FIELD OF THE INVENTION

The present invention generally relates to a nano particle generator and to a method for generating nano particles by said device.

BACKGROUND OF THE INVENTION

Micro particle manufacturing is currently provided by various solvent-ousting techniques, some of them are known as "wet technology". These techniques are of only limited use in the preparation of sub-micron and nano-particles, due to filtration problems and agglomeration of the colloid systems.

Air, electric field, and ultrasound sputtering are commonly used to obtain finely dispersed aerosols. Air dispersion involves with a vigorous mixing of a thin liquid jet and a high-energy gas flow. This cost effective and high capacity technique can be implemented for the sputtering of relatively viscous liquids. However, the aerosol is widely dispersed and hence rarely provided thus for nano-particle manufacture.

Ultrasound (i.e., US) technique for dispersion of liquids is being widely used in various engineering branches, including air humidity control in premises, painting, mass spectrometric devices, etc. Aerosol US generators are being large-scale manufactured, and possess wide capacity and particle size range. WO Pat. Appl. No. 02056866 to Watanabe et al., discloses a method for the preparation of nano-particles an aerosol ultrasound generator, hereinafter denoted by the term 'nebulizer', which is equipped with aerosol drying system, having optimal configuration and temperature field intensity. However, this method and means is characterized by an insufficient capacity and resulted with only inadequate yield of particle size of the nano range.

It is known in the art that ultrasonic dispersion provides for an aerosol of low bipolar electric charge, characterized by a Coulomb interaction, coagulation, and thus by a wide dispersion of the particle size. US Pat. No. 5,247,842 to Kaufman et al. introduces an electro-spray nebulizer generates an aerosol comprised of sub micron and uniform droplets. A liquid sample is supplied at a controlled rate to a capillary needle of the nebulizer, and droplets are formed due to an electrical field in the region about the needle discharge. The tendency of the droplets to disintegrate due to Coulomb forces is counteracted by sources of ionizing radiation within the nebulizer. The ions reduce the charge in each droplet while solvent evaporation reduces the diameter of the droplet. To further ensure against Coulomb disintegration, a controlled air sheath was introduced to the nebulizer for transporting droplets more rapidly downstream.

SUMMARY OF THE INVENTION

The particle size comprising a known liquid composition is determined by the electric field applied on said particle and by stabilizing forces, such as the particle's surface tension etc. By applying an efficient corona discharge, homogenous small-sized particles are obtained, wherein their size is determined solely by said liquid characteristics. A novel and most effective nano-particle generator according to the present invention is adapted to apply near to maximal corona discharge, so that homogenous small-sized particles are obtained.

BRIEF DESCRIPTION OF THE FIGURES

In order to understand the invention and to see how it may be implemented in practice, a preferred embodiment will now be described, by way of non-limiting example only, with reference to the accompanying drawing, in which

Fig. 1 schematically shows a lateral cross section one of the embodiments of nano particle generation device, wherein the ultrasound nebulizer is used as the aerosol source;

Fig. 2 displays an alternative device embodiment with a modified charging unit;

Fig. 3 displays alternative device embodiment with a modified charging unit and aerosol source; and, Figure 4 schematically presents a design of the setup comprising two parallel operating nano-particle generators.

DETAILED DESCRIPTION OF THE INVENTION

The following description is provided, alongside all chapters of the present invention, so as to enable any person skilled in the art to make use of said invention and sets forth the best modes contemplated by the inventor of carrying out this invention. Various modifications, however, will remain apparent to those skilled in the art, since the generic principles of the present invention have been defined specifically to provide a novel nano particle generator and an effective method for generating nano particles by said device.

It is in the scope of the present invention to provide a cost effective and highly efficient nano-particle generator comprising an aerosol generation unit comprising an ultrasonic nebulizer adapted to generate an aerosol from gas feed; and an aerosol charging unit comprising a porous, gas-permeable, electrically conductive sleeve, and at least one corona electrode; said sleeve is grounded and barriered in its both sides by a plurality of insulation rings and contact discs connected to a high-voltage source; said electrode is adapted to be electrically communicated with a high-voltage source via said contact discs. The generator is provided in the manner that when the setup is energized, gas flow and/or gas flow enriched with solvent fumes enters said charging unit so that large aerosol drops are deposited and returned to the stock solution, an efficient corona discharge is generated between said corona electrode and the walls of said sleeve, generated ions are combined with the particles of the aerosol in the manner that a homogeneously dispersed aerosol are obtained. Moreover, wherein interactions between said charged aerosol particles and the applied electric field are resulted by the drift of the obtained nano particles towards said sleeve walls; gas- solvent mixture that filters through the porous walls of said sleeve creates an air dynamic barrier, which prevents particle sedimentation. It is in also the scope of the present invention wherein the gas fed to the nebulizer is enriched with solution-constituent solvent fumes; and/or wherein the sleeve is made of materials selected from sintered stainless steel, bronze, metal-covered polymers or any combination thereof. The aforesaid corona electrode may be a needle. It is also in the scope of the present invention wherein the hereto-defined generator additionally comprising an aerosol-drying unit comprising a heat source. The drying unit is possibly made of materials selected from aluminum or aluminum-containing alloys. The heat source is selected in a non-limiting manner from a heated gas stream; an ohmic heat module or any combination thereof. It is also in the scope of the present invention wherein means for encapsulating nano or micro particles comprising a mixing chamber in communication with a first nano- particle generator and a second nano-particle generator; said first generator is adapted for the preparation of aerosol particles with relatively large diameter and negative charge and comprising a aerosol generation unit and a aerosol charging unit; said second generator is adapted to produce dry positively charged nano-particles with respectively smaller diameter, and it comprising a aerosol generation unit, a aerosol charging unit and an aerosol drying unit; wherein particle fluxes obtained from said first and second generator are directed to said mixing chamber in the manner that particles of reciprocal surface charges are forced to merge together so that liquid particles comprising polymer solution are incorporated with solid particles.

