Structural Characterization and Thermoelectric Properties of Hot-Pressed CoSi Nanocomposites

Fabrication of nanocomposites by introduction of SiO₂ metal oxide nanoparticles into a cobalt silicide thermoelectric matrix is studied. The CoSi matrix material was prepared through solid-state synthesis, and the nano-SiO₂ metal oxide was introduced by mechanical grinding. The mixed powders were hot pressed to fabricate nanocomposites. The structural and morphological modifications were studied by powder x-ray diffraction analysis and scanning electron microscopy. The thermoelectric properties of the materials were followed through the Hall effect, Seebeck coefficient, and electrical and thermal conductivities in the temperature range from 300 K to 1000 K.     

Properties of SiO2 grown on Ti,Co, Ni,Pd, and Pt silicides

Successful utilization of silicides for VLSI applcations depends strongly on the formation of electrically insulating oxide on top of the silicide (1) . It is found that almost all silicides on a Si substrate can be oxidized to form an SiO₂ layer on their surface. In this paper, we present some of the properties of such SiO₂ layers formed on TiSi₂, CoSi₂, NiSi₂, Pd₂Si, and PtSi on a <Si> substrate following dry and wet oxidation. Electrical parameters that were investigated are the dielectric constant, dielectric strength (breakdown field), and pinhole density. The dielectric constant was found to be 3.49 ± 0.24, which is similar to the values reported for SiO₂ grown on Si. The dielectric strength of the oxide layers depends on the polarity of the applied voltage, as is the case for oxide grown on poly-Si. Pinhole density in this oxide was also estimated and is less than 40 per cm². The oxide density and stoichiometry were evaluated using Rutherford Backscattering Spectrometry (RBS) and DEKTAK, and compared to SiO₂ grown on <Si>. The conclusion we have reached is that oxides grown on almost all the silicides investigated (except PdSi), hold promise for integrated circuit application. The main problem is the suicide roughness, induced by the thermal oxidation, that reduces the dielectric breakdown field.     

Thermoelectric Properties of Cobalt Monosilicide and Its Alloys

Semiconductors - Thermoelectric properties of cobalt monosilicide CoSi and (Co1 –xMxSi, M = Fe, Ni) alloys are studied. Alloy compositions with an iron content of up to 10 at % and nickel...     

Electronic Structure and Thermoelectric Properties of Al-Mn-Fe-Si Alloys

In order to develop practical thermoelectric materials consisting solely of environmentally friendly elements, we investigated the thermoelectric properties of the Al₁₀Mn₃-type (P6₃/mmc, hp26) Al_77−x Mn₂₃Si _x alloys and the Al₁₀₂Mn₂₄Si₁₂-type (Pm-3, cP138) Al_82−x Mn_5.5Fe_12.5Si _x alloys, both of which possess a pseudogap at the Fermi level. The formation range in which the single phase is obtained was determined for these two phases. The electrical resistivity, Seebeck coefficient, and thermal conductivity of the samples involving no secondary phase were measured over the temperature range of 2 K to 300 K. It is found that the thermoelectric properties of these phases are qualitatively accounted for in terms of the pseudogap at the Fermi level in the electronic density of states and the disordering in local atomic arrangements.     

Thermoelectric Properties of (Pb,Sn,Ge)Te-Based Alloys

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 [(PbTe)(SnTe)(Bi₂Te₃)] _x (GeTe)_1−x alloys for thermoelectric applications using spark plasma sintering (SPS). Varying the carrier concentration of GeTe was achieved by alloying of PbTe, SnTe, and/or Bi₂Te₃. The rhombohedral to cubic phase transition temperature, T _c, was found to be sensitive to the degree of alloying. Highest power factor values (P ≤ 33 μW/cm K²) were obtained for (GeTe)_0.95(Bi₂Te₃)_0.05 composition.     

