JPH0672020B2 - Method for producing acicular hexagonal ferrite particles - Google Patents

Description

The present invention relates to a method for producing hexagonal ferrite particles having a needle-like particle shape. More specifically, it relates to a method of heating a mixture of hydroxide such as barium and acicular goethite by microwave plasma to generate acicular hexagonal ferrite particles having an easy axis of magnetization in a direction perpendicular to the major axis. It is a thing.

The acicular hexagonal ferrite particles obtained by the present invention,
It is particularly useful as a material for perpendicular magnetic recording.

[Prior Art] Hexagonal ferrites such as barium ferrite are excellent as magnetic recording media because their coercive force and squareness ratio are larger than those of γ-Fe ₂ O ₃ which has been widely used in the past.

Conventionally, there are the following two main methods for producing barium ferrite particles. The first method is a vitrification crystallization method. This method uses barium oxide (BaO) and diiron trioxide (F
In addition to e ₂ O ₃ ), boron oxide (B ₂ O ₃ ) as a glass component and excess barium oxide (BaO) were added and heated and melted. Make a ribbon. When this is reheated, barium ferrite particles are precipitated. Unnecessary barium oxide (BaO) and boron oxide (B
₂ O ₃ ) is dissolved and removed to obtain barium ferrite particles.

The second method is a hydrothermal reaction method. In this method, iron chloride (FeCl ₂ ) and barium chloride (BaCl ₂ ) are placed in an autoclave, the alkalinity is adjusted with sodium hydroxide (NaOH), and hydrothermal treatment is performed to form a precipitate. This was heated to 600 to 800 ° C to produce barium ferrite particles.

However, these methods have drawbacks in that it is difficult to obtain uniform barium ferrite particles, and since the number of steps is large and the efficiency is low, mass productivity is poor.

Further, since these barium ferrite particles have a hexagonal plate shape, it is difficult to apply a magnetic field to align the c-axis, which is the easy axis of magnetization, in the same direction, and apply it flatly on a plastic film or the like. For this reason, the magnetic characteristics originally possessed by barium ferrite are not fully utilized, and the recording density is not sufficiently high.

In order to solve this problem and use it as a recording medium for magnetic cards and flexible magnetic disks, the barium ferrite particles are uniaxially oriented so that the easy axis of magnetization of each particle is perpendicular to the base film surface. There is a need to.

One of the inventors of the present invention was that acicular goethite (α-FeOOH)
Sintering of a mixture of barium hydroxide and barium hydroxide at 820 to 880 ° C. succeeded in producing acicular barium ferrite particles in which the easy axis of magnetization was oriented in the direction perpendicular to the long axis (JP-A-61-104602). (See the official gazette). Here, a small amount of low-melting oxides such as boron oxide (B ₂ O ₃ ) and / or bismuth oxide (Bi ₂ O ₃ ) is added in order to perform firing at a predetermined relatively low temperature.

[Problems to be Solved by the Invention] However, in the above-mentioned method, even if the temperature is low, the needle-shaped goethite particles are heated from the surface and locally melt, so that the needle-shaped structure is broken or the needle-shaped structure is broken. Particles are bound to each other and the ratio of the long axis to the short axis decreases.

By the way, it is known that the acicular ratio of a magnetic recording medium affects the SN ratio of recording / reproducing, and a method with a large acicular ratio improves the SN ratio. Therefore, the reduction of the acicular ratio due to the above factors leads to the degradation of the recording / reproducing characteristics.

Although a small amount, a non-magnetic material such as boron oxide (B ₂ O ₃ ) and / or bismuth oxide (Bi ₂ O ₃ ) is mixed, which also causes a decrease in magnetic characteristics.

The object of the present invention is to solve the problems of the above-described conventional techniques, to obtain acicular hexagonal ferrite particles having excellent magnetic properties, while maintaining a high acicular ratio, efficiently in a short time. It is to provide a method that can be manufactured.

[Means for Solving the Problems] The present invention that can achieve the above-mentioned object is barium hydroxide (Ba
(OH) ₂ ), strontium hydroxide (Sr (OH) ₂ ), lead hydroxide (Pb (OH) ₂ ), and a mixture containing one or more kinds and acicular goethite (α-FeOOH) Is irradiated with microwaves to generate plasma, which is then heated to cause a ferrite reaction to generate acicular hexagonal ferrite particles.

