Cell stacking for beta-voltaic batteries and associated manufacturing process

Abstract

The invention relates to a stack of cells (100) for a beta-voltaic battery, comprising at least two cells (10,20) assembled via an interface (40), each cell (10,20) being based on a rectifier junction and comprising: - a support layer (11,21), - a first contact layer (12,22), of p- or n-doped semiconductor material, disposed on the support layer (11,21), - an interlayer layer (13,23), of weakly doped semiconductor material, disposed on the first contact layer (12,22), - a second contact layer (14,24), composed of one or more semiconductor and / or conductor material(s), disposed on the interlayer layer (13,23), - optionally, a reservoir layer disposed on the second contact layer (14,24). In the stack (100), an intermediate layer (30) is interposed between the two cells (10, 20) and has open porosity. The invention also relates to a method for manufacturing said stack.Figure to be published with the abbreviation: Fig.1a.

Description

Title of the invention: Cell stacking for beta-voltaic batteries and associated manufacturing process. FIELD OF THE INVENTION

[0001] The present invention relates to the field of beta-voltaic batteries. It relates in particular to a cell stack for such batteries, and its manufacturing process.

[0002] TECHNOLOGICAL BACKGROUND OF THE INVENTION

[0003] A beta-voltaic battery, like a photovoltaic battery, is designed to use a radiation source (here, beta radiation emitted by a radioactive isotope) to form electron-hole pairs in a semiconductor diode, thereby producing electricity. In a silicon photovoltaic device, the wavelength range of the absorbable solar spectrum is between 300 nm and 1100 nm, corresponding to radiation with energies between 4.13 eV and 1.27 eV, respectively. In the case of a beta-voltaic device, the radiation is much more energetic, with, for example, a maximum energy of 18 keV for emissions from 3H (hydrogen-3, also called tritium) or 200 keV for 147Pm (promethium-147). Beta particles interact with the moderator (lightly doped semiconductor layer) to form numerous electron-hole pairs.These electron-hole pairs are subsequently separated in the diode (Schottky diode, pn Schottky diode, pn diode, etc.).

[0004] A beta-voltaic battery has the advantage of generating energy in total autonomy with a constant and predictable discharge, solely a function of the half-life of the radionuclide.

[0005] Environmental operating conditions have little impact on its performance over typical temperature ranges from -150°C to 200°C, and pressures ranging from vacuum to a few bar. In a beta-voltaic battery, the longer the half-life of the radionuclide used as the beta radiation source, the longer the battery life; since the discharge is continuous, this allows for continuous use over periods on the order of decades, thus overcoming the problem of the short lifespan of chemical batteries.

[0006] A beta-voltaic battery is usually composed of a stack of several cells (diodes) packaged in a casing (packaging) that allows the battery to be used in the application environment. Each cell is associated with a layer that is a source of beta radiation. It is therefore necessary to adapt the battery manufacturing process to the handling of radioactive material, from the formation of the beta-ray source layer to the final packaging stages. in order to protect operators and avoid cross-contamination of radioactive materials on equipment that would be used for the production of other products.

[0007] SUBJECT OF THE INVENTION

[0008] The present invention provides a cell stack designed to undergo a radioactive isotope infusion step to form a beta-voltaic battery. The use of such a stack reduces the handling of hazardous materials and improves the safety of the production line. The invention also relates to a method for manufacturing such a cell stack.

[0009] BRIEF DESCRIPTION OF THE INVENTION

[0010] The invention relates to a stack of cells for a beta-voltaic battery, comprising at least two cells assembled via an interface, each cell being based on a semiconductor rectifier junction and comprising:

[0011] - a support layer,

[0012] - a first contact layer, made of p- or n-doped semiconductor material, arranged on the support layer,

[0013] - an interlayer, made of lightly doped semiconductor material, disposed on the first contact layer,

[0014] - a second contact layer, composed of one or more material(s) semiconductor(s) and / or conductor(s), arranged on the interlayer.

[0015] The cell stack further comprises an intermediate layer having an open porosity and interposed between the second contact layers of the two cells.

