Note: Descriptions are shown in the official language in which they were submitted.
~Z35~
This invention relates to a process for making a
silicon aluminum oxynitride refractory material and, more
particularly, a process wherein at least a portion of initial
reactants are converted to at least a portion of effective
reactants in a first heating step in the presence of nitrogen
and effective reactants are converted to a silicon aluminum
o~ynitride refractory material in a second heating step.
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 me~hods 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.
~ ,
~L2~5~
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
Journal of Material Sciences, 11, (1976) at pages 1135-1158.
Compositions 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
high density refractory by conventional sintering techniques.
To produce high density refractories from silicon nitride or
silicon oxynitride requires the use of pressure sintering
techniques.
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 struc~ure similar to that of
beta Si3N4 but with an expanded lattice structure. Weaver's
method of making the described product is by hot pressing
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Si2ON2 (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
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.
Further, in the process as taught by Cutler excess carbon and
unreacted silicon dioxide must be removed from the mixture
after the mixture is nitrided.
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
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828-1210
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 does
not employ a two-step heating process in producing beta'-SiAlON.
It would be advantageous, therefore, to provide a
process whereby readily available and relatively inexpensive
initial reactant materials comprising A12O3 and SiO2 are nitrided
to make silicon aluminum oxynitride materials without the
necessity of further processing in removing excess carbon
and /or silica.
This invention is for a process for producing an
essentially beta'-SiAlON refractory material from a uniform
mixture of SiO2, A12O3 and C initial reactants. These reactants
are placed in a reactor and nitrided at temperatures between
1200C and 1450C for a time sufficient to convert at least
a portion of the initial reactants to at least a portion of
effective reactants. The effective reactants are then heated
in the presence of nitrogen at a temperature higher than the
nitriding temperature and within a range between 1400C and
1650C for a time sufficient to convert the effective reactants
to an essentially beta'-SiAlON refractory material.
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 which
is essentially beta'-SiAlON is intended to mean a material
having approximately 80% or more of beta'-SiAlON therein.
Alternatively, compounds which yield silica or alumina under
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~235710
828-1210
the temperatures employed in the praetiee of this invention
may be used as sourees of siliea or alumina. Such sourees
include silicates such as quartz, cristabolite, tridymite
and amorphous silica as silica sources, for example, and aluminum
earbonate, aluminum nitrate, aluminum hydroxide or gibbsite
(aluminum trihydrate), for example, as alumina sources. References
hereinafter to siliea (SiO2) and alumina (A12O3) are intended
to inelude, but are not limited to, the foregoing materials
cited as examples. A process for producing beta'-SiAlON from
discrete partieles of A12O3 and SiO2 is the subjeet of an
applieation for a U.S. patent by Phelps et al filed eoneurrently
herewith. Other initial reaetants may inelude sources of
silicon dioxide and aluminum oxide as disclosed in Cutler
U. S. Patent No.
- 4a -
~:3~i710
828-1210
3,960,581.
If the initial reactants are discrete particles, they
are mixed to uniformly distribute the particles thoughout
the mixture and the mixture is then combined with enough water
to plasticize the mixture for forming into shapes. If the
initial reactants are intimately combined, as disclosed in
U.S. Patent No. 3,960,581, the reactants are simply finely
ground if necessary to adapt the reactants for forming. 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 nitrided to convert the initial reactants
into transitory or effective reactants, and in a further heating
step in a nitrogen atmosphere, the effective reactants are
converted to beta'-SiAlON.
It is an object of the invention to provide a method
of producing ~eta'-SiAlON from economical, readily available
initial reactants comprising A12O3 and SiO2.
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 the initial reactants to produce beta'-SiAlON by a process
of this invention.
Fig. 2 is a graph showing the composition 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 de-fined
as a solid solution of A12O3 within an Si3N4 matrix and
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~L2357~
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 Si~AlON3, 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 Gther
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, if necessary. The particles are then
combined with enough water by mixing either during blending or
subsequent 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 of the reactants may vary, but
generally, the smaller the particle size, the more complete the
reaction when fired, as will be discussed later. The preferred
median particle size of A12O3 is less than 3.5 microns and
preferably less than 0.5 micron. The preferred SiO2 source is
fumed silica having a median particle size of 0.1 micron.
After mixing and molding the initial reactants into
pellets, the pellets are dried at a low temperature, such as
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110C, for example, to drive off any excess moisture. The
pellets are then charged into a reaction chamber adapted to
nitride and heat the pellets in a two-stage heating cycle.
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 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 a first charge of pellets into the reactor into an
upper heat zone 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 in the upper heating zone of the reactor. It is
believed that by maintaining the reactants at a given tempera-
ture 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.
1;~357~
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:
N2
(a) SiO2 + C ~ Si3N4 + CO
N2
(b) A12O3 ~ C -~ AlN -~ CO.
It may be noted that in addition to Si3N4 and AlN,
A12O3 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 first charge of pellets is moved downwardly to a second heat
zone and 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. Con-
currently with the movement of the first charge of pellets into
the second heat zone, additional initial reactants are charged
into the first heat zone. 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 tempera-
ture is increased, less time is required to effect an essen-
tially 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. Thus, the time of
1;2357~L~
residence of the reactants in each heat zone can be controlled
to be essentially the same and the process can be operated on a
continuous batch by-batch basis. In an alternate method of
operating the process continuously, the initial reactants may be
fed into the first heat zone at a rate suitable to traverse the
first heat zone and effect the conversion to effective reactants.
The effective reactants then move continuously into the second
heat zone and traverse the second zone a time sufficient to
convert the reactants to essentially beta'-SiAlON. It may be
seen that the extent of the heat zones may be adjusted to insure
that the pellets remain in each heat zone a sufficient length of
time as they advance at a uniform rate. 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.
In the foregoing description the two-step nitriding
and heating cycle of the reactants is accomplished successively
and continuously in a vertical shaft 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.
~23~7~
The following example is offered to illustrate the
production of beta'-SiAlON by a 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 provlded 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 diameter x 18.75 mm length were produced
by extruding.
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
lQ
0
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-Fel 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.
