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Calcium hexaaluminate synthesis and its influence on the properties of CA2–Al2O3-based refractories
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ARTICLE IN PRESS
JECS-10259; No. of Pages 5
Journal of the European Ceramic Society xxx (2015) xxx–xxx
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Calcium hexaaluminate synthesis and its influence on the properties of CA2 –Al2 O3 -based refractories ˛ ´ Agnieszka Pieta, Mirosław M. Bucko, Marta Janu´s, Jerzy Lis, Stanisława Jonas ∗ AGH University of Science and Technology, Faculty of Materials Science and Ceramics, al. Mickiewicza 30, 30-059 Cracow, Poland
a r t i c l e
i n f o
Article history: Received 2 June 2015 Received in revised form 28 August 2015 Accepted 30 August 2015 Available online xxx Keywords: Thermal expansion Refractories Microstructure-final Calcium aluminates
a b s t r a c t The investigations of low thermal expansion coefficient refractories, made of calcium dialuminate (CA2 ) and corundum, have been presented. Classifying them into a group of reactive materials means that there is a chemical reaction at the contact area of CA2 and Al2 O3 grains, accompanied by a formation another compound CaAl12 O19 (CA6 ) during firing. The conversion degree of the reaction as a function of temperature and time has been determined, including the formulation of kinetic equations, the assessment of apparent activation energy and a presumable mechanism of CA6 formation has been proposed. It has been proved that the formation of plate-shaped CA6 crystals at the contact area of Al2 O3 grains with the surrounding CA2 matrix changes pore size distribution and total porosity of the materials. It is also related with local changes of thermal expansion coefficients, which in turn influences total expansion of the tested samples. © 2015 Published by Elsevier Ltd.
1. Introduction Calcium dialuminate CaAl4 O7 (CA2 ) is a compound which can be broadly used in the industry due to its optical, electrical, thermal and mechanical properties. It is an object of basic research of the lattice at elevated temperatures [1,2]. In ceramic technologies it is the material used mainly as a component of the high-aluminate hydraulic cements-matrix of refractory concretes [3–5]. It was observed [6–9] that among all compounds in the CaO–Al2 O3 system, calcium dialuminate is characterized by a high melting temperature of about 1762 ◦ C and very low, up to 500 ◦ C by an even negative thermal expansion coefficient [9]. This feature is considered to be the basis for designing new refractory materials of high resistance to thermal shocks. The basic assumption in designing of these materials comprises such a selection of initial components that the matrix composed of relatively fine CA2 grains (below 100 m) is surrounded by coarser grains of some oxide or carbide materials. According to these assumptions, the obtained composites should be characterized by considerably low thermal expansion coefficients under the condition that the microstructure is developed in such a manner that CA2 is formed as a continuous phase. The bodies consisting of fine-grained matrix of calcium dialuminate with coarser corundum grains should be classified as
∗ Corresponding author. Fax: +48 12 617 24 93. E-mail address: [email protected] (S. Jonas).
a group of reactive bodies. It means that the chemical reaction at the area contact of Al2 O3 and CA2 grains, accompanied by the formation of calcium hexaaluminate (hibonite, CA6 ), will take place according to the equation: CaAl4 O7 + 4Al2 O3 → CaAl12 O19
(1)
Calculations of free enthalpy changes, G, of this reaction in the function of temperature are difficult because of the lack of suitable thermodynamic data in the literature. However, in practice, only a general assessment of the sign of G function on the basis of the equation: G = H − TS without executing detailed calculations, conditioned by accessibility of thermodynamic data of all reacting substances, is satisfactory. The case in question concerns the exothermic reaction (H < 0) between solid bodies progressing without discrete changes of the state of aggregation of substances participating in the process (thermal distribution, crystallization, melting, evaporation). Thus, we can assume that it belongs to a group of processes progressing without entropy changes (S ∼ = 0). So, the G value of this reaction is negative within full temperature ranges, which are essential for a technologist. In the light of these assumptions, resistances of a kinetic nature will determine the reaction course. So, from a technological point of view kinetics and mechanism of the reaction in question a in possibly broad temperature range is very essential. Because the total mole volume of substrates is twice smaller than the product mole volume, it should be expected that the formation of new CA6 grains at the contact area of Al2 O3 and CA2 will result in a development of tensile
http://dx.doi.org/10.1016/j.jeurceramsoc.2015.08.034 0955-2219/© 2015 Published by Elsevier Ltd.