[0016] According to other advantageous and non-limiting features of the invention, taken alone or in any technically feasible combination: • the semiconductor rectifier junction is a pn junction, and the second contact layer of each cell comprises a layer of n- or p-doped semiconductor material, with doping opposite to that of the first contact layer, disposed on the interlayer layer, and a metallic ohmic contact layer disposed on the n- or p-doped semiconductor material layer; • the semiconductor rectifier junction is a Schottky junction, and the second contact layer of each cell comprises a metallic Schottky contact layer disposed on the interlayer layer; • the intermediate layer has a porosity between 15% and 60%, and a proportion of open pores between 70% and 100%; • the intermediate layer comprises open pores with an average size between 0.5 sqm and 10 sqm; • the intermediate layer has a thickness between 5 pm and 50 pm; • the intermediate layer is formed from a material selected from titanium, vanadium, zirconium, niobium, molybdenum, tantalum, tungsten, and a corresponding carbide or nitride; • the semiconductor materials of each cell are chosen from diamond, silicon carbide, gallium nitride and silicon; • the cell stack includes, between the second contact layer of each cell and the intermediate layer, a metallic layer formed from a material capable of trapping atoms of a radioactive isotope, called the reservoir layer; • the reservoir layer is formed from a metallic material selected from titanium, scandium, magnesium, zirconium, calcium, uranium, chromium, hafnium, vanadium, niobium, molybdenum, tantalum and tungsten; • the radioactive isotope is tritium 3H or carbon 14 14C; • The cell stacking comprises at least four cells, and the stacking is such that: • an intermediate layer is interposed between a first and a second cell, and • another intermediate layer is interposed between a third and a fourth cell, • the second cell and the third cell share a common support layer and are respectively formed on a front face and on a back face of said support layer.

[0017] The invention relates to a method for manufacturing a cell stack as described above, comprising the following steps:

[0018] a) the development of two cells,

[0019] b) the formation of all or part of the intermediate layer on one or both cells, or separately from said cells, the intermediate layer having open porosity,

[0020] c) the assembly of the two cells via at least one interface, to form the stack, the intermediate layer being interposed between the second contact layers of the two cells.

[0021] According to other advantageous and non-limiting features of the invention, taken alone or in any technically feasible combination:

[0022]

[0023]

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[0036]

[0037] • step c) includes bringing the two cells into contact, with the application of a temperature between 200°C and 700°C and a force less than or equal to 500 N; • the manufacturing process includes a step d) of infusing radioactive isotope atoms, applied to the stack of cells, said atoms migrating through the porous intermediate layer and being at least stored in semiconductor materials of the semiconductor rectifier junction of each cell and / or in the second contact layer and / or in a reservoir layer when said reservoir layer is included in each cell; • step d) is carried out at a temperature between 400°C and 1000°C; • after step d), the reservoir layer exhibits an M3HX stoichiometry or M14CX with x between 0.1 and 2. BRIEF DESCRIPTION OF THE FIGURES Other features and advantages of the invention will become apparent from the detailed description of the invention which follows with reference to the accompanying figures in which: [Fig. aa] The [Fig. aa] presents a stacking of cells according to the present invention; [Fig.lb] The [Fig.lb] shows two variants (i), (ii) of a semiconductor rectifier junction for cells of a stack according to the present invention; [Fig.2a] [Fig.2a'] [Fig.2b] [Fig.2b'] [Fig.2c] [Fig.2c'] [Fig.2d] Fig.2a, Fig.2a', Fig.2b, Fig.2b', Fig.2c, Fig.2c' and Fig.2d present steps of a process for manufacturing a cell stack according to the present invention; [Fig.3a] [Fig.3a'] [Fig.3b] [Fig.3b'] [Fig.3c] [Fig.3c']

[0038] [Fig.3d] Fig.3a, Fig.3a', Fig.3b, Fig.3b', Fig.3c, Fig.3c' and Fig.3d present steps of a process for manufacturing a stack of cells according to an advantageous embodiment;

[0039] [Fig.4] Fig.4 shows a stacking of cells according to another mode of realization of the present invention.

[0040] The same reference numerals in the figures may be used for elements of the same type. Some figures are schematic representations which, for the sake of clarity, are not to scale. In particular, the thicknesses of the layers along the z-axis are not to scale with respect to the lateral dimensions along the x and y axes; and the relative thicknesses of the layers are not necessarily to scale in the figures. DETAILED DESCRIPTION OF THE INVENTION

[0041] The present invention relates to a 100-cell stack for a beta-voltaic battery ([Fig. 1a]). The 100-cell stack comprises at least two 10,20 cells, each based on a semiconductor rectifier junction. The rectifier junction can typically consist of a pn junction or a Schottky diode. As a reminder, the Schottky diode is a unipolar component that uses a metal-semiconductor junction. The pn junction is a bipolar component that uses a semiconductor-semiconductor junction.

[0042] Several semiconductor materials can be considered for forming this rectifier junction, including silicon, silicon carbide, gallium nitride, diamond, etc. However, wide bandgap materials remain the most promising. Indeed, the Shockley-Queisser equations show that the conversion efficiency is strongly influenced by the bandgap width of the semiconductor. Moreover, group IV semiconductors (Si, SiC, C) are particularly advantageous because they exhibit longer charge carrier diffusion lengths than group IILV semiconductors such as GaN. Diamond is thus a very promising material for fabricating high-efficiency beta-voltaic batteries.

