Plasma synthesis of metal oxide nanopowder

<u>FIELD OF THE INVENTION</u>

The present invention relates to a process for the synthesis of metal-containing powders. In particular but not exclusively, the present invention relates to the synthesis of nanosized particles of titanium dioxide by the oxidation of titanium tetrachloride in the vapour phase via induction plasma followed by rapid cooling.

<u>BACKGROUND OF THE INVENTION</u>

Pigments that contribute light-scattering properties to coatings are generally known as white, or hiding, pigments. They act by scattering all wavelengths of light, owing to their relatively high refractive index, so that they are perceived as white to the human eye. The most widely used white pigment is titanium dioxide (TiO₂), a polymorphous substance that exists in three modifications or crystal structures, rutile, anatase or brookite. Only the anatase and rutile modifications are of any note, technically or commercially.

The high demand for titanium dioxide based pigments is driven by a combination of a high refractive index and a reasonable manufacturing cost. Additionally, titanium dioxide based pigment does not suffer from the same environmental considerations as earlier white pigments such as Lead carbonate, which had a high toxicity and were readily released into the environment when placed in contact with water.

The anatase phase of titanium dioxide has a lower refractive index and is generally less durable than the rutile form, which makes it less desirable as a coating pigment. However, as will be seen below, both the lower refractive index and lower durability are highly desirable in some applications.

Although the most important use for titanium dioxide is as a pigment, the material is in fact colourless. To reveal its special properties, the titanium dioxide must first be processed to a certain particle size. For example, for pigment applications the particle size would be one half the wavelength of visible light or about 0.3 microns.

Aside from its excellent properties as a pigment, titanium dioxide has dielectric properties, high ultraviolet absorption and high stability which allows it to be used in speciality applications, such as Electro-ceramics, glass and as an insulator.

Titanium dioxide pigments are used in man-made fibres, such as polyester, viscose and rayon to name a few. As man made fibres have an undesirable glossy and translucent appearance, the pigment is incorporated into the fibre during the spinning process either for brightening the fibre or reducing the fibre's lustre. For this application the anatase phase is advantageous since as it has a more neutral white tone than the rutile modification and is also less abrasive. This latter property is very important as the process for spinning fibres is very delicate and would be adversely affected by the addition of the rutile form of titanium dioxide to the fibres. Anatase, on the other hand, is a photo catalyst that is activated by ultraviolet radiation resulting in the rapid degradation of the man made fiber when exposed to sunlight.

Titanium dioxide is also used for adding opacity and brightness to plastics. The opaqueness and high brightness help mask the poor natural colour of many plastics. Additionally, some grades of titanium dioxide absorb ultraviolet light which can accelerate the ageing of plastics.

Additionally, titanium dioxide is added as a filler to the pulp in paper manufacturing processes to enhance brightness and opaqueness. This allows, for example, for the production of highly opaque lightweight papers. For this application titanium dioxide in its anatase phase can be used.

In order to manufacture titanium dioxide, a source of titanium is required. Although titanium ranks ninth in abundance among elements found in the crust of the earth, it is never found in the pure state. Rather, it occurs as an oxide in the minerals ilmenite (FeTiO₃), rutile (TiO₂) or sphene (CaO-TiO₂-SiO₂).

The production of titanium dioxide pigments is a two step process. The first step is to purify the ore, and is basically a refinement step. This may be achieved by either the sulphate process, which uses sulphuric acid as a liberating agent or the chloride process, which uses chlorine as the liberating agent.

In the sulphate process, the titanium containing ore is dissolved in sulphuric acid, yielding a solution of titanium, iron, and other metal sulphates. Through a series of steps including chemical reduction, purification, precipitation, washing, and calcination, pigment size TiO₂ is produced.

Alternatively, the chloride process includes high-temperature, anhydrous vapour phase reactions. Titanium ore is reacted with chlorine gas under reducing conditions to obtain titanium tetrachloride (TiCl₄) and metallic chloride impurities, which are subsequently removed. Highly purified TiCl₄ is then oxidized at high temperature to produce intermediate TiO₂. The oxidation step in the chloride process permits control of particle size distribution and crystal type, making it possible to produce high quality pigment grade TiO₂.

