NANOPARTICLES

Field of the Invention

The present invention relates to an apparatus and a method for large scale manufacturing of nanoparticles. In particular, the present invention relates to an apparatus and method utilizing electric discharge for large scale manufacturing of nanoparticles.

Background of the Invention

Nanoparticles may exhibit completely new or improved properties based on specific characteristics (size, distribution, morphology, phase and chemical composition) when compared with larger particles of the bulk material they are made of. Therefore, the study of nanoparticles has spread to various fields, such as mechanic, electronics, optics, magnetics, and catalysis. However, large scale production of nanoparticles is still a bottleneck for widespread application of nanoparticles.

To overcome difficulties related to large scale manufacturing,

US2014/0255716 describes an apparatus where a first and a second electrode define a "plasma cloud zone" described as a volume of sustained arc-like discharge. A linear actuator coupled to a controller and at least one of the first and second electrode is used to advance the first or second electrode corresponding to a signal representative of the plasma zone, such that a plasma cloud is maintained. US2014/0255716 further discloses that the apparatus may comprise a pair of anodes and a pair of cathodes arranged to form a plasma cloud zone.

However, there may be difficulties related to the generation of a stable plasma cloud zone, leading to a manufacturing process which is complex and difficult to control. Accordingly, there is a need for a simplified apparatus and method for large-scale manufacturing of metal nanoparticles. Summary

In view of above-mentioned and other drawbacks of the prior art, it is an object of the present invention to provide an improved apparatus and method for manufacturing of nanoparticles, simplifying large scale

manufacturing of nanoparticles.

According to a first aspect of the invention, it is therefore provided an apparatus for manufacturing nanoparticles comprising: a container comprising a liquid; an anode and a cathode arranged facing each other in the container, wherein the anode is movable in relation to the cathode such that a distance between the anode and the cathode is controllable; a power supply

connected to the anode and the cathode to form an electric field

therebetween; a current transducer control system comprising a current sensor arranged and configured to detect a discharge current from an electrostatic discharge between the anode and the cathode, wherein the discharge is configured to result in formation of metal nanoparticles through melting and evaporation of the anode, which nanoparticles are subsequently dispersed in the liquid; a motor configured to control the movement of the anode to control the distance between the anode and the cathode; and a control unit configured to control the motor to move the anode towards the cathode until a discharge current is detected by the current transducer control system, and when a discharge current is detected, to stop movement of the anode towards the cathode, and to determine a new movement speed and movement direction of the anode based on a consumption rate of the anode such that the discharge is interrupted.

The present invention is based on the realization that a current transducer control system can be used to address the issue of large scale production of nanoparticles by means or arc discharge. In particular, the described apparatus comprising the current transducer control system enables controlled large scale, and scalable, manufacturing of metallic nanoparticles.

The general principle of arc discharge is that an "explosion", i.e. a discharge, will be triggered when the electric field strength in the dielectric liquid between two high voltage loaded electrodes reaches the breakdown voltage. When the explosion happens, a plasma channel will be formed between the electrodes. This channel together with the resulting high energy and high pressure can increase the surface temperature of the electrodes to around 10 000 K in a few microseconds. Under such high temperature, the electrode materials will be melted and evaporated. The molten part will quench in the dielectric liquid to form microparticles. Meanwhile, the evaporated part will become nanoparticles when it solidifies.

After each explosion, a gap will be formed between electrodes due to the consumption of the anode and cathode electrodes. A feeding system is therefore needed to reduce the gap for triggering the next explosion.

However, the electrode consumption of each explosion, which is proportional to the explosion intensity, is not necessarily stable. To overcome the variability and to provide a reliable process, the anode is stopped after a discharge is detected, and a new movement speed and movement direction of the anode is determined based on a consumption rate of the anode such that the discharge is interrupted. Accordingly, a linear movement of the anode is possible in both directions (up-down) in order to stop the arc discharge, which is important for uniform particle formation with high yield.

Accordingly, the consumption rate of the electrodes, and in particular the consumption rate of the anode, influences how the anode is moved after a discharge has been detected in order to interrupt the discharge, and to initiate a new discharge. Thereby, a DC current source is used to apply an

intermittent arc discharge method for manufacturing nanoparticles.

