Lanthanum thermal expansion coefficients

The perovskite lanthanum chromite (LaCr03) is one of the exceptional ceramic compounds that are chemically very resistant to both oxidizing and reducing ambients. Moreover, being an electronic conductor, it is eminently suitable as a bipolar connector in solid oxide fuel cells. It is evident that the thermal expansion coefficients of the different components in a fuel cell (electrolyte, electrodes, bipolar connector) must be closely matched. Doping the chromite with strontium or magnesium ions is necessary to increase its electronic conductivity as well as its sinter activity. 

Perovskite-type Sr-doped LaMnOs (Lanthanum Strontimn Manganites- LSM) has particularly attracted substantial interest as a promising material for cathode in SOFCs. This material has good properties such as chemical and thermal stability, and high catalytic activity for oxygen reduction. Additionally, it has a thermal expansion coefficient similar to that of a solid electrolyte (YSZ), and high electronic conductivity [2]. 

In FeCr 18 steels containing 7-10 wt% tungsten [59], increasing tungsten content results in thermal expansion coefficients close to that of YSZ electrolyte and small additions of Ce, La or Zr lead to reduced corrosion rates. Also, in contact with lanthanum manganite, a Cr-Mn spinel/CraOs double layer is formed. 

FIGURE 4.4 Coefficients of thermal expansion of calcium- and strontium-doped lanthanum... 

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. 

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