CA1195096A - Sintering process for refractory articles using direct-heated gases - Google Patents
Sintering process for refractory articles using direct-heated gasesInfo
- Publication number
- CA1195096A CA1195096A CA000409008A CA409008A CA1195096A CA 1195096 A CA1195096 A CA 1195096A CA 000409008 A CA000409008 A CA 000409008A CA 409008 A CA409008 A CA 409008A CA 1195096 A CA1195096 A CA 1195096A
- Authority
- CA
- Canada
- Prior art keywords
- silicon
- green body
- oxygen
- gas
- furnace
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
Links
Classifications
-
- C—CHEMISTRY; METALLURGY
- C04—CEMENTS; CONCRETE; ARTIFICIAL STONE; CERAMICS; REFRACTORIES
- C04B—LIME, MAGNESIA; SLAG; CEMENTS; COMPOSITIONS THEREOF, e.g. MORTARS, CONCRETE OR LIKE BUILDING MATERIALS; ARTIFICIAL STONE; CERAMICS; REFRACTORIES; TREATMENT OF NATURAL STONE
- C04B35/00—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products
- C04B35/622—Forming processes; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products
- C04B35/64—Burning or sintering processes
- C04B35/65—Reaction sintering of free metal- or free silicon-containing compositions
-
- C—CHEMISTRY; METALLURGY
- C04—CEMENTS; CONCRETE; ARTIFICIAL STONE; CERAMICS; REFRACTORIES
- C04B—LIME, MAGNESIA; SLAG; CEMENTS; COMPOSITIONS THEREOF, e.g. MORTARS, CONCRETE OR LIKE BUILDING MATERIALS; ARTIFICIAL STONE; CERAMICS; REFRACTORIES; TREATMENT OF NATURAL STONE
- C04B35/00—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products
- C04B35/515—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products based on non-oxide ceramics
- C04B35/56—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products based on non-oxide ceramics based on carbides or oxycarbides
- C04B35/565—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products based on non-oxide ceramics based on carbides or oxycarbides based on silicon carbide
-
- C—CHEMISTRY; METALLURGY
- C04—CEMENTS; CONCRETE; ARTIFICIAL STONE; CERAMICS; REFRACTORIES
- C04B—LIME, MAGNESIA; SLAG; CEMENTS; COMPOSITIONS THEREOF, e.g. MORTARS, CONCRETE OR LIKE BUILDING MATERIALS; ARTIFICIAL STONE; CERAMICS; REFRACTORIES; TREATMENT OF NATURAL STONE
- C04B35/00—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products
- C04B35/515—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products based on non-oxide ceramics
- C04B35/58—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products based on non-oxide ceramics based on borides, nitrides, i.e. nitrides, oxynitrides, carbonitrides or oxycarbonitrides or silicides
- C04B35/584—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products based on non-oxide ceramics based on borides, nitrides, i.e. nitrides, oxynitrides, carbonitrides or oxycarbonitrides or silicides based on silicon nitride
- C04B35/591—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products based on non-oxide ceramics based on borides, nitrides, i.e. nitrides, oxynitrides, carbonitrides or oxycarbonitrides or silicides based on silicon nitride obtained by reaction sintering
Abstract
A SINTERING PROCESS FOR REFACTORY ARTICLES USING
DIRECT-HEATED GASES
ABSTRACT OF THE DISCLOSURE
This invention comprises a process for firing or sintering a shaped silicon containing refactory article by preheating an oxygen-free,inert,or noble gas and introducing it into a furnace containing a formed particulate refractory green body. If a bonded silicon nitride article is the desired product, silicon in the green body is reacted with direct heated nitrigen. Silicon may be admixed with silicon carbide in the green body which after nitriding yields a silicon carbide - silicon nitride bonded article.
DIRECT-HEATED GASES
ABSTRACT OF THE DISCLOSURE
This invention comprises a process for firing or sintering a shaped silicon containing refactory article by preheating an oxygen-free,inert,or noble gas and introducing it into a furnace containing a formed particulate refractory green body. If a bonded silicon nitride article is the desired product, silicon in the green body is reacted with direct heated nitrigen. Silicon may be admixed with silicon carbide in the green body which after nitriding yields a silicon carbide - silicon nitride bonded article.
Description
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 -! ¦
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 ~
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 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 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~
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 ~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 ~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.
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 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~
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
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 -! ¦
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 ~
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 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 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~
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 ~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 ~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.
