Cermet Electrode Material

Abstract

A cermet material includes as mass percentages, at least: 50% to 90% of a metallic phase containing an alloy of copper (Cu) and nickel (Ni), and 10% to 50% of an oxide phase containing at least iron, nickel and oxygen with the following proportion by mass of Ni: 0.2%Ni17%.

An electrode, preferably an anode, may include this cermet material.

Claims

1. Cermet material comprising as mass percentages, at least: 50% to 90% of a metallic phase which contains at least one alloy of copper (Cu) and nickel (Ni), said alloy comprising as percentages by mass: 35% to 75% of nickel, 25% to 65% of copper, 10% to 50% of an oxide phase containing at least iron, nickel and oxygen with the following proportion by mass of nickel: 0.2%Ni17%.

2. Cermet material according to claim 1, characterized in that the alloy of copper (Cu) and nickel (Ni) contains iron (Fe), the mass percentage of iron in said alloy not exceeding 20%.

3. Cermet material according to claim 1, characterized in that the oxide phase further contains at least one metal (M) selected from aluminum (Al), cobalt (Co), chromium (Cr), copper (Cu), manganese (Mn), titanium (Ti), zirconium (Zr), tin (Sn), vanadium (V), niobium (Nb), tantalum (Ta), yttrium (Y), and hafnium (Hf).

4. Cermet material according to claim 1, characterized in that said oxide phase comprises: a monoxide phase of composition Ni.sub.xM.sub.yFe.sub.1-x-yO with the following proportions by mass: 0.3%Ni17%, 60%Fe78%, 0M10%, and/or a nickel ferrite oxide phase of composition Ni.sub.xM.sub.yFe.sub.3-x-yO.sub.4 with the following proportions by mass: 0.2%Ni13%, 60%Fe72%, 0M8%, M being a metal selected from aluminum (Al), cobalt (Co), chromium (Cr), copper (Cu), manganese (Mn), titanium (Ti), zirconium (Zr), tin (Sn), vanadium (V), niobium (Nb), tantalum (Ta), yttrium (Y), hafnium (Hf) or a combination of these metals.

5. Cermet material according to claim 4, characterized in that when said oxide phase comprises a nickel ferrite oxide phase, said nickel oxide ferrite phase is of composition Ni.sub.xM.sub.yFe.sub.3-x-yO.sub.4 with the following mass proportions: 0.2%Ni10%, 63%Fe72%, 0M4%.

6. Cermet material according to claim 4, characterized in that when the oxide phase of the cermet material comprises a monoxide phase, said monoxide phase is of composition Ni.sub.xM.sub.yFe.sub.1-x-yO with the following mass proportions: 0.3%Ni13%, 65%Fe78%, 0M4%.

7. Cermet material according to claim 1, characterized in that the metallic phase further comprises at least one rare earth element selected from yttrium (Y), cerium (Ce), lanthanum (La) and neodymium (Nd).

8. Cermet material obtained after a pre-oxidation treatment of a cermet material according to claim 1.

9. Cermet material according to claim 8, characterized in that the pre-oxidation treatment is carried out in air between 900 C. and 1000 C. for a time between 2 and 10 hours.

10. Cermet material, comprising a cermet material according to claim 2 which is coated completely or partially with a protective layer of composition Ni.sub.0.9M.sub.yFe.sub.2,1-yO.sub.4.

11. Cermet material according to claim 10, characterized in that a thickness of the protective layer is between 15 and 30 m.

12. (canceled)

13. Electrode comprising a metallic core covered wholly or partially by at least one layer comprising a cermet material comprising: 50% to 90% of a metallic phase which contains at least one alloy of copper (Cu) and nickel (Ni), said alloy comprising as percentages by mass; 35% to 75% of nickel, 25% to 65% of copper, 10% to 50% of an oxide phase containing at least iron, nickel and oxygen with the following proportion by mass of nickel: 0.2%Ni17%.

14. Electrode according to claim 13, characterized in that said metallic core comprises at least one alloy of nickel (Ni) and iron (Fe), with proportions by mass of Ni and Fe being the following: 40%Ni85%, 15%Fe60%.

15. Electrode according to claim 14, characterized in that said metallic core further comprises copper (Cu) in the following mass proportion: 5%Cu40%.

16. (canceled)

17. Electrode according to claim 13, characterized in that the metallic core of the electrode material comprises at least one metal A chosen from aluminum (Al), cobalt (Co), chromium (Cr), manganese (Mn), molybdenum (Mo), titanium (Ti), zirconium (Zr), tin (Sn), vanadium (V), niobium (Nb), tantalum (Ta), and hafnium (Hf) or a combination of these metals, the proportion by mass of metal A in the metallic core being as follows: 0.5%A30%.

