Electrolytes: Stabilized Zirconia

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1 Laurea Magistrale in Scienza dei Materiali Materiali Inorganici Funzionali Electrolytes: Stabilized Zirconia Prof. Antonella Glisenti - Dip. Scienze Chimiche - Università degli Studi di Padova

2 Bibliography 1. N.Q. Minh, T. Takahashi: Science and technology of ceramic fuel cells Elsevier O. Yamamoto et al. Ionics, 4 (1998) V.V. Kharton et al. Solid State Ionics 174 (2004) S.P.S. Badwal et al. Solid State Ionics (2000) 91-99

3 Electrolyte: requirements Functions: To conduct ions between the anode and cathode To separate the fuel from the oxidant in the fuel cell Requirements: Stability chemical, morphological, dimensional stability at the dual atmosphere and at the operating and fabrication temperatures Ionic conductivity in the dual atmosphere (at the FC operating temperature) to minimize ohmic losses (negligible electronic conductivity) Compatibility chemical compatibility with the other cell components Thermal expansion must match (from RT to the operating and fabrication temperatures) that of other components; thermal coefficient stable in the dual atmosphere Porosity high density or no connected porosity to prevent gas cross leakage

4 Electrolytes Electrolyte is a solid, non-porous metal oxide (Y 2 O 3 -stablilized 3, 8 or 10% ZrO 2 ). Nernst 1890s = certain perovskites, stabilized zirconias, conducted ions in a certain temperature range. Baur and Preis 1943 = such electrolytes could be used as (oxygen) ion conductors in fuel cells. YSZ = high ionic conductivity at T > 700 C negligible electronic conductivity (above 1500 C it becomes an electronic conductor). FC 250 ma/cm 2 at 1000 C electrolyte = 200 µm thickness, resistance loss = 50 mv. 800 C = electrolyte thickness µm to maintain a similar ohmic loss ZrO 2 = stable in SOFC oxidizing and reducing atmospheres; only under highly reducing conditions (< atm at 1000 C) it is reduced to ZrO 2-δ ZrO 2 is stabilized by direct substitution of divalent or trivalent cations for Zr 4+ ; this substitution creates oxygen vacancies = high oxygen-ion mobility. Y 2 O 3 ZrO 2 2 Y Zr + V O + 3O x O

5 Zirconia and stabilized zirconia ZrO 2 : From RT to 1170 C monoclinic From 1170 C to 2370 C tetragonal From 2370 C to 2680 C (melting point) cubic CaO, Y 2 O 3, MgO, Sc 2 O 3 and certain rare-earth oxides stabilize ZrO 2 in cubic fluorite structure These oxides exhibit a relatively high solubility in ZrO 2 Crystal structure for (ZrO 2 ) 1-x (Ln 2 O 3 ) x

6 Influence of dopant and dopant concentration The conductivity shows a maximum for a narrow range of dopant concentrations: the defect complexes (Y Zr V O and Y V OY Zr Zr ) Variation of ionic conductivity of stabilized ZrO 2 with dopant concentration at 1080 K The conductivity of stabilized ZrO 2 depends on the size of dopant cation ( Nd 3+, Sm 3+, Gd 3+, Y 3+, Yb 3+, Sc 3+ = 0.104, 0.097, 0.092, 0.086, nm Zr 4+ = nm): lattice strain, steric blocking effect

7 Zirconia and stabilized zirconia ZrO 2 Ln 2 O 3 (trivalent rare earths) Y 2 O 3, Sc 2 O 3 ZrO 2 AO (A = divalent alkaline earth metal) corresponding conductivity (processing history and microstructural features): dopant segregation, impurities, kinetically limited phase transitions and formation of ordered microdomains. Dependence of the electrical conductivity of Zr 0.90 R 0.10 O 1.95 ceramics (R = rare earth element) on radii of the cations R 3+ at 1000 C (1) and 800 C (2). (ceramics prepared by coprecipitation of hydroxides + sintering). Maximum ionic conductivity when the concentration of acceptor-type dopant(s) is close to the low stabilization limit Zr 1-x Y x O 2x-2 x= Zr 1-x Sc x O 2x-2 x = conductivity depends on the difference between the host and dopant cation radii

