WO2024094847A1

WO2024094847A1 - Process for producing a fused mayenite product

Info

Publication number: WO2024094847A1
Application number: PCT/EP2023/080658
Authority: WO
Inventors: Nadia Frederich; Caroline Tardivat
Original assignee: Saint-Gobain Centre De Recherches Et D'etudes Europeen
Priority date: 2022-11-03
Filing date: 2023-11-03
Publication date: 2024-05-10
Also published as: FR3141690A1

Abstract

The invention relates to a process for producing mayenite, comprising the following sequence of steps: a) making a feedstock suitable for obtaining, at the end of step c), a product comprising more than 90% of a mayenite comprising calcium and/or strontium, aluminium and oxygen, as a percentage by weight based on the weight of the crystallised phases; b) melting said feedstock until a molten material is obtained; c) cooling said molten material so as to solidify it and obtain a fused polycrystalline product; said melting being carried out with an electric arc furnace and in a reducing medium.

Description

Title: Process for manufacturing a molten mayenite product

Technical area

The present invention relates to a molten mayenite electride product and a method of manufacturing such a product. The invention finally relates to devices comprising a melted product according to the invention or a melted product manufactured by a process according to the invention.

Prior art

EP 1 717 217 describes mayenites in the form of an electride, or “mayenite electrides” comprising calcium and/or strontium, aluminum and oxygen, in particular mayenites of formulas ((Ca24A12sO64) ^{4 +} )(4e') and ((Sr24A12sO64) ⁴⁺ )(4e'). These mayenites are manufactured by melting a starting charge, maintaining the molten material in an atmosphere having an oxygen partial pressure less than or equal to 10 Pa, then cooling.

The article "Mayenite-based electride C 12A7e': an innovative synthetic method via plasma arc melting", Materials Chemistry Frontiers, 2021, pages 1301-1314, by Weber Sebastian et Al, describes a process for manufacturing an electride based of mayenite by fusion using a plasma furnace.

A mayenite in the form of an electride can also be obtained by solid phase sintering then replacement of oxygen ions present in the nano-cages with electrons. This replacement can, for example, result from exposure to reducing metal vapors.

It is also known that the electrons of a mayenite in the form of an electride can be partly or totally replaced by anions, arranged in nano-cages of the mayenite. If the replacement is partial, the electrons keep the mayenite electrically conductive, that is to say in the form of an electride. Otherwise the mayenite becomes electrically insulating, and is therefore no longer an electride.

The processes for manufacturing mayenite in the form of an electride comprising calcium and/or strontium, aluminum and oxygen are complex and expensive. They only allow small quantities of mayenite to be manufactured. There is therefore a continuing need for a simpler manufacturing process that makes it possible to manufacture mayenite, particularly in the form of an electride, in large quantities. An aim of the invention is to respond, at least partially, to this need.

Summary of the invention

According to the invention, this goal is achieved by means of a process for manufacturing mayenite in the form of an electride, said process comprising the following successive steps: a) making a starting charge; b) melting said starting charge until a molten material is obtained, c) cooling said molten material so as to solidify it and obtain a polycrystalline molten product, said starting charge being adapted, to step a), to obtain, at the end of step c), a product comprising more than 90% of a mayenite comprising calcium and/or strontium, aluminum and oxygen, in mass percentage based on the mass of the crystallized phases; d) optionally, grinding the molten product so as to give it the form of a powder; e) optionally, particle size selection on the powder; f) optionally, replacement of only part of the free electrons of the mayenite of said molten product by anions, arranged in nano-cages of the mayenite, preferably chosen from F", Cl', OH", H", O2 ', O", CC ² ' ¹ , N ³ ', NH ² ', NH2', C2 ^<2-> , S', CN', NO2', S2', and their mixtures, an anion in a nano-cage which may be different depending on the nano-cage considered or, preferably, being identical whatever the nano-cage considered, said fusion being carried out with an electric arc furnace and in a reducing medium.

In other words, fusion is carried out with the heat produced by an electric arc passing through the starting charge.

As will be seen in more detail in the remainder of the description, the inventors have discovered that the above process, simple and inexpensive due to the fact that it only involves a single melting step, makes it possible to produce a molten product comprising a very large quantity of mayenite electride at the end of step c).

The manufacturing process according to the invention may also have one or more of the following optional characteristics:

- in step b), the electrodes of the electric arc furnace soak in the molten material;

- in step a), the starting charge contains more than 0.5% of a reducing agent, in mass percentage based on the mass of the starting charge;

- in step a), the starting charge contains less than 7% of a reducing agent, in mass percentage based on the mass of the starting charge;

- the reducing agent is chosen from a carbon source, a metal, and their mixtures;

- in step a), the starting charge contains less than 50% of mayenite comprising

- calcium and/or strontium,

- aluminum and

- oxygen;

- in step c), the cooling is adapted so that the molten material is completely solidified in less than 3 minutes.

In one embodiment, step f) is replaced by a step f) which differs from step f) in that, substantially all the free electrons of the mayenite of said molten product are replaced by anions, arranged in mayenite nano-cages, preferably chosen from F, Cl', OH', H', Of, O', O ₂ ⁽² ' ^} , N ³ ', NH ² ', NHf, C ₂ ⁽² ' ⁾ , S', CN', NOf, Sf, and their mixtures, an anion in a nano-cage being able to be different depending on the nano-cage considered or, preferably, being identical whatever the nano-cage considered. f) thus makes mayenite electrically insulating.

