Influence of particle size on the crystallization kinetics of glasses produced from waste materials

Journal of Non-Crystalline Solids 357 (2011) 211–219

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Journal of Non-Crystalline Solids j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / j n o n c r y s o l

Influence of particle size on the crystallization kinetics of glasses produced from waste materials M. Erol ⁎, S. Küçükbayrak, A. Ersoy-Meriçboyu Department of Chemical Engineering, Chemical&Metallurgical Engineering Faculty, Istanbul, Technical University, Maslak 34469, Istanbul, Turkey

a r t i c l e

i n f o

Article history: Received 4 August 2009 Received in revised form 29 August 2010 Available online 20 October 2010 Keywords: Glass; Crystallization; Glass-ceramic; Differential thermal analysis

a b s t r a c t The crystallization behavior and kinetics of glasses produced from coal fly ashes, red mud and silica fume were investigated by using differential thermal analysis, X-ray diffraction and scanning electron microscopy techniques. The kinetic parameters of the glass-crystallization transformation were estimated under nonisothermal conditions applying three different equations, namely, Kissinger, Matusuta-Sakka and Ozawa. Non-isothermal differential thermal analysis curves were obtained using both coarse and fine glass samples. The crystallization activation energies of coarse glasses are in the range of 233–439 kJ/mol while the activation energies of fine glasses change in the range of 369–450 kJ/mol. Avrami exponent, n, values of coarse glasses indicated the three-dimensional bulk crystallization. This result is in well agreement with the crosssectional scanning electron microscopy investigations. The values of the n obtained experimentally are in the range of 1.24–1.36 for fine glasses which show the one-dimensional surface crystallization. The crystallized phase of the glass-ceramic samples produced from waste glasses by applying the controlled heat treatment process was identified as diopside by X-ray diffraction analysis. © 2010 Elsevier B.V. All rights reserved.

1. Introduction An increasingly urgent problem for the near future of human kind is the recycling of industrial wastes. One of the major wastes is coal fly ash, which is produced in significant amounts in Turkey. Fly ash is a major source for environmental pollution since it is in a fine powder form and its heavy metal content is very high. Because of increasingly stringent environmental regulations, coal fly ash is regarded as hazardous material in most countries. Therefore, it must be recycled or, at least, be deposited in standard landfill sites with minimum risk. Vitrification technology is one of the most promising solutions for the utilization of industrial wastes. It is feasible to use coal fly ash as a raw material source to develop glass materials since it contains large amounts SiO2 and Al2O3, which are main glass network formers. The glass materials produced from industrial wastes can also be converted into glass-ceramic materials possessing outstanding mechanical, physical and chemical properties. In recent years, the production of glass-ceramic materials made by recycling industrial wastes such as, coal fly ash, iron blast furnace slags, municipal incinerator fly ash and even red mud from aluminum production have been investigated by many researchers [1–6]. Glassceramic production is achieved through a two step process namely nucleation and crystallization stages. In the nucleation stage, small nuclei are formed within the parent glass. After the formation of stable

⁎ Corresponding author. Tel.: + 90 212 285 3351; fax: + 90 212 285 2925. E-mail address: [email protected] (M. Erol). 0022-3093/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2010.09.027

nuclei, crystallization proceeds by growth of a new crystalline phase. Knowledge of nucleation and crystallization parameters is important in the preparation of glass-ceramics with desired microstructure and properties [7]. Thermal analysis is widely used in studying the crystallization kinetics of glasses because of providing rapid and convenient means [8]. Isothermal and non-isothermal methods are applied for thermal analysis. Most of the studies are connected with non-isothermal investigations of the nucleation, the crystal growth and the kinetics of crystallization [8,9]. Several equations have been proposed, attempting to interpret the non-isothermal data. Most of these equations are available to analyze the DTA data and to determine the activation energy for crystallization. These equations shed important light on the understanding the nature of the crystallization mechanisms of different glass forming systems. Recently, there were reports on crystallization kinetics of glasses produced from industrial wastes [1–5,10,11]. The activation energy of the glasses produced from coal fly ash was calculated as 370 kJ/mol by Cioffi et al. [3]. The crystallization behavior of glasses made of mixture of coal ash and soda lime glass cullet was investigated by Francis et al. [4]. Obtained results showed that the crystallization mechanism is diffusion controlled crystallization with a decreasing nucleation rate. More recently, Romero et al. [12] have investigated the effect of glass particle size on the crystallization kinetics of an iron-rich glass from a nickel leaching waste. It was reported that the activation energies of the glass were in the range of 349–423 kJ/mol and crystallization in the glass occured by different mechanisms depending on the glass particle size. It was also stated that coarse particles showed three-

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dimensional crystal growth controlled by diffusion while the fine particles led to an interface reaction mechanism with two-dimensional growth of crystals. In this study, the crystallization behavior of glasses produced from mixtures of coal fly ashes obtained from two different thermal power plants, red mud and silica fume was investigated by using DTA. For this purpose, Kissinger [13], Matusita & Sakka [14,15] and Ozawa [16] equations were used to determine the crystallization mechanism and the activation energy for crystallization. Particle size effect on the crystallization kinetics of waste glasses was also studied.

