SEMI-HARD MAGNETIC POWDER HAVING A HIGH VALUE AND METHOD FOR SYNTHESIZING SAME

Abstract

An object of the present invention is to provide a semi-hard magnetic white powder having characteristics suitable as a security pigment, such as the magnetic powder contained in magnetic inks used for MICR. The white powder includes base particles made of a semi-hard magnetic Alnico alloy, the base particles having a titanium oxide film and a metallic silver film in this order on the surfaces thereof.

Claims

1. A white powder comprising: base particles made of a semi-hard magnetic Alnico alloy; and a titanium oxide film and a metallic silver film that are disposed in this order on surfaces of the base particles made of the Alnico alloy outwardly from the surfaces.

2. The white powder according to claim 1, wherein the base particles made of the Alnico alloy have a composition, relative to the total weight of the base particle made of the Alnico alloy, of: 7% to 13% by weight of Al; 14% to 25% by weight of Ni; 0% to 38% by weight of Co; 0% to 4% by weight of Cu; 0% to 8% by weight of Ti; and the balance consisting of Fe and inevitable impurities.

3. The white powder according to claim 1, wherein the base particles made of the Alnico alloy are particles of atomized powder formed by atomization.

4. The white powder according to claim 1, wherein the white powder has semi-hard magnetism with a residual magnetic moment of 15 emu/g or more.

5. The white powder according to claim 4, wherein the white powder further has semi-hard magnetism with a coercivity of less than 500 Oe in addition to the residual magnetic moment of 15 emu/g or more.

6. The white powder according to claim 1, wherein the white powder has an average particle diameter of 5 μm to 20 μm, a specific surface area of 0.01 m.sup.2/g to 20 m.sup.2/g, and a lightness L* of 75 or more.

7. A magnetic ink comprising the white powder according to claim 1.

8. A method for producing a white powder made of an Alnico alloy, the method comprising: forming a titanium oxide film and a metallic silver film in this order on surfaces of Alnico alloy powder particles outwardly from the surfaces.

9. The method for producing a white powder according to claim 8, wherein the Alnico alloy powder has a composition, relative to the total weight of the Alnico alloy powder, of: 7% to 13% by weight of Al; 14% to 25% by weight of Ni; 0% to 38% by weight of Co; 0% to 4% by weight of Cu; 0% to 8% by weight of Ti; and the balance consisting of Fe and inevitable impurities.

10. The method for producing a white powder according to claim 8, wherein the Alnico alloy powder is produced by atomization.

11. The method for producing a white powder according to claim 10, wherein the Alnico alloy powder formed by the atomization has an average particle diameter of 1 μm to 100 μm and a specific surface area of 0.01 m.sup.2/g to 5.0 m.sup.2/g.

12. The method for producing a white powder according to claim 8, wherein the Alnico alloy powder is subjected to heat treatment to be turned into a semi-hard magnetic powder with a residual magnetic moment of 15 emu/g or more, followed by forming the titanium oxide film and the metallic silver film in this order.

13. The method for producing a white powder according to claim 12, wherein the heat treatment is performed by heating for 0 to 30 minutes at 750° C.; to 1000° C.; in an inert gas atmosphere.

14. The white powder according to claim 1, wherein the white powder has a specific surface area of 0.01 m.sup.2/g to 5.0 m.sup.2/g.

Description

BRIEF DESCRIPTION OF DRAWING

[0032] FIG. 1 is an SEM photograph of an Alnico alloy powder coated with titanium oxide and silver.

DESCRIPTION OF EMBODIMENTS

[0033] (Semi-Hard Magnetism)

[0034] The present invention is greatly characterized by using a semi-hard magnetic powder as a magnetic powder used for MICR or the like.

[0035] In general, ferromagnetic materials are classified into hard magnetic materials and soft magnetic materials depending on the magnetization characteristics. Whether a magnetic property of a magnetic material is hard magnetic or soft magnetic is generally determined depending on the magnitude of the coercivity. Hard magnetic materials have high coercivity and high residual magnetic moment, and soft magnetic materials have low coercivity and low residual magnetic moment.

