Method for manufacturing an electrolyte for solid oxide cells by magnetron cathode sputtering

Abstract

A method of manufacturing by magnetron cathode sputtering an electrolyte film for use in solid oxide cells (SOC). This method comprises the steps consisting of heating a substrate to a temperature ranging from 200° C. to 1200° C.; followed by subjecting the substrate to at least two treatment cycles, each treatment cycle comprising: 1) depositing one layer of a metal precursor on the substrate by magnetron cathode sputtering of a target made up of the metal precursor, the sputtering being carried out under elemental sputtering conditions; followed by 2) oxidation-crystallisation of the metal precursor forming the layer deposited on the substrate in the presence of oxygen to obtain the transformation of the metal precursor into the electrolyte material; and in that the substrate is kept at a temperature ranging from 200° C. to 1200° C. for the entire duration of each treatment cycle.

Claims

1. A method for manufacturing an electrolyte in the form of a film for solid oxide electrochemical cells, comprising the steps of: heating a substrate to a temperature ranging from 200° C. to 1,200° C.; then subjecting the substrate to at least two treatment cycles, each treatment cycle comprising: a) depositing one layer of a metal precursor on the substrate by reactive magnetron cathode sputtering of a target consisting of the metal precursor, the sputtering being carried out under elemental sputtering conditions; then b) oxidation-crystallisation of the metal precursor forming the layer deposited on the substrate in the presence of oxygen to obtain the transformation of the metal precursor into the electrolyte material; the substrate being kept at a temperature ranging from 200° C. to 1,200° C. for the duration of each treatment cycle; wherein each treatment cycle is carried out in a magnetron cathodic sputtering chamber in which the heated substrate and the target are located and in which a pressure prevails, and wherein each treatment cycle comprises the steps of: supplying the sputtering chamber with a plasma gas at a flow rate D.sub.P1; adjusting the pressure prevailing in the sputtering chamber to a value ranging from 0.1 Pa to 10 Pa; starting the sputtering of the target; supplying the sputtering chamber with oxygen at a flow rate D.sub.R1 lower than the flow rate D.sub.P1 while keeping the pressure prevailing in the sputtering chamber at the value ranging from 0.1 Pa to 10 Pa, in order to create elemental sputtering conditions; keeping the elemental sputtering conditions for a time sufficient to obtain the formation of the layer of the metal precursor on the substrate; stopping the sputtering of the target; increasing the flow rate D.sub.R1 to obtain a supply flow rate of the sputtering chamber with oxygen D.sub.R2, stopping the supplying of the sputtering chamber with plasma gas or reducing the flow rate D.sub.P1 to obtain a supply flow rate of the sputtering chamber with plasma gas D.sub.P2 at most equal to ½ of D.sub.R2 and adjusting the pressure prevailing in the sputtering chamber to a value at least equal to 10 Pa, to create oxidation-crystallisation conditions of the metal precursor forming the layer deposited on the substrate; keeping the oxidation-crystallisation conditions for a time sufficient to obtain the transformation of the metal precursor forming the layer deposited on the substrate into the electrolyte material by oxidation-crystallisation of the metal precursor; and stopping the supplying of the sputtering chamber with oxygen; whereby the electrolyte for solid oxide electrochemical cells is obtained.

2. The method of claim 1, wherein the substrate is heated and kept at a temperature ranging from 450° C. to 850° C. for the duration of each treatment cycle.

3. The method of claim 1, wherein the substrate is kept at a constant temperature for the duration of each treatment cycle.

4. The method of claim 1, wherein the flow rate D.sub.P1 is comprised between 1 sccm and 500 sccm.

5. The method of claim 1, wherein the flow rate D.sub.R1 is at most equal to 100 sccm.

6. The method of claim 1, wherein the flow rate D.sub.R2 is comprised between 10 sccm to 500 sccm.

7. The method of claim 1, wherein the elemental sputtering conditions are kept for between 1 minute and 120 minutes and the oxidation-crystallisation conditions are kept for between 1 minute and 60 minutes.

