The NASA enhanced MMRTG (eMMRTG) project seeks to enhance the performance of the Multi-Mission Radioisotope Thermoelectric Generator by exchanging the heritage thermoelectric couple materials and design with that of more efficient skutterudite (SKD)-based thermoelectric (TE) couples. Both at the Jet Propulsion Lab—where the SKD materials were developed—and at Teledyne Energy Systems, Inc.—where the technology is being matured and optimized for fabrication under the Skutterudite Technology Maturation (SKD Tech Mat) Project—many SKD material batches have been produced. A strong correlation has been observed between the TE performance of the materials and the particular lot numbers (same producer and supplier) of various raw materials used to produce the SKD materials. An analysis of the differences between lots, their effects on performance, and the current plan to mitigate the risk posed by these results is discussed.

Calcium cobaltate (Ca3Co4O9) remains one of the most promising p-type oxide materials for high-temperature thermoelectric energy conversion. While much progress has been made in refining our understanding of the unique structure of the material, as well as optimization of the transport properties for thermoelectric efficiency, there remains a gap in the knowledge, both experimental and theoretical, of the thermodynamics of the system. Presented herein is an analysis of the heat of formation of the Ca3Co4O9 phase from sol–gel precursors using a highly sensitive differential scanning calorimeter, as well as observations of its decomposition into the Ca3Co2O6 phase. The reaction enthalpy of forming Ca3Co4O9 from CaCO3 and Co3O4 sol–gel precursors was determined to be +284 (±2%) kJ/mol, leading to a standard enthalpy of Ca3Co4O9 of −3307 (±3.5%) kJ/mol.

The Multi-Mission Radioisotope Thermoelectric Generator (MMRTG) developed for NASA by Aerojet Rocketdyne and Teledyne Energy Systems, Inc. (TESI) has been providing reliable power to the Mars Curiosity Rover since its launch in 2011 (Mission on Mars began in August 2012). An improvement of the performance of the MMRTG is underway at TESI through a technology maturation program where higher efficiency skutterudite materials developed on the laboratory scale at NASA's Jet Propulsion Laboratory (JPL) are being further developed at the production level into "flight-ready" materials and components. The success of this project will result in an enhanced MMRTG (eMMRTG) with an anticipated improvement of 25-30% in the beginning-of-life power and even greater improvement by end-of-life due to improved thermoelectric life properties. The status of the program, including engineering challenges, successes and current production capabilities will be presented.

The National Aeronautics and Space Administration’s Mars Science Laboratory terrestrial rover, Curiosity, has recently completed its first Martian year (687 Earth days) during which it has provided a wealth of information and insight into the red planet’s atmosphere and geology. The success of this mission was made possible in part by the reliable electrical power provided by its onboard thermoelectric power source—the multi-mission radioisotope thermoelectric generator (MMRTG). In an effort to increase the output power and efficiency of these generators, a newly designed enhanced MMRTG (eMMRTG) that will utilize the more efficient skutterudite-based thermoelectric materials has been conceptualized and modeled, and is now being developed. A discussion of the motivations, modeling results and key design factors are presented and discussed.

A systems integrated thermal model of an enhanced Multi-Mission Radioisotope Thermoelectric Generator (eMM-RTG) was performed utilizing Sinda 2014 from MSC Software. A comprehensive physics model was added to the Sinda solver which includes, among others, the Seebeck effect, Peltier effect, Thomson effect, Joule heating, and thermal radiation in all the applicable components. The added physics enables the computation of the power output of the eMMRTG. A custom tool was developed to rapidly and efficiently create the necessary Sinda input files allowing for parameterization and optimization of geometries and materials with ease. The methods and select results are presented and discussed.

We report a high-performance thermoelectric (TE) oxide-based module using the segmentation of half-Heusler Ti_(0.3)Zr_(0.35)Hf_(0.35)CoSb_(0.8)Sn_(0.2) and misfit-layered cobaltite Ca_3Co_4O_(9+δ) as the p-leg and 2 % Al-doped ZnO as the n-leg. The maximum output power of a 4-couple segmented module at ΔT=700 K attains a value of approximately 6.5 kW m^(−2), which is three times higher than that of the best reported non-segmented oxide module. The TE properties of individual legs, as well as the interfacial contact resistances, were characterized as a function of temperature. Numerical modeling was used to predict the efficiency and to evaluate the influence of the electrical and thermal losses on the performance of TE modules. Initial long-term stability tests of the module at the hot and the cold side temperatures of 1073 K and 444 K, respectively, showed a promising result with 4 % degradation for 48 h operating in air.

