WO2008080887A1

WO2008080887A1 - Industrial ultrasonic spraying device

Info

Publication number: WO2008080887A1
Application number: PCT/EP2007/064388
Authority: WO
Inventors: Gilles Boursier; Philippe Vaxelaire
Original assignee: Heraeus Psp France Sas
Priority date: 2007-01-02
Filing date: 2007-12-20
Publication date: 2008-07-10
Also published as: TWI421129B; TW200841936A; FR2910825B1; FR2910825A1

Abstract

The invention relates to an ultrasonic spraying device, comprising a generator connected to an electro-acoustic transducer fixed to a pulveriser constituted by an axial rod (41) around which is arranged a radial crown (42) having at its outer edge a spraying surface (44), in which the material and dimensions of the pulveriser are selected such that the pulveriser vibrates in resonance at the frequency F, in a vibration mode where the rod (41) and the radial crown (42) simultaneously contract then dilate alternatively, any point of the spraying surface being animated by an homogeneous vibration orthogonal to said surface.

Description

Industrial ultrasonic spraying device

The invention relates to the general technical field of ultrasonic spraying devices and processes.

GENERAL PRESENTATION OF THE PRIOR ART

It should first be recalled that, in the field of power ultrasound, there are existing systems orthogonally transforming the direction of propagation of vibrations, that is, from ultrasonic vibration spreading in a first direction and entering the system, a vibration of the same frequency spreading in a second direction, orthogonal to the first, emerges from the system.

This can involve either longitudinal vibrations spreading in straight parts, or radial vibrations spreading in circular parts.

Systems of this type are described especially in the document US 3 696 259 dated 03/10/1972.

These systems are constructed as follows: they integrate, in general in the same part, two orthogonal parts relative to one another ; the length of each of these parts is determined to correspond to a half-wavelength (in the case of a longitudinal part) or to a quarter of a wavelength (taken between the axis of revolution and the vibrating outer edge, in the case of a circular part) in the particular and specific mode of vibration of the part concerned at the frequency F considered ;

- both ends of each of these parts correspond to a vibration displacement antinode, while the middle

(in the case of a longitudinal part) or the centre (in the case of a circular part) corresponds to a displacement node of the vibration ; the two parts are arranged relative to one another such that the middle, or the centre, of one coincides with the middle, or the centre, of the other. All these systems operate as follows: the ultrasonic vibratory energy of frequency F is communicated to the input location of the part ; that part then starts to vibrate at this same frequency F, its central point being a displacement node of the vibration and, outgoing, a stress antinode; from the effect of the Poisson coefficient, the central point of the input part contracts and dilates laterally in phase opposition with the ends of said input part ; as the output part is attached to the input part orthogonally and since their central points coincide, the vibratory energy is thus transmitted to the output part by means of this « Poisson » effect ; the output part then starts to vibrate in its own mode in keeping with the same frequency F, the vibration being in phase opposition with that of the input part, that is, when the input part contracts, the output part dilates and vice and versa .

The two input and output parts of the vibration are thus orthogonal to one another and the coupling between their respective vibration modes then occurs at their central point by means of the Poisson coefficient, in phase opposition.

It should also be remembered that the principle of ultrasonic spraying of fluids has been known for some time and described especially in works such as the journal « Ultrasonics » (October 1967, E .G. Lierke and R. Pohlman and E. G. Lierke and A. Griesshammer; January 1974, R. Pohlman, K. Heisler, M. Cichos) .

Finally, it should be recalled that the combination of the two principles, such as described in European patent EP 0 588 609, utilises a radial crown vibrating in phase opposition with a straight axial rod arranged at the centre of the crown, that is, the crown is in extension radial phase when the rod itself is in axial contraction phase and vice and versa.

The outer edge of the crown is the surface whereof the vibration enables fluid spraying.

The production capacity of such a pulveriser is proportional to the surface of the outer edge of the crown and, thus, all things being equal, is also proportional to the radius/diameter of this crown.

However, for a determined frequency F and a particular material, such a pulveriser whereof the crown vibrates in a quarter of a wavelength has a radius also well determined.

