US20110209578A1

US20110209578A1 - Nanoparticle manufacturing device and nanoparticle manufacturing method and method of manufacturing nanoparticle-dispersed liquid alkali metal

Info

Publication number: US20110209578A1
Application number: US13/014,120
Authority: US
Inventors: Kuniaki Ara; Junichi Saito; Hiroyuki Sato; Nobuki Oka; Masahiko Nagai; Koichi Fukunaga
Original assignee: Mitsubishi Heavy Industries Ltd; Japan Atomic Energy Agency
Current assignee: Mitsubishi Heavy Industries Ltd; Japan Atomic Energy Agency
Priority date: 2010-02-26
Filing date: 2011-01-26
Publication date: 2011-09-01
Also published as: FR2956826B1; FR2956826A1

Abstract

A nanoparticle manufacturing device capable of particle size control of nanoparticles made of a raw material metal powder and control of the occurrence condition of chaining of nanoparticles and of necking. The device 1 is provided for manufacturing nanoparticles by heating and melting a mixture of a raw material metal powder and a carrier gas in a heating space, cooling the mixture in a cooling space and collecting the mixture in a collection space. The heating space, the cooling space and the collection space form a continuous flow path without a back flow, and the cross-sectional area of the collection space is set at a large value compared to the cross-sectional area of the heating space and the cooling space. Further, there is provided a method of manufacturing a nanoparticle-dispersed liquid alkali metal by dispersing nanoparticles in a liquid alkali metal. A liquid alkali metal obtained by dispersing nanoparticles in the liquid alkali metal is manufactured by performing a rough dispersion step of stirring nanoparticles in the liquid alkali metal by a physical effect and a dispersion step of dispersing nanoparticles in the liquid alkali metal by irradiating the liquid alkali metal with ultrasonic waves after the rough dispersion step.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention
The present invention relates to. (A) a nanoparticle manufacturing device and a nanoparticle manufacturing method which are suitable for manufacturing nanoparticles of single metals, alloys and the like.
Further, the present invention relates to (B) a method of manufacturing a nanoparticle-dispersed liquid alkali metal which is used to manufacture a liquid metal obtained by uniformly dispersing nanoparticles in a liquid alkali metal, such as reactor-cooling liquid sodium.
2. Description of the Related Art
Regarding (A) described above, particles having sizes of the order of nanometers, what is called nanoparticles, have unique properties which manifest themselves due to the size effect or have a large specific surface area and, therefore, in recent years studies aimed at applying nanoparticles have been carried out actively in many fields.
Various manufacturing methods of such nanoparticles as described above have been proposed. For example, Japanese Patent Laid-Open No. 2007-84849 discloses a manufacturing method of metal ultrafine particles which involves causing a raw material metal powder to fall onto a heating-controlled evaporation surface from above in an inert gas under reduced pressure and causing the raw material metal powder to evaporate instantaneously, whereby the raw material metal powder is ultra-micronized and condensed and is caused to adhere to a collection surface in an upper part of the device.
In conventional manufacturing methods, because the heating space in which a raw material metal powder is evaporated, the cooling space in which the evaporated raw material metal powder is cooled and particulated, and the space in which the cooled raw material metal powder is collected, are formed as the same space, the following phenomena occur. That is, in the process of particulating the raw material metal powder evaporated in the cooling space by cooling the evaporated raw material metal powder, particles become mixed as liquid particles or solid particles in the heating space and the mixed liquid particles or solid particles aggregate together a raw material metal powder which has been newly evaporated in the heating space by serving as nuclei or chaining of particles and of necking occur, with the result that coarse particles are formed; also, in the cooling space, in the process of particulating the raw material metal powder evaporated in the cooling space by cooling the evaporated raw material metal powder, the evaporated raw material metal powder becomes mixed as liquid particles in the heating space and these liquid particles aggregate together and become chained, with the result that coarse particles are formed; and furthermore, the cooled raw material metal powder which is present in the collection space becomes mixed in the heating space in the same manner as described above, and the mixed solid raw material powder aggregate together a raw material metal powder which has been newly evaporated in the heating space by serving as nuclei or becomes chained, with this raw material metal powder which has been newly evaporated in the heating space, with the result that coarse particles are formed. Such behavior are hereinafter referred to as “reaggregation behavior”. Because of the occurrence of these behavior, the ratio in which coarse particles having particle diameters of not less than 20 nm are produced is very high, thereby posing a problem. Chaining of particles refers to the behavior that particles aggregate together simply, and necking is defined as the behavior that particles adhere.
In addition, regarding (B) described above, because liquid alkali metals of sodium or the like have high thermal conductivity and for other reasons, these liquid metals are named as major candidates for the coolant used in a fast breeder reactor (FBR).
On the other hand, however, liquid alkali metals of sodium or the like have high chemical activity and provide such properties that they can cause severe chemical reactions leading to explosions when they come into contact with air and water.
Accordingly, in an FBR, due to contact of liquid alkali metals of sodium or the like with water in an SG (steam generator), reactions between liquid alkali metals of sodium or the like and water as described above will occur and it is expected that nonconformity events (high-temperature rupture, the wastage behavior) occur. For this reason, measures are taken from structural viewpoints, for example, by using double-wall piping. However, this poses the problems that high-level technologies are required and that the construction cost is high.
Therefore, there has been made a proposition to disperse ultrafine particles (nanoparticles: particles whose particle diameters are on the order of nanometers) in a liquid alkali metal of sodium or the like, whereby the high chemical activity of the liquid alkali metal of sodium or the like is suppressed.
For example, Japanese Patent No. 3930495 discloses a method of adjusting a liquid alkali metal of sodium or the like in which nanoparticles as described above are dispersed. This Japanese Patent No. 3930495 describes a method which involves putting nanoparticles in liquid sodium and dispersing the nanoparticles while they are being stirred by use of a stirrer.
However, in the method of Japanese Patent No. 3930495 which involves putting nanoparticles in liquid sodium, which is a liquid alkali metal, and dispersing the nanoparticles while they are being stirred by use of a stirrer, it is possible that some kinds of particles reaggregate and settle.
That is, there are various kinds of nanoparticles to be input, and for example, in the case of a particle whose primary particle diameter is not more than several tens of nanometers, because of the small particle diameter, the proportion of the number of atoms on the surface to the number of atoms forming a nanoparticle increases and the surface energy increases, causing secondary aggregation, with the result that the dispersibility to liquid sodium worsens.
Also, in the case of nanoparticles whose primary particles are chained to generate the necking behavior, secondary aggregation occurs due to tangling and the like and the dispersibility to liquid sodium tends to worsen.
In handling such nanoparticles, simply putting nanoparticles into liquid sodium and stirring the nanoparticles posed the problem that many particles settle although part of the particles are dispersed, resulting in low dispersibility to liquid sodium.
The challenge that the present invention takes up is to obtain a nanoparticle-dispersed liquid alkali metal in which nanoparticles do not aggregate or settle in dispersing nanoparticles in a liquid alkali metal of sodium or the like and which maintains the dispersion of nanoparticles stably even after a lapse of time.

