Interconnector for a stack of solid oxide cells of the SOEC/SOFC type including different elements in relief

Abstract

An interconnector for a stack of solid oxide cells of the SOEC/SOFC type, intended to be arranged between two adjacent electrochemical cells, which includes a flat face whereon at least one first group of identical first elements in relief and a second group of identical second elements in relief are formed, the first elements in relief having different geometric features with respect to the second elements in relief, the height of each first element in relief being different from the height of each second element in relief, the contact width of each first element in relief being different from the contact width of each second element in relief.

Claims

1. An interconnector for a stack of solid oxide cells of the SOEC/SOFC type operating at high temperature, intended to be arranged between two adjacent electrochemical cells of the stack, each electrochemical cell being formed of a cathode, of an anode and of an electrolyte intercalated between the cathode and the anode, the interconnector including: a flat face whereon at least one first group of identical first elements in relief with respect to the flat face and a second group of identical second elements in relief with respect to the flat face are formed, the first elements in relief having different geometric features with respect to the second elements in relief, the height of each first element in relief, measured as being the largest vertical dimension of the first element in relief with respect to the flat face, being different from the height of each second element in relief, measured as being the largest vertical dimension of the second element in relief, the contact width of each first element in relief, measured as being the largest horizontal dimension with respect to the flat face of the outer contact end of each first element in relief, opposite the inner end in contact with the flat face and intended to be in contact with an electrochemical cell, being different from the contact width of each second element in relief, measured as being the largest horizontal dimension of the with respect to the flat face of the outer contact end of each second element in relief, opposite the inner end in contact with the flat face and intended to be in contact with an electrochemical cell; and a metal alloy substrate having two main flat faces, one of the main flat faces comprising a first coating layer forming a first contact layer with an electrochemical cell, the other of the main flat faces comprising a second coating layer forming a second contact layer with an electrochemical cell, the first coating layer and/or the second coating layer comprising a flat face and elements in relief formed thereon.

2. The interconnector according to claim 1, wherein the contact width of each first element in relief is between 0.5 and 5 mm, preferably equal to 1 mm.

3. The interconnector according to claim 1, wherein the contact width of each second element in relief is between 0.005 mm and 0.5 mm, preferably equal to 100 m.

4. The interconnector according to claim 1, wherein the height of each first element in relief is between 200 m and 1,000 m, preferably equal to 350 m.

5. The interconnector according to claim 1, wherein the height of each second element in relief is between 250 m and 1,050 m, preferably equal to 400 m.

6. The interconnector according to claim 1, wherein the difference between the height of each second element in relief and the height of each first element in relief is between 5 m and 500 m, particularly in the order of 50 m.

7. The interconnector according to claim 1, further including a number N, N being a whole number greater than or equal to 2, preferably between 2 and 50, also preferably equal to 5, groups of elements in relief, formed on the flat face, the elements in relief of the same group all being identical, and the elements in relief of different groups having different geometric features, namely different heights and different contact widths.

8. The interconnector according to claim 1, wherein the elements in relief are in the form of teeth or grooves, disposed parallel with one another, the spaces between the elements in relief forming gas circulation channels.

9. The interconnector according to claim 1, wherein the elements in relief are in the form of pads, particularly of cylindrical shape, the spaces between the elements in relief forming a single serpentine gas circulation channel.

10. The interconnector according to claim 1, wherein the elements in relief are evenly distributed over the flat face, being particularly spaced the same distance from one another, particularly between 50 m and 5 mm, preferably equal to 750 m, according to at least one horizontal direction (DH) on the flat face.

11. The interconnector according to claim 1, wherein at least one area of the flat face, particularly a central area, is devoid of elements in relief.

12. The interconnector according claim 1, wherein the elements in relief having the largest width are located around the periphery of the flat face, at a distance from the other elements in relief and from the gas circulation channel(s) formed by the spaces between the other elements in relief.

13. The interconnector according to claim 1, wherein the metal alloy substrate is of the chromia-forming type the base element of which is iron (Fe) or nickel (Ni), and in that the elements in relief formed on said flat face are formed by machining.

14. The interconnector according claim 1, wherein the first coating layer is a thick ceramic coating layer, porous or not, the ceramic material being particularly selected from a lanthanum manganite of formula La.sub.1-xSr.sub.xMO.sub.3 with M (transition metals)=nickel (Ni), iron (Fe), cobalt (Co), manganese (Mn), chrome (Cr), alone or a mixture thereof, or materials of lamellar structure such as the lanthanide nickelates of formula Ln.sub.2NiO.sub.4 (Ln=lanthanum (La), neodymium (Nd), praseodymium (Pr)), or another electrically conductive perovskite oxide.

