| A
newly developed sensor based on semi-conducting materials makes
an existing DSC system approximately 15 times more sensitive than
conventional instruments. Applications
described below demonstrate the significantly higher performance. |
Figure 1. The new highly
sensitive semi-conductor DSC sensor |
Knowledge of thermal properties is critical
for development, processing and application of most materials and products.
Endothermal effects such as softening of amorphous portions or melting
of crystalline components, as well as exothermal reactions such as crystallization
during cooling, play a decisive role in characterization of substances.
Understanding thermal properties in the fields of polymer technology,
pharmaceutical, chemical and food technologies is also on demand, as
it is in the automobile, space and aircraft industries.
Differential Scanning Calorimetry
For determination of quantitative thermal
parameters (properties) in research and development, quality assurance
and failure analysis, Differential Scanning Calorimetry (DSC) according
to DIN 51007 [1] or ASTM E 793 [2] and ASTM E 1356 [3]
has established its firm place within the analytical lab [4].
Until now, the more capable DSC instruments on the market have been
differentiated mainly by things like the user-interface, operating software,
or perhaps peripheral devices and accessories (e.g. different kinds
of controlled cooling systems, automatic sample changers, etc.).
However, after years of stagnation, the
sensor - the actual heart of the DSC cell - has recently regained importance
in order to approach the emerging challenges of micro calorimetry and
biotechnology, yet maintaining the original advantages of DSC regarding
ease-of-operation, speed, and sample preparation. Thus, the major goal
in this new development was to create a considerably more sensitive
DSC sensor while simultaneously maintaining fast response time. These
seemingly contradictory requirements for heat-flow sensors were challenging
to be overcome. Additionally, the new sensor material had to have not
only good temperature shock resistance, but also an outstanding stability
against chemical corrosion.
The new DSC sensor
This "work of art" was achieved by joint
cooperation with a renowned German institute in the field of aerospace
technology. The result is a sensor approximately 15 times more sensitive
than conventional sensors - with a short time constant of less than
3 seconds. This unique, new heat-flux sensor (figure 1) is based on
a semi-conducting material and can be installed directly into the proven
DSC 204 Phoenix.
The wide temperature range where the sensor
can be applied spans from ñ150 to 400¡C. Heating and cooling
rates from 5 to 20 ¡ C/min can be employed. Since the Phoenix
is equipped with an exchangeable sensor plate, the sensor can be installed
into already existing measuring cells. Due to its flexibility, the Phoenix
with the new sensor can even be operated with an automatic sample changer.
Performance in detail
Secondary transitions, e.g. glass transition
temperature Tg, are usually better detected in a DSC when high sample
weights and higher heating rates are used [5, 6]. As far as
highly-filled or semi crystalline polymers with a very low amorphous
component, such as POM (polyoxymethylene), PE (polyethylene) or PP (polypropylene),
are concerned - this is limited using a conventional DSC. Instead, dynamic
mechanical analysis is commonly preferred in practice, where the Tg
can be determined by means of the peak maximum of the loss modulus or
the mechanical loss factor.
Due to the high sensitivity of this new
sensor, however, it is now possible to detect 2nd order phase
transitions with virtually no difficulty. Figure 2 depicts glass transitions
for a resin-coated, high-gloss paper in the blue DSC curve. It is possible
to determine not only the Tg for the paper (cellulose) at 24¡C - but
also the Tg for the 40 µm thick coating at 69¡C with a change in
specific heat (D cp) of 0.03 J/g◊ K with the help of the
derivated DSC curve (DDSC, red dash-dotted curve) ñ and, interestingly
enough, with a sample weight of only 2.74 mg and heating rate of 20
K/min.
Figure 2. DSC curve and its 1st
derivative for detection of the glass transitions
of a resin-coated paper
In the field of food technology, e.g.
the thermal behavior of hydrous starch solutions, is often analyzed.
