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Preparation of novel reticulated porous ceramics with hierarchical pore structures
Journal of Alloys and Compounds 806 (2019) 596e602
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Preparation of novel reticulated porous ceramics with hierarchical pore structures Ruoyu Chen a, Wenbao Jia a, b, Dong Lao a, Shujing Li c, Daqian Hei a, * a
College of Materials Science and Technology, Nanjing University of Aeronautics and Astronautics, Nanjing, 211106, China Collaborative Innovation Center of Radiation Medicine of Jiangsu Higher Education Institutions, Nanjing, China c The State Key Laboratory of Refractories and Metallurgy, Wuhan University of Science and Technology(WUST), Wuhan, 430081, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 19 April 2019 Received in revised form 23 July 2019 Accepted 24 July 2019 Available online 24 July 2019
Al2O3-ZrO2 reticulated porous ceramics (RPCs) with hierarchical pore structures were prepared by vacuum infiltration. Al(OH)3 was applied to provide the pore structure. Vacuum infiltration using an alumina/aluminum hydroxide slurry via a pre-sintering cycle was performed to fill up the hollow struts generated due to burnout of the polyurethane foam template and the coating on the Al2O3-ZrO2 ceramic struts. The results showed that as the Al(OH)3 content in the infiltration slurry was increased from 5% to 13%, the pore size distribution of the ceramic struts widened and the major pore size decreased. In addition, a compressive residual stress developed within the multilayered struts because of the difference in thermal expansion of the Al(OH)3-Al2O3 coating layer and Al2O3-ZrO2 ceramic struts. Moreover, with increasing Al(OH)3 content in the infiltration slurry, the porosity of the ceramic struts gradually increased, resulting in a decrease in Young's modulus and thermal conductivity, which influenced the compressive residual stress and thermal stress within the ceramic struts. Furthermore, the potential of the fabricated RPCs as catalyst supports was demonstrated. © 2019 Elsevier B.V. All rights reserved.
Keywords: Al2O3-ZrO2 reticulated porous ceramics Hierarchical pore structures Residual stress Aluminum hydroxide Catalyst support
1. Introduction Porous ceramics with high porosity are the preferred materials for a wide range of applications such as high-temperature catalyst supports, lightweight structural components, and radiant burners owing to their high inner geometric surface areas, permeability, and excellent chemical stability at high temperatures [1e3]. Notably, optimization of their pore sizes and structures is important for widening their practical applications. Several methods have been developed for the fabrication of porous ceramics with hierarchical pore structures, such as the direct foaming technique, pyrolysis of various organic additives, and gel casting [4e6]. Although these techniques yield high-surface-area porous ceramics with hierarchical pore structures, the mechanical properties of the obtained porous ceramics are significantly weak [7,8]. Currently, the polymer sponge replica technique is one of the most commonly used method to produce porous ceramics with high porosity, low density, and open three-dimensional network structures from polyurethane foams [9,10]. In general, the pore size
* Corresponding author. Tel.: 86þ18068800965, fax: 86þ025 52112626. E-mail address: [email protected] (D. Hei). https://doi.org/10.1016/j.jallcom.2019.07.287 0925-8388/© 2019 Elsevier B.V. All rights reserved.
of the polyurethane foam determines the macropore size and macrostructure of the porous ceramics. Notably, Furler and coworkers successfully fabricated ceria with a reticulated porous ceramic (RPC) structure and dual scale porosity in the millimeter and micrometer ranges by adding a pore-forming agent to the ceramic slurry [11]. Chen et al. fabricated Al2O3-ZrO2 RPCs with hierarchical pore structures and excellent mechanical properties by vacuum infiltration, CaCO3 decomposition, and calcium hexaluminate grain growth [12]. The high porosity and strong grain bonding of these materials were attributed to the addition of Al(OH)3, which underwent 60% volume contraction during decomposition, producing fine Al2O3 grains [13e15]. Chen and coworkers reported that alumina underwent g-to-a transformation, accelerating densification of alumina particles and generating a large number of surface pores on the particles [16]. It is necessary to develop an effective and simple process for the fabrication of RPCs with hierarchical pore structures and excellent properties. In this study, we prepared Al2O3-ZrO2 RPCs with hierarchical pore structures and excellent properties by vacuum infiltration with Al2O3/Al(OH)3 slurries, based on in situ pore formation by Al(OH)3. The mechanical properties of the as-prepared RPCs were investigated; moreover, the effect of the addition amount of
