Calcium-doped Lanthanum Chromites Research Articles

Tape Casting, Lamination and Sintering of Calcium-Doped Lanthanum Chromite for SOFC Interconnects

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環境・エネルギー問題と膜 メンブレーンリアクターとしての固体酸化物形燃料電池

Characteristic features of solid oxide fuel cells have been reviewed with an emphasis on their features as membrane reactor. In particular, the mass transfer characteristics of electrolytes and interconnect materials have been discussed to clarify the appropriateness as SOFC electrolyte and interconnects. For electrolyte, the low electron conductivity in addition to the high oxide conductivity is essential. Since the relatively large electron contribution for doped ceria, these materials cannot be used as SOFC electrolyte, whereas lanthanum gallate-based electrolyte exhibits excellent properties particularly at low temperatures. For interconnect materials, lanthanum chromite exhibits excellent properties. The oxygen permeation through the calcium doped lanthanum chromite has been identified as surface reaction limited process, which allows to use as thin interconnect.

Chemical and Thermal Expansion of Calcium-Doped Lanthanum Chromite

Atomistic free-energy minimization techniques were used to simulate three simultaneous volumetric shrinkage/expansion phenomena in calcium-doped lanthanum chromite solid oxide fuel cell (SOFC) interconnect materials. Four sets of interatomic potentials were developed and tested over the temperature range 0–1273 K. The predicted unit-cell volumes, elastic properties, volumetric shrinkage due to A-site doping of the ABO3 perovskite (La1−xCax)CrO3, defect-induced volumetric expansion due to reducing atmospheres, and thermal expansion were in reasonable agreement with experiment, though not all concurrently with a single set of potentials. Potentials based either on simple oxides or on partial charge models appeared to give the best overall predictions. Additional experimental data are needed to improve the potentials.

Influence of bond valence on bond covalency in calcium doped lanthanum chromite

AbstractThe influence of bond valence on bond covalence in Lal‐xCaxCrO3(x = 0.0, 0.1, 0.2, 0.3) has been studied by using semi empirical method. This method is the extortion of the dialectic description theory proposed by Phillips, Van Vetches, Levine and Tanaka (PVLT). In the calculation of bond valence, two schemes were adopted. The first is the equal‐valence scheme, and the second is Bond Valence Sums (BVS) scheme. Bath schemes suggest that for the title compound bond covalence be mainly influenced by bond valence, and insensitive to the Ca doping level. Generally speaking, larger bond valences usually result in higher bond covalence's.

The effect of reducing environments on the stability of acceptor substituted yttrium chromite

The dimensional stability of calcium-substituted yttrium chromite was evaluated over a wide range of temperatures, oxygen partial pressures, and compositions, and compared to data collected for calcium-doped lanthanum chromite. Sample expansion was dependent on the concentrations of the acceptor dopant and CaCrO 4, a secondary phase observed in samples with greater than 25 mole% calcium, as well as measurement temperature and the oxygen partial pressure. The instability of yttrium chromite in reducing environments was directly related to the loss of lattice oxygen and the simultaneous reduction of Cr 4+ to Cr 1+ to maintain electroneutrality. The evolution of oxygen and the reduction of chromium resulted in a lattice expansion, caused by the size change associated with the Cr 4+ to Cr 3+ transition and electrostatic repulsion between adjacent oxygen octahedra. The measured expansion of yttrium chromites was 42% less than that for similarly doped lanthanum chromites. The reduction in expansion is a result of the decreased size of the unit cell (6.5% smaller than calcium-doped lanthanum chromite) which suppresses expansion by hindering the reduction of Cr 4+ and oxygen evolution during reduction.

Reactions at the Calcium Doped Lanthanum Chromite–Yttria Stabilized Zirconia Interface

Solid oxide fuel cell materials calcium doped lanthanum chromite (La,Ca)CrO3and yttria stabilized zirconia (YSZ) are known to interact with each other. This interaction causes interruption of cosintering processes and the formation of secondary phases which degrade fuel cell performance. The present study shows by X-ray diffraction and microscopic analysis that Ca migrates via an exsolved Ca–Cr–O liquid phase emanating from (La,Ca)CrO3and reacts with YSZ to obtain an intermediate layer of CaZrO3. The width of the CaZrO3reaction region was found to increase with increasing Ca content in the exsolved Ca–Cr–O second phase. Calcium zirconate was observed to react with YSZ along the YSZ grain boundaries, eventually digesting YSZ grains into a CaZrO3matrix. The results demonstrate the improbability of cosintering (La,Ca)CrO3and YSZ in the presence of the Ca–Cr–O liquid phase and illustrate the importance of obtaining single phase (La,Ca)CrO3to achieve compatibility with YSZ.

Liquid phase behaviour in nonstoichiometric calcium-doped lanthanum chromites

The sinterability of (La,Ca)CrO3 is analysed by comparing the liquid phase behaviour in A-site excess and A-site deficient compositions (referring to the ABO3 formula). The analysis shows that a series of A-site excess and deficient compositions form distinct liquid phases belonging to the CaO-Cr2O3 phase system. Although both series experience grain growth and densification due to the presence of a liquid phase, the A-site excess compositions exhibit greater shrinkage and sinter to closed porosity, whereas the A-site deficient compositions remain porous. During the final stages of sintering, surplus liquid from the A-site excess material exudes to the free-surface forming a layer of uniform thickness. By comparison, the liquid in the A-site deficient composition segregates to the free-surface forming islands and leaving porous regions in the bulk matrix. It is concluded that the different liquid phase compositions in the A-site excess and A-site deficient (La,Ca)CrO3 have dissimilar wetting characteristics during the later stages of sintering. The A-site deficient liquid forms a premature solid phase, which accounts for the difference in sintering behaviour.

