Papers by Keyword: MgAl₂O₄

Abstract: The purpose of this study was the research for a non-standardized method of measuring the shