JP2894661B2 - Tantalum material and method for producing the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a tantalum material used in a high temperature range.

[0002]

2. Description of the Related Art Tantalum materials are often used in high-temperature atmospheres, but their main applications are divided into two types: when they are repeatedly used under vacuum and high temperature as lead wires for tantalum capacitors and general plate tubes. . Both have the property that the material becomes brittle due to the coarsening of the crystal and the presence of oxygen. Tantalum is generally potassium fluoride tantalate (K ₂ Ta).
F ₇ ) is obtained by reduction with sodium (Na). FIG.
Shows a flow sheet of a general manufacturing method. For a capacitor, this reduced powder is adjusted to a capacitor powder, but the powder is becoming thinner year by year in order to increase the capacitance and reduce the size, and the oxygen content is correspondingly increasing.

[0003] On the other hand, processed products such as tantalum plate tubes are not limited to a method of plastic working of a sintered body by a conventional powder metallurgy technique. In recent years, as shown in a flow sheet of FIG. It is manufactured by briquetting and electron beam melting to form an ingot, and the ingot is subjected to plastic working. For materials such as tantalum wire and plate tubes, heating at 1000 ° C or higher causes recovery from removal of processing strain, then new crystal grains are generated and recrystallization starts, and at higher temperatures, grain growth coarsens. You. In the case of tantalum, the recrystallization temperature is said to be around 1000 ° C., but varies depending on the amount of impurities, the processing method, the processing rate, and the like.
If the crystal grains are coarse and a large amount of oxygen is present in the crystal, the material becomes brittle, which causes a problem. In order to prevent this, a tantalum material that is difficult to recrystallize and has a slow crystal growth is desired. In order to suppress grain growth, the second
It is said that it is good to uniformly disperse a dispersion such as phase particles and impurities such as solute atoms in a matrix.

[0004] The tantalum material according to the present invention relates to a material for this processed product, and is mainly intended for a lead wire for a tantalum capacitor and a high temperature material. The following describes the case where tantalum is used for these two purposes.

When tantalum is used as a lead wire for a tantalum capacitor, first, the tantalum powder produced by the method shown in FIG. 1 is formed into a pellet, and at that time, a separately prepared tantalum wire is charged. Since the pellets require strength in the subsequent capacitor manufacturing process as shown in FIG. 3, they are usually sintered at a high temperature of about 1500 ° C. for about 30 minutes or more. This sintering is often completed only once,
It may be performed more than once. By the way, in the sintering process of the capacitor manufacturing process, oxygen in the tantalum powder diffuses into the tantalum wire, and the oxygen concentration in the wire increases. Under the above sintering conditions, the wire undergoes recrystallization and causes crystal growth. At that time, the oxygen contained tends to collect at the crystal grain boundary, which is a collection of crystal defects. Oxygen present at the grain boundary is in a solid solution state while its concentration is low, so there is no problem of embrittlement. However, it is known that when the oxygen concentration is high, the wire becomes brittle after sintering. In addition, when the crystal grains of the tantalum wire are remarkably coarsened, the yield as a product deteriorates. In order to reduce such a phenomenon, it is important to reduce the oxygen in the tantalum powder, but on the other hand, a lead wire that does not cause crystal grains to become coarse during sintering is desired.

Next, the case of using as a furnace material for a sintering furnace for manufacturing a tantalum capacitor or a high-temperature annealing furnace used for annealing a high-temperature material will be described. As the furnace material, a high melting point metal such as tungsten, molybdenum, and tantalum is usually used. Among them, a tantalum plate, a round bar, or the like is often used in terms of workability and strength. For example, the sintering conditions for tantalum capacitors are shifting to low temperatures year by year with the increase in capacity of the capacitors, but they are still often performed at around 1500 ° C. or higher. When the sintering operation of the capacitor is repeated under such conditions, crystal grains of the tantalum material grow. On the one hand, every time the furnace is opened and closed, the surface of the activated tantalum material comes into contact with air, so that oxygen adheres and a natural oxide film is formed each time. As a result, the oxygen concentration accumulated in the base material increases, segregating into coarse crystal grain boundaries, causing grain boundary destruction. Such a phenomenon also occurs when used as an annealing furnace for a high-temperature material, and the life of the tantalum material is exhausted.

