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NaxCoO2 has a particularly high contact resistance because it forms an insulated layer of NaHCO3 and Na2CO3, which are produced in a chemical reaction with carbon dioxide and water in air on the surface. In this study, we tried to improve the interface resistance between NaxCoO2 and Ag sheet electrodes by connecting these materials with the spark plasma sintering (SPS) technique. The interface resistance between NaxCoO2 and Ag sheet electrodes connected by SPS is compared with that connected with Ag paste. In an experiment, the interface resistance of a sample treated by decrease to less than 1/600 of the former value. It is thought that the NaHCO3 and Na2CO3 insulated layer is decomposed through the application of a large value of applied DC current by using the SPS technique.
Literature data of the Mn-Si system is analyzed and discordances are pointed out. First principles calculations are performed to clarify the enthalpies of formation of the intermetallic phases. Especially the enthalpies of formation of the various possible structures of the MnSix are discussed. On the basis of these new data, a thermodynamic description of the Gibbs energy of the phases is performed using the Calphad method. The system Ge-Mn is also assessed using the Calphad method for the first time.
The mixing enthalpy in the D88 solid solution is calculated between Mn5Ge3 and Mn5Si3 by DFT calculations.
Finally a thermodynamic description of the ternary system is suggested. Especially the solubility of germanium in MnSix is modeled.
The germanium-manganese system has been experimentally studied but no Calphad description is available yet. After a critical review of the literature concerning the phase diagram and the thermodynamic properties, a thermodynamic description of the Gibbs energy of the phases is performed using the Calphad method. The liquid phase is described with an associated model and the variation to the stoichiometry of the solid phases is taken into account.
Thermoelectric energy recovery is an important technology for recovering waste thermal energy in high-temperature industrial, transportation and military energy systems. Thermoelectric (TE) power systems in these applications require high performance hot-side and cold-side heat exchangers to provide the critical temperature differential and transfer the required thermal energy to create the power output. Hot-side and cold-side heat exchanger performance is typically characterized by hot-side and cold-side thermal resistances, Rh,th and Rc,th, respectively. Heat exchanger performance determines the hot-side temperature, Th, and cold-side temperature, Tc, conditions when operating in energy recovery environments with available temperature differentials characterized by exhaust temperatures, Texh, and ambient temperature, Tamb. This work analytically defined a crucially important design relationship between (P/Pmax) and (Rh,th / Rc,th) in TE power generation systems to determine the optimum ratio of (Rh,th / Rc,th) maximizing TE system power. A sophisticated integrated TE device / heat exchanger analysis was used, which simultaneously integrates hot- and cold-side heat exchanger models with TE device optimization models incorporating temperature-dependent TE material properties for p-type and n-type materials, thermal and electrical contact resistances, and hot side and cold side heat loss factors. This work examined the (P/Pmax) - (Rh,th / Rc,th) relationship for system designs employing single-material and segmented-material TE couple legs with various TE material combinations, including bismuth telluride alloys, skutterudite compounds, and skutterudite / bismuth telluride segmented combinations. This work defined the non-dimensional functional relationships and found the optimum thermal resistance condition: (Rh,th / Rc,th)opt > 10 to 30 created the maximum power output in TE optimized designs for various TE material combinations investigated. The non-dimensional relationships were investigated for various electrical contact resistances, differing thermal loss factors, and at various hot-side/cold-side temperature conditions. This work showed that the non-dimensional functional relationships were invariant under these differing conditions. It was determined that a condition of (Rh,th / Rc,th) = 1 creates power output far below maximum power conditions. The (P/Pmax) - (Rh,th / Rc,th) relationship also dictated certain temperature profile conditions, defined by the parameter, (Th – Tc) / (Texh – Tamb), which were directly associated with design points in this relationship including maximum power points. The value of (Th – Tc) / (Texh – Tamb) was generally less than 0.5 at maximum power conditions in TE energy recovery designs using TE materials investigated here. The wide-ranging ramifications on TE energy recovery systems and their design optimization for industrial and transportation-related applications are discussed.
In the present work, a comparative study is attempted, dealing with the influence of the grain size distribution on the microstructure and the free carrier concentration in Mg2SnXSi1-X (x=0.2) ternary compounds doped with Sb. Structural in-homogeneities were monitored by using Transmission Electron Microscopy (TEM), Scanning Electron Microscopy (SEM) as well as Fourier transform infrared spectroscopy (FTIR) in the reflectivity mode.
We present an improved AC (Alternating Current) method for the determination of the Thomson coefficient, which can be used for obtaining the absolute Seebeck coefficient. While previous work has focused on DC (Direct Current) methods, we analyze the influence of an AC current in order to derive the Thomson coefficient of a thin wire from measurable quantities. Our expression requires five parameters including AC current, resistance, temperature gradient, and the temperature changes due to the Thomson and Joule effects. Thus, a prior determination of thermal conductivity and sample geometry is not required, unlike DC methods. In order to validate our analysis, the Thomson coefficient of a thin Pt wire has been measured at several frequencies. The results agree with those obtained from a conventional DC method.
A RF magnetron sputtering method was used to prepare Mg2Si films at 300-400oC on (001) Al2O3 substrates from a Mg disc target with Si chips. Mg deposition was not detected at 400°C from a pure Mg disc target without Si chips due to the high vapor pressure of Mg. However, the amount of Mg deposition increased with the increase in Si/(Mg+Si) area ratio of the target surface together with the increase of the Si deposition. The obtained films had a stoichiometric composition of Si/(Mg+Si)=0.33 that consisted of the well crystalline Mg2Si single phase regardless of Si/(Mg+Si) area ratio of the target surface. This showed the existence of a “process window” against supply ratio of Si/(Mg+Si) for Mg2Si single phase films with a stoichiometric composition. This is considered to be due to the vaporization of the excess Mg prepared under the Mg excess condition as reported by Mahan et al. for Mg2Si films prepared at 200°C by ultra-high vacuum evaporation.
Mg-Si thin films were systematically studied using combinatorial approach by co-sputtering with Mg and Si targets. Single phase of Mg2Si appeared around the stoichiometric composition region, and in Mg-rich region (Mg/Si>4) Mg2Si and Mg phases coexisted. The transition of electrical conduction type from n-type to p-type occurred near the stoichiometric composition region where the strongest peak of Mg2Si appeared in the XRD patterns and the Raman scattering spectra. The p-type conduction was observed in Mg-poor region near the stoichiometric composition region. The results of first principle calculation suggest that Mg vacancy may cause p-type conduction.
Dimagnesium silicide (Mg2Si) is an eco-friendly material useful for thermoelectric generation using waste heat of temperature range of 600 to 900 K. To improve the thermoelectric performance of the Mg2Si compound, we made the Al-added compounds under magnesium-rich condition (with 67.0 at% of Mg) using a liquid-solid phase reaction and using a pulse discharge sintering. The thermoelectric performance of each sample containing Al of 0 to 2.0 at% was measured during 50 h air exposure with temperature difference. The temperature difference was given by contacting the hot side of a sample with a hot plate kept at 773 K and by contacting the cold side with a heat sink with cooling fan. The electrical resistivity of the Al-free sample increased with air exposure time by internal oxidation. All the Al-added samples kept the low resistivity during the air exposure test. We confirmed the resistance to deterioration in thermoelectric performance of the Al-added samples during air exposure with temperature difference.
Recently there have been reports of hot carrier thermoelectric response in nanostructured materials like graphene and MoS. We report observing that thermoelectric nanowire junctions detect light. In these experiments we employed devices composed of bismuth nanowire arrays which are capped with a transparent indium tin oxide electrode. The incident surface features very low optical reflectivity and enhanced light trapping. The unique attributes of the thermoelectric arrays are the combination of strong temporal and optical wavelength dependences of the photocurrent. Under infrared illumination, the signal can be completely described by “quasi-equilibrium” thermoelectric effects considering cooling rates given by heat diffusion through the array. The thermal diffusivity is found to be less (by a factor of 3.5) than in the bulk, a result that we discuss in terms of phonon confinement effects. In addition to a thermoelectric response, under visible illumination, we observe a photovoltaic response.