The invention also relates to a polycrystalline melt comprising more than 90% mayenite, in mass percentage based on the crystallized phases, said mayenite comprising

- calcium and/or strontium,

- aluminum and

- oxygen, said molten product comprising carbon in an amount greater than 15 ppm and less than or equal to 5%, by mass based on the mass of the molten product, said mayenite being:

- preferably in the form of an electride, or

- electrically insulating.

In a polycrystalline product, a quantity of carbon greater than 15 ppm and less than or equal to 5% and a quantity of mayenite greater than 90%, in mass percentage based on the crystallized phases are considered as a signature of a processing step. fusion in an electric arc furnace and in a reducing environment. The product according to the invention can therefore be manufactured according to a process according to the invention. In a preferred embodiment, said product is obtained according to a process according to the invention. The molten product obtained directly by a process according to the invention, that is to say without having undergone additional treatment, has a color representative of its electrical conductivity. The color of an insulating mayenite is white. The color of a mayenite in the form of an electride is between creamy yellow color and black color, preferably between green color and black color. Preferably, the mayenite in the form of an electride is dark green or black in color, preferably black in color. The closer the color is to black, the higher the electrical conductivity.

As described in EP 3 489 197, a color between creamy yellow and yellow corresponds to an electrical conductivity less than 1.10' ⁴ S/cm, a green color corresponds to an electrical conductivity between 1.10' ⁴ S/cm and 1 S/cm and a black color corresponds to an electrical conductivity greater than 1 S/cm.

In one embodiment of a mayenite in the form of an electride, the total quantity of the elements Ca, Sr, Al, C and O, Ca+Sr+Al+O+C is greater than 97%, as a percentage in mass based on the mass of the melt.

A polycrystalline melt according to the invention may also include one or more of the following optional characteristics:

- the product presents more than 95%, preferably more than 98% of mayenite in the form of an electride, in percentage by mass on the basis of the crystallized phases;

- the product has a carbon content greater than 300 ppm, preferably greater than 700 ppm, and/or less than 3%, preferably less than 1.5%;

- the product has a total quantity of the elements Ca, Sr, Al, C and O, Ca+Sr+Al+O+C, greater than 97%, preferably greater than 98%, in percentage by mass on the basis of the mass of melted product;

- the product has a ratio (CaO+SrOj/AhCh, CaO, SrO, and AI2O3 being molar contents on the basis of the oxides, greater than 1.60, preferably greater than 1.68, and/or less than 2, preferably less than 1.80;

- the mayenite comprises at least one element chosen from Mg, K, Na, Li, Ba, Ga, Si, Ge, Sn, Ti, Zr, Cr, Mn, Fe, Co, Ni, Cu, Zn.

The invention finally relates to a device comprising a product according to the invention and/or a product manufactured by a process according to the invention.

When the product is in the form of an electrure, said device is chosen from:

- a catalyst support, in particular for the synthesis of NH3 or the decomposition of C0 ₂ ;

- a solid oxide fuel cell (or “SOFC” in English), in particular an electrode, in particular a cathode;

- an organic light-emitting diode (or “OLED” in English), in particular a cathode of such a diode;

- a Hall effect thruster, in particular a cathode of such a thruster;

- a grid ion thruster, (or “ion thruster” in English), in particular a cathode of such a thruster;

-a proton conductive ceramic fuel cell (or “PCFC” in English).

When the product is electrically insulating, the device is chosen from:

- a solid oxide fuel cell (or “SOFC” in English), in particular an electrolyte of such a cell;

- a solid oxide electrolysis cell (or “SOEC” in English);

- a CO ₂ capture device.

The device can also be a device for storing and/or transporting hydrogen, the product being in the form of an electroplating or being electrically insulating.

Definitions

An “electride” (or “electride” in English) is classically an ionic compound in which the trapped electrons play the role of negatively charged ions.

By “mayenite” is conventionally meant a compound of chemical formula 12CaO.7AhO3 or 12SrO.7AhO3 or 12(Ca,Sr)O.7AhO3, in which a part of Ca and/or Sr and/or Al can be optionally substituted by another element, in particular by Mg, K, Na, Li, Ba, Ga, Si, Ge, Sn, Ti, Zr, Cr, Mn, Fe, Co, Ni, Cu, Ir, V, P, Er, Yb, Eu, Au, Tb or Zn. A mayenite crystal has a crystal lattice forming nano-cages. A mayenite is electrically neutral.

An electrically insulating mayenite can conventionally be considered to contain, per lattice unit, 12 nano-cages and two O ² ' ions in said nano-cages, said O ² ' ions being conventionally called "free oxygens". Said electrically insulating mayenite can be expressed by the formula ((Ca24AhsO64) ⁴⁺ ) (O ² ')2.

It is known that all or part of the free oxygens can be conventionally replaced by electrons, called "free electrons", and considered to be localized in the nano-cages: the initially electrically insulating mayenite then becomes electrically conductive, that is to say, it presents itself in the form of an electrure. In the particular case where all the free oxygens are replaced by free electrons, we obtain a mayenite in the form of an electride classically expressed by the formula ((Ca24AhsO64) ⁴⁺ ) (4e').

