Modification And Mechanism Of Second Phase And Acceptor Doping For Yb_0.3Co₄Sb₁₂

Thermoelectric materials,as a kind of new energy materials,can realize the direct conversion between heat and electricity,which have a great application prospect in the field of waste heat power generation and thermoelectric refrigeration.In intermediate-temperature range,Co Sb₃ based skutterudites have been considered to be one of the most commercial application due to their excellent thermoelectric properties and structural stability.Yb_0.3Co₄Sb₁₂ alloy is the most representative n-type single filled skutterudite,while the relatively high thermal conductivity exists the space for further reduction.Therefore,one of the most effective ways for improving the thermoelectric performance of the alloy is to suppress the total thermal conductivity without damaging the electrical properties.In this paper,second phase and acceptor doping were adopted in Yb_0.3Co₄Sb₁₂ alloy to respectively reduce the lattice thermal conductivity and electronic thermal conductivity,which effectively inhibited the total thermal conductivity and boosted ZT value.The effects of the second phase and acceptor doping on microstructure,thermoelectric transport performance and mechanical properties were systematacially investigated by scanning electron microscope,transmission electron microscope,X-ray diffraction instrument,laser thermal conductivity instrument,Seebeck coefficient/electrical conductivity test system,Hall coefficient measurement system,nano indentation instrument and Vickers hardness tester.A large number of dispersive Co Si nanoparticles were constructed in Yb_0.3Co₄Sb₁₂ alloy by melt spinning and hot-press sintering method with adding additional equiatomic Co and Si,which could enhance phonon scattering and reduce lattice thermal conductivity.Simultaneously,the low energy carriers are effectively filtered out by the schottky barrier of interfaces between Co Si and skutterudite,leading to the increased Seebeck coefficient and the boosted power factor.With the increase of Co Si nanoparticles,When the Co Si content was up to 0.05,ZT_max reached1.5@873 K and ZT_ave reached 0.96(300?873 K),deriving from the maximum power factor and the lowest lattice thermal conductivity.In addition,the hardness and fracture toughness of the alloy were synergistically enhanced by the participation of hard phase Co Si nanoparticles based on precipitation strengthening.The submicron scale Co Al phase in ohmic contact with the matrix was successfully constructed by in-situ method with adding additional Al in Yb_0.3Co₄Sb₁₂alloy.With the increase of Al content,Yb filling fraction gradually decreased,while both carrier concentration and electrical conductivity decreased first and then slightly increased.Due to the reduced carrier concentration,the seebeck coefficient were significantly boosted,leading to the improved power factor in low temperature range.In addition,the lattice thermal conductivity first increased and then decreased,while the total thermal conductivity was remarkably suppressed.The maximum of ZT reached 1 36@873 K and ZT_ave reached 0.96(300?873 K)when Al doping amount was 0.3.Through adding equiatomic Co and Al to ensure the invariable Yb filling fraction,it was found that a part of electrons of Co Al phase were injected into the matrix,leading to increased carrier concentration and electrical conductivity.Simultaneously,the lattice thermal conductivity was effectively reduced due to the enhanced phonon scattering.Furthermore,the mechanical properties of the alloy were strengthened by the introduction of hard phase Co Al particles.The Sn or Ge acceptor doping of IV group elements boosted the thermoelectric properties of Yb_0.3Co₄Sb₁₂ alloy.With the increased IV group elements,the carrier concentration was observably decreased,combining with the role of ionized impurity scattering,resulting in that the electrical conductivity was significantly decreased,while the absolute value of Seebeck coefficient was expectedly increased.Although the bipolar thermal conductivity was gradually prominent,the total thermal conductivity was effectively suppressed by the significantly reduced electronic thermal conductivity and the relatively low lattice thermal conductivity.When Sn doping was up to 0.15,the maximum of ZT value reached 1.42@873 K and ZT_avereached 0.98(300?873 K).Comparatively,Ge doping not only synergistically modulates Fermi energy level and strength of ionized impurity scattering to an optimal range in the determination of a benign power factor,but also offers a valuable oppprtunity to further suppress?_e for classic Yb_0.3Co₄Sb₁₂ alloy.Thus,the ZT_ave of1.02(300?873 K)in the Yb_0.3Co₄Sb_11.85Ge_0.15sample were obtained.The theoretical output power density and the conversion efficiency of single leg are calculated by assuming the cold-side temperature 300 K,hot-side temperature 850K,leg length of 4 mm.The results indicated that Yb_0.3Co₄Sb₁₂/0.05Co Si and Yb_0.3Co₄Sb₁₂/0.3Al alloys possessed the maximum theoretical output power density,was 9.61 W cm^-2 and 9.53 W cm^-2 respectively.Under the condition of unlimited heat source,the two alloys have great advantages.However,the Yb_0.3Co₄Sb_11.75Ge_0.25,Yb_0.3Co₄Sb_11.75Ge_0.25/0.05Co Si,Yb_0.3Co₄Sb_11.85Sn_0.15/0.05Co Si alloy had the maximum conversion efficiency?15%.Under the condition of limited heat source,the three alloys have the greatest practical application potential.