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Definitions

• FIELD OF THE INVENTION relates to superconductors and particularly high temperature superconductors and to an improved preparation of high temperature superconductors and fabrication of articles containing high temperature superconductors.
• High temperature superconductors were first disclosed in 1987 and are characterised by having superconductor transition temperatures (Tc) above the temperature of liquid helium and many with a Tc above the temperature of liquid nitrogen (77.3K).
• Tc superconductor transition temperatures
• HTSC high temperature superconductors
• Solution co-precipitation is a technique to prepare precursor (for example of HTSC compounds) compounds so that they are mixed in a very intimate manner, perhaps as closely as on a molecular level.
• precursor for example of HTSC compounds
• the various precursor components are initially dissolved in an aqueous solution (usually containing an acidifying agent such as nitric acid).
• the dissolved precursors are precipitated from solution by addition of a precipitating agent.
• These agents have included carbonates, acetates, formates, hydroxides and oxalates.
• the resultant co-precipitate is filtered usually by a centrifuge and subjected to a low temperature calcination process to remove or decompose the anion to provide a mixture of the respective cation oxides.
• the technique may be used to co-precipitate precursors to all copper oxide based ceramics, such as semiconductor, insulator and superconductor phases in YBCO, BSSCO and thallium based systems.
• the technique involves the surprising and unexpected discovery that minimising the variation in the pH of the solution during the co-precipitation process can result in the co-precipitate precursors having a high and consistent phase purity.
• the co-precipitates may also exhibit a narrow variation in particle size ranges.
• the invention resides in a method for co-precipitating superconducting precursor components, the method comprising the steps of - dissolving the superconducting precursor components into solution, adding a precipitating agent to the aqueous solution, and maintaining the pH of solution substantially constant during the co-precipitation process.
• the precursor components may be those which form copper, bismuth and thallium based HTSC compounds.
• aqueous solutions are used. It is found that the pH of the aqueous solution does not significantly vary (ie. variations of greater than 0.1) upon initial dilution of the solution. Therefore, while not wishing to be bound by theory, there appears to be a buffer effect present.
• the co-precipitation process suitably is caused to occur within the buffer range or just outside of the buffer range. That is, it is preferred that the initial aqueous solution is diluted until just before the pH begins to significantly vary and that the co- precipitation process is carried out at this dilution value. We believe that this maximises the buffer effect during the precipitation process and therefore minimises the pH variation during co-precipitation.
• the superconductor precursor components are those which form high temperature superconductors.
• the superconductor precursor components are preferably dissolved in an acidic aqueous solution.
• the solution may be acidified by an organic or inorganic acid and nitric acid is a typical acidifying agent.
• the initial pH may vary depending on the type of superconducting components dissolved therein and various other variants such as volume of water and acidifying agent used but typically, the initial pH range is between 0.1 to 3 and suitably about 0.7 (for the yttrium based family) and about 0.45 (for the bismuth based family).
• the precipitating agent is a dicarboxylic acid or derivatives thereof.
• the derivatives may include salts, esters anhydrides, amides and the like.
• Ammonium oxalate is one suitable derivative.
• a suitable dicarboxylic acid is oxalic acid.
• co-precipitated precursor oxalates used by our technique can be calcined to the precursor oxides at a fraction of the time currently used to form precursor oxides by other processes. While again not wishing to be bound by theory, we believe that the improved times results from the high phase purity we achieve by our technique, the intimate mixing, and possibly the constant particle size obtained.
• the advantage of being able to calcine the co- precipitate precursors at a much quicker rate than hitherto achieved, is that conventional high through volume driers such as drop tube furnaces, fluidized bed furnaces and inclined furnaces can be used to form the precursor oxides in commercial quantities.
• the invention resides in a method for forming shaped articles or products which contain HTSC material.
• HTSC high temperature super conductors
• Calcined 123 powder is placed into suspension by using toluene as a solvent and other organics as dispersants. It is necessary to add dispersants so as to deflocculate or disperse the slurry because agglomeration of the slurry results in the poor quality of formed products. This slurry can be cut into shapes.
• the additives such as dispersants need to be burned out in a very controlled manner from the "green" product. This results in increased porosity and lack of product density and can also result in the formation of cracks, faults and defects.
• thickening agents such as cellulose to aqueous solutions in order to form a slurry.
• a further known technique of forming wire/coils is by extrusion which forces a ceramic paste through an orifice.
