NOZZLE AND METHOD FOR FORMING MICRODROPLETS

Abstract

The invention relates to a nozzle for producing microdroplets of metal using gas flow, to a nozzle for producing microdroplets using electrodispersion, to a combination of a melt spinner for forming elongate metal fibers with a nozzle and to a method of forming microdroplets using at least one of a gas flow and electrodispersion.

Claims

1.-18. (canceled)

19. A nozzle for producing microdroplets of metal, the nozzle comprising a reservoir for molten metal, a nozzle opening for directing the molten metal in a flow direction out of the reservoir and a channel connecting the reservoir with the nozzle opening, wherein the nozzle further comprises an external force generating device configured to apply an external force on a molten metal flow flowing in said channel with a force per unit area generated by the external force generating device at the molten metal being larger than a surface tension of the molten metal.

20. A nozzle for producing microdroplets of metal using gas flow, the nozzle comprising a reservoir for molten metal, a nozzle opening for directing the molten metal in a flow direction out of the reservoir and a channel connecting the reservoir with the nozzle opening, wherein the nozzle further comprises a gas flow generating device for generating and directing the gas flow to the molten metal through at least one supply opening into the channel, wherein the gas supply opening is located at the nozzle opening, wherein a force per unit area generated by the gas flow at the molten metal is larger than the surface tension of the molten metal.

21. The nozzle according to claim 20, wherein the gas flow generating device is configured to direct the gas flow perpendicular to or at an angle to the flow direction of the channel.

22. The nozzle according to claim 20, wherein the channel comprises two or more supply openings to receive the gas flow from more than one side around the circumference of the nozzle.

23. The nozzle according to claim 20, wherein the gas in the gas flow is air, argon, Helium, N2, Ar2, CO2 or combinations of the foregoing.

24. A nozzle for producing microdroplets of metal using electrodispersion, the nozzle comprising a reservoir for molten metal, a nozzle opening for directing the molten metal in a flow direction out of the reservoir and a channel connecting the reservoir with the nozzle opening, wherein the nozzle further comprises a first electrode such as a metal piece and a device to apply an electric field between the first electrode and the molten metal with a force per unit area generated by the electric field at the molten metal being larger than a surface tension of the molten metal.

25. The nozzle according to claim 24, wherein the first electrode comprises an essentially cuboid shape.

26. The nozzle according to claim 24, wherein the electric field generated between the first electrode and the molten metal has a field strength which lies in the range of 1 V/cm to 1000 V/cm.

27. The nozzle according to claim 24, wherein the electric field is generated by one of an alternating current and a direct current.

28. The nozzle according to claim 20, wherein a cross-section of the channel in the flow direction of the molten metal comprises one of a square shape, a rectangular shape, a round shape, an oval shape, a polygonal shape and a triangular shape.

29. The nozzle according to claim 20, wherein the cross-section of the channel in a plane perpendicular to the flow direction of the molten metal comprises a circular, rectangular, triangular, oval, or polygonal shape.

30. The nozzle according to claim 20, wherein the channel comprises a length in the range of 0.1 to 100 mm,

31. The nozzle according to claim 20, wherein the nozzle opening comprises a circular, oval, square, rectangular, triangular, polygonal or any other shaped cross-section.

32. The nozzle according to claim 31, wherein a rectangular nozzle opening comprises a length in the range of 0.5 to 10 cm; or wherein a circular nozzle opening comprises a diameter of 10 to 500 μm, preferably 20 to 200 μm, in particular 30 to 100 μm.

33. The nozzle according to claim 20, wherein the reservoir comprises an inner shape, which is connected with the channel via a channel opening in the inner shape, wherein the inner shape of the reservoir is rounded or sloped at the channel opening such that the molten metal is guided to the nozzle opening.

