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Microstructural evolution and phase transformation of different sintered Kaolins powder compacts
Materials Chemistry and Physics 112 (2008) 876–885
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Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Microstructural evolution and phase transformation of different sintered Kaolins powder compacts Atidel Ghorbel ∗ , Mohieddine Fourati, Jamel Bouaziz Laboratory of Industrial Chemistry (LCI), National School of Engineering, BPW 3038 Sfax, Tunisia
a r t i c l e
i n f o
Article history: Received 23 January 2008 Received in revised form 13 June 2008 Accepted 21 June 2008 Keywords: Ceramics Differential thermal analysis (DTA) Nuclear magnetic resonance (NMR) Microstructure
a b s t r a c t Four kinds of Kaolins (K1 , K2 , K3 and K4 ) from various origins were studied using X-ray diffraction, 29 Si and 27 Al MAS-NMR spectroscopy and scanning electron microscopy (SEM). Mineralogical and morphological characteristics of these samples are given. Kaolinte is the principal mineral but other minerals are present in small quantities: Illite, Muscovite and Quartz. The thermal behaviour of K1 , K2 , K3 and K4 was studied. The transformation heats during heating were quantified from DTA measurements and phase changes were followed by X-ray diffraction analyses and 29 Si and 27 Al MAS-NMR spectroscopy. Results indicated that a series of phase transformations take place as the Kaolin is fired at elevated temperature. Mullite is first formed at a temperature as low as 1100 ◦ C. Microstructural evolution of this specimen and their mechanical properties are investigated. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Kaolin, relatively pure clay, has been widely used in ceramic industries for centuries [1]. Kaolin is, therefore, one of the most important raw materials for ceramic industries. Though Kaolin has been used for many years, to explore the complexities involved in its phase transformation and microstructural evolution at elevated temperature is still a challenging task [2]. The main product phase after firing Kaolin at high temperature is Mullite (3Al2 O3 , 2SiO2 ). This type of Mullite is designated as “Old Mullite” in comparison to the term “New Mullite” which represents high purity Mullite (Chemical Mullite) [1]. Mullite is an important and widely studied ceramic material. Mullite itself is very stable at high temperature. Furthermore, its thermal expansion coefficient and dielectric loss are low. It is, therefore, widely used in a diverse number of applications, including structural and refractory ceramics, microelectronic packaging, high-temperature protective coatings, microwave dielectrics, and infrared-transmitting materials [3,4]. During heating, the removal of the hydroxyl group from Kaolinite is accompanied by a reorganization of the octahedral layer of Kaolinite to a tetrahedral configuration in metakaolin (400–500 ◦ C) [5]. Different mechanisms [6,7] were proposed to take into account the nature of decomposition of metakaolin (around 980 ◦ C). Metakaolin decomposes, upon heating to Aluminium–Silicon
∗ Corresponding author. Tel.: +216 97 58 85 73; fax: +216 74 67 69 08. E-mail address: gh [email protected] (A. Ghorbel). 0254-0584/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2008.06.047
spinel and additional Silica. A high degree of Silicon incorporation into ␥-Alumina was also proposed [8]. Mullite begins to form around 980 ◦ C and the amount of Mullite increases with increasing temperature. This process has been investigated by many scientists. Some authors affirm that Mullite forms through an intermediate spinel-type phase, others suggest that Mullite forms direct from metakaolinite [9,10]. The aim of this study was to determine, by using 29 Si and 27 Al MAS-NMR spectroscopy, scanning electron microscopy (SEM) and differential thermal and thermogravimetric analysis (DTA and TGA), the microstructural evolution of the Kaolin during heat treatment and the effect of the presence of mica-phase and Quartz on the fired densification of the fired products. For this study, 4 kinds of Kaolins (K1 , K2 , K3 and K4 ) from various origins have been used. The chemical, physical and mineralogical properties of the Kaolins have been characterized by X-ray diffraction (XRD), 29 Si and 27 Al MAS-NMR spectroscopy and scanning electron microscopy. 2. Experimental procedures In this study, different varieties of Kaolins are selected (K1 , K2 , K3 and K4 ). Kaolin particles are usually flaky in shape. The powder compacts were prepared by applying the die-pressing technique. The Kaolin flakes tend to lie down on the plane, which is perpendicular to the die-pressing direction. The firing was carried out at a temperature varied from room temperature to 1300 ◦ C for 1 h. The chemical, physical and mineralogical properties of the four kinds of Kaolins are characterized by the following techniques: • XRD analysis of the powder are carried out at room temperature by means of a diffractometer using Cu K␣ radiation (K␣1 = 1.5406 Å)
A. Ghorbel et al. / Materials Chemistry and Physics 112 (2008) 876–885 Table 1 Chemical compositions of the four parent Kaolins samples Sample
SiO2
Al2 O3
K2 O
CaO
MgO
Na2 O
SO3
SiO2 /Al2 O3
K1 K2 K3 K4
40.12 44.36 41.75 58.27
40.24 35.03 39.66 24.46
2.05 2.69 1.08 0.1
– – – –
– 0.08 0.15 –
– 0.039 – –
– – – –
0.997 1.26 1.05 2.38
• DTA and TGA of samples are conducted in reduced atmosphere until 1200 ◦ C at a heating rate of 10 ◦ C min−1 using a setaram set sys 24 apparants. • 29 Si and 27 Al cp MAS-NMR spectra of sample are measured with a Bruker 300 WB NMR spectrometer solid-state high resolution, respectively. • Scanning electron microscope, Model PHILIPS XL30: is working with an accelerating voltage going from 0.2 to 40 kV, a source (filament tungsten), and a power data. This microscope is equipped with an analyzer of dispersion’s energy X(EDX).
