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Classroom
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classroom situation. We may suggest strategies for dealing with them, or invite responses,
or both. “Classroom” is equally a forum for raising broader issues and sharing personal
experiences and viewpoints on matters related to teaching and learning science.
Kevin Lukas and Peter K LeMaire
Differential Scanning Calorimetry: Fundamental Overview
This paper discusses the fundamentals of Differential Scanning
Calorimetry (DSC) including functions and outputs of a
Differential Scanning Calorimeter along with common uses
in academia and industry.
Department of Physics, Central
Connecticut State University,
New Britain, CT 06050.
Email: [email protected]
Introduction
A solid material can be amorphous, crystalline, or a combination
of the two. An amorphous material is one with no discernable
orientation for its atoms or molecules; they are arranged in a
random fashion. Crystals on the other hand are arranged in a
specific pattern, which is repeatable and predictable. A material
may also be a combination of amorphous and crystal structure;
this material could be crystalline in one area, and amorphous in
another, or islands of crystalline arrangements which are randomly
distributed in an amorphous system. The thermodynamic state
affects the structure of a material which in turn affects its properties.
Variations in the thermodynamic state of a material thus lead to
variations in its properties. Using Differential Scanning
Calorimetry (DSC), one can better understand phase transitions
and reactions in materials, and how they contribute to the properties
and characteristics of the material.
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Keywords
Heat flow, phase transitions, latent heat.
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DSC
DSC is a method of thermal analysis that is widely used to study thermal transitions, i.e., solidsolid transitions as well as solid-liquid and various other transitions and reactions. A solid-solid
phase transition would be if the material had its structure altered, but not gain enough energy to
become a liquid. Using thermal analysis, it is possible to understand what is happening in a
material, even if there is no visual evidence that a change has occurred. For instance, it is easy
to see when an ice cube melts into water and when water boils into steam; these are visible
changes. There are however several different phase changes within water in a solid state. Ice
at colder and colder temperatures can have several different crystal structures and undergo
many solid-solid phase transitions, and in each of these phases, the ice has different properties
ranging from brittleness to conductivity. By understanding the technique and instrumentation
of DSC, it is possible to understand what the materials go through during energy gain or loss.
Earlier versions of the DSC were known as Differential Thermal Analyzers (DTA).
Instrumentation Basics
In a DSC, there are two pans isolated from the ambient environment in a chamber. One pan
contains the sample to be analyzed, and the other pan is empty and is used as a reference pan.
Both pans have heaters underneath them that are used to raise or lower the temperature. Each
pan also has a sensor that indicates what the temperatures of the pans are at any given moment.
Using computer controlled sensors and logic, the heaters are set to heat the pans at a constant
rate (T/t), that is the rate of temperature change is the same, for example 5 degrees celsius per
minute. The instrumentation is also designed to ensure that the pans heat at the same rate as each
other. Having the pans heat at a constant rate, but also the same rate as each other might seem
repetitious; however the heat flow, which is the rate at which thermal energy (Q/t) is supplied
to the pans, does differ, due to the fact that there is material in one of the pans, and nothing in
the other pan. Consider, for example, two pots, one with water and one empty, and both pots
start out at the same initial temperature and are then placed on stoves. If each pot is heated at
the same rate (both stove settings are placed on low, for example) the pot with the water in it will
heat up slower, that is the rate of temperature change will be smaller than the pot with nothing.
This is because heat capacity is different for water than it is for air. The same thing happens in
the pans of our instrument, the pan with the material/sample in it will heat up slower than the
reference pan which contains nothing. Going back to our example of the two pots, the computer
would control the stoves and set the pots to be heated at a constant 5 degrees per minute, and also
monitor the temperature of each pot and ensure that the two heat at the same rate. Since the pot
with the water in it will heat up slower than the empty pot, the computer will have to supply a
higher heat flow to the pot containing water in order to force the pots to heat at the same rate.
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Figure 1 (left). DSC profile. In this profile, exothermic heat flow is measured versus temperature.
