Time-temperature-transformation diagrams for crystallization of the oxyfluoride glass system

Results in Physics 10 (2018) 356–359

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Time-temperature-transformation diagrams for crystallization of the oxyfluoride glass system M. Soleymani Zarabad, M. Rezvani

T

⁎

Department of Material Engineering, University of Tabriz, Tabriz, Iran

A B S T R A C T

Isothermal crystallization studies were performed on the glass 37.26SiO2-28.11Al2O3-7.73CaO-26.89CaF24.5K2O (wt%) in the crystallization region between the glass transition (Tg) and end of crystallization temperature (Tf), resulting in a time-temperature-transformation (T.T.T.) diagram for crystallization. The crystallized fraction calculated by “Ohlberg-Strickler” method. By fitting Avrami curves on experimental data kinetic properties of isothermal crystallization of CaF2 has been determined. It is concluded that the crystallization of the mentioned glass is a process controlled by Avrami nucleation and three-dimensional diffusion controlled growth. The (T.T.T.) ‘‘C’’ shape diagrams with a nose at 690 °C resulted in from fitted Avrami “S” shapes Diagrams.

Introduction Wang and Ohwaki [1–6] proposed oxyfluoride-based glass-ceramics consisting of fluoride nanocrystals with RE3+incorporated ions into fluoride nanocrystals. Usually, glass-ceramics are prepared using controlled heat treatment. Nucleation and growth of fluoride crystals such as pbF2, LaF3, CaF2 and etc occur in the base glass during heat treatment and RE3+ prefers to segregate into the fluoride nanocrystals [1]. Time-temperature-transformation (T.T.T.) diagrams indicate the transformation rate of equal phase transformation [7]. T.T.T. diagrams are used mostly for steels and alloys [8], metallic-glasses [9,10], glass and glass ceramics [11–15]. In the processing of glass-based materials, T.T.T. diagrams help to predict the characteristics of crystallization, state of nucleation and conversion to a certain degree of crystallinity. In general, T.T.T. diagrams are “C-shape” curves for metals and glassbased materials [10,13]. In order to propose a T.T.T. diagram, degree of crystallinity should be measured. Most conventional method for determination of crystallization percentage in glass-ceramics is the investigation of X-ray diffraction (XRD) patterns. The Ohlberg and Strickler [16] XRD based method is used especially to study the crystallization procedure on glasses with different heat treatment conditions. It is based on the XRD patterns comparison of heat treated glass–ceramic samples with totally amorphous and mechanical mixture specimens. Intensities of X-ray patternsin2θ which the nanocrystalline scattering intensity is high for fully amorphous glass and in the same time is not overlapped with the crystalline peaks of both heat-treated glass and mechanical mixture ⁎

sample. This paper proposes primary T.T.T. diagrams for the crystallization of an oxyfluoride-based glass. These T.T.T. diagrams can now be used to predict the crystallization degree of an oxyfluoride-based glass for isothermal annealing and can give suitable guidance for isothermal heating steps. In this study, we report on crystallization study of 37.26SiO2-28.11Al2O3-7.73CaO-26.89CaF2-4.5K2O (%wt). Heat treating this glass in different temperature and times then measuring the crystallinity enabled us to measure the complete T.T.T. diagrams for crystallization of parent glass. Experimental procedures 37.26SiO2-28.11Al2O3-7.73CaO-26.89CaF2-4.5K2O (wt%) glasses with some dopants were prepared using precursor powders. The composition of initial glass which is the most typical one, have been used by other scientists [17–25] has been shown in Table 1.The main starting materials are high purity leached SiO2(99% purity), Al2O3 (Merck 101077) and CaF2 (Dae Jung 2508145), CaCO3 (Merck 102069) and K2CO3 (Sigma-Aldrich P5833) were used to introduce CaO and K2O. To avoid the bubbles in the samples, Sb2O3 and As2O3 were used as refining agents. Pr2O3, Y2O3, and V2O5 were added as dopants to improve crystallization and optical properties. K2O was added to the batch to have a melt with favorable viscosity [17]. 50 g of the mixed batch in alumina crucibles was melted at 1450 °C for 1 h in an electric furnace. The prepared molten glass was poured into molds which preheated at 500 °C. Finally obtained glassy discs with 0.5 cm thickness were

Corresponding author at: Department of Materials Science and Engineering, Faculty of Mechanical Engineering, University of Tabriz, 29 Bahman Blvd., Tabriz, Iran. E-mail addresses: [email protected] (M.S. Zarabad), [email protected] (M. Rezvani).

https://doi.org/10.1016/j.rinp.2018.06.041 Received 20 May 2018; Received in revised form 17 June 2018; Accepted 18 June 2018 Available online 21 June 2018 2211-3797/ © 2018 Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).

