Laurea Magistrale in Scienza dei Materiali. Materiali Inorganici Funzionali. Electrolytes: Stabilized bismuthsesquioxide

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1 Laurea Magistrale in Scienza dei Materiali Materiali Inorganici Funzionali Electrolytes: Stabilized bismuthsesquioxide 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 P. Shuk et al. Solid State Ionics 89 (1996) A. Watanabe Solid State Ionics 176 (2005) A. Watanabe et al. Solid State Ionics 176 (2005) V. Fruth et al J. Europ. Ceram. Soc. 26 (2006)

3 Stabilized bismuthsesquioxide Conductivity data for Bi 2 -M 2 Bi 2 -MO 2 (M = W, Mo); Bi 2 -M 2 O 5 (M = V, Nb, Ta); Bi 2 -M 6 O 11 (M = Pr); Ni 2 -Y 2 -Nb 2 O 5 ; Bi 2 -Ln 2 -TeO 2 (Ln = La, Sm, Gd, Er); Bi 4 V 2 - x M x O 11-δ (M = divalent or pentavalent cation) BIMEVOX; BICUVOX (Cu doped) BINIVOX (Ni doped) BIPBVOX (Pb doped) BIZNOVOV (Zn doped) BICOVOX (Co-doped)

4 Conductivity in stabilized bismuthsesquioxide Arrhenius plots of oxygen ion conductivity of M 2 stabilized bismuth oxides show a characteristic kink at transition temperature ca. 600 C with a lower activation energy above and a higher activation energy below The conductivity shows a continuous decay with time on isothermal annealing in air below this transition temperature (the mobile oxygen vacancies are consumed in the growing ordered clusters and become unavailable for conduction) This process is reversible

5 Bismuthsesquioxide Bismuth (III) oxide = monoclinic (α) and fcc (δ) α δat K (purity of samples, thermal prehistory and oxygen stoichiometry). Cooling down of the hightemperature -phase is accompanied by formation of an intermediate metastable tetragonalβ-phase and a bccγ-polymorph (silenite phase) δ βwith a sudden large volume change α δentropy gain is 75% of the entropy gain observed for melting Survey of the temperature regions of stable and metastable phases encountered in B 2

6 Conductivity of bismuthsesquioxide Ionic Mixed Electronic Conductivity vs T For Bi 2

7 Bismuthsesquioxide: Bi 2 Unlike stabilized zirconia, doping only serves to stabilize the high temperature phase at lower temperatures Advantage: great ionic conductivity = possibility to operate at reduced temperature Disadvantage: Small oxygen partial pressure range of ionic conduction: stabilized Bi 2 is reduced under low oxygen partial pressures and decomposes into bismuth metal at a P O2 = atm at 600 C (= material has to be protected against reducing atmosphere) Development of solid solutions with appropriate composition and improved properties = flexibility with which bismuth oxide forms compounds with widely variable compositions and many different substitutions

8 Conductivity in bismuthsesquioxide Conductivity of Bi 2 and Bi 2 - Y 2 as a function of yttria content and temperature 25% mol Y 2 = no jump in conductivity; < 25% mol = abrupt increase in conductivity due to phase transition

9 Structure of bismuthsesquioxide High temperature δ-bi 2 (1 Ω -1 cm -1 near the melting point: 825 C); Sillen (1937) = cubic fluorite structure with ordered defects in the oxygen sublattice in the <111> direction Gattow and Schroeder (1962 -High T XRD) = cubic fluorite structure with an oxygen sublattice with a statistical occupation of the sites (75%) Willis (1963) = each anion sites replaced by four equivalent sites displaced in the <111> direction from the ideal position. Oxide ion occupy these sites statistically with an occupacy of 3/16 Battle (neutron diffraction) = 43 % of the regular oxide ion sites are randomly occupied; 1.28 oxide ions per unit cell being displaced from their ideal positions along the <111> direction. High level of disorder = high ion conductivity δ- Phase is stable only in a temperature range close to the melting point

10 Structure of bismuthsesquioxide β and γ -Bi 2 β phase (metastable) = distorted defect-fluorite structure with ordered vacant sites in the anion sublattice γ-phase = bcc phase affected by traces of impurities

11 Thermal Expansion The averadge thermal expansion coefficients α of the Bi 2 phases The transition from the δ to the β, γ phases is accompanied by a large sudden volume change and a deterioration of the mechanical properties of the material

12 Doped bismuthsesquioxide Takahashi, Iwahara, Nagai (1972) The high temperature phase with high ionic conductivity can be stabilised to lower temperatures by substitution of aliovalent cations for Bi The stability region of the high ionic conductivity phases can be extended to room temperature by incorporation of: mol% W mol% Y mol% Gd mol% Er mol%dy mol% Sm mol% Nb 2 O 5 and mol% Ta 2 O mol% Pr mol% Tb 2.5

