Thermoelectric And Mechanical Properties Of Nano/Micro Composites

Recent years, nano thermoelectric composites prepared by nano-technology, have a bright prospect in improving ZT value and mechanical properties. CoSb3 is one of the most promising thermoelectric materals in the moderate temperature range, and it becomes a popular reseach spot in the thermoelectric materials. In this thesis, two different nanoparticles (nanoα-Al2O3, single-walled carbon nanotubes) were included into matrix CoSb3 separately. With combination of solid-state reaction method, ball-milling mixing, and spark plasma sintering, the bulk composites were synthesized with homogeneous dispersion and good relative density. The research focused on the effects of this two different nano particles on the thermoelectric and mechanical properties of P-type CoSb3.For the nanoα-Al2O3/P-type CoSb3 composite, when the content of nano particles reached 0.6wt%, porosity decreased compared with non-inclusion. A small amount of nano-particles can well fill the gap of micron CoSb3 particles, thus relative density was increased. Thermoelectric test results revealed that compared with pure CoSb3, as the content of nanoα-Al2O3 increased, the electrical conductivity of composites decreased, Seebeck coefficient increased, and thermal conductivity decreased significantly. As a result, ZT of the composites mainly remains unchanged. Mechanical test results displayed a small amount addition of nanoα-Al2O3 had no impact on Young's modulus. But the failure mechanism was changed from intergranular cracking to transgranular cracking, then the strength was improved greatly. When the content of nanoα-Al2O3 was lwt%, compressive strength and bending strength of the composite reached 630MPa and 179MPa, increased by 42.7% and 52.0%.When a small amount of single-walled carbon nanotubes(SCNT) was added into P-type CoSb3, the composites would have bad relative density because SCNT looks threadlike and was difficultly mixed. At the same time, compound powders were easily oxidized in Milling, P-type CoSb3 was changed into N-type with electrical conductivity decreased. When the content of SCNT reached 3wt%, there came a change from P-type to N-type in high temperatures. Because of SCNT as the second phase, thermal conductivity of the composites had a significant reduction. When the the content of SCNT was only lwt%, it decreased by 34% compared with pure CoSb3 at room temperature. As a result, ZT had decreased slightly with no importance. A small amount of SCNT as additive which warped micron CoSb3 particles can improve fracture toughness because of carbon nanotubes with excellent mechanical properties. When the content of SCNT was 0.5wt%, the fracture toughness reached the maximum 1.7MPa*m1/2, increased by 13%.CoSb3 as the matrix with a small amount of nano particles as additive can be improved in mechanical strength, without thermoelectric decreased. But the preparation technology of nano/micro composites yet has to be improved, in order to obtaining better thermoelectric and mechanical performance.