Patent Application
20090197771
The present invention relates to a method of manufacturing an oxide superconducting wire, a method of modifying an oxide superconducting wire and an oxide superconducting wire, and more particularly, it relates to a method of manufacturing an oxide superconducting wire capable of improving the critical current density, a method of modifying an oxide superconducting wire and an oxide superconducting wire.
In general, a method of obtaining an oxide superconducting wire by heat-treating a wire formed by charging a metal tube with raw material powder of an oxide superconductor and thereafter wiredrawing and rolling the metal tube for sintering the raw material powder of the oxide superconductor is known as a method of manufacturing an oxide superconducting wire. However, the wire is blistered in the aforementioned heat treatment step for sintering, to disadvantageously reduce the superconductivity of the obtained oxide superconducting wire.
Japanese Patent Laying-Open No. 5-101723 (Patent Document 1) proposes a method of manufacturing an oxide superconducting wire by heat-treating a metal tube filled with powder of an oxide superconductor or a flattened body thereof in a pressurized atmosphere for sintering the powder of the oxide superconductor. The aforementioned gazette describes that a wire having excellent superconductivity is obtained according to this method by performing a pressure heat treatment.
More specifically, it is attempted to store a metal tube charged with powder of an oxide superconductor in a heat-resistant/pressure-resistant closed vessel for preventing blistering in sintering by increasing the internal pressure following heat-up in the closed vessel. The aforementioned gazette describes that the current internal pressure can be obtained from a state equation of gas or the like, and an internal pressure of about 4 atm. can be obtained with a heating temperature of about 900° C., for example.
Japanese Patent No. 2592846 (Japanese Patent Laying-Open No. 1-30114) (Patent Document 2) proposes a method of manufacturing an oxide superconducting conductor by holding a metal tube filled with oxide superconducting powder or the like in a high-pressure state at least either in a beat treatment or after the heat treatment. The aforementioned gazette describes that partial separation on the interface between the oxide superconductor and the metal tube caused in sintering can be prevented according to this method by setting the metal tube in the high-pressure state.
More specifically, the metal tube filled with the oxide superconducting powder can be press-fitted to a sintered body by holding the metal tube in a high-pressure state of 500 to 2000 kg/cm2 (about 50 to 200 MPa) at least either in the heat treatment or after the heat treatment. Thus, when the superconductor partially causes quenching, heat resulting from this quenching can be quickly removed. In addition, it is also possible to prevent deterioration of the superconductivity resulting from a separation part forming a stress concentration part causing distortion.
In Japanese Patent Laying-Open No. 5-101723, however, the internal pressure obtained following heat-up in the closed vessel is about 4 atm (0.4 MPa). Thus, voids are formed between oxide superconducting crystals in sintering, to disadvantageously reduce the critical current density.
Further, the oxide superconducting wire cannot be sufficiently inhibited from blistering caused in sintering due to the internal pressure of about 4 atm (0.4 MPa), and hence the critical current density is also disadvantageously reduced.
In the method according to Japanese Patent No. 2592846, it is difficult to control the partial oxygen pressure in the heat treatment due to application of the excessively high pressure of 500 to 2000 kg/cm2 (about 50 MPa to 200 MPa), to reduce the critical current density.
Accordingly, an object of the present invention is to provide a method of manufacturing an oxide superconducting wire capable of improving the critical current density by suppressing formation of voids between oxide superconducting crystals and blisters of the oxide superconducting wire while simplifying partial oxygen pressure control in a heat treatment, a method of modifying an oxide superconducting wire and an oxide superconducting wire.
The method of manufacturing an oxide superconducting wire according to the present invention comprises a step of preparing a wire formed by covering raw material powder of an oxide superconductor with a metal and a heat treatment step of heat-treating the wire in a pressurized atmosphere having a total pressure of at least 1 MPa and less than 50 MPa in the heat treatment. At a heat-up time before the heat treatment in the heat treatment step, pressurization is started from a temperature reducing 0.2% yield strength of the metal below the total pressure in the heat treatment.
The method of modifying an oxide superconducting wire according to the present invention comprises a heat treatment step of heat-treating an oxide superconducting wire formed by covering an oxide superconductor with a metal in a pressurized atmosphere having a total pressure of at least 1 MPa and less than 50 MPa in the heat treatment. At a heat-up time before the heat treatment in the heat treatment step, pressurization is started from a temperature reducing 0.2% yield strength of the metal below the total pressure.
According to the inventive method of manufacturing or modifying an oxide superconducting wire, a pressure is applied to the wire in a state where the 0.2% yield strength of the metal is smaller than the total pressure of the pressurized atmosphere in the heat treatment. Thus, the portion of the metal is readily compressed through compressive force resulting from pressurization due to an effect similar to that of hot working. Therefore, the wire is compressed before pressurizing gas penetrates into the wire through pinholes, whereby formation of voids and blisters can be sufficiently suppressed by the pressurization. Consequently, the sintering density of the oxide superconductor can be improved, and the critical current density of the oxide superconducting wire can be improved.
Another method of manufacturing an oxide superconducting wire according to the present invention comprises a step of preparing a wire formed by covering raw material powder of an oxide superconductor with a metal including silver and a heat treatment step of heat-treating the wire in a pressurized atmosphere having a total pressure of at least 1 MPa and less than 50 MPa in the heat treatment. At a heat-up time before the heat treatment in the heat treatment step, pressurization is started after the temperature of the atmosphere exceeds 400° C.
Another method of modifying an oxide superconducting wire according to the present invention comprises a heat treatment step of heat-treating an oxide superconducting wire formed by covering an oxide superconductor with a metal including silver in a pressurized atmosphere having a total pressure of at least 1 MPa and less than 50 MPa in the heat treatment. At a heat-up time before the heat treatment in the heat treatment step, pressurization is started after the temperature of the atmosphere exceeds 400° C.
