JP2511043B2 - Manufacturing method of contact alloy for vacuum valve - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Object of the Invention] (Industrial field of application) The present invention relates to a method for producing Cu and Ag-Cr alloys having few gas and pores (holes), and particularly to the frequency of re-ignition. The present invention relates to a method for manufacturing a contact alloy for a vacuum valve, which has improved low surge characteristics.

(Prior Art) The main characteristics required for a contact for a vacuum valve are welding resistance, withstand voltage, and high breaking property.

However, since these three requirements require contradictory physical properties, it is difficult to ideally balance these requirements. At present, we are responding with a slight sacrifice.

For example, conventionally, there is known a high-breakdown-voltage, large-capacity vacuum circuit breaker equipped with a Cu alloy containing 5% by weight or less of a welding prevention component (Bi, Te, Pb, etc.) as an electrode contact (Japanese Patent Publication No. 41-12131).

However, in recent years, the demand for higher voltage has not been sufficient in terms of withstand voltage.

In other words, the vacuum circuit breaker has advantages such as small size, light weight, and maintenance-free environmental harmony as compared with other circuit breakers, so its application range is expanding year by year, and the circuit of 36KV or less, which has been generally used conventionally, is used. Since it is applied to a higher voltage circuit, the demand for opening and closing a special circuit, for example, a capacitor circuit is rapidly increasing. Therefore, higher withstand voltage is required.

The reignition phenomenon is one of the important factors that hinder the achievement of the reignition phenomenon.

Although the re-ignition phenomenon is regarded as important from the viewpoint of improving the reliability of products, neither the prevention technology nor the direct cause has been clarified.

With the increase in the withstand voltage, the contact material is also required to have a higher withstand voltage and a characteristic that the re-ignition phenomenon is less likely to occur.

In order to achieve high withstand voltage and non-re-ignition of contact materials, minimize the amount of fragile anti-welding components that are defects in pressure resistance, avoid excessive concentration, and avoid gas impurities and pins. It is desirable to minimize holes and the like, increase the strength of the contact alloy itself, and so on.

From these viewpoints, the aforementioned Cu-Bi alloy is not satisfactory.

Although other contact materials that have been used conventionally, Cu-W contacts or Cu-WC contacts, are quite excellent in terms of withstand voltage, this sintered contact alloy has bubbles remaining due to the manufacturing method. In addition, it has a drawback that the re-ignition phenomenon is likely to occur because the emission of thermoelectrons is active.

On the other hand, in fields requiring high breakdown voltage and large current interruption,
Cu-Cr alloys are being applied. The Cu-Cr alloy has the advantage that, unlike other contact materials, the difference in vapor pressure between the constituent elements is small and uniform performance can be expected. Depending on how it is used, its characteristics can be fully utilized. Is.

This Cu-Cr diameter contact alloy is generally manufactured as follows. For example, according to Japanese Examined Patent Publication No. 59-30761, Cr powder and a small amount of Cu powder are mixed, the mixed powder is filled in a die mold, and a small pressure is applied to press-mold the molded product from the die mold. After taking it out, there is a method of vacuum-sintering this to form a Cr skeleton and finally infiltrating Cu.

Recently, Cr powder was poured into the mold and
A method is also disclosed in which Cu pellets are placed, the whole is degassed, and then infiltration treatment is performed under reduced pressure (Japanese Patent Laid-Open No. 59-2.
(See Japanese Patent No. 5903).

Furthermore, a method of obtaining a Cu-Cr alloy by mixing Cu and Cr, which are the final target values from the beginning, and by solid-phase sintering the molded body obtained from this at the melting point of Cu or below .

On the other hand, the Ag-Cr alloy is also produced by a method similar to that of the Cu-Cr alloy. That is, Cr skeleton and Ag as infiltrant
This is a method in which the voids in the skeleton are filled with Ag by heating while closely contacting and.

(Problems to be solved by the invention) However, in the above-mentioned Ag-Cr alloy in particular, it is generally manufactured by the powder metallurgy method, and its raw material powder management, sintering technology, and melting that are involved in re-ignition generation are involved. There is a problem that the frequency of re-ignition is not yet satisfactory because the immersion technology is not well established.

That is, in the material of the Ag-Cr alloy, undesired residual pores and a large amount of residual gas due to this are unavoidably observed, and as a result, the above-mentioned re-ignition occurs and variations in the cutting characteristics are recognized. Be done.

