Raheleh Nikonam Mofrad

During the in-situ nitridation of Si in the presence of CaO, MgO and Al₂O₃ formation of reaction bonded silicon nitride ceramics (RBSN) having various microstructures indicated the possibility of different nitridation mechanisms operating. Experimental evidence suggested that, whereas the morphology of pores was controlled by nitridation of Si(g) in CaO-RBSN and SiO(g) in Al₂O₃-RBSN, a combination of these two reactions occurred during the nitridation of the MgO-RBSN. By inhibiting the growth of whiskers and maximizing the α/β ratio, the CaO addition led to the formation of matte grains creating clean spherical cavities with d < 40 µm and flexural strength of 0.8 MPa. In contrast, when using Al₂O₃ additions, a microstructure with a very low α/β ratio, fine inter-particle pores and 2.4 MPa flexural strength, reinforced with interlocking whiskers, was produced. The highest porosity (85%) and the lowest strength (0.3 MPa) occurred in the MgO-RBSN, which was composed of both matte grains and fine whiskers. Local supersaturation and low content of β-nuclei led to the formation of anisotropic β-grains with a bimodal microstructure in heat-treated CaO-RBSN while a unimodal microstructure was observed in heat-treated MgO-RBSN. No porosity loss or β-grain growth occurred in the heat-treated Al₂O₃-RBSN.

We determined the mechanism of mechanochemical synthesis of fluorapatite from CaO, CaF₂ and P₂O₅ by characterisation of the intermediate compounds. We used atomic absorption spectroscopy, X-ray diffraction, field emission scanning electron microscopy, FTIR spectroscopy and transmission electron microscopy to find the transitional compounds. Investigation of the binary and ternary powder mixtures revealed the appearance of H₃PO₄, Ca(OH)₂, Ca₂P₂O₇ and CaCO₃ as the intermediate compounds. At early stages of the milling, conversions of P₂O₅ to H₃PO₄ and CaO to Ca(OH)₂ occurred in the wet atmosphere. Later, a combination of Ca(OH)₂ and H₃PO₄ formed C a₂P₂O₇ while the unreacted CaO was converted to CaCO₃ by CO₂ of the ambient atmosphere. Spherical crystalline Ca₁₀ (PO₄)₆F₂ particles formed after 48 hours of milling due to the reaction between Ca₂P₂O₇, CaCO₃ and CaF₂.

Nanostructured copper-doped fluorapatite (Cux.Ca(10-x).(PO4)6.F2) having crystallite sizes of 19, 29 and 34 nm at x = 0.9, 0.4 and 0.0, respectively, was synthesized by planetary ball milling of CaO, P2O5, CaF2 and CuO powders. Specifications of the products were determined by Fourier transform infrared spectroscopy, field emission scanning electron microscopy, transmission electron microscopy and X-ray diffraction analysis. In-vitro studies and Mossman's Tetrazole Test (MTT) assays were also conducted by incubating Cux.Ca(10-x).(PO4)6.F2 powder into Kokubo’s simulated body fluid (SBF) and against BT-20 cell, respectively, to determine bioactivity and biocompatibility of the material. Antibacterial effects toward Staphylococcus aureus was assessed by the disc diffusion test method. Measurements showed that the rate of formation of fluorapatite was lowered by Cu content. Besides, in vitro experiments showed the same SBF interacted apatite precipitation for all samples. In contrast, MTT assays revealed different behavior for pure fluorapatite and apatite with x=0.9 Cu against BT-20 cell after 24 h of incubation. This highlights increase of fluorapatite cytotoxicity when Cu ion is present in the apatite structure. Copper-doped fluorapatite was, however, desirably antibacterial. This stemmed from copper ions interactions with the bacterial metabolism which resulted in enzymes neutralization and copper-doped fluorapatite antibacterial behavior

The exothermic reactions in Al+Ni system were used as a power source for welding of Al1100 alloy so that the obtained joint was reinforced with in situ produced Al3Ni intermetallic compound, simultaneously. Due to the combustion heat, the surfaces of the plates were melted down which, in turn, eventuated in formation of a strong surficial bond. The cross sections of joints were analyzed using optical microscopy, EDS-equipped scanning electron microscopy (SEM), and X-ray diffraction experiments. XRD and EDS results represented the Al3Ni intermetallic compound as the reaction product. The compressive shear strength of the joint was 20 MPa.