Synthesis, morphology, thermal stability and magnetic properties of α^″-Fe₁₆N₂ nanoparticles obtained by hydrogen reduction of γ-Fe₂O₃ and subsequent nitrogenation

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• 作者：I. Dirba^a ; ^{dirba@fm.tu-darmstadt.de" class="auth_mail" title="E-mail the corresponding author} ; C.A. Schwö ; bel^a ; L.V.B. Diop^a ; M. Duerrschnabel^b ; L. Molina-Luna^b ; K. Hofmann^c ; P. Komissinskiy^a ; H.-J. Kleebe^b ; O. Gutfleisch^a
• 关键词：Reduction ; Nanoparticles ; Permanent magnets ; Fe16N2 ; Magnetic properties
• 刊名：Acta Materialia
• 出版年：2017
• 出版时间：15 January 2017
• 年：2017
• 卷：123
• 期：Complete
• 页码：214-222
• 全文大小：4031 K

文摘

Typical synthesis of α^″-Fe₁₆N₂ nanoparticles involves reduction of iron oxides by hydrogen at elevated temperatures which is disadvantageous due to the particle coalescence. Here we report on a process for reduction of iron oxides at elevated pressures and show that by increasing hydrogen pressure from atmospheric to 53 MPa, it is possible to reduce the reaction temperature from 663 K down to 483 K, resulting in phase-pure α-Fe nanoparticles without noticeable particle growth. By subsequent nitrogenation in an ammonia flow, fine, 99% phase-pure α^″-Fe₁₆N₂ nanoparticles could be synthesized. The reduction temperature and the respective particle size has a significant influence on the nitrogenation step. α^″-Fe₁₆N₂ nanoparticles exhibit semi-hard magnetic properties with M_s(0) = 215 Am² kg⁻¹, μ₀H_c = 0.22 T, T_C = 634 K and exchange stiffness A_c = 6.84 pJ m⁻¹, A_a,b = 7.53 pJ m⁻¹. Synthesis conditions, microstructure, chemical composition and thermal stability of the nanoparticles are systematically studied and correlated with the observed magnetic properties.