Preparation And Thermoelectric Properties Of Doped Cage-like Materials

Thermoelectric(TE) materials, as one of functional materials, can directly converse heat energy into electric energy, and vice versa. Co Sb3–based skutterudites and type-I clathrates, with cage-like structure, are two of the most promising TE materialssatisfying the concept of “phonon glass-electron crystal”. Thermoelectric performance can be manipulated independently in this type of structure via doping or filling. In this thesis, high TE performance cage-like TE materials have been synthesized through doping or filling by using high-pressure synthesis technique.High purity powder elements were used as raw materials, and high-pressure synthesis combined with spark plasma sintering was carried out to prepare the samples. X-ray diffraction measurements, scanning electron microscope, Raman scattering spectra and PPMS system analysis were used to analyze the composition, structure and carrier concentration of samples. The electrical resistivity and Seebeck coefficient were measured with ZEM-3 and the thermal conductivity was measured with TC-9000 H.Two groups of Co Sb3 based samples were synthesized according to the stoichiometric compositions Co Sb3-xSex(x = 0.05, 0.1, 0.05, 0.1) and Co Sb2.85-yTe0.15Sey(y= 0.025, 0.05, 0.025). Se are doped into the poison of Co Sb3 lattice frame through the method of high-pressure synthesis which is impossible under the conduction of atmospheric pressure, and the biggest doping fraction of Se can reach to 3% under the condition of high pressure 5 GPa. The thermal conductivity of the sample of x=0.3 in the whole range of temperatures is below 1.83W/Km. The co-doping amount of Se/Te are improved obviously compared the conduction of high pressure with atmospheric pressure. The ZT value of sample y=0.5 at 780 k can reach 1.29.Three Ba-Al-Ge based samples were synthesized according to the stoichiometric compositions CexBa8.2-xNi0.62Al13.8Ge31.58(x=0.5,1.2)and Ce1.2Ba7.0Ni0.44Al13.8Ge31.76. X-ray diffraction shows that the samples are of the single phase, but the back scattered electron imaging shows that there are two kinds of second phase in them. Surface scanning imaging shows that Ce can uniformly distribute in matrix phases and second phases. The final samples are the type I clathrates with Ce filling in hole of lattice frame, and there are some second phases in them. The increase of Ce content significantly increased the ZT value of samples. The ZT value of sample Ce1.2Ba7.0Ni0.44Al13.8Ge31.76 can reach 0.26 at 900 k.A group of Ba-Ga-Ge based samples were synthesized according to the stoichiometric compositions Ba8-xYbxNi0.1Zn0.54Ga13.8Ge31.56(x = 0, 0.1, 0.3 and 0.5). X-ray shows that these samples are single phase samples. EDS shows that the actual Se doping contents of samples of x=0.3 and x=0.5 are 0.21 and 0.32 respectively. However, all of Yb-samples contain a small amount of Yb2O3, which may be attributed to to the inevitable oxidation of Yb. The thermal conductivity of sample of x=0.3 at 800 k can be as low as 1.23 W/Km, which lead to the big ZT value of 0.91.