Search

The effect of a galia (Ga2O3) addition on the crystallographic phase evolution and the crystal growth behavior of zirconia (ZrO2) rich powder were investigated. Binary compositions of galia-zirconia powder (on the zirconia-rich side of the quasi-binary phase diagram) were obtained by the sol-gel method and calcined at different temperatures for 2 h. Rietveld refinement (FullProf program) was used to determine the crystallographic phase composition. Crystal size was approximated based on X-ray diffractions analysis combined with estimates of average crystal size obtained by processing high-resolution scanning electron microscopy images. The surface area of the powders obtained was measured via the Brunauer-Emmett-Teller method. Zirconia-galia sol-gel powders were found to form tetragonal or mixed tetragonal-monoclinic solid solution phases depending on both composition and calcination temperature. It was shown that galia suppressed crystal growth process in zirconia, producing finer nano-powders with higher surface area compared to pure zirconia. [ABSTRACT FROM AUTHOR]

Thermoelectrics is gaining increased attention as a renewable direct energy conversion method from heat to electricity. The most efficient and up-to-date thermoelectric materials for temperatures of up to 250°C are (BiSb) (TeSe) alloys. In the current research, to discover practical thermoelectric power generation devices capable of operation at such temperatures, BiSb alloy was considered as a lead-free high-temperature (<250°C) solder composition for bonding of n-type BiTeSe and p-type BiSbTe legs into Cu, Ag, Ni and Fe metallic bridges. In the case of Cu, fine contacts with low electrical contact resistance of ∼1.5 ± 0.5 mΩ mm were observed upon soldering at 350°C. [ABSTRACT FROM AUTHOR]

Thermoelectric materials as a means of direct conversion of heat into electricity are attracting increasing attention. The most efficient n-type thermoelectric materials for temperatures up to 300°C are currently BiTeSe-based. To compete with other methods of energy conversion, the efficiency of such materials should be further increased. The possibility of thermoelectric enhancement of such alloys by introduction of the layered structured MoSe phase has been investigated. On introduction of large amounts (up to 15 wt.%) of this phase, no effects of doping were observed, emphasizing the potential of sub-micron homogeneously distributed MoSe in such composites to effectively scatter phonons without any adverse contribution to the optimized electronic properties. [ABSTRACT FROM AUTHOR]

The higher manganese silicides (HMS) are regarded as very attractive p-type thermoelectric materials for direct conversion of heat to electricity. To compete with other thermodynamic engines (e.g. the Stirling and Rankine cycles), however, the thermoelectric figure of merit of such silicides must be improved. HMS follow a complicated solidification reaction on cooling from the melt, which leads to formation of undesired secondary phases. Furthermore, the electronic carrier concentration of HMS is much higher than the optimum for thermoelectric applications and should be compensated by introduction of doping agents. In this research, the electronic donor action associated with substitution of HMS by FeSi was investigated. The effects of excess Si on phase distribution and thermoelectric properties are also discussed in detail. [ABSTRACT FROM AUTHOR]

Cost-effective highly efficient nanostructured n-type MgSiSn and p-type higher manganese silicide (HMS) compositions were prepared for the development of practical waste heat generators for automotive and marine thermoelectric applications, in the frame of the European Commission (EC)-funded PowerDriver project. The physical, mechanical, and structural properties were fully characterized as part of a database-generation exercise required for the thermoelectric converter design. A combination of high maximal ZT values of ∼0.6 and ∼1.1 for the HMS and MgSiSn compositions, respectively, and adequate mechanical properties was obtained. [ABSTRACT FROM AUTHOR]

The present challenges in the energy crisis require finding new ways to reduce consumption of fossil fuels. Thermoelectrics can help reduce fuel consumption by producing electricity from waste heat. The higher manganese silicides (HMS) have shown promise in this field as inexpensive, nontoxic, and highly stable p-type thermoelectric materials. One of the production techniques for HMS is mechanical alloying by ball milling. In this research the effect of the ball-milling duration and speed on the phases produced was studied. Mn and Si powders were milled at speeds of 200 RPM to 800 RPM for 1 h to 7 h. X-ray diffraction (XRD) results of the samples prepared using mechanical alloying show deterioration into the MnSi phase. The sample that underwent 5 h of milling at 800 RPM showed the greatest amount of HMS phase and was subsequently spark plasma sintered. The sample showed insufficient thermoelectric properties ( ZT ≈ 0.1 at 450°C), compared with either solid-state reaction samples showing ZT ≈ 0.4 or cast samples showing ZT ≈ 0.63 at 450°C. The reduced ZT values of the mechanically alloyed and spark-plasma-sintered samples were attributed to the high relative amount of MnSi phase. The correlation between the relative amount of MnSi and the transport properties is described in detail. [ABSTRACT FROM AUTHOR]

Since the 1960s, the TAGS system, namely (GeTe)(AgSbTe), with two specific compositions x = 0.8 and 0.85, known as TAGS-80 and TAGS-85, respectively, was identified as containing highly efficient p-type thermoelectric materials. Recently, another highly efficient p-type GeTe-rich composition, namely 3% BiTe-doped GePbTe, achieving thermoelectric properties comparable to TAGS-based solid solutions, was also reported. Since all of these compositions were obtained by different manufacturing approaches, a comparison between the transport and mechanical properties of these alloys, prepared by the same manufacturing techniques, is required to identify the advantages and disadvantages of these compositions for practical thermoelectric applications. In the current research, the thermoelectric and mechanical properties of three highly efficient GeTe-rich alloys, TAGS-80, TAGS-85, and 3% BiTe-doped GePbTe, following hot pressing, were investigated and compared. Maximal ZT values of ∼1.75, ∼1.4, and ∼1.6 at 500°C were found for these compositions, respectively. Improvement of the mechanical properties was observed by increasing the GeTe content. The influence of the GeTe relative amount on the transport and mechanical properties was interpreted by means of the phase-transition temperatures from the low-temperature rhombohedral to the high-temperature cubic phases. [ABSTRACT FROM AUTHOR]

