US3188236A

US3188236A - Cathodes and method of manufacture

Info

Publication number: US3188236A
Application number: US860130A
Authority: US
Inventors: Dimitrios M Speros
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1959-12-17
Filing date: 1959-12-17
Publication date: 1965-06-08
Anticipated expiration: 1982-06-08
Also published as: GB967273A

Description

June 8, 1965 D. M. SPEROS CATHODES AND METHOD OF MANUFACTURE Filed Dec. 17, 1959 Life (hours) INVENTOR. Dimizrios M. Spams BY l-h's Attorney United States Patent 3,138,236 QATHGEJES AND METHGD til Dimitrios M. Spores, Painesvllle, @hio, assignor to General Electric Qoinpany, a corporation of New York Filed Dec. 17, E59, Ser. No. 869,139 S Qlaims. (Cl. 117-228) This invention relates to cathodes for electric discharge devices and more particularly to improved electron emissive materials and to methods of preparing and activating cathodes incorporating such materials. The invention is especially concerned with cathodes suitable for supporting relatively high discharge'currents and adapted for use in discharge lamps for continuous operation.

The compounds and compositions which have been proposed as electron emitters or activating materials for cathodes have been extremely numerous. To mention but classes of materials, borides, carbides, hydrides, oxides and silicides of various metals including the alkaline earths and the rare earths have been proposed, both alone and in combinations. There have also been proposed at various times a large variety of binders, protective coatings, interfaces and base materials. It is apparent, from a review of the literature in this field, that cathodes and electron emitters have been developed in the past largely as an art on a trial and error basis. Even so however, good cathodes have been produced. For instance the alkaline earth oxide type of cathode commonly used in fluorescent lamps provides good results in its own field. However this type of cathode, sometimes referred to as the triple carbonate mix because it is made by dipping a tungsten coil into a suspension of the carbonates of barium, strontium and calcium, requires rather elaborate processing to decompose the carbonates to oxides in order to activate the cathode. Moreover the alkaline earth oxide cathode is limited in its operating temperature and in the current density which it can withstand without excessive vaporization and consequent blackening of the lamp envelope. To prevent or reduce such blackening, the practice has been adopted or" adding a refractory oxide such as zirconium oxide to the alkaline earth oxides but this has only a limited value.

in higher temperature or higher current density lamps such as high pressure mercury vapor lamps, the cathode which has been most commonly used is in the form of a tungsten coil wound on a tungsten shank or inlead with a small piece or sliver of thorium metal inserted within the coil and lying alongside the tungsten shank. These cathodes are not as etficient as the alkaline earth oxide cathodes inasmuch as they operate with a higher cathode voltage drop. Also they require a higher starting voltage, and gradual vaporization or sputtering of metal from the cathode causes lamp blackening during life.

Therefore the general object of the invention is to provide improved electron emissive materials for the oathodes of electric discharge devices and which give improved results from the point of view of performance or which are easier to process.

A more specific object of the invention is to provide new electron emissive materials particularly suitable for thermionic or discharge lamp cathodes and permitting operation at relatively high current densities or temperatures with a much lesser degree of envelope darkening during life than has been possible heretofore.

Another object of the invention is to provide improved manufacturing or processing methods for cathodes which are readily adaptable to factory use.

A further object of the invention is to elaborate some scientific principles providing guidance in establishing the final design or making the ultimate choice of cathode material w hin the purview of this invention according to the intended use or field of application and the opcrating characteristics desired.

In accordance with the invention, I have discovered that the interoxide systems consisting of barium thorate BaThO and barium zirconate BaZrO form excellent electron emissive substances provided they are activated by an excess of bariummetal firmly bound into the interoxide lattice. Further these interoxides are stable in air prior to activation when they contain equal molecular proportions of emissive oxide, namely BaO and refractory oxide, that is Th0 or ZrO as the case may be.

