- Email: [email protected]
Analysis of properties of partially stabilized zirconia-doped Na+-beta-alumina prepared by calcining-cum-sintering process
Accepted Manuscript Title: Analysis of properties of partially stabilized zirconia-doped Na+ -beta-alumina prepared by calcining-cum-sintering process Authors: Dae-Han Lee, Sung-Tae Lee, Jin-Sik Kim, Sung-Ki Lim PII: DOI: Reference:
S0025-5408(16)31589-6 http://dx.doi.org/doi:10.1016/j.materresbull.2017.05.003 MRB 9318
To appear in:
MRB
Received date: Revised date: Accepted date:
18-10-2016 13-4-2017 1-5-2017
Please cite this article as: Dae-Han Lee, Sung-Tae Lee, Jin-Sik Kim, SungKi Lim, Analysis of properties of partially stabilized zirconia-doped Na+beta-alumina prepared by calcining-cum-sintering process, Materials Research Bulletinhttp://dx.doi.org/10.1016/j.materresbull.2017.05.003 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Analysis of properties of partially stabilized zirconia-doped Na+-beta-alumina prepared by calcining-cum-sintering process Dae-Han Lee, Sung-Tae Lee, Jin-Sik Kim, and Sung-Ki Lim* Department of Materials Chemistry and Engineering, Konkuk University, 120 Neungdong-ro, Gwangjin-gu, Seoul 143-701, Republic of Korea Corresponding author: Sung-Ki Lim Address: Department of Materials Chemistry and Engineering, Konkuk University, 120 Neungdong-ro, Gwangjin-gu, Seoul 143-701, Republic of Korea Telephone: 82-2-450-3500 Fax: 82-2-444-3490 E-mail: [email protected] Graphical Abstract
BA BA_3YSZ BA_10YSZ BA_3CSZ BA_10CSZ
-1
Conductivity(S cm )
0.1 0.01 1E-3 1E-4 1E-5 1E-6 1.5
2.0
2.5 -1
1000/T(K )
3.0
3.5
Highlights
The PSZ(Partially Stabilized Zirconia: YSZ or CSZ)-doped Na+-β/β"-Al2O3 as a Na+ solid electrolyte were successfully fabricated using a calcining-cum-sintering method as a simplified process. The β"-phase fraction of PSZ-doped Na+-β/β"-Al2O3 was maintained compared to the undoped Na-β/β"-Al2O3. The relative density and the ionic conductivity of PSZ-doped Na+-β/β"-Al2O3 were higher than those of the undoped Na+-β/β"-Al2O3. The ionic conductivity values of the specimens prepared using the calcining-cumsintering method were approximately 7–10 Ω·cm, which were as good as those produced by the conventional process. The maximum and minimum ionic conductivities, i.e., 1.31 × 10−1 and 1.41 × 10−2 S/cm at 350 °C, were obtained in the BA_3CSZ and BA_10CSZ specimens, respectively. In case of CSZ 10wt%-doped Na+-β/β"-Al2O3, however, the ionic conductivity was significantly decreased owing to the formation of secondary phase of Na4Ca3(AlO2)10.
Abstract To overcome the disadvantages of a conventional process, including its complex steps, Na+-β/β″-Al2O3 for use as a Na+-conductive solid electrolyte was prepared using a calcining-cum-sintering process. Further, to improve the sintering properties, partially CaO- or Y2O3-stabilized zirconia (CSZ or YSZ, respectively) was added as a sintering additive. To analyze the properties of the samples, the XRD patterns, sintered density, microstructure, and ionic conductivity of each partially stabilized zirconia (PSZ)-doped Na+-β/β″-Al2O3 specimen were investigated. The ionic conductivity of each specimen was measured using AC impedance spectroscopy, and the highest ionic conductivity of 1.3 ×10−1 S/cm was observed for the BA_3CSZ specimen. The specific resistance values of the PSZ-doped Na+-β/β″-Al2O3 specimens were approximately 7–10 Ω·cm and were as good as those produced by the conventional process, with the exception of the BA_10CSZ specimen. The ionic conductivity of BA_10CSZ was significantly decreased, owing mainly to the formation of the secondary phase, Na4Ca3(AlO2)10.
Keywords: Na+-beta-alumina; beta-alumina; stabilized zirconia; solid electrolyte 1. Introduction Na+-β/β″-Al2O3 has been widely utilized as a solid electrolyte in sodium beta batteries for electric vehicles and storage of excess electricity [1–3], where the Na+ ions act as charge carriers. Na+-β/β″-Al2O3 is characterized by a highly ionic nature and low electronic conductivity, and extensive research has been performed to exploit these features for practical purposes. There are two parent phases: β-alumina has the theoretical formula Na2O·11Al2O3 or NaAl11O17, and β″-alumina has the theoretical formula Na2O·5Al2O3 or NaAl5O8 [4–6]. According to the Na2O–Al2O3 phase diagram proposed by Fally et al. [7], the β + β″ phases coexist in a region corresponding to the formula Na2O·nAl2O3 (5.33 ≤ n ≤ 8.5). β″-alumina has a rhombohedral structure with an R3m space group and lattice constants of a = 5.614 and c = 33.85 Å. Generally, its a axis is similar to that of β-alumina; its c axis is 1.5 times longer, and the concentration of alkaline earth metal ions on its conduction plane is higher. Therefore, β″-alumina shows a much higher ionic conductivity [8, 9]. Of the beta-alumina solid electrolytes, a MgO (or Li2O)-stabilized beta-alumina solid electrolyte has conventionally been produced as follows. A stabilizer, a sodium compound, and α-alumina are mixed at an appropriate ratio. The mixture is calcined to obtain beta-alumina. Then, after grinding, the ground material is granulated, molded into a desired shape, and sintered to obtain a beta-alumina solid electrolyte. In this conventional process, the materials are calcined in advance to obtain beta-alumina because when the production of beta-alumina by direct firing is attempted without calcination, severe volume expansion occurs in the phase transition from α-alumina to
beta-alumina, making it difficult to obtain a beta-alumina solid electrolyte with uniform quality and high strength. However, in the conventional process, the preliminary calcination step to obtain beta-alumina increases the complexity of the steps and the total cost. Hence, in the production of sodium–sulfur- or sodium-based batteries (e.g., the Zeolite Battery Research Africa Project battery), it would be very beneficial to shorten the number of production steps and produce a beta-alumina solid electrolyte more efficiently. Further, because preliminary calcination (to obtain beta-alumina), grinding, granulation, molding, and sintering are conducted in the conventional process described above, the crystals constituting the resulting beta-alumina solid electrolyte are highly oriented. Moreover, the electrolyte contains large crystals because the beta-alumina formed by calcination acts as a nucleus during crystal growth. Consequently, a calciningcum-sintering process has advantages over the conventional process, including a simpler fabrication process, and cost and energy reductions. Fig. 1 shows schematic illustrations of the conventional and calcining-cum-sintering processes for Na+-β/β″-Al2O3 sintered specimens [10–12]. Fig. 1 Schematic illustrations of (a) conventional and (b) calcining-cum-sintering processes Pure zirconia (ZrO2) has a cubic fluorite structure at temperatures between its melting point (2680 °C) and 2370 °C. It is transformed into a tetragonal phase below 2370 °C and a monoclinic phase below 1170 °C [13]. Cations with less than four valence electrons, such as Y3+, Ca2+, and Mg2+, create oxygen vacancies and can be used to stabilize the high-temperature zirconia phases over the entire temperature range. Partially stabilized zirconia (PSZ) has been widely used for applications such as oxygen sensors, advanced
