JPS5925729B2 - Manufacturing method of MnZn ferrite - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing MnZn ferrite, which has extremely high sintered density, extremely uniform crystal grain size, and extremely excellent magnetic properties, especially at high frequencies.

BACKGROUND ART In recent years, with the expansion and progress of the field of application of magnetic recording technology, magnetic recording media, mainly magnetic tapes, have also made remarkable progress, and high-density recording has become possible.

In addition to recording media, magnetic heads, which are indispensable for magnetic recording, are also required to have various characteristics that are required to meet the demands for higher-density recording. Under these circumstances, ferrite magnetic heads have excellent frequency characteristics and wear resistance, but recently there has been a trend in which high-density magnetic recording materials with excellent frequency characteristics are required, especially in the MH1 band. In order to satisfy these required properties, in addition to rigorous compositional considerations, the crystal grain size must be uniform and small, and the sintered body must have an extremely high density (fine) with a sintered density almost equal to the theoretical density. Must be. In order for a sintered body obtained by a normal dry method to have a relative density of 99% or more and at the same time satisfy magnetic properties, it is necessary to sinter under optimal sintering conditions. For MnZn ferrite, this temperature is 360 ~
The average crystal grain size of the obtained sintered body is about 80-100μ, and the magnetic permeability at high frequencies is significantly reduced. Further, the sintered density is also limited to about 99.2%. The conventional method for producing high-density ferrite is to sinter powder produced by a wet method such as a coprecipitation method or an air oxidation method, and calcined powder obtained by a dry method using a hot press. There is a way. This method is superior to the usual dry method, and can produce sintered bodies with an average particle size of about 20 μm and a relative sintered density of about 99.0 to 99.8%. However, the problem with the hot pressing method is that the pressing direction is not hydrostatic, and therefore a sintered body with a complicated shape cannot be obtained. Furthermore, it is difficult to obtain a large sintered body due to equipment limitations, and even if continuous hot pressing is used, it is not industrially advisable. A further disadvantage of this method is that it is necessary to coat ferrite with a powder such as alumina and then perform hot pressing, and it is impossible to prevent the solid phase reaction that occurs between ferrite and alumina during sintering. This is possible. Therefore, there is a large compositional variation between the center of the sintered body and the area near the surface, and other compounds are generated especially near the surface, so a considerable portion must be removed from the surface of the sintered body. Not only that, but the yield is also extremely poor. As a method for eliminating the above-mentioned drawbacks, a method using hot isostatic pressing is known from JP-A-49-128296.

However, the details still need to be considered.
The inventor has arrived at the present invention by conducting detailed studies, especially regarding the conditions of preliminary sintering. The outline of the method of the present invention is to advance the preliminary sintering of the molded body to the extent that no pores (open bores) communicating from the inside of the sample to the surface are formed, and then to perform hot isostatic pressing (hereinafter referred to as HIP). This is a method of achieving densification using hydrostatic pressure.

