Purification apparatus 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 (5a, 5b) field to the flow of the powder.

Description: The present invention relates to an apparatus and a method for purifying a superconductor fine particles. It enables desired superconductor particles to be obtained selectively from mixtures of fine particles of different diameters and comprised of superconductors, normal conductors or insulators having different critical temperatures and critical magnetic fields. In recent years it has been discovered that by sintering ceramic materials with particular compositions there can be obtained a sinter that exhibits superconductivity (or is superconducting) at 77°K or above, in some instances near room temperature. However, the crystal structure and phase of these superconductors has not been sufficiently elucidated and usually they coexist with non-superconducting crystal phases. It is very difficult to separate superconducting crystal phases from non-superconducting crystal phases where these phases coexist. Furthermore a technique of controlling heat treatment conditions has not been established that enables only superconducting crystalline materials to be formed. In recently available ceramic superconductors, there also often coexists a plurality of superconducting crystal phases which differ in their critical temperature or critical magnetic field. No method has been established for selectively separating the superconducting crystal phases having a particular desired critical temperature range and critical magnetic field range from amongst the various superconducting phases present. In addition, superconductive sinter generally comprises a mass or aggregate of fine crystals and its superconductivity characteristics vary greatly depending on the state of the crystal grain boundaries. In order to produce a sinter having stable characteristics, the crystal grain boundaries must be made uniform. It has been proposed to produce a superconducting sinter of uniform crystal grain boundaries by re-sintering superconductor fine particles of uniform particle diameter. However, up to now no suitable method for classifying such superconductor fine particles of uniform particle diameter has been discovered, and only ordinary particle classification methods are available as described in from Funtai Kogaku Handobukku (Particle Technology Handbook) (edited by Koichi Iitani, Asakura Publishing Co). Ordinary particle classification methods involve screening by means of sieves having different opening sizes which are stacked up in order from those having the largest openings to enable classification to be carried out. A sedimentation method is known in which the terminal velocity at which particles in a fluid settle is used to carry out the classification. However, in the screening method it is impossible to prepare screens whose sizes are several micrometres or below, thus preventing particles of small diameter from being classified. Furthermore, it is often the practice to apply a load to the fine particles in order to produce a pressure tending to force them through the openings in the screen. This practice gives rise to the problems that classification cannot be carried out in a vacuum, as would otherwise be preferred because of its greater precision. In the sedimentation method described above, the settling velocity depends both on the diameter of the particles and on the specific gravity thereof, and accurate classification cannot be achieved. Furthermore, in liquid phase sedimentation methods it is laborious to separate fine particles from liquid and in general the settling velocity is so low that the time required for classification is very lengthy. The method also suffers from the problems that it is inherently not a vacuum method. A method for separating superconducting particles from non-superconducting particles by means of magnetic levitation is disclosed in a paper by S Vieira et al, "A Simple Device for Quick Separation of High-Tc Superconducting Materials", J. Phys. E: Sci Instrumen. 20 (1987) 1292-1293. Another magnetic separation method is disclosed in a paper by Barstoum et al, "Use of the Meissner Effect to Separate, Purify and Classify Superconducting Powders", Appl. Phys. Lett. 51(23), 7 December 1987, page 1954. Patent Publication WO88/08619 discloses a method of separating superconductive materials from other materials by allowing the material to fall and applying a magnetic field to the falling material. The apparatus and method of this invention are defined in the accompanying claims. Forms of the above defined apparatus enable superconductor fine particles to be separated and purified when they are present as a powder containing other particles. Further forms of the invention enable superconductor fine particles to be classified and purified according to the desired characteristics, when there is present in a powder a plurality of such fine particles having different characteristics e.g. particle diameter, critical temperature and critical magnetic field. Various forms of the invention will now be described, by way of example only, with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Figs. 1, 4, 7, 12, 13, 14, 15, and 16 each illustrate schematic constitution of an example for the purification apparatus for superconductor fine particles according to the present invention; Fig. 2 is a graph showing an X-ray diffraction 25 pattern of a superconductor