Influence of Sedimentation of Atoms on Structural and Thermoelectric Properties of Bi-Sb Alloys

Functionally graded thermoelectric materials (FGTMs) have been prepared by sedimentation of atoms under a strong gravitational field. Starting samples of Bi _x Sb_1?x alloys with different composition x were synthesized by melting of metals and subsequent annealing of quenched samples. The thermoelectric properties (Seebeck coefficient, electrical conductivity) of the starting materials were characterized over the temperature range from 300 K to 525 K. Strong gravity experiments were performed in a unique ultracentrifuge apparatus under acceleration of over 0.5 × 10⁶ G at temperatures of 538 K and 623 K. Changes of the microstructure and chemical composition were analyzed using scanning electron microscopy with energy-dispersive x-ray spectroscopy analysis. The distribution of the Seebeck coefficient of the Bi-Sb alloys was characterized by scanning thermoelectric microprobe. As a result of sedimentation, large changes in chemical composition (x = 0.45 to 1) were obtained. It was found that the changes in chemical composition were correlated with alterations of the Seebeck coefficient. The obtained experimental data allowed the development of a semiempirical model for the selection of optimal processing parameters for preparation of Bi-Sb alloys with required thermoelectric properties.     

Effect of Processing Route on the Microstructure and Thermoelectric Properties of Bismuth Telluride-Based Alloys

Considerable work has been done to engineer materials with high efficiencies of thermoelectric heat-to-electricity conversion and the mechanical strength necessary to withstand the demands of practical applications. In particular, in the bismuth telluride system, extrusion pressing has been found to be effective for improving the mechanical strength of alloys via grain refinement. We review some of the literature relating to processing approaches for the bismuth telluride system. We also present preliminary data for a series of samples obtained by incorporating C₆₀ via ball milling and spark plasma sintering into a matrix consisting of a (Bi,Sb)₂Te₃ alloy, with a focus on the texture of the composites and its relation to thermoelectric transport properties, in comparison to the parent material. The viability of improving the thermoelectric performance of bismuth telluride alloys by the insertion of nanoparticles into a composite is also considered.     

Ni,Pd, or Pt as contact materials for GaSb and InSb semiconductors: Phase diagrams

As processing, and even use, of semiconductor devices usually includes the exposure to elevated temperatures, interface reactions often occur, especially during metallization and further heat treatment. It is thus important to understand the corresponding phase equilibria of the involved elements. We present here the phase diagrams of Ni, Pd, and Pt with GaSb and InSb; experimental results in the systems Ga-M-Sb and In-M-Sb (M = Ni, Pd, Pt) are summarized and are discussed in the context of contact chemistry. For GaSb and InSb, it is found that, from a thermodynamic point of view, binary and ternary compounds in equilibrium with the corresponding semiconductor would be the best choice for contact materials as these contacts will remain stable even after long exposure to elevated temperatures.     

Thermoelectric Properties and n- to p-Type Conversion of Co-Doped ZrNiSn-Based Half-Heusler Alloys

Half-Heulser thermoelectric materials ZrNi_1?y Co _y Sn (y?=?0, 0.02, 0.04, 0.08, 0.12) were prepared by a time-efficient levitation melting and spark plasma sintering procedure. X-ray diffraction analysis and electron probe microanalysis showed that single-phase half-Heusler compounds without compositional segregation have been obtained. The effects of Co doping on the electrical conductivity, Seebeck coefficient, and thermal conductivity of ZrNiSn-based half-Heusler alloys have been investigated from 300?K to 900?K. The Seebeck coefficient displayed a change from negative to positive values above nominal Co doping content of y?=?0.02, indicating a transition in the conduction behavior from n-type to p-type. The maximum dimensionless figure of merit ZT of undoped ZrNiSn sample reached 0.5 at 870?K.     

Effect of Nanostructuring on the Thermoelectric Properties of Co0.97Pd0.03Sb3

We synthesized n-type CeO₂/Co_0.97Pd_0.03Sb₃ composites with nanometric grain sizes (200 nm to 300 nm) by spark plasma sintering in order to promote phonon scattering at grain boundaries. Powdered samples were initially obtained by ball milling Co_0.97Pd_0.03Sb₃ together with x vol.% (x = 0, 0.5, 1, 2) of CeO₂ nanoparticles. This additive slows down the grain size growth of the skutterudite matrix which occurs during sintering, thereby contributing to phonon scattering. The nanostructured samples display reduced Hall electron concentration compared with that of the reference Co_0.97Pd_0.03Sb₃ because of Fe contamination by the steel balls and vials. However, the electronic transport properties are nearly identical to those of Co_0.98Pd_0.02Sb₃, which allows for comparison with this latter compound. The lattice thermal conductivity is strongly decreased in nano-Co_0.97Pd_0.03Sb₁₂ (?40% at 300 K). This results in an enhanced (+32%) ZT value peaking at 0.65 at 650 K in nano-Co_0.97Pd_0.03Sb₁₂ + 2% CeO₂ when compared with micro-Co_0.98Pd_0.02Sb₃.     

