WO1999018032A1

WO1999018032A1 - Method and apparatus for vapor phase manufacture of nanoparticles

Info

Publication number: WO1999018032A1
Application number: PCT/US1998/020861
Authority: WO
Inventors: Gerhard Kappler; Christian Artelt; Wolfgang Peukert
Original assignee: Hosokawa Micron International, Inc.
Priority date: 1997-10-06
Filing date: 1998-10-05
Publication date: 1999-04-15
Also published as: AU1066799A

Abstract

A method and apparatus for the manufacture of nanometer-sized particles using vapor phase process in a reaction chamber (102) with electrodes (104) to impart electric charge to the particles formed by the combustion of reactants, which includes the introduction of a cool, inert gas of sufficient amount and velocity to flush the particles from the reaction chamber (102) to prevent agglomeration of the particles at or near the electrodes (104). The electrodes (104) are preferably arranged about a rectangular burner mouth (109) for creating a stable electric field over the entire cross section of the reaction chamber (102).

Description

METHOD AND APPARATUS FOR VAPOR PHASE MANUFACTURE OF NANOPARTICLES

FIELD OF THE INVENTION This invention relates to the manufacture of materials with nanometer-sized particles, such as titanium dioxide, carbon black and others, using vapor phase process wherein the particles are formed by combusting reactant gases in a reaction chamber under controlled reaction conditions.

BACKGROUND OF THE INVENTION Various physical, chemical and mechanical methods have been devised for the synthesis of nanometer-sized particles. These are described in detail in the Patent Cooperation Treaty ("PCT") application No. WO 98/02241. Of particular relevance in the present invention is the prior art on the synthesis of nanometer-sized particles (1) using flame aerosol reactors, and (2) charging particles during their formation.

Flame technology is routinely used in large scale manufacture of carbon blacks and ceramic commodities such as fumed silica and specialty chemicals such as zinc oxide. This technology involves oxidizing reactants in an exothermic reaction to form particles of different submicron sizes depending on process conditions as well as the materials used. Manufacture of certain substances reguires catalysts with uniformly precise surface areas and their crystalline structure. For example, titanium dioxide (Ti0₂) has been found to be an effective photo-catalyst at submicron particle sizes. For this reason, numerous methods have been suggested to reduce the particle sizes to achieve better results in the uniformity of the particle density and size.

Usage of the vapor phase process for the manufacture of finely powdered particles is generally known to practitioners in the art. The many advantages of the vapor phase process over other methods are widely known and have resulted in substantial work in this area. It is widely known to persons skilled in the art that regulation of process conditions—such as process temperature, gases mixed in the reactor, reactants and precursors selected, time spent by the reactants in the reaction area and the electrical fields applied to the reaction area—varies the physical characteristics of particles formed using the vapor phase process.

U.S. patent 5,698,177 teaches the preparation of titanium dioxide powder by mixing vapor phase reactants in a reaction area, externally heating the mixture in the reaction area, and collecting the titanium dioxide powder formed. This patent also teaches that dopants, when added to the reaction mixture, affect the desirable properties of titanium dioxide produced. This patent also teaches that reactants be added to the reaction by bubbling an inert gas, such as argon, through liquid reactants and by directing that gas to the reaction area. Additionally, according to the patent, a corona electric field located across the reaction area where the combustion takes place maximizes production of particles having high surface area and low rutile (high anatase) content . While there are advantages to the vapor phase process, there also are problems, including the depositing of reactants on reactor walls, unsatisfactory mixing of the reactants, problems with heat transfer and particle agglomeration on electrodes resulting in particles of larger and uneven sizes. Accordingly, there is a need for a method and an apparatus to prevent agglomeration of the particles on the electrodes and flush them out of the reaction chamber as soon as they are formed.

