Sudakar, C and Subbanna, GN and Kutty, TRN (2003) Wet chemical synthesis of multicomponent hexaferrites by gel-to-crystallite conversion and their magnetic properties. In: Journal of Magnetism and Magnetic Materials, 263 (3). pp. 253-268.
Restricted to Registered users only
Download (1346Kb) | Request a copy
The hexaferrites of different compositions from the $BaO-NiO-Fe_2O_3$ ternary phase diagram were prepared by a wet chemical process of gel-to-crystallite conversion. Herein, the coprecipitated reactive gels of hydrated $nFe(OH)_3\cdotpNi(OH)_2\cdotxH2O$; 70<x<110 with n=12–36 and p=0–2 were reacted with $mBa(OH)_2\cdot8H_2O$ (m=0–4) in aqueous ethanol medium at 80°C yielding the precursors of the general formula $Ba_mFe_nNi_pO_q(OH)_r\cdot_sH_2O$ (q=13–35; r=6–50; s=5–30) which are crystalline by electron diffraction. Dehydroxylation of the precursor is accomplished by continuous weight loss as evidenced from thermogravimetry and is complete at $\sim$650°C by isothermal conditions. Differential thermal analysis showed exothermic peaks arising from the recrystallization followed by grain growth of the particles at $\sim$750°C. Direct phase evolution of hexaferrites are evidenced from X-ray diffraction studies. Monophasic nanoparticles of hexaferrites were obtained on heat-treating the precursor at 850°C. HREM of these crystallites show defect-free, regular, coherent growth of structural blocks. The evolution of para- to ferrimagnetic phase on heat-treatment is continuous and is through the superparamagnetic state. This is associated with a continuous increase in magnetization with heat-treatment temperature. Samples heat-treated below 850°C show diffuse magnetic transition near $T_c$, which results from the intergrowths observed in the lattices. Hysteresis loop for various compositions showed significant differences from hard ferrite characteristics for M-phase $(\sigma_s=52 emu/g; H_c=4520 Oe)$ to soft ferrite $(\sigma_s=25.5 emu/g; H_c=205 Oe)$ for Y-phase. Crystallographic domains of complex stacking sequences such as YMYM4, $M_2SM_3S$, MMYY, etc. are observed when the hexaferrites are heat-treated above 1350°C.
|Item Type:||Journal Article|
|Additional Information:||Copyright of this article belongs to Elsevier.|
|Keywords:||Gel-to-crystallite conversion;Hexaferrite;Multicomponent oxides;Nanoparticles|
|Department/Centre:||Division of Chemical Sciences > Materials Research Centre|
|Date Deposited:||02 Jun 2006|
|Last Modified:||19 Sep 2010 04:28|
Actions (login required)