Instrumentation
A thermal analysis system contains a graphite
resistor furnace that can heat a sample from
room temperature to 1750°C. The atmosphere
in the system can be either inert (vacuum,
nitrogen, helium, argon) to protect the
sample from oxidation or active (air, oxygen,
hydrogen) to react with the sample.To determine
small weight changes of the sample during
heating a microbalance is used (fig.1) that
has a resolution of 0.03 µg.
Options
Thermal analysis is a general name for a group
of analytical techniques used to monitor the
behavior of a material as a function of temperature
(or time at a specific temperature).
Table 1 shows an overview of the different
techniques that will be discussed in this note.
To characterize a variety of samples, different
sample holders are available (fig. 2).
Homemade sample holders are also available
that allow the detailed characterization of thin
layers.
Thermogravimetric Analysis (TGA)
In TGA the exact mass of a sample is
determined while it undergoes a temperature
treatment. It is possible to keep the sample
at a constant temperature or a linear temperature
gradient can be applied.
Alternatively, the sample can be heated until a
constant mass is obtained.
Example I: Dehydration and decomposition
of calcium oxalate
Starting materials in production can contain
impurities like an unknown quantity of water.
This water can reduce reactivity or add
unwanted weight to the material – both of
which can cause production problems. To
prevent this from happening a TGA analysis can
help to identify the state of the material.
In this example calcium oxalate is analyzed
using TGA (fig. 3). The first loss of sample
weight is due to the dehydration of water
(12 %).The loss of sample weight is equal to the
quantity of one mole of crystal water.
The second loss of sample weight is due to loss
of CO (19 %) and the third step to loss of CO2
(30 %).When the sample would have been contaminated
with e.g. barium or strontium
oxalate more peaks would be visible. Absence
of these peaks indicates that the material is a
monohydrate and no other major contamination
is present in this material.
Fig. 2: Sample holders as used in TGA (right)
DSC (middle) and for thin films (left).
Technique
Thermogravimetric
Analysis
Differential
Thermal Analysis
Differential
Scanning
Calorimetry
Abbreviation
TGA
DTA
DSC
Applications
Sample purity
Decomposition
Dehydration
Oxidation
Phase changes
Dehydration
Decomposition
Reactions
Heat capacity
Phase changes
Reactions
Calorimetry
Property
Mass
difference
Temperature
difference
Energy
difference
Table 1: Different TA techniques and their
applications.
Example II: The combustion rate of carbon
In production processes, gas purity can be
critical.When carbon parts are heated to high
temperatures, the presence of oxygen can
decrease lifetime. With TGA it is possible to
determine what oxygen concentration can be
tolerated without degrading the carbon parts.
To do so, a carbon sample is heated to
1100°C in a few minutes using a nitrogen
atmosphere with different oxygen concentrations
(<2, 200, 500 vol.ppm). The weight loss
is monitored during 2 hours at 1100°C. The
combustion rate is calculated by measuring
the loss of weight during a specified time.The
results of the TGA measurements are shown
in figure 4, where the sample mass is plotted
as function of time. The combustion rate is
determined from the slope of the curves. By
plotting the combustion rate of carbon at
1100°C against the oxygen concentration, a
linear relation is observed in this concentration
range. The maximum oxygen concentration
in nitrogen that can be tolerated in a
process can be determined on the basis of
this information.
Differential Thermal Analysis (DTA)
Differential Thermal Analysis is an analytical
method in which the sample and an inert
reference material are heated concurrently,
each having its own temperature sensing and
recording apparatus.The temperature changes
that occur in the course of heating are
plotted.This thermogram provides data on the
chemical and physical transformations that
have occurred, such as melting, sublimation,
glass transitions, crystal transitions, and
crystallization.
Fig. 3: Thermogravimetric analysis of calcium
oxalate, the first derivative signal (DTG) shows
how fast the weight change occurs.
Differential Scanning Calorimetry
(DSC)
DSC is used to determine the temperature
and enthalpy of a phase transformation. This
type of information is very useful for quality
control as phase transformations are very
sensitive for purity, thermal stability and
compositions of mixtures. DSC uses additional
metal calibration standards which make it
possible to calculate the energy emitted or
absorbed by a sample (enthalpy).
Hyphenated techniques
The above described thermal analysis
techniques can be performed stand-alone or
combined. The most common hyphenated
techniques are TGA-DSC and TGA-DTA.
