Melt-Spun SiGe Nano-Alloys: Microstructural Engineering Towards High Thermoelectric Efficiency

Silicon-germanium (SiGe) alloys are prominent high-temperature thermoelectric (TE) materials used as a powering source for deep space applications. In this work, we employed rapid cooling rates for solidification by melt-spinning and rapid heating rates for bulk consolidation employing spark plasma sintering to synthesize high-performance p-type SiGe nano-alloys. The current methodology exhibited a TE figure-of-merit (ZT) approximate to 0.94 at 1123 K for a higher cooling rate of similar to 3.0 x 10(7) K/s. This corresponds to approximate to 88% enhancement in ZT when compared with currently used radioisotope thermoelectric generators (RTGs) in space flight missions, approximate to 45% higher than pressure-sintered p-type alloys, which results in a higher output power density, and TE conversion efficiency (eta) approximate to 8% of synthesized SiGe nano-alloys estimated using a cumulative temperature dependence (CTD) model. The ZT enhancement is driven by selective scattering of phonons rather than of charge carriers by the high density of grain boundaries with random orientations and induced lattice-scale defects, resulting in a substantial reduction of lattice thermal conductivity and high power factor. The TE characteristics of synthesized alloys presented using the constant property model (CPM) and CTD model display their high TE performance in high-temperature regimes along with wide suitability of segmentation with different mid-temperature TE materials

Enhanced thermoelectric performance in p-type ZrCoSb based half-Heusler alloys employing nanostructuring and compositional modulation

ZrCoSb based half-Heusler (HH) alloys have been widely studied as a p-type thermoelectric (TE) material for power generation applications in the mid-temperature regime. However, their intrinsically high thermal conductivity has been found to be detrimental for the improvement in their thermoelectric figure-of-merit (ZT), which presently is far below unity. In the current work, a state-of-the-art ZT ∼1.1 at 873 K was realized in an optimized composition of nanostructured Zr1-xHfxCoSb0.9Sn0.1 HH alloys by employing compositional modulation i.e. grain-by-grain compositional variations, which leads to a substantial increase in its power factor coupled with a concurrent decrease in its thermal conductivity. Significant reduction in the phonon mean-free-path is observed on Hf substitution, which is comparable to the average crystallite size (∼25 nm), thus leading to a very low thermal conductivity of ∼2.2 W m−1K−1 at 873 K, which is amongst the lowest reported in HH alloys. The TE device characteristics, estimated using cumulative temperature dependence model for quantitative evaluation of TE performance, yielded an output power density of ∼10 Wcm−2 with a leg efficiency of ∼10% in the optimized composition of nanostructured Zr1-xHfxCoSb0.9Sn0.1 HH alloys, which is comparable to the reported efficiencies of other state-of-the-art TE materials. Keywords: Half Heuslers, Iso-electronic, Waste heat recovery, Mid-temperature, Efficiency, Compositional modulatio

Facile fabrication of p- and n-type half-Heusler alloys with enhanced thermoelectric performance and low specific contact resistance employing spark plasma sintering

Half-Heusler (HH) materials have been actively explored for thermoelectric (TE) based power generation applications in the mid-temperature regime. In the current work, we demonstrate the defect engineering in non-stoichiometric HH alloys to realize a state-of-the-art ZT 1 at 873 K for both n-type Zr0.5Hf0.5Ni1-xSn and p-type Zr0.5Hf0.5Co1+xSb0.8Sn0.2 (x = 0.04) compositions. This enhanced ZT leads to high conversion efficiency of -9% with a high output power density similar to 9 Wcm(2) in both the synthesized n and p-type HH alloys, estimated using cumulative temperature dependence model. A time efficient fabrication route of HH device thermo-elements with Ti electrical contacts, employing spark plasma sintering process, is also demonstrated in the optimized defect engineered HH compositions. A low specific contact resistivity of <10 m Omega.cm(2) in both n-type and p-type HH/Ti thermo-elements was realized, which suggests exciting opportunities and possibilities for achieving highly efficient TE based power generation

Compositional tuning of ZrNiSn half-Heusler alloys: Thermoelectric characteristics and performance analysis

Nanostructuring has rejuvenated great interest in thermoelectric (TE) based power generation by enabling enhanced performance in nano-grained TE materials over their bulk counterparts. In ZrNiSn based half-Heusler (HH), a promising n-type TE materials, we examine the prospects of nanostructuring by synthesizing nano crystalline iso-electronically substituted n-type Zr1-xHfxNiSn HH alloys to achieve a state-of-the-art TE figure-of-merit ZT similar to 1.2 at 873 K, which is similar to 20% higher than its bulk counterparts and corresponds to high conversion efficiency of similar to 9% with output power density similar to 7 Wcm(-2), estimated using cumulative temperature dependence model. Enhanced phonon scattering at nano-scale grain boundaries and crystal defects, arising from nanostructuring and mass defect fluctuation in Hf substituted ZrNiSn alloys resulted in significantly reduced thermal conductivity. It was found that the partial substitution at Zr site with its heavier homologue Hf causes variation in the carrier effective mass and carrier concentration, which at lower Hf concentration results in an enhancement in the power factor

Twelve-month observational study of children with cancer in 41 countries during the COVID-19 pandemic

Childhood cancer is a leading cause of death. It is unclear whether the COVID-19 pandemic has impacted childhood cancer mortality. In this study, we aimed to establish all-cause mortality rates for childhood cancers during the COVID-19 pandemic and determine the factors associated with mortality