3. Results and discussions
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In addition, K1 and K2 contains minor Muscovite mica (Mu) and K3 shows minor Illite mica (I) [11,12]. The results are in agree with those of MAS-NMR. Fig. 3 shows the experiment 27 Al MAS-NMR and 29 Si MAS-NMR spectra of the four Kaolins investigated. 27 Al MAS-NMR spectra show that the different values of signals prove that the Kaolinite is not the only phase present. Indeed, in the structure of the Kaolinite, the Aluminium occupies octahedral sites, the presence of the characteristic peaks of tetrahedral sites in K1, K2 and K3 can be explained by the presence of mica-phase. 29 Si MAS-NMR spectra show that the signal at −91 ppm correspond to Kaolinite and Illite [13]. The free amorphous Silica (Quartz) is clearly identified in K2, K3 and K4 by a characteristic board resonance at −107 ppm [13]. The 29 Si signal of Quartz is much stronger in K4 . In addition, the signal at −86 ppm observed in 29 Si spectra of K1 and K2 is characteristic of Muscovite [13]. This signal is much stronger in K2 . Evaluated from XRD, MAS-NMR, scanning electron microscopy and chemical compositions, the nature of major phases is:
3.1. Characterization of Kaolins The chemical compositions of the four parent Kaolins (K1 , K2 , K3 and K4 ) are shown in Table 1. The ratio SiO2 /Al2 O3 in K2 and K4 higher than the value for the Kaolinite (1.178 = 46.55/39.49), is due to the presence of Quartz. The high percentage of K2 O (2.05% for K1 , 2.69 wt.% for K2 and 1.08% for K3 ) is due to the existence of a small amount of a mica-phase (Muscovite, Illite). Scanning electron microscopy showed the Kaolinites particles having an average diameter of 2 m in K1 , K2 , K3 and 5 m in K4 . Fig. 1 shows the morphology of the Kaolins particles. The Kaolins particles are flaky in shape, Fig. 1(a) and (b). Some large Kaolin flakes are stacked together to form agglomerates, Fig. 1(c) and (d). The XRD pattern of the Kaolins powders is shown in Fig. 2. Apart from the Kaolinite phase, a small amount of Quartz is detected in K2 , K3 and K4 by the XRD analysis, how is the main impurity in K4 .
• • • •
Kaolinite and Muscovite in K1 . Kaolinite, Muscovite and Quartz in K2 . Kaolinite, Illite and Quartz in K3 . Kaolinite and Quartz in K4 .
3.2. Densification behaviour of Kaolins The powder compacts were prepared by applying the diepressing technique. The strength pressure applied was 35 MPa. Cylindrical samples are made up from different varieties of Kaolins (K1 , K2 , K3 and K4 ) and subjected to thermal treatment at temperatures ranging from 1000 to 1300 ◦ C for 1 h in the presence of the air. The heating rate and cooling rate were 10 ◦ C min−1 . The heated powders were characterized using 29 Si and 27 Al MAS-NMR spectroscopy, scanning electron microscopy and DTA and TGA in order
Fig. 1. Scanning electron micrograph of powder of Kaolins: (a) K4 (14,928×); (b) K2 (7464×); (c) K1 (933×); (d) K2 (933×).
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A. Ghorbel et al. / Materials Chemistry and Physics 112 (2008) 876–885
Fig. 2. XRD pattern of Kaolins powder: (a) K1 ; (b) K2 ; (c) K3 ; (d) K4 .
to understand the microstructural evolution of the Kaolins during heat treatment and the effect of the presence of mica-phase and Quartz on the fired densification and the mechanical properties of the fired products. The Brazilian test has been employed to characterize the mechanical resistance of the fired samples. The specimen is loaded in compression along one of their diameters and the failure appears when the tension loads generated reach the limiting value of the tensile strength. The Brazilian test is carried out in a testing machine Loyd EZ50 (temperature: 25 ◦ C and displacement rate: 0.5 mm min−1 ). The specimen geometry used was a disc of 6 mm thickness and 30 mm in diameter. Fig. 4 shows the variation of mechanical resistance with the variation of temperature (1000, 1100, 1200 and 1300 ◦ C). This figure shows clearly the influence that content of the mica-phase and Quartz (SiO2 ) has on mechanical resistance. In the first case, the presence of mica-phase in Kaolin leads increase the mechanical resistance and decreases the maximum temperature densification. By the way, in this case, Kaolin K2 present the best strength and the maximum temperature densification is at 1100 ◦ C. The densification is more pronounced for the Kaolin containing an important quantity of mica-phase (K2 ). At higher temperatures the mechanical resistance decreases. Otherwise, the presence of SiO2 decreases the mechanical resistance: the densification of K4 is not finished at 1300 ◦ C. The results are in agreed with those of scanning electron microscopy and relative density of the fired compacts. The fracture surfaces of the specimens are shown in Figs. 5 and 6 at the different temperatures. These figures shows that the densification of K4 is not finished at 1300 ◦ C due to the presence of grains SiO2 inside Kaolin. At 1100 ◦ C, the densification of K2 is more pronounced compared to the rest of Kaolins. Densification of the compact can, therefore, take place through viscous flow. The presence of nearly spherical pores in the sintered specimens, Fig. 6, shows evidences the existence of
the viscous flow mechanism [14]. Furthermore, the fracture path around the large pores is very much flat. The observation of the fresh fracture surface reveals also the increases of the diameter of pores (K2 ) at higher temperatures, when the temperature of the beginning of densification is exceeded. This phenomenon results in a decrease in mechanical properties (Fig. 4). The relative density of the powder compacts is shown as a function of firing temperature in Fig. 7. This figure also shows that the relative density of K2 increases with the increase of firing temperature until 1100 ◦ C. The density drops above 1100 ◦ C. It is due to the presence of larges pores formed within the fired compact. The density is, therefore, decreased. In addition, Kaolin K4 presents the lows relative densities until 1300 ◦ C: that confirm that the densification of K4 is not finished. We can conclude that: • The increase in the strength corresponds to an evolution of the microstructure: structure in increasingly dense. • Densifications of the compact take place through viscous flow. • The densification is more marked in the case of the samples containing a significant quantity of mica-phase (Kaolin K2 ). That is due to the formation of an intergranular liquid resulting from the mica-phase. • The increase in the size of the pores after densification causes a reduction of the mechanical properties. On the other hand, Fig. 8 shows the XRD patterns of Kaolin (K2 ) after heating at 1150 ◦ C in the presence of the air. It reveals the formation of Mullite (M) and Quartz (Q) [15–17]. This indicates that Mullite can be formed at lower temperature. The weight losses (TGA) observed during heating of fired Kaolins are reported in Fig. 9. DTA curves recorded during heating of Kaolins are presented in Fig. 10. The characteristic of the endo and exothermic peaks are reported in Table 2 for each Kaolins.
A. Ghorbel et al. / Materials Chemistry and Physics 112 (2008) 876–885
Fig. 3.
27
Al and 29 Si MAS-NMR spectra of K1 , K2 , K3 and K4 .
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Fig. 4. Variation of mechanical resistance with sintering temperature.
The DTA curves shows two endothermic peaks at 100 and 570 ◦ C and an exothermic one at 970 ◦ C. In TG trace, a two-step weight loss is observed. The weight loss of the first step at 100 ◦ C is about 1.0 wt.%, corresponding entirely to the first endothermic peak in DTA. The weight loss of the second step as the heating temperature at 450–850 ◦ C is between 7.0 wt.% (K4 ) and 11.0 wt.% (K3 ), corresponding with the second endothermic effect at 570 ◦ C. On the other hand, the weight loss does not change significantly during the heating temperature at above 850 ◦ C. The results show that the first endothermic peak of low intensity corresponds to the loss of the weakly bound water, and the second, much larger, endothermic peak corresponds to the loss of structural hydroxyl groups and the transformation of Kaolinite to metakaolinite. Lastly, the intense exothermic peak observed between 930 and 970 ◦ C, may
Fig. 5. Scanning electron micrograph of fracture surfaces of Kaolins powder (K1 , K2 , K3 and K4 ) at 1000 and 1100 ◦ C.
