WO1991004951A1

WO1991004951A1 - Process for making a lightweight cellular inorganic material

Info

Publication number: WO1991004951A1
Application number: PCT/US1990/005550
Authority: WO
Inventors: Gary L. Messing; Takamitsu Fujiu
Original assignee: Research Corp Technologies Inc
Current assignee: Research Corp Technologies Inc
Priority date: 1989-09-29
Filing date: 1990-09-28
Publication date: 1991-04-18
Anticipated expiration: 1992-03-29
Also published as: AU6522290A; CA2026559A1

Abstract

A process for making a cellular inorganic material is disclosed. In this process a sol is formed by dispersing inorganic particles having a size of less than 1000 Å in a dispersant liquid. A liquid, immiscible in the dispersant liquid, is added to the sol. A stabilizing effective amount of surfactant is added to the sol prior to, concurrently with or subsequent to the introduction of the immiscible liquid. The stabilized sol is converted into a gel by increasing the viscosity of the sol. The gel is next treated such that the immiscible liquid is dispersed into a plurality of droplets or vaporized into an interconnected gaseous network wherein the gel becomes cellular. The dispersant liquid and the immiscible liquid are removed from the cellular gel followed by removal of the surfactant and sintering to form the cellular inorganic material. The above process may be supplemented by the additional inclusion of an inorganic material selected from the group consisting of inorganic whiskers and inorganic fibers introduced into the dispersant liquid along with the inorganic particles.

Description

PROCESS FOR MAKING A LIGHTWEIGHT CELLULAR INORGANIC MATERIAL

The importance of lightweight, inorganic materials has been recognized in recent years what with its growing use as an acoustic and thermal insulator, a catalyst support, an electronic substrate and the like. These applications derive from the unique physical properties of lightweight inorganic materials which include low thermal conductivity, low density and low dielectric constant. Furthermore, its bulk

properties, which accrue from its size, shape and number of pores, also contribute to its use in these and other

applications.

There are two general classes of lightweight, inorganic materials. The first class are those fabricated from fibers, that is, shaped masses of fibers or whiskers, of which fiberglass is a prime example. Although these

lightweight materials have all the advantages associated with lightweight inorganic materials, they suffer from a major disadvantage. That is, when formed solely from fibers, the physical properties of the fabricated product are

anisotropic.

The second class of lightweight inorganic materials are those materials which possess pores in an otherwise dense inorganic matrix. Examples of this type of material include lightweight cements, foamed glass, replicated foams and sintered hollow spheres. Lightweight cements are made by stirring a composition of a surfactant and a cement. The bubbles created by the stirring action are stabilized by the surfactant until the cement hardens. Foamed glasses are formed by adding a decomposable compound to a glass powder which is heated. The compound decomposes with the resultant formation of a gas while the glass softens and melts. The bubbles created by the gas in the viscous, molten glass are stabilized and preserved by cooling. Replicated foams are prepared by adding a slurry of a powder into a polymeric foam. The thus infiltrated foam is dried then heated to decompose the polymer and sinter the powder.

The lightweight inorganic materials of the prior art, characterized by interspersed pores, like lightweight materials fabricated solely from fibers, suffer from

important disadvantages. These prior art materials are usually characterized by relatively large pore sizes.

Generally, lightweight cements, foamed glass, replicated foams and sintered hollow spheres all include pores greater than one millimeter in diameter. The lightweight,

pore-containing materials of the prior art also are limited by the number of material systems that could be used in their formation. Oftentimes, a glass, a cement or the like is not desired. However, the teaching of the prior art does not disclose porous inorganic materials other than porous glass or cement. Moreover, there are a number of defects specific to each class of interspersed porous material. For instance, replicated foams are characterized by hollow struts that lower their mechanical strength.

The above remarks suggest the general characteristics of a process needed to advance the art of lightweight cellular inorganic materials formation. A new process for making a lightweight, inorganic material should permit tailoring of the of the material's microstructure and

properties. For example, desired pore size, bulk density strength properties, electrical properties and the like should be obtainable within narrow limits. The processes of the prior art resulting in the formation of porous, lightweight inorganic materials do not meet this requirement.

The prior art includes a plurality of teachings directed to cellular inorganic materials relevant to the desired product discussed above. That is, the identified prior art is directed to processes which produce inorganic cellular materials whose pores are formed from a gas

generating agent.

U.S. Patent 3,136,645 to Dess and U.S. Patent 3,150,988 to Dess et al . define a process wherein silica, silicon, an aqueous sodium silicate solution, sodium

fluorosilicate and a quaternary ammonium halide surface active agent are intimately mixed with the resultant

generation of a gas from the reaction of silicon and sodium fluorosilicate. A mineral foam is formed from the foamed product of this reaction.

U.S. Patent 4,112,032 to Blaszyk et al. describes a process for making a porous silica-containing article. In this process a colloidal silica solution is reacted with an organic compound selected from the group consisting of formaldehyde, paraformaldehyde, formamide, glyoxal,

methyl formate, ethyl formate, methyl acetate, ethyl acetate and mixtures thereof at a temperature consistent with the maintenance of the liquid solution for a time sufficient to gel the porous product of this reaction.

