DSC- Basics PPT.ppt

2/7/2023 Dr.Anand, MME, NITK 1
Thermal Analysis
Differential Scanning
Calorimetry (DSC)

Dr.Anand, MME, NITK 2
2/7/2023
Differential Scanning Calorimetry
(DSC)
 Principle
DSC measures the differences in heat flow into
a substance and a reference as a function of
sample temperature while both are subjected to
a controlled temperature program
 DSC provides access to accurate
thermodynamic data as well as information
regarding reactivity and phase
transformations

-0.4
-0.3
-0.2
-0.1
0.0
0.1
Heat
Flow
(W/g)
0 25 50 75 100 125 150
Temperature (°C)
Exo Up
Endothermic Heat Flow
 Heat flows into the sample as a result of either
Heat capacity (heating)
Glass Transition (Tg)
Melting
Evaporation
Other endothermic processes
Endothermic

-0.1
0.0
0.1
Heat
Flow
(W/g)
0 20 40 60 80 100 120 140 160
Temperature (°C)
Exo Up
Exothermic Heat Flow
 Heat flows out of the sample as a result of either
 Heat capacity (cooling)
 Crystallization
 Curing
 Oxidation
 Other exothermic processes
Exothermic

DSC Heat Flow
t)
(T,
dt
dT
Cp
dt
dH
f


signal
flow
heat
DSC
dt
dH

Weight
Sample
Heat x
Specific
Sample
Capacity
Heat
Sample
Cp


Rate
Heating
dt
dT

(kinetic)
re
temperatu
absolute
an
at
time
of
function
is
that
flow
Heat
t)
(T, 
f

2/7/2023
 DSC is the most sophisticated and
advanced of the thermal methods.
 There are two principal types:
power compensated DSC
heat-flux DSC
DSC - types

2/7/2023
Power Compensated DSC-
principles
 Temperature difference is maintained
zero, i.e., ΔT = 0, by supplying heat into
the sample or reference according to heat
emission or absorption
 Electrical power is proportional to heat
change in the sample
i.e., P = I2.R

2/7/2023
Power Compensated DSC-
principles
 Rate of change of power input is plotted
against average S & R temperature
 x-axis (abscissa) is temperature and the y-
axis (ordinate) is difference in power input
(which is proportional to the heat change
i.e., enthalpy)

2/7/2023
Instrumentation

2/7/2023
Furnace and sample holder

2/7/2023
 Small, flat samples are contained in shallow
pans, with the aim of making a good
thermal contact between sample, pan and
heat flux plate.
 Symmetrical heating of the cell, and
therefore S and R, is achieved by
constructing the furnace from a metal of
high thermal conductivity – for example,
silver
How does it work?

2/7/2023
How does it work?
 Sample and reference material are heated
by separate heaters in two independent
furnaces
 The furnaces are imbedded in a large
temperature-controlled heat sink
 Sample holders are above the furnaces
 Pt resistance thermometers are imbedded
in the furnaces to monitor the
temperatures of sample and reference
continuously

2/7/2023
How does it work?
 There is provision for establishing gas
flow through the cell, to sweep away
volatiles, provide the required atmosphere,
and to assist in heat transfer.
 Control of the furnace, signal acquisition,
and data storage and analysis are
handled by a computer.

2/7/2023
How does it work?
 Two control circuits are used to obtain
differential thermograms
One for average temperature control
One for ΔT control
 S and R temp. signals are fed into a
differential amplifier via a comparator circuit
that determines which is greater
 The amplifier output then adjusts the power
input to the two furnaces in such a way that
their temperatures are kept identical

2/7/2023
How does it work?
 Throughout the experiment, S and R are
isothermal
 A signal proportional to difference in power
input to the S and R furnaces is
transmitted to the data acquisition system
 The power differences are plotted as a
function of the sample temperature and its
unit is milli Watts (mW)

2/7/2023
Heat flux DSC
Constantan disk
Chromel disk
Chromel wire
Alumel wire

2/7/2023
How does it work?
 S and R are heated by a single heater
 Differential heat flow into the S and R pans is
monitored by chromel disk/constantan
thermocouple
 Differential heat flow in to S and R pans is
directly proportional to the difference in output
of the two thermocouple junctions
 Sample temp. is estimated by chromel/alumel
junction under the sample disk

2/7/2023
Purge gases
 Typical purge gases are air/N2
 He is useful for efficient heat transfer and
removal of volatiles.
 Ar is preferred as an inert purge when
examining samples that can react with
nitrogen.
 The experiment can also be carried out
under a vacuum or under high pressure.

2/7/2023
Crucibles
Choice of crucible is critical.
• Thermal properties of crucible.
• Reactive properties with samples.
• Catalytic behaviour with samples.
Aluminum: inexpensive, low temp
Copper: used as catalyst (testing polymers)
Gold: higher temp, expensive
Platinum: still higher temp, expensive.
Alumina (Al2O3): very high temp
Sapphire: crystalline alumina, more
chemically resistant than amorphous Al2O3.

