WO1995012899A1

WO1995012899A1 - Preparation of htsc conductors by deformation-induced texturing of superconductors without slip system

Info

Publication number: WO1995012899A1
Application number: PCT/NZ1994/000122
Authority: WO
Inventors: Robert George Buckley; Donald Mark Pooke; Jeffery Lewis Tallon; Alexander Otto; Noel Flower; Michael Staines
Original assignee: Industrial Research Limited; American Superconductor Corporation
Priority date: 1993-11-03
Filing date: 1994-11-02
Publication date: 1995-05-11
Also published as: NZ275163A

Abstract

A process for preparing high temperature superconductor components comprises loading powders of a high temperature superconductor material, which powders have a platy or highly aspected morphology, into a metal tube such as a silver tube, and then subjecting the powders to deformation to align the highly aspected plates with the deformation. Before loading into the silver tube the HTSC powders are preferably mixed with silver metal and/or other less aspected HTSC material. Preferably the silver tube is drawn down to reduce its diameter and then pressed or rolled into a tape. High current densities conductors may be produced with crystallographically aligned grains for systems including the Tl-Sr-Ca-Cu-O and Y-Ba-Cu-O systems.

Description

PREPARATION OF HTSC CONDUCTORS BY DEFORMATION-INDUCED . TEXTURING OF SUPERCONDUCTORS WITHOUT SLIP SYSTEM

FIELD OF INVENTION The present invention comprises processes, and composite materials thereby produced, for deformation- induced texturing of high-T_c superconducting cuprates (HTSC) which do not possess an active slip system at the temperature of deformation wherein the HTSC is initially in the form of a platey powder with highly aspected grains which is optionally mixed with silver metal then by means of deformation the preexisting platey grains are progressively tilted to align with the deformation.

BACKGROUND High-T_c superconducting cuprates (HTSC) are known to have superconducting transition temperatures, T_c exceeding the temperature at which liquid nitrogen boils, 77 K. As such they have a potentially large number of applications ranging from power generation, distribution, transformation and control, to high-field magnets, motors, body scanners, telecommunication and electronics. T_c values may be of the order of 92 K for example for YBa₂Cu₃0₇_₆, 107 K for example for Bi₂Sr₂Ca₂Cu₃O₁₀ or as high as 128 K for Tl₂Ba₂Ca₂Cu₃O₁₀. For many of these applications such T_c values alone do not guarantee the utility of these HTSC at 77K or higher temperatures. Often these applications require large critical currents in the HTSC and this is not achieved unless the grains of the HTSC are crystallographically aligned, otherwise known as textured, and well sintered together. It is common, for example, in the case of superconducting wires which use the HTSC material Bi₂Sr₂Ca-Cu₃O₁₀ to utilise a combination of heat treatment and deformation in order to grow well-aligned well-sintered grains thus allowing critical currents at 77 K and zero field of up to 60,000 Amps/cm². This is the basis of the powder-in-tube (PIT) technique wherein precursor powder is loaded into a silver tube which is drawn down in diameter then subjected to a succession of cycles of reaction followed by rolling or pressing deformation to form high current-density tapes containing textured Bi₂Sr₂Ca₂Cu₃O₁₀. A key element of this ability to texture is the fact that Bi₂Sr₂Ca₂Cu₃0₁₀ has an active slip system which allows quasi- plastic deformation of individual grains at room temperature.

However, other HTSC such as TlBa₂CaCu₂0₇ or Tl₀.₅Pb₀._sSr₂CaCu₂O₇ (Tl-1212) , TlBa₂Ca₂Cu₃0₉ or Tl₀.₅Pb₀.₅Sr₂Ca₂Cu₃O₉ (Tl-1223), Yba₂Cu₃0₇_₆ (RBC0-123) or Y₂Ba₄Cu₇0₁₅._δ (RBC0-247), while they have exceptionally good flux pinning properties making these materials potentially far more attractive for wire technology than Bi₂Sr₂Ca₂Cu₃O₁₀, do not possess any active slip system. As a consequence, simple deformation coupled with heat treatment fails to induce sufficient texturing to obtain good transport properties. Up to the present no group has successfully made high current-density wires or tapes using the PIT technique with any HTSC other than the bismuth HTSC: Bi₂Sr₂CaCu₂0₈ and Bi₂Sr₂Ca₂Cu₃0₁₀.

SUMMARY OF THE INVENTION The present invention comprises a process for preparing HTSC components, referred to here as "steric texturing", wherein HTSC powders having a platey or highly aspected morphology are optionally mixed with silver metal and/or other less aspected HTSC material or precursor HTSC material, then contained and subjected to deformation so as to align the highly aspected plates with the deformation.

Preferably the platey HTSC material together with any optionally-added silver metal and other less aspected HTSC material are contained by loading into a metal tube, which is then reduced to a smaller diameter by drawing down or extruding for example, and then preferably pressed or rolled to a tape. The deformation associated with the drawing, extruding, pressing, rolling, or like processes is effective in aligning the large flat surfaces of the grains with these surfaces orthogonal to the local direction of the negative strain.