Courtesy: TA Instruments, Inc.
Figure 2 (right). DSC profile. Here the endothermic heat flow is measured versus temperature.
© 2006-2008, PerkinElmer, Inc. All rights reserved. Printed with permission
The computer keeps track of the starting temperature, heating rate, and heat flow, and records
the difference in heat flow between the reference pan and the sample pan (water pot in our
example), which is the heat flow for the sample, and plots it against temperature as shown in
Figures 1 and 2.
As seen from Figures 1 and 2, the heat flow may be measured as exothermic, or endothermic and
plotted against temperature. The slope of the curve is the rate of change of heat capacity,
Cp/t. As previously stated, the heating rate (T/t) is set, and the heat flow (Q/t) is
measured. The heat flow may be divided by the heating rate to obtain a value for heat capacity,
i.e., (Q/t) /(T/t) = Q/T = Cp. Knowing the mass (m) of the sample, the specific heat
capacity (cp= Cp/m), as a function of temperature may be obtained, thus yielding information on
the variations in the physical properties of the material.
Phase Transitions
Every solid state material when continuously heated will come to a point where it changes
phase. For example ice, when heated, at standard atmospheric pressure, will undergo a phase
change from solid to liquid (melt) at a transition temperature (melting point) of 0 oC, and when
the resulting water is heated, it will eventually undergo another phase transition from liquid to
gas (vaporize) at a transition temperature (boiling point) of 100 oC. We are also relatively
familiar with solid to gas transitions in solids such as dry ice. As a material is heated, the atoms
and/or molecules gain more energy. At the solid to liquid transition temperature, the atoms/
molecules have enough energy to break away from their rigid structure to a less restricted state
which is the liquid state. The energy supplied simply goes into converting the solid to liquid, that
is, the energy is spent in breaking down the rigid solid structure into the much less rigid liquid
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state, leading to gain in energy of the material (an endothermic transition). The energy per unit
mass used in bringing about such a phase change is known as the latent heat. Transitions that
have latent heats associated with them are said to be first order transitions. The energy supplied
does not lead to a temperature change until the phase transformation is complete. A similar
process takes place during the phase transformation from liquid to gas.
As discussed earlier a DSC measures the heat absorbed or released by a material as a function
of temperature or time (isothermally). This then allows the determination of important thermal
characteristics such as transition temperatures, for example, melting and boiling points as well
as other transition temperatures such as glass transition temperature (TG), and crystallization
temperature (Tc), which will be discussed later in this article. It also allows for the determination
of quantitative measurements such as the specific heat capacity, heat of fusion, heat of
vaporization, heat of polymerization, heat of crystallization, and heat of reaction. Practically all
DSC instruments allow such measurements to be done under various atmospheres.
Glass Transitions
Glass transitions can only take place in an amorphous material. A glass transition has no latent
heat associated with it, and such transitions are referred to as second order transitions. The heat
flow vs. temperature for a glass transition is as shown in Figure 3.
In this case, the heat capacity has increased and the molecules have become more mobile, and
so the material is now in a state where its atoms/molecules have more energy. In order to keep
the heat rate constant, the sample pan heater needed to increase heat flow. The material is still
amorphous but has undergone a transition from a lower energy state to a higher energy state.
The temperature at which the material undergoes a glass transition is known as the glass
transition temperature (TG), and as seen in Figure 3, the TG is assigned to the mid point
temperature of the range of temperature during the transition. A simple example of a glass
Figure 3. Glass transition
temperature (TG) of Silicon
Rubber.
© 2006–2008, PerkinElmer, Inc.
All rights reserved. Printed with
permission.
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transition can be seen in plastics. When a plastic or rubber material is placed in a freezer, it
becomes very brittle and can easily be shattered, the plastic or rubber is then in a ‘glass’ state.