34. The nozzle according to claim 20, wherein the formed microdroplets comprise a size in the range of 0.010 to 500 mm.

35. A combination of a melt spinner for forming elongate metal fibers with a nozzle according to claim 20, wherein the melt spinner further comprises a rotatable wheel with a circumferential surface, at least one rotating planar surface and collection means for collecting solidified fibers formed on one of the circumferential surface and the rotating planar surface of the rotatable wheel from the molten metal and separated from the rotatable wheel by forces generated by the rotation of the rotatable wheel.

36. A method of forming microdroplets using at least one of an external force field, a gas flow and electrodispersion, wherein the method comprises the following steps: providing a flow of molten metal at a nozzle opening; and applying a force per unit area at said nozzle opening by means of one of the on said flow of molten metal, with said force per unit area being larger than a surface tension of said flow of molten metal.

37. The nozzle according to claim 19, wherein the cross-section of the channel in a plane perpendicular to the flow direction of the molten metal comprises a circular, rectangular, triangular, oval, or polygonal shape.

38. The nozzle according to claim 19, wherein the nozzle opening comprises a circular, oval, square, rectangular, triangular, polygonal or any other shaped cross-section.

Description

[0054] The invention will now be described in further detail by way of example only with reference to the accompanying drawings. In the drawings there are shown:

[0055] FIGS. 1a to 1c: different examples of nozzles according to the invention using gas flow;

[0056] FIG. 1d: a further example of a nozzle according to the invention;

[0057] FIGS. 2a to 2d: different examples of nozzles according to the invention using electrodispersion;

[0058] FIG. 3: an example of a horizontal melt spinner;

[0059] FIG. 4: an example of a vertical melt spinner;

[0060] FIGS. 5a and 5b: experimental results and pictures of produced microdroplets;

[0061] FIG. 6 scanning electron micrographs of a produced fibre;

[0062] FIG. 7: a photograph of a cross section of a produced fiber;

[0063] FIGS. 8a to 8c: experimental results for distributions of fiber thicknesses and widths;

[0064] FIGS. 9a to 9c: experimental results for distributions of fiber thicknesses and widths;

[0065] FIG. 10: a photograph of a produced bronze fiber;

[0066] FIG. 11: a photograph of a plurality of produced bronze fibers;

[0067] FIGS. 12a to 12c: experimental results for distributions of thicknesses and widths of produced fibers; and

[0068] FIG. 13: experimental results for the variation of metal droplet volume by controlling the gas pressure which chops the continuous flow of metals in metal droplets.

[0069] FIGS. 1a to 1c show different examples nozzles 10, each comprising a reservoir 12 filled with molten metal 14, a channel 16 and a nozzle opening 18. It can be seen that the nozzle opening relates to the last opening in a flow direction F of the molten metal 14, where the molten metal 14 actually leaves the nozzle 10.

[0070] The nozzles 10 shown in FIGS. 1a to 1c each furthermore comprise two supply openings 20 for a gas flow, which is generated by a gas flow generating device (not shown). As can be seen the supply openings can either supply said gas flow in a direction perpendicular to the flow direction F of the molten metal 14 (see FIG. 1a) or at an angle smaller than 90° between the flow direction F and a gas flow direction G (see FIGS. 1b and 1c).

[0071] Regardless of the angle, the gas flow crosses the flow of molten metal 14 right at the nozzle opening 18 or right before the molten metal 14 exits the nozzle opening 18 such that a force per unit area generated by the gas flow at the molten metal 14 exceeds the surface tension of the molten metal 14 to form microdroplets 22.

[0072] In this connection it is noted that the gas flow can either be a continuous flow of gas or a pulsed flow of gas. The used gas can for example be N.sub.2, Ar.sub.2 or another gas such as CO.sub.2.

[0073] The supply openings 20 shown in FIGS. 1a to 1c each comprise a diameter in the range of 0.005 to 0.015 mm, whereas the channel 16 comprises a diameter in the range of 0.050 to 0.250 mm. Even though the expression “diameter” is used, it is clear that the cross section of both the supply openings 20 and the channel 16 do not necessarily have to be circular but can also be polygonal, triangular, rectangular, oval or any other shape.