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Al(OH)3 on the morphology and thermal shock resistance was studied. Finally, the potential of these materials as highperformance catalyst supports was demonstrated. 2. Experimental procedure 2.1. Raw materials The main raw materials used were active alumina(AMA-40, d50 < 5 mm, Al2O3 99.3%, Hubei Smile New Materials Co., Ltd., China.), zirconia(MSZ-DM, 5mol% MgO, d50 ¼ 0.98 mm, Jiangxi Size Materials Co., Ltd., China.), nano-Al(OH)3(Al2O3 99.9%), and opencell polyurethane foam sponge (20 ppi; 20 mm 50 mm 50 mm). 2.2. Experimental setup and method Typically, an Al2O3-ZrO2 slurry (solid content ¼ 76 wt%) was prepared using alumina powder, zirconia powder, polycarboxylate (dispersant), polyvinyl alcohol 20-90 (binder), carboxymethyl cellulose (thickening agent), and absolute ethyl alcohol (antifoam agent) by ball milling for use as a strut material. An open-cell polyurethane foam sponge (20 ppi; 20 mm 50 mm 50 mm) was impregnated with the Al2O3-ZrO2 slurry and dried at 80 C for 24 h, followed by sintering at 800 C to prepare an Al2O3 preform. Then, Al2O3/Al(OH)3 infiltration slurries with a solid content of 50% were prepared by mixing the dry powders (Table 1) for use as porous coating materials; the mixtures were stirred for 60 min. The fabricated Al2O3-ZrO2 preforms were then immersed into the asprepared infiltration slurries and a vacuum of 0.5 Pa was applied for 10 min. Then, the Al2O3-ZrO2 preforms were dried at 80 C for 24 h and sintered at different temperatures (1380, 1480, and 1580 C) for 3 h to obtain Al2O3-ZrO2 RPCs. In addition, the fabricated Al2O3-ZrO2 RPCs were coated with a TiO2 layer by immersing them into TiO2 infiltration slurries, followed by vacuum drying in an oven under a vacuum of 0.5 Pa. The as-prepared RPCs were dried at 80 C and heated at 650 C for 1 h to obtain TiO2-coated Al2O3-ZrO2 RPCs. 2.3. Characterization techniques The bulk densities of the RPC specimens were calculated from their mass-to-volume ratios. The apparent porosity of the RPCs was measured by Archimedes' method. The RPC compressive strength (CS) was tested using a hydraulic universal testing machine (ETM, Wance, China) at a crosshead speed of 0.1 mm/min. The median diameter of the pores within the ceramic struts was determined by mercury porosimetry (Quantachrome PM60GT-18, Quantachrome Instruments Ltd., USA). The thermal shock test was performed as follows: the prepared RPCs were heated from 25 C to 1100 C, and held at this temperature for 30 min. Then, the samples were taken out and immersed in water for 20 min [17,18]. After the thermal shock test, the residual strength (RS) of the RPCs was measured. The thermal shock resistance of the RPCs was evaluated from the RS/CS ratio. Moreover, the RPC microstructure was characterized by fieldemission scanning electron microscopy (FESEM, Quanta 400; FEI Company, Hillsboro, OR, USA). The microstructure of RPC with TiO2
597
layer was characterized by scanning electron microscopy(SEM, Phenom ProX; Phenom Scientific Company, Netherlands). The phase compositions of the RPCs were determined by X-ray diffraction (XRD, X'Pert PRO, Philips, Netherlands). Furthermore, the simulative calculation of residual stress distribution in the ceramic struts was performed using the finite element method. Finally, the photocatalytic activity of the RPCs with TiO2 coating layer were measured as previously reported(Fig. 1) [12]. 3. Results and discussion 3.1. Microstructure and pore size distribution of Al2O3-ZrO2 RPCs The SEM images of fractured surfaces of Al2O3-ZrO2 RPC samples are shown in Fig. 2. Notably, a large number of pores existed on the surface of the ceramic struts. The pores were formed due to the decomposition of Al(OH)3, phase transformation of alumina, and particle packing. As observed from Fig. 2(a)e(c), the calcination temperature significantly influenced the size of the pores within the ceramic struts. As observed from Fig. 3, in the low-temperature range (<700 C), Al(OH)3 decomposed into H2O and g-Al2O3. Upon heating, g-Al2O3 converted into a-Al2O3, as follows: Al(OH)3 / gAl2O3 / q-Al2O3 / a-Al2O3, creating pores with different sizes within the ceramics struts, as shown in Fig. 2. In addition, with an increase in the calcination temperature from 1380 C to 1480 C, densification occurred, and the pore size gradually decreased. At the same time, vermicular colonies were formed (Fig. 2(b)) via coalescence of nuclei during the phase transformation of alumina instead of growth by grain boundary migration, which resulted in the formation of pores [19], as schematically shown in Fig. 4. Moreover, with an increase in the Al(OH)3 content to 13% (specimen A), the major pore size decreased to the sub-micron level. as shown in Fig. 2(b). Nevertheless, with further increase in the calcination temperature from 1480 C to 1580 C, the small pores within the ceramic struts disappeared due to densification and grain growth, but the stable pores grew (Fig. 2(b) and (c)). Thus, the increase in the calcination temperature led to coarsening of pores. This was attributed to the fact that normally in porous ceramics, with increasing calcination temperature, the sizes of pores and grains increase, while their amounts decrease, as observed in specimen C [20]. On the basis of the above results, we selected the Al2O3-ZrO2 RPC samples with different Al(OH)3 contents sintered at 1480 C for
Table 1 The formulations of the Al(OH)3-Al2O3 infiltration slurries(wt%). Specimens
Al2O3
Al(OH)3
FS
PVA
CMC
Slurry-A Slurry-B Slurry-C Slurry-D
95 91 87 /
5 9 13 /
0.2 0.2 0.2 /
/ / / /
/ / / /
Fig. 1. Experimental setup measure the photocatalytic activity of the different RPCs.
