- Email: [email protected]
Fabrication and properties of porous anorthite ceramics with modelling pore structure
Materials Letters 190 (2017) 95–98
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/mlblue
Fabrication and properties of porous anorthite ceramics with modelling pore structure Cuiwei Li a,⇑, Yao Han a, Linghao Wu a, Kepi Chen b, Li-nan An c a
School of Mechanical, Electronic and Control Engineering, Beijing Jiaotong University, Beijing 100044, PR China School of Energy, Power and Mechanical Engineering, North China Electric Power University, Beijing 102206, PR China c Department of Materials Science and Engineering, University of Central Florida, Orlando, FL 32816, USA b
a r t i c l e
i n f o
Article history: Received 31 October 2016 Received in revised form 4 December 2016 Accepted 30 December 2016 Available online 31 December 2016 Keywords: Porous materials Ceramics Microstructure Thermal properties Compressive strength
a b s t r a c t A single-phased anorthite porous ceramics was fabricated by foam-gelcasting technique using a-Al2O3, CaCO3 and SiO2 as starting material. Through adjusting foaming agent concentration and solid content in the slurry, tailored porosity (62–91%), pore size (7–350 lm), thermal conductivity (0.04–0.27 W/ mK) and compressive strength (0.27–13.39 MPa) were obtained. The experimental thermal conductivity was in good agreement with the universal model. The dependence of compressive strength on porosity can be described by the Gibson model. Ó 2017 Elsevier B.V. All rights reserved.
1. Introduction Anorthite has many good properties, such as low theoretical density (about 2.76 g/cm3), low thermal conductivity (about 3.67 W/mK) and low thermal expansion coefficient (4.8 106/ K), making it a very promising material for thermal insulating applications [1–4]. Although anorthite has already been used as one of the main component in many commercialized refractory bricks, the relatively low purity and poor mechanical properties of these bricks limit their applications. Therefore, preparation of porous anorthite ceramics with high-purity, high strength and low thermal conductivity has become an area of focused research. In past years, several techniques have been developed to prepare porous anorthite ceramics, including direct foaming [1], addition of foaming agent [4], organics burned [5], and foam-gelcasting [6]. Among those, foam-gelcasting leads to porous ceramics with good properties. Through this method, porous anorthite ceramics with high purity and ultra-low thermal conductivity (lowest to 0.018 W/mK in vacuum) has been fabricated using c-Al2O3, CaCO3 and SiO2 powders as raw materials [6]. But the particle properties of c-alumina make it impossible to prepare porous anorthite ceramics with low porosity (less than 69.9%) and high strength.
⇑ Corresponding author. E-mail address: [email protected] (C. Li). http://dx.doi.org/10.1016/j.matlet.2016.12.131 0167-577X/Ó 2017 Elsevier B.V. All rights reserved.
Thus, in this paper, a-Al2O3 powder was selected as alumina resource to make porous anorthite ceramics by foam-gelcasting technique. Phase composition, pore structure and properties were thoroughly investigated. Thermal conductivity and compressive strength were compared with theoretical models, and discussed in terms of microstructure.