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%.

Preferably the metal tube is of silver metal and the optionally-added silver metal is in the form of silver powder which is mixed with the HTSC powder before loading into the tube. The silver powder is preferably added in the range 10 to 100% and most preferably 16 to 50% by weight of the total HTSC powder excluding the silver powder. Preferably the silver powder has a particle size in the range 0.1 to 20 μm and most preferably 0.1 to 0.7 μm.

After texturing as described above the tape may be heat treated to sinter together the aligned HTSC grains and/or to react any other precursor HTSC material, included for this purpose, lying about the aligned grains and thus sinter together the aligned grains. Sintering may be carried out at a temperature in the range 800°C to 950°C and most preferably 850°C to 925°C for Tl-1201, Tl-1212 and Tl- 1223, 850°C to 950°C for RBCO-123, and 850°C to 880°C for RBCO-247.

The process of the invention may be carried out with platey powders of any HTSC including in particular those mentioned above, including any small variations in stoichiometry from these compositions or any substituted form generally recognised in the art as being of Tl-1201, Tl-1212, Tl-1223, RBC0-123, or RBCO-247 composition.

By 1201 is meant TlSr₂Cu0₅ where Tl is optionally partially substituted by Pb and/or Bi, Sr is optionally partially substituted by La or Ba including Tl₀.₅Pb₀.₅Sr₂__xLa_xCuO₅, Tl₀.₅Pb₀.₅Sr₂__wBa_wCuO₅ and Tl-__yBi_ySr₂._u. _vLa_.B -.CuO₅ where 0≤x,w≤2 and 0≤u,v,y≤0, or more generally (Tl,Pb,Bi)i(Sr,Ba,La)₂Cu0₅.

By 1212 is meant the nominal composition TlSr₂CaCu₂0₇ where Tl is optionally partially substituted by Pb and/or Bi, Sr is optionally partially substituted by La or Ba, and Ca is optionally partially substituted by R, including Tl₀.₅Pb₀.₅Sr₂_--Ba_v-CaCu₂O₇, Tl₀._sPb₀.₅Sr₂_._xLa-_:CaCu₂O₇, Tl-.sPbo.sSr_.Ca-._yR_yCu.O,, Tl₁__yBi_ySr₂__u__vLa_uBa_vCaCu₂0₇, and Tl.__yBi_ySr₂__vBa_vCa₁__uR_uCu₂0₇ where 0≤x,w≤2, 0≤u,v,y≤l and R is Y or any of the lanthanide rare earth elements, or more generally (Tl,Pb,Bi)₁(Sr,Ba,La)₂(Ca,R)-Cu₂0₇. Also, La may be partially or completely substituted by Nd.

By 1223 is meant the nominal composition TlSr₂Ca₂Cu₃0₉ where Tl is optionally partially substituted by Pb and/or Bi and Sr is optionally partially substituted by Ca or Ba including Tl₀.₅Pb₀.₅Sr₂_-.Ca_xCa₂Cu₃0₉, Tl₀.₅Pb₀.₅Sr₂__w_-.Ba_wCa_xCa₂Cu₃θ₉, and Tl₁._yBi_ySr₂__wBa_wCa₂.₁.R_uCu₃0₉ where 0≤x,y,u,w≤l or more generally (Tl,Pb,Bi)₁(Sr,Ba,Ca)₂Ca₂Cu₃0₉. By 123 is meant the nominal composition RBa₂Cu₃0₇__d where O≤d≤l and R is Y or any lanthanide rare earth element. Also R may be partially substituted by Ca and Ba may be partially substituted by Sr, La or Nd.

By 247 is meant R₂Ba₄Cu₇0₁₅._d where -0.3≤d≤l and R is Y or any lanthanide rare earth element. Also R may be partially substituted by Ca and Ba may be partially substituted by Sr, La or Nd.

In all of these Tl compounds, as is known in the art, there may exist some degree of oxygen non-stoichiometry so that 0₅, 0₇ and 0₉ may be interpreted as 0_5+δ, 0_7+δ and 0₉₊₅ where -0.3≤5≤+0.3 for example.

The HTSC may also be a mercury-based HTSC comprising HgBa₂Cu0_4+δ, HgBa₂CaCu₂0_6+l5 or HgBa₂Ca₂Cu₃0₈₊₆ or any small variations in stoichiometry from these compositions or any substituted form generally recognised in the art as being of 1201, 1212 or 1223 composition, including any of or combination of Tl, Bi, Pb, or Cd being partially substituted for Hg and partial substitution of Ba by Sr.