[0043] As illustrated in [Fig.lb], each cell 10,20 comprises a support layer 11,21, the role of which is to give a certain mechanical resistance to the cell 10,20 and to allow its handling during its manufacture and that of the stack 100. The support layer 11,21 is advantageously composed of a semiconductor material.

[0044] The cell 10,20 also includes a set of active layers 12,13,14;22,23,24 forming the semiconductor rectifier junction. A first contact layer 12,22, made of p- or n-doped semiconductor material (preferably A heavily doped semiconductor material (11, 21) is disposed on the support layer and is intended to be connected to a terminal such as an anode or cathode. An interlayer layer (13, 23), made of a lightly p- and / or n-doped semiconductor material, is disposed on the first contact layer (12, 22). A second contact layer (14, 24), composed of a semiconductor and / or conductive material, is disposed on the interlayer layer (13, 23) and is intended to be connected to the other terminal such as the cathode or anode. The semiconductor material of the second contact layer (14, 24) has a doping level opposite to that of the first contact layer (12, 22), advantageously with a high doping level.

[0045] The cell 10,20 has a rear face 10b,20b on the side of the supporting substrate 11,21, and a front face 10a,20a on the side of the active layers 12,13,14;22,23,24.

[0046] The semiconductor materials of the cells 10,20 can advantageously be chosen from diamond, silicon carbide, gallium nitride, silicon.

[0047] According to a particular embodiment, the first contact layer 12,22 is heavily p-doped. By heavily p-doped, we mean a doping of acceptor atoms such that the resistivity of the layer is less than 20 mΩ·cm. Such a resistivity can be achieved with various dopants and concentrations, depending on the nature of the semiconductor material. For example, for diamond, the boron concentration can be greater than 1 × 10⁻³ at / cm³, or even greater than 3 × 10⁻³ at / cm³, and up to 1.5 × 10⁻³ at / cm³ (implying a resistivity typically on the order of 1 mΩ·cm).

[0048] The interlayer 13,23 may be lightly doped p-type, lightly doped n-type, or composed of two sublayers, one lightly doped p-type in contact with the first contact layer 12,22, and the other lightly doped n-type in contact with the second contact layer 14,24. Lightly doped means doping of acceptor or donor atoms such that the resistivity of the layer is greater than or equal to 1000 ohm·cm. In the case of diamond, the interlayer 13,23 or a lightly doped p-type sublayer may comprise a boron dopant concentration of less than 2 × 10¹⁶ at / cm³; the interlayer 13,23 or a lightly doped n-type sublayer may comprise a nitrogen or phosphorus dopant concentration typically less than 1 × 10¹⁶ at / cm³.

[0049] In a first embodiment, the rectifier junction is a pn junction ([Fig. 1b] (i)). Considering a first contact layer 12,22 of heavily p-doped semiconductor material, the second contact layer 14,24 of the cell 10,20 comprises a layer of n-doped semiconductor material 141,241, disposed on the intercalated layer 13,23, and a metallic ohmic contact layer 142,242 disposed on the n-doped semiconductor material layer 141,241. The semiconductor layer 141,241 is preferably heavily n-doped, i.e., having a high donor atom doping, in particular, for diamond, phosphorus doping at a concentration between 1.10 at / cm and 1.10 at / cm, or even more, and a resistivity typically less than 1000 ohm.cm, or even 100 ohm.cm.

[0050] The ohmic contact metal layer 142,242 can, for example, be formed of an ohmic metal such as W, Ta, Nb, Ti, Ru, Rh, Pd, Re, Os or Ir. This metal layer 142,242 can advantageously be surmounted by one or more noble metals with high electrical conductivity, resistance to oxidation and compatibility with wiring typically based on platinum and gold.

[0051] In a second embodiment, the rectifier junction is a Schottky diode ([Fig. 1b] (ii)). Considering a first contact layer 12,22 of heavily p-doped semiconductor material, the second contact layer 14,24 of the cell 10,20 comprises a Schottky contact metal layer 143,243 disposed on the interlayer layer 13,23. The Schottky contact metal layer 143,243 can, for example, be formed of a metal selected from W, Ta, Mo, Nb, Cr, Zr, Pd, Ni, Mn, Ru, Ir, Tb, and Er. This metal layer 143,243 is advantageously surmounted by one or more noble metals with high electrical conductivity, resistance to oxidation, and compatibility with wiring typically based on platinum and gold.