The chloride process is inherently cleaner than the sulphate process and requires a smaller investment on behalf of the manufacturer in terms of waste treatment facilities. Additionally, titanium dioxide produced using the chlorine process is generally of higher purity, more durable and has a particle size distribution which is narrower, the latter improving brightness, gloss and opacity.

As stated above, the chloride process includes high-temperature anhydrous vapour phase reactions where liquid titanium tetrachloride is vaporised and superheated after which it is reacted with hot oxygen to produce titanium dioxide. The superheating and subsequent reaction phase can be carried out either by a refractory process, where the reactants are heated by refractory heat exchangers and combined. Alternatively, carbon monoxide can be purified and then mixed with the titanium tetrachloride and oxidizing agent and then the mixture subject to a controlled combustion. Finally, the titanium tetrachloride can be vaporised in a hot plasma flame along with the oxidizing agent. This final method has proven to be the most efficient.

A number of technical approaches are available for generating the plasma. For example, the plasma may be generated by passing the working gas between a pair of electrodes whereby an arc discharge ionizes the gas as it passes between. A drawback of this approach is that the electrode is bound to contaminate the working gas, either by trace chemical reaction between the electrode and the working gas, or by degradation of the electrodes. This drawback is particularly acute when the working gas is an inert, reducing or oxidizing gas.

United States Patent No. <patcit id="pcit0001" dnum="US5935293A"><text>5,935,293</text></patcit>, entitled "Fast Quench Reactor Method" issued to Detering et al. on August 10, 1999 described a method for producing ultra-fine solid particles in an electrode-generated plasma reactor. The reactor is configured so as to cause a metal halide reactant stream introduced in the reactor to expand after reaching a predetermined reaction temperature thereby causing rapid cooling thereof. The expansion results from the stream passing through a quench zone where the stream reaches supersonic velocity. The quench zone is intended to prevent back reaction and promote completion of the reaction.

A major drawback of the Detering method, in addition to the above-mentioned contamination problem, is that it does not lend itself for a reactant dilution sufficiently high for the generation of nanopowders, and to avoid powder agglomeration. Indeed, electrode-generated plasma are known to be relatively high-energy and to yield non-uniform temperature in the reactor. Those two conditions prevent the use of important dilution of reactant and render difficult control on the particle size distribution. It is to be noted that the Detering method, when used in the synthesis of TiO₂, does not promote the production of its anatase phase.

In other known methods, the working gas may be passed through a high frequency electrostatic field. According to other known methods, the working gas may be passed through a high frequency induction coil whereby the electromagnetic field ionizes the gas as it passes within the coil. It is to be noted that induction plasma torches are characterized by a volume discharge larger than direct current plasma source, and a longer residence time. Indeed, for comparable power rating, an induction plasma torch would operate with more than 100 standard liters per minute of plasma gas, compared with 20-30 standard liters per minute of plasma gas with electrode-generated plasma reactor.

The synthesis of pigment grade titanium dioxide through the oxidation of titanium tetrachloride in a plasma flame formed by passing a working gas through a high frequency induction coil is well known in the art as for example from <patcit id="pcit0002" dnum="WO02086179A"><text>WO 02/086179</text></patcit> and has been used industrially for some time for the commercial production of such powders for the paint industry.

Traditionally, the product obtained in this case is composed of relatively large opaque particles with a particle size in the range of 0.2 to 2.0 micrometers or more. Such powders are used as a base material for the production of a wide range of paints and surface modification coatings.

There has always been an interest in obtaining finer powders in the nanometer range for a wide variety of other applications including ultraviolet protection and the sunscreen industry as well as for advanced catalyst development. However, the development of a process to produce large quantities of titanium dioxide nanopowders has proved difficult to attain. The main obstacle has been the method to achieve such an important reduction in the size of distribution of the powder and control its chemistry and surface properties.

<u>SUMMARY OF THE INVENTION</u>

The present invention addresses the above limitations by providing an improved process for the production of metal oxide nanopowders.

More specifically, in accordance with a first aspect of the present invention, there is provided a process for the synthesis of a metal oxide nanopowderfrom a metal compound vapour. This process comprises the steps of bringing the metal compound vapour to a reaction temperature, reacting the metal compound vapour at the reaction temperature with an oxidizing gas to produce a metal oxide vapour, producing a highly turbulent gas quench zone, and producing the metal oxide nanopowder by cooling the metal oxide vapour in the quench zone.

