Thermionic tungsten/scandate cathodes and method of making the same

Abstract

A thermionic dispenser cathode having a refractory metal matrix with scandium and barium compounds in contact with the metal matrix and methods for forming the same. The invention utilizes atomic layer deposition (ALD) to form a nanoscale, uniform, conformal distribution of a scandium compound on tungsten surfaces and further utilizes in situ high pressure consolidation/impregnation to enhance impregnation of a BaO—CaO—Al.sub.2O.sub.3 based emissive mixture into the scandate-coated tungsten matrix or to sinter a tungsten/scandate/barium composite structure. The result is a tungsten-scandate thermionic cathode having improved emission.

Claims

1. A process for making a thermionic dispenser cathode, the process including the steps of: providing a starting powder comprising particles of a refractory metal and/or metal alloy; placing the starting powder inside a furnace having a controlled atmosphere and heating the starting powder in the flow of hydrogen or hydrogen/inert gas mixture to reduce surface oxides to produce a cleaned starting powder; making a porous preformed compact from the cleaned starting powder; without exposing the cleaned starting powder to an external atmosphere, placing the porous preformed compact inside a particle atomic layer deposition (ALD) reactor and controllably depositing a conformal nanometer-scale film of a scandium compound on all available surfaces inside and outside of the porous preformed compact to produce a scandium compound-coated compact with a conformal nanometer-scale scandium film having a predetermined thickness uniformly deposited on all of the surfaces thereof; without exposing the scandium compound-coated compact to air, placing the scandium compound-coated compact in contact with an emissive mixture comprising a barium compound; and without exposing the scandium compound-coated compact in contact with the emissive mixture to air, subjecting the compact with the contacted emissive mixture to a predetermined pressure P and temperature T greater than a melting point of the emissive mixture so that the emissive mixture becomes a molten emissive mixture that infiltrates the scandium compound-coated compact to form a barium-impregnated scandium-coated compact; wherein the impregnated compact forms the cathode.

2. The process according to claim 1, wherein the refractory metal and/or metal alloy is tungsten.

3. The process according to claim 1, wherein the scandium compound is scandium oxide.

4. The process according to claim 1, wherein the barium compound is barium-calcium-aluminate.

5. The process according to claim 1, wherein the pressure P is between about 0.1 and 5 GPa.

6. The process according to claim 1, wherein the temperature T is between 1,500° C. and 2,100° C.

7. A process for making a thermionic dispenser cathode, the process including the steps of: providing a sample of a porous refractory metal and/or metal alloy; placing the sample inside a furnace having a controlled atmosphere and heating the starting powder in the flow of hydrogen or hydrogen/inert gas mixture to reduce surface oxides to produce a cleaned sample; without exposing the cleaned sample to an external atmosphere, placing the cleaned sample inside an atomic layer deposition (ALD) reactor and controllably depositing a conformal nanometer-scale film of a scandium compound on all available surfaces inside and of the cleaned sample to produce a scandium compound-coated sample with a conformal nanometer-scale scandium film having a predetermined thickness uniformly deposited on all of the surfaces thereof; without exposing the scandium compound-coated sample to air, placing the scandium compound-coated sample in contact with an emissive mixture comprising a barium compound; and without exposing the scandium compound-coated sample with contacted emissive mixture to air, subjecting the scandium compound-coated sample with contacted emissive mixture to a predetermined pressure P and temperature T greater than a melting point of the emissive mixture so that the emissive mixture becomes a molten emissive mixture that infiltrates the scandium compound-coated sample to form a barium-impregnated scandium-coated sample; wherein the impregnated sample forms the cathode.