containing impurities; Fig. 3 is a graph showing an X-ray diffraction pattern of a purified superconductor; Fig. 5 is a graph showing temperature dependency of the electric resistance of a superconductor; Fig. 6 illustrates constitution of another example of a magnet and a partition panel part of the apparatus of the present invention; Fig. 8 illustrates constitution of another example of a powder-feeding means in the apparatus of the present invention; Figs. 9 and 10 each illustrate constitution of another example of a means for applying magnetic field in the apparatus of the present invention; Fig. 11 illustrates constitution of another example of a powder-collecting means in the apparatus of the present invention; Figs. 28 and 29 respectively illustrate a block diagram and a time chart of a controlling system in the apparatus of the present invention; Figs. 30 and 31 respectively illustrate a block diagram and a time chart of 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 abovementioned 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 a gaseous carrier medium along a path, including, for example, a means for directly blowing a carrier gas to the powder. 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 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, 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. 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 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 having a means for horizontally ejecting powder containing superconductor fine particles to form a flow of the powder, a partition means horizontally provided, having one or plural slit(s) at the position with a specified distance from an ejecting outlet, and a means provided beneath said respective slit(s), for applying a magnetic field in the vertical direction to the flow of said powder According to the present embodiment, the powder to be purified containing superconductor fine particles maintained to a temperature at which their superconductivity can be sufficiently exhibited are horizontally ejected from a nozzle or the like together with a carrier gas, whereupon the powder particles larger in particle diameter and heavier among the powder to be purified begin to fall at a position near to the above nozzle, and the particles smaller in particle diameter and lighter among them begin to fall at a position far from the above nozzle, so that the groups of the falling powder particles form the particle size distribution in the horizontal direction. Classification can be carried out by selectively separating them by the partition means horizontally provided, having one or plural slit(s) at the position with an appropriate distance from the nozzle. Here, the various conditions such as the kind of the above-mentioned carrier gas, the flow rate, the flow quantity, the slit width may be appropriately selected according to the desired particle diameter range. Only the superconductor fine particles can also be separated by providing a plate-shaped magnet beneath the above slit, having a width and shape sufficient for making purification under the above temperature, in a vertical state or a state slightly inclined than vertical, applying the magnetic field previously mentioned to the powder to be purified that have been classified and passed the slit as described above to let the powder fall in the vicinity of the plate-shaped magnet, under the temperature previously mentioned, to change the falling locus of the superconductor fine particles in the powder to be purified, and collecting them by use of a collecting means each provided at the falling position of the superconductor fine particles and at the falling position of the other fine particles. Thus, the apparatus of the present embodiment can carry out the purification and classification simultaneously. In the above apparatus, the means for applying a magnetic field may be any of a permanent magnet and an electromagnet, without any particular limitation also in its shape. However, when, for example, a sheet of the above plate-like magnet is constituted of a plurality of electromagnets to give a means for applying magnetic fields that can repeat on-off in succession in the powder-falling direction and with a specified period, the falling locus of the superconductor fine particles can be changed with good efficiency to bring about the advantages such that the purification process can be simplified. To attempt to further simplify this purification process, a plate of non-magnetic material may be provided on the surface of the magnet and may be vibrated (as exemplified by an ultrasonic vibration plate), thus making it possible to smooth the flow of powder. A second 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. 28. In Fig. 28, the numeral 101 denotes a synchronizing computer; 102, a shutter to open and close an inlet (Position A exemplified in Fig. 14 and Fig. 15 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. 14 and Fig. 15 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. 14 and Fig. 15 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. 29. In Fig. 29. 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 t₁ 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 t₂, 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 t₃ 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. 