- Email: [email protected]
Energy efficient synthesis of porous ZrO2 with fine closed pores by microwave irradiation
Materials Letters 93 (2013) 293–296
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Energy efficient synthesis of porous ZrO2 with fine closed pores by microwave irradiation Shinobu Hashimoto a,n, Tomoya Umeda a,b, Kiyoshi Hirao b, Naoki Kondo b, Hideki Hyuga b, Y Zhou b, Sawao Honda a, Yuji Iwamoto a a b
Nagoya Institute of Technology, Department of Environmental Materials and Engineering, Gokiso-cho, Showa-ku, Nagoya 466-8555, Japan National Institute of Advanced Industrial Science and Technology (AIST), Chubu Center, Anagahora, Shimo-shidami, Moriyama-ku, Nagoya 463-8560, Japan
a r t i c l e i n f o
abstract
Article history: Received 3 October 2012 Accepted 24 November 2012 Available online 2 December 2012
Partially stabilized ZrO2 balls with a diameter of 5 mm and a relative density of over 99%, containing 3.7 mass% Y, 0.11 mass% Si, 590 ppm of Ti, 580 ppm of P, and 150 ppm of Ca as impurities were used. When the ZrO2 balls were placed in a BN crucible and heated by microwave irradiation at 2.45 GHz to 1400 1C for 10 min, each ball expanded by 25% in volume due to the formation of numerous fine closed pores and connected with strong linkage and strong bonds formed between the balls. The surface of each ZrO2 ball was a dense layer approximately 100 mm thick, and closed pores 5–20 mm in diameter formed inside the balls. After heating, the open pore porosity was 2% and the closed pore porosity was 22.2% (total: 24.2%). Superplastic behavior of ZrO2 and generated gas species in the system P–O with a high pressure during heating contribute to show the strong bonds and the closed pores, respectively. & 2012 Elsevier B.V. All rights reserved.
Keywords: Ceramics Porous materials Thermodynamics and kinetics in processes in materials
1. Introduction Porous ceramics are useful as lightweight, insulating, and machinable materials. Since ZrO2 has a relatively high density (6.05 103 kg/m3), porous ZrO2 would be more useful in a wide range of applications, such as refractory, energy, and environmental materials. Usually, porous ZrO2 can be fabricated by heating at a temperature below the sintering temperature of raw powder compact [1]. However, the fabrication of porous ZrO2 with fine closed pores is difficult. If porous ZrO2 with fine closed pores is fabricated, the chemical corrosion resistance would increase due to suppressed infiltration of chemical etchant. This effect should increase the lifetime of porous ZrO2 refractory materials. Recently, we have developed a method for forming porous ZrO2 with fine closed pores using an electric carbon furnace [2]. The addition of impurities such as hydroxyapatite, TiO2, and SiO2 increased the pressures of gaseous species in the P–O system to more than the atmospheric pressure, and superplastic behavior of the resulting ZrO2 [3] occurred at 1700 1C. However, this fabrication technique requires considerable energy and processing time. In this study, we attempted to fabricate porous ZrO2 with fine closed pores using less energy by employing a microwave irradiation technique. Generally, ZrO2 has a low dielectric loss, and is therefore not easily heated by microwave irradiation [4].
n
Corresponding author. Tel./fax: þ 81 52 735 5291. E-mail address: [email protected] (S. Hashimoto).
0167-577X/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2012.11.111
Therefore, an SiC susceptor was used to heat the ZrO2 at lower than 800 1C. The suitability of microwave heating for the fabrication of porous ZrO2 with fine closed pores was investigated and compared to that of conventional electrical resistance heating.
