United States Patent [191
`Roy et al.
`
`[54] METHOD FOR MANUFACTURING A
`TRANSPARENT CERAMIC BODY
`[75] Inventors: Donald W. Roy; Franklin J.
`Stermole, both of Golden, Colo.
`[73] Assignee: Coors Porcelain Company, Golden,
`Colo.
`Oct. 10, 1972
`[22] Filed:
`[21] Appl. No.: 296,420
`Related U.S. Application Data
`[63] Continuation~inapart of Ser. No. 72,846, Sept. I6,
`1970, abandoned.
`
`[52] U.S. Cl .................................... .. 264/65; 264/66
`[51] Int. Cl.2 ....................... .. F27B 9/04; F27B 9/10
`[58] Field of Search ...................... .. 264/66. 65, 332;
`106/62, 73.4, 65; 65/18
`
`[56]
`
`References Cited
`UNITED STATES PATENTS
`
`3,026,2l0
`3.2l2.40l
`3,530,209
`3,531,308
`3,768,990
`3,862,846
`
`3/]962 Coble .................................. .. 106/65
`l0/l965
`9/l970
`9/1970
`Sellers et al. .......... ..
`lO/l973
`l/l975 Smoak et al. ....................... .. 106/62
`OTHER PUBLICATIONS
`W. T. Bakker et al., “Reactor Magnesia Spine], Prepa
`ration and Properties,” Ceramic Bulletin, Nov. 1967,
`pp. 1094-1097.
`
`3,974,249
`[111
`[451 Aug. 10, 1976
`
`P. E. Hart et al., “Densi?cation Mechanisms in Hot
`Pressing of Magnesia with a Fugitive Liquid," Jour.
`Amer. Cer. Soc., Feb. ‘70 pp. 83-86.
`‘
`‘
`
`Primary E.\'aminer—Donald J. Arnold
`Attorney, Agent, or Firm-Reising, Ethington.
`Barnard, Perry & Brooks
`
`ABSTRACT
`[57 1
`The present invention provides a high heat resistant,
`light transmitting, thermal shock resistant and high
`strength sintered ceramic body containing at least
`about 98% by weight magnesia-alumina spinel and a
`method of manufacturing the same which comprises
`forming a uniform mixture containing approximately
`equal molar amounts of magnesia and alumina plus
`lithium fluoride in an amount of about 2% by weight
`of the mixture, the alumina in the mixture having an
`average particle size of less than 0.5 microns; calcin
`ing the mixture at such a temperature and under such
`conditions as will result in substantially less than total
`reaction to form spinel during calcining, i.e. less than
`80% and preferably less than 50%; pressing the mix
`ture into a self-sustaining compact having a density of
`at least about 2 grams per cubic centimeter; and then
`?ring the compact, preferably in a wet hydrogen at
`mosphere, to a temperature of from about 1600° to
`1900°C.
`
`11 Claims, No Drawings
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`1
`
`METHOD FOR MANUFACTURING A
`TRANSPARENT CERAMIC BODY
`
`ation.
`
`’
`
`.
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`5
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`3,974,249
`tages'of the invention will appear more clearly from the
`following more detailed description thereof.
`PREPARATION OF THE MIXTURE
`First, a mixture is made of approximately equal molar
`amounts of magnesia and alumina together wth lithium
`?uoride in an amount of about 2% by weight of the
`mixture. The amount of lithium ?uoride included can
`vary from the amounts speci?ed above, though it
`should not generally be less than about 0.2% or more
`than about 5% by weight of the mixture. The alumina
`should have an‘ average particle size of less than 0.5
`microns and preferably less than'0.3 microns. The in
`gredients used to form the mixture should preferably be
`free of water and, to this end, it is desirable to precal
`cine the magnesium oxide at about 500°C for from 1 to
`4 hours to drive off surface water. To assure uniformity
`of the mixture, the ingredients are dry milled together,
`as in a conventional ball mill, for about 3 hours and
`then screened.
