Classifications

• • 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
  • H01F1/04—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 metals or alloys
  • H01F1/06—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 metals or alloys in the form of particles, e.g. powder
  • H01F1/065—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 metals or alloys in the form of particles, e.g. powder obtained by a reduction
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B22—CASTING; POWDER METALLURGY
  • B22F—WORKING METALLIC POWDER; MANUFACTURE OF ARTICLES FROM METALLIC POWDER; MAKING METALLIC POWDER; APPARATUS OR DEVICES SPECIALLY ADAPTED FOR METALLIC POWDER
  • B22F9/00—Making metallic powder or suspensions thereof
  • B22F9/16—Making metallic powder or suspensions thereof using chemical processes
  • B22F9/18—Making metallic powder or suspensions thereof using chemical processes with reduction of metal compounds
  • B22F9/20—Making metallic powder or suspensions thereof using chemical processes with reduction of metal compounds starting from solid metal compounds
  • B22F9/22—Making metallic powder or suspensions thereof using chemical processes with reduction of metal compounds starting from solid metal compounds using gaseous reductors
• • 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
  • H01F1/04—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 metals or alloys
  • H01F1/06—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 metals or alloys in the form of particles, e.g. powder
  • H01F1/061—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 metals or alloys in the form of particles, e.g. powder with a protective layer

