Civil Engineering Reference
In-Depth Information
Oscillating DSC (ODSC).
The characterizing ability of DSC can
be greatly enhanced using dynamic DSC measurements known as modu-
lated (MDSC™),
[33]-[35]
oscillating (ODSC),
[36]
or dynamic (DDSC).
[37]
The dynamic DSC measurement is a fairly new technique, which was first
developed jointly by TA Instrument and ICI Paints in the early 1990s and
followed
by
Seiko Instruments (ODSC), and Perkin-Elmer (DDSC). It
combines aspects of both DSC and AC Calorimetry. In this technique, the
temperature program (linear heating or isothermal or cooling) is modulated
by some form of perturbation. This approach provides new information on
reversing (
C
p
) and non-reversing (kinetic) characteristics of thermal events.
This information helps to interpret thermal events and provides unique
insights into the structure and behavior of materials.
[38]
As mentioned earlier in DSC, thermocouples are utilized to mea-
sure the quantity or heat flow difference (
∆
Q
) between the sample and the
reference. For example, when a sample melts during heating, energy is
absorbed by the sample. If a material crystallizes or cures, its temperature
becomes greater than that of the reference, and heat evolves.
[36]
This heat
flow can be mathematically expressed by the equation:
[39]
Eq. (3)
dQ
/
dt
= -
C
p
• (
dT
/
dt
) +
f
(
T
,
t
)
where
dQ
/
dt
= heat flow out of the sample
C
p
= thermodynamic heat capacity
dT
/
dt
= heating rate
T
= temperature
t
= time
f
(
T
,
t
) = the function governing the kinetic response of
any physical or chemical transformation
With the dynamic DSC technique, the same DSC furnace assembly
and cell is utilized, but a different heating or cooling profile is applied to the
sample and reference by the same furnace assembly. An oscillating time/
temperature sinusoidal signal is superimposed onto the conventional linear
heating ramp. This yields a heating profile where the sample temperature
profile, on the average, is still increasing in a constant manner with respect
to time. However, on a short-term examination, the increase is not linear,
but sinusoidal in nature.
[36]
The temperature increases at a rate which is sometimes faster than
the average, underlying heating ramp. The time-temperature profile ob-
tained via dynamic DSC is continuously accelerating and decelerating
during the course of a heating experiment. There are three parameters that
