Processing of Ceramic Foams for ThermalProtection

Sujith Vijayan, Praveen Wilson, and Kuttan Prabhakaran

ContentsIntroduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1Thermal Conductivity of Ceramic Foams . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2Mechanical Strength . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4Thermal Shock Resistance of Ceramic Foams . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5Processing of Ceramic Foams . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6Polymer Foam Template Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8Sacrificial Template Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8Ceramic Foams by Freeze Casting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9Emulsion Templating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11Direct Foaming . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13Gelcasting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16Particle-Stabilized Foams . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17Thermo-foaming . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19Ceramic Foams from Pre-ceramic Polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21Thermal Conductivity of Ceramic Foams Prepared by Various Methods . . . . . . . . . . . . . . . . . . . . . . . 24Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

KeywordsCeramic foams · Processing · Microstructure · Porosity · Thermal conductivity ·Compressive strength

S. Vijayan · P. Wilson · K. Prabhakaran (*)Department of Chemistry, The Indian Institute of Space Science and Technology (IIST),Thiruvananthapuram, Kerala, Indiae-mail: kp2952002@gmail.com

© Springer Nature Switzerland AG 2019Y. Mahajan, R. Johnson (eds.), Handbook of Advanced Ceramics and Composites,https://doi.org/10.1007/978-3-319-73255-8_34-1

1

Introduction

The rapid industrialization and increased space exploration activities necessitate thedevelopment of materials of low thermal conductivity capable of withstanding veryhigh temperatures. These materials are called thermal protection materials. They areused either to protect the surrounding personals from the heat flux of high-temperature heat treatments in industries or protect the instruments and astronautswithin the crew cabin of a space vehicle from the intense heat flux generated duringits reentry into the atmosphere. The high-temperature ceramics such as alumina,silica, zirconia, mullite, SiC, silicon nitride, and silicon oxycarbide are capableof withstanding high temperatures. The materials of choice at extremely hightemperatures (above 2000 �C) are either carbon or ultrahigh-temperature ceramics.They include borides and nitrides such as TiB2, ZrB2, HfB2, TiN, ZrN, etc. Thesematerials in their dense state though withstand high temperatures exhibit relativelyhigh thermal conductivity. In addition to the high-temperature capability, the thermalprotection materials should be light in weight and have low thermal conductivity.The aspect of low density is utmost important in thermal protection materials usedfor space applications as an increase in weight increases the amount of fuel requiredfor takeoff and subsequent reentry. The way to achieve lightweight and low thermalconductivity in ceramic materials is by making them porous. The porous ceramicswith porosity greater than 70 vol% are called ceramic foams.

A ceramic foam can be defined as a continuous ceramic matrix consisting ofa three-dimensional array of pores (called cells) interconnected to each other viastruts with an inherent porosity of>70 vol%. In addition to low thermal conductivityand density, the ceramic foams have low gas absorption, low thermal capacity andhigh thermal stability, high thermal cycling capacity, and very good shock resistance.Another important aspect is their easy production of different sizes and shapes.Ceramic foams can be divided into three categories based on their cell morphologyand cell interconnectivity. They are open-cellular, closed-cellular, and reticulatedceramics. In the case of open-cellular ceramics, the neighboring cells areinterconnected through one or more cell windows which facilitate the transportof materials between them. In the case of closed-cellular ceramic foams, the cellsare isolated from each other preventing the transport of matter. Reticulated ceramicscan be considered as an extension of open-cellular ceramics, where the entire foam isan interconnected network of pore struts. Typically, reticulated ceramics have a veryhigh porosity (>95 vol%) and low mechanical strength. In addition to these threetypes, there are foams with a combination of open and closed pores. Typicalmicrostructures of closed-cellular, open-cellular, and reticulated ceramic foams aregiven in Fig. 1.

Thermal Conductivity of Ceramic Foams

Thermal conductivity k (Wm�1K�1) of a material is the measure of its ability toconduct heat. It can be defined as the amount of heat transferred by conductionthrough a unit cross-sectional area of the material, with the temperature gradient

2 S. Vijayan et al.

perpendicular to the area. The thermal conductivity can be expressed in terms ofFourier’s law as shown in Eq. (1) below:

q0 ¼ �k@T=@x (1)

where q0 is heat flux and @T/@x is temperature gradient along one direction.Ceramic foams are composite materials, in which one phase is a gas, usually air.

The heat transfer through gas must also be considered in addition to the thermalconduction through solids. Gases are capable of conduction, convection, and radi-ative transfer of heat. Therefore, the thermal conductivity of ceramic foam has fourcontributions: conduction through the solid (ks), conduction through the gas (kg),convection within the cell walls (kc), and radiation through the cell walls and acrossthe cell voids (κr). The total thermal conductivity, k, can be now defined as

k ¼ ks þ kg þ kc þ kr (2)

Fig. 1. Microstructure of typical (a) closed-cellular, (b) open-cellular, and (c) reticulated ceramicfoams

Processing of Ceramic Foams for Thermal Protection 3

The convection contribution to thermal conductivity is suppressed in cellularsolids due to their smaller cells. The heat conduction through the foams is mainlyattributed to the conduction through the solid and through the gas filled in the cells.Ashby proposed an equation which is completely dependent on the relative densityas shown below [1]:

k ¼ 1

3

ρ

ρ0

� �þ 2

ρ

ρ0

� �3=2" #

ks þ 1� ρ

ρ0

� �� �kg (3)

Where ρ0 is the theoretical density of the dense ceramic and ρ is the bulk density ofthe foam. The term associated with the gas, often negligible, becomes important infoams intended for thermal insulation since these have a low relative density and aconductivity approaching that of a gas.

Mechanical Strength

The mechanical strength of brittle foam materials like ceramic foam is generallymeasured in terms of its compressive strength or crushing strength. Figure 2 showsa typical compressive stress-strain graph of the elastic-brittle solid foam. The stress-strain graph generally contains an initial linear elastic region followed by a plateaustress region and a region of densification. The initial linear elastic region dependson the cell size, relative density, and nature of the solid. After the elastic region, thecells start collapsing, and the cell collapse continues throughout the plateau region.The debris formed by the cell collapse fill the cells in the remaining foam thatresulted in an increase of foam density which is responsible for the increase of stressin the densification region. The stress corresponding to the plateau region is taken asthe compressive strength, and the slope of the initial elastic region in the stress-straingraph is taken as Young’s modulus. Generally, the increase in relative density of thefoam increases Young’s modulus and compressive strength and decreases the strainat which densification starts.

