V. Ponnambalam

Fe-based bulk metallic glasses with diameter thickness larger than one centimeter

Fe–Cr–Mo–(Y,Ln)–C–B bulk metallic glasses (Ln are lanthanides) with maximum diameter thicknesses reaching 12 mm have been obtained by casting. The high glass formability is attained despite a low reduced glass transition temperature of 0.58. The inclusion of Y/Ln is motivated by the idea that elements with large atomic sizes can destabilize the competing crystalline phase, enabling the amorphous phase to be formed. It is found that the role of Y/Ln as a fluxing agent is relatively small in terms of glass formability enhancement. The obtained bulk metallic glasses are non-ferromagnetic and exhibit high elastic moduli of approximately 180–200 GPa and microhardness of approximately 13 GPa.

Synthesis of iron-based bulk metallic glasses as nonferromagnetic amorphous steel alloys

Iron-based amorphous metals are investigated as nonferromagnetic amorphous steel alloys with magnetic transition temperatures well below ambient temperatures. Rod-shaped amorphous samples with diameters reaching 4 mm are obtained using injection casting. Amorphous steel alloys are designed by considering atomistic factors that enhance the stability of the amorphous phase, coupled with the realization of low-lying liquidus temperatures. The present alloys are found to exhibit superior mechanical strengths. In particular, the elastic moduli are comparable to those reported for super austenitic steels.

Effect of Sb doping on the thermoelectric properties of Ti-based half-Heusler compounds, TiNiSn1−xSbx

Half-Heusler alloys (MgAgAs type) with the general formula MNiSn where M is a group IV transition metal (Hf, Zr, or Ti) are currently under investigation for potential thermoelectric materials. These materials exhibit a high negative thermopower (−40 to −250 μV/K) and low electrical resistivity values (0.1–8 mΩ cm) both of which are necessary for a potential thermoelectric material. Results are presented in this letter regarding the effect of Sb doping on the Sn site (TiNiSn1−xSbx). The Sb doping leads to a relatively large power factor of (0.2–1.0) W/m K at room temperature for small concentrations of Sb. These values are comparable to that of Bi2Te3 alloys, which are the current state-of-the-art thermoelectric materials. The power factor is much larger at T≈650 K where it is over 4 W/m K making these materials very attractive for potential power generation considerations.

Thermoelectric properties of semimetallic (Zr, Hf)CoSb half-Heusler phases

Unlike semiconducting TiCoSb, ZrCoSb and HfCoSb half-Heusler phases are semimetallic below room temperature and exhibit small Seebeck coefficients of ∼−10 μV/K at 300 K. However, upon substituting (doping) the Co and Sb sites with Pt and Sn, respectively, much larger thermopowers (S) are obtained. For ZrCoSb, S reaches −110 and +130 μV/K while resistivity ρ decreases from ∼5×104 μΩ cm in the undoped phase to 1–2×103 μΩ cm in the substituted phases at 300 K. The lowest thermal conductivity obtained in the substituted alloys is ∼3.0 W/m K at 300 K, which is among the lowest reported for this class of structural phases. There are indications that the thermoelectric properties have not been optimized in these multinary alloys.

Grain structure effects on the lattice thermal conductivity of Ti-based half-Heusler alloys

Half-Heusler alloys with the general formula TiNiSn1−xSbx are currently being investigated for their potential as thermoelectric (TE) materials. A systematic investigation of the effect of Sb doping on the Sn site and Zr doping on the Ti site on the electrical and thermal transport of the TiNiSn system has been performed. Unexpectedly, lattice thermal conductivity κL appears to increase somewhat randomly with small amounts (x<5%) of Sb doping. Subsequently, an investigation of grain structure in these Sb-doped materials has been found to correlate with the anomalous behavior of κL. Furthermore, effects of submicron grain sizes on κL in ball milled and shock compressed samples are also presented.

Electrical transport properties of TiCoSb half-Heusler phases that exhibit high resistivity

Electrical transport measurements have been performed on doped and undoped TiCoSb half-Heusler phases. The semiconducting properties are found to be more robust than those reported for MNiSn (M = Ti, Zr, Hf ). Undoped TiCoSb phases exhibit large n-type Seebeck coefficients and high resistivities that reach -500 µV K-1 at 300 K and ~1500 Ω cm at 4.2 K, respectively. A tendency towards carrier localization is seen in several disordered phases. The effects due to n-type and p-type dopants are readily manifested in the thermopower, from which moderately heavy electron and hole band masses are inferred. The unusual properties measured are consistent with the prediction of a wide bandgap for the TiCoSb phase. A resistivity minimum is observed at 500-600 K for undoped and V-doped TiCoSb. Consequently, the semiconducting gap has not been determined.