• This process can be adapted for HTSC compounds.
• a powder of Calcined 123 compound is mixed with a binder, dispersant, plasticizer and a solvent.
• the formed slurry is extruded through a die forming flexible green wires. At this green or unfired state the wires can be coiled or wrapped around a former.
• the wires are then heat treated to burn out the additives and to sinter the HTSC form.
• Australian Patent 603001 discloses a method of forming super conductive products using casting or screen printing HTSC powders to form bulk shapes or coatings.
• a calcined HTSC powder is used which is already inherently brittle. Therefore, in order to form a stable suspension, additives are necessary such as deflocculants and plasticizers and these must be in relatively large amounts - up to 40% in order to provide a sufficiently plastic/flowing mass.
• optimizing parameters of slip casting process we may remove substantially voids, cracks or faults which can adversely affect the superconducting nature of the article or product.
• Ceramic materials including HTSC compounds are known to be inherently non-plastic. Therefore in order to form shapes such as wires and crucibles, plasticity providing agents such as binders, dispersants, lubricants and plastxcxzers must be added.
• the present invention can produce a precursor mix with no additives, and which is plastic. This plastic mix may be added to HTSC compounds or their precursors, metals and other ceramics thereby providing a mix which may be shaped by conventional techniques.
• the invention resides in a method for shaping HTSC containing materials comprising the steps of - dissolving HTSC precursor components into solution, adding a pecipitant which will cause the precursor components to precipitate, the precipitant being selected to provide plastic properties to the co-precipitated product, subjecting the resultant co-precipitated product to a shaping step, and, removing the precipitant from the shaped product.
• the precipitant is an organic dicarboxylic acid or derivative thereof.
• a suitable organic dicarboxylic acid is oxalic acid and the derivatives may include salts, esters, anhydrides, and amides.
• the resultant co-precipitated product may be treated to increase or decrease its viscosity depending on the particular type of shaping step required. Water can be added or removed to adjust the viscosity and therefore the plasticity of the precipitate.
• plasticity is meant that the precipitate or slurry is of sufficient viscosity to allow it to be moulded, extruded, cast, coated and the like.
• additives may be added to the precipitated product.
• HTSC compounds/precursors may include HTSC compounds/precursors, inert fillers and the like.
• Various additives may also be added to the solution prior to precipitation to allow intimate mixing to occur.
• the precipitant can be removed by a heating step, sufficient to burn off or decompose the precipitant. If the precipitant is an oxalate, the heating step should be sufficient to reduce the oxalate to form the various cation oxides.
• the heating step may be continued or adjusted to result in the shaped product having HTSC properties.
• the initial precipitation technique is similar or identical to that disclosed above with buffering of the pH of the medium during the co- precipitation process.
• Figure 1 is a schematic view of process scheme.
• Figure 2 is a schematic flow sheet of Example 1.
• Figure 3 are pH values of Example 1.
• Figure 4 is an XRD plot of Y-123 powder.
• Figure 5 shows XRD plots of HTSC materials formed at different times from oxides.
• Figure 6 is an XRD plot of HTSC formed directly from oxalates.
• Figure 7 is a susceptibility v temperature for wire prepared by Example 4.
• Figure 8 is an XRD plot of wire prepared by Example 4.
• Figure 9 is a susceptibility v temperature for Y-123 wire prepared by example 5.
• Figure 10 is a J v sintering temperature of Y-123 wire prepared by Example 5.
• Figure 11 is a plot of resistivity v temperature of a Y-
• Figure 12 is a susceptibility v temperature for the samples prepared by Example 7.
• Figure 13a is a transmission electron micrograph of
• Figure 13b is a transmission electron micrograph image formed from calcined precursors.
• Figure 14 is an XRD plot of BSSCO powder of Example 11.
• Figures 15a and 15b are scanning electron micrographs of
• Figure 16 is a susceptibility v temperature for wire made by Example 8.
• Figure 17 is an XRD of Pb doped BSSCO powder of process 2 in Example 10.
• Figure 18a is an XRD plot of Bi-2223 precursor powder of process 1 of Example 10.
• Figure 18b is an SEM micrograph of the powder of process 1 of Example 10.
• Figure 18c is an SEM micrograph of the powder of Example
• Figure 19 is an XRD plot of Y-211 powder made by Example
• Figures 20a and 20b are SEM photos of sample of process
• Figure 20c is a magnetic susceptibility v temperature of sample of process 2, Example 10.
