KR101044890B1 - FABRICATION METHOD OF MgB2 SUPERCONDUCTING WIRE - Google Patents

Abstract

Disclosed is a method for producing a magnesium diboron superconducting wire. The method for producing a magnesium diboron superconducting wire may include preparing milled boron powder and unmilled magnesium powder, respectively; Filling a metal tube with a mixed powder mixed with boron powder and magnesium powder; Drawing a metal tube filled with the mixed powder; And heat treating the drawn metal tube below 650 ° C.

Description

Manufacturing Method of Magnesium Diboron Superconducting Wire {FABRICATION METHOD OF MgB2 SUPERCONDUCTING WIRE}

The present invention relates to the production of superconducting wires, and more particularly to a method for producing a magnesium diboron superconducting wire.

Magnesium diborate (MgB _2, T _c = 39 K), a superconductor with relatively high critical temperature ( T _c ), is a low temperature superconductor, Nb-Ti ( T _c = 9 K) and Nb ₃ Sn Can operate at temperatures higher than ( T _c = 18 K). In addition, since the magnesium diboron is prepared using a cheap magnesium (Mg) powder and a boron (B) powder as a starting material as compared to Nb-Ti or Nb ₃ Sn, there is an advantage in practical applications.

Unlike high temperature superconductors having a high critical temperature, the magnesium diboron has no weak link between grain boundaries and has a long coherence length. Because of this, if the magnesium diboron has a smaller grain size, the magnesium diboron, like other low temperature superconductors, has a critical grain boundary of magnesium diboron as the density of the grain boundary acts as a flux pinning center. Can improve the characteristics.

One embodiment of the present invention provides a method for producing a magnesium diboron superconducting wire having a small grain size using a low temperature heat treatment process and milled boron powder.

One embodiment of the present invention is a duplex capable of improving superconducting characteristics such as upper critical field ( H _c2 ), irreversibility field ( H _irr ), critical current density ( J _c ), and the like. Provided is a method for producing a small magnesium superconducting wire.

One embodiment of the present invention provides a method for producing a magnesium diboron superconducting wire that can shorten the reaction time of magnesium and boron at low temperatures.

Method for producing a magnesium diboron superconducting wire according to an embodiment of the present invention comprises the steps of preparing milled boron powder and unmilled magnesium powder, respectively; Filling a metal tube with a mixed powder of the boron powder and the magnesium powder; Drawing the metal tube filled with the mixed powder; And low temperature heat treatment of the drawn metal tube below 650 ° C.

According to one embodiment of the present invention, since the reactivity of the boron powder is improved by milling, the boron powder and magnesium powder can be reacted in a short time even at low temperatures. In addition, since the low-temperature heat treatment and milled boron powder are used, magnesium diboron may have a small grain size, and superconducting characteristics such as upper critical magnetic field, irreversible magnetic field, and critical current density may be improved.

Hereinafter, a method of manufacturing a magnesium diboron superconducting wire according to an aspect of the present invention will be described in detail.

To prepare the magnesium diboron superconducting wire according to one aspect of the present invention, first, milled boron powder and unmilled magnesium powder are prepared.

The unmilled boron powder may be dry milled or wet milled to prepare the milled boron powder. However, when dry milling the boron powder, the boron powder is easily oxidized to form a boron oxide (B ₂ O ₃ ) powder. Therefore, it is preferable to perform wet milling using, for example, a solvent free from oxygen, in which an anti-oxidation atmosphere is formed during milling of the boron powder. Milling the boron powder may improve the reactivity of the boron powder. On the other hand, the reason for using the unmilled magnesium powder to manufacture the magnesium diboron superconducting wire according to an aspect of the present invention is as follows. That is, for example, when the magnesium powder and the boron powder are milled simultaneously, the magnesium powder may be easily oxidized during milling to form magnesium oxide (MgO) powder due to the high activity of the magnesium powder. At this time, if the fraction of the produced magnesium oxide phase is increased, the superconducting phase fraction of magnesium diborate is reduced, and thus the superconducting characteristics of magnesium diborate may be reduced. Therefore, as described above, the use of unmilled magnesium powder is used.