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the mercury intrusion porosimetry test. The pore size distributions of different specimens are presented in Fig. 5. The major pore diameter of the Al2O3-ZrO2 RPCs with 5% Al(OH)3 content was 1.33 mm. However, with an increase in the Al(OH)3 content to 13%, the major pore size decreased and the peak height increased due to the decomposition of Al(OH)3 and phase transformation of alumina. Moreover, the pore size distribution widened due to Al2O3 particle packing, resulting in the formation of larger pores. However, with increasing Al(OH)3 content, the number of large pores gradually decreased. This phenomenon was attributed to the fact that nano-Al(OH)3 filled the interspace between the large Al2O3 particles, transforming the large pores into small pores. In addition, the apparent porosity of the Al2O3-ZrO2 RPCs increased with increasing Al(OH)3 content (Table 2). From these results, it could be deduced that a high Al(OH)3 content was beneficial to improving the porosity of the RPCs, which could promote the loading capacity of the catalyst. 3.2. Mechanical properties and thermal shock resistance of Al2O3ZrO2 RPCs According to the microstructure and pore size distribution analysis, among the Al2O3-ZrO2 RPC samples, those sintered at 1480 C were the most suitable products for industrial applications; the linear shrinkage of the Al2O3-ZrO2 RPCs sintered at 1580 C was too large to ensure product stability and the strength of the products sintered at 1380 C was not sufficiently high to ensure product quality [17]. Fig. 6 shows the physical characteristics of different Al2O3-ZrO2 RPCs, revealing a marginal increase in linear shrinkage, bulk density, and CS with increasing amount of nano-Al(OH)3. This result was inconsistent with that reported by Deng et al. [13], owing to the large specific surface area of nano-Al(OH)3, which accelerated the sintering reaction of the alumina particles. In addition, the decomposition of nano-Al(OH)3 caused 60% volume reduction, leading to the formation of a large number of pores within the ceramic struts, which negatively affected the mechanical properties of the Al2O3-ZrO2 RPCs. Therefore, the CS of specimens A and C were different. The thermal shock resistance of the as-prepared specimens is presented in Fig. 6(b). Interestingly, the Al2O3-Al(OH)3 coating layer was beneficial to improving the thermal shock resistance of the specimens, because the coating layer caused an increase in the thickness of the ceramic struts and repaired the defects and flaws in the ceramic struts. Moreover, with increasing Al(OH)3 content in the infiltration slurry, the residual strength ratio first increased, and then decreased, because of the presence of micropores within the ceramics struts and thermal conductivity of the coating layer [21]. When the Al(OH)3 content in the infiltration slurry was less than 13%, the residual strength ratio was high. On the other hand, at high Al(OH)3 contents, a large number of pores were formed within the ceramic struts due to the decomposition of Al(OH)3 and phase transformation of alumina, which led to a decrease in the thermal conductivity of the ceramic struts (Eq. (1)), resulting in the generation of thermal stress within the coating layer [22].
l ¼ ls ð1 PÞ
Fig. 2. The microstructure of the Al2O3-ZrO2 reticulated porous ceramics(a) sintered at 1380 C, (b) sintered at 1480 C and (c) sintered at 1580 C.
(1)
where l is the thermal conductivity of the ceramic struts, ls is the thermal conductivity of the solid phase, and P is the volume fraction of pores in the ceramic struts. As shown in Fig. 7, a large thermal stress was introduced at the surface and edge of the struts of the specimens. Particularly, the thermal stress within the ceramic struts increased with the addition of Al(OH)3, and the maximum thermal stress was located at the edge of the inner layer
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Fig. 3. XRD patterns of products obtained from Al(OH)3 after heat treatment at 700, 900, 1000 and 1480 C.
of specimen C, negatively affecting the mechanical properties of the RPCs. This was attributed to the presence of a large amount of micropores within the coating layer. Moreover, it has been reported that a difference in thermal expansion and sintering shrinkage of the coating layer and matrix causes a residual stress within the product [23,24]. Moreover, the porosity of the materials significantly influenced Young's modulus, which in turn affected the residual stress. The residual stresses within the multilayered struts were calculated according to Eq. (2) [25]. Tð0
!,
sR ¼ Ei ti ðai ao ÞdT
Fig. 4. Illustration of the in-situ pore forming of Al(OH)3.
