FR2750975A1 - PROCESS FOR THE PREPARATION OF A SODIUM ALUMINA-BETA PRODUCT COMPOUND, PRECURSOR MIXTURE FOR THE PREPARATION AND COMPOUND OBTAINED THEREBY - Google Patents

Abstract

The invention relates to a process for making a compound produced by sodium beta-alumina, a precursor mixture intended to be heated to produce such a product and the compound obtained by such a process from the precursor mixture. Particulate comprising a doping component and a sodium beta-alumina component is formed and heated to a temperature of at least 1200 deg.C. The doping component comprises a mixture of the oxides of titanium and tantalum, respectively, and the sodium beta "alumina component is sodium beta" alumina stabilized with lithium oxide or a precursor thereof. . The oxides of titanium and tantalum respectively are present in specific proportions amounting respectively to 0.3-0.5% by weight of sodium beta-alumina and 0.05-0.15% by weight of sodium alumina. beta "of sodium. Application to electrochemical accumulator elements.

Description

A process for the preparation of a sodium alumina product compound,

  precursor mixture for this preparation and

compound obtained by this method.

  This invention relates to a process for the preparation of a sodium alumina compound, more particularly to a process for the preparation of such a compound in the form of a polycrystalline worker product, or product

  worked when manufactured according to the process.

  The sodium alumina-P 'compounds form a family of polyaluminates which have the general formula: Na 2 O x AlO 3 O 3 wherein x has a value expressed as 4 x x 6. Metastable P phase with rhombohedral symmetry The 5% sodium alumina compounds, for stability at elevated temperatures at which they can be sintered, must have their P phase stabilized by a spinel stabilizing oxide dispersed therein. a concentration of 2-3 mol% (as oxide) when the spinel stabilizing oxide has a monovalent cation such as lithium An example of a sodium oxide-stabilized sodium alumina compound Lithium is LiNA5Al32O051. The sodium alumina compounds have a structural backbone comprising spinel blocks in the form of aluminum / oxygen blocks and these blocks are serially spaced apart from one another by

  series of conduction planes occupied by sodium.

  According to the invention, there is provided a process for preparing a product compound of sodium alumina, the process comprising the steps of: forming a particulate precursor mixture comprising a doping component and a constituent of sodium alumina-P "; and heating the precursor mixture to a temperature of at least 1200 ° C, characterized in that the doping component comprises a mixture of the oxides (respectively in the form of TiO 2 and Ta 2 O 5 or in the form of TiO 2 and Ta 2 O 5 precursors) respectively of titanium and tantalum, and the sodium alumina-P "component being selected, on the one hand, from sodium 3-alumina compounds in the form of spinel stabilized with lithium oxide (Li 2 O) and on the other hand from mixtures of sodium oxide (in the form of Na2O or in the form of an Na2O precursor) and aluminum oxide calcine (in the form of Al2O3 or as a precursor of A1203), having lithium oxide (in the form of Li 2 O or in the form of a Li 2 O precursor) as a spinel stabilizer dispersed therein, the proportions on a weight basis of the various constituents, in the form of said oxides and with the proportion of the content of alumina-3 "being taken equal to 10 0, being as follows: i Component Proportion (as oxide) (parts by weight) 3 "alumina or its precursor (as Li 2 O + Na 2 O + Al 2 O 3): 100 dopant or its precursor (in the form of TiO 2 + Ta 2 O 5): 0.35-0.65 titanium oxide or its precursor (in the form of TiO 2): 0.3-0.5 tantalum oxide or its precursor (as Ta 2 O 5): 0.05 -0.15 sodium oxide or its precursor (in the form of Na 2 O): 8.6-9.6 lithium oxide or its precursor (in the form of Li 2 O): 0.58-0.83 By doping constituent, under Oxide mixture form means the oxides of the titanium and tantalum metals of the doping component in their fully oxidized state, i.e. TiO 2 and Ta 2 O 5; and the mass ratio that the dopant constituent bears relative to the 3-alumina-sodium component is with the component of alumina-o "(with all its component oxides, that is, the Al 2 O 3, the

  Na 2 O and Li 2 O) being in their fully oxidized state.

