Magnetic properties of the multiferroic double perovskite lead iron niobate: Role of disorder

The peculiarities of the magnetism of one of the first known and most studied multiferroic compounds remain a challenge for solid-state theory. The antiferromagnetic ordering of PbFe1/2Nb1/2O3(PFN) occurs at T-N approximate to 150 K. On the one hand, this value is much larger than the T-N value for most of the other double perovskites. On the other hand, it is significantly lower than theoretical estimates. The temperature dependence of magnetic susceptibility is substantially different from that one expected for conventional antiferromagnets. In order to find the solution of these puzzles, several density functional theory calculations of the magnetic interactions in PFN are reported. The interactions between first-, second-, third- and fourth-neighbor Fe3+ ions (S = 5/2) are calculated. The magnetic response of Fe spins is shown to depend both on the interaction value and the mutual spin arrangement. The magnetic susceptibility of various spin lattices is calculated using the tenth-order high-temperature expansion method and compared with the experiment. A substantial role of disorder for the understanding of the magnetic properties is discussed

Optimal exergy-based control of internal combustion engines

© 2016 Elsevier Ltd Exergy or availability is defined as the maximum useful work during a process. This metric has been used to analyze and understand loss mechanisms of Internal Combustion Engines (ICEs). In this paper, an optimal control method based on exergy is introduced for transient and steady state operation of ICEs. First, an exergy model is developed for a single cylinder Ricardo engine. The ICE exergy model is based on the Second Law of Thermodynamics (SLT) and characterizes irreversibilities. Such quantifications are not identified in the First Law of Thermodynamics (FLT) analysis. For steady-state operation of the ICE, a set of 175 different operating conditions is used to construct the SLT efficiency maps. Two different SLT efficiency maps are generated depending on the applications whether work, or Combined Power and Exhaust Exergy (CPEX) is the desired output. To include transient ICE operation, a model to predict exergy loss/destruction during engine transients is developed. The sources of exergy destruction/loss are identified for a Homogeneous Charge Compression Ignition (HCCI) engine. Based on the engine operating conditions (i.e., steady-state or transient) SLT efficiency contour maps or predicted exergy losses are determined at every given engine load. An optimization algorithm is proposed to find the optimum combustion phasing to maximize the SLT efficiency. Application of the optimization algorithm is illustrated for combustion phasing control. The results show that using the exergy-based optimal control strategy leads to an average of 6.7% fuel saving and 8.3% exergy saving compared to commonly used FLT based combustion control in which a fixed combustion phasing (e.g., 8°aTD) is used