[0074] The same applies also to the nozzle opening 18, which can have a circular cross section as well as a rectangular, triangular, oval or polygonal one.

[0075] The reservoir 12 shown in FIGS. 1a to 1c is formed as a hollow space, which is configured to accommodate the metal melt. The hollow space comprises a channel opening 24 through which the metal melt 14 can flow into the channel 16. An inner shape of the reservoir 12 is rounded at the channel opening 24 such that the melt 14 can flow easily into the channel 16.

[0076] The reservoir 12 can either be a tank, which holds a bigger volume of melt 14, or a connecting piece, which is configured to be attached to a separate tank. Hence, the hollow space can either be big enough to hold a bigger volume of melt 14 or just as big to act as a connecting piece between the channel 16 and a separate tank.

[0077] Another example of a nozzle 10 using a gas flow to produce the microdroplets 22 is shown in FIG. 1d. Also in this embodiment a reservoir 12 for the molten metal is provided. The reservoir 12 further comprises a channel opening 24 through which a channel 16 is connected to the reservoir. At the far end of the channel 16 the nozzle 10 comprises a nozzle opening 18 through which the molten metal 14 can flow. Furthermore, one can see in FIG. 1d that the channel 16 comprises two gas flow channels 17 with respective supply openings through which one can direct a gas to the channel 16, which then chops the continuous flow of metal into a noncontinuous flow such that droplets exit the opening 18.

[0078] It should also be noted that the supply openings 20 could be arranged at the nozzle opening 18 in order to separate the flow of molten metal 14 at the nozzle opening 18.

[0079] In the design shown in FIG. 1d, the two gas flow channels 17 first run in parallel to the channel 16 until they make a turn in the direction of the channel 16 such that they meet the channel at the respective supply openings 20 of the channel 16. They can either meet the channel 16 such that the gas, which flows through the gas flow channels 17 “cuts” the molten metal 14 perpendicular to the flow direction F or at another defined angle.

[0080] In this connection it should be noted that the two gas flow channels 17 could also be arranged in a different manner and extend e.g. obliquely with respect to the channel 16 from their starting point.

[0081] Thus, it can be seen that the flow of gas can be provided at the nozzle at the reservoir 12 and in flow direction F. Such an embodiment can help to reduce the space needed for the nozzle 10 since the gas flow generating device can be provided at the reservoir 12 and thus, does not need any additional space next to the channel 16.

[0082] It is also possible that only one air flow channel 17 or more than two air flow channels 17 are provided. Hence, it is noted that the embodiment of FIG. 1d describes only an example and does not restrict the invention in any way.

[0083] A typical diameter of said air flow channel is about 1 mm. Depending on the type of metal used said diameter can also vary. The typical gas pressure, with which the gas flows through the air flow channel 17 and through the supply opening 18, lies in the range from 100 to 10000 mbar, preferably in the range of 800 to 1500 mbar. Said pressure can be dependent on the precise shape of the cross section of the air flow channel as well as the channel for the molten metal.

[0084] FIGS. 2a to 2d show different examples of nozzles 10, which all use the concept of electrodispersion to form microdroplets 22 of molten metal 14. The shown nozzles comprise generally the structure as the nozzles 10 of FIGS. 1a to 1c expect for the part with the supply opening 20 since the nozzles 10 from FIGS. 2a to 2d do not need a supply opening of any kind.

[0085] However, said nozzles 10 also comprise a reservoir 12 filled with molten metal 14, a channel 16 and a nozzle opening 18. Furthermore, said nozzles 10 comprise a first electrode 26, which can be designed in several different ways. As can be seen in FIGS. 2c and 2d said first electrode 26 is a separate piece of metal, which is placed near the nozzle opening 18. A typical value for the spacing between the nozzle opening 18 and the first electrode 26 lies in the range from 4 to 6 mm. FIG. 2d additionally shows a second piece of metal, which is used as a second electrode 30 such that the flow of molten metal 14 is guided through a space between said two electrodes 26, 30 such that the microdroplets 22 are formed therein.