15 of the apparatus of an Example described later), equipped is a shutter 106 (at Position C in Fig. 15). A block diagram and a time chart in a controlling system for that instance are shown in Fig. 30 and Fig. 31, 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. As described previously, the ultrasonic vibration plate that may be provided on the surface of the magnet in the first embodiment of the present invention employing a piezoelectric material (such as ZnO, AlN and PZT). There is no particular limitation in the frequency of the vibration, but in general it may range approximately from 1 Hz to 20 kHz. 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. 1 illustrates an example of the apparatus of the present invention. The numeral 9 indicates superconductor fine particles having relatively large particle diameter; 10, superconductor fine particles having relatively small particle diameter of about 0.1 µm or less; and 7 and 8, non-superconductor fine particles having relatively large particle diameter and non-superconductor fine particles having relatively small particle diameter. In the present Example, the superconductive material to be classified and purified is YBa₂Cu₃O 7-δ (0 ≦ δ ≦ 0.5). Y₂O₃, BaCO₃ 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. 2. In Fig. 2, the peaks in the intended superconductor YBa₂Cu₃O 7-δ are indicated by "S". As will be clear from this figure, this sample contains a non-superconductor. The mixture was ground in a mortar, and thereafter the settling velocity was measured in toluene to reveal that the settling velocity differs depending on the particle diameter, but the particles having the same particle diameter as a whole settled substantially in the same velocity. Accordingly, the superconductors and the impurity non-superconductors are considered to have substantially the same specific gravity. First, powder containing superconductor fine particles and a carrier gas (as exemplified by He gas) having a temperature not higher than the critical emperature are mixed to make a mixed powder 1. This is ejected from an opening 2. The ejecting rate is selected depending on the specific gravity of powder or the particle diameter range as desired. The position of slits 11a and 11b of a partition panel 3 is made movable, and the width of the slits is selected by the particle diameter range as desired. Fine particles having a larger particle diameter are transported near the opening 2, and smaller fine particles having a smaller particle diameter, up to a distant place by the carrier gas. The powder having uniform particle diameter pass the slits and then fall by gravity, but at this time, because of a plate-shaped permanent magnet, the superconductor fine particles 9 having relatively large particle diameter become apart from the surface of the magnet to pass over a partition panel 4a to fall. However, it may not occur that normal conductors, i.e., the fine particles 7 of non-superconductor (including conductors, semiconductors and insulators) pass over the partition panel 4a. The superconductor fine particles passed over the partition panel 4a are collected by a collection receptacle 6a. Similarly, the superconductor fine particles 10 having relatively small particle diameter are also separated from the non-superconductor fine particles 8 by a permanent magnet 5b and a partition panel 4b, and collected in a collection receptacle 6b. In the present Example, the classification and purification are carried out under the following conditions. In Fig. 1, the mixed powder 1 is ejected from the opening 2 at a flow rate of about 200 ml/min with use of the carrier gas comprising He gas. The He gas is beforehand cooled to a temperature of 70 K or less by use of a cooling unit (not shown). The slit 11a has a space of 3 mm, and the slit 11b, 4 mm. The distance from the opening 2 to the slit 11a is 50 cm, and that from the opening 2 to the slit 11b, 150 cm. The permanent magnets are all comprised of Sm-Co and cooled to a temperature of 77 K using a a cooling unit (not shown). The partition panels 4a and 4b protrude by 4 mm from the surface of the magnets 5a and 5b. Under the working conditions as described above, superconductors were able to be collected from about 5 g of raw material powder in the collection receptacle 6a in an amount of about 2.5 g, and in the collection receptacle 6b, about 2 g. The powder in this collection receptacle showed the X-ray diffraction pattern as shown in Fig. 3, resulting in disappearance of all the diffraction peaks of the non-superconductor fine particles in Fig. 2. Thus it was able to confirm that the present apparatus can make purification of superconductors. An electron microscope also revealed that superconductors of about 100 to 200 µm in particle diameter have been collected in the collection receptacle 6a, and those of about 10 to 50 µm in particle diameter, in the collection receptacle 6b, thus confirming the effect of classification. Example 2 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 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 pressures 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 high efficiency like in Example 1 even if the powder contains those having a particle diameter of about 100 µm or more. Example 3 It may often occurs that superconductor phases having different critical temperatures coexist if ceramics having the composition such as YBa₂Cu₃O x (x = 6.00 to 7.00) or Bi₂ (Sr, Ca)₃ Cu₂O 12-x (x > 0) are sintered under the same conditions. In such an instance, used is an apparatus having functions as