Cellular Ceramics / p2

2.1 Ceramic Foams 41

(a)

(b)

Fig. 7 Relation between window size and a) mean cell size and b) internal surface area per unit volume for reticulated ceramics; after Ref. [16]. (L.J. Gauckler and M.M. Waeber, in Light Metals 1985, Proc. 114th Ann. Meet. Metal. Soc. AIME, 1985, pp. 1261–1283.) Reprinted with permission of The Minerals, Metals and Materials Society.

42Part 2 Manufacturing

2.1.3

Foaming Techniques

The foaming of ceramic slurries involves dispersing a gas in the form of bubbles into a ceramic suspension. There are two basic approaches for achieving this: 1) incorporating an external gas by mechanical frothing, injection of a gas stream, or application of an aerosol propellant, and 2) evolution of a gas in situ. In most cases the addition of a surfactant is required to reduce the surface tension of the gas–liquid interfaces and thus stabilize the gas bubbles developed within the slurry. Such stabilization only works for a limited period of time, however, since several transformations in the bubble structure may subsequently occur due to thinning of their surrounding lamellae. This necessitates a further mechanism to provide a longer term form of stabilization, since any changes in the foam structure prior to solidification influence the final cell size distribution, wall thickness, and microstructure of the solid foams. These in turn have a major role in determining properties such as permeability and strength. For example, when the films surrounding bubbles remain intact until solidification, a closed-cell foam is formed. Open-celled foams are produced when the films partially rupture. In extreme cases, excessive film rupture can lead to foam collapse.

The sections below examine each of the options associated with the different foaming techniques in turn. The process of foaming itself is a well-documented science and although the majority of the literature available relates to the production of foams from surfactant solutions, the same principles apply when foaming ceramic slurries. Therefore, a discussion of the important factors necessary in producing a stable foam, such as the effect of surfactants on surface tension, film elasticity, and foam persistence, will not be reviewed here. Interested readers are referred to treatises such as that by Rosen [17].

2.1.3.1

Incorporation of an External Gas Phase

Various processes have been patented for the manufacture of foam ceramics by entraining an external gas phase [18–20]; all of them incorporate a material that orientates itself at the gas–water interface, thereby stabilizing the gas phase, and one or more additives that cause the structure to set prior to drying and firing.

One of the earliest, patented in 1967 by Du Pont [18], focused on the production of foams from aqueous suspensions of very fine (< 200 nm), negatively charged colloidal silica particles and a cationic surfactant. Other additives included materials that served as binding agents to stabilize the foams, and the wide range listed in the patent includes water-soluble polymers, sugars, starches, resins, and gums. Once formed, the slurries were converted to foams by mechanical agitation by a wide range of approaches, including beateror blender-type mixers, injection of a stream of gas, and introduction of an aerosol propellant. Once formed into the desired shape of product, the final step involved drying the foam prior to sintering.

A slightly more sophisticated approach was patented by Mitsubishi Chemical Industries [19] for the production of alumina-based foams. The process consisted of

2.1 Ceramic Foams 43

mixing gibbsite powder and at least one other powder selected from a group consisting of pseudoboehmite, amorphous aluminum hydroxide, and/or alumina cement. These powders were ground and then mixed with water to form a slurry. A thickener and, if necessary, a surfactant or a binder, were also added if required, depending on the desired structure of the foam and how likely it was to collapse before drying could be completed. While various types of binders were listed, those preferred by Mitsubishi included Portland cement, magnesia cement, and gypsum. A slight variation of the process involved reversing the order by producing the aqueous suspension of the water, thickener, and surfactant first and then adding the ceramic materials. Either way, the actual foaming process was achieved by mechanical agitation to produce the desired quantity of foam, which depended on the required bulk density and the shape of the product. After the foamed slurry had solidified in the mold, it was subjected to hydrothermal treatment to precipitate boehmite crystals from the boehmite-forming compounds and thus develop increased strength.

