Note: Descriptions are shown in the official language in which they were submitted.
ACKGROUND OF rEE INVENTION
The present invention relates to a sir~terin~ proce~s
:~or making shaped refractory articles Iby using preheated
oxygen-freer noble or inert gases (in some instances nitrogen);
ancl ln particular r it: relates 'co makiny silicon carbide
(SiC) and ~;licon nitride (Si3N4~ bonded refractory articles.
If a ~ilicc)n carbide sintered product :is desired and 211
the sills:on in the green body is in the form of silicon
carbider then any oxy~en-free ga~ (ine.r'c), includirlg nitrogen~
1~ can be u~d to heat the ~reen body to the ~inteEillg or
f iring t~mperature .
If silicon nitride or ~ili on nitride bonded product~
are desired then el emental ~ilicon must bs~ included in
the greerl body and heated ni trogen must be used to conver~c
the ~ilico~D to ~ilicc)D nitrideO
5ilicon carbide has several physical and chemicaï
properties which make it an excellent mat2rial for higl
11~5096 .
temperature structural use Because of its high thermal
conductivity, silicon carbide can reduce fuel costs and
: is an excellent material for muffle type furnaces, gas~
turbine engines and retorts in the carbothermic production
5 1l and distillation o zinc. Silicon carbide is also used
: in electrial resistance elemen~s, ceramic ~iles, boilers,
around tapping holes~ in hea~ treating, annealing and
forging furnaces, in gas producers, and in other places
where strength at high temperatuxes, shock resistance
and sl.a~ resistance are required. Other propexties associated
with silicon carbide are superior strength, reractoriness,
corrosion resistance, abrasion resistance, thermal shock
resistance, and high specific gravity.
Silicon nitride has some advantages ovex silicon carbide,
such a~ a lower thermal expansion and higher fracture
toughnessO Other properties associated with ~ilicon nitride
are high thermal-shock resistancer high thermal conductivity,
strength at high temperatures, and corxosion resistance.
20 li
Most ceramic or refractory articles axe formed by
combining fine powders of a reractory material with binders
at low temperatures, then sintering th.is formed green
body at high temperatuxes. Refxactory articles are usually
I formed by conventional procedures such as dry pressing,
I air-hammering, or vibratlng (jo1~ing)~ These formed "green9'
(unsintered) bodies are then sintered at high temperatures
(over 1000 C) to develop de;irable physical and chemical
--3--
-
! ¦
proper~ies such as high strength, low porosi~y, or low
~, chemic~l reactivi~y.
.l In practice~ many ceramic or refractory material~
~ ~uch as those con5isting of alumina and silica are heated
in kilns which are ired by fos~il fuels and air or oxygen.
If the ceramic material can be exposed to air and/or the
" products of combustiont then the kiln may be dlrectly
fired, in which case the heating and utili~ation of energy
~ may be reasonably efficient. However, for certain ceramic
materials, including the carbides, the firing must be
done in the absence of oxygen or oxygen-bearing gases,
such as water and carbon dioxide, ~o prevent formation
I of oxides, which may have inferior physical or chemical
I properties. Under such conditions~ fossil fuel-fired
,I furnaces may be use~ but the ceramic parts must be kept
.in a controlled environment isolated from the combustion
products of the fuelO Because the shaped gree~ bodies
must be heated indirectly, the heating is inefficient
and slow. On a commiercial scale such a process, using
a tunnel ki].n, ~or example, requires about 84 hours (including
the cooling cycle).
¦¦ Electric kilns are also used to sinter ceramic or
refractory materials, but also tend to be energy inef~icient
and slowO In the case of a kiln e~uipped with graphite
electrodes, the ~olta~e ean be controlled and the ki:ln
can be heated to fairly high ~emper~tu~esr yet there are
Il ~
--4--
09~
several diSadV-ffntages: 1~ The graphite electrodes h~faVe
a limited size and must be kept under a strictly controlled
atmosphere to maintain a long life, and~ 2~ ~urnace size
is limited and i~ is difficult to achieve a uniform temperature
in this type of kiln because only the graphite electrodes
are the source of the radian~ hea~. Because of ~his radiant
heat tran~er as well as a size limit for graphi~e electrodes,
the kiln ha~ a limited productivity and poor energy efficiency,
f
Several patents reveal slow bonding times for ~ilicon
carbide or silicon nitride-bonded refractoriesO U~S.
