Semi-vitrified porous kyanite mullite ceramics: Young modulus, microstructure and pore size evolution
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Microporous porcelain formulations are successfully carried out through sintering processing. During the thermal treatment of ceramic products, it was found that the addition of kyanite together with ϕ- and γ-Al2O3 allowed to enhance interconnected pores network with micrometric size from 0.1 to 9 µm in a semi-vitrified composite. Between 1200 and 1350 °C, the mullitization of kyanite hindered the extension of vitrification and the growth of acicular mullite from the transformation of metakaolin. The main pores size decreased from 4.33 to 1.54 µm for the formulation containing 32 wt% of kyanite. In this interval the specific pore area increased from 0.64 to 8.75 m2 g−1 due to the total conversion of the kyanite to fibrous and acicular mullite that reduced the voids provided by the earlier mullitization. The improvement in the mullitization without extensive vitrification and grain growth and the reduction of the pores size with the increase in the specific pore area contributed to the formation of a microporous matrix with the Young’s modulus increased from 7 to > 20 GPa. The microstructure of the microporous porcelain, their specific pore area and pores size as well as the interconnection of pores was found innovative for the applications in the field of engineering filtration where high mechanical strength, strain, stiffness and pressure resistance are required.
KeywordsMicroporous porcelain Kyanite Interconnected pores Microstructure Elastic modulus
Porcelain materials are characterized by important technological features like high mechanical and chemical stability, high hardness, wear resistance and durability [1, 2, 3]. Therefore, porous porcelain matrices can be recommended and promising for filtering applications (hot gas filters, heat exchangers for turbine engines and gas separators) [4, 5, 6]. Their final microstructure is controlled by the amount of glassy phase formed upon vitrification, porcelain being densified by viscous flow sintering mechanisms, and in particular, the viscosity of the liquid/glassy phase affects the pore network. Porous porcelain used for filtering applications like catalyst supports, hot gases filter, liquid food productions, membrane reactors and heat exchangers for turbines should possess limited glassy phase [4, 5, 6, 7]. It is also important to control the amount and consistency of the glassy phase produced upon sintering to guarantee the presence of the fine well-interconnected pores network. In a previous work , the presence of interconnected pores with the size between 3 nm and 4.5 µm was achieved by producing a glassy phase with low thermal expansion coefficient starting from kaolin, bauxite, feldspar and kyanite . It was shown that the presence of kyanite delayed the vitrification at relatively low temperature and improved the formation of mullite at high temperature, thus allowing the maintenance of certain porosity in a mechanically stable matrix. In order to understand the properties of this type of porous semi-vitrified matrices, it is necessary to investigate their formation mechanisms. Firstly, the difference exists between the vitrification range of the feldspar and the decomposition of kyanite in the mullite appeared. Kyanite as starting materials is a mixture of an alumina-rich and silica-rich material with the molar ratio of Al2O3/SiO2 = 1. In addition, the sillimanite group minerals include kyanite, sillimanite and andalusite, all of which can be represented by the chemical formula Al2O3·SiO2, which corresponds to the content 62.9% Al2O3 and 37.1% SiO2. There is, however, a marked difference in their crystal structures and properties. Above 1250 °C, kyanite is converted to mullite (3Al2O3·SiO2) and free silica (SiO2), and SiO2 provides the structure of the viscoelastic toughening [9, 10]. K2O, Na2O and CaO oxides of natural kyanite were known to alter the viscosity and the glass transition temperature of silicates. At lower temperature in the aluminosilicates refractory, K2O, Na2O and CaO promote the formation of viscous glassy phase and strengthen the viscous ligaments formed . Kyanite crystallizes in triclinic system and starts decomposition only at temperature > 1100 °C. With the presence of both free silica and reactive Al2O3 at high temperature, the formation of the acicular mullite is favored through atomic diffusion [11, 12, 13]. Additionally, the microstructural evolution of the kyanite during sintering is particular with the development of fibrous and acicular mullite between 1200 and 1350 °C far from the large size and elongated secondary mullite developed in the semi-vitrified matrix of porcelain considering the same range of temperature [14, 15, 16, 17]. During the process of the decomposition of the kyanite particle, multitude mode of pores appeared in the matrices. Highly reactive alumina (α-alumina) derived from the calcined bauxite [18, 19, 20] contributes to the secondary mullite formation with the ceramic linkages developed; this enhances the apparent porosity by consuming the glassy silica. Thus, the secondary mullitization limited the formation of the excess of liquid phase.
