EP0560436A1 - Cathode with solid element - Google Patents

Abstract

The invention relates to a cathode having a solid element (4) which contains metallic components and oxidic components. High emission flow densities with a long life are achieved, even at low operating temperatures, in that the structure of the components and the volume ratio vm of the metallic components relative to the total volume of the solid element is selected in such a manner that the resistivity rho has a value in the range rho o.10<-4>> rho > rho m.10<2>, rho 0 and rho m being the resistivities, defined at 20 DEG C of the pure oxidic components and of the pure metallic components respectively. <IMAGE>

Description

The invention relates to a cathode with a solid element, which contains metallic components and oxidic components.

Replenishment cathodes consist of a porous metal matrix with more than 70% metal volume, which ensures good electrical conductivity, as well as an oxide component such as Alkaline earth oxides BaO or CaO or 4BaO.CaO.Al₂O₃, which is located in the pores of the metal matrix or in a storage area. When such a cathode is operated at 900 to 1000 ° C., atomic films consisting of the metal (s) (Ba) and atomic oxygen (O) contained in the oxide form on the metal cathode surface (W) and ensure a low work function. Known cathodes of this type are the I cathode (cf. EP-A 0333 369) and the scandate cathode (cf. EP-A 0442 163). Such cathodes have the features mentioned above.

At operating temperatures between 900 ° C and 1000 ° C, saturation current densities between 10 and 150A / cm² are achieved. Such cathodes require relatively high heating temperatures, which lead to limitation of the service life due to the destruction of the W heating coil.

Oxide cathodes (cf. EP-A 0395 157) consist of a relatively thick porous oxide layer made of alkaline earth oxides (for example BaO.SrO) and other oxide dopants (for example Sc₂O₃, Eu₂O₃) on a metal support such as nickel. They allow significantly lower operating temperatures of approx. 730 to 850 ° C with emission current densities of 10 to 50A / cm², but only in the µsec range. Due to the 50A / cm², but only in the µsec range. Due to the low electrical conductivity of the oxide components, the permanent load capacity is limited to 1-3 A / cm².

The invention has for its object to design a solid element of the type mentioned in such a way that high emission current densities result in a long service life even at low operating temperatures.

hat, wobei ρ_O bzw. ρ_m die bei 20°C bestimmten spezifischen Widerstände der reinen oxidischen Bestandteile bzw. der reinen metallischen Bestandteile sind.The solution is achieved in that the structure of the components and the volume ratio v _{m of} the metallic components relative to the total volume of the solid element are chosen such that the specific resistance ρ is a value in the range ${\text{ρ}}_{\text{O}}{\text{· 10⁻⁴>ρ> ρ}}_{\text{m}}\text{· 10²}$

has, where ρ _O or ρ _{m are} the specific resistances of the pure oxidic components or the pure metallic components determined at 20 ° C.

The term "percolation" is used in connection with the behavior of granular metals in "Adv. Physics 24 (1975), page 424 ff.

The specific electrical resistance ρ of a solid-state element according to the invention has a value in the range of the so-called percolation threshold. Cathodes with solid-state elements according to the invention can therefore be referred to as percolation cathodes.

