Dislocation Array Construction And Thermoelectric Properties Of Filled Skutterudites

Medium-temperature thermoelectric materials Co4Sb12 based skutterudites possess high power factor,excellent mechanical properties and thermal stability,which have great potential for applications.However,their high lattice thermal conductivity leads to the low ZT values.Therefore,reducing the lattice thermal conductivity without the sacrifice of the electrical properties is one of the most effective ways to enhance their thermoelectric performance.In this thesis,liquid phase compaction and spinodal decomposition were used to construct dislocation arrays to enhance phonon scattering and reduce lattice thermal conductivity and thus improve ZT values of n-type Yb filled Co4Sb12 skutterudites and p-type Ce and La filled Fe3 Co Sb12 based skutterudites.The influence of dislocation arrays and filled atom content on the microstructure and thermoelectric properties was investigated by X-ray diffraction,scanning electron microscopy,transmission electron microscopy,physical property measurement system,the Seebeck coefficient and electrical conductivity test system and laser flash diffusivity apparatus.Liquid phase compacted n-type Yb filled Co4Sb12 skutterudites were prepared by melt spinning and hot-press sintering with adding additional Sb.A large number of dislocation arrays were constructed in the liquid phase compaction process,which could filter the low energy carrier and improve the power factor.Simultaneously,the phonon scattering was enhanced by dislocation arrays and the lattice thermal conductivity was reduced.With the increase of additional Sb content,both ZT value and average ZT value(ZTavg)increased first and then decreased.Finally,the maximum value of ZT reached 1.45@823 K and ZTavg reached 1.08 from 300 K to 823 K when the additional Sb content was 20%.In this basis,the thermoelectric properties of the liquid phase compacted Ybx Co4Sb12 alloys were further optimized by adjusting the content of filled element Yb.When the content of filled Yb was less than 0.5,the electrical conductivity increased while the absolute value of Seebeck coefficient decreased with the content of filled Yb increasing,which resulted in the power factor first increased and then decreased.Meanwhile,the optimum carrier concentration increased with the increase of temperature,and the thermal conductivity of lattice decreased.Ultimately,the ZT value increased first and then decreased,and the maximum ZT value was up to 1.54@873 K when x=0.45.It was revealed that a great deal of dislocation arrays existed in liquid phase compacted p-type Cex Fe3 Co Sb12 alloys,which could increase the phonons scattering and decrease the lattice thermal conductivity but have little effect on the electrical properties.With the content of filler atom Ce increasing,the carrier concentration and electrical conductivity decreased,while Seebeck coefficient increased and power factor was almost unchanged.Simultaneously,the lattice thermal conductivity first decreased then stayed basically unchanged,and the total thermal conductivity was reduced.Finally,with the increase of the content of filler atom Ce,ZT value of liquid phase compacted Cex Fe3 Co Sb12 alloys increased first and then decreased,and the maximum ZT value was reached 1.1@723 K when the content of filler atom Ce was 0.8,which was 22% higher than the hot-pressed alloy with the same content of Ce filler.Moreover,the liquid phase compacted Ce0.8Fe3 Co Sb12 alloys also had the largest ZTavg value from 300 K to 823 K,which was up to 0.77.Research showed that there was spinodal decomposition structure in both annealed-hot-pressed and spun-hot-pressed La0.8Ti0.1Ga0.1Fe3 Co Sb12 alloys,which consisted of La rich and La poor skutterudite phases and dislocation arrays in phase boundaries.Compared with annealed-hot-pressed alloy,the number of phase boundaries of spun-hot-pressed alloy was increased leading to the increase of dislocation arrays density,which produced stronger strain field and improved the DOS near the Fermi level and the Seebeck coefficient with little effect on electrical conductivity,thus the power factor was improved.Simultaneously,the lattice thermal conductivity decreased significantly due to the enhanced phonon scattering by the increased dislocation arrays.Finally,the maximum ZT value of spun-hot-pressed La0.8Ti0.1Ga0.1Fe3 Co Sb12 alloy reached 1.13@823 K,which was 40% higher than annealed-hot-pressed alloy.Furthermore,after annealing 100 h at 973 K,the thermoelectric properties of the spun-hot-pressed alloy were almost unchanged,showing an excellent thermal stability.