Xps Study of Ni-Fe Manganite Thermistor Material
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x-ray diffraction verified their respective identities and showed there were no extraneous oxides or other second phases present. Pure metal standards of Ni (99.998%), Mn (99.99%), and Fe (99.999%) from Geller Microanalytical Laboratory, Peabody, MA (Standard UHV-EL) were also analyzed under the same operating conditions. Argon sputtering at 4 kV was used to remove normal surface contamination. In analyzing the XPS spectra, binding energies were referenced to 285.0 eV for the C Is peak of hydrocarbon contamination due to air exposure (adventitious carbon). Curve-fitting of the spectra was conducted using Gaussian/Lorentzian line shapes after removal of the background. RESULTS Results of the XPS analyses are summarized in Fig. 1. The 2 p3/2 peaks for the pure metal standards of Mn, Fe, and Ni (Fig. la) and starting oxide powders (Fig. lb) are shown at the bottom of each figure. The same peaks for each respective ion in the spinel are shown for the asfabricated thermistor material (Fig. Ic), and after aging at 300"C for three (Fig. Id) and nine days (Fig. le). Binding energies (BE) for Mn, Fe, and Ni for each material and condition are listed in Table 1 under the column labeled 2p3/2. The full width half maximum (FWHM) for each peak is also listed, along with data for the satellite structure and the 2 py2-2 pl/2 separation. The O-Is BE of 529.6 eV, for the spinel, did not change for any of the conditions and is not listed. The 2 p3/2 peak for Mn metal had a BE of 638.6 eV and a 2P3/2- 2 p,/2 separation of 11.1 eV (Table 1) which compares favorably with the values of 639.0 and 11.05 eV, respectively, listed in the handbook.' The same Mn peak in the Mn 20 3 powder had a BE of 641.6 eV with a shake-up satellite of 645.7 eV and a 2p3/2- 2 pl/2 separation of 11.6 eV. The Mn peaks in the as-fabricated spinel were virtually identical to those in the Mn 2 0 3 powder and thermal aging for 3 or 9 days did not affect the peaks (Fig. 1 and Table 1). A 2 p3/ BE of 706.38 eV and a 2p3/2- 2pl/2 separation of 13.2 eV was measured for Fe metal (Table 1) which, again, compared well with handbook values of 707.0 and 13.10 eV. 9 The same two parameters for the Fe 20 3 powder were 710.1 and 13.7 eV (Table 1) while handbook values were 710.9 and 13.6 eV.1 As in the case for Mn, the 2 p3/2 peak shape and BE's for Fe were the same for the starting oxide, the as-fabricated disks, and the thermally aged material (Fig. 1 and Table 1). The two shake-up satellite peaks in the left portion of the broadened 2p3/2 peak of the oxide were also found in the spinel. The 2p.3/2 peak for Ni metal had a slightly lower BE of 852.0 eV (Table 1) compared to the 852.7 eV handbook value. 9 However, the experimental value for the 2 p312- 2pl/2 separation of 17.2 eV agreed closely with that in the handbook, 17.27 eV.9 A weak, broad peak due to plasmon losses was observed at 857.8 eV. The 2p3/2 Ni peak in the starting NiO powder has a complex structure as shown in Fig. 1. Its measured BE of 852.8 eV is I eV lower than the 853.8 eV handbook value,9 but the 2 P3/2- 2
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