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, _.

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• 1988
  • 1988-10-18 FR FR8814221A patent/FR2637888B1/fr not_active Expired - Fee Related
• 1989
  • 1989-10-17 ES ES89420399T patent/ES2058585T3/es not_active Expired - Lifetime
  • 1989-10-17 JP JP1511169A patent/JPH04503938A/ja active Pending
  • 1989-10-17 AT AT89420399T patent/ATE91675T1/de active
  • 1989-10-17 WO PCT/FR1989/000540 patent/WO1990004570A1/fr not_active Application Discontinuation
  • 1989-10-17 EP EP89912113A patent/EP0439524A1/fr active Pending
  • 1989-10-17 EP EP89420399A patent/EP0369897B1/fr not_active Expired - Lifetime
  • 1989-10-17 DE DE89420399T patent/DE68907696T2/de not_active Expired - Fee Related
• 1991
  • 1991-06-10 US US07/678,251 patent/US5135733A/en not_active Expired - Lifetime

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