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Influence of rare earth oxides on the non-isothermal crystallization of phosphosilicate melts during cooling
Journal of Non-Crystalline Solids 385 (2014) 75–80
Contents lists available at ScienceDirect
Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/ locate/ jnoncrysol
Influence of rare earth oxides on the non-isothermal crystallization of phosphosilicate melts during cooling Shujiang Liu a,⁎, Zhitao Shan a, Guizhen Fu a, Yuanzheng Yue a,b,⁎⁎ a b
Key Laboratory of Processing and Testing Technology of Glass & Functional Ceramics of Shandong Province, Qilu University of Technology, Jinan 250353, China Section of Chemistry, Aalborg University, Aalborg DK-9000, Denmark
a r t i c l e
i n f o
Article history: Received 9 August 2013 Received in revised form 25 October 2013 Available online 23 November 2013 Keywords: Phosphosilicate glasses; Transparent glass ceramics; Melt-crystallization kinetics
a b s t r a c t We report a detailed calorimetric study concerning the influence of Yb2O3 and Er2O3 on the non-isothermal crystallization in phosphosilicate melts. The results show that Yb3+/Er3+ ions promote the Zn2SiO4 crystal formation, but suppress the Na3PO4 and AlPO4 formation during cooling. The non-isothermal melt-crystallization kinetics can be well described by the Avrami model. The activation energy Ee of crystallization in both the undoped and Yb3+/Er3+ codoped samples during cooling is determined using the differential iso-conversional method of Friedman. The Ee value decreases with crystallinity (θ = 0.1 to 0.9) during cooling for both glasses, and introducing Yb2O3 and Er2O3 leads to an increase in energy barrier of crystallization upon cooling. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Transparent glass ceramics (TGCs) doped with rare earth (RE) have shown wide applications such as optical data storage, laser and optical amplifier due to their high transparency [1–3]. The transparency depends on several factors such as the type, size and fraction of crystals, and the difference in refractive index between crystalline phases and residual glass. These factors are determined by the preparation method and crystal-formation route. Post heat-treatment is the most popular way for preparing TGCs. However, we have recently developed another approach for preparing TGCs from phosphosilicate system, i.e., the melt–cooling–devitrification (MCD) approach [4]. The key of this approach is to control the cooling rate of melt in such way that the nano-sized crystals can spontaneously form and thereby the high transparency of the glass ceramics can be achieved. To achieve the optimum effect of the MCD it is important to understand the crystallization kinetics in melt. The kinetics can be studied by means of differential scanning calorimetry (DSC) under either isothermal or non-isothermal (dynamic heating) conditions [5–8]. However, the non-isothermal one is relatively fast compared to the isothermal method. In the present work, we apply DSC to study how Yb2O3 and Er2O3 affects spontaneous crystallization behavior of phosphosilicate glass melts under non-isothermal condition, i.e., during cooling. The kinetics of the melt-crystallization process will be analyzed based on the Avrami model. The activation energy for non-isothermal melt-
⁎ Corresponding author. Tel.: +86 89631232; fax: +86 89631226. ⁎⁎ Correspondence to: Y. Yue, Key Laboratory of Processing and Testing Technology of Glass & Functional Ceramics of Shandong Province, Qilu University of Technology, Jinan 250353, China. Tel.: +86 89631232; fax: +86 89631226. E-mail addresses: [email protected] (S. Liu), [email protected] (Y. Yue). 0022-3093/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jnoncrysol.2013.11.009
crystallization process will be evaluated using the differential isoconversional method of Friedman. 2. Experimental A phosphosilicate glass system studied in this work has the following composition (mol%): 60SiO2, 3Al2O3, 3.5P2O5, 16ZnO, 17.5Na2O, with additional 0.5 mol% Yb2O3 and 0.125 mol% Er2O3. The mixture of reagent grade chemicals (SiO2, Al2O3, (NH4)2HPO4, ZnO, and Na2CO3) was preheated at 673 K for 12 h and at 1273 K for 2 h to remove NH3 and CO2, and then completely melted in a lidded platinum crucible placed in an electric furnace at 1773 K for 2 h. Subsequently, homogeneous melt was poured onto a stainless steel mold to obtain bulk sample with a thickness of 7.0 mm and naturally cooled to room temperature in air. For comparison, a sample without Yb2O3/Er2O3 was also prepared using the same method. In order to identify the crystalline phases in the samples, X-ray powder diffraction (XRD) measurements were performed using an X-ray diffractometer (BRUKER AXS D8-Advance) with graphite monochromatized Cu Kα1 radiation. A field emission scanning electron microscope (FE-SEM) (FEI Quanta 200) was used to observe the morphology of the samples. The FE-SEM measurements were made on fresh fracture surfaces of the bulk samples, which had been etched in 5 wt.% HNO3 for 15 s. To compare the characteristic temperatures such as the glass transition temperatures Tg, the crystallization peak temperatures Tp and liquidus temperatures TL of the undoped and RE-doped glasses, we prepared amorphous samples for isobaric heat capacity (Cp) measurements. These amorphous samples were obtained by a fast quenching process in which the melts were poured into water. The Cp measurements were performed in a platinum crucible in N2 (see the experimental details
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Fig. 1. The heat capacity curves of fast quenched undoped (a) and Yb3+/Er3+ codoped (b) samples. Insets: Cp curve during DSC down-scan that follows the DSC up-scan. The heating and cooling rates are 10 K min−1.
in ref. [4]). To study the non-isothermal melt-crystallization behavior of both glasses, differential scanning calorimetry (DSC, Setaram Labsys evo, France) was conducted on five samples of each glass. Each of the samples was individually upscanned in the DSC from 333 to 1623 K at 20 K min−1. To ensure complete melting before cooling, each of them was kept at 1623 K for 20 min and then cooled to room temperature at 20, 30, 40, 50 to 60 K min−1, respectively.