15. The interconnector according claim 1, wherein the second coating layer is a thick metal coating layer, the metal material being particularly selected from nickel (Ni) and its alloys or chromia-forming alloys the base element of which is iron (Fe).

16. A stack of solid oxide cells of the SOEC/SOFC type operating at high temperature, including a plurality of electrochemical cells each formed of a cathode, of an anode and of an electrolyte intercalated between the cathode and the anode, and a plurality of interconnectors according to claim 1, each arranged between two adjacent electrochemical cells.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention may be better understood upon reading the following detailed description, non-limiting examples of implementation thereof, as well as upon examination of the figures, schematic and partial, of the drawing appended, wherein:

(2) FIG. 1 is a schematic view showing the operating principle of a high temperature solid oxide electrolyser (SOEC),

(3) FIG. 2 is an exploded schematic view of a portion of a high temperature solid oxide electrolyser (SOEC) comprising interconnectors according to the prior art,

(4) FIG. 3 is a schematic front view of an interconnector according to the prior art of a stack of a high temperature solid oxide electrolysis cell (SOEC) or of a fuel cell (SOFC) operating at high temperature,

(5) FIG. 3A is a detailed sectional view of an interconnector according to [FIG. 3],

(6) FIG. 3B is a view similar to that of [FIG. 3] showing the current lines passing through the interconnector,

(7) FIG. 4 illustrates, in graph form, the height, expressed in mm, of the teeth of an interconnector depending on the length, expressed in mm, for a configuration before clamping and a configuration after clamping of a stack of a high temperature electrolysis cell (SOEC) or of a high temperature fuel cell (SOFC),

(8) FIG. 5 shows, graphically, the polarisation curves for three different configurations with two different teeth geometries and two different clamping forces,

(9) FIG. 6 is a sectional view of two teeth and of a channel of a conventional interconnector of a stack of high temperature solid oxide cells of the SOEC/SOFC type,

(10) FIG. 7 is a sectional view of five teeth and of four channels of an interconnector in accordance with the invention for a stack of high temperature solid oxide cells of the SOEC/SOFC type, before clamping,

(11) FIG. 8 is a sectional view of the configuration of [FIG. 7], after clamping,

(12) FIG. 9 is a top view of the configuration of [FIG. 7] and [FIG. 8],

(13) FIG. 10 is an alternative embodiment of the configuration of [FIG. 7],

(14) FIG. 11 is a top view of the configuration of [FIG. 10],

(15) FIG. 12 is an alternative embodiment of the configuration of [FIG. 11],

(16) FIG. 13 is a geometric alternative embodiment of the configuration of [FIG. 9],

(17) FIG. 14 is an alternative embodiment of the configuration of [FIG. 13], and

(18) FIG. 15 shows, in perspective and by observation from the top, an assembly comprising a stack of solid oxide cells of the SOEC/SOFC type with interconnectors in accordance with the invention and a clamping system of the stack.

(19) In all of these figures, identical references may designate identical or similar elements.

(20) In addition, the various portions shown in the figures are not necessarily according to a uniform scale, to make the figures more readable.

DETAILED DESCRIPTION OF PARTICULAR EMBODIMENTS

(21) FIGS. 1 and 2 have already been described above in the part relating to the prior art. It is specified that, for FIGS. 1 and 2, the symbols and the H.sub.2O steam supply arrows, for distributing and recovering dihydrogen H.sub.2, oxygen O.sub.2, air and electric current, are shown for the purposes of clarity and accuracy, to illustrate the operation of the devices shown.

(22) Furthermore, it should be noted that all of the constituents (anode/electrolyte/cathode) of a given electrochemical cell are preferably ceramics. The operating temperature of a stack of the high temperature SOEC/SOFC type is moreover typically between 600 and 1,000 C.

(23) In addition, the possible terms upper and lower are to be understood here according to the normal direction of orientation of a stack of the SOEC/SOFC type when in its use configuration.

