JPH0359114A - Production of superconducting fiber - Google Patents

Abstract

PURPOSE:To obtain uniform, dense superconducting fiber suitable for cable and wire, having high critical current density, excellent handleability by adding an organic acid and spinning auxiliary to acetates of specific metals to give an aqueous solution, concentrating, spinning and burning. CONSTITUTION:Acetates of Ln, X and Cu (Ln is Sc, Y, La, Ce, Pr, Nd, pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu; X is Ba, Sr and Ca) are blended with an organic acid (preferably propionic acid) and a spinning auxiliary (preferably polyvinyl alcohol-based compound) to give an aqueous solution, which is concentrated and spun to give precursor fiber. The precursor fiber is burned (preferably in an air or in an inert gas atmosphere in a lowtemperature range at <= the decomposition temperature of the organic substance at 0.2-10 deg.C/minute rate of heating and then in an air or in an oxygen atmosphere in a temperature range at >= the decomposition temperature at 1-40 deg.C/minute rate of heating) to give superconducting fiber.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a method for producing superconducting fibers. More specifically, the present invention relates to a method for producing oxide-based superconducting fibers that have a fineness of fiber diameter of several hundred micrometers or less, are easy to handle, and exhibit excellent superconducting performance, particularly high critical current density.

[Prior art and problems to be solved by the invention] Conventionally,
To manufacture oxide-based superconducting fibers, silver or copper pipes are filled with oxidized powder prepared in advance by solid-phase reaction or coprecipitation, heated and stretched to form fibers, and then fired to form superconducting fibers. Wire drawing method to obtain wire; oxide powder, dispersant, and binder are mixed and wet-spun in a suspended state to form fibers.
Next, a suspension spinning method was used. but,
In all of these methods, the starting material is oxide powder, so the fiber diameter largely depends on the particle size of the raw material powder, and even when fine particles are used, it is difficult to obtain fine yarn with a fiber diameter of 100 μm or less, and The subsequent density did not increase sufficiently and the strength was brittle, so a high critical current density could not be expected.

In addition, there is a token method in which a polymeric fibrous material is impregnated with a solution containing ceramic elements and then fired; and a sol-gel method in which the polymer is made into a fiber by hydrolysis of a metal alkoxide solution, which is then fired after being made into fibers. In all of these methods, the content of inorganic substances is low, the viscosity and concentration of the raw solution before fiberization cannot be sufficiently increased, and it is impossible to produce homogeneous long fibers. This method has problems in practical use because it does not produce composite fibers and shows only a low critical current density, similar to the wire drawing method and suspension spinning method.

Therefore, the purpose of the present invention is to first facilitate the formation of homogeneous and stable fibers, then improve the toughness, homogeneity, and handleability of the fibers after firing, and furthermore, significantly improve the critical current density, which is the most important factor. It is an object of the present invention to provide a method for producing superconducting fibers with high performance and fineness.

[Means for Solving the Problems] The present inventors believe that the reason why conventional superconducting fibers exhibit only a low critical current density is that the fiber structure is not sufficiently densified. Focusing on homogeneous raw materials that are expected to improve properties, we developed a raw material composition that can be concentrated to a high concentration while maintaining a homogeneous system, has spinnability, and can therefore yield precursor fibers with uniform and stable fineness. As a result of various studies regarding firing, the present invention was arrived at.

That is, the present invention involves adding an organic acid and a spinning aid as a spinning agent to acetates of Ln, X, and Cu, which are constituent metals of an LrrX-Cu-based oxide superconductor, to form an aqueous solution, and concentrating the aqueous solution. A method for producing superconducting fibers, characterized in that: a homogeneous spinning dope is obtained, the spinning dope is spun into a precursor fiber, and the precursor fiber is fired; Pr,
Nd, Pm, Sm, Eu, Gd, Tb.

At least one element selected from Dy, Ho, Er, Tm, Yb and Lu, and X is Ba, Sr
and at least one element selected from Ca].

The present invention will be explained in more detail below.

A method for manufacturing ceramic fibers that starts from a homogeneous stock solution and spins it in solution is generally known, but in the case of superconducting compositions, the stock solution is a multi-component system, which poses problems in terms of spinning performance. It is difficult to obtain long fibers, and therefore, the critical current density is often so low that it cannot be evaluated, and the critical current density is only as high as < 1 OOA/cm'' (77K).

In the present invention, by specifying the solution composition and additives, the concentration performance of the solution is significantly improved, and a thick spinning stock solution that is uniform, stable, and has excellent spinnability can be prepared. It can be easily made into fibers using the spinning method used to produce synthetic fibers, and by firing it, a fired yarn with homogeneity and fineness that far exceeds current standards can be produced.
As a result, a high critical current density of 10"A/Cm" (77K) or more can be expected.

Among the metal elements constituting the Ln-X-Cu-based oxide superconductor, Ln is generally a rare earth element sc, y, L
a.

