CA1185081A - Process for freezing an inorganic particulate slurry or suspension - Google Patents
Process for freezing an inorganic particulate slurry or suspensionInfo
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- CA1185081A CA1185081A CA000446158A CA446158A CA1185081A CA 1185081 A CA1185081 A CA 1185081A CA 000446158 A CA000446158 A CA 000446158A CA 446158 A CA446158 A CA 446158A CA 1185081 A CA1185081 A CA 1185081A
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Abstract
ABSTRACT
This invention relates to a process of freezing an inorganic particulate or a ceramic slurry containing a freeze sensitive ceramic colloidal sol in the presence of lithium ions contained in the slurry or suspension in an amount sufficient to inhibit ice crystal growth, with or without supercooling. The invention further includes composite inorganic structures which are in the nature of laminates where at least one inorganic structure, such as a zirconia plate, is adhered to a different inorganic structure, such as an alumina plate.
This invention relates to a process of freezing an inorganic particulate or a ceramic slurry containing a freeze sensitive ceramic colloidal sol in the presence of lithium ions contained in the slurry or suspension in an amount sufficient to inhibit ice crystal growth, with or without supercooling. The invention further includes composite inorganic structures which are in the nature of laminates where at least one inorganic structure, such as a zirconia plate, is adhered to a different inorganic structure, such as an alumina plate.
Description
5~
~ 1 --INORGANIC COMPOSITE STRVCTVRES
-TECHNICAL FIELD
Inorganic products, such as refractory bodies, and method of producing such products by freezing aqueous slurries of the inorganic particles of which the inorganic products are composed.
BACKGROUND ART
Freezing a slurry of particulate inorganic materials to form refractory products has been disclosed in U.S.
Patents Nos. 3,177,161; 3,5i2,571; 3,816,~72 and 3,885,005.
The production of refractory structures in accordance with the prior art resulted in products which often contain large voids which adversely affect the strength of the ceramic product, which in turn adversely affects their properties such as thermal conductivity and thermal shock resistance.
These deficiencies in the resulting ceramic products are believed to be caused by nucleation at one of the cooling surfaces due to contact with an ice crystal or with the mold surface that has a nucleation temperature higher than that of the slurry itself as the dispersion is cooled to its freezing point. This nucleation is believed to cause large ice crystals to grow out from these points and eventually entrap the last remaining liquid, resulting n rupture, cracking~ weak, and non-uniform ceramic structures.
~iany attempts have been made to overcome this problem, such as thorough waxing of the molds, the use of different mold materials, thorough cleaning of mold surfaces, even 50~3~1L
with acids, but -the problem still persis-ted.
DISCLOSURE OF T~E INVENTION
This invention rela-tes to a process which substantially eiiminates the problems discussed above by using an inorganic particulate slurry or suspension containing a freeze sensitive ceramic colloidal sol and freezing the slurry or suspension in the presence of lithium ions contained in the slurry or suspension in an amount sufficient to inhibit iee crystal growth.
The invention may inelude the addition of lithium ions to the freezing media with or without supercooling. The invention may be used to make composite inorganic strue-tures which are in the nature of laminates where at least one inorganic structure, sueh as a zirconia plate, is adhered to a different inorganic structure, such as an alumina plate.
The various inorganic particles that can be used according to this invention include, without limitation, aluminas such as mullite and tabular alumina, silicas sueh as fused silica, magnesia, chromite, spinels such as chromite spinel, kyanite, carbomul, zirconia, mica, carbon, graphite, molydisulfide, uranium oxide, thoria, titania, clays, etc.
The invention, however J iS broadly applicable to suspen-sions of inorganie partieles in general ineluding other metal compounds. Mixtures may be used if desired.
The inorganic particle size is not critical. Best results seem to have been obtained to date when the majority of the particles, or 40-50~ of them, are below about 200 mesh. Small particle or grain size seems to be particularly advantageous when using zirconia. Much 3 :~L
smaller particle sizes can also be used even in a colloidal size. The ultimate particle size and size distribution will depend to some extent on the end use of the structures and the properties desired therein.
The inorganic products produced according to this inven-tion are porous and the size of the grains or particles employed in the slurries will to a large extent determine the degree of porosity. The products of the invention have a wide variety of uses depending to some extent on the type of particle being employed in the process. For example, if ceramic particles are employed, the products can be used in the same manner as ceramic and refractories are used such as fire brick, linings for furnaces in the steel, glass and other industries. They can also be used as filters, carriers for catalysts, thermal shock resis-tant dinnerware, grinding wheels, etc. When particles such as graphite and molydisulfide are employed their uses can include many of the above but adding thereto the lubrica-tion properties of these mater:ials. Generally the prod-ucts are useful in any area where porosity is desired or in areas where porosity is not desired but is not detri-mental.
.
The freeze-sensitive colloidal ceramic sols useful accord-ing to this invention are well known and include colloidal ceramic sols, such as disclosed in the Smith-Johannsen U.S. Patent 3,177,161 and U.S. Patents 3,512,571 to Phelps, 3,816,572 to Roelofs and 3,885,005 to Downing et al. A freeze-sensitive sol is one which, when frozen, will break down and no longer exist as a sol or colloidal suspension when thawed. Both cationic and anionic silica sols can be used with the anionic preferred at least with alumina and zirconia refractories. Ammonia stabilized silica sols, such as Dupont's AM LUDOX*, may be advantageous *Trade Mark ~ 3~
where elimination of sodium is desired. Other ~reeze-sensitive colloidal ceramic sols, such as zirconia and magnesia sols, can also be used. Silica sols have been used because they are readily available on the market.
Although not necessarily preferable due to insufficient expeximental data to date, most present experiments mainly utilize a freeze-sensitive sodium stabilized colloidal silica sol having about 30% colloidal silica supplied by Nalco Chemical Company.
The total amount of sol stabilizer such as sodium, ammonium and/or lithium should be sufficient to stabilize the sol but not be so high as to render the sol non-freeze sen--sitive or to lower the strength of the fired silica or other ceramic contained in the sol when fused or fired or to lower the strength of the fused or fired product to an unacceptable level. This can readily be determined by routine experimentation by one skilled in the art. For example a mole ratio of silica to lithia of about 85 in a lithium stabilized silica sol works quite well but when the ratio is lowered to about 48 the sol appears to lose some freeze sensitivity resulting in weaker bonds. The optimum amounts have not as yet been determined.
Generally, the sodium stabilized silica sol are quite adequate to practice the invention disclosed herein. With some inorganic particles, namely zirconia and magnesia, some adjustments can be made to improve the results.
These adjustments are desirable to improve pot li~e and to preserve the distribution of particle sizes during the filling operation so that an optimum degree o~ uniform packing can be obtained.
When zirconia, for example is mixed with a negatively charged sodium stabilized silica sol (DuPont Ludox HS-40*) it is well *Trade Mark 3~
wetted and the particles quickly segregate. It is be-lieved that this segregation occurs because the sol particles and the zirconia particles are so charged to prevent or minimize particle association. The zirconia particles generally have a charge of about -20 m.v. (zeta poten~ial) in deionized water while the above Ludox partlcles are even more highly charged. To overcome particle segregation problem, the zeta potential of the particles comprising the various mixes can be altered.
One way of altering the zeta potential is to reduce the pH
of ~he above Ludox from about 10 to about 8 by adding dilute HCl which lowers the zeta potential of the silica sol particles and rendering them less stable. Under these conditions, the silica particles begin to precipitate onto the ceramic tzirconia) particles creating a degree of association between all of the particles of the mix and segregation of coarse and fine ceramic particles such as zirconia is greatly inhibited. The acid can also be added to the inorganic particles or to a mixture of the sol and particles. The amount of acid is that which is sufficient to prevent settling or segragation of the particles. In practice, it is advantageous to add the acid directly to the sol. With zirconia, the amount of acid found practi-cal to accomplish the result is about 0.6 percent by ~5 weight (based on the total weight of zirconia) of a 37 percent HCl solution. If such a problem is encountered with other inorganic particles the zeta potential can be measured and appropriate adjustment with acid ox alkali to alter the ze~ potential can be made in such a manner as to insure particle association.