(24) An interconnector 5 may have a particular geometry, particularly grooved with the presence of teeth and channels. For example, as described in the French patent application FR 2 996 065 A1, the interconnector 5 may consist of a component including a metal alloy substrate, particularly of the chromia-forming type, the base element of which is iron or nickel, in particular, ferritic steels of the Uginox K41 type or of the VDM Crofer type, this substrate having two main flat faces, one of the faces being coated with a coating including a thick ceramic layer, grooved by delimiting channels for the distribution and/or the collection of gases and teeth, this layer also being referred to as contact layer. Thus, the teeth and channels may be formed on the contact layer. In addition, in the following description, it is understood that the teeth and channels, or more generally the reliefs, of an interconnector 5 may be formed of a contact layer of this interconnector 5.

(25) FIGS. 3, 3A and 3B show an interconnector 5 commonly used in a stack of the high temperature SOEC/SOFC type. The conveyance or the collection of the current at the electrode is performed by the teeth 10, or ribs, which are in direct mechanical contact with the electrode concerned. The conveyance of steam at the cathode or of draining gas at the anode in an HTE electrolyser, the conveyance of dioxygen at the cathode or of hydrogen at the anode in an SOFC cell is symbolised by the arrows F1 that can be seen in FIG. 3.

(26) The collection of the hydrogen produced at the cathode or of the oxygen produced at the anode in an HTE electrolyser, the collection of the water produced at the cathode or of the excess hydrogen at the anode in an SOFC cell is carried out by the channels 11 that open into a fluidic connection, commonly known as manifold, which is common to the stack of cells. The structure of these interconnectors 5 is made to achieve a compromise between the two functions of conveyance and of collection (gas/current).

(27) To obtain a good electrical conductivity between an interconnector 5, particularly the contact layer, and an electrochemical cell, the teeth 10 should be spaced fairly close together. However, this then tends to have a small passage surface for the gases, which may lead to significant pressure losses during the operation.

(28) Moreover, the interconnector 5 should make it possible to correctly circulate the gases and to have low pressure losses, which may be achieved using wide channels 11. Nevertheless, this leads to having teeth 10 spaced apart from one another, which is detrimental to the electrical conductivity.

(29) Furthermore, the geometry of the teeth 10 and of the channels 11 should make it possible to accommodate the surface defects, particularly of the cells and of the interconnector 5. For this, they must be able to crush easily. This may, for example, be obtained by producing narrow teeth 10. However, if the teeth 10 crush significantly, the height of the channels 11 will reduce significantly and the passage surface for the gases will thereby be reduced, which will lead to greater pressure losses. By way of example, FIG. 4 illustrates, in graph form, the height H, expressed in mm, of the teeth 10 depending on the length L, expressed in mm, for a configuration before clamping C1 and a configuration after clamping C2.

(30) The force applied to a stack of solid oxide cells of the SOEC/SOFC type, or stack, makes it possible to calculate the local clamping stress. Thus, if a force F of 1,000 N is applied and the bearing surface S is 100 cm.sup.2, then the F/S stress will be 0.2 MPa. If the contact is carried out using the teeth 10 of an interconnector 5, particularly of its contact layer, which represent half of the surface, then the local stress will be 0.4 MPa.

(31) Three real experiments (E1, E2, E3) for producing hydrogen from a stack of the SOEC type with five cells of surface area of 100 cm.sup.2 were performed with two geometries (tooth A and tooth B) of interconnectors and two different clamping forces (force A and force B). A total flow of 12 Nml/min/cell/cm.sup.2 of mixture of steam and of hydrogen were sent. The H.sub.2O/H.sub.2 mixture is 90% of H.sub.2O and 10% of H.sub.2. The temperature of the stack is 800 C.

(32) A polarisation curve (E1 for Tooth A, Force A; E2 for Tooth B, Force A; E3 for Tooth B, Force B) is carried out each time by progressively increasing the current i, expressed in A/cm.sup.2, and by measuring the voltage E, expressed in V, of associated cells. These curves make it possible to measure the maximum use rate t of steam as well as the Area Specific Resistance (ASR) coming from the cells, interconnectors, interfaces, connection systems, etc.

(33) The reference interconnector geometry includes an interconnector, particularly a contact layer, with teeth 10 of width A (tooth A). A second interconnector geometry was produced with teeth 10 of width B (tooth B), three times narrower than the width A. The force applied may be the reference force A (force A) or the force B, three times less than the force A.

(34) FIG. 5 illustrates, in graph form, the polarisation curves obtained E1, E2, E3 for three stacks with two interconnector geometries (Tooth A, Tooth B) and two different forces (Force A, Force B). In addition, Table 1 below shows the relative pressure losses obtained for the O.sub.2 chamber.