As can be seen from these two examples, when a pure tantalum material or a tantalum wire is used at a high temperature, the increase in oxygen causes the crystal coarsening to become brittle, thereby shortening the life of the material or breaking the wire. Occurs
In the case of a capacitor, the product yield is reduced. In order to prevent these phenomena, it is desired that the tantalum material be slow in crystal growth during high-temperature treatment.

In order to suppress crystal growth, it is well known to utilize the effect of adding a dopant and dispersing it uniformly and finely in tantalum to prevent the movement of crystal grain boundaries. For example, USP 1,054,049 (1967) and USP
No. 3,268,328 (1967) describes a material to which 10 ppm to 10,000 ppm of yttrium metal is added. Also, Journal of The
Less-Common Metals: 9 (196
5), International Symposiu
montantalum and Niobium:
November 1988, KOBELCO TEC
HNOLOGY Review No. 11: June
1991 and the like disclose a method of adding a rare earth oxide of 5 ppm or more. As a common feature of the techniques disclosed in these documents, the lower limit of the amount of added elements is 5 ppm in all cases. As for the addition method, a powder metallurgy method or a production method by arc melting in an inert atmosphere is known. As described above, conventionally, a method of adding a trace amount of an additive to a high melting point metal such as tantalum is generally performed by arc melting or powder metallurgy.

[0009]

The conventional rare earth element is 5
A tantalum material containing more than ppm causes an increase in leakage current when used as a lead wire of a capacitor, and has problems such as deterioration in workability when processing into a fine wire or foil. preferable. But,
A tantalum material having high purity and less likely to cause high-temperature embrittlement by using a small amount of a crystal growth inhibiting element has not been obtained. According to conventional manufacturing methods, the method of adding a grain growth inhibitor at a high temperature is limited to a powder metallurgy method or argon or vacuum arc melting. According to these methods, in the former case, sintering is usually performed with a solid lower than the additive of the element. In the latter case, the impurities are removed in an inert atmosphere or in a low vacuum, and in either case, the impurities do not fully escape and remain in the product, especially when used as tantalum capacitor lead wires. Cause.

[0010] On the other hand, a method of using an ingot manufactured by electron beam melting as a material is also known. According to this method, the purity is good and the influence on the leakage current is small,
Because of melting under high vacuum and high temperature, it has been generally impossible to dope additives having a high vapor pressure. If a rare earth element which is a crystal growth inhibiting element having a higher vapor pressure than the base material can be added by electron beam melting, the purity as a whole will be high,
In addition, a material having excellent functionality can be obtained. The present invention relates to a technique for efficiently adding a crystal growth-inhibiting element, which is conventionally considered to be difficult to add, by electron beam melting. That is, according to the present invention, a tantalum material having a crystal growth inhibiting effect even with an amount much smaller than that required in the conventional technology, having very few impurities, and having high quality and excellent workability can be obtained. It is.

[0011]

According to the present invention, the oxygen concentration in tantalum is suppressed to 100 ppm or less, and the rare earth element is contained at 0.01 to 1.0 ppm. The reason for limiting the oxygen to 100 ppm or less is to keep the rare earth element in tantalum effectively and to suppress grain boundary embrittlement of the material.
Rare earth elements include yttrium (Y) and scandium (S
c), lanthanum-based lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd),
Promethium (Pm), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (T
b), dysprosium (Dy), holmium (Ho),
Erbium (Er), thulium (Tm), ytterbium (Yb), and lutetium (Lu) are applicable. The reason for limiting the amount of the rare earth element to 1.0 ppm or less is that an increase in the amount of the rare earth element causes an increase in leakage current when used for a capacitor lead wire, or deteriorates workability. On the other hand, in order to obtain the effect of preventing the coarsening of the crystal grains by the rare earth element, it is necessary to contain 0.01 ppm or more.