½ti Ei ð1 no Þ=Eo þ 2to ð1 ni Þ
(2)
T
where a is the thermal expansion coefficient, n is Poisson's ratio, E is Young's modulus, and t is the layer thickness; To and T are 1480 C and 25 C, respectively; and the subscripts i and o stand for the matrix and coating layers, respectively. According to the equation, as Young's modulus of the coating layer (Eo) decreases (with all other parameters fixed), the residual stress decreases. Notably, the elastic properties of porous materials over an entire porosity range can be described using the power-law empirical relationship proposed by Phani and Niyogi (Eq. (3)) [26]:
Eo ¼ Eð1 P=Pc Þf
Fig. 5. The pore size distribution of the different Al2O3-ZrO2 reticulated porous ceramics sintered.
Table 2 The apparent porosity of the different ceramic struts. Specimens
A
B
C
Apparent porosity, %
36.72
41.33
46.10
(3)
where Eo is the effective Young's modulus of the porous material (Al(OH)3 coating layer) with porosity P, E is Young's modulus of the solid material (Al2O3), Pc is the porosity at which the effective Young's modulus becomes zero, and f is a parameter dependent on the morphology of grains and pores. It has been reported that for porous materials (porosity range from 0 to 85%), f is 1.66 ± 0.07 and Pc is nearly equal to 1 [27]. According to a previous study, Young's modulus of Al2O3 materials is 232.68 GPa [28]. Therefore, on the basis of the porosity results, Young's modulus values for the different coating layers were calculated using Eq. (3); the values are listed in Table 3. Because the thermal expansion coefficient of the coating layer was lower than that of the Al2O3-ZrO2 matrix, calcination at high temperature led to the development of a residual compressive stress in the coating layer, which was beneficial to improving the compressive stress and thermal shock resistance of
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Fig. 6. The properties of the different Al2O3-ZrO2 reticulated porous ceramics (a) mechanical properties and (b) residual strength ratio.
Fig. 7. The thermal stress distribution in the ceramic struts of different specimens.
The mechanical properties and thermal conductivity of the RPCs with hierarchical pore structures fabricated in this study were considerably superior to those previously reported.
Table 3 The Young's modulus and Poisson ratio of the different ceramic struts. Specimens
A
B
C
Young's modulus, GPa Poisson ratio
120.52 0.32
109.95 0.32
99.36 0.32
the specimens [17]. The compressive residual stress of the specimens was calculated using Eq. (2), as listed in Table 4. It was very interesting to find that with increasing porosity, the compressive residual stress gradually decreased, significantly influencing the crack closure and growth rate, and thereby, affecting the mechanical properties and thermal shock resistance of the RPCs [12,29,30].
Table 4 The compressive residual stress of the RPCs treated at 1480 C. Specimens
A
B
C
Residual stress, MPa
50.79
47.48
44
3.3. Photocatalytic activity of Al2O3-ZrO2 RPCs with TiO2 layer The Al2O3-ZrO2 RPCs with TiO2 layer were used as catalysts for the photodegradation of methylene orange (MO) solution (Fig. 8). The photocatalytic activity and degradation rates of these materials increased with increasing Al(OH)3 content (Fig. 9) (Table 5). The concentration of MO solution catalyzed with sample C was the lowest, and it was significantly lower than that previously reported. This indicated that the addition of Al(OH)3 as a pore-forming agent in infiltration slurries was highly beneficial to improving the photocatalytic activity of RPCs with TiO2 layer [12]. This was attributed to the fact that Al(OH)3, as a pore-forming agent, significantly increased the apparent porosity of the RPCs. Moreover, the specific surface areas of the ceramic struts prepared in this study were larger than those previously reported, resulting in enhanced catalyst loading.
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Funding for Outstanding Doctoral Dissertation in NUAA (No. BCXJ19-09). References
Fig. 8. The microstructure of the Al2O3-ZrO2 RPCs-C with TiO2 layer.
Fig. 9. UVevis adsorption spectra of MO solution catalyzed by the different RPCs.
Table 5 The degradation rates of the different RPCs with TiO2 layer. Specimens
A
B
C
Degradation rates, %
36.43
66.31
82.06
4. Conclusion We fabricated novel Al2O3-ZrO2 RPCs with dual-scale porosity for catalyst carrier applications. The Al(OH)3 content in the infiltration slurry significantly influenced the mechanical properties, thermal shock resistance, and pore size distribution of the Al2O3ZrO2 RPCs, and this effect was attributed to the decomposition of Al(OH)3, phase transformation of alumina, and particle packing. As the Al(OH)3 content increased from 5% to 13%, the pore size distribution widened and the major pore size decreased. In addition, the photocatalytic activity of the RPCs with TiO2 layer increased with increasing Al(OH)3 content, because of an increase in the specific surface area of the ceramic struts. Therefore, this result indicated that the our method for the preparation of Al2O3ZrO2 RPCs with hierarchical pore structures, based on in situ pore formation by Al(OH)3, could be potentially used for the manufacture of high-temperature catalyst carriers with excellent properties.