A. Ghorbel et al. / Materials Chemistry and Physics 112 (2008) 876–885
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Fig. 6. Scanning electron micrograph of fracture surfaces of Kaolins powder (K1 , K2 , K3 and K4 ) at 1200 and 1300 ◦ C.
be attributed to Mullite nucleation. Some others [16–20] attribute this reaction to spinel formation. According to Fig. 7, the exothermic event is irrefutable evidence for the Mullite formation. Figs. 11 and 12 show, respectively the 27 Al NMR spectra and 29 Si NMR spectra of the different Kaolins heat-treated from 1000 to 1300 ◦ C. 27 Al MAS-NMR spectra (Fig. 11) display only two signals at ca. 53.76 and −3.23 ppm in K1 , at ca. 52.26 and −0.65 ppm in K2 at ca. 56.11 and 2.38 ppm in K3 and at ca. 55.51 and 1.28 ppm in K4 . The first signal corresponds to tetra-coordinated Aluminium and the other signal to hexa-coordinated Aluminium. The signal
Table 2 Endothermic and exothermic phenomena observed for K1 , K2 , K3 and K4 heated from room temperature to 1200 ◦ C Material
K1 K2 K3 K4
Endothermic phenomenon
Exothermic phenomenon
Temperature (◦ C)
Temperature (◦ C)
Temperature (◦ C)
101 101 101 101
570 574 568 560
982 971 967 975
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A. Ghorbel et al. / Materials Chemistry and Physics 112 (2008) 876–885
Fig. 7. Variation of relative density with sintering temperature.
correspond to tetra-coordinated Aluminium is much stronger than that hexa-coordinated Aluminium signal. This phenomenon is less pronounced in K4 . It proves that the Mullite in K4 is relatively richer in Silicon than those in K1 , K2 and K3 . The different 27 Al NMR spectra reveal the transformation process of Aluminium(VI) in Kaolins to Aluminium(IV). These results are in agree with those of literature. In the Kaolinite–Mullite reaction sequence, Mullite begins to form around 980 ◦ C and the amount of Mullite increases with increasing temperature. Sanz et al. [21] reported three 27 Al resonances at 0 ppm (Al(VI)), 42 ppm (Al(V)), and 60 ppm (Al(IV)). Other researchers [22–24] revealed only two 27 Al signals corresponding to Al(IV) and Al(VI). 29 Si MAS-NMR spectra (Fig. 12) display in K and K two sig1 2 nals centered at ca. −106 and −90 ppm. The signal at ca. −106 ppm is much stronger than that at −90 ppm. The signal at ca. −90 ppm increases gradually with increasing temperature. 29 Si signal at −90 and −106 ppm are related to Mullite and to Q4 Si (Silica), respectively. 29 Si MAS-NMR spectra show the presence in K and K three 3 4 signals centered at ca. −87, −107 and −111 ppm. 29 Si signal at −111 ppm is related to Cristobalite. The signals at ca. −111 and −87 ppm increases gradually with increasing temperature wile that at ca. −107 ppm decreases. This reflects increased Mullite and Cristobalite formation with Silica consumption. Sheriff and Grundy [25] have reported a correlation between 29 Si MAS-NMR chemical shift data and the local geometry around Silicon (Si) in Silicate structures. Applying that correlation, they calculated a chemical
Fig. 8. XRD patterns of Kaolin K2 after heating at 1150 ◦ C.
shift of −86.1 ppm for Sillimanite, which is in good agreement with our experimental results. Therefore, the signal at −87 ppm observed in K3 and K4 suggests a Sillimanite-type Al/Si ordering scheme existing in Mullite derived from these Kaolinites. This also confirms the well-known structural similarity between Mullite and Sillimanite. The Mullite structure can be derived from that of Sillimanite by operation of the substitution: O2− + 2Si4+ → 2Al3+ + . This operation results in removing away some oxygens from the structure [26–28]. Mullite derived from K3 and K4 is richer in Silicon than from K1 and K2 because Sillimanite generally is richer in Silicon than Mullite. This explanation is supported by the 27 Al NMR spectra, as shown less amount of Aluminium atoms in tetrahedral sites of Mullite in K3 and K4 than that in K1 and K2 . This analysis suggests that during heating, the removal of the hydroxyl group from Kaolinite is accompanied by a reorganization of the octahedral layer of Kaolinite to a tetrahedral configuration in metakaolin (400–500 ◦ C). From 1000 ◦ C, there is formation of:
Fig. 9. Thermogravimetric analysis of Kaolins during heating.
A. Ghorbel et al. / Materials Chemistry and Physics 112 (2008) 876–885
Fig. 10. Differential thermal analysis of Kaolins during heating.
Fig. 11.
27
Al MAS-NMR spectra of K1 , K2 , K3 and K4 .
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• Mullite and amorphous Silica in K1 and K2 (the quantity of Mullite increases with the temperature). • Sillimanite-type Al/Si, amorphous Silica and Cristoballite. According to the temperature, the quantity of Cristoballite increases which depends on the quantity of amorphous Silica according to the reaction [2]: 3Al2 O3 2SiO2 + Mullite
T>1200 ◦ C
4SiO2 amorphous Silica
−→
3Al2 O3 2SiO2 + Mullite
4SiO2 Cristoballite (1)
From our results and the previous studies [2,29], the Kaolin powder compacts underwent a series of phase transformations as the temperature was raised from room temperature to 1300 ◦ C. The phase transformations are expressed in the form of chemical reactions for the ease of explanation. However, the above equations are not exactly balanced for many ceramic products and are more or less non-stoichiometric. These phase transformations are:
Fig. 12.
29
When the Kaolinite is heated, the adsorbed water is liberated at above 100 ◦ C and the weakest part of the chemical bond is broke or perturbed. When T = 400–500 ◦ C, the dehydroxylation takes place: 2SiO2 Al2 O3 2H2 O → 2SiO2 Al2 O3 + 2H2 O Kaolinite
(2)
metakaolinite
For Kaolinite, dehydroxylation might result in the disturbance of the Al(O,OH)6 octahedral sheet by the outer hydroxyls, but does not have much effect on the SiO4 tetrahedral sheet due to the more stable inner hydroxyls groups. The outer hydroxyls of octahedral sheets may be more easily removed by heating than inner ones that will maintain a more ordered SiO4 tetrahedral group in structure during dehydroxylation. When T ∼ 950 ◦ C, the metakaolinite is transformed to a spinel structure or a Si-containing ␥-Al2 O3 and amorphous Silica. The SiO4 groups combined with AlO6 group to form the Al–Si spinel
Si MAS-NMR spectra of K1 , K2 , K3 and K4 .