ceramics, and solid oxygen fuel cells. Moreover, the addition of PSZ to α-alumina enhances its mechanical strength [14, 15]. There have been many studies on the mechanical and electrochemical properties of α-alumina with yttria-stabilized zirconia (YSZ) [16–20] and calcia-stabilized zirconia (CSZ) [21, 22] additives. Moreover, the addition of YSZ to Na+-β/β″-Al2O3 has been shown to improve both its fracture toughness and strength without compromising the electrical properties of this superionic material [23, 24]. The enhancement in fracture toughness leads to an obvious improvement in its critical current density [25], which has significant economic advantages with regard to battery performance. For example, in addition to extending the battery life, the increase in the charging current density could dramatically shorten the charging time [26]. On the other hand, the addition of CSZ to Na+-β/β″-Al2O3 has not been reported recently. In this study, PSZ (YSZ or CSZ)-doped Na+-β/β″-Al2O3 specimens stabilized with MgO were fabricated using a calcining-cum-sintering process, and the relationship between the YSZ or CSZ content and the properties of the doped Na+-β/β″-Al2O3 samples were investigated. 2. Materials and methodology Undoped Na+-β/β″-Al2O3 specimens and Na+-β/β″-Al2O3 specimens doped with YSZ or CSZ were synthesized via a calcining-cum-sintering process using commercial αAl2O3 (99.99%, High Purity Chemicals, Japan), Na2CO3 (99+%, Sigma-Aldrich, USA), Mg(OH)2 (99+%, Sigma-Aldrich, USA), YSZ (99.9%, High Purity Chemicals, Japan), and CSZ (99+%, High Purity Chemicals, Japan) powders as the starting materials. The synthesized Na+-β/β″-Al2O3 specimens contained 1.6 wt% MgO (as a stabilizer), Na2O at
an [Na2O]/[Al2O3] molar ratio of 1:5, and YSZ or CSZ amounts ranging from 3.0 to 10 wt%. Table 1 lists the compositions and designations of the evaluated specimens. Table 1. Compositions and designations of sintered specimens with and without PSZ The raw materials were ball-milled for 5 h. The slurries used in the granulation step contained 0.6 wt% polyvinyl alcohol (Mw 13,000–23,000, Sigma-Aldrich, USA) as a binder for the composite powder, 0.8 wt% ammonium polymethacrylate (DARVAN® CN, R.T. Vanderbilt, USA) as a dispersant, and 0.8 wt% octyl alcohol (99+%, SigmaAldrich, USA) as an antifoaming agent in distilled water. Before spray-drying, the slurry formulations were ball-milled. Then, they were spray-dried using a disk-type spray dryer, where the inlet and outlet temperatures were set at 200 and 110 °C, respectively. After the spray-drying was complete, only granules that passed through a 100 mesh (149 μm) sieve were collected. To prepare the sintered specimens, the granules were uniaxially pressed at 100 MPa and sintered for 30 min at 1600 °C. Although sintering is typically conducted at 1650 °C, a temperature of 1600 °C was used in this experiment to maximize the advantages of the process. The sintered density of each specimen was measured using the Archimedes method. The sintered specimens’ microstructures and phase fractions were assessed using scanning electron microscopy (SEM; Model JSM-6380, JEOL, Japan) and X-ray diffraction (XRD, D/max 2200, Rigaku, Japan) measurements. XRD analysis was conducted at 40 kV and 30 mA using Cu Kα radiation. The relative phase composition was determined by calculating the line intensities of each phase’s wellseparated peaks [27, 28]: % 𝐨𝐟 𝛂 =
𝐟(𝛂) × 𝟏𝟎𝟎 𝐟(𝛂) + 𝐟(𝛃) + 𝐟(𝛃")
(𝟏)
% 𝐨𝐟 𝛃 =
𝐟(𝛃) × 𝟏𝟎𝟎 ( ) 𝐟 𝛂 + 𝐟(𝛃) + 𝐟(𝛃")
% 𝐨𝐟 𝛃" =
𝐟(𝛃") × 𝟏𝟎𝟎 ( ) 𝐟 𝛂 + 𝐟(𝛃) + 𝐟(𝛃")
(𝟐)
(𝟑)
𝐟(𝛂) =
𝟏 𝟏𝟎 { 𝐈𝛂(𝟏𝟎𝟒) × + 𝐈𝛂(𝟏𝟏𝟑) } 𝟐 𝟗
𝐟(𝛃) =
𝟏 𝟏𝟎 𝟏𝟎 𝟏𝟎 {𝐈𝛃(𝟏𝟎𝟐) × } (𝟓) + 𝐈𝛃(𝟐𝟎𝟔) × + 𝐈𝛃(𝟏𝟎𝟕) × 𝟑 𝟑 𝟑. 𝟓 𝟓. 𝟓
𝐟(𝛃") =
𝟏 𝟏𝟎 𝟏𝟎 {𝑰𝜷"(𝟏𝟎𝟏𝟏) × + 𝑰𝛃"(𝟐𝟎𝟏𝟎) × } (𝟔) 𝟐 𝟒 𝟖
(𝟒)
where Iα(104) and Iα(113) denote the X-ray intensities of the (104) and (113) planes of the αalumina phase, respectively; Iβ(102), Iβ(206), and Iβ(107) represent the X-ray intensities of the (102), (206), and (107) planes of the β-alumina phase, respectively; and Iβ"(1011) and Iβ"(2010) indicate the X-ray intensities of the (1011) and (2010) planes of the β"-alumina phase, respectively. The microstructures and densities of the sintered specimens were determined using SEM (Model JSM-6380, Japan) and the Archimedes method (ASTM 373-88), respectively. The ionic conductivities of the sintered specimens were measured using a four-probe AC impedance analyzer (IM6, Zahner) with blocking silver electrodes in a frequency range of 1 Hz to 3 MHz and temperature range of 25–350 °C. The sodium conductivities were calculated using Eq. (7). 𝛔=
𝑳 (𝟕) 𝑹𝒔 × 𝑨
where σ, L, RS, and A denote the ionic conductivity, specimen thickness, measured resistance, and electrode area, respectively.
3. Results and discussion
Fig. 2 schematically shows the structures of β-Al2O3 and β″-Al2O3, which contain a tightly packed spinel block and loosely arranged conduction slab where the sodium ions can be rapidly transported. Three spinel blocks are contained in each β″-alumina unit cell, whereas β-Al2O3 has two spinel blocks per unit cell. In the beta-alumina structure, aluminum ions are found in both the octahedral and tetrahedral sites. Generally, divalent cations (Ca2+, Mg2+, Co2+, Mn2+, Zn2+, and Fe2+) strongly prefer a tetrahedral coordination environment in the beta-alumina structure [29].
Fig. 2 Schematic representations of idealized structures of (a) Na+-β-Al2O3 and (b) Na+β″-Al2O3 [8, 9]
Fig. 3 shows the XRD patterns of the undoped and PSZ-doped Na+-β/β″-Al2O3 sintered specimens. The XRD pattern of the BA sintered specimen shows that a phase transformation of α-Al2O3 to Na+-β/β″-Al2O3 occurred without any α-Al2O3 remaining. The XRD patterns of all the sintered specimens conform to the crystalline phases of NaAl5O8 (JCPDS 31-1262) and NaAl11O17 (JCPDS 31-1263). All of the XRD patterns of the sintered specimens exhibited growth of the (107) and (206) planes, and the (1011) and (2010) planes, which correspond to β- and β″-alumina, respectively. The β″-alumina peaks had higher intensities than the β-alumina peaks, and the results showed that the
sintered specimens show successful synthesis of Na+-β/β″-Al2O3, with the exception of the BA_10CSZ specimen. The secondary Na4Ca3(AlO2)10 phase (JCPDS 02-1003) was clearly observed in the BA_10CSZ specimen. The CSZ phase was considered to be clearly destabilized to the monoclinic phase by the reaction between Al from the Al2O3 and Ca from the CSZ [13, 18]. As a result, the peak intensity of the tetragonal phases for the peaks in a 2θ range of 30.4° decreased with increasing CaO content, and the peak for BA_10CSZ at 28.2° was assigned to the monoclinic phase. The tetragonal phase of CSZ was destabilized with Na+-β/β″-Al2 O3 during the calcining-cum-sintering step. Fig. 4 shows the phase diagram of the ternary system Na2O–CaO–Al2O3, which was produced by Brownmiller and Bogue [30].
