Process for the manufacture of nano-sized powders
This invention pertains in general to methods and processes for the
synthesis of nano-sized particles. In particular, the present
invention concerns the manufacture of metal-bearing nano-sized
powders starting from a refractory or high-boiling precursor.
Nanoparticles generally indicate particles with at least one
dimension less than 100 run. The small size of these particles can be
beneficial for specific mechanical, optical, electrical, chemical,
magnetic and/or electronic properties compared to bulk materials.
A wide range of synthesis routes are currently being developed to
make nanopowders. Thereby, so-called top-down approaches are an
extension of traditional methods for the production of
nanocrystalline powders by reducing the particle size from micron-
scale down to nanopowders. These processes generally involve some
form of high-energy milling. A major disadvantage of this high-energy
milling process is the long milling times, from several hours up to
many days. Due to the wear of the milling media, especially with
high-energy milling, contamination of the end product is a serious
risk.
The most recent developments in nanopowder production techniques are
situated in bottom-up approaches. In all these processes, the major
issue to overcome consists in controlling nucleation and growth of
the particles. The different techniques can be classified in solid,
liquid, and vapour techniques, including sol-gel processes, colloidal
precipitation, hydrothermal processes, and gas phase synthesis
routes.
Known gas phase synthesis routes for the manufacture of nanopowders
involve the injection of a volatile precursor in a hot gas stream,
followed by the reaction of this precursor with a gaseous substance,
thereby forming the desired compound. This compound condenses and
nucleates, either in the hot gas stream if it is refractory or high-
boiling, or else upon cooling of the gas.

US2004/0009118 describes a continuous method for producing metal
oxide nanoparticles by means of a microwave plasma with a hot zone of
at least 3500 °C. In US2004/0005485, a process for the manufacturing
of nano-scaled powders is described, wherein the step of providing
thermal energy raises the peak processing temperature to at least
3000 K. The vaporization of solid precursors may imply extremely high
temperatures, above 3000 K, in particular when the precursor compound
happens to be refractory or high-boiling. Such extreme temperatures
result in high energy losses. The production apparatus itself becomes
expensive as it has to withstand extreme conditions.
In US6669823, stoichiometric nano-sized materials, as for example
cerium oxide powders, are synthesized through introduction of an
oxidising gas into the plasma. US6254940 describes the production of
nanoparticles starting from boric acid, BC13, SiCl4, silane, metal
halide, metal hydride, metal alcoholate, metal alkyl, metal amide,
metal azide, metal boronate, metal carbonyl and combinations of these
materials. These materials are heated in a flame reactor and passed
between plate electrodes. In US5788738, nano-sized oxide powders are
synthesized through injection of metal powders together with an inert
carrier gas. However, the injection of metal powders as precursor may
cause safety problems during handling. These safety problems can be
overcome by using inert refractory powders as injection material.
Additionally, partially melting of the precursor may occur during
injection which may be harmful for the process.
The present invention solves the above-mentioned problems by
proposing a reaction scheme that includes the preliminary conversion
of a less volatile metal-bearing precursor into a more volatile
intermediate. This conversion can be realized at relatively low
temperatures, well below the volatilization temperature of the
precursor.
Accordingly, a new process for the production of a nano-sized metal-
bearing powder is divulged, comprising the steps of:
(a) providing a hot gas stream at a temperature of 1000 K to 3000 k,
wherein