A. Ghorbel et al. / Materials Chemistry and Physics 112 (2008) 876–885
phase that in a short range order structure. The Al–Si spinel phase appears at 920 ◦ C and persists until at least 1100 ◦ C. At this temperature, whether a spinel or a Silicon-containing ␥-Al2 O3 is formed is still under debate: 2SiO2 Al2 O3 → SiAl2 O5 + metakaolinite
spinel
SiO2 amorphous Silica
(3)
or 2SiO2 Al2 O3 → Al2 O3 metakaolinite
+
␥−Alumina
SiO2 amorphous Silica
(4)
When T > 1100 ◦ C, Mullite phase first appears at a temperature around 1100 ◦ C, its amount increases with the increase of temperature: the Mullite formation increase with the heating temperature increase from 1050 to 1300 ◦ C as shown in Fig. 11: SiAl2 O5 + spinel
SiO2 amorphous Silica
→
1 3Al2 O3 2SiO2 3 Mullite
+
4 3
SiO2 amorphous Silica
(5)
or Al2 O3 + SiO2 amorphous Silica ␥−Alumina
→
1 3Al2 O3 2SiO2 3 Mullite
+
4 3
SiO2 amorphous Silica (6)
T > 1200 ◦ C,
When the amorphous SiO2 changes to Cristobalite according to reaction (1). Furthermore, the impurities in the starting powder (mica-phase) can induce a liquid phase during firing (Fig. 6). The presence of the liquid phase can shift slightly the formation temperature of each phase and its amount. 4. Conclusion The various characterizations (using XRD, chemical analyses, scanning electron microscopy, thermal analyses and NMR) allowed the detection and identification of the constituent minerals in the samples of the four Kaolins studied. Kaolinite is the dominant mineral, but all also contain differing amounts of other minerals (Quartz, Illite, and Muscovite). These “impurities” affect their physico-chemical and morphological properties. The transformation heats during heating have quantified from TG-DTA and phase changes have followed by XRD and NMR analyses. Results indicated that the different kinds of Kaolins powder compacts underwent a series of phase transformations during heating from room temperature to 1300 ◦ C: Kaolinite dehydroxylation, metakaolinite structure change, exothermic structural organization
885
and Mullite formation. The impurities in the starting powder can induce a liquid phase during firing and can affect the mechanical resistance. Therefore, the advantages of using Kaolinite powder as the starting material for Mullite preparation are low mullitization temperature and unique microstructure. Given their mineralogy, physicochemical characteristics and mechanical properties, these Kaolins have been used for the fabrication of ceramic-metal composites (Kaolin–Aluminium) [30]. References [1] F.H. Norton, Fine Ceramics, Technology and Applications, Robert E. Krieger, USA, 1978, p. 142. [2] W.M. Carty, U. Senapati, J. Am. Ceram. Soc. 81 (1998) 3. [3] P. Kansal, R.M. Laine, F. Babonneau, J. Am. Ceram. Soc. 80 (1997) 2597. [4] R. Soundararajan, G. Kuhn, R. Atisivan, S. Bose, A. Bandyopadhyay, J. Am. Ceram. Soc. 84 (2001) 509. [5] S. Iwai, H. Tagai, T. Shimamune, Acta Crystallogr. B 27 (1971) 248. [6] A.K. Chakrabarty, D.K. Ghosh, J. Am. Ceram. Soc. 61 (1978) 90. [7] K. Srikrishn, G. Thomas, R. Martinez, M.P. Corral, S. De Aza, J.S. Moya, J. Mater. Sci. 25 (1990) 607. [8] I.M. Low, R. McPherson, J. Mater. Sci. 24 (1989) 926. [9] B. Sonuparlak, M. Sarikaya, I.A. Aksay, J. Am. Ceram. Soc. 70 (1987) 837. [10] A. Gualtieri, M. Belloto, G. Artioli, S.M. Clark, Phys. Chem. Miner. 22 (1995) 207. [11] H. Bingqiang, L. Nan, Ceram. Int. 31 (2005) 227. [12] J. Temuujin, K.J.D. MacKenzie, M. Schmücker, H. Schneider, J. McManus, S. Wimperis, J. Eur. Ceram. Soc. 20 (2000) 413. [13] J. F. Stebbins, Nuclear magnetic resonance spectroscopy of Silicates and oxides in geochemistry and geophysics, mineral physics and crystallography, A Handbook of Physical Constants, copyright 1995 by the American Geophysical Union. 303. [14] J.K. Mackenzie, R. Shuttleworth, Phenomenological theory of sintering, Proc. Phys. Soc. Lond. 62 (1949) 633–652. [15] L. Ya-Fei, L. Xing-Qin, W. Hui, M. Guang-Y, Ceram. Int. 27 (2001) 1. [16] O. Castelein, B. Soulestin, J.P. Bonnet, P. Blanchart, Ceram. Int. 27 (2001) 517. [17] M.A. Sainz, F.J. Serrano, J.M. Amigo, J. Bastida, Caballero, J. Eur. Ceram. Soc. 20 (2000) 403. [18] M. Bellotto, A. Gualtieri, G. Artioli, S.M. Clark, Phys. Chem. Miner. 22 (1995) 207. [19] Z. Tatli, A. Demir, R. Yilmaz, F. Caliskan, A.O. Kurt, J. Eur. Ceram. Soc. 27 (2007) 743. [20] C.Y. Chen, G.S. Lan, W.H. Tuan, J. Eur. Ceram. Soc. 20 (2000) 2519. [21] J. Sanz, A. Madani, J.M. Serratosa, J. Am. Ceram. Soc. 71 (1988) 418. [22] J. Rocha, J. Klinowski, Phys. Chem. Miner. 17 (1990) 179. [23] D. Massiot, P. Dion, J.F. Alcover, F. Bergay, J. Am. Ceram. Soc. 78 (1995) 2940. [24] W.M. Brown, K.J.D. Mackenzie, M.E. Bowden, R.H. Meinhold, J. Am. Ceram. Soc. 68 (1985) 293. [25] B.L. Sherriff, H.D. Grundy, Nature 332 (1988) 819. [26] W.E. Cameron, Am. Miner. 62 (1977) 747. [27] J. Angel, C.T. Prewitt, Am. Miner. 71 (1986) 1476. [28] U.C. Bertram, V. Heine, I.L. Jones, G.D. Price, Phys. Chem. Miner. 17 (1990) 326. [29] G.W. Brindley, M. Nakahira, J. Am. Ceram. Soc. 40 (1957) 346. [30] A. Ghorbel, J. Bouaziz, M. Fourati, I. RE. M. E. 2 (2008) 386.