  By a precursor mixture is meant a particulate mixture which comprises compounds which, when heated to 1000 C in air, produce the above aluminum / oxygen spinel blocks, with the sodium oxide which produces the mobile sodium in the conduction planes, with lithium oxide as a spinel stabilizer, the precursor mixture, when heated in air up to 1200 C, undergoing a reaction of its constituents, thanks to which the The structural skeleton of alumina-p "is formed by an aluminum oxide precursor means an element or compound which, when heated to 1000 C in air, produces Al 2 O 3, and the word or the term "precursor", with respect to TiO2 and Ta2O5 and with respect to Na20 and Li20, has a corresponding meaning, although heating at a temperature below 1000 C, for example 250-1000 C, may naturally in some cases be enough to get the oxide in quest By calcined aluminum oxide (or a precursor thereof) is meant the calcined obtained by heating the aluminum oxide (or a precursor thereof) in the air at a temperature of 50.degree.

  maximum temperature which is between 250 C and 1200 C.

  Titanium oxide (or its precursor) is preferably used in the P-alumina precursor mixture in a proportion whereby, as mentioned above, it (as fully oxidized oxide) forms 0.3-0.5% by weight of the alumina-f "component, the tantalum oxide (or its precursor) being preferably used in a proportion by which it (as fully oxidized oxide) The titanium oxide and tantalum oxide in question can, however, be used in any of their permitted oxidation states. and,

  when they are not completely oxidized, they will be the precursors respectively of TiO2 and Ta205.

  As noted above, two different routes may

  be followed according to the method of the present invention.

  According to one of these routes, an undoped sodium spinel-stabilized sodium alumina compound is first produced, typically in the form of a powder, in which the sodium p-alumina is stabilized by lithium oxide as a spinel stabilizer by heating in air at a temperature of at least 1200 ° C, which sodium alumina-P "is then used as the alumina component -P "sodium in the precursor mixture. According to the other of these routes, instead of the compound of sodium alumina 3 "as constituent of sodium alumina-P", a mixture of sodium oxide (or a precursor thereof) is used. ) with an aluminum oxide calcine (or precursor thereof) having a lithium oxide (or a precursor thereof) as a stabilizer of

spinels dispersed in it.

  Accordingly, an undoped spinel-stabilized sodium alumina compound is first produced in a stabilized Li 2 O form as a spinel stabilizer by heating the constituents in air at a temperature of 100.degree. at least 1200 ° C, the undoped 3-alumina compound of spinel-stabilized sodium being then used as a constituent of sodium alumina-P1 'in the precursor mixture, or, instead, as the 3-alumina component, a mixture of Na 2 O (or a precursor thereof) can be used with a calcine of Al 2 O 3 (or a calcine of a precursor of Al 2 O 3), the mixture of A1203 or its precursor and Na20 or its precursor having Li20 (or a precursor thereof) dispersed therein

spinel stabilizer.

  The calcined can be A1203 (or a precursor thereof)

  chosen to produce, when heated in air at 1000 C, A1203 with a grain size of 0.5-010pm.

  The precursors of the various metal oxides in question may be chosen from the metals themselves or from hydroxides, oxyhydroxides, peroxides, nitrates, carbonates, sulphates, chlorates, halides, 30) oxyhalides and organometallic compounds, such as alkoxides, from such metals, or among oxides of such metals in which the metals are not completely in their oxidized state. Such precursors may be selected from water-soluble salts of the metals in question, for example nitrates and halides which, when subjected to a change in pH, precipitate to form insoluble species with average dimensions. particles smaller than 0, lpm; or they may be selected from alkoxides which, when dissolved in alcohols such as methanol or ethanol and then contacted with water, precipitate as oxyhydroxides with an average size of particles less than 0, lpm. Preferred precursors are selected from the alkoxides or halides of the metals in question. The sodium alumina-P "used in the first route is preferably selected to have a well-ordered aluminum oxide subarray and any aluminum oxide (or its precursor) used in the other route is preferably chosen to produce, when heated to 1000 ° C., an aluminum oxide grain size of 0.5-0.0 μm, preferably 5-50 μm, having a well-defined aluminum oxide sub-network. Boehmite and bayerite aluminum oxide precursors are described in US-A-4,732,741 and US Pat.