18. (canceled)

19. Electrode according to claim 13, characterized in that the metallic core further comprises at least one rare earth element selected from yttrium (Y), cerium (Ce), lanthanum (La ) and neodymium (Nd).

20. Electrode according to claim 13, characterized in that said layer comprising the cermet material is an intermediate layer arranged between the metallic core and a layer of oxide-rich cermet material or pure oxide.

21. (canceled)

22. (canceled)

23. Method of manufacturing a cermet material comprising: 50% to 90% of a metallic phase which contains at least one alloy of copper (Cu) and nickel (Ni), said allow comprising as percentages by mass: (Ni), said alloy comprising as percentages by mass: 35% to 75% of nickel, 25% to 65% of copper, 10% to 50% of an oxide phase containing at least iron, nickel and oxygen with the following proportion by mass of nickel: 0.2%Ni17%; wherein the method comprises forming the cermet material using a powder metallurgy method or a thermal spraying technique, characterized in that said manufacturing method uses as raw materials comprising: iron in metallic form or as an alloy, and optionally copper and nickel in metallic form or as an alloy, an oxide selected from nickel ferrite oxides Ni.sub.xFe.sub.3-xO.sub.4, NiO, Fe.sub.2O.sub.3, CuO, Cu.sub.2O, CuFeCO.sub.2, the spinel of type Cu.sub.xFe.sub.3-xO.sub.4 with 0x1.

24. Manufacturing method according to claim 23, characterized in that between 30% and 100% of the copper is supplied in the form of an oxide.

25. Manufacturing method according to claim 23, characterized in that between 30% and 100% of the iron is supplied in the form of metallic iron.

26. Cermet material according to claim 1, characterized in that the mass percentage of the metallic phase is 60% to 80% and the mass percentage of the oxide phase is 20% to 40%.

27. Cermet material according to claim 1, characterized in that the percentage by mass of nickel is 40% to 60% and the percentage by mass of copper is 40% to 55% in the alloy of copper (Cu) and nickel (Ni).

28. Cermet material according to claim 2, characterized in that the percentage by mass of iron in the alloy of copper (Cu) and nickel (Ni) is between 2% and 15%.

29. Cermet material according to claim 5, characterized in that said nickel oxide ferrite phase has the following mass proportions: 0.2%Ni5%, 68%Fe72%, 0M4%.

30. Cermet material according to claim 6, characterized in that said monoxide phase has the following mass proportions: 0.3%Ni8%, 70%Fe78%, 0M4%.

31. Electrode according to claim 13, characterized in that the proportions by mass of Ni and Fe in the at least one alloy of nickel (Ni) and iron (Fe) are the following: 55%Ni80%, 20%Fe45%.

Description

DESCRIPTION OF THE FIGURES

[0164] FIG. 1 is a photograph of an observation by backscattered electron SEM of a portion at the core of a monolithic anode made up of a cermet material according to the invention after pre-oxidation treatment.

[0165] FIG. 2 is a photograph of an observation by backscattered electron SEM of a portion on the surface of the monolithic anode made up of a cermet material according to the invention shown in FIG. 1.

[0166] FIG. 3 is a photograph of an observation by backscattered electron SEM of a portion on the surface of the monolithic anode made up of a cermet material according to the invention which is shown in part in the photographs in FIGS. 1 and 2, after 96 hours of electrolysis at a current of 0.6 A/cm.sup.2.

[0167] FIG. 4 is a photograph of an observation by backscattered electron SEM of a portion on the surface of the monolithic anode made up of a cermet material according to the invention which is shown in part in the photographs in FIGS. 1 and 2, after 506 hours of electrolysis at a current of 0.8 A/cm.sup.2.

[0168] FIG. 5 is a graph showing the affected internal thickness of the monolithic anode shown in part in FIGS. 1 to 4 as a function of electrolysis time.

[0169] FIG. 6 is a graph showing the nickel content expressed in terms of the content x of the nickel ferrite oxide phases of composition Ni.sub.xAl.sub.yFe.sub.3-x-yO.sub.4 and monoxide of composition Ni.sub.xFe.sub.1-xO, and the content by mass of nickel in the metallic phase, as a function of the distance from the surface of the monolithic anode shown in part in FIGS. 1 to 4 after 211 hours of electrolysis.