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 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~
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
Claims (3)
PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. A process for firing a shaped green body of silicon carbide refractory material comprising the steps of:
a. forming a shaped green body of refractory material;
b. placing the shaped green body in a furnace that is flushed free of oxygen or oxygen-bearing gases;
c. preheating an oxygen-free gas to at least 1500°C; and d. introducing the preheated gas directly into the furnace con-taining the shaped green body, causing direct heat transfer from the preheated gas to the shaped green body, for a minimum time necessary to complete a bonding reaction.
a. forming a shaped green body of refractory material;
b. placing the shaped green body in a furnace that is flushed free of oxygen or oxygen-bearing gases;
c. preheating an oxygen-free gas to at least 1500°C; and d. introducing the preheated gas directly into the furnace con-taining the shaped green body, causing direct heat transfer from the preheated gas to the shaped green body, for a minimum time necessary to complete a bonding reaction.
2. A process in accordance with claim 1 wherein the gas in step d is nitrogen.
3. A process for making a silicon nitride bonded shaped refractory material, comprising the steps of:
a. forming a shaped green body of an admixture of a particulate refractory material and elemental silicon, b. placing the shaped green body in a furnace flushed free of oxygen or oxygen-bearing gases;
c. preheating nitrogen gas to at least 1500°C; and d. introducing the preheated nitrogen gas into the furnace whereby the preheated nitrogen gas causes direct heat transfer and, additionally, a reaction with the silicon in the shaped green body to form silicon nitride.
a. forming a shaped green body of an admixture of a particulate refractory material and elemental silicon, b. placing the shaped green body in a furnace flushed free of oxygen or oxygen-bearing gases;
c. preheating nitrogen gas to at least 1500°C; and d. introducing the preheated nitrogen gas into the furnace whereby the preheated nitrogen gas causes direct heat transfer and, additionally, a reaction with the silicon in the shaped green body to form silicon nitride.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US29151381A | 1981-08-10 | 1981-08-10 | |
| US291,513 | 1981-08-10 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1195096A true CA1195096A (en) | 1985-10-15 |
Family
ID=23120601
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000409008A Expired CA1195096A (en) | 1981-08-10 | 1982-08-09 | Sintering process for refractory articles using direct-heated gases |
Country Status (7)
| Country | Link |
|---|---|
| JP (1) | JPS5874578A (en) |
| BR (1) | BR8204669A (en) |
| CA (1) | CA1195096A (en) |
| DE (1) | DE3229701A1 (en) |
| FR (1) | FR2510986B1 (en) |
| GB (1) | GB2106142B (en) |
| MX (1) | MX158079A (en) |
Families Citing this family (12)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US4559312A (en) * | 1983-09-19 | 1985-12-17 | Kennecott Corporation | Sintering or reaction sintering process for ceramic or refractory materials using plasma arc gases |
| US4707583A (en) * | 1983-09-19 | 1987-11-17 | Kennecott Corporation | Plasma heated sintering furnace |
| DE3337025A1 (en) * | 1983-10-12 | 1985-05-02 | Feldmühle AG, 4000 Düsseldorf | METHOD FOR PRODUCING A SILICON NITRIDE COMPONENT |
| JPS60166264A (en) * | 1984-02-10 | 1985-08-29 | 科学技術庁無機材質研究所長 | Method of sintering silicon carbide |
| US4649002A (en) * | 1985-04-01 | 1987-03-10 | Kennecott Corporation | System for preventing decomposition of silicon carbide articles during sintering |
| US4666775A (en) * | 1985-04-01 | 1987-05-19 | Kennecott Corporation | Process for sintering extruded powder shapes |
| US4698481A (en) * | 1985-04-01 | 1987-10-06 | Kennecott Corporation | Method for preventing decomposition of silicon carbide articles during high temperature plasma furnace sintering |
| US4676940A (en) * | 1985-04-01 | 1987-06-30 | Kennecott Corporation | Plasma arc sintering of silicon carbide |
| DE3519612A1 (en) * | 1985-05-31 | 1986-12-04 | Hutschenreuther Ag, 8672 Selb | DEVICE FOR BURNING CERAMIC MOLDED PARTS, IN PARTICULAR PORCELAIN PLATEWARE |
