Journal of Materials Science & Technology  2019 , 35 (7): 1255-1260 https://doi.org/10.1016/j.jmst.2019.03.038

Orginal Article

Preparation and properties of a new porous ceramic material used in clean energy field

Shuming Wanga*, Xiaofang Zhanga, Fenghua Kuangb, Jiangshan Lia, Yanxin Wanga, Ruiping Wanga, Yanru Wanga, Xin Linc, Jianming Lid

a Department of Materials Science and Engineering, University of Science & Technology Beijing, Beijing, 100083, China;
b Ceramic Science Institute China Building Materials Academy, Beijing, 100024, China
c State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi’an, 710072, China;
dBohai Shipyard Group Co., Ltd., Huludao, 125004, China

Corresponding authors:   *Corresponding author.E-mail address: wangshuming@ustb.edu.cn (S. Wang).

Received: 2019-01-27

Revised:  2019-03-2

Accepted:  2019-03-18

Online:  2019-07-20

Copyright:  2019 Editorial board of Journal of Materials Science & Technology Copyright reserved, Editorial board of Journal of Materials Science & Technology

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Abstract

At high temperature, the oxide redox reactions of ceria can split H2O and CO2 to produce H2 and CO, so porous ceria with high temperature resistance and high specific surface area has an important foreground in clean energy applications. In this work, a reticulated porous ceria ceramic material with interconnected porous structure was prepared by the impregnation technique using organic polyurethane sponges as template. The influences of pretreated sponge, dipping time length, pore size and sintering temperature on the porosity and strength of the porous ceria ceramics were systematically studied. With the increasing sintering temperature, the glass phase occurred and led to an increase in strength, but an decrease in porosity. Eventually, we analyzed the relationships between porosity and strength of these porous materials, aiming to provide theoretical and practical references for its application in clean-energy field.

Keywords: Porous ceramics ; Porous ceria ; Impregnation ; Clean-energy

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Shuming Wang, Xiaofang Zhang, Fenghua Kuang, Jiangshan Li, Yanxin Wang, Ruiping Wang, Yanru Wang, Xin Lin, Jianming Li. Preparation and properties of a new porous ceramic material used in clean energy field[J]. Journal of Materials Science & Technology, 2019, 35(7): 1255-1260 https://doi.org/10.1016/j.jmst.2019.03.038

1. Introduction

Oxidation-reduction reactions of metal oxides can be used to split H2O and CO2 into a mixture of H2 and CO, and this mixture gas can be compressed into a precursor of conventional liquid fuels. The products after combustion of the liquid fuel can be fully recycled to effectively mediate the emission of carbon dioxide [[1], [2], [3]]. In addition, compared with the traditional thermal decomposition methods, using metal oxide oxidation reduction to prepare H2 and CO doesn’t have the problem of CO/O2 and H2/O2 separation, so it is an efficient and environment-friendly cleaning energy source preparation method, which provides a valuable way for the preparation of H2/CO clean energy. Compared with ferrite or other oxygen-deficient metal oxides, ceria has a better redox activity for its rapid O2- production capacity, so it is a promising material for this application [[4], [5], [6]].

The principle of splitting H2O and CO2 with ceria is as fellows [7,[1], [2], [3], [4], [5], [6], [7], [8], [9]]:

High temperature reduction reaction:

CeO2 = CeO2-δ + 1/2O2

Low temperature reduction reaction:

CeO2-δ + δH2O = CeO2 + δH2

CeO2-δ + δCO2 = CeO2 + δCO

In the reduction reaction of δ = 0.06, temperature is higher than 1500 °C, and oxygen partial pressure is higher than 10-5 bar. When temperature is lower than 1400 °C, the reduction reaction stops and oxidation reaction starts. Meanwhile, to maintain this reduction reaction and improve the splitting efficiency, H2O and CO2 should fully contact with ceria. So preparing reticulated porous ceria ceramics with high porosity and high temperature strength is the key factors to realize its application [[10], [11], [12]]. But, usually high porosity will lead to decline in strength of the pours ceramics. Therefore, to prepare high-porosity and high-strength reticulated porous ceria ceramics, it is necessary to explore the relationships between porosity and strength.

Due to limited references on preparation and application of porous ceria ceramics in clean-energy field, it is essential to carried out further theoretical and practical studies [[13], [14], [15]]. In this work, we prepared reticulated porous ceria ceramics by foam dipping, studied the influences of sponges’ pretreatment, dipping times, pore size and sintering temperature on the porosity and strength of porous ceria ceramics, and systematically analyzed the relationships between porosity and strength.

