JP2689937B2 - Method for producing mercury-based superconductor - Google Patents

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 mercury-based oxide superconducting material used in various superconducting devices and superconducting devices.

[0002]

2. Description of the Related Art Metal and alloy type superconductors and compound superconductors are widely used as Josephson devices, superconducting magnet wire rods and magnetic shield materials. Liquid helium is still the mainstream as the cooling means in these practical application devices. If the superconducting transition temperature (Tc) of the superconducting material being used rises and cheap liquid nitrogen or refrigerator can be used instead of liquid helium, it is expected that the applicable range of applied equipment will increase dramatically. Has been done.

Since the discovery of a high temperature superconductor in the La-Ba-Cu-O system in 1986, copper oxide superconductors have attracted attention, and many researchers have been actively conducting research activities. The announcement of various oxide superconductors has been made. Among them, the materials for which the highest Tc is currently reported are a group of copper oxide materials called mercury-based materials. Therefore,
It can be said that the mercury-based oxide superconductor is the most important substance in considering the application of the superconductor.

However, the mercury-based superconductor having such excellent characteristics also has a problem in synthesis. It is considered that a mercury-based superconductor has a non-stoichiometric ratio to mercury, that is, a compound having a slight mercury deficiency due to its stoichiometric composition.
Although such a slight defect does not significantly change the crystal structure or phase stability, it has a great influence on the electrical and magnetic properties, so controlling the amount of mercury deficiency in this substance is very practical. This is an important technology.

However, the temperature range (7
Since the vapor pressure of mercury at temperatures above 00 ° C is extremely high, it is inevitable that most of the mercury will escape from the sample during sintering in a normal firing furnace, and in the worst case, all mercury will be removed. In some cases, the target phase itself cannot be achieved at all. Therefore, a method in which a sample is enclosed in a quartz tube or the like and sintering is performed in a sealed state has been used so far.

[0006]

With the above-mentioned synthesis method in the sealed state, it is difficult to synthesize a large amount of samples at one time, and the synthesis conditions are determined only by the temperature, so the quality of the sample is improved. Since it is difficult to set the condition optimization to achieve this, it was not possible to completely suppress the mercury deficiency even by this method.

As a measure against mercury deficiency, a method of considering the evaporation of mercury in advance and synthesizing it from a raw material in which the amount of mercury is larger than the target composition can be considered, but although it is possible, the reproducibility of the composition is poor and control is difficult. In these prior art techniques, synthesis has been carried out at conditions far away from the chemical equilibrium for mercury evaporation. Further, as another measure against mercury deficiency, it is possible to increase the mercury vapor pressure in the gas phase. However, it is difficult to adopt the method in which the highly toxic mercury vapor is fired under high pressure.

The present invention has been made in view of the above circumstances, and an object thereof is to provide a method for producing a mercury-based superconductor which is free from mercury deficiency and suitable for mass production. To do.

[0009]

In order to achieve the above object, according to the present invention, there is provided a mercury-based superconductor for synthesizing a superconductor by sintering a mixture of a plurality of oxides including mercury oxide. In the manufacturing method, there is provided a method characterized in that the sintering is performed in a container having an increased oxygen partial pressure.

[0010]

In the synthesizing method of the present invention, firing is performed under a high oxygen partial pressure. At the time of firing, mercury oxide has an equilibrium state of HgO (s) ← → Hg (g) + (1/2) O ₂ (where s is a solid and g is a gas). it is conceivable that. Therefore, the reaction to the right side can be suppressed under the condition that the oxygen partial pressure is high. That is, the phenomenon of hydrogen desorption can be suppressed by the method of the present invention.

Further, according to this method, it is possible to control minute mercury deficiency by controlling the oxygen partial pressure and the firing temperature. Further, application of a high oxygen partial pressure can be easily performed by using, for example, a HIP (High Isostatic Press) device. Therefore, according to the present invention, it becomes possible to mass-produce a high-quality mercury-based superconductor by a simple method.

[0012]

Next, embodiments of the present invention will be described with reference to the drawings. [First Embodiment] FIG. 1 is a flow chart showing the procedure of sample synthesis according to the present invention. First, powders of HgO, BaO, CaO, and CuO oxides of mercury, barium, calcium, and copper are mixed in a metal element ratio of 1: 2: 2: 3. After press-molding this mixture, wrap it in gold foil and HI
Sinter with P equipment. The synthesis condition is argon: oxygen = 4:
In a mixed gas atmosphere having a ratio of 1, the total pressure is 100 to 100.
0 atm (optimum value: 700 atm), firing temperature 700-95
Firing was performed for 3 hours at 0 ° C (optimum value: 850 ° C).

