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Influence of mechanical activation on DC conductivity of kaolin
Applied Clay Science 154 (2018) 36–42
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Research paper
Influence of mechanical activation on DC conductivity of kaolin a,⁎
T
Ján Ondruška , Štefan Csáki , Viera Trnovcová , Igor Štubňa , František Lukáč , Jaroslav Pokornýd, Libor Vozára, Patrik Dobroňb a,b
a
a
b,c
a
Department of Physics, Constantine the Philosopher University, Nitra 949 74, Slovakia Faculty of Mathematics and Physics, Charles University, Ke Karlovu 3, Prague 121 16, Czech Republic Institute of Plasma Physics, Czech Academy of Sciences, Prague 182 00, Czech Republic d Department of Materials Engineering and Chemistry, Czech Technical University, Thákurova 7, Prague 166 29, Czech Republic b c
A R T I C L E I N F O
A B S T R A C T
Keywords: Kaolin Mechanical activation DC conductivity
In this study, the effect of dry milling of kaolin (92 mass% of kaolinite) on its physical properties and microstructure development during firing was investigated using thermal analyses, X-ray diffraction, scanning electron microscopy, and DC conductivity measurements. X-ray diffraction showed a decrease in the intensity of reflections of kaolinite with rising milling time. Moreover, formation of agglomerates from kaolinite particles was observed. A longer milling time led to a lower dehydroxylation temperature and contributed to a more substantial overall contraction of samples. After dehydroxylation, the relative bulk density increased with an increasing milling time. Below the temperature 450 °C (start of dehydroxylation), the highest values of the DC conductivity of raw samples were observed for the longest milling times. The main charge carriers are the H+ and OH– ions, originated from dissociation of the adsorbed water and from the coordinated water that was formed during mechanical dehydroxylation of kaolinite, complemented with alkali ions Na+ and K+ which are present as impurities in kaolin. The presence of coordinated water was proven by increasing values of the DC conductivity and by decreasing values of conduction activation energy from 1.73 eV to 0.85 eV with increasing milling time. A similar trend of conduction activation energy was observed in the temperature range 650–750 °C, where the values of conduction activation energy changed from 0.79 eV to 0.52 eV with increasing milling time. After dehydroxylation (above 750 °C), the DC conductivity of raw samples slightly decreased with increasing milling time.
1. Introduction Kaolin is one of the most widely used clay with number of use in a range of industries, including paper, paints and coatings, cosmetics, chemical, agricultural, concrete and ceramics among many others. Therefore, the processing of raw kaolin is of special importance. Milling plays a key role in the preparation of ceramic raw materials, because the particle size reduction shifts the sintering process to lower temperatures (Nakahara et al., 1999; Rahaman, 2003). Likewise, longer milling time reduces the activation energy of dehydroxylation process and the process occurs at lower temperatures (Horváth et al., 2003; Ptáček et al., 2013; Sánchez-Soto et al., 1994, 2000; Vágvölgyi et al., 2008; Vizcayno et al., 2010). According to Frost et al. (2001a), Horváth et al. (2003), Kristóf et al. (2002), and Mako et al. (2001) mechanochemically treated (mechanically activated) kaolinite contains coordinated water. The coordinated water is formed from hydroxyl groups as a result of mechanical dehydroxylation of kaolinite.
⁎
After 4 h of milling, the dehydroxylation minimum in DTA curve of kaolinite almost vanishes (Juhász, 1998) and after 9 h of high energy ball milling, this minimum disappears completely (Hamzaoui et al., 2015). Milling promotes the absorption of water molecules, thus the mass loss during thermal treatment increases with an increasing milling time (Frost et al., 2001a; Vágvölgyi et al., 2008; Vizcayno et al., 2010). Mechanical activation leads to a structural decomposition of the kaolinite structure (Ding et al., 2012; Frost et al., 2001a; Hamzaoui et al., 2015; Leonel et al., 2014; Mitrović and Zdujić, 2013; Vágvölgyi et al., 2008; Vdovic et al., 2010) and the crystalline phase is transformed into an amorphous one (Hamzaoui et al., 2015; Juhász, 1998) losing its plate-like shape. The time required for a complete amorphization of kaolinite varies for various types of kaolin (Vizcayno et al., 2010). Quartz particles, present in the kaolin as impurities, act as grinding bodies, thus shortening the time required for complete amorphization. Vizcayno et al. (2010) showed, that when the quartz content was ~50%, the mechanochemical amorphization of kaolinite was reached
Corresponding author. E-mail address: [email protected] (J. Ondruška).
https://doi.org/10.1016/j.clay.2017.12.038 Received 16 June 2017; Received in revised form 18 December 2017; Accepted 22 December 2017 0169-1317/ © 2017 Elsevier B.V. All rights reserved.
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2. Experimental
after 30 min of milling, while in the case of kaolin with 8% of quartz, this time was significantly longer (105 min). Moreover, an increase in the pore size due to a rising milling time was also observed (Vágvölgyi et al., 2008). Since kaolin very often contains quartz, the influence of quartz on milling was also investigated. Mako et al. (2001) reported that quartz grains help accelerate mechanically induced amorphization of the kaolinite structure. The crystalline order of kaolinite was completely destroyed after grinding a sample containing 75 mass% of quartz for 4 h. The results indicate that quartz grains act as grinding bodies during the intensive dry grinding of kaolin. The kaolinite surface is significantly modified by milling, and surface hydroxyls are replaced with water molecules, as was confirmed by thermogravimetric analyses. Changes in the molecular structure of the surface hydroxyls of the kaolinite/quartz mixtures were followed by infrared spectroscopy. Kaolinite hydroxyls were lost after 2 h of grinding as evidenced by the decrease in intensity of the Al–OH stretching vibrations at 3695 and 3619 cm− 1 and the deformation modes at 937 and 915 cm− 1 (Frost et al., 2001b). The DC conductivity of green ceramic materials has not been widely studied yet. An ionic character of the DC conductivity during thermal treatment of kaolin has been reported by Podoba et al. (2014) and Trnovcová et al. (2012). An increase in the electrical conductivity at the lowest temperatures can be explained by the ionization of the residual physically bound water (transport of H+ and OH− ions). The amount of these ions is reduced and the conductivity decreases when the water evaporates above 120 °C. The successive increase in the conductivity with an increasing temperature results from the migration of Na+ and K+ ions originated from impurities contained in kaoline samples (Table 1) (Podoba et al., 2014; Štubňa et al., 2015). During dehydroxylation, half of the H+ ions leaves their parent hydroxyl ions, which form O2 − ions, and then recombines with the other half of the OH− ions forming H2O molecules according to the relation OH− + OH− → O2 − + H2O (Mackenzie, 1973). It was also found that a collapse of the metakaolinite structure at 950 °C is accompanied by a decrease in AC resistivity from 108 to 106 Ω cm− 1 (Freund, 1967). Thus, the measurement of the DC conductivity during thermal treatment helps to reveal a connection between processes in clays and creation of free ions (H+, OH−, Na+ and K+ in clays) as well as conditions for their transport. The presence of free ions and their concentrations are closely linked to the microstructure of the kaolin and the structure of kaolinite crystals. In this study, the effect of dry milling of kaolin (92 mass% of kaolinite) on its physical properties and microstructure development during firing was investigated. The samples were studied during thermal treatment using in-situ techniques (thermodilatometric analysis (DIL), DTA, thermogravimetry (TG, DTG)) and special attention was paid to the DC conductivity. Development of the particle size after various milling times was studied using particle size distribution plots. Microstructure of the samples was observed using X-ray diffraction (XRD) and scanning electron microscopy (SEM). Measurement of the DC conductivity allows to determine whether the coordinated water has a significant effect on the electrical properties during firing or not. To the authors` knowledge, the influence of dry milling on the DC conductivity of kaolin has not been studied before. Understanding conduction behavior of raw ceramics at higher temperatures enables more accurate control of the chemical composition of the material and furnace temperature in flash sintering which is an energy-saving modern method of ceramic production.
