Keywords

1 Introduction

Recently, the utilization of porous ceramics has become great consideration due to its outstanding mechanical strength, chemical, abrasion resistance, etc. (Eom et al. 2013; Colombo 2008). Some benefits of porous ceramics are illustrated in Fig. 1. The broadly considering porous ceramics are silica, alumina, titania, magnesium oxide, zirconia, etc. (Colombo 2002). The pore structure, morphology, pore size, pore wall, porosity, the density of the pore strut, and interconnection of pores have a significant role in the properties of porous ceramics (Sakka 2005; Liu and Chen 2014). The particle size distribution of ceramic powders, the concentration of binder, types of binders, fabrication, etc., can influence the porosity and pore size (Ohji and Fukushima 2012). Typically, the particle size of raw ceramic powder is about two to five times greater than pores, for acquiring the needed pore size. And, the greatest service temperature of porous ceramics is around 1000–2000 °C. The pores are averted in ceramic materials due to intrinsically brittle behaviour than the polymeric or metallic products. The enhanced usage of porous ceramics in environments such as elevated temperatures, wear, and corrosive media (Studart et al. 2006; Guzman 2003; Gauckler et al. 2009). The applications of porous ceramics are mainly implemented in the field of environmental protection, energy and in chemical engineering as liquid gas filters, catalysis supports (Gao and Shi 1999; Scheffler and Colombo 2005). In addition, porous ceramics are widely considering in biological applications, due to their eminent melting point, resistance to surface damage, and electronic properties (Pokhrel et al. 2013). The porous ceramics can be used in water treatment by decorating the silver nano-particles (Uthaman et al. 2021b) on porous ceramic composites (Lv et al. 2009). The porous ceramics itself does not have the ability to prevent the growth of bacteria. The incorporation of metal nanoparticles such as silver nanoparticles that possess excellent antibacterial properties (Lal et al. 2021a, b; Uthaman et al. 2021c) by coating on the ceramic surface induce antibacterial properties. Furthermore, the porous ceramics can either have interrelated voids surrounded through ceramic or have a closed void within an uninterrupted ceramic matrix. Interrelated or reticulated porous ceramics are utilised in industrial filters, diesel engine exhaust filters, etc. (Al-Naib 2018).

Fig. 1
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Benefits or properties of porous ceramics

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2 Classification of Porous Ceramics

In the case of porous ceramics, there is not a commonly admitted classification because these are mainly depending on the application purposes and principles. However, the fundamental characteristics of porous ceramics is depending on the classification of pores. The classification of porous ceramics (Al-Naib 2018) is shown in Fig. 2. These classifications aim to arrange the pores, based on structure, shape, etc. (Ohji and Fukushima 2012).

Fig. 2
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Classification of porous ceramics: reproduced with permission from Al-Naib (2018)

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  1. 1.

    According to chemical components: The chemical components of the initial materials such as silicate, oxide, aluminosilicate, etc. (Guzman and Sysoev 1975).

  2. 2.

    According to inner structure: Fibrous, cellular, and granular.

  3. 3.

    According to porosity: Based on porosity, super-high, high, and moderate. Super-high is above 75%, high is in between 60 and 75%, and moderate is between 30 and 50% (Guzman and Sysoev 1975). These types contain two phases as solid ceramic and gas-filled porous phase (German 2013). The closed pores comprised of gases are independent of the environment (Misyura 2016). The porosity of ceramic body can be divided into two such as open and closed porosity. Open porosity means that can access from the outside and are of two types such as open dead-end pores and open pore channels. The addition of open and closed porosity is called total porosity. The open porosity enhances frictional porosity. The closed porosity can dominate if the materials have less frictional porosity.

  4. 4.

    According to nature of porosity, size and volume fraction of pores (Gaydardzhiev 2008), there are two kinds of nature of porosities are found; natural and synthetic ceramics. The porosity nature of natural ceramics depends on their genesis, while in the case of synthetic ceramics, it depends on their production, which could control.

