DE1130070B - Cathode for gas and / or vapor discharge lamps and process for their manufacture - Google Patents

Description

Cathode for gas and / or vapor discharge lamps and processes for their Manufacture The invention relates to cathodes for electrical discharge devices and in particular an emissive material and method of making and Activation of cathodes containing such a material. The cathodes according to of the invention are capable of delivering high discharge currents and are particularly suitable for continuous gas discharge lamps.

It is already a very large number of compounds and compositions known for emissive activators of cathodes. One knows, for example, just to name a few compounds, borides, carbides, hydrides, oxides, silicides of various metals including alkaline earths and rare earth metals, both on their own and in combinations. Besides, you know them various binders, protective layers, interlayer and base materials. The prior art in this field shows that cathodes and cathode pastes so far generally developed empirically. Even so, there is already pretty good cathodes. For example, cathodes based on alkaline earth oxides have proven themselves Well proven with fluorescent lamps. This type of cathode, sometimes used as a three carbonate mixture is called because it is produced by immersing a tungsten spiral in a suspension the carbonates of barium, strontium and calcium is produced, however a rather arduous treatment to decompose the carbonates to the oxides the formation of the cathode. In addition, the operating temperature and current density are at Alkaline earth oxide cathodes limited, otherwise vaporization and no longer acceptable a corresponding piston blackening occur. To prevent blackening of the piston or at least to push them back, the alkaline earth oxides are already resistant Oxide, such as zirconium oxide or thorium oxide, mixed in; however, this only creates remedial action to a limited extent.

In lamps that operate at higher temperatures and with higher current densities to be operated, like high-pressure mercury lamps, has mostly been the case up to now Cathodes in the form of a tungsten filament attached to a tungsten shaft or lead-in wire is wrapped and which contains a small piece of silver or thorium metal that is inside the filament lies along the tungsten shaft, is used. Such cathodes have does not have as high an efficiency as alkaline earth oxide cathodes, because the cathode fall is higher. They also require a higher ignition voltage and during operation A gradual blackening of the lamp bulb occurs due to vaporizing of the lamp or sputtering cathode metal.

The invention is therefore intended to provide an improved, emissive Material for cathodes of electric discharge lamps can be specified, the better Has operational characteristics and can be more easily manufactured and formed. In particular, an emissive material is intended for those who work with glow emission Lamp cathodes are specified that operate at relatively high current densities or temperatures with a simultaneous lower blackening of the lamp bulb during the life of the lamp allowed. The manufacturing process is said to be suitable for a suitable for industrial production. Furthermore, the invention is intended to provide scientific Guidelines for the composition and manufacture of cathode pastes are given that allows the properties of the cathodes to be adapted to the intended field of application and to adapt the desired operating characteristics as desired.

It has now been shown that the intermediate oxide systems or mixed oxides cathode materials made from barium thorate Ba Th03 and barium zirconate BaZr03 for discharge lamps, if they are affected by an excess of barium metal, the is firmly bound in the mixed oxide lattice, activated will. These mixed oxides are also stable in air prior to formation or activation if they are the same molar proportions of emissive oxide, namely Ba O and heat-resistant oxide, contain depending on Th02 or Zr02. The excess of emissive alkaline earth metal (Barium) can be caused by a reaction of the mixed oxide taking place in the solid state an excess of stable metal, i.e. thorium or zirconium, which is in powder form can be introduced, are formed. According to another feature of the invention is a very convenient and practical method of manufacture and formation of cathodes according to the invention in that a base or a cathode support in any suitable way with the emissive mixture of the mixed oxide and powdered, resistant metal provided or coated and then is heated so that a solid-state reaction takes place in which the desired forms excess of emissive barium metal in the mixed oxide lattice.

A cathode for gas and / or vapor discharge lamps with a conductive core and an emissive material which contains a heat-resistant metal oxide, barium oxide and free barium is characterized according to the invention that the emissive material according to the formula a Ba O; R0 ,; b Ba, where R02 represents the heat-resistant metal oxide that provides transparent sublimation products and the values for and preferably 0.15 <β <0.30 if the starting mixture has the composition BaO + R02 + βR. The invention will now be explained in more detail with reference to the drawing, FIG. 1 means a high pressure mercury lamp with cathodes according to the invention, FIG. 2 an enlarged partial view of a cathode and FIG Invention and on the other hand cathodes according to the prior art included.

A well-known emissive material with an excess of barium in the lattice is obtained by a reduction of barium aluminate carried out in the solid state with aluminum. Barium aluminate which contains an excess of barium is suitable excellent for cold cathodes and therefore especially for intermittently operated ones Flashbulbs. It has now been shown that in a certain respect one can compare them Can achieve results with barium thorate and barium zirconate and that the so obtained cathode pastes surprisingly for a glow emission at relatively high temperatures and thus for gas discharge lamps that work continuously suitable.