Fig. 2. XRD patterns of undoped and Yb3+/Er3+ codoped bulk samples obtained by casting method (a) and iso-thermally heat-treated fast quenched samples (b). Inset: Optical images of undoped (left) and RE-doped (right) bulk samples. The thickness of the samples is around 7.0 mm.
observed in a DSC thermogram for the non-isothermal crystallization data can be transformed into the time domain. The Avrami model is widely used for describing not only the isothermal [9–11], but also the non-isothermal crystallization behavior of
3. Theoretical background In DSC, the energy released during a non-isothermal crystallization process is a function of temperature. The temperature dependence of the crystallinity of vol.%, θ(T), can be formulated as Z θðT Þ ¼
T To
ðdHc =dT ÞdT ΔH c
ð1Þ
where To and T represent the onset for crystallization and a given temperature, respectively. dHc is the enthalpy of crystallization released in the temperature range of dT, and ΔHc is the total enthalpy of crystallization for a specific cooling condition. To use Eq. (1) in analyzing non-isothermal crystallization data obtained by DSC, the relation between the crystallization time t and the temperature T can be written as t¼
T r −T α
ð2Þ
where Tr is an arbitrary reference temperature and α is the cooling rate of the glass melt. According to Eq. (2), the horizontal temperature axis
Fig. 3. The DSC cooling curves exhibiting the non-isothermal melt-crystallization peaks of undoped (a) and Yb3+/Er3+ codoped (b) phosphosilicate glasses for different cooling rates, ranging from 20 to 60 K min−1.
S. Liu et al. / Journal of Non-Crystalline Solids 385 (2014) 75–80
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Table 1 Characteristic values for the non-isothermal melt-crystallization for both undoped and Yb3+/Er3+ codoped samples, respectively. α (K min−1)
Undoped sample
60 50 40 30 20
RE-doped sample
Δtinc (min)
Tonset (K)
Tpeak (K)
Toffset (K)
Δtinc (min)
Tonset (K)
Tpeak (K)
Toffset (K)
3.20 3.36 3.80 4.68 5.28
1071 1095 1111 1122 1157
1047 1069 1083 1096 1116
957 970 983 1015 1070
4.28 4.86 5.85 7.47 10.65
1106 1120 1129 1139 1150
1071 1086 1093 1100 1102
1004 1013 1029 1035 1048
± ± ± ± ±
0.03 0.02 0.05 0.05 0.03
± ± ± ± ±
2 1 2 2 1
± ± ± ± ±
1 1 1 2 1
glasses [12,13]. The time dependence of the crystallinity, θ(t), can be expressed as
± ± ± ± ±
3 1 2 1 2
± ± ± ± ±
0.01 0.04 0.02 0.04 0.05
± ± ± ± ±
1 2 1 1 1
± ± ± ± ±
1 1 1 1 2
± ± ± ± ±
1 2 2 1 2
4. Results and discussion 4.1. Crystalline phases formed during cooling
n θðt Þ ¼ 1− exp −ðKt Þ
ð3Þ
where n is the Avrami exponent associated with the crystallization mechanism, K is the overall rate constant, which represents the temperature dependence of the overall crystallization rate and is frequently parameterized in terms of the Arrhenius equation. E K ¼ A exp − RT
ð4Þ
where R is the gas constant, A is the pre-exponential factor, and E is the activation energy. In particular, Eq. (4) is the basis of the Kissinger method [14] for evaluating the effective activation energy of nonisothermal crystallization. However, the Kissinger method provides invalid results in some cases, e.g., melt-crystallization during cooling [15]. In contrast, the differential iso-conversional method of Friedman [16] or the iso-conversional method of Vyazovkin [15] can give reliable values of the effective energy barrier for non-isothermal crystallization in melts during cooling. In the present work, we use the Friedman method and the equation:
ln ½Φðt Þ ¼ A−
Ee RT
ð5Þ
where Φ(t) is the instantaneous crystallization rate for a given crystallinity θ, A is an arbitrary pre-exponential parameter, and Ee is the effective energy barrier of the process for a given θ.