2. Experimental procedure For anorthite synthesis, commercial powders a-Al2O3 (99.99%, Beijing boyu gaoke new material technology Co. Ltd., China), CaCO3 (99.5%, Aladdin, China) and SiO2 (99.4%, Shanghai fengchen powder material Co. Ltd., China) with particle size of about 0.3, 0.8 and 10 lm are used as raw materials. Materials used for aqueous foam-gelcasting processing is same as that used in reference [6] and [7]. The foam-gelcasting procedure used herein was performed as follows, which is similar to that described in reference [6] and [7]. First of all, ceramics powders in stoichiometric ratio of anorthite and premixed slurry were ball-milled for about 20 h to prepare even ceramics slurry. Secondly, Foaming agent was added into the slurry followed by rapidly mechanical stirring to form foamed slurries. Thirdly, the catalyst and initiator (the amount of catalyst and initiator were adjusted to ensure the solidification time was within 30 min.) were slowly added into the foamed slurries and then mixed rapidly. After adequately mixed, foamed
96
C. Li et al. / Materials Letters 190 (2017) 95–98
Table 1 Processing and properties of porous anorthite ceramics made by foam-gelcasting. Sample
Solid content (vol.%)
Foaming agent concentration (g/L)
Porosity (%)
Pore size (lm)
Compressive strength (MPa)
Thermal conductivity (W/mK)
1 2 3 4 5 6 7 8 9 10 11
35
0.25 0.5 1 2 4 8 1
62.16 ± 0.69 71.39 ± 1.18 78.12 ± 0.05 85.09 ± 0.40 85.42 ± 0.50 85.49 ± 0.20 90.70 ± 1.85 85.70 ± 0.19 83.45 ± 0.42 78.12 ± 0.05 77.50 ± 0.11
7.77 14.91 21.62 68.69 63.23 59.71 82.10 69.67 59.70 21.62 23.52
13.39 ± 2.451 4.98 ± 1.054 3.13 ± 1.110 1.03 ± 0.099 1.50 ± 0.035 2.96 ± 0.290 0.27 ± 0.087 1.43 ± 0.080 1.65 ± 0.630 3.13 ± 1.110 5.28 ± 1.483
0.272 ± 0.0197 0.192 ± 0.0064 0.137 ± 0.0029 0.065 ± 0.0093 0.062 ± 0.0019 0.060 ± 0.0043 0.040 ± 0.0029 0.059 ± 0.0019 0.086 ± 0.0073 0.137 ± 0.0029 0.156 ± 0.0069
20 25 30 35 40
slurries were poured into molds and solidified at room temperature for 30 min. Then green bodies were removed from mold and dried in a microwave oven. Finally, the dried green bodies were sintered at 1400 °C for 5 h. To prepare porous anorthite ceramics with varied pore structure, the solid content and foaming agent concentration were tailored, as shown in Table 1. Phase composition was identified by XRD (D/MAX-IIIB, Rigaku, Japan) and microstructure was observed using SEM (JSM 6700F, JEOL, Tokyo, Japan). Archimedes method in distilled water was used to test porosity. Pore size distribution was analyzed using mercury intrusion method (Auto Pore IV 9510, Micrometrics, USA). Room temperature thermal conductivity was measured on 5 5 3 mm3 machined specimens using Thermal Transport Option (TTO) parts of Physical Properties Measurement System (PPMS, Model 6000, Quantum Design, USA) in high vacuum environment. Compressive strength was measured with cylindrical specimens of 20 mm diameter and of 20 mm height by a mechanical testing machine (WDW3020, Changchun, China) with a crosshead speed of 0.5 mm/min. At least three specimens were measured for each material.
3. Results and discussion Fig. 1 shows typical XRD patterns of the prepared materials. It is seen that all diffraction peaks can be identified as anorthite, and no
other phase is detected. XRD patterns for other samples are almost the same, which are not shown here. This result indicates that a single-phased porous anorthite ceramics can be prepared by this technique. SEM images and pore size distribution curves (Fig. 2) show the pore characteristics of the ceramics fabricated using different foaming agent concentrations (Table 1 samples 1–6). SEM images show that materials mainly contain two types of pores: large spherical pores generated by foaming process and small cellular holes within the internal walls derived from the removal of organic matter. The images also reveal that pore size and porosity vary with foaming agent concentration. These results consist with the pore size distribution curves. As the foaming agent concentration increases from 0.25 to 2 g/L, the size of the large spherical pores and the porosity increases. However, upon further increasing it to 8 g/L, the size of the large spherical pores decreases and the porosity remains nearly the same. Observation (SEM image not show) of the ceramics prepared with different solid phase loads revealed that the size of the large pores decreases upon increasing the load from 20 to 35 vol%, whereas the size remains the same when the solid load further increases to 40 vol%. Effects of foaming agent concentration and solid content on pore structure can be understood by considering foaming mechanism. Surface energy and viscosity of the slurry are two main factors during foaming. Foaming agent is a kind of surfactant which can lower surface energy, and solid content may change the viscosity of the slurry. Low surface energy and viscosity is helpful for bubble to form and grow. The thermal conductivity of porous anorthite ceramics was measured at room temperature in a vacuum (Table 1). The results demonstrate that the thermal conductivity can be tailored within a very large range by changing foaming agent concentration or solid content, suggesting that the technique employed herein is very useful for preparing thermal insulating porous ceramics. Further analysis of the thermal conductivity data was performed by comparing the experimental data to theoretical models (Fig. 3). From Fig. 3, the experimental data cannot be described by the five classic models (series, parallel, Maxwell-Eucken-I, Maxwell-Eucken-II, and Effective Medium Theory) but fit well with the universal model (Eq. (1)) when di = 3 and k’ = 0.26. High porosity and complex pore structure are the main reason for low thermal conductivity.