US Patent 4,946,664.

  Accordingly, at least one of the constituents of the precursor mixture may be a precursor of a metal oxide selected from TiO 2, Ta 2 O 5, Li 2 O, Na 2 O and Al 2 O 3, each metal oxide precursor being selected from the group of compounds consisting of oxides in which the metal is in a permissible oxidation state in which it is not completely oxidized, hydroxides, oxyhydroxides, peroxides, nitrates, carbonates, sulfates, chlorates, halides, oxyhalides and compounds organometallics. In addition, both titanium and tantalum oxides can be used in the precursor mixture as precursors of TiO 2 and Ta 2 O 5, respectively, which are titanium oxides and

  tantalum which are respectively, as indicated above, in permissible oxidation states of titanium and tantalum respectively in which titanium and tantalum are not completely oxidized respectively to TiO 2 and Ta 2 O 5.

  More particularly, as indicated above, at least one of said precursors of a metal oxide may be a water-soluble salt capable of being precipitated from an aqueous solution by a pH change to form a water-insoluble precipitate which has an average particle size of less than 0.1 μm. Instead, at least one of said precursors of a metal oxide may be an organometallic compound selected from alkoxides that are capable of being precipitated from an alcohol solution by adding water to the solution in a solution. an alcohol, to form an oxyhydroxide precipitate with a

  average particle size less than 0.1 μm.

  Typically, the precursor mixture will be milled, for example wet milled into an aqueous or organic sludge, to a particle size of at most 10 μm, preferably 0.2-5 μm, to make it substantially homogeneous, followed by drying, for example drying under vacuum or by spraying, to a moisture content of at most 2% by weight, preferably 0.5-1% by weight, by consolidation into a semi-finished product. compressed by compression (for example uniaxial and / or isostatic compression) to a maximum pressure of at least 10 MPa,

  preferably 150-300 MPa, and by air sintering to a temperature of at least 1580 C, preferably 1600-

1620 C.

  In particular, the precursor mixture may be milled to a particle size of at most 10 μm, to make it substantially homogeneous, followed in turn by drying the milled mixture to a moisture content of at most 2 μm. % by weight, by consolidating the dried mixture into a semi-worked product by compression at a pressure of at least 10mpa and by sintering the consolidated mixture in air to a temperature of at least

1580 C.

  The alumina-P "component or its precursor and the doping component or its precursor may together form at least 95%, typically 99%, preferably at least 99.5%, for example 99.9% by weight of the product. precursor mixture, such that the precursor mixture of alumina-3 "essentially comprises a mixture of the oxides of sodium, lithium, aluminum, titanium and tantalum

  in their completely oxidized states.

