A differential scanning calorimeter (DSC) Netzsch DSC 404 was added to our setup and incorporated into the basic measurement routines for data in the temperature range of about 500 to 1500 K. The DSC is able to perform accurate specific heat capacity measurements in the above mentioned temperature range. The results are combined with those of the pulse-heating experiments by using the enthalpy versus temperature dependence of the DSC to expand the temperature range of the pulse-heating data. Thus, temperature dependences of all thermophysical properties can now be extended down to the DSC onset temperature of about 500 K.
The DSC can be used primarily for measurements of the heat capacity of the sample (5.2 mm diameter and 0.5 mm height) in the temperature range from 500 to 1500 K. The sample is measured relative to a second, inert sample of approximately the same heat capacity. One experiment consists usually of three separate runs: a scan with two empty pans, a scan with one pan containing a sapphire reference sample, and finally a scan with the sample in the same pan where the reference sample was previously. The heat capacity as a function of temperature of the sample under investigation, cp(T), is obtained by using the following equation:
where D1, D2, D3 are the three DSC signals with empty pans, the signal of the reference, and the signal of the sample. mr and m are the masses of the reference and the sample, respectively, and crp is the heat capacity of the reference. Using this heat capacity cp(T) obtained with DSC measurements, one is able to calculate the enthalpy of the specimen by integrating the heat capacity signal with respect to temperature and adding the room temperature enthalpy H(298) to the result:
where H(T) is the enthalpy and cp is the specific heat capacity. The assumption in this equation of a constant heat capacity between 298 K and 473 K is taken into consideration for the uncertainty of the enthalpy values. Therefore, the enthalpy versus temperature dependence for a given material can be calculated directly from DSC measurements. This enthalpy - temperature dependence can further be used to obtain the inverse dependence, temperature versus enthalpy. With this result, we are able to extend our electrical measurements (i.e., enthalpy or electrical resistivity) of the pulse-heating experiment to lower temperatures by combining the temperature scale from the DSC (temperature versus enthalpy) with the electrical measured properties versus enthalpy. It has to be noted that the above mentioned procedure is only applicable as long as there are no phase transitions in the solid sate of the material under investigation. Phase transitions can easily be observed with DSC measurements, but can be wholly or partially suppressed under pulse-heating conditions as applied within this experiment, due to the extreme high heating rates of 108K×s-1. This procedure enables us to extend the results for enthalpy versus temperature and resistivity versus temperature to lower temperature regions, starting now at the onset temperature of the DSC (500 K). Up to now access to these temperature regions when using pulse-heating techniques was only possible by experiments with millisecond time resolution.
Institut of Experimental Physics
Graz University of Technology
ao. Univ.-Prof. Dr.
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