JPH0747508B2 - High-density oxide sintering method - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an improved method for high density sintering of oxides, in particular hot isostatic pressing.

Prior art and its problems In recent years, the so-called rubber press method, in which powder is placed in a flexible bag such as rubber or plastic, sealed and molded by uniformly applying pressure from the surroundings, instead of the molding method by a die press Has become widely used. However, in the rubber pressing method, the powder is simply compacted at room temperature, and a step of further sintering the compact is required to form the sintered compact.

As a method of performing these two steps at the same time, a hot isostatic pressing method (hereinafter referred to as HI
The abbreviation "P" was developed, and recently, the use of HIP for the production of ceramic materials has been rapidly progressing.

On the other hand, among various oxide-based ceramic materials such as ferrite, those requiring sintering in an oxidizing atmosphere, or
In many cases, the characteristics change depending on the oxygen concentration in the atmosphere during sintering.

As a means for coping with this, a method has conventionally been adopted in which a pre-sintered body of an oxide ceramics material is embedded in the same material powder to adjust the necessary oxygen concentration and HIP treatment is performed (Japanese Patent Laid-Open No. Sho. 48-70717, 53-51217, etc.).

However, in such a method, the cost of the powder itself is not negligible, embedding in the powder, it takes time to take out from the powder, the feeding of the pressure medium gas, the powder is scattered at the time of recovery, the heating element, There are problems such as damage to furnace materials.

Various methods for HIP treatment in an inert gas partially containing oxygen have been studied as means for solving this, but none of them have been successful.

This is because, for example, in the sintering of Mn-Zn ferrite, in addition to the target spinel structure, an altered layer such as a hematite structure is generated, or a crack is generated in the sintered body, which is not practical.

II Object of the Invention The object of the present invention is to carry out HIP of an oxide ceramic material that is sensitive to the atmospheric oxygen concentration during sintering.
It is an object of the present invention to provide a high-density sintering method capable of obtaining a sintered body having extremely good physical properties.

III DISCLOSURE OF THE INVENTION Such an object is achieved by the present invention described below.

That is, the present invention is a high-density oxide sintering method in which a pre-sintered body is housed in a container and is sintered under pressure in an atmosphere gas. The oxygen partial pressure (%) is set to be smaller than the equilibrium oxygen partial pressure (%) at the holding temperature under normal pressure, and the oxygen concentration in the atmosphere gas at the temperature raising and lowering stages is set to the atmospheric gas at the temperature holding stage. This is a method for high-density sintering of oxides, which is characterized in that the oxygen concentration is not more than the inside.

IV Specific Structure of the Invention Hereinafter, the specific structure of the present invention will be described in detail.

The high-density sintering method of the present invention is carried out using a hot isostatic pressing apparatus (hereinafter abbreviated as HIP apparatus) 1 as shown in FIG. 1, for example.

In this case, the powder is molded into a predetermined shape in the high-pressure container 2 and the pre-sintered body 3 pre-sintered is carried and heated by the heating element 4,
The heating temperature is detected and controlled by the thermocouple 5.

Then, a high pressure gas pressure inlet 6 and a high pressure gas discharge port 7 are used to press a predetermined atmosphere gas into the high pressure container 2 or discharge the gas from the high pressure container 2 to control the atmosphere during sintering.

The high-density sintering method of the present invention, as shown in FIG.
A series of sintering operations are divided into three stages to adjust the ambient temperature and the ambient gas.

The first stage is a temperature raising stage. Here, the temperature inside the high-pressure container 2 is raised from room temperature to near the conventional holding temperature. The atmosphere gas is an inert gas or the like, and has an oxygen concentration equal to or lower than the oxygen concentration in the temperature holding step described later.

The inert gas may be composed of one kind or two or more kinds of Ar, He, N _{2 and the} like, and is preferably Ar alone.

The oxygen partial pressure is preferably 0.005 vol% or less, and particularly 0.001 vol% or less in the case of Mn-Zn type ferrite.

This is because if it exceeds 0.005 vol%, the pre-sintered body is oxidized.

The temperature rising rate in the high-pressure vessel 2 in this first stage is 2 to 7
C./min., Especially in the case of Mn-Zn type ferrite, etc., about 3-5.degree. C./min. In this case, the temperature rise may or may not be linear as shown.

Further, the pressure here may be equal to or lower than the pressure at the temperature holding stage described later. More specifically, generally, the temperature is started to be normal pressure or 300 Kgf / cm ² or less, and at the time of shifting from the first step to the second step, an atmospheric gas having a predetermined oxygen concentration, which will be described in detail later, is pressed in to a predetermined value. Will be the pressure of.

If the pressure in the first stage exceeds 300 Kgh / cm ² , cracking will occur.

In such a case, the rate of pressure increase may be about ³ Kgf / cm ² · min.

The end temperature of the first stage is generally about 800 to 1000 ° C, and is about 800 to 1000 ° C for Mn-Zn type ferrite.

Then, as a second step, the temperature holding step is performed.

Here, as described above, the pressure is applied in the atmosphere having a predetermined oxygen concentration.

Pressurization pressure is generally about 300-1500 Kgf / cm ² , especially 500-100.
It should be about 0 kgf / cm ² .

In the temperature profile in the second stage, the temperature is raised from the end temperature of the first stage to maintain the temperature, and then the temperature is lowered.

The rate of temperature rise is generally about 2 to 5 ° C./minute.

The holding temperature, that is, the sintering temperature may be appropriately determined depending on the material and application, but is generally about 1100 to 1300 ° C, particularly 11000 to 1250 ° C for ferrite, and about 1150 to 1200 ° C for Mn-Zn ferrite.

The holding time is generally 1 to 4 hours, especially 2 to 2 for ferrite.
3 hours.

Further, the temperature lowering rate is generally about 2 to 5 ° C./minute.

The atmosphere in the second stage is the above-mentioned inert gas, and the oxygen partial pressure (%) is equal to or less than the equilibrium oxygen partial pressure (%) at the holding temperature at normal pressure (1 atm).

The equilibrium oxygen concentration at 1 atm for each holding temperature has been extensively studied for each material and composition, and is described in various documents.

For example, regarding ferrite having a spinel structure such as magnetite, Mn-ferrite, Mn-Zn ferrite, Ni-ferrite, and Ni-Zn ferrite, the stoichiometric composition and the composition deviated by a predetermined amount from it are described in various documents in various documents. Has been done.

For example, US Patent No. 3027327, IEEE Transactoins on
Magnetics, vol.Mag-11 No.5 September 1975 P1312 ~ 13
14, Joernal of the American Ceramic Society vol.64
No. 7 P419-421 etc.

The equilibrium oxygen partial pressure can be easily determined by using the numerical values directly described in these documents, by calculation from the values described therein, or by the experiment according to the method described therein.

During the second stage, pressurization is performed with an atmosphere gas of an inert gas containing oxygen at an oxygen partial pressure lower than the equilibrium oxygen partial pressure. In such a case, since oxygen is consumed during sintering, it is preferable to replace the atmospheric gas in order to keep the amount of oxygen constant.

As a method of gas replacement, depressurize by degassing to, for example, 10 to 20% of the pressurization pressure in the second step, and then depressurize the inert gas containing oxygen near the initial oxygen concentration to the initial pressurization pressure. Just press down.

In this case, the number of times of gas replacement may be appropriately determined depending on the container capacity, the amount of sintered body, the pressure, the temperature holding time, etc., and is usually about 1 to 5 times.

When replacing the gas, the amount of oxygen in the container may be monitored and controlled. The oxygen partial pressure in the second stage is preferably controlled within a fluctuation range of about ± 30%.

Further, in order to control the oxygen gas partial pressure, a solid electrolyte cell used as a so-called oxygen pump may be arranged in the container.

Therefore, in such a case, the oxygen partial pressure in the second stage is preferably 0.3 to 5%, more preferably 1% to 4% of the equilibrium oxygen partial pressure under normal pressure described above.

If this value exceeds 5%, oxidation of the pre-sintered body occurs, and
If less than 0.3%, it is reduced.

Therefore, the oxygen partial pressure in the second stage is generally 0.01 to 0.05 vo
It is about l%. More specifically, for example, for Mn-Zn ferrite, the oxygen partial pressure is more preferably about 0.03 to 0.05 vol% and the oxygen pressure is about 0.3 to 0.5 Kgf / cm ² .

After this, the process moves to the third stage.

The third stage is a temperature lowering stage, and the temperature is generally lowered from the final temperature of the second stage to room temperature to about 300 ° C. The rate of temperature decrease is about 3 to 10 ° C / min.

The atmosphere gas in the third stage should be an inert gas alone or the like with an oxygen concentration equal to or lower than that in the temperature holding stage.

The inert gas in this case may be the same as the inert gas used in the first stage. And the oxygen partial pressure is 0.
005vol% or less, especially 0.0 for Mn-Zn series ferrite
01 vol% or less is suitable. If it exceeds 0.005 vol%, oxidation of the sintered body occurs.

The atmospheric pressure should be below the pressure of the temperature holding stage, and
Normal pressure to 500 Kgf / cm ² or less.

If it exceeds 500 Kgf / cm ² , cracks may occur.

The pressure drop rate is about 4 Kgf / cm ² · min.

The temperature lowering rate in the high-pressure container 2 is about 2 to 5 ° C / minute.

Thus, in particular, by setting the oxygen partial pressure in the atmosphere to be smaller than the equilibrium oxygen partial pressure of atmospheric pressure sintering in the second step,
For example, in the sintering of oxides such as ferrite such as Mn-Zn system, which is sensitively affected by the oxygen content in the atmosphere during sintering,
Very good Mn-Zn with no altered layer or cracks
Ferrite is obtained.

In the present invention, prior to such sintering, the powder is first subjected to pressure molding, mud firing, etc., according to a conventional method. And
This is pre-sintered. At the time of molding, various known pretreatments according to each material are performed.

The pre-baking is performed by a method generally used as a pre-baking method for HIP ceramics.

The pre-sintered body that can be sintered according to the present invention is an oxide or the like, and various ferrites, titanic acid-based ceramics and the like are available.

The ferrite can be applied to various materials such as Mn-Zn series and Ni-Zn series.

In addition, although Mo is usually used for the heating element 4 of the apparatus, Fe-Cr-Al alloy or the like may be used.

A known device may be used for the HIP method.

V Specific Actions and Effects of the Invention In the high-density sintering method of the present invention, a series of sintering operations is divided into three stages, and in particular, in the second stage in which the sintering temperature is maintained, the oxygen partial pressure in the atmosphere is sintered under normal pressure. It is regulated to be lower than the equilibrium oxygen partial pressure in.

As a result, it is possible to obtain a very good sintered body that does not have an altered layer or cracks even if it is, for example, Mn-Zn ferrite that is sensitively affected by the oxygen content in the atmosphere during sintering. The unexpected effect was realized.

Further, as a process before and after sintering, it is not necessary to embed the pre-sintered body in the powder and take out the sintered body from the powder as in the conventional process. In addition, a heating element due to the scattering of powder,
The furnace material is not damaged. Therefore, according to the present invention, high-density sintering can be made extremely easy and at low cost.

VI Specific Examples of the Invention Hereinafter, the present invention will be described in more detail with reference to Examples of the present invention.

Example As a powder material, 54 mol% in terms of Fe ₂ O ₃ , 30 mol% in terms of MnO, Zn
16 mol% O-converted Mn-Zn ferrite powder was prepared. This powder was molded by isostatic pressing, and then pre-sintered by a normal atmosphere pre-baking method.

Then, it was sintered in a hot isostatic pressing apparatus as shown in FIG. 1 according to the temperature profile shown in FIG.

In this case, the first stage heating rate is 4.5 ° C / min, the second stage heating temperature is 2.5 ° C / min, the temperature holding time is 120 minutes, the second stage cooling rate is 2.5 ° C / min, and the third stage cooling rate is 4.5 ° C / min. ℃ / min, finish temperature 300 ℃. The other conditions were as shown in Table 1. The initial pressure in the first stage and the final pressure in the third stage were 250 Kgf / cm ² .

For sample Nos. 6 to 10, the oxygen partial pressure in the second stage is ± 20%.
Controlled within.

As for the magnetic characteristics of the Mn-Zn ferrite sintered compacts sample Nos. 1 to 10 obtained by sintering in this way, the residual magnetic flux density (B
r), coercive force (Hc), frequency 500Hz, permeability at 5MHz (μ
₅₀₀ , (μ _5M ) was measured. Also, as physical properties, apparent density (SD), fracture toughness value ( _Kic ), bending strength (F)
Was measured. In addition, the presence or absence of an altered layer other than the spinel structure of the sintered body and the occurrence of cracks were examined.

The results are shown in Table 2.

From the results shown in Table 2, the effect of the present invention is clear.

[Brief description of drawings]

FIG. 1 is a sectional view showing an example of a hot isostatic pressing apparatus used in the high-density sintering method of the present invention. FIG. 2 is a diagram showing the change with time of the atmospheric temperature in the high-density sintering method of the present invention. Explanation of symbols 1 ... Hot isostatic pressing device 2 ... High pressure container 3 ... Preliminary sintered body 4 ... Heating element 5 ... Thermocouple 6 ... High pressure gas inlet 7 ... High pressure gas outlet

Claims (6)

[Claims] 1. A method for high-density sintering of an oxide in which a pre-sintered body is housed in a container and sintered under pressure with an atmospheric gas, wherein the temperature of the atmosphere during the temperature holding step during sintering is kept in the atmosphere gas. The oxygen partial pressure (%) is set to be smaller than the equilibrium oxygen partial pressure (%) at the holding temperature under normal pressure, and the oxygen concentration in the atmosphere gas at the temperature raising and lowering stages is set to the atmospheric gas at the temperature holding stage. A method for high-density sintering of oxides, characterized in that the oxygen concentration is not more than the inside. 2. The oxygen partial pressure (%) in the atmosphere of the temperature-holding stairs.
Is the equilibrium oxygen partial pressure of 0.3 to 5 at the holding temperature under normal pressure.
% High density sintering method of oxide according to claim 1. 3. The method for high-density sintering of oxide according to claim 1, wherein the oxide has a spinel structure. 4. The high-density calcination of an oxide according to claim 1, wherein the atmosphere gas is replaced with an atmosphere gas containing oxygen at a predetermined concentration in the temperature holding step. How to tie. 5. The method for high-density sintering of an oxide according to any one of claims 1 to 4, wherein the pressure in the temperature holding step is 300 to 1500 kgf / cm ² . 6. The method for high-density sintering of an oxide according to claim 1, wherein the holding temperature in the temperature holding step is 1100-1300 ° C.