JP4054343B2 - Pure silver material and method for producing the same - Google Patents

Description

  The present invention relates to a pure silver material, and more particularly to a pure silver material that hardly undergoes crystal grain growth even when subjected to heat treatment and does not impair subsequent workability.

Silver is a metal having high thermal and electrical conductivity, high ductility and malleability, and good workability, and is a material used for various industrial applications. For example, silver has recently been used as an auxiliary material for wire-like high-temperature superconducting materials. A wire-like high-temperature superconducting material using silver as an auxiliary material is formed by first forming a pipe made of silver, and then adding a high-temperature superconducting material (for example, Bi ₂ Sr ₂ Ca ₂ Cu ₃ O _x , Bi ₂ Sr ₂ CaCu ₂ O _x ) powder or green compact is filled and sealed, and the silver pipe is drawn and rolled. The reason for using such an auxiliary material is that the high-temperature superconducting material is ceramic, and shape control by plastic working is extremely difficult. Since silver has good processability, it is suitable as an auxiliary material for shape control of the high-temperature superconducting material and does not impair the characteristics of the high-temperature superconducting material.
Japanese Patent Application Laid-Open No. 07-057569

  By the way, in the manufacture of the wire-shaped high-temperature superconducting material described above, after filling the silver pipe with the high-temperature superconducting material powder, in order to remove the gas adsorbed on the high-temperature superconducting material powder and the pipe inner surface, It is common to heat-process and then process. The heat treatment for degassing is more effective as the temperature is higher, and the heat treatment time is shortened. On the other hand, however, when silver is heat-treated at a high temperature of 500 ° C. or higher, crystal grain growth occurs, and processing cracks are likely to occur during the subsequent wire drawing.

  Such problems when performing processing after heat treatment of pure silver can occur in addition to the auxiliary material for the high-temperature superconducting material described above, as well as when forming and processing after pure silver is clad and pressed. is there. In such an application, the heat treatment temperature may be set to a low temperature, but it is not the best measure considering the efficiency of the heat treatment. Attempts have also been made to shift the silver crystal grain growth temperature to a higher temperature side by adjusting the material structure, etc., but this is also limited and not stable.

  The present invention has been made based on the background as described above, and crystal grain growth is suppressed even when heat treatment is performed at a high temperature of 500 ° C. or higher, particularly 600 ° C. or higher. It is an object of the present invention to provide a pure silver material that is not easily cracked and a method for producing the same.

  The inventors of the present invention have studied to solve the above problems, and found that the inevitable impurities in pure silver form an oxide, thereby finding that this has an effect of suppressing grain growth, and have arrived at the present invention.

  That is, the present invention is a pure silver material composed of silver and an inevitable impurity of a metal element, and a part or all of the inevitable impurity element forms an oxide, and is at a temperature of 500 ° C. or higher and a melting point or lower. It is a pure silver material in which the average value of the crystal grain size after heating is 100 μm or less.

  Not only silver but a general metal material has impurities in the order of ppm that cannot be avoided even if it is called pure metal after refining. However, this inevitable impurity does not affect the characteristics of the entire material, and the pure metal substantially exhibits the original characteristics of the metal. The present inventor actively uses this inevitable impurity as an oxide. A metal oxide finely dispersed in pure silver has a so-called pinning effect and can prevent crystal grain growth that occurs under heating. And in the pure silver material which concerns on this invention, even if it heats at the temperature below 500 degreeC or more and melting | fusing point or less, the coarsening of a crystal grain diameter does not arise, but the average value can maintain 100 micrometers or less. Therefore, according to the present invention, it is possible to perform product processing with a good yield without causing processing cracks even when processing is performed after heat treatment.

  In the present invention, the concentration of inevitable impurities is preferably 5 to 500 ppm. If the amount is less than 5 ppm, the amount of oxide to be dispersed is small and the effect of suppressing the grain growth is not sufficient. If the amount exceeds 500 ppm, the hardness is significantly increased by the oxide, and mechanical properties are higher than that of pure silver. This is because it changes greatly and the workability deteriorates. A particularly preferable inevitable impurity concentration is 10 to 100 ppm. The concentration of these inevitable impurities is based on the analysis result obtained by ICP emission analysis.

  The type of inevitable impurities is not particularly limited as long as it is other than a metal element that is difficult to oxidize. However, as a result of studies by the present inventors, the inevitable impurity elements effective from the viewpoint of suppressing crystal grains include at least one of calcium, magnesium, aluminum, copper, manganese, iron, lead, silicon, palladium, and rare earth metals. It is a waste. In particular, those containing copper, manganese, and palladium have remarkable effects.

  And since the pure silver material which concerns on this invention includes an oxide, it is the characteristics that oxygen content is higher than normal pure silver. The oxygen content is preferably 3 to 80 ppm. This is because when the amount of oxygen is less than 3 ppm, sufficient oxide is not formed and the effect of suppressing grain growth is not exhibited. On the other hand, if the oxygen content exceeds 80 ppm, oxygen more than the amount necessary for oxidizing inevitable impurities is introduced, which is not preferable. A particularly preferable oxygen content is 5 to 20 ppm.