PREPARATION OF POWDERS HIGH ASPECT RATIO Preferably discrete aspected powders of thallium- based HTSC are prepared by reacting suitable stoichiometric precursor materials, preferably oxides, in a flux, preferably molten at the temperature of reaction, which can be dissolved away after cooling. The flux material is preferably soluble in the solvent while the HTSC material is not.

Preferably the flux either accelerates the atomic mobility of the reactants or alters surface energies, so as to cause a platey and highly aspected morphology of the grains of the superconducting cuprate thus formed, which grains may be separated into free discrete particles by dissolving away the flux after reaction resulting in a powder comprising discrete highly-aspected grains. Preferred fluxes include alkali carbonates, alkali halides, thallium halides, thallium oxides, alkaline earth halides, alkali oxides or hydroxides and alkaline earth hydroxides. Preferred solvents include water, ethanol, methanol, acetone, glycerine and liquid ammonia. Preferred flux/solvent combinations include Na₂C0₃/water, K₂C0₃/water, K₂C0₃/ethanol, Na₂C0₃/ethanol, KCl/ethanol, Kcl/glycerine, NaCl/glycerine, Kl/ethanol, CaCl₂/acetone, BaBr₂/ethanol, Tll/liq NH₃, TlBr/ethanol, Tl₂0₃ ^■* TlO(875^βC, lbar0₂)/ethanol and Ca(OH)₂/liq NH₃.

After dissolving away the flux the residue platey HTSC material may still be partially aggregated. Therefore additionally, though optionally, the residue material may be further separated into discrete particles by agitating using, for example, an ultrasonic bath and optionally further washing in an iso-electric fluid which may separate residual fine particles. Preferred isoelectric fluids include acids such as nitric acid, sulphuric acid at pH - 1.8 ± 0.2.

According to the known art of separation and grading of powders the residual powders may also optionally be graded in a cyclone, air flow, by sedimentation or other such processes in order to separate out only those plates within a desired range of sizes. Additionally, such graded powders may be subsequently mixed in whatever desired proportion in order to improve flow properties or ability to pack densely.

Apart from carrying out the reaction in the presence of the flux the reaction from precursor materials is carried out at known temperatures and other reaction conditions for preparing Tl-based HTSC, and using conventional precursor materials. For example, for the 1201, 1212 or 1223 compounds the precursor materials may be primary oxides optionally obtained from decomposed nitrates, oxalates, or other such decomposable compounds or may be a combination of primary oxides and other pre-reacted oxides such as %T10₃+ijPbO+Ba₂Cu0₃ where the Ba₂Cu0₃ is pre-reacted for example. These precursor materials are mixed in stoichiometric proportions, and the reaction may be carried out at a temperature in the range 750°C to 950°C at an oxygen partial pressure ≤l bar.

Alternatively where the HTSC is Tl-1212 the HTSC may be prepared by mixing 1201 in stoichiometric proportions with material of nominal composition 0011 (as herein defined) and reacting at between 750°C and 950°C and most preferably 750°C and 870°C in an oxygen partial pressure less than 1 bar and preferably less than 0.2 bar in order to form 1212 in which the grains so formed are highly aspected. Alternatively where the HTSC is Tl-1223 the HTSC may be prepared by mixing 1212 + 0011 or 1201 + 2 (0011) in stoichiometric proportions and reacting at between 750°C and 950°C and most preferably 810°C and 900°C in an oxygen partial pressure less than 1 bar and preferably less than 0.2 bar to form highly aspected 1223.

By 0011 is meant any suitable combination of compounds with nominal composition CaCu0₂ or, if y>0, Ca-. _yR_yCu0_2> This may therefore be the combination of compounds ³5Ca₂Cu0₃+35CuO. When preparing 1212 preferably x or y is 0.2±0.02. When preparing 1223 most preferably 0.2≤x≤0.4 and 0.3≤w≤l.

Powders of the superconducting material prepared by the methods described above have a platey or highly aspected morphology. They may have an aspect ratio of four or more. By aspect ratio is meant the ratio of the square root of the large surface area of the grains to the thickness of the grains perpendicular or generally perpendicular to the large surface area.

DETAILED DESCRIPTION OF EXAMPLES The invention will be further described with reference to the accompanying examples and figures. In the figures shown:

Figure 1 shows the magnetic field dependence of the critical current for (a) a sterically-textured thallium- 1212 silver-clad tape and (b) an untextured tape. The plots also show the magnetic field dependence of the product of critical current and magnetic field.

Figure 2 shows an x-ray diffraction pattern (using Co Kα radiation) for (a) the core of a sterically-textured thallium-1212 silver clad tape and (b) untextured thallium- 1212.

Figure 3 shows two scanning electron micrographs of platey grains of Tl₀.₅Pb₀.₅Sr₂_-.La-.CaCu₂O₇ grown in a K₂C0₃ flux and washed and separated as described above. Figure 4 shows x-ray diffraction patterns for Tl_o.₅Pb_o.₅Sr₂_-.La-.CaCu_.O-. (a) made in platey form as described above and showing strong 00i textuπ.. :j because of the platey-ness; and (b) made in conventional form with equiaxed grains and consequent absence of texture.