˛ et al., Calcium hexaaluminate synthesis and its influence on the properties of CA2 –Al2 O3 -based Please cite this article in press as: A. Pieta, refractories, J Eur Ceram Soc (2015), http://dx.doi.org/10.1016/j.jeurceramsoc.2015.08.034
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2
stresses in the matrix. In consequence, it can lead to induce microfractures and porosity increase, including open porosity, which can stimulate changes of the material thermal expansion coefficient. The present study is aimed at examining the kinetics of the calcium hexaaluminate formation during the reaction between calcium dialuminate and corundum, including an assessment of the chemical stability of CA2 –Al2 O3 composites. 2. Experimental The materials used for the examination of kinetic reactions of calcium hexaaluminate comprised the mixtures of CA2 powder, prepared according to the procedure described in the previous works [7,8] and the commercial corundum powder (Alcoa A-17). The mole ratios between the precursors resulted from stoichiometry of Eq. (1). The powders were mixed in a ball mill for two hours, dried overnight at 80 ◦ C, and then cylinder-shaped samples of 10 mm diameter and 30 mm high were formed from the uniform mixture with the hydraulic press under pressure of 80 MPa. Grain size distribution for the powder was measured by dynamic light scattering on Nanosizer-ZS of Malvern Instruments. The samples were fired at temperatures ranging from 1300 ◦ C to 1550 ◦ C,the temperature rate was 10 ◦ C/min, and they were kept at the maximum temperature from 30 min to 48 h. The phase composition of fired materials was determined by the X-ray diffraction analysis (CuK␣1 , Empyrean, Panalytical) and the amounts of particular phases were determined by the Rietveld method. Relative amounts of the CA6 phase in the tested materials after firing in different periods at the defined temperature, expressed in a form of mass fraction, corresponded to the reaction conversion degree, ␣. A separate series of samples was made of mixtures of CA2 powder of the 150 m grain size and alumina powder of the 0.2/0.5 mm grain size. The samples of pure CA2 and powders containing 10–30% of corundum, formed in the same way as described above, were fired at 1600 ◦ C for 3 h. The grain size distributions for all powder mixtures were qualitatively comparable. The microstructure of such materials was observed with the FEI microscope Nova Nano SEM200 with the analyzer of chemical composition EDAX – Genesis. Poremaster 60 of Quantachrom was used to determine the pore size distribution, whereas the measurements of the linear thermal expansion coefficient were made with the dilatometer Netzsch DIL 402C within the temperature ranges from 20 ◦ C to 1200 ◦ C, with temperature rate 5 ◦ C/min. 3. Results and disscusion Fig. 1 shows grain size distribution for the CA2 and Al2 O3 powders (as an inset) and for their mixture. The grain size distribution is bi-modal and the mode at about 10 m corresponds mainly to the CA2 powder while the second mode at about 70 m to the alumina powder. The determination of kinetics of the formation of CA6 in the tested system was preceded by measurements of the conversion degree of such a reaction as the function of firing temperature. The changes of the amount of particular phases in the materials, fired for 4 h at various temperatures, are shown in Fig. 2. Kinetic resistances of this reaction up to the temperature of about 1300 ◦ C are so large that no effects can be seen and the mass fraction of CA6 is as small as 0.07. An essential increase of the reaction conversion degree takes place above this temperature, what in turn causes that the determination of the reaction of CA6 development was based on the data obtained for the temperature ranging from 1350 ◦ C to 1500 ◦ C. In such a temperature range, the conversion degree is changed from about 0.12 to about 0.95. At 1550 ◦ C the reaction proceeds so fast that the variability range of the conversion degree is too narrow for a reliable analysis. According to the assumed attitude applied in
Fig. 1. Grain size distribution for the mixture of the CA2 and Al2 O3 powders. Inset reveals grain size distributions of the component powders.
Fig. 2. Changes of phase composition of the materials fired for 4 h at various temperatures. Marker size corresponds to determination error.
the examination of the kinetics of chemical reactions, the function of the reaction conversion degree F(˛), was determined, which was linearly dependent on time, according to the equation: F(˛) = k × t
(2)
The form of the function depends on the reaction mechanism. In the tested system, this function satisfies the above condition if we assume the reaction model in which the growth of nuclei constitutes a stage determining the total rate of the process. This model has the best statistical compatibility among basic equations describing the reaction kinetics in the solid phase (in this case a correlation factor is the highest)—Fig. 3. The kinetics of such a reaction is described with Avrami–Jerofiejew’s equation: F(˛) = ln
1 1−˛
= kt
m
(3)
where: k—constant nucleation rate, m—index exponent depending on reaction mechanism and geometrical factor. The applied model describes reactions in the solid phase via the formation of three-dimensional (spherical), two-dimensional (plate-like) and one-dimensional (needle-like) nuclei. The processes limiting the growth of nuclei can be related to the surface reaction or diffusion in the solid phase. For a constant nucleation rate it was assumed that the value of exponent m in this equation for the tested reaction is 3, which means that growing crystal seeds of
˛ et al., Calcium hexaaluminate synthesis and its influence on the properties of CA2 –Al2 O3 -based Please cite this article in press as: A. Pieta, refractories, J Eur Ceram Soc (2015), http://dx.doi.org/10.1016/j.jeurceramsoc.2015.08.034
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Fig. 3. The kinetics of the formation of calcium hexaaluminate in mixture of CaAl4 O7 –Al2 O3 powders.
Fig. 4. SEM image of the material fired at 1450 ◦ C for 3 h. SEM image for the system: coarse corundum grain in CA2 matrix.
CA6 are plate-like and their growth is limited by diffusion. Microscope observations proved the pertinence of these assumptions. Fig. 4 shows the SEM images presenting hexagonal, plate-like CA6 particles formed on the Al2 O3 –CA2 grain boundary during firing at 1450 ◦ C for 3 h. On the basis of SEM image analysis and general thermochemical properties of calcium dialuminate and corundum we can expect that the particles of calcium hexaaluminate are formed at the contact area of grains of both substrates as a result of diffusion of Ca2+ ions from the CA2 phase to the grain boundary. So, the formation of the CA6 seeds and their further growth are limited by diffusion of Ca2+ ions. Indirect evidence for such a type of mechanism is proved by the fact that the CA6 plates directly contacting with the corundum grain have a smaller size than those from the side of CA2 (see Fig. 4). On the basis of the reaction-rate constants calculated from Eq. (3), i.e.: 4.91 × 10−3 s−1 at 1350 ◦ C, 7.56 × 10−3 s−1 at 1400 ◦ C, 11.55 × 10−3 s−1 at 1450 ◦ C and 14.94 × 10−3 s−1 at 1500 ◦ C, the temperature dependence of the reaction-rate constant was plotted in the Arrhenius coordinates (Fig. 5). Apparent activation energy of the CA6 formation in the investigated system calculated from the plot is about 134 kJ/mol, which is in good agreement comparable
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Fig. 5. Temperature dependence of the reaction-rate constant in tested system.
with the values of activation energy of similar processes of some solid state synthesis. The results show that the formation of CA6 in the studied system is proceeding intensively at the temperature over 1350 ◦ C. Thus it can be assumed that in a series of CA2 based materials with the addition of 10%, 20% or 30% corundum, fired at 1600 ◦ C for 3 h, this reaction will also take place. It was proved both by XRD and SEM examinations. The investigation of phase composition showed that sintered bodies, obtained from the mixtures of the CA2 and corundum powders, are composed of three compounds of high refractoriness (melting temperature of CA6 is about 1850 ◦ C). The amounts of CA6 formed in the described conditions are 9.7, 21.2 and 29.4 mass% in the materials obtained from the powder mixture containing 10–30% of Al2 O3 , respectively. SEM images of the material obtained from the powder mixture with 20% of corundum are shown in Fig. 6. Similarly, as it was observed in the materials previously described, in the contact area of corundum grains with CA2 matrix, plate-shaped grains of calcium hexaaluminate are visible. The results of measurements of apparent density and open porosity of the sintered bodies are shown in Table 1. It can be observed that the larger content of corundum in the initial mixture, i.e., the larger the content of CA6 in the sintered bodies the more porous they are. The deterioration of sinterability of the CA2 powder, connected with the presence of corundum grains, results from the formation of the CA6 phase which occurs during sintering. The molar volume of calcium hexaaluminate is over twice larger than the sum of the molar volume of the substrates and some effects of reactive sintering might be expected, the formation of CA6 grains must lead to a formation of cracks and separation of CA2 grains. The changes in the image of pore size distribution prove the influence of the reaction between corundum and CA2 on powder sinterability (Fig. 7). The increase of corundum content in the initial mixture, i.e., the intensification of the reaction with CA2 , results in forming larger and larger pores while pore size distributions are wider and wider. Such larger pores can be formed during pressing of powders and their elimination during sintering is hindered by the formation of CA6 , or they can be formed as secondary pores, related to the changes of the system volume, caused by the same reaction. The temperature dependence of linear thermal expansion coefficient (TEC), obtained on the basis of dilatometric curves, is shown in Fig. 8. The pure CA2 material has a negative expansion (it shrinks) up to the temperature of about 500 ◦ C. Above this temperature TEC is relatively low when compared with other refractory materials. In multi-phase materials, the increase of the amount of corundum in the initial mixture results in a decreasing tendency for initial
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Table 1 Apparent density and porosity of materials obtained via sintering of CA2 powder and its mixtures with corundum. Sample
Apparent density, da (g/cm3 )
CA2 CA2 + 10% Al2 O3 CA2 + 20% Al2 O3 CA2 + 30% Al2 O3
2.31 2.37 2.42 2.38
± ± ± ±
0.02 0.05 0.04 0.05
True density, dt (g/cm3 ) 2.66 2.84 2.96 3.06
± ± ± ±
0.02 0.02 0.03 0.03
Porosity, P (%) 13.01 16.58 18.13 22.46
± ± ± ±
0.39 0.41 0.54 0.58
Median (m)
Mode (m)
5.14 5.37 6.12 7.25
6.00 5.33 5.23 6.92
Fig. 6. SEM images of the sintered body obtained from the mixture of CA2 and 20% Al2 O3 powders.