13 Structure of doped bismuthsesquioxide Most of the ion conducting solid solutions of Bi 2 form with the fcc structure of the high temperature delta phase. In addition a rhombohedral (Bi 2 -SrO type) structure was found

14 Structure of doped bismuthsesquioxide Bi 2 -M 2 fcc and rhombohedral phase formation depends on the ionic radius of the rare-earth cation and the rare-earth oxide content Range of fcc and rhombohedral phase formation in (Bi 2 ) 1-x -(Ln 2 ) x systems Rhombohedral phase: Large rare earth ion radii and relatively low x fcc phase: Cations with smaller cationic radii than Bi(III) and relatively high x Double doping favours the δ- phase stabilization

15 Structure of doped bismuthsesquioxide The high T δ-phase structure is stabilized by contraction of the structure > r between Bi 3+ - Ln 3+ > distortion < required amount of dopant Double doping favours the δ-phase stabilization down at room temperature: a cooperative effect due to the entropy increase favouring the defect properties

16 Conductivity in stabilized bismuthsesquioxide the minimum dopant content without jump in conductivity corresponds to the lowest content of added oxides required to stabilize the fcc phase Conductivity of (Bi 2 ) 1-x - x min, the minimum value of x (Ln 2 ) x as a function of ionic required to stabilize the fcc radius phase in (Bi 2 ) 1-x (Ln 2 ) x vs the ionic radius (r ion ) of Ln(III) Lattice parameter maps of the various fluorite oxides

17 Conductivity in stabilized bismuthsesquioxide (Verkerk model) Short-range ordered units consisting of three Bi(III) ions and one Ln(III) ion (Bi3Ln)- tetrahedron (oxide ions at ¾) Model of the ordered microdomain unit for (Bi 2 ) 0.75 (Ln 2 ) 0.25 Low Temperature range Short Ln-O bond strength Preferential paths High Temperature (870 K) range Increment of the Ln-O bonds All oxide ions are mobile No preferential paths

18 Typical solid electrolytes based on Bi2O3

19 Stabilized bismuthsesquioxide (Bi 2 ) 1-x (Me 2 ) x (Bi 2 ) 1-x-α (Me 2 ) x + 2αBi + 3α/2 O 2 P O2 > Pa at 973 K P O2 > Pa at 873 K Oxygen partial pressure dependence of the hole and electron conductivities of Bi 2 -M 2 together with the oxygen-ion conductivity at 500, 600, and 700 C.

20 Synthesis, structure and properties of doped Bi 2 J. Europ. Ceram. Soc. 26 (2006) 3011 Fruth. Et al. The influence of selected ions (Fe 3+, Sb 3+ /Sb 5+, Ta 5+ ) introduced as dopant in α-bi 2 and their effect on the structure and properties on the oxide polymorph forms obtained at high temperature Bi 2 and Sb 2, Fe 2 and Ta 2 O 5 mixed in stoichiometric proportions to form Bi 1.9 M 0.1 O x (M = Fe, Sb or Ta) mixtures. The samples were repeatedly ground in a mortar and paste to remove any agglomerates present, then pressed into discs of 10 mm diameter, sintered at different temperatures up to 850 C and cooled in the furnace till RT.

21 δ-phase stability range Bi 2 : C 0.95(Bi 2 ):0.05(Fe 2 ): C 0.95(Bi 2 ):0.05(Sb 2 ): C 0.95(Bi 2 ):0.05(Ta 2 ): C Mixed conduction mechanism: n-type Temperature dependence of the total electrical conductivity

22 Density and shrinkage of the ceramic bodies ad different temperatures

23 Effect of the annealing at C β*phase superstructures with 2x2x1 relationships to Bi 2 γ*phase mixture of phases related to the presence of a liquid phase in the system

24 Two phases, one well formed with large particles (> 10 µm), the other one with small particles (γ) Sustituted oxide forms a thin coating on the surface of Bi 2 powder particles 10 h 850 C 0.95(Bi 2 ):0.05(Fe 2 ) after 10 h at 830 C, in air Melting point decreases with decreasing particle size T m (D) = T m ( ) a/z where T m ( ) = bulk melting temperature, Z = particle diameter or layer thickness a = constant. 0.95(Bi 2 ):0.05(Ta 2 O 5 ) 10 h 870 C

25 after 10 h at 830 C, in air 0.95(Bi 2 ):0.05(Sb 2 ) after 20 h at 820 C, in air. Bi 1.42 Sb 0.58 Bi 1.78 Sb 0.22