After preparing the milled boron powder and the unmilled magnesium powder, respectively, the milled boron powder and the unmilled magnesium powder are mixed to prepare a mixed powder, and then the mixed powder is filled into a metal tube. As the material of the metal tube, it is possible to use a metal which does not deteriorate the properties of the magnesium diboron superconducting wire according to an aspect of the present invention and which is not reactive with magnesium diborate and has drawability.

After the mixed powder is filled in the metal tube, the metal tube is drawn to reduce the outer diameter of the metal tube and to lengthen the metal tube. Through this process, the mixed powder in the metal tube can be strongly compressed.

After drawing out the metal tube, the mixed powder and the drawn metal tube may be prevented from being oxidized as necessary. For example, the drawn metal tube may be sealed using another metal tube, but the present invention is not limited or limited thereto. In the sealing, the entire drawn metal tube may be sealed, or the drawn metal tube may be cut to a predetermined size and sealed.

Next, the drawn or anti-oxidized metal tube is heat treated at less than the melting point (650 ° C.) of magnesium, for example, in an inert atmosphere at 500 to 600 ° C., and then cooled to diboron according to an aspect of the present invention. Magnesium superconducting wire can be produced. Here, as the heat treatment temperature is closer to the melting point of the magnesium, the heat treatment time required for sufficiently solid phase reaction of the boron powder and the magnesium powder may be shortened, while the heat treatment time may be longer as the heat treatment temperature decreases. At this time, if the heat treatment temperature is less than 450 ℃, it may vary depending on various variables of the boron powder itself used, for example, purity and powder diameter, but even if the heat treatment for a long time is insufficient or almost no magnesium diboron phase formation You may not. Therefore, in consideration of the above points, the heat treatment is preferably carried out at 450 ℃ or less than 650 ℃.

By the low temperature heat treatment as described above, the boron powder and the magnesium powder in the metal tube may be in a solid phase reaction. At this time, the milled boron powder has an improved reactivity compared to the unmilled boron powder, so that the solid phase reaction may occur more quickly and smoothly. In addition, the solid phase reaction by the low temperature heat treatment may form a magnesium diboron phase having a small grain size due to the slow grain growth rate. In addition, the crystallinity of magnesium diboride decreases due to the milling of the boron powder, thereby lowering the critical temperature ( T _c ) of the superconducting wire. Rather can be improved. Here, the term “low temperature” refers to a temperature at or above the magnesium melting point, that is, a concept in contrast to the high temperature, and a temperature at which the boron powder and the magnesium powder can react in a solid phase.

Since the low-temperature heat treatment may be performed so that the boron powder and the magnesium powder sufficiently react with the solid phase, the heat treatment time is not particularly limited, but the heat treatment may be performed for several hours to several hundred hours, for example, 5 to 100 hours. can do. In addition, the heat treatment may be performed in an inert gas, for example, argon gas atmosphere that is not reactive with the magnesium diboron superconducting wire.

Hereinafter, the present invention will be described in detail with reference to the following examples. However, the technical spirit of the present invention is not limited or limited thereto.

[ Example ]

One. Magnesium diborate  Superconductivity Wire rod  Produce

Example  One

Magnesium powder (99%, 4-6 μm, Tangshan Weihao Magnesium Powder, China) and low purity boron powder (95-97%, 1 μm or less, Tangshan Weihao Magnesium Powder, China) were prepared. Subsequently, using a planetary ball milling machine (Pulverisette 5, Fritsch, Germany) equipped with 1.9 mm diameter zirconia balls, boron powder with toluene (C ₇ H ₈ ) solvent at 200 rpm planetary disc speed for 2 hours. Wet milling and then dried at 100 ° C. in a vacuum oven. The milled boron powder was then mixed with magnesium powder in a ratio of Mg: B = 1: 2. Thereafter, a mixed powder mixed with magnesium and boron (Mg + 2B) was filled into an iron tube (Fe tube) having an outer diameter of 8 mm and an inner diameter of 6 mm. The composite tube was then drawn to have an outer diameter of 1.58 mm. Thereafter, the magnesium diborate / iron wire, which was cut to a suitable length to prevent oxidation of the magnesium diborate / iron wire, was sealed in a titanium (Ti) tube, and then at 500 ° C. at a heating rate of 5 ° C./min under high purity argon gas. After heat treatment for 40 hours, the furnace (furnace) was cooled to room temperature to prepare a magnesium diboron superconducting wire.

