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Electromotive Force and Measurement in Several Systems Part 11

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Tham khảo tài liệu 'electromotive force and measurement in several systems part 11', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 140 Electromotive Force and Measurement in Several Systems Fitzner et al 1993 that Gibbs free energy of formation of Ln 123 phase Ln is a symbol of any lanthanide element cannot be obtained directly from the e.m.f s produced by the cell with zirconia electrolyte. More sophisticated strategy must be adopted to achieve this aim and another cell with CaF2 solid electrolyte must be employed. The measurements require three steps which are schematically shown in Fig. 11. Fig. 11. Scheme of phase equilibria leading of subsequent cells construction. Cell I is assembled to measure the stability of phases existing in CuO-Ln2O3 systems. It is based on zirconia electrolyte and essentially in its construction does not differ much from the cell employed to study liquid dilute solutions Fig.10 . The only substantial difference consists in the working electrode which in this case is a mixture of oxides. Depending on the size of the lanthanide atom there are two phases which can exist at high oxygen pressure CuLn2O4 and Cu2Ln2O5. Under decreasing oxygen pressure some of them can be reduced to delafossite - type CuLnO2 .In a series of experiments we determined Gibbs free energy of formation of a number of phases resulting from the reaction between CuO and respective lanthanide oxides. The results of these investigations are summarized in Table 2. Having established AG0 oxides in CuO - Ln2O3 systems we could proceed with cell II. It operates with the working electrode CuO- CuO Ln2O3 phase -Ln2BaCuO5 phase designated as Ln 211 . To put this cell into operation CaF2 solid electrolyte is required. The application of CaF2 - type electrolyte in e.m.f. cells employed to the study of oxide systems was described in details by Levitskii in 1978. The cell construction used in our study followed that used by Alcock and Li in 1990. The cell assembly and the whole experimental setup are schematically shown in Fig. 12 Electromotive Force Measurements in High-Temperature Systems 141 Ln AG0f oxide A