Note: Descriptions are shown in the official language in which they were submitted.
7V~
REFRACTORY
Silicon nitride, aluminum nitride and aluminum
oxide in the form of fine powders when thoroughly and
uniformly mixed in suitable proportions, and heated at
elevated temperatures, can provide ceramics which have
relatively good high temperature properties and application
in excess of 1400C. Nitride compounds referred to as
sialon compounds have been synthesized by mixing alpha
and/or beta silicon nitride with alpha and/or gamma alumina
powder. Sialon generally means an intimate dispersion of
alumina oxide throughout a silicon nitride matrix. It is
believed that upon sinteringJ the material becomes a solid
solution of aluminum oxide in silicon nitride. The letters
which make up the term "sialon" are the letters taken from
the chemical abbreviation for the elements therein, that is,
silicon, aluminum, oxygen and nitrogen.
Considerable effort has been directed to the
development of ceramic articles containing 80~ and more of
silicon nitride, silicon oxynitride and/or sialon. These
articles consist predominantly of single phase nitrides and
display good thermal shock resistance, strength and
corrosion resistance. Little information exists in the
utilization of these nitride phases as the bonding agent in
conventional refractories. Several limiting factors which
have retarded large scale development of nitride bonded
refractories include the high cost of purchased silicon
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7~55
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nitride, the instability of certain oxynitrides at high
temperature, and the hydrolizing tendency of possible
starting materials, such as, aluminum nitride and magnesium
nitride. To overcome these obstacles, it would be
advantageous to form, in situ, nitride phases by the
addition of a single metallic metal powder which can react
with gaseous nitrogen to produce a crystalline nitride phase
capable of ceramic bonding to relatively inexpensive
refractory grains. This approach will greatly lower the
cost of nitride articles and couple the distinct advantages
of nitride compounds to the established advantages of
conventional refractory grains.
It is an object of the present invention to
produce nitride bonded refractories with improved physical
properties compared to refractories made with the addition
- of two or more reactive metal powders.
Another object of the invention is to join a
sialon and other nitride phases with conventional refractory
grains which are typically bonded by oxides which can be
readily decomposed by certain metals to provide properties,
such as, non-wetability by molten metals, resistance to
chlorine attack and low thermal expansion.
A further object of the invention is to provide
nitride bonded refractories having improved porosity and
~5 relatively good room temperature and elevated temperature
strength.
In accordance with the present invention, there is
provided a method for producing nitride bonded refràctory
shapes in situ. A mixture is prepared comprising about 1 to
25%, by weight, aluminum, about 1 to 25%, by
weight, substantially pure silica and the balance a
refractory brick making size graded refractory aggregate.
The mixes are pressed into refractory shapes and burned at
elevated temperatures in a nitriding atmosphere to form the
nitride bond.
In a preferred embodiment, the aluminum comprises
about 1 to 16% and the substantially pure silica comprises
about 1.5 to 20~, by weight, of the mix. The shapes are
preferably burned at a temperature between about 1090 and
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1750C and the nitriding~atmosphere is composed o~ either
gaseous nitrogen, industrial annealing gas, or ammonia gas.
I'he refractory aggregate is preferably selected from silicon
carbide, fused mullite and magnesite.
In a nitrogen atmosphere, at elevated
temperatures, aluminum reduces silica forming silicon,
alumina, aluminum nitride and gamma aluminum oxynitride.
With additional treatment at elevated temperatures, silicon
is nitrided to form beta silicon nitride and the alumina,
aluminum nitride and aluminum oxynitride enters into the
silicon nitride structure as a solid solution to form beta
prime sialon. During firing it is always possible that
minor levels of oxygen may enter into the chamber confining
the refractories. In such an event, the formation of a pure
beta prime sialon is hampered and the so-called "X", "J" or
aluminum nitride polytypes may also form.
During nitriding, the metallic phase undergoes a
gas-metal reaction and forms minute crystals surrounding the
metal nucleus. Maintaining a hold during the firing process
ensures drainage of the metal from the nucleus through the
pores of the crystalline mat which allows additional
nitridization of the metal. During the end of the hold
period, true ceramic bonding is achieved with the coarse
refractory grains by virtue of their solubility in the
nitride phases.
To successfully achieve nitridization and also an
economical firing schedule, it is preferred that the
starting metal powder be as fine as possible. Generally,
the aluminum powder should have an average particle diameter
of about 34 microns with 90% of the particles being finer
than 70 microns. The silica used in the mixes may have one
or more ranges of particle size. For instance, extremely
fine silica can be used which has an average particle
diameter of less than about 1 micron. However,
incorporation of large quantities of this exceedingly fine
material to a refractory mix, often results in pressing
difficulties. It is advantageous to add the very fine
silica with a coarser form of silica to obtain the large
amount of silica needed in the mix.
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It is also preferred that the reactive material
not exceed about 20~ of the mix for economic reasons. Also,
larger quantities do not result in articles with materially
improved physical properties.
In the following examples, illustrated below,
aluminum powder was mixed with silica and either silicon
carbide, fused mullite or magnesite. A solution of dextrin
and/or lignin liquor and water was used as a temporary
binder. The mixes were formed into shapes by power pressing
to about 18,000 psi. The brick were then fired in the
presence of flowing nitrogen to a temperature of about
2600F with a holding time of about four hours. Mixes were
also prepared containing a combination of both aluminum and
silicon metal powders. The overall results indicated that
the mixes made with only a single metal addition was
stronger and less porous than mixes made with the two metal
additions. The various bonding phases are also shown in
Table I.
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In the above mixes, the refractory aggregate was
sized such that about 5 to 22% was retained on a 10 mesh
screen, about 23 to 36% was -10+28 mesh, about 8 to 25% was
-28+65 mesh, about 7 to 10% was -65+200 mesh and about 30 to
35% passed a 200 mesh screen. All of the above mesh sizes
are based upon the Tyler standard series.
As to the raw materials used above, the aluminum
powder was pure aluminum metal, and the silica analyzed in
excess of 98% SiO2. The refractory aggregate used in the
examples have the approximate chemical analysis as shown in
Table II below.
TABLE II
Silicon Fused Deadburned
Carbide Mullite Magensite
SiO2 - 22.9% 0.7
A123 0 4 76.4 0.2
TiO2 0.1 0.1 -
Fe2O3 0.8 0.3 0.2
CaO 0.2 - 0.6
MgO 0.02 - 98.3
Alk. 0.03 0.35
Calculated SiC 96.5 - -
All of the chemical analyses are based on an oxide analysis
which would not indicate the carbon content of the silicon
carbide.
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