Abstract:
A mixture including at least one iron oxide and a mon-magnetic matrix material is melted or vaporized and then heat is rapidly removed from the material. The resulting magnetic oxide precipitates are densely packed in the non-magnetic matrix. The precipitates have a narrow particle size distribution which results in a high signal-to-noise ratio when the oxides are used for magnetic recording purposes. The non-magnetic matrix can be removed to yield homogeneous, small particle iron oxide containing magnetic powder. Alternatively, the non-magnetic matrix/iron oxide material can be processed to yield a shaped recording medium.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to methods for producing iron oxide particles in a solid non-magnetic matrix by rapid solidification of an iron oxide precursor mixed with a non-magnetic matrix material. 
     Oxide particles for magnetic recording purposes are currently synthesized using a very complex and lengthy procedure. This procedure requires the precipitation of synthetic α-(FeO)OH(goethite) from aqueous solutions, the dehydration of α-Fe 2  O 3  to Fe 3  O 4 , and finally the careful oxidation of Fe 2  O 4 , to form γ-Fe 2  O 3 . (Akashi, Ferrites: Proceedings of the International Conference, Sept.-Oct. 1980 Japan p. 548-552). 
     Another wet chemistry technique for preparation of iron oxide (spinel ferrite) powders consists of precipitation of the spinel ferrite from aqueous solutions containing ferrous ions and other divalent metallic ions. The solution pH is controlled formed by oxidation of the aqueous solution in air above 50° C. (Takada, Ferrites: Proceedings of the International Conference. Sept.-Oct., 1980, Japan, p. 3-6). 
     Another wet chemical method for synthesis of iron oxide powders involves the reduction of ferrous ions with sodium borohydride, a very expensive reducing agent, in the presence of a magnetic field (Akashi, Ferrites: Proceedings of the International Conference, Sept.-Oct., 1980, Japan, p. 548-552). 
     It is also possible to prepare cobalt-ferrite iron oxide powders using wet chemistry methods. According to one such method, acicular γ-Fe 2  O 3  particles are suspended in an alkaline solution containing Co 2+  and then treated at 90° C. for 10 hours (Hayama, Ferrites: Proceedings of the International Conference, Sept.-Oct., 1980, Japan, p. 521-525). 
     Alternative preparation methods for iron oxide powders include condensation of vaporized metal in a low pressure inert gas atmosphere in the presence of a magnetic field. Powders produced in this manner exhibit low noise levels and excellent stability against oxidation; however, this method is extremely expensive (Akashi, Ferrites: Proceedings of the International Conference, Sept.-Oct., 1980, Japan, p. 548-552). 
     Cobalt modified iron oxides may be prepared using pyrolytic decomposition (chemical vapor deposition) of cobalt-acetylacetonate vapor on the surface of iron oxide fluidized acicular particles. Powders prepared using this process are suitable for magnetic recording applications. They exhibit high coercivities (550-600 Oe) at relatively low Co 2+  (2-3) and Fe 2+  (8-9) wt % (Monteil et al. Ferrites: Proceedings of the International Conference, Sept.-Oct., 1980, Japan, p. 532-536). 
     Complex oxides for ferrites, consisting of iron oxide and zinc oxide, are prepared by spraying aqueous chloride solutions of the respective oxide metallic constituents onto a fluidized roasting furnace (Hirai et al. WO88-00925). 
     The iron oxide and doped iron oxide particles produced according to these methods are incorporated into magnetic discs, tapes and other devices for magnetic recording. Such magnetic recording media usually consist of a non-magnetic support for a magnetic recording layer of ferromagnetic powder (i.e. iron oxide or doped iron oxide) dispersed in an organic binder material (Saito et al., U.S. Pat. No. 4,820,581, JP 62125533, JP 57078631, JP 62241134, JP 62162228, and Funahashi et al. U.S. Pat. No. 4,820,565) where the non-magnetic support or substrate may be a polyester film or tape. 
     Efforts have also been directed towards improvement of durability of magnetic recording media. One approach involves the combination of cobalt doped γ-Fe 2  O 3  and Cr 2  O 3  with non-magnetic α-iron oxide in an organic binder by ball milling subsequent coating of a PET film (JP 55129935). 