According to this inventive method of manufacturing or modifying an oxide superconducting wire, a pressure is applied to the wire in a state where 0.2% yield strength of the metal including silver is reduced to a level substantially identical to the total pressure of the pressurized atmosphere in the heat treatment. Thus, the portion of the metal is readily compressed through compressive force resulting from pressurization due to an effect similar to that of hot working. Therefore, the wire is compressed before pressurizing gas penetrates into the wire through pinholes, whereby formation of voids and blisters can be sufficiently suppressed by the pressurization. Consequently, the sintering density of the oxide superconductor can be improved, and the critical current density of the oxide superconducting wire can be improved.
Preferably in the aforementioned manufacturing and modifying methods, the pressurization is started after the temperature of the atmosphere exceeds 600° C. at the heat-up time before the heat treatment in the heat treatment step.
Thus, the pressure is applied to the wire in a state where the 0.2% yield strength of the metal including silver is reduced to about half the total pressure of the pressurized atmosphere in the heat treatment. Therefore, the portion of the metal is further readily compressed through the compressive force resulting from the pressurization. Consequently, the sintering density of the oxide superconductor can be further improved, and the critical current density of the oxide superconducting wire can be further improved.
Preferably in the aforementioned manufacturing and modifying methods, the speed of pressurization is at least 0.05 MPa/min.
The inventors have found that the speed of the pressurizing gas penetrating into the wire through pinholes is less than about 0.05 MPa/min. in the heat treatment step. When the total pressure of the atmosphere is controlled to continuously increase at a speed of at least 0.05 MPa/min. at the heat-up time before the heat treatment, therefore, the pressure in the atmosphere can be regularly kept higher than the pressure in the wire. Thus, compressive force can be applied to the wire at the heat-up time before the heat treatment whether or not the wire has pinholes before the heat treatment step, whereby formation of voids and blisters is suppressed. Consequently, the sintering density of the oxide superconductor can be effectively improved due to the heat treatment in the pressurized atmosphere of at least 1 MPa and less than 50 MPa, and the critical current density of the oxide superconducting wire can be effectively improved.
Preferably in the aforementioned manufacturing and modifying methods, the speed of pressurization is at least 0.1 MPa/min.
Thus, the pressure in the atmosphere can be kept further higher than the pressure in the wire. Therefore, the compressive force can be further largely applied to the wire at the heat-up time before the heat treatment whether or not the wire has pinholes before the heat treatment step, whereby formation of voids and blisters is suppressed. Consequently, the sintering density of the oxide superconductor can be further effectively improved due to the heat treatment in the pressurized atmosphere of at least 1 MPa and less than 50 MPa, and the critical current density of the oxide superconducting wire can be further effectively improved.
Preferably in the aforementioned manufacturing and modifying methods, the heat treatment step is carried out in an oxygen atmosphere, and the partial oxygen pressure is at least 0.003 MPa and not more than 0.02 MPa.
When the partial oxygen pressure is kept in the range of at least 0.003 MPa and not more than 0.02 MPa in the heat treatment, a stable oxide superconducting phase is formed and the critical current density can be improved. A non-superconducting phase is formed if the partial oxygen pressure exceeds 0.02 MPa, while the oxide superconducting phase is hardly formed and the critical current density is reduced if the partial oxygen pressure is less than 0.003 MPa.
Preferably in the aforementioned manufacturing method, the raw material powder of the oxide superconductor includes a Bi2223 phase, and the oxide superconducting wire is annealed in an oxygen-containing atmosphere of a temperature of at least 300° C. and not more than 600° C. in the heat treatment step.
Preferably in the aforementioned modifying method, the oxide superconducting wire includes a Bi2223 phase, and the oxide superconducting wire is annealed in an oxygen-containing atmosphere of a temperature of at least 300° C. and not more than 600° C. in the heat treatment step.
The inventors have noted that a Bi2212 phase is included in an oxide superconductor mainly composed of a Bi2223 phase and have made deep studies to find that the oxygen content of the Bi221.2 phase changes when the oxide superconductor is annealed in an oxygen atmosphere to improve the critical current density at a low temperature of about 20 K. The principle of the improvement of the critical current density at the low temperature of about 20 K is now described.
An oxide superconductor (oxide superconductor filaments) of an oxide superconducting wire includes a Bi2212 phase in addition to a main phase of a Bi2223 phase (no wire of an oxide superconductor consisting of a 2223 phase by 100% is implemented at present). When this wire is annealed in an oxygen atmosphere so that the Bi2212 phase absorbs oxygen, low-temperature characteristics of the wire are improved due to the following properties:
(1) As to Bi2212 Phase
In the Bi2212 phase, the oxygen content remarkably changes when the wire is annealed in an oxygen atmosphere. In other words, a value z in (BiPb)2Sr2Ca1Cu2O8+z changes due to the annealing in the oxygen atmosphere, to change the critical temperature (Tc) and the critical current density (Jc) of the Bi2212 phase. More specifically, the critical temperature Tc is reduced (changes in the range of 70 K to 90 K) when the value z is increased. Further, the critical current density Jc is increased at a low temperature of about 20 K, while the critical current density Jc is reduced at a high temperature of about 77 K.
This change is caused since the concentration of carriers (holes) bearing conduction is increased when the oxygen content in the Bi2212 phase is increased. In other words, the critical temperature Tc is reduced if oxygen is excessively introduced since there is an optimum hole concentration increasing Tc in relation to the critical temperature Tc, while the critical current density Jc is improved since electrical conduction is improved as the carrier concentration is increased in relation to the critical current density Jc at a temperature sufficiently lower than the critical temperature Tc. In relation to the critical current density Jc at a high temperature, the critical temperature Tc (77 K, for example: since the critical temperature Tc of the Bi2212 phase is close to or not more than the same) is reduced and hence the critical current density Jc is also reduced.
