Comparison of terahertz and infrared spectroscopy for degraded hydrocarbon liquid

Terahertz (THz) spectroscopy has high sensitivity for the interaction between molecules and vibration of large molecules. Besides, it is well known that infrared (IR) spectroscopy can be used to detect the vibration of functional groups. Hence, the comparison of the conventional diagnosis method using IR spectroscopy and THz spectroscopy is expected for more potential diagnosis. In this study, the comparison between THz spectroscopy and IR spectroscopy for deteriorated hydrocarbon liquids has been conducted for the degraded insulating oils. The surveyed samples are pure dodecane and mineral oil. The degraded samples are made by heating in the oxygen atmosphere. In addition, we also measured the breakdown voltage and the volume resistivity to know the states of degraded insulating oil. The experimental results show that oxidation causes spectral changes in the both of THz and IR regions. The spectral change in the IR region is due to carbonyl group and hydroxyl group vibrations. Whereas, in the THz region, the spectral change originates from the modification of intermolecular interaction. The degradation phenomena of the insulating oil contribute to higher carbonyl/hydroxyl content and associated by stronger intermolecular interaction that leads to its lower breakdown voltage.