Zirconia, and especially magnesia, also appear -to react with the sodium stabilizer (and also ammonia somewhat) to cause limited pot life. In fact, the reaction with ~ 3~3 magnesia is so rapid -that mixing itself becomes difficult.
The use of a li-thium stabilized silica sol was found to eliminate this reac-tion -to the extent tha-t magnesia dispension could be readily mixed and cast without concern of short pot life. The use of lithium s-tabilized sols also overcomes the particle segregatlon problem referred to above wi-th respect to zirconia. Thus it may not be necessary to adjust the zeta potentials of the particles if a sol having the requisite zeta potential ean initially be used.
When the lithium stabilized silica sol was used with magnesia and zirconia another new and very signifiean-t property was observed. These sols inhibited ice crystal growth even in the absenees of supereooling. In faet, when nueleation was delibera~ely initiated, in the ease of magnesia dispersion containing lithium ions, from the surface with an iee erystal, no maero or large erystal growth was detectable for more than two millimeters from the initiation site. Thus the use of lithium stabilizea ceramie sols not only solved the pot life problems and partiele segregation problems of zireonia and magnesia but has been found extremely advantageous for produeing small uniform iee erystals durina the freezing step with regard to all inorganie dispersions. The use of a lithium stabilized eeramie sol in eombination with supereooling has been found most advantageous.
When using lithium in the inorganie particle slurries in accordance with the invention sol, it is of eourse most praetieable to employ a lithium stabilized eeramie sol available on the market. A silica sol having a silica to lithia ratio of 85 worked quite well, however this sol, DuPont's Lithium Polysilieate 85*, is not being marketed today. One lithium stabilized siliea sol whieh is avail-*Trade Mark able today contains a silica to lithia ratio of about ~8 (DuPont's Lithium Polysilicate 48*). This amoun-t of lithia however minimizes the freeze sensitivity of the sol and when used alone produced fired products having weaker bonds. This commercial lithium stabilized sol can be used however by using it in admixture with a sodium or prefer-ably an ammonia stabilized sol. A 50-50 mixture has worked well but the optimum has not as yet been determined.
I-t is the presence of the lithium ion which produces the surprising ice crystal growth inhibition rather than the absence of sodium or ammonia. Thus lithium ions can be added to the slurries by the àddition of ionizable lithium compounds such as lithium chloride, lithium hydroxide, lithium sulfate, lithium succinate and so forth. It is preferred to add th~ lithium ions to the ceramic sol. The amount of lithium ions added to an inorganic particle slurry should be sufficient to inhibit ice crystal growth to the desired degree but insufficient to adversely affect the freeze sensitivity of the sol. This can be determined by routine experimentation with respect to any particular system being frozen. Only a very small amount of lithium ion is necessary to inhibit ice crystal growth. Higher amounts may however be required to increase the pot life when magnesia is used as can be observed in Examples below.
To accomplish the supercooling and substantial instan-taneous freezing, it is not a simple matter of inserting a mold filled with the slurries into a cold freezing media even at -40 or -60F. This invention includes a process of insuring supercooling of the ceramic slurries by treating the mold with a hydrophobic liquid such as xylene, mineral spirits, or perchloroethylene to cover at least the entire working surface of the mold, and inserting the slurries or suspensions into the mold while *Trade ~ark ~L i it is still wet. This can be accomplished by simply dipping the mold in the hydrophobic liquid. The mold can then be closed and the slurry frozen. It is also advantageous to cover the aqueous slurry or suspension in the mold with a thin layer of the hydrophobic liquid. The mold itself is preferably of light weight and of low mass relative to the freezing media and the ceramic or partic-ulate slurry being frozen. The mold and freezing media should also ha~e a high thermal conductivity. Although the freezing temperature can be varied, it should be sufficiently low to insure supercooling and a rapid freeze. Temperature of about ~5 to -50F can be used.
When lithium i9 used temperatures of about -10F can advantageously be used.
When producing large articles, supercooling may only occur to a certain depth from the mold surface toward the center of the slurry because all of the heat within the center cannot be removed before freezing of a portion of the slurry closer to the mold. When large articles are to be made it is thus advantageous to cool the entire slurry to near the freezing point before inserting it into the freezing media to insure complete supercooling. The presence of lithium ions in large slurries is also advan~
- tageous.
The supercooling can also be carried out without the use of mold such as by extruding cylinders, sheets or films of the aqueous slurry or suspension on a belt treated with the hydrophobic liquid and then into the hydrophobic freezing media. The transport through the freezing media can be between a pair of belts and the process can be continuous. The terms "mold" as used herein is intended to include any structure for supporting and/or encompassing the slurries or suspensions.
Supercooling of the slurry to a temperature where it spontaneously nucleates results in a structure that is uniform throughout. At the time of nucleation not all the water freezes because the heat of fusion raises the 5 temperature back to the freezing point. However, as cooling proceeds further, ice crystal growth is completed from all of these nucleation sites at substantially the same time. The structure that develops is therefore much more uniform and fine grained regardless of the thickness of the structure to be produced or frozen.
Random tests made on some of the freezing steps set forth herein indicate that the temperature of supercooling is about 4 degrees below the freezing temperatures of the aqueous slurry.
Various freezing media can be used to freeze the slurry structures such as those described in the above-mentioned patents. A hydrophobic freezing media such as Freon* or perchloroethylene is advantageously used to prevent penetration of the freezing media into the aqueous slur-ries to prevent the growth of large or variable sizedice crystals, and to insure supercooling.
The various inorganic materials or ceramics useful according to this invention have different and known firing or sintering temperatures in conventional refractory pro-cesses. For example, alumina is generally fired at atemperature of 1400C or slightly above, and zirconia at about 1700C, in conventional refractory processes. As a general rule, freeze-cast ceramics are most advantage-ously fired at about 50~ to 65% of their melting temper-ature. Thus, when firing freeze-cast ceramics, alumina is advantageously fired at about 1250C while zirconia is advantageously fired at about 1400C.
Other inorganic structures can be sintered at their known or determined sintering temperatures. The temperature *Trade Mark used and time of heating should be sufficient to bond the particles together into a strong integral structure but insufficient to significantly reduce or adversely aEfect the desired porosity or uniformity of the product.
5 E~amples of such temperatures are given, for example, in U.S. Patent 3,177,161.
The molds or patterns are usually made of lightweight steel or aluminum if more thermal conductivity is desired, The wetting of a mold with a hydrophobic liquid to aid in supercooling can also act as a release agent.
The slurries should be as free from entrapped air as practical. Entrapped air can be avoided to some extent by the manner by which the slurries are first mixed, and any entrapped air can be removed in various known manners, such as using long-periods of holding time, vibration, or vacuum treatment techniques.
After freezing, the frozen slurry structures are removed from the mold, thawed and dried. Although various manners of thawing and drying can be employed, the thawing and drying can be accelerated by the use of heat. The use of a conventional drying o~en has been round satisfactory for this purpose.