und d³logρ/dV_m³<0 mathematisch eingegrenzt werden. In diesem Bereich liegt der spezifische Widerstand ρ zwischen ρ_O10⁻⁴ und ρ_m10², vorzugsweise im Bereich zwischen 10³ Ωcm und 10⁻³ Ωcm. Zur näheren Erläuterung des erfindungsgemäß vorzusehenden Bereichs wird auf Fig. 2 Bezug genommen. Dort ist der spezifische elektrische Widerstand ρ (gemessen bei Raumtemperatur) eines aus BaO- und W-Partikeln der mittleren Größe 30 nm zusammengesetzten Festkörpers in logarithmischem Maßstab in Abhängigkeit des prozentualen Metallvolumenanteils v_m dargestellt. Im Bereich v_m = 0 ergibt sich der hohe spezifische Widerstand ρ_o eines BaO-Festkörpers, im Bereich v_m = 100% der spezifische Widerstand ρ_m von Wolfram. Im Bereich 0<v_m<v_ma wird ein oxidisches, im Bereich v_mb<v_m 100% ein metallisches Leitwertverhalten festgestellt. Ein Mischverhalten ergibt sich im Bereich v_ma<v_m<V_mb der Perkolationsschwelle. Im steilen Kennlinienbereich p zwischen den Grenzwerten v_ma und v_mb ist die relative Volumenzusammensetzung eines erfindungsgemäßen Festkörperelements gewählt, wobei Volumenanteile im schraffierten Bereich für Kathoden besonders günstig sind. Für diesen schraffierten Bereich gilt in etwa als zusätzliche Bedingung, daß d⁴logρ/d V_m⁴ positiv ist. Die Grenzwerte v_ma und v_mb können den Bereich von v_m = 20% bis v_m = 80% einschließen. Die Steilheit der Kennlinie P hängt in starkem Maße von der Struktur des erfindungsgemäßen Festkörperelements ab, nämlich von der Größe der metallischen und/oder oxidischen Partikel sowie von der Homogenität ihrer Verteilung. Eine vorteilhafte Ausführung ist dadurch gekennzeichnet, daß der oxidische Volumenanteil größer als der metallische ist. Partikel im Sinne der vorliegenden Erfindung sind Teilchen, die separat gebildet (Laserablation, Sputtern von einem Target) und zu einem Festkörperelement verbunden wurden, oder auch Körner, die auf einem Substrat durch chemischen Niederschlag aus der Dampfphase (CVD) gebildet wurden. Ferner können zwischen CVD-Körnern auch separat gebildete weitere Teilchen eingemischt werden (vgl. EP-A 0442 163), so daß beispielsweise dem Substrat über einen Gasstrom zugeführte BaO-Partikel in eine per CVD auf dem Substrat gebildete Wolfram-Matrix eingelagert werden.In the area of the percolation threshold of a material composition formed from metallic and oxidic fine particles, the metallic conductivity changes to the oxide conductivity. Depending on the percentage volume of the metal (v _m ) of the solid element, the specific resistance ρ changes in the range between v _m = 0 to v _m = 1 with a typical S-shaped course, the range of the percolation threshold being defined by the steep range of characteristic curves with average values of v _m . This area can also pass through ${\text{d²logρ / <figure-callout id="2" label="dV m" filenames="imgf0001.png" state="{{state}}">dV </figure-callout>}}_{\text{m}}\text{² = 0}$

and d³logρ / dV _m ³ <0 can be mathematically limited. In this range the specific resistance ρ lies between ρ _O 10⁻⁴ and ρ _m 10², preferably in the range between 10³ Ωcm and 10⁻³ Ωcm. For a more detailed explanation of the area to be provided according to the invention, reference is made to FIG. 2. There, the specific electrical resistance ρ (measured at room temperature) of a solid composed of BaO and W particles of average size 30 nm is represented on a logarithmic scale as a function of the percentage metal volume fraction v _m . The high specific resistance ρ _{o of} a BaO solid results in the range v _m = 0, and the specific resistance ρ _m of tungsten in the range v _m = 100%. In the range 0 <v _m <v _ma an oxidic conductance behavior and in the range v _mb <v _m 100% a metallic conductance behavior is determined. A mixing behavior results in the range v _ma <v _m <V _{mb of} the percolation threshold. In the steep characteristic curve range p between the limit values v _ma and v _mb , the relative volume composition of a solid-state element according to the invention is selected, with volume fractions in the hatched area being particularly favorable for cathodes. For this shaded area, the additional condition is that d⁴logρ / d V _m ⁴ is positive. The limit values v _ma and v _mb can include the range from v _m = 20% to v _m = 80%. The steepness of the characteristic curve P depends to a large extent on the structure of the solid element according to the invention, namely on the size of the metallic and / or oxidic particles and on the homogeneity of their distribution. An advantageous embodiment is characterized in that the oxidic volume fraction is larger than the metallic one. Particles in the sense of the present invention are particles which are formed separately (laser ablation, sputtering of a target) and to a solid element, or grains, which were formed on a substrate by chemical precipitation from the vapor phase (CVD). Furthermore, separately formed further particles can be mixed in between CVD grains (cf. EP-A 0442 163), so that, for example, BaO particles supplied to the substrate via a gas stream are embedded in a tungsten matrix formed on the substrate by CVD.

Solid-state elements according to the invention consist of fine and homogeneously mixed structures of individual chemically different types of solid-state elements, a spatial network of metallic particles being nested in a spatial network of oxidic components or vice versa and possibly including tunnel current paths. Furthermore, both the oxidic and the metallic constituents can also be present as particles or grains.

)³ die Anzahl der Partikel um weniger als +-20% von dem entsprechenden Volumenanteil im gesamten Festkörperelement unterscheidet, wobei $\overline{\text{d}}$

der mittlere Durchmesser der Partikel ist. Große lokale Agglomarationen von Partikeln sind dabei zu vermeiden.Particularly high emission current densities are achieved in that the metallic constituents or the oxidic constituents in the form of particles in the other constituent are distributed so homogeneously that volume ranges of size (20 $\overline{\text{d}}$

) ³ the number of particles differs by less than + -20% from the corresponding volume fraction in the entire solid element, whereby $\overline{\text{d}}$

is the average diameter of the particles. Large local agglomerations of particles should be avoided.

