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Conventional DSC is an analytical technique in which the difference in heat flow between a sample and an inert reference is measured as a function time and temperature as both the sample and reference are subjected to a controlled environment of time, temperature, atmosphere and pressure. In conventional DSC, the temperature regime seen by sample and reference is linear heating or cooling at rates from as fast as 200 0° C/min to rates as slow as 0 ° C/min (isothermal).
Modulated DSC (MDSC) is a technique which also measures the difference in heat flow between a sample and an inert reference as a function of time and temperature. In addition, the same “heat flux” cell design is used. However, in MDSC a different heating profile (temperature regime) is applied to the sample and reference. Specifically, a sinusoidal modulation (oscillation) is overlaid on the conventional linear heating ramp to yield a heating profile in which the average sample temperature still continuously increases with time but not in a linear fashion. The net effect of imposing this more complex heating profile on the sample is the same as if two experiments were run simultaneously on the material – one experiment at the traditional linear (average) heating rate and one at a sinusoidal heating rate.
In MDSC, since the material is being subjected to a more complex temperature profile, the operator must be careful in choosing experimental parameters (see Manual).
A simplified equation which describes the resultant heat flow at any point in a DSC or MDSC experiment is:
dH/dt = CpdT/dt + f(T,t)
where: dH/dt = DSC heat flow signal (J/s).
Cp = Sample Heat Capacity (J/ °C).
dT/dt = heating rate (°C).
F(T,t) = Heat flow that is function of time at an absolute temperature (kinetic) (J/s).
As can be seen from the equation, the total heat flow (dH/dt) which is the only heat flow measured by conventional DSC, is composed of two parts. One part is a function of the sample’s heat capacity and rate of temperature change, and the other is a function of absolute temperature and time.
Modulated DSC determines the total, as well as these two individual heat flow components, to provide increased understanding of complex transitions in materials. MDSC is able to do this because it effectively uses two heating rates – the average heating rate which provides total heat flow information and a sinusoidal heating rate which provides heat capacity information from the heat flow that responds to the rate of temperature change.
All MDSC flow signals are calculated from the measured signals: time, modulated heat flow, and modulated heating rate (the derivative of modulated temperature). The Heat Capacity (Cp) of the sample is continuously determined by dividing the modulated heat flow amplitude by the modulated heating rate amplitude.
*TA Inst. Patent, US Patent no. 5,224,775; 5,248,199; 5,346,306. Canadian Patent no. 2,089,225 and Japanese Patent no. 2,966,691.
| Equipment |
Instrument: TA Instruments Q2000 MDSC (modulated differential scanning calorimeter) with 50 position autosampler and mass flow control. Sensitivity < 0.2 µW, and baseline drift < 10 µW. The Q2000 provides temperature modulation capability with amplitude and period selections. Tzero technology provides direct heat capacity measurements.
Temperature: from – 90 °C to 400 °C. Refrigerated Cooling System (RCS90).
Calibration: Indium for both temperature transitions and the heats of fusion. Sapphire was used for heat capacity constant. |
Please contact Krystyna Brzezinska (kbrzez@mrl.ucsb.edu) to schedule training. Before training starts please read the online MANUAL.
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