The above limitations were determined based on the following experimental results. That is, the tantalum powder obtained by the Na reduction method is deoxygenated with calcium so that the oxygen level becomes 2,000.
0, 2,500, 2,800, 3,000, 3,50
A powder of 0, 4,000 ppm was obtained. To these tantalum powders, Y ₂ O ₃ was added in an amount of 1 wt% in terms of Y pure content, press-molded, and sintered at 1,500 ° C. for 30 minutes. Each of the sintered pellets was used as a raw material and melted by an electron beam to form an ingot, which was then forged and rolled to form a 1 mm thick plate. The content of Y was analyzed for each ingot.
Further, the plate is heated at 1600 ° C. in a vacuum of about 1 × 10 ⁻⁴ Torr.
For 30 minutes, and the crystal grain size after annealing was measured. Table 1 shows the results.

[0013]

[Table 1]

In electron beam melting, 10 ^-3 to 10 ^-5 Torr
Dissolve the tantalum under high vacuum on the stage. Since the melting point of tantalum is 3,000 ° C., ordinary elements and compounds
If the temperature is higher than 000 ° C., the vapor pressure is so high that it evaporates and does not remain in the tantalum ingot. Conventionally, it has been considered difficult to add rare earth elements to tantalum ingots. However, when performing electron beam melting, lowering the oxygen of the raw material tantalum, mixing the rare earth oxide, pressing it, and sintering it at 1,500 ° C or higher for 30 minutes or more diffuses the tantalum and the rare earth element. It was found that the rare earth elements remained in the matrix in a small but dispersed form even after the electron beam melting, when they were bonded.

Oxides of rare earth elements are more stable in terms of free energy of formation than oxides of tantalum. Therefore,
It is considered that a trace amount of a rare earth element is present in the matrix in the form of an oxide regardless of whether it is added as a pure substance or as an oxide. The form of addition may be elemental, but it is desirable to use a more stable and easily available oxide. First, the tantalum powder and the rare earth oxide are uniformly mixed. The finer the oxide, the better. This mixed powder is press-molded in accordance with an electron beam melting furnace. The molding pressure may be about ² to 4 ton / cm ² . Next, this compact is heated to 1,500 ° C. or more in a vacuum and sintered for 30 minutes or more. The sintering conditions are appropriately adjusted depending on the size of the compact.

In electron beam melting, the evaporation of additives is controlled by the degree of vacuum and the temperature of the molten metal. However, in order to stably emit an electron beam, the degree of vacuum needs to be a certain value or more, and in order to eliminate surface defects and internal defects of the ingot, a certain temperature of the molten metal, that is, an electric input is required. . In particular, when processing a plate, a pipe, or the like, since the above-mentioned defect becomes a defect of the product, an electric input enough to completely dissolve is applied. As described above, it was found that the amount of the finally remaining additive was almost constant under the conditions for the electron beam melting. According to the technique of the present invention, the amount of residual rare earth elements in tantalum in electron beam melting is often between 1 ppm and 0.01 ppm, and even this amount is effective in suppressing the growth of tantalum grains. I found that there was.

[0017]

By simply mixing the rare earth element oxide and the tantalum powder, they do not stay in the tantalum during the electron beam melting and evaporate, so that they do not easily remain. According to the present invention, these mixed powders are press-formed in advance and sintered at a high temperature in a vacuum, so that tantalum and rare earth element oxides are diffusion-bonded and energy is required for their separation. Therefore, it is considered that the ratio of the residual is increased. Also, if the raw material tantalum powder has a large amount of oxygen, the surface layer also has a large amount of oxygen, which hinders the diffusion bonding with the rare earth element oxide. Therefore, it is important to keep the oxygen concentration in the tantalum powder low. Since rare earth elements are very small and very finely dispersed in the structure, their existence cannot be observed with the current technology, and it is difficult to accurately explain the reason why the effect is increased, but the low effect where the effect was not obtained in the past The effect of suppressing the crystal growth at the concentration is that the existence ratio of the rare earth element in the effective form in the smelting ingot is much higher than in the past by introducing the vacuum sintering process of the briquette containing the rare earth at the raw material stage. It is considered that