[0086] As can be seen FIG. 2c, on the other hand, it is not necessary to provide a second separate electrode 30 since the molten metal 14 itself can act as the second electrode, meaning that the electric field is generated between the first electrode 26 and the molten metal 14.

[0087] In FIGS. 2a and 2b, on the other hand, the first electrode is realized by a (metal) surface 28, onto which the formed microdroplets 22 are directed. As will be described later in connection with FIGS. 4 and 5, said surface 28 can be a circumferential or tangential surface of a rotating wheel of a so called melt spinner.

[0088] It should be noted that the channel 16 may comprises an approximate length selected in the range of 5 to 30 mm, in particular in the range of 8 to 15 mm, whereas the diameter (or length) of the nozzle opening 18 lies in that range of 0.005 to 0.100 mm.

[0089] FIG. 3 shows a typical horizontal melt spinner 32 for producing elongate metal strands comprising a nozzle 10 with a nozzle opening 18, which deposits drops of molten metal 14 in a deposition direction D onto a rotating planar surface 34 of a rotating wheel 36. In order to be able to deposit molten metal, the nozzle 10 comprises a heating device 38, which heats the metal inside the nozzle 10 to a temperature where the metal is in its liquid state.

[0090] The nozzle opening 18 may be of any geometry, usually circular, oval, rectangular, quadratic or triangular. The opening width can lie in the range of 10 μm to 500 μm. The nozzle direction N may vary from 90° with respect to the planar surface 34, i. e. it may be selected to lie in the range from 0° to 90°. Hence, the nozzle 10 could also be aligned parallel to the rotating planar surface 34 and still have a deposition direction D which is perpendicular, or any other angle, to the planar surface 34.

[0091] The diameter of the wheel 36 can range from centimeters to meters and the wheel material maybe of any choice, which withstands the metal molt deposition and fast rotation speed, in particular metal alloys such as copper, copper alloys, brass, nickel, iron, iron oxide, stainless steel or carbon based material such as graphite or carbide, ceramic materials. It is also possible that the wheel 36 is a wheel of a base material having a layer made of a metal or of a metal alloy of a ceramic material or of graphite or a vapor deposited carbon, for example a copper wheel 36 having a layer of graphite.

[0092] Because of the rotation of the wheel 36, the molten metal drops 22, which come into contact with the surface are entrained and thereby elongated by the wheel 36 to form elongate metal strands 40. These strands 40 remain on the surface until they are cooled down enough to solidify. For this purpose the rotating wheel 36 can be cooled by a cooling device to for example room temperature or even below by cooling with liquid nitrogen in order for the molten metal drops 22 to be able to solidify to metal strands 40. If the wheel 36 was not cooled at all it would eventually heat up because of its contact with the (hot) molten metal 14 and hence prevent the molten metal 14 to cool down sufficiently to solidify. Heating of the wheel can also affect its mechanical stability. The cooling device C is shown inside the rotatable wheel 36, but it is noted that does not necessarily have to be located inside the wheel. There are sufficiently many methods known to cool such devices.

[0093] Once the metal fibers 40 are solidified the centrifugal forces which act on the metal fibers 40 due to the rotation of the wheel 36 will suffice in order to move the metal fibers 40 away from the planar surface. As the adhesion force between the solidified metal fibers 40 and the planar surface is less than the force acting on the metal fiber 40 due to the rotation of the planar surface. Thus, the solidified metal fibers 40 fly away from the wheel 36 in a direction transverse to the circumference of the wheel 36.

[0094] For collection of the solidified fibers 40 collection means 42 are provided, which basically catch the fibers 40 flying away from the rotating wheel 36.

[0095] A typical vertical melt spinner is shown in FIG. 4. Since the vertical melt spinner comprises several components, which are identical to the ones from the horizontal melt spinner, only the differences between these two will now be described.

