Progress in material selection for solid oxide fuel cell technology: A review

This review article summarized the recent understanding of resistance switching (RS) behavior in several binary oxide thin film systems. Among the various RS materials and mechanisms, TiO2 and NiO thin films in unipolar thermo-chemical switching mode are primarily dealt with. To facilitate the discussions, the RS was divided into three parts; electroforming, set and reset steps. After short discussions on the electrochemistry of 'electrolytic' oxide materials, the general and peculiar aspects of these RS systems and mechanism are elaborated. Although the RS behaviors and characteristics of these materials are primarily dependent on the repeated formation and rupture of the conducting filaments (CFs) at the nanoscale at a localized position, this mechanism appears to offer a basis for the understanding of other RS mechanisms which were originally considered to be irrelevant to the localized events. The electroforming and set switching phenomena were understood as the process of CF formation and rejuvenation, respectively, which are mainly driven by the thermally assisted electromigration and percolation (or even local phase transition) of defects, while the reset process was understood as the process of CF rupture where the thermal energy plays a more crucial role. This review also contains several remarks on the outlook of these resistance change devices as a semiconductor memory.

https://iopscience.iop.org/article/10.1088/0957-4484/22/25/254002/pdf

Nanofilamentary resistive switching in binary oxide system; a review on the present status and outlook

The stabilities of Pt/Ti bilayer metallizations in an oxidizing atmosphere have been investigated with several thicknesses of interfacial Ti‐bonding layers. Reactions in the Pt/Ti/SiO2/Si interface were examined as a function of various annealing conditions in the temperature range 200–800 °C by using Rutherford backscattering spectrometry, Auger electron spectroscopy, x‐ray diffraction, and transmission electron microscopy. Thermal treatment in oxygen was found to cause rapid oxidation of the Ti layer, accompanied by the migration of Ti into the Pt film. Diffusion of oxygen through the Pt grain boundaries was mainly responsible for the adverse reactions at the interface and loss of mechanical integrity. Thin Ti (10 nm) layers resulted in the depletion of the interfacial bonding layer causing serious adhesion problems, whereas thicker Ti films (100 nm) caused the formation of TiO2−x in the Pt‐grain boundaries, ultimately encapsulating the Pt surface with an insulating TiO2 layer. Improved stability and adhesion in the Pt/Ti bilayer metallization compatible with ferroelectric thin film processing, were achieved by incorporating well reacted thin TiO2 layers in situ, and depositing Pt films at a high temperature.

Investigation of Pt/Ti bilayer metallization on silicon for ferroelectric thin film integration

We suggest a possible mechanism for bipolar switching in a $\text{Pt}/{\text{TiO}}_{2}/\text{Pt}$ resistive switching cell in terms of electrochemical reactions involving oxygen ions/vacancies. The electrochemical reactions are considered to take place at an interface between Pt and ${\text{TiO}}_{2}$ solid electrolyte, and they modulate the Schottky barrier height at the interface. Calculation results using this proposed mechanism can explain a bipolar switching behavior and semiquantitatively describe experimental data.

Mechanism for bipolar switching in a Pt / TiO 2 / Pt resistive switching cell

Deposition processes of chromium (Cr) species were investigated for the O2 reduction on (La,Sr)MnO3 (LSM), Pt and (La,Sr)(Co,Fe)O3 (LSCF) electrodes in the presence of chromia-forming alloy metallic interconnect at 900oC under air flow. For the reaction on LSM electrodes, deposition of Cr species preferentially occurred on the zirconia electrolyte surface, forming a distinct deposit ring at the edge of the LSM electrode while at LSCF electrodes, Cr species deposited on the electrode and electrolyte surface, forming isolated Cr particles. In contrast, there was no detectable deposition of Cr species either on the electrode or electrolyte surface for the O2 reduction reaction on Pt electrodes. The results clearly demonstrated that deposition of Cr species in solid oxide fuel cells is not an electrochemical reduction of high valent Cr vapor species to Cr2O3 in competition with O2 reduction. Cr deposition at SOFC cathodes is basically a chemical dissociation reaction and is controlled by the nucleation reaction between the nucleation agent and the gaseous Cr species. The nature of the nucleation agent strongly depends on the electrode material and impurities which may be introduced during electrode and electrolyte fabrication processes.

A comparative investigation of chromium deposition at air electrodes of solid oxide fuel cells

The diffusivity and solubility of oxygen in metal specimens were determined from measurements of permeation through thin wall tubes containing oxygen and heated by electrical resistance. The permeating oxygen desorbed in vacuum as monoatomic oxygen and the flux was monitored mass-spectrometrically. A known helium leak rate and experimentally-determined sensitivities of the two gases were used for flow calibration. The diffusion coefficient of oxygen in pure platinum, calculated from the time lag to reach a steady state flux, is: {fx065-01} in the temperature range of 1435∮ to 1504°. The solubility of oxygen in pure platinum was obtained from the steady-state flux using the previously determined diffusivity. The solubility is proportional to p1/2O2 and at 1 atm of oxygen the solubility is: CsO = (0.2 ± 0.1) × 1012 exp {fx065-02}, wt pct. Small amounts of nickel, less than required for internal oxidation, had a negligible effect on the oxygen solubility and diffusivity in platinum alloys.

L. R. Velho

Papers

Diffusivity and solubility of oxygen in platinum and Pt-Ni alloys