Washed kaolin from Sedlec (Czech Republic) was used (Table 1). According to the supplier, this kaolin consisted of 92 ± 2 mass% of kaolinite, 2 ± 1 mass% of quartz, and 6 ± 2 mass% of illite. Crushed kaolin (95 g) was inserted into a planetary ball mill Retsch PM100 and dry milled for 0, 30, 60, and 120 min in 250 ml alumina vessel with 12 alumina balls (with diameter ~19.9 mm and mass ~16.1 g, each) with 350 rpm. During the longer milling times (longer, than 30 min) a 5 min break was kept after the each 30 min to prevent the powder to reach temperatures above 60 °C (Ptáček et al., 2013). Such prepared powders and samples were labelled as K0, K30, K60, and K120, respectively. The wet plastic mass was prepared by mixing the as-prepared powder (70 mass%) with distilled water (30 mass%). Rectangular samples were obtained by pressing the mass into a gypsum form. To reach the same conditions for a release of the physically bound water and dehydroxylation in samples used in DTA, TG and DC conductivity, samples prepared by the same technology were used. The samples were freely air-dried, then dried at 120 °C for 1 h and stored in a desiccator. Thermal analyses DTA and TG were carried out in static air using the upgraded Derivatograph analyzer (MOM Budapest) (Podoba et al., 2012) in which compact samples of rectangular shape 10 × 10 × 20 mm were used. As a reference sample for DTA, a prism of the same size made from pressed Al2O3 was used. Thermal analyses were performed from laboratory temperature up to 1050 °C with heating rate 5 °C/min. Measurement of the DC conductivity was performed on rectangular samples (prepared using a plastic form designed especially for these samples) with dimensions 10 × 10 × 20 mm in which two parallel platinum wires (∅ 0.4 mm) served as electrodes, Fig. 1 (Štubňa et al., 2015). The distance between the electrodes was 3 mm. This arrangement was chosen in order to minimize the surface electric current of the sample. After free drying in ambient atmosphere and to obtain equal initial conditions for all measurements, the samples were preheated at 120 °C for 1 h and then measured up to 1100 °C with heating rate 5 °C/ min in static air. The measuring circuit (serial connection of the voltage source, the sample, and the electrometer) was fed with DC power supply Tesla BS 525 with voltage of 10 V. Electrical current was measured and recorded using electrometer Keithley 6514 directly connected to the computer (Fig. 1). Thermodilatometry was carried out on rectangular samples 8 × 8 × 22 mm using the Netzsch DIL 402C horizontal push rod dilatometer in a dynamic nitrogen atmosphere with a flow rate of 40 ml/
Table 1 Chemical composition of Sedlec kaolin (in mass%). LOI
SiO2
Al2O3
Fe2O3
TiO2
CaO
MgO
K2O
Na2O
12.95
45.80
37.31
0.98
0.17
0.58
0.46
1.17
0.58
Fig. 1. Electrical scheme with location of electrodes in the sample. PS – power supply, EM – electrometer.
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min. The measurements were performed in the temperature range from 120 °C to 1100 °C, with heating rate of 5 °C/min. Relative bulk density was derived from the TG and DIL results as
( (
Δm (t )
1− m ρ (t ) 0 = Δl (t ) ρ0 1+3 l 0
) )
(1)
where ρ0 = m0/V0 is the initial bulk density in which m0 and V0 are the mass and the volume of the sample at room temperature. The values of − Δm(t)/m0 and Δl(t)/l0 are the relative mass loss and the relative expansion measured by the TG analyzer and dilatometer at temperature t. The particle size distribution of the milled kaolin was determined by the Analysette 22 Micro Tec plus (Fritsch) laser diffraction device with the measuring range up to 2 mm. To ensure the repeatability of the measurements, all analyses were carried out on 3 different samples and then an average value was taken. The differences between the measurements did not exceed 2%. Structural observations were made using the FEI Quanta 200F electron microscope in low vacuum mode (100 Pa) with an accelerating voltage 10 kV. Micrographs were taken from fracture areas both on raw and fired samples. Phase compositions were determined from X-ray diffraction (XRD) patterns obtained at room temperature by Co Kα (wavelength 0,179 nm, 2 Θ range from 10° to 80°, 0.03° step size, 288 s time per step, divergent beam with opening degree of 0.5°) and 1D LynxEye detector (Fe ß filter in front of the detector) mounted on Bruker D8 Discover and subsequent Rietveld refinement (Rietveld, 1967) was performed in TOPAS 5 (Coelho, 2016). Powders were prepared in a standard PMMA sample holder with a cavity diameter of 25 mm. Fundamental parameters approach was used for calculation of profile broadening due to instrumental effects. Sizes of coherently diffracting domains (CDD) of the kaolinite phase were determined from broadening of the Lorentzian component of pseudo-Voigt function and calculated as volume weighted average derived from full width at half maximum (FWHM) (Scardi et al., 2004).
Fig. 3. XRD patterns of samples milled for 0, 30, 60, and 120 min.
Aglietti et al. (1986). The XRD reflections of the kaolinite phase gradually broadened during milling with an increasing milling time (Fig. 3). This is in accordance with the expected partial degradation of the kaolinite lattice as reported by Ding et al. (2012), Frost et al. (2001a), Hamzaoui et al. (2015), Leonel et al. (2014), Mitrović and Zdujić (2013), and Vdovic et al. (2010). Sizes of the CDD obtained by fitting the whole pattern continuously decreased with an increasing milling time from 36 nm to 22 nm. This indicates structural degradation of kaolinite (Frost et al., 2001a). However, these results also show that the applied milling regime was not sufficient for significant amorphization of kaolinite. The SEM micrographs of raw samples K0 and K120 are presented in Fig. 4. The sample K0 exhibited plate-like kaolinite particles. The milling resulted in the formation of agglomerates of small particles. A similar result was obtained by Aglietti et al. (1986). Mass loss (Fig. 5), caused by the removal of the physically bound water as well as coordinated water, was observed at temperatures lower than 450 °C (at which dehydroxylation started) for all samples. After milling, the samples retained more water (added during the sample preparation), and its removal was not completed even at 450 °C. The increase in the sample mass loss rose from 0.54% (K0) to 2.56% (K120) with prolonged milling. Thus, the amount of the physically bound water and the coordinated water in the samples increased with the increasing milling time. The DIL curves (Fig. 6) exhibited a slow expansion in the lowtemperature region (< 450 °C) caused by heating of the samples K0, K30, and K60, while sample K120 exhibited a modest contraction owing to the higher amount of the released physically bound and coordinated water. The contraction at temperatures lower than 450 °C is caused by the kaolinite particles moving closer to each other when the physically bound water is released. The mass loss in the temperature interval from 450 °C to 750 °C is caused by kaolinite dehydroxylation. This process is accompanied by a significant mass loss and destruction of the kaolinite lattice (Chakraborty, 2014). The mass loss during dehydroxylation was almost the same (~ 12.1 mass%) for samples K0, K30, and K60; only K120 had a slightly lower mass loss (10.9 mass%). Therefore, the used milling treatments were not sufficient for complete mechanochemical dehydroxylation of kaolinite. The shifts of the TG lines to lower temperatures with increasing milling times are obvious, and the same trend is observed for the shifts of DTA and DTG peak temperatures (Table 2). The kinetics of dehydroxylation depends (except other influences such as, for example, temperature, partial pressure of H2O vapor, and other) on the defect density. The less perfect the structure of the kaolinite crystals, the lower the temperature of dehydroxylation. A
3. Results and discussion The influence of milling on the particle size of kaolinite is shown in Fig. 2. The particle size distribution of all samples exhibited two maxima – around 50 μm and 3 μm. The increasing milling time increased the volume fraction of the particles with the radii between 10 and 100 μm, thus milling promoted formation of agglomerates of small particles. The agglomerates were also found in ground kaolin by
Fig. 2. Particle size distribution in samples K0, K30, K60 and K120.
