Olivia A. Graeve
Abstract Zirconia is a very important industrial ceramic for structural applications because of its high toughness, which has proven to be superior to other ceramics. In addition, it has applications making use of its high ionic conductivity. The thermodynamically stable, room temperature form of zirconia is baddeleyite. However, this mineral is not used for the great majority of industrial applications of zirconia. The intermediate-temperature phase of zirconia, which has a tetragonal structure, can be stabilized at room temperature by the addition of modest amounts (below ~8 mol%) of dopants such as Y3+ and Ca2+. This doped zirconia has mechanical toughness values as high as 17 MPa • m1/2. On the other hand, the high-temperature phase of zirconia, which has a cubic structure, can be stabilized at room temperature by the addition of significant amounts (above ~8 mol%) of dopants. This form of zirconia has one of the highest ionic conductivity values associated with ceramics, allowing the use of the material in oxygen sensors and solid-oxide fuel cells. Research on this material actively continues and many improvements can be expected in the years to come.
Zirconia (ZrO2) is an extremely versatile ceramic that has found use in oxygen pumps and sensors, fuel cells, thermal barrier coatings, and other high-temperature applications, all of which make use of the electrical, thermal, and mechanical properties of this material. Proof of the interest and usefulness of zirconia can be seen from the voluminous literature found on this material. This chapter is intended to provide a concise summary of the physical and chemical properties of all phases of zirconia that underlie the appropriate engineering applications.
The three low-pressure phases of zirconia are the monoclinic, tetragonal, and cubic, which are stable at increasingly higher temperatures. Calculated energy vs. volume data at zero absolute temperature confirms the higher stability of the monoclinic phase (Fig. 1). However, most engineering applications make use of the tetragonal and cubic phases, even though their stability at low temperatures is quite low. In fact, the engineering use of all three phases of zirconia in pure form is rare. Generally,
J. F. Shackelford and R. H. Doremus (eds.), Ceramic and Glass Materials: 169
Structure, Properties and Processing.
© Springer 2008
Fig. 1 Computed energy vs. volume data for cubic, tetragonal, and monoclinic phases from (a) Stapper et al.  and (b) Dewhurst and Lowther  (reprinted with permission)
zirconia is doped with oxides such as Y2O3 that stabilize the high-temperature phases at room temperature. This has enormous consequences for both the mechanical and electrical properties of zirconia, even though the local atomic and electronic structure of Zr 4+ in all three polymorphs is for the most part dopant independent .
Doping of zirconia results in stabilization of the tetragonal phase at lower dopant concentrations (for mechanical toughness) or the cubic phase at higher dopant concentrations (for high ionic conductivity) at room temperature. The stabilization of the tetragonal phase at room temperature can result in the following common forms of zirconia: (1) partially stabilized zirconia (PSZ) – zirconia consisting of a matrix of a brittle ceramic and a dispersion of tetragonal precipitates, where the tetragonal precipitates can either be in pure form or doped with Ca2+ (Ca-PSZ) or Mg2+ (Mg-PSZ); and (2) tetragonal zirconia polycrystals (TZP) – zirconia consisting of a matrix of stabilized ZrO2 that has been stabilized in the tetragonal form by the addition of dopants such as Ce4+ (Ce-TZP) and Y3+ (Y-TZP). Fully-stabilized zirconia (FSZ) refers to a material that has been completely stabilized in the cubic form.
Stabilized zirconia in thermal barrier coatings (TCB) is ubiquitous, finding itself in combustor liners, transition sections, nozzle guide vanes, and rotor blades. It is one of the most used ceramics for TCB applications because of its low thermal conductivity, high-temperature stability in oxidizing and reducing environments, coefficient of thermal expansion similar to iron alloys, high toughness, and cost-effectiveness by which it can be applied onto metal surfaces. Its use allows a 200°C increase in the operational temperature of the engine, resulting in a much higher efficiency .
The second, well-known use of stabilized zirconia is in oxygen sensors. These types of devices make use of the very high ionic conductivity of Y2O3- or CaO-doped cubic zirconia. The sensor assembly consists of a zirconia tube with one end closed. The inside of this tube is exposed to air and the outside is exposed to the gas that requires measurement of oxygen levels. When there is a difference in oxygen partial pressure between the inside and outside, oxygen is transported across the ceramic tube. This transport results in a measurable voltage.
A solid-oxide fuel cell (SOFC) functions similar to an oxygen sensor. An SOFC converts the chemical energy of a fuel directly to electrical energy and heat and consists of two electrodes that sandwich an electrolyte, allowing ions to pass while blocking electrons. The air electrode allows oxygen to pass through to the electrolyte. At the electrolyte interface, the oxygen dissociates into ions that travel across the electrolyte via ionic conduction. Typical SOFC’s consist of an Y2O3-doped ZrO2, with about 8 mol% yttrium, as the electrolyte. At the fuel electrode, the oxygen ions that have traveled across the electrolyte react with the fuel forming H2O and possibly other gases, depending on the type of fuel used. During the reaction, at the fuel electrode/ electrolyte interface, electrons are generated that travel through an external circuit, thus generating electrical current that can be used for doing external work. This technology will become increasingly important as a “clean” source of electricity as pressures on the environment from the use of coal and petroleum continue to increase.