Patent 3,206,318 teaches a process which is repres@ntative
; of the prior art. Specifically, it teaches the bonding
'I or nitriding of particulate sillcon and silicon carbide
refractory materials, by placing the green ceramic body
in a nitrogen akmoshpere within a muffle furnace and then
heating the furnace contents to 1300 1420C, whereby
the silicon react~ with the nitrogen to form silicon nitride
Il bonds, The examples reveal that the entire process (includin~
l the cooling cycle~ re~uire~ about 16 hours.
, I
U.S. Patent 4,127f63n diseloses a process for nitriding
a refractory article ormed from an elemental silicon
If powder. ~xample 1 describes t~.ffe use of a double-wallffad f~
25 1l gas tight silicon car~idfe box~ into which the green ~ody
j~ is placedfO The box is then 1Ooaed with nitro~ffffarff and
placed within an electric furnare whereby the contentYf
of the box are incrementally heated t~ 1450C. The example
_5_
11~5~
!I reveals that the total heating cycle is 63 hour~
~, V,S. Patent 3,222~438 xel~tes to a 19-20 hour proce~s
~ for nitriding a formea ~ilicon refrac~ory ar~icle, and
~ U.S. Patent 3~926,~57 reveals a 20 hour bonding proce~s
j fvr silicon, carbon, and nitrogen reacting to form silicon
carbide and silicon nitride. UOS. Patent 2,618~538 teaches
the use of a fluoride catalyst to speed up the nitriding
~ reaction between silicon and nitrogen ~o form silicon
ll nitride.
!~ Thus, prior art processes for forming oxygen~fr@e
bonded refractory articles~ in general~ require tedious
~ techniques for providing an oxygen free a!mosphere, have
ll a low productivity, are time consuming and energy inefficient.
Another problem associated specifically with nitrlding
Il silicon arises when the silicon nitride forms a layer
,1 on the silicon material; this layer i~ fairly impervious
to nitrogen. Thu.sl a longer reaction time i~ required
for co~ver~ion of the silicon to silicon nitride bonds~
SUMMARY OF THE IN~NTION
The present invention pxovides a solution to several
o the aforementioned problems associated with the prior
proces~es for producing oxygen-free bonded refractory
articl2s. SpeciEically, in accordance with the present
invention, a shaped green body of refractory raterial
--6--
is sintered or fired comprising the steps of: a) forming
a shaped geen body of a particulate refractory material
by conventional means; b) placing the shaped green body
in a furnace that can be flushed free of exygen or oxygen-
bearing gases by introducing an oxygen-free, inert or noble
gas (including nitrogen); c) preheating the oxygen-free
or noble gas to at lest 1500°C and preferably higher,
and, d) introducing the preheated gas directluy into
the furnace containing the shaped green body, causing
direct heat transfer from the preheated gas to the shaped
green boby, for a minimum time necessary to complete a
bonding reaction.
More specifically, in accordance with the present
invention, a green refractory body is formed from silico
carbide particles, place in a furnace which can be flushed
free of oxygen-bearing gases, and subjected to argon or
inert plasma gases preheated to greater than 4000°C, resulting
in a faster bonding time than that required by prior art
processes. The resulting temperature of the green refractory
body caused by direct heat transfer from the preheated
gases is around 1900-2200°C, which is below the melting
point of silicon carbide.
