WO1994029906A1

WO1994029906A1 - Process for preparation of 2-4-7 superconductor

Info

Publication number: WO1994029906A1
Application number: PCT/NZ1994/000061
Authority: WO
Inventors: Jeffery Lewis Tallon
Original assignee: Industrial Research Limited
Priority date: 1993-06-15
Filing date: 1994-06-15
Publication date: 1994-12-22

Abstract

A process for preparing a high temperature 2-4-7 superconductor of formula Ra-pBb-qAp+qCucO¿15-δ? comprises intimately mixing precursor materials with silver as a reaction rate enhancer and then reacting the precursor materials to form the 2-4-7 supercondcutor, the mole fraction of silver satisfying the ratio x:a:b:c where x is the mole fraction of silver and 0 < x « 1.0 and a, b and c are approximately 2, 4, 7 and are the cation mole fractions of the resulting 2-4-7 superconductor. The reaction may be carried out in a silver tube into which the precursor materials and silver are loaded as powders. In the presence of the silver powder reaction rate enhancer 2-4-7 is formed more rapidly and subsequent processing to load oxygen into the 2-4-7 structure and increase Tc may not be required. R is Y, La, Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm, or Y, or any combination thereof, B is Ba or Ba plus a minor amount of Sr or La or a combination thereof, and A is Ca, Li, Na, K, Cs or Rb or any combination thereof, or La or La in combination with any of Ca, Li, Na, K, Cs and Rd, 1.9 < a < 2.1, 3.9 < b < 4.1, 6.8 < c « 7.2, 0 « p,q < 1, 0 « p+q < 1, 1.0 < δ < -0.3.

Description

PROCESS FOR PREPARATION OF 2-4-7 SUPERCONDUCTOR FIELD OF INVENTION The present invention comprises a process for preparing the high-Tc superconducting cuprate R₂Ba₄Cu₇0₁₅__δ, where R is Y or one of the lanthanide rare earth elements.

BACKGROUND The high temperature superconducting cuprate (HTSC) compound R₂Ba₄Cu₇0₁₅_₆ (referred to as 2-4-7) is known to be a superconductor with T_c exceeding 9OK when δ is close to zero or less - see Tallon et al, Phys. Rev. B41 (1990) 7220 and US patent 5,140,000. This compound may be synthesized at high oxygen pressure and high temperatures or it may be conveniently synthesized at 1 bar (1 atmosphere or 10⁵Pa) oxygen pressure in a narrow temperature range of 860 to 875°C provided a reaction rate enhancer or catalyst is utilised. For a practical industrial process, synthesis at 1 atmosphere is distinctly preferable to high pressure synthesis. The most convenient reaction rate enhancer is known to be an alkali oxide derived from NaN0₃ or KN0₃. The volatility of the alkali especially for the potassium oxide, ensures that a substantial fraction of the alkali evaporates over the period of synthesis leaving essentially single phase 2-4-7 material.

Where a reaction rate enhancer is utilised although most of the alkali is evaporated during the reaction small residual quantities generally remain rendering the material vulnerable to atmospheric degradation. Another disadvantage is that if the synthesis is carried out in a restricted environment such as in a silver-clad tube then there is no opportunity for efflux of the alkali, thus rendering the process unsuitable for the common powder-in-silver-tube route for making superconducting wires - see for example M.P. Maley et al, Phys. Rev.B45 (1992) 7566.

Preparation of 2-4-7 at atmospheric pressure with alkali reaction rate enhancers requires extended reaction times, typically of the order of three to five days for example. After synthesis the material requires to be slow cooled in an oxygen- containing atmosphere in order to reduce the oxygen deficiency, 6 by loading oxygen into the crystal structure. It is necessary to reduce δ to less than about 0.05 for the transition temperature of 2-4-7 to be above 90K (Tallon et al, Phys. Rev. B41 (1990) 7220 and US patent 5,140,000) and usually the slow cooling, to 400°C or lower requires to be carried out over many hours, typically more than 10 hours.

US patent 4,826,808 discloses a process for preparing a superconducting oxide-metal composite material where a noble metal such as silver is mixed with the precursor materials to form the superconducting oxide and the materials are then reacted, with the presence of the noble metal providing improved mechanical properties to the resulting material. Sufficient noble metal is used to act as a "skeleton" in the composite material, giving improved ductility and strength. Relatively large amounts of the noble metal are required to form such a skeleton giving improved mechanical properties.

Kogure et al, Physica C 156 (1988) 45-56 disclose a process for producing Yb₂Ba₄Cu₇O_x HTSC material by oxidising metallic precursors wherein approximately 33% by weight of silver is added to the starting precursors, again to form a silver skeleton for the 2-4-7 material to improve the mechanical properties of the material including ductility.

