Investigation Of Preparation Of β-FeSi₂Thermoelectric Material By Laser Sintering

β-FeSi₂-based thermoelectric material is a kind of thermoelectric material applicationwithin300K^-1200K. It has advantages of good thermoelectric capability, excellentoxidation resistance, nontoxicity and low price and rich resources of raw materials. It canbe widely applied in the thermoelectric generation and thermoelectric cooling. However,many difficulties have to be overcome during preparation of this material due to its narrowrange of single phase. A new method based on laser sintering and vacuum annealing isintroduced for preparation of β-FeSi₂-based thermoelectric material. Meanwhile,influences of different processes on its phase transformation, microstructure andthermoelectric properties are investigated.YAG laser with maximum output power of1000W is selected as heat source forsintering. Spot size and laser power density are respectively12mm and <10³W/cm². Lasersintering experiments are all carried out in specially designed vacuum laser sinteringfurnace. Finite Element Analysis （FEA） method is applied to simulate laser sintering fieldtemperature distribution as well as influences of different laser power and sintering timeon temperature field distribution. The simulation results indicate that the most appropriatelaser power and sintering time is500W and80s respectively. By contrasting influences ofsintering field on temperature before and after application of beam uniformization device,it is found that laser sintering field temperature is distributed more evenly afteruniformization, which is in favor of samples alloying, components uniformization andsintering.Influences of ball milling on synthesizing FeSi₂alloy are studied. Through XRD testand SEM analysis, the best parameters for ball milling are obtained as follows: rotationspeed of250r/min, ball material ratio of20:1, ball milling impregnant of30%, ball milling time of6h. Influences of sintering through common sintering furnace on FeSi₂alloy areanalysed and the results indicate that samples sintered at1300℃for10h are basicallyalloyed. Microstructure of these samples can be greatly improved after they arelaser-sintered for40s. Compared with samples obtained by traditional sintering, thesesamples exhibit smaller grains with an average size of10μm. Direct laser sintering is thenapplied to prepare alloy and research results indicate that samples laser sintered for90scan be basically alloyed. According to SEM and EDS analysis, the grey background isα-Fe2Si5, the white is γ-FeSi and the black are pores and cracks. The density is about93%and grain size of γ-FeSi is about10μm. Laser sintering is an unbalanced thermodynamicprocess. Due to its fast heating speed and high cooling rate, liquid-solid interface isevolved quickly upon solidification, long distance diffusion of Fe and Si atoms isprohibited, therefore, it leaves little space for the the grain to grow. Influences ofannealing temperature and annealing time on alloy phase transformation are alsoresearched. The results show that samples laser sintered for90s can achieve completephase transformation after being annealed for only15h at1073K in vacuum furnace filledwith argon. Through SEM test, it is found that these samples exhibit equal hexagon grainswith an average size of5μm as well as good density （95%） and homogeneity. Besides, themaximum Seebeck coefficient at room temperature can reach115μV/K. Influences of Cuaddition on annealing are also studied. Through Cu addition, phase transformation rate canbe greatly improved and it only takes5h for annealing to complete phase transformation.Influences of Al and Co dopant on thermoelectric properties of β-FeSi₂are studied.Laser sintering based method is applied to prepare FeAl_xSi₂-0.1at.%Cu （X=0.03、0.05、0.07） alloy and Fe1-xCoxSi₂-0.1at.%Cu （X=0.03、0.04、0.05） alloy. The results indicate:the whole electric power factor of samples with Al addition （X=0.05） is the highest, that is,123.5μWm^-1K^-1; comprehensive electric power factor of samples with Co addition（X=0.05） is higher than others. The laser sintering based method could be proved as a promising method forpreparing β-FeSi₂. It can generate finer grains due to its advantages of rapid heating rateand high cooling rate. Besides, this method saves more preparation time and exhibitsunpolluted sintering process with high rate.