The present invention has been made in view of the problems associated with the above-mentioned prior art, and provides a method for producing a contact alloy for a vacuum valve in which re-ignition occurrence frequency is significantly reduced and cutting characteristics are improved. The purpose is to do.

[Structure of Invention]

(Means and Actions for Solving Problems) The inventors of the present invention have made a skeleton material and an infiltrant material in order to reduce the frequency of re-ignition of contact alloys for vacuum valves and to stabilize the cutting characteristics. As a result of examining the highly conductive material to be used as the material and further examining the sintering conditions and the infiltration conditions, it was confirmed that the skeleton composed of the arc-resistant material was previously mixed with the Ag-Cu-based material having the eutectic ratio. It was found that the wettability of the conductive material component to the skeleton in the infiltration step is significantly improved. The present invention has been made based on the above findings.

That is, the method for producing a contact alloy for a vacuum valve according to the present invention comprises a conductive material component 25 to 75% by weight consisting of Cu and Ag, and an arc resistant material component 75 consisting of Cr and / or Ti.
.About.25% by weight, and Cu of the conductive material component.
A method for manufacturing a contact alloy for a vacuum valve, wherein Ag and Ag form a eutectic structure in the alloy, comprising the steps of (a) to
It is characterized by including (c).

(B) Ag-C having a eutectic ratio in the arc-resistant material powder
a step of forming a mixed powder obtained by previously mixing u-based powder in an amount of 2 to 30% by weight based on the total amount of the conductive material component and the arc-resistant material component, (b) the obtained formed body A step of sintering to obtain a skeleton, and (c) a step of infiltrating a conductive material component into the voids in the obtained skeleton.

According to the method of the present invention, Ag-Cr (and / or Ti)
In the production of alloys based on Cr (and / or Ti), since Ag-Cu, which is a eutectic component, is added to the skeleton of Cr instead of pure Ag as a pre-mixing material, the wettability to Cr is only Ag. It is possible to produce Ag-Cr based contact alloys with less pores and less gas, which is improved over time.

The Cu-50wt% Cr alloy currently used as a contact material for high withstand voltage and large current interruption has a drawback that the breaking current value is high and its variation range is large although the frequency of re-ignition is low.

On the other hand, the Ag-Cu alloy exhibits better breaking properties than the Cu-Cr alloy due to the contribution of about two orders of magnitude higher vapor pressure difference that Ag has compared with Cu.

However, it is difficult to manufacture this Ag-Cr alloy on an industrial scale by the method of infiltrating Ag into a pure Cr skeleton.

Further, in order to improve this problem, a method of infiltrating Ag into a Cr skeleton in which Ag is pre-blended in advance is conceivable, but by this method as well, similar to the above, the production of Ag-Cr alloy is industrially performed. Tend to be difficult.

Thus, the Ag-Cr alloy, which is superior to the Cu-Cr alloy in cutting properties, has a problem in industrial production because it is difficult to infiltrate Ag into the Cr skeleton.

According to the findings of the inventors, it has been found that the Cr surface is contaminated by gas released from Ag at the time of heating, and that wetting of Cr and Ag is inhibited.

Furthermore, the present inventors have made a detailed observation on the total amount of gas released in the process of heating the contact material and the morphology of the release, and there is an important correlation between these factors and the occurrence of the re-ignition phenomenon, In particular, it has been found that the reignition phenomenon can be effectively suppressed by controlling the release of these gases, particularly the release of gas that is suddenly generated near the melting point, for each of the raw materials that make up the contact material.

That is, when the contact material is heated, most of the adsorbed gas is degassed below the melting point and the solid solution gas is released near the melting point. Although it is (for example, about several milliseconds), a pulse-like burst gas release (several to hundreds of bursts) is observed.

These burst gases contain a small amount of C ₂ H ₂ , CH _4, etc., but since these are mainly oxygen systems such as CO, CO ₂ , O ₂ etc., these burst gases are oxides contained in the contact material. It is considered to be released by decomposition of.

According to the research conducted by the present inventors, the contact material that frequently causes the re-ignition phenomenon also releases a large amount of sudden gas.

Therefore, based on the above knowledge, it is possible to reduce the occurrence of the re-ignition phenomenon by holding the contact material at a temperature equal to or higher than its melting point and releasing this sudden gas in advance.

