WO1993002787A1

WO1993002787A1 - Process for the production of ultra-fine powdered materials

Info

Publication number: WO1993002787A1
Application number: PCT/GB1992/001301
Authority: WO
Inventors: Hélène AGEORGES; Jean-Marie Baronnet; Charles Peter Heanley; John Kenneth Williams
Original assignee: Tetronics Research & Development Co. Limited; Universite De Limoges
Priority date: 1991-07-31
Filing date: 1992-07-16
Publication date: 1993-02-18
Also published as: GB9116446D0

Abstract

A process for the production of powdered materials having a particle size in the range of from 10 to 200 nanometers, which process comprises coupling at least two plasma arcs of opposite polarity in a reactor to form a plasma arc coupling zone, subjecting a target material to heating by means of the plasma arc coupling zone thereby causing the target material to fume, optionally subjecting the target material to a chemical reaction to form a product material, entraining the fumed target material or product material in a gas stream, cooling the said gas stream and collecting the powdered target material or product material.

Description

PROCESS FOR THE PRODUCTION OF ULTRA-FINE POWDERED MATERIALS

The present invention relates to a process for the production of nano-sized powders and, in particular, to a process for the production of very pure nano-sized aluminium nitride. However, the process of the present invention may be used to produce many types of powders, such as Zrθ₂, ^v ₂°₃' ^A12°₃ ^and additionally metals such as

Ti, Fe, , Cr, Si and nitrides of these and other metals.

Conventional aluminium nitride powder preparation methods and subsequent thermal processing produce substrates which possess thermal conductivities in the range of from 110 to 170 watts/metre/°Kelvin. These are significantly lower than the empirically predicted value for pure single crystals (320 watts/metre/°Kelvin) . High purity powders in the 10/100 nanometre (nm) size range (ultrafine) , in principle, allow substrate thermal conductivities to approach the theoretical limit for single crystals of aluminium nitride. We have now developed a process for the preparation of ultra-fine powders, having a particle size in the range of from 10 to 200 nanometres, which is based upon a twin torch plasma system which has the merits of both transferred and non-transferred arc plasmas in as much as the gas flows are low and there is no product contaminating return electrode. Accordingly, the present invention provides a process for the production of powdered materials having a particle size in the range of from 10 to ₂00 nanometres, which process comprises coupling at least two plasma arcs of opposite polarity in a reactor to form a plasma arc coupling zone, subjecting a target material to heating by means of the plasma arc coupling zone thereby causing the target material to fume, optionally subjecting the target material to a chemical reaction to form a product material, entraining the fumed target material or product material in a gas stream, cooling the said gas stream and collecting the powdered target material or product material.

The plasma arcs are usually generated by a system comprising at least two plasma electrodes of opposite polarity, one electrode acting as the cathode and one electrode acting as the anode. A plurality of electrodes of opposite polarity may also be used, if desired. The plasma electrodes are preferably inclined at an angle to one another, preferably in a symmetrical disposition. A wide range of electrode angles are possible ranging from the torches being parallel to each other to pointing at each other. It is preferred to have the electrodes pointing downstream with the angle between the electrodes being in the range of from 80° to 130°, i.e. the electrodes being at an angle of from 40° to 65° to the vertical. The electrode tips may be in close proximity or they may be widely spaced depending upon the electrode sizing and input power. whilst all gases can be ionized to form a plasma, preferred gases for use when no chemical reaction of the fumed target material is being effected are He, Ne, Ar and N2, or mixtures or combinations thereof, with argon and nitrogen being the most preferred gases for use.

The current to the plasma arc torches used in the present invention may conveniently be in the range of from 100 to 1000 amps at a voltage of from 50 to 300 volts. The gas flows through the anode plasma torch and the cathode plasma torch may conveniently be in the range of from 10 to 80 litres per minute.

The fumed product produced according to the process of the invention is of high purity and is ultra-fine, of a particle size in the range of from 10 to 200nm in diameter. The degree of purity depends upon the plasma gases and initial feedstock purity and, when a chemical reaction is being carried out, upon the purity of the reactant.

Using the arrangement of plasma electrodes described above, it will be apparent that there is no requirement for a non-plasma torch return electrical connection, as the conductive path is via the plasma gases through the surrounding gaseous medium.

Accordingly, there are no contamination problems using the process of the invention.

