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Experimental determination of the liquidus temperatures of the binary (SiO2–ZnO) system in equilibrium with air
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Experimental determination of the liquidus temperatures of the binary (SiO2 –ZnO) system in equilibrium with air Longgong Xia a,b , Zhihong Liu a , Pekka Antero Taskinen a,b,∗ a b
Central South University, School of Metallurgy and Environment, Changsha 410083, China Aalto University, School of Chemical Technology, Metallurgical Thermodynamics and Modeling Research Group, Espoo 02150, Finland
a r t i c l e
i n f o
Article history: Received 16 March 2015 Received in revised form 2 July 2015 Accepted 5 July 2015 Available online xxx Keywords: Phase equilibrium Liquidus ZnO SiO2 Zn2 SiO4
a b s t r a c t The phase equilibria and liquidus temperatures in the binary SiO2 –ZnO system in equilibrium with air (PO2 = 21278.25 Pa) have been experimentally determined at temperatures between 1430 ◦ C and 1690 ◦ C. Equilibration at target temperature in an appropriate container material was achieved, followed by rapid quenching. The microstructure of the specimen was observed by a scanning electron microscope from polished sections. The phase compositions were collected by an energy-dispersive X-ray spectroscopy (EDS), and confirmed by an electron probe X-ray microanalysis (EPMA). The only binary compound was found to be Zn2 SiO4 . Two binary eutectics were determined at 1442 ± 2 ◦ C and 0.52 ± 0.01 mole fraction of ZnO, and at 1497 ± 2 ◦ C and 0.71 ± 0.01 mole fraction of ZnO, respectively. The results were compared with the earlier studies available for this system. © 2015 Elsevier Ltd. All rights reserved.
1. Introduction SiO2 -ZnO contained glass-ceramics are one of the most important materials due to their electrical, optical and physical properties [1–3]. Glass-ceramics with ZnO are promising candidates in phosphors [4,5], industrial glazes [6]. Pure and doped Znx SiO4-x ceramics exhibit a low electric permittivity and a high Q × f value (Q, dielectric loss; f, resonant frequency), which fit well with the requirements of low temperature co-fired ceramics (LTCC) [7,8] and dielectric devices [9,10]. Accurate phase diagram and thermodynamic information of the SiO2 –ZnO system are important factors in enhancing the performance of those applications. SiO2 –ZnO contained slags are formed during the pyrometallurgical processing of zinc [11,12], lead [13,14], secondary copper [15] and steel [16]. Reliable thermodynamic descriptions of the SiO2 –ZnO system can contribute to optimizing and improving the control of processing temperatures and slag fluxing, reducing operating costs and to developing novel pyrometallurgical extraction processes. Zinc silicate (Zn2 SiO4 ), which is well-known by its mineral name willemite, exists in three different crystal structures, ␣-, - and ␥-Zn2 SiO4 [17,18]. It is reported that, ␣-Zn2 SiO4 is the most common stable crystalline phase, and is the sole thermodynamically stable compound at temperatures between 800 ◦ C and the liquidus [1,17,18]. -Zn2 SiO4 and ␥-Zn2 SiO4 are thermodynamically
∗ Corresponding author.
metastable phases and proved to be devitrifications of zinc silicate glasses through equilibrium and quenching techniques [19]. Taylor [20] proved that the orthorhombic -Zn2 SiO4 will transform into ␣-Zn2 SiO4 by reheating and cooling either rapidly or slowly. ␥-Zn2 SiO4 is also sensitive to reheating at subsolidus temperatures, and will disappear rapidly on holding at 800–1000 ◦ C [19]. Reyes and Gaskell [21] measured the Gibbs energy of formation of SiO2 –ZnO melts and the activity of ZnO in SiO2 –ZnO system, using a transportation technique with CO–CO2 as the carrier gas. Their study indicated that ZnO is a relatively basic oxide. The first complete phase diagram of this system was presented by Bunting [22] in 1930s, using an equilibrium-quenching experimental technique, followed by a visual microscope analysis. The diagram was determined on the basis of Gibbs phase rule. The reported diagram formed two eutectics with solid tridymite (at 1432 ◦ C and 0.491 mol percentage of ZnO) and with solid wurtzite (at 1507 ◦ C and 0.775 mol percentage of ZnO). The melting point of Zn2 SiO4 was reported to be 1512 ± 3 ◦ C. The region of two immiscible liquids in equilibrium with cristobalite (at a monotectic temperature of 1695 ◦ C) was determined to extend from 2% to 35% mole percentage of ZnO. However, impurities in the raw materials and the vaporization of zinc oxide (ZnO(s) → Zn(g) + ½ O2(g) ) at high temperatures [23,24] brought a lot of uncertainties to the results. Williamson [19] re-determined the liquidus temperatures in the SiO2 –ZnO system through an equilibrium-quenching technique in 1964. Agreement with the earlier study was satisfactory, the liquidus temperatures being 10 to 15 ◦ C lower than that in Bunting’s
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Please cite this article in press as: L. Xia, et al., Experimental determination of the liquidus temperatures of the binary (SiO2 –ZnO) system in equilibrium with air, J Eur Ceram Soc (2015), http://dx.doi.org/10.1016/j.jeurceramsoc.2015.07.007
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work. The compound (Zn2 SiO4 ) was reported to melt at 1498 ◦ C. The eutectics were reported to be at 1421 ± 4 ◦ C and 0.502 mole fraction of ZnO, and at 1491 ± 4 ◦ C and 0.752 mole fraction of ZnO. Weber [25,26] determined the eutectic systems of SiO2 –Zn2 SiO4 and Zn2 SiO4 –ZnO by a melting- controlled cooling- PEEM (photoemission electron microscopy) and XRD (X-ray diffraction) analysis technique in 1975. The eutectic points were reported to be at 1440 ◦ C and 0.49 mole fraction of ZnO, and at 1475 ◦ C and 0.708 mole fraction of ZnO, respectively. The thermodynamic data for the binary system, including its liquid and solid phases, were assessed and optimized by a CALPHAD-type thermodynamic fitting by Hansson [27] in 2005. They also reinvestigated the phase equilibia of SiO2 –ZnO system using an equilibrium-quenching-EPMA (electron probe X-ray microanalysis) technique. An important advantage of his research was the elimination of the inaccuracies associated with sample composition changes, resulting from the vaporization of zinc oxide. Two binary eutectics involving the congruently melting willemite (Zn2 SiO4 ) were reported at 1448 ± 5 ◦ C and 0.52 ± 0.01 mole fraction of ZnO and at 1502 ± 5 ◦ C and 0.71 ± 0.01 mole fraction of ZnO, respectively. The obtained melting temperature of the congruently melting willemite was in good agreement with previously reported data (1512 ± 3 ◦ C, according to Bunting [22]). No other liquidus data or assessed thermodynamic descriptions of this system were found in the literature. The development of experimental apparatus, temperature measurement and analytical technique enables accurate information on phase equilibrium to be obtained. In present study, the phase equilibrium of binary SiO2 -ZnO system in equilibrium with air has been investigated to confirm the results reported by previous researchers, as well as to determine the temperatures of eutectic reactions. The evaporation of ZnO has been overcome by the application of a new experimental technique. The technique used in this study involved the preparation of a synthetic slag of predetermined composition. Slag equilibrated with air at target temperature for certain period of time. Then rapid quenching, the silica-containing liquid converts to glass so that the phase assemblage that exists at a high temperature is frozen. The crystalline solid solutions were also readily quenched. The compositions of the solid and liquid phase were then measured by an energy dispersive spectrometer (EDS) and confirmed by an electron-probe X-ray microanalysis (EPMA) from polished sections. Each experiment provides information on the liquidus composition and the compositions of the solid phases formed in equilibrium. This greatly increases the productivity of the research and gives information on the solid solutions, which are of particular importance for further thermodynamic modeling. In addition, this work is part of the efforts to consolidate experimental phase equilibria data from binary SiO2 –ZnO system toward characterization of Cu–O–ZnO–SiO2 slags. It ultimately opens up possibilities to extract property data from molten silica and copper rich melts to develop different silicate fluxes in secondary copper pyrometallurgical processes as well as to better control their industrial furnace operations.
