EP0005279B1 - Hot cathode - Google Patents

Hot cathode Download PDF

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Publication number
EP0005279B1
EP0005279B1 EP79200056A EP79200056A EP0005279B1 EP 0005279 B1 EP0005279 B1 EP 0005279B1 EP 79200056 A EP79200056 A EP 79200056A EP 79200056 A EP79200056 A EP 79200056A EP 0005279 B1 EP0005279 B1 EP 0005279B1
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EP
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Prior art keywords
diffusion
layer
platinum
additive
hot cathode
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German (de)
French (fr)
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EP0005279A3 (en
EP0005279A2 (en
Inventor
Charley Dipl.-Ing. Buxbaum
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BBC Brown Boveri AG Switzerland
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BBC Brown Boveri AG Switzerland
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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
    • H01J1/00—Details of electrodes, of magnetic control means, of screens, or of the mounting or spacing thereof, common to two or more basic types of discharge tubes or lamps
    • H01J1/02—Main electrodes
    • H01J1/13—Solid thermionic cathodes
    • H01J1/20—Cathodes heated indirectly by an electric current; Cathodes heated by electron or ion bombardment
    • H01J1/28—Dispenser-type cathodes, e.g. L-cathode
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
    • H01J1/00—Details of electrodes, of magnetic control means, of screens, or of the mounting or spacing thereof, common to two or more basic types of discharge tubes or lamps
    • H01J1/02—Main electrodes
    • H01J1/13—Solid thermionic cathodes
    • H01J1/14—Solid thermionic cathodes characterised by the material

