GB2079036A - Electron emitting coating in metal halide arc lamp - Google Patents

Abstract

The present invention relates to a high pressure metal halide arc discharge lamp. The lamp is a mercury halide lamp and has arc sustaining electrodes (12, 13) the cathode being tungsten having electron emissive material on it. The electron emissive material is an oxide of scandium or dysprosium, or a boride of thorium, scandium or lanthanum. <IMAGE>

Description

SPECIFICATION Electron emitting coatings in metal halide arc lamp The invention relates to electron emissive coatings or activating materials for the electrodes of metal halide arc lamps.

In electric discharge lamps the cathodes usually comprise a base or support member of refractory metal, ordinarily tungsten, and electron-emissive material supported thereby, such material being more active than the base metal. However in metal halide lamps, the vaporized fill gives a highly reactive atmosphere and the use of electron-emitting cathode materials in metal halide discharge lamps has been severly limited by such atmosphere.

In present day commercially available metal halide lamps, the fill most generally used include sodium iodide and scandium iodide.

The alkaline earth oxides such as BaO SrO commonly used in fluorescent lamps, or the combination alkaline earth-refractory metal oxide Ba2CaWO6 commonly used in high pressure sodium vapor lamps, react with these halides causing devitrification or blackening of the lamp walls. Consequently, for such metal halide lamps, the lighting industry has resorted to cathodes consisting of one of the following alternatives: 1. Bare tungsten electrodes, 2. Tungsten in some combination with thorium, 3. Coatings of ThO2 Y203.

While cathodes of the foregoing kind avoid the devitrification and blackening problems, they are inferior by comparison with cathodes used in other kinds of discharge lamps and impose a heavier burden on the ballast.

The object of the invention is to provide new and improved electron-emitting materials and coatings suitable for use in metal halide lamps and particularly in such lamps utilizing sodium iodide and scandium iodide in the fill.

We have determined that the oxides of scandium and dysprosium and the borides of thorium, scandium and lanthanum give superior performance in metal halide arc lamps of the foregoing kind. The performance is particularly improved with respect to reduction of the glow voltage and reduction in duration of the glow. The glow condition occurs immediately after ignition of the discharge and is characterized by a higher voltage than the normal operating voltage of the lamp. This high voltage accelerates ions, bombarding the cathode and causing destructive sputtering.

Therefore it is desirable that the glow voltage be as low as possible and that the glow period be as short as possible.

Our improved emitters are particularly suitable for use in the low wattage metal halide lamps of improved efficacy described in patent 4,161,672-Cap and Lake, 1979.

The present invention will be further described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 shows a miniature metal halide arc lamp in which the invention may be embodied.

Figures 2 to 6 are voltage-time and currenttime traces measured on lamps similar to that of Fig. 1. In Figs. 2 and 3 prior art cathodes are used, while in Figs. 4 to 6 a variety of electron-emitting coatings embodying the invention are used.

A miniature arc tube 1 in which the invention may be embodied is illustrated in Fig. 1 and may be of the kind disclosed in patent 4,161,672 < ap et al., July 1979. The envelope is of fused silica and comprises a thin walled central bulb portion 2 having neck portions projecting on opposite sides. The inlead seals are made by collapsing through heat softening, assisted by vacuum if desired, the quartz of the necks 3, 4 upon the molybdenum foi portions 5, 6 of the electrode inlead assemblies. Leads 7, 8 welded to the foils project externally of the necks while electrode shanks 9, 10 of tungsten welded to the opposite sides of the foils extend through the necks into the bulb portion. The lamp is intended for unidirectional current operation and the shank 10 terminated by a balled end 11 suffices for an anode.The cathode comprises a helix 1 2 of tungsten wire (about 6 turns) close wound on shank 9 up to its balled end. The shank is terminated at its distal end in a mass or ball 1 3 which may be formed by initially leaving the shank slightly longer than shown and then melting it back. The electronemitting materials to which the invention pertains are coated on the turns of helix 1 2 or fill the interstices between the turns and the shank 9.

A typical miniature metal halide arc tube intended for a lamp of 35 watt size may have an ellipsodal bulb about 7 millimeters in outer diameter; the inner diameter is about 6 mm and the internal length, disregarding irregularities, about 7 mm, giving a discharge volume from 0.1 to 0.15 cc. A suitable filling for the envelope comprises argon or other inert gas at a pressure of several tens of torr to serve as a starting gas, and a charge comprising mercury and the metal halides Na and Sic13. The charge may be introduced into the arc chamber through one of the necks before sealing in the second electrode.To make a complete lamp, the illustrated arc tube is usually mounted within an outer protective envelope or jacket (not shown) which is either evacuated or filled with an inactive gas and which is provided with a base having contact terminals to which the inleads 7, 8 are connected.