SUMMARY OF THE INVENTION

(A) Nanoparticle manufacturing device and method
To solve such problems as described above, the first aspect of the present invention provides a nanoparticle manufacturing device for manufacturing nanoparticles by heating and melting a mixture of a raw material metal powder and a carrier gas in a heating space, cooling the mixture in a cooling space and collecting the mixture in a collection space. In this nanoparticle manufacturing device, the heating space, the cooling space and the collection space form a continuous flow path without a back flow, and the cross-sectional area of the collection space is set at a large value compared to the cross- sectional area of the heating space and the cooling space.
In the second aspect of the present invention, the nanoparticle manufacturing device according to the first aspect is such that the heating and melting temperature of the heating space is maintained at a first temperature which is not less than the melting point of the raw material metal powder, the cooling space is maintained at a second temperature which is lower than the melting point of the raw material metal powder, and the collection space is maintained at a third temperature which is lower than the second temperature of the cooling space.
The third aspect of the present invention provides a nanoparticle manufacturing method for manufacturing nanoparticles by subjecting a mixture of a raw material metal powder and a carrier gas to a heating and melting treatment in a heating space, subjecting the mixture to a cooling treatment in a cooling space and subjecting the mixture to a collection treatment in a collection space. In this nanoparticle manufacturing method, the heating and melting treatment in the heating space, the cooling treatment in the cooling space and the collection treatment in a collection space form a continuous flow path without a back flow, and the cross-section area of the collection space is set at a large value compared to the cross-sectional area of the heating space and the cooling space, whereby nanoparticles are manufactured.
In the fourth aspect of the present invention, the nanoparticle manufacturing method according to the third aspect is such that the treatment temperature of the heating and melting treatment in the heating space is maintained at a first temperature which is not less than the melting point of the raw material metal powder, the temperature of the cooling treatment in the cooling space is maintained at a second temperature which is lower than the melting point of the raw material metal powder, and the temperature of the collection treatment in the collection space is maintained at a third temperature which is lower than the second temperature of the cooling space.
In conventional manufacturing methods, as described in the summary above, because of the occurrence of “re-reaggregation behavior”, the ratio in which coarse particles having particle diameters of not less than 20 nm are produced is very high, thereby posing a problem. According to the nanoparticle manufacturing device and nanoparticle manufacturing method of the present invention, because the heating space, the cooling space and the collection space form a continuous flow path without a back flow, and because the cross-sectional area of the collection space is set at a large value compared to the cross-sectional area of the heating space and the cooling space, it is possible to substantially reduce the possibility that the above-described “reaggregation behavior” occur and it becomes possible to perform the manufacture of nanoparticles composed of single metals having particle diameters of the order of 5 nm to 10 nm.
(B) Nanoparticle-dispersed liquid alkali metal
In addition to the above, to solve such problems as described above, the fifth aspect of the present invention provides a method of manufacturing a nanoparticle-dispersed liquid alkali metal by dispersing nanoparticles in a liquid alkali metal, which includes a rough dispersion step of stirring nanoparticles in the liquid alkali metal by a physical action and a dispersion step of dispersing nanoparticles in the liquid alkali metal by irradiating the liquid alkali metal with ultrasonic waves after the rough dispersion step.
In the sixth aspect of the present invention, in addition to the first aspect, the liquid alkali metal is any of lithium, sodium, potassium and an alloy whose main component is any of the metals.
In the seventh aspect of the present invention, in addition to any of the first and second aspect, the nanoparticles are made of any of the metals selected from the group consisting of titanium, vanadium, chromium, iron, cobalt, nickel and copper.
According to the method of manufacturing a nanoparticle-dispersed liquid alkali metal of the present invention, even by using nanoparticles aggregating together and nanoparticles having necking, it is possible to manufacture a nanoparticle-dispersed liquid alkali metal which has good dispersibility of nanoparticles and can maintain the state of dispersion even after a lapse of time.
As a result of this, by uniformly dispersing nanoparticles in a liquid alkali metal, it is possible to improve the suppression of the thermal conductivity and chemical activity (reactions to water and the like) of the liquid alkali metal. Furthermore, it is possible to stably maintain the fluidity and the like which liquid alkali metals essentially have.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing the schematic of a nanoparticle manufacturing device 1 in an embodiment of the present invention;

FIG. 2 is a diagram to explain the principle of nanoparticle manufacture performed by use of the nanoparticle manufacturing device 1 in the embodiment of the present invention;

FIGS. 3(A) and 3(B) are diagrams each showing an example of component of a raw material supply section 10 in the nanoparticle manufacturing device according to the embodiment of the present invention;

FIGS. 4(A) and 4(B) are diagrams each showing an example of component of the cooling space in the nanoparticle manufacturing device 1 according to the embodiment of the present invention;

FIGS. 5(A) and 5(B) are diagrams each showing an example of component of the cooling space in a nanoparticle manufacturing device 1 according to another embodiment of the present invention;

FIG. 6 is a diagram showing the schematic of a nanoparticle manufacturing device 1 in another embodiment of the present invention;

FIG. 7 is a diagram showing an example of a nanoparticle manufacturing device 1 in an embodiment of the present invention; and

FIGS. 8(A) to 8(C) are transmission electron microscope photographs of nanoparticles produced by a nanoparticle manufacturing device 1 in an embodiment of the present invention.

Further, FIG. 9 is a diagram showing a processing procedure of the method of manufacturing a nanoparticle-dispersed liquid alkali metal in an embodiment of the present invention;

FIG. 10 is a diagram showing how nanoparticles are dispersed in a liquid alkali metal by using an ultrasonic irradiation device in an embodiment of the present invention;

FIG. 11 is a graph showing static state characteristics of a nanoparticle-dispersed liquid alkali metal in an embodiment of the present invention;

FIG. 12 is a graph showing the relationship between the ultrasonic irradiation time by an ultrasonic irradiation device and dispersion characteristics in an embodiment of the present invention; and

