The stability of the disordered glassy phase in the relaxors PbMg 1/3 Nb 2/3 O 3 and ͑PbMg 1/3 Nb 2/3 O 3 ͒ 0.88 ͑PbTiO 3 ͒ 0.12 , called PMN and PMN-PT, was investigated by preparing partially polarized samples and allowing them to age at zero field in the temperature range for which the phase is history dependent. The PMN-PT polarization would spontaneously increase until long-range order formed, first appearing as giant polarization noise. Thus, the thermodynamically stable phase in PMN-PT appears to be ferroelectric. In contrast, a PMN sample lacking the sharp first-order field-driven transition found in some other samples spontaneously depolarized, consistent with its glassy state being thermodynamically stable. Detailed thermal depolarization results in PMN showing two distinct broad peaks, as well as a small fraction of material with a distribution of abrupt melting transitions.

Fe3O4-Ce0.75Zr0.25O2 (FeCZ) is an oxygen carrier material aimed to produce syngas through methane partial oxidation in absence of oxygen gas feed. The objective of the present research is to study the catalytic effect of Ni on FeCZ using an evaluation of the global kinetics (activation energy, reaction rate, order and constant) of its reaction with methane for syngas production. FeCZ and 0.05NiFeCZ (Ni/Fe = 0.05 molar ratio) were synthesized through co-precipitation of their precursor nitrate salts, while 2NiFeCZ was prepared by impregnation of FeCZ with a nickel nitrate solution to obtain a 2 %W Ni material. Samples were calcined at 950°C during 4 hours in air. Kinetic study of oxygen carriers (FeCZ, 0.05NiFeCZ and 2NiFeCZ) reduction with methane was followed through thermogravimetric analysis (TGA) at 5, 7.5 and 10% CH4/Ar and 600, 650 and 700°C. Initial reaction rate was obtained from the slope of the linear region of the weight change signal as a function of time. Results indicate a first order global reaction rate for all materials. Activation energies for samples FeCZ, 0.05NiFeCZ and 2NiFeCZ were 52.2, 39.5 and 28.3 Kcal/mol, respectively. Thus, reflecting the catalytic effect of Ni over the FeCZ global reaction rate.

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