Figure 5 shows the phase fractions, as determined from x-ray diffraction, of thallium-1201, thallium-1212 and Ca₂Cu0₃ present plotted as a function of time when 1201 + 0011 is reacted at 820°C in an oxygen partial pressure of 0.02bar.

Figure 6 shows the phase fractions, as determined from x-ray diffraction, of thallium-1201, thallium-1212, Ca₂Pb0₄ and Ca₂Cu0₃ present plotted as a function of time when 1201 + 0011 is reacted at 800°C in an oxygen partial pressure of 0.02bar.

Figure 7 shows the phase fractions, as determined from x-ray diffraction, of thallium-1201, thallium-1212, Ca₂Pb0₄ and Ca₂Cu0₃ present plotted as a function of time when 1201 + 0011 is reacted at 790°C in an oxygen partial pressure of 0.02bar.

Figure 8 shows the phase fraction, plotted as a function of time, of thallium-1212 formed when 1201 + 0011 is reacted in an oxygen partial pressure of 0.02bar at the various temperatures shown in °C.

Figure 9 shows a scanning electron micrograph of the thallium-1212 formed when 1201 + 0011 is reacted for 8 hours at 800°C in an oxygen partial pressure of 0.02bar.

Figure 10 shows the phase fractions, as determined from x-ray diffraction, of thallium-1201, thallium-1212 and Ca₂Pb0₄ present plotted as a function of time of reaction when the primary oxides of each of the constituent cations are reacted in an oxygen partial pressure of 1 bar at 950°C, 850°C and 750°C.

Figure 11 shows the phase fractions, as determined from x-ray diffraction, of thallium-1201, thallium-1212 and Ca₂Pb0₄ present plotted as a function of time of reaction when the primary oxides of each of the constituent cations are reacted in an oxygen partial pressure of 1 bar at 650°C and 550°C.

Figure 12 shows the phase fraction, plotted as a function of time, of thallium-1212 formed when 1201 + 0011 is reacted in an oxygen partial pressure of 1 bar at the various temperatures shown in °C. The dashed curve annotated (La) is for 0.2La substituted for Sr in the thallium-1212 compound. CONDUCTOR PRODUCTION Example 1

A stoichiometric mixture of Tl_o.₅Pb_o.₅Sr₂_-.La-.CuO₅ with x=0.2 and a nominal 0011 mixture 0.5Ca₂CuO₃+0.5CuO were reacted together for 2 hours at 850°C in a gas mixture of 2% oxygen and 98% nitrogen. The result was a sintered mass of highly platey 1212 material Tl_o.₅Pb₀.₅Sr₂_-_:La_J-CaCu₂0₇ with x=0.2 having aspect ratios of about 20:1. This was lightly ground to give a free powder with particle aspect ratios of approximately 5:1, mixed and milled with an equal weight of silver powder (0.7μm diameter particles) then loaded and lightly tamped down into a 5mm outer diameter silver tube having wall thickness 0.5mm. The wire was then drawn down to a diameter of 1.2 mm, cut into two equal lengths one of which was rolled down to 0.2mm thickness and the other pressed to 0.2mm thickness. The wire received no heat treatment except intermittent anneals at 400°C to anneal the work-hardened silver. Some of the resultant tapes were then sectioned for scanning electron microscopy and the silver surface of the tape was cut and peeled back. Surface Lotgering factors for both the pressed and rolled tapes were around 0.35 where a factor of 1.0 denotes 100% c-axis alignment and of 0.0 denotes complete lack of texture. Tapes were mounted in epoxy and polished back into the depth of the_ HTSC core. In the core both pressed and rolled tapes showed Lotgering factors of about 0.3. Other tapes were pressed to 0.14mm thickness but these showed the same degree of texturing which appears to be maximised for the particular aspected HTSC powders which were used. By way of comparison, conventional reaction and sintering of 1212 using the PIT technique results in cores with Lotgering factors -0.02≤f_L≤+0.02 reflecting the complete lack of texture. No attempt was made to explore optimum conditions for sintering the textured cores but one tape was heat treated at 870°C for 2.5 hours in flowing oxygen. This tape had a self field critical current I_c=21 Amps or J_c=6000Amps/cm². About 30% of the cross section is silver so this is equivalent to a superconductor J_c=8600 Amps/cm². Figure la shows the magnetic-field dependence of the critical current for this tape and this is compared in Figure lb with the field dependence of an identical, though untextured, tape. The critical current I_c has a low-field plateau that extends out to 10 times the field that the untextured wire has. This is best illustrated by the pinning force obtained from the product of I_c and the field strength H. This product is also plotted in Figures la and b and it can be seen that this product peaks at more than 10 times the value seen in the untextured tape. Also the textured tape shows a marked anisotropy depending upon whether the field is aligned in the plane of the tape or normal to the tape. In the former case the I_c value is 2.3 times that for the field normal to the tape at the highest field. This anisotropy reflects the texturing which has been introduced to the microstructure of the tape. Example 2