Mixed oxides a (M #, 0y) (p; (R, OH,); where M, Oy is an emissive oxide, e.g. barium oxide, and R # 0 "means a heat-resistant or resistant oxide, such as Th02, result in poor cathodes when used directly, even if a is less than 1. The emission properties, however, improve by leaps and bounds if an excess of emissive metal M is introduced according to the following reaction equation: the left side of the equation corresponds to the starting mixture, the right side to the formed cathode or its emissive cathode paste. With a = 1, x = 1, y = 1, z = 1, w = z we get for the curly bracket expression: ie the relationship specified in the claim. The reducing agent R is in all cases the same as the metal of the stable oxide component R, 0W, namely thorium in the case of thorate and zirconium in the case of zirconate. With this method, the chemically simplest of the cathode materials according to the invention are obtained after formation.

A wide variety of cathode materials can be obtained using the general formula given above. The nature of the resulting cathode can be influenced by the following variables: first, choice of the mixed oxide system; second, choice of reducing agent; third, amount of the reducing agent; and fourth, distribution state of the reducing agent. Extensive investigations and series of tests have shown that the mixed oxides barium thorate Ba Th 03 (or B a O - Th 02) and barium zirconate Ba Zr 03 (or Ba O - Zr 02) have the best combination properties for hot cathodes or cathode pastes with regard to the simplicity of the Manufacture to show the efficiency in operation and stability, ie the absence of blackening of the bulb during the life of the lamp. If barium thorate or barium zirconate is used as the starting material in a stoichiometric ratio, ie 1 mole of BaO per mole of Th 02 or 1 mole of BaO per mole of Zr 02, the mixtures are stable in air before activation and can be stored in a sealed bottle for a long time . In tests, such materials were stored for up to a year before use and formation, and there were no adverse effects of storage. With regard to production, this represents a particular advantage. According to the invention, the starting materials are therefore mixtures of barium thorate or barium zirconate with barium oxide and the heat-resistant oxide in stoichiometric proportions. When barium oxide and the stable oxide are in stoichiometric proportions, the general equation (I) given above is simplified to the following, much simpler form: R stands for Th or Zr, and ß denotes the number of moles of R that enter into the reaction per mole of Ba O - R OZ.

If the formula given above is applied to two specific quantitative ratios, which represent the preferred quantitative ratios for the two materials, as will be explained in more detail below, one obtains: Barium thorate with 0.2 mol Th: 750 ° C BaTh03 -E- 0.2 Th D- 1.2 (0.5 Ba0 - 1 Th 02 '0.33 Ba) (III) Barium zirconate with 0.25 mol Zr: 700 ° C BaZr 03 -I- 0.25 Zr 1.25 (0.4 Ba O - 1 Zr 02 - 0.4 Ba) (IV) The left-hand side of the reaction equations given above corresponds to the emission mixture that is applied to the electrode arrangement or pad prior to formation. The electrode assembly may consist of tungsten wire, e.g. B. from a double coil with a third overwinding, as is common in many fluorescent lamps and other low-pressure gas discharge lamps. The electrode can also contain a single or double tungsten filament wrapped around a tungsten shaft, as in many high pressure mercury lamps. In such arrangements, the cathode paste can be applied to the electrode either by immersing the electrode in a suspension of the mixture in a volatile carrier such as butyl acetate or ethyl alcohol. For some areas of application, a simpler arrangement with a pill made of pressed material can also be used. The right-hand sides of the reaction equations given above again correspond to the activated, emissive material that forms at the given temperatures.

In the above-described method of making electrodes according to the invention, the formation was carried out by a solid state chemical reaction that does not release gases or any special gas atmosphere is necessary for implementation. The reaction also runs at proportionate low temperatures, so prolonged heating or heating up very high temperatures is not required. Even heating in a reducing Atmosphere, for example in a hydrogen stream, can be omitted, which is the case with some Cathode pastes for high-pressure mercury lamps are required that consist of mixtures Contains barium carbonate and thorium oxide. The method according to the invention provides so a significant advance in the manufacture of lamps, in particular the formation of the cathode.

Another advantage of the solid state reaction according to the invention is to see in it that no gases arise and are released. This creates the pumping system relieved during formation. When gases are released from the cathode paste In addition, there is always a loss of volume or weight, so that in the end less active substance remains on a given electrode array. All these disadvantages are avoided in the method according to the invention. Used you especially with the materials described a sponge, compact or Composite made of heat-resistant metal, this results in a less porous structure, and the risk of poisoning from the ingress of water or carbon dioxide the environment after formation is reduced.

