O. B. Karlsen

Phase relations and crystal structures in the Ag–Ga system

Abstract The Ag–Ga phase diagram and the ζ-Ag2Ga↔ζ′-Ag2Ga transformation have been investigated by use of optical, scanning and transmission electron microscopy combined with X-ray powder diffraction and thermal differential analysis. On the basis of the present study a new equilibrium phase diagram for the Ag–Ga system is proposed. Only two intermetallic phases are found in the Ag–Ga system: ζ′ and ζ. The compositional change of Ga in ζ′ is described by replacing Ga atoms in 1b position in space group P 6 2m with Ag atoms. In alloys having 25–40 wt.% Ga, the high temperature ζ-Ag2Ga is found to decompose into a Ga-rich liquid and solid ζ′-Ag2Ga at 302°C. In Ga-poor alloys (15 wt.%), the high temperature ζ-Ag2Ga is found to decompose into α-Ag and liquid at 611°C and into α-Ag and ζ′-Ag2Ga at 395°C.

Nanoscale inclusions in the phonon glass thermoelectric material Zn4Sb3

We have investigated the thermoelectric material Zn4Sb3 using transmission electron microscopy (TEM). Nanoscale inclusions with a diameter of about 10 nm were observed, constituting on the order of 1% by volume of the material. Studies using energy filtered imaging, electron diffraction, and high-angle annular dark-field STEM indicate that the inclusions consist of Zn. These inclusions are expected to scatter the medium and long-wavelength phonons effectively, thus contributing to phonon glass behavior which results in the exceptionally low thermal conductivity for this thermoelectric material.

Impurity band conduction in the thermoelectric material ZnSb

ZnSb is favourable as a thermoelectric material, from both an environmental and a global resources point of view. Its efficiency can possibly be improved by the reduction of the thermal conductivity through nanostructuring and optimization of doping. These tasks require a better understanding of the material and, in particular, of the interplay between preparation techniques and material properties. We have prepared bulk polycrystalline samples and report on low-temperature electrical transport measurements (6 K to room temperature). The data have been interpreted in terms of hole impurity band conduction: intrinsic acceptor defects creating bands that are conducting when there are also compensating donors. Modelling the transport reveals qualitatively good agreement. Quantitative differences are discussed in terms of the structure of the samples, which has been studied by using x-ray diffraction, a scanning electron microscope and an electron microprobe. The systematics of adding different amounts of Mn and Cr to ZnSb has been studied and has the effect of varying the density of states in impurity bands and varying the hole concentrations.

High temperature Seebeck coefficient and resistance measurement system for thermoelectric materials in the thin disk geometry.

A versatile apparatus to measure the cross-plane Seebeck coefficient and the resistivity of bulk samples shaped as disks or thin plates, over a temperature range of 300 K-620 K with possible extension to higher temperatures, is presented. It is constructed from readily available equipment and instrumentation with parts that are easily manufactured. The Seebeck coefficient is measured over an average region of the sample under steady-state conditions. The sample resistance is measured using a four-point alternating current method and scaled to room temperature measurements with known geometry to calculate resistivity. A variety of sample shapes are supported. Most importantly, the support of the thin disk geometry allows for the very same samples to be used in a laser flash instrument. The design allows for rough vacuum, high vacuum, or purging with inert gases in the sample chamber. Measurements on thermoelectric ZnSb and a Ni reference material are presented.

The ternary system Au-Cr-Sn and the crystal structure of Au1.15Cr2Sn8.85

Abstract The phase relations in the ternary system Au–Cr–Sn have been studied by powder X-ray diffraction, metallography, electron microprobe analysis and thermal analysis. The condensed phases occurring, tie-lines and tie-triangles are presented for an isothermal section of the phase diagram at 300°C. Most of the binary phases show no significant solid solubility in the ternary dimension at 300°C. The present study has not been able to confirm or rule out the existence of the Au4Cr phase. The earlier reported Cr2Sn3 phase is shown to have the composition CrSn2. A new, genuine ternary phase, Au1.15Cr2Sn8.85 (Au0.540CrSn4.460–Au0.690CrSn4.310), has been uncovered. Its unit cell is tetragonal (space group P4/mcc) with a=687.7(1) and c=566.0(2) pm at room temperature, and the crystal structure is determined by direct methods from single crystal data (150(2) K). There is a random distribution of 58% Au and 42% Sn in the 2b position, Cr holds the 2c position and Sn the 8m position with x=0.1774(1) and y=0.3763(1). The Au1.15Cr2Sn8.85 structure is related to the AlCu2- and CoNb4Si-type structures and its atomic arrangement is discussed in relation to these prototypes. Au1.15Cr2Sn8.85 exhibits weak (Pauli-like) paramagnetism with a slight temperature dependence (5–300 K).