The same platey 1212 + 0011 + silver powder mixture described in Example 1 was loaded into a silver tube with outer diameter 5.5mm and inner diameter 2.8mm, that is, with wall thickness approximately equal to the inner radius. This was drawn down to 90% total area reduction then rolled to a thickness reduction of 50%, 75%, 85% and 90%. The silver on the flat surface of the resulting tape was cut and peeled off and the exposed HTSC surface was analysed for texture using x-ray diffraction collected between 2Θ values of 20° and 45°. A typical diffraction pattern for a rolling thickness reduction of 75% is shown in Figure 2a and compared with a diffraction pattern for untextured 1212 shown in Figure 2b. The enhancement of the <001> lines <003>, <004> and <005> resulting from steric texturing is readily apparent. Lotgering factors calculated from rationing the <003> reflection intensity with that for <103> confirm significant texturing with values of f_L ranging from 0.45 to 0.6 consistently obtained for all rolling strains investigated. Lotgering factors calculated from rationing the <003>, <004> and <005> reflection intensities with all reflections with 2Θ between 20° and 45° consistently give higher values of f_L ranging from 0.55 to 0.7. These values are summarised in Table 1.

Table I. Lotgering factors, f_L for cores of sterically textured 1212 tape subjected to 90% drawing strain followed by rolling thickness reductions e_r of 50, 75, 85 and 90%. f_L is calculated by rationing the diffraction intensity of the <003> to the <103> reflections and also by rationing the sum of the <00_2> intensities for i=3, 4 and 5 with the sum of intensities of all reflections between 20°≤2Θ≤45° using Co Kα radiation. ^*Ground surface. *Top surface of peeled core.

PRODUCTION OF HIGH ASPECT RATIO HTSC Example 3

A stoichiometric mixture of 1.8Sr(N0₃)₂ + 0.lLa₂O₃ + Ca(N0₃)₂ + 2CuO was decomposed by ramping from 600°C to 800°C over 2 hours in air. This was mixed with 0.25 Tl₂0₃ + 0.5PbO then milled with double the quantity by weight of K₂C0₃. The powder was placed in a gold bag, which was filled with oxygen gas then sealed by crimping the open end tightly shut. This was placed in a tube furnace at 940°C under flowing oxygen gas for 2 hours, so reacting the contained materials through to nearly single phase Tl_n._sPb_o._sSri.gLa_o.j-CaCUjO, then air quenched. The fused mass of material was washed in distilled water to dissolve away the K₂C0₃. The residual material was agitated in ethanol in an ultrasonic bath for 10 minutes to further separate the grains of powder then washed in HN0₃ at Ph = 1.8 to achieve isoelectric separation of fine particles from the plates, again using an ultrasonic bath. The powders were separated by settling in a column of isopropyl alcohol. Plates of size 8 to 20μm settled within 30 min and the remaining fluid suspension containing smaller plates and sub-micron fines was decanted. The settled residues were dried by evaporation and oven heating at 200°C for 2 hours. Figure 3 shows two scanning electron micrographs of grains from the first category of settled powder. The aspect ratio of these grains can be seen to be of the order of 10:1. The desirable tendency of these powders to texture when settling, or where deformed in any way as a powder bed is illustrated in Fig. 4. The powder with grain size between 8 and 20 μ was placed in an x-ray diffraction sample holder and pressed flat with a glass slide using finger pressure. This pressing process caused the highly aspected grains to substantially align. Figure 4(a) shows that the 00i reflections are substantially enhanced over those shown in Figure 4(b) for conventionally prepared untextured powders of Tl₀.₅Pb₀.₅Sr_.₈La_o.₂CaCu₂0₇ with approximately equiaxed grains. Analysis shows a calculated Lotgering factor of F = 0.55 where F = 0 denotes untextured and F = 1 denotes 100% c-axis alignment.

The process was repeated using several other combinations of flux and solvent. These resulted in powders comprising plates up to 10 μm in size and included the following flux/solvent combinations: Kl/ethanol, Til/liquid NH₃, Ca(OH)₂/liquid NH₃.

