Surface-modified iron-based oxide magnetic particle powder

Abstract

A surface-modified iron-based oxide magnetic particle powder has good solid-liquid separation property in the production process, has good dispersibility in a coating material for forming a coating-type magnetic recording medium, has good orientation property, and has a small elution amount of a water-soluble alkali metal, and to provide a method for producing the surface-modified iron-based oxide magnetic particle powder. The surface-modified iron-based oxide magnetic particle powder can be obtained by neutralizing a solution containing dissolved therein a trivalent iron ion and an ion of the metal, by which the part of Fe sites is to be substituted, with an alkali aqueous solution, so as to provide a precursor, coating a silicon oxide on the precursor, heating the precursor to provide e-type iron-based oxide magnetic powder, and adhering a hydroxide or a hydrous oxide of one kind or two kinds of Al and Y thereto.

Claims

1. A surface-modified iron-based oxide magnetic particle powder comprising iron-based oxide magnetic particle powder containing ε-Fe.sub.2O.sub.3 or ε-Fe.sub.2O.sub.3, a part of Fe sites of which is substituted by another metal element, having an average particle diameter measured with a transmission electron microscope of 5 nm or more and 30 nm or less, having adhered to a surface thereof a hydroxide or a hydrous oxide of at least one of Al and Y forming a precipitate of a hydroxide in an aqueous solution having pH of 7 or more and 12 or less, and wherein the surface-modified iron-based oxide magnetic particle powder has a tap density of 0.30 g/cm.sup.3 or more and 1.60 g/cm.sup.3 or less.

2. The surface-modified iron-based oxide magnetic particle powder according to claim 1, wherein the surface-modified iron-based oxide magnetic particle powder has a molar ratio of the at least one of Y and Al/M of 0.02 or more and 0.10 or less, wherein M represents a sum of Fe and the substituting metal element contained therein.

3. The surface-modified iron-based oxide magnetic particle powder according to claim 1, wherein the iron-based oxide is ε-A.sub.xB.sub.yC.sub.zFe.sub.2-x-y-zO.sub.3 (wherein A represents at least one divalent metal element selected from Co, Ni, Mn, and Zn; B represents at least one tetravalent metal element selected from Ti and Sn; C represents at least one trivalent metal element selected from In, Ga, and Al; and 0<x<1, 0<y<1, and 0<z<1).

Description

EXAMPLES

Example 1

(1) As starting substances of Fe, Ga, Co, and Ti, 3,296.53 g of iron(III) nitrate nonahydrate, 854.72 g of a Ga(III) nitrate aqueous solution having a Ga concentration of 10.70 mass %, 74.27 g of cobalt(II) nitrate hexahydrate, and 77.96 g of titanium(IV) n-hydrate having a Ti concentration of 15.2 mass were used respectively, to which 20.31 kg of pure water was added to prepare a mixed aqueous solution. 2.78 kg of a 22.35 mass % ammonia solution was added to the raw material solution at a liquid temperature of 30° C. under mechanically stirring to neutralize the solution, and then the solution was continuously stirred for 0.5 hour to prepare a slurry having the precursor dispersed therein (Procedure 1).

(2) To the slurry obtained in Procedure 1, 5.65 kg of 97.1 mass % tetraethoxysilane was added dropwise over 35 minutes under mechanically stirring, and the mixture was continuously stirred at 30° C. for 20 hours in the air, thereby providing a slurry containing the precursor coated with a silanol derivative formed through hydrolysis. The slurry was rinsed and subjected to solid-liquid separation, so as to recover the precursor coated with the silanol derivative as a cake (Procedure 2).

(3) After drying the cake obtained in Procedure 2, the dried powder thereof was subjected to a heat treatment in a furnace with an air atmosphere at 1,068° C. for 4 hours, so as to provide iron-based oxide magnetic particle powder coated with a silicon oxide, which was then stirred in a 20 mass % NaOH aqueous solution at approximately 60° C. for 24 hours to remove the silicon oxide on the surface of the particles, and rinsed with an ultrafiltration membrane until the electroconductivity reached 5 mS/m or less, thereby providing a slurry containing ε-type iron-based oxide magnetic powder, a part of Fe sites of which was substituted by Ga, Co, and Ti (Procedure 3).