Fig. 3 XRD patterns of sintered specimens with and without PSZ powder
Fig. 4 Na2O–CaO–Al2O3 ternary phase diagram presented by Brownmiller and Bogue [30], where C = CaO, A = Al2O3, and N = Na2O
The phase fractions of β″-Al2O3 were calculated using Eqs. (1)–(6) (Fig. 5). Regardless of the type of PSZ, the β″-Al2O3 phase fraction was maintained at that of the BA sample, with the exception of the BA_10CSZ sintered specimen. The β″-Al2O3 phase fractions of the BA and BA_10CSZ specimens were 61% and 44%, respectively. In the BA_10CSZ specimen, the dissolved Ca from the destabilized CSZ phase in the Na+-β/β″-Al2O3 led to excessive volatilization of the Na2O to adjust the charge balance of the Na+-β/β″-Al2 O3,
and the β″-Al2O3 phase fraction decreased. In the YSZ-doped Na+-β/β″-Al2O3, the relative density (RD) increased with the addition of YSZ powder. Doping with CSZ powder also improved the relative sintered density of the Na+-β/β″-Al2O3 compared to that of the BA sintered specimen. The sintered BA and PSZ-doped Na+-β/β″-Al2O3 specimens showed relative densities in the range of 95%–99.5%. In general, in the range of pure Na+-β/β″-Al2O3 to 21 wt% zirconia, the sintered density of the doped Na+-β/β″Al2O3 increased with the zirconia content, indicating that the addition of zirconia particles allowed the densification of the alumina matrix, and the volume expansion from tetragonal to monoclinic associated with the phase transformation improved the relative sintered density [31]. The relative sintered density increased slightly with the addition of CSZ than YSZ-doped specimens. The increase in the Ca content of CSZ caused the formation of liquid phases during the sintering process [36], and this aided the densification of the Na+-β/β″-Al2O3 specimens.
Fig. 5 Relative sintered density and β″-phase fraction of Na+-β/β″-Al2O3 sintered specimens with and without PSZ powder
Fig. 6 shows SEM images of the sintered Na+-β/β″-Al2O3 specimens. Platelet-shaped or anisotropic grain growth clearly occurred without and with the addition of PSZ powder. The dispersed PSZ grains, which appear as relatively white regions in Fig. 6(b), are located at the grain boundaries of the Na+-β/β″-Al2O3 grains. As a result, the Na+-β/β″Al2O3 grains were entrapped by the dispersed PSZ, which inhibited the rapid and abnormal grain growth of Na+-β/β″-Al2O3 crystallites [32, 33]. The intergranular liquid
at the BA_10CSZ specimen surface penetrated into the grain boundaries in the surface region compared to the BA_3YSZ specimen. This suggests that the intergranular liquid is not uniformly distributed throughout the specimen but is dynamically redistributed during sintering, as has been reported in many other systems [34–36]. Indeed, the progress of densification and grain growth during sintering of the CSZ-doped Na+-β/β″-Al2O3 was comparable to that of the YSZ-doped Na+-β/β″-Al2 O3 [22].
Fig. 6 SEM images of sintered (a) BA (RD: 95.1%), (b) BA_3YSZ (RD: 98.5%), (c) BA_10YSZ (RD: 98.6%), (d) BA_3CSZ (RD: 99.2%), and (e) BA_10CSZ (RD: 99.5%) specimens
Fig. 7 shows the relationship between the ionic conductivity and the temperature of the YSZ- and CSZ-doped Na+-β/β″-Al2O3 specimens compared to that of the BA specimen at 25–350 °C. The previous results and ionic conductivities of undoped and PSZ-doped Na+-β/β″-Al2O3 sintered specimens at 350 °C are listed in Table 2. The ionic conductivity of the BA sintered specimen was 6.57 × 10−2 S/cm at 350 °C. In the YSZ-doped Na+β/β″-Al2O3, the ionic conductivity improved with increasing YSZ content compared to that of the BA sintered specimen. An ionic conductivity of 1.26 × 10−1 S/cm at 350 °C was realized in the BA_3YSZ specimen. In the BA_10YSZ specimen, however, the ionic conductivity decreased compared to that of the BA_3YSZ specimen. While potentially enhancing the mechanical strength, the addition of ZrO2 to Na+-β/β″-Al2O3 might degrade the electrical performance because of its poor sodium ionic conductivity. Heavens studied the strength improvement in Na+-β/β″-Al2O3 with the addition of ZrO2 and found that the
resistivity did not increase significantly until the ZrO 2 content reached 10 wt% [37]. In the CSZ-doped Na+-β/β″-Al2O3, doping with 3 wt% CSZ powder enhanced the ionic conductivity. More specifically, the maximum and minimum ionic conductivities, i.e., 1.31 × 10−1 and 1.41 × 10−2 S/cm at 350 °C, were obtained in the BA_3CSZ and BA_10CSZ specimens, respectively. The improvement in the ionic conductivity of the CSZ-doped Na+-β/β″-Al2O3 specimens resulted from the increase in the relative sintered density via the liquid-phase sintering process, which promotes densification of Na+-β/β″Al2O3. Addition of excess CSZ, for example, in BA_10CSZ, however, decreased the ionic conductivity, although doping with a small amount of CSZ improves the ionic conductivity of the BA specimens. The addition of 10 wt% CSZ powder reduced the ionic conductivity owing to the formation of Na4Ca3(AlO2)10 as a secondary phase by a reaction between Al from the Al2O3 and Ca from the CSZ and the resulting decrease in the β″Al2O3 phase fraction.
Fig. 7 Ionic conductivities of sintered specimens as function of addition of PSZ powder at 25–350 °C.
Table 2. Ionic conductivities, relative sintered densities, and β″-phase fractions of undoped and PSZ-doped Na+-β/β″-Al2O3 specimens at 350 °C
4. Conclusion
YSZ- and CSZ-doped Na+-β/β″-Al2O3 specimens were successfully fabricated using a calcining-cum-sintering method, a simplified process that could reduce the production time and cost, and their properties were investigated. The β″-phase fraction was maintained at that of a BA specimen, and the relative sintered density increased with the addition of PSZ powder, with the exception of the BA_10CSZ specimen. In the BA_10CSZ specimen, the Ca dissolved from the destabilized CSZ phase in the Na+-β/β″Al2O3 led to excessive volatilization of the Na2O to adjust the charge balance of the Na+β/β″-Al2O3, and therefore the β″-Al2O3 phase fraction decreased. The sintered BA and PSZ-doped Na+-β/β″-Al2O3 specimens had RD values in the range of 95%–99.5%, which were close to the theoretical density of Na+-β/β″-Al2O3. The addition of 10 wt% PSZ powder led to a decrease in the ionic conductivity because the addition of ZrO 2 to Na+β/β″-Al2O3 might degrade the electrical performance as a result of its poor sodium ionic conductivity. In this work, the maximum and minimum ionic conductivities, i.e., 1.31 × 10−1 and 1.41 × 10−2 S/cm at 350 °C, were obtained in the BA_3CSZ and BA_10CSZ specimens, respectively. In the CSZ-doped Na+-β/β″-Al2O3, doping with 3 wt% CSZ powder enhanced the ionic conductivity. The improvement in the ionic conductivity of CSZ-doped Na+-β/β″-Al2O3 specimens resulted from the increase in the relative sintered density via the liquid-phase sintering process, which promotes densification of Na+-β/β″Al2O3. In BA_10CSZ, for example, the ionic conductivity was significantly decreased, owing mainly to the formation of the secondary phase, Na4Ca3(AlO2)10. The specific resistance values of the PSZ-doped Na+-β/β″-Al2O3 specimens prepared using the calcining-cum-sintering method were approximately 7–10 Ω·cm and were as good as those produced by the conventional process, with the exception of the sintered BA_10CSZ specimen.
Acknowledgments
This study was supported by Konkuk University in 2016.
References
1. S. Barison, S. Fasolin, C. Mortalo, S. Boldrini and M. Fabrizio, J. Eur. Ceram. Soc. 35, 2099 (2015). 2. L. P. Yang, S. J. Shan, X. L. Wei, X. M. Liu, H. Yang and X. D. Shen, Ceram. Int. 40, 9055 (2014). 3. T. Oshima, M. Kajita and A. Okuno, Int. J. Appl. Ceram. Tec. 1, 269 (2004). 4. C. A. Beevers and M. A. S. Ross, Z. Krist., 97, 59 (1937). 5. Y. Y. Yung-Fang and J. T. Kummer, J. Inorg. Nucl. Chem., 29, 2453 (1967).
