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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.

In waste heat recovery applications, thermoelectric (TE) generators are subjected to thermal gradients and thermal transients, creating mechanical stresses in the TE legs. Such stresses are functions of the elastic moduli of the TE material. For SnTe1±x matrices (where x = 0.0 or 0.016) composite specimens with 0–4 vol% SiC nanoparticle (SiCNP) additions, the elastic moduli (Young’s modulus, shear modulus, and Poisson’s ratio) were measured by resonant ultrasound spectroscopy from room temperature (RT) to 663 K. The effects of matrix composition and the SiCNP additions on the RT intercepts and the slopes of the elastic modulus as a function of temperature are also discussed.

The coefficient of thermal expansion (CTE) is a key design parameter for thermoelectric (TE) materials, especially in energy harvesting applications since stresses generated by CTE mismatch, thermal gradients, and thermal transients scale with the CTE of the TE material. For the PbTe–PbS-based TE material (Pb0.95Sn0.05Te)0.92(PbS)0.08—0.055 % PbI2 over the temperature ranges of 293–543 and 293–773 K, a CTE, αavg, of 21.4 ± 0.3 × 10−6 K−1 was measured using (1) dilatometry and (2) high-temperature X-ray diffraction (HT-XRD) for powder and bulk specimens. The CTE values measured via dilatometry and HT-XRD are similar to the literature values for other Pb-based chalcogenides. However, the processing technique was found to impact the thermal expansion such that bloating (which leads to a hysteresis in thermal expansion) occurred for hot pressed billets heated to temperatures >603 K while specimens fabricated by pulsed electric current sintering and as-cast specimens did not show a bloating-modified thermal expansion even for temperatures up to 663 K. The relationship of bloating to the processing techniques is discussed, along with a possible mechanism for inhibiting bloating in powder processed specimens.

Lead-antimony-silver-tellurium (LAST) thermoelectric materials are candidates for waste-heat recovery applications. However, rapid heating and cooling (thermal shock) imposes thermomechanical stresses that can cause microcracking. Waste-heat recovery applications involve thermal fatigue, in which a series of hundreds or thousands of individual thermal shock events can lead to accumulation of microcrack damage in brittle thermoelectrics such as LAST. Microcracking in turn leads to a decrease in transport properties, such as electrical conductivity and thermal conductivity, and mechanical properties, including elastic modulus and strength. Thus, microcracking can affect both thermoelectric performance and mechanical integrity. In this study, LAST specimens were rapidly cooled (quenched) into a fluid (water or silicone oil) in order to compare the results with the vast majority of thermal shock studies of brittle materials that are quenched in a similar manner. Decreases in elastic modulus, E, with accumulating microcrack damage were measured using resonant ultrasound spectroscopy (RUS). The evolution of thermal fatigue damage observed in this study is also described well by an equation that successfully describes thermal fatigue damage in a variety of brittle materials.

Cubic garnet Li6.24La3Zr2Al0.24O11.98 (LLZO) is a candidate material for use as an electrolyte in Li–Air and Li–S batteries. The use of LLZO in practical devices will require LLZO to have good mechanical integrity in terms of scratch resistance (hardness) and an adequate stiffness (elastic modulus). In this paper, the powders were fabricated by powder processing of cast ingots. All specimens were then densified via hot pressing. The room temperature elastic moduli (Young’s modulus, shear modulus, bulk modulus, and Poisson’s ratio) and hardness were measured by resonant ultrasound spectroscopy, and Vickers indentation, respectively. For volume fraction porosity, P, the Young’s modulus was 149.8 ± 0.4 GPa (P = 0.03) and 132.6 ± 0.2 GPa (P = 0.06). The mean Vickers hardness was 6.3 ± 0.3 GPa for P = 0.03 and 5.2 ± 0.4 for P = 0.06.

Mg2Si is of interest as a thermoelectric (TE) material in part due to its low materials cost, lack of toxic components, and low mass density. However, harvesting of waste heat subjects TE materials to a range of mechanical and thermal stresses. To understand and model the material’s response to such stresses, the mechanical properties of the TE material must be known. The Mg2Si specimens included in this study were powder processed and then sintered via pulsed electrical current sintering. The elastic moduli (Young’s modulus, shear modulus, and Poisson’s ratio) were measured using resonant ultrasound spectroscopy, while the hardness and fracture toughness were examined using Vickers indentation. Also, the Vickers indentation crack lengths were measured as a function of time in room air to determine the susceptibility of Mg2Si to slow crack growth.

Lead chalcogenides such as (Pb0.95Sn0.05Te)0.92(PbS)0.08-0.055%PbI2 have received attention due to their encouraging thermoelectric properties. For the hot pressing (HP) and pulsed electric current sintering (PECS) techniques used in this study, decomposition reactions can generate porosity (bloating). Porosity in turn can degrade electrical, thermal, and mechanical properties. In this study, microstructural observations (scanning electron microscopy) and room-temperature elasticity measurements (resonant ultrasound spectroscopy) were used to characterize bloating generated during post-densification anneals. Although every HP specimen bloated during post-densification annealing, no bloating was observed for the PECS specimens processed from dry milled only powders. The lack of bloating for the annealed PECS specimens may be related to the electrical discharge intrinsic in the PECS process, which reportedly cleans the powder particle surfaces during densification.

Room temperature Young's modulus, shear modulus and Poisson's ratio were measured by Resonant Ultrasound Spectroscopy (RUS) for a total of 29 skutterudite specimens cut from four hot pressed billets: (i) two p-type (Ce 0.9 Fe 3.5 Co 0.5 Sb 12 ) billets, (ii) one n-type (Co 0.95 Pd 0.05 Te 0.05 Sb 3 ) billet with 0.1 atomic% Ce addition and (iii) one n-type billet without Ce addition. Within a given hot pressed billet, the elastic moduli were very uniform from specimen to specimen. Also, Ce-doping of the n-type specimens did not result in a significant shift in the elastic modulus. For n-type specimens with 98% relative density, the mean Young's modulus, shear modulus and Poisson's ratio were 140.6 ± 0.2 GPa, 57.34 ± 0.05 and 0.226 ± 0.001 GPa, respectively. For the p-type specimens of similar relative density, the mean Young's and shear moduli were approximately 7% lower than the n-type, while there was no significant difference between the mean Poisson's ratio values of the n-type and p-type 98% dense specimens. The measured elastic moduli agreed relatively well with the literature values for other Sb-based skutterudite compositions. In addition, nanoindentation measurements on three mutually orthogonal specimen faces showed no appreciable anisotropy in Young's modulus.