Synthesis And Properties Of Doped Oxide Thermoelectrical Materials

With the developing of Chinese economy,the problems of energy and environment have become more and more serious.Thermoelectric materials(TE) have become a fashion in the research field of materials due to the special functions of the utilization of energy and environmental protection.TE materials convert the energy between the heat and electricity depending on migratory energy by cavities and electrons.TE materials were widely used in the important field,such as low temperature refrigeration and TE generation.Oxide systems such as NaCo₂O₄, Ca₃Co₄O₉ and La_xCa_yMnO₃ exhibit high performance as TE materials.It has been reported that some metallic elements doped into these TE materials could improve their thermoelectric performance.So we except to obtain the high power factor TE materials through doping different content of Cu and Ag element into Co site of Na₍1.7_Co₂O₄.On the other hand,we hoped to make sure the value of x and y of the La_xCa_yMnO₃,in order to make a thermoelectrical devise.In this dissertation,Na_xM_yCo_2-yO₄(M=Cu,Ag)and La_xCa_yMnO₃ thermoelectric oxide ceramics were prepared by auto-ignition combining with the conventional powder metallurgy techniques.X-ray diffraction,TEM and SEM studies were used to demonstrate the crystal structure and morphology of products.The content of elements and density have some effects on electrical conductivity,seebeck coefficient,thermal conductivity,power factor and ZT of Na_xM_yCo_2-yO₄(M=Cu,Ag) and La_xCa_yMnO₃ thermoelectric materials.The main achievements of the work are listed as followings:1.Self-ignition route can be used to obtain single-phase Na_xCo₂O₄ powder with an average diameter of about 2μm in the ultrafine powders combined with the following process of airflow crushed.Over 20 per cent NaNO₃ content was added to make up for Na evaporation.The traditional powder metallurgy process can be used to prepare the ceramic oxide thermoelectric materials with a high degree of directional crystallization of When x=1.7.The thermoelectrical properties of product were improved,and the ceramic has no impurity,which can be chosen as matrix to dope with Cu and Ag.2.Cu and Ag dopents can also promote the hexagonal Na_xCo₂O₄ crystalline grains growing along the C-axis orientation.Especially,Na_1.7CU_yCO_2-yO₄ and Na_1.7Ag_0.2Co_1.8O₄ were most obvious,Lotgering factor reached 0.975.Doping can reduce the grain size, further affect the density of samples.The maximum density of Na_1.7Cu_yCo_2-yO₄ is 4.219 g/cm³ and Na_1.7Ag_yCo_2-yO₄ is 4.638 g/cm³.3.The thermoelectric properties of Na_xCo₂O₄ can be improved by the appropriate density and content of doping.Na_1.7Ag_0.2Co_1.8O₄ has best micromechanism and thermoelectric properties among all the composition.When the measurement temperature was increased to 873K,its value of ZT reached the maximum:0.316.4.The materials prepared by self-ignition route and conventional PM method have single phase,homogeneous composition and no impurity.La_0.6Ca_0.4MnO₃ performed good thermoelectric properties.When the temperature was 873K,its value of ZT reached 0.074.This study provided a new route to research the oxide thermoelectric materials.The self-ignition route and conventional PM method can be used to prepare the doped oxide thermoelectric materials which have single phase,homogeneous composition and fine thermoelectric properties.