  5. 5.

    According to pore size, there are three types, such as microporous (<2 nm), macroporous (>50 nm), and mesoporous (2–50 nm). The pore size is determined through mercury intrusion porosimetry technique.

2.1 Different Methods for Enhancing the Porosity of Porous Ceramic Materials

  1. 1.

    For the primary moulding mixture, the choosing of granular composition (Berkmam and Mel’nikova 1969).

  2. 2.

    Incorporating the synthesized or natural filler grains into the mixture (Kitaitsev 1970).

  3. 3.

    Incorporating additives in the primary mixture and then followed removal through the evaporation, sublimation, dissolution, etc.

  4. 4.

    By thermal treatment, swelling of the individual or total mixture.

  5. 5.

    Through mechanical treatment, the involvement of air into ceramics suspensions.

  6. 6.

    Polymer cellular matrix and ceramic suspension were impregnated and followed by squeezing, drying, and thermal treatments.

  7. 7.

    Addition of ceramic fibres in to a mixture and moulding with binder and thermal treatment of moulded materials (Kashcheev and Strelov 1992).

  8. 8.

    The formation of a honeycomb structure by extrusion moulding of plasticised ceramic mixture.

3 Fabrication of Porous Ceramics

3.1 Particle Stacking Sintering

In this process, the porous ceramics are made through the sintering of the aggregate particles. Similar particle sizes as the aggregate particles are incorporated for the interconnection. The additives containing thermal expansion coefficient (TEC) capable with aggregate particles, solid reactions with the aggregate particles, and wettable in the liquid phases at high temperatures (Zhang and Li 2003). The formation of porous ceramics can happen through the stacking of ceramic particle sintering because of the ceramic particles sinterability. Initially, tiny similar-type particles are imparted after that sintered, in order to connect high aggregate particles (Zhu and Su 2000). The aggregate particles are joined with others by a number of points to produce great number of three-dimensional interconnected networks. Generally, the higher aggregate particles can have higher average pore sizes, and besides, the lesser the aggregate particles can have greater pore distribution. Moreover, through calcination or low-density ceramic powders can produce solid-sintered pores (Sepulveda 1997). Usually, the sintering process is partially implemented and isothermal static pressing could be considered for ceramics having greater than 50 nm pore sizes (Nettleship 1996).

The porous ceramics having great strain bearing capability, low weight and young’s modulus are commonly utilised in structural portions (Yang et al. 2001). The low porosity (<0.4) is produced through pressing ceramic powders to the needed size and density could control via the amount of powders. In addition, the improvement in contact strength could protect the porous materials from mechanical collapsing. The schematic representation of the changes occurring with particles under sintering (Dorozhkin 2013) is shown in Fig. 3.

Fig. 3
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Schematic representation of the changes occurring with particles under sintering: reproduced with permission from Dorozhkin (2013)

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3.2 Addition of Pore-Forming Agent

3.2.1 Powders

There are several applications that are considered with the addition of a pore-forming agent to produce porous ceramics. In the case of ceramic powders, the volatile-type pore-forming agents are incorporated and the formation of pores, later the volatilisation at high temperatures, and subsequently, the products can be developed with complicated shapes and various pore structures (Zhu and Su 2000). This method is almost same as to the common ceramic preparation. To attain the porous ceramics, the ceramic powders were combined with carbon powders, fibres, wood scraps and organic powders such as flour, naphthalene and later pressed and sintered. The content of agent has a significant role in the pore size, volume and pore distribution. Furthermore, the open-cell porosity enhances with the content of the agents (Li and Wu 2000; Sepulveda 1997). The ceramic powders can combine not only with organic powders but also with inorganic salts. However, the inorganic salts are difficult to melt though dissolvable after sintering (Li and Wu 2000). Incrementing the sintering temperature and expanding, the time can decrease the porosity and improve the density, and thereby, the pore wall and porous ceramics strength can enhance (Zhang et al. 2002). The pore structures contain tiny and big pores and can prevent the content of the polymer powders to recognize the accurate control of the permeability of the final product. The schematic representation of the pore-forming agent method for porous ceramic production (Ribeiro et al. 2019) is shown in Fig. 4.