Nucleation and crystallization experiments were carried according to the results obtained in the other studies [18,19]. For this purpose, CR and ORS glasses were heated at a rate of 10 K/min to the nucleation temperatures of 963 and 988 K, respectively. Maximum nucleation temperature and time were determined in the previous studies [18,19]. CR and ORS glass samples were held at maximum nucleation temperatures for 4 and 2 h, respectively to obtain fully nucleated glasses. Following nucleation, the temperature was raised to the crystallization temperatures of 1135 and 1188 K, for CR and ORS samples, respectively. Samples were held at these temperatures for 15 min and cooled in the furnace.

2. Experimental procedure 2.3. Microstructural characterizations 2.1. Starting materials and glass preparation The raw materials used in this study were coal fly ashes obtained from the Çayırhan and Orhaneli thermal power plants in Turkey, red mud from aluminum production and silica fume. Chemical compositions of these wastes were given in Table 1 [6,17]. Five percent red mud was added to Çayırhan coal fly ash to produce glass samples, while 20% red mud and 20 % silica fume were added to Orhaneli coal fly ash to increase the SiO2 and Fe2O3 content of it. Glass samples were prepared from the mixed wastes. In each batch 20 g of waste was melted in a platinum crucible for 2 h in an electrically heated furnace at 1773 K. To ensure homogeneity, the melt was poured into water. The cast glasses were crushed, pulverized and remelted at the same temperature for 3 h to remove the air bubbles from the melt. Following this procedure, the refined melt was cast in a preheated graphite mould (673 K) to form cylinders of approximately 0.8 to 1 cm in diameter and 1 to 4 cm in length. The cylinders were cooled to room temperature. To remove thermal residual stress, the cast glasses were annealed in a furnace at 873 K for 2 h followed by slow cooling to room temperature. The annealing temperature and time were chosen on the basis of the results obtained in a previous study [11]. CR and ORS codes were given to the glasses produced from Çayırhan fly ash + red mud and Orhaneli fly ash + red mud + silica fume, respectively. 2.2. Differential thermal analysis and heat treatment Differential thermal analysis (DTA) experiments were performed by heating 20 mg glass samples in a Pt-crucible and using Al2O3 as a reference material in the temperature range between 293 and 1373 K at the heating rates of 5, 10, 15 and 20 K/min. The glasses were ground to a coarse particle size (800–1000 μm) to be a representative of a bulk sample and to a fine particle size (b180 μm) to be a representative of powder samples. The variation of the crystallization peaks with different DTA heating rates can be used to estimate the activation energy for crystallization and to determine the crystallization mechanism. The crystallization behavior of glasses was determined by using nonisothermal methods. DTA was performed on both coarse and fine glass samples to determine the particle size effect on the crystallization mechanism. As non-isothermal methods Kissinger, Matusita–Sakka and Ozawa methods were used. Table 1 Chemical compositions of the waste materials [6,16]. Waste materials

SiO2 (%)

Al2O3 (%)

CaO (%)

MgO (%)

Fe2O3 (%)

Na2O (%)

K2O (%)

SO3 (%)

LOI⁎ (%)

Çayirhan fly ash Orhaneli fly ash Silica fume Red mud

41.53 32.83 90.80 10.40

17.77 13.34 1.02 28.50

12.52 30.35 2.55 3.90

4.46 4.51 0.94 7.70

9.93 5.61 1.93 35.1

2.57 2.15 – 3.80

2.43 1.37 – 1.60

8.03 5.85 0.50 4.65

0.68 3.67 1.57 3.90

⁎LOI: loss of ignition.