[0018]

EXAMPLES As tantalum powder, a powder as obtained by reducing K ₂ TaF ₇ with Na (hereinafter referred to as powder A) and a powder obtained by further deoxidizing the powder with Ca (hereinafter referred to as powder B) were prepared. . Powder A had an oxygen content of 4,500 ppm and powder B had an oxygen content of 2,900 ppm. As rare earth element compounds, yttrium oxide (Y ₂ O ₃ ), neodymium oxide (Nd ₃ O ₂ ), and cerium oxide (Ce ₂ O ₃ ) were prepared. Each oxide had a purity of 99.9% or more and a particle size of 200 mesh or less.

Next, tantalum powder B having a low oxygen content
Then, yttrium oxide, neodymium oxide, and cerium oxide were blended so that each rare earth element was 1 wt% in pure content, and were uniformly mixed by a mixer. 1. Put the mixed powder in a mold.
Forming by applying pressure of 9 ton / cm ² , 55mmφ × 1
It was a 0 mml briquette. This briquette is 10 ^-4
Heat treatment was performed at 1,500 ° C. for 30 minutes in a vacuum of Torr or less to obtain a sintered briquette.

Each sintered briquette was melted twice by electron beam melting and cast into a 150 mmφ mold to obtain an ingot. At this time, if the output of the electron beam was too weak, defects were generated on the surface and inside. If the output was too strong, the beam was turned into plasma and scattered and could not be melted. Under these conditions, the temperature of the molten metal was constant at a temperature higher than the melting point by 100 ° C. or more, and the behavior of the degree of vacuum in the chamber was the same for each melting.

The ingot manufactured by such a process was processed into a plate and a wire by the flow sheet shown in FIG. Referring to the process of FIG. 2, each of the ingots was surface-removed with a lathe to remove defects on the surface, and formed into a 30 mm thick slab by a 1000 ton press. The slab was roughly rolled into a plate having a thickness of 6 mm, and was annealed to remove processing strain.
This annealed material was rolled into a 1 mm thick plate. This plate was subjected to an evaluation test. Separately chamfered ingot was forged into a 50 mmφ billet, and after annealing, swaged into a 9 mmφ wire rod.
A wire of 1 mmφ was formed by die drawing. This wire was used for a lead wire of a tantalum capacitor and subjected to a bending test.

Each sample was subjected to comparative evaluation of coarsening of the grain of the plate material, and press sintering of the wire together with the tantalum powder to produce a sintered pellet for a tantalum capacitor. While applying a load to the wire, the wire was repeatedly bent 90 degrees in the horizontal direction and then returned to the original position (once), and then returned to the original position by bending it 90 degrees in the opposite direction, and the number of times of bending was counted. The plate material is 10 ^-5 To
Anneal at 1600 ° C in vacuum of rr level, wire is 4,
Pressed with tantalum powder containing 000 ppm oxygen,
Sintering was performed at 1,700 ° C. and 1,800 ° C. for 30 minutes each to obtain pellets for tantalum capacitors with lead wires. Table 2 shows the rare earth oxide content, the sintered particle size after annealing of the sheet material, and the results of the wire bending test at each ingot stage.

Comparative Example For comparison, a powder A having a high oxygen content was used, a rare earth element was added in the same manner as in the example, molded into a briquette, sintered, and then melted with an electron beam to form an ingot. Using this ingot, the same plate and wire as in the example were processed, and the same evaluation test as in the example was performed. Powder A
A plate material was processed in the same manner as in Example except that sintering of the briquette was omitted using the powder B and the powder B, and an evaluation test was performed. FIG. 4 shows the adjustment history of the materials subjected to these evaluation tests. Table 2 also shows the evaluation results.

