Preparation of cathodes for thin film SOFCs

SOFCs are expected to become competitive devices for electrical power generation, but successful application is dependent on decreasing working temperature from 1000 to 800 °C, without detrimental effects on resistance and on electrode processes. This requires a reduction of the stabilized zirconia electrolyte thickness and an optimization of the electrodes, especially the cathode, where losses are higher. Strontium doped lanthanum manganites are the most common materials tested as cathodes for SOFCs working at high temperature (1000 °C). This cathode material presents high electronic and oxygen-ion conductivities, a thermal expansion coefficient compatible with stabilized zirconia and good catalytic activity. For thin film SOFC devices working at intermediate temperatures (less than 800°C), we have studied the optimization of this type of cathode. Strontium doped lanthanum manganite has been deposited on yttria stabilized zirconia electrolyte substrates by spray-pyrolysis and by RF sputtering. The electrode performances depend strongly on cathode microstructure, influenced by processing conditions. With spray-pyrolysis processes, large porosity is expected. This is important for the supply of oxygen, via O₂ molecules through the pores to the triple phase boundaries, where the gas, the cathode and the electrolyte are in contact and where oxygen reduction may occur. However, large porosity can have a nefaste effect on electronic conductivity. With RF sputtering, denser films with higher electronic conductivity are obtained. But, in that case, the supply of oxygen occurs via adsorbed O-atoms in a diffusion process through the cathode to the electrolyte. Spraypyrolysis and RF sputtering have been compared relative to electrode properties. Paper presented at the 4th Euroconference on Solid State Ionics, Renvyle, Galway, Ireland, Sept. 13–19, 1997     

Influence of voltage on the degradation of channels formed in thin film MDM cathodes

The influence of voltage on the rate of degradation of formed MDM systems with various upper electrode materials and various dielectric film thicknesses is examined. It is shown that the degradation rate of formed chanels is determined by complex influence of MDM system parameters and deposition conditions. An increase in dielectric thickness permits a significant increase in operating voltage, positioning the maximum of the N-shaped volt-ampere characteristic in a region of higher voltare.Tomsk Institute of Automated Systems, Control and Radioelectronics. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 10, pp. 44–47, October, 1992.     

Theory of resonant tunneling through the insulator conduction band of a thin film metal-insulator-metal (MIM) heterojunction

An “atomic” model of an insulating barrier between two free-electron model metals is used to investigate resonant tunneling across the insulator in the presence of a medium to large, externally applied electric field (bias). The exact numerically calculated tunneling current exhibits a pronounced oscillatory bias dependence superposed on the dominant roughly exponential tunneling characteristic. The interpretation of these results in terms of an internal field emission or Fowler-Nordheim type tunneling subject to “periodic deviations” (or interferences) seems plausible and was suggested by Maserjian. To test this conjecture, a trapezoidal barrier model of our “atomic” model analyzed numerically. As expected, the trapezoidal barrier model could only qualitatively reproduce the oscillatory bias dependence of the barrier transmissivity and of the current. Furthermore this limited agreement depends on allowing the effective mass in the barrier to become a strictly adjustable parameter. This failure of the conventional model of the junction can be interpreted as follows: (i) For moderate external (bias) fields the trapezoidal barrier fails to account for the correct position dependence of the Blochwave vector in the insulator's conduction band, hence the correct interference conditions cannot be reproduced. (ii) For large external fields the band model itself begins to fail. An explanation of oscillatory bias dependence at the tunneling current in terms of splitting of the insulator's conduction band into a set of discrete Stark levels is suggested. It is demonstrated that a fit of the oscillatory tunneling characteristics in the “Fowler-Nordheim regime” is not a reliable technique to determine the effective mass in the thin insulating film of tunneling junctions over the energy interval containing the forbidden gap and the adjoining conduction-band.     

Conductivity of a steadily glowing shock-compressed channel in a film metal-insulator-metal structure

The action of particles accelerated in an electrostatic accelerator on film metal-insulator-metal structures is described. The effect of steady glow of a shock-compressed conducting channel is studied. The temperature of the shock-compressed channel, the through conductivity of the metal-insulator-metal structure, and the related ion mass spectra are found. The applications of the results in various fields of technology are shown.     

The impedance of thin dense oxide cathodes

The impedance is derived for a dense layer electrode of a mixed conducting oxide, assuming that the electronic resistance may be ignored. The influence of layer thickness, oxygen diffusion and surface exchange rate on the ‘General Finite Length Diffusion’ expression is evaluated. The thickness dependence is tested for a series of thin, dense layer electrodes of La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ (LSCF) deposited on a Ce_0.9Gd_0.1O_1.95 electrolyte by pulsed laser deposition (PLD). A minimum thickness is required to avoid the influence of contact points of the contacting Pt-gauze and sheet resistance, which is about 1 μm for the studied LSCF electrodes. LEISS surface analysis indicates that PLD deposition process easily leads to a significant Cr contamination of the LSCF surface. Electrochemical impedance spectroscopy analysis indicates that the influence on the exchange rate of this Cr-contamination is still negligible.     

