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Microstructural evolution and crystallization mechanism of zircon from frit glaze
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ScienceDirect Journal of the European Ceramic Society 35 (2015) 2671–2678
Microstructural evolution and crystallization mechanism of zircon from frit glaze Shaohua Wang a,b , Cheng Peng a , Huiyin Xiao b , Jianqing Wu a,∗ a
School of Materials Science and Engineering, South China University of Technology, Guangzhou 510640, China b Dongguan Wonderful Ceramic Industry Co., Ltd., Dongguan 523281, China Received 28 January 2015; received in revised form 6 March 2015; accepted 11 March 2015 Available online 30 March 2015
Abstract Zircon (ZrSiO4 ) is widely used as a glaze opacifier in the ceramic industry. The crystallization mechanism of ZrSiO4 from a CaO-riched zirconiumbased frit glaze was studied by XRD, SEM, and DSC in this work. Isothermal and non-isothermal heat treatments were used to study the phase development and microstructural evolution during the firing process. It was found that t-ZrO2 and Ca2 ZrSi4 O12 were first crystallized at a relative low temperature and then converted to ZrSiO4 with the increasing of temperature. ZrSiO4 originating from t-ZrO2 has an inward growth and an outward growth, while ZrSiO4 transformed from Ca2 ZrSi4 O12 only has an outward growth. SEM showed that ZrSiO4 appeared to be a rod shape in fired samples. The calculated activation energy for ZrSiO4 is 311.2 kJ/mol, and the Avrami index (n) is 0.35–0.39. This indicates the one-dimensional diffusion-controlled growth of ZrSiO4 , which explains the formation of rod-shape ZrSiO4 crystal. © 2015 Elsevier Ltd. All rights reserved. Keywords: Zircon crystallization; Crystal growth; Activation energy; Opaque glaze
1. Introduction Zirconium glaze is the most widely used opaque glaze due to its advantages of easy preparation, low cost and good decorative effect [1–3]. ZrSiO4 crystal, with high refractive index and Mohs hardness, can not only produce a high opacity, but also impart excellent mechanical properties to the glaze [4]. Therefore, for optimizing the appearance and properties of final product, it is important to understand the crystallization mechanism of ZrSiO4 . The introducing of ZrSiO4 into glazes is either milling addition in raw glaze or reaction product in frit glaze [5,6]. The materials used for milling addition can be ZrSiO4 or other Zrcompounds such as ZrO2 , CaZrSiO6 , ZnZrSiO6 , while ZrSiO4 is the main opacifier in the final glaze regardless of the resource of Zr [7]. In a frit glaze, ZrSiO4 crystallizes during firing and this process has been studied extensively [8–12]. Grum-Grzhimailo and Kvyatkovskaya et al. [9,10] studied the crystallization
∗
Corresponding author. Tel.: +86 20 87111669; fax: +86 20 87110273. E-mail address: [email protected] (J. Wu).
http://dx.doi.org/10.1016/j.jeurceramsoc.2015.03.011 0955-2219/© 2015 Elsevier Ltd. All rights reserved.
behavior of ZrSiO4 in boron glaze and found that t-ZrO2 was always prior to the formation of ZrSiO4 and the intensity of tZrO2 decreased with the increase of ZrSiO4 , so they considered that the crystallization of ZrSiO4 was due to the reaction between t-ZrO2 and amorphous SiO2 . Escardino et al. [8,11] made an indepth study on the mechanism of t-ZrO2 transformed to ZrSiO4 , and it involved in two simultaneous and consecutive processes: the crystallization of tetragonal zirconia (t-ZrO2 ) and the chemical reaction between t-ZrO2 and amorphous SiO2 . The detailed process can be described as follows [11]. Zrintheglassyphase → ZrO2 units + ZrO2 activenuclei
(1)
ActiveZrO2 → t-ZrO2 crystal
(2)
t-ZrO2 + SiO2 → ZrSiO4 crystal
(3)
Compositions such as ZnO, flux content, and the ratio of ZrO2 :SiO2 have been reported to have an important effect on ZrSiO4 crystallization [6,12–14]. Moreover, the effect of nucleating agent (Fe2 O3 ) on ZrSiO4 formation was also studied [3]. However, to our knowledge, little work has been done regarding the microstructural evolution of ZrSiO4 in a frit glaze. Therefore,
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S. Wang et al. / Journal of the European Ceramic Society 35 (2015) 2671–2678 Table 1 Characteristic temperatures of the frit determined by DSC and thermalmicroscope (◦ C). Sample
DSC Tg
Frit glaze 760
Thermal-microscope Tc
T1
T2
T3
T4
T5
T6
1140
810
939
1016
1068
1228
1300
the purposes of this investigation were to (i) study ZrSiO4 formation from a zirconium-based frit glaze, (ii) explore the phase development and microstructural evolution during firing, and (iii) calculate the activation energy of ZrSiO4 crystallization.
18, Oberkochen, Germany), and the chemical compositions of different phases were obtained by an attached energy dispersive spectrometer (EDS; INCA Energy 300, Oxford instruments, High Wycombe, UK). Glaze surfaces were etched by 5 vol% HF solution for 30 s before observation. The thermal analysis of samples was carried out using a thermal analyzer with alumina crucible (Netzsch STA 449 C, Selb, Germany) from 20 ◦ C to 1200 ◦ C at various heating rates in an O2 atmosphere. In addition, the sintering behavior of the frit was observed by a thermal-microscope (TOMMI, Würzburg, Germany). The frit powder was pressed into cylinder with size of ∅ 3 mm × 3 mm and heated from room temperature to 1300 ◦ C at a heating rate of 10 ◦ C/min.
2. Experimental procedure
3. Results and discussion
2.1. Sample preparation
3.1. Characteristic temperatures of frit
The starting raw materials were feldspar, quartz, zircon, kaolin, calcium carbonate, talc, zinc oxide, and frit in industrial grade and their compositions are shown in Ref [15]. The weighed and thoroughly mixed materials were melted in an alumina crucible in an electric furnace at 1500 ◦ C for 1 h. Then the melt was quenched in water and wet-ball milled to pass a 200-mesh sieve. After drying, the frit powder had the following composition (wt%): 5.38% R2 O, 1.28% RO, 2.46% ZnO, 10.60% CaO, 9.87% Al2 O3 , 62.67% SiO2 , and 7.65% ZrO2 (R2 O = Na2 O + K2 O; RO = MgO + BaO + PbO). The frit powders were placed on a platinum plate, heat treated isothermally at temperatures from 950 ◦ C to 1250 ◦ C for 10–160 min, and then quenched in water. The as-prepared samples were crushed into fine powders for XRD analysis. For microstructral observation, the frit powder was first mixed with water and their mass ratio was 2:1. 0.2 wt% sodium carboxyl methyl cellulose (CMC) and 0.2 wt% sodium tripolyphosphate (STP) were added as binder and dispersing agent, respectively. Then the prepared slurries were directly coated on the green porcelain tiles, and fired in the same way as the powder.
Fig. 1 shows the DSC curve of the frit powder. It has a glass transition temperature (Tg ) at 760 ◦ C, a typical Tg of common glass–ceramic glazes [16]. The strongly endothermic valley beginning at about 870 ◦ C is the result of liquid phase formation. The exothermal peak at 1140 ◦ C (Tc ) is caused by the crystallization of ZrSiO4 according to the XRD analysis in the next section. The sintering behavior characterized by thermal-microscope is shown in Fig. 2. The frit cylinder experiences the following high-temperature process during the firing period: the contraction of the cylinder (T1 and T2 are the onset and ending temperatures of the contraction, respectively); the rounding of the corner (T3); the swelling to sphere in the lateral direction (T4); and hemisphere formation (T5). At T6, the height of the sample is close to 1/3 of its initial height, which means T6 is its flow temperature. Normally, the firing temperature of a glaze is located in the range of T3–T6. In this study, T3 = 1016 ◦ C and T6 = 1300 ◦ C (as shown in Table 1), so the frit has a wide range of melting temperature.
Fig. 1. DSC curve of the frit. Tg is the glass transition temperature.
3.2. Crystallization sequence 2.2. Characterization Crystalline phases were determined by X-ray diffraction analysis (XRD; PANalytical X’pert PRO, Almelo, Netherlands) using Cuk␣ radiation (40 kV and 40 mA). XRD patterns were recorded in the 2θ range from 10◦ to 60◦ with a step of 0.017◦ and a duration time of 8 s for each step. Secondary electron (SE) images and back-scattered electron (BSE) images were observed by a scanning electron microscopy (SEM; ZEISS EVO
In order to study the crystallization behavior of the frit, it was fired to different temperatures at a heating rate of 10 ◦ C/min and then quenched in water after soaking for 10 min. XRD patterns (Fig. 3) show that there is no crystalline phase at 900 ◦ C, and a small amount of t-ZrO2 and quartz (SiO2 ) appear at 1000 ◦ C. After firing at 1050 ◦ C, the diffraction peaks of calcium zirconium silicate (Ca2 ZrSi4 O12 ) are obvious, but completely disappear at 1250 ◦ C. The crystallization of ZrSiO4 starts at
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Fig. 2. High-temperature softening behavior of the frit (at a heating rate of 10 ◦ C/min).
1050 ◦ C and its intensity increases with the increasing of temperature, indicating a higher temperature is favor for ZrSiO4 crystallization. For a more visual comparison, the variation of the characteristic peak area of each phase versus temperature is shown in Fig. 4. It shows that t-ZrO2 and Ca2 ZrSi4 O12 first crystallize at low temperatures but their intensities gradually decrease as the temperature increases, and eventually disappear. In contrast, the intensity of ZrSiO4 increases rapidly at 1050 ◦ C to 1200 ◦ C, which suggests that the crystallization of ZrSiO4 is transformed from t-ZrO2 and Ca2 ZrSi4 O12 . The process for the transformation of t-ZrO2 to ZrSiO4 in glaze with low CaO content has been detailed described [9–11]. Nevertheless, the frit in this study contains a large quantity of CaO, so lots of Ca2 ZrSi4 O12 is crystallized as an intermediate. Ca2 ZrSi4 O12 glass–ceramic is widely used because of its advantages of low thermal expansion coefficient and excellent mechanical properties [17–19], but its conversion to ZrSiO4 , to our knowledge, is rarely been reported. The conversion of Ca2 ZrSi4 O12 to ZrSiO4 may have two ways: one is directly decomposed into ZrSiO4 by solid state reaction; the other is that it first decomposes into t-ZrO2 and then the t-ZrO2 reacts with amorphous SiO2 to form ZrSiO4 . Jacobs [7] found that ZrSiO4 was the main phase in the final glazes when various zirconium-type crystals (ZrSiO4 , ZrO2 , BaZrSiO5 , ZnZrSiO5 , CaZrSiO5 , MgZrSiO5 and Zr spinel) were used as opacifers. The elevated-temperature XRD detected that ZrSiO4 was gradually formed in glaze during the heating process without the formation of transition phase [7]; we can therefore speculate these Zr-compounds directly break down into ZrSiO4 . Similarly, this
process occurs for Ca2 ZrSi4 O12 and is available in the following chemical formula:
Fig. 3. XRD patterns of the frit fired at different temperatures for 10 min.