The excess of emissive alkali earth metal (barium) may be formed through a solid state reaction of the interoxide with an excess or" the refractory metal, that is thorium or zirconium as the case may be, which may be introduced in powdered form. Therefore according to a further aspect of the invention, a very convenient and practical process for making and activating cathodes in accordance with the invention is to coat or otherwise provide on the base member or cathode substrate, a quantity of the emission mixture consisting of the interoxide and powdered refractory metal which is then heated to cause the solid state reaction to proceed and form the desired excess of emissive barium metal in the interoxide lattice.

For further objects and advantages and for a better understanding of the invention, attention is now directed to the following detailed description of the invention taken in conjunction with the accompanying drawing. The

features of the invention believed to be novel will be more particularly pointed out in the appended claims.

In the drawing:

FlG. 1 shows a high pressure mercury Vapor discharge lamp utilizing improved cathodes in accordance with the invention. V

1G. 2 shows in greater detail the cathode construction.

FIG. 3 is a graph comparing the performance of high pressure mercury vapor lamps utilizing the improved cathodes in accordance with the invention with that of other lamps using prior art cathodes.

In my Patent 2,871,196, Cathode and Emissive Material Therefor, I have described and claimed the improved electron emissive material which results from the solid state reduction of barium aluminate by aluminum in order to achieve an excess of barium in the lattice. Barium aluminate with an excess of barium provides an excellent emitter under cold cathode operating conditions and for this reason is particularly suitable for flashtubes intended for intermittent operation. somewhat similar results may be achieved with barium thorate and barium zirconate and that the resulting electron emitters are adapted to thermionic operation at relatively high temperatures making them particularly suitable for discharge lamps intended for continuous operation.

Interoxide systems (M O (R Where M O represents an emissive oxide, BaO for instance, and R 0 a refractory oxide such as Th0 form poor cathodes when used di ectly even when on is less than 1. Their performance as cathodes improves radically however when an excess of the emissive metal M is introduced. In accordance with the invention, this is done by chemically reducing part of the ivl O component as follows:

( i y 1) (12.0.) M (I) where the left hand side of the equation represents the emission mixture, while the right hand side represents the activated cathode or emissive substance. The reducing l used' each case is preferably but not necessarily I have now discovered that .view' of ease of manufacture.

3,1ss,2se

the same as the component of the refractory oxide R O namely thorium in the case of the thorate and zirconium in the case of the zirconate. By so doing, the simplest possible chemical species is obtained after activation.

A great many cathodes or electron emissive substances may be obtained by applying the above generic formula to specific materials. The type of cathode produced will be controlled by the following variables: (1) selection of the interoxide system; (2) selection of the reducing agent; (3) the amount of the reducing agent; (4) the ,state of subdivision of the reducing agent.

As a result of extensive tests and experimentation, I have discovered that the intcroxides barium thorate BaThO (or BaO-ThO and barium zirconate BaZrO (or BaO-ZrO offer the best combination characteristics in a thermionic cathode or emitting material from the to activation are stable in air and can be stored for long periods of time in stoppered bottles. This has been fully established through tests using materials prepared as much as one year prior to use and activation, and no adverse effects of any account were encountered. This of course is a very important advantage from the point of Therefore, in accordance with the invention, the starting materials for the emission mixture are barium thorate or barium zirconate with the where R stands for Th or Zr, and [3 stands for the number of mols of R entering the reaction per mol of BaO'RO Applying the above formula to two specific proportions which, as will be discussed more fully hereinafter, are-preferred proportions for the two materials, there is obtained:

Barium thorate using 0.2 mol of thorium- 150 C. BaThOs 0.2 Th 1.2 (0.5 BaO-l Thor-0.33 Ba) (III) Barium zirconate with 0.25 mol of zirconium 700 C. BaZrO 0.25 Z1 1.25 (0.4BaO-1 ZrOz-0.4 13a) (IV) The left hand side of the above equations shows the emission mixture which is placed on the electrode structure or substrate material prior to activation. The electrode structure may consist of a tungsten wire filament, for instance a coiled coil with a triple overwound as used in many fluorescent lamps or low pressure discharge lamps. Or else the electrode structure may consist of a single or double coil of tungsten wound around a tungsten shank as used in many forms of high pressure mercury vapor lamps. For such structures, the emission mixture may be placed on the electrode either by dipping the electrode in a suspension of the mixture in a readily vaporized vehicle such as butyl acetate or ethyl alcohol. For some applications, a simpler structure incorporating a pellet of the pressed material may be used. The right hand side of the above equation of course shows the resulting activated emission material which proceeds to completion at the temperatures indicated.

