EP0439524A1 - Method for preparing magnetic particles of hexaferrites, particles obtainted and product containing them - Google Patents

Method for preparing magnetic particles of hexaferrites, particles obtainted and product containing them

Info

Publication number
EP0439524A1
EP0439524A1 EP89912113A EP89912113A EP0439524A1 EP 0439524 A1 EP0439524 A1 EP 0439524A1 EP 89912113 A EP89912113 A EP 89912113A EP 89912113 A EP89912113 A EP 89912113A EP 0439524 A1 EP0439524 A1 EP 0439524A1
Authority
EP
European Patent Office
Prior art keywords
barium
particles
salt
process according
strontium
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Pending
Application number
EP89912113A
Other languages
German (de)
French (fr)
Inventor
Jean-Christophe Kodak-Pathé ROBERT
Pierre-André MARI
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Kodak Pathe SA
Original Assignee
Kodak Pathe SA
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Kodak Pathe SA filed Critical Kodak Pathe SA
Publication of EP0439524A1 publication Critical patent/EP0439524A1/en
Pending legal-status Critical Current

Links

Classifications

    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01G—COMPOUNDS CONTAINING METALS NOT COVERED BY SUBCLASSES C01D OR C01F
    • C01G49/00—Compounds of iron
    • C01G49/0018—Mixed oxides or hydroxides
    • C01G49/0036—Mixed oxides or hydroxides containing one alkaline earth metal, magnesium or lead
    • G—PHYSICS
    • G11—INFORMATION STORAGE
    • G11B—INFORMATION STORAGE BASED ON RELATIVE MOVEMENT BETWEEN RECORD CARRIER AND TRANSDUCER
    • G11B5/00—Recording by magnetisation or demagnetisation of a record carrier; Reproducing by magnetic means; Record carriers therefor
    • G11B5/62—Record carriers characterised by the selection of the material
    • G11B5/68—Record carriers characterised by the selection of the material comprising one or more layers of magnetisable material homogeneously mixed with a bonding agent
    • G11B5/70—Record carriers characterised by the selection of the material comprising one or more layers of magnetisable material homogeneously mixed with a bonding agent on a base layer
    • G11B5/706—Record carriers characterised by the selection of the material comprising one or more layers of magnetisable material homogeneously mixed with a bonding agent on a base layer characterised by the composition of the magnetic material
    • G11B5/70626—Record carriers characterised by the selection of the material comprising one or more layers of magnetisable material homogeneously mixed with a bonding agent on a base layer characterised by the composition of the magnetic material containing non-metallic substances
    • G11B5/70642—Record carriers characterised by the selection of the material comprising one or more layers of magnetisable material homogeneously mixed with a bonding agent on a base layer characterised by the composition of the magnetic material containing non-metallic substances iron oxides
    • G11B5/70678—Ferrites
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01F—MAGNETS; INDUCTANCES; TRANSFORMERS; SELECTION OF MATERIALS FOR THEIR MAGNETIC PROPERTIES
    • H01F1/00—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties
    • H01F1/01—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties of inorganic materials
    • H01F1/03—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties of inorganic materials characterised by their coercivity
    • H01F1/032—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties of inorganic materials characterised by their coercivity of hard-magnetic materials
    • H01F1/10—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties of inorganic materials characterised by their coercivity of hard-magnetic materials non-metallic substances, e.g. ferrites, e.g. [(Ba,Sr)O(Fe2O3)6] ferrites with hexagonal structure
    • H01F1/11—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties of inorganic materials characterised by their coercivity of hard-magnetic materials non-metallic substances, e.g. ferrites, e.g. [(Ba,Sr)O(Fe2O3)6] ferrites with hexagonal structure in the form of particles
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01P—INDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
    • C01P2004/00—Particle morphology
    • C01P2004/20—Particle morphology extending in two dimensions, e.g. plate-like
    • C01P2004/22—Particle morphology extending in two dimensions, e.g. plate-like with a polygonal circumferential shape
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01P—INDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
    • C01P2006/00—Physical properties of inorganic compounds
    • C01P2006/42—Magnetic properties

