Aluminium oxide powder produced by flame hydrolysis and
having a large surface area
The present invention relates to an aluminium oxide powder
produced by flame hydrolysis and having a large surface
area, and to its production and use.
It is known to produce aluminium oxide powder by means of
pyrogenic processes. Pyrogenic processes include flame
hydrolysis, in which an aluminium halide, generally
aluminium chloride, is hydrolysed at high temperatures with
the formation of aluminium oxide and hydrochloric acid
according to Eq. 1
2 AlCl3 + 3 H2O -> Al2O3 + 6 HC1 (Eq. 1)
4 AlCl3 + 3 O2 -> 2 A12O3 + 6 Cl2 (Eq. 2) .
Aluminium oxide C, Degussa AG, for example, is produced in
this manner. Aluminium oxide C has a surface area of
approximately 90 m2/g.
A further aluminium oxide powder produced by flame
hydrolysis is the powder from Cabot. It has a BET surface
area of 55 m2/g and comprises about 56 % theta and 20 %
delta crystal modifications as well as 24 % amorphous
constituents.
EP-A-1083151 describes an aluminium oxide powder having a
BET surface area of more than 115 m2/g, which powder at the
same time has a Sears index of more than 8 ml/2 g and the
dibutyl phthalate absorption of which cannot be determined.
The example describes a powder having a BET surface area of
121 m2/g and a Sears index of 9.38 ml/2 g.
US 3663283 describes a process for the production of metal
oxide powders by flame hydrolysis. Although an example
relating to aluminium oxide is given, the aluminium oxide

is described only as being finely divided with narrow
particle distribution. Further details are not given.
In US 5527423 there is claimed a dispersion that contains
precipitated aluminium oxide or aluminium oxide produced by
flame hydrolysis having a BET surface area of from 40 to
430 m2/g. The manner in which such aluminium oxide powders
are obtained is not disclosed, however. In the examples,
aluminium oxide powders having a BET surface area only
within a narrow range of from 55 to 100 m2/g are disclosed.
In EP-A-1256548, aluminium oxide particles having a mean
primary particle diameter of from 5 to 100 nm and a mean
aggregate diameter of from 50 to 80 nm are [sic]. The
particles may be amorphous or crystalline. The proportion
of particles larger than 45 µm is preferably 0.05 wt.% or
less. These aluminium oxide particles are said to be
obtainable by a gas-phase reaction of aluminium chloride
with oxygen and/or steam, wherein the reactants are pre-
heated, at temperatures of about 800°C and subsequent
separation of the aluminium oxide that is formed from
gaseous substances. Oxygen, water and oxygen/water mixtures
are to be used as the oxidising agents in the reaction.
This reaction is, however, a gas-phase reaction, not flame
hydrolysis or flame oxidation. The powder obtained in
accordance with EP-A-1256548 has a different structure and
different properties than a powder obtained by flame
hydrolysis or flame oxidation. For example, the proportion
of chloride may be up to several wt.%. The powder can have
an undesirable grey colour, which may be attributable to
aluminium oxychloride constituents resulting from the
incomplete reaction of aluminium chloride.
Many possible uses of aluminium oxide powders are known.
They are used in the paper industry, in particular in ink-
jet papers. Aluminium oxide powders affect, inter alia, the
gloss, the brilliance of colour, the adhesion and the ink

absorption. The increasing demands that are made of ink-jet
papers require the values of these parameters to be
improved.
Aluminium oxide powders are also used as an abrasive in
dispersions for polishing oxidic and metallic coatings in
the electronics industry (chemical mechanical polishing,
CMP). Here too, the continued miniaturisation of the
components requires customised abrasives which allow
surfaces in the nanometre range to be polished without
scratching.
The object of the invention is to provide an aluminium
oxide powder that meets the increased demands in the fields
of ink-jet and CMP. In particular, it should be possible to
incorporate the powder into dispersions easily and with
high degrees of filling. A further object of the invention
is a process for the production of such a powder.
The invention provides an aluminium oxide powder produced
by flame hydrolysis and consisting of aggregates of primary
particles, which powder is characterised in that
it has a BET surface area of from 1,00 to 250 m2/g,
the dibutyl phthalate absorption is from 50 to
450 g/100 g of aluminium oxide powder, and
it shows only crystalline primary particles on high-
resolution TEM pictures.
The aluminium oxide powder according to the invention
preferably has an OH density of from 8 to 12 OH/nm2.
The chloride content of the aluminium oxide powder
according to the invention is preferably less than
1.5 wt.%.
It is also preferred for the proportion of particles having
a diameter greater than 45 µm to be in a range of from
0.0001 to 0.05 wt.%.

Preference may also be given to an aluminium oxide powder
according to the invention which in the X-ray diffractogram
exhibits an intensity, expressed as. the counting rate, of
more than 50 at an angle 2 theta of 67°.
Such an aluminium oxide powder may exhibit the signals of
gamma-, theta- and/or delta-aluminium oxide in the X-ray
diffractogram, the signal of gamma-aluminium oxide
generally being the most intense.
It is also possible for the aluminium oxide powder
according to the invention to exhibit in the X-ray
diffractogram an intensity, expressed as the counting rate,
of less than 50 at an angle 2 theta of 67°. Such a powder
is X-ray amorphous to the greatest possible extent.
Preference may be given to an aluminium oxide powder
in which the BET surface area is from 120 to 200 m2/g,
the dibutyl phthalate absorption is from 150 to
350 g/100 g of aluminium oxide powder, the OH density
is from 8 to 12 OH/nm2, and which
shows only crystalline primary particles in high-
resolution TEM pictures, and
in the X-ray diffractogram has signals with an
intensity, expressed as the counting rate, of more
than 50 at an angle 2 theta of 67°, and
exhibits signals of gamma-, theta- and/or delta-
aluminium oxide.
In such a powder, a BET surface area of from 125 to
150 m2/g is particularly preferred.
Preference may further be given to an aluminium oxide
powder
in which the BET surface area is from 120 to 200 m2/g,
the dibutyl phthalate absorption is from 150 to
350 g/100 g of aluminium oxide powder, the OH density is
from 8 to 12 OH/nm2, which powder

shows only crystalline primary particles in high-
resolution TEM pictures and
in the X-ray diffractogram exhibits an intensity,
expressed as the counting rate, of less than 50 at an
angle 2 theta of 67°.
In the case of such a powder, a BET surface area of from
135 to 190 m2/g is particularly preferred.
The invention further provides a process for the production
of the aluminium oxide powder according to the invention,
in which
aluminium chloride is vaporised, the vapour is
transferred by means of a carrier gas to a mixing
chamber and,
separately therefrom, hydrogen, air (primary air), which
may optionally be enriched with oxygen and/or may
optionally be pre-heated, are supplied to the mixing
chamber, then
the mixture of aluminium chloride vapour, hydrogen and
air and is ignited in a burner and the flame burns into
a reaction chamber that is separated from the
surrounding air,
the solid material is subsequently separated from
gaseous substances, and
the solid material is then treated with steam and
optionally with air,
the discharge rate of the reaction mixture from the
mixing chamber into the reaction chamber being at least
10 m/s, and
the lambda value being from 1 to 10 and
the gamma value being from 1 to 15.
The structure of the aluminium oxide powders according to
the invention in respect of their X-ray crystalline or X-
ray amorphous state can be controlled by varying the
aluminium chloride concentration in the gas stream. High

aluminium oxide concentrations in the gas stream yield an
X-ray crystalline powder.
The definition of a high aluminium chloride concentration
is dependent on the structure of the reactor; a range from
0.2 to 0.6 kg of AlCl3/m3 of gas can be used as a reference
point for a production installation.
If the aluminium chloride concentration in the same
production installation is multiplied by a factor of from
0.4 to 0.6, a powder that is X-ray amorphous to the
greatest possible extent is obtained.
In addition to adjustments that yield X-ray crystalline
powders or powders that are X-ray amorphous to the greatest
possible extent, it is also possible, by varying the
aluminium concentration in the gas stream, to obtain
powders that, for example, contain a defined proportion of
X-ray amorphous aluminium oxide.
In a particular embodiment of the process according to the
invention, a secondary gas consisting of air and/or
nitrogen can be introduced into the reaction chambers The
ratio primary air/secondary gas preferably has values of
from 10 to 0.5. The introduction of a secondary gas can
help to avoid caking in the reaction chamber.
The invention relates also to the use of the aluminium
oxide powder according to the invention as an ink-absorbing
substance in ink-jet media.
The invention relates also to the use of the aluminium
oxide powder according to the invention as an abrasive.
The invention relates also to the use of the aluminium
oxide powder according to the invention in dispersions.
The invention relates also to the use of the aluminium
oxide powder according to the invention as a filler, as a
carrier, as a catalytically active substance, as a ceramics

base, in the electronics industry, in the cosmetics
industry, as an additive in the silicone and rubber
industry, for adjusting the rheology of liquid systems, for
heat stabilisation, in the surface coatings industry.
Examples
Analysis
The BET surface area of the particles is determined in
accordance with DIN 66131.
The X-ray diffractograms are determined by means of a
transmission diffractometer from Stoe & Cie Darmstadt,
Germany. The parameters are: CuK alpha radiation,
excitation 30 mA, 45 kV, OED.
The dibutyl phthalate absorption is measured with a
RHEOCORD 90 device from Haake, Karlsruhe. For that purpose,
16 g, accurate to 0.001 g, of the aluminium oxide powder
are introduced into a kneading chamber, the chamber is
closed with a lid, and dibutyl phthalate is metered in
through a hole in the lid at a predetermined metering rate
of 0.0667 ml/s. The kneader is operated at a motor speed of
125 revolutions per minute. When the maximum torque is
reached, the kneader and the DBP metering are automatically
switched off. From the amount of DBP that has been consumed
and the quantity of particles weighed in, the DBP
absorption is calculated as follows:
DBP number (g/100 g) = (DBP consumption in g / weighed
portion of particles in g) x 100.
The hydroxyl group density is determined in accordance with
the method published by J. Mathias and G. Wannemacher in
Journal of Colloid and Interface Science 125 (1988), by
reaction with lithium aluminium hydride.