shrinkage during heating; for the material obtained from the mixture containing 30% of corundum TEC is positive already from about 200 ◦ C. The presence of CA6 in the sintered materials causes that the changes of TEC are non-monotonic with the increase of temperature. At the temperature over 900 ◦ C the drop of TEC is observed for all materials obtained from the two-phase powders, and this drop increases for the higher amount of CA6 in the material. This fact is difficult to explain and requires further studies. We can, however, assume that these phenomena are connected with relaxation of stresses generated by differences and structural anisotropy of elasticity moduli and thermal expansion coefficients of all phases occurring in the sintered materials. 4. Conclusions
Fig. 7. Pore size distribution in the materials obtained from the mixtures of CA2 powders with: (1) 10% Al2 O3 , (2) 20% Al2 O3 , (3) 30% Al2 O3 .
Fig. 8. Changes of linear thermal expansion coefficients of CA2 sintered body in the materials obtained from the mixtures of CA2 powder and corundum.
Firing of the mixture of calcium dialuminate and corundum powders is accompanied by a chemical reaction leading to calcium hexaaluminate formation. Measurable effects are visible just at the temperatures exceeding 1300 ◦ C. The product of the CA6 synthesis crystallizes in a form of plate-like particles, located between CA2 and Al2 O3 grains. Among the basic equations describing kinetics of solid state reactions the best statistical compatibility was observed in the case of the model, in which the growth of nuclei constitutes a stage determining the total process rate. Apparent activation energy of CA6 formation in the described system is about 134 kJ/mole, which is in agreement with some values of activation energy in solid state synthesis. It is supposed that the intensification of diffusion of Ca2+ ions to the grain boundary of Al2 O3 grain via the lattice of the formed CA6 is a process limiting the growth of nuclei. The development of plate-shaped CA6 grains causes an increase of material porosity and growth of pore sizes. These effects are more and more intense with the growth of the amount of corundum in the initial powder mixture. Sintered materials, made of pure CA2 , have a negative thermal expansion up to about 500◦ C, and above this temperature the thermal expansion coefficient is low when compared to other refractories. The formation of CA6 reduces the tendency to initial shrinkage during heating. The increase of temperature results in non-monotonic changes of TEC. At the temperature exceeding 900◦ C there is a drop of its value for all materials
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obtained from the two-phase powders. This drop is larger when the content of CA6 in the materials is higher. These phenomena are probably related to relaxation of internal stresses, generated in the obtained three-phase CA2 –CA6 –Al2 O3 materials. Acknowledgement This work was supported by the grant of The National Centre of Research and Development: INNOTECHK1/IN1/71/153131/NCBR/12. References [1] Y. Suzuki, T. Ohji, Anisotropic thermal expansion of calcium dialuminate (CaAl4 O7 ) simulated by molecular dynamics, Ceram. Int. 30 (2004) 57–61. [2] A. Altaya, C.B. Carter, P. Rulis, W.-Y. Ching, I. Arsland, M.A. Gülgüne, Characterizing CA2 and CA6 using ELNES, J. Solid State Chem. 183 (2010) 1776–1784.
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[3] I.R. Oliveira, F.S. Ortega, V.C. Pandolfelli, Hydration of CAC cement in a castable refractory matrix containing processing additives, Ceram. Int. 35 (2009) 1545–1552. [4] F.A. Cardoso, M.D.M. Innocentini, M.M. Akiyoshi, V.C. Pandolfelli, Effect of curing time on the properties of CAC bonded refractory castables, J. Eur. Ceram. Soc. 24 (2004) 2073–2078. [5] A.P. Luz, V.C. Pandolfelli, CaCO3 addition effect on the hydration and mechanical strength evolution of calcium aluminate cement for endodontic applications, Ceram. Int. 38 (2012) 1417–1425. [6] E. Criado, S. De Aza, Calcium hexaalminates as refractory material, in: Proceedings of UNITECR Congress, Aachen, 1991, p. 403. [7] S. Jonas, F. Nadachowski, D. Szwagierczak, A new non-silicate refractory of low thermal expansion, Ceram. Int. 24 (1998) 211–216. [8] S. Jonas, F. Nadachowski, D. Szwagierczak, Low thermal expansion refractory composites based on CaAl4 O7 , Ceram. Int. 25 (1999) 77–84. [9] S. Jonas, F. Nadachowski, D. Szwagierczak, G. Wójcik, Thermal expansion of CaAl4 O7 -based refractory compositions containing MgO and CaO additions, J. Eur. Ceram. Soc. 26 (2006) 2273–2278.