An examination of the results carried out on a large number of different cathodes with the change of β using various resistive oxides revealed, as a result, the possibility that the work function e-99, or more precisely the slope of the curve of the Richardson equation between about 750 and 850 ° C can be kept approximately the same and at around 1.6 eV, regardless of the constant oxide (A12 03, Th 02, ZrO2) and with the exception of extreme mixing ratios, regardless of the BaO content of the mixed oxide system. Emissive layers were applied to cathode arrangements or substrates made of tungsten, molybdenum and nickel and investigations showed that the exit potential 99 is practically independent of the nature of the substrate. It was also found that the factor A in the Richardson equation seems to depend on the amount of barium which is present in excess over the stoichiometric ratio of the mixed oxide system; A is very small for pure substances and grows with the barium content.

These results lead to the hypothesis that the exit potential 99 and thus the cathode fall of the mixed oxide system due to the nature of the existing ones Emission centers (in this case BaO-Ba) are determined while the saturation current density on the number of emission centers per unit area of the emittable surface depends. From the above it seems further that the role of the permanent Component (Th 02 or Zr 02) simply consists in the stability of the BaO-Ba emission centers and the amount of barium that is made up of that consisting of barium oxide and the permanent oxide Grids can be combined to increase.

The foregoing suggests that there is a wide range of different mixtures with different molar ratios of emissive oxide and emissive metal with respect to the stable oxide in which electron emission occurs; this was actually found in the present case. The range of optimal proportions can be determined relatively easily that the starting point before the formation always forms a stoichiometric compound, ie a compound which per mole of Ba 0 1 mole 1h02 in the case of Ba Th 03 or 1 mole B a O per mole of Zr 02 in the case of BaZr03. This condition is of course based on the requirement that the emission mixture should be stable in air before formation, as has already been explained above. In addition, the final, formed and activated compounds result from a solid-state reaction of the stoichiometric barium thorate or barium zirconate with ß mol Ba or Zr necessarily in the form aBa0-1R02 * bBa, the number of moles of the end product are. The composition of the final formed mixture is therefore determined by the choice of ß.

The table below shows the values for x and y for various values of β in relation to the solid-state reaction of barium thorate with thorium or barium zirconate with zirconium. _ i -2ß _ 2ß ß a 1 + ß b 1 + ß 0.05 0.86 0.095 0.1 0.73 0.18 0.2 0.5 0.33 0.25 0; 4 0.4 0.3 0.30 0.46 0.4 0; 14 0.57 0.5 0 0.67 It can be seen that β cannot become equal to or smaller than 0.5, since then no Ba0 would remain in the compound and no BaO-Ba emission centers would be formed. A large excess of barium would then be obtained, and although the emissivity is considerable, there would be a fear of rapid blackening of the bulb as a result of excessive evaporation of barium. This has also been confirmed in practice.

At the other end of the range, when β = 0.05, there is a relationship from barium (b = 0.095) to barium oxide (a = 0.86) of about 1: 9, and one can expect that the emissivity becomes too low; The investigations have also confirmed this.

So it turns out that at an acceptable electron-emitting Material according to the invention is the molar ratio of to the stoichiometric barium thorate added thorium or zirconium added to the stoichiometric barium zirconate should be greater than 0.05 and less than 0.5. Within these limits resulted in one thermogravimetric analysis that a for optimal stability in the vicinity of Should be 0.5, provided that the dissolving power of barium in the lattice is not exceeded. Experiments have shown that the preferred range of β for thermally emitting cathodes, which have proven themselves well in practice, between about 0.15 and 0.25 with an optimum of about 0.2 in the case of thorium and im Range from about 0.20 to 0.30 with an optimal value of about 0.25 in the case of zircon lies. In the case of a cathode containing thorate, this results in the formed State the formula 0.5 BaO-Th02-0.33 Ba.

In the case of barium zirconate, results for the formed compound the formula 0.4BaO-Zr02 * 0.4Ba.

The thorate has a slightly lower barium content than the zirconate preferable, as the solubility of barium in the thorate lattice is somewhat smaller appears.

Fig. 1 of the drawing shows a high pressure mercury lamp 1 with a Discharge vessel 2 made of quartz, which is enclosed by a transparent bulb 3 which ends in a screw sock14. The discharge vessel 2 contains mercury and an inert ignition gas. The main electrodes 5, 6 at the two ends of the quartz tube consist of two approximately concentric tungsten filaments 7, 8 (Fig. 2), of which one is tightly wound around a tungsten shaft 9 forming the lead-in, while the other spiral is screwed onto the smaller one, as shown in FIG. 2. The inner one Helix? has a steeper slope in the middle, creating cavities formed that can be filled with cathode paste. Ignition at lower Voltage is relieved by an auxiliary electrode 10 which is close to the main electrode 5 is arranged.