By the way, when such observations were made on Cu and Ag, it was interesting that a difference was observed in the gas release behavior between the two. That is, most of the gas contained in Cu is recognized near the melting point of Cu, but the behavior of gas discharge of Cu and Ag is different such that the gas continues to be released even above the melting point of Ag.

Based on these findings described above, the inventors of the present invention are a pre-blending material and an infiltrant for improving the wetting between Ag and Cr.
It was found that by substituting a part of Ag for Cu, Cu can be improved by the effect of wetting Cr.

Furthermore, by making the ratio of Ag and Cu eutectic, the wettability was improved and the flowability was also improved, making it possible to manufacture a sound alloy and improving the breaking characteristics.
Further, the above-mentioned improvements were similarly observed when the arc-resistant material component was composed of Ti.

The arc-resistant material component in the present invention is made of Cr, but if necessary, a part or all of it can be replaced with Ti.

The contact alloys obtained by the method of the invention are Cu and
25 to 75% by weight of a conductive material component made of Ag and 75 to 25% by weight of an arc resistant material component made of Cr and / or Ti, and Cu and Ag of the conductive material components are contained in the alloy. It is composed of crystal structure.

 Hereinafter, the method of the present invention will be described according to steps.

First, the arc-resistant material powder has a eutectic ratio in advance.
Ag-Cu powder is blended to obtain a mixed powder. Ag- in this case
The Cu-based powder may be a mixed powder of Ag powder and Cu powder or a powder of Ag-Cu alloy. Further, the mixing ratio of the Ag-Cu-based powder needs to be 2 to 30% by weight with respect to the total amount of the contact alloy material, that is, the total total amount of the conductive material component and the arc resistant material component. If it is less than 2%, a sufficiently uniform mixed state with the arc resistant material cannot be obtained due to the mixing technique, and the expected effect of improving the leak resistance to Cr is small. On the other hand, when it is 30% or more, unevenness occurs in the material after infiltration, which is not preferable.

It is preferable to form a molded body from the prepared metal powder according to a conventional method under an external pressure of, for example, 8 ton / cm ² or less or the pressure of the weight of the metal powder.

The molded body thus obtained is placed in a heating furnace together with a sintering container and sintered. The sintering atmosphere needs to be a non-oxidizing atmosphere, such as vacuum or hydrogen. Among these atmospheres, a vacuum (1 × 10 ⁻⁵ Torr or more) atmosphere is preferable in terms of removing filled metal powder, oxygen and nitrogen stored in a pressed compact or container.

Sintering is preferably performed in the temperature range of 600 to 1000 ° C. Next, the conductive material component is infiltrated into the voids in the obtained skeleton. That is, Cu and Ag, which are infiltration materials, are placed on the upper surface and / or the lower surface of the skeleton, and the whole is heated, for example, in a vacuum (1 × 10 ⁻⁴ to 1 × 10 ⁻⁶ TOrr).
Infiltrate Cu and Ag into the skeleton voids.

The temperature at the time of infiltration is a temperature above the melting points of Cu and Ag. Further, the infiltration time is set to a time sufficient for completely impregnating the voids in the skeleton with these melts.

In the case of the present invention, infiltration is preferably carried out in the temperature range of 900 to 1200 ° C. The infiltration step is preferably performed in a non-oxidizing atmosphere.

After the above infiltration, the contact piece is cooled and further processed into a predetermined shape according to a conventional method.

(Examples) Hereinafter, the present invention will be described based on Examples. First, the structure of a vacuum valve to which a contact material according to the method of the present invention is applied will be described with reference to FIGS. 1 and 2.