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Fig. 4. SEM micrographs of the fracture surfaces of the raw samples K0 (a) and K120 (b).
Table 2 DTA and DTG dehydroxylation peak temperatures. DTA peak temperatures Sample name Temperature (°C)
K0 615
K30 608
K60 605
DTG peak temperatures K120 595
K0 600
K30 595
K60 590
K120 580
dehydroxylation (Fig. 6). The contraction increased with an increasing duration of the milling up to ~960 °C (metakaolinite → Al-Si spinel transformation). Since no liquid phase is created at these temperatures, sintering runs in a solid-state. This is supported by results reported by Monteiro and Vieira (2004) where successful sintering of small-particle kaolinitic-clay was found at temperatures below 700 °C. In the narrow temperature interval around 960 °C, at which the transformation metakaolinite → Al-Si spinel takes place (Chakraborty, 2014), a significant contraction was observed (Fig. 6). The temperature of this transformation was not influenced by the milling. A longer milling time led to a more intense shrinkage of the samples (Fig. 6). The overall contraction of the sample K0 was 2.5%, while the sample K120 exhibited 7% shrinkage. Development of the relative bulk density can be derived using the results of TG and DIL (Fig. 7). After a continuous decrease in the relative bulk density caused by the removal of the physically bound and coordinated water, dehydroxylation influences the bulk density to a large extent (Freund, 1967). During dehydroxylation, the density steeply decreases. This decrease is less significant for longer milling times. Between ~600 °C and ~900 °C, a moderate increase in the relative bulk density was observed. This increase depends on the milling time. Since development of the relative bulk density is directly linked to
Fig. 5. TGA curves of the samples K0, K30, K60, and K120. Embedded graph: Mass loss between 20 °C and 450 °C.
Fig. 6. Thermodilatometric curves of the samples K0, K30, K60, and K120. The embedded graph shows an expansion rate near the metakaolinite → Al-Si spinel transformation.
decrease in the size of the crystals and an increase in the defect concentration lead to a lower activation energy of dehydroxylation. Therefore, dehydroxylation can run easier in such damaged crystals (Štubňa et al., 2006). A contraction of samples is observed in the temperature interval of
Fig. 7. Development of the relative bulk density for the samples K0, K30, K60, and K120.
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Fig. 8. SEM micrographs of the fracture surfaces of the samples K0 (a) and K120 (b) after heating at 1100 °C.
sintering, a more intensive sintering was found in long-milled samples. The final relative bulk density (at 1050 °C) is reached after metakaolinite → Al-Si spinel transformation and is also determined by the milling time. The highest relative bulk density was obtained after the longest milling time. After heating to 1100 °C, a compact microstructure having relatively well-formed particles was built in K0, contrary to the microstructure consisting of small and damaged particles in K120 (Fig. 8). The difference between both microstructures did not vanish after heating to 1100 °C. The factors that influence the ionic DC conductivity in solids are the concentration of charge carriers, temperature, the density of defects and/or the structural disorder, and the facility with which an ion can jump into a neighboring vacant site. This facility is controlled by the activation energy, i.e. the free energy barrier, which an ion has to overcome for a successful jump between the sites. When the charge of the jumping ions is larger, they find higher energy barrier for their jumps between the sites. In the simplest case, the conduction activation energy (CAE) can be determined using the Arrhenius relation
σ = σ0 exp(−Eσ / kT )
(2)
where σ is the conductivity at temperature T, k is Boltzmann constant, Eσ is the conduction activation energy, and σ0 is the pre-exponential factor. The ionic conductivity relates to the self-diffusion coefficient D of dominant charge carrying ions by Nernst-Einstein equation
σ / D = nq2 / kT
Fig. 9. Temperature dependence of DC conductivity for the samples K0, K30, K60 and K120.
highest milling time it vanished. This behavior proved the influence of coordinated water on electrical properties of mechanically activated kaolin. The initial amount of the physically bound and coordinated water in the samples increased with the milling time (Fig. 5), consequently, more H+ and OH− ions were present in the longer milled samples. The second stage of the removal of physically bound water (reported at ~300 °C in (Podoba et al., 2014)) was observed only in non-milled kaolin with the lowest concentration of the physically bound water and no coordinated water. Part of the physically bound and coordinated water remained in samples up to start of dehydroxylation (at ~450 °C), so the migration of these ions in the electric field was still contributing to the conductivity. In addition to the H+ and OH− ions, Na+ and K+ ions, which originate from contamination of kaolin, contributed to the conductivity. Na+ and K+ ions got dominant charge carriers when water molecules were removed. Temperature dependences of the DC conductivity exhibit two parts, which obey Arrhenius' law. The first temperature interval (from 320 °C to 500 °C) gave the CAE of 1.73 eV for K0. The high value belongs to the migration of Na+ and K+ ions, as no PBW is present in the non-milled samples at these temperatures. A clear dependence of the conductivity on the milling-time was visible below the temperature of dehydroxylation (Fig. 9): the longer milling time the higher conductivity and smaller conduction activation energy. The CAE was linearly decreasing from 1.73 eV to 0.85 eV with an increasing milling time (Fig. 10). The decrease in the CAE can be related to the increasing content of the
(3)
where n is the concentration of dominant charge carrying ions and q is their charge. Temperature dependences of the DC conductivity for green samples are shown in Fig. 9. At the lowest temperatures, an increase in the conductivity with an increasing temperature resulted from the release of weakly bound charge carriers (H+ and OH– ions), which are the products of self-dissociation of water molecules in water layers located on the crystal surfaces (Podoba et al., 2014) and of coordinated water in milled kaolin. When the water was evaporating, above 120 °C, the amount of these ions decreased and, consequently, the conductivity also decreased. Only H+ and OH– ions, which are in a direct contact with charged surface defects can be charge carriers at higher temperatures together with K+ and Na+ ions that originate from contamination of kaolin. Moreover, OH– ions are also released during mechanochemical treatment which is partially equivalent to dehydroxylation. Kristóf et al. (2002) proposed that mechanochemical treatment of kaolinite causes localization of H+ when the long-range order is lost. At ~200 °C, the conductivity reached its minimum, corresponding to the minimal amount of the charge carriers (H+ and OH− ions). This minimum of the conductivity was shifted to higher temperature with an increasing milling time due to an increasing initial content of coordinated water, the concentration of which rose owing to longer milling times. At the same time, the minimum got shallower and at the 40
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• Milling promotes formation of agglomerates and of coordinated water. • Dehydroxylation is shifted to lower temperatures by prolongation of milling. • Overall shrinkage of samples is more intense for longer milling times. • Below the temperature of dehydroxylation, the DC conductivity of • • • •
Fig. 10. Conduction Activation Energies of kaolin samples after different milling times in temperature region 320–500 °C (full circle) and in temperature region 650–750 °C (empty circle).
