Preparation And Properties Of The Ca-co-o System Oxide Thermoelectric Materials Research

Thermoelectric materials are the functional materials that can convert heat energy to electric energy directly due to thermoelectric effects and have immense prospects with many applications thermoelectric power generation and refrigeration. Ca-Co-O-based thermoelectric materials have attracted much attention in recent years because of high Seebeck coefficient and electrical conductivity, the advantages of good thermal stability, being difficult to be oxidized, non-toxic and so on. At present, the research of Ca-Co-O-based thermoelectric materials focused on the preparation of cubic materials. Recently because of the quantum size effect, these oxide materials had been provided a new low-dimensional research in the field, on the one hand, greatly reducing the thermal conductivity, on the other hand, in maintaining a high level of electrical conductivity while making small changes in the Seeback coefficient. The sol-gel method and solid reaction method were used to synthesize the three kinds of Ca-Co-O-based thermoelectric material ceramics for the first time. Then Ca-Co-O-based thermoelectric thin films were grown on different kind substrates by pulsed-laser deposition techniques (PLD).These work provided a new way to improve the thermoelectric properties of Ca-Co-O-based thermoelectric materials.First the development of the history of the thermoelectric effect, thermoelectric materials, as well as the application of thermoelectric materials to improve the way was described.Then the classification of thermoelectric materials and thermoelectric materials preparation methods have been done in detail. The preparation process, microstructure and electrical properties of Ca3CoO9, Ca3Co2O6 and Ca2Co2O5 were investigated by x-ray diffraction, metallograph microscopy, scanning electron microscopy, electron spectrometer, four probe standard technique and so on.XRD analysis showed that the crystallization quality of Ca3Co4O9 prepared by sol-gel method were better than by solid reaction method, the single phase Ca3Co2O6 ceramics were prepared by Sol-Gel method, the single phase Ca2Co2O5 ceramics were prepared by solid reaction method. Ca3Co4O9 thin films and Ca2Co2O5 thin films appear to be obviously c-axis-oriented. Microstructure analysis showed that the grains of Ca3Co4O9, Ca3Co2O6 and Ca2CO2O5 ceramic samples were small and relatively compact. The grains of Ca3Co4O9, Ca3Co2O6 and Ca2Co2O5 prepared by solid reaction method were larger than by sol-gel method. SEM analysis showed that the surface roughness of Ca3Co4O9 and Ca2Co2O5 thin films were higher, uniform grain size, and Ca3Co2O6 thin films were not formed on the surface, there ere a lot of holes distribution.Electron spectroscopy analysis showed that Ca3Co4O9, Ca3Co2O6 and Ca2Co2O5 thin films all exhibited Co elements volatilization at different temperatures. The relationship between resistance and temperature test results showed that the better crystallization quality of thin films and ceramic samples, the more stable trend of the relationship between resistance and temperature can be acquired. Ca3Co4O9, Ca3Co2O6 and Ca2Co2O5 thin films exhibited obviously semiconductor behaviour.The better preparation process of Ca3Co4O9 and Ca2Co2O5 thin films were established by the phase, microstructure, composition and electrical properties. Ca3Co4O9 were deposited on were deposited on A12O3 (0001) substrate at a substrate temperature of 800℃, under an deposition oxygen pressure 17Pa, with a laser energy of 450mJ, under an annealing oxygen pressure of 1×104Pa. This experiment had also obtained the better preparation process of the Ca2Co2O5 thin film for the first time. Ca2Co2O5 thin films were deposited on Si (100) at a substrate temperature of 700℃, under an deposition oxygen pressure 50Pa, with a laser energy of 300mJ, under an annealing oxygen pressure of 1×104Pa, on A12O3 (0001) at a substrate temperature of 725℃, under an deposition oxygen pressure 50Pa, with a laser energy of 300mJ, under an annealing oxygen pressure of 5x104Pa,at an annealing temperature of 800℃.