`'
`-
`It will be appreciated by those.v skilled in this art that
`various grades of alumina and magnesia may be em
`ployed. Also, various salts of magnesia and alumina,
`which salts are converted to the‘ oxide upon heating
`with the anion being driven off thus leaving no residue,
`may be substituted for all or part of the magnesia or
`alumina. Although a number of aluminas are suitable,
`we have found that those derived from ammonium
`alum such as Linde A or Ugine give best results for the
`purposes of‘this invention. We have also employed
`various grades of magnesia, including U.S.P. and re
`agent grade, with best results being obtaind when the
`U.S.P. ‘magnesia is used. Among the salts which may be
`employed in place of magnesia or alumina are alumi
`num sulfate (Al2SO4), magnesium sulfate (MgSO4) and
`magnesium carbonate (MgCOa). However, best results
`are obtained 'when magnesia and alumina are used
`rather than salts thereof.
`‘
`t
`I
`"
`-
`
`This patent application is a continuation-in-part of
`US. patent application Ser. No. 72,846, ?led Sept. 16,
`1970, now abandoned.
`The present invention relates to an improved heat
`resistant transparent ceraminc body and method for
`manufacture thereof. More particularly, the invention
`relates to an improved ceramic body and method of
`manufacture wherein the body is of sintered magnesia
`alumina spinel (MgAl2O4) and has excellent light trans
`mission characteristics in both the visible and infrared
`regions of the electromagnetic.‘spectrum. One of the
`signi?cant advantages ‘of the ceramic bodies of the .
`present invention is their relatively low cost which is
`possible because their manufacture is accomplished
`without the requirement for a hot-pressing operation.
`The ceramic bodies of the present invention ?nd utility
`as high strength, high temperature resistant windows
`for the transmission of visible and infrared light, as
`encapsulations for ionized alkali vapor lamps and other
`applications where there is a requirement for a material ‘
`having transparency in combination with high heat
`resistance, mechanical strength and good resistance to
`thermal shock.
`~
`Accordingly, it is a principle object of the present
`invention to provide a ceramic body which has high
`temperature resistance, high mechanical strength, high
`resistance to thermal shock and good light transpar
`ency in both the visible' and infrared wave length re
`gions of the electromagnetic spectrum. A further and
`attendant object object the invention is the provision of
`a relatively low cost method for manufacturing such
`bodies without the requirement for a hot-pressing oper
`
`Brie?y, the transparent ceramic bodiesof the present
`invention are manufactured by cold pressing and then
`sintering a mixture ‘containing approximately equal
`molar amounts of magnesia and alumina, said mixture
`having been precalcined in the presence of lithium
`?uoride under such conditions as will result in less than
`about 80% spinel formation during the calcining step.
`The magnesia and alumina in the precalcined mixture
`totally react during the sintering operation to form
`spinel whereby the ?nal sintered body contains
`magnesia-alumina spine] in an amount in excessof 98%
`by weight. The mixture which is cold pressed may be
`formed by calcining, at a temperature of from about
`800° to l050°C, preferably from about 900° to about
`950°C, in ambient atmosphere a mixture containing
`approximately equal molar amounts of magnesia and
`alumina together with a small amount, preferably about
`2% by weight of the mixture, of lithium ?uoride. The
`alumina used in the mixture should have a particle size
`of less than about 0.5 microns and ideally not in excess
`of 0.3 microns; The precise role played by the lithium
`?uoride is not presently known with certainty; how
`ever, what has been established is that when’ a mixture
`containing a small amount of lithium ?uoride is ca]
`‘cined as aforesaid'such that the magnesia and alumina
`do not totally react to form spinel, but instead are left
`to‘ undergo substantial spinel-forming reaction during
`the subsequent sintering operation, the resultant ?nal
`sintered bodies demonstrate greatly improved light
`' transparency as well as other excellent physical‘ proper
`ties. Other objects and important featurestand advan
`
`40
`
`45
`
`CALCINING
`‘Next, the mixture is calcined in ordinary ambient’
`atmosphere at a temperature from about 800° to
`1050°C for from V2 to 5 hours, and more preferably at
`a temperature of about 900° to 950°C for about 3
`hours. It is preferred, although notnecessary, to press
`the mixture into a self-sustaining powder compact by a
`cold pressing operation prior' to calcining. A pressure
`of about 15,000 psi is satisfactory for this pressing
`operation. The resulting powder compact is then cal
`cined at the temperature and for the time described.
`We have found that the calcining operation is a criti
`cal stage in the process of ‘forming the improved heat
`resistant transparent ceramic bodies of this’ invention.