The process of the present invention enables ultra-fine powders 10 to 200 nanometres in diameter to be produced. The process may be operated as a batch process in which the target material is placed below the plasma arc coupling zone in a reactor, or as a continuous process in which granules of the target material are fed to a reactor to replenish the target material consumed.

The target material may optionally be subjected to a chemical reaction during the process of the present invention by reaction with a reactive gas. For example, various metals may be reacted with a source of N atoms to form the metal nitrides. It is preferred that the source of N atoms for the formation of nitrides is nitrogen and/or ammonia. The reactor in which the process of the present invention is carried out preferably comprises a water cooled outer shell. The fumed target material or product material is entrained in a gas stream which is then cooled and the particulate material collected therefrom. The gas stream will comprise the gases from the plasma formation and any gases which are used for reaction with the fumed target material. The gas stream is exhausted from the reactor and cooled, for example in a water-cooled heat exchanger.

In the continuous production of, for example, aluminium nitride, granules of the aluminium target material are fed to the reactor. In a particular embodiment of this process the aluminium target material is fed in the form of granules into a crucible which can be moved downwardly in the vertical direction. Not only is the aluminium target material consumed in the reaction to form ultra-fine powdered aluminium nitride, but as the crucible is moved vertically downwards, an aluminium nitride log is formed in the crucible.

The present invention also includes within its scope an apparatus for the production of powdered materials which comprises: i) a water-cooled reactor; ii) at least one anodic plasma arc electrode and at least one cathodic plasma arc electrode, contained within the reactor, the electrodes being arranged in such a manner that the plasma arcs produced when the electrodes are in use couple together in a coupling zone; iii) a crucible contained within the reactor adapted to contain a target material; iv) means to exhaust gases from the reactor; v) a heat exchanger for cooling the exhaust gases from the reactor; and vi) means for the collection of the powdered material from the exhaust gases.

Preferably the apparatus of the present invention is provided with means to adjust the angle of inclination of the plasma arc torches.

Furthermore, the apparatus will also preferably include means for the introduction of a reactive gas into the reactor.

The means for the collection of the powdered material will generally comprise at least one bag filter, more preferably a plurality of bag filters arranged in parallel. The apparatus may also comprise means for moving the crucible vertically in an upwards or downwards direction.

The process of the present invention is particularly suitable for the production of ultra- fine aluminium nitride. In carrying out this process the cathode and anode arcs are coupled above a target of aluminium, for example an aluminium ingot. The gases used in the formation of the plasma arcs are preferably nitrogen at the cathode and argon at the anode. Preferably an ammonia stream is directed at the aluminium surface in order to increase the rate of reaction. The plasma arcs are transferred onto the aluminium target and due to the thermal energy of the plasma, the metal is melted and metal atoms are vaporised from the metal surface. The nitrogen used as the plasma gas gives N atoms, N⁺ ions, electrons and small amounts of N₂. The aluminium melt becomes supersaturated with nitrogen atoms which escape out of the arc and react into the vapour phase with aluminium vapor to produce aluminium nitride.

The fumed product is then entrained in the gas stream and collected, for example in a bag filter.

The present invention will be further described with reference to the accompanying drawings, in which: Figure 1 is a schematic diagram of the reactor for producing aluminium nitride; Figure 2 is a partial view of the apparatus of Figure 1 during operation;

Figure 3 is a partial view of the apparatus of Figure 1 after operation for a further period of time; Figure 4 is a schematic layout of the powder production operation, and

Figures 5a to 5f are schematic diagrams of the use of a moving crucible in the production of an aluminium nitride log as well as ultra-fine aluminium nitride powder.

Referring to the drawings, two plasma arc torches, 1 and 2 are housed within a stainless steel reactor shell generally shown at 3, the reactor shell being water cooled. Plasma arc torch 1 acts as the anode and plasma arc torch 2 acts as the cathode. The arcs from the plasma arc torches 1 and 2 are coupled together in a coupling zone 4.A graphite crucible 5 is placed beneath the coupling zone 4, the graphite crucible 5 containing a source 6 of the target material.