Example 4

The compound Tlo.s b_o.sSr-.gLa_o.jCuOs (1201) was synthesized by mixing stoichiometric quantities of Sr(NO)₂, La(N0₃)₃.6H₂0 and CuO and milling. The mixture was decomposed at 750°C in air for 2 hours. Stoichiometric quantities of Tl₂0₃ and PbO were added to the calcined residue, mixed, milled and die pressed into 13 mm diameter pellets of about 1 girt wt. These were placed in a small gold bag which was sealed under oxygen by crimping the end closed, then placed in a tube furnace at 900°C in flowing oxygen. After 1 hour the pellets were essentially single-phase 1201 material. The pellets were ground and Ca₂Cu0₃ and CuO added in the following stoichiometric proportions

1201 + 3gCa₂CuO + JgCuO. The mixture was well mixed and milled then die- pressed into 13 mm diameter pellets and up to six of these pellets were placed in a gold bag, flushed with a gas mixture of 2% oxygen + 98% nitrogen and the gold bag was sealed by crimping. The gold bag containing the pellets was placed in a tube furnace set at a fixed temperature lying between 750°C and 850°C through which was flowing a gas mixture of 2% oxygen in 98% nitrogen and reacted for a period of time. The bag was then removed, one sample was removed from the bag which was then sealed as described above and further heat treated under the same gas mixture. This process was repeated until the last sample was removed in some cases (at the lower temperatures) after a total reaction time of several days. The pellets progressively reacted through to the 1212 compound. Each pellet was ground and subjected to x-ray diffraction to determine the phases present and their proportions. If this reaction is carried out at higher temperatures and in oxygen as is typical in the art of synthesis of 1212 and 1223 then Ca₂Pb0₄ always appears as an impurity phase. Notably this phase is absent for all syntheses above 800°C. It is produced in small quantities in syntheses below this temperature. Figures 5, 6 and 7 show the reaction products as a function of time for reactions at 820°C, 800°C and 790°C respectively showing, at the higher temperatures, a decline in 1201 and Ca₂Cu0₃ phases and the growth of the 1212 phase. Figure 8 summarises the fraction of 1212 formed plotted against time for the different temperatures shown beside each curve, namely 850°C, 830°C, 820°C, 800°C, 790°C and 780°C. The reaction is very rapid at the higher temperatures and progresses to completion, while it is very slow and incomplete at the lower temperatures. Such conditions at the lower temperatures, where the fraction reacted after 1 hour is less than 1%, are highly suitable for staged-growth processing as referred to above.

Pieces of each pellet were also investigated using scanning electron microscopy. At all temperatures the growth of 1212 is in the form of highly aspected plates as shown in Figure 9 for reaction at 800°C. These have an aspect ratio of up to 20:1. Similar microstructure was observed at all temperatures investigated from 790 to 850°C.

The experiment was repeated with silver powder added to the extent of 16% by weight. The silver powder grains are of the order of 0.7 μm in diameter. The reaction rate is found to be greatly enhanced. At 800°C the reaction is complete in 1 hour. Again highly aspected grains with an aspect ratio of up to 20:1 are obtained. At a temperature of 750°C the reaction does not appear to proceed significantly even after reaction for 24 hours. Example 5

The reaction of primary oxides to form the 1212 compound was studied using several techniques differing from that described in Example 4. These included (i) reaction from primary oxides in oxygen at 1 bar, and (ii) reaction from 1201+"0011" in oxygen at 1 bar. It was found from reaction (i) that under no conditions was the morphology of the so-formed 1212 platey or highly aspected and at no temperature was the reaction to 1212 slow enough to allow use of the principles of staged growth as described above. In reaction (ii) it was found that lower temperature conditions allowed a sufficiently slow conversion to 1212 but the grain morphology was never platey. We conclude from these studies in combination with Example 4 that reacting 1212 from 1201+"0011" is essential to allow a sufficiently slow reaction rate for staged growth and that reaction at low oxygen partial pressure is essential to allow the platey morphology. Reaction from 1201+"0011" also has the added benefit that with the thallium being chemically bound in the 1201 compound thallium volatility is greatly diminished to the extent that thallium loss was not significant in these syntheses. Reaction from primary oxides including Tl₂0₃ always presents difficulties with thallium evaporative loss. Reaction (i) The reaction

0.25Tl₂O₃ + 0.5PbO + 1.8Sr0 + 0. lLa₂0₃ + CaO + 2Cu0 —> Tl_o.₅Pb_o.₅Sr-.₈La₀.₂CaCuO₇ was studied at various temperatures under 1 bar of oxygen. The SrO and CaO were obtained by decomposing nitrate. The precursor material was mixed, milled, die-pressed to 13mm diameter pellets and sealed in a small gold bag under oxygen by crimping closed the open end. After reaction for a given length of time the sample was removed, ground, examined by x-ray diffraction (XRD) then repressed as a 13mm diameter pellet and reacted in the same way in a gold bag for a further length of time. Figure 10 shows the fraction of each compound present as determined from XRD plotted as a function of total time reacted at 950°C, 850°C and 750°C and Figure 11 shows the fractions of each compound for reaction at 650°C and 550°C. 1212 forms at the first three temperatures, though more slowly at the lower temperatures, but does not form at all at 650°C or 550°C. At no temperature did 1212 form slowly enough and in progressive fashion as might be suitable for staged-growth texturing. At the temperatures between 650 and 750°C where the total fraction of 1212 formed was small the reaction to this fraction took place within minutes and did not proceed further with time. Clearly those regions in the precursor where the local composition was suitable for formation of 1212 reacted immediately and further reaction was limited by solid state diffusion.