A ceramic foam material is formed in a process disclosed in U.S. Patent 4,610,832 to Brockmeyer. The '832 patent involves a process in which an organic polymer foam, preferably polyurethane foam, is impregnated with an aqueous slurry of a thixotropic ceramic composition, preferably a gelled alumina hydrate. A colloidal dispersion of the gelled alumina hydrate is formed by adding an acid to a slurry of the alumina in water. The polymeric foam, impregnated with the colloidal dispersion, is fired at a temperature of at least 2,000°F for a period of time sufficient to volatilize the organic constituents and sinter the refractory material resulting in the formation of the ceramic foamed product.

Several additional references disclose the addition of organic foaming agents to ceramic colloids, slurries and the like. These foaming agents generate gases which cause foaming. In addition, other foaming materials are added to ceramic sols, solutions, colloids and the like wherein the reaction product is a foam generating gas. In these cases inorganic materials are reacted with reactive components of a ceramic solution, especially alkali metal silicates. These teachings result in the formation of large pore size foamed inorganic ceramic materials. These products, although having high strength to weight ratios, do not possess sufficient strength to be useful in high strength applications.

A new process has now been discovered which allows tailoring of the properties of lightweight cellular inorganic materials. This process permits formation of lightweight cellular inorganic materials having desired physical

structure and properties. For example, an inorganic cellular material characterized by pores which are considerably smaller than the cellular products formed from the known prior art processes for making cellular inorganic materials can be produced. As such, the process of the present invention is far more flexible in producing desirable inorganic cellular materials than are the processes for making cellular inorganic products produced by the processes of the prior art.

In accordance with the present invention, a process for making a cellular inorganic material is disclosed. In this process finely divided particles, having a size of less than about 1,000 Angstroms, of an inorganic material are disposed in a dispersant material, a liquid having the structural formula ROH, where R is hydrogen or lower alkyl. An organic material in the liquid state, immiscible in said sol, is then introduced into said sol. A stabilizing

effective amount of a surfactant is added to the sol prior to, concurrently with or subsequent to the introduction of the immiscible organic liquid. The viscosity of the

so-treated sol is increased such that a gel is formed. The gel is treated to disperse the immiscible organic material into a plurality of droplets or vaporized into an

interconnected gaseous network wherein the gel becomes cellular. The cellular gel is next treated to remove the dispersant material and the immiscible organic material. The surfactant is then removed. Finally, the cellular gel is sintered to form the cellular inorganic material.

In further accordance with the present invention, a process is provided which includes the steps of the process recited above with the additional step of including, in addition to the inorganic particles, an inorganic material selected from the group consisting of inorganic whiskers and inorganic fibers. The inorganic material is present in a concentration such that the inorganic material comprises between about 1% to about 50% by volume on a dry volume basis. The process of the present invention involves the formation of a cellular inorganic material. In this process, a finely divided inert inorganic material having a particle size of less than about 1,000 8. is disposed in a liquid.

Those skilled in the art are aware that such inorganic oxides as silica, alumina, zirconia, magnesia, beryllia, titania and mixtures thereof are within the contemplation of this

inorganic genus. However, these inorganic oxides are only illustrative of preferred species within this generic class and any inorganic material having a particle size of less than about 1000 A may be employed. In addition, mixtures of these materials, whether natural ores or formulated

compounds, can be utilized in this application. For example, mullite, a natural mineral, a compound having the structural formula 3Al₂O₃.2SiO₂, as may formulated compounds such as ferrite, barium titanate, BaTiO₃, the complex oxide,

Pb(Zr,Ti)O₃ and the silicon nitride, Si₃N₄, can be used alone, with each other or with one or more of the

aforementioned inorganic oxides.

The dispersant material into which the finely divided inorganic particles are disposed is, in one preferred embodiment, a liquid having the structural formula ROH, where R is hydrogen or lower alkyl. An obvious preferred

embodiment of this class of liquids is water. However, lower alkanols such as methanol, ethanol, n-propanol, isopropanol, n-butanol, isobutanol, tertiary butanol and the like may also be utilized as the dispersant liquid.

To this sol or hydrosol is added a liquid, an organic material immiscible in the liquid dispersant. Any organic material in the liquid state immiscible in the dispersant liquid, a liquid having the structural formula ROH, whether having a higher or lower boiling point than the liquid dispersant may be employed.

Because immiscibility is critical, the organic liquid, in this preferred embodiment, is usually non-polar in that the dispersant liquid, a liquid having the structural formula ROH, is polar. Those skilled in the art are aware that polar and non-polar liquids are oftentimes immiscible in each other.

Among the non-polar organic liquids within the contemplation of the immiscible material of this invention are saturated and unsaturated hydrocarbons substituted or unsubstituted with halogen which are liquid at ambient conditions. Of these, halogen substituted alkanes are preferred. For example, fluoro-chloroalkanes, particularly fluorochloromethanes and fluorochloroethanes, find utility in this application. Thus, trichlorofluoromethane, difluorodichloromethane, trifluorochloromethane and other fluorochloro-substituted methanes are oftentimes used in this application. Other immiscible hydrocarbons that may be used in this process include alkanes, such as butane, pentane, hexane and the like, alkenes, which include pentenes and hexenes, and aryl halides such as chlorobenzene and

chlorotoluenes among others.

