| Misfit layered Ca3Co4O9 with the feature of "phonon glass-electron crystal"consists of the conductive CoO2 layers and insulating Ca2CoCO3layers,the Co4+(3d5,t2g5 eg,0 D= 1/2)with low spin state in CO2 layers provide hole carriers with high consentration(～1020 cm-3)responsing for the high electrical conductivity,the misfit arranged Ca2CoO3 layers along b-axis serve as phonon block resulting in the low lattice thermal conductivity(～3.5W/mK).Meanwhile Ca3Co4O9 has high thermopower and large transport anisotropy along and within the layers.In this thesis systematical experiments have been carried out on the preperation of highly c-axis oriented Ca3Co4O9 polycrystalline bulks,and growth of c-axis epitaxial films by pulsed laser deposition(PLD),the thermoelectric transport properties and laser-induced transverse voltage(LITV)effect in which have been exprementally investigated,and the structure anisotropy related thermoelectric transport anisotropy along ab-plane and c-axis as well as the mechanism of which have been preliminarily investigated.Ca3Co4O9 polycrystalline bulks with c-axis orientatation have been prepared by conventional pressless sintering in air.Bulks with(00l)lotgering factor of 0.74 can be attained after sintering at 800℃/24h+870℃/24h with pre-pressing of 20MPa uniaxially.The samples exhibit metallic and insulator electrical transport behaviors in the temperature ranges of 80～300K and 336～826K,respectively,which mainly derive from the lattice scatter and intrinsic heat excitation of Co4+ in the different temperature ranges;Seebeck coefficient was about 130μV/K at 336K and increase with the increasing temperature which due to the extra thermopower provided by phonon dragging.Ca3Co4Og polycrystalline bulks prepared by hot-press sintering have poorer crystalline quality and less preferred orientations,which may due to the insufficient stoichiometric of oxygen in the samples.Under the irradiation of pulsed laser(λ＝248nm,pulse width=28ns,energy density=17mJ/cm),LITV response was obtained along the inclined direction with peak voltage of 42mV,which mainly due to the intrinsic Seebeck tensor anisotropy△S＝Sab-Sc of the material was strongly neutralized by the grains with orientations deviated from(00l).Epitaxial Ca3Co4O9 films have been prepared on LaAlO3(001)、SrTiO3(001)、Al2O3(0001)single crystal substrates with different tilted angles.The deposition rates of～0.163A/pulse abd～0.256A/pulse have been deduced by AFM surface morphology observations of films with different thickness and growth stages.Ca3CO4O9 films exhibit Stranski-Krastanovs growth mode on planar LaAlO3 and SrTiO3 substrates,while Step-flow growth mode on tilted LaAlO3 substrates.Furthuremore,with the increasing tilted angles of Al2O3 substrates,the films morphology gradually transfer from smooth surface into island bumps.The Ca3Co4O9 films show metallic and insulating electrical transport behaviors along the plain and tited directions in the temperature range of 100～300K,the electrical transport anisotropy ρc/ρab≈300 has been deduced by tensor transformation of anisotropic resistivity and the origin of which has been discussed;The thinner and tilted films exhibit larger Seebeck coefficients when≥700K,which may derive from the transformation from low spin state to intermediate spin state of Co4＋carriers by defects and stacking faults.LITV singnal of 416mV with normalized response rate of 173mvcm/mJ have been detected under the laser radiation with energy density of 2.4mJ/cm2,in the range of 44～117nm and 0～10°,the LITV peak values increase with increasing film thickness and tilted angles.From the LITV singals,the intrinsic thermopower anisotropy △S＝Sab-Sc≈20pμV/K has been deduced,the origin of Sab and Sc in Ca3Co4O9 have been preliminarily discussed. |
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