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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.

Twenty-six (Pb0.95Sn0.05Te)0.92(PbS)0.08–0.055% PbI2–SiC nanoparticle (SiCnp) composite thermoelectric specimens were either hot pressed or pulsed electric current sintered (PECS). Bloating (a thermally induced increase in porosity, P, for as-densified specimens) was observed during annealing at temperatures >603 K for hot-pressed specimens and PECS-processed specimens from wet milled powders, but in contrast seven out of seven specimens densified by PECS from dry milled powders showed no observable bloating following annealing at temperatures up to 936 K. In this study, bloating in the specimens was accessed via thermal annealing induced changes in (i) porosity measured by scanning electron microscopy on fractured specimen surfaces, (ii) specimen volume and (iii) elastic moduli. The moduli were measured by resonant ultrasound spectroscopy. SiCnp additions (1–3.5 vol.%) changed the fracture mode from intergranular to transgranular, inhibited grain growth, and limited bloating in the wet milled PECS specim...

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.

Harvesting of waste heat may lead to macrocrack and/or microcrack damage accumulation in thermoelectrics. No studies in the open literature address the thermal fatigue of any thermoelectric material. This study characterizes the thermal fatigue behavior for two PbTe-based thermoelectric materials, n-type LAST (lead–antimony–silver–tellurium) and p-type LASTT (lead–antimony–silver–tellurium–tin). The mechanical properties (fracture strength, elastic moduli) were evaluated for up to 200 thermal fatigue cycles. In addition, the electrical and thermal transport properties were evaluated for n- and p-type specimens for thermal cycling. The elastic moduli were relatively insensitive to thermal fatigue treatment. The fracture strength, σf, of the thermally fatigued LASTT specimens was in a band of from 25 to 40 MPa while σf of the thermally fatigued LAST ranged from 15 to 38 MPa. The thermopower and electrical conductivity of LASTT samples showed small deviations from the low temperature trend near 600 K and the data repeated well after the first temperature cycle for all samples. For the n-type LAST samples, the electrical conductivity and thermopower showed larger deviations from the low temperature trend near 500 K with some samples requiring several temperature cycles before showing repeatability in the data, suggesting a possible secondary phase in the samples.

During waste heat recovery applications, thermoelectric (TE) materials experience thermal gradients and thermal transients, which produce stresses that scale with the TE material's coefficient of thermal expansion (CTE). Thus, the temperature-dependent CTE is an important parameter for the design of mechanically robust TE generators. For three skutterudite thermoelectric compositions, n-type Co0.95Pd0.05Te0.05Sb3 (with and without 0.1 at. % cerium doping) and p-type Ce0.9Fe3.5Co0.5Sb12, the CTE was measured using two methods, i.e. X-ray diffraction on powder and bulk specimens and dilatometry on bulk specimens. Each bulk specimen was hot pressed using powders milled from cast ingots. Between 300 K and 600 K, the mean CTE values were 9.8–10.3 × 10−6 K−1 for the non-cerium-doped n-type, 11.6 × 10−6 K−1 for the 0.1 at. % cerium-doped n-type and from 12.7 to 13.3 × 10−6 K−1 for the p-type. In the literature, similar CTE values are reported for other Sb-based skutterudites. For temperatures >600 K, an unrecove...

Effective models of the mechanical behavior of thermoelectric materials under device conditions require knowledge of temperature-dependent elastic properties. Between room temperature and 600 K, resonant ultrasound spectroscopy measurements of three skutterudite thermoelectric materials, i.e. n-type Co0.95Pd0.05Te0.05Sb3 (both with and without 0.1 at.% cerium dopant) and p-type Ce0.9Fe3.5Co0.5Sb12, showed that the Young's and shear moduli decreased linearly with temperature at a rate of −0.021 GPa/K to −0.032 GPa/K, and −0.011 GPa/K to −0.013 GPa/K, respectively. In contrast, the Poisson's ratio was approximately 0.22 for the three materials and was relatively insensitive to temperature. For temperatures >600 K, the elastic moduli decreased more rapidly and resonance peaks broadened, indicating the onset of viscoelastic behavior. The viscoelastic relaxation of the moduli was least for Ce-doped n-type material, for which grain boundary precipitates may inhibit grain boundary sliding which in turn has impor...

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.

Two-phase PbTe–PbS materials, in which PbS is a nanostructured phase, are promising thermoelectric materials for the direct conversion of heat energy into electricity. In this study, a Vickers indentation mean hardness of 1.18 ± 0.09 GPa was measured for hot pressed specimens Pb 0.95 Sn 0.05 Te–PbS 8% while the mean hardness of cast specimens was 0.68 ± 0.07 GPa. The mean fracture toughness of the not pressed specimens was estimated as 0.35 ± 0.04 MPa m 1/2 via Vickers indentation. Resonant Ultrasound Spectroscopy (RUS) measurements on hot pressed specimens gave mean values of Young's modulus, shear modulus and Poisson's ratio of 53.1 GPa, 21.4 GPa and 0.245, respectively while for the cast specimens the Young's and shear moduli were about 10% lower than for the hot pressed, with a mean value of Poisson's ratio of 0.245. The differences between the hardness and elastic moduli values for the cast and hot pressed specimens are discussed.