By way of example, such a high-performance steel pulveriser vibrating at 20 kHz has a straight axial rod in half-wavelength of the order of 120 mm in length and a first crown radius of the order of 65 mm, constituting the base crown, vibrating radially at a quarter of a wavelength considered between the axis of the crown and its outer edge.

Quite obviously, the expert knows that he can augment the radius of such a pulveriser, at the same time preserving its resonant character, by adding around the base crown a ring whereof the width is determined to comprise a whole number of half-wavelengths, in radial vibration mode at the frequency considered.

The whole constituted by the base crown and the ring is frequently made in a single part. This whole, constituted by the base crown and optionally the ring, will be named « crown » or « radial crown » hereinbelow.

Within the scope of the present invention « width » of a ring is understood as the difference between the external radius and the internal radius of the ring considered.

So, if a ring of width corresponding to a half- wavelength, in radial vibration is added to the base crown previously described, the result is a radius of the assembly constituted by the base crown and of the ring of around 125 mm.

Adding a ring of width corresponding to two half- wavelengths would extend this same radius (base crown + ring) to around 175 mm.

However, the expert is also aware that the more the ring comprises half-wavelengths, the more the amplitude of the displacement antinode situated at the end of the assembly constituted by the base crown and the ring is reduced, such that this amplitude rapidly becomes insufficient to yield a satisfactory spraying phenomenon.

Accordingly, pursuant to figure 1, if the amplitude of the input vibration is 100%, then the base crown of 65 mm already has an amplitude 1 at its outer edge of only 50%, the crown of 125 mm offers no more than an amplitude 2 of 33% and that of 175 mm offers no more than an amplitude 3 of 28%.

This can partially be compensated by increasing the amplitude of the input vibration in the axial part of the pulveriser. All the same, this is not always possible, especially in the case of high-temperature atomisation of molten products where the mechanical losses of heat in the materials used for making the acoustic chain should be added.

Consequently, the expert is lead to the problem of increasing the radius of the assembly constituted by the base crown and the ring without excessively decreasing the amplitude of the vibration of its outer edge.

PRESENTATION OF THE INVENTION

To this effect, an ultrasonic spraying device is provided, comprising a generator outputting an electric signal of ultrasonic frequency F, at least an electro- acoustic transducer and a pulveriser constituted by an axial rod enclosed by a radial crown, positioned around the rod orthogonally to the axis of the rod and having on its outer edge a spraying surface, the transducer being attached solidly to the pulveriser through one of the ends of the rod, the material and dimensions of the pulveriser being selected such that the pulveriser vibrates in resonance at the frequency F, in a vibration mode where the rod and the crown simultaneously contract then dilate alternatively, all points of the spraying surface being animated by a homogeneous vibration orthogonal to said surface. According to a preferred though non-limiting aspect of the device according to the invention, a ring, whereof the width is determined to comprise a whole number of half-wavelengths in the radial vibration mode at the frequency F, is added around the base crown corresponding to the first resonant radius.

Preferably, the base crown and the ring are made from a single part.

In this case, the assembly constituted by the base crown and of the ring can advantageously be cascaded, having variations in thickness located on the nodal circles of the vibration, so as to create at least a transformation in amplitude of the radial vibration.

Optionally, an axial recess is made in the rod.

Also, one (or a set of) part(s) - whereof the frequency F is a particular resonance frequency - can be inserted in between the transducer and the pulveriser. This allows either to remove the pulveriser from the immediate vicinity of the transducer, or to attach the resulting acoustic assembly to a support, or to modify the amplitude of the vibration emitted by the transducer, or again several of these occurrences to happen simultaneously.

The ultrasonic power dissipated in the pulveriser can be between 50 and 1000 Watt. Further, the ultrasonic frequency F can be between 20 and 200 kHz.

According to a variant, the spraying assembly constituted by the transducer, the inserted part, or the set of parts, and the radial pulveriser is set in slow rotation so as to optimise distribution of the liquid to be pulverised on the upper flat surface of the radial crown . In certain applications, the device can be installed inside a sealed atomisation chamber.