2. Experimental procedure Commercial 3 mol % of Y2O3 addition partially stabilized ZrO2 (3Y-PSZ) balls with a 5 mm diameter were used. According to GD mass spectroscopy analysis, 3.7 mass% of Y, 1.7 mass% of Hf, 0.19 mass% of Al, 0.11 mass% of Si, 590 ppm of Ti, 580 ppm of P, and 150 ppm of Ca were present as impurities. The relative density of the ZrO2 balls was above 99.0%. The ZrO2 balls were loaded into a 24-mm-diameter BN crucible without a lid, and the crucible was placed in a microwave heating apparatus, as shown in Fig. 1. The maximum power of the microwave heating apparatus (magnetron) was 6 KW (2.45 GHz). The BN crucible was filled with N2 at a flow rate of 6 L/min. The magnetron power was controlled by checking the temperature at the top surface of the ZrO2 balls using a pyrometer. The change in power during heating was recorded to calculate the total energy consumption. The maximum temperature, duration, and heating rate were 1400 1C, 10 min, and 70 1C/ min, respectively. Furthermore, the intrinsic temperature of the cavity containing the ZrO2 balls was determined by inserting a K-type thermocouple among the ZrO2 balls, and the temperature difference between the surface and interior ZrO2 balls was
294
S. Hashimoto et al. / Materials Letters 93 (2013) 293–296
Pyrometer N gas outlet
Thermal insulation BN crucible ZrO balls (φ 5 mm)
SiC susceptor
Thermocouple
Quartz tube
Magnetron
Magnetron
(Frequency : 2.45 GHz)
(Frequency : 2.45 GHz)
N gas inlet
High temperature behavior of the ZrO2 balls: Fig. 3 shows optical and SEM photographs of the porous ZrO2 balls after the microwave heating treatment. When a single ZrO2 ball was heated, its volume expansion was 25%. The open pore porosity was 2%, whereas the closed pore porosity was 22.2% (total: 24.2%). In contrast, when many balls were heated in the BN crucible, all of the ZrO2 balls expanded, and strong connections were formed among the ZrO2 balls. Analysis of sample cross-sections after cutting with a diamond wheel indicated that strong chemical bonds were formed between the balls. Furthermore, each ball had a dense layer approximately 100 mm thick at the surface, and the interior contained spherical pores 5–20 mm in diameter. Closed pore formation mechanism: A BN crucible was used because BN has a low dielectric constant. In addition, the use of a BN crucible decreased the partial pressure of oxygen: 10 4 to 10 5 atm in N2(g) (impurity) by the following reaction: 4BN(s)þ 3O2(g)-2B2O3(l)þ2N2(g)
Fig. 1. Microwave heating apparatus used.
(1)
As shown in Fig. 2, the true temperature of the ZrO2 balls during heating at 1400 1C (pyrometer) could be as high as 1600 1C (1900 K). At this temperature, B2O3 is a liquid, so the equilibrium constant of Eq. (1), Kp, could be described as follows: 2 2 3 PN2 =½ðABN Þ4 P O2 ð2Þ Kp ¼ ½ AB2 O3 where, AB2O3 and ABN are liquid- and solid-phase activities, respectively, so both values can be set as 1. Furthermore, Eq. (2) can be converted to logarithmic notation, as follows: logK P ¼ 2log P N2 3log PO2
ð3Þ
Under N2 flow, PN2 ¼1; that is: logKp ¼ 3log PO2
ð4Þ
In contrast, at 1900 K, log Kp of Eq. (1) can also be calculated from the log Kp of each substance using JANAF thermochemical data; [7,8] logKp ¼ 36:366 Fig. 2. Temperature profiles of the thermocouple and pyrometer during processing.
measured, as shown in Fig. 1. Finally, the energy savings realized by the use of microwave heating was estimated.
3. Results and discussion Temperature difference: In the early stage of microwave irradiation, the SiC was heated due to its excellent dielectric characteristics, and this heat was conducted from the SiC to the ZrO2 balls [5] via the BN crucible. However, after heating to 800 1C, the ZrO2 began to absorb microwaves, and self-heating of the ZrO2 occurred due to its high dielectric loss [4]. As a result, ZrO2 balls can be easily heated by microwave irradiation at temperatures above 800 1C due to the increase of dielectric loss. In general, the surface and interior temperatures of a sample heated by microwave irradiation will differ [6]. In order to clarify this temperature difference, a K-type thermocouple ( 1000 1C) was inserted among the ZrO2 balls to directly measure the temperature. Fig. 2 shows the temperature profiles of the thermocouple and pyrometer during processing. The temperature among the ZrO2 balls was higher, and the temperature difference was 200 1C at around 1000 1C after heating for 20 min. It is possible that intrinsic temperature has a 200 1C higher value when 1400 1C was measured by pyrometer at the surface of ZrO2 balls. In addition, when microwave irradiation was employed, over and above heating by dielectric loss would occur in the zirconia.