P2 Ke ¼
dk v i ðd 1Þk þk P2 dk v i i¼1 ðd 1Þk þk i¼1 ki
i
i
Fig. 1. Typical XRD patterns obtained from three different ceramics: sample 1, (b) sample 3, (c) sample 6.
0
i
i
0
0
0
i
ð1Þ
i
where k and v are thermal conductivity and volume fraction [8]. Here, thermal conductivity of high vacuum and dense anorthite ceramics are chosen as 1 109 and 3.67 W/mK.
C. Li et al. / Materials Letters 190 (2017) 95–98
97
Fig. 2. SEM images and pore size distribution curves of porous anorthite ceramics fabricated from different foaming agent concentrations: (a) 0.25, (b) 0.5, (c) 1, (d) 2, (e) 4 and (f) 8 g/L.
Fig. 3. Plots of thermal conductivity as a function of porosity: comparison between experimental data and theoretical predictions of different effective models. The open circles represent the data measured from samples 1–6 and the solid triangles represent the data measured from the samples 7–11.
The compressive strength of the obtained porous anorthite was also measured to illustrate the effectiveness of the foam-gelcasting technique (Table 1). The compressive strength of the materials also varies over a very large range. Previous studies have revealed that the compressive strength of porous materials strongly depends on the porosity and pore size [9,10]. Fig. 4a shows the comparison of experimental data with theoretical prediction (the Gibson and Ashby model [9]). The results show that most of data are aligned very well with the Gibson and Ashby model. The m is obtained by the best fitting experimental data to be 2.46, which is much higher than 1.5 predicted by Gibson for brittle materials, but is similar with those in other studies [11,12]. In the original assumption of Gibson model, the material contains uniform cubic cell structure, but in this studied materials pores are not cubic, and the pore size is not uniform but in a large range (from 7 to 350 lm). Fig. 4b shows the relationship between compressive strength and pore size, which is not similar with that in reference [10]. The reason may be the combination role of both pore size and porosity.
98
C. Li et al. / Materials Letters 190 (2017) 95–98
Fig. 4. Plots of compressive strength as a function of (a) porosity and (b) pore size. The open circles represent the data measured from samples 1–6 and the solid triangles represent the data measured from the samples 7–11.
4. Conclusion A single-phased porous anorthite ceramics were synthesized using the foam-gelcasting technique. The porosity and pore size were tailored over a large range by varying the foaming agent concentration or solid phase content. These materials showed a large range of thermal conductivities (0.04–0.27 W/mK) and compressive strengths (0.27–13.39 MPa). The thermal conductivity of the materials was in good agreement with the universal model when di = 3 and k0 = 0.26. The compressive strength of the materials also followed the predicted dependencies on porosity and pore size. The current results demonstrate that materials with high strength and low thermal conductivity can be fabricated by the foam-gelcasting technique. Acknowledgments This work was supported by the National Natural Science Foundation of China (Grant Nos. 51102012 and 51472023). References [1] Y. Li, X.D. Cheng, L.L. Gong, J.J. Feng, W. Cao, R.F. Zhang, H.P. Zhang, Fabrication and characterization of anorthite foam ceramics having low thermal conductivity, J. Eur. Ceram. Soc. 35 (2015) 267–275.
