Metal treatment – Process of modifying or maintaining internal physical... – Treating loose metal powder – particle or flake
Patent
1993-09-17
1995-08-22
Walsh, Donald P.
Metal treatment
Process of modifying or maintaining internal physical...
Treating loose metal powder, particle or flake
252 6251, B22F 100, H01F 103
Patent
active
054436664
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to acicular fine particles containing metal iron, iron carbide and free carbon. More particularly the invention relates to acicular fine particles having magnetic characteristics effectively usable for metal tapes for use in audio cassette recorders, a process for preparing the same, and a magnetic coating composition and magnetic recording medium which contain such particles.
BACKGROUND ART
Magnetic powders usable in metal tapes for audio cassette recorders need to have such magnetic characteristics that they are about 1000 to about 1300 Oe in coercive force (Hc) so as to conform to the metal tape selector position.
Conventional magnetic powders include a metal iron magnetic powder which is termed metal magnetic powder and obtained by reducing an iron oxide with hydrogen. The powder generally has a coercive force higher than the above value and is therefore not usable as it is for audio metal tapes. Accordingly, the powder is made larger in particle size, for example, at least 0.5 .mu.m in the length of long axis so as to exhibit a coercive force within the above range. However, the powder of relatively large particle sizes has the drawback that the tape prepared with use of the powder is rough-surfaced and is great in spacing loss and noise, failing to satisfactorily deliver the input signal. As an alternative measure, it has been practice to add Ni powder or Co to the metal iron magnetic powder, but the material then has the drawback of being costly because of the production step and the material additionally needed.
A metal iron magnetic powder prepared from iron carbide and having a high coercive force is described in Japanese Patent Application No. 126291/1990, which nevertheless mentions nothing about such a powder which is 1000 to 1300 Oe in coercive force. Although a comparative example given in the same application affords an Fe/Fe.sub.5 C.sub.2 composition obtained by an interrupted reduction reaction, the composition is poor in SFD (Switching Field Distribution) characteristics.
Magnetic powders in which metal iron, iron carbide and free carbon can be present are disclosed in prior-art literature such as JP-A-211625/1985, JP-A-212821/1985, JP-A-196502/1986 and U.S. Pat. No. 4,794,042.
JP-A-211625/1985 and JP-A-212821/1985, which are different from the present invention in object, disclose mixtures of two different kinds of primary magnetic particles. The former provides a mixture of metal magnetic particles and metal carbide magnetic particles, while the latter provides a mixture of metal magnetic particles and magnetic metal particles having a coating of iron carbide over the metal surface. These mixtures differ from the acicular fine particles of the present invention in that the material of the invention contains iron carbide, metal iron and carbon within primary particles. The present invention employs a different production process in that the material is obtained by the reduction of iron carbide. JP-A-196502/1986 relates to magnetic particles containing iron carbide and much smaller proportion of iron oxide and carbon as by-products than had previously been prepared by improving a known process for preparing iron carbide. The particles of this publication have a lower carbon content than the fine particles of this invention, are therefore different in this respect and also differ from the fine particles of the invention in magnetic characteristics. The production process of the invention is intended to cause a predetermined amount of metal iron to be present in the particles, and is different from that of this publication. U.S. Pat. No. 4,794,042 discloses a magnetic recording medium containing iron carbide and iron oxide, or a magnetic recording medium further containing metal iron. The powder of this patent has FenC at least in the surface layer and contains .alpha.-Fe in the central portion. With FenC present in the surface layer, the powder has the drawback of being low in dispersibility in coating compositions. Furthermore, the powder
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Arase Takuya
Daimon Shigeo
Kamiya Fumihiro
Kiatamura Ikuo
Daikin Industries Ltd.
Greaves John N.
Walsh Donald P.
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