[0024]

[Table 2]

As can be seen from the results in Table 2, when the oxygen content of the raw material tantalum powder for electron beam melting is 4500 ppm (powder A), the added rare earth elements are all detected at 0.01 ppm or less even in the case of press sintering. It is below the limit and does not remain. Further, all the crystal grains after annealing the 1 mm plate material at 1600 ° C. were 280 μm or more. In addition, the bending test of the tantalum wire was found to be brittle from 0 to 2 times. On the other hand, even when a powder having a low oxygen content of 2900 ppm (powder B) is used as a raw material, the effect of suppressing the grain growth is clearly exhibited only in a sample obtained by blending a rare earth element oxide and vacuum sintering. You can see that it is. In addition, when the tantalum wire compared at the same time was also annealed at 1700 ° C. and 1800 ° C. for 30 minutes, the grain size and wire bending were measured. Only the sinter coarsening is delayed. In the bending test, all the samples were 136 times or more.
From this, it was found that the high-temperature embrittlement of the tantalum material does not occur only with the coarsening of the crystal grains, but occurs only with oxygen. As for the tendency of embrittlement, when using tantalum wire as a capacitor lead wire,
It is known that the higher the oxygen of the tantalum powder pressed together and the higher the sintering temperature, the easier the embrittlement.

In the case of a tantalum plate, since it is used alone,
Embrittlement does not appear remarkably in one annealing. Normally, treatment is performed at a high temperature, and after cooling, the cycle of leaking with air and treating again at a high temperature is repeated many times, and activated at a high temperature to diffuse oxygen in the surface layer into the inside. While repeating this phenomenon several times, the oxygen in the matrix increases. In addition, since the crystals are coarsened, the situation is similar to that of a wire for a tantalum capacitor. In the case of a plate material, the coarse grain boundaries are finally broken and deformed. For the evaluation of the sheet material, considerable annealing must be repeated to check the life, but the tendency can be estimated from a bending test with a wire. It is also known that if there is a difference in crystal grain growth even in one annealing, the difference will continue even in several repetitions.

[0027]

As can be seen from these examples, in a tantalum wire for a tantalum capacitor and a tantalum plate for a high-temperature furnace, etc., when an oxide of a rare earth element is added by electron beam melting, even a very small amount of sintering in a high-temperature treatment is performed. Effective for suppressing grain coarsening. In addition, in the addition method of the electron beam melting, the oxygen of the raw material tantalum powder is 3000 ppm or less, and a mixture with the rare earth element oxide is press-molded and processed at a temperature sufficient for sintering and bonding (preferably 1500 ° C. or more). It tends to remain. As described above, the technology in which the crystal grains of the tantalum material are not coarsened even by the high-temperature treatment makes it possible to control the crystal grain size. Conventionally, the crystal grain size distribution is controlled by the working ratio and the annealing temperature, but it is easier to control by adding a rare earth element. New applications of such materials include molding bullet materials, high-speed armor plate penetration materials, and the like.

[Brief description of the drawings]

FIG. 1 is a view showing a manufacturing process of a tantalum powder for a capacitor.

FIG. 2 is a view showing a manufacturing process of a tantalum material.

FIG. 3 is a view showing a manufacturing process of the tantalum capacitor.

FIG. 4 is a diagram showing a history of samples subjected to an evaluation test.

Claims (3)

(57) [Claims] 1. A tantalum wrought material characterized by containing 100 ppm or less of oxygen and 0.01 to 1.0 ppm of a rare earth element. 2. The wrought tantalum material according to claim 1, wherein the rare earth element is any one of yttrium, neodymium, and cerium. 3. A rare earth element or an oxide thereof is added to a tantalum powder having an oxygen content of 3,000 ppm or less to form a uniform mixed powder, and the mixed powder is subjected to press molding, high-temperature vacuum sintering, and sintering. A method for producing a tantalum wrought material containing a rare earth element, wherein electron beam melting is performed using a briquette as a raw material.