The electrodes are placed in the discharge vessel before they are melted down the lamp into a mixed oxide suspension of barium thorate with additional thorium. immersed or in a suspension made of barium zirconate with additional zirconium. the In addition to the cathode materials according to the invention, a suspension can be relatively contain volatile organic liquids such as butyl acetate or ethyl alcohol. The material fills the cavities and spaces between the turns of the coils 7, B. Activation of the cathodes can be achieved simply by heating them to a temperature over 700 to 800 ° C during the melting of the electrodes into the quartz tube. The discharge vessel is then evacuated and filled with an inert ignition gas, such as argon, under a relatively low pressure and a small amount of mercury, which exceeds the amount that evaporates during normal operation of the lamp, filled.

Fig. 3 shows in a diagram the improvements in the operating characteristics and stability, which is achieved by using mixed oxide cathode pastes according to Invention can be achieved, in particular by a cathode paste by a Reaction of 0.2 Mot Th with stoichiometric barium thorate to a compound with of the formula 0.5 Ba0 - 1 Th02-0.33 Ba was formed. In the diagram is on the Ordinate is the light output of the lamp in lumens as a percentage of the light output at start-up of the lamp and the operating time in hours on the abscissa. The dashed Line 12 shows the light output of a known lamp as a function of the operating time; the lamp contained familiar and widely used Cathodes with a core of tungsten wire with thoriated silver, which is between the Tungsten lead-in wire wedged into the electrode and the tungsten filament was. The solid curve 13 shows the light output of a lamp with the one shown in Fig. 2 shown cathode arrangement activated with barium thorate according to the invention was. The significant improvement in the stability of the lamp is evident.

One in terms of maintaining the light output of the lamp during The service life of lamps with improved mixed oxide cathodes is a noteworthy feature according to the invention consists in that the sublimation products that are during the operation of the lamp on the inner wall of the lamp envelope, whitish and are translucent and not black and light-absorbing as before. this stems from the fact that the sublimation products produced by the cathode pastes according to Invention given are transparent.

Another important advantage is the lowering of the ignition voltage, which allows the lamps to be ignited at a much lower voltage. So humiliated z. B. the ignition voltage of the lamp shown when using barium thorate mixed oxides according to the invention from about 300 V to about 225 V.

The embodiments described and those described and illustrated Details are for explanation purposes only and are not to be interpreted as restrictive.

Claims (7)

PATENT CLAIMS: 1. Cathode for gas and / or vapor discharge lamps with a conductive core and an emissive material which contains a heat-resistant metal oxide, barium oxide and free barium, characterized in that the emissive material according to the formula aBaO; R02; b Ba is built up, where R O2 represents the heat-resistant metal oxide that supplies transparent suplimation products and the values for and 0.05 <β <0.5, preferably 0.15 <β <0.30, if the starting mixture has the composition (p BaO + R02 + β R). 2. Cathode according to claim 1, characterized in that that the emissive material contains thorium oxide in a manner known per se and that the molar proportion of barium is between 0.15 and 0.25. 3. Cathode after Claim 1 and 2, characterized in that the emissive material approximately is built up according to the formula 0.5 BaO-Th02 * 0.33 Ba. 4. cathode according to claim 1, characterized in that the emissive material in. Per se known manner Contains zirconium oxide and that the molar proportion of barium is between 0.20 and 0.30. 5. Cathode according to claim 4, characterized in that the emissive material is built up approximately according to the formula 0.4BaO-Zr02.0.4Ba. 6. Method of manufacture a cathode according to one of claims 1 to 5, characterized in that a conductive cathode core is coated with a mixture consisting of a stoichiometric Compound of the formula BaO-R02 (R = Th or Zr) and Thonum or zircon in powdered form Form in a molar proportion between 0.05 and 0.5 per 1 mole of the compound, and that the cathode assembly so produced in a neutral atmosphere for Formation is heated to a relatively low temperature. 7. Procedure according to claim 6, characterized in that as a mixture to 1 mole of barium thorate powdered according to the formula Ba O - Th 02. Thorium in a molar proportion between 0.15 and 0.25 is used. B. The method according to claim 6, characterized in that that as a mixture of 1 mol of barium zirconate according to the formula Ba O - Zr 02 powdered Zircon is used in a molar proportion between 0.20 and 0.30. Into consideration printed publications: German Patent No. 597 580; British patent specification No. 526 064; U.S. Patent No. 2,843,801.