[0052] By way of example, the support layer 11, 21 may consist of a solid substrate of single-crystal diamond (doped or undoped) or of a composite substrate comprising a layer of single-crystal diamond (doped or undoped) deposited on a base substrate of diamond or other lower-quality material. The semiconducting layers 12, 13, 141; 22, 23, 241 are then produced by successive diamond epitaxies on the support substrate 11, in a temperature range of approximately 800°C - 1000°C. The first contact layer 12, 22 may be doped with boron atoms (p-type), with a concentration greater than 3 x 10²⁰ atoms / cm³, and have a thickness between 30 nm and 1500 nm. The weakly doped intercalated layer 13.23 has as low a doping level as possible (for example, a boron concentration of less than 2.1016 at / cm3), so as to extend the depth of the charge space zone, which corresponds to the volume of semiconductor stripped of charge carriers. Its thickness can be between 0.5 pm and 20 pm.

[0053] According to the first embodiment in which the rectifier junction is a pn junction, the semiconducting layer 141,241 of the second contact layer 14,24 can be doped with phosphorus atoms (n-type), with a concentration on the order of 4 x 10¹⁷ at / cm³, and have a thickness between 30 and 1500 nm. The ohmic contact metallic layer 142,242 of the second contact layer 14,24 can be formed by depositing 50 nm of titanium onto the semiconducting layer 141,241. diamond; the ohmic contact is then established by annealing the 10,20 cell between 400°C and 700°C.

[0054] According to the second variant in which the rectifier junction is a Schottky diode, the Schottky contact metal layer 143,243 of the second contact layer 14,24 of the cell 10,20 can be produced by depositing 50nm of molybdenum on the intercalated layer 13,23.

[0055] For semiconductor layers 12, 13, 141; 22, 23, 241 made of silicon carbide (epitaxially grown on a bulk single-crystal SiC substrate or on a composite substrate comprising a single-crystal SiC layer), a first contact layer 12, 22 (disposed on the support layer 11, 21) is preferably heavily p-doped, typically with an aluminum concentration greater than 1.1021 at / cm3. The weakly doped intercalated layer 13, 23 has the lowest possible doping level. The semiconductor layer 141, 241 of the second contact layer 14, 24 (in the case of the pn junction) can be heavily n-doped, typically with a nitrogen concentration greater than 1.1019 at / cm3; the ohmic contact metallic layer 141,242 of the second contact layer 14,24 can be formed in Ni, Al, Ti or Pt.In the case of a Schottky rectifier junction, the Schottky contact metal layer 143,243 of the second contact layer 14,24 of the cell 10,20 can be formed in Mo, Pt, Cr, W, Zr, Ni or Ir. .

[0056] In the case of silicon semiconductor layers 12, 13, 141; 22, 23, 241, the first contact layer 12, 22 and the semiconductor layer 141, 241 of the second contact layer 14, 24 (in the case of the pn junction) can both be easily doped above the metal-insulator transition. For example, a high p-type doping corresponds to a boron concentration greater than 4 × 10²⁰ at / cm³ and a high n-type doping corresponds to a phosphorus concentration greater than 3.5 × 10¹⁸ at / cm³. The ohmic contact metal layer 141,242 of the second contact layer 14,24 (in the case of the pn rectifier junction) can be formed from Ni, Al, Ti, or Pt. The Schottky contact metal layer 143,243 of the second contact layer 14,24 of the cell 10,20 (in the case of the Schottky rectifier junction) can be formed from Mo, Pt, Cr, W, Zr, Ni, or Ir. As previously mentioned, the intercalated layer 13,23, on the other hand, exhibits the lowest possible doping level.

[0057] Although the first contact layer 12,22 has been described as being p-doped in a particular embodiment, it can alternatively be n-doped; this implies adjustments of doping types at the level of the interlayer 13,23 and the second contact layer 14,24, to realize a semiconducting rectifier junction (pn or Schottky).

[0058] The development of the two cells 10,20 falls within the first step (step a) of the manufacturing process of the stack 100 according to the invention ([Fig.2a]).

[0059] According to an advantageous embodiment, each cell 10,20 further comprises a metallic layer 15,25, disposed on the second contact layer 14,24 ([Fig. 3a]). The metallic layer 15,25 has the particularity of being capable of trapping atoms of a radioactive isotope, which is why it is called the reservoir layer 15,25. We will see later in the description that a step of infusing radioactive isotope atoms aims to introduce and store said atoms within the stack 100, in particular in this reservoir layer 15,25.

[0060] In the case of tritium atoms 3H, the reservoir layer 15,25 will form a hydride MHX with x between 0.1 and 2. In the case of carbon atoms 14 14C the metal can form a carbide MCX with x between 0.1 and 2.