shown in Fig. 4. In Fig. 4, the numeral 7a indicates non-superconductor fine particles having relatively large particle diameter; 8a, non-superconductor fine particles having relatively small particle diameter; and 9a, 9b, 10a and 10b each, superconductor fine particles, where 9 indicates a higher critical temperature than 10, and a indicates those having relatively large particle diameter, and b, those having relatively small particle diameter. The temperature dependence on the electric resistance of the superconductors used as samples of the present invention is shown in Fig. 5. As will be seen from Fig. 5, there exists a crystal phase that exhibits superconductivity at 107 K or less. In a mortar, about 10 g of raw material powder is ground, and this is ejected from an opening 2 of 5 mm in diameter at a flow rate of about 300 ml/min by using He gas cooled to about 60 K as a carrier gas. Sm-Co magnets 5a and 5b at the upper stage are beforehand cooled to 50 K by means of a cooler (not shown) and Sm-Co magnets 5c and 5d at the lower stage are similarly beforehand cooled to 90 K. The partition panels 4a and 4b at the upper stage protrude by 4 mm from the surface of the magnets and the partition panels 4c and 4d at the lower stage protrude by 3 mm. As a result, superconductors (critical temperature: 107 K) having a particle diameter of about 100 to 400 µm were collected in the one collection receptacle 6a and those having a particle diameter of about 30 to 50 µm were collected in the other collection receptacle 6b in an amount of about 0.05 g and 0.08 g, respectively. On the partition panels 4c and 4d, also collected were 3 g and 5 g, respectively, of superconductors haing a critical temperature of 80 K. Meanwhile, the slits 11a and 11b are both 5 mm in width, and the slit 11 is 70 cm distant and the slit 11b is 200 cm distant, from the opening 2. Example 4 In Examples 1 to 3, the magnets and the partition panels at the bottom parts thereof, as exemplified by the magnets 5a and 5b and the partition panels 4a and 4b in Fig. 1 had the shapes of flat plates. The partition panels 4a and 4b are made to have the shapes having edges fitted on their both ends as shown in Fig. 6, and in some instances the partition panels are made continuously movable according to a belt conveyor system. This makes it possible to make separation of a large quantity of powder. Example 5 In the instances where the powder containing superconductor fine particles have relatively uniform grain size and no further classification is required as in Examples 1 to 4, no carrier gas is required, and the superconductor fine particles can be separated by allowing the powder to fall on a magnet 5 as shown in Fig. 7. More specifically, the powder is allowed to fall from a container 12 holding the powder containing superconductor fine particles, and the powder may be brought to slip down the surface of the magnet 5 by appropriately selecting the inclination of the magnet 5 from a vertical position according to the particle diameter of the powder, so that only the superconductors 9 kept apart from the magnet surface pass over the partition panel owing to the Meissner effect and are collected in a collection receptacle 6. Example 6 As shown in Fig. 8, powder is allowed to fall from the container 12 holding the superconductors 9 to make the apparent shape of the powder falling on a slip board 13 to be of thin plate, to which a carrier gas is blown through a gas-introducing pipe 14. The classification of the superconductor fine particles can be carried out in the same manner as in Example 1 except for transporting the powder in this manner. Example 7 In Examples 1 to 5, the means for applying the magnetic field is constituted of a plurality of electromagnets 5I, 5II, 5III, ... and 5n as shown in Fig. 9, so that the on-off of the above electromagnets may be repeated (in the order of 5I → 5II → 5III → ... 5n → 5I) by a means (not shown) for controlling the application of magnetic fields, in succession in the falling directions of from 5I to 5n and with an appropriate period. This makes it possible to attempt to simplify the purification process and separate a large quantity of powder. The period of the on-off of the magnetic fields may be selected according to the velocity of the powder flow, and also the magnetic fields may be made stronger in succession from 5I toward 5n. Example 8 In Examples 1 to 5 and 7, provided is a vibration plate that ultrasonically vibrates by use of an ultrasonic oscillator (not shown) as shown in Fig. 10. This makes it possible to prevent the powder to be purified from being deposited on the magnets, attempt to make efficient the purification process, and separate a large quantity of the powder. Example 9 In instances where the powder containing superconductor fine particles have relatively a uniform particle diameter so that no further classification may be required, a collecting means as shown in Fig. 11, may be provided after the same classification and purification as in Example 1, so that the superconductor fine particles can be separated according to the difference in the specific gravity. In Fig. 11, the numeral 19a denotes superconductor fine particles having relatively high specific gravity; 19, superconductor fine particles having relatively low specific gravity; and 7, non-superconductor fine particles, where all of these fine particles have substantially the same particle diameter. The collecting means