2. Material and methods 2.1. Preparation of initial mixtures High-purity oxide powders of SiO2 and ZnO were employed as starting materials, see Table 1. In the equilibrium experiments, pure powders were calcined at 110 ◦ C, weighted in desired proportions, mixed thoroughly in an agate mortar, and pelletized to 5 Mpa. Total weight of the pellet was 0.2 g. The small size of the pellet was an advantage in quenching of the molten phase. The pellet was
Table 1 Mass fraction purity and sources of materials used in the present study Chemical
Purity (mass%)
Supplier
Silicon Dioxide Zinc Oxide SiO2 Crucible Platinum wire Platinum foil
99.99% 99.99% 99.98% 99.99% 99.99%
Umicore (Liechtenstein) Alfa Aesar (Germany) OM Lasilaite Oy (Finland) Johnson-Matthey Noble Metals (UK) Johnson-Matthey Noble Metals (UK)
equilibrated at a fixed temperature for certain period of time in air, and then quenched in ice-cold water. Platinum wires and foils, which were inert in experimental conditions of this study, were used as suspending and holding basket to preserve the purity of the system. 2.2. Selection of container material When conducting experiments on the SiO2 saturated side of the system, small quartz crucibles were employed as the container (Fig. 1 left). The purity and supplier of the crucible were listed in Table 1. Zn2 SiO4 and ZnO substrates were involved in experiments on the Zn2 SiO4 and the ZnO saturated side respectively (Fig. 1 right). The sample mixtures were placed on those substrates, and then equilibrated in an envelope made from 0.05-mm-thick platinum foil. In the preparation of Zn2 SiO4 substrates, 1.0 g mixture of SiO2 powder and ZnO powder in mole ration of 1:2 was mixed thoroughly in an agate mortar, pelletized at 10 Mpa, and then the pellets were manually shaped into a ‘crucible’ that can contain molten phases without spreading. The shaped substrates were sintered in a chamber furnace at a temperature of 1400 ◦ C in air and holding for 48 h. The ZnO substrates were prepared in the same procedure. However, the shaped ZnO pellets were sintered at 1500 ◦ C in air and holding for 60 h. Both heating and cooling rates in the sintering procedures were kept at 4 ◦ C/min. Substrates prepared in this way could be used as supports for the melts because of low porosity, and proper density. 2.3. Equilibration technique The samples were equilibrated in a vertical tube furnace (RHTV 40-250/18, Nabertherm, Germany) within electrical resistance of silicon carbide heating elements (Fig. 2). The uniform temperature hot zone of the furnace was first confirmed by measuring its thermal profiles at sevsral temperatures from 1200 ◦ C to 1700 ◦ C. The temperature of the hot zone was determined with a calibrated S-type thermocouple (Johnson-Matthey Noble Metals, UK), which was connected to a Keithley 2010 DMM multimeter. The working thermocouple in a recrystallized alumina sheath was positioned immediately next to the specimen and periodically tested against a calibrated thermocouple. The cold junction compensation was performed by a Pt100 resistance thermometer (SKS-group, Finland, tolerance class B 1/10), connected to a Keithley 2000 DMM multimeter. Special attention was paid to the accuracy of the equilibrium temperature, and the temperature accuracy was estimated to be ±2 ◦ C for the entire experimental temperature range of interest. The temperature data during the experiments were collected with a NI LabVIEW temperature logging program. The specimen hooked to a pure platinum wire (0.5-mm diameter) was introduced into the furnace from the bottom of the working tube and was suspended in the hot zone. Specimen was kept in the hot zone at the target temperature for the required equilibration period. Particular attention was paid to ensure the achievement of equilibrium, by approaching the final equilibrium compositions with variation of the equilibration time. Achievement
Please cite this article in press as: L. Xia, et al., Experimental determination of the liquidus temperatures of the binary (SiO2 –ZnO) system in equilibrium with air, J Eur Ceram Soc (2015), http://dx.doi.org/10.1016/j.jeurceramsoc.2015.07.007
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Fig. 1. Schematic of specimen and crucible suspension techniques (left);
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Specimen, substrate and envelop combination techniques (right).
single-phase solid solution. Three dedicated sets of experiments were conducted to determine the equilibrium time. A longer equilibrium time is a positive factor to approaching the real equilibrium. However, the vaporization of ZnO turned to be a great challenge at elevated temperatures. It is found that in the SiO2 primary field, the glassy places were homogenous throughout the specimen within 4 h when equilibrated at 1450 ◦ C. Equilibrium was achieved within 1 h when temperature was raised up to 1540 ◦ C. In the Zn2 SiO4 and ZnO saturated sides, equilibrium could be achieved in 15 min, and specimen vaporized vanishing in 2 h. The experiments were conducted with the bottom and the top of the reaction tube open to the ambient atmosphere in order to ensure the composition of air. Once the equilibrium time was reached, a bottle of water and ice mixture was attached to the bottom of the tube and the specimen was then rapidly quenched by dropping into the ice-cold water bath (0 ◦ C). It was dried and mounted in epoxy resin. A polished cross section of the mounted specimen was prepared using the conventional metallographic grinding and polishing technique. 2.4. Analysis
Fig. 2. Schematic vertical section of furnace and auxiliaries.
