Polyethylene Nanocomposites for the Next Generation of Ultralow-Transmission-Loss HVDC Cables: Insulation Containing Moisture-Resistant MgO Nanoparticles

The use of MgO nanoparticles in polyethylene for cable insulation has attracted considerable interest, although in humid media the surface regions of the nanoparticles undergo a conversion to a hydroxide phase. A facile method to obtain MgO nanoparticles with a large surface area and remarkable inertness to humidity is presented. The method involves (a) low temperature (400 °C) thermal decomposition of Mg(OH)2, (b) a silicone oxide coating to conceal the nanoparticles and prevent interparticle sintering upon exposure to high temperatures, and (c) heat treatment at 1000 °C. The formation of the hydroxide phase on these silicone oxide-coated MgO nanoparticles after extended exposure to humid air was assessed by thermogravimetry, infrared spectroscopy, and X-ray diffraction. The nanoparticles showed essentially no sign of any hydroxide phase compared to particles prepared by the conventional single-step thermal decomposition of Mg(OH)2. The moisture-resistant MgO nanoparticles showed improved dispersion and interfacial adhesion in the LDPE matrix with smaller nanosized particle clusters compared with conventionally prepared MgO. The addition of 1 wt % moisture-resistant MgO nanoparticles was sufficient to decrease the conductivity of polyethylene 30 times. The reduction in conductivity is discussed in terms of defect concentration on the surface of the moisture-resistant MgO nanoparticles at the polymer/nanoparticle interface.