The compressive strength of ceramic foams depends on the porosity, cell size, andmicrostructure of the struts. In general, the compressive strength of brittle foamincreases with the decrease of porosity and pore size. The presence of pores in thestruts of the foam decreases its compressive strength. Due to the porous struts, thereticulated ceramics exhibits very low compressive strength. The closed-cell foamsshow better mechanical properties than their open-cell counterparts at similar poros-ity. The compressive strength of ceramic foams is in the order closed cellular> opencellular >> reticulated. In order to understand the mechanical behavior of ceramicfoams, it is necessary to understand the relationship between mechanical propertiesand porosity or relative density. Many models have been proposed that containexpressions which describe these relationships. The Young’s modulus of brittleopen-cellular solids is best modeled using the Eq. (1) proposed by Gibson andAshby [1].

4 S. Vijayan et al.

E

E0¼ C

ρ

ρ0

� �n

(4)

Verma et al. [2] used a similar Eq. (2) to model the compressive strength of open-cellular ceramics.

σ

σ0¼ C

ρ

ρ0

� �n

(5)

where E and E0 are Young’s modulus, while σ and σ0 are the compressive strength ofthe ceramic foam and fully dense ceramic, respectively. ρ and ρ0 are the densities ofthe foam and fully dense ceramic, respectively. The values of C and n for the relationbetween E/E0 and ρ/ρ0 are expected to be ~ 1 and ~ 2, respectively, for brittle open-cellular foams with a cubic array of cells. However, in the majority of the reportedceramic foam materials, the C and n values deviate from the theoretical ones due totheir deviation from the cubic morphology and broad distribution of cell sizes.

Thermal Shock Resistance of Ceramic Foams

Thermal stresses arise in a body when its surface experiences a sudden change intemperature resulting in the formation of cracks within the body. The thermal stressarising due to the rapid cooling of a body can be expressed as Eq. (6) [1, 3, 4]

σt ¼ E α ΔT1� ν

(6)

0 20 40 60 800

1

2

3

Stre

ss (M

Pa)

Strain (%)

Fig. 2 Typical compressivestress-strain graph of aceramic foam

Processing of Ceramic Foams for Thermal Protection 5

where E is Young’s modulus, α is the coefficient of thermal expansion, ν is thePoisson’s ratio, and ΔT is the temperature difference. When the thermal stressexceeds the fracture strength, a crack develops and propagates leading to thefailure [1]. The thermal shock resistance of a ceramic solid can be expressed inthe terms of temperature and fracture stress as Eq. (7)

Rs ¼ σf 1� νð ÞE α

(7)

where RS is thermal shock resistance of solid and σf is fracture or flexural strength.Open-cellular ceramic foams find applications in molten metal filtration, burners,

solid oxide fuel cells, and catalytic converters due to low weight, high-temperaturestability and high permeability [5, 6]. In all these applications, foams are subjected toelevated temperatures (>800 �C), fast heating and cooling rates, and temperaturegradients which results in the formation of thermal stresses in the body over a timeperiod. Hence, the thermal shock resistance is an important factor determining theuse of ceramic foams for high-temperature applications.

By substituting the value of Young’s modulus [E = C1Es(ρ/ρs)2] and fracture

strength [σf = C2σfs(ρ/ρs)1.5] [7] of foams in Eq. (7), the thermal shock resistance of

an isotropic foam material is obtained as

Rb ¼ Rs C2=C1ð Þ ρ=ρsð Þ�0:5 (8)

That is the thermal shock resistance of a foam which increases with an increase inthermal shock resistance of the solid and increase in porosity.

Processing of Ceramic Foams

The ceramic foams are prepared from respective ceramic fine powders orpre-ceramic polymers. The conversion of ceramic powder to ceramic foams isachieved through either dry powder pressing or colloidal processing. In the drypressing method, a mixture of ceramic powder and pore-forming agent is consoli-dated by pressing. Removal of the pore template by burnout followed by sinteringresults in the formation of ceramic foam. The powder pressing method is mostlylimited to ceramic foams of lower porosity. On the other hand, the colloidalprocessing route uses a well-dispersed suspension of the ceramic powder in anaqueous or nonaqueous medium. The colloidal processing produces ceramic foamsof higher porosity compared to the powder pressing using pore templates. Moreover,the ceramic foams produced from colloidal suspensions have dense cell walls andstruts that result in better mechanical strength. Therefore, the majority of the foamproduction methods in the literature are through the colloidal route. The processingof ceramic foams from powder suspension requires a thorough understanding ofceramic powder dispersion in a liquid medium.

6 S. Vijayan et al.

Ceramic powder dispersion in a liquid involves the manipulation and control ofthe interparticle interactions such as van der Waals attraction and electrostaticand polymer-induced steric repulsions. The well-dispersed powder suspension isachieved when the electrostatic or polymer-induced steric repulsion between theparticles in the liquid medium overcomes the van der Waals attractive forcesnaturally existing between them. The understanding of the interaction between thesolvent molecules and the powder surface is also very important. In the case of oxideceramics, due to the more ionic nature of the bonds, the particle surface is easilycharged when coming in contact with protic solvent molecules. Due to the interac-tion of ionic species in solution with its surface, an oxide ceramic particle obtainsnegative or positive surface charge. Water is the most preferred solvent for thedispersion of the oxide ceramics with H+ and OH- as the ionic species. In suchcase, the amount of positive or negative site on the particle surface is determined bythe pH of the aqueous solution. By adjusting the pH, a net positive or negative chargecan be obtained on the particle surface. The counterions from the solution areattracted to the interface from a diffused ion cloud adjacent to the particle surface.This is called as an electrical double layer. The thickness of this double layer isdetermined by the concentration and valency of the ions in the solution. When theconcentration of the ions in the solution is high, it results in a thin double layerleading to poor stability of the dispersions. The thickness of the double layer can bedefined in the terms of Debye length (1/κ) which is given by Eq. (9) [8].