Acknowledgments This work was supported by the Open Foundation of the State Key Laboratory of Refractories and Metallurgy (No.G201905) and
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Preparation of novel reticulated porous ceramics with hierarchical pore structures Ruoyu Chen a, Wenbao Jia a, b, Dong Lao a, Shujing Li c, Daqian Hei a, * a
College of Materials Science and Technology, Nanjing University of Aeronautics and Astronautics, Nanjing, 211106, China Collaborative Innovation Center of Radiation Medicine of Jiangsu Higher Education Institutions, Nanjing, China c The State Key Laboratory of Refractories and Metallurgy, Wuhan University of Science and Technology(WUST), Wuhan, 430081, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 19 April 2019 Received in revised form 23 July 2019 Accepted 24 July 2019 Available online 24 July 2019
Al2O3-ZrO2 reticulated porous ceramics (RPCs) with hierarchical pore structures were prepared by vacuum infiltration. Al(OH)3 was applied to provide the pore structure. Vacuum infiltration using an alumina/aluminum hydroxide slurry via a pre-sintering cycle was performed to fill up the hollow struts generated due to burnout of the polyurethane foam template and the coating on the Al2O3-ZrO2 ceramic struts. The results showed that as the Al(OH)3 content in the infiltration slurry was increased from 5% to 13%, the pore size distribution of the ceramic struts widened and the major pore size decreased. In addition, a compressive residual stress developed within the multilayered struts because of the difference in thermal expansion of the Al(OH)3-Al2O3 coating layer and Al2O3-ZrO2 ceramic struts. Moreover, with increasing Al(OH)3 content in the infiltration slurry, the porosity of the ceramic struts gradually increased, resulting in a decrease in Young's modulus and thermal conductivity, which influenced the compressive residual stress and thermal stress within the ceramic struts. Furthermore, the potential of the fabricated RPCs as catalyst supports was demonstrated. © 2019 Elsevier B.V. All rights reserved.
Keywords: Al2O3-ZrO2 reticulated porous ceramics Hierarchical pore structures Residual stress Aluminum hydroxide Catalyst support
1. Introduction Porous ceramics with high porosity are the preferred materials for a wide range of applications such as high-temperature catalyst supports, lightweight structural components, and radiant burners owing to their high inner geometric surface areas, permeability, and excellent chemical stability at high temperatures [1e3]. Notably, optimization of their pore sizes and structures is important for widening their practical applications. Several methods have been developed for the fabrication of porous ceramics with hierarchical pore structures, such as the direct foaming technique, pyrolysis of various organic additives, and gel casting [4e6]. Although these techniques yield high-surface-area porous ceramics with hierarchical pore structures, the mechanical properties of the obtained porous ceramics are significantly weak [7,8]. Currently, the polymer sponge replica technique is one of the most commonly used method to produce porous ceramics with high porosity, low density, and open three-dimensional network structures from polyurethane foams [9,10]. In general, the pore size
* Corresponding author. Tel.: 86þ18068800965, fax: 86þ025 52112626. E-mail address: [email protected] (D. Hei). https://doi.org/10.1016/j.jallcom.2019.07.287 0925-8388/© 2019 Elsevier B.V. All rights reserved.
of the polyurethane foam determines the macropore size and macrostructure of the porous ceramics. Notably, Furler and coworkers successfully fabricated ceria with a reticulated porous ceramic (RPC) structure and dual scale porosity in the millimeter and micrometer ranges by adding a pore-forming agent to the ceramic slurry [11]. Chen et al. fabricated Al2O3-ZrO2 RPCs with hierarchical pore structures and excellent mechanical properties by vacuum infiltration, CaCO3 decomposition, and calcium hexaluminate grain growth [12]. The high porosity and strong grain bonding of these materials were attributed to the addition of Al(OH)3, which underwent 60% volume contraction during decomposition, producing fine Al2O3 grains [13e15]. Chen and coworkers reported that alumina underwent g-to-a transformation, accelerating densification of alumina particles and generating a large number of surface pores on the particles [16]. It is necessary to develop an effective and simple process for the fabrication of RPCs with hierarchical pore structures and excellent properties. In this study, we prepared Al2O3-ZrO2 RPCs with hierarchical pore structures and excellent properties by vacuum infiltration with Al2O3/Al(OH)3 slurries, based on in situ pore formation by Al(OH)3. The mechanical properties of the as-prepared RPCs were investigated; moreover, the effect of the addition amount of