Definitions

  • the invention relates to a hot cathode based on a high-melting carrier metal, an activation substance present as an oxide of a Group IIIb metal, a reducing agent present as a carbide of the carrier metal, and an additive which promotes the diffusion of the activation substance onto the active surface.
  • Glow cathodes for electron tubes are known in the art in numerous embodiments and material combinations. They range from classic oxide cathodes with a low operating temperature and low emission current density but a long service life to complex multi-material systems, particularly the so-called reaction cathodes. Among the latter, the cathode materials of type W / W 2 C / Th0 2 (“thoriated tungsten cathodes”), which work from the inside with chemical conversion and subsequent delivery of the activating substance and have a relatively high operating temperature, are used. They are characterized by a long service life with a moderate emission current density. It was also possible to show that the work function of the electrons can be reduced by adding a platinum metal to the aforementioned system and its emission properties can be improved (e.g.
  • the invention is based on the object of developing a hot cathode which, regardless of the shape and production process and structure, enables optimum material utilization with a high emission current density and ensures the longest possible service life even under the toughest operating conditions.
  • this is achieved in that, in the initially defined hot cathode to reduce the intrinsic diffusion of said diffusion-promoting additive from the effective cathode surface into the interior on the side of the layer which contains the diffusion-promoting additive or is formed by the latter, facing away from the active surface, a further layer of one for the said addition of diffusion-inhibiting substance is provided.
  • the guiding principle underlying the invention is based on the knowledge that the temperature dependence of the self-diffusion of the diffusion-promoting additive (for example platinum) into the interior of the carrier metal (for example molybdenum) is substantially steeper than the temperature dependence of the corresponding evaporation measure.
  • the higher the operating temperature the higher the percentage of the additive diffusing from the coated surface into the cathode body. It is precisely at the higher operating temperatures required to achieve a high emission current density that a disproportionately large percentage of diffusion-promoting additive is lost for the cathode surface.
  • concentration of this additive which promotes the diffusion of the activating substance in the structure of the carrier metal, can be chosen to be significantly lower than was previously assumed. The service life of the cathode can therefore be significantly increased by switching on an additional layer which inhibits the self-diffusion of the additive to the interior of the body.
  • Such a barrier layer is subject to the condition that it does not adversely affect the chemical-thermodynamic equilibria and the corresponding reaction kinetics of the substances involved. In particular, it allows diffusion of the activation substance on the cathode surface. Furthermore, the barrier layer must not form an alloy with the diffusion-promoting additive which could chemically and physically change the activating substance and reduce its diffusion to the cathode surface.
  • the above-mentioned conditions are particularly sufficient for elements whose grain boundary diffusion inside the cathode body is smaller than that of the diffusion-promoting additive used at the same time for the respective activation substance.
  • those elements will have an advantageous effect whose vapor pressure at the operating temperature is lower than the vapor pressure of the diffusion-promoting additive used at the same time.
  • the diffusion-promoting additive consists of a platinum metal, another platinum metal or another suitable metal with a lower vapor pressure can be used for the barrier layer.
  • molybdenum powder was mixed with 2% lanthanum oxide powder, pressed and sintered.
  • a plate was separated from the heat-treated sintered body, its surface polished and electrolytically coated with rhenium.
  • the thickness of the barrier layer averaged 5 ⁇ . and can vary between 1 and 20 ⁇ .
  • the platelet was then provided with a platinum layer 10 ⁇ m thick, which was also applied electrolytically. This surface layer can advantageously 1 to 50 u. be fat.
  • the sintered plate was then carburized in a benzene-hydrogen mixture.
  • 1 shows the structure of the cathode in section.
  • 1 represents the sintered body consisting of carrier metal (e.g. molybdenum) and activation substance, which is held in a holder 4 made of a high-temperature material (e.g. also molybdenum) and is indirectly heated by a heating coil 5.
  • the barrier layer 3 made of diffusion-inhibiting material (for example rhenium) is located on the effective side of the cathode, directly below the surface layer 2 made of platinum metal.
  • the cathode can also be designed to be directly heated, as a result of which the heating coil 5 and the holder 4 are eliminated or are replaced by other components.
  • the sintered body can also have a shape other than that outlined in the figure.
  • Fig. Shows the cross section through such a cathode wire with a layered structure of the activation substance.
  • the core zone 6 of the carrier metal e.g. molybdenum
  • the boundary layer with a low content of activating substance adjoins the core zone 6.
  • the barrier layer 3 made of diffusion-inhibiting material (e.g. rhenium) and, to the extreme, the surface layer 2 made of platinum metal (platinum in the present case).
  • Example 2 According to the procedure given in Example 2, a core and a shell were created prepared, both of which had the same concentration of activating substance lanthanum oxide of 3%.
  • the core was provided with an electrolytically deposited rhenium layer of 20 ⁇ thickness and a platinum layer of 100 ⁇ thickness.
  • the layer thicknesses can vary between 5 ⁇ and 50 ⁇ or 20 ⁇ and 200 ⁇ .
  • the body was first brought down to a diameter of 3 mm by round hammering and then pulled down to a diameter of 1 mm and then carburized. Finally, the wire was provided with an electroplated rhenium and platinum layer of 3 ⁇ and 15 ⁇ , respectively.
  • the finished wire cross section can be seen in FIG. 3. 6.
  • 6 represents the core zone with a high content of activation substance and 8 the corresponding, also highly doped edge layer.
  • the barrier layer 10 made of diffusion-inhibiting material (for example rhenium) is located directly under the intermediate layer 9 made of platinum metal.
  • the structure of the surface layers 2 and 3 corresponds to that of FIG. 2.
  • the surface layer 2 made of platinum metal may also be missing in the delivery state, the barrier layer 3 initially taking on the role of the diffusion-promoting additive in the edge zone until sufficient platinum metal from the inside to the surface is diffused. Thanks to numerous alloy formation options between 2 and 3, further material combinations with differently graduated functions can be carried out anyway.
  • f1 represents the core of diffusion-promoting additive (platinum metal, in the present case platinum), while 12 denotes the jacket made of carrier metal (here molybdenum) doped with a high content of activation substance.
  • the surface layer 2 made of platinum metal (platinum) lies on the barrier layer 3 made of diffusion-inhibiting material (rhenium).
  • a core and a shell were produced in each case according to the process given in Example 2, but an additional 0.5% of platinum peat was added to the powdery starting materials for the core.
  • the further processing was carried out analogously to the process steps mentioned in Example 2.
  • the round hammered wire provided with a 4 ⁇ thick rhenium and a 25 ⁇ thick platinum layer had a diameter of 3.5 mm.
  • the core zone 13 of the carrier metal e.g. molybdenum
  • the edge layer 7 with a low content of activation substance bears the surface layers 2 and 3 corresponding to FIG. 2.
  • FIG. 6 shows a diagram which shows the service life as a function of the layer thickness of the diffusion-promoting additive for a round hammered cathode wire according to FIG. 2.
  • the influences of the barrier layer and the operating temperature are characterized by different lines.
  • the curves "a” and “b” refer to an operating temperature of 2050 K, the curves "c” and “d” to one of 1880 ° K.
  • the layer thickness of the diffusion-promoting additive platinum
  • the life of the cathode is plotted in h on a logarithmic scale.
  • a and “c” are the comparison curves for hot cathodes without a barrier layer, based on molybdenum as a carrier metal, molybdenum carbide as a reducing agent and platinum as a diffusion-promoting additive.
  • B and “d” represent the corresponding curves for hot cathodes with an additional rhenium barrier layer of 5 ⁇ thickness.
  • the operating pressure of the electron tube was 5. 10- 4 torr. The comparison of the curves shows that the lifespan of the rhenium coating could be increased on average to 2.5 times the original value.
  • the invention is not restricted to the embodiments explained in the aforementioned examples. It can also be advantageously applied to combinations of materials other than those described above.
  • the W / W 2 C / Th0 2 / Ru and Ta / Ta 2 C / Y 2 O 3 / Pd systems are also considered.
  • the proposed barrier layer can be applied in general to all material combinations of reaction cathodes of the diffusion delivery type doped with a diffusion-promoting additive, in order to reduce or completely prevent an undesirable self-diffusion of the additive in an undesired direction prevent.
  • the new hot cathodes according to the invention have created components for electron tubes which, with a high emission current density, enable the best use of the available materials and ensure a long service life.
  • the barrier layer allows the chemical-physical processes to be largely directed in the desired direction and to effectively limit unfavorable side effects during operation.

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  • Solid Thermionic Cathode (AREA)

Description

Die Erfindung betrifft eine Glühkathode auf der Basis eines hochschmelzenden Trägermetalls, einer als Oxyd eines Metalls der Gruppe Illb vorliegenden Aktivierungssubstanz, eines als Karbid des Trägermetalls vorliegenden Reduktionsmittels und eines die Diffusion der Aktivierungssubstanz an die wirksame Oberfläche fördernden Zusatzes.The invention relates to a hot cathode based on a high-melting carrier metal, an activation substance present as an oxide of a Group IIIb metal, a reducing agent present as a carbide of the carrier metal, and an additive which promotes the diffusion of the activation substance onto the active surface.

