Lanthanum conductivity-type

At the other end of the conduction spectrum, many oxides have conductivities dominated by electron and positive hole contributions to the extent that some, such as Re03, SnC>2 and the perovskite LaCrCb have conductivities at the level of metallic conduction. High levels of p-type semiconduction are found in some transition metal perovskites especially those containing alio-valent ions. Thus the lanthanum-based perovskites containing transition metal ions, e.g. LaM03 (M-Cr, Mn, Fe, Co, Ni) have enhanced p-type semiconduction due to the dependence of the transition metal ion valencies on the ambient... 

These incorporate membranes fabricated from insoluble crystalline materials. They can be in the form of a single crystal, a compressed disc of micro-crystalline material or an agglomerate of micro-crystals embedded in a silicone rubber or paraffin matrix which is moulded in the form of a thin disc. The materials used are highly insoluble salts such as lanthanum fluoride, barium sulphate, silver halides and metal sulphides. These types of membrane show a selective and Nemstian response to solutions containing either the cation or the anion of the salt used. Factors to be considered in the fabrication of a suitable membrane include solubility, mechanical strength, conductivity and resistance to abrasion or corrosion. 

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... 

The fluoride electrode is a typical example of an ion selective electrode. Its sensitive element is a crystal of lanthanum trifluoride that allows fluorine atoms to migrate into the network formed by lanthanum atoms (Fig. 18.3). Other electrodes use a mineral membrane obtained as agglomerates of crystalline powders (for measurement of Cl-, Br-, I , Pb++, Ag+ and CN ). Generally, the internal electrolyte can be eliminated (by dry contact). However, it is preferable to insert a polymer layer with a mixed-type conductivity to ensure the passage of electrons from the ionic conductivity membrane to the electronic conductivity electrode (Fig. 18.3). 

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). 

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. 

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. 

CoTi03 is a p-type semiconductor and was introduced by Chu et al. for the detection of ethanol.36 Lanthanum volume doped CoTi03 showed higher conductivity and better ethanol sensitivity of the material as shown in Siemons.29 The use of HT-IS enabled the analysis of surface doping with different amounts of Au, Ce, Pd, Pt, Rh, and Ru in order to find out the highest selectivity and sensitivity of the respective material composition. Measurements of CoTi03/La samples at different temperatures and under different atmospheres will be shown in order to illustrate the high reproducibility of the impedance measurements and sensitivities of identical samples on different positions. 

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