of the equilibrium was determined by measuring the composition of the samples equilibrated for different times. In order to confirm the achievement of equilibrium, the equilibration experiments were repeated starting from both pure oxide powders and from a
The compositions and purities of the substrates were determined by raw materials and the sintering procedure. ZnO substrates prepared in air, using analytical ZnO powder as raw material could meet the requirement of equilibrium experimental work in the ZnO saturated side. The Zn2 SiO4 substrates used in Zn2 SiO4 saturated experiments were characterized by a Philips X-ray diffractometer (XRD), using Cu K␣ radiation. The recorded patterns of as-prepared Zn2 SiO4 substrates (appendix A) showed strong diffraction peaks, which were flawlessly assigned to willemite (JCPDS card #37-1485) [28,29], indicating that the main phase of the substrate was willemite. No other phases were detected. All samples were carbon-coated with a Leica EM SCD050 Coater (supplied by Leica Mikrosysteme GmbH, Vienna). The microstructure and phase composition of the samples were firstly examined by Scanning Electron Microscopy (SEM) and Energy Dispersive
Please cite this article in press as: L. Xia, et al., Experimental determination of the liquidus temperatures of the binary (SiO2 –ZnO) system in equilibrium with air, J Eur Ceram Soc (2015), http://dx.doi.org/10.1016/j.jeurceramsoc.2015.07.007
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Table 2 Experimentally determined phase compositions for the (SiO2 –ZnO) system equilibrated with air (PO2 = 21278.25 Pa) No.
Temperature (◦ C)
Equilibrium time (h)
1
(premelting 1480) 1430
(15min) 60
2
1440
12
3
1450
8
4
1480
8
5
1510
8
6
1540
8
7
1570
8
8
1600
8
9
1630
4
10
1662
4
11
1688
4
12
1445
2
13
1502
1
14
1502
1
15
1495
1
16
1500
2
17
1515
0.5
18
1540
0.5
19
1570
0.5
20
1595
0.5
Phase
EDS (mol%)
EPMA (mol%)
SiO2
ZnO
SiO2
ZnO
Standard deviation ()
‘Liquid’
45.67
54.33
45.82
54.18
0.12
SiO2 Zn2 SiO4 SiO2 Liquid SiO2 Liquid SiO2 Liquid SiO2 Liquid SiO2 Liquid SiO2 Liquid SiO2 Liquid SiO2 Liquid SiO2 Liquid SiO2 Liquid Zn2 SiO4 Liquid Zn2 SiO4 Liquid Zn2 SiO4 Zn2 SiO4 ZnO Liquid ZnO Liquid ZnO Liquid ZnO Liquid ZnO Liquid ZnO
98.52 33.74 98.83 47.92 98.70 48.74 98.81 50.23 98.70 50.87 98.90 50.95 98.99 53.86 98.61 55.81 98.91 60.33 98.87 62.50 96.93 47.85 33.88 42.21 33.84 34.75 26.06 33.71 0.11 28.74 0.13 28.02 0.17 27.59 0.09 26.81 0.29 25.64 0.24
1.48 66.26 1.17 52.08 1.30 51.26 1.19 49.77 1.30 49.13 1.10 49.05 1.01 46.14 1.39 44.19 1.09 39.67 1.13 37.50 3.07 52.15 66.12 57.79 66.16 65.25 73.94 66.29 99.89 71.26 99.87 71.98 99.83 72.41 99.91 73.18 99.71 74.36 99.76
98.53 33.92 98.42 47,37 98.51 48.61 98.76 49.70 98.49 51.30 98.57 52.45 98.54 52.67 98.59 55.98 98.56 59.80 98.85 62.27 96.89 47.66 33.54 42.43 34.01 33.63 26.36 33.02 0.20 28.38 0.20 27.97 0.22 27.46 0.15 26.50 0.25 25.87 0.20
1.47 66.08 1.58 52.63 1.49 51.39 1.24 50.30 1.51 48.70 1.43 47.55 1.46 47.33 1.41 44.02 1.44 40.20 1.15 37.73 3.11 52.34 66.46 57.57 65.99 66.37 73.64 66.98 99.80 71.62 99.80 72.03 99.78 72.54 99.85 73.50 99.75 74.13 99.80
0.06 0.05 0.07 0.11 0.07 0.26 0.07 0.20 0.03 0.14 0.02 0.06 0.04 0.20 0.05 0.21 0.40 0.20 0.03 0.10 0.08 0.06 0.07 0.15 0,09 0.08 0.12 0.12 0.03 0.16 0.04 0.19 0.03 0.10 0.06 0.16 0.02 0.18 0.04
*Standard uncertainty: (T) = ± 2 ◦ C; (X) = ± 0.01 mol%. **Standard deviation: (EDS) = 0.34; (EPMA) = 0.11.
Spectrometer (EDS). A LEO 1450 (Carl Zeiss Microscopy GmbH, Jena, Germany) scanning electron microscope was employed with a linked Inca X-Sight 7366 Energy EDS analyzer (Oxford Instruments plc, Abingdon, Oxfordshire, UK). The accelerating voltage employed was 15 kV and employed standards were zinc sulfide (ZnS) for Zn and quartz for Si and O. The spectral lines used for Zn, Si, O were L␣, K␣ and K␣, respectively. The compositions of the solid and liquid phases were then measured by Electron-Probe X-ray Microanalysis (EPMA). The analysis was performed by a CAMECA SX100 instrument, which was equipped with five wavelength dispersive spectrometers. An accelerating voltage of 15 kV, and beam current and diameter of 10 nA and (1-5) m were utilized in the analysis. The standards used in this work were natural minerals (zinc sulfide for Zn, L␣; quartz for Si, K␣). All standards were purchased from Astimex Scientific ltd, Canada. The raw measurement results were corrected using the PAP on-line correction program [30]. 3. Results and discussion All the sample were carefully analyzed, and each of them contained two homogeneous phases, no concentration gradients were detected within them. The obtained EDS and EPMA results of the
Fig. 3. Isopleth diagram of the binary (SiO2 –ZnO) system in air (PO2 = 21278.25 Pa).