coordinated water as the milling time increases. In the second interval (from 650 °C to 750 °C) the CAE decreased, reaching the value of 0.79 eV in non-milled samples. This value corresponded to the migration of alkali ions in metakaolinite structure. However, the dehydroxylation was still running, thus the contribution of OH˗ and H+ ions had to be also considered. CAE linearly decreased in the temperature interval 650 °C–750 °C from 0.79 eV to 0.52 eV (Fig. 10) as a consequence of longer milling time; at the same time, the DC conductivity slightly decreased, probably due to increased density of kaolin. An anomaly in the temperature dependence of the DC conductivity near 500 °C indicates the dehydroxylation process. The OH− ions released during dehydroxylation got associated with mobile alkali ions into neutral complexes which did not contribute to the DC conductivity and, for a short time period, the increase of the conductivity with increasing temperature decelerated. This deceleration was more significant for longer milling times. At temperatures above 650 °C, all samples showed very close temperature dependences of the DC conductivity with only slight decrease with an increasing milling time. This dependence of the conductivity on the milling time could result from the increasing density with an increasing milling time because then the moving ions have a lower free volume for their jumps. At temperatures above 700 °C, a deceleration of the rise in the conductivity with an increasing temperature was observed. The reason for this is not clear. However, the migration of Na+ and K+ ions toward the negative electrode in the DC electric field leads to a decrease in the internal electric field and, thus, to a reduction of the mobility of alkali ions (Ondruška et al., 2015, 2017). Sample K0 had the highest conductivity and sample K120 exhibited the lowest one in this temperature region. This influence of the milling time was probably a consequence of different bulk densities of the samples. The small narrow peak at ~960 °C was caused by shifts of Al3 + ions when the metakaolinite lattice transforms into Al-Si spinel lattice. This transformation runs in a narrow temperature interval as the DTA results show (Chakraborty, 2014; Podoba et al., 2014). When this transformation was over, the conductivity decreased. In the spinel lattice, the dominant charge carriers were K+ and Na+ ions.
•
raw samples increases with an increasing milling time. The main charge carriers are H+ and OH– ions originated from self-dissociation of water molecules (at the lowest temperatures) and coordinated water originated from mechanochemical treatment. Below the temperature of dehydroxylation, alkali ions Na+ and K+, present as impurities in kaolin, also contribute to the DC conductivity. During dehydroxylation, the OH– ions, released from the structure of kaolinite, contribute to the DC conductivity; however, the association of these ions with alkali ions into neutral complexes decelerates the increase of the conductivity with increasing temperature. Above the temperature of dehydroxylation, the relative bulk density increases with the increasing milling time. Above the temperature of dehydroxylation, the DC conductivity decreases with increasing milling time. The charge carriers are Na+ and K+ ions from contaminants in kaolin. The conduction activation energies linearly decrease with the increasing milling time in the temperature interval 320–500 °C from 1.73 eV for K0 to 0.85 eV for K120, and in temperature range 650–750 °C from 0.79 eV for K0 to 0.52 eV for K120.
Acknowledgements This work was financially supported by the grant VEGA 1/0162/15 from the Ministry of Education of Slovak Republic and by the Czech Science Foundation (Project No. P105/12/G059). References Aglietti, E.F., Porto Lopez, J.M., Pereira, E., 1986. Mechanochemical effects in kaolinite grinding. I. Textural and physicochemical aspects. Int. J. Miner. Process. 16, 125–133. http://dx.doi.org/10.1016/0301-7516(86)90079-7. Chakraborty, A.K., 2014. Phase Transformation of Kaolinite Clay. Springer India, New Delhi. http://dx.doi.org/10.1007/978-81-322-1154-9. Coelho, A.A., 2016. TOPAS Version 5 (Computer Software). Ding, S., Zhang, L., Ren, X., Xu, B., Zhang, H., Ma, F., 2012. The characteristics of mechanical grinding on kaolinite structure and thermal behavior. Energy Procedia 1237–1240. http://dx.doi.org/10.1016/j.egypro.2012.01.197. Freund, F., 1967. Kaolinite-metakaolinite, a model of a solid with extremely high lattice defect concentrations. Berichte der Dtsch Keramischen Gesellschaft 44, 5–13. Frost, R.L., Makó, E., Kristóf, J., Horváth, E., Kloprogge, J.T., 2001a. Mechanochemical treatment of kaolinite. J. Colloid Interface Sci. 239, 458–466. http://dx.doi.org/10. 1006/jcis.2001.7591. Frost, R.L., Makó, É., Kristóf, J., Horváth, E., Kloprogge, J.T., 2001b. Modification of kaolinite surfaces by mechanochemical treatment. Langmuir 17, 4731–4738. http:// dx.doi.org/10.1021/la001453k. Hamzaoui, R., Muslim, F., Guessasma, S., Bennabi, A., Guillin, J., 2015. Structural and thermal behavior of proclay kaolinite using high energy ball milling process. Powder Technol. 271, 228–237. http://dx.doi.org/10.1016/j.powtec.2014.11.018. Horváth, E., Frost, R.L., Makó, É., Kristóf, J., Cseh, T., 2003. Thermal treatment of mechanochemically activated kaolinite. Thermochim. Acta 404, 227–234. http://dx.doi. org/10.1016/S0040-6031(03)00184-9. Juhász, A.Z., 1998. Aspects of mechanochemical activation in terms of comminution theory. Colloids Surf. A Physicochem. Eng. Asp. 449–462. http://dx.doi.org/10. 1016/S0927-7757(98)00245-3. Kristóf, J., Frost, R.L., Kloprogge, J.T., Horváth, E., Makó, É., 2002. Detection of four different OH-groups in ground kaolinite with controlled-rate thermal analysis. J. Therm. Anal. Calorim. 69, 77–83. Leonel, E.C., Nassar, E.J., Ciuffi, K.J., Calefi, P.S., De Franca, U., 2014. Effect of highenergy ball milling in the structural and textural properties of kaolinite. Cerâmica 60, 267–272. Mackenzie, K.J.D., 1973. Thermal reactions of inorganic hydroxy-compounds under applied electric fields. J. Therm. Anal. 5, 19–32. Mako, E., Frost, R.L., Kristof, J., Horvath, E., 2001. The effect of quartz content on the mechanochemical activation of kaolinite. J. Colloid Interface Sci. 244, 359–364. http://dx.doi.org/10.1006/jcis.2001.7953. Mitrović, A., Zdujić, M., 2013. Mechanochemical treatment of Serbian kaolin clay to obtain a highly reactive pozzolana. J. Serbian Chem. Soc. 78, 579–590. http://dx.doi.
