Method for preparing pressed scandia-doped dispenser cathodes using microwave sintering

Abstract

The present disclosure discloses a preparation method of pressed Scandia-doped dispenser cathode using microwave sintering. Embodiments of the present disclosure include dissolving some nitrates and ammonium metatungstate with deionized water to prepare a homogeneous solution. Precursor powder with uniform size is obtained by spray drying, the precursor powder is decomposed, and two-step reduction may be proceeded to form doped tungsten powder with uniform element distribution. The cathode is prepared by one-time microwave sintering. One-time forming of cathode sintering is realized, and sintering shrinkage and sintering time are reduced significantly. The method has excellent repeatability, and the cathode has a homogeneous structure and excellent emission performance at 950° C.

Claims

1. A method of preparing a pressed scandia-doped dispenser cathode using microwave sintering, the method comprising: preparing precursor powders by: dissolving ammonium metatungstate hydrate, scandium nitrate, aluminum nitrate, barium nitrate and calcium nitrate in water under a room temperature to obtain solutions, respectively, mixing a solution of the ammonium metatungstate hydrate and a solution of the scandium nitrate to obtain a mixture, adding the nitrate solution slowly while performing constant agitation to the mixture, and obtaining precursor powder by spray drying, wherein a feeding rate is 200 ml/h-600 ml/l, a blast rate is 0.4 m.sup.3/min-0.6 m.sup.3/min, an inlet temperature is 150° C., and an outlet temperature is 90-96° C.; performing decomposition and reduction of the precursor powder by: decomposing the precursor powder in a muffle furnace for 3 hours to remove powder containing C and N in the precursor powder under a condition including a temperature of 550° C. and air or oxygen atmosphere to obtain oxide powder, and performing a reduction process of the oxide powder in a tube furnace under hydrogen atmosphere by: keeping the temperature at 450-550° C. maintain 2-3 h, and raising the temperature to 800-850° C. and maintaining 2-3 h to obtain doped tungsten powder; and pressing and sintering a cathode by: pressing the decomposed powder under a certain pressure using molds, placing green bodies in an auxiliary heating and insulation combining device, placing the auxiliary heating and insulation combining device in a microwave cavity of a microwave source, turning on the microwave source and raising the temperature in the microwave cavity with a constant rate until to 800-850° C., maintaining the temperature for 5 minutes, raising the temperature with a rate of 10˜15° C./min until to 1400-1500° C., maintaining the temperature for 10-30 min, and obtaining the cathode after cooling the cathode to the room temperature.

2. The method of claim 1, wherein a proportion of the ammonium metatungstate hydrate and the scandium nitrate b is: W (85%), Sc.sub.2 O.sub.3(5%), BaO, CaO and Al.sub.2O.sub.3(10%), and a molar ratio of Ba:Ca:Al is 4:1:1.

3. The method of claim 1, wherein a composition of the reduced powder generated is W, Sc.sub.2 O.sub.3, Bao, CaO, and Al.sub.2O.sub.3.

4. The method of claim 1, further comprising: filtering the powder through a 200-mesh screen after the preparing of the precursor powders and the performing of the decomposition and reduction of the precursor powder, respectively.

5. The method of claim 1, wherein a pressing pressure in the pressing and sintering of the cathode is 0.8-1.2 t/cm.sup.2.

6. The method of claim 1, wherein an auxiliary heating material of the auxiliary heating and insulation combining device is SiC, and an insulation material of the insulation combining device is Mullite Fiber.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0015] To describe the technique embodiments in detail, the appended drawings used in embodiments is introduced below. Apparently, appended drawings below are only a part of drawings of the present disclosure, and as for normal technician in this area, they can obtain other drawings based on this kind of appended drawings without creative labor.

[0016] FIG. 1 is a flow chart of the preparation method of the present disclosure.

[0017] FIG. 2 is a schematic diagram of microwave sintering device.

[0018] FIG. 3 is XRD results of powders after reduction.

[0019] FIG. 4 shows SEM photo and EDAX energy spectrum analysis of the present disclosure.

[0020] FIG. 5 shows cathode real photo and SEM photo of the present disclosure (a: cathode real photo, b: SEM photo of cathode surface of embodiments 1, c: SEM photo of cathode surface of embodiments 2, d: SEM photo of cathode surface of embodiments 3).

[0021] FIG. 6 is a cathode thermal emission curve (Log U-Log I) of three embodiments of the present disclosure.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0022] The present disclosure will be described in more detail.

