Nanostructuring, compositional fluctuations, and atomic ordering in the thermoelectric materials AgPb(m)SbTe(2+m). The myth of solid solutions

The nature of the thermoelectric materials Ag(1-x)Pb(m)SbTe(m+2) or LAST-m materials (LAST for Lead Antimony Silver Tellurium) with different m values at the atomic as well as nanoscale was studied with powder/single-crystal X-ray diffraction, electron diffraction, and high-resolution transmission electron microscopy. Powder diffraction patterns of different members (m = 0, 6, 12, 18, infinity) are consistent with pure phases crystallizing in the NaCl-structure-type (Fmm) and the proposition that the LAST family behaved as solid solutions between the PbTe and AgSbTe2 compounds. However, electron diffraction and high resolution transmission electron microscopy studies suggest the LAST phases are inhomogeneous at the nanoscale with at least two coexisting sets of well-defined phases. The minority phase which is richer in Ag and Sb is on the nanosized length scale, and it is endotaxially embedded in the majority phase which is poorer in Ag and Sb. Moreover, within each nanodomain we observe extensive long range ordering of Ag, Pb, and Sb atoms. The long range ordering can be confirmed by single crystal X-ray diffraction studies. Indeed, data collections of five different single crystals were successfully refined in space groups of lower symmetry than Fmm including P4/mmm and Rm. The results reported here provide experimental evidence for a conceptual basis that could be employed when designing high performance thermoelectric materials and dispel the decades long belief that the systems (AgSbTe2)(1-x)(PbTe)x are solid solutions.     

Crystallization kinetics of amorphous

The active sites of hydrogen-exchanged Y zeolite (HY) and dealuminated (HDY) zeolites are investigated by TPD of carbon monoxide. Only the high temperature TPD spectra of CO (TM»620-690°C) were observed, meaning that CO molecules interact with very strong acid sites. The amounts of CO bonded on these sites are small (less than 1 molecule per unit cell). The strong influence of dealumination on the coverage degree is found. The calculated values for kinetic parameters indicate chemisorption of CO in the investigated systems (Edes»240 kJ mol^-1, A»1011 s^-1).     

A combined characterization of zinc hot-dip galvanized wireswith DSC, XRD and SEM

The effect of Pb and Sn on the structure of zinc hot-dip galvanized coatings on steel wires is examined. The coating quality is often low because of the Sandelin effect. An improvement could be expected if 1.0 mass% Pb is added to the Zn bath. In this case the η phase formation is enhanced, while the coating thickness is reduced. The Sn addition at the same concentration promotes the formation of the η phase but it does not affect the thickness.     

Inducing bioactivity in dental porcelain through bioglass®

Dental materials restore morphology and function of lost or destroyed teeth, but cannot completely rebuild the structural relationship with soft periodontal tissues. The induction of bioactivity on classic dental porcelain can be achieved through the addition of bioactive glass. The aim of this study was to investigate the effect of Bioglass^® on the thermal properties of dental porcelain in order to correlate the proportions of mixtures with the changes in thermal properties. Differential thermal analysis was performed in order to determine the characteristic temperatures of the mixtures. The increase of bioactive glass concentration in mixtures induces a shift to lower temperatures of the melting point temperature. This observation is attributed to the substitution of silicon ions by aluminium ions and to the formation of Al–O bond, which is weaker than Si–O bond. Mixtures heated at 950°C were examined also by the transmission electron microscopy (TEM) in order to be studied the microstructure of these samples at this critical temperature. The observed microstructural changes, confirm the process of substitution of Si ⁴⁺ ions by Al ³⁺ ions.     

A differential scanning calorimetric study on phase transformations of HgI2

The influence of stoichiometry on the mercuric iodide to transformation temperature was studied in a limited region around the exact stoichiometry. It is concluded that although this influence is small, the transformation temperature has its highest value at the exact stoichiometry composition.

---

Zusammenfassung In einem begrenzten Intervall in der Nähe der exakten Stöchiometrie von Quecksilberjodid wurde der Einfluß der Stöchiometrie auf die/ß Umwandlungstemperatur untersucht. Es konnte festgestellt werden, daß dieser Einfluß zwar gering ist, die Umwandlungstemperatur ihren größten Wert jedoch bei der exakten Stöchiometrie erreicht.

     

Incorporation of group III,IV and V elements in 3C–SiC(1 1 1) layers grown by the vapour–liquid–solid mechanism

We report on a comparative investigation of the incorporation of group III, IV and V impurities in 3C–SiC heteroepitaxial layers grown by the vapour–liquid–solid (VLS) mechanism on on-axis α-SiC substrates. To this end, various Si-based melts have been used with addition of Al, Ga, Ge and Sn species. Homoepitaxial α-SiC layers grown using Al-based melts were used for comparison purposed for Al incorporation. Nitrogen incorporation depth profile systematically displays an overshoot at the substrate/epilayer interface for all the layers. Ga and Al incorporations follow the same distribution shape as N whereas this is not the case for the isoelectronic impurities Ge and Sn. This suggests some interaction between Ga/Al and N coming from the high bonding energy between the group III and V elements, which does not exist with Ge and Sn. This is why both incorporate as a cluster. A model of incorporation is proposed taking into account metal-N and metal-C bonding energies together with the solid solubility of the corresponding nitrides.     

On the Surface Quality and Micromorphology of HgBrI Single Crystals

Mercuric Bromoiodide (HgBrI) is the only compound in the HgI₂—HgBr₂ system of solid solutions and it is a candidate material for solid state nuclear radiation detector applications, operating at room temperature. In the present paper we report on the surface morphology of the crystals, grown from the vapour phase, using optical microscopy and optical spectroscopy. This examination aims at the determination of the factors that influence the surface quality of the crystals.     

On the comparative study of three silver coins of the IIIrd century B.C. minted in Korkyra, Dyrrachion and by the Illyrian king Monounios

The microstructure of three silver coins dating back to the IIIrdcentury B.C. from the Illyrian King Monounios, the ancient Greek city of Dyrrachion and the ancient Greek city of Korkyra was studied with XRF, PIXE, microhardness measurements, and light and electron microscopy. From this investigation it turned out that these coins have different chemical compositions and different microstructures. The as-collected data allow the determination of a possible minting method. The Korkyra coin was probably hot-worked or cold-worked and annealed, while after cooling, it was cold-worked again. The coin of King Monounios is composed by equi-axed grains which are overrun by slip lines. Consequently it is likely that this coin was casted and then cold-struck. Finally the coin from Dyrrachion is characterized by small-sized grains and strain lines and as a result this coin was probably cold-struck. PACS 07.60.Pb; 78.70.En; 68.37.Ef; Hk; Lp; Nq; 61.66.Bi; 61.72.Mm     

Phase transformation in the system TlBi(Te1‐xSex)2

TlBiTe₂ and TlBiSe₂ ‐ that are ternary analogs of the IV‐VI semiconductors ‐, although they crystallize in the same space group R m (D⁵_3d), exhibit different behaviour during heating. The observed phase transformation depends on Se content (x) in the system TlBi(Te_1‐xSe_x)₂ and the transformation disappears by increasing Se content after a certain value (x = 0.25). This dependence is examined through the analysis of the DSC non‐isothermal measurements and an attempt for the explanation of the observed behaviour is undertaken through the consideration of off‐center atoms. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)