الملخص الإنجليزي
In this thesis we report on our attempts to mechanosynthesize single-phased nanocrystalline pure and Ti-doped BiFeO; (bismuth ferrite) in a bid to solve the documented problem that it is difficult to synthesize single phases of these materials via thermally-assisted routes. We also report on the structural and magnetic properties of the best single-phased nanocrystalline particles of both materials that we managed to synthesize. Mechanosynthesis was attempted with a planetary ball mill machine by subjecting an initial 1: I molar mixture of a-Bi,O, and c-Fe,0, (or Ti" -doped a-Fe,Os) to two distinct milling regimes. In the first, Milling Regime (1), a milling speed of 300 rpm and a ball-to-mass ratio of 15: I were used, whereas in the second, Milling Regime (2), the values of 600 rpm and 22: 1 were used, respectively. To our knowledge, our study is the first where Tie-doped and a-Fe, was used as a precursor to produce Ti-doped BiFeO, rather than using stoichiometric amounts of a-Fe,0, and TiO, (separately). With Milling Regime (1), an almost single(-97%) rhombohedral BiFeO; nanocrystalline phase can form after 100 h of milling followed by heating at 780 °C (7.5 min). The average crystallite size is found to be 82 nm and particle size ranges between 50 nm and 150 nm. Almost singlephased (98%) cubic Ti-doped BiFeO3 nanocrystalline particles form using the Milling Regime (1) for 60 h followed by heating at 850 °C (7.5 min). The average crystallite size 41 nm and average grain size of -50-300 nm. With Milling Regime (1), while our results are generally better than many published ones, it was not possible to get rid of the very small amounts (< 2%) impurity sillenite (Bisfe04/BiasFeOs), mullite (BizFe 0,) and 6-Bi20, phases that develop with heating. Milling Regime (2) has facilitated the formation of BiFeO, without passing through the intermediate phases. With this regime, single-phased ( 98%) nanocrystalline cubic BiFeO, particles were formed by heating reactants at 400 °C for lh after milling for 48 h. The crystallite size was-19 nm and the particle size varies in range-5-40 nm. Cubic Ti-doped BiFeO, single-phased ( 98% ) nanocrystalline particles, with average crystallite and particle size of less than 10 nm, were found to form by milling the reactants using Milling Regime (2) for 32 h at room temperature. The Mössbauer spectra of the nanocrystalline BiFeO, particles prepared with Milling Regime (2) show the presence of a -12% superparamagnetic component with blocking temperatures less than 78 K. The almost zero quadrupole splittings in the spectral component of the larger "magnetic" nanocrystalline BiFeO; particles indicate the cubic nature of their structure in agreement with the XRD data. That no Mössbauer signal was detected from the Ti-doped BifcO; nanoparticles could be related to their very small particle (crystallite) sizes (<10 nm) that lead to collective magnetic excitations resulting in broadening and a poor resolution of the Mössbauer lines. The magnetization of the BiFeO, nanoparticles prepared using Milling Regime (2) is -3 times and their magnetic hardness is -14 times larger than those of the corresponding bulk. The Ti-doped BiFeO; nanocrystalline particles show a complex weak ferromagnetic behaviour. These results are mainly attributed to the reduction of particle size below a critical characteristic length and the weakening of the magnetic exchange interaction due to the presence of the nonmagnetic TT ion in the magnetic lattice.