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Microstructural evolution and phase transformation of different sintered Kaolins powder compacts Atidel Ghorbel ∗ , Mohieddine Fourati, Jamel Bouaziz Laboratory of Industrial Chemistry (LCI), National School of Engineering, BPW 3038 Sfax, Tunisia
a r t i c l e
i n f o
Article history: Received 23 January 2008 Received in revised form 13 June 2008 Accepted 21 June 2008 Keywords: Ceramics Differential thermal analysis (DTA) Nuclear magnetic resonance (NMR) Microstructure
a b s t r a c t Four kinds of Kaolins (K1 , K2 , K3 and K4 ) from various origins were studied using X-ray diffraction, 29 Si and 27 Al MAS-NMR spectroscopy and scanning electron microscopy (SEM). Mineralogical and morphological characteristics of these samples are given. Kaolinte is the principal mineral but other minerals are present in small quantities: Illite, Muscovite and Quartz. The thermal behaviour of K1 , K2 , K3 and K4 was studied. The transformation heats during heating were quantified from DTA measurements and phase changes were followed by X-ray diffraction analyses and 29 Si and 27 Al MAS-NMR spectroscopy. Results indicated that a series of phase transformations take place as the Kaolin is fired at elevated temperature. Mullite is first formed at a temperature as low as 1100 ◦ C. Microstructural evolution of this specimen and their mechanical properties are investigated. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Kaolin, relatively pure clay, has been widely used in ceramic industries for centuries [1]. Kaolin is, therefore, one of the most important raw materials for ceramic industries. Though Kaolin has been used for many years, to explore the complexities involved in its phase transformation and microstructural evolution at elevated temperature is still a challenging task [2]. The main product phase after firing Kaolin at high temperature is Mullite (3Al2 O3 , 2SiO2 ). This type of Mullite is designated as “Old Mullite” in comparison to the term “New Mullite” which represents high purity Mullite (Chemical Mullite) [1]. Mullite is an important and widely studied ceramic material. Mullite itself is very stable at high temperature. Furthermore, its thermal expansion coefficient and dielectric loss are low. It is, therefore, widely used in a diverse number of applications, including structural and refractory ceramics, microelectronic packaging, high-temperature protective coatings, microwave dielectrics, and infrared-transmitting materials [3,4]. During heating, the removal of the hydroxyl group from Kaolinite is accompanied by a reorganization of the octahedral layer of Kaolinite to a tetrahedral configuration in metakaolin (400–500 ◦ C) [5]. Different mechanisms [6,7] were proposed to take into account the nature of decomposition of metakaolin (around 980 ◦ C). Metakaolin decomposes, upon heating to Aluminium–Silicon
∗ Corresponding author. Tel.: +216 97 58 85 73; fax: +216 74 67 69 08. E-mail address: gh [email protected] (A. Ghorbel). 0254-0584/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2008.06.047
spinel and additional Silica. A high degree of Silicon incorporation into ␥-Alumina was also proposed [8]. Mullite begins to form around 980 ◦ C and the amount of Mullite increases with increasing temperature. This process has been investigated by many scientists. Some authors affirm that Mullite forms through an intermediate spinel-type phase, others suggest that Mullite forms direct from metakaolinite [9,10]. The aim of this study was to determine, by using 29 Si and 27 Al MAS-NMR spectroscopy, scanning electron microscopy (SEM) and differential thermal and thermogravimetric analysis (DTA and TGA), the microstructural evolution of the Kaolin during heat treatment and the effect of the presence of mica-phase and Quartz on the fired densification of the fired products. For this study, 4 kinds of Kaolins (K1 , K2 , K3 and K4 ) from various origins have been used. The chemical, physical and mineralogical properties of the Kaolins have been characterized by X-ray diffraction (XRD), 29 Si and 27 Al MAS-NMR spectroscopy and scanning electron microscopy. 2. Experimental procedures In this study, different varieties of Kaolins are selected (K1 , K2 , K3 and K4 ). Kaolin particles are usually flaky in shape. The powder compacts were prepared by applying the die-pressing technique. The Kaolin flakes tend to lie down on the plane, which is perpendicular to the die-pressing direction. The firing was carried out at a temperature varied from room temperature to 1300 ◦ C for 1 h. The chemical, physical and mineralogical properties of the four kinds of Kaolins are characterized by the following techniques: • XRD analysis of the powder are carried out at room temperature by means of a diffractometer using Cu K␣ radiation (K␣1 = 1.5406 Å)
A. Ghorbel et al. / Materials Chemistry and Physics 112 (2008) 876–885 Table 1 Chemical compositions of the four parent Kaolins samples Sample
SiO2
Al2 O3
K2 O
CaO
MgO
Na2 O
SO3
SiO2 /Al2 O3
K1 K2 K3 K4
40.12 44.36 41.75 58.27
40.24 35.03 39.66 24.46
2.05 2.69 1.08 0.1
– – – –
– 0.08 0.15 –
– 0.039 – –
– – – –
0.997 1.26 1.05 2.38
• DTA and TGA of samples are conducted in reduced atmosphere until 1200 ◦ C at a heating rate of 10 ◦ C min−1 using a setaram set sys 24 apparants. • 29 Si and 27 Al cp MAS-NMR spectra of sample are measured with a Bruker 300 WB NMR spectrometer solid-state high resolution, respectively. • Scanning electron microscope, Model PHILIPS XL30: is working with an accelerating voltage going from 0.2 to 40 kV, a source (filament tungsten), and a power data. This microscope is equipped with an analyzer of dispersion’s energy X(EDX).