According to one embodiment of the invention, the control unit is configured to move the anode away from the cathode. If the anode is comprised of a material having a high melting point, less of the anode will be consumed compared to for an anode having a lower melting point.

Accordingly, to avoid welding between the anode and the cathode, the anode is moved away from the cathode. Once a discharge current is detected, the anode can be moved away either by a predetermined distance or to a predefined position sufficiently far away from the cathode such that the discharge current between anode and cathode is interrupted to avoid damaging the anode and the cathode.

According to another embodiment of the invention, the control unit is configured to stop movement of the anode, wait for a predetermined period of time, and to start movement of the anode towards the cathode. For an anode having a lower melting point, and thereby a higher consumption rate, the consumption of the anode during a discharge is such that the resulting gap is sufficiently large to prevent further discharge. Thereby, the new movement speed and movement direction of the anode.

Moreover, it has been observed that it is advantageous to wait for a predetermined period of time after a detected discharge before the next discharge is initiated. By allowing the electrodes, and in particular the anode, to cool down between consecutive discharges, the proportion of formed nanoparticles compared to microparticles is increased. Furthermore, the new speed may be lower than the previous speed.

According to one embodiment of the invention, the cathode may be arranged vertically in the container, extending substantially perpendicularly from a bottom of the container, and wherein the anode is arranged vertically in the container, above the cathode and facing said cathode. By vertically arranging the anode and the cathode, the motor controlling the movement of the anode can be located above the container, which provides a simple and scalable system. In comparison, a horizontal alignment would require either that the motor is located within the container comprising the liquid, or that the movement of the anode and/or cathode would require connections to a motor located outside of the container through passages in the sidewall of the container.

According to one embodiment of the invention, the cathode may advantageously comprise graphite or tungsten, both of which have a melting temperature which is high compared to typical melting temperatures of the metallic anode, thereby reducing the consumption of the cathode which in turn leads to that the process can be run for a longer period of time before the cathode needs replacing. In one embodiment of the invention, the anode may advantageously consist of a combination of two materials. The anode material may for example be selected from the group comprising using Bi ₂ Te ₃ , Ag ₃ Sn, Cu ₆ Sn ₅ , Cu ₃ Sn, SiC, GaN, InP, SiGe, MoS ₂ , WS ₂ , BN, P, Fe, Ferrous alloys, stainless steel, Ti, Cu, Al, Sn, In and their alloys or WSe ₂ . This allows for the formation of nanoparticles from a wide range of material combinations. It is also possible to form an anode using a combination of materials not explicitly mentioned herein such as other 2D materials.

According to one embodiment of the invention, the liquid may advantageously comprise a surfactant for preventing agglomeration of said nanoparticles. The surfactant may for example be Polyvinylpyrrolidone (PVP). Other possible surfactant materials which may be used are water soluble and protective materials such as gelatin, starch, DNA, pyrene, APTES,

polycarboxylate ether (PCEs).

According to one embodiment of the invention, the liquid may comprise water soluble graphene configured to form a protective layer for the

nanoparticles. The protective graphene layer improves the dispersability and prevents further oxidation of the nanoparticles.

According to one embodiment of the invention, the anode and the cathode may advantageously be cylindrical and arranged such that a base surface of the anode faces a base surface of the cathode. The circular configuration of the anode and cathode results in a more uniform electric field therebetween.

According to one embodiment of the invention the apparatus may comprise a plurality of anodes and corresponding cathodes arranged in parallel in the container, each anode being arranged to face a corresponding cathode. As can be understood from the above description of the apparatus, the manufacturing method can easily be scaled by arranging a plurality of anode/cathode pairs in parallel in the container. Moreover, the anodes may be moved individually with a motor and current transducer control system for each anode and cathode pair. Alternatively, all of the anodes may be moved simultaneously by a shared motor, where a current transducer control system is arranged to detect a discharge between any of the anode cathode pairs. Thereby, even if the resulting melting of evaporation of each individual discharge may vary, such that the nominal distance between one

anode/cathode pair may differ from the next, it will be the anode and cathode pair with the shortest distance that will discharge. Accordingly, a self regulating system is provided where all anodes are equally consumed over time.

