Synthesis And Characterization Of Cobalt-Base Thermoelectric Oxide Ceramics

The energy shortage and environment population are two major challenges of the21th century owing to the rapid urbanization and industrialization. In order to solve these two problems, altemative energy and new energy materials are needed.Thermoelectric material is an emerging functional material that can convert the heat into electricity directly, which have attracted a wide range of attention. Among all thermoelectric materials, cobalt-base oxides are particularly promising, because they are nontoxic, taintless, and very stable at high temperature, which is attractive for application in waste heat recovery, solar energy harversting, and heat management.In this thesis, the research progress and application of thermoelectric materials was illuminated firstly. Then cobalt-base ceramic oxide of thermoelectric materials was studied as the main object. The research was fouced on the fabrication, fundamental physical properties, and characterization the thermoelectric properties. Doping, nanocomposites and low-dimension method were used to improve the thermoelectric properties. The main research works and key results are summarized as follows.1. The preparation of samples and experimental equipments used for testing the thermoelectric properties were introduced first. A custom-designed system to measure Seebeck coefficient and electrical conductivity was built, which can measure the Seebeck coefficient and electrical conductivity stably. It was used to examine the corresponding performance of samplesprocessed by cold-pressing sintering.2. Ca3Co4O9powders with Mg doped in Ca site or Fe, Mn, and Cu doped in Co site were synthesized by sol-gel method, and then suppressed to bulk ceramic by cold-pressing sintering. The topography and properties were studied, and the effects of phase, topography on the thermoelectric properties were analyzed. The results showed that：The density of the samples is very low, which results in a lot of pore in samples. Although it increased the phonon scattering, and decreased the thermal conductivity, it also decreased the electrical conductivity substantially, resulting in a lower figure of merit ZT. For samples with Fe doped in Co site, the Seebeck coefficient and thermal conductivity were increased, while the electrical conductivity decreased. The figure of merit was increased initially after doping, and then decreased with the increased doping of Fe. Ca3Co3.95Fe0.05O9had the highest figure of merit ZT, which was reached0.12at973K, while ZT of Ca3Co4O9was0.102at the same temperature. For Mn doped samples, the Seebeck coefficient increased, while the electrical conductivity and thermal conductivity decreased when comparing with pure Ca3Co4O9sample. The figure of merit ZT was chaged little. For the Cu doped sample, the electrical conductivity decreased sharply, while the Seebeck coefficient did not change much, which results in very low ZT.3. Dual doping Ca3-xNxCo4-yMyO9powders were prepared by sol-gel method, and then consolidated into bulk ceramics by spark plasma sintering (SPS). The phase structure, topography and thermoelectric properties were examined. The results showed as following: The powders showed obvious layer structure. After SPS sintering, the density was increased with grain growth. For La and Cu dual-doped bulk samples, the average particle size was decreased to less than1μm owing to increased La element. After SPS, the grain size increased with obvious layer structure.The Seebeck coefficient of Na and Cu co-doped samples was increased, while corresponding La and Cu co-doped sample decreased. CNCCO-3showed the highest Seebeck coefficient in all samples, which reached168μV/K at873K. At the same time, CNCCO-3showed the lowest electrical conductivity, but CLCCO-3presented the highest electrical conductivity at873K, reached178.4S/cm. CLCCO-1had the highest power factor and lowest thermal diffusivity, power factor was3.84×10-4Wm-1K-2. And corresponding thermal diffusivity was4.70×10-3cm2/s. All doping samples showed lower thermal conductivity than pure CCO. CLCCO-2had the highest ZT approaching0.203at773K.4. Doped nanofibers were processed by electrospinning based on sol-gel method. The effect of major processing parameters on the topography was studied, and the thermoelectric properties of these samples were measured. It was found that the raw materials, collecting baseline and collecting time were very important to the morphology of fibers. The phase was no obvious changes with doping. Mg doped fiber showed sheet shape, Mn and Cu doped fibers showed platy structure. Thermal conductivities of all fibers samples were lower than the powders sample，which confirmed that decreasing the particle size by nanofibers is useful for reducing the thermal conductivity.5. Ca3Co4O9/Ag composites were prepared by sol-gel, and then pressed to bulk samples with cold-press sintering and SPS. The phase, morphology and thermoelectric properties were studied. It is found that the density is lower with the cold-press sintering. When increasing Ag, the density decreased and the crack increased. The thermoelectric properties were not examined because the samples were corrupted during cutting. For SPS samples, densities increased, and thermal conductivity increased as Ag increase.In summary, novelty duel-doping and original electrospinning methods were used to improve the thermoelectric properties and decrease the particle size of samples in this thesis. However, the time required for sample preparation is long by electrospinning. So how to shorten the time is a remaining issue. Meanwhile, the crystal sizes of fibers would increase when the fibers flack away from the collecting baseline after sintering. Therefore, an appropriate baseline should be choosen to keep the crystal small.