JPH0417909B2 - - Google Patents

Description

[Detailed description of the invention]

B. Field of industrial application of the invention This invention relates to a superconducting material whose electrical resistance disappears at low temperatures. Conventional technology The superconductors known so far require cooling with extremely low temperature liquid helium (boiling point 4.2K), and the high cost of cooling and the uneven distribution of helium resources hinder their widespread use. was. The superconducting material that has been confirmed to have the highest critical temperature is Nb ₃ Ge.
Tc (transition onset temperature) was 23.6K, which did not reach a level that could be used under cooling with liquid hydrogen (boiling point 20.3K) or liquid neon (boiling point 27.1K). A barium-lanthanum-copper-oxygen oxide was pointed out by JGB'ednorz and KAMuller as a substance whose critical temperature could exceed 30K (Zeit.Phys.B-Condensed Matter vol.64,
P.189-193 (1986)), it is claimed that a decrease in resistivity is observed from around 30K, and that the resistance disappears at 13K. However, this has not yet reached the level where it can be used under cooling with liquid hydrogen or liquid neon. Problems to be Solved by the Invention An object of the present invention is to provide a superconducting material whose electrical resistance disappears at a higher temperature than conventionally known superconducting materials. B. Means for solving the structural problems of the invention The superconducting material according to the present invention is mainly composed of a composition of the general formula (La _1-x M _x ) ₂ CuO ₄ (1), and has a composition of K ₂ NiF ₄ type. It is characterized by having a crystal structure. Here, M represents Sr or Ca, x is 0.04 to 0.20,
Preferably it is a numerical value of 0.05 to 0.15. Cu mostly exists in a divalent state, La is trivalent,
As a result of M being divalent, the theoretical value of O in the composition of formula (1) above is 4-X, but depending on the firing temperature and atmosphere when crystallizing the composition, a part of Cu may be 3-X. In some cases, things that have become a state of valence coexist;
In that case, the overall molar ratio of O appears to be slightly higher than that of 4-X and approaches 4. That is, the molar ratio of O is a value that balances the total valence of the metal components, and should be understood as approximately expressed as 4. Further, a part of M may be a component other than Sr or Ca. In the present invention, the term "mainly composed" refers to a state in which the above-mentioned composition occupies the majority, and as long as it does not adversely affect the crystal structure and the achievement of the object of the present invention, compositions other than the above-mentioned composition may not be used. This does not exclude cases where metals coexist. Effect A material mainly composed of the composition of the above general formula and having a K ₂ NiF ₄ type crystal structure starts superconducting transition at a higher temperature than conventionally known superconducting materials. EXAMPLES Hereinafter, the present invention will be specifically explained with reference to Examples, but the present invention is not limited to these Examples. Example 1 In the above general formula, X is 0.05, 0.10 and 0.15
A lanthanum-strontium-copper-oxygen composition was prepared. Calculated reagent special grade La ₂ (CO ₃ ) ₂ , SrCO ₃ and CuO
Mix each powder in ethanol in an agate pot,
Place it in a crucible and raise the temperature to 700℃ in about 30 minutes.
The temperature was further raised to 1000℃ over 3 hours, and then heat treated at 1000℃.
Pressed into pellets at a pressure of Kg/cm ² and heated to 1000°C.
It was sintered in a furnace for 5 hours. It was confirmed by X-ray diffraction that each sintered product had a K ₂ NiF ₄ type crystal structure. For each, superconducting transition start temperature Tc (temperature at which magnetic susceptibility or electrical resistivity begins to show superconducting transition) and transition temperature width △Tc (when magnetic susceptibility or electrical resistivity changes from the normal value (near Tc)) Table 1 shows the results of measuring the temperature interval when the rate of change of 90% to 10%.

[Table] Example 2 Calculated amount of reagent special grade La ₂ (CO ₃ ) ₂ , CaCO ₃ and CuO
Lanthanum-calcium-copper-oxygen compositions in which X in the general formula is 0.05 and 0.15 were prepared from each powder in the same manner as in Example 1, and sintered in the same manner as in Example 1. It was confirmed by X-ray diffraction that each sintered product had a K ₂ NiF ₄ type crystal structure. Table 2 shows the results of measuring the superconducting transition start temperature Tc and transition temperature width ΔTc for each.

[Table] Example 3 In the general formula, M is a mixture of Sr and Ca and x
Table 3 shows the results of preparing samples according to Examples 1 and 2 and measuring the superconducting transition starting temperature Tc for the cases where M = 0.1 and when a part of M was replaced with Ba.

【table】

[Table] Based on this result, superconducting transition starting temperature Tc in Sr, Ca, Ba ternary system (total atomic ratio = 0.1)
is shown in Figure 1. Example 4 In the general formula, M is a mixture of Sr and a rare earth element, and the atomic ratio of Sr is 0.1 and the atomic ratio of the rare earth element is
Table 4 shows the results of preparing samples according to Example 1 and measuring the superconducting transition start temperature Tc for the case where the temperature was 0.01 to 0.05.

