Phase boundaries, zirconia

In thick-film technology (layer thickness > 1 pm), roll or tampon printing methods well known in the porcelain industry are suitable for applying layers of defined geometry on the green ceramic. The thickness is controlled by precise adjustment of the paste viscosity and solid content. To provide an adequate three-phase boundary, zirconia is added to the platinum to form a cermet (ceramic-metal compound). 

Electrocatalytic reactions, such as the transformation of O2 from the zirconia lattice to oxygen adsorbed on the film at or near the three-phase-boundaries, which we denote by 0(a), have been found to take place primarily at these three phase boundaries.5 8 This electrocatalytic reaction will be denoted by ... 

The hydrogen oxidation within a fuel cell occurs partly at the anode and the cathode. Different models were supposed for the detailed reaction mechanisms of the hydrogen at Ni-YSZ (yttria stabilised zirconia) cermet anodes. The major differences of the models were found with regard to the location where the chemical and electrochemical reactions occur at the TPB (three-phase boundary of the gaseous phase, the electrode and the electrolyte). However, it is assumed that the hydrogen is adsorbed at the anode, ionised and the electrons are used within an external electrical circuit to convert the electrical potential between the anode and the cathode into work. Oxygen is adsorbed at the cathode and ionised by the electrons of the load. The electrolyte leads the oxide ion from the cathode to the anode. The hydrogen ions (protons) and the oxide ion form a molecule of water. The anodic reaction is... 

High-temperature stabilized NO-, zirconia potentiometric sensors are also being utilized [187], The electrochemical reactions on zirconia devices take place at the triple-phase boundary, that is, the junction between the electrode, electrolyte, and gas [186], It has been reported that sensors composed of a W03 electrode, yttria-stabilized zirconia electrolyte, and Pt-loaded zeolite filters demonstrate high sensitivity toward NO,, and are free from interferences from CO, propane, and ammonia, and are subject to minimal interferences from humidity and oxygen, at levels typically present in combustion environments [188], In this sensor, a steady-state potential arises when the oxidation-reduction reaction [186,188]... 

A third path, namely the ionization of the oxygen on the electrolyte surface followed by a direct incorporation into the electrolyte, can also not be excluded. In this case the electronic charge carriers, which are required in the oxygen reduction reaction, have to be supplied from the electrolyte. In solid electrolytes with very low electronic conductivity (e.g. zirconia), it can therefore be expected that the active zone is restricted to a region very close to the three-phase boundary. Hence, this path is, from a geometrical point of view, similar to the surface path discussed above. 

The electrolyte in an SOFC must consist of a good ion conductor, which has essentially no electronic conductivity. Otherwise the cell will be internally short-circuited. An often-used electrolyte material is yttria-stabilised zirconia (YSZ). The electrodes must pos.scss good electron conductivity in order to facilitate the electrochemical reaction and to collect the current from the cell. The fuel electrode usually contains metallic nickel for this purpose. The anodic oxidation of the fuel (H or CO) can only take place in the vicinity of the so-called three-phase boundary (TPB), where all reactants (oxide ions, gas molecules and electrons) are present. Thus, it is advantageous to extend the length and width of the TPB zone as much as possible. One way to do this is by making a composite of Ni and YSZ called a Ni-YSZ-cermet. Another way is to use a mixed ionic and electronic conductor, which in principle can support the electrochemical reaction all over the surface as illustrated in Fig. 15.1. Partially reduced ceria is a mixed ionic and electronic... 

Zirconia also undergoes high pressure phase transitions from the monoclinic to two orthorhombic structures. A thermochemical study of these phases, combined with phase equilibrium observations (Ohtaka et al. 1991) suggests that surface energy changes the apparent position of phase boundaries, as well as enhances the kinetics of transformation. However Ohtaka et al. (1991) were unable to quantify these effects. 

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. 

A very important aspect of gas sensors in automotive exhaust-gas environments is aftertreatment of the electrodes to control a specific sensor behavior. For example, to measure nonequilibrium raw emissions, the sensor needs excellent catalytic ability. Various methods are known to improve electrodes in Zr02-based sensors. One well known method is to increase the active platinum surface area and the three-phase boundary area by partial reduction of zirconia close to the electrode. This occurs when the ceramic is exposed to a reducing atmosphere at high temperatures or when an electrical cathodic current is applied through the electrode and electrolyte. A similar effect can be achieved by chemical etching of the elec-... 

The usual anode is a cermet (composite material made of a ceramic and a metal). Porous Ni-YSZ (yttria-stabilised zirconia) is the state-of-the-art electrode, presenting electronic and ionic conductivities in order to increase the number of reaction sites, called triple phase boundaries. It corresponds to the area where 0 , e and H2 are all present for the time required for the oxidation reaction to occur. No single phase has been found to completely fit all the requirements for an anode thermal and chemical compatibilities with the electrolyte, mixed ionic and electronic conductivity, high electro-catalytic activity and stability in reductive atmosphere. 

Due to this process, a concavo-convex surface is formed on the sensor element and it is possible that robust adhesion and effective surface area of Pt electrode increases. Pt electrode has several small pores because of heat treatment. A large amount of three-phase boundary, which is called triple point of Pt electrode material, zirconia solid electrolyte, and atmosphere gases, is created, which allows easy electrode reaction (see Figure 3.1.6). 

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