6. G. Yamaguchi, J. Electrochem. Soc. Japan, 18, 260 (1943). 7. J. Fally, C. Lasne, Y. Lazennec, Y. Le Cars, and P. Margotin, J. Electrochem. Soc. 120, 1296 (1973). 8. G. Yamaguchi and K. Suzuki, Bull. Chem. Soc. Japan, 41, 93 (1968). 9. A. P. De Kroon, G. W. Schafer and F. Aldinger, Chem. Mater., 7[5], 878 (1995). 10. J. L. Sudworth and A. R. Tilley, The sodium sulfur battery, Champman and Hall Ltd., London, 1985, Chap. 2, 21. 11.T. Kitagawa, M. Kajita. U.S. Patent 6,419,875 (2002) 12. M. Kajita, T. Kajihara, T. Totoki, Eupean Patent Application 675,558 (1995) 13. B. Kim, J. Ryu, S. Jeon, S. Jo and H. Lee, Int. J. Appl. Ceram. Technol., 13[4] 697 (2016). 14. J.G.P. Binner, R. Stevens and S.R. TAN, J. Microscopy, 140, 183 (1985). 15. R. H. J. Hannink, R. M. Kelly and B. C. Muddle, J. Am. Ceram. Soc., 83 [3] 461 (2000). 16. M. Mon, T. Abe, H. Itoh, O. Yamamoto, Y. Takeda, T. Kawahara, Solid State Ionics, 74, 157 (1994). 17. K. Oe, K. Kikkawa, A. Kishimoto, Y. Nakamura, H. Yanagida, Solid State Ionics, 91, 131(1996). 18. M.J. Verkerk, A.J. Winnubst, A.J. Burggraaaf, J. Mater. Sci. 17, 3113 (1982). 19. M. Mori, M. Yoshikawa, H. Itoh, T. Abe, J. Am. Ceram. Soc. 77, 2217 (1994).
20. S. Rahandran, J. Drennan, S.P.S. Badwal, J. Mater. Sci. Lett. 6, 1431 (1987). 21. A. Peters, C. Korte, D. Hesse, N. Zakharov and J. Janek, Solid State Ionics. 178, 67 (2007). 22. J.H. Lee, J.H. Lee, Y.S. Jung and D.Y. Kim J. Am. Ceram. Soc., 86 [9] 1518 (2003). 23. F.F. Lange, B.I. Davis, D.O. Raleigh, J. Am. Ceram. Soc., 66, C50 (1983). 24. D.J. Freen, J. Mater. Sci. 20, 2639 (1985). 25. A.V. Virkar, J. Mater. Sci., 16, 1142 (1981). 26. X. Lu, G. Xia, J.P. Lemmon and Z. Yang, J. Power Sources 198, 2431 (2010). 27. C. Schimid, J. Mater. Sci. Lett. 5, 263 (1986). 28. S. T. Lee, S. G. Kim, M. H. Jang, S. H. Hwang, J. R. Haw and S. K. Lim, J. Ceram. Pro. Res. 11, 86 (2010). 29. L. Xie and A. N. Cormack, Mater. Lett., 9, 474 (1990). 30. L.T. Brownmiller and R. H. Bogue, Am. J. Sci., 23, 501 (1932). 31. M.C.C.S. Moraes, C. N. Elias, J. D. Filho and L. G. Oliveira, Mat. Res. 7[4] 643 (2004). 32. D. Olson, and L.C. De Jonghe, J. Mater. Sci., 21, 4221 (1986). 33. D. Casellas, M. M. Nagl, L. Llanes, and M. Anglada, J. Am. Ceram. Soc., 88[7], 1958 (2005). 34. T. M. Shaw, J. Am. Ceram. Soc., 69[1], 27 (1986).
35. Y. S. Yoo, J. J. Kim, and D. Y. Kim, J. Am Ceram. Soc., 70[11], C-322 (1987). 36. R. Brydson, P. C. Twigg, F. Loughran and F. L. Riley, J. Mater. Res., 16[3], 652 (2001). 37. S. N. Heavens, J. Mater. Sci. 23, 3515 (1988).
Table captions
Table 1. Compositions and designations of sintered specimens with and without PSZ
Table 2. Ionic conductivities, relative sintered densities, and β″-phase fractions of undoped and PSZ-doped Na+-β/β″-Al2O3 specimens at 350 °C
Figure captions
Fig. 1 Schematic illustrations of (a) conventional and (b) calcining-cum-sintering processes
Fig. 2 Schematic representations of idealized structures of (a) Na+-β-Al2O3 and (b) Na+β"-Al2O3 [8, 9]
Fig. 3 XRD patterns of sintered specimens with and without PSZ powder
Fig. 4 Na2O–CaO–Al2O3 ternary phase diagram presented by Brownmiller and Bogue [30], where C = CaO, A = Al2O3, and N = Na2O
Fig. 5 Relative sintered densities and β″-phase fractions of Na+-β/β″-Al2O3 sintered specimens with and without PSZ powder
Fig. 6 SEM images of sintered (a) BA (RD: 95.1%), (b) BA_3YSZ (RD: 98.5%), (c) BA_10YSZ (RD: 98.6%), (d) BA_3CSZ (RD: 99.2%), and (e) BA_10CSZ (RD: 99.5%) specimens
Fig. 7 Ionic conductivities of sintered specimens as function of addition of PSZ powder at 25–350 °C
List of Figures Fig. 1 Schematic illustrations of (a) conventional and (b) calcining-cum-sintering processes
Fig. 2 Schematic representations of idealized structures of (a) Na+-β-Al2O3 and (b) Na+β″-Al2O3 [8, 9]
Fig. 3 XRD patterns of sintered specimens with and without PSZ powder
t-ZrO2 m-ZrO2
''(1011)
''(2010)
Na4Ca3(AlO2)10
BA_10CSZ
BA_3CSZ
Intensity
BA_10YSZ
BA_3YSZ
BA
+
JCPDS 31-1262 (Na - ''-Al2O3)
+
JCPDS 31-1263 (Na - -Al2O3)
0
10
20
30
40
2 Theta (deg.)
50
60
70
Fig. 4 Na2O–CaO–Al2O3 ternary phase diagram presented by Brownmiller and Bogue [30], where C = CaO, A = Al2O3, and N = Na2O
Fig. 5 Relative sintered density and β″-phase fraction of Na+-β/β″-Al2O3 sintered
100
100
90
90
80
80
70
70
60
60
50
50
40
40 BA
BA_3YSZ
BA_10CSZ
BA_3CSZ
BA_10CSZ
Sintered specimens with and without PSZ
''-phase fraction(%)
Relative density(%)
specimens with and without PSZ powder
Fig. 6 SEM images of sintered (a) BA (RD: 95.1%), (b) BA_3YSZ (RD: 98.5%), (c) BA_10YSZ (RD: 98.6%), (d) BA_3CSZ (RD: 99.2%), and (e) BA_10CSZ (RD: 99.5%) specimens
(a)
(b)
(c)
(d)
(e)
Fig. 7 Ionic conductivities of sintered specimens as function of addition of PSZ powder at 25–350 °C.
BA BA_3YSZ BA_10YSZ BA_3CSZ BA_10CSZ
-1
Conductivity(S cm )
0.1 0.01 1E-3 1E-4 1E-5 1E-6 1.5
2.0
2.5 -1
1000/T(K )
3.0
3.5
List of Tables.