Reaction (ii)

The reaction 1201 + 0011 was studied at various temperatures in oxygen at 1 __ar pressure. Figure 12 shows the fraction of 1212 formed (as determined from XRD) plotted as a function of time of reaction. Much of the data is for unsubstituted 1201, thai, is Tlo.₅Pbo.₅Sr₂Cu0₅, while the curve annotated (La) denotes La-substituted 1201, namely

Evidently around 800°C the rate of formation of 1212 is sufficiently slow that over a period of 15 to 60 minutes a fraction of the order of 1% or less is formed. This is very suitable for staged growth processing. Unfortunately, the grain morphology as shown by scanning electron microscopy reveals predominantly equiaxed grains of size between 3 and lOμm. The absence of aspected grains removes any prospect of staged growth texturing.

Example 6

The '1223' superconductor having composition Tl₀.₉Bi₀._Sr_.₂BaCa_.₈Cu₃O₉ was synthesized by prereacting a nominal composition BaCu₂0₃ and adding stoichiometric proportions of 0.45Tl₂O₃ and Bi₀.₁Sr₁.₂Ca_1>8CuO_y. The choice of BaCu₂0₃ in the composition was motivated by its relatively low melting point. The composition was reacted in oxygen at 870°C for two hours. The resultant material was essentially single phase 1223 compound with a very platey microstructure with typical plates of dimension lOμm x lOμm x lμm. The highly platey morphology is attributable to the occurrence of a melt during synthesis.

Example 7

The 1223 composition was synthesised by reacting oxides of Tl, Bi, Cu mixed with decomposed nitrates of Ba, Sr and Ca in the stoichiometric proportions corresponding to Tl₀.₉Bi₀._SrBaCa₂Cu₃O₉, the reaction being carried out in an equal weight of K₂C0₃ at 880°C in oxygen at 1 bar pressure for 20 hours. The resulting material was washed to remove the K₂C0₃, ground and sedimented to remove the large agglomerates of material. The resultant suspension was drawn off and sieved in a silicon wafer sieve. The particles of the obtained powder were platey with dimensions approximately 10 to 20 μm square by 4 to 8 μm thick.

Example 8

The powders of example 3 were sedimented from isopropyl alcohol onto a silver substrate and then dried and pressed. The resulting Lotgering factor was 97% showing a very high degree of texture.

Example 9 The powders of example 7 were sedimented from isopropyl alcohol onto a silver substrate and then dried and pressed. The resulting Lotgering factor was 92% showing a very high degree of texture.

Example 0

The powders of example 7 were mixed with 30% by weight of silver powder then loaded into a 6mm outer diameter thick-walled silver tube, drawn down to 1mm diameter then rolled to a thickness of 0.35mm. The wire tapes were opened up to expose the surface of the superconductor powder. X-ray diffraction showed a significant degree of texture with a Lotgering factor of 30%. It is straightforward to improve this texture by improving the plateyness of the superconductor powder by for example using powders obtained from the process of example 6.

The foregoing describes the invention and preferred forms and examples thereof. Alterations and modifications as will be obvious to those skilled in the art are intended to be incorporated within the scope hereof.

Claims

CLAIMS 1. A process for preparing high temperature superconductor components, wherein powders having a plate¬ like or highly aspected morphology predominantly of a high temperature superconductor of nominal composition TlSr₂Cu0₅ where Tl may be partially substituted by Pb and/or Bi and Sr may be partially substituted by La or Ba; TlSr₂CaCu₂0₇ where Tl may be partially substituted by Pb and/or Bi, Sr may be partially substituted by La or Ba, and Ca may be partially substituted by Y or any lanthanide rare earth element; or TlSr₂Ca₂Cu₃0₉ where Tl may be partially substituted by Pb and/or Bi and Sr may be partially substituted by Ca or Ba; are contained and subjected to deformation so as to align the highly aspected plates with the deformation.

2. A process according to claim 1, wherein the said HTSC powders are contained by loading into a metal tube, and are then subjected to deformation while in the metal tube by drawing down or extruding the metal tube.

3. A process according to claim 2, wherein the metal tube is subjected to deformation by pressing or rolling effective to align the larger flat surfaces of a major portion of the grains with these surfaces orthogonal to the local direction of the negative strain.

4. A process according to claim 2, wherein said drawing or extruding achieves a cross-sectional reduction of the metal tube between 70 to 95%.

5. A process according to claim 3, wherein said pressing or rolling achieves a cross-sectional reduction of the metal tube between 70 to 95%.

6. A process according to claim 2, wherein the metal tube is a silver tube and silver is mixed with the HTSC in powder form before loading into the silver tube.