The crystallization tendency of both fast quenched undoped and REdoped glasses is reflected in the Cp curves as shown in Fig. 1. The exothermic crystallization peaks occur near glass transition in both samples, i.e., at 873 K and 916 K in undoped sample, in contrast, only one crystallization peak occurs in RE-doped sample at 895 K. This indicates that the addition of RE oxides leads to precipitation of different crystalline types compared to the undoped sample. The insets of Fig. 1 show the Cp curves of the undoped and RE-doped samples during downscan, where a sharp endothermic peak is seen at 800 K in undoped sample, and the endothermic peak could be related to polymorphic transformation [17]. At the high temperature region, the endothermic melting peaks are observed, from which the TL is determined to be 1263 K and 1363 K for undoped and RE-doped samples, respectively. Relative transparency is seen for both the undoped and RE-doped bulk samples, both of which were obtained by casting (see the inset of Fig. 2), implying that Yb2O3 and Er2O3 oxides are related to the increases in transparency. XRD patterns of both bulk samples are shown in Fig. 2(a) and sharp diffraction peaks can be observed for both samples. However, the difference in the identity of crystal types between the RE-doped and undoped samples is pronounced, indicating that RE oxide has a strong chemical impact on the crystallization. The main crystalline phases in the undoped sample are identified to be Na3PO4 and AlPO4, whereas the main phase in the Yb2O3 and Er2O3 doped sample is Zn2SiO4. This could be attributed to the fact that RE ions tend to be associated with Al\O− forming RE\O\Al bonds in RE-doped glass melt [18]. On the other hand, it is likely that RE ions are associated with P\O− bonds instead of Al3 + ions as a next nearest neighbor. Therefore, the precipitation of AlPO4 and Na3PO4 is hindered due to the decreased mobility of Al3+ and Na+ ions during cooling. Fig. 2(b) shows the XRD patterns of fast quenched samples, which were isothermally heat-treated at Tp for 3 h. It can be found that
Fig. 4. Relative crystallinity as a function of time of undoped (a) and Yb3+/Er3+ codoped (b) samples observed for different cooling rates, ranging from 20 to 60 K min−1. The raw experimental data are shown as various geometrical points; whereas the model predictions based on the Avrami model are shown as dashed lines.
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Table 2 Parameters of the non-isothermal melt-crystallization kinetics for both undoped and Yb3+/Er3+ codoped samples, respectively, based on the model fits of the Avrami equation. α (K min−1)
60 50 40 30 20
Undoped sample
RE-doped sample
K
n
r2
K
n
r2
1.26 1.11 0.84 0.65 0.40
1.49 1.63 1.66 1.71 1.98
0.9993 0.9998 0.9997 0.9996 0.9989
1.20 1.12 0.92 0.61 0.37
2.12 2.10 2.02 2.06 2.13
0.9997 0.9996 0.9993 0.9988 0.9996
Note: Model parameter values are reported for independent fits of each cooling rate α for both samples. The correlation factor r2 describing the fitting quality is also reported.
isothermally heat-treated samples have the same crystalline types as those of bulk samples cooled in air. In other words, thermal history has little effect on crystalline types for the glass compositions studied in this work. The comparison of the two undoped samples suggests that the heat treatment at lower temperatures leads to formation of Na3PO4 as the main crystalline phase, whereas the heat treatment at higher temperature leads to formation of AlPO4 as the main crystalline phase. In addition, the diffraction peaks of isothermally heat-treated samples have unexpectedly lower intensities than those of bulk samples, although the heat-treatment duration for the former is greatly longer than that for the bulk samples. Therefore, the MCD approach for preparing the TGCs from phosphosilicate systems is more effective than the conventional post heat-treatment on parent glasses. 4.2. Non-isothermal crystallization Fig. 3 shows the DSC downscan curves of both the undoped and REdoped glasses cooled at 20, 30, 40, 50 to 60 K min−1, respectively. It is seen that there are two crystallization peaks for the undoped glass for all the cooling rates. According to the Cp and XRD results, Na3PO4 and AlPO4 crystals are associated with the low temperature and the high temperature peaks, respectively. In contrast, there exists a single crystallization peak for all five cooling rates in RE-doped glasses, meaning that only Zn2SiO4 crystals precipitate during cooling. The temperature dependence of the crystallinity can be determined from the crystallization peaks in Fig. 3 by using Eq. (1). The onset and offset temperatures for the crystallization of the non-isothermal meltcrystallization processes, Tonset and Toffset, respectively, are obtained at the crystallinities of 1% and 99%, respectively. The Tonset and Toffset values, and the crystallization peak temperature Tpeak, for both glasses are listed in Table 1. Apparently, the Tonset, Tpeak, and Toffset values are all shifted towards higher temperatures with decreasing the cooling rate α. In addition, apparent incubation period Δtinc is also listed in
Table 1, which is defined as a time period during which the glass is still in its molten state, viz. Δtinc = (TL − Tonset) / α, where TL is the liquidus temperature. The Δtinc value is found to monotonically increase with decreasing the cooling rate. This could be ascribed to the fact that the undercooling degree, ΔT, is inversely proportional to the energy barrier of nucleation. A higher cooling rate corresponds to a larger ΔT, and hence Δtinc becomes shorter. The crystallinity data can be further analyzed by converting the temperature scale of the θ(T) curves into the time scale, using Eq. (2). Thus the time dependence of the crystallinity is obtained, and the converted θ(t) curves for both glasses are illustrated in Fig. 4. Both the Avrami crystallization rate constant K and the Avrami exponent n are obtained by fitting the θ(t) data to Eq. (3) as shown in Table 2. For each cooling rate α of both samples, the fitting error of parameters is determined using the standard deviation method, and it is estimated to be less than 0.01 for K value and 0.03 for n value, respectively. From the correlation coefficient r2 values, the Avrami model describes the nonisothermal melt-crystallization data of both glasses very well. However, for all the cooling rates, the n value is found to be smaller for the undoped glass than for the RE-doped glass, implying that introduction of RE oxides might tend to change the crystallization mechanism from two-dimensional to three-dimensional growth of crystals in the melts upon cooling [19,20]. In order to quantify the kinetics of the non-isothermal meltcrystallization process, the crystallization time at an arbitrary crystallinity (i.e. tθ) is determined from the θ(t) curves. For the crystallinity values of θ = 0.2, 0.4, 0.6 and 0.8, ln(tθ) (after exclusion of the Δtinc value) is plotted in Fig. 5 against the cooling rate. Clearly, the ln(tθ) value for a given value of θ is found to decrease with increasing cooling rate, suggesting that non-isothermal melt-crystallization proceeds faster with increasing cooling rate. In an attempt to further analyze the relation between Δtinc and cooling rate, ln(Δtinc) is plotted against ln(α) in the inset of Fig. 5, which exhibits a linear relationship for both glasses. Thus, we can use this relation to estimate the Δtinc values. According to our previous work, the cooling rate is about 160 K s−1 when casting the glass melts onto a steel mold [21]. This cooling rate is used to estimate the incubation periods for both glasses, and the Δtinc values are calculated to be 0.26 min for undoped glass and 0.06 min for REdoped glass, respectively. The latter value is obviously much smaller than the former one, indicating that RE oxides can greatly enhance the spontaneous crystallization in melts during cooling. To apply the differential iso-conversional method of Friedman, the θ(T) function should be converted into the θ(t) function. The latter is then differentiated with respect to t to obtain the instantaneous crystallization rate Φ(t). According to Eq. (5), Φ(t) can then be plotted at various crystallinities by using the data obtained from both θ(t) and θ(T)
Fig. 5. Relationship between the crystallization time at various crystallinities and the cooling rate for both undoped (a) and Yb3+/Er3+ codoped (b) glasses. Inset: Apparent incubation period as a function of cooling rate.