8. The process according to claim 7, wherein the refractory metal and/or metal alloy is tungsten.

9. The process according to claim 7, wherein the scandium compound is scandium oxide.

10. The process according to claim 7, wherein the barium compound is barium-calcium-aluminate.

11. The process according to claim 7, wherein the pressure P is between about 0.1 and 5 GPa.

12. The process according to claim 7, wherein the temperature T is between 1,500° C. and 2,100° C.

13. A process for making a thermionic dispenser cathode, the process including the steps of: providing a starting powder comprising particles of a refractory metal and/or metal alloy; placing the starting powder inside a furnace having a controlled atmosphere and heating the starting powder in the flow of hydrogen or hydrogen/inert gas mixture to reduce surface oxides to produce a cleaned starting powder; making a porous preformed compact from the cleaned starting powder; without exposing the cleaned starting powder to an external atmosphere, placing the porous preformed compact inside a particle atomic layer deposition (ALD) reactor and controllably depositing a conformal nanometer-scale film of a scandium compound on all available surfaces inside and outside of the porous preformed compact to produce a scandium compound-coated compact with a conformal nanometer-scale scandium film having a predetermined thickness uniformly deposited on all of the surfaces inside and outside thereof; with the scandium compound-coated compact still in the ALD reactor and without exposing the compact to air, depositing a conformal layer of a barium compound on all available surfaces inside and outside of scandium compound-coated compact; and without exposing the scandium compound-coated compact with deposited barium layer to air, subjecting the scandium compound-coated compact with deposited barium layer to a predetermined pressure P and temperature T to sinter the compact to full density; wherein the sintered compact forms the cathode.

14. The process according to claim 13, wherein the refractory metal and/or metal alloy is tungsten.

15. The process according to claim 13, wherein the scandium compound is scandium oxide.

16. The process according to claim 13, wherein the deposited barium layer is barium oxide.

17. The process according to claim 13, wherein the pressure P is between about 0.1 and 5 GPa.

18. The process according to claim 13, wherein the temperature T is between 800° C. and 2,100° C.

19. A process for making a thermionic dispenser cathode, the process including the steps of: providing a sample of a porous refractory metal and/or metal alloy; placing the sample inside a furnace having a controlled atmosphere and heating the starting powder in the flow of hydrogen or hydrogen/inert gas mixture to reduce surface oxides to produce a cleaned sample; without exposing the cleaned sample to an external atmosphere, placing the cleaned sample inside an atomic layer deposition (ALD) reactor and controllably depositing a conformal nanometer-scale film of a scandium compound on all available surfaces inside and outside of the cleaned sample to produce a scandium compound-coated sample with a conformal nanometer-scale scandium film having a predetermined thickness uniformly deposited on all of the surfaces thereof; with the scandium compound-coated sample still in the ALD reactor and without exposing the compact to air, depositing a conformal layer of the barium compound on all available surfaces inside and outside of scandium compound-coated sample; and with the scandium compound-coated sample with deposited barium layer still in the ALD reactor and without exposing the scandium compound-coated sample with deposited barium layer to air, subjecting the scandium compound-coated sample with deposited barium layer to a predetermined pressure P and temperature T to sinter the sample to full density; wherein the sintered sample forms the cathode.

20. The process according to claim 19, wherein the refractory metal and/or metal alloy is tungsten.

21. The process according to claim 19, wherein the scandium compound is scandium oxide.

22. The process according to claim 19, wherein the deposited barium layer is barium oxide.

23. The process according to claim 19, wherein the pressure P is between about 0.1 and 5 GPa.

24. The process according to claim 19, wherein the temperature T is between 800° C. and 2,100° C.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a transmission electron microscopy (TEM) image of a tungsten particle coated with a 10-nm thick film of scandium oxide.

(2) FIG. 2 is a plot illustrating the results of an energy-dispersive X-ray spectroscopy (EDX) characterization conforming scandium on tungsten.

(3) FIGS. 3A-3E are flow diagrams illustrating aspects of a method for making a thermionic tungsten/scandate cathode in accordance with the present invention.

DETAILED DESCRIPTION

(4) The aspects and features of the present invention summarized above can be embodied in various forms. The following description shows, by way of illustration, combinations and configurations in which the aspects and features can be put into practice. It is understood that the described aspects, features, and/or embodiments are merely examples, and that one skilled in the art may utilize other aspects, features, and/or embodiments or make structural and functional modifications without departing from the scope of the present disclosure.

(5) The present invention provides a dispenser cathode comprising a refractory metal matrix with scandium and barium compounds in contact with metal matrix and methods for making the same.

(6) The method of present invention provides a universal approach for making bulk nanostructures of ceramics, semiconductors and metal using traditional sintering based techniques, including but not limited to Spark Plasma Sintering, microwave sintering, and high pressure sintering that have not previously demonstrated successes in producing fully dense bulk materials with grain sizes <50 nm.

(7) As described in more detail below, the present invention provides a novel two-step fabrication method that creates a uniform and nano-scale scandate film on sub-micron tungsten powders and subsequently consolidates the powder while retaining the architectured microstructure in the bulk cathode. This two-step method utilizes an in situ high pressure consolidation/impregnation technique that enhances impregnation of scandate into tungsten powder.