The present work deals with the understanding of the mechanism formation of porous semi-vitrified matrices. This includes the descriptive microstructure and the pore size development. The works successfully demonstrated the ability of the kyanite particles to control the mullitization process and hinder the feldspar vitrification during the sintering processing. Therefore, it describes in detail the pore volume, pore size behavior and their interconnection within the matrices, giving information regarding the changes that appear following the action of kyanite at low and high temperature. The viscoelastic phase developed, mullitization and the simultaneous pore network forming with consequent changes in the elastic behavior of the porous matrices are discussed using microstructural approach.
2 Experimental procedures
2.1 Preparation of the samples
Chemical composition of the used raw materials (metakaolin, calcined bauxite, kyanite and feldspar)
Chemical composition of the three formulations
Calcined bauxite (wt%)
All the ingredients were mixed, wet and ball milled for 60 min in rapid ball mill at 1200 rpm. The obtained paste was dried at room temperature down to 10–15% water. Cylindrical pellets (diameter = 50 mm, thickness = 4–6 mm) were prepared by uniaxial pressing using a maximum pressure of 40 MPa. The specimens were dried at 105 °C for 24 h before firing. This was carried out in an electric furnace (Borel FP 1600, Standard Furnaces & Ovens) at 1200, 1275, 1350 and 1400 °C for 1 h using a heating rate of 5 °C/min. The samples were then cooled down naturally in the furnace.
2.2 Characterization of fired specimens
The microstructure of the fired samples was analyzed by SEM (JEOL-JSM5500). Microchemical analyses were carried out with the same instrument fitted with an energy-dispersive X-ray detector (EDS, Joel IT300). The samples were mounted on aluminum stubs and sputter coated with Pt/Pd; some of them were preliminarily etched in 5% HF-HNO3 water solution for 30 s to reveal the crystalline phases better.
3 Results and discussion
3.1 Formation of highly viscoelastic phase
When the temperature increased between 1200 and 1400 °C, there coexists in the system the decomposition of the kyanite with the formation of mullite and glassy viscous phase from the feldspar–metakaolin mix where the mullitization started earlier. In standard porcelain formulations, at that stage, it is expected shrinkage which here is compensated with the expansion linked to the expansive action of silica from kyanite decomposition. The combined action of glassy phase and alumina from calcined bauxite, in the matrices, enhances the formulation of secondary mullite (Fig. 2a–c) and the reduction of the extension of the formation and expression of glassy phase is demonstrated in the next paragraph. The expansion contributed to maintain the porosity inducing low sinter ability and low densification. Moreover, there were no microcracks observed in the matrices as it is the case with conventional triaxial porcelain as from the thermal coefficient expansion mismatch. In the latter the extension of liquid phase makes a gap between the thermal expansion coefficient of liquid phase and the crystalline more important particularly when quartz particles are concerned [1, 2, 24, 25].
The expression of the glassy phase decreased with the temperature and the kyanite content (Figs. 1 and 2). This is completely different from the general behavior of the porcelain and semi-vitrified ceramic systems [26, 27, 28]. The principal explanation is the presence of low-temperature formation of alumina (ρ- or α-Al2O3) that will still in the matrix and reacts subsequently with the reactive SiO2 from the decomposition of kyanite that is continued between 1100 and 1300 °C. Amorphous silica is not transformed into liquid phase but reacts to form mullite. The extension of the formation of viscous phase at low temperature (1200–1275 °C) compared to high temperature is from the low eutectic (~ 950 °C) that characterizes the K2O-Al2O3–SiO2 systems. However, the simultaneous presence of reactive Al2O3 and SiO2 contributed to hinder the vitrified phase.