4. Conclusions Influence of milling (mechanochemical treatment) on the mass loss, thermal expansion, relative bulk density, and DC electrical conductivity of kaolin was studied. The main results are: 41
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Sci. 29, 1276–1283. http://dx.doi.org/10.1007/BF00975075. Sánchez-Soto, P.J., Carmen Jiménez de Haro, M., Pérez-Maqueda, L.A., Varona, I., PérezRodríguez, J.L., 2000. Effects of dry grinding on the structural changes of kaolinite powders. J. Am. Ceram. Soc. 83, 1649–1657. http://dx.doi.org/10.1111/j.11512916.2000.tb01444.x. Scardi, P., Leoni, M., Delhez, R., 2004. Line broadening analysis using integral breadth methods: a critical review. J. Appl. Crystallogr. 37, 381–390. http://dx.doi.org/10. 1107/S0021889804004583. Štubňa, I., Varga, G., Trník, A., 2006. Investigation of kaolinite dehydroxylations is still interesting. Építőanyag 58, 6–9. http://dx.doi.org/10.14382/epitoanyag-jsbcm. 2006.2. Štubňa, I., Trnovcová, V., Vozár, L., Csáki, Š., 2015. Uncertainty of the measurement of dc conductivity of ceramics at elevated temperatures. J. Electr. Eng. 66, 33–38. http:// dx.doi.org/10.2478/jee-2015-0005. Trnovcová, V., Podoba, R., Štubňa, I., 2012. DC conductivity of kaolin-based ceramics in the temperature range 20-600°C. Építőanyag 2012, 46–49. Vágvölgyi, V., Kovács, J., Horváth, E., Kristóf, J., Makó, É., 2008. Investigation of mechanochemically modified kaolinite surfaces by thermoanalytical and spectroscopic methods. J. Colloid Interface Sci. 317, 523–529. http://dx.doi.org/10.1016/j.jcis. 2007.09.085. Vdovic, N., Jurina, I., Škapin, S.D., Sondi, I., 2010. The surface properties of clay minerals modified by intensive dry milling – revisited. Appl. Clay Sci. 48, 575–580. http://dx. doi.org/10.1016/j.clay.2010.03.006. Vizcayno, C., de Gutiérrez, R.M., Castello, R., Rodriguez, E., Guerrero, C.E., 2010. Pozzolan obtained by mechanochemical and thermal treatments of kaolin. Appl. Clay Sci. 49, 405–413. http://dx.doi.org/10.1016/j.clay.2009.09.008.
org/10.2298/JSC120829107M. Monteiro, S.N., Vieira, C.M.F., 2004. Solid state sintering of red ceramics at lower temperatures. Ceram. Int. 30, 381–387. http://dx.doi.org/10.1016/S0272-8842(03) 00120-2. Nakahara, M., Kondo, Y., Hamano, K., 1999. Effect of particle size of powders ground by ball milling on densification of cordierite ceramics. J. Ceram. Soc. Jpn. 107, 308–312. Ondruška, J., Štubňa, I., Trnovcová, V., Medveď, I., Kaljuvee, T., 2015. Polarization and depolarization currents in kaolin. Appl. Clay Sci. 114, 157–160. http://dx.doi.org/10. 1016/j.clay.2015.05.022. Ondruška, J., Štubňa, I., Trnovcová, V., Vozár, L., Bačík, P., 2017. Polarization currents in illite at various temperatures. Appl. Clay Sci. 135, 414–417. http://dx.doi.org/10. 1016/j.clay.2016.10.024. Podoba, R., Trník, A., Podobník, Ľ., 2012. Upgrading of TGA/DTA analyzer Derivatograph. Építőanyag 64, 28–29. http://dx.doi.org/10.14382/epitoanyagjsbcm.2012.5. Podoba, R., Štubňa, I., Trnovcová, V., Trník, A., 2014. Temperature dependence of DC electrical conductivity of kaolin. J. Therm. Anal. Calorim. 118, 597–601. http://dx. doi.org/10.1007/s10973-013-3533-1. Ptáček, P., Opravil, T., Soukal, F., Wasserbauer, J., Másilko, J., Baráček, J., 2013. The influence of structure order on the kinetics of dehydroxylation of kaolinite. J. Eur. Ceram. Soc. 33, 2793–2799. http://dx.doi.org/10.1016/j.jeurceramsoc.2013.04.033. Rahaman, M.N., 2003. Ceramic Processing and Sintering, 2nd ed. CRC Press, New York. Rietveld, H.M., 1967. Line profiles of neutron powder-diffraction peaks for structure refinement. Acta Crystallogr. 22, 151–152. http://dx.doi.org/10.1107/ S0365110X67000234. Sánchez-Soto, P.J., Justo, A., Pérez-Rodríguez, J.L., 1994. Grinding effect on kaolinitepyrophyllite-illite natural mixtures and its influence on mullite formation. J. Mater.
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Research paper
Influence of mechanical activation on DC conductivity of kaolin a,⁎
T
Ján Ondruška , Štefan Csáki , Viera Trnovcová , Igor Štubňa , František Lukáč , Jaroslav Pokornýd, Libor Vozára, Patrik Dobroňb a,b
a
a
b,c
a
Department of Physics, Constantine the Philosopher University, Nitra 949 74, Slovakia Faculty of Mathematics and Physics, Charles University, Ke Karlovu 3, Prague 121 16, Czech Republic Institute of Plasma Physics, Czech Academy of Sciences, Prague 182 00, Czech Republic d Department of Materials Engineering and Chemistry, Czech Technical University, Thákurova 7, Prague 166 29, Czech Republic b c
A R T I C L E I N F O
A B S T R A C T
Keywords: Kaolin Mechanical activation DC conductivity
In this study, the effect of dry milling of kaolin (92 mass% of kaolinite) on its physical properties and microstructure development during firing was investigated using thermal analyses, X-ray diffraction, scanning electron microscopy, and DC conductivity measurements. X-ray diffraction showed a decrease in the intensity of reflections of kaolinite with rising milling time. Moreover, formation of agglomerates from kaolinite particles was observed. A longer milling time led to a lower dehydroxylation temperature and contributed to a more substantial overall contraction of samples. After dehydroxylation, the relative bulk density increased with an increasing milling time. Below the temperature 450 °C (start of dehydroxylation), the highest values of the DC conductivity of raw samples were observed for the longest milling times. The main charge carriers are the H+ and OH– ions, originated from dissociation of the adsorbed water and from the coordinated water that was formed during mechanical dehydroxylation of kaolinite, complemented with alkali ions Na+ and K+ which are present as impurities in kaolin. The presence of coordinated water was proven by increasing values of the DC conductivity and by decreasing values of conduction activation energy from 1.73 eV to 0.85 eV with increasing milling time. A similar trend of conduction activation energy was observed in the temperature range 650–750 °C, where the values of conduction activation energy changed from 0.79 eV to 0.52 eV with increasing milling time. After dehydroxylation (above 750 °C), the DC conductivity of raw samples slightly decreased with increasing milling time.
1. Introduction Kaolin is one of the most widely used clay with number of use in a range of industries, including paper, paints and coatings, cosmetics, chemical, agricultural, concrete and ceramics among many others. Therefore, the processing of raw kaolin is of special importance. Milling plays a key role in the preparation of ceramic raw materials, because the particle size reduction shifts the sintering process to lower temperatures (Nakahara et al., 1999; Rahaman, 2003). Likewise, longer milling time reduces the activation energy of dehydroxylation process and the process occurs at lower temperatures (Horváth et al., 2003; Ptáček et al., 2013; Sánchez-Soto et al., 1994, 2000; Vágvölgyi et al., 2008; Vizcayno et al., 2010). According to Frost et al. (2001a), Horváth et al. (2003), Kristóf et al. (2002), and Mako et al. (2001) mechanochemically treated (mechanically activated) kaolinite contains coordinated water. The coordinated water is formed from hydroxyl groups as a result of mechanical dehydroxylation of kaolinite.
