Note: Descriptions are shown in the official language in which they were submitted.
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This invention relates to a process for making a
silicon aluminum oxynitride refractory material and, more
particularly, a process which includes the use of A12O3 and
SiO2 in a discrete particle form as initial reactants.
Silicon aluminum oxynitride refractory materials, and
more particularly materials in the Si3N4-AlN-A12O3-SiO2 system,
are of ever-increasing interest for refractory applications.
For ease of identification, compositions within this system are
referred to as SiAlON, and a number of different phases of
SiAlON have been produced and identified. For example, Jack et
al U.S. Patent No. 3,991,166 describes one phase and methods of
making it, the phase having the general formula Si6 zAlzOzN8 z
where z is greater than zero and less than or equal to five.
Various compositions within the bounds of the general formula
taught by Jack et al may be produced and each has a crystalline
structure similar to beta-Si3N4 and is consequently identified
as beta'-SiAlON. Beta'-SiAlON can be defined as a solid
solution of A12O3 within a matrix of Si3N4. The compositional
limits of reactants, referred to as effective reactants, to
produce beta'-SiAlON may be seen by referring to Fig. 2. The
compositional amounts of Si3N4, AlN and A12O3 for any
beta'-SiAlON formulation may be determined by referring to line
AB which is a plot of the compositions of the aforesaid
compounds to produce a beta'-SiAlON having the general formula
Si6 zAlzOzN8 z where z is greater than zero and less than or
equal to five.
Another phase, known as y-phase SiAlON represented by
the formula SiA14O2N4, is described in an article entitled
"Review: SiAlONs and Related Nitrogen Ceramics", published in
~ s
.
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Journal of Material Sciences, 11, (1976) at pages 1135-1158.
Compcsitions of SiAlON within a given phase and from phase to
phase demonstrate varying characteristics, for example,
variances in density, which effect their preferential use in a
given application.
Thus far, of all the SiAlON materials, the
beta'-SiAlONs have generated the greatest interest because
their refractory properties and corrosion resistance
characteristics are similar to other nitride refractories such
as silicon nitride and silicon oxynitride. Beta'-SiAlON
compositions offer a distinct advantage over silicon nitride
and silicon oxynitride for making a refractory, however,
because beta'-SiAlON material can be used to produce a
refractory by conventional sintering techniques, whereas a
refractory produced from silicon nitride or silicon oxynitride
requires the use of a pressure sintering technique.
A number of processes for making silicon aluminum
oxynitride refractories and refractory materials have been
suggested. Weaver U.S. Patent No. 3,837,871 describes a method
for producing a product having a substantial amount of what the
patentee believes to be the quaternary compound silicon
aluminum oxynitride which has a structure similar to that of
beta Si3N4 but with an expanded lattice structure. Weaver's
method of making the described product is by hot pressing
Si20N2 (silicon oxynitride) in the presence of varying amounts
of aluminum.
Kamigaito et al U.S. Patent No. 3,903,230 describes a
method of making a silicon aluminum oxynitride ceramic by
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sintering or hot pressing a mixture of finely divided powders
of silicon nitride, alumina and aluminum nitride.
Cutler U.S. Patent No. 3,960,581 describes a process
for producing SiAlON by reacting silicon and aluminum compounds
in the presence of carbon and nitrogen. Cutler teaches and
stresses the importance of using a reactant material having the
silicon and aLuminum compounds intimately combined prior to
nitriding in order that aluminum oxide is intimately dispersed
throughout silicon nitride in the final product. Suggested
reactant materials are clay, rice hulls having a solution
containing a dissolved aluminum salt absorbed therein, and a
precipitate of aluminum and silicon salts. In each case Cutler
emphasizes that the silicon and aluminum compound reactants are
intimately combined prior to nitriding to produce SiAlON.
Jack et al U.S. Patent No. 3,991,166 describes a
beta'-SiAlON product produced by sintering a mixture of alumina
or a compound which decomposes to produce alumina and silicon
nitride. Another method of producing beta'-SiAlON as described
by Jack et al is nitriding silicon powder in the presence of
alumina powder.