[2] J.S. Goldsmith, The melting and breakdown reactions of anorthite at high pressures and temperatures, Am. Mineral. 65 (1980) 222–284. [3] V.V. Primachenko, V.V. Martynenko, L.A. Dergaputskaya, Anorthite lightweight material with microporous structure, in: Proceedings of UNITECR’01, Cancun, Mexico. 1(2001) 193–195. [4] V.V. Martynenko, V.V. Primachenko, L.A. Dergaputskaya, High performance heat insulating castable with microporous anorthite aggregate, in: Proceeding of UNITECR’07, Dresden, Germany. (2007) 125–128. [5] M. Sutcu, S. Akkurt, A. Bayram, U. Uluca, Production of anorthite refractory insulating firebrick from mixtures of clay and recycled paper waste with sawdust addition, Ceram. Int. 38 (2012) 1033–1041. [6] Y. Han, C.W. Li, C. Bian, S.B. Li, C.A. Wang, Porous anorthite ceramics with ultralow thermal conductivity, J. Eur. Ceram. Soc. 33 (2013) 2573–2578. [7] C.W. Li, C. Bian, Y. Han, C.A. Wang, L.N. An, Mullite whisker reinforced porous anorthite ceramics with low thermal conductivity and high strength, J. Eur. Ceram. Soc. 36 (2016) 761–765. [8] J.F. Wang, J.K. Carson, M.F. North, D.J. Cleland, A new approach to modelling the effective thermal conductivity of heterogeneous materials, Int. J. Heat Mass Transf. 49 (2006) 3075–3083. [9] L.J. Gibson, M.F. Ashby, Cellular Solids: Structure and Properties, Cambridge University Press, UK, 1997. [10] M. Ashizuka, E. Ishida, T. Matsushita, M. Hisanaga, Elastic modulus strength and fracture toughness of alumina ceramics containing pores, J. Ceram. Soc. Jpn. 110 (2002) 554–559. [11] P. Sepulveda, J.G.P. Binner, Processing of cellular ceramics by foaming and in situ polymerisation of organic monomers, J. Eur. Ceram. Soc. 19 (1999) 2059–2066. [12] X.J. Mao, S.W. Wang, S. Shimai, Porous ceramics with tri-modal pores prepared by foaming and starch consolidation, Ceram. Int. 34 (2008) 107–112.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/mlblue
Fabrication and properties of porous anorthite ceramics with modelling pore structure Cuiwei Li a,⇑, Yao Han a, Linghao Wu a, Kepi Chen b, Li-nan An c a
School of Mechanical, Electronic and Control Engineering, Beijing Jiaotong University, Beijing 100044, PR China School of Energy, Power and Mechanical Engineering, North China Electric Power University, Beijing 102206, PR China c Department of Materials Science and Engineering, University of Central Florida, Orlando, FL 32816, USA b
a r t i c l e
i n f o
Article history: Received 31 October 2016 Received in revised form 4 December 2016 Accepted 30 December 2016 Available online 31 December 2016 Keywords: Porous materials Ceramics Microstructure Thermal properties Compressive strength
a b s t r a c t A single-phased anorthite porous ceramics was fabricated by foam-gelcasting technique using a-Al2O3, CaCO3 and SiO2 as starting material. Through adjusting foaming agent concentration and solid content in the slurry, tailored porosity (62–91%), pore size (7–350 lm), thermal conductivity (0.04–0.27 W/ mK) and compressive strength (0.27–13.39 MPa) were obtained. The experimental thermal conductivity was in good agreement with the universal model. The dependence of compressive strength on porosity can be described by the Gibson model. Ó 2017 Elsevier B.V. All rights reserved.
1. Introduction Anorthite has many good properties, such as low theoretical density (about 2.76 g/cm3), low thermal conductivity (about 3.67 W/mK) and low thermal expansion coefficient (4.8 106/ K), making it a very promising material for thermal insulating applications [1–4]. Although anorthite has already been used as one of the main component in many commercialized refractory bricks, the relatively low purity and poor mechanical properties of these bricks limit their applications. Therefore, preparation of porous anorthite ceramics with high-purity, high strength and low thermal conductivity has become an area of focused research. In past years, several techniques have been developed to prepare porous anorthite ceramics, including direct foaming [1], addition of foaming agent [4], organics burned [5], and foam-gelcasting [6]. Among those, foam-gelcasting leads to porous ceramics with good properties. Through this method, porous anorthite ceramics with high purity and ultra-low thermal conductivity (lowest to 0.018 W/mK in vacuum) has been fabricated using c-Al2O3, CaCO3 and SiO2 powders as raw materials [6]. But the particle properties of c-alumina make it impossible to prepare porous anorthite ceramics with low porosity (less than 69.9%) and high strength.