˛ et al., Calcium hexaaluminate synthesis and its influence on the properties of CA2 –Al2 O3 -based Please cite this article in press as: A. Pieta, refractories, J Eur Ceram Soc (2015), http://dx.doi.org/10.1016/j.jeurceramsoc.2015.08.034
ARTICLE IN PRESS
JECS-10259; No. of Pages 5
Journal of the European Ceramic Society xxx (2015) xxx–xxx
Contents lists available at www.sciencedirect.com
Journal of the European Ceramic Society journal homepage: www.elsevier.com/locate/jeurceramsoc
Calcium hexaaluminate synthesis and its influence on the properties of CA2 –Al2 O3 -based refractories ˛ ´ Agnieszka Pieta, Mirosław M. Bucko, Marta Janu´s, Jerzy Lis, Stanisława Jonas ∗ AGH University of Science and Technology, Faculty of Materials Science and Ceramics, al. Mickiewicza 30, 30-059 Cracow, Poland
a r t i c l e
i n f o
Article history: Received 2 June 2015 Received in revised form 28 August 2015 Accepted 30 August 2015 Available online xxx Keywords: Thermal expansion Refractories Microstructure-final Calcium aluminates
a b s t r a c t The investigations of low thermal expansion coefficient refractories, made of calcium dialuminate (CA2 ) and corundum, have been presented. Classifying them into a group of reactive materials means that there is a chemical reaction at the contact area of CA2 and Al2 O3 grains, accompanied by a formation another compound CaAl12 O19 (CA6 ) during firing. The conversion degree of the reaction as a function of temperature and time has been determined, including the formulation of kinetic equations, the assessment of apparent activation energy and a presumable mechanism of CA6 formation has been proposed. It has been proved that the formation of plate-shaped CA6 crystals at the contact area of Al2 O3 grains with the surrounding CA2 matrix changes pore size distribution and total porosity of the materials. It is also related with local changes of thermal expansion coefficients, which in turn influences total expansion of the tested samples. © 2015 Published by Elsevier Ltd.
1. Introduction Calcium dialuminate CaAl4 O7 (CA2 ) is a compound which can be broadly used in the industry due to its optical, electrical, thermal and mechanical properties. It is an object of basic research of the lattice at elevated temperatures [1,2]. In ceramic technologies it is the material used mainly as a component of the high-aluminate hydraulic cements-matrix of refractory concretes [3–5]. It was observed [6–9] that among all compounds in the CaO–Al2 O3 system, calcium dialuminate is characterized by a high melting temperature of about 1762 ◦ C and very low, up to 500 ◦ C by an even negative thermal expansion coefficient [9]. This feature is considered to be the basis for designing new refractory materials of high resistance to thermal shocks. The basic assumption in designing of these materials comprises such a selection of initial components that the matrix composed of relatively fine CA2 grains (below 100 m) is surrounded by coarser grains of some oxide or carbide materials. According to these assumptions, the obtained composites should be characterized by considerably low thermal expansion coefficients under the condition that the microstructure is developed in such a manner that CA2 is formed as a continuous phase. The bodies consisting of fine-grained matrix of calcium dialuminate with coarser corundum grains should be classified as
∗ Corresponding author. Fax: +48 12 617 24 93. E-mail address: [email protected] (S. Jonas).
a group of reactive bodies. It means that the chemical reaction at the area contact of Al2 O3 and CA2 grains, accompanied by the formation of calcium hexaaluminate (hibonite, CA6 ), will take place according to the equation: CaAl4 O7 + 4Al2 O3 → CaAl12 O19
(1)
Calculations of free enthalpy changes, G, of this reaction in the function of temperature are difficult because of the lack of suitable thermodynamic data in the literature. However, in practice, only a general assessment of the sign of G function on the basis of the equation: G = H − TS without executing detailed calculations, conditioned by accessibility of thermodynamic data of all reacting substances, is satisfactory. The case in question concerns the exothermic reaction (H < 0) between solid bodies progressing without discrete changes of the state of aggregation of substances participating in the process (thermal distribution, crystallization, melting, evaporation). Thus, we can assume that it belongs to a group of processes progressing without entropy changes (S ∼ = 0). So, the G value of this reaction is negative within full temperature ranges, which are essential for a technologist. In the light of these assumptions, resistances of a kinetic nature will determine the reaction course. So, from a technological point of view kinetics and mechanism of the reaction in question a in possibly broad temperature range is very essential. Because the total mole volume of substrates is twice smaller than the product mole volume, it should be expected that the formation of new CA6 grains at the contact area of Al2 O3 and CA2 will result in a development of tensile
http://dx.doi.org/10.1016/j.jeurceramsoc.2015.08.034 0955-2219/© 2015 Published by Elsevier Ltd.
