arXiv subjects

Ana Alvarez

Publications and source records attributed to Ana Alvarez.

3 recordsLinked to original sources

arXiv2020

DC Resistance Degradation of SrTiO$_3$: The Role of Virtual-Cathode Needles and Oxygen Bubbles

This study of highly accelerated lifetime tests of SrTiO$_3$, a model semiconducting oxide, is motivated by the interest in reliable multilayer ceramic capacitors and resistance-switching thin-film devices. Our analytical solution to oxygen-vacancy migration under a DC voltage -- the cause of resistance degradation in SrTiO$_3$ -- agrees with previous numerical solutions. However, all solutions fail to explain why degradation kinetics feature a very strong voltage dependence, which we attribute to the nucleation and growth of cathode-initiated fast-conducting needles. While they have no color contrast in SrTiO$_3$ single crystals and are nominally invisible, needles presence in DC-degraded samples -- in silicone oil and in air -- was unambiguously revealed by in-situ hot-stage photography. Observations in silicone oil and thermodynamic considerations of voltage boundary conditions further revealed a cooccurrence of copious oxygen bubbling and the onset of final accelerating degradation, suggesting sudden oxygen loss is a precursor of final failure. Remarkably, both undoped and Fe-doped SrTiO$_3$ can emit electroluminescence at higher current densities, thus providing a vivid indicator of resistance degradation and a metal-to-insulator resistance transition during cooling. The implications of these findings to thin ceramic and thin film SrTiO$_3$ devices are discussed, along with connections to similar findings in likewise degraded fast-ion yttria-stabilized zirconia.

cond-mat.mtrl-sci↗

arXiv2019

DC Electrical Degradation of YSZ: Voltage Controlled Electrical Metallization of A Fast Ion Conducting Insulator

DC electrical degradation as a form of dielectric and resistance breakdown is a common phenomenon in thin-film devices including resistance-switching memory. To obtain design data and to probe the degradation mechanism, highly accelerated lifetime tests (HALT) are often conducted at higher temperatures with thicker samples. While the mechanism is well established in semiconducting oxides such as perovskite titanates, it is not in stabilized zirconia and other fast oxygen-ion conductors that have little electronic conductivity. Here we model the mechanism by an oxygen-driven, transport-limited, metal-insulator transition, which finds support in rich experimental observations - including in situ videos and variable temperature studies - of yttria-stabilized zirconia. They are contrasted with the findings in semiconducting titanates and resistance memory, and provide new insight into ceramic processing with extremely rapid heating and cooling such as flash sintering and melt processing.

cond-mat.mtrl-sci↗

arXiv2018

Potential jumps at transport bottlenecks cause instability of nominally ionic solid electrolytes in electrochemical cells

Normal operations of electrochemical devices such as solid oxide fuel cells (SOFC), solid oxide electrolyzer cells (SOEC) and lithium ion batteries (LIB) sometimes fail because of unexpected formation of internal phases. These phases include oxygen bubbles at grain boundaries inside the zirconia electrolyte of SOEC, isolated Li metal islands inside the (garnet type) Li7La3Zr2O12 electrolyte of all-solid-state LIB, and similar Na metal islands inside the Na-beta-alumina and NASICON electrolytes of Na-S batteries. Remarkably, although the devices can operate in both polarities, the propensity for failure depends on the polarity. Here we explain these and other phenomena in nominally ionic solid electrolytes and mixed-conducting electrodes in simple thermodynamic and kinetic terms: the unexpected internal phases are caused by a large potential jump that is needed to push a constant ion or electron flow through its internal transport bottleneck. Definite rules for internal phase formation including its polarity dependence are formulated to help predict and mitigate it, which leads to microstructural instability, efficiency deterioration and breakdown.

cond-mat.mtrl-sci↗