Particularly preferably, M (where M is 1 selected from Ba, Sr and Pb)
Or a mixture of 2 or more) having a molar ratio of Fe of Fe / M = 9 to 13 is irradiated with microwaves under an oxygen pressure of 5 to 40 Torr to generate oxygen plasma, and then heated.
The plasma is heated by irradiation with microwaves under a reduced pressure of 50 Torr. In order to maintain a stable plasma, it is preferable to set the range of 15 to 40 Torr under the oxygen pressure, and 3 to 50 Torr under the reduced pressure, although it depends on the apparatus configuration.

[Function] When a compound having a hydroxyl group is irradiated with microwaves, the hydroxyl group is excited and heat is generated from the inside. Further, in a specific atmosphere, plasma is generated and heated by microwave irradiation. The present invention utilizes these phenomena. That is, when a mixture of a hydroxide such as barium and acicular goethite is irradiated with microwaves, its hydroxyl group vibrates and is uniformly heated from the inside. In addition, plasma is generated to generate active chemical species, and plasma heating is also applied to cause a ferritization reaction to directly generate acicular hexagonal ferrite particles.

When microwave irradiation is performed under an oxygen pressure of 5 to 40 Torr, oxygen plasma is generated and heat is generated. When microwave irradiation is performed under a reduced pressure of 1 to 50 Torr, plasma is generated by the residual gas and the gas generated by thermal decomposition of the hydroxyl group, and heat is generated. The generated activated oxygen ions also contribute to the ferritization reaction.
Comparing the reactions under oxygen pressure and under reduced pressure, better plasma is easier to occur under oxygen pressure, and the ferrite generation efficiency is higher.

For example, when barium hydroxide is used, barium hydroxide adheres to the surface of the acicular goethite particles, and the barium ions are diffused and combined by heating to generate barium ferrite fine crystals. At that time, in terms of energy, the axes of easy magnetization are not arranged so as to be stacked in the long axis direction of the acicular goethite, but are generated so as to be oriented in the direction perpendicular to the long axis. As a result, the barium ferrite particles maintain the acicularity of the goethite base material, and the easy axis of magnetization is oriented in the direction perpendicular to the long axis thereof.

In the microwave plasma heating, the particles are heated uniformly from the inside, so that the particles are unlikely to bond with each other, and the original acicularity is unlikely to be broken.

[Example] The average particle shape has a major axis length of about 2.6 μm and a minor axis length of about 0.15 μm.
Using needle-shaped goethite and barium hydroxide
It was compounded at a molar ratio of Fe / Ba = 12. It was stirred and mixed in warm water to dissolve barium hydroxide, brushed on a stainless steel plate and dried, and dried powder was collected to obtain a sample. This adjustment is for sufficiently uniform mixing and for preventing mutual fusion of needle-shaped particles.

The above sample is made of quartz glass with a diameter of 12mmφ and a length of 200mm.
Put it in the bottom of the letter-shaped reaction tube and pull it out from the waveguide.
The microwave of 2.45 GHz was irradiated with an output of 50 to 150 W. The microwave plasma heating was performed for 10 minutes under a reduced pressure of 20 Torr. The relationship between the microwave irradiation output and the generation rate (%) is shown in FIG. The production rate is the magnetization value of barium ferrite.
It was set to 65 emu / g, and expressed as a ratio with the magnetization value of the generated sample.

Also, for strontium hydroxide and lead hydroxide, samples were prepared by the same method, and a plasma heating experiment was conducted under the same conditions. The generation rate is 66 emu / g for the strontium ferrite magnetization value and 52 emu / g for the lead ferrite magnetization value.
It was expressed as a ratio with the magnetization value of the generated sample.

In all samples, the production rate increased as the microwave irradiation output increased. When compared with the same microwave irradiation output, lead ferrite was most likely to be generated, followed by barium ferrite and strontium ferrite in that order.

FIG. 2 shows the production rate with respect to the irradiation time of the mixture of needle-shaped goethite and barium hydroxide prepared in the same manner as above, while keeping the microwave irradiation output constant at 150 W under a reduced pressure of 20 Torr. As the heating time increases, the production rate increases,
It approached 100% in about 8 minutes.

FIG. 3 shows the dependence of magnetization and coercive force on the degree of reduced pressure for a sample compounded at a ratio of Fe / Ba = 10. Both the magnetization and the coercive force increase between 5 and 50 Torr, but tend to decrease in the degree of pressure reduction before and after that. In the apparatus used for the experiment, microwave plasma was not efficiently generated at less than 5 Torr, the barium ferrite formation reaction was insufficient, and the magnetization and coercive force were small. Further, when it exceeds 50 Torr, goethite (α-FeOOH) is oxidized to produce diiron trioxide (Fe ₂ O ₃ ), so that the magnetization and the coercive force also decrease. The microwave irradiation output was 150 W and the irradiation time was constant for 10 minutes.

