ELECTROCHEMICAL PROCESS FOR TREATMENT OF A MATERIAL IN OXIDE FORM.

Abstract

L'invention concerne le traitement d'un matériau sous forme d'oxyde en vue de faire varier sa stoechiométrie onionique ainsi que les produits obtenue par celui-ci. Le traitement selon l'invention consiste à utiliser le matériau sous frome d'électrode lors d'une électrolyse, ledit matériau correspondant à la formule suivante: (CF DESSIN DANS BOPI) dans laquelle. A1 A'sont des éléments choisis dans les colonnes Ia, IIa,ou parmi les terres rares. . B est un élément choisi parmi les prmière, deuxième et troisième séries de transition. . x et y varient entre 0 et 1 inclus.

Description

ELECTROCHEMICAL TREATMENT PROCESS OF A PHASE IN OXIDE FORM This application is an addition to the patent application French NO 8915856. It concerns the electrochemical treatment of a phase in oxide form in order to vary its anionic stoichiometry. Until now, known processes were carried out under temperature conditions ranging from 600 to 15,000 C and of oxygen pressure varying from 1 bar to 60 kbars. This was notably the case for the preparation of calcium ferrite (CaFeO3), obtained by a treatment of 60 kbars of oxygen at 9000 C. However, it was found that obtaining electrochemically phases in oxide form were perfectly suited and more particularly linked in the case of stoichiometric perovskite-type phases such as Ca Fe O 3, Sr Fe O 3 in particular. The advantage of such a method is that it allows you to working in mild conditions, particularly at low temperatures erasure, while effectively controlling the stoichiometry of the treated materials. Thus, the process according to the invention for treating a material in oxide form in order to vary its anionic stoichiometry includes the use of said material as an electrode in electrolysis in a liquid medium according to any one of the claims 1 to 6 and 11 to 14 of the main claim are characterized by sterized in that a material of formula A 1-x A'x B 03 y (I) is used, in which: A and A' are chosen from the elements of the columns Ia, l Ia, or rare earths. B is chosen from among the elements of the first, second or third transition series. x and y vary between 0 and 1 inclusive. But other advantages and features will appear at reading the description and example that will follow. The process according to the invention is suitable for the preparation of stoichiometric perovskite-type phases or perovskite derivatives and is particularly suitable for the preparation of phases corresponding to formula (I). Regarding the elements in column Ia that are suitable for implementing the invention, we can mention in particular the potassium and sodium. The material to be treated may include, as an element of column I Ia, calcium, strontium or barium. The said material may further comprise an element selected from among the rare earth elements. This term refers to elements whose atomic number varies between 57 and 71, as does yttrium. However, according to a particular embodiment of the invention, a material corresponding to formula (I) is treated and including strontium as an element of column Ia. As elements of the first series of transitions, we Examples include iron, cobalt, copper, and nickel. As an element of the second transition series, we can For example, consider money. As part of the third series of transitions, we gold can be mentioned. According to a particular embodiment, the invention consists of treating a material corresponding to formula (I) and including iron as an element of the first transition series. According to another embodiment of the invention, a material of formula (I) is treated, in which y is between 0.45 and 0.5. Finally, according to a last embodiment of the process, treats a material of formula (1) in which x = 0. The material that we wish to process is therefore used as a working electrode, regardless of its shape. First, it can be used in bulk form or as a powder shaped by compression. This powder compressed material may have been sintered. The material can also be applied as a thin or thick layer on a substrate that will be chosen to be inert in the environment used, particularly in a neutral or alkaline environment. can also possibly be used in single crystal form. Electrical contact between the material and the electrical carrier trode will be ensured in a manner known in itself. Electrolysis can be carried out in any suitable installation. An electrolysis cell with two compartments, separated for example by a sinter. We can work with two electrodes, the electrode of work including the material to be treated and a counter electrode. However, in order to better control the potential, a three-electrode assembly, the third electrode being an electro- of reference. We will use the counter electrode and the electrode of reference any suitable material adapted to the work environment. The reference electrode could be a mercuric oxide (Hg₂O / Hg) electrode, a hydrogen electrode, or a calomel electrode, for example. The counter electrode will have a large surface area and could be made of vitreous carbon, platinum, gold in particular. It can be advantageous to use a working electrode rotating disc. Electrolysis takes place in a liquid medium, and more precisely in a liquid medium at the temperatures at which the These reactions are generally below 100°C, more specifically... particularly at or near ambient temperature (20-250°C). Therefore, molten salt environments are excluded. Generally, the electrolytic medium of the process of The invention is a neutral or basic medium. According to a particular embodiment of the invention, the The environment is an aqueous and, in particular, alkaline environment. Usually, an alkali or alkaline earth hydroxide such as Na₂OH, KOH, Li₂OH, NH₄OH, or possibly Ba(OH)₂ or Ca(OH)₂ is used as the base to form the medium. a mixture of these bases. The pH of the medium must be chosen so that the material being treated can withstand the medium. It usually varies between 7 and included, preferably between 13 and 15. Electrolysis is done by applying a polarization anodic or cathodic to the working electrode. In the case of anodic polarization, the value of this potential (determined with respect to a mercuric oxide (Hg₂O / Hg) reference electrode) depends on the material being treated and is generally at least 200 mV. This value is usually between 200 and 1500 mV, and more specifically between 200 and 500 mV. Generally, the upper value of the potential remains lower than that at which oxygen evolution occurs. on said material. The process is carried out at low temperature. As mentioned above, this means a temperature below 1000°C. Typically, work is done at ambient temperature (20-250°C) or close to it, for example, between 10 and 400°C. Finally, work can be carried out in air, in an atmosphere of nitrogen, argon, or oxygen at atmospheric pressure or at a pressure slightly higher than this. The invention also relates to materials obtained by the process described above. These materials, according to one of claims 17 to 19 of the The main demand is obtained by processing a first material corresponding to the following general formula: Al.x A'x BO 3 +y, in which A, A', B, x, and y have the same meanings as before. Thus, everything said above concerning the The process remains valid for defining these products. A concrete example will now be given. E Xr'MPLE The material to be treated has a composition of Sr Fe O 2.50. It is formed by the solid-state reaction of SrCO3 and Fe2Q3 at 9500 C for 12 hours, then at 12000 C for 6 hours, under air. before getting soaked. The resulting product is then annealed at 1300°C for 48 hours in the open air. Finally, the SrFe 02.50 phase is obtained by reducing the annealed product at 1000°C under an argon current for 18 hours followed of an argon-enriched quench. Next, a pellet measuring 8 mm in diameter and 2 mm in thickness (corresponding to a product mass of 400 mg) is produced, and then it is annealed under an argon current at 1000°C. for 4 hours. The chemical analysis of the powder indicates a product of composition Sr Fe O 2.53 0.02- X-ray diffraction analysis shows that the unit cell is orthorhombic with the following parameters: a = 5.519 ± 0.005 A, b = 15.54 ± 0.01 A, c = 5.662 ± 0.005 A. Mässbauer spectroscopy analysis indicates that the iron is 94% in the form of iron (III). The M5 ssbauer spectrum consists of two magnetic sextuplets. The Mässbauer parameters are as follows: Site % Fe (III)* 6 (mm s-1) H (T) octahedral 50 0.376 49.7 (Oh) tetrahedral 50 0.189 41.1 (Td) with θ: isomeric shift H: hyperfine magnetic field. *: the percentage of iron is expressed as a percentage of the total amount of iron trivalent. The electrolysis conditions are as follows: a two-compartment cell with a three-electrode setup: a reference electrode (Hg₂O / Hg, KOH 1 N) and a counter electrode (Au). The working electrode is a rotating disc electrode. It is equipped with a cylindrical metal tip onto which is... fixed the pellet of material to be treated. Ohmic contact is ensured by a silver lacquer. A coating is applied using a thermosetting resin to ensure electrolyte contact with the material's disc. only one side of it and to guarantee the watertightness of the whole. The setup is potentiostatic. The polarization E is equal to 400 mV / (Hg₂O / Hg). The electrolyte is a 1 N KOH solution. The temperature is 200°C and the partial pressure of oxygen is... of 0.2. Electrolysis takes place in air for 60 hours. After electrolysis, a material is obtained exhibiting a cubic structure with parameter a = 3.845 + 0.007 &. The phase is pure and well crystallized. Chemical analysis indicates a phase of composition Sr Fe O 3.00 + 0.02. Mässbauer spectroscopy analysis indicates that all the iron is in the form Fe (IV). The Messbauer spectrum consists of a para- singlet isomeric displacement magnetic characteristic of iron tetravalent (δ = 0.082 mm s-1). The width at half maximum of the peak is relatively small <r = 0.494 mm s-1) and confirms the presence of Fe (IV) in a single site. Since the quadrupole splitting is zero, the environment is perfectly symmetrical and corresponds to a stacking of regular octahedra.

