KR900003545B1 - Fabrication method of contact compound metal of vacuum valve - Google Patents

Abstract

A contact forming alloy for a vacuum valve or circuit breaker is prepd. by (a) forming a structure from chromium powder under an external pressure of no more than 8 tons per square centimeter or a pressure due to its own wt. This chromium powder has an average particle size of from 5 to 250 micron and the content of each of aluminium sillicon, vanadium and calclum present in the chromium powders as impurities is not more than 100 ppm; (b) sintering the formed structure contained in a vessel for sintering, in a nonoxidising atmos. together with the vessel for sintering in order to obtain a chromium skeleton; (c) inflltrating the voids in the resulting chromium skeleton with copper and or silver; and (d) cooling the infiltrated alloy material to adjust its conductivity.

Description

Manufacturing Method of Contact Alloy for Vacuum Valve

1 is a cross-sectional view of a vacuum circuit breaker to which the contact material of the present invention is applied.

2 is an enlarged cross-sectional view of a contact portion.

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing Cu and / or Ag-Cr alloys with less gas and pores, and more particularly, to a method of manufacturing a contact alloy for vacuum valves that can reduce the frequency of re-ignition.

Characteristics required for the contact for the vacuum valve are welding, withstand voltage and high breaking resistance.

However, since these three requirements are required to have opposite physical properties, it is difficult to make them ideally compatible, and the present condition is that the priority requirements of the circuit to be applied are the first and other requirements are slightly sacrificed.

For example, it is known that a Cu alloy containing 5% by weight or less of a welding prevention component (Bi, Te, Pb, etc.) is provided as an electrode contact point in a high breakdown voltage and a large capacity vacuum circuit breaker. 41-12131).

However, in recent years, the demand for higher voltage is not sufficient in terms of withstand voltage. That is, the vacuum circuit breaker has superior characteristics compared to other circuit breakers such as small size, light weight, easy maintenance, and environmental conditioning, so the scope of application is expanded every year, and it is applied to the circuit of higher voltage from the circuit of 36KV or less, which has been generally used in the past. In addition, the demand for opening and closing special circuits, such as capacitor circuits, is also increasing rapidly.

One of the important factors that hinders its achievement is re-calling and re-calling.

Although the re-ignition phenomenon is considered to be important in terms of improving the reliability of the product, it is still not clear about the prevention technology as well as the cause of the direct occurrence.

In connection with the high withstand voltage, the contact materials are required to have a higher withstand voltage and a lower generation variability in the re-ignition shape.

In order to achieve high pressure resistance and non-intercalation of the contact materials, avoid the extremely low or excessive concentration of fragile welding prevention components that are pressure-resistant defects, and extremely low gas impurities or pinholes. It is desired to increase the strength of the alloy itself.

In this respect, the above-described Cu-Bi alloy is not satisfactory.

In addition, Cu-W contacts or Cu-Wc contacts, which are conventionally used as contact materials, are quite excellent in terms of withstand voltage. However, the sintered contact alloy is easy to retain bubbles in the manufacturing method, and also has many hot electron emission. There is a drawback that re-ignition is likely to occur.

On the other hand, application of Cu-Cr alloy is performed in the field | area which is high breakdown voltage and requires a large current interruption. The Cu-Cr alloy has the advantage of expecting uniform performance due to the small vapor pressure difference between the members of different contact materials, and its characteristic is that it is a fully available contact alloy.

This Cu-Cr based gradual alloy is manufactured as follows substantially. For example, according to Japanese Patent Publication No. 59-30761, Cr powder and a small amount of Cu powder are mixed, and the mixture is filled into a mold and subjected to pressure molding with little pressure, and the molded body is taken out of the mold. Vacuum sintering forms Cr skeleton and finally Cu infiltrates.

Also recently, a method of injecting powder into a mold, placing a pellet thereon, degassing the whole, and then performing infiltration treatment under reduced pressure is also disclosed (see Japanese Patent Laid-Open No. 59-25903).

Moreover, the method of obtaining Cu-Cr alloy is also performed by mixing Cu and Cr of a final target value from the beginning, and solidifying and sintering the molded object obtained by this at the melting point of Cu or less.

However, as these alloys are generally manufactured by powder metallurgy as described above, raw powder control, sintering, and infiltration techniques that are involved in re-ignition have not been sufficiently established. no.

An object of the present invention is to provide a method for manufacturing a contact alloy for a vacuum valve, in which the occurrence frequency of re-ignition is significantly reduced.

The present inventors have studied and studied the suntic, sintering conditions, infiltration conditions and cooling conditions of the raw material Cr of this manufacturing method to reduce the re-ignition frequency and stabilize the conductivity characteristics of the contact alloy for vacuum valves. Came to complete. That is, the manufacturing method of the contact alloy for vacuum valves of this invention by a 1st aspect is characterized by including following process (a)-(d).

(a) Cr powders having an average particle diameter in the range of 5-250 μm and containing Al, Si, V, and Ca as impurities in an amount of 100 ppm or less, respectively, having an external pressure of ⁸ ton / cm ² or less, or the weight of the Cr powder To make a molded body under pressure.

(b) sintering the molded body contained in the sintering container in a non-oxidizing atmosphere together with the sintering container in order to obtain a Cr skeleton.

(c) A step of infiltrating Cu or / and Ag in the voids in the obtained Cr skeleton.

(d) cooling the infiltrated alloy material to adjust the conductivity.

The manufacturing method of the vacuum alloy contact alloy according to the second aspect of the present invention includes the following steps (a) to (f).

(a) Process of heat-processing raw material Cr at least 1 time in the non-oxidizing atmosphere in the temperature range of 1300 degreeC near melting | fusing point.

(b) A step of grinding the raw material Cr subjected to the heat treatment to obtain Cr powder having an average particle diameter of 5-250 µm and contents of oxygen and nitrogen of 200 ppm or less, respectively.

(c) A step of molding the Cr powder thus obtained at an external pressure of ⁸ ton / cm ² or less or a pressure of the weight of the Cr powder.

(d) A step of sintering the Cr powder compacts contained in the sintering vessel together with the sintering vessel in a non-oxidizing atmosphere to obtain Cu skeleton.

(e) A step of infiltrating Cu or / and Ag in the voids in the obtained Cr skeleton.

(f) cooling the infiltrated alloy material so that the conductivity is adjusted.

Hereinafter, the present invention will be described in detail according to the above steps.