It is also possible to couple a gas analyzer to
the thermal analysis system to identify the
emitted vapors, for example to unravel mechanisms
of transformations. Typical analyzers
are a mass-spectrometer (MS) or a fouriertransform
infrared instrument (FT-IR). Emitted
vapors can also be absorbed in gas-trapping
bottles or organic sampling tubes. In this way
analyses can be performed off-line.
Example III: Dehydration of copper
sulphate pentahydrate
Figure 5 shows the results of a TGA-DSC
measurement of copper sulphate pentahydrate.
The different dehydration steps that
occur during heating of the material are
difficult to distinguish in the TGA-signal, but
are clearly visible in the DSC-result. The
majority of the water is lost below 150°C,
but the monohydrate is stable until 244°C.
The loss of water from the monohydrate
corresponds to a weight loss of 7.0%.
Fig. 4: Carbon oxidation as studied at different
oxygen concentrations.
Sample temperature (°C)
TGA
DTG
Sample weight (wt.%)
Weight change (wt.%/min)
Ca(COO)2-H2O
Ca(COO)2
CaCO3
CaO
-6
-5
-4
-3
-2
-1
0
1 110
100
90
80
70
60
50
40
30
0 100 200 300 400 500 600 700 800 900 1000 1100
Time (sec)
Sample mass (mg)
combustion rate: 0.13 µg/min
combustion rate: 4.73 µg/min
combustion rate: 11.05 µg/min
2 vol.ppm O2
229 vol.ppm O2
537 vol.ppm O2
155.1
154.9
155.3
154.7
154.5
153.3
154.1
153.9
153.7
153.5
0 1000 2000 3000 4000 5000 6000 7000 8000 9000
Philips Research Materials Analysis
offers a full range of analytical methods and
expertise to support both research and
manufacturing, serving customers by taking an
integral, solution-oriented approach.
For more information:
Tel./fax: +31 -40-27 43210/43075
E-mail: materialsanalysis@philips.com
http://www.philips.com/materialsanalysis
http://pww.natlab.research.philips.com:25222/
Technical Note 18
December 2005
Jeroen van den Berg
making the invisible visible
Fig. 5: The analysis of copper sulphate pentahydrate
with TGA-DSC.
Example IV: Characterization of epoxy
resins with TGA-DSC
TGA-DSC can also be used to investigate the
thermal and chemical properties of a material.
In this example a customer wanted to replace
a standard epoxy resin with a new epoxy
resin. The most obvious difference between
the old and new epoxy resin is the color, due
to the addition of another pigment. It is
important to know the thermal and chemical
differences between both epoxy resins before
this change is made in the factory. By using
the coupled TGA-DSC the thermal effect of
the hardening reaction was determined using
DSC and the ash residue was measured simultaneously
by TGA.
The samples are first put in a nitrogen atmosphere
at 30°C. Then, the temperature is
increased to 300°C and held there for 15 minutes.
During this period the atmosphere is
changed from nitrogen to air, which allows
combustion of the sample. Subsequently, the
temperature is increased to 1000°C.The TGA
and DSC signals are monitored during the
whole procedure.The results of the TGA and
DSC measurements are shown in figure 6.The
DSC result of the black epoxy is comparable
with that of the orange epoxy resin. An
exothermic reaction is observed at 155°C.
The enthalpy is 17 J/g for both materials.
The TGA results are comparable too. For both
epoxy resins an ash residue of 59 wt.% is
found. The TGA-DSC results show that relevant
properties of the epoxy resins are the
same and that the process parameters do not
have to be changed. Fig. 6: Epoxy resin analysis with TGA-DSC.
Sample temperature (°C)
TGA
DSC
Sample weight (wt.%)
Related heat flow (µV)
Ca(COO)2-H2O
CuSO4-H2O
CuSO4 ?m = -7.0%
-20
-15
-10
-5
0
5 105
95
85
75
65
55
45
0 50 100 150 200 250 300 350 400 450
Sample temperature (°C)
Sample weight (wt.%)
Heat flow (µV)
DSC Epoxy Orange
-5
-4
-3
-2
-1
0
1
2
3
4 110.00
100.00
90.00
80.00
70.00
60.00
50.00
0 100 200 300 400 500 600 700 800 900 1000
DSC Epoxy Black
TGA Epoxy Orange
TGA Epoxy Black