فهرست مطالب محسن نوری خضراباد

Prediction of the failure lifetime of thermal barrier coatings (TBCs) is usually considered as one of the important challenges in the hot section components of gas turbines. In this study, the effective stresses in determining the lifetime of new-generation single-layer Gd2Zr2O7 (GdZO) and dual-layer ZrO2-8Y2O3 (YSZ) / GdZO TBCs were investigated. For this purpose, singlelayer GdZO and dual-layer YSZ / GdZO TBCs were applied on IN738LC / CoNiCrAlY by atmospheric plasma spraying technique. Then, their cyclic oxidation behavior was studied at 1100°C with 4-h cycles. The morphologies of the TBCs (before and after oxidation) were investigated by field emission scanning electron microscopy. In addition, the mechanical behavior of the TBCs was examined by the Nanoindentation test. The variation in the energy release rate of the TBCs during oxidation cycles was calculated. The results showed that the changes in the energy release rate of dual-layer YSZ / GdZO TBC compared to single-layer GdZO TBC are less, and, therefore, have a better high-temperature performance. Applying an intermediate YSZ layer between the bond and top coats enhances the mechanical properties of new-generation TBCs and extends their lifetime by improving their resistance to high-temperature oxidation.

a-Al2O3 is the most important raw material for synthesizing modern ceramics and high alumina refractories. Active Alumina (also known as Hydratable Alumina and r-Al2O3) is a kind of transition alumina which is produced by flash-calcining of Gibbsite at 500-600 oC. In this research, the effect of Ammonium bi-fluoride additive (NH4HF2), on the formation of a-Al2O3 from active alumina was investigated. Suspensions containing active alumina and different amounts of fluoride additive (0-3 wt%) were stirred for 1 hour. The water of each suspension was removed by filter paper and the resultant powders were dried at 110 oC for 24 hrs. Powders were calcined up to 1400 oC for DSC and XRD analysis. The DSC test results showed that by having 1 wt% fluoride additive, a sharp peak appeared at 1100 oC indicating the formation of a-Al2O3. Increasing the additive amount to 3 wt%, shifted the transformation peak to as low temperatures as 925 oC. FESEM studies showed that in samples containing fluoride additive, the particles possessed hexagonal morphology. However, the ordering of the platelets and their thickness was a function of additive content. The higher the fluoride additive, the more ordered hexagonal shape as well as more thickness of the platelets.

Although Hydroxyapatite (HA) is one of the biocompatible and insulating ceramics, Carbon Nanotube (CNT) is superior due to its high thermal conductivity. This research aims to compare the thermal conductivity of HA/CNT composite in two fluids, i.e., water and Simulated Body Fluid (SBF), to achieve simultaneous biocompatibility and thermal conductivity for dental coatings. To this end, followed by synthesizing the hydroxyapatite, the HA nanofluid was made, and the prepared carbon nanotube was added to the HA nanofluid (volume ratio of HA:CNT was 1:1) to achieve the hybrid nanofluid with different volume fractions (0.2 to 1.0 Vol %). The thermal conductivity was then measured for the two water and SBF fluids in the temperature range of 20 to 50 °C. The results showed that 1.0 Vol % sample of the hybrid nanofluid had the maximum thermal conductivity enhancement compared to the SBF-based hybrid and water-based hybrid nanofluids with 32 % and 20 % enhancement, respectively, at 50 °C.