It is another object of the present invention to present a novel method for generating nano-particles comprising the steps of generating an aerosol by means of a an aerosol generation unit comprising an ultrasonic nebulizer adapted to generate an aerosol from gas feed; charging the aerosol obtained in the previous step by a means of an aerosol charging unit comprising a porous, gas-permeable, electrically conductive sleeve, and at least one corona electrode, wherein said sleeve is grounded and barriered in its both sides by a plurality of insulation rings and contact discs connected to a high-voltage source, and wherein said electrode is adapted to be electrically communicated with a high-voltage source via said contact discs. The aforesaid method for generating nano- particles is especially preferably provided by means of the aerosol generator as defined in any of the above.

The method may additionally comprise of the step of drying the charged aerosol by a means of an aerosol-drying unit comprising a heat source. Additionally or alternatively, a method for encapsulating nano or micro particles is hereto provided useful by a means of a device comprising a mixing chamber in communication with a first nano-particle generator and a second nano-particle generator; said first generator is adapted for the preparation of aerosol particles with relatively large diameter and negative charge and comprising a aerosol generation unit and a aerosol charging unit; said second generator is adapted to produce dry positively charged nano-particles with respectively smaller diameter, and it comprising a aerosol generation unit, a aerosol charging unit and an aerosol drying unit. The method hence mat comprise of the steps of directing particle fluxes obtained from the first and second generator towards the mixing chamber so that particles of reciprocal surface charges are forced to merge together so that liquid particles comprising polymer solution are incorporated with solid particles.

It is acknowledged in this respect that the smaller is the drop diameter, the more intricate is its further separation. Moreover, surface tension is altered when electric repulsion forces are applied during electric sputtering, in the way that extreme electric force must be introduce in order to separate such a small liquid drop. Aerosol obtained in this manner is generally characterized by significant wide fraction size distribution, wherein average particle size is relatively large. Narrow distribution is obtained however, when maximal charging level is introduced for a sufficient long period and particles are of a single charge density.

When a particle is charged in an electric field with unipolar three-dimensional charge, the increase of the particle charge may be defined by the number of ions attracted to the particle per time unit:

dq/dt = e τ f ds (1 )

wherein q is the particle charge; e is electron charge; f is ion flow density vector; s is surface area of the particle whereupon the ions are deposited.

Flow of ions hitting the particle is determined by the electric-field- induced ion movement, and by the ion-diffusion-caused movement due to ion concentration gradient:

f = n k E - D grad n (2)

wherein E is electric field strength at particle surface; n, k are ion concentration and mobility; D is diffusion coefficient. In steady-state case, ion spatial distribution around the particle is set much faster than the variation in particle charge. Equation (I ) shows that the diffusion mechanism predominates over impact mechanism, provided that D grad n » n k E. Assuming that at particle surface n = 0, and its perturbing effect on ion concentration expands to the distance equal to 2a, we have grad n ~no/2a. According to the molecular kinetic theory of gases, under atmospheric pressure D ~ 0,025 k. Hence, one can write E « 0,025/(2a). For the conditions observed in nano-particle generation setup, when ~(l-3)105kW/cm, we see that with particle sizes of 2a « 0,1 μm, the diffusion mechanism of particle charging prevails. Impact charging prevails in these conditions for the particles of 2a » 1 μm.

Reference is made now to figure 1 , wherein the device according to one embodiment of the present invention is schematically presented in its lateral cross section view. Aforesaid elongated and typically rounded tube-like assembly comprises inter alia the following three functional segments: aerosol generation unit 10, aerosol charging unit 20, and aerosol drying unit 30. All above specified elements are preferentially cylindrical and are situated consecutively inside common housing 40.