[0096] The difference lies in the alignment of the rotating wheel 36 and hence the corresponding surface onto which the microdroplets 22 are guided. While the rotating wheel 36 of the horizontal melt spinner of FIG. 3 is aligned such that the microdroplets are being guided on one of its planar lateral surfaces 34, in the vertical melt spinner the rotating wheel 36 is aligned such that the micropdroplets 22 are guided onto the circumferential surface 35 of the wheel. Hence, a rotation axis A of the rotating wheel 36 is aligned perpendicular to the flow direction F of the molten metal 14, whereas the rotation axis A of the horizontal melt spinner is aligned parallel to the flow direction F of the melt spinner.

[0097] Regardless of the alignment of the rotating wheel 36, the microdroplets 22 are elongated by the rotating wheel 36 just as described before in connection with FIG. 4.

[0098] FIGS. 5a to 12c show different photographs and experimental results of the produced microdroplets 22 as well as the therewith produced fibers 40.

[0099] FIG. 5a shows a photograph of microdroplets 22, which are composed of bronze, whereas FIG. 5b shows experimental results for a size distribution of the diameter of microdroplets 22, which are composed of a cobalt-alloy. The diameter for both materials was constant throughout the experiment and in the range of 0.060 to 0.250 mm. It has further shown that the ejection of the droplets 22 can also be held constant in the range of 1 to 10 ms depending on the precise experimental settings. An increase of pressure, for example, has shown to have a minor influence on the microdroplet diameter, but a notable influence on the time laps between the ejection of two microdroplets.

[0100] The solidified fiber, which results from guiding the cobalt-alloy microdroplet 22 on a rotating wheel 36 of a horizontal or vertical melt spinner is shown in FIG. 6. It is observed that a small droplet remains at the very end of the produced fiber with a width of approximately 60 μm. Said remaining droplet is shown in detail in the three bottom photographs. The width of the produced fiber is approximately 12 μm.

[0101] A cross section of a produced bronze fiber is shown in FIG. 7. It can be seen that a typical cross section is asymmetric and comprises a straight part 44, which is contact with the wheel surface as well as a curved part 46 at the opposite side of the fiber 40. The parts with the highest curvature (left and right on the picture) result of poor wetting of the rotating wheel 36 by the melt 14. The maximal height of the fiber in this photograph is about 6 μm.

[0102] Size distributions of the fiber thicknesses and widths are shown in FIGS. 8a to 9c. The distributions are quite narrow and usually either Gaussian or log-normal distributions. Experiments have shown that parameters such as the wheel speed, roughness of the wheel surface, temperature of the melt and so on, can influence the landing of the microdroplet 22 on the wheel surface and thus the formation of the fiber. For a cobalt-alloy, the comparison of the size distributions indicates that the wheel surface speed influences the fiber geometry significantly. If the droplet diameter is kept constant as well as other experimental parameters, the increase of the wheel surface speed from 25 m/s to 50 m/s results in a decrease of thickness of 50% and decrease of width of 30% (comparison of FIGS. 8a to 8c with 9a to 9c and FIG. 12).

[0103] If the microdroplet 22 diameter is reduced to 60 μm (see FIG. 5a), the width of the fabricated fiber at standard experimental conditions is significantly below 10 μm (see FIG. 10). A picture of a large amount of fibers produced with the said experimental settings is shown in FIG. 11.

[0104] The dropping process and its stability have shown to depend on the physical properties of the materials in contact at the microscopic scale, i.e. viscosity and surface tension of the melt, wetting of the nozzle surfaces by the melt (often sharply depending on the temperature) and the mechanical properties of the nozzle surfaces.

[0105] At the minimum required pressure difference and slightly above, three qualitative experimental observations are of crucial importance to support the dropping of the microdroplets: Firstly, the melt should not wet the nozzle (i. e. wetting angle>>90° but between contact angles from 0° to 90°);

[0106] Secondly, a reduced roughness of the surface of the rotating wheel is of advantage to improve the process stability. If the roughness of the nozzle surface is in the range of 0.05 mm, it introduces a heterogeneous flow of melt. Hence, polishing the surface with a polishing paper, such as sandpaper with a grit size of down to a grain size of 0.003 mm has shown to be beneficial. In this connection sand paper with a grit size selected in the range of 20 to 500 can be selected preferably with a grit size of around 200 to 350.