Impurity management of Cr powder

The present inventors have observed in detail the total amount of gas emitted during the heating of the contact material and the form of the emission, and as a result, these factors and the occurrence of re-ignition phenomena have an important correlation, and in particular, the raw material constituting the contact material It has been found that re-emergence can be effectively suppressed by controlling the release of gas which occurs suddenly near the melting point, and releases gas one by one. In other words, when the contact material is heated, most of the adsorbed gas is reached at the melting point or lower, and the gas dissolved in the vicinity of the melting point is released, but when further heated above the melting point, it is extremely short time (for example, several m sec) but pulsed. Release of the abrupt gas (several to hundreds of times) is observed.

These abrupt gases include a little C ₂ H ₂ , CH _4, etc., but the main body is oxygen, such as CO, CO ₂ , O ₂ , and thus these abrupt gases are thought to be released by decomposition of the oxide contained in the contact material.

According to the research of the present inventors, the contact material which generate | occur | produces much re-ignition phenomenon also releases abrupt gas.

Therefore, it is conceivable from the above-mentioned knowledge that the occurrence of re-ignition will be reduced by releasing the gas in advance.

However, the contact materials for vacuum circuit breakers contain a considerable amount of Cu, and in order to decompose and remove these oxides, for example, a temperature of about 1200 ° C. or higher at 10 ^-3 -10 ^-4 Torr vacuum degree is required, so that the high altitude of Cu, etc. is high. The heat treatment as described above with respect to the contact material containing the malleable material or the adhesion preventing material such as Bi, Te, etc. causes variation of the components and is inconvenient in terms of management of the contact characteristics.

For example, when Bi is heated as an anti-deposition material, a plurality of kinds of gases are released very rapidly in the vicinity of 400-550 ° C.

Some of these emissions are combined with Cu, which is in the process of temperature rise, to make relatively well-recognized compounds, some of which decompose during dissolution but others remain to be one of the causes of abrupt gases.

Such emission of abrupt gas can still be seen, for example, even if Bi is used as a raw material with a purity of 99.9999% when left to undergo oxidation or gas adsorption.

The observation as described above indicates the necessity of removing in advance the ball impurities causing the abrupt gas by the separate heat treatment of the highly conductive material such as Cu and the anti-deposition component material in the contact material including the anti-deposition material. In the manufacture or heat treatment process of contact alloys, it is suggested that the contamination of contact alloys by the emission gas from crucibles, boards and plates directly contacting the liquid phase of contact alloys, which are partially or entirely in liquid state, is also suggested.

With respect to the former knowledge, the inventors have found that the individual heat treatment of member elements for the reduction of the abrupt gas is effective to some extent, and the probability of the occurrence of re-ignition also decreases with this.

Regarding the latter knowledge, the present inventors acknowledge that the material of the crucible and the like in contact with the liquid phase and the physical and chemical state of the surface affect the emission form of the abrupt gas and are also related to the redemption probability. It was found that the latter effect is essential to reliably and efficiently improve the effect of abrupt gas emission by management at the level.

2,3 knowledge of the above-mentioned re-calling is valid for the reduction, but it recognizes the necessity of further improvement for the demand of further re-calling hardening and blocking capacity. It is desired to develop other measures to achieve this.

For example, when the Cu-Cr alloy is produced by superimposing the knowledge of 2 and 3, the effect is greatly increased than when alone, thus suggesting the necessity of totally managing a series of processes. In particular, raw material technology and cooling technology need to be sufficiently grasped. That is, as one of the causes of the abrupt gas described above, it is considered that the contents (impurity) and the state (surface oxidation, presence of a mixture) of the raw materials Cr and Cu are important.

Foreign substances such as oxides which have an oxide form from the beginning and are only mixed in the raw material powder are removed by sedimentation method using specific gravity difference with the raw material powder or sieved by using a difference in particle diameter and sieved in advance. The infiltration process at the time of infiltrating highly conductive material in a tone can be performed in one direction, and the foreign material, such as an oxide, can be gathered in one place. These works have shown good results in reducing the occurrence of the same redemption phenomenon.

However, the problem is impurities present in the raw material that are dissolved or precipitated. They could not be sifted, removed by specific gravity or infiltration, and could be thought of as having one factor of potential re-ignition. However, it was found that the raw material powder (Cr powder) is sufficiently examined as one means of solving the problem, and the raw material powder having less impurities is selected, and the occurrence of the re-ignition phenomenon tends to be further reduced.

As described above, the inventors have found that the selection of raw material powders containing few impurities (mainly oxides) is effective in reducing the re-interaction phenomenon. That is, although a lot containing substantially no impurities in the Cr powder was selected, and this was used as a Cr raw material, and the same lot was used as the raw material for Cu, the presence of precipitates and presence of precipitates in the alloy was observed. As a result of comparing the recursion incidences, it was found that a large number of beams were generated in the solenoid valve using the alloy in which the electron precipitates were present.

Such a precipitate is presumed to be an impurity produced by the reaction of an element of some kind dissolved in Cr powder with an atmosphere during sintering and / or infiltration. Therefore, in order to further improve the re-interlocking characteristics, it is necessary to pay attention to any element (particularly not microscopically detected and confirmed to be in an employment state) in the raw material other than impurities such as oxides mixed only in the raw material. It is showing that there is. In other words, especially in the production of Cu-Cr alloy, there is a need to manage through a series of processes. A series of processes are the selection of raw materials with certain conditions in the process of producing Cu-Cr alloys by sintering and / or infiltration, and the preparation, sintering and / or infiltration conditions of Cr or / and Cu skeletons. Means the technology of control.

In carrying out the method of the present invention, the type and amount of impurities in the Cr powder are extremely important because they affect other processes. As previously described, sintering with oxides (A1 ₂ O ₃ , SiO ₂ , CaO, V ₂ O ₅ ) or metals dissolved in Cr powder (Al, Si, Ca, V), which causes sudden gas, and / or Oxides newly produced by reaction with the atmosphere during infiltration are particularly problematic. As described above, Cu-Cr alloys with high incidence of re-ignition have a large amount and their correlation in Cr.