The nanoparticles of Al-doped ZnO were successfully grown into rod-like and platelet-like morphologies by soft chemical routes. These powders were consolidated using spark plasma sintering (SPS) technique. The samples consolidated from rods and platelets exhibited characteristic structures with preferential orientation while the sample consolidated from nanoparticles did not show any sign of preferential orientation. The measured ZT values along the preferred orientation directions were found to be 0.16 and 0.25 at 1223 K for the samples consolidated from rods and platelets, respectively. The sample consolidated from nanoparticles exhibited fine grains and highly distributed nanoprecipitates, resulting in a ZT value of 0.3 at 1223 K due to the lower thermal conductivity resulting from nanostructuring. Using the simple parabolic band model and Debye-Callaway thermal transport model, the anisotropic properties of the nanostructured samples were elucidated and the influence of the grain size and nanoprecipitates on the electron and phonon transport was analyzed and discussed in detail.

A rapid method for the synthesis of Ca3Co4O9+δ powder is introduced. The procedure is a modification of the conventional citric-nitrate sol–gel method where an auto-combustion process is initiated by a controlled thermal oxidation–reduction reaction. The resulting powders inherit the advantages of a wet chemical synthesis, such as morphological and compositional homogeneity, and fine, well-defined particle sizes coming from the controlled nature of the auto-combustion. Optimized spark plasma sintering (SPS) processing conditions were determined and used to fabricate dense and highly c-axis oriented samples. The microstructure and thermoelectric transport properties were determined both parallel (||) and perpendicular (⊥) to the SPS pressure axis in order to investigate any possible anisotropy variations in the transport properties. At 800 °C, power factors of 506 μW/m K2 (⊥) and 147 μW/m K2 (||), thermal conductivities values of 2.53 W/m K (⊥) and 1.25 W/m K (||), and resulting figures-of-merit, ZT, of 0.21 (⊥) and 0.13 (||) were observed.

A customized Fe-Cr alloy that has been optimized for high temperature applications in oxidizing atmospheres has been interfaced via spark plasma sintering (SPS) with a p-type thermoelectric oxide material: calcium cobaltate (Ca3Co4O9). The properties of the alloy have been analyzed for its compatibility with the Ca3Co4O9 in terms of its thermal expansion and transport properties. The thermal and electrical contact resistances have been measured as a function of temperature, and the long term electronic integrity of the interface analyzed by measuring the resistance vs. time at an elevated temperature. The kinetics of the interface have been analyzed through imaging with scanning electron microscopy (SEM), elemental analysis using energy dispersive spectroscopy (EDS), and phase identification with X-ray diffraction (XRD). The results reveal the formation of an intermediate phase containing calcium and chromium in the interface that is highly resistive at room temperature, but conducting at the intended thermoelectric device hot-side operating temperature of 800 degrees C. As the alloy is well matched in terms of its thermal expansion and highly conducting compared to the Ca3Co4O9, it may be further considered as an interconnect material candidate at least with application on the hot-side of an oxide thermoelectric power generation module. (c) 2013 Elsevier B.V. All rights reserved.

Wheat gluten from ethanol production is presented as flame-retardant silica hybrid biofoams for insulation. The porosity of 90% and self-extinguishing nature make them an attractive alternative to petroleum-based foams.

Practical implementation of oxide thermoelectrics on an industrial or commercial scale for waste heat energy conversion requires the development of chemically stable interfaces between metal interconnects and oxide thermoelements that exhibit low electrical contact resistances. A commercially available high-chrome iron alloy (i.e., Crofer® 22 APU) serving as the interconnect metal was spray coated with LaNi_0.6Fe_0.4O_3 (LNFO) or (Mn,Co)_3O_4 spinel and then interfaced with a p-type thermoelectric material—calcium cobaltate (Ca_3Co_4O_9)—using spark plasma sintering. The interfaces have been characterized in terms of their thermal and electronic transport properties and chemical stability. With long-term exposure of the interfaced samples to 800 °C in air, the cobalt–manganese spinel acted as a diffusion barrier between the Ca_3Co_4O_9 and the Crofer® 22 APU alloy resulting in improved interfacial stability compared to that of samples containing LNFO as a barrier layer, and especially those without any barrier. The initial area specific interfacial resistance of the Ca_3Co_4O_9/(Mn,Co)_3O_4/Crofer® 22 APU interface at 800 °C was found to be ∼1 mΩ·cm^2.

Ca3Co4O9+δ samples were synthesized by solid-state (SS) and sol–gel (SG) reactions, followed by spark plasma sintering under different processing conditions. The synthesis process was optimized and the resulting materials characterized with respect to their microstructure, bulk density, and thermoelectric transport properties. High power factors of about 400 μW/m·K2 and 465 μW/m·K2 (at 800°C) were measured for SS and SG samples, respectively. The improved thermoelectric performance of the SG sample is believed to originate from the smaller particle sizes and better grain alignment. The SG method is suggested to be a beneficial means of obtaining high-performance thermoelectric materials of Ca3Co4O9+δ type.

