Zirconia stabilised

Despite earlier doubts, (Willmann, 1993) zirconia materials, in particular tetragonal zirconia partially stabilised with yttria (Y-PSZ), magnesia (Mg-PSZ) and calcia (Ca-PSZ), have found various applications in biomedical devices, most importantly as hard and tough structural ceramic material for femoral balls in hip endoprostheses (Cales and Stefani, 1995) and as material for restorative 

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A more complicated, and more effective, mechanism operates in partially stabilised zirconia (PSZ), which has general application to other ceramics. Consider the analogy of a chocolate bar. Chocolate is a brittle solid and because of this it is notch-sensitive notches are moulded into chocolate to help you break it in a fair, controlled way. Some chocolate bars have raisins and nuts in them, and they are less brittle a crack, when it... 

Electrochemistry plays an important role in the large domain of. sensors, especially for gas analysis, that turn the chemical concentration of a gas component into an electrical signal. The longest-established sensors of this kind depend on superionic conductors, notably stabilised zirconia. The most important is probably the oxygen sensor used for analysing automobile exhaust gases (Figure 11.10). The space on one side of a solid-oxide electrolyte is filled with the gas to be analysed, the other side... 

The activation energy represents the ease of ion hopping, as already indicated above and shown in Fig. 2.5. It is related directly to the crystal structure and in particular, to the openness of the conduction pathways. Most ionic solids have densely packed crystal structures with narrow bottlenecks and without obvious well-defined conduction pathways. Consequently, the activation energies for ion hopping are large, usually 1 eV ( 96 kJ mole ) or greater and conductivity values are low. In solid electrolytes, by contrast, open conduction pathways exist and activation energies may be much lower, as low as 0.03 eV in Agl, 0.15 eV in /S-alumina and 0.90 eV in yttria-stabilised zirconia. 

CeOj and ThOj have the cubic fluorite structure. Fig. 2.15, and can be doped with large amounts of, for example, Ca, La or Gd to give extensive ranges of cubic solid solutions. ZrOj is cubic only above 2400°C, however, and requires 8% of dopant to stabilise the cubic form to room temperature (as in YSZ, yttria-stabilised zirconia). 

A key factor in the possible applications of oxide ion conductors is that, for use as an electrolyte, their electronic transport number should be as low as possible. While the stabilised zirconias have an oxide ion transport number of unity in a wide range of atmospheres and oxygen partial pressures, the BijOj-based materials are easily reduced at low oxygen partial pressures. This leads to the generation of electrons, from the reaction 20 Oj + 4e, and hence to a significant electronic transport number. Thus, although BijOj-based materials are the best oxide ion conductors, they cannot be used as the solid electrolyte in, for example, fuel cell or sensor applications. Similar, but less marked, effects occur with ceria-based materials, due to the tendency of Ce ions to become reduced to Ce +. 

The most thoroughly developed sensor based on a solid electrolyte is the oxygen sensor using a stabilised zirconia electrolyte. This type of sensor is one of the most successful commercial sensors to date. They are widely used in industry, especially in the analysis of exhaust gases from combustion engines. The following configuration is used in the Oj sensors ... 

Several oxygen sensors based on oxygen pumping with stabilised zirconia have been reported (Hetrick, Fate and Vassell, 1981). This type of oxygen sensor is able to measure the oxygen partial pressure in the exhaust gas from the engine in lean burn. The operating principle of the... 

Both liquid and solid electrolytes can be used, ranging from molten halides, such as a eutectic mixture of LiCl and KCl, to very sophisticated solid-state electrolytes such as calcia or yttria stabilised zirconia, CSZ, YSZ, which are conductors of oxygen ions. 

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... 

The stack electrolytes are scandia-stabilised zirconia, about 140 (im thick. The air-side electrodes (anode in the electrolysis mode) are a strontium-doped manganite. The electrodes are graded, with an inner layer of manganite/zirconia ( 13 pm) immediately adjacent to the electrolyte, a middle layer of pure manganite ( 18 pm), and an outer bond layer of cobaltite. The steam/hydrogen electrodes (cathode in the electrolysis mode) are also graded, with a nickel-zirconia cermet layer ( 13 pm) immediately adjacent to the electrolyte and a pure nickel outer layer ( 10 pm). 

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,... 

For the HTE process, the electrochemical cell consists of a tri-layer ceramic, well known for its brittleness, which limits applied loads. In addition, the relatively low ionic conduction properties of the electrolyte materials (3% yttrium-stabilised zirconia) requires an operating temperature above 700°C to reduce ohmic losses. This creates difficulties for the involved metallic materials, including bipolar plates and seals. 

The oxygen electrode of the cells under analyses in our project is a perovskite material Ao.8Sr0.2Mn03 (element A is not disclosed because it is proprietary information). Scandia-stabilised zirconia (SSZ) is used as the electrolyte. The cathode consists of a Ni-SSZ cermet. Also, a lanthanum strontium cobaltite (La0.8Sr0.2CoO3, also known as LSC) is used as the bond layer. 

Probes for measuring oxygen potential in liquid copper have been successfully developed and used in laboratory scale Abraham, 1967). Many industrial copper smelting and refining units in Europe have been successfully using magnesia-stabilised zirconia probes using air as the reference electrode. 

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... 

Kilo, M. Cation Transport in Stabilised Zirconias, Trans Tech Publications LTD, Zurich, 2005, pp. 185-253. 

Hendriks, M.G.H.M., ten Elshof, J.E., Bouwmeester, H.J.M., Verweij, H. The defect structure of the double layer in yttria-stabilised zirconia. Solid State Ionics 2002,154, 467-72. 

G.Z. Cao, H.W. Brinkman, J. Meijerink, K.J. de Vries and A.J. Burggraaf, Pore narrowing and formation of ultra thin yttria-stabilised zirconia layers in ceramic membranes by chemical vapor deposition. /. Am. Cer. Soc., 76 (1993) 2201-2208. 

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