[0061] The reservoir layer 15,25 must also be stable at high temperatures, typically at temperatures greater than or equal to 800°C, or even 1000°C.

[0062] For tritium retention, it is advantageously composed of a material selected from titanium, scandium, magnesium, zirconium, calcium, or uranium. These metals are the most capable of forming hydrides that store a significant amount of tritium.

[0063] If the radioactive isotope is carbon-14, the 15,25 reservoir layer will preferably be chosen from among the d-block metals in the left-hand region of the periodic table: these are metals known for their ability to form carbides. The most interesting group of atoms corresponds to the elements of the periodic table located around niobium, namely chromium, hafnium, titanium, vanadium, zirconium, niobium, molybdenum, tantalum, and tungsten.

[0064] The metallic layer 15,25 can be produced by a conventional microelectronics deposition technique such as evaporation or sputtering, on the second contact layer 14,24. It can have a thickness between 0.1 qm and 5 qm.

[0065] Advantageously, an anode 110 can be provided on each of the first 10 and second 20 cells, in contact with the first contact layer 12,22 (as illustrated in [Fig. 2a]' and [Fig. 3a]') or the second contact layer 14,24, depending on the type of doping of these layers. This can be achieved using conventional lithography and etching steps. For example, etching allows the metallic layer 15,25 (if present), the second contact layer 14,24, and the intercalated layer 13,23 to be locally removed; a metallic film, for example titanium, is then locally deposited (using masking) to establish ohmic contact with the first contact layer 11,21 and form the anode 110. Advantageously, the anode 110 can extend onto the side of cell 10,20 (as illustrated in Figures 2a' and 3a') to facilitate its subsequent connection. Finally, the deposition of a dielectric film 115, for example of Al2O3, SiO2 or Si3N4, makes it possible to effectively insulate the anode 110 from the future cathode 120, which will be electrically connected to the other contact layer of cell 10,20. This deposition can be carried out by evaporation, sputtering, atomic layer deposition (ALD) or molecular beam epitaxy (MBE).

[0066] The next step b) of the manufacturing process of the stack 100 corresponds to the formation of all or part of an intermediate layer 30 on the metal layer 15,25 (if present) or on the second contact layer 14,24 of one or both of the cells 10,20. In other words, the intermediate layer 30 can be formed entirely on one or the other of the cells 10,20 ([Fig.2b], [Fig.3b]), or partly on the first cell 10 and partly on the second cell 20 ([Fig.2b]', [Fig.3b]').

[0067] In the stack 100 according to the invention, this intermediate layer 30 will be interposed between the second contact layers 14,24 of the two assembled cells 10,20. In the particular case where the cells 10,20 comprise metallic layers 15,25, the intermediate layer 30 is more precisely interposed between said metallic layers 15,25.

[0068] The intermediate layer 30 has an open porosity and can be composed of a metal, an alloy, or a ceramic (metal carbide or nitride). The metals are preferably chosen from the d-block metals, known as transition metals. For technical reasons, the melting temperature of the material composing the intermediate layer 30 is advantageously chosen to be above 1300°C; for economic reasons, precious metals are of little interest. Ideally, the native oxide of the materials considered should be passivating in order to preserve the integrity of the assembly over time. The most suitable group of atoms corresponds to the elements of the periodic table located around niobium, excluding chromium and hafnium, namely titanium, vanadium, zirconium, niobium, molybdenum, tantalum, and tungsten. The intermediate layer 30 can also be formed from a carbide or nitride of these metals.

[0069] The size and density of the pores of the intermediate layer 30 must allow gaseous diffusion at high temperature, while ensuring the mechanical strength of the stack 100 that will be manufactured.

[0070] Preferably, the intermediate layer 30 has a porosity of between 15% and 60%, and an open pore proportion of 70% to 100%. The average pore size is between 500 nm and 10 pm. The porosity can be measured by sonic conduction in a non-destructive manner and validated at regular intervals on microscopic studies of slices of the intermediate layer 30. The porosity opening can be measured by the BET (Brunauer, Emett, and Teller) method.

[0071] The thickness of the intermediate layer 30 is advantageously between 5 sqm and 50 sqm.

[0072] The shaping of the intermediate layer 30 can be carried out using a precursor in powder form, with a particle size ranging from 500 nm to 20 pm. The powder is applied to the front face 10a,20a of the cell 10,20, either by dry deposition, or by spreading a liquid solution in which the powder is suspended (in a solution based on water, ethanol, glycol, or other), or by spreading a gel comprising the powder particles, known as the "sol-gel" process. The spreading can be done by spin coating, deep coating, or spraying.

