Electrodeposition of Ni and Te‐doped Cobalt Triantimonide in Citrate Solutions

Skutterudite compounds form a new class of potential candidates for thermoelectric applications. Cobalt triantimonide (CoSb₃) shows good thermoelectric properties at medium and high temperatures. Doping this system with substitution elements, for either Co or Sb or both, may result in an increase of the thermoelectric figure of merit (ZT). This work focused on the electrochemical doping and characterization of films and nanowires of Co‐Sb system in citrate solutions using gold‐coated PCTE templates. The electrodeposition was performed on gold surface that was pre‐treated electrochemically to ensure reproducible results. The electrochemical treatment acted as an annealing process for the surface, which resulted in an increase in Au(111) as demonstrated by XRD. Detailed electrochemical studies including deposition‐stripping experiments was performed in order to develop a better understanding of the co‐deposition kinetics and a better control over the composition of doped Co‐Sb system. Scanning electron microscopy (SEM/EDS) helped study the morphology and the composition of the doped and undoped Co‐Sb system. Co‐deposition of Co‐Sb showed that the amount of Co is higher in nanowires than in film or mushroom caps due to the slow Sb deposition rate dictated by slow Sb(III) complex diffusion. Doped nanowires have been also obtained. Both Ni and Te electrochemical doping of the Co‐Sb system affected the composition of the deposit but there was no effect on nanowire morphology.     

Determining the effect of Ru substitution on the thermal stability of CeFe4−xRuxSb12

The ternary, rare-earth filled (RE) Skutterudites (REM₄Pn₁₂; M = Fe–Os; Pn = P–Sb) have been proposed for use in high-temperature thermoelectric devices to convert waste heat to useful power. CeFe₄Sb₁₂ has been one of the most popular materials proposed for this application; however, it oxidizes at relatively low temperatures. The thermal stability of Skutterudites can be enhanced by selective substitution of the constituent elements and Eu(Fe,Ru)₄Sb₁₂ variants have been found to oxidize at temperatures above that of CeFe₄Sb₁₂. Unfortunately, these materials have poor thermoelectric properties. In this study, the thermal stability of CeFe_4−xRu_xSb₁₂ was examined depending on the value of x. (These compounds have similar thermoelectric properties to those of CeFe₄Sb₁₂.) It has been found by use of TGA and XANES that the temperature at which point CeFe_4−xRu_xSb₁₂ oxidizes increases with greater Ru substitution. XANES was also used to confirm the general charge assignment of Ce³⁺Fe_4−x²⁺Ru_x²⁺Sb₁₂¹⁻.     

Structure,microstructure and microhardness of rapidly solidified Smy(FexNi1-x)4Sb12 (x = 0.45, 0.50, 0.70, 1) thermoelectric compounds

Skutterudites are interesting compounds for thermoelectric applications. The main drawback in the synthesis of skutterudites by solidification of the melt is the occurrence of two peritectic reactions requiring long annealing times to form a single phase. Aim of this work is to investigate an alternative route for synthesis, based on rapid solidification by planar flow casting. The effect of cooling rate on phases formation and composition, as well as on structure, microstructure and mechanical properties of the filled Sm_y(Fe_xNi_1-x)₄Sb₁₂ (x = 0.45, 0.50, 0.70, 1) skutterudites was studied. Conversely to slowly cooled ingots, rapidly quenched ribbons show skutterudite as the main phase, suggesting that deep undercooling of the liquid prevents the nucleation of high temperature phases, such as (Fe,Ni)Sb and (Fe,Ni)Sb₂. In as-quenched samples, a slightly out of equilibrium Sm content is revealed, which does not alter the position of the p/n boundary; nevertheless, it exerts an influence on crystallographic properties, such as the cell parameter and the shape of the Sb₄ rings in the structure. As-quenched ribbons show a fine microstructure of the skutterudite phase (grain size of 2–20 μm), which only moderately coarsens after annealing at 873 K for 4 days. Vickers microhardness values (350–400 HV) of the skutterudite phase in as-quenched ribbons are affected by the presence of softer phases (i.e. Sb), which are homogeneously and finely dispersed within the sample. The skutterudite hardens after annealing as a consequence of a moderate grain growth, which limits the matrix effect due to the presence of additional phases.     

Crystal Structure and Properties of Some Filled and Unfilled Skutterudites: GdFe4P12, SmFe4P12, NdFe4As12, Eu0.54Co4Sb12, Fe0.5Ni0.5P3, CoP3, and NiP3

The new cubic compound Fe_0.5Ni_0.5P₃ (a = 775.29(5) pm) as well as the known compounds CoP₃ and NiP₃ were synthesized from the elemental components using tin as a flux. Their skutterudite (CoAs₃) type structures were refined from single‐crystal X‐ray data. The new compound GdFe₄P₁₂ was prepared by reaction of an alloy Gd_1/3Fe_2/3 with phosphorus in a tin flux. Its cubic “filled” skutterudite (LaFe₄P₁₂ type) structure was refined from single‐crystal X‐ray data: a = 779.49(4) pm, R = 0.019 for 304 structure factors and 11 variable parameters. SmFe₄P₁₂ shows Van Vleck paramagnetism while GdFe₄P₁₂ is a soft ferromagnet with a Curie temperature of T_C = 22(5) K. Both are metallic conductors. The new isotypic polyarsenide NdFe₄As₁₂ (a = 830.9(1) pm) was obtained by reacting NdAs₂ with iron and arsenic in the presence of a NaCl/KCl flux. The new isotypic polyantimonide Eu_0.54(1)Co₄Sb₁₂ (a = 909.41(8) pm) was prepared by reaction of EuSb₂ with cobalt and antimony. Its structure was refined from single‐crystal X‐ray data to a residual of 0.024 (137 F values, 12 variables). A comparison of the Fe–P and P–P bond lengths in the compounds AFe₄P₁₂, where the A atoms (A = Ce, Eu, Gd, Th) have differing valencies, suggests that the Fermi level cuts through Fe–P bonding and P–P antibonding bands.     

Reaction mechanism and thermoelectric properties of In0·22Co4Sb12 prepared by magnesiothermy

The magnesioreduction synthesis of In_0·22Co₄Sb₁₂ with high In rattler concentration from Sb₂O₄ and In-doped Co₃O₄ precursors is reported. This process directly yields a submicronic powder in a single step of 96 h at 810 K. The reaction mechanism has been investigated by stopping the reaction every 12 h and quantifying the existing phases by X-ray diffraction and Rietveld refinements. The precursors are first reduced in CoO and Sb₂O₃ lower oxides, then form CoSb₂O₆ and CoSb₂O₄ intermediates which are finally reduced in In_xCo₄Sb₁₂. A powder with 350 nm average size and mostly composed of In-filled skutterudite phase with composition close to In_0·17Co₄Sb₁₂ is obtained. Upon spark plasma sintering, small residual amount of InSb reacts with the skutterudite matrix to form a single-phase densified pellet with composition close to In_0·22Co₄Sb₁₂. The resulting densified material with 1.8 μm average grain size shows a figure of merit ZT_max of 0.95 at 750 K.