Accordingly, the process of the invention enables the production of a metal oxide nanopowder with a controlled particle size distribution and surface reactivity.

Also in accordance with the present invention, there is provided a process for the synthesis of a metal oxide nanopowder from a metal compound vapour, comprising: <ul id="ul0001" list-style="none" compact="compact"><li>generating an induction plasma jet by passing a working gas through a high frequency electromagnetic field;</li><li>introducing the metal compound vapour and the induction plasma jet through a first axial end of a reactor; the plasma jet causing the metal compound vapour to reach a reaction temperature and to react with the working gas, yielding nanosized metal oxide particles;</li><li>rapidly cooling the nanosized metal oxide particles in a quench zone of said reactor located downstream from the first axial end, thereby stopping the growth process of the nanosized metal oxide particles, yielding metal oxide nanopowder; and</li><li>collecting the metal oxide nanopowder downstream from the quench zone;</li><li>whereby, the combination of a) reacting the metal oxide compound with the induction plasma; induction plasma allowing for sufficiently large volume discharge and sufficiently long residence time in the reactor, and b) rapidly cooling the yielded nanosized particles in a quench zone, allows to control the metal oxide particles sizes.</li></ul>

Additionally, in accordance with another embodiment, a doping agent may be mixed with the metal chloride prior to injecting said metal chloride in the plasma.

In accordance with still another embodiment, the step of reacting the metal chloride vapour with an working gas may further comprise injecting a doping agent into the plasma after the metal chloride has reacted with the oxidizing gas.

In accordance with yet another embodiment, the metal oxide nanopowder may be coated with a doping agent.

In accordance with a second aspect, there is provided an apparatus for synthesising a metal oxide nanopowder from a metal compound vapour. The apparatus comprises the following elements: <ul id="ul0002" list-style="none" compact="compact"><li>a reaction chamber including a vertically disposed generally tubular chamber section and a taper chamber section mounted at a lower end of the vertically disposed generally tubular chamber section for collecting synthesized metal oxide nanopowder;</li><li>an induction plasma assembly including a reactant mixing chamber mounted to an upper end of the vertically disposed generally tubular chamber section so as to be in fluid communication with the reaction chamber; the induction plasma assembly further including an inductive coil surrounding the reactant mixing chamber for generating a high frequency magnetic field in the reactant mixing chamber, a first inlet for receiving a first working gas and a second inlet for receiving the metal compound vapour; the first and second inlet being connected to the generally tubular reactant mixing chamber; and</li><li>a plurality of substantially coplanar quench gas nozzles connected to the periphery of the vertically disposed generally tubular chamber section below the upper end thereof for creating a quench zone in the reaction chamber;</li><li>whereby, in operation, an induction plasma jet is created by passing a working gas through the high frequency electromagnetic field in the reactant mixing chamber; the induction plasma jet and the metal compound vapour being introduced in the reaction chamber; the plasma jet causing the metal compound vapour to reach a reaction temperature and to react with the working gas, yielding nanosized metal oxide particles; the nanosized metal oxide particles being rapidly cooled in the quench zone of the reactor, thereby stopping the growth process of the nanosized metal oxide particles, yielding metal oxide nanopowder.</li></ul>

The foregoing and other objects, advantages and features of the present invention will become more apparent upon reading of the following non-restrictive description of an illustrative embodiment thereof, given by way of example only with reference to the accompanying drawings.