[0073] The intermediate layer 30 then requires a sintering step at a temperature between 200°C and 700°C, with or without pressure (applied force less than 500 N); this step can advantageously be carried out during step c) of assembly, the next step in the manufacturing process of the stack 100.

[0074] The intermediate layer 30 can alternatively be manufactured separately, in the form of a pellet, and then attached to the front face 10a,20a of one and then the other cell 10,20, or to the front faces 10a,20a of both cells 10,20 simultaneously. Note that in this case, two assembly interfaces will be present in the stack 100 of two cells 10,20, at the end of the next step c) of the process.

[0075] The shaping of this pellet can be carried out using conventional ceramic manufacturing techniques such as a uniaxial or isostatic press, followed by sintering at a temperature below 1200°C. The sintering temperature can be higher than the bonding step, since the intermediate layer 30 is produced separately from the cells 10, 20. The thickness of the pellet can then be on the order of 20 µm to 50 µm. Subsequently, the intermediate layer 30, in the form of a pellet, is bonded to the cells 10, 20, advantageously by bringing them into contact, heating them (at a temperature below 700°C), and compressing them.

[0076] Open porosity can be obtained directly via the technique of developing the intermediate layer 30 (layer formed from compacted and sintered particles), the intervals between the sintered particles, called pores, will then be advantageously preserved to the maximum.

[0077] According to one embodiment, the open porosity can be obtained subsequently by implementing a micro-structuring step of said layer 30. In this latter case, the intermediate layer 30 is advantageously formed from two powders of different materials: a powder with small-diameter particles, composed of a first material susceptible to chemical attack, and a powder with large-diameter particles, composed of the structural material of the layer Intermediate 30 is titanium, vanadium, zirconium, niobium, molybdenum, tantalum, tungsten, or a nitride or carbide of these materials. After sintering or brazing the intermediate layer 30, the first material is dissolved by chemical etching. The chemical etching method is selected to dissolve the first material more rapidly than the second (for example, by applying an oxidant ineffective against the oxide formed on the surface of the second material), thus leaving the intermediate layer 30 with a network of open pores.

[0078] The next step c) of the manufacturing process of the stack of cells 100 corresponds to the assembly of the first 10 and the second 20 cells, via an assembly interface 40. The interface 40 can extend between the intermediate layer 30 and a second contact layer 24 ([Fig.2c]), or between the intermediate layer 30 and a metallic layer 25 when it is present ([Fig.3c]); it can alternatively extend to the heart of the intermediate layer 30 ([Fig.2c]', [Fig.3c]').

[0079] This assembly can implement various known techniques, for example thermocompression bonding.

[0080] When the intermediate layer 30 is in its compact solid form at the end of step b), the cells 10,20 can be brought into contact at the level of their respective front faces 10a,20a and the assembly is preferably held in a press with a compression typically between 10 N and 500 N, and which can be heated to a temperature between 400°C and 700°C, to carry out sintering at the level of the assembly interface 40.

[0081] When the intermediate layer 30 has not reached a compact solid form after step b) (slurry, gel), the cells 10,20 can also be brought into contact at their front faces 10a,20; the assembly is then subjected to a first heat treatment to evaporate the liquids, and then to a second heat treatment, with or without pressure, to sinter the intermediate layer 30 and assemble the cells 10, 20; pressurization is preferred in order to close the assembly interface 40.

[0082] The stack 100 according to the invention is formed after step c) of the process.

[0083] Of course, the stack of layers 100 can comprise more than two cells. For example, it can comprise at least four cells and involve an intermediate layer 30 interposed between a first 10 and a second 20 cell, and another intermediate layer 31, interposed between a third 50 and a fourth 60 cell. As illustrated in [Fig. 4], the second cell 20 and the third cell 50 advantageously share a common support layer 21 and are formed respectively on a front face and on a back face of said support layer 21.

[0084] Once the stacking of layers 100 has been carried out, the manufacturing process preferably includes a step d) of infusion of radioactive isotope atoms (for example, tritium or carbon 14), applied to the stacking of cells 100: said atoms then migrate through the intermediate layer(s) 30,31 with open porosity and are trapped in the active layers, in particular in the second contact layer 14,24,54,64 ([Fig.2d]), and / or absorbed in the reservoir layers 15,25,55,65 of the cells 10,20,50,60 when these reservoir layers are present ([Fig.3d]).

