الفهرس 
LITERATURE SURVEYOnly 14 pages are availabe for public view
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Abstract
Polycrystalline ferrites with general formula Li0.5+0.5xSixFe2.5-1.5xO4 (where; x=0.0, 0.1, 0.2, 0.3, 0.4, 0.5 and 0.6) were prepared by high energy ball milling (HEBM) technique. The high purity materials Li2Co3, SiO2 and Fe2O3 were mixed and grounded into a very fine powder using ball milling machine, where milling was continued for 20h. Afterwards, the obtained powder was pressed under 3x108 Pa into disc and toroid shapes. The obtained samples were annealed at three different temperatures (700, 900 and 1000oC) for 2h and then slowly cooled down to room temperature with cooling rate of 2oC/min.
Several techniques have been used to investigate the prepared compositions such as: X-ray diffraction (XRD), transmission electron microscope (TEM), scanning electron microscope (SEM) and infra-red (IR) spectra to study the structure, B-H loops, magnetization and initial permeability to study the magnetic properties and dc measurements to investigate the electrical properties of the prepared samples.
X-ray diffraction analysis confirmed the formation of single phase cubic spinel structure at annealing temperature 1000oC. The lattice parameter was calculated for each composition at every annealing temperature. It was found that the lattice parameter values have a slight reduction with Si content, however have no significant change by increasing the annealing temperature.
The morphology and particle size values were estimated for x=0.0, 0.3 and 0.6 thermally annealed at two different temperatures 700 and 900oC by using TEM micrographs. It was observed that the compositions annealed at 700oC showed irregular microstructure with particle size values in the range of 90-180 nm. As the annealing temperature increased to 900oC, the ferrite powders showed a uniform coarse structure with an average particle size values about 200 nm.
The grain diameter (D) was estimated from SEM micrographs for the samples annealed at 1000oC. It was found that the grain diameter decrease with increasing Si concentration, where the grain diameter values vary from 2 to 10 µm. Also the SEM micrographs indicated the presence of intergranular pores and the absence of internal pores.
The IR spectra were performed in the wave number range from 200 to 1500 cm-1 for the samples annealed at 700, 900 and 1000oC. Two prominent bands were observed; high frequency band ( ) is assigned to tetrahedral complexes and low frequency band ( ) is assigned to octahedral complexes. These bands were changed slightly by the increment of annealing temperature.
The hystersis loops were firstly measured for all compositions annealed at 700, 900 and 1000oC at high magnetic field up to 20 kG by using vibrating sample magnetometer (VSM). from these measurements, it was observed that the saturation magnetization (Ms) decreases with silicon content. This decrement was explained in terms of Neèl’s two sublattice model in the light of the proposed cation distribution. Also it was found that the saturation magnetization of the samples has been increased, meanwhile the coercivity has been decreased with the increase of the annealing temperature.
The hystersis loops were measured again at low magnetizing field ranging from 200 to 1590 Am-1 for the samples annealed at 1000oC. from the obtained loops, some parameters such as: coercive field HC (Am-1), remanence induction Br(T), saturation induction Bs(T), and the relative magnetization (R= Br/Bs) were determined as a function of silicon concentration. It was found that all these parameters are greatly affected by Si substitution and matched well with the assumed cation distribution.
The magnetization M of Li0.5+0.5xSixFe2.5-1.5xO4 annealed at 1000oC was measured at room temperature in the range of magnetizing field up to 6000Am-1. It was noticed that the magnetization increases gradually with increasing the applied magnetic field. The variation of M with compositions x was studied at three different values of H (1500, 3000 and 5000 Am-1). from the obtained results, it was found that the magnetization has been decreased with Si substitution.
The thermal spectra of initial permeability (µi) of Li-Si ferrite annealed at two different temperatures (900 and 1000oC) were measured in a wide range of temperature from room temperature up to 925 K. It was found that µi increases with increasing the temperature and falling abruptly close to Curie temperature, TC. The Curie temperature was determined from µi-T curves and it was observed that it is not affected by Si4+ ions substitution and the annealing temperature. Moreover, it is observed that the permeability has showed increasing trend with the increase of annealing temperature since the permeability depends on microstructure.
Dc electrical conductivity was studied in a wide range of temperature from 300 up to 960 K for the samples annealed at 1000oC. the relation ln σ vs. T-1 shows mainly one region for all values of x except at x=0.0 and 0.1, shows two regions with different slopes. The activation energies values were calculated for the two different regions. The activation energy at region I (for x=0.0 and 0.1) equals 0.27 eV which suggests that the conduction is due to the hopping of electrons from Fe3+ to Fe2+ ions through the crystal lattice. Meanwhile, the activation energies at region II for all compositions are greater than 0.3 eV, which clearly indicates that the conduction is due to small polaron hopping. It is also found that the dc conductivity of lithium-silicon ferrites decreases as Si content increases.