(4) To 2,802 g of the slurry containing 1.37 mass % of the iron-based oxide magnetic particle powder obtained in Procedure 3, a NaOH aqueous solution was added under mechanically stirring at a rotation number of 391 rpm at a liquid temperature 40° C. to control pH 11.7, then a 1.72 mass % aluminum sulfate aqueous solution was added, and then a NaOH aqueous solution was added dropwise until pH 8.5, followed by continuously stirring for 10 minutes, so as to adhere a hydroxide of Al to the iron-based oxide magnetic particle powder. Subsequently, the slurry was rinsed until the electroconductivity reached 1 mS/m or less, filtered with hardened filter paper 4A, and then dried, thereby providing surface-modified iron-based oxide magnetic particle powder (Procedure 4).

(5) The surface-modified iron-based oxide magnetic particle powder obtained in Procedure 4 was subjected to the chemical analysis of the composition, the TEM observation, the measurement of the magnetic characteristics, and the like, and a magnetic tape was produced therewith according to the “Formation of Magnetic Sheet” shown above and measured for the magnetic characteristics of the magnetic tape according to the “Measurement of Magnetic Hysteresis Curve (Sheet B-H Curve)” shown above. In the production of the tape, the dispersion time was 10 minutes, and the composition was dried in a magnetic field with an orientation magnetic field of 5.5 kOe (438 kA/m).

(6) The properties of the resulting surface-modified iron-based oxide magnetic particle powder, the bulk magnetic characteristics thereof, and the magnetic characteristics of the tape are shown in Table 1.

(7) It is found that the surface-modified iron-based oxide magnetic particle powder having Al adhered thereto obtained in this example has a TEM average particle diameter that is the same as that of the iron-based oxide magnetic particle powder having no Al adhered thereto of Comparative Example 1 described later, but the tap density thereof is decreased due to the adherence of Al. It is estimated that this is because the silicon oxide remaining in a slight amount on the surface of the iron-based oxide magnetic particle powder is hidden to prevent the iron-based oxide magnetic particle powder from being aggregated. As a result, it is considered that the tape produced with the surface-modified iron-based oxide magnetic particle powder in the form of a coating material has an enhanced SQx.

Example 2

(8) Surface-modified iron-based oxide magnetic particle powder having Y adhered thereto was obtained in the same procedures as in Example 1 except that a 2.13 mass % yttrium sulfate aqueous solution was used as the aqueous solution containing the element to be adhered.

(9) The properties of the resulting surface-modified iron-based oxide magnetic particle powder, the bulk magnetic characteristics thereof, and the magnetic characteristics of the tape are shown in Table 1.

(10) In this example, the tap density is decreased due to the adherence of Y, and the SQx is increased, as similar to Example 1.

Comparative Example 1

(11) The same procedures as until Procedure 3 of Example 1 were performed, and then the slurry was filtered with hardened filter paper 4A (retaining particle diameter: 1 μm), but the iron-based oxide magnetic particles as a solid content were passed through the filter paper, and solid-liquid separation was not able to be performed by filtration. By using Omnipore membrane filter (model No.: JGWP09025, pore diameter: 0.2 μm) as filter paper, the same result was obtained, and solid-liquid separation was not able to be performed. The slurry was then subjected to solid-liquid separation and drying by entirely evaporating the water content with a dryer, thereby providing iron-based oxide magnetic particle powder. The properties of the resulting surface-modified iron-based oxide magnetic particle powder obtained in Comparative Example 1, the bulk magnetic characteristics thereof, and the magnetic characteristics of the tape are shown in Table 1. This comparative example provides such results that the tap density is large, and the SQx is low since the silicon oxide remaining on the surface of the particles on drying acts to aggregate the particles.