Definitions

• the invention relates to a process for producing acicular ferromagnetic iron particles by annealing a provided with a shape-stabilizing surface coating goethite to o6-iron (III) oxide and reducing with hydrogen at 275-425 0 C and the use of the iron particles thus obtained, as the magnetic material in the preparation of of magnetic recording media.
• ferromagnetic metal powders and metal thin layers are of particular interest for the production of magnetic recording media, since in this way the energy product and the information . ration density can be increased considerably and such recording media result in narrower signal widths and better signal amplitudes compared to the current standard.
• the mechanical properties of such information media can be achieved by a suitable selection of the polymeric organic material influence solvent systems within wide limits, however - apart from the magnetic properties, further requirements with regard to shape, size and dispersibility of the metal particles have to be made.
• the corresponding metal particles must show magnetic single-range behavior; moreover, the anisotropy that is present or can additionally be achieved by the magnetic alignment in the strip should be only slightly impaired by external influences, such as temperature or mechanical stress, that is, the small particles should preferably be shape-anisotropic Case should be acicular and generally should be between 10 2 and 10 4 ⁇ in size.
• the catalytic acceleration of the reduction of preferably needle-shaped starting compounds generally results in needles which are much smaller than the starting product and which also has a low length / thickness ratio.
• the end product has a fairly large particle size spectrum.
• the particle size dependence of coercive force and remanence in magnetic substances is very strong in the order of magnitude of the single-range particles. If there are also the influences which arise from a proportion of superparamagnetic particles which can arise as fragments in the above-mentioned procedure, such magnetic materials are unsuitable for use in the production of magnetic recording media. With such heterogeneous mixtures, the magnetic field strength, which is necessary to remagnetize the particles, is very different, and the distribution of the remanent magnetization as a function of the applied external field also results in a less steep remanence curve.
• the object of the invention was therefore to provide a method for producing acicular ferromagnetic iron particles, with which it is easy to produce distinctly shape-anisotropic particles with high values for coercive field strength and in particular remanence and relative remanence.
• acicular ferromagnetic iron particles are formed by reacting an aqueous solution of an iron (II) salt with aqueous solutions of alkali metal hydroxides, oxidizing the resulting suspensions of iron (II) hydroxide with oxygen-containing gases to goethite, and applying a shape-stabilizing coating on the surface of the goethite, tempering the goethite treated in this way to form ⁇ -iron (III) oxide and then reducing it with hydrogen at 275 to 425 ° C to acicular ferromagnetic iron particles with the required properties if the goethite provided with a shape-stabilizing coating is used 250 to 450 ° C in a water vapor-containing atmosphere with a water vapor partial pressure of at least 30 mbar is annealed for 10 minutes to 10 hours.
• the goethite provided with a shape-stabilizing coating is annealed for 10 minutes to 10 hours at 250 to 450 ° C. in a water vapor-containing atmosphere with a water vapor partial pressure of 30 to 1013 mbar.
• goethite used in the process according to the invention by the so-called alkaline process is known and is described in detail, for example, in DE-ASen 12 04 644, 25 50 225, 25 50 307 and 25 50 308.
• These goethite needles have a BET specific surface area of 20 to 75 m 2 jg, an average particle length between 0.2 and 1.5 and preferably between 0.3 and 1.2 ⁇ m and a length-to-thickness ratio characterized by at least 10, suitably 10 to 40.
• goethite particles required for the process according to the invention are now provided in a known manner with a shape-stabilizing surface coating, which contributes to the preservation of the outer shape during the further reworking steps.
• the treatment of goethite with an alkaline earth metal and a carboxylic acid or another organic compound which has at least two groups capable of chelating with the alkaline earth metal is suitable for this purpose.
• Also known and described in DE-OS 26 46 348 is the shape-stabilizing finish of goethite on its surface with hydrolysis-resistant oxygen acids of phosphorus, their salts or esters and aliphatic mono- or polybasic carboxylic acids.
• Possible hydrolysis-resistant substances are phosphoric acid, soluble mono-, di- or triphosphates such as potassium, ammonium dihydrogen phosphate, disodium or dilithium orthophosphate, trisodium phosphate, sodium pyrophosphate and metaphosphates such as sodium metaphosphate.
• the compounds can be used alone or as a mixture with one another.
• esters of phosphoric acid with aliphatic monoalcohols with 1 to 6 carbon atoms such as Use tert-butyl ester of phosphoric acid.
• Carboxylic acids in the process are saturated or unsaturated aliphatic carboxylic acids with up to 6 carbon atoms and up to 3 acid groups, it being possible for one or more hydrogen atoms in the aliphatic chain to be substituted by hydroxyl or amino radicals.
• Oxidic and oxitricarboxylic acids such as oxalic acid, tartaric acid and citric acid are particularly suitable.
• the goethite which has been given a shape-stabilizing effect in the manner described, is then annealed for 10 minutes to 10 hours at temperatures between 250 to 450 ° C. in a water vapor-containing atmosphere with a water vapor partial pressure of at least 30 mbar.
• the end product is one with the corresponding to the previous equipment trained surface coating provided acicular o6-iron (III) oxide.
• This tempering can be carried out discontinuously or continuously.
• Reactors such as muffle furnaces, rotary tube furnaces or swirl furnaces are suitable for batch drainage.
• air, inert gases or air-inert gas mixtures can be passed over or through the stationary or moving iron oxide, these gases being loaded beforehand with the appropriate amount of water vapor.