Table 1. Compositions and designations of sintered specimens with and without PSZ Y2O3 Denotation
or CaO stabilized
Composition zirconia addition(wt.%)
BA
[Na2O] : [Al2O3] = 1 : 5, MgO 1.6 wt.% as stabilizer
none
BA_3YSZ
1 : 5, MgO 1.6 wt.%
YSZ 3 wt%
BA_10YSZ
1 : 5, MgO 1.6 wt.%
YSZ 10 wt%
BA_3CSZ
1 : 5, MgO 1.6 wt.%
CSZ 3 wt%
BA_10CSZ
1 : 5, MgO 1.6 wt.%
CSZ 10 wt%
Table 2. Ionic conductivities, relative sintered densities, and β″-phase fractions of undoped and PSZ-doped Na+-β/β″-Al2O3 specimens at 350 °C
Ionic conductivity at Denotation
350 oC(S/cm)
β"-phase R.D(%) fraction(%)
BA
0.0657
95.1
64
BA_3YSZ
0.1265
98.5
61
BA_10YSZ
0.0943
98.6
61
BA_3CSZ
0.1315
99.2
61
BA_10CSZ
0.0141
99.5
44
S0025-5408(16)31589-6 http://dx.doi.org/doi:10.1016/j.materresbull.2017.05.003 MRB 9318
To appear in:
MRB
Received date: Revised date: Accepted date:
18-10-2016 13-4-2017 1-5-2017
Please cite this article as: Dae-Han Lee, Sung-Tae Lee, Jin-Sik Kim, SungKi Lim, Analysis of properties of partially stabilized zirconia-doped Na+beta-alumina prepared by calcining-cum-sintering process, Materials Research Bulletinhttp://dx.doi.org/10.1016/j.materresbull.2017.05.003 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Analysis of properties of partially stabilized zirconia-doped Na+-beta-alumina prepared by calcining-cum-sintering process Dae-Han Lee, Sung-Tae Lee, Jin-Sik Kim, and Sung-Ki Lim* Department of Materials Chemistry and Engineering, Konkuk University, 120 Neungdong-ro, Gwangjin-gu, Seoul 143-701, Republic of Korea Corresponding author: Sung-Ki Lim Address: Department of Materials Chemistry and Engineering, Konkuk University, 120 Neungdong-ro, Gwangjin-gu, Seoul 143-701, Republic of Korea Telephone: 82-2-450-3500 Fax: 82-2-444-3490 E-mail: [email protected] Graphical Abstract
BA BA_3YSZ BA_10YSZ BA_3CSZ BA_10CSZ
-1
Conductivity(S cm )
0.1 0.01 1E-3 1E-4 1E-5 1E-6 1.5
2.0
2.5 -1
1000/T(K )
3.0
3.5
Highlights
The PSZ(Partially Stabilized Zirconia: YSZ or CSZ)-doped Na+-β/β"-Al2O3 as a Na+ solid electrolyte were successfully fabricated using a calcining-cum-sintering method as a simplified process. The β"-phase fraction of PSZ-doped Na+-β/β"-Al2O3 was maintained compared to the undoped Na-β/β"-Al2O3. The relative density and the ionic conductivity of PSZ-doped Na+-β/β"-Al2O3 were higher than those of the undoped Na+-β/β"-Al2O3. The ionic conductivity values of the specimens prepared using the calcining-cumsintering method were approximately 7–10 Ω·cm, which were as good as those produced by the conventional process. The maximum and minimum ionic conductivities, i.e., 1.31 × 10−1 and 1.41 × 10−2 S/cm at 350 °C, were obtained in the BA_3CSZ and BA_10CSZ specimens, respectively. In case of CSZ 10wt%-doped Na+-β/β"-Al2O3, however, the ionic conductivity was significantly decreased owing to the formation of secondary phase of Na4Ca3(AlO2)10.
Abstract To overcome the disadvantages of a conventional process, including its complex steps, Na+-β/β″-Al2O3 for use as a Na+-conductive solid electrolyte was prepared using a calcining-cum-sintering process. Further, to improve the sintering properties, partially CaO- or Y2O3-stabilized zirconia (CSZ or YSZ, respectively) was added as a sintering additive. To analyze the properties of the samples, the XRD patterns, sintered density, microstructure, and ionic conductivity of each partially stabilized zirconia (PSZ)-doped Na+-β/β″-Al2O3 specimen were investigated. The ionic conductivity of each specimen was measured using AC impedance spectroscopy, and the highest ionic conductivity of 1.3 ×10−1 S/cm was observed for the BA_3CSZ specimen. The specific resistance values of the PSZ-doped Na+-β/β″-Al2O3 specimens were approximately 7–10 Ω·cm and were as good as those produced by the conventional process, with the exception of the BA_10CSZ specimen. The ionic conductivity of BA_10CSZ was significantly decreased, owing mainly to the formation of the secondary phase, Na4Ca3(AlO2)10.
Keywords: Na+-beta-alumina; beta-alumina; stabilized zirconia; solid electrolyte 1. Introduction Na+-β/β″-Al2O3 has been widely utilized as a solid electrolyte in sodium beta batteries for electric vehicles and storage of excess electricity [1–3], where the Na+ ions act as charge carriers. Na+-β/β″-Al2O3 is characterized by a highly ionic nature and low electronic conductivity, and extensive research has been performed to exploit these features for practical purposes. There are two parent phases: β-alumina has the theoretical formula Na2O·11Al2O3 or NaAl11O17, and β″-alumina has the theoretical formula Na2O·5Al2O3 or NaAl5O8 [4–6]. According to the Na2O–Al2O3 phase diagram proposed by Fally et al. [7], the β + β″ phases coexist in a region corresponding to the formula Na2O·nAl2O3 (5.33 ≤ n ≤ 8.5). β″-alumina has a rhombohedral structure with an R3m space group and lattice constants of a = 5.614 and c = 33.85 Å. Generally, its a axis is similar to that of β-alumina; its c axis is 1.5 times longer, and the concentration of alkaline earth metal ions on its conduction plane is higher. Therefore, β″-alumina shows a much higher ionic conductivity [8, 9]. Of the beta-alumina solid electrolytes, a MgO (or Li2O)-stabilized beta-alumina solid electrolyte has conventionally been produced as follows. A stabilizer, a sodium compound, and α-alumina are mixed at an appropriate ratio. The mixture is calcined to obtain beta-alumina. Then, after grinding, the ground material is granulated, molded into a desired shape, and sintered to obtain a beta-alumina solid electrolyte. In this conventional process, the materials are calcined in advance to obtain beta-alumina because when the production of beta-alumina by direct firing is attempted without calcination, severe volume expansion occurs in the phase transition from α-alumina to
beta-alumina, making it difficult to obtain a beta-alumina solid electrolyte with uniform quality and high strength. However, in the conventional process, the preliminary calcination step to obtain beta-alumina increases the complexity of the steps and the total cost. Hence, in the production of sodium–sulfur- or sodium-based batteries (e.g., the Zeolite Battery Research Africa Project battery), it would be very beneficial to shorten the number of production steps and produce a beta-alumina solid electrolyte more efficiently. Further, because preliminary calcination (to obtain beta-alumina), grinding, granulation, molding, and sintering are conducted in the conventional process described above, the crystals constituting the resulting beta-alumina solid electrolyte are highly oriented. Moreover, the electrolyte contains large crystals because the beta-alumina formed by calcination acts as a nucleus during crystal growth. Consequently, a calciningcum-sintering process has advantages over the conventional process, including a simpler fabrication process, and cost and energy reductions. Fig. 1 shows schematic illustrations of the conventional and calcining-cum-sintering processes for Na+-β/β″-Al2O3 sintered specimens [10–12]. Fig. 1 Schematic illustrations of (a) conventional and (b) calcining-cum-sintering processes Pure zirconia (ZrO2) has a cubic fluorite structure at temperatures between its melting point (2680 °C) and 2370 °C. It is transformed into a tetragonal phase below 2370 °C and a monoclinic phase below 1170 °C [13]. Cations with less than four valence electrons, such as Y3+, Ca2+, and Mg2+, create oxygen vacancies and can be used to stabilize the high-temperature zirconia phases over the entire temperature range. Partially stabilized zirconia (PSZ) has been widely used for applications such as oxygen sensors, advanced
