JPH0574550B2 - - Google Patents

Description

[Detailed description of the invention]

Field of Application The present invention relates to a method for producing raw material powder for superconducting ceramics, and particularly to a method for calcination of raw material powder for superconducting ceramics to obtain a raw material powder for calcination consisting of an orthorhombic low-temperature stable phase. BACKGROUND ART Conventionally, as a method for calcining starting materials for producing BaO-Y ₂ O ₃ -CuO-based superconducting ceramics, the starting materials have a particle size of 3 μm or less and a purity of 99.9%.
The above BaCO ₃ , Y ₂ O ₃ , and CuO powders were blended and mixed according to the required composition ratio, and then calcined in the air at a temperature of 900° C. to 1000° C. However, in the above calcining method, for example, when the raw material powder is filled in a container and heated, the raw material powder is formed into calcined powder depending on the conditions for filling the raw material powder into the container and the calcination conditions. Changes occurred in the crystal structure. Furthermore, in the conventional calcining method, contact with the atmosphere tends to be insufficient, and a green insulating phase (BaCuY ₂ O ₃ phase) may be formed in the calcined powder. Furthermore, a trace amount of BaCO ₃ may remain in the calcined powder, and when this powder is used for sintering, cracks and cracks are observed in the sintered body due to the generated CO ₂ gas, resulting in a dense sintered body. I had a problem with not being able to form a solid. Additionally, if the calcination temperature is 1020℃ or higher, CuO undergoes a decomposition reaction of 2CuO〓2Cu+O ₂ , and if calcination is performed at 900℃ to 1020℃, calcination of the mixed powder and grain growth may proceed. There were many other problems. Purpose of the Invention In view of the current situation, the present invention solves the problems in the conventional calcination method and develops an orthorhombic crystal that is optimal for producing BaO-Y ₂ O ₃ -CuO ceramics having excellent superconductivity. The object of the present invention is to provide a method for calcination of raw material powder for superconducting ceramics, which can yield raw material powder for calcination having a low-temperature stable phase structure. Summary of the Invention This invention uses BaCO ₃ , Y ₂ O ₃ ,
After mixing the required amount of CuO powder, the temperature is raised to 650℃ to 800℃ in a vacuum, and then calcined by holding at 650℃ to 900℃ for 10 to 20 hours in an O ₂ 100vol% atmosphere. Furthermore, the present invention is a method for calcination of raw material powder for superconducting ceramics, characterized by cooling at a cooling rate of 4° C./min or less. To explain this invention in more detail, the starting raw material powder is
It is preferable that the purity is 99.9% or more and the particle size is 3 μm or less, and the starting raw material powders of BaCO ₃ , Y ₂ O ₃ , and CuO having such properties are blended and mixed in a desired composition ratio. Next, in a vacuum of 1 × 10 ^-2 Torr or less, in order to completely decompose BaCO ₃ in the raw material powder,
Raise the temperature to 800℃. Furthermore, the atmosphere is changed to an O ₂ 100 vol % atmosphere, and the temperature is maintained at 650° C. to 900° C. for 10 hours to 20 hours for calcination. Thereafter, it is cooled in an atmosphere of 100 vol % O ₂ at a cooling rate of 4° C./min or less to obtain a calcined raw material powder consisting of an orthorhombic low-temperature stable phase. Reason for limitation In this invention, the raw material powder after blending is heated at 650°C to 800°C in a vacuum of 1 × 10 ^-2 Torr or less.
However, in order to completely decompose BaCO ₃ in the raw material powder, the higher the degree of vacuum, the better. If the temperature is less than 650℃, the decomposition of BaCO ₃ will be insufficient, and if it exceeds 800℃, sintering will occur. As this progresses, the temperature should be raised between 650°C and 800°C in vacuum. The reason for changing the atmosphere from vacuum to O ₂ 100% atmosphere after heating above 650℃ following the temperature increase in vacuum is that in calcination in vacuum, YBa ₂ Cu ₃ O _7-x (x=