1

κ¼ e e0 k T

e2P

iniz2i

� �(9)

where e is the electronic charge, k is the Boltzmann constant, T is the temperature, niis the concentration of ions with charge zi, ε is the dielectric constant of the liquid,and ε0 is the permittivity of vacuum. The energy due to the repulsion of sphericalparticles due to the double layer can be given by Eq. (10) [8]

V elect ¼ 2πere0aΨ20e

�κh (10)

where a is particle radius, his minimum separation distance between particle sur-faces, and Ψ0 is surface potential.

In the case of nonaqueous solvents, controlling the pH does not yield stabledispersions. In such cases, suitable polymeric dispersants are added to prepare stablecolloidal suspensions. The polymeric molecules adsorb on the surface of the parti-cles. Upon the close approach of two particles covered with adsorbed polymerlayers, the interpenetration of the polymer layers gives rise to a repulsive forceknown as steric stabilization. To be effective, the adsorbed polymer layers must be ofsufficient thickness and density in order to overcome the van der Waals attractionbetween particles. The steric stabilization can be explained in terms of osmoticeffect. When two ceramic particles with the adsorbed polymer layer come together,it creates an area of low solvent concentration between them. Due to the osmoticpressure, the solvent molecules are force into this area separating the ceramicparticles. A combination of electrostatic and steric stabilization is used for the

Processing of Ceramic Foams for Thermal Protection 7

preparation of ceramic powder suspensions of high solid loadings in an aqueousmedium. The dispersion mechanism in these cases is termed electrosteric. In this,polyelectrolytes are widely used as dispersants. Polyelectrolytes are moleculeswhich contain one or more ionizable groups, either a carboxylic or sulfonic group,and have structures ranging from homopolymers to block copolymers. Polyelectro-lyte adsorption is strongly influenced by the chemical and physical propertiesof the solid surfaces and solvent medium. The adsorption behavior and conformationof polyelectrolyte species can be modulated by tailoring the pH and temperature.The various methods for the preparation of ceramic foams from the colloidaldispersion of powders are discussed in the subsequent sections. More detaileddiscussion on powder dispersions in the aqueous and nonaqueous medium isfound elsewhere.

Polymer Foam Template Method

In the polymer foam template method, the structure of polymer foam is replicated inceramic. The cell size of the ceramic foams is decided by the cell size of the polymerfoam template. The method uses flexible polyurethane foams of desired pore size asthe template. In this, the polymer foam is impregnated with the aqueous ceramicpowder suspension containing a suitable binder. The excess ceramic powder sus-pension filled in the pores is removed by squeezing the slurry impregnated polymerfoam by rolling. Thus obtained polymer foam coated with the ceramic powder on thecell walls and webs is dried, binder removed and sintered to produce the ceramicfoam. A pictorial representation of the polymer foam replication method is shownin Fig. 3. Multiple impregnations are used to increase the cell wall and strutthicknesses. The reticulated ceramic foams are generally prepared by the polymerfoam template method. The main drawback of this method is that the ceramic foamsproduced have porous struts. The reticulated ceramic foams produced by the poly-mer foam template method have low crushing strength due to their porous struts.Impregnation of the reticulated ceramic foams with the ceramic powder suspensionis used to increase their crushing strength [9, 10].

Sacrificial Template Method

The sacrificial template method involves the use of fugitive particles which act as thepore-forming agents. These fugitives are either mixed with the ceramic powder ordispersed within the ceramic slurry or precursor solution to ensure homogenousmixing. The dry ceramic powder-fugitive mixture is conformed to shape by com-paction in a mold. On the other hand, fugitive particle-dispersed ceramic powdersuspensions and precursor solutions are consolidated by casting in a mold followedby setting and drying. The fugitives are removed either by evaporation, pyrolysis, orleaching out using a suitable solvent. The green body obtained is then sintered to getthe ceramic foam. A schematic of the fugitive template-based process for thepreparation of ceramic foams is shown in Fig. 4.

8 S. Vijayan et al.

The porosity can be controlled by adjusting the amount of the sacrificial phase.The size and shape of the fugitives affect the pore size and pore morphology of theporous ceramic. The pore-forming agents can be generally classified into organic,inorganic, and metallic. Synthetic and natural organics used as pore templates areoften extracted through pyrolysis by applying long thermal treatments at tempera-tures between 200 �C and 600 �C. On the other hand, salts used as pore templateare removed by leaching with hot water. The typical microstructure of ceramic foamprepared using NaCl as pore template is shown in Fig. 5. The long periods requiredfor complete pyrolysis of the organic component and the extensive amountof gaseous by-products generated during this process are the main disadvantagesof using organic materials as the sacrificial phase. In addition, fugitive template-based process produces ceramic foams with relatively low porosity. The porosity ofceramic achieved by various sacrificial templates is given in Table. 1.

Ceramic Foams by Freeze Casting

Freeze casting is a versatile method widely studied for the preparation of ceramicfoams with a wide range of porosity and pore structure [11]. In this, the frozensolvent crystals function as pore template. In the freeze casting process, the ceramicpowder suspension cast in a mold is set by freezing the dispersion medium. Theceramic particles in the suspension are expelled to the boundaries of crystallizedsolvent. The frozen dispersion medium is subsequently removed by sublimation, andthen the resulting green body is sintered to produce the ceramic foam. The spacecreated by the removal of the crystallized dispersion medium remained as the poresin the sintered ceramics. The anisotropic growth of the solidifying dispersing

Fig. 3 Pictorial representation of polymer foam template method

Fig. 4 Schematic of the sacrificial template-based process

Processing of Ceramic Foams for Thermal Protection 9

medium to produce unidirectional aligned pores is favored by creating a temperaturegradient. This is achieved by extracting heat from the suspension from one directionusing specially designed molds having well-insulated surfaces except one. Thethermally conducting surface of the mold containing the ceramic powder suspensionis placed on a platform cooled by liquid nitrogen to achieve the directional cooling.During solidification of the ceramic powder suspension by extracting heat fromone side, the ceramic particles expelled that are arranged in a specific way result in

Fig. 5 Microstructure of ceramic foam prepared using NaCl as pore template

Table 1 The maximum porosity achieved in ceramics with various sacrificial pore templates

Sl. no Sacrificial phase Ceramic system Porosity (%)

1. Polymethyl methacrylate (PMMA) Yttria-stabilized zirconia (YSZ) 51.6

2. PMMA Silicon carbide (SiC) 77

3. PMMA β-Tricalcium phosphate 80

4. PMMA Alumina 77.2

5. Graphite powder Mullite-SiC 43.4

6. Starch Silicon nitride (Si3N4) 23.8

7. Starch Mullite 60

8. Phosphoric acid Si3N4 63

9. Poppy seed Alumina 37.6

10. Potato starch, almond crust Hydroxyapatite 59.2

11. Wax spheres Hydroxyapatite 42.5

12. Cotton thread Alumina 35

13. Wheat particles Alumina 76.7

14. Rayon fibers Mullite 59.3

15. Carbon fibers Alumina 38

10 S. Vijayan et al.

well-defined directional (lamellar) microstructures. Extraction of heat from theopposite directions is also attempted to control the lamellar microstructure [12].On the other hand, the freezing conditions can be controlled to achieve uniformtemperature and freezing rate throughout the sample to inhibit the directional growthof the solidified dispersion medium [13, 14]. This results in the homogeneouscellular microstructure of the ceramic foams with open-pore structure. Figure 6shows the schematic of the freeze casting technique. There are four major steps inthe freeze casting process: preparation of the slurry, controlled solidification of thedispersion medium, sublimation of the frozen dispersion medium, and finallysintering of the green body. The dispersion medium is the most important factor inthe freeze casting process as it affects both the process and the microstructure of thefinal product. Other parameters controlling the pore structure are substrate temper-ature, cooling rate, and an additive which control the crystal growth [15, 16].Nevertheless, the porosity of the ceramic foam is decided by the slurry concentra-tion. That is, the porosity increases with a decrease in ceramic powder concentrationin the slurry. Water, camphene, naphthalene, t-butyl alcohol, etc. are the commonlyused dispersion medium [17–22].

Water is the most commonly used dispersion medium for freeze casting dueto its environmentally friendly and abundant nature. Ceramic foams with lamellarpore structure and open-cellular morphology are produced from aqueous powdersuspensions by freeze casting. The additives such as glycerol, PVA, and gelatinare used to control the growth of ice crystals and thereby control the foam micro-structure [16, 23, 24]. The use of camphene instead of water gives advantages interms of higher (60 �C) freezing temperature and easy removal of the frozen mediumby sublimation at room temperature. However, the preparation of the slurry forfreeze casting needs to be done above the melting point of camphene. The aqueous-and camphene-based freeze casting is attempted for various ceramic systems such asalumina, mullite, zirconia, hydroxyapatite, silicon nitride, and silicon carbide. Poros-ity as high as 89 and 95 vol% is achieved with the aqueous- and camphene-basedfreeze casting, respectively. The typical microstructure (along the freezing direction)of ceramic foam produced by freeze casting showing the unidirectional alignment ofpores is shown in Fig. 7.

Emulsion Templating

The ceramic foams prepared using fugitive particles as pore template are limited tolower porosity. Just like the fugitive particles, the immiscible solvent dropletsdispersed in a ceramic powder suspension can function as a template for pores.The principle of this method is adopted from the preparation of polymer foams usinghigh internal phase emulsion. In this, an organic liquid (oil) immiscible with water isdispersed in an aqueous ceramic powder suspension using the principle of emul-sions. That is, a suitable emulsifying agent is used for the dispersion of the oil phasein the aqueous ceramic powder suspension. Subsequently, the oil droplets areremoved after gelation and drying of the emulsion. The oil-removed emulsion bodies

Processing of Ceramic Foams for Thermal Protection 11

are sintered to produce the ceramic foams. The emulsion templating methodis originally reported for the preparation of macroporous ceramics with orderedpores through sol-gel route [25, 26]. In this, nonaqueous emulsion fractionatedinto a narrow droplet size was mixed with sols prepared by the hydrolysis andcondensation of the metal alkoxides. Subsequent gelation by further condensation ofthe prepolymer sol followed by removal of the emulsion droplets and sinteringresulted in ceramic foams with ordered pores. However, the gels undergo largeshrinkage during drying and sintering. Moreover, the preparation starting fromceramic powders is more economically viable than the sol-gel route.

Recently, a method based on a high alkane phase emulsified suspensions(HAPES) using decane as the oil phase [26–28] has been studied for the preparationof ceramic foams with interconnected spherical cell structure. In this, the emulsionscast in an open mold could be removed from the mold only after partial or completedrying which hampers the production rate. This problem was resolved by using anoil phase which becomes solid on cooling. Vitorino et al. [29] reported the prepara-tion of alumina foams by emulsion templating method using liquid paraffin as the oilphase. In this, the aqueous ceramic powder suspension is emulsified by dispersing

Fig. 6 Schematic of freeze casting technique

Fig. 7 Typical microstructureof freeze-cast ceramic foamsshowing unidirectionalaligned pores

12 S. Vijayan et al.

molten wax at a temperature above its melting point. The emulsion cast in a moldundergoes setting during cooling due to the solidification of wax and gelation of theaqueous ceramic powder suspension. Gelling agents such as gelatin, agar, andcarrageenan which dissolve in hot water and form a strong gel on cooling due tophysical cross-linking are used for the setting of the aqueous powder suspension inthe emulsion. The molten naphthalene is also used as an oil phase which is a solid atroom temperature. The advantage of naphthalene over wax is the ability of theformer to expel from the body by sublimation. However, the toxic nature ofnaphthalene prevents its use for the production of ceramic foams. Moreover, thefoams prepared from naphthalene-based emulsions have highly distorted pore struc-ture against the interconnected spherical cellular structure obtained for otheremulsion-based processes. The typical microstructure of alumina foams preparedusing naphthalene as an oil phase is shown in Fig. 8.

The hydrogenated vegetable oil (HVO) is used as an eco-friendly oil phase for thepreparation of ceramic foams by emulsion templating. In this, the molten hydroge-nated vegetable oil is dispersed in an aqueous ceramic powder suspension containingcarrageenan gelling agent using sodium dodecyl sulfate emulsifying agent ata temperature of ~85 �C to form medium and high internal phase emulsions. Thethus obtained stable emulsions with low viscosities are cast in suitable molds andsubsequently set by cooling. The gelled bodies are strong enough to remove fromthe mold without any deformation. The gelled bodies dried at ambient temperatureare extracted with either toluene or petroleum ether to remove the HVO. TheHVO-removed bodies are sintered to produce ceramic foams. The flowchart of theHVO-based emulsion templating method is shown in Fig. 9.

The porosity of the ceramic foams is decided by the volume fraction of aqueousalumina slurry and HVO to alumina slurry volume ratio. On the other hand, the cellsize depends on the concentration of the emulsifying agent and stirring speed usedfor emulsion preparation. The porosity of alumina foams could be modulated in therange of 70.7–92.5 vol% by changing the hydrogenated vegetable oil to aqueousalumina slurry volume ratios in the range of 1.3–2.7 and using aqueous slurries ofalumina concentrations in the range of 10–30 vol%. The foams produced have aninterconnected cellular structure. The HVO-based emulsion route produces ceramicfoams with relatively lower cell size. The cell sizes obtained are in the range of5–27 μm. The compressive strength and Young’s modulus of the alumina ceramicfoams are in the ranges 2.74–39.6 and 64.5–2350 MPa, respectively, at porosity inthe range of 70.7–92.5 vol% [30, 31]. The photograph of an alumina foam body andits microstructure is shown in Fig. 10.

Direct Foaming

In the direct foaming method, wet foams are produced by incorporating air intoceramic powder suspensions by mechanical frothing or by the injection of gases. Thewet foam cast in a mold is subsequently set, dried, and then sintered at elevatedtemperatures to get the ceramic foam. The total porosity of the ceramic foamdepends on the number of air bubbles incorporated in the ceramic suspension. The

Processing of Ceramic Foams for Thermal Protection 13

pore morphology and pore size of the foams depend on the stabilization of the airbubbles in the wet foams. Several transformations in the bubble structure, whichdestabilize the foams, occur within the interval between foam generation and foamsetting [32]:

1. Ostwald ripening, i.e., smaller bubbles shrink, while bigger ones grow in size.2. Creaming of bubbles, i.e., a foam layer forms on top of bulk liquid.3. Deformation of bubbles.4. Drainage of liquid from foam to bulk.5. Coalescence due to rupture of bubbles.

Fig. 8 Microstructure ofalumina foams prepared usingnaphthalene as an oil phase

Fig. 9 Flowchart of HVO-based emulsion templating method

14 S. Vijayan et al.

The liquid foams are thermodynamically unstable, due to their high interfacialfree energy. Here, the interfacial free energy is the minimum amount of workrequired to create the air-liquid interface in the bubbles. The driving force for theabove transformations occurring in foamed suspensions is to reduce the interfacialfree energy. The above transformations can be minimized by the addition of suitablesurface-acting agents or surfactants to the suspensions before foaming. A surfactantis a substance that alters the surface or interfacial free energy by adsorbingonto the surfaces or interfaces [33]. Amphiphilic molecules with a nonpolar hydro-phobic portion, called a tail attached to a hydrophilic group termed head, are usedas surfactants. The hydrophobic part is mostly hydrocarbon chain consisting of 8–18carbon atoms. The surfactants are classified into four categories based on their head.They are cationic, anionic, nonionic, and amphoteric or zwitterionic [33, 34].The cationic and anionic surfactants have positively and negatively charged headgroups, whereas the nonionic surfactants have a neutral head group. The zwitterionicsurfactant possesses both positive and negative groups in their structure. The com-monly used surfactants for the preparation of ceramic foams are given in Fig. 11.In addition to the above class of surfactants, proteins such as collagen, egg white,and whey protein are also used to stabilize bubbles generated in ceramic powdersuspensions for the preparation of ceramic foams. The selection of surfactant is veryimportant as the stability of the wet foam depends on the nature of surfactants.Therefore, the porosity and foam microstructure are directly dependent on the natureof the surfactant used. In many cases, a mixture of surfactant is proved to producefoams with better porosity and microstructure.

The surfactant-stabilized wet foams have only interim stability. They tendto collapse during drying. Therefore, an additional stabilization by setting thefoamed ceramic powder suspension is essential. The in situ polymerization oforganic monomers and the cross-linking agent is one of the ways to set thesurfactant-stabilized wet foams. The alternative methods include gelation by

Fig. 10 Microstructure ofalumina foam prepared byHVO-based emulsiontemplating method

Processing of Ceramic Foams for Thermal Protection 15

coagulation of proteins and physical cross-linking of certain carbohydrate polymers.The process utilizing in situ polymerization of organic monomers is calledgelcasting.

Gelcasting

Gelcasting, a colloidal processing method originally developed for the fabrication ofhigh-quality, complex-shaped dense ceramic bodies by means of in situ polymeri-zation, is extended for the preparation of ceramic foams. In this, the ceramic powdersuspension containing the organic monomer and cross-linking agent is foamed usinga suitable surfactant by incorporating a gas. The foamed suspension cast in a mold isset by in situ polymerization. The gelled foam is removed from the mold, dried,binder removed, and sintered to produce the ceramic foams. The flowchart of thegelcasting process for the preparation of ceramic foams is shown in Fig. 12. A rangeof monomer and cross-linker systems are studied for the preparation of a largenumber of ceramic foam materials (Table 2).