Porosity in thermoelectric materials affects the thermal, electrical and mechanical properties of the materials. In this study, the resonant ultrasound spectroscopy technique was used to determine the Young's modulus, E, shear modulus, G, and Poisson's ratio, ν, of 12 hot pressed and five cast polycrystalline specimens of the thermoelectric material LAST (lead–antimony–silver–tellurium) as a function of volume fraction porosity, P, for P ranging from 0.01 to 0.14. A least-squares fit of the Young's and shear moduli data to the relationships E = ED(1 − bPEP) and G = GD(1 − bPGP), respectively yielded ED = 58.4 ± 0.6 GPa and GD = 23.0 ± 0.2 GPa, where ED and GD are the estimated Young's modulus and shear modulus at room temperature for theoretically dense specimens, respectively. The unitless, material-dependent constants bPE and bPG were bPE = 3.5 ± 0.2 and bPG = 3.5 ± 0.2.

In this study, we focus on the room-temperature mechanical properties of LAST (Pb–Sb–Ag–Te) materials fabricated using casting with slow-cool temperature profiles (cooling rate 5°C h−1). The higher biaxial flexural strength is potentially beneficial for the mechanical integrity of thermoelectric devices.

In the open literature, reports of mechanical properties are limited for semiconducting thermoelectric materials, including the temperature dependence of elastic moduli. In this study, for both cast ingots and hot-pressed billets of Ag-, Sb-, Sn- and S-doped PbTe thermoelectric materials, resonant ultrasound spectroscopy (RUS) was utilized to determine the temperature dependence of elastic moduli, including Young's modulus, shear modulus and Poisson's ratio. This study is the first to determine the temperature-dependent elastic moduli for these PbTe-based thermoelectrics, and among the few determinations of elasticity of any thermoelectric material for temperatures above 300 K. The Young's modulus and Poisson's ratio, measured from room temperature to 773 K during heating and cooling, agreed well. Also, the observed Young's modulus, E, versus temperature, T, relationship, E(T) = E 0(1–bT), is consistent with predictions for materials in the range well above the Debye temperature. A nanoindentation study o...

For the material (Pb0.95Sn0.05Te)1-x(PbS)x nanostructuring from nucleation and growth and spinodal decomposition were reported to enhance the thermoelectric figure of merit over bulk PbTe, producing ZT of 1.1 - 1.4 at 650 K for x = 0.08[1]. Thermoelectric modules made from (Pb0.95Sn0.05Te)1-x(PbS)x materials with various hot-side metal electrodes were fabricated and tested. Short circuit current was measured on unicouples of Pb0.95Sn0.05Te – PbS 8% (n-type) legs and Ag0.9Pb9Sn9Sb0.6Te20 (p-type) legs over 10 (A) for a hot side temperature of 870K, and a cold side of 300K. Hot pressed (Pb0.95Sn0.05Te)1-x(PbS)x materials were also investigated for module fabrication. Investigations of the electrical properties of hot-pressed (Pb0.95Sn0.05Te)1-x(PbS)x materials are presented along with the latest advancements in the fabrication and characteristics of modules based on the processing of these materials.

PbTe-based thermoelectric (TE) materials exhibit promising thermoelectric properties and have potential applications in waste heat recovery from sources such as truck engines and shipboard engines. TE components designed for these applications will be subject to mechanical/thermal loading and vibration as a result from in-service conditions, including mechanical vibration, mechanical and/or thermal cycling, and thermal shock.In the current study, we present and discuss the mechanical properties of several PbTe-based compositions with different dopants and processing methods, including n-type and p-type specimens fabricated both by casting and by powder processing. Room temperature hardness and Young's modulus are studied by Vickers indentation and nanoindentation while fracture strength is obtained by biaxial flexure testing. Temperature dependent Young's modulus, shear modulus, and Poisson's ratio are studied via resonant ultrasound spectroscopy (RUS).

Thermoelectric modules are of great interest for power generation applications where temperature gradients of approximately 500K exist, and hot side temperatures near 800K. The fabrication of such modules requires optimization of the material compositions, low contact resistivities, and low thermal loss.AgPbmSbTe2+m (LAST) and Ag(Pb1-xSnx)m SbTe2+m (LASTT) compounds are among the best known materials appropriate for this temperature range. Various measurement systems have been developed and used to characterize bulk samples in the LAST and LASTT systems within this operating temperature range. From the characterized data, modeling of modules based on these materials and segmented legs using LAST(T) with Bi2Te3 have been used to identify the optimal geometry for the individual legs, and the length of the Bi2Te3 segments. We have segmented LAST(T) with Bi2Te3 and achieved contact resistivities of less than 10 μΩ•cm2.Here we give a detailed presentation on the procedures used in the fabrication of thermoelectric generators based on LAST, LASTT, and segmented with Bi2Te3 materials. We also present the output data on these generators.