B. J. Beaudry

Preparation of γ‐LaSy (1.33<y<1.50) alloys by the pressure‐assisted reaction sintering method and their thermoelectric properties

The pressure‐assisted reaction sintering (PARS) method was used to prepare thermoelectric elements of the γ‐phase lanthanum sesquisulfide solid solution alloys which exist over a wide homogeneity range: LaSy (1.33<y<1.5). A mixture of powders of stoichiometric lanthanum sesquisulfide (La2S3) and lanthanum trihydride (LaH3) was reaction sintered in a hot‐press to make the desired composition of γ‐LaSy. This method produced black dense single‐phase γ‐LaSy pellets with a theoretical density of 97% or better. The thermoelectric properties (electrical resistivity and Seebeck coefficient) of the samples prepared by the PARS technique were measured. The room temperature electrical resistivity and Seebeck coefficient of γ‐LaSy varied in a systematic manner with the electron concentration indicating that γ‐LaSy alloys behave like a simple metal. Although the current carriers appear to be quite heavy (drift mobility ∼3 cm2/V s), the temperature dependence (up to 1000 °C) of these properties of LaS1.42 were found to...

Thermoelectric properties of lanthanum sulfide with Sm, Eu, and Yb additives

The Seebeck coefficient and electrical resistivity have been measured for the La3−x Mx S4 alloys with the Th3 P4 ‐type structure for M=Sm, Eu, Yb, and x=0.1 to 0.9 in order to determine their potential as high‐temperature thermoelectric energy conversion materials. An analysis of the lattice parameters of these alloys suggests that Sm has a valence of 2.2 over the entire composition range. Furthermore, the variation of the Seebeck coefficient and electrical resistivity as a function of electron concentration is different for the Sm‐doped alloys than for the Eu‐ and Yb‐doped alloys, which are all different from that of the pure binary LaSy alloys. A maximum in the electrical power factor at 1000 °C is found to occur at x ≂ 0.3, but when a reasonable estimate is made of the thermal conductivity the maximum in the figure‐of‐merit at 1000 °C shifts to larger x values (∼0.7) in all alloy systems. The long term stability of the Sm and Eu ternaries is also reported.

Structure of LaNi2.286 and the LaNi system from LaNi1.75 to LaNi2.50

Abstract Seven alloys near the LaNi2 composition were studied by metallographie and X-ray techniques. Alloys that were cooled slowly were found to consist of two phases: from x = 1.75 to 2.25 and from x = 2.40 to 2.50, where x is the Ni:La ratio. A single-phase alloy was found near x = 2.30. A complete single-crystal structure analysis showed that the true composition of this tetragonal phase ( a = 7.355 A and c = 14.51 A ) is LaNi2.286. The space group is I42m and the unit cell contains 32 nickel atoms and 14 lanthanum atoms. This tetragonal structure is related to the cubic C15 Laves phase such that at ≈ ac and ct ≈ 2ac and two lanthanum atoms are removed from the tetragonal structure. The formation of this phase is thought to result from the large disparity between the lanthanum and nickel radii ( r La r Ni = 1.506 ) relative to that expected for the ideal constituent atoms of the AB2 cubic Laves phase ( r A r B = 1.225 ), which prevents the formation of the ideal C15 Laves phase.

Thermophysical properties of the lanthanide sesquisulfides. I. Schottky functions and magnetic and electronic properties of γ‐La2S3, γ‐Ce2S3, γ‐Nd2S3, and γ‐Gd2S3

Heat capacity measurements of four lanthanide sesquisulfides La2S3, Ce2S3, Nd2S3, and Gd2S3, prepared in the γ phase, have been obtained between 6 and 350 K by adiabatic calorimetry. The total heat capacity has been resolved into lattice, electronic, magnetic, and Schottky components. The Schottky contributions agree well with the calculated values based on the observed splitting of the ground‐state manifold of the rare earth ions occupying sites of S4 symmetry in the Th3P4 structure. The observed splitting is obtained from an analysis of the hot bands in the absorption spectrum and from direct observation of the Stark levels in the far infrared. The Stark levels (all doublets) for Ce2S3 (2F5/2) are 0, 185, and 358 cm−1; for Nd2S3(4I9/2), they are 0, 76, 150, 180, and 385 cm−1. For La2S3, which has no Schottky or magnetic contributions to the heat capacity, the thermal data can be extrapolated to 0 K. The entropy for La2S3 at 298.15 K (as S0/R) is 19.51. Schottky and magnetic ordering at lower temperature...

