Ceramic foams with highly open channel structure from direct foaming method in combination with hollow spheres as pore-former

To satisfy the ever-increasing demands for high-performance ceramic foams that could be applied in catalysts loader, ltrations and adsorptions, it is critical to develop technologies for ceramic foams with open channel structure. In this work, we fabricated ceramic foams with three-dimensional porous structure specially with open channels by combining direct foaming method with adding pore-forming agent method. There are two levels of length scale present in this hierarchically porous structure, that is, foam structure with spherical pores evolved from bubbles, and open pores on the cell wall derived from silica hollow spheres with thin shell as the pore-former. Hierarchical ZrO 2 based foams with porosity of 86.5%-95.1% and compressive strength of 2.05-5.67 MPa, and Al 2 O 3 based foams with porosity of 86.2%-91.0% and compressive strength of 6.8-13.2 MPa were fabricated. The prepared ceramic foams characterized by this open channel structure are promising to perform outstandingly in the abovementioned elds due to their uniform pore size, low density as well as high mechanical strength.


Introduction
Owing to their excellent properties involving unique pore structure, low density, high speci c surface area, outstanding speci c strength as well as low thermal conductivity etc., ceramic foams are playing indispensable role in numerous elds as either structural or functional materials [1][2] . Therefore, it has become a research hotspot to develop novel porous materials with low cost, low bulk density and various microstructures to satisfy the ever-increasing application demands. From topological standpoint, porous ceramic materials can be categorized into closed pore structure and open pore structure. Closed pore structure usually endows porous materials with high strength and better thermal insulation property, while open pore structure promotes better permeability [3] . So far, direct foaming method, replica method and adding pore-forming agent method are mostly selected three methods for the preparation of porous materials [4][5][6][7] . As is well known, direct foaming method is featured by high-porosity level and closed pore structure, and usually chosen for the elds of thermal insulations and light-weight structural components [8] . In comparison to ceramic foams with closed pores, ceramic foams with open channel structure play important roles due to their high accessible speci c surface area, and they are mainly applied in catalysis supports, selective separation, biomaterial engineering, etc., leading to more and more attention [9] . The replica method is one of the commonly employed routes to achieve highly open pores. A primary disadvantage of this technique is that the mechanical strength of the porous scaffolds is not favorable, as it is di cult to avoid the formation of microcracks in the struts during the template removal process [3] .
Moreover, the burning out process of the polymer sponges would cause serious energy consumption and increase burden on the environment protection.
To meet the urgent requirements for multifunctionalities, the design and synthesis of hierarchically porous ceramics with three-dimensional porous frameworks and open channels become of great importance in recent years. In the current climate of sustainable development, we fabricated ceramic foams with hierarchically porous and open-channel structure by combining two methods in this study, i. containing particles and hollow spheres are prepared via direct foaming method, which would generate foam structure. And hollow spheres serving as pore-former could favor forming secondary pores of open windows on the cell wall. Such a strategy employing bubbles as foam template and hollow spheres as pore-former offers several advantages, such as high porosity level, ordered pore structure, free of binder removal step, environmental friendliness and high speci c strength. Both ZrO 2

Materials
In this study, ZrO 2 particles with average particle size of 0.48 μm and boehmite nanoparticles with average particle size below 100 nm were employed to prepare ZrO 2 based foams and Al 2 O 3 based foams, which were provided from Guangdong Orient Zirconia Co., Ltd., China, and Hangzhou Zhihuajie Technology Co., Ltd., China, respectively. Silica hollow spheres with diameter of 5-80 μm were commercially purchased from Forsman Scienti c (Beijing) Co., Ltd., China. Long-chain surfactant sodium dodecyl sulfate (abbreviated to SDS) which was employed as foaming agent, was purchased from Sinopharm Chemical Reagent Co., Ltd., Shanghai, China. Agar was also purchased from Sinopharm Chemical Reagent Co., Ltd., China. The pH of colloidal suspension was tailored via 2 mol/L HCl solution.

Preparation
In the case of ZrO 2 based foams, homogeneously colloidal suspension containing ZrO 2 particles and deionized water was prepared via ball milling for 4 h. The suspension with pH of 7.0 was heated in 80°C water bath, followed by adding hot concentrated agar solution with temperature of 80°C. Then, SDS was added into the above suspension, followed by frothing via mechanical stirring with high speed of 2000 rpm. During mechanical stirring, silica hollow spheres were added little by little. The concentration of ZrO 2 particles, silica hollow spheres, agar and SDS in the nal suspension were 30 wt%, 6 wt%, 1 wt%, 0.05 wt%, respectively. The foamed suspension was cooled down rstly with temperature of 2°C for 5 mins, and then dried at room temperature.
In the case of Al 2 O 3 based ceramic foams, homogeneously colloidal suspension containing 30 wt% boehmite nanoparticles and deionized water was prepared by mixing them under 80°C water bath for 24 h. Subsequently, SDS was added followed by mechanical stirring in room temperature with high speed of 2000 rpm. During mechanical stirring, sieved silica hollow spheres (with nal concentration of 6 wt%, size smaller than 20 μm) were added little by little, and then HCl solution was added drop by drop, which decreased pH below 4.0 to promote the gelation of boehmite foams. The obtained foams were dried in a humidity chamber with temperature of 30°C and humidity of 90%. Both dried Al 2 O 3 based foams and ZrO 2 based foams were sintered in a furnace with a heating rate of 3°C /min and a dwelling time of 2 h.
The apparent densities were calculated by the ratio between mass and dimensions of machined specimens. Total porosity was calculated according to equation , where P is the total porosity, and D b are the true density and bulk density of ceramic foams, respectively. The microstructure of ZrO 2 based foams and Al 2 O 3 based foams was observed by scanning electron microscope (SEM, MERLIN VP Compact, Carl Zeiss, Jena, Germany). The compressive strength was measured using a universal material testing machine (AG-IC 20 kN, Shimadzu, Japan) with loading speed of 1 mm/min, and at least three specimens with cubic shape and dimensions in the range of 10-20 mm were measured to calculate average values.