FIG. 4 shows the oxygen pressure dependence of magnetization and coercive force using the same sample. The magnetization increases as the oxygen pressure increases, but decreases rather than 40 Torr. The device used in the experiment was difficult to obtain a stable oxygen plasma below 20 Torr, so the magnetization decreased. The coercive force hardly changed under any oxygen pressure. The microwave irradiation output is 150
W, irradiation time is constant for 10 minutes.

FIG. 5 shows the change in the production rate of barium ferrite with respect to the Fe / Ba compounding ratio. The microwave irradiation output is 150W,
The irradiation time is constant for 10 minutes. Almost 100% of barium ferrite was formed between Fe / Ba = 9 to 13. In the compounding ratio on both sides, in addition to barium ferrite, a compound of barium oxide (BaO) and diiron trioxide (Fe ₂ O ₃ ) exists as a different phase.

When observing the samples obtained in the above experimental examples with a transmission electron microscope, the average shape of the hexagonal ferrite particles is
It became slightly shorter and thicker than the original acicular goethite particles, the major axis length was about 2.0 μm, the minor axis length was about 0.2 μm, and the acicular ratio was about 10.

As described above, the production rate was reduced to less than 15 Torr under oxygen pressure and less than 5 Torr under reduced pressure, because the apparatus used in the experiment is unlikely to generate stable plasma in that region. Depending on the device, 5 Torr under oxygen pressure
Up to 1 Torr can be used under reduced pressure. In the above embodiment, the case of mainly producing barium ferrite was systematically explained, but the same tendency also exists in the case of strontium ferrite and lead ferrite, and they can be manufactured under almost the same conditions.

In addition, the ferrite particles of the above-mentioned embodiment are completely hexagonal (M
Type) ferrite (MFe ₁₂ O ₁₉ ), but the present invention is also applicable to the production of W-type ferrite (MMe ₂ Fe ₁₆ O ₁₇ ) (where Me is various divalent metal ions) having a similar hexagonal structure. Applicable.

[Advantages of the Invention] The present invention is a method of irradiating a mixture of acicular goethite and barium hydroxide, etc. with microwaves for plasma heating to cause a ferrite reaction, as described above. It is possible to directly and efficiently produce acicular hexagonal ferrite particles having a high acicularity and a large acicular ratio in a short time. Since these acicular hexagonal ferrite particles have an easy axis of magnetization in a direction perpendicular to the long axis, magnetic tapes and magnetic disks can be manufactured by using conventional coating techniques as they are, and particularly, perpendicular magnetic recording media. Is industrially extremely useful.

[Brief description of drawings]

FIG. 1 is a graph showing the microwave irradiation output dependence of the production rate of acicular hexagonal ferrite particles, FIG. 2 is a graph showing the relationship of the production rate of barium ferrite particles with respect to the microwave irradiation time, and FIG. 3 is Fig. 4 is a graph showing the dependence of the magnetization and coercive force of barium ferrite on the pressure reduction degree. Fig. 4 is a graph showing the dependence of the magnetization and coercive force of barium ferrite on the oxygen pressure. Fig. 5 is barium ferrite particles with respect to the Fe / Ba composition ratio. 5 is a graph showing the generation rate of

Claims (3)

[Claims] 1. Barium hydroxide, strontium hydroxide,
A mixture containing one or more kinds selected from lead hydroxide and acicular goethite is irradiated with microwaves to generate plasma and heat to cause a ferrite reaction to generate acicular hexagonal ferrite particles. A method for producing acicular hexagonal ferrite particles, comprising: 2. Barium hydroxide, strontium hydroxide,
The molar ratio of Fe to M (where M is one or more selected from Ba, Sr, Pb) in one or more selected from lead hydroxide and acicular goethite is Fe / M = The method according to claim 1, wherein the mixture of 9 to 13 is irradiated with microwaves under an oxygen pressure of 5 to 40 Torr to generate oxygen plasma and heat the mixture. 3. Barium hydroxide, strontium hydroxide,
The molar ratio of Fe to M (where M is one or more selected from Ba, Sr, Pb) in one or more selected from lead hydroxide and acicular goethite is Fe / M = The production method according to claim 1, wherein the mixture of 9 to 13 is irradiated with microwaves under reduced pressure of 1 to 50 Torr to perform plasma heating.