JPH0246642B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a stabilization treatment for metal powder, and particularly to a stabilization treatment for a ferromagnetic metal powder whose main component is iron. In recent years, acicular ferromagnetic metal powder containing iron as a main component has been attracting attention and being used as a magnetic material for magnetic recording media. This acicular ferromagnetic powder is obtained by heating, dehydrating, and reducing goethite iron oxide, and has superior coercive force and saturation magnetization compared to conventional iron oxide magnetic materials, making it possible to achieve high-density recording. The biggest drawback is poor oxidation stability. In particular, as recording equipment has recently become more compact and recorded for longer periods of time, magnetic recording tapes are required to have improved characteristics, higher output, and lower nozzles, and there is a trend toward finer particles of magnetic metals. However, such fine metal particles have a large specific surface area and are extremely chemically active, and when taken out into the atmosphere, heat generation and ignition occur due to rapid oxidation reactions.
Therefore, a method has been conventionally used in which fine metal particles are brought into contact with an oxygen-containing gas in a liquid phase or a gas phase to form an oxide film on the surface of the fine metal particles, thereby performing a stabilization treatment. However, if the metal magnetic powder treated by the above method is left in the atmosphere for a long time, its magnetic properties will deteriorate.
Furthermore, when taken out into the atmosphere after being stored in a sealed container for a long period of time, the metal magnetic powder, which was supposed to have been stabilized by forming an oxide film, becomes reactivated and generates heat or ignites, and the above stabilization treatment alone is not sufficient.
including handling and storage issues. The cause of this is not clear, but it may be due to changes in the oxide film over time during sealed storage, such as diffusion of oxygen in the film layer within the surface of the magnetic powder particles, or reactivation due to diffusion of metal atoms inside the particles to the surface of the film. Conceivable. The present inventors have improved this by suppressing reactivation during sealed storage and preventing the oxide film from changing over time during storage in the atmosphere. As a result of research into a method for stabilizing the metal magnetic powder that does not significantly deteriorate the magnetic properties of the metal magnetic powder, it was discovered that this problem could be solved by the following method, and the present invention was achieved. In the present invention, a metal magnetic powder containing iron as a main component is brought into contact with an oxygen-containing gas in an organic solvent or in a gas phase to form an oxide film on the metal powder for primary stabilization, and then in an inert gas atmosphere in a gas phase. In this method, the metal magnetic powder containing iron as a main component is stabilized by heating and annealing the powder and then bringing it into contact with an oxygen-containing gas again in an organic solvent or gas phase. The most important feature of the present invention is that the oxide film of the metal magnetic powder is prevented from changing over time and from being reactivated during sealed storage. Although the cause of reactivation is not clear,
For example, diffusion of film layer oxygen in the surface layer of the magnetic powder particles due to non-uniformity of the oxide film, or diffusion of metal atoms inside the particles to the film surface, etc., can be considered. It is thought that such atomic diffusion proceeds very slowly at room temperature. In the present invention, the aging of the oxide film is promoted by heating and annealing, and after reactivation, it is brought into contact with an oxygen-containing gas in an organic solvent or gas phase to form a uniform and dense oxide film. It is characterized by providing a stable metal magnetic powder that has little change over time, does not undergo reactivation after long-term sealed storage, and does not generate heat or ignite even when taken out into the air. The method of the present invention can be applied to all ferromagnetic metal powders containing iron as a main component used as magnetic recording media. Examples of the ferromagnetic metal powder containing iron as a main component include iron oxyhydroxide, hematite, maghemite, magnetite, etc., and ferromagnetic metal powder obtained by reducing various alloy types of iron oxide. When the oxygen-containing gas and the ferromagnetic metal powder are brought into contact with each other in an organic solvent in the primary stabilizing oxide film forming treatment, the ferromagnetic metal powder may be in the form of molded pellets or in the form of a slurry coarsely ground in the organic solvent. If the particles are in the form of pellets, the thickness of the oxide film on the particles will be uneven between the inside and the surface of the granules, and the subsequent heat annealing treatment will take a long time, so it is preferably in the form of a slurry. When the primary stabilization treatment is carried out in the gas phase, it is preferable to use pellet-shaped particles obtained by molding and granulating magnetic particles for convenience of handling. The temperature of heating annealing needs to be 80°C to 600°C. Preferably the temperature is 90°C to 200°C. If it is below 80℃, it will take a long time to obtain the effect of heat annealing, and