S. Liu et al. / Journal of Non-Crystalline Solids 385 (2014) 75–80
functions. Finally the effective energy barrier Ee for non-isothermal melt-crystallization for a given crystallinity can be determined from the slope of the plot. As an example, the determination of Ee at θ = 0.1 for both the undoped and RE-doped glasses is shown in the inset of Fig. 6, where ln(Φ(t)) is plotted against 10,000 / T at θ = 0.1. The Ee values for various crystallinities (θ = 0.1 to 0.9) are listed in Table 3 and also plotted in Fig. 6 to illustrate the dependence of Ee on crystallinity. The error bar represents the standard deviation of the data, calculated from the kinetic data at different cooling rates. It is seen that Ee linearly decreases with the crystallinity as shown in Fig. 6. In addition, the Ee of RE-doped glass is greatly larger than that of undoped one for all crystallinity values, i.e., the melt-crystallization effective energy barrier of silicate is larger than that of phosphate.
79
Table 3 Effective energy barriers Ee for the non-isothermal melt-crystallization of undoped and Yb3+/Er3+ codoped samples, based on the linear fits of Friedman equation for various crystallinities. Crystallinity θ
Undoped sample −1
Ee (kJ mol 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
86.5 83.3 80.9 74.8 69.4 62.4 59.1 49.3 47.8
)
RE-doped sample r
Ee (kJ mol−1)
r2
0.963 0.975 0.968 0.953 0.944 0.934 0.946 0.956 0.926
289.6 275.9 270.1 266.4 255.3 237.1 229.2 210.2 201.0
0.933 0.922 0.928 0.893 0.906 0.895 0.916 0.931 0.896
2
4.3. Micromorphology of bulk samples Fig. 7(a) and (b) shows the SEM micrographs of the fracture surface of both bulk samples. It is seen that the volume fraction of crystallite in the RE-doped sample is larger than that in the undoped sample by comparing crystal number in unit area for both samples. Many dense spherical pits exist in the residual glassy phases for both samples. This indicates that the phase separation (phosphate and silicate-rich phase) takes place during cooling of both glass melts, and erosion of phosphate-rich phase by H3NO3 leads to formation of pits in the residual glass matrix. The phase-separation plays a critical role in spontaneous crystallization, i.e., enhances heterogeneous nucleation and subsequent crystalgrowth [22,23]. The difference in crystallite concentration between both bulk samples suggests that RE oxides promote the migration of elements involving in Zn2SiO4 crystal. This is related to the fact that RE-doped glass has the smaller Δtinc value compared to undoped one. There are three phases in both samples according to the SEM images (Fig. 7), i.e., crystalline phase, separated phosphate- and silicate-rich glass phases. Both the crystallite and the separated phosphate-rich glass phases determine the transparency of the samples via the scattering light with visible wavelengths [24,25]. According to the scattering theory [26], if the sizes of particles are much less than the typical wavelength of the visible light, only Rayleigh scattering takes place and the scattered intensity depends mainly on the ratio of the particle-size to the wavelength of light. In this work, the size of crystallite and separated phosphate-rich glass phase is smaller than the minimum wavelength of visible light; this ensures the high transparency of both bulk samples.
Fig. 6. Effective activation energy for crystallization at various crystallinities for both undoped and Yb3+/Er3+ codoped glasses. Inset: determination of effective activation energy for crystallization at θ = 0.1 according to Eq. (5).
5. Conclusions The Yb3+/Er3+ codoped transparent glass ceramics with good transparency were obtained using MCD method. Non-isothermal meltcrystallization behavior for undoped and RE-doped phosphosilicate glasses was investigated using DSC. Both the crystallization time and the apparent incubation period are a function of cooling rate. Introduction of Yb2O3 and Er2O3 into the glass composition greatly influences spontaneous crystallization behavior, e.g. crystal type, fraction and energy barrier for crystallization. Separation of phosphate and silicaterich glassy phases is an important step for melt-crystallization, which enhances heterogeneous nucleation and subsequent crystal growth.