<u>BRIEF DESCRIPTION OF THE DRAWINGS</u>

In the appended drawings: <ul id="ul0003" list-style="none"><li><figref idref="f0001">Figure 1</figref> is a schematic elevation view of an apparatus for the production of a metal oxide nanopowder;</li><li><figref idref="f0002">Figure 2</figref> is a cross sectional view, taken along line 2-2 of <figref idref="f0001">Figure 1</figref>, of the apparatus for the production of a metal oxide nanopowder;</li><li><figref idref="f0002">Figure 3</figref> is a graph illustrating the photocatalytic degradation of phenol in water in the presence of doped and non treated TiO₂ nanopowder;</li><li><figref idref="f0003">Figure 4</figref> is a stream line graph illustrating turbulence in the reactor of <figref idref="f0001">Figure 1</figref> for a gas flow rate in the quench zone of 225 slpm;</li><li><figref idref="f0003">Figure 5</figref> is a stream line graph illustrating turbulence in the reactor of <figref idref="f0001">Figure 1</figref> for a gas flow rate in the quench zone of 375 slpm;</li><li><figref idref="f0004">Figure 6</figref> is a graph illustrating temperature iso-contours in the reactor of <figref idref="f0001">Figure 1</figref> for a gas flow rate in the quench zone of 225 slpm;</li><li><figref idref="f0004">Figure 7</figref> is a graph illustrating velocity iso-contours in the reactor of <figref idref="f0001">Figure 1</figref> for a gas flow rate in the quench zone of 225 slpm;</li><li><figref idref="f0005">Figure 8</figref> is a graph illustrating temperature iso-contours in the reactor of <figref idref="f0001">Figure 1</figref> for a gas flow rate in the quench zone of 375 slpm;</li><li><figref idref="f0005">Figure 9</figref> is a graph illustrating velocity iso-contours in the reactor of <figref idref="f0001">Figure 1</figref> for a gas flow rate in the quench zone of 375 slpm;</li><li><figref idref="f0006">Figure 10</figref> is a graph illustrating the turbulent intensity iso-contours in the reactor of <figref idref="f0001">Figure 1</figref> for a quench gas flow rate of 225 slpm;</li><li><figref idref="f0006">Figure 11</figref> is a graph illustrating the turbulent intensity iso-contours in the reactor of <figref idref="f0001">Figure 1</figref> for a quench gas flow rate of 375 slpm;</li><li><figref idref="f0007">Figure 12</figref> is a graph illustrating the turbulence intensity longitudinal distribution along the centerline of the reactor of <figref idref="f0001">Figure 1</figref> for a gas flow rate in the quench zone of 225 slpm;</li><li><figref idref="f0008">Figure 13</figref> is a graph illustrating the cooling rate longitudinal distribution along the centerline of the reactor of <figref idref="f0001">Figure 1</figref> for a gas flow rate in the quench zone of 225 slpm;</li><li><figref idref="f0009">Figure 14</figref> is a graph illustrating the axial temperature profile along the centerline of the reactor of <figref idref="f0001">Figure 1</figref> for a gas flow rate in the quench zone of 225 slpm;</li><li><figref idref="f0010">Figure 15</figref> is a graph illustrating the turbulence intensity longitudinal distribution along the centerline of the reactor of <figref idref="f0001">Figure 1</figref> for a gas flow rate in the quench zone of 375 slpm;</li><li><figref idref="f0011">Figure 16</figref> is a graph illustrating the cooling rate longitudinal distribution along the centerline of the reactor of <figref idref="f0001">Figure 1</figref> for a gas flow rate in the quench zone of 375 slpm; and</li><li><figref idref="f0012">Figure 17</figref> is a graph illustrating the axial temperature profile along the centerline of the reactor of <figref idref="f0001">Figure 1</figref> for a gas flow rate in the quench zone of 375 slpm.</li></ul>

<u>DETAILED DESCRIPTION OF THE ILLUSTRATIVE EMBODIMENT</u>

According to an illustrative embodiment of the present invention, titanium dioxide nanopowder is manufactured by heating titanium tetrachloride to a reaction temperature using an induction plasma, reacting the obtained titanium tetrachloride vapour with an oxidizing gas to form titanium dioxide vapour and rapidly cooling the titanium dioxide vapourto promote homogeneous nucleation of a fine aerosol and stop the growth process of the resulting particles.

Referring now to the drawings, <figref idref="f0001">Figure 1</figref> illustrates a reactor 2 and a filter unit 4. The reactor 2 includes a sealed reaction chamber 6 comprising a vertically disposed generally tubular chamber section 8 closed at the upper end by an induction plasma jet assembly 10. The sealed reaction chamber 6 also comprises a taper chamber section 12 integrally mounted to the vertically disposed generally tubular section 8 at the lower end thereof. This taper chamber section 12 defines a region 14 for receiving titanium dioxide nanopowder or any other resulting metal oxide nanopowder. The pressure in the reaction chamber 6 is generally uniform and ranges from about 400 to 500 Torr.