SUMMARY OF THE INVENTION

The present invention is directed to a method and apparatus for manufacturing nanometer-sized particles using vapor phase process. The apparatus comprises a reaction chamber bounded by walls where gaseous reactants are burned to produce finely sized particles. The walls of the reaction chamber has openings through which electrodes are introduced into the chamber to impart electric charge to the particles formed in the chamber. The electrically charged particles move toward electrodes of opposite polarity causing an agglomeration at the electrodes. Such particle agglomeration at the electrodes interferes with the electric field, causing unevenly sized particles to be formed. In the present invention, agglomeration of particles near electrodes is overcome by introducing a gas that is cooler than the reactants into the main gas stream in an amount and with a velocity sufficient to dislodge particles from the electrodes. Specifically, the present invention relates to an apparatus for the manufacture of nanometer-sized particles, comprising a reaction chamber which includes at least one wall; means for combusting at least one gaseous reactant in the reaction chamber to form particles; a plurality of electrodes disposed within the reaction chamber to impart an electrical charge to the particles; and means associated with the electrodes to provide a gas thereto in an amount and at a velocity sufficient to dislodge particle agglomeration from the electrodes. In one aspect of a preferred embodiment, the gas providing means includes a plurality of holes in the reaction chamber wall and wherein electrodes pass through each hole to extend into the reaction chamber. Another aspect of a preferred embodiment includes a conduit arranged around each needle tip electrode and extending into the reaction chamber through the wall to direct the gas around the electrodes. In yet another aspect of a preferred embodiment, the needle tip electrode extends into the reaction chamber beyond the conduit end. The electrodes may include pairs of wires or porous fibers made of conductive material, and are preferably mounted along the wall. It is also possible for the reaction chamber wall to be porous to allow gas passage therethrough. Preferably, needle tip electrodes extend through the porous wall into the reaction chamber. Also, the gas providing means can include an antechamber for allowing entry of gas into the reaction chamber, preferably with the antechamber filled with a plurality of beads or similar members to further provide for an even flow of gas.

In another embodiment of the apparatus, the combusting means the combusting means includes a burner mouth having a substantially rectangular shaped cross-section and the electrodes are arranged around the burner mouth to create a uniform and stable electric field over the entire cross section of the reaction chamber to control the growth of the particles. Preferably, the burner mouth includes four sides, two of which have a length greater than the other two, the burner mouth is positioned within the reaction chamber wall, and the rows of electrodes are arranged along the greater length sides of the burner mouth.

Another aspect of the invention relates to a method for the manufacture of nanometer-sized particles comprising the steps of burning at least one gaseous reactant in a reaction zone to form particles; imparting an electric charge to the particles by disposing a plurality of electrodes in the reaction zone; and providing a gas adjacent to the electrodes in an amount and at a velocity sufficient to dislodge particle agglomeration from the electrodes.

Preferably, the gas is inert and is cooler than the burning reactant (s) and said electric charge is unipolar, bipolar or of alternating polarity. The gas may include a precursor or dopant to change the composition of the particles. Advantageously, gas is introduced around the electrodes and the electrodes are disposed around the burning gaseous reactant (s) in a manner to create a stable electric field over the entire cross section of the reaction zone to control the growth of the particles.

Another method relates to the manufacture of nanometer- sized particles comprising the steps of burning at least one gaseous reactant in a reaction zone to form particles; disposing a plurality of electrodes around the burning gaseous reactant (s) in a manner to create a uniform and stable electric field over the entire cross section of the reaction zone so as to impart an electric charge to the particles for control of particle growth; and collecting the particles. In this method, the gaseous reactants are burned in a zone of substantially rectangular cross-section and the electrodes are disposed around the substantially rectangular cross-sectional zone to establish the electric field.

