Modulation Of Thermoelectric Properties For Ca₃Co₄O₉ Oxide Ceramics

Thermoelectric materials can realize the direct conversion of thermal and electrical energy based on the thermoelectric effect,which is a new type of functional material with a wide range of applications in the fields of power generation and refrigeration.Layered cobalt-based oxide Ca₃Co₄O₉ is a promising high-temperature thermoelectric material with the advantages of high temperature resistance,oxidation resistance and no pollution.Ca₃Co₄O₉ with excellent thermoelectric properties requires both highly conductive and Seebeck coefficient,as well as lower thermal conductivity,and single optimization of one of these parameters often leads to non-synergistic changes in the other parameters.Based on the thermoelectric transport theory and combined with the characteristics of Ca₃Co₄O₉ layered structure,the competition between Seebeck coefficient and conductivity is weakened by using doping ions,introducing intrinsic vacancies,and composite nanomaterials to modulate material carrier conduction,impurity energy levels,and Co electron spin entropy.Meanwhile,the introduction of point defects,interfacial thermal resistance,and porosity reduces the thermal conductivity and ultimately improves the ZT value.The specifics are as follows:The effects of doping with K ions,Gd ions and Co intrinsic vacancies on the thermoelectric properties of Ca₃Co₄O₉ were investigated.The results show that the introduction of environmentally friendly monovalent K ions,which undergoes substitution doping mainly at the Ca site,increases the carrier concentration and conductivity significantly;the Seebeck coefficient does not decrease with the introduction of impurity energy levels.The doping of heavy element trivalent Gd ions and the introduction of Co intrinsic vacancies mainly affected the Co sites,increased the sample conductivity,and induced a Co4+to Co3+electron spin transition,generating additional spin entropy and enhancing the Seebeck coefficient.The doping of K ions,Gd ions and the introduction of Co vacancies all resulted in enhanced phonon scattering and reduced thermal conductivity.The ZT of Ca2.95K0.05Co4O9 reaches 0.22,which is about 35%higher than the original sample.the ZT value of Ca3Co3.94Gd0.06O9 reaches 0.19 at 773 K,which is about 27%higher than the original sample.The ZT value of Ca3Co3.925O9 reaches 0.10 at 1026 K.which is nearly 46%higher.The effect of compounding MoSi2 nanoparticles and carbon nanotubes in Ca₃Co₄O₉ on the thermal properties was investigated.The composite MoSi2 samples increased the electrical conductivity;formed impurity energy levels and improved the Seebeck coefficient.The addition of MoSi2 increased the phonon scattering center,and the difference in particle size between MoSi2 and the matrix caused stress field scattering,which decreased the lattice thermal conductivity.Theoretical analysis shows that the introduction of interfacial thermal resistance and porosity by adding MoSi2 has an important effect on reducing the thermal conductivity of the composites.The composite carbon nanotubes in Ca₃Co₄O₉ deteriorated the sample conductivity and Seebeck coefficient,but the thermal conductivity was significantly reduced.The total thermal conductivity of Ca₃Co₄O₉/0.16 MoSi2 and Ca₃Co₄O₉/10 wt.%CNTs samples were reduced by 27%and 73%,respectively,compared with the original samples.The ZT of Ca₃Co₄O₉/0.05 MoSi2 reaches 0.26 at 1080 K,which is 44%higher than the original sample.the ZT value of Ca₃Co₄O₉/3 wt.%CNTs sample is 29%higher than the original sample at 625 K.The ZT value of Ca₃Co₄O₉/0.05 MoSi2 sample reaches 0.26 at 1080 K,which is 44%higher than the original sample.