(35) TABLE-US-00001 TABLE 1 pressure losses Tooth A Tooth B Force A 100 180 Force B 60

(36) Thus, when the teeth are thinner (tooth B) by maintaining the same clamping force (force A), the ASR are lower, therefore the performances are improved, but the pressure losses are increased. The crushing of the teeth has reduced the passage surface for the gases. When the teeth are thinner (tooth B) but the force is reduced (force B), the performances are degraded (higher ASR and reduced maximum use rate) but the pressure losses are significantly reduced.

(37) The principle of the invention, which will now be described with reference to FIGS. 7 to 14, thus aims to optimise these aspects, and particularly obtain a design of interconnector, in particular of contact layer, making it possible to both have good crushing of the teeth 10 and maintain gas circulation channels 11 with a significant volume.

(38) An interconnector 5 for a stack of solid oxide cells of the SOEC/SOFC type operating at high temperature, intended to be arranged between two adjacent electrochemical cells 1 of the stack, each cell being formed of a cathode, of an anode and of an electrolyte intercalated between the cathode and the anode, usually has a regular geometry. In particular, the contact layer forming a coating on one of the faces of a metal alloy substrate of the interconnector 5 conventionally includes teeth 10 and channels 11 of regular geometry. Thus, the teeth 10 all have the same dimensions (height and width) and all of the channels 11 have the same width. The main features of the teeth 10 and of the channels 11 are detailed in the sectional view in FIG. 6. Thus, the contact width of a tooth 10 is noted D, the width of the top of the channel 11 is noted C-h whereas the width of the bottom of the channel 11 is noted C-b, and the height of the teeth 10 is noted H.

(39) In accordance with the invention, the geometry of the interconnector 5, particularly of the contact layer, is modified to obtain an inhomogeneity enabling both an optimal electrical contact and a gas distribution offering only little resistance during the passage of the gases, and therefore little overpressure. In particular, an inhomogeneous machining is performed to obtain teeth and channels with different features on the same interconnector 5, particularly on the same contact layer of this interconnector 5.

(40) Thus, an interconnector 5 according to the invention includes a flat face P whereon at least a first group of identical first elements in relief 10a and a second group of identical second elements in relief 10b are formed, the first 10a and second 10b elements in relief having different geometric features.

(41) FIGS. 7 and 8 show, before and after crushing, an example of embodiment with two types of machining geometry. However, a large number of different geometries may be provided for the interconnector 5 within the scope of the invention.

(42) Thus, the height H1 of each first element in relief 10a, measured as being the largest vertical dimension of the first element in relief 10a with respect to the flat face P, is different from the height H2 of each second element in relief 10b, measured as being the largest vertical dimension of the second element in relief 10b. Likewise, the contact width D1 of each first element in relief 10a, measured as being the largest horizontal dimension with respect to the flat face P of the outer contact end 10ae of each first element in relief 10a, opposite the inner end 10ai in contact with the flat face P and intended to be in contact with an electrochemical cell 1, is different from the contact width D2 of each second element in relief 10b, measured as being the largest horizontal dimension with respect to the flat face P of the outer contact end 10be of each second element in relief 10b, opposite the inner end 10bi in contact with the flat face P and intended to be in contact with an electrochemical cell 1.

(43) In particular, the contact width D1 of each first element in relief 10a is between 0.5 and 5 mm, preferably being equal to 1 mm. This wide width makes it possible to support the clamping constraints and to act as a crush limiter.

(44) The contact width D2 of each second element in relief 10b is between 0.005 mm and 0.5 mm, preferably being equal to 100 m. This narrow width makes it possible to have regular contact points around the entire contact surface of the electrochemical cell 1 without hindering fluid flows.

(45) Moreover, the height H1 of each first element in relief 10a is lower than the height H2 of each second element in relief 10b, being respectively 350 and 400 m. Thus, the elements in relief 10b of narrow width D2 ensure the electrical contact.

(46) It should be noted that in this example of FIGS. 7 and 8, as for FIGS. 10 to 12, the elements in relief 10a, 10b, 10c are in the form of teeth or grooves, disposed parallel with one another. However, the elements in relief may take any shape that guarantees ensuring the electrical contact and the circulation of gases. Thus, the spaces between the elements in relief 10a, 10b, 10c form gas circulation channels 11.