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Al(OH)3 on the morphology and thermal shock resistance was studied. Finally, the potential of these materials as highperformance catalyst supports was demonstrated. 2. Experimental procedure 2.1. Raw materials The main raw materials used were active alumina(AMA-40, d50 < 5 mm, Al2O3 99.3%, Hubei Smile New Materials Co., Ltd., China.), zirconia(MSZ-DM, 5mol% MgO, d50 ¼ 0.98 mm, Jiangxi Size Materials Co., Ltd., China.), nano-Al(OH)3(Al2O3 99.9%), and opencell polyurethane foam sponge (20 ppi; 20 mm 50 mm 50 mm). 2.2. Experimental setup and method Typically, an Al2O3-ZrO2 slurry (solid content ¼ 76 wt%) was prepared using alumina powder, zirconia powder, polycarboxylate (dispersant), polyvinyl alcohol 20-90 (binder), carboxymethyl cellulose (thickening agent), and absolute ethyl alcohol (antifoam agent) by ball milling for use as a strut material. An open-cell polyurethane foam sponge (20 ppi; 20 mm 50 mm 50 mm) was impregnated with the Al2O3-ZrO2 slurry and dried at 80 C for 24 h, followed by sintering at 800 C to prepare an Al2O3 preform. Then, Al2O3/Al(OH)3 infiltration slurries with a solid content of 50% were prepared by mixing the dry powders (Table 1) for use as porous coating materials; the mixtures were stirred for 60 min. The fabricated Al2O3-ZrO2 preforms were then immersed into the asprepared infiltration slurries and a vacuum of 0.5 Pa was applied for 10 min. Then, the Al2O3-ZrO2 preforms were dried at 80 C for 24 h and sintered at different temperatures (1380, 1480, and 1580 C) for 3 h to obtain Al2O3-ZrO2 RPCs. In addition, the fabricated Al2O3-ZrO2 RPCs were coated with a TiO2 layer by immersing them into TiO2 infiltration slurries, followed by vacuum drying in an oven under a vacuum of 0.5 Pa. The as-prepared RPCs were dried at 80 C and heated at 650 C for 1 h to obtain TiO2-coated Al2O3-ZrO2 RPCs. 2.3. Characterization techniques The bulk densities of the RPC specimens were calculated from their mass-to-volume ratios. The apparent porosity of the RPCs was measured by Archimedes' method. The RPC compressive strength (CS) was tested using a hydraulic universal testing machine (ETM, Wance, China) at a crosshead speed of 0.1 mm/min. The median diameter of the pores within the ceramic struts was determined by mercury porosimetry (Quantachrome PM60GT-18, Quantachrome Instruments Ltd., USA). The thermal shock test was performed as follows: the prepared RPCs were heated from 25 C to 1100 C, and held at this temperature for 30 min. Then, the samples were taken out and immersed in water for 20 min [17,18]. After the thermal shock test, the residual strength (RS) of the RPCs was measured. The thermal shock resistance of the RPCs was evaluated from the RS/CS ratio. Moreover, the RPC microstructure was characterized by fieldemission scanning electron microscopy (FESEM, Quanta 400; FEI Company, Hillsboro, OR, USA). The microstructure of RPC with TiO2
597
layer was characterized by scanning electron microscopy(SEM, Phenom ProX; Phenom Scientific Company, Netherlands). The phase compositions of the RPCs were determined by X-ray diffraction (XRD, X'Pert PRO, Philips, Netherlands). Furthermore, the simulative calculation of residual stress distribution in the ceramic struts was performed using the finite element method. Finally, the photocatalytic activity of the RPCs with TiO2 coating layer were measured as previously reported(Fig. 1) [12]. 3. Results and discussion 3.1. Microstructure and pore size distribution of Al2O3-ZrO2 RPCs The SEM images of fractured surfaces of Al2O3-ZrO2 RPC samples are shown in Fig. 2. Notably, a large number of pores existed on the surface of the ceramic struts. The pores were formed due to the decomposition of Al(OH)3, phase transformation of alumina, and particle packing. As observed from Fig. 2(a)e(c), the calcination temperature significantly influenced the size of the pores within the ceramic struts. As observed from Fig. 3, in the low-temperature range (<700 C), Al(OH)3 decomposed into H2O and g-Al2O3. Upon heating, g-Al2O3 converted into a-Al2O3, as follows: Al(OH)3 / gAl2O3 / q-Al2O3 / a-Al2O3, creating pores with different sizes within the ceramics struts, as shown in Fig. 2. In addition, with an increase in the calcination temperature from 1380 C to 1480 C, densification occurred, and the pore size gradually decreased. At the same time, vermicular colonies were formed (Fig. 2(b)) via coalescence of nuclei during the phase transformation of alumina instead of growth by grain boundary migration, which resulted in the formation of pores [19], as schematically shown in Fig. 4. Moreover, with an increase in the Al(OH)3 content to 13% (specimen A), the major pore size decreased to the sub-micron level. as shown in Fig. 2(b). Nevertheless, with further increase in the calcination temperature from 1480 C to 1580 C, the small pores within the ceramic struts disappeared due to densification and grain growth, but the stable pores grew (Fig. 2(b) and (c)). Thus, the increase in the calcination temperature led to coarsening of pores. This was attributed to the fact that normally in porous ceramics, with increasing calcination temperature, the sizes of pores and grains increase, while their amounts decrease, as observed in specimen C [20]. On the basis of the above results, we selected the Al2O3-ZrO2 RPC samples with different Al(OH)3 contents sintered at 1480 C for
Table 1 The formulations of the Al(OH)3-Al2O3 infiltration slurries(wt%). Specimens
Al2O3
Al(OH)3
FS
PVA
CMC
Slurry-A Slurry-B Slurry-C Slurry-D
95 91 87 /
5 9 13 /
0.2 0.2 0.2 /
/ / / /
/ / / /
Fig. 1. Experimental setup measure the photocatalytic activity of the different RPCs.