Fig. 4
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Schematic representation of the pore-forming agent method for porous ceramic production. Reprinted from Ribeiro et al. (2019), with permission

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In the case of porous hydroxyapatite, the manufacturing process is occurred by the addition of pore-forming agents like olefin, naphthalene and PMMA. While heating, the TEC of the pore-forming agents may raise and can achieve 10 times than the hydroxyapatite at thermal decomposition temperature. Furthermore, the higher variation in TEC could results a great number of cracks and decrease in strength (Yao et al. 2000). However, the carbon powders TEC is around near to the hydroxyapatite and less cracks are formed and growth in mechanical strength can be found. The porous hydroxyapatite having improved sinterability and strength was developed via the incorporation of carbon powder agents and biological glass. The hydroxyapatite is a type of biological active material. The development of green body can take place through the combining of binders, hydroxyapatite powders, biological glass, carbon powders and dispersants together and followed ball milling, drying, pressing and isostatic moulding. The commonly considering pore-forming agents, particle size is about 124–147 μm and around 30 volume % content. For the green product, the disperser is organic phosphate and about 1200 °C sintering temperature. Yao et al. reported that PMMA can use as pore-forming agent for producing porous hydroxyapatite ceramics (Yao et al. 2001). It was observed that porous structure was similar and pore size and porosity are <200 μm and <50% respectively (Yao et al. 2000).

3.2.2 Slurry

The carbon powders or organic volatile pore-forming agents are implemented into the ceramic slurry. While sintering, these agents may burn and remains several pores are formed as porous ceramic. Nowadays, the eco-friendly type of porous ceramics is developed by using starch as the pore-forming agent. The starch and the aqueous slurry of the ceramic powders are produced and subsequently added into the mould (infiltration is not required) and heated at a temperature of 50–70 °C (Bowden and Rippey 2002). Then, the starch powders interacted with water can extend the expansion of the powders and the absorption of water. And the liquid slurry can convert to a solid body as the shape of the mould. After demoulding, drying, and sintering, the final product can be achieved. The porosity and the pore size are depending on the starch powder content and size since, after sintering agents burned and remains pores. However, this method has several benefits such as easy to operate, less expensive, and porosity can be controlled. Instead of starch, the rice, potato powders, or the combination of this also can be considered as the binder and forming agent.

For the application purposes, some significant properties of starch powders are binding, gel, densification and generate membranes. Moreover, these are insoluble in water below 50 °C so the starch powders could be use and not influence on the structure. At the same time, above 55–80 °C can destroy the intermolecular bond and by absorbing the water, and the powders may expand. The thermal insulation components, infiltration components, gas combustors, and biological ceramics, etc., are produced by using this method and some products with different shapes.

3.3 Polymeric Sponge Impregnation Process

In 1963, the organic foam impregnation was patented (Schwartzwalder and Somers 1963). This is the most commonly considering method for producing the porous ceramics. These are most widely utilised to produce open-cell three-dimensional reticulated porous ceramic materials. Here, the organic foam is impregnated as preformed ceramic slurry and followed burning, for attaining porous ceramics. Rather than ceramic slurry, the colloidal solution or sol–gel could be applied (Li and Wu 2000). The schematic representation of polymeric sponge impregnation method is shown in Fig. 5. Impregnating organic slurry process can attain great porosity of around 70–95% moreover and is simple and cheap fabricating method (Zhu and Jiang 2000). Compared with other process, the speciality of this method is that the products pore structure, i.e., open-cell three-dimensional reticulated, is about similar to that of organic foam precursors. The pores in the organic foam can influence the pore size and also the coated layer thickness, slurry drying and shrinkage while sintering.

