Thermoelectric and mechanical properties of melt spun and spark plasma sintered n-type Yb- and Ba-filled skutterudites

Here we present thermoelectric and mechanical properties of n-type filled-skutterudites produced by a combination of melt spinning of pre-melted charges with subsequent consolidation by spark plasma sintering, a process we refer to as MS-SPS. This combination of processing steps leads to phase-pure n-type filled-skutterudites and obviates more energy and time intensive annealing steps. We show that both the thermoelectric properties and the tensile fracture strength compare favorably to materials made by traditional methods. The process is scalable to at least 80 g billets, such that the transport properties measured on test bars harvested from these larger billets compare favorably to those measured on lab-scale billets (5 g total billet mass). ZT values approaching 1.1 at 750 K were observed in materials made by MS-SPS. In addition, the tensile fracture strength of test bars cut from an 80 g billet is ∼128 MPa at room temperature and decreases with increasing temperature. Fractography of the test bars reveals that the majority failed due to surface and edge flaws with few failures due to volume type flaws. This indicates that the powder metallurgical methods employed to produce these samples is mature.     

Focused ion beam processing to fabricate ohmic contact electrodes on a bismuth nanowire for Hall measurements

Ohmic contact electrodes for four-wire resistance and Hall measurements were fabricated on an individual single-crystal bismuth nanowire encapsulated in a cylindrical quartz template. Focused ion beam processing was utilized to expose the side surfaces of the bismuth nanowire in the template, and carbon and tungsten electrodes were deposited on the bismuth nanowire in situ to achieve electrical contacts. The temperature dependence of the four-wire resistance was successfully measured for the bismuth nanowire, and a difference between the resistivities of the two-wire and four-wire methods was observed. It was concluded that the two-wire method was unsuitable for estimation of the resistivity due to the influence of contact resistance, even if the magnitude of the bismuth nanowire resistance was greater than the kilo-ohm order. Furthermore, Hall measurement of a 4-μm-diameter bismuth microwire was also performed as a trial, and the evaluated temperature dependence of the carrier mobility was in agreement with that for bulk bismuth, which indicates that the carrier mobility was successfully measured using this technique.

PACS

81.07.Gf     

Effect of heat exchanger material and fouling on thermoelectric exhaust heat recovery

Thermoelectric devices are being investigated as a means of improving fuel economy for diesel and gasoline vehicles through the conversion of wasted fuel energy, in the form of heat, to useable electricity. By capturing a small portion of the energy that is available with thermoelectric devices can reduce engine loads thus decreasing pollutant emissions, fuel consumption, and CO₂ to further reduce green house gas emissions. This study is conducted in an effort to better understand and improve the performance of thermoelectric heat recovery systems for automotive use. For this purpose an experimental investigation of thermoelectrics in contact with clean and fouled heat exchangers of different materials is performed. The thermoelectric devices are tested on a bench-scale thermoelectric heat recovery apparatus that simulates automotive exhaust. It is observed that for higher exhaust gas flowrates, thermoelectric power output increases from 2 to 3.8 W while overall system efficiency decreases from 0.95% to 0.6%. Degradation of the effectiveness of the EGR-type heat exchangers over a period of driving is also simulated by exposing the heat exchangers to diesel engine exhaust under thermophoretic conditions to form a deposit layer. For the fouled EGR-type heat exchangers, power output and system efficiency is observed to be 5-10% lower for all conditions tested.     

Characterization of a β-FeSi2 p-n junction formed by the PECS method

We have fabricated semiconducting β-FeSi₂ bulks without doping and with Mn and Co doping by using a pulse electric current sintering (PECS) method, and explored the possibility of a direct bonding of n-type and p-type β-FeSi₂ bulks to form a p-n junction structure. P-type Mn-doped and n-type Co-doped β-FeSi₂ bulks were obtained by an annealing process at 800-850 °C for 100 h. The PECS was applied to bond the n-type and p-type bulks together for forming a p-n junction structure. We confirmed that the bonding was processed without any change in the β-FeSi₂ phase and was strongly joined with each other. Although we could not obtain the electrical characteristics of the p-n junction, Seebeck coefficients for n-type and p-type β-FeSi₂ in the bonded sample were determined to be −356 and 778 μV/K, respectively. We propose that these results should lead to an expanded use of the sintered β-FeSi₂ bulks in thermoelectric devices.     

