A method to compare the thermal to magnetic transduction efficiencies of different thermomagnetic systems was developed. The efficiencies of operating about a spin reorientation transition and the alternative ferromagnetic to paramagnetic transformation at the Curie point were directly compared. A case study was performed comparing Gd operating about its spin reorientation temperature and its Curie point. Additionally, a case study on NdCo5 operating about its spin reorientation temperature using experimentally derived values of the materials' temperature dependent magnetic properties was conducted. Analysis suggests that choosing the appropriate material and operating it about its transition produces considerable efficiencies (∼22%) as well as large harvestable energy densities (∼2.6 MJ/m3), which is an order of magnitude larger than Gd single domains operating about their Curie point (∼100 kJ/m3).

The abundance of low-grade waste heat necessitates energy harvesting devices to convert thermal energy to electrical energy. Through magnetic transduction, thermomagnetics can perform this conversion at reasonable efficiencies. Thermomagnetic materials use thermal energy to switch between magnetic and non-magnetic states and convert thermal energy into electrical energy. In this study, we numerically analyzed an active thermomagnetic device for thermal energy harvesting composed of gadolinium (Gd) and neodymium iron boron (NdFeB). A parametric study to determine the device efficiency was conducted by varying the gap distance, heat source temperature, and Gd thickness. Furthermore, the effect of the thermal conductance and applied field was also evaluated. It was found that the relative efficiency for smaller gap distances ranges from ∼15% to 28%; the largest allowable volume of Gd should be used and higher applied field leads to higher efficiencies.

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The distribution of the backscattered radiation of a long straight conductor shows a sharp return lobe perpendicular to its orientation. In horizontal observation such a ``rope'' would therefore give an appreciable echo only if it were almost exactly vertical. In practice no such effect will be observed since, because of wind and other disturbances, the rope will never be straight but actually over its whole length will pass through regions of space where the phase of the incident wave will change many times through its full range 2π. The effect of these irregularities will be to make the sharp dependence of the cross section upon the orientation of the rope disappear, and to give a cross section proportional to the total length L of the rope. In Section I, the wave-length λ is assumed to be large compared to the transverse dimensions of the rope. In Section II, the results are extended to include the case where the wave-length λ of the incident radiation is not necessarily large but is comparable with or even small compared to the width d of a ribbon-shaped rope. It is found that the cross section of such a rope can be written generally in the form σ = KAf(πd/λ), where K is a numerical constant depending upon the deviations of the rope from the vertical and where A = Ld is the area of the ribbon. The length L is assumed to be much greater than λ. The function f is evaluated by the use of a method developed by Morse and Rubenstein.