˛ et al., Calcium hexaaluminate synthesis and its influence on the properties of CA2 –Al2 O3 -based Please cite this article in press as: A. Pieta, refractories, J Eur Ceram Soc (2015), http://dx.doi.org/10.1016/j.jeurceramsoc.2015.08.034
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2
stresses in the matrix. In consequence, it can lead to induce microfractures and porosity increase, including open porosity, which can stimulate changes of the material thermal expansion coefficient. The present study is aimed at examining the kinetics of the calcium hexaaluminate formation during the reaction between calcium dialuminate and corundum, including an assessment of the chemical stability of CA2 –Al2 O3 composites. 2. Experimental The materials used for the examination of kinetic reactions of calcium hexaaluminate comprised the mixtures of CA2 powder, prepared according to the procedure described in the previous works [7,8] and the commercial corundum powder (Alcoa A-17). The mole ratios between the precursors resulted from stoichiometry of Eq. (1). The powders were mixed in a ball mill for two hours, dried overnight at 80 ◦ C, and then cylinder-shaped samples of 10 mm diameter and 30 mm high were formed from the uniform mixture with the hydraulic press under pressure of 80 MPa. Grain size distribution for the powder was measured by dynamic light scattering on Nanosizer-ZS of Malvern Instruments. The samples were fired at temperatures ranging from 1300 ◦ C to 1550 ◦ C,the temperature rate was 10 ◦ C/min, and they were kept at the maximum temperature from 30 min to 48 h. The phase composition of fired materials was determined by the X-ray diffraction analysis (CuK␣1 , Empyrean, Panalytical) and the amounts of particular phases were determined by the Rietveld method. Relative amounts of the CA6 phase in the tested materials after firing in different periods at the defined temperature, expressed in a form of mass fraction, corresponded to the reaction conversion degree, ␣. A separate series of samples was made of mixtures of CA2 powder of the 150 m grain size and alumina powder of the 0.2/0.5 mm grain size. The samples of pure CA2 and powders containing 10–30% of corundum, formed in the same way as described above, were fired at 1600 ◦ C for 3 h. The grain size distributions for all powder mixtures were qualitatively comparable. The microstructure of such materials was observed with the FEI microscope Nova Nano SEM200 with the analyzer of chemical composition EDAX – Genesis. Poremaster 60 of Quantachrom was used to determine the pore size distribution, whereas the measurements of the linear thermal expansion coefficient were made with the dilatometer Netzsch DIL 402C within the temperature ranges from 20 ◦ C to 1200 ◦ C, with temperature rate 5 ◦ C/min. 3. Results and disscusion Fig. 1 shows grain size distribution for the CA2 and Al2 O3 powders (as an inset) and for their mixture. The grain size distribution is bi-modal and the mode at about 10 m corresponds mainly to the CA2 powder while the second mode at about 70 m to the alumina powder. The determination of kinetics of the formation of CA6 in the tested system was preceded by measurements of the conversion degree of such a reaction as the function of firing temperature. The changes of the amount of particular phases in the materials, fired for 4 h at various temperatures, are shown in Fig. 2. Kinetic resistances of this reaction up to the temperature of about 1300 ◦ C are so large that no effects can be seen and the mass fraction of CA6 is as small as 0.07. An essential increase of the reaction conversion degree takes place above this temperature, what in turn causes that the determination of the reaction of CA6 development was based on the data obtained for the temperature ranging from 1350 ◦ C to 1500 ◦ C. In such a temperature range, the conversion degree is changed from about 0.12 to about 0.95. At 1550 ◦ C the reaction proceeds so fast that the variability range of the conversion degree is too narrow for a reliable analysis. According to the assumed attitude applied in
Fig. 1. Grain size distribution for the mixture of the CA2 and Al2 O3 powders. Inset reveals grain size distributions of the component powders.
Fig. 2. Changes of phase composition of the materials fired for 4 h at various temperatures. Marker size corresponds to determination error.
the examination of the kinetics of chemical reactions, the function of the reaction conversion degree F(˛), was determined, which was linearly dependent on time, according to the equation: F(˛) = k × t
(2)
The form of the function depends on the reaction mechanism. In the tested system, this function satisfies the above condition if we assume the reaction model in which the growth of nuclei constitutes a stage determining the total rate of the process. This model has the best statistical compatibility among basic equations describing the reaction kinetics in the solid phase (in this case a correlation factor is the highest)—Fig. 3. The kinetics of such a reaction is described with Avrami–Jerofiejew’s equation: F(˛) = ln
1 1−˛
= kt
m
(3)
where: k—constant nucleation rate, m—index exponent depending on reaction mechanism and geometrical factor. The applied model describes reactions in the solid phase via the formation of three-dimensional (spherical), two-dimensional (plate-like) and one-dimensional (needle-like) nuclei. The processes limiting the growth of nuclei can be related to the surface reaction or diffusion in the solid phase. For a constant nucleation rate it was assumed that the value of exponent m in this equation for the tested reaction is 3, which means that growing crystal seeds of
˛ et al., Calcium hexaaluminate synthesis and its influence on the properties of CA2 –Al2 O3 -based Please cite this article in press as: A. Pieta, refractories, J Eur Ceram Soc (2015), http://dx.doi.org/10.1016/j.jeurceramsoc.2015.08.034
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Fig. 3. The kinetics of the formation of calcium hexaaluminate in mixture of CaAl4 O7 –Al2 O3 powders.
Fig. 4. SEM image of the material fired at 1450 ◦ C for 3 h. SEM image for the system: coarse corundum grain in CA2 matrix.