What should be noted in this method is that since pressure is applied hydrostatically, densification will not proceed if open bores exist in the sample. Therefore, it is important not to generate orthobores during preliminary sintering. However, it is not so difficult to prevent the formation of open bores during pre-sintering, and the relative density of the pre-sintered body (hereinafter d
/Dx. If d: density of sintered body, Dx: theoretical density) is about 93% or more, no open bore will be generated. Furthermore, although the detailed mechanism by which HIP densifies a sintered body with extremely poor malleability such as MnZn ferrite is not clear, from a thermodynamic point of view it can be considered that the following relationship holds true as a rough approximation: can. Here, Pext is the external pressure, that is, the HIP pressure (500-1500 atmospheres),
P, O, and e are the pressures of the pores inside the sintered body, and γ is the interfacial tension of the pores. By the way, according to theoretical calculations, the energy given to a substance by pressurization reaches a value corresponding to the energy of chemical bonds for the first time at a pressure of 106 atmospheres or more. Furthermore, since the pressure corresponding to the energy of phase transition in solids is usually 104 to 105 atm or higher, the pressure of about 103 atm due to HIP is 1000℃.
It is thought that even at a certain temperature, ions do not have a significant effect on the crystal grains arranged by chemical bonding force. Therefore, when pressure is applied, most of the densification progresses as the crystal grains rotate along the grain boundaries, which are balanced by a binding force (that is, interfacial tension) that is much weaker than the chemical bonding force. Presumed. Of course, since the pressure is applied in a heated state, the pores in the crystal grains will also separate to the grain boundaries to some extent due to the diffusion of pores, and if the gas in the pores is 02. , its diffusion rate is about two orders of magnitude higher than that of N2. In fact, the more grain boundaries there are, that is, the smaller the grain size, the better, unless the pre-sintered body is accompanied by the formation of open bores. Densification progresses easily during HIP. This fact is extremely advantageous for MnZn ferrite for high frequency magnetic materials.
By the way, if densification by HIP occurs according to equation (1), γ in equation (1) corresponds to the interfacial tension at the grain boundary. In addition, P,Ore is about 1 atm to 10 atm when MnZn ferrite is pre-sintered in air, etc., but after HIP, even considering the term 2γ/r, PpO
r8 is expected to be around several hundred atmospheres. Now, assuming that the gas in the hole is an ideal gas, Re
latmlHIP conditions to 1200'ClOOOO atmosphere, H
If P, Ore after IP is 500 atm and no gas diffusion or desorption occurs, the volume of the pores after HIP is approximately 1/1
It decreases to 00. Of course, 02, which has a high diffusion rate in the oxide, will diffuse away from the grain boundaries to the sintered body surface during HIP, but when primary sintering is performed in air, N2, which has a low diffusion rate, , as mentioned above, at least a portion of it is confined in the grain boundaries under high pressure. When such a sample is annealed for a long time in order to obtain predetermined magnetic properties as described below, the volume of the compressed pores often expands again, causing the sintered density to decrease again. Therefore, by performing part of the pre-coldification process in a vacuum,
It is necessary to keep the gas pressures P, O, and O inside the holes as low as possible. In addition, the sintered body after HIP mainly accumulates the strain stress generated during cooling, and the magnetic properties, especially the magnetic permeability (hereinafter referred to as μ
) has decreased significantly, so strain relief sintering is necessary. Even when this annealing is performed in air, since the sintered body has an extremely high density, only a very small amount of α-Fe2O is precipitated on the surface layer of the sintered body, so it is extremely easy. Embodiments of the present invention will be illustrated below based on Examples.

EXAMPLE The MnZn ferrite produced by the method of the present invention is suitable for high μ materials used in the high frequency region, that is, 1 MHz or higher, especially 5
The main objective is to produce high-density materials with μ≧800 at MHz.

For this reason, three types of compositions shown in Table 1 were selected from the composition range of MnZn ferrite in consideration of the snake's limit, magnetostriction constant, and anisotropy constant. All compositions below are indicated by sample numbers. Table 2 shows the raw material ZnO for producing 1.5k9 of these MnZn ferrites. Mn
An example of weighing cO3 and α-Fe2O3 is shown. Next, a preliminary sintered body was produced by a conventional dry method. In addition to the following, the raw materials may be those that can be easily converted into oxides by heating, such as oxalates, nitrates, sulfates, oxyhydroxides, chlorides, and hydroxides. The manufacturing conditions are as follows. Calcining was carried out in air at 900°C ± 1'Cll time. Mixing and grinding were carried out in a vibratory mill using pure water as a medium for 4 and 6 hours, respectively. Molding is done using a hydraulic press at 5t/~30m7! LWX4OmlLXl
27n7! A block of L was molded. Pre-sintering in air from room temperature to 800℃, 1200 to 130℃ above 800℃
Raise the temperature to 0℃ in vacuum (~104u1Hg) or raise the temperature and hold it, then continue to raise the temperature in N2 or air and then hold it in air.If the sintering temperature is low, hold it in vacuum and then hold it in the same temperature atmosphere. was then switched to air and continued under each holding condition. Cooling was done in N2.
The holding time during preliminary sintering is 4 to 10 hours, and the temperature variation at this time is ±5°C or less. Of the pre-sintered bodies thus obtained, the microstructure photograph of composition 1 is
As shown in the figure.

