Microstructure and Thermal Stability of Fe, Ti, and Ag Implanted Yttria-Stabilized Zirconia

Yttria-stabilized zirconia (YSZ) was implanted with 15 keV Fe or Ti ions up to a dose of 8 x 1016 at cm -2. The resulting "dopant" concentrations exceeded the concentrations corresponding to the equilibrium solid solubility of Fe2Oa or TiO2 in YSZ. During oxidation in air at 400 ° C, the Fe and Ti concentration in the outermost surface layer increased even further until a surface layer was formed of mainly Fe20 3 and TiO2, as shown by XPS and ISS measurements. From the time dependence of the Fe and Ti depth profiles during anneal treatments, diffusion coefficients were calculated. From those values it was estimated that the maximum temperature at which the Feand Ti-implanted layers can be operated without changes in the dopant concentration profiles was 700 and 800 ° C, respectively. The high-dose implanted layer was completely amorphous even after annealing up to 1100 ° C, as shown by scanning transmission electron microscopy. Preliminary measurements on 50 keV Ag implanted YSZ indicate that in this case the amorphous layer recrystallizes into fine grained cubic YSZ at a temperature of about 1000 ° C. The average grain diameter was estimated at 20 nm, whereas the original grain size of YSZ before implantation was 400 nm. This result implies that the grain size in the surface of a ceramic material can be decreased by ion beam amorphisation and subsequent recrystallisation at elevated temperatures. PACS: 61.70.Tm, 66.30.Jt Yttria-stabilized zirconia (YSZ) is an important oxygen ion conducting solid electrolyte. It is used in applications like oxygen sensors, oxygen pumps, and fuel cells [1]. Mixed conducting oxides, which show both ionic and electronic conductivity, are considered to be attractive candidates for use as electrode material on top of YSZ [2-4]. The major objective of the present study is to check the possibility to obtain by ion implantation a mixed conducting surface layer in YSZ [5-9] and in this way to investigate the possibility to increase the rate of electrochemical reactions at the gas-solid interface, Fe and Ti have been chosen as dopants since the corresponding oxides are semiconductors at the operating temperature of YSZ [10,11]. Some studies on the electronic properties of YSZ doped with transition metal oxides have shown that mixed conductivity is obtained. These results, however, depend strongly on temperature, oxygen partial pressure, and dopant concentration [5, 12-15]. Ion implantation has been used as a surface modification technique because the transport of oxygen ions from the ion implanted surface layer into the solid electrolyte can take place very gradually. By, e.g., sputtering of doped YSZ a sharp interface would exist between the sputtered thin film and the solid electrolyte. Such an interface may hinder the transport of oxygen from the mixed conducting surface layer into the solid electrolyte, especially if the mixed conducting layer and the solid electrolyte are not iso-structural. Transition metal oxides have only limited solid solubility in YSZ. The concentration of the implanted ion that can be obtained by ion implantation is independent of its equilibrium solid solubility level. Concentrations of the transition metal oxide far above its solubility level may be necessary in order to obtain the desired effects. In this paper we report on the chemical composition of the surface, the thermal stability of the dopant depth profile and the microstructure of the implanted layer. With respect to the microstructure also preliminars, results on Ag-implanted YSZ, indicating the influence of the type of implanted ion, are reported. In subsequent papers the electrochemical properties of the ionimplanted layer will be discussed [16].