Also, in accordance with the present invention, a
shaped green body of admixed particulate refractory and
elemental silicon is formed, placed in an oxygen-free
atmosphere furnace, and exposed to preheated nitrogen
-7-
11~350~6
yas (at 1500C and preerably higher). This efiE2CtE; a
faster nitriding reaction to form silicon ni~ride bonds
! than that required by previous prior art processes because
l~ of the nitrogen gase~ incxea~2d reactivity and direct
1 heat transer. The resulting ~emperature of ~he green
refractory body during nitriding is 1000 - 1900C, which
is below the melting point of silicon nitrideO
'il ~
Il Preheating the oxy~en-free ga~ to 1500C or higher
10 j' is preferably achieved by using an electric arc and more
preferably, a plasma arc fired directly into the kilnO
,' Electric arc or plasma arc fired gases differ greatly
from ordinary furnace heated gases in ~ha~ ~hey contain
I electrically charged particles capable of transferring
5 11 electricity and heat, and become ionized; or as in the
case of nitrogen become dissociated and highly reactiveO
Theses phenomena greatly increase the reaction rates for
bonding refractory materialsO Nitrogen~ for example,
Il which dissoci tes at around 5700C and 1 atmosphere pressure f
0 ~ would not dissociate under the normal furnace heatiny
conditions of around 1450C required for a silicon nitrid.ing
reaction. Even if a furnace could reach the high dissociation
temperature of nitrogenl it would be undesirable for the
refractory green body to be at thi~ high temperature because
.5 the silicon nitride bonds would decompose at lgOOOC~
Thus~ ~:he "plasma" gas can be superheated to effect ionization
or di~sociation~ ~hile the refractory green bs:dy can be
directly heai:ed by the preheated gas to a much lower temperature~
-8--
Nitrogen gas dissociates into a highly reactive mixture
on N2-molecules, N-atoms, N+-ions and electrons. Argon
ionizes rather than dissociates when used with a plasma
arc.
Another important difference resulting from direct
heating of a refractory green body by preheated gases
is the quality of the product. Refractory products which
are fired by a rapid heating rate and a short soaking
time have good mechincal strenght, density and alkali-
resistance. However, a short-reation time tends to produce
a matrix of soft crystals, whereas a long-reation time
tends to produce a hard-crystal struture. Densities tend
to be the same using either slow or fst heating rates.
Quality of the product must be considered when selecting
a fast of slow heating rate for a refractory bonding process.
Accordingly, it is an object of the presetn invention
to provide a sintering process for producing a bonded
refractory article by preheating an oxygen-free, noble,
or inert gas and contacting a formed particulate refractory
body with it.
It is andother object of the present invention to reduce
the reaction time for the refractory green body to form
bonds because of the higher reactivity of the prefheated
gas and direct heat transfer to the refractory green body;
thereby providing higher furnace productivity, and minimizing
-9-
~i capital and operating costs.
. I, ,
1~ Still another object of the present invention is to
provide a preci~ion method for controlling the atmosphere
surrounding the refractory ~reen body during ~he bondin~
reaction, whereby each r~frac~ory article may be expo~ed
to identical, reproducible condi~ion~ including heating
Il rate, dwell time within the furnace, composition of gaseous
environment and heating temperatures.
Other objects and further scope of applicability of
the present invention will become apparen~ from the detailed
description to follow, ~aken in conjunction with the accom- ¦
I panying drawiny.
', B EF D SCRIPTI(:iN OF THE DRAWIN(;
,,
! Figure l is a schematic illu~tration of a prior art
Il indirect heating process for producing bonded refractroy
20 11 articles in a conventional fuel-fired ceramic kiln; and,
Il
Figure 2 is a schematic illustration of an embodiment
of the present invention wherein the gaseous atmosphere
Il surrounding and in intim~te contact with thP refractory
green body is directly heated by an electric arc.
,1
--10--
~5~
DESCRIPTION OF T~E PREFERRED EMBODIMENTS
! At the out~et r the process of the pre~en~ in~entivn
1~ is described in i~s brsades~ overall aspects with a mor@
1 detailed de~cription following. The present invention
' is a proces~ for sintering or firin~ c2ramic ~r refractory
!l articles; particularly non-oxide materials such as silicon
and silicon carbide, using gases electrically preheated
to at least 1500C~ and preferably higher, for efficient
, and rapid sintering. Particular applications are in the
manufacture of self~bonded silicon carbide, silicon nitride-
bonded silicon carbide and silicon-bonded refractGry articles.
