A METHOD FOR PRODUCING NANO-ENGINEERED PRECURSORS

Abstract: A method for producing nano-engineered precursors, in particular superconductor precursors is disclosed, said method comprising the steps of: dissolving at least two metal compounds, precipitating agents and optionally pH-adjusting agents to form feed solutions (60) of known concentrations; mixing the feed solutions (60) by adding them, preferably into a reactor (70); co-precipitating a solid from the mixed solutions (60), preferably in the reactor (70) under controlled operating conditions; thickening the co-precipitated solid; separating the precipitated solid from the mother liquor, for instance by filtration, which mother liquor preferably is recycled to the reactor (70), and; optionally washing the solid during filtration to remove impurities; wherein said at least two metal compounds are dissolved to form separate solutions (40, 50) before being mixed together. The co-precipitated solid is then filtered, washed, dried and calcined to obtain a desired super conducting oxide.

Description: A method for producing nano-engineered precursors Field of the invention The present invention relates to a method for producing nano-engineered precursors, and more particularly precursors of superconductors. Background art Methods for producing precursors of the aforesaid kind are well known within this field. The two main techniques to obtain to precursors such as High Temperature Superconductors (HTSC) are grinding/mixing the respective HTSC components together or co-precipitating the respective components in liquid solution. One well known problem with grinding/mixing technique is that it is a lengthy process, requiring several grinding and sorting stages to be able to obtain a mixture which will be suitable for the production of the HTSC. However, it is difficult to obtain phase purity and reproducibility, especially in large scale. Phase purity is critical as regards the efficiency of the HTSC:s, since even slight deviations from the correct ratio of the different HTSC components will result in degradation or loss of HTSC properties. Co-precipitation in solution is a technique for preparation of precursors, so that they are very well mixed. In solution precipitation, the different components, such as metal salts, are initially dissolved in aqueous solution, thereby forming ions. The dissolved components are precipitated from the solution by the addition of precipi- taring agents, such as carbonates, formates, hydroxides and oxalates. Thereafter, the resultant precipitate is filtered and thereafter calcined to remove or decompose the anion to obtain a mixture of the respective cation oxide. Further heating at high temperature, above 900 °C result in transformation of the cation oxides to a high temperature super conducting material (HTSC). The co-precipitation technique of HTSC precursors is generally based on the solid formation reactions between desired metals ions (M 3+ and M 2+ ) and a specified precipitant ligand (L n- ) as follows, nM + +3L n " → M n L (s) (Eq. la) and M 2+ + 2/n L n " → ML 2 / n (s) (Eq lb) where M + =Y + , Bi 3+ , etc; M 2+ = Ba 2+ , Sr 2+ , Ca 2+ , Cu 2+ , Pb 2+ , etc.; n = 1 or 2, L- = OH", I = C0 3 2 ", C 2 0 4 2 -, etc. The conventional co-precipitation based method generally comprises the following steps: -Dissolving the metal salts in water and/or acid to obtain the respective aqueous solutions of single-metal ions. However, it is also possible to obtain a solution mixture containing all desired metal ions at a defined molar ratio; -Dissolving a precipitating agent in water or acid; -Precipitating the desired metal ions, according to Eq. la and lb, by mixing the solutions of the precipitating agent and the metal ions, preferably under stirring. The solution mixing can also be obtained by adding the solution(s) of metal ions into the precipitant solution or vice versa. In order to enhance the precipitation, ad- dition of the precipitant solution in excess is always required. The pH of the mixed solution is adjusted, for instance by adding an alkaline solution, to the optimal level for co-precipitation during, or after, the third step. Subsequently, the co-precipitated solid is then filtered, washed, dried and calcined to obtain the desired super conducting oxide. Compared to the traditional reaction method based on intensively mechanical grinding/mixing, a co-precipitation method described as above can provide improved precursors. With a more uniform and smaller particle size (< 1 urn), such a co-precipitated powder is not only better in mixing the different metal components, but also more reactive to form the superconductive oxide. However, there are several serious drawbacks when performing the above method, since co-precipitation of multiple-metal components is generally a complicated chemical process. The precipitation rates, i. e. the kinetics, as well as the thermodynamics, of individual components are significantly different from each other and depending on many parameters. In addition to the two mostly recognised parame- ters, type of the precipitant and respective pH range of the reaction media, the following parameters are also of importance: initial concentrations of the reactants, solution addition/mixing sequence, addition rates, ratio of the reactants, and residence time of the precipitating suspension. As the control and optimisation