Rapid synthesis and thermoelectric properties of clathrate Ba8Cu6Ge15Si25 by high temperature and high pressure

Ba8Cu6Ge[Formula: see text]Si[Formula: see text] were successfully synthesized by a simple high pressure and high temperature (HPHT) method to investigate the microstructures and thermoelectric (TE) properties. After high pressure synthesis, a highly dense bulk material with lots of fine-layered structure, lattice defects and disorders has been obtained. As expected, the thermal conductivity decreased greatly and the ZT value has been improved significantly, which reaches up to 0.43 at around 773 K. Comparing with other methods, HPHT could shorten the synthesis time from several days to half an hour. It reveals that the HPHT method will become an effective approach for optimizing the TE performance of these materials.

Dependence of the Yield and Fatigue Strength of the Thread Rolled Mild Steel on Dislocation Density

Dependence of the yield and fatigue strength of steel bolts with composition in accordance to AISI 1035 manufactured by thread rolling and machining process on dislocation density were investigated. The results indicate that the fatigue strength of the rolled bolts are 55% higher than the machined bolts and by full annealing at 850°C, it reduced to the extent of machined specimen. Partial annealing of the thread rolled bolts at 680°C caused a reduction of fatigue strength by approximately 61% due to reduction in the dislocation density. Fatigue strength was improved by deformation rate (i.e., rolling speed), which is also due to the increasing dislocation density. Yield stress of the studied specimens followed the same pattern as fatigue strength. Considering the obtained results from the low and high speed, partial and full annealed thread rolled specimens, yield stress of the thread rolled bolts has been modeled based on the dislocation density. The obtained results from the model are in good agreement with the experimental results. The contribution to fatigue strength by thread rolling stems from the strain hardening effect which would facilitate the formation of compressive residual stress near the surface layer. The strengthening may be attributed to increasing dislocation density in the ferrite phase (i.e., substructure formation), in addition to the formation of a fine layered structure consisting of elongated pearlite colonies and ferrite grains.

Experimental determination of the elastic coefficients of an orthorhombic material

In terms of elastic anisotropy, many rocks may be considered to have orthorhombic symmetry. Experimentally determining the nine independent elastic coefficients required for this case remains challenging. Elastic coefficients are most often found from measurements of the phase velocity in a variety of directions throughout a material, but finding this plane‐wave velocity is problematic. Here, quasi‐P and quasi‐S phase speeds are found using the τ‐p transformation through a composite material of orthorhombic symmetry. Arrays of specially constructed transducers (0.65 MHz) with different modes of vibration were placed on a rectangular prism of the material. More than 620 individual measures of phase speed were obtained at different directions and subsequently used in a generalized least‐squares inversion that yields the required elastic coefficients. The analysis does not account for the effects of wave‐speed dispersion evident in the waveforms acquired in the composite material. This dispersion is particularly severe for the in‐plane, quasi‐S polarization and is possibly a consequence of the fine layered structure of the material.