(2) As to Bi2223 Phase
The Bi2223 phase extremely hardly absorbs or discharges oxygen, and the oxygen content thereof hardly changes when the wire is annealed in the oxygen atmosphere. In other words, the value z in (BiPb)2Sr2Ca2Cu3O10+z substantially remains at zero. Therefore, the critical temperature Tc and the critical current density Jc of the Bi2223 phase remain unchanged when the wire is annealed in the oxygen atmosphere.
As understood from Table 1 showing the aforementioned results, the properties of the Bi2223 phase are not changed by the annealing in the oxygen atmosphere, while the Bi2212 phase contains oxygen to change the properties thereof and hence the critical current density Jc is improved at the low temperature of about 20 K in the overall wire.
The annealing temperature is so set to at least 300° C. and not more than 600° C. that the Bi2212 phase can effectively contain oxygen and decomposition of the Bi2223 phase can be prevented. In other words, no oxygen is introduced into/discharged from the Bi2212 phase if the annealing temperature is less than 300° C., while the main Bi2223 phase is decomposed if the annealing temperature exceeds 700° C.
Preferably, the aforementioned manufacturing method further comprises a step of twisting the wire in advance of the heat treatment step. Thus, the twisted oxide superconducting wire can be inhibited from formation of blisters, and the critical current density can be improved.
Preferably in the aforementioned manufacturing method, the wire is not rolled. Thus, a round oxide superconducting wire can be inhibited from formation of blisters.
Preferably in the aforementioned manufacturing method, a wire formed by covering a ceramic-covered rod, obtained by covering the raw material powder with ceramic, with the metal is prepared in the step of preparing the wire formed by covering the raw material powder of the oxide superconductor with the metal. Thus, the oxide superconducting wire having a ceramic covering layer can be inhibited from formation of blisters.
Preferably, the aforementioned manufacturing method further comprises a step of molding the wire into a coil in advance of the heat treatment step. Thus, deterioration of the critical current value in the coil of the oxide superconducting wire can be effectively suppressed while inhibiting the wire from blistering.
Preferably in the aforementioned manufacturing method, the wire is held under a decompressed atmosphere before the pressurization in the heat treatment step is started.
Preferably in the aforementioned modifying method, the oxide superconducting wire is held under a decompressed atmosphere before the pressurization in the heat treatment step is started.
Thus, the pressure in the atmosphere does not exceed the pressure in the wire in the state before the pressurization in the heat treatment is started, whereby gas hardly penetrates into the wire, and formation of blisters on the wire can be further suppressed.
In the oxide superconducting wire according to the present invention, the sintering density of an oxide superconductor is at least 95%, preferably at least 99%.
When employing the method of manufacturing an oxide superconducting wire or the method of modifying an oxide superconducting wire according to the present invention, it is possible to obtain an oxide superconducting wire having an oxide superconductor exhibiting a high sintering density, which has been conventionally unmanufacturable. Further, the critical current density of the oxide superconducting wire can be improved by increasing the sintering density of the oxide superconductor in the oxide superconducting wire.
Throughout the specification, the term “Bi2223 phase” denotes a Bi—Pb—Sr—Ca—Cu—O oxide superconducting phase containing bismuth, lead, strontium, calcium and copper in atomic ratios of (bismuth and lead):strontium:calcium:copper approximately expressed as 2:2:2:3, more specifically a (BiPb)2Sr2Ca2Cu3O10+z superconducting phase.
Further, the term “Bi2212 phase” denotes a Bi—Pb—Sr—Ca—Cu—O oxide superconducting phase containing bismuth, lead, strontium, calcium and copper in atomic ratios of (bismuth and lead):strontium:calcium:copper approximately expressed as 2:2:1:2, more specifically a (BiPb)2Sr2Ca1Cu2O8+z superconducting phase.
According to the inventive method of manufacturing an oxide superconducting wire, the pressure is applied to the wire while the 0.2% yield strength of the metal is smaller than the total pressure of the pressurized atmosphere in the heat treatment. Thus, the portion of the metal is readily compressed through the compressive force resulting from the pressurization due to an effect similar to that of hot working. Therefore, the wire is compressed before the pressurizing gas penetrates into the wire through pinholes, whereby formation of voids and blisters can be sufficiently suppressed by the pressurization. Consequently, the sintering density of the oxide superconductor can be improved, and the critical current density of the oxide superconducting wire can be improved.
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1, 1a, 1b oxide superconducting wire, 2, 2a to 2c oxide superconductor filament, 3, 3a to 3c sheath part, 4 gas inlet, 5 top cover, 6 pressure vessel cylinder, 7 thermal barrier, 8 treated object, 9 heater, 10 bearer, 11 bottom cover, 12 superconducting crystal, 13 apparatus, 14 pinhole, 21 ceramic covering layer, 22 ceramic-covered rod, 25 rod.
Embodiments of the present invention are now described with reference to the drawings.
A multifilamentary oxide superconducting wire, for example, is described with reference to
While a multifilamentary wire has been described in the above, an oxide superconducting wire having a single-filamentary wire structure comprising a single oxide superconductor filament 2 covered with a sheath part 3 may alternatively be employed.
A method of manufacturing the aforementioned oxide superconducting wire is now described.
Referring to
Silver or a silver alloy having high heat conductivity is preferably employed for the metal tube. Thus, heat generated when the superconductor partially causes quenching can be quickly removed from the metal tube.