- a solid metal-bearing precursor compound is dispersed; and
- a first volatile reactant is introduced,
whereby a gaseous metal intermediate compound is formed, said
compound being volatile at a temperature lower than the
volatilisation temperature of the precursor;
(b) introducing a second volatile reactant into the gas stream
whereby the gaseous metal intermediate compound is converted into a
nano-sized metal-bearing powder; and
(c) separating the nano-sized metal-bearing powder from the gas
stream.
This process is particularly suitable when the solid metal-bearing
precursor is non-volatile at the temperature of the hot gas stream.
The production of nano-sized powder is further enhanced by quenching
the gas stream after the step of introducing the second volatile
reactant and before separating the nano-sized metal-bearing powder;
alternatively, quenching can be combined with the introduction of the
second volatile reactant in the gas stream.
The above process can be used for the manufacture of mixed or doped
oxides by starting from a solid metal-bearing precursor powder
mixture containing at least two metals.
A nano-sized mixed or doped oxide can also be prepared starting from
a solid metal-bearing precursor powder which is dispersed in a second
metal-bearing liquid or gaseous precursor.
The hot gas stream can be generated by means of either one of a gas
burner, a hydrogen burner, an RF plasma, or a DC arc plasma.
The process is specially adapted for the use of one or more of ZnO,
GeO2, In2O3, indium-tin-oxide, MnO2, Mn2O3 and A12O3 as solid precursor.
The first volatile reactant advantageously comprises either one or
more of hydrogen, nitrogen, chlorine, CO, or a volatile hydrocarbon
such as methane or ethane. The second volatile reactant preferably
comprises oxygen or nitrogen, such as air.

According to a preferred embodiment, the precursor comprises a
micron-sized or submicron-sized ZnO powder, the first volatile
reactant is methane, and the second volatile reactant is air.
In a further preferred embodiment, the second volatile reactant is
used to quench the hot gas stream to a temperature below 250°C.
In a further preferred embodiment, the solid precursor is a mixture
of ZnO and either one or more of A12O3, Al and MnCl3 powder.
By volatilisation temperature of the metal-bearing precursor is meant
the temperature at which the precursor either evaporates or
decomposes into at least one metal-bearing gaseous species.
The details of the invention are illustrated in Figures 1 to 4:
- Fig. 1 is a calculated phase preponderance diagram showing, as a
function of temperature, the volatilization of zinc oxide in the
presence of oxygen;
- Fig. 2 is a calculated phase preponderance diagram showing, as a
function of temperature, the volatilization of zinc oxide in the
presence of methane;
- Fig. 3 shows a SEM picture of nano-sized zinc oxide obtained
according to the invention;
- Fig. 4 shows an X-ray diffraction spectrum of a nano-sized Al-doped
ZnO powder obtained according to the invention.
One advantage of this invention is that a wider range of potentially
cheap or easy to handle precursor compounds becomes usable, including
in particular those that are refractory or high-boiling.
A further advantage of this invention is that the process can be
carried out at relatively low temperature: between 1000 and 3000 K,
or preferably between 2000 and 3000 K. This mitigates both energy
loss and construction materials requirements.

Moreover, the residence time of the precursor in the gas stream can
be short, allowing the process to be carried out in a compact
apparatus.
To ensure fast kinetics, a gas stream temperature of preferably at
least 500 K, and more preferably at least 800 K above the
volatilisation temperature of the intermediate could be used. The
resulting kinetics then allow for the nearly complete O99.9 wt%)
conversion of the precursor into nanopowder with a residence time of
the precursor in the hot gas stream of only 100 ms or less.
According to thermodynamic calculations, many relevant metal-bearing
precursors show volatilization points above 2000 K. In the presence
of a volatile reactant (such as hydrogen, methane, ethane, propane,
chlorine or combinations thereof) a thermodynamic environment is
created which favours the formation of an intermediate metal-bearing
compound with a significantly lower volatilization temperature. It is
advisable to form an intermediate metal-bearing compound having a
volatilisation temperature that is at least 500 K lower than that of
the precursor.
Fig. 1 shows the result of thermodynamic calculations for the
volatilization of zinc oxide in the presence of oxygen. It is shown
that under normal atmospheric (oxidic) conditions, solid ZnO will
completely form zinc gas only at temperatures above 2200 K.
Fig. 2 shows the thermodynamic data for the volatilization of zinc
oxide in the presence of methane. A gaseous zinc compound will be
formed at the much lower temperature of about 1100 K. Re-oxidation of
this zinc gas in combination with fast quenching results in the
formation nano-sized ZnO powder.
In a similar way, GeO2, which has a volatilisation point higher than
2000 K will, in the presence of a reducing atmosphere of e.g.
methane, form a sub-stoichiometric oxide GeO with a volatilization
point lower than 1500 K. Oxidation of this GeO, followed by fast
quenching, will finally result in nano-sized GeO2 powder. Analogue
thermodynamic calculations have been performed for In2O3, Mn2O3, MnO2
and A12O3, as shown in Table 1.


