JPH04285058A - Production of bismuth-containing oxide superconducting material - Google Patents

Abstract

PURPOSE:To produce a Bi-contg. oxide superconducting material having an increased critical temp. at which transition to superconductivity is attained an improved superconducting characteristics. CONSTITUTION:Powdery starting materials for a Bi-contg. oxide superconducting material are heated in a low oxygen atmosphere under <=0.1 atm partial pressure of oxygen and the heated starting materials are molded and fired to produce a Bi-contg. oxide superconducting material.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a bismuth-based oxide superconducting material having an improved transition initiation temperature.

0002

[Prior Art] Conventionally, a method for producing bismuth-based oxide superconducting materials uses solid-phase reaction powder or co-precipitated powder as raw material powder, and synthesizes the superconducting material by pre-calcining it in the atmosphere. Generally, a process is adopted in which the material is molded and then fired.

The bismuth-based oxide superconducting material thus obtained has a critical temperature (Tc) for transitioning to superconductivity of 8.
There are at least two types of superconducting phases: a low Tc phase around 0K and a high Tc phase with a critical temperature around 110K.

Among these, low Tc with a critical temperature of around 80K
It is known that the superconducting properties of the phase change very sensitively to the amount of oxygen it contains, similar to what is seen in yttrium-based superconducting materials, for example. Contrary to yttrium-based superconductors, bismuth-based superconductors tend to have a higher critical temperature as the oxygen content decreases. Studies are underway to control the amount of oxygen in the body and raise the critical temperature.

[0005]

However, no matter how much the firing atmosphere is controlled in the above-mentioned conventional manufacturing method, it has not been possible to sufficiently improve the critical temperature of the resulting sintered body. This is believed to be because in the conventional method described above, the superconducting phase in the raw material powder after calcination has a high oxygen content, that is, the critical temperature is low.
When this was molded and fired as it was, it was difficult to reduce the oxygen content only by controlling the firing atmosphere.

[0006] Furthermore, when producing a superconducting material mainly consisting of a high Tc phase, lead is often added to the raw material powder in order to promote its formation. However, because the amount of lead dissolved in the high-Tc phase was low, much of the added amount was dissipated as vapor during the sintering process, so the effect of adding lead on improving superconducting properties could not be expected to be significant. .

The present invention has been made in view of the above-mentioned conventional problems, and is intended to produce a bismuth-based oxide superconducting material sintered body with improved superconducting properties by increasing the critical temperature at which it transitions to superconductivity. The purpose is to provide a method.

[0008]

[Means for Solving the Problems] The method for producing a bismuth-based oxide superconducting material of the present invention is to process raw material powder for a bismuth-based oxide superconducting material in a low-oxygen atmosphere with an oxygen partial pressure of 0.1 atmosphere or less. It is characterized by consisting of a heat treatment step in which the powder is heated, and a firing step in which the obtained heat-treated powder is molded and then fired.

In the present invention, the bismuth-based oxide superconducting material includes at least bismuth (Bi) strontium (
It is a superconducting material made of an oxide containing Sr), calcium (Ca), and copper (Cu). The method for producing the raw material powder is not particularly limited, but solid phase reaction powder, co-precipitated powder, etc. can be used. When manufacturing a superconductor mainly consisting of a high Tc phase, lead (Pb) is often added, but the amount of lead added in this case is in the range of 15 to 50% of the amount of bismuth. It is desirable to do so.

In the heat treatment step, this raw material powder is heated in a low oxygen atmosphere with an oxygen partial pressure of 0.1 atm or less. When the oxygen partial pressure exceeds 0.1 atm, the effect of improving the transition start temperature is small. Usually, this can be achieved by using a gas mixture of argon (Ar), nitrogen (N2), etc. and oxygen (O2), or by reducing the pressure of the entire heat treatment system.

[0011] The heat treatment temperature is preferably in the range of 400 to 850°C. Although oxygen enters and exits from around 200℃,
Below 0°C, the amount is small and the effect is small. 85 again
If the temperature exceeds 0°C, the reducing action becomes strong and the decomposition of the superconductor increases, which is not preferable.

[0012] After the powder that has undergone the heat treatment process is molded,
Subsequently, it is subjected to a firing process. The firing temperature is 750~
A range of 900°C is desirable. If the temperature is lower than 750°C, the sintering effect will be small and the crystallinity of the superconductor will also be poor. 900℃
If the value exceeds 100%, melting will become intense and the superconducting phase will disappear. The sintering atmosphere is not particularly limited, but when producing a superconductor mainly consisting of a low Tc phase, sintering should be performed under low oxygen pressure, or measures should be taken to prevent the sintered body from absorbing oxygen from the atmosphere. It becomes necessary. For example, if raw material powder is packed in a silver-Ag tube, densified through plastic processing, and then fired, oxygen absorption can be reduced due to the presence of the silver sheath material on the surface, so even when fired in the atmosphere, the critical temperature can be maintained. Deterioration can be prevented.

