MECHANOCHEMISTRY AND MECHANICAL ALLOYING 2003 JOURNAL OF MATERIALS SCIENCE 39 (2 0 0 4 ) 5151 – 5154 Characterization of nanocrystalline Mn-Zn ferrites obtained by mechanosynthesis T. VERDIER ∗ , V. NIVOIX, M. JEAN, B. HANNOYER LASTSM Universit ´ e de Rouen, Institut des Mat ´ eriaux, BP12, 76801 Saint Etienne du Rouvray Cedex, France E-mail: thomas.verdier@univ-rouen.fr Different Zn and Mn-Zn ferrites have been obtained by High Energy Ball Milling (HEBM) from elemental oxides. By varying the rotation speed of vials and main disk, several modes of milling can be obtained. In this paper, the materials prepared by two modes of milling, called shock (S) and high shock (HS), are studied by XRD and M ¨ ossbauer spectroscopy. Several mechanisms for spinel phase formation have been proposed. In particular, a reduction of Fe(III) occurs for the S mode, which leads to the formation of a w ¨ ustite-type phase which disappears after a long milling time. C  2004 Kluwer Academic Publishers 1. Introduction The High-Energy Ball Milling technique (HEBM) is applied in the field of synthesis of spinel ferrites. In some reports, after the HEBM, a thermal treatment is done to ensure the formation of fine-grained strained particles [1, 2]. This way of synthesis can induce changes in the distribution of cations in the spinel lattice [3, 4], which leads to unusual magnetic properties. Mixed manganese zinc ferrites are materials of tech- nological importance, widely used as transformers and inductors. They are usually synthesized at high tem- perature by solid state method. In this paper, their pro- cessing is performed in a high-energy planetary mill (Vario-Mill, Fritsch), which allows the vials and main disk velocities (ω and  (in rpm)) to be chosen inde- pendently. By varying the ratio of the velocities, it is possible to control the movements and trajectories of milling balls and consequently the nature of the end product. Based on two kinematic studies by Gaffet [5, 6], the different milling parameters are chosen to pro- duce direct shock mode, intermediate mode or friction mode. 2. Experimental The synthesis of Zn, Mn or Mn-Zn ferrite was ac- complished by combining stoichiometric mixtures of powdered reactants such as ZnO, α-Fe 2 O 3 and MnO obtained from ALFA AESAR (99% purity or better). The milling process was carried out under air atmo- sphere in a tempered steel vial (80 ml) with steel balls. A ball-to-powder weight ratio of 1:20 was used for two grinding modes, ( = 500, ω =−500) and ( = 600, ω =−300), which are called shock mode (S) and high shock (HS) mode, respectively. X-ray diffraction patterns were collected using a D-5000 pow- der diffractometer (Siemens) with CoK α radiation. The ∗ Author to whom all correspondence should be addressed. M¨ ossbauer measurements were performed in transmis- sion geometry using 57 Co/Rh as γ -ray source. Mosfit software was used to analyse spectra [7]. 3. Results 3.1. Mechanosynthesized zinc ferrite Synthesis of zinc ferrite (ZnFe 2 O 4 ) has been performed using several grinding conditions. The Fig. 1 shows the evolution of the ZnO/α-Fe 2 O 3 mixture with the milling time for the S and HS modes. For the HS mode, a pro- gressive disappearance of reactant oxide powders oc- curs in favour of a spinel phase which becomes pure after 12 h of milling. With S mode, disappearance of ZnO and α-Fe 2 O 3 leads to simultaneous formation of a spinel phase and a w ¨ ustite-type phase (Fe, Zn)O. The latter phase disappears between 6 and 9 h of milling. The w¨ ustite-type phase clearly shows the appearance of Fe(II) in the powders during the milling. The zinc ferrite phases are obtained for both sets of grinding conditions, with a small amount of metallic iron originating from the wear of the milling equip- ment. However, the S mode is more efficient than the HS mode: using the S mode, the total disappearance of the starting oxides is reached within 9 h, whereas 12 h are necessary for the HS mode. The crystallite sizes of the obtained spinel phase (7 and 11 nm) and its cell pa- rameters (0.8439 and 0.8438 nm) are similar for HS and S mode, respectively. These values are slightly lower than the cell parameter of a ceramic ZnFe 2 O 4 (0.84411 nm , JCPDS file 22-1012). In Fig. 2, the room temperature M ¨ ossbauer spectra of the materials obtained using the two different grinding modes are shown and compared with the spectrum of a ceramic ZnFe 2 O 4 . In the latter case, the spinel for- mula is (Zn 2+ ) A (Fe 3+ Fe 3+ ) B O 2− 4 , where A and B rep- resent tetrahedral and octahedral sites, respectively. The 0022–2461 C  2004 Kluwer Academic Publishers 5151