MnZn ferrites with different composition have been synthesized by soft chemistry routes using oxalate precursors with the partial replacement of water by acetone as the solvent. Toroids were sintered at 1250 °C for 3 h in O2/N2 atmosphere. Sintering conditions significantly changed macroscopic characteristics as density and initial permeability for different composition. Variation of the complex permeability with frequency have been measured over a wide range of frequency, up to 1.8 GHz where dispersion is attributed to spin resonance. No additives for increasing densification were employed. Samples exhibit the highest permeability values for Mn0.8Zn0.2Fe2O4. An anomalous effect is observed when copper is added to MnZn ferrites. The initial permeability (μi) values do not exhibit much variation with temperature, except near Tc where it falls sharply. Curie point and magnetic losses are related with composition.
We have analyzed entropy production in chemically reacting systems and extended previous results to the two limiting cases of ideal reactors, namely continuous stirred tank reactor (CSTR) and plug flow reactor (PFR). We have found upper and lower bounds for the entropy production in isothermal systems and given expressions for non-isothermal operation and analyzed the influence of pressure and temperature in entropy generation minimization in reactors with a fixed volume and production. We also give a graphical picture of entropy production in chemical reactions subject to constant volume, which allows us to easily assess different options. We show that by dividing a reactor into two smaller ones, operating at different temperatures, the entropy production is lowered, going as near as 48 % less in the case of a CSTR and PFR in series, and reaching 58 % with two CSTR. Finally, we study the optimal pressure and temperature for a single isothermal PFR, taking into account the irreversibility introduced by a compressor and a heat exchanger, decreasing the entropy generation by as much as 30 %.
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