⇑ Corresponding author. E-mail address: [email protected] (C. Li). http://dx.doi.org/10.1016/j.matlet.2016.12.131 0167-577X/Ó 2017 Elsevier B.V. All rights reserved.
Thus, in this paper, a-Al2O3 powder was selected as alumina resource to make porous anorthite ceramics by foam-gelcasting technique. Phase composition, pore structure and properties were thoroughly investigated. Thermal conductivity and compressive strength were compared with theoretical models, and discussed in terms of microstructure.
2. Experimental procedure For anorthite synthesis, commercial powders a-Al2O3 (99.99%, Beijing boyu gaoke new material technology Co. Ltd., China), CaCO3 (99.5%, Aladdin, China) and SiO2 (99.4%, Shanghai fengchen powder material Co. Ltd., China) with particle size of about 0.3, 0.8 and 10 lm are used as raw materials. Materials used for aqueous foam-gelcasting processing is same as that used in reference [6] and [7]. The foam-gelcasting procedure used herein was performed as follows, which is similar to that described in reference [6] and [7]. First of all, ceramics powders in stoichiometric ratio of anorthite and premixed slurry were ball-milled for about 20 h to prepare even ceramics slurry. Secondly, Foaming agent was added into the slurry followed by rapidly mechanical stirring to form foamed slurries. Thirdly, the catalyst and initiator (the amount of catalyst and initiator were adjusted to ensure the solidification time was within 30 min.) were slowly added into the foamed slurries and then mixed rapidly. After adequately mixed, foamed
96
C. Li et al. / Materials Letters 190 (2017) 95–98
Table 1 Processing and properties of porous anorthite ceramics made by foam-gelcasting. Sample
Solid content (vol.%)
Foaming agent concentration (g/L)
Porosity (%)
Pore size (lm)
Compressive strength (MPa)
Thermal conductivity (W/mK)
1 2 3 4 5 6 7 8 9 10 11
35
0.25 0.5 1 2 4 8 1
62.16 ± 0.69 71.39 ± 1.18 78.12 ± 0.05 85.09 ± 0.40 85.42 ± 0.50 85.49 ± 0.20 90.70 ± 1.85 85.70 ± 0.19 83.45 ± 0.42 78.12 ± 0.05 77.50 ± 0.11
7.77 14.91 21.62 68.69 63.23 59.71 82.10 69.67 59.70 21.62 23.52
13.39 ± 2.451 4.98 ± 1.054 3.13 ± 1.110 1.03 ± 0.099 1.50 ± 0.035 2.96 ± 0.290 0.27 ± 0.087 1.43 ± 0.080 1.65 ± 0.630 3.13 ± 1.110 5.28 ± 1.483
0.272 ± 0.0197 0.192 ± 0.0064 0.137 ± 0.0029 0.065 ± 0.0093 0.062 ± 0.0019 0.060 ± 0.0043 0.040 ± 0.0029 0.059 ± 0.0019 0.086 ± 0.0073 0.137 ± 0.0029 0.156 ± 0.0069
20 25 30 35 40
slurries were poured into molds and solidified at room temperature for 30 min. Then green bodies were removed from mold and dried in a microwave oven. Finally, the dried green bodies were sintered at 1400 °C for 5 h. To prepare porous anorthite ceramics with varied pore structure, the solid content and foaming agent concentration were tailored, as shown in Table 1. Phase composition was identified by XRD (D/MAX-IIIB, Rigaku, Japan) and microstructure was observed using SEM (JSM 6700F, JEOL, Tokyo, Japan). Archimedes method in distilled water was used to test porosity. Pore size distribution was analyzed using mercury intrusion method (Auto Pore IV 9510, Micrometrics, USA). Room temperature thermal conductivity was measured on 5 5 3 mm3 machined specimens using Thermal Transport Option (TTO) parts of Physical Properties Measurement System (PPMS, Model 6000, Quantum Design, USA) in high vacuum environment. Compressive strength was measured with cylindrical specimens of 20 mm diameter and of 20 mm height by a mechanical testing machine (WDW3020, Changchun, China) with a crosshead speed of 0.5 mm/min. At least three specimens were measured for each material.
