Thin-Film Calorimetry (TFC) allows for qualitative and quantitative in-situ-analysis of thermal properties of thin-film systems consisting of e.g. electrochemical cells or metal layer sequences. It is based on highly sensitive piezoelectric resonators which serve as planar temperature sensors. The application of piezoelectric langasite crystals ensures high-temperature stability. Thermodynamic and gravimetric data like phase transformations and mass loss/gain of the films deposited on the resonators can be extracted from disturbances in its resonance frequency. Deviations from the undisturbed resonance frequency are caused by the generation or consumption of heat and/or mass changes.
The TFC system presented here includes recent improvements and extensions, including an improved temperature control. Constant temperature settings show deviations of ± 0.1 K from set values. Constant temperature ramps of 1 K min-1 show less than 3 % deviation from room temperature (RT) up to 1000 °C.
The calibration of the resonator/active layer arrangement is done via defined energy pulses at 231.9 ± 0.1 K by resistive heating. Thereby, Al2O3 layers serve as electrical insolation for Pt heating structures on one side and as diffusion barriers for Sn active layers on the reverse side of the resonators. The temperature is cycled between 180 and 250 °C to determine the resonance frequency shifts from the solid-liquid phase transformation of Sn. Resulting frequency shifts ΔfR and applied energies during calibration are quantitatively compared. Melting of 64 µg Sn results in an average ΔfR = 73.1 Hz which corresponds to an enthalpy change of ΔH = 59.3 J/g. Energy pulses of 3.86 mJ (required energy to melt 64 µg Sn) give an average value of ΔfR= 73.8 Hz [Fig.1]. Time constants τ for cooling (τ = 2.87 ± 0.17 s) are compared to values calculated from measured thermal diffusivities (2.99 s, see Fig. 2).
An application example are measurements of sublimation temperatures of C60 fullerenes. The films are deposited on the resonators via drop-solution with chlorobenzene as a solvent. Deposited film masses range from 19 to 96 µg. Sublimation of the films is investigated from RT up to 550 °C in 0.2 % H2/Ar and ambient air. Measurements in ambient air show sublimation starting at approx. 290 °C and completing at about 450 °C. In 0.2 % H2/Ar sublimation ranges from 330 to 450 °C. In both cases the fullerenes sublimate completely, as the resonance frequency returns to the value prior to the application of the drop solution.