Then, the aforementioned wire is wiredrawn thereby forming a single-filamentary wire having a core material of a precursor covered with a metal such as silver (step S1a). Then, a large number of such single-filamentary wires are bundled and fitted into a metal tube of a metal such as silver, for example (multifilamentary fitting: step S1b). Thus, a wire of a multifilamentary structure having a large number of core materials of the raw material powder is obtained. Then, the wire of the multifilamentary structure is wiredrawn thereby forming a multifilamentary wire of the raw material powder embedded in a sheath part of silver or the like, for example (step S2). Thus, a multifilamentary wire formed by covering the raw material powder of the oxide superconductor with the metal is obtained.
Primary rolling is performed on this wire (step S3), followed by a first heat treatment (step S4). An oxide superconducting phase is generated from the raw material powder due to these operations. Secondary rolling is performed on this heat-treated wire (step S5). Thus, voids resulting from the first heat treatment are removed. A second heat treatment is performed on the secondarily rolled wire (step S6). Sintering of the oxide superconducting phase progresses while the oxide superconducting phase is simultaneously converted to a single phase through the second heat treatment.
The oxide superconducting wire shown in
In this embodiment, at least either the first heat treatment (step S4) or the second heat treatment (step S6) is performed in a pressurized atmosphere to which a pressure of at least 1 MPa and less than 50 MPa is applied as the total pressure.
The heat treatment in this pressurized atmosphere is performed by hot isostatic pressing (HIP), for example. This hot isostatic pressing is now described.
Referring to
According to this embodiment, the bearer 10 supports the wire obtained by filling the raw material powder into the metal tube and thereafter wiredrawing/rolling the same as the treated object 8 in the pressure vessel cylinder 6. In this state, prescribed gas is introduced into the pressure vessel cylinder 6 through the gas inlet 4 thereby forming a pressurized atmosphere of at least 1 MPa and less than 50 MPa in the pressure vessel cylinder 6 and heating the wire 8 with the heater 9 to a prescribed temperature under this pressurized atmosphere. This heat treatment is preferably performed in an oxygen atmosphere, and the partial oxygen pressure is preferably at least 0.003 MPa and not more than 0.02 MPa. Thus, the wire 8 is subjected to the heat treatment by hot isostatic pressing.
According to this embodiment, the heat treatment is performed in the pressurized atmosphere of at least 1 MPa and less than 50 MPa as hereinabove described, to mainly attain the following three effects:
First, the number of voids formed between oxide superconducting crystals in the heat treatment can be reduced.
The inventors have found that the number of voids formed between oxide superconducting crystals mainly in a heat treatment can be remarkably reduced by performing the heat treatment in a pressurized atmosphere of at least 1 MPa as compared with a case of less than 1 MPa.
a) to (d) are conceptual diagrams showing behavior of voids between oxide superconducting crystals stepwise.
Referring to
To pass a current through a superconducting wire is to pass a current between superconducting crystals constituting the superconducting wire. What limits the quantity of a current passable while keeping a superconducting state (causing no electric resistance) in a refrigerant (e.g., liquid nitrogen or helium, or a refrigerator) for employing a superconducting wire is generally the junction between superconducting crystals having a weak superconducting state (the superconducting crystals have stronger superconductivity than the junction between the crystals). Voids inevitably remain in the junction between the superconducting crystals in normal atmospheric sintering. When the number of voids between the superconducting crystals is reduced, therefore, the performance of the superconducting wire is so improved that reduction of the critical current density can be prevented.
More specifically, the sintering density of an oxide superconductor heat-treated in the atmospheric pressure was 80 to 90% as to an oxide superconducting wire containing a Bi2223 phase, while the sintering density of an oxide superconductor prepared by setting the total pressure of a pressurized atmosphere to 10 MPa was 93 to 96%, and reduction of the number of voids formed between oxide superconducting crystals was observed.
Second, the oxide superconducting wire can be prevented from blisters formed in the heat treatment.
The inventors have investigated the number of blisters formed in a heat-treated wire when varying the total pressure for heat-treating an oxide superconducting wire in a pressurized atmosphere.
Referring to
Powder of an oxide superconductor is generally filled into a metal tube at a filling factor of about 80% of the theoretical density before sintering, and hence gas is present in voids of the powder. The gas in the voids of the powder causes cubic expansion when reaching a high temperature in a heat treatment, to blister the wire. According to this embodiment, however, the heat treatment is performed in the pressurized atmosphere of at least 1 MPa, and hence the pressure outside the metal tube can increase beyond that in the metal tube. Thus, the wire is conceivably prevented from blisters caused by the gas in the voids of the powder.
The inventors have further studied the cause for blisters of the wire, to also recognize that adsorbates such as carbon (C), water (H2O) and oxygen (O2) adhering to the raw material powder of the oxide superconductor are vaporized during sintering and the volume in the metal tube expands due to this gas to blister the wire. However, such blisters of the wire resulting from vaporization of the adsorbates to the powder can also conceivably be prevented since the external pressure can increase beyond the intermetallic internal pressure by performing the beat treatment in the pressurized atmosphere of at least 1 MPa.
Thus, it is conceivably possible to substantially remove not only blisters resulting from the gas present in the voids of the raw material powder of the oxide superconductor but also blisters resulting from vaporization of the adsorbates adhering to the surfaces of particles thereof. The blisters of the oxide superconducting wire cause reduction of the critical current density, and hence reduction of the critical current density can be prevented by preventing the wire from blisters.
Third, the partial oxygen pressure can be readily controlled in the heat treatment.
The inventors have found that a 2223 phase of a Bi-based oxide superconductor is stably formed when the partial oxygen pressure is controlled to at least 0.003 MPa and not more than 0.02 MPa regardless of the total pressure. A non-superconducting phase such as Ca2PbO4 is formed if the partial oxygen pressure exceeds 0.02 MPa, while the Bi2223 phase is hardly formed and the critical current density is reduced if the partial oxygen pressure is less than 0.003 MPa.