2/7/2023
DSC thermograms
S – glass transition
Ex – exothermic reaction
En – endothermic reaction
Exo
Endo
DSC thermogram of poly(ethylene terephthalate)

2/7/2023
Enthalpy changes
 The DSC curve may show an exothermic or
endothermic peak
 The enthalpy changes associated with the events
occurring are given by the area under the peaks.
 Peaks may be characterized by:
 Position (i.e., start, end, extrapolated onset and peak
temperatures)
 Size (related to the amount of material and energy of
the reaction)
 Shape (which can be related to the kinetics of the
process)

2/7/2023
Process Exotherm Endotherm
Solid-solid transition * *
Crystallization *
Melting *
Vaporisation *
Sublimation *
Adsorption *
Desorption *
Desolvation (drying) *
Decomposition * *
Solid-solid reaction * *
Solid-liquid reaction * *
Solid-gas reaction * *
Curing *
Polymerization *
Catalytic reactions *
Some possible processes giving enthalpic peaks

2/7/2023
Exo
Endo
7.96 Cal/g
8.04 Cal/g
crystallization
melting
Measurement of ΔH of thermal
transitions

2/7/2023

2/7/2023
Calibration of DSC
 Temperature calibration is carried out by
running standard materials, usually very
pure metals with accurately known
melting points.
 Energy calibration may be carried out by
using either known heats of fusion for
metals, commonly indium, or known heat
capacities.

2/7/2023

-30
-25
-20
-15
-10
-5
0
Heat
flow
(mW)
140 145 150 155 160 165 170 175 180
Temperature (°C)
Indium as a Measure of Sensitivity & Resolution
Height
Width at Half-Height
Peak Height Increases
Peak Width Decreases
Height/Width Increases

Melting of Indium
157.01°C
156.60°C
28.50J/g
Indium
5.7mg
10°C/min
-25
-20
-15
-10
-5
0
Heat
Flow
(mW)
150 155 160 165
Temperature (°C)
Exo Up Universal V4.0B TA Instruments
Peak Temperature
Extrapolated
Onset
Temperature
Heat of
Fusion
For pure, low molecular
weight materials
(mw<500 g/mol) use
Extrapolated Onset as
Melting Temperature

Melting of PET
249.70°C
236.15°C
52.19J/g
PET
6.79mg
10°C/min
-7
-6
-5
-4
-3
-2
-1
Heat
Flow
(mW)
200 210 220 230 240 250 260 270
Temperature (°C)
Exo Up Universal V4.0B TA Instruments
Extrapolated
Onset
Temperature
Peak Temperature
Heat of
Fusion
For polymers, use Peak as Melting Temperature

2/7/2023
Effect of heating rate
Exo
Endo
Temperature (0 C)
Heat
flow
(mW)

2/7/2023
Effect of heating rate
 Slower heating rates will more accurately
depict the onset temperature of
transformation
 Two transformations which are very close
in temperature range may be more
distinctly seen as separate peaks, whereas
they may be mistaken for a single
transformation under a rapid heating rate

Comparison of First and Second Heating Runs
0 5
0 1
0
0 1
5
0 2
0
0 2
5
0 3
0
0
-
0
.
2
4
-
0
.
2
0
-
0
.
1
6
-
0
.
1
2
-
0
.
0
8
-
0
.
0
4
T
e
m
p
e
r
a
t
u
r
e
(
°
C
)
H
e
a
t
F
l
o
w
(
W
/
g
)
T
g
T
g
1
5
5
.
9
3
°
C
1
0
2
.
6
4
°
C
2
0
.
3
8
J
/
g
R
e
s
i
d
u
a
l
C
u
r
e
F
i
r
s
t
S
e
c
o
n
d

Method Design Rules
 Start Temperature
 Generally, the baseline should have two (2) minutes to
completely stabilize prior to the transition of interest.
Therefore, at 10°C/min., start at least 20°C below the
transition onset temperature
 End Temperature
 Allow a two (2) minute baseline after the transition of
interest in order to correctly select integration or analysis
limits
 Don’t Decompose sample in DSC Cell

Selecting Optimum Experimental Conditions
 "Always" run a TGA experiment before beginning DSC tests
on new materials
 Heat approximately 10mg sample in the TGA at 10°C/min
to determine:
 Volatile content
Unbound water or solvent is usually lost over a broader
temperature range and a lower temperature than a
hydrate/solvate
 Decomposition temperature
DSC results are of little value once the sample has lost
5% weight due to decomposition (not desolvation)
Decomposition is a kinetic process (time + temperature
dependent). The measured decomposition temperature
will shift to lower temperatures at lower heat rates

2/7/2023 Dr.Anand, MME, NITK 37
Applications of DSC in
materials analysis

2/7/2023
 Here is the DSC curve for a polymeric material such
as high density polyethylene (HDPE). We see
three phase transition temperatures: glass
transition temperature (Tg), crystallization
temperature (Tc), and the melting temperature (Tm)
Tg
Tc
Tm
Polymer characterization

2/7/2023

Glass Transitions
 The change in heat capacity at the glass transition is a
measure of the amount of amorphous phase in the sample
 Enthalpic recovery at the glass transition is a measure of
order in the amorphous phase. Annealing or storage at
temperatures just below Tg permit development of order as
the sample moves towards equilibrium
 The glass transition is a step change in molecular mobility
(in the amorphous phase of a sample) that results in a step
change in heat capacity
 The material is rigid below the glass transition temperature
and rubbery above it.
 Amorphous materials flow, they do not melt (no DSC melt
peak)

Measuring/Reporting Glass Transitions
 The glass transition is always a temperature range
 The molecular motion associated with the glass transition is
time dependent. Therefore, Tg increases when heating
rate increases or test frequency (MDSC®, DMA, DEA, etc.)
increases.
 When reporting Tg, it is necessary to state the test method
(DSC, DMA, etc.), experimental conditions (heating rate,
sample size, etc.) and how Tg was determined
 Midpoint based on ½ Cp or inflection (peak in derivative)

Glass Transition Analysis
Polystyrene
9.67mg
10°C/min

Step Change in Cp at the Glass Transition
% Amorphous = 0.145/0.353= 41%
PET
9.43mg

What Affects the Glass Transition?
Heating Rate
Heating & Cooling
Aging
Molecular Weight
Plasticizer
Filler
Crystalline Content
Copolymers
Side Chains
Polymer Backbone
Hydrogen Bonding
Anything that affects the
mobility of the molecules,
affects the Heat Capacity and,
in turn, the Glass Transition

2/7/2023
 Tg may be used to identify polymers
 The amount or effectiveness of a plasticizer may
be judged by how much it reduces Tg or affects
the shape of the transition.
 Examination of the transitions in polymer blends
gives information as to their compatibility.
 Curing reactions result in an increase in Tg and
measurements can be used to monitor the extent
of cure.

2/7/2023
It is well known that Tg increases with increasing molecular weight, M. This is
expressed by the Fox and Flory equation: Tg = Tg() - Kg/M
where Tg() is the limiting Tg at a very high molecular weight and
Kg is a constant.

2/7/2023

2/7/2023
 Thermogram of the blend shows two distinct Tg.
 Therefore, the components of this blend are
immiscible in each other
exo
endo
Polymer A
Polymer B
Blend of A+B

2/7/2023
 Tg also varies with chain length for a related
group of polymers.
 Additional features occurring in the glass
transition region, often a superimposed
endothermic peak, are related to the aging
undergone by the material in the glassy state,
and can sometimes obscure the transition,
making precise temperature measurement
difficult.

2/7/2023
This is due to aging

2/7/2023
Analysis of explosives
 Ammonium perchlorate is an important
component of high explosives. The
stability of this material is critical to their
safe handling. Mechanism of
decomposition was investigated.

2/7/2023
solid-solid phase transition
to the cubic phase
decomposition
Literature values for Ea 37 - 260 kJ/mol with different
mechanisms proposed. This work clarified the mechanism
and identified the activation energy as 115 kJ/mol.

2/7/2023
Analysis of lubricants
 An important test in the automotive
industry is to determine the stability of
lubricating oils at elevated temperatures
and pressures. This will impact its utility
as a lubricant in motors. In this case, the
oil is brought to a high operating
temperature and held there under an
oxygen atmosphere.

2/7/2023
At some point, the oil begins to oxidize and then
quickly decomposes exothermically. Note how the
synthetic oil has a much longer oxidation induction
time (OIT) than does the mineral oil.

Guidelines for interpreting data
2/7/2023
It is better to have some idea of what transitions to look for and why.
For example:
1. What type of sample is it?
2. What type of transitions can it undergo?
3. What would any changes appear as?
4. What is the temperature range of interest?
5. What data are available from complimentary techniques, e.g. TGA?
6. Has there been any previous analysis?
Then examine the data:
1. Is the event an endotherm or exotherm?
2. Is the event repeatable on a fresh sample or on a reheat?
3. What happens on cooling?
4. Is the event the same in a sealed and unsealed pan?
5. Is the transition sharp or gradual, large or small?
6. Does the event look real? Thermal events are not normally excessively sharp.

2/7/2023
Text book
 D. A. Skoog et al., Principles of
instrumental analysis, fifth edition,
Harcourt Publishers, 2001.
 R. F. Speyer, Thermal analysis of
materials, Marcel Dekker, 1994.
Reference

DSC- Basics PPT.ppt

Recommended

Recommended

More Related Content

What's hot

What's hot (20)

Similar to DSC- Basics PPT.ppt

Similar to DSC- Basics PPT.ppt (20)

Recently uploaded

Recently uploaded (20)

DSC- Basics PPT.ppt