Gelcasting produces foams with interconnected cellular structure with highporosity. The ceramic foams produced by gelcasting exhibit nonporous strutsresulting in superior crushing strength. The typical microstructure of alumina foamproduced by gelcasting is given in Fig. 13. Gelcasting techniques utilizing environ-mentally friendly biopolymers as gel formers for the manufacturing of ceramicfoams are intensively studied. Different biopolymers such as gelatin, proteins likeovalbumin and bovine serum albumin, agar, and starch have recently been appliedas nontoxic components for the fabrication of porous ceramics [35–39]. In this,the setting of the foamed suspensions is not due to in situ polymerization but dueto physical cross-linking through hydrogen-bonding interactions. The variousgelcasting systems and achieved porosity of the foams are presented in Table 2.A continuous process for gelcasting fabrication of ceramic foams has beenreported [40]. In this, preformed N2 bubbles from a separate canister are mixedwith the gelcasting slurry instead of bubbling nitrogen gas in conventional method to

Fig. 11 Surfactants used for foaming (a) cationic, (b) neutral, (c) zwitterionic, and (d) anionic

16 S. Vijayan et al.

produce the wet foam. Compared to conventional methods using N2 gas to generatebubbles, porous ceramic produced using the new method displayed lower density,higher open and total porosities, and broad pore size distribution.

Particle-Stabilized Foams

The aqueous foams produced using a surfactant are thermodynamically unstable.Therefore, the foamed ceramic powder suspensions undergo either coalescenceor Ostwald ripening unless timely set by gelation. This leads to either foam collapseor the formation of large cells within the final foam microstructure. Colloidalparticles have been used instead of surfactants for the stabilization of liquid-liquidand gas-liquid interfaces to produce emulsions and foams [41–43]. The foamsproduced by stabilization of liquid-air interface by colloidal particles are calledparticle-stabilized foams. In this, the particles adsorb on the liquid-gas interfaceand thereby reduce the interfacial free energy. The critical parameter deciding theadsorption of colloidal particles on the liquid-gas interface is the wetting behavior(contact angle) of the particle with the liquid. The ideal contact angle for producing

Fig. 12 Flowchart of the gelcasting process for the preparation ceramic foams

Processing of Ceramic Foams for Thermal Protection 17

particle-stabilized foams is in the range of 60–80� [42]. That is, the particles are onlypartially wetted by the fluid. The contact angle of colloidal particles at fluid inter-faces depends on the surface chemistry, roughness, impurities, and particle size. Theadvantage of particle-stabilized foams over the surfactant-stabilized ones is their

Table 2 Various gelcasting systems and achieved porosity of the foams

Sl.no Gel system Ceramic system

Porosity(%)

1. 2-Hydroxyethyl methacrylate and poly(ethylene glycol1000) dimethacrylate

Mg0.9Co0.1Al2O4 98

2. Acrylamide (AM), N,N0-methylenebisacrylamide(MBAM), and ammonium persulfate (APS)

Alumina 82

3. AM and MBAM Alumina 88

4. N,N,N0,N0-tetramethylethylene diamine and APS Hydroxyapatite 90

5. AM and MBAM Mullite 73.5

6. Starch Alumina 73

7. Starch Hydroxyapatite 70

8. Starch Silica 82

9. Agar SiC 88

10. Agar Alumina andmullite

83

11. Agarose Hydroxyapatite 90

12. Agarose Alumina 90

13. Egg white protein Hydroxyapatite 75

14. Ovalbumin Alumina 96

15. Bovine serum albumin Alumina 92.3

16. Gelatin and polyvinyl alcohol Alumina 95

Fig. 13 Microstructure ofalumina foam prepared bygelcasting

18 S. Vijayan et al.

superior thermodynamic stability [43]. Therefore, aqueous ceramic powder suspen-sions foamed using colloidal particles can be dried without coalescence or Ostwaldripening even in the absence of an additional setting mechanism such as in situpolymerization or coagulation.

The preparation of particle-stabilized foams is rather simple. The processinvolves agitation of aqueous ceramic powder suspension containing short-chainamphiphilic (<8 carbon atoms) molecules to form foamed ceramic powder suspen-sion [44]. The amphiphilic molecules convert the hydrophilic oxide ceramic parti-cles to partially hydrophobic by adsorbing on their surface. That is, the ceramicparticles are partially hydrophobized in situ by the amphiphilic molecules. Theamphiphilic molecules such as propionic, valeric, butyric, and enanthic acid areused for partial hydrophobization [45]. The air bubbles incorporated in the powdersuspension by agitation are stabilized by the adsorption of partially hydrophobizedparticles on the air-liquid interface. The foamed ceramic powder suspensions aresubsequently dried and sintered to produce the ceramic foam. The schematic repre-sentation of particle-stabilized foam is shown in Fig. 14. A combination of particlestabilization and gelcasting using calcium aluminate as a setting agent is also usedfor the preparation of ceramic foams [46]. Alumina, silica, zirconia, zirconia tough-ened alumina, and silicon nitride foams are prepared through the particle-stabilizedfoaming method. The advantages of this method are that it produces ceramic foamswith high porosity, relatively low cell size, uniform pore size distribution, andsuperior crushing strength. Ceramic foams with porosity as high as 95 vol% areproduced by the particle-stabilized foam route [45].

Thermo-foaming

The principle of preparation of polymer foams involves foaming by generating gasbubbles in the polymer medium or polymerizable organic material using a blowingagent. The foamed polymer is subsequently stabilized by setting by further poly-merization and cross-linking reactions. The blowing agent can be a liquid whichevaporates at the foaming temperature or a solid which decomposes and generatesthe gas at the foaming temperature. The principle of polymer foam preparation isadapted for the preparation of ceramic foams. In this, a sufficient amount of theceramic powder is incorporated into the polymer or polymerizable organic materialbefore foaming and setting. The solid foam is subsequently heated to remove theorganic matter and sintered to produce the ceramic foam.

Sucrose is studied as an organic medium for the preparation of ceramic foams bythermo-foaming. The oxide ceramic powders such as alumina, silica, etc. disperse inmolten sucrose due to the hydrophilic interaction between the powder surface andsucrose hydroxyl groups. The ceramic powder dispersed molten sucrose undergoesfoaming when heated at temperatures in the range of 120–150 �C due to the watervapor generated by the hydroxyl condensation between glucose and fructose anhy-dride formed from the sucrose. The bubbles created by the water vapor are stabilizedby an increase in viscosity due to polymerization and ceramic particles adsorbed on

Processing of Ceramic Foams for Thermal Protection 19

the molten sucrose-air interface by the particle stabilization mechanism. The settingof the foamed dispersions into solid bodies is by the continued polymerization(caramelization) through the hydroxyl condensation. The foaming time, foam settingtime, and foam rise depend on the alumina powder to sucrose weight ratio andfoaming temperature. Sucrose polymer burnout and sintering produces aluminafoam bodies of smaller sizes without any cracks. On the other hand, an intermediatecarbonization step is inevitable to prevent the cracking of larger bodies. A consid-erable shrinkage of the body occurs during carbonization, and the carbon producedfrom the sucrose polymer binds the alumina particles that prevent cracking. Theprocess flowchart and photograph of an alumina foam body produced by thermo-foaming are shown in Fig. 15.

The porosity of alumina and mullite ceramic foams produced using thermo-foaming depends on the ceramic powder to sucrose weight ratio and foamingtemperature. The alumina and mullite foams produced by the thermo-foamingprocess have interconnected cellular structure with cell sizes in the rangesof 0.4–1.6 and 0.49–1.58 mm, respectively. The foaming time and foam settingtime in thermo-foaming of powder dispersions in molten sucrose can be consider-ably decreased by using magnesium nitrate as a blowing agent. The faster foamingand foam setting is by the faster –OH condensation due to the catalytic effect of nitricacid produced by the hydrolysis of magnesium nitrate. The magnesium nitrate notonly modulates the porosity and cell size but also changes the foam microstructurefrom cellular- to reticulate-like. Figure 16 shows the typical foam microstructureproduced with and without magnesium nitrate blowing agent.

Fig. 14 Schematic representation of (a) particles at the air-liquid interface and (b) particlestabilization of wet foams

20 S. Vijayan et al.

Low-density silicon carbide foams are also prepared by the thermo-foamingof silicon powder dispersions in molten sucrose. In this, the carbon requiredfor reaction with silicon to form silicon carbide is derived from the sucrose. Thethermo-foaming of silicon powder dispersions in molten sucrose at appropriatesilicon to sucrose weight ratio followed by carbonization at 900 �C in inertatmosphere produces stoichiometric carbon-silicon composite foams. Subsequenthigh-temperature (~1500 �C) heat treatment in inert atmosphere results in thereaction between carbon and silicon to form SiC foams. The density of the SiCfoams is controlled in the range of 0.08–0.168 by controlling the foaming temper-ature and magnesium nitrate concentration. The SiC foams have interconnectedcellular structure with cell sizes in the range of 0.55–1.12 mm. Cell walls of theSiC foams are decorated with SiC nanowires grown by vapor-liquid-solid and vapor-solid mechanisms. The SEM photograph showing the cellular structure and SiCnanowires on the cell wall surface is shown in Fig. 17.

Ceramic Foams from Pre-ceramic Polymers

A pre-ceramic polymer is a polymer which on pyrolysis yields a ceramic. Theceramic materials prepared from pre-ceramic polymers are known as polymer-derived ceramics (PDC). They include silicon carbide (SiC), silicon oxycarbide(SiOC), silicon nitride (Si3N4), SiO2, SiNC, etc. The earliest known pre-ceramicpolymers are organosilicon compounds. They were used for the preparation ofsilicon-based ceramics such as SiC and SiOC. The pre-ceramic polymers are widelystudied for the preparation of SiC and SiOC ceramic foams. The general structure ofsome of the pre-ceramic polymers used for the preparation of ceramic foams isgiven in Fig. 18.

Fig. 15 Flowchart of thermo-foaming process and photograph of alumina foam

Processing of Ceramic Foams for Thermal Protection 21

The methods for the preparation of foams from pre-ceramic polymer includedirect foaming, polymer foam replication, and sacrificial template. The directfoaming involves two techniques, the melt foaming of the precursor and foamingof the precursor solution using blowing agents. The melt foaming method employsthe use of a powder precursor. In this, the pre-ceramic polymer powder is melted andthen transferred to a preheated mold at temperatures in the range of 250–270 �C. Themolten polymer starts to cross-link; the gases evolved during this process foamthe molten polymer which is then set by the completion of the cross-linking, forminga thermoset foam. The green foam is then pyrolyzed in inert conditions or airatmosphere to get the ceramic foam. In some of the cases, the foaming is dueto blowing agents mixed with the precursor prior to the melt foaming stage.

Fig. 16 Foam microstructure of alumina foam produced (a) with and (b) without magnesiumnitrate blowing agent

Fig. 17 SEM showing (a) cellular structure and (b) nanowire growth on cell walls of thermo-foamed SiC foam

22 S. Vijayan et al.

In the second case, the pre-ceramic precursor solution is foamed at room tempera-tures using blowing agents and set by the cross-linking of the polymer precursor.The green foam is dried and then pyrolyzed or sintered to get the final product.The polymer foam replication method uses polyurethane foam template. Thepre-ceramic polymer solution is impregnated into the polyurethane foam till maxi-mum coverage is ensured. The excess solution is then drained, the impregnated foamis dried, and the template is removed by pyrolysis at 600 �C to get the green.The sintering of the green foam in inert atmosphere results in the ceramic foam.In the sacrificial template method, pore templates such as polymer beads, expandedmicrospheres, microbeads, etc. are dispersed within the pre-ceramic polymer andthen cross-linked by heating at temperatures between 180 �C and 250 �C. The greenbody is then dried, pyrolyzed to remove the template, and sintered in an inertatmosphere to get the ceramic foam. The SEM micrograph of SiC foam preparedfrom a polycarbosilane by polymer foam replication method is shown in Fig. 19.

The freeze casting and emulsion templating methods are also studied for thepreparation of ceramic foams from pre-ceramic polymers. In the former, apre-ceramic polymer solution is frozen to crystallize the solvent, and then the solventcrystals are removed by sublimation to form pre-ceramic polymer foamswith aligned pores. Subsequent pyrolysis and sintering produce ceramic foams.The emulsion route uses emulsification of pre-ceramic polymer solution in a non-aqueous solvent using a suitable emulsifying agent followed by drying, cross-linkingof the pre-ceramic polymer, and pyrolyzing to produce ceramic foams withinterconnected cellular structure. The maximum porosity of SiC and SiOC foamsachieved by various processing methods from different pre-ceramic polymers issummarized in Table 3.