Claims

DEMANDS A process for the electrochemical treatment of a material in oxide form in order to vary its anionic stoichiometry, comprising the use of said material as an electrode in electrolysis. in a liquid medium, according to any one of claims 1 to 6 and 11 to 14 of the main application, characterized in that a material of the following general formula is used: Al-X A'x BO 3 +y (I) in which: A and A' are chosen from elements of columns Ia, IIa, or rare earths; B is chosen from elements of the first, second, or third transition series x and y vary between 0 and 1 inclusive.

2. A method according to claim 1, characterized in that a material corresponding to formula (I) and comprising the is used strontium as an element of column I Ia.

3. A method according to claim 1 or 2, characterized in that that a material corresponding to formula (I) and including iron as an element of the first transition series is used.

4. A method according to any one of claims 1 to 3, character- sterized in that a material corresponding to the formula is used (I) in which y varies from 0.45 to 0.

50. A method according to any one of the preceding claims characterized in that a material corresponding to the is used formula (I) and in which x equals 0.

6. Material obtained by electrochemical treatment of a first material in oxide form, in which the first material is used as an electrode during electrolysis in a liquid medium, according any one of claims 17 to 19 of the main claim, characterized in that the first material corresponds to the following formula: Al-x A'x BO 3 +y (I) in which: A, A' represent elements of columns Ia, I Ia, or rare earths; B represents an element of the first, second, or third transition series x, y vary between 0 and 1 inclusive.

7. Material according to claim 5 characterized in that the element A is strontium.

8. Material according to claim 5 or 6 characterized in this that element B is iron.

9. Material according to any one of claims 5 to 7 characterized in this that y varies between 0.45 and 0.

5. Material according to any one of claims 8, characterized in that x equals 0.