[0013]

[Operation] The low Tc phase in the bismuth-based oxide superconducting material is a system in which holes are overdoped, and when oxygen is removed from the composition, the hole concentration decreases and the critical temperature increases. In the present invention, the raw material powder is heat-treated in a low-oxygen atmosphere, which makes it possible to easily control the oxygen in the superconducting phase, reduce the amount of oxygen contained, and increase the critical temperature. becomes. In addition, homogenization of the entire sintered body,
It is also useful for removing weakly bonded phases and leads to an improvement in critical current density (Jc).

On the other hand, when lead is included in the composition, the solid solubility limit of lead is increased by performing the heat treatment step under low oxygen pressure, and more lead can be dissolved in the solid solution. Furthermore, by increasing the amount of solid solution in the heat treatment stage, dissipation due to evaporation of lead in the firing process is suppressed. As a result, the amount of solid solution of lead increases,
Critical temperature improves. Although the reason why the critical temperature is improved by solid solution of lead in the high Tc phase is not well understood, it is expected that it is indirectly involved in the hole concentration at the Cu-O surface.

[0015]

[Example] Solid phase reaction powder blended so that Bi:Sr:Ca:Cu=1:1:1:2 was heated at 840°C and O2/A.
Heat treatment was performed in a mixed atmosphere. The O2/Ar mixing ratio was varied within the range of 0 to 1 as shown in Table 1. The heat-treated powder was pressed with a mold to obtain a molded body of 5 x 2 x 20 mm, and further fired at 850° C. for 4 hours under an oxygen partial pressure of 0.1 atm to obtain a fired body. The superconductivity of the obtained bismuth-based superconducting material was evaluated, and the results are shown in Table 1. Superconductivity was evaluated by measuring alternating current magnetic susceptibility, and the starting point of rapid change in magnetic susceptibility was defined as the critical temperature Tc.

[0016] In addition, the powder heat-treated in the same manner was sieved through #80 mesh and packed into a 10 mmφ Ag tube.
Swaged to This material was heated to 860℃ in the atmosphere.
It was baked for 4 hours. Superconductivity was similarly evaluated, and the results are also listed in Table 1.

[0017]

[Table 1]

As shown in the table, the heat treatment was carried out at an oxygen partial pressure of 0.
．． In Examples 1 to 3, which were conducted at 1 atm or less, the critical temperature Tc was significantly improved. In addition, Ag in Example 2 and Comparative Example 2
The critical current density Jc of the sheath material was measured using a four-terminal current in liquid nitrogen N2.
104 A/cm2 (Example 2), 1.5×103
A/cm2 (Comparative Example 2), and it can be seen that the critical current density Jc of the bismuth-based oxide superconducting material obtained by the method of the present invention is also improved compared to the conventional one.

Next, Bi:Sr:Ca:Cu=2:2:
A bismuth-based oxide superconducting material was produced by the same method using a raw material powder obtained by calcining coprecipitated powder at 750°C with a ratio of 2:3 and replacing bismuth with lead in a range of 15 to 50%. was manufactured. The heat treatment was performed at 840° C. in an O2/Ar mixed gas with an oxygen partial pressure of 0.05 atm. Firing was performed in the same manner as in the above example to produce a normal fired body and Ag sheath material. Firing conditions are oxygen partial pressure 0.1 atm, 8
The temperature was 40°C for 4 hours. For comparison, superconducting materials were manufactured in the same manner except that the heat treatment was performed under an oxygen partial pressure of 0.2 atm, and the superconducting properties of each were evaluated.

As a result, the oxygen partial pressure in the heat treatment step was reduced to 0.2
The critical temperature Tc of all samples set to atmospheric pressure was below 110K. On the other hand, the bismuth-based superconducting material of the present invention heat-treated at an oxygen partial pressure of 0.05 atm has a critical temperature of T
c was 115 to 120K, which is 5 to 10K higher than that of the conventional model.

[0021]

As described above, according to the method of the present invention, the critical temperature of a bismuth-based oxide superconducting material can be significantly improved compared to the conventional method. Therefore, the usable temperature will be significantly increased, and it can be expected to be used in a wider range of fields. Furthermore, it is possible to produce a bismuth-based oxide superconducting material with excellent superconducting properties, such as improved critical current density, which is an important element of superconducting properties.

Claims (1)

[Claims] [Claim 1] A heat treatment step in which raw material powder for bismuth-based oxide superconducting material is heated in a low-oxygen atmosphere with an oxygen partial pressure of 0.1 atmosphere or less, and the resulting heat-treated powder is shaped and then fired. 1. A method for producing a bismuth-based oxide superconducting material, comprising a firing step.