Enameled wire is composed of conductive copper or aluminum core surrounded by an insulation layer made of polymer. The lifespan of the electric motor is strongly dependent on thermal and electrical properties of the insulation layer. In a high speed switching inverter-fed motor, especially, when instantaneous overlapping voltage called surge voltage is higher than partial discharge inception voltage (PDIV), the insulation layer is degraded by partial discharge (PD), which leads to insulation breakdown. For this reason, surge-resistant enameled wire has attracted attention for prolonging the lifespan of various inverter-fed motor installed electric devices such as electric vehicles. For protection from the degradation, nanosized inorganic fillers were incorporated in the insulation layer where corona evolved by PD can be trapped on or reflected from the surface of the inorganic filler so that the degradation would be retarded.

Polyesterimide (PEI) was frequently chosen for the insulation layer of high temperature enameled wires because it meets both requirements of high thermal index and flexibility sufficient to withstand severe stress while winding. There are many reports regarding the enhanced breakdown time of PEI enameled wire by adding inorganic nanoparticles such as silica, alumina, and clay in the insulation layer. In 2002, Hitachi Cable Ltd. developed silica nanoparticle-PEI nanocomposite enameled wire having breakdown time of 42 min compared to 1 min for one without silica in the PD condition of 2.8 kV and 10 kHz. Since then, surge-resistant properties of nanocomposite enameled wire have been systematically studied. As expected, the breakdown time increased with nanoparticle volume fraction on the condition that the nanoparticles are uniformly dispersed.

Because it is difficult to disperse nanoparticle in a viscous varnish solution, the sol-gel method, which has a great advantage for dispersion, has attracted attention. In the sol-gel method, nanoparticles are easily dispersed in the insulation layer as soon as they are synthesized from the precursors premixed in the varnish during the wire annealing process accompanied by a coating process. However, serious problems were revealed; for example, the enameled wire production cost increased because the process was too slow and the breakdown strength of the enameled wire was reduced due to impurities remaining from the sol-gel process . In order to avoid such problems, nanoparticles need to be directly dispersed in the varnish and therefore should be properly surface-modified.

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