Three-dimensional Space Charge Microscopy Using a Focused Ultrasound Transducer

The space charge measurement for insulating systems usually has one-dimensional spatial resolution, using film samples. However, in the future, measurement for complex structures such as cable joints and high voltage power devices may be required. The authors have developed a space charge microscope using the pulsed electro-acoustic method combined with a focused ultrasonic transducer. A 100 μm-thick polymer film, on which a tapered metallic electrode was placed, was mounded on the polymer substrate coated with metal as the ground electrode. The gap between the tapered electrode and film was filled with silicone rubber, creating a triple junction. A bias voltage of about 5 kV and a pulse voltage of about 2 kV and 20 ns in width were applied between the tapered electrode and the ground plane. Acoustic signals were sequentially acquired while mechanically scanning the transducer at the rear side of the substrate. The frequency response of the acoustic signal was corrected by deconvolution, by referring an appropriate signal corresponding to the impulse response of the system. Electric field analysis was performed to calculate the conversion coefficients between local charge intensity and acoustic signal detected by the transducer. The space charge distribution along the heterogeneous insulating interface was successfully obtained.