Fig. 5
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Schematic representation of polymeric sponge impregnation method

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3.3.1 Choosing Organic Foam and Pretreatment

While choosing an organic foam, the pore size must have an importance since; plastic foams pore sizes can decide the porous ceramic products pore size. So, the organic plastic foam should carefully determine according to the necessity of pore sizes and porosity of the products (Zhu and Jiang 2000; Wang 1997). Besides, the foam contains sufficient resilience and their vaporization temperature is less than the sintering temperature. Some fundamental factors for the organic foams are

  • Enough rebound resilience to remove excess slurry.

  • Open-cell reticulated structure to assure the impregnation of ceramic slurry and inter bonding.

  • Hydrophilicity to ingest the ceramic slurry.

  • Volatilisation at lower sintering temperature.

The organic foams having these fundamental factors are polymerised sponges after exposed to some foaming methods. For example, polyurethane, cellulose, polyvinyl chloride, etc. (Zhu and Jiang 2000; Montanaro et al. 1998). The morphology of the porous silica ceramics with different pore sizes is shown in Fig. 6 (Wen et al. 2007). And also, these are open-cell reticulated structure, and the cells are linked by solid 3D skeletons forming connected open channels. The template of organic polymer sponge can influence the cell size of porous ceramics.

Fig. 6
figure 6

Morphology of the porous silica ceramics with different pore sizes a 5.7 mm pore size and 78 vol.% porosity, b 5.3 mm pore size and 77.8 vol.% porosity, c 4.4 mm pore size and 77.3 vol.% porosity, d 3.2 mm pore size and 76.2 vol.% porosity, e 2.4 mm pore size and 74.9 vol.% porosity, f 2.2 mm pore size and 74.6 vol.% porosity. Reproduced from Wen et al. (2007) with permission of Elsevier

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3.3.2 Preparation of Slurry and Impregnation

The proper selection of ceramic powders is a significant factor for the application purposes (Zhu and Jiang 2000). The slurry contains liquid solvents, ceramic powders and additives. Commonly, the liquid solvent is aqueous and also organic solvent, such as ethanol, is considered. The additives usually used are plasticizers, surfactants, binders, defoamers, and dispensers. The solid part of the slurry must be as much as possible higher, in order to control the shrinkage and cracking that can happen after molding and drying and also for enhancing the coating content in the organic foam. The solid part is about 50–70%, and water is around 10–40% (Zhu and Jiang 2000).

The use of binders could increase the strength of a green body after drying and also protect breakage when removing the organics to assure the mechanical strength of the sintered product. Different types of binders are found such as organic and inorganic binders. Some examples of organic binders are poly vinyl alcohol, and inorganic binders are potassium/sodium silicate, phosphate, borate, etc. The type of binder can influence the properties of the final products.

Another important factor is the rheological properties of the slurry. The slurry should be flowable and is thixotropic. The flowability can assure the impregnation of the slurry into the organic foam and homogeneous coating on the pore wall. Thixotropy can decrease the viscosity and boost the moulding process. After moulding, the slurry viscosity can improve and the flowability reduces and adhesion of slurry on the pore wall solidifies.

The surfactant also has a significant role in the preparation of slurry. In the case of liquid slurry, if the organic foam has very less wettability, then the slurry will be thick in the area of cross section of the foam. This can induce cracking in sintering and decrease the strength of the porous ceramics. The usage of surfactant can increase the adhesion and properties of the materials (Uthaman et al. 2021a).

After preparation of the slurry, next section is the impregnation of slurry. The organic foam contains air that must be moved out through vacuum adsorption, hand rubbing, or any other methods before impregnation. After impregnation of the organic foam, the excessive slurry needs to removed (Wang 1997). In the organic foam impregnating method, the moulding of the slurry dipped organic foam have an important role.