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Fig. 4. Backscattered SEM images of quenched SiO2 –ZnO slags in equilibrium with air: (a) specimen equilibrated at 1440 ◦ C for 14 h, starting with 50 mol% ZnO; (b) specimen equilibrated at 1445 ◦ C for 8 h, starting with 55 mol% ZnO; (c) specimen equilibrated at 1495 ◦ C for 2 h, starting with 90 mol% ZnO; (d) specimen equilibrated at 1500 ◦ C for 2 h, starting with 90 mol% ZnO.
phase equilibration experiments of the binary SiO2 –ZnO system in equilibrium with air in the SiO2 , Zn2 SiO4 and ZnO primary phase fields at temperatures range from 1430 ◦ C to 1690 ◦ C were listed in Table 2. The individual standard deviations of the EPMA results were calculated and presented in the table. Overall standard deviations of the EDS and EPMA results were listed below the table. The agreement between these two measurement series was good. For the temperature range of interest, small variations of ZnO solubility in the solid phases of SiO2 , it is 3.11 mol% at 1688 ◦ C. SiO2 solubility in the solid phase of Zn2 SiO4 is small as shown in Table 2, 1.01 mol% SiO2 in Zn2 SiO4 phase. Zn2 SiO4 has smaller practical solubility in solid ZnO phase, it is below 0.25 mol% at the temperature range of interest. The Results in present study have been compared with previous work and the comparison is depicted graphically in Fig. 3. Silicate exists two different polymorphs in the studied temperature range. Tridymite transfers into cristobalite at 1470 ◦ C in air [31]. The calculated phase diagram is based on the work of Bunting in 1930s [22], when the experimental equipment and raw materials affected the results a lot. Evaporation of ZnO at Zn2 SiO4 and ZnO saturated field would contribute to the uncertainty of the data in Bunting’s work. The results of present work are in good agreement with the recent work of Hansson et al. [27]. The concentration of ZnO in the liquid oxide phase at the SiO2 primary field is a little lower than that reported by Hansson et al. [27] when temperature is above 1600 ◦ C, but slightly higher in the other regions. The impurities in Hansson’s starting materials may have accounted for the experimental differences compared to the current study. The eutectic reaction between tridymite, willemite and liquid was found in this study to occur at 1442 ± 2 ◦ C, and at 0.52 ± 0.01 mole fraction of ZnO. As shown in Fig. 4(a), two solid phases (SiO2 and Zn2 SiO4 ) were observed in the sample equilibrated at 1440 ◦ C with a bulk composition between tridymite and willemite (experiment 2). In the similar way, the sample equilibrated at 1442 ◦ C in Hansson’ work [22] had the same results. Specimen with a
bulk composition in the willemite primary-phase equilibrated at 1445 ◦ C (experiment 12) was found to contain a liquid phase and a solid phase, as shown in Fig. 4 (b). The rate of eutectic reacftion (liquid → tridymite + willemite) was obvious slow. Experiment equilibrated at 1430 ◦ C (experiment 1, with 15 min premelting treatment at 1480 ◦ C) for 60 h was found to be consisted with one solid phase and one homogeneous phase. These experiments confirmed both the temperature and the composition of the invariant point. The eutectic temperature between willemite, wurtzite and liquid was found to be 1497 ± 2 ◦ C, and at 0.71 ± 0.01 mole fraction of ZnO in this study. Two solid phases (willemite and wurtzite) were observed in the specimen (experiment 15) equilibrated at 1495 ◦ C (shown in Fig. 4(c)). Primary and secondary wurtzite detected are evidence of the eutectic reaction. The sample (experiment 16) equilibrated at 1500 ◦ C (shown in Fig. 4(d)) with a bulk composition between willemite and wurtzite was found to contain one homogeneous phase and one solid phase.
4. Summary and conclusions The equilibration/quenching/EDS-EPMA analysis technique has been successfully employed to determine the phase equilibria and liquidus temperatures of the binary SiO2 –ZnO system in air. The effect of ZnO on the compositions of liquidus lines of the SiO2 , Zn2 SiO4 and ZnO primary phases of the system in equilibrium with air at temperatures between 1430 ◦ C and 1690 ◦ C has been measured. The obtained liquidus lines are in good agreement with the earlier work of Hansson et al. [22], but show significant differences with the older studies [19,21,25,26]. The temperatures and compositions of the two eutectics between tridymite, willemite and liquid, and willemite, wurtzite and liquid equilibrated in air were estimated to be 1442 ± 2 ◦ C and 0.52 ± 0.01 mole fraction of ZnO, and 1497 ± 2 ◦ C and 0.71 ± 0.01 mole fraction of ZnO, respectively.
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Appendix A
Fig. A1. XRD patterns of Zn2 SiO4 mixtures and substrates.
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[10] P.W. McMillan, G. Patridge, The dielectric properties of certain ZnO–Al2 O3 –SiO2 glass-ceramics, J. Mater. Sci. 7 (8) (1972) 847–855. [11] F. Habashi, Handbook of Extractive Metallurgy, 2, WILEY-VCH, Heidelberg, Germany, 1997. [12] G.A. Assis, Emerging pyrometallurgical processes for zinc and lead recovery from zinc-bearing waste materials, in: 37th Zinc & Lead Processing Symposium (CIM meeting), 16-19Calgary, Canada, August, 1998, pp. 243–265. [13] B. Errington, J.K. Wang, P. Arthur, Y. Dong, The ISA-YMG lead smelting process Proceeding of the International Symposium on Lead and Zinc Processing, Kyoto, Japan, 2005, pp. 587–599. [14] A.F.S. Schoukens, L.R. Nelson, N.A. Barcza, Plasma-arc treatment of steel-plant dust and zinc-containing slag ?theoretical and practical considerations, in: Recycling of Lead and Zinc, the Challenge of the 1990, Proceedings of an international conference organized by the International Lead and Zinc