The characterization of the produced glass-ceramic samples was carried out using both electron microscopy and X-ray diffraction (XRD) techniques. Scanning electron microscopy (SEM) investigations were operated at 25 kV to observe the microstructure of the produced samples. For the SEM investigations, optical mount specimens were prepared using standard metallographic techniques followed by chemical etching them in an HF solution (5%) for 1.5 min. The etched glass-ceramic samples were coated with carbon prior to examination. X-ray diffraction was utilized to determine that the crystalline phases occurred in the glass-ceramic samples. The X-ray diffraction investigations were carried out using CuKα radiation at 40 kV and 30 mA settings in the 2θ range from 10° to 80°. In all cases, samples which were analyzed by X-ray diffraction were ground to fine powder form. 3. Results 3.1. Activation energy determination The kinetic parameters of the glass-crystallization transformation were estimated under non-isothermal conditions applying three different equations, namely, Kissinger [13], Matusita–Sakka [14,15] and Ozawa [16]. Crystallization mechanisms and activation energies for crystallization were calculated in each method from the heating rate dependence of the crystallization peak temperatures. Apparent activation energies for crystallization may be determined employing non-isothermal methods to the crystallization peak. For example, in the Kissinger equation [13], the crystallization peak temperature is monitored as a function of the heating rate; the following relationship is then applied α ln Tp2

! =

−Eck RTp

! + cons tan t

ð1Þ

where α is the heating rate, Tp is the crystallization peak temperature, R is the gas constant, and Eck is activation energy for crystallization, determined by the Kissinger equation. A plot of ln (α/T2p) vs. 1/Tp should be a straight line, from the slope of which Eck can be determined. Matusita–Sakka [14,15] have stated that Eq. (1) is valid only when crystal growth occurs on a fixed number of nuclei. Incorrect values for the activation energy are obtained if a majority of the nuclei are formed during the DTA measurement, so that the number of nuclei continuously varies with α. They have proposed a modified form of the Kissinger equation as given before (Eq. (1))

αn ln Tp2

! =

−mEc RTp

! + cons tan t

ð2Þ

M. Erol et al. / Journal of Non-Crystalline Solids 357 (2011) 211–219 Table 2 Values of n and m for different crystallization mechanisms in the heating process [9,14]. Crystallization mechanism Bulk crystallization with a constant number of nuclei (i.e., the number of nuclei is independent of the heating rate) Three-dimensional growth of crystals Two-dimensional growth of crystals One-dimensional growth of crystals Bulk crystallization with an increasing number of nuclei (i.e., the number of nuclei is inversely proportional to the heating rate) Three-dimensional growth of crystals Two-dimensional growth of crystals One-dimensional growth of crystals Surface crystallization

n

m

3 2 1

3 2 1

4 3 2 1

3 2 1 1

where Ec is the correct activation energy for crystallization, n is a constant known as the Avrami parameter and m represents the dimensionality of crystal growth. When surface crystallization predominates, m = 1 and when the crystallization is predominantly bulk, m = 3 (from Table 2). The value of m is related to n as m = n when crystallization at different heating rates occurs on a fixed

213

number of nuclei (i.e., the number of nuclei is constant during DTA runs at different values of α), and m = n − 1 when nucleation occurs during DTA and the number of nuclei in the glass is inversely proportional to α. In addition, when surface crystallization predominates, m = n = 1 and Eq. (2) essentially reduces to the Kissinger equation which will yield the correct value for the activation energy, i.e., Eck = Ec. In the presence of bulk crystallization, Eck does not necessarily to be equal to Ec. Rather, a close inspection of Eqs. (1) and (2) shows that Ec = ðn = mÞEck −2ððn−1Þ = mÞRTp

ð3Þ

For most oxide glass systems, Ec ≥ 20 RTp typically [20]. Therefore, the neglect of 2 ((n −1 )/m)RTp ≤ 2 RTp in Eq. (3) will result in an error less than only 10 % in the value of Ec. This error is within the error range of the DTA experiment. Then we obtain Ec ≅ðn = mÞEck

ð4Þ

For m = n, i.e., when crystallization occurs on a fixed number of nuclei, Eck = Ec. Thus, for predominantly surface crystallization or for

Coarse

Coarse

ΔT Exothermic

(d) (c)

ΔT Exothermic

(d)

(c)

(b)

(b)

(a)

(a) 400

800

1200

400

600

T (K)

800

1000

1200

1400

T (K) Fine

Fine

(d) (c)

ΔT Exothermic

ΔT Exothermic

(d) (c) (b) (b) (a) (a) 400

800

1200

T (K) Fig. 1. DTA plots of the coarse and fine CR glasses scanned at the heating rates of: a) 5 K/min, b) 10 K/min, c) 15 K/min and d) 20 K/min.

400

800

1200

T (K) Fig. 2. DTA plots of the coarse and fine ORS glasses scanned at the heating rates of: a) 5 K/min, b) 10 K/min, c) 15 K/min and d) 20 K/min.