Example  2

A magnesium diboron superconducting wire was prepared through the same process as in Example 1 except that the heat treatment was performed at 500 ° C. for 80 hours.

Example  3

Magnesium diboron superconducting wire was prepared through the same process as Example 1 except that heat treatment was performed at 550 ° C. for 10 hours.

Example  4

Magnesium diboron superconducting wire was prepared through the same process as in Example 1 except that heat treatment was performed at 550 ° C. for 20 hours.

Example  5

Magnesium diboron superconducting wire was prepared in the same manner as in Example 1 except that heat treatment was performed at 550 ° C. for 40 hours.

Example  6

Magnesium diboron superconducting wire was prepared in the same manner as in Example 1 except that heat treatment was performed at 550 ° C. for 80 hours.

Example  7

Magnesium diboron superconducting wire was prepared through the same process as Example 1 except that heat treatment was performed at 600 ° C. for 10 hours.

Example  8

Magnesium diboron superconducting wire was prepared in the same manner as in Example 1 except that heat treatment was performed at 600 ° C. for 20 hours.

Example  9

Magnesium diboron superconducting wire was prepared in the same manner as in Example 1 except that heat treatment was performed at 600 ° C. for 40 hours.

Comparative example  One

A magnesium diboron superconducting wire was prepared in the same manner as in Example 1 except that the boron powder was used without milling.

Comparative example  2

Magnesium diboron superconducting wire was prepared through the same process as in Example 1 except that the boron powder was used without milling and heat-treated at 500 ° C. for 80 hours.

Comparative example  3

Magnesium diboron superconducting wire was prepared through the same process as Example 1 except that the boron powder was used without milling and heat-treated at 550 ° C. for 10 hours.

Comparative example  4

Magnesium diboron superconducting wire was prepared in the same manner as in Example 1 except that the boron powder was used without milling and heat-treated at 550 ° C. for 20 hours.

Comparative example  5

Magnesium diboron superconducting wire was manufactured through the same process as Example 1 except that the boron powder was used without milling and heat-treated at 550 ° C. for 40 hours.

Comparative example  6

Magnesium diboron superconducting wire was manufactured through the same process as Example 1 except that the boron powder was used without milling and heat-treated at 550 ° C. for 80 hours.

Comparative example  7

Magnesium diboron superconducting wire was manufactured through the same process as Example 1 except that the boron powder was used without milling and heat-treated at 600 ° C. for 10 hours.

Comparative example  8

Magnesium diboron superconducting wire was prepared through the same process as in Example 1 except that the boron powder was used without milling and heat-treated at 600 ° C. for 20 hours.

Comparative example  9

Magnesium diboron superconducting wire was prepared through the same process as in Example 1 except that the boron powder was used without milling and heat-treated at 600 ° C. for 40 hours.

Comparative example  10

Magnesium diboron superconducting wire was prepared in the same manner as in Example 1 except that heat treatment was performed at 800 ° C. for 0.5 hour.

Comparative example  11

A magnesium diboron superconducting wire was prepared through the same process as in Example 1 except that the boron powder was used without milling and heat-treated at 800 ° C. for 0.5 hour.