ð5Þ
Finally, the partial pressure of oxygen Po2 can be calculated from Eqs. (4)–(5): ð6Þ logP O2 ¼ 12:122 P O2 ¼ 1012:122 atm At 1900 K, in the CaO–P2O5 system [9], Ca3(PO4)2 (3CaO P2O5) which is the most stable compound due to the highest melting point was supposed to be in the ZrO2 ball in order to estimate the lowest partial pressure of PO(g):log PPO. Under these conditions, the log PPO can be calculated from the following equation under condition (6): 2Ca3(PO4)2 ¼6CaO(s)þ4PO(g)þ3O2(g)
(7)
logP PO ¼ 6:901
ð8Þ
This log PPO is much lower than the atmospheric pressure at 1600 1C (1900 K). The SiC susceptor further decreased the partial pressure of oxygen: log PO2 ¼ 17.002 at 1400 1C (1700 K), and this temperature was actually detected at the SiC surface using a pyrometer. SiCþO2 ¼SiO2 þC
(9)
Log PPO was calculated to be 0.997 using (7). This value is close to the atmospheric pressure. Furthermore, if other compounds from the CaO–P2O5 system such as 2CaO P2O5 and CaO P2O5 were present, PPO would increase even more. Thus, many closed pores formed inside the ZrO2 balls, and were filled with PO(g) during heating at 1600 1C (1900 K). At the same time, superplastic behavior contributed to expanding the volume fraction, forming a dense surface and connecting the ZrO2 balls with
S. Hashimoto et al. / Materials Letters 93 (2013) 293–296
295
Strong connection
Single ball Before
After
between two balls
Volume expansion: 25.0% Many balls inside BN crucible
1 cm 50 μm Cutting
5 μm
1 cm
Integrated power consumption / KWh
Fig. 3. Optical and SEM photographs of the porous ZrO2 balls after microwave heating treatment.
20 18 16 14 12 10 8 6 4 2 0
Energy consumption: Fig. 4 shows the integrated electric energy consumption difference between microwave heating and electric resistance element heating. Although a SiC susceptor was used, when microwave irradiation was used for the fabrication of porous ZrO2 with fine closed pores, the total energy consumption decreased to one tenth that of conventional electric element heating. Microwave heating is therefore a much more energy efficient method for the fabrication of porous ZrO2.
Conventional electric element heating
Microwave heating
0
20
40
60
80
100
120
140
Time / min Fig. 4. Integrated electric energy consumption difference between microwave heating and electric resistance element heating.
strong chemical bonds. In order to lead this superplastic expansion, tensile strength needs more than atmospheric pressure at the wall of each closed pore, but so far the intrinsic pressure value could not be clarified.
4. Summary When partially stabilized ZrO2 balls with 3.7 mass% Y, 0.11 mass% Si, 590 ppm of Ti, 580 ppm of P, and 150 ppm of Ca as impurities were heated by microwave irradiation to 1400 1C for 10 min, porous ZrO2 balls containing close pores 5–20 mm in diameter were formed. The open pore porosity was 2%, whereas the closed pore porosity was 22.2% (total: 24.2%). The total energy consumption for microwave heating was one tenth that of conventional electric resistance element heating.
296
S. Hashimoto et al. / Materials Letters 93 (2013) 293–296
References [1] Perko S, Dakskobler A, Kosmac T. J Eur Ceram Soc 2012;32:2633. [2] Hashimoto S, Umeda T, Hirao K, Kondo N, Zhou Y, Hyuga H, et al. J Eur Ceram Soc 2013;33:61. [3] Wakai F, Sakaguchi S, Matsuno Y. Adv Ceram Mater 1986;1:259. [4] Charmond S, Carry CP, Bouvard D. J Eur Ceram Soc 2010;30:1211. [5] Li Z, Zhou W, Lei T, Luo F, Huang Y, Cao Q. J Alloy Compd 2009;475:506.
[6] Hashimoto S, Ohashi S, Hirao K, Zhou Y, Hyuga H, Honda S, et al. J Ceram Soc Jpn 2011;119:740. [7] Chase Jr. MW, Davies CA, Downey Jr. JR, Frurip DJ, McDonald RA, Syverud AN. JANAF Thermochemical Tables. 3rd Ed. Columbus, Ohio: American Ceramic Society; 1985. [8] Barin I. Thermochemical Data of Pure Substances. 3rd Ed. Weinheim: VCH; 1995. [9] Levin EM, Robbins CR, McMurdie HF. Phase Diagram for Ceramists. 2nd Ed. Columbus, Ohio: American Ceramic Society; 1969 Fig. 246.