Fig. 4. The characteristic peak area of crystalline phase versus temperature.
Ca2 ZrSi4 O12 ↔ 2CaO + 3SiO2 + ZrSiO4
(4)
The excess CaO and SiO2 dissolve into the melt. 3.3. Phase transformation and microstructural development An isothermal heat treatment was taken to study the phase transformation and microstructural evolution of the frit glaze. Fig. 5 gives the variation of characteristic peak area of three main crystalline phases (Ca2 ZrSi4 O12 (0 2 1), t-ZrO2 (1 0 1), and ZrSiO4 (2 0 0)) with temperature. It shows that the conversion of t-ZrO2 to ZrSiO4 mainly occurs at temperatures below 1100 ◦ C, while the conversion of Ca2 ZrSi4 O12 occurs at higher temperatures. At 1000 ◦ C and 1050 ◦ C (Fig. 5a and b), two changes occur as the soaking time increases: the continuous crystallization of Ca2 ZrSi4 O12 and the transformation of t-ZrO2 to ZrSiO4 . As the temperature increases to 1100 ◦ C and 1150 ◦ C, XRD results in Fig. 5c and d show that ZrSiO4 is mainly transformed from Ca2 ZrSi4 O12 . Comparing the crystallization curves, it can be found that temperature is more important than soaking time for ZrSiO4 crystallization. The crystallization rate in Fig. 5d is more rapid than that in Fig. 5c though the temperature is only increased by 50 ◦ C. The ZrSiO4 yield at 1150 ◦ C for 80 min is 1.7 times than that at 1100 ◦ C for 80 min. The corresponding microstructural evolutions at 1000 ◦ C are shown in Figs. 6 and 7. After soaking for 10 min (Fig. 6), many tiny crystals are observed but their outlines are not very clear. Glaze surface are full of crystals after soaking for 40 min at
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Fig. 5. The relationship between characteristic peak area of crystalline phase and soaking time. (a) 1000 ◦ C, (b) 1050 ◦ C, (c) 1100 ◦ C, and (d) 1150 ◦ C. Arrows indicate the change of ZrSiO4 content.
Fig. 6. Back-scattered electron (BSE) image of the glaze fired at 1000 ◦ C for 10 min.
1000 ◦ C (Fig. 7a) and two kinds of crystal region with different brightness are distinguished in back-scattering mode: the bright and small cross-shaped crystals (indicated by rings in the figure), and gray radial crystals. The area of gray crystal is much larger than that of bright one. The difference in brightness results ¯ the from the difference in mean atomic number of crystal (Z): ¯ larger the Z, the brighter the crystal [20]. According to the XRD result in Fig. 5a, t-ZrO2 and Ca2 ZrSi4 O12 are the main crystals in the glaze after firing at 1000 ◦ C for 40 min, and their mean ¯ ZrO2 = 18.67 and Z ¯ Ca2 ZrSi4 O12 = 12.21, atomic numbers are Z respectively. Thus the bright and small cross-shaped crystals in Fig. 7a should be t-ZrO2 , and the gray radial crystals should be Ca2 ZrSi4 O12 . The microstructures of enlarged t-ZrO2 rich area is shown in Fig. 7b, c and d. Except for cross-shaped t-ZrO2 , a little amount of small radial/fibrous crystals is also present (indicated by arrows) and they should be ZrSiO4 . The fibrous shape of ZrSiO4 is due to its growth mechanism. If ZrSiO4 is transformed from t-ZrO2 , it contains an inward growth and an outward growth [21]. After the surface of t-ZrO2 is wrapped by a layer of ZrSiO4 , the inward growth starts and is controlled by diffusion process. Since the radius of Si4+ (0.026 nm) is smaller
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Fig. 7. Back-scattered electron (BSE) images of the glaze fired at 1000 ◦ C for different times: (a) 40 min (1000×), (b) 40 min (5000×), (c) 80 min (5000×), and (d) 120 min (5000×). The cross-shaped crystals are t-ZrO2 , the crystals indicated by arrows are ZrSiO4 , and the crystal indicated by rectangle is Ca2 ZrSi4 O12 .