The plasma jet assembly 10 comprises a generally tubular reactant mixing chamber 16 and an inductive coil 18 coaxial with and surrounding the mixing chamber 16. Of course, the reactant mixing chamber 16 is in fluid communication with the sealed reaction chamber 6. The plasma 20 used to heat the titanium tetrachloride is produced by the plasma jet assembly 10 by passing a gas, referred to in the art as a working gas, through a high frequency electromagnetic field, such as a radio frequency field. This electromagnetic field should have a power level sufficient high to cause, by induction, the gas to ionize and thereby produce and sustain plasma. The working gas could be any gas which will ionize when subject to the high frequency electromagnetic field and which remains inert when in the presence of titanium tetrachloride or any other reactant used. Examples of suitable working gases include helium, argon, carbon monoxide, oxygen, and air or a mixture thereof. By supplying a high frequency electric current to the inductive coil 18 the mixture of gases in the reactant mixing chamber 16 is ionized and plasma created.

In the illustrative embodiment, the working gas is formed of a mixture of oxygen and argon (with oxygen also acting as the oxidizing agent). Oxygen is introduced into the reactant mixing chamber 16 via a first inlet 22 and argon via a second inlet 24. A high frequency electric current is applied to the inductive coil 18; the power level of this electric current is sufficiently high to ionize the oxygen/argon mixture and create the plasma 20. The minimum power level applied to the inductive coil 18 necessary for self sustained induction plasma discharge is determined by the gas, pressure and frequency of the magnetic field. The minimum power necessary for sustaining an induction plasma discharge may be lowered by reducing the pressure or by adding ionising mixtures. Power can vary from 20 to 30 kW all the way up to hundreds of kilowatts depending on the scale of operation. The frequency of the current supplied to the inductor coil 18 can be of the order of 3 MHz, although successful operation can be demonstrated at typical frequencies as low as 200 kHz or as high as 26.7 MHz. It should also be apparent to a person of ordinary skill in the art that frequencies outside the range of 200 kHz to 26.7 MHz may be used. In the illustrative embodiment a sinusoidal 30kW electrical current of 3MHz is applied to the inductive coil 18 whereby the oxygen/argon mixture in the reactant mixing chamber 16 is ionized to create the induction plasma 20.

Titanium tetrachloride is introduced axially into the reactant mixing chamber 16 via a third inlet 26. In an alternative illustrative embodiment the titanium tetrachloride is introduced radially into the plasma 20 immediately below the reactant mixing chamber 16 via a fourth inlet 28. In a second alternative illustrative embodiment a combination of axial introduction of titanium tetrachloride via the third inlet 26 and radial introduction of titanium tetrachloride via the fourth inlet 28 is used.

Additionally, a doping agent can react with the oxidizing gas to modify the bulk and/or surface properties of the nanopowders produced. In a first alternative illustrative embodiment the doping agent is mixed with the titanium tetrachloride prior to the titanium tetrachloride being brought to the reaction temperature by the plasma 20. Bringing the mixture to reaction temperature causes both the titanium tetrachloride and the doping agent to simultaneously under go oxidization thus modifying the bulk properties of the titanium dioxide formed, its surface properties, or both.

In a second alternative illustrative embodiment, the doping agent is injected into the plasma 20 after the titanium tetrachloride has reacted with the oxidizing gas and the titanium dioxide formed. Similar to the first alternative illustrative embodiment described above, provided the doping agent is vaporised at the reaction temperature, the doping agent will react with the oxidizing gas, modifying the bulk properties of the titanium dioxide, its surface properties, or both.

Doping agents introduced into the process at this stage may include volatile metal compounds, such as Silicon Tetrachloride and Zinc Chloride.

It should be noted that once the plasma 20 has been established it may be sustained solely by the flow of titanium tetrachloride. Indeed, the plasma 20 may be initiated and established by the flow of titanium tetrachloride alone. Also, by mixing a readily ionized working gas such as argon with the titanium tetrachloride, ignition of the plasma is greatly simplified.

As the titanium tetrachloride comes into contact with the plasma 20 it vaporises and the oxidation reaction proceeds almost instantaneously giving rise to the formation of titanium dioxide and free chlorine. The reaction is estimated as taking place at a temperature between 1500°C and 3000°C although it should be apparent to one of ordinary skill in the art that lower or higher temperatures can also be used depending on plasma loading and input power to the inductor coil 18.

The process involves a high intensity turbulent quench technique which has been developed for the ultra rapid cooling of the products of the reaction and the hindrance of the particle growth process normally associated with the formation of aerosol particles through vapour condensation. The rapid quench technique contributes to the formation of the nanopowder and the predominance (experimental results reveal over 80%) of the anatase phase in this powder. The quench technique aims to bring the temperature of the titanium dioxide vapours down from the reaction temperature of between 1500°C to 3000°C to a temperature in the range of 100°C and 500°C. Experimental tests carried out using an apparatus in accordance with the illustrative embodiment yielded cooled temperatures of approximately 120°C.

Referring now to <figref idref="f0002">Figure 2</figref> in addition to <figref idref="f0001">Figure 1</figref>, a highly turbulent gas quench zone 30 is produced by injecting an intense turbulent stream of compressed quench gas into the plasma discharge 32. This is made via coplanar fine quench gas nozzles such as 34 oriented in respective directions having both radial and tangential components to produce respective high speed jets of quench gas in the same radial/tangential direction.

In turbulent flow, as it is well known to a person skilled in the art, the level of turbulence is measured in terms of the intensity of turbulence of the flow which is defined as the ratio of the root mean square (rms) of the fluctuating fluid velocity to the time mean fluid velocity. In laminar flows the turbulence intensity is zero, since the local fluid velocity is stable and does not change with time. In turbulent flows, the intensity of turbulence depends on the nature of the flow. For example, in turbulent pipe flows, the intensity of turbulence is in the 5 to 7% range while in free and confined jet, and in turbulent shearflows, the turbulence intensity can be in the 10 to 20 % range or higher. In the context of the present invention, the term "high turbulent flow" refers to the use of internal jets and shear flows in the quench zone with turbulence intensities in the 20 to 30% range or higher.

As better shown in <figref idref="f0002">Figure 2</figref>, the nozzles 34 are equally spaced apart from each other around the periphery of the reactor 2. This results in rapid cooling of the product vapour and the immediate halting of the particle growth process. It has been found that introducing turbulence of at least 20 to 30 % in the quench zone promotes the rapid mixing of the quench gas with the reactant products and allows for cooling the reaction products at rates that can be as high as 10⁶°C/s which is sufficiently rapid to stop their growth process.

The combination of the highly turbulent quench zone 30 and of the use of an induction plasma, allowing for a large volume discharge and a long residence time of the reactant in the plasma zone, is largely responsible for the control achieved by this process on the particle size distribution and the nanosized mean particle diameter of the titanium dioxide powder obtained.

The above-described high intensity turbulent quench technique allows controlling the flow pattern in the quench zone 30 through the use of an array of high velocity jets directed at an angle with the normal to the periphery of the reaction chamber 8 (see <figref idref="f0002">Figure 2</figref>), towards the center of the reactor at the quench level. The localized pinch effect thus produced in the center of the quench zone 30, combined with the high intensity turbulence level and the associated aerosol dilution resulting from the addition of a significant amount of gas into the flow, contributes to control the particle size distribution of the formed nanopowder, reduces chances of its agglomeration and even allows for the control of the crystal structure of the nanopowder obtained.

More specifically, the quench technique used in the illustrative embodiment is comprised of a circular air channel which is located below the plasma discharge 32 in the reactor 2. The location of the quench zone 30, depending on the process requirement, may vary between a few centimetres to about 20 centimetres downstream of the plasma discharge 32. Although air is used as a quench gas in the illustrative embodiment in accordance with the present invention, it should be apparent to one of ordinary skill in the art that selection of the quench gas is dictated to some degree by the chemistry of the process, and that other gases such as for example pure oxygen and nitrogen may also be used as a quench gas.

