X-Ray Diffraction

Uploaded by cmditr on 19.05.2011

Narration My name is Tuesday Kuykendall and I’m the
scientific instructional technician for the Materials Science and Engineering Department
at the University of Washington. This our Focus D8 x-Ray Diffractometer
The Bruker D8 Focus x-Ray Spectrometer is used to measure the crystal structure of powders
or thin film materials. The X-rays are produced in an evacuated tube
similar to a cathode ray tube. An applied current heats up a tungsten filament
which liberates electrons. The liberated electrons are accelerated by a high voltage and hit
a copper target where copper x-rays are generated. The x-rays exit the tube and are incident
on the sample from which they are diffracted into a scintillating type detector.
The beam passes through a slit which determines the angular width of the beam. Wider slits
gives more energy but have wider peaks, smaller slits give less energy but better resolution.
The beam also passes through a nickel filter. The nickel filter removes the K-beta energy
before the diffracted beam enters the detector. X-rays at angle theta are reflected from internal
crystal planes separated by distance d. Bragg diffraction results from constructive wave
interference when the quantity 2d sine theta is an integral number of wavelengths.
A fine powder material contains many crystals located at random angles. Certain crystals
happen to be oriented so that the x-ray beam, crystal and detector satisfy Bragg’s equation.
This causes a signal spike at specific detector angles.
It is important to have a sufficient number of crystals to have an even distribution at
all possible crystal orientations. During a scan the detector is rotated over
a range of angles to detect bands of diffracted x-rays produced by the correctly aligned crystals
within the sample. So now we are going discuss sample preparation.
For x-ray diffractometry powder diffraction it is important to have a well-defined polycrystalline
material and the individual particle size of your material should below 45 microns.
You need enough to fill one of these wells and it should be packed in. What is important
is to have enough of each individual crystalline plane so that when the beam hits it at a particular
angle Braggs equation is satisfied and you get a peak. This is a statistics issue so
you want to make sure to have plenty of sample so that all those planes are represented.
There are two sizes of sample holders here. This is preferred, this large well here because
as I said the more particles you have represented on the surface in all the various crystal
planes the better your data is going to be. But often most people don’t have this much
material and this will work. If your material has been prepared correctly it’s just really
important to make sure you have a very fine powder.
We have sample holders in the lab and once your sample has been prepared properly you
are welcome to come in and use our sample holders, but we ask that you clean up after
you are finished and that they should never leave the lab.
Press your sample in the well and just estimate how much you are going to need. You are going
be pressing it down and you want to make sure it’s filling the
well but is still flat with the surface of the sample holder and just gently scrape off
whatever comes out of the well. You want to keep it cleaned up because keep
in mind that you going to put this in and there is going to three pins on three places
on the sample holder and if there is a lot of material underneath there that is going
alter the angle that it is sitting at and you are going to get and your positions will
be off. Flatten it down and that’s ready to go.
Even though it’s loose powder if you have prepared it properly it will actually hold
together on its own. To verify that you have that just kind of tip it up at a 45 degree
angle and make sure that it’s holding. The sample stage is held in place with a pressure
plate. The position of the beam is fixed and therefore the position in the z-axis must
be very accurate. The plane of the sample must be the same as the plane of the sample
holder that is defined by three pins. Once your sample is loaded you can close the
doors. The doors are leaded glass and are part of the safety circuit in the instrument.
So we are generating x-rays and those are radiation and could be dangerous.
Radiation doesn’t travel very far and doesn’t travel through thick objects and it doesn’t
travel through lead, so these are leaded glass. And they have little pins in the handles that
will engage mechanical switches right in here in the door and there are also electronic
switches in the door in the bottom and top. What this does is that it ensures that when
you close the door all the switches are engaged correctly and the that the door is correctly
closed. If the door is not closed correctly it will
cause the safety circuit to trip and it will shut everything down.
There is a control panel here and the exact same set of control panels over there. And
there are LED lights; ready, on, alarm and busy, they’re all important to note what
state they are in. These buttons up here you must never touch. The only button that you
ever want to push on this panel is this big green square button and that’s the door
open button. To check to make sure you have done it correctly just check down here and
verify that the alarm LED has gone off. Two of the mechanical things that are really
important to watch for is how you open and close the door because that can trigger the
safety circuit and also how you load and unload your sample.