Regarding the frequency of re-ignition in relation to other processes, the content of Al, Si, and Ca should be 100ppm or less, V should be 10ppm or less, and the suppression of phenomena occurring in a very short time after the contact is opened and extinguished. In consideration of the above, it is necessary to manage Al, Si, Ca, and V more strictly, and it should be less than 10ppm, 20ppm, 10ppm and 10ppm, respectively. The content of oxygen and nitrogen in Cr is also preferably at least 1000 ppm or less, respectively. When Al, Si, Ca, V and the like exceed these values, the amount of product generated by the reaction with the atmospheric gas in the sintering and / or infiltration process in the subsequent process is also large, and it is not preferable for reignition and callout. not.

[Adjustment of Raw Material Cr]

Currently, industrially supplied metal Cr refining method is to reduce Cr ores such as FeCr ₂ O ₄ , MgCr ₂ O ₄ to other metals such as Al or Si to obtain metal Cr (reduction method) and dissolve the Cr ore There are two main methods of separating the undissolved nonmetallic impurities and electrolyzing them as electrolytes to obtain metal Cr (electrolytic method).

However, Cr obtained by the electron reduction method contains about 1,000 ppm of gas amount (oxygen, nitrogen), and about 1,000 ppm-10,000 ppm of impurities such as Al, Si and Fe. On the other hand, Cr by the latter electrolytic method, on the other hand, has a large amount of gas (oxygen, nitrogen) of 1,000 ppm to 10,000 ppm, relatively small impurities such as Al, and for example, contains about 100 ppm or less.

In view of such circumstances, in the method of the present invention, it is preferable to perform a specific treatment on the raw material Cr before turning the raw material Cr into a powder.

According to the research of the present inventors, it is unpreferable to remove the impurity in Cr after a powder process. This is because if the Cr powder is further heated (even if it is heated to a sufficient temperature for degassing), the Cr particles aggregate and become larger and require a crushing process again, and the specific surface area of Cr is large. This is because the amount of coating is increased and the gas efficiency is lowered. Further, additional heat treatment for removing impurities during sintering (skeleton formation) is also undesirable because it causes variation in composition.

In the present invention, for example, at least one step of each step of processing the metal Cr obtained by the reduction method or the electrolytic method into a metal Cr aggregate, a step during the processing, and a step after the processing (before the powdering step) is given. In a non-oxidizing atmosphere such as vacuum or hydrogen, heat treatment is carried out at least once at a temperature not lower than 1300 占 폚 and near the melting point of the reactor Cr. This makes it possible to avoid the drawbacks and disadvantages of performing the heat treatment after the Cr in powder form, and to obtain a metal Cr effective for reducing the re-ignition.

At a temperature below 1300 ° C., the degassing effect of the metal Cr aggregate is dropped, and the effect is small for the reduction of re-ignition. At 1500 DEG C or higher, the effect is large and particularly effective, but the evaporation also becomes severe. Therefore, the atmosphere during the treatment is advantageous to select the atmospheric gas rather than the vacuum. By performing the heat treatment before the powdering step in this manner, it is possible to advantageously reduce the re-ignition rather than to form Cr powder. In order to further improve the performance of the vacuum valve, it is effective not only to perform the previous step of processing the metal Cr aggregate but also to superimpose the above-described heat treatment in the subsequent step.

[Cr powder adjustment]

The heat treated metal Cr aggregate is made of Cr powder having a predetermined particle size without contaminating the metal Cr aggregate. The particle diameter of the Cr powder is limited by the contact characteristics as a vacuum valve and the contact manufacturing technique such as sintering. As for the average particle diameter of Cr powder, 5-250 micrometers is preferable. When the average particle diameter of the Cr powder is less than 5 μm, undesirable pores are easily generated in the skeleton or / and the contact material after sintering and / or immersion, and accordingly, there is a tendency that a large amount of gas also remains. It is in an unfavorable state with respect to a characteristic (for example, re-ignition characteristic).

Further, in the Cr particle diameter exceeding 250 μm, severe variations can be seen in any of the welding resistance, the breakdown voltage characteristic and the breaking characteristic. Segregation can be seen in the contact material, which is undesirable from the viewpoint of the reliability of the vacuum valve.

On the other hand, it is preferable that oxygen and nitrogen gas in Cr powder are suppressed to 200 ppm or less, respectively. These gases are composed of the total amount of gas contained in Cr and adsorbed on the gas. Since the former gas is minimized in the heat treatment step of the raw material Cr, it is particularly important to reduce the latter adsorption gas. That is, it is important to grind without contaminating, and it is important that Cr grinding | pulverization generate | occur | produces excessively the grinding | pulverization energy by the grinding energy, and oxidation does not progress. Therefore, crushing such as large frictional heat generated should be avoided. In addition, grinding in a non-oxidizing atmosphere is also effective. When Cr particle diameter becomes 100 micrometers or less, it is necessary to fully consider such a point.

When the amount of gas is 200 ppm or more, it is difficult to maintain the amount of gas in the alloy at a preferable low level (for example, 200 ppm or less, preferably 100 ppm or less) even when using Cu-Cr alloy using these raw material Cr powders. That is, at the heat treatment temperature selected at the time of sintering and / or infiltration of the contact sound for the vacuum valve, the purification of the raw material Cr is slightly insufficient. When the amount of gas (in this case, oxygen) in the Cu—Cr alloy is 200 ppm or more, a plurality of recursive phenomena may occur.

By adopting the heat treatment step of the raw material Cr as described above, it is possible to eliminate the scattering of Cr powder which can be seen in the case of degassing heat treatment after powdering, and to prevent equipment contamination.

Additional Ingredients

Other elements in Cr, that is, Fe, Co, Mo, W, v, Nb, Ta, Ti, Zr, are beneficial for improving the breakdown voltage characteristics of the Cu-Cr alloy made through the process of the present invention. If these metals are less than 50% with respect to Cr, the function of Cr in a Cu-Cr alloy will not be impaired. Since Cr has evaporation property equivalent to that of Cu, it has a function of leveling the contact surface after current interruption and has a favorable effect on the electric resistance, and the ratio between Cr and other elements in Cr is preferably present at 50% or more. .

In the present invention, a mixed powder of Cr powder and Cu or / and Ag powder may be used as the raw material powder of the molded product. In this case, the amount of Cu or Ag powder in the mixed powder is preferably 30% by weight or less.

[Molding]

The Cr powder is formed with an external pressure of ⁸ ton / cm ² or less, or a pressure of its own weight.

The molding pressure at the time of obtaining a molded object is a factor which determines the amount of Cr in a Cu-Cr alloy, and is one of the characteristic points in the method of this invention.