FIG. 1 shows an example of the structure of a vacuum breaker to which the contact material according to the present invention is applied. In FIG. 1, reference numeral 1 denotes a breaking chamber, and this breaking chamber 1 is made of an insulating material to have a substantially cylindrical shape. Formed insulating container 2 and sealing metal fittings 3a, 3b on both ends
It is configured to be vacuum-tight with the metallic lids 4a and 4b provided via. In the shut-off chamber 1, the conductive rod 5,
A pair of electrodes 7 and 8 attached to opposite ends of 6 are arranged, with the upper electrode 7 being a fixed electrode and the lower electrode 8 being a movable electrode. A bellows 9 is attached to the electrode rod 6 of the movable electrode 8 to enable the electrode 8 to move in the axial direction while keeping the inside of the cut-off chamber 1 vacuum-tight. Further, a metallic arc shield 10 is provided above the bellows 9 to prevent the bellows 9 from being covered with arc vapor. Reference numeral 11 denotes a metallic arc shield provided in the shut-off chamber 1 so as to cover the electrodes 7 and 8 to prevent the insulating container 2 from being covered with arc vapor. Further, as shown in an enlarged manner in FIG. 2, the electrode 8 is fixed to the conductive rod 6 by a brazing portion 12 or is crimp-connected by caulking. The contact 13a is brazed to the electrode 8 1
It is fixed at 4. In FIG. 1, reference numeral 13b denotes a fixed contact.

The contact material of the present invention is suitable for forming both or either of the contacts 13a and 13b as described above.

Comparative Example-1 Cr with an average particle size of 125 μm was molded at a pressure of ² ton / cm ² ,
The obtained molded body is housed in a carbon container, and is temporarily sintered in vacuum at 850 ° C. for 1 hour.

An infiltrant made of Ag-Cu, which is a eutectic component, was placed on the upper side of the pre-sintered body, and then the alloy was obtained by moving to an infiltration step of holding it at 1100 ° C for 1 hour in vacuum.

As shown in Table 1, the alloy of Comparative Example-1 was difficult to infiltrate and could not be electrically evaluated.

Comparative Examples-2 to 3 and Examples-1 to 3 As in Comparative Example-1, Cr powder having an average particle diameter of 125 μm is prepared. Eutectic Ag-Cu alloy powder with a similar particle size was added to this, mixed with a ball mill, and the molding pressure was applied by pressing, and the resulting compact was stored in a carbon container and kept in vacuum at 850 ° C for 1 hour. Temporary sintering is performed by holding. An infiltrant consisting of Ag-Cu, which is a eutectic component, is placed on the upper side of this pre-sintered body, and then moved to the infiltration step, which is held at 1100 ° C for 1 hour in vacuum, and the Got

Comparative Example-4 Cr powder having an average particle size of 125 μm is prepared in the same manner as above. C to this
After u is added and mixed in a ball mill, a molding pressure is applied by a press, the obtained molded body is housed in a carbon container, and temporary sintering is performed by holding at 1000 ° C. for 1 hour in a vacuum. A Cu infiltration material is placed on the upper side of this pre-sintered body, and then the infiltration process is performed by holding it at 1100 ° C. for 1 hour in vacuum.
An alloy was obtained.

The alloys produced in Comparative Examples-2 to 4 and Examples-1 to 3 were processed into a predetermined contact shape, which was attached to a detachable test device, and the breaking current characteristic and the re-ignition characteristic were evaluated.

 The results are shown in Table 1.

In the production of Ag-50wt% Cr alloy, when the proportion of highly conductive material pre-blended in the arc resistant material is less than 2wt% (Comparative Examples-1 and 2), the breaking current value and re-ignition frequency The value was large and the variation range was large. Especially, the proportion of highly conductive material pre-blended in arc resistant material is 0 wt%
In the case of (Comparative Example-1), Ag-Cu in the Cr skeleton
Is difficult to infiltrate, and alloy production is not possible.

On the other hand, when the proportion of the highly conductive material pre-blended in the arc resistant material is 2 to 30 wt% (Examples 1-3), the breaking current value is Cu-50wt% Cr (Comparative Example-4). ) And the width of variation is small, which is preferable. The frequency of restriking is not as small as that of Cu-50wt% Cr, but tends to be favorable.

On the other hand, when the ratio of the highly conductive material to be pre-blended in the arc resistant material is 30 wt% or more (Comparative Example-3), the value of the breaking current value tends to be large and the variation width tends to be large.

From these facts, the optimum proportion of the highly conductive material to be pre-blended in the arc resistant material is 2 to 30 wt%.

Comparative Examples-5 to 8 and Examples-4 to 5 The contact alloys shown in Table 2 were manufactured in the same manner as Examples-4 to 5, and Ag powder and Cu powder in the pre-compounded highly conductive material were prepared. Were manufactured by changing the respective ratios. The evaluation results are shown in Table 2 of the same.

In the production of Ag-50wt% Cr alloy, when the ratio of Ag in the high-conductivity material to be pre-blended is 9: 1 and 10: 1 (Comparative Examples 5 and 6), the value of the breaking current is somewhat large. There is a tendency, and the range of variation is large. Further, the re-ignition frequency shows a large value due to the influence of the built-in gas due to the large number of pores generated in the material, and the variation range is large.

On the other hand, when the ratio of Ag in the highly conductive material to be pre-blended is 8 to 6 (Example-2, 4, 5), the value of the breaking current is Cu-.
It is smaller than 50 wt% Cr (Comparative Example-4) and has a small variation width, which is preferable. The frequency of re-ignition is also low, and the variation width is small, which is a preferable tendency.

On the other hand, when the ratio of Ag in the high-conductivity material to be pre-blended is higher than the above (Comparative Examples-7 and 8), the breaking current value and the re-ignition frequency are large, and the fluctuation range tends to be large. .

From these, Ag in the high conductivity material to be pre-blended and
The optimum Cu ratio is 8: 2 to 6: 4.

Comparative Examples-9-10, Examples-6-7 The Examples-1-5, Comparative Examples 1-3, Comparative Examples-5-5
Regarding No. 8, an example in which an (Ag-Cu) -50 wt% Cr alloy was manufactured as a contact material was described, but the technique of the method of the present invention, regardless of this ratio, is 25 wt% Cr (Example-6) and 75 wt% Cr.
It was also found to be effective for the production of Cr (Example-7) alloy.

That is, under the conditions shown in Table 3, 25 wt% Cr (Example-
6), a 75 wt% Cr (Example-7) alloy was manufactured, and the cutting characteristics and restriking characteristics were evaluated, but stable results were obtained in both cases.

On the other hand, in the alloy containing 15 wt% of Cr in the contact material (Comparative Example-9), there are variations in the breaking and re-ignition characteristics.

On the other hand, in the 90 wt% Cr (Comparative Example-10) alloy, not only is the frequency of re-ignition high, although the breaking characteristics are excellent,
It is recognized that the production of the material is industrially difficult, and the contact resistance of the contact is likely to be high, so that the current flow failure occurs.

Therefore, in the (Ag-Cu) -Cr alloy, C
It is effective to apply the method of the present invention to an alloy having an r amount in the range of 25 to 75 wt%.

Comparative Examples-11 to 15, Examples-8 to 11 The Examples-1 to 7, Comparative Examples-1 to 3, and Comparative Examples-5 to 5
For No. 10, the sintering temperature was 85 when manufacturing the contact alloy.
An example in which the manufacturing is carried out at 0 ° C. and an infiltration temperature of 1100 ° C. will be described. The technique of the method of the present invention has sintering temperatures of 600 ° C. (Example-8) and 1000 ° C. (Example- It has also been found to be effective for the production of alloys according to 9).

Further, it was found that the same effect was obtained for producing alloys having infiltration temperatures of 900 ° C (Example-10) and 1200 ° C (Example-11).

That is, when the sintering temperature is 600 ° C (Example-8), 1000 ° C (Example-9), and the infiltration temperature is 1100 ° C shown in Table 4. Also, the sintering temperature is 850 ℃, and the infiltration temperature is 900 ℃.
(Example-10) and 1200 ° C (Example-11) respectively (Ag-C
u) -50wt% Cr alloy was manufactured and the breaking and re-ignition characteristics were evaluated, but low and stable results were obtained.

On the other hand, the alloy produced at a sintering temperature of 500 ° C. (Comparative Example-11) has a poor skeleton state and has many pores in the material. Therefore, the frequency of re-ignition is high and the variation range is large.

On the other hand, the alloy produced at a sintering temperature of 1200 ° C (Comparative Example-12) had a poor skeleton state because of the large evaporation loss of the highly conductive material premixed in the skeleton, and the generation of pores in the material Many. For this reason, although not as high as in Comparative Example-11, the frequency of re-ignition is high and the variation range is large.
Also, the breaking current value tends to be high.

Subsequently, the materials produced at the infiltration temperatures of 700 ° C (Comparative Example-13) and 800 ° C (Comparative Example-14) showed that the infiltration material was poorly infiltrated into the skeleton and the re-ignition frequency was high. became.