The reason that we are concerned about the safety circuit is that inside our evacuated
tube is our tungsten filament and this filament has a really high power across it. And just
like with a regular lightbulb over time that filament will become brittle and break and
we want to extend that amount of time as much as possible. And so when we bring the current
and voltage down and when we bring the current and voltage up we are doing it slowly so that
this filament has a chance to soak and to slowly get up to power and this will help
to extend its lifetime. Now that we have looked at the physical part
of the instrument we are now going to go the computer and this is where we do the controlling
and we control the ramp and where we control the power to the tube.
This is XRD commander, this application should always be on. One of the reasons is that the
XRD commander is what is controlling the x-Ray diffractometer and it’s also controlling
the power. This is where the power is changed by the user, where it says set. And you will
see that right now the black is 40 and 40 kilovolts and milliamps . But the blue indicated
here is the status, that is the current status of the power. At 20 kilovolts and 5 milliamps
is standby status. If XRD commander is not open it will not go into this standby status
and the tube will be at maximum power for too long. And we want to extend the life of
the tube as much as possible so we want it to be able to allow it to go into standby
mode. It will do this automatically after 30 minutes of non use.
When you do want to begin you want to ramp your power up to 40 and 40 kilovolts. It’s
important to not just hit set right here, you want to ramp it up slowly so what you
want to do is input the values first. So we’re going to ramp up by 10 kilovolts first leaving
the current the same and then we are going to ramp up the current. I’m going to put
in 30 kilovolts and 5 milliamps and then press the set button. We want to wait approximately
30 seconds to a minute between each step to give it time to soak at that power.
Alright it’s been about 30 seconds and I am going to ramp this up to 40 kilovolts and
5 milliamps. And now I’m going to go ahead and ramp it go up to 20, so now let’s go
ahead up to 30 milliamps. We could run a scan in this window very easily.
We just put in the start value and the x axis here refers to position of the detector and
we call it 2theta. And the start button here is this set already at 25 2theta and the stop
is at 85 2theta. We have an increment here in values of 2theta. And it’s set right
now at 0.03 2theta. And the scan speed the setting here is 2 seconds per step.
If you want to do a standard scan that is similar to what you see in most of the literature
you would have a start value of 10, and end of 90, your increment would be 0.02 and your
time would be one second and that is a standard scan.
Now the other thing to note is the shutter and x-ray. The shutter is closed and it’s
green which indicates that you can open and close the door. And the power to the x-ray
is on. There are other buttons here open, close, on and off that you do not want to
select. If you select these manually it there can be problems with the timing in the computer,
it is set up to do this automatically. Up here we have another place where you can
actually change the positions of the detector and the sample. Right now the blue, just like
with the power, the blue is the current status of the position and the black is where you
want to change it to. Right now it is at 42 and 85 and I am going to change it to 30 and
60. And there is a little icon up heat that says
“move drives”, just press that and the drives will move to that position. This is
not necessary to do to run your scan. When you hit the start button it’s just going
to go to whatever value you have in here but it is convenient to move the detector and
sample exchanger if it is at a weird angle to start with.
So if I run a scan from this window I can get a perfectly good scan, I can run the standard
or do a quick scan or whatever I want, however, this data will not automatically be saved.
In order to automatically save data I need to set up a parameters file and then call
that parameters file in the Jobs tab. If you come down here at the bottom you see
a number of tabs, in the Jobs tab you this is where we create our job. Go to the top
and you can either select menu create a jobs or you can select the icon that is the same
as the Jobs tab down here, create jobs. And this dialog has a number of fields in it but
the only three that you care about is the sample ID, parameter file, and raw file.
The sample ID is similar to a header, this is just information that will go in your actual
sample. Your parameter file is the parameter file that I will show you how to set up in
just a second. The raw file is the file where your data will be saved. Running a scan from
this window and from your parameters and hitting start here ensures that your data will be
saved automatically. What happens is people will run a scan and then go to lunch, not
get back in time, and someone who is signed up to run the instrument right after them
will run as scan over it and your data is gone. That doesn’t happen if you do it from
the jobs tab. The way to set up a parameters file is in
a separate application and the link to it is on the toolbar down here and it’s called
the XRD wizard. When you open up the wizard you get the click wizard page which is fine.
The only thing you really care about here is to putting your name in here, and determining
your scan definition and verifying your generator voltage and current. Your scan type is locked
coupled and continuous this the standard way to do it. What this means is that your detector
will be following at 2theta from your sample. You put in your start value and stop value
in 2theta. Your step size is also in 2theta, it calculates the number of steps. You put
in your time and it will calculate the total scan time and this is important because it
will calculate your total scan time and this important because you want to know how long
your scan is going to take and this is an important to know when you are reserving your
time for the instrument. Also see that generator voltage and current
the default is 40 and 40 which is the maximum and if you want to change that you can do
that here. I’m just going to open a file that is for
training so you see what one of those looks like.
This is a very quick scan. I’ve got it from 50 to 55 and my step size is 0.5. For purposes
of demonstration that is perfectly fine. If I wanted to do any analysis though that would
be unreasonable. Most analysis software need a number of points for a peak to even recognize
that the peak exists. So the maximum step size recommended is 0.1 but 0.02 is preferred.
Once you have filled this all this in you can just save it as your name and it will
automatically put the .dql extension it and save it in the scan parameters file.
And I’m going to create a job. There is a little button here that will take
me to the file folders. I want to go up one into parameters file and select my training
and then my raw file, select that. Most people have a folder they have created for themselves.
Now once I’ve done that everything is loaded and ready to go and my sample is in the chamber
but the reason I’m not running it right away is that the parameters file power is
for 40 kilovolts and 40 milliamps but the power I have in the actual instrument is still
40 and 30. I want to always make sure that the blue, which is the status, matches whatever
is in the parameters file before I hit the start button because the values that are in
the parameters file takes precedence. So if I’m at 20 and 5 here and I hit start for
a parameter file that is at 40 and 40 it will automatically ramp the power up to 40 and
40 without going through the steps and this is not healthy for the tube.
Now it’s actually up to maximum power and I can go ahead and run my sample.
I recommend that you wait 5 minutes or so while it’s at maximum power. I have run
a number scans at different times after getting it to maximum power and have noticed a small
change in maximum intensity. But for now since we just doing a demo I will just hit the start
button here. The sample ID is now changed to “demonstration”,
it has automatically loaded my start and stop and increment values and its running a scan.
It has moved the stage and detector automatically so everything is being done automatically.
You can see a number of peaks already. This is alumina which is a NIST standard.
Right now it is just drawing lines, if I right click in here you and select dots I can actually
see the datapoints and hopefully you can see right away that this would not be something
that would be recognized in analysis software there is only one data point here and you
want to have a minimum of at least three on its way up and on its way down.
That click indicates that it is finished. The shutter is now closed which means you
can open the door and the power is still on and it’s still at 40 and 40. It is not necessary
to ramp the power down when you are finished. We want to be able to keep it up if someone
is right behind, it’s better for it to stay than it is to go up down. But if no one is
using it for the rest of the day we do want it to go into standby mode. So we just leave
this open and leave it like it is. Minimize it. Get your data out of your scans. Make
sure you remember to fill in the log. And you may remove your sample and you’ve finished
your scan. Your scan file is automatically saved to a
scan folder and we have a shortcut to it on the desktop.
What people do is they find their data when it’s finished and with a USB drive, we also
have a 3 and half inch floppy and CD, whatever you have, find your sample and save it to
whatever drive you have. This sample can be opened in X-diffraction software designed
for that. To open the door there is a big square open
door button here it’s the only button you ever want push.
Press it, the alarm light starts to flash, pull the handles out towards you and away.
When you are removing your sample you want to make sure first that you have hold of your
sample. There is a little switch under here, put your thumb outside of there and then flip
the switch towards you. Resist the temptation to squeeze the switch and the pressure unit
together. This pressure unit is hollow aluminum and if you squeeze it like that you will bend
it. Hang on to your sample, flip the switch and
remove your sample. When your sample has been removed you can close the doors, no need to
slam them. Verify that the doors are closed by checking
the alarm light and you’re finished.