The amount of Cr in the Cu (or / and Ag) -Cr alloy may be selected in the range of 20-30% by weight. Molding pressure for yireogi is better if 8ton / cm ² or less, 7.5ton / cm ² or less if desired, and 7ton / cm ² or less. This is because the amount of Cr after infiltration exceeds 80% at a pressure exceeding 8ton / cm ² , thus leaving the well known in the present invention. Although it is possible to cope with one Cr, in order to secure a low Cr alloy around 20%, it is impossible to select pure Cr as a skeleton, and it is achieved by employing a Cr + Cu mixed powder in which Cu is appropriately blended with Cr. At this time, the molding pressure may be freely selected to a pressure of 8ton / cm ² or less in accordance with the amount of the Cu powder to be mixed.

When the molding pressure exceeds 8 ton / cm ₂ , cracks may occur in the molded body during heating, which is not preferable.

[Sintering]

The molded article thus obtained is installed in a heating furnace together with the sintering container and sintered. The sintering atmosphere needs to be in a non-oxidizing atmosphere, for example in vacuum or hydrogen.

A vacuum (1 × 10 ^-5 Torr or more) atmosphere is suitable for removing the oxygen and nitrogen stored in the Cr powder, the molded molded body or the container, and the like filled in these atmospheres.

The sintering temperature and the sintering time to be applied affect the density of the sintered skeleton, and conversely, the porosity of the skeleton.

For example, the porosity should be 40-50% in order to approximate the relationship between the Cr skeleton and the amount of Cu infiltrated into the principal metal in a weight ratio of 50:50, in order to achieve a sintering temperature of 800 ° C-1050 ° C. Preferably it is 900 degreeC-950 degreeC, the sintering period is 0.25-2 hours, and the range of 0.5-1 hour is preferable. The above conditions are suitably selected according to the ratio of Cr and Cu.

[Invasion]

Cu or / and Ag, which is an infiltration agent, is placed on the upper or / and lower surface of the obtained skeleton and the whole is heated, for example, in vacuum (1 × 10 ⁻⁴ −1 × 10 ⁻⁶ Torr) to form Cu or / and Ag. Is infiltrated during the Skelton donation

The temperature at the time of infiltration is the temperature above the melting point of Cu or / and Ag. In the case of Cu, the range is 1100 ° C-1300 ° C, and in the case of Ag, the range of 1000 ° C-1100 ° C is appropriate. The soaking time also sets a time sufficient for the impregnation of these solutions to the voids in the skeleton.

In the above immersion step, it is possible to obtain excellent contact bonding properties (when brazing the conductive rod) of the contact alloy obtained by simultaneously forming a layer of immersion metal on at least part of the surface of the skeleton.

[Cooling]

The alloying material infiltrated in the process is cooled to adjust the conductivity.

Cooling conditions after sintering and infiltration are factors that determine the basic properties of the material of the Cu—Cr alloy, in particular, the conductivity, and are one of the features of the method of the present invention.

Since Cr is a metal which is extremely easy to oxidize, it goes without saying that the management of the raw material powder or the molded body is important, and the conditions of the atmosphere during sintering and infiltration also influence the material properties.

However, even in the Cu-Cr alloy obtained by sufficiently managing the temperature and time during sintering and infiltration, there are variations and instabilities in specific resistance, contact resistance or temperature rise characteristics, and it is desired to eliminate these variations and to have stability.

Studies have shown that the instability of Cu-Cr-based contact materials is present in (1) variation in composition in Cu-Cr alloys, (2) particle diameters of Cr particles, particle size distribution, degree of segregation, and (3) It turns out that it depends on the degree of the hollow hole, and further (4) the quality of the raw material Cr. Although these solutions have been found to be effective in the selection of raw material Cr and management of the sintering technique, in order to further improve the stability, the management of sophisticated sintering techniques in addition to the above (1), (2) (3) and (4) I knew it was necessary. That is, it was found that the instability of the above characteristics was correlated with the difference in the amount of Cr contained in a small amount in Cu. After all, when the amount of Cr contained in the Cu portion of the Cu-Cr alloy is estimated by the semi-quantitative method by X-ray microanalysis, it is generally in the range of 0.2-0.5Wt% in the Cu-Cr alloy, which exhibits unstable values. On the other hand, the variation of the Cu-Cr alloy exhibiting stable characteristics by the present invention described below was 0.1% or less as a representative value of 0.2% or less. Recognizing that this difference is dependent on the thermal history after sintering or infiltration of the Cu-Cr alloy, it is clarified that this effect is very effective in improving the conductivity and reducing the deviation width of the Cu-Cr alloy. In addition, the thermal history after sintering or immersion can be represented by the cooling rate characteristic which a contact body receives substantially. In other words, it refers to managing the cooling rate that is varied by the size of the contact point and the characteristics of the furnace under predetermined conditions.

The aspect of cooling which improves the electrical conductivity of Cu-Cr alloy is as follows.

Cooling of the raw material obtained in the infiltration step is preferably performed at a temperature difference of at least 100 ° C. at a cooling rate of 0.6-6 ° C./min among the temperature sections of 300 ° C.-400 ° C. Here, at a cooling rate of less than 0.6 ° C / min, there are no drawbacks for the conductivity characteristics, but the manufacturing time becomes longer and becomes economically disadvantageous. At cooling rates exceeding 6 ° C./min, the amount of Cr dissolved in the Cu phase in the Cu—Cr alloy increases, leading to a decrease in electrical conductivity, which is undesirable. For example, if the amount of Cr in the Cu phase of the Cu-50% Cr alloy exceeds about 0.5%, the conductive wool drops to 1/2 of 0.1%. (In 0.1%, the conductivity is 40% IACS. In% it drops to 20% IACS or less).

As another aspect, in the cooling process of the method of the present invention, cooling from 400 ° C to room temperature is preferably quenched by blowing inert gas. In general, the time required for cooling within the above range by quenching is determined by the heat capacity of the sample and the like and requires a long time. Therefore, the production efficiency is improved by quenching.

Further, in the cooling step of the method of the present invention, heating is performed at least 1 hour for at least 0.25 hours at any temperature in the 300-400 ° C temperature section. By maintaining the heating in this way, it is easy to regenerate (improving the recovery of the conductivity) after sintering and the end of the infiltration, especially when a contact with low conductivity is found.