  According to another aspect of the invention, there is provided a precursor mixture for heating to produce a product compound of 3 "alumina, the precursor mixture comprising a doping component and a P-alumina component. of sodium, a precursor mixture characterized in that the doping component comprises a mixture of the oxides (respectively in the form of TiO 2 and Ta 2 O 5 or in the form of TiO 2 and Ta 2 O 5 precursors) respectively of titanium and of tantalum and the alumina P "of sodium is chosen, firstly, from lithium alumina-fP" compounds in the form of spinel stabilized by lithium oxide (Li 2 O) and, secondly, from oxide mixtures. of sodium (as Na 2 O or as a precursor of Na 2 O) and aluminum oxide calcine (as Al 2 O 3 or as a precursor of Al 2 O 3), having lithium oxide (as Li20 or as a precursor of Li2O) as stabilizer The amount of spinels dispersed therein, the proportions on a weight basis of the various constituents, in the form of said oxides and with the proportion of the alumina content-P "being taken to be equal to 100, being as follows: Proportion Proportion (in the form of oxide) (parts by weight) sodium alumina or its precursor (in the form of Li 2 O + Na 2 O + Al 2 O 3): 100 dopant or its precursor (in the form of TiO 2 + Ta 2 O 5): 0.35-0.65 oxide titanium or its precursor (in the form of TiO2): 0.3-0.5 tantalum oxide or its precursor (in the form of Ta2O5): 0.05-0.15 () sodium oxide or its precursor (in the form of Na 2 O): 8.6-9.6 lithium oxide or its precursor (in the form of Li 2 O): 0.58 -0t. The sodium alumina-P "component can be a sodium alumina-P" compound stabilized in the form of spinel, stabilized with Li 2 O. Instead, the 3% alumina component may be a mixture of Na 2 O (or a precursor thereof) with a calcined Al 2 O 3 (or a) calcined with a precursor of Al 2 O 3), the mixture having Li 2 O (or a precursor thereof) dispersed therein as a spinel stabilizer and, in this case, the calcine may be an Al 2 O 3 (or a precursor thereof) selected to produce, when heated to air up to 1000 C, from

  A1203 with a grain size of 0.5-100lpm.

  At least one of the constituents of the precursor mixture may be a precursor of a metal oxide selected from TiO 2, Ta 2 O 5, Li 2 O, Na 2 O and Al 2 O 3, each metal oxide precursor being selected from the group of compounds consisting of oxides in which the metal is in a permissible oxidation state in which it is not completely oxidized, hydroxides, oxyhydroxides, peroxides, nitrates, carbonates, sulphates, chlorates, halides, oxyhalides and compounds organometallics. The oxides of both titanium and tantalum can be precursors respectively of TiO 2 and Ta 2 O 5, which are titanium and tantalum oxides 1 () which are respectively in permissible oxidation states of titanium and tantalum in which titanium and tantalum are not completely oxidized respectively to TiO2 and Ta2O5. At least one precursor of a metal oxide may be a water-soluble salt capable of being precipitated from an aqueous solution thereof by a change in pH, to form a water-insoluble precipitate with an average size of particles less than 0.1 pm. Instead, at least one precursor of a metal oxide may be an organometallic compound selected from alkoxides which are capable of being precipitated from a solution in an alcohol by adding water to the solution in an alcohol. form an oxyhydroxide precipitate with an average particle size

less than O, lym.

  Each of the constituents may have a particle size of at most 10 μm; and the P-alumina component or its precursor and the doping component or its precursor can together form at least 95%, typically 99%, preferably at least 99.5%, for example 99.9%. by weight of the precursor mixture, such that the precursor mixture of alumina-P "essentially comprises a mixture of the oxides of sodium, lithium, aluminum,

  titanium and tantalum in their completely oxidized states.

  The invention also relates to a sodium alumina compound having a spinel structure which is stabilized by Li 2 O which acts as a spinel stabilizer, the sodium alumina compound having TiO 2 and Ta 2 O 5 dispersed therein as a dopant, the sodium alumina-P "compound having constituents which are respectively lithium, sodium, aluminum, titanium and tantalum oxides in their fully oxidized Li 2 O form. , Na 2 O, Al 2 O 3, TiO 2 and Ta 2 O 5, the proportions of these oxides, with Li 2 O, Na 2 O and Al 2 O 3 being present in the form of Li 2 O-stabilized sodium alumina, being as follows: II Proportionate Proportion (in the form of oxide) (parts by weight) sodium alumina stabilized with Li 2 O (Li 2 O + Na 2 O + Al 2 O 3) 100 dopant (TiO 2 + Ta 2 O 5): 0.35-0.65 titanium oxide (TiO 2): 0.30- 0.50 tantalum oxide (Ta2O5): 0.05-0.15 [l sodium oxide (Na2O): 8.6-9.6 lithium oxide (Li2O): 0.58-0.83 Li2O, Na2O and A1 203 (in the form of spinel-stabilized sodium alumina) and TiO0 and Ta205 can together form at least 95%, typically 99%, preferably at least 99.5%, for example 99.9% by weight of the compound such that the compound essentially comprises said lithium, sodium oxides,

  of aluminum, titanium and tantalum.