     Other approaches coat the magnetic iron oxide particles with a layer of silica (SiO 2 ) by sputtering (JP 58006528) or by immersing the metal oxide particle in a ph-controlled suspension with an amorphous, powdered silicate which is subsequently made crystalline by adjustment of the suspension ph. An alumina (Al 2  O 3 ) protective coating may also be e-beam deposited on the surface of magnetic iron oxide particles (JP 58006528). A non-magnetic zinc oxide (ZnO) protective coating has also been electro-deposited on a magnetic iron oxide layer which had previously been electro-deposited on an aluminum or aluminum alloy substrate (JP 621255126). 
     Glass-forming additives such as B 2  O 3 , Bi 2  O 3 , P 2  O 5 , MoO 3  and V 2  O 5  have been used in the solid state reaction of α-FeOOH particles and colloidal BaCo 3  or SrCO 3 . 
     Addition of approximately 0.5 wt % B 2  O 3  best accelerated ferrite formation without adhesion of particles, resulting in thin-plate hexagonal ferrites with good magnetic recording properties (Sugimoto, Fourth International Conference on Ferrites, Part 2, Oct.-Nov. 1984, San Francisco, Calif., U.S.A. p. 273-279). 
     Amorphous cobalt ferrite (CoFe 2  O 4 ) films have also been prepared using a two-source vacuum evaporation technique with CoFe 2  alloy and P 2  O 5  as the source materials. These amorphous cobalt ferrite films display high perpendicular anisotropy (Hiratsuka et al., Electronics and Communications in Japan, Part 2, 71, 95-102 (1988) and IEEE Transactions on Magnetics, MAG-23, 3326-3328 (1987)). 
     Japanese patent, JP 61080618, also describes sputtering of Co-γ-Fe 2  O 3  on a non-magnetic disc substrate. Japanese patent, JP 62095735, describes a process for conversion of sputtered α-Fe 2  O 3  to Fe 3  O 4  on a non-magnetic substrate such as a drum or disc, using laser beam irradiation. 
     Methods exist for precipitation or nucleation of magnetic crystallites in glassy matrices. 
     U.S. Pat. No. 4,083,727 to Andrus et al. discloses a method for production of glass-ceramic articles having integral magnetic magnetite (Fe 3  O 4 ) crystals. According to this method, a Li 2  O--FeO--Al 2  O 3  --SiO 2  glass article nucleated with TiO 2  is heat-treated to induce crystalline nucleation within the article body, resulting in a glass-ceramic article which is subsequently exposed to a reducting atmosphere to convert hematite crystals in its surface layer to magnetite, yielding films with high coercivities and saturation magnetizations which compare favorably to those of magnetite and other ceramic ferrite materials. The magnetic recording medium disclosed in JP 62042315 consists of a layered structure, one layer of which is a magnetic recording medium made from α-FeO 2  O 3 , Al 2  O 3 , SiO 2 , B 2  O 3  or Co 3  O 4 . 
     The nucleation of inhomogeneous precipitates having ferrimagnetic cores within antiferromagnetic skins has been observed in B 2  O 3  --BaO--Fe 2  O 3  glass matrices prepared by slow quenching between stainless steel slabs (Fahmy et al., Physics And Chemistry Of Glasses, 13, 21-26 (1972) and MacCrone, in Amorphous Magnetism, H. O. Hooper and A. M. deGraaf, eds., Plenum Press, New York-London 1973 p. 77). 
     Magnetic phases have been precipitated in Li 2  B 2  O 4  --LiFe 5  O 8  glass systems prepared by splat quenching, roller quenching and gun quenching techniques (Chaumont et al., Mat. Res. Bull. 15, 771-776 (1980); Chaumont et al., Rapidly Quenched Metals III, Third International Conference, University of Sussex, Brighton, July 1978 p. 401; Chaumont et al., Rev. Int. Htes. Temp. et Refract. 15, 23-32 (1978)). 