BRIEF DESCRIPTION OF THE DRAWINGS Preferred features of the present invention are disclosed in the accompanying drawings, wherein similar reference characters denote similar elements throughout the several views, and wherein:

Fig. 1 is a top view of an arrangement of the burner and the charging device;

Fig. 2 is a side view of the arrangement shown in Fig. 1 including the burner hood, the burner mouth and flushing gas inlets on either side of the gas delivering chamber;

Fig. 3 is a detailed view of the gas delivering chamber and the charge providing unit;

Fig. 4 is a front view of a multiple-needle arrangement of the charge providing unit shown in Fig. 3;

Fig. 5 is a side view of a plurality of needle electrodes extending through a porous sintered metal plate arrangement;

Fig. 6 is a cross-sectional view of a porous plate arrangement;

Fig. 7 is a front view of a porous sintered metal plate arrangement with wire electrodes;

Fig. 8 is a side view of the porous plate/wire electrode arrangement of Fig. 7; Fig. 9 is a cross-sectional view of the needle electrode featuring an outlet for flushing gas;

Fig. 10 is a cross-sectional view of one embodiment of the needle core and the sheathing assembly;

Fig. 11 is a cross-sectional view of the needle core and the sheathing assembly;

Fig. 12 is a side view which depicts the insulation of the needle electrode; Fig. 13 is an elevational view of a rectangular burner mouth for premixed gaseous reactants; and

Fig. 14 is a top view of the rectangular burner mouth of Fig. 13.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention is directed to a method and apparatus for the manufacture of nanometer-sized particles in a reaction zone using vapor phase process. In general, it is known that the physical characteristics such as size and crystal structure of a particle formed using vapor phase process can be controlled by regulating process conditions such as process temperature, gas mixing, selection of reactants and precursors, residence time in the reaction area and by applying electrical fields to the reaction area. By the present invention, however, using a single device, independent control of the process temperature and the application of electric fields is achieved to control particle synthesis more precisely. In particular, in the present invention, particle growth is controlled by means of charging the particles generated due to a reaction between reactant gases and vapors, while process temperature is influenced by blowing gas into the reaction zone.

As illustrated in Figures 1 and 3 , the present invention employs an apparatus which comprises a pair of opposing gas delivery chambers 105, charge providing units 107 and reactor walls 101. The process temperature is controlled by introducing gas into the reaction zone as soon as possible after particle generation has taken place. Preferably, such gas is cooler than the reaction zone temperature and is inert.

As used herein, the term "inert" means a non-reactive, non- combustible gas such as nitrogen or argon, which is inert to the chemical reactions and reactants employed to form the desired nanometer-sized materials. The introduction of the cold gas, in relation to the temperature of the particle, cools or quenches the particle, thereby effectively freezing the vapor phase reaction and accordingly, stopping further particle size growth. Thus, the flow of cold gas can be employed to control the size and shape of the particles formed by vapor phase reaction.

The gas delivery chamber 105 may optionally be filled with beads 106 made of a non-conductive, heat-resistant material such as glass or a ceramic to promote equal gas flow distribution throughout the gas delivery chamber 105. The gas delivery chamber 105 is also made of a non-conductive heat resistant material, preferably a ceramic, in order to provide insulation for the charge providing unit 107. A grid 108 separates the charge providing unit 107 and the gas delivery chamber 105 in order to prevent the beads in the gas delivery chamber from interfering with the charge providing unit. The gas passes through the charge providing unit and exits to a reaction zone through a plurality of openings in the reactor wall 101, near an electrode of the charge providing unit. As illustrated in Figures 3 to 8, the openings 122 through which the cooling gas exits can be formed by a variety of means. An additional embodiment includes a tube made of a heat resisting material surrounding the electrodes forming the outlet for the flushing gas. (Figures 1-4, 9 and 10). Alternatively, as shown in Figures 5-7 , the gas may exit through a porous sintered metal or ceramic plate 130 which forms all or a portion of the reactor wall 101. Suitable materials for plate 130 include nickel-chromium alloys such as stainless steel, hastelloy and inconel or ceramics such as A1₂0₃, Si₂0₃ and Sic.