(8) By using particle atomic layer deposition (ALD) of scandium oxide in the first step, the method of the present invention will bring unmatched conformal control and unprecedented uniformity of the scandate material in addition to allowing the thickness to be tailored from angstroms to 100s of nanometers.

(9) By using high pressure sintering at 0.1-5 GPa and moderate temperatures in the second step, the method of the present invention will allow complete (i.e., to full density) consolidation of the cathode while retaining the nanostructure of the ALD-processed material.

(10) These processes have not been employed previously individually or in tandem and such a combination will revolutionize scandate cathode production by allowing high emission cathodes to be produced on an industrial scale with unprecedented microstructural control and reproducibility.

(11) In some embodiments, a method for making a dispenser cathode comprising a refractory metal matrix with scandium and barium compounds in contact with metal matrix in accordance with the present invention includes the steps of coating a metal surface with scandium and barium compounds.

(12) In some embodiments, a method for making a dispenser cathode comprising a refractory metal matrix with scandium and barium compounds in contact with metal matrix in accordance with the present invention includes the steps of coating a metal surface with scandium and barium compounds as a conformal coating on the metal surface.

(13) In some embodiments, a method for making a dispenser cathode comprising a refractory metal matrix with scandium and barium compounds in contact with metal matrix in accordance with the present invention includes the steps of coating a metal surface with scandium and barium compounds as a conformal coating on a metal surface with a coating thickness at nanometer scale.

(14) These and other aspects of processes for making a thermionic dispenser cathode in accordance with the present invention can be achieved by means of any one or more of the embodiments described below.

First Embodiment

(15) In this first embodiment, a refractory metal and/or metal alloy powder is provided and treated to provide a scandium-coated and barium-impregnated cathode.

(16) In the description below, the refractory metal and/or metal alloy powder used as a starting material is tungsten (W) powder, typically of micron or sub-micron size, but other refractory metal and/or metal alloy powders can be used as appropriate.

(17) The first step in this embodiment is cleaning tungsten oxides from the surface of the W powder by reducing the W powder in a hydrogen atmosphere at an elevated temperature. This step is preferably conducted in a furnace, which will permit the transfer of the reduced (i.e., cleaned) W powder to a deposition chamber without exposing the reduced W powder to the atmospheric air.

(18) In the second step of this embodiment, the W powder is transferred to a deposition chamber and all particles of the cleaned W powder are coated with a conformal nanometer-thick film of a scandium compound. The TEM image in FIG. 1 illustrates an exemplary coated tungsten particle in accordance with this aspect of the present invention, where the tungsten particle is coated with a 10-nm thick film of a scandium compound to form a scandium compound-coated W powder (W/Sc). The energy-dispersive X-ray spectroscopy shown in FIG. 2 collected from the region shown in FIG. 1 illustrates that the transition metal compounds comprising the coated powder are scandium and tungsten. The film can be continuous or discontinuous. This step requires precise control of the nanoscale thickness or amount of the deposited scandium compound as well as uniform distribution of the scandium compound on the surface of all particles of the powder. Although any film deposition process or technique including CVD, sputtering, electro deposition, etc. can be used for deposition of the scandium compound on the W powder, particle atomic layer deposition (pALD) is preferred because it provides superior conformal control and unprecedented uniformity of the scandate material in addition to allowing the thickness to be tailored from angstroms to 100s of nanometers. Scandium oxide is the preferred scandium compound, but other suitable scandium compounds may be used as appropriate.

(19) In a third step of this embodiment, the scandium compound-coated W powder (W/Sc) is contacted with an emissive mix usually comprising, but not limited to BaO, CaO, and Al.sub.2O.sub.3. The emissive mix is preferably Ba—CaO—Al.sub.2O.sub.3 but other suitable compounds including BaO, CaO, and/or Al.sub.2O.sub.3 may be used as appropriate.

(20) In a fourth step in this embodiment, pressure is then applied at room temperature to create a W/Sc compact from the W/Sc powder, the W/Sc compact being in contact with the emissive mixture. The pressure should be high enough to break the thin film of Sc compound so as to make electrical contact between the W particles but should not exceed a level that would cause the W/Sc compact to become so densified that it doesn't have open porosity. It is preferable that this fourth step be conducted without exposing the W/Sc powder to air.