In the foregoing methods of preparing electrodes in accordance with the invention, the process of activation formation or liberation of gases.

is a solid state chemical reaction and it involves no evolution of gases nor does it necessitate any particular gas atmosphere for its progress. Furthermore the reaction takes place at a relatively low temperature with the result that prolonged heating or heating to a very high. temperature is not necessary. Nor is heating in a reducing at mosphere, for instance in a flow of hydrogen, necessary as is the case with some emission mixtures comprising barium carbonate and thorium oxides which have more recently found favor abroad for use in high pressure mercury vapor lamps. Thus the process in accordance with the invention affords a great simplification in the activation schedule step in lamp manufacturing.

Another advantage of the solid state reactions in accordance with the invention is that they do not involve This is advantageous in itself because it reduces the load on the exhaust system during the activation scheduled in lamp manufacture. Furthermore where the emission mixture liberates gases on activation, it shrinks in volume or weight so that a lesser amount of activated material is left on a given electrode structure or substrate. This drawback is avoided by the materials and processing in accordance with the invention. In particular, when the materials are used in conjunction with a refractory metal sponge or compact, a less porous structure is achieved whereby the danger of contamination as a result of penetration by atmospheric water and carbon dioxide after activation, is reduced.

A study of the results obtained with many different cathodes using variations in ,8 with different refractory oxides brings out the possibility that the value of the work function 95, or more precisely the slope of the Richardson plots, is approximately the same and in the vicinity of 1.6 electron volts at 750 to 850 C. regardless of the refractory oxide (A1 0 ThO ZrO and, except at extremes of composition, regardless of the B210 content of the interoxide system. Emission coatings were applied on cathode structures or substrates made of tungsten, molybdenum and nickel and tests indicated that the value of 5 is substantially independent of the cathode base or substrate. Furthermore, the value of A in the Richardson equation seems dependent on the amount of Ba present in excess of the stoichiometric formula for the interoxide system, being very small for pure substances and increasing with the Ba content.

This evidence points to the hypothesis that the work function (and therefore the cathode voltage drop) in interoxide systems is determined by the nature of the emissive sites involved (BaOBa in this case), while the saturated current density is determined by the number of these sites per unit area of emissive surface. From the foregoing. it would appear that the role of the refrac tory component (T or ZrO is simply to increase the stability of the BaO-Ba emissive sites, and to increase the amount of Ba that may be combined with the BaO-refractory oxide matrix.

The foregoing suggests that there is a wide range of composition in regards to the molar ratios of emissive oxide and emissive metal relative to refractory oxide, throughout which electron emission will take place; this in fact has been found to be the case. However in determining an optimum range of composition in accordance with the invention, the problem is considerably simplified by reason of the fact that the starting point in the activation schedule is always a stoichiometric compound, that is a compound containing 1 mol of BaO per mol of ThO in the case of BaThO or 1 mol of BaO per mol of ZrO in the case of BaZrO This condition of course follows from the requirement for stability in air of the emission mixture prior to activation, as previ- 3,1ss,2se

ously explained. Furthermore the final activated compounds which will result from the solid state reaction of the stoichiometric barium thorate or barium zirconate with ,8 mols of thorium or zirconium as the case may be,

Therefore, by defining 3, the composition of the final activated mixture is determined. The following table gives the values of x and y for various values of ,8 in respect of the solid state reaction of barium thorate with thorium, or barium zirconate with zirconium.

Table I Q5 86 O95 1 73 18 2 5 33 25 4 4 3 46 l l4 57 .5 0 .67

It is clear from the above that 18 cannot be equal to or greater than 0.5 because then there is no 2210 remaining in the compound and there would not be any BaO-Ba emissive sites. In such event there would be a great excess of barium and although the electron emissivity might be substantial, rapid darkening would be expected from the excessive vaporization of barium. This in fact has been found to be the case.

At the other end of the. scale, if ,8 is made 0.05, it is observed that the ratio of barium :.095) to barium oxide (.r=.86) is rou hly 1 in 9 and it might therefore be expected that the electron emissivity would be too low; such in fact has been found to be the case.

lt'follows ther from that for an acceptable electron emitter in accordance with the invention, the molar proportion of thorium added to the stoichiometric barium thorate, or of zirconium added to the stoichiometric barium zirconate, should be greater than 0.05 and less than 0.5. Within these limits, thermogravimetric analysis has indicated that for maximum stability x should be in the vicinity of 0.5 provided at the same time the solubility of barium in the lattice is not exceeded. By actual experiments, I have established that for a good practical thermionic cathode or electron emitter, p; is desirably in the range or" 0.15 to 0.25, preferably about 0.2, for thorium, and in the range of 0.20 to 0.30, preferably about 0.25, for zirconium. This results in the case of the tho ate in an activated compound having the formula ammo-r110 0.3mm

A slightly lower proportion of barium is preferred in the thorate than in the zirconate because the solubility of barium in the thorate lattice appears to be slightly less.

PEG. 1 of the drawing illustrates a high pressure mercury vapor lamp 1 comprising a quartz arc tube 2 enclosed within a vitreous jacket 3 provided with a screw base 4. The are tube contains mercury and an inert starting gas. The main electrodes 5, 6 at opposite ends of the arc tube comprise a pair of generally concentric coils or helices of tungsten wire 7, 3, the former being tightly wound around the tungsten shank or inlead 9 and the latter being screwed on the former as shown in EEG 2; Th inside helix 7 6 has a greater winding pitch in its middle portion whereby to leave cavities which are filled with the activated electron emitting mixture. An auxiliary electrode 10 close to main electrode 5 facilitates starting at a lower voltage.

Prior to assembly into the arc tube of the lamp, the electrodes are dipped into an interoxide suspension of barium thorate with added thorium, or barium zirconate with added zirconium, in accordance with the invention. The suspension may consist of the material in a comparatively volatile organic liquid such as butyl acetate or ethyl alcohol. The material fills the cavities and spaces between the turns of the coils 7, 8. The activation of the cathode may occur merely as a result of the heating to a temperature exceeding about 700 to 800 C. during the operation of pinch sealing the electrode inleads into the quartz arc tube. The lamp is completed by evacuating the arc tube and providing a filling of an inert starting gas such as argon at a low pressure, along with a small quantity of mercury exceeding the amount vaporized during normal operation of the lamp.

FIG. 3 illustrates the improvements in performance obtained by the use of the interoxide electron emitters in accordance with the invention, specifically that formed by reacting 0.2 mol of thorium with stoichiometric barium thorate to provide a compound having the formula in the graph, dotted line curve 12 illustrates the performance of prior art lamps wherein the cathodes were of the Well known and widely used type comprising a core of tungsten wire with a thorium silver wedged between the tungsten shank or electrode inlead andthe tungsten coil. Solid curve 13 illustrates the lumen maintenance characteristic of lamps having the cathode structure illustrated in PEG. 2 of the drawing and activated with barium thorate in accordance with the invention. The remarkable improvement in maintenance is self-evident.

A noteworthy feature in regards to the lumen maintenance of lamps having the improved interoxide cathodes in accordance with the'invention is that the products of sublimation which deposit on the arc tube wall during life are whitish and transparent rather than black and light absorbing as heretofore. This is due to the transparent nature of the products of sublimation resulting from the use of the nstant interoxide emitters.

Another important advantage is the lowering of the breakdown voltage so that the lamps are enabled to start at much lower temperatures. For instance, in the illusti 'at d lamp, use of barium thorate interoxide material in accordance with the invention reduces the starting voltage from about 300 volts to about 225 volts.