3. Results and discussion Fig. 1 shows typical XRD patterns of the prepared materials. It is seen that all diffraction peaks can be identified as anorthite, and no
other phase is detected. XRD patterns for other samples are almost the same, which are not shown here. This result indicates that a single-phased porous anorthite ceramics can be prepared by this technique. SEM images and pore size distribution curves (Fig. 2) show the pore characteristics of the ceramics fabricated using different foaming agent concentrations (Table 1 samples 1–6). SEM images show that materials mainly contain two types of pores: large spherical pores generated by foaming process and small cellular holes within the internal walls derived from the removal of organic matter. The images also reveal that pore size and porosity vary with foaming agent concentration. These results consist with the pore size distribution curves. As the foaming agent concentration increases from 0.25 to 2 g/L, the size of the large spherical pores and the porosity increases. However, upon further increasing it to 8 g/L, the size of the large spherical pores decreases and the porosity remains nearly the same. Observation (SEM image not show) of the ceramics prepared with different solid phase loads revealed that the size of the large pores decreases upon increasing the load from 20 to 35 vol%, whereas the size remains the same when the solid load further increases to 40 vol%. Effects of foaming agent concentration and solid content on pore structure can be understood by considering foaming mechanism. Surface energy and viscosity of the slurry are two main factors during foaming. Foaming agent is a kind of surfactant which can lower surface energy, and solid content may change the viscosity of the slurry. Low surface energy and viscosity is helpful for bubble to form and grow. The thermal conductivity of porous anorthite ceramics was measured at room temperature in a vacuum (Table 1). The results demonstrate that the thermal conductivity can be tailored within a very large range by changing foaming agent concentration or solid content, suggesting that the technique employed herein is very useful for preparing thermal insulating porous ceramics. Further analysis of the thermal conductivity data was performed by comparing the experimental data to theoretical models (Fig. 3). From Fig. 3, the experimental data cannot be described by the five classic models (series, parallel, Maxwell-Eucken-I, Maxwell-Eucken-II, and Effective Medium Theory) but fit well with the universal model (Eq. (1)) when di = 3 and k’ = 0.26. High porosity and complex pore structure are the main reason for low thermal conductivity.
P2 Ke ¼
dk v i ðd 1Þk þk P2 dk v i i¼1 ðd 1Þk þk i¼1 ki
i
i
Fig. 1. Typical XRD patterns obtained from three different ceramics: sample 1, (b) sample 3, (c) sample 6.
0
i
i
0
0
0
i
ð1Þ
i
where k and v are thermal conductivity and volume fraction [8]. Here, thermal conductivity of high vacuum and dense anorthite ceramics are chosen as 1 109 and 3.67 W/mK.
C. Li et al. / Materials Letters 190 (2017) 95–98
97
Fig. 2. SEM images and pore size distribution curves of porous anorthite ceramics fabricated from different foaming agent concentrations: (a) 0.25, (b) 0.5, (c) 1, (d) 2, (e) 4 and (f) 8 g/L.
Fig. 3. Plots of thermal conductivity as a function of porosity: comparison between experimental data and theoretical predictions of different effective models. The open circles represent the data measured from samples 1–6 and the solid triangles represent the data measured from the samples 7–11.