(21) In a fifth step in this embodiment, the W/Sc compact in contact with the emissive mixture is heated to a temperature exceeding the melting point of the emissive mixture so as to cause the molten emissive mixture to impregnate the porous W/Sc compound compact. Impregnation under pressure creates an additional force for more efficient and complete impregnation and allows to use W powder with particle size less than 1 micron.

(22) In exemplary cases, the pressure can be between about 0.1-5 GPa and the temperature can be between 1,500° C. and 2,100° C., but other appropriate pressures and temperatures can also be used.

Second Embodiment

(23) In a second exemplary embodiment, a porous preformed compact is formed from the refractory metal and/or metal alloy powder and is placed inside an atomic layer deposition reactor.

(24) As in the first embodiment, in the description below, the refractory metal and/or metal alloy powder used as a starting material is tungsten (W) powder, typically of micron or sub-micron size, but other refractory metal and/or metal alloy powders can be used as appropriate.

(25) In this second embodiment, the first step is the same as in the first embodiment, i.e., cleaning tungsten oxides from the surface of the W powder by reducing the W powder in a hydrogen atmosphere at an elevated temperature. This step is preferably conducted in a furnace, which will permit the transfer of the reduced (i.e., cleaned) W powder to the deposition chamber without exposing the reduced W powder to the atmospheric air.

(26) The second step is making a porous tungsten compact with connected porosity (W compact) from the W powder. The compact can be made by any suitable technique but is preferably made without exposing the cleaned W powder to air.

(27) In a third step, the W compact is transferred to a deposition chamber and all available surfaces of the porous W compact are coated with a conformal nanometer-thick film of a scandium compound to produce a W/Sc compact. The film can be continuous or discontinuous. This step requires precise control of the nanoscale thickness or amount of the deposited scandium compound as well as uniform distribution of the scandium compound on all available surfaces in pores inside of W compact. Although any film deposition process or technique including CVD, sputtering, electro deposition, etc. can be used for deposition of the scandium compound on the W compact, particle atomic layer deposition (pALD) is preferred because it provides superior conformal control and unprecedented uniformity of the scandate material in addition to allowing the thickness to be tailored from angstroms to 100s of nanometers. Scandium oxide is the preferred scandium compound, but other suitable scandium compounds may be used as appropriate.

(28) In a fourth step, the W/Sc compact is contacted with an emissive mixture usually comprising, but not limited to BaO, CaO, and Al.sub.2O.sub.3. The emissive mix is preferably Ba—CaO—Al.sub.2O.sub.3 but other suitable compounds including BaO, CaO, and/or Al.sub.2O.sub.3 may be used as appropriate.

(29) In a fifth step in this embodiment, pressure is applied at room temperature, with the pressure not exceeding a level at which the W/Sc compact becomes so densified that it doesn't have open or connected porosity. It is preferable that this fifth step be conducted without exposing the W/Sc compact to air.

(30) In a sixth step, the W/Sc compact in contact with emissive mix is heated to a temperature exceeding the melting point of the emissive mix so as to cause the molten emissive mix to impregnate the porous W/Sc compact. Impregnation under pressure creates an additional force for more efficient and complete impregnation and allows the use of a W compact having pore sizes of less than 1 micron.

(31) The pressure P can be between about 0.1-5 GPa and the temperature can be between 1,500° C. and 2,100° C., but other appropriate pressures and temperatures can also be used.

Third Embodiment

(32) In a third exemplary embodiment, there is provided a porous refractory metal and/or metal alloy, with the porous refractory metal and/or metal alloy being coated with a scandium compound and being placed in contact with an emissive mixture.

(33) In the description below, the sample porous refractory metal and/or metal alloy with connected porosity is a porous tungsten (W) metal sample but other suitable metals and/or metal alloys may be used as appropriate.

(34) The first step in this embodiment is cleaning tungsten oxides from the surface of the porous W metal sample by reducing the sample in a hydrogen atmosphere at an elevated temperature to produce a reduced (i.e., cleaned) porous W sample. This step is preferably conducted in a furnace, which will permit the transfer of the reduced porous W sample to a deposition chamber for the next step without exposing the porous W sample to the atmospheric air.

