Control and Separation of Ultra Fine Particles using Eleectrostatic Foree

The feasibility of the high temperature electrostatic precipitation method was demonstrated for separation of the ultra-fine particles synthesized by thermally activated CVD(Chemical Vapor Deposition)technique from a reaction gas stream。Silicon nitride particles were produced in a tubular flow reactor by using the NH_3/SiCl_4reaction system at 1200°C and atmospheric pressure.The reaction system produces the NH_4Cl by-product that is in solid state at room temperature.Separation under room temperature presents，therefore，a mixture of silicon nitride and ammonium chloride particles。Using the electrostatic precipitation technique above about340°C，the sublimation temperature of NH_4Cl，it is expected that Si_3N_4particles as the product may be directly collected from the reaction gas stream，without particles of the NH_4Cl by-product.The new electrode system using Ac-corona surface discharge was developed to charge the particles efficiently becaaDuse-corona discharge dnique above about face discharge was developed to charge the particles of the NH_4ClA high AC-voltage with peak voltage above 10 KV and frequency of 10 KHz was applied for the electrodes system to generate a high frequency SC-corona surface discharge。Negative high voltage about1KV was used as a grid voltage to take out monopolar ions from the AC-corona in order to charge the particles monopolar and as DC-field voltage to migrate the charged particles to a collection zone.The silicon nitride particles prepared at 1200℃by the CVD method were under submicron size and completely collected by the electrostatic separation system.The X-ray diffraction analysis indicated that the particles collected under high temperature condition of 500°C contains no particle the NH_4Cl by-product。

The feasibility of the high temperature electrostatic precipitation method was demonstrated for separation of the ultra-fine particles synthesized by thermally activated CVD (Chemical Vapor Deposition) technique from a reaction gas stream. Silicon nitride particles were produced in a tubular flow reactor by using the NH_3/SiCl_4 reaction system at 1200゜C and atmospheric pressure. The reaction system produces the NH_4Cl by-product that is in solid state at room temperature. Separation under room temperature presents, therefore, a mixture of silicon nitride and ammonium chloride particles. Using the electrostatic precipitation technique above about 340゜C, the sublimation temperature of NH_4Cl, it is expected that Si_3N_4 particles as the product may be directly collected from the reaction gas stream, without particles of the NH_4Cl by-product.The new electrode system using Ac-corona surface discharge was developed to charge the particles efficiently because Dc-corona discharge did not stably occur in complex gas composition of the reaction system and at high temperature. A high AC-voltage with peak voltage above 10KV and frequency of 10KHz was applied for the electrodes system to generate a high frequency SC-corona surface discharge. Negative high voltage about 1KV was used as a grid voltage to take out monopolar ions from the AC-corona in order to charge the particles monopolar and as DC-field voltage to migrate the charged particles to a collection zone.The silicon nitride particles prepared at 1200゜C by the CVD method were under submicron size and completely collected by the electrostatic separation system. The X-ray diffraction analysis indicated that the particles collected under high temperature condition of 500゜C contains no particle the NH_4Cl by-product.