By measuring the elastic slow dynamics behavior of various consolidated granular materials and metallic alloys, we show that the evolution of the wave velocity as a function of time can be reproduced by using a continuous relaxation-time spectrum, whose main features can be extracted from the experiments. Here we propose to adopt a Weibull distribution of the relaxation times, that we find to display a maximum, which is related to the material grain size and independent of the conditioning amplitude. These results show that the role of the grain-size distribution is fundamental to quantify slow dynamics effects, as well as to clarify its mesoscopic physical origin. Moreover, they can pave the way for the implementation of advanced characterization methods or nondestructive testing techniques capable of retrieving information on the different spatial scales or defects in the materials under investigation.

Nanostructures play an important role in thermoelectric materials. Their thermal stability, such as phase change and evolution at elevated temperatures, is thus of great interest to the thermoelectric community. In this study, in situ neutron diffraction was used to examine the phase evolution of nanostructured bulk PbTe-PbS materials fabricated using hot pressing and pulsed electrical current sintering (PECS). The PbS second phase was observed in all samples in the as-pressed condition. The temperature dependent lattice parameter and phase composition data show an initial formation of PbS precipitates followed by a redissolution during heating. The redissolution process started around 570–600 K, and completed at approximately 780 K. During cooling, the PECS sample followed a reversible curve while the heating/cooling behavior of the hot pressed sample was irreversible.

Introducing nanostructural second phases has proved to be an effective approach to reduce the lattice thermal conductivity and thus enhances the figure of merit for many thermoelectric materials. Studies of the formation and evolution of these second phases are essential to understanding material temperature dependent behaviors, improving thermal stabilities, as well as designing new materials. In this study, powder samples of the PbTe-PbS thermoelectric material were examined using in situ neutron diffraction and small angle neutron scattering (SANS) techniques between room temperature and elevated temperature up to 663 K, to explore quantitative information on the structure, weight fraction, and size of the second phase. Neutron diffraction data showed that the as-milled powder was primarily a solid solution prior to heat treatment. During heating, a PbS second phase precipitated out of the PbTe matrix around 500 K, while re-dissolution started around 600 K. The second phase remained separated from the matrix upon cooling. Furthermore, SANS data indicated that there are two populations of nanostructures. The size of the smaller nanostructure increased from initially 5 nm to approximately 25 nm after annealing at 650 K, while the size of the larger one remained unchanged. This study demonstrated that in situ neutron techniques are effective means to obtain quantitative information on temperature-dependent nanostructural behavior of thermoelectrics and likely other high-temperature materials.

A sufficient mechanical response of thermoelectric materials (TEMats) to structural loadings is a prerequisite to the exploitation of any candidate TEMat's thermoelectric efficiency. If a TEMat is mechanically damaged or cracks from service-induced stresses, then its thermal and electrical functions can be compromised or even cease. Semiconductor TEMats tend to be quite brittle and have a high coefficient of thermal expansion; therefore, they can be quite susceptible to mechanical failure when subjected to operational thermal gradients. Because of this, sufficient mechanical response (vis-a-vis, mechanical properties) of any candidate TEMat must be achieved and sustained in the context of the service-induced stress state to which it is subjected. This report provides an overview of the mechanical responses of state-of-the-art TEMats; discusses the relevant properties that are associated with those responses and their measurement; and describes important, nonequilibrium phenomena that further complicate their use in thermoelectric devices. For reference purposes, the report also includes several appendixes that list published data on elastic properties and strengths of a variety of TEMats.

For thermoelectric generators, the individual thermoelectric elements are subjected to significant stresses under in-service conditions, due to thermal cycles, transients, and gradients, as well as thermal expansion mismatch and externally applied mechanical stresses. Most thermoelectric materials are brittle, possessing a low fracture toughness that is typically no more than 1.5 MPa m1/2 and is often less than 0.5 MPa m1/2. The combination of the stresses encountered in the device application environment and the materials’ low fracture toughness constitute a severe challenge to the viability of thermoelectric generators. The addition of silicon carbide nanoparticles (SiCNP) may provide a route to improving the fracture toughness for a wide range of thermoelectric materials. This study examines the mechanical properties, including elastic modulus, hardness, and fracture toughness for 0–4 vol% SiCNP incorporated into Mg2Si thermoelectric matrices.