(35) In the second step, all surfaces of the porous W sample are coated with a conformal nanometer-thick film of a scandium compound. The film can be continuous or discontinuous. This step requires precise control of nanoscale thickness or amount of the deposited scandium compound as well as uniform distribution of a scandium compound on all available surfaces in pores inside of the porous W sample to produce a scandium compound-coated porous W sample (porous W/Sc sample). Although any film deposition process or technique including CVD, sputtering, electro deposition, etc. can be used for deposition of the scandium compound on the W sample, particle atomic layer deposition (pALD) is preferred because it provides superior conformal control and unprecedented uniformity of the scandate material in addition to allowing the thickness to be tailored from angstroms to 100s of nanometers. Scandium oxide is the preferred scandium compound, but other suitable scandium compounds may be used as appropriate.

(36) In a third step in this embodiment, the scandium compound-coated porous W sample (porous W/Sc sample) is contacted with an emissive mixture usually comprising, but not limited to, BaO, CaO, and Al.sub.2O.sub.3. The emissive mix is preferably Ba—CaO—Al.sub.2O.sub.3 but other suitable compounds including BaO, CaO, and/or Al.sub.2O.sub.3 may be used as appropriate.

(37) In a fourth step, pressure is then applied to the porous W/Sc sample contacted with the emissive mixture at room temperature. The pressure should be high enough to break the thin film of Sc compound so as to make electrical contact between the W particles but should not exceed a level that would cause the porous W/Sc to become so densified that it doesn't have open or connected porosity. It is preferred that this fourth step be conducted without exposing the porous W/Sc sample to air.

(38) In a fifth step, the porous W/Sc sample in contact with the emissive mixture is heated to a temperature that exceeds the melting point of the emissive mix so as to cause the molten emissive mix to impregnate porous W/Sc sample. Impregnation under pressure creates an additional force for more efficient and complete impregnation and allows to use porous W with pore sizes of less than 1 micron.

(39) The pressure P can be between about 0.1-5 GPa and the temperature can be between 1,500° C. and 2,100° C., but other appropriate pressures and temperatures can also be used.

Fourth Embodiment

(40) In a fourth exemplary embodiment, a refractory metal and/or metal alloy powder is coated with conformal nanometer-scale film of a scandium compound and a conformal layer of barium compound.

(41) In the description below, the refractory metal and/or metal alloy powder used as a starting material is tungsten (W) powder, typically of micron or sub-micron size, but other refractory metal and/or metal alloy powders can be used as appropriate.

(42) In a first step of this embodiment, the W powder is cleaned as described above with respect to the first embodiment.

(43) In a second step of this embodiment, the cleaned W powder is transferred to a deposition chamber and all particles of the cleaned W powder are coated with a conformal nanometer-thick film of a scandium compound to form a scandium compound-coated W (W/Sc) powder. The film can be continuous or discontinuous. This step requires precise control of the nanoscale thickness or amount of the deposited scandium compound as well as uniform distribution of the scandium compound on the surface of all particles of the powder. Although any suitable film deposition process or technique including CVD, sputtering, electro deposition, etc. can be used for deposition of the scandium compound on the W powder, particle atomic layer deposition (pALD) is preferred because it provides superior conformal control and unprecedented uniformity of the scandate material in addition to allowing the thickness to be tailored from angstroms to 100s of nanometers. Scandium oxide is preferred scandium compound, but other suitable scandium compounds may be used as appropriate.

(44) In a third step of this embodiment, the particles of the W/Sc powder are further coated with a conformal nanometer-thick film of a barium (Ba) compound to form a scandium- and barium-coated (W/Sc/Ba) W powder, where the Ba film on any given particle can be continuous or discontinuous. As with the scandium compound deposited in the previous step, this step requires precise control of the nanoscale thickness or amount of the deposited scandium compound as well as uniform distribution of the barium compound on the surface of all particles of the powder. As with the deposition of the scandium compound, although any suitable film deposition process or technique including CVD, sputtering, electro deposition, etc. can be used for deposition of the barium compound on the W/Sc powder, particle atomic layer deposition (pALD) is preferred particle atomic layer deposition (pALD) is preferred because it provides superior conformal control and unprecedented uniformity of the barium material in addition to allowing the thickness to be tailored from angstroms to 100s of nanometers. Barium oxide is the preferred barium compound, but other suitable barium compounds can be used as appropriate.

(45) In a fourth step, pressure is applied to the W/Sc/Ba powder at room temperature and without exposing the W/Sc/Ba powder to the atmosphere to create a W/Sc/Ba compact from the W/Sc/Ba powder. The pressure should be high enough to break the Sc/Ba thin film on the particles so as to permit electrical contact between the W particles but should not exceed a level that would cause the W/Sc/Ba compact to become so densified that it doesn't have open porosity.

