ELECTRON-EMITTING CERAMIC

Abstract

Embodiments are directed to the field of ceramics and relate to electron-emitting ceramics such as those which can be used as cathode material for electron emissions in space flight systems, for example. Embodiments specify an electron-emitting ceramic which has an improved temperature conductivity with a simultaneously continuous electron emission. The electron-emitting ceramic contains at least>70 vol. % C12A7 electride and a proportion of Zr, Hf, V, Nb, Ta, Cr, Mo, W, Mn, Re, Fe, Ru, Os, Ni, Pd, Pt, Cu, Ag, Au, Zn, Cd, In, Sn, Sb, Te, Tl, Pb, or Bi as metal and/or with Ti, wherein the proportion of the metals lies between>0 and<30 vol. %, and the ceramic has a density of at least 85% of the theoretical density of the ceramic and the ceramic contains 0 to maximally 10 vol. % production-specific impurities.

Claims

1. An electron-emitting ceramic which contains at least>70 vol. % C12A7 electride and a proportion of Zr, Hf, V, Nb, Ta, Cr, Mo, W, Mn, Re, Fe, Ru, Os, Ni, Pd, Pt, Cu, Ag, Au, Zn, Cd, In, Sn, Sb, Te, Tl, Pb, or Bi as metal individually or as a mixture or compound or alloy of said metals with one another and/or with Ti, wherein the proportion of the metals lies between>0 and<30 vol. %, and the ceramic has a density of at least 85% of the theoretical density of the ceramic and the ceramic contains 0 to maximally 10 vol. % production-specific impurities, dopants, auxiliary materials, and/or additives.

2. The electron-emitting ceramic according to claim 1 in which 70 to 90 vol. %, advantageously 75 to 90 vol. %, C12A7 electride is present in the ceramic.

3. The electron-emitting ceramic according to claim 1 in which 5 to<30 vol. %, advantageously 5 to 20 vol. %, more advantageously 10 to 15 vol. %, of metals are present.

4. The electron-emitting ceramic according to claim 1 in which a percolation network of the metals is formed.

5. The electron-emitting ceramic according to claim 1 in which the density of the ceramic is >95% of the theoretical density of the ceramic.

6. The electron-emitting ceramic according to claim 1 in which inert metals, advantageously Mo, W, Nb, Ta, Re, Au, Pt, Pd, are present as metal.

7. The electron-emitting ceramic according to claim 1 in which individual metals or alloys of metals are present as metal.

8. The electron-emitting ceramic according to claim 1 in which alkaline earth elements such as Sr and/or Ba are present as production-specific impurities, dopants, auxiliary materials, and/or additives.

Description

Example 1

[0052] CaCO.sub.3 and Al.sub.2O.sub.3 powders are mixed at an amount-of-substance ratio of 12 to 7 and melted at a temperature of 1450° C. The melt is quenched on a brass block, and is comminuted in a vibratory disc mill and by means of wet grinding. During the wet grinding, 29.9 mass % Mo powder, which corresponds to 10 vol. % Mo powder, is added and the mixture is further homogenized. The ground material is then dried, and the powder obtained is pressed into cylindrical discs. The discs are sintered under nitrogen atmosphere in a furnace with a graphite heater at 1350° C. with a holding time of 10 h.

[0053] The electron-emitting ceramic obtained has a density of>95% of the theoretical density and, after a dry polishing, can be directly used as cathode for an electron emitter.

[0054] To determine the work function of the cathode material, said material is heated under vacuum at 10.sup.−6 Pa to a temperature of 300° C. to 950° C. and the current that flows through the emitted electrons onto an opposing plate is measured at a maximum electric field of 40 V/cm. The work function determined was 2.4-2.8 eV at a measuring temperature of at least 800° C.

[0055] The temperature conductivity of the ceramic was 1.5 mm.sup.2/s (25° C.) and 1.1 mm.sup.2/s (300° C.).

[0056] During use of the ceramic as cathode material in a hollow cathode in a satellite propulsion device, it was possible to establish an improvement of the long-term stability with a simultaneously continuous electron emission.

Example 2

[0057] CaCO.sub.3, SrCO.sub.3, and Al.sub.2O.sub.3 powders are mixed at a CaO:SrO:Al.sub.2O.sub.3 amount-of-substance ratio of 11.5:0.5:7 and melted at a temperature of 1450° C. The melt is quenched on a brass block, and is comminuted in a vibratory disc mill and by means of wet grinding. During the wet grinding, 65 mass % W powder, which corresponds to 20 vol. % W powder, is added and the mixture is further homogenized.

[0058] The ground material is then dried, and the powder obtained is pressed into cylindrical discs. The discs are sintered under nitrogen atmosphere in a furnace with a graphite heater at 1350° C. with a holding time of 10 h.

[0059] The electron-emitting ceramic obtained thus contains as a doping approximately 3.4mass % SrO and has a density of 98% of the theoretical density and, after a dry polishing, can be directly used as cathode for an electron emitter.

[0060] To determine the work function of the cathode material, said material is heated under vacuum at 10.sup.−6 Pa to a temperature of 300° C. to 950° C. and the current that flows through the emitted electrons onto an opposing plate is measured at a maximum electric field of 40 V/cm. The work function determined was 2.5 eV at a measuring temperature of at least 800° C.

[0061] The temperature conductivity of the ceramic was 2.5 mm.sup.2/s (25° C.) and 1.9 mm.sup.2/s (300° C.).

[0062] During use of the ceramic as cathode material in a satellite propulsion device, it was possible to establish an improvement of the long-term stability with a simultaneously continuous electron emission.

Example 3

[0063] CaCO.sub.3 and Al.sub.2O.sub.3 powders are mixed at an amount-of-substance ratio of 12 to 7 and melted at a temperature of 1450° C. The melt is quenched on a brass block, and is comminuted in a vibratory disc mill and by means of wet grinding. During the wet grinding, 17 mass % Ti-15 Mo alloy powder, which corresponds to 10 vol. % Ti-15 Mo alloy powder, is added and the mixture is further homogenized. The ground material is then dried, and the powder obtained is pressed into cylinders with a length of 20 mm. From the cylinders, hollow cylinders with an outer diameter of 4.5 mm and an inner diameter of 1 mm are fabricated by means of dry green machining. The hollow cylinders are sintered under nitrogen atmosphere in a furnace with a graphite heater at 1350° C. with a holding time of 10 h.

[0064] The electron-emitting ceramic obtained has a density of>95% of the theoretical density.

[0065] For the generation of an electric plasma, one of the ceramic hollow cylinders is installed in a hollow cathode. The hollow cathode is essentially composed of the hollow cylinder insert, a holder, a gas connection, an insulator, and a keeper. The hollow cathode is positioned with a Hall-effect thruster in a high-vacuum chamber. 100

[0066] 691 For the operation of the cathode, krypton gas is conducted through the cathode and therefore the hollow cylinder. By applying a potential difference between the hollow cylinder emitter and the keeper, a plasma state is excited in the cathode, wherein a plasma is ignited. The plasma thruster is ignited and operated by applying a positive potential at the anode of the Hall-effect thruster.

[0067] During operation of the hollow cathode, a temperature near the electron-emitting hollow cylinder body of approximately 150° C. is reached, which is significantly lower than with conventional electron-emitting materials.

[0068] During use of the ceramic as hollow cathode in a satellite propulsion device, it was possible to establish a reduced temperature of the electron source with a simultaneously continuous plasma generation.