Example 3

(12) In a reaction tank, 4,659.28 g of iron(III) nitrate nonahydrate having a purity of 99.7%, 1,421.39 g of a Ga (III) nitrate aqueous solution having a Ga concentration of 12.9%, 157.83 g of cobalt (II) nitrate hexahydrate having a purity of 97%, and 119.13 g of titanium(IV) n-hydrate having a Ti concentration of 15.1% were dissolved in 23.64 g of pure water in an air atmosphere under a condition of 40° C. under mechanically stirring with a stirring blade. The charged solution had a molar ratio of metal ions of Fe/Ga/Co/Ti=1.530/0.350/0.070/0.050. The numbers in parentheses following the reagent names are the valencies of the metal elements.

(13) Under mechanically stirring with a stirring blade in the air atmosphere at 40° C., 2,698.88 g of a 23.31% ammonia solution was added thereto at one time, and the mixture was stirred for 2 hours.

(14) 2,887.51 g of a citric acid solution having a citric acid concentration of 20 mass % was then continuously added thereto under a condition of 40° C. over 1 hour, then 1,470.86 g of a 23.31% ammonia solution was added thereto at one time, and the mixture was retained under a condition of a temperature of 40° C. under stirring for 1 hour, thereby forming crystals of iron oxyhydroxide containing the substituting element, which was a precursor as an intermediate (Procedure 1).

(15) Thereafter, in an air atmosphere at 40° C., to the precursor slurry obtained in Procedure 1 under stirring, tetraethoxysilane in an amount of approximately 700% by weight based on ε-Fe.sub.2O.sub.3c was added, and 8,553.94 g of tetraethoxysilane was added to the slurry liquid over 35 minutes. The mixture was further stirred for approximately 1 day, so as to coat with a silanol derivative formed through hydrolysis. Thereafter, the resulting solution was subjected to rinsing and solid-liquid separation, so as to recover as a cake (Procedure 2).

(16) After drying the cake obtained in Procedure 2, the dried powder thereof was subjected to a heat treatment in a furnace with an air atmosphere at from 1,040° C. to 1,050° C. for 4 hours, so as to provide iron-based oxide magnetic particle powder coated with a silicon oxide, which was then stirred in a 20 mass % NaOH aqueous solution at approximately 60° C. for 24 hours to remove the silicon oxide on the surface of the particles, and rinsed with an ultrafiltration membrane until the electroconductivity reached 5 mS/m or less, thereby providing a slurry containing ε-type iron-based oxide magnetic powder, a part of Fe sites of which was substituted by Ga, Co, and Ti (Procedure 3).

(17) To 4,000 g of the slurry containing 1.50 mass % of the iron-based oxide magnetic particle powder obtained in Procedure 3, a NaOH aqueous solution was added under mechanically stirring at a rotation number of 391 rpm at a liquid temperature 40° C. to control pH 11.9, then 19.07 g of a 1.77 mass % aluminum sulfate aqueous solution and 55.14 g of a 1.98 mass % yttrium sulfate solution were added, and then a NaOH aqueous solution was added dropwise until pH 8.5, followed by continuously stirring for 10 minutes, so as to adhere a hydroxide of Al and Y to the iron-based oxide magnetic particle powder. Subsequently, the slurry was rinsed until the electroconductivity reached 1 mS/m or less, filtered with hardened filter paper 4A, and then dried, thereby providing surface-modified iron-based oxide magnetic particle powder (Procedure 4).