The hot gas stream used in the invention may be generated by a flame
burner, a plasma torch such as a microwave plasma, an RF or DC plasma
arc, an electric heating or conductive heating furnace. In the former
case, it may be useful to produce combustion gasses that already
contain the volatile reactant needed to convert the metal in the
precursor into a volatile intermediate. A lean combustion mixture
could be used, thus introducing the reducing gas through the burner.
This process can be applied to the manufacture of nano-sized ZnO,
starting from coarse ZnO powder. In this case, the non-volatile
metal-bearing precursor powder is (relatively coarse) ZnO; the first
volatile reactant is a reducing gas; the volatile metal compound is
metallic Zn; the second volatile reactant is air; and the nano-sized
metal-bearing powder is again ZnO. Although refractory, ZnO is indeed
chosen as a precursor because it is both cheap and widely available
as a powder.
The process can further advantageously be applied for the production
of mixed oxides such as indium-tin-oxide. The first volatile reactant
may comprise hydrogen gas, nitrogen, chlorine, carbon monoxide, a
volatile hydrocarbon such as methane or ethane, or others. The second
volatile reactant may comprise air, oxygen and nitrogen.
Once the final reaction product is formed, it is useful to quench the
reactant as well as the gas. Quenching is hereby defined as to cool
the hot gas and powder rapidly, so as to avoid the aggregation,
sintering and growth of the nanoparticles. This quenching can e.g. be

performed by injecting a relatively large amount of cold air into the
mixture of gas and nanoparticles. The nanoparticles are readily
entrained by the gas flow and can be separated, e.g. with filters.
The invention will now be further illustrated by following examples:
Example 1
A 500 kW DC plasma torch is used, with nitrogen as plasma gas. The
gasses exiting the plasma at a rate of 160 NmVhour are at about 2500
k. Relatively coarse ZnO powder with a specific surface area of 9
mVg is injected behind the plasma at an injection rate of 30
kg/hour, together with a flow of 17.5 NmVhour natural gas. In this
zone, the coarse ZnO powder is reduced to volatile metallic Zn
vapour. Thereafter, air is blown, thereby oxidizing the Zn vapour
gas. Subsequently, air is blown at a flow rate of 15000 NmVhour to
quench the gas/solids flow and produce nano-sized ZnO powder. After
filtering, nanopowder is obtained with a specific surface area of 30
mVg. A FEG-SEM micrograph of the particles is shown in Fig. 3,
illustrating nano-sized ZnO powder with average primary particle size
well below 100 run.
Examples 2 and 3
The same apparatus as in Example 1 was operated according to the
conditions shown in Table 2. It can be concluded that increasing the
precursor throughput from 30 to 4 0 kg/h still results in a ZnO
nanopowder. In both experiments, a nano-sized ZnO powder is obtained
with an average primary particle size well below 100 nm.
Example 4
This Example is similar to Example 2. The quench air is however
injected in 2 steps. Straight behind the oxidation air inlet, a first
quench step is performed with an airflow of 500 NmVh, in order to
cool the gases as well as the ZnO powder to a temperature of about
600 °C. Afterwards the particles stay at this temperature during a
period between 1 and 10 s. Subsequently, a second quench step by
means of an airflow of 14500 NmVh applied, down to a temperature
below 250 °C. As shown in Table 2, this 2 step quench enables to