When the gas is preheated to at leas~ 1500C, and pre~erably
higher, the gas causes direct hea~ transfer ~o th~ refractory
ll green body, thus effecting bond formation. In the case
of elemental silicon exposed to preheated nitrogen gas,
a silicon nitride bond i~ formed~
I¦ It has been found that by usin~ an electric arc~ and
ll more preferably, a plasma arc, gases become ionized or
dissociated, making them highly reactive, thus increasing
the bonding reaction rate. Plasma arc systems which can
~ire directly into a kiln can be fitted to conventional
I ~eriodic tbatch~ kilns ~s shown in Figure 2, or continuous
kilns. Thu~, the proces~ of the pre~ent invention may
be operated as either a batch process or a~ a continuous
process.
--11--
1195096
The shaped green refractory body, which is ~re~ed
~1l in ac~ordance with the present invention, is formed from
¦~ powders of refrac~ory materials in a conventional manner.
The furnace used in accordance with ~he present invention
may be specifically constructed for the purpose of the
present invention, or a~ noted above, may be any conventional
furnace including ba~ch and con~inuous type furnaces,
modified by using electric-arc or plasma-arc devices instead
o fuel burners or electrodes.
The g~ses employ@d in ~he process o the present invention
',l should be completely free of oxygen, water, carbon dioxide
or other oxygen-bearing gases to prevent oxidation of
I the refrac~ory product. To effect ~he bonding reaction/
15 ll the oxygen-free gas is electrically heatea to a high temperature
j which may vary anywhere from 1500 20~000~C.
The green refractory bodies are directly contacted
with the preheated oxygen--free gases, or a sufficient
~0 1 time to heat the green articles to bonding te~peratures
(usually within the range of 1000 - 20000C) and to coMplete
l! the bonding reactionO It has been found that the u~e
¦ of nitrogen gas heated to about 3000C will bring green
Il bricks of silicon and silicon carbide powder~ up to nitrid3 ng
ll temperatures (1000 -- 1600C) in two to e1ght hours, depending
on the design of the furnace; and the use of argon or
nitrogen gas heated to above 4000aC will bring green bricks
of silicon carbide powders up to bonding temperatures
-12-
5~
Il (1900 - 2200C) in the same time period.
il l
Il ~ E~
Powdexed silicon carbide is admixed with parki~ulate
elemental silicon and a carbonaceous binder and pressformed
into green bricks in a conven~ional ~a~nerO One hundred
pounds of ~uch green bricks are placed wi~hin an insulated
I retort (batch kiln) such as ifi illustra~ed in Fig.2.
0 I Electric-arc heaters, operating at 3.75 kilowatt~, are
provided in each of the inlet ports located at the bottom
of the insulated retort. Nitrogen gas is introduced through
` the retort inlet pvrts, pa~sing thrnugh the electric arcs
, and thereby heated to about 2000~C. The preheated, rea~tive
5 ~I nitrogen ~as is allowed to circulate through the green
bricks for eight hours to br.ing the green bricks up to
j nitriding tempera~ure a 1200C, which is below the melting
point of silicon (1450C), and for an additional period
~,l of time to complete the tr~nsformation of the elementa.l
0 l s.ilicon into sil.icon nitride tthereby foxm.ing the desired
silic4n nitr.ide bond~)O The overall thermal efi~iency
I i~ calculated to be 67 percent.
1l .
ll ~
Powdered ~ilicon carbide admi~ed with particulate
elemental silicon and a phenolic resin binder is formed
into green refxactory brick~ in a CoDVentional manneE~
-13
09ti
Plasma-fired ni~rogen gas at ~bove 3000C is introduced
into the furnace, heating the refrac~ory bricks ~o at
temperature of 1400 ~ 1600C, which is above the melting
l~ point of silicon (1450C). A phenolic resin instead of
',1 a conventional carbonaceous binder is used ~o efect formation
of beta silicon carbide by reaction of the silicon carbid~
powder with carbon, present as graphite inclusions and
~, in the phenolic resin. It is believed that beta silicon
l carbide increases the refractory~s alkali resistance,
'' without impairing the refractory's mechanical or physical
' proper~ies. ~itriding is determined to be completed within
I one hour. The total cycle time, including cooling, is
about eight hours.
'I Althou~h the invention has been described with reference
to these preferred embodiments, other embodiments can
I acheive the same results. Variations and modifications
of the present invention will be obvious to those sk.illed
Il in the art and it is intended to cover in the appended
claims al.l such modification~ and equivalentsO
.5
I
-14-
