Metallurgical Processing for Impregnated Thermionic Cathodes Manufacturing

Zanella, Glauco P.; Sousa, Gabriel G. J. de; Motta, Cláudio C.

doi:10.1590/1980-5373-MR-2021-0485

Glauco P. Zanella

Universidade de São Paulo, São Paulo, SP, Brasil.

Gabriel G. J. de Sousa

Universidade de São Paulo, São Paulo, SP, Brasil.

Cláudio C. Motta ^**e-mail: ccmotta@usp.br

Universidade de São Paulo, São Paulo, SP, Brasil.

https://orcid.org/0000-0002-2508-7320

*e-mail: ccmotta@usp.br

Figures (16)
Tables (7)

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Figure 1
Pressing matrix: (1) top die punch; (2) die; (3) cathode button; and (4) bottom die punch.

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Figure 2
Induction heating jig for pre-sintering and decopperization steps: (1) cathode button; (2) quartz tube; (3) molybdenum crucible; (4) 3-inch coil with four turns; (5) alumina disk; and (6) dry hydrogen entrance.

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Figure 3
Flowchart for the Mo-Ru brazing paste.

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Figure 4
Schematic drawing for the brazing paste application.

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Figure 5
Schematic jig drawing for hydrogen brazing process.

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Figure 6
Arrangement to the strength test: 1 - cathode sample; 2 - clamp; and 3 - bottom punch.

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Figure 7
Cumulate mass losses along matrix manufacturing process.

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Figure 8
Arrangement of the sample holder with the cathode buttons.

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Figure 9
Optical micrograph of a decoppered sample taken with the microscope porosity analysis module.

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Figure 10
Sample prepared to analyze the melting of eutectic alloy of Mo-Ru system, (a) before melting (b) after melting.

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Figure 11
(a) optical micrograph of an 1890°C brazed sample, and (b) optical micrograph of a 1967°C brazed sample.

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Figure 12
Scanning electron micrograph of a 1967°C brazed sample.

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Figure 13
(a) SEM with a 150x magnification in the junction area, (b) EDS map counting Mo in the junction area, (c) EDS map counting Ru in the junction, and (d) EDS map counting W in the same region.

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Figure 14
EDS full-scale counting spectrum taken in micrograph analysis in SEM micrograph with a 150x magnification in the junction area.

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Figure 15
(a) cathode sample 1 after strength test, and (b) cathode sample 3 after strength test.

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Figure 16
SEM fractography of brazed cathode in the (a) overview surface, (b) cracks propagation origin region and (c) grains brittle fracture.

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Table 1
Parameters for the tungsten cathode matrix pre-sintering step.

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Table 2
Improved parameters for the tungsten cathode matrix decopperization step.

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Table 3
Chemical composition of cathode pellet after chemical etching obtained by an EDX analysis.

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Table 4
Apparent porosity calculated by the Archimedes’ method.

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Table 5
EDX analysis of the Mo-Ru alloy surface before and after the melting process.

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Table 6
SEM quantitative EDS specimens results.

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Table 7
Strength test results from 5 cathode samples brazed at 1967°C.

Parameter
Maximum coil current	333.9 A
Induction power	1900-2000 W
Maximum temperature	1200 °C
Heating and cooling times	60 min (total)
Plateau time	5 min
Heating chamber pressure	760 Torr
Hydrogen dry flow (H₂)	3 l/min

Parameter
Maximum coil current	478.8 A
Induction power	3980 - 4040 W
Maximum temperature	1620 °C
Heating and cooling times	60 min (total)
Plateau time	6 h
Heating chamber pressure	760 Torr
Hydrogen dry flow (H₂)	3 l/min

Sample	W (± 0.01 mol %)	Cu (± 0.01 mol %)	Others (± 0.01 mol %)
1	98.64	0.05	< 1.31
2	98.48	0.06	< 1.46
3	98.44	0.07	< 1.49
4	98.49	0.10	< 1.40
5	98.17	0.17	< 1.66
6	98.92	0.07	< 1.01
7	98.39	0.08	< 1.53
8	99.29	0.05	< 0.66
9	98.53	0.05	< 1.43
10	96.73	0.06	< 3.20
11	97.68	0.06	< 2.26
12	99.39	0.04	<0.57
Average	98.43	0.07

Sample	Apparent porosity (± 0.5%)	Relative density (± 0,1 g/cm³)	Bulk density (± 0,1 g/cm³)
01	34.9	10.1	11.7
02	34.7	9.5	11.2
03	38.5	9.3	11.9

Element	Before (± 0.01 mol %)	After (± 0.01 mol %)
Mo	69.89	57.82
Ru	29.90	42.18
Others	< 0.21	-

Element	Weight %	Weight % Error	Norm. Wt % Err	Norm. Wt. % Err	Atom %	Atom % Error
Ge	0.97	+/- 0.83	0.97	+/- 0.83	1.62	+/- 1.39
Mo	49.99	+/- 0.85	49.99	+/- 0.85	62.95	+/- 1.07
Ru	5.96	+/- 0.52	5.96	+/- 0.52	7.12	+/- 0.62
W	43.08	+/- 2.24	43.08	+/- 2.24	28.31	+/- 1.47
Total	100.00		100.00		100.00

Sample	Fracture stress (±1 MPa)	Fracture point observed
1	62	porous matrix
2	78	porous matrix
3	195	cathode body
4	117	porous matrix
5	256	cathode body
Average	142

[1] *e-mail: ccmotta@usp.br

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Metallurgical Processing for Impregnated Thermionic Cathodes Manufacturing

Abstract