of these parameters have not been well established, reproducibility of the conventional co- precipitation routines has been always questionable, especially when the quality of chemical composition of the co-precipitated solid is of great importance. In an oxalate co-precipitation method for precursors of Y-Ba-Cu-0 superconductors, for instance, the conventional method of adding excess oxalate precipitant re- suits in increasing the solubility of copper oxalate though enhancing the precipitation of others. Therefore, a certain amount of excess addition of copper ions is always required. To maintain a constant ratio of dissolved to solid copper species, such a process becomes very sensitive regarding the operating conditions as mentioned above, and a quality control of the overall chemical composition of the co- precipitated powder is always a problem. On the other hand, the conventional solu- tion addition/mixing process, as described above in the third step, could hardly accomplish a simultaneous co-precipitation of all the desired metal components but "precipitation-in-sequence" of the individual components. In case of a Y-Ba-Cu-0 precursor, for instance, the precipitation sequence of the metal ions is Y> Ba »Cu, if the solution of metal ions is added into the precipitant solution. If the precipitant solution is added into the solution of metal ions, the precipitation sequence is Y>Cu» Ba. In either way, the precipitate could hardly be a uniform mixture of various oxalates formed in the respective particles of individual components. Large scale production implies more serious drawbacks, which could even overcome the advantages of the co-precipitation technique. To obtain HTSC precursors having high efficiency and high transition temperature, it is of great importance to provide that the components are present in the correct ratio. The ratio must be extremely carefully controlled to obtain the high phase pu- rity as required. Thus, it is generally desirable from several aspects to be able to control the ratio of the components. Unfortunately, however, this is not taught by any known technique. There is thus a need of a solution which will allow the control of the components. Summary of the invention The aforesaid problems are avoided essentially completely by the present invention. Thus, an object of the present invention is to provide a new co-precipitation based method that will overcome the drawbacks of the conventional methods, while enhancing their advantages as mentioned above. In other words, the object of the invention is to provide a method that can easily control the quality in overall chemical composition of the co-precipitated solid, as well as the uniformity in compositional distribution among the individual particles, while reducing the particles size down to the nanometer scale < 100 nm, for producing nano-particle size precursors. An inventive method is primarily characterised in that the metal components, i. e. the metal compounds, are dissolved to form separate solutions before being mixed together, or a solution of known concentrations before being mixed together Furthermore, another object of the invention is to improve the quality of powder products to obtain high reactivity, particularly for meta-stable co-precipitated powders for HTSC precursors. The process claimed in this invention can be used to produce high quality precur- sors of rare earth oxide and/or alkaline earth oxide and/or copper oxide based ceramics in general, and oxide superconductors in particular. Moreover, the method involves the knowledge that the dissolution of the metal compounds to form a feed solution of all the desired metal components at a fixed ratio of known concentrations, or two separate feed solutions: one containing a metal component(s) that can only be soluble at relatively high acidity, and another containing the other metal components at a fixed ratio of known concentrations, result in a co-precipitate solid having a high and consistent purity and a narrow variation in particle size. According to one embodiment of the invention, a method for producing nano- particle size precursors, in particular superconductor precursors, comprises the steps of: -dissolving at least two metal compounds, precipitating agents and optionally pH- adjusting agents to form feed solutions of known concentrations; -mixing the feed solutions by adding them, preferably into a reactor; -co-precipitating a solid from the mixed solutions, preferably in the reactor under controlled operating conditions; -thickening the co-precipitated solid; -separating the precipitated solid from the mother liquor, for instance by filtration, whereby the mother liquor preferably is recycled to the mixer/reactor, and; -optionally washing the solid during filtration to remove impurities; wherein said at least two metal compounds are dissolved to form separate solutions before being mixed simultaneously together, in the mixer/reactor (70). According to a preferred embodiment of the invention, the feed solutions are added to a buffer solution under controlled conditions, e. g. pH. The buffer solution is preferably added to the reactor and re-circulated by means of a buffer tank and a corresponding pump. Suitably, the HTSC components are those which form high temperature superconductors. Such compounds are described for instance in WO- A 1-94/00385. The metal compounds