FIG. 13 is a graph showing, the relationship between the bond energy and bias of charge in the elements for nanoparticles.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(A) Nanoparticle manufacturing device and method
Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a diagram showing the schematic of a nanoparticle manufacturing device in an embodiment of the present invention. In FIG. 1, reference numeral 1 denotes a nanoparticle manufacturing device, reference numeral 3 an input section, reference numeral 4 a main controller, reference numeral 5 a pump controller, reference numeral 6 a valve controller, reference numeral 7 a carrier gas regulation valve, reference numeral 8 a flow meter, reference numeral 9 a carrier gas induction pipe, reference numeral 10 a raw material supply section, reference numeral 15 a raw material supply conduit, reference numeral 20 a heating section, reference numeral 21 a first induction pipe section, reference numeral 22 a second induction pipe section, reference numeral 41 a third induction pipe section, reference numeral 23 a protective layer section, reference numeral 24 a heat source section, reference numeral 40 a cooling section, reference numeral 50 a collection section, reference numeral 51 an outer circumferential portion, reference numeral 52 an inner circumferential portion, reference numeral 53 an inner wall surface, reference numeral 55 an exhaust pipe, reference numeral 58 a pressure gauge, reference numeral 59 a vacuum pump, reference numeral 60 a coolant path, reference numeral 61 a coolant induction pipe, and reference numeral 62 denotes a coolant discharge pipe.
The nanoparticle manufacturing device 1 is roughly provided with the raw material supply section 10 which supplies a raw material metal powder along with a carrier gas to the treatment process, the heating section 20 which causes the raw material metal powder supplied by this raw material supply section 10 to pass and vaporizes the raw material metal powder, the cooling section 40 which causes the raw material metal powder vaporized in the heating section 20 to pass and cools the raw material metal powder, and the collection section 50 which collects the raw material metal powder which has passed through the cooling section 40.
The raw material supply section 10 causes a raw material metal powder to be carried by an inert gas, such as Ar, He and N₂, which is inducted from the carrier gas induction pipe 9, and supplies the raw material metal powder to the heating process, which is a first treatment step. Raw material metal powders from which nanoparticles are capable of being manufactured by the nanoparticle manufacturing device 1 of the present invention, are single metals, alloys and intermetallic compounds, and the raw material supply section 10 is configured to be able to cause a raw material metal powder made of any one of single metals, alloys and intermetallic compounds to fall in minute amounts. In the nanoparticle manufacturing device 1 according to an embodiment of the present invention, the average particle diameter of a raw material metal powder supplied by the raw material supply section 10 is preferably not more than 500 μm, more preferably not more than 100 μm.
The construction of the interior of the raw material supply section 10 will be described here. FIGS. 3(A) and 3(B) are diagrams each showing an example of component of the raw material supply section 10 in the nanoparticle manufacturing device according to the embodiment of the present invention. FIG. 3(A) shows a schematic component of the raw material supply section 10 for which the low-frequency type feed method is adopted, and FIG. 3(B) shows a schematic component of the raw material supply section 10 for which the hammering type feed method is adopted.
In the low-frequency type feed method of FIG. 3(A), a hopper 11 which is provided with a nozzle 12 and has almost the shape of a syringe is connected to a vibrator 13. This vibrator 13 performs low-frequency vibrations by use of a piezoelectric element. The tip portion of the nozzle 12 is adapted to be inserted into the raw material supply conduit 15. In the raw material supply section 10 having such a construction, when a raw material metal powder is charged into the hopper 11 and the hopper 11 is vibrated by low-frequency vibrations generated in the vibrator 13, it is ensured that as a result of this, the raw material metal powder is put from the nozzle 12 into the raw material supply conduit 15. It is possible to realize very stable, good raw material supply due to the raw material supply section 10 in which the low-frequency type feed method is adopted.
In the hammer type feed method of FIG. 3(B), a raw material metal powder is charged into a slit hopper 16 provided with slits which are not shown in the drawing and the slit hopper 16 is periodically hammered by use of a stainless steel hammer rod 17, and it is ensured that the raw material metal powder is supplied to the raw material supply conduit 15 from the slits of the slit hopper 16 due to the impact of the hammer. Also by this hammer type feed method, it is possible to realize good raw material supply.
Between a carrier gas source (not shown in the drawing) and the raw material supply section 10 in the carrier gas induction pipe 9, there are provided the carrier gas regulation valve 7 which regulates the flow rate of a carrier gas supplied to the raw material supply section 10 and the flow meter 8 which measures the flow rate of a carrier gas supplied to the raw material supply section 10.
The opening of the carrier gas regulating valve 7 is controlled by the valve controller 6 and it is ensured that the flow rate of a carrier gas supplied to the raw material supply section 10 can be regulated. Also, it is ensured that the valve controller 6 controls the opening of the carrier gas regulating valve 7 under instructions from the main controller 4 at a level higher than the valve controller 6. Furthermore, it is ensured that the flow rate data obtained by the flow meter 8 is sent to the higher-level main controller 4.
The raw material supply conduit 15 is provided so as to pass a carrier gas and a raw material metal powder vertically downward from the raw material supply section 10, and the raw material metal powder falling from this raw material supply conduit 15 is guided to a pipe-shaped portion through the inner circumferential portion 52, which is composed of the first induction pipe section 21, the second induction pipe section 22 and the third induction pipe section 41.
The heat source section 24 is arranged around the second induction pipe section 22 in this pipe-shaped portion, and it is ensured that the temperature of the space in the second induction pipe section 22 is maintained at the first temperature of not less than the melting point of the raw material metal powder by heating this heat source section 24. That is, the space in the second induction pipe section 22 functions as a heating space X in which the raw material metal powder is vaporized when the raw material metal powder falling from the raw material supply conduit 15 passes through this space.
In the heat source section 24, it is possible to select and use an appropriate heating type from various heating types, such as the resistance heating type, such as a carbon heater and a tungsten heater, (capable of heating to hundreds of degrees Celsius to 2000° C.), the plasma heating type (capable of heating to thousands of degrees Celsius to tens of thousands of degrees Celsius) and the induction heating type (capable of heating to hundreds of degrees Celsius to 1500° C.). In any of the heating types, it is possible to configure a stable heat source section 24 by providing an insulating or heat insulating structure between the heat source section 24 and a heat source section holding structure (not clearly specified).
The protective layer section 23 made of a ceramics material having good corrosion resistance (for example, P-BN) is provided on the inner surface of the second induction pipe section 22, and this protective layer section 23 has the function of protecting the surface layer on the inner surface of the second induction pipe section 22 so that the inner surface of the second induction pipe section 22 is not melted and vaporized by heating by the heat source section 24 and does not become mixed with the raw material metal powder. It is preferred that thermocouples (not shown in the drawing) for temperature control be attached to the inner surface of the second induction pipe section 22.