⁎
After 4 h of milling, the dehydroxylation minimum in DTA curve of kaolinite almost vanishes (Juhász, 1998) and after 9 h of high energy ball milling, this minimum disappears completely (Hamzaoui et al., 2015). Milling promotes the absorption of water molecules, thus the mass loss during thermal treatment increases with an increasing milling time (Frost et al., 2001a; Vágvölgyi et al., 2008; Vizcayno et al., 2010). Mechanical activation leads to a structural decomposition of the kaolinite structure (Ding et al., 2012; Frost et al., 2001a; Hamzaoui et al., 2015; Leonel et al., 2014; Mitrović and Zdujić, 2013; Vágvölgyi et al., 2008; Vdovic et al., 2010) and the crystalline phase is transformed into an amorphous one (Hamzaoui et al., 2015; Juhász, 1998) losing its plate-like shape. The time required for a complete amorphization of kaolinite varies for various types of kaolin (Vizcayno et al., 2010). Quartz particles, present in the kaolin as impurities, act as grinding bodies, thus shortening the time required for complete amorphization. Vizcayno et al. (2010) showed, that when the quartz content was ~50%, the mechanochemical amorphization of kaolinite was reached
Corresponding author. E-mail address: [email protected] (J. Ondruška).
https://doi.org/10.1016/j.clay.2017.12.038 Received 16 June 2017; Received in revised form 18 December 2017; Accepted 22 December 2017 0169-1317/ © 2017 Elsevier B.V. All rights reserved.
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2. Experimental
after 30 min of milling, while in the case of kaolin with 8% of quartz, this time was significantly longer (105 min). Moreover, an increase in the pore size due to a rising milling time was also observed (Vágvölgyi et al., 2008). Since kaolin very often contains quartz, the influence of quartz on milling was also investigated. Mako et al. (2001) reported that quartz grains help accelerate mechanically induced amorphization of the kaolinite structure. The crystalline order of kaolinite was completely destroyed after grinding a sample containing 75 mass% of quartz for 4 h. The results indicate that quartz grains act as grinding bodies during the intensive dry grinding of kaolin. The kaolinite surface is significantly modified by milling, and surface hydroxyls are replaced with water molecules, as was confirmed by thermogravimetric analyses. Changes in the molecular structure of the surface hydroxyls of the kaolinite/quartz mixtures were followed by infrared spectroscopy. Kaolinite hydroxyls were lost after 2 h of grinding as evidenced by the decrease in intensity of the Al–OH stretching vibrations at 3695 and 3619 cm− 1 and the deformation modes at 937 and 915 cm− 1 (Frost et al., 2001b). The DC conductivity of green ceramic materials has not been widely studied yet. An ionic character of the DC conductivity during thermal treatment of kaolin has been reported by Podoba et al. (2014) and Trnovcová et al. (2012). An increase in the electrical conductivity at the lowest temperatures can be explained by the ionization of the residual physically bound water (transport of H+ and OH− ions). The amount of these ions is reduced and the conductivity decreases when the water evaporates above 120 °C. The successive increase in the conductivity with an increasing temperature results from the migration of Na+ and K+ ions originated from impurities contained in kaoline samples (Table 1) (Podoba et al., 2014; Štubňa et al., 2015). During dehydroxylation, half of the H+ ions leaves their parent hydroxyl ions, which form O2 − ions, and then recombines with the other half of the OH− ions forming H2O molecules according to the relation OH− + OH− → O2 − + H2O (Mackenzie, 1973). It was also found that a collapse of the metakaolinite structure at 950 °C is accompanied by a decrease in AC resistivity from 108 to 106 Ω cm− 1 (Freund, 1967). Thus, the measurement of the DC conductivity during thermal treatment helps to reveal a connection between processes in clays and creation of free ions (H+, OH−, Na+ and K+ in clays) as well as conditions for their transport. The presence of free ions and their concentrations are closely linked to the microstructure of the kaolin and the structure of kaolinite crystals. In this study, the effect of dry milling of kaolin (92 mass% of kaolinite) on its physical properties and microstructure development during firing was investigated. The samples were studied during thermal treatment using in-situ techniques (thermodilatometric analysis (DIL), DTA, thermogravimetry (TG, DTG)) and special attention was paid to the DC conductivity. Development of the particle size after various milling times was studied using particle size distribution plots. Microstructure of the samples was observed using X-ray diffraction (XRD) and scanning electron microscopy (SEM). Measurement of the DC conductivity allows to determine whether the coordinated water has a significant effect on the electrical properties during firing or not. To the authors` knowledge, the influence of dry milling on the DC conductivity of kaolin has not been studied before. Understanding conduction behavior of raw ceramics at higher temperatures enables more accurate control of the chemical composition of the material and furnace temperature in flash sintering which is an energy-saving modern method of ceramic production.
Washed kaolin from Sedlec (Czech Republic) was used (Table 1). According to the supplier, this kaolin consisted of 92 ± 2 mass% of kaolinite, 2 ± 1 mass% of quartz, and 6 ± 2 mass% of illite. Crushed kaolin (95 g) was inserted into a planetary ball mill Retsch PM100 and dry milled for 0, 30, 60, and 120 min in 250 ml alumina vessel with 12 alumina balls (with diameter ~19.9 mm and mass ~16.1 g, each) with 350 rpm. During the longer milling times (longer, than 30 min) a 5 min break was kept after the each 30 min to prevent the powder to reach temperatures above 60 °C (Ptáček et al., 2013). Such prepared powders and samples were labelled as K0, K30, K60, and K120, respectively. The wet plastic mass was prepared by mixing the as-prepared powder (70 mass%) with distilled water (30 mass%). Rectangular samples were obtained by pressing the mass into a gypsum form. To reach the same conditions for a release of the physically bound water and dehydroxylation in samples used in DTA, TG and DC conductivity, samples prepared by the same technology were used. The samples were freely air-dried, then dried at 120 °C for 1 h and stored in a desiccator. Thermal analyses DTA and TG were carried out in static air using the upgraded Derivatograph analyzer (MOM Budapest) (Podoba et al., 2012) in which compact samples of rectangular shape 10 × 10 × 20 mm were used. As a reference sample for DTA, a prism of the same size made from pressed Al2O3 was used. Thermal analyses were performed from laboratory temperature up to 1050 °C with heating rate 5 °C/min. Measurement of the DC conductivity was performed on rectangular samples (prepared using a plastic form designed especially for these samples) with dimensions 10 × 10 × 20 mm in which two parallel platinum wires (∅ 0.4 mm) served as electrodes, Fig. 1 (Štubňa et al., 2015). The distance between the electrodes was 3 mm. This arrangement was chosen in order to minimize the surface electric current of the sample. After free drying in ambient atmosphere and to obtain equal initial conditions for all measurements, the samples were preheated at 120 °C for 1 h and then measured up to 1100 °C with heating rate 5 °C/ min in static air. The measuring circuit (serial connection of the voltage source, the sample, and the electrometer) was fed with DC power supply Tesla BS 525 with voltage of 10 V. Electrical current was measured and recorded using electrometer Keithley 6514 directly connected to the computer (Fig. 1). Thermodilatometry was carried out on rectangular samples 8 × 8 × 22 mm using the Netzsch DIL 402C horizontal push rod dilatometer in a dynamic nitrogen atmosphere with a flow rate of 40 ml/
Table 1 Chemical composition of Sedlec kaolin (in mass%). LOI
SiO2
Al2O3
Fe2O3
TiO2
CaO
MgO
K2O
Na2O
12.95
45.80
37.31
0.98
0.17
0.58
0.46
1.17
0.58
Fig. 1. Electrical scheme with location of electrodes in the sample. PS – power supply, EM – electrometer.