3. Results and discussions
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In addition, K1 and K2 contains minor Muscovite mica (Mu) and K3 shows minor Illite mica (I) [11,12]. The results are in agree with those of MAS-NMR. Fig. 3 shows the experiment 27 Al MAS-NMR and 29 Si MAS-NMR spectra of the four Kaolins investigated. 27 Al MAS-NMR spectra show that the different values of signals prove that the Kaolinite is not the only phase present. Indeed, in the structure of the Kaolinite, the Aluminium occupies octahedral sites, the presence of the characteristic peaks of tetrahedral sites in K1, K2 and K3 can be explained by the presence of mica-phase. 29 Si MAS-NMR spectra show that the signal at −91 ppm correspond to Kaolinite and Illite [13]. The free amorphous Silica (Quartz) is clearly identified in K2, K3 and K4 by a characteristic board resonance at −107 ppm [13]. The 29 Si signal of Quartz is much stronger in K4 . In addition, the signal at −86 ppm observed in 29 Si spectra of K1 and K2 is characteristic of Muscovite [13]. This signal is much stronger in K2 . Evaluated from XRD, MAS-NMR, scanning electron microscopy and chemical compositions, the nature of major phases is:
3.1. Characterization of Kaolins The chemical compositions of the four parent Kaolins (K1 , K2 , K3 and K4 ) are shown in Table 1. The ratio SiO2 /Al2 O3 in K2 and K4 higher than the value for the Kaolinite (1.178 = 46.55/39.49), is due to the presence of Quartz. The high percentage of K2 O (2.05% for K1 , 2.69 wt.% for K2 and 1.08% for K3 ) is due to the existence of a small amount of a mica-phase (Muscovite, Illite). Scanning electron microscopy showed the Kaolinites particles having an average diameter of 2 m in K1 , K2 , K3 and 5 m in K4 . Fig. 1 shows the morphology of the Kaolins particles. The Kaolins particles are flaky in shape, Fig. 1(a) and (b). Some large Kaolin flakes are stacked together to form agglomerates, Fig. 1(c) and (d). The XRD pattern of the Kaolins powders is shown in Fig. 2. Apart from the Kaolinite phase, a small amount of Quartz is detected in K2 , K3 and K4 by the XRD analysis, how is the main impurity in K4 .
• • • •
Kaolinite and Muscovite in K1 . Kaolinite, Muscovite and Quartz in K2 . Kaolinite, Illite and Quartz in K3 . Kaolinite and Quartz in K4 .
3.2. Densification behaviour of Kaolins The powder compacts were prepared by applying the diepressing technique. The strength pressure applied was 35 MPa. Cylindrical samples are made up from different varieties of Kaolins (K1 , K2 , K3 and K4 ) and subjected to thermal treatment at temperatures ranging from 1000 to 1300 ◦ C for 1 h in the presence of the air. The heating rate and cooling rate were 10 ◦ C min−1 . The heated powders were characterized using 29 Si and 27 Al MAS-NMR spectroscopy, scanning electron microscopy and DTA and TGA in order
Fig. 1. Scanning electron micrograph of powder of Kaolins: (a) K4 (14,928×); (b) K2 (7464×); (c) K1 (933×); (d) K2 (933×).
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Fig. 2. XRD pattern of Kaolins powder: (a) K1 ; (b) K2 ; (c) K3 ; (d) K4 .
to understand the microstructural evolution of the Kaolins during heat treatment and the effect of the presence of mica-phase and Quartz on the fired densification and the mechanical properties of the fired products. The Brazilian test has been employed to characterize the mechanical resistance of the fired samples. The specimen is loaded in compression along one of their diameters and the failure appears when the tension loads generated reach the limiting value of the tensile strength. The Brazilian test is carried out in a testing machine Loyd EZ50 (temperature: 25 ◦ C and displacement rate: 0.5 mm min−1 ). The specimen geometry used was a disc of 6 mm thickness and 30 mm in diameter. Fig. 4 shows the variation of mechanical resistance with the variation of temperature (1000, 1100, 1200 and 1300 ◦ C). This figure shows clearly the influence that content of the mica-phase and Quartz (SiO2 ) has on mechanical resistance. In the first case, the presence of mica-phase in Kaolin leads increase the mechanical resistance and decreases the maximum temperature densification. By the way, in this case, Kaolin K2 present the best strength and the maximum temperature densification is at 1100 ◦ C. The densification is more pronounced for the Kaolin containing an important quantity of mica-phase (K2 ). At higher temperatures the mechanical resistance decreases. Otherwise, the presence of SiO2 decreases the mechanical resistance: the densification of K4 is not finished at 1300 ◦ C. The results are in agreed with those of scanning electron microscopy and relative density of the fired compacts. The fracture surfaces of the specimens are shown in Figs. 5 and 6 at the different temperatures. These figures shows that the densification of K4 is not finished at 1300 ◦ C due to the presence of grains SiO2 inside Kaolin. At 1100 ◦ C, the densification of K2 is more pronounced compared to the rest of Kaolins. Densification of the compact can, therefore, take place through viscous flow. The presence of nearly spherical pores in the sintered specimens, Fig. 6, shows evidences the existence of
the viscous flow mechanism [14]. Furthermore, the fracture path around the large pores is very much flat. The observation of the fresh fracture surface reveals also the increases of the diameter of pores (K2 ) at higher temperatures, when the temperature of the beginning of densification is exceeded. This phenomenon results in a decrease in mechanical properties (Fig. 4). The relative density of the powder compacts is shown as a function of firing temperature in Fig. 7. This figure also shows that the relative density of K2 increases with the increase of firing temperature until 1100 ◦ C. The density drops above 1100 ◦ C. It is due to the presence of larges pores formed within the fired compact. The density is, therefore, decreased. In addition, Kaolin K4 presents the lows relative densities until 1300 ◦ C: that confirm that the densification of K4 is not finished. We can conclude that: • The increase in the strength corresponds to an evolution of the microstructure: structure in increasingly dense. • Densifications of the compact take place through viscous flow. • The densification is more marked in the case of the samples containing a significant quantity of mica-phase (Kaolin K2 ). That is due to the formation of an intergranular liquid resulting from the mica-phase. • The increase in the size of the pores after densification causes a reduction of the mechanical properties. On the other hand, Fig. 8 shows the XRD patterns of Kaolin (K2 ) after heating at 1150 ◦ C in the presence of the air. It reveals the formation of Mullite (M) and Quartz (Q) [15–17]. This indicates that Mullite can be formed at lower temperature. The weight losses (TGA) observed during heating of fired Kaolins are reported in Fig. 9. DTA curves recorded during heating of Kaolins are presented in Fig. 10. The characteristic of the endo and exothermic peaks are reported in Table 2 for each Kaolins.
A. Ghorbel et al. / Materials Chemistry and Physics 112 (2008) 876–885
Fig. 3.
27
Al and 29 Si MAS-NMR spectra of K1 , K2 , K3 and K4 .
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Fig. 4. Variation of mechanical resistance with sintering temperature.
The DTA curves shows two endothermic peaks at 100 and 570 ◦ C and an exothermic one at 970 ◦ C. In TG trace, a two-step weight loss is observed. The weight loss of the first step at 100 ◦ C is about 1.0 wt.%, corresponding entirely to the first endothermic peak in DTA. The weight loss of the second step as the heating temperature at 450–850 ◦ C is between 7.0 wt.% (K4 ) and 11.0 wt.% (K3 ), corresponding with the second endothermic effect at 570 ◦ C. On the other hand, the weight loss does not change significantly during the heating temperature at above 850 ◦ C. The results show that the first endothermic peak of low intensity corresponds to the loss of the weakly bound water, and the second, much larger, endothermic peak corresponds to the loss of structural hydroxyl groups and the transformation of Kaolinite to metakaolinite. Lastly, the intense exothermic peak observed between 930 and 970 ◦ C, may
Fig. 5. Scanning electron micrograph of fracture surfaces of Kaolins powder (K1 , K2 , K3 and K4 ) at 1000 and 1100 ◦ C.