Definitions

  • the present invention relates to a process for the preparation of barium or strontium hexaferrites which are particularly useful in magnetic recording processes.
  • Barium and strontium hexaferrites are well known for their magnetic properties. In particular, they have a high coercive field, which makes them useful in all applications where such a high coercive field is desirable.
  • the particles obtained after grinding are of non-homogeneous size, difficult to disperse in the binders used for magnetic media and tend to to form agglomerates during the calcination treatment. These drawbacks can make the particles unsuitable for the preparation of magnetic recording layers. Many methods have been proposed recently to overcome these drawbacks, such as hydrothermal synthesis, or improved methods of coprecipitation. Common methods of synthesis of Ba and Sr hexaferrites lead to the production of particles in the form of hexagonal platelets. It is therefore desirable to improve the dispersibility and the magnetic properties of the hexaferrite particles at the same time.
  • a barium or strontium salt is used in the presence of carbonate, so as to coprecipitate the carbonate of Ba or Sr and iron hydroxide.
  • the barium and strontium hydroxides are in fact partially soluble, which results, if they are used in the coprecipitate, that one cannot obtain the desired stoichiometry of the hexaferrite, unless significant excesses are used. compared to stoichiometry. Rather than hydroxides, therefore, insoluble carbonates are used, even at neutral pH.
  • the quantities of reagents used are generally such that the stoichiometry is equal to 1 to 1.2.
  • a weak SFD is also an indication of the narrower particle size distribution, non-agglomeration of these particles and a purer chemical composition.
  • the subject of the present invention is a process for the preparation of Ba or Sr hexaferrites, making it possible to obtain magnetic particles which disperse well in the binders usually used for magnetic media, and which have improved magnetic properties, in particular a narrow distribution of particle sizes, better magnetization and a weaker distribution of switching fields.
  • the process according to the invention consists in making an iron salt and a barium or strontium salt coprecipitate in an alkaline medium, in filtering, washing and drying the precipitate of hydroxides obtained, then in heating to temperatures between 750 ° C. and 950 ° C, and preferably between 800 and 850 ° C, to crystallize the hexaferrite, and it is characterized in that:
  • the medium is neutralized until a pH of between 7 and 10 is obtained
  • the coprecipitate of iron hydroxide and of Ba or Sr hydroxide is washed with a buffered aqueous solution at pH between 7 and 10 of a barium or strontium salt, having a concentration between 2 x 10 ⁇ 3 and 0.2 M, preferably between 4 x 10 -3 and 40 x 10 ⁇ 3 M.
  • a dopant metal such as Ni, Co, Ti, Cu, Sn, or any other dopant known in the art.
  • the present invention therefore also relates to particles of barium hexaferrite or strontium, doped or not, which, compared to those obtained by a co-precipitation method of the prior art, have a higher coercive field and a magnetization and a ratio of the distribution of the switching fields to the coercive field (SFD / Hc) at less equivalent, as shown in the examples below.
  • the subject of the present invention is in particular particles of undoped Ba or Sr hexaferrites which have an SFD / Hc ratio equal to or less than 0.45 and a saturation magnetization greater than 55 emu / g, and more advantageously a SFD / Hc ratio equal to or less than 0.40, a saturation magnetization greater than 60 emu / g and a coercive field greater than 5000 Oe (400 kA / m).
  • the subject of the invention is also a magnetic recording product containing particles of Ba or Sr hexaferrite which have the characteristics defined above.
  • the recording product can take any known form such as tape, disc or card.
  • a ferric iron salt such as a nitrate, a chloride, a bromide, an iodide, a fluoride, etc.
  • a salt of Ba or Sr such that a chloride, which is made to precipitate in an alkaline medium free of carbonate having a pH equal to or greater than 12.
  • the precipitate formed of iron hydroxide and barium hydroxide or strontium is washed with a solution of a Ba or Sr salt, such as a chloride or a nitrate, having a concentration of between 2 ⁇ 10 and 0.2 M and preferably between 4 x 10 ⁇ 3 and 40 x 10 ⁇ 3 M.
  • a Ba or Sr salt such as a chloride or a nitrate
  • the coprecipitate is dried, then annealed at a temperature between 750 ° C and 950 ° C and preferably between 800 and 850 ° C. Particles of hexaferrites are obtained in the form of hexagonal plates.
  • a ferrous salt is used as the starting product; by coprecipitation with a Ba or Sr salt, then by oxidation, hydrated ferric oxide alpha (goethite) containing Ba or Sr is obtained, which serves as a precursor.
  • This embodiment makes it possible to obtain hexaferrites having the shape of goethite serving as a precursor, needles or nodules.
  • the method described in European patent 45246 is used.
  • a reducing agent is used in the alkaline precipitation medium, before oxidation, so as to avoid the presence of ferric ions at the start, and then to control the oxidation. This avoids the formation of undesirable by-products.
  • the appropriate reducing agent should not cause an insoluble precipitate and should be active under the process pH conditions.
  • the reduction can be done by any means, including electrochemically using appropriate electrodes.
  • a compound is used whose redox potential is less than 1 volt.
  • reducing agent mention may be made of stannous chloride, a dihydroxybenzene or a hydroxylamine.
  • the amount of reducing agent is adjusted according to the concentration of ferric ions in the solution, as indicated below.
  • a ferrous salt which can be a chloride, a bromide, an iodide, a fluoride, a nitrate, etc. is dissolved. Then, the reducing agent is added, the proportions of which will be such that a colorimetric test of the Fe ions (KSCN for example) is negative. Then added to this solution, a Ba or Sr salt such as a chloride or a nitrate for example; slight acidification with an acid may promote dissolution. This solution is preferably added slowly
  • alkali metal hydroxide sodium or potassium for example
  • the temperature of the solutions can be between 0 and 100 ° C, and the pH is equal to or greater than 12.
  • a device for finely dispersing the salts in the aqueous alkali metal hydroxide solution without contact with an oxidizing agent.
  • a device for finely dispersing the salts in the aqueous alkali metal hydroxide solution without contact with an oxidizing agent.
  • Such a device can be, for example, that described in French patent 1,157,156, called "Rotating saucer”.
  • the ferrous hydroxide is then oxidized by passing an oxidizing gas stream so as to obtain the particles of the precursor.
  • Air is a very good oxidizing agent.
  • the oxidation temperature can vary between 0 and 100 ° C at ambient pressure, depending on the shape of the particles that are desired. Preferably, it is less than 60 ° C, and even more advantageously it is less than 40 ° C.
  • the air flow rate for a liter of solution is generally between 5 and 60 l / h, but higher flow rates can also be used.
  • the end of the oxidation is measured by following the evolution of the potential of the Fe 2 + / Fe 3 + couple; when the Fe 2+ concentration becomes zero, the potential changes rapidly, according to the well-known redox equations.
  • the precursor concentrations are those described in European patent 45246, namely between 0.1 and 0.5 mol / l of iron, in general.
  • the alkaline excess is between 25% and 600%, generally from 100 to 200%.
  • the solution is then filtered.
  • the precipitate is re-dispersed in a minimum of water and the alkaline excess is neutralized with a dilute acid solution (HN0 3 or HC1) so as to bring the pH of the solution between 7 and 10.
  • HN0 3 or HC1 dilute acid solution
  • the salts contained in this water are then removed by successive washing with a dilute solution of a Ba or Sr salt, such as a chloride, either by the partial recirculation technique, or by the lost water technique.
  • a Ba or Sr salt such as a chloride
  • the concentration of this Ba or Sr salt solution can vary between 2 x 10 ⁇ 3 M and 0.2 M, preferably between 4 x 10 ⁇ 3 and 40 x 10 ⁇ 3 M.
  • the optimal concentration is adjusted according to the solubility coefficient of Ba (OH) or Sr (OH), so that there is equilibrium between the barium or strontium ions redissolved by the washing solution and those provided by this same solution. If this concentration is too low, barium or strontium is lost, and the desired saturation magnetization is not obtained.
  • This washing technique eliminates the dissolution of Ba or Sr in the washing water, and therefore makes it possible to keep the stoichiometry used for the starting reagents, which is generally between 1 and 1.2.
  • the product is ⁇ then filtered and dried, then heated to a temperature realized "- - between 750 ° C and 950 ° C, and preferably between 800 and 850 ° C for 2 to 4 hours, so as to crystallize the Ba or Sr. hexaferrite
  • the particles obtained after heat treatment, without subsequent washing treatment retain the shape of the initial precursor. Their diameter is between 0.02 and 0.1 ⁇ m, their length between 0.02 and 0.5 ⁇ m. It is possible, during the growth of the precursor in aqueous solution, to modify the shape of the particles by adding crystal growth modifiers, as described in patent application WO 86/05026, such as pyrogallol. Without a growth modifier, the particularity of the needles obtained, their diameter, their length depends on the oxidation temperature, the concentration and the flow rate of the oxidizing gas.
  • the magnetization and the coercive field are measured with the VSM (Vibrating Sample Magnetometer) under a maximum magnetizing field of 20,000 Oe (1590 kA / m). These values depend on the oxidation temperature and the alkaline excess. By adjusting these two parameters appropriately, it is possible to obtain undoped hexaferrite particles having both a magnetization at saturation greater than 60 emu / g, a coercive field greater than 5000 Oe (400 kA / m) and an SFD / Hc ratio equal to or less than 0.40.
  • An alkaline aqueous solution is prepared by dissolving 44 g of NaOH in 0.6 1 of distilled water (i.e. 1.1 moles). This preparation is brought to 55 ° C.
  • 190 ml of distilled water acidified with 10 ml of IN hydrochloric acid 53.7 g of FeCl 2 , 4H 2 0 is dissolved, ie 0.27 mole; 0.2 g of SnCl 2 , 2H 2 0, or 8.8 ⁇ 10 -4 mole, are added to this latter solution. It is easy to check with KSCN that there are no more ferric ions in solution.
  • 5.5 g of BaCl 2 , 2H 2 0 are then dissolved (22.5 x 10 ⁇ 3 mole). This quantity corresponds to the stoichiometry of the final product BaFe 12 0, g .
  • This solution previously heated to 55 ° C., is poured gently (10 min) with vigorous stirring into the alkaline solution, at a pH greater than 12. A precipitate forms. This is then oxidized by a stream of air (30 l / h) which is distributed uniformly in the solution by means of a sintered glass, at 55 ° C. and at a pH greater than 12. The potential Fe ++ / Fe +++ of the solution is checked. After 3 hours, this potential changes rapidly from around -800 mV to around -200 mV. The precipitate is then filtered, then washed with distilled water until the pH of the water leaving the filter has dropped to around 8 to 9. After drying, the latter is treated for 3 hours at 830 ° C. .
  • Example 1 The procedure of Example 1 is repeated, except that the precipitate is not washed, but redispersed in 150 cm 3 of water. The excess soda is then neutralized with IN HCl (the pH of the solution has dropped to 7). The solution is filtered and washed with 500 cm of distilled water, in order to remove the residual salts (concentration of Na + ions less than 10 -3 M).
  • Example 1 The procedure of Example 1 is repeated. The precipitate is not washed, but redispersed in 150 cm of water. The excess sodium hydroxide is neutralized with IN HCl so as to lower the pH to 8, then the precipitate is filtered and washed with 500 cm of an 8 ⁇ 10 -3 M solution of BaCl 2 , 2H 2 0, the pH is adjusted to 8.0 __: 0.1 with NH.OH. After drying, the precipitate is treated for 3 hours at 830 ° C.
  • An aqueous alkaline solution is prepared by dissolving 29 g (0.72 mole) of NaOH in 0.6 1 of distilled water and the mixture is brought to 35 ° C. 36 g of FeCl 2 , 4H 2 0 (0.181 mole) are dissolved in 190 ml of distilled water acidified with 10 ml of IN hydrochloric acid, and 0.2 g of SnCl 2 , 2H 2 0 (8.8) is added. x 10 ⁇ 4 mole).
  • KSCN aqueous alkaline solution
  • Example 4 The procedure of Example 4 is repeated.
  • the precipitate is not washed, but redispersed in 150 cm of water.
  • the excess sodium hydroxide is neutralized with IN HCl so as to lower the pH to 8, then the precipitate is filtered and washed with 500 cm 3 of an 8 ⁇ 10 ⁇ 3 M solution of BaCl 2 , 2H 2 0 of which the pH is adjusted to 8.0 ⁇ 0.1 with NH 4 OH. After drying, the precipitate is treated for 3 hours at 830 ° C.
  • This example is an example of synthesis on a semi-industrial scale. 21.4 kg of 50% NaOH are dissolved in 210 liters of water. It is brought to 25 ° C. A solution is prepared by adding, in order, in 50 liters of acidified water with 0.75 1 of N HCl, 12.06 kg of FeCl, 4H O, 48 g of SnCl, 2H O, and 1, 48 kg of BaCl, 2H O. This solution heated to 25 ° C is poured with vigorous stirring into the alkaline solution.
  • a precipitate is formed which is oxidized by a stream of air and which is treated as described in Example 6, except that it is washed 3 times with a quantity of solution of BaCl2.2H2O 8 x 10 _ M, sufficient to put the precipitate back into dispersion, and calcined in a rotary oven.
  • the barium hexaferrite particles obtained have a saturation magnetization of 64 emu / g, a coercive field of 5,000 Oe (398 kA / m), an SFD of 1,800 Oe (143 kA / m) and a SFD / Hc ratio 0.36.
  • EXAMPLE 8 (Comparative)
  • Example 1 36.7 g of NaOH, 44.7 g (0.22 mole) of FeCl 2 , 4H 2 0 and 3.6 g (0.018 mole) of SrCl, 2H O (stoichiometric amount for SrFe 20o) are used.
  • the oxidation temperature is 25 ° C.
  • the precipitate neutralization step is carried out until a pH of 10 is reached.
  • the washing after filtration is carried out with a solution of 16 ⁇ 10 -3 mole / 1 of SrCl 2 , 2H 2 0, the pH of which has been adjusted to 10.0 ⁇ 0.1 with NH 4 OH. After drying, the precipitate is treated for 2 hours at 830 ° C.
  • a solution of 13.8 g of K 2 CO 2 and 11.2 g " of OH in 70 ml of water is prepared in parallel.
  • the reaction is exothermic, and the temperature is adjusted to 60 ° C.
  • the saline solution is poured into the basic solution and a mixture of iron hydroxide and barium hydroxide is precipitated. Agitation must be such that the solution always remains homogeneous.
  • the product is dried and then treated for 3 hours at 800 ° C to crystallize barium ferrite BaFe 12 0, _.

Landscapes

  • Chemical & Material Sciences (AREA)
  • Organic Chemistry (AREA)
  • Life Sciences & Earth Sciences (AREA)
  • General Life Sciences & Earth Sciences (AREA)
  • Geology (AREA)
  • Inorganic Chemistry (AREA)
  • Engineering & Computer Science (AREA)
  • Power Engineering (AREA)
  • Hard Magnetic Materials (AREA)
  • Compounds Of Iron (AREA)
  • Soft Magnetic Materials (AREA)

Abstract

La présente invention concerne un procédé de préparation de particules magnétiques d'hexaferrites de baryum ou de strontium. Le procédé consiste à faire coprécipiter un sel de fer et un sel de baryum ou de strontium, à filtrer et neutraliser ce coprécipité, et à le laver avec une solution d'un sel de baryum ou de strontium ayant une concentration comprise entre 2 x 10-3 et 0,2 M. Application à l'obtention de particules magnétiques d'hexaferrites de baryum ou de strontium ayant une bonne dispersabilité et des propriétés magnétiques améliorées.The present invention relates to a process for the preparation of magnetic particles of barium or strontium hexaferrites. The process consists in coprecipitating an iron salt and a barium or strontium salt, filtering and neutralizing this coprecipitate, and washing it with a solution of a barium or strontium salt having a concentration of between 2 × 10 -3 and 0.2 M. Application to obtaining magnetic particles of barium or strontium hexaferrites having good dispersibility and improved magnetic properties.