[Reaction prevention material]

In the sintering step and the infiltration step, it is preferable to insert a reaction preventing material between the molded body and the sintering container and between the skeleton and the sintering container in order to reduce the reaction and / or the wetting between these members. By preventing the reaction and wetting as described above, the properties of the alloy can be further improved.

Examples of such reaction preventing material is preferably made of at least one kind of heat-resistant inorganic material in particulate or fiber form selected from the pre-heating Al ₂ O _3, SiO ₂ of at least 400 ℃. For example, the reaction inhibitor may be made of fibrous ceramics.

As another preferable aspect, an inhibitor can be made into a ceramic fiber bundle.

As another aspect, an inhibitor can be made into the knitted fabric of ceramic fiber.

As a preferable aspect in this invention, the said fibrous ceramic may be obtained from the long fiber of 5-100 micrometers in diameter length.

Preferred reaction inhibitors used in the present invention are fibrous ceramics, which are preferably Al ₂ O ₃ or / and SiO ₂ as the main component. This is because Al ₂ O ₃ and SiO ₂ are excellent in heat resistance and workability. The content of Al ₂ O ₃ and SiO ₂ is preferably 50% or more, more preferably 90% or more. Other acceptable components other than Al ₂ O ₃ and SiO ₂ include TiO ₂ , MgO, CaO, Fe ₂ O ₃ , B ₂ O ₃ , and SrO. Their content is at most 20%.

In the present invention, the fibrous ceramics is used as a direct contact member as a ceramic fiber bundle obtained by bundling a plurality of fibers or a knitted fabric obtained from ceramic fibers. Although the member is used in various heat treatment methods, for example, the ceramic fiber bundle and / or the ceramic knitted fabric are sandwiched between the heat treatment container and the heat-treatment material, and the ceramic knitted fabric is formed into a container and heat treated. In some cases, the heat-treatment material is in direct contact with the ceramic fibers.

The fibrous ceramics are obtained from 0.1-300 μm in diameter, preferably 5-100 μm in long fibers. The use of the fibers in such a range is because the flexibility as a container is worse at a thicker diameter than this and the workability is impaired.

[The furtherance costs of alloy]

The following range is preferable for each component of the contact alloy finally obtained.

Cu or / and Ag: 80-20% by weight, Cr: 20-80% by weight

When the amount of Cr in the alloy is larger than 80%, Joule welding occurs a lot, which is not preferable for the surface roughness that is deeply related to re-ignition, and it is difficult to block 40KA at a voltage of 7.2 KV. On the contrary, when the amount of Cr is less than 20%, for example, when blocking 40 KV, arc resistance is not maintained and large arc consumption is not preferable.

In addition, the amount of Cr dissolved in the Cu or / and Ag phase which is a high conductive component in the above composition range should be 0.01-0.35% by weight, which is preferable in terms of stabilizing the conductive characteristics.

[Atmosphere]

The treatment in each of the above steps is preferably carried out in a non-oxidizing atmosphere, and is specifically carried out in an inert gas such as argon gas, H ₂ gas N ₂ gas or in vacuum.

[Vacuum valve]

Next, a vacuum valve (vacuum breaker) applied to the alloy obtained by the method of the present invention will be described with reference to the accompanying drawings.

1 shows an example of the construction of a vacuum circuit breaker to which a contact material according to the present invention is applied, in which FIG. 1 shows a shielding chamber, and the shielding chamber 1 is formed of an insulating material having a substantially cylindrical shape with an insulating material. The metal covers 4a and 4b provided at both ends via the sealing holes 3a and 3b are constituted by a vacuum seal. However, the pair of electrodes 7, 8 mounted at the opposite ends of the conductive rods 5, 6 are disposed in the blocking chamber 1, and the upper electrode 7 is fixed and the lower electrode A is movable. In addition, a bellows 9 is attached to the electrode rod 6 of the movable electrode 8 to allow the electrode 8 to move in the axial direction while keeping the inside of the shutoff chamber 1 under vacuum. In addition, the upper portion of the bellows 9 is provided with a metallic arc shield 10 to prevent the bellows 9 from being covered by the arc steam. Further, l1 is a metallic arc shield covering the electrodes 7, 8 and provided in the shielding chamber 1 to prevent the insulation vessel 2 from being covered with arc steam. In addition, the electrode 8 is fixed by the portion 12 brazed to the conductive rod 6, or is crimped and connected by a lever as shown in an enlarged view in FIG. The contact 13a is fixed to the electrode 8 by brazing 14. In addition, 13b in FIG. 1 is a fixed side contact.

The contact member of the present invention is suitable for forming both or both of the contacts 13a and 13b as described above. The present invention will be described below by way of examples.

Example A-1

Cu-Cr alloy was adjusted using the following Cr powder for raw materials.

Average particle size 74μm

Al content 8ppm

Si content 15ppm

V content 〈2ppm

Ca content 〈2ppm

Oxygen content 100ppm

Nitrogen content 30ppm

High-purity Cu was heated in a graphite crucible at about 1500 ° C. in a vacuum for 1 hour, and subjected to directional solidification to form infiltrated Cu ingots.

A graphite container preheated to about 2000 ° C. or higher in vacuum was prepared as a heat treatment container for sintering and / or infiltration, and Al ₂ O ₃ powder having a particle diameter of 1-300 μm preheated to about 400 ° C. or higher in vacuum or hydrogen was prepared. It was prepared as a reaction or / and wetting agent.

Next, a molded article was formed from the Cr powder for the raw material at a pressure of 5 ton / cm ² and placed in the previously heat-treated sintering container.

The molded body was sintered together with the container at a vacuum degree of 2 × 10 ⁻⁶ Torr at a temperature of 1100 ° C. for 1 hour to form a Cr skeleton having a porosity of 20%.

Cr Skelton was placed in the pre-heated graphitized infiltration container, and the Al ₂ O ₃ powder prepared above was placed between the skeleton and the container so that both were not reacted or / and wet. The previously prepared infiltration Cu ingot was placed on the skeleton, and the pores of the skeleton were infiltrated with Cu by maintaining the vacuum at 2 × 10 ⁻⁶ Torr for 1 hour at a temperature of 1200 ° C.

After completion of the infiltration, cooling was performed at a cooling rate of 1.1 ° C./min in the temperature range of 800 ° C.-400 ° C. to obtain 80% Cr-Cu material having high conductivity.

