Formation of Alumina Insulation Films for a CRT Heater by Constant Current Electrophoretic Deposition Method

Abstract

Electrophoretic coating by a constant current deposition method (CCDM) was conducted to form alumina insulation layers on the tungsten double helical filaments of a CRT heater. Alumina particles (average diameter of about 4μm) were dispersed in an ethanol-water suspension containing dehydrated Al(NO3)3+Mg(NO3)2⋅6H2O. Positively charged alumina particles in the suspension (43 mass% solid) were deposited on tungsten double helical filaments by passing a direct current from 0.015 to 0.32A. The thickness and packing ratio of Al2O3 film formed within the spaces between the coils of the double helical filaments were measured after deposition under various CCDM conditions. A gel-like substance with a uniform thickness was deposited on the overall surface of the electrode of the tungsten helical heater by passing the constant current. Better current conditions of the CCDM for achieving Al2O3 packing ratio above 55% were 0.05 A<I<0.12A. The thickness of the Al2O3 film did not reach 75μm because a long range deposition time for the CCDM caused surface roughness above 20μm. Use of a double-step CCDM for a practical process was examined to give an Al2O3 film thickness over 100μm and Al2O3 packing ratio of over 55%. A typical condition for double-step CCDM was I=0.08A for 10s as the first step and I=0.5A for 3.0s as the second step.