7. A process according to claim 6, wherein silver powder is added in the range 16 to 50% by weight of the HTSC powder.

8. A process according to claim 7, wherein the silver powder has a particle size in the range 0.1 to 20 μm.

9. A process according to claim 7, wherein the silver powder has a particle size in the range 0.1 to 0.7 μm.

10. A process according to claim 6, wherein after said deformation the metal tube containing the aligned platey HTSC grains is sintered to react together the aligned HTSC grains.

11. A process according to claim 10, wherein said sintering is carried out at a temperature in the range 800°C to 950°C.

12. A process according to claim 10, wherein said sintering is carried out at a temperature in the range 850°C or 925°C.

13. A process according to claim 2, wherein the HTSC is Tlo.₅Pb₀.₅Sr₂_-_<La_:.Cuθ₅__δ or Tl₀.₅Pb₀.₅Sr₂__wBa_wCuO₅__δ where 0≤x,w≤2 and -0.3≤δ≤+0.3, or Tl___yBi_ySr₂-._u__vLa_uBa_vCu0₅__δ where 0<u,v,y≤l and -0.3≤δ≤+0.3.

14. A process according to claim 2, wherein the HTSC is Tlo.₅Pb₀.₅Sr₂__wBa_wCaCu₂θ₇__δ, Tlo.₅Pbo.₅Sr₂__xLa-_:CaCu₂0₇__δ, Tl₀.₅Pb₀.₅Sr₂Ca___yR_yCu₂O₇__δ, Tl₁__yBi_ySr₂__u__vLa_llBa_vCaCu₂0₇__δ or Tl__ _yBi_ySr₂_.-Ba_vCa₁__uR_uCu₂0₇__δ where 0≤x,w≤2, 0≤u,v,y≤l, -0.3≤δ≤+0.3 and La may be partially or completely substituted by Nd and R is Y or any lanthanide rare earth element.

15. A process according to claim 2, wherein the HTSC is Tlo.₅Pbo.₅Sr₂_._:Ca_xCa₂Cu₃0₉__fi, Tl₀.₅Pb₀.₅Sr₂__w_„Ba_wCa-_:Ca₂Cu₃O₉__l5 or Tl₁__yBi-.Sr₂__wBa_wCa₂__R_uCu₃0₉__δ where 0≤x,y,u,w≤l and -0.3≤δ≤+0.3.

16. A process according to claim 2, wherein the HTSC powders are prepared prior to loading into the metal tube by reacting precursor materials in stoichiometric proportions in a flux which can be dissolved away after cooling to separate the reacted plate-like HTSC into free discrete particles.

17. A process according to claim 16, wherein the flux either accelerates the atomic mobility of the reactants or alters surface energies, so as to cause a platey and highly aspected morphology of the HTSC grains.

18. A process according to claim 16, wherein the flux is selected from alkali carbonates, alkali halides, thallium halides, thallium oxides, alkaline earth halides, alkali oxides or hydroxides and alkaline earth hydroxides.

19. A process according to claim 18, wherein the flux is dissolved away with a solvent selected from water, ethanol, methanol, acetone, glycerine t.-d liquid ammonia.

20. A process according to claim 16, including agitating the reacted HTSC after dissolving away the flux to further separate the reacted HTSC into discrete particles.

21. A process according to claim 2, wherein the HTSC has been reacted from a compound of nominal composition TlSr₂CuO_s where Tl may be partially substituted by Pb and/or Bi and Sr may be partially substituted by La or Ba, mixed with a compound or combination of compounds of nominal composition CaCu0₂ or Ca___yR_yCu0₂ where y>0 and R is Y or any lanthanide rare earth element, in stoichiometric proportions to produce said HTSC.

22. A process according to claim 2, wherein the HTSC has been reacted from a compound of nominal composition TlSr₂Cu0₅ where Tl may be partially substituted by Pb and/or Bi and Sr may be partially substituted by La or Ba, mixed with a compound or combination of compounds of nominal composition 2CaCu0₂ or 2Ca___yR-,Cu0₂ where y>0 and R is Y or any lanthanide rare earth element, in stoichiometric proportions to produce said HTSC.

23. A process according to claim 2, wherein the HTSC has been reacted from a compound of nominal composition TlSr₂CaCu0₅ where Tl may be partially substituted by Pb and/or Bi and Sr may be partially substituted by La or Ba, mixed with a compound or combination or compounds of nominal composition CaCu0₂ or Ca₁__yR_yCu0₂ where y>0 where R is Y or any lanthanide rare earth element in stoichiometric properties to produce said HTSC.

24. A process according to claim 21, wherein the reaction has been carried out at a temperature between 750°C and 870°C, most preferably 750°C and 830°C and an oxygen partial pressure of less than 0.2 bar and preferably less than 0.02 bar.

25. A process according to claim 22, wherein the reaction has been carried out at a temperature between 800°C and 925°C and an oxygen partial pressure of less than 0.2 bar and preferably ≤ 0.1 bar.