After selection of the specific particles to be formed into a structure, they can be mixed in the con~entional manner having due regard to particle size, and the freeze-sensitive colloidal ceramic added to each of the dried ceramic materials selected. The freeze-sensitive col-loidal ceramics are contained in water and the solid colloidal content may range from 15~ to 50% solids. Thus, the addition of the freeze-sensitive colloids -to the dried ceramic material usually automatically adds the neces-sary water for handling. For example`, a mix commonly used in slip cas-ting containing up to about 10% water, having a consistency somewhat like pancake batter can be poured into a mould or injected by simple means. This can readily be accomplished by maintaining the proper consistency of the ceramic slurries either by using high solid content freeze-sensitive colloidal ceramic sol or by removing water prior to freezing. One manner of accomplishing this mixing is to dry mix the ceramic ~rain in a ribbon blender and add the freeze-sensitive ceramic sol toge~her with its liquid component, slowing the ribbon blender and continuing until thorough mixing is obtained. The particulate suspensions or slurries, should have a particle content sufficient to insure particle to particle contact during the freezing step as described in U.S. patent 3,177~161~ If the particles are dispersed too thinly, no structure will be formed when the ice melts. The amount of water is desirably held to a minimum practical mount for eeonomic reasonsO
The amount of the freeze-sensitive eeramic sol ean be as reported in the above~noted U.S. patents. The most suitable percentage appears to be about 15% by weight - of the colloidal ceramic sol (30% solids) based on the weight of the dried inorganic particles.
BEST MODE FOR CARRYING OUT THE INVENTION
. . ~
Example 1 Alumina Mix (19 kilograms) - crucible Tabular Alumina Alcoa T61 - 28 + 48 Mesh 55%
- 100 Mesh 25~
- 325 Mesh 20%
Sol 30% Colloidal Silica (NALCO)*
14.3% by weight pH about 10 *Trade Mark - 12 ~
2irconia Mix (367.5 grams) - plate Monoclinic Zirconia - 100 " 50%
- 325 " 50%
Sol 30% Colloidal Silica (NALCO) 12.5% by weight Modified by 0.6% HCL. to pH 7.5 Example _ ~ircon (380.5 grams) - plate - 80 Mesh 70~
- 325 Mesh 30%
Sol 30~ Silica (REMASIL SP-30), pH about 10 11.7% by weight Example 4 Mullite-Remasil ~60 (1180 grams) - plate - 20 Mesh 20%
- 70 Mesh 20%
- 200 Mesh 40~
325 Mesh 20%
Sol 30~ Silica (REMASIL SP-30), pH about 10 14.6% by weight The above mixes are thoroughIy blended and each mix has the consistency of thick pancak~ batter and can be poured and placed in a mold with the aid of a spatula.
The molds are thoroughly wet with perchloroethylene.
Each mix is placed in the mold while it is still wet with the perchlorethylene to the desired depth, about two inches. The top layers of each mix is then covered with a layer of perchloroethylene freezing liquid, the molds closed and molds inserted entirely in perchloroethylene freezing liquid at a temperature of -48F. The liquid slurries are supercooled and then frozen. The frozen structures are then removed from the mold while frozen, thawed and dried in a radiant heated oven at a temperature of about 120F. No significant large voids were detected and the products aftex freezing were extremely uniform.
After drying ~he structures are each fired in a conven tional kiln at a temperature of 1250C for about 4 hours, after which the moldings were allowed to slowly cool to ambient temperature. The moldings were then subjected to heat and thermal shock by applications of a torch (about 6000F) directly to the moldings at ambient temperature.
The moldings remained substantially unaffected after the application of heat with no visible or physical defects except for surface melting where the torch was applied.
Repeated tests such as those described above gave con-sistent and repeatable results.
Example 5 60 parts by weight of magnesia (50%-14 mesh and 50~-48 mesh) were mixed with 12 parts by weight of a lithium stabilized 30% aqueous silica sol having a silica-lithium ratio of 85, formerly marketed by DuPont under the designa-tion Lithium Polysilicate 85. The mixture was supercooledand frozen in the same manner as set forth in the above examples. No ini~ial reaction was noted and the pot life of the mix was very goodO
Example 6 60 parts by weight of alumina (50%-28 ~ 48 mesh and 20%-325 mesh) were mixed together with 2.4 parts by weight of a lithium stabilized 30% aqueous silica sol having a - 14 ~
silica-lithium ra-tio of 4~, marketed by DuPont under the designation Lithium Polysilicate 48, and 9.6 parts by weight of an ammonia stabilized 30% aqueous silica sol marketed by DuPont under the designation Ludox AS-40 and the mixture supercooled and frozen in the same manner as set forth in the above examples.
Example 7 Example 6 was repeated using 12 parts by weight of the ammonia stabilized sol and 0.025 parts by weight of a saturated solution of lithium hydroxide substituted for the lithium stabilized sol.
The ice formed during the freezing in examples 5, 6 and 7 was much finer than the ice formed in examples 1-4. The unfired products were more uniform and were stronger than those produced in examples 1-4.
The thawed products can also be broken up into particles and then fired for various uses such as fillers for resins or plastics~ The particles have a unique shape which is advantageous for use as fillers. A clay slurry, for example, was frozen, thawed, and driéd according to this invention, broken up or ground into small particles; their shape was particularl~ suitable for filler particles. The jagged nature of these particles makes them particularly suited to mixing with small amounts of powdered thermo-plastic or thermosetting resins to form, under heat and pressure, ceramic-like products such as roofing tiles.
Composite inorganic structures such as ceramics, can be considered as being in the nature of laminates where one inorganic structure, such as a zirconia plate, is adhered -30 to a different structure, such as an alumina plate. Prior attempts to produce such composite ceramics by simul-taneous firing of the ceramics in a single mold or by firing the ceramics separately and cementing them together have not been successful to applicant's knowledge. The interfacial bonds between the different ceramics obtained in these manners have not been adequate to withstand the high temperatures and thermal shock to which they are frequently subjected. This interfacial bond failure is mainly due to the differences in the thermal coefficients of expansion of the clifferent refractories or ceramics.
A further problem involved in producing composite ceramics is the difference in the firing temperatures required of the different ceramic materials.
The composite inorganic articles produced in accordance with this invention, particularly ceramics, may be com-posed of different inorganic materials adhered or lamin-ated together and having an interfacial bond of extra-ordinary strength sufficient to withstand extremely high temperatures of thermal shock, such as experienced in the metals industry. Composite ceramics produced according to this invention have been subjected to temperatures as high as 6000F without any noticeable effect on the inter-facial bonds.
This invention also includes a process ~or producing composite ceramics in a single mold by inserting the different ceramic materials into the mold in layered fashion and in contact with each other, freezing the layered ceramics while in contact with each other, thawing the frozen composite, and then firing the thawed composite.
There are a number of important and surprising results obtained when composite ceramic articles are made ac-cording to this invention. The bond between the ~wo different ceramics is at least as strong as the bond between the particles of the individual ceramics employed.
- 16 - 3~
The composi-te frozen structure can be fired at a single temperature despite the fact the ceramics individually are known to re~uire different firing temperatures.
The differences in the thermal coefficients or the mass thermal coefficients of expansion of the different ceramics does not cause disruption or weakening of the interfacial freeze bonding during the firing operation and the fired bond is not significantly affected during high temperature use or by subjecting the composite ceramic to extreme thermal shock. It is further surprising that experiments to date indicate that a very wide range o~
different ceramics can be employed to make the composite ceramics of this invention having wide differences in their firing temperatures and thermal coefficients of expansion.
The thermal coefficient of expansion refers to the ex-pansion of the individual particles or crystals of the refractory. When these particles are bonded or sintered together in a mass and heat is applied, it is the individual particles that expand causing the dimensions of the entire mass to change due to the cumulative effect of the expansion of the individual particles. This phenomenon is referred to herein as the mass thermal co-efficient of expansion.
When refractories are prepared by the conventional pro-cess, they act as a single body and when the body is sub-jected to heat the entire body expands due to the cumu-lative effect of the individual particles or crystals making up the body. In the composites of this invention, the body acts as indi~idual particles and although the individual particles or crystals expan~ when the body is heat2d.