in the present embodiment comprises passages 18a and 18b and superconductors collection receptacles 6a and 6b, and is constituted such that the powder to be purified fall on the slant of a magnet 5, and the non-superconductor fine particles 7 in the powder continue to come into contact with the slant until they slip down in an impurities receptacle 17. It is also constituted such that the superconductor fine particles 19b having relatively light specific gravity become greatly apart from the magnet 5 owing to the magnetic field applied, and the superconductor fine particles 19a having relatively heavy specific gravity are not so much apart from it so that the distribution of the flows according to the specific gravity can be formed. It is also constituted such that each of the superconductor fine particles 19a and 19b in that distribution can be collected by superconductors collection receptacles 6a and 6b through the passages 18a and 18b. The distance of the passages 18a and 18b from the magnet 5 in this apparatus and the inclination of the magnet 5 may be appropriately selected to find an optimum value according to the desired particle diameter and specific gravity. To carry out the classification and separation with further precise particle diameter and specific gravity, the passages 18a and 18b may be made to have a narrower width and the number of the passages may be increased. Example 10 If the spacing of the slits become broader, it can be contemplated, for example, in Fig. 1 that the non-superconductors pass over the partition panels 4a and 4b to mix into the collection receptacle. In such an instance, a flow-deflecting device 1 (as exemplified by a baffle) may be provided, so that the precision of the classification and purification can be improved. Example 11 The apparatus illustrated in Fig. 13 comprises a container 20, a diaphragm 21, a partition panel 3, a nozzle 1, and a funnel 23. The inside of the container is kept at about 70 K, and its lower part is filled with liquid nitrogen. The upper part thereof is filled with helium gas blown in from the nozzle together with sample powder. The sample powder blown out from the nozzle falls from slits 11a, 11b and 11c corresponding to the respective superconductor to enter into the liquid nitrogen. Magnets 5a, 5b and 5c are provided in the vicinity of the powder-falling orbital path, and the superconductor fine particles are deflected in their orbital path by the repulsion owing to the Meissner effect and gathered in saucers 6a, 6b and 6c. The non-superconductor particles fall straight near the magnets and selected and separated from the superconductors. In the present Example, the heat capacity of the liquid is larger by far than that of the gas so that the samples which fell into the liquid immediately come to have the same temperature as the liquid. Thus, this is characterized by having good precision for the temperature at the time of purification. In the above Example, used magnets are all permanent magnets, but the magnetic fields may be applied to the powder by use of electromagnets to obtain quite the same effect. The slits are provided on the partition panels 3 at three points, but the slits may be made small in width and large in the number to effect precise classification of particle diameter. There are no particular limitations in the width and number of the slits. Alternatively, a vessel that can be moved by a belt conveyor or the like may be used in place of the slits, and this may, for example, be moved in the direction perpendicular to the paper surface of Fig. 13, and thereafter may be allowed to fall on the magnet areas. Moreover, it is needless to say that the particle size distribution of the superconductor fine particles collected in the collection receptacle 6 may not be affected at all even if there is a great difference in the specific gravity between the superconductors and non-superconductors. Example 12 Fig. 14 is a schematic view illustrating another example of the apparatus of the present invention. In Fig. 14, 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. 14, 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 t₁, t₂ and t₃ previously mentioned, the flow rate is 300 ml/min; t₁, 20 seconds; t₂, 5 seconds; and t₃, 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; t₁, 20 seconds; t₂, 5 seconds; and t₃, 20 seconds, in approximation, which are typical values. Example 13 Fig. 15 is a schematic view illustrating an apparatus constituted by providing in the apparatus shown in Fig. 14 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 t₁, t₂ and t₃ are the same as in the case of Example 12. Example 14 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. 14 or Fig. 15 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 like in Example 1 even if the powder contain those having a particle diameter of about 100 µm or more. Example 15 Fig. 16 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. 14. or Fig. 15. 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 16 In the apparatus shown in Fig. 14, Fig. 15 and Fig. 16, 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. Also, using the apparatus shown in Fig. 14, about 5 g of powder (showing the same X-ray diffraction pattern as in Fig. 2) containing the superconductor fine particles obtained in the same manner as in Example 1 were 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. 3. 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.