0 to 0.25) cannot be obtained; in other words, the low-temperature stable phase (superconducting phase), orthorhombic, has a higher oxygen content than the high-temperature stable phase, tetragonal, and therefore cannot transition to the superconducting phase. This is because the calcination temperature is limited to 650°C to 900°C. Temperatures below 650°C are unfavorable because the calcination reaction takes a long time; This is not preferable because the sintering progresses and the next sintering temperature becomes high. The reason for limiting the holding time at the calcination temperature to 10 to 20 hours is that if it is less than 10 hours, a sufficient calcination reaction will not be obtained and unreacted materials will remain, and if it exceeds 20 hours, the effect will be saturated and the cost will be reduced. This is not preferable because it causes rise in temperature, and 10 to 20 hours are required. This is because if the cooling rate from the calcination temperature exceeds 4°C/min, the phase transition from tetragonal to orthorhombic crystals will be insufficient, and more preferably 1°C/min or less. The cooling atmosphere is preferably O ₂ 100vol%. Effects of the Invention According to the method of this invention, after completely decomposing BaCO ₃ in the raw material powder, it is calcined in an atmosphere of 100 vol% O ₂ and cooled to room temperature. No phase remains, and a uniform powder consisting entirely of superconducting phases is obtained. The resulting calcined powder was finely pulverized to a desired particle size, and the crystal structure of the finely pulverized calcined powder was investigated by X-ray diffraction, and it was found that it consisted of an orthorhombic low-temperature stable structure. The finely pulverized calcined powder obtained by the method of this invention is
At 550℃~600℃ in an atmosphere of _O2 20vol% or more
By heating and sintering at a pressure of 200Kg/cm ² to 2000Kg/cm ² and then cooling in a furnace, the entire sintered body becomes an excellent BaO−Y ₂ O ₃ −CuO consisting of an orthorhombic low-temperature stable phase.
system superconducting ceramics can be obtained. Example: BaCO ₃ , Y ₂ with a purity of 99.9% or more and a particle size of 1 μm or less
O ₃ and CuO powders were blended at a molar ratio of 2:1:3 and mixed for 6 hours in a ball mill containing alcohol. 2 up to 700℃ in a vacuum of 1×10 ^-3 Torr or less
After raising the temperature at a temperature increase rate of °C/min, the temperature was maintained at 700 °C for 1 hour. Then, switch to O ₂ 100vol% atmosphere and heat at 700℃.
The temperature was raised from 1 to 850°C at a rate of 2°C/min. Then, hold at 850℃ for 10 hours, and then
It was cooled to room temperature in an O ₂ atmosphere at a cooling rate of 1° C./min. The obtained calcined powder was finely pulverized to obtain calcined powder with a particle size of 1 μm or less. The crystal structure of this was measured by X-ray diffraction, and the result was that it was a low-temperature stable phase consisting of orthorhombic crystals. Then, the calcined powder was
Using a die made of SiC material, 100vol of O ₂ at 5 atm.
% atmosphere at 920° C. and a pressure of 1000 Kg/cm ² for 10 hours. Thereafter, it was cooled at a cooling rate of 1° C./min to obtain a sintered body with dimensions of 30 mm in diameter and 5 mm in height. The structure and crystal structure of the obtained sintered body were examined using X-ray diffraction and a microscope, and the results showed that the sintered body had an orthorhombic low-temperature stable phase and exhibited the Meissner effect at a Tc of 89°K. Superconducting ceramics were obtained. Comparative example Particle size with purity of 99.9% or more with the same composition ratio as the example
After mixing BaCO ₃ , Y ₂ O ₃ and CuO powders of 1 μm or less,
As shown in Table 1, after calcination under the conditions of elevated temperature in vacuum and calcination in O ₂ atmosphere, the calcined powder was obtained by cooling, and then finely pulverized to produce calcined powder with a particle size of 1 μm or less. The properties were investigated using X-ray diffraction and a microscope, and the results are shown in Table 2.

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Claims (1)

[Claims] 1. After mixing the required amounts of BaCO ₃ , Y ₂ O ₃ , and CuO powder as starting materials, the mixture was heated to 650°C to 800°C in vacuum, and then placed in an atmosphere of 100 vol% O ₂ . 1. A method for calcination of raw material powder for superconducting ceramics, characterized by calcination by holding at 650°C to 900°C for 10 to 20 hours, and further cooling at a cooling rate of 4°C/min or less.