In order to fabricate durable and efficient thermoelectric generators (TEG) for applications like automobile waste heat recovery, where thermal stress is a major concern, one needs to assess the mechanical performance of the TE materials. This work reports the hardness and elastic modulus of the moderate temperature range (200-1000 8C) TE materials, including halfHeusler, skutterudites, bismuth telluride, silicon germanium alloys, and lead selenide, using nanoindentation and atomic force microscopy (AFM). The p-type half-Heusler exhibits considerably higher hardness and modulus values, and lower brittleness as compared with other materials, which may be indicative of its more robust mechanical performance.

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a b s t r a c tRecent works have established the critical role of flexoelectricity in a variety of size-dependent physical phenomena related to ferroelectrics including giant piezoelectricity at the nanoscale, dead-layer effect in nanocapacitors, dielectric properties of nanostructures among others. Flexoelectricity couples strain gradients to polarization in both ordinary and piezoelectric dielectrics. Relatively few experimental works exist that have determined flexoelectric properties and they all generally involve some sort of bending tests on micro-specimens. In this work, we present a straightforward method based on nanoindentation that allows the evaluation of flexoelectric properties in a facile manner. The key contribution is the development of an analytical model that, in conjunction with indentation load-displacement data, allows an estimate of the flexoelectric constants. In particular, we confirm the experimental results of other groups on BaTiO 3 which differ by three orders of magnitude from atomistic predictions. Our analytical model predicts (duly confirmed by our experiments) a strong indentation size-effect due to flexoelectricity.

The fracture resistance of a fully dense, transparent, polycrystalline magnesium aluminate spinel was measured from room temperature to 1400°C using the chevron-notched beam and the straight-notched beam macroflaw techniques, as well as the indentation-induced, controlled-microflaw test method, all in three-point bending. Flexural strengths were also measured for the same range of temperatures to compare with the fracture toughness measurements. From the load vs load-line displacement (P-u) curves of the chevronnotched test specimens, the crack growth resistance curves (R-curves) and the total work-of-fracture were determined. It was observed that polycrystalline MgAI2O4 exhibits rising R-curve behavior which increases with increasing test temperature. The R-curve increases are attributed to the geometric constraints due to grain bridging and grain wedging phenomena as well as secondary grain boundary microcracking processes, all of which occurred in the wake region behind the advancing crack. The work-of-fracture and the R-curves increased rapidly above 800°C coincident with the onset of increased secondary grain boundary microcracking.