Investigation on the Cracking Mechanism of Melt Growth Alumina/Aluminum Titanate Ceramics Prepared by Laser Directed Energy Deposition

Oxide melt growth ceramics(OMGCs)exhibit excellent performance and microstructure stability near their melt-ing point and are expected to become a new structural material for long-term stable service in extremely high-temperature water-oxygen environments.Owing to its unique advantages of high efficiency,flexible manufac-turing,and near-net shaping,laser directed energy deposition(LDED)has become a promising technology for the rapid preparation of high-performance OMGCs.However,owing to the limited understanding of the crack-ing mechanism,the severe cracking problem that hinders OMGCs-LDED towards engineering applications has not been resolved.Alumina/aluminum titanate(Al_(2)O_(3)/Al_(x)Ti_(y)O_(z),A/AT)ceramics are prepared using an LDED system and their cracking characteristics are investigated.Subsequently,numerical simulations are conducted to reveal the dominant factors and influencing mechanisms of the cracking behavior.The results demonstrate that the cracking nucleation process is mainly controlled by solidification defects,whereas the cracking propagation process is determined primarily by both the microstructure and stress level.This study provides a theoretical basis for the development of appropriate cracking suppression methods for OMGCs-LDED.

Investigation on micro-mechanism of palm oil as natural ester insulating oil for overheating thermal fault analysis of transformers

In order to explore the pyrolysis mechanism of palm oil as the natural ester insulating liquid from the molecular aspect,the ReaxFF reactive force field was used to simulate the pyrolysis process of palm oil.Firstly,the molecular models of the four main molecules of palm oil-tripalmitin,trilinolein,triolein,and tristearin-are constructed via density functional theory(DFT).Secondly,the vibrational frequencies of the four molecules are calculated,so as to calculate the infrared(IR)spectra.Thirdly,molecular dynamics(MD)simulations employing the ReaxFF method are performed to simulate the pyrolysis reaction of the palm oil system under different temperatures,so as to observe the generation pathways of major products used in the dissolved gas analysis(DGA)in the transformer thermal fault analysis.Finally,hydrogen bonds in the pyrolysis process are counted.It is found that the number of hydrogen bonds gradually increased with the increase of pyrolysis time and pyrolysis temperature.It reveals why the life of the insulating paper immersed with natural esters is prolonged,and why the breakdown voltage is reduced slowly in the actual experiment from another perspective.This study can provide theoretical guidance and effective reference for the thermal fault analysis of an insulating liquid using palm oil as raw material.