It is also known in the production of YBa₂Cu₃0₇__δ and related 1-2-3 superconductors and superconductors in the Bi- Sr-Ca-Cu-0 system to add silver metal to produce composite HTSC materials again having improved mechanical properties such as ductibility.

DISCLOSURE OF INVENTION In broad terms the invention comprises a process for preparing a high temperature 2-4-7 superconductor of formula

wherein

1 . 9<a<2 . 1 3 . 9<b<4 . 1 6 . 8<c≤7 .2 1 . 0<δ<-0 . 3 0≤p,q<l 0≤p+q<l

R is Y, La, Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb, or Lu, or any combination thereof,

B is Ba or Ba plus a minor amount of Sr or La or a combination thereof, and

A is Ca, Li, Na, K, Cs or Rb or any combination thereof, or La or La in combination with any of Ca, Li, Na, K, Cs and Rb,

comprising intimately mixing precursor materials with silver metal or a compound of silver and then reacting the precursor materials in an oxygen containing atmosphere to form said 2-4-7 superconductor, the mole fraction of silver satisfying the ratio x:a:b:c where x is the mole fraction of silver and 0<x≤1.0 and a, b and c are as defined above and are the cation mole fractions of said cation components R, B and Cu of the resulting 2-4-7 superconductor.

Preferably the mole fraction of silver which is intimately mixed with the precursor materials satisfies the ratio x:a:b:c where x is the mole fraction of silver and 0.2≤x≤0.4 and a, b and c are as defined above. Preferably the cation mole fractions of R, Ba and Cu in the starting precursor materials are in said ratio a:b:c but this is not essential as even when the precursors are present outside of this ratio 2-4-7 will form along with other non- superconducting compounds, but in any case silver is added in a mole fraction taken in relation to the 2-4-7 content of the reacted material excluding other non-superconducting compounds.

In the process of the invention silver is used in small amounts as a reaction rate enhancer, and it has surprisingly been found that the formation of the 2-4-7 superconductor from the reactants is accelerated. Also, 2-4-7 HTSC material prepared by the process of the invention may be oxygen loaded to maximise T_c more rapidly, in a matter of seconds or minutes instead of hours. We have also found that 2-4-7 can be synthesized at or about 1 bar pressure providing a dense material comprising the 2-4-7 superconductor which superconducts at or above 9OK with oxygen loading taking place in a matter of minutes or seconds during air quenching from the furnace. 2-4-7 material can also be prepared by processes of the invention to have a T_c of up to 97K which is higher than that previously reported for 2-4-7. The process of the invention also enables production of 2-4-7 material by enclosing the reactants in a silver-clad container such as for example in the powder-in-silver-tube process for producing practical superconducting wires including melt processing of the 2-4-7 material to densify and texture the 2-4-7 material and remove weak links between HTSC grains in the silver tube. The silver or silver compound is intimately mixed as a powder with powders of precursor materials which are subsequently reacted to form the 2-4-7 superconductor. Such precursor materials may be fine powders of compounds which provide the cation content of the 2-4-7 superconductor such as oxides, nitrates, carbonates, or similar of Y, Ba and Cu and optionally Ca or similar or fine powders of the pure metals R, Ba and Cu and optionally Ca for example. Preferably the silver powder has an average particle size in the range 0.1 to 20 μm and most preferably less than 7μm, and preferably the precursor materials comprise powders of a similar size.

Where a compound of silver is used this may be Ag₂0, Ag N0₃, or any other suitable compound of silver.

For example, precursor materials in the form of oxides, nitrates, carbonates for example or other suitable precursor compounds of Y or a rare earth element R, Ba and Cu may be intimately mixed with the silver metal or a compound of silver as fine powders, by milling together for example.

The precursor materials may also be Y or the rare earth element R, Ba, and Cu as pure metals which are again intimately mixed with the silver metal or a compound of silver as fine powders by milling together for example. After mixing the precursor materials and silver are then reacted in oxygen or an oxygen containing atmosphere such as air. The reaction may be carried out as is known for preparing 2-4-7 with alkali catalysts as described in US patent 5,140,000 for example, or at elevated pressures as is also known in the art. Preferably the precursor materials are reacted at a temperature T in °C and oxygen partial pressure Po₂ in Pa satisfying the equation

1210 - 180L + 21L² < T < 2320 - 581.5L + 58.5L²

where L = log₁₀Po₂. For example at about atmospheric pressure i.e. about 10⁵Pa the reaction may be carried out at a temperature typically in the range 865°C to 880°C.