Embodiment 1

[0023] Dissolving barium nitrate (20.34 g), calcium nitrate tetrahydrate (4.60 g), aluminum nitrate nonahydrate (14.61 g), nitrate hexahydrate scandium (36.84 g) and ammonium metatungstate (160.668 g) in deionized water respectively, stirring until they are fully dissolved. Then preparing “411” solution, which includes barium nitrate, calcium nitrate and aluminum nitrate. Mix nitrate scandium solution and ammonium metatungstate solution, adding “411” solution slowly with constantly agitation to realize intensive mixing. Precursor powder is obtained by spray-drying, feeding rate is 600 ml/l, blast rate is 0.4 m.sup.3/min-0.6 m.sup.3/min, the inlet temperature is 150° C., the outlet temperature is 90-96° C. The precursor powder is decomposed in muffle furnace, C and N contains in precursor powder is removed under the conditions of 550° C., air atmosphere (or oxygen atmosphere) and maintain 3 hours, oxide powder contains scandium oxide, and tungsten oxide is obtained; proceeding reduction process of the oxide powder in tube furnace under hydrogen atmosphere, which includes two steps, first, maintaining the temperature at 500° C. maintain 2-3 h, then, raising the temperature to 900° C. and maintain 2-3 h to obtain, doped tungsten powder. At last, using molds to press the decomposed powder under pressure of 1.2 t/cm.sup.2, putting green bodies in auxiliary heating and insulation combining device, then putting them in microwave cavity, turning on the microwave source, raising the temperature to 850° C. with a constant rate of 20° C./min, then raising the temperature to 1500° C. with a constant rate of 13° C./min, maintaining the temperature for 20 min, and cooling to room temperature.

Embodiment 2

[0024] Dissolving barium nitrate (6.78 g), calcium nitrate tetrahydrate (1.53 g), aluminum nitrate nonahydrate (4.78 g), nitrate hexahydrate scandium (12.28 g) and ammonium metatungstate (56.61 g) in deionized water respectively, stirring until they are fully dissolved. Then preparing “411” solution, which includes barium nitrate, calcium nitrate and aluminum nitrate. Mixing nitrate scandium solution and ammonium metatungstate solution, and adding “411” solution slowly with constantly agitation to realize intensive mixing. Precursor powder is obtained by spray-drying, feeding rate is 600 ml/l, blast rate is 0.4 m.sup.3/min-0.6 m.sup.3/min, the inlet temperature is: 150° C., the outlet temperature is 90-96° C. The precursor powder is decomposed in muffle furnace, C and N contains in precursor powder is removed under the conditions of 550° C., air atmosphere (or oxygen atmosphere) and maintaining 3 hours, oxide powder contains scandium oxide, and tungsten oxide is obtained; proceeding reduction process of the oxide powder in tube furnace under hydrogen atmosphere, includes two steps, first, maintaining the temperature at 500° C. with 2-3 h, then, raising the temperature to 900° C. and maintain 2-3 h to obtain doped tungsten powder. At last, using molds to press the decomposed powder under pressure of 1.2 t/cm.sup.2, putting green bodies in auxiliary heating and insulation combining device, then putting them in microwave cavity, turning on the microwave source, raising the temperature to 850° C. with a constant rate of 20° C./min, then raising the temperature to 1500° C. with a constant rate of 15° C./min, maintaining the temperature for 15 min, and cooling to room temperature.

Embodiment 3

[0025] Dissolving barium nitrate (20.34 g), calcium nitrate tetrahydrate (4.60 g), aluminum nitrate nonahydrate (14.61 g), nitrate hexahydrate scandium (36.84 g) and ammonium metatungstate (160.668 g) in deionized water respectively, stirring until they are fully dissolved. Then prepare “411” solution, which includes barium nitrate, calcium nitrate and aluminum nitrate. Mixing nitrate scandium solution and ammonium metatungstate solution, add “411” solution slowly with constantly agitation to realize intensive mixing. Precursor powder is obtained by spray-drying, feeding rate is 300 ml/l, blast rate is 0.4 m.sup.3/min-0.6 m.sup.3/min, the inlet temperature is: 150° C., the outlet temperature is 90-96° C. The precursor powder is decomposed in muffle furnace, C and N contains in precursor powder is removed under the conditions of 550° C., air atmosphere (or oxygen atmosphere) and maintain 3 hours, oxide powder contains scandium oxide, and tungsten oxide is obtained; proceeding reduction process of the oxide powder in tube furnace under hydrogen atmosphere, includes two steps, first, maintaining the temperature at 500° C. with 2 h, then, raising the temperature to 900° C. and maintain 2 h to obtain doped tungsten powder. At last, using molds to press the decomposed powder under pressure of 1.2 t/cm.sup.2, putting green bodies in auxiliary heating and insulation combining device, then putting them in microwave cavity, turn on the microwave source, raising the temperature to 800-850° C. with a constant rate of 20° C./min, then raising the temperature to 1550° C. with a constant rate of 15° C./min, maintaining the temperature for 30 min, and cooling to room temperature.

[0026] The embodiments mentioned above just are a further description of the present disclosure, but the present disclosure is more illustrative than that. Any modifications, similar substitutions, and improvements based on the present disclosure should be included in the protective range of the present disclosure.