M. Erol et al. / Journal of Non-Crystalline Solids 357 (2011) 211–219

Table 3 DTA results of coarse and fine CR glass samples. Heating rate (K/min)

Crystallization peak temperatures for coarse particles (K)

Crystallization peak temperatures for fine particles (K)

5 10 15 20

1157 1187 1205 1222

1080 1091 1108 1113

Table 4 DTA results of coarse and fine ORS glass samples. Heating rate Crystallization peak temperatures Crystallization peak (K/min) for coarse particles (K) temperatures for fine particles (K) 5 10 15 20

1200 1222 1237 1254

1082 1097 1108 1110

crystal growth that occurs on a fixed number of nuclei, the analysis of DTA data by the Kissinger equation (Eq. (1)) yields the correct value of Ec. When the number of nuclei changes during the DTA measurements, either, Eq. (2) should be used or Eck determined from Eq. (1) should be multiplied by the term (n/m) to obtain the correct activation energy [8]. From the exothermic peak, the Avrami parameter, n, can be obtained by using the modified Ozawa equation [16]:

j

dðlnð− lnð1−xÞÞÞ d ln α

j

= −n

ð5Þ

and 2, respectively. The crystallization peak temperatures of CR and ORS glasses of both coarse and fine particles measured by DTA were given in Tables 3 and 4, respectively. As can be seen in Figs. 1 and 2 and Tables 3 and 4, crystallization peak temperatures of all glass samples increased with the increase in heating rates. Tp for coarse particles was significantly higher than that for fine particles and this difference in Tp increased consistently with increasing heating rate. At a particular heating rate, the Tp for a glass depends on the total concentration of surface and bulk nuclei present in the glass and decreases with increasing concentration of nuclei. Since the surface area increases with decreasing particle size, the concentration of surface nuclei is expected to be higher for fine particles. If a glass is not initially saturated with internal nuclei during a DTA scan, the concentration of it will be higher for slower heating rates, since the glass spends a longer time in the temperature range where nucleation can occur. The effect of these internal nuclei on crystallization is more pronounced in coarse particles, as they provide larger effective volumes for internal nucleation. It is expected that at low heating rates, all the different particle sizes are highly nucleated (surface or bulk). At higher heating rates, the concentration of nuclei in the coarse particles is less than that in the fine particles so that crystallization for

-2 Coarse

-4

ln(αn/Tp2)

214

-6

T

where x is the volume fraction crystallized at a fixed temperature T with the heating rate of α. x is the ratio of the partial area at a certain temperature to the total area of a crystallization exotherm. Non-isothermal DTA curves were obtained with selected heating rates (5, 10, 15 and 20 K/min) using both coarse and fine glass samples. Typical DTA graphs for coarse and fine samples of CR and ORS glasses recorded at 4 different heating rates were shown in Figs. 1

-8

-10 8.0

8.2

8.4

8.6

8.8

1/Tp *10-4 (K-1) -9.5

2

Fine

Coarse Fine -10.0

ln(αn/Tp2)

ln(-ln(1-x))

0 -10.5

-11.0

-2 -11.5

-12.0

-4 1.6

2.0

2.4

2.8

lnα Fig. 3. The Ozawa plots of the coarse and fine CR glasses.

3.2

8.9

9.0

9.1 -4

9.2 -1

1/Tp *10 (K ) Fig. 4. The Matusita–Sakka plots of the coarse and fine CR glasses.

9.3

M. Erol et al. / Journal of Non-Crystalline Solids 357 (2011) 211–219

coarse particles occurs at a higher temperature [21]. This is due to the particle size effect on heat transfer. Temperature gradients developed between the surrounding air and the surface of the glass sample and inside the glass sample. Temperature gradients can arise when the glass samples' dimensions or the heating rate are relatively large. The temperature rises gradually from inside the glass sample to the surface of it since the heat transfer coefficient on the surface of the sample is small. At a given heating rate, coarse particles have greater heat transfer resistance, so it takes longer for the center of the particle to reach the furnace temperature and a higher observed crystallization temperature results. The concentration of nuclei in coarse particles is expected to decrease with increasing heating rate (less time in the nucleation range), so the difference in nuclei concentration between coarse and fine particles will increase with increasing heating rate. This will cause the difference in Tp between coarse and fine particles with increasing heating rate. This case has also been observed by several authors [22–24]. Assuming that the nucleation and growth processes had occurred simultaneously in as-quenched samples during the DTA measurements, the data for as-quenched glasses were analyzed by the