It is also known that all or part of the free oxygens and/or free electrons can be replaced by anions such as for example F" (the number of anions F" making it possible to ensure electroneutrality).

To express composition or content "on an oxide basis", all elements, other than carbon, are converted to the form of their most stable oxide, according to usual industry convention. For example, the contents of the elements calcium, strontium and aluminum are expressed after conversion of these elements in the form of CaO, SrO and Al2O3, respectively. The content of an element other than carbon thus converted is expressed on the basis of all the elements other than carbon thus converted.

In particular, the contents of CaO, SrO and Al2O3 relate to the overall contents for each of the corresponding chemical elements Ca, Sr and Al. The contents of CaO, SrO and AI2O3 mentioned in this description therefore express the contents of the elements Ca, Sr and Al not only in the form of CaO, SrO and AI2O3 phases, but also in other forms. The contents of the elements Ca, Sr and Al are therefore included in particular in the form of suboxides and possibly nitrides, oxynitrides, carbides, oxycarbons, carbonitrides, or even metals.

When reference is made to an oxide phase, for example to a phase of CaO, SrO or AI2O3, this is specified by indicating “phase” CaO, SrO or AI2O3.

The total quantity of the elements Ca, Sr, Al, C and O, Ca+Sr+Al+O+C, is expressed as a percentage by mass based on the mass of the molten product, without conversion of the metals in the form of l the most stable oxide.

By “impurities” we mean the inevitable constituents, necessarily introduced with the raw materials. In particular the compounds belonging to the group of oxides, nitrides, oxynitrides, carbides, oxycarbides, carbonitrides and metallic species of silicon, sodium and other alkalis, iron, and vanadium are impurities, in particular when the substitution of part of the elements Ca and/or Sr and/or Al of mayenite is not sought.

By “precursor” of an oxide is meant a constituent capable of providing said oxide during the manufacture of a molten product according to the invention. By “molten product” is meant a solid product obtained by solidification by cooling of a molten material.

A “molten material” is a mass made liquid by heating a starting charge, which may contain some solid particles, but in an insufficient quantity for them to be able to structure said mass. To maintain its shape, a molten material must be contained in a container.

An electric arc furnace is a well-known furnace, classically used for the manufacture of molten products, in particular refractory blocks or abrasive grains. It has electrodes between which an electric arc is produced. This arc releases a large amount of heat, which helps melt the starting charge.

An electric arc furnace produces, between classically graphite electrodes, typically in the open air, an electric arc which passes through the starting charge in order to melt it. An electric arc furnace (EAF) is distinguished in particular from a plasma furnace (PAF), in which a plasma melts the starting charge , said plasma being generated by the excitation of a gas using an electric arc (which therefore does not serve to directly melt the starting charge). The heating dynamics are also very different, which can change the phases of the manufactured product.

It is conventionally said that the electrodes soak “in the bath of molten material” when they are arranged so that their free end is embedded in said liquid bath during fusion.

A “reducing medium” is a medium which leads to an extraction of oxygen atoms from the molten material. Carrying out a fusion in a reducing medium poses no difficulty. In particular, a reducing agent can be added to the starting charge or, in an electric arc furnace, the process conditions can be adjusted to ensure reduction, in particular by bringing the electrodes closer to the bath of molten material, or even by quenching. the electrodes in the bath.

When we talk about “all free electrons”, or “substantially all free electrons”, we mean all free electrons, with a few exceptions, which do not modify the measurable properties of mayenite. When we talk about “a part of the free electrons”, we exclude “substantially all the free electrons”.

We call the “median size” of a powder the size dividing the particles into first and second populations equal in mass, these first and second populations not comprising only particles having a size greater than or equal to, or less than, respectively, the median size. The median size of a powder can be determined using a particle size distribution carried out using a laser particle size analyzer.

“Behave”, “understand” or “present” must be interpreted in a non-limiting manner.

detailed description

The following description is provided for illustrative purposes and does not limit the invention.

Process for manufacturing mayenite in the form of an electride

The process according to the invention can be a process as described for the examples below. It includes steps a) to f).

In step a), raw materials are conventionally dosed so as to obtain the starting charge having the desired composition. Preferably, said raw materials are mixed.

Choosing the raw materials for the starting charge so that the molten product, that is to say the solid mass obtained at the end of step c), has a composition conforming to that desired, poses no difficulty for man of career. He knows how to adapt the composition of the starting charge, in particular depending on the flight of some of the raw materials from the starting charge during melting.

The starting charge is preferably in the form of a particulate mixture. The median size of at least one raw material of the particulate mixture, preferably of each raw material, is preferably less than 1 mm, preferably less than 0.5 mm.

Preferably, the starting charge contains less than 70%, preferably less than 50%, preferably less than 30%, preferably less than 10% of mayenite comprising calcium and/or strontium, aluminum and oxygen.

More preferably, the starting charge does not contain mayenite containing calcium and/or strontium, aluminum and oxygen. Advantageously, the process is simplified.

