EP0588451B1

EP0588451B1 - Purification apparatus for superconductor fine particles

Info

Publication number: EP0588451B1
Application number: EP93203439A
Authority: EP
Inventors: Fumio C/O Canon K.K. Kishi; Masatake Akaike; Keisuke Yamamoto; Taiko C/O Canon K.K. Motoi; Norio Kaneko; Fuji Iwatate; Kazuaki Ohmi; Takehiko Kawasaki; Atsuko Shinjou
Original assignee: Canon Inc
Current assignee: Canon Inc
Priority date: 1987-12-09
Filing date: 1988-07-08
Publication date: 1997-10-22
Anticipated expiration: 2008-07-08
Also published as: EP0320083A2; EP0588450A3; DE3856037D1; EP0588452A2; JP2656550B2; EP0588451A3; EP0320083A3; DE3854520D1; EP0588450A2; EP0320083B1; DE3856053T2; JPH02265661A; DE3854520T2; EP0588452B1; DE3856053D1; DE3856037T2; EP0588452A3; EP0588451A2

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

In 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 shieves 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 7.
The features in common with the teaching of S. Vieira et al. appear in the preambles to these claims.
Other embodiments of the present invention will become apparent from the following descriptions.

BRIEF DESCRIPTION OF THE DRAWINGS

Figs. 1 and 4 each illustrate schematic constitution of an example for the purification apparatus for superconductor fine particles according to the present invention;
Fig. 2 illustrates the constitution of another example of a partition means in the apparatus of the present invention;
Fig. 3 illustrates the constitution of another example of a means for applying magnetic field in the apparatus of the present invention;
Figs. 5 and 6 respectively illustrate a block diagram and a time chart of a 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.
An 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. 5 illustrates a block diagram of a controlling system (The illustrated example premises the apparatus exemplified in Fig. 1 and Fig. 2 described later). The numeral 107 denotes a controlling computer; 108, a shutter to open or close a nozzle (Position N in Fig. 1 ); 109, an electric source for the electromagnet; 110, an electromagnet to apply a first magnetic field (Position M₁ in Fig. 1); and 111, an electromagnet to apply a second magnetic field (Position M₂ in Fig.1). In an instance where two paths exist for the fall of powder (the apparatus exemplified in Fig. 2), there are added a shutter 112 to open or close the slit belonging to a first path (Positions S₁, S₁' and S₁'') and a shutter 113 to open or close the slit belonging to a second path (Postions S₂, S₂' and S₂''.
The above shutters and electromagnets are driven, and synchronized as a whole, for example, according to the time chart as shown in Fig. 6. 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: $T_{o} = \frac{2 ℓ}{V_{f}}$
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. 1 is a cross section to explain the principle of an example of the apparatus of the present invention. In this Fig. 1, 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. 1, 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 2

Fig. 2 illustrates an apparatus constituted by modifying the partition panel 44 in the apparatus of the present invention shown in Fig. 1, 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. 1, 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 1.

Example 3

Fig. 3 illustrates an apparatus same as in Examples 1 and 2 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 4

Fig. 4 illustrates another modification of Example 1. 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 (42, 43) 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 (45, 46) 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 on the basis of the Meissner effect; and

partition means (44) for dividing the superconductor fine particles separated by said means for applying a magnetic field (45, 46) from the remainder of the powder;
characterised in that said partition means (44) is provided across the flow path of said powder containing superconductor fine particles and provided with at least one slit at a certain part thereof; and
said means (45, 46) for applying a magnetic field to said flow of said powder is adapted to apply an alternating magnetic field so as to make specific superconductor fine particles having a desired property deflect from the said flow path to a direction perpendicular to said flow and pass through said slit generating a path in a zig zag fashion.
The apparatus of claim 1, wherein said means (45, 46) for applying a magnetic field comprises an electromagnet.
The apparatus of claim 1 or 2, further comprising collecting means (47) for collecting the superconductor fine particles in a flow path deflected by said means (45, 46) for applying the magnetic field.
The apparatus of claim 1, 2 or 3, further comprising means (41) for retaining a coolant so that said flow path passes through said coolant.
The apparatus of claim 1, 2, 3 or 4, wherein said partition means (44) includes a plurality of partitions disposed in the falling direction of the powder.
The apparatus of any of the preceding claims wherein said desired characteristics comprise any of particle diameter, critical temperature, critical magnetic field and specific gravity.
A method of 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

dividing the superconductor fine particles separated by said magnetic field from the remainder of the powder;
characterised by the provision of partition means across the flow path of said powder containing superconductor fine particles with at least one slit at a certain part thereof; and
controlling the application of said magnetic field so as to make specific superconductor fine particles having a desired property deflect from the said flow path to a direction perpendicular to said flow and pass through said slit generating a path in a zig zag fashion.
A powder comprising superconductor fine particles of desired characteristics obtained by the method of claim 7.
A method of creating a superconducting sinter having desired characteristics comprising the steps of:
purifying a powder containing superconducting fine particles by using the method of claim 7;

collecting said superconductor fine particles having desired characteristics divided from the remainder of the powder; and

sintering said powder.
A method in accordance with any of claims 7 or 9
wherein said desired characteristics comprise any of particle diameter, critical temperature, critical magnetic field and specific gravity.