than that of Zr4+ (0.074 nm), Si4+ has a faster diffusion rate than Zr4+ and thus the inward growth rate is controlled by the diffusion of Si4+ through the ZrSiO4 layer [22–25]. This process continues until ZrO2 is completely consumed. If ZrSiO4 only has inward growth, its shape would be limited to the shape of tZrO2 and difficult to grow into fibrous crystal, so ZrSiO4 should also have a mechanism to outward growth [21]. The outward growth is controlled by the diffusion of Zr4+ and Si4+ in the melt, which uses the formed ZrSiO4 crystal as nucleating agent. This growth mechanism can well explain why ZrSiO4 crystal maintains the similar morphology of t-ZrO2 , and is longer than the latter. Fig. 8 shows the microstructural evolutions at 1150 ◦ C. There are two main kinds of crystal in the glaze: bright rod-like crystals and gray bulk crystals. XRD in Fig. 5d shows ZrSiO4 and Ca2 ZrSi4 O12 are predominant at 1150 ◦ C. On the basis of the ¯ ZrSiO4 = previous discussion and the mean atomic number Z ¯ 14.33 > ZCa2 ZrSi4 O12 = 12.21, we can infer that the rod-like crystal is ZrSiO4 , and the gray bulk crystal is Ca2 ZrSi4 O12 . Unlike the morphology in Fig. 7, almost all of the ZrSiO4 is single, rod-like crystal in Fig. 8. At 1150 ◦ C, the formation of ZrSiO4 is attributed to the decomposition of Ca2 ZrSi4 O12 . As discussed in Section 3.2, Ca2 ZrSi4 O12 may directly decompose into ZrSiO4 during the heating process and the reactions can be described as follows: at first, Ca2 ZrSi4 O12 breaks down into fine ZrSiO4 crystals, and excess CaO and SiO2 . Then, the newly
formed ZrSiO4 acts as a nucleating agent to promote the further crystallization of ZrSiO4 , which is similar to the outward growth generated by the conversion of t-ZrO2 to ZrSiO4 . Thus, a single rod-like crystal is formed. 3.4. Devitrification kinetic of ZrSiO4 The Kissinger model is often used to calculate the activation energy of crystallization [26–29]. The following relationship exists between exothermic peak on DTA curve and heating rate: Φ Ec =− ln + constant Tp2 RTp where Ec is activation energy, Tp is exothermic peak temperature, Φ is heating rate, and R is the gas constant. The activation energy can be obtained from the slope of the plot of ln(Φ/Tp2 ) vs. RT1 p . Fig. 9 is the DTA curves of the frit powder heat treated at different heating rates. The endothermic valleys around 1000 ◦ C are due to the formation of liquid phase, and the exothermic peaks after 1100 ◦ C are corresponding to the crystallization of ZrSiO4 . Crystallization peaks shift to a higher temperature with an increase in heating rate, due to the lag effect of heat transfer. The plot of ln((Φ/Tp2 ) − (1/RTp )) is shown in Fig. 10.
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Fig. 8. Back-scattered electron (BSE) images of the glaze fired at 1150 ◦ C for different times: (a) 10 min, (b) 40 min, (c) 80 min, and (d) 120 min.
The calculated activation energy for ZrSiO4 crystallization is about 311.2 kJ/mol, which is different from the activation energy (443.5 kJ/mol) calculated by Escardino et al. [11]. Reasons may be the different experimental compositions and calculation methods. Previous studies have shown that different materials [30,31], different calculation methods [32,33], and even different particle size of raw materials [32] would result in different activation energy values.
Fig. 9. DSC curves of sample heat treatment at different heating rates. (a) 5 ◦ C/min, (b) 10 ◦ C/min, (c) 15 ◦ C/min, and (d) 20 ◦ C/min.
The crystal growth mechanism can be estimated by the following formula [27,28]: n=
Tp2 2.5 · T (Ec /R)
where n is the Avrami index, and T is the full-width at half maximum of the exothermic peak, which can be obtained by Gaussian fitting. The calculated n at 5 ◦ C/min, 10 ◦ C/min, 15 ◦ C/min and ◦ 20 C/min are 0.35, 0.38, 0.37 and 0.39, respectively, which
Fig. 10. Kissinger plot for the crystallization of ZrSiO4 .
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agree well with the results calculated by Sun et al. [34]. Small n values mean that ZrSiO4 is one-dimensional diffusion controlled growth [35,36], and will eventually grows into rods [37], which is consistent with its morphology in electron micrographs. Since the exothermic peaks of t-ZrO2 and Ca2 ZrSi4 O12 are not observed, their crystallization activation energy cannot be calculated. Escardino et al. [11,38] has established a series of theoretical formula for simulating and calculating the nucleation and growth of crystal on the surface of frit particles. Using these formulas and their deformations, they successfully calculated the activation energy, 253.5 kJ/mol for t-ZrO2 and 443.5 kJ/mol for ZrSiO4 [11]. The lower activation energy for t-ZrO2 explains why the t-ZrO2 always precedes ZrSiO4 crystallization. Based on this result and the crystallization sequence, we can deduce that the activation energy for Ca2 ZrSi4 O12 crystallization should be intermediate between that for t-ZrO2 and that for ZrSiO4 . 4. Conclusion The crystallization mechanism and microstructural evolution of ZrSiO4 from a frit glaze was studied and following conclusions are obtained. (1) ZrSiO4 can form from the conversion of t-ZrO2 and Ca2 ZrSi4 O12 . The transformation of t-ZrO2 to ZrSiO4 mainly occurs at low temperature, and the growth of ZrSiO4 has two mechanisms: the inward growth and the outward growth. The former is the migration of Si4+ from the melt to the ZrSiO4 –ZrO2 interface, and the latter is the result of nucleation-growth process. Ca2 ZrSi4 O12 may directly break down into ZrSiO4 , which acts as a nucleating agent to promote the further crystallization of ZrSiO4 . SEM images show that ZrSiO4 are rod-shaped (or fibrous) crystals in the final glazes. (2) The activation energy for ZrSiO4 is 311.2 kJ/mol. The Avrami index (n) is 0.35–0.39, which means the crystallization of ZrSiO4 is one-dimensional diffusion-controlled growth. This can explain why ZrSiO4 eventually grows into a rod shape. (3) The formation of liquid phase absorbs large amounts of heat to cover up the exothermic peaks of t-ZrO2 and Ca2 ZrSi4 O12 , not allowing to calculate their activation energy. However, based on the crystallization sequence, we can infer that the relationship between the three activation energy should be Et−ZrO2 < ECa2 ZrSi4 O12 < EZrSiO4 . Acknowledgment This work is supported by the National Natural Science Foundation of China (Grant No. 51472092). References [1] Parmelee CW. Ceramic Glazes; 1973. [2] Nosova Z. Substituting zirconium for stannous oxide in opaque glazes. Glass Ceram 1959;16:442–8.