When it is warmed the plastic or rigid rubber becomes very pliable, or goes into a ‘rubbery’
state. The glass transition temperature of Polyethylene Terephthalate (PET), the plastic material
that various beverage containers are made of, has a TG of about 70 oC, and the TG for glass used
as containers is about 550 oC. A word of caution as to the glass transition temperatures: they
can vary significantly depending on the rate at which the material is heated or cooled. However,
under continuous heating, the next step for an amorphous material will be for the material to
become crystalline.
Crystallization
When a material is crystallized, it becomes ordered. Crystallization is a first order transition,
and thus a latent heat is associated with the transition. As shown in Figure 2, the process is
exothermic, that is, thermal energy is released and the sample gets hotter, and so during
crystallization, the sample heater supplies less energy than the reference heater to maintain the
constant set heat rate. As can be seen from the shape of the crystallization profile in Figure 2,
it takes a while for all the material to go from the amorphous state to the crystalline state, and
so the presence of mixed phases during the crystallization process. The shape is also affected by
constraints of current technology.
The crystallization temperature (Tc) is at the peak of the depression in the graph. The area of the
depression can be obtained and using the heating rate and the mass of the sample, the latent heat
of crystallization may be calculated. At or near the crystallization temperature, the material has
gained enough energy so that its atoms/molecules could re-arrange into a more stable and
ordered state. By nature, materials tend toward the lowest energy state, and the lowest energy
state possible is one of order. A crystalline material can undergo solid-solid phase transitions.
For example Cesium Nitrate, undergoes a solid-solid phase transition at 154 oC [1,2].
Melting
When a material melts, it becomes disordered in the form of a liquid. In materials that are made
up of either several different phases or different components, a DSC may show multiple melting
transitions. There are various theories as to what causes a material to melt and what is actually
happening on the molecular level as it melts. General observations about melting include three
major components. The first is that melting always occurs with an increase in disorder. Second,
there is always a change in volume (positive or negative). Third, there is always a change in heat
capacity. A simple theory of melting backed by experiments, is that of Lindemann (1910)
[3,4,5]. He theorized that melting is caused by instability in the crystal lattice structure due to
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vibrations. As the temperature increases, the vibrational amplitudes increase, finally reaching
a critical fraction of the lattice distance that renders the crystal unstable [5]. A material cannot
melt unless it is a crystal. Therefore, any material that melts must first crystallize. An example
of the heat flow vs. temperature graph during melting can be seen in Figures 1 and 2, showing
melting as an endothermic process.
Similar to when the material gave off heat/energy as it became a crystal, at the melting point
(Tm), it must now absorb energy to go to higher energy state. In this case the sample heater
supplies more energy per unit time as compared to the reference heater, and the temperature of
the material at the melting point will stay constant until the material goes from the solid phase
to the liquid phase as discussed earlier. Once the solid has become a liquid, the liquid can then
proceed to heat up. So in the graph of the heat flow vs. temperature, the heat flow to the sample
must increase dramatically in order for the sample to melt and still keep the same heating rate
as the reference pan. The peak of the curve is the melting temperature (Tm) and the area under
the curve may be used to find the latent heat of fusion (melting).
Figure 1 shows the range of transitions. An amorphous material will first undergo a glass
transition where its heat capacity is increased, but no latent heat is present. The material will
then crystallize and release energy. The material can undergo solid-solid phase transitions by
re-crystallizing into different arrangements; this is known as polymorphism, followed eventually
by melting and may undergo additional phase changes in the liquid phase. Inherently crystalline
materials thus may only undergo solid to solid transitions and then melt. As mentioned earlier,
depending on the thermodynamic state of the material it may undergo a solid to gas transition
in a process known as sublimation, and the latent heat of sublimation may be measured.
Thermogravimetric Analysis (TGA)
A thermogravimetric analyzer (TGA) is used to complement a DSC, and measures mass
changes of a sample as a function of temperature or time. The measurements also can be done
under various atmospheres just like the DSC. The TGA thus allows, among others, the
determination of thermal and oxidative stability, and optimum processing conditions. The TGA
can be used as a stand alone instrument, but it is often used in conjunction with other thermal
equipment such as the DSC. For example, if one sees an endothermic or exothermic reaction
using a DSC, the TGA measurements will help determine whether the change is physical or
chemical in nature. The reaction is chemical in nature if a mass change is associated with it and
physical if no mass change occurs. An example of a physical change is melting of a solid to
liquid, and iron rusting in air is an example of a chemical change.