The temperature of the space on the inner side of the third induction pipe section 41 in the pipe-shaped portion is maintained at the second temperature lower than the melting point of the raw material metal powder, and this space functions as a cooling space Y which cools the raw material metal powder vaporized in the heating space X within the second induction pipe section 22 by causing this raw material metal powder to pass. In the nanoparticle manufacturing device 1 of the present invention, because the layout is such that the third induction pipe section 41 constituting the cooling space Y is positioned vertically under the second induction pipe section 22 constituting the heating space X, a raw material metal powder evaporated on the evaporation surface does not cause “reaggregation behavior” due to stagnation and the like for the duration in which the evaporated raw material gas reaches the collection surface after passing through the cooling space, which occurred in conventional manufacturing methods. For this reason, it is possible to reduce the proportion in which coarse particles having particle diameters of not less than 20 nm are produced.
Furthermore, it is preferred that the heating space X and the cooling space Y have almost the same cross-sectional area and cross-sectional shape.
It is possible to adopt various arrangements for maintaining the temperature of the third induction pipe section 41 at the second temperature lower than the melting point of a raw material metal powder.
FIGS. 4(A) and 4(B) are diagrams each showing an example of component of the cooling space Y in the nanoparticle manufacturing device according to the embodiment of the present invention. In the third induction pipe section 41 constituting the cooling space shown in FIG. 4(A), there are provided a coolant induction pipe 42 which causes a coolant to flow into a coolant path 44 and a coolant discharge pipe 43 which discharges the coolant from the coolant path 44, and it is ensured that the temperature of the cooling space Y is maintained stably by causing water or the like to flow by using this component.
FIG. 4(B) shows that it is ensured that the temperature gradient in the above-described cooling space is positively formed. By the arrangement of the coolant path 44 or by setting the kind of a coolant which is caused to flow through the coolant path 44, it is possible to produce various kinds of temperature gradients, such as the temperature gradient (a) or the temperature gradient (b), in the cooling space. In the nanoparticle manufacturing device 1 according to the embodiment of the present invention, it is possible to perform the control of the properties of nanoparticles in various ways by positively forming such temperature gradients in the cooling space.
Furthermore, it is also possible to adopt methods of natural cooling, cooling methods achieved by providing radiation fins on the outer circumference of the third induction pipe section 41, and the like.
This time, the present inventors were able to obtain the knowledge that the passing speed at which a raw material metal powder vaporized in the heating space X passes through the cooling space Y in the third induction pipe 41, has a great effect on the particle diameter control of nanoparticles manufactured, the control of the particle diameter distribution width, and the control of the occurrence condition of chaining of nanoparticles and of necking. Therefore, in the nanoparticle manufacturing device 1 of the present invention, control means which controls the speed at which a raw material metal powder passes through the cooling space Y is provided, whereby the properties of nanoparticles made of a raw material metal powder collected in the collection section 50 are controlled.
In the nanoparticle manufacturing device, the cross-sectional area of the collection space Z is drastically as large as not less than 5 times the cross-sectional area of the heating space X and cooling space Y and, therefore, when a raw material metal powder enters the collection space Z from the cooling space Y, the flow velocity of the raw material metal powder decreases abruptly, whereby it is ensured that a back flow of the raw material metal powder does not occur between the heating space X and the collection space Z.
The collection section 50 through the third induction pipe section 41 is provided vertically under the third induction pipe section 41. The temperature of this collection section 50 is maintained at the third temperature lower than the above-described second temperature in the cooling space Y, and the collection section 50 collects the raw material metal powder which has passed through the cooling space Y as nanoparticles. The collection section 50 has a double structure composed of the outer circumferential portion 51 in contact with the atmosphere side and the inner circumferential portion 52 which surrounds the space for which the atmospheric pressure is controlled, and it is ensured that nanoparticles which have passed through the third induction pipe section 41 (the air cooling space Y) are collected. The space surrounded by the inner circumferential portion 52 is referred to as the collection space Z.
The coolant path 60 is formed between the outer circumferential portion 51 and the inner circumferential portion 52 in the collection section 50. The collection section 50 is provided with the coolant induction pipe 61 which causes a coolant to flow into this coolant path 60 and the coolant discharge pipe 62 which causes the coolant to flow out of the coolant path 60. Examples of coolants caused to flow into such a coolant path 60 formed in the collection section 50 include temperature-controlled water and liquid nitrogen. It is ensured that the temperature of the inner wall surface 53 of the inner circumferential portion 52 is maintained by a coolant flowing through the coolant path 60 between the inner circumferential portion 52 and the inner wall surface 53.
The exhaust pipe 55 is arranged from the collection space Z within the inner circumferential portion 52 in the collection section 50, and it is ensured that the gas in the inner circumferential portion 52 can be exhausted by use of the vacuum pump 59. Although any type can be used as the vacuum pump 59, it is possible to use, for example, a vacuum pump system arrangement composed of a rotary pump and a turbo-molecular pump.
In the nanoparticle manufacturing device 1 according to this embodiment, the above-described component of the collection section 50 was adopted. However, as a method of collecting nanoparticles, it is also possible to adopt a method in which a bag filter is used, the inertia force method, the electric precipitation method and the like.
In the nanoparticle manufacturing device 1 according to the present invention, it is ensured that through the use of such a vacuum pump 59, the degree of vacuum at which a raw material metal powder is vaporized in the heating space X is maintained. That is, when the raw material metal powder which has fallen from the raw material supply conduit 15 passes through the heating space X (the space in the second induction pipe section 22), the raw material metal powder comes to not only a liquid state, but also a further advanced gas state.
The vacuum pump 59 which performs evacuation from the inner circumferential portion 52 in the device can control the air volume displacement under instructions from the pump controller 5. The main controller 4 issues instructions to the pump controller 5, and the pump controller 5 receives the instructions and regulates the air volume displacement of the vacuum pump 59. The pressure in the device is measured by the pressure gauge 58 and measured values of the pressure are sent to the high-level main controller 4.
The main controller 4 is an all-purpose information processing mechanism composed of a microcomputer, a ROM which holds programs working on this microcomputer, a RAM which is a work area of the microcomputer, and the like, and can perform prescribed data processing. The main controller 4 has the input section 3 from which instructions from the user of the device can be inputted, and it is ensured that the speed at which a raw material metal powder passes through the cooling space Y can be specified from this input section 3.
When the passing speed of a raw material metal powder in the cooling space Y has been indicated from the input section 3, the main controller 4 performs computations on the basis of the flow rate data obtained from the flow meter 8 and the data on the pressure in the inner circumferential portion 52 obtained from the pressure gauge 58, and controls the valve controller 6 and the pump controller 5 so that the target passing speed inputted from the input section 3 is obtained. That is, the main controller 4 issues instructions to the valve controller 6 to regulate the opening of the carrier gas regulating valve 7 so that the passing speed of the raw material metal powder in the cooling space Y obtains the target value indicated from the input section 3 and at the same time, the main controller 4 issues instructions also to the pump controller 5 to regulate the air volume displacement of the vacuum pump 59.