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the mercury intrusion porosimetry test. The pore size distributions of different specimens are presented in Fig. 5. The major pore diameter of the Al2O3-ZrO2 RPCs with 5% Al(OH)3 content was 1.33 mm. However, with an increase in the Al(OH)3 content to 13%, the major pore size decreased and the peak height increased due to the decomposition of Al(OH)3 and phase transformation of alumina. Moreover, the pore size distribution widened due to Al2O3 particle packing, resulting in the formation of larger pores. However, with increasing Al(OH)3 content, the number of large pores gradually decreased. This phenomenon was attributed to the fact that nano-Al(OH)3 filled the interspace between the large Al2O3 particles, transforming the large pores into small pores. In addition, the apparent porosity of the Al2O3-ZrO2 RPCs increased with increasing Al(OH)3 content (Table 2). From these results, it could be deduced that a high Al(OH)3 content was beneficial to improving the porosity of the RPCs, which could promote the loading capacity of the catalyst. 3.2. Mechanical properties and thermal shock resistance of Al2O3ZrO2 RPCs According to the microstructure and pore size distribution analysis, among the Al2O3-ZrO2 RPC samples, those sintered at 1480 C were the most suitable products for industrial applications; the linear shrinkage of the Al2O3-ZrO2 RPCs sintered at 1580 C was too large to ensure product stability and the strength of the products sintered at 1380 C was not sufficiently high to ensure product quality [17]. Fig. 6 shows the physical characteristics of different Al2O3-ZrO2 RPCs, revealing a marginal increase in linear shrinkage, bulk density, and CS with increasing amount of nano-Al(OH)3. This result was inconsistent with that reported by Deng et al. [13], owing to the large specific surface area of nano-Al(OH)3, which accelerated the sintering reaction of the alumina particles. In addition, the decomposition of nano-Al(OH)3 caused 60% volume reduction, leading to the formation of a large number of pores within the ceramic struts, which negatively affected the mechanical properties of the Al2O3-ZrO2 RPCs. Therefore, the CS of specimens A and C were different. The thermal shock resistance of the as-prepared specimens is presented in Fig. 6(b). Interestingly, the Al2O3-Al(OH)3 coating layer was beneficial to improving the thermal shock resistance of the specimens, because the coating layer caused an increase in the thickness of the ceramic struts and repaired the defects and flaws in the ceramic struts. Moreover, with increasing Al(OH)3 content in the infiltration slurry, the residual strength ratio first increased, and then decreased, because of the presence of micropores within the ceramics struts and thermal conductivity of the coating layer [21]. When the Al(OH)3 content in the infiltration slurry was less than 13%, the residual strength ratio was high. On the other hand, at high Al(OH)3 contents, a large number of pores were formed within the ceramic struts due to the decomposition of Al(OH)3 and phase transformation of alumina, which led to a decrease in the thermal conductivity of the ceramic struts (Eq. (1)), resulting in the generation of thermal stress within the coating layer [22].
l ¼ ls ð1 PÞ
Fig. 2. The microstructure of the Al2O3-ZrO2 reticulated porous ceramics(a) sintered at 1380 C, (b) sintered at 1480 C and (c) sintered at 1580 C.
(1)
where l is the thermal conductivity of the ceramic struts, ls is the thermal conductivity of the solid phase, and P is the volume fraction of pores in the ceramic struts. As shown in Fig. 7, a large thermal stress was introduced at the surface and edge of the struts of the specimens. Particularly, the thermal stress within the ceramic struts increased with the addition of Al(OH)3, and the maximum thermal stress was located at the edge of the inner layer
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Fig. 3. XRD patterns of products obtained from Al(OH)3 after heat treatment at 700, 900, 1000 and 1480 C.
of specimen C, negatively affecting the mechanical properties of the RPCs. This was attributed to the presence of a large amount of micropores within the coating layer. Moreover, it has been reported that a difference in thermal expansion and sintering shrinkage of the coating layer and matrix causes a residual stress within the product [23,24]. Moreover, the porosity of the materials significantly influenced Young's modulus, which in turn affected the residual stress. The residual stresses within the multilayered struts were calculated according to Eq. (2) [25]. Tð0
!,
sR ¼ Ei ti ðai ao ÞdT
Fig. 4. Illustration of the in-situ pore forming of Al(OH)3.