Study Group Sub-Committee on Recycling, Rome, June, 1991, pp. 361–370. [15] M.E. Schlesinger, M.E. Davenport, M.J. King, K.C. Sole, W.G. Davenport, Extractive Metallurgy of Copper, 5th ed., Elsevier Science Ltd, Oxford, UK, 2011, ISBN: 978-0-08-096789-9. [16] A.R. Marder, The metallurgy of zinc-coated steel, Prog. Mater. Sci. 45 (2000) 191–271. [17] A.E. Ringwood, A. Major, High pressure transformations in zinc germanates and silicates, Nature 215 (1967) 1367–1368. [18] A.M. Doroshev, M. Olesch, V.M. Logvinov, I.J. Malinovsky, High pressure stability of zinc clinopyroxene ZnSiO3 and the occurrence of a new polytype of zinc orthosilicate Zn2 SiO4 as a breakdown product, N. Jb. Miner. Mh. H6 (1983) 277–288. [19] J. Williamson, F.P. Glasser, Crystallisation of zinc silicate liquids and glasses, Phys. Chem. Glasses 5 (1964) 52–59. [20] H.F.W. Taylor, The dehydration of hemimorphite, Am. Miner. 47 (1962) 932–944. [21] R.A. Reyes, D.R. Gaskell, The thermodynamic activity of ZnO in the silicate melts, Metall. Mater. Trans. B 14B (1983) 725–731. [22] E.N. Bunting, Phase equilibria in the system SiO2 –ZnO, J. Res. Natl. Bur. Stand. 4 (1930) 131–136. [23] H.A. Wriedt, The O–Zn (Oxygen–Zinc) system, Bull. Alloy Phase Diagrams 8 (2) (1987) 166–176. [24] B.J. Kim, R.E. Garcia, E.A. Stach, Kinetics of congruent vaporization of ZnO islands, Phys. Rev. Lett. 107 (14) (2011), Article ID 146,101, 4 pages. [25] L. Weber, H.R. Oswald, Investigation of phase intergrowth morphologies in the system Zn2 SiO4 –SiO2 by photo-emission electron microscopy, J. Mater. Sci. 10 (1975) 973–982. [26] L. Weber, H.R. Oswald, E. Grauer-Carstensen, Investigation of chemical microstructures in the system ZnO-Zn2 SiO4 by photo-emission electron microscopy and X-ray diffraction, Micron 6 (1975) 129–139. [27] R. Hansson, B.J. Zhao, P.C. Hayes, E. Jak, A Reinvestigation of phase equilibria in the system Al2 O3 –SiO2 –ZnO, Metall. Mater. Trans. B 36B (2) (2005) 187–193. [28] Y. Syono, S.I. Akimoto, High pressure transformation in zinc silicates, J. Solid. State. Chem. 3 (1971) 369–380. [29] L.E. Mir, A. Amlouk, C. Barthou, S. Alaya, Synthesis and luminescence properties of ZnO/Zn2 SiO4 /SiO2 composite based on nanosized zinc oxide-confined silica aerogels, Phys. B: Condensed Matter. 388 (2007) 412–417. [30] J.L. Pouchou, F. Pichoir, Basic Expression of PAP Computation for Quantitative EPMA, in 11th International Congress on X-ray Optics and Microanalysis (ICXOM), in: J.D. Brown, R.H. Packwood (Eds.), ICXOM-11, Ontario, Canada, 1987. [31] K. Kihara, T. Matsumoto, M. Imamura, Structure change of orthorhombic-I tridymite with temperature: a study based on second order thermal-vibrational parameters, Z. Kristallogr. 177 (1986) 27–38.
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Experimental determination of the liquidus temperatures of the binary (SiO2 –ZnO) system in equilibrium with air Longgong Xia a,b , Zhihong Liu a , Pekka Antero Taskinen a,b,∗ a b
Central South University, School of Metallurgy and Environment, Changsha 410083, China Aalto University, School of Chemical Technology, Metallurgical Thermodynamics and Modeling Research Group, Espoo 02150, Finland
a r t i c l e
i n f o
Article history: Received 16 March 2015 Received in revised form 2 July 2015 Accepted 5 July 2015 Available online xxx Keywords: Phase equilibrium Liquidus ZnO SiO2 Zn2 SiO4
a b s t r a c t The phase equilibria and liquidus temperatures in the binary SiO2 –ZnO system in equilibrium with air (PO2 = 21278.25 Pa) have been experimentally determined at temperatures between 1430 ◦ C and 1690 ◦ C. Equilibration at target temperature in an appropriate container material was achieved, followed by rapid quenching. The microstructure of the specimen was observed by a scanning electron microscope from polished sections. The phase compositions were collected by an energy-dispersive X-ray spectroscopy (EDS), and confirmed by an electron probe X-ray microanalysis (EPMA). The only binary compound was found to be Zn2 SiO4 . Two binary eutectics were determined at 1442 ± 2 ◦ C and 0.52 ± 0.01 mole fraction of ZnO, and at 1497 ± 2 ◦ C and 0.71 ± 0.01 mole fraction of ZnO, respectively. The results were compared with the earlier studies available for this system. © 2015 Elsevier Ltd. All rights reserved.
1. Introduction SiO2 -ZnO contained glass-ceramics are one of the most important materials due to their electrical, optical and physical properties [1–3]. Glass-ceramics with ZnO are promising candidates in phosphors [4,5], industrial glazes [6]. Pure and doped Znx SiO4-x ceramics exhibit a low electric permittivity and a high Q × f value (Q, dielectric loss; f, resonant frequency), which fit well with the requirements of low temperature co-fired ceramics (LTCC) [7,8] and dielectric devices [9,10]. Accurate phase diagram and thermodynamic information of the SiO2 –ZnO system are important factors in enhancing the performance of those applications. SiO2 –ZnO contained slags are formed during the pyrometallurgical processing of zinc [11,12], lead [13,14], secondary copper [15] and steel [16]. Reliable thermodynamic descriptions of the SiO2 –ZnO system can contribute to optimizing and improving the control of processing temperatures and slag fluxing, reducing operating costs and to developing novel pyrometallurgical extraction processes. Zinc silicate (Zn2 SiO4 ), which is well-known by its mineral name willemite, exists in three different crystal structures, ␣-, - and ␥-Zn2 SiO4 [17,18]. It is reported that, ␣-Zn2 SiO4 is the most common stable crystalline phase, and is the sole thermodynamically stable compound at temperatures between 800 ◦ C and the liquidus [1,17,18]. -Zn2 SiO4 and ␥-Zn2 SiO4 are thermodynamically
∗ Corresponding author.
