Novel Composite Electrolytes of Zr0.92Y0.08O2-α(8YSZ)-Low Melting Point Glass Powder for Intermediate Temperature Solid Oxide Fuel Cells Novel Composite Electrolytes of Zr0.92Y0.08O2-α(8YSZ)-Low Melting Point Glass Powder for Intermediate Temperature Solid Oxide Fuel Cells https://orcid.org/0000-0002-2482-6320WangHongtao*DuRuifengShiRuijuanLiuJunlong School of Chemical and Material Engineering, Fuyang Normal College, Anhui Provincial Key Laboratory for Degradation and Monitoring of Pollution of the Environment, Fuyang 236037, China; ruifengdu1@163.com (R.D.); rjshi@fync.edu.cn (R.S.); jlliu@fync.edu.cn (J.L.)
*Correspondence: hongtaoking3@163.com; Tel.: +86-558-2596-249; Fax: +86-558-2596-703 1221 In this study, Zr0.92Y0.08O2-α(8YSZ) powders were synthesized by the sol-gel method.
The chemical physics changes and phase formation temperature of 8YSZ crystal were determined by thermogravimetry analysis and differential scanning calorimetry (TGA-DSC). 8YSZ-low melting point glass powder (8YSZ-glass) composite electrolytes with various weight ratios were prepared and calcined at different temperatures.
The X-ray diffraction (XRD) patterns of the composite electrolytes were tested.
The effects of synthesis temperature, weight ratio, test temperature, and oxygen partial pressure on the conductivities of 8YSZ-glass composite electrolytes, were also investigated at 400–800 °C.
The result of the logσ ~ log(pO2) plot indicates that the 8YSZ-20% glass (700 °C) is almost a pure ionic conductor.
The oxygen concentration discharge cell illustrates that the 8YSZ-20% glass (700 °C) composite electrolyte is a good oxygen ion conductor. defects electrolyte fuel cell ceramics sol-gel preparation 1.
Introduction ZrO2-based electrolytes doped, with rare earth metallic cations, are excellent oxide ionic conductors, and they are widely used in oxygen sensors and solid oxide fuel cells (SOFCs) [1,2,3,4] due to their good mechanical strength and high ionic transport numbers.
During the past decades, continuous research concentrating on Y2O3-stabilized ZrO2 (YSZ) has been done [5,6].
For example, Caruso et al. investigated the influence of different parameters on the morphology and microstructure of YSZ powders synthesized by the sol-gel method [5].
However, SOFCs using YSZ as electrolyte membrane usually run at high temperatures (800–1000 °C).
Therefore, researchers have focused on two ways to lower the operating temperature of YSZ.
One strategy is to use YSZ films, and the other way is to construct composite electrolytes, which may have the combined advantages of each component [7,8,9,10,11].
A few research reports have shown that the thin film fuel cells using YSZ as membrane electrolytes generated maximum power output densities of 200–400 mW·cm−2 at 800 °C [12,13,14].
Singh et al. reported that the YSZ-SDC (samarium doped ceria) composite electrolyte with a weight ratio 8.5:1.5 has a higher electrical conductivity than single material YSZ at 400–700 °C [9].
It is well known that silicate, borate, mica, and other glass systems are commonly used as sealing materials in fuel cell systems [15,16,17,18].
It may be expected that using silicate low melting point glass powder, as a sintering aid, as well as composite electrolytes with improved gas tightness, durability, and better component matching, could be synthesized.
In this study, novel composite electrolytes of Zr0.92Y0.08O2-α(8YSZ)-low melting point glass powder were synthesized.
The morphology, structure, and intermediate temperature electrochemical properties of the composite electrolytes were investigated by a variety of methods. 2.
Experimental We initially synthesized Zr0.92Y0.08O2-α(8YSZ) electrolyte via a sol-gel method using citric acid as a chelating agent as reported previously [19].
All the reagents used are analytical-grade.
Firstly, Y2O3 was dissolved in nitric acid and Zr(NO3)4·5H2O was dispersed into distilled water.
The solution was then mixed with citric acid and NH4OH and evaporated at 90 °C to get a gel.
After gelation and ashing treatment, the obtained ash was calcined at 700 °C, 1200 °C and 1550 °C for 6 h, respectively, to get Zr0.92Y0.08O2-α(8YSZ) powder.
