Lanthanum chromites

10 000 h. LaCrC 3 was considered a prime candidate for this application since it combines a melting point of 2500 °C with high electronic conductivity (about lOOSrn-1 at 1400 °C) and resistance to corrosion. There is now little interest in MHD systems but LaCrC 3 is established as a specialized heating element. 

LaCrC 3 is one of the family of lanthanide perovskites RTO3, where R is a lanthanide and T is a period 4 transition element. In the cubic unit cell R occupies the cube corners, T the cube centre and O the face-centre positions. The coordination numbers of T and R are 6 and 8 respectively. LaCrC 3 loses chromium at high temperatures, leaving an excess of O2- ions. The excess charge is neutralized by the formation of Cr4+ which results in p-type semiconductivity with hole hopping via the localized 3d states of the Cr3+ and Cr4+ ions. The concentration of Cr4+ can be enhanced by the substitution of strontium for lanthanum. A 1 mol.% addition of SrO causes the conductivity to increase by a factor of approximately 10 (see Section 2.6.2). 

Satisfactory conductivity is maintained up to 1800 °C in air but falls off at low oxygen pressures so that the upper temperature limit is reduced to 1400 °C when the pressure is reduced to 0.1 Pa. A further limitation arises from the volatility of Cr2C 3 which may contaminate the furnace charge. The combination of high melting point, high electronic conductivity and resistance to corrosion has led to the adoption of lanthanum chromite for the interconnect in high temperature solid oxide fuel cells (see Section 4.5.3). 

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The result is the formation of a dense and uniform metal oxide layer in which the deposition rate is controlled by the diffusion rate of ionic species and the concentration of electronic charge carriers. This procedure is used to fabricate the thin layer of soHd electrolyte (yttria-stabilized 2irconia) and the interconnection (Mg-doped lanthanum chromite). 

The anode material in SOF(7s is a cermet (rnetal/cerarnic composite material) of 30 to 40 percent nickel in zirconia, and the cathode is lanthanum rnanganite doped with calcium oxide or strontium oxide. Both of these materials are porous and mixed ionic/electronic conductors. The bipolar separator typically is doped lanthanum chromite, but a metal can be used in cells operating below 1073 K (1472°F). The bipolar plate materials are dense and electronically conductive. 

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... 

Lanthanum chromite is the most common base for SOFC interconnects, but chromites of other lanthanide elements have also been used [43, 45, 46, 48, 54, 55], Although the conductivity of calcium-doped gadolinium chromite for low calcium contents is in the upper range of conductivities for lanthanum chromite, other nonlanthanum chromites typically have lower conductivities. However, the use of other lanthanides provides benefits in controlling the phase transformation temperature and in potential cost savings [48],... 

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