Glühkathoden für Elektronenröhren sind in der Technik in zahlreichen Ausführungsformen und Werkstoffkombinationen bekannt. Sie reichen von den klassischen Oxydkathoden mit tiefer Betriebstemperatur und geringer Emissionstromdichte aber langer Lebensdauer bis zu komplizierten Vielstoffsystemen insbesondere der Gattung der sogenannten Reaktionskathoden. Unter letzteren werden vor allem die mit chemischer Umsetzung und Nachlieferung der Aktivierungssubstanz aus dem Innern arbeitenden und eine verhältnismäßig hohe Betriebstemperatur aufweisenden Kathodenmaterialien des Typs W/W2C/Th02 (»thorierte Wolframkathoden«) eingesetzt. Sie zeichnen sich durch lange Lebensdauer bei mäßiger Emissionsstromdichte aus. Es konnte ferner gezeigt werden, daß die Austrittsarbeit der Elektronen durch Hinzufügen eines Platinmetalls zum vorgenannten System gesenkt und dessen Emissionseigenschaften verbessert werden können (z. B. DE-OS 1614541). Es sind außerdem noch andere Werkstoffkombinationen bekannt, welche bei mittlerer Lebensdauer eine Steigerung der Emissionsstromdichte erlauben, worunter vor allem die Systeme Mo/Mo2C/La203 (nach DE-AS 2 344 936) und Mo/Mo2C/La203/Pt-Metall und ähnliche (nach DE-AS 2454569) genannt seien. Es sind außerdem Glühkathoden bekannt, welche auf der Basis von aus Pulvermischungen eines hochscmelzenden Metalls mit einem Platinmetall hergestellten porösen Sinterkörpern aufgebaut sind, deren Poren mit einem die Aktivierungssubstanz enthaltenden Material ausgefüllt sind (z. B. DE-OS 2 727 187). Derartige Kathoden zeichnen sich vor allem durch hohe Emissionsstromdichte bei verhältnismäßig niedrigen Betriebstemperaturen aus.Glow cathodes for electron tubes are known in the art in numerous embodiments and material combinations. They range from classic oxide cathodes with a low operating temperature and low emission current density but a long service life to complex multi-material systems, particularly the so-called reaction cathodes. Among the latter, the cathode materials of type W / W 2 C / Th0 2 (“thoriated tungsten cathodes”), which work from the inside with chemical conversion and subsequent delivery of the activating substance and have a relatively high operating temperature, are used. They are characterized by a long service life with a moderate emission current density. It was also possible to show that the work function of the electrons can be reduced by adding a platinum metal to the aforementioned system and its emission properties can be improved (e.g. DE-OS 1614541). There are also other combinations of materials known which allow an increase in the emission current density with a medium service life, including in particular the systems Mo / Mo 2 C / La 2 0 3 (according to DE-AS 2 344 936) and Mo / Mo 2 C / La 2 0 3 / Pt metal and the like (according to DE-AS 2454569) may be mentioned. Glow cathodes are also known, which are constructed on the basis of porous sintered bodies produced from powder mixtures of a high-melting metal with a platinum metal, the pores of which are filled with a material containing the activating substance (e.g. DE-OS 2 727 187). Such cathodes are characterized above all by a high emission current density at relatively low operating temperatures.

Die Lebensdauererwartung der vorgenannten Kathoden ist in Anbetracht der meist noch als relativ hoch anzusehenden Betriebstemperaturen für viele Anwendungszwecke ungenügend. Bei den ein Platinmetall als diffusionsfördernden Zusatz enthaltenden Kathodenwerkstoffen (z. B. System Mo/Mo2C/La203/Pt) hat sich nun gezeigt, daß sowohl Lebensdauer wie Emissionsvermögen der monoatomaren Aktivatorschicht maßgend von der Konzentration bzw. Lebensdauer des diffusionsfördernden Zusatz in der wirksamen Oberflächenschicht bzw. Randzone der Kathode abhängt. Der Bestand einer dichten Schicht dieses Zusatzes ist einesteils von dessen Abdampfmaß an der Oberfläche, andernteils von dessen Diffusionsgeschwindigkeit ins Innere des Kathodenkörpers abhängig. Beide Faktoren sind stark temperaturabhängig, so daß der Kathodenlebensdauer schlußendlich auch von dieser Seite her Grenzen gesetzt sind. Bei den herkömmlichen Reaktionskathoden wird die Funktionsfähigkeit durch den ins Innere des Trägermetalls diffundierten und nicht kontrollierbaren Anteil an diffusionsförderndem Zusatz vorzeitig beeinträchtigt, so daß sich der Vorrat des letzteren in der Oberflächenschicht frühzeitig erschöpft.The life expectancy of the aforementioned cathodes is insufficient for many applications in view of the operating temperatures, which are usually still considered to be relatively high. With the cathode materials containing a platinum metal as diffusion-promoting additive (e.g. system Mo / Mo 2 C / La 2 0 3 / Pt), it has now been shown that both the service life and emissivity of the monoatomic activator layer are dependent on the concentration or service life of the diffusion-promoting agent Additive depends in the effective surface layer or edge zone of the cathode. The existence of a dense layer of this additive depends partly on its degree of evaporation on the surface and partly on its rate of diffusion into the interior of the cathode body. Both factors are strongly temperature-dependent, so that the cathode service life is ultimately also limited from this side. In the case of the conventional reaction cathodes, the functionality is prematurely impaired by the proportion of diffusion-promoting additive diffused and not controllable into the interior of the carrier metal, so that the supply of the latter in the surface layer is exhausted at an early stage.

Der Erfindung liegt die Aufgabe zugrunde, eine Glühkathode zu entwickeln, welche unabhängig von Form und Herstellungsverfahren sowie Gefügebeschaffenheit bei hoher Emissionsstromdichte eine optimale Werkstoffausnutzung ermöglicht und eine möglichst hohe Lebensdauer selbst unter den härtesten Betriebsbedingungen gewährleistet.The invention is based on the object of developing a hot cathode which, regardless of the shape and production process and structure, enables optimum material utilization with a high emission current density and ensures the longest possible service life even under the toughest operating conditions.

Erfindungsgemäß wird dies dadurch erreicht, daß bei der eingangs definierten Glühkathode zur Herabsetzung der Eigendiffusion des besagten diffusionsfördernden Zusatzes von der wirksamen Kathodenoberfläche weg ins Innere auf der der wirksamen Oberfläche abgewandten Seite der den diffusionsfördernden Zusatz enthaltende oder durch letzteren gebildeten Schicht eine weitere Schicht eines für den besagten Zusatz diffusionshemmenden Stoffes vorgesehen ist.According to the invention, this is achieved in that, in the initially defined hot cathode to reduce the intrinsic diffusion of said diffusion-promoting additive from the effective cathode surface into the interior on the side of the layer which contains the diffusion-promoting additive or is formed by the latter, facing away from the active surface, a further layer of one for the said addition of diffusion-inhibiting substance is provided.