Fe–Mn–Cr–Mo–(Y,Ln)–C–B (Ln = Lanthanides) bulk metallic glasses as formable amorphous steel alloys

The glass formability of high-manganese amorphous steel alloys reported earlier by us has been found to improve upon additions of yttrium and lanthanide elements, enabling the formation of bulk glassy samples with diameter thicknesses reaching 7 mm by casting. Based on extensive studies using different Ln additions and systematic measurements of alloy oxygen contents, the intrinsic roles of Y/Ln in attaining good glass formability in both the high-Mn alloys and previously reported high-Cr alloys are revealed. The yield strengths of the non-ferromagnetic glassy alloys obtained are estimated to be three times those of high-strength stainless steel alloys, and high elastic moduli in the range 150–200 GPa are measured. Furthermore, in the supercooled liquid regions, the glassy rods can be bent into various configurations by hand without fracturing. The observed plastic behavior together with the measured high mechanical strengths suggests that the present Fe–based bulk metallic glasses can potentially be developed as formable non-ferromagnetic amorphous steel alloys.

Grain structure and thermal transport properties of TiNiSn/sub 1-x/Sb/sub x/ and Ti/sub 1-y/Zr/sub y/NiSn/sub 0.95/Sb/sub 0.05/ half-Heusler alloys

Thermal conductivity of two series of Ti-based half-Heusler alloys have been investigated and compared. A significant reduction in the lattice thermal conductivity (/spl kappa//sub L/) in Ti/sub 1-y/Zr/sub y/NiSn/sub 0.95/Sb/sub 0.05/ via mass fluctuation scattering (y>20%) is observed. However, small amounts of Sb (x/spl les/5%) in TiNiSn/sub 1-x/Sb/sub x/ do not yield similar results. A non-systematic increase in /spl kappa//sub L/ in the TiNiSn/sub 1-x/Sb/sub x/ series was observed, even with the amounts (x/spl les/5%) of Sb-doping. Extensive investigations of the grain structure in these materials show that there is a direct correlation between /spl kappa//sub L/ and the average grain size in these materials. These results are in good agreement with the theoretical predictions of Goldsmid et al. Theoretical calculations relating to the phonon mean free path in both the series of compounds would also be presented and discussed.

Half-Heusler phases as prospective p-type thermoelectric materials

It is well known that most of the half-Heuslers with potential thermoelectric properties are n-type due to electron domination in the transport. However, p-type half-Heusler alloys can be prepared by appropriate combination of elements. These alloys are multi-component and exhibit resistivity values 1000-3000 /spl mu//spl Omega/ cm and thermopower values hundreds of /spl mu/V/K. Such values are typical for state-of-the-art thermoelectric materials and the power factor S/sup 2//spl sigma/ can be in the range of 10-15 /spl mu/W cm/sup -1/ K/sup -2/ at room temperature. In addition to the high power factors, low thermal conductivities are expected due to the presence of heavy elements. We have studied a series of such alloys and the details are presented in this paper.

Thermoelectric properties of Sb-doping in the TiNiSn/sub 1-x/Sb/sub x/ half-Heusler system

Half Heusler alloys, MNiSn (M=Zr, Hf, Ti) systems, have recently been studied for their potential as new thermoelectric materials. They have shown both high thermopower (/spl alpha/) values (40-250 /spl mu/V/K) and reasonable values of electrical resistivity, /spl rho/ (0.1-8 m/spl Omega/-cm). However, the thermal conductivity in these systems is high for a potential thermoelectric material, on the order of 4-10 W/m-K. In an effort to reduce the thermal conductivity through alloy scattering, Sb is substituted on the Sn site with compositions TiNiSn/sub 1-x/Sb/sub x/ where x=0 to 0.1. With this substitution, the thermopower is only slightly reduced while the resistivity is reduced by approximately one order of magnitude resulting in marked improvement in the power factor (/spl alpha//sup 2/T//spl rho/). Thermopower, resistivity, and thermal conductivity have been measured on a series of Sb doped TiNiSn samples from 10 K<T<300 K. Heat capacity and Hall measurements on these same samples are measured from 2 K to 350 K and will be discussed. A room temperature power factor in this system has been calculated to be as high as 1.4 W/m-K, making these materials interesting for potential thermoelectric applications.