(47) Moreover, the elements in relief 10a, 10b are here distributed regularly over the flat face P. Precisely, they are spaced the same distance C-b apart from one another, particularly between 50 m and 5 mm, and preferably equal to 750 m, according to at least one horizontal direction DH over the flat face P. The spacings between elements in relief 10a, 10b are therefore constant and make possible good distribution of the current within the electrode of the electrochemical cell 1. The value of the spacing may be dependent on the electrochemical cell 1 used.

(48) During the clamping, the elements in relief 10b will crush first because they are higher. The crushing will be significant because the contact width D2 is narrow. This will then enable good accommodation of the geometric defects.

(49) This crushing will continue until the height H2 of the elements in relief 10b reaches the height H1 of the elements in relief 10a. The contact surface will therefore increase rapidly, which will stop the crushing. This stopping of the crushing makes it possible to preserve significant spaces for the gas circulation channels 11. Thus, the pressure losses can remain low. In addition, as the spacing C-b between the elements in relief 10a, 10b is fairly small, a good electrical conductivity is obtained.

(50) Advantageously, the invention does not require precisely adjusting the clamping force to stop the crushing. Indeed, the significant increase in surface when the contact with the elements in relief 10a is established makes it possible to significantly reduce the stress, limiting the effect of the initial force.

(51) FIG. 9 makes it possible to view the regular distribution of the first 10a and second 10b elements in relief of the example of FIGS. 7 and 8.

(52) As the manufacturing of interconnectors 5 and of electrochemical cells 1 is not regular, it may further be advantageous to have an interconnector 5, particularly a contact layer of the interconnector 5 comprising the elements in relief, the crushing of which may be modulated during the operation.

(53) Thus, by creating a number N of different geometries, easily accessible crushing stages can be obtained, even during a test. In other words, the interconnector may more generally include a number N, N being a whole number greater than or equal to 2, preferably between 2 and 50, also preferably equal to 5, of groups of elements in relief, formed on the flat face P, the elements in relief of the same group all being identical, and the elements in relief of different groups having different geometric features, namely different heights and different contact widths.

(54) FIGS. 10 and 11 make it possible to illustrate the case for N=3, which is only an illustrative and non-limiting example of the invention. Thus, the interconnector 5 comprises first elements in relief 10a of contact width D1 and of height H1, second elements in relief 10b of contact width D2 and of height H2, and third elements in relief 10c of contact width D3 and of height H3. The values chosen are such that D3>D1>D2 and H2>H1>H3.

(55) This possibility makes it possible to have a plurality of possible crushing levels. Thus, it is possible to have a crushing with a force 1 that will only crush the elements in relief 10b. If this is not sufficient, because the geometric defects to be compensated are significant, it is possible to change to a force 2, greater than the force 1, to crush the elements in relief 10a to the height H3 of the elements in relief 10c. The crushing and the contact can thus be regulated as required.

(56) It is therefore possible to envisage, if necessary, having N different geometries of increasing contact width. The continuous increase of the clamping force would make it possible to crush by stages the elements in relief, then stop as soon as the contact is good and for an optimal crushing. Thus, an interconnector 5, or a contact layer thereof, which adapts to all geometries, is obtained.

(57) FIG. 12 illustrates the possibility of having the elements in relief 10c having the largest contact width D3 that are located around the periphery Pi of the flat face P, at a distance from the other elements in relief 10a, 10b and from the gas circulation channels 11.

(58) These elements in relief 10c form the crushing limiters since they have the greatest contact width D3. They may be located outside of active areas. In this manner, a maximum of surface is reserved for the passage of the gases.

(59) Furthermore, any shape is possible for the elements in relief 10a, 10b, 10c. They are not necessarily in the shape of teeth as described above.

(60) Thus, FIGS. 13 and 14 illustrate the possibility of having elements in relief 10a, 10b in the form of pads, and particularly of cylindrical shape. Other shapes are also possible, for example a parallelepiped shape. The spaces between the elements in relief 10a, 10b then form a single serpentine gas circulation channel 11.

(61) Advantageously, this may make it possible to more accurately regulate the stresses with a suitable surface and optimise the passage of the gases.

(62) Moreover, FIG. 14 illustrates the possibility of having at least one area Z of the flat face P, here the central area Z, which is devoid of elements in relief. Indeed, the heating due to surfaces of electrochemical cells 1 that are too large may create overheating problems, and particularly at the centre of the cells 1 where the heat has difficulty in being evacuated. Thus, it is possible to intentionally limit the reactions at the core of the cells 1 by reducing the conductivity in the specific central area Z, which is thus intentionally devoid of electrical contacts.