After reacting the reacted material may optionally be slow cooled in an oxygen containing atmosphere and further optionally annealed at for example 350 to 400°C to load oxygen into the crystal structure to reduce the oxygen deficiency δ and raise Tc. Preferably slow cooling and/or annealing is carried out to reduce δ to 0.6<δ<-0.2 and most preferably to 0.05<δ<-0.2. However, such slow cooling and annealing may not be necessary as on removal from the furnace in which the synthesis reaction is carried out, after air quenching and a cool down (in air) of less than 60 minutes or less than 20 minutes or even only a few minutes or seconds the 2-4-7 material may be found to have a Tc of up to 9OK or more. The precursor materials including the silver may be intimately mixed as powders and then loaded into a silver clad container such as a silver tube and then reacted in the silver tube in the presence of for example flowing oxygen to prepare HTSC 2-4-7 conductors by the powder-in-silver tube process. Alternatively the precursor materials may be partially reacted and then the partially reacted product reground and the resulting ground material loaded into the silver tube and the reaction completed with the material in the silver tube. In both cases the precursor materials are reacted in the silver tube under the temperature and pressure conditions described above as suitable for forming 2-4-7.

After loading the precursor powders into the silver tube and before or intermittently during the synthesis reaction the silver tube (and contents) is preferably reduced to a smaller diameter by drawing down or extruding for example and/or pressing or rolling to a tape. Preferably the total drawing strain lies between 10 and 97% area reduction and most preferably 70 to 95%, and the total pressing or rolling strain lies between 10 and 95% area reduction and most preferably 70 to 90%.

While in the silver tube the 2-4-7 HTSC material may also be subjected to melt processing to densify and texture the 2-4-7 material and remove weak links between HTSC grains. After carrying out the synthesis reaction in the silver tube as described above, the 2-4-7 material in the silver tube is heated to a temperature and oxygen partial pressure in excess of the melting point of 2-4-7. For example, at atmospheric pressure in oxygen the 2-4-7 material in the silver tube may be heated to a temperature of about 875°C or above but below the melting point of the silver tube which for silver in oxygen is 930°C. Also, melt processing may be carried out at an oxygen partial pressure lower than 10⁵ Pa where the melting point of the 2-4-7/silver composite is lower while the melting point of the silver metal is higher than 930°C.

After melt processing as described above the silver tube may be heat treated to ensure complete reaction of the oxide material to 2-4-7. Sintering may be carried out at a temperature in the range 850°C to 880°C at atmospheric pressure for example.

DESCRIPTION OF DRAWINGS The invention is further illustrated in the following examples which refer to the accompanying drawings. In the drawings:

Figs IA to 1C show x-ray diffraction patterns for (a) a 2-4-7 sample synthesized at 872°C for 2 days with x=0.4 silver; (b) the same sample without silver (x=0) wherein only 1-2-3 material has formed; and (c) a model pattern for the 2-4-7 compound; Fig. 2 shows the temperature dependence of the resistance of a 2-4-7 sample synthesized at 872°C for 2 days with x=0.4 silver and then air quenched from the furnace at 872°C without slow cooling to load oxygen, showing a zero resistance T_c of 89K;

Fig. 3 shows the AC susceptibility of a 2-4-7 sample synthesized at 872°C for 2 days with x=0.4 silver and then air quenched from the furnace at 872°C without slow cooling to load oxygen, showing an onset T_c of 92K; and

Fig. 4 shows the AC susceptibility of the same 2-4-7 sample shown in Figs 2 and 3 after cooling to, and annealing at, 350°C in oxygen at 1 bar, showing an onset T_c of 97K.

DESCRIPTION OF EXAMPLES Example 1

Precursor powders of Y₂0₃ + 4Ba(N0₃)₂ + 7CuO + xAg, where the Ag powder is 0.7μm in size and x=0, 0.4 and 0.8, were weighed out and mixed by milling then decomposed for 1 hour at 750°C in air. The result was milled and die-pressed into 13 mm diameter pellets. These sets of samples were reacted in flowing oxygen at three different temperatures: 865, 872 and 880^βC overnight, ground, die-pressed and each set reacted again at the same temperature overnight. Fig. IA shows an x-ray diffraction pattern for the result of the complete reaction at 872°C using x=0.4. The resultant pattern is for single-phase 2-4-7 material with the only significant additional diffraction lines associated with free silver metal. In the absence of silver (x=0) the resultant material was YBa₂Cu₃0₇_₆ i.e. 1-2-3 together with CuO as indicated by the x-ray diffraction pattern shown in Fig. IB. The calculated 2-4-7 diffraction pattern is shown for reference in Fig. 1C.