The invention likewise relates to a production process for metallic powders utilising a spraying device such as described hereinabove.

The invention likewise relates to powders obtained by utilising the process according to the invention described hereinabove.

PRESENTATION OF FIGURES

Other characteristics, aims and advantages of the present invention will emerge from the following description, which is purely illustrative and non- limiting and must be viewed in conjunction with the attached diagrams, in which:

• figure 1 shows the positioning of the different successive resonant radii, as well as the amplitude of the vibration at the outer edge of the assembly constituted by the base crown and, optionally, of the ring, in the case of a pulveriser according to the prior art,

• figure 2 shows the positioning of the different successive resonant radii, as well as the amplitude of the vibration at the outer edge of the assembly constituted by the base crown and, optionally, of the ring, in the case of a pulveriser according to the invention,

• figures 3a and 3b show modelling of a pulveriser according to the prior art, of « Poisson » type, in the two positions of extreme amplitude,

• figure 4 shows a radial liquid pulveriser 40 according to the invention, • figures 5a and 5b show modelling of a pulveriser according to the invention, of « Poumon » type, in the two positions of extreme amplitude,

• figure 6 shows another radial pulveriser 40 according to the invention,

• figures 7a and 7b show modelling of the pulveriser illustrated in figure 6, in the two positions of extreme amplitude,

• figures 8 and 9 show a complete liquid spraying device according to the invention.

DESCRIPTION OF THE INVENTION

As previously described, the aim of the present invention especially is to contribute a solution to the problem of increasing the radius of a base crown and ring assembly of a pulveriser without excessive decrease in the amplitude of the vibration of its outer edge.

For this, the invention proposes a type of ultrasonic radial pulveriser utilising a novel physical principle for vibration radically different to that used in European patent EP 0 588 609.

The European patent EP 0 588 609 has a pulveriser composed of a straight axial rod 41 resonating in half wavelength, around which is arranged a radial crown 42 resonating in quarter of a wavelength at the same frequency. The coupling between the vibrations of the rod 41 and the crown 42 is done by the Poisson coefficient, the two vibrations being in phase opposition.

Figures 3a and 3b show modelling of a pulveriser such as that described in the document EP 0 588 609, of « Poisson » type, in the two positions of extreme amplitude . During the period of axial contraction of the rod 41 (according to the axis A-A' ) , the crown 42 dilates radially, as illustrated in figure 3a (the dotted lines correspond to the position of the part at rest) . During the period of axial dilation of the rod 41, the crown 42 contracts radially (Poisson effect) , as illustrated in figure 3b .

Accordingly, radial dilation (respectively contraction) of the crown is in phase opposition with axial dilation (respectively contraction) of the rod: this illustrates the Poisson effect.

The pulveriser 40 according to the invention also has a radial crown 42 arranged around an axial rod 41 as illustrated in figure 4. The rod 41 and the crown 42 are coaxial, the rod 41 extending on either side of the crown 42.

The pulveriser comprises a spraying surface 44 located on the outer edge of the crown 42, and a longitudinal recess 45 made in the rod 41 and extending according to the axis of revolution of the rod 41. In the case illustrated in figure 4, the radius of the crown 42 is equal to the first resonant radius referenced 4 in figure 2.

The material and geometry of the pulveriser 40 are selected such that the rod 41 vibrates longitudinally in resonance at the frequency F and the crown 42 vibrates radially in resonance at the same frequency.

The outer edge 44 of the crown 42 constitutes a spraying surface. In fact, any point of the spraying surface is animated by a vibration orthogonal to said spraying surface, allowing homogeneous and industrially productive spraying of liquid by this outer edge. A particular feature of the pulveriser 40 according to the invention (vis-a-vis EP 0 588 609) relates to the fact that instead of presenting different and independent vibration modes and coupled only by means of the Poisson effect and its own identical frequency, the rod 41 and the crown 42 are going to vibrate according to a unique vibration mode, which can be revealed only by modelling by finite elements followed by making a prototype.