The compressive strength of the obtained porous anorthite was also measured to illustrate the effectiveness of the foam-gelcasting technique (Table 1). The compressive strength of the materials also varies over a very large range. Previous studies have revealed that the compressive strength of porous materials strongly depends on the porosity and pore size [9,10]. Fig. 4a shows the comparison of experimental data with theoretical prediction (the Gibson and Ashby model [9]). The results show that most of data are aligned very well with the Gibson and Ashby model. The m is obtained by the best fitting experimental data to be 2.46, which is much higher than 1.5 predicted by Gibson for brittle materials, but is similar with those in other studies [11,12]. In the original assumption of Gibson model, the material contains uniform cubic cell structure, but in this studied materials pores are not cubic, and the pore size is not uniform but in a large range (from 7 to 350 lm). Fig. 4b shows the relationship between compressive strength and pore size, which is not similar with that in reference [10]. The reason may be the combination role of both pore size and porosity.
98
C. Li et al. / Materials Letters 190 (2017) 95–98
Fig. 4. Plots of compressive strength as a function of (a) porosity and (b) pore size. The open circles represent the data measured from samples 1–6 and the solid triangles represent the data measured from the samples 7–11.
4. Conclusion A single-phased porous anorthite ceramics were synthesized using the foam-gelcasting technique. The porosity and pore size were tailored over a large range by varying the foaming agent concentration or solid phase content. These materials showed a large range of thermal conductivities (0.04–0.27 W/mK) and compressive strengths (0.27–13.39 MPa). The thermal conductivity of the materials was in good agreement with the universal model when di = 3 and k0 = 0.26. The compressive strength of the materials also followed the predicted dependencies on porosity and pore size. The current results demonstrate that materials with high strength and low thermal conductivity can be fabricated by the foam-gelcasting technique. Acknowledgments This work was supported by the National Natural Science Foundation of China (Grant Nos. 51102012 and 51472023). References [1] Y. Li, X.D. Cheng, L.L. Gong, J.J. Feng, W. Cao, R.F. Zhang, H.P. Zhang, Fabrication and characterization of anorthite foam ceramics having low thermal conductivity, J. Eur. Ceram. Soc. 35 (2015) 267–275.
[2] J.S. Goldsmith, The melting and breakdown reactions of anorthite at high pressures and temperatures, Am. Mineral. 65 (1980) 222–284. [3] V.V. Primachenko, V.V. Martynenko, L.A. Dergaputskaya, Anorthite lightweight material with microporous structure, in: Proceedings of UNITECR’01, Cancun, Mexico. 1(2001) 193–195. [4] V.V. Martynenko, V.V. Primachenko, L.A. Dergaputskaya, High performance heat insulating castable with microporous anorthite aggregate, in: Proceeding of UNITECR’07, Dresden, Germany. (2007) 125–128. [5] M. Sutcu, S. Akkurt, A. Bayram, U. Uluca, Production of anorthite refractory insulating firebrick from mixtures of clay and recycled paper waste with sawdust addition, Ceram. Int. 38 (2012) 1033–1041. [6] Y. Han, C.W. Li, C. Bian, S.B. Li, C.A. Wang, Porous anorthite ceramics with ultralow thermal conductivity, J. Eur. Ceram. Soc. 33 (2013) 2573–2578. [7] C.W. Li, C. Bian, Y. Han, C.A. Wang, L.N. An, Mullite whisker reinforced porous anorthite ceramics with low thermal conductivity and high strength, J. Eur. Ceram. Soc. 36 (2016) 761–765. [8] J.F. Wang, J.K. Carson, M.F. North, D.J. Cleland, A new approach to modelling the effective thermal conductivity of heterogeneous materials, Int. J. Heat Mass Transf. 49 (2006) 3075–3083. [9] L.J. Gibson, M.F. Ashby, Cellular Solids: Structure and Properties, Cambridge University Press, UK, 1997. [10] M. Ashizuka, E. Ishida, T. Matsushita, M. Hisanaga, Elastic modulus strength and fracture toughness of alumina ceramics containing pores, J. Ceram. Soc. Jpn. 110 (2002) 554–559. [11] P. Sepulveda, J.G.P. Binner, Processing of cellular ceramics by foaming and in situ polymerisation of organic monomers, J. Eur. Ceram. Soc. 19 (1999) 2059–2066. [12] X.J. Mao, S.W. Wang, S. Shimai, Porous ceramics with tri-modal pores prepared by foaming and starch consolidation, Ceram. Int. 34 (2008) 107–112.