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Fig. 6. Scanning electron micrograph of fracture surfaces of Kaolins powder (K1 , K2 , K3 and K4 ) at 1200 and 1300 ◦ C.
be attributed to Mullite nucleation. Some others [16–20] attribute this reaction to spinel formation. According to Fig. 7, the exothermic event is irrefutable evidence for the Mullite formation. Figs. 11 and 12 show, respectively the 27 Al NMR spectra and 29 Si NMR spectra of the different Kaolins heat-treated from 1000 to 1300 ◦ C. 27 Al MAS-NMR spectra (Fig. 11) display only two signals at ca. 53.76 and −3.23 ppm in K1 , at ca. 52.26 and −0.65 ppm in K2 at ca. 56.11 and 2.38 ppm in K3 and at ca. 55.51 and 1.28 ppm in K4 . The first signal corresponds to tetra-coordinated Aluminium and the other signal to hexa-coordinated Aluminium. The signal
Table 2 Endothermic and exothermic phenomena observed for K1 , K2 , K3 and K4 heated from room temperature to 1200 ◦ C Material
K1 K2 K3 K4
Endothermic phenomenon
Exothermic phenomenon
Temperature (◦ C)
Temperature (◦ C)
Temperature (◦ C)
101 101 101 101
570 574 568 560
982 971 967 975
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Fig. 7. Variation of relative density with sintering temperature.
correspond to tetra-coordinated Aluminium is much stronger than that hexa-coordinated Aluminium signal. This phenomenon is less pronounced in K4 . It proves that the Mullite in K4 is relatively richer in Silicon than those in K1 , K2 and K3 . The different 27 Al NMR spectra reveal the transformation process of Aluminium(VI) in Kaolins to Aluminium(IV). These results are in agree with those of literature. In the Kaolinite–Mullite reaction sequence, Mullite begins to form around 980 ◦ C and the amount of Mullite increases with increasing temperature. Sanz et al. [21] reported three 27 Al resonances at 0 ppm (Al(VI)), 42 ppm (Al(V)), and 60 ppm (Al(IV)). Other researchers [22–24] revealed only two 27 Al signals corresponding to Al(IV) and Al(VI). 29 Si MAS-NMR spectra (Fig. 12) display in K and K two sig1 2 nals centered at ca. −106 and −90 ppm. The signal at ca. −106 ppm is much stronger than that at −90 ppm. The signal at ca. −90 ppm increases gradually with increasing temperature. 29 Si signal at −90 and −106 ppm are related to Mullite and to Q4 Si (Silica), respectively. 29 Si MAS-NMR spectra show the presence in K and K three 3 4 signals centered at ca. −87, −107 and −111 ppm. 29 Si signal at −111 ppm is related to Cristobalite. The signals at ca. −111 and −87 ppm increases gradually with increasing temperature wile that at ca. −107 ppm decreases. This reflects increased Mullite and Cristobalite formation with Silica consumption. Sheriff and Grundy [25] have reported a correlation between 29 Si MAS-NMR chemical shift data and the local geometry around Silicon (Si) in Silicate structures. Applying that correlation, they calculated a chemical
Fig. 8. XRD patterns of Kaolin K2 after heating at 1150 ◦ C.
shift of −86.1 ppm for Sillimanite, which is in good agreement with our experimental results. Therefore, the signal at −87 ppm observed in K3 and K4 suggests a Sillimanite-type Al/Si ordering scheme existing in Mullite derived from these Kaolinites. This also confirms the well-known structural similarity between Mullite and Sillimanite. The Mullite structure can be derived from that of Sillimanite by operation of the substitution: O2− + 2Si4+ → 2Al3+ + . This operation results in removing away some oxygens from the structure [26–28]. Mullite derived from K3 and K4 is richer in Silicon than from K1 and K2 because Sillimanite generally is richer in Silicon than Mullite. This explanation is supported by the 27 Al NMR spectra, as shown less amount of Aluminium atoms in tetrahedral sites of Mullite in K3 and K4 than that in K1 and K2 . This analysis suggests that during heating, the removal of the hydroxyl group from Kaolinite is accompanied by a reorganization of the octahedral layer of Kaolinite to a tetrahedral configuration in metakaolin (400–500 ◦ C). From 1000 ◦ C, there is formation of:
Fig. 9. Thermogravimetric analysis of Kaolins during heating.
A. Ghorbel et al. / Materials Chemistry and Physics 112 (2008) 876–885
Fig. 10. Differential thermal analysis of Kaolins during heating.
Fig. 11.
27
Al MAS-NMR spectra of K1 , K2 , K3 and K4 .
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• Mullite and amorphous Silica in K1 and K2 (the quantity of Mullite increases with the temperature). • Sillimanite-type Al/Si, amorphous Silica and Cristoballite. According to the temperature, the quantity of Cristoballite increases which depends on the quantity of amorphous Silica according to the reaction [2]: 3Al2 O3 2SiO2 + Mullite
T>1200 ◦ C
4SiO2 amorphous Silica
−→
3Al2 O3 2SiO2 + Mullite
4SiO2 Cristoballite (1)
From our results and the previous studies [2,29], the Kaolin powder compacts underwent a series of phase transformations as the temperature was raised from room temperature to 1300 ◦ C. The phase transformations are expressed in the form of chemical reactions for the ease of explanation. However, the above equations are not exactly balanced for many ceramic products and are more or less non-stoichiometric. These phase transformations are:
Fig. 12.
29
When the Kaolinite is heated, the adsorbed water is liberated at above 100 ◦ C and the weakest part of the chemical bond is broke or perturbed. When T = 400–500 ◦ C, the dehydroxylation takes place: 2SiO2 Al2 O3 2H2 O → 2SiO2 Al2 O3 + 2H2 O Kaolinite
(2)
metakaolinite
For Kaolinite, dehydroxylation might result in the disturbance of the Al(O,OH)6 octahedral sheet by the outer hydroxyls, but does not have much effect on the SiO4 tetrahedral sheet due to the more stable inner hydroxyls groups. The outer hydroxyls of octahedral sheets may be more easily removed by heating than inner ones that will maintain a more ordered SiO4 tetrahedral group in structure during dehydroxylation. When T ∼ 950 ◦ C, the metakaolinite is transformed to a spinel structure or a Si-containing ␥-Al2 O3 and amorphous Silica. The SiO4 groups combined with AlO6 group to form the Al–Si spinel
Si MAS-NMR spectra of K1 , K2 , K3 and K4 .
