Machhindra Koirala

Thermoelectric property enhancement by Cu nanoparticles in nanostructured FeSb2

We present the thermoelectric figure-of-merit (ZT) improvement in nanostructured FeSb2 by Cu nanoparticles of ∼5 nm as a modulation dopant. Because of the similar work functions between FeSb2 and Cu and the high electrical conductivity of Cu, the Kondo insulator-like electrical resistivity of FeSb2 at low temperatures was dramatically reduced. Both carrier concentration and mobility of the nanocomposites were improved over pure FeSb2 without degrading the Seebeck coefficient. Overall, an improvement of ∼90% in power factor was achieved for the optimized nanocomposite FeSb2Cu0.045. Combined with the reduced thermal conductivity by Cu/FeSb2 interfaces, ZT was improved by ∼110%. These results clearly demonstrate the potential of modulation doping to enhance the thermoelectric performance of FeSb2. A similar approach could be applied to other Kondo insulators or previously known thermoelectric materials to improve ZT.

Role of surface electromagnetic waves in metamaterial absorbers

Metamaterial absorbers have been demonstrated across much of the electromagnetic spectrum and exhibit both broad and narrow-band absorption for normally incident radiation. Absorption diminishes for increasing angles of incidence and transverse electric polarization falls off much more rapidly than transverse magnetic. We unambiguously demonstrate that broad-angle TM behavior cannot be associated with periodicity, but rather is due to coupling with a surface electromagnetic mode that is both supported by, and well described via the effective optical constants of the metamaterial where we achieve a resonant wavelength that is 19.1 times larger than the unit cell. Experimental results are supported by simulations and we highlight the potential to modify the angular response of absorbers by tailoring the surface wave.

Bi-layer metamaterials as fully functional near-perfect infrared absorbers

In this letter, we discuss the design, fabrication, and experimental characterization of a bi-layer fully functional near-perfect metamaterial absorber (MMA) in the long-wavelength infrared (LWIR), which is broadband and generally insensitive to polarization up to a 60° incidence angle. A spectral absorptance of ≥99% was attained simultaneously at multiple LWIR wavelengths, with a bandwidth of 2 μm where the absorptance is ≥90%. This remarkable behavior is attributed to the strong mixing of coupling modes between the two resonators and the ground plane in the presence of a lossy dielectric, in which single layer structures do not exhibit. Furthermore, we show, by comparing two different MMA structures, how the absorption can be tailored by design within and across several IR subdivisions through a slight change in geometrical parameters. The bi-layer MMA has the immediate application of a functionally versatile, low-profile thermal sensor or emitter.

Nanostructured YbAgCu4 for Potentially Cryogenic Thermoelectric Cooling

We have studied the thermoelectric properties of nanostructured YbAgCu4 materials. A high power factor of ∼131 μW cm(-1) K(-2) has been obtained at 22 K for nanostructured samples prepared by ball milling the arc melted ingot into nanopowder and hot pressing the nanopowder. The implementation of nanostructuring method decreased the thermal conductivity at 42 K by 30-50% through boundary scattering comparing with the previously reported value of polycrystalline YbAgCu4. A peak dimensionless thermoelectric figure-of-merit, ZT, of 0.11 has been achieved at 42 K, which may find potential applications for cryogenic cooling below 77 K. The nanostructuring approach can be extended to other heavy Fermion materials to achieve high power factor and low thermal conductivity and ultimately higher ZT.

Boron-10 nanoparticles filled silicon trenches for thermal neutron detection application

This paper reports on the use of 10B nano/microparticles in order to fill microstructures of deep trenches fabricated in n-type Si (110) bulk wafers for the development of solid-state thermal neutron detectors. The high aspect-ratio trenches were fabricated in the wafer by wet etching, with a trench width of 3.5 to 6 μm and a maximum depth of 120 μm. Boron was diffused at a temperature of ∼1000 °C in order to convert the entirety of the delicate Si microstructures into a p+-n junction diode. The deep trenches of the diode were completely filled with 10B nanoparticles using a simple room-temperature process involving the pumping and venting of a vacuum chamber containing the etched wafer with 10B nanoparticles atop. The simple filling process was reproduced consistently, and the best 2.5 × 2.5 mm2 device demonstrated an intrinsic thermal neutron (En < 0.5 eV) detection efficiency of 32.2 ± 1.5% under a self-biased condition. This result is promising as it demonstrates a complete, low-cost fabrication proce...