Precise Determination of the Specific Heat by Means of DSC

Materials with a high specific heat change their temperature only slightly when a given amount of energy is put in.

We will show how the specific heat can be accurately and successfully determined by means of Differential Scanning Calorimetry. The method is described in various standards including ASTM E 1269, DIN 51 007 or ISO 11357-4 (especially for plastics).

Number of Measurements

Every determination of the specific heat by means of DSC – and also temperature-modulated (TM-) DSC – is comprised of three measurements: baseline, standard and sample measurement. The crucible on the reference side always remains empty. Usually sapphire (α-Al2O3 single crystal) is suggested as a standard, but other materials with known cp values can also be used. For special applications, like in the aviation sector, standards also include data for other materials (e.g. n-heptane in ASTM D 4816 or diphenyl ether in ASTM D 3947).

Fig. 1. Setup of a measurement series for determination of the specific heatFig. 1. Setup of a measurement series for determination of the specific heat

All three measurements should be carried out under identical conditions. The most important points here are:

1. Crucibles

1. Crucibles

In order to obtain signals as high as possible, it is recommended to employ crucible and lid materials with a high thermal conductivity (such as aluminum, platinum or also graphite) as well as a relatively high heating rate of 10 or 20 K/min. Aluminum oxide crucibles are not ideally suited for cp determinations since Al2O3 becomes transparent for thermal radiation at higher temperatures.

2. Samples

2. Samples

It is important that there be a good heat coupling of the sample, crucible bottom and sensor. This requires a plane crucible bottom together with as large a contact area between the sample and crucible as possible. Ideal are disk-shaped samples with a diameter of, e.g., 5 to 6 mm and a height of approx. 1 mm. If there is a risk of a contact reaction between the sample and metal crucible, platinum crucibles with Al2O3 liners, for example, can be used as an alternative. These liners are considerably thinner-walled than conventional Al2O3 crucibles and only have a minor effect on the sensitivity of the system. For end temperatures close to 1400°C or more, it is advisable to use an Al2O3 disk as a support for the Pt/Rh crucibles in order to prevent sticking of the crucibles to the support areas of the sample carriers. More background information on this phenomenon can be found here.

For powdery samples, it is recommended to seal the powder with a stamp.

With regard to the sample mass, it is advantageous when the “thermal masses” of the sapphire and the sample are approximately equivalent, i.e

According to ASTM E 1269, the sample mass should not have changed by more than 0.3% upon re-weighing of the sample after termination of the experiment. Otherwise, the measurement must be rejected. This statement is based upon the fact that only the initial mass of the sample is taken into consideration in the calculation formula. If greater mass changes occur, to be strictly correct the specific heat would have to be recalculated in the following part of the curve with a changed initial mass.

3. Position of the Crucible

A reproducible crucible position between the baseline, sapphire and sample runs should be ensured. Significantly different crucible positions on the sensor can have a negative influence on the uncertainty of the results.

4. Temperature Profile of the Furnace

For cp determinations, high demands are made on the temperature profile of the furnace in order to guarantee the most homogeneous temperature conditions possible in the interior. Along with the compact, fast furnace of the DSC 204 F1 Phoenix®, best suited for the DSC 404/STA 449 are the Pt/Rh furnace up to 1500°C, the Rh furnace up to 1650°C, the low-temperature furnace up to 675°C (silver heating element) and the low-temperature furnace up to 1000°C (stainless steel heating element).

5. Temperature Program

a) Dynamic

Two different temperature programs are available for the classical cp determination (figure 2). What the two temperature programs have in common – prior to the heating of interest – is a 10- to 15-minute isothermal phase for stabilization of the start temperature.

Fig. 2. Classical temperature programs for determination of the specific heatFig. 2. Classical temperature programs for determination of the specific heat

b) Isothermal

The ASTM International Technical Commitee is currently working on a new standard (ASTM E 37; the 3rd draft was published in August 2008) for the determination of specific heat by means of temperature-modulated DSC. The temperature range here is limited to -100°C to 600°C. Evaluation is carried out in the isothermal segments (example of a temperature program in figure 3).In both the dynamic and isothermal modes, it is usually best to switch off the STC (sample temperature controller) during the measurement.

Fig. 3. Example of a step program for the isothermal determination of the specific heat by means of TM-DSCFig. 3. Example of a step program for the
isothermal determination of the specific heat by means of TM-DSC

In both the dynamic and isothermal modes, it is usually best to switch off the STC (sample temperature controller) during the measurement.

6. Calibration of the Instrument

A new sensitivity calibration is established via the baseline and sapphire measurement within a cp triplet. Integration of a sensitivity curve into the measurements is therefore not necessary; it is sufficient to activate the relevant temperature calibration. Up to a temperature of approx. 1200°C, the accuracy of the determined cp data is generally approx. +/- 2.5%. For even higher temperatures, it is a bit greater. To verify the measured values, it has proved favorable to measure two sapphires of different masses, i.e. to use one sapphire as a standard and the second one as a sample. The results should correspond to the theoretical cp values for sapphire.