Fig. 7. SEM images of both the undoped (a) and Yb3+/Er3+ codoped (b) bulk samples obtained by the casting method.
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Contents lists available at ScienceDirect
Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/ locate/ jnoncrysol
Influence of rare earth oxides on the non-isothermal crystallization of phosphosilicate melts during cooling Shujiang Liu a,⁎, Zhitao Shan a, Guizhen Fu a, Yuanzheng Yue a,b,⁎⁎ a b
Key Laboratory of Processing and Testing Technology of Glass & Functional Ceramics of Shandong Province, Qilu University of Technology, Jinan 250353, China Section of Chemistry, Aalborg University, Aalborg DK-9000, Denmark
a r t i c l e
i n f o
Article history: Received 9 August 2013 Received in revised form 25 October 2013 Available online 23 November 2013 Keywords: Phosphosilicate glasses; Transparent glass ceramics; Melt-crystallization kinetics
a b s t r a c t We report a detailed calorimetric study concerning the influence of Yb2O3 and Er2O3 on the non-isothermal crystallization in phosphosilicate melts. The results show that Yb3+/Er3+ ions promote the Zn2SiO4 crystal formation, but suppress the Na3PO4 and AlPO4 formation during cooling. The non-isothermal melt-crystallization kinetics can be well described by the Avrami model. The activation energy Ee of crystallization in both the undoped and Yb3+/Er3+ codoped samples during cooling is determined using the differential iso-conversional method of Friedman. The Ee value decreases with crystallinity (θ = 0.1 to 0.9) during cooling for both glasses, and introducing Yb2O3 and Er2O3 leads to an increase in energy barrier of crystallization upon cooling. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Transparent glass ceramics (TGCs) doped with rare earth (RE) have shown wide applications such as optical data storage, laser and optical amplifier due to their high transparency [1–3]. The transparency depends on several factors such as the type, size and fraction of crystals, and the difference in refractive index between crystalline phases and residual glass. These factors are determined by the preparation method and crystal-formation route. Post heat-treatment is the most popular way for preparing TGCs. However, we have recently developed another approach for preparing TGCs from phosphosilicate system, i.e., the melt–cooling–devitrification (MCD) approach [4]. The key of this approach is to control the cooling rate of melt in such way that the nano-sized crystals can spontaneously form and thereby the high transparency of the glass ceramics can be achieved. To achieve the optimum effect of the MCD it is important to understand the crystallization kinetics in melt. The kinetics can be studied by means of differential scanning calorimetry (DSC) under either isothermal or non-isothermal (dynamic heating) conditions [5–8]. However, the non-isothermal one is relatively fast compared to the isothermal method. In the present work, we apply DSC to study how Yb2O3 and Er2O3 affects spontaneous crystallization behavior of phosphosilicate glass melts under non-isothermal condition, i.e., during cooling. The kinetics of the melt-crystallization process will be analyzed based on the Avrami model. The activation energy for non-isothermal melt-
⁎ Corresponding author. Tel.: +86 89631232; fax: +86 89631226. ⁎⁎ Correspondence to: Y. Yue, Key Laboratory of Processing and Testing Technology of Glass & Functional Ceramics of Shandong Province, Qilu University of Technology, Jinan 250353, China. Tel.: +86 89631232; fax: +86 89631226. E-mail addresses: [email protected] (S. Liu), [email protected] (Y. Yue). 0022-3093/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jnoncrysol.2013.11.009
crystallization process will be evaluated using the differential isoconversional method of Friedman. 2. Experimental A phosphosilicate glass system studied in this work has the following composition (mol%): 60SiO2, 3Al2O3, 3.5P2O5, 16ZnO, 17.5Na2O, with additional 0.5 mol% Yb2O3 and 0.125 mol% Er2O3. The mixture of reagent grade chemicals (SiO2, Al2O3, (NH4)2HPO4, ZnO, and Na2CO3) was preheated at 673 K for 12 h and at 1273 K for 2 h to remove NH3 and CO2, and then completely melted in a lidded platinum crucible placed in an electric furnace at 1773 K for 2 h. Subsequently, homogeneous melt was poured onto a stainless steel mold to obtain bulk sample with a thickness of 7.0 mm and naturally cooled to room temperature in air. For comparison, a sample without Yb2O3/Er2O3 was also prepared using the same method. In order to identify the crystalline phases in the samples, X-ray powder diffraction (XRD) measurements were performed using an X-ray diffractometer (BRUKER AXS D8-Advance) with graphite monochromatized Cu Kα1 radiation. A field emission scanning electron microscope (FE-SEM) (FEI Quanta 200) was used to observe the morphology of the samples. The FE-SEM measurements were made on fresh fracture surfaces of the bulk samples, which had been etched in 5 wt.% HNO3 for 15 s. To compare the characteristic temperatures such as the glass transition temperatures Tg, the crystallization peak temperatures Tp and liquidus temperatures TL of the undoped and RE-doped glasses, we prepared amorphous samples for isobaric heat capacity (Cp) measurements. These amorphous samples were obtained by a fast quenching process in which the melts were poured into water. The Cp measurements were performed in a platinum crucible in N2 (see the experimental details
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Fig. 1. The heat capacity curves of fast quenched undoped (a) and Yb3+/Er3+ codoped (b) samples. Insets: Cp curve during DSC down-scan that follows the DSC up-scan. The heating and cooling rates are 10 K min−1.
in ref. [4]). To study the non-isothermal melt-crystallization behavior of both glasses, differential scanning calorimetry (DSC, Setaram Labsys evo, France) was conducted on five samples of each glass. Each of the samples was individually upscanned in the DSC from 333 to 1623 K at 20 K min−1. To ensure complete melting before cooling, each of them was kept at 1623 K for 20 min and then cooled to room temperature at 20, 30, 40, 50 to 60 K min−1, respectively.