According to a second aspect of the invention, there is provided a method for manufacturing nanoparticles in an apparatus comprising: a container; an anode and a cathode arranged facing each other in the container, wherein the anode is movable in relation to the cathode such that a distance between the anode and the cathode is controllable; a power supply connected to the anode and the cathode to form an electric field

therebetween; a current transducer control system comprising a current sensor arranged to detect a discharge current from an electrostatic discharge between the anode and the cathode, wherein the discharge is configured to result in formation of metal nanoparticles through melting and evaporation of the anode, which nanoparticles are subsequently dispersed in the liquid; a motor configured to control the movement of the anode to control the distance between the anode and the cathode, the method comprising: providing a liquid in the container; providing a surfactant in the container; controlling the motor to move the anode towards the cathode until a discharge current is detected by the current transducer control system, and when a discharge current is detected, to move the anode away from the cathode by a predetermined distance.

Effects and features of the second aspect of the invention are largely analogous to those described above in connection with the first aspect of the invention.

Further features of, and advantages with, the present invention will become apparent when studying the appended claims and the following description. The skilled person realizes that different features of the present invention may be combined to create embodiments other than those described in the following, without departing from the scope of the present invention.

Brief Description of the Drawings

These and other aspects of the present invention will now be described in more detail, with reference to the appended drawings showing an example embodiment of the invention, wherein:

Fig. 1 schematically illustrates an apparatus according to an

embodiment of the invention;

Fig. 2 schematically illustrates a control system for an apparatus according to an embodiment of the invention;

Figs. 3A-D are TEM images illustrating nanoparticles manufactured by an apparatus according to embodiments of the invention; and

Fig. 4 schematically illustrates a method according to an embodiment of the invention.

Detailed Description of Example Embodiments

In the present detailed description, various embodiments of the apparatus and method for manufacturing nanoparticles according to the present invention will be described.

Fig. 1 schematically illustrates an apparatus 100 for manufacturing nanoparticles. The apparatus comprises a container 102 configured to contain a dielectric liquid 104 and a cathode 106 arranged in the container 102 such that the cathode 106 is covered by the liquid 104 when the apparatus 100 is in use. The liquid 104 is a dielectric liquid such that the dielectric breakdown strength of the liquid 104 determines the field strength required to cause electric discharge between the anode 108 and the cathode 106. A conductive liquid could not be used since it would short-circuit the anode 108 and the cathode 106.

An anode 108 is arranged facing the cathode 106, wherein the anode

108 is movable in relation to the cathode 106 such that a distance between the anode 108 and the cathode 106 is controllable. A power supply 1 10 is connected to the anode and the cathode to form an electric field therebetween. The power supply 1 10 is controllable such that the applied voltage between the anode 108 and the cathode 106, and thereby the electric field, can be controlled. Moreover, a DC-current transducer control system 1 12 comprising a current sensor 1 14 is arranged and configured to detect a discharge current from an electrostatic discharge between the anode 108 and the cathode 106, wherein the discharge is configured to result in formation of metal nanoparticles 120 through melting and evaporation of the anode 108. The current transducer control system 1 12 is arranged between the power supply 1 10 and either of the anode and the cathode. In Fig. 1 , the current transducer control system 1 12 is arranged between the power supply 1 10 and the cathode 106.

The anode 108 and cathode 106 are here illustrated as being

substantially cylindrical, which results in a homogenous electric field

therebetween, which in turn provides a well controlled discharge reaction. The anode may have a smaller diameter than the cathode. By using a larger cathode, the erosion of the cathode 106 can be reduced by moving or rotating the anode 108 over the surface of the cathode 106. Thereby, localized heating and hole formation in the cathode can be avoided. The movement in the xy-plane of the anode 108 in relation to the cathode 106 can be

performed manually, or automatically using a motor.