metastable phases and proved to be devitrifications of zinc silicate glasses through equilibrium and quenching techniques [19]. Taylor [20] proved that the orthorhombic -Zn2 SiO4 will transform into ␣-Zn2 SiO4 by reheating and cooling either rapidly or slowly. ␥-Zn2 SiO4 is also sensitive to reheating at subsolidus temperatures, and will disappear rapidly on holding at 800–1000 ◦ C [19]. Reyes and Gaskell [21] measured the Gibbs energy of formation of SiO2 –ZnO melts and the activity of ZnO in SiO2 –ZnO system, using a transportation technique with CO–CO2 as the carrier gas. Their study indicated that ZnO is a relatively basic oxide. The first complete phase diagram of this system was presented by Bunting [22] in 1930s, using an equilibrium-quenching experimental technique, followed by a visual microscope analysis. The diagram was determined on the basis of Gibbs phase rule. The reported diagram formed two eutectics with solid tridymite (at 1432 ◦ C and 0.491 mol percentage of ZnO) and with solid wurtzite (at 1507 ◦ C and 0.775 mol percentage of ZnO). The melting point of Zn2 SiO4 was reported to be 1512 ± 3 ◦ C. The region of two immiscible liquids in equilibrium with cristobalite (at a monotectic temperature of 1695 ◦ C) was determined to extend from 2% to 35% mole percentage of ZnO. However, impurities in the raw materials and the vaporization of zinc oxide (ZnO(s) → Zn(g) + ½ O2(g) ) at high temperatures [23,24] brought a lot of uncertainties to the results. Williamson [19] re-determined the liquidus temperatures in the SiO2 –ZnO system through an equilibrium-quenching technique in 1964. Agreement with the earlier study was satisfactory, the liquidus temperatures being 10 to 15 ◦ C lower than that in Bunting’s
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work. The compound (Zn2 SiO4 ) was reported to melt at 1498 ◦ C. The eutectics were reported to be at 1421 ± 4 ◦ C and 0.502 mole fraction of ZnO, and at 1491 ± 4 ◦ C and 0.752 mole fraction of ZnO. Weber [25,26] determined the eutectic systems of SiO2 –Zn2 SiO4 and Zn2 SiO4 –ZnO by a melting- controlled cooling- PEEM (photoemission electron microscopy) and XRD (X-ray diffraction) analysis technique in 1975. The eutectic points were reported to be at 1440 ◦ C and 0.49 mole fraction of ZnO, and at 1475 ◦ C and 0.708 mole fraction of ZnO, respectively. The thermodynamic data for the binary system, including its liquid and solid phases, were assessed and optimized by a CALPHAD-type thermodynamic fitting by Hansson [27] in 2005. They also reinvestigated the phase equilibia of SiO2 –ZnO system using an equilibrium-quenching-EPMA (electron probe X-ray microanalysis) technique. An important advantage of his research was the elimination of the inaccuracies associated with sample composition changes, resulting from the vaporization of zinc oxide. Two binary eutectics involving the congruently melting willemite (Zn2 SiO4 ) were reported at 1448 ± 5 ◦ C and 0.52 ± 0.01 mole fraction of ZnO and at 1502 ± 5 ◦ C and 0.71 ± 0.01 mole fraction of ZnO, respectively. The obtained melting temperature of the congruently melting willemite was in good agreement with previously reported data (1512 ± 3 ◦ C, according to Bunting [22]). No other liquidus data or assessed thermodynamic descriptions of this system were found in the literature. The development of experimental apparatus, temperature measurement and analytical technique enables accurate information on phase equilibrium to be obtained. In present study, the phase equilibrium of binary SiO2 -ZnO system in equilibrium with air has been investigated to confirm the results reported by previous researchers, as well as to determine the temperatures of eutectic reactions. The evaporation of ZnO has been overcome by the application of a new experimental technique. The technique used in this study involved the preparation of a synthetic slag of predetermined composition. Slag equilibrated with air at target temperature for certain period of time. Then rapid quenching, the silica-containing liquid converts to glass so that the phase assemblage that exists at a high temperature is frozen. The crystalline solid solutions were also readily quenched. The compositions of the solid and liquid phase were then measured by an energy dispersive spectrometer (EDS) and confirmed by an electron-probe X-ray microanalysis (EPMA) from polished sections. Each experiment provides information on the liquidus composition and the compositions of the solid phases formed in equilibrium. This greatly increases the productivity of the research and gives information on the solid solutions, which are of particular importance for further thermodynamic modeling. In addition, this work is part of the efforts to consolidate experimental phase equilibria data from binary SiO2 –ZnO system toward characterization of Cu–O–ZnO–SiO2 slags. It ultimately opens up possibilities to extract property data from molten silica and copper rich melts to develop different silicate fluxes in secondary copper pyrometallurgical processes as well as to better control their industrial furnace operations.
2. Material and methods 2.1. Preparation of initial mixtures High-purity oxide powders of SiO2 and ZnO were employed as starting materials, see Table 1. In the equilibrium experiments, pure powders were calcined at 110 ◦ C, weighted in desired proportions, mixed thoroughly in an agate mortar, and pelletized to 5 Mpa. Total weight of the pellet was 0.2 g. The small size of the pellet was an advantage in quenching of the molten phase. The pellet was
Table 1 Mass fraction purity and sources of materials used in the present study Chemical
Purity (mass%)
Supplier
Silicon Dioxide Zinc Oxide SiO2 Crucible Platinum wire Platinum foil
99.99% 99.99% 99.98% 99.99% 99.99%
Umicore (Liechtenstein) Alfa Aesar (Germany) OM Lasilaite Oy (Finland) Johnson-Matthey Noble Metals (UK) Johnson-Matthey Noble Metals (UK)
equilibrated at a fixed temperature for certain period of time in air, and then quenched in ice-cold water. Platinum wires and foils, which were inert in experimental conditions of this study, were used as suspending and holding basket to preserve the purity of the system. 2.2. Selection of container material When conducting experiments on the SiO2 saturated side of the system, small quartz crucibles were employed as the container (Fig. 1 left). The purity and supplier of the crucible were listed in Table 1. Zn2 SiO4 and ZnO substrates were involved in experiments on the Zn2 SiO4 and the ZnO saturated side respectively (Fig. 1 right). The sample mixtures were placed on those substrates, and then equilibrated in an envelope made from 0.05-mm-thick platinum foil. In the preparation of Zn2 SiO4 substrates, 1.0 g mixture of SiO2 powder and ZnO powder in mole ration of 1:2 was mixed thoroughly in an agate mortar, pelletized at 10 Mpa, and then the pellets were manually shaped into a ‘crucible’ that can contain molten phases without spreading. The shaped substrates were sintered in a chamber furnace at a temperature of 1400 ◦ C in air and holding for 48 h. The ZnO substrates were prepared in the same procedure. However, the shaped ZnO pellets were sintered at 1500 ◦ C in air and holding for 60 h. Both heating and cooling rates in the sintering procedures were kept at 4 ◦ C/min. Substrates prepared in this way could be used as supports for the melts because of low porosity, and proper density. 2.3. Equilibration technique The samples were equilibrated in a vertical tube furnace (RHTV 40-250/18, Nabertherm, Germany) within electrical resistance of silicon carbide heating elements (Fig. 2). The uniform temperature hot zone of the furnace was first confirmed by measuring its thermal profiles at sevsral temperatures from 1200 ◦ C to 1700 ◦ C. The temperature of the hot zone was determined with a calibrated S-type thermocouple (Johnson-Matthey Noble Metals, UK), which was connected to a Keithley 2010 DMM multimeter. The working thermocouple in a recrystallized alumina sheath was positioned immediately next to the specimen and periodically tested against a calibrated thermocouple. The cold junction compensation was performed by a Pt100 resistance thermometer (SKS-group, Finland, tolerance class B 1/10), connected to a Keithley 2000 DMM multimeter. Special attention was paid to the accuracy of the equilibrium temperature, and the temperature accuracy was estimated to be ±2 ◦ C for the entire experimental temperature range of interest. The temperature data during the experiments were collected with a NI LabVIEW temperature logging program. The specimen hooked to a pure platinum wire (0.5-mm diameter) was introduced into the furnace from the bottom of the working tube and was suspended in the hot zone. Specimen was kept in the hot zone at the target temperature for the required equilibration period. Particular attention was paid to ensure the achievement of equilibrium, by approaching the final equilibrium compositions with variation of the equilibration time. Achievement
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Fig. 1. Schematic of specimen and crucible suspension techniques (left);
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Specimen, substrate and envelop combination techniques (right).