The synthesized sample was analyzed by powder x-ray diffraction method (Rietveld analysis method) and EPMA (E
lectron Probe Micro Analizer), the obtained sample was HgBa ₂ Ca ₂ Cu ₃ O.
_{It was 8 + d} , which proved to be a good quality with no precipitation of impurities. This sample was found to exhibit a sharp superconducting transition at an absolute temperature of 137K.

[Second Embodiment] HgO, BaO, Ca
O and CuO in Hg: Ba: Ca: Cu = 1: 2: 1: 2
The mixture was mixed so as to form a mixture, pressed into a predetermined shape, wrapped in gold foil, and set in a HIP device. Argon: Oxygen =
The mixed gas mixed at a ratio of 4: 1 was heated to 800 ° C. and calcined for 3 hours while adjusting the total pressure to 500 atm.

In the powder x-ray diffraction pattern of the obtained sample, it was found that peaks from phases other than HgBa ₂ CaCu ₂ O _{6 + d} could not be observed and almost a single phase was obtained. In addition, this sample shows a good superconducting transition having a superconducting start temperature of 123 K (temperature at which electric resistance begins to decrease) and a zero resistance temperature of 121 K (temperature at which electric resistance completely disappears) in resistance measurement, and Meissner From the measurement of the diamagnetic susceptibility by the effect, a superconductor volume fraction of almost 100% was obtained. These facts indicate that the precipitation of the impurity phase and the deterioration of the superconducting properties due to the evaporation of Hg did not occur.

[Comparative Example] For comparison, a material having the same mixing ratio as in the second example was fired at the same temperature in oxygen at 1 atm. When the obtained sample was analyzed,
The x-ray diffraction pattern has almost no peaks from the HgBa ₂ CaCu ₂ O _{6 + d} phase, and the sample does not show superconductivity.
This is because almost all of the mercury has evaporated from the sample, which also indicates that a high oxygen pressure is useful for suppressing the evaporation of Hg.

[Third Embodiment] HgO, BaO, Ca
O and CuO are metal ratio Hg: Ba: Ca: Cu = 1: 1.
Mixed to be 2: 2: 3. After pressing this mixture into a predetermined shape, it was wrapped in a gold foil and put in a pressure-resistant container made of stainless steel. After filling the inside with pure oxygen at 9 atm, it was sealed and heated in an electric furnace at 850 ° C. for 3 hours. The internal oxygen pressure during firing is estimated to be 35 atm.

From the x-ray diffraction pattern, the obtained sample was H
It was revealed to be gBa ₂ Ca ₂ Cu ₃ O _{8 + d} . This sample was a good superconductor having a superconducting start temperature of 135K and a zero resistance temperature of 133K, and having a superconductor volume fraction estimated from magnetic susceptibility measurement of almost 100%.

[Fourth Embodiment] HgO, BaO, CuO
Were mixed so that the metal ratio was Hg: Ba: Cu = 1: 2: 1. After pressing this mixture into a predetermined shape, it was wrapped in gold foil and set in a HIP device. Argon: Oxygen =
The mixed gas mixed at a ratio of 4: 1 was heated to 800 ° C. and calcined for 3 hours while adjusting the total pressure to 500 atm.

In the powder x-ray diffraction pattern of the obtained sample, it was found that peaks from phases other than HgBa ₂ CuO _{4 + d} could not be observed and almost a single phase was obtained. Also, this sample shows a good superconducting transition with a superconducting onset temperature of 92K and a zero resistance temperature of 90K in resistance measurement,
Almost 1 from the measurement of diamagnetic susceptibility by the Meissner effect.
A superconductor volume fraction of 00% was obtained. These things are
It shows that the precipitation of the impurity phase and the deterioration of the superconducting properties due to the evaporation of Hg did not occur.

[0021]

As described above, according to the present invention, a mercury-based superconductor is synthesized in an atmosphere having a high oxygen partial pressure ratio. Therefore, according to the present invention, mercury in a sample during firing is Evaporation can be completely suppressed within the range where the superconductor is synthesized, and a large amount of samples exhibiting good superconducting properties can be obtained at one time.

[Brief description of the drawings]

FIG. 1 is a flowchart for explaining an embodiment of the present invention.

Claims (4)

(57) [Claims] 1. In a method for producing a mercury-based superconductor, which comprises synthesizing a superconductor by sintering a mixture of a plurality of oxides including mercury oxide, the sintering is performed in a container having an increased oxygen partial pressure. A method for producing a mercury-based superconductor characterized by the above. 2. The method for producing a mercury-based superconductor according to claim 1, wherein sintering is performed in an atmosphere having an oxygen partial pressure of 20 atm or more. 3. The method for producing a mercury-based superconductor according to claim 1, wherein sintering is performed in a mixed gas atmosphere of oxygen and an inert gas. 4. The method for producing a mercury-based superconductor according to claim 1, wherein the sintering is performed in a hot isostatic press or a closed pressure-resistant container.