Description

PROCEDE DE PREPARATION DE PARTICULES MAGNETIQUES D'HEXAFERRITES. PARTICULES OBTENUES ET PRODUIT LES CONTENANT La présente invention concerne un procédé de préparation d'hexaferrites de baryum ou de strontium particulièrement utiles dans les procédés d'enregistrement magnétique. PROCESS FOR THE PREPARATION OF MAGNETIC PARTICLES OF HEXAFERRITES. The present invention relates to a process for the preparation of barium or strontium hexaferrites which are particularly useful in magnetic recording processes.
Les hexaferrites de baryum et de strontium sont bien connues pour leurs propriétés magnétiques. Elles possèdent en particulier un champ coercitif élevé, ce qui les rend utiles dans toutes les applications où un tel champ coercitif élevé est souhaitable.Barium and strontium hexaferrites are well known for their magnetic properties. In particular, they have a high coercive field, which makes them useful in all applications where such a high coercive field is desirable.
Depuis quelques années, on a développé des produits magnétiques, tels que des disques, des cartes et des bandes, pour des applications dans lesquelles une bonne conservation de l'enregistrement et/ou une bonne résistance à la falsification ou à l'effacement accidentel sont souhaitées. Ceci concerne notamment les cartes de crédit, les cartes d'identité, ou les cartes d'accès contrôlé, etc. à piste magnétique. Les hexaferrites offrent des possibilités pour de telles applications. En outre, elles permettent une densité élevée d'enregistrement.In recent years, magnetic products, such as discs, cards and tapes, have been developed for applications in which good preservation of the recording and / or good resistance to forgery or accidental erasure are desired. This particularly concerns credit cards, identity cards, or controlled access cards, etc. magnetic strip. Hexaferrites offer possibilities for such applications. In addition, they allow a high recording density.
L'obtention des hexaferrites est connue depuis longtemps, en particulier par coprécipitation en solution alcaline d'un sel ferrique et d'un sel de Ba ou Sr, puis calcination à haute température du coprécipité obtenu. CD. Mee et J.C. Jeschke "Single-Domain Properties in Hexagonal Ferrites" J. of Applied Physics, Vol. 34, n°4, 1271-2, 1963, décrivent la préparation d'hexaferrites. Ce procédé ancien qui est assez facile à mettre en oeuvre, présente cependant l'inconvénient d'aboutir à un produit sous forme de blocs très durs, difficiles à broyer. Les particules obtenues après broyage sont de taille non homogène, difficilement dispersables dans les liants utilisés pour les milieux magnétiques et ont tendance à former des agglomérats lors du traitement de calcinâtion. Ces inconvénients peuvent rendre les particules inaptes à la préparation des couches d'enregistrement magnétique. De nombreux procédés ont été proposés récemment pour remédier à ces inconvénients, tels que la synthèse hydrothermale, ou des procédés améliorés de coprécipitation. Des procédés de synthèse courants des hexaferrites de Ba et de Sr conduisent à l'obtention de particules sous forme de plaquettes hexagonales. II est donc souhaitable d'améliorer en même temps la dispersabilité et les propriétés magnétiques des particules d'hexaferrites.Obtaining hexaferrites has been known for a long time, in particular by coprecipitation in an alkaline solution of a ferric salt and a Ba or Sr salt, then calcination at high temperature of the coprecipitate obtained. CD. Mee and JC Jeschke "Single-Domain Properties in Hexagonal Ferrites" J. of Applied Physics, Vol. 34, n ° 4, 1271-2, 1963, describe the preparation of hexaferrites. This old process which is fairly easy to implement, however has the disadvantage of resulting in a product in the form of very hard blocks, difficult to grind. The particles obtained after grinding are of non-homogeneous size, difficult to disperse in the binders used for magnetic media and tend to to form agglomerates during the calcination treatment. These drawbacks can make the particles unsuitable for the preparation of magnetic recording layers. Many methods have been proposed recently to overcome these drawbacks, such as hydrothermal synthesis, or improved methods of coprecipitation. Common methods of synthesis of Ba and Sr hexaferrites lead to the production of particles in the form of hexagonal platelets. It is therefore desirable to improve the dispersibility and the magnetic properties of the hexaferrite particles at the same time.
Dans les procédés connus par coprécipitation, on utilise un sel de baryum ou de strontium en présence de carbonate, de façon à coprécipiter le carbonate de Ba ou de Sr et l'hydroxyde de fer. Les hydroxydes de baryum et de strontium sont en effet partiellement solubles, ce qui a pour résultat, si on les utilise dans le coprécipité, qu'on ne peut obtenir la stoechiometrie souhaitée de l'hexaferrite, à moins d'utiliser des excès importants par rapport à la stoechiometrie. Plutôt que les hydroxydes,on utilise donc des carbonates insolubles, même à pH neutre. Au cours du lavage du coprécipité, il n'y a aucune perte de baryum ou de strontium et les quantités de réactifs engagées sont en général tels que la stoechiometrie est égale à 1 à 1,2.In known methods by coprecipitation, a barium or strontium salt is used in the presence of carbonate, so as to coprecipitate the carbonate of Ba or Sr and iron hydroxide. The barium and strontium hydroxides are in fact partially soluble, which results, if they are used in the coprecipitate, that one cannot obtain the desired stoichiometry of the hexaferrite, unless significant excesses are used. compared to stoichiometry. Rather than hydroxides, therefore, insoluble carbonates are used, even at neutral pH. During the washing of the coprecipitate, there is no loss of barium or strontium and the quantities of reagents used are generally such that the stoichiometry is equal to 1 to 1.2.
Cependant, les grains coprécipités de carbonate de Ba ou de Sr étant plus gros que les grains d'hydroxyde de fer, le mélange fer-Ba/Sr n'est pas intime, et la structure du produit final après calcination est hétérogène. La demande de brevet japonais 62052133 décrit l'obtention de particules d'hexaferrite de Ba par coprécipitation à partir d'un sel ferreux en l'absence d'ions carbonate. L'exemple comparatif ci-après montre qu'on n'obtient pas par ce procédé les propriétés avantageuses des particules obtenues selon la présente invention, et en particulier les propriétés magnétiques telles que l'aimantation et la répartition des champs de commutation. La répartition des champs de commutation, dite ci-après "SFD" pour Switching Field Distribution, caractérise la largeur de la distribution des champs de commutation. A partir du cycle d'hystérésis, (flux magnétique M en fonction du champ magnétisant H), on trace la courbe dM/dH en fonction de H, dérivée du cycle, et on calcule la largeur à mi-hauteur ΔH du pic observé quand H = Hc ; cette valeur est»appelée SFD. On divise cette valeur de SFD par le champ coercitif des particules, ce qui donne le rapport sans dimension SFD/Hc. Le champ coercitif et le SFD dépendent de la nature des particules, c'est pourquoi on utilise le rapport SFD/Hc qui est une meilleure valeur comparative que le SFD et qui varie dans le même sens.However, the co-precipitated grains of Ba or Sr carbonate being larger than the grains of iron hydroxide, the iron-Ba / Sr mixture is not intimate, and the structure of the final product after calcination is heterogeneous. Japanese patent application 62052133 describes obtaining particles of Ba hexaferrite by coprecipitation from a ferrous salt in the absence of carbonate ions. The comparative example below shows that the properties are not obtained by this process. advantageous particles obtained according to the present invention, and in particular the magnetic properties such as the magnetization and the distribution of the switching fields. The distribution of the switching fields, hereinafter referred to as "SFD" for Switching Field Distribution, characterizes the width of the distribution of the switching fields. From the hysteresis cycle, (magnetic flux M as a function of the magnetizing field H), the curve dM / dH is plotted as a function of H, derived from the cycle, and the width at mid-height ΔH of the peak observed is calculated. H = Hc; this value is »called SFD. This SFD value is divided by the coercive field of the particles, which gives the dimensionless SFD / Hc ratio. The coercive field and the SFD depend on the nature of the particles, this is why we use the SFD / Hc ratio which is a better comparative value than the SFD and which varies in the same direction.
