JPH0574548B2 - - Google Patents

Description

[Detailed description of the invention] Industrial field of application The present invention relates to a method for manufacturing superconducting ceramics exhibiting the Meissner effect, in which the entire obtained sintered body consists of a low-temperature stable phase having superconductivity.
The present invention relates to a method for producing BaO-Y ₂ O ₃ -CuO-based superconducting ceramics. Background technology Conventionally, superconducting materials include Nb-Ti, Nb-Sn,
Alloy materials such as Nb ₃ Sn or intermetallic compound materials are known. The superconducting material exhibits the Meissner effect when the critical temperature Tc at which the electrical resistance becomes zero is at most 30K. However, recently, high-temperature superconducting materials that exhibit the Meissner effect at a critical temperature Tc of around 90K have been developed.
BaO-Y ₂ O ₃ -CuO-based superconducting ceramics have been proposed, and many research studies have begun. This BaO−Y ₂ O ₃ −CuO based superconducting ceramic,
For example, YBa ₂ Cu ₃ O _7-x ceramics undergoes a phase transition at around 550°C to 600°C, and in this case, the tetragonal structure of the high temperature phase does not exhibit a superconducting phase.
It is known that the orthorhombic structure of the low-temperature stable phase exhibits a superconducting phase. On the other hand, regarding the firing of such superconducting ceramics, the firing method using conventional powder metallurgy is not possible at 950°C.
The method used was to sinter the material nearby for five days and then cool it. During this sintering, oxygen is absorbed during transformation from a high-temperature phase with a tetragonal structure to a low-temperature stable phase with an orthorhombic structure. However, if the oxygen supply during metamorphosis is insufficient,
Even at low temperatures, the tetragonal structure exists as a metastable phase, and in particular, in dense sintered bodies, oxygen cannot be supplied to the inside, and the entire sintered body is transformed into a superconducting orthorhombic phase. It was difficult to transform the organization. Purpose of the Invention The object of the present invention is to provide a method for manufacturing superconducting ceramics that can transform the entire obtained sintered body into an orthorhombic structure of a superconducting phase when manufacturing BaO-Y ₂ O ₃ -CuO-based superconducting ceramics. . Summary of the Invention This invention provides orthorhombic crystal formation by mixing required amounts of BaCO ₃ , Y ₂ O ₃ , and CuO as calcined raw material powders, and calcining the mixture at 700°C to 900°C for more than 1 hour in the air. YBa ₂ Cu ₃ O _7-x with a grain size of 1 μm or less, consisting of a mixed structure of a tetragonal structure and a tetragonal structure
Raw material powder with the composition (x = 0 to 0.25) is heated at a temperature of 600°C to 1050°C, 300Kg/cm 2 to 2000Kg/in an atmosphere of ₁ atm to 10 atm and 20vol% or more of O ² .
After pressure sintering for 2 to 20 hours at a pressure of ^cm2 ,
This is a method for producing superconducting ceramics characterized by slow cooling at a cooling rate of 2° C./min or less. Structure of the Invention To explain this invention in detail, first, BaO−Y ₂ O ₃ −
It is a starting material powder for CuO-based superconducting ceramics.
After mixing the required amounts of BaCO ₃ , Y ₂ O ₃ and CuO, calcination is performed in the air at 700° C. to 900° C. for 1 hour or more. Furthermore, it is finely pulverized to a particle size of 1 μm, consisting of a mixed structure of an orthorhombic low-temperature stable phase and a tetragonal high-temperature unstable phase.
The following calcined powder is obtained. This calcined powder is maintained at a pressure of 300 Kg/cm ² to 2000 Kg/cm ² in an O ₂ atmosphere of 1 atm to 10 atm and 20 vol% or more, and heated at 600°C to 1050°C for 2 to 20 hours. The material is then pressure sintered, followed by slow cooling at a rate of 2°C/min or less, resulting in a structure consisting of an orthorhombic, low-temperature stable phase.
The purpose is to manufacture BaO-Y ₂ O ₃ -CuO-based superconducting ceramics. In this invention, the pressure sintering method may be a hot isostatic pressing method or a hot pressing method. Reason for limitation In this invention, the particle size of the calcined raw material powder is 1 μm.
The reason for limiting the grain size to the following is that if the grain size exceeds 1 μm, high temperatures are required to obtain a dense sintered body, and the crystal grains become large, making it difficult to undergo phase transition to orthorhombic crystals inside the grains. do not have. The pressure sintering conditions in this invention are as follows. The atmosphere during pressure sintering must be at least 20 vol% O ₂ atmosphere in order to generate only orthorhombic crystals. Further, heating conditions are not preferable since sintering is difficult to proceed if it is less than 600°C, and CuO decomposes if it exceeds 1050°C, so it is necessary to set the heating condition to 600°C to 1050°C. In addition, if the pressure condition is less than 300Kg/ ^cm2 , the powder will not become densified and the effect of pressure sintering will not be recognized.
If it exceeds 2000Kg/cm ² , the device itself is not practical, so it is limited to 300Kg/cm ² to 2000Kg/cm ² . In addition, the pressure sintering conditions are 1 atm to 10 atm.
The reason for using an O ₂ atmosphere of 20 vol % or more is that if it is less than 1 atm, the amount of oxygen will decrease, which is undesirable, and if it exceeds 10 atm, it will require high-pressure equipment, which is not preferred. In addition, the reason for limiting the holding time to 2 to 20 hours is that densification will not be completed if it is less than 2 hours, and if it exceeds 20 hours, it is not practical in terms of equipment or safety, and it is not effective for improving properties. Due to the lack of. In this invention, the cooling from the above-mentioned heating temperature is performed slowly, and the lower the cooling rate, the more preferable it is.
C/min or less is desirable. Effects of the Invention This invention provides that the calcined powder of the blended raw material powder is a mixed phase of orthorhombic crystals (superconducting phase) and tetragonal crystals (non-superconducting phase) that are easy to manufacture, but the blended raw material powder has a stable superconducting phase. Because the sintering is carried out under pressure under conditions that produce the same conditions, all the tetragonal crystals transform into orthorhombic crystals, and the entire sintered body consists of a homogeneous superconducting phase, making it possible to obtain a superconducting material with a high critical temperature Jc. . Example: BaCO ₃ , Y ₂ with a purity of 99.9% or more and a particle size of 2 μm or less
O ₃ and CuO powder were mixed in a molar ratio of 2:1:3, mixed for 6 hours in a ball mill containing alcohol, and then dried. Thereafter, it was calcined in the air at 900°C for 20 hours, and then finely pulverized to obtain calcined powder with a particle size of 1 μm or less. As a result of examining the crystal structure of the calcined powder using X-ray diffraction, it was found that it consisted of a mixed composition of a low-temperature stable phase consisting of orthorhombic crystals and a high-temperature unstable phase consisting of tetragonal crystals. This calcined finely pulverized powder is made of material SiC and
Using a die with dimensions of 20 mmφ x 30 mm height, heat to 920°C at a heating rate of 2°C/Hr in an atmosphere of 5 atm O ₂ 100vol%, then at 920°C and a pressure of 1000 Kg/cm ²
After holding for 10 hours, hot pressing was performed. Thereafter, the sintered body was cooled at a cooling rate of 1° C./min to obtain a sintered body having dimensions of 20 mm in diameter and 5 mm in height. As a result of examining the structure and crystal structure of the obtained sintered body using an X-ray diffraction method and a microscope, it was clear that the sintered body exhibited a low-temperature stable phase structure of orthorhombic crystals.
Superconducting ceramics exhibiting the Meissner effect were obtained at Tc of 90°K. (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 powder of 2 μm or less,
After calcining under the same calcining conditions as in the example, the calcined powder was finely pulverized to an average particle size of 1 μm or less, and the calcined powder was investigated by X-ray diffraction and found to be a low-temperature stable phase consisting of orthorhombic crystals and squares. It was found that the structure consists of a mixed structure of high-temperature unstable phases consisting of crystals. After charging the calcined powder into a die with the same dimensions and material as in the example, it was pressure sintered under the heating conditions and pressure sintering conditions shown in Table 1, and then slowly cooled to form a sintered body. Obtained. Table 1 shows the results of examining the obtained sintered body using an X-ray diffraction method and a microscope. 【table】

Claims (1)

[Claims] 1. After mixing the required amounts of BaCO ₃ , Y ₂ O ₃ , and CuO as calcination raw material powder, calcination is performed at 700°C to 900°C in the air for 1 hour or more, and then an orthorhombic structure and a tetragonal structure are formed. YBa ₂ Cu ₃ with a particle size of less than 1 μm consisting of a mixed structure of
Raw material powder with O _7-x composition is exposed to O ₂ at 1 atm to 10 atm.
In an atmosphere of 20vol% or more, at a temperature of 600℃ to 1050℃ and a pressure of 300Kg/cm ² to 2000Kg/cm ² for 2 hours or more
A method for producing superconducting ceramics, which comprises performing pressure sintering for 20 hours and then gradually cooling at a cooling rate of 2°C/min or less.