Strontium doping

Conceptually elegant, the SOFC nonetheless contains inherently expensive materials, such as an electrolyte made from zirconium dioxide stabilized with yttrium oxide, a strontium-doped lanthanum man-gaiiite cathode, and a nickel-doped stabilized zirconia anode. Moreover, no low-cost fabrication methods have yet been devised. 

Nickel containing YSZ (Ni cermet) Strontium-doped LaMn03 (LSM)... 

Another application is in the oxidation of vapour mixtures in a chemical vapour transport reaction, the attempt being to coat materials with a thin layer of solid electrolyte. For example, a gas phase mixture consisting of the iodides of zirconium and yttrium is oxidized to form a thin layer of yttria-stabilized zirconia on the surface of an electrode such as one of the lanthanum-strontium doped transition metal perovskites Lai Sr MO --, which can transmit oxygen as ions and electrons from an isolated volume of oxygen gas. 

Cheng Z, Zha S, Aguilar L, and Liu M. Chemical, electrical, and thermal properties of strontium doped lanthanum vanadate. Solid State Ionics 2005 176 1921-1928. 

The interconnect material is in contact with both electrodes at elevated temperatures, so chemical compatibility with other fuel cell components is important. Although, direct reaction of lanthanum chromite based materials with other components is typically not a major problem [2], reaction between calcium-doped lanthanum chromite and YSZ has been observed [20-24], but can be minimized by application of an interlayer to prevent calcium migration [25], Strontium doping, rather than calcium doping, tends to improve the resistance to reaction [26], but reaction can occur with strontium doping, especially if SrCr04 forms on the interconnect [27],... 

Lanthanum chromite is a p-type conductor so divalent ions, which act as electron acceptors on the trivalent (La3+ or Cr3+) sites, are used to increase the conductivity. As discussed above, the most common dopants are calcium and strontium on the lanthanum site. Although there is considerable scatter in the conductivities reported by different researchers due to differences in microstrucure and morpohology, the increase in conductivity with calcium doping is typically higher than that with strontium doping [4], The increase in conductivity at 700°C in air with calcium additions is shown in Figure 4.1 [1, 2, 28-44], One of the advantages of the perovskite structure is that it... 

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

Meadowcroft DB. Some properties of strontium-doped lanthanum chromite. Brit. J. Appl. Phys. 1969 D2 1225-1233. 

Tai TW and Lessing PA. Modified resin-intermediate of perovskite powders. Part II. Processing for fine, nonagglomerated strontium-doped lanthanum chromite powders. J. Mater. Res. 1992 7 511-519. 

Deshpande K, Mukasyan A, and Varma A. Aqueous combustion synthesis of strontium-doped lanthanum chromite ceramics. J. Am. Ceram. Soc. 2003 86 1149-1154. 

Paulik SW, Hardy J, Stevenson JW, and Armstrong TR. Sintering of non-stoichiomet-ric strontium doped lanthanum chromite, J. Mater. Lett. 2000 19 863-865. 

For some applications, such as the cathode materials in solid oxide fuel cells (see Section 5.4.4), a material is needed that can conduct both ions and electrons. The strontium-doped perovskites LaMn03 (LSM), and LaCr03 (LSC) have both these properties. 

The cathode materials used have to conduct both oxide ions and electrons satisfactorily, but, in addition, for compatibility, they must have similar thermal expansion coefficients as the electrolyte. The strontium-doped perovskite, LSM (see Section 5.4.2), is one of the materials of choice. 

The fuel cell analyzed in the present section is a disk-shaped anode-supported SOFC, currently produced by H.C. Starck/InDEC B.V As illustrated in Figure 4.1 [1], the anode material is a cermet of nickel oxide doped with yttrium stablilized zir-conia (NiO/8YSZ). The cathode is composed of two layers one made of 8YSZ with strontium-doped LaMnC>3 (8YSZ/LSM) and one of LSM. The electrolyte consists of a dense 8YSZ material. 

Yasuda, I. and Hishinuma, M., Electrical conductivity and chemical diffusion coefficient of strontium-doped lanthanum manganites, Journal of the Electrochemical Society 143, 1996, 1583. 

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). 

The favoured material is modified lanthanum manganite (e.g. La0 8Sr0 2Mn03+x) which has the perovskite structure. It is a p-type semiconductor the electron transport occurring by electron-hopping (see Section 2.6.2) between the +3 and +4 states of the Mn ion. The strontium-doping enhances the conductivity. 

Fig. 13.22. The monolithic SOFC concept of Argonne National Laboratory. Anode nickel-yttria-stabilized zirconia. Cathode strontium-doped lanthanum manganite. Interconnect doped lanthanum chromite, a, Interconnection b, electron-ion path c, anode d, electrolyte e, cathode. (Reprinted from K. Kordesch,...

It has been seen in the previous section that the ratio of the onsite electron-electron Coulomb repulsion and the one-electron bandwidth is a critical parameter. The Mott-Hubbard insulating state is observed when U > W, that is, with narrow-band systems like transition metal compounds. Disorder is another condition that localizes charge carriers. In crystalline solids, there are several possible types of disorder. One kind arises from the random placement of impurity atoms in lattice sites or interstitial sites. The term Anderson localization is applied to systems in which the charge carriers are localized by this type of disorder. Anderson localization is important in a wide range of materials, from phosphorus-doped silicon to the perovskite oxide strontium-doped lanthanum vanadate, Lai cSr t V03. 

---