Stock material or miscellaneous articles – Composite – Of inorganic material
Reexamination Certificate
2001-10-04
2003-04-08
Le, H. Thi (Department: 1773)
Stock material or miscellaneous articles
Composite
Of inorganic material
C423S138000, C423S140000, C423S148000, C423S151000, C428S402000
Reexamination Certificate
active
06544673
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to spindle-shaped magnetic metal particles containing iron as a main component and a process for producing the spindle-shaped magnetic metal particles, and more particularly, to spindle-shaped magnetic metal particles containing iron as a main component, which exhibit an adequate coercive force, and are excellent in dispersibility, oxidation stability and coercive force distribution despite fine particles, especially notwithstanding the particles have an average major axis diameter as small as 0.05 to 0.15 &mgr;m.
In recent years, recording-time prolongation, miniaturization and lightening of audio, video or computer magnetic recording and reproducing apparatuses for various magnetic recording media such as digital audio tapes (DAT) for people's livelihood use, 8-mm video tapes, Hi-8 tapes, VTR tapes for business use, computer tapes or discs thereof have proceeded more rapidly. In particular, VTRs (video tape recorders) are now widespread, so that there have been intensely developed VTRs aiming at the transfer of analog recording types into digital ones in addition to the above recording-time prolongation, miniaturization and lightening thereof. On the other hand, with such recent tendencies, the magnetic recording media have been required to have high image quality and high output characteristics, especially high frequency characteristics. To meet these requirements, it is necessary to reduce noise due to the magnetic recording media themselves and enhance residual magnetic flux density, coercive force, dispersibility, filling property and tape-surface smoothness thereof. Therefore, it has been further required to improve S/N ratio of the magnetic recording media.
These properties of the magnetic recording media have a close relation to magnetic particles used therein. In recent years, magnetic metal particles containing iron as a main component have been noticed, because such particles can show a higher coercive force and a larger saturation magnetization as compared to those of conventional magnetic iron oxide particles, and such magnetic metal particles containing iron as a main component have been already used as magnetic particles for magnetic recording media such as DAT, 8-mm video tapes, Hi-8 tapes, video tapes for business use, computer tapes or discs. The magnetic metal particles containing iron as a main component conventionally used in DAT, 8-mm video tapes, Hi-8 tapes or the like have been required to be further improved in output characteristics and weather resistance. In addition, the magnetic metal particles containing iron as a main component must fulfill applicability to existing format and good economy at the same time. Therefore, it has been strongly required to provide magnetic metal particles containing iron as a main component capable of satisfying the above requirements while minimizing amounts of various metals added thereto.
Various properties of magnetic recording media are detailed below.
In order to obtain high image quality in video magnetic recording media, it has been required to enhance S/N ratio and video frequency characteristics thereof. For this reason, it is important to improve a surface smoothness of the magnetic recording media. For improving the surface smoothness, it is also required to improve a dispersibility of magnetic particles in coating composition as well as orientation and filling properties thereof in coating film. In addition, in order to enhance the video frequency characteristics, the magnetic recording media have been required to exhibit not only a high coercive force and a large residual magnetic flux density, but also an excellent S.F.D. (Switching Field Distribution), i.e., a small coercive force distribution.
Further, it is important that the magnetic recording media can show good repeat-running property, good still property as well as high recording reliability even when used under severe environmental conditions, i.e., high durability.
As to the magnetic metal particles containing iron as a main component for magnetic recording media capable of satisfying the above various properties, those having a larger particle size are preferable from the standpoint of improvement in dispersibility and oxidation stability. On the contrary, the magnetic metal particles containing iron as a main component, having a smaller particle size are preferable from the standpoint of improvement in surface smoothness and reduction of noise. However, the smaller the particle size of the magnetic metal particles, the poorer the dispersibility and oxidation stability thereof. Also, when the particle size becomes smaller, the coercive force thereof is generally increased. Therefore, it is necessary to appropriately control the particle size for attaining aimed magnetic properties. Further, it is preferable to incorporate a large amount of cobalt into the magnetic metal particles containing iron as a main component in the consideration of chemical composition thereof since as well known in the arts, cobalt forms a solid solution with iron and contributes to improvement in oxidation stability. However, the use of a large amount of expensive cobalt is disadvantageous from economical viewpoint. Consequently, it has been demanded to provide magnetic metal particles containing iron as a main component exhibiting an adequate coercive force as well as excellent dispersibility and oxidation stability despite minimizing contents of expensive metal elements such as cobalt and reducing the particle size.
As known in the arts, the magnetic metal particles containing iron as a main component are produced by heat-treating either goethite particles, hematite particles obtained by heat-dehydrating the goethite particles, or particles obtained by incorporating different metal elements other than iron into the above goethite or hematite particles as starting material, in a non-reducing atmosphere, if required; and then heat-reducing the resultant particles in a reducing atmosphere. In this case, it is required that the obtained magnetic metal particles containing iron as a main component can analogously maintain shape and size of the goethite particles as starting material by appropriately controlling the shape and size of the goethite particles and further by preventing heat fusion between particles upon heat-treatments such as heat-dehydration and heat-reduction, or deformation and shape-breakage of each particle.
The goethite particles as starting material are classified into two kinds of goethite particles according to configurations thereof, i.e., acicular goethite particles produced by using alkali hydroxide, and spindle-shaped goethite particles produced by using alkali carbonate. The acicular goethite particles tend to generally have a large aspect ratio, but tend to be deteriorated in particle size distribution and become large in size as compared to spindle-shaped goethite particles. The particle size distribution is an index of uniformity of primary particles and, therefore, has a close relationship with coercive force distribution and oxidation stability of the magnetic metal particles containing iron as a main component. Consequently, spindle-shaped goethite particles having an excellent particle size distribution are preferably used as the starting materials of the magnetic metal particles containing iron as a main component.
On the other hand, as heat-reducing apparatuses, there are known fluidized a bed-type reducing apparatus for heat-reducing the starting material while fluidizing the material in the form of particles, a fixed-bed-type reducing apparatus for heat-reducing a fixed-bed formed of the starting material which is granulated into granular particles, a moving bed-type reducing apparatus for heat-reducing the starting material while moving the fixed-bed thereof, or the like.
With the increasing demand for developing mass-production techniques in association with the increase in amount of the used magnetic metal particles containing iron as a main component, among the above
Okinaka Kenji
Uegami Masayuki
Le H. Thi
Nixon & Vanderhye P.C.
Toda Kogyo Corporation
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