Der der Erfindung zugrunde liegende Leitgedanke beruht auf der Erkenntnis, daß die Temperaturabhängigkeit der Eigendiffusion des diffusionsfördernden Zusatzes (beispielsweise Platin) ins Innere des Trägermetalls (beispielsweise Molybdän) wesentlich steiler verläuft als die Temperaturabhängigkeit des entsprechenden Abdampfmaßes. Mit anderen Worten, der von der beschichteten Oberfläche in den Kathodenkörper hineindiffundierende prozentuale Anteil des Zusatzes ist umso höher, je höher die Betriebstemperatur ist. Es geht also gerade bei den zur Erlangung einer hohen Emissionsstromdichte geforderten höheren Betriebstemperaturen ein unverhältnismäßig großer Prozentsatz an diffusionsförderndem Zusatz für die Kathodenoberfläche verloren. Andererseits hat sich gezeigt, daß die Konzentration dieses, die Diffusion der Aktivierungssubstanz im Gefüge des Trägermetalls fördernden Zusatzes wesentlich kleiner gewählt werden darf, als bisher angenommen wurde. Die Lebensdauer der Kathode kann daher durch Einschaltung einer zusätzlichen, die Eigendiffusion des Zusatzes nach dem Inneren des Körpers hemmenden Schicht wesentlich erhöht werden.The guiding principle underlying the invention is based on the knowledge that the temperature dependence of the self-diffusion of the diffusion-promoting additive (for example platinum) into the interior of the carrier metal (for example molybdenum) is substantially steeper than the temperature dependence of the corresponding evaporation measure. In other words, the higher the operating temperature, the higher the percentage of the additive diffusing from the coated surface into the cathode body. It is precisely at the higher operating temperatures required to achieve a high emission current density that a disproportionately large percentage of diffusion-promoting additive is lost for the cathode surface. On the other hand, it has been shown that the concentration of this additive, which promotes the diffusion of the activating substance in the structure of the carrier metal, can be chosen to be significantly lower than was previously assumed. The service life of the cathode can therefore be significantly increased by switching on an additional layer which inhibits the self-diffusion of the additive to the interior of the body.

An eine derartige Sperrschicht wird die Bedingung gestellt, daß sie im übrigen die chemisch-thermodynamischen Gleichgewichte sowie die entsprechende Reaktionskinetik der beteiligten Stoffe nicht in ungünstiger Weise beeinflußt. Insbesondere darf sie die Diffusion der Aktivierungssubstanz an die Kathodenoberfläche nicht beeinträchtigen. Die Sperrschicht darf ferner mit dem diffusionsfördernden Zusatz keine derartige Legierung eingehen, welche die Aktivierungssubstanz chemisch-physikalisch verändern und deren Diffusion an die Kathodenoberfläche herabsetzen könnte.Such a barrier layer is subject to the condition that it does not adversely affect the chemical-thermodynamic equilibria and the corresponding reaction kinetics of the substances involved. In particular, it allows diffusion of the activation substance on the cathode surface. Furthermore, the barrier layer must not form an alloy with the diffusion-promoting additive which could chemically and physically change the activating substance and reduce its diffusion to the cathode surface.

Den oben genannten Bedingungen genügen vor allem Elemente, deren Korngrenzendiffusion im Innern des Kathodenkörpers kleiner ist als diejenige des gleichzeitig zur Anwendung kommenden diffusionsfördernden Zusatzes für die jeweilige Aktivierungssubstanz. Darüber hinaus werden sich diejenigen Elemente in vorteilhafter Weise auswirken, deren Dampfdruck bei der Betriebstemperatur niedriger ist als der Dampfdruck des gleichzeitig verwendeten diffusionsfördernden Zusatzes. Besteht letzterer beispielsweise aus Platin oder einer hauptsächlich dieses Metall enthaltenden Legierung, so können für die Sperrschicht vor allem die Elemente Hafnium, Rhenium, Ruthenium, Osmium oder Iridium sowie Legierungen dieser Elemente wie auch eine Palladium/Rhenium-Legierung Verwendung finden. Es kann ganz allgemein gesagt werden daß, falls der diffusionsfördernde Zusatz aus einem Platinmetall besteht, für die Sperrschicht ein anderes Platinmetall oder ein sonstwie geeignetes Metall mit niedrigerem Dampfdruck genommen werden kann.The above-mentioned conditions are particularly sufficient for elements whose grain boundary diffusion inside the cathode body is smaller than that of the diffusion-promoting additive used at the same time for the respective activation substance. In addition, those elements will have an advantageous effect whose vapor pressure at the operating temperature is lower than the vapor pressure of the diffusion-promoting additive used at the same time. If the latter consists for example of platinum or an alloy mainly containing this metal, the elements hafnium, rhenium, ruthenium, osmium or iridium as well as alloys of these elements as well as a palladium / rhenium alloy can be used for the barrier layer. It can generally be said that if the diffusion-promoting additive consists of a platinum metal, another platinum metal or another suitable metal with a lower vapor pressure can be used for the barrier layer.

Weitere Einzelheiten der Erfindung ergeben sich aus den durch Figuren näher erläuterten Ausführungsbeispielen.Further details of the invention result from the exemplary embodiments explained in more detail by the figures.

