Oxygen Ion Mobility And Ionic Conductivity Prediction In Cubic Yttria-Stabilized Zirconia Single Crystals

Zirconia (Credit: diamondnexus.com)

Cubic yttria-stabilized zirconia (c-YSZ) is a ceramic material which is applied in Solid Oxide Fuel Cells (SOFCs). There are a considerable number of vacancies in the structure of this material which makes it a great choice for the electrolyte in SOFCs. Electrolyte in SOFC should be able to transport ions but not electrons. The oxygen ions can travel through the vacancies of the material. However, the rate of oxygen ions traveling through the material, or in other words the ionic conductivity of c-YSZ, varies by temperature and also the composition of the material.

There are two main parameters affect the ionic conductivity of c-YSZ, including the concentration of vacancies and the mobility of the ions in the material’s structure. We developed two maps to show these two main parameters at various temperatures and different compositions of the c-YSZ single crystals. Based on these two maps, then, we developed a predictive map to show the ionic conductivity of the material in a range of temperature and at various compositions of the material.

Significance Of The Work

Since the oxygen ion mobility in c-YSZ and also the ionic conductivity of c-YSZ can directly affect the efficiency of SOFCs or oxygen sensors, the developed predictive diagrams in this project can provide valuable information to wisely design a SOFC or oxygen sensor. The developed diagrams can show the ionic conductivity and oxygen ion mobility of c-YSZ single crystals with a specific composition in a range of temperature. Therefore, choosing the proper material composition and determining the efficient working temperature can be easily and precisely done by the help of the diagrams we developed in this project.

Finding the conductivity or oxygen ion mobility in the material through doing experiment can be very expensive and time-consuming. It is worth mentioning that the developed diagrams in this project can also considerably help to decrease the cost of production by removing the need for doing experiments.

Technical Breakdown Of Research

In this project, we developed predictive diagrams to show the ionic conductivity and oxygen ion mobility in Cubic yttria-stabilized zirconia (c-YSZ) single crystals. This material is a well-known candidate as solid state electrolyte for solid oxide fuel cells (SOFCs). c-YSZ can be also applied in oxygen sensors and catalytic membrane reactors. It is attractive due to its high ionic conductivity, which is related to its high oxygen vacancy concentration. Doping zirconia with yttria can make c-YSZ stable at low temperatures by substitution of Zr+4 ions with Y+3 ions. This substitution produces oxygen vacancies since three O-2 ions replace four O-2 ions.

Due to the critical role of c-YSZ as an electrolyte in SOFCs, it is crucial to know its ionic conductivity at different conditions. Extensive studies have been carried out to measure the conductivity of c-YSZ with different compositions at various temperatures. In addition, several investigations tried to discover the relationship between ionic conductivity of c-YSZ and the temperature, oxygen vacancy concentration, and concentration of yttria.

However, a quantitative mobility diagram to show the effect of yttria concentration and also temperature on the oxygen ion mobility in c-YSZ was not available. Furthermore, there was no oxygen vacancy concentration quantitative diagram at various temperatures and YSZ compositions. Both of these two parameters greatly affect the ionic conductivity of c-YSZ. Moreover, it is not easily possible to experimentally measure the conductivity of low yttria content c-YSZ, since cubic-tetragonal phase transformation interferes. Therefore, an important and critical portion of information required to effectively design SOFCs and oxygen sensors was missing.

In this study, applying the CALPHAD (calculation of phase diagrams) approach, a very useful predictive conductivity diagram was developed, in which the ionic conductivity of c-YSZ single crystals is predicted versus its composition in a range of temperature. We also provided a predictive mobility diagram, in which the oxygen ion mobility for various compositions of c-YSZ single crystals is shown at different temperatures. A similar diagram was also developed to show the oxygen ion concentration in c-YSZ single crystals at various conditions.

These findings are described in the recently published article entitled Oxygen ion mobility and conductivity prediction in cubic yttria-stabilized zirconia single crystals, in the Journal of Materials Science. This work was led by Mohammad Asadikiya from Florida International University.

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