• This example illustrates the co-precipitation process to manufacture pre-cursors and HTSC powders of the compound Y 1 Ba 2 Cu 3 0 7 _ d (123).
• An overall schematic process is illustrated in figure 1. 78.Og of reagent grade yttrium oxide, Y 2 0 3 , and
• the pH shows a perceptible increase.
• the pH of the solution is about 0.9. (The volume of the solution is now about 17 litres).
• the aqueous solution may be buffered and then surpassed by substituting water addition with addition of other solvents in the final stage. These solvents may be those that do not cause immediate precipitation such as acetic acid. However, this addition shows no observable improvement in precipitate properties.
• nitrates and oxalic acid have the same volume; for example, in this case, 17 litre.
• the two containers with their respective solutions are connected to peristaltic pumps which drive the two solutions at identical speeds into a reaction tank.
• This reaction tank also acts as a convenient medium for quality control measurements.
• the mixed solutions rapidly form a blue precipitate which consists of the respective cation-oxalate co-precipitates.
• these co-precipitates are yttrium-oxalate, barium-oxalate and copper oxalate as well as mixed oxalates such as barium and copper and oxalate.
• the mixture may then be transferred to a centrifuge for filtration.
• the precipitated oxalate mix in the reaction tank may be sufficiently dried and used directly for forming.
• a schematic flow sheet of this arrangement is shown in Figure 2.
• the reaction tank is placed on a magnetic stirrer and constantly stirred. About 2000ml of the reacted mix is maintained in the reaction tank by controlling the flow rate out of the second peristaltic pump. Such a volume in the reaction tank increases the homogeneity of the final co-precipitate and also serves as another location for quality control measurements.
• a pH meter is used to constantly read the pH of the reacted mix as well as that of the filtrate. It is observed that both pH values are constant during the course of the reaction.
• pH of the reaction mix gives an indication of the consistency of the co-precipitation process and values are shown in Figure 3 for a number of runs.
• the pH of the slurry is generally observed to be about 1.3, with an average variation of +0.1.
• the slurry at this stage can be used for at least three different applications and may require at least three different viscosity treatments. These are respectively:
• a spray drying process has been used to dry a slurry of carbonate co-precipitated powders which then required long calcination times with intermittent slurrying to obtain Y 1 Ba 2 Cu 3 0 7 _ d (123) powder.
• a spray dried nitrate solution of Y 1 Ba 2 Cu 3 0 7 _ d (123) cations to obtain powders which were then calcined for long hours to obtain Y 1 Ba 2 Cu 3 0 7 . d (123) powder has also been used.
• the coprecipitated oxalate as obtained from the centrifuge of Example 1 is dried in a vacuum oven at 100°C. After about 12 hours, the precipitate is a dry mass which can be easily ground in a mortar and pestle or in a food processor to give a loose fluffy powder. This powder may then be heat treated by two different methods. The end products of both methods are pure Y 1 Ba 2 Cu 3 0 7 _ d (123) powders with identical physical and chemical properties. Method 1
• the powder is transferred to a refractory beaker (eg alumina) and heat treated in a muffle furnace to decompose the oxalates to the oxides of the respective cations.
• a temperature may be selected that is below the first binary eutectic but above the decomposition temperature of each oxalate.
• a temperature of 500C is selected.
• the powder in the alumina beaker is transferred to the muffle furnace and heat treated at a rate of 5C/min to 500C at which it is held for 15 hours. The powder is then cooled down to room temperature at 5C/min.
• Figure 13a shows transmission electron micrograph of the calcined powder and it is clearly seen from the extent of lattice fringe images with individual crystals that the particle size is less than 20 nm and the various particles are intimately mixed.
• Figure 4 shows an XRD of Y-123 powder made from these calcined powders and
• Figure 13b shows a transmission electron micrograph of the YBa 2 Cu 3 0 7 _ d (123) powder. The unit cell dimensions of this micrograph indicate the HTSC superconducting phase is present.
• FIG. 5 shows XRD patterns of oxide powders as obtained above after heat treatment for various times at 950C. It is observed that the oxides have reacted to form the HTSC material in about 5 minutes. This time is considerably reduced when compared with conventional firing periods of several hours.
• the advantage with the above two step firing is that we may conveniently store powders which have been heat treated at 500C. In comparison, the dried oxalate powders tend to absorb water while the 123 HTSC compound slowly degrades under humid conditions.