     Partially recrystallized glasses in the Li 2  O--Fe 2  O 3  SiO 2  system, prepared by slow quenching between steel plates exhibit some ferrimagnetic properties at high Fe 2  O 3  contents (Weaver et al., American Ceramic Society Bulletin 52, 467-472 (1973)). 
     Ferrimagnetic amorphous cobalt ferrites have been prepared by rapid quenching to liquid nitrogen temperatures of cobalt ferrite combined with P 2  O 5  glass network former (Sugimoto et al., Jpn. J. Apl. Phys. 21, 197-198 (1982)). 
     Splat quenching techniques have been applied to the BaO--Fe 2  O 3  and SrO--Fe 2  O 3  systems where BaFe 12  O 19  crystals have been observed along with weak ferromagnetism in the glass matrix (Monteil et al., Mat. Res. Bull. 12, 235-240 (1977); Monteil et al., Journal of Solid State Chemistry, 25, 1-8 (1978); Chaumont et al., Mat. Res. Bull. 15, 771-776 (1980); Chaumont et al., Rapidly Quenched Metals III, Third International Conference, University of Sussex, Brighton, July 1978 p. 401; Chaumont et al., Rev. Int. Htes. Temp. et Refract. 15, 23-32 (1978)). 
     SUMMARY OF THE INVENTION 
     According to one aspect of the invention, the simplified method for producing iron oxide containing magnetic oxide precipitates embedded in a non-magnetic matrix includes forming a mixture including at least one iron oxide precursor and a non-magnetic matrix material. This mixture is vaporized or melted and then subjected to rapid heat removal, by either condensation from the vapor in sputtering processes or rapid solidification in quenching from a melt. Such materials are well suited for applications in magnetic recording media and other magnetic components. 
     In one embodiment, rapid heat removal occurs by rapid solidification specifically by a double-roller quenching technique where the cooling rate is greater than 10 3  K per second and is preferably in the range 10 5  -10 6  K per second. The mixture may be made by comminuting the iron oxide precursor and matrix material to form a powder and then pressing and sintering the powder before it is melted and rapidly solidified. The mixture can include α-Fe 2  O 3  and SiO 2  Other glass network formers such as B 2  O 3 , P 2  O 5 , GeO 2 , As 2  O 5 , Sb 2  O 5  or Zr 2  O 5  can be matrix materials. 
     In another embodiment, an oxide such as Fe 3  O 4  and a matrix material such as SiO 2  are ball-milled in acetone, dried, pressed into rods, and sintered in flowing 02 to produce the starting mixture. CoO or other transition metal oxides such as Sc 2  O 3 , TiO 2 , V 2  O 5 , CrO 3 , MnO 7 , NiO, CuO or ZnO or rare earth oxides may also be included in the mixture. 
     In another embodiment, thin film cobalt ferrite within a glass former matrix may be sputtered so that the film material rapidly condenses from the vapor phase at heat removal rates of typically 10 12  °/second, rates much greater than those possible by rapid solidification from a liquid melt. 
     According to another aspect of the invention, iron oxide containing magnetic oxide powder is produced by forming a mixture which includes at least one iron oxide and a matrix material. This mixture is melted and then rapidly solidified to form an iron oxide/matrix solid. The matrix material is then removed and the iron oxide containing magnetic oxide powder is collected. 
     The oxide precipitates collected after rapid solidification and subsequent removal of the non-magnetic matrix material are equiaxed, isotropic particles of γ-Fe 2  O 3 . 
     According to another aspect of the invention, a shaped recording medium may be fabricated from magnetic oxide precipitates embedded in a non-magnetic matrix. The magnetic oxide precipitate containing iron oxide embedded in a non-magnetic matrix flakes can be crushed to a size suitable for powder processing and subsequently formed into thick self-supporting shaped articles which are sintered to yield magnetic recording media. 
     Alternatively, according to another aspect of the invention, shaped recording media are produced by removing the magnetic oxide containing iron oxide precipitates from the non-magnetic matrix and mixing them with binders and elastomers to make an unsintered sheet which can then be applied to a substrate. Such a structure can then be sintered to yield a thick recording film having the shape of the supporting substrate. 