The addition of the silver has clearly accelerated the formation of the 2-4-7 material from the reactants and in its absence the 2-4-7 material does not form, at least on the time scale of the present experiment. These results show that the silver accelerates the reaction at least as much as the alkali rate enhancers. The reaction was not quite complete and some 1-2-3 was present for x=0.8 and also for the syntheses at 865°C and at 880°C. The temperature of 872°C gave the best result. A silver content of x=0.8 appears to be excessive for best performance. The preferred silver content thus lies between x=0.2 and 0.4.

The samples thus synthesized were removed from the furnace and air-quenched. The 2-4-7 samples synthesized with silver addition were superconducting around 9OK upon removal and air quenching from the furnace at the temperature of synthesis, namely around 870°C. The approximately 20 second cool down after removal from the furnace was sufficient to load oxygen and reduce oxygen deficiency to near zero. Fig. 2 shows the temperature dependence of the resistance of the sample synthesized at 872°C with x=0.4 silver as removed from the furnace without slow cooling or annealing. Measurements were made using the standard 4-point technique. Zero resistance occurs about 89K. Fig. 3 shows the AC susceptibility of the same sample. The diamagnetic onset transition temperature is 92K. Evidently these 2-4- 7/silver composite materials load oxygen extremely rapidly providing a very significant advantage in the processing of Y/Ba/Cu/O HTSC.

The samples were then slow cooled and annealed at 350°C in flowing oxygen and transition temperatures were found to be raised further. Fig. 4 shows the AC susceptibility for the same sample displayed in Figs 2 and 3 after oxygen loading. The transition temperature has risen to 97K.

Example 2

Differential thermal analysis (DTA) was carried out on a 2-4-7 stoichiometric composition given by 1-2-3 + 0.5CuO + 0.4Ag(powder) (x=0.8) in flowing oxygen at 1 bar pressure. A large melting endotherm and freezing exotherm was observed centred on 875°C. In the absence of silver melting occurs at 960°C in oxygen at 1 bar. The presence of a melt phase appears to be the reason for the accelerated kinetics of formation of 2- 4-7. This also shows that 2-4-7 can be subjected to melt processing in a silver tube in order to obtain textured material free of weak links. Melt processing of 1-2-3 is not compatible with powder-in-silver-tube technology as the melt phase does not appear until about 1015°C, well above the melting point of the silver tube containing the 1-2-3 material. The appearance of a melt phase at about 875°C in the process of the invention, well below the melting point, 930°C, of silver in oxygen enables the combining of melt processing with powder-in-silver-tube technology.

Example 3

2-4-7 materials were reacted essentially as described in Example 1 with x=0.4 for a period of 5 days at 872°C in oxygen at 1 bar with the sample being air quenched after 1 day, reground and repressed as a pellet and further reacted for 1 day, reground, repressed as a pellet and further reacted for the remaining 3 days. The sample was annealed in oxygen at 380°C for 1 day in oxygen at 1 bar. The resulting material was single phase material with no impurity phases detectable by x-ray or neutron diffraction. Structural refinements showed δ=0.03 and resistivity and susceptibility showed a sharp superconducting transition with zero resistance at 93K.

Example 4

Gd₂Ba₄Cu₇0₁₅_₆ was reacted much as described in Example 3. A sharp superconducting transition was observed with zero resistance at 96K.

The forgoing describes the invention including a preferred form thereof. Alterations and modifications as will be obvious to those skilled in the art are intended to be incorporated in the scope hereof, as defined in the following claims.

Claims

1. A process for preparing a high temperature 2-4-7 superconductor of formula

R_a._pB_b._qA_p+qCu_c0₁₅.₆ wherein

1.9<a<2.1

3.9<b<4.1

6.8<c≤7.2

0≤p,q<l

0≤p+q<l

1.0<δ<-0.3

R is Y, La, Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm, or Y, or any combination thereof,

B is Ba or Ba plus a minor amount of Sr or La or a combination thereof, and

comprising intimately mixing precursor materials with silver metal or a compound of silver as a reaction rate enhancer and then reacting the precursor materials in an oxygen containing atmosphere to form the 2-4-7 superconductor, the mole fraction of silver satisfying the ratio x:a:b:c where x is the mole fraction of silver and 0<x≤1.0 and a, b and c are as defined above and are the cation mole ractions of said cation components R, B and Cu of the resulting 2-4-7 superconductor.

2. A process according to claim 1, wherein 0.2≤x≤0.4.

3. A process according to either one of claims 1 and 2, wherein the silver metal or compound of silver is mixed with the precursor materials as a powder of average particle size less than about 7 microns.