Referring to
In practice, the partial oxygen pressure is controlled by monitoring the total pressure and the oxygen concentration. In other words, the partial oxygen pressure is calculated by multiplying the value of the total pressure by the oxygen concentration. Therefore, if the total pressure is 50 MPa, for example, the oxygen concentration is 0.01% when the heat treatment is performed with a partial oxygen pressure of 0.005 MPa. Therefore, the injected gas mixture must be controlled by measuring the oxygen concentration of 0.01%. However, the oxygen concentration of 0.01% is substantially identical to a measurement error, and hence it is difficult to control the oxygen gas in the injected gas mixture by correctly measuring this oxygen concentration. According to this embodiment, the total pressure in the pressurized atmosphere is set to less than 50 MPa so that the concentration of the oxygen gas in the injected gas mixture can be kept high to some extent by reducing influence by a measurement error of the oxygen concentration, whereby the partial oxygen pressure can be readily controlled.
While this embodiment has been with reference to the case of constituting the pressurized atmosphere of nitrogen and oxygen, the pressurized atmosphere may be constituted of rare gas and oxygen. Therefore, the pressurized atmosphere may be constituted of argon and oxygen, for example.
It has been recognized that formation of voids and blisters can be effectively suppressed by performing a heat treatment in the aforementioned pressure range (at least 1 MPa and less than 50 MPa) when a wire formed by covering raw material powder of an oxide superconducting wire with a metal has no pinholes while formation of voids and blisters cannot be sufficiently suppressed by merely performing a heat treatment in the aforementioned pressure range when the wire has pinholes.
Referring to
Referring to
Thus, it has been recognized that formation of voids and blisters can be effectively suppressed by performing the heat treatment in the aforementioned pressure range (at least 1 MPa and less than 50 MPa) when there are no pinholes while formation of voids and blisters cannot be sufficiently suppressed by merely performing the heat treatment in the aforementioned pressure range when there are pinholes.
In the heat treatment in the pressurized atmosphere according to the present invention, plastic flow and creep deformation take place in the superconducting crystals formed in the heat treatment due to the high pressure of at least 1 Mia outside the wire, whereby voids between the oxide superconducting crystals formed in the heat treatment are suppressed. Further, the gas in the voids of the oxide superconducting crystal powder formed in the heat treatment or the gas adhering to the oxide superconducting crystal powder formed in the heat treatment can be inhibited from expansion in the heat treatment due to the pressure from outside the metal tube, whereby the oxide superconducting wire is inhibited from formation of blisters. Consequently, reduction of the critical current density resulting from voids and blisters is prevented.
In a wire having pinholes, however, pressurizing gas penetrates into the wire through the pinholes also when the aforementioned heat treatment in the pressurized atmosphere is performed. Therefore, no difference is caused between the internal and external pressures of the wire and formation of voids and blisters is not sufficiently suppressed by the heat treatment in the pressurized atmosphere. Consequently, the effect of preventing reduction of the critical current density is reduced.
Referring to
According to this technique, pressurization is performed at a speed of at least 0.05 MPa/min. at a heat-up time before a heat treatment in at least either a first heat treatment (step S4) or a second heat treatment (step S6). In the heat treatment, the total pressure in the atmosphere is controlled to continuously increase. In cool-down time immediately after the heat treatment, further, control is made to compensate for reduction of the pressure resulting from temperature reduction (to add a pressure).
Referring to
The inventors have found that the speed of pressurizing gas penetrating into a wire through pinholes is less than about 0.05 MPa/min. when an oxide superconducting wire having the pinholes is heat-treated in a pressurized atmosphere. Therefore, the pressure in the atmosphere can be kept higher than that in the wire at the heat-up time before the heat treatment by controlling the total pressure of the atmosphere to continuously increase at a speed of at least 0.05 MPa/min. at the heat-up time before the heat treatment.
Thereafter the temperature is kept at 830° C., for example, in the heat treatment. On the other hand, the pressure in the atmosphere is continuously increased. While the pressing speed in the heat treatment is preferably as high as possible, the total pressure exceeds 50 MPa if the pressing speed is excessively high and hence the pressure must be continuously increased at such a proper pressing speed that the total pressure in the heat treatment does not exceed 50 MPa. Referring to
Thereafter in cool-down time immediately after the heat treatment, the pressure starts to lower according to the state equation of gas following reduction of the temperature in the atmosphere. At this time, the pressure is controlled to compensate for reduction of the pressure resulting from temperature reduction (to add a pressure). In order to form a stable oxide superconducting phase, the partial oxygen pressure is controlled to be regularly in the range of 0.003 to 0.02 MPa.
According to this technique, the pressure in the atmosphere increases beyond that in the wire at the heat-up time before the heat treatment, whereby compressive force can be applied to the wire. Further, the state where the pressure in the atmosphere is higher than that in the wire can be continuously kept longer in the heat treatment. Consequently, formation of voids and blisters is suppressed at the heat-up time before the heat treatment and at the heat-up time, whereby reduction of the critical current density can be effectively suppressed due to the heat treatment in the pressurized atmosphere of at least 1 MPa and less than 50 MPa.
The inventors have further made deep studies, to find that the critical current density of an oxide superconducting wire can be further improved by employing a technique described below.