PbTe-based thermoelectric (TE) materials have been extensively investigated as TE generator materials, however, the tellurium content limits the application potential due to both availability and cost. Replacing the tellurium with selenium or sulfur produces an isomorphic TE material with very good reported figure of merit, ZT, values of 1.3–1.6, but the effect of the material changes designed to increase ZT (doping, nano- and micro-precipitate additions) on mechanical properties has not been reported. In order to effectively incorporate these new materials into TE devices, it is important to understand materials’ response to thermally and mechanically imposed loads, which in turn requires knowledge of the mechanical properties. In this study, the hardness was determined by Vickers indentation and elastic modulus and Poisson’s ratio were measured using resonant ultrasound spectroscopy on PbSe- and PbS-based TE specimens as a function the addition of 0–4 at.% of CdS or ZnS. With 2.0 or 2.5 at.% Na doping, the hardness of PbSe- or PbS-based TE materials increased by about 30 % and the elastic moduli decreased by 5–10 %. In addition, PbS may be effectively sintered at 723 K when doped with 2.5 at.% Na, but requires a higher sintering temperature when undoped. This study shows that the hardness and moduli of PbSe- or PbS-based TE materials are not strong functions of the addition of CdS or ZnS precipitates.

The heat capacity of natural (Cu12-x (Fe, Zn, Ag) x(Sb, As)(4)S-13) and synthetic (Cu12-xZnxSb4S13 with x = 0, 1, 2) tetrahedrite compounds was measured between 2K and 380 K. It was found that the temperature dependence of the heat capacity can be described using a Debye term and three Einstein oscillators with characteristic temperatures that correspond to energies of similar to 1.0 meV, similar to 2.8 meV, and similar to 8.4 meV. The existence of localized vibrational modes, which are assigned to the displacements of the trigonally coordinated Cu atoms in the structure, is discussed in the context of anharmonicity and its effect on the low lattice thermal conductivity exhibited by these compounds. (C) 2014 AIP Publishing LLC.

This letter reports the thermoelectric properties of Bi-doped Mg2Si0.4Sn0.6 thermoelectric materials. It was found that the ZTs of this material could be greatly enhanced by Bi-doping. Analyses on the transport properties showed that the power factors of the material were enhanced while the lattice thermal conductivities were reduced by Bi-doping. The reduction of the lattice thermal conductivity was likely caused by the interstitial Bi impurities. A peak ZT approximate to 1.55 at 773K was obtained. (C) 2014 AIP Publishing LLC.

Microcracks are present in bone and can result from fatigue damage due to repeated, cyclically applied stresses. From a mechanical point, microcracks can dissipate strain energy at the advancing tip of a crack to improve overall bone toughness. Physiologically, microcracks are thought to trigger bone remodeling. Here, we examine the effect of microcracks specifically on osteoblasts, which are bone-forming cells, by comparing cell responses on microcracked versus non-microcracked hydroxyapatite (HA) specimens. Osteoblast attachment was found to be greater on microcracked HA specimens (p<0.05). More importantly, we identified the preferential alignment of osteoblasts in the direction of the microcracks on HA. Cells also displayed a preferential attachment that was 75 to 90 μm away from the microcrack indent. After 21 days of culture, osteoblast maturation was notably enhanced on the HA with microcracks, as indicated by increased alkaline phosphatase activity and gene expression. Furthermore, examination of bone deposition by confocal laser scanning microscopy indicated preferential mineralization at microcrack indentation sites. Dissolution studies indicate that the microcracks increase calcium release, which could contribute to osteoblast responses. Our findings suggest that microcracks signal osteoblast attachment and bone formation/healing.

The thermoelectric skutterudite Ba0.3Co4Sb12 is a promising candidate for waste heat recovery applications. Recently, it was demonstrated that the addition of silver nanoparticles (AgNP) to Ba0.3Co4Sb12 increases both the thermoelectric figure of merit and electrical conductivity. This study is the first to examine the effect of AgNP addition on the material’s mechanical properties. This study also found that the Young’s modulus, E, shear modulus, G, and bulk modulus, B, decreased linearly with increasing volume fraction porosity, P. Resonant ultrasound spectroscopy was employed to measure the elastic moduli, and Vickers indentation was used to determine the hardness, H, and fracture toughness, K C. Trends in the mechanical properties as a function of grain size, porosity, and the AgNP are discussed in terms of the pertinent literature. While K C was independent of AgNP addition, porosity, and grain size, both E and H decreased linearly with increasing porosity. In addition, this study is the first to identify (i) the Ag3Sb phase formed and (ii) the enhanced densification that occurs when the AgNP is sintered with Ba0.3Co4Sb12 powders, where both effects are consistent with the eutectic and peritectic reactions observed in the binary phase diagram Ag–Sb. These eutectic/peritectic reactions may also be linked to the enhancement of electrical conductivity previously observed when Ag is added to Ba0.3Co4Sb12. Also, similar beneficial eutectic/peritectic reactions may be available for other systems where conductive particles are added to other antimonides or other thermoelectric systems.

