Power compensation DSC

Power compensation DSC has at its heart two small identical furnaces, one for the sample and one for the reference (normally an empty pan), the reference being the right-hand furnace; Figure 1. These are both heated at a pre-programmed heating (or cooling) rate and power compensations are applied to either furnace as required to maintain this rate. In the resulting DSC trace the difference in energy flowing into the sample furnace is compared to the inert reference and plotted as a function of temperature or time. This design measures flow of energy directly in mW or J/s.

Figure 1. Diagram of a power compensation DSC. In this system, both the sample and reference furnaces are heated at a programmed heating/cooling rate. In order to maintain this rate when transitions occur in the sample, a power compensation circuit increases or reduces power to either furnace as required in order to maintain the heating rate. The power compensation circuit therefore reflects the energy changes occurring in the sample and is presented on the screen as a function of temperature or time. This technique measures energy changes directly.

The fundamental equation of DSC is 

Therefore, the raw heat flow signal can be viewed as a form of heat capacity. In practice, it reflects the changes occurring in heat capacity, and the absolute value is obtained when the method used takes into account the contribution of the empty pans and reference together with the scan rate.

The small furnaces of this system can be heated or cooled at very low rates to very high rates and are ideal for a range of different techniques, particularly fast scan DSC. Power compensated DSC also permits true isothermal operation, since under constant temperature conditions both the sample and furnace are held isothermally. The temperature range of use is from liquid nitrogen temperatures to around 730◦C.

Heat flux DSC

Heat flux DSC is of a single furnace design with a temperature sensor (or multiple sensors) for each of the sample and reference pans located within the same furnace; see Figure 2. Sample and reference pans are placed in their required positions and the furnace heated at the pre-programmed heating (or cooling) rate. When transitions in the sample are encountered a temperature difference is created between sample and reference. On continued heating beyond the transition this difference in temperature decreases as the system reaches equilibrium in accordance with the time constant T of the system. It is the difference in temperature or △t signal that is the basic parameter measured. Modern analyzers are carefully calibrated so that the △t signal is converted to a heat flow equivalent and this is displayed as a function of temperature or time. The reason a difference in temperature is created is easily understood if melting is considered. When melting of a single crystal occurs the resulting mixture of solid and liquid remains at the melting point until melting is complete, so the temperature of the sample will fall behind that of the reference. Typical heat flux DSC analyzers can be used from liquid nitrogen temperatures to a maximum of around 700◦C similar to power compensation DSC, though modern high-temperature DTA analyzers normally offer a calibrated DTA (heat flow) signal giving a measurement derived from the heat flux to significantly higher temperatures.

Figure 2. Diagram of a heat flux DSC. In this system, both the sample and reference experience the same heat flux, but as energy demands differ, the heating or cooling effect will differ resulting in a difference in temperature between sample and reference. This difference in temperature is converted to an energy equivalent by the analyzer giving the familiar DSC signal in mW.

Differential thermal analysis DTA

This design principle is similar to heat flux DSC, except that the △t signal remains as a microvolt signal and is not converted to a heat flow equivalent. This was the original instrument approach used before quantitative energy measurements were established using DSC. Often instruments capable of heating to around 1500◦C or higher use this principle and are referred to as DTA analyzers. The furnace design is usually quite different to that of lower temperature systems, though modern equipment may offer a choice of heat flow or microvolt signals.

Differential photocalorimetry DPC

Reactions not only occur as a function of temperature but may also be initiated by irradiation, specifically ultraviolet (UV) light for materials that are photosensitive. UV systems have therefore been attached to DSC analysers to provide DPC systems. This can be done fairly crudely by shining a light on a material using fibre optics, but formal accessories are available from a number of manufacturers. Isothermal control of these systems is important, and the light source should not adversely affect the calorimetry. Applications are found in curable materials in the composites and manufacturing industries, dentistry and dental materials together with films, coatings and printing inks. The wavelength chosen and the intensity of the light are significant factors, which together with the temperature of reaction and length of exposure can be used to define a method. The effects of temperature, light intensity, and wavelength can be investigated on different materials and additives, and the kinetics of reaction investigated using isothermal kinetics models.

The effect of infrared light can also be investigated in a similar manner. These authors also highlight the ability to measure a simultaneous non-contact TMA signal from a sample placed in the DSC cell.

Many manufacturers provide pressure cells as an accessory to the DSC. These normally work to fairly modest pressures of around 50–100 bar and are designed with a view to the suppression of volatiles in OIT tests involving oils and greases. A high-pressure cell capable of working to much higher pressures has been constructed by Hohne and co-workers and can be operated at pressures from 0.1 to 500MPa over a temperature range from20 to 300◦C and is potentially capable of being added to some commercial analyzers.

Standard DSC Methods

ISO Standards

• 11357-1:1997 Plastics – Differential Scanning Calorimetry (DSC)

Part 1: General Principles

• 11357-2:1999 Plastics – Differential Scanning Calorimetry (DSC)

Part 2: Determination of Glass Transition Temperature

• 11357-3:1999 Plastics – Differential Scanning Calorimetry (DSC)

Part 3: Determination of Temperature and Enthalpy of Melting and Crystallisation

• 11357-4:2005 Plastics – Differential Scanning Calorimetry (DSC)

Part 4: Determination of Specific Heat Capacity

• 11357-5:1999 Plastics – Differential Scanning Calorimetry (DSC)

Part 5: Determination of Characteristic Reaction Curve Temperatures and Times Enthalpy of Reaction and Degree of Conversion

• 11357-6:2002 Plastics – Differential Scanning Calorimetry (DSC)

Part 6: Determination of Oxidative Induction Time

• 11357-7:2002 Plastics – Differential Scanning Calorimetry (DSC)

Part7: Determination of Crystallisation kinetics