(18) The surface-modified iron-based oxide magnetic particle powder obtained in Procedure 4 was subjected to the chemical analysis of the composition, the TEM observation, the measurement of the magnetic characteristics, and the like, and a magnetic coating material was prepared according to “Preparation of Magnetic Coating Material” shown above, and a magnetic tape was produced therewith according to the “Formation of Magnetic Sheet” shown above, except that in the production of the tape, the dispersion time was 60 minutes, and the coating material was dried in a magnetic field with an orientation magnetic field of 5.5 kOe (438 kA/m). The tape was measured for the magnetic characteristics of the magnetic tape according to the “Measurement of Magnetic Hysteresis Curve (Sheet B-H Curve)” shown above. It is found that the surface-modified iron-based oxide magnetic particle powder having Al and Y adhered thereto obtained in this example has a TEM average particle diameter that is the same as that of the iron-based oxide magnetic particle powder having no Al or Y adhered thereto of Comparative Example 2 described later, but the tap density thereof is decreased due to the adherence of Al and Y. It is estimated that this is because the silicon oxide remaining in a slight amount on the surface of the iron-based oxide magnetic particle powder is hidden to prevent the iron-based oxide magnetic particle powder from being aggregated. As a result, it is considered that the tape produced with the surface-modified iron-based oxide magnetic particle powder in the form of a coating material has an enhanced SQx as compared to Comparative Example 2.

Comparative Example 2

(19) The same procedures as until Procedure 3 of Example 3 were performed except that the iron-based oxide magnetic particle powder was rinsed with an ultrafiltration membrane until the electroconductivity reached 1 mS/m or less, thereby providing a slurry containing ε-type iron-based oxide magnetic powder, a part of Fe sites of which was substituted by Ga, Co, and Ti. The slurry was then filtered with hardened filter paper 4A (retaining particle diameter: 1 μm), but the iron-based oxide magnetic particles as a solid content were passed through the filter paper, and solid-liquid separation was not able to be performed by filtration. By using Omnipore membrane filter (model No.: JGWP09025, pore diameter: 0.2 μm) as filter paper, the same result was obtained, and solid-liquid separation was not able to be performed. The slurry was then subjected to solid-liquid separation and drying by entirely evaporating the water content with a dryer, thereby providing iron-based oxide magnetic particle powder. The properties of the resulting surface-modified iron-based oxide magnetic particle powder obtained in Comparative Example 2, the bulk magnetic characteristics thereof, and the magnetic characteristics of the tape are shown in the table. This comparative example provides such results that the tap density is large, and the SQx is low since the silicon oxide remaining on the surface of the particles on drying acts to aggregate the particles.

(20) Filtration Characteristics

(21) As described above, for the slurries containing the iron-based oxide magnetic particle powder having no hydroxide adhered thereto of Comparative Examples, the iron-based oxide magnetic particles as a solid content were passed through the hardened filter paper 4A (retaining particle diameter: 1 μm) and Omnipore membrane filter (model No.: JGWP09025, pore diameter: 0.2 μm), and solid-liquid separation was not able to be performed by filtration. Accordingly, the slurry was subjected to solid-liquid separation and drying by entirely evaporating the water content with a dryer.

(22) In Examples, on the other hand, for the slurries containing the iron-based oxide magnetic particle powder having one kind or two kinds of Al and Y adhered thereto, the iron-based oxide magnetic particle powder was able to be recovered with the hardened filter paper 4A (retaining particle diameter: 1 μm).

(23) In the slurry containing the iron-based oxide magnetic particle powder having the hydroxide adhered thereto according to the invention, it is considered that the aggregation property of the particles in an aqueous medium is enhanced by adhering the hydroxide on the surface of the particles, and thus can be subjected to solid-liquid separation by filtration.

(24) As described herein, the iron-based oxide magnetic particle powder that has a hydroxide adhered thereto can be subjected to solid-liquid separation by filtration, so as to save the energy required for solid-liquid separation and drying, and thus is preferred in industrial production.

(25) Dispersibility in Coating Material

(26) It is considered that the aggregation property of the particles in an aqueous medium is enhanced by adhering the hydroxide on the surface of the particles, but the magnetic tapes produced with the surface-modified iron-based oxide magnetic particle powder in the form of a coating material obtained in the invention showed excellent magnetic characteristics. It is considered that this is because the problem of aggregation of the particles does not occur in the medium of an organic solvent used for forming the coating material, and thus the dispersibility thereof in the coating material is rather improved.

(27) Water-Soluble Alkali Metal

(28) By the solid-liquid separation that can be performed by filtration, the impurities, such as an alkali metal, contained in a slight amount in the liquid of the slurry containing the iron-based oxide magnetic particle powder having a hydroxide adhered thereto can be removed.