The elements Ca and/or Sr, and Al are preferably introduced into the starting charge in the form of oxides CaO and/or SrO, and Al2O3. They can also be conventionally introduced in the form of precursors of these oxides, for example in the form of CaCCL phase and/or SrCOa. The element Al is preferably at least partly introduced into the starting charge in the form of Al2O3 phase and/or in the form of precursors of this oxide, for example in the form of aluminum hydroxide and/or boehmite. Preferably, the element Al is introduced into the starting charge partially in the form of the AI2O3 phase and partially in a metallic form. In one embodiment, the element Al is introduced into the starting charge entirely in the form of the AI2O3 phase.

In a preferred embodiment, the starting charge comprises a reducing agent, that is to say creating a reducing medium during fusion, preferably chosen from a carbon source, a metal, and their mixtures. Preferably, the reducing agent comprises, preferably consists of, a carbon source, preferably chosen from carbon, petroleum coke, pitch, coal and their mixtures, preferably petroleum coke. Preferably, the metal is aluminum.

Those skilled in the art know how to determine an appropriate quantity of reducing agent. Preferably, the quantity of reducing agent in the starting charge is greater than 0.5%, preferably greater than 1%, preferably greater than 1.5% and, preferably, less than 7%, preferably less at 6%, preferably less than 5%, preferably less than 4%, preferably less than 3.5%, as a percentage by mass based on the starting charge.

In step b), an electric arc furnace is used, preferably of the Hérault type with graphite electrodes, but all known electric arc furnaces are possible, provided that they allow the starting charge to be melted in a reducing medium.

Fusion in a reducing medium is preferably obtained by the presence, in the starting charge, of a reducing agent and/or by the proximity of the graphite electrodes to the bath of molten material.

The melting of said starting charge is carried out with the electrodes arranged at a distance from the starting charge adapted to obtain fusion in a reducing medium, preferably in grazing mode, the distance between the electrodes and the starting charge being preferably less at 2 cm, or in a mode in which the electrodes soak in the bath of molten material.

Preferably, the electrodes are in grazing mode or soak in the bath of molten material, preferably soak in the bath of molten material. Preferably, the starting charge contains a reducing agent. Preferably, the electrodes are soaked in the bath of molten material and the starting charge contains a reducing agent.

Preferably, the raw materials are melted at atmospheric pressure. Preferably, the raw materials are melted in an uncontrolled gas environment, preferably in air.

Preferably, an electric arc furnace is used, comprising a 160 liter tank, with a melting energy before casting greater than 1 kWh per kg of raw materials and preferably less than 6 kWh per kg of raw materials, for a power preferably greater than 200 kW, or an electric arc furnace of different capacity operated under equivalent conditions. Those skilled in the art know how to determine such equivalent conditions.

In step c), the molten material is cooled so as to solidify it and obtain the molten product.

Preferably, the cooling is rapid, preferably so that the molten material is completely solidified in less than 3 minutes, preferably in less than 2 minutes, preferably in less than 1 minute, preferably in less than 40 seconds , preferably in less than 30 seconds, and preferably in more than 1 second. Rapid cooling can in particular result from casting in molds as described in US 3,993,119.

Steps a) to c) make it possible in particular to manufacture the molten product comprising a high quantity of mayenite electride, comprising free electrons and possibly free oxygen housed in the nano-cages.

In optional step d), the melted product is ground to obtain a powder. Said grinding can be carried out by any conventional technique.

In step e), optional, a particle size selection is made on the powder, so as to adapt the particle size of the powder to the intended application. A particle size selection, for example by sieving or cycloning, can be implemented.

In optional step f), only part of the free electrons (and possible free oxygens) of the mayenite of the molten product, possibly in the form of a powder, are replaced by at least anions, arranged in nano- mayenite cages, preferably chosen from F, Cl', OH', H', Of, O', O ₂ ⁽² ' ^} , N ³ ', NH ² ', NHf, C ₂ ⁽² ' ⁾ , S ', CN', NOf, Sf, and their mixtures, an anion possibly being different depending on the nano-cage considered or, preferably, being identical whatever the nano-cage considered.

The methods for replacing free electrons (and possible free oxygens) present in the mayenite of the molten product with anions are well known to those skilled in the art. For example, H' anions can replace at least part of the free electrons present in the mayenite of the melted product using a heat treatment at 1250°C for 2 hours in a 100% H2 atmosphere.

Process for manufacturing an insulating mayenite

In an embodiment which is not preferred, the process comprises, instead of step f), an optional step F), during which substantially all the free electrons (and possible free oxygens) of the mayenite of the molten product, possibly in the form of a powder, by anions, arranged in nano-cages of the mayenite, the electroneutrality of the mayenite being preserved. The anions are preferably chosen from F, Cl', OH', H', O, O', O ₂ ⁽² - ^} , N ³ ', NH ² ', NH ^, C _2,2 ' ⁾ , S', CN', NO ₂ ', S ₂ ', and mixtures thereof. The anions can be different depending on the nano-cage considered or, preferably, are identical regardless of the nano-cage considered.

The mayenite, in the form of an electride before step f), thus becomes electrically insulating.