In this respect, use can be made of various calculation software by finite elements as aid to the geometric definition of the different realisations according to the invention ; example include ATILA version 5.22 edited by ISEN, 41 bd VAUBAN of 59046 LILLE Cedex, France and marketed by CEDRAT, 10 chemin du Pre Carre at 38246 GRENOBLE Cedex, France, resolution software by finite elements dedicated to modelling 2D/3D structures, inter alia.

The expert knows how to utilise such software in which he will first enter the novel geometry, the vibratory characteristics of which he wishes to examine ; he will then proceed with appropriate meshing of this geometry and he will finally enter the significant physical characteristics of the material considered (Young's modulus, Poisson coefficient, volumic mass).

According to the invention, the axial (rod) and radial (crown) parts of the pulveriser 40 vibrate in phase .

During the period of axial dilation of the rod 41 (according to the axis A-A' ) , the crown 42 dilates radially, as illustrated in figure 5a. During the period of axial contraction of the rod 41 (according to the axis A-A' ) , the crown 42 contracts radially, as illustrated in figure 5b .

Accordingly, radial dilation (respectively contraction) of the crown 42 is in phase with axial dilation (respectively contraction) of the rod 41. In other terms, the rod 41 and the crown 42 contract and dilate simultaneously.

The existence of the Poisson coefficient translates, during the length contraction phase, by simultaneous thickening of the different parts of the pulveriser and, during the dilation phase, by thinning.

The longitudinal recess 45 enables the material in excess located at the centre of the pulveriser 40 to find its place during the length contraction phase, so as not to impair the good vibration of the pulveriser.

This vibration mode has two specific advantages : first, the radius of the smallest resonating radial crown 42 is (all things being equal) substantially twice as big as in the case of the similar pulveriser using the Poisson effect, second, the level of the amplitude of the vibration is substantially the same at the outer edge of the crown 42 of smallest resonating radius, than at the ends of the axial part, such that the pulveriser 40 according to the invention has an input/output ratio of the order of 1/1 (that is without attenuation) .

All the interest shown in the pulveriser 40 will consequently be understood, which, relative to the prior art represented by European patent EP 0 588 609, allows effectively to double the atomising capacity in terms of flow rate, by using at the same time a pulveriser requiring a source of axial vibration of two times less amplitude, thus reducing considerably the stresses in the materials used.

Obtaining a pulveriser 40 operating according to this principle depends especially on the dimensions of the crown and the rod, the material used to make the pulveriser, and the frequency utilised to make the pulveriser vibrate.

These parameters can be determined empirically by means of modelling software by finite elements similar to those described hereinabove.

By way of non-limiting examples, there is for example a pulveriser 40 made of high-performance steel, for example 55 NCDV5, and vibrating at 20 kHz, having an axial rod of the order of 200 mm in length, enclosed by a resonant radial crown whereof the outer radius, of the order of 130 mm, is the first resonant radius.

In this example and in the present case of a liquid such as water, the rod 41 should be excited with an amplitude of the order of 8 to 10 microns crest to crest to obtain an amplitude of mechanical vibrations of the crown 42 at its outer edge of the same order, that is 8 to 10 microns, which is ideal for atomising water at a high rate, that is, up to several hundreds of litres of water per hour.

Obviously, the radius of the radial crown 42 of the pulveriser 40 according to the invention can be augmented by adding, around the base crown corresponding to the first resonant radius, a ring whereof the width is determined to comprise a whole number of half- wavelengths, in radial vibration mode at the frequency considered ; this increases the admissible spraying rate of the device according to the invention, while retaining its resonating character.

In the event where this number of half-wavelengths is even, the outer edge of the radial crown 42 constituted by the base crown and the ring vibrates in phase with the rod 41.

In the event where this number of half-wavelengths is odd, the outer edge of the radial crown 42 constituted by the base crown and the ring vibrates in phase opposition with the rod 41, as illustrated in figures 6, 7a and 7b.

The expert can adapt (from modelling software such as ATILA), the dimensions of the pulveriser 40 to gain such an effect (dilation and contraction in phase of the rod 41 and of the base crown; whole number of additional half-wavelengths) depending on the material employed and the resonance frequency F.