A description will be given of the manufacturing procedure of metal nanoparticles by the nanoparticle manufacturing device 1 of the present invention, which is configured as described above. First, a raw material metal powder is charged into the raw material supply section 10. It is necessary that the volume of the raw material metal powder be reduced and the heat-receiving surface area of the raw material metal powder be increased in the heating space X (the space in the second conduction pipe section 22) in order to cause instantaneous evaporation. Concretely, it is preferred that the particle diameter be reduced to not more than 500 μm, particularly to the order of 100 μm.
Cooling water is supplied to the nanoparticle collection section 50 from the coolant induction pipe 61, and the cooling water circulates in the interior and is discharged from the coolant discharge pipe 62. This enables the temperature of the inner wall surface 53 of the inner circumferential portion 52 to be maintained at low temperatures during operation. The interior of the inner circumferential portion 52 is evacuated by use of the vacuum pump 59, thereafter an inert gas (usually, argon gas) is introduced from the carrier gas induction pipe 9 while performing evacuation, and the interior of the inner circumferential portion 52 is replaced with the inert gas so that the atmospheric pressure is set at an as-manufactured prescribed pressure (for example, 3 torr). The flow of the carrier gas is such that at this point in time, the carrier gas is introduced from the carrier gas induction pipe 9 in an upper part of the device and is exhausted from the exhaust pipe 55.
When the inner pressure of the inner circumferential portion 52 has became stable, the power of the heat source 24 is turned on and the space in the second induction pipe section 22 is heated. When the temperature of the space in the second induction pipe section 22 has reached a set temperature, the raw material metal powder is caused to fall in minute amounts from the raw material supply section 10. The raw material metal powder is caused to fall continuously or intermittently into the heating space X within the second conduction pipe section 22.
Hereinafter, a description will be given as to how the raw material metal powder which has fallen changes into nanoparticles after passing through the heating space X, the cooling space Y and the collection space Z. FIG. 2 is a diagram to explain the principle of nanoparticle manufacture performed by use of the nanoparticle manufacturing device 1 in the embodiment of the present invention, and schematically shows the manufacturing process of nanoparticles. In FIG. 2, the stages (1) to (4) schematically show the condition of nanoparticles according to positions within the device. In the following, the stages (1) to (4) will be described.
(1) When a raw material metal powder passes through the heating space X in the second induction pipe section 22 while falling, the raw material metal powder is evaporated instantaneously.
(2) After passing through the heating space X, the vaporized raw material metal powder becomes condensed gradually and nuclei are formed.
(3) In the cooling space Y within the third conduction pipe section 41, the nuclei formed in (2) above gather together and grow into particles.
(4) The grown nanoparticles move to the collection space Z, float in the inert gas, and are collected on the inner wall surface 53 of the collection space Z of the collection section 50. The nanoparticles adhere to the inner wall surface 53 as loose particles and become condensed without aggregating together.
The phenomenon of thermophoresis has a great effect on the collection of nanoparticles in the collection space Z of the collection section 50 of (4) above. When particles are floating in a gas, the molecules of the gas constantly collide against the particles due to the thermal motions thereof. When there is a large temperature gradient in the gas, the momentum of the gas molecules is larger on the high-temperature side of the particles than on the low-temperature side thereof, with the result that the particles receive a force which moves toward the low-temperature side. This force is called a thermophoretic force and the phenomenon that the particles move toward the low-temperature side due to this force is called thermophoresis. Fine particles in a high-temperature gas move toward a low-temperature solid wall and the like due to this phenomenon and usually adhere to the wall. For example, the phenomenon that soot adheres to an inner wall of a stack is substantially caused by the thermophoresis phenomenon.
When the supply of the raw material metal powder is finished and after the above-described series of stages (1) to (4) of the nanoparticle manufacturing process are finished, the temperature of the heat source section 24 is lowered. When the temperature has been lowered to room temperature, the internal pressure of the inner circumferential portion 52 of the collection section 50 is returned to a normal pressure (an atmospheric gas is introduced). When the pressure of the interior of the inner circumferential portion 52 has returned to a normal pressure, the device is disassembled and the nanoparticles adhering to the inner wall surface 53 are collected.
According to the nanoparticle manufacturing device and nanoparticle manufacturing method described above, there is provided the control means which controls the speed at which the raw material metal powder vaporized in the heating space within the second induction pipe section 22 passes through the cooling space Y within the third induction pipe section 41, and the cooling of the raw material metal powder appropriately vaporized by this control means is controlled. Therefore, it becomes possible to perform the particle diameter control of nanoparticles manufactured from a raw material metal powder, the control of the particle diameter distribution width, and the control of the occurrence condition of tying together of nanoparticles in a row and of necking, and hence it becomes possible to manufacture nanoparticles made of single metals which have particle diameters of the order of 5 nm to 10 nm.
Next, a description will be given of another embodiment which is such that in the nanoparticle manufacturing device 1 of the present invention, the passing speed of a raw material metal powder in the cooling space Y is regulated. FIGS. 5(A) and 5(B) are diagrams each showing an example of component of the cooling space Y in a nanoparticle manufacturing device 1 according to another embodiment of the present invention. FIG. 5(A) shows an example in which the third induction pipe section 41 constituting the cooling space Y has an inside diameter r₀. FIG. 5(B) shows an example in which the third induction pipe section 41 constituting the cooling space Y has an inside diameter r₁, which is different from the inside diameter r₀. As one method for regulating the passing speed of a raw material metal powder in the cooling space Y, the configuration of this embodiment is such that the third induction pipe section 41 shown in FIG. 5(A) and the third induction pipe section 41 shown in FIG. 5(B) are exchangeable with each other.
When the volume of a carrier gas supplied from the carrier gas induction pipe 9 is constant and the volume of a gas exhausted from the vacuum pump 59 is constant, it becomes possible to change the third induction pipe section 41 as shown in FIGS. 5(A) and 5(B). In this embodiment, the third induction pipe section 41 is configured to be exchangeable, whereby a desired passing speed of a raw material metal powder in the cooling space Y is obtained and hence this makes it possible to perform the particle diameter control of nanoparticles manufactured, the control of the particle diameter distribution width, and the control of the occurrence condition of chaining of nanoparticles and of necking.
Next, another embodiment of the present invention will be described. FIG. 6 is a diagram showing the schematic of a nanoparticle manufacturing device 1 in another embodiment of the present invention. The embodiment shown in FIG. 6 differs from the embodiment shown in FIG. 1 in the point that a classification device 80 is provided in part of the collection section 50. In this embodiment, collected nanoparticles are classified by the classification device 80. In the classification device 80, classification data as to particles of what particle diameter are present to what degree is obtained and the data is sent to a higher main controller 4. Upon receiving the classification data, the main controller 4 controls the valve controller 6 and the pump controller 5 so as to obtain a desired particle diameter distribution and causes the passing speed of the raw material metal powder in the cooling space Y to be changed.
For the classification device 80, any method can be adopted so long as the device can measure the distribution of particles in a short time. It is possible to use “the particle distribution measuring device” of Japanese Patent No. 4204045 and the like.

