, shape, internal stress of small crystalline structures
What are the advantages of XRD?
It is a non-destructive method that is well studied and mathematically based, the signal is averaged over a large volume
What are the disadvantages of XRD?
Weak interaction, there is noise, imaging is not possible
What should happen to XRD samples to be prepared?
the surfaces should be smooth after polishing, they should be thermally annealed to get rid of surface deformations
What does TEM stand for? What is it based on?
Transmission Electron Microscope, based on the physics principle that moving electrons act as waves
What is TEM patterned after and what is the difference?
TEM is similar to the light microscope yet electrons are used instead of light
How are electrons in TEM focused?
By one or two electron lenses that focus the electrons
What dimensions are seen in TEM and what does brightness demonstrate?
TEM is a 2 dimensional method, brightness corresponds to the electron-dense region of the sample
What dimensions are seen in SEM, how is this image made?
SEM is 3 dimensional, it uses 2-3 spots of electrons, it is then reflected off of the specimen to give the image, due to the method, it is often lower resolution.
What is STEM?
A combination of the two methods, it determines the presence of atomic elements in a sample, STEM can distinguish columns of atoms in a crystal.
What is the difference between HRTEM and HAADF?
HRTEM uses a conventional coherent beam of electronics while HAADF uses incoherently scattered electrons due to thermal diffusion
What happens when XRD is done on crystals smaller than 100 nm?
Peak broadening can occur and this is not good data for finding the angle 2theta
How does an electron microscope work?
Light source makes a vibrational motion that sends energy towards the object, after this the object generates a new series of waves towards due to diffraction, the new waves interact with the original and this is what creates the image
What does the edge of an electron microscope image look like?
It appears to have a band or many fringes that are called fresnel fringes
Where does spherical aberration occur and what is it from?
It occurs in electron microscopes, it occurs in both glass and spherical lenses, it happens when the outer rays are passing more than the ones along the axis
Where does chromatic aberration occur?
It occurs in electron microscopes and it bends the lower energy electrons more strongly