single-phase solid solution. Three dedicated sets of experiments were conducted to determine the equilibrium time. A longer equilibrium time is a positive factor to approaching the real equilibrium. However, the vaporization of ZnO turned to be a great challenge at elevated temperatures. It is found that in the SiO2 primary field, the glassy places were homogenous throughout the specimen within 4 h when equilibrated at 1450 ◦ C. Equilibrium was achieved within 1 h when temperature was raised up to 1540 ◦ C. In the Zn2 SiO4 and ZnO saturated sides, equilibrium could be achieved in 15 min, and specimen vaporized vanishing in 2 h. The experiments were conducted with the bottom and the top of the reaction tube open to the ambient atmosphere in order to ensure the composition of air. Once the equilibrium time was reached, a bottle of water and ice mixture was attached to the bottom of the tube and the specimen was then rapidly quenched by dropping into the ice-cold water bath (0 ◦ C). It was dried and mounted in epoxy resin. A polished cross section of the mounted specimen was prepared using the conventional metallographic grinding and polishing technique. 2.4. Analysis
Fig. 2. Schematic vertical section of furnace and auxiliaries.
of the equilibrium was determined by measuring the composition of the samples equilibrated for different times. In order to confirm the achievement of equilibrium, the equilibration experiments were repeated starting from both pure oxide powders and from a
The compositions and purities of the substrates were determined by raw materials and the sintering procedure. ZnO substrates prepared in air, using analytical ZnO powder as raw material could meet the requirement of equilibrium experimental work in the ZnO saturated side. The Zn2 SiO4 substrates used in Zn2 SiO4 saturated experiments were characterized by a Philips X-ray diffractometer (XRD), using Cu K␣ radiation. The recorded patterns of as-prepared Zn2 SiO4 substrates (appendix A) showed strong diffraction peaks, which were flawlessly assigned to willemite (JCPDS card #37-1485) [28,29], indicating that the main phase of the substrate was willemite. No other phases were detected. All samples were carbon-coated with a Leica EM SCD050 Coater (supplied by Leica Mikrosysteme GmbH, Vienna). The microstructure and phase composition of the samples were firstly examined by Scanning Electron Microscopy (SEM) and Energy Dispersive
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Table 2 Experimentally determined phase compositions for the (SiO2 –ZnO) system equilibrated with air (PO2 = 21278.25 Pa) No.
Temperature (◦ C)
Equilibrium time (h)
1
(premelting 1480) 1430
(15min) 60
2
1440
12
3
1450
8
4
1480
8
5
1510
8
6
1540
8
7
1570
8
8
1600
8
9
1630
4
10
1662
4
11
1688
4
12
1445
2
13
1502
1
14
1502
1
15
1495
1
16
1500
2
17
1515
0.5
18
1540
0.5
19
1570
0.5
20
1595
0.5
Phase
EDS (mol%)
EPMA (mol%)
SiO2
ZnO
SiO2
ZnO
Standard deviation ()
‘Liquid’
45.67
54.33
45.82
54.18
0.12
SiO2 Zn2 SiO4 SiO2 Liquid SiO2 Liquid SiO2 Liquid SiO2 Liquid SiO2 Liquid SiO2 Liquid SiO2 Liquid SiO2 Liquid SiO2 Liquid SiO2 Liquid Zn2 SiO4 Liquid Zn2 SiO4 Liquid Zn2 SiO4 Zn2 SiO4 ZnO Liquid ZnO Liquid ZnO Liquid ZnO Liquid ZnO Liquid ZnO
98.52 33.74 98.83 47.92 98.70 48.74 98.81 50.23 98.70 50.87 98.90 50.95 98.99 53.86 98.61 55.81 98.91 60.33 98.87 62.50 96.93 47.85 33.88 42.21 33.84 34.75 26.06 33.71 0.11 28.74 0.13 28.02 0.17 27.59 0.09 26.81 0.29 25.64 0.24
1.48 66.26 1.17 52.08 1.30 51.26 1.19 49.77 1.30 49.13 1.10 49.05 1.01 46.14 1.39 44.19 1.09 39.67 1.13 37.50 3.07 52.15 66.12 57.79 66.16 65.25 73.94 66.29 99.89 71.26 99.87 71.98 99.83 72.41 99.91 73.18 99.71 74.36 99.76
98.53 33.92 98.42 47,37 98.51 48.61 98.76 49.70 98.49 51.30 98.57 52.45 98.54 52.67 98.59 55.98 98.56 59.80 98.85 62.27 96.89 47.66 33.54 42.43 34.01 33.63 26.36 33.02 0.20 28.38 0.20 27.97 0.22 27.46 0.15 26.50 0.25 25.87 0.20
1.47 66.08 1.58 52.63 1.49 51.39 1.24 50.30 1.51 48.70 1.43 47.55 1.46 47.33 1.41 44.02 1.44 40.20 1.15 37.73 3.11 52.34 66.46 57.57 65.99 66.37 73.64 66.98 99.80 71.62 99.80 72.03 99.78 72.54 99.85 73.50 99.75 74.13 99.80
0.06 0.05 0.07 0.11 0.07 0.26 0.07 0.20 0.03 0.14 0.02 0.06 0.04 0.20 0.05 0.21 0.40 0.20 0.03 0.10 0.08 0.06 0.07 0.15 0,09 0.08 0.12 0.12 0.03 0.16 0.04 0.19 0.03 0.10 0.06 0.16 0.02 0.18 0.04
*Standard uncertainty: (T) = ± 2 ◦ C; (X) = ± 0.01 mol%. **Standard deviation: (EDS) = 0.34; (EPMA) = 0.11.