`We have found that for the manufacture of transparent
`spinel bodies it is necessary that‘ the mixture be cal
`cined under conditions which will result in less than
`total reaction to spinel during the calcining operation;
`Ideally we would prefer to calcine under conditions
`which result in substantially no spinel formation. Ex
`60
`perimental data have shown that if the spinel reaction
`proceeds to greater than about 80% spinel formation
`during calcining, the transparency of the ?nal product
`is reduced.
`-
`It'will be appreciated by those skilled in this art that
`there are various parameters involved in determining
`optimal calcining conditions. Thus, the calcining tem
`perature necessary is dependentupon the time of’cal
`cining, the types and grades of reactants, and the atmo- ‘
`
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`3,974,249
`4
`ever, vegetable oils or animal fats are preferred. Of the
`animal fats about 3% by weight of neofat, which is
`chicken fat, is particularly preferred; and cotton seed
`oil is an example of a suitable vegetable oil. It will be
`appreciated by those skilled in the art that the lubricant
`should be such as will not react with the reaction com
`ponents or otherwise adversely affect the same.
`The particulate mixture is dry milled, with or without
`an organic lubricant, until the average particle size of
`the mixture is less than about 0.5 microns. Generally
`about 10 hours of dry milling in a conventional ball mill
`is sufficient.
`The organic lubricant, although primarily useful dur~
`ing the grinding operation, also serves as a lubricant
`during the subsequent cold pressing operation, allow
`ing the particles to slide one with respect to the other
`and thereby contribute to the attainment of high den
`sity in the green compact form. Thus, in those situa
`tions wherein a lubricating material is not employed, it
`is necessary to increase the pressure employed in the
`cold pressing operation to compensate for the absence
`of the lubricant and attain a sufficiently high green
`density.
`Thus, the calcined particulate material, whether with
`or without an organic lubricant, is dry pressed to a
`self-sustaining green compact, of the shape desired, the
`pressure used being suf?cient that the resulting com
`pacts have a density of at least about 2 grams per cubic
`centimeter and preferably in excess of about 2.3 grams
`per cubic centimeter. Generally, the shape desired will
`be that of a ?at plate and the pressing can be performed
`in conventional matched metal dies. A pressure of
`about 10,000 psi is generally satisfactory where the
`compact being pressed is a relatively thin plate as will
`usually be the case where the ?nal bodies being manu
`factured are to be used as light transmitting windows.
`Since there is shrinkage, generally about 20%, during
`the sintering operation, the compact should be appro
`priately larger than the ?nal sintered bodies desired.
`
`20
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`25
`
`sphere in which calcining is carried out. Although the
`various parameters can be varied during the calcining
`operation, we have found that in the production of
`transparent spinel bodies, the best results are obtained,
`if the reactants are calcined at 800° to lO50°C, prefer
`ably 900° to 950°C as noted above, in normal ambient
`atmosphere for such a time as will result in substantially
`less than total reaction to form spinel. We have found
`by X-ray defraction studies that when calcining takes
`place in normal ambient atmosphere within the pre
`ferred temperature range of 900°~950°C for l to 3
`hours the reaction to form spinel proceeds to less than
`50%. The spinel body ultimately formed from these
`calcined reactants, it has been found, demonstrates
`superior transparency. However, products having good
`transparency have been produced also by using starting
`materials, i.e. alumina, magnesia, lithium ?uoride,
`which have been calcined at l025°C in normal ambient
`atmosphere. On the other hand, calcining at l075°C
`results in excess spinel formation during calcining and,
`therefore, in a ?nal product having poor transparency.
`We have not dtermined with certainty what occurs
`during this critical calcining stage which ultimately aids
`in poducing a superior ?nal product. However, we do
`believe that an advantage of the particular calcining
`procedure employed herein is that it provides a much
`more reactive system than would be obtained by con‘
`ventional calcining techniques wherein substantially
`total spinel formation occurs. Although, the precise
`role played by lithium ?uoride during calcining is not
`known, we believe that its use does have a signi?cant
`effect on the reactivity of the calcined mixture, thereby
`making possible a faster reaction rate and, at least
`theoretically, more complete spinel formation during
`sintering. In any event, regardless of the reason there
`for, wehave found that by calcining in the presence of
`lithium ?uoride under the conditions mentioned above
`such as will produce substantially less than total spinel
`formation, i.e. less than about 80% and preferably less
`than about 50%, we are able ‘to produce a superior
`transparent spinel product.