According to this technique, pressurization is started when the temperature of the atmosphere exceeds 400° C., preferably 600° C. at the heat-up time before the heat treatment in at least either the first heat treatment step (step S4) or the second heat treatment step (step S6) shown in
Referring to
Referring to
Referring to
The sintering densities of the oxide superconductor filaments shown in
F
t
=M
t
−W (1)
V
t
=F
t/ρ (2)
Then, the oxide superconducting wire is dissolved in nitric acid and silver is determined by performing ICP (inductive coupled plasma) emission spectroscopy on the solution, for calculating the ratio (Y) of silver occupying the weight of the oxide superconducting wire. The weight (Mf (g)) of an oxide superconductor filament part and the weight (Ms (g)) of a sheath part are calculated from the weight of the oxide superconducting wire according to the following equations (3) and (4):
M
s
=M
t
×Y (3)
M
f
=M
t
−M
s (4)
Then, the volume ((Vs (cm3)) of the sheath part is calculated from the known silver gravity (10.5 (g/cm3)), and the volume ((Vf (cm3)) of oxide superconductor filaments is calculated from the volume of the sheath part. The density ρf of the oxide superconductor filaments is calculated from the volume of the oxide superconductor filaments. More specifically, the density ρf is calculated according to the following equations (5) to (7):
V
s
=M
s/10.5 (5)
V
f
=V
t
−V
s (6)
ρf=Mf/Vf (7)
On the other hand, a value 6.35 g/cm3 is employed as the theoretical density of the oxide superconductor filaments. This value is calculated by the following method. The atomic ratio of a Bi2223 phase in the oxide superconductor filaments is calculated by ICP emission spectroscopy and EDP (energy dispersive X-ray spectroscopy) analysis. The lattice constant of the Bi2223 phase is obtained by X-ray analysis, for calculating the values of a- and c-axes. The theoretical density is calculated from these values.
The sintering density of the oxide superconductor filaments is calculated from the ratio between the density of the oxide superconductor filaments obtained by the aforementioned method and the theoretical density of the oxide superconductor filaments. More specifically, the sintering density is calculated according to the following equation (8):
Sintering Density(%)=(ρf/6.35)×100 (8)
Referring to
It is understood from the aforementioned results shown in
According to the method of manufacturing the oxide superconducting wire of this embodiment, a pressure is applied to the wire when the 0.2% yield strength of the sheath part is reduced to a level substantially identical to the total pressure of the pressurized atmosphere in the heat treatment. Thus, the sheath part is readily compressed through compressive force resulting from the pressurization due to an effect similar to that of hot working. Therefore, the wire is compressed before the pressurizing gas penetrates into the wire through pinholes, whereby formation of voids and blisters can be sufficiently suppressed due to the pressurization. Consequently, the sintering density of the oxide superconductor filaments can be improved, and the critical current density of the oxide superconducting wire can be improved.
Preferably in the aforementioned manufacturing method, the pressurization is started after the temperature of the atmosphere exceeds 600° C. at the heat-up time before the heat treatment in the heat treatment step.
Thus, a pressure is applied to the wire while the 0.2% yield strength of the sheath part is reduced to about half the total pressure of the pressurized atmosphere in the heat treatment. Therefore, the sheath part is further readily compressed through the compressive force resulting from the pressurization. Consequently, the sintering density of the filaments of the oxide superconducting wire can be further improved, and the critical current density of the oxide superconducting wire can be further improved.
Preferably in the aforementioned manufacturing method, the speed of the pressurization is at least 0.05 MPa/min. more preferably at least 0.1 MPa/min.
Thus, the sintering density of the oxide superconductor filaments can be further improved, and the critical current density of the oxide superconducting wire can be further improved.
Preferably in the aforementioned manufacturing method, the heat treatment step is carried out in an oxygen atmosphere, and the partial oxygen pressure is at least 0.003 MPa and not more than 0.02 MPa.
Thus, a stable oxide superconducting wire is formed, and the critical current density can be improved. A hetero phase is formed if the partial oxygen pressure exceeds 0.02 MPa, while the oxide superconducting phase is hardly formed and the critical current density is reduced if the partial oxygen pressure is less than 0.003 MPa.
Each of the first to third embodiments has been described with reference to the method of improving the critical current density (method of manufacturing an oxide superconducting wire) by performing a prescribed heat treatment method in at least either the first heat treatment step (step S4) or the second heat treatment step (step S6) shown in
Further, each of the first to third embodiments has been described with reference to the case of heat-treating the oxide superconducting wire having the sheath part of silver in the pressurized atmosphere having the total pressure of at least 1 MPa and less than 50 MPa in the heat treatment and starting pressurization after the temperature of the atmosphere exceeds 400° C. at the heat-up time before the heat treatment in the heat treatment step. However, the present invention is not restricted to this case but is applicable to all oxide superconducting wires formed by covering oxide superconductors with metals. In this case, the heat treatment is performed in a pressurized atmosphere having a total pressure of at least 1 MPa and less than 50 MPa in the heat treatment, and pressurization is started from a temperature reducing at least 0.2% yield strength of the metal below the total pressure (at least 1 MPa and less than 50 MPa) in the heat treatment at a heat-up time before the heat treatment in the heat treatment step. Thus, a pressure is applied to the wire in a state where the 0.2% yield strength of the metal is smaller than the total pressure of the pressurized atmosphere in the heat treatment, whereby the portion of the metal is readily compressed through compressive force resulting from the pressurization. Thus, the sintering density of the oxide superconductor can be improved and the critical current density of the oxide superconducting wire can be improved due to reasons similar to those in the aforementioned oxide superconducting wire having the sheath part of silver.
In general, a bismuth (Bi)-based oxide superconducting wire is known as one of oxide superconducting wires, This Bi-based oxide superconducting wire is usable at the liquid nitrogen temperature, and can obtain a relatively high critical current density. Further, this Bi-based oxide superconducting wire, which is relatively easy to elongate, is expected for application to a superconducting cable or magnet. However, a conventional Bi-based oxide superconducting wire has been disadvantageously unsuitable to application requiring high performance under a low temperature, due to a low critical current density (Jc) at a low temperature of about 20 K.
In relation to this, the inventors have found that the critical current density of a Bi-based oxide superconducting wire under a low temperature of about 20 K can be improved by combining the following technique with the technique according to each of the first to third embodiments. This technique is now described.