Dabei zeigt

  • Fig. 1 den Querschnitt durch ein als Kathode wirkendes Sinterplättchen,
  • Fig. 2 den Querschnitt durch einen Kathodendraht mit schichtweisem Aufbau der Aktivierungssubstanz,
  • Fig. 3 den Querschnitt durch einen Kathodendraht mit diffusionsförderndem Zusatz als Zwischenschicht,
  • Fig. 4 den Querschnitt durch einen Kathodendraht mit diffusionsförderndem Zusatz als Kern,
  • Fig. 5 den Querschnitt durch einen Kathodendraht mit feinverteiltem diffusionsförderndem Zusatz in der Kernzone,
  • Fig. ein Diagramm der Lebensdauer in Funktion der Dicke des diffusionsfördernden Zusatzes für einen rundgehämmerten Kathodendraht mit und ohne Sperrschicht.
It shows
  • 1 shows the cross section through a sintered plate acting as a cathode,
  • 2 shows the cross section through a cathode wire with a layered structure of the activation substance,
  • 3 shows the cross section through a cathode wire with a diffusion-promoting additive as an intermediate layer,
  • 4 shows the cross section through a cathode wire with a diffusion-promoting additive as the core,
  • 5 shows the cross section through a cathode wire with a finely divided diffusion-promoting additive in the core zone,
  • Fig. A diagram of the service life as a function of the thickness of the diffusion-promoting additive for a round hammered cathode wire with and without a barrier layer.

Ausführungsbeispiel 1Embodiment 1 Siehe Fig. 1See Figure 1

98% Molybdänpulver wurden mit 2% Lanthanoxydpulver gemischt, gepreßt und gesintert. Vom wärmebehandelten Sinterkörper wurde ein Plättchen abgetrennt, seine Oberfläche poliert und auf elektrolytischem Wege mit Rhenium beschichtet. Die Dicke der Sperrschicht betrug durchschnittlich 5 u. und kann in den Grenzen von 1 bis 20 µ variieren. Hierauf wurde das Plättchen mit einer Platinschicht von 10 µ Dicke versehen, die ebenfalls elektrolytisch aufgebracht wurde. Diese Oberflächenschicht kann vorteilhafterweise 1 bis 50 u. dick sein. Das Sinterplättchen wurde hierauf in einem Benzol-Wasserstoffgemisch karburiert.98% molybdenum powder was mixed with 2% lanthanum oxide powder, pressed and sintered. A plate was separated from the heat-treated sintered body, its surface polished and electrolytically coated with rhenium. The thickness of the barrier layer averaged 5µ. and can vary between 1 and 20 µ. The platelet was then provided with a platinum layer 10 μm thick, which was also applied electrolytically. This surface layer can advantageously 1 to 50 u. be fat. The sintered plate was then carburized in a benzene-hydrogen mixture.

Den Aufbau der Kathode zeigt Fig. 1 im Schnitt. 1 stellt den aus Trägermetall (z. B. Molybdän) und Aktivierungssubstanz bestehenden Sinterkörper dar, welcher in einer Fassung 4 aus einem Hochtemperatur-Werkstoff (z. B. ebenfalls Molybdän) gehalten und von einem Heizwendel 5 aus mittelbar erwärmt wird. Die Sperrschicht 3 aus diffusionshemmendem Stoff (beispielsweise Rhenium) befindet sich auf der wirksamen Seite der Kathode, direkt unterhalb der Oberflächenschicht 2 aus Platinmetall.1 shows the structure of the cathode in section. 1 represents the sintered body consisting of carrier metal (e.g. molybdenum) and activation substance, which is held in a holder 4 made of a high-temperature material (e.g. also molybdenum) and is indirectly heated by a heating coil 5. The barrier layer 3 made of diffusion-inhibiting material (for example rhenium) is located on the effective side of the cathode, directly below the surface layer 2 made of platinum metal.

Die Kathode kann auch direkt beheizt ausgeführt werden, wodurch der Heizwendel 5 und die Fassung 4 in Wegfall kommen bzw. durch andere Bauelemente ersetzt sind. Dabei kann der Sinterkörper auch eine andere als die in der Figur skizzierte Form aufweisen.The cathode can also be designed to be directly heated, as a result of which the heating coil 5 and the holder 4 are eliminated or are replaced by other components. The sintered body can also have a shape other than that outlined in the figure.

Ausführungsbeispiel 2Embodiment 2 Siehe Fig. 2See Figure 2

99% Molybdänpulver und 1% Lanthanoxydpulver wurden zur Herstellung einer Hülle gemischt, gepreßt und gesintert und anschließend mechanisch zu einem Hohlzylinder verarbeitet. Zur Herstellung eines Kerns wurden 97% Molybdänpulver mit 3% Lanthanoxydpulver nach dem gleichen Verfahren zu einem Stab verarbeitet. Letzterer wurde mit der Hülle zusammengefügt und durch Rundhämmern auf einen kleineren Durchmesser reduziert. Hierauf wurde der auf diese Weise hergestellte Draht von 3 mm Durchmesser in einem Benzol-Wasserstoffgemisch karburiert, zunächst mit einer galvanisch abgeschiedenen Rheniumschicht von 5 µ Dicke und zum Schluß mit einer 10 µ dicken Platinschicht versehen.99% molybdenum powder and 1% lanthanum oxide powder were mixed to form a shell, pressed and sintered and then mechanically processed to form a hollow cylinder. To make a core, 97% molybdenum powder with 3% lanthanum oxide powder was made into a rod by the same method. The latter was put together with the casing and reduced to a smaller diameter by round hammers. The wire of 3 mm diameter produced in this way was then carburized in a benzene-hydrogen mixture, first provided with an electrodeposited rhenium layer 5 μm thick and finally with a 10 μm thick platinum layer.