Figure 2 is a graphic representation of the amplitude of the vibration of the outer edge of the radial crown, in % of the input amplitude, as a function of the outer radius of the base crown assembly and, optionally, ring, for a pulveriser according to the invention resonating at 40 kHz (that is calculated to vibrate at 40 kHz) .

Accordingly, pursuant to figure 2, if the amplitude of the input vibration is 100%, then the base crown radius equal to 65 mm has an amplitude 4 at its outer edge of substantially 100%, the crown of radius 115 mm (augmented by a ring of width corresponding to a half- wavelength) has an amplitude 5 of 66% and that of radius 165 mm (augmented by a ring of width corresponding to two half-wavelengths) has an amplitude 6 of 58%. On this basis, another particular embodiment of a pulveriser according to the invention can be cited, vibrating at 40 kHz, with a base crown and ring corresponding to an half-wavelength, thus constructed according to the second resonant radius of figure 2, the assembly crown plus ring having an outer radius of 115 mm for an axial length of 100 mm.

An illustration of this example of embodiment is given in figure 6. The radial crown 42 (base crown + ring) is made in a single part, of outer radius 115 mm.

The radial crown 42 has two zones, 42' and 42'', of different thickness.

The zone 42', closer to the rod 41, is thicker than the zone 42'', which is more distant from it.

The increase in amplitude obtained by decreasing the thickness between the two zones is advantageously done at the level of a nodal amplitude circle 46, whereof the radius is referenced 7 in figure 2.

Just as for the embodiment illustrated in figure 4, the rod 41 comprises a cylindrical recess 45 coaxial to the axis of revolution of the rod 41.

This type of pulveriser will preferably be made from high-performance steel, for example 55 NCDV5 and could advantageously be utilised for the production of alloy powders having low melting points, such as those used for soldering electronic components.

The reader will have understood that the fact of increasing the radius of the base crown and ring assembly effectively: increases the external surface 44 of the pulveriser, thus 5

increasing the spraying surface of the liquid and thus increasing the atomising capacity.

Figure 8 illustrates a complete liquid-spraying device .

The device comprises a sealed enclosure 60 in which any appropriate atmosphere can be set up, from the viewpoint of composition, temperature and pressure, with the aim of constituting a spraying device of spray/cooling type, or of spray/drying type, or even any other type.

The sealed enclosure 60 can have heating or cooling means .

The device comprises the pulveriser 40 described in reference to figure 6.

The device likewise comprises an electro-acoustic transducer 20, equipped with two ceramics.

This transducer 20 is connected to a straight cylindrical coupling rod 30 allowing to remove the pulveriser from the immediate vicinity of the transducer as suitable.

The device further comprises a speed transformer, or booster, 35 at its fixing point (vibration antinode) on the pulveriser 40.

The device also comprises a feeding pipe 90 of the liquid substance to be atomised.

In the event where the liquid substance is a metal alloy, it is previously brought to a temperature greater than its melting point, then atomised in (a « spray » of) fine droplets 100 emitted by the outer edge 44 of the radial crown 42 (base crown + ring) . These droplets then cool as they fall inside the enclosure 60 and dry to produce the preferred powders.

The operating principle of the spraying device illustrated in figure 8 is as follows.

The generator 10 produces an alternative electric signal with narrow passband (100 to 200 Hz) around an ultrasonic frequency F. This signal is converted into mechanical vibratory energy of the same frequency in the transducer 20. The booster 35 transforms the vibration by increasing its amplitude (for example in a ratio of 2/1 or 1.5/1) .

This vibratory energy of appropriate amplitude is then transmitted to the pulveriser 40, whereof the outer edge 44 atomises the liquid guided by the feeding pipe 90 into droplets 100.

It is also noted that fixing and guiding in rotation the device with the aim of optimising, if necessary, distribution of the liquid on the upper surface of the radial crown 42, will be advantageously completed in the vicinity of an amplitude node 70, as shown in figure 9, which illustrates a device comprising the electro- acoustic transducer 20, the coupling rod 30, the booster 35 and the pulveriser 40, the conveying of liquid 90, the spray of droplets 100 and a motor-reducer 80 ensuring rotation of the device.