Ce, Pr, Nd, Pa, Sm, Eu, Gd, Tb
, Dy, Ho, Er, Tm, Yb, Lu, X is alkaline earth metal Ba, Sr, Ca, Ln
,

In addition, the nonmetallic element in the composition of the fired body is mainly oxygen, but
For the purpose of improving superconducting performance, elements such as fluorine may be added as long as a homogeneous system is maintained.

The compound form of these gold j[Ln, X, Cu is acetate. Considering only water solubility, solutions can be prepared with chlorides, nitrates, etc., but in this case, the compatibility with other additives decreases during the concentration process to make the spinning stock solution, and crystals may precipitate or compatibility may occur. This is inconvenient as it causes separation.

By adding an organic acid, firstly, the compatibility is improved, and as a result, the concentration performance for turning a homogeneous solution into a spinning dope is improved.

Second, it is possible to prevent chelation between metal and a spinning aid (for example, polyvinyl alcohol (hereinafter abbreviated as PVA)) as a threading agent. For example, PVA used as a spinning aid tends to chelate with the constituent metals, especially Cu ions.
This chelation causes phase separation and loss of spinnability as a spinning aid. Therefore, the pH of the solution must be lowered by addition of an organic acid to a range that can prevent chelation. Here, the pH of the solution is preferably 5 or less, more preferably 4.5 to 3.0.

As the organic acid, a low-molecular compound is preferable because even if it is water-soluble, a relatively high-molecular compound causes a decrease in compatibility and tends to cause phase separation.

Specific examples include lactic acid, propionic acid, and acetic acid, among which propionic acid is preferred from the viewpoint of concentration performance and ease of handling.

Note that inorganic acids such as hydrochloric acid, nitric acid, and sulfuric acid other than organic acids can also be used if only for the purpose of pH adjustment, but in reality, if inorganic acids are used, the compatibility will be insufficient and it will not be possible to concentrate uniformly. .

In addition, as a spinning aid that can prevent chelation, PvA is most suitable, judging from its thickening properties, spinnability, handling properties, etc. If other water-soluble polymers such as polyethylene oxide, polyacrylic acid, or polyvinylpyrrolidone are used, the concentration limit will be significantly lowered due to lack of compatibility with any metal compound, including acetate, chloride, and nitrate. Furthermore, even if it is possible to concentrate, the spinnability and handling properties of the spinning stock solution will be significantly inferior.

The average polymerization degree and saponification degree of PVA are not particularly limited, but due to water solubility, the average polymerization degree is 5000 or less, and the saponification degree is 7.
The average degree of polymerization is preferably 0 to 10,010 Os%, more preferably 4,000 to 1,000 in terms of spinnability and handling properties. The degree of saponification is 80 to 95 mo1%.

Other modified PVAs can also be used as long as they are water-soluble.

On the other hand, the amount of these organic acids and spinning aids to be added is usually 1O10N150 per total acetate.
%, and a spinning aid of 15 wt % or more can yield a precursor fiber with good handling properties, but preferably the organic acid is 30 to 30 wt %.
100 wt%, and the spinning aid is 20 wt% or more. As the amount of these additives increases, the content of inorganic substances in the precursor fiber decreases, which tends to make it difficult to densify during the subsequent firing process.In order to improve superconducting performance, the amount added should be as low as possible. This is desirable.

The purpose of the organic acid and the spinning aid as additives is achieved only when they coexist, and if either one is missing, a uniform, stable, and concentrated spinning stock solution cannot be obtained.

In a system containing only a metal acetate and an organic acid, metal salt crystals tend to precipitate during or after concentration, resulting in an extremely unstable system, and the system does not exhibit spinnability as a spinning stock solution.

Furthermore, in a system containing only a metal acetate and a spinning aid such as PVA, chelation occurs simultaneously with solution preparation, leading to phase separation.

As a result, by an appropriate combination of organic acids and spinning aids,
Although it was previously impossible to achieve high concentration, solid content concentration of 60~
Even if concentrated to 70%, a stable spinning dope can be obtained.

In the present invention, any of the conventional spinning methods for organic synthetic fibers, such as dry spinning, wet spinning, and dry-wet spinning, can be used as the spinning method.

However, in the case of wet spinning, elution of metal acetates may be observed depending on the coagulation bath composition, so dry spinning is more advantageous in this respect.

Since the present invention uses a homogeneous stock solution, it is possible to make the fiber finer with any spinning method, and generally the precursor fiber diameter can be obtained from Iooo to several μm, and after firing, it is several hundred to several μm. It becomes about ILLm.

The spun precursor fibers are then fired to become the desired superconducting fibers.

It is preferable to perform the calcination under conditions in which the decomposition rate is relatively slow in a low temperature range below the temperature at which the organic substance decomposes, and conversely the decomposition rate is accelerated in a high temperature range after the decomposition, since better densification can be achieved.

Specifically, for example, below 300 to 500°C, the temperature increase rate is 0.2 to b in air or an inert gas atmosphere, and at higher temperatures, the temperature increase rate is 1 in air or oxygen atmosphere.
Heat at ~40°C/win.

The reason why there is a considerable range in heating rate and overlap between low temperature and high temperature regions is that the conditions change greatly depending on the diameter of the precursor fiber, and this tendency becomes stronger as the fiber diameter increases. The conditions may be selected appropriately depending on the diameter of the .

