Internal cracks distribution in hot-pressed and spark plasma sintered nanostructured Bi2Te3-based thermoelectrics: acoustic microscopy versus optical microscopy

We have produced the samples of Bi2Te3-based nanostructured thermoelectric (TE) alloys by the method of high energy mechanotreatment from the mixture of the nano- and micro-sized powders (ball milling) followed by two methods of bulk samples preparing: (a) by the method of hot pressing sintering (HPS) and (b) by the method of spark plasma sintering (SPS). The microstructure of the obtained pellets was investigated by methods of acoustic and optical microscopy. With the crack/pore detection in acoustic microscopy images we marked that spot and cut the pellet through that spot perpendicular to the pellet's plane. The cut section, after polishing and etching, was examined on Olympus BX-51D microscope. The obtained pictures for the HP sintering samples shows, that the planes of all macro- and of micro-cracks are oriented perpendicularly to the axis of the load of the pellet. By using acoustic microscopy we investigate cracks and pores distributions in bulk of nanostructured Bi0.4Sb1.6Te3 alloy sintered by HPS and SPS methods.