[0170] FIG. 7 is a graph showing the nickel content expressed in terms of the content x of the nickel ferrite oxide phases of composition Ni.sub.xAl.sub.yFe.sub.3-x-yO.sub.4 and monoxide of composition Ni.sub.xFe.sub.1-xO, and the content by mass of nickel in the metallic phase, as a function of the distance from the surface of the monolithic anode shown in part in FIGS. 1 to 4 after 506 hours of electrolysis.

[0171] FIG. 8 is a graph showing the tracking of electrolysis potential over a period of 506 hours of the monolithic anode shown in part in FIGS. 1 to 4.

[0172] FIG. 9 is a photograph of an observation by backscattered electron SEM of a portion of the layer of a cermet material according to the invention that an anode made of a metallic core covered with said cermet material contains after sintering.

[0173] FIG. 10 is a photograph of an observation by backscattered electron SEM of the same portion of the layer of cermet material shown in FIG. 9 after 230 hours of electrolysis.

EXPERIMENTAL SECTION

[0174] Cermet materials according to the invention were prepared by mixing powders in the following mass proportions: [0175] 12% of NiFe.sub.2O.sub.4; [0176] 3% of Cu; [0177] 32% of CuO; [0178] 5% of Ni; [0179] 48% of an NiFe alloy (the mass contents of nickel and iron in this alloy were 50%).

[0180] Sintering was then carried out in argon at 1250 C. so as to obtain monolithic anodes consisting of a cermet material according to the invention.

[0181] Finally, the resulting monolithic anodes were subjected to a pre-oxidation treatment in air at 930 C. for 9 hours.

[0182] For all the experiments performed on said anodes, the electrolysis conditions were as follows: a cryolite bath with an initial cryolitic ratio of 2.2 and containing as mass percentages 5% of CaF.sub.2, and 7.5% of alumina. The cryolitic ratio is the ratio in molar percentages of NaF to AlF.sub.3.

[0183] The bath temperature was kept at 960 C. with a current of 0.6 A/cm.sup.2 to 0.8 A/cm.sup.2. The electrolysis potential was stable with a standard deviation of about 0.25V including variations in the metal layer throughout the testing period.

[0184] Observations of sections of anodes after electrolysis, coating and cutting showed that the inside of said anodes was basically unchanged, after 96 hours, 211 hours and 506 hours of electrolysis.

[0185] These observations demonstrate the excellent performance of the cermet material according to the invention when used as an inert anode during electrolysis, over quite remarkable time periods (up to 506 hours). These experiments have demonstrated the value of the cermet material according to the invention from an industrial standpoint.

[0186] FIG. 1 is a photograph of an observation by backscattered electron SEM of a portion at the core of a monolithic anode made up of a cermet material according to the invention which was obtained from the powder mixture and sintering described above and after pre-oxidation treatment.

[0187] FIG. 2 is a photograph of an observation by backscattered electron SEM of a portion on the surface of the monolithic anode shown in part in FIG. 1.

[0188] In the photographs in FIGS. 1 and 2, the different phases in the presence of the cermet material can be seen: [0189] the metallic phase 1 of nickel and copper alloy (white areas), [0190] the phase 2 of nickel ferrite oxide Ni.sub.xFe.sub.3-xO.sub.4 (dark gray areas), [0191] porosities 3 (black spots),

[0192] In addition, the photograph in FIG. 2 shows: [0193] a monoxide phase 5 of Ni.sub.xFe.sub.1-xO (light gray areas), [0194] a nickel ferrite phase 4 of composition Ni.sub.0.9Fe.sub.2.1O.sub.4 at the surface of the cermet material (dark gray areas) corresponding to the protective layer mentioned above, [0195] a copper-rich oxide phase 6.

[0196] As explained above, the nickel ferrite layer that forms on the surface of the cermet material according to the invention is particularly advantageous because it is adherent and coherent, which contributes to the excellent performance of said cermet material, even under the aggressive conditions found for example in a cryolite bath used during electrolysis for manufacturing aluminum.

[0197] In addition, the nickel ferrite layer will be continuously renewed during electrolysis, as is shown in the photographs in FIGS. 3 and 4, which are photographs of an observation by backscattered electron SEM of a portion of the monolithic anode which is shown in part in the photographs in FIGS. 1 and 2, after 96 hours and 506 hours of electrolysis respectively.

[0198] In FIGS. 3 and 4 the protective layer of nickel ferrite which adheres on the periphery of the anode can be made out. The thickness of this protective layer is about 20 to 30 m. Therefore after 96 hours and even 506 hours of electrolysis, the protective layer of nickel ferrite is still present on the surface of the anode with a substantially identical thickness.

[0199] FIG. 5 is a graph showing the affected internal thickness of the monolithic anode shown in part in FIGS. 1 to 4 over a period of 506 hours of electrolysis.