After the dielectric discharge has taken place, the resulting

nanoparticles 120 are subsequently dispersed in the liquid 104.

The apparatus 100 further comprises a motor 1 16 configured to control the movement of the anode 108 to control the distance between the anode 108 and the cathode 106, and a control unit 1 18 configured to control the motor 1 16 to move the anode 108 towards the cathode 106 until a discharge current is detected by the current sensor 1 14 of the current transducer control system 1 12, and when a discharge current is detected, to move the anode 108 away from the cathode 106 such that the discharge is interrupted. The discharge current may typically be in the range of 5 to 70A. Accordingly, the motor 1 16 operates based on the discharge current detected by the current transducer control system 1 12, and the motor control unit 1 18 can thus be considered to be a part of the control system 1 12.

The current transducer control system 1 12 is illustrated in further detail in the schematic illustration of Fig. 2. The control system 1 12 includes a current sensor 202, i.e. a current transducer 202, a comparator chip 204, a voltage supply V3 for the components of the control system and a voltage divider sub-circuit 206 comprised of resistors R1 and R4.

When the discharge between the anode 108 and cathode 106 occur, the current is sensed by the current sensor 1 14 and the motor 1 16 controlling the movement of the anode 108 is stopped. Furthermore, the current transducer 202 transforms the amplitude of the current into a voltage which is used as input to a comparator chip 204. If the voltage crosses a

predetermined threshold value of the comparator 204, the feeding speed of the anode 108 can be changed once the motor receives a control signal from the controller 1 18 based on the output of the comparator 204, otherwise the speed of the motor 1 16 may remain at the previously set speed.

Moreover, both the feeding speed and the feeding direction of the anode 108 may be changed depending on the consumption rate of the anode 108. If the consumption rate of the anode 108 is known beforehand, the desired behavior of the motor can be pre-programmed by an operator of the apparatus 100.

Alternatively, or in combination, the control system controlling the motor may be self learning, where the consumption rate of the anode is learned, either during a dedicated initialization step or during operation. Such a self learning method may comprise, after a discharge, identifying how far back the motor is moved, and the identifying how far towards the cathode 106 the motor 1 16 must move the anode 1 18 before the next discharge is detected, thereby learning the consumption rate of the anode (and of the cathode, in case of noticeable cathode consumption).

Thereby, for a comparatively low consumption rate of the anode 108, the anode 108 may need to be moved back to avoid a continued current between and resulting welding of the anode 108 and cathode 106. For a higher consumption rate of the anode 108, it is sufficient to stop the anode, and possibly to wait for a predetermined time before again commencing movement of the anode 108 towards the cathode 106. The predetermined time to wait may be a few seconds, such as 1 -5 seconds.

In addition to the material choice of the anode 108, the consumption can also be controlled to some degree by setting the maximum discharge current, where a lower maximum discharge current typically leads to a lower consumption rate. Moreover, it has also been observed that a lower current gives rise to a discharge creating a higher proportion of nanoparticles.

As further illustrated in Fig. 1 , the cathode 106 is arranged vertically in the container 102, extending substantially perpendicularly from the bottom of the container, and the anode 108 is also arranged vertically in the container, above the cathode 106 and facing the cathode 106. The cathode 106 may for example consist of graphite or tungsten which are both conductive so that a discharge current can flow through the cathode.

The anode typically consists of a combination of two materials in order to form nanoparticles comprising the same combination of material. Such combined materials can also be referred to as 2D-materials. Examples of suitable materials for the anode comprise Bi ₂ Te ₃ , Ag ₃ Sn, Cu ₆ Sn ₅ , Cu ₃ Sn, SiC, GaN, InP, SiGe, MoS ₂ , WS ₂ , BN, P, Fe, Ferrous alloys, stainless steel, Ti, Cu, Al, Sn, In and their alloys or WSe ₂ . However, it is also possible to use other metallic and semi-electrical conducting material combinations. The size of the resulting nanoparticles is typically in the range of 1 nm to 100 nm.