(46) Finally, in a fifth step, the W/Sc/Ba compact is heated to a temperature high enough to sinter the W/Sc/Ba compact to a dense compact at the applied pressure, where the dense compact doesn't have a connected porosity or a porosity less than 15%.

(47) The pressure P can be between about 0.1-5 GPa and the temperature can be between 800° C. and 2,100° C., but other appropriate pressures and temperatures can also be used.

Fifth Embodiment

(48) In a fifth embodiment, a porous preformed compact is formed from a refractory metal and/or metal alloy powder and is coated with conformal nanometer-scale film of a scandium compound and a conformal layer of barium compound.

(49) In the description below, the refractory metal and/or metal alloy powder is tungsten (W) powder, typically of micro or sub-micron size, but other refractory metal and/or metal alloy powders can be used as appropriate.

(50) In a first step of this embodiment, the W powder is cleaned as described above with respect to the first embodiment.

(51) In a second step, a porous tungsten compact (W compact) having connected porosity is made from the cleaned W powder. The compact can be made by any suitable technique but is preferably made without exposing the cleaned W powder to air.

(52) In a third step, the W compact is transferred to a deposition chamber and all available surfaces of the W compact are coated with a conformal nanometer-thick film of a scandium compound to produce a W/Sc compact. The film can be continuous or discontinuous. This step requires precise control of the nanoscale thickness or amount of the deposited scandium compound as well as uniform distribution of the scandium compound on all available surfaces in pores inside of W compact. Although any suitable film deposition process or technique including CVD, sputtering, electro deposition, etc. can be used for deposition of the scandium compound deposition on the W compact, particle atomic layer deposition (pALD) is preferred because it provides superior conformal control and unprecedented uniformity of the scandate material in addition to allowing the thickness to be tailored from angstroms to 100s of nanometers. Scandium oxide is the preferred scandium compound, but other suitable scandium compounds can be used as appropriate.

(53) In a fourth step of this embodiment, the W/Sc compact is further coated with a conformal nanometer-thick film of a barium (Ba) compound to form a scandium- and barium-coated W compact (W/Sc/B compact), where the Ba film on any given particle can be continuous or discontinuous. As with the scandium compound deposited in the previous step, this step requires precise control of the nanoscale thickness or amount of the deposited scandium compound as well as uniform distribution of the barium compound on the surface of all particles of the powder. Although any suitable film deposition process or technique including CVD, sputtering, electro deposition, etc. can be used for deposition of the barium compound on the W/Sc compact, particle atomic layer deposition (pALD) is preferred because it provides superior conformal control and unprecedented uniformity of the barium material in addition to allowing the thickness to be tailored from angstroms to 100s of nanometers. Barium oxide is the preferred barium compound, but other suitable barium compounds can be used as appropriate.

(54) In a fifth step, pressure is applied to the W/Sc/B compact at room temperature and without exposing the W/Sc/B compact to the atmosphere.

(55) Finally, in a sixth step, still without exposing the W/Sc/Ba compact to air, the W/Sc/Ba compact is heated to a temperature high enough to sinter the W/Sc/Ba compact to a dense compact at the applied pressure, where the dense compact doesn't have a connected porosity or a porosity less than 15%.

(56) The pressure P can be between about 0.1-5 GPa and the temperature can be between 800° C. and 2,100° C., but other appropriate pressures and temperatures can also be used.

Sixth Embodiment

(57) The sixth embodiment is similar to the fifth embodiment, but the starting material is a sample of porous refractory metal and/or metal alloy with connected porosity, with the metal sample being coated with conformal nanometer-scale film of a scandium compound and a conformal layer of barium compound.

(58) As with the other embodiments described herein, in the description below, the porous refractory metal and/or metal alloy with connected porosity used as a starting material in this embodiment is a porous tungsten (W) metal but other suitable metals and/or metal alloys may be used as appropriate.

(59) The first step in this embodiment is cleaning tungsten oxides from the surface of the porous W sample by reducing the sample in a hydrogen atmosphere at an elevated temperature to produce a reduced (i.e., cleaned) porous W sample. This step is preferably conducted in a furnace, which will permit the transfer of the reduced porous W sample to a deposition chamber for the next step without exposing the porous W sample to the atmospheric air.