Results in Physics 10 (2018) 356–359

M.S. Zarabad, M. Rezvani

Table 1 The composition of the initial glass (in wt.%). SiO2

Al2O3

CaF2

CaO

K2 O

Pr2O3

V2O5

Y2O3

37.26

28.11

26.89

7.73

4.5

1.5

1.5

1.5

annealed at 500 °C for 30 min to release the internal stresses due to thermal shock. In order to construct the T.T.T. diagram for glass-oxyfluoride nanocrystals, the parent glass was heat treated at temperatures of 620, 640, 660, 690 and 720 °C (between Tg and Tf) in the time range of 1 until 18 h with the step of 2 h. Differential thermal analysis of 50 mg powdered glass samples was done with the rate of 10(°C/min) by DTG-60AH Shimadzu to determine crystallization temperatures. Crystal phase analysis of glass-ceramics was carried out by X-ray diffraction (XRD, Siemens D-500). In this study X-ray diffraction used to identify the phase analysis and measure the crystallinity of mechanical mixture of glass, initial amorphous glass, and partially crystallized glass to plot T.T.T. diagrams. Results and discussion Fig. 1 shows the conventional DTA curves of 37.26SiO2-28.11Al2O37.73CaO-26.89CaF2-4.5K2O (wt%) at a heating rate of 10(°C/min). The glass transition temperature Tg, the end of the first crystallization Tf and especially the crystallization temperature of desired CaF2 phase, Tp1 is shown. The temperatures for crystallization of CaF2 nanocrystals was selected in the region of Tg to Tf (where the crystallization completes). To investigate the crystallization mechanism and plotting T.T.T. diagrams, several glass samples were heat treated at various temperatures of 620, 640, 660, 690 and 720 °C which selected based on DTA results, for various times. All the samples heat treated in selected temperatures crystallized partially. But with increasing temperature, the time needed for definite crystallized fraction decreases. In Fig. 2 XRD patterns of the two glass-ceramic samples which heat-treated at 690 °C for 6 h and 9 h, the parent glass and the mechanical mixture of the crystalline compound with chemical composition equivalent to parent glass are shown. Based on Ohlberg & Strickler model [16] the XRD pattern of the mechanical mixture is used for correction of background. The values of 2θ are selected in the regions that the scattering intensity for amorphous glass is high and is not overlapped with the crystalline peaks in both partially crystallized sample and mechanical mixture. The Ohlberg & Strickler equation which shows the crystallinity of partially crystallized is as following:

Fig. 2. XRD patterns of: (a) the parent glass (fully-amorphous sample) and partially heat-treated glasses at 690 °C for different times with their backgrounds (b) mechanical mixture of crystalline compounds.

Ig−Ix ⎞ %C = ⎜⎛ ⎟ × 100 ⎝ Ig−Ib ⎠ In this equation, Ig, Ix, and Ib are the XRD intensity scattered by the parent glass, the partially crystallized glass and a mechanical mixture of crystalline powders. It is clear from Fig. 2 that crystalline phase was formed in the expense of amorphous phase and the scattering intensity of amorphous phase proportionally was reduced because of crystallization. The calibration curve for this study was obtained using the mixture of CaF2 and the parent glass at different ratios. Parent glass, parent glass-CaF2 mixtures and pure crystalline CaF2 were used to ensure Ig, Ix and Ib intensities. The results are plotted in Fig. 3 and summarized in Table 2. It can be seen that the amounts of determined and calculated crystallinity are in good agreement with each other. The isothermal reaction kinetics of nucleation and growth in solidsolid phase transformations can be quantified using the Johnson-MehlAvrami equation [26–28], and we will also employ this description in this study. In this approach, the transformed fraction, At, for samples after a time t is given by:

At = 1−exp(−(kt )n)