If the temperature is above ℃, sintering of the particles and destruction of the oxide film due to the difference in expansion coefficient between the inside of the magnetic particle and the oxide film occur, resulting in a decrease in coercive force and squareness ratio. The heating annealing time is determined depending on the temperature, but it needs to be 0.5 to 24 hours. More preferably 1 to 5 hours. If it is less than 0.5 hours, the effect of heat annealing will hardly be obtained, and if it is more than 24 hours, no further effect will be obtained and it is not economical. The organic solvent that can be used in the treatments before and after heat annealing is preferably one that is inert to the ferromagnetic powder, and includes hydrocarbons such as benzene, toluene, xylene, hexane, heptane, and cyclohexane, acetone, methyl ethyl ketone, and methyl isobutyl. ketones, ketones such as cyclohexanone,
Examples include esters such as ethyl acetate, butyl acetate, and dibutyl phthalate, alcohols such as methanol, ethanol, and n-butanol, and fluorocarbon solvents such as perfluorobutylhydrofuran and perfluoroxylene. . The amount used is preferably at least twice the weight of the ferromagnetic metal powder. Oxygen or air can be used as the oxygen-containing gas that can be used for the oxidation treatment before and after heat annealing in the present invention.
Air is diluted with at least one inert gas such as N ₂ , He, Ar, or Ne to reduce the oxygen concentration to 5% by volume or less, but usually air is diluted with N ₂ to reduce the oxygen concentration. It is inexpensive and practical to use a gas mixture containing 5% by volume or less. The temperature during the contact treatment with oxygen-containing gas before and after the heat annealing is 10 to 90°C.
is preferred. The contact treatment time with the oxygen-containing gas before the heat annealing is preferably 5 to 24 hours. The contact time with the oxygen-containing gas after the heat annealing is preferably 1 to 12 hours. As the inert gas that can be used during the heat annealing, any of the above inert gases may be used, but it is industrially inexpensive and practical to use _N2 . Although the cause of the effect in the present invention is not necessarily clear, by performing heat annealing,
The uniformity and crystallinity of the oxide film are improved by promoting the diffusion of oxygen within the particle surface and the diffusion of metal atoms inside the particle to the film surface, and then oxidizing the surface again. The result is a stable ferromagnetic metal powder that has improved ferromagnetic properties, forms a uniform and dense oxide film, has little deterioration in coercive force and saturation magnetization even in the atmosphere, and does not generate heat or ignite even when taken out into the atmosphere after long-term sealed storage. It will be done. Examples are shown below, but the present invention is not limited thereto. Example 1 Coercive force (Hc) 1501Oe, saturation magnetization (σs)
1.5 kg of iron-based ferromagnetic metal powder with magnetic properties of 150 emu/g and a squareness ratio (Rs) of 0.513 was slurried in 30 kg of toluene and placed in a reaction vessel equipped with a stirring device, heating device, and aeration device. Get in
While stirring at 130 rpm, a 50°C gas containing 5% by volume of oxygen, which is air diluted with N ₂ , was blown from the bottom at 90/min through a preheater, and the reaction was carried out at 50°C for 12 hours. Thereafter, toluene was excessively removed, and N ₂ gas at 150°C was blown from the bottom at 60/min through a preheater to dry it, followed by annealing at 150°C for 3 hours. After cooling, add 30Kg of toluene to make a slurry. While stirring at 130rpm, air is passed through a preheater to _N2.
90°C gas containing 5% oxygen diluted with
/min from the bottom and the reaction was carried out at 50°C for 2 hours. The magnetic properties of this magnetic powder are Hc: 1502Oe,
σs: 126emu/g, Rs: 0.510. In addition, the oxidation resistance stability is determined by the rate of decrease in σs after being left in the air for 3 days at 60℃ and 90% RH [(σs after stabilization - σs after stability test) × 100/(after stabilization The value was 5.3%. Also, an opening test (1 kg of the magnetic powder after the above stabilization treatment is dried, placed in a sealed container, the initial atmosphere is air, and stored at 35°C for 10 days, and then opened to check for temperature rise or ignition.)
