Optimization On Microstructures And Transport Properties Of Iron Disilicide Based Thermoelectrics Materials

Thermoelectric (TE) materials are semiconducting functional materials, which can convert heat energy directly to electricity or reversely. They are of interest for applications in TE cooling devices and power generators. Iron disilicide (β-FeSi₂) based alloys have been selected as the subject of this study due to its non-toxicity, low cost of raw materials and high oxidation resistance.In the present work, FeSi₂ based alloys with various doping elements, and composed of submicron α-Fe₂Si₅ and nanostructured ε-FeSi, were prepared by rapid solidification. Phase transformations in the rapidly solidified FeSi₂ alloys during annealing under pressures were studied by DSC and in-situ high-temperature and high-pressure X-ray diffraction measurements (EDXRD) using synchrotron radiation. Doping modification, pre-annealing treatment and nitriding treatment has been designed to integrate the hot uniaxial pressing (HUP) process, to prepare n type and p type bulk β-FeSi₂ with fine grain sizes and improved thermoelectric properties. Microstructures of the rapidly solidified powders and the hot pressed bulk FeSi₂ alloys were studied by means of XRD, SEM, and EDX observations. Transport properties of the hot pressed bulk samples were measured, and correlations between microstructures and transport properties of all pressed samples have been discussed.It is found through the measurements of EDXRD of DSC that in the heating rate range of 0-20℃/min and the pressure range of 0-5 GPa, the phase transformation (α + ε→ β) of the rapidly solidified (RS) FeSi₂ based powders during continuous heating takes place between 600-700℃, and β phase decomposes reversely in the temperature range of 960-1100℃. It is also found that a lower heating rate and a higher pressure are not only beneficial for the β phase transformation, and also delay the decomposition of the formed β phase. These results are very important for the optimization of the HUP procedure to prepare bulk β-FeSi₂, since EDXRD has actually simulated the process of HUP. Based on the results of EDXRD, the old HUP process has been modified to a new one, which decreases the heating rate for the β phase transformation, and prolongs the duration where β phase is under pressures. The new HUP process is not only propitious to the β phase transformation and retardation of β phase decomposition, can also improve densities of the pressed β-FeSi₂.Aimed to control the grain growth of β-FeSi₂, a new process flow of "rapid solidification → preannealing → HUP" has been designed, based on the dynamic investigation of grain growth of β phase. Both n-type and p-type bulk β-FeSi₂ with submicronstructures and improved TE properties are achieved. Average grain sizes of pre-annealed Fe_0.94Co_0.06Si_2.00 have been successfully controlled to be less than 1μm, by pressing the pre-annealed powders below 920℃. The pre-annealed Fe_0.94Co_0.06Si_2.00 pressed at 850℃ has an average grain size of about 300 nm, hence the thermal conductivity is remarkably reduced by the pre-annealing process. The maximal dimensionless figure of merit ZT_max of pre-annealed Fe_0.94Co_0.06Si_2.00 sample pressed at 850℃ reaches 0.28 at 930 K, which is among the best values ever reported for bulk β-FeSi₂ based TE materials. The average grain size of pre-annealed pressed Fe_0.94Mn_0.06Al_0.02Si_1.98 is also less than 1μm, with the ZT_max of 0.19 at 930 K, which is 20%higher than that of the Fe_0.94Mn_0.06Al_0.02Si_1.98 alloy hot pressed directly from the rapidly solidified powders.The intention of introducing dispersive nitrides is performed on β-FeSi₂ based alloys, by nitriding the RS powders before they were hot pressed. Improved thermoelectric properties are achieved for Fe_0.92Mn_0.08Si_2.00 alloy, which were prepared by HUP following nitriding. ZT_max of nitrided Fe_0.92Mn_0.08Si_2.0 reaches 0.21 at 973 K, which is also among the best values ever reported for p-type bulk β-FeSi₂ based TE materials, and is 24% higher than that of the directly pressed Fe_0.92Mn_0.08Si_2.00 without nitriding treatment. It is found by systematic investigations that during the nitriding treatment of Fe_0.92Mn_0.08Si_x alloys, N reacts with Si to form Si₃N₄. The dispersive Si₃N₄ acts as the scattering center, hence decreases the thermal conductivity without worsening the Seebeck coefficient and the electrical conductivity. But when the Si/Fe ratio is above 2.1, N also acts as an n-type dopant of the excessive Si. Large amounts of n-type Si decrease the Seebeck coefficient while increase the thermal conductivity, so the ZT value of Fe_0.92Mn_0.08Si_x alloys with x > 2.1 is decreased after nitriding. In the nitrided FeAl_xSi_2.0 (x = 0.05, 0.10) alloys, N is easy to react with Al into AlN. Though the thermal conductivity is decreased by AlN, the formation of AlN decreases the actual doping concentration of Al, which results in a strong decrement of the electrical conductivity. As a total effect, nitriding decreases the figure of merit of the Al-doped FeSi₂ alloys.Doping modification is also attempted to improve the properties of β-FeSi₂. Influence of various dopants on the microstructures and TE properties of β-FeSi₂ have been studied, such as single Co- or Mn- or Al- doping, compensated Al+Ni or Al+Co doping, and double Mn+Al doping. Average grain sizes of β-FeSi₂ doped by various contents and prepared by the process route "rapid solidification → HUP → annealing" is in the range of 1-5μm. The best figure of merits for n- and p- type β-FeSi₂ is obtained at around 900 K for singly doped Feo.94Coo.06Si2.oo with ZT_max = 0.24 and Fe_0.92Mn_0.08Si_2.00 with ZT_max = 0.17, respectively. Mn+Al double doping is only beneficial with less Al content. The compensated doping of Al+Ni and Al+Co increases the thermal conductivity and decrease the thermoelectric properties of the samples, due to ambipolar diffusion caused by the existence of large amounts of compensated charge carriers with opposite types.It is reported that the electrical conductivity of β-FeSi₂ increases with the deviation of lattice constants from the stoichiometric ones. Calculated results obtained from EDXRD and XRD show that β-FeSi₂ has tetragonal structure, instead of the typical stoichiometrically orthorhombic lattice structure for both single crystal and polycrystalline of β-FeSi₂. It is also found that the processing parameters such as pressure and pressing temperature can easily change the lattice constants of β-FeSi₂, especially the lattice constant along α-axis. So the modification of crystal structure on β-FeSi₂ through processing alternation may become a sally port of improving the thermoelectric properties of β-FeSi₂.