½ti Ei ð1 no Þ=Eo þ 2to ð1 ni Þ
(2)
T
where a is the thermal expansion coefficient, n is Poisson's ratio, E is Young's modulus, and t is the layer thickness; To and T are 1480 C and 25 C, respectively; and the subscripts i and o stand for the matrix and coating layers, respectively. According to the equation, as Young's modulus of the coating layer (Eo) decreases (with all other parameters fixed), the residual stress decreases. Notably, the elastic properties of porous materials over an entire porosity range can be described using the power-law empirical relationship proposed by Phani and Niyogi (Eq. (3)) [26]:
Eo ¼ Eð1 P=Pc Þf
Fig. 5. The pore size distribution of the different Al2O3-ZrO2 reticulated porous ceramics sintered.
Table 2 The apparent porosity of the different ceramic struts. Specimens
A
B
C
Apparent porosity, %
36.72
41.33
46.10
(3)
where Eo is the effective Young's modulus of the porous material (Al(OH)3 coating layer) with porosity P, E is Young's modulus of the solid material (Al2O3), Pc is the porosity at which the effective Young's modulus becomes zero, and f is a parameter dependent on the morphology of grains and pores. It has been reported that for porous materials (porosity range from 0 to 85%), f is 1.66 ± 0.07 and Pc is nearly equal to 1 [27]. According to a previous study, Young's modulus of Al2O3 materials is 232.68 GPa [28]. Therefore, on the basis of the porosity results, Young's modulus values for the different coating layers were calculated using Eq. (3); the values are listed in Table 3. Because the thermal expansion coefficient of the coating layer was lower than that of the Al2O3-ZrO2 matrix, calcination at high temperature led to the development of a residual compressive stress in the coating layer, which was beneficial to improving the compressive stress and thermal shock resistance of
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Fig. 6. The properties of the different Al2O3-ZrO2 reticulated porous ceramics (a) mechanical properties and (b) residual strength ratio.
Fig. 7. The thermal stress distribution in the ceramic struts of different specimens.
The mechanical properties and thermal conductivity of the RPCs with hierarchical pore structures fabricated in this study were considerably superior to those previously reported.
Table 3 The Young's modulus and Poisson ratio of the different ceramic struts. Specimens
A
B
C
Young's modulus, GPa Poisson ratio
120.52 0.32
109.95 0.32
99.36 0.32
the specimens [17]. The compressive residual stress of the specimens was calculated using Eq. (2), as listed in Table 4. It was very interesting to find that with increasing porosity, the compressive residual stress gradually decreased, significantly influencing the crack closure and growth rate, and thereby, affecting the mechanical properties and thermal shock resistance of the RPCs [12,29,30].
Table 4 The compressive residual stress of the RPCs treated at 1480 C. Specimens
A
B
C
Residual stress, MPa
50.79
47.48
44
3.3. Photocatalytic activity of Al2O3-ZrO2 RPCs with TiO2 layer The Al2O3-ZrO2 RPCs with TiO2 layer were used as catalysts for the photodegradation of methylene orange (MO) solution (Fig. 8). The photocatalytic activity and degradation rates of these materials increased with increasing Al(OH)3 content (Fig. 9) (Table 5). The concentration of MO solution catalyzed with sample C was the lowest, and it was significantly lower than that previously reported. This indicated that the addition of Al(OH)3 as a pore-forming agent in infiltration slurries was highly beneficial to improving the photocatalytic activity of RPCs with TiO2 layer [12]. This was attributed to the fact that Al(OH)3, as a pore-forming agent, significantly increased the apparent porosity of the RPCs. Moreover, the specific surface areas of the ceramic struts prepared in this study were larger than those previously reported, resulting in enhanced catalyst loading.
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Funding for Outstanding Doctoral Dissertation in NUAA (No. BCXJ19-09). References
Fig. 8. The microstructure of the Al2O3-ZrO2 RPCs-C with TiO2 layer.
Fig. 9. UVevis adsorption spectra of MO solution catalyzed by the different RPCs.
Table 5 The degradation rates of the different RPCs with TiO2 layer. Specimens
A
B
C
Degradation rates, %
36.43
66.31
82.06
4. Conclusion We fabricated novel Al2O3-ZrO2 RPCs with dual-scale porosity for catalyst carrier applications. The Al(OH)3 content in the infiltration slurry significantly influenced the mechanical properties, thermal shock resistance, and pore size distribution of the Al2O3ZrO2 RPCs, and this effect was attributed to the decomposition of Al(OH)3, phase transformation of alumina, and particle packing. As the Al(OH)3 content increased from 5% to 13%, the pore size distribution widened and the major pore size decreased. In addition, the photocatalytic activity of the RPCs with TiO2 layer increased with increasing Al(OH)3 content, because of an increase in the specific surface area of the ceramic struts. Therefore, this result indicated that the our method for the preparation of Al2O3ZrO2 RPCs with hierarchical pore structures, based on in situ pore formation by Al(OH)3, could be potentially used for the manufacture of high-temperature catalyst carriers with excellent properties.
Acknowledgments This work was supported by the Open Foundation of the State Key Laboratory of Refractories and Metallurgy (No.G201905) and
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