Spectrometer (EDS). A LEO 1450 (Carl Zeiss Microscopy GmbH, Jena, Germany) scanning electron microscope was employed with a linked Inca X-Sight 7366 Energy EDS analyzer (Oxford Instruments plc, Abingdon, Oxfordshire, UK). The accelerating voltage employed was 15 kV and employed standards were zinc sulfide (ZnS) for Zn and quartz for Si and O. The spectral lines used for Zn, Si, O were L␣, K␣ and K␣, respectively. The compositions of the solid and liquid phases were then measured by Electron-Probe X-ray Microanalysis (EPMA). The analysis was performed by a CAMECA SX100 instrument, which was equipped with five wavelength dispersive spectrometers. An accelerating voltage of 15 kV, and beam current and diameter of 10 nA and (1-5) m were utilized in the analysis. The standards used in this work were natural minerals (zinc sulfide for Zn, L␣; quartz for Si, K␣). All standards were purchased from Astimex Scientific ltd, Canada. The raw measurement results were corrected using the PAP on-line correction program [30]. 3. Results and discussion All the sample were carefully analyzed, and each of them contained two homogeneous phases, no concentration gradients were detected within them. The obtained EDS and EPMA results of the
Fig. 3. Isopleth diagram of the binary (SiO2 –ZnO) system in air (PO2 = 21278.25 Pa).
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Fig. 4. Backscattered SEM images of quenched SiO2 –ZnO slags in equilibrium with air: (a) specimen equilibrated at 1440 ◦ C for 14 h, starting with 50 mol% ZnO; (b) specimen equilibrated at 1445 ◦ C for 8 h, starting with 55 mol% ZnO; (c) specimen equilibrated at 1495 ◦ C for 2 h, starting with 90 mol% ZnO; (d) specimen equilibrated at 1500 ◦ C for 2 h, starting with 90 mol% ZnO.
phase equilibration experiments of the binary SiO2 –ZnO system in equilibrium with air in the SiO2 , Zn2 SiO4 and ZnO primary phase fields at temperatures range from 1430 ◦ C to 1690 ◦ C were listed in Table 2. The individual standard deviations of the EPMA results were calculated and presented in the table. Overall standard deviations of the EDS and EPMA results were listed below the table. The agreement between these two measurement series was good. For the temperature range of interest, small variations of ZnO solubility in the solid phases of SiO2 , it is 3.11 mol% at 1688 ◦ C. SiO2 solubility in the solid phase of Zn2 SiO4 is small as shown in Table 2, 1.01 mol% SiO2 in Zn2 SiO4 phase. Zn2 SiO4 has smaller practical solubility in solid ZnO phase, it is below 0.25 mol% at the temperature range of interest. The Results in present study have been compared with previous work and the comparison is depicted graphically in Fig. 3. Silicate exists two different polymorphs in the studied temperature range. Tridymite transfers into cristobalite at 1470 ◦ C in air [31]. The calculated phase diagram is based on the work of Bunting in 1930s [22], when the experimental equipment and raw materials affected the results a lot. Evaporation of ZnO at Zn2 SiO4 and ZnO saturated field would contribute to the uncertainty of the data in Bunting’s work. The results of present work are in good agreement with the recent work of Hansson et al. [27]. The concentration of ZnO in the liquid oxide phase at the SiO2 primary field is a little lower than that reported by Hansson et al. [27] when temperature is above 1600 ◦ C, but slightly higher in the other regions. The impurities in Hansson’s starting materials may have accounted for the experimental differences compared to the current study. The eutectic reaction between tridymite, willemite and liquid was found in this study to occur at 1442 ± 2 ◦ C, and at 0.52 ± 0.01 mole fraction of ZnO. As shown in Fig. 4(a), two solid phases (SiO2 and Zn2 SiO4 ) were observed in the sample equilibrated at 1440 ◦ C with a bulk composition between tridymite and willemite (experiment 2). In the similar way, the sample equilibrated at 1442 ◦ C in Hansson’ work [22] had the same results. Specimen with a
bulk composition in the willemite primary-phase equilibrated at 1445 ◦ C (experiment 12) was found to contain a liquid phase and a solid phase, as shown in Fig. 4 (b). The rate of eutectic reacftion (liquid → tridymite + willemite) was obvious slow. Experiment equilibrated at 1430 ◦ C (experiment 1, with 15 min premelting treatment at 1480 ◦ C) for 60 h was found to be consisted with one solid phase and one homogeneous phase. These experiments confirmed both the temperature and the composition of the invariant point. The eutectic temperature between willemite, wurtzite and liquid was found to be 1497 ± 2 ◦ C, and at 0.71 ± 0.01 mole fraction of ZnO in this study. Two solid phases (willemite and wurtzite) were observed in the specimen (experiment 15) equilibrated at 1495 ◦ C (shown in Fig. 4(c)). Primary and secondary wurtzite detected are evidence of the eutectic reaction. The sample (experiment 16) equilibrated at 1500 ◦ C (shown in Fig. 4(d)) with a bulk composition between willemite and wurtzite was found to contain one homogeneous phase and one solid phase.
4. Summary and conclusions The equilibration/quenching/EDS-EPMA analysis technique has been successfully employed to determine the phase equilibria and liquidus temperatures of the binary SiO2 –ZnO system in air. The effect of ZnO on the compositions of liquidus lines of the SiO2 , Zn2 SiO4 and ZnO primary phases of the system in equilibrium with air at temperatures between 1430 ◦ C and 1690 ◦ C has been measured. The obtained liquidus lines are in good agreement with the earlier work of Hansson et al. [22], but show significant differences with the older studies [19,21,25,26]. The temperatures and compositions of the two eutectics between tridymite, willemite and liquid, and willemite, wurtzite and liquid equilibrated in air were estimated to be 1442 ± 2 ◦ C and 0.52 ± 0.01 mole fraction of ZnO, and 1497 ± 2 ◦ C and 0.71 ± 0.01 mole fraction of ZnO, respectively.
Please cite this article in press as: L. Xia, et al., Experimental determination of the liquidus temperatures of the binary (SiO2 –ZnO) system in equilibrium with air, J Eur Ceram Soc (2015), http://dx.doi.org/10.1016/j.jeurceramsoc.2015.07.007
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Appendix A
Fig. A1. XRD patterns of Zn2 SiO4 mixtures and substrates.
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Please cite this article in press as: L. Xia, et al., Experimental determination of the liquidus temperatures of the binary (SiO2 –ZnO) system in equilibrium with air, J Eur Ceram Soc (2015), http://dx.doi.org/10.1016/j.jeurceramsoc.2015.07.007