4. A process according to any one of claims 1 to 3, wherein the cation mole fractions of said R, Ba and Cu in the precursor materials are present in said ratio a:b:c.

5. A process according to any one of claims 1 to 4, wherein said reaction is carried out at a temperature T in °C and oxygen partial pressure Po₂ in Pa satisfying the equation 1210 - 180L + 21L² < T < 2320 - 581.5L + 58.5L² where L = log₁₀ Po₂.

6. A process according to any one of claims 1 to 4, wherein said reaction is carried out at about 10⁵Pa at a temperature in the range 865°C to 880°C.

7. A process according to any one of claims 1 to 6, wherein said reaction is carried out in oxygen.

8. A process according to any one of claims 1 to 7, wherein the precursor materials are pure metals R, B and Cu.

9. A process according to any one of claims 1 to 8, where the precursor materials are mixed by milling together.

10. A process according to any one of claims 1 to 9, wherein the precursor materials and the silver metal or compound of silver are loaded as powders into a silver tube and said reaction is carried out in the silver tube.

11. A process according to claim 10, wherein the silver tube containing the precursor and silver powders is reduced to a smaller cross-sectional dimension.

12. A process according to either one of claims 10 and 11, wherein said silver tube and the contents thereof are heated to the melting point of the 2-4-7 material or above but below the melting point of the silver tube to densify and texture the 2-4-7 material and remove weak links between grains of material in the silver tube.

13. A process according to any one of claims 1 to 12, wherein the 2-4-7 superconductor is prepared to exhibit a Tc above 85K without cooling or annealing to increase the oxygen content of the material for more than 60 minutes.

14. A process according to any one of claims 1 to 12, where the 2-4-7 superconductor is prepared to exhibit a Tc above 85K without cooling or annealing to increase the oxygen content of the material for more than 20 minutes.

15. A process according to any one of claims 1 to 12, wherein after the synthesis reaction the material is slowly cooled in any oxygen containing atmosphere to load oxygen into the 2-4-7 material to reduce the oxygen deficiency δ thereof to 0.6<δ<-0.2.

16; A process according to any one of claims 1 to 12, wherein after the synthesis reaction the material is slowly cooled in any oxygen containing atmosphere to load oxygen into the 2-4-7 material to reduce the oxygen deficiency δ thereof to 0.05<δ<-0.2.

17. A process for preparing a conductor composed of 2-4-7 superconductor of formula

R_a-_pB_b-_qA_p+q^_cC¹ ₁₅__δ wherein

1.9<a<2.1

3.9<b<4.1

6.8<c≤7.2

0≤p,q<l

0≤p+q<l

1.0<δ<-0.3 R is Y, La, Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm, or Y, or any combination thereof,

B is Ba or Ba plus a minor amount of Sr or La or a combination thereof, and

comprising:

intimately mixing precursor materials as powders with a powder of silver metal or a compound of silver as a reaction rate enhancer,

loading the intimately mixed precursor and silver powders into a silver tube,

carrying out operations on the silver tube containing the precursor powders to reduce the tube to a smaller cross- sectional dimension,

heating the silver tube and precursor materials therein in an oxygen containing atmosphere to react the precursor materials to form said 2-4-7 superconductor. further heating the silver tube and contents thereof to a temperature above the melting point of said 2-4-7 superconductor but below the melting point of the silver tube to densify and texture the 2-4-7 material within the silver tube and remove weak links between grains in the material in the silver tube, and

cooling the silver tube containing the 2-4-7 superconductor material in an oxygen containing atmosphere.

18. A process according to claim 17, wherein said cooling comprises cooling the silver tube and contents thereof sufficiently slowly in said oxygen containing atmosphere to reduce the oxygen deficiency of the 2-4-7 superconductor material to 0.6<δ<-0.2.

19. A process according to claim 17, wherein said cooling comprises cooling the silver tube and contents thereof sufficiently slowly in said oxygen containing atmosphere to reduce the oxygen deficiency of the 2-4-7 superconductor material to 0.05<δ<-0.2.

20. A process according to any one of claims 17 to 19, wherein the precursor materials and silver powder are intimately mixed by milling together.

21. A process according to any one of claims 17 to 20, wherein the 2-4-7 superconductor is prepared to exhibit a Tc above 85K without cooling or annealing to increase the oxygen content of the material for more than 60 minutes.

22. A process according to any one of claims 17 to 20, where the 2-4-7 superconductor is prepared to exhibit a Tc above 85K without cooling or annealing to increase the oxygen content of the material for more than 20 minutes.

23. A process for preparing a high temperature 2-4-7 superconductor substantially as herein described.