2. Magnesium diborate  Superconductivity Wire rod  Characterization

Before analyzing the properties of the magnesium diboron superconducting wire, each measurement method or analysis method will be described in detail. Image formation and crystal structure were obtained through X-ray diffraction (XRD) pattern. In addition, the critical temperature of FIGS. 3A and 3B was measured by the magnetization method using diamagnetism. Meanwhile, the critical temperature, the upper critical magnetic field and the irreversible magnetic field of FIG. 4 use a superconducting physical property measurement system (Physical Properties Measurement System, PPMS, up to 9 T, Quantum Design) by the 4-probe method in the range of 5 to 300 K. Was measured via a magneto-resistivity curve. The upper critical and irreversible magnetic field values were defined as 90% and 10% of steady state resistance, respectively, from the curves of resistance (R) versus temperature (T) at various magnetic fields, respectively. In addition, the critical current density of magnesium diboron was measured using a DC superconducting quantum interference device (SQUID) magnetic field in a time-varying magnetic field with a maximum size of 7T. In addition, the magnetic critical current density was measured by applying a magnetic field parallel to the longitudinal direction of the wire rod at 5K and 20K. At this time, the critical current density values were obtained from the empty threshold model width (△ M) of the magnetic loop with (Bean critical model).

1 is a view showing the X-ray diffraction pattern of the magnesium diboron superconducting wire according to the Examples and Comparative Examples of the present invention, Figure 1a shows the X-ray diffraction patterns of Example 3 and Comparative Example 3, Figure 1b The X-ray diffraction patterns of Example 5 and Comparative Example 5, the X-ray diffraction patterns of Example 7 and Comparative Example 7 in Figure 1c, the X-ray diffraction patterns of Comparative Examples 10 and 11 in Figure 1d Each is shown.

As shown in FIGS. 1A-1C, when unheated at a heating temperature below the melting point of magnesium, unreacted magnesium peaks were detected, whereas as shown in FIG. 1D, heat treatment at a heating temperature above the melting point of magnesium was observed. In this case, no unreacted magnesium peaks were detected. This is because the kinetics of the solid phase diffusion reaction in the formation of magnesium diboron at low temperatures are significantly slower than in the solid (B) -liquid (Mg) process. On the other hand, in both the Example and Comparative X-ray diffraction patterns shown in Fig. 1, a small amount of magnesium oxide phase was present at a similar level as the main impurity phase.

1A to 1C, unreacted magnesium remained relatively large in comparison with the examples in the comparative examples using the unmilled boron powder. In other words, the kinetics of magnesium diboron phase formation was found to be greater than those using the milled boron powder compared to the comparative examples using the unmilled boron powder, which indicates why there is much magnesium remaining in the comparative examples. . In addition, as the heat treatment temperature increased from 550 ° C. to 600 ° C., magnesium diboron phase formation increased. It was found that Example 7, which was heat treated at 600 ° C. for 10 hours using milled boron powder, contained less unreacted magnesium, ie, a more improved magnesium diboron phase.

Referring to FIG. 1D, for the solid-liquid reaction at 800 ° C., the peak shape and the amount of magnesium diborate phase were all similar in Comparative Examples 10 to 11.

As shown in Table 1, under the same heat treatment conditions, the milling of the boron powder was performed in the case where the milling of the boron powder was less than the full width at half-maximum (FWHM) value. I could see this greater. The increase in the half width value may be attributed to the decrease in grain size and the decrease in crystallinity of magnesium diboron by milling. The half-width values were used to Williamson-Hall analysis to predict the calculated grain size shown in Table 1 below. When comparing Examples and Comparative Examples at the same temperature conditions, the calculated grain size of the Examples was smaller. In other words, it was found that the grain boundary density increased. In addition, in the case where the boron powder is milled, the calculated grain size increases as the heat treatment temperature increases. Likewise, in the case of not milling the boron powder, the calculated grain size increases as the temperature increases. It was.

FIG. 2 is a diagram showing the magnesium diborate fraction of the magnesium diboron superconducting wire according to the Examples and Comparative Examples of the present invention, and shows the magnesium diborate fractions of Examples 1 to 9 and Comparative Examples 1 to 11. FIG.

Referring to Figure 2, when the heat treatment at 500 ℃ the amount of magnesium diboron formed in both the Example and Comparative Example was small despite the heat treatment for 80 hours. Also, at 550 ° C., the fraction of magnesium diboron phase increased with increasing heat treatment time in both Examples and Comparative Examples. In addition, at 600 ° C., ie in Examples 7-9, the fraction of magnesium diboron phase showed a high value of almost 88%. In addition, it was found that, under the same heat treatment temperature and time, the amount of magnesium diborate was greater in the case of Examples than in Comparative Example. This can be analyzed because the reactivity of the boron powder is improved after milling. For Example 7, the levels were similar to those of Comparative Examples 10 and 11, which were heat-treated at 800 ° C. for a solid-liquid reaction.

