In this research, two methods, hydrothermal and sol-gel, have been investigated to adopt a suitable method. First, lithium ferrite (Li0.5Fe2.5O4) and magnesium ferrite (MgFe2O4) have been synthesized by these methods. Phase analysis has been performed by using X-ray diffraction (XRD). The results showed that there are very impurities in the samples synthesized by hydrothermal method while the samples synthesized by sol-gel method showed desirable purity. Magnetic measurement and microstructure have been evaluated using a vibrating sample magnetometer (VSM) and scanning electron microscopy (SEM). The results of these tests showed desirable magnetic properties of lithium ferrite produced by sol-gel method. So, the sol-gel method has been chosen for the rest of this research. Lithium-Magnesium ferrite has been produced by sol-gel method with the composition of (Li0.5Fe0.5)1-xMgxFe2O4 (where x=0, 0.2, 0.4, 0.6, 0.8, 1). A porous structure with crystallite size of 30-50 nm is observed. A single stage heat treatment process has been performed at 1000˚C for eliminating partial impurities. VSM results before and after heat treatment indicate that changes of saturation magnetization (Ms) with composition showed a desirable consistent with theoretical viewpoint. Electromagnetic and dielectric properties at room and higher temperature have been investigated by a LCR-meter. In the initial samples, dielectric constant, tanδ and electrical conductivity increases by increasing the amount of magnesium in composition while saturation magnetization, electrical modulus and impedance decreases. After heat treatment process a different behavior is observed. Saturation magnetization, dielectric constant, tanδ and electrical conductivity decreases by increasing the amount of Mg, while electrical modulus increases. Dielectric and electromagnetic Curie temperature is calculated in the range 85- 350˚C and 320 – 550˚C, respectively.

In this work, lithium ferrite and bismuth titanate have been synthesized by combustion method to fabricate a multiferroic composite. The effect of fuel types such as citric acid, glycine, acetylacetone and urea has been investigated individually and also in each fuel the effect of different molar ratio of fuel-to-nitrate has been studied. Since the powders prepared by glycine fuel exhibits the lowest impurity and loss factor, this fuel was chose as the best fuel. The glycine also has been used for lithium ferrite preparation. Multiferroic composites with the formula (where x=0, 0.1, 0.3, 0.5, 0.7, 0.9, 1) were prepared. X-ray diffraction patterns show that bismuth titanate with perovskite structure has been synthesized successfully for all fuels except citric acid. According to the XRD patterns, nanostructured lithium ferrite powders with spinel structure have been produced successfully. Microstructural examinations carried out by scanning electron microscope(SEM), indicate nanoparticles of bismuth titanate powder with the particles size of 10-60 nm. Dielectric and electromagnetic properties were examined by a LCR meter. Magnetic properties were evaluated using a vibrating sample magnetometer (VSM). Dielectric constant increases with increasing lithium ferrite content in the composites. Since high dielectric constant and low dielectric losses were chose as the criteria for the optimum composition, the composite containing x=0.7 is introduced as the best composition. The effect of temperature on dielectric and electromagnetic properties was examined in the range of 25-400 ºC. The Results show that the properties vary significantly by temperature. Although the maximum permeability is observed at room temperature in x=0.7 sample for all frequencies, this situation does not remain any more for further temperatures and at 50 ºC, Li ferrite sample shows the optimum properties. From 50 to 200 ºC, other composite samples indicate the better behavior and at the highest temperature, Li ferrite is the best again. In fact, the coupling between ferroelectric and ferrimagnetic properties leads to the change of electromagnetic behavior. 

In this work, NiFe2O4 ferrite was prepared by plasma arc discharge method, also NiZnFe2O4 ferrite was prepared by plasma arc discharge and a subsequently heat treatment process. Appropriate molar ratio of zinc, iron and nickel powders were mixed and formed into the cylindrical shape electrodes. After establishing an arc between the electrodes, the as-synthesized powders were collected and their structure and magnetic properties were examined by XRD, FESEM and VSM, respectively. In addition, the electromagnetic and dielectric properties were determined by a LCR meter. The effects of pressure, atmosphere and electric current on the different properties of as-prepared powders were evaluated. According to the Results, air atmosphere with a pressure of 1 atm and an electrical current of 400 A were identified as the appropriate conditions. In order to achieve the pure ferrite phase in the as-synthesized powders, different combinations of raw materials were investigated. It was found that the pure Ni ferrite is formed through removal of zinc from initial powders. Also, a subsequently heat treatment process is needed for the production of pure Ni-Zn ferrite phase in such away the quantitative amount of spinel phase increases by increasing heat treatment temperature and a pure of Ni-Zn ferrite phase is successfully achieved at 1250 °¬C. The average particle size of as-synthesized powders was found to be in the range 25-150 nm. Saturation magnetization and magnetic permeability increased with increasing of ferrite phase. In conclusion, Ni-Zn ferrite prepared by plasma arc discharge method and heat treated at 1250 °¬C with the highest saturation magnetization (35 emu/g), the lowest coercivity (5 Oe), the highest magnetic permeability (≈6 up to 5 MHz) and the highest dielectric constant (frequency range 20 Hz up to 2 kHz) was identified as the best sample.

 Friction stir welding (FSW) process is an emerging solid state joining process in which the material that is being welded does not melt and recast. Today, this process is used to create joints of aluminum alloys. The welding parameters such as tool rotational speed, welding speed, shoulder diameter and pin diameter play a major role in deciding the joint strength. In this investigation an attempt was made to develop empirical relationship to predict the tensile strength of the friction stir welded AA7075T6. This empirical relationship incorporates process parameters, namely rotational speed, welding speed, shoulder diameter and pin diameter. The empirical relationship is developed by response surface methodology (RSM). Response Surface Methodology (RSM) is a collection of mathematical and statistical techniques for establishing an empirical relationship. Four factors, five levels central composite design have been used to minimize number of experiments. Microstructural evolution was evaluated with optical microscopy, scanning electron microscopy (SEM) and differential scanning calorimeter (DSC). Mechanical property evolution was evaluated with tensile test and microhardness. Compared to the parent material that contains mainly η′, the precipitates have coarsened and more η was detected in the heat affected zone (HAZ). The distributions of microhardness exhibited a typical ‘‘W’’ shape and the minimum hardness zone was located at the heat affected zone (HAZ) of the weld joints. Compared to unwelded base metal, samples tested transverse to the weld showed reduction in strength and failure in the heat affected zone (HAZ), where coarsened precipitates caused localized softening. The developed empirical relationship can be effectively used to predict the tensile strength of FSW joints of AA7075T6 aluminum alloy at 93.21% confidence level. The optimum values of design parameters were achieved by using the empirical relationship. It was determined that the maximum tensile strength can be achieved in rotational speed 513.65 rpm, welding speed 95 mm/min, shoulder diameter 16.12 mm and pin diameter 5mm.