Although many different inorganic materials can be used to form composites, some of the more commercially important composites are ceramic or refractory composites such as zirconia-alumina, zircon-alumina, magnesia-alumina, silica-alumina, zircon-silica, zirconia-magnesia and zircon-magnesia composites. The zirconia alumina com-posite, for example, is particularly valuable to the steel industry. Zirconia has a low ther~.al conductivity when compared to alumina, and is very corrosive and errosi~e resistant to molten steel. Zirconia, however, is not very strong when compared to alumina (especially in its soft form) and is also much more expensive than alumina. Thus, a zirconia-alumina composite brick or slab having about one-quarter inch of zirconia adhered to one side of two to three inches of alumina gives a product of high strength and at a significantly reduced cost while retaining all of the advantages of zirconia in its most desirable soft form. The advantages of other combinations of ceramics will depend somewhat upon their intended use and variations can be produced to meet the requirements of any particular - end use. Alumina is presently preferred as a basic ceramic to which other ceramics are bonded because it is inex-pensive, strong, easy to work with, and has a convenient firing -temperature.
Various shapes can be produced; for example, alumina can be faced on one or more sides with zirconia or magnesia, the internal surface of alumina cylinders can be lined with zirconia, alumina noz71es and ladles can be llned with zirconia and so forth.
In firing the composite ceramics, it has been found to be ad~antageous to fire at a temperature for the ceramic having -the lower firing temperature. For example, when firing a composite composed of alumina and zirconia, it has been found advantageous to fire it at about 1250C.
- 18 ~ 5~
Such a firing temperature, however, is below that norma]ly used for zirconia and also below that which is used for firing the individually freeze-cast zirconias, namely 1400C. When firing such a composite at 1250C, the zirconia would normally be considered as somewhat under-fired. Soft zirconia is even more corrosive and errosive resistant to molten uranium than hard zirconia fired at its normal temperature; and the fact that the zirconia is bound or adhexed to the alumina renders the composite as a whole very strong, thus permitting one to take maximum advantage of the properties of zirconia while maintaining excellent strength~ If desired, the zirconia portion of such a substancs could be locally fired to increase its hardness.
The formation of the composite structures can take place in a single mold. The different ceramics are formed into slurries containing the freeze-sensitive colloidal ceramic sols. One ceramic slurry can then be placed at the bottom of the mold and the different ceramic slurry placed on top thereof, the composite frozen, thawed, and subse~
quently fired. To prevent any substantial mixing of the different ceramics, the slurries can be formed in a viscous state or other techniques can be used to prevent significant mixing, such as inserting a thin separating slip or shield between the ceramic slurries and pouring . the other slurry on top or on the other side of the slip to temporarily effect a physical separation of the dif-ferent slurries and then removing the separating slip ~ust prior to the freezing of the slurries.
Example 8 Laminate Zirconia-Alumina (ll" melting crucible) Alumina Mix (15 kilograms) Tabular Alumina Alcoa* T61 - 28 + 48 Mesh 55 *Trade Mark .9~
- 100 Mesh 25%
- 325 Mesh 20%
Sol 30% solids Colloidal Silica (NALCO) pH about 10 14.3% by weight Zirco Ili a Mix (7 kilograms) Monoclinic Zirconia - 100 Zirconia 50%
- 325 Zirconia 50~
Sol 30% solids Colloidal Silica- (NALCO) 12.4% by weight ModiEied by 0.6% HCL. to p~l 7.5 Example 9 Laminated Zircon-Alumina (plate) Alumina Mix (1060 grams) - Tabular Alumina Alcoa T61 - 28 ~ 48 Mesh 55%
- 100 Mesh 25%
- 325 Mesh 20%
Sol 30% solids Colloidal Silica (NALCO) pH about 10 14.3% by weight Zircon (600 grams) - 80 Mesh 70%
- 325 Mesh 30%
Sol 30% solids Silica (NALCO) pH about 10 11.7~ by weight Example 10 Laminated Alumina & Mullite (plate) Tabular Alumina Alcoa T61 (300 grams) - 14 Mesh 40%
- 48 Mesh 44%
- 325 Mesh 16%
Sol 30% Silica (NALCO), pH about 10 14.7% by weight - 20 ~
Mullite-Remasil #60 (89 grams) - 20 Mesh 30%
- 70 Mesh 30%
~ 200 Mesh 30%
- 325 Mesh 10~
Sol 30~ Silica (NALCO), pH about 10 14.6% by weight The above mixes were thoroughly blended and each mix had the consistency of thick pancake batter and could be poured and placed in a mold with the aid of a spatula.
The molds were thoroughly waxed and highly polished.
The zirconia mix was first placed in each mold to the desired depth, about 1/4 inch in these examples, and the alumina, zircon, and mullite in each of the above examples placed in the same molds on top of the zirconia to a depth of about two inches. The top layers of alumina, zircon and mullite are each then covered with a layer o hydrophobic freezing liquid, the molds closed and molds inserted entirely in per hloroethylene freezing liquid at a temperature of ~48F.
The frozen composite bodies were then removed from the mold while frozen, thawed and dried in a radiant heated oven at a temperature of about 120F. After drying the composite bodies were each fired in a conventional kiln at a temperature of 1250C for about 4 hours, after which the moldings were allowed to slowly cool to ambient temperature. The moldings were then subjected to heat - and thermal shock by applications of a 3000F torch directly to the moldings at ambient temperature. The mold-ings remained substantially unaffected after the appli-cation of heat with no visible or physical effect on the interfaced bonds. The zirconia-alumina composite was also hit with a gas torch at about 6000F with no apparent damage to the interfacial bond although it melted the alumina.
3~:~
Example 8 was repeated using a mold previously immersed in perchloroethylene and the mold filled while still wet. Supercooling occurred quite readily when the com-posite ceramics were inserted into the freezing liquid and a fine uniform grained composite structure obtained.
Example 12 Example 11 was repeated substituting for the NALCO, a 50-50 mixture of the sodium stabilized NALCO silica sol and a 30 percent solids lithium stabilized silica sol having a siiicalithia ratio of about 48.
Example 13 Example 8 was repeated in which about 0.6 percent by weight a 3.7 percent HCl solution base on the weight of the zirconia was added to the zirconia grains and about 15.6 percent by weight of the NAL~O silica sol (30~
solids) having a pH of about 10 was used. The amount of acid used is about the same amount tha-t would be re-quired to bring the pH of the sodium stabilized silica sol to about 7-8.
- Example 14 60 parts by weight of magnesia (50~-14 mesh and 50%-48 mesh) were mixed with 12 parts by weight of a lithium stabilized 30% aqueous silica sol having a silica-lithium ratio of 85, formerly marketed by DuPont under the desig-nation Lithium Polysilicate 85. The mixture was super-cooled and frozen in the same manner as set forth in the above examples. No initial reaction was noted and the pot life of the mix was very good.
~ 1 --INORGANIC COMPOSITE STRVCTVRES
-TECHNICAL FIELD
Inorganic products, such as refractory bodies, and method of producing such products by freezing aqueous slurries of the inorganic particles of which the inorganic products are composed.
BACKGROUND ART
Freezing a slurry of particulate inorganic materials to form refractory products has been disclosed in U.S.
Patents Nos. 3,177,161; 3,5i2,571; 3,816,~72 and 3,885,005.
The production of refractory structures in accordance with the prior art resulted in products which often contain large voids which adversely affect the strength of the ceramic product, which in turn adversely affects their properties such as thermal conductivity and thermal shock resistance.
These deficiencies in the resulting ceramic products are believed to be caused by nucleation at one of the cooling surfaces due to contact with an ice crystal or with the mold surface that has a nucleation temperature higher than that of the slurry itself as the dispersion is cooled to its freezing point. This nucleation is believed to cause large ice crystals to grow out from these points and eventually entrap the last remaining liquid, resulting n rupture, cracking~ weak, and non-uniform ceramic structures.