3.3 Pores size distribution and their interconnection
Variation of the average pore size and pore surface area of the fired samples with the kyanite content and temperature evolution
PSA (m2 g−1)
PSA (m2 g−1)
PSA (m2 g−1)
In fact, at 1200 °C, the pores size distribution shows a bimodal distribution with peaks centered at 1.7 and 3.4 µm for PK3. The increase in the temperature shows a certain pore coarsening behavior at 1400 °C (Fig. 4a). In PK4, there is a bimodal distribution at 1200 °C with peaks at 1.6 and 4.4 µm for the temperature of 1200 °C. The increase in the temperature up to 1275 °C conducted to a unimodal distribution of pores with the peak centered at 3.4 µm. Increasing the temperature up to 1400 °C, three bands with peaks at 1.3, 4.1 and 4.5 µm appeared as indications of the pores coarsening from the improvement in crystallization. The global behavior of the porous mullite-based matrices shows a less influence of the action of kyanite for relatively low temperature (1200 °C) due to the fact that at this level the decomposition of kyanite was not yet effective. The high temperature corresponds to more crystallization of mullite with additional pores bands. The reduction of the size of the first band can be attributed to the fact that the decomposition of kyanite produces acicular mullite as those from the transformation of metakaolin make more packing particles with consequence of the reduction of the intergranular pores size. In the absence of feldspar [19, 20], the pore size was between 2.4 and 9.5 µm. Results are in line with the pore network of sintered kyanite-based ceramics .
The porous mullite matrices as designed in the present work have nano- and micrometric pores that formed the pore network. The extension of crystallization with a significant reduction of the expression of the glassy phase allowed good percolation of the pores. At 1200 °C, with 37% wt. of the content of kyanite, the water permeability was > 500 L m−2 h−1 kPa−1 for PK5 and increase with the reduction of the kyanite content (28 % wt.) up to 750 L m−2 h−1 kPa−1 for PK3. This can be explained by the fact that kyanite is a stable and non-porous material at low temperature so its presence reduces the permeability as well as the pores connectivity of the matrix . However, those grains of kyanite contributed to hinder the sintering and maintain at 1200 °C the porous systems in comparison with conventional semi-vitrified matrix with lower porosity at that level of temperature . The temperature development enhanced the crystallization of mullite; however, the balance with the reduction of the percolation probability seems to dominate all the matrices as the action of liquid phase even small as can be significant when considering semi-vitrified products. However, PK5 appeared as the matrix with the most connectivity of pores following by PK4 and PK3 : in agreement with the kyanite content.
In the fully vitrified porcelain, open porosity is generally absent making the water absorption near zero [26, 33]. The closed porosity has pore size concentrated under 1 µm with peaks between 0.01 and 0.1 µm . The bands of pores with size under 1 µm in the porous mullite composites can be nucleated from the vitrified phases, while those with size between 3.4 and 4.5 µm are intergranular pores characterizing the spaces between grains of mullite, the so-called intergranular pores. It was generally observed that the extension of crystallization was the main reason of the domination of this class of pores. The spaces generally filled by glassy phase in porcelain remain empty due to the limited expression of the vitrification. By the way as it can be appreciated with the cumulative pore volume curves , the connectivity of those matrices was significant in the range of the pore sizes between 1 and 10 µm.
3.4 Relation porous microstructure: structural strength
A suitable membrane support must provide high porosity and large pores size to decrease the resistance to the fluid flow [36, 37, 38, 39]. Membranes for gas separation purposes need a support with low resistance to the filtrate flow and a smooth surface together with high strength and permeability. The porous mullite composites PK3, PK4 and PK5 offer characteristics in line with the above requirements. The final matrix is homogeneous enough to be considered as isotropic elastic and stress free as no microcracks were observed and the glassy phase was low and highly viscous to not induce expansion coefficient mismatch with the crystalline phase (Figs. 1, 2, 5, 6, 7). The pore size distribution of the porous semi-vitrified composites indicated pores band between 0.01 and 0.1 µm (10–100 nm) and between 1 and 10 µm (1000–10,000 nm) much similar to the pore distribution of zeolite NaA membranes supported on alumina hollow fibers described by Shao et al. . Our solution to produce this class of materials is energy efficient, environmentally friendly with the use of low-cost natural mineral resources and relatively sustainable process and low-temperature processing compared to that of zeolite NaA. The semi-vitrified porous matrices were designed using metakaolin, kyanite, Al2O3 from Al(OH)3 and potassium feldspar. While metakaolin is directly transformed to acicular mullite, feldspar and kyanite particles that are not ready to react in the range of the temperature of primary mullite formation (950–980 °C) act to hinder vitrification. Amorphous SiO2 from metakaolin can readily react with Al2O3. As from 1170 °C, the feldspar is decomposed producing amorphous silica that will bind to Al2O3 to form secondary mullite. In the main time kyanite starts to develop fibrous mullite as from 1200 °C. The residual SiO2 once again is going to form long grains of mullite II. The limitation of the growth of mullite from metakaolin and the production of the fine grains of acicular mullite from kyanite contributed to the densification of the interpores spaces and improved the flexural strength as well as the Young’s modulus. As it can be observed in Table 3 and Fig. 4, the mean pore size reduced with the increase in temperature up to 1350 °C. The pore network and their interconnectivity that can be controlled during the process offer a suitable possibility for the production of membrane supports for emerging engineering filtration particularly in the context where pressure resistance and mechanical strength are required.