The quench gas is injected into the reactor 2 with a velocity on the order of several hundred meters per second up to sonic velocity. In the illustrative embodiment the velocity of the injected quench gas is 260 metres per second. The higher the difference between the velocity of the injected gas and the velocity of the injected plasma jet, the higher is the resulted turbulence. As will be demonstrated hereinbelow, the cooling rate increases with the turbulence. The injected quench gas results in the formation of a high intensity turbulent flow zone 30 in the center the vertically disposed generally tubular section 8 of the reaction chamber 6 of the reactor 2 at the level of the quench gas nozzles 34. The formation of this flow zone 30 gives rise to the rapid cooling of the products of the reaction and their condensation in the form of a nanometer sized aerosol particles. The rapid cooling of the products of the reaction also favours the formation of the TiO₂ nanopowder in the anatase phase which is the predominant phase formed at high temperature.

The direction of the quench gas nozzles 34 can be adjusted in the plane in which these nozzles 34 are lying in order to control the turbulence characteristics in the center of the quench zone 30 which, in turn, has an influence on the nature of the nanopowders obtained.

A conduit 36 interposed between the reactor 2 and the filter unit 4 is affixed at the lower, smaller-diameter end of the taper section 12 of the reaction chamber 6 of the reactor 2, and is used for transporting the cooled nanopowderto the filter unit 4 for filtering. A fifth inlet 38 is located in the wall of the conduit 36. A suitable doping agent may possibly be introduced through this fifth inlet 38 for coating the cooled nanopowder. By coating the powder, properties of the powder can be modified to adapt them to particular applications. For example, as stated above the process produces TiO₂ with a proportionally higher content of the anatase phase. Adding the anatase phase to man made fibres combined with exposure to ultraviolet radiation can lead to auto-degradation of the fibres (due to the catalytic behaviour of the anatase phase when in the presence of ultraviolet radiation). By first coating the powder with the polymer methyl methylacrylate, prior to its addition to man made fibres, the auto degradation can be effectively halted thereby extending the life of the fibres.

Another aspect to be considered in the coating process is the temperature of the powder to be coated. Traditionally, TiO₂ powders are left to cool for some time before an additional and separate coating process is applied to modify the surface characteristics of the powder. The rapid cooling of the powder provided by the highly turbulent gas quench technique means that the powder can be coated immediately following quenching with a range of materials which would other wise be destroyed or negatively effected by the heat of the powder. Additionally, for a number of coatings an accurate control of the cooled temperature is necessary, especially polymers if polymerisation is to take effect. Experiments have revealed, for example, that the coating of a TiO₂ powder with the polymer methyl methylacrylate can be carried out at a temperature of 120°C, a temperature which can be readily achieved and controlled through the use of the highly turbulent gas quench technique.

This coating of the nanopowder after cooling by the quench zone is herein referred to as inline doping. Although in this regard reference is made to the coating of a cooled nanopowder, it should be evident to one of ordinary skill in the art that the inline coating process could also be applied to a powder with a particle size larger than a nanopowder.

Depending on the intended use of the nanopowder (or powder, in the case a powder with a particle size greater than a nanopowder is being coated), many surface coating agents may be considered. The surface coating agent controls the surface properties of the nanopowder. For example, as stated above, the use of methyl methylacrylate as surface coating agent resulted in a significant reduction of the catalytic properties of the predominantly anatase TiO₂ nanopowder produced. Referring to <figref idref="f0002">Figure 3</figref> the photocatalytic degradation of a normalised concentration phenol in water in the presence of a TiO₂ nanopowder doped with methyl methylacrylate ("doped powder") is displayed versus that of a non-treated powder. The process is not limited, however, to one specific surface coating agent. Other potential surface coating agents are known to those of ordinary skill in the art and may include, for example, Teflon monomer, Diethyl Zinc, chloro-fluorocarbons and metallic vapours.

The filter unit 4 is comprised of an upper, vertically disposed generally tubular section 40. A taper section 43 is mounted on the lower end of the generally tubular section 40 and defines a region 44 for receiving filtered titanium dioxide nanopowder. A porous filter medium 42, such as Goretex™, capable of capturing the nanopowder is mounted axially and centrally within the generally tubular section 40 and has porosity such that the nanopowders cannot pass there through and are removed from the exhaust gases which are expelled via the exhaust 46. Nanopowder received in the region 44 is collected through a bottom vertical conduit 48.