Fig. 2. XRD patterns of undoped and Yb3+/Er3+ codoped bulk samples obtained by casting method (a) and iso-thermally heat-treated fast quenched samples (b). Inset: Optical images of undoped (left) and RE-doped (right) bulk samples. The thickness of the samples is around 7.0 mm.
observed in a DSC thermogram for the non-isothermal crystallization data can be transformed into the time domain. The Avrami model is widely used for describing not only the isothermal [9–11], but also the non-isothermal crystallization behavior of
3. Theoretical background In DSC, the energy released during a non-isothermal crystallization process is a function of temperature. The temperature dependence of the crystallinity of vol.%, θ(T), can be formulated as Z θðT Þ ¼
T To
ðdHc =dT ÞdT ΔH c
ð1Þ
where To and T represent the onset for crystallization and a given temperature, respectively. dHc is the enthalpy of crystallization released in the temperature range of dT, and ΔHc is the total enthalpy of crystallization for a specific cooling condition. To use Eq. (1) in analyzing non-isothermal crystallization data obtained by DSC, the relation between the crystallization time t and the temperature T can be written as t¼
T r −T α
ð2Þ
where Tr is an arbitrary reference temperature and α is the cooling rate of the glass melt. According to Eq. (2), the horizontal temperature axis
Fig. 3. The DSC cooling curves exhibiting the non-isothermal melt-crystallization peaks of undoped (a) and Yb3+/Er3+ codoped (b) phosphosilicate glasses for different cooling rates, ranging from 20 to 60 K min−1.
S. Liu et al. / Journal of Non-Crystalline Solids 385 (2014) 75–80
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Table 1 Characteristic values for the non-isothermal melt-crystallization for both undoped and Yb3+/Er3+ codoped samples, respectively. α (K min−1)
Undoped sample
60 50 40 30 20
RE-doped sample
Δtinc (min)
Tonset (K)
Tpeak (K)
Toffset (K)
Δtinc (min)
Tonset (K)
Tpeak (K)
Toffset (K)
3.20 3.36 3.80 4.68 5.28
1071 1095 1111 1122 1157
1047 1069 1083 1096 1116
957 970 983 1015 1070
4.28 4.86 5.85 7.47 10.65
1106 1120 1129 1139 1150
1071 1086 1093 1100 1102
1004 1013 1029 1035 1048
± ± ± ± ±
0.03 0.02 0.05 0.05 0.03
± ± ± ± ±
2 1 2 2 1
± ± ± ± ±
1 1 1 2 1
glasses [12,13]. The time dependence of the crystallinity, θ(t), can be expressed as
± ± ± ± ±
3 1 2 1 2
± ± ± ± ±
0.01 0.04 0.02 0.04 0.05
± ± ± ± ±
1 2 1 1 1
± ± ± ± ±
1 1 1 1 2
± ± ± ± ±
1 2 2 1 2
4. Results and discussion 4.1. Crystalline phases formed during cooling
n θðt Þ ¼ 1− exp −ðKt Þ
ð3Þ
where n is the Avrami exponent associated with the crystallization mechanism, K is the overall rate constant, which represents the temperature dependence of the overall crystallization rate and is frequently parameterized in terms of the Arrhenius equation. E K ¼ A exp − RT
ð4Þ
where R is the gas constant, A is the pre-exponential factor, and E is the activation energy. In particular, Eq. (4) is the basis of the Kissinger method [14] for evaluating the effective activation energy of nonisothermal crystallization. However, the Kissinger method provides invalid results in some cases, e.g., melt-crystallization during cooling [15]. In contrast, the differential iso-conversional method of Friedman [16] or the iso-conversional method of Vyazovkin [15] can give reliable values of the effective energy barrier for non-isothermal crystallization in melts during cooling. In the present work, we use the Friedman method and the equation:
ln ½Φðt Þ ¼ A−
Ee RT
ð5Þ
where Φ(t) is the instantaneous crystallization rate for a given crystallinity θ, A is an arbitrary pre-exponential parameter, and Ee is the effective energy barrier of the process for a given θ.