Such a device for example atomises metal alloys of melting range between 100⁰C and 600⁰C and produce, after solidification of the resulting droplets, spherical powders featuring a narrow size distribution.

This device allows to reach flow rates in a range of 100 to 150 kg per hour of alloy in optimal amplitude conditions (9 to 12 microns at input into the rod 41, for 7

6 to 8 microns at the outer edge 44 of the radial crown 42, base crown + ring) and thus to produce a non-oxidised spherical powder having a Gaussian size distribution centred around 45/50 microns.

The reader will appreciate that numerous modifications can be brought to the invention such as described hereinabove without materially departing from the teaching of the present document.

For example, the pulveriser according to the invention can naturally be employed with any type of ultrasound generators.

The radius of the radial crown, constituted by the base crown and the ring can, in certain embodiments of the present invention, be more or less significant, as soon as it enables radial vibration of said radial crown in resonance. In particular, this radius comprises a first quarter of a wavelength, then a whole number of half wavelengths.

Claims

1. An ultrasonic spraying device, comprising a generator (10) outputting an electric signal of ultrasonic frequency F, at least an electro-acoustic transducer (20) and a pulveriser (40) constituted by an axial rod (41) around which is arranged a radial crown (42), positioned around the rod (41) orthogonally to the axis of the rod and having at its outer edge a spraying surface (44) , the transducer (20) being attached solidly to the pulveriser (40) by means of one of the ends (43) of the rod (41), characterised in that the radial crown (42) comprises at least one base crown corresponding to the smallest resonant radius, and in that the material and dimensions of the pulveriser (40) are selected such that the pulveriser vibrates in resonance at the frequency F, in a vibration mode where the rod (41) and the base crown simultaneously contract then dilate alternatively, any point of the spraying surface (44) being animated by an homogeneous vibration orthogonal to said surface.

2. The ultrasonic spraying device as claimed in Claim 1, characterised in that the radial crown (42) is constituted by the base crown, around which is arranged a ring of width corresponding to a whole number of half- wavelengths in the radial vibration mode at the frequency F.

3. The ultrasonic spraying device as claimed in Claim 2, characterised in that the radial crown (42), constituted by the base crown and the ring, is made of a single part.

4. The ultrasonic spraying device as claimed in Claim 2, characterised in that the radial crown (42), constituted by the base crown and the ring, is made cascaded having variations in thickness situated on the nodal circles of the vibration, so as to create at least one amplitude transformation of the radial vibration.

5. The ultrasonic spraying device as claimed in any one of Claims 1 to 4, characterised in that an axial recess (45) is made in the rod (41) .

6. The ultrasonic spraying device as claimed in any one of Claims 1 to 5, characterised in that inserted in between the transducer (20) and the pulveriser (40) is a part (30), or an assembly of parts, whereof the frequency F is a resonance frequency and which allows either to remove the pulveriser (40) from the immediate vicinity of the transducer (20), or to attach the resulting acoustic assembly to a support, or to modify the amplitude of the vibration emitted by the transducer, or again several of these occurrences to happen simultaneously.

7. The device as claimed in any one of Claims 1 to 6, characterised in that the ultrasonic power dissipated in the pulveriser (40) is between 50 and 1000 Watt.

8. The device as claimed in any one of Claims 1 to 7, characterised in that the ultrasonic frequency F is between 20 and 200 kHz.

9. The device as claimed in any one of Claims 1 to

8 characterised in that the spraying assembly constituted by the transducer 20, the part (30) or the assembly of parts, and the radial pulveriser (40) is set in slow rotation so as to optimise distribution of the liquid to be pulverised on the upper surface of the radial crown

(42) .

10. The device as claimed in any one of Claims 1 to

9 characterised in that the device is installed inside a sealed atomisation chamber (60).

11. A production process for metallic powders using a spraying device as claimed in any one of Claims 1 to 10.

12. Powders obtained by using the process as claimed in Claim 11.