On the other hand, the material manufactured at the infiltration temperature of 1300 ° C (Comparative Example-15) tends to have a large breaking current value and a large variation range because the evaporation loss of the infiltration material during infiltration is large. .

From these facts, it is effective to apply the method method of the present invention in which the sintering temperature is 600 to 1000 ° C and the infiltration temperature is 900 to 1200 ° C in the production of the (Ag-Cu) -Cr alloy. .

Examples-12 to 14 The above Examples-1 to 11, Comparative Examples-1 to 3, and Comparative Examples-5 to 5.
With regard to No. 15, the contact alloy was described as a method for producing an (Ag-Cu) -Cr alloy.
wt% Ti (Example-12), similarly 25 wt% Ti (Example-13)
When replaced with. In addition, 25 wt% Cr-25 wt% Ti (Example-1
It was also found to be effective for the production of alloys substituted for 4).

That is, 50 wt% Ti shown in Table 5 (Example-1
2), 25 wt% Ti (Example-13), and 25 wt% Ti-25 wt% Cr
Alloys were manufactured by substituting each of them with (Example-14), and the breaking characteristics and the re-ignition characteristics were evaluated, but low and stable results were obtained.

Thus, (Ag-Cu) -50wt% Ti alloy, (Ag-Cr)-
It is also effective to apply the method and technique of the present invention to the production of 25 wt% Ti alloy and (Ag-Cu) -25 wt% Ti-25 wt% Cr alloy.

Evaluation Method / Conditions The conditions for evaluating the contact material produced by the method of the present invention are shown below.

(1) Breaking current characteristics A pair of test contacts having a diameter of 10 mm and a thickness of 4 mm, one of which is a flat surface and the other of which has a spherical surface of 200 mmR, are baked at 400 ° C and are subjected to discharge aging, and then inserted coaxially in series. The voltage drop of the shunt was observed with a synchroscope. AC is applied to the contact through L, C, and circuit, and contact pressure is 10kg.
The table below shows the variation range of the breaking current value for 300 switching operations.

This breaking current value determines the degree of low surge 1
On one of the important scales, the smaller the value and the smaller the variation range, the better the low surge property.

(2) Re-ignition characteristics A disc-shaped contact piece with a diameter of 30 mm and a thickness of 5 mm was attached to a demountable vacuum valve, and the frequency of re-ignition was measured when the circuit of 6 KV x 500 A was cut 2000 times. However, the variation width (maximum and minimum) of 2 circuit breakers (6 valves)
Indicated by. Baking heating (45
(0 ° C., 30 minutes) only, and no brazing material was used and no accompanying heating was performed.

〔The invention's effect〕

As described in detail above, according to the present invention, it is possible to provide a contact material for a vacuum valve, which is excellent in the stability of the current cutting characteristic and the restriking characteristic.

[Brief description of drawings]

FIG. 1 is a sectional view of a vacuum valve to which a contact material for a vacuum valve according to the present invention is applied, and FIG. 2 is an enlarged sectional view of an electrode portion of the vacuum valve shown in FIG. 1 ... shut-off chamber, 2 ... insulating container, 5, 6 ... conductive rod, 13
a, 13b …… Contacts.

Claims (4)

(57) [Claims] 1. A conductive material component 25 to 75 composed of Cu and Ag.
For a vacuum valve, which is composed of 75% to 25% by weight of an arc resistant material component composed of Cr and / or Ti, and Cu and Ag of the conductive material components form a eutectic structure in an alloy. A method for producing a contact alloy, comprising the following steps (a)
(C) is included, The manufacturing method of the contact alloy for vacuum valves characterized by the above-mentioned. (B) Ag-C having a eutectic ratio in the arc-resistant material powder
a step of forming a mixed powder obtained by preliminarily mixing the u-based powder in an amount of 2 to 30% by weight based on the total amount of the conductive material component and the arc resistant material component, (b) Obtained forming A step of sintering the body to obtain a skeleton, and (c) a step of infiltrating a conductive material component into the voids in the obtained skeleton. 2. In the step (b), the sintering is 600-100.
The method according to claim 1, which is carried out in a temperature range of 0 ° C. 3. In the step (c), the infiltration is 900-120.
The method according to claim 1 or 2, which is carried out in a temperature range of 0 ° C. 4. The method according to claim 1, wherein the sintering in the step (b) and the infiltration in the step (c) are performed in a non-oxidizing atmosphere.