The crystallization tendency of both fast quenched undoped and REdoped glasses is reflected in the Cp curves as shown in Fig. 1. The exothermic crystallization peaks occur near glass transition in both samples, i.e., at 873 K and 916 K in undoped sample, in contrast, only one crystallization peak occurs in RE-doped sample at 895 K. This indicates that the addition of RE oxides leads to precipitation of different crystalline types compared to the undoped sample. The insets of Fig. 1 show the Cp curves of the undoped and RE-doped samples during downscan, where a sharp endothermic peak is seen at 800 K in undoped sample, and the endothermic peak could be related to polymorphic transformation [17]. At the high temperature region, the endothermic melting peaks are observed, from which the TL is determined to be 1263 K and 1363 K for undoped and RE-doped samples, respectively. Relative transparency is seen for both the undoped and RE-doped bulk samples, both of which were obtained by casting (see the inset of Fig. 2), implying that Yb2O3 and Er2O3 oxides are related to the increases in transparency. XRD patterns of both bulk samples are shown in Fig. 2(a) and sharp diffraction peaks can be observed for both samples. However, the difference in the identity of crystal types between the RE-doped and undoped samples is pronounced, indicating that RE oxide has a strong chemical impact on the crystallization. The main crystalline phases in the undoped sample are identified to be Na3PO4 and AlPO4, whereas the main phase in the Yb2O3 and Er2O3 doped sample is Zn2SiO4. This could be attributed to the fact that RE ions tend to be associated with Al\O− forming RE\O\Al bonds in RE-doped glass melt [18]. On the other hand, it is likely that RE ions are associated with P\O− bonds instead of Al3 + ions as a next nearest neighbor. Therefore, the precipitation of AlPO4 and Na3PO4 is hindered due to the decreased mobility of Al3+ and Na+ ions during cooling. Fig. 2(b) shows the XRD patterns of fast quenched samples, which were isothermally heat-treated at Tp for 3 h. It can be found that
Fig. 4. Relative crystallinity as a function of time of undoped (a) and Yb3+/Er3+ codoped (b) samples observed for different cooling rates, ranging from 20 to 60 K min−1. The raw experimental data are shown as various geometrical points; whereas the model predictions based on the Avrami model are shown as dashed lines.
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Table 2 Parameters of the non-isothermal melt-crystallization kinetics for both undoped and Yb3+/Er3+ codoped samples, respectively, based on the model fits of the Avrami equation. α (K min−1)
60 50 40 30 20
Undoped sample
RE-doped sample
K
n
r2
K
n
r2
1.26 1.11 0.84 0.65 0.40
1.49 1.63 1.66 1.71 1.98
0.9993 0.9998 0.9997 0.9996 0.9989
1.20 1.12 0.92 0.61 0.37
2.12 2.10 2.02 2.06 2.13
0.9997 0.9996 0.9993 0.9988 0.9996
Note: Model parameter values are reported for independent fits of each cooling rate α for both samples. The correlation factor r2 describing the fitting quality is also reported.
isothermally heat-treated samples have the same crystalline types as those of bulk samples cooled in air. In other words, thermal history has little effect on crystalline types for the glass compositions studied in this work. The comparison of the two undoped samples suggests that the heat treatment at lower temperatures leads to formation of Na3PO4 as the main crystalline phase, whereas the heat treatment at higher temperature leads to formation of AlPO4 as the main crystalline phase. In addition, the diffraction peaks of isothermally heat-treated samples have unexpectedly lower intensities than those of bulk samples, although the heat-treatment duration for the former is greatly longer than that for the bulk samples. Therefore, the MCD approach for preparing the TGCs from phosphosilicate systems is more effective than the conventional post heat-treatment on parent glasses. 4.2. Non-isothermal crystallization Fig. 3 shows the DSC downscan curves of both the undoped and REdoped glasses cooled at 20, 30, 40, 50 to 60 K min−1, respectively. It is seen that there are two crystallization peaks for the undoped glass for all the cooling rates. According to the Cp and XRD results, Na3PO4 and AlPO4 crystals are associated with the low temperature and the high temperature peaks, respectively. In contrast, there exists a single crystallization peak for all five cooling rates in RE-doped glasses, meaning that only Zn2SiO4 crystals precipitate during cooling. The temperature dependence of the crystallinity can be determined from the crystallization peaks in Fig. 3 by using Eq. (1). The onset and offset temperatures for the crystallization of the non-isothermal meltcrystallization processes, Tonset and Toffset, respectively, are obtained at the crystallinities of 1% and 99%, respectively. The Tonset and Toffset values, and the crystallization peak temperature Tpeak, for both glasses are listed in Table 1. Apparently, the Tonset, Tpeak, and Toffset values are all shifted towards higher temperatures with decreasing the cooling rate α. In addition, apparent incubation period Δtinc is also listed in
Table 1, which is defined as a time period during which the glass is still in its molten state, viz. Δtinc = (TL − Tonset) / α, where TL is the liquidus temperature. The Δtinc value is found to monotonically increase with decreasing the cooling rate. This could be ascribed to the fact that the undercooling degree, ΔT, is inversely proportional to the energy barrier of nucleation. A higher cooling rate corresponds to a larger ΔT, and hence Δtinc becomes shorter. The crystallinity data can be further analyzed by converting the temperature scale of the θ(T) curves into the time scale, using Eq. (2). Thus the time dependence of the crystallinity is obtained, and the converted θ(t) curves for both glasses are illustrated in Fig. 4. Both the Avrami crystallization rate constant K and the Avrami exponent n are obtained by fitting the θ(t) data to Eq. (3) as shown in Table 2. For each cooling rate α of both samples, the fitting error of parameters is determined using the standard deviation method, and it is estimated to be less than 0.01 for K value and 0.03 for n value, respectively. From the correlation coefficient r2 values, the Avrami model describes the nonisothermal melt-crystallization data of both glasses very well. However, for all the cooling rates, the n value is found to be smaller for the undoped glass than for the RE-doped glass, implying that introduction of RE oxides might tend to change the crystallization mechanism from two-dimensional to three-dimensional growth of crystals in the melts upon cooling [19,20]. In order to quantify the kinetics of the non-isothermal meltcrystallization process, the crystallization time at an arbitrary crystallinity (i.e. tθ) is determined from the θ(t) curves. For the crystallinity values of θ = 0.2, 0.4, 0.6 and 0.8, ln(tθ) (after exclusion of the Δtinc value) is plotted in Fig. 5 against the cooling rate. Clearly, the ln(tθ) value for a given value of θ is found to decrease with increasing cooling rate, suggesting that non-isothermal melt-crystallization proceeds faster with increasing cooling rate. In an attempt to further analyze the relation between Δtinc and cooling rate, ln(Δtinc) is plotted against ln(α) in the inset of Fig. 5, which exhibits a linear relationship for both glasses. Thus, we can use this relation to estimate the Δtinc values. According to our previous work, the cooling rate is about 160 K s−1 when casting the glass melts onto a steel mold [21]. This cooling rate is used to estimate the incubation periods for both glasses, and the Δtinc values are calculated to be 0.26 min for undoped glass and 0.06 min for REdoped glass, respectively. The latter value is obviously much smaller than the former one, indicating that RE oxides can greatly enhance the spontaneous crystallization in melts during cooling. To apply the differential iso-conversional method of Friedman, the θ(T) function should be converted into the θ(t) function. The latter is then differentiated with respect to t to obtain the instantaneous crystallization rate Φ(t). According to Eq. (5), Φ(t) can then be plotted at various crystallinities by using the data obtained from both θ(t) and θ(T)
Fig. 5. Relationship between the crystallization time at various crystallinities and the cooling rate for both undoped (a) and Yb3+/Er3+ codoped (b) glasses. Inset: Apparent incubation period as a function of cooling rate.