(1)

where At is the transformed fraction mentioned in the, n is the Avrami exponent and k is the reaction rate constant that includes nucleation

Fig. 1. DTA curves of the oxy-fluoride glass measured at different heating rates. 357

Results in Physics 10 (2018) 356–359

M.S. Zarabad, M. Rezvani

Table 3 The Avrami kinetic parameters of n and k as a function of isothermal heat treatment temperature determined by fitting Eq. (2) to the experimental data. Heat treatment temperature (°C)

n

k

620 640 660 690 720

1.64 1.66 1.67 1.69 1.88

0.000772 0.00103 0.00135 0.00197 0.0017

The Avrami exponent can be expressed in terms of nucleation and growth parameters as below [29,30]: (3)

n = a + bm

where a is nucleation index that shows the time dependence of the number of nuclei per unit volume for parent (untransformed) phase (a = 0 for zero nucleation rate, a = 1 for the constant nucleation rate, 0 < a < 1 for decreasing and a > 1 for increasing nucleation rate respectively). b is the dimensionality of growth (b = 1, 2 or 3 for one-dimensional, two dimensional or three-dimensional growth respectively). m is a growth index which depends on the type of transformation (m = 0.5 is for a parabolic growth as in diffusion controlled growth mode and m = 1 is for linear growth as in interface controlled growth mode). The Avrami exponent, n for this oxy-fluoride system varies from 1.64 to 1.88 with the average of about 1.71 can gives b = 3, m = 0.5 and 0 < a < 1 which implies a main diffusion controlled three-dimensional growth with an Avrami decreasing nucleation rate during the crystallization which is in agreement with other researches[31]. The fitted Avrami curves were then extrapolated to establish the definite transformation times for each temperature. In this study, the temperature (T) is plotted as a function of time (t) at fixed transformation fraction. The curves in T.T.T. diagrams for the oxy-fluoride glass system exhibit a “C-type” form with respect to the temperature-time ordinates, as shown in Fig. 5. Thereby, this “C-type” curve shape is given by the increase in driving force for crystallization with decreasing temperature of the amorphous phase and the concurrent decrease in atomic mobility of the material. Thus, the nose defines the temperature Tn, the minimum cooling rate for definite phase transformation occurs.

Fig. 3. Experimentally determined crystallinity vs. calculated crystallinity for mechanical mixtures of CaF2 and parent glass. Table 2 Calculation of crystallinity using Ohlberg and Strickler’s method. Mixture

I −I

Glass

CaF2

100 70 50 30 0

0 30 50 70 100

⎛ g x ⎞ × 100 ⎝ Ig − Ib ⎠

0 31.4 49.28 68.56 100

and growth. Eq. (1) may be rewritten in a form that is more convenient for analysis as follows:

ln(−ln(1−At )) = nln (k ) + nln (t )

(2)

Fig. 4 shows the fitted Avrami curves of the samples at 620, 640, 660, 690 and 720 °C for various times. The Avrami parameters n and k were determined by fitting Eq. (2) to the experimental data. The results are summarized in Table 3. With increasing heat treating temperature from 620 to 720 °C, the kinetic parameter n slightly increases from 1.64 to 1.88 (indicating the little influence of heat treating temperature on the growth mechanism [26–28]) whereas k varies from 0.000772 to 0.00197.

Conclusion The

Fig. 4. Fitted Avrami curves on experimental data of Transformed fraction as a function of time at temperatures of at 620, 640, 660, 690 and 720 °C.

time-temperature-transformation

(T.T.T.)

diagram

of

Fig. 5. Time-temperature-transformation (TTT) diagram of the oxy-fluoride glass system derived from fitted Avrami curves. 358

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M.S. Zarabad, M. Rezvani

37.26SiO2-28.11Al2O3-7.73CaO-26.89CaF2-4.5K2O (wt%) glass system was plotted based on isothermal crystallization studies. The crystallized fraction calculated by “Ohlberg-Strickler” method. Fitting Avrami curves on experimental data resulted in determining kinetic properties and a time-temperature-transformation (T.T.T.) diagram for CaF2crystallization. It is concluded that the crystallization of the mentioned glass is a process controlled by Avrami nucleation and threedimensional diffusion controlled growth. The T.T.T. “C” shape diagrams with a nose at 690°Cresulted in from fitted Avrami “S” shape Diagrams.

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