Microporous mullite with multimodal pore size distribution was successfully designed avoiding extension of vitrification by using kyanite that acted to hinder vitrification at relatively low temperature and improve crystallization between 1200 and 1350 °C. Consequently, the semi-crystalline matrix maintained nano- and micrometric pores with good interconnection. In this range of temperature, optimum interconnectivity, lower average pore size (1.54 µm), higher specific pore area and elastic modulus (> 20 GPa) as the results of the control of the mullite grain growth and optimum densification and packing of acicular, fibrous and secondary mullite developed. At temperature above 1350 °C, the volume of nanometric pores reduced although there is microstructure coarsening. The final matrices combined porosity, pores connectivity and high strength promising for filtrations where mechanical strength and pressure resistance are required.
The authors acknowledge the staff of the Ceramics and Glass Laboratory of the University of Trento, Italy, for their assistance in the characterization of samples. They also recognize the assistance from MIPROMALO for the realization of this project.
Compliance with ethical standards
Conflict of interest
The authors declare that they have no conflict of interest.
- 2.Iqbal Y, Lee WE (1999) Fired porcelain microstructures revisited. J Am Ceram Soc 82:3584–3590. https://doi.org/10.1111/j.1151-2916.1999.tb02282.x CrossRefGoogle Scholar
- 3.Kamseu E, Bakop T, Djangang C, Melo UC, Hanuskova M, Leonelli C (2013) Porcelain stoneware with pegmatite and nepheline syenite solid solutions: pore size distribution and descriptive microstructure. J Eur Ceram Soc 33:2775–2784. https://doi.org/10.1016/j.jeurceramsoc.2013.03.028 CrossRefGoogle Scholar
- 9.Guo H, Li W (2018) Effects of Al2O3 crystal types on morphologies, formation mechanisms of mullite and properties of porous mullite ceramics based on kyanite. J Eur Ceram Soc 38:679–686. https://doi.org/10.1016/j.jeurceramsoc.2017.09.003 CrossRefGoogle Scholar
- 20.Deutou JGN, Mohamed H, Nzeukou NA, Kamseu E, Melo UC, Beda T, Leonelli C (2016) The role of kyanite in the improvement in the crystallization and densification of the high strength mullite matrix: phase evolution and sintering behaviour. J Therm Anal Calorim 126:1211–1222. https://doi.org/10.1007/s10973-016-5686-1 CrossRefGoogle Scholar
- 23.ASTM C1161 (2003) Standard test method for flexural strength of advanced ceramics at ambient temperature. ASTM Int., West ConshohockenGoogle Scholar
- 26.Lerdprom W, Chinnam RK, Jayaseelan DD, Lee WE (2016) Porcelain production by direct sintering. J Eur Ceram Soc 36:4319–4325. https://doi.org/10.1016/j.jeurceramsoc.2016.07.013 CrossRefGoogle Scholar
- 34.Kamseu E, Ngouloure ZNM, Ali BN, Zekeng S, Melo UC, Rossignol S, Leonelli C (2015) Cumulative pore volume, pore size distribution and phases percolation in porous inorganic polymer composites: relation microstructure and effective thermal conductivity. Energy Build 88:45–56. https://doi.org/10.1016/j.enbuild.2014.11.066 CrossRefGoogle Scholar