According to this technique, the wire is annealed in an atmosphere containing oxygen at a temperature of at least 300° C. and not more than 600° C. in at least either the first heat treatment step (step S4) or the second heat treatment step (step S6) shown in
Referring to
In order to confirm the effect of the aforementioned annealing, the inventors have made the following experiment:
The inventors have investigated the degrees of improvement of critical current values Ic at 20 K as to a case of performing annealing and a case of performing no annealing in heat treatment steps. The annealing was performed for various annealing times under various partial oxygen pressures. Table 2 shows the averages of ratios of increase of critical current values Ic(22 K) at 22 K to critical current values Ic (77 K) at 77 K after the heat treatment steps as to respective samples. The critical current values were measured in a magnetic field of 3 T.
Referring to Table 2, the averages of the ratios of increase of the critical current values at 22 K in the case of performing no annealing are 1.6, 1.7 and 1.5 respectively. On the other hand, the averages of the ratios of increase of the critical current values at 22 K in the case of performing annealing are 2.1, 1.9 and 2 respectively. Thus, it is understood that the critical current value at 20 K can be improved in the case of performing annealing as compared with the case of performing no annealing.
In order to confirm the effect of annealing the wire in the atmosphere containing oxygen at the temperature of at least 300° C. and not more than 600° C., the inventors have made the following experiment:
First, a tape-like Bi-based oxide superconducting wire of a multifilamentary structure having 61 filaments with external sizes of 4.2 mm in width and 0.24 mm in thickness and a silver ratio of 1.5 was prepared. Further, this oxide superconducting wire was heat-treated and annealed in this heat treatment. The annealing was performed in an oxygen jet for an annealing time of 20 hours under various annealing temperatures shown in Table 3. The quantity of a Bi2212 phase in the oxide superconductor was also varied. Table 3 also shows respective critical current values Ic at 77 K and 20 K before the annealing and respective critical current values Ic at 77 K and 20 K after the annealing as to respective samples.
Used wires were selected from the same lot, and it is assumed that the sectional areas of superconducting portions of respective wires are identical to each other. Therefore, the magnitude of the critical current value Ic in the following Table 3 is proportionate to the critical current density Jc (Jc=Ic/sectional area of superconducting portion).
It is understood from the results shown in Table 3 that the critical current value Ic (critical current density Jc) at the low temperature (20 K) is improved as compared with that before the annealing due to the annealing performed in the oxygen atmosphere at the temperature of at least 300° C. and not more than 600° C. It is also understood that the critical current value Ic after the annealing exceeds 530 A and the absolute value of the critical current value Ic (critical current density Jc) is increased due to the quantity of the Bi2212 phase in the oxide superconductor set to at least 5 mol % and not more than 20 mol %.
The inventors have also investigated the critical current values Ic of oxide superconducting wires at respective temperatures (K) before performing annealing and after performing the annealing at a temperature of 500° C.
It is understood from the results shown in
In a method of manufacturing an oxide superconducting wire according to this embodiment, the oxide superconducting wire includes a Bi2223 phase, and is annealed in an oxygen-containing atmosphere at a temperature of at least 300° C. and not more than 600° C. Thus, the critical current density of the oxide superconducting wire at a low temperature of about 20 K is improved.
This embodiment has been described with reference to a method of improving the critical current density by performing a prescribed heat treatment method in at least either the first heat treatment step (step S4) or the second heat treatment step (step S6) shown in
Each of the first to fourth embodiments has been described with reference to the manufacturing method in the case where the superconductor filaments 2 extend in the longitudinal direction of the oxide superconducting wire 1 and the oxide superconducting wire 1 is in the form of a tape, as shown in
The twisted oxide superconducting wire can effectively reduce AC loss. When the present invention is applied to a method of manufacturing a twisted oxide superconducting wire, the twisted oxide superconducting wire can be inhibited from formation of blisters, and the critical current density can be improved.
The inventors have made a test in order to confirm the aforementioned effect. This test is now described.
Powder having composition ratios of Bi:Pb:Sr:Ca:Cu=1.82:0.33:1.92:2.01:3.02 was prepared from Bi2O3, PbO, SrCO3, CaCO3 and CuO. This powder was heat-treated at a temperature of 750° C. for 10 hours, and thereafter heat-treated at a temperature of 800° C. for 8 hours. Thereafter a sintered body obtained through the heat treatments was pulverized in an automatic mortar. Powder obtained through the pulverization was heat-treated at a temperature of 850° C. for 4 hours, and a sintered body obtained through the heat treatment was pulverized in an automatic mortar. The powder obtained through the pulverization was heat-treated, and thereafter filled into a silver pipe of 36 mm in outer diameter and 30 mm in inner diameter (step S1). Then, the silver pipe filled with the powder was wiredrawn for obtaining a single-filamentary wire (step S1a). Further, 61 such single-filamentary wires were bundled and fitted into a silver pipe of 36 mm in outer diameter and 31 mm in inner diameter (step S1b). Then, the silver pipe fitted with the plurality of single-filamentary wires was wiredrawn for obtaining a multifilamentary wire of 1.5 mm in diameter (step S2). Then, this multifilamentary wire was twisted at twisting pitches of 20 mm, 15 mm, 10 mm and 5 mm (step S2a). Thereafter primary rolling (step S3) was performed for obtaining a tape-shaped wire having a thickness of 0.26 mm, a width of 3.7 mm and a length of 100 m. Then, this wire was heat-treated at a temperature of 840° C. in an atmosphere having an oxygen concentration of 8% for 30 hours as a first heat treatment (step S4). Then, secondary rolling (step S5) was performed for drafting the wire by 8%. Then, this wire was heat-treated at a temperature of 820° C. in an atmosphere having a total pressure of 25 Pa and a partial oxygen pressure of 8 kPa for 50 hours as a second heat treatment (step S6). At a heat-up time before the second heat treatment (step S6), pressurization was started from a temperature reducing the 0.2% yield strength of the silver pipe below 25 MPa. A sample 1 was prepared from the twisted oxide superconducting wire 1a obtained through the aforementioned steps.