The contact points from the obtained Cr-Cu materials were tested for the probability of re-ignition, firing and conductivity. The results are shown in the first table below.

[A-1 in comparison]

The following Cr powder for raw materials was used, Cu ingots by ordinary vacuum dissolution were used as Cu ingots, and cooling was carried out in the same manner as in Example A-1 except for cooling at a natural cooling rate in a furnace. . The results are shown in the first table.

Al content 1410ppm

Si content 620ppm

V content 78ppm

Ca content 45ppm

Oxygen Content 4670ppm

Nitrogen Content 269ppm

Example A-2

As in Example A-1, Cu ingots for infiltration of Cr powder for raw materials and wetting materials were prepared. Subsequently, the Cr powder for raw materials was naturally charged in the above-mentioned heat-treated sintering container without pressurizing. The Cr powder was sintered at a vacuum of 3 × 10 ⁻⁶ Torr at a temperature of 900 ° C. for 1 hour as a container to form Cr skeleton having a porosity of about 50%.

Cu was infiltrated into Cr Skeleton in the same manner as in Example A-1 except that the temperature was infiltrated at 1150 ° C in hydrogen for 0.5 hour. When cooling to the end of the infiltration was cooled to about 1000 ℃ by blowing sufficiently dry argon gas to obtain a 50% Cr-Cu material. The material was maintained at a vacuum degree of 2 × 10 ⁻⁶ Torr and a temperature of 750 ° C. for 3 hours and reheated to obtain a material having adjusted conductivity.

This material was tested in the same manner as in Example A-1.

The results are shown in the first table.

Comparative Example A-2

It carried out similarly to Example A-2 except having used the Cr powder for raw materials of the comparative example A-1, using the Cu ingot by normal vacuum dissolution as Cu ingot, and cooling by in a natural cooling rate in a furnace. And tested.

The results are shown in the first table.

Example A-3

As in Example A-1, Cr powder for a raw material and a wetting agent were prepared. Moreover, the Cu powder for mixing of 5-44 micrometers of particle diameters which carried out the reduction | restoration process by maintaining the dew point at 350 degreeC in hydrogen below 70 degrees C or less for 1 hour was adjusted. 700 g of the Cr powder and 2800 g of the Cu powder for mixing were mixed and pressurized at 4 ton / cm ² to form a molded body.

The molded body was accommodated in the previously heat-treated sintering container, and the molded body was sintered in hydrogen like a container at a temperature of l000 ° C. for 1 hour to obtain a skeleton.

The skeleton is again pressurized to 5 ton / cm ² , the dew point is heated to a temperature of 1000 ° C. for 1 hour in hydrogen below minus 70 ° C. and then to 7 ton / cm ² again, and l.5 × 10 ⁻⁶ Torr of Vacuum degree, heated to 1030 ℃ temperature for 1 hour.

In the cooling after heating, the furnace power was turned off, and the furnace was cooled to a natural cooling rate determined by the heat capacity of the heat-treated material such as the furnace and the heat-treated material, and the heating was maintained at 700 ° C. for 3 hours to improve the conductivity. Afterwards it was cooled again naturally to obtain a 20% Cr-Cu material.

This material was tested in the same manner as in Example A-1. The results are shown in the first table.

Comparative Example A-3

It tested like Example 3 except having used Cr powder for a raw material of Comparative Example A-1, and using normal Cu powder as a mixing Cu powder. The test results are shown in the first table.

TABLE 1

The above test was conducted after confirming by chemical analysis that the amount of Cr in each material was controlled by an error of 1% or less with respect to the target value.

As apparent from the comparison between the examples of Table 1 and the comparative examples, not only the electrical conductivity of the material was maintained but also the occurrence of re-ignition and reduction of firing were found.

[Examples B-1 to B-8, Comparative Examples B-1 to B-3]

The high-carbon ferrochrome was dissolved in sulfuric acid, electrolytically reduced, and the metal Cr plate obtained by pulverizing was pulverized into particles of about 0.5-2 mm in size (sample 1).

Sample 1 was heated at a temperature of 1450 ° C. for 2 hours in a vacuum of 2 × 10 ⁻⁵ Torr (Sample 2). The mixture was treated at 350 DEG C for 24 hours in the same vacuum (Sample 11).

Sample 1 was collected and molded into a Briquel machine at room temperature to produce a Briquel metal Cr aggregate having a size of about 20 mm x 25 mm and a thickness of about 8 mm (Sample 3).

Sample 2 was collected and molded into a Briquel machine to make a Briquel-like metal Cr aggregate having a size of about 8 mm by about 20 mm x 25 mm (Sample 4).

Sample 3 was heated at 1450 ° C. for about 2 hours in a vacuum of 2 × 10 ⁻⁵ Torr (Sample 7)

Sample 4 was heated at 1450 ° C. for about 2 hours in a vacuum of 2 × 10 ⁻⁵ Torr (Sample 8).

Sample 1 was collected, placed in a heat-resistant refractory ceramic container, and heated to 1300 ° C. in a vacuum of 2 × 10 ⁻⁵ Torr to form a metal Cr aggregate having a diameter of about 20 mm and a height of 8 mm (sample 5). The same heat treatment was again performed on the sample 5 cooled to normal temperature (sample 9).

Sample 2 was collected, placed in a heat-resistant refractory ceramic container, and heated to 1300 ° C. in a vacuum of 2 × 10 −5 Torr to form a metal Cr aggregate having a diameter of about 20 mm and a height of 8 mm (sample 6). The same heat treatment was again performed on the sample 6 cooled to normal temperature (sample 10).

Subsequently, about the obtained sample 1-11, it grind | pulverized about 6 hours in the ball mill packed with argon gas, and the particle | grains of the range of 5-250 micrometers were sifted out. At this point, all of the Cr powder except for sample 1 contained 200 ppm or less of oxygen and nitrogen. Al and Si were each 100 ppm or less, and Ca was 10 ppm or less.

After screening 74-105 μm particles in Cr powder of Sample 1-11 and naturally charging them in the graphite container, each of Sample 1-11 was heated at 950 ° C. in hydrogen for one hour with hydrogen, and the pore volume was about 55%. Each Cr Skelton was obtained.

Cu vacuum degassed separately in this Cr skeleton was immersed at 1150 ° C. for 1 hour in a vacuum of 1 × 0 ⁻⁵ Torr to obtain about 50% by weight of a Cu—Cr alloy. In this infiltration process, a reaction prevention sheet woven between 85% Al ₂ O ₃ -15% SiO ₂ ash and fibrous ceramic fibers in the form of a sheet was placed between the sample 1-8 and the infiltration heat treatment container. Put in.