(29) Comparative Examples provided such results that the water-soluble Na amounts eluted from the obtained iron-based oxide magnetic particle powder were as high as 92 ppm and 10 ppm as shown in Table 1. In Comparative Example 2, in particular, the iron-based oxide magnetic particle powder was obtained by drying the slurry after rinsing the slurry with an ultrafiltration membrane until the electroconductivity reached 1 mS/m or less, which was the same as Examples where the electroconductivity was lowered to 1 mS/m or less, but there were such results that the iron-based oxide magnetic particle powder had a large water-soluble Na amount.

(30) It is considered that this is because in Comparative Examples, the solid-liquid separation and drying are performed by subjecting the slurry containing the iron-based oxide magnetic particle powder to a dryer since the solid-liquid separation cannot be performed by filtration, and thus Na contained in a slight amount in the liquid is not removed, but is concentrated and remains on the surface of the particles.

(31) Furthermore, as the factor of the large water-soluble Na amount in Comparative Examples described above, it is also considered that the Na component adhered to the surface of the particles of the iron-based oxide magnetic particle powder is not removed by the rinsing by ultrafiltration performed in Procedure 3, but remains on the surface of the particles.

(32) On the other hand, Examples 1 to 3 provided such results that the water-soluble Na amount of the resulting surface-modified iron-based oxide magnetic particle powder was as low as 3 ppm or less as shown in the table. It is considered that this is because, for example, the impurities contained in a slight amount in the liquid can be removed by performing solid-liquid separation by filtration, such a state is provided that the Na component amount on the surface of the particles is decreased by adhering a hydroxide of the metal S to the surface of the iron-based oxide magnetic particle powder, and further the surface is modified by the adhered material into such a state that prevents the Na component from remaining, resulting in effective removal of the Na component on the surface of the surface-modified iron-based oxide magnetic particle powder by ultrafiltration, and consequently it is considered that the elution amount of the water-soluble Na amount is decreased.

(33) Due to the small eluted Na amount, the surface-modified iron-based oxide magnetic particle powder of the invention can decrease the formation amount of the fatty acid Na precipitate formed on the surface of the coating-type magnetic recording medium, and thus is favorably applied to the purpose of a coating-type magnetic recording medium.

(34) While Na hydroxide is used as the alkali hydroxide in the production process in Examples of the invention, the same results may be obtained by using other alkali hydroxides, such as K hydroxide and Ca hydroxide.

(35) TABLE-US-00001 TABLE 1 Adhering Ultrafiltration Properties of powder step Electro- TEM average BET specific Adhered conductivity Solid-liquid particle diameter surface area TAP density Water-soluble Na element S (mS/m) separation (nm) (m.sup.2/g) (g/cm.sup.3) (ppm) Example 1 Al 1 filtered with hardened 16.4 71 1.51 3 filter paper 4A Example 2 Y 1 filtered with hardened 16.4 66 1.30 2 filter paper 4A Comparative none 5 drying slurry 16.4 73 1.84 92 Example 1 Example 3 Al and Y 1 filtered with hardened 17.2 66 1.44 <1 filter paper 4A Comparative none 1 drying slurry 17.2 73 1.77 10 Example 2 Bulk magnetic characteristics Adhered (VSM 10 kOe) Tape Compositional ratio amount of S Hc Hc σs characteristics Fe Ga Co Ti (S/M) (Oe) (kA/m) (Am.sup.2/kg) SQ SQx Example 1 1.64 0.241 0.051 0.064 0.030 3352 267 14.9 0.53 0.67 Example 2 1.65 0.241 0.050 0.064 0.032 3384 269 14.4 0.53 0.66 Comparative 1.64 0.242 0.051 0.064 — 3376 269 15.2 0.53 0.65 Example 1 Example 3 1.56 0.343 0.043 0.053 0.034 2661 212 15.2 0.52 0.62 Comparative 1.56 0.333 0.048 0.057 2718 216 15.8 0.53 0.60 Example 2