2. Experimental

2.1. Preparation of porous ceria ceramics

The preparation processes of porous ceramic are shown in Fig. 1.

Fig. 1.   Flow chart for preparing reticulated porous ceria ceramics.

In the sponge matrix pretreatment [16], polyurethane sponge with aperture of 10 ppi and 35 ppi was used. The pretreatment was to increase the porosity of the ceramics and the amount of slurry on the skeleton, including immersion of sodium hydroxide solution (10%, 60 °C, 15 min) and carboxymethyl cellulose solution (CMC; 1.5%, 12 h). Those two solutions played roles of hydrolysis and surface modification, respectively [[9], [10], [11]]. During the preparation of slurry, ceria powder (particle size < 5 μm, purity 99.99%) was mixed with deionized water at a ratio of 5:2, followed by a dispersant, a binder (PVA) and a defoamer (glycerol trifoliate) with constant stirring, and the proportion of the three additives were 1.2%, 1.5% and 0.05%. Then ball milling was performed for 3 h to mix ceria and additives evenly. The pretreated polyurethane sponge was immersed in the ceramic slurry, and excess slurry of the sponge skeleton was removed by extrusion after fully absorbed. Then it was dried in air. After 1 h, the sponge was immersed in the ceramic slurry again, and repeated for 2-4 times. Then the immersed sponge was placed in an oven for drying (60 °C, 24 h). The dried sponge was heated in a box-type resistance furnace (0.5 °C/min heating rate up to 600 °C, 1 °C/min heating rate up to 850 °C, 2 °C/min heating rate up to 1050 °C, followed by furnace cooling), then the green ceramic without sponge matrix was final sintered (5 °C /min heating rate up to 200 °C, 2 °C/min heating rate up to 1200 °C, 1 °C/min heating rate up to 1600-1650 °C, holding for 3 h, followed by furnace cooling).

2.2. Characterization techniques

Size and distribution of the preheated powder were measured with a Mastersizer 3000 laser diffraction particle size analyzer (Malvern Instruments Co. Ltd.). The specific surface area of the powder was measured by BET method using an IQ2 type specific surface area and porosity analyzer (Contac, USA). A Zetasizer NanoZS90 (Malvern Instruments Co., Ltd.) was used to measure Zeta potential of the powder by electrophoretic light scattering.

Rheological properties of the slurry were measured using an NXS-11B viscometer (Chengdu Instrument Factory, China) and shear rate is in the range of 0-800 s-1. Compressive strength of the ceramic samples was measured with a CDW-5, 5 kN fine ceramic tester (Changchun Chaoyang Test Instrument Co., China). Microscopic morphology of the powder was observed by scanning electron microscopy (SEM, LEO1450, USA). An X-ray diffractometer (DMAX-RB, Japan) with Cu Kα radiation at 0.1548 nm was used to identify the phase structure.

3. Results and discussion

3.1. Effects of sponge pretreatment

Polyurethane sponge is an organic matter, and its adhesion to the ceramic slurry is poor, so it is hard to be completely infiltrated on the surface of the untreated sponge skeleton even after multiple immersions [[17], [18], [19]]. And in the following processes, when the sponge matrix was removed, the sponge skeleton was collapsed, making it hard to get a complete porous ceria ceramic. In order to increase the amount of ceramic slurry on the sponge skeleton, we pretreated the polyurethane sponge. The process involved hydrolysis of sodium hydroxide solution (10%, 60 °C, 15 min.) and surface modification of CMC solution (1.5%, 12 h). The influences of pretreatment are shown in Fig. 2, and the role of sodium hydroxide is: 1. Polyurethane sponge is a high polyester compound. When it encounters alkaline oxide under certain conditions (10%, 60 °C, 15 min), hydrolysis reaction occurs. Thus the membrane between sponge holes is removed and the porosity of the sponge is increased. 2. The surface roughness is increased, which is conducive to the attachment of the slurry, while its hydrophilic performance is not fundamentally changed [20,21]. The role of CMC solution is a surface modifier, which provides sponge surface with strong hydrophilic in order to achieve the purpose of improving the adhesion between the polyurethane sponge and slurry [22,23].

Fig. 2.   Raw body that the sponge (a) was not pre-treated and (b) was pre-treated.

3.2. Influences of dipping times and sponge aperture

When the sponge immersed in the slurry just one time, the slurry adhered to the sponge skeleton was very limited, which decreased the strength of sintered ceramic. In the process of removing the sponge matrix, the phenomenon of collapse occurred [24]. So it is necessary to immerse the sponge in the slurry for multiple times. Too much immersion time, however, may cause the phenomenon of plugging, leading in decreasing of the porosity of sintered ceramic. Therefore, it is necessary to find the appropriate dipping time frames. Fig. 3 shows the sintered ceramic (35 ppi) under different dipping times. We can see that the appropriate number of dipping cycles is three. In addition, changing the solid content and the flowability of the slurry can also effectively improve the pulp hanging effect, thereby improving the performance of the sintered body.