The low melting point glass powder was used as a sintering aid to form composite.
The composition of the low melting point glass powder is Na2O-CaO-SiO2-ZnO (Taizhou Xinhai Special Materials Factory, 300 mesh, m.p. is 550 °C). 8YSZ and low melting point glass powder were mixed with a weight ratio of four to one and heated at 700 °C, 1200 °C and 1550 °C for 2 h, correspondingly.
The composites with weight ratio of 8YSZ: low melting point glass powder = 9:1 and 7:3 were also synthesized at 700 °C.
These results are summarized in Table 1.
The chemical physics changes and phase formation temperature of 8YSZ crystal were determined by thermogravimetry analysis and differential scanning calorimetry (TGA-DSC) (TGA-DSC, Universal V 3.7A, TA Instruments, New Castle, DE, USA).
The X–ray diffraction (XRD) (XRD, X’pert Pro MPD, Amsterdam, Netherlands)patterns of the above electrolytes were tested with a Panalytical X′Pert Pro MPD diffractometer.
The morphology of the 8YSZ-20% glass (700 °C) was observed using a scanning electron microscope (SEM, S-4700, Hitachi, Tokyo, Japan) [20,21].
The conductivities vs. different synthesis temperature, test temperature, oxygen partial pressure and weight ratio in nitrogen atmosphere were tested with an electrochemical analyzer (CHI660E, Shanghai, China) at 400–800 °C.
All the samples were ground into thin slices of 1.0–1.2 mm.
A 20% palladium-80% silver paste with silver wires was used to fabricate the electrodes (area: 0.5 cm2).
Oxygen concentration discharge fuel cell and H2/O2 fuel cell using the 8YSZ-20% glass (700 °C) as electrolyte were constructed [22,23]. 3.
Results and Discussion The TGA and DSC curves of the Zr0.92Y0.08O2-α(8YSZ) gel heated at 15 °C·min−1 in nitrogen atmosphere up to 1000 °C are shown in Figure 1.
It can be seen that the TGA curve shows a weight loss about 7% from 35 °C to 130 °C corresponding to two weak endothermic peaks in DSC curve, which is attributed to the residual water in the 8YSZ gel [24,25].
About seventy percent of weight loss of 8YSZ gel occurred up to c.a. 500 °C.
The first calcined temperature was fixed at 700 °C because there is almost no weight loss at 520 °C and above [26,27].
The XRD patterns of 8YSZ and 8YSZ-glass obtained with different weight ratio and calcined at different synthesis temperature are shown in Figure 2.
Figure 2a shows the XRD patterns of the 8YSZ-glass with different weight ratio calcined at 700 °C, i.e., 0%, 10%, 20% and 30%.
All the samples possess coexisting tetragonal and monoclinic phases, where tetragonal is the major phase.
The XRD angles at 30.14°, 34.72° and 35.04° belong to the (101), (002), and (110) crystal planes of t-Zr0.9Y0.1O1.95 (JCPDS 82-1241), respectively.
From Figure 2b, when the synthesis temperature reaches 1200 °C and 1550 °C, the XRD patterns of 8YSZ are merely tetragonal structures.
However, there are still a few obvious diffraction peaks of monoclinic structure in 8YSZ-20% glass calcined at 1200 °C and 1550 °C, respectively.
Mori et al. observed that the Ti4+-doped 8YSZ electrolyte transform from a pure cubic structure to two-phase compound containing small amount of tetragonal phase with increasing Ti content [28].
And a monoclinic-to-tetragonal phase transformation was found in 9 mol% MgO doped ZrO2 above 1300 °C [29].
Therefore, it is probably the high synthesis temperature and 20% weight ratio of low melting point glass leads to the appearance of monoclinic phase.
Besides, a diffraction peak is observed at 2θ ≈ 26° may be indexed to the SiO2 (JCPDS 13-0026) or Na2Si3O7 (JCPDS 38-0019).
This indicates that the Na2O-SiO2 in low melting point glass changes from amorphous to crystalline at high temperature.
The conductivities vs. different synthesis temperature and weight ratio were tested at 400–800 °C in nitrogen atmosphere as shown in Figure 3.
It is clear that the conductivities of composite electrolytes increase with the increase in glass concentration.