ceramics, and solid oxygen fuel cells. Moreover, the addition of PSZ to α-alumina enhances its mechanical strength [14, 15]. There have been many studies on the mechanical and electrochemical properties of α-alumina with yttria-stabilized zirconia (YSZ) [16–20] and calcia-stabilized zirconia (CSZ) [21, 22] additives. Moreover, the addition of YSZ to Na+-β/β″-Al2O3 has been shown to improve both its fracture toughness and strength without compromising the electrical properties of this superionic material [23, 24]. The enhancement in fracture toughness leads to an obvious improvement in its critical current density [25], which has significant economic advantages with regard to battery performance. For example, in addition to extending the battery life, the increase in the charging current density could dramatically shorten the charging time [26]. On the other hand, the addition of CSZ to Na+-β/β″-Al2O3 has not been reported recently. In this study, PSZ (YSZ or CSZ)-doped Na+-β/β″-Al2O3 specimens stabilized with MgO were fabricated using a calcining-cum-sintering process, and the relationship between the YSZ or CSZ content and the properties of the doped Na+-β/β″-Al2O3 samples were investigated. 2. Materials and methodology Undoped Na+-β/β″-Al2O3 specimens and Na+-β/β″-Al2O3 specimens doped with YSZ or CSZ were synthesized via a calcining-cum-sintering process using commercial αAl2O3 (99.99%, High Purity Chemicals, Japan), Na2CO3 (99+%, Sigma-Aldrich, USA), Mg(OH)2 (99+%, Sigma-Aldrich, USA), YSZ (99.9%, High Purity Chemicals, Japan), and CSZ (99+%, High Purity Chemicals, Japan) powders as the starting materials. The synthesized Na+-β/β″-Al2O3 specimens contained 1.6 wt% MgO (as a stabilizer), Na2O at
an [Na2O]/[Al2O3] molar ratio of 1:5, and YSZ or CSZ amounts ranging from 3.0 to 10 wt%. Table 1 lists the compositions and designations of the evaluated specimens. Table 1. Compositions and designations of sintered specimens with and without PSZ The raw materials were ball-milled for 5 h. The slurries used in the granulation step contained 0.6 wt% polyvinyl alcohol (Mw 13,000–23,000, Sigma-Aldrich, USA) as a binder for the composite powder, 0.8 wt% ammonium polymethacrylate (DARVAN® CN, R.T. Vanderbilt, USA) as a dispersant, and 0.8 wt% octyl alcohol (99+%, SigmaAldrich, USA) as an antifoaming agent in distilled water. Before spray-drying, the slurry formulations were ball-milled. Then, they were spray-dried using a disk-type spray dryer, where the inlet and outlet temperatures were set at 200 and 110 °C, respectively. After the spray-drying was complete, only granules that passed through a 100 mesh (149 μm) sieve were collected. To prepare the sintered specimens, the granules were uniaxially pressed at 100 MPa and sintered for 30 min at 1600 °C. Although sintering is typically conducted at 1650 °C, a temperature of 1600 °C was used in this experiment to maximize the advantages of the process. The sintered density of each specimen was measured using the Archimedes method. The sintered specimens’ microstructures and phase fractions were assessed using scanning electron microscopy (SEM; Model JSM-6380, JEOL, Japan) and X-ray diffraction (XRD, D/max 2200, Rigaku, Japan) measurements. XRD analysis was conducted at 40 kV and 30 mA using Cu Kα radiation. The relative phase composition was determined by calculating the line intensities of each phase’s wellseparated peaks [27, 28]: % 𝐨𝐟 𝛂 =
𝐟(𝛂) × 𝟏𝟎𝟎 𝐟(𝛂) + 𝐟(𝛃) + 𝐟(𝛃")
(𝟏)
% 𝐨𝐟 𝛃 =
𝐟(𝛃) × 𝟏𝟎𝟎 ( ) 𝐟 𝛂 + 𝐟(𝛃) + 𝐟(𝛃")
% 𝐨𝐟 𝛃" =
𝐟(𝛃") × 𝟏𝟎𝟎 ( ) 𝐟 𝛂 + 𝐟(𝛃) + 𝐟(𝛃")
(𝟐)
(𝟑)
𝐟(𝛂) =
𝟏 𝟏𝟎 { 𝐈𝛂(𝟏𝟎𝟒) × + 𝐈𝛂(𝟏𝟏𝟑) } 𝟐 𝟗
𝐟(𝛃) =
𝟏 𝟏𝟎 𝟏𝟎 𝟏𝟎 {𝐈𝛃(𝟏𝟎𝟐) × } (𝟓) + 𝐈𝛃(𝟐𝟎𝟔) × + 𝐈𝛃(𝟏𝟎𝟕) × 𝟑 𝟑 𝟑. 𝟓 𝟓. 𝟓
𝐟(𝛃") =
𝟏 𝟏𝟎 𝟏𝟎 {𝑰𝜷"(𝟏𝟎𝟏𝟏) × + 𝑰𝛃"(𝟐𝟎𝟏𝟎) × } (𝟔) 𝟐 𝟒 𝟖
(𝟒)
where Iα(104) and Iα(113) denote the X-ray intensities of the (104) and (113) planes of the αalumina phase, respectively; Iβ(102), Iβ(206), and Iβ(107) represent the X-ray intensities of the (102), (206), and (107) planes of the β-alumina phase, respectively; and Iβ"(1011) and Iβ"(2010) indicate the X-ray intensities of the (1011) and (2010) planes of the β"-alumina phase, respectively. The microstructures and densities of the sintered specimens were determined using SEM (Model JSM-6380, Japan) and the Archimedes method (ASTM 373-88), respectively. The ionic conductivities of the sintered specimens were measured using a four-probe AC impedance analyzer (IM6, Zahner) with blocking silver electrodes in a frequency range of 1 Hz to 3 MHz and temperature range of 25–350 °C. The sodium conductivities were calculated using Eq. (7). 𝛔=
𝑳 (𝟕) 𝑹𝒔 × 𝑨
where σ, L, RS, and A denote the ionic conductivity, specimen thickness, measured resistance, and electrode area, respectively.
3. Results and discussion
Fig. 2 schematically shows the structures of β-Al2O3 and β″-Al2O3, which contain a tightly packed spinel block and loosely arranged conduction slab where the sodium ions can be rapidly transported. Three spinel blocks are contained in each β″-alumina unit cell, whereas β-Al2O3 has two spinel blocks per unit cell. In the beta-alumina structure, aluminum ions are found in both the octahedral and tetrahedral sites. Generally, divalent cations (Ca2+, Mg2+, Co2+, Mn2+, Zn2+, and Fe2+) strongly prefer a tetrahedral coordination environment in the beta-alumina structure [29].
Fig. 2 Schematic representations of idealized structures of (a) Na+-β-Al2O3 and (b) Na+β″-Al2O3 [8, 9]
Fig. 3 shows the XRD patterns of the undoped and PSZ-doped Na+-β/β″-Al2O3 sintered specimens. The XRD pattern of the BA sintered specimen shows that a phase transformation of α-Al2O3 to Na+-β/β″-Al2O3 occurred without any α-Al2O3 remaining. The XRD patterns of all the sintered specimens conform to the crystalline phases of NaAl5O8 (JCPDS 31-1262) and NaAl11O17 (JCPDS 31-1263). All of the XRD patterns of the sintered specimens exhibited growth of the (107) and (206) planes, and the (1011) and (2010) planes, which correspond to β- and β″-alumina, respectively. The β″-alumina peaks had higher intensities than the β-alumina peaks, and the results showed that the
sintered specimens show successful synthesis of Na+-β/β″-Al2O3, with the exception of the BA_10CSZ specimen. The secondary Na4Ca3(AlO2)10 phase (JCPDS 02-1003) was clearly observed in the BA_10CSZ specimen. The CSZ phase was considered to be clearly destabilized to the monoclinic phase by the reaction between Al from the Al2O3 and Ca from the CSZ [13, 18]. As a result, the peak intensity of the tetragonal phases for the peaks in a 2θ range of 30.4° decreased with increasing CaO content, and the peak for BA_10CSZ at 28.2° was assigned to the monoclinic phase. The tetragonal phase of CSZ was destabilized with Na+-β/β″-Al2 O3 during the calcining-cum-sintering step. Fig. 4 shows the phase diagram of the ternary system Na2O–CaO–Al2O3, which was produced by Brownmiller and Bogue [30].