are preferably selected from those which form high temperature superconductors, including for example compounds of Y, Bi, Ba, Sr, Ca, Cu, Pb. Compounds of other metals having similar or suitable properties are also included in the scope of the invention. According to another preferred embodiment of the invention, the separated solid is heated after the filtration to remove water and carbon contents and form a precursor of oxides, preferably being superconducting. Suitably, the precipitating agent is selected from a carboxylic acid, or its salt, an oxalic acid, an oxalate salt, or a carbonate salt. Preferably, the pH-adjusting agents are a solution of nitric acid to decrease the pH, and a solution of an alkali hydroxide, such as sodium hydroxide to increase the pH. These and other objectives, advantaged and features of features of the present invention will be better understood and appreciated by reference to the written specification. Brief description of the drawings The present invention will now be described in more detail with reference to the accompanying drawings, in which: Fig. 1 is a schematic process scheme of the method according to the invention. Fig. 2 is a more detailed process scheme of the method according to a preferred embodiment of the invention. Detailed description of preferred embodiments Fig. 1 shows a general process scheme. The overall operation can be generalised as follows: The precipitating agent and pH-adjusting agents (acid and base) are separately dissolved in water in stirred tanks forming separate solutions 10, 20, 30, while the metal compounds are dissolved in water or acid in other tanks, forming solutions 40, 50. The prepared solutions 60, referred to as "feed solutions" are sampled for analysis of the accurate concentrations of the respective chemicals. The feed solutions 60 of metal components can also be combined in one tank (not illustrated). Then, a defined molar ratio of the metal ions should be fixed if the feed solution contains more than one metal ion. Moreover, some metal compounds may have to be dissolved in an acid solution, instead of water. The feed solutions 60 of the reactants, i. e. the precipitating agent 20 and metal ion 40, 50 solutions, are pumped at a defined flow rate into a buffer solution 85 in a batch reactor 70 under stirring. A recycled flow from a buffer tank 80 is also simul- taneously pumped into the reactor 70. The pH of the buffer solution in the reactor 70 is conveniently monitored via a combined glass electrode or the like to assure it is within a defined range. The pH of the buffer solution is adjusted by adding a certain amount of the base 30 (or acid 20) solution into the reactor 70. The overflow of the buffer solution in the reactor 70 is conducted to the buffer tank 80 and recycled. After defined quantities of the reactants have been added into the reactor 70, the feeding pumps 90 are stopped. The precipitated solid is sampled for analysis in a given time interval until its composition does not change. The stirring, as well as the pumping is then stopped to allow the suspension to sedimentate. After a certain period, the suspension is thickened at the bottom of the reactor 70, and the clear solution is drained off through a conduit to the buffer tank 80. The stirring is restarted and the bottom valve 100 is turned on to let the suspension flow to a thickener 110. The suspension from the reactor 70 is mixed with the filtrate solution in the thickener 110 under stirring for a certain period of time. The stirring and pumping are then stopped to allow the suspension to sedimentate. After a certain settling period, the suspension is thickened at the bottom of the thickener 110 and the clear upper solution is drained off to the buffer tank 80 via a valve. The stirring is restarted and the bottom valve 111 is turned on to let the suspension flow to a filter 120. During the filtration, the solid product 130 is separated from its mother liquor, washed with water and then dried. The filtrate solutions comprising the mother liquor and wash- ing water are returned to the thickener 110. The controlled operating conditions for mixing and co-precipitation include the overall addition ratios between the feed solutions, the respective flow rates of the feed solutions, the stirring rates and the pH ranges of the reaction mixture, and its residence period in both the reactor and thickener. The overall addition ratios between the feed solutions are predetermined according to the chemical equilibrium calculation for said co-precipitation. The actual overall addition ratios between the metal components and precipitating agent are continuously monitored and kept constant within 0.1% derivation by controlling the respective flow rates of the feed so- lutions during operation. Preferably, the operational conditions of the respective flow rates of the feed solutions are designed according to the predetermination of the overall addition ratios, as well as the optimal residence period in the reactor. The operational condition of the stirring rates of the reaction mixture in the reactor is designed according to the predetermination for its influence on the solution mixing and particle growth/dispersion. According to another embodiment of the invention, the actual stirring rate is con- tinuously monitored and kept constant within 5% variation during operation. The actual pH values of the