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ScienceDirect Journal of the European Ceramic Society 35 (2015) 2671–2678
Microstructural evolution and crystallization mechanism of zircon from frit glaze Shaohua Wang a,b , Cheng Peng a , Huiyin Xiao b , Jianqing Wu a,∗ a
School of Materials Science and Engineering, South China University of Technology, Guangzhou 510640, China b Dongguan Wonderful Ceramic Industry Co., Ltd., Dongguan 523281, China Received 28 January 2015; received in revised form 6 March 2015; accepted 11 March 2015 Available online 30 March 2015
Abstract Zircon (ZrSiO4 ) is widely used as a glaze opacifier in the ceramic industry. The crystallization mechanism of ZrSiO4 from a CaO-riched zirconiumbased frit glaze was studied by XRD, SEM, and DSC in this work. Isothermal and non-isothermal heat treatments were used to study the phase development and microstructural evolution during the firing process. It was found that t-ZrO2 and Ca2 ZrSi4 O12 were first crystallized at a relative low temperature and then converted to ZrSiO4 with the increasing of temperature. ZrSiO4 originating from t-ZrO2 has an inward growth and an outward growth, while ZrSiO4 transformed from Ca2 ZrSi4 O12 only has an outward growth. SEM showed that ZrSiO4 appeared to be a rod shape in fired samples. The calculated activation energy for ZrSiO4 is 311.2 kJ/mol, and the Avrami index (n) is 0.35–0.39. This indicates the one-dimensional diffusion-controlled growth of ZrSiO4 , which explains the formation of rod-shape ZrSiO4 crystal. © 2015 Elsevier Ltd. All rights reserved. Keywords: Zircon crystallization; Crystal growth; Activation energy; Opaque glaze
1. Introduction Zirconium glaze is the most widely used opaque glaze due to its advantages of easy preparation, low cost and good decorative effect [1–3]. ZrSiO4 crystal, with high refractive index and Mohs hardness, can not only produce a high opacity, but also impart excellent mechanical properties to the glaze [4]. Therefore, for optimizing the appearance and properties of final product, it is important to understand the crystallization mechanism of ZrSiO4 . The introducing of ZrSiO4 into glazes is either milling addition in raw glaze or reaction product in frit glaze [5,6]. The materials used for milling addition can be ZrSiO4 or other Zrcompounds such as ZrO2 , CaZrSiO6 , ZnZrSiO6 , while ZrSiO4 is the main opacifier in the final glaze regardless of the resource of Zr [7]. In a frit glaze, ZrSiO4 crystallizes during firing and this process has been studied extensively [8–12]. Grum-Grzhimailo and Kvyatkovskaya et al. [9,10] studied the crystallization
∗
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http://dx.doi.org/10.1016/j.jeurceramsoc.2015.03.011 0955-2219/© 2015 Elsevier Ltd. All rights reserved.
behavior of ZrSiO4 in boron glaze and found that t-ZrO2 was always prior to the formation of ZrSiO4 and the intensity of tZrO2 decreased with the increase of ZrSiO4 , so they considered that the crystallization of ZrSiO4 was due to the reaction between t-ZrO2 and amorphous SiO2 . Escardino et al. [8,11] made an indepth study on the mechanism of t-ZrO2 transformed to ZrSiO4 , and it involved in two simultaneous and consecutive processes: the crystallization of tetragonal zirconia (t-ZrO2 ) and the chemical reaction between t-ZrO2 and amorphous SiO2 . The detailed process can be described as follows [11]. Zrintheglassyphase → ZrO2 units + ZrO2 activenuclei
(1)
ActiveZrO2 → t-ZrO2 crystal
(2)
t-ZrO2 + SiO2 → ZrSiO4 crystal
(3)
Compositions such as ZnO, flux content, and the ratio of ZrO2 :SiO2 have been reported to have an important effect on ZrSiO4 crystallization [6,12–14]. Moreover, the effect of nucleating agent (Fe2 O3 ) on ZrSiO4 formation was also studied [3]. However, to our knowledge, little work has been done regarding the microstructural evolution of ZrSiO4 in a frit glaze. Therefore,
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S. Wang et al. / Journal of the European Ceramic Society 35 (2015) 2671–2678 Table 1 Characteristic temperatures of the frit determined by DSC and thermalmicroscope (◦ C). Sample
DSC Tg
Frit glaze 760
Thermal-microscope Tc
T1
T2
T3
T4
T5
T6
1140
810
939
1016
1068
1228
1300
the purposes of this investigation were to (i) study ZrSiO4 formation from a zirconium-based frit glaze, (ii) explore the phase development and microstructural evolution during firing, and (iii) calculate the activation energy of ZrSiO4 crystallization.
18, Oberkochen, Germany), and the chemical compositions of different phases were obtained by an attached energy dispersive spectrometer (EDS; INCA Energy 300, Oxford instruments, High Wycombe, UK). Glaze surfaces were etched by 5 vol% HF solution for 30 s before observation. The thermal analysis of samples was carried out using a thermal analyzer with alumina crucible (Netzsch STA 449 C, Selb, Germany) from 20 ◦ C to 1200 ◦ C at various heating rates in an O2 atmosphere. In addition, the sintering behavior of the frit was observed by a thermal-microscope (TOMMI, Würzburg, Germany). The frit powder was pressed into cylinder with size of ∅ 3 mm × 3 mm and heated from room temperature to 1300 ◦ C at a heating rate of 10 ◦ C/min.