| JPS62260773A (en) * | 1986-05-06 | 1987-11-13 | 科学技術庁無機材質研究所長 | High density silicon carbide sintered body and manufacture |
| DE3617428A1 (en) * | 1986-05-23 | 1987-11-26 | Krupp Gmbh | Process and apparatus for preparing electrically conductive refractory building materials and use of these building materials |
| BR9901512A (en) * | 1999-05-27 | 2001-01-09 | Lupatech S A | Binding plasma extraction process |
Family Cites Families (5)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US2618565A (en) * | 1948-02-26 | 1952-11-18 | Carborundum Co | Manufacture of silicon nitride-bonded articles |
| US3291623A (en) * | 1965-04-29 | 1966-12-13 | Electro Refractories & Abrasiv | Refractory body and method of making same |
| US3926857A (en) * | 1971-11-08 | 1975-12-16 | Atomic Energy Authority Uk | Electrically conducting material containing silicon carbide in a matrix of silicon nitride |
| US3980467A (en) * | 1973-02-16 | 1976-09-14 | Camacho Salvador L | Method of operating a batch type annealing furnace using a plasma heat source |
| DE2548983C2 (en) * | 1975-11-03 | 1977-12-01 | Schneider Gmbh & Co, 5020 Frechen | Method and device for the production of a ceramic-bonded building material |
-
1982
- 1982-08-06 GB GB08222721A patent/GB2106142B/en not_active Expired
- 1982-08-09 FR FR8213874A patent/FR2510986B1/en not_active Expired
- 1982-08-09 MX MX19394282A patent/MX158079A/en unknown
- 1982-08-09 BR BR8204669A patent/BR8204669A/en not_active IP Right Cessation
- 1982-08-09 CA CA000409008A patent/CA1195096A/en not_active Expired
- 1982-08-10 JP JP57138062A patent/JPS5874578A/en active Pending
- 1982-08-10 DE DE19823229701 patent/DE3229701A1/en not_active Ceased
Also Published As
| Publication number | Publication date |
|---|---|
| BR8204669A (en) | 1983-08-02 |
| JPS5874578A (en) | 1983-05-06 |
| GB2106142A (en) | 1983-04-07 |
| MX158079A (en) | 1989-01-05 |
| DE3229701A1 (en) | 1983-03-03 |
| FR2510986B1 (en) | 1986-06-27 |
| FR2510986A1 (en) | 1983-02-11 |
| GB2106142B (en) | 1985-05-22 |
Similar Documents
| Publication | Publication Date | Title |
|---|---|---|
| CA1195096A (en) | Sintering process for refractory articles using direct-heated gases | |
| US4147911A (en) | Method for sintering refractories and an apparatus therefor | |
| Janney et al. | Microwave processing of ceramics: Guidelines used at the Oak Ridge National Laboratory | |
| US4676940A (en) | Plasma arc sintering of silicon carbide | |
| US4389355A (en) | Sintering UO2 and oxidation of UO2 with microwave radiation | |
| NO154728B (en) | METHOD AND DEVICE FOR THERMAL PRODUCTION OF ALUMINUM. | |
| US4649002A (en) | System for preventing decomposition of silicon carbide articles during sintering | |
| CA1266378A (en) | Plasma heated sintering furnace | |
| Aguilar et al. | Microwaves as an energy source for producing magnesia-alumina spinel | |
| CN115231580B (en) | Method for preparing forsterite and magnesia by sintering fine-particle magnesite flotation tailings | |
| PĂUNESCU et al. | Comparative analysis of the own experimental techniques of producing the foamed glass-ceramic | |
| US4698481A (en) | Method for preventing decomposition of silicon carbide articles during high temperature plasma furnace sintering | |
| JPH0232233B2 (en) | ||
| EP0275614A1 (en) | System for preventing decomposition of silicon carbide articles during sintering | |
| Nagata et al. | Production of silicon by microwave heating | |
| Ault | Raw materials for refractories: SiC and Si3N4 | |
| Sakashita et al. | Development and application of ultra-high-temperature HIP by means of an optical temperature measurement system | |
| Anthony et al. | The adaptation to space applications of a 2000° C furnace in an oxidizing atmosphere | |
| JPS63195168A (en) | Method of preventing decomposition of silicon carbide products during sintering | |
| JPH0345110Y2 (en) | ||
| RU2065425C1 (en) | Method for roasting ceramic articles | |
| Thomas et al. | Microwave Nitrudation of Silicon Compacts Utilizing a Temperature Gradient | |
| Criss | Fused silica refractories for industrial applications | |
| JPH10122512A (en) | Ceramic diffuser cone | |
| Thornton et al. | Sintering UO 2 and oxidation of UO 2 with microwave radiation |
Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| MKEX | Expiry |