`MILLING AND DRY-PRESSING OPERATIONS
`If, as is preferred, the initial mixture is pressed into a
`powder compact prior to the calcining operation, the
`mixture resulting from the calcining operation will be
`in the form of a calcined compact or billet. At the
`conclusion of the calcining operation the compact or
`billet is crushed to provide a mixture of relatively small
`particles. Of course, this crushing step is not necessary
`where the reactants have been calcined in powder form
`rather than in compact or billet form.
`Particularly in the case wherein the reactants have
`been calcined in billet or compact form and subse
`quently crushed, it will be necessary to subsequently
`dry mill the crushed particles in order to break down
`agglomerates of the body components (alumina, mag
`nesia, lithium ?uoride, if any) and to thoroughly mix
`the same._ In situations wherein the reactants have been
`calcined in powder form it is also preferable to dry mill
`to break down any agglomerates which may have
`formed during calcining.
`Preparatory to dry milling it is preferable to add a
`small amount of organic lubricating material which acts
`as a grinding aid during the milling operation. Any of
`the various well‘known organic lubricants useful in
`milling operations may be employed in amounts rang
`ing from 2-5% by weight of the entire mixture; how
`
`SINTERING OPERATION
`The compact is ?red in a reducing atmosphere to
`cause substantially complete reaction of all the magne
`sia and alumina present to form spinel, and to sinter the
`spinel to a transparent dense body. The sintering and
`the spinel formation are all part and parcel of the total
`reaction and probably occur largely simultaneously
`rather than in complete step-wise fashion. The ?nal
`?ring should preferably be in a wet reducing atmo
`sphere, a wet hydrogen atmosphere being ideal. The
`?ring temperature should be at least 1600°C, and up to
`about 1900°C, with a substantial soak period or pe—
`riods, at a temperature within this range. Though it will
`be understood that there can be variations in the tem
`perature-time schedule, the following schedule in a wet
`hydrogen atmosphere is particularly excellent. Heat to
`1800°C with a 100°C per hour temperature rise and
`soak 24 hours at that temperature. The moisture con
`tent in the wet hydrogen atmosphere may be varied
`over a wide range but room temperature dew points in
`the 30° to 60°F give fired parts with best transparency.
`Re?ring parts under the same ?ring conditions or dou
`bling the soak time signi?cantly improves transmission.
`In the case wherein a lubricant was present during
`cold pressing, the pressed powder compact, prior to the -
`?nal ?ring or sintering operation, is first air fired at a
`temperature in the l000° to 1300°C range for l to 5
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`hours to burn out the organic lubricant and start the
`initial sintering phase.
`'
`>
`
`3,974,249
`,
`.6
`The embodiments of the invention in which an exclu
`sive property or privilege is claimed are de?ned as
`
`10
`
`COMPOSITION AND PROPERTIES OF THE
`SINTERED BODIES
`The sintered bodies resulting from the ?ring opera
`tion contain at least about 98% by weight magnesia
`alumina spinel (MgAl2O4) and have a density of at least
`about 3.57 grams per cubic centimeter which is in
`excess of 99.5% of theoretical for the spinel. Light
`transmission measurements taken on the bodies, in the
`form of disks of 1.25 millimeter thickness, and after
`optical polishing thereof, show the transmission of visi
`ble light having a wavelength of 0.4 microns to be 68%
`and the transmission of visible light-having a wave
`length of 0.7 microns to be 72%. Applying Beer’s Law
`and Correcting to a one millimeter thickness, the trans
`mission is 72% at 0.4 microns and 75% at 0.7 microns.
`Thus, transmission is in excess of 70 in the visible light
`region of the electromagnetic spectrum. The polished
`1.25 millimeter thick disk also shows a transmission in
`excess of 80% in the infrared region (3 microns to 5
`microns wavelength) of the electromagnetic spectrum.