It may be noted that several of the foregoing
processes employ silicon nitride or silicon oxynitride as
reactants. Neither of these compounds is found in nature and
they are relatively expensive to produce. Cutler's process
provides for the use of reactants found in nature but also
requires that the reactants be intimately combined before being
converted to SiAlON.
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It would be advantageous, therefore, to provide a
process whereby readily available and relatively inexpensive
react:ant materials are nitrided to make silicon aluminum
oxynitride materials.
Discrete particles of silica, alumina and carbon may
be used as initial reactants in producing essentially
beta'-SiAlON. For purposes of this invention, a material that
is described as essentially beta'-SiAlON is intended to mean a
material having a beta'-SiAlON content greater than
approximately 80%. Alternatively, compounds which yield silica
or alumina under the temperatures employed in the practice of
this invention may be used as sources of silica or alumina.
Such sources include silicates such as quartz, cristabolite,
tridymite and amorphous silica as silica sources, for example,
and aluminum carbonate, aluminum nitrate, aluminum hydroxide or
gibbsite (aluminum trihydrate), for example, as alumina
sources. References hereinafter to silica (SiO2) and alumina
(A1203) are intended to include, but are not limited to, the
foregoing materials cited as examples. The discrete particles
of silica, alumina and carbon are mixed to uniformly distribute
the particles throughout the mixture which is then combined
with enough water to plasticize the mixture for forming into
shapes. Forming may be by extruding or other molding methods
familiar to those skilled in the art to shape the mixture into
pellets. The pellets are then heated in a nitrogen atmosphere
to convert the reactants to a beta'-SiAlON material.
It is an object of the invention to provide a method
of producing beta'-SiAlON from economical, readily available
reactants.
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This and other objects and advantages will be more
fully understood and appreciated with reference to the
following description and associated drawings.
Fig. 1 is a graph showing the compositional limits of
theinitial reactants to produce beta'-SiAlCN by a process of
this invention.
Fig. 2 is a graph showing the compositional limits of
transitional or effective reactants to produce beta'-SiAlON by
a process of this invention.
As has been noted previously, beta'-SiAlON may be
defined as a solid solution of A12O3 within an Si3N4 matrix and
is represented by the general formula Si6 zAlzOzN8 z where z is
greater than zero and less than or equal to five. To produce
beta'-SiAlON by a process of this invention, initial reactants
A12O3, SiO2 and C are provided in compositional ratios as
indicated by the line AB in Fig. 1. To produce a beta'-SiAlON
when z = 2 with a formula of Si2AlON3, for example, would
require 23% by weight A12O3, 24% by weight C and 53% by weight
SiO2. Although not essential, it is advantageous to add iron
in a form such as Fe2O3 as a catalyst in promoting the
formation of beta'-SiAlON. It is believed that oxides of other
transitional metals such as nickel, chrome or manganese, for
example, may also be used as catalysts in the practice of this
invention. Only a small percentagae of catalyst, such as 2%
for example, is added.
The SiO2, A12O3 and C initial reactants are
mechanically mixed by any suitable mixing method to uniformly
blend the particles. The particles are then combined with
enough water by mixing either during blending or subsequent
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thereto, preferably subsequent thereto, to render the mixture
plastic for extruding or other molding methods familiar to one
skilled in the art to produce a pellet suitable for nitriding.
The particle size and particle distribution of the reactants
may vary, but generally, the finer the particles, the more
complete is the reaction when fired, as will be discussed
later.