215

Matusita–Sakka equation (Eq. (2)) to determine Ec. Since Eq. (2) included the n and m, first of all their values were determined using the Ozawa equation (Eq. (5)). Plots of ln(−ln(1 − x)) vs. lnα for CR glass of coarse and fine particles are shown in Fig. 3. x values were determined at the fixed temperatures of 1185 K and 1195 K for coarse and fine particles, respectively. The values of n determined from the slops of these plots are 3.42 ± 0.21 for coarse particles (correlation coefficient r = 0.92) and 1.36 ± 0.10 (r = 0.98) for fine particles. These values indicate that bulk and surface crystallizations are dominant in the coarse and fine particles, respectively. The m value for the coarse particles should be equal to n − 1, i.e., 2.42 ± 0.21 from Table 2. Fine particles have a larger effective surface area so that the number of internal nuclei formed during the DTA run could be neglected. This means that crystal growth for glasses of fine particles should have occurred on a fixed number of nuclei during the DTA measurements. Therefore, we can assume that n is equal to m for the fine glasses. n = m =1.36 ± 0.10 for CR glass of fine particles and this value is very close to 1 which means surface crystallization. Using n, m and R (R value was taken as 8.3144 J/mol K) values, Matusita–Sakka plots, ln(αn/T2p) vs. 1/Tp, for CR sample yield Ec values

-10.8

2

Coarse

Coarse

-11.2

ln(-ln(1-x))

ln(α/Tp2)

0

-11.6

-2

-12.0 -4 -12.4

8.0

8.2

8.4

8.6

-6

8.8

1.6

1/Tp *10-4 (K-1)

2.0

2.4

2.8

3.2

lnα

-10.8

0.5

Fine

Fine 0.0

ln(-ln(1-x))

ln(α/Tp2)

-11.2

-11.6

-0.5

-1.0

-12.0 -1.5

-12.4

-2.0 8.9

9.0

9.1

9.2

1/Tp *10-4 (K-1) Fig. 5. The Kissinger plots of the coarse and fine CR glasses.

9.3

1.6

2.0

2.4

2.8

lnα Fig. 6. The Ozawa plots of the coarse and fine ORS glasses.

3.2

216

M. Erol et al. / Journal of Non-Crystalline Solids 357 (2011) 211–219

as 350 ± 11 (r = 0.97) and 372 ± 17 (r = 0.91) kJ/mol for coarse and fine particles, respectively (Fig. 4). Also using the Kissinger equation (Eq. (1)), Eck values of as-quenched glasses of coarse and fine particles can be determined from the slope of the plots ln(α/T2p) vs. 1/Tp (Fig. 5). For coarse particle Eck is 233 ± 9 kJ/mol (r = 0.96) that is lower than the Ec values determined by using the Matusita–Sakka equation. By using Eq. (4), i.e., multiplying the Eck value for coarse particles by the factor n/m (=3.42/2.42) yields the Ec value of 330 ± 9 kJ/mol. For fine particles Eck value is 369 ± 14 kJ/mol (r = 0.92) that is close to the Ec values determined from the Matusita–Sakka equation. For n = m, i.e., when crystallization occurs on a fixed number of nuclei, Eck = Ec. This case shows that the crystal growth for fine particles occurred on a fixed number of nuclei during the DTA measurements. The Ozawa plots of ln(−ln(1− x)) vs. lnα for the coarse and fine particle sizes for ORS glass are linear according to Eq.(5) with n= 3.68± 0.18 (r =0.89) and n= 1.24 ±0.11 (r =0.93), respectively (Fig. 6). x values were determined at the fixed temperatures of 1227 K and 1102 K for coarse and fine particles, respectively. n =3.68 ±0.18 value for coarse particles indicates the bulk crystallization while n= 1.24±0.11 value for fine particles shows the surface crystallization. For coarse particles m is equal to 2.68±0.18 while m is equal to 1.24±0.11 for fine particles.

By plotting ln(αn/T2p) vs. 1/Tp, Ec values for coarse and fine particles were estimated from the slope of straight lines as 439 ± 12 (r = 0.98) and 450 ± 18 (r = 0.94) kJ/mol, respectively (Fig. 7). The Kissinger equation was used to determine the Eck values of as-quenched ORS glasses. Plots of ln(α/T2p) vs. 1/Tp, for coarse and fine particles are shown in Fig. 8 and the values of Eck obtained from the slopes of the plots are 305 ± 10 (r = 0.98) and 444 ± 12 (r = 0.95) kJ/mol, respectively. The activation energy of crystallization for fine glass obtained from Kissinger equation is close to the Ec values of fine glass estimated from the Matusita–Sakka equation. n value of fine ORS glass was found 1.24 ± 0.11 which indicates the surface crystallization. We know that the Kissinger equation (Eq. (1)) is valid only when the crystallization occurs on a fixed number of nuclei during the DTA runs or when the surface crystallization mechanism is dominant in the glass. Therefore, the Eck value of fine ORS glass is more accurate than the Eck value of coarse ORS glass. If we multiply Eck value of coarse particles with n/m (=3.68/2.68) we obtained the Ec value as 414 ± 10 kJ/mol by using Eq. (4). Tables 5 and 6 show the Ec, n and m values of CR and ORS glasses of coarse and fine particles, respectively. As seen from Tables 5 and 6, n and m values are different for coarse and fine glasses. It is important to

-2

Coarse

Coarse

-11.2

-4

ln(α/Tp2)

ln(αn/Tp2)

-11.6

-6

-12.0

-12.4 -8

-12.8 7.9

8.0

8.1

8.2

8.3

8.4

7.9

8.0

1/Tp *10-4 (K-1)

8.1

8.2 -4

8.3

8.4

9.2

9.3

-1

1/Tp *10 (K )

-10.0

-10.8

Fine

Fine -10.5

-11.0

ln(α/Tp2)

ln(αn/Tp2)

-11.2

-11.6

-11.5 -12.0

-12.0

-12.4 9.0

9.1

9.1

9.2 -4

9.2

-1

1/Tp *10 (K ) Fig. 7. The Matusita–Sakka plots of the coarse and fine ORS glasses.

9.3

9.0

9.1

9.1

9.2 -4

-1

1/Tp *10 (K ) Fig. 8. The Kissinger plots of the coarse and fine ORS glasses.

M. Erol et al. / Journal of Non-Crystalline Solids 357 (2011) 211–219

217

Table 5 Ec, n and m values of the coarse glasses.

CR ORS

Activation energy (kJ/mol) Matusita–Sakka method

Kissinger method

350 ± 11 439 ± 12

233 ± 9 305 ± 10

n

m

3.42 ± 0.21 3.68 ± 0.18

2.42 ± 0.21 2.68 ± 0.18

note that during the DTA run, surface and bulk crystallization may proceed simultaneously but if there is a control of particle size in glass, the crystallization may be governed by particle size. The difference in activation energies obtained in this study could result from a different crystallization mechanism, which is strongly influenced by the particle size in glasses. As it was also observed in CR and ORS glass samples a large Ec is associated with surface crystallization while a small Ec is associated with bulk crystallization in the same glass.

Intensity (a.u.)

Sample name

Diopside

20

40

60

80

2θ Fig. 9. XRD pattern of the CR glass-ceramic sample.

3.2. XRD studies of the produced glass-ceramic samples 4. Discussions

3.3. SEM studies of the produced glass-ceramic samples Cross-sectional SEM investigations were carried out to characterize the crystalline morphology in the bulk of the sample. The microstructural studies were performed on the bulk glass-ceramic samples using SEM. Fig. 11 shows the cross-sectional SEM micrograph of the CR glass-ceramic sample. It can be seen from Fig. 11 that tiny equiaxed crystallites uniformly dispersed in the cross-sectional area of the CR glass-ceramic sample with an average crystalline size of 0.25 μm. Fig. 12 is a representative SEM micrograph of the cross-sectional area of the ORS glass-ceramic sample. As can be seen from Fig. 12 that both locally oriented dendritic crystalline growth and a significant number of leaf-shaped crystals occurred in the ORS glass-ceramic sample. SEM investigations confirm that bulk crystallization is a predominant mechanism in both CR and ORS glass-ceramic samples as predicted by the Ozawa equation.

In this study, the kinetic parameters of both coarse and fine glass samples obtained from waste materials were estimated under nonisothermal conditions applying three different equations, namely, Kissinger [13], Matusita–Sakka [14,15] and Ozawa [16]. For CR and ORS glass samples, the Eck values of coarse particles obtained from Kissinger equation are lower than the Ec values of coarse particles determined from the Matusita–Sakka method. However, activation energy values of crystallization for fine glasses estimated from Kissinger and Matusita–Sakka equations are close to each other. The difference observed in activation energy values for coarse glasses is discussible when the Kissinger equation is used for the estimation of Ec. Matusita–Sakka stated that the Kissinger equation can be used only when crystallization occurs on a fixed number of nuclei as in the case of fine glass samples. This result confirms that Ec value will be correct if the crystallization occurred on a fixed number of nuclei however, the analysis of crystallization data using the Kissinger equation, yields incorrect values for Ec if nucleation and crystallization occur simultaneously. This result further confirms the necessity of using the Matusita–Sakka equation in determining Ec if the glass is initially unsaturated with nuclei. The Kissinger equation can be used to determine Ec only when the glass is fully nucleated with surface or bulk nuclei prior to crystallization so that crystal growth at a different α occurs always on a fixed number of nuclei. All equations gave similar results for the crystallization activation energy of fine glasses (allowing for experimental errors, Ec values of all equations are close to each other).