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min. The measurements were performed in the temperature range from 120 °C to 1100 °C, with heating rate of 5 °C/min. Relative bulk density was derived from the TG and DIL results as
( (
Δm (t )
1− m ρ (t ) 0 = Δl (t ) ρ0 1+3 l 0
) )
(1)
where ρ0 = m0/V0 is the initial bulk density in which m0 and V0 are the mass and the volume of the sample at room temperature. The values of − Δm(t)/m0 and Δl(t)/l0 are the relative mass loss and the relative expansion measured by the TG analyzer and dilatometer at temperature t. The particle size distribution of the milled kaolin was determined by the Analysette 22 Micro Tec plus (Fritsch) laser diffraction device with the measuring range up to 2 mm. To ensure the repeatability of the measurements, all analyses were carried out on 3 different samples and then an average value was taken. The differences between the measurements did not exceed 2%. Structural observations were made using the FEI Quanta 200F electron microscope in low vacuum mode (100 Pa) with an accelerating voltage 10 kV. Micrographs were taken from fracture areas both on raw and fired samples. Phase compositions were determined from X-ray diffraction (XRD) patterns obtained at room temperature by Co Kα (wavelength 0,179 nm, 2 Θ range from 10° to 80°, 0.03° step size, 288 s time per step, divergent beam with opening degree of 0.5°) and 1D LynxEye detector (Fe ß filter in front of the detector) mounted on Bruker D8 Discover and subsequent Rietveld refinement (Rietveld, 1967) was performed in TOPAS 5 (Coelho, 2016). Powders were prepared in a standard PMMA sample holder with a cavity diameter of 25 mm. Fundamental parameters approach was used for calculation of profile broadening due to instrumental effects. Sizes of coherently diffracting domains (CDD) of the kaolinite phase were determined from broadening of the Lorentzian component of pseudo-Voigt function and calculated as volume weighted average derived from full width at half maximum (FWHM) (Scardi et al., 2004).
Fig. 3. XRD patterns of samples milled for 0, 30, 60, and 120 min.
Aglietti et al. (1986). The XRD reflections of the kaolinite phase gradually broadened during milling with an increasing milling time (Fig. 3). This is in accordance with the expected partial degradation of the kaolinite lattice as reported by Ding et al. (2012), Frost et al. (2001a), Hamzaoui et al. (2015), Leonel et al. (2014), Mitrović and Zdujić (2013), and Vdovic et al. (2010). Sizes of the CDD obtained by fitting the whole pattern continuously decreased with an increasing milling time from 36 nm to 22 nm. This indicates structural degradation of kaolinite (Frost et al., 2001a). However, these results also show that the applied milling regime was not sufficient for significant amorphization of kaolinite. The SEM micrographs of raw samples K0 and K120 are presented in Fig. 4. The sample K0 exhibited plate-like kaolinite particles. The milling resulted in the formation of agglomerates of small particles. A similar result was obtained by Aglietti et al. (1986). Mass loss (Fig. 5), caused by the removal of the physically bound water as well as coordinated water, was observed at temperatures lower than 450 °C (at which dehydroxylation started) for all samples. After milling, the samples retained more water (added during the sample preparation), and its removal was not completed even at 450 °C. The increase in the sample mass loss rose from 0.54% (K0) to 2.56% (K120) with prolonged milling. Thus, the amount of the physically bound water and the coordinated water in the samples increased with the increasing milling time. The DIL curves (Fig. 6) exhibited a slow expansion in the lowtemperature region (< 450 °C) caused by heating of the samples K0, K30, and K60, while sample K120 exhibited a modest contraction owing to the higher amount of the released physically bound and coordinated water. The contraction at temperatures lower than 450 °C is caused by the kaolinite particles moving closer to each other when the physically bound water is released. The mass loss in the temperature interval from 450 °C to 750 °C is caused by kaolinite dehydroxylation. This process is accompanied by a significant mass loss and destruction of the kaolinite lattice (Chakraborty, 2014). The mass loss during dehydroxylation was almost the same (~ 12.1 mass%) for samples K0, K30, and K60; only K120 had a slightly lower mass loss (10.9 mass%). Therefore, the used milling treatments were not sufficient for complete mechanochemical dehydroxylation of kaolinite. The shifts of the TG lines to lower temperatures with increasing milling times are obvious, and the same trend is observed for the shifts of DTA and DTG peak temperatures (Table 2). The kinetics of dehydroxylation depends (except other influences such as, for example, temperature, partial pressure of H2O vapor, and other) on the defect density. The less perfect the structure of the kaolinite crystals, the lower the temperature of dehydroxylation. A
3. Results and discussion The influence of milling on the particle size of kaolinite is shown in Fig. 2. The particle size distribution of all samples exhibited two maxima – around 50 μm and 3 μm. The increasing milling time increased the volume fraction of the particles with the radii between 10 and 100 μm, thus milling promoted formation of agglomerates of small particles. The agglomerates were also found in ground kaolin by
Fig. 2. Particle size distribution in samples K0, K30, K60 and K120.
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Fig. 4. SEM micrographs of the fracture surfaces of the raw samples K0 (a) and K120 (b).
Table 2 DTA and DTG dehydroxylation peak temperatures. DTA peak temperatures Sample name Temperature (°C)
K0 615
K30 608
K60 605
DTG peak temperatures K120 595
K0 600
K30 595
K60 590
K120 580
dehydroxylation (Fig. 6). The contraction increased with an increasing duration of the milling up to ~960 °C (metakaolinite → Al-Si spinel transformation). Since no liquid phase is created at these temperatures, sintering runs in a solid-state. This is supported by results reported by Monteiro and Vieira (2004) where successful sintering of small-particle kaolinitic-clay was found at temperatures below 700 °C. In the narrow temperature interval around 960 °C, at which the transformation metakaolinite → Al-Si spinel takes place (Chakraborty, 2014), a significant contraction was observed (Fig. 6). The temperature of this transformation was not influenced by the milling. A longer milling time led to a more intense shrinkage of the samples (Fig. 6). The overall contraction of the sample K0 was 2.5%, while the sample K120 exhibited 7% shrinkage. Development of the relative bulk density can be derived using the results of TG and DIL (Fig. 7). After a continuous decrease in the relative bulk density caused by the removal of the physically bound and coordinated water, dehydroxylation influences the bulk density to a large extent (Freund, 1967). During dehydroxylation, the density steeply decreases. This decrease is less significant for longer milling times. Between ~600 °C and ~900 °C, a moderate increase in the relative bulk density was observed. This increase depends on the milling time. Since development of the relative bulk density is directly linked to
Fig. 5. TGA curves of the samples K0, K30, K60, and K120. Embedded graph: Mass loss between 20 °C and 450 °C.
Fig. 6. Thermodilatometric curves of the samples K0, K30, K60, and K120. The embedded graph shows an expansion rate near the metakaolinite → Al-Si spinel transformation.
decrease in the size of the crystals and an increase in the defect concentration lead to a lower activation energy of dehydroxylation. Therefore, dehydroxylation can run easier in such damaged crystals (Štubňa et al., 2006). A contraction of samples is observed in the temperature interval of
Fig. 7. Development of the relative bulk density for the samples K0, K30, K60, and K120.