Fig. zeigt den Querschnitt durch einen derartigen Kathodendraht mit schichtweisem Aufbau der Aktivierungssubstanz. Die Kernzone 6 des Trägermetalls (z. B. Molybdän) ist mit einem verhältnismäßig hohen Gehalt an Aktivierungssubstanz (z. B. Lanthanoxyd) dotiert. An die Kernzone 6 schließt sich die Randschicht mit niedrigem Gehalt an Aktivierungssubstanz an. Darauf folgt die Sperrschicht 3 aus diffusionshemmendem Stoff (z. B. Rhenium) und zu äußerst die Oberflächenschicht 2 aus Platinmetall (im vorliegenden Fall Platin).Fig. Shows the cross section through such a cathode wire with a layered structure of the activation substance. The core zone 6 of the carrier metal (e.g. molybdenum) is doped with a relatively high content of activating substance (e.g. lanthanum oxide). The boundary layer with a low content of activating substance adjoins the core zone 6. This is followed by the barrier layer 3 made of diffusion-inhibiting material (e.g. rhenium) and, to the extreme, the surface layer 2 made of platinum metal (platinum in the present case).

Ausführungsbeispiel 3Embodiment 3 Siehe Fig. 3See Figure 3

Nach dem unter Beispiel 2 angegebenen Verfahren wurde je ein Kern und eine Hülle hergestellt, wobei beide die gleiche Konzentration an Aktivierungssubstanz Lanthanoxyd von 3% aufwiesen. Vor dem Zusammenfügen wurde der Kern mit einer elektrolytisch abgeschiedenen Rheniumschicht von 20 µ Dicke und einer Platinschicht von 100 µ Dicke versehen. Die Schichtdicken können zwischen 5 µ und 50 µ bzw. 20 µ und 200 µ variieren. Hierauf wurde der Körper zunächst durch Rundhämmern auf einen Durchmesser von 3 mm und dann durch Ziehen auf einen solchen von 1 mm heruntergebracht und anschließend karburiert. Der Draht wurde zum Schluß mit je einer galvanisch aufgebrachten Rhenium- und Platinschicht von 3 µ bzw. 15 µ versehen.According to the procedure given in Example 2, a core and a shell were created prepared, both of which had the same concentration of activating substance lanthanum oxide of 3%. Before the assembly, the core was provided with an electrolytically deposited rhenium layer of 20 μ thickness and a platinum layer of 100 μ thickness. The layer thicknesses can vary between 5 µ and 50 µ or 20 µ and 200 µ. The body was first brought down to a diameter of 3 mm by round hammering and then pulled down to a diameter of 1 mm and then carburized. Finally, the wire was provided with an electroplated rhenium and platinum layer of 3 µ and 15 µ, respectively.

Der fertige Drahtquerschnitt ist aus Fig. 3 ersichtlich. 6 stellt die Kernzone mit hohem Gehalt an Aktivierungssubstanz und 8 die entsprechende, ebenfalls hoch dotierte Randschicht dar. Die Sperrschicht 10 aus diffusionshemmendem Stoff (beispielsweise Rhenium) befindet sich unmittelbar unter der Zwischenschicht 9 aus Platinmetall. Der Aufbau der Oberflächenschichten 2 und 3 entspricht demjenigen der Fig. 2. Die Oberflächenschicht 2 aus Platinmetall kann gegebenenfalls im Anlieferungszustand auch fehlen, wobei zunächst die Sperrschicht 3 die Rolle des diffusionsfördernden Zusatzes in der Randzone übernimmt, bis genügend Platinmetall aus dem Innern an die Oberfläche diffundiert ist. Dank zahlreicher Legierungsbildungsmöglichkeiten zwischen 2 und 3 sind ohnehin weitere Werkstoffkombinationen mit unterschiedlich abgestuften Funktionen ausführbar.The finished wire cross section can be seen in FIG. 3. 6 represents the core zone with a high content of activation substance and 8 the corresponding, also highly doped edge layer. The barrier layer 10 made of diffusion-inhibiting material (for example rhenium) is located directly under the intermediate layer 9 made of platinum metal. The structure of the surface layers 2 and 3 corresponds to that of FIG. 2. The surface layer 2 made of platinum metal may also be missing in the delivery state, the barrier layer 3 initially taking on the role of the diffusion-promoting additive in the edge zone until sufficient platinum metal from the inside to the surface is diffused. Thanks to numerous alloy formation options between 2 and 3, further material combinations with differently graduated functions can be carried out anyway.

Ausführungsbeispiel4Embodiment4 Siehe Fig. 4See Figure 4

96% Molybdänpulver und 4% Lanthanoxydpulver wurden zur Herstellung eines Mantels gemischt, gepreßt, gesintert und anschließend mechanisch zu einem zylindrischen Stab mit einer Bohrung verarbeitet. In die Bohrung wurde ein Platindraht stramm passend eingeschoben und das Ganze durch Rundhämmern auf einen Durchmesser von 4 mm reduziert. Der auf diese Weise gefertigte Draht wurde nach dem unter Beispiel 2 angegebenen Verfahren mit einer Rheniumschicht von 8 µ Dicke und einer Platinschicht von 20 µ Dicke versehen und anschließend karburiert.96% molybdenum powder and 4% lanthanum oxide powder were mixed, pressed, sintered and then mechanically processed into a cylindrical rod with a bore to produce a jacket. A platinum wire was inserted tightly into the hole and the whole thing was reduced to a diameter of 4 mm by round hammering. The wire produced in this way was provided with a rhenium layer of 8 .mu.m thick and a platinum layer of 20 .mu.m thick according to the method given in Example 2 and then carburized.