It has been observed in the practice of a process of
this invention using fumed silica, petrole~m carbon and A12O3
as reactants that approximately 70% of the A12O3 reactant goes
into solution within an Si3N4 matrix with a distribution of
A12O3 particles as follows: 90% less than 136 microns, 50%
less than 77 microns, 30% less than 62 microns, and 10% less
than 42 microns. If the particle size distribution of A1203 is
changed to 90% less than 7 microns, 70% less than 4.4 microns,
50% less than 3.3 microns, 30% less than 2.3 microns, and 10%
less than 1.3 microns, the amount of Al2O3 which goes into
solution within the Si3N4 matrix increases to 81%. With yet a
further reduction in particle size and distribution of 90% less
than 1.1 microns, 70% less than 0.52 micron, 50% less than 0.37
micron, 30% less than 0.29 micron, and 10% less than 0.20
micron, the percentage of Al2O3 in solid solution within the
Si3N4 matrix increased to more than 95%.
To determine a preferred median particle size and
part~cle distribution for SiO2, tests were performed in
practicing a process of this invention using petroleum carbon,
A12O3 having a median particle size of 0.37 micron, and SiO2
having a particle size distribution of 90% less than 88
microns, 70% less than 44 microns, 50% less than 27.6 microns,
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30% l.ess than 11 microns, and 10~ less than 3.9 microns. With
the foregoing SiO2 particle distribution, 92.3~ of the SiO2 was
nitrided and 86% of the A12O3 went into solid solution within
an Si3N4 matrix. With a change in SiO2 particle distribution
to 90% less than 24.7 microns, 70% less than 11 microns, 50%
less than 5.7 microns, 30% less than 3.9 microns, and 10% less
than 2.3 microns, 100% of the SiO2 was nitrided and 86% of the
A12O3 went into solid solution within an Si3N4 matrix.
On the basis of the above observations, a preferred
median particle size for A12O3 in the practice of a process of
this invention is less than 3.5 microns, and a more preferred
median particle size of A12O3 is less than 0.37 micron. A
preferred median particle size for SiO2 is less than 27.6
microns and a more preerred median particle size is less than
5.7 microns.
After mixing and molding the initial reactants into
pellets, the pellets are dried at a low temperature, such as
110~C, for example, to drive off any excess moisture.
Conversion of the reactants into beta'-SiAlON is
accomplished by heating the pellets in a nitrogen atmosphere.
A preferred method of conversion as hereinafter described is
the subject of an improved method for producing SiAlON in an
application for a U.S. patent by Phelps et al filed concurrently
herewith. To convert the reactants to beta'-SiAlON by the
referenced preferred method, the pellets are charged into a
reactor adapted to maintain the pellets in a nitrogen
atmosphere. Nitrogen may be provided as a gas or a compound,
such as ammonia, for example, that will reduce to nitrogen gas
at the reaction temperature. It is preferred that the nitrogen
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be provided continuously under a positive pressure to insure
that the nitrogen will uniformly contact all of the reactants
during the reaction cycle. A suitable reactor to accomplish the
above purposes is a fluid bed reactor or packed bed reactor
provided with a nitrogen gas dispersing means near the bottom of
the reactor and a nitrogen and off-gas outlet near the top.
After charging the pellets into the reactor to form a suitable
bed, nitrogen is dispersed through the bed under a positive
pressure to purge the reactor of its normal atmosphere.
After establishing a nitrogen atmosphere within the
reactor, temperature of the reactants is elevated by a suitable
heating means to a temperature of at least 1200C, preferably
at least 1400C. It is believed that by maintaining the
reactants at a given temperature of at least 1200C for a
sufficient period of time, a portion of the initial reactants
are reduced to a portion of the effective reactants necessary
for producing beta'-SiAlON. The period of time required to
accomplish this initial reaction will vary with the temperature
employed. It has been discovered that heating at a temperature
of 1400C for 1-1/2 hours, for example, is sufficient to
accomplish the initial reaction in the process.
It is believed that the above-described initial
nitriding step yields Si3N4, AlN and CO as off-gas and may be
represented by the equations:
ta) SiO2 + C ~2 Si3N4 + CO
(b) A12O3 + C ~2 AlN + CO.
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It may be noted that in addition to Si3N~ and AlN,
A12O,3 is also required as an effective reactant in producing
beta^-SiAlON, and A12O3 is provided in a quantity in excess of
the amount needed for production of the necessary AlN so that a
portion of the A12O3 remains as an effective reactant after the
initial reaction.