In the production of aluminium nitride according to the process of the invention, the plasma arc torch 1 preferably comprises an argon plasma arc column with a secondary stream of nitrogen injected via the shroud of the plasma arc torch into the argon plasma arc column. The use of nitrogen as a primary anode gas was found to give excessive erosion and hence product contamination. The nitrogen torch shroud produced a satisfactory product without consumption of the electrode. The cathode torch 2 preferably uses nitrogen as its plasma gas and, for symmetry, incorporates a nitrogen shroud. Gas and water connections to the plasma arc torches 1 and 2 are made via ports 30 and 31, respectively. The plasma arc torches 1 and 2 are pivoted in the reactor shell by pivot means 7 and 8. The pivot means 7 and 8 enable the angles of the plasma arc torches to be varied, as required. As illustrated the torches are at an angle of approximately 120° to one another. In the production of aluminium nitride, ammonia gas is injected into the reactor 3 via two water cooled stainless steel nozzles: a water cooled stainless steel nozzle 9 and a toroidal tube nozzle 10. The primary flow of ammonia is via nozzle 9, with secondary ammonia flow being injected through the toroidal tube 10 positioned below the reactor throat 12. Nozzle 9 is positioned directly above the plasma arc coupling zone and ammonia is directed at the intercept of the centres of the twin plasma arc torches, as shown.

The plasma to plasma centre distance is set at about 75mm for plasma start and transfer of the twin plasma arcs.

The stainless steel reactor 3 may be considered to be divided into three sections - zone A contains the target material, preferably aluminium, in a graphite crucible. In zone B, the twin plasma arcs couple and impinge upon the target to produce a fumed product which expands into zone B where additional ammonia jets impinge directly across the reactor throat. The fumed product is then carried in the gas streams via the reactor throat into an expansion zone C (see Figure 4) where the chamber is expanded to allow an increased residence time of the fumed products.

The course of the reaction may be examined at any time by. means of a sight port 13 introduced in the wall of the stainless steel reactor shell 3.

It will be seen that a growth of aluminium nitride crystals 14 is shown on the surface of the molten aluminium 6, the aluminium nitride crystals 14 being formed shortly after the plasma arc reaction begins.

The next stage in the reaction to produce aluminium nitride is illustrated in Figure 2. As the reaction commences a mushroom growth 20 from the surface of the aluminium reservoir is formed. The mushroom growth 20 was found to consist of aluminium nitride in an aluminium matrix. The mushroom development ceased at a distance of about 100 mm from the plasma torch centres. The fume rate increased as the mushroom approached the plasma arc coupling zone and then decreased until a balance of growth to vaporisation occurred.

Figure 3 illustrates the reaction after a further period of time when a cavity 21 is formed in the mushroom 20 so that a type of "cocoon" is formed. The cavity 21 formed in the mushroom 20 develops as the aluminium reservoir is consumed and it develops around the tail flame of the plasma arcs. Fuming continues as the plasma arc columns impinge on and within the growing cocoon.

Figure 4 of the accompanying drawings shows a schematic layout for the production of ultra-fine powder. Reference numerals, where appropriate, correspond to the reference numerals in Figure 1.

The water cooled reactor 3 is positioned on a trolley 40, or other suitable means of support. The reactor has an inlet 41 for the additional supply of gas to the reactor, for example a supply of nitrogen. The reactor is equipped with an exhaust 42 which is connected to a water-cooled heat exchanger 43. The heat exchanger 43 is provided with a baffle plate 44 which prevents the escape of large particles from the heat exchanger. The heat exchanger is connected via pipe 45, flow valve 46 and ball valve 47 to two bag filters 48 which are arranged in parallel. The bag filters are gas permeable and may be made, for example, from Gortex or from Nomex fibre coated with a fine layer of polytetrafluoroethylene. Twin bag filters are used in parallel to allow off-line collection of the product. The fumed product which exits from the reactor via exhaust 42 is about 300°C lower in temperature than the product in zone B of the reactor. This is the result of the expansion in zone C, as previously discussed. Gases are vented from the bag filter collection unit 48 via line 49. The vented gases comprise approximately 30% hydrogen, 7% argon and the remainder nitrogen. They are then vented via a water trap (not shown) to the atmosphere. Referring to Figures 5a and 5f, Figure 5a shows plasma arc torches 50 and 51 coupling above an aluminium reservoir 52 in a crucible 53. After some minutes of operation, as shown in Figure 5b, a cavity 54 forms in the aluminium reservoir 52. Aluminium feedstock material is then fed in a stream 55 into the cavity 54 and the crucible 53 is moved downwardly, as shown by the arrow in Figure 5c. As shown in Figure 5d the crucible 53 continues to be lowered in a downwardly direction, whilst the aluminium feedstock continues to be fed to the crucible 53 as a stream 55. An aluminium nitride log begins to be formed at 56.