In Fig. 4 ist der Querschnitt des Kathodendrahtes schematisch dargestellt. f1 stellt den Kern aus diffusionsförderndem Zusatz (Platinmetall, im vorliegenden Fall Platin) dar, während mit 12 der mit hohem Gehalt an Aktivierungssubstanz dotierte Mantel aus Trägermetall (hier Molybdän) bezeichnet ist. Die Oberflächenschicht 2 aus Platinmetall (Platin) liegt auf der Sperrschicht 3 aus diffusionshemmendem Stoff (Rhenium) auf.4, the cross section of the cathode wire is shown schematically. f1 represents the core of diffusion-promoting additive (platinum metal, in the present case platinum), while 12 denotes the jacket made of carrier metal (here molybdenum) doped with a high content of activation substance. The surface layer 2 made of platinum metal (platinum) lies on the barrier layer 3 made of diffusion-inhibiting material (rhenium).

Ausführungsbeispiel 5Embodiment 5 Siehe Fig. 5See Fig. 5

Nach dem unter Beispiel 2 angegebenen Verfahren wurde je ein Kern und eine Hülle hergestellt, wobei jedoch den pulverförmigen Ausgangsmaterialien für den Kern zusätzlich noch 0,5% Platinmoor zugegeben wurde. Die Weiterverarbeitung erfolgte analog den in Beispiel 2 erwähnten Verfahrensschritten. Der mit einer 4 µ dicken Rhenium- und einer 25 µ dicken Platinschicht versehene rundgehämmerte Draht wies einen Durchmesser von 3,5 mm auf.A core and a shell were produced in each case according to the process given in Example 2, but an additional 0.5% of platinum peat was added to the powdery starting materials for the core. The further processing was carried out analogously to the process steps mentioned in Example 2. The round hammered wire provided with a 4 µ thick rhenium and a 25 µ thick platinum layer had a diameter of 3.5 mm.

Fig. 5 zeigt den schichtweisen Aufbau eines derartigen Kathodendrahtes. Die Kernzone 13 des Trägermetalls (z. B. Molybdän) weist einen verhältnismäßig hohen Gehalt an Aktivierungssubstanz (Lanthanoxyd) auf und ist überdies mit feinverteiltem Platin als diffusionsförderndem Zusatz dotiert. Die Randschicht 7 mit niedrigem Gehalt an Aktivierungssubstanz trägt ihrerseits die der Fig. 2 entsprechenden Oberflächenschichten 2 und 3.5 shows the layered structure of such a cathode wire. The core zone 13 of the carrier metal (e.g. molybdenum) has a relatively high content of activating substance (lanthanum oxide) and is also doped with finely divided platinum as a diffusion-promoting additive. The edge layer 7 with a low content of activation substance in turn bears the surface layers 2 and 3 corresponding to FIG. 2.

In Fig. 6 ist ein Diagramm dargestellt, welches die Lebensdauer in Funktion der Schichtdicke des diffusionsfördernden Zusatzes für einen rundgehämmerten Kathodendraht gemäß Fig. 2 dargestellt. Die Einflüsse der Sperrschicht sowie der Betriebstemperatur sind durch verschiedene Linienzüge gekennzeichnet. Die Kurven »a« und »b« beziehen sich auf eine Betriebstemperatur von 2050 K, die Kurven »c« und »d« auf eine solche von 1880° K. Auf der Abszisse ist die Schichtdicke des diffusionsfördernden Zusatzes (Platin) in µ im natürlichen, auf der Ordinate die Lebensdauer der Kathode in h im logarithmischen Maßstab aufgetragen. »a« und »c« sind die Vergleichskurven für Glühkathoden ohne Sperrschicht, auf der Basis von Molybdän als Trägermetall, Molybdänkarbid als Reduktionsmittel und Platin als diffusionsförderndem Zusatz. »b« und »d« stellen die entsprechenden Kurven für Glühkathoden mit einer zusätzlichen Rhenium-Sperrschicht von 5 µ Dicke dar. Der Betriebsdruck der Elektronenröhre betrug 5 . 10-4 Torr. Aus dem Kurvenvergleich geht hervor, daß die Lebensdauer durch die Rheniumbeschichtung durchschnittlich auf den 2,5fachen Wert des ursprünglichen erhöht werden konnte.FIG. 6 shows a diagram which shows the service life as a function of the layer thickness of the diffusion-promoting additive for a round hammered cathode wire according to FIG. 2. The influences of the barrier layer and the operating temperature are characterized by different lines. The curves "a" and "b" refer to an operating temperature of 2050 K, the curves "c" and "d" to one of 1880 ° K. On the abscissa, the layer thickness of the diffusion-promoting additive (platinum) is in µ im natural, on the ordinate the life of the cathode is plotted in h on a logarithmic scale. "A" and "c" are the comparison curves for hot cathodes without a barrier layer, based on molybdenum as a carrier metal, molybdenum carbide as a reducing agent and platinum as a diffusion-promoting additive. "B" and "d" represent the corresponding curves for hot cathodes with an additional rhenium barrier layer of 5 µ thickness. The operating pressure of the electron tube was 5. 10- 4 torr. The comparison of the curves shows that the lifespan of the rhenium coating could be increased on average to 2.5 times the original value.

Die Erfindung ist nicht auf die in den vorgenannten Beispielen erläuterten Ausführungsformen beschränkt. Sie läßt sich in vorteilhafter Weise auch auf andere als die oben beschriebenen Werkstoffkombinationen anwenden. Außer dem System Mo/Mo2C/La2O3/Pt kommen vor allem auch die Systeme W/W2C/ Th02/Ru und Ta/Ta2C/Y2O3/Pd in Betracht. Die vorgeschlagene Sperrschicht läßt sich ganz allgemein auf alle Werkstoffkombinationen von mit einem diffusionsfördernden Zusatz dotierten Reaktionskathoden des Diffusions-Nachlieferungstyps anwenden, um eine unliebsame Eigendiffusion des Zusatzes in eine unerwünschte Richtung zu verringern oder gänzlich zu unterbinden.The invention is not restricted to the embodiments explained in the aforementioned examples. It can also be advantageously applied to combinations of materials other than those described above. In addition to the Mo / Mo 2 C / La 2 O 3 / Pt system, the W / W 2 C / Th0 2 / Ru and Ta / Ta 2 C / Y 2 O 3 / Pd systems are also considered. The proposed barrier layer can be applied in general to all material combinations of reaction cathodes of the diffusion delivery type doped with a diffusion-promoting additive, in order to reduce or completely prevent an undesirable self-diffusion of the additive in an undesired direction prevent.