The pre-sintering conditions at this time were: 1250°C x 4 hours in vacuum;
1300°C in air for 5 hours. From the figure, it can be seen that there are a large number of pores. Next, these preliminary sintered bodies were heated in a temperature range of 55 to 75% of the melting point of MnZn ferrite ('::100'C-130'C) and 500 to 15% of the melting point of MnZn ferrite.
Hot isostatic pressing was carried out in an Ar atmosphere under a pressure of 0.000 atm. The holding time was 1 hour in all cases. The results for Sample 2 are shown in FIG.

As is clear from the figure, densification of 99.5% or more can be achieved at 1250°C or higher at 500 atm, 1150°C or higher at 1000 atm, and 11000C or higher at 1500 atm. Samples 1 and 3 also show almost the same tendency as shown in FIG. 3, and the achieved sintered densities are also comparable.
However, the maximum holding temperature during pre-sintering is 135
At temperatures above 0°C, pores remain in the center of the sample, especially at 100°C.
When HIP is performed at 0 atmospheric pressure and above 1100° C., grain growth near the sample surface is remarkable, and the grain size reaches 2 to 31 m on the sample surface. Also, the pre-sintering temperature is 122
If the temperature is 0° C. or higher, the same tendency as shown in FIG. 3 is exhibited, and sufficient densification can be achieved. On the other hand, the magnetic properties after HIP deteriorate significantly at μ, and μ? between 100 KHz and 5 MHz. It is about 300 to 500, and strain relief annealing is required. The reason why d/Dx is 100% or more in FIG. 3 is that the portion near the sample surface is partially reduced. As is clear from FIG. 3, when Pext pressure is 500 atm, the theoretical density (Dx=5.
A densification corresponding to 0.085 g/~, AO28.4898) can be achieved.

On the other hand, when Pext=1500 atm, densification clearly progresses from about 850° C., which corresponds to about 50% of the melting point. Also, if you want to hold it for a long time. It is expected that densification will proceed even if the HIP temperature is lower than the above. The temperature during HIP has an extremely large effect on the crystal grain size of MnZn ferrite, similar to the sintering temperature, so compositions other than those mentioned in the present invention may need to be held at a low temperature for a long time during HIP. It makes me think of a child enough. In the composition of the present invention, when limited to Pext'::500 to 1500 atmospheres and a holding time of 1 hour, the optimum HIP temperature is 6
1000-126'C, which corresponds to 5% to 75%. As a result of detailed study on temperature, atmosphere, etc. for strain relief annealing, it was found that the material of the present invention has a temperature of 1050 to 11
After being kept in air for 2-5 hours at a temperature of 50°C, N2
It was found that by cooling inside, it has extremely excellent magnetic properties. However, the strain relief annealing conditions for obtaining excellent magnetic properties vary slightly depending on the composition, pre-sintering conditions, and HIP conditions, and therefore must be selected in accordance with the pretreatment conditions. In addition, the sintered body obtained by the method of the present invention has the following properties in the preliminary sintering process:
Since the material undergoes vacuum sintering, the pressure in the internal pores varies considerably; therefore, even after strain relief annealing, no expansion of pore volume due to residual gas pressure is observed, and d/Dx does not change before and after annealing.
Figures 2A and 2B show the pore distribution state of sample 3 and the microstructure of sample 2 after strain relief annealing, respectively. Preliminary sintering of sample 3 was conducted at 1250°C x 4h in vacuum, 1300'C x 5h1 in air, and 1250°C x 100°C for sample 2.
4h1 air 125'C x 5h each. 1st
As is clear from the figure and FIG. 2, the sample subjected to post-HIP annealing is extremely densified compared to the sample before HIP, and the relative density is almost 100%. Of the materials produced under various manufacturing conditions by the method of the present invention, pre-sintering conditions (125"CX4 in vacuum")
time + 1300°C in air x 5 hours), HIP conditions (Ar
Average grain size 20-30 μ, d/DxC: 99.8 obtained during strain relief annealing (1100°C in air for 3 hours, cooling in N2)
The frequency characteristics of μ for materials 1, 2 and 3 with a content of ~99.9% are shown in FIG.