Fig. 3 XRD patterns of sintered specimens with and without PSZ powder
Fig. 4 Na2O–CaO–Al2O3 ternary phase diagram presented by Brownmiller and Bogue [30], where C = CaO, A = Al2O3, and N = Na2O
The phase fractions of β″-Al2O3 were calculated using Eqs. (1)–(6) (Fig. 5). Regardless of the type of PSZ, the β″-Al2O3 phase fraction was maintained at that of the BA sample, with the exception of the BA_10CSZ sintered specimen. The β″-Al2O3 phase fractions of the BA and BA_10CSZ specimens were 61% and 44%, respectively. In the BA_10CSZ specimen, the dissolved Ca from the destabilized CSZ phase in the Na+-β/β″-Al2O3 led to excessive volatilization of the Na2O to adjust the charge balance of the Na+-β/β″-Al2 O3,
and the β″-Al2O3 phase fraction decreased. In the YSZ-doped Na+-β/β″-Al2O3, the relative density (RD) increased with the addition of YSZ powder. Doping with CSZ powder also improved the relative sintered density of the Na+-β/β″-Al2O3 compared to that of the BA sintered specimen. The sintered BA and PSZ-doped Na+-β/β″-Al2O3 specimens showed relative densities in the range of 95%–99.5%. In general, in the range of pure Na+-β/β″-Al2O3 to 21 wt% zirconia, the sintered density of the doped Na+-β/β″Al2O3 increased with the zirconia content, indicating that the addition of zirconia particles allowed the densification of the alumina matrix, and the volume expansion from tetragonal to monoclinic associated with the phase transformation improved the relative sintered density [31]. The relative sintered density increased slightly with the addition of CSZ than YSZ-doped specimens. The increase in the Ca content of CSZ caused the formation of liquid phases during the sintering process [36], and this aided the densification of the Na+-β/β″-Al2O3 specimens.
Fig. 5 Relative sintered density and β″-phase fraction of Na+-β/β″-Al2O3 sintered specimens with and without PSZ powder
Fig. 6 shows SEM images of the sintered Na+-β/β″-Al2O3 specimens. Platelet-shaped or anisotropic grain growth clearly occurred without and with the addition of PSZ powder. The dispersed PSZ grains, which appear as relatively white regions in Fig. 6(b), are located at the grain boundaries of the Na+-β/β″-Al2O3 grains. As a result, the Na+-β/β″Al2O3 grains were entrapped by the dispersed PSZ, which inhibited the rapid and abnormal grain growth of Na+-β/β″-Al2O3 crystallites [32, 33]. The intergranular liquid
at the BA_10CSZ specimen surface penetrated into the grain boundaries in the surface region compared to the BA_3YSZ specimen. This suggests that the intergranular liquid is not uniformly distributed throughout the specimen but is dynamically redistributed during sintering, as has been reported in many other systems [34–36]. Indeed, the progress of densification and grain growth during sintering of the CSZ-doped Na+-β/β″-Al2O3 was comparable to that of the YSZ-doped Na+-β/β″-Al2 O3 [22].
Fig. 6 SEM images of sintered (a) BA (RD: 95.1%), (b) BA_3YSZ (RD: 98.5%), (c) BA_10YSZ (RD: 98.6%), (d) BA_3CSZ (RD: 99.2%), and (e) BA_10CSZ (RD: 99.5%) specimens
Fig. 7 shows the relationship between the ionic conductivity and the temperature of the YSZ- and CSZ-doped Na+-β/β″-Al2O3 specimens compared to that of the BA specimen at 25–350 °C. The previous results and ionic conductivities of undoped and PSZ-doped Na+-β/β″-Al2O3 sintered specimens at 350 °C are listed in Table 2. The ionic conductivity of the BA sintered specimen was 6.57 × 10−2 S/cm at 350 °C. In the YSZ-doped Na+β/β″-Al2O3, the ionic conductivity improved with increasing YSZ content compared to that of the BA sintered specimen. An ionic conductivity of 1.26 × 10−1 S/cm at 350 °C was realized in the BA_3YSZ specimen. In the BA_10YSZ specimen, however, the ionic conductivity decreased compared to that of the BA_3YSZ specimen. While potentially enhancing the mechanical strength, the addition of ZrO2 to Na+-β/β″-Al2O3 might degrade the electrical performance because of its poor sodium ionic conductivity. Heavens studied the strength improvement in Na+-β/β″-Al2O3 with the addition of ZrO2 and found that the
resistivity did not increase significantly until the ZrO 2 content reached 10 wt% [37]. In the CSZ-doped Na+-β/β″-Al2O3, doping with 3 wt% CSZ powder enhanced the ionic conductivity. More specifically, the maximum and minimum ionic conductivities, i.e., 1.31 × 10−1 and 1.41 × 10−2 S/cm at 350 °C, were obtained in the BA_3CSZ and BA_10CSZ specimens, respectively. The improvement in the ionic conductivity of the CSZ-doped Na+-β/β″-Al2O3 specimens resulted from the increase in the relative sintered density via the liquid-phase sintering process, which promotes densification of Na+-β/β″Al2O3. Addition of excess CSZ, for example, in BA_10CSZ, however, decreased the ionic conductivity, although doping with a small amount of CSZ improves the ionic conductivity of the BA specimens. The addition of 10 wt% CSZ powder reduced the ionic conductivity owing to the formation of Na4Ca3(AlO2)10 as a secondary phase by a reaction between Al from the Al2O3 and Ca from the CSZ and the resulting decrease in the β″Al2O3 phase fraction.
Fig. 7 Ionic conductivities of sintered specimens as function of addition of PSZ powder at 25–350 °C.
Table 2. Ionic conductivities, relative sintered densities, and β″-phase fractions of undoped and PSZ-doped Na+-β/β″-Al2O3 specimens at 350 °C
4. Conclusion
YSZ- and CSZ-doped Na+-β/β″-Al2O3 specimens were successfully fabricated using a calcining-cum-sintering method, a simplified process that could reduce the production time and cost, and their properties were investigated. The β″-phase fraction was maintained at that of a BA specimen, and the relative sintered density increased with the addition of PSZ powder, with the exception of the BA_10CSZ specimen. In the BA_10CSZ specimen, the Ca dissolved from the destabilized CSZ phase in the Na+-β/β″Al2O3 led to excessive volatilization of the Na2O to adjust the charge balance of the Na+β/β″-Al2O3, and therefore the β″-Al2O3 phase fraction decreased. The sintered BA and PSZ-doped Na+-β/β″-Al2O3 specimens had RD values in the range of 95%–99.5%, which were close to the theoretical density of Na+-β/β″-Al2O3. The addition of 10 wt% PSZ powder led to a decrease in the ionic conductivity because the addition of ZrO 2 to Na+β/β″-Al2O3 might degrade the electrical performance as a result of its poor sodium ionic conductivity. In this work, the maximum and minimum ionic conductivities, i.e., 1.31 × 10−1 and 1.41 × 10−2 S/cm at 350 °C, were obtained in the BA_3CSZ and BA_10CSZ specimens, respectively. In the CSZ-doped Na+-β/β″-Al2O3, doping with 3 wt% CSZ powder enhanced the ionic conductivity. The improvement in the ionic conductivity of CSZ-doped Na+-β/β″-Al2O3 specimens resulted from the increase in the relative sintered density via the liquid-phase sintering process, which promotes densification of Na+-β/β″Al2O3. In BA_10CSZ, for example, the ionic conductivity was significantly decreased, owing mainly to the formation of the secondary phase, Na4Ca3(AlO2)10. The specific resistance values of the PSZ-doped Na+-β/β″-Al2O3 specimens prepared using the calcining-cum-sintering method were approximately 7–10 Ω·cm and were as good as those produced by the conventional process, with the exception of the sintered BA_10CSZ specimen.
Acknowledgments
This study was supported by Konkuk University in 2016.
References
1. S. Barison, S. Fasolin, C. Mortalo, S. Boldrini and M. Fabrizio, J. Eur. Ceram. Soc. 35, 2099 (2015). 2. L. P. Yang, S. J. Shan, X. L. Wei, X. M. Liu, H. Yang and X. D. Shen, Ceram. Int. 40, 9055 (2014). 3. T. Oshima, M. Kajita and A. Okuno, Int. J. Appl. Ceram. Tec. 1, 269 (2004). 4. C. A. Beevers and M. A. S. Ross, Z. Krist., 97, 59 (1937). 5. Y. Y. Yung-Fang and J. T. Kummer, J. Inorg. Nucl. Chem., 29, 2453 (1967).