Fig. 18 Structures of pre-ceramic polymers used for the preparation of SiC and SiOC ceramicfoams (R1, R2, R3, and R4 stand for alkyl or aryl groups)

Processing of Ceramic Foams for Thermal Protection 23

Thermal Conductivity of Ceramic Foams Prepared by VariousMethods

Thermal insulation helps to reduce power consumption during thermal processingof materials and also protects the surrounding space from thermal hazard. Generally,ceramic foam materials possess high-temperature stability, low thermal conductivity,and thermal shock resistance, and these are key requirements for thermal insula-tors [6, 47]. Both open-cellular and closed-cellular foams with high porosity areexcellent candidates for thermal insulation. However, closed-cell ceramic foamshave lower thermal conductivity values compared to open-cell foams due to theirisolated pore structure and hence are more suitable for thermal insulation. The foamsneeded for thermal insulation must have near identical pore size distribution andhigh porosity. Normally, gelcasting and direct foaming techniques are used for thepreparation of such foams. Table 4 lists the thermal conductivity of ceramic foamsprepared by various processing routes. The ceramic foams prepared by variousroutes exhibited thermal conductivity in the range of 0.027–0.37 W/mK. Thethermal conductivity of ceramic foams is further decreased by filling their poreswith aerogels [48, 49]. The ceramic foam skeleton provides adequate strength for theaerogels which are otherwise known to be highly fragile. On the other hand,the aerogel filled in the cells of ceramic foam materials provides very high thermalinsulation.

Summary

Ceramic foams are ideal candidates for thermal protection materials due to their lowthermal conductivity, high-temperature stability and thermal shock resistance, andlow density. The ceramic foams are prepared from the respective ceramic powders or

Fig. 19 Microstructure ofSiC foam prepared bypolymer foam replicationusing polycarbosilane

24 S. Vijayan et al.

pre-ceramic polymers. A wide range of processing methods starting from powderpressing using fugitive pore-forming additives to freeze casting of powder suspen-sions is used for the development of ceramic foams. The colloidal processingmethods provide better control over the porosity, pore size, pore morphology, andpore interconnectivity. Direct foaming, freeze casting, and emulsion templating arethe major colloidal processing routes for ceramic foam production. The wet foamsproduced by particle stabilization mechanism have more thermodynamic stabilitywhich enables their drying without foam collapse even without setting. On the otherhand, the surfactant-stabilized foams require an additional setting mechanism by insitu polymerization or coagulation to avoid the foam collapse. The emulsion-basedmethods use immiscible liquid droplets dispersed in ceramic powder suspension,whereas in freeze casting, the crystals of dispersion medium produced on freezingact as pore templates. The freeze casting produces ceramic foams with homogenouspore structure as well as aligned lamellar pore structure depending on the freezingcondition and additives used to control solvent crystallization. The thermo-foamingusing powder dispersions in molten sucrose enables the production of alumina,mullite, and SiC foam bodies with interconnected cellular structure. SiC and SiOCfoams are prepared from pre-ceramic polymers using most of the abovementionedprocessing routes.

Table 3 Maximum porosity of ceramic foams prepared from pre-ceramic polymers by variousprocessing methods

Sl. no Pre-ceramic polymer Process Foam Porosity (%)

1. Methylhydroxylsiloxane Direct foaming SiOC 94

2. Methylhydroxylsiloxane Replication SiOC 90

3. Methylpolysiloxane Replication SiC 92

4. Methylpolysiloxane/PMMA Sacrificial template SiC 85

5. YR3370 polysiloxane/PMMA Sacrificial template SiOC 80

6. Methylpolysiloxane/rice bran Sacrificial template SiOC 46

7. Polymethylsiloxane/PMMA Sacrificial template SiC 76

8. Polyallylhydridocarbosilane Emulsion templating SiC 35

9. Poly(methylsilsesquioxane) Emulsion templating Si/C/O

40

10. Poly(methylsilsesquioxane) Emulsion templating SiOC 80

11. Polycarbosilane/HIPE Emulsion templating SiC 92

12. PCS Nippon type S Direct foaming SiC 45

13. Methyl phenyl poly(silsesquioxane) Melt foaming SiOC 87

14. Silres(polysiloxane)/TBA-camphene

Freeze casting SiOC 94.8

15. Polymethylsiloxane Freeze casting SiOC 74

Processing of Ceramic Foams for Thermal Protection 25

Table

4Therm

alcond

uctiv

ityof

ceramicfoam

sprepared

byvariou

sprocessing

routes

Sl.no

.Ceram

icfoam

Processingroute

Porosity

(vol%)

Therm

alcond

uctiv

ity(W

/mK)

Reference

1.Mullite

Gelcasting

860.09

[50]

2.Alumina

Foam

gelcastin

g97

.50.12

[51]

3.Ano

rthite

Foam

gelcastin

g91

0.01

8[52]

4.Y2SiO

5Foam

gelcastin

g92

0.05

4[53]

5.Alumina

Directfoam

ing

910.05

[54]

6.Ano

rthite

Directfoam

ing

940.04

2[55]

7.Zirconia

Particle-stabilized

foam

ing

97.9

0.02

7[56]

8.Mullite-Corun

dum

Directfoam

ing

73.7

0.28

7[57]

9.Siliconcarbide

Therm

o-foam

ing

97.5

0.07

6[58]

10Kaolin

Directfoam

ing

950.23

[59]

11Mullite

Foam

gelcastin

g81

0.14

[60]

12Glass

ceramic

Directfoam

ing

800.37

[61]

13Diatomite

Foam

gelcastin

g84

.50.09

7[62]

14Kaolin

Directfoam

ing

900.05

4[63]

15MgA

l 2O4

Foam

gelcastin

g75

.10.24

[64]

16γ-Y2Si 2O7

Foam

gelcastin

g84

.90.23

[65]

17Mullite

Directfoam

ing

77.3

0.37

[66]

18YSZ

Freezecasting

39.25

0.04

5[48]

19Y2SiO

5Freezecasting

86.4

0.06

9[49]

20Mullite

Sacrificialtemplate

790.114

[67]

21Mullite

Freezecasting

910.23

[68]

26 S. Vijayan et al.

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