• the dried oxalate powder as obtained after drying in a vacuum oven may be directly calcined to produce the HTSC powder.
• 80gm of oxalate powder was placed in a shallow ( ⁇ lcm height) alumina boat and fired at 950C for various times in a muffle furnace. Results obtained from this procedure are shown in Figure 6.
• Method 1 or Method 2 may be used to convert the oxalates into the oxides; the high reactivity of the precursor powders to firing and subsequent transformation to HTSC material is due to (i) the proper co-precipitation of oxalate precursors and (ii) the fine, intimately-mixed nature of the co-precipitated powders. Since the reaction time for these co-precipitated oxalate powders is in the order of a few minutes, we are able to use a variety of industrial equipment for calcinations. This equipment includes a tunnel kiln, drop tube furnace, a fluidised bed furnace and a horizontal zone furnace.
• calcined Y 1 Ba 2 Cu 3 0 7 _ d (123) powder is mixed with about 5g of co-precipitated oxalate paste.
• the oxalate paste had a viscosity about that of toothpaste.
• the mix was ground in a mortar and pestle by hand to increase homogeneity and break any possible agglomeration. It was then extruded through a 0.6 mm die using about 400kPa applied pressure.
• the wires thus formed were dried at room temperature for about 2 days. After adequately drying, the wires were placed on an alumina boat and transferred into a tube furnace.
• This example illustrates that a more conventional process of adding binder to the HTSC compound may be used to fabricate shapes such as wires.
• less material which is non-superconducting need be removed from the sintered body and thus, we are able to obtain dense wires with high current carrying capacity.
• Wires have been extruded using the more conventional route in which a binder is added.
• lOg of Y 1 Ba 2 Cu 3 0 7 _ d (123) powder as made by calcination of the co-precipitated YBCO oxalate precursor was mixed with 0.2g of commercial HPMC (binder) and 50ml of water by means of a magnetic stirrer in a glass beaker. In about 40 minutes, sufficient water had evaporated to leave a residue that had viscosity similar to toothpaste. This viscous mass was then extruded and processed as in Example 2 listed above. Magnetic susceptibility and phase purity results obtained from this sintered wire are similar to those for Example 2.
• the co-precipitation process gives an oxalate mixture that is inherently plastic and can stay in suspension without the use of defloculants.
• the oxalate powder used in this process once dried, can be re-suspended in water without the addition of any organic agent. Therefore, it becomes possible to use, for forming, an aqueous mix of oxalates either by themselves or with varying amounts of calcined HTSC powder and other additives such as silver and/or superconducting compounds. It further becomes possible to mix these additives in solution, thereby obtaining intimate mixing at an extremely fine-scale (e.g. sub-micrometer).
• a slurry is prepared by addition of water to oxalate co-precipitate powder in a glass beaker.
• the slurry is continuously mixed by a magnetic stirrer and water is added till the slurry pours evenly.
• a mold was prepared from plaster of paris using a preformed crucible or some other suitable shape.
• the slurry is poured into the mold, and after about 5 seconds, drained out. In this manner, a coating of the slurry mixture (e.g. YBCO precursor oxalates) remained on the mold. After about 2 hours, the coating releases itself from the mould and can be easily extracted and left to dry in open air.
• a coating of the slurry mixture e.g. YBCO precursor oxalates
• oxide powder or HTSC compounds may be added to the slurry.
• the oxide powder is obtained by calcining oxalates at 500C. Both calcined oxide powders and HTSC compounds behave similarly in this type of oxalate slurry, and therefore, provide similar products upon suitable firing.
• the example illustrates the use of a co-precipitated oxalate slurry, made by dissolution of a dried oxalate powder, with an addition of HTSC oxides for the slip-casting of shapes which show superconducting properties and a critical current density >10 2 A.cm -2 .
• oxalate powder 6.0 g is added to 60ml of water in a glass beaker and mechanically stirred by magnetic stirrer for 3 days. After this time, it is observed that the mixture had become quite viscous and poured like honey. To this viscous mixture was added 12.Og of oxide powder. This oxide powder had been prepared by calcining dry oxalate powder to 500C for lOh in a muffle furnace. A homogenous mix is obtained in about 5 minutes, and then the slurry is poured into a pre-formed plaster of paris mould. After about 30 seconds, the slurry is poured out, thereby leaving a coating about 1.5mm thick on the inside of the mould. After about 2hours, the cast mixture of oxalate.
