What Is the DSC Used For

Izmir Institute of Technology Mechanical Engineering Department

ME 206 Thermodynamics-I

DETERMINATION OF SPECIFIC HEAT (Cp)

Lab: Center for Geothermal Research & Application Researcher: Ebru KUZGUNKAYA What is DSC used for?

Differential scanning calorimetry (DSC) is a thermal analysis technique that measures the temperatures and heat flows associated with transitions in materials as a function of time and temperature. Such measurements provide qualitative and quantitative information about physical and chemical changes that involve endothermic or exothermic processes or changes in heat capacity. DSC is a valuable analytical technique for polymers, organic chemicals, pharmaceuticals, oils and greases, foods and biological materials, as well as metals and some inorganic materials. Typical measurements include:

 Glass transitions in amorphous/semi-crystalline materials  Melting points, boiling points  Crystallization time and temperature, percent crystallinity  Heats of fusion and reactions  Oxidative stability  Purity  Rate of cure, degree of cure  Reaction kinetics  Specific heat and heat capacity  Thermal stability

Sample Types

The DSC can be used to analyze virtually any material that can be put into a DSC sample pan. This includes:

 Films  Fibers  Powders  Solutions  Composites

How Does a DSC Work?

In a "heat flux" DSC, the sample material, encapsulated in a pan, and an empty reference pan sit on a thermoelectric disk surrounded by a furnace. As the temperature of the furnace is changed (usually by heating at a linear rate), heat is transferred to the sample and reference through the thermoelectric disk. The differential heat flow to the sample and reference is measured by area thermocouples using the thermal equivalent of Ohm's Law. Fig. 1. Principle work of DSC .

T q  (W) (1) R q = sample heat flow (W)

T = temperature diffrence between sample and reference (ºC or K)

R = resistance of the thermoelectric disk (K/W)

This simple relationship, however, does not take into account extraneous heat flow within the sensor or between the sensor and sample pan.

The cell sensor consists of a constantan body with separate raised platforms or vessel to hold the sample and reference (Fig. 1). The platforms are connected to the heating block (base) by thin-walled tubes that create thermal resistances between the platforms and the base. Area detectors (thermocouples) on the underside of each platform measure the temperature of the sample and reference. A third thermocouple measures the temperature at the base. The equation 2 shows the thermal network model which represents this cell arrangement, and the resultant heat flow expression that describes this cell arrangement (designated the To [Tzero™] cell) is

T  R  R  dT dT  r s  s q   T0    Cr  Cs   Cr (2) Rr  Rr Rs  dt dt

The first term in this expression is the equivalent of the conventional single-term DSC heat flow expression. The second and third terms account for differences between the sample and reference resistances and capacitances respectively. These terms have their largest impact during regions of the thermal curve where the heat capacity of the sample is the predominant contributor to heat flow. The fourth term accounts for the difference in heating rate between the sample and reference. This term has its largest impact during enthalpic events (e.g., melting). This equation can be further modified to account for pan heat flow effects.

Specific Heat Calculation

The continuous method consists in performing two runs with two vessels at the same scanning rate: the measuring vessel and the reference vessel. The first run is performed with the two empty vessels (blank test), the second run with a mass ms of sample in the measuring vessel. Each run consists in a ramp between an initial temperature Ti and a final temperature Tf at constant heating rate (Fig. 2). If Qb and Qs are the amplitudes measured with the blank and with the sample respectively, the value of specific heat Cp is:

Q  Q Cp  s b .60 (J/gK) (3) hr .m

Q = sample heat flow (mW) hr = heating rate (°C/min) m = sample weight (mg) b = blank

Fig. 2. Continuous method to calculate Cp. Results

a) Give the procedure of the experiment.

b) Calculate Cp depending on temperature.

c) Plot a graph which shows Cp (y axis) versus temperature (x axis).