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Energy efficient synthesis of porous ZrO2 with fine closed pores by microwave irradiation Shinobu Hashimoto a,n, Tomoya Umeda a,b, Kiyoshi Hirao b, Naoki Kondo b, Hideki Hyuga b, Y Zhou b, Sawao Honda a, Yuji Iwamoto a a b
Nagoya Institute of Technology, Department of Environmental Materials and Engineering, Gokiso-cho, Showa-ku, Nagoya 466-8555, Japan National Institute of Advanced Industrial Science and Technology (AIST), Chubu Center, Anagahora, Shimo-shidami, Moriyama-ku, Nagoya 463-8560, Japan
a r t i c l e i n f o
abstract
Article history: Received 3 October 2012 Accepted 24 November 2012 Available online 2 December 2012
Partially stabilized ZrO2 balls with a diameter of 5 mm and a relative density of over 99%, containing 3.7 mass% Y, 0.11 mass% Si, 590 ppm of Ti, 580 ppm of P, and 150 ppm of Ca as impurities were used. When the ZrO2 balls were placed in a BN crucible and heated by microwave irradiation at 2.45 GHz to 1400 1C for 10 min, each ball expanded by 25% in volume due to the formation of numerous fine closed pores and connected with strong linkage and strong bonds formed between the balls. The surface of each ZrO2 ball was a dense layer approximately 100 mm thick, and closed pores 5–20 mm in diameter formed inside the balls. After heating, the open pore porosity was 2% and the closed pore porosity was 22.2% (total: 24.2%). Superplastic behavior of ZrO2 and generated gas species in the system P–O with a high pressure during heating contribute to show the strong bonds and the closed pores, respectively. & 2012 Elsevier B.V. All rights reserved.
Keywords: Ceramics Porous materials Thermodynamics and kinetics in processes in materials
1. Introduction Porous ceramics are useful as lightweight, insulating, and machinable materials. Since ZrO2 has a relatively high density (6.05 103 kg/m3), porous ZrO2 would be more useful in a wide range of applications, such as refractory, energy, and environmental materials. Usually, porous ZrO2 can be fabricated by heating at a temperature below the sintering temperature of raw powder compact [1]. However, the fabrication of porous ZrO2 with fine closed pores is difficult. If porous ZrO2 with fine closed pores is fabricated, the chemical corrosion resistance would increase due to suppressed infiltration of chemical etchant. This effect should increase the lifetime of porous ZrO2 refractory materials. Recently, we have developed a method for forming porous ZrO2 with fine closed pores using an electric carbon furnace [2]. The addition of impurities such as hydroxyapatite, TiO2, and SiO2 increased the pressures of gaseous species in the P–O system to more than the atmospheric pressure, and superplastic behavior of the resulting ZrO2 [3] occurred at 1700 1C. However, this fabrication technique requires considerable energy and processing time. In this study, we attempted to fabricate porous ZrO2 with fine closed pores using less energy by employing a microwave irradiation technique. Generally, ZrO2 has a low dielectric loss, and is therefore not easily heated by microwave irradiation [4].
n
Corresponding author. Tel./fax: þ 81 52 735 5291. E-mail address: [email protected] (S. Hashimoto).
0167-577X/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2012.11.111
Therefore, an SiC susceptor was used to heat the ZrO2 at lower than 800 1C. The suitability of microwave heating for the fabrication of porous ZrO2 with fine closed pores was investigated and compared to that of conventional electrical resistance heating.