3.3.3 Drying and Sintering of Green Bodies

After extrusion of the excess slurry, the porous green bodies ensure to be dried. The drying can be done in shade, oven drying, or any other way. For sintering, the green bodies are added to a kiln; during this, the content of water is less than 1% (Zhu and Jiang 2000). The sintering can be occurred in two phases such as high-temperature and low-temperature phase. In low temperature phase, the green bodies are slowly heated for completely removing the organic foam. And if fast heating, the decomposition of organic ejects higher amount of gas to break and pulverize the green bodies. The organic foam slurry impregnation method, the continuous impregnating, spraying, drying, and fibre reinforcement can be implemented to increase the structure and properties of the final products (Zhu and Jiang 2000). The presintering treatment needs to be considered for the green bodies to acquire the strength.

3.3.4 High Strength of the Ceramic Foam

The strength of the porous ceramics is an important factor. The strength of the ceramic foams could be improved through a higher content of coated slurry, modified sintering, and siliconizing (Zhai et al. 2008). Another method to enhance the properties of ceramic foams is by second coating, second phase toughening, siliconizing, and sintering modification (Zhai et al. 2008). The method of impregnation of slurry is illustrated in Fig. 7.

Fig. 7
figure 7

Slurry impregnating method

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3.4 Foaming Process

In 1970s, this method was discovered for the production of porous ceramics with the foaming agents such as calcium hydroxide, aluminium sulphate, hydrogen peroxide, etc. (Wang 1997). The pre-processed raw materials are put into the mould and heated at around 900–1000 °C. Upon the pressure and in the oxidizing atmosphere, the powders were bonded and eject gas from the foaming agents to fill in the mold and after cooling the porous ceramics were incurred. There are different types of process; green body foaming, slurry foaming etc. The slurry foaming process is the cheap and much better way to produce high-strength porous ceramics (Sepulveda 1997). This method is impelled via the gaseous phases in the ceramic slurry, comprised of water, ceramic powders, polymer binders, gels, and surfactants. In addition, the anionic surfactants are more stable and have great foaming ability. The generation of foam can be prepared by mechanical foaming, exothermic reaction gas releasing, foaming agent, and evaporating low melting point solvent, etc.

The foaming method was discovered in 1974, for the manufacturing of polyurethane foam and ceramic foaming were conducted together to form a homogeneous distribution of ceramic powders in the organic foam (Wang 1997; Wood et al. 1974). The fabrication procedure of foam (Ishikawa 2010) is shown in Fig. 8.

Fig. 8
figure 8

Procedure for the fabrication of foam from Ishikawa (2010)

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The porous ceramics were produced at room temperature, and atmospheric pressure was invented in 1978. The materials used are acids and phosphates, alkali metal silicates, foam stabilizers, etc. (Wang 1997). The ceramic foams could produce through foaming by blending aqueous solution of the metal carbonates having multivalences and metal phosphates (Hirschfeld et al. 1996; Yoshino and Iwami 1980). The slurry formed by the ejection of carbonates as CO2 and the oxides plays as a curing agent. The viscosity can measure by the content of water; if the water content is very less, the developed foaming structure is protected from shrinkage. Another type is partly closed-cell porous ceramics, produced via blending surfactants, silica gel, and methanol. Then the foaming agent of freon is imparted to the foam and solidified followed by sintering (Fujiu et al. 1990). The increase in solid content can enhance the mechanical properties of the product. However, the greater amount of solid content can raise the slurry viscosity and pretends the formability and making cracks in the materials. Some of the benefits of foaming gel method is that it is easy to prevent the shape, density, and the compositions (Chen et al. 2009). Based on (Xu et al. 2016) report, Fig. 9a–c represents the SEM images of the pore structure of Al2O3 ceramics and magnified images of pore ridge, pore wall. And Fig. 9d, e represents the good permeability of porous Al2O3 ceramics to water and ethanol.

