Classification of thermal ageing impact of ester fluid-impregnated pressboard material adopting LIBS

This article reports the thermal ageing of ester fluid-impregnated pressboard material along with its performance evaluation using optical emission spectroscopy(OES)and classifi-cation using machine-learning algorithms adopting laser-induced breakdown spectroscopy(LIBS).The surface discharge analysis on ester-impregnated pressboard(EIP)is studied using OES and the plasma temperature was evaluated based on the Cu I emission lines which were higher under the negative DC compared with the positive DC and AC voltages.The LIBS analysis was performed on the EIP material operated at different energy levels in order to acquire the optimal energy required to be used for its classification algorithm.The intensity ratio and electron density evaluated from LIBS studies correlated well with the plasma temperature.The lower limit of detection(LOD)calculated based on linear regression analysis for copper peak was around 3.5 times higher than the identification of carbon peak.The machine-learning techniques like principal component analysis and neural network algorithm have been performed on the LIBS spectral dataset in order to classify the ageing of EIP material.Artificial neural network adopting LIBS provided a better classi-fication accuracy on all the aged samples compared with principal component analysis which classified only for the samples aged at higher temperatures.

^(1)H NMR tests on damaged and undamaged XLPE and SiR samples

Cross-linked polyethylene(XLPE)and silicone rubber(SiR)samples were subjected to a high-voltage AC stress plane-plane configuration and inclined plane test,respectively.The voltage was applied such that discharge was observed across the surface of the XLPE test sample for several hours and for visible damage to occur on SiR samples also after several hours.Selected stressed samples together with virgin samples from the same manufactured batch were tested using nuclear magnetic resonance(NMR)spectroscopy.Specifically,^(1)H NMR spin-lattice(T_(1))and spin-spin(T_(2))relaxation time measurements were employed to examine potential changes in the chemical bonding of undamaged and damaged XLPE and SiR samples.Preliminary results show that there may be a moderate increase in the T_(1)and T_(2)values of the damaged samples in comparison with the undamaged ones.This raises the possibility that NMR can be a useful additional experimental tool in characterising material degradation.