     The magnetic oxide precipitates made according to the methods of the invention exhibit superior magnetic properties excellently suited for magnetic recording applications or fabrication of ferrite magnetic recording heads. In particular, the material consists of densely-packed, isolated individual Particles that do not clump, permitting high-density information storage. The materials are magnetically isotropic and are thus easy to align. Further, the material exhibits a narrow distribution of particle sizes which results in a high signal-to-noise ratio in a magnetic recording application. 
     The magnetic oxide Precipitates produced according to the methods of the invention may be removed from the non-magnetic matrix material by dissolution or crushing. They are collected and incorporated into conventional binders such as organic compounds for application to appropriately shaped substrates in fabrication of magnetic tapes, hard disks, floppy disks or drums usable for double sided recording. 
     The method of the present invention represents a major simplification over conventional methods for making iron oxide powders or shaped articles for magnetic recording purposes. The method is a relatively easily-controlled process with fewer steps than the presently known procedure. The raw materials are inexpensive and readily available. Further, the method is flexible, allowing the addition of transition metal elements such as cobalt or chromium, and rare earth elements. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is a schematic illustration of the double-roller rapid solidification method utilized in the practice of the invention; 
     FIG. 2 is an SEM micrograph at 22,000× magnification of Fe 2  O 3  --Fe 3  O 4  crystallites growing from glassy strands produced from a starting composition of 82 wt % Fe 3  O 4  +SiO 2  prepared according to the method of the invention; and 
     FIG. 3 is an SEM micrograph at 80,000× magnification of the same material as shown in FIG. 2. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     EXAMPLE 1 
     The samples used in the double roller quenching process to produce crystallites were made from powder mixtures that had been ball-milled for approximately eight hours in acetone, dried, and pressed into rods. These rods had a diameter of 5/8 inch and had lengths that varied from 1.5 to 2 inches. The resulting rods were sintered under flowing oxygen for a period of approximately five hours. The starting powder compositions are listed below in Table I along with the sintering temperatures and the phases present after sintering. 
     
                       TABLE I______________________________________Initial     Sintering  Phases PresentComposition Temperature                  After Sintering______________________________________82 wt % Fe.sub.3 O.sub.4 +       1435° C.                  α - Fe.sub.2 O.sub.3 + Fe.sub.3 O.sub.4 +18 wt % SiO.sub.2      cristobalite77 wt % Fe.sub.3 O.sub.4 +        950° C.                  α - Fe.sub.2 O.sub.3 + SiO.sub.2 +18 wt % SiO.sub.2 +    Fe.sub.3 O.sub.45 wt % CoO46 wt % Fe.sub.3 O.sub.4 +        950° C.                  CoFe.sub.2 O.sub.4 + (Co,Fe).sub.2 O.sub.3 +26 wt % SiO.sub.2 +    glass28 wt % CoO______________________________________ 
    
     It should be noted that the present invention can also be used to produce the magnetic precipitates directly from a starting mixture of α-Fe 2  O 3  and SiO 2 . 
     The method according to the invention is illustrated in FIG. 1. Rod 10 of sintered precursor material was suspended above tool-steel rollers 12 that were held against one another by spring loaded bearings (not shown). The distance from the bottom of rod 10 to the rollers&#39; surface was 10 centimeters. Rollers 12 were 10 centimeters in length and 5.5 centimeters in diameter and had a maximum rotation rate of 6,000 rpm in the direction given by arrows 15. Suspended rod 10 was melted in air using H 2  --O 2  torches 13. Drops melted from rod 10 one-by-one were quenched into flakes as they passed through roller nip 14. Flakes 16 were collected in an aluminum basket 17 placed below rollers 12. Other rapid solidification techniques such as splat quenching, single roller quenching, the gun technique, and melt extraction can be used. 
     X-ray diffraction studies have been performed on the as-quenched samples. The flakes of the samples that contain zero- and 5-wt % CoO consisted of γ-Fe 2  O 3 , Fe 3  O 4 , or Berthollide oxide. The sample that contained 28 wt % CoO was composed of (Co,Fe)O and (Co,Fe) 2  O 3 . The magnetic properties of the as-quenched flakes are listed below in Table 2. Table 2 also lists two common magnetic recording materials for comparison. Note that the magnetic properties of the sample that included 5 wt % CoO in its starting mixture are comparable to those of currently used magnetic oxide media. Table 2 also lists the average size of the precipitates produced according to the method of the present invention. 