Example 5
A 100 kW RF inductively coupled plasma (ICP) torch is used, using an
argon/natural gas plasma with 3 Nm3/h argon and 0.3 NmVhour natural
gas. Relatively coarse ZnO powder is injected at rate of 500 g/hour
in the downstream region of the ICP torch, where the plasma reaches a
temperature of about 2000 k. As described above, the ZnO powder is
totally reduced to metallic Zn, which volatilizes. Air is blown
further downstream of the torch, thereby oxidizing the Zn and
producing nano-sized ZnO. More air is blown at a rate of 20 mVh to
quench the gas/solids flow. After filtering, nano-sized ZnO powder is
obtained having a specific surface area of 20 m2/g.
Example 6
The ICP torch of Example 5 is used, with a 15:1 argon:hydrogen gas
mixture as plasma gas. Relatively coarse GeO2 powder, having an
average particle size of 0.5um is injected in the ICP torch at rate
of 500 g/hour. The GeO2 powder is thereby reduced to a GeO sub-oxide,
which volatilizes. Oxygen is blown at the plasma torch output,
thereby oxidizing the GeO and producing nano-sized GeO2. More air is
blown at a rate of 30 mVhour to quench the gas/solids flow. After

filtering, nano-sized GeO2 powder is obtained having a specific
surface area of 35m2/g, which corresponds to an average spherical
particle size of 30 nm.
Example 7
The DC plasma torch of Examples 1 to 4 is used, with nitrogen as
plasma gas. The gasses exiting the plasma at a rate of 160 NmVhour
are at about 2500 K. Relatively coarse ZnO powder, with a specific
surface area 9m2/g, is premixed with micron-sized aluminium powder.
This powder mixture is injected behind the plasma at an injection
rate of 40 kg/h, together with a flow of 17.5 Nm3/h of natural gas.
In this zone, the coarse Al/ZnO powder mixture is reduced to a
volatile metallic Zn and Al vapour. Thereafter, air is blown, thereby
oxidizing the vapour. Subsequently, air is blown at a flow rate of
15000 mVh to quench the gas/solids flow and produce a nano-sized Al-
doped ZnO powder with 1 wt% Al. After filtering, nanopowder is
obtained with a specific surface area of 27 m2/g. The XRD spectrum
shown in Fig. 4 reveals the hexagonal crystal structure of ZnO with a
small peak shift, indicating that Al is embedded in the crystal
lattice of ZnO. Alloying levels of 0.1, 0.5, 1, 2, 5, 10 and 15 wt%
were obtained by varying the relative amount of Al in the feed.
Example 8
The DC plasma torch of Examples 1 to 4 is used, with nitrogen as
plasma gas. The gasses exiting the plasma at a rate of 160 NmVhour
are at about 2500 K. Relatively coarse ZnO powder, with a specific
surface area 9m2/g, is premixed with micron-sized aluminum oxide
(A12O3) powder. This powder mixture is injected behind the plasma at
an injection rate of 40 kg/h, together with a flow of 17.5 Nm3/h of
natural gas. In this zone, the coarse A12O3 and ZnO powder mixture is
reduced to a volatile metallic Zn and Al vapour. Thereafter, air is
blown, thereby oxidizing the vapour. Subsequently, air is blown at a
flow rate of 15000 mVh to quench the gas/solids flow and produce a
nano-sized Al-doped ZnO powder with 1 wt% Al. After filtering,
nanopowder is obtained with a specific surface area of 26 mz/g. The
XRD spectrum reveals the hexagonal crystal structure of ZnO with a
small peak shift, indicating that Al is embedded in the crystal
lattice of ZnO.