8. A method for manufacturing an electrolyte in the form of a film for solid oxide electrochemical cells, comprising the steps of: heating a substrate to a temperature ranging from 200° C. to 1,200° C.; then subjecting the substrate to at least two treatment cycles, each treatment cycle comprising: a) depositing one layer of a metal precursor on the substrate by reactive magnetron cathode sputtering of a target consisting of the metal precursor, the sputtering being carried out under elemental sputtering conditions; then b) oxidation-crystallisation of the metal precursor forming the layer deposited on the substrate in the presence of oxygen to obtain the transformation of the metal precursor into the electrolyte material; the substrate being kept at a temperature ranging from 200° C. to 1,200° C. for the duration of each treatment cycle; wherein each treatment cycle is carried out in two different chambers, the depositing being carried out in a first chamber which is a magnetron cathodic sputtering chamber in which elemental sputtering conditions prevail, and the oxidation-crystallisation being carried out in a second chamber in which oxidation-crystallisation conditions prevail; wherein each treatment cycle further comprises a transfer of the substrate from the first chamber to the second chamber; wherein the elemental sputtering conditions comprise a pressure prevailing in the first chamber comprised between 0.1 Pa and 10 Pa, a supplying of the first chamber with a plasma gas at a flow rate D.sub.P1 and a supplying of the first chamber with oxygen at a flow rate D.sub.R1 lower than the flow rate D.sub.P1; and wherein the oxidation-crystallisation conditions comprise a pressure prevailing in the second chamber at least equal to 10 Pa and a supplying of the second chamber with oxygen at a flow rate D.sub.R2 greater than the flow rate D.sub.R1; whereby the electrolyte for solid oxide electrochemical cells is obtained.

9. The method of claim 8, wherein the flow rate D.sub.P1 is comprised between 1 sccm and 500 sccm.

10. The method of claim 9, wherein the flow rate D.sub.R1 is at most equal to 100 sccm.

11. The method of claim 8, wherein the flow rate D.sub.R2 is comprised between 10 sccm to 500 sccm.

12. The method of claim 8, wherein the substrate is kept in the first chamber for between 1 minute and 120 minutes and is kept in the second chamber for between 1 minute and 60 minutes.

13. The method of claim 1, comprising from 2 to 20 treatment cycles.

14. The method of claim 1, wherein the target comprises a mixture M-M′ wherein M is zirconium or cerium and M′ is yttrium, scandium or a lanthanide.

15. The method of claim 14, wherein the target comprises a mixture of zirconium and yttrium.

16. The method of claim 8, wherein the substrate is heated and kept at a temperature ranging from 450° C. to 850° C. for the duration of each treatment cycle.

17. The method of claim 8, comprising from 2 to 20 treatment cycles.

18. The method of claim 8, wherein the target comprises a mixture M-M′ wherein M is zirconium or cerium and M′ is yttrium, scandium or a lanthanide.

19. The method of claim 18, wherein the target comprises a mixture of zirconium and yttrium.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) FIG. 1A, already commented on, diagrammatically shows the operating principle of a SOFC.

(2) FIG. 1B, already commented on, diagrammatically shows the operating principle of a SOEC.

(3) FIG. 2 diagrammatically shows an embodiment of the method of the invention wherein each treatment cycle is carried out in a single chamber which is a magnetron sputtering.

(4) FIG. 3A shows the pressure (curve P), current (curve C) and temperature (curve T) curves as a function of time, noted as t and expressed in min, such as measured during the implementation of the method of the invention according to the mode shown in FIG. 2; in this figure, the pressure values, noted as P and expressed in Pa, and the current values, noted as C and expressed in A, are indicated on the left y-axis while the temperature values, noted as T and expressed in ° C., are indicated on the right y-axis.

(5) FIG. 3B is an enlargement of the portion located between the dotted line in FIG. 3A; in this figure, are shown the pressure (curve P), argon flow rate (curve D.sub.P) and oxygen flow rate (curve D.sub.R) curves as a function of time, noted as t and expressed in min, such as measured on this zone; in this figure, the pressure values, noted as P and expressed in Pa, are indicated on the left y-axis while the flow rate values, noted as D and expressed in sccm, are indicated on the right y-axis.

(6) FIGS. 4A and 4B are images taken with the scanning electron microscope (or SEM), at two different magnifications, of an YSZ film manufactured by the method of the invention on a silicon wafer, FIG. 4A being an image of the film seen as a cross-section and FIG. 4B being an image of the film seen from above.

(7) FIGS. 5A and 5B are images taken with the SEM, at two different magnifications, of a YSZ film carried out by conventional deposition by magnetron sputtering on a silicon wafer, FIG. 5A being an image of the film seen as a cross-section and FIG. 5B being an image of the film seen from above.