After completion of the infiltration process, the sample 1-8 was cooled under the condition of controlling the cooling rate from 800 ° C to 400 ° C to 3 ° C / min after naturally cooling the infiltration work temperature from 1150 ° C to 800 ° C.

Contact pieces were made from these contact materials and mounted in a prefabricated vacuum valve.

The amount of gas of the contact material thus obtained was compared, and the firing characteristics were evaluated under the evaluation method and conditions described later, in which the contact of this sample 1-11 was attached to the assembled vacuum valve.

As shown in the second table, the Cu-Cr alloy was made of Cr powder as it was after electrolytic Cr was pulverized (Comparative Example B-1), and the metal Cr aggregates were subjected to a Briquel machine without treating the same Cr powder. When Cu-Cr alloy was made from Cr powder obtained by pulverizing it (Comparative Example B-2, the occurrence frequency of re-ignition was more than 2% in all cases. In the case where the heat treatment was performed during molding (during the production of the metal Cr aggregate), the occurrence of re-ignition was greatly reduced in all cases (Examples B-1 to B-8). In this case, the effect is not exerted and re-emergence can be seen (Comparative Example B-3).

TABLE 2

Examples B-9 to 18 and Comparative Examples B-4 and 5

Metal Cr ingots obtained by Al reduction of chromic anhydride were evenly made into particles of about 0.5-2 mm in size. In addition, impurities such as Al and Si in the metal Cr mass were sufficiently removed (sample 21).

Sample 21 was heated at a temperature of 1450 ° C. for 2 hours in a vacuum of 2 × 10 ⁻⁵ Torr (sample 22). It heat-processed at 350 degreeC for 24 hours in the same vacuum (Sample 31).

Sample 21 was collected and molded at room temperature with a pressurized fleece to produce a Briquel metal Cr aggregate having a size of about 20 mm x 25 mm and a thickness of 8 mm (sample 23).

Sample 22 was collected and molded into a compression placen to produce a Briquel-like metal Cr aggregate having a size of about 20 mm x 25 mm and a thickness of about 8 mm (sample 24).

Sample 23 was heated to 1450 ° C. for about 2 hours in a vacuum of 2 × 10 ⁻⁵ Torr (Sample 27).

Sample 24 was heated to 1450 ° C. for about 2 hours in a vacuum of 2 × 10 ⁻⁵ Torr (Sample 28).

Sample 21 was collected, placed in a heat-resistant refractory ceramic container, and heated to 1300 ° C. in a 2 × 10 ⁻⁵ Torr vacuum to produce a metal Cr aggregate having a diameter of about 20 mm and a height of 8 mm (sample 25).

The same heat treatment was again performed on the sample 25 cooled to normal temperature (sample 29).

Sample 22 was collected, placed in a heat-resistant refractory ceramic container, and heated to 1300 ° C. in a vacuum of 2 × 10 ⁻⁵ Torr to form a metal Cr aggregate having a diameter of about 20 mm and a height of 8 mm (sample 26).

The same heat treatment was again performed on the sample 26 cooled to normal temperature (sample 30).

Subsequently, the obtained sample 21-31 was ground in a ball mill packed with argon gas for 12 hours and sieved to sift particles in the range of 5-250 μm.

At this point, each of the Cr powders except Sample 21 had 200 ppm or less of oxygen and nitrogen. Al and Si were each 100 ppm or less, and Ca was 10 ppm or less.

74-105 μm particles were selected from the Cr powders of Samples 21-31. Electrolytic copper having approximately the same particle diameter subjected to hydrogen reduction treatment at 400 ° C. was prepared. The Cr powder and the electrolytic Cr powder were mixed to have 25% Cr, molded at ⁴ ton / cm ² , and then calcined in a hydrogen stream up to 800 ° C., and then sintered under vacuum at 1040 ° C. for 2 hours to form a contact material. . Since the ratio of Cr and Cu can be arbitrarily selected because of the solid state sintering method, the above-described 25% Cr to Cu alloy is shown as a representative example.

The sintered material after vacuum sintering at 1040 ° C. was cooled to near room temperature, and then reheated at 550 ° C. for 90 minutes in vacuum to obtain a contact material. The contact pieces were made from these contact materials, the amount of gas of the contact materials was compared, and the contact points of this specimen piece 21-31 were attached to the assembled vacuum valve, and the re-firing characteristics were evaluated by the following evaluation method and conditions.

As a result, when the Cu-Cr alloy is made by sintering method with Cr powder obtained by crushing Al-reduced Cr (Comparative Example B-4), a metal Cr aggregate is formed by a compression molding machine without treating the same Cr powder, and then crushed. In the case where the Cu-Cr alloy was made from the obtained Cr powder (Comparative Example B-5), the occurrence of re-ignition was much higher than 5% in any case. In addition, the amount of gas in Cr content was 770-1400 ppm of oxygen and 300-420 ppm of nitrogen, and the gas amount in 25% Cr-Cu was 110-200 ppm of oxygen and 88-180 ppm of nitrogen.

On the other hand, when the heat treatment was carried out before and after the step of forming the metal Cr aggregate or during the forming (vacuum hot press), the frequency of re-ignition was greatly reduced and decreased to 0.5% or less (Example B). -9-17: sample 22, samples 24-30). However, when the heat treatment conditions were not preferable, the effect was not exerted and re-ignition of 2% or more was observed. (Example B-18; Sample 3l).

Although the example of 1450 degreeC as heat processing temperature was mainly described, when this processing temperature is higher, the re-ignition suppression effect aimed at by this invention improves further, On the other hand, it is time to grind in a 2nd process. This tends to take. Moreover, the effect of re-ignition reduction tends to be inferior at the process temperature lower than 1300 degreeC.

In addition, Cr containing 420ppm of Al, 170ppm of Si, and 70ppm of Ca was collected and pulverized into a metal Cr aggregate to obtain a Cr-50% Cr alloy by a sintering / sintering method. In this case, the heat treatment was performed at 1450 ° C. for 2 hours, but the incidence of re-ignition was not reduced, and the frequency was 5% or more.

Moreover, in the alloy for vacuum valves obtained by the present invention, not only the frequency of re-ignition was reduced but also the variation in the generation rate for each vacuum valve was reduced.

