The influence of different compositional modifications on the magnetic entropy change and refrigerant capacity of Finemet, Nanoperm, HiTperm, and bulk amorphous alloys is presented. For all the studied alloys, the field dependence of the magnetic entropy change exhibits a quadratic dependence in the paramagnetic regime, a linear dependence in the ferromagnetic temperature range, and a potential law with a field exponent ϳ0.75 at the Curie temperature. This exponent can be explained using the critical exponents of the Curie transition. It is shown that for alloys of similar compositional series, the magnetic entropy change follows a master curve behavior. © 2007 American Institute of Physics. ͓DOI: 10.1063/1.2709409͔Magnetic refrigeration is a field of current interest due to the enhanced performance and reduced environmental impact of these systems when compared with those based on the gas compression-expansion cycle. 1-3 Although current prototypes are based on rare-earth-based materials, 4 soft magnetic amorphous alloys have been recently proposed as low cost candidates for high temperature magnetic refrigeration. [5][6][7][8][9][10][11][12][13] Albeit the maximum magnetic entropy change, ͉⌬S M pk ͉, for these alloys is modest when compared to that of rareearth-based materials, 14,15 the remarkable difference in material costs is an incentive for studying their suitability as magnetic refrigerants. Besides their reduced magnetic hysteresis ͑virtually negligible͒, higher electrical resistivity ͑which would decrease eddy current losses͒, tunable Curie temperature and, in the case of bulk amorphous alloys, outstanding mechanical properties are beneficial characteristics for a successful application of the material. The field dependence of the magnetic entropy change ⌬S M in materials with a second order phase transition has also been recently studied, demonstrating that a master curve behavior is fulfilled for ⌬S M curves measured up to different maximum fields. 16 The aim of this work is twofold: to present a comparison of the relevant characteristics of the magnetocaloric response of selected Finemet, Nanoperm, HiTperm, and bulk amorphous alloys ͑BAAs͒ prepared in ribbon shape and to demonstrate that ⌬S M ͑T͒ curves of different alloys from the same compositional series can also collapse in a master curve.Amorphous alloys from different compositional series have been prepared in ribbon shape ͑ϳ25 m thick͒ by melt spinning: Fe x Co

This report was prepared as an account of work sponsored by the United States Government. AbstractThe hotside operating temperatures for many projected thennophotovoltaic (TPV) conversion system applications are approximately 10oO 'C, which sets an upper limit on the TPV diode bandgap of 0.6 eV from efficiency and power density considerations. This bandgap requirement has necessitated the development of new diode material systems, never previously considered for energy generation. To date, InGaAsSb quaternary diodes grown lattice-matched on GaSb substrates have achieved the highest performance. This report relates observed diode performance to electrooptic properties such as minority carrier lifetime, diffusion length and mobility and provides initial links to microstructural properties. This analysis has bounded potential diode performance improvements. For the 0.52 eV InGaAsSb diodes used in this analysis the measured dark current is 2 x Ncm2 (no photon recycling), and an absolute thermodynamic limit of 1.4 x A/cm2. These dark currents are equivalent to open circuit voltage gains of 20 mV (7%), 60 mV (20%) and 140 mV (45%), respectively.

This report presents an assessment of the efficiency and power density limitations of thermophotovoltaic (TPV) energy conversion systems for both ideal (radiative-limited) and practical (defect-limited) systems. Thermodynamics is integrated into the unique process physics of TPV conversion, and used to define the intrinsic tradeoff between power density and efficiency. The results of the analysis reveal that the selection of diode bandgap sets a limit on achievable efficiency well below the traditional Carnot level. In addition it is shown that filter performance dominates diode performance in any practical TPV system and determines the optimum bandgap for a given radiator temperature. It is demonstrated that for a given radiator temperature, lower bandgap diodes enable both higher efficiency and power density when spectral control limitations are included. The goal of this work is to provide a better understanding of the basic system limitations that will enable successfiil long-term development of TPV energy conversion technology.

The FeCoSiAlGaPCB alloys can be prepared as bulk amorphous materials, with outstanding mechanical properties and increased electrical resistivity. These features can be beneficial for their application as a magnetic refrigerant. The influence of Co addition on the magnetic entropy change of the alloy has been studied. This compositional modification displaces the temperature of the peak entropy change closer to room temperature, but reduces the refrigerant capacity of the material. For the Co-free alloy, the peak entropy change is increased with respect to a Finemet alloy containing Mo, but its refrigerant capacity is not enhanced. © 2006 American Institute of Physics. ͓DOI: 10.1063/1.2188385͔Magnetic refrigeration is a field of research that has gained increasing attention, as it is considered an alternative to the gas compression-expansion cycle. At temperatures close to room temperature, rare-earth-based materials are among the most relevant ones. [1][2][3][4] To display a big magnetocaloric response, two requirements have to be fulfilled: the material needs to exhibit a big magnetic moment and, also, a strong temperature dependence of magnetization close to the working temperature. This second condition, related to a magnetic phase transition, can be achieved in two different ways. Either by a first-order phase transition, which produces an abrupt temperature change of the magnetic moment and, therefore, a remarkable peak in the magnetic entropy change ͑⌬S M ͒ at the transition temperature, or by a second order phase transition, which causes a more smeared peak in ⌬S M . However, the remarkable hysteresis that appears in some materials, associated to first order phase transitions, may reduce the actual efficiency of the cooling process. 5 It has been pointed out, nevertheless, that in order to compare the characteristics of different materials as candidates for magnetic refrigerants, their refrigerant capacity ͑RC͒ in a reversible cycle, connected to the entropy absorbed by the refrigerant at the cold end of the cycle and its temperature span, should be used. 6 The search for low-cost materials for high-temperature magnetic refrigeration is a field of current interest. 7-12 Recently it has been shown that some soft-magnetic amorphous alloys are good candidates for this application, 11 with a refrigerant capacity that is comparable to that of low-hysteretic Gd-based materials. 4 It has also been shown that the nanocrystallization of the alloy, although broadening the ⌬S M

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