And the highest conductivities are obtained for the 8YSZ-20% glass (700 °C), 8YSZ-20% glass (1200 °C), and 8YSZ-20% glass (1550 °C) to be 5.7 × 10−2 S·cm−1, 4.1 × 10−3 S·cm−1, and 2.3 × 10−2 S·cm−1 at 800 °C, respectively.
A recent investigation by Lee et al. [29] reported that a single cubic phase of 8YSZ showed higher conductivity than 9 mol% MgO doped ZrO2 which has a mixed phase.
Similarly, the conductivities of the 8YSZ-20% glass (700 °C) (Figure 3a) and 8YSZ-20% glass (1550 °C) (Figure 3b) are higher than that of 8YSZ-20% glass (1200 °C) (Figure 3b) which has evidently tetragonal and monoclinic biphasic structure in Figure 2b.
The conductivities of the 8YSZ-20% glass (700 °C) are lower than that of 8YSZ-30% glass (700 °C) composite electrolyte as shown in Figure 3a.
However, the 8YSZ-30% glass (700 °C) composite electrolyte is unstable because it will cause segregation and reduce the mechanical hardness in the molten state when the glass powder is too high in percentage.
Figure 4 shows the variation of conductivity of 8YSZ-30% glass (700 °C) composite electrolyte with time in nitrogen atmosphere at 800 °C.
The conductivity reaches a steady state in the first hour.
However, with increasing time, the conductivity of 8YSZ-30% glass (700 °C) composite electrolyte gradually decreased.
This suggests that it cannot be used for long period at 800 °C.
The external (a) and cross-sectional (b) surface SEM images of the 8YSZ-20% glass (700 °C) composite electrolyte are displayed in Figure 5.
The 8YSZ agglomerated with low melting point glass powder, few pores are observed and the microstructure is homogeneous after heating at 700 °C, which is attributed to high fluidity of molten glass.
Figure 5 shows that the two components are evenly dispersed and intimately connected and do not react with each other due to their high chemical stability [3,5,9,11].
In order to investigate ionic conduction of the 8YSZ-20% glass (700 °C), the relationship between the oxygen partial pressure (pO2) and conductivities was studied.
As shown in Figure 6, there is almost a straight line within the whole pO2 range.
The result indicates that the 8YSZ-20% glass (700 °C) is almost a pure ionic conductor [20,21,22,23].
In the pO2 range of 10−20~10−15 atm, the curve is slightly upwarped, indicating that there is a trace electron conduction in the 8YSZ-20% glass (700 °C) in reducing atmosphere.
It is well known that ZrO2-based electrolyte is a good oxygen ion conductor.
To study the oxide ionic conduction of the 8YSZ-20% glass (700 °C) composite electrolyte, an oxygen concentration discharge cell was tested at 800 °C as shown in Figure 7.
The calculational electromotive forces (EMFcal) could be obtained from EMFcal = R T 4 F tO ln[pO2 (A)/pO2 (B)] when tO = 1.
The air (pO2 (B)) and pure O2 (pO2 (A)) are introduced into the anode and cathode, correspondingly.
From Figure 7, the open circuit voltage is 35.6 mV, which is close to the calculated EMF (36.1 mV).
Moreover, a stable discharge line could be seen in Figure 7.
All the results illustrate that the 8YSZ-20% glass (700 °C) composite electrolyte is a good oxygen ion conductor.
The H2/O2 fuel cell electrochemical performance was tested at 800 °C for the 8YSZ-20% glass (700 °C) as shown in Figure 8.
It can be seen that the 8YSZ-20% glass (700 °C) reveals a high open circuit voltage (1.09 V) which means the composite electrolyte is dense [5].
The maximum power density of the 8YSZ-20% glass (700 °C) is 72.7 mW·cm−2 (thickness = 1.1 mm) at 800 °C.
The result is lower than that previous reported cathode supported thin film fuel cell with 200–400 mW·cm−2 at 800–850 °C [12,13,14,30], which can be attributed to the electrolyte thickness and the electrode/electrolyte interface.
Further work is in progress to optimize the composition of composite electrolytes and develop the stable and high performance solid oxide fuel cell [31,32]. 4.
Conclusions In this study, low melting point glass powder was chosen as a sintering aid to prepare novel Zr0.92Y0.08O2-α(8YSZ)-low melting point glass composite electrolytes.
The results of XRD indicate that the major phase in composite electrolytes is tetragonal and no diffraction peaks of low melting point glass are found.