Figure 3 shows the gelatinization at 63¡C and decomposition of the amylose-lipid
complex of a solution consisting of 5 mg noodles and 10 mg water during
the 1st heating (blue curve). The 2nd heating
(green curve) provides an endothermic effect at 65¡C with a change in
specific heat of 0.05 J/g× K due to the glass transition. The measurement
was carried out in closed aluminum crucibles at a heating rate of 5
K/min. For measurements to higher temperatures, medium pressure crucibles
are recommended. These pressure crucibles made of stainless steel have
a higher heat capacity and thermal inertia, but only slightly reduce
the sensitivity of the new sensor. The resolution performance at a correspondingly
lower heating rate is comparable to the aluminum crucibles.
Figure 3. Comparison of the 1st
(blue curve) with the 2nd heating (green)
of a water-solved noodle sample
Figure 4 depicts the considerably better
sensitivity of the new sensor (red DSC curve) compared to a conventional
heat-flux sensor (green curve) with the example of an amorphous PET
(polyethylene terephthalate). Presenting the y-axis in µV/mg (direct
sensor signal, standardized sample weight) an approximately 18 times
larger melting peak area and peak height can be realized with the new
sensor. The signal-time-constant (peak shape) is approximately the same.
The short time constant, important for separation of effects which are
close together, is therefore of the same magnitude.
Figure 4. Comparison of the sensitivity
on thermal effects of PET
(red curve: new DSC sensor, green curve: conventional sensor)
The considerably better signal-to-noise
ratio (red DSC curve) in comparison with a conventional sensor (blue
curve) is shown in figure 5 with the example of a multiple, endothermal
phase transition of the liquid crystal MHPOBC. The presented raw data
was recorded at a high data acquisition rate at a heating rate of 2
K/min in a nitrogen atmosphere, without using a temperature calibration.
The sample weight for the standard sensor was 4.0 mg and 4.6 mg for
the new sensor, respectively.
Figure 5. Presentation of the
signal-to-noise-ratio with the example of two DSC measurements
on one liquid crystal. New
sensor (red curve), conventional sensor (blue curve)
Finally, for exact determination of the
specific heat cp, a stable baseline as well as high reproducibility
are necessary. Compared to literature values for standard materials,
smallest deviations from 0.1 to 1% could be achieved with the new DSC
sensor. Future test measurements will certainly confirm the outstanding
performance of this sensor.
Concluding Comments
The presented application examples clearly
demonstrate the capability of the DSC 204 Phoenix equipped with
a newly developed sensor made of semi-conducting material compared to
a conventional DSC. In contrast to a micro-calorimeter, a considerably
wider temperature range can be achieved. Faster heating and cooling
rates as well as simpler sample handling also guarantee higher sample
throughput.
Literature
[1] DIN 51007: Differenzthermoanalyse
(DTA), Grundlagen, Beuth Verlag, Berlin 1994.
[2] ASTM E 793-95: Enthalpies
of Fusion and Crystallization by Differental Scanning
Calorimetry, American Society for Testing
and Materials 1995.
[3] ASTM E 1356-98: Glass Transition
Temperatures by Differential Scanning Calorimetry
or Differential Thermal Analysis, American
Society for Testing and Materials, 1998.
[4] Hˆhne, G., Hemminger,
W., Flammersheim, H.-J.: Differential Scanning Calorimetry,
Springer-Verlag Berlin Heidelberg 1996.
[5] DIN 53765: Thermische Analyse,
Dynamische Differenzkalorimetrie (DDK), Pr¸fung von
Kunststoffen und Elastomeren, Beuth Verlag,
Berlin 1994.
[6] ISO 11357-2: Plastics ñ
Differential scanning calorimetry (DSC), Part 2: Determination of
glass transition temperature, ISO Geneve,
1999.
Authors
Stephan Knappe, Erwin Kaisersberger and
Martin Schmidt
NETZSCH-Geraetebau GmbH, Selb/Germany
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