• the "green-body” in the shape of a crucible is dried in air for 3 days. Once dried, it is then fired to sinter the body, thereby forming the superconducting compound (e.g. Y 1 Ba 2 Cu 3 0 7 _ d (123)) in the shape of the crucible.
• This sintered body also maintains sufficient mechanical strength to withstand handling and to contain liquids, such as liquid nitrogen.
• a suitable firing scheme for these slip-cast bodies may involve firing the "green-body" crucible at 0.5C/minute to 500C, where it is held for 2h. This first firing allows the water to escape very slowly, thereby limiting the formation and propagation of cracks. After the hold at 500C, the furnace is heated at 5C/minute to 960C and held for 2h, after which the furnace is cooled to 475 C and held for 5h. The hold at 475C is essential for converting a maximum proportion of the tetragonal phase (YBCO) to the superconducting orthorhombic phase.
• the complete firing scheme may be undertaken in a flowing oxygen atmosphere, or, as also demonstrated, in air.
• FIG. 12 An electron micrograph illustrates the fine grained morphology of the surface of slip cast bodies and the high bulk density (>90%).
• An optimum selection of the firing scheme may be used to texture (i.e. grain orientation) or to improve individual grain morphology.
• Figure 12 shows magnetic susceptibility measurements on pieces of slip cast crucible fired at 960C (curve "a"), 930C (curve “b” ) and 910C (curve "c”) for 10 hours in flowing 0 2 . It is observed that there is a superconducting transition at 91K as is expected for Y 1 Ba 2 Cu 3 0 7 . d (123) material.
• This example shows the use of a slurry to add Y 2 BaCu0 5 (211), which is non-superconducting, in a defined fashion - in terms of size, amount and shape - to use as part of flux pinning experiments in which the objective is to surround small grains of 211 phase with 123 material.
• calcined Y 1 Ba 2 Cu 3 0 7 _ d (123) powder is mixed with about 5g of co-precipitated Y 2 BaCu0 5 (211), oxalate paste.
• the oxalate paste has a viscosity about that of toothpaste.
• the mix was ground in a mortar and pestle, or by a mechanised mixer under mild vacuum. These latter mixing methods allow controlled mixing conditions as well as de-airing of the oxalate+HTSC mixture. This mixture was then extruded through a 1.0 mm die using about 400kPa applied pressure.
• the wires thus formed were dried at room temperature for about 2 days. After adequate drying, the wires were placed on an alumina boat and transferred into a tube furnace.
• This example illustrates the application of the process of co-precipitation to the manufacture of the compound Y 2 Ba 1 Cu 1 0 5 . It further illustrates that our process may be used to successfully manufacture copper oxide-based compounds other than the 123 stoichiometry. Whilst the compound Y 2 Ba 1 Cu 1 0 5 is not known to be superconducting, it is used for a variety of HTSC-related applications, including those for doping and as a substrate. The chemicals are taken in the molar stoichiometry Y:Ba:Cu :: 2:1:1
• yttrium oxide 45.16g yttrium oxide is added to a 1 litre capacity flask. To the flask are also added 100ml concentrated nitric acid and the volume is made to 1 litre by addition of de-ionized water. The mixture is mechanically stirred till it becomes clear. A typical pH value of the clear solution thus obtained is 0.15.
• a typical pH value of this clear solution is 1.0.
• the two nitrate solutions when mixed will have a typical pH value of 0.7. If further water is added to a maximum of 500ml, the buffering effect may be seen).
• oxalic acid To a 2 litre container is added 126.07g of oxalic acid and the volume is made to 2 litre by addition of de-ionized water. The mixture is mechanically mixed till it becomes clear. A typical pH of this solution is 3.5. To the oxalic acid solution is added 150ml of ammonium hydroxide. The pH is now typically at 3.9. Alternatively a mixture of ammonium oxalate and oxalic acid may be used to achieve the same pH.
• the nitrate solution is mixed with the oxalic acid solution, thereby obtaining co-precipitation of yttrium, barium, copper oxalates and mixed cation oxalates.
• a typical pH of the reacted mix is 1.3.
• the slurry thus obtained is dried in a vacuum oven at the boiling point of water.
• the dry mass is extracted in about 12 h and ground by a food processor.
• the loose powder may then be further ground by a mortar and pestle.