The solid-state element according to the invention is preferably characterized in that the metallic particles are arranged in such a way that - possibly via tunnel sections - tracks with metallic conductivity through the oxidic braid exist.

der Partikel kleiner als 800 nm, vorzugsweise im Bereich von 0,5 bis 100 nm und insbesondere im Bereich von 1 bis 20 nm gewählt ist.Heavy-duty cathodes were also obtained in that the average diameter $\overline{\text{d}}$

the particle is selected to be smaller than 800 nm, preferably in the range from 0.5 to 100 nm and in particular in the range from 1 to 20 nm.

With small particle dimensions, solid-state elements according to the invention can be produced particularly reliably with the desired percolation properties. The solid properties (for example electrical resistance) are sufficiently isotropic when the particles are thoroughly mixed.

der Partikel im Bereich von 0,5 bis 4 nm gewählt ist.In the case of dimensioning outside the area hatched in FIG. 2, it is advantageous that the specific electrical resistance ρ is set in the range from 10 2 to 10 12 Ωcm and that the mean diameter $\overline{\text{d}}$

the particle is selected in the range from 0.5 to 4 nm.

der Partikel monomodal verteilt sind und eine Halbwertsbreite von ≦ 50% und den Mittelwert $\overline{\text{d}}$

besitzen.The desired data can be advantageously achieved while at the same time being economically viable in that the diameters $\overline{\text{d}}$

the particles are monomodal and have a half-width of ≦ 50% and the mean $\overline{\text{d}}$

have.

₁ der Partikel des einen Bestandteils kleiner als etwa 100 nm und die mittleren Durchmesser $\overline{\text{d}}$

₂ der Körper der Partikel des anderen Bestandteils kleiner als das 10-fache des Wertes $\overline{\text{d}}$

₁ gewählt sind, und daS die Partikel beider Bestandteile derart homogen verteilt sind, daß in einem Volumenbereich der Größe (20 $\overline{\text{d}}$

₂)³ die Anzahlen der Partikel eines jeden Bestandteiles um weniger als ± 20% vom entsprechenden Volumenanteil im gesamten Festkörperelement abweichen.According to a preferred solution, it is provided that both the metallic and the oxidic constituents are in the form of particles, the mean diameter $\overline{\text{d}}$

₁ the particles of a component less than about 100 nm and the average diameter $\overline{\text{d}}$

₂ the body of the particles of the other component is less than 10 times the value $\overline{\text{d}}$

₁ are selected, and that the particles of both components are distributed so homogeneously that in a volume range of size (20 $\overline{\text{d}}$

₂) ³ the number of particles of each component deviate by less than ± 20% from the corresponding volume fraction in the entire solid element.

Then there is an overall granular solid which, particularly when the diameters of all the particles are in the range from 0.5 to 100 nm, has particularly isotropic solid properties, the properties of which can be maintained even in series production with a narrow spread.

Percolation cathodes constructed with solid-state elements according to the invention can be subjected to higher loads than oxide cathodes, lower operating temperatures being required than with subsequent delivery cathodes.

The following material combinations are particularly suitable: Oxidic content Metallic part Ba0 Ca0 Al₂O₃ Sc₂O₃ W BaO SrO W, Ni, Mg Ba0 Sr0 Sc₂O₃ W Ni ThO₂ W * re La₂O₃ Mon * Pt *: An admixture of W₂C or Mo₂ to W or Mo can be advantageous.

Solid-state elements according to the invention require only relatively low operating temperatures in the range from 730 to 850 ° C. Since neither a high-temperature impregnation step at temperatures of more than 1500 ° C. nor a longer activation at temperatures of about 1100 ° C. are necessary, the structure of a solid-state element structured according to the invention remains largely stable, even when components are used whose solid solubility in one another is not negligible is.

A solid-state element according to the invention can be heated by direct current passage. Such a solution is advantageously characterized in that the proportions and / or the particle sizes of the oxidic (negative temperature coefficient) and / or metallic (positive temperature coefficient) components are selected such that the specific resistance in the range from room temperature to operating temperature is less than 5 %, preferably 1%, changes. This has the advantage that when the solid-state element is heated directly, the heating current and voltage do not have to be readjusted, or only slightly, when heating up to a certain operating temperature.

Solid-state elements according to the invention can be produced in any known manner. For example, suitable processes are described in EP-A 0442 163 or in EP-A 0333 369.

The advantageous properties of a solid-state element according to the invention are not only achieved with a compact and 100% leakproof construction. A porosity of up to about 20% is even advantageous because it facilitates the subsequent delivery of the emitting film components to the surface. The electrical conductivity is nevertheless only insignificantly determined by electron gas conduction, but almost exclusively by the percolation structure.