[0107] As a third point experiments have shown that the borders of the nozzle opening should be as sharp as possible, i. e. rounded borders favor droplets of lager diameter. Hence, the sharper the borders, the smaller the microdroplets can get.

[0108] Finally, FIG. 13 shows how the microdroplet volume varies if the gas pressure of the gas flow, which chops the continuous flow of metal in droplets is controlled to different values. One can clearly see that a control of the gas pressure is crucial in order to produce microdroplets with a volume down to several nanoliters.

[0109] Generally speaking, the volume of the microdroplets lies in the range of 0.1 to 20 nanoliters, in particular in the range of 2 to 9 nanoliters. In this connection a nozzle pressure, i. e. the pressure, with which the gas, e.g. Argon, is supplied in the channel to the molten metal, can be selected in the range of 900 mbar to 2000 mbar, in particular in the range of 1000 to 1500 mbar for a crucible pressure of 880 to 1050 mbar, in particular in the range of 900 to 1000 mbar, with the specific values being indicated in the drawing of FIG. 13. The term “crucible pressure” relates to the pressure exerted on the molten metal in the channel of the molten metal. In this connection it is noted that the nozzle pressure should not be much greater than the crucible pressure, as otherwise the molten metal can flow back into the gas passage for the gas flow.

Control of the Liquid Metal Droplet Volume by Electrodispersion

[0110] As already mentioned above, the interfacial tension of liquid metals is very large, i.e. >400 mN/m. Therefore, liquid metals tend to form droplets in gas atmosphere or liquids to minimize their surface energy.

[0111] Electrodispersion techniques utilize a high electrical voltage to overcome the surface tension of a liquid meniscus at a orifice, allowing the breaking of the liquid into either monodisperse or polydisperse fine droplets.

[0112] Breakup of emerging liquid metal droplet occurs when the disruptive forces, i.e. the Coulomb force induced by the high voltage, overcome the interfacial tension that resists deformation of the droplet.

[0113] The liquid metal phase acts as the second electrode while for example a piece of solidified metal surface is the first electrode. In between the two electrodes an electric field is generated by a (not shown) device. Said electric field then generates a Coulomb force, which can overcome the interfacial tension that resists deformation of the droplet. Increasing the voltage or decreasing the distance between the two electrodes increases the electric field and thus also the Coulomb force, which is acting at the liquid interface.

Electrodispersion in Combination with Melt Spinning

[0114] The control of a liquid metal droplet volume, which exits the nozzle of a crucible by electric fields can directly be applied to control the dimension of ultrafine metal fibers. Therefore, in some embodiments of the invention the liquid metal is contacted by one electrode while the second electrode is the rotating metal wheel or an electrode, which is brought close to the exit of the nozzle. The thereby formed droplets with controlled volume are brought in contact with the rotating wheel, as described above, and an ultrafine metal fiber is pulled out of the droplet, which is in contact with the fast rotating wheel.

Control of Liquid Metal Droplet Volume by Gas Flow

[0115] A strong gas flow, which is brought close to the exit of a nozzle and crosses the linear flow of liquid metal from the exit, can overcome the surface tension of a fluid meniscus at an orifice, allowing the breaking of the liquid into either monodisperse or polydisperse fine droplets. The possible set up for a nozzle as used in this setup has been explained above in connection with FIGS. 1 to 4.

[0116] Generally, it is imaginable that both described methods, i.e. electrodispersion and gas flow, are realized in one single nozzle. With such a nozzle, the used method could for example be chosen according to the composition of the used molten metal. In some cases it can also be useful to apply both methods at the same time. Hence, the option to choose is can be given.