[Table] Example 5 In the general formula, M is Sr and x = 0.1, and a sample in which part of Cu was replaced with the metal shown in Table 5 was prepared according to Example 1, and the superconducting transition starting temperature Tc The results of the measurements are shown in Table 5.

【table】

[Table] Example 6 An experiment was conducted to determine the optimum Sr atomic ratio x in the (La _1-x Sr _x ) ₂ CuO ₄ system. After weighing the raw materials containing La ₂ O ₃ , SrCO ₃ and CuO and varying the ratio of x, they were mixed in a mortar and 870
After calcination for 6 hours at ℃, further crushing and mixing, 1
The pellets were made into 1 cmφ pellets at a pressure of ton/cm ² and heated at 1100°C.
Main firing was performed for 22 hours, followed by annealing at 800°C in oxygen for 20 hours. AC magnetic susceptibility (AC-
susceptibility) was measured. Figure 2 shows the measurement results for samples with Sr atomic ratios x of 0.06, 0.08, and 0.10. In Figure 2, the horizontal axis represents absolute temperature (K), and the vertical axis represents AC magnetic susceptibility (△L/w). The point at which the line bends in Figure 2 (the temperature at which magnetic susceptibility begins to drop) is Tc. Figure 3 shows the relationship between x and Tc when the value of x is changed more finely. In Figure 3, the horizontal axis is the value of x, and the vertical axis is
Represents Tc. Tc shows the highest value around x=0.08. Example 7 Based on the results of Example 6, the optimal conditions for oxygen annealing after firing in the composition of (La _0.92 Sr _0.08 ) ₂ CuO ₄ were determined. FIG. 4 shows the results of AC magnetic susceptibility measurements of samples obtained by firing the pellets produced in accordance with Example 6 at 1100° C. for 22 hours, annealing and pulverizing at varying temperatures and times. In FIG. 4, the horizontal axis represents absolute temperature (K), and the vertical axis represents alternating current magnetic flux (ΔL/w). It is shown that the annealing temperature is preferably 600 to 800°C. Furthermore, the results of measuring the electrical resistivity are shown in Table 6 and FIG. In Table 6, Tc ^zero is the temperature at which electrical resistance becomes 0, and Tc ^mid is... Also, in Figure 5, the horizontal axis is absolute temperature (K) and the vertical axis is electrical resistivity (10 ^-3 S ^-1 cm). represents.

[Table] It is shown that the annealing temperature is preferably 600 to 800°C. Example 8 La ₂ (CO ₃ ) ₃ , SrCO ₃ and CuO were used as raw materials, weighed to give (La _0.92 Sr _0.08 ) ₂ CuO ₄ and dissolved in nitric acid. This was dropped into an oxalic acid aqueous solution to cause coprecipitation. The coprecipitated powder obtained by filtration after being left overnight was calcined in the air at 900℃ for 5 hours and at 1000℃ for 24 hours.
Pellets with a diameter of 1 cm were formed at a pressure of n/cm ² and heated to 22
Sinter for 1 hour and then heat the pellets at 800℃ for 2 hours → 600℃
Annealing treatment was carried out for 2 hours → 48 hours at 400°C → the power was turned off in the furnace and allowed to cool to room temperature. The results of measuring the electrical resistivity of this sample are shown in FIG.
The electrical resistance became 0 at 38.2K. C. Effects of the Invention The superconducting material of the present invention has the following advantages. The high critical temperature makes cooling much easier than before. Because it is stable even when heated to a high temperature of around 1100°C in air, it has great flexibility in manufacturing as a superconducting wire or electronic device material. Because it is a ceramic-based superconductor, its electrical, magnetic, and mechanical properties are thought to be different from conventional metal-based superconductors. , increasing the diversity of their properties. In particular, the La-Sr-Cu-O type is liquid hydrogen (boiling point 20.3K) or liquid neon (boiling point
Can be used under cooling (27.1K).

[Brief explanation of drawings]

Figure 1 shows the ternary system of Sr, Ca, and Ba (total atomic ratio =
A diagram showing the superconducting transition starting temperature Tc at 0.1),
Figure 2 shows the relationship between temperature and AC magnetic susceptibility for samples with Sr atomic ratios of 0.06, 0.08 and 0.10, Figure 3 shows the relationship between Sr atomic ratio and Tc, and Figure 4 shows the annealing temperature and time. Figure 5 is a diagram showing the relationship between sample temperature and AC magnetic susceptibility when the annealing temperature and time are changed. Figure 6 is a diagram showing the relationship between sample temperature and electrical resistivity when the annealing temperature and time are changed. Figure 6 is the temperature of the sample of Example 8. FIG. 2 is a diagram showing the relationship between electrical resistivity and

Claims (1)

[Claims] 1 Mainly composed _of the general formula (La _1-x M _x ) ₂ CuO ₄ where M ₌ Sr, Ca A superconducting material. 2. The superconducting material according to claim 1, wherein M is Sr and X is 0.08.