Effect of Ce-Doping on Thermoelectric Properties in PbTe Alloys Prepared by Spark Plasma Sintering

Ce-doped Pb_1−x Ce _x Te alloys with x = 0, 0.005, 0.01, 0.015, 0.03, and 0.05 were prepared by induction melting, ball milling, and spark plasma sintering techniques. The structure and thermoelectric properties of the samples were investigated. X-ray diffraction (XRD) analysis indicated that the samples were of single phase with NaCl-type structure for x less than 0.03. The lattice parameter a increases with increasing Ce content. The lower Ce-doped samples (x = 0.005 and 0.01) showed p-type conduction, whereas the pure PbTe and the higher doped samples (x = 0, 0.015, 0.03, and 0.05) showed n-type conduction. The lower Ce-doped samples exhibited a much higher absolute Seebeck coefficient, but the higher electrical resistivity and higher thermal conductivity compared with pure PbTe resulted in a lower figure of merit ZT. In contrast, the higher Ce-doped samples exhibited a lower electrical resistivity, together with a lower absolute Seebeck coefficient and comparable thermal conductivity, leading to ZT comparable to that of PbTe. The lowest thermal conductivity (range from 0.99 W m⁻¹ K⁻¹ at 300 K to 0.696 W m⁻¹ K⁻¹ at 473 K) was found in the alloy Pb_0.95Ce_0.05Te due to the presence of the secondary phases, leading to a ZT higher than that of pure PbTe above 500 K. The maximum figure of merit ZT, in the alloy Pb_0.95Ce_0.05Te, was 0.88 at 673 K.     

Low-Temperature Physical and Thermoelectric Properties of Ba8Ni5Ge41

Resistivity, Hall resistivity, thermopower, thermal conductivity, and magnetization are reported for polycrystalline Ba₈Ni₅Ge₄₁. Ba₈Ni₅Ge₄₁ is diamagnetic with susceptibility χ _dia = (?2.4 to ?2.82) × 10^?7 emu/g. Semiconductor-like behavior was observed for the resistivity. The thermopower shows positive values for a wide temperature range. The Hall resistivity indicates the dominance of electrons, suggesting the existence of multiband conductance. At room temperature, the thermal conductivity is 1.78(5) W/K m. The highest ZT of Ba₈Ni₅Ge₄₁ is 0.0016 at about 278 K.     

Nanograin Effects on the Thermoelectric Properties of Poly-Si Nanowires

In this work we perform a theoretical analysis of the thermoelectric performance of polycrystalline Si nanowires (NWs) by considering both electron and phonon transport. The simulations are calibrated with experimental data from monocrystalline and polycrystalline structures. We show that heavily doped polycrystalline NW structures with grain size below 100 nm might offer an alternative approach to achieve simultaneous thermal conductivity reduction and power factor improvements through improvements in the Seebeck coefficient. We find that deviations from the homogeneity of the channel and/or reduction in the diameter may provide strong reduction in the thermal conductivity. Interestingly, our calculations show that the Seebeck coefficient and consequently the power factor can be improved significantly once the polycrystalline geometry is properly optimized, while avoiding strong reduction in the electrical conductivity. In such a way, ZT values even higher than the ones reported for monocrystalline Si NWs can be achieved.     