(8) FIGS. 6A and 6B are images taken with the SEM, at two different magnifications, of a YSZ film manufactured by the method of the invention on a film of a porous electrode material NiO-YSZ, FIG. 6A being an image of the film seen as a cross-section and FIG. 6B being an image of the film seen from above.

(9) FIGS. 7A and 7B are images taken with the SEM, at two different magnifications, of a YSZ film carried out by conventional deposition by magnetron sputtering on a film of a porous electrode material NiO-YSZ, FIG. 7A being an image of the film seen as a cross-section and FIG. 7B being an image of the film seen from above.

(10) FIG. 8 shows in the form of a bar chart the leakage rate, noted as Q and expressed in hPa.Math.L/s, such as obtained in a nitrogen permeability test for a YSZ film manufactured by the method of the invention (bar on the right) and for a YSZ film of the same thickness carried out by a conventional deposition by magnetron sputtering (bar on the left).

(11) FIG. 9 shows the optical transmission, noted as R and expressed in %, for a wavelength, noted as X and ranging from 350 nm to 1,000 nm, such as obtained for YSZ films manufactured by the method of the invention (curves C, D and E), for a YSZ film carried out by a conventional deposition by magnetron sputtering (curve A), for a film prepared by a method that comprises only one treatment cycle (curve B) for a completely oxidised YSZ film (curve F).

DETAILED DISCLOSURE OF PARTICULAR EMBODIMENTS

I—Block Diagram of an Embodiment

(12) Reference is made to FIG. 2 which diagrammatically shows an embodiment of the method according to the invention wherein each treatment cycle to which the substrate is subjected is carried out in the sputtering chamber of a PVD deposition machine by magnetron cathode sputtering.

(13) As can be seen in this figure, the first step of the method consists of heating the substrate, fastened beforehand onto the substrate holder of the sputtering chamber, to bring the temperature of this substrate to a temperature ranging from 200° C. to 1,200° C. and, preferably, from 450° C. to 850° C. This heating can be, for example, carried out by means of a heating module with which the sputtering chamber is provided.

(14) Once the desired temperature of the substrate is reached, then begins the first treatment cycle of the substrate with successively and in this order:

(15) a) supplying the sputtering chamber with a plasma gas at a flow rate D.sub.P1, the plasma gas being typically a neutral gas, preferably argon, and the flow rate D.sub.P1 being, preferably, comprised between 1 sccm and 500 sccm and, even better, between 5 sccm and 50 sccm;

(16) b) adjusting the pressure prevailing in the sputtering chamber to a value ranging from 0.1 Pa to 10 Pa, with this adjustment being carried out by means of the pumping system with which the sputtering chamber is provided;

(17) c) starting the sputtering of the target formed from the metal precursor that is to be deposited onto the substrate by starting the electric generator associated with the target;

(18) d) supplying the chamber with oxygen at a flow rate D.sub.R1 lower than the flow rate D.sub.P1, the flow rate D.sub.R1 being, preferably, at most equal to 100 sccm and, even better, at most equal to 10 sccm, while keeping the pressure prevailing in the chamber at the value to which it was adjusted in step b), in such a way as to institute elemental sputtering conditions in the sputtering chamber;

(19) e) keeping elemental sputtering conditions for a time that is sufficient to obtain the formation of the layer of the metal precursor on the substrate, this time able to range from 1 min to 120 min according to the nature of the metal precursor and of the thickness of the layer that it sought to be deposited;

(20) f) stopping the sputtering of the target;

(21) g) increasing the supply flow rate D.sub.R1 of the sputtering chamber with oxygen—this flow rate being, preferably, brought to a value D.sub.R2 ranging from 10 sccm to 500 sccm and, even better, ranging from 20 sccm to 200 sccm—, stopping the supplying of the sputtering chamber with plasma gas or reducing the flow rate D.sub.P1 in order to obtain a supply flow rate of the sputtering chamber with plasma gas D.sub.P2 at most equal to half of D.sub.R2 and, preferably, at most equal to one-tenth of D.sub.R2, and adjusting the pressure prevailing in the sputtering chamber to a value at least equal to 10 Pa, typically from 10 Pa to 500 Pa, in order to create oxidation-crystallisation conditions of the metal precursor forming the layer deposited on the substrate, the adjusting of the pressure being carried out by reducing, even stopping the pumping;

(22) h) keeping the oxidation-crystallisation conditions for a time that is sufficient to transform the metal precursor forming the layer deposited on the substrate into the electrolyte material by oxidation-crystallisation of the metal precursor, this time being able to range from 1 min to 60 min; and

(23) i) stopping the supplying of the sputtering chamber with oxygen.