The charge is applied to the particles formed by vapor phase reactions by forming an electric field (s) using one or more pairs of electrodes preferably placed on opposing sides of the reaction zone 102. A potential is provided by an HV ground supply 110 to a plurality of suitable charge emitters that provide a corona-type discharge. Such suitable charge emitters include, but are not limited to, needle tip electrodes 104 (as shown in Figures 1-4) , wire(s) 131 (as shown in Figures 7 and 8) and the surface of any porous conductive material made of small grain sizes or fibers.130. The corona type discharge introduces new ions to the reaction zone, wherein such ions attract and thereby separate the charged particles that are formed in the flame. In particular, the charge emitters attract and effectively separate the positive and negative ions that are formed in the flame as the flame ions move rapidly towards each electrode of opposite polarity. These ions charge the newly formed particles and slow down collisions between particles, as well as slowing growth thereof. The mechanism by which a corona discharge electric field facilitates the removal of the particles formed by vapor phase reaction from the reaction area and limits the particle size thereof is more fully disclosed in PCT Application No. PCT/US95/10841, published March 7, 1996, the disclosure of which is hereby incorporated by reference in its entirety. As illustrated in Figures 1-3, in one embodiment, the present invention employs a pair of opposing gas delivery chambers and charging devices each of which comprises a charge providing unit 107 formed from a plurality of needle electrodes 104 which extend through the reactor walls 101. The needles from each charge providing unit oppose one another so as to provide a corona discharge electric field across burner mouth 109. The reactor walls 101 are formed so as to provide an opening around each needle electrode through which the cool, inert flushing gas 111 is introduced into the reaction zone. In another embodiment, a precursor or dopant (e.g., another gaseous reactant) 112 can be introduced instead of an inert gas when composite powder particles are to be formed. Suitable precursors include any of the conventionally used materials, including halogenides such as A1C1₃, VC1₄, SiCl₄, TiCl₄, FeCl₂, and organic compounds such as the silanes, e.g., Si(CH₃)₄.

As shown in Figure 4 , which depicts an embodiment of the invention, each charge emitting unit comprises a plurality of needle electrodes 104, each surrounded by a flushing gas outlet 122. The inert gas passing through the outlets 122 also cools the burner flame to help influence the size and shape of the resultant particles that are formed. As illustrated in Figure 5, the needle electrodes 104 extend through a porous sintered metal plate 130. Instead of having discrete flushing gas outlets 122, which are preferably tubular outlets as shown, surrounding each needle electrode, in this embodiment of the invention, the flushing gas flows directly through a porous plate 130 of a sintered metal, thus flowing by the needle electrodes and into the reaction zone. Although Figure 5 illustrates that plate 130 is a sintered metal plate, this plate can be made from any porous material. Preferably, a porous non-conductive material which is capable of withstanding exposure to the elevated temperatures of the reaction zone 102 is used. Typically, porous ceramics, such as A1₂0₃, Si0₂ or mixtures thereof, are useful as this plate. Figure 6 illustrates another embodiment of the present invention wherein the charge emitter is formed from a porous conductive sintered metal plate. In this embodiment of the invention, the electric field is provided across the reaction zone by corona discharges emitted from grain or fibers on the surface of the conductive plate. The flushing gas flows directly through the conductive plate and into the reaction zone 102.

Figures 7 and 8 provide front and side views of an another alternative embodiment of the present invention, wherein the charge providing unit comprises a plurality of wires 131. Suitable materials for wire 131 include nickel- chromium-iron alloys such as stainless steel, hastelloy and inconel. A porous plate 130 is employed to introduce the flushing gas across the wires 131 and into the reaction zone 102. The porous plate may be made from any non-conducting material capable of withstanding the elevated temperature of the reaction zone, such a ceramic or sintered metal.

Additionally, Figures 9-11 provide detailed illustrations of a needle electrode and sheathing assembly for electrodes which are suitable for use in the present invention. As illustrated in these figures, the needle electrode is surrounded by a sheathing which extends along substantially the entire length of the needle electrode, thereby forming the flushing gas outlet 122 which allows for the introduction of the cool, inert gas into the reaction zone. Preferably, the needle electrode and the sheathing are formed from stainless steel or other high temperature, corrosion resistant alloys. Suitable materials for the needle electrode and sheathing include nickel-chromium-iron alloys such as stainless steel, hastelloy and inconel.