Fig. 9
figure 9

a SEM images of pore structure of Al2O3 ceramics, and magnified images of pore ridge (b) and pore wall (c); d porous Al2O3 ceramics having good permeability to water; and e porous Al2O3 ceramics with (floating) and without (sank) a silicone layer outside the ceramic showing different floating status in ethanol. Reproduced from Xu et al. (2016) with permission of Elsevier

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Compared with the organic foam impregnation, the slurry foaming could be considered for the preparation of open-cell and closed-cell foamed bodies. The organic foam impregnation can be used for developing only open-cell materials. The importance of surfactant usage is that, can keep the stability either in the liquid-gas interface of the slurry or the solution in the slurry foaming method. And also, the type of surfactant can affect the density and pore of the ceramic foams.

3.5 Sol–Gel Process

The microporous ceramic materials are commonly developed with the sol–gel process having nanometre range pore size (Zhang and Li 2003). Besides, these can consider for producing the porous materials contain high regularity (Zhu and Lu 2001). In this method, there are mainly three steps (Nettleship 1996) such as

  • The metal oxide sol needs to be attained through the hydrolysis of the metal alkoxides dissolved in less alcohol mixed with water.

  • The amorphous gel could be achieved via the polycondensation of the nanosized metal oxide particles through regulating the pH.

  • The porous metal oxide ceramics can obtain through drying the gel and then organic material decomposing by heating.

The foam products could develop via the sol–gel process through stabilizing the bubbles since the viscosity gets incremented while the transformation of sol to gel (Sepulveda 1997). By foaming the SiO2 gel system, many ceramic systems such as silica colloid, zirconia, and boehmite can be produced. In this method, the porous structures developed in the green body can be considered as the three categories such as templating, foaming, and solvent evaporation method (Li et al. 2020) as schematic represented in Fig. 10.

Fig. 10
figure 10

Schematic representation of sol–gel process (Li et al. 2020)

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3.5.1 Templates

The templates play an important role in the case of sol–gel method. There are various types of templates found.

  1. (a)

    Stacking with Uniform Particles Template

Recently, the products having same structure as to the natural opals are synthesized, and a process to produce this type of new material is by replicating the colloidal crystal structure. These materials have a broad range of periodic structures produced with template as the aggregate of colloidal crystalline grains and subsequently transfer the sol into the interspaces and taking out the colloidal particles (Velev and Kaler 2000). To develop the template, the colloidal particles are filtered by a smooth filter membrane, and after accumulating, the particles are crystallized. Then, by using a cationic surfactant, the latex layer is washed. The zirconia, porous titania, silica, etc., were developed using sol–gel process having the template of the colloidal crystal through the agglomeration of latex particles.

  1. (b)

    Organic Foam Template

The materials or products have around 97% porosities, <200 μm pore sizes can obtain from the sol–gel process or coating of a colloidal solution on organic foam (Brown et al. 1993; Hirschfeld et al. 1996). The range of pore size for organic foam is about 100–45 pores per inch and about 8–10 μm pore wall thickness. The organic foam is dipped in to the sol–gel solution, and to absorb the solution, the foam is enlarged. This enlarged foam is rolled in order to eliminate the excess solution. After that, it is dried at room temperature for two days and aged at temperatures around 40–60 °C for two days, then at 110 °C for 10 h, and lastly sintered at a temperature of 1200 °C for 24 h.

3.6 Other Processing Process of Porous Ceramics

3.6.1 Gel Casting

This method is applied for developing near reticulated ceramic materials (Kazumichi et al. 1997). In this method, by internal chemical interactions and in-situ solidification, a green body having uniform microstructure with great density can be produced (Jiao and Zhu 2007; Omatete and Janney 1991). The slurry could be foamed and solidified from the in-situ polymerization, and the reticulated green body has high strength. The premixing, casting, drying, and sintering are the main steps (Zhu and Lu 2001). Generally, this method was generated for the compact materials, and this was used to produce the porous ceramics to enhance the ceramic slurry foaming method. The monomer in the slurry could be in-situ polymerised to produce gel structures; can protect from collapsing of the foamed body. This foamed body attains high strength by the combination of gel and foaming (Sepulveda 1997). The schematic representation of gel‐casting process (Hooshmand et al. 2019) is shown in Fig. 11.