Figur 1

zeigt den Prinzipaufbau einer Kathode mit einem erfindungsgemäßen Festkörperelement

Figur 2

zeigt den spezifischen Widerstand ρ in Abhängigkeit des prozentualen Volumenanteils v_m metallischer Bestandteile einer nanostrukturierten, aus metallischen und oxidischen Bestandteilen bestehenden Festkörpers.

Figur 3

zeigt den Strukturaufbau eines Volumenelements des Festkörperelements nach Figur 1.

Figur 4

zeigt einen alternativen Strukturaufbau für ein Festkörperelement nach Fig. 1.

The invention is explained in more detail with reference to the description of exemplary embodiments shown in the drawing.

Figure 1

shows the basic structure of a cathode with a solid element according to the invention

Figure 2

shows the specific resistance ρ in dependence the percentage by volume v _{m of} metallic components of a nanostructured solid consisting of metallic and oxidic components.

Figure 3

shows the structure of a volume element of the solid element according to Figure 1.

Figure 4

shows an alternative structure for a solid element according to FIG. 1.

The percolation cathode indicated in cross section in FIG. 1 consists of a tungsten heating coil 1, a molybdenum heating cap 2, a metal base 3 made of tungsten or nickel and a solid element 4 structured according to the invention, the specific electrical resistance ρ of which in the area of the percolation threshold on the characteristic branch P. Figure 2 lies.

= 3 nm.A volume element of the solid element 4 is shown greatly enlarged in cross section in FIG. One can see a relatively compact structure made up of interconnected particles with a low pore content of about 10% by volume. The metallic particles 5 (hatched) consist of tungsten (28 vol.%). The oxide particles 6 (closely hatched) consist of scandium oxide Sc₂O₃ (2 vol.%), While the oxidic particles 7 (not hatched) consist of barium oxide / strontium oxide (BaO / SrO) with a share of 60 vol.% Of the total volume. The mean diameter of the particles is 5.6 and 7 $\overline{\text{d}}$

= 3 nm.

At an operating temperature of 730 ° C and an ambient pressure of 10⁻⁸ Torr, a pulse emission (5 µsec) of 25 A / cm² was achieved. A permanent load of 10A / cm² was possible in the space charge restricted area, i.e. 4 times higher values than with oxide cathodes despite the low operating temperature.

At an operating temperature of 880 ° C, pulse emission current densities of more than 160A / cm² and permanent loads of 20A / cm² were measured. The specified values for permanent load capacity apply for a service life of more than 10⁴ hours. Similar good values were achieved with a modified pore-free structure according to FIG. 4, in which otherwise the same proportions for the constituents W, Sc²O₃ or BaO / SrO were provided as for the structure according to FIG. 3. However, here W and Sc₂O₃ are particles 8 or 9 with an average diameter of 10 nm embedded in a solid matrix 10 of BaO / SrO.

Claims (10)

Cathode with a solid element (4), which contains metallic components as well as oxidic components,
characterized in that the structure of the components and the volume ratio v _{m of} the metallic components relative to the total volume of the solid element are chosen such that the specific resistance ρ is a value in the range ${\text{ρ}}_{\text{O}}{\text{· 1O⁻⁴>ρ> ρ}}_{\text{m}}\text{· 10²}$

has, where ρ _O or ρ _{m are} the specific resistances of the pure oxidic components or the pure metallic components determined at 20 ° C. Cathode according to claim 1,
characterized in that the resistivity is between 10³ Ω cm and 10⁻³ Ω cm. Cathode according to claim 1 or 2,
characterized in that the metal content v _{m is} in the range from 20 to 80% by volume. Cathode according to one of claims 1 to 3,
characterized in that the metallic volume fraction is smaller than the oxidic, and is preferably in the range of 33-50%. Cathode according to one of claims 1 to 4,
characterized in that the metallic particles are arranged in such a way that - possibly via tunnel sections - there are tracks with metallic conductivity through the oxidic mesh. Cathode according to one of claims 1 to 5,
characterized in that the mean diameter $\overline{\text{d}}$

the particle (5 to 9) is selected to be less than 800 nm, preferably in the range from 0.5 to 100 nm and in particular in the range from 1 to 20 nm. Cathode according to one of claims 1 to 6,
characterized in that means are provided by which a heating current can be conducted through the solid element. Cathode according to claim 7,
characterized in that the proportions and / or the particle sizes of the oxidic and / or metallic constituents are selected such that the specific electrical resistance ρ changes in the range from room temperature to operating temperature by less than 5%, preferably by less than 1%. Cathode according to claim 1,
characterized in that each metallic component contains at least one representative of the group W, Ni, Mg, Re, Mo, Pt. Cathode according to claim 1 or 9,
characterized in that each oxidic component contains at least one representative of the group BaO, CaO, Al₂O, Sc₂O₃, ThO, La₂O₃.

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