Referring now to <figref idref="f0003 f0004 f0005 f0006 f0007 f0008 f0009 f0010 f0011 f0012">Figures 4 to 17</figref>, the impact of the quench zone 30 on the particles cooling rate will be illustrated.

Experiments and computer simulations have been performed to model the flow, temperature and concentration fields in the reactor quench zone 30 under the following conditions: <ul id="ul0004" list-style="none"><li>▪ plasma torch model PL-35 with a 35 mm internal diameter plasma confinement tube; reactor diameter at the level at which the plasma jet enters the reactor is 260mm;</li><li>▪ axial distance between the level at which the quench gas is introduced into the reactor 2 and the reactor inlet is 160mm;</li><li>▪ quench ring geometry made up of 16 equally spaced, radially oriented, holes each with a diameter of 1.5 mm; and</li><li>▪ plasma torch operating conditions: <ul id="ul0005" list-style="bullet"><li>sheath gas = 125 slpm (02);</li><li>central gas = 30 slpm (Ar)</li><li>reactant gas = 18.5 slpm (Ar);</li><li>quench gas flow rate = 225 or 375 slpm (Air);</li><li>reactor pressure = 425 Torr;</li><li>oscillator frequency = 3 MHz; and</li><li>plasma power = 28.4 kW.</li></ul></li></ul>

Results are provided for two quench gas flow rates of 225 slpm (Air) and 375 slpm (Air). These results show the importance of introducing a high level of turbulence in order to achieve the high cooling rate necessary for the formation of nanosized powders.

The respective flow pattern in the reactor 2 for each of these two quench gas flow rates are given in <figref idref="f0003">Figure 4 and Figure 5. Figure 4</figref> corresponds to a quench gas flow rate of 225 slpm while <figref idref="f0003">Figure 5</figref> corresponds to the case of a quench gas flow rate of 375 slpm. The significant influence of the quench gas flow rate on the gas recirculation pattern can thus be observed.

The corresponding data for the temperature and velocity fields are shown respectively in <figref idref="f0004">Figures 6 and 7</figref> for a quench gas flow rate of 225 and on <figref idref="f0005">Figures 8 and 9</figref> for a quench gas flow rate of 375 slpm (Air).

Turbulence intensity iso-contours for these conditions are given in <figref idref="f0006">Figures 10 and 11</figref> for quench gas flow rate of 225 and 375 slpm respectively.

Actual turbulent intensity profiles along the center line of the reactor 2 and the corresponding reaction products cooling rates are given in <figref idref="f0007">Figures 12</figref> and <figref idref="f0010">15</figref> for quench gas flow rates of 225 and 375 slpm (Air) respectively. These Figures show turbulence intensity levels as high as 80% and 270% in the quench zone which is significantly higher than any turbulence values in known reactor configurations.

<figref idref="f0008">Figures 13</figref> and <figref idref="f0011">16</figref> show reaction product cooling rates achieved using high intensity quench technique from the illustrative embodiments as high as 6x10⁶K/s which plays a key role in the formation of the nanopowders. These two Figures also show how an increase in turbulence in the quench zone results in the increase in the plasma cooling rate. By comparing <figref idref="f0009">Figures 14</figref> and <figref idref="f0012">17</figref>, which illustrates the temperature profile in the reactor 2 for quench gas flow rate of 225 and 375 slpm respectively, one can also see the positive impact of the increase in turbulence in the quench zone on the cooling rate.

It is to be noted that a production capacity of a 30 kW induction plasma installation, which is in the hundreds of grams per minute (above 100g/min) of titanium dioxide nanopowder, can be achieved using a process according to the present invention, compared to that of a few grams per minute associated with known direct current/supersonic quench process.

Through experimentation, the production rate of TiO₂ nanopowder of near 150 g/min was achieved, which is significantly higher than what can be achieved using known techniques. The corresponding BET (Bruaner Emett and Teller) specific area analysis for the produced nanopowder showed 34.7 m²/g, with a corresponding mean particle diameter of 43.3 nm. X-ray diffraction (XRD) analysis of the produced powder showed that it is constituted of 84%wt of the anatase phase.

Although the present invention has been described by way of reference to the synthesis of titanium dioxide nanopowder by heating titanium tetrachloride, the present invention can be used to manufacture other metal oxide nanopowder such as Zinc oxide or Zirconium oxide.