Study On The Microstructure Evolution And Thermoelectric Performance Of La-Fe-Co-Sb Peritectic Alloys Fabricated In The Non-equilibrium Processes

Skutterudite is a kind of excellent thermoelectric materials in medium temperature. Previous researchs have found that the coherency strain fields from spinodal decomposition can make a significant reduction in the lattice thermal conductivity and increase power factor. However, the theoretical basis, formation mechanism and the influence of processsing parameters to the spinodal decomposition was not yet clear. In this paper, intermediate organizations were acquired through the furnace cooling and rapid solidification, and a dense, single phase of for La0.8Ga0.1Ti0.1FexCo4-xSb12 was obtained by the fast hot-pressing later. The thermodynamic theory and formation mechanism of spinodal decomposition were studied. In addition, the influence of parameters on the spinodal decomposition was analyzed, including the temperature and pressure in hot pressing, original and intermediary composition.In the first, the mixture energy of La-Fe-Co-Sb systems was calculated with the first principles method. The calculation confirmed the spinodal decomposition exists in these systems under 1200 K, moreover, Fe/Co ratio and La fill rate differences can be driving forces. The original composition La0.8Ga0.1Ti0.1FexCo4-xSb12 was located in the metastable zone, where the nucleation and growth process were necessary for spinodal decomposition.Secondly, under near-equilibrium conditions, the process of peritectic reaction and spinodal decomposition in skutterdite were studied by melting + annealing(MS+AN). In the annealing process under 973 K, peritectic transformation occurred first, and then chemical reaction produced, leading to formation of skutterdite. After annealing for 100 h, the organization of La0.8Ga0.1Ti0.1Fe3 Co Sb12 was skutterudite phase, and bending boundary of spinodal decomposition phase was observed, where Fe/Co ratio x and La fill rates are related. Subsequently, the melting + annealing + hot-pressing tests were done, the different and the initial Fe/Co ratio x influence on spinodal decomposition organization was studied. It was found that the composition differences was the biggest in x=3.3 sample, in which two-phases were La-rich area with high Fe/Co ratio and La-poor area with low Fe/Co.Thirdly, under non-equilibrium conditions, the process of peritectic reaction and spinodal decomposition in skutterdite were studied the melt-spunning + hot-pressing(MS+HP). The effects of temperature and pressure in hot pressing on spinodal decomposition were studied. At 473 K, the transition to skutterudite began, which was far lower than the equilibrium phase transition temperature of 1147 K. In, when pressure was more than 60 MPa, the microstructure of MS+HP samples tend to be uniform composition, the microporous disappeared basically and spinodal decomposition area decreased. Then, the initial Fe/Co ratio x influence on spinodal decomposition organization was studied in MS+HP samples. With the increase of initial Fe/Co ratio x, the driving force of spinodal decomposition changed from Fe/Co- La fill rates to La fill rates alone. In x = 3.3 sample, two-phase area obtained maximum differences of La filling rate. In spinodal decomposition zone, fine La-poor areas were uniformly distributed in La-rich matrix, and the La-poor particles were close to 100 nm. In x = 4.0 sample, the size of La-poor particles were further reduced to 50 nm.Finally, combined with calculating of density functional theory and thermoelectric tests, the effect of two-phases’ distribution on thermoelectric performance was analyzed. The dendritic segregation and spinodal decomposition could also cause regional composition differences, which improved the TE properties. In contrast to Fe/Co ratio differences of two-phases, La fill rate differences were more significant to promote good TE properties. The presence of spinodal decomposition improved the Seebeck coefficient and reduced the lattice thermal conductivity. Dendritic segregation was conducive to reduce the lattice thermal conductivity, but had little impact on performance. In AN+HP, x = 3.3 sample owed the maximum differences of two-phase composition, which lead to the good TE performance, the maximum of ZT = 0.75. For MS+HP, x = 3.3 sample obtained high La filling rate and large difference between two-phases, the biggest ZT was 1.34, the transfer efficiency achieved 16%. Due to the effect of nanometer La-poor phase, low thermal conductivity 1.47 Wm-1K-1 was found in x = 4.0 sample(MS+HP), and achieved ‘phonon glass’ effect for TE materials.