E.H.P. Cordfunke

Chemical reactivity and interdiffusion of (La, Sr)MnO3 and (Zr, Y)O2, solid oxide fuel cell cathode and electrolyte materials

The chemical reactivity and interdiffusion of (La, Sr)MnO3 and (Zr, Y)O2 were studied from 1110 to 1755 K. Reaction of LaMnO3 with (Zr, Y)O2 was already observed at 1170 K, whereas reactions between (La, Sr)MnO3 with 30 at.% Sr and (Zr, Y)O2 with 8 at.% Y were not observed at 1365 K. The reaction products observed in the experiments are La2Zr2O7 and/ or SrZrO3. It is proposed that reaction layers are formed by diffusion of La and/or Sr into (Zr, Y)O2 via a vacancy diffusion mechanism. The composition of the layers depends on the La2O3 and SrO activities in (La, Sr)MnO3. The activation energy for the formation of a La2Zr2O7 reaction layer was determined to be 17.5 ± 1.8 kJ · mol−1, the activation energy for the formation of a SrZrO3 reaction layer was determined to be 18.8 ± 1.9 kJ·mol−1. On the basis of the experiments it was calculated that at 1273 K it would take about 29100 h to grow a reaction layer of SrZrO7 from (La0.5Sr0.5)MnO3 and (Zr0.97Y0.03)O1.985, and about 82000 h to grow a reaction layer of La2Zr2O7 from LaMnO3 and (Zr0.92Y0.08)01.96, with a thickness of 1 μm for both layers. It is propose d reaction layers might result in both ohmic and polarization losses of the SOFC.

A new defect model to describe the oxygen deficiency in perovskite-type oxides

Abstract The defect chemistry of LaMO3−δ (M =Mn, Fe, Co) perovskite-type oxides has been studied in order to describe the composition versus oxygen partial pressure phase diagrams in these systems. It is proposed that these systems can be described with extended defects. Literature data on LaMnO3−δ and LaCoO3−δ indicate that the phase diagrams can be described by a simple cluster model. The simple cluster model is suggested to be the building block for a number of highly defective perovskite-type phases that are known from the literature, like La4Mn4O11 and La4Ni4O11, that have different coordination sites for the transition metal ions (tetragonal and square planar, respectively). Other oxygen-deficient perovskite-type oxides probably can be described by similar, but more complicated cluster models, as will be discussed.

Sinter behaviour of (La, Sr)MnO3

Abstract The sinter behaviour of (La, Sr)MnO 3 has been studied on samples with 0, 15, 30, and 50 at.% Sr prepared by coprecipitation. With increasing strontium content the sinter curves are shifted to higher temperatures. This shift can be explained by a combined effect of the difference in ionic radii between La 3+ and Sr 2+ , and by a decrease of the number of defects that are formed in (La, Sr)MnO 3 . The sinter behaviour of coprecipitated samples is compared with samples obtained from citrate pyrolysis.

On the structure of SrZrO3

Abstract The structure of SrZrO 3 was studied from room temperature up to 1373 K. It was found that there are only minor structural changes. At room temperature SrZrO 3 was found to crystallize with either pseudocubic or orthorhombic symmetry, probably due to differences in preparation temperature and impurities. The sample studied could be indexed on the basis of a doubled cubic perovskite cell, with a = 0.8206(1)nm; the space group is P 2 1 3. Around 1023 K there is a transformation to space group P 2/n3, with a = 0.8270(1)nm at 1063 K. Another, very smooth phase transformation, which probably results in space group P m3m, starts around 1273 K, with a = 0.8287(2)nm at 1323 K.

The standard molar enthalpy of formation of Cs2ZrO3

The enthalpy of Cs2ZrO3 has been derived from its enthalpy of solution in HF · 100H2O, as measured calorimetrically, in combination with auxiliary values. For the standard molar enthalpy of formation of Cs2ZrO3 the value ΔfHmo(298.15 K) = −(1584.8 ± 1.9) kJ · mol−1 has been found.

The standard enthalpies of formation of hydroxides IV. La(OH)3 and LaOOH

The standard molar enthalpies of solution of La(OH)3 and LaOOH have been measured. From these values the standard molar enthalpies of formation have been derived: ΔfHmo(La(OH)3, s, 298.15 K) = −(1415.5±1.4) kJ·mol−1 and ΔfHmo(LaOOH, s, 298.15 K) = −(1078.6±1.4) kJ·mol−1.

Sr3U11O36: Crystal structure and thermal stability

Abstract The crystal structure of Sr3U11O36 has been determined by X-ray, electron, and neutron diffraction. The structure is related to α-U3O8, with uranium in both octahedral and pentagonal bipyramidal coordination. The enthalpy of formation has been determined and is discussed in relation to the other strontium uranium oxides.

Preparation and properties of the violet “U3O8 hydrate”

Abstract The preparation of two non-stoichiometric UO3 hydrates UO2·86·1·5H2O and UO2·86·0·5H2O is described. The so-called “U3O8 hydrate”, described in the literature, is identical with the amorphous form of these non-stoichiometric UO3 hydrates. The heats of formation and the water vapour pressures of these hydrates have been measured.

Standard enthalpies of formation of tellurium compounds III. Cs2TeO3, Cs2Te2O5, Cs2Te4O9, and Cs2TeO4

Abstract The enthalpies of solution of Cs2TeO3, Cs2Te2O5, Cs2Te4O9, and Cs2TeO4 in 3.11 mol·dm−3 (3.0H2SO4 + 0.1K2Cr2O7 + 0.01MnSO4)(aq) have been measured calorimetrically. Combination with values from auxiliary reactions gave for the standard enthalpies of formation: ΔfHmo(s, 298.15 K)/(kJ·mol−1): Cs2TeO3, −(993.2 ± 3.4); Cs2Te2O5, −(1352.4 ± 6.4); Cs2Te4O9, −(2033.3 ± 12.8); Cs2TeO4, −(1118.9 ± 4.8).

Standard enthalpies of formation of uranium compounds XIII. Cs2UO4

Abstract The enthalpy of formation of Cs 2 UO 4 (s) has been obtained by solution calorimetry using two independent thermochemical cycles: the first, in 1.505 mol · dm −3 H 2 SO 4 (aq), based on the standard enthalpies of formation of U 3 O 8 and γ-UO 3 , and the second, in 0.5 mol · dm −3 HF(aq), based on the standard enthalpy of formation of UF 6 . The routes give identical results: Δ f H ° m o ( Cs 2 UO 4 , s , 298.15 K )/( kJ · mol −1) = −(1928.15 ± 1.10) and −(1928.89 ± 3.09) , respectively.