Harlan U. Anderson

Structure and electrical properties of La1−xSrxCo1−yFeyO3. Part 1. The system La0.8Sr0.2Co1−yFeyO3

Abstract Crystal structure, thermal expansion, thermogravimetry, thermoelectricity, and electrical conductivity of compositions in the system La0.8Sr0.2Co1−yFeyO3 with 0 ≤ y ≤ 1 were studied as function of Co Fe ratio and temperature, in air. The electrical conduction mechanism is attributed to the adiabatic-hopping of p-type small polarons. At high temperatures, oxygen deficiency causes lattice expansion and a reduction in electrical conductivity. The observed temperature dependence of the Seebeck coefficient is attributed to changes in carrier concentration caused by a thermally excited charge disproportionation of Co3+ ions and by the ionic compensation of induced oxygen vacancies. The measured electrical conductivity and Seebeck coefficient as a function of the Co Fe ratio is interpreted using a two-site hopping and the site-percolation model. It is suggested that a preferential electronic compensation of Fe ions over Co ions may occur in this system.

Structure and electrical properties of La1 − xSrxCo1 − yFeyO3. Part 2. The system La1 − xSrxCo0.2Fe0.8O3

Abstract Crystal structure, thermal expansion, oxygen stoichiometry, thermoelectricity, and electrical conductivity of compositions in the system La 1 − x Sr x Co 0.2 Fe 0.8 O 3 with 0 ≤ x ≤ 0.6 were studied as function of temperature and Sr content, in air. The solubility of Sr in the sintered perovskite-type oxide (ABO 3 ) was limited to x ≤ 0.4. The observed p -type electrical conduction appeared to occur via a small-polaron hopping mechanism. The thermally-induced oxygen loss caused a lattice expansion plus decreases in both the carrier concentration and the carrier mobility. A semi-empirical model was developed which takes into account the thermally activated disproportionation of Co 3+ ions into Co 4+ and Co 2+ pairs, and the ionic compensation of oxygen vacancies formed at high temperatures. The concentrations of B-site ions (Co or Fe) in different valence states were calculated using this model and experimental data.

RAMAN SCATTERING AND LATTICE DEFECTS IN NANOCRYSTALLINE CEO2 THIN FILMS

Abstract The results of Raman scattering studies of nanocrystalline CeO 2 thin films are presented. The spectra have been described using the spatial correlation model from which the correlation length has been determined as a function of grain size. A direct comparison between the concentration of defects related to correlation length and CeO 2 non-stoichiometry has been achieved. The relationship between the lattice disorder and the form of the Raman spectra in nanocrystalline CeO 2 is discussed.

Oxidation-reduction behavior of undoped and Sr-doped LaMnO3 nonstoichiometry and defect structure

Undoped and Sr-doped LaMnO{sub 3} showed reversible oxidation-reduction behavior. These perovskites can be excess, stoichiometric or deficient in oxygen content depending on the specific conditions. Under very reducing conditions decomposition into new phases occurs. Phase stabilities for these oxides were determined. The results showed that Sr doping caused the LaMnO{sub 3} to dissociate at higher oxygen activities than those necessary for undoped LaMnO{sub 3}. Defects models are proposed to interpret the thermogravimetric results in which metal vacancies are assumed for the oxygen excess condition and oxygen vacancies are assumed for the oxygen deficient condition. Thermodynamic properties were calculated which support the model.

Oxidation-reduction behavior of undoped and Sr-doped LaMnO3: Defect structure, electrical conductivity, and thermoelectric power

Abstract Seebeck coefficient and electrical conductivity measurements were performed for undoped and Sr-doped LaMnO 3 as a function of temperature and oxygen partial pressure. The results of electrical conductivity showed typical p -type behavior. As reduction proceeded, the electrical conductivity of these LaMnO 3 -based perovskites decreased with P 1 4 O 2 . The analysis of the electrical conductivity data was performed by extending the defect model from a previous thermogravimetric (TG) study. The measured Seebeck coefficients were found to be positive except for the most reducing conditions when the decomposition into multiple phases occurred. The Heikes formula was adopted to interpret the Seebeck coefficient results and to calculate the mobility. These results indicated that the conduction was due to p -type carriers of a localized nature. It was also suggested that the conductivity for these perovskites was dominated by the mobility rather than by the carrier concentration.

Electrical conductivity of nanocrystalline ceria and zirconia thin films

Abstract The results of studies of the preparation, structure and electrical conductivity of ZrO 2 :16% Y and CeO 2 thin films are presented. Dense films with grain size controlled in the region of 1–400 nm have been obtained on monocrystalline sapphire and polycrystalline Al 2 O 3 substrates using a polymeric precursor spin coating method. The electrical conductivity of nanocrystalline thin films has been studied as a function of oxygen activity and temperature and correlated with the microstructure. Nanocrystalline specimens are characterized by enhanced electrical conductivity and different stoichiometry compared with microcrystalline material.

Microstructure–electrical conductivity relationships in nanocrystalline ceria thin films

A study of nanocrystalline oxide thin film processing and influence of microstructure on the electrical properties of nanocrystalline Gd3+-doped CeO2 thin films was reported. Nanocrystalline films on sapphire substrate were prepared using a polymeric precursor spin coating technique. The grain size of these films depends upon the annealing temperatures and the dopant content, where higher content of dopant realized smaller grain size. The electrical conductivity of nanocrystalline Gd3+-doped CeO2 thin films was studied as a function of temperature and oxygen activity, and correlated with the grain size. The results show that the electronic conductivity of CeO2 increases, whereas the ionic conductivity increases in doped samples as the grain size decreases. From these results, the enthalpy of oxygen vacancy formation was determined as a function of grain size. For CeO2 sample, an enhancement of electronic conductivity was observed with decreasing grain size below 100 nm. In the case of Gd3+-doped CeO2, the electrical conductivity results show that an increase of the ionic conductivity was observed as the grain size decreased, which is related to a decrease in the activation energy for the ion mobility.

Review of p-type doped perovskite materials for SOFC and other applications

Abstract p-type perovskite-type oxides are candidates for use as components of high temperature fuel cells and as oxygen separation membranes. The particular properties that these applications require are reviewed. The characteristics that these oxides have which allow them to satisfy many of these requirements are discussed and a defect model presented. The status of the utilization of these oxides and of the areas which need to be addressed such as thermal expansion and sintering characteristics are reviewed.

Band gap energy in nanocrystalline ZrO2:16%Y thin films

The results of optical absorption measurements on nanocrystalline ZrO2:16%Y thin films are presented. Dense 0.7 μm thick films with 1–300 nm grain size have been obtained on sapphire substrate using a polymeric precursor spin coating technique. The relationship between the energy gap and microstructure of ZrO2:16%Y has been determined and discussed. The quantum confinement effect was observed at the grain size lower than 100 nm with the band gap energy shift of 0.25 eV when the microstructure was changed up to 1 nm. Some limitation of the model has been observed and discussed. The band gap energy of 5.62±0.05 eV has been determined as microstructure independent value.

Room-temperature homogeneous nucleation synthesis and thermal stability of nanometer single crystal CeO2

Nanometer (about 4∼5 nm) CeO2 single crystals were first synthesized by room-temperature homogeneous nucleation; the size was determined by electron microscopy and specific surfaced area of the particles. Modeling revealed that the surface energy of as-synthesized nanometer single crystals was in the range of 2.8–3.7 J/m2. Crystal growth mechanisms change over the temperature regimes, from boundary diffusion over low-temperature regime (Ea=0.16 eV) to bulk diffusion (Ea=0.50 eV) over high-temperature region.