The charge emitters are preferably arranged in one elevation in order to provide electric fields with uniform and stable conditions. Such electric fields are unipolar (either polarity) , bipolar or alternating. It is very common in the art of vapor phase reaction to employ a gas burner having a substantially circular gas discharge port or mouth through which the combustion gas flows and is ignited. For this invention, however, it is more desirable for the gas burner discharge port to be rectangular in shape, as opposed to the conventional circular port, because it is very difficulty to obtain a uniform electric field across the reaction zone when the charge emitters must be configured around a substantially circular burner discharge port.

For the control of particle growth in the entire reaction chamber, it is important to maintain a stable electric field over the full cross section of the reaction chamber. By using a burner having a discharge mouth 109 with a substantially rectangular cross-section, as shown in Figure 1, the charge emitters can readily be positioned in such a configuration so as to provide a relatively uniform electric field across substantially all of the reaction zone. The resulting electric field is nearly homogenous and can be precisely controlled, thereby providing precise control of the particle size of the powders formed therein.

In addition, a rectangular cross section burner allows one to establish a uniform and stable electric field both in laboratory size reactors as well as for large production size reactors. The length of the short burner side depends upon the maximum distance between the oppositely arranged tips of the electrodes. This maximum distance between these electrode tips depends primarily upon the applied voltage to the electrodes. As an example, for an applied voltage of lOOkV on an array of needle tip electrodes surrounding a rectangular burner mouth, the maximum distance between the electrode tips would be about 20 cm, and the length of the burner mouth, in theory, could be as long as possible. For a burner of 100 cm length, the area would be 2,000 cm². In comparison, a circular burner mouth must also provide a maximum distance of 20 cm, but this would be its radius. As the area of a circular burner mouth would be 314.6 cm², the rectangular burner mouth provides an increase in area of about 635%. This allows a much greater scale up range than a circular burner mouth. Although it is possible to use multiple circular burner mouths to try to increase the coverage area, the use of multiple burner mouths makes operation more difficult. Even so, the 100 cm long rectangular burner mouth provides an increase in area of 27% over the use of five circular 20 cm diameter circular burner mouths, along with simpler control of particle size growth. Further, in addition to controlling the particle size growth of the particles resulting from the vapor phase reaction, the relatively cool gas introduced into the reaction zone also acts as flushing gas which prevents the deposition of the particles on the surface of the charge emitters, as well as reducing the temperature and corrosion of the charge emitting devices. Without the introduction of such flushing gas, the particles rapidly deposit on the surface of the charge emitters, reducing and eventually completely eliminating the electric field in the reaction zone. The introduction of cool, inert gas also reduces the temperature of the burner flame with a corresponding reduction in the size of the particles that are formed. The gas introduced into the reaction zone provides additional benefits, including, diluting the reaction gases, assistance in transporting the particles out of the reaction zone, as well as optionally containing additional precursors in order: a) to produce composite powders; b) to control phase composition; and/or c) to sinter by means of additives.

The present invention is employed to form a wide variety of fine powdered materials, including but not limited to nanometer-sized metal or ceramic powders, carbides, nitrides, oxides, borides, suicides, phosphites, sulphides and/or combinations thereof of the elements selected from the group consisting of Group 4b, 5b, 6b, 7b and 8 transition metals of the Periodic Table. In particular, examples of materials that the present invention is employed to produce includes nanometer sized powders of pyrogenic oxides. Examples of specific materials that can be made by the present invention include, but are not limited to, titanium dioxide and carbon black. Further, it should be understood that variations and modifications within the spirit and scope of the invention may occur to those skilled in the art to which the invention pertains. Accordingly, all expedient modifications readily attainable by one versed in the art from the disclosure set forth herein that are within the scope and spirit of the present invention are to be included as further embodiments of the present invention. The scope of the present invention is accordingly to be defined as set forth in the appended claims.

Claims

THE CLAIMSWhat is claimed is:

1. An apparatus for the manufacture of nanometer-sized particles comprising: a reaction chamber which includes at least one wall; means for combusting at least one gaseous reactant in the reaction chamber to form particles; a plurality of electrodes disposed within the reaction chamber to impart an electrical charge to the particles; and means associated with the electrodes to provide a gas thereto in an amount and at a velocity sufficient to dislodge particle agglomeration from the electrodes.