In addition, the inner diameter is 4mmφ and the outer diameter is 8m! φ, thickness 0.16
A ring-shaped sample measuring 5m71 with a mirror finish on both sides was wound 10 times, and the test was carried out at a temperature of 20°C. For comparison, FIG. 5 shows the frequency characteristics of μ for a material of the same composition densified to about d/Dx=98 to 99% by ordinary vacuum sintering. In addition, the average crystal grain size at this time is 80 to 100μ
It is. As is clear from a comparison of FIGS. 4 and 5, the MnZn ferrite obtained by the method of the present invention has a larger μ value especially at frequencies of 1 MHz or higher than those produced by ordinary sintering. This tendency is remarkable in composition 2, and 5MH
μ at z shows an extremely large value of 1050 to 1100. Further, since the optimum manufacturing conditions according to the method of the present invention differ depending on the composition, it is expected that the μ characteristics of compositions 1 and 3 can be sufficiently improved if manufacturing conditions suitable for each composition are strictly selected. Figure 6 shows the B of each composition shown in Figure 4.
-H curve. In the figure, 61 is the value of magnetic flux density measured in a magnetic field of 150e, and 62 is a curve for measuring coercive force. As is clear from the figure, B: 14800-52
In particular, composition 2, which was obtained under conditions extremely close to the optimum manufacturing conditions, is a high frequency, high μ material, and at the same time has a high B
You can see that it is made of wood. Furthermore, if the method of the present invention is used, the average crystal grain size is smaller at 20 to 30μ compared to 80 to 100μ in the normal sintering method, so it can be used in higher frequency ranges where the shape effect of crystal grains becomes a problem. It can be seen that it is extremely promising as a material to be used. Incidentally, the material of the present invention has an extremely high value of μ≧500 even at 15 MHz. From the above, if the method of the present invention is used, MnZn ferrite having a small crystal grain size and extremely excellent magnetic properties can be produced in a state almost equal to the theoretical density.

In addition, the raw materials used in the present invention are ordinary inexpensive industrial raw materials, and the ordinary dry method is applied for sintering and annealing, and the preliminary sintering temperature is 100°C compared to the ordinary sintering temperature.
A moderately low temperature is sufficient. Furthermore, there is no need to cover the molded body with alumina or the like, unlike in normal hot pressing, and solid phase reactions that often lead to property deterioration do not occur during hot pressing.
In addition, since the hot pressing is hydrostatic, it is possible to easily densify complex-shaped materials and large-sized materials without causing cracks or the like. It has many features as described above, and if ordinary commercially available HIP is used, it is possible to manufacture about 15 kg of material at a time, making it suitable for mass production as a material for extremely small magnetic heads, etc. The material yield is also very good.

Therefore, the method of the present invention makes it possible to easily mass-produce MnZn ferrite having excellent properties from inexpensive raw materials, and has extremely great industrial value from the standpoint of resource conservation.

[Brief explanation of the drawing]

Figure 1 is a microstructure photograph of composition 1 of MnZn ferrite obtained by pre-sintering including vacuum sintering in the method of the present invention, and Figure 2 is a microstructure photograph of MnZn ferrite produced by the method of the present invention. Among them, vacancy distribution A of composition 3 and microstructure photograph B1 of composition 2 Figure 3 shows the MnZn produced by the method of the present invention.
Relative density d of ferrite under HIP conditions for composition 2
FIG. 4 is a diagram showing changes in μ of MnZn ferrite produced by the method of the present invention, and FIG.
The figure is a frequency characteristic diagram of μ of MnZn ferrite manufactured by the normal sintering method, and Figure 6 is the μ frequency characteristic diagram of MnZn ferrite manufactured by the method of the present invention.
It is a BH curve diagram of n-ferrite.

Claims (1)

[Claims] 1. A preliminary sintering process in which part of the temperature raising process or holding process is carried out in a vacuum, and a densification process by hot isostatic pressing under a pressure of 500 to 1500 atm and in a temperature range of 1000 to 1260°C. , and a sintering process carried out at the hot isostatic pressing temperature or a temperature lower than that.
Method for manufacturing nZn ferrite.