Results And Discussion
As illustrated in Fig. 1, the foams are subjected to solidi cation with the assistance of agar molecules in the case of ZrO 2 based foams [11] , and rapid gelation of boehmite nanoparticles was triggered by decreasing pH in the case of Al 2 O 3 based foams [12] . Therefore, the dried foams exhibit favorable strength, that keep the whole porous green body holding together and be free of breaking. The hollow spheres exhibit excellent spherical morphology, large-volume cavity, very thin shell and low softening temperature. It is found that these spheres melt when sintering temperature increases to around 1000°C, which is much lower than that of Al 2 O 3 and ZrO 2 . In the direct foaming process, close assembly of particles at water/air interfaces would cause the mostly closed pores after sintering [13] . By contrast, hollow spheres inserting in the cell wall would occupy the space rstly, exactly acting as pore former, and thus facilitate the achievement of open windows on the cell wall after their melting during heat treatment process.
As shown in Fig. 2, the formation of open windows on the cell wall of ZrO 2 foams sintered above 1000°C leads to the interconnection of pores. The obtained ZrO 2 based foams exhibit hierarchically porous structure in two levels of length scale, that is, spherical pores evolved from frothed bubbles, the pore diameter of which are in the range of 100-300 μm and open windows on the cell wall with diameter around 20-80 μm, which are derived from the hollow spheres. Such a strategy combines the advantage of direct foaming method and adding pore-forming agent method, and consequently endows the obtained ZrO 2 based foams with high porosity level, uniform pore size and open channel structure, further contributing to the enhancement of strength. These features render ceramic foams with an easy penetration of gas or liquid, which expands their application as functional materials in a broad range of speci c hi-tech elds, including catalytic loader, ltration and adsorption, etc.
As can be seen from Fig. 3(a), in addition to m-ZrO 2 , ZrSiO 4 and very small amount of stishovite are detected in the ZrO 2 based foams. Fig. 3(b) demonstrates the compressive strength of ZrO 2 based foams prepared at different sintering temperatures. Highly porous ZrO 2 based foams with porosity ranging from 86.5% to 95.1% are synthesized in this work, which possessing compressive strength of 2.05-5.67 MPa.
The shrinkage, porosity and strength of ZrO 2 based foams can be controlled by sintering temperature. It is noted that high sintering temperature above 1400℃ is normally needed for ZrO 2 ceramics to realize densi cation of grains and high strength [14][15] , while the foams reported here sintered at 1000℃ still display excellent mechanical strength, up to around 2 MPa. This value is very similar to pure ZrO 2 ceramic foams prepared with only ZrO 2 particles, which possesses porosity of 95.4% and compressive strength of 2.1 MPa [15] . The achievement of both low sintering temperature and high strength is attributed to the silica hollow spheres with thin shell and low sintering temperature that would melt and ll in the space among insu ciently sintered ZrO 2 grains, acting as high-temperature binder, and therefor contributing to the formation of dense wall without gaps between grains, as can be seen clearly in Fig.  2(b).
In addition to ZrO 2 based foams, we also demonstrate the fabrication of Al 2 O 3 based foams with open channel via the approach illustrated in Fig. 1(b). Similar structure is observed, as shown in Fig. 4(a-d).
After sintering, boehmite would transform to Al 2 O 3 via losing bound water and hydroxyl groups [13] . It is  Fig. 4(f)). Therefore, the combination of direct foaming method and inorganic hollow spheres with low melting point as pore-former is a universal and feasible approach to fabricate ceramic foams with open pores. Such a strategy is featured by no discharge of hazardous substances, low energy consumption due to low sintering temperature, which is a promising and eco-friendly approach for the fabrication of ceramic foams with open channels, low density, and uniform pore size.

Conclusions
A simple, versatile, low-cost and eco-friendly approach has been proposed to fabricate ceramic foams with hierarchically porous structure as well as open channels. The overall three-dimensional porous structure was built by employing bubbles as templates, which is bene ted from the direct foaming method and contributes to the uniform pore distribution. Both ZrO 2 and Al 2 O 3 based foams are fabricated based on the gelation of foams, which could be achieved by temperature-transformation of agar-included suspension or pH-initiated gelation of boehmite nanoparticles, respectively. The secondary pores were designed via selecting silica hollow spheres as pore-former. Hollow spheres with low melting point, thin shell wall, high volume cavity enable the formation of open windows during sintering process. The prepared ZrO 2 and Al 2 O 3 based foams with three-dimensional porous skeleton and interconnected pores behaves low bulk density and relatively high compressive strength, enlarging their potential applications in the elds of gas/liquid ltrations, lightweight structural components, catalyst supports and the like.