  2 () The compound may be in the form of a semi-worked product

polycrystalline fused.

  The invention also relates to a product compound of sodium alumina, in particular in the form of a semi-worked sintered product when it is obtained by the

method described above.

  The invention will now be described, by way of illustration

  non-limiting, with reference to the following example.

EXAMPLE

  Alumina-B "sodium stabilized with lithium oxide

15 and doped with TiO 2 and Ta 2 O 5.

  Two precursor mixtures were formed by different routes according to the method of the present invention. In each case, a lithium aluminum stabilized sodium alumina component was prepared according to the process, from the highly crystalline boehmite cera hydrate described in US-A-4,946,664, to teach in each case 8.95% by weight of Na 2 O and 0.6% by weight of Li 2 O, the remainder being Al 2 O 3, and was mixed with a TiO 2 powder and a Ta 2 O 5 powder to form a particulate precursor mixture. TiO 2 was present in a proportion of 0.4% by weight of the product and 1) the Ta 2 O 5 powder in a proportion of 0.1% by weight.

  product weight, on a dry basis.

  According to the first route, the above sodium alumina-P ", TiO 2 and Ta 2 O 5 components were wet-milled in a friction mill using a ZrO 2 grinding medium and isopropanol as the liquid (at 20 ° C.). n-pentanol can be used for a period of 75 minutes to form a homogenous sludge having a particle size of at most 5 pm The slurry was dried in a rotary vacuum dryer and sieved. through

  a sieve having apertures 0.25mm in diameter.

  According to the second route, said components were mixed in water for a period ranging from 2 to 4 hours, to form a homogeneous slurry having a particle size of at most 5 μm which was dried using

a spray dryer.

  Each of the powders was uniaxially compressed to 20 MPa () to form a semi-worked product which was then isostatically compressed at a pressure of

  and 300 MPa to obtain a crude semi-worked product for sintering.

  Each crude semi-worked product was then air-sintered at 1620 ° C. in a magnesia crucible for 15 minutes to obtain a sintered ceramic semi-worked product with a high P-phase content greater than 90% by weight. phase ["and with high density

  greater than 96% of the theoretical total density.

  The ionic resistivities of the semi-worked products were measured with that of a compressed and sintered control semi-workpiece in the same manner as described above from the sodium alumina-P "component of the precursor mixture, by an impedance method using Al blocking electrodes at temperatures between

  l) 20 C and 350 C in the frequency range of 1 Hz-5x105 Hz.

  The semi-worked product produced by the first road had resistivities of 2.6 Qcm at 300 C and that produced by the second road had resistivities of 2.7 Qcm at 300 C. For the control, the resistivity values were substantially higher, being equal to 5.0 Qcm at

300 C.

  In lithium oxide stabilized sodium alumina products known to the Applicant, which have highly mobile sodium ions in their conduction plane and are exploited in electrochemical detectors and electrochemical accumulators, for example as the solid electrolyte conducting the sodium ions in the electrochemical sodium / sulfur storage elements or the electrochemical sodium / metal chloride accumulators The products are typically used in the form of substantially single-phase dense single-phase polycrystalline ceramics. A1203.3 () The applicant believes, without being limited by theory, that in these known products small proportions of impurity metal cations, such as calcium cations, may occupy the conduction planes of sodium ions under the form of oxides thereof such as calcium oxide to provide a mixed ion effect that interferes with the mobility and transport of sodium ions along such conduction planes and reduces the conductivity of the sodium ions of the product. Moreover, without being limited by theory, the applicant believes that other impurities such as silica are capable of forming glassy phases, such as sodium silicate phases and sodium aluminosilicate phases that can increase intragranular impedance. such products, helping to lower the ionic conductivity of such products. Impurities of iron oxide in the product may have effects

similar undesirable effects.