Figure 4 shows a simultaneous DSC and TGA thermal profile of Barium Yttrium Oxides,
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Figure 4. Thermal profile of
Barium Yttrium Oxide
showing TGA (weight changes) and DSC (heat flow).
showing loss of adhered and hydrated water in the sample followed by decomposition into
component crystalline and amorphous materials. The latter was confirmed by X-ray Diffraction
measurements [6].
Applications of DSC
In addition to materials research applications, Thermal Analysis (DSC and TGA) have found
uses in a large number of industries, such as tools manufacturing, semiconductor technology,
battery safety, food technology, explosives manufacturing, metallurgy, cosmetics, textiles,
energy companies – petroleum and coal. Others include applications in quality control, testing
and catalysis, for example, for the screening of catalysts for CO oxidation in the automotive
industry. For explosives development, a DSC may be used to measure safely, the energy
released by a small amount of a sample without any catastrophic consequences.
DSC is used for many industrial applications which include finding polymorphic transitions,
melting points/profiles, glass transition (softening point), thermal history/processing conditions,
crystallization temperature, percent crystallinity, thermal safety/stability, effect of additives
(plasticizers, etc.), polymer blends, degree of cure, and protein denaturation.
In the study of Copper Iodide (CuI) nanocrystals, crystallographic phase transitions from
“zincblende (-phase) to distorted wurtzite (-phase) to disordered zincblende (-phase)” were
investigated using DSC, obtaining transition temperatures, enthalpies and other thermodynamic
information to help explain the process for the formation of these nanocrystals [7]. In materials
research DSCs and TGAs are often used to verify and/or corroborate results from other research
methods. In the case of the mixed conductor Ag3.0CS2 [8], an amorphous material that conducts
both electrons and ions, electrical conductivity data showed an abrupt change at about 178 oC.
DSC showed what seems to be a crystallization transition at that temperature in support of the
conductivity results.
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Figure 5. Industrial raw
material inspection.
© 2006–2008, PerkinElmer,
Inc. All rights reserved.
Printed with permission.
One major purpose of DSC is to identify a material. Similar DSC profiles may suggest similar
materials. For example, by running samples and finding the value for Tg and Tm, the material can
be identified and qualified by running a sample of an unknown content and comparing the DSC
profile to that of a known material. In the example in Figure 5, DSC is used in quality assurance
and control, where “incoming Nylon’s melting point is compared to the quality standard for
melting temperature”. In this case it turns out that the incoming nylon turns out to be Nylon 11
instead of the required Nylon 6 which has a higher melting point.
There are many practical applications for DSC during the fabrication of a component or
material. No matter how much planning goes into design and fabrication there may still be
problems that may need to be addressed: a material may melt when it should not break when it
should not or not act as it was designed, etc. Every time that a component fails or does not act
as expected, an investigative analysis is run called ‘failure analysis’. During failure analysis,
DSC is a powerful tool that can be used to see if there is something going on in the material that
may not be visible to the naked eye, under a microscope, or during other tests.
Figure 6 shows another example of the use of DSC in industry and research. In this case the
effect of additives on the oxidative stability of polyethylene is studied by measuring the
Oxidative Induction Time (OIT). As seen in the figure, the “failed polyethylene sample lasted
less than 5 minutes in an oxidative atmosphere before decomposing”.
With improvements in DSC technology more information can be obtained using DSC. In
Figure 7, Perkin Elmer’s Jade DSC is used to study liquid crystal phase transitions by running
a cooling and heating cycle between 30 oC and 155 oC. The high resolution and sensitivity of the
instrument allows otherwise barely visible transitions to be observed. Advancements in cooling
accessories have also improved the application of DSCs at sub-ambient temperatures, usually
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Figure 6. Oxidative Induction Time measurements
on polyethylene.
© 2006–2008, PerkinElmer, Inc.
All rights reserved. Printed with
permission.
down to liquid nitrogen temperatures. For example, the “CryoFill” automated liquid nitrogen
cooling DSC accessory, allows easier determination of transitions, such as TG, even at
extremely low temperatures. As indicated earlier, the shape of thermal profiles measured by
DSC is affected by the state of current technology. In Figure 8, Advanced TzeroTM technology
is shown to improve the profile of the melting peak of Indium to be closer to the true shape,
which is supposed to be a well-defined sharp peak.
Employee health and safety issues are high priority, especially in today’s work place. DSC is
used to analyze samples for potential health issues. For example, long-term overexposure to
respirable crystalline silica and other particulates may cause silicosis and other respiratory
diseases [9]. Worker safety regulatory agencies such as the Occupational Safety and Health
Agency (OSHA) of the United States Department of Labor has ruled that “an employee’s
exposure to particulates, not otherwise regulated, should not exceed 5 mg/m3 (respirable dust),
Figure 7. Liquid crystal phase transitions.
© 2006-2008, PerkinElmer, Inc. All rights reserved. Printed with permission.
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Figure 8. Melting of Indium
showing TzeroTM technology.
Courtesy: TA Instruments, Inc.
15 mg/m3 (total dust), 8-hr time weighted average” [9]. By properly identifying the percentage
of toxic material, the safety and well being of all employees can be ensured.
DSC plays a critical role in the study of explosive material. In a search for “more powerful, safer
and environmentally friendly explosives” DSC was used to study the solid-solid phase transition
kinetics of 1,1-diamino-2,2-dinitroethene (FOX-7) and to measure the enthalpies associated
with the transitions [10].
As a material ‘ages’, its thermal profile changes. In pharmaceutical industry, DSC is used to
study ‘aging’ and shelf life of pharmaceuticals, as well as other basic research and development
[11,12]. An example is in the study of the two polymorhs of the drug ‘carbamazepine’ and to
accurately determine associated enthalpy [12].
In the Food Industry, DSC has numerous applications to monitor thermal events discussed
earlier such as melting, crystallization, etc, as well as decomposition, denaturation, dehydration,
polymorphism, oxidation, etc [13,14,15,16,17]. For example “the effect of storage temperature
on phase transitions of the lipid components” in apple and capsicum cuticles was studied using
DSC, and it was found that at “low temperatures, the cuticular membrane underwent a change
in structure and the melting enthalpy of the waxes present decreased” [13].
DSC finds use in both industry and academia. It has a range of practical applications, and its
fundamental principles are important in understanding material structure.
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Acknowledgements
The authors are indebted to Prof. Sadanand Nanjundiah for his encouragement in getting this
article prepared, and to TA Instruments Inc., and Perkin Elmer Inc., for providing high quality
thermograms and figures.
Suggested Reading
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[3]
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[4]
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[5]
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of FOX-7, University of California, Lawrence Livermore National Laboratory, 2005 NATAS Annual Conference Universal City, CA, United States.
[11]
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[13]
[14]
P Aggarwal, Thermochimica Acta, Vols.367–368, 9–13, 2001.
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[15]
Semih Otles, Serkan Otles, Electronic Journal of Polish Agricultural Universities (EJPAU), Vol.8, No. 4,
2005. or www.ejpau.media.pl/volume8/issue4/art-69.html
[16]
http://www.setaram.com/files/documents/Characterization-Food-products.pdf
[17]
Rachit Suwapanich and Methinee Haewsungcharern, Effect of Storage Temperature on Thermal Properties of
mango cv. Nam Dok Mai Si Thong, Conference on International Agricultural Research for Development,
Tropentag 2005, Stuttgart Hohenheim, October 11–13, 2005.
[18]
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[19]
Differential Scanning Calorimetry, Glass Transition, Department of Polymer Science, University of Southern
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