~iany attempts have been made to overcome this problem, such as thorough waxing of the molds, the use of different mold materials, thorough cleaning of mold surfaces, even 50~3~1L
with acids, but -the problem still persis-ted.
DISCLOSURE OF T~E INVENTION
This invention rela-tes to a process which substantially eiiminates the problems discussed above by using an inorganic particulate slurry or suspension containing a freeze sensitive ceramic colloidal sol and freezing the slurry or suspension in the presence of lithium ions contained in the slurry or suspension in an amount sufficient to inhibit iee crystal growth.
The invention may inelude the addition of lithium ions to the freezing media with or without supercooling. The invention may be used to make composite inorganic strue-tures which are in the nature of laminates where at least one inorganic structure, sueh as a zirconia plate, is adhered to a different inorganic structure, such as an alumina plate.
The various inorganic particles that can be used according to this invention include, without limitation, aluminas such as mullite and tabular alumina, silicas sueh as fused silica, magnesia, chromite, spinels such as chromite spinel, kyanite, carbomul, zirconia, mica, carbon, graphite, molydisulfide, uranium oxide, thoria, titania, clays, etc.
The invention, however J iS broadly applicable to suspen-sions of inorganie partieles in general ineluding other metal compounds. Mixtures may be used if desired.
The inorganic particle size is not critical. Best results seem to have been obtained to date when the majority of the particles, or 40-50~ of them, are below about 200 mesh. Small particle or grain size seems to be particularly advantageous when using zirconia. Much 3 :~L
smaller particle sizes can also be used even in a colloidal size. The ultimate particle size and size distribution will depend to some extent on the end use of the structures and the properties desired therein.
The inorganic products produced according to this inven-tion are porous and the size of the grains or particles employed in the slurries will to a large extent determine the degree of porosity. The products of the invention have a wide variety of uses depending to some extent on the type of particle being employed in the process. For example, if ceramic particles are employed, the products can be used in the same manner as ceramic and refractories are used such as fire brick, linings for furnaces in the steel, glass and other industries. They can also be used as filters, carriers for catalysts, thermal shock resis-tant dinnerware, grinding wheels, etc. When particles such as graphite and molydisulfide are employed their uses can include many of the above but adding thereto the lubrica-tion properties of these mater:ials. Generally the prod-ucts are useful in any area where porosity is desired or in areas where porosity is not desired but is not detri-mental.
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The freeze-sensitive colloidal ceramic sols useful accord-ing to this invention are well known and include colloidal ceramic sols, such as disclosed in the Smith-Johannsen U.S. Patent 3,177,161 and U.S. Patents 3,512,571 to Phelps, 3,816,572 to Roelofs and 3,885,005 to Downing et al. A freeze-sensitive sol is one which, when frozen, will break down and no longer exist as a sol or colloidal suspension when thawed. Both cationic and anionic silica sols can be used with the anionic preferred at least with alumina and zirconia refractories. Ammonia stabilized silica sols, such as Dupont's AM LUDOX*, may be advantageous *Trade Mark ~ 3~
where elimination of sodium is desired. Other ~reeze-sensitive colloidal ceramic sols, such as zirconia and magnesia sols, can also be used. Silica sols have been used because they are readily available on the market.
Although not necessarily preferable due to insufficient expeximental data to date, most present experiments mainly utilize a freeze-sensitive sodium stabilized colloidal silica sol having about 30% colloidal silica supplied by Nalco Chemical Company.
The total amount of sol stabilizer such as sodium, ammonium and/or lithium should be sufficient to stabilize the sol but not be so high as to render the sol non-freeze sen--sitive or to lower the strength of the fired silica or other ceramic contained in the sol when fused or fired or to lower the strength of the fused or fired product to an unacceptable level. This can readily be determined by routine experimentation by one skilled in the art. For example a mole ratio of silica to lithia of about 85 in a lithium stabilized silica sol works quite well but when the ratio is lowered to about 48 the sol appears to lose some freeze sensitivity resulting in weaker bonds. The optimum amounts have not as yet been determined.
Generally, the sodium stabilized silica sol are quite adequate to practice the invention disclosed herein. With some inorganic particles, namely zirconia and magnesia, some adjustments can be made to improve the results.
These adjustments are desirable to improve pot li~e and to preserve the distribution of particle sizes during the filling operation so that an optimum degree o~ uniform packing can be obtained.
When zirconia, for example is mixed with a negatively charged sodium stabilized silica sol (DuPont Ludox HS-40*) it is well *Trade Mark 3~
wetted and the particles quickly segregate. It is be-lieved that this segregation occurs because the sol particles and the zirconia particles are so charged to prevent or minimize particle association. The zirconia particles generally have a charge of about -20 m.v. (zeta poten~ial) in deionized water while the above Ludox partlcles are even more highly charged. To overcome particle segregation problem, the zeta potential of the particles comprising the various mixes can be altered.
One way of altering the zeta potential is to reduce the pH
of ~he above Ludox from about 10 to about 8 by adding dilute HCl which lowers the zeta potential of the silica sol particles and rendering them less stable. Under these conditions, the silica particles begin to precipitate onto the ceramic tzirconia) particles creating a degree of association between all of the particles of the mix and segregation of coarse and fine ceramic particles such as zirconia is greatly inhibited. The acid can also be added to the inorganic particles or to a mixture of the sol and particles. The amount of acid is that which is sufficient to prevent settling or segragation of the particles. In practice, it is advantageous to add the acid directly to the sol. With zirconia, the amount of acid found practi-cal to accomplish the result is about 0.6 percent by ~5 weight (based on the total weight of zirconia) of a 37 percent HCl solution. If such a problem is encountered with other inorganic particles the zeta potential can be measured and appropriate adjustment with acid ox alkali to alter the ze~ potential can be made in such a manner as to insure particle association.
Zirconia, and especially magnesia, also appear -to react with the sodium stabilizer (and also ammonia somewhat) to cause limited pot life. In fact, the reaction with ~ 3~3 magnesia is so rapid -that mixing itself becomes difficult.
The use of a li-thium stabilized silica sol was found to eliminate this reac-tion -to the extent tha-t magnesia dispension could be readily mixed and cast without concern of short pot life. The use of lithium s-tabilized sols also overcomes the particle segregatlon problem referred to above wi-th respect to zirconia. Thus it may not be necessary to adjust the zeta potentials of the particles if a sol having the requisite zeta potential ean initially be used.
When the lithium stabilized silica sol was used with magnesia and zirconia another new and very signifiean-t property was observed. These sols inhibited ice crystal growth even in the absenees of supereooling. In faet, when nueleation was delibera~ely initiated, in the ease of magnesia dispersion containing lithium ions, from the surface with an iee erystal, no maero or large erystal growth was detectable for more than two millimeters from the initiation site. Thus the use of lithium stabilizea ceramie sols not only solved the pot life problems and partiele segregation problems of zireonia and magnesia but has been found extremely advantageous for produeing small uniform iee erystals durina the freezing step with regard to all inorganie dispersions. The use of a lithium stabilized eeramie sol in eombination with supereooling has been found most advantageous.
When using lithium in the inorganie particle slurries in accordance with the invention sol, it is of eourse most praetieable to employ a lithium stabilized eeramie sol available on the market. A silica sol having a silica to lithia ratio of 85 worked quite well, however this sol, DuPont's Lithium Polysilieate 85*, is not being marketed today. One lithium stabilized siliea sol whieh is avail-*Trade Mark able today contains a silica to lithia ratio of about ~8 (DuPont's Lithium Polysilicate 48*). This amoun-t of lithia however minimizes the freeze sensitivity of the sol and when used alone produced fired products having weaker bonds. This commercial lithium stabilized sol can be used however by using it in admixture with a sodium or prefer-ably an ammonia stabilized sol. A 50-50 mixture has worked well but the optimum has not as yet been determined.