S. Liu et al. / Journal of Non-Crystalline Solids 385 (2014) 75–80
functions. Finally the effective energy barrier Ee for non-isothermal melt-crystallization for a given crystallinity can be determined from the slope of the plot. As an example, the determination of Ee at θ = 0.1 for both the undoped and RE-doped glasses is shown in the inset of Fig. 6, where ln(Φ(t)) is plotted against 10,000 / T at θ = 0.1. The Ee values for various crystallinities (θ = 0.1 to 0.9) are listed in Table 3 and also plotted in Fig. 6 to illustrate the dependence of Ee on crystallinity. The error bar represents the standard deviation of the data, calculated from the kinetic data at different cooling rates. It is seen that Ee linearly decreases with the crystallinity as shown in Fig. 6. In addition, the Ee of RE-doped glass is greatly larger than that of undoped one for all crystallinity values, i.e., the melt-crystallization effective energy barrier of silicate is larger than that of phosphate.
79
Table 3 Effective energy barriers Ee for the non-isothermal melt-crystallization of undoped and Yb3+/Er3+ codoped samples, based on the linear fits of Friedman equation for various crystallinities. Crystallinity θ
Undoped sample −1
Ee (kJ mol 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
86.5 83.3 80.9 74.8 69.4 62.4 59.1 49.3 47.8
)
RE-doped sample r
Ee (kJ mol−1)
r2
0.963 0.975 0.968 0.953 0.944 0.934 0.946 0.956 0.926
289.6 275.9 270.1 266.4 255.3 237.1 229.2 210.2 201.0
0.933 0.922 0.928 0.893 0.906 0.895 0.916 0.931 0.896
2
4.3. Micromorphology of bulk samples Fig. 7(a) and (b) shows the SEM micrographs of the fracture surface of both bulk samples. It is seen that the volume fraction of crystallite in the RE-doped sample is larger than that in the undoped sample by comparing crystal number in unit area for both samples. Many dense spherical pits exist in the residual glassy phases for both samples. This indicates that the phase separation (phosphate and silicate-rich phase) takes place during cooling of both glass melts, and erosion of phosphate-rich phase by H3NO3 leads to formation of pits in the residual glass matrix. The phase-separation plays a critical role in spontaneous crystallization, i.e., enhances heterogeneous nucleation and subsequent crystalgrowth [22,23]. The difference in crystallite concentration between both bulk samples suggests that RE oxides promote the migration of elements involving in Zn2SiO4 crystal. This is related to the fact that RE-doped glass has the smaller Δtinc value compared to undoped one. There are three phases in both samples according to the SEM images (Fig. 7), i.e., crystalline phase, separated phosphate- and silicate-rich glass phases. Both the crystallite and the separated phosphate-rich glass phases determine the transparency of the samples via the scattering light with visible wavelengths [24,25]. According to the scattering theory [26], if the sizes of particles are much less than the typical wavelength of the visible light, only Rayleigh scattering takes place and the scattered intensity depends mainly on the ratio of the particle-size to the wavelength of light. In this work, the size of crystallite and separated phosphate-rich glass phase is smaller than the minimum wavelength of visible light; this ensures the high transparency of both bulk samples.
Fig. 6. Effective activation energy for crystallization at various crystallinities for both undoped and Yb3+/Er3+ codoped glasses. Inset: determination of effective activation energy for crystallization at θ = 0.1 according to Eq. (5).
5. Conclusions The Yb3+/Er3+ codoped transparent glass ceramics with good transparency were obtained using MCD method. Non-isothermal meltcrystallization behavior for undoped and RE-doped phosphosilicate glasses was investigated using DSC. Both the crystallization time and the apparent incubation period are a function of cooling rate. Introduction of Yb2O3 and Er2O3 into the glass composition greatly influences spontaneous crystallization behavior, e.g. crystal type, fraction and energy barrier for crystallization. Separation of phosphate and silicaterich glassy phases is an important step for melt-crystallization, which enhances heterogeneous nucleation and subsequent crystal growth.
Fig. 7. SEM images of both the undoped (a) and Yb3+/Er3+ codoped (b) bulk samples obtained by the casting method.
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