A. Ghorbel et al. / Materials Chemistry and Physics 112 (2008) 876–885
phase that in a short range order structure. The Al–Si spinel phase appears at 920 ◦ C and persists until at least 1100 ◦ C. At this temperature, whether a spinel or a Silicon-containing ␥-Al2 O3 is formed is still under debate: 2SiO2 Al2 O3 → SiAl2 O5 + metakaolinite
spinel
SiO2 amorphous Silica
(3)
or 2SiO2 Al2 O3 → Al2 O3 metakaolinite
+
␥−Alumina
SiO2 amorphous Silica
(4)
When T > 1100 ◦ C, Mullite phase first appears at a temperature around 1100 ◦ C, its amount increases with the increase of temperature: the Mullite formation increase with the heating temperature increase from 1050 to 1300 ◦ C as shown in Fig. 11: SiAl2 O5 + spinel
SiO2 amorphous Silica
→
1 3Al2 O3 2SiO2 3 Mullite
+
4 3
SiO2 amorphous Silica
(5)
or Al2 O3 + SiO2 amorphous Silica ␥−Alumina
→
1 3Al2 O3 2SiO2 3 Mullite
+
4 3
SiO2 amorphous Silica (6)
T > 1200 ◦ C,
When the amorphous SiO2 changes to Cristobalite according to reaction (1). Furthermore, the impurities in the starting powder (mica-phase) can induce a liquid phase during firing (Fig. 6). The presence of the liquid phase can shift slightly the formation temperature of each phase and its amount. 4. Conclusion The various characterizations (using XRD, chemical analyses, scanning electron microscopy, thermal analyses and NMR) allowed the detection and identification of the constituent minerals in the samples of the four Kaolins studied. Kaolinite is the dominant mineral, but all also contain differing amounts of other minerals (Quartz, Illite, and Muscovite). These “impurities” affect their physico-chemical and morphological properties. The transformation heats during heating have quantified from TG-DTA and phase changes have followed by XRD and NMR analyses. Results indicated that the different kinds of Kaolins powder compacts underwent a series of phase transformations during heating from room temperature to 1300 ◦ C: Kaolinite dehydroxylation, metakaolinite structure change, exothermic structural organization
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and Mullite formation. The impurities in the starting powder can induce a liquid phase during firing and can affect the mechanical resistance. Therefore, the advantages of using Kaolinite powder as the starting material for Mullite preparation are low mullitization temperature and unique microstructure. Given their mineralogy, physicochemical characteristics and mechanical properties, these Kaolins have been used for the fabrication of ceramic-metal composites (Kaolin–Aluminium) [30]. References [1] F.H. Norton, Fine Ceramics, Technology and Applications, Robert E. Krieger, USA, 1978, p. 142. [2] W.M. Carty, U. Senapati, J. Am. Ceram. Soc. 81 (1998) 3. [3] P. Kansal, R.M. Laine, F. Babonneau, J. Am. Ceram. Soc. 80 (1997) 2597. [4] R. Soundararajan, G. Kuhn, R. Atisivan, S. Bose, A. Bandyopadhyay, J. Am. Ceram. Soc. 84 (2001) 509. [5] S. Iwai, H. Tagai, T. Shimamune, Acta Crystallogr. B 27 (1971) 248. [6] A.K. Chakrabarty, D.K. Ghosh, J. Am. Ceram. Soc. 61 (1978) 90. [7] K. Srikrishn, G. Thomas, R. Martinez, M.P. Corral, S. De Aza, J.S. Moya, J. Mater. Sci. 25 (1990) 607. [8] I.M. Low, R. McPherson, J. Mater. Sci. 24 (1989) 926. [9] B. Sonuparlak, M. Sarikaya, I.A. Aksay, J. Am. Ceram. Soc. 70 (1987) 837. [10] A. Gualtieri, M. Belloto, G. Artioli, S.M. Clark, Phys. Chem. Miner. 22 (1995) 207. [11] H. Bingqiang, L. Nan, Ceram. Int. 31 (2005) 227. [12] J. Temuujin, K.J.D. MacKenzie, M. Schmücker, H. Schneider, J. McManus, S. Wimperis, J. Eur. Ceram. Soc. 20 (2000) 413. [13] J. F. Stebbins, Nuclear magnetic resonance spectroscopy of Silicates and oxides in geochemistry and geophysics, mineral physics and crystallography, A Handbook of Physical Constants, copyright 1995 by the American Geophysical Union. 303. [14] J.K. Mackenzie, R. Shuttleworth, Phenomenological theory of sintering, Proc. Phys. Soc. Lond. 62 (1949) 633–652. [15] L. Ya-Fei, L. Xing-Qin, W. Hui, M. Guang-Y, Ceram. Int. 27 (2001) 1. [16] O. Castelein, B. Soulestin, J.P. Bonnet, P. Blanchart, Ceram. Int. 27 (2001) 517. [17] M.A. Sainz, F.J. Serrano, J.M. Amigo, J. Bastida, Caballero, J. Eur. Ceram. Soc. 20 (2000) 403. [18] M. Bellotto, A. Gualtieri, G. Artioli, S.M. Clark, Phys. Chem. Miner. 22 (1995) 207. [19] Z. Tatli, A. Demir, R. Yilmaz, F. Caliskan, A.O. Kurt, J. Eur. Ceram. Soc. 27 (2007) 743. [20] C.Y. Chen, G.S. Lan, W.H. Tuan, J. Eur. Ceram. Soc. 20 (2000) 2519. [21] J. Sanz, A. Madani, J.M. Serratosa, J. Am. Ceram. Soc. 71 (1988) 418. [22] J. Rocha, J. Klinowski, Phys. Chem. Miner. 17 (1990) 179. [23] D. Massiot, P. Dion, J.F. Alcover, F. Bergay, J. Am. Ceram. Soc. 78 (1995) 2940. [24] W.M. Brown, K.J.D. Mackenzie, M.E. Bowden, R.H. Meinhold, J. Am. Ceram. Soc. 68 (1985) 293. [25] B.L. Sherriff, H.D. Grundy, Nature 332 (1988) 819. [26] W.E. Cameron, Am. Miner. 62 (1977) 747. [27] J. Angel, C.T. Prewitt, Am. Miner. 71 (1986) 1476. [28] U.C. Bertram, V. Heine, I.L. Jones, G.D. Price, Phys. Chem. Miner. 17 (1990) 326. [29] G.W. Brindley, M. Nakahira, J. Am. Ceram. Soc. 40 (1957) 346. [30] A. Ghorbel, J. Bouaziz, M. Fourati, I. RE. M. E. 2 (2008) 386.