Vladimir Petrovsky

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.

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.

Performance of a Porous Electrolyte in Single-Chamber SOFCs

A cell which consists of a porous 18 μm thick Y-doped ZrO 2 (YSZ) electrolyte (23 ′ 3 vol % open porosity) on a NiO-YSZ anode substrate and a cathode using (La, Sr)(Co, Fe)O 3 has been investigated in the single-chamber configuration. The cell performance and catalytic activity of the anode was measured in a flowing air-methane gas mixture with various flow rates. The results showed that the open-circuit voltage and the power density increased as the gas flow rate increased. The cell generated an open-circuit voltage of about 0.78 V, which was only about 0.1 V lower than that observed with dense electrolyte specimens. A maximum power density of 660 mW cm - 2 (0.44 V) was obtained at set temperature = 606°C (cell temperature = 744°C) in the flow rate of 900 cm 3 min - 1 , where the current efficiency was about 5% determined from fuel consumption.

Anode Supported Single Chamber Solid Oxide Fuel Cell in CH 4-Air Mixture

In this study, yttrium-stabilized zirconia (YSZ) thin films 1-2 μm thick electrolyte have been prepared using NiO-YSZ anode as substrates. Fuel cell test was conducted with the single chamber configuration in methane-air gas mixture using (La,Sr)(Co,Fe)O 3 (LSCF) as the cathode. Test results showed that the open-circuit voltage to be >0.8 V, with power density as high as 0.12 W cm -2 . It was also shown that gas flow rate has a large influence on the performance of the fuel cell, which indicates the importance of the geometrical design for anode support fuel cell system.

Impedance Studies of Diffusion Phenomena and Ionic and Electronic Conductivity of Cerium Oxide

In this study, the electrical conductivity of undoped cerium oxide is evaluated using impedance spectroscopy. We have observed a new phenomenon, which is not related to electrode response but probably to diffusion processes in the grain boundary. An equivalent circuit has been constructed based on the observation, and the electronic bulk, ionic grain, and ionic grain boundary conductivities are calculated. Evaluation of fitting results to the proposed equivalent circuit suggests that diffusion-related phenomena are caused by blocking effects in the grain boundaries due to impurity segregation. Investigation of the grain boundary composition by energy dispersive X-ray spectroscopy shows that it contains detectable amounts of impurities in comparison to the grain, which supports the suitability of the proposed equivalent circuit.

Optical properties of CeO2 films prepared from colloidal suspension

Optical absorption of nanocrystalline thin films can be influenced by the presence of both porosity and grain size effects. If both are present simultaneously, their effects are difficult to separate. In this study it is shown that the combination of uv-vis transmittance and reflectance measurements on porous CeO2 films provides enough data to make this separation. The CeO2 films were prepared by deposition of nanosized (∼5 nm) particles from a water colloidal suspension onto sapphire and subjecting these films to sintering temperatures sufficiently high to provide a series of films with a typical thickness of 0.6 μm with a wide range of grain sizes and porosity. X-ray diffraction, scanning electron microscopy, ellipsometry, and profilometry were used to characterize the films and to compare the observed grain sizes and porosity with that obtained from optical measurements. All of the techniques used gave results on porosity and grain size which were in good agreement, from 15% to 50% and 5 to 65 nm, resp...

Optical properties of undoped and Gd-doped CeO2 nanocrystalline thin films

The results of studies of the preparation, structure, and optical properties of undoped and Gd-doped CeO2 thin films are presented. Dense films with 4–150 nm grain size have been obtained on monocrystalline sapphire substrates using a polymeric precursor spin coating method. The results of the optical measurements are presented and correlated with the microstructure of the films. The transmission spectra have been used to determine the energy dependence of the refractive index, n and the extinction coefficient, k. Both n and k of the thin films decreased as grain size decreased and these results showed that this change could be related to the transition from crystalline to amorphous CeO2. The effect of dopant has little influence on n, <5%, but doping resulted in about a 30%–40% reduction of k compared to undoped specimens, which could be related to the decrease of absorption centers due to the replacement of Ce by Gd.

Electrical Properties of YSZ Films Prepared by Net Shape Technology

The preparation of dense electrolyte films for most electrochemical devices is a crucial technological process. Net shape technology is a new approach, which uses a combination of colloidal suspensions and polymer precursor techniques, to obtain the dense electrolyte layers. It allows the overlapping of the thickness range from 1 to 10 mm in which other preparation techniques experience difficulties. Net shape processing is a low-temperature technology ~preparation temperature can be as low as 400°C! and it eliminates shrinkage of the film during the densification stage, so chemical reactions between the substrate and the film can be minimized. In this study two types of dense substrates were used to confirm these features of the net shape technology: single-crystal sapphire and platinum foil. It was shown that dense yttria-stabilized zirconia ~YSZ! layers can be obtained on both types of substrates at temperatures as low as 400°C. Moreover, further higher annealing temperature does not produce either shrinkage or cracking of the film. Electrical properties of YSZ films were measured in plane~on sapphire! and through the film~on platinum! using impedance spectroscopy and two-probe dc methods.

Nanocomposite Nickel Ceria Cermet with Low Nickel Content for Anode-Supported SOFCs

Ni-ceria cermets with 7.5, 11, and 14 vol % of Ni manufactured by a novel method, called net-shape technology, are presented and evaluated. The method is a combination of conventional ceramic sintering and a polymer precursor impregnation technique. Ceria powder with a carbon pore former is sintered, constructing a rigid and stable oxide skeleton, which is next backfilled with Ni polymer precursor. The electrical conductivity of the cermets as a function of Ni content has been investigated.

Modeling and Characterization of Electrical Transport in Oxygen Conducting Solid Electrolytes

A model for the electrical conductivity in acceptor-doped oxides which involves an association between the acceptor-dopants and oxygen vacancies resulting in donor centers is considered. The model relates the behavior of the electrical conductivity with the temperature, ambient atmosphere and band structure. The predictions of the model are compared to experimental data for ZrO2:16% Y and SrCeO3:5% Yb oxygen conductors and some band structure parameters have been determined.