  During the production of sodium alumina products, impurities containing calcium, silicon and iron, present in the raw material used or introduced during the treatment, can lead to dispersion and a lack of reproducibility of the structural properties and the resistivity / conductivity of the sodium ions of such products The present invention, at least with reference to the example, provides a method of manufacturing a product of 3-alumina of sodium, in particular in the form of a 2) sintered ceramic semi-worked product in which the high phase content P ", the density, the resistance and the conductivity of the sodium ions are improved, In particular, the process improves a reduction of the dispersion. above-mentioned and stimulates the reproducibility of the conductivity of sodium ions, independently of the variation of its iron content and more particularly of its calcium oxide and silica contents. The applicant believes that this, without being limited by theory, can be achieved by the tight binding of the impurity metals or their oxides by the doping oxides and / or their metals so that they are kept captive and immobilized. in phases other than the "phase of the product compound and are prevented from interfering with ionic conduction in the conduction planes of the f" phase and / or are prevented from reacting with sodium-containing glassy phases in the

compound produced.

Claims (23)

  1. A process for producing a sodium alumina product compound, comprising the steps of: forming a particulate precursor mixture comprising a doping component and a sodium alumina-1 'component and Heating the precursor mixture to a temperature of at least 1200 ° C, characterized in that the doping component comprises a mixture of the oxides (respectively in the form of TiO 2 and Ta 2 O 5 or in the form of TiO 2 and Ta 2 O 5 precursors respectively) titanium and tantalum, and the constituent of alumina-g "of sodium being chosen, firstly among the compounds of sodium alumina-P" stabilized in the form of spinel by lithium oxide (Li 2 O) and 2 () on the other hand from mixtures of sodium oxide (in the form of Na 2 O or in the form of an Na 2 O precursor) and of an aluminum oxide calcine (in the form of Al 2 O 3 or in the form of of a precursor of Al 2 O 3), having lithium oxide (in the form of Li 2 O or in the form of a Li 2 O precursor) as a spinel stabilizer dispersed therein, the proportions on a weight basis of the various constituents, in the form of said oxides and with the proportion of the alumina content-g "being taken as 100, being as following: 3o) Component Proportion (as oxide) (parts by weight) sodium alumina or its precursor (as Li20 + Na2O + Al2O3): 100 dopant or its precursor (as TiO2 + Ta2O5 ): 0.35-0.65 titanium oxide or its precursor 166 (as TiO2) 0.3-0.5 tantalum oxide or its precursor (as Ta205): 0.05-0.15 sodium oxide or its precursor (in the form of Na2O): lithium 8.6-9.6 or its precursor (in the form of Li2O): 0.58-0.83 2. A process according to claim 1, characterized in that a sodium alumina compound is first produced in a stabilized Li 2 O form as a spinel stabilizer by heating its components to the air at a temperature of at least 1200 ° C, the undoped spinel stabilized sodium alumina compound being then used as the sodium alumina component in the precursor mixture. 3. A process according to claim 1, characterized in that a mixture of Na 2 O (or a precursor) is used as the constituent of sodium alumina-P ".   of it) with a calcine of Al 2 O 3 (or a calcine of a precursor of Al 2 O 3), the mixture of the calcine of Al 2 O 3 or its precursor and Na 2 O or its precursor having Li 2 O 25 (or one of its precursors) dispersed therein as a spinel stabilizer.   