Molten mayenite product in the form of an electride

A molten product according to the invention comprising more than 90% of a mayenite in the form of an electride preferably has one or more of the following optional characteristics:

- the content of oxide compounds is greater than 95%, preferably greater than 96%, preferably greater than 97%, preferably greater than 98%, in percentages by mass based on the mass of the melted product;

- the carbon and the oxide compounds together represent more than 99%, preferably more than 99.5%, preferably 100% of the mass of the melted product;

- the carbon content is greater than 15 ppm, preferably greater than 20 ppm, preferably greater than 100 ppm, preferably greater than 300 ppm, preferably greater than 500 ppm, preferably greater than 700 ppm, and/or preferably less than 4%, preferably less than 3%, preferably less than 2%, preferably less than 1.5%, in percentages by mass based on the mass of the melt;

- the quantity of mayenite is greater than 92%, preferably greater than 93%, preferably greater than 94%, preferably greater than 95%, preferably greater than 96%, preferably greater than 97%, preferably greater than 98%, preferably greater than 99%, as a mass percentage based on the crystallized phases;

- the quantity of amorphous phase, measured as described in the examples, is less than 30%, preferably less than 20%, preferably less than 15%, preferably less than 10%, as a mass percentage based on the mass of the melted product;

- in a preferred embodiment, preferably for a mayenite in the form of an electride, the total quantity of the elements Ca, Sr, Al, C and O, Ca+Sr+Al+O+C is greater than 97% , preferably greater than 98%, preferably greater than 99%, as a percentage by mass based on the mass of the melt;

- the ratio (CaO+SrO)/Al ₂ O3, CaO, SrO, and AI2O3 being molar contents on the basis of the oxides, is greater than 1.60, preferably greater than 1.64, preferably greater than 1, 68, and preferably less than 2, preferably less than 1.94, preferably less than 1.86, preferably less than 1.80, preferably less than 1.76, preferably less than 1.74;

- in one embodiment, the mayenite comprises at least one element chosen from Mg, K, Na, Li, Ba, Ga, Si, Ge, Sn, Ti, Zr, Cr, Mn, Fe, Co, Ni, Cu, Zn, preferably in substitution for Ca and/or Sr and/or Al;

- the Raman spectrum of a molten product according to the invention comprises a band at 1870 cm' ¹ , the Raman spectrum being determined using a monochromatic source of wavelength equal to 532 nm;

- part of the mayenite nano-cages are occupied by anions, preferably chosen from F', Cl', OH', H', Of, O', O ₂ ⁽² ' ^} , N ³ ', NH ² ', NHf, C ₂ ⁽² ' ^} , S', CN', NOf, Sf, and mixtures thereof, said mayenite further comprising free electrons;

- part of the mayenite nano-cages are occupied by O ² ' and part of the mayenite nanocages are occupied by anions chosen from F', Cl", OH', H', Of, O', O ₂ ⁽² 'N ³ ', NH ² ', NHf, C ₂ ⁽² ' ^} , S', CN', NOf, Sf, a said anion which may be identical or different depending on the nano-cage considered, said mayenite comprising in besides free electrons;

- different anions can be housed in different nano-cages of the mayenite or, preferably, all the nano-cages occupied by an anion are occupied by identical anions, said mayenite also comprising free electrons.

In a preferred main embodiment, said melted product according to the invention presents:

- a content of oxide compounds greater than 95%, preferably greater than 96%, preferably greater than 97%, preferably greater than 98%, in percentages by mass based on the mass of the melted product, and

- a carbon content greater than 15 ppm, preferably greater than 20 ppm, preferably greater than 100 ppm, preferably greater than 300 ppm, preferably greater than 500 ppm, preferably greater than 700 ppm and less than 4%, preferably less than 3%, preferably less than 2%, preferably less than 1.5%, in percentages by mass based on the mass of the melt, and

- a quantity of mayenite phase greater than 90%, preferably greater than 92%, preferably greater than 93%, preferably greater than 94%, preferably greater than 95%, preferably greater than 96%, preferably greater than 97%, preferably greater than 98%, preferably greater than 99%, as a mass percentage based on the crystallized phases, and

- a quantity of amorphous phase, measured as described in the examples, in percentages by mass based on the mass of the melt, less than 30%, preferably less than 25%, preferably less than 20%, preferably less at 15%, preferably less than 10%, and

- a total quantity of the elements Ca, Sr, Al, C and O, Ca+Sr+Al+O+C, greater than 97%, preferably greater than 98%, preferably greater than 99%, in percentage by mass on the basis of the mass of the melt, and

- a ratio (CaO+SrO)/AhO3, CaO, SrO, and AI2O3 being molar contents on the basis of the oxides, greater than 1.60, preferably greater than 1.64, preferably greater than 1.68, and preferably less than 2, preferably less than 1.94, preferably less than 1.86, preferably less than 1.80, preferably less than 1.76, preferably less than 1.74.

Preferably, the color of the mayenite of said melted product is between green color and black color, preferably dark green or black color, preferably black color.

In one embodiment, the molten product is in the form of a powder, preferably having a median size greater than 0.1 μm and less than 1 mm.