Diopside

Intensity (a.u.)

In order to identify the crystalline phase(s), X-ray scans were carried out on both CR and ORS glass-ceramic samples. In the XRD scan of CR glass-ceramic sample, the d-values matched the card values of the diopside (Ca(Mg,Al)(Si,Al)2O6) phase. As seen in the representative XRD pattern of CR glass sample nucleated at 963 K for 4 h and crystallized at 1135 K for 15 min (Fig. 9), all the diffraction peaks can be indexed as arising from the reflection planes of the diopsidealumina phase. Fig. 10 is the X-ray pattern of ORS glass sample nucleated at 988 K for 2 h and crystallized at 1188 K for 15 min. X-ray diffraction analysis revealed that the main crystalline phase that occurred in ORS glassceramic sample was diopside (Ca(Mg,Al)(Si,Al)2O6). In ORS glassceramic sample, not only the coal fly ash was used, but also the red mud and silica fume were used to produce glass samples. With the addition of red mud and silica fume, the chemical composition of ORS glass-ceramic sample was closed to the chemical composition of the CR glass-ceramic sample. Therefore, the diopside phase was also expected in the ORS glass-ceramic sample.

Table 6 Ec, n and m values of the fine glasses. Sample name CR ORS

Activation energy (kJ/mol)

n

Matusita–Sakka method

Kissinger method

372 ± 17 450 ± 18

369 ± 14 444 ± 12

m

20 1.36 ± 0.10 1.24 ± 0.11

1.36 ± 0.10 1.24 ± 0.11

40

60

2θ Fig. 10. XRD pattern of the ORS glass-ceramic sample.

80

218

M. Erol et al. / Journal of Non-Crystalline Solids 357 (2011) 211–219

Fig. 11. Cross-sectional SEM micrograph of CR glass-ceramic sample.

As seen from Tables 5 and 6, crystallization of fine glasses occurs with higher activation energy of crystallization than coarse glasses for all glass samples. This result may be understood that the surface strain of the grain boundaries of the glasses effects on the nucleation and growth of crystals inside the glass. The applied energy from DTA may be exhausted in the part for the crystallization inside the glass and the other part for the surface strain from the grain boundaries of the glass particles. Since the small sized glasses have large surface grain energy compared with the large sized glass particles, thus the more energy is needed relatively for the crystallization in the small sized glass sample [23]. It can be clearly seen that activation energy is dependent on temperature, i.e., the higher crystallization temperature is correlated with lower activation energy. In the case of surface nucleation, smaller particle size with its relatively large specific surface area helps the occurrence of crystallization and thus decreases the crystallization temperature. Consequently, Ec is larger for fine glass samples than that observed for coarse glasses. The explanation for different n and m values obtained for coarse and fine glasses can be discussed in terms of glass particle size. Large particles have much less surface area and few nuclei are formed. In this case, the dominant process would be the growth of nuclei. On heating above Tg, the nuclei are surrounded by liquid and their growth would be three-dimensional. As seen from Table 5, n values of CR and ORS coarse glasses indicated the three-dimensional bulk crystallization. The larger surface area of smaller particles will contain larger numbers of nuclei. Supposing that glass surface is completely crystallized, crystal growth proceeds one-dimensionally from the surface to the interior of the glass [25]. This statement can be clearly

Fig. 12. Cross-sectional SEM micrograph of ORS glass-ceramic sample.