Four years later, in 1989, a patent application by BASF [20] outlined yet another similar approach to the production of foam ceramics from an aqueous slurry, though the patent was subsequently withdrawn in 1992. This approach involved the desired ceramic powder, water, polymer binder, and, optionally, a surfactant, gelling agents, and a rheology-control agent. An extensive list of polymeric binders was discussed in the patent; those used were all film-forming below room temperature, capable of deformation under pressure, and able to cross-link or react with a crosslinking agent on input of energy from a range of sources such as heating, an electron beam, ultraviolet light, or X-ray irradiation. Foaming was carried out continuously in a high-shear foaming head such as an Oakes mixer or batchwise with a conventional rotary beater such as a Hobart mixer. The patent indicated that for stable foams the slurry should be foamed to between three and five times its original volume, although experiments were conducted with a wider range of volume increases. The foam produced was shaped by placing it in a mold or by spreading it to form a sheet of material that could be cut into slabs. Setting could be achieved with or without the addition of gelling agents, the latter giving greater green strength and allowing thicker foams to be achieved; the patent talks of increasing the thickness possible from 3.75 to 7.5 cm by using a gelling agent. It also indicated that it was possible to produce thicker foams, but these led to problems with sintering, since foams are extremely effective insulators and thus large thermal gradients resulted. It was also noted that foams which were gelled tended to shrink, whereas systems without gelling agents tended to expand. Drying was accomplished by heating; for foams less than 3.75 cm thick this typically involved 100–120 C for 15–35 min. After drying, the green body could be sintered at the appropriate temperature.

In the mid-1990s a new process was developed and patented that utilized the in situ polymerization of an organic monomer to stabilize foams produced from aqueous ceramic powder suspensions and prevent them from draining and ultimately collapsing [21, 22]. The monomer had the added advantage of significantly improving the green strength of the foam to such an extent that it was readily machineable after drying.

The process for fabricating the ceramic foams, which has now been commercialized by a new company, Hi-Por Ceramics (http://www.hi-por.com/), is given by the

44 Part 2 Manufacturing

flowchart in Fig. 8. A stable, well-dispersed, high-solids, aqueous ceramic suspension is prepared that also incorporates an acrylate monomer together with an initiator and catalyst to provide in situ polymerization. After the addition of a foaming agent, a high-shear mixer is used to provide mechanical agitation that results in the formation of a wet ceramic foam. Process variables include the degree of mechanical agitation and the pressure in the system. The latter affects the degree to which the bubbles formed can expand and hence can be used to control pore size if required. One of the advantages of the in situ polymerization method is that it is common to observe a period of inactivity between the addition of reagents and the actual beginning of the polymerization reaction. This is known as the induction period or idle time ti and is beneficial since it allows the casting of the fluid foam into a mold prior to polymerization and stabilization of the foam structure. However, on the negative side, the induction period also allows time for bubble enlargement and lamella thinning. These can result in the presence of flaws in the cell walls if excessive disruption of the films occurs before polymerization takes place. Good control of the induction time is primarily achieved by altering the concentration of initiator and catalyst, although other parameters are also very important in determining the induction time, such as temperature and pH.

Ceramic powder, organic monomers, water and dispersents

Ceramic suspension

Surfactant

Foaming

Initiator, catalyst

Polymerisation

Removal from mould

Drying and sintering

Fig. 8 Process flowchart for the production of ceramic foams by mechanical agitation and the in situ polymerization of organic monomers; after Ref. [22].

The manufacture of a ceramic foam begins with selection of the appropriate ceramic powder. This depends on the end use of the foam and the physical and chemical properties required. However the heart of the process is the creation of a stable ceramic slurry, which necessitates the use of fine ceramic powders, typically D50 < 3 mm. Materials have been processed with larger particle sizes, but the quality of the dispersion is inferior and the ability of the powder to densify within the foam struts during sintering is reduced. Once selected, the powder is mixed with deionized water containing dispersants, initiator, catalyst, and a premix solution containing about 6 wt % organic monomers. High-shear mixing is used to destroy any

2.1 Ceramic Foams 45

agglomerates. The organic monomer solution provides a low-viscosity liquid medium that can be readily polymerized to form a strong cross-linked polymer–water gel. The polymerization reaction traps both the ceramic powder and the water in the foamed structure. A foaming agent (surfactant) is then added to the slurry, the amount of which depends on the viscosity of the slurry and the final density of the foam desired, prior to transfer to a continuous foaming unit. This is then configured to produce a foam with the required density and cell size consistently and repeatably. Usually the slurry is foamed to between 2 and 7 times its original volume. The onset of polymerization is controlled to allow enough time for the foam to exit the foaming unit and be cast into the appropriate mold. After polymerization is complete the structure is strong enough to be demolded and transferred to an oven for drying. The polymeric binder is burnt out as a controlled step in the sintering cycle to leave a mechanically strong, highly porous ceramic. Foam densities in the range of 7–50 % of theory can be produced with cell sizes varying from about 30 to 2000 mm. In general, the lower the density, the larger the cell size, but there has been substantial work in recent years to obtain foams with densities of 30 % of theory or more and cell sizes greater than 1000 mm to provide a combination of high permeability and strength.

The gel-foaming method is able to produce parts with a high degree of complexity and excellent mechanical properties. The complexity can be achieved in part due to the casting process, which makes it possible to shape forms without additional machining. If further complexity is required, such as holes or slots, the dried green foams are strong enough to be clamped and easy to machine. The good mechanical properties result from the foam structure (Fig. 9); the struts between the cells can be fully sintered and typically contain minimal defects. However, it is also possible, at the sacrifice of some of the strength, to only partially sinter the foams and thus obtain porosity in the foam struts. It has been shown [23] that when used for the

Strut

Cell wall

Fig. 9 Structure of a 30 % dense alumina ceramic foam produced by gel casting; after Ref. [64]. (J.G.P. Binner, Brit. Ceram. Trans. 1997 96 [6] 247–249.) Reprinted with permission from the Institue of Materials, Minerals and Mining.

46 Part 2 Manufacturing

Fig. 10 A human long bone cell growing on the surface of an hydroxyapatite ceramic foam. The presence of microporosity in the foam walls allows the cell to attach easily.

bioceramic hydroxyapatite this results in foams that are extremely good for encouraging bone ingrowth (Fig. 10).

2.1.3.2

In Situ Gas Evolution

The common theme in foaming ceramics by this approach is the presence of a foaming agent that decomposes due to heat or a chemical reaction to generate a gas within a ceramic slurry.

An early patent by A.C.I. Operations in 1973 described a method of producing a foam ceramic of predominantly open-celled porosity as a result of internal reactions to liberate hydrogen [24]. In the process, an amphoteric metal powder, which could be aluminum, zinc, or tin, reacted with sodium silicate to liberate hydrogen and hence foam a ceramic slurry. The mix was then cured by heating at a temperature of at least 80 C to produce a lightweight foamed product.

A patent for a similar process was obtained by the Duriron company [25] in 1989. The initial slurry was a mixture of the appropriate ceramic powder, a water-soluble source of silicate and aluminate, a particulate metal, a surfactant system, and a gel strengthening agent. On mixing, an aluminosilicate hydrogel was formed that served to bind together the components in a generally self supporting structure over a brief period of time. During this in situ setting reaction, an additional reaction took place in which the particulate metal reacted with the alkali compounds present, which were typically sodium-based, to produce hydrogen gas in situ. As the internal evolution of the gas expanded the slurry, the hydrogel reaction was timed to coincide with the end of hydrogen gas evolution and thus set the porous structure formed. The surfactant present in the composition served to break up the bubbles of the evolving gas into suitably small bubbles as well as stabilizing them and thus ensuring that the porosity developed in the structure was predominantly of an open-celled nature.

2.1 Ceramic Foams 47

Around the same time, Fujiu et al. [26] used the rapid increase in viscosity that can be induced in sol–gels to stabilize ceramic foams. Commercially available 40 wt% silica sols were combined with Freon (CCl3F) as foaming agent, which was used because of its low boiling point of 23.8 C and limited solubility in water. As a result of these properties, Freon could be dispersed in water as an emulsion and then heated to induce foaming by its vaporization. The precursor solution was first stored in a bath at 30 C until the sol viscosity had increased to the desired level for foaming, typically 20–40 min. Foaming was then promoted by stirring the gel, the pH being adjusted to increase the gelation rate. The foam was then stored for 12 h at 30 C followed by 6 d at room temperature before being dried at 70 C for a further 7 d, heated to 400 C for a further 2 d, and finally sintered in air at 1000–1100 C. Production was therefore not fast by any standard, but porous bodies were produced with a minimum cell size of 90 mm at 31 % density and 400 mm at 17 % density.

Just two years later, Minnear [5] produced foamed ceramics by a much faster process combining a prepolymer and acetone solution with a suspension of deionized water, alumina, hydrochloric acid, and a surfactant. The two systems were stirred vigorously until a creamy consistency was achieved, and then the mixture allowed to rise as a result of the carbon dioxide produced. Mixing typically took only about 15 s, the rise time was 2–3 min, curing took about 5 min, and the drying time was 24 h in a laboratory hood. Examples of the ceramics produced are shown in Fig. 11.

10 mm

Fig. 11 Typical examples of foamed ceramics produced by in situ gas evolution; after Ref. [5]. (W.P. Minnear, Ceram. Trans., 1992 26 149–156.) Reprinted with permission of The American Ceramic Society, www.ceramics.org. Copyright [1992]. All rights reserved.

More recently, Colombo and Bernardo [27] produced macrocellular silicon oxycarbide (SiOC) open-cell ceramic foams by in situ gas evolution in solutions of preceramic polymers. They mixed a methyl polysiloxane with precursors for polyurethane (polyols and isocyanates) in dichloromethane, together with appropriate surfactants and catalysts. Blowing was started by vigorously stirring the mixture and inserting the sample into an oven at a controlled temperature in the range 25–40 C. The expansion was caused by a combination of evaporation of the solvent as a result of the exothermic reactions occurring in the solution (physical blowing) and chemical blowing arising from the reaction between the water generated by condensation of

48 Part 2 Manufacturing

the SiOH groups in the silicone resin with the isocyanate to form carbon dioxide gas [28].

The choice of polyurethane precursors and the type and amount of surfactants influenced the viscosity of the mixture and hence the final foam architecture and characteristics, while the type and amount of catalysts controlled the rising profile of the foam. Due to immiscibility with the preceramic polymer, during the blowing stage the polyurethane phase separated into small islands, typically measuring 50–300 mm in diameter, embedded within the preceramic polymer matrix. This controlled the resulting green foam morphology and characteristics [28]. Specifically, by using polyurethane precursors as both blowing aids and structural templates for the preparation of the ceramic foams, a wide range of bulk densities and cell sizes could be fabricated. Depending whether the precursors were the basis for so-called flexible, semirigid, or rigid polyurethane foams, the foams produced ranged from completely open-celled, owing to the presence of windows in the cell walls, to completely closed-celled.

Work by Greil, Scheffler and co-workers [29–32] has shown the possibility of foaming poly(silsesquioxane) without an additional foaming agent. A specific poly (phenyl methyl silsesquioxane) containing small amounts (ca. 7 mol %) of ethoxyl and hydroxyl groups was foamed by an in situ blowing technique. When heated above 200 C, condensation reactions involving the functional groups released water and ethanol, which triggered pore formation in the polymer melt. Simultaneously, an increase in the number of ”Si–O–Si” intermolecular cross-links led to an increase in the viscosity of the polymer melt and thus prevented collapse of the nascent foam structure. The resulting stabilized, thermoset preceramic polymer foam could be easily machined, and subsequent heat treatment in an inert atmosphere above 1000 C led to the formation of amorphous or partly crystalline Si–C–O ceramic. The properties of the open-celled ceramic foams could be controlled by mixing the preceramic polymer with inert or reactive fillers prior to the foaming step and by pyrolysis parameters such as atmosphere and temperature [33].

A single-stage process has recently been developed and commercialized for the production of ultralight cellular ceramics by Grader et al. [34–37]. The foams are generated by the simple heat treatment of a single-crystalline precursor that contains all the necessary foaming functions. These are crystals of an aluminum chloride isopropyl ether complex [AlCl3(iPr2O)], which are obtained by mixing concentrated solutions of AlCl3, iPr2O, and CH2Cl2. The foaming mechanism is based on the decomposition of the precursor crystals, which yield polymerizing species dissolved in liquid isopropyl chloride. As long as the solvent and growing AlOx–- Cly(OiPr)z species are mixed homogeneously, the boiling point of the solution is above that of pure isopropyl chloride and its vapor pressure is low. The inventors believe that polymerization takes place in the liquid until a critical polymer size is attained, whereupon phase separation into polymer-rich and solvent-rich regions occurs. Since the expelled solvent is suddenly above its boiling point, bubbles start forming instantly and rise to the surface. The foam is stabilized as a result of gelation in the polymer-rich regions, creating the cell walls. The resultant foams have porosities that are typically in the range 94–99 % of theoretical and consist of an

2.1 Ceramic Foams 49

arrangement of closed cells, 50–300 mm in diameter, having cell walls about 1–2 mm thick. The very high surface area of the foams, 200 m2 g–1 at 650 C, suggests that the cell walls contain nanometer-sized pores. While the cellular structure is retained during heating to 1500 C, the surface area of the foams decreases rapidly with increasing sintering temperature [31]. Products made by this process are now available from the company Cellaris in Israel (http://www.cellaris.com).

A different approach by Matthews et al. has yielded a process that is based on the use of supercritical carbon dioxide (scCO2) and which allows the extrusion or injection molding of both openand closed-cell porous ceramics from ceramic–polymer mixes [38, 39]. A supercritical fluid is a substance that exists above its critical temperature and pressure, that is, the liquid and gaseous phases are in equilibrium and the fluid has the properties of both phases. In this process, specific functional materials are dissolved in scCO2 and this mixture is then incorporated into the bulk poly- mer–ceramic mix during processing where the solution of functional material dissolves in the bulk material. Altering the solution conditions of temperature and pressure then facilitates the creation of a cellular structure from the system due to its inherent thermal instability. The polymer can be burnt out to leave behind a porous green ceramic that can subsequently be sintered.

There are a number of advantages to using a supercritical fluid, for example, it has a plasticizing effect on polymers, reducing the viscosity by about 50 % on average, which in turn allows lower processing temperatures and injection pressures to be used. This lowers clamping forces and decreases cycle times. Thus, the addition of scCO2 to ceramic–polymer mixes significantly alters the rheological behavior, and this allows loadings of ceramic powder in the mix to be increased and more complex shaped parts to be molded. In addition, the supercritical fluid can be recycled and used again. Preliminary patent applications have been made [34] and the process is now being commercialized by SCF Processing Ltd in Ireland (http://www.scf.ie).

2.1.3.3

Gelation

Foam stability can be increased by various factors. For foams with thick lamellae, the major influence on foam stability is the bulk viscosity of the solution or suspension. This is the main reason why thickeners or gelling agents such as high molecular weight polymers were often reported as being used in the patents discussed above. They have been well reviewed by Moreno [40] and hence will not be discussed further here.

A number of approaches have been adopted for the setting of fluid suspensions, regardless of whether they have been previously foamed. They include the incorporation of gelling substances such as cellulose derivatives [40] and alginates [41], the use of compositions that can intrinsically form a gel [42], and in situ polymerization of monomers [43].

The most commonly used gelling additives are cellulose and its derivatives. These include methyl cellulose, hydroxycellulose and carboxymethyl cellulose. Usually binders gel on cooling, but methyl cellulose derivatives gel on heating due to hydropho-

50 Part 2 Manufacturing

bic self-aggregation processes to form a three-dimensional network with an interstitial liquid. Although cellulose-derived materials and agar are members of the same general family, their gelation behavior is quite different. Agar and its purified derivative agarose are soluble in hot water and gel on cooling. Agar is a naturally occurring polysaccharide, derived from the red algae class of seaweed by a series of extraction and bleaching operations, and has been used in the production of hydroxyapatite foams for wastewater filters [44]. Foams with densities of less than 20 % of theoretical and cell sizes from 50 to 1000 mm could be produced; the presence of agar allowed lower solids suspensions to be used and reduced the risk of formation of drying cracks. Agarose is composed of alternating units of two forms of the sugar molecule galactose. The polymer chains aggregate to form rigid bundles of twisted helical chains, yielding a cage network that immobilizes water in the cages by strong interaction between the hydroxyl groups along the polymer backbone and the water molecules. Agarose has found use in ceramic processing as a gelling agent in the production of injection-molded ceramic pieces [45]. Other polymers that can fulfil this role include poly(vinyl alcohol), poly(acrylic acid), polyacrylamide, and polyvinylpyrrolidone [46].

The use of polymerizable materials was proposed by Landham et al. [47] in the context of tape casting, and the idea was further extended for bulk ceramic bodies in a series of papers by Omatete et al. during the development of the process of gel casting [43, 48–51]. The concept involves using a monomeric solvent that is polymerized after casting to form a rigid body. The benefits compared to using a gelling agent that is already polymerized are the ability to formulate slurries with a lower viscosity, since the size of the organic molecule is smaller, so that higher solids loadings can be achieved while still achieving a good packing density, and the excellent green strengths that can be achieved. The organic monomers used must be soluble in water and retain high reactivity. Commonly used materials include methyl methacrylate, butyl acrylate, acrylamide, and other acrylates. The polymerization of these vinyl monomers can be brought about by a variety of initiating systems that have a marked influence on the rate at which the reaction proceeds. For example, chemical initiation with systems such as peroxide–amine, as initiator and catalyst, respectively, can be very effective in producing fast polymerization [43].

The primary disadvantage of all systems that are based on the addition of gelling agents, whether they be polymeric when used or polymerize in situ during processing, is that they need burning out at the end of the process, either in a separate stage or by including a medium-temperature (typically 400–500 C) hold into the sintering cycle. While this can be a significant problem for the manufacture of dense ceramics due to the relatively long time periods required for the potentially large volumes of gas to escape, for ceramic foams it is usually little more than an inconvenience, especially for the more open structures.