• the gases or gas mixtures at temperatures between 40 0 C and the boiling point of water saturated in particular between 50 ° C and the boiling point of the water with steam and introduced in this state into the annealing reactors.
• the water can of course also be used in the form of steam itself or in a mixture with other gases.
• the tempering can be carried out particularly advantageously in continuous reactors, for example in a continuous rotary tube furnace, since here, in addition to the water vapor in the gas passed through, water vapor from the tempering reaction of goethite is always supplied in the same amount. It is therefore also possible here to work with little or no inert gas flows or air flows. After a short setting time, the corresponding required water vapor partial pressure of preferably 70 to 1013 mbar in the reaction space is reached.
• the ⁇ -iron (III) oxide provided with a shape-stabilizing surface coating is reduced in a manner known per se with hydrogen at 275 to 425, preferably at 300 to 400 ° C. It is advisable to passivate the finely divided iron powders obtained in this way by passing an air or oxygen-inert gas mixture over them, since this changes the pyrophoric character of the needle-shaped iron particles "a length between 0.1 to 0.8 ⁇ m and a length-to-thickness ratio of 5 to 25: 1 can be controlled.
• acicular ferromagnetic iron particles which are distinguished by a pronounced shape anisotropy. This is achieved in that the starting products are both largely free of dendrites and treated to maintain the outer shape and, in addition, result in a well-crystallized iron (III) oxide for the subsequent reduction reaction due to the inventive tempering.
• the resulting iron particles are characterized by markedly improved values for coercive force, specific remanence and relative remanence.
• the needle-shaped particles can be magnetically oriented particularly easily, and important electroacoustic values such as depth and height controllability are improved.
• the iron particles produced according to the invention are dispersed in a known manner in polymeric binders.
• Known compounds such as homopolymers and copolymers of polyvinyl derivatives, polyurethanes, polyesters and the like are suitable as binders for this purpose.
• the binders are used in solutions in suitable organic solvents, which may contain further additives, for example to increase the conductivity and the abrasion resistance of the magnetic layers.
• suitable organic solvents which may contain further additives, for example to increase the conductivity and the abrasion resistance of the magnetic layers.
• the nitrogen surface S N determined according to BET was used primarily to characterize the acicular iron (III) oxide hydroxides used. Electron microscope images provide information about the appearance and dimensions (LID ratio) of the iron oxide hydrate particles.
• the magnetic values of the iron powder were measured with a vibration magnetometer at a magnetic field of 160 or 800 kA / m.
• Specific remanence (M r / ⁇ ) and saturation (M m / ⁇ ) are given in nTm 3 / g.
• remanent coercive field strength H R is an important assessment variable.
• H R in the case of constant field demagnetization, half of the particles are remagnetized at the field strength H R in terms of volume. It thus represents a characteristic quantity for recording processes, which in particular determines the operating point in magnetic recording.
• a value h 5 for the total width of the remanence curve and h 25 for the steepness of the remanence curve is determined from the constant field demagnetization curve. The values are determined according to
• the number index at the letter H indicates how many of the particles are magnetized in percent.
• 70 parts of a goethite finished on the surface according to Example 1 are also annealed in a tube furnace at 350 ° C. for one hour, but under a pressure of 25 mbar.
• the negative pressure in the reaction chamber is generated by a vacuum pump and kept constant by metering in air dried over silica gel via a vacuum valve.
• the resulting surface-finished ⁇ -iron (III) oxide with a surface S N2 of 50 m 2 / g is then reduced to metal in the same way as described in Example 1.
• the magnetic values measured on the acicular iron particles are shown in Table 1.
• Example 2 The procedure is as described in Example 2, but the surface finish of goethite is reduced without additional treatment as in Example 2.
• the magnetic properties of the resulting pyrophoric and passivated iron particles are listed in Tables 1 and 2.
• 800 parts of the passivated iron particles produced according to Example 2 are in a 600-volume steel cylinder mill, which contains 9000 parts of steel balls with a diameter between 4 and 6 mm, with 456 parts of a 13 percent solution of a thermoplastic polyester urethane from adipic acid, 1,4-butanediol and 4,4'-diisocyanatodiphenylmethane in a solvent mixture of equal parts of tetrahydrofuran and dioxane, 296 parts of a 10 percent solution of a polyvinylformal binder, containing 82 percent vinyl formal, 12 percent vinyl acetate and 6 percent vinyl alcohol units, in the solvent mixture mentioned, 20 parts butyl stearate and mixed another 492 parts of the solvent mixture mentioned and dispersed for 4 days.
• the electroacoustic properties of these tapes are measured in accordance with DIN 45 512 with a tape speed of 4.75 cm / sec, a bias current J HF of 23 mA and an equalization of 70 / usec.
• Table 3 shows the values for the modulation at 333 Hz (A T ) and at 10 kHz (A H ).
• the values for the magnetic tape were set to 0 dB according to comparative experiment 7.

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• Engineering & Computer Science (AREA)
• Power Engineering (AREA)
• Chemical & Material Sciences (AREA)
• Chemical Kinetics & Catalysis (AREA)
• General Chemical & Material Sciences (AREA)
• Hard Magnetic Materials (AREA)
• Magnetic Record Carriers (AREA)
• Manufacture Of Metal Powder And Suspensions Thereof (AREA)
• Paints Or Removers (AREA)

Applications Claiming Priority (2)

Publications (3)

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• 1979
  • 1979-09-01 DE DE19792935358 patent/DE2935358A1/de not_active Withdrawn
• 1980
  • 1980-07-31 US US06/174,267 patent/US4344791A/en not_active Expired - Lifetime
  • 1980-08-21 DE DE8080104979T patent/DE3065058D1/de not_active Expired
  • 1980-08-21 EP EP80104979A patent/EP0024694B1/fr not_active Expired
  • 1980-08-29 JP JP11854280A patent/JPS5638405A/ja active Pending

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