(60) In the second step, all surfaces of the porous W sample are coated with a conformal nanometer-thick film of a scandium compound. The film can be continuous or discontinuous. This step requires precise control of nanoscale thickness or amount of the deposited scandium compound as well as uniform distribution of a scandium compound on all available surfaces in pores inside of the porous W sample to produce a scandium compound-coated porous W sample (porous W/Sc sample). Although any suitable film deposition process or technique including CVD, sputtering, electro deposition, etc. can be used for deposition of the scandium compound on the porous W sample, particle atomic layer deposition (pALD) is preferred because it provides superior conformal control and unprecedented uniformity of the scandate material in addition to allowing the thickness to be tailored from angstroms to 100s of nanometers. Scandium oxide is preferred scandium compound but other suitable scandium compounds may be used as appropriate.

(61) In a third step of this embodiment, the scandium compound-coated porous W sample (porous W/Sc sample) is further coated with a conformal nanometer-thick film of a barium (Ba) compound to form a scandium- and barium-coated porous W/Sc (W/Sc/Ba) sample, where the Ba film on the sample can be continuous or discontinuous. As with the scandium compound deposited in the previous step, this step requires precise control of the nanoscale thickness or amount of the deposited scandium compound as well as uniform distribution of the barium compound on all surfaces of the porous W/Sc sample. Although any suitable film deposition process or technique including CVD, sputtering, electro deposition, etc. can be used for deposition of the barium compound on the porous W sample, particle atomic layer deposition (pALD) is preferred because it provides superior conformal control and unprecedented uniformity of the barium material in addition to allowing the thickness to be tailored from angstroms to 100s of nanometers. Barium oxide is the preferred barium compound, but other suitable barium compounds may be used as appropriate.

(62) In a fourth step, pressure is applied to the W/Sc/Ba sample at room temperature and without exposing the W/Sc/Ba sample to the atmosphere.

(63) Finally, in a fifth step, the W/Sc/Ba sample is heated to a temperature high enough to sinter the W/Sc/Ba sample at the applied pressure, where the sintered W/Sc/Ba sample doesn't have a connected porosity or a porosity less than 15%.

(64) The pressure P can be between about 0.1-5 GPa and the temperature can be between 800° C. and 2,100° C., but other appropriate pressures and temperatures can also be used.

EXAMPLE

(65) FIG. 3 is a flow diagram illustrating a process flow used in this Example and shows the final structure of the scandate cathode made in this example.

(66) Tungsten powder 4-8 micron was placed in a tube furnace and was heated at about 900° C. for 1 hour in a hydrogen atmosphere to clean the particles and reduce tungsten oxide on their surface (FIG. 3A). After the treatment, the cleaned tungsten powder was transferred to a rotary atomic layer deposition (ALD) reactor without exposing the powder to air. Inside ALD reactor tungsten powder was exposed to 100 cycles of alternative pulses of scandium precursor (Sc(thd).sub.3, thd=2,2,6,6-tetramethyl-3,5-heptanedione) and ozone (FIG. 3B). As a result, all of the tungsten particles were coated with scandium oxide film having thickness of about 10 nm to form a W/Sc.sub.2O.sub.3 powder. In the next step (FIG. 3C), the W/Sc.sub.2O.sub.3 powder was placed in a die and was compacted without exposure to air into a cylinder having a diameter of 10 mm diameter and a height of 2 mm. In addition, a cylinder of an emissive mixture comprising BaO, CaO, and Al.sub.2O.sub.3 was compacted from an emissive mixture powder, and the two compacted cylinders were placed in contact with each other inside a high pressure cell, which was placed inside a high pressure apparatus (FIG. 3D). Pressure of 0.5 GPa was applied to the samples and they were heated to temperature of about 1750° C. to cause the emissive mixture to melt and impregnate the porous W/Sc.sub.2O.sub.3 compact. The sample was then cooled and the pressure released. The resulting sample of scandate cathode had a diameter of 9.5 mm and a height of 1.8 mm and had the structure shown in FIG. 3E, i.e., a W/Sc.sub.2O.sub.3 compact impregnated with the BaO—CaO—Al.sub.2O.sub.3 mixture. The resulting structure provided good uniformity of electron emission.

(67) Although particular embodiments, aspects, and features have been described and illustrated, it should be noted that the invention described herein is not limited to only those embodiments, aspects, and features but also contemplates any and all modifications within the spirit and scope of the underlying invention described and claimed herein that may be made by persons skilled in the art, and all such embodiments are within the scope and spirit of the present disclosure.