If the decomposition conditions of organic matter are deviated from the above conditions by increasing the decomposition conditions in a low temperature range or slowing it down in a high temperature range, phenomena such as hollowing, porosity, cracking, deformation, etc. will appear in the fibers after firing. The compactness decreases significantly, making it difficult to obtain a high critical current density as a superconducting performance.

The firing temperature is preferably 900°C or higher, and the temperature cooling rate also requires attention in order to improve the superconducting properties.In general, it is important to incorporate oxygen, so it is preferable that the temperature cooling rate be as slow as possible. is also required.

[Example] Hereinafter, the present invention will be further explained by showing examples.

Example 1 Average degree of polymerization 1700. Partially saponified PVA with a saponification degree of 90.5 mat% was prepared in advance as a 10% aqueous solution, and 40 g of propionic acid and 140 g of water were added to 360 g of the aqueous solution.
g to make an organic acid-PVA aqueous solution, and to this solution Y (CHsCOO) 14HJ, Ba (CHsCOO
)i, Cu(CHsCOO)1HJ to Y:Ba:Cu
A total of 86 g was added so that the atomic ratio was 1:2:3 and dissolved at 50°C to prepare a homogeneous metal-containing aqueous solution with a pH of 4.2. Subsequently, the aqueous solution was concentrated using a rotary evaporator until the solution viscosity reached approximately 500 poise (80°C), taken out into a small spin tank, and left overnight at 80°C in a pressurized closed system to degas the spinning solution. And so.

Next, dry spinning was performed using the spinning dope using a conventional method using a spinneret with a diameter of 0.1 mm to obtain a precursor fiber having a dogbone-shaped cross section with a major axis of 53 μm and a minor axis of 33 μm.

This precursor fiber was heated under a nitrogen atmosphere from room temperature to 400°C at a heating rate of 3°C/win, and from 400 to 930°C under an oxygen atmosphere at a heating rate of 10°C/min, and then heated to 930°C under the same oxygen atmosphere. When the fibers were held for 5 hours at room temperature and cooled down to room temperature at a rate of 1.7° C./min without changing the atmosphere, a strong superconducting fiber having a dogbone-shaped cross section with a major axis of 24 μm and a minor axis of 15 μm was obtained.

The critical temperature (off 5et) of this superconducting fiber is 89, and the critical current density at 77K is 1300 A/cm.
"showed that.

Example 2 Using the spinning dope prepared in Example 1, a diameter of 0.5 m was obtained.
Dry spinning was carried out at room temperature using a spindle of m, with a long diameter of 305 μm,
A precursor fiber having a dogbone-shaped cross section with a short diameter of 120 um was obtained.

The fiber was heated under a nitrogen atmosphere from room temperature to 400°C at a heating rate of 1"C/win, and from 400 to 930°C under an oxygen atmosphere at a heating rate of 20°C/win, and then heated to 930°C under the same oxygen atmosphere. After holding at 500° C. for 8 hours, the temperature was lowered to room temperature at a rate of 1.7° C./min without changing the atmosphere, and annealing was performed by holding at 500° C. for 5 hours.

The finally obtained fiber has a long axis of 152 μm and a short axis of 48 μm.
It was a strong superconducting fiber with a dogbone-shaped cross section.

The critical temperature (off 5et) of this superconducting fiber is 90, and the critical current density at 77K is 1860A/c.
m” was shown.

[Effects of the Invention] Since the superconducting fiber obtained by the present invention becomes a homogeneous and more dense long fiber, a high critical current density is expected.
Because it is easy to handle, it can be applied to large-scale power generation transportation using magnetic coils, medical devices, and cable lines where energy loss is a problem such as power transmission and communication, and can be made even thinner. As a result, the field of direct current has shifted to the field of alternating current, the range of applications has greatly expanded, and the value of industrial use is enormous.

Claims (1)

[Claims] 1. Ln as a constituent metal of the Ln-X-Cu-based oxide superconductor
, an organic acid and a spinning aid as a spinning agent are added to the acetate of A method for producing a superconducting fiber, comprising firing the precursor fiber, [wherein Ln is Sc, Y, La, Ce, Pr, Nd, P
m, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm
, Yb, and Lu, and X is at least one element selected from Ba, Sr, and Ca]. 2. The method for producing superconducting fibers according to claim 1, wherein the organic acid is lactic acid, propionic acid or acetic acid. 3. The method for producing superconducting fibers according to claim 1, wherein the spinning aid is a polyvinyl alcohol compound. 4. The method for producing superconducting fibers according to claim 1, wherein the spinning dope has a pH of 5 or less. 5. In the low temperature range below the decomposition temperature of organic matter, the firing rate of the precursor fiber is 0 in air or inert gas atmosphere.
．． 2 to 10°C/min, and in a high temperature region higher than the decomposition temperature, the heating rate is 1 to 40°C/min in an air or oxygen atmosphere.
The method for producing a superconducting fiber according to claim 1, which is carried out by: