This is somewhat misleading. As I will explain below, the amount of heat transferred back into the DSC machine
can be determined using the exotherm trace.
Jones does not state if he used pans with lids or open pans [a peer review would have noted this and had him be specific].
I will explain how the use of lids is irrelevant to the experiment and the conclusions drawn.
The diagrams used in this thread are drawn by me and may be used as a reference when quoting my text within this summary.
The image below is a basic block diagram of what you might see in the typical DSC machine:
- A platen
- Some sort of heating element
- Sensitive temperature monitor
As you will see, there are two plates; one is empty and one holds the object to be tested. The empty chamber on the top left is used as the control
plate. You can monitor the amount of heat sourced to the plate; the amount of heat stored in the plate; the amount of heat leaving the plate.
This is possible because you have a known temperature for the heating source as well as the ambient temperatures. Since heat cannot be created out of
thin air, we can conclude that any heat leaving the control platen is released into the atmosphere.
In essence, you can subtract the platen temperature from the sourced heat and determine the rate at which the ambient air is cooling the platen, and
also the degree of change (delta) between the two.
This is a typical function of Thermal Vacuum Chambers (TVAC) and Hotplates which I use daily in my lab at work. I’ll be happy to show pictures for
those that would like to see one in action.
On the top right you see the exact same configuration, but this is where you place your sample to be tested. Normally, you start testing with a
sample that is at room temperature along with the platen. Therefore there is very little heat transfer occurring at the beginning of the test.
As you heat up the platen, some heat will escape into the atmosphere, or inside of the chamber in the case with the DSC. If the lid is missing, or
installed it wont matter because both sides will lose the same amount of heat at equal rates. This is why we don’t concern ourselves with this
minor attribute. If it becomes a concern, I can e-mail Dr. Jones and ask whether he used the lids, or not. My estimate is that he setup the machine
exactly as LLNL to produce the most accurate results.
Moving forward, I will now explain how the exotherm trace is produced and how we can interpret the basic dips and peaks.
On the bottom right, we have a platen which is heating up along with the device under test (D.U.T.). Just as with the control side, heat is leaving
the plate into the atmosphere but it is also transferring into the D.U.T. Since the control side does not have a mass sitting on top, there will be
no heat lost to a mass. This is where the “Differential” comes into play. The trace is drawn based on the differences between the control platen
and the test platen.
If we were to pick up the DUT from the test side of the platen, it would feel warm (or hot) because the heat from the plate has transferred to the
DUT. Since we know the rate of heating, and the amount of heat lost to the ambient environment, we can conclude that any dip in the trace is heat
stored in the DUT , or additional heat radiated by surface area.
The TVAC and plates that I use at work can sense temperature stability and delta down to 0.00001 of a degree Celsius. This is extremely precise;
about the resolution to measure the change in temperature of a pool if you were to spit in it. The DSC machines are probably more accurate, I would
have to look up the specifications to know for sure.
Now we know how a dip in the trace is produced. So how about the peaks? Since heat cannot be created out of thin air, the peak shown in Jones’
graph must have been additional heat from the chip which began ignition around the 420’C point on the x-axis.
Pteridine contradicts himself when he says you can calculate the amount of energy per unit mass, but there is no way to tell how much temperature is
created from the reaction. Really?
So if we know the rate of heating (10’C), and we know the specifications of the platen, and we know the control temperatures, can we not deduce the
amount of heat put back into the plate by the energy release by the reaction of the chip?
Pteridine, are you stating for the record that the height of the peak, and the width of peak cannot display the amount of energy used to raise the
temperature of the platen?
The temperature shown on the x-axis is the furnace temperature, not the sample temperature, so there is no way of directly measuring flame
So, “Pteridine” if the x-axis is just the furnace temperature and not the sample temperature where did all of that extra heat come from in the
exotherm trace? Watts/gram remains close to zero until 420'C; can you not convert
Watts/gram into heat?
We know the rate of heating was 10’C per minute. Please clarify this point so we can move forward, or concede that the reaction temperature
indeed does show up in the trace.
edit on 4-12-2010 by turbofan because: Clarify exotherm question