CA6 are plate-like and their growth is limited by diffusion. Microscope observations proved the pertinence of these assumptions. Fig. 4 shows the SEM images presenting hexagonal, plate-like CA6 particles formed on the Al2 O3 –CA2 grain boundary during firing at 1450 ◦ C for 3 h. On the basis of SEM image analysis and general thermochemical properties of calcium dialuminate and corundum we can expect that the particles of calcium hexaaluminate are formed at the contact area of grains of both substrates as a result of diffusion of Ca2+ ions from the CA2 phase to the grain boundary. So, the formation of the CA6 seeds and their further growth are limited by diffusion of Ca2+ ions. Indirect evidence for such a type of mechanism is proved by the fact that the CA6 plates directly contacting with the corundum grain have a smaller size than those from the side of CA2 (see Fig. 4). On the basis of the reaction-rate constants calculated from Eq. (3), i.e.: 4.91 × 10−3 s−1 at 1350 ◦ C, 7.56 × 10−3 s−1 at 1400 ◦ C, 11.55 × 10−3 s−1 at 1450 ◦ C and 14.94 × 10−3 s−1 at 1500 ◦ C, the temperature dependence of the reaction-rate constant was plotted in the Arrhenius coordinates (Fig. 5). Apparent activation energy of the CA6 formation in the investigated system calculated from the plot is about 134 kJ/mol, which is in good agreement comparable
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Fig. 5. Temperature dependence of the reaction-rate constant in tested system.
with the values of activation energy of similar processes of some solid state synthesis. The results show that the formation of CA6 in the studied system is proceeding intensively at the temperature over 1350 ◦ C. Thus it can be assumed that in a series of CA2 based materials with the addition of 10%, 20% or 30% corundum, fired at 1600 ◦ C for 3 h, this reaction will also take place. It was proved both by XRD and SEM examinations. The investigation of phase composition showed that sintered bodies, obtained from the mixtures of the CA2 and corundum powders, are composed of three compounds of high refractoriness (melting temperature of CA6 is about 1850 ◦ C). The amounts of CA6 formed in the described conditions are 9.7, 21.2 and 29.4 mass% in the materials obtained from the powder mixture containing 10–30% of Al2 O3 , respectively. SEM images of the material obtained from the powder mixture with 20% of corundum are shown in Fig. 6. Similarly, as it was observed in the materials previously described, in the contact area of corundum grains with CA2 matrix, plate-shaped grains of calcium hexaaluminate are visible. The results of measurements of apparent density and open porosity of the sintered bodies are shown in Table 1. It can be observed that the larger content of corundum in the initial mixture, i.e., the larger the content of CA6 in the sintered bodies the more porous they are. The deterioration of sinterability of the CA2 powder, connected with the presence of corundum grains, results from the formation of the CA6 phase which occurs during sintering. The molar volume of calcium hexaaluminate is over twice larger than the sum of the molar volume of the substrates and some effects of reactive sintering might be expected, the formation of CA6 grains must lead to a formation of cracks and separation of CA2 grains. The changes in the image of pore size distribution prove the influence of the reaction between corundum and CA2 on powder sinterability (Fig. 7). The increase of corundum content in the initial mixture, i.e., the intensification of the reaction with CA2 , results in forming larger and larger pores while pore size distributions are wider and wider. Such larger pores can be formed during pressing of powders and their elimination during sintering is hindered by the formation of CA6 , or they can be formed as secondary pores, related to the changes of the system volume, caused by the same reaction. The temperature dependence of linear thermal expansion coefficient (TEC), obtained on the basis of dilatometric curves, is shown in Fig. 8. The pure CA2 material has a negative expansion (it shrinks) up to the temperature of about 500 ◦ C. Above this temperature TEC is relatively low when compared with other refractory materials. In multi-phase materials, the increase of the amount of corundum in the initial mixture results in a decreasing tendency for initial
˛ et al., Calcium hexaaluminate synthesis and its influence on the properties of CA2 –Al2 O3 -based Please cite this article in press as: A. Pieta, refractories, J Eur Ceram Soc (2015), http://dx.doi.org/10.1016/j.jeurceramsoc.2015.08.034
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Table 1 Apparent density and porosity of materials obtained via sintering of CA2 powder and its mixtures with corundum. Sample
Apparent density, da (g/cm3 )
CA2 CA2 + 10% Al2 O3 CA2 + 20% Al2 O3 CA2 + 30% Al2 O3
2.31 2.37 2.42 2.38
± ± ± ±
0.02 0.05 0.04 0.05
True density, dt (g/cm3 ) 2.66 2.84 2.96 3.06
± ± ± ±
0.02 0.02 0.03 0.03
Porosity, P (%) 13.01 16.58 18.13 22.46
± ± ± ±
0.39 0.41 0.54 0.58
Median (m)
Mode (m)
5.14 5.37 6.12 7.25
6.00 5.33 5.23 6.92
Fig. 6. SEM images of the sintered body obtained from the mixture of CA2 and 20% Al2 O3 powders.
shrinkage during heating; for the material obtained from the mixture containing 30% of corundum TEC is positive already from about 200 ◦ C. The presence of CA6 in the sintered materials causes that the changes of TEC are non-monotonic with the increase of temperature. At the temperature over 900 ◦ C the drop of TEC is observed for all materials obtained from the two-phase powders, and this drop increases for the higher amount of CA6 in the material. This fact is difficult to explain and requires further studies. We can, however, assume that these phenomena are connected with relaxation of stresses generated by differences and structural anisotropy of elasticity moduli and thermal expansion coefficients of all phases occurring in the sintered materials. 4. Conclusions
Fig. 7. Pore size distribution in the materials obtained from the mixtures of CA2 powders with: (1) 10% Al2 O3 , (2) 20% Al2 O3 , (3) 30% Al2 O3 .
Fig. 8. Changes of linear thermal expansion coefficients of CA2 sintered body in the materials obtained from the mixtures of CA2 powder and corundum.
Firing of the mixture of calcium dialuminate and corundum powders is accompanied by a chemical reaction leading to calcium hexaaluminate formation. Measurable effects are visible just at the temperatures exceeding 1300 ◦ C. The product of the CA6 synthesis crystallizes in a form of plate-like particles, located between CA2 and Al2 O3 grains. Among the basic equations describing kinetics of solid state reactions the best statistical compatibility was observed in the case of the model, in which the growth of nuclei constitutes a stage determining the total process rate. Apparent activation energy of CA6 formation in the described system is about 134 kJ/mole, which is in agreement with some values of activation energy in solid state synthesis. It is supposed that the intensification of diffusion of Ca2+ ions to the grain boundary of Al2 O3 grain via the lattice of the formed CA6 is a process limiting the growth of nuclei. The development of plate-shaped CA6 grains causes an increase of material porosity and growth of pore sizes. These effects are more and more intense with the growth of the amount of corundum in the initial powder mixture. Sintered materials, made of pure CA2 , have a negative thermal expansion up to about 500◦ C, and above this temperature the thermal expansion coefficient is low when compared to other refractories. The formation of CA6 reduces the tendency to initial shrinkage during heating. The increase of temperature results in non-monotonic changes of TEC. At the temperature exceeding 900◦ C there is a drop of its value for all materials
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obtained from the two-phase powders. This drop is larger when the content of CA6 in the materials is higher. These phenomena are probably related to relaxation of internal stresses, generated in the obtained three-phase CA2 –CA6 –Al2 O3 materials. Acknowledgement This work was supported by the grant of The National Centre of Research and Development: INNOTECHK1/IN1/71/153131/NCBR/12. References [1] Y. Suzuki, T. Ohji, Anisotropic thermal expansion of calcium dialuminate (CaAl4 O7 ) simulated by molecular dynamics, Ceram. Int. 30 (2004) 57–61. [2] A. Altaya, C.B. Carter, P. Rulis, W.-Y. Ching, I. Arsland, M.A. Gülgüne, Characterizing CA2 and CA6 using ELNES, J. Solid State Chem. 183 (2010) 1776–1784.
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[3] I.R. Oliveira, F.S. Ortega, V.C. Pandolfelli, Hydration of CAC cement in a castable refractory matrix containing processing additives, Ceram. Int. 35 (2009) 1545–1552. [4] F.A. Cardoso, M.D.M. Innocentini, M.M. Akiyoshi, V.C. Pandolfelli, Effect of curing time on the properties of CAC bonded refractory castables, J. Eur. Ceram. Soc. 24 (2004) 2073–2078. [5] A.P. Luz, V.C. Pandolfelli, CaCO3 addition effect on the hydration and mechanical strength evolution of calcium aluminate cement for endodontic applications, Ceram. Int. 38 (2012) 1417–1425. [6] E. Criado, S. De Aza, Calcium hexaalminates as refractory material, in: Proceedings of UNITECR Congress, Aachen, 1991, p. 403. [7] S. Jonas, F. Nadachowski, D. Szwagierczak, A new non-silicate refractory of low thermal expansion, Ceram. Int. 24 (1998) 211–216. [8] S. Jonas, F. Nadachowski, D. Szwagierczak, Low thermal expansion refractory composites based on CaAl4 O7 , Ceram. Int. 25 (1999) 77–84. [9] S. Jonas, F. Nadachowski, D. Szwagierczak, G. Wójcik, Thermal expansion of CaAl4 O7 -based refractory compositions containing MgO and CaO additions, J. Eur. Ceram. Soc. 26 (2006) 2273–2278.
˛ et al., Calcium hexaaluminate synthesis and its influence on the properties of CA2 –Al2 O3 -based Please cite this article in press as: A. Pieta, refractories, J Eur Ceram Soc (2015), http://dx.doi.org/10.1016/j.jeurceramsoc.2015.08.034