The specific embodiments and details of the invention which have been described and illustrated herein are intended as exemplary and not as limitative of the invention whose scope is to be determined from the appended claims.

What I claim as new and desire to secure by Letters Patent of the United States is:

l. A cathode comprising a conducting core and an emissive material applied thereto corresponding to the formula xBaO- IRO yBa wherein and and ,8 is less than 0.5 and greater than 0.05 and wherein R stands for a material selected from the group consisting of thorium, zirconium, and mixtures thereof.

2. A cathode comprising a conducting core and an emissive material applied thereto corresponding to the formula xlEaO- lThOyyBa wherein 7 and fl is in the range of 0.15 to 0.25.

3. A cathode comprising a conducting core and an emissive material applied thereto corresponding approximately to the formula (0.5)BaO-1ThO -(0.33)Ba.

4. A cathode comprising a conducting core and an emissive material applied thereto corresponding to the formula xBaO- lZrO 'yBa wherein 12B 2B +s 1+B and ,6 is in the range of 0.20 to 0.30.

5. A cathode comprising a conducting core and an emissive material applied thereto corresponding approximately to the formula (0.4)BaO-1ZrO -(0.4)Ba.

6. The process of making an activated electron emissive cathode which comprises coating a substrate with a mixture of a compound having a stoichornethric formula BaO-RO wherein R stands for a material selected from the group consisting of thorium, zirconium, and mixtures thereof and having added R in powdered form in a molar proportion 18 in the range of 0.05 to 0.5, and heating said substrate and coating to a relatively low temperature in a neutral atmosphere sufficient to allow the solid state reaction to proceed to completion whereby to produce an activated material.

7. The process of making an activated electron emissive. cathode which comprises coating a substrate with a mixture of barium thorate having the stoichiometric formula BaO-ThO and having added thorium in powdered form in a molar proportion in the range of 0.15 to 0.25, and heating said substrate and coating to a relaitvely low temperature in a neutral atmosphere sufiicient to allow the solid state reaction to proceed to completion whereby to produce an activated material.

8. The process of making an activated electron emissive cathode which comprises coating a substrate with a mixture of barium zirconate having the stoichiometric formula BaO-Zr0 and having added zirconium in powdered form in a molar proportion in the range of 0.20 to 0.30, and heating said substrate and coating to a relatively low temperature in a neutral atmosphere suflicient to allow the solid state reaction to proceed to completion whereby to produce an activated material.

References Cited by the Examiner UNITED STATES PATENTS 1,946,603 2/34 Von Wedel 117221 2,106,753 2/38 Lederer et a1. 117-220 2,394,095 2/46 Noble et a1 117220 2,647,067 7/53 Vfiliiams 117-220 2,687,489 8/54 Anderson et a1. 313346.1 2,726,178 12/55 Nelson 11722O 2,871,196 1/59 Speros 252512 2,878,409 3/59 Levi 313-346 2,902,621 9/59 7 Winter 3l3346 RICHARD D. NEVLIUS, Primary Examiner. RALPH G. NELSON, ARTHUR GAUSS, Examiners.

Claims

7. THE PROCESS OF MAKING AN ACTIVATED ELECTRON EMISSIVE CATHODE WHICH COMPRISES COATING A SUBSTRATE WITH A MIXTURE OF BARIUM THORATE HAVING THE STOICHIOMETRIC FORMULA BAO-THO2 AND HAVING ADDED THORIUM IS POWDERED FORM IN A MOLAR PROPORTION IN THE RANGE OF 0.15 TO 0.25, AND HEATING SAID SUBSTRATE AND COATING TO A RELAITVELY LOW TEMPERATURE IN A NEUTRAL ATMOSPHERE SUFFICIENT TO ALLOW THE SOLID STATE REACTION TO PROCEED TO COMPLETION WHEREBY TO PRODUCE AN ACTIVATED MATERIAL.