Reaction between Calcium‐Doped Lanthanum Chromite and Silica

Calcium‐doped lanthanum chromite reacted with silica to form silicates and chromium oxide at 1473‐1773 K. A reaction mechanism was proposed at each temperature. At 1473 K, a reaction zone was observed in two areas: one was the bulk reaction area where silicates and chromium oxide were formed near the reaction interface; the other was the grain boundary reaction area where chromium oxide and calcium silicate spots were formed far from the reaction interface. Above 1673 K, silicates in the La2O‐CaO‐SiO, system and chromium oxide were formed simultaneously in the reaction zone with no grain boundary reaction. No layered reaction products were observed in the reaction zone of the lanthanum chromite side. Formation of melt silicates above 1723 K at the reaction interface resulted in strong bonding between calcium‐doped lanthanum chromite and silica.

Chemical Diffusion in Polycrystalline Calcium-Doped Lanthanum Chromites

The chemical diffusion coefficient of polycrystalline calcium-doped lanthanum chromites, La 1- x Ca x CrO 3-δ, was measured as a function of oxygen partial pressure, composition, and temperature by means of electrical conductivity relaxation. The measured chemical diffusivity increased with increasing oxygen vacancy concentration, the mechanism of which was discussed in light of defect chemical equilibrium. The derived vacancy diffusion coefficient was independent of the vacancy concentration, suggesting that oxygen vacancies were randomly distributed.

The effect of sintering on the mechanical properties of SOFC ceramic interconnect materials

Solid oxide fuel cell (SOFC) interconnect materials, strontium and calcium doped lanthanum chromite, were synthesized to investigate the effect of dopant content and sintering temperature on their sinterability. The results show that approximately 96% of the theoretical density could be achieved when LaCrO3, doped with 30 mol% Ca, was sintered in air at 1400°C. However, to get the same sintered density for the strontia doped material, a 1700°C sintering temperature had to be used. The effect of sintering temperature on the fracture strength was also investigated. A maximum fracture strength of 234 MPa for La0.7Sr0.3CrO3−δ and 256 MPa for La0.7Ca0.3CrO3−δwere obtained for both samples sintered at 1700°C.

Formation of calcium chromate hydroxylapatite on the surface of a calcium-doped lanthanum chromite sintered body

Sintered bodies of highly sinterable calcium-doped lanthanum chromites with chromium deficiency have been found to deteriorate and form calcium chromate hydroxyl apatite on their surfaces when exposed to high-temperature water-containing atmospheres. Microstructure analysis, composition measurement, and phase identification were performed on the surface of the samples after annealing in various conditions. The mechanism of the surface deterioration is thus deduced: traces of liquid phase which are formed in the sintering process migrate to the external surface and then react with water vapour to yield an apatite-type compound. Electrical conductivity has also been measured. The poorly conductive nature of the surface will cause some problems when the subject materials are used as high-temperature electronic conductors.

Liquid phase sintering, electrical conductivity, and chemical stability of lanthanum chromite doped with calcium and nickel

Abstract The substitution of 10 mol% nickel for chromium in calcium doped lanthanum chromite has been shown to promote rapid densification of the compound at low temperatures in air by the formation of a transient liquid phase. Liquids were generated via the decomposition of CaCrO 4 second phase material present in calcined powders. During electrical conductivity measurements at 1000°C, severe microstructural degradation occurred in atmospheres of H 2 and CO 2 . The processes leading to chemical degradation are attributed to the precipitation of Ni from the (La,Ca) (Cr,Ni)O 3 solid solution and to the decomposition of small amounts of residual CaCrO 4 which remains at grain boundaries and triple points after sintering. DTA studies on CaCrO 4 in CO 2 and H 2 atmospheres suggested that the degradation due to CaCrO 4 decomposition was primarily a result of the reaction of CaO with CO 2 to form CaCO 3 . The presence of H 2 gas is thought to catalyse this reaction via the formation of large amount of CaO during the decomposition of CaCrO 4 . Degradation is more severe in atmospheres containing a mixture of CO 2 and H 2 gases than in either gas on its own.

Sinterability and electrical conductivity of calcium-doped lanthanum chromites

Calcium-doped lanthanum chromites, (La1−xCax) (Cr1−y Cay O3, have been synthesized to investigate effects of calcium doping on sinterability and electrical conductivity. X-ray diffractometric results have revealed that in addition to normal perovskites (La1−xCaxCrO3), chromium-deficient perovskites can exist as a single phase in the composition region 0.1 <x < 0.3, although the deficit of chromium is small. These chromium-deficient perovskites show a good sinterability even in air at 1873 K. Electrical conductivity of these perovskites has been measured as functions of temperature and oxygen potential. It has been found that electrical conductivity of the chromium-deficient perovskites increases almost linearly with total calcium content. The magnitudes of electrical conductivity are comparable to those of strontium-doped lanthanum chromites.

Thermal expansion of some chromium deficient lanthanum chromites

The dense calcium-doped lanthanum chromites were prepared by liquid mixing method as a SOFC interconnector. A slight, nominal deficit of chromium drastically enhanced sinterability of lanthanum chromite even in air. These samples consisted of perovskite phase and CaO (or La 2O 3). The thermal expansion coefficients of the chromium deficient samples were found to increase with calcium content, from 8.56×10 −6K −1 to 10.84×10 −6K −1.