Download What is the DSC used for

Izmir Institute of Technology
Mechanical Engineering Department
ME 206 Thermodynamics-I
DETERMINATION OF SPECIFIC HEAT (Cp)
Lab: Center for Geothermal Research & Application
Researcher: Ebru KUZGUNKAYA
What is DSC used for?
Differential scanning calorimetry (DSC) is a thermal analysis technique that measures the
temperatures and heat flows associated with transitions in materials as a function of time and
temperature. Such measurements provide qualitative and quantitative information about
physical and chemical changes that involve endothermic or exothermic processes or changes
in heat capacity. DSC is a valuable analytical technique for polymers, organic chemicals,
pharmaceuticals, oils and greases, foods and biological materials, as well as metals and some
inorganic materials. Typical measurements include:
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Glass transitions in amorphous/semi-crystalline materials
Melting points, boiling points
Crystallization time and temperature, percent crystallinity
Heats of fusion and reactions
Oxidative stability
Purity
Rate of cure, degree of cure
Reaction kinetics
Specific heat and heat capacity
Thermal stability
Sample Types
The DSC can be used to analyze virtually any material that can be put into a DSC sample pan.
This includes:
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Films
Fibers
Powders
Solutions
Composites
How Does a DSC Work?
In a "heat flux" DSC, the sample material, encapsulated in a pan, and an empty reference pan
sit on a thermoelectric disk surrounded by a furnace. As the temperature of the furnace is
changed (usually by heating at a linear rate), heat is transferred to the sample and reference
through the thermoelectric disk. The differential heat flow to the sample and reference is
measured by area thermocouples using the thermal equivalent of Ohm's Law.
Fig. 1. Principle work of DSC .
q
T
R
(W)
(1)
q
= sample heat flow (W)
T
= temperature diffrence between sample and reference (ºC or K)
R
= resistance of the thermoelectric disk (K/W)
This simple relationship, however, does not take into account extraneous heat flow within the
sensor or between the sensor and sample pan.
The cell sensor consists of a constantan body with separate raised platforms or vessel to hold
the sample and reference (Fig. 1). The platforms are connected to the heating block (base) by
thin-walled tubes that create thermal resistances between the platforms and the base. Area
detectors (thermocouples) on the underside of each platform measure the temperature of the
sample and reference. A third thermocouple measures the temperature at the base. The
equation 2 shows the thermal network model which represents this cell arrangement, and the
resultant heat flow expression that describes this cell arrangement (designated the To
[Tzero™] cell) is
q
 R  Rs
T
 T0  r
Rr
 Rr Rs

dT
dT
  Cr  C s  s  Cr
dt
dt

(2)
The first term in this expression is the equivalent of the conventional single-term DSC heat
flow expression. The second and third terms account for differences between the sample and
reference resistances and capacitances respectively. These terms have their largest impact
during regions of the thermal curve where the heat capacity of the sample is the predominant
contributor to heat flow. The fourth term accounts for the difference in heating rate between
the sample and reference. This term has its largest impact during enthalpic events (e.g.,
melting). This equation can be further modified to account for pan heat flow effects.
Specific Heat Calculation
The continuous method consists in performing two runs with two vessels at the same scanning
rate: the measuring vessel and the reference vessel. The first run is performed with the two
empty vessels (blank test), the second run with a mass ms of sample in the measuring vessel.
Each run consists in a ramp between an initial temperature Ti and a final temperature Tf at
constant heating rate (Fig. 2). If Qb and Qs are the amplitudes measured with the blank and
with the sample respectively, the value of specific heat Cp is:
Cp 
Qs  Qb
.60
hr .m
(J/gK)
Q
= sample heat flow (mW)
hr
= heating rate (°C/min)
m
= sample weight (mg)
b
= blank
Fig. 2. Continuous method to calculate Cp.
(3)
Results
a) Give the procedure of the experiment.
b) Calculate Cp depending on temperature.
c) Plot a graph which shows Cp (y axis) versus temperature (x axis).