2. Experimental procedure Commercial 3 mol % of Y2O3 addition partially stabilized ZrO2 (3Y-PSZ) balls with a 5 mm diameter were used. According to GD mass spectroscopy analysis, 3.7 mass% of Y, 1.7 mass% of Hf, 0.19 mass% of Al, 0.11 mass% of Si, 590 ppm of Ti, 580 ppm of P, and 150 ppm of Ca were present as impurities. The relative density of the ZrO2 balls was above 99.0%. The ZrO2 balls were loaded into a 24-mm-diameter BN crucible without a lid, and the crucible was placed in a microwave heating apparatus, as shown in Fig. 1. The maximum power of the microwave heating apparatus (magnetron) was 6 KW (2.45 GHz). The BN crucible was filled with N2 at a flow rate of 6 L/min. The magnetron power was controlled by checking the temperature at the top surface of the ZrO2 balls using a pyrometer. The change in power during heating was recorded to calculate the total energy consumption. The maximum temperature, duration, and heating rate were 1400 1C, 10 min, and 70 1C/ min, respectively. Furthermore, the intrinsic temperature of the cavity containing the ZrO2 balls was determined by inserting a K-type thermocouple among the ZrO2 balls, and the temperature difference between the surface and interior ZrO2 balls was
294
S. Hashimoto et al. / Materials Letters 93 (2013) 293–296
Pyrometer N gas outlet
Thermal insulation BN crucible ZrO balls (φ 5 mm)
SiC susceptor
Thermocouple
Quartz tube
Magnetron
Magnetron
(Frequency : 2.45 GHz)
(Frequency : 2.45 GHz)
N gas inlet
High temperature behavior of the ZrO2 balls: Fig. 3 shows optical and SEM photographs of the porous ZrO2 balls after the microwave heating treatment. When a single ZrO2 ball was heated, its volume expansion was 25%. The open pore porosity was 2%, whereas the closed pore porosity was 22.2% (total: 24.2%). In contrast, when many balls were heated in the BN crucible, all of the ZrO2 balls expanded, and strong connections were formed among the ZrO2 balls. Analysis of sample cross-sections after cutting with a diamond wheel indicated that strong chemical bonds were formed between the balls. Furthermore, each ball had a dense layer approximately 100 mm thick at the surface, and the interior contained spherical pores 5–20 mm in diameter. Closed pore formation mechanism: A BN crucible was used because BN has a low dielectric constant. In addition, the use of a BN crucible decreased the partial pressure of oxygen: 10 4 to 10 5 atm in N2(g) (impurity) by the following reaction: 4BN(s)þ 3O2(g)-2B2O3(l)þ2N2(g)
Fig. 1. Microwave heating apparatus used.
(1)
As shown in Fig. 2, the true temperature of the ZrO2 balls during heating at 1400 1C (pyrometer) could be as high as 1600 1C (1900 K). At this temperature, B2O3 is a liquid, so the equilibrium constant of Eq. (1), Kp, could be described as follows: 2 2 3 PN2 =½ðABN Þ4 P O2 ð2Þ Kp ¼ ½ AB2 O3 where, AB2O3 and ABN are liquid- and solid-phase activities, respectively, so both values can be set as 1. Furthermore, Eq. (2) can be converted to logarithmic notation, as follows: logK P ¼ 2log P N2 3log PO2
ð3Þ
Under N2 flow, PN2 ¼1; that is: logKp ¼ 3log PO2
ð4Þ
In contrast, at 1900 K, log Kp of Eq. (1) can also be calculated from the log Kp of each substance using JANAF thermochemical data; [7,8] logKp ¼ 36:366 Fig. 2. Temperature profiles of the thermocouple and pyrometer during processing.
measured, as shown in Fig. 1. Finally, the energy savings realized by the use of microwave heating was estimated.
3. Results and discussion Temperature difference: In the early stage of microwave irradiation, the SiC was heated due to its excellent dielectric characteristics, and this heat was conducted from the SiC to the ZrO2 balls [5] via the BN crucible. However, after heating to 800 1C, the ZrO2 began to absorb microwaves, and self-heating of the ZrO2 occurred due to its high dielectric loss [4]. As a result, ZrO2 balls can be easily heated by microwave irradiation at temperatures above 800 1C due to the increase of dielectric loss. In general, the surface and interior temperatures of a sample heated by microwave irradiation will differ [6]. In order to clarify this temperature difference, a K-type thermocouple ( 1000 1C) was inserted among the ZrO2 balls to directly measure the temperature. Fig. 2 shows the temperature profiles of the thermocouple and pyrometer during processing. The temperature among the ZrO2 balls was higher, and the temperature difference was 200 1C at around 1000 1C after heating for 20 min. It is possible that intrinsic temperature has a 200 1C higher value when 1400 1C was measured by pyrometer at the surface of ZrO2 balls. In addition, when microwave irradiation was employed, over and above heating by dielectric loss would occur in the zirconia.
