Study of iron based magnetocaloric nanomaterials

Abstract

Magnetic materials experience a change in temperature when they are adiabatically magnetized and demagnetized, this phenomena is known as the magnetocaloric effect (MCE). The MCE can be employed in environmentally friendly, green and novel energy efficient cooling systems as this technique does not have hydrofluorocarbons or ozone depleting gases, unlike conventional gas compression cooling systems. The giant MCE in rare earth based materials has motivated magnetocaloric research in the last two decades. However, the systems studied so far, i.e., gadolinium based materials are very expensive, corrode easily and have limited availability. Developing a new, affordable, readily available and corrosion resistant material is desired for commercial use. Low relative cooling power (RCP) is often another challenge in developing a magnetic cooling system.Nanoparticles can increase the working temperature span, therefore we developed transition metal based magnetocaloric nanoparticles which are environmentally friendly, affordable and possess RCP higher than those of gadolinium nanoparticles.The MCE of (Fe70Ni30)100-xAx nanocrystalline powders with A = B, Mn and Cr produced by high energy ball milling has been investigated. Binary Fe70Ni30 nanoparticles show high magnetization and low coercivity but they are not useful for room temperature cooling applications because of their high Curie temperature (TC ~ 443 K). Boron, manganese and chromium, were individually used to tune the TC closer to room temperature.(Fe70Ni30)89B11 nanoparticles were found to exhibit very high RCP up to 640 J-kg -1 for a field change ΔH of 5 T with TC ~ 381 K. Broad operating temperature range along with moderate change in entropy and very high RCP make these nanoparticles potential candidates for magnetic cooling applications in low grade waste heat recovery. Critical analysis of the magnetic phase transition using the modified Arrott plot, Kouvel-Fisher method and critical isotherm plots yields critical exponents of β = 0.364, γ = 1.319, δ = 4.623 and α = -0.055, which are close to the theoretical exponents obtained from the 3D-Heisenberg model.