Embodiments

FIG. 7 shows another embodiment in which the embodiment shown in FIG. 1 is further developed on the basis of the concept that the vaporized raw material metal powder is caused to pass at a prescribed speed between the heating space X and the cooling space Y. Components bearing the same symbols as in the devices described above are the same components as used in these devices. What is different from the above-described devices is that the heat source section 24 is provided on the bottom surface side of the device and that the heating space is formed in a region just above the heat source section 24. The space shifted sideways from the heat source section 24 functions as the cooling space Y.
A forward end portion 18 of the raw material supply conduit 15 is provided with an angle that permits the supply of a raw material metal powder in the direction in which the raw material metal powder passes across the heating space X and cooling space Y formed as described above. The raw material metal powder to be carried by a carrier gas, which is supplied from the above-described forward end portion 18 of the raw material supply conduit 15 flows in a trajectory roughly indicated by a dotted line and passes across the heating space X and the cooling space Y. By passing across the cooling space Y, nanoparticles formed from the raw material metal powder are collected on the inner wall surface 53 on the left side of the collection section 50.
A shielding plate 19 is provided in order to prevent a raw material metal powder which has passed through the heating space X from entering again the heating space X and the cooling space Y. Therefore, the layout of the arrangement of the nanoparticle manufacturing device 1 of the present embodiment shown in FIG. 7 prevents the occurrence of the phenomenon that a raw material metal powder evaporated on the evaporation surface is re-vaporized after passing through the cooling space Y before reaching the collection surface, as in conventional manufacturing methods. For this reason, according to the nanoparticle manufacturing device 1 of the present embodiment, it is possible to reduce the proportion in which coarse particles having particle diameters of not less than 20 nm are produced.
In the nanoparticle manufacturing device 1 configured as described above, a Ti powder having particle diameters not more than 45 μm and an average particle diameter of 20 μm (a titanium powder made by Kojundo Chemical Laboratory Co., Ltd. (99.9%)) was used as the raw material metal powder. Argon gas was used as the carrier gas, and the pressure of the inner circumferential portion 52 was regulated to 3 torr. A carbon heater (resistance heating) was used in the heat source section 24, and a current was caused to flow so that the temperature of the heat source section 24 became 2000° C.
The low-frequency feed method was adopted in the raw material supply section 10, and the Ti powder was supplied from this raw material supply section 10 at a rate of 3 g/hour.
After a prescribed amount of the raw material Ti powder was caused to fall, the heating of the heat source section 24 was finished, the temperature of the heat source section 24 was caused to drop to room temperature and the collection section 50 was opened. After the opening, nanoparticles were collected from the inner surface wall 53.
Nanoparticles were produced at three different speeds V₀, 2V₀and 4V₀at which the raw material metal powder passes through the cooling space Y, which were obtained by regulating the supply volume of argon gas through the use of the carrier gas regulating valve 7 and regulating the exhaust volume through the use of the vacuum pump 59. The nanoparticles thus produced were observed under a transmission electron microscope. FIGS. 8(A) to 8(C) are transmission electron micrographs of the nanoparticles. FIG. 8(A) is a micrograph of nanoparticles produced at the cooling space passing speed of V₀, FIG. 8(B) is a micrograph of nanoparticles produced at the cooling space passing speed of 2V₀, and FIG. 8(C) is a micrograph of nanoparticles produced at the cooling space passing speed of 4V₀.
Vo denotes “the flow velocity of a carrier gas at which a raw material metal powder is appropriately evaporated in the heating space,” and is appropriately fixed beforehand by experiments and the like for each raw material metal powder.
As shown in FIGS. 8(A) to 8(C), it is apparent that the higher the cooling space passing speed, the larger the particle diameter of Ti nanoparticles and that chaining of Ti nanoparticles and of necking increase accordingly. For the nanoparticles generated at the cooling space passing speed of V₀, it was able to produce nanoparticles having exceedingly small particle diameters of the order of 5 to 10 nm.
In summary, in conventional manufacturing methods, the configuration is such that a raw material metal powder evaporated on the evaporation surface is re-vaporized after passing through the cooling space before reaching the collection surface, whereas according to the nanoparticle manufacturing device and nanoparticle manufacturing method of the present invention, there is provided the control means which controls the speed at which a raw material metal powder vaporized in the heating space passes through the cooling space and the cooling of the appropriately vaporized raw material metal powder is controlled by this control means. Therefore, it becomes possible to perform the particle diameter control of nanoparticles manufactured from a raw material metal powder, the control of the particle diameter distribution width, and the control of the occurrence condition of chaining of nanoparticles and of necking, and hence it becomes possible to manufacture nanoparticles made of single metals which have particle diameters of the order of 5 nm to 10 nm.
The above description was given on the precondition that control is performed by the main controller 4. However, it is possible to perform nanoparticle manufacture without using the main controller 4.
In FIG. 1, for the raw material metal powder from which nanoparticles are to be manufactured, the regulation quantity of the carrier gas flow rate in the carrier gas regulating valve 7, the raw material supply amount in the raw material supply section 10, the heating temperature in the heat source section in of the heating section 20, the regulation quantity of the cooling temperature of the cooling section 40, and the cooling temperature and degree of vacuum in the collection section 50 are set beforehand and the nanoparticle manufacturing device is operated under these set conditions, whereby it is possible to realize the manufacture of desired nanoparticles without using the main controller 4.
The above description was given of the case where a Ti powder is used as a raw material metal powder. The present invention is intended for manufacturing nanoparticles by melting, vaporizing and cooling a raw material metal powder, and not intended for manufacturing nanoparticles by the oxidation, reduction and other chemical actions of a raw material metal powder. Therefore, it is possible to manufacture nanoparticles by using, as raw materials, 3d elements of the periodic table of elements (Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Ge, As, Se, Br, Kr),4d elements (Sr, Y, Zr, Nb, Mo, Tc, Ru, Rh, Pd, Ag, Cd, In, Sn, Sb, Te, I, Xe), 5d elements (Ba, Hf, Ta, W, Re, Os, Ir, Pt, Au), and the like. Preferably, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni and Cu, which are the 3d elements of the periodic table of elements, Sr, Y, Zr, Nb, Mo, Tc, Ru, Rh, Pd and Ag, which are 4d elements, and Ba, Hf, Ta, W, Re, Os, Ir, Pt and Au, which are 5d elements are used, and more preferably, Ti, Cr, Fe, Co, Ni and Cu, which are 3d elements, Zr, Nb, Mo, Pd and Ag, which are 4d elements, and Pt and Au, which are 5d elements, are used.
(B) Nanoparticle-dispersed liquid alkali metal
In addition to the above, the method of manufacturing a nanoparticle-dispersed liquid alkali metal of the present invention includes a rough dispersion step of stirring nanoparticles in the liquid alkali metal by a physical effect and a dispersion step of dispersing nanoparticles in the liquid alkali metal by irradiating the liquid alkali metal with ultrasonic waves after the rough dispersion step. The liquid alkali metals to be dispersed with nanoparticles are lithium, sodium, potassium and an alloy whose main component is any of the metals, and the nanoparticles are made of any of the metals selected from the group consisting of titanium, vanadium, chromium, iron, cobalt, nickel and copper.
Unlike water and the like, liquid alkali metals have a very large surface tension. Therefore, if nanoparticles are dispersed by general stirring and the like, nanoparticles gather together only on a surface layer of a liquid alkali metal and the dispersion of nanoparticles into the liquid alkali metal is difficult. Therefore, the dispersion of nanoparticles to the liquid alkali metal is ensured by the two stages consisting of the rough dispersion step by physical stirring and the dispersion step by ultrasonic irradiation.
The method of manufacturing a nanoparticle-dispersed liquid alkali metal will be described in detail on the basis of FIG. 1.
First, in Step S1, nanoparticles to be dispersed in a liquid alkali metal are uniformly dispersed, and furthermore, the nanoparticles are classified into prescribed particle diameters and nanoparticles of the desired particle diameters are used in order to stabilize the thermal conductivity and chemical activity of the liquid alkali metal after the dispersion of nanoparticles. The particle diameters of nanoparticles to be dispersed are in the range of several nanometers to hundreds of nanometers.
The classification method is appropriately carried out by well-known techniques described in Japanese Patent No. 3506947 “ULTRAFINE PARTICLE CLASSIFICATION DEVICE”, Japanese Patent Laid-Open No. 2007-64893 “ELECTRIC MOVING CLASSIFICATION DEVICE AND MEASURING SYSTEM OF MICROPARTICLE COMPONENTS” and the like.
In Step S2, the nanoparticles classified in Step S1 are rough-dispersed in a liquid alkali metal through the use of a stirring blade and the like.
The surface tension of a liquid alkali metal is very large compared to water and the like. Therefore, if nanoparticles are simply put into a liquid alkali metal, the nanoparticles float only on the surface and the dispersion into the liquid alkali metal is difficult. For this reason, the nanoparticles are forcedly dispersed in the liquid alkali metal by use of a physical force by the rotation of a stirring blade. In this stage, in the liquid alkali metal, the nanoparticles are spreading while bunching up together and the state of dispersion is not reached as yet.
In Step S3, the liquid alkali metal including the nanoparticles in Step S2 is irradiated with ultrasonic waves while being circulated in a prescribed vessel, whereby the nanoparticles in the liquid alkali metal are uniformly dispersed throughout the whole liquid alkali metal. The ultrasonic irradiation intensity, the ultrasonic irradiation time and the like are appropriately fixed according to the kind of the liquid alkali metal and the material for the nanoparticles.
By performing the above-described processing, it is possible to disperse the nanoparticles uniformly throughout the whole liquid alkali metal.