4. A process according to claim 3, characterized in that the calcine is a calcine of Al 2 O 3 (or a precursor thereof) selected to produce, when heated in air up to 1000 C, Al 2 O 3 with   a grain size of 0.5-10 μm.   5. A method according to any one of claims 1 to   3j 4, characterized in that at least one of the constituents of the precursor mixture is a precursor of a metal oxide chosen from TiO 2, Ta 2 O 5, Li 2 O, Na 2 O and Al 2 O 3, each metal oxide precursor being chosen from the group of compounds consisting of of oxides in which the metal is in a permissible oxidation state, in which it is not completely oxidized, hydroxides, oxyhydroxides, peroxides, nitrates, carbonates, sulphates, chlorates, halides, oxyhalides and organometallic compounds.   6. A process according to claim 5, characterized in that the oxides of both titanium and tantalum are used in the precursor mixture as precursors respectively of TiO 2 and Ta 2 O 5, which are oxides of titanium and tantalum which are respectively in permitted oxidation states of titanium   and tantalum in which titanium and tantalum are not completely oxidized respectively to TiO 2 and Ta 2 O 5.   7. A process according to one of claims 5 and 6, characterized in that at least one of said precursors   of metal oxides is a water-soluble salt capable of being precipitated from an aqueous solution thereof by a change in pH to form a water-insoluble precipitate with an average particle size of less than 0, lpm .25   8. A method according to one of claims 5 and 6, characterized in that at least one of said precursors   of metal oxides is an organometallic compound selected from alkoxides which are capable of being precipitated from an alcohol solution by adding water to the solution in an alcohol to form a precipitate   of oxyhydroxide with an average particle size of less than 0.1 μm.   9. A process according to any one of claims 1   at 8, characterized in that the precursor mixture is ground to a particle size of at most 10 μm to make it substantially homogeneous, followed in turn by drying the ground mixture at a moisture content of at most 2% by weight, by consolidating the dried mixture into a semi-worked product by compression at a pressure of at least 10 MPa and by sintering the consolidated mixture in air at a temperature of temperature of at least 1580 C.   10. A process according to any one of claims 1   to 9, I) characterized in that the p-alumina component or its precursor and the doping component or its precursor   together form at least 95% by weight of the precursor mixture, such that the precursor mixture of alumina-P "essentially comprises a mixture of the oxides of sodium, lithium, aluminum, titanium and tantalum in their complete states. oxidized.   11. A precursor mixture for heating to produce a sodium p-alumina product compound, the precursor mixture comprising a doping component, and a sodium alumina-1 "component, a precursor mixture characterized in that that the doping component comprises a mixture of the oxides (respectively in the form of TiO 2 and Ta 2 O 5 or in the form of TiO 2 and Ta 2 O precursors) respectively of titanium and of tantalum, and the component of sodium alumina is chosen from one of sodium 3-alumina compounds stabilized as spinel by lithium oxide (Li 2 O) and, on the other hand, of mixtures of sodium oxide (as Na 2 O or in the form of a Na 2 O) precursor and an aluminum oxide calcine (as Al 2 O 3 or as a precursor of Al 2 O 3), having lithium oxide (as Li 2 O or as a form of a Li 2 O precursor as dispersed spinel stabilizer in the proportions on a weight basis of the various constituents, in the form of said oxides and with the proportion of the alumina content being equal to being as follows: Component Proportion (as oxide) (parts by weight) alumina Sodium or its precursor I (in the form of Li 2 O + Na 2 O + Al 2 O 3): 100 dopant or its precursor (in the form of TiO 2 + Ta 2 O 5): 0.35-0.65 titanium oxide or its precursor (in the form of TiO2 form): 0.3-0.5 tantalum oxide or its precursor (as Ta2O5): 0.05-0.15 sodium oxide or its precursor (as Na2O): 8.6-9 , 6 lithium oxide or its precursor (as Li 2 O): 0.58-0.83 12. A mixture according to claim 11, characterized in that the component of alumina-3 "sodium is a compound of alumina-P" sodium stabilized in form   of spinel, stabilized with Li 2 O.   13. A mixture according to claim 11, characterized in that the sodium alumina-3 "component is a mixture of Na 2 O (or a precursor thereof) with a calcine of Al 2 O 3 (or a calcined dimer). a precursor of Al 2 O 3), the mixture having Li 2 O (or a precursor of   this one) dispersed in as a spinel stabilizer.   