Electrical conductivity stabilized zirconia

Four solid oxide electrolyte systems have been studied in detail and used as oxygen sensors. These are based on the oxides zirconia, thoria, ceria and bismuth oxide. In all of these oxides a high oxide ion conductivity could be obtained by the dissolution of aliovalent cations, accompanied by the introduction of oxide ion vacancies. The addition of CaO or Y2O3 to zirconia not only increases the electrical conductivity, but also stabilizes the fluorite structure, which is unstable with respect to the tetragonal structure at temperatures below 1660 K. The tetragonal structure transforms to the low temperature monoclinic structure below about 1400 K and it is because of this transformation that the pure oxide is mechanically unstable, and usually shatters on cooling. The addition of CaO stabilizes the fluorite structure at all temperatures, and because this removes the mechanical instability the material is described as stabilized zirconia (Figure 7.2). 

Many oxygen ion conducting electrolytes are available for sensor applications. These include mainly solid solutions of Zr02, HFO, Th02, or CeO. Of these, stabilized zirconia has been found to have the best combination of cost, mechanical, chemical, and electrical properties for this type of application and has been the most widely used. Various stabilizers are available and have a strong effect on the properties obtained, particularly the electrical conductivity. 

Figure 1.43 shows the electrical conductivity o versus oxygen pressure Po. curves at fixed temperatures for Zro gjCao ijOj 5 or (Zr02)o.85(CaO)o,i5 (called stabilized zirconia), as an example. In the oxygen pressure range... 

Fig. 1.43 Electrical conductivity of the stabilized zirconia (Zr02)o.85(CaO)o.i5 4s a function of Po, at different temperatures. Pq was controlled by the use of a two phase mixture such as CU-CU2O, Ni NiO etc. Open and closed marks are data obtained from different samples.

The zirconia sensor operates primarily on the principle of a concentration cell. It consists of a non-porous solid electrolyte layer fabricated from zirconia stabilized with yttria or calcia and exhibits high oxygen ion mobility. This layer is sandwiched between two porous and electrically conductive electrodes. 

A controlled modification of the rate and selectivity of surface reactions on heterogeneous metal or metal oxide catalysts is a well-studied topic. Dopants and metal-support interactions have frequently been applied to improve catalytic performance. Studies on the electric control of catalytic activity, in which reactants were fed over a catalyst interfaced with O2--, Na+-, or H+-conducting solid electrolytes like yttrium-stabilized zirconia (or electronic-ionic conducting supports like Ti02 and Ce02), have led to the discovery of non-Faradaic electrochemical modification of catalytic activity (NEMCA, Stoukides and Vayenas, 1981), in which catalytic activity and selectivity were both found to depend strongly on the electric potential of the catalyst potential, with an increase in catalytic rate exceeding the rate expected on the basis of Faradaic ion flux by up to five orders of... 

Haering, C., A. Roosen, H. Schichl, M. Schnoller (2005), Degradation of the Electrical Conductivity in Stabilized Zirconia System Part II Scandia-stabilised Zirconia , Solid Sate Ionics, Vol. 176, No. 3-4,... 

The electrical conductivity of rare-earth oxide fluorides was first investigated for the development of a binary anion conductive solid electrolyte. As a result, it was found that the binary rare-earth oxide fluorides exhibited oxide ion conductivity. Among them, the conductivity of neodymium europium oxide fluoride, Nd2Eu203F6, was reported to be much higher than that of yttria-stabilized zirconia, YSZ, practically used as an oxygen sensor [34]. The electrical conductivities of the binary rare-earth oxide fluorides vary not only with the combination of Ln203 with Ln F3... 

Fig. 22. Arrhenius plots of the electrical conductivities of several rare-earth fluoride stabilized zirconias measured under an oxygen partial pressure of 1.33 x 10 1 Pa.

Scandia-stabilized zirconia electrolyte with standard SOFC electrodes is used to construct the cells. Stacks are constructed using stainless steel interconnects. The surfaces of the stainless steel are treated to provide an electrically conductive scale with low-scale growth rate. The performance characteristics of a single cell (2.5 cm active area) and a 25-cell stack (active area of 64 cm per cell) are shown in hgure 3.5 and hgure 3.6. The performance stability of the stack is shown in hgure 3.7. 

Zhou, X.D., Scarfino, B., and Anderson, H.U., Electrical conductivity and stability of Gd-doped ceria/Y-doped zirconia ceramics and thin films. Solid State Ionics, 2004, 175 19-22. 

The peculiarity of stabilized zirconia is its relatively high electrical conductivity, especially in combination with Y2O3, so that the material is suitable for the manufacture of ceramic heating elements for high temperatures (the so-called Nernst mass). Above 1000 °C, the mains voltage is sufficient to ensure suitable passage of current. The elements have to be heated to this temperature in another way. 

Historically, stabilized (and partially stabilized) zirconia ceramics were prepared from powders in which the component oxides are mechanically blended prior to forming and sintering. Because solid state diffusion is sluggish, firing temperatures in excess of 1800°C are normally required. Furthermore, the dopant was nonuniformly distributed, leading to inferior electrical properties. Trace impurities in the raw materials can also lead to enhancement of electronic conductivity in certain temperature ranges, which is also undesirable. To overcome these problems, several procedures have been developed to prepare reactive (small particle size) and chemically pure and homogeneous precursor powders for both fully stabilized and partially stabilized material. Two of these are alkoxide synthesis and hydroxide coprecipitation. 

Electrodes The anodes of SOFC consist of Ni cermet, a composite of metallic Ni and YSZ, Ni provides the high electrical conductivity and catalytic activity, zirconia provides the mechanical, thermal, and chemical stability. In addition, it confers to the anode the same expansion coefficient of the electrolyte and renders compatible anode and electrolyte. The electrical conductivity of such anodes is predominantly electronic. Figure 14 shows the three-phase boundary at the interface porous anode YSZ and the reactions which take place. The cathode of the SOFC consists of mixed conductive oxides with perovskite crystalline structure. Sr doped lanthanum manganite is mostly used, it is a good /7-type conductor and can contain noble metals. 

G.Z. Cao, Electrical conductivity and oxygen semipermeability of terbia and yttria stabilized zirconia. /. Appl. Electrochem., 24 (1994) 1222-1224. 

Badwal, S.P.S., Electrical conductivity of single crystal and polycrystalline yttria-stabilized zirconia, J. Mater. Sci. 19 (1984) 1767-1776. 

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