Example 9
The DC plasma torch of Examples 1 to 4 is used, with nitrogen as
plasma gas. The gasses exiting the plasma at a rate of 160 Nm3/h are
at about 2500 K. Behind the plasma, relatively coarse ZnO powder,
with a specific surface area 9ma/g, is injected together with MnCl3
at an injection rate of 40 kg/h, together with a flow of 17.5 NmVh
of natural gas. In this zone, the coarse MnCl3/ZnO mixture is reduced
to a volatile metallic Zn and Mn vapour. Thereafter, air is blown,
thereby oxidizing the vapour. Subsequently, air is blown at a flow
rate of 10000 m3/h to quench the gas/solids flow and produce a nano-
sized Mn-doped ZnO powder. After filtering, nanopowder is obtained
with a specific surface area of 29 m2/g. The XRD spectrum reveals the
hexagonal crystal structure of ZnO with a small peak shift,
indicating that Mn is embedded in the crystal lattice of ZnO.
Alloying levels of 0.1, 0.5, 1, 2, and 5 wt% were obtained by varying
the relative amount of MnCl3 in the feed.
Example 10
The DC plasma torch of Examples 1 to 4 is used, limiting the power
input to 250 kW, and using nitrogen as plasma gas. The gasses exiting
the plasma at a rate of 160 Nm3/h are at about 1900 K. Relatively
coarse ZnO powder is injected behind the plasma at an injection rate
of 25 kg/h, together with natural gas. In this zone, the coarse ZnO
powder is reduced to a volatile metallic Zn vapour. Thereafter, air
is blown, thereby oxidizing the Zn vapour. Subsequently, air is blown
at a flow rate of 15000 Nm3/h to quench the gas/solids flow and
produce nano-sized ZnO powder. After filtering, nanopowder is
obtained with a specific surface area of 35 m2/g.

Claims
1. A process for the production of a nano-sized metal-bearing powder
comprising the steps of:
(a) providing a hot gas stream at a temperature of 1000 K to 3000 K,
wherein
- a solid metal-bearing precursor compound is dispersed; and
- a first volatile reactant is introduced,
whereby a gaseous metal intermediate compound is formed, said
compound being volatile at a temperature lower than the
volatilisation temperature of the precursor;
(b) introducing a second volatile reactant into the gas stream
whereby the gaseous metal intermediate compound is converted into a
nano-sized metal-bearing powder; and
(c) separating the nano-sized metal-bearing powder from the gas
stream.

2. Process according to claim 1, wherein the solid metal-bearing
precursor is non-volatile at the temperature of the hot gas stream.
3. Process according to claims 1 or 2, wherein between the step of
introducing the second volatile reactant and separating the nano-
sized metal-bearing powder, the gas stream is quenched.
4. Process according to claims 1 or 2, wherein during the step of
introducing the second volatile reactant the gas stream is quenched.
5. Process according to anyone of claims 1 to 4, whereby a nano-sized
doped metal bearing powder is produced starting from a solid metal-
bearing precursor powder mixture containing at least two metals.
6. Process according to anyone of claims 1 to 4, whereby a nano-sized
doped metal bearing powder is produced starting from a solid metal-
bearing precursor powder which is dispersed in a second metal-bearing
liquid or gaseous precursor.

7. Process according to anyone of claims 1 to 6, whereby a hot gas
stream is provided by means of either one of a gas burner, a hydrogen
burner, an RF plasma, or a DC arc plasma.
8. Process according to anyone of claims 1 to 7, wherein the solid
precursor comprises either one or more of ZnO, GeO2, In2O3, indium-
tin-oxide, MnO2, Mn2O3 and AI2O3.
9. Process according to anyone of claims 1 to 8, wherein the first
volatile reactant comprises either one or more of hydrogen, nitrogen,
chlorine, CO, or a volatile hydrocarbon such as methane or ethane.

10. Process according to anyone of claims 1 to 9, wherein the second
volatile reactant is air, or comprises oxygen or nitrogen.
11. Process according to anyone of claims 1 to 10, wherein the solid
precursor comprises a micron-sized or sub-micronsized ZnO powder, the
first volatile reactant is methane, and the second volatile reactant
is air.
12. Process according to claim 11, wherein the second volatile
reactant is used to quench the hot gas stream to a temperature below
250°C.
13. Process according to claims 11 or 12, wherein the solid precursor
is a mixture of ZnO and either one or more of A12O3, Al and MnCl3
powder.

Nano-sized metal-bearing powders and doped-powders are synthesized by means of a process whereby a non-volatile metal-bearing precursor powder or powder mixture is dispersed in a hot gas stream at relatively low temperatures. A first volatile reactant is added, converting the metal in the precursor into a volatile metal compound. Subsequently a second volatile reactant is injected into the gas stream, converting the volatile metal compound into a solid, which condenses as a nano-sized metal-bearing powder upon quenching. Finally, the vapour/metal-bearing powder mixture is separated from the gas stream.