Study On Synthesis And Thermoelectric Properties Of Layered Calcium Cobalt Oxides

Thermoelectric materials are a kind of semiconducting functional materials, which can be used to interconvert heat energy and electricity directly. They offer a reliable, fully solid-state means of power generation and cooling. Recently layered calcium cobalt oxides attract considerable attention due to their large Seebeck coefficient and low thermal conductivity. Layered calcium cobalt oxides are promising thermoelectric materials.Ca₂Co₂O_5+δ, Ca₃Co₄O_9+δ and (Ca_0.85OH)_1.16CoO₂ are hot layered calcium cobalt oxides. Ca₂Co₂O_5+δ, as a raw compound, can decompose into Ca₃Co₄O_9+δ at a high temperature and into (Ca_0.85OH)_1.16CoO₂ under an alkaline hydrothermal condition, respectively. Here, Ca₂Co₂O_5+δ was prepared by molten salt method and improved sol-gel method. Ca₃Co₄O_9+δ was prepared by improved sol-gel method.(Ca_0.85OH)_1.16CoO₂ was prepared from Ca₂Co₂O_5+δ by hydrothermal method. The structure, morphology and thermoelectric properties of the oxides were characterized by TG-DTA,XRD,SEM,FT-IR,XPS. Furthermore, ions doping was used to improve the thermoelectric properties of three oxides and the electron transport mechanism was studied by measuring electrical conductivity.Single-phase Ca₂Co₂O_5+δ was successfully synthesized by molten salt method and improved sol-gel method. The XRD analysis showed that the crystal structure of Ca₂Co₂O_5+δ oriented along the ab-plane during the process of molten salt synthesis. The formation process of the grains is formed along dissolution-precipitation process, and the growth of the grains is controled by interface growth. The bulk specimens of the Ca₂Co₂O_5+δ powders were prepared by no-press sintering. The thermoelectric properties were compared each other for samples prepared by two methods. The results indicated that the power factor of the sample synthesized by molten salt method is higher than that synthesized by improved sol-gel method. But the ZT value of the latter is larger than that of the former. Furthermore, Ca_2-xNa_xCo₂O_5+δ (x=0-0.3) were synthesized by improved sol-gel method. The effects of Na doping on thermoelectric properties were investiged. The results indicated that the electrical conductivity increases while Seebeck coefficient and thermal conductivity decreases, the ZT value increases with Na-doped content. The maximal ZT value (0.116) can be obtained at 973 K.The bulk specimens of Ca₃Co₄O_9+δ, Ca3-xErxCo4O9+δ(x=0-0.5) and Ca_3-x-yNd_xNa_yCo₄O_9+δ (x=0-0.3; y=0, 0.1) were prepared by improved sol-gel method combined hot-press sintering technique. The results indicated that the electrical conductivity decrease/increase with Er, Nd/Na contents, while the Seebeck coefficient changes reversely because the carrier concentration is changed due to substitution of Er, Nd/Na ions for Ca²⁺. On the other hand, the thermal conductivity decreases with Er or Nd content due to the scattering of heavy atoms of Er or Nd. The ZT value increases with Er or Nd content. Futhermore, Nd and Na co-substitution can further increase the ZT value of Ca₃Co₄O_9+δ by changing the carrier concentration and by increasing the disorder of the atoms. For Ca2.6Nd0.3Na0.1Co4O9+δsamples, the ZT value reaches 0.29 at 1073 K.The electrical conductivity behavier for Ca₃Co₄O_9+δ and doped Ca₃Co₄O_9+δ samples fit the small polaron hopping conduction model in the temperature range of 550-1073 K. The difference of activation energies are insignificant (not larger than 0.02 eV), indicating that the conduction path of the CoO2 layer is not changed by doping Er, Nd or Na in Ca₃Co₄O_9+δ system.Single-phase (Ca_0.85OH)_1.16CoO₂ was synthesized from Ca₂Co₂O_5+δ by hydrothermal method. The electronic calculation indicated that (Ca_0.85OH)_1.16CoO₂ is a direct-gap semiconductor material. The density of state analysis showed that the top of valence-band and the bottom of conduction-band are mainly consisted of Co 3d-t2g states and Co 3d-eg states, respectively. The conductive mechanism is d-d transition of Co.The effects of Na and Nd substitution of Ca on the thermoelectric properties of (Ca_0.85OH)_1.16CoO₂ were investigated at the temperature range of 323～573 K. The results showed that the electrical conductivity increases with Na-doped content, but first decreases then increases with Nd-doped content. The seebeck coefficient decreases/increase with Na/Nd-doped content. Furthermore, the thermoelectic properties of (Ca_0.85OH)_1.16CoO₂ were not improved by co-substituting of Nd and Na for Ca. The maximal PF values are the same for (Ca_0.7Na_0.1OH)_1.16CoO₂, (Ca_0.7Nd_0.1OH)_1.16CoO₂ and (Ca_0.75Na_0.05Nd_0.05OH)₍1.16_CoO₂ samples at 573 K.There is an interesting metal-semiconductor change around 370 K on the curve of electrical conductivity for both Ca₂Co₂O_5+δ, Ca₃Co₄O_9+δ and (Ca_0.85OH)_1.16CoO₂ samples. The change is caused by a spin-state transition from low spin (LS, t³_2ge²_g) to intermediate spin (IS, t⁵_2ge⁰_g) for Co⁴⁺ and Co³⁺. This change gradually disappears when the ratio of Co³⁺/Co⁴⁺ is increased by substitution of Ca²⁺ by ions of higher valance of Co⁴⁺ and Co³⁺.