Plus le SFD est faible, plus les moments magnétiques des particules commutent pour un champ proche du champ coercitif. Les transitions de flux lors d'un enregistrement magnétique sont donc mieux définies, l'épaisseur de la couche enregistrée peut être plus faible et mieux déterminée, contribuant ainsi à un bruit plus faible, donc à un rapport signal/bruit plus élevé, particulièrement lorsque la densité d'information est élevée. Pour résoudre le problème de l'augmentation du rapport signal/bruit, on doit donc chercher à diminuer la répartition des champs de commutation. Un SFD faible est aussi une indication de la répartition plus étroite de la taille des particules, d'une non agglomération de ces particules et d'une composition chimique plus pure. La présente invention a pour objet un procédé de préparation d'hexaferrites de Ba ou de Sr, permettant d'obtenir des particules magnétiques qui se dispersent bien dans les liants habituellement utilisés pour les milieux magnétiques, et qui ont des propriétés magnétiques améliorées, en particulier une répartition étroite des tailles de particules, une meilleure aimantation et une répartition plus faible des champs de commutation. Le procédé selon l'invention consiste à faire coprécipiter un sel de fer et un sel de baryum ou de strontium en milieu alcalin, à filtrer, laver et sécher le précipité d'hydroxydes obtenu, puis à chauffer à des températures comprises entre 750°C et 950°C, et de préférence entre 800 et 850°C, pour faire cristalliser l'hexaferrite, et il est caractérisé en ce que :The lower the SFD, the more the magnetic moments of the particles commute for a field close to the coercive field. The flux transitions during magnetic recording are therefore better defined, the thickness of the recorded layer can be smaller and better determined, thus contributing to lower noise, therefore to a higher signal / noise ratio, particularly when the information density is high. To solve the problem of increasing the signal / noise ratio, we must therefore seek to reduce the distribution of the switching fields. A weak SFD is also an indication of the narrower particle size distribution, non-agglomeration of these particles and a purer chemical composition. The subject of the present invention is a process for the preparation of Ba or Sr hexaferrites, making it possible to obtain magnetic particles which disperse well in the binders usually used for magnetic media, and which have improved magnetic properties, in particular a narrow distribution of particle sizes, better magnetization and a weaker distribution of switching fields. The process according to the invention consists in making an iron salt and a barium or strontium salt coprecipitate in an alkaline medium, in filtering, washing and drying the precipitate of hydroxides obtained, then in heating to temperatures between 750 ° C. and 950 ° C, and preferably between 800 and 850 ° C, to crystallize the hexaferrite, and it is characterized in that:
1) on coprécipité en l'absence d'ions carbonate,1) it is coprecipitated in the absence of carbonate ions,
2) après la coprécipitation, on neutralise le milieu jusqu'à l'obtention d'un pH compris entre 7 et 10, 3) après la filtration, on lave le coprécipité d'hydroxyde de fer et d'hydroxyde de Ba ou de Sr avec une solution aqueuse tamponnée à pH compris entre 7 et 10 d'un sel de baryum ou de strontium, ayant une concentration comprise entre 2 x 10~3 et 0,2 M, de préférence entre 4 x 10-3 et 40 x 10~3 M. Dans le cas où l'on souhaite abaisser le champ coercitif des particules obtenues, il est possible d'ajouter un métal dopant tel que Ni, Co, Ti, Cu, Sn, ou tout autre dopant connu dans la technique. Il suffit par exemple d'ajouter un sel de métal dopant dans le milieu de précipitation, à raison de la quantité souhaitée, pour coprécipiter l'hydroxyde de ce métal avec les hydroxydes de fer et de baryum ou de strontium. Ce procédé permet d'obtenir des particules ayant une répartition étroite des champs de commutation, sans perte de baryum ou de strontium, donc sans diminution de 1"aimantation.2) after the coprecipitation, the medium is neutralized until a pH of between 7 and 10 is obtained, 3) after filtration, the coprecipitate of iron hydroxide and of Ba or Sr hydroxide is washed with a buffered aqueous solution at pH between 7 and 10 of a barium or strontium salt, having a concentration between 2 x 10 ~ 3 and 0.2 M, preferably between 4 x 10 -3 and 40 x 10 ~ 3 M. In the case where it is desired to lower the coercive field of the particles obtained, it is possible to add a dopant metal such as Ni, Co, Ti, Cu, Sn, or any other dopant known in the art. It suffices for example to add a salt of doping metal in the precipitation medium, at a rate of the desired quantity, to coprecipitate the hydroxide of this metal with the hydroxides of iron and barium or strontium. This process makes it possible to obtain particles having a narrow distribution of the switching fields, without loss of barium or strontium, therefore without reduction of the magnetization.
La présente invention a donc aussi pour objet des particules d'hexaferrite de baryum ou de strontium, dopées ou non, qui, par rapport à celles obtenues par un procédé de co-précipitation de la technique antérieure, ont un champ coercitif et une aimantation plus élevés et un rapport de la répartition des champs de commutation au champ coercitif (SFD/Hc) au moins équivalent, comme le montrent les exemples ci-après.The present invention therefore also relates to particles of barium hexaferrite or strontium, doped or not, which, compared to those obtained by a co-precipitation method of the prior art, have a higher coercive field and a magnetization and a ratio of the distribution of the switching fields to the coercive field (SFD / Hc) at less equivalent, as shown in the examples below.
La présente invention a en particulier pour objet des particules d'hexaferrites de Ba ou de Sr non dopées qui ont un rapport SFD/Hc égal ou inférieur à 0,45 et une aimantation à saturation supérieure à 55 emu/g, et plus avantageusement un rapport SFD/Hc égal ou inférieur à 0,40, une aimantation à saturation supérieure à 60 emu/g et un champ coercitif supérieur à 5000 Oe (400 kA/m) . L'invention a aussi pour objet un produit d'enregistrement magnétique contenant les particules d'hexaferrite de Ba ou de Sr qui présentent les caractéristiques définies ci-dessus. Le produit d'enregistrement peut prendre toute forme connue telle que bande, disque ou carte. II est particulièrement utile pour obtenir le milieu magnétique d'une carte à mémoire magnétique à haute capacité décrite dans la demande de brevet français 8814217 intitulée "Carte à mémoire magnétique à grande capacité et procédé de fabrication". Selon un mode de réalisation de l'invention, on utilise comme produit de départ un sel de fer ferrique tel qu'un nitrate, un chlorure, un bromure, un iodure, un fluorure etc., et un sel de Ba ou de Sr tel qu'un chlorure, qu'on fait coprécipiter dans un milieu alcalin exempt de carbonate ayant un pH égal ou supérieur à 12. Après neutralisation et filtration, on lave le précipité formé d'hydroxyde de fer et d'hydroxyde de baryum ou de strontium avec une solution d'un sel de Ba ou de Sr, tel qu'un chlorure ou un nitrate, ayant une concentration comprise entre 2 x 10 et 0,2 M et de préférence entre 4 x 10~3 et 40 x 10~3 M. Le coprécipité est séché, puis recuit à une température comprise entre 750°C et 950°C et de préférence entre 800 et 850°C. On obtient des particules d'hexaferrites sous forme de plaquettes hexagonales.The subject of the present invention is in particular particles of undoped Ba or Sr hexaferrites which have an SFD / Hc ratio equal to or less than 0.45 and a saturation magnetization greater than 55 emu / g, and more advantageously a SFD / Hc ratio equal to or less than 0.40, a saturation magnetization greater than 60 emu / g and a coercive field greater than 5000 Oe (400 kA / m). The subject of the invention is also a magnetic recording product containing particles of Ba or Sr hexaferrite which have the characteristics defined above. The recording product can take any known form such as tape, disc or card. It is particularly useful for obtaining the magnetic medium of a high-capacity magnetic memory card described in French patent application 8814217 entitled "Large-capacity magnetic memory card and manufacturing process". According to one embodiment of the invention, a ferric iron salt such as a nitrate, a chloride, a bromide, an iodide, a fluoride, etc., is used as starting material, and a salt of Ba or Sr such that a chloride, which is made to precipitate in an alkaline medium free of carbonate having a pH equal to or greater than 12. After neutralization and filtration, the precipitate formed of iron hydroxide and barium hydroxide or strontium is washed with a solution of a Ba or Sr salt, such as a chloride or a nitrate, having a concentration of between 2 × 10 and 0.2 M and preferably between 4 x 10 ~ 3 and 40 x 10 ~ 3 M. The coprecipitate is dried, then annealed at a temperature between 750 ° C and 950 ° C and preferably between 800 and 850 ° C. Particles of hexaferrites are obtained in the form of hexagonal plates.
Selon un mode de réalisation préféré de l'invention, on utilise comme produit de départ un sel ferreux ; par coprécipitation avec un sel de Ba ou de Sr, puis par oxydation, on obtient de l'oxyde ferrique hydraté alpha (goethite) contenant du Ba ou du Sr, qui sert de précurseur. Ce mode de réalisation permet d'obtenir des hexaferrites ayant la forme de la goethite servant de précurseur, aiguilles ou nodules.According to a preferred embodiment of the invention, a ferrous salt is used as the starting product; by coprecipitation with a Ba or Sr salt, then by oxidation, hydrated ferric oxide alpha (goethite) containing Ba or Sr is obtained, which serves as a precursor. This embodiment makes it possible to obtain hexaferrites having the shape of goethite serving as a precursor, needles or nodules.