Preparation and characterization of Li2CoMn3O8 thin film cathodes for high energy lithium batteries

An ion layer gas reaction (ILGAR) dip-coating process for the deposition of homogeneous spinel structured Li₂CoMn₃O₈ thin layers has been developed. Thin film cathodes for use in high-energy density lithium batteries with thicknesses of about 200 nm have been prepared. The films were found to be X-ray amorphous after preparation. After annealing at 700°C in air for 2 h, the spinel structure of Li₂CoMn₃O₈ was observed by X-ray diffraction analysis. The composition of the surface was studied by XPS, which indicated enhanced Li and Mn concentrations as a result of the rinsing process and different solubilities of the precursor salts. The electrochemical behavior was investigated by separating the annealed electrode sample from a conventional organic lithium ion-conducting electrolyte by a layer of LiPON solid electrolyte and using elemental lithium as counter electrode. A capacity of 110.8 mAh/g was observed which is related to the valence changes of Mn and Co in the spinel structure.     

ZnS thin film prepared through a self-assembled thin film precursor route

Poly(zinc 1,6-hexanedithiolate) thin film, a precursor to prepare ZnS thin film, was self-assembled on a quartz substrate. The UV-vis spectra monitored the annealing process of the poly(zinc 1,6-hexanedithiolate) film, which revealed that the ZnS thin film began to form at approximately 515 K. The result of XRD confirmed the crystallinity of ZnS. With increase of annealing temperature, a red shift of the emission spectra was observed.     

Superconductivity in Pb-PbTe thin film structures

The critical temperatures of thin superconducting films of Pb in intimate contact with PbTe have been measured resistively. The films were formed by sequential deposition and by co-evaporation of Pb and PbTe. The results show a depression of the critical temperature.     

FMR in thin ferromagnetic granular film

The ferromagnetic resonance is considered for a thin granular film consisting of spherical metal particles. It is shown that the interparticle dipole interactions lead to a noticeable shift of the resonance field and a strong asymmetry of absorption line. The free induction signal is time modulated and its envelopem _ind(t) obeys the law ln (m _ind) ≈t ^2/3. The role of the particle shape in considered system is also discussed.     

Tunable laser thin film interrogation

Film thickness is not only a crucial parameter in producing processes, such as semiconductor and optics production, but also a monitored variable in chemistry and biology, for example for tissue microscopy. Many working principles have been demonstrated and are in use in different fields due to their different limitations (observation film thickness, accuracy, measurement speed, etc.). One of these working principles is thin film reflectometry (TFR). One method is based on a laser source and monitors the reflected intensity over growing film time. Another one employs a spectrally broad light source and measures the reflected intensity using a spectrometer. We introduce and demonstrate a measurement system based on a tunable laser stage. There are several different setups for laser wavelength tuning. One of the most promising solutions is based on monolithic laser diodes. Rapid tuning of the lasers wavelength is crucial for achieving high measurement rates. Monolithic laser diodes offer highest tuning rates and hence high performance. On the other hand, mechanically tunable lasers show broadband spectra that result in higher thickness accuracy in this particular application. Hence, we show a comparison of thin film measurements with a monolithic and a mechanically tunable laser source. This comparison shows that the measurement accuracy of the monolithic laser diode can compete with mechanical tuning. Furthermore, it is a promising approach when measurement tuning speed is an issue.     

Efficiency of thin film photocells

We propose a new concept for the design of high-efficiency photocells based on ultra-thin (submicron) semiconductor films of controlled thickness. Using a microscopic model of a thin dielectric layer interacting with incident electromagnetic radiation we evaluate the efficiency of conversion of solar radiation into the electric power. We determine the optimal range of parameters which maximize the efficiency of such photovoltaic element.     

Meniscus instability in a thin elastic film

A new kind of meniscus instability leading to the formation of stationary fingers with a well-defined spacing has been observed in experiments with elastomeric films confined between a plane rigid glass and a thin curved glass plate. The wavelength of the instability increases linearly with the thickness of the confined film, but it is remarkably insensitive to the compliance and the energetics of the system. However, lateral amplitude (length) of the fingers depends on the compliance of the system and on the radius of curvature of the glass plate. A simple linear stability analysis is used to explain the underlying physics and the key observed features of the instability.     

Hydrogen-induced blistering mechanisms in thin film coatings

We report on the mechanisms of hydrogen-induced blistering of multilayer coatings. Blister formation is a result of highly localized delamination occurring at the two outermost metal-on-silicon interfaces. The number, size, and type of blisters formed varied depending on the composition and ion energy of the incident flux. The results are explained in terms of the multilayer structure being simultaneously susceptible to blistering via two independent mechanisms. A high density of small blisters developed when relatively energetic (several 100 eV) ions were present. Independently, a hydrogenation process that was facilitated by the presence of a small flux of low energy ions (≤ 50 eV) induced a low density of large blisters.     

Imaging excitations in magnetic thin film microstructures

We show how a photoemission electron microscope (PEEM) installed at a synchrotron can be used to image magnetic objects with very high spatial and temporal resolution. Sub-nanosecond magnetic field pulses are used to excite the fundamental magnetic modes of micron sized permalloy particles. The time evolution of the magnetization is imaged using a pump-probe technique where the magnetic contrast is given by X-ray magnetic circular dichroism (XMCD). Depending on the shape and size of the magnetic object we can observe modes related to either the homogenously magnetized domains, to the domain walls or to the vortex. All of these can be analyzed quantitatively yielding their frequencies, amplitudes and damping time constants. For objects with controlled defects we show that the magnetic vortex can be switched between defects using magnetic field pulses.