[Evaluation Conditions]

Evaluation of the contact material for a vacuum valve evaluated re-ignition by the evaluation conditions shown below. A disc-mounted contact piece with a diameter of 39 mm and a thickness of 5 mm was mounted on a demountable vacuum valve, and the frequency of re-ignition when the circuit of 6KV × 500A was cut off 2000 times was measured, and two circuit breakers (6 valves) were used. The deviation width (maximum and minimum) of is shown. When attaching a contact, only baking heating (450 degreeC, 30 minutes) was performed, but the use of the brazing material and the heating according to it were not performed.

Claims (22)

(a) The external pressure of ⁸ ton / cm ² or less from Cr powder whose average diameter of particles is in the range of 5-250 μm and the Al, Si, V and Ca contents as impurities are 100 ppm or less, respectively, or the magnetic weight of this Cr powder. Forming a molded article at a pressure of (b) a factory in which the molded article placed in a sintering container is sintered in a non-oxidizing atmosphere together with a sintering container in order to obtain Cr skeletal, and (c) a void in the obtained Cr skeleton. A method of manufacturing a contact alloy for a vacuum valve, comprising the steps of infiltrating Cu or / and Ag, and (d) cooling the infiltrated alloy material so that its conductivity is adjusted. The manufacturing method of the contact alloy for vacuum valve according to claim 1, wherein the cooling of the step (d) is performed at a cooling rate of 0.6-6 ° C / min between a temperature difference of at least 100 ° C in a temperature section of 800-400 ° C. Way. The method for manufacturing a contact alloy for vacuum valve according to claim 1, wherein in the cooling of the step (d), inert gas is blown to perform quenching. The method of manufacturing a contact alloy for a vacuum valve according to claim 1, wherein in the cooling of the step (d), heating is maintained at least once for 0.25 hours at any temperature in a temperature range of 800 ° C to 400 ° C. . The Cr powder used in the step (a) is characterized in that the Cr powder contains 10 ppm or less of Al, 20 ppm or less of Si, 10 ppm or less of V, 10 ppm or less of Ca, 1000 ppm or less of oxygen, and nitrogen. A method for producing a contact alloy for vacuum valve, characterized in that it contains 1000ppm or less. The method of manufacturing a contact alloy for vacuum valve according to claim 1, wherein the molded article of step (a) is made of a mixture of Cr powder and Cu or / and Ag powder. The vacuum valve according to claim 1, wherein in the sintering of the step (b), a reaction preventing material is inserted between the molded body and the container for sintering to reduce the reaction and / or the wetness of both. Method for producing a contact alloy. The vacuum valve according to claim 1, wherein in the infiltration of step (c), a reaction preventing material is inserted between the Cr skeleton and the infiltration vessel to reduce the reaction and / or the wetness of both. Method of Manufacturing Contact Alloys The contact alloy of a vacuum valve according to claim 7 or 8, wherein the reaction preventing material is at least one of a particulate or fibrous heat-resistant inorganic material selected from Al ₂ O ₃ and SiO ₂ heated to at least 400 ° C. Manufacturing method. (a) heating the raw material Cr at least once in a non-oxidizing atmosphere at a temperature in the vicinity of 1300 ° C.-melting point; (b) grinding the heated raw material Cr to obtain an average particle diameter of 5-250 μm, oxygen and nitrogen. To obtain a Cr powder having a content of 200 ppm or less, (c) molding the Cr powder thus obtained at an external pressure of ⁸ ton / cm ² or a pressure of its own weight, and (d) a container for sintering. Sintering the encapsulated Cr powder compact together with a sintering vessel in a non-oxidizing atmosphere to obtain Cr skeleton, (e) infiltrating Cu or / and Ag into voids in the obtained Cr skeleton, (f) A method of manufacturing a contact alloy for a vacuum valve, characterized in that it comprises the step of cooling the infiltrated alloy material to adjust the conductivity. The method of manufacturing a contact alloy for vacuum valve according to claim 10, wherein the sintering of step (d) is performed at a temperature of at least 800 ° C. The contact alloy for vacuum valves according to claim 10, wherein the cooling in the step (f) is performed at a cooling rate of 0.6-6 ° C / min between a temperature difference of at least 100 ° C in the 800 ° C-400 ° C temperature range. Manufacturing method. The method of manufacturing a contact alloy for vacuum valve according to claim 10, wherein in the cooling of the step (f), an inert gas is blown and quenched. The method of manufacturing a contact alloy for a vacuum valve according to claim 10, wherein in the cooling of the step (f), heating is maintained at least once at a certain temperature in a temperature range of 800 ° C to 400 ° C. The powder obtained in the step (b) is characterized by containing 100 ppm or less of Al, 100 ppm or less of Si, 10 ppm or less of Ca, 200 ppm or less of oxygen, and 200 ppm or less of nitrogen as impurities in the Cr powder. Method for manufacturing a contact alloy for vacuum valve. The method of manufacturing a contact valve for a vacuum valve according to claim 10, wherein the molded article obtained in the step (c) is a mixture of Cr powder and Cu or / and Ag powder. 11. The vacuum according to claim 10, wherein in the sintering of the step (d), a reaction preventing material is inserted between the molded body and the container for sintering to reduce the reaction and / or the wetness of both. Method for manufacturing contact alloy for valve. 18. The vacuum alloy contact alloy according to claim 16 or 17, wherein the reaction preventing material is at least one of a particulate or fibrous heat-resistant inorganic material selected from Al ₂ O ₂ and SiO ₂ preheated to at least 400 ° C. Manufacturing method. The Cr group according to claim 10, wherein the raw material Cr in the step (a) contains less than 50 wt% of at least one selected from the group consisting of Fe, Co, Mo, W, V, Nb, Ta, Ti and Zr. Method for producing a contact alloy for a vacuum valve, characterized in that made of an alloy. The method for manufacturing a contact alloy for vacuum valve according to claim 10, wherein the Cr raw material of the step (a) is obtained by an electrolytic method or a reduction method. The method of claim 10, wherein the non-oxidizing atmosphere in the steps (a) and (b) is H ₂ gas, N ₂ gas rare gas, or vacuum. The method of manufacturing a contact valve for a vacuum valve according to claim 10, wherein the amount of Cr dissolved in the Cu or / and Ag phase of the obtained contact alloy is controlled to be 0.01-0.35% by weight.

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