Iron-based low-cost magnetocaloric materials for near room temperature magnetic cooling

• Main Author: Sharma, Vinay Kumar
• Other Authors: Raju V. Ramanujan
• Format: Theses and Dissertations
• Language: English
• Published: 2019
• Subjects: DRNTU::Engineering::Materials
• Online Access: https://hdl.handle.net/10356/104887; http://hdl.handle.net/10220/47800
• Institution: Nanyang Technological University

Magnetic cooling relies on magnetocaloric materials (MCM). It is an emerging energy efficient, “green” thermal management technology which has the potential to replace conventional gas-compressor based cooling systems. Current research on MCM aims at developing low-cost, environmentally friendly materials which can be manufactured by simple processes and exhibit superior magnetocaloric effect (MCE) near room temperature. Many materials have been developed over the years which exhibit high MCE, however they suffer from disadvantages such as high cost of rare earth (RE) elements, low corrosion and oxidation resistance, tedious synthesis routes, strategic limitations etc. Hence, good performance and readily available MCM with high corrosion resistance and established manufacturing technology are urgently needed for cooling applications near room temperature. In this thesis, the magnetocaloric properties of (Fe, Cr)3Al and (Fe, Mn)3Al alloys have been systematically explored with detailed investigation of their structure, magnetic and magnetocaloric properties. Binary Fe3Al alloys exhibit a high Curie temperature (Tc), which is not suitable for near room temperature magnetic cooling. Addition of antiferromagnetic elements (Mn and Cr) and application of external hydrostatic pressure can be used to tune the MCE properties of Fe3Al alloys by modifying the transition temperature, saturation magnetization and relative cooling power. Subsequently, magnetic fluids of (Fe, Cr)3Al based alloys were also prepared and their magnetic cooling performance was demonstrated. Fe75-xMnxAl25 bulk alloys, prepared by arc melting, were found to exhibit high values of relative cooling power (RCP), which is an important figure-of-merit for a good MCM. Fe60Mn15Al25 and Fe57.5Mn17.5Al25 alloys exhibited RCP values of 395 Jkg-1 and 430 Jkg-1 at an applied field of 5 T, respectively and Tc values of 290 K and 250 K, respectively. Scaling analysis and local exponent analysis were performed on the thermodynamic data of these samples to study the nature of relevant phase transitions. We investigated the influence of annealing and external pressure on the structure, magnetic and MCE properties of Fe75-xMnxAl25 alloys. The Tc of Fe75-xMnxAl25 ribbons was tuned in a wide range, at an annealing temperature of 1000 °C and optimum annealing durations, yielding technologically attractive values of Tc in the range of 277 K to 290 K. Optimal annealing conditions increased the MCE properties. Increasing hydrostatic pressure from 0 kbar to up to 11.5 kbar resulted in Tc and saturation magnetization (Ms) decreasing at a rate of dTc/dP ~ 1.8 K/kbar and dMs/dP ~ 0.8 emu/g/kbar. A magneto-structural transition was not observed in Fe2MnAl alloys even at applied pressure utpo 11.5 kbar. The effect of Cr alloying on the MCE properties of Fe3Al bulk alloys was explored. The Tc was tuned to near room temperature values. These samples exhibited RCP/US$ values ~ 15 times higher than Gd2Si2Ge2 alloys and ~24 times higher than the RCP/US$ values of the “benchmark”, Gd. Synthesis routes can change the structure of these alloys, which is reflected in their magnetic and MCE properties. A notable enhancement of 54% in RCP and 33% in isothermal magnetic entropy change (-∆Sm) was observed for Fe54Cr21Al25 ribbons compared to the bulk counterpart. This enhancement was attributed to a change in structural order and microstructure due to high cooling rates during melt spinning. The critical behavior of Fe54Cr21Al25 ribbons conformed to a short-range order 3D Heisenberg model, with the values of critical exponents: β = 0.345, γ = 1.316 and δ = 4.63. Fe-Cr-Al buttons were milled at high speeds for short durations and found to exhibit an unusual cyclic change in structural order. A thermodynamic reaction rate model was developed to calculate deduce the stored energy during high energy milling. An enhancement of 50 % in the Ms and 23 % in the RCP was observed in the Fe-Cr-Al samples milled for only 15 min. Magnetic fluids of Fe75-xCrxAl25 nanoparticles were prepared by surfactant-assisted ball milling. These fluids were used to demonstrate magnetic cooling in a thermomagnetic cooling device (TMCD). Cooling was evaluated by optimizing several parameters such as Cr content, volume concentration and heat load. A maximum cooling of 5.4 °C was achieved at a heat load of 4 W, volume concentration of 0.8% and applied field of 0.3T for Fe50Cr25Al25 magnetic fluids. The self-regulating and self-pumping nature of the TMCD was evident. These results were in good agreement with the modelling results.