Papers by Keyword: MgO

Abstract: In this paper, pure phase Nd-doped YAG powders were prepared by co-precipitation method with different amount of MgO (0~ 0.6 wt %) as additive. Transparent Nd:YAG ceramics were fabricated by vacuum sintering at 1750°C for 10 hours using the powders. Microstructure of the surfaces of thermally etched Nd:YAG ceramics was observed by SEM. Transmittance of the transparent Nd:YAG ceramics was measured over the wavelength region from 600nm to1200nm. The results showed that MgO as sintering aid can restrain abnormal grain growth and reduce pores in grains and at the same time, optical transmittance of the ceramics was increased. The optimal weight percentage of MgO added can be determined as 0.3wt % for the fabrication of transparent Nd:YAG ceramics.

Abstract: Wet gel obtained by sol-gel technique was dried in supercritical CO2 to prepare hydrated form of magnesium oxide. Calcination at 723 K under vacuum yielded nanocrystalline MgO aerogel. Structure studies were performed by X-ray diffraction, scanning and transmission electron microcopies. Electron microscopy images reveal rough, unfolded and ramified structure of solid skeleton. Specific surface area SBET was equal to 238 m2/g. X-ray pattern reveals the broadened diffraction lines of periclase, the only crystalline form of magnesium oxide. The gamma crystallite size distribution was determined using FW 5 4 / 5 1 M method proposed by R. Pielaszek. The obtained values of and σ (measure of polydispersity) of particle size parameters are equal to 6.5 nm and 1.8 nm, respectively, whereas the average crystallite size estimated by Williamson-Hall procedure was equal to 6.0 nm. The obtained at Rietveld refinement Rwp, and S fitting parameters equal to 6.62% and 1.77, respectively, seem to be satisfactory due to the nanosize of MgO crystallites and because of the presence of amorphous phase.

Abstract: Cubic shaped Magnesium oxide nanoparticles were successfully synthesized by sol-gel method using magnesium nitrate and sodium hydroxide at room temperature. Hydrated Magnesium oxide nanoparticles were annealed in air at 300 and 500°C. X-ray diffraction patterns indicate that the obtain nanoparticles are in good crystallinity, pure magnesium oxide periclase phase with (200) orientation. Morphological investigation by FESEM reveals that the typical sizes of the grown nanoparticles are in the range of 50-70nm. Powder composition was analyzed by the FTIR spectroscopy and the results confirms that the conversion of brucite phase magnesium hydroxide in to magnesium oxide periclase phase was achieved at 300°C.The Thermo-gravimetric analysis showed the phase transition of the synthesized magnesium oxide nanoparticles occurs at 280-300°C.

Abstract: High purity MgO is a common ceramic material used as a protecting layer in an alternating current plasma display panel (AC-PDP). In this work, pure MgO was added by CaO for a better protecting layer. The effect of CaO addition was discussed according to the changes in the microstructure, mechanical strength and the ability of the sample to withstand surface breakdown (flashover) with various CaO addition in the powder mixture. In addition, it was found that 2.5 wt% CaO-addition shows a better electrical discharge. Since secondary electron emission coefficient contributes in the electrical discharge, it could be predicted that the 2.5 wt% CaO added MgO has a higher secondary electron emission constant and becomes a promising candidate for a protecting layer in AC-PDP.

Abstract: Silicon nitride samples were pressureless sintered with up to 5 w/o MgO to give densities in the range 98-99% of theoretical. After pressureless sintering, selected samples were placed in a vacuum heat treatment furnace surrounded by a carbon bed in a carbon crucible at a pressure of less than 4x10-4 mbar, and vacuum heat treated at different temperatures and times to remove grainboundary glass. The results showed that this was substantially achieved at 1575oC for 3h and that increasing the time to 5 hours gave still further improvement. SEM images, EDX analysis and oxidation tests provided additional evidence for the removal of Mg from the samples.

Abstract: Effects of MgO coating on electric properties of BaTiO3 have been studied in terms of microstructural development and substitution of Mg ion. MgO coated BaTiO3 particles were prepared by a homogeneous precipitation method and other powders by a conventional mixing method for a parallel study. MgO coated BaTiO3 exhibited homogeneous microstructure compared with mixed samples. XRD analysis revealed that Mg substitution for the Ti site in the MgO mixed sample was much greater than in the coated one. MgO coated BaTiO3 showed higher dielectric constants at the Curie temperature whereas mixed ones had higher dielectric constants at 60
~80
. Electrical properties of MgO mixed and coated BaTiO3 were affected by the diffusion behavior of Mg in BaTiO3 lattice, which was confirmed by leakage currents.

Abstract: Characteristics of MgO layer deposited under hydrogen atmosphere were investigated. Hydrogen gas was introduced during e-beam evaporation coating process of MgO layer and its effects on microstructure, cathode luminescence spectra, discharge voltages and effective yield of secondary electron emission were examined. The results indicated that the hydrogen influences the concentration and energy levels of defects in MgO layer and that affects the luminance efficiency and discharge delays of the panels significantly.

Abstract: Vacuum infiltration is one of the composite production method. Reinforcement volume ratio, vacuum value, molten matrix composition and temperature, infiltration atmosphere and time are important parameters in infiltration of molten metal into preformed reinforcement. In this study, MgO powder and Al were used as reinforcement and matrix respectively. Mixture of MgO and Al powders with –105 μm particle size were filled in quartz tubes freely to form 30 % reinforcement volume ratio. Liquid pure Al with varying Mg contents were vacuum infiltrated into the MgO powder under 550 mmHg vacuum at two different temperatures in normal atmosphere for 3 min. After vacuuming, infiltration height and density of produced composite have been determined. Fracture behavior of composites has also been determined by three point fracture test. Microstructure of composites and fracture surface were investigated by SEM analysis. It has been found that fracture strength of composites decreased although infiltration height and density increased with increasing Mg content of liquid Al. It has been also determined that molten metal temperature facilitates infiltration.

Abstract: MgO insulation coating with thickness of ca. 50nm was evaporated on the surface of iron powder and fabricated soft magnetic composite(SMC) with low iron loss. The MgO insulation coating had greater heat resistance than conventional phosphate insulation coating, which enabled stress relieving annealing at higher temperature (600C). Electrical and magnetic properties of SMC was examined. The electrical resistivity of the SMC annealed at 700C was 21μΩm. The iron loss at 50Hz for Bm = 1.5T was 5.34W, which was 40% of conventional SMC and was at the same level with laminated steel plate (t0.35). These results show that MgO insulation coating has enough heat resistance and adhesiveness to powder surface to obtain SMC with low iron loss.