(63) It should be noted that, advantageously, the interconnector 5 according to the invention may include a metal alloy substrate, particularly of the chromia-forming type the base element of which is iron (Fe) or nickel (Ni), in particular ferritic steels of the Uginox K41 type or of the VDM Crofer type, having two main flat faces, as described in the French patent application FR 2 996 065 A1.

(64) One of the main flat faces comprises a first coating layer forming a first contact layer with an electrochemical cell 1, and the other of the main flat faces comprises a second coating layer forming a second contact layer with an electrochemical cell 1.

(65) The first coating layer and/or the second coating layer may comprise the flat face P and the elements in relief 10a, 10b, 10c formed thereon, particularly by machining, such as described above.

(66) These elements in relief may or may not be identical on the first and second coating layers, and their distribution may or may not be identical on the first and second coating layers, when these two coating layers are provided with such elements in relief.

(67) The first coating layer may particularly be a first thick ceramic contact layer, porous or not, particularly based on strontium-doped lanthanum manganite. It may be provided on the oxygen electrode side.

(68) The second coating layer may particularly be a second thick metal, particularly based on nickel, contact layer. It may be provided on the hydrogen electrode side.

(69) This second layer may in particular include at least two different types of nickel grids. On these grids, the number of meshes per cm.sup.2 and the wire diameter may be modulated. It is for example possible to use a grid A of height Ha with a number of meshes Na, forming elements in relief, enabling it to crush significantly, and a second grid B of height Hb, less than the height Ha, with a number of meshes Nb, forming elements in relief, less than the number of meshes Na, so as to act as a crush limiter.

(70) Moreover, FIG. 15 shows a stack 20 of solid oxide cells of the SOEC/SOFC type operating at high temperature in accordance with the invention.

(71) More precisely, FIG. 15 shows an assembly 80 comprising the stack 20 of solid oxide cells of the SOEC/SOFC type and a clamping system 60.

(72) This assembly 80 has a structure similar to that of the assembly described in the French patent application FR 3 045 215 A1.

(73) The stack 20 includes a plurality of electrochemical cells 1 each formed of a cathode, of an anode and of an electrolyte intercalated between the cathode and the anode, and a plurality of interconnectors 5 in accordance with the invention each arranged between two adjacent electrochemical cells 1. This assembly of electrochemical cells 1 and of interconnectors 5 may also be designated as stack.

(74) In addition, the stack 20 includes an upper terminal plate 43 and a lower terminal plate 44, respectively also known as upper stack terminal plate 43 and lower stack terminal plate 44, between which the plurality of electrochemical cells 1 and the plurality of interconnectors 5 are clamped, or between which the stack is located.

(75) Moreover, the assembly 80 also includes a clamping system 60 of the stack 20 of solid oxide cells of the SOEC/SOFC type, including an upper clamping plate 45 and a lower clamping plate 46, between which the stack 20 of solid oxide cells of the SOEC/SOFC type is clamped.

(76) Each clamping plate 45, 46 of the clamping system 60 includes four clamping orifices 54. In addition, the clamping system 60 further includes four clamping rods 55, or tie rods, extending through a clamping orifice 54 of the upper clamping plate 45 and through a corresponding clamping orifice 54 of the lower clamping plate 46 to make it possible to assemble the upper 45 and lower clamping plates 46 to each other. In addition, the clamping system 60 includes clamping means 56, 57, 58 at each clamping orifice 54 of the upper 45 and lower clamping plates 46 cooperating with the clamping rods 55 to make it possible to assemble the upper 45 and lower clamping plates 46 to each other. More precisely, the clamping means include, at each clamping orifice 54 of the upper clamping plate 45, a first clamping nut 56 cooperating with the corresponding clamping rod 55 inserted through the clamping orifice 54. In addition, the clamping means include, at each clamping orifice 54 of the lower clamping plate 46, a second clamping nut 57 associated with a clamping washer 58, the latter cooperating with the corresponding clamping rod 55 inserted through the clamping orifice 54. The clamping washer 58 is located between the second clamping nut 57 and the lower clamping plate 46.

(77) Of course, the invention is not limited to the examples of embodiments that have just been described. Miscellaneous modifications may be made by the person skilled in the art.