High-Performance Low-Cost n‑Type Se-Doped Mg₃Sb₂‑Based Zintl Compounds for Thermoelectric Application

Thermoelectric materials, capable of converting heat directly into electricity without moving parts, provide a promising solid-state solution for waste heat harvesting. However, currently available commercial thermoelectric materials PbTe and Bi₂Te₃ are based on tellurium, an extremely scarce and expensive element, which prohibits large scale applications. Herein, we present a systematic study on a new low-cost Te-free material, n-type Se-doped Mg₃Sb_1.5Bi_0.5, by combining the structure and property characterization with electronic structure and electrical transport modeling. Compared with pure Mg₃Sb₂, Se-doped Mg₃Sb_1.5Bi_0.5 shows the considerably enhanced power factor as well as much lower thermal conductivity. The excellent electrical transport originates from a nontrivial near-edge conduction band with six conducting carrier pockets and a light conductivity effective mass as well as the weak contribution from a secondary conduction band with a valley degeneracy of 2. The accurate location of the conduction band minimum is revealed from the Fermi surface, which appears to be crucial for the understanding of the electronic transport properties. In addition, the total thermal conductivity is found to be reasonably low (∼0.62 W m^–1 K^–1 at 725 K). As a result, an optimal <i>zT</i> of 1.23 at 725 K is obtained in Mg_3.07Sb_1.5Bi_0.48Se_0.02. The high <i>zT</i>, as well as the earth-abundant constituent elements, makes the low-cost Se-doped Mg₃Sb_1.5Bi_0.5 a promising candidate for the intermediate-temperature thermoelectric application. Moreover, the systematic electronic structure and transport modeling provide an insightful guidance for the further optimization of this material and other related Zintl compounds

Development of a Dual-Stage Continuous Flow Reactor for Hydrothermal Synthesis of Hybrid Nanoparticles

This paper provides a comprehensive description of the design and commissioning of a dual-stage flow reactor for hydrothermal synthesis, notably heterogeneous nanomaterials such as core–shell particles or nanocomposites. The design is based on the hypothesis that the next frontier of studies within continuous, hydrothermal synthesis lies as much with scalability as it does with the materials properties and performance in applications. Therefore, this reactor belongs to the up-scaled end of a laboratory system with a synthesis capacity of up to 50 g/h. Commissioning was accomplished with TiO₂ nanoparticles as a model material. Results comply with earlier ones obtained from single-stage reactors. Dual-stage synthesis of a TiO₂@SnO₂ nanocomposite was performed by adding a SnCl₄ solution to newly formed 9 nm TiO₂ nanoparticles, yielding deposition of 2 nm rutile SnO₂. Synthesis of pure SnO₂ produced much larger nanocrystals, indicating that TiO₂ nanoparticles provide the nucleation sites for SnO₂ and impede the growth beyond 2 nm

Hydrothermal Liquefaction of Dried Distillers Grains with Solubles: A Reaction Temperature Study

The effect of the reaction temperature on hydrothermal liquefaction of dried distillers grains with solubles (DDGS) was investigated using a novel stop-flow reactor system at varying temperatures (300–400 °C), fixed pressure (250 bar), and fixed reaction time (15 min). The stop-flow reactor provides rapid heating of biomass feeds and the option of performing multiple sequential repetitions. This bypasses long, uncontrollable temperature gradients and unintended changes in the reaction chemistry. The product, a crude bio-oil, was characterized in terms of yield, elemental composition, and chemical composition. Higher reaction temperatures resulted in improved bio-oil yields, less char formation, and higher heating values of the bio-oil. A supercritical reaction temperature of 400 °C was found to produce bio-oil in the highest yields and of the best quality

TiO₂ Nanoparticles for Li-Ion Battery Anodes: Mitigation of Growth and Irreversible Capacity Using LiOH and NaOH

TiO₂ anatase and rutile nanoparticles with various sizes and morphologies have been synthesized by very facile and scalable methods, involving common acids as catalysts for room-temperature precipitations. A post-treatment including addition of LiOH or NaOH to the particles followed by heating at 180 °C in air or autoclave suppressed crystallite growth of both rutile and anatase. Furthermore, the treatment with LiOH or NaOH consistently increased the first-cycle Coulombic efficiency in half-cells from ∼0.77 to ∼0.90 on average and even to ∼1.00 in some cells. Whether LiOH or NaOH was used, or the amount, did not appear to affect the electrochemical properties significantly. The structural properties were investigated by Rietveld refinement of powder X-ray diffractograms and related to the electrochemical performance in half-cells. The crystal structure, sizes, and morphologies of the TiO₂ nanoparticles were found to depend on the synthesis conditions, e.g., hydrolysis ratio and the type and concentration of the acid catalyst. Furthermore, increasing the size of rutile crystallites from ∼6 to 11 nm decreased the maximal capacity and rate ability of the half-cells. The anatase crystallites showed optimal electrochemical performance for crystallite sizes of ∼5–8 nm