Purification apparatus and method for superconductor fine particles

Abstract: A purification apparatus for superconductor fine particles is provided which comprises a means for forming a flow of powder containing the superconductor fine particles, and a means for applying a magnetic field to the flow of the powder.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to an apparatus and a method for classifying and purifying fine particles to obtain only the desired superconductor particles from among mixtures of fine particles of different particle diameters comprised of superconductors, normal conductors, insulators or the like having different critical temperatures, critical magnetic fields, etc. Related Background Art Recent years, it has been discovered that the sintering of ceramic materials with certain definite composition can give a sinter that exhibits superconductivity (or superconducting) at 77 K or more, or, in some instances, near room temperature. However, the crystal structure and the phase of these superconductors have not been sufficiently elucidated yet, and usually there coexist non-superconducting crystal phases. In the instance where the non-superconducting crystal phases coexist, it is very difficult to separate them from the superconducting crystal phases. Moreover, no technique has been established that will form only the superconducting crystalline material by controlling heat-treatment conditions. In recently available ceramic superconductors, there also often coexist a plurality of superconducting crystal phases different in the critical temperature or critical magnetic field, and no method has been established to separate only the superconducting crystal phases having any desired critical temperature range and critical magnetic field range from among them. In addition, the sinter having superconductivity usually comprises a mass or aggregate of fine crystals, and its superconductivity characteristics depend greatly on the state of crystal grain boundaries so that its crystal grain boundaries must be made uniform to obtain a sinter having stable characteristics. A proposal has ever been made to obtain a superconducting sinter having uniform crystal grain boundaries by re-sintering superconductor fine particles having uniform particle diameter. However, any suitable method for classifying such superconductor fine particles having uniform particle diameters has not been discovered, and nothing has been available except for the method in which the generally practiced particle classification methods as described in Funtai Kogaku Handobukku (Particle Technology Handbook) (edited by Koichi Iitani, Asakura Publishing Co.) are applied in the superconductor fine particles. Known as the conventional generally practiced particle classification methods are a screening method in which sieves having different openings are piled up in the order from those having larger opening diameter to carry out classification, a sedimentation method in which the terminal settling velocity of the particles settling in a fluid is utilized to carry out the classification, and the similar methods. For example, however, in the screening method, it is impossible to prepare those having a screen opening of several micrometers or less, thus enabling no classification for particles of very small diameter. Moreover, it is often practiced to apply pressure loading to the fine particles to force them to pass through the screen openings, and in such an instance the problems occur such that the classification cannot be carried out in vacuo as a means for classification with higher precision. Also, in the sedimentation method, where the settling velocity depends not only on the diameter of particles but also the specific gravity thereof, no strict classification can be carried out. In instances where a liquid phase sedimentation method is used, it requires much labor to separate fine particles from liquid, and also the settling velocity is so low in general as to take much time for the classification. This method also involves the problems such that it cannot naturally be carried out in vacuo. J. Phys. E. Sci. Instrum., Vol. 20, 6 October 1987, pages 1292-1293; S. Vieira et al: "A simple device for quick separation of high-Tc superconducting materials", discloses an experimental apparatus for separating superconducting particles from non-superconducting ones. The apparatus disclosed comprises a permanent magnet with two pole pieces which produce a high magnetic field in a restricted zone within a glass tube. A second tube containing glass trays at different heights surrounds the first tube. This whole assembly is then immersed in a liquid nitrogen bath. Powder containing superconducting particles is then introduced into the first tube. If the powder is superconducting it is levitated by the magnet as a result of the Meissner effect, otherwise it falls into the lowest tray. When the inner tube is removed, the superconducting grains fall sideways into the different upper trays depending on the levitation height. It is therefore possible to separate samples which are superconducting to a lesser or greater degree. WO88/08619 an earlier patent document published after the priority date of the current application, discloses a method and means for classifying and purifying a material having a superconducting component. The equipment comprises means for cooling the material to the range in which superconductivity occurs, means for applying a magnetic field to the sample and means for effecting relative movement between the sample and the magnetic field generated. At a defined temperature the material is subjected to a pulsed magnetic field which causes the superconducting portion to separate from the non-superconducting portion, in accordance with the Meissner effect. It is possible to vary the temperature or the strength of the magnetic field and collected portions can be classified accordingly. SUMMARY OF THE INVENTION An object of the present invention is to provide an apparatus for purifying superconductor fine particles that is capable of separating and purifying only the superconductor fine particles from among powder to be purified containing superconductor fine particles. Another object of the present invention is to provide an apparatus for purifying superconductor fine particles that is capable of classifying and purifying only the superconductor fine particles having desired characteristics from among powder to be purified in which a plurality of superconductor fine particles having differences in the characteristics such as the particle diameter, critical temperature and critical magnetic field coexist. The above objects can be achieved by the invention described below. According to an aspect of the present invention there is provided a purification apparatus for superconductor fine particles in accordance with claim 1. According to another aspect of the present invention there is provided a method for purifying a powder containing superconductor fine particles in accordance with claim 10. The features in common with the teaching of S. Vieira et al appear in the preamble of these claims. Other embodiments of the present invention will become apparent from the following description. In the accompanying drawings: Figs. 1 and 2 are graphs showing respective X-ray diffraction patterns of a superconductor containing impurities and a purified superconductor; Fig. 3 is a graph showing a typical temperature dependency of the electrical resistance of a superconductor; Figs. 4 to 6 are schematic drawings showing purification apparatus of a first embodiment of the present invention, and variants thereof; Fig. 7 is a schematic drawing showing purification apparatus of a second embodiment of the present invention; Fig. 8 shows alternative partition means for use in the apparatus of the preceding figure; Fig. 9 shows alternative means of applying magnetic field for use in the apparatus of the second embodiment; Fig. 10 is a schematic drawing showing a variant of the second embodiment; Figs. 11 and 12 respectively illustrate a block diagram and a time chart of a controlling system in the apparatus of the present invention; Figs. 13 and 14 respectively illustrate a block diagram and a time chart of another controlling system in the apparatus of the present invention; and Figs. 15 and 16 respectively illustrate a block diagram and a time chart of still another controlling system in the apparatus of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention is based on the utilization of the Meissner effect which is attributable to magnetic properties inherent in superconductors. The Meissner effect is meant to be the effect that the superconductor fine particles become perfectly diamagnetic when a magnetic field is applied to the fine particles at the temperature at which the superconductor fine particles exhibit superconductivity. More specifically, at the above-mentioned temperature, application of a magnetic field by means of a magnet to the powder containing superconductor fine particles produces repulsion to the magnet owing to the Meissner effect with respect to those having a particle diameter of about 0.01 µm or more. On the other hand, no repulsion is produced since the Meissner effect is not brought about with respect to those having a particle diameter less than that and the fine particles of normal conductors or insulators. According to this principle it is possible to separate and purify with a high precision only the superconductor fine particles from among the powder mixed with normal conductors or insulators. For example, a flow of powder to be purified, mixed with normal conductors, insulators, etc. is formed, and a magnetic field with a strength by which the superconductivity can be effectively utilized is applied to the powder to be purified, under the temperature of the degree at which the superconductor fine particles in the powder to be purified exhibit the superconductivity, so that the repulsion produced as a result owing to the Meissner effect causes positional separation of the flow of the superconductor fine particles in the powder to be purified, from the flow of the particles other than the same, thus effecting the purification. The locus of the flow of a superconductor-containing fine particle that shifts according to Meissner effect depends on the proportion of the superconductor contained in the fine particle. This is because the force by which the superconductor fine particle is moved is produced by Meissner effect. Namely, even if the superconductors have the same particle diameters, the repulsion becomes small when the proportion of the superconductors is small. In other words, the low purity thereof results in a small change in the locus of the flow of particles. For example, in instances where a magnetic field having the distribution such that the magnetic flux density becomes smaller from a lower part toward an upper part is applied, the particles having a small proportion of superconductors may be given small Meissner effect, resulting in a small height of floating. On the contrary, the particles having a large proportion of superconductors can float higher. The floating height depends on the balance between the weight of particles and the greatness of the Meissner effect. By selectively collecting portions at a certain height of the thus floated superconductor fine particles, it is possible to take out only the superconductor fine particles having desired purity, i.e., desired values for the proportion of superconductors. Here may be used any means for forming the flow of powder in the apparatus of the present invention, including, for example, a means for directly blowing a carrier gas to the powder, and a means for naturally allowing the powder to fall in a fluid such as helium gas and liquid nitrogen. The carrier gas used in the apparatus of the present invention may include, for example, helium gas. Also preferred is a gas that may not liquified even at a temperature sufficiently lower than the critical temperature of the superconductors. The means for applying a magnetic field to the flow of the above powder include, for example, a permanent magnet and an electromagnet, which magnets may have any shape so long as there can be applied a magnetic field by which the superconductor fine particles can deflect their flying path. Accordingly, it may include plate-shaped, column-shaped or concave-shaped magnets, or those arranged with a plurality of these magnets. In instances where the flow of powder is formed by gravitational fall, the magnets may be shaped or positioned such that a magnetic field that deflects their falling orbital path can be applied. In instances where a desired particle diameter or particle size distribution is to be obtained, various types of classifying means can also be used in combination according to the range of the desired particle diameter. However, since the Meissner effect for the effective purification can be obtained usually in respect of the superconductor fine particles having a particle diameter of 0.01 µm or more, the fine particles having a particle diameter of 0.01 µm or more and the fine particles having a less particle diameter can be readily classified, which have ever been classified not easily by conventional classification methods. It is also possible to carry out classification of particles of the particle diameters other than that. More specifically, if powder having uniform specific gravity are treated in the apparatus of the present invention, the difference in flying distance or floating height of the particles owing to the carrier gas or the difference in terminal settling velocity, for example, depends on their particle diameter. Therefore, they may be collected selectively by zones, so that it becomes also possible to classify the superconductor fine particles included in a desired particle diameter range from among the superconductor fine particles having a particle diameter of 0.01 µm or more. Superconductor fine particles having the same weight and different specific gravity, which have differences in their settling velocity, can also be separated according to the specific gravity by sedimentation in a liquid. To carry out the powder classification with higher precision in the apparatus of the present invention, a partition means having one or plural slit(s) may preferably be provided additionally in the same apparatus. In the apparatus of the present invention, superconductor fine particles having a desired critical temperature range or critical magnetic field range can also be obtained from among the powder in which a plurality of superconductor fine particles each different in the critical temperature (superconductive transition temperature) or critical magnetic field (superconductive transition magnetic field). For example, in instances where the superconductor fine particles having a desired critical temperature range are to be obtained in the above purification apparatus, the above purification apparatus may be operated while appropriately selecting the temperatures of a powder storing vessel, a carrier gas, a powder flow path, etc. according to the desired critical temperature range. Also in instances where the superconductor fine particles having a desired critical magnetic field range are to be obtained, the above purification apparatus may be operated while appropriately selecting the magnetic field applied to the powder to be purified, according to the desired critical magnetic field range. In the apparatus of the present invention, it is further possible to obtain only the powder having a desired specific gravity, not to speak of the above purification and classification of the superconductor fine particles. More specifically, if the powder have a uniform particle diameter, the difference in flying distance or floating height of the powder owing to the carrier gas, the difference in terminal settling velocity, and the degree of changes in the flow direction of superconductor fine particles owing to the application of a magnetic field depend on their specific gravity. Therefore, they may be selectively collected by zones; so that it becomes also possible to separate only the superconductor fine particles having a desired specific gravity. The above purification apparatus for superconductor fine particles of the present invention will be described below by giving several preferred embodiments. A first embodiment of the apparatus of the present invention is characterized by a means for floating the powder containing superconductor fine particles, in a carrier-gas flow path, a means for applying a magnetic field that may move the superconductor fine particles in the vertical direction to the carrier gas flow formed by the first-mentioned means, and a partition means vertically provided, having one or plural slit(s) at the position facing said magnetic field applying means. According to the present embodiment, the powder containing superconductor fine particles are floated by the floating means using a carrier gas, so that, if the powder have a uniform particle diameter, the powder having a smaller specific gravity float higher and the powder having a larger specific gravity float only up to a lower position. Accordingly, the purification and separation can be simultaneously carried out by applying a magnetic field to the floating powder, separating selectively to the outside of the carrier gas flow only the superconductor fine particles from among the powder by the repulsion owing to the Meissner effect, through means of a partition panel provided with one or plural slit(s) and according to the height of the slit(s), and collecting the particles using a collecting means provided at the outside of the carrier gas flow. Here, the floating means may be any means so long as it is a means capable of blowing the powder by use of the carrier gas, and may include, for example, a means for directly blowing the carrier gas to the powder, a means for putting the powder in a container and introducing the carrier gas in that container, a means for allowing the powder to fall from a container and blowing the carrier gas to that falling powder, and a means for making suction from the direction of the magnetic field applying means. The magnetic field applying means may also be any of a permanent magnet and an electromagnet also in the present embodiment, without any particular limitation in its shape. When the magnet comprises, for example, an electromagnet to give a means for applying a magnetic field that can repeat on-off synchronously with the powder floating means and with a specified period, the classification can be carried out more precisely. In this instance, the respective sections can be synchronized by electrical control with use of a controlling system as shown in Fig. 11. In Fig. 11, the numeral 101 denotes a synchronizing computer; 102, a shutter to open and close an inlet (Position A exemplified in Fig. 4 and Fig. 5 of the apparatus of an Example described later) for injecting a carrier gas for blowing up the powder; 103, a shutter fitted to an inlet (Position B exemplified in Fig. 4 and Fig. 5 of the apparatus of an Example described later) for introducing the superconductor fine particles discharged from the carrier-gas flow path by the repulsion owing to the Meissner effect from among the given floated fine particles; 104, an electric source for the magnet; 105, the electromagnet (which is provided at Position M exemplified in Fig. 4 and Fig. 5 of the apparatus of an Example described later) that produces the magnetic field necessary for purifying the superconductor fine particles owing to the Meissner effect. The above shutters and magnet are driven by being synchronized as shown in Fig. 12. In Fig. 12. the abscissas indicate the time, and the ordinates indicate the driving pulse in respect of the actuation of the shutters 102 and 103, and the electric current to be flowed in respect of the magnet 105. The shutters 102 and 103 turn "open" by the rise of the pulse and turn "close" by the decay of the pulse. First, the shutter 102 turns "open" to bring the powder particles to float in the carrier-gas flow path together with the carrier gas. The shutter 103 turns "open" when the time t 1 lapses and the floating positions have been settled for each particle diameter, and at the same time the magnet turns "on". Thereafter, during the time t 2 , the superconductor fine particles are discharged from the carrier-gas flow path through the introducing inlet. Thereafter the shutters 102 and 103 turn "close" and the magnet turns "off". After the time t 3 lapsed and the non-superconductor particles having remained in the carrier-gas flow path have fallen, the shutter 102 again turns "open", thus repeating the above operations. In an instance where a means for collecting non-superconductors is provided (exemplified in Fig. 5 of the apparatus of an Example described later), equipped is a shutter 106 (at Position C in Fig. 5). A block diagram and a time chart in a controlling system for that instance are shown in Fig. 13 and Fig. 14, respectively. Operations are the same as described above. The shutter 106 turns "close" when the shutter 102 is "open", and the shutter 106 turns "open" when the shutter 102 is "close". In the above apparatus, the various conditions such as the type of the above carrier gas, the flow rate, the flow quantity, the width of the slit may be suitably selected according to the desired particle diameter range. A second embodiment of the apparatus of the present invention is a purification-classification apparatus for superconductor fine particles, comprising a container filled with a fluid such as helium gas or liquid nitrogen, a means for allowing the powder containing superconductor fine particles to fall in said container, a means for maintaining said fluid and said powder to a temperature not higher than the superconductive transition temperature of the desired superconductors, a partition means horizontally provided in singularity or in plurality in a path for allowing said powder to fall and provided with a slit at a certain part, and a means for alternately applying, with an appropriate period, magnetic fields having inclinations in the two directions facing each other in the plane rectangular to the falling direction of said powder, where said slit is suitablly disposed, and made to be opened or closed or moved so that only the superconductor fine particles having a particular settling velocity may be passed through the slit to be selectively taken out only the superconductor fine particles having the desired particle diameter. According to the present embodiment, the powder to be purified, kept at a temperature not higher than the critical temperature Tc are allowed to fall in a fluid such as helium gas or liquid nitrogen, and, in a zone at which the fine particles having the desired particle diameter have reached the terminal settling velocity, the magnetic fields having the inclinations opposite directed each other are made to be alternately applied with an appropriate period to the fine powder that are settling in the above zone, so that only the fine powder existing in the superconducting state are made to generate a settling orbital path in a zigzag fashion by the repulsion caused by the Meissner effect. At the same time, an suitable number of partition panels are disposed in a suitable number in the above zone, and slits are provided at certain parts of the partition panels, which slits are suitably arranged or the slits are made open or close with an appropriate period so that only the superconductor fine particles having a particular settling velocity may be passed through them, thus carrying out the purification and classification simultaneously. In the above apparatus, the magnetic field applying means may be any of a permanent magnet and an electromagnet, without any particular limitation also in its shape. These opening and closing of the nozzle and the shutters for slits and the on-off of the electromagnets are electrically controlled and synchronized, for example, in the following manner. Fig. 15 illustrates a block diagram of a controlling system (The illustrated example premises the apparatus exemplified in Fig. 7 and Fig. 8 described later). The numeral 107 denotes a controlling computer; 108, a shutter to open or close a nozzle (Position N in Fig. 17); 109, an electric source for the electromagnet; 110, an electromagnet to apply a first magnetic field (Position M 1 in Fig. 7); and 111, an electromagnet to apply a second magnetic field (Position M 2 in Fig. 7). In an instance where two paths exist for the fall of powder (the apparatus exemplified in Fig. 18), there are added a shutter 112 to open or close the slit belonging to a first path (Positions S 1 , S 1 ' and S 1 '') and a shutter 113 to open or close the slit belonging to a second path (Postions S 2 , S 2 ' and S 2 ''. The above shutters and electromagnets are driven, and synchronized as a whole, for example, according to the time chart as shown in Fig. 16. The abscissas indicate the time. The driving pulse is indicated in respect of each shutter, which turns "open" by the rise of the pulse and turns "close" by the decay of the pulse. Electric currents are indicated in respect of the electromagnets. The period T o for driving the shutters and electromagnets has the following relationship with the terminal settling velocity V f of the desired superconductor fine particles and the distance ℓ of the partition panels: T o = 2ℓ V f Provided that this relationship can be precisely established when the falling locus of the superconductor fine particles is not so much deviated from the straight line, thus actually somewhat requiring experiential correction. The deviation T d in the timing between the opening and closing of the shutter of the nozzle and the driving of other parts corresponds to the time by which the powder to be purified are released from the nozzle to reach the zone at which the purification is effected, or the time remaining when the time of integer times of T o has been deducted from that time, and can be experientially found so that the quantity of the superconductor fine particles collected after purification may become maximum. As having described above, employment of the apparatus of the present invention makes it possible to simultaneously and readily carry out the purification, classification and separation of superconductor fine particles having the desired purity, particle diameter, critical temperature range and critical magnetic field range from among the powder to be purified, and the apparatus used in the process can be of small size and simple, with the course of the process capable of being visually observed. Moreover, the process can be carried out under a low pressure, and yet the above process is proceeded while forming the flow of the powder to be purified. Accordingly, a large quantity of powder can be purified in a high rate and high precision. The apparatus of the present invention is also very useful in enhancing the purity of a superconductive sinter that contains impurities. More specifically, since the present invention can carry out the purification and classification in the order of a µm unit, the sinter can be very finely grounded and purified to the extent such that a superconductivity part and an impurity part may not coexist in its one fine particle. As a result, there can be obtained superconductive powder with high purity. In the apparatus of the present invention, it is also further possible to obtain superconductor fine particles having uniform specific gravity, and thus possible to obtain superconductors with less intermixing of superconductors having different composition. EXAMPLES The present invention will be described below in greater detail by giving Examples and also with reference to the drawings. Example 1 Fig. 4 is a schematic view illustrating another example of the apparatus of the present invention. In Fig. 4, the numeral 38a denotes superconductor fine particles having relatively large particle diameter; 39a, superconductor fine particles having a particle diameter of about 0.1 µm or more but a relatively small particle diameter; and 37a and 37b, non-superconductor fine particles. The floating means in this Example comprises a powder container 31, a opening 32 and a carrier gas ejector (not shown). The magnetic field applying means comprises a magnet 36. This apparatus is so constituted that the powder to be purified contained in the powder container 31 can be floated by the carrier gas from the powder container 31 to a vertical passage 33 through the opening 32. The fine particles 37a and 38a having relatively large particle diameter among the powder floated in the vertical passage 33 float only up to a lower position, but the superconductor fine particles 37a and 38b having relatively small particle diameter float up to a higher position. In this Fig. 4, all the superconductor fine particles and non-superconductor fine particles are assumed to have substantially constant specific gravities. The ejection rate at this time may be selected according to the specific gravity of the powder or the desired particle diameter range. The position, width and number of the vertical passage 33 and horizontal passages 34a and 34b may also be selected according to the desired specific gravity and particle diameter. The apparatus is further so constituted that once a magnetic field is applied by a magnet 36 to the powder inside the vertical passage 33 which have floated to different heights depending on the particle diameter, the respective superconductor fine particles 38a and 38b move to the respective horizontal passages 34a and 34b by the action of the repulsion caused by the Meissner effect, and are collected in the collection receptacles 35a and 35b for the respective superconductor fine particles. It is also so constituted that the non-superconductor fine particles 37a and 37b staying inside the vertical passage 33 fall into the powder container 31 by stopping ejecting the carrier gas or by turning down the gas. Employing this apparatus makes it possible to simultaneously and readily carry out the classification-separation according to the particle diameter and/or specific gravity by floating the powder with use of a carrier gas, and the purification by applying the magnetic field to the powder. In regard to the flow rate of the carrier gas and the driving timing t 1 , t 2 and t 3 previously mentioned, the flow rate is 300 ml/min; t 1 , 20 seconds; t 2 , 5 seconds; and t 3 , 1 minute in instances where, for example, the superconductor fine particles to be classified contains even the fine particles of about 1 to 3 µm in particle diameter. In instances where the particles having a relatively large particle diameter of about 10 to 30 µm are classified, the flow rate is 1 lit/min; t 1 , 20 seconds; t 2 5 seconds; and t 3 , 20 seconds, in approximation, which are typical values. Example 2 Fig. 5 is a schematic view illustrating an apparatus constituted by providing in the apparatus shown in Fig. 4 an impurities collection receptacle 39 and a closing flap 40, and arranging the opening 32 in the horizontal direction. This apparatus is so constituted that when the non-superconductor fine particles 37 and 38 staying inside the vertical passage 33 are allowed to fall by stopping ejecting the carrier gas or by turning down the gas, they fall into the impurities collection receptacle 39 if the closing flap 40 is opened. Accordingly, the non-superconductor fine particles 37 and 38 may not return to the inside of the powder container 31 to enable the purification with good efficiency. Since moreover the direction of flow of the carrier gas containing the powder greatly changes, the powder tend to be uniformly distributed inside the vertical passage 33. The flow rate of the carrier gas and the values for t 1 , t 2 and t 3 are the same as in the case of Example 1. Example 3 In the case when the powder containing superconductor fine particles have a large particle size distribution and at the same time contain large particles of about 100 µm or more in particle diameter, the opening 2 of the apparatus illustrated in Fig. 4 or Fig. 5 is made to have the shape of a nozzle, and powder flow is ejected from the nozzle-shaped opening 2 so that the ratio of the pressure of the carrier gas before and after passing the nozzle-like opening 2 may be 10 or more. This operation enables the classification of the superconductor fine particles with a good efficiency even if the powder contain those having a particle diameter of about 100 µm or more. Example 4 Fig. 6 illustrates an apparatus constituted by providing ultrasonic vibration plates 41a and 41b respectively at the bottom surfaces of the horizontal passages 34a and 34b of the apparatus illustrated in Fig. 4. or Fig. 5. In this apparatus, there is no friction resistance between the superconductor fine particles having moved from the vertical passage 33 to the horizontal passages 34a and 34b, and the bottom surfaces of the horizontal passages 34a and 34b, so that the superconductor fine particles can be efficiently transported to the collection receptacles 35a and 35b without stagnating on the way of the horizontal passages 34a and 34b. Example 5 In the apparatus shown in Fig. 4, Fig. 5 and Fig. 6, the magnet 36 was made to comprise an electromagnet, and the electromagnet was so provided that it can repeat on-off with a certain period while being synchronized with the powder-floating means by using a means (not shown) for controlling the application of magnetic fields. More specifically, the magnet was so provided that the magnetic field may be applied after lapse of an appropriate time by which the particle size distribution of the powder formed in the vertical direction inside the vertical passage comes to a steady state after ejection of the carrier gas, and further that this operation may be repeated with a specified period. Employment of such an apparatus further improves the precision in the classification of the superconductor fine particles. In the present Example, the superconductive material to be classified and purified is YBa 2 Cu 3 O 7-δ (0 ≤ δ ≤ 0.5). Y 2 O 3 , BaCO 3 and CuO were mixed in a ratio of Y : Ba : Cu = 1 : 2 : 3, and the mixture was treated by heating for 2 hours at 950°C in an atmosphere. The X-ray diffraction pattern obtained here is shown in Fig. 1 . In Fig. 1, the peaks in the intended superconductor YBa 2 Cu 3 O 7-δ are indicated by "S". As will be clear from this figure, this sample contains a non-superconductor. Using the apparatus shown in Fig. 4, about 5 g of powder was added to the flow of a carrier gas of a temperature not higher than the critical temperature (He gas; 70 K or less), and this was ejected from the opening 32 at a flow rate of about 200 ml/min. The horizontal passage 34a was made to be 4 mm in width, and the horizontal passage 34b, 3 mm in width. The horizontal passage 34a was also made 5 cm apart from the opening 32, and 10 cm apart from the horizontal passage 34b. Used for the magnet 36 was a permanent magnet (Sm-Co), and the inside of the apparatus was cooled to 77 K by a cooling means (not shown) to carry out the process. With respect to about 5 g of the raw material powder the process of the present invention was carried out. As a result, it was able to collect about 2.5 g of superconductor fine particles in the collection receptacle 5a, and about 1.3 g of superconductor fine particles in the collection receptacle 5b. The X-ray diffraction pattern of the superconductor fine particles in the collection receptacles 35a and 35b resulted in the same as in Fig. 2. From these results, it was able to confirm that the apparatus of the present invention can make purification of superconductors in good precision. Observation of the superconductor fine particles in the collection receptacles 35a and 35b using an electron microscope also revealed that the fine particles in the collection receptacle 35a had a particle diameter of about 100 to 200 µm, and the fine particles in the collection receptacle 35b, about 10 to 50 µm. From these results it was able to confirm that the apparatus of the present invention can make classification of superconductors having a very small particle diameter. Example 6 Fig. 7 is a cross section to explain the principle of another example of the apparatus of the present invention. In this Fig. 7, a helium gas chamber 41 is filled in its inside with helium gas maintained at a temperature not higher than Tc with an appropriate pressure. At an upper part of the helium gas chamber 41, a powder tank 42 equipped with a nozzle 43 at its lower end is provided, and powder to be purified and kept at a temperature not higher than Tc are held in the powder tank 42. The nozzle 43 opens or closes in agreement with the on-off period of a magnet described later. Inside the helium gas tank 41, partition panels 44 are disposed in plural stages that are spaced above and below, and a saucer 47 is provided at a lower part of a lowermost partition panel 44. In each of partition panels 44, slits 44a, 44b, 44c, etc. are formed at alternately shifted positions. At both outsides of the helium gas tank 41, a first magnet 45 and a second magnet 46 are provided facing each other. In Fig. 7, the powder to be purified, contained in the powder tank 42 are allowed to fall from the nozzle 43 that opens or closes in agreement with the on-off period of the first and second magnets 45 and 46. When the powder fall inside the helium gas tank 41, its velocity reaches the terminal settling velocity which depends on the particle diameter. Thereafter the powder approaches the partition panels 44 having the slits in an alternate fashion. Here, the first magnet is excited, and the superconductor fine particles are subject to the repulsion owing to the Meissner effect to suffer deflection of the falling orbital path and pass the first slit 44a. Naturally, the powder that are not in a superconducting state can not pass the slit. Subsequently, after the magnetic intensity of the first magnet was lowered, the second magnet 46 is excited and the superconductor fine particles are deflected to opposite side to pass the next slit 44b. At this time, only the powder having a specific settling velocity can pass successively the slits 44a, 44b, 44c, etc. by appropriately selecting the on-off periods of both the magnets 45 and 46 by using a means (not shown) for controlling the application of magnetic fields. As to the particles in which the superconductor phases and impurity phases coexist, even though having the same settling velocity, the repulsion owing to the Meissner effect is so small as compared with its mass that the moving distance to the lateral direction becomes small. For this reason, the postions of the slits 44a, 44b, 44c, etc. may be suitably selected to enable removal of such powder. In this manner, the superconductor fine particles having been purified and classified are finally collected in the saucer 47. In the present Example the part that must be mechanically driven is only the nozzle 43 for allowing the powder to fall, and thus what are aimed can be achieved by a very simple mechanism. As to the distance between the partition panels and the values for T o and T d in instances where, for example, the superconductor fine particles of 5 µm in particle diameter are to be obtained, T o was 4.5 seconds and T d was 1.3 seconds when the distance is 5 cm. Example 7 Fig. 8 illustrates an apparatus constituted by modifying the partition panel 44 in the apparatus of the present invention shown in Fig. 7, wherein a shutter 48 is provided on every two slits 44p and 44q provided on each partition panel 44. Making this shutter 48 appropriately open and close makes it possible to pass both the powder passing through a settling path 49 and the powder passing through another settling path 50. In the apparatus of Fig. 7, it was possible to open the nozzle only once with respect to one on-off period of the magnets 45 and 46, but, it is possible according to this embodiment to open it twice, making double the treatment capacity. The distance of the partition panels and the values for T o and T d are the same as those in Example 6. Example 8 Fig. 9 illustrates an apparatus same as in Examples 6 and 7 but comprising a disc-like barrier 44 having one opening 44a, and a permanent magnet 51 mounted in place of the electromagnet, both of which are rotated on a common rotating shaft with an appropriate period. In this instance, the powder can be continuously allowed to fall to carry out the purification and classification. Example 9 The present Example, illustrated in fig. 11, operates like Example 6. The inside of the apparatus is maintained at about 70 K, whose upper half is filled with helium gas, and lower half, with liquid nitrogen. The sample placed in a powder tank 42 falls in the liquid nitrogen when a nozzle 43 is opened, and settles at a terminal settling velocity according to the particle diameter of each particle. Magnetic fields having the inclination opposite each other may be alternately applied to this settling particles with an appropriate period, so that only the superconductor fine particles having the desired particle diameter can be allowed to pass slits 44a, 44b and 44c provided on partition panels 44 and gathered on a saucer 47. As compared with the case when only the liquid is used, what is characteristic are that the apparatus can be made small in size and there can be used even the powder having relatively large particle diameter.

Claims: A purification apparatus for superconductor fine particles comprising: means (31, 32) for forming a flow of powder containing superconductor fine particles along a flow path; means for cooling said powder to a temperature at or below the superconductive transition temperature of said superconductor fine particles; means (36) for applying a magnetic field to the flow of said powder for separating superconductor fine particles having desired characteristics from the remainder of the powder with the aid of the Meissner effect; and collection means (35a, 35b) for collecting the superconductor fine particles deflected by a magnetic field applied by said means for applying a magnetic field (36); characterised by shutter means for partitioning said flow path and said collection means (35a, 35b); and electrical control means for controlling means for applying a magnetic field and opening and closing of said shutter means wherein the timing of the on and off of the application of the magnetic field is synchronised with the timing of the opening and closing of said shutter means. The apparatus of claim 1, wherein said means (36) for applying a magnetic field comprises an electromagnet. The apparatus of claim 1 or 2, wherein said means (36) for applying a magnetic field is provided with a plurality of magnets disposed along the flow path of the powder with the plurality of magnets optionally producing fields of different strength. The apparatus of any preceding claim, wherein said means for forming a flow of powder includes means for directly blowing a carrier gas to the powder. The apparatus of claim 4, wherein said carrier gas serves as a coolant for the powder. The apparatus of any of claims 1 to 3, wherein said means for forming a flow of powder allows the powder to fall due to the force of gravity. The apparatus of any preceding claim, further comprising partition means (44) having at least one slit (44a, 44b) provided in the flow path of the powder. The apparatus of any preceding claim, further comprising collecting means (39) for collecting fine particles in another flow path not deflected by said means for applying the magnetic field. The apparatus of any of the preceding claims wherein said desired characteristics comprises any of particle diameter, critical temperature, critical magnetic field and specific gravity. A method for purifying a powder containing superconductor fine particles comprising the steps of: forming a flow of powder containing superconductor fine particles along a flow path; cooling said powder to a temperature at or below the superconductive transition temperature of said superconductor fine particles; applying a magnetic field to the flow of said powder to separate superconductor fine particles having desired characteristics from the remainder of the powder on the basis of the Meissner effect; and collecting superconductor fine particles having desired characteristics that are separated by said magnetic field, in collection means; characterised by: controlling the timing of on and off application of the magnetic field so that it is synchronous with on and off operations of shutter means interposed between said flow path and said collecting means. A method of producing a superconductor material sinter having desired characteristics comprising the steps of: purifying a powder containing superconducting fine particles in accordance with claim 10; collecting said superconductor fine particles having desired characteristics divided from the remainder of the powder; and sintering said powder. A method in accordance with either of claims 10 or 11 wherein said desired characteristics comprise any of particle diameter, critical temperature, critical magnetic field and specific gravity.

Inventors: AKAIKE MASATAKE, IWATATE FUJIO, KANEKO NORIO, KAWASAKI TAKEHIKO, KISHI FUMIO, MOTOI TAIKO, OHMI KAZUAKI, SHINJOU ATSUKO, YAMAMOTO KEISUKE, AKAIKE, MASATAKE, IWATATE, FUJIO, KANEKO, NORIO, KAWASAKI, TAKEHIKO, KISHI, FUMIO, MOTOI, TAIKO, OHMI, KAZUAKI, SHINJOU, ATSUKO, YAMAMOTO, KEISUKE, KISHI, FUMIO,C/O CANON K.K., MOTOI, TAIKO,C/O CANON K.K.
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1988
Main classification: B03C1
IPC codes: B03C1