3 is a view showing the critical temperature of the magnesium diboron superconducting wire according to the Examples and Comparative Examples of the present invention, Figure 3a, the critical temperature of Comparative Examples 2 to 9, Figure 3b the critical temperature of Examples 2 to 9 Is shown.

Referring to FIG. 3, when Examples 2, 3, and 4 and Comparative Examples 2, 3, and 4 are compared with each other under the same heat treatment conditions, the critical temperatures of Examples 2, 3, and 4 are respectively Comparative Examples 2, 3, and 4 Higher than each critical temperature. This can be analyzed because, as described above, when the milled boron powder is used, the formation of the magnesium diboron phase is more smooth. Thus, when annealed for a short time at too low a heat treatment temperature, the use of milled boron powder may have a higher critical temperature than when using unmilled boron powder. On the other hand, the amounts of magnesium diboron phases of Examples 6, 7, 8, 9, and Comparative Examples 6, 7, 8, and 9 were similar, but the critical temperature value of the Example was lower than that of the comparative example. This can be attributed to the deterioration of crystallinity due to the milling process and the increased lattice disorder (or strain).

4 is a view showing the temperature-resistance of the magnesium diboron superconducting wire according to the Examples and Comparative Examples of the present invention, the magnetic field is 0 for Examples 5, 7, and Comparative Examples 5, 7, 10, 11 The resistive transition and the critical temperature at is shown.

4 and Table 1, Comparative Examples 10 and 11 show a higher critical temperature because the crystallinity is improved by heat treatment at an elevated temperature, whereas Examples 5, 7, and Comparative Examples 5, 7 are relative Low threshold temperature transitions. At this time, the critical temperature values of Examples 5 and 7 were lower than those of Comparative Examples 5 and 7. On the other hand, under the same heat treatment conditions, as shown in Table 1 below, the specific resistance (ρ) value of the example was higher than that of the comparative example. This can be analyzed not only due to the deterioration of crystallinity due to the milling process but also to the increase in grain boundary density, which is why the critical temperature is relatively low.

5 is a view showing the upper critical magnetic field and irreversible magnetic field of the magnesium diboron superconducting wire according to the embodiment and comparative example of the present invention, Figure 5a of Examples 5, 7, and Comparative Examples 5, 7, 10, 11 The upper critical magnetic field is shown in FIG. 5B, irreversible magnetic fields of Examples 5, 7, and Comparative Examples 5, 7, 10, and 11, respectively.

Referring to FIG. 5, under the same heat treatment conditions, the upper critical magnetic field and the irreversible magnetic field were both higher in the case of using the milled boron powder than in the case of the unmilling boron powder. This is explained by an increase in grain boundary scattering due to small grains, with low crystallinity and defects resulting from the milling process, as indicated by the reduction of the residual resistivity ratio (RRR) described in Table 1 below. can do.

6 is a view showing the critical current density of the magnesium diboron superconducting wire according to the Examples and Comparative Examples of the present invention, Figure 6a shows the critical current density of Comparative Examples 2 to 9, Figure 6b of Examples 2 to 9 6C shows the critical current densities of Examples 5, 7, and Comparative Examples 5, 7, 10, and 11, respectively.