Thermoelectrics are widely explored as a renewable and environmentally friendly method for converting waste heat to electrical power. Higher manganese silicides (HMS) have been identified as promising, nontoxic, inexpensive, p-type materials for thermoelectric applications. To mass-produce practical thermoelectric converters, an inexpensive, effective, and simple production method should be applied for these materials. In the frame of the current research, HMS have been synthesized using furnace melting followed by powdering and spark plasma sintering. Highly dense samples were obtained and measured for their thermoelectric properties. The samples were also characterized using x-ray diffraction, scanning electron microscopy, and Hall-effect measurements. Homogeneous samples were obtained with small inclusions of Si, reaching a figure of merit of about 0.6 at 450°C. [ABSTRACT FROM AUTHOR]

Half-Heusler (HH) and especially TiNiSn-based alloys have shown high potential as thermoelectric (TE) materials for power generation applications. The reported transport properties show, however, a significant spread of results, due mainly to the difficulty in fabricating single-phase HH samples in these multicomponent and multiphased systems. In particular, little attention has been paid to the influence of the various minority phases on the TE performance of these compounds. A clear understanding of these issues is mandatory for the design of improved and stable TE HH-based composites. This study examines the structural and compositional influence of the residual metallic (Sn) and intermetallic phases (mainly Ti6Sn5 and the Heusler compound TiNi2Sn) on the TE properties of the TiNiSn HH compounds processed by spark plasma sintering. [ABSTRACT FROM AUTHOR]

The search for alternative energy sources is at the forefront of applied research. In this context, thermoelectricity, i.e., direct conversion of thermal into electrical energy, plays an important role, particularly for exploitation of waste heat. Materials for such applications should exhibit thermoelectric potential and mechanical stability. PbTe-based compounds include well-known n-type and p-type compounds for thermoelectric applications in the 50°C to 600°C temperature range. This paper is concerned with the mechanical and transport properties of p-type PbSnTe:Te and PbTe samples, both of which have a hole concentration of ∼1 × 10 cm. The ZT values of PbTe were found to be higher than those of PbSnTe:Te, and they exhibited a maximal value of 0.8 compared with 0.5 for PbSnTe:Te at 450°C. However, the microhardness value of 49 H found for PbSnTe:Te was closer to that of the mechanically stable n-type PbTe (30 H) than to that of PbTe (71 H). Thus, although lower ZT values were obtained, from a mechanical point of view PbSnTe:Te is preferable over PbTe for practical applications. [ABSTRACT FROM AUTHOR]

The search for alternative energy sources is presently at the forefront of applied research. In this context, thermoelectricity for direct energy conversion from thermal to electrical energy plays an important role. This paper is concerned with the development of highly efficient p-type GePbTe alloys for thermoelectric applications, using spark plasma sintering. The carrier concentration of GeTe was varied by alloying of PbTe and/or by BiTe doping. Very high ZT values up to ~1.8 at 500°C were obtained by doping PbGeTe with 3 mol% BiTe. [ABSTRACT FROM AUTHOR]

Pseudoternary (Ge,Sn,Pb)Te compounds display favorable thermoelectric properties. Spinodal decomposition in the quasiternary (Ge,Sn,Pb)Te system is at the origin of a wide solubility gap at low Sn concentrations. The structural evolution of the spinodal decomposition was investigated as a function of aging time at 500°C, using x-ray diffraction, electron microscopy, and scanning electron microscopy. The evolution of the structure at 500°C consists initially of a short diffusion-controlled demixing stage into Pb- and Ge-rich coherent areas, with compositions corresponding to the inflection points of the free-energy curve. The Pb-rich areas adopt configurations associated with the directions of the soft elastic moduli of the cubic compound. Both the Pb- and Ge-rich areas are supersaturated and undergo in a second stage a nucleation and growth process and give rise to a biphased structure with equilibrium compositions corresponding to the boundaries of the miscibility gap. The resulting Pb-rich areas display a relatively stable microstructure suggesting the presence of long-range interactions between the Pb-rich precipitates in the Ge-rich matrix. [ABSTRACT FROM AUTHOR]

The effect of a galia (Ga2O3) addition on the crystallographic phase evolution and the crystal growth behavior of zirconia (ZrO2) rich powder were investigated. Binary compositions of galia-zirconia powder (on the zirconia-rich side of the quasi-binary phase diagram) were obtained by the sol-gel method and calcined at different temperatures for 2 h. Rietveld refinement (FullProf program) was used to determine the crystallographic phase composition. Crystal size was approximated based on X-ray diffractions analysis combined with estimates of average crystal size obtained by processing high-resolution scanning electron microscopy images. The surface area of the powders obtained was measured via the Brunauer-Emmett-Teller method. Zirconia-galia sol-gel powders were found to form tetragonal or mixed tetragonal-monoclinic solid solution phases depending on both composition and calcination temperature. It was shown that galia suppressed crystal growth process in zirconia, producing finer nano-powders with higher surface area compared to pure zirconia. [ABSTRACT FROM AUTHOR]