Following the above-described initial nitriding step.
the reactant temperature is increased to a maximum of 1650C,
preferably within a range of 1550 to 1600C, and maintained
within that temperature range for a time sufficient to convert
the effective reactants to beta'-SiAlON. It is believed that
some conversion of the effective reactants begins to occur at
temperatures as low as 1200C, but it has been discovered that
if the temperature is increased, less time is required to
effect an essentially complete conversion of the effective
reactants to beta'-SiAlON. Within the range of 1550 to
1600C, a time of heating of approximately 1-1/2 hours is
sufficient to yield an essentially single phase beta'-SiAlON.
Although raising the temperature after the initial heating step
to produce effective reactants is advantageous in effecting a
conversion of the transitional or effective reactants into an
essentially single phase beta'-SiAlON, raising the temperature
above approximately 1650C promotes the formation of other
SiAlON phases which is detrimental to the purposes of the
invention.
During the final heating step after nitriding, a
nitrogen atmosphere is maintained in the reactor to preserve a
stoichiometric balance as expressed in the equation:
Si3N4 + A12O3 + AlN ~ beta'-SiAlON.
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In the foregoing description the two-step nitriding
and h,eating cycle of the reactants was accomplished
successively and continuously in a reactor. If desired, the
process may be interrupted after the initial nitriding step in
making the effective reactants, and the effective reactants can
then be transferred to an alternate reactor to make the
ultimate conversion to beta'-SiAlON.
The following example is offered to illustrate the
production of beta'-SiAlON by a preferred process of this
invention.
Example
500 g of beta'-SiAlON having a formula Si2AlON3 were
prepared from discrete particles of A12O3, fumed SiO2,
petroleum carbon and an Fe2O3 catalyst.
The above-mentioned initial reaction particles of
A12O3, fumed silica and Fe2O3 were provided having median
particle sizes as follows: A12O3 - approximately 1 micron,
SiO2 - 0.1 micron, and Fe2O3 - 2.5 microns. By reference to
Fig, 1, the portions of reaction materials required to produce
500 g of Si2AlON3 were determined to be: 115 g A12O3, 265 g
SiO2 and 120 g of carbon.
The reaction materials in the above-stated portions
plus 2~ or 10 g of Fe2O3 catalyst material were charged into a
4.9 liter ceramic ball mill where the materials were uniformly
mixed. The resultant mixture was then mixed with enough water
to render the mixture plastic, and pellets having dimensions of
approximately 3.1 mm in diameter x 18.75 mm long were produced
by extruding.
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The pellets were then dried to drive off excess water
and were charged into an enclosed reactor vessel provided with
an inlet below the pellet bed to permit uniform circulation of
gaseous nitrogen through the pellets and an outlet near the top
of the vessel to permit discharge of nitrogen and reaction gas
products.
The vessel having the pellets therein was enclosed in
a heating chamber and nitrogen was charged into the vessel at a
pressure sufficient to maintain a flow of nitrogen through the
vessel throughout the subsequent heating cycles.
When it was determined that the reaction vessel had
been purged of air, temperature within the heating chamber was
increased an amount necessary to raise the temperature of the
pellets to 1400C and that pellet temperature was maintained
for 1-1/2 hours.
The pellet temperature was then increased to 1600C
and maintained thereat for 1-1/2 hours. The pellets were then
cooled to room temperature and analyzed for composition. It
was determined by X-ray diffraction that the processed material
was comprised of beta'-Si2AlON3 in excess of 90% and 3A12O3 '
2SiO2 (mullite), alpha-Fe, SiC and other unidentified phases
making up the balance.
Various modifications may be made in the invention
without departing from the spirit thereof, or the scope of the
claims, and therefore, the exact form shown is to be taken as
illustrative only and not in a limiting sense, and it is
desired that only such.limitations shall be placed thereon as
are :imposed by the prior art, or are specifically set forth in
the appended claims.
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