[0085] Advantageously, in the case of tritium, the intermediate layer(s) 30,31 is / are chosen from a material that is intrinsically chemically indifferent to the radioactive isotope atoms or covered with a native oxide that is chemically indifferent to and inpermeable to these radioactive isotope atoms, so as to minimize the net amount of isotope used, said amount being mostly absorbed by the nearest active layers 14,24,54,64 and / or the reservoir layers 15,25,55,65. This is not necessarily the case for carbon atoms 14C, which, even if accumulated in the intermediate layer(s) 30,31, should emit sufficiently penetrating radiation to produce current in the cells 10,20,50,60.

[0086] Infusion is usually carried out at high temperatures (typically 400°C < T < 1000°C): this is why the stack 100 as a whole must be compatible with these temperature ranges. The stack 100 is placed in a controlled atmosphere chamber between 10⁴ and 10⁸ Pa, containing radioactive isotope atoms in gaseous form, in particular tritium ³H₂, or carbon ¹⁴C, for example in the form of a light hydrocarbon (methane, propane, butane, etc.).

[0087] The infusion step typically lasts between 10 min and 10 h, and can allow the formation of M3HX hydrides or M14CX carbides in the reservoir layers 15,25,55,65 with x between 0.1 and 2. When the stack 100 does not include reservoir layers 15,25,55,65, the radioactive isotope atoms (at least when it is tritium 3H) can be trapped mainly in the crystal lattice of the active layers closest to the intermediate layer 30, namely the second contact layer 14,24,54,64 or the intercalated layer 13, 23, 53, 63.

[0088] The advantages of this process lie in the application of infusion to cells 10,20 assembled in a stack 100. First, this prevents any release of the radioactive isotope, any modification of the composition, or any unwanted crystallographic change in the system, since all the high-temperature heat treatments have been carried out before the infusion. Infusing the stack (after assembly) also improves safety and logistics. In terms of production: by reducing the portion of the manufacturing chain using radioactive material, the exposure time for operators is limited; the amount of production equipment and the factory floor space dedicated to handling hazardous materials is also reduced, resulting in economic gains, lower costs for qualified personnel, and should facilitate obtaining government permits. By directly infusing an assembly (rather than each cell individually as is conventionally done), the free infused surface area is more limited before packaging: since the stack of 100 is a single unit, there is no subsequent assembly step, and therefore no handling of cells infused with radioactive atoms. This limits exposure time, as well as human error and potential radioactive material spills (incidents, particles, waste).

[0089] The cell stack 100 according to the invention is particularly suitable for manufacturing a beta-voltaic battery. A cathode 120 can be brought into contact with the second contact layer 14, 24, 54, 64 or the metal layer 15, 25, 55, 65 (if present) of each cell in the stack 100, preferably before assembly. The wiring is typically carried out with metal ribbons. Their thickness is advantageously less than the thickness of the intermediate layer 30. The ribbons are soldered to the second contact layer 14, 24, 54, 64 or the metal layer 15, 25, 55, 65 by a conventional microelectronics technique, such as ultrasonic microsoldering.

[0090] Alternatively, if the intermediate layer 30 is chosen to be electrically conductive, a cathode 120 can contact each intermediate layer 30, 31, as illustrated in [Fig. 4]. In this example, a wire or metallic ribbon 120 with a thickness less than that of the intermediate layer was placed in the precursor of the intermediate layer 30, 31, just before step c) of assembly, to form the cathode 120.

[0091] A wire or metallic ribbon can also be connected to the anode 110 for reconnection.

[0092] In the stack 100 according to the invention, the intercalated layer 13,23 forms the moderator of the generator and serves to convert the radiation emitted by the radioactive atoms trapped in an active layer (in particular the second contact layer 14,24,54,64 and possibly the moderator itself 13, 23, 53, 63) and / or in a reservoir layer 15,25,55,65, into charge carriers.

[0093] The intrinsic robustness of the stack 100 (consolidated assembly interface(s) 40) can make it possible to simplify the battery packaging, in particular to make it thinner and lighter.

[0094] Beta-voltaic batteries deliver a low electric current in constant discharge over long periods. The half-life of tritium is twelve years. Therefore, twenty-four years after its assembly, the battery's power will be divided by four. The half-life of carbon-14 is 5730 years; consequently, the power loss will be negligible even after a century of use. Beta-voltaic batteries offer highly desirable qualities such as long-term reliability, autonomy, resilience to variations in environmental conditions (pressure and temperature), resistance to radiation, and a long operating life.

[0095] The applications are numerous, in very diverse fields such as the space sector, industry, drilling and mining exploration, the medical field, civil engineering or even defense.

[0096] Of course, the invention is not limited to the embodiments and examples described, and alternative embodiments can be made without departing from the scope of the invention.