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 >603K 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 936K. 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 specimens. Inhibition of bloating likely occurs due to cleaning of contamination from powder particle surfaces via PECS processing which has been reported previously in the literature.

Gadolinia doped ceria (GDC) is commonly used in chemical reactors, gas sensors, gas separation membranes, and solid oxide fuel cells (SOFCs). In the present study, the room temperature elastic properties of GDC10 (Ce0.9Gd0.1O1.95) were measured as a wide function of porosity for the first time. GDC10 specimens with volume fraction porosities ranging from 0.07 to 0.60 were produced by hard die pressing and sintering GDC10 powders in air at temperatures ranging from 825 to 1475°C. The room temperature Young's modulus, shear modulus, bulk modulus and Poisson's ratio were measured using resonant ultrasound spectroscopy. The elastic moduli decreased exponentially with increasing porosity.

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.

Low cost, highly efficient thermoelectric materials for waste heat recovery applications can be made by combining the naturally occurring thermoelectric mineral tetrahedrite (Cu10Zn2As4S13) and the synthetic compound Cu12Sb4S13. To better utilize this material in waste heat harvesting applications, it is essential to characterize the material’s mechanical properties including elastic modulus, hardness, and fracture toughness. In this study, powders of Cu10Zn2As4S13 were mixed with varying amounts of Cu12Sb4S13 and then densified by hot pressing. The room temperature mechanical properties were investigated as a function of (i) composition and (ii) ball milling time. Elastic moduli were measured using resonant ultrasound spectroscopy. Hardness and fracture toughness were determined by Vickers indentation technique.

This is the first report of fracture toughness analysis of a new ceramic electrolyte (Li7La3Zr2O12). The fracture toughness was measured using an indentation technique and subsurface crack propagation was analyzed using a novel focused ion beam/scanning electron microscope (FIB–SEM) technique. The fracture toughness of polycrystalline Li7La3Zr2O12 was between 0.86–1.63MPa√m. FIB–SEM analysis of the indentation cracks suggested that both half-penny and Palmquist cracks were present. The fracture toughness of Li7La3Zr2O12 is comparable to other state of the art ceramic electrolytes such as Li0.33La0.57TiO3 and Li1.3Al0.3Ti1.7(PO4)3.

Porous hydroxyapatite (HA) is used in a variety of applications including biomedical materials such as engineered bone materials and microbe filters. Despite the utility of the Weibull modulus, m, as a gauge of the mechanical reliability of brittle solids, there have been very few studies of m for porous HA. A recent study of porous HA that included the current authors (Fan, X., Case, E.D., Ren, F., Shu, Y., Baumann, M.J., 2012a. Journal of the Mechanical Behavior of Biomedical Materials. 8, 21–36) showed increases in m for porosity, P, approaching PG, the porosity of the green (unfired) specimen. In this paper, 18 groups of highly porous HA specimens (12 groups fabricated in this study and 6 groups from Fan et al., 2012a) were analyzed with P values from 0.59 to 0.62, where PG=0.62. The partially sintered HA specimens were fractured in biaxial flexure using a ring-on-ring test fixture. The fracture strength decreased monotonically with decreasing sintering temperature, Tsinter, from 4.8MPa for specimens sintered at 1025°C–0.66MPa for specimens sintered at 350°C. However, the Weibull modulus remained surprisingly high, ranging from 6.6 to 15.5. In comparison, for HA specimens with intermediate values of P, from about 0.1–0.55, the Weibull modulus tended to be lower (ranging from about 4 to 11) than the highly porous specimens included in this study.

Mg2(Si,Sn) compounds are promising candidate low-cost, lightweight, nontoxic thermoelectric materials made from abundant elements and are suited for power generation applications in the intermediate temperature range of 600 K to 800 K. Knowledge on the transport and mechanical properties of Mg2(Si,Sn) compounds is essential to the design of Mg2(Si,Sn)-based thermoelectric devices. In this work, such materials were synthesized using the molten-salt sealing method and were powder processed, followed by pulsed electric sintering densification. A set of Mg2.08Si0.4−x Sn0.6Sb x (0 ≤ x ≤ 0.072) compounds were investigated, and a peak ZT of 1.50 was obtained at 716 K in Mg2.08Si0.364Sn0.6Sb0.036. The high ZT is attributed to a high electrical conductivity in these samples, possibly caused by a magnesium deficiency in the final product. The mechanical response of the material to stresses is a function of the elastic moduli. The temperature-dependent Young’s modulus, shear modulus, bulk modulus, Poisson’s ratio, acoustic wave speeds, and acoustic Debye temperature of the undoped Mg2(Si,Sn) compounds were measured using resonant ultrasound spectroscopy from 295 K to 603 K. In addition, the hardness and fracture toughness were measured at room temperature.

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.