The temperature was 1°C 2 hours after opening. Example 2 The treatment was carried out in the same manner as in Example 1 except that ferromagnetic metal powder having the same magnetic properties as in Example 1 was used and the heat annealing conditions were 150° C. for 20 hours. Its magnetic properties etc. are shown in Table 1. Example 3 A treatment was carried out in the same manner as in Example 1 except that ferromagnetic metal powder having magnetic properties of Hc1510Oe, σs149emu/g, and Rs0.510 was used, and the heat annealing conditions were 80° C. for 24 hours. Its magnetic properties etc. are shown in Table 1. Comparative Example 1 The treatment was carried out in the same manner as in Example 1 except that ferromagnetic metal powder having the same magnetic properties as in Example 3 was used and the heating annealing conditions were 300°C and 0.4 hours. Its magnetic properties etc. are shown in Table 1. Comparative Example 2 Using ferromagnetic metal powder with magnetic properties of Hc1507Oe, σs153, Rs0.509, heating annealing conditions were 650℃,
The same treatment as in Example 1 was performed except that the heating time was 1 hour. Its magnetic properties etc. are shown in Table 1. It is clear from Table 1 that after each treatment, the coercive force and squareness ratio of the ferromagnetic metal powder significantly decreased. Comparative Example 3 1.5 kg of ferromagnetic metal powder having the same magnetic properties as in Example 1 was made into a slurry in 30 kg of toluene, placed in a reaction vessel similar to Example 1, and while stirring at 130 rpm, air was bubbled through the preheater. A gas at 50° C. containing 5% oxygen diluted with N ₂ was blown from the bottom at a rate of 90/min, and the reaction was carried out at 50° C. for 12 hours. After that, toluene was excessively removed, and the mixture was dried. The magnetic properties of this magnetic powder are Hc1500Oe,
It was σs130emu/g and Rs0.509. The oxidation resistance stability was Δσs24%. The result of the opening test was that the product became red hot within 1 to 2 minutes after opening, leading to an oxidation runaway reaction. Comparative Example 4 The same treatment as in Comparative Example 3 was carried out except that ferromagnetic metal powder having the same magnetic properties as in Example 3 was used and the reaction temperature was 90°C. Its magnetic properties etc. are shown in Table 1. Comparative Example 5 Comparative Example 4 except that ferromagnetic metal powder having the same magnetic properties as Example 3 was used and the reaction time was 48 hours.
The same process was performed. Its magnetic properties etc. are shown in Table 1. Comparative Example 6 Magnetic powder was obtained in the same manner as in Example 1 up to heating annealing. When this powder was dried and taken out into the air for an open test, it immediately caught fire. 【table】

Claims (1)

[Scope of Claims] 1. Ferromagnetic metal powder mainly composed of iron, which has been brought into contact with an oxygen-containing gas having an oxygen concentration of 5% by volume or less in an organic solvent to oxidize the surface layer, is heated in a gas phase in an inert gas atmosphere. A method for producing a ferromagnetic metal powder having an oxide film, which comprises heating and annealing the powder at 80 to 600°C for 0.5 to 24 hours, and then contacting the powder with an oxygen-containing gas having an oxygen concentration of 5% by volume or less in an organic solvent. 2. The method according to claim 1, wherein the heating annealing temperature is 90°C to 200°C. 3. The method according to claim 1 or 2, wherein the heating annealing time is 1 to 5 hours. 4. Any one of claims 1 to 3, characterized in that the amount of the organic solvent in the treatments before and after the heating annealing is at least twice the weight of the ferromagnetic metal powder containing iron as a main component. Method described in Crab. 5. The method according to any one of claims 1 to 4, characterized in that an organic solvent inert to the ferromagnetic metal powder is used as the organic solvent. 6. Claims characterized in that the oxygen-containing gas before and after the heating annealing is a mixture of air or oxygen gas with an inert gas consisting of at least one of nitrogen, helium, argon, and neon as a diluent gas. Paragraphs 1 to 5
The method described in any of the paragraphs. 7. The method according to any one of claims 1 to 6, wherein nitrogen, helium, argon, or neon is used as the inert gas for the heat annealing.