De préférence, pour préparer le précurseur, on utilise le procédé décrit au brevet européen 45246. Dans ce procédé, on utilise un agent réducteur dans le milieu alcalin de précipitation, avant l'oxydation, de façon à éviter la présence d'ions ferriques au départ, et à contrôler ensuite l'oxydation. Ceci permet d'éviter la formation de sous-produits indésirables. L'agent réducteur approprié ne doit pas provoquer de précipité insoluble et doit être actif dans les conditions de pH du procédé. La réduction peut se faire par tout moyen, y compris par voie électrochimique à l'aide d'électrodes appropriées. De préférence, on utilise un composé dont le potentiel redox est inférieur à 1 volt. Comme exemples d'agent réducteur, on peut citer le chlorure stanneux, un dihydroxybenzene ou une hydroxylamine. La quantité d'agent réducteur est ajustée en fonction de la concentration des ions ferriques dans la solution, selon les indications ci-dessous.Preferably, to prepare the precursor, the method described in European patent 45246 is used. In this method, a reducing agent is used in the alkaline precipitation medium, before oxidation, so as to avoid the presence of ferric ions at the start, and then to control the oxidation. This avoids the formation of undesirable by-products. The appropriate reducing agent should not cause an insoluble precipitate and should be active under the process pH conditions. The reduction can be done by any means, including electrochemically using appropriate electrodes. Preferably, a compound is used whose redox potential is less than 1 volt. As examples of reducing agent, mention may be made of stannous chloride, a dihydroxybenzene or a hydroxylamine. The amount of reducing agent is adjusted according to the concentration of ferric ions in the solution, as indicated below.
Suivant ce procédé, on met en solution un sel ferreux qui peut être un chlorure, un bromure, un iodure, un fluorure, un nitrate, etc. Puis, on ajoute l'agent réducteur dont les proportions seront telles qu'un test colorimétrique des ions Fe (KSCN par exemple) soit négatif. On ajoute alors à cette solution, un sel de Ba ou de Sr tel qu'un chlorure ou un nitrate par exemple ; une légère acidification à l'aide d'un acide peut favoriser la dissolution. Cette solution est ajoutée de préférence lentementAccording to this process, a ferrous salt which can be a chloride, a bromide, an iodide, a fluoride, a nitrate, etc. is dissolved. Then, the reducing agent is added, the proportions of which will be such that a colorimetric test of the Fe ions (KSCN for example) is negative. Then added to this solution, a Ba or Sr salt such as a chloride or a nitrate for example; slight acidification with an acid may promote dissolution. This solution is preferably added slowly
(10 mn en général) à une solution aqueuse d'hydroxyde de métal alcalin (sodium ou potassium par exemple) en excès de 25 à 600%, de préférence de 100 à 200% par rapport à l'ensemble des sels. La température des solutions peut être comprise entre 0 et 100°C, et le pH est égal ou supérieur à 12.(10 min in general) to an aqueous solution of alkali metal hydroxide (sodium or potassium for example) in excess of 25 to 600%, preferably 100 to 200% relative to all of the salts. The temperature of the solutions can be between 0 and 100 ° C, and the pH is equal to or greater than 12.
Il est préférable d'utiliser un dispositif permettant de disperser finement les sels dans la solution aqueuse d'hydroxyde de métal alcalin, sans qu'il y ait contact avec un agent oxydant. Un tel dispositif peut être, par exemple, celui décrit au brevet français 1 157 156, appelé "Soucoupe rotative".It is preferable to use a device for finely dispersing the salts in the aqueous alkali metal hydroxide solution, without contact with an oxidizing agent. Such a device can be, for example, that described in French patent 1,157,156, called "Rotating saucer".
L'hydroxyde ferreux est ensuite oxydé par passage d'un courant gazeux oxydant de façon à obtenir les particules du précurseur. L'air est un très bon agent d'oxydation.The ferrous hydroxide is then oxidized by passing an oxidizing gas stream so as to obtain the particles of the precursor. Air is a very good oxidizing agent.
Les meilleurs résultats sont obtenus si le gaz est réparti finement dans l'ensemble de la solution. La température d'oxydation peut varier entre 0 et 100°C à pression ambiante, selon la forme des particules que l'on souhaite. De préférence, elle est inférieure à 60°C, et encore plus avantageusement elle est inférieure à 40°C.The best results are obtained if the gas is distributed finely throughout the solution. The oxidation temperature can vary between 0 and 100 ° C at ambient pressure, depending on the shape of the particles that are desired. Preferably, it is less than 60 ° C, and even more advantageously it is less than 40 ° C.
Le débit d'air pour un litre de solution est généralement compris entre 5 et 60 1/h, mais des débits plus forts peuvent aussi être utilisés. La fin de l'oxydation est mesurée en suivant l'évolution du potentiel du couple Fe 2+/Fe3+ ; quand la concentration en Fe 2+ devient nulle, le potentiel change rapidement, selon les équations d'oxydo-réduction bien connues.The air flow rate for a liter of solution is generally between 5 and 60 l / h, but higher flow rates can also be used. The end of the oxidation is measured by following the evolution of the potential of the Fe 2 + / Fe 3 + couple; when the Fe 2+ concentration becomes zero, the potential changes rapidly, according to the well-known redox equations.
Après obtention du précurseur, on peut éventuellement effectuer une maturation à 90°C pendant 1/2 heure àAfter obtaining the precursor, it is possible to carry out a maturation at 90 ° C. for 1/2 hour at
1 heure. Les concentrations en précurseur sont celles décrites dans le brevet européen 45246, à savoir entre 0,1 et 0,5 mole/1 en fer, en général. L'excès alcalin est compris entre 25 % et 600 %, en général de 100 à 200 %. La solution est alors filtrée. Dans un système travaillant en discontinu, le précipité est remis en dispersion dans un minimum d'eau et l'excès alcalin est neutralisé avec une solution d'acide dilué (HN03 ou HC1) de façon à ramener le pH de la solution entre 7 et 10. Dans un système en continu, il est possible, après élimination de la majorité de la phase aqueuse, d'opérer une recirculation d'eau dont le pH sera maintenu entre 7 et 10.1 hour. The precursor concentrations are those described in European patent 45246, namely between 0.1 and 0.5 mol / l of iron, in general. The alkaline excess is between 25% and 600%, generally from 100 to 200%. The solution is then filtered. In a discontinuous system, the precipitate is re-dispersed in a minimum of water and the alkaline excess is neutralized with a dilute acid solution (HN0 3 or HC1) so as to bring the pH of the solution between 7 and 10. In a continuous system, after elimination of the majority of the aqueous phase, it is possible to recirculate water, the pH of which will be maintained between 7 and 10.
Les sels contenus dans cette eau sont alors éliminés par lavages successifs avec une solution diluée d'un sel de Ba ou de Sr, tel qu'un chlorure, soit par la technique de recirculation partielle, soit par la technique en eau perdue. La concentration de cette solution en sel de Ba ou de Sr peut varier entre 2 x 10~3 M et 0,2 M, de préférence entre 4 x 10~3 et 40 x 10~3 M. La concentration optimale est ajustée en fonction du coefficient de solubilité de Ba (OH) ou de Sr(OH) , de façon à ce qu'il y ait équilibre entre les ions baryum ou strontium redissous par la solution de lavage et ceux apportés par cette même solution. Si cette concentration est trop faible, on perd du baryum ou du strontium, et on n'obtient pas l'aimantation à saturation souhaitée. Si elle est trop élevée, il se produit une imprégnation de Ba ou de Sr à la surface, ce qui, dans le produit final, entraîne une chute du champ coercitif par apparition d'une deuxième population de particules à plus bas champ coercitif. Cette concentration ne dépend pas de la quantité de fer mise en jeu dans la synthèse. La quantité de solution de lavage n'est pas critique ;. elle doit seulement être suffisante pour remettre le précipité en dispersion. Le pH de cette solution est maintenu à une valeur comprise entre 7 et 10 par adjonction d'une certaine quantité de NH,OH. La valeur optimale est ajustée suivant la nature des particules. On a trouvé qu'on obtenait de bons résultats quand elle était égale à 8,0 ± 0,5 pour l'hexaferrite de Ba et à 10,0 ± 0,5 pour l'hexaferrite de Sr.The salts contained in this water are then removed by successive washing with a dilute solution of a Ba or Sr salt, such as a chloride, either by the partial recirculation technique, or by the lost water technique. The concentration of this Ba or Sr salt solution can vary between 2 x 10 ~ 3 M and 0.2 M, preferably between 4 x 10 ~ 3 and 40 x 10 ~ 3 M. The optimal concentration is adjusted according to the solubility coefficient of Ba (OH) or Sr (OH), so that there is equilibrium between the barium or strontium ions redissolved by the washing solution and those provided by this same solution. If this concentration is too low, barium or strontium is lost, and the desired saturation magnetization is not obtained. If it is too high, an impregnation of Ba or Sr occurs on the surface, which, in the final product, causes a fall in the coercive field by the appearance of a second population of particles with a lower coercive field. This concentration does not depend on the amount of iron involved in the synthesis. The amount of washing solution is not critical; it should only be enough to put the precipitate back in dispersion. The pH of this solution is kept at a value between 7 and 10 by adding a certain amount of NH, OH. The optimal value is adjusted according to the nature of the particles. It has been found that good results are obtained when it is equal to 8.0 ± 0.5 for hexaferrite from Ba and 10.0 ± 0.5 for hexaferrite from Sr.
Cette technique de lavage supprime la dissolution du Ba ou du Sr dans les eaux de lavage, et permet donc de garder la stoechiometrie utilisée pour les réactifs de départ, qui est en général comprise entre 1 et 1,2.This washing technique eliminates the dissolution of Ba or Sr in the washing water, and therefore makes it possible to keep the stoichiometry used for the starting reagents, which is generally between 1 and 1.2.