Durch die erfindungsgemäßen neuen Glühkathoden wurden Bauelemente für Elektronenröhren geschaffen, die bei hoher Emissionsstromdichte beste Ausnutzung der verfügbaren Materialien ermöglichen und eine lange Lebensdauer gewährleisten. Die Sperrschicht gestattet, die chemisch-physikalischen Vorgänge weitgehend in die gewünschte Richtung zu dirigieren und unvorteilhafte Nebenerscheinungen im Betrieb wirksam einzuschränken.The new hot cathodes according to the invention have created components for electron tubes which, with a high emission current density, enable the best use of the available materials and ensure a long service life. The barrier layer allows the chemical-physical processes to be largely directed in the desired direction and to effectively limit unfavorable side effects during operation.

Claims (9)

1. Hot cathode based on a support metal having a high melting point, an activating substance which is present in the form of the oxide of a metal of group lllb, a reducing agent which is present in the form of the carbide of the support metal, and an additive which promotes the diffusion of the activating substance to the active surface, characterised in that, in order to decrease the self-diffusion of the said diffusion-promoting additive away from the active surface of the cathode into the interior, at least one additional layer (3, 10), of a material which inhibits the diffusion of the said additive is provided on that side of the layer (2) containing the diffusion-promoting additive, or of the layer formed by the said additive, which faces away from the active surface.
2. Hot cathode according to Claim 1, characterised in that the diffusion-inhibiting material has a lower vapour pressure, at the operating temperature of the cathode, than the diffusion-promot- ingadditive.
3. Hot cathode according to Claims 1 and 2, characterised in that the diffusion-promoting additive consists of a platinum metal, or of an alloy of at least two platinum metals, and that the diffusion-inhibiting material is composed of another platinum metal, or of hafnium or rhenium, or of an alloy of at least two of the abovementioned elements.
4. Hotcathode according to Claim 3, characterised- in that the diffusion-inhibiting material consists of at least one of the elements hafnium, rhenium, ruthenium, osmium or iridium.
5. Hot cathode according to Claim 3, characterised in that the diffusion-promoting additive- consists of platinum, and the diffusion-inhibiting material consists of rhenium.
6. Hot cathode according to Claim 1, characterised in that the support metal is molybdenum and the reducing agent is molybdenum carbide, that the activating substance consists of lanthanum oxide, the diffusion-promoting additive consists of platinum or of an alloy of platinum, and the diffusion-inhibiting material consists of rhenium.
7. Hot cathode according to Claim 1, characterised in that the layer of the diffusion-promoting additive has a thickness of 1 to 50 µ, and the layer of the diffusion-inhibiting material has a thickness of 1 to 20 µ.
8. Hot cathode according to Claims 1 and 7, characterised in that it is present in the form of a sintered body, the content of activating substance being distributed essentially uniformly across the cross-section of the support metal and the active surface being provided with a layer of a diffusion-inhibiting material, over which layer a layer of the diffusion-promoting additive is present.
9. Hot cathode according to Claims 1 and 7, characterised in that it is present in the form of a wire, a strip, or a plate, that the cross-section possesses a layered structure with respect to the concentration and/or the arrangement of the activating substance included within the support metal, and that the diffusion-promoting additive forms the surface layer and/or an intermediate layer, below which, in each case, a layer of the diffusioninhibiting material is present.
EP79200056A 1978-05-05 1979-02-02 Hot cathode Expired EP0005279B1 (en)

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DE2822665A1 (en) * 1978-05-05 1979-11-08 Bbc Brown Boveri & Cie GLOW CATHODE MATERIAL
FR2445605A1 (en) * 1978-12-27 1980-07-25 Thomson Csf DIRECT HEATING CATHODE AND HIGH FREQUENCY ELECTRONIC TUBE COMPRISING SUCH A CATHODE
GB2050045A (en) * 1979-05-29 1980-12-31 Emi Varian Ltd Thermionic cathode
EP0143222B1 (en) * 1983-09-30 1987-11-11 BBC Aktiengesellschaft Brown, Boveri & Cie. Thermionic cathode capable of high emission for an electron tube, and method of manufacture
KR950012511A (en) * 1993-10-05 1995-05-16 이헌조 Impregnated Cathode for Cathode Ray Tubes
ATE167755T1 (en) * 1993-10-28 1998-07-15 Philips Electronics Nv STORAGE CATHODE AND PRODUCTION PROCESS
JPH0850849A (en) * 1994-05-31 1996-02-20 Nec Kansai Ltd Cathode member and electronic tube using it
DE4421810C2 (en) * 1994-06-22 1996-08-29 Siemens Ag Thermionic electron emitter and method for its production
US6815876B1 (en) * 1999-06-23 2004-11-09 Agere Systems Inc. Cathode with improved work function and method for making the same
KR20020063396A (en) * 2001-01-29 2002-08-03 삼성에스디아이 주식회사 Metal cathode for electron tube
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FR1381553A (en) * 1962-10-31 1964-12-14 Thomson Houston Comp Francaise Improvements to electronic tubes
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CH629033A5 (en) 1982-03-31
DE2822614A1 (en) 1979-11-08
EP0005279A2 (en) 1979-11-14
JPS54146951A (en) 1979-11-16
AT367567B (en) 1982-07-12
ATA82879A (en) 1981-11-15
DE2960432D1 (en) 1981-10-08
US4274030A (en) 1981-06-16

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