Measurement of the Sears index is described in EP-A-717008.
gamma = H2 supplied/stoichiometrically required H2
lambda = O2 supplied/stoichiometrically required O2
Example 1:
2.76 kg/h of A1C13 are vaporised in a vaporiser. The
vapours are transferred by means of an inert gas
(2.00 Nm3/h) to a mixing chamber. Separately therefrom,
3.04 Nm3/h of hydrogen and 10.00 Nm3/h of air are
introduced into the mixing chamber. In a central pipe, the
reaction mixture is fed to a burner and ignited. The
discharge rate of the reaction mixture from the burner is
31.4 m/s. The flame burns into a water-cooled flame tube.
20 Nm3/h of secondary air are additionally introduced into
the reaction chamber. The powder that forms is separated
off in a downstream filter and then treated
countercurrently with air and steam at about 600°C. The
physico-chemical data of the powder are shown in Table 2.
Examples 2 to 8 are carried out analogously to Example 1.
The process parameters and the physico-chemical data of the
powders are to be found in Table 1.
Figure 1A shows the X-ray diffractogram of the powder from
Example 1, Figure IB shows that of the powder from
Example 4. The X-ray diffractogram of the powder from
Example 1 clearly shows the signals of aluminium oxide
modifications. The powder from Example 4, on the other
hand, shows only a very weak signal at 2 theta = 67° and is
to be characterised as X-ray amorphous to the greatest
possible extent. The primary particles of both powders
consist of crystalline primary particles. Figure 2 shows a
high-resolution TEM picture of the powder from Example 4,
which shows this situation.

WE CLAIM :
1. Process for the production of the aluminium oxide powder produced
by flame hydrolysis comprising
- aluminium chloride is vaporised, the vapour is transferred by means
of a carrier gas to admixing chamber and,
- separately therefrom, hydrogen, air (primary air), which may
optionally be enriched with oxygen and/or may optionally be pre-
heated, are supplied to the mixing chamber, then
- the mixture of aluminium chloride vapour, hydrogen and air is
ignited in a burner and the flame burns into a reaction chamber
that is separated from the surrounding air,
- the solid material is subsequently separated from the gaseous
substances, and
- the solid material is then treated with steam and optionally with air,
- the discharge rate of the reaction mixture from the mixing chamber
into the reaction chamber being at least 10 m/s, and
- the lambda value being from 1 to 10 and the gamma value being
from 1 to 15 and
- wherein a secondary gas consisting of air and/or nitrogen is
introduced into the reaction chamber, the ratio primary
air/secondary gas is from 10 to 0.5 and

the aluminium chloride concentration ranges from 0.08 to 0.36 kg
of AlCl3/m3 of gas, wherein the said aluminium oxide has a BET
surface area of from 100 to 250 m2/g,
the dibutyl phthalate absorption is from 50 to 450g/100g of
aluminium oxide powder, and
it shows only crystalline primary particles on high-resolution TEM
pictures and wherein
the X-ray diffractogram it exhibits an intensity, expressed as the
counting rate, of less than 50 at an angle 2 theta of 67°.

Abstract

Aluminium oxide powder produced by flame hydrolysis and
having a large surface area
Aluminium oxide powder produced by flame hydrolysis and
consisting of aggregates of primary particles, having a BET
surface area of from 100 to 250 m2/g, a dibutyl phthalate
absorption of from 50 to 450 g/100 g of aluminium oxide
powder, which powder shows only crystalline primary
particles in high-resolution TEM pictures.
It is prepared by vaporising aluminium chloride,
transferring the vapour by means of a carrier gas to a
mixing chamber and, separately therefrom, supplying
hydrogen, air (primary air), which may optionally be
enriched with oxygen and/or may optionally be pre-heated,
to the mixing chamber, then igniting the mixture of
aluminium chloride vapour, hydrogen, air and in a burner
and burning the flame into a reaction chamber that is
separated from the surrounding air, subsequently separating
the solid material from the gaseous substances and then
treating the solid material with steam and optionally with
air, the discharge rate of the reaction mixture from the
mixing chamber into the reaction chamber being at least 10
m/s, and the lambda value being from 1 to 10 and the gamma
value being from 1 to 15.
It can be used as an ink-absorbing substance in ink-jet
media.