Improved Thermoelectric Properties of p-Type Bismuth Antimony-Based Alloys Prepared by Spark Plasma Sintering

Bi₈₅Sb_15?x Pb _x (x = 0, 0.5, 1, 2, 3) alloys have been prepared by the mechanical alloying–spark plasma sintering (MA-SPS) method. X-ray diffraction and scanning electron microscopy were used to characterize the microstructure of the alloys. The effect of Pb content on the thermoelectric properties was investigated in the temperature range 77–300 K. The results showed that the electrical transport properties of the Bi–Sb alloys changed from n-type to p-type with substitution of Sb by Pb. The maximum power factor reached 1.6 × 10^?3 W/mK² at 190 K, a significant improvement on values reported elsewhere. This study demonstrated that high-performance p-type thermoelectric Bi–Sb materials can be obtained by spark plasma sintering.     

Properties of Au,Pt, Pd and Rh levels in silicon measured with a constant capacitance technique

Measurements of emission rates and majority carrier capture cross-sections of Au, Pt, Pd and Rh centres in silicon are reported, and the activation energies associated with the different levels of these centres are determined. Where appropriate, our results are compared with values reported in the literature; other results have not been previously reported. The measurement depends on the emission and capture of majority carriers on the centres in the depletion layer of a p-n junction or Schottky barrier. The change in charge state of the centres is monitored by measuring the change in reverse bias applied to the junction necessary to keep the junction capacitance constant. The advantage of this technique, compared with the usual method of keeping the bias voltage constant and measuring the change in capacitance, is demonstrated.     

Interplay between Structural and Thermoelectric Properties in Epitaxial Sb2+xTe3 Alloys

In recent years strain engineering is proposed in chalcogenide superlattices (SLs) to shape in particular the switching functionality for phase change memory applications. This is possible in Sb₂Te₃/GeTe heterostructures leveraging on the peculiar behavior of Sb₂Te₃, in between covalently bonded and weakly bonded materials. In the present study, the structural and thermoelectric (TE) properties of epitaxial Sb_2+xTe₃ films are shown, as they represent an intriguing option to expand the horizon of strain engineering in such SLs. Samples with composition between Sb₂Te₃ and Sb₄Te₃ are prepared by molecular beam epitaxy. A combination of X‐ray diffraction and Raman spectroscopy, together with dedicated simulations, allows unveiling the structural characteristics of the alloys. A consistent evaluation of the structural disorder characterizing the material is drawn as well as the presence of both Sb₂ and Sb₄ slabs is detected. A strong link exists among structural and TE properties, the latter having implications also in phase change SLs. A further improvement of the TE performances may be achieved by accurately engineering the intrinsic disorder. The possibility to tune the strain in designed Sb_2+xTe₃/GeTe SLs by controlling at the nanoscale the 2D character of the Sb_2+xTe₃ alloys is envisioned.     

Reduced Grain Size and Improved Thermoelectric Properties of Melt Spun (Hf,Zr)NiSn Half-Heusler Alloys

Half-Heusler thermoelectric materials Hf(/Zr)NiSn were prepared by levitation melting followed by melt-spinning to refine the boundary structures, and then they were consolidated by spark plasma sintering. X-ray diffraction analysis and scanning electron microscopy showed that single phased half-Heusler compounds without compositional segregation had been obtained. It was found that the thermoelectric properties, especially the thermal conductivity, depended strongly on the boundary structures. The melt-spinning samples with refined boundary structures had a lower thermal conductivity but a power factor comparable to that of the sample prepared by levitation melting, thus providing good thermoelectric properties.     

Effects of Annealing on Microstructure and Thermoelectric Properties of Nanostructured CoSb3

Nanocrystallization of thermoelectric materials is an effective way to reduce their thermal conductivity, but so far the thermal stability of nanostructured thermoelectric materials has been little studied. Effects of annealing treatment on the microstructure and the thermoelectric properties of nanostructured CoSb₃ were investigated in this work. Samples with average grain size of 300 nm were prepared by spark plasma sintering of high-energy ball-milled nanosized CoSb₃ powders. The study shows that annealing has a very significant impact on the grain size of the samples. The grain size of the sample with 100 h annealing is three times greater than before annealing. The major phase in the 150-h-annealed sample is still skutterudite, except for a trace amount of Sb phase. With increasing annealing time, the density reduces slightly. In addition, the power factor of the sample decreases, thus resulting in a decrease of the thermoelectric figure of merit.