In another preferred embodiment of the present invention the dispersant and the immiscible organic liquids are reversed. That is, the dispersant material is a

non-polar organic liquid and the immiscible material is a polar liquid. In this embodiment the preferred and more preferred embodiments of the polar liquid used as the

dispersant liquid in the embodiment described above are utilized as the preferred and more preferred embodiments of the immiscible liquid. Thus, the immiscible liquid is preferably characterized by the structural formula ROH, where R is hydrogen or lower alkyl, with water being particularly preferred. Similarly, the preferred and more preferred embodiments of the dispersant liquid correspond to the preferred and more preferred embodiments of the materials utilized as the immiscible organic liquid in the first recited preferred embodiment. Therefore, the dispersant liquid is preferably a saturated or unsaturated hydrocarbon which may be halogen substituted.

To aid in the later gelation of the finely divided particles dispersed in the dispersant liquid, often referred to as a sol, or a hydrosol when the dispersant liquid is water, a gelation aiding agent is optionally added to the sol or hydrosol. The gelation aiding agent is added to the sol or hydrosol for the obvious purpose of increasing the rate of gelation formation. Those skilled in the art are aware, for example, that hydrosols are alkaline, characterized by a pH in excess of 7. Reduction of hydrosol alkalinity to a pH below 7, that is, acidifying the hydrosol, results in more rapid gelation. Thus, in a preferred embodiment, the

optional step of introducing a gelation aiding agent involves the addition of an acid to the sol. Preferably, the acid utilized is a mineral acid. Among the preferred mineral acids for use in this application are hydrochloric acid, sulfuric acid and nitric acid. Of these, sulfuric acid is particularly preferred.

In other sols the gelation aiding agent may be alkaline. In summary, therefore, the gelating aiding agent is just that, an agent that aids in gelling the sol. In certain sol systems the chemistry is such that higher pH enhances gelation. Thus, an alkalinating agent such as NaOH,

KOH, NH₃ and the like may be employed in this application.

In order to form a cellular material it is required that the immiscible liquid be uniformly dispersed in the dispersant liquid. To accomplish this a stabilizing

effective amount of surfactant is added to the sol of finely divided particles in the dispersant liquid. The surfactant may be added to the sol prior to, concurrently with or subsequent to the introduction of the immiscible organic liquid. Alternatively, the surfactant may be added in part. That is, the surfactant is partially added at two or all three of these points in the process of this invention. Any surfactant which disperses the immiscible liquid in the dispersant liquid may be utilized. Thus, the surfactant may be cationic, anionic, non-ionic or amphoteric. Of these, anionic and cationic surfactants have been found to be especially effective. Anionic surfactants useful in this application include sodium dodecyl sulfate (SDS),

polyalkoxycarboxylates and the like. SDS is particularly preferred. Among the cationic surfactants within the

contemplation of the present invention are aliphatic mono-, di- and polyamines, amine oxides, alkylamine alkoxylates, especially alkylamine ethoxylates, quaternary ammonium salts and the like. For example, cetyltrimethylammonium bromide (CTAB) is particularly preferred for use as a cationic surfactant.

An optional additional step may be included in the process of the present invention. In this step a

co-surfactant, different from the surfactant, is added to the sol or hydrosol. A co-surfactant, for example, is employed to better emulsify the preferred embodiment wherein an immiscible organic liquid is added to a polar dispersant liquid to aid in the stabilization of gas bubbles that may subsequently form in this process. That is, the

co-surfactant supplements the activity of the surfactant.

The use of a co-surfactant is particularly advisable in those cases where, although the surfactant adequately disperses the organic immiscible liquid in the dispersant liquid, the surfactant does not provide adequate stabilization of the immiscible organic liquid. For example, in the preferred embodiment wherein a halogenated alkane is employed as the immiscible organic liquid it is known that alkanols aid in its dispersion. This is especially the case when the dispersant liquid is water. Of course, if an alkanol is utilized as the dispersant liquid, it is obvious that the alkanol used as the co-surfactant is not identical to the alkanol employed as the dispersant liquid. In the particularly preferred embodiment wherein a fluorochlorocarbon is utilized as the immiscible organic liquid in the dispersant liquid water, and wherein the surfactant is an anionic or cationic surfactant, it is particularly desirable to use an alkanol as a co-surfactant. Of the alkanols, methanol is particularly preferred. The use of methanol as a co-surfactant significantly improves the emulsification of the fluorochlorocarbon in the sol. This, in turn, increases the cellular uniformity of the final product.

The optional step of adding a co-surfactant to the sol or hydrosol preferably occurs simultaneously with the addition of the surfactant. Thus, the co-surfactant is added prior to the introduction of, simultaneously with or

subsequent to the addition of the immiscible organic liquid. Alternatively, the co-surfactant may be introduced into the sol in part, that is, in two or in all three points where the co-surfactant may be added.

The thus treated sol or hydrosol is next converted into a gel. In this step the viscosity of the sol or

hydrosol is increased to a level at which sol becomes a gel.

In a preferred embodiment this occurs by increasing the viscosity of the sol to between about 80 cp and about

1500 cp. To accomplish gelation, as those skilled in the art are aware, the sol may be aged, that is, the sol is allowed to remain untreated for a period of time. However, gelation by aging may take an unacceptably long period of time.

Therefore, to accelerate this procedure, the sol may be heated, its pH changed or both.

Cell generation is initiatd immediately subsequent to gelation. Cell generation to convert the thus formed gel to a cellular gel occurs by treatment of the gel. In one preferred embodiment, this treatment involves heating the gel to a temperature below the boiling point of the dispersant liquid. This either results in the formation of dispersed liquid droplets or of a gaseous network of the immiscible organic in the dispersant liquid.

In another preferred embodiment, cell generation, resulting from dispersed liquid droplets or a vaporized gaseous network of the immiscible material, can be

effectuated by change in pressure. That is, the pressure can be reduced to a level which produces the same result as that provided by temperature increase.

Whether cellular formation results in an open- celled network or a closed-celled structure can depend on whether the above discussed step results in the formation of liquid droplets or a gaseous network. This, in turn, depends upon whether the immiscible liquid or the dispersant liquid has the higher vapor pressure. Those skilled in the art will appreciate that formation of a multiplicity of droplets of immiscible liquid can result in a closed cellular product, while the formation of interconnected gaseous network usually effects the formation of an open celled product. Those skilled in the art will understand, of course, that even if the immiscible liquid has a higher vapor pressure than the dispersant liquid a sufficiently high concentration of dispersed liquid droplets will result in interconnection of these droplets causing the formation of an interconnected open- celled network. Similarly, a small enough

concentration of vaporized immiscible material will not provide enough pathways to insure an open-celled structure.

The above remarks suggest the preferred embodiment of the present process wherein the steps of gelation and cellular formation occur substantially simultaneously. In this combined step, the sol is either heated to a temperature below the boiling temperature of the dispersant liquid at atmospheric pressure or the temperature of the sol is

maintained constant while the pressure is reduced to a pressure above the vapor pressure of the dispersant liquid at ambient temperature. This not only increases the viscosity of the sol, preferably to between about 80 cp and about 1500 cp, and thus effects gelation, but, in addition, disperses or vaporizes the immiscible organic liquid to form a closed or open celled gel, respectively.

The step of cellular formation is coincident with the step of immiscible organic liquid removal in the

preferred embodiment wherein the immiscible liquid has a lower vapor pressure than the dispersant liquid. That is, the vaporized immiscible organic material escapes through the interconnected network. Preferably, the coincident step of cellular formation and immiscible organic compound removal occurs at a temperature in the range of ambient and about

200°C, more preferably, between ambient and about 150°C and most preferably, between ambient and about 100°C.

The thus formed cellular gel is next treated to remove the dispersant liquid. This step is accomplished, in one preferred embodiment, at atmospheric pressure by exposing the cellular gel containing dispersant to a temperature in the range of between about 20°C and about 100°C. Preferably, this step occurs at a temperature of between about 25°C and about 75°C. More preferably, the dispersant-containing cellular gel is exposed to a temperature in the range of between about 35°C and about 50°C. This procedure occurs over a period of between about 1 day and about 1 week.

Preferably, the time period over which drying occurs is between about 2 days and about 3 days.

In another preferred embodiment, the dispersant- containing cellular gel is exposed to a reduced pressure environment wherein the same dispersant removal result is obtained as in the above-discussed embodiment.

in the preferred embodiment wherein the vapor pressure of the immiscible organic liquid is greater than that of the dispersant liquid the step subsequent to cellular gel formation is the removal of the dispersant liquid. The removal of the dispersant liquid is substantially identical with the step of dispersant liquid removal in the preferred embodiment wherein the dispersant liquid has a higher vapor pressure than the immiscible liquid. Thus, the preferred procedures mentioned therein apply to this embodiment.

The following step, in this embodiment wherein the immiscible organic liquid has a higher vapor pressure than the dispersant liquid, occurs under conditions very similar to the removal of the dispersant liquid. That is, it occurs at a temperature in the range of between about 20°C and about 100°C. Preferred embodiments generally in accordance with the preferred procedures utilized in the dispersant removal step albeit, in many cases, at slightly higher temperatures or slightly lower pressures over slightly longer periods of time are used in this immiscible material removal step.

The cellular gel, free of dispersant liquid, whether open or closed cell, is next treated to remove the surfactant constituent therefrom, thus converting the

cellular gel to a cellular inorganic gel. This step of removing the surfactant involves thermodynamic treatment of the foamed gel. In a preferred embodiment, the cellular gel is heated to a temperature in the range of between about 300°C and about 500°C over a period of about 1 day to about 3 days. More preferably, the surfactant removal step involves exposing the foamed gel to a temperature in the range of between about 350°C and about 450°C for a period in the range of about 1½ to 2 ½ days. More preferably, the surfacant removal step involves heating the foamed gel at a temperature in the range of between about 375°C and about 425°C for about 2 days.

This thermodynamic treatment which constitutes the surfactant removal step is be-ieved to effect a chemical reaction of the organic compound which serves as the

surfactant, with oxygen to form the carbon oxides, carbon monoxide and/or carbon dioxide. This gaseous carbon oxides product easily escapes from the gel leaving a surfactant- free, i.e., organic-free gel. Of course, this theory

explaining surfactant removal is just that and the invention is independent of any theory explaining its operability.

The cellular, organic-free gel resulting from the organic constituent removal step is, in a last step, sintered to produce the final cellular inorganic product. The

sintering step involves heating the organic-free gel at a sintering effective temperature. As those skilled in the art are aware, a sintering effective temperature is one at which densification of the material being sintered begins. In a preferred embodiment, where the inorganic particles are silica, the sintering step occurs at a temperature in the range of between about 900°C and about 1200°C. More

preferably, sintering occurs at a temperature in the range of between about 1000°C and about 1100°C to produce the cellular inorganic product of the present invention.