On the other hand, the steps S1 to S5 shown in
The inventive manufacturing method is also applicable to a method of manufacturing a round oxide superconducting wire, for example.
The round oxide superconducting wire 1b is manufactured by not carrying out the primary rolling (step S3) and the secondary rolling (step S5) in the method of manufacturing an oxide superconducting wire shown in
The round oxide superconducting wire can effectively reduce AC loss. When the present invention is applied to a method of manufacturing a round oxide superconducting wire, the round oxide superconducting wire can be inhibited from formation of blisters, and the critical current density can be improved.
Each of the first to fourth embodiments has been described with reference to the manufacturing method in the case where the superconductor filaments extend along the longitudinal direction of the oxide superconducting wire and the oxide superconducting wire 1 has a tape-like shape. The fifth embodiment has been described with reference to the manufacturing method in the case where the oxide superconducting wire is a twisted wire or a round wire. In place of these methods of manufacturing oxide superconducting wires, the inventive manufacturing method is also applicable to a method of manufacturing an oxide superconducting wire having a ceramic covering layer, for example.
First, raw material powder is molded for forming a rod 25 of the raw material powder, as shown in
The oxide superconducting wire having a ceramic covering layer can effectively reduce AC loss. When the present invention is applied to a method of manufacturing an oxide superconducting wire having a ceramic covering layer, the oxide superconducting wire having a ceramic covering layer can be inhibited from formation of blisters, and the critical current density can be improved.
In order to confirm the aforementioned effect, the inventors have prepared an oxide superconducting wire having a ceramic covering layer through the manufacturing method according to this embodiment, and measured the critical current density. Consequently, the critical current density was improved to 1.4 times as compared with a case of performing both of two heat treatments in the atmosphere.
When an oxide superconducting wire is applied to a magnet or the like, employed is a substance prepared by winding the oxide superconducting wire in the form of a track reel-shaped coil as shown in
The wire not yet subjected to the second heat treatment (step S6) has stronger flexural strength than the oxide superconducting wire completed through the second heat treatment (step S6). A flexural load is applied to the wire in the step of molding the same into a coil, and hence the coil of the oxide superconducting wire prepared through the wind and react technique is advantageously smaller in deterioration of the critical current value as compared with a coil molded after completion of an oxide superconducting wire. Deterioration of the critical current value can be effectively suppressed by employing the wind and react technique particularly in preparation of a coil of an oxide superconducting wire having a diameter of not more than 100 mm.
On the other hand, the wind and react technique has such a disadvantage that the completed coil of the oxide superconducting wire is unusable if the wire is blistered in the second heat treatment (step S6). Therefore, the wind and react technique is not frequently employed for manufacturing a coil of an oxide superconducting wire in practice.
When the inventive heat treatment method is applied in the second heat treatment (step S6), however, a coil of an oxide superconducting wire can be prepared through the wind and react technique while suppressing blisters of the wire. Thus, deterioration of the critical current value can be effectively suppressed in the case of molding the wire into a coil. The remaining method of manufacturing an oxide superconducting wire is identical to the method of manufacturing an oxide superconducting wire according to the first embodiment shown in
The first embodiment has been described with reference to the case of gently increasing the pressure in the atmosphere from the atmospheric pressure according to the state equation of gas before starting the pressurization in the heat treatment (A in
As described with reference to the first embodiment, the gas in the atmosphere penetrates into the wire through pinholes when the pressure in the atmosphere exceeds the pressure in the wire. Therefore, the wire is held under a decompressed atmosphere before the pressurization in the heat treatment is started, so that the pressure in the atmosphere does not exceed the pressure in the wire. Thus, gas hardly penetrates into the wire also in the state before the pressurization in the heat treatment is started, and formation of blisters on the wire can be further suppressed.
The inventors have investigated the effect of holding the wire under a decompressed atmosphere before the pressurization in the heat treatment is started. More specifically, they prepared oxide superconducting wires while setting pressures to about 0.1 MPa (atmospheric pressure) and 10 Pa respectively before starting pressurization in heat treatments. These oxide superconducting wires were dipped in a vessel charged with liquid nitrogen pressurized to 1 MPa, and left for 24 hours. Thereafter the numbers of blisters formed on the respective oxide superconducting wires were examined. Consequently, the oxide superconducting wire prepared by setting the pressure to about 0.1 MPa (atmospheric pressure) before starting the pressurization in the heat treatment caused a blister per 1000 m. On the other hand, the oxide superconducting wire prepared by setting the pressure to about 10 Pa before starting the pressurization in the heat treatment exhibited absolutely no blisters. Thus, it is understood that blisters of the oxide superconducting wire can be further suppressed by holding the wire under a decompressed atmosphere before the pressurization in the heat treatment is started.
This embodiment has been described with reference to the case of holding the wire under a decompressed atmosphere before the pressurization in the heat treatment is started, as a method of manufacturing an oxide superconducting wire. In place of this case, however, the present invention is also applicable as a heat treatment step carried out on a manufactured oxide superconducting wire, i.e., a method of modifying an oxide superconducting wire. Also when the inventive heat treatment is applied as a method of modifying an oxide superconducting wire, the critical current density of the oxide superconducting wire can be improved.
The embodiments disclosed in the above must be considered illustrative in all points and not restrictive. The range of the present invention is shown not by the aforementioned embodiments but by the scope of claim for patent, and intended to include all corrections and modifications within the meaning and range equivalent to the scope of claim for patent.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2003-304536(P) | Aug 2003 | JP | national |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 10540550 | Jun 2005 | US |
| Child | 12358133 | US |