Claims: Apparatus for purifying superconductor fine particles comprising means for cooling powder to be purified and containing the fine particles to below its critical temperature, means (2; 32) for forming a flow of the powder along a path, partition means having a plurality of openings (11a, 11b ; 34a, 34b) at different spaced-apart locations along the path for receiving particles of different properties, and means (5a, 5b ; 36) for applying a magnetic field to the flow of the powder to separate the fine particles by means of their superconductivity, characterised in that: (a) the powder flow forming means is arranged to cause the powder to flow in a gaseous carrier medium along the path so that particles of different size separate according to their size and mass as they travel along the path; and (b) the locations at which the openings (11a, 11b ; 34a, 34b) are provided are where gravitationally induced separation of particles of different sizes has taken place. Apparatus according to claim 1, wherein the means for forming the flow of powder is arranged to blow the powder in a horizontal direction, the partition means is horizontally directed and has a plurality of openings at different distances from the blowing means, and means is provided beneath the respective openings for applying a magnetic field to the powder as it travels along the path. Apparatus according to claim 2, wherein the blowing means comprises a nozzle. Purification apparatus according to claim 2, wherein the magnetic field providing means comprises first magnets located relatively close beneath the respective openings and cooled to a first relatively low temperature and second magnets for receiving material that has passed from the first magnets and located at a relatively greater distance beneath the respective openings, the second magnets being cooled to a relatively high temperature, the arrangement being such that only material whose critical temperature for superconductivity is greater than the second temperature passes the second magnets. Apparatus as claimed in claim 2,3 or 4, wherein there is provided beneath a respective opening a baffle facing oppositely to the blowing direction of the powder for directing the powder towards the magnetic field applying means. The apparatus of any preceding claim, wherein the means for applying a magnetic field comprises a permanent magnet. The apparatus of any of claims 1 to 5, wherein the means for applying a magnetic field comprises an electromagnet. Apparatus according to any preceding claim, further comprising electronic control means for controlling the means for applying the magnetic field and the means for blowing the powder, the electronic control means being arranged to synchronize the application of the magnetic field with the flow of the powder. Apparatus according to any preceding claim, further comprising collecting means for collecting the superconductor fine particles after they have been deflected by the magnetic field applying means. Apparatus according to any of claims 1 to 8, further comprising first collecting means for collecting superconductor fine particles having a flow path deflected by the magnetic field applying means and second collecting means for collecting fine particles having a different flow path not deflected by the magnetic field applying means. Apparatus according to any of claims 1 to 10, wherein the gaseous carrier medium serves as a coolant for the powder. The apparatus according to any preceding claim, wherein a body of liquid is provided beneath the openings into which particles passing through the openings fall, the body of liquid serving as a coolant for the particles. Apparatus according to claim 1, wherein the powder flow forming means is arranged to produce a flow of particles floated in the carrier gas, the partition means is vertically directed and is formed with a plurality of openings at different heights above the powder flow forming means, means is provided at locations facing the respective openings for applying a magnetic field to the flow of powder, and electrical control means is provided for controlling the means for forming the flow of powder and the means for applying a magnetic field. Apparatus according to claim 13, wherein the means for applying a magnetic field comprises an electromagnet. The apparatus of claims 13 or 14, further comprising collecting means for collecting superconductor fine particles which have travelled along the flow path and have been deflected by the magnetic field applying means. Apparatus according to claim 13 or 14, further comprising first collecting means for collecting superconductor fine particles that have travelled along the flow path and have been deflected by the magnetic field applying means and second collecting means for collecting fine particles that have travelled along the flow path but have not been deflected by the magnetic field applying means. The apparatus of claim 16, wherein the first collecting means and the second collecting means are controlled by the electrical control means. The apparatus of any of claims 13 to 17, wherein the carrier gas serves as a coolant for the powder. A method for purifying superconductor fine particles comprising: (a) cooling the powder containing the fine particles to be purified to below its critical temperature; (b) forming a flow of the powder along a flow path; (c) causing the powder travelling along the flow path to enter openings at different spaced-apart locations along the path as a result of differences in the properties of the particles, and (d) applying a magnetic field to the powder to separate the fine particles by means of their superconductivity, characterised in that: (e) the powder flow forming means causes the powder to flow in a gaseous carrier medium along the path so that particles of different size separate according to their size and mass as they travel along the path to separate locations; and (f) the flow of particles is caused to enter openings at different locations where gravitionally induced separation of particles of different sizes has taken place, whereby there can be obtained via the different openings purified superconductive particles having different sizes. A method according to claim 19, wherein the particles are blown in a horizontal direction along a horizontal partition having slits spaced at different distances from the blowing means, and magnetic fields are applied to the particles which fall through the slits to bring about separation of the particles according to their superconductivity. A method according to claim 19, wherein the powder flow forming means brings about flotation of the fine particles in an upward stream of the carrier gas, the carrier gas flows along vertically directed partition means having a plurality of slits at different heights above the flotation means, and particles of different size which separate at different heights are deflected into respective slits by magnetic field applying means which is arranged to apply a magnetic field to the powder as it flows along the partition means. A method of purifying superconductive fine particles which comprises treating the particle in apparatus as claimed in any of claims 1 to 18 and recovering the purified superconductive powder.

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
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1988
Main classification: H01L39
IPC codes: C01G1, B03C1, H01B12, H01L39, C01B13