Evolution of Thermoelectric beta-FeSi2 Phase by Cryo Milling and Sintering

 

Cryo milling refers to milling processes that take place at very low temperatures. In the powder metallurgical technique, milling is the top to bottom approach utilised to make tiny powder. In order to obtain such a low temperature (below 123K subzero), N2 is used, which assists in maintaining the low temperature. In milling, particles are mixed mechanically, but in the sintering process, particles are mixed chemically. Sintering is a procedure that ensures proper bonding and strength of materials.

Let's take a closer look at thermoelectric materials. This stuff is intriguing and attracting attention from all across the world. possessing the ability to convert heat energy into electrical energy Many instances of energy loss in the form of heat can be found in everyday life. We all know that conserving energy is the best approach to grow it. We can notice the energy shortage all around us in the recent circumstances. The thermoelectric effect is based on the Seebeck principle, and its advantages and applications are as follows:

·       There is no residue or disturbances in material during operation

         Light weight and high efficiency ZT value

·       High conductivity inside crystal and lattice thermal conductivity

 

Let us look at the procedure of the experiment :

 

Results of the experiment showed that the size distribution reduces as milling period increases. Specifically, after 6 h, the particle size range is from nearly 0.15–14 um.Increasing milling period to 6 h gives rise to b-FeSi2 phase formation itself. This indicates that Si atoms disperse into Fe lattice and supersaturated solid solution of BETA-FeSi2 is formed

§  In DTA analysis of sample. During the heating cycle, an exothermic peak is observed at around 596–600C in all the three powders. Broadening of exothermic peak is observed, which extends over around 50C temperature range. This temperature range is believed to indicate eutectoid transformation (a-Fe2- Si5 gives b-FeSi2 + Si) and secondary transformation of e-FeSi + Si gives b-FeSi2. This shows that actual eutectoid reaction temperature i.e. 932C then decreases to 600 C due to Cryo milling.

§  We saw very less beta-Fes₂ in CM4HT6 but in it was more in CM6HT6 with some amount of Si this show that Si atoms have been dispersed into Fe lattice and supersaturated solid solution of beta-FeSi2 has formed

the terms D10, D50, D90 represents respectively the percentage of powder particle whose diameter is smaller than the corresponding observed diameter.

The conclusion obtained from the above result was that because the milling temperature was below the ductile to brittle transition temperature, the power brittle, which made milling easier, the usual b-FeSi2 formation temperature was 932 °C, which had been reduced to roughly 600 C. In CM6, beta-FeS2 production was seen, followed by 6 hours of sintering at 800°C, which revealed significant transformation to beta-FeS2 and traces of silicon. More than 6 hours of milling resulted in agglomeration of FE particles, which acted as Si deprived zones, resulting in the creation of allied phases, which obstruct phase transformation of beta-Fes2 and alter its properties.

 

References :

https://www.researchgate.net/publication/301598459_Evolution_of_Thermoelectric_b-FeSi2_Phase_by_Cryo_Milling_and_Sintering

     

      Credits:

      Om Gawande (TY Metallurgy)

      (Team R&D)

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