The influences of amount of additive, synthesis temperature, test temperature, and oxygen partial pressure on the electrical conductivities of the composite electrolytes were investigated at 400–800 °C.
The results of the XRD and conductivities show that the 8YSZ-20% glass (700 °C) is a suitable choice.
The oxygen concentration discharge cell illustrates that the 8YSZ-20% glass (700 °C) composite electrolyte is a good oxygen ion conductor.
The maximum power density of the 8YSZ-20% glass (700 °C) is 72.7 mW·cm−2 (thickness = 1.1 mm) at 800 °C.
Author Contributions H.W. and R.D. conceived and designed the experiments; R.S. and J.L. performed the experiments; H.W. and R.D. analyzed the data; R.S. and J.L. contributed the used materials and analysis tools; H.W. and R.S. wrote the paper. Funding This work was supported by the National Natural Science Foundation (No. 51402052) of China. The Natural Science Project of Anhui Province (No. KJ2018A0337). Excellent Youth Foundation of Anhui Educational Committee (No. gxyq2018046). Research and innovation team horizontal cooperation project of Fuyang municipal government and Fuyang Normal College (No. XDHX2016019, XDHXTD201704. XDHX201739). Excellent Youth Foundation of Fuyang Normal College (rcxm201805). Conflicts of Interest The authors declare no conflicts of interest. References 1. ChenY.OrlovskayaN.PayzantE.A.GrauleT.KueblerJ. A search for temperature induced time-dependent structural transitions in 10 mol% Sc2O3-1 mol% CeO2-ZrO2 and 8 mol% Y2O3-ZrO2 electrolyte ceramics J. Eur. Ceram. Soc. 2015 35 951 958 10.1016/j.jeurceramsoc.2014.08.030 2. LiJ.ZhangH.GaoM.LiQ.BianW.TaoT.ZhangH. High-temperature wettability and interactions between Y-containing Ni-based alloys and various oxide ceramics Materials 2018 11 749 10.3390/ma11050749 29735958 3. FondardJ.BertrandP.BillardA.FourcadeS.BatocchiP.MauvyF.BertrandG.BrioisP. Manufacturing and testing of a metal supported Ni-YSZ/YSZ/La2NiO4 ITSOFC synthesized by physical surface deposition processes Solid State Ion. 2017 310 10 23 10.1016/j.ssi.2017.07.027 4. XiaC.QiaoZ.FengC.KimJ.WangB.ZhuB. Study on zinc oxide-based electrolytes in low-temperature solid oxide fuel cells Materials 2018 11 40 10.3390/ma11010040 29283395 5. MamanaN.Díaz-ParralejoA.OrtizA.L.Sánchez-BajoF.CarusoR. Influence of the synthesis process on the features of Y2O3-stabilized ZrO2 powders obtained by the sol-gel method Ceram. Int. 2014 40 6421 6426 10.1016/j.ceramint.2013.11.090 6. LiuZ.ZhaoZ.ShangL.OuD.CuiD.TuB.ChengM. LSM-YSZ nano-composite cathode with YSZ interlayer for solid oxide fuel cells J. Energy Chem. 2017 26 510 514 10.1016/j.jechem.2016.11.019 7. DankeawA.PoungchanG.PanapoyM.KsapabutrB. In-situ one-step method for fabricating three-dimensional grass-like carbon-doped ZrO2 films for room temperature alcohol and acetone sensors Sens. Actuators B Chem. 2017 242 202 214 10.1016/j.snb.2016.11.055 8. LiuX.Y.XuZ.H.LiangG.Y. Comparative study of the sintering behaviors between YSZ and LZ/YSZ composite Mater. Lett. 2017 191 108 111 10.1016/j.matlet.2016.12.105 9. RaghvendraSinghP. Electrical conductivity of YSZ-SDC composite solid electrolyte synthesized via glycine-nitrate method Ceram. Int. 2017 43 11692 11698 10.1016/j.ceramint.2017.05.359 10. BuetaF.R.ImperialJ.F.CerveraR.B. Structure and conductivity of NiO/YSZ composite prepared via modified glycine-nitrate process at varying sintering temperatures Ceram. Int. 2017 43 16174 16177 10.1016/j.ceramint.2017.08.193 11. ElmouwahidiA.Bailón-GarcíaE.Pérez-CadenasA.F.Maldonado-HódarF.J.Castelo-QuibénJ.Carrasco-MarínF. Electrochemical performances of supercapacitors from carbon-ZrO2 composites Electrochim. Acta 2018 259 803 814 10.1016/j.electacta.2017.11.041 12. MoonH.KimS.D.HyunS.H.KimH.S. Characteristics of SOFC single cells with anode active layer via tape casting and cofiring Int. J. Hydrogen Energy 2008 33 2826 2833 10.1016/j.ijhydene.2008.03.024 13. SongJ.H.ParkS.I.LeeJ.H. Fabrication characteristics of an anode-supported thin-film electrolyte fabricated by the tape casting method for IT-SOFC J. Mater. Process. Technol. 2008 198 414 418 10.1016/j.jmatprotec.2007.07.030 14. RathM.K.JungY.M.ParkJ.H.JohD.W.LeeK.T. Heterogeneous nanograin structured NiO-YSZ anodes via a water-in-oil microemulsion route for solid oxide fuel cells J. Alloys Compd. 2017 723 681 688 10.1016/j.jallcom.2017.06.294 15. HabibniaM.ShakeriM.NourouziS. Determination of the effective parameters on the fuel cell efficiency, based on sealing behavior of the system Int. J. Hydrogen Energy 2016 41 18147 18156 10.1016/j.ijhydene.2016.06.258 16. GödekeaD.DahlmannU. Study on the crystallization behaviour and thermal stability of glass-ceramics used as solid oxide fuel cell-sealing materials J. Power Sources 2011 196 9046 9050 10.1016/j.jpowsour.2010.12.054 17. RautanenM.PulkkinenV.TallgrenJ.HimanenO.KiviahoJ. Effects of the first heat up procedure on mechanical properties of solid oxide fuel cell sealing materials J. Power Sources 2015 284 511 516 10.1016/j.jpowsour.2015.03.012 18. ErtugrulT.Y.CelikaS.MatM.D. Optimum processing parameters to improve sealing performance in solid oxide fuel cells Ceram. Int. 2015 41 9834 9842 10.1016/j.ceramint.2015.04.057 19. ShiR.LiuJ.WangH.WuF.MiaoH. Intermediate temperature fuel cell durability of Eu-doped SrCeO3-SrZrO3 solid solution/NaCl-KCl composite electrolyte Ceram. Int. 2017 43 16931 16935 10.1016/j.ceramint.2017.09.097 20. SunL.WangH.ShengL.LiH. Gadolinium doped strontium cerate prepared by citric-nitrate auto-combustion process and intermediate temperature electrical properties of its composite electrolyte Int. J. Electrochem. Sci. 2017 12 9689 9696 10.20964/2017.10.37 21. GuanQ.WangH.MiaoH.ShengL.LiH. Synthesis and conductivity of strontium cerate doped by erbium oxide and its composite electrolyte for intermediate temperature fuel cell Ceram. Int. 2017 43 9317 9321 10.1016/j.ceramint.2017.04.094 22. ShiR.LiuJ.WangH.WuF.MiaoH.CuiY. Low temperature synthesis of SrCe0.9Eu0.1O3−α by sol-gel method and SrCe0.9Eu0.1O3−α-NaCl-KCl composite electrolyte for intermediate temperature fuel cells Int. J. Electrochem. Sci. 2017 12 11594 11601 10.20964/2017.12.81 23. ZhangW.YuanM.WangH.LiuJ. High-performance intermediate temperature fuel cells of new SrCe0.9Yb0.1O3−α-inorganic salt composite electrolytes J. Alloys Compd. 2016 677 38 41 10.1016/j.jallcom.2016.03.245 24. MatsudaA.OhS.NguyenV.H.DaikoY.KawamuraG.MutoH. Anhydrous proton conductivity of KHSO4-H3PW12O40 composites and the correlation with hydrogen bonding distance under ambient pressure Electrochim. Acta 2011 56 9364 9369 10.1016/j.electacta.2011.08.019 25. SooM.T.PrastomoN.MatsudaA.KawamuraG.MutoH.NoorA.F.M.LockmanZ.CheongK.Y. Elaboration and characterization of sol-gel derived ZrO2 thin films treated with hot water Appl. Surf. Sci. 2012 258 5250 5258 10.1016/j.apsusc.2012.02.008 26. MirandaM.I.G.BicaC.I.D.NachtigallS.M.B.RehmanN.RosaS.M.L. Kinetical thermal degradation study of maize straw and soybean hull celluloses by simultaneous DSC–TGA and MDSC techniques Thermochim. Acta 2013 565 65 71 10.1016/j.tca.2013.04.012 27. HuangS.ShengJ.J. An innovative method to build a comprehensive kinetic model for air injection using TGA/DSC experiments Fuel 2017 210 98 106 10.1016/j.fuel.2017.08.048 28. MoriM.HieiY.ItohH.TompsettG.A.SammesN.M. Evaluation of Ni and Ti-doped Y2O3 stabilized ZrO2 cermet as an anode in high-temperature solid oxide fuel cells Solid State Ion. 2003 160 1 14 10.1016/S0167-2738(03)00144-9 29. YoonS.NohT.KimW.ChoiJ.LeeH. Structural parameters and oxygen ion conductivity of Y2O3-ZrO2 and MgO-ZrO2 at high temperature Ceram. Int. 2013 39 9247 9251 10.1016/j.ceramint.2013.05.032 30. MengX.GongX.YangN.TanX.YinY.MaZ. Fabrication of Y2O3-stabilized-ZrO2 (YSZ)/La0.8Sr0.2MnO3−δ-YSZ dual-layer hollow fibers for the cathode-supported micro-tubular solid oxide fuel cells by a co-spinning/co-sintering technique J. Power Sources 2013 237 277 284 10.1016/j.jpowsour.2013.03.026 31. RoehrensD.PackbierU.FangQ.BlumL.SeboldD.BramM.MenzlerN. Operation of thin-film electrolyte metal-supported solid oxide fuel cells in lightweight and stationary stacks: Material and microstructural aspects Materials 2016 9 762 10.3390/ma9090762 28773883 32. KusnezoffM.TrofimenkoN.MüllerM.MichaelisA. Influence of electrode design and contacting layers on performance of electrolyte supported SOFC/SOEC single cells Materials 2016 9 906 10.3390/ma9110906 28774024 Figure 1 TGA-DSC test of the Zr0.92Y0.08O2−α(8YSZ) gel heated at 15 °C·min−1. Figure 2 (a) XRD patterns of the 8YSZ, 8YSZ-10% glass, 8YSZ-20% glass and 8YSZ-30% glass calcined at 700 °C for 6 h; (b) XRD patterns of the 8YSZ and 8YSZ-20% glass calcined at 1200 °C and 1550 °C, respectively. Figure 3 The conductivities vs. (a) different weight ratio of the 8YSZ-10% glass, 8YSZ-20% glass and 8YSZ-30% glass after calcined at 700 °C; (b) different synthesis temperature of the 8YSZ-20% glass (1200 °C, 1550 °C) in nitrogen atmosphere at 400–800 °C. Figure 4 The variation of conductivity of 8YSZ-30% glass (700 °C) with time in nitrogen atmosphere at 800 °C. Figure 5 The external (a) and cross-sectional (b) surface SEM images of the 8YSZ-20% glass (700 °C) composite electrolyte. Figure 6 The conductivities of the 8YSZ-20% glass (700 °C) composite electrolyte as a function of pO2 at 750 °C is almost a pure ionic conductor. Figure 7 The oxygen concentration discharge cell: air, Pd-Ag|8YSZ-20% glass (700 °C)|Pd-Ag, O2 at 800 °C. Figure 8 H2/O2 fuel cell of the 8YSZ-20% glass (700 °C) at 800 °C. materials-11-01221-t001_Table 1 Table 1 The samples synthesized with different synthesis temperature and weight ratio. Sample Synthesis Temperature Abbreviation Zr0.92Y0.08O2−α-10 wt% low melting point glass 700 °C 8YSZ-10% glass 700 °C Zr0.92Y0.08O2−α-20 wt% low melting point glass 700 °C 8YSZ-20% glass 700 °C Zr0.92Y0.08O2−α-30 wt% low melting point glass 700 °C 8YSZ-30% glass 700 °C Zr0.92Y0.08O2−α-20 wt% low melting point glass 1200 °C 8YSZ-20% glass 1200 °C Zr0.92Y0.08O2−α-20 wt% low melting point glass 1550 °C 8YSZ-20% glass 1550 °C