6. G. Yamaguchi, J. Electrochem. Soc. Japan, 18, 260 (1943). 7. J. Fally, C. Lasne, Y. Lazennec, Y. Le Cars, and P. Margotin, J. Electrochem. Soc. 120, 1296 (1973). 8. G. Yamaguchi and K. Suzuki, Bull. Chem. Soc. Japan, 41, 93 (1968). 9. A. P. De Kroon, G. W. Schafer and F. Aldinger, Chem. Mater., 7[5], 878 (1995). 10. J. L. Sudworth and A. R. Tilley, The sodium sulfur battery, Champman and Hall Ltd., London, 1985, Chap. 2, 21. 11.T. Kitagawa, M. Kajita. U.S. Patent 6,419,875 (2002) 12. M. Kajita, T. Kajihara, T. Totoki, Eupean Patent Application 675,558 (1995) 13. B. Kim, J. Ryu, S. Jeon, S. Jo and H. Lee, Int. J. Appl. Ceram. Technol., 13[4] 697 (2016). 14. J.G.P. Binner, R. Stevens and S.R. TAN, J. Microscopy, 140, 183 (1985). 15. R. H. J. Hannink, R. M. Kelly and B. C. Muddle, J. Am. Ceram. Soc., 83 [3] 461 (2000). 16. M. Mon, T. Abe, H. Itoh, O. Yamamoto, Y. Takeda, T. Kawahara, Solid State Ionics, 74, 157 (1994). 17. K. Oe, K. Kikkawa, A. Kishimoto, Y. Nakamura, H. Yanagida, Solid State Ionics, 91, 131(1996). 18. M.J. Verkerk, A.J. Winnubst, A.J. Burggraaaf, J. Mater. Sci. 17, 3113 (1982). 19. M. Mori, M. Yoshikawa, H. Itoh, T. Abe, J. Am. Ceram. Soc. 77, 2217 (1994).
20. S. Rahandran, J. Drennan, S.P.S. Badwal, J. Mater. Sci. Lett. 6, 1431 (1987). 21. A. Peters, C. Korte, D. Hesse, N. Zakharov and J. Janek, Solid State Ionics. 178, 67 (2007). 22. J.H. Lee, J.H. Lee, Y.S. Jung and D.Y. Kim J. Am. Ceram. Soc., 86 [9] 1518 (2003). 23. F.F. Lange, B.I. Davis, D.O. Raleigh, J. Am. Ceram. Soc., 66, C50 (1983). 24. D.J. Freen, J. Mater. Sci. 20, 2639 (1985). 25. A.V. Virkar, J. Mater. Sci., 16, 1142 (1981). 26. X. Lu, G. Xia, J.P. Lemmon and Z. Yang, J. Power Sources 198, 2431 (2010). 27. C. Schimid, J. Mater. Sci. Lett. 5, 263 (1986). 28. S. T. Lee, S. G. Kim, M. H. Jang, S. H. Hwang, J. R. Haw and S. K. Lim, J. Ceram. Pro. Res. 11, 86 (2010). 29. L. Xie and A. N. Cormack, Mater. Lett., 9, 474 (1990). 30. L.T. Brownmiller and R. H. Bogue, Am. J. Sci., 23, 501 (1932). 31. M.C.C.S. Moraes, C. N. Elias, J. D. Filho and L. G. Oliveira, Mat. Res. 7[4] 643 (2004). 32. D. Olson, and L.C. De Jonghe, J. Mater. Sci., 21, 4221 (1986). 33. D. Casellas, M. M. Nagl, L. Llanes, and M. Anglada, J. Am. Ceram. Soc., 88[7], 1958 (2005). 34. T. M. Shaw, J. Am. Ceram. Soc., 69[1], 27 (1986).
35. Y. S. Yoo, J. J. Kim, and D. Y. Kim, J. Am Ceram. Soc., 70[11], C-322 (1987). 36. R. Brydson, P. C. Twigg, F. Loughran and F. L. Riley, J. Mater. Res., 16[3], 652 (2001). 37. S. N. Heavens, J. Mater. Sci. 23, 3515 (1988).
Table captions
Table 1. Compositions and designations of sintered specimens with and without PSZ
Table 2. Ionic conductivities, relative sintered densities, and β″-phase fractions of undoped and PSZ-doped Na+-β/β″-Al2O3 specimens at 350 °C
Figure captions
Fig. 1 Schematic illustrations of (a) conventional and (b) calcining-cum-sintering processes
Fig. 2 Schematic representations of idealized structures of (a) Na+-β-Al2O3 and (b) Na+β"-Al2O3 [8, 9]
Fig. 3 XRD patterns of sintered specimens with and without PSZ powder
Fig. 4 Na2O–CaO–Al2O3 ternary phase diagram presented by Brownmiller and Bogue [30], where C = CaO, A = Al2O3, and N = Na2O
Fig. 5 Relative sintered densities and β″-phase fractions of Na+-β/β″-Al2O3 sintered specimens with and without PSZ powder
Fig. 6 SEM images of sintered (a) BA (RD: 95.1%), (b) BA_3YSZ (RD: 98.5%), (c) BA_10YSZ (RD: 98.6%), (d) BA_3CSZ (RD: 99.2%), and (e) BA_10CSZ (RD: 99.5%) specimens
Fig. 7 Ionic conductivities of sintered specimens as function of addition of PSZ powder at 25–350 °C
List of Figures Fig. 1 Schematic illustrations of (a) conventional and (b) calcining-cum-sintering processes
Fig. 2 Schematic representations of idealized structures of (a) Na+-β-Al2O3 and (b) Na+β″-Al2O3 [8, 9]
Fig. 3 XRD patterns of sintered specimens with and without PSZ powder
t-ZrO2 m-ZrO2
''(1011)
''(2010)
Na4Ca3(AlO2)10
BA_10CSZ
BA_3CSZ
Intensity
BA_10YSZ
BA_3YSZ
BA
+
JCPDS 31-1262 (Na - ''-Al2O3)
+
JCPDS 31-1263 (Na - -Al2O3)
0
10
20
30
40
2 Theta (deg.)
50
60
70
Fig. 4 Na2O–CaO–Al2O3 ternary phase diagram presented by Brownmiller and Bogue [30], where C = CaO, A = Al2O3, and N = Na2O
Fig. 5 Relative sintered density and β″-phase fraction of Na+-β/β″-Al2O3 sintered
100
100
90
90
80
80
70
70
60
60
50
50
40
40 BA
BA_3YSZ
BA_10CSZ
BA_3CSZ
BA_10CSZ
Sintered specimens with and without PSZ
''-phase fraction(%)
Relative density(%)
specimens with and without PSZ powder
Fig. 6 SEM images of sintered (a) BA (RD: 95.1%), (b) BA_3YSZ (RD: 98.5%), (c) BA_10YSZ (RD: 98.6%), (d) BA_3CSZ (RD: 99.2%), and (e) BA_10CSZ (RD: 99.5%) specimens
(a)
(b)
(c)
(d)
(e)
Fig. 7 Ionic conductivities of sintered specimens as function of addition of PSZ powder at 25–350 °C.
BA BA_3YSZ BA_10YSZ BA_3CSZ BA_10CSZ
-1
Conductivity(S cm )
0.1 0.01 1E-3 1E-4 1E-5 1E-6 1.5
2.0
2.5 -1
1000/T(K )
3.0
3.5
List of Tables.
Table 1. Compositions and designations of sintered specimens with and without PSZ Y2O3 Denotation
or CaO stabilized
Composition zirconia addition(wt.%)
BA
[Na2O] : [Al2O3] = 1 : 5, MgO 1.6 wt.% as stabilizer
none
BA_3YSZ
1 : 5, MgO 1.6 wt.%
YSZ 3 wt%
BA_10YSZ
1 : 5, MgO 1.6 wt.%
YSZ 10 wt%
BA_3CSZ
1 : 5, MgO 1.6 wt.%
CSZ 3 wt%
BA_10CSZ
1 : 5, MgO 1.6 wt.%
CSZ 10 wt%
Table 2. Ionic conductivities, relative sintered densities, and β″-phase fractions of undoped and PSZ-doped Na+-β/β″-Al2O3 specimens at 350 °C
Ionic conductivity at Denotation
350 oC(S/cm)
β"-phase R.D(%) fraction(%)
BA
0.0657
95.1
64
BA_3YSZ
0.1265
98.5
61
BA_10YSZ
0.0943
98.6
61
BA_3CSZ
0.1315
99.2
61
BA_10CSZ
0.0141
99.5
44