Studies On The Structures And Thermoelectric Properties Of Sb-based Zintl Phases

The compounds containing phosphorus element(fifteenth main families)have been widely studied because of their large number of structures,their rich structures and diverse properties.In the continuous research,we found that the superconducting properties of some iron based arsenic compounds,and the thermoelectric properties were found in some compounds containing Sb.Due to the aggravation of environmental pollution and energy shortage,thermoelectric materials,which can convert waste heat into electricity,have been paid more and more attention.Due to their complex crystal structure and electronic band structure,Zintl phase compounds are considered to be potential excellent thermoelectric materials.In this thesis,the synthesis,structure and thermoelectric properties of 9-4-9 and 1-1-1 structure compounds in Zintl phase were studied.1,Compounds of Ca9Zn4+xSb9 structure,due to its large single cell and the complex crystal structure,indicate that they may have a low lattice thermal conductivity,and then better thermoelectric properties can be expected.Through the synthesis in muffle furnace,the pure phase of Ca9Zn4.5Sb9 can be received,and its highest ZT,about 0.37 at 823K.The main reason for the low ZT is the higher resistivity of the compound,which hinders the improvement of its thermoelectric properties.2,In order to carry on the doping modification,we first design a series of experiments of Ca9Zn4.5-xAl2x/3Sb9(x=0,0.5,1)and the pure phase of these compounds can be received.Thermoelectric properties are also studied.In these compounds,with not too much change in the carrier concentration,we replaced Zn atoms by 2/3A1.In this way,we can only explore the influence of on the properties.And more interstitial defects,the lower carrier mobility,thereby reducing the conductivity of materials.After the discovery of this phenomenon,we design a series of new experiments of several compounds Ca9Zn4.5-xCuxSb9(x=0.05,0.1,0.15,0.2).The interstitial defects do not increase under the condition of doping significantly and the electrical resistivity of the material reduced.Although the carrier concentration increases the thermal conductivity,the thermoelectric properties of these compounds have increased dramatically because of the sharp rise in power factor.At 873K,ZT of Ca9Zn4.35Cu0.15Sb9 is 0.72,which is almost twice of undoped Ca9Zn4.5Sb9(ZT=0.37).In previous reports,there has been no correlation between the interstitial defects and the carrier transport.After that,we have carried out the research on the different doping atoms and obtained better thermoelectric properties.3,in order to study on the influence of cation doping in the Ca9Zn4+xSb9 compounds,we use Eu atom with greater radius replacing in the position of Ca and through XRD diffraction analysis,we confirm the results that we achieve our objective,namely "stretching the lattice".Then we find that the carrier concentration decreases slightly with the increase of doping concentration and the carrier mobility increases obviously in the hall measurement.It is also shown that the interstitial atoms hold more Zn atoms,which is caused by the larger lattice parameter after doping.Moreover,we have obtained the results that they are consistent with the changes of the electrical properties and thermodynamic properties.Based on the tests,the effective mass and Lorenz constant are studied through the calculation.Finally considering the various properties of materials,the Ca8.2Eu0.8Zn4.5Sb9 materials gains its highest value of ZT(ZT?0.81),which is also the reported highest thermoelectric properties in this kind of materials.4,Through doping rare earth elements in the YbAgSb,we receive two new crystals and the structural formula of Yb1.76La0.24Ag1.79Sb2 and Yb1.67Ce0.33Ag1.74Sb2 are determined by X-ray diffraction.According to the proportion of elements,we receive a series of new compounds Yb0.85RE0.15Ag0.85Sb(RE=La,Ce,Pr,Nd,Sm)in the method of arc smelting furnace.After that,thermoelectric properties test has been carried on with these compounds and we obtain the best figure of merit in this series of compounds.For the Yb0.85Ce0.15Ag0.85Sb material,ZT reaches 0.4 at 1023K.Based on the existing compounds,through the analysis of the influence of carrier concentration on the thermoelectric properties,we design a series of experiments.Through Yb atoms replaced by Ca,we get a series of new compounds Yb0.85-xCaxCe0.15Ag0.85Sb(x=0.1,0.2,0.3,0.4,0.5,0.6)and study their thermoelectric properties.Before 900K,the thermoelectric properties of the materials have been improved.