Fig. 3.   Sintered body under different dipping runs: (a) 2; (b) 3; and (c) 4.

Fig. 4(a) and (b) shows a sintered ceramic body made by foamed sponges with pore sizes of 35 ppi and 10 ppi respectively. When the sintering temperature is 1600 °C and the aperture is 35 ppi, the sintering shrinkage is about 8%; when the aperture is 10 ppi, the sintering shrinkage is about 10%. Therefore, as aperture increases, the sintering shrinkage gradually increases. The sintered powder was subjected to XRD analysis, and the results are shown in Fig. 5. The results show that ceria did not undergo valence state transformation during sintering. In addition, the diffraction peak of the powder after the sintering is sharper, and the half-width is narrower, which shows that after sintering ceria was well crystallized and accompanied with crystalline grain growth. Table 1 shows the porosity and compressive strength of porous ceramics with different apertures. It can be seen that the porosity increases with the increasing in sponge aperture, which leads to splitting efficiency improving of the ceria at high temperature, but the high temperature strength of ceramic decreases with the increase in the sponge aperture. Therefore, in order to suppress the decrease in the strength of the porous ceria ceramic while maintaining the high porosity, it is possible to take appropriate measures such as increasing the amount of solid phase of the ceramic slurry, increasing the sponge dipping times, increasing the sintering temperature and time length, etc.

Fig. 4.   Sintered body with (a) 35 ppi and (b) 10 ppi.

Fig. 5.   XRD spectra of green powder and the powder of sintered body.

Table 1   Performance of reticulated porous ceria ceramics with different pore size.

Pore sizePerformance
Porosity (%)Comprehensive strength (MPa)
10 ppi72.520.13
35 ppi63.790.24

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It can be seen from SEM results shown in Fig. 6 that, at high temperature the end of sponge skeleton has been volatilized, leaving a clear hole with the size of the millimeter level. This is an important reason of the low porosity of the obtained ceramic. Fig. 7(a) and (b) shows the skeletal fracture and the skeletal surface, respectively. It can be seen that there are many micro-sized holes in the fracture and the surface, which is beneficial to improve the porosity of the porous ceramic, so as to improve the splitting efficiency of the ceria porous ceramics [25].

Fig. 6.   Microphotographs of fracture section of the sintered body: (a) 10 ppi, (b) 35 ppi.

Fig. 7.   Microphotographs of (a) skeleton surface and (b) fracture section of sintered body.

3.3. Relationships of sintering temperature and strength

Through scanning electron microscopy photographs, it is found that the ceria ceramic skeleton has many micropores, which is the reason why the strength of sintered ceramic is low [26]. In order to improve the sintering property of the ceramic, we increased the sintering temperature to 1650 °C, and the obvious glass phase appeared in individual area (Fig. 8). Since the glass phase exists between the crystalline grains, it increases the density of the ceramic, and thus improves the strength of the porous ceramic skeleton.

Fig. 8.   Glass phase on the surface of sintered body.

Fig. 9 shows the cerium oxide ceramic strength changing with sintering temperature. It can be seen that, as sintering temperature increased, the strength of the sintered body was obviously improved, because the content of the glass phase obviously increased with sintering temperature. However, as we know, the increasing in glass phase will decrease the porosity of the porous ceramics, so just the optimal sintering temperature can make a balance between porosity and strength.

Fig. 9.   Comprehensive strengths at different temperatures.

4. Conclusion

A new porous ceramic material with high porosity and good strength at high temperature were prepared using the foam impregnation approach. The pretreatment of sponge is an important part during the preparation of the porous ceria ceramic because of the poor adhesion of the ceramic slurry to the sponge matrix. As the sponge aperture increases, the sintering shrinkage and porosity of the porous ceria ceramics increase gradually, and the compressive strength decreases correspondingly. The glass phase occurred gradually with increasing the sintering and pretreatment temperature and time, which can increase the compressive strength of the sintered body. So, the high sponge aperture with reasonable sintering and pretreatment process are crucial to fabricate high performance porous ceria ceramics used in clean energy producing.

Acknowledgement

The authors gratefully acknowledge the financial supports from the fund of the State Key Laboratory of Solidification Processing in NWPU (SKLSP201704).

The authors have declared that no competing interests exist.


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