Fig. 11
figure 11

Schematic representation of gel‐casting process. Republished from Hooshmand et al. (2019)

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The uniform slurry is produced with a solution of ceramic powder, dispersers, waters, and monomers. Then in the container, without the presence of oxygen, the surfactants are incorporated; can form the slurry. Also, the initiators and catalyst are imparted to boost the polymerisation. Following, the gel (rubber structure) is dried and burned to move out the polymer, and the dense ceramics are incurred. The ceramic powders like hydroxyapatite, alumina, zirconia, etc., could consider for this method (Sepulveda 1997). This type of processing can be broadly considered in the automobile, electronics, and defence industries.

The addition of surfactants into the gas–liquid interface of bubbles in the slurry for stabilization. The surfactants contains hydrophilic and hydrophobic parts (Uthaman et al. 2021a). While sintering, the surfactants transmit to the gas–liquid interface and the hydrophobic component is tailored towards the gas surface in order to decrease the surface tension and also to stabilize the bubbles. Non-ionic surfactants are commonly considered for this method.

The gel casting method have many characteristic properties such as controllability in casting and solidifying steps, high amount of solid components having large particle powders, high strength and processability of green body, and low shrinkage while drying and sintering (Wang et al. 1995).

3.6.2 Wood Ceramics

When woods undergo physical and chemical treatments, for attaining a porous carbon material, porous oxide, and porous ceramic base composites and to the final products are generated by the process are known as wood ceramics (Qian et al. 2003). The materials such as wood, bamboo, waste paper, etc., are used. These are classified into two types such as carbon base and SiC base ceramics. The carbon base ceramics are the porous carbon materials achieved by carbonisation at elevated temperature of soaked wood or wood powders with thermosetting resin as phenolic resin followed by drying and solidifying (Zeng et al. 2007, 2008). The amorphous carbon is attained via the carbonisation of porous wood powders, and its high strength assures that the wood ceramics contain high mechanical properties. The SiC base ceramics are formed through the elevated temperature pyrolysis of natural wood in an inert atmosphere and then infiltration into liquid silicon at 1600 °C.

For high-temperature applications, the materials need the properties such as high mechanical strength, thermal, and corrosion resistance (Lal et al. 2020; Uthaman et al. 2020). Porous SiC ceramics contains all these properties and are broadly applied as structural materials in high-temperature applications such as filters and carriers for catalyst (Liu and Chen 2014; Sieber et al. 2002). Nowadays, the production of porous SiC ceramics by transmitting of biological organic materials as wood is popular.

3.6.3 Freeze-Drying Process

The benefit of freeze-drying process is that the freezing effect of aqueous slurry, prevents the direction of ice formation and results the sublimation of ice by drying in low pressure, leading in a green body. And then, the porous ceramics having complex pore structure are formed after sintering. The sintering temperature and freezing temperatures can affect the pore size distribution and microstructure. Compared with freezing of chemical solutions, these have better properties like low shrinkage, high mechanical properties, wide range of porosity control, and eco-friendly (Fukasawa et al. 2001). The microstructure of porous ceramic fabricated by using freeze-drying method is shown in Fig. 12.

