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Module
7.1 - Thermal
Analysis of Materials Lab
Objectives
Upon completing this lab exercise you should:
- Understand what
DSC is and have a basic understanding of how it works
- Be able to operate
a DSC
- Be able to analyze
some types of DSC data
Introduction
Properties of materials vary with temperature. Plastics that are stiff and
electrically insulating at room temperature can become malleable and loose
their insulating qualities at high temperature. Metals can melt, and change
in size, stiffness, and electrical resistivity as temperature is varied. Liquids
can evaporate or freeze. Polymers can crystallize or become amorphous.
For scientists and engineers in a wide range of disciplines it is important
to understand the thermal behavior of materials being studied. Various thermal
analysis techniques have been developed in response to this need, one of which
is Differential Scanning Calorimetry. DSC is an experimental method of determining
the heat absorbed or released by a material as it is heated, cooled, or held
at a constant temperature. From this information it is possible to learn a
lot about the properties of a material, and thoughtful analysis of the data
can often yield information beyond what the DSC can provide directly. Phase
changes, endothermic and exothermic reactions, and changes in specific heat
can be identified using DSC. In some cases it is possible to determine sample
composition as well.
In this lab exercise you will perform at least two DSC runs. First you will
run a sample of pure meal and determine its melting point and latent heat of
fusion from the DSC data. Second you will determine the glass transition temperature
of a polymer and examine the thermodynamics of the transition. It may be necessary
to perform other runs as indicated in the experimental procedure.
Theory
DSC Principle of Operation
To begin most DSC runs two sample pans are placed into a closed furnace. They
are identical except that one contains a sample of the material being studied
and the other is empty. The two pans are then taken through a thermal cycle
set by the DSC operator during which they can be heated, cooled or held at
a constant temperature. During most of the run this takes a nearly equal amount
of energy for both pans. When a physical or chemical change that is endothermic
or exothermic (a ‘thermal transition’) occurs, the energy required
to keep both pans at the same temperature differs significantly. The DSC records
this energy difference and outputs the information to a computer where it can
be plotted as a function of temperature or time.
Latent Heat and Glass Transition
The energy necessary to accomplish a change of phase is referred to as the
latent heat of the phase transition. For example, the amount of heat Q required
to melt a sample of tin of mass m is given by Q = ml where l is the latent
heat of melting of tin. Latent heat has the SI units of Joules/gram. When a
DSC sample melts, there is a corresponding peak in the plotted data due to
the increase in energy being absorbed by the sample. Integration of this melting
peak with respect to time yields the latent heat of melting for the sample
if the sample mass is known.
For amorphous (non-crystalline) polymers there is a temperature above which
they are flexible and below which they are brittle. This is known as the glass
transition temperature or Tg. There is no latent heat associated with the glass
transition, however the heat capacity of a polymer is greater after the glass
transition than it was beforehand. As a result there is no peak in the DSC
data but there is a change in the slope of the trace corresponding to the greater
amount of energy necessary to heat the sample once its heat capacity has increased.
Procedure
The procedure for both runs is essentially the same. If it is possible that
the polymer you are using will evolve liquid or gas during the run, do not
hermetically seal the sample in step 4 and put a small pinhole in the cover
of the pan.
- Obtain a sample
of a pure metal such as Sn, Bi, In, Pb or Zn, or of an amorphous
or semi-crystalline polymer such as polycarbonate.
- Cut a small piece
of the material that weighs between two and ten mg. Larger samples
yield better defined peaks but are more subject to thermal gradients
than smaller samples.
- Place the sample
into a DSC sample pan and then into a crimping press.
- Hermetically
seal the sample pan. Put the crimping press, with sample pan, into
a vacuum chamber and pump down the chamber with a mechanical vacuum
pump. Backfill the chamber with an inert gas such as argon and pump
down again. Backfill and pump down several more times to remove as
much air as possible from the chamber. Finally, with the pan still
under vacuum, seal the pan while it is still inside the vacuum chamber.
Your instructor can provide more precise instructions applicable
to the specific equipment you are using.
- Inspect your
sample pan to make sure that it has been sealed and has a flat bottom.
If the bottom is not flat you can try to flatten it, but reseal the
sample in a new pan if the pan becomes significantly distorted. Flat
pans provide the best thermal contact with the surface of the furnace
and yield the cleanest data. If you have time, consider running a
sample in a pan without a flat bottom for comparison.
- Prepare an empty
reference pan in the same manner as the sample pan.
- Using tweezers,
not your fingers, remove the DSC cell cover and place both pans into
the DSC cell, making sure that they rest flat on the surface of the
furnace.
- Write a procedure
and start the run, following the instructions provided by your instructor
or in the manual included with the DSC software you are using. For
a pure metal a reasonable procedure to start with is equilibrating
at room temperature, heating at 5-10 deg C per min to 30 degrees
above the literature melting temperature, holding the sample at this
temperature for 2 min, and cooling at 5 deg C per min. Possible cooling
rates will be determined by the capabilities of your DSC.
Data
Analysis
Analysis of DSC data is generally done using computer software. Consult your
instructor or the manual of the software package you are using for detailed
instructions and use the questions below to guide your analysis.
Latent Heat of
Melting
Open the data for the pure metal and create a plot with heat flow on the y
axis and time on the x axis. You should notice two peaks, one positive and
one negative, corresponding to the melting and solidification of the metal.
If there are more than two peaks or they are not clearly defined you may need
to take another run. Identify the melting peak and the cooling peak. Zoom in
on the melting peak and use the computer software to find the area under it.
This is your experimental value for the latent heat of melting of your sample.
How does it compare with the literature value? What are possible sources of
error and how can they be minimized?
Notice that the melting and solidification peaks do not occur at the same temperature.
This is due to an effect called undercooling of the metal. Undercooling is
a consequence of the nature of the solidification process and can be explained
using Gibbs free energy considerations. For more information on undercooling
consult chapter four of Porter and Easterling2 or a similar materials science
text.
Glass Transition
Open the data for the polymer sample and create another plot of heat flow v.
time. Identify where the glass transition occurs on the plot. Theoretically
the only thermal event at the glass transition is a change in the heat capacity
of the sample. Is this all that is shown on the DSC trace at the glass transition?
Notice that the glass transition does not occur at a discrete temperature but
the computer software has a function that returns a single glass transition
temperature. How is this done? Use the software to find the Tg for your sample.
How does your value compare with the literature value?
References
1. The University of Southern Mississippi Department of Polymer Science has
an informative and accessible polymer website that includes pages on the
glass transition and DSC at:
http://www.psrc.usm.edu/macrog/index.htm
Most relevant are levels 3 and 5 on the site.
2. D.A. Porter and
K.E. Easterling, Phase Transformations in Metals and Alloys, 2nd ed.
Nelson Thornes, Cheltenham, 2001.
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2.1
Introduction