seen from Table 6. The n values of fine CR and ORS glasses obtained experimentally are 1.36 ± 0.10 and 1.24 ± 0.11, respectively. n values of fine samples showed that the crystallization mechanism is a onedimensional surface crystallization. The crystallization mode of a glass has a practical importance in the usage of it and also in the fabrication of a glass-ceramic since the surface crystallization mode may introduce huge thermal expansion difference at the boundary between the glass phase and crystallized phase, building up high tensile stress. This high tensile stress at the interface mostly causes total failure of the glass. On the other hand, in the case of the bulk crystallization mode where the crystal growth occurs at the finely distributed precursor nuclei in the glass, the huge thermal expansion coefficient gradient across the whole body does not occur and the glass body is safe against the thermal failure [26]. Therefore, from this perspective the bulk crystallization is desirable compared to the surface crystallization. The effect of glass particle size on the crystallization kinetics of an iron-rich glass from a nickel leaching waste was investigated by Romero et al. [12]. Similar results for n and m values both coarse and fine particles were reported. It was stated that the crystallization mode and the dimensionality of crystals are strongly dependent of the glass particle size. In their study, coarse particles showed three-dimensional crystal growth controlled by diffusion while the fine particles led to an interface reaction mechanism with two-dimensional growth of crystals. The studied glasses are in the SiO2–Al2O3–Fe2O3–CaO quartet system since the chemical compositions of mixed wastes are mainly composed of SiO2, Al2O3, Fe2O3 and CaO. With the solid-state reactions and the rearrangement of these structures, crystalline phases occur in the glassy matrix when the heat treatment process was applied on the glass samples. This must require breaking and reforming of the Si–O, Al–O, Fe–O and Ca–O bonds. The single bond strengths of the Si–O, Al– O, Fe–O and Ca–O bonds are 445, 423, 335 and 128 kJ/mol, respectively [27–29]. The activation energies of crystallization for ORS and CR glasses are close to Si–O, Al–O and Fe–O bond strengths. This result is consistent with the mechanism requiring the breaking of Si–O, Al–O and Fe–O bonds and also rules out their possibilities as a reaction controlling kinetics in forming crystalline phases in the glassy matrix. The activation energy of crystallization for ORS glass is higher than the Ec values of CR glass. It was observed that the increase in Fe2O3 content in the glass compositions caused a decrease in the crystallization activation energies of the glasses. Since the Fe2O3 content of CR glass is higher than that of ORS glass, Ec value of it is lower than the Ec value of ORS glass. This result was also observed in the other studies [30,31]. Romero et al. [12] also reported that high iron oxide content resulted to the high crystallization tendency of glasses. Iron oxides lower glass viscosity and consequently increase the crystal growth rate, which also lowers the crystallization temperatures. Despite that it is usually an intermediate glass network ion, the Fe3+ could act as a modifier of the glass structure, breaking the Si–O bonds [30]. Therefore, the Ec value of ORS glass is higher than that of the CR glass sample. Only a few researchers studied on crystallization kinetics of coal fly ash based glasses. Activation energy values of crystallization for glasses produced from coal fly ashes were found 283 and 318 kJ/mol in the previous studies [11,32] which are lower than the Ec values obtained in this study since the chemical compositions of the studied glassy systems are different from each other. The calculated activation energy value of CR glass is close to the value of 370 kJ/mol found by Cioffi et al. [3] for a glass produced from coal fly ash while the Ec values of ORS glass are higher than that value. The activation energy values for crystallization are lower for the incinerator fly ashes than for the coal fly ash containing glasses, which are in the SiO2–Al2O3–Fe2O3– CaO quartet glassy systems. The activation energy values of crystallization for glasses obtained from incinerator fly ashes are in the range of 293–468 kJ/mol [2,10], while the Ec values of glasses produced from industrial wastes (such as coal fly ash, glass cullet and

M. Erol et al. / Journal of Non-Crystalline Solids 357 (2011) 211–219

soda-lime) are in the range of 534–545 kJ/mol [1,4]. Romero et al. [12] calculated that the activation energy of the glass produced from nickel leaching waste is in the 349–423 kJ/mol interval which is very close to the Ec value of the CR glass obtained in this study. 5. Conclusions From the experimental results the following conclusions can be drawn: (1) DTA results showed that the crystallization peak temperatures increased with the increase in particle size. (2) The crystallization behavior of glass samples has been investigated under non-isothermal conditions. Using the Ozawa equation, the Avrami constants (n) were calculated as 3.42 ± 0.21 for coarse CR glasses and 3.68 ± 0.18 for coarse ORS glasses, indicating that bulk nucleation occurs in both glasses by three-dimensional growth. n values of fine samples showed that the crystallization mechanism is one-dimensional surface crystallization. (3) The crystallization activation energies of coarse glasses are in the range of 233–439 kJ/mol while the activation energies of fine glasses change in the range of 369–450 kJ/mol. The activation energy values for crystallization increased with the decrease in particle size of the waste glasses. (4) XRD results revealed that the CR and ORS glass-ceramic samples produced from the different mixed wastes which had similar chemical compositions have the same crystalline phase determined as the diopside phase. (5) SEM investigations revealed that crystallites were uniformly dispersed in the microstructure for both glass-ceramic samples. The cross-sectional SEM investigations on the bulk CR and ORS glass-ceramic samples indicated that this result is in well agreement with those determined by the Ozawa equation.

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