Moreover, both graphite and tungsten have a melting temperature which is high compared to typical melting temperatures of the metallic anode 108, thereby reducing the consumption of the cathode 106 during operation of the apparatus 100, which in turn leads to that the manufacturing process can be run for a longer period of time before the cathode 106 needs replacing. Furthermore, replacing the anode 108 is comparatively straightforward since the anode 108 can be easily removed from its holder. In an example embodiment of the invention, the liquid comprises a surfactant for preventing agglomeration of said nanoparticles. Here, the liquid is deionized water and the surfactant is Polyvinylpyrrolidone (PVP). However, water soluble and protective materials such as gelatin, starch, DNA, pyrene, APTES, PVP, sodium dodecyl benzene sulfonate may also be used as a surfactant. The surfactant thus facilitates a uniform dispersion of

nanoparticles in the dielectric liquid. The liquid may also be wax, oil or liquid nitrogen. The PH-value of the solution is controlled in order to control the charge balance thus creating uniform dispersion.

According to one embodiment, water soluble functionalized graphene is used as a protective layer for the metal nanoparticles, where it is attached onto the nanoparticles by van-der Waals force. The graphene can thereby wrap the nanoparticle surface and form a gas-impermeable coating for the nanoparticle. A nanoparticle having a graphene protection layer can be used as a composite for strengthening and reinforcement and a gas-impermeable coating can also prevent oxidation of the nanoparticles.

Even though the present description relates to an apparatus having one anode and one cathode, the apparatus is easily scalable by arranging a number of anodes and cathodes in parallel in the container. In a parallel application, all anodes could be controlled by the same motor, or they could be individually controlled.

Figs. 3A-D are transmission electron microscope (TEM) images of nanoparticles manufactured using the above described apparatus.

Fig. 3A shows Fe nanoparticles having a size in the range of about 30- 50 nm.

Fig. 3B shows Bi ₂ Te ₃ semiconductor nanoparticles having a size of about 20 nm.

Fig. 3C shows Ag ₃ Sn nanoparticles having a size in the range of about 10-30 nm.

Fig. 3D shows Cu ₆ Sn ₅ semiconductor nanoparticles having a size in the range of about 30-50 nm. As can be seen from Figs. 3A-D, various metallic and semiconducting nanoparticles can be manufactured using the described apparatus.

There is also provided a method for manufacturing metal nanoparticles using the above described apparatus 100. The method comprises providing 402 a liquid in the container 102, providing 404 a surfactant in the container 102, controlling 406 the motor 1 16 to move the anode 108 towards the cathode 106 until a discharge current is detected 408 by the current transducer control systeml 14, and when a discharge current is detected, to stop movement 410 of the anode 108. In a final step 412, a new movement speed and movement direction is determined, based on the consumption rate of the anode 108, to interrupt the discharge current.

The discharge, which can be likened with an explosion, will be triggered when the electric field in the dielectric liquid between two high voltage loaded electrodes (anode and cathode) reaches breakdown. When the discharge happens, a plasma channel will be formed between the electrodes. This channel together with the high energy and high pressure can increase the surface temperature of the electrodes to around 10,000 K in a few microseconds. Under such high temperatures, the electrode materials will be melted and evaporated. The molten part will quench in the dielectric liquid to form micron particles. Meanwhile, the evaporated part will become nanoparticles when the vapor solidifies in the liquid. The dielectric liquid is passivated with chemicals to prevent oxidation. Nitrogen is also continuously supplied to the liquid. Moreover, after each discharge, a gap will be formed between the electrodes due to the consumption of the electrode material. A feeding system therefore is needed to reduce the gap for triggering next explosion. However, the electrode consumption of each discharge is directly proportional to the discharge intensity and is as such not stable. Accordingly, by means of the described method and apparatus, a well controlled process for manufacturing nanoparticles is provided.

Even though the invention has been described with reference to specific exemplifying embodiments thereof, many different alterations, modifications and the like will become apparent for those skilled in the art. Also, it should be noted that parts of the apparatus and method may be omitted, interchanged or arranged in various ways, the apparatus and method yet being able to perform the functionality of the present invention.

Additionally, variations to the disclosed embodiments can be understood and effected by the skilled person in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.