26 Aging of stabilized bismuth sesquioxides Of all others, Er 2 yielded the best results: the fcc phase was stabilized over a compositional range of mol% Er 2 in the system Bi 2 Er 2 (Bi 2 ) 0.8 (Er 2 ) 0.2 showed the highest oxide-ion conduction, 2.3x10-2 and 3.7x10-1 S cm -1 at 500 and 700 C, respectively. Bi 2 + Er 2 (prefired for dehydration at 700 C in air) Heated in a gold crucible at C

27 Aging of stabilized bismuth sesquioxides 1 st 374 h 650 C δ-bi 2 7 th 140 h 602 C Darkened reflections: unknown phase, the other reflections: Bi-Sr-O type hexagonal phase. DTA curve: two endothermic peaks at 700 (hexagonal/fcc) and 740 C (unknown phase/fcc). 12 th 301 h 650 C solid circles: α-bi 2 1. Repetitive annealing at C led to a gradual transformation of the fcc phase to the hexagonal phase; the transformation was incomplete despite a long heat treatment as reported 2. The hexagonal phase is metastable

28 Aging of stabilized bismuth sesquioxides Bi 2 Er 2 intermediate phases: fcc phase, BiSrO-type hexagonal phase, triclinic (Bi 2 ) 0.51 (Er 2 ) 0.49 orthorhombic (Bi 2 ) 0.72 (Er 2 ) 0.28 the hexagonal phase disappears and the low-temperature orthorhombic phase stable (Er 2 < 28%) or triclinic phase (Er 2 > 28%) is generated. Electrical conductivity of the fcc and orthorhombic phases of (Bi 2 ) 0.72 (Er 2 ) 0.28 and the triclinic phase (Bi 2 ) 0.51 (Er 2 ) 0.49.

29 Stability in ternary systems Effect of double doping Stabilizedδ-Bi 2 phase in the system Bi 2 Er 2 W good oxide-ion conduction, ((Bi 2 ) (Er 2 ) 0.21 (W ) at 550 C, σ =0.05 S cm -1 ) Bi 2 + W + Er 2. (Er 2 prefired at 700 C in air before use for dehydration) The mixture was heated in a covered platinum crucibles at 825 C for 10 h or more and quenched to room temperature by an air stream.

30 Stability in ternary systems 1 st 6 th 7 th The first heat treatment generates the δ-phase This y-phase has been kept stably during subsequent six times ~300 h annealing at 600 C On further annealing (7-8) a part of the δ-phase gradually decomposes 8 th XRD of Er and W doped Bi 2 solid triangle: phase different than delta In the present system, since the solid state reaction was extremely sluggish, the several times repetitive long-term annealing was needed to reach true equilibrium at 600 C.

31 Stability in ternary systems Stabilized δ-bi 2 phase in the system Bi 2 Er 2 W solid circles = fcc single phase open squares = mixture Conductivity as a function of temperature and time for Er and W doped Bi 2.

32 Stabilized bismuthsesquioxide Bi 2 -MO (M = Ca, Sr, Ba) solid solutions tend to form rhombohedral phase that shows high oxygen-ion conductivity Sharp jump = β 1 β 2 transition (Bi 2 ) 0.84 (BaO) 0.16 highest oxygen-ion conductivity (0.88 Ω -1 cm -1 at 600 C) Conductivity of Bi 2 -MO systems

33 BIMEVOX (BIsmuth MEtal dopant Vanadium OXide) Solid solutions with Bi 2 between % α-bi 2 VO 5.5 (orthorhombic) transforms, through the β-phase ( K - tetragonal) to the tetragonal γ-phase. Perovskite slabs containing oxygen vacancies The ideal structure of γ-type Bi 2 VO 5.5 Aurivillius-type phase: [Bi 2 O 2 ] 2+ layers sandwiched between defect [V.5 (V O ) 0.5 ] 2- perovskite-like slabs slabs contain oxygen vacancies responsible for the high oxide ion conductivity

34 STABILIZED-BIMEVOX (Bi 2 V 1-x M x O 5.5-y ) M = Cu, Ni, Zn, Pb, Mo, W, Co, Ti, Zi, Sn, Pb B 2 VO 5.5 = relatively large oxide ion conductivity in the high-temperature γ-phase ( K) Many elements can substitute for V to form type solid solutions BIMEVOX: by substituting other cations for vanadium Depending on the chemical nature of the sostituent M the critical concentration values x min limiting the solid solution domain differ and have a maximum value (x = 0.5) with Nb and Sb. Frequently the minumum value is around 0.10