• the ground oxalate powder is transferred to a refractory container and charged into a muffle furnace.
• the furnace is heated at 5C/minute to 500C and held for lOh.
• the furnace is then cooled to room temperature.
• the powder, now black is hand ground in mortar and pestle, and charged into the furnace as before.
• the furnace is heated to 980C and held for 12 h. It is then cooled at 5C/minute to room temperature.
• the green powder obtained is characterized by x-ray diffraction and electron microscopy to be pure Y j Ba j CU j O g , as shown in Figure 19.
• EXAMPLE 10 This example illustrates the process of co-precipitation as applied to the manufacture of bismuth- and lead-based HTSC precursor and compounds of the formula (BiPb) 2 Ca 2 Sr 2 Cu 3 O 10 . The chemicals are weighed in the molar stoichiometry (Bi:Pb):Sr:Ca:Cu :: (1.65:0.35):2:2:3.
• the solution of ammonium oxalate and the solution of nitrates are then mixed together using the peristaltic pumps and filtered using the centrifuge as described in Example 1.
• a typical pH of the precipitated mixture is 1.6, with an average variation of less than +0.2.
• the precipitate is dried in a vacuum drying oven at the boiling point of water and in about 12 hours the precipitate is dry. It is then ground by a food processor followed by mortar and pestle to obtain a fine light blue coloured powder.
• the powder is then calcined to convert the oxalates to a fine mixture of oxides. This may be done in a muffle furnace where the powder, in a refractory container, is heated at 5C/minute to 500C for lOh, and cooled to room temperature at 5C/minute.
• High resolution transmission electron micrographs of this calcined powder show that the powder consists of intimately mixed oxide grains of size less than 20nm. X-ray microanalysis shows that these grains are typically oxides of copper, bismuth, calcium, strontium or lead, depending on the copper-oxide family under consideration.
• the powder obtained above is then converted to HTSC precursors or HTSC materials as described in Process 1 and Process 2.
• Process 1 This process of heat treatment gives a precursor which is ready to be converted into the three layer HTSC superconductor (Bi,Pb) 2 Ca 2 Sr 2 Cu 3 0 10 .
• the powder obtained after calcining at 500C is transferred into a muffle furnace.
• the furnace is then heat treated at 5C/minute to 800C and held at this temperature for 2h. It is then cooled to room temperature at 5C/minute.
• the powder is ground up by hand and then placed into the muffle furnace.
• the muffle furnace is heat treated at 5C/minute to 840C and held for lOh. The furnace is then allowed to cool down to room temperature and the powder taken out.
• Process 2 This process converts the precursor obtained in Process 1 to the superconductor phase (BiPb) 2 Ca 2 Sr 2 Cu 3 O 10 .
• the precursor powder obtained in Process 1 is pressed into a disk, for example, less than 2cm diameter and about 0.5cm thick.
• the disk is placed on a refractory setter such as MgO and transferred into the muffle furnace.
• the muffle is heated at 5C/minute to 860C and held for 15h.
• the sample is then cooled to room temperature at 5C/minute.
• Figures 17, 20A-C illustrates, by XRD, SEM and magnetic susceptibility data, the development of the three layer HTSC compound (Bi,Pb) 2 Ca 2 Sr 2 Cu 3 0 10 via this process. Note that a high phase purity (about 99.9%) is obtained by this process and the superconducting transition has an onset temperature of 108K.
• EXAMPLE 11 EXAMPLE 11:
• This example illustrates the process of co-precipitation as applied to the 1 manufacture of bismuth- and lead-based HTSC precursor and compounds of the formula Bi 2 Sr 2 Ca 1 Cu 2 O ⁇ (or 2212).
• Reagent grade chemicals are weighed according to the molar stoichiometry : Bi:Sr:Ca:Cu : : 2:2:1:2.
• lOOg bismuth nitrate is added to a 1 litre flask. To this flask is added 50ml nitric acid and the mixture shaken to dissolve most of the bismuth nitrate.
• De-ionized water is added to make the volume to 1 litre and the mixture is mechanically stirred till it becomes clear.
• a typical pH value of the clear solution is about 0.15.
• the two nitrate solutions are mixed and mechanically stirred for 10 minutes.
• a typical pH of the clear solution is 0.4.
• 142.llg ammonium oxalate is added to a 4 litre container and the volume made up to 4 litres by adding de-ionized water. The mixture is mechanically stirred till it becomes clear.
• a typical pH of the clear solution is 6.7.
• the nitrate solution and the oxalate solutions are mixed to form the co-precipitate and the slurry collected in the centrifuge.
• a typical pH of the reactant mix is 1.2, with an average variation of +_ 0.1.
• the precipitated oxalates thus obtained are dried for 12h in a vacuum oven at the boiling point of water.
• the dried mass is ground in a food processor followed by fine grinding by hand in a mortar and pestle.
• the blue ground powder is then transferred to a refractory container and charged into a muffle furnace for heat treatment.
• the furnace is heated to 500C at 5C/minute and held for lOh. It is then cooled to room temperature at 5C/minute to room temperature.
• the powder, now black, is ground by mortar and pestle and re-charged as before into the furnace.
• the furnace is heated to 800C at 5C/minute and held for 2h. It is then cooled to room temperature at 5C/minute.
• the powder is removed, re-ground and then re-charged into the furnace.
• the furnace is heated to 840C at 5C/minute and held for lOh.
• the sample is then cooled to room temperature at 5C/minute.
• the powder is removed, re-ground, and recharged into the furnace as before.
• the furnace is heated at 5C/minute to 840C and held as before for lOh. It is then cooled to room temperature at 5C/minute.
• X-ray diffraction data in Figure 14 illustrate that the powder consists of the HTSC phase At such stage of the production of the 2212 phase, various modifications to the average particle size distribution can be made by appropriate modifications to the heat treatment.
• EXAMPLE 12 Due to the inherent plasticity present in our co-precipitated powder it becomes possible to add cationic mixtures (or compounds) containing barium, yttrium, copper, or silver (as may be required for the YBCO system) or strontium, bisumth, calcium, lead or silver (as may be required for the BSSCO system). The addition may be made to dried co-precipitated powder which may contain one, few or all of the cations required in the desired final compound. The process may be facilitated through water medium to permit a simple drying operation.
• the already dried co-precipitates are ground up and transferred to water where they are stirred by a mixer. This mixing makes a finely dispersed suspension.
• a separate container is prepared a solution of the new or additional cation. This solution is then mixed to the co-precipitate solution, which is then stirred and evaporated to dryness. Because the co-precipitated powders have the ability to form a fine suspension with water, it allows remixing of dried powders with fresh powders/precipitates/solution.
• the starting co-precipitate is made by the process described earlier in the text and consists of yttrium, barium, and copper oxalates. It is determined by I.C.P. (Inductively coupled plasma atomic emission spectroscopy) results that the cationic ratio of Y:Ba:Cu is 1:1.5:3.0. (Such a composition finds application in bulk HTSC shapes as it yields a fraction of the insulating compound Y-211 which is expected to act as flux pinning centres). To 800 g of the dried co- precipitate powder is added 500 ml of water and the mixture is stirred vigorously. In another container 79.2g of barium nitrate is weighed out and dissolved in 1 litre of water.
• the barium nitrate solution is added to the co-precipitate mix and stirred vigorously for 30 minutes in a blender.
• the blended mix is transferred to a hot plate where it is continuously stirred till almost dry. _.
• At this stage it is transferred to a vacuum oven where the mixture is allowed to dry at the boiling point of water.
• Transmission electron microscopy on the dried powder indicated that the powder consists of well mixed cations on a nanometer level similar to powders obtained earlier.
• TEM on this powder after calcinating at 500C (to eliminate oxalate anion) shows that the cations are well mixed on a nanometer level.
• the new mix prepared may be treated in a manner similar to that described earlier in the text for co- precipitated powders. Thus for example, we may fire it in the same manner to form superconducting Y-123. XRD and I.C.P. results confirm that the cation stoichiometry of the resulting powder is 1:2:3. EXAMPLE 13:
• sputtering targets were fabricated. Calcined powders were ground and pressed into discs and then sintered at between 920C and 980C where the superconducting phase is stable. Since dense targets are required for thin film fabrication via a well-known sputtering process, the density of those targets were measured. It was found that a 500C calcined powder (mixture of oxides, no YBa 2 Cu 3 0 7 _ d phase) gave 40-60% higher density than high temperature (>800 ⁇ C, mainly YBa 2 Cu 3 0 7 _ d ) calcined powders after sintering the final products. This difference in density can be attributed to the intermediate phase/s which assist the densification during transformation from 500C calcined oxide phases to a superconducting phase.

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