Embodiments

For the method of manufacturing a nanoparticle-dispersed liquid alkali metal of the present invention, manufacturing method for the case where the liquid alkali metal is liquid sodium and the nanoparticles are made of titanium will be described in more detail as embodiments.
FIG. 10 is a diagram showing an outline of the method for manufacturing a nanoparticle-dispersed liquid alkali metal in an embodiment of the present invention.
In FIG. 10, liquid sodium and titanium nanoparticles to be dispersed are put in the interior of a vessel 103 installed in a heater 104. The step of dispersing titanium nanoparticles in the liquid sodium is carried out in an inert gas atmosphere of a glove box 105. The liquid sodium in the vessel 103 was heated by use of the heater 104 and ultrasonic irradiation by an ultrasonic irradiation device 101 was carried out, with the temperature maintained at 350° C. to 500° C.
A homogenizer tip 102 of an ultrasonic irradiation device 101 is inserted into the liquid sodium in the vessel 103 and is set so as to be able to irradiate the liquid sodium in the vessel 103 with ultrasonic waves. Examples of the ultrasonic irradiation device 101 include, for example, UP-400S made by Hielscher Ultrasonics GmbH (oscillation frequency 24 kHz, rated output 400 W) and US-300 made by Nippon Seiki Co., Ltd. (oscillation frequency 20 kHz, rated output 300 W). In this embodiment, the former ultrasonic irradiation device was used.
Titanium nanoparticles having necking although the particle diameter of a single particle (hereinafter referred to as the primary particle diameter) is as small as 5 to 10 nm, were used as the titanium nanoparticles, and an ultrasonic irradiation experiment was conducted in liquid sodium.
For the stable dispersion of the nanoparticles by the ultrasonic irradiation method adopted in the manufacturing method of the present invention, the mechanism described in (a) to (e) below is assumed.
(a) When a strong ultrasonic wave is radiated into a liquid, pressurization and depressurization occur in the liquid, because an ultrasonic wave is a compressional wave (a longitudinal wave which is such that the medium vibrates in the same direction as the traveling direction of the wave).
(b) The liquid is torn by this depressurization and cavities are formed. This phenomenon is called cavitation.
(c) At this time, bubbles composed of the vapor of the liquid and the gas dissolved in the liquid are formed. The bubbles vibrate while pressurization and depressurization are repeated by the ultrasonic wave.
(d) However, the bubbles may sometimes get crushed during the vibration and on that occasion, a shock wave is generated or in the vicinity of the wall, a micro jet is generated, giving a local large force to the liquid.
(e) When this shock wave acts on the nanoparticles in the liquid, nanoparticles aggregating together not strongly are easily unloosened and also nanoparticles aggregating together strongly become gradually dispersed.
The space in which shock waves effective in dispersing nanoparticles act is called the ultrasonic irradiation region. The time in which nanoparticles are present in this ultrasonic irradiation region is called the residence time.

Embodiment 1 for the Liquid Alkali Metal

As Embodiment 1 for the liquid alkali metal, a description will be given of the case where the liquid alkali metal is liquid sodium and the nanoparticles are made of titanium.
In this embodiment, the inventors adopted a method in which a slurry flows only by a propulsive force by ultrasonic waves and the slurry flows efficiently into the ultrasonic irradiation region (the forward end portion of the homogenizer tip 102; a tip 102 having a diameter of 3 mm was adopted here), and they ascertained that the titanium nanoparticles tend to decrease in size.
Concretely, the particle diameters were measured by use of the particle diameter measuring device LB-550 made by HORIBA, Ltd. and it was ascertained that particle diameters as median size (D50: based on the number of particles), which were several micrometers only by stirring before the ultrasonic irradiation processing, decreased to 109 nm after a lapse of 60 minutes.
In order to stably disperse nanoparticles in liquid sodium, it is necessary to loosen secondary aggregation nanoparticles as far as possible and to reduce the particle diameters to average particle diameters of the order of 100 nm, preferably to the sizes of primary particles (several nanometers to tens of nanometers) as far as possible.
For this reason, in the manufacturing method of the present invention, it is ensured that by radiating ultrasonic waves into a solvent including nanoparticles, the nanoparticles are dispersed by the shock waves generated by the ultrasonic waves, whereby the state in which the nanoparticles are stably dispersed for a longer time in the liquid alkali metal is generated.
In the conventional method described in Japanese Patent No. 3930495 by which nanoparticles are dispersed in liquid sodium, because the nanoparticles whose primary particle diameters are not more than tens of nanometers easily undergo secondary aggregation due to the van der Waals force, it is possible that secondary aggregation has already occurred in the stage where the nanoparticles are put in the liquid sodium and, therefore, it was difficult to cause the secondary aggregating nanoparticles to get loosened only by the force by stirring after the nanoparticles are put in the liquid sodium. In the case of particles having necking, it is difficult to cut thick necking portions only by stirring, with the result that the particles settle easily in liquid sodium.
That is, although the stirring operation is effective means in dispersing particles which do not aggregate and particles having no necking, and is effective in improving the wettability between nanoparticles and liquid sodium and in causing nanoparticles to sink into liquid sodium, the stirring operation has not become effective means in stably dispersing particles aggregating together and particles having necking.
In contrast to this, in the manufacturing method of the present invention, even in the case of such nanoparticles aggregating together and nanoparticles having necking, it becomes possible to manufacture a nanoparticle-dispersed liquid alkali metal which has good dispersibility of nanoparticles and can maintain the state of dispersion even after a lapse of a time.

Embodiment 2 for the Liquid Alkali Metal

A dispersion test in liquid sodium was carried out using a sample prepared by the manufacturing method of the present invention which is such that the liquid alkali metal of Embodiment 1 is liquid sodium and the nanoparticles are made of titanium.
The dispersibility of titanium nanoparticles was evaluated by a method which involves causing a sample subjected to ultrasonic irradiation processing to stand. Concretely, a sample irradiated with ultrasonic waves was caused to stand, with the temperature of liquid sodium maintained at a liquid temperature of 350° C., and the titanium concentration remaining in a supernatant liquid of the sample was measured by use of ICP (inductively coupled plasma) (the high-frequency plasma emission spectrometer ICPS-8100 made by Shimazu Corporation) immediately after the ultrasonic irradiation processing, after a lapse of 24 hours and after a lapse of 96 hours. FIG. 11 is a graph showing the results of a standing test of nanoparticle-dispersed liquid sodium produced by the manufacturing method in an embodiment of the present invention. Even if time elapses, the dispersibility is good when the titanium concentration in the supernatant liquid is high.
The dispersion processing by ultrasonic irradiation in liquid sodium was performed and dispersibility was verified by causing a sample to stand, which provides the severest conditions on an FBR plant, and it was ascertained that even the sample is caused to stand for 100 hours or so, not less than 80% of titanium nanoparticles remain in the supernatant liquid. That is, it was ascertained that the manufacturing method of the present invention can be applied even to particles having necking although their primary particle diameters are as small as 5 to 10 nm.

