ABSTRACT
Bulb-shaped field emission lamps (FELs) with a helical cathode filament were simulated and fabricated in this research. The light bulbs comprised a helical stainless steel filament cathode grown with carbon nano-coils (CNCs) and an Al anode deposited on the bottom hemisphere of a 60-mm-diameter glass bulb. White light was generated when the field-emitted electrons bombarded a layer of three-color phosphor coated on the anode. A numerical simulation model for the helical-cathode FELs was constructed, and the field emission (FE) performance was carefully studied. Due to the screening effect, the electric field strength as well as the FE current density on the inner side of the helix dramatically decreased with decreasing helical pitch. Real FELs using cathodes with various helical radii and pitches were fabricated and their FE currents were measured. The theoretical and experimental results were in good agreement. A maximum total FE current was found at a pitch of 16 mm (helical radius = 2 mm), where the optimum trade-off between a large total surface area and a small screening effect was obtained. The optimized FEL showed a total luminous flux of about 220 lm at an applied voltage of 8 kV and a color rendering index of 94. Compared to a straight filament cathode, a helical cathode offered a higher total FE current or, alternatively, a lower current density and a longer cathode life, if we fix the total current by using a lower voltage.
ABSTRACT
Bulb-shaped field emission lamps (FELs) using carbon nano-coil (CNC) cathodes were fabricated and their performances were characterized. A straight steel wire grown with CNCs was placed on the symmetry axis in the bulb as the cathode. The anode was defined by a Ni film deposited on the bottom hemisphere of a 60-mm-diameter glass bulb. And a phosphor layer was coated on the Ni film for light generation via cathodoluminescence. A numerical simulation model for this geometry also was constructed. The simulated current-voltage curves agree well with the experimental results and reproduce the trends observed in the experiment. The effects of varying the length and diameter of the cathode were studied. Both experiment and simulation showed that the total field emission current increases with the cathode length and decreases with the cathode diameter. Although the design is simple and inherently low-cost, good uniformity of light emission over the entire anode was demonstrated.