  As a method for producing a pure silver material according to the present invention, a method in which pure silver containing inevitable impurities is subjected to an internal oxidation treatment is preferable. The internal oxidation treatment is a treatment in which a material to be treated is heat-treated in an oxidizing atmosphere, and an additive element (impurity) inside the material is oxidized to form an oxide. In the present invention, it is required to oxidize inevitable impurities in order to form an oxide sufficient to suppress crystal grain growth.

  In the present invention, the heating temperature for the internal oxidation treatment needs to be 300 to 700 ° C. If the temperature is less than 300 ° C., the progress of internal oxidation is slow, surface oxidation is preferential, and an oxide cannot be formed effectively. On the other hand, when the temperature exceeds 700 ° C., crystal grain growth occurs simultaneously with internal oxidation, and the material structure becomes non-uniform.

  In order to oxidize inevitable impurities without excess or deficiency in the above temperature range, a balance between the heating temperature and the heating time is important. This heat treatment time is preferably set according to the following formula.

Here, D ₀ is a frequency term (mm ² / s), Q is activation energy (kJ / mol), R is a gas constant (J / K · mol), T is a heat treatment temperature (K), and X Represents the thickness of the silver subjected to the heat treatment, and t represents the heat treatment time (sec). For example, when the heat treatment atmosphere is air, D ₀ is 340 (mm ² / s), and Q is 96000 (kJ / mol). D ₀ and Q are numerical values that change depending on the heat treatment atmosphere, and the heat treatment time is shortened by increasing the oxygen partial pressure. In addition, when adjusting oxygen partial pressure, it is preferable that the range shall be 0.02 Mpa-1 Mpa. This is because the oxygen partial pressure of less than 0.02 MPa is lower than that of the atmosphere, and the heat treatment time cannot be shortened, and even if it exceeds 1 MPa, the effect of shortening the heat treatment time is small. The conditional expression can be applied without depending on the shape of the sample, and the above expression can be applied to a plate-like or pipe-like one.

  Although the timing for performing the internal oxidation treatment is not particularly limited, it is preferably performed before the finishing process in the process of processing from the ingot to the shape to be used. For example, in the use of the high-temperature superconducting auxiliary material, it is preferably performed in the middle of the process of forming into a pipe shape.

  As described above, the pure silver material according to the present invention is suppressed in crystal grain growth even when subjected to a heat treatment at 500 ° C. or higher, and it is difficult for processing cracks to occur during subsequent processing. Therefore, when used as a high temperature superconducting auxiliary material, the temperature of the degassing heat treatment after filling the high temperature superconducting material can be set to 500 ° C. or more, and the efficiency can be improved. The form of the pure silver material according to the present invention is not particularly limited, and can be used in various shapes such as a pipe shape and a plate shape.

  Hereinafter, preferred embodiments of the present invention will be described. In this embodiment, first, various pure silver materials having different impurity concentrations shown in Table 1 were formed into a pipe having a diameter of 20 mm, then subjected to internal oxidation treatment, and further processed into a pipe having a diameter of 5 mm. Then, the processed pipe was heat-treated at 600 ° C., and the change in the material structure and the presence or absence of crystal grain growth were examined.

  The internal oxidation treatment performed here was performed in an air atmosphere at a heating temperature of 500 ° C. and a heating time of 5 hours. The heat treatment after the pipe formation was performed in vacuum at 600 ° C. for 10 hours. The presence or absence of crystal grain growth after the heat treatment was evaluated by observing the material structure after the heat treatment with a microscope, and the appearance and average particle diameter before and after the heat treatment. Moreover, the oxygen content after the internal oxidation treatment was measured with an oxygen analyzer.

  And about the sample after material structure observation etc., the wire drawing which makes a diameter 1mm was performed, and the presence or absence of the crack generation by processing was evaluated. Table 2 shows the examination results of the grain growth and crack generation. Table 2 also shows the results of testing the sample 1 that was heat-treated without the internal oxidation treatment as a comparison.

  As can be seen from Table 2, it was confirmed that in each sample subjected to the internal oxidation treatment, crystal grain growth by the subsequent heat treatment was suppressed. FIG. 1 shows the material structure before and after the heat treatment of Sample 1 and Sample 1 that was not subjected to internal oxidation treatment. From this photograph, the sample subjected to the internal oxidation treatment undergoes crystal grain growth due to the subsequent heat treatment. It can be confirmed that there is not. And from Table 2, the suppression effect of a processing crack can also be confirmed. However, in Sample 5 having a low inevitable impurity concentration, the effect of suppressing the crystal grain growth was non-uniform, the average grain size was large, and a slight amount of work cracking occurred. Therefore, in order to expect a sufficient effect, it was confirmed that the inevitable impurities are 10 ppm or more. On the other hand, it was confirmed that the crystal growth by the heat treatment was remarkable in the comparative sample not subjected to the internal oxidation treatment.

Photograph showing material structure before and after heat treatment of Sample 1 and Sample 1 that was not subjected to internal oxidation treatment

Claims (1)

A pure silver material containing at least one of calcium, magnesium, aluminum, copper, manganese, iron, lead, silicon, palladium, and rare earth metals in a total concentration of 5 to 500 ppm, with the balance consisting of silver and inevitable impurities in an oxidizing atmosphere. Oxidized pure silver material,
A pure silver material in which a part or all of calcium, magnesium, aluminum, copper, manganese, iron, lead, silicon, palladium, and rare earth metal forms an oxide and has a dissolved oxygen content of 3 to 80 ppm by the internal oxidation treatment.

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