Fig. 12
figure 12

Microstructures of porous ceramics fabricated by freeze-dry process a cross-sectional perpendicular to the macroscopic ice growth direction and b cross-sectional parallel to the macroscopic ice growth direction. Reproduced from Fukasawa et al. (2001) with permission of Springer Nature

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3.6.4 Hollow Sphere Sintering

This method is widely used for producing closed-cell ceramics and these are simple and easiest process for producing the porous ceramics (Green 1988; Hirschfeld et al. 1996). The schematic representation of hollow sphere fabrication (Zhang et al. 2019) process is shown in Fig. 13. In this method, the hollow spheres were added in to the mould under pressure to accomplish a green body. Microwave heating is used for sintering the hollow spheres. The blending of glycerol and binders could increase the energy absorption, and then sintering can carried out in air. The SEM image of silica hollow spheres and fly ash hollow spheres (Zhang et al. 2019) are shown in Fig. 14.

Fig. 13
figure 13

Schematic representation of a fabrication of hollow sphere ceramic process, b SEM image of green body, and c SEM image of porous ceramics and pore size distribution. Reproduced from Zhang et al. (2019) with permission of Springer Nature

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Fig. 14
figure 14

SEM image of hollow spheres of a silica and b fly ash. Reproduced from Zhang et al. (2019) with permission of Springer Nature

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4 Porous Ceramic Honeycombs

The honeycomb structure is created by using extrusion moulding technique; contains raw materials, extrusion, drying and burning (Chen et al. 2009). The materials such as binder, plasticiser, ceramics, and lubricants were added into the pug and mix homogenously in a stirrer. Then, the pug needs to under ageing in order to form uniformly distributed liquid phases. By extrusion using a reticulated mould, the ceramic honeycomb green body is produced. Followed by drying and sintering, the regular-shaped porous ceramic honeycomb can develop (Pu et al. 2004). The limitations of these ceramic honeycomb are poor strength, fast deformation, bubbles, cracks can happen. The topologies of extruded and sintered ceramic honeycomb (Zhang et al. 2015) and FeCrAl honeycomb (Zhou et al. 2007) are shown in Fig. 15. Generally, the ceramic honeycombs have square, triangle, hexagonal parallel channels. The materials like aluminium titanate, cordierite, zeolite, etc., are used to produce the ceramic honeycombs. The honeycomb ceramics have high surface area, corrosion resistance and low thermal expansion. However, the metallic-type honeycombs are better than ceramic honeycomb since, they have high thermal conductivity, fracture toughness, etc. (Zhang et al. 2015).

Fig. 15
figure 15

Topologies of extruded and sintered a ceramic honeycombs and b FeCrAl honeycombs. Reproduced from Zhang et al. (2015) with permission of Elsevier

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5 Porous Ceramic Composites

Porous ceramic composites are divided into two kinds such as porous-reinforced ceramic composites and porous ceramic composite materials. Porous-reinforced ceramic composite is a composite generated from the infiltration of other materials in to the open-cell porous ceramics (Mclean 1997; Ning et al. 2001). Porous ceramic composites contain a base ceramics and other materials. Some examples of porous ceramic composites are Cu, α-SiC and graphite added to Si–O–C base ceramics for generating a conductive modified porous material (SiC and C are conductive materials, and conductivity is about 1 × 102–2.2 × 107 Ω−1 m−1) (Liu 2010).

6 Conclusion

Porous ceramics are highly considering in many applications due to their attractive properties. The porosity of the material has a significant role in porous ceramics. The fabrication methods such as particle stacking sintering, the addition of pore foaming agents, polymeric sponge impregnation, sol–gel, foaming, gel casting, freeze-drying, etc., are profoundly emphasized in this chapter. Among these, the most commonly using fabrication technique is the polymeric sponge method. Each method possesses its advantages and disadvantages. The porous ceramics are widely considering in industrial application, due to the inherent properties such as cost effectiveness, mechanical strength, corrosion resistance, etc. Several problems are still required to be solved based on the preparative techniques of porous ceramic materials. In addition, the need for eco-friendly fabrication of porous ceramic materials is in higher demand. The future works on the porous ceramic material need to solve all these related problems.