     
                       TABLE 2______________________________________Sample                SpecificStarting    Coercivity                 Magnetization                             Edge LengthComposition (OE.)     (EMU/G)     (In A)______________________________________82 wt % Fe.sub.3 O.sub.4 +       240       52          150018 wt % SiO.sub.277 wt % Fe.sub.3 O.sub.4 +       530       50          200018 wt % SiO.sub.2 +5 wt % CoO46 wt % Fe.sub.3 O.sub.4 +       650       10          160026 wt % SiO.sub.2 +28 wt % CoOCo.sub.x Fe.sub.2-x O.sub.3         515 -   62          Prior Art(x = 0.06)  600                   Materials(Co,Fe).sub.2 O.sub.3 --         580 -   60(Co,Fe).sub.3 O.sub.4       700______________________________________ 
    
     Scanning electron microscope studies have revealed that all of the samples consist of uniformly sized, equiaxed precipitates that are embedded in a glassy matrix. In the case of the rapidly solidified Fe 3  O 4  +SiO 2 , the precipitates are actually cubes. The edges of the precipitates are rounded, indicating that they are probably encased in glass. In many of the samples, strands of glass were present; these strands were sometimes twisted and usually had crystallites on them or within them, growing in an oriented manner along the strand. FIGS. 2 and 3 show strings of cubic γ-Fe 2  O 3  particles in a matrix of SiO 2  -rich glass. Each particle is approximately 0.1 mm on edge. 
     EXAMPLE 2 
     Iron oxide containing magnetic oxide powder can be produced from the flakes of Example 1 by chemical removal of the matrix material and collection of the remaining magnetic particles. Flakes may be reacted with a suitable etch solution selected to etch away the glass matrix while leaving intact the magnetic particles. The etch may be heated to an elevated temperature near the glass transition temperature of the glass matrix but well below the melting point of the magnetic oxide particles to accelerate the kinetics of the dissolution reaction. 
     The undissolved magnetic oxide particles can then be collected, washed to remove traces of etchant solution, and subjected to further processing. Such processing can include sintering or mixing with conventional organic binder materials, well known in the art. 
     EXAMPLE 3 
     Iron oxide powder may also be produced from the flakes of Example 1 by mechanically removing the glass matrix. The brittle flakes can be crushed and the magnetic iron oxide precipitate particles separated from the group glass matrix magnetically. 
     The resulting magnetic iron oxide precipitate particles can be processed further as described in Example 2. 
     EXAMPLE 4 
     Shaped magnetic recording media can be fabricated using the flakes prepared according to Example 1. The flakes may be crushed to a suitable size for powder processing i.e. reduced to micron sized particles. These particles can be formed into thick, self-supporting shaped articles, such as disks or drums and sintered. Disks made by this method offer the advantage of addressability from either side. 
     EXAMPLE 5 
     Alternatively, magnetic recording media can be made by applying inks containing magnetic iron oxide containing precipitate particles to shaped substrates. Iron oxide containing powders prepared according to the method outlined in Example 2 or 3 can be used to produce inks which can be painted on substrates having the desired shape and sintered to produce a thin solid recording layer. 
     Thick recording films can be made by combining the iron oxide containing powders of Example 2 or 3 with binders and elastomers to make a &#34;green&#34; i.e. unsintered sheet which can be applied to a substrate of the desired shape and then sintered. 
     EXAMPLE 6 
     Thin film magnetic recording media can be sputtered onto substrates selected for their shape and optical properties. Such condensation of materials from the vapor phase results in extremely high cooling rates on the order of 10 12  °/second and can be employed to deposit smooth surfaced 0.1 micron-10.0 micron cobalt ferrite glass matrix films. 
     Sputtering techniques can also be used to deposit magnetic iron oxide containing precipitates embedded in a glass matrix on metallic substrates having reflective properties suitable for optical recording based on the Faraday and Kerr effects.