Embodiment 3 for the Liquid Alkali Metal

Next, an ultrasonic irradiation test was conducted in a case where the resistance time of ultrasonic waves in liquid sodium is changed. FIG. 12 is a graph showing the results of the dispersibility test in which the dispersibility of nanoparticles of a nanoparticle-dispersed liquid sodium produced by the manufacturing method in an embodiment of the present invention changes as a result of changes in the residence time of ultrasonic waves.
It was ascertained that when the resistance time of ultrasonic waves is changed in liquid sodium, the particles aggregate and the dispersibility becomes worsen when the residence time is too long. In order to improve the dispersibility, it is preferred that the residence time be 5 to 20 seconds when the ultrasonic output density is 460 W/cm². On the other hand, the output density may take on small values of the order of several to several tens of watts per cm². In this case, because the surface of the ultrasonic oscillator is large, the dispersibility is good if the resistance time is several tens of seconds. Because of the small output density, the progress of particle aggregation is very slow even when the residence time is extended.
That is, when particles whose primary particle diameters are as small as 5 to 10 nm are adopted, reaggregation behavior due to ultrasonic waves may sometimes occur and, therefore, it is necessary that short-time irradiation be carried out when dispersion processing is carried out under large output density conditions. Although in the case of particles having necking although their primary particle diameters are 5 to 10 nm, it is necessary to cut the necking as far as possible, in the case of particles having no necking, it is unnecessary to cut necking and hence the output density may be smaller (figures at the second decimal place to several watts per cm²).
If the residence time is extended at an ultrasonic output density of 460 W/cm², particles dispersed once due to strong shock waves generated by ultrasonic waves undergo reaggregation and the dispersibility worsens.
Although in the embodiments described above, the description was given of the method by which titanium nanoparticles are stably dispersed in liquid sodium, it is possible to stably disperse nanoparticles of metals other than titanium. When also the suppression of the sodium-water reaction is considered, it is important that when sodium and a target atom approach each other, electric charge transfer occurs between the two, and metal nanoparticles which are such that the bond energy between sodium and a target atom is large and that the bias of charge is large are preferable. Such metal nanoparticles will be described below.
The bond energy and bias of charge occurring when sodium and metals approach each other were calculated by using the theoretical calculation software developed by Vienna University, Austria, VASP (Vienna Ab-initio Simulation Package). For example, in the case of titanium used in the above embodiments, the bond energy is larger than in a sodium-sodium bond and the bias of charge was also ascertained, and from the standpoint of reaction suppression it became apparent that the use of titanium is suitable. For other atoms, it became apparent that vanadium, chromium, iron, cobalt, nickel, copper and the like are suitable. Furthermore, for other atoms, even if they are not effective in suppressing reactions, so long as they form particles which do not react with sodium, liquid sodium is diluted with these particles dispersed in liquid sodium and the absolute amount of sodium which reacts with water decreases, and hence it is possible to suppress sodium-water reactions. Examples of such atomic species include aluminum and zirconium.
In the case of titanium, the standard free energy of its oxides is lower than that of sodium and hence when oxygen is present, the oxygen is trapped on the titanium side. If an element for which oxygen is trapped on the sodium side is used, sodium and oxygen react, sodium oxides are generated, and the properties as a fluid are lost. Therefore, it is necessary to perform thorough purity control of an element to be used. In this respect, in the case of titanium, oxygen is not trapped on the sodium side and even oxides have good dispersibility and have also the reaction suppressing effect. Therefore, this is advantageous.
Examples of elements whose oxides have lower standard free energy than sodium include vanadium and aluminum in addition to titanium. FIG. 13 is a graph showing examples of calculations of the intermetallic bonding force with sodium and bias of charge. As metals capable of stably dispersing nanoparticles in liquid sodium, metals in which the sodium-metal bond energy is larger than the sodium-sodium bond energy and the bias of charge is large are suitable, and according to FIG. 13, examples of these metals include vanadium, nickel and copper.

Claims

1. A nanoparticle manufacturing device for manufacturing nanoparticles by heating and melting a mixture of a raw material metal powder and a carrier gas in a heating space, cooling the mixture in a cooling space and collecting the mixture in a collection space,

wherein the heating space, the cooling space and the collection space form a continuous flow path without a back flow, and the cross-sectional area of the collection space is set at a large value compared to the cross-sectional area of the heating space and the cooling space.

2. The nanoparticle manufacturing device according to claim 1,

wherein the heating and melting temperature of the heating space is maintained at a first temperature which is not less than the melting point of the raw material metal powder,

wherein the cooling space is maintained at a second temperature which is lower than the melting point of the raw material metal powder, and

wherein the collection space is maintained at a third temperature which is lower than the second temperature of the cooling space.

3. A nanoparticle manufacturing method for manufacturing nanoparticles by subjecting a mixture of a raw material metal powder and a carrier gas to a heating and melting treatment in a heating space, subjecting the mixture to a cooling treatment in a cooling space and subjecting the mixture to a collection treatment in a collection space,

wherein the heating and melting treatment in the heating space, the cooling treatment in the cooling space and the collection treatment in the collection space form a continuous flow path without a back flow, and the cross-section area of the collection space is set at a large value compared to the cross-sectional area of the heating space and the cooling space, whereby nanoparticles are manufactured.

4. The nanoparticle manufacturing method according to claim 3,

wherein the treatment temperature of the heating and melting treatment in the heating space is maintained at a first temperature which is not less than the melting point of the raw material metal powder,

wherein the temperature of the cooling treatment in the cooling space is maintained at a second temperature which is lower than the melting point of the raw material metal powder, and

wherein the temperature of the collection treatment in the collection space is maintained at a third temperature which is lower than the second temperature of the cooling space.

5. A method of manufacturing a nanoparticle-dispersed liquid alkali metal by dispersing nanoparticles in a liquid alkali metal, comprising:

a rough dispersion step of stirring nanoparticles in the liquid alkali metal by a physical effect; and

a dispersion step of dispersing nanoparticles in the liquid alkali metal by irradiating the liquid alkali metal with ultrasonic waves after the rough dispersion step.

6. The method of manufacturing a nanoparticle-dispersed liquid alkali metal according to claim 5, wherein the liquid alkali metal is any of lithium, sodium, potassium and an alloy whose main component is any of the metals.

7. The method of manufacturing a nanoparticle-dispersed liquid alkali metal according to claim 5, wherein the nanoparticles are made of any of the metals selected from the group consisting of titanium, vanadium, chromium, iron, cobalt, nickel and copper.

8. The method of manufacturing a nanoparticle-dispersed liquid alkali metal according to claim 6, wherein the nanoparticles are made of any of the metals selected from the group consisting of titanium, vanadium, chromium, iron, cobalt, nickel and copper.