High-temperature heat contents, thermal diffusivities, densities, and thermal conductivities of n-type SiGe(GaP), p-type SiGe(GaP), and p-type SiGe alloys

The high‐temperature heat contents of n‐type SiGe(GaP), p‐type SiGe(GaP), and p‐type SiGe alloys were measured from 400 to 1400 K by using a copper block drop calorimeter. The heat capacity and related thermodynamic functions were calculated. Smoothed values of H0T−H0298.15, Cp, S0T−S0298.15, and −(F0T−H0298.15)T−1 were tabulated at 100‐K intervals. Thermal diffusivities were measured by using a laser technique. Thermal conductivities were calculated from the heat capacity, thermal diffusivity, and density.

Magnetic Properties of Gd–Sc Alloys

Magnetic properties of some Gd–Sc alloys have been measured. Paramagnetic to antiferromagnetic to ferromagnetic transitions were observed in the 69 at. % Gd alloy. Alloys of higher Gd content were ferromagnets and those of lower Gd content were antiferromagnets. The paramagnetic Curie temperatures decreased with decreasing Gd concentration. The effective number of Bohr magnetons per Gd atom showed a nearly linear increase with decreasing Gd concentration. Also the saturation magnetic moment per Gd atom increased with decreasing Gd concentration in the case of the ferromagnets.

Preparation of the metastable high pressure γ-R2S3 phase (REr, Tm, Yb and Lu) by mechanical milling

The preparation of the metastable crystalline high pressure polymorphs of R 2 S 3 , where R≡Er, Tm, Yb and Lu, was investigated at room temperature by mechanical milling (MM). For Er 2 S 3 and Yb 2 S 3 the pure metastable high pressure γ-phases were obtained by MM whereas for the Tm 2 S 3 and Lu 2 S 3 samples the metastable high pressure γ-phases coexisted with the corresponding equilibrium ambient polymorphic phase.

The ultrapurification of rare earth metals: Gadolinium and neodymium

Abstract Purification using zone refining followed by solid state electrotransport has been assessed for two moderate vapour pressure rare earth metals, gadolinium and neodymium. The highest purity attained for gadolinium was 99.94 at.% and for neodymium was 99.97 at.% with respect to all impurities. It is concluded that by following similar practices and scales of operation to those described, it may be possible to refine similar rare earth metals to 99.99 at.% overall purity. However, to achieve even higher purities changes in procedures, aimed at increasing the effectiveness of both techniques, would be required.

Parasitic effects of oxygen on the thermoelectric properties of Si80Ge20 doped with GaP and P

The presence of oxygen in silicon‐germanium can have a significant effect on the sintering, microstructural, and transport properties of these alloys. A neutron activation study was performed to follow the total oxygen content during the preparation of mechanically alloyed n‐type Si‐20 at. % Ge alloys using either fine powders, polycrystalline Czochralski chunk, or single‐crystal float zoned starting materials. The alloys were prepared with a nominal composition of Si0.747Ge0.187(GaP)0.016P0.034. The total oxygen concentration was measured in the starting materials, after 6 h of mechanical alloying in a helium environment, after hot pressing, and after a 3 h, 1100 °C heat treatment in fused silica ampoules. Alloys that contained oxygen levels in excess of 2.0 at. % exhibited low carrier mobility and low thermal conductivity, whereas those that contained less than 0.6 at. % oxygen possessed high mobility and thermal conductivity. The microstructure, as observed by optical metallography and scanning electro...

Effects of vial shape on the rate of mechanical alloying in Si80Ge20

The rate of mechanical alloying for doped Si80Ge20 alloys was studied using flat and concave ended hardened tool steel vials. Alloying was found to occur at significantly higher rates using a flat ended vial. It was also found by X-ray diffraction that the homogeneity of the alloy remained constant between 9 and 30 h for the flat ended vial and between 1.5 and 30 h for the round ended vial.