Temperature and Enthalpy Adjustment of the High Pressure DSC827e - METTLER TOLEDO

Temperature and Enthalpy Adjustment of the High Pressure DSC827e

Experimental Results

The measurements described here were performed with a high pressure DSC827 e that had been adjusted at standard atmospheric pressure using the “total calibration” method. The melting peaks of indium, tin and zinc under nitrogen were then measured at different pressures. The corresponding curves for indium together with the values for the enthalpy of fusion and the melting temperature are displayed in Figure 1. The results show that with increasing pressure the enthalpy of fusion shows a slight decrease and the melting temperature a slight increase.

Influence of pressure on melting of indium 

In a DSC, increased pressure results in better thermal contact between the crucible (and therefore the sample) and its surroundings (furnace, furnace lid). As a result, the heat flow measured by the sensor decreases, which is equivalent to a reduction in sensor sensitivity. One therefore expects the measured enthalpy of fusion to decrease with increasing pressure. This is confirmed by the results presented in Figure 1.

Figure 2 shows the relative enthalpies of fusion of indium, tin and zinc measured at different pressures using the enthalpy of fusion at standard pressure as the reference value (100%). 

The data shows that the influence of pressure only becomes noticeable at pressures above about 50 bar (5 MPa). The enthalpy of fusion is of course also pressure dependent for physical reasons: increased pressure changes the lattice dimensions slightly, which in turn affects the enthalpy of fusion. For example, for indium, a pressure increase of 1 kbar causes the enthalpy of fusion to change by about −0.24% [1]. Such a small effect cannot be detected in the pressure range within which the HP DSC827 e operates and can therefore be neglected.

Figure 2 shows that if the enthalpy adjustment is performed at standard pressure, then at high pressures, enthalpy values are obtained that are systematically about 5% too low, depending on the particular pressure and temperature.

Figure 3 shows the differences between the pressure-dependent melting temperatures (literature values) and the melting temperatures measured at different pressures for In, Sn and Zn. For example, from the literature it is known that the melting temperature of indium increases by about 4.7 mK when the pressure is increased by 1 bar [1]. 

Conclusions

The adjustment of the high pressure DSC827 e turns out to be slightly pressure dependent both with respect to enthalpy and to temperature. An analysis of melting temperatures showed that the melting temperature measured at a particular pressure compared with the “true” melting temperature at the same pressure was about 1.5 mK/bar too low. If measurements are performed at maximum pressure (100 bar = 10 MPa) in the DSC827e, the temperature is consequently about 0.15 K too low. For most intents and purposes, this deviation is of no practical importance.

At pressures up to about 50 bar, enthalpy adjustment is also practically independent of pressure. Above 50 bar, pressure dependence increases significantly. Depending on the temperature and pressure, the measured enthalpies are systematically too low by up to about 5%. The results presented here refer to nitrogen as the fill gas. Compared with nitrogen, other gases may have markedly different temperature-dependent and pressure-dependent thermal conductivity properties, for example helium. The results found here therefore cannot be directly applied to other gases.

Temperature and Enthalpy Adjustment of the High Pressure DSC827e | Thermal Analysis Application No. UC 285| Application published in METTLER TOLEDO Thermal Analysis UserCom 28