Claims

Demands

1. A cell stack (100) for a beta-voltaic battery, comprising at least two cells (10, 20) assembled via an interface (40), each cell (10, 20) being based on a semiconductor rectifier junction and comprising: - a support layer (11, 21), - a first contact layer (12, 22), made of p- or n-doped semiconductor material, disposed on the support layer (11, 21), - an interlayer layer (13, 23), made of lightly doped semiconductor material, disposed on the first contact layer (12, 22), - a second contact layer (14, 24), composed of one or more semiconductor and / or conductor material(s), disposed on the interlayer layer (13, 23), the cell stack (100) further comprising an intermediate layer (30) having an open porosity and interposed between the second contact layers (14,24) of the two cells (10,20).

2. Stack of cells (100) according to the preceding claim, wherein the semiconductor rectifier junction is a pn junction, and the second contact layer (14,24) of each cell (10,20) comprises a layer of n- or p-doped semiconductor material (141,241), with doping opposite to that of the first contact layer (12,22), disposed on the interlayer layer (13,23), and an ohmically contacting metallic layer (142,242) disposed on the n- or p-doped semiconductor material layer (141,241).

3. Stack of cells (100) according to claim 1, wherein the semiconductor rectifier junction is a Schottky junction, and the second contact layer (14,24) of each cell (10,20) comprises a Schottky contact metal layer (143,243) disposed on the interlayer layer (13,23).

4. Cell stack (100) according to any one of the preceding claims, wherein the intermediate layer (30) has a porosity of between 15% and 60%, and a proportion of open pores of between 70% and 100%.

5. Cell stack (100) according to any one of the preceding claims, wherein the intermediate layer (30) comprises open pores having an average size between 0.5 µm and 10 µm.

6. Stack of cells (100) according to any one of the preceding claims, wherein the intermediate layer (30) has a thickness between 5 µm and 50 µm.

7. Stack of cells (100) according to any one of the preceding claims, wherein the intermediate layer (30) is formed of a material selected from titanium, vanadium, zirconium, niobium, molybdenum, tantalum, tungsten, and a corresponding carbide or nitride.

8. Stack of cells (100) according to any one of the preceding claims, wherein the semiconductor materials of each cell (10,20) are selected from diamond, silicon carbide, gallium nitride and silicon.

9. Stack of cells (100) according to any one of the preceding claims, comprising, between the second contact layer (14,24) of each cell (10,20) and the intermediate layer (30), a metallic layer (15,25) formed of a material suitable for trapping atoms of a radioactive isotope, called reservoir layer (15,25).

10. Stack of cells (100) according to the preceding claim, wherein the reservoir layer (15,25) is formed of a metallic material selected from titanium, scandium, magnesium, zirconium, calcium, uranium, chromium, hafnium, vanadium, niobium, molybdenum, tantalum, tungsten.

11. Stack of cells (100) according to one of the two preceding claims, wherein the radioactive isotope is tritium 3H or carbon 14 14C.

12. Stack of cells (100) according to any one of the preceding claims, comprising at least four cells and wherein: - an intermediate layer (30) is interposed between a first (10) and a second (20) cell, and - another intermediate layer (31) is interposed between a third (50) and a fourth (60) cell, the second cell (20) and the third cell (50) sharing a common support layer (21) and being respectively formed on a front face and on a back face of said support layer (21).

13. A method for manufacturing a stack of cells (100) according to any one of the preceding claims, comprising the following steps: a) the fabrication of two cells (10, 20), b) the formation of all or part of the intermediate layer (30) on one or both of the cells (10,20), or apart from said cells (10,20), the intermediate layer (30) having an open porosity, c) the assembly of the two cells (10,20) via at least one interface (40), to form the stack (100), the intermediate layer (30) being interposed between the second contact layers (14,24) of the two cells (10,20).

14. A manufacturing method according to the preceding claim, wherein step c) comprises bringing the two cells (10,20) into contact, with the application of a temperature between 200°C and 700°C and a force less than or equal to 500 N.

15. A manufacturing method according to one of the two preceding claims, comprising a step d) of infusing radioactive isotope atoms, applied to the stack of cells (100), said atoms migrating through the porous intermediate layer (30) and being at least stored in semiconductor materials of the semiconductor rectifier junction of each cell (10,20) and / or in the second contact layer (14,24) and / or in a reservoir layer (15,25) when said reservoir layer (15,25) is included in each cell (10,20).

16. A manufacturing process according to the preceding claim, wherein step d) is carried out at a temperature between 400°C and 1000°C.

17. A manufacturing method according to one of the two preceding claims, wherein, after step d), the reservoir layer (15,25) has an M3HX or M14CX stoichiometry with x between 0.1 and 2.

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