I-t is the presence of the lithium ion which produces the surprising ice crystal growth inhibition rather than the absence of sodium or ammonia. Thus lithium ions can be added to the slurries by the àddition of ionizable lithium compounds such as lithium chloride, lithium hydroxide, lithium sulfate, lithium succinate and so forth. It is preferred to add th~ lithium ions to the ceramic sol. The amount of lithium ions added to an inorganic particle slurry should be sufficient to inhibit ice crystal growth to the desired degree but insufficient to adversely affect the freeze sensitivity of the sol. This can be determined by routine experimentation with respect to any particular system being frozen. Only a very small amount of lithium ion is necessary to inhibit ice crystal growth. Higher amounts may however be required to increase the pot life when magnesia is used as can be observed in Examples below.
To accomplish the supercooling and substantial instan-taneous freezing, it is not a simple matter of inserting a mold filled with the slurries into a cold freezing media even at -40 or -60F. This invention includes a process of insuring supercooling of the ceramic slurries by treating the mold with a hydrophobic liquid such as xylene, mineral spirits, or perchloroethylene to cover at least the entire working surface of the mold, and inserting the slurries or suspensions into the mold while *Trade ~ark ~L i it is still wet. This can be accomplished by simply dipping the mold in the hydrophobic liquid. The mold can then be closed and the slurry frozen. It is also advantageous to cover the aqueous slurry or suspension in the mold with a thin layer of the hydrophobic liquid. The mold itself is preferably of light weight and of low mass relative to the freezing media and the ceramic or partic-ulate slurry being frozen. The mold and freezing media should also ha~e a high thermal conductivity. Although the freezing temperature can be varied, it should be sufficiently low to insure supercooling and a rapid freeze. Temperature of about ~5 to -50F can be used.
When lithium i9 used temperatures of about -10F can advantageously be used.
When producing large articles, supercooling may only occur to a certain depth from the mold surface toward the center of the slurry because all of the heat within the center cannot be removed before freezing of a portion of the slurry closer to the mold. When large articles are to be made it is thus advantageous to cool the entire slurry to near the freezing point before inserting it into the freezing media to insure complete supercooling. The presence of lithium ions in large slurries is also advan~
- tageous.
The supercooling can also be carried out without the use of mold such as by extruding cylinders, sheets or films of the aqueous slurry or suspension on a belt treated with the hydrophobic liquid and then into the hydrophobic freezing media. The transport through the freezing media can be between a pair of belts and the process can be continuous. The terms "mold" as used herein is intended to include any structure for supporting and/or encompassing the slurries or suspensions.
Supercooling of the slurry to a temperature where it spontaneously nucleates results in a structure that is uniform throughout. At the time of nucleation not all the water freezes because the heat of fusion raises the 5 temperature back to the freezing point. However, as cooling proceeds further, ice crystal growth is completed from all of these nucleation sites at substantially the same time. The structure that develops is therefore much more uniform and fine grained regardless of the thickness of the structure to be produced or frozen.
Random tests made on some of the freezing steps set forth herein indicate that the temperature of supercooling is about 4 degrees below the freezing temperatures of the aqueous slurry.
Various freezing media can be used to freeze the slurry structures such as those described in the above-mentioned patents. A hydrophobic freezing media such as Freon* or perchloroethylene is advantageously used to prevent penetration of the freezing media into the aqueous slur-ries to prevent the growth of large or variable sizedice crystals, and to insure supercooling.
The various inorganic materials or ceramics useful according to this invention have different and known firing or sintering temperatures in conventional refractory pro-cesses. For example, alumina is generally fired at atemperature of 1400C or slightly above, and zirconia at about 1700C, in conventional refractory processes. As a general rule, freeze-cast ceramics are most advantage-ously fired at about 50~ to 65% of their melting temper-ature. Thus, when firing freeze-cast ceramics, alumina is advantageously fired at about 1250C while zirconia is advantageously fired at about 1400C.
Other inorganic structures can be sintered at their known or determined sintering temperatures. The temperature *Trade Mark used and time of heating should be sufficient to bond the particles together into a strong integral structure but insufficient to significantly reduce or adversely aEfect the desired porosity or uniformity of the product.
5 E~amples of such temperatures are given, for example, in U.S. Patent 3,177,161.
The molds or patterns are usually made of lightweight steel or aluminum if more thermal conductivity is desired, The wetting of a mold with a hydrophobic liquid to aid in supercooling can also act as a release agent.
The slurries should be as free from entrapped air as practical. Entrapped air can be avoided to some extent by the manner by which the slurries are first mixed, and any entrapped air can be removed in various known manners, such as using long-periods of holding time, vibration, or vacuum treatment techniques.
After freezing, the frozen slurry structures are removed from the mold, thawed and dried. Although various manners of thawing and drying can be employed, the thawing and drying can be accelerated by the use of heat. The use of a conventional drying o~en has been round satisfactory for this purpose.
After selection of the specific particles to be formed into a structure, they can be mixed in the con~entional manner having due regard to particle size, and the freeze-sensitive colloidal ceramic added to each of the dried ceramic materials selected. The freeze-sensitive col-loidal ceramics are contained in water and the solid colloidal content may range from 15~ to 50% solids. Thus, the addition of the freeze-sensitive colloids -to the dried ceramic material usually automatically adds the neces-sary water for handling. For example`, a mix commonly used in slip cas-ting containing up to about 10% water, having a consistency somewhat like pancake batter can be poured into a mould or injected by simple means. This can readily be accomplished by maintaining the proper consistency of the ceramic slurries either by using high solid content freeze-sensitive colloidal ceramic sol or by removing water prior to freezing. One manner of accomplishing this mixing is to dry mix the ceramic ~rain in a ribbon blender and add the freeze-sensitive ceramic sol toge~her with its liquid component, slowing the ribbon blender and continuing until thorough mixing is obtained. The particulate suspensions or slurries, should have a particle content sufficient to insure particle to particle contact during the freezing step as described in U.S. patent 3,177~161~ If the particles are dispersed too thinly, no structure will be formed when the ice melts. The amount of water is desirably held to a minimum practical mount for eeonomic reasonsO
The amount of the freeze-sensitive eeramic sol ean be as reported in the above~noted U.S. patents. The most suitable percentage appears to be about 15% by weight - of the colloidal ceramic sol (30% solids) based on the weight of the dried inorganic particles.
BEST MODE FOR CARRYING OUT THE INVENTION
. . ~
Example 1 Alumina Mix (19 kilograms) - crucible Tabular Alumina Alcoa T61 - 28 + 48 Mesh 55%
- 100 Mesh 25~
- 325 Mesh 20%
Sol 30% Colloidal Silica (NALCO)*
14.3% by weight pH about 10 *Trade Mark - 12 ~
2irconia Mix (367.5 grams) - plate Monoclinic Zirconia - 100 " 50%
- 325 " 50%
Sol 30% Colloidal Silica (NALCO) 12.5% by weight Modified by 0.6% HCL. to pH 7.5 Example _ ~ircon (380.5 grams) - plate - 80 Mesh 70~
- 325 Mesh 30%
Sol 30~ Silica (REMASIL SP-30), pH about 10 11.7% by weight Example 4 Mullite-Remasil ~60 (1180 grams) - plate - 20 Mesh 20%
- 70 Mesh 20%
- 200 Mesh 40~
325 Mesh 20%
Sol 30~ Silica (REMASIL SP-30), pH about 10 14.6% by weight The above mixes are thoroughIy blended and each mix has the consistency of thick pancak~ batter and can be poured and placed in a mold with the aid of a spatula.
The molds are thoroughly wet with perchloroethylene.
Each mix is placed in the mold while it is still wet with the perchlorethylene to the desired depth, about two inches. The top layers of each mix is then covered with a layer of perchloroethylene freezing liquid, the molds closed and molds inserted entirely in perchloroethylene freezing liquid at a temperature of -48F. The liquid slurries are supercooled and then frozen. The frozen structures are then removed from the mold while frozen, thawed and dried in a radiant heated oven at a temperature of about 120F. No significant large voids were detected and the products aftex freezing were extremely uniform.
After drying ~he structures are each fired in a conven tional kiln at a temperature of 1250C for about 4 hours, after which the moldings were allowed to slowly cool to ambient temperature. The moldings were then subjected to heat and thermal shock by applications of a torch (about 6000F) directly to the moldings at ambient temperature.
The moldings remained substantially unaffected after the application of heat with no visible or physical defects except for surface melting where the torch was applied.
Repeated tests such as those described above gave con-sistent and repeatable results.
Example 5 60 parts by weight of magnesia (50%-14 mesh and 50~-48 mesh) were mixed with 12 parts by weight of a lithium stabilized 30% aqueous silica sol having a silica-lithium ratio of 85, formerly marketed by DuPont under the designa-tion Lithium Polysilicate 85. The mixture was supercooledand frozen in the same manner as set forth in the above examples. No ini~ial reaction was noted and the pot life of the mix was very goodO
Example 6 60 parts by weight of alumina (50%-28 ~ 48 mesh and 20%-325 mesh) were mixed together with 2.4 parts by weight of a lithium stabilized 30% aqueous silica sol having a - 14 ~
silica-lithium ra-tio of 4~, marketed by DuPont under the designation Lithium Polysilicate 48, and 9.6 parts by weight of an ammonia stabilized 30% aqueous silica sol marketed by DuPont under the designation Ludox AS-40 and the mixture supercooled and frozen in the same manner as set forth in the above examples.
Example 7 Example 6 was repeated using 12 parts by weight of the ammonia stabilized sol and 0.025 parts by weight of a saturated solution of lithium hydroxide substituted for the lithium stabilized sol.
The ice formed during the freezing in examples 5, 6 and 7 was much finer than the ice formed in examples 1-4. The unfired products were more uniform and were stronger than those produced in examples 1-4.
The thawed products can also be broken up into particles and then fired for various uses such as fillers for resins or plastics~ The particles have a unique shape which is advantageous for use as fillers. A clay slurry, for example, was frozen, thawed, and driéd according to this invention, broken up or ground into small particles; their shape was particularl~ suitable for filler particles. The jagged nature of these particles makes them particularly suited to mixing with small amounts of powdered thermo-plastic or thermosetting resins to form, under heat and pressure, ceramic-like products such as roofing tiles.
Composite inorganic structures such as ceramics, can be considered as being in the nature of laminates where one inorganic structure, such as a zirconia plate, is adhered -30 to a different structure, such as an alumina plate. Prior attempts to produce such composite ceramics by simul-taneous firing of the ceramics in a single mold or by firing the ceramics separately and cementing them together have not been successful to applicant's knowledge. The interfacial bonds between the different ceramics obtained in these manners have not been adequate to withstand the high temperatures and thermal shock to which they are frequently subjected. This interfacial bond failure is mainly due to the differences in the thermal coefficients of expansion of the clifferent refractories or ceramics.
A further problem involved in producing composite ceramics is the difference in the firing temperatures required of the different ceramic materials.
The composite inorganic articles produced in accordance with this invention, particularly ceramics, may be com-posed of different inorganic materials adhered or lamin-ated together and having an interfacial bond of extra-ordinary strength sufficient to withstand extremely high temperatures of thermal shock, such as experienced in the metals industry. Composite ceramics produced according to this invention have been subjected to temperatures as high as 6000F without any noticeable effect on the inter-facial bonds.
This invention also includes a process ~or producing composite ceramics in a single mold by inserting the different ceramic materials into the mold in layered fashion and in contact with each other, freezing the layered ceramics while in contact with each other, thawing the frozen composite, and then firing the thawed composite.
There are a number of important and surprising results obtained when composite ceramic articles are made ac-cording to this invention. The bond between the ~wo different ceramics is at least as strong as the bond between the particles of the individual ceramics employed.
- 16 - 3~
The composi-te frozen structure can be fired at a single temperature despite the fact the ceramics individually are known to re~uire different firing temperatures.
The differences in the thermal coefficients or the mass thermal coefficients of expansion of the different ceramics does not cause disruption or weakening of the interfacial freeze bonding during the firing operation and the fired bond is not significantly affected during high temperature use or by subjecting the composite ceramic to extreme thermal shock. It is further surprising that experiments to date indicate that a very wide range o~
different ceramics can be employed to make the composite ceramics of this invention having wide differences in their firing temperatures and thermal coefficients of expansion.
The thermal coefficient of expansion refers to the ex-pansion of the individual particles or crystals of the refractory. When these particles are bonded or sintered together in a mass and heat is applied, it is the individual particles that expand causing the dimensions of the entire mass to change due to the cumulative effect of the expansion of the individual particles. This phenomenon is referred to herein as the mass thermal co-efficient of expansion.
When refractories are prepared by the conventional pro-cess, they act as a single body and when the body is sub-jected to heat the entire body expands due to the cumu-lative effect of the individual particles or crystals making up the body. In the composites of this invention, the body acts as indi~idual particles and although the individual particles or crystals expan~ when the body is heat2d.
Although many different inorganic materials can be used to form composites, some of the more commercially important composites are ceramic or refractory composites such as zirconia-alumina, zircon-alumina, magnesia-alumina, silica-alumina, zircon-silica, zirconia-magnesia and zircon-magnesia composites. The zirconia alumina com-posite, for example, is particularly valuable to the steel industry. Zirconia has a low ther~.al conductivity when compared to alumina, and is very corrosive and errosi~e resistant to molten steel. Zirconia, however, is not very strong when compared to alumina (especially in its soft form) and is also much more expensive than alumina. Thus, a zirconia-alumina composite brick or slab having about one-quarter inch of zirconia adhered to one side of two to three inches of alumina gives a product of high strength and at a significantly reduced cost while retaining all of the advantages of zirconia in its most desirable soft form. The advantages of other combinations of ceramics will depend somewhat upon their intended use and variations can be produced to meet the requirements of any particular - end use. Alumina is presently preferred as a basic ceramic to which other ceramics are bonded because it is inex-pensive, strong, easy to work with, and has a convenient firing -temperature.
Various shapes can be produced; for example, alumina can be faced on one or more sides with zirconia or magnesia, the internal surface of alumina cylinders can be lined with zirconia, alumina noz71es and ladles can be llned with zirconia and so forth.
In firing the composite ceramics, it has been found to be ad~antageous to fire at a temperature for the ceramic having -the lower firing temperature. For example, when firing a composite composed of alumina and zirconia, it has been found advantageous to fire it at about 1250C.
- 18 ~ 5~
Such a firing temperature, however, is below that norma]ly used for zirconia and also below that which is used for firing the individually freeze-cast zirconias, namely 1400C. When firing such a composite at 1250C, the zirconia would normally be considered as somewhat under-fired. Soft zirconia is even more corrosive and errosive resistant to molten uranium than hard zirconia fired at its normal temperature; and the fact that the zirconia is bound or adhexed to the alumina renders the composite as a whole very strong, thus permitting one to take maximum advantage of the properties of zirconia while maintaining excellent strength~ If desired, the zirconia portion of such a substancs could be locally fired to increase its hardness.
The formation of the composite structures can take place in a single mold. The different ceramics are formed into slurries containing the freeze-sensitive colloidal ceramic sols. One ceramic slurry can then be placed at the bottom of the mold and the different ceramic slurry placed on top thereof, the composite frozen, thawed, and subse~
quently fired. To prevent any substantial mixing of the different ceramics, the slurries can be formed in a viscous state or other techniques can be used to prevent significant mixing, such as inserting a thin separating slip or shield between the ceramic slurries and pouring . the other slurry on top or on the other side of the slip to temporarily effect a physical separation of the dif-ferent slurries and then removing the separating slip ~ust prior to the freezing of the slurries.
Example 8 Laminate Zirconia-Alumina (ll" melting crucible) Alumina Mix (15 kilograms) Tabular Alumina Alcoa* T61 - 28 + 48 Mesh 55 *Trade Mark .9~
- 100 Mesh 25%
- 325 Mesh 20%
Sol 30% solids Colloidal Silica (NALCO) pH about 10 14.3% by weight Zirco Ili a Mix (7 kilograms) Monoclinic Zirconia - 100 Zirconia 50%
- 325 Zirconia 50~
Sol 30% solids Colloidal Silica- (NALCO) 12.4% by weight ModiEied by 0.6% HCL. to p~l 7.5 Example 9 Laminated Zircon-Alumina (plate) Alumina Mix (1060 grams) - Tabular Alumina Alcoa T61 - 28 ~ 48 Mesh 55%
- 100 Mesh 25%
- 325 Mesh 20%
Sol 30% solids Colloidal Silica (NALCO) pH about 10 14.3% by weight Zircon (600 grams) - 80 Mesh 70%
- 325 Mesh 30%
Sol 30% solids Silica (NALCO) pH about 10 11.7~ by weight Example 10 Laminated Alumina & Mullite (plate) Tabular Alumina Alcoa T61 (300 grams) - 14 Mesh 40%
- 48 Mesh 44%
- 325 Mesh 16%
Sol 30% Silica (NALCO), pH about 10 14.7% by weight - 20 ~
Mullite-Remasil #60 (89 grams) - 20 Mesh 30%
- 70 Mesh 30%
~ 200 Mesh 30%
- 325 Mesh 10~
Sol 30~ Silica (NALCO), pH about 10 14.6% by weight The above mixes were thoroughly blended and each mix had the consistency of thick pancake batter and could be poured and placed in a mold with the aid of a spatula.
The molds were thoroughly waxed and highly polished.
The zirconia mix was first placed in each mold to the desired depth, about 1/4 inch in these examples, and the alumina, zircon, and mullite in each of the above examples placed in the same molds on top of the zirconia to a depth of about two inches. The top layers of alumina, zircon and mullite are each then covered with a layer o hydrophobic freezing liquid, the molds closed and molds inserted entirely in per hloroethylene freezing liquid at a temperature of ~48F.
The frozen composite bodies were then removed from the mold while frozen, thawed and dried in a radiant heated oven at a temperature of about 120F. After drying the composite bodies were each fired in a conventional kiln at a temperature of 1250C for about 4 hours, after which the moldings were allowed to slowly cool to ambient temperature. The moldings were then subjected to heat - and thermal shock by applications of a 3000F torch directly to the moldings at ambient temperature. The mold-ings remained substantially unaffected after the appli-cation of heat with no visible or physical effect on the interfaced bonds. The zirconia-alumina composite was also hit with a gas torch at about 6000F with no apparent damage to the interfacial bond although it melted the alumina.
3~:~
Example 8 was repeated using a mold previously immersed in perchloroethylene and the mold filled while still wet. Supercooling occurred quite readily when the com-posite ceramics were inserted into the freezing liquid and a fine uniform grained composite structure obtained.
Example 12 Example 11 was repeated substituting for the NALCO, a 50-50 mixture of the sodium stabilized NALCO silica sol and a 30 percent solids lithium stabilized silica sol having a siiicalithia ratio of about 48.
Example 13 Example 8 was repeated in which about 0.6 percent by weight a 3.7 percent HCl solution base on the weight of the zirconia was added to the zirconia grains and about 15.6 percent by weight of the NAL~O silica sol (30~
solids) having a pH of about 10 was used. The amount of acid used is about the same amount tha-t would be re-quired to bring the pH of the sodium stabilized silica sol to about 7-8.
- Example 14 60 parts by weight of magnesia (50~-14 mesh and 50%-48 mesh) were mixed with 12 parts by weight of a lithium stabilized 30% aqueous silica sol having a silica-lithium ratio of 85, formerly marketed by DuPont under the desig-nation Lithium Polysilicate 85. The mixture was super-cooled and frozen in the same manner as set forth in the above examples. No initial reaction was noted and the pot life of the mix was very good.
Claims (2)
1. A process of freezing an inorganic particulate slurry or suspension containing a freeze-sensitive ceramic sol which comprises freezing the slurry or suspension in the presence of lithium ions contained in the slurry or suspension in an amount sufficient to inhibit ice crystal growth.
2. A process according to claim 1 in which the slurry or suspension is supercooled before it is frozen.
Applications Claiming Priority (5)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US1715479A | 1979-03-02 | 1979-03-02 | |
| US06/016,871 US4246209A (en) | 1979-03-02 | 1979-03-02 | Freezing inorganic particulate slurries |
| US017,154 | 1979-03-02 | ||
| US016,871 | 1979-03-02 | ||
| CA000346007A CA1161238A (en) | 1979-03-02 | 1980-02-18 | Inorganic composite structures |
Related Parent Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000446158A Division CA1185081A (en) | 1979-03-02 | 1984-01-26 | Process for freezing an inorganic particulate slurry or suspension |
Related Child Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000446158A Division CA1185081A (en) | 1979-03-02 | 1984-01-26 | Process for freezing an inorganic particulate slurry or suspension |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1185081A true CA1185081A (en) | 1985-04-09 |
Family
ID=27166589
Family Applications (5)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000446155A Expired CA1188079A (en) | 1979-03-02 | 1984-01-26 | Composite inorganic structure |
| CA000446158A Expired CA1185081A (en) | 1979-03-02 | 1984-01-26 | Process for freezing an inorganic particulate slurry or suspension |
| CA000446159A Expired CA1185082A (en) | 1979-03-02 | 1984-01-26 | Process for producing composite ceramics |
| CA000446157A Expired CA1185080A (en) | 1979-03-02 | 1984-01-26 | Process for freezing an inorganic particulate slurry or suspension |
| CA000446156A Expired CA1185079A (en) | 1979-03-02 | 1984-01-26 | Process for freezing an inorganic particulate slurry or suspension |
Family Applications Before (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000446155A Expired CA1188079A (en) | 1979-03-02 | 1984-01-26 | Composite inorganic structure |
Family Applications After (3)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000446159A Expired CA1185082A (en) | 1979-03-02 | 1984-01-26 | Process for producing composite ceramics |
| CA000446157A Expired CA1185080A (en) | 1979-03-02 | 1984-01-26 | Process for freezing an inorganic particulate slurry or suspension |
| CA000446156A Expired CA1185079A (en) | 1979-03-02 | 1984-01-26 | Process for freezing an inorganic particulate slurry or suspension |
Country Status (1)
| Country | Link |
|---|---|
| CA (5) | CA1188079A (en) |
Families Citing this family (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| DE102021102491A1 (en) | 2021-02-03 | 2022-08-04 | Fraunhofer-Gesellschaft zur Förderung der angewandten Forschung eingetragener Verein | Green body or sintered body made therefrom and method for their production |
-
1984
- 1984-01-26 CA CA000446155A patent/CA1188079A/en not_active Expired
- 1984-01-26 CA CA000446158A patent/CA1185081A/en not_active Expired
- 1984-01-26 CA CA000446159A patent/CA1185082A/en not_active Expired
- 1984-01-26 CA CA000446157A patent/CA1185080A/en not_active Expired
- 1984-01-26 CA CA000446156A patent/CA1185079A/en not_active Expired
Also Published As
| Publication number | Publication date |
|---|---|
| CA1185080A (en) | 1985-04-09 |
| CA1185079A (en) | 1985-04-09 |
| CA1185082A (en) | 1985-04-09 |
| CA1188079A (en) | 1985-06-04 |
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Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| MKEX | Expiry | ||
| MKEX | Expiry |
Effective date: 20020409 |