A novel two-phase ceramic composite (cercer) material consisting of a solid solution of barium cerate and -zirconate doped with yttrium (BaCe0.2Zr0.7Y0.1O3-delta : BCZY27), together with niobium-doped strontium titanate (Sr0.95Ti0.9Nb0.1O3-delta : STN95), has been synthesized by solid-state reaction and sintered conventionally (CS) at 1350-1500 A degrees C, as well as by spark plasma sintering (SPS) at 1300-1350 A degrees C. CS samples were porous and exhibited high degrees of inter-phase reaction. Nickel oxide sintering aids did not improve CS sample density. In contrast, samples made by SPS were significantly denser (> 95 %) and showed less reaction between phases. A pseudo-optimum SPS profile was developed, accounting for the effects of thermal expansion mismatch between BCZY27 and STN95. X-ray diffraction indicated secondary phases exist, but there was no indication of their presence at grain boundaries based on thorough study of these regions with high-resolution transmission electron microscopy and selective area electron diffraction. We thus suggest that these phases are present as independent grains in the bulk. It is believed these secondary phases inhibit electronic conductivity in the composite.

A systematic investigation on the microstructure and thermoelectric properties of Al-doped ZnO using α- and γ-Al2O3 as dopants was conducted in order to understand the doping effect and its mechanism. The samples were prepared by the spark plasma sintering technique from precursors calcined at various temperatures. Clear differences in microstructure and thermoelectric properties were observed between the samples doped with α- and γ-Al2O3. At any given calcination temperature, γ-Al2O3 resulted in the formation of a larger amount of the ZnAl2O4 phase in the Al-doped ZnO samples. The average grain size was found to be smaller for the γ-Al2O3-doped samples than that for the α-Al2O3-doped ones under the same sintering condition. It is proposed that the ZnAl2O4 phase is the reason for the observed suppression of grain growth and also for the slightly reduced lattice thermal conductivity exhibited by these samples. The γ-Al2O3 promoted the substitution for donor impurities in ZnO, thus resulting in shrinkage of the unit cell volume and an increase in the electrical conductivity compared with the α-Al2O3-doped ZnO. At a calcination temperature of 1173 K, the γ-Al2O3-doped sample showed a ZT value of 0.17 at 1173 K, which is 27% higher than that of the α-Al2O3-doped sample.

About 18g of Ni0.05Mo3Sb5.4Te1.6 were prepared by heating the elements in the stoichiometric ratio at 1000K. The product was divided into four parts, and then C60 was added to three of these four parts at 1, 2, and 3mass%, respectively. Each part was hot-pressed at 150MPa and 923K. The sample with 1% C60 was characterized via a Rietveld refinement and TEM analyses. Measurements of the three thermoelectric key properties revealed that the Seebeck coefficient barely depends on the carbon amount added, while both the electrical and the thermal conductivity decrease with increasing amount of carbon. Depending on the amount of C60 used and on the temperature, the thermoelectric performance was either enhanced or decreased, depending on whether the electrical conductivity decreased less or more than the thermal conductivity. At the highest temperature measured, all carbon-containing samples performed better than the unmodified bulk sample, namely up to 14%. These improvements are within the error margin, however.

Incorporating oxide thermoelectric (TE) materials into TE power generation modules necessitates study of the interfaces between the oxide TE elements and the interconnect materials used to transfer current between them. In this study, interfaces between pure nickel and undoped calcium cobaltate (Ca3Co4O9) have been formed directly by spark plasma sintering (SPS). An intermediate NiO phase is formed during the SPS processes, which grows during post-heating with Co entering from the cobaltate side to form a graded Ni1−x Co x O interfacial layer. The electrical and thermal transport across these interfaces, as well as the long-term chemical stability of the intermediate layers, have been studied and are discussed.

Zirconium diselenide (ZrSe2) is one of many members of the layer-structured transition-metal dichalcogenide family. The structure of these materials features a weakly bonded van der Waals gap between covalently bonded CdI2-type atomic layers that may host a wide range of intercalants. Intercalation can profoundly affect the structural, thermal, and electronic properties of such materials. While the thermoelectric potential of layer-structured transition-metal dichalcogenides has been formerly studied by several groups, to our best knowledge, neither the thermoelectric properties of ZrSe2 nor the impact of intercalation on its thermoelectric properties have been reported (specifically, the full evaluation of the dimensionless figure of merit, ZT, which includes the thermal conductivity). In this proof-of-principle study, ZrSe2 single crystals have been synthesized using an iodine-assisted vapor transport method, followed by a wet-chemistry lithium intercalation process. The results of resistivity, thermopower, and thermal conductivity measurements between 10 K and 300 K show that Li intercalation induced additional charge carriers and structural disorder that favorably affected the thermoelectric properties of the material. As a result, a dimensionless figure of merit ZT ≈ 0.26 has been attained at room temperature in a Li-intercalated sample, representing nearly a factor of three improvement compared with the pristine sample. These improvements, along with the abundance, relatively low toxicity, and low cost of such materials, merit further thermoelectric investigations of intercalated zirconium diselenide, especially in conjunction with a substitutional doping approach.