reaction mixture in both the reactor and thickener, may be continuously monitored and controlled within a relatively insensitive range of two pH units (pH=3-5, or 10-12) by occasionally modifying the flow rate of one or more pH-adjusting agent solution(s) during operation. The metastable product (powder) as obtained is specified by its well-defined composition, high uniformity in compositional distribution among the individual grains on nanometer scale, its low crystalinity or amorphous phase, and its high uniformity in phase distribution. The metastable product (powder) is specified by its ultrafine particle size, or nanometer grain size, and its high uniformity in size distribution. The product, i. e. the nano-engineered precursor is specified by its nano-sized grains with high reactivity for further processing toward high performance oxide superconductors. The invention will now be described by means of examples. Example 1 : This example illustrates the production of oxalate precursors if BiPbSrCaCuO superconductors; BiPbSrCaCuO actually represents a homological series of superconducting phases of (BIl-xPbx)2Sr2Can-lCun02n+4+5with x =0-0.3 and n =1,2,3,...Among them, a stable high-Tc (1 10 K) phase like Bil.6Pb0.4Sr2Ca2Cu3O10+8 is mostly preferred. For simplicity, BiPbSrCaCuO is only referred to as Bil.6Pb0.4Sr2Ca2Cu3O10 in the following. As shown in Fig. 2, the co-precipitation procedure compromises three major steps: 1. Dissolution — preparation of the feed solutions The precipitating agent solution 10 is prepared by dissolving oxalic acid or sodium oxalate in water at a concentration of 0.1-0.5 M. The accurate concentration should be analysed. The acid solution 20 is approximately 1 M nitric acid in water. The basic solution 30 is 1-5 M sodium hydroxide in water. The solutions of metal components of Bi,Pb,Sr,Ca and Cu are prepared, either by dissolving the respective oxides, hydroxides or carbonates in 4 M nitric acid, or by dissolving the respective nitrates in water or acid. Excess nitric acid of 1-3 M is required in the solution of 0.1-1 M Bi to avoid the precipitation. The concentrations of other metal components in the respective solutions are 0.5-2 M. The accurate concentration should be analysed; The solutions of Pb, Sr, Ca, and Cu are mixed and stored at a molar ratio of Pb:Sr:Ca:Cu = 0.4:2x:2.0;3.0 (x=l.1-2.0) in one tank, while the solution 50 of Bi is in another tank. 2) Co-precipitation of the oxalate solid. The reactor 70 is half-filled with a recycled solution 85 pumped from a buffer tank 80. The pH of the solution 85 should be between 4-5; The feed solutions 60 of 20, 30, 10, 40 and 50, are simultaneously and continuously pumped at the respective flow rates into the reactor 70 under constant stirring. The flow rates of the feed solutions 60 are continuously monitored and controlled at such a constant ratio that the molar ratio of the added reactants in the reactor 70 should always be maintained at C 2 0 4 :Cu:Bi = 9.8:3.0: 1.6; The pH of the solution mixture 75 is continuously momtored and controlled controlled within a range of 3-5 by adjusting the flow rate of 30. After the addition of the reactants 60 is complete , the stirring of the suspension should continue until the bulk composition of the precipitated oxalate precursor is independent on time. The final composition of the precursor should be Bi:Pb:Sr:Ca:Cu=* 1.6:0.4:2:2:3 with a derivation <2%. 3) Separation of the oxalate precursor from the process flows The residence time of the suspension in the thickener 110 is about one hour. The operational condition to filtration are mostly dependent upon the filter 120 to be used. The washing operation should lower the impurity to <0.1% in the oxalate product. The process claimed has the following technological features: On-line operation/analysis facilities , sampling and adjusting the process operating conditions; Computerised data acquisition & control facilities for process control, optimisation and automation; Operational variability for both batch and continuous processing regardless the scale; Product variability for synthesising various types and compositions of the precursors; Quality control availability for tailoring the metastable powder precursors with well-defined composition, low crystalinity, ultrafine particle size and high uniformity in their distributions. The nano-engineered precursors are synthesised via a controlled co-precipitation procedure that produces the metastable solids of multiple metal components of well- defined composition, such as low crystalinity, ultrafine particle size, and high uniformity in their distributions within the nanometer scale (< 100 n ). The actual residence period is controlled by adjusting the flow rate of the recycled solution from the buffer tank during operation. The operational criteria of the average residence period of the reaction mixture in the reactor can be designed according to the predetermination for completion of more than 99% of co-precipitation equilibrium. The actual residence period is regularly verified by on-line analysis of the soluble metal component that is known to have the slowest precipitation rate, and adjusted accordingly during operation. The invention shall therefore not be considered limited to the aforedescribed exemplifying embodiments thereof, since other embodiments are conceivable within the scope of the following claims.

Claims: Claims 1. A method for producing nano-engineered precursors, in particular superconductor precursors, said method comprising the steps of: -dissolving at least two metal compounds, precipitating agents and optionally pH- adjusting agents to form feed solutions (60) of known concentrations; -mixing the feed solutions (60) by adding them, preferably into a reactor (70); -co-precipitating a solid from the mixed solutions, preferably in the reactor (70) un- der controlled operating conditions; -thickening the co-precipitated solid, preferably in a thickener (110); -separating the precipitated solid from the mother liquor, for instance by filtration, whereby the mother liquor preferably is recycled to the mixer/reactor (70), and; -optionally washing the solid during filtration to remove impurities; characterised in that said at least two metal compounds are dissolved to form separate solutions (40, 50) before being mixed simultaneously together in the mixer/reactor (70). 2. A method according to claim 1, wherein said feed solutions (60) are added to a buffer solution under controlled conditions. 3. A method according to claim 1 or 2, wherein the separated solid is heated to remove water and carbon contents and form a precursor of oxides, preferably super conducting. 4. A method according to any one of the claims 1-3, wherein the metal compounds are selected from those which form high temperature superconductors, such as compounds of Y, Bi, Ba, Sr, Ca, Cu or Pb. 5. A method according to any one of the claims 1-4, wherein the precipitating agent is selected from a carboxylic acid, or its salt, an oxalic acid, an oxalate salt, or a carbonate salt. 6. A method according to any one of the preceding claims, wherein the pH- adjusting agents are a solution (20) of nitric acid to decrease the pH, and a solution (30) of an alkali hydroxide, such as sodium hydroxide to increase the pH. 7. A method according to any one of the preceding claims, wherein the controlled operating conditions for mixing and co-precipitation include the overall addition ratios between the feed solutions (60), the respective flow rates of the feed solutions (60), the stirring rates and the pH ranges of the reaction mixture, and its residence periods in the reactor (70) and thickener (110). 8. A method according to claim 7, wherein the overall addition ratios between the feed solutions (60) are predetermined according to the chemical equilibrium calculation for said co-precipitation. 9. A method according to any one of the preceding claims, wherein the actual over- all addition ratios between the metal compounds (40, 50) and precipitating agent are continuously monitored and kept constant within 0.1% derivation by controlling the respective flow rates of the feed solutions (60) during operation. 10. A method according to any one of the preceding claims, wherein the operational conditions of the respective flow rates of the feed solutions (60) are designed according to the predetermination for the overall addition ratios, as well as the optimal residence period in the reactor (70). 11. A method according to any one of the preceding claims, wherein the operational condition of the stirring rate of the reaction mixture in the reactor (70) is de- signed according to the predetermination for its influence on the solution mixing and particle growth/dispersion. 12. A method according to claim 11, wherein the actual stirring rate is continuously monitored and kept constant within 5% variation during operation. 13. A method according to any one of the preceding claims, wherein the actual pH values of the reaction mixture in the reactor (70) and thickener (110) are continuously momtored and controlled within a range of two pH units (pH=3-5, or 10- I 2 ) by occasionally modifying the flow rate of one or more-pH-adjusting agent solution (20, 30) during operation. 14. A solid produced by the method according to any one of the claims 1-13. 15. A precursor produced by the method according to claim 3, wherein the solid is heated to a temperature sufficient to convert the co-precipitated solid to a HTSC.

Inventors: MUHAMMED MAMOUN, WANG LINGNA, ZHANG YU, MUHAMMED, MAMOUN, WANG, LINGNA, ZHANG, YU
Applicants: MAMOUN MUHAMMED, WANG LINGNA, ZHANG YU, MUHAMMED, MAMOUN, WANG, LINGNA, ZHANG, YU
Application year: 2000
Main classification: C04B35
IPC codes: C01B13, C01G21, C01G29