2. The apparatus of claim 1 wherein said electrodes further comprise porous fibers made of conductive material.

3. The apparatus of claim 1 wherein said electrodes further comprise pairs of wires.

4. The apparatus of claim 1 wherein said electrodes further comprise pairs of needle tip electrodes.

5. The apparatus of claim 4 wherein said gas providing means includes a plurality of holes in the reaction chamber wall and wherein a needle tip electrode passes through each hole to extend into the reaction chamber.

6. The apparatus of claim 5 wherein said gas providing means includes a conduit arranged around each needle tip electrode and extending into the reaction chamber through the wall to direct the gas around the electrodes.

7. The apparatus of claim 6 wherein the needle tip electrode extends into the reaction chamber beyond the conduit end.

8. The apparatus of claim 3 wherein the electrodes are mounted along the wall.

9. The apparatus of claim 1 wherein the reaction chamber wall is porous to allow gas passage therethrough and the electrodes comprise needle tip electrodes which extend through the porous wall into the reaction chamber.

10. The apparatus of claim 1 wherein said gas providing means further comprises an antechamber for allowing entry of gas into the reaction chamber.

11. The apparatus of claim 10 wherein said antechamber is filled with a plurality of beads to further provide for an even flow of gas.

12. The apparatus of claim 1 wherein the combusting means includes a substantially rectangular shaped burner mouth and the electrodes are arranged in rows around the burner mouth to create a uniform and stable electric field over the entire cross section of the reaction chamber to control the growth of the particles.

13. An apparatus for the manufacture of nanometer-sized particles comprising: a reaction chamber which includes at least one wall; means for combusting at least one gaseous reactant in the reaction chamber to form particles; and a plurality of electrodes disposed within the reaction chamber to impart an electrical charge to the particles; wherein the combusting means includes a burner mouth having a substantially rectangular shaped cross-section and the electrodes are arranged around the burner mouth to create a uniform and stable electric field over the entire cross section of the reaction chamber to control the growth of the particles.

14. The apparatus of claim 13 wherein the burner mouth includes four sides, two of which have a length greater than the other two, the burner mouth is positioned within the reaction chamber wall, and the rows of electrodes are arranged along the greater length sides of the burner mouth.

15. A method for the manufacture of nanometer-sized particles comprising the steps of: burning at least one gaseous reactant in a reaction zone to form particles; imparting an electric charge to the particles by disposing a plurality of electrodes in the reaction zone; and providing a gas adjacent to the electrodes in an amount and at a velocity sufficient to dislodge particle agglomeration from the electrodes.

16. The method of claim 15 wherein said gas is inert and is cooler than the burning reactant (s) and said electric charge is unipolar, bipolar or of alternating polarity.

17. The method of claim 15 wherein said gas includes a precursor which can be combusted with the burning reactant (s) to form composite particles.

18. The method of claim 15 wherein the gas is introduced around the electrodes and into the reaction zone.

19. The method of claim 15 wherein the electrodes are disposed around the burning gaseous reactant (s) in a manner to create a stable electric field over the entire cross section of the reaction zone to control the growth of the particles.

20. The method of claim 19 wherein the gaseous reactants are burned in a zone of substantially rectangular cross- section and the electrodes are disposed around the substantially rectangular cross-sectional zone to establish a uniform and stable electric field therearound.

21. A method for the manufacture of nanometer-sized particles comprising the steps of: burning at least one gaseous reactant in a reaction zone to form particles; disposing a plurality of electrodes around the burning gaseous reactant (s) in a manner to create a uniform and stable electric field over the entire cross section of the reaction zone so as to impart an electric charge to the particles for control of particle growth; and collecting the particles.

22. The method of claim 21 wherein the gaseous reactants are burned in a zone of substantially rectangular cross- section and the electrodes are disposed around the substantially rectangular cross-sectional zone to establish the electric field.