Le produit est^alors filtré et séché, puis porté à une température comprit»-;-, entre 750°C et 950°C, et de préférence entre 800 et 850°C pendant 2 à 4 heures, de façon à cristalliser l'hexaferrite de Ba ou de Sr.The product is ^ then filtered and dried, then heated to a temperature realized "- - between 750 ° C and 950 ° C, and preferably between 800 and 850 ° C for 2 to 4 hours, so as to crystallize the Ba or Sr. hexaferrite
Les particules obtenues après traitement thermique, sans traitement de lavage ultérieur, conservent la forme du précurseur initial. Leur diamètre est compris entre 0,02 et 0,1 um, leur longueur entre 0,02 et 0,5 um. II est possible, lors de la croissance du précurseur en solution aqueuse, de modifier la forme des particules en ajoutant des modificateurs de croissance cristalline, comme il est décrit à la demande do brevet WO 86/05026, tel que le pyrogallol. Sans modificateur de croissance, 1'acicularité des aiguilles obtenues, leur diamètre, leur longueur dépend de la température d'oxydation, de la concentration et du débit du gaz oxydant.The particles obtained after heat treatment, without subsequent washing treatment, retain the shape of the initial precursor. Their diameter is between 0.02 and 0.1 µm, their length between 0.02 and 0.5 µm. It is possible, during the growth of the precursor in aqueous solution, to modify the shape of the particles by adding crystal growth modifiers, as described in patent application WO 86/05026, such as pyrogallol. Without a growth modifier, the particularity of the needles obtained, their diameter, their length depends on the oxidation temperature, the concentration and the flow rate of the oxidizing gas.
L'aimantation et le champ coercitif sont mesurés au VSM (Vibrating Sample Magnetometer) sous un champ magnétisant maximum de 20 000 Oe (1590 kA/m) . Ces valeurs dépendent de la température d'oxydation et de l'excès alcalin. En ajustant ces deux paramètres de façon appropriée, on peut obtenir des particules d'hexaferrite non dopées ayant à la fois une aimantation à saturation supérieure à 60 emu/g, un champ coercitif supérieur à 5 000 Oe (400 kA/m) et un rapport SFD/Hc égal ou inférieur à 0,40.The magnetization and the coercive field are measured with the VSM (Vibrating Sample Magnetometer) under a maximum magnetizing field of 20,000 Oe (1590 kA / m). These values depend on the oxidation temperature and the alkaline excess. By adjusting these two parameters appropriately, it is possible to obtain undoped hexaferrite particles having both a magnetization at saturation greater than 60 emu / g, a coercive field greater than 5000 Oe (400 kA / m) and an SFD / Hc ratio equal to or less than 0.40.
Comme on l'a déjà mentionné ci-dessus, on peut modifier l'aimantation et/ou le champ coercitif en introduisant des ions dopants, comme il est connu dans la technique. Les exemples suivants illustrent l'invention. EXEMPLE 1 (Témoin)As already mentioned above, the magnetization and / or the coercive field can be modified by introducing doping ions, as is known in the art. The following examples illustrate the invention. EXAMPLE 1 (Witness)
Une solution aqueuse alcaline est préparée en dissolvant 44 g de NaOH dans 0,6 1 d'eau distillée (soit 1,1 mole). Cette préparation est portée à 55°C. Dans 190 ml d'eau distillée acidifiée avec 10 ml d'acide chlorhydrique IN, on dissout 53,7 g de FeCl2,4H20 soit 0,27 mole ; 0,2 g de SnCl2,2H20, soit 8,8 x 10-4 mole, sont ajoutés à cette dernière solution. Il est facile de vérifier avec KSCN qu'il n'y a plus d'ions ferriques en solution. 5,5 g de BaCl2,2H20 sont alors mis en solution (22,5 x 10~3 mole). Cette quantité correspond à la stoechiometrie du produit final BaFe120,g.An alkaline aqueous solution is prepared by dissolving 44 g of NaOH in 0.6 1 of distilled water (i.e. 1.1 moles). This preparation is brought to 55 ° C. In 190 ml of distilled water acidified with 10 ml of IN hydrochloric acid, 53.7 g of FeCl 2 , 4H 2 0 is dissolved, ie 0.27 mole; 0.2 g of SnCl 2 , 2H 2 0, or 8.8 × 10 -4 mole, are added to this latter solution. It is easy to check with KSCN that there are no more ferric ions in solution. 5.5 g of BaCl 2 , 2H 2 0 are then dissolved (22.5 x 10 ~ 3 mole). This quantity corresponds to the stoichiometry of the final product BaFe 12 0, g .
Cette solution préalablement chauffée à 55°C est versée doucement (10 mn) sous forte agitation dans la solution alcaline, à un pH supérieur à 12. Il se forme un précipité. Celui-ci est ensuite oxydé par un courant d'air (30 1/h) qui est réparti uniformément dans la solution au moyen d'un verre fritte,à 55°C et à pH supérieur à 12. Le potentiel Fe++/Fe+++ de la solution est contrôlé. Au bout de 3 heures, ce potentiel change rapidement pour passer de -800 mV à -200 mV environ. Le précipité est alors filtré, puis lavé à l'eau distillée jusqu'à ce que le pH de l'eau sortant du filtre soit descendu aux environs de 8 à 9. Après séchage, celui-ci est traité 3 heures à 830°C.This solution, previously heated to 55 ° C., is poured gently (10 min) with vigorous stirring into the alkaline solution, at a pH greater than 12. A precipitate forms. This is then oxidized by a stream of air (30 l / h) which is distributed uniformly in the solution by means of a sintered glass, at 55 ° C. and at a pH greater than 12. The potential Fe ++ / Fe +++ of the solution is checked. After 3 hours, this potential changes rapidly from around -800 mV to around -200 mV. The precipitate is then filtered, then washed with distilled water until the pH of the water leaving the filter has dropped to around 8 to 9. After drying, the latter is treated for 3 hours at 830 ° C. .
Les résultats sont donnés au tableau I. EXEMPLE 2 (Témoin)The results are given in Table I. EXAMPLE 2 (Control)
On reprend le mode opératoire de l'exemple 1, sauf que le précipité n'est pas lavé, mais remis en dispersion dans 150 cm3 d'eau. L'excès de soude est alors neutralisé par HCl IN (le pH de la solution est descendu à 7). La solution est filtrée et lavée avec 500 cm d'eau distillée, afin d'éliminer les sels résiduels (concentration en ions Na+ inférieure à 10-3 M) .The procedure of Example 1 is repeated, except that the precipitate is not washed, but redispersed in 150 cm 3 of water. The excess soda is then neutralized with IN HCl (the pH of the solution has dropped to 7). The solution is filtered and washed with 500 cm of distilled water, in order to remove the residual salts (concentration of Na + ions less than 10 -3 M).
Après séchage, le précipité est traité 3 heures à 830°C.After drying, the precipitate is treated for 3 hours at 830 ° C.
Les résultats sont donnés au tableau I. EXEMPLE 3 (Invention)The results are given in Table I. EXAMPLE 3 (Invention)
On reprend le mode opératoire de l'exemple 1. Le précipité n'est pas lavé, mais remis en dispersion dans 150 cm d'eau. On neutralise l'excès de soude avec HCl IN de façon à abaisser le pH à 8, puis on filtre le précipité et on le lave avec 500 cm d'une solution 8 x 10~3 M de BaCl2,2H20 dont le pH est ajusté à 8,0 __: 0,1 avec NH.OH. Après séchage, le précipité est traité 3 heures à 830°C.The procedure of Example 1 is repeated. The precipitate is not washed, but redispersed in 150 cm of water. The excess sodium hydroxide is neutralized with IN HCl so as to lower the pH to 8, then the precipitate is filtered and washed with 500 cm of an 8 × 10 -3 M solution of BaCl 2 , 2H 2 0, the pH is adjusted to 8.0 __: 0.1 with NH.OH. After drying, the precipitate is treated for 3 hours at 830 ° C.
Les résultats sont indiqués au tableau I. On peut constater que l'aimantation à saturation est nettement plus élevée que pour les témoins des exemples 1 et 2. TABLEAU IThe results are shown in Table I. It can be seen that the saturation magnetization is significantly higher than for the controls of Examples 1 and 2. TABLE I
Im = Aimantation à saturationIm = Saturation magnetization
** Hc = Champ coercitif EXEMPLE 4 (Témoin)** Hc = Coercive field EXAMPLE 4 (Witness)
On prépare une solution alcaline aqueuse en dissolvant 29 g (0,72 mole) de NaOH dans 0,6 1 d'eau distillée et on porte à 35°C. On dissout 36 g de FeCl2,4H20 (0,181 mole) dans 190 ml d'eau distillée acidifiée avec 10 ml d'acide chlorhydrique IN, et on ajoute 0,2 g de SnCl2,2H20 (8,8 x 10~4 mole). On vérifie avec KSCN qu'il n'y a plus d'ions ferriques en solution. On ajoute alors 4,4 g de BaCl2,2H20 (18 x 10~3 mole). Cette quantité correspond à un excès de Ba de 20% par rapport à la stoechiometrie du produit final BaFe120.„. Cette solution préalablement chauffée à 35°C est versée doucement (10 mn) sous forte agitation dans la solution alcaline. On reprend ensuite le mode opératoire de l'exemple 1, sauf qu'on maintient la température à 35°C pendant l'oxydation. Les résultats sont indiqués au tableau II. EXEMPLE 5 (Témoin)An aqueous alkaline solution is prepared by dissolving 29 g (0.72 mole) of NaOH in 0.6 1 of distilled water and the mixture is brought to 35 ° C. 36 g of FeCl 2 , 4H 2 0 (0.181 mole) are dissolved in 190 ml of distilled water acidified with 10 ml of IN hydrochloric acid, and 0.2 g of SnCl 2 , 2H 2 0 (8.8) is added. x 10 ~ 4 mole). We check with KSCN that there are no more ferric ions in solution. Then added 4.4 g of BaCl 2 , 2H 2 0 (18 x 10 ~ 3 mole). This amount corresponds to an excess of Ba of 20% relative to the stoichiometry of the final product BaFe 12 0. „. This solution, previously heated to 35 ° C., is poured gently (10 min) with vigorous stirring into the alkaline solution. The procedure of Example 1 is then resumed, except that the temperature is maintained at 35 ° C during the oxidation. The results are shown in Table II. EXAMPLE 5 (Witness)