(24) After a waiting time, typically, from 30 s to 600 s, in order to allow for a deoxidation of the target, the substrate is subjected to a new treatment cycle identical to the one that has just been described, and this, one or more times, preferably, from 1 to 20 times according to the thickness that the electrolyte film must have in the end, this thickness ranging typically from 1 μm to 10 μm, and/or according to the stoichiometry in oxygen or the degree of oxidation-crystallisation that is sought to be conferred to this film (cf. point II.2 hereinafter).

(25) As known per se, a protective cover is advantageously disposed between the target and the substrate so as to protect this substrate during all the steps other than the step of deposition of the layer of the precursor material on the substrate, i.e. the step e) of each cycle.

II—Experimental Validation

(26) II.1—Manufacture of YSZ Films:

(27) YSZ films are manufactured by applying the embodiment described in point I hereinabove in a magnetron sputtering chamber with a volume of 35 litres (APRIM VIDE COMPANY) provided with a 4-inch magnetron, a Pinnacle™+(ADVANCED ENERGY) pulse generator and a metal target comprised of zirconium and yttrium in a mass ratio of 82/18 and purity equal to 99.9%.

(28) The chamber is connected via a slide valve (HVA) to a pumping system comprised of a dry pump XDS-5 (EDWARDS Limited) and of a turbo-molecular pump ATH 400M (ADIXEN).

(29) The gas flow rates are controlled by mass flow rate regulators (BRONKHORST) and the pressure prevailing in the sputtering chamber is measured by a Pirani/cold cathode (ALCATEL ACC 1009) combined gauge and a capacitive (PFEIFFER CCR 375) gauge.

(30) The substrates used are, on the one hand, silicon wafers and, on the other hand, films of a porous electrode material that are films of a yttria-(or NiO-YSZ)-doped nickel and zirconia cermet manufactured conventionally by tape casting and screen printing.

(31) Each one of these substrates is subjected to 3 treatment cycles by using the following conditions:

(32) For the Deposition of the Layers of ZrY (Step e): Temperature at the substrate holder: 550° C.±10° C. Distance between the target and the substrate: 127 mm Intensity of the discharge current on the target: 1.25 A Voltage of the discharge on the target: −205 V-−160 V Pulses: 5 μs at 50 kHz Argon flow rate (D.sub.P1): 20 sccm Oxygen flow rate (D.sub.R1): 1 sccm Pressure prevailing in the chamber: 1 Pa Deposition time: 20 min for each layer, which is a total deposition time of 60 min for the 3 layers

(33) For the Oxidation-Crystallisation of the Layers of ZrY (Step h): Temperature at the substrate holder: 550° C.±10° C. Argon flow rate: 0 sccm Oxygen flow rate (D.sub.R2): 15 sccm Pressure prevailing in the chamber: 10 Pa Oxidation-crystallisation time: 5 min for each layer, i.e. a total crystallisation-oxidation time of 15 min for the 3 layers.

(34) FIG. 3A shows the curves of pressure in Pa (curve P), of current in A (curve C) and of temperature in ° C. (curve T) as a function of time, noted as t and expressed in min, such as measured over the 250 min following the starting of the heating of the substrate (which corresponds to the point 0 of the x-axis). Note that, as this figure shows, the heating of the substrate has been stopped and the pressure prevailing in the sputtering chamber brought to 1 Pa at the end of the third treatment cycle.

(35) FIG. 3B is an enlargement of the portion located between the dotted line in FIG. 3A, which shows the curves of pressure in Pa (curve P), of argon flow rate in sccm (curve D.sub.P) and of oxygen flow rate in sccm (curve D.sub.R) as a function of time, expressed in minutes, such as measured on this zone. In this figure, the drop in pressure that can be seen in the dotted circle corresponds to a consumption of oxygen by the metal film deposited beforehand and proves that the metal film is in the process of oxidation

(36) The films thus manufactured are subjected to examinations under a scanning electron microscope and the images obtained are compared with those obtained for YSZ films that were manufactured by using the same types of substrates but by proceeding with a conventional depositing by magnetron sputtering. This conventional depositing was carried out in the following elemental sputtering conditions:

(37) Temperature at the substrate holder: not controlled and <200° C.