2. Experimental procedure
3. Results and discussion
2.1. Sample preparation
3.1. Characteristic temperatures of frit
The starting raw materials were feldspar, quartz, zircon, kaolin, calcium carbonate, talc, zinc oxide, and frit in industrial grade and their compositions are shown in Ref [15]. The weighed and thoroughly mixed materials were melted in an alumina crucible in an electric furnace at 1500 ◦ C for 1 h. Then the melt was quenched in water and wet-ball milled to pass a 200-mesh sieve. After drying, the frit powder had the following composition (wt%): 5.38% R2 O, 1.28% RO, 2.46% ZnO, 10.60% CaO, 9.87% Al2 O3 , 62.67% SiO2 , and 7.65% ZrO2 (R2 O = Na2 O + K2 O; RO = MgO + BaO + PbO). The frit powders were placed on a platinum plate, heat treated isothermally at temperatures from 950 ◦ C to 1250 ◦ C for 10–160 min, and then quenched in water. The as-prepared samples were crushed into fine powders for XRD analysis. For microstructral observation, the frit powder was first mixed with water and their mass ratio was 2:1. 0.2 wt% sodium carboxyl methyl cellulose (CMC) and 0.2 wt% sodium tripolyphosphate (STP) were added as binder and dispersing agent, respectively. Then the prepared slurries were directly coated on the green porcelain tiles, and fired in the same way as the powder.
Fig. 1 shows the DSC curve of the frit powder. It has a glass transition temperature (Tg ) at 760 ◦ C, a typical Tg of common glass–ceramic glazes [16]. The strongly endothermic valley beginning at about 870 ◦ C is the result of liquid phase formation. The exothermal peak at 1140 ◦ C (Tc ) is caused by the crystallization of ZrSiO4 according to the XRD analysis in the next section. The sintering behavior characterized by thermal-microscope is shown in Fig. 2. The frit cylinder experiences the following high-temperature process during the firing period: the contraction of the cylinder (T1 and T2 are the onset and ending temperatures of the contraction, respectively); the rounding of the corner (T3); the swelling to sphere in the lateral direction (T4); and hemisphere formation (T5). At T6, the height of the sample is close to 1/3 of its initial height, which means T6 is its flow temperature. Normally, the firing temperature of a glaze is located in the range of T3–T6. In this study, T3 = 1016 ◦ C and T6 = 1300 ◦ C (as shown in Table 1), so the frit has a wide range of melting temperature.
Fig. 1. DSC curve of the frit. Tg is the glass transition temperature.
3.2. Crystallization sequence 2.2. Characterization Crystalline phases were determined by X-ray diffraction analysis (XRD; PANalytical X’pert PRO, Almelo, Netherlands) using Cuk␣ radiation (40 kV and 40 mA). XRD patterns were recorded in the 2θ range from 10◦ to 60◦ with a step of 0.017◦ and a duration time of 8 s for each step. Secondary electron (SE) images and back-scattered electron (BSE) images were observed by a scanning electron microscopy (SEM; ZEISS EVO
In order to study the crystallization behavior of the frit, it was fired to different temperatures at a heating rate of 10 ◦ C/min and then quenched in water after soaking for 10 min. XRD patterns (Fig. 3) show that there is no crystalline phase at 900 ◦ C, and a small amount of t-ZrO2 and quartz (SiO2 ) appear at 1000 ◦ C. After firing at 1050 ◦ C, the diffraction peaks of calcium zirconium silicate (Ca2 ZrSi4 O12 ) are obvious, but completely disappear at 1250 ◦ C. The crystallization of ZrSiO4 starts at
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Fig. 2. High-temperature softening behavior of the frit (at a heating rate of 10 ◦ C/min).
1050 ◦ C and its intensity increases with the increasing of temperature, indicating a higher temperature is favor for ZrSiO4 crystallization. For a more visual comparison, the variation of the characteristic peak area of each phase versus temperature is shown in Fig. 4. It shows that t-ZrO2 and Ca2 ZrSi4 O12 first crystallize at low temperatures but their intensities gradually decrease as the temperature increases, and eventually disappear. In contrast, the intensity of ZrSiO4 increases rapidly at 1050 ◦ C to 1200 ◦ C, which suggests that the crystallization of ZrSiO4 is transformed from t-ZrO2 and Ca2 ZrSi4 O12 . The process for the transformation of t-ZrO2 to ZrSiO4 in glaze with low CaO content has been detailed described [9–11]. Nevertheless, the frit in this study contains a large quantity of CaO, so lots of Ca2 ZrSi4 O12 is crystallized as an intermediate. Ca2 ZrSi4 O12 glass–ceramic is widely used because of its advantages of low thermal expansion coefficient and excellent mechanical properties [17–19], but its conversion to ZrSiO4 , to our knowledge, is rarely been reported. The conversion of Ca2 ZrSi4 O12 to ZrSiO4 may have two ways: one is directly decomposed into ZrSiO4 by solid state reaction; the other is that it first decomposes into t-ZrO2 and then the t-ZrO2 reacts with amorphous SiO2 to form ZrSiO4 . Jacobs [7] found that ZrSiO4 was the main phase in the final glazes when various zirconium-type crystals (ZrSiO4 , ZrO2 , BaZrSiO5 , ZnZrSiO5 , CaZrSiO5 , MgZrSiO5 and Zr spinel) were used as opacifers. The elevated-temperature XRD detected that ZrSiO4 was gradually formed in glaze during the heating process without the formation of transition phase [7]; we can therefore speculate these Zr-compounds directly break down into ZrSiO4 . Similarly, this
process occurs for Ca2 ZrSi4 O12 and is available in the following chemical formula:
Fig. 3. XRD patterns of the frit fired at different temperatures for 10 min.