Aforementioned drying unit (30) outlet aperture comprising a circular electrode 50 adapted to generate external electric field. Electrode 50 may alternatively be knife-like or needle-shaped and is under positive or negative polarity potential.

Aerosol generation unit 10 comprises disc-shaped US nebulizer 1 1 mounted on sealed membrane 12, which may be an integrated part of housing 40. Nebulizer 1 1 forms aerosol at chamber 13 that comprises a plurality (e.g., two) of lateral orifices. Orifice 14 serves for solution feeding inlet and is in communication with a feeder. A pump may be used, wherein syringe pump, peristaltic pump or hydrostatic feeder are applicable. Orifice 15 is the gas-feeding inlet of chamber 13. Air, nitrogen, or any alternative inert gases can be used, preferably at ambient temperature. It is in the scope of the present invention wherein a gas, e.g., nitrogen, is enriched with solution- constituent solvent fumes.

Such a nebulizer is thus an aerosol generation of high capacity and admissible disperse phase features; nevertheless, alternative solutions are acceptable. Fig. 3 presents the device comprising electrostatic atomizer 16 with corona capillary 17. Electrical sputtering with pre-charging, air sputtering, etc. can be implemented as well. Aerosol charging unit 20 comprises cylindrical sleeve 21 made of porous, gas- permeable, electrically conductive, and chemically resistant to organic-solvent-fume- medium agents. Sintered stainless steel, bronze and/or metal-covered polymers are preferable. Sleeve 21 is grounded and barriered in its both sides by insulation rings 22 and contact discs 23 connected to high-voltage source.

Contact discs 23 are shaped as a ring comprising one or more thin diameter and centered ribs. Thus, contact discs 23 have wide central orifice and exhibit low air dynamic resistance to aerosol flow but at the same time they can be used for corona electrode 24 mounting between them, its location coinciding with sleeve 21 longitudinal symmetry axis.

Corona electrode 24 is preferably made of thin metal wire, platinum for example, and connected to the high-voltage source via contact discs. The surfaces of sleeve 21 , housing 40, and insulation discs 22 form a supercharge chamber 25 having an inlet 26 for gas feeding.

Reference is made now to Fig. 2, displaying yet another embodiment of the aerosol charging unit, wherein needle 27 utilizes as the corona electrode. Drying unit 30 comprises inter alia tube 31 which is a metal thin-walled, i.e., aluminum or aluminum-containing alloys. The exterior surface of tube 31 is comprised of a plurality of ribs 32 adapted to form a spiral-shaped heat exchange chamber 33 in housing wall 40. Heat exchange chamber comprising inlet 34 and outlet 35 for hot air passage, the air being heat carrier. Although this embodiment is the most convenient from the viewpoint of the unit high-voltage section insulation, alternative heating versions are also feasible, e.g. heating by ohmic heat.

When the setup is energized, gas flow enriched with solvent fumes is fed via orifice 15 and enters the charging unit 20. At this stage, large aerosol drops are deposited and returned to the stock solution. A strong corona discharge is generated between the electrode and the walls of grounded cylinder sleeve 21 as voltage of about 10 kW is supplied to corona electrode 24. Generated ions are combined with the particles of the aerosol in the manner that a homogeneously dispersed aerosol is obtained. The medium is saturated with solvent fumes so the drops are not dried. Interactions between charged aerosol particles and the applied electric field are resulted by the drift of the particles towards sleeve 21 walls. Gas-solvent mixture that filters through the porous walls of sleeve 21 creates an air dynamic barrier, which prevents particle sedimentation, and carries them, together with the gas supplied from chamber 13, into the drying chamber 30 inner volumes.

A 10-fold to 100-fold decrease in particle sizes usually occurs during drying, and the disperse phase dry fraction is produced. The excessive electric charge ionizes air-gas mixture and subsequently removed outside.

The particles obtained can be of immediate use in coating of diversified surfaces, or can be trapped by different-design filters, cyclones, scrubbers, electrostatic graders, etc.

It is another embodiment of the present invention wherein the device described above is used for the capsulation of nano- or micro particles. Reference is made thus to Fig. 4 that is schematically presenting a setup comprising two parallel operating nano- particle generators. One of the generators (A) is adapted for the preparation of aerosol particles with relatively large diameter and negative charge out of polymer solution (e.g., cellulose acetate phthalate). Generator does not include a drying unit. A second generator (B) produces dry positively charged nano-particles (e.g., paracetamol) with respectively smaller diameter. Both particle fluxes are directed to a mixing chamber so that flux tabulation and curving by means of deflectors are obtained. Under these conditions, particles of reciprocal surface charges are forced to merge together so that liquid particles comprising polymer solution are incorporated with solid particles. After a partial neutralization, the aerosol cloud remains negatively charged, in the manner that subsequent coagulation of particles is prevented. Out of mixing chamber, the aerosol flow moves to the drying chamber, where the solvent is removed and the finished product is recovered.