Molten product of electrically insulating mayenite

In one embodiment, which is not preferred, the mayenite is electrically insulating. Said mayenite contains substantially no free electrons, part of the nano-cages being occupied by anions, preferably chosen from F", Cl', OH", H", O2', O", O2 ⁽² ' ⁾ , N ³ ', NH ² ', NH , C ₂ ⁽² ' ⁾ , S', CN', NO ₂ -, S ₂ ', an anion in a nano-cage being identical or different depending on the nano-cage considered. Preferably, part of the mayenite nano-cages are occupied by free oxygen O ² 'and part of the mayenite nano-cages are occupied by anions chosen from F", Cl', OH", H", O2', O", O2 ⁽² ' ⁾ , N ³ ', NH ² ', NH ₂ -, C2 ⁽² ' ^} , S', CN', NO ₂ -, S ₂ -, a said anion which may be identical or different depending on the nanocage considered. The invention therefore relates to a polycrystalline molten product comprising more than 90% of electrically insulating mayenite, in percentage by weight on the basis of the crystallized phases, said mayenite comprising

- calcium and/or strontium,

- aluminum and

- oxygen, said molten product comprising carbon in a quantity greater than 15 ppm and less than or equal to 5%, by mass based on the mass of the molten product.

Said melted product has a white color.

The other characteristics described above for the mayenite melt in the form of an electride, in particular the characteristics relating to the content of oxide compounds, the content of carbon and oxide compounds, the carbon content, the quantity of mayenite, the quantity of amorphous phase, the total quantity of the elements Ca, Sr, Al, C and O, the ratio (CaO+SrO)/AhO3 (CaO, SrO, and AI2O3 being molar contents on the basis of oxides), to the elements which may be present in the mayenite (in particular in substitution for Ca and/or Sr and/or Al), and to the anions present in the nano-cages, are applicable, optionally, to this embodiment .

Examples

The following non-limiting examples are given for the purpose of illustrating the invention.

Measurement protocols

To determine the composition of a melt, a bead is made by melting a powder of the melt. The content of elements other than carbon is measured by X-ray fluorescence, with oxygen being considered as the 100% mass complement.

The carbon content of the molten product is measured using a carbon-sulfur analyzer model CS744, marketed by the company LECO.

The median size of a powder is conventionally measured using a laser particle size analyzer model LA950V2 marketed by the company Horiba.

The measurement of the quantities of the different crystallized phases present in the molten product is carried out on samples ground dry in an RS 100 grinder marketed by the company Retsch, equipped with a bowl and a tungsten carbide roller, so as to what the samples are in the form of a powder having a refusal at 40 μm of less than 5% by mass.

The acquisitions are carried out using a D8 Endeavor type device from the company Bruker, over an angular range 29 between 5° and 80°, with a step of 0.01°, and a counting time of 0.34 s/not. The front optic has a 0.3° primary slit and a 2.5° Soller slit. The sample rotates on itself at a speed equal to 15 rpm, using the automatic knife. The rear optics include a 2.5° Soller slit, a 0.0125 mm nickel filter and a 1D detector with an aperture equal to 4°.

The diffraction patterns are then analyzed qualitatively using EVA version 6.0 software and the COD database.

The COD 4308076 file from the COD database (“Crystallography Open Database”) allows the mayenite phase to be identified. The peaks of the mayenite phase present in the melt may present a slight shift compared to the data sheets used, depending on the presence of the element Sr, and the elements chosen from Mg, Ga, Si, Ge, Sn, Ti, Zr, Cr, Mn, Fe, Co, Ni, Cu, Zn, and mixtures thereof.

Once the phases present have been identified, the mass quantities of mayenite and other crystallized phases are evaluated by Rietveld refinement using HighScore Plus software.

The quantity of amorphous phase present in a molten product is measured by X-ray diffraction, using a D8 Endeavor type device from the Bruker company according to the following method. The acquisition of the diffraction pattern is carried out using this equipment, in the same way as for the determination of the crystallized phases, the analyzed sample being in the form of a powder having a refusal at 40 pm of less than 5%. in mass.

After opening the diffraction pattern obtained with the EVA 6.0 software, click on the “define background” icon: a “define background” window appears on the screen.

In the “Set Background” window, select “Original Measurement” and “Add Background as Scan”, then check the “Auto Curvature and Threshold” box.

In the “Properties” window of the diffraction pattern, check the “Calculate crystallinity” and “Show amorphous” boxes.

The value for the amount of amorphous phase is the “%-Amorphous” value in the “Properties” window of the diffraction pattern, in mass percent based on the mass of the sample. Manufacturing protocol

The examples were prepared from the following raw materials: an alumina powder with a purity greater than 99.8% by mass, and having a median size equal to 90 μm, a calcium carbonate powder with a purity by mass greater than 99 .3%, and having a median size equal to 1.6 pm, petroleum coke.

The molten product of Example 1*, comparative, was prepared according to the teaching of

EP 1 717 217, as follows: 37.3 g of the alumina powder and 62.7 g of the calcium carbonate powder were mixed in a jar mixer for 30 minutes, then the mixture was placed in a carbon crucible, said crucible being closed using a carbon lid. Finally, the crucible thus filled was placed in the quartz tube of a tubular furnace, an argon atmosphere being maintained in said quartz tube during the entirety of the following heat treatment: rise from ambient temperature to 1650°C at a speed equal to 400°C/h, maintaining at 1650°C for 9h30, lowering from 1650°C to ambient temperature at a speed equal to 400°C/h.

The molten product of Example 2*, comparative, was prepared in the same way as Example 1*, the starting charge containing an additional 3.1 g of petroleum coke (3%).