On reprend le mode opératoire de l'exemple 4, sauf que le précipité n'est pas lavé, mais remis en solution et neutralisé comme à l'exemple 2.The procedure of example 4 is repeated, except that the precipitate is not washed, but redissolved and neutralized as in example 2.
Les résultats sont indiqués au tableau II. EXEMPLE 6 (Invention)The results are shown in Table II. EXAMPLE 6 (Invention)
On reprend le mode opératoire de l'exemple 4. Le précipité n'est pas lavé, mais remis en dispersion dans 150 cm d'eau. On neutralise l'excès de soude avec HCl IN de façon à abaisser le pH à 8, puis on filtre le précipité et on le lave avec 500 cm3 d'une solution 8 x 10~3 M de BaCl2,2H20 dont le pH est ajusté à 8,0 ± 0,1 avec NH4OH. Après séchage, le précipité est traité 3 heures à 830°C.The procedure of Example 4 is repeated. The precipitate is not washed, but redispersed in 150 cm of water. The excess sodium hydroxide is neutralized with IN HCl so as to lower the pH to 8, then the precipitate is filtered and washed with 500 cm 3 of an 8 × 10 −3 M solution of BaCl 2 , 2H 2 0 of which the pH is adjusted to 8.0 ± 0.1 with NH 4 OH. After drying, the precipitate is treated for 3 hours at 830 ° C.
Les résultats sont indiqués au tableau II. Dans ce cas aussi, on observe une aimantation à saturation plus élevée que pour les témoins des exemples 3 et 4. TABLEAU IIThe results are shown in Table II. In this case too, there is a magnetization at higher saturation than for the controls of Examples 3 and 4. TABLE II
Im = Aimantation à saturationIm = Saturation magnetization
****
Hc = Champ coercitif EXEMPLE 7Hc = Coercive field EXAMPLE 7
Cet exemple est un exemple de synthèse à l'échelle semi-industrielle. On dissout 21,4 kg de NaOH à 50 % dans 210 litres d'eau. On porte à 25°C. On prépare une solution en ajoutant, dans l'ordre, dans 50 litres d'eau acidifiée avec 0,75 1 d'HCl N, 12,06 kg de FeCl ,4H O, 48 g de SnCl ,2H O, et 1,48 kg de BaCl ,2H O. Cette solution chauffée à 25°C est versée sous forte agitation dans la solution alcaline. Il se forme un précipité qu'on oxyde par un courant d'air et qu'on traite comme décrit à l'exemple 6, sauf qu'on lave 3 fois avec une q -_.uantité de solution de BaCl2,2H2O 8 x 10_ M, suffisante pour remettre le précipité en dispersion, et qu'on calcine dans un four rotatif.This example is an example of synthesis on a semi-industrial scale. 21.4 kg of 50% NaOH are dissolved in 210 liters of water. It is brought to 25 ° C. A solution is prepared by adding, in order, in 50 liters of acidified water with 0.75 1 of N HCl, 12.06 kg of FeCl, 4H O, 48 g of SnCl, 2H O, and 1, 48 kg of BaCl, 2H O. This solution heated to 25 ° C is poured with vigorous stirring into the alkaline solution. A precipitate is formed which is oxidized by a stream of air and which is treated as described in Example 6, except that it is washed 3 times with a quantity of solution of BaCl2.2H2O 8 x 10 _ M, sufficient to put the precipitate back into dispersion, and calcined in a rotary oven.
Les particules d'hexaferrite de baryum obtenues ont une aimantation à saturation de 64 emu/g, un champ coercitif de 5 000 Oe (398 kA/m), un SFD de 1800 Oe (143 kA/m) et un rapport SFD/Hc de 0,36. EXEMPLE 8 (Comparatif)The barium hexaferrite particles obtained have a saturation magnetization of 64 emu / g, a coercive field of 5,000 Oe (398 kA / m), an SFD of 1,800 Oe (143 kA / m) and a SFD / Hc ratio 0.36. EXAMPLE 8 (Comparative)
Cet exemple est réalisé suivant le mode opératoire de l'exemple 1 de la demande de brevet japonais 6252133. On dissout 9,16 g de BaCl2,2H20 (37,5 x 10-3 mole) dans 240 ml d'une solution contenant 59,6 g de FeCl2,4H20 (0,3 mole), ce qui correspond à un excès de Ba de 50 % par rapport à la stoechiometrie. On ajoute 60 g de NaOH et de l'eau pour faire 1 1. On chauffe la suspension alcaline de Fe(OH)2 et on fait passer un courant d'air dont on a éliminé le C02, à 40°C et à une vitesse de 250 1/heure. Au bout de 5 heures, les ions ferreux sont totalement transformés en ions ferriques. On filtre le précipité, on le lave à l'eau et on le sèche à 80°C. On le traite ensuite 3 heures à 830°C.This example is carried out according to the procedure of Example 1 of Japanese patent application 6252133. 9.16 g of BaCl 2 , 2H 2 0 (37.5 x 10 -3 mole) is dissolved in 240 ml of a solution containing 59.6 g of FeCl 2 , 4H 2 0 (0.3 mole), which corresponds to a 50% excess of Ba compared to stoichiometry. 60 g of NaOH and water are added to make 1 1. The alkaline suspension of Fe (OH) 2 is heated and a stream of air is passed from which the CO 2 has been removed, at 40 ° C. and at a speed of 250 l / hour. After 5 hours, the ferrous ions are completely transformed into ferric ions. The precipitate is filtered, washed with water and dried at 80 ° C. It is then treated for 3 hours at 830 ° C.
Les résultats sont les suivants : Im = 52,9 emu/g Hc = 5028 Oe (400 kA/m) SFD = 2820 Oe (225 kA/m) SFD/Hc = 0,56The results are as follows: Im = 52.9 emu / g Hc = 5028 Oe (400 kA / m) SFD = 2820 Oe (225 kA / m) SFD / Hc = 0.56
On constate que le procédé décrit dans la demande de brevet japonais 6252133 ne permet pas d'obtenir à la fois un faible rapport SFD/Hc et une forte aimantation si on le compare aux résultats obtenus avec le procédé de l'invention.It can be seen that the process described in Japanese patent application 6252133 does not make it possible to obtain both a low SFD / Hc ratio and a strong magnetization if it is compared to the results obtained with the process of the invention.
EXEMPLE 9 (Invention)EXAMPLE 9 (Invention)
On reprend le mode opératoire de. l'exemple 1; On utilise 36,7 g de NaOH, 44,7 g (0,22 mole) de FeCl2,4H20 et 3,6 g (0,018 mole) de SrCl , 2H O (quantité stoechiométrique pour SrFe 20o) . La température d'oxydation est de 25°C. L'étape de neutralisation du précipité est effectuée jusqu'à atteindre un pH de 10. Le lavage après filtration est réalisé avec une solution de 16 x 10~3 mole/1 de SrCl2,2H20 dont le pH a été ajusté à 10,0 ± 0,1 avec NH4OH. Après séchage, le précipité est traité 2 heures à 830°C.We resume the operating mode of. Example 1; 36.7 g of NaOH, 44.7 g (0.22 mole) of FeCl 2 , 4H 2 0 and 3.6 g (0.018 mole) of SrCl, 2H O (stoichiometric amount for SrFe 20o) are used. The oxidation temperature is 25 ° C. The precipitate neutralization step is carried out until a pH of 10 is reached. The washing after filtration is carried out with a solution of 16 × 10 -3 mole / 1 of SrCl 2 , 2H 2 0, the pH of which has been adjusted to 10.0 ± 0.1 with NH 4 OH. After drying, the precipitate is treated for 2 hours at 830 ° C.
Les caractéristiques sont les suivantes : Im = 61.7 emu/g Hc = 470 kA/m (5900 Oe) SFD = 169 kA/m (2120 Oe)The characteristics are as follows: Im = 61.7 emu / g Hc = 470 kA / m (5900 Oe) SFD = 169 kA / m (2120 Oe)
SFD/Hc = 0,36 EXEMPLE 10 (Témoin)SFD / Hc = 0.36 EXAMPLE 10 (Witness)
On disperse 25 g de Fe( 03)3,9H20 dans 70 ml d'eau distillée et on chauffe à 60°C. Puis on ajoute 1,26 g de BaCl2,2H20 dont la dissolution est rendue possible par le pH acide de la solution et la température.25 g of Fe (0 3 ) 3 , 9H 2 0 are dispersed in 70 ml of distilled water and the mixture is heated to 60 ° C. Then 1.26 g of BaCl 2 , 2H 2 0 is added, the dissolution of which is made possible by the acid pH of the solution and the temperature.
On prépare parallèlement une solution de 13,8 g de K2CO_ et 11,2 g "de OH dans 70 ml d'eau. La réaction est exothermique, et on ajuste la température à 60°C. On verse la solution saline dans la solution basique et on précipite un mélange d'hydroxyde de fer et d'hydroxyde de baryum. L'agitation doit être telle que la solution reste toujours homogène. Après filtration et lavage, le produit est séché, puis traité 3 heures à 800°C pour cristalliser la ferrite de baryum BaFe120,_. On obtient les résultats suivants : Im = 60 emu/g Hc = 421 kA/m (52S0 Oe) SFD = 199 kA/m (2500 t)e) SDF/Hc = 0,47A solution of 13.8 g of K 2 CO 2 and 11.2 g " of OH in 70 ml of water is prepared in parallel. The reaction is exothermic, and the temperature is adjusted to 60 ° C. The saline solution is poured into the basic solution and a mixture of iron hydroxide and barium hydroxide is precipitated. Agitation must be such that the solution always remains homogeneous. After filtration and washing, the product is dried and then treated for 3 hours at 800 ° C to crystallize barium ferrite BaFe 12 0, _. The following results are obtained: Im = 60 emu / g Hc = 421 kA / m (52S0 Oe) SFD = 199 kA / m (2500 t) e) SDF / Hc = 0.47
EXEMPLE 11 (Invention)EXAMPLE 11 (Invention)
On procède comme précédemment, sauf que la solution basique se compose de 12,5 g de KOH dans 70 ml d'eau. D'autre part, le lavage des précipités est effectué (comme expliqué précédemment) avec une solution deThe procedure is as above, except that the basic solution consists of 12.5 g of KOH in 70 ml of water. On the other hand, the washing of the precipitates is carried out (as explained above) with a solution of
BaCl2,2H20 (2 g/1) jusqu'à neutralité. On obtient les résultats suivants :BaCl 2 , 2H 2 0 (2 g / 1) until neutral. The following results are obtained:
Im = 62 emu/g Hc = 395 kA/m (4900 Oe) SFD = 135 kA/m (1700 Oe)Im = 62 emu / g Hc = 395 kA / m (4900 Oe) SFD = 135 kA / m (1700 Oe)
SFD/Hc = 0,34 On observe aussi dans ce mode de réalisation, à la fois une augmentation de l'aimantation à saturation et une diminution de la répartition du champ de commutation des particules obtenues. SFD / Hc = 0.34 Also observed in this embodiment is both an increase in the saturation magnetization and a decrease in the distribution of the switching field of the particles obtained.

Claims

REVENDICATIONS 1 - Procédé de préparation d'hexaferrites de baryum ou de strontium, consistant à faire coprécipiter un sel de fer et un sel de baryum ou de strontium en milieu alcalin, à filtrer, laver et sécher le précipité d'hydroxydes obtenu, puis à chauffer à des températures comprises entre 750°C et 950°C pour faire cristalliser l'hexaferrite, caractérisé en ce que : 1) On coprécipité en l'absence d'ions carbonate,CLAIMS 1 - Process for the preparation of barium or strontium hexaferrites, consisting of making an iron salt and a barium or strontium salt coprecipitate in an alkaline medium, filtering, washing and drying the precipitate of hydroxides obtained, then heating to temperatures between 750 ° C and 950 ° C to crystallize the hexaferrite, characterized in that: 1) Coprecipitate in the absence of carbonate ions,
2) après la coprécipitation, on neutralise le milieu jusqu'à l'obtention d'un pH compris entre 7 et 10,2) after the co-precipitation, the medium is neutralized until a pH of between 7 and 10 is obtained,
3) après filtration, on lave le coprécipité d'hydroxyde de fer et d'hydroxyde de Ba ou de Sr, avec une solution aqueuse tamponnée à pH compris entre 7 et 10 d'un sel de baryum ou de strontium, ayant une concentration comprise entre 2 x 10 "3 et 0,2 M.3) after filtration, the coprecipitate of iron hydroxide and of hydroxide of Ba or of Sr is washed with an aqueous buffered solution at pH between 7 and 10 of a barium or strontium salt, having a concentration comprised between 2 x 10 "3 and 0.2 M.
2 - Procédé conforme à la revendication 1, dans lequel la concentration de la solution de sel de Ba ou de Sr est comprise entre 4 x 10 ~3 et 40 x 10 ~3 M.2 - Process according to claim 1, wherein the concentration of the Ba or Sr salt solution is between 4 x 10 ~ 3 and 40 x 10 ~ 3 M.
3 - Procédé conforme à l'une des revendications 1 ou 2, dans lequel le précipité d'hydroxyde obtenu est chauffé à une température comprise entre 800 et 850°C. 4 - Procédé conforme à l'une des revendications 1 à 3, dans lequel le sel de fer est un sel ferrique. 5 - Procédé conforme à l'une des revendications 1 à 3, dans lequel le sel de fer est un sel ferreux, et on oxyde le coprécipité d'hydroxydes obtenu. 6 - Procédé conforme à la revendication 5, dans lequel on oxyde par un courant d'air, à une température comprise entre 0 et 100°C. 7 - Procédé conforme à la revendication 6, dans lequel on oxyde par un courant d'air à une température inférieure à 60°C. - Procédé conforme à la revendication 6, dans lequel on oxyde par un courant d'air à une température inférieure à 40°C. - Procédé conforme à l'une des revendications 5 à 8, dans lequel on ajoute un réducteur dans le milieu de précipitation. - Procédé conforme à la revendication 9, dans lequel le réducteur est du chlorure stanneux, un dihydroxybenzene ou une hydroxylamine. - Particules d'hexaferrites de baryum ou de strontium, dopées ou non, préparées suivant le procédé d'une quelconque des revendications 1 à 10. - Particules d'hexaferrites de baryum ou de strontium non dopées, ayant à la fois un rapport de la répartition des champs de commutation au champ coercitif égal ou inférieur à 0,45 et une aimantation à saturation supérieure à 55 emu/g. - Particules d'hexaferrites de baryum ou de strontium de la revendication 12 ayant à la fois un rapport de la répartition des champs de commutation au champ coercitif égal ou inférieur à 0,40, une aimantation à saturation supérieure à 60 emu/g, et un champ coercitif supérieur à 5 000 Oe (400 kA/m) . - Produit d'enregistrement magnétique contenant les particules d'hexaferrites de Ba ou de Sr des revendications 11 à 13. 3 - Process according to one of claims 1 or 2, wherein the hydroxide precipitate obtained is heated to a temperature between 800 and 850 ° C. 4 - Process according to one of claims 1 to 3, wherein the iron salt is a ferric salt. 5 - Process according to one of claims 1 to 3, wherein the iron salt is a ferrous salt, and the coprecipitate of hydroxides obtained is oxidized. 6 - Process according to claim 5, wherein it is oxidized by a stream of air, at a temperature between 0 and 100 ° C. 7 - Process according to claim 6, wherein it is oxidized by a stream of air at a temperature below 60 ° C. - Method according to claim 6, wherein it is oxidized by a stream of air at a temperature below 40 ° C. - Process according to one of claims 5 to 8, in which a reducing agent is added to the precipitation medium. - Process according to claim 9, in which the reducing agent is stannous chloride, a dihydroxybenzene or a hydroxylamine. - Particles of barium or strontium hexaferrites, doped or not, prepared according to the process of any one of claims 1 to 10. - Particles of undoped barium hexa or ferrites, having both a ratio of distribution of the switching fields in the coercive field equal to or less than 0.45 and a saturation magnetization greater than 55 emu / g. - Barium or strontium hexaferrite particles of claim 12 having both a ratio of the distribution of the switching fields to the coercive field equal to or less than 0.40, a saturation magnetization greater than 60 emu / g, and a coercive field greater than 5,000 Oe (400 kA / m). - Magnetic recording product containing particles of Ba or Sr hexaferrites of claims 11 to 13.
EP89912113A 1988-10-18 1989-10-17 Method for preparing magnetic particles of hexaferrites, particles obtainted and product containing them Pending EP0439524A1 (en)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
FR8814221 1988-10-18
FR8814221A FR2637888B1 (en) 1988-10-18 1988-10-18 PROCESS FOR THE PREPARATION OF MAGNETIC PARTICLES OF HEXAFERRITES, PARTICLES OBTAINED AND PRODUCT CONTAINING THEM

Publications (1)

Publication Number Publication Date
EP0439524A1 true EP0439524A1 (en) 1991-08-07

Family

ID=9371446

Family Applications (2)

Application Number Title Priority Date Filing Date
EP89912113A Pending EP0439524A1 (en) 1988-10-18 1989-10-17 Method for preparing magnetic particles of hexaferrites, particles obtainted and product containing them
EP89420399A Expired - Lifetime EP0369897B1 (en) 1988-10-18 1989-10-17 Process for the preparation of magnetic hexaferrite particles, particles obtained and product containing them

Family Applications After (1)

Application Number Title Priority Date Filing Date
EP89420399A Expired - Lifetime EP0369897B1 (en) 1988-10-18 1989-10-17 Process for the preparation of magnetic hexaferrite particles, particles obtained and product containing them

Country Status (8)

Country Link
US (1) US5135733A (en)
EP (2) EP0439524A1 (en)
JP (1) JPH04503938A (en)
AT (1) ATE91675T1 (en)
DE (1) DE68907696T2 (en)
ES (1) ES2058585T3 (en)
FR (1) FR2637888B1 (en)
WO (1) WO1990004570A1 (en)

Families Citing this family (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
FR2638000B1 (en) * 1988-10-18 1990-12-14 Kodak Pathe LARGE CAPACITY MAGNETIC MEMORY CARD AND MANUFACTURING METHOD
US5306592A (en) * 1992-10-29 1994-04-26 Eastman Kodak Company Method of preparing electrographic magnetic carrier particles
US5268249A (en) * 1992-10-29 1993-12-07 Eastman Kodak Company Magnetic carrier particles
US5660772A (en) * 1993-09-27 1997-08-26 Alfred University Process for making ultra-fine barium hexaferrite particles
US20030124052A1 (en) * 2001-12-28 2003-07-03 Mauldin Lloyd Ballard Production of alkaline earth metal ferrites

Family Cites Families (7)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4025449A (en) * 1975-10-21 1977-05-24 The Dow Chemical Company Process to produce barium and strontium ferrite
US4120807A (en) * 1976-08-30 1978-10-17 Dowa Mining Co., Ltd. Process for producing hexagonal-system ferrite powder
US4120806A (en) * 1976-08-30 1978-10-17 Dowa Mining Co., Ltd. Hexagonal-system ferrite powder, composite plastic-ferrite magnet comprising same and process for production thereof
WO1982000287A1 (en) * 1980-07-24 1982-02-04 Kodak Co Eastman Process for preparing acicular iron oxide particles
DE3042959A1 (en) * 1980-11-14 1982-07-08 Basf Ag, 6700 Ludwigshafen METHOD FOR THE PRODUCTION OF FINE-PARTICALLY HIGH-COCITIVE BARIUM-FERRITE, AND ITS USE FOR THE PRODUCTION OF MAGNETIC RECORDING CARRIERS
JPH0647465B2 (en) * 1985-08-29 1994-06-22 戸田工業株式会社 BaO.9 / 2Fe (2) O (3) O (3) Microcrystalline powder manufacturing method
JPS62100419A (en) * 1985-10-24 1987-05-09 Nippon Zeon Co Ltd Production of magnetic powder for magnetic recording

Non-Patent Citations (1)

* Cited by examiner, † Cited by third party
Title
See references of WO9004570A1 *

Also Published As

Publication number Publication date
WO1990004570A1 (en) 1990-05-03
ATE91675T1 (en) 1993-08-15
EP0369897A1 (en) 1990-05-23
DE68907696D1 (en) 1993-08-26
FR2637888B1 (en) 1990-12-07
DE68907696T2 (en) 1994-03-31
ES2058585T3 (en) 1994-11-01
EP0369897B1 (en) 1993-07-21
US5135733A (en) 1992-08-04
FR2637888A1 (en) 1990-04-20
JPH04503938A (en) 1992-07-16

Similar Documents

Publication Publication Date Title
EP0717397B1 (en) Spindle-shaped magnetic ironbased alloy particles containing cobalt and iron as the main ingredients and process for producing the same
EP0466338B1 (en) Spindle-shaped magnetic iron based alloy particles and process for producing the same
JPH06100317A (en) Magnetic iron oxide grain powder and its production
FR2487326A1 (en) PROCESS FOR THE PREPARATION OF ACIDIC PARTICLES OF MAGNETIC IRON OXIDE
US3897354A (en) Cobalt-containing acicular ferrimagnetic iron oxide of improved remanence stability
EP0369897B1 (en) Process for the preparation of magnetic hexaferrite particles, particles obtained and product containing them
JPS5853688B2 (en) Method for producing acicular alloy magnetic particle powder mainly composed of Fe-Mg
EP0240529B1 (en) Compositions of particulate magnetic oxides with a structure of the lacunar spinel type, preparation and application thereof
JPS608606B2 (en) Manufacturing method of ferromagnetic powder
JPH0134935B2 (en)
FR2549821A1 (en) WET PROCESS FOR PREPARING FINE FERRITE PARTICLES HAVING A MAGNETOPLUMBITE STRUCTURE
FR2660650A1 (en) PROCESS FOR PREPARING HEXAFERRITE PARTICLES
EP0707738A1 (en) Doped magnetic iron oxide particles and method of preparing the same
US20060292403A1 (en) Spinel ferrimagnetic particles and magnetic recording medium
JP3603926B2 (en) Spindle-shaped goethite particle powder and method for producing the same, spindle-shaped hematite particle powder and method for producing the same, and spindle-shaped metal magnetic particle powder containing iron as a main component and method for producing the same
JPS59107924A (en) Manufacture of magnetic iron oxide powder containing cobalt
JPS6149251B2 (en)
FR2462222A1 (en) PROCESS FOR PRODUCING METAL IRON PARTICLES FOR MAGNETIC RECORDING AND PRODUCT OBTAINED
JPS6135135B2 (en)
EP0371384B1 (en) Process for producing magnetic iron oxide particles for magnetic recording
JP4378763B2 (en) Method for producing compound particle powder containing iron as its main component
JP3264374B2 (en) Method for producing spindle-shaped iron-based metal magnetic particle powder
JPH0471012B2 (en)
JPS62158801A (en) Magnetic metallic particle powder essentially consisting of iron having spindle shape and production thereof
KR930011235B1 (en) Method for producing cobalt-containing ferromagnetic iron oxide powder

Legal Events

Date Code Title Description
PUAI Public reference made under article 153(3) epc to a published international application that has entered the european phase

Free format text: ORIGINAL CODE: 0009012

STAA Information on the status of an ep patent application or granted ep patent

Free format text: STATUS: REQUEST FOR EXAMINATION WAS MADE

17P Request for examination filed

Effective date: 19910409

AK Designated contracting states

Kind code of ref document: A1

Designated state(s): AT BE CH DE FR GB IT LI LU NL SE

XX Miscellaneous (additional remarks)

Free format text: VERFAHREN ABGESCHLOSSEN INFOLGE VERBINDUNG MIT 89420399.1/0369897 (EUROPAEISCHE ANMELDENUMMER/VEROEFFENTLICHUNGSNUMMER) VOM 30.12.91.