Phase transformation enthalpies are determined using the recently developed
measurement technique Thin-Film Calorimetry (TFC), which is based on
piezoelectric resonators vibrating in thickness shear mode. They are applicable
up to at least 1000 °C. To the best of our knowledge, no comparable TFC
systems for such high temperatures exist.
The experimental part is divided into two subsections. The first is addressed to
a thermodynamic investigation on piezoelectric langasite crystals (LGS,
La3Ga5SiO14) which are the key component of
the TFC system. The specific heat capacity is measured on LGS crystals of three
different manufacturers. It ranges from about 0.45 J g-1
K-1 at 40 °C to about 0.60 J g-1 K-1
at 1000 °C. Thereby, deviations of up to 5 % between the different
crystals are detected. Thermal diffusivity data for Y-cut LGS crystals are
determined as well. Here, a constant decrease with temperature is detected
ranging from 0.48 mm2 s-1 at room temperature to 0.38
mm2 s-1 at 700 °C.
The second part presents thin-film calorimetric investigation on thin films of
the family Li-Ni-Mn-Co-Al-Oxide (NMC/NMCA). These cathode materials are
investigated and compared when annealed in ambient air or 0.5 % H2/Ar
up to 860 °C. Three stoichiometries are chosen:
The samples show three or four phase transformations. In air, the samples
crystallize in the range of 250-350 °C. In 0.5 % H2/Ar, the
transformations occur at higher temperatures. Especially in air, stoichiometric
NMC crystallizes at lower temperatures compared to Ni-rich compositions.
Additional doping with Al enhances the thermal stability which shifts all phase
transformations to higher temperatures in both atmospheres.