6A and 6B, in the case of magnesium diboron wires according to comparative examples using unmilled boron powders, the superconducting fraction of magnesium diboron increased with heat treatment temperature and time, and as a result, 5 K and 20 The critical current density increased at K. The critical current density values of the magnesium diboron wires according to the embodiments using milled boron powders over the whole area of the magnetic field were higher than the comparative examples using the unmilled boron powders. This is due to an increase in grain boundary pinning with increasing superconducting fraction and decreasing grain size. In addition, the critical current density tendency of the examples was similar to the comparative examples, but Example 7, which was heat-treated at 600 ° C. for 10 hours using milled boron powder, had the highest critical current density value. Although the magnesium diborate fractions of Examples 7 to 9 are similar, as shown in FIG. 2, the density of the grain boundaries serving as the flux fixation point may be higher in Example 7 due to the shorter heat treatment time. This resulted in an improved critical current density. In addition, although the magnesium diborate fractions of Example 7, and Comparative Examples 10 and 11 were similar, as shown in Fig. 2, the grain boundary density was high in Example 7, and as a result, as shown in Fig. 6c. Likewise, the critical current density value of Example 7 could be higher. This can be attributed to the improvement of grain boundary pinning of magnesium diboron through the low temperature solid phase reaction as compared with the high temperature heat treatment.

As described above, the solid-phase diffusion reaction using milled boron powder and performed at a temperature lower than the melting point of magnesium reduces the grain size of magnesium diborate and lowers the crystallinity to improve the upper critical magnetic field and irreversible magnetic field. In addition, it is possible to increase the critical current density.

(100)
Half width
(deg.) (002)
Half width
(deg.) Calculated
Crystal grain
size
(nm) Critical temperature
(K) ρ _{300 K}
(μΩ · cm) ρ _{40 K}
(μΩ · cm) Residual resistivity ratio Comparative Example 5 0.343 0.299 73 36.4 120 60.4 1.99 Example 5 0.423 0.48 47 36.1 151 97.6 1.55 Comparative Example 7 0.36 0.394 103 36.4 100 56.2 1.78 Example 7 0.411 0.522 53 35.9 142 91.2 1.56 Comparative Example 11 0.344 0.311 162 36.7 101 50 2.02 Comparative Example 10 0.374 0.339 70 36.5 93 55 1.69

Although the embodiments of the present invention have been described above with reference to the accompanying drawings, those skilled in the art to which the present invention pertains can realize that the present invention can be implemented in other specific forms without changing the technical spirit or essential features. I can understand that.

Therefore, it should be understood that the above-described embodiments are provided so that those skilled in the art can fully understand the scope of the present invention. Therefore, it should be understood that the embodiments are to be considered in all respects as illustrative and not restrictive, The invention is only defined by the scope of the claims.

1A to 1D are diagrams showing X-ray diffraction patterns of magnesium diboron superconducting wires according to Examples and Comparative Examples of the present invention.

2 is a view showing the magnesium diborate fraction of the magnesium diboron superconducting wire according to the Examples and Comparative Examples of the present invention.

3a and 3b are views showing the critical temperature of the magnesium diboron superconducting wire according to the comparative example and the embodiment of the present invention.

4 is a view showing the resistance-temperature of the magnesium diboron superconducting wire according to the Examples and Comparative Examples of the present invention.

5A and 5B illustrate upper critical magnetic fields and irreversible magnetic fields of magnesium diboron superconducting wires according to examples and comparative examples of the present invention.

Figure 6a is a comparative example of the present invention, Figure 6b is according to an embodiment of the present invention, Figure 6c is a view showing the critical current density of the magnesium diboron superconducting wire according to the embodiment and the comparative example of the present invention.

Claims (6)

Preparing wet milled boron powder and unmilled magnesium powder, respectively; Filling a metal tube with a mixed powder of the boron powder and the magnesium powder; Drawing the metal tube filled with the mixed powder; And Heat-treating the drawn metal tube below 650 ° C. Method for producing a magnesium diboron superconducting wire comprising a. The method of claim 1, Preventing oxidation of the mixed powder and the metal tube before heat treatment of the drawn metal tube Method for producing a magnesium diboron superconducting wire further comprising. delete The method of claim 1, The heat treatment is a method for producing a magnesium diboron superconducting wire, characterized in that carried out under an inert gas atmosphere. The method of claim 1, The heat treatment temperature is 500 to 600 ℃ method for producing a magnesium diboron superconducting wire. The method of claim 1, The heat treatment time is a method for producing a magnesium diboron superconducting wire, characterized in that 5 to 100 hours.

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