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Fig. 8. SEM micrographs of the fracture surfaces of the samples K0 (a) and K120 (b) after heating at 1100 °C.
sintering, a more intensive sintering was found in long-milled samples. The final relative bulk density (at 1050 °C) is reached after metakaolinite → Al-Si spinel transformation and is also determined by the milling time. The highest relative bulk density was obtained after the longest milling time. After heating to 1100 °C, a compact microstructure having relatively well-formed particles was built in K0, contrary to the microstructure consisting of small and damaged particles in K120 (Fig. 8). The difference between both microstructures did not vanish after heating to 1100 °C. The factors that influence the ionic DC conductivity in solids are the concentration of charge carriers, temperature, the density of defects and/or the structural disorder, and the facility with which an ion can jump into a neighboring vacant site. This facility is controlled by the activation energy, i.e. the free energy barrier, which an ion has to overcome for a successful jump between the sites. When the charge of the jumping ions is larger, they find higher energy barrier for their jumps between the sites. In the simplest case, the conduction activation energy (CAE) can be determined using the Arrhenius relation
σ = σ0 exp(−Eσ / kT )
(2)
where σ is the conductivity at temperature T, k is Boltzmann constant, Eσ is the conduction activation energy, and σ0 is the pre-exponential factor. The ionic conductivity relates to the self-diffusion coefficient D of dominant charge carrying ions by Nernst-Einstein equation
σ / D = nq2 / kT
Fig. 9. Temperature dependence of DC conductivity for the samples K0, K30, K60 and K120.
highest milling time it vanished. This behavior proved the influence of coordinated water on electrical properties of mechanically activated kaolin. The initial amount of the physically bound and coordinated water in the samples increased with the milling time (Fig. 5), consequently, more H+ and OH− ions were present in the longer milled samples. The second stage of the removal of physically bound water (reported at ~300 °C in (Podoba et al., 2014)) was observed only in non-milled kaolin with the lowest concentration of the physically bound water and no coordinated water. Part of the physically bound and coordinated water remained in samples up to start of dehydroxylation (at ~450 °C), so the migration of these ions in the electric field was still contributing to the conductivity. In addition to the H+ and OH− ions, Na+ and K+ ions, which originate from contamination of kaolin, contributed to the conductivity. Na+ and K+ ions got dominant charge carriers when water molecules were removed. Temperature dependences of the DC conductivity exhibit two parts, which obey Arrhenius' law. The first temperature interval (from 320 °C to 500 °C) gave the CAE of 1.73 eV for K0. The high value belongs to the migration of Na+ and K+ ions, as no PBW is present in the non-milled samples at these temperatures. A clear dependence of the conductivity on the milling-time was visible below the temperature of dehydroxylation (Fig. 9): the longer milling time the higher conductivity and smaller conduction activation energy. The CAE was linearly decreasing from 1.73 eV to 0.85 eV with an increasing milling time (Fig. 10). The decrease in the CAE can be related to the increasing content of the
(3)
where n is the concentration of dominant charge carrying ions and q is their charge. Temperature dependences of the DC conductivity for green samples are shown in Fig. 9. At the lowest temperatures, an increase in the conductivity with an increasing temperature resulted from the release of weakly bound charge carriers (H+ and OH– ions), which are the products of self-dissociation of water molecules in water layers located on the crystal surfaces (Podoba et al., 2014) and of coordinated water in milled kaolin. When the water was evaporating, above 120 °C, the amount of these ions decreased and, consequently, the conductivity also decreased. Only H+ and OH– ions, which are in a direct contact with charged surface defects can be charge carriers at higher temperatures together with K+ and Na+ ions that originate from contamination of kaolin. Moreover, OH– ions are also released during mechanochemical treatment which is partially equivalent to dehydroxylation. Kristóf et al. (2002) proposed that mechanochemical treatment of kaolinite causes localization of H+ when the long-range order is lost. At ~200 °C, the conductivity reached its minimum, corresponding to the minimal amount of the charge carriers (H+ and OH− ions). This minimum of the conductivity was shifted to higher temperature with an increasing milling time due to an increasing initial content of coordinated water, the concentration of which rose owing to longer milling times. At the same time, the minimum got shallower and at the 40
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• Milling promotes formation of agglomerates and of coordinated water. • Dehydroxylation is shifted to lower temperatures by prolongation of milling. • Overall shrinkage of samples is more intense for longer milling times. • Below the temperature of dehydroxylation, the DC conductivity of • • • •
Fig. 10. Conduction Activation Energies of kaolin samples after different milling times in temperature region 320–500 °C (full circle) and in temperature region 650–750 °C (empty circle).
coordinated water as the milling time increases. In the second interval (from 650 °C to 750 °C) the CAE decreased, reaching the value of 0.79 eV in non-milled samples. This value corresponded to the migration of alkali ions in metakaolinite structure. However, the dehydroxylation was still running, thus the contribution of OH˗ and H+ ions had to be also considered. CAE linearly decreased in the temperature interval 650 °C–750 °C from 0.79 eV to 0.52 eV (Fig. 10) as a consequence of longer milling time; at the same time, the DC conductivity slightly decreased, probably due to increased density of kaolin. An anomaly in the temperature dependence of the DC conductivity near 500 °C indicates the dehydroxylation process. The OH− ions released during dehydroxylation got associated with mobile alkali ions into neutral complexes which did not contribute to the DC conductivity and, for a short time period, the increase of the conductivity with increasing temperature decelerated. This deceleration was more significant for longer milling times. At temperatures above 650 °C, all samples showed very close temperature dependences of the DC conductivity with only slight decrease with an increasing milling time. This dependence of the conductivity on the milling time could result from the increasing density with an increasing milling time because then the moving ions have a lower free volume for their jumps. At temperatures above 700 °C, a deceleration of the rise in the conductivity with an increasing temperature was observed. The reason for this is not clear. However, the migration of Na+ and K+ ions toward the negative electrode in the DC electric field leads to a decrease in the internal electric field and, thus, to a reduction of the mobility of alkali ions (Ondruška et al., 2015, 2017). Sample K0 had the highest conductivity and sample K120 exhibited the lowest one in this temperature region. This influence of the milling time was probably a consequence of different bulk densities of the samples. The small narrow peak at ~960 °C was caused by shifts of Al3 + ions when the metakaolinite lattice transforms into Al-Si spinel lattice. This transformation runs in a narrow temperature interval as the DTA results show (Chakraborty, 2014; Podoba et al., 2014). When this transformation was over, the conductivity decreased. In the spinel lattice, the dominant charge carriers were K+ and Na+ ions.
•
raw samples increases with an increasing milling time. The main charge carriers are H+ and OH– ions originated from self-dissociation of water molecules (at the lowest temperatures) and coordinated water originated from mechanochemical treatment. Below the temperature of dehydroxylation, alkali ions Na+ and K+, present as impurities in kaolin, also contribute to the DC conductivity. During dehydroxylation, the OH– ions, released from the structure of kaolinite, contribute to the DC conductivity; however, the association of these ions with alkali ions into neutral complexes decelerates the increase of the conductivity with increasing temperature. Above the temperature of dehydroxylation, the relative bulk density increases with the increasing milling time. Above the temperature of dehydroxylation, the DC conductivity decreases with increasing milling time. The charge carriers are Na+ and K+ ions from contaminants in kaolin. The conduction activation energies linearly decrease with the increasing milling time in the temperature interval 320–500 °C from 1.73 eV for K0 to 0.85 eV for K120, and in temperature range 650–750 °C from 0.79 eV for K0 to 0.52 eV for K120.
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4. Conclusions Influence of milling (mechanochemical treatment) on the mass loss, thermal expansion, relative bulk density, and DC electrical conductivity of kaolin was studied. The main results are: 41
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