In another preferred embodiment, wherein the inorganic particles are silicon nitride, sintering occurs at a temperature in the range of between about 1800°C and about 2200°C, more preferably, between about 1900°C and about

2100°C.

In still another preferred embodiment, the preferred embodiment wherein the inorganic particles are alumina, the sintering effective temperature is in the range of between about 1200°C and about 1800°C, more preferably, between about 1400°C and about 1600°.

The above discussion of sintering temperature emphasizes that sintering preferably occurs at a temperature in the range of between about 900°C and about 2200°C with the above specific ranges preferred depending upon the identity of the inorganic particles.

Sintering occurs over a period of about 0.5 hour to about 4 hours. More preferably, the term over which

sintering occurs is in the range of between about 0.75 hour and about 2 hours. Most preferably, sintering occurs over a period of between about 0.8 hour and about 1.5 hours. The sintering step can occur in any environment. That is, sintering can be conducted in a vacuum or in the presence of a gas which may be reactive or inert. Among the reactive environments, the most commonly utilized is air. Of the inert gases, sintering in nitrogen is most preferred, although an atmosphere of argon is also often employed. Of all these possibilities, sintering in air is most preferred.

The above detailed description is applicable to another preferred embodiment of the present invention. This preferred process is identical with the processes defined above with the additional inclusion of an inorganic material in the dispersant liquid along with the inorganic particles. The inclusion of an inorganic material, in addition to inorganic particles, allows for refinement and tailoring of properties of the resultant cellular product to meet

requirements particular to specific applications including those requiring higher tensile and/or compressive strength, improved ductility and toughness and necessary dielectric properties.

The inorganic material, selected from the group consisting of inorganic whiskers and inorganic fibers, is added to the dispersant liquid along with the inorganic particles in a concentration such that the organic material represents between 1% and about 50% by volume on a dry volume basis. That is, the inorganic material represents between about 1% and about 50% by volume based on the total volume of the inorganic particles and the inorganic material.

Preferably, the inorganic material is present in a

concentration representative of a concentration of between about 5% and about 50% by volume on a dry volume basis. More preferably, the inorganic material constitutes between about 10% and about 40% by volume on a dry volume basis. Still more preferably, the inorganic material is found in a

concentration in the range of between about 15% and about 30% by volume on a dry valume basis. It is particularly

preferable that the inorganic material represents about 20% by volume on a dry volume basis. The inorganic material, as stated above, is selected from the group consisting of inorganic whiskers and inorganic fibers. Whether whiskers or fibers, the inorganic material is preferably characterized by an aspect ratio, i.e., a length to width ratio, of at least more than about 5:1. Those skilled in the art are aware that whiskers are defined as mono-crystalline materials whereas fibers are polycrystalline. Inorganic materials within the

contemplation of the present invention include inorganic whiskers and inorganic fibers formed from silicon carbide, silicon nitride, alumina and mullite, among others. Of these preferred compounds and natural occurring ores, the use of silicon carbide, especially silicon carbide whiskers, is particularly applicable for use in this preferred embodiment of the present invention.

It should be appreciated that the process of preparing inorganic cellular products, which includes an inorganic material, comprises all the steps recited

hereinbefore except for the inclusion of the additional step of adding the inorganic material to the dispersant liquid prior to, simultaneously with or subsequent to the

introduction of the inorganic particles to the dispersant liquid.

The present invention not only contemplates the processes described above but also the products produced in these processes. That is, the cellular inorganic materials produced in accordance with the above-defined processes are within the contemplation of the present invention.

The following examples are given to illustrate the scope of the present invention. Because these examples are given for illustrative purposes only, the process of the present invention should not be limited thereto. EXAMPLE 1

Preparation of a Cellular Material from a Sol

A silica sol comprising 40% by weight silica in water, stabilized with 0.41% by weight sodium oxide, was utilized. Although the silica sol could have been produced by adding finely divided silica to water, a commercially available silica sol formulation, Ludox [trademark] HS-40, was employed. To this silica sol was added sulfuric acid in a concentration sufficient to reduce the original pH of the silica sol, 9.7, to a pH of 5.25. The sol was allowed to indubate for a period of 30 minutes.

The increase in acidity of the silica to a pH of 5.25 and incubation for 30 minutes resulted in a

corresponding increase in the viscosity of the sol to 80 cp. A surfactant, sodium dodecyl sulfate (SDS), was introduced into the acidified sol. The amount of SDS added to the sol was such that after its addition the SDS concentration in the silica sol was 5.8 x 10^-3 moles of SDS per liter of silica sol. An immiscible organic liquid, trichlorofluoromethane, better known by its trademark, Freon-11, was next introduced into the silica sol. The Freon-11 was added in a 50%

methanol solution, with the methanol serving not only as a Freon-11 solvent but also as a co-surfactant. It was

provided in a volume such that the concentration of the solution constituted 3.3% by volume, based on the total volume of the sol. That is, a volume of the 50% solution was added so that the Freon-11 solution constituted 3.3% by volume of the total volume. Thus, Freon-11 was present in the sol in a volume concentration of 1.65%, based on the total volume of the sol. All of the above steps were conducted at a

temperature of 20°C. Therefore, the hydrosol was at a temperature of 20°C. At this point the temperature of the hydrosol was increased to 30°C and allowed to age at this temperature for 30 minutes. To insure that this 30°C

temperature was maintained constant, the beaker containing the sol was maintained in a temperature bath. At the end of this period gelation of the sol, a viscosity of 80 cp was initiated. At this point, stirring of the gel with a

magnetic stirrer was begun. Stirring promoted foaming which reached its maximum height in about 1 minute.

The foamed gel, disposed in a beaker, which was covered to prevent evaporation, was stored at 30°C for 12 hours . The foamed gel was stored for an additional five days at room temperature with the beaker still covered to reduce gel drying. The cover was thereupon removed and the foamed gel was allowed to remain at 30°C for an additional 24 hours.

The partially dried foamed material resulting from the above procedure, was disposed in an oven and remained heated at a temperature of 70°C for one week. Thereupon, the foamed material was transferred to a higher temperature oven, an oven maintained at 400°C, and therein heated for two days. Finally, the foamed material was sintered in air at a

temperature of approximately 1100°C to produce a cellular inorganic material.

The thus formed cellular inorganic material was cut into disk-shaped samples. The average pore diameter of the disk-shaped samples was measured on diamond sawed surfaces and the average pore size calculated by multiplying the average measured diameter by 1.22 in accordance with the procedure enumerated in R.L. Fullman, "Measurement of

Particle Sizes in Opaque Bodies," Journal of Medals, 565-575 (1953). The quantity and size of the microporosity were determined by mercury porosimetry. The results of this measurement established that the cellular material had a relative bulk density of 17% Relative bulk density, as those skilled in the art are aware, is a ratio, expressed in percent, of the total weight to the total volume of the material divided by the theoretical density of the material. The inorganic cellular product was determined to have an average cell size of 400 microns. This example is summarized in the Table below.

EXAMPLES 2 TO 5 Preparation of Additional Cellular Materials

The procedure of Example 1 was repeated in four additional examples. These examples, denoted as Examples 2 to 5, differed from Example 1 in that the viscosity of the sol at the onset of gelation was varied by changing the pH, caused by the addition of different concentration of sulfuric acid, and the period of incubation. These variables were adjusted so that the viscosity of the hydrosol of each example was in excess of the viscosity of lower numbered example. It is emphasized, however, that each of these examples was within the scope of the instant invention.

The results of these examples are summarized in the Table.

COMPARATIVE EXAMPLE 1

Preparation of a Cellular Material

Example 1 was repeated except that the amount of sulfuric acid added to the silica sol decreased the pH of the sol to 5.80. Moreover, the incubation period was limited to 18 minutes. This combination of factors resulted in an increase of viscosity of the silica sol to only 30 cp, below the minimum sol viscosity required by the present invention.

At this viscosity, the subsequent steps, conducted in accordance with the procedure of Example 1, did not result in foaming and gelation and no cellular product could be formed.

For completeness, this example is included in the summary provided by the Table.

COMPARATIVE EXAMPLE 2

Preparation of a Cellular Material

Example 1 was repeated but for the steps of hydrosol acidification and incubation. Acidification, by adding sulfuric acid to the hydrosol, was reduced to produce a hydrosol having a pH of 5.80, instead of 5.25. Moreover, the period of incubation was reduced from 30 minutes to 29 minutes. The combined effect of these changes resulted in increase of the viscosity of the hydrosol to 15,000 cp.

Subsequent processing of the hydrosol into a cellular material in accordance with Example 1 resulted in the formation of a cellular material having a cell size of 180 microns and a relative bulk density of 40%. This density is far too higher to be considered a lightweight cellular material having the desirable properties of inorganic materials sought in the process of the present invention.

EXAMPLE 6

Preparation of a Cellular Material from Inorganic Particles and Whiskers

The process of Example 3 was repeated but for the addition of silicon carbide whiskers to the silica sol Ludox [trademark] HS-40, which in this example was stabilized with Na₂SO₄. The SiC whiskers (F-9, manufactured by Composite Material Corp., Greer, S.C.) were added to the silica sol in a concentration such that the SiC whiskers represented 20% by volume, based on the total volume of silica particles and silicon carbide whiskers (20 vol % on a dry solid basis).

Upon addition of the SiC whiskers, the mixture was milled in a plastic jar for eight hours. Thereupon the procedure of Example 1 was substantially repeated but for the presence of the silicon carbide whiskers.

The cellular material formed in accordance with this example was characterized by approximately the same relative density, 20%, as in Example 3.

The cellular product of this example was tested to determine its flexural strength. Flexural strength was obtained by the three point bending method, as defined by F.I. Baratta, "Requirements for Flexure Testing of Brittle Materials," AMMRC TR 82-20, Watertown, MA, April, 1982, which technical paper is incorporated herein by reference, at a loading rate of 0.5 mm per minute. It was found that the flexural strength of the sample was about 7.0 MPa. For comparison the sample of Example 3 was tested and found to possess a flexural strength of about 2.9 MPa. The above embodiments and examples are given to illustrate the scope and spirit of the present invention. These embodiments and examples will make apparent, to those skilled in the art, other embodiments and examples. These other embodiments and examples are within the contemplation of the present invention. Therefore, the present invention should be limited only by the appended claims.

Claims

WHAT IS CLAIMED IS:

1. A process for making a cellular inorganic material comprising:

(a) forming a sol by disposing inorganic particles having a size of less than about 1000 Å in a dispersant liquid, said dispersant liquid having the structural formula ROH, where R is hydrogen or lower alkyl;

(b) adding an organic material, a liquid immiscible in said dispersant liquid, to said sol;

(c) introducing a stabilizing effective amount of a surfactant to said sol, said surfactant added to said sol pri"br to, concurrently with or subsequent to said

introduction of said immiscible organic liquid;

(d) increasing the viscosity of said sol wherein a gel is formed;

(e) treating said gel such that said immiscible organic liquid is dispersed into a plurality of liquid droplets or vaporized into an interconnected gaseous network wherein the gel becomes cellular;

(f) removing said immiscible organic material and said dispersant liquid from said cellular gel with the proviso that the first removed of said immiscible organic and said dispersant liquid has the lower yapor pressure;

(g) removing said surfactant from said immiscible organic-free and dispersant-free cellular gel; and

(h) sintering said dispersant surfactant-free cellular gel.

2. A process in accordance with Claim 1 including the step of acidifying said sol prior to the introduction of said surfactant, whereby the pH of the sol is reduced to a range of between about 5 and about 6.

3. A process in accordance with Claim 2 wherein said acidification occurs by adding a mineral acid to said sol.

4. A process in accordance with Claim 3 wherein said mineral acid is selected from the group consisting go hydrochloric acid, sulfuric acid and nitric acid.

5. A process in accordance with Claim 4 wherein said mineral acid is sulfuric acid.

6. A process in accordance with Claim 1 comprising the step of introducing a co-surfactant into said sol simultaneous with the introduction of said surfactant, simultaneously with the introduction of said immiscible organic liquid, prior to the introduction of said surfactant and said immiscible organic liquid or subsequent to the introduction of said surfactant and said immiscible liquid.

7. A process in accordance with Claim 6 wherein said co-surfactant is an alkanol.

8. A process in accordance with Claim 1 wherein said particles having a size of less than about 1000 A is an inert inorganic material.

9. A process in accordance with Claim 8 wherein said inert inorganic particles are selected from the group consisting of silica, alumina, zirconia, magnesia, beryllia, mullite, ferrite, barium titanate, Pb(Zr,Ti)O₃, silicon nitride and mixtures thereof.

10. A process in accordance with Claim 9 wherein said inorganic particles are silica.

11. A process in accordance with Claim 1 wherein said surfactant is selected from the group consisting of anionic and cationic surfactants.

12. A process in accordance with Claim 11 wherein said surfactant is sodium dodecyl sulfate.

13. A process in accordance with Claim 11 wherein said surfactant is cetyltrimethylammonium bromide.

14. A process in accordance with Claim 1 wherein said immiscible organic liquid is selected from the group consisting of a saturated hydrocarbon, an unsaturated

hydrocarbon, a saturated hydrocarbon substituted with

halogen and an unsaturated hydrocarbon substituted with halogen.

15. A process in accordance with Claim 1 wherein the viscosity of the sol in step (d) is increased to a range of between about 80 cp and 1,500 cp.

16. A process in accordance with Claim 1 wherein said step of forming a cellular gel, step (e), comprises heating said gel to a temperature below the boiling point of said dispersant liquid.

17. A process in accordance with Claim 16 wherein said immiscible organic has a lower vapor pressure than said dispersant.

18. A process in accordance with Claim 17 wherein said immiscible organic liquid is a fluorochlorocarbon.

19. A process in accordance with Claim 16 wherein said immiscible organic has a higher vapor pressure than said dispersant.

20. A process in accordance with Claim 18 wherein said step of removing said immiscible organic from said cellular gel comprises heating said cellular gel to a

temperature in the range of between ambient and about 200°C.

21. A process in accordance with Claim 1 wherein said step of removing said dispersant liquid from said cellular gel comprises heating said cellular gel to a

temperature in the range of between about 20°C and about 100°C for a period of between about 1 day and 1 week.

22. A process in accordance with Claim 1 wherein said step of removing said surfactant from said cellular gel, step (g), comprises heating said cellular gel to a

temperature in the range of between about 300°C and about

500°C over a period of time in the range of about 1 day and about 3 days.

23. A process in accordance with Claim 1 wherein said sintering step, step (h), comprises heating said sur±actant-free cellular gel at a sintering effective temperature.

24. A process in accordance with Claim 23 wherein said sintering step occurs at a temperature in the range of between about 900°C and about 2400°C.

25. A process for making a cellular inorganic material comprising:

(a) forming a silica hydrosol by disposing silica particles, having a size less than about 1000 Å, in water;

(b) adding an organic liquid, immiscible in water, to said silica hydrosol;

(c) introducing a stabilizing effective amount of a surfactant to said hydrosol, said surfactant added prior to, simultaneously with or subsequent to said introduction into said hydrosol of said immiscible organic liquid;

(d) adding a co-surfactant to said hydrosol

concurrent with or subsequent to said introduction of said immiscible organic liquid;

(e) increasing the viscosity of said hydrosol to a range of between about 80 cp and about 1500 cp wherein said hydrosol is converted into a gel;

(f) converting said gel intp a cellular gel by heating said gel to a temperature below the boiling point of the dispersant liquid;

(g) heat treating said cellular gel to remove said immiscible organic liquid;

(h) removing said dispersant liquid from said immiscible organic-free cellular gel;

(i) removing said surfactant from said dispersant liquid-free cellular gel; and

(j) forming a cellular inorganic material by sintering said surfactant-free cellular gel in air.

26. A process in accordance with Claim 25 wherein said step (e), said step of increasing the viscosity of said hydrosol, is effectuated by acidification and incubation of said hydrosol.

27. A process in accordance with Claim 26 wherein said acidification results in a decrease in the pH of said hydrosol to between about 5 to about 6.

28. A process in accordance with Claim 25 wherein said surfactant added in step (c) is sodium dodecyl sulfate.

29. A process in accordance with Claim 28 wherein said immiscible organic liquid added in step (b) is a fluorochlorocarbon.

30. A process in accordance with Claim 29 wherein said fluorochlorocarbon is trichlorofluoromethane.

31. A process in accordance with Claim 28 wherein said co-surfactant, added in step (d), is an alkanol.

32. A process in accordance with Claim 31 wherein said alkanol is methanol.

33. A process in accordance with Claim 32 wherein said methanol is introduced into said hydrosol concurrently with the addition of said immiscible organic liquid.

34. A process in accordance with Claim 25 wherein step (g), said step of heat removing said immiscible organic from said foamed gel, comprises heating said cellular gel to a temperature in the range of between ambient and about 200°C.

35. A process in accordance with Claim 25 wherein step (h), said step of removing said dispersant liquid from said foamed gel, comprises heating said cellular gel to a temperature in the range of between about 25°C and 75°C over a period of between about 2 days and about 3 days.

36. A process in accordance with Claim 25 wherein said step (i), said step of removing said surfactant, comprises heating said cellular gel to a temperature in the range of between about 350°C and about 450°C for a period in the range of between about 1½ days and about 2½ days.

37. A process in accordance with Claim 25 wherein step (j), said sintering step, occurs at a temperature in the range of between about 1000°C and about 1100°C.

38. A process for making a cellular inorganic material comprising:

(a) forming a sol by disposing inorganic particles, having a size of less than about 1000 Å and an inorganic material selected from the group consisting of inorganic whiskers and inorganic fibers, said inorganic material present in a concentration such that said inorganic material comprises between about 1% and about 50% by volume on a dry volume basis, in a dispersant liquid, said dispersant liquid having the structural formula ROH, where R is hydrogen or lower alkyl;

(b) adding an organic liquid, immiscible in said dispersant liquid, to said sol;

(c) introducing a stabilizing effective amount of a surfactant to said sol, said surfactant added to said sol prior to, concurrently with or subseqμent to said

introduction of said immiscible organic liquid;

(d) increasing the viscosity of said sol wherein a gel is formed;

(e) treating said gel such that said immiscible organic liquid is dispersed into a plurality of liquid droplets or vaporized into an interconnected gaseous network wherein a cellular gel is formed;

(f) removing said immiscible organic and said dispersant liquid from said cellular gel;

(g) removing said surfactant from said immiscible organic-free and dispersant liquid-free cellular gel;

(h) sintering said surfactant-free cellular gel.

39. A process in accordance with Claim 28 wherein said inorganic material comprises between about 5% and about 50% by volume, based on total volume of said inorganic particles and said inorganic material.

40. A process in accordance with Claim 39 wherein said inorganic material is present in a concentration such that it represents between about 10% and about 40% by volume, based on the total volume of said inorganic particles and said inorganic material.

41. A process in accordance with Claim 40 wherein said inorganic material is present in a concentration such that it represents between about 15% and about 30% by volume, based on the total volume of said inorganic particles and said inorganic material.

42. A process in accordance with Claim 38 wherein said inorganic material has an aspect ratio of at least more than about 5:1.

43. A process in accordance with Claim 38 wherein said inorganic material is selected from the group consisting of silicon carbide, alumina, silicon nitride, mullite and mixtures thereof.

44. A process in accordance with Claim 43 wherein said inorganic material is monocrystalline inorganic

whiskers.

45. A process in accordance with Claim 44 wherein said inorganic material is silicon carbide whiskers.

46. A process in accordance with Claim 43 wherein said inorganic material is polycrystalline inorganic fiber.

47. A product made in accordance with the process of Claim 1.

48. A product made in accordance with the process of Claim 25.

49. A product made in accordance with the process of Claim 38.

50. A process for making a cellular inorganic material comprises:

(a) forming a sol by disposing inorganic particles having a size of less than 1000 51 in a non-polar organic liquid;

(b) adding a polar material, in the liquid state, immiscible in said non-polar organic dispersant liquid, to said sol;

(c) introducing a stabilizing effective amount of a surfactant to said sol, said surfactant added to said sol prior to, concurrently with or subsequent to said

introduction of said immiscible polar liquid;

(d) increasing the viscosity of said sol wherein a gel is formed;

(e) treating said gel such that said immiscible polar liquid is dispersed into a plurality of liquid droplets or vaporized into an interconnected gaseous network wherein said gel becomes cellular;

(f) removing said immiscible polar material and said non-polar dispersant from said cellular gel;

(g) removing said surfactant from said cellular gel; and

(h) sintering said cellular gel.

51. A product made in accordance with the process of Claim 50.