14. A mixture according to claim 13, characterized in that the calcined is a calcined A1203 (or a precursor thereof) chosen to produce, when heated in air up to 1000 C, A1203 with a grain size of 0.5-100um.   15. A mixture according to one of claims 11 to 14,   characterized in that at least one of the constituents of the precursor mixture is a precursor of a metal oxide selected from TiO 2, Ta 2 O 5, Li 2 O, Na 2 O and Al 2 O 3, each metal oxide precursor being selected from the group of compounds consisting of oxides in which the metal is in a permissible oxidation state in which it is not completely oxidized, hydroxides, oxyhydroxides, peroxides, nitrates, carbonates, sulfates, chlorates, halides, oxyhalides and organometallic compounds. 16. A mixture according to claim 15, characterized in that the oxides of both titanium and tantalum are precursors respectively of TiO 2 and Ta 2 O 5, which are oxides of titanium and tantalum which are respectively in permissible oxidation of titanium and tantalum in which titanium and tantalum are not   not completely oxidized respectively to TiO2 and Ta205.   17. A mixture according to one of claims 15 and 16,   characterized in that at least one precursor of a metal oxide is a water-soluble salt capable of being   precipitated from an aqueous solution thereof by a pH change to form a water-insoluble precipitate with an average particle size of less than 0.1 μm.   18. A mixture according to one of claims 15 and 16, characterized in that at least one precursor of an oxide   metal is an organometallic compound selected from alkoxides which are capable of being precipitated from a solution in an alcohol by adding water to the solution in an alcohol, to form an oxyhydroxide precipitate with an average particle size of less than 0 , lpm.   19. A mixture according to any one of the claims 11 to 18,   characterized in that each of the constituents has a   particle size of at most 10pm.   20. A mixture according to one of claims 11 to 19,   ) characterized in that the 5'-alumina component or its precursor and the dopant component or its precursor   together form at least 95% by weight of the precursor mixture, such that the precursor alumina-f "mixture essentially comprises a mixture of the oxides of sodium, lithium, aluminum, titanium and tantalum in their complete states. oxidized.   21. A sodium alumina compound having a Li20 stabilized spinel structure acting as spinel stabilizer, composed of sodium alumina-P, characterized in that it has TiO 2 and Ta 2 O 5 dispersed therein. as the dopant, the sodium alumina compound having components which are respectively lithium, sodium, aluminum, titanium and tantalum oxides in their fully oxidized states in the form of Li 2 O, Na 2 O , Al 2 O 3, TiO 2 and Ta 2 O 5, the proportions of these oxides, with Li 2 O, Na 2 O and Al 2 O 3 being present in the form of Li 2 O-stabilized sodium alumina, being as follows: Proportion Proportion (as oxide) (parts by weight) 1-alumina stabilized with Li 2 O (Li 2 O + Na 2 O + Al 2 O 3): 100 dopant (TiO 2 + Ta 2 O 5): 0.35-0.65 titanium oxide (TiO 2): 0.30-0, 50 tantalum oxide (Ta2O5): 0.05-0.15 22 sodium oxide (Na2O): 8.6-9.6 lithium oxide (Li2O): 0.58-0.83 22. A compound according to claim 21, characterized in that Li 2 O, Na 2 O and Al 2 O 3 (in the form of spinel stabilized sodium alumina), and TiO 2 and Ta 2 O 5 together form at least 95% by weight of the compound such that the compound essentially comprises said lithium oxides,   sodium, aluminum, titanium and tantalum.   23. A compound according to one of claims 21 and 22, characterized in that it is in the form of a product   semi-processed polycrystalline fuse.15