ð5Þ
Finally, the partial pressure of oxygen Po2 can be calculated from Eqs. (4)–(5): ð6Þ logP O2 ¼ 12:122 P O2 ¼ 1012:122 atm At 1900 K, in the CaO–P2O5 system [9], Ca3(PO4)2 (3CaO P2O5) which is the most stable compound due to the highest melting point was supposed to be in the ZrO2 ball in order to estimate the lowest partial pressure of PO(g):log PPO. Under these conditions, the log PPO can be calculated from the following equation under condition (6): 2Ca3(PO4)2 ¼6CaO(s)þ4PO(g)þ3O2(g)
(7)
logP PO ¼ 6:901
ð8Þ
This log PPO is much lower than the atmospheric pressure at 1600 1C (1900 K). The SiC susceptor further decreased the partial pressure of oxygen: log PO2 ¼ 17.002 at 1400 1C (1700 K), and this temperature was actually detected at the SiC surface using a pyrometer. SiCþO2 ¼SiO2 þC
(9)
Log PPO was calculated to be 0.997 using (7). This value is close to the atmospheric pressure. Furthermore, if other compounds from the CaO–P2O5 system such as 2CaO P2O5 and CaO P2O5 were present, PPO would increase even more. Thus, many closed pores formed inside the ZrO2 balls, and were filled with PO(g) during heating at 1600 1C (1900 K). At the same time, superplastic behavior contributed to expanding the volume fraction, forming a dense surface and connecting the ZrO2 balls with
S. Hashimoto et al. / Materials Letters 93 (2013) 293–296
295
Strong connection
Single ball Before
After
between two balls
Volume expansion: 25.0% Many balls inside BN crucible
1 cm 50 μm Cutting
5 μm
1 cm
Integrated power consumption / KWh
Fig. 3. Optical and SEM photographs of the porous ZrO2 balls after microwave heating treatment.
20 18 16 14 12 10 8 6 4 2 0
Energy consumption: Fig. 4 shows the integrated electric energy consumption difference between microwave heating and electric resistance element heating. Although a SiC susceptor was used, when microwave irradiation was used for the fabrication of porous ZrO2 with fine closed pores, the total energy consumption decreased to one tenth that of conventional electric element heating. Microwave heating is therefore a much more energy efficient method for the fabrication of porous ZrO2.
Conventional electric element heating
Microwave heating
0
20
40
60
80
100
120
140
Time / min Fig. 4. Integrated electric energy consumption difference between microwave heating and electric resistance element heating.
strong chemical bonds. In order to lead this superplastic expansion, tensile strength needs more than atmospheric pressure at the wall of each closed pore, but so far the intrinsic pressure value could not be clarified.
4. Summary When partially stabilized ZrO2 balls with 3.7 mass% Y, 0.11 mass% Si, 590 ppm of Ti, 580 ppm of P, and 150 ppm of Ca as impurities were heated by microwave irradiation to 1400 1C for 10 min, porous ZrO2 balls containing close pores 5–20 mm in diameter were formed. The open pore porosity was 2%, whereas the closed pore porosity was 22.2% (total: 24.2%). The total energy consumption for microwave heating was one tenth that of conventional electric resistance element heating.
296
S. Hashimoto et al. / Materials Letters 93 (2013) 293–296
References [1] Perko S, Dakskobler A, Kosmac T. J Eur Ceram Soc 2012;32:2633. [2] Hashimoto S, Umeda T, Hirao K, Kondo N, Zhou Y, Hyuga H, et al. J Eur Ceram Soc 2013;33:61. [3] Wakai F, Sakaguchi S, Matsuno Y. Adv Ceram Mater 1986;1:259. [4] Charmond S, Carry CP, Bouvard D. J Eur Ceram Soc 2010;30:1211. [5] Li Z, Zhou W, Lei T, Luo F, Huang Y, Cao Q. J Alloy Compd 2009;475:506.
[6] Hashimoto S, Ohashi S, Hirao K, Zhou Y, Hyuga H, Honda S, et al. J Ceram Soc Jpn 2011;119:740. [7] Chase Jr. MW, Davies CA, Downey Jr. JR, Frurip DJ, McDonald RA, Syverud AN. JANAF Thermochemical Tables. 3rd Ed. Columbus, Ohio: American Ceramic Society; 1985. [8] Barin I. Thermochemical Data of Pure Substances. 3rd Ed. Weinheim: VCH; 1995. [9] Levin EM, Robbins CR, McMurdie HF. Phase Diagram for Ceramists. 2nd Ed. Columbus, Ohio: American Ceramic Society; 1969 Fig. 246.