`The transparent, sintered ceramic bodies’ produced
`by the preferred method of this invention also exhibit
`the following properties:
`
`20
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`
`follows: >
`
`'
`
`‘
`
`l. A method for manufacturing a ceramic body com
`prising the steps of forming a uniform mixture contain
`ing approximately equal molar amounts of magnesia
`and alumina and from about 0.2% to 5% by weight
`lithium fluoride, the alumina in said mixture having a
`particle size of less than about 0.5 microns, calcining
`said mixture at a temperature of at least about 800°C
`for a time that there is less than about 80% reaction of
`the alumina and magnesiav to form spineLforming the
`calcined mixture into a self-sustaining compact and
`?ring the compact in a reducing atmosphere to sinter
`ingtemperature to cause the reaction of the magnesia
`and alumina in the calcined mixture to form spinel and
`thereby form a sintered body consisting'predominately
`of magnesia-alumina spinel. >
`2. a method as set forth in claim 1 wherin the mixture I
`is calcined at a temperature of at least about 800°C for
`a time that there is less than about 50% reaction of the
`alumina and magnesia to form spinel.
`3. A method as set‘forth in claim 1, wherein mixture,
`after said calcining thereof, is milled in admixture with
`from about 2% to about 5% by weight‘ of the mixture of
`an organic lubricant and wherein the mixture is formed
`into the self-sustaining compact by a pressing operation
`during which the organic lubricant remains in the mix
`
`Melting Point
`Knoop Hardness
`Surface Finish, Polished
`4-P0int Bending Strength
`Tensile Strength
`Compression Strength
`Modulus of Elasticity
`Shear Modulus
`Bulk Modulus
`Poisson's Ratio
`Coef?cient of Linear Expansion
`25-200°C
`25—500°C
`25-1000°C
`Thermal Conductivity, g-cal
`100°C
`I200°C
`Speci?c Heat, g-cal
`20°C
`l040°C
`Transmittance,
`.5-5 Microns
`(0.040" — thick test specimen)
`Index of Refraction
`Dielectric Constant
`l0“I-lz
`
`Dissipation Factor
`
`Loss Index
`
`2135°C
`I300°C
`l microinch AA
`33,400 psi — 40,000 psi
`23,000 psi
`390,000 psi
`35 X 10“ psi static
`15.89 X 10“ psi dynamic
`27.93 X 10“ psi dynamic
`0.2608
`
`0.0357
`0.0130
`
`0.200
`0.2 l4
`
`70 - 80%
`
`0.00003
`0.00002
`0.000013
`
`0.00025
`0.0002
`0.0001
`
`It will be understood thatwhile the invention has
`been described speci?cally with reference to a pre
`ferred embodiment thereof, various changes and modi-,
`?cations may be made all within the full and intended
`scope of the claims which follow.
`
`30
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`ture.
`
`-
`
`4. A method as set forth in claim 1, wherein said
`compact is ?red in a wetreducing atmosphere to a
`temerature of from about l600° to l900°C.
`5. A method as set forth in claim 1, wherein the
`compact is ?red by ?rst heating it to a temperature of
`from about 1000°C to about 1300°C in an oxidizing
`atmosphere and is then heated in a wet reducing atmo
`sphere to a temperature of about 1800°C with a soak at
`that temperature of about 24 hours.
`‘
`6. A method as set forth ‘in claim 1, wherein the
`amount of lithium ?uoride included in said mixture is
`approximately 2% by weight of the mixture.
`‘
`7. A method as set forth in claim 1, wherein the
`aluminum oxide in the mixture wich is calcined has an
`average particle size of less than about 0.3 microns.
`8. A method as set forth in claim 1, wherein said
`compact is formed so as to have a density greater than
`2 grams per cubic centimeter.
`9. A method as set forth in’claim l, whereinsaid
`mixture, prior to the calcining step, is formed into a
`self-sustaining compact and wherein said compact,
`after said calcining step is crushed and thereafter
`milled to a reduced particle size after from about 2% to
`about 5% by weight of the mixture of an organic lubri
`cant has been added thereto.
`‘
`10. A method as set forth in claim 1 wherein said
`mixture is calcined at a temperature of from about 800°
`to 1050°C.
`,
`’
`11. A method as set forth in claim 1 wherein said
`mixture is calcined at a temperature of from about
`900°C to 950°C for approximately 3 hours.
`
`* * *
`
`* *
`
`‘
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`>
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