[0036] In this regard, when a magnetic powder is used as a security pigment for MICR or the like, hard magnetic powder having high coercivity requires that large magnetization energy be applied from the outside and thus requires a large-scale magnetization apparatus. In contrast, soft magnetic powder having small coercivity reduces the magnetism of the medium after being magnetized to a level lower than the read limit of the magnetic information reader, resulting in a disadvantageous effect of failing to accurately read magnetic information.

[0037] Accordingly, the inventors of the present invention focused on semi-hard magnetic materials having intermediate magnetic properties between hard magnetism and soft magnetism as a magnetic powder.

[0038] Semi-hard magnetic materials have intermediate coercivity between hard and soft magnetic materials and can avoid the above-described disadvantageous effect. Additionally, since the coercivity and residual magnetic moment of semi-hard magnetic materials are lower than those of hard magnetic materials, semi-hard magnetic materials are less likely to aggregate in magnetic ink, which is in a liquid form, and hence, the dispersibility and dispersion stability of their magnetic particles can be improved.

[0039] The Alnico alloy powder of the present invention has a low coercivity but an intermediate residual magnetic moment between hard and soft magnetic materials and is, thus, a magnetic powder having characteristics suitable for use as a security pigment.

[0040] (Alnico Alloy Powder Produced by Atomization)

[0041] The Alnico alloy powder of the present invention is produced by atomization, so that it is possible to form the spherical powder particles with similar particle diameters to some extent.

[0042] Thus, the dispersibility in magnetic ink can be further improved. Additionally, the fluidity can be improved, and the printing characteristic with a printer is improved accordingly. Also, the spherical powder particles enable the formation of uniform thin TiO.sub.2 and metallic silver films in the whitening step, which is a post-step in the present invention, thus favorable in view of magnetic properties.

[0043] (Particle Size of Alnico Alloy Powder)

[0044] The volume average particle diameter D50 of the Alnico alloy powder measured by a laser diffraction/scattering method is preferably 1 μm to 100 μm and more preferably 5 μm to 20 μm.

[0045] Also, the Alnico alloy powder preferably has a specific surface area of 0.01 m.sup.2/g to 5.0 m.sup.2/g in view of whitening the powder in a subsequent step. When the specific surface area is larger than 5.0 m.sup.2/g, a large amount of metallic silver is required to hide the surfaces of the magnetic particles. Using a large amount of metallic silver is undesirable because it degrades the magnetization characteristics of the magnetic powder. When the specific surface area is smaller than 0.01 m.sup.2/g, the particle size of the magnetic particles increases. This is unsuitable for printing security materials and thus undesirable.

[0046] To control the magnetic powder to a preferred specific surface area, it is preferable to classify the Alnico alloy powder after it is produced by atomization.

[0047] (Composition of Alnico Alloy Powder)

[0048] In general, it is believed that when the Ni content of Alnico alloy is reduced, the residual flux density increases and the coercivity decreases, while when the Al content is increased, the coercivity decreases. In the present invention, the Alnico alloy powder has an alloy composition in the following range. This enables the Alnico alloy to have a coercivity with a value suitable for use in magnetic inks.

[0049] The composition of the Alnico alloy powder used in the present invention, relative to the total weight of the Alnico alloy powder, is 7% to 13% by weight of Al, 14% to 25% by weight of Ni, 0% to 38% by weight of Co, 0% to 4% by weight of Cu, 0% to 8% by weight of Ti, and the balance consisting of Fe and inevitable impurities.

[0050] (Magnetization Treatment of Alnico Alloy Powder)

[0051] The thus formed Alnico alloy powder is magnetized to a desired magnetic material by aging heat treatment. The heat treatment is performed by heating for 0 to 30 minutes at 750° C. to 1000° C. in an inert gas atmosphere.

[0052] Such aging heat treatment can increase the residual magnetic moment to a desired value.

[0053] (Whitening Method)

[0054] To whiten the Alnico alloy powder, a titanium oxide film and a metallic silver film are formed in this order on the surfaces of the powder particles.

[0055] The titanium oxide film is formed of mainly tetravalent titanium oxide TiO.sub.2, but divalent or trivalent titanium oxide may be used. For forming the film, titanium alkoxide may be hydrolyzed, or an aqueous film may be formed from a titanium salt aqueous solution and then oxidized.

[0056] Next, in metallic silver film formation, a known method such as electroless plating may be used without particular limitation.

[0057] Such a whitening method can significantly increase the lightness of the magnetic powder because of the presence of the titanium oxide film between the magnetic powder and the silver film. Additionally, the metallic silver film can be formed to a smaller thickness than that of known metallic silver film-coated white powder, improving the magnetic properties of the white powder.

[0058] For the magnetic properties of the thus produced white powder, preferably, the residual magnetic moment Mr is 15 emu/g or more and the coercivity Hc is less than 500 Oe. For the shape, preferably, the average particle diameter (volume average D50) is 1 μm to 100 μm and the specific surface area is 0.01 m.sup.2/g to 20 m.sup.2/g. Also, the lightness L* is preferably 75 or more.

EXAMPLES

[0059] (Alnico Alloy Powder)

[0060] Iron-aluminum-nickel-cobalt alloy of Epson Atmix Corporation produced by atomization (Al: 12.9 wt %, Ni: 20.9 wt %, Co: 4.9 wt %, Cu: 3.0 wt %, Mr: 2.1 emu/g, Hc: 0.02 Oe, lightness L*: 52) was classified by an air flow classifier, and a metal powder having the particle size distribution of D.sub.10: 4.3 μm, D.sub.50: 9.6 μm, and D.sub.95: 24.7 μm was obtained. The specific surface area was 0.08 m.sup.2/g.

[0061] (Magnetization Treatment)

[0062] The metal powder obtained by the classification was heat-treated at a temperature increase rate of 15° C./min in a nitrogen atmosphere. After the central temperature of the powder reached 850° C., the powder was air-cooled to yield a magnetic powder with Mr of 20.4 emu/g and He of 354 Oe.

[0063] (Whitening)

[0064] A transparent yellow peroxotitanic acid solution was prepared by mixing 2.2 mL of a titanium tetrachloride solution (16.0% to 17.0% in terms of Ti), 5.84 g of aqueous ammonia, and 10.0 g of hydrogen peroxide solution with 19.8 g of deionized water. In 535.81 g of deionized water, 9.92 g of boric anhydride, 11.72 g of potassium chloride, and 2.55 g of sodium hydroxide were dissolved. In this solution, 167.5 g of the above magnetic powder was suspended. The peroxotitanic acid solution was dropped into and mixed with the suspension with stirring, and then the suspended matter was dried to yield titanium oxide film-coated powder.

[0065] A reducing solution was prepared by dissolving 1.2 g of glucose, 0.12 g of tartaric acid, and 2.12 g of ethanol in 26.56 g of deionized water. A silver ammine complex solution was prepared by mixing 1.25 g of sodium hydroxide, 1.75 g of silver nitrate, and 3 g of aqueous ammonia with 90 g of deionized water, and 10.0 g of the titanium oxide film-coated powder was suspended in the complex solution. The reducing solution was mixed with the suspension being irradiated with ultrasonic waves, and the suspended matter was dried to yield silver film-coated powder.

[0066] (White Powder)

[0067] The resulting white powder had a lightness L* of 80.0, Mr of 21.6 emu/g, He of 362 Oe, D.sub.50 of 13.5 μm, and a specific surface area of 0.1 m.sup.2/g.

INDUSTRIAL APPLICABILITY

[0068] The white powder made of the Alnico alloy powder produced according to the present invention has semi-hard magnetic properties and significantly increased lightness, accordingly expected to be used as a highly useful security pigment in industry.