P3G-2 Effect of Ultrasound Exposure in Standing Wave Sound Field on Isoelectric Point of Nanometer Sized Diamond Particles for Abrasive Agent

We reported the improvement of dispersion on nanometer sized diamond particles to obtain abrasive agents for precision polishing by ultrasound exposure. The diamond particles aggregated to the particles with size of a few microns. Zeta potential on the diamond particles was increased to absolute value of 35 mV by ultrasound exposure. Furthermore, average particle size of the particles was decreased to about 100 nm. Effect of ultrasound exposure on dispersion stability of the diamond particles was investigated in this study. At first, sound pressure distribution in water tank of our ultrasound exposure system was measured by scanning a hydrophone. As the results, maximum sound pressure was about 300 kPa in central region. Average sound pressure was about 70 kPa at operating frequency of 150 kHz. Dispersion stability of the particles was improved in standing wave acoustic field. Then, the change of isoelectric points of the particles by ultrasound exposure was measured. Isoelectirc point is pH of solution when their zeta potential is 0 mV. Dispersion stability of the particles is depended on difference between pH at isoelectric point and pH of dispersion medium. A larger magnitude of the difference makes better dispersion stability of the particles. Therefore, the change of isoelectric point is effective for improvement of dispersion stability. Distilled water of pH 6.8 was used as dispersion medium. The distilled water with the particles was exposed to ultrasound for 20 min. The pH was controlled with hydrochloric acid and sodium hydrate. The pH of 2 at isoelectric point before ultrasound exposure was changed to pH of 1 by ultrasound exposure. Dispersion stability of the particles was improved by ultrasound exposure. It was proved that surface modification of the particles by active oxygen species generated by ultrasound exposure caused to the change of isoelectirc point of the particles.