The product of Example 3, according to the invention, was prepared according to the following manufacturing process, in accordance with the invention: a) production, by mixing, of a starting charge consisting of 36.2% of the alumina powder, 60.8% of calcium carbonate powder and 3% of petroleum coke, in mass percentages based on the mass of the starting charge; b) melting in a reducing medium of said starting charge in a single-phase electric arc furnace of the Hérault type with graphite electrodes, with a furnace tank of 0.8 m in diameter, a voltage of 80 V, an intensity of 2250 A , and a specific electrical energy supplied of 3 kWh/kg loaded, the electrodes soaking in the bath of molten material; c) sudden cooling of the molten material by means of a casting device between thin metal plates such as that presented in patent US-A-3, 993,119, so that the molten material is entirely solidified under the form a plate in less than 3 minutes. The product of Example 4, according to the invention, was prepared in the same way as the product of Example 3, only step c) being different: the molten material was poured into a graphite mold with dimensions 220*200*180 mm ³ so as to obtain the molten product. The cooling of the molten material is slower than that carried out for Example 3. The product of Example 5, according to the invention, was prepared according to the following manufacturing process, in accordance with the invention: a) production, by mixing, of a starting charge consisting of 37.3% of the alumina powder and 62.7% of the calcium carbonate powder, in mass percentages based on the mass of the charge of departure, b) melting in a reducing medium of said starting charge in a single-phase electric arc furnace of the Hérault type with graphite electrodes, with a furnace tank of 0.8 m in diameter, a voltage of 88 V, an intensity of 2250 A, and a specific electrical energy supplied of 6 kWh/kg loaded, the electrodes soaking in the bath of molten material; c) sudden cooling of the molten material by means of a casting device between thin metal plates such as that presented in patent US-A-3, 993,119, so that the molten material is entirely solidified under the form a plate in less than 3 minutes.

The following table 1 summarizes the results obtained.

[Table 1]

nd: not determined *: excluding invention

The products of Examples 1 and 2 contain little or no mayenite.

The products of Examples 3 and 4 have a black color, characteristic of a mayenite electride having high electrical conductivity.

The product of Example 5 has a yellow color, characteristic of a mayenite electride having low electrical conductivity.

The products of Examples 3 to 5 have a quantity of amorphous phase less than 10%, as a mass percentage based on the mass of the melted product.

In example 1, the fusion was carried out in a closed carbon crucible placed in a neutral argon atmosphere, the starting charge not containing any reducing agent. It can be seen that Example 1 does not contain mayenite, whereas a process according to the invention leads to a quantity of mayenite electride greater than 90%, as a percentage by mass based on the mass of the crystallized phases. The virtual absence of mayenite is also contrary to the teaching of EP 1 717 217.

A comparison of Examples 1* and 2* shows that, under the same manufacturing conditions, an addition of 3% petroleum coke as a reducing agent in the starting charge (Example 2*) increases the quantity of mayenite, but is not sufficient. not to obtain a quantity of a mayenite phase electride greater than 90%, in percentage by mass based on the mass of the crystallized phases.

A comparison of Examples 2* and 3 shows that, surprisingly, with the same content of petroleum coke (reducing agent) in the starting charge (3%), the melting being carried out in a reducing medium, the use of an electric arc furnace makes it possible to obtain a molten product comprising more than 90% of a mayenite phase electride, in percentage by mass based on the mass of the crystallized phases.

The considerable increase in the quantity of mayenite, multiplied by 2.9, does not result only from the additional carbon provided by the electrodes, the quantity of which is marginal. It does not result either from a difference in the quantity of reducing agent, these quantities being identical for the two examples. Surprisingly, and without it currently being possible to theoretically explain it, this increase is therefore attributed to the use of an electric arc furnace.

A comparison of Examples 3 and 4 shows that rapid cooling was favorable to the quantity of mayenite. Example 5 finally shows that the presence of a reducing agent is not essential to obtain a high electride content of mayenite, provided that the fusion is carried out with an electric arc furnace adjusted so as to ensure fusion in medium reducer.

Remarkably, the Raman spectrum of Example 3 according to the invention includes a band at 1870 cm' ¹ , the Raman spectrum being determined using a monochromatic source of wavelength equal to 532 nm.

As shown in the examples, the process according to the invention, simple and inexpensive due to the fact that it only involves a single melting step, makes it possible to manufacture a molten product comprising a very large quantity of mayenite electride. In other words, it is not necessary to carry out several heating steps before obtaining the desired quantity of mayenite phase.

Of course, the present invention is not limited to the embodiments described provided by way of illustrative and non-limiting examples.

In particular, the melted products according to the invention are not limited to particular shapes or dimensions.

Claims

Claims Process for manufacturing mayenite in the form of an electride, said process comprising the following successive steps: a) making a starting charge; b) melting said starting charge until a molten material is obtained, c) cooling said molten material so as to solidify it and obtain a polycrystalline molten product, said starting charge being adapted, to step a), to obtain, at the end of step c), a product comprising more than 90% of a mayenite comprising

- calcium and/or strontium,

- aluminum and

- oxygen, in mass percentage based on the mass of the crystallized phases; d) optionally, grinding the molten product so as to give it the form of a powder; e) optionally, particle size selection on the powder; said fusion being carried out with an electric arc furnace and in a reducing medium. Method according to the preceding claim, in which, in step b), the electrodes of the electric arc furnace soak in the molten material. Method according to any one of the preceding claims, wherein in step a), the starting charge comprises more than 0.5% of a reducing agent, in mass percentage based on the mass of the starting charge . Method according to any one of the preceding claims, wherein in step a), the starting charge comprises less than 7% of a reducing agent, in mass percentage based on the mass of the starting charge. Process according to any one of the two immediately preceding claims, in which the reducing agent is chosen from a carbon source, a metal, and mixtures thereof. Process according to any one of the preceding claims, wherein in step a), the starting charge contains less than 50% mayenite comprising calcium and/or strontium, aluminum and oxygen. Method according to any one of the preceding claims, in which the electric arc furnace comprises graphite electrodes and, in step b), the fusion of said starting charge is carried out with the electrodes arranged at a distance from the charge starting point adapted to obtain fusion in a reducing medium. Method according to any one of the preceding claims, wherein in step c), the cooling is adapted so that the molten material is completely solidified in less than 3 minutes. Method according to any one of the preceding claims, comprising, after step c), and/or, when the method comprises steps d) and/or e), after said steps d) and/or e), a step f) in which only part of the free electrons of the mayenite of said molten product are replaced by anions, preferably all identical. Process according to the immediately preceding claim, in which the anions are chosen from F, CP, OH', H', Of, O', O ₂ ^(2_) , N ³ ', NH ² ', NHf, C ₂ ⁽² ' ^} , S', CN', NOf, Sf, and mixtures thereof, said process comprising.

- a process according to any one of claims 1 to 8, and

- after step c), and/or, when said method according to any one of claims 1 to 8 comprises steps d) and/or e), after said steps d) and/or e), a step f ) in which, substantially all the free electrons of the mayenite of said melt are replaced by anions. Process according to the immediately preceding claim, in which the anions are chosen from F, CP, OH', H', Of, O', O ₂ ⁽² ' ^} , N ³ ', NH ² ', NHf , C ₂ ⁽² ' ^} , S', CN', NOf, Sf, and mixtures thereof. Polycrystalline molten product comprising more than 90% mayenite in the form of an electride, in mass percentage on the basis of the crystallized phases, said mayenite comprising

- calcium and/or strontium,

- aluminum and - oxygen, said molten product comprising carbon in an amount greater than or equal to 15 ppm and less than or equal to 5%, by mass based on the mass of the molten product. Melted product according to the immediately preceding claim,

- presenting more than 95% mayenite in the form of an electride, in percentage by mass on the basis of the crystallized phases; and or

- having a carbon content greater than 300 ppm and less than 3%; and or

- having a total quantity of the elements Ca, Sr, Al, C and O, Ca+Sr+Al+O+C greater than 97%, in percentage by mass based on the mass of the melted product; and or

- having a ratio (CaO+SrO)/AhO3, CaO, SrO, and AI2O3 being molar contents on the basis of the oxides, greater than 1.60 and less than 2; and/or in which the mayenite comprises at least one element chosen from Mg, K, Na, Li, Ba, Ga, Si, Ge, Sn, Ti, Zr, Cr, Mn, Fe, Co, Ni, Cu, Zn. Melted product according to the immediately preceding claim,

- presenting more than 98% mayenite in the form of an electride, in mass percentage based on the crystallized phases; and or

- having a carbon content greater than 700 ppm and less than 1.5%; and or

- having a total quantity of the elements Ca, Sr, Al, C and O, Ca+Sr+Al+O+C greater than 98%, in percentage by mass based on the mass of the melted product; and or

- having a ratio (CaO+SrO)/AhO3, CaO, SrO, and AI2O3 being molar contents on the basis of the oxides, greater than 1.68 and less than 1.80. Melted product according to any one of the three immediately preceding claims, having an electrical conductivity between 1.10' ⁴ S/cm and 1 S/cm or an electrical conductivity greater than 1 S/cm. Melted product according to any one of the four immediately preceding claims, having a Raman spectrum comprising a band at 1870 cm ⁴ , the Raman spectrum being determined using a monochromatic source of wavelength equal to 532 nm. Molten product according to any one of the four immediately preceding claims, manufactured by a process according to any one of claims 1 to 10. Polycrystalline melted product of white color comprising more than 90% mayenite of formula ((Ca24AhsO64) ⁴⁺ ) (O ² ')2, in mass percentage on the basis of the crystallized phases, said mayenite comprising

- calcium and/or strontium,

- aluminum and

- oxygen, said molten product comprising carbon in an amount greater than or equal to 15 ppm and less than or equal to 5%, by mass based on the mass of the molten product. Molten product according to the immediately preceding claim, manufactured by a process according to any one of claims 11 to 12. Device comprising

- a product according to any one of claims 13 to 18, said device being chosen from:

- a catalyst support;

- an electrode of a solid oxide fuel cell;

- an organic light-emitting diode;

- a Hall effect thruster;

- a grid ion thruster;

- a proton-conductive ceramic fuel cell;

- a hydrogen storage and/or transport device, or

- a product according to any one of claims 19 to 20, said device being chosen from:

- an electrolyte of a solid oxide fuel cell;

- a solid oxide electrolysis cell;

- a CO2 capture device;

- a hydrogen storage and/or transport device.