Fig. 4. The characteristic peak area of crystalline phase versus temperature.
Ca2 ZrSi4 O12 ↔ 2CaO + 3SiO2 + ZrSiO4
(4)
The excess CaO and SiO2 dissolve into the melt. 3.3. Phase transformation and microstructural development An isothermal heat treatment was taken to study the phase transformation and microstructural evolution of the frit glaze. Fig. 5 gives the variation of characteristic peak area of three main crystalline phases (Ca2 ZrSi4 O12 (0 2 1), t-ZrO2 (1 0 1), and ZrSiO4 (2 0 0)) with temperature. It shows that the conversion of t-ZrO2 to ZrSiO4 mainly occurs at temperatures below 1100 ◦ C, while the conversion of Ca2 ZrSi4 O12 occurs at higher temperatures. At 1000 ◦ C and 1050 ◦ C (Fig. 5a and b), two changes occur as the soaking time increases: the continuous crystallization of Ca2 ZrSi4 O12 and the transformation of t-ZrO2 to ZrSiO4 . As the temperature increases to 1100 ◦ C and 1150 ◦ C, XRD results in Fig. 5c and d show that ZrSiO4 is mainly transformed from Ca2 ZrSi4 O12 . Comparing the crystallization curves, it can be found that temperature is more important than soaking time for ZrSiO4 crystallization. The crystallization rate in Fig. 5d is more rapid than that in Fig. 5c though the temperature is only increased by 50 ◦ C. The ZrSiO4 yield at 1150 ◦ C for 80 min is 1.7 times than that at 1100 ◦ C for 80 min. The corresponding microstructural evolutions at 1000 ◦ C are shown in Figs. 6 and 7. After soaking for 10 min (Fig. 6), many tiny crystals are observed but their outlines are not very clear. Glaze surface are full of crystals after soaking for 40 min at
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Fig. 5. The relationship between characteristic peak area of crystalline phase and soaking time. (a) 1000 ◦ C, (b) 1050 ◦ C, (c) 1100 ◦ C, and (d) 1150 ◦ C. Arrows indicate the change of ZrSiO4 content.
Fig. 6. Back-scattered electron (BSE) image of the glaze fired at 1000 ◦ C for 10 min.
1000 ◦ C (Fig. 7a) and two kinds of crystal region with different brightness are distinguished in back-scattering mode: the bright and small cross-shaped crystals (indicated by rings in the figure), and gray radial crystals. The area of gray crystal is much larger than that of bright one. The difference in brightness results ¯ the from the difference in mean atomic number of crystal (Z): ¯ larger the Z, the brighter the crystal [20]. According to the XRD result in Fig. 5a, t-ZrO2 and Ca2 ZrSi4 O12 are the main crystals in the glaze after firing at 1000 ◦ C for 40 min, and their mean ¯ ZrO2 = 18.67 and Z ¯ Ca2 ZrSi4 O12 = 12.21, atomic numbers are Z respectively. Thus the bright and small cross-shaped crystals in Fig. 7a should be t-ZrO2 , and the gray radial crystals should be Ca2 ZrSi4 O12 . The microstructures of enlarged t-ZrO2 rich area is shown in Fig. 7b, c and d. Except for cross-shaped t-ZrO2 , a little amount of small radial/fibrous crystals is also present (indicated by arrows) and they should be ZrSiO4 . The fibrous shape of ZrSiO4 is due to its growth mechanism. If ZrSiO4 is transformed from t-ZrO2 , it contains an inward growth and an outward growth [21]. After the surface of t-ZrO2 is wrapped by a layer of ZrSiO4 , the inward growth starts and is controlled by diffusion process. Since the radius of Si4+ (0.026 nm) is smaller
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Fig. 7. Back-scattered electron (BSE) images of the glaze fired at 1000 ◦ C for different times: (a) 40 min (1000×), (b) 40 min (5000×), (c) 80 min (5000×), and (d) 120 min (5000×). The cross-shaped crystals are t-ZrO2 , the crystals indicated by arrows are ZrSiO4 , and the crystal indicated by rectangle is Ca2 ZrSi4 O12 .