(38) Distance between the target and the substrate: 127 mm

(39) Intensity of the discharge current on the target: 1.25 A

(40) Voltage of the discharge on the target: ≈−230 V

(41) Pulses: 5 μs at 50 kHz

(42) Argon flow rate: 20 sccm

(43) Oxygen flow rate: 3 sccm

(44) Pressure prevailing in the chamber: 1 Pa

(45) Deposition time: 60 min.

(46) The SEM images of the films manufactured by the method of the invention are shown in FIGS. 4A and 4B on the one hand, and 6A and 6B on the other hand, while the SEM images of the films manufactured by conventional deposition are shown in FIGS. 5A and 5B on the one hand, and 7A and 7B on the other hand.

(47) As these figures show, a continuous columnar structure over the entire thickness is observed for the films manufactured by conventional deposition, while, in the case of the films manufactured by the method of the invention, the deposition/oxidation-crystallisation steps can clearly be distinguished (cf. FIGS. 4A and 4B).

(48) In addition, the films manufactured by the method of the invention have a crystallised structure with the presence of many facets (cf. in particular FIG. 6B) that are not found in the films manufactured by conventional deposition.

(49) Moreover, the permeability to the gases of the films is appreciated by a nitrogen permeability test consisting in imposing an initial pressure difference of 3.Math.10.sup.4 Pa (300 mbar) between the anode and cathode sides of electrochemical cells measuring 60 mm in diameter and having for electrolyte one of these films and in measuring the drop in pressure over time.

(50) The results of this test are shown in FIG. 8 which shows the leakage rate, noted as Q and expressed as hPa.Math.L/s, such as obtained for two films of YSZ each measuring 1.75 μm thick: one film that was manufactured by the method of the invention (bar on the right) and one film that was manufactured by conventional depositing (bar on the left).

(51) As can be seen in this figure, the leakage rate is 10.sup.−2 hPa.Math.L/s for the film that was manufactured by conventional depositing while this leakage rate is only 6.66.Math.10.sup.−4 hPa.Math.L/s for the film that was manufactured by the method of the invention.

(52) The leakage rate obtained for the film that was manufactured by the method of the invention is close to that which typically has a film of YSZ electrolyte for SOC 8 μm thick obtained by screen printing (10.sup.−4 hPa.Math.L/s).

(53) II.2 Demonstration of the Influence of the Number of Cycles on the Degree of Oxidation-Crystallisation of the Films:

(54) Given that the optical transmission of a film is characteristic of its degree of oxidation-crystallisation, the optical transmission is measured between 350 nm and 1,000 nm for YSZ films that were manufactured by the method of the invention as described in point II hereinabove but by subjecting the substrates to 2, 3 or 4 treatment cycles.

(55) For the purposes of comparison, the optical transmission is also measured in the same wavelength range: for a YSZ film that was manufactured by a conventional deposition by magnetron sputtering such as described in point II.1 hereinabove, for a YSZ film that was manufactured by a method that is differentiated from the method of the invention in that it comprises only a single treatment cycle, and for an entirely oxidised YSZ film, this film was manufactured by a method that is differentiated from the method of the invention in that is comprises only a single treatment cycle, and then thermally treated in an oven under air at 500° C. for 1 min.

(56) The results of these measurements are shown in FIG. 9 which shows the change in the optical transmission, noted as R and expressed in %, according to the wavelength, noted as X and expressed in nm. In this figure, curve A corresponds to the film that was manufactured by conventional deposition; curve B corresponds to the film that was manufactured by the method that comprises only one treatment cycle; curves C, D and E correspond to the films that were manufactured by the method of the invention, while curve F corresponds to the entirely oxidised film.

(57) As this figure shows, the optical transmission and, therefore, the degree of oxidation-crystallisation of the films that were prepared by the method of the invention (curves C, D and E) is much greater than that of the film that was manufactured by the method that comprises only one treatment cycle (curve B) which itself is much greater than that of the film that was manufactured by conventional deposition (curve A).

(58) Moreover, the optical transmission and, therefore, the degree of oxidation-crystallisation of the films that were manufactured by the method of the invention increases with the number of treatment cycles implemented in this method.

(59) It is therefore possible to confer to an electrolyte film the stoichiometry in oxygen and/or the degree of oxidation desired by adjusting the number of treatment cycles implemented in the method of the invention just as it is possible to adapt the number of treatment cycles according to the tendency that the material forming the film can have to delaminate.

REFERENCES MENTIONED

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