The crucible 53 continues to be lowered as shown in Figure 5e and the aluminium nitride log increases in size. The final aluminium nitride log produced is shown in Figure 5f.

During all of the stages 5a to 5e detailed above ultra-fine aluminium nitride powder is produced by the method as particularly described with reference to Figures 1 and 4. The aluminium nitride log is thus produced as a by-product of the production of ultra-fine aluminium nitride powder.

The present invention will be further described with reference to the following Example.

EXAMPLE

Using the apparatus as described with reference to Figures 1 and 4, aluminium nitride was produced under the following conditions.

Argon was used as the primary gas for the anode torch, with a secondary shroud of nitrogen, whilst nitrogen was used as both the primary anode gas and secondary shroud for the cathode torch. The gas flows were as follows:

40 N litres/minute each shroud (nitrogen) 35 N litres/minute anode electrode (argon) 75 N litres/minute cathode electrode gas (nitrogen) .

The apex angle between the plasma torches was 120° and the plasma centre to centre distance was set at 75mm for plasma start and transfer of the twin arcs. Ammonia was directed at the intercept of the centres of the twin torches as shown in Figure 1, the ammonia flow rate being 70 N litres/minute. Secondary ammonia was injected across the throat of the reactor from nozzles 10 and 11 at a rate of 70 N 1itres/minute.

The plasma current was in the range of from 350 to 420 amps, although a higher current was used initially to heat up the aluminium target material. Under these conditions, aluminium nitride was produced and collected in the bag filters. The aluminium nitride had a particle size in the range of from 12 to 100 nanometres.

Using this method the aluminium nitride was produced at a rate of approximately 3 kilograms per hour.

Claims

CLAIMS :

1. A process for the production of powdered materials having a particle size in the range of from 10 to 200 nanometres, which process comprises coupling at least two plasma arcs of opposite polarity in a reactor to form a plasma arc coupling zone, subjecting a target material to heating by means of the plasma arc coupling zone thereby causing the target material to fume, optionally subjecting the target material to a chemical reaction to form a product material, entraining the fumed target material or product material in a gas stream, cooling the said gas stream and collecting the powdered target material or product material.

2. A process as claimed in claim 1 wherein the plasma arcs are generated by at least two plasma arc electrodes, a least one electrode acting as the cathode and at least one electrode acting as the anode.

3. A process as claimed in claim 2 wherein the plasma electrodes are inclined at an angle to the vertical.

4. A process as claimed in claim 3 wherein the plasma electrodes are symmtrically disposed, each being at an angle of 40° to 65° to the vertical.

5. A process as claimed in any one of the preceding claims wherein the plasma arcs are formed using argon, helium, neon or nitrogen.

6. A process as claimed in any one of the preceding claims wherein the target material is subjected to nitridation by reaction of the target material with a source of N atoms.

7. A process as claimed in claim 6 wherein the source of N atoms is ammonia and/or nitrogen.

8. A process as claimed in claim 6 or claim 7 wherein the target material is aluminium.

9. A process as claimed in any one of the preceding claims wherein the reactor has a water-cooled outer shell.

10. A process as claimed in any one of the preceding claims wherein the gas stream entraining the fumed target material or product material is exhausted from the reactor and cooled in a heat exchanger.

11. Apparatus for the production of powdered materials which comprises:

i) a water-cooled reactor; ii) at least one anodic plasma arc electrode and at least one cathodic plasma arc electrode, contained within the reactor, the electrodes being arranged in such a manner that the plasma arcs produced when the electrodes are in use couple together in a coupling zone; iii) a crucible contained within the reactor adapted to contain a target material; iv) means to exhaust gases from the reactor; v) a heat exchanger for cooling the exhaust gases from the reactor; and vi) means for the collection of the powdered material from the exhaust gases.

12. Apparatus as claimed in claim 11 wherein the plasma arc torches are provided with means for adjustment of the angle of inclination.

13. Apparatus as claimed in claim 11 or claim 12 which includes means for the introduction of a reactive gas into the reactor.

14. Apparatus as claimed in any one of the claims 11 to 13 wherein the means for the collection of the powdered material comprises at least one bag filter.

15. Apparatus as claimed in any one of claims 11 to 13 which additionally comprises means for moving the crucible vertically in an upwards or downwards direction.