Molecular dynamics simulations of lattice thermal conductivity and spectral phonon mean free path of PbTe: Bulk and nanostructures

In this work, molecular dynamics (MD) simulations are performed to predict the lattice thermal conductivity of PbTe bulk and nanowires. The thermal conductivity of PbTe bulk is first studied in the temperature range 300-800 K. Excellent agreement with experiments is found in the entire temperature range when a small vacancy concentration is taken into consideration. By studying various configurations of vacancies, it is found that the thermal conductivity in PbTe bulk is more sensitive to the concentration rather than the type and distribution of vacancies. Spectral phonon relaxation times and mean free paths in PbTe bulk are obtained using the spectral energy density (SED) approach. It is revealed that the majority of thermal conductivity in PbTe is contributed by acoustic phonon modes with mean free paths below 100 nm. The spectral results at elevated temperatures indicate molecular scale feature sizes (less than 10 nm) are needed to achieve low thermal conductivity for PbTe. Simulations on PbTe nanowires with diameters up to 12 nm show moderate reduction in thermal conductivity as compared to bulk, depending on diameter, surface conditions and temperature.     

Thermoelectric properties and power generation characteristics of sintered undoped n-type Mg2Si

Fabrication of sintered undoped n-type Mg₂Si, initiated from pre-synthesized all-molten polycrystalline Mg₂Si powder, was carried out using a Plasma Activated Sintering technique. The thermoelectric properties were evaluated from room temperature up to 861 K. The maximum dimensionless figure of merit was estimated to be 0.60 at 861 K. For output power measurements, the length and cross-sectional area of Mg₂Si elements equipped with Ni electrodes were varied from 6 to 15 mm and 2 × 2 to 4 × 4 mm², respectively. For the 3 × 3 × 7.5 mm³ element, the maximum output power density was 1.43 Wcm^-2 with the temperatures of the cool and hot sides being 373 K and 873 K (ΔT = 500 K), while the highest output power was 203 mW for the sample of 4 × 4 × 7.5 mm³ at ΔT = 500 K. The results of aging tests for 11,000 h with the hot side at 873 K and the cool side at 373 K under atmospheric conditions showed that the fabricated device elements possess sufficient durability at high power-generation operating temperatures.     

Thermoelectric Characterization of (Ga,In)<Subscript>2</Subscript>Te<Subscript>3</Subscript> with Self-Assembled Two-Dimensional Vacancy Planes

The key properties for the design of high-efficiency thermoelectric materials are a low thermal conductivity and a large Seebeck coefficient with moderate electrical conductivity. Recent developments in nanotechnology and nanoscience are leading to breakthroughs in the field of thermoelectrics. The goal is to create a situation where phonon pathways are disrupted due to nanostructures in “bulk” materials. Here we introduce promising materials: (Ga,In)₂Te₃ with unexpectedly low thermal conductivity, in which certain kinds of superlattice structures naturally form. Two-dimensional vacancy planes with approximately 3.5-nm intervals exist in Ga₂Te₃, scattering phonons efficiently and leading to a very low thermal conductivity.     

Al-doped ZnO and N-doped CuxO thermoelectric thin films for self-powering integrated devices

With the use of a thermoelectric material, terrestrial heat can be harvested then converted to electrical power. The advent of these devices has led to the idea of self-powering wherein devices are driven by heat from their working environment. The focus of this study is to fabricate low cost thermoelectric materials, such as aluminum-doped ZnO (ZnO:Al) and nitrogen-doped Cu_xO (Cu_xO:N) that can effectively harvest heat for power generation.ZnO:Al (n-type) and Cu_xO:N (p-type) thin films with nanocrystallites were deposited in (1.27×0.64) cm² glass substrates via spray pyrolysis technique. These materials exhibit significantly high thermoelectric properties, which is comparable to previous works on thermoelectric materials. ZnO:Al showed to have a maximum Seebeck coefficient (S) of 448 μV/K ranging from 300 to 330 K. Cu_xO:N exhibited a significantly much larger |S| of 1002 μV/K at the same temperature range. A prototype of a thermoelectric device was constructed based from these grown thin films and showed to generate a maximum of 32.8 mV at 28 K temperature difference.