In Figs. 2 to 6, the upper traces show the glow voltage and the lower traces show electrode current, both plotted on a time scale, from various cathodes in arc tubes such as illustrated in Fig. 1. The traces were obtained by photographing the screen of a long persistence cathode ray oscilloscope connected into the arc tube's operating circuit in the manner described in patent 3,249,859-Speros et al., Method and Apparatus for Measuring the Starting Characteristics of Gas-Filled Discharge Lamps, 1966. In each photograph, a second trace is superimposed on the first and was obtained by a second starting of the arc tube after it had been aliowed to cool to ambient temperature. The coincidence or lack thereof in the traces is indicative of reproducibility or erratic performance as the case may be.

The principles gbverning the measurements made are fully described in the above-mentioned patent and may be summarized as follows. When voltages versus time traces are taken of the discharge through a lamp across which a relatively low overvoltage sufficient to ignite it is applied, three distinct voltage levels of varying time duration may be observed.

The first level, termed plateau A in the patent, corresponds to a formative time lag for breakdown, sometimes known as emissive deadtime. It is usually short and practically disappears when the applied open-circuit voltage substantially exceeds the breakdown voltage of the lamp. In Figs. 2 to 6, the first level is not seen and the second level appears to begin at the left margin and is the first observed. It is termed plateau B in the patent and corresponds to a period of generalized emission from the cathode, often referred to as the glow emission phase. The third level, which is the second observed in Figs. 2 to 6 is termed plateau C in the patent. It is characterized by constriction of the discharge at the cathode to a hot spot and is often referred to as the arc phase.The arc phase is indefinite in duration, being the normal mode of operation of the lamp, and continues as long as the lamp is turned on. The glow phase which follows immediately after ignition is characterized by a higher voltage than the normal operating voltage of the lamp. The high glow voltage accelerates ions bombarding the cathode and casuing destructive sputtering. Therefore it is desirable that the glow phase be reduced in time duration as much as possible and that the glow voltage be as low as possible.

Referring to the drawings, Fig. 2 pertains to a cathode comprising a tightly wound tungsten coil left uncoated and bare. The glow voltage is observed to exceed 1 90 volts and the glow duration extends almost to 300 milliseconds; the uncoated tight coil tungsten electrode may be looked upon as the worst case situation. In Fig. 3, the same electrode is conventionally coated with ThO2-Y203 emission material. The improvement relative to the uncoated cathode of Fig. 2 is primarily in the reduction of the glow duration to about 250 milliseconds and one of the traces shows a minor reduction in the glow voltage to about 1 75 volts.Fig. 3 represents the best perfor- mance that the prior art could achieve; it is somewhat erratic, as evidenced by the lack óf coincidence in the traces.

Figs. 4 to 6 show that the glow voltage or the glow duration or both are substantially reduced by the use of the oxides and borides in accordance with the invention. Fig. 4 shows the behaviour when the tight coil tungsten electrode is coated with Dy203 emission material; the improvement is in the reduction of glow voltage which drops to 1 20 volts.

Glow duration at about 260 milliseconds is substantially unchanged.

In Fig. 5, the tight coil tungsten electrode is coated with thorium tetraboride ThB4 emission material. The glow voltage is reduced to about 11 5 volts and the duration of the glow phase to about 210 milliseconds. Other tests using thorium hexaboride, lanthanum hexaboride and scandium diboride show similar improvements in reduction of glow voltage and glow duration.

In Fig. 6, the tightly wound tungsten coil is coated with Sc203 emission material; the glow voltage is reduced to about 105 volts and the duration of the glow phase is shortened to about 1 80 milliseconds.

In all of the foregoing examples, the activating materials were prepared as powders and then made into suspensions in a solution of nitrocellulose in amyl acetate. The electrodes were coated by dipping into the suspension; thereafter the coated electrodes were baked in air for approximately one hour at 1 00 C and then fired in argon for 10 minutes at 740"C.

No special precautions were taken after the electrodes were fired; they were freely exposed to the atmosphere and lamps were made using standard manufacturing techniques.

The reduction in flow voltage and glow duration achieved by our invention leads to the expectation of substantial improvement in maintenance, particularly in miniature arc lamps subject to frequency starting, and preliminary life tests indicate that these expectations will be borne out.

Claims (5)

1. A high pressure metal halide arc discharge lamp comprising: an envelope of refractory light-transmitting material containing a fill of mercury and metal halide adapted to be vaporized during operation, and inert gas at a low pressure to facilitate starting, and a pair of opposed arc-sustaining elec- .

trodes sealed into said envelope, at least one of said electrodes serving as cathode comprising a base member of tungsten and electronemissive material supported thereby, said electron-emissive material being se lected from the group consisting of the oxides of scandium oxides of dysprosium, borides of thorium, borides of scandium and borides of lanthanum,

2. A lamp as claimed in claim 1 wherin said cathode base member comprises a coil of tungsten wire wound around a tungsten shank, and said material forms a coating on the coil or is present in the interstices of the coil.

3. A lamp as claimed in claim 1 or claim 2, wherein the metal halide in the fill comprises Nal and 8c13.

4. A lamp as claimed in any one of the preceding claims for unidirectional current operation wherein the envelope is fused silica and has a volume not exceeding 1 cc, and the metal halide in the fill comprises Nal and Sol3.

5. A lamp as claimed in claim 1, substantially as hereinbefore described, with reference to the accompanying drawings.