[0200] Affected internal thickness means the thickness in which the composition of the material is different from the composition at the core of the anode, the core of the anode corresponding to the initial composition of the cermet material before electrolysis.

[0201] Given the graph in FIG. 5, it is noted that the affected internal thickness changes linearly and only slightly at a rate of 12 m/hour over a period of 506 hours of electrolysis. This graph reflects the excellent stability of the cermet material according to the invention which is resistant to the aggressive conditions found in a cryolite bath during electrolysis.

[0202] The wear of the anode material is very low, less than 0.2 mm after 506 hours of electrolysis.

[0203] FIG. 6 is a graph showing, after 211 hours of electrolysis, the nickel content expressed in terms of the content x of the nickel ferrite oxide phases of composition Ni.sub.xAl.sub.yFe.sub.3-x-yO.sub.4 and monoxide of composition Ni.sub.xFe.sub.1-xO, and the content by mass of nickel in the metallic phase, as a function of the distance from the surface of the monolithic anode shown in part in FIGS. 1 to 4.

[0204] FIG. 7 is a graph showing, after 506 hours of electrolysis, the nickel content expressed in terms of the content x of the nickel ferrite oxide phases of composition Ni.sub.xAl.sub.yFe.sub.3-x-yO.sub.4 and monoxide of composition Ni.sub.xFe.sub.1-xO, and the content by mass of nickel in the metallic phase, as a function of the distance from the surface of the monolithic anode shown in part in FIGS. 1 to 4.

[0205] Given the graphs in FIGS. 6 and 7 it is noted that the profiles of the amounts of nickel in the oxide phases and in the metallic phase are similar but offset towards the core of the anode. There is therefore a movement of a certain amount of nickel from the metallic phase to the oxide phases, all the more deeply from the surface of the anode as the electrolysis time increases.

[0206] The composition of the anode core remains unchanged. The modification front of the cermet material tends to move slowly from the surface to the core of the anode and a stable composition plateau forms on the surface of the anode.

[0207] These two graphs in FIGS. 6 and 7 show that the anode made up of a cermet material according to the invention remains stable during electrolysis and is therefore perfectly suitable for industrial use.

[0208] As shown in the graph in FIG. 7, the composition of the oxide phase stabilizes on the surface of the anode with a higher nickel content, in particular a nickel ferrite phase Ni.sub.xAl.sub.yFe.sub.3-x-yO.sub.4 with x close to 0.9, making this layer more resistant to corrosion by the cryolite bath. The metallic nickel phase enters either the nickel ferrite, or the monoxide phase. Nickel dissolves only slightly in the cryolite bath, which confirms the graph in FIGS. 6 and 7.

[0209] FIG. 8 is a graph showing the tracking of electrolysis potential over a period of 506 hours of the monolithic anode shown in part in FIGS. 1 to 4. It can be seen that the anode's behavior is stable. The advance of the modification front does not affect the electrolysis potential, in particular because of the large amount of metallic phase in the cermet material which maintains high conductivity.

[0210] FIG. 9 is a photograph of an observation by backscattered electron SEM from the interface between the metallic core of composition Ni.sub.65Fe.sub.25Cu.sub.10 and a cermet material according to the invention of an anode which was obtained after sintering at 1200 C. The cermet material comprised 68% of nickel ferrite oxide of composition Ni.sub.0.04Fe.sub.2.96O.sub.4 and 32% of metal comprising 50% of Ni, 40% of Cu and 10% of Fe.

[0211] FIG. 10 is a photograph of an observation by backscattered electron SEM of this interface shown in FIG. 9 after 230 hours of electrolysis.

[0212] In FIGS. 9 and 10, phase 7 of the metallic core can be made out.

[0213] By comparing the two photographs in FIGS. 9 and 10, it is noted that the interface between the metallic core and the cermet material according to the invention of the anode is very similar and has therefore remained stable after 230 hours of electrolysis. The interface is cohesive and there was no infiltration of the bath after 230 hours of electrolysis. This demonstrates the stability during electrolysis of an anode made of a metallic core coated with a cermet material according to the invention.

[0214] Furthermore, no fluorine was identified at the interface. This means that the anode was not attacked by corrosion after 230 hours of electrolysis.

[0215] These experimental results also demonstrate the value from an industrial standpoint of the cermet material according to the invention when used as a coating for the metallic core of an electrode, and in particular an anode. Such an embodiment may be particularly advantageous to increase the service life of the anode because of the regeneration of the cermet material by means of migration of iron from the metallic core to the cermet material.