than that of Zr4+ (0.074 nm), Si4+ has a faster diffusion rate than Zr4+ and thus the inward growth rate is controlled by the diffusion of Si4+ through the ZrSiO4 layer [22–25]. This process continues until ZrO2 is completely consumed. If ZrSiO4 only has inward growth, its shape would be limited to the shape of tZrO2 and difficult to grow into fibrous crystal, so ZrSiO4 should also have a mechanism to outward growth [21]. The outward growth is controlled by the diffusion of Zr4+ and Si4+ in the melt, which uses the formed ZrSiO4 crystal as nucleating agent. This growth mechanism can well explain why ZrSiO4 crystal maintains the similar morphology of t-ZrO2 , and is longer than the latter. Fig. 8 shows the microstructural evolutions at 1150 ◦ C. There are two main kinds of crystal in the glaze: bright rod-like crystals and gray bulk crystals. XRD in Fig. 5d shows ZrSiO4 and Ca2 ZrSi4 O12 are predominant at 1150 ◦ C. On the basis of the ¯ ZrSiO4 = previous discussion and the mean atomic number Z ¯ 14.33 > ZCa2 ZrSi4 O12 = 12.21, we can infer that the rod-like crystal is ZrSiO4 , and the gray bulk crystal is Ca2 ZrSi4 O12 . Unlike the morphology in Fig. 7, almost all of the ZrSiO4 is single, rod-like crystal in Fig. 8. At 1150 ◦ C, the formation of ZrSiO4 is attributed to the decomposition of Ca2 ZrSi4 O12 . As discussed in Section 3.2, Ca2 ZrSi4 O12 may directly decompose into ZrSiO4 during the heating process and the reactions can be described as follows: at first, Ca2 ZrSi4 O12 breaks down into fine ZrSiO4 crystals, and excess CaO and SiO2 . Then, the newly
formed ZrSiO4 acts as a nucleating agent to promote the further crystallization of ZrSiO4 , which is similar to the outward growth generated by the conversion of t-ZrO2 to ZrSiO4 . Thus, a single rod-like crystal is formed. 3.4. Devitrification kinetic of ZrSiO4 The Kissinger model is often used to calculate the activation energy of crystallization [26–29]. The following relationship exists between exothermic peak on DTA curve and heating rate: Φ Ec =− ln + constant Tp2 RTp where Ec is activation energy, Tp is exothermic peak temperature, Φ is heating rate, and R is the gas constant. The activation energy can be obtained from the slope of the plot of ln(Φ/Tp2 ) vs. RT1 p . Fig. 9 is the DTA curves of the frit powder heat treated at different heating rates. The endothermic valleys around 1000 ◦ C are due to the formation of liquid phase, and the exothermic peaks after 1100 ◦ C are corresponding to the crystallization of ZrSiO4 . Crystallization peaks shift to a higher temperature with an increase in heating rate, due to the lag effect of heat transfer. The plot of ln((Φ/Tp2 ) − (1/RTp )) is shown in Fig. 10.
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Fig. 8. Back-scattered electron (BSE) images of the glaze fired at 1150 ◦ C for different times: (a) 10 min, (b) 40 min, (c) 80 min, and (d) 120 min.
The calculated activation energy for ZrSiO4 crystallization is about 311.2 kJ/mol, which is different from the activation energy (443.5 kJ/mol) calculated by Escardino et al. [11]. Reasons may be the different experimental compositions and calculation methods. Previous studies have shown that different materials [30,31], different calculation methods [32,33], and even different particle size of raw materials [32] would result in different activation energy values.
Fig. 9. DSC curves of sample heat treatment at different heating rates. (a) 5 ◦ C/min, (b) 10 ◦ C/min, (c) 15 ◦ C/min, and (d) 20 ◦ C/min.
The crystal growth mechanism can be estimated by the following formula [27,28]: n=
Tp2 2.5 · T (Ec /R)
where n is the Avrami index, and T is the full-width at half maximum of the exothermic peak, which can be obtained by Gaussian fitting. The calculated n at 5 ◦ C/min, 10 ◦ C/min, 15 ◦ C/min and ◦ 20 C/min are 0.35, 0.38, 0.37 and 0.39, respectively, which
Fig. 10. Kissinger plot for the crystallization of ZrSiO4 .
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agree well with the results calculated by Sun et al. [34]. Small n values mean that ZrSiO4 is one-dimensional diffusion controlled growth [35,36], and will eventually grows into rods [37], which is consistent with its morphology in electron micrographs. Since the exothermic peaks of t-ZrO2 and Ca2 ZrSi4 O12 are not observed, their crystallization activation energy cannot be calculated. Escardino et al. [11,38] has established a series of theoretical formula for simulating and calculating the nucleation and growth of crystal on the surface of frit particles. Using these formulas and their deformations, they successfully calculated the activation energy, 253.5 kJ/mol for t-ZrO2 and 443.5 kJ/mol for ZrSiO4 [11]. The lower activation energy for t-ZrO2 explains why the t-ZrO2 always precedes ZrSiO4 crystallization. Based on this result and the crystallization sequence, we can deduce that the activation energy for Ca2 ZrSi4 O12 crystallization should be intermediate between that for t-ZrO2 and that for ZrSiO4 . 4. Conclusion The crystallization mechanism and microstructural evolution of ZrSiO4 from a frit glaze was studied and following conclusions are obtained. (1) ZrSiO4 can form from the conversion of t-ZrO2 and Ca2 ZrSi4 O12 . The transformation of t-ZrO2 to ZrSiO4 mainly occurs at low temperature, and the growth of ZrSiO4 has two mechanisms: the inward growth and the outward growth. The former is the migration of Si4+ from the melt to the ZrSiO4 –ZrO2 interface, and the latter is the result of nucleation-growth process. Ca2 ZrSi4 O12 may directly break down into ZrSiO4 , which acts as a nucleating agent to promote the further crystallization of ZrSiO4 . SEM images show that ZrSiO4 are rod-shaped (or fibrous) crystals in the final glazes. (2) The activation energy for ZrSiO4 is 311.2 kJ/mol. The Avrami index (n) is 0.35–0.39, which means the crystallization of ZrSiO4 is one-dimensional diffusion-controlled growth. This can explain why ZrSiO4 eventually grows into a rod shape. (3) The formation of liquid phase absorbs large amounts of heat to cover up the exothermic peaks of t-ZrO2 and Ca2 ZrSi4 O12 , not allowing to calculate their activation energy. However, based on the crystallization sequence, we can infer that the relationship between the three activation energy should be Et−ZrO2 < ECa2 ZrSi4 O12 < EZrSiO4 . Acknowledgment This work is supported by the National Natural Science Foundation of China (Grant No. 51472092). References [1] Parmelee CW. Ceramic Glazes; 1973. [2] Nosova Z. Substituting zirconium for stannous oxide in opaque glazes. Glass Ceram 1959;16:442–8.
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