8 Zirconia and stabilized zirconia High conductivity in solid oxide ion conductors = low E att for conduction E att for the conduction depends on the dopant ion radius. > difference between the host and dopant cation radii > association of oxygen vacancies and dopant cations into complex defects of low mobility. The ZrO 2 -MO 2 -Ln 2 O 3 (M = Ce, Hf) ternary systems have been examined to change the ZrO 2 host lattice parameter. CeO 2 was selected to expand the ZrO 2 lattice and HfO 2 to reduce it.

9 Scandia Stabilized Zirconia (SSZ) ZrO 2 Sc 2 O 3 Sc 2 O 3 ZrO > 5.0 mol% Sc 2 O 3 = show tetragonal structure with tetragonality decreasing with increasing Sc 2 O 3 content 9 mol% Sc 2 O 3 = structure is nearly cubic mol% Sc 2 O 3 = Ordered β-phase (Sc 2 Zr 7 O 17 ). depending on the preparation conditions the homogeneity of the material, the thermal history and the temperature of investigation, this phase has been reported in compositions containing much lower level of Sc 2 O 3

10 Compositions with or below 9.5 mol% Sc 2 O 3 content showed the normal behaviour observed for such materials: a continuously changing slope towards lower activation energy with an increase in the measurement temperature. However, both 10.0 and 11.0 mol% Sc 2 O 3 ZrO 2 compositions showed a clear jump in the conductivity curves. Rhombohedralβ-phase (Sc 2 Zr 7 O 17 )

11 Electrolyte: preparation Preparation of fully dense polycrystalline layers 1. Powder technology Involves compaction of ZrO 2 powder into the desired shape (tape casting, tape calendering) and densification at elevated temperatures 2. Deposition procedure Involves the formation of a thin layer (on a substrate or support) by a chemical or physical process Electrochemical vapour deposition (EVD) Chemical vapour deposition (CVD) Rf sputtering Rf ion plating Spray pyrolysis Sol-gel Pulse laser deposition

12 Influence of temperature The conductivity of stabilized ZrO 2 as a function of temperature typically follows Arrhenius-type behaviour. where σ = conductivity, T = temperature, k = Boltzmann constant, A σ = preexponential constant, α and β = positive constants this equation holds for single crystal and polycrystalline YSZ in a certain temperature range, α+βt -1 can be approximated as a constant (= activation energy for conduction, E σ ), and: σt = A σ exp(-e σ /kt) Arrhenius resistivity plots for Y 2 O 3 -doped ZrO 2

13 Influence of atmosphere The conductivity of stabilized ZrO 2 is described by empirical relations; (ZrO 2 ) 0.92 (Y 2 O 3 ) 0.08 between 800 and 1050 C with partial pressures of oxygen (0.21 to atm) Ionic conductivity Electronic conductivity

14 Influence of atmosphere The conductivity of stabilized ZrO 2 is usually independent of oxygen partial pressure over several orders of magnitude Conductivities of stabilized zirconia at 1000 C as a function of oxygen partial pressure Conductivities of (ZrO 2 ) 0.9 (Y 2 O 3 ) 0.10 as a function of oxygen partial pressure only at very low oxygen partial pressure the electronic conductivity becomes significant and the total conductivity starts to increase with decreasing oxygen partial pressure The oxygen partial pressure at which the electronic conductivity becomes significant is higher at higher temperatures

15 Influence of grain boundary Conductivity polycrystalline YSZ = bulk (BC) + grain-boundary (GBC) Equivalent circuits and schematic complex impedance plot of polycrystalline ZrO 2 Complex impedance plot of polycrystalline (ZrO 2 ) 0.9 (Y 2 O 3 ) 0.1 at 800 C in air

16 Influence of grain boundary Conductivity polycrystalline YSZ = bulk (BC) + grain-boundary (GBC) GBC: impurities or second phases introduced via the raw materials or during the fabrication processes SiO 2 and Al 2 O 3 (commonly present as impurities in commercial YSZ powders or added to starting powders as sintering aids). impurity/grain size Small grains (< 2-4 µm) GBC is independent of the grain size and is 100 times lower than that of the bulk Large grains (> 2-4 µm) GBC decreases with increasing grain size impurity level GBC: poor contribution for high-density pure polycrystalline materials; Significant influence at low and intermediate temperatures (< 700 C) (high E att )

17 Influence of time/temperature: aging ρ(t) = A - B 1 exp (-K 1 t) - B 2 exp (-K 2 t) BC GBC ρ= resistivity, t = time, A, B 1, K 1, B 2, and K 2 = constants BC of fully stabilized zirconia = in a short time the steady value is reached (due to the crystal reorganization) BC of partially stabilized zirconia = aging effect due to the precipitation of tetragonal phase from the cubic matrix GBC aging due to the surface segregation of impurities

18 Influence of time/temperature: aging ρ plots as a function of the dopant content both before and after annealing at 850 and 1000 C. ρ change at 850 C in 9.0 mol% SSZ as a function of time and % increase in the ρ per 5000 min. Impedance spectra recorded at 350 C showing the effect of annealing 7.0 and 9.3 mol% Sc 2 O 3 ZrO 2 at 850 and 1000 C (5000 min). Formation of t -phase: a distorted fluorite-type phase Formation of microdomains of ordered phase rich in Sc

19 Chemical interaction YSZ at the FC operating temperature (600 to 1000 C) = little or no chemical interaction with other components. YSZ with LaMnO 3 at higher temperatures: insulating phases (La 2 Zr 2 O 7, as an example) at the interface at T > 1100 C. These phases cause cell performance to degrade significantly!!!

20 Thermal expansion The thermal expansion coefficient of undoped ZrO 2 single crystal is 8.12 x 10-6 cm/cmk in the temperature range of 20 to 1180 C Doped materials typically have higher thermal expansion coefficients TEC of partially stabilized zirconia is very similar to that of fully stabilized and is essentially unaffected by the presence of tetragonal precipitates Thermal expansion coefficients of YSZ Thermal expansion coefficients of YSZ at different temperatures

21 Thermal expansion Without material tailoring or modification significant thermal expansion mismatch can exist YSZ electrolyte is selected as the baseline material TEC of perovskite cathode and interconnect may be adjusted by tailoring the dopant element and concentration TEC of nickel cermet anode may be tailored by modifying the nickel content and ZrO 2 concentration or by using additives. Thermal expansion curves for several stabilized ZrO 2 and perovskite oxide materials

22 Mechanical properties: YSZ YSZ (8% - at RT): bending strength = MPa; Fracture Toughness = 3 MNm 3/2 Mechanical properties depend on the starting powders and fabrication route large agglomerates (up to 100 µm in diameter) = defects in the prepared electrolyte (= poor strength of the component) YSZ sheets produced by tape calendering have superior mechanical properties with a mean strength about 15% higher than that of the material made by tape casting Mechanical properties depend on temperature: mean strength of YSZ at 900 C = 280 MPa (vs 368 MPa at RT); a bending strength of about 225 MPa has been reported for YSZ at 1000 C.

23 Mechanical properties: YSZ Inclusions to improve mechanical properties: Partially stabilized zirconia (30% wt. fracture toughness 2.95 MPam 1/2 = 200% higher than that of YSZ fabricated under similar process conditions, ionic conductivity 0.15 Ω - 1 cm -1 = 17% lower than that of YSZ) Al 2 O 3 (20% wt. bending strength = 323 MPa compared with 235 MPa for YSZ), ionic conductivity 0.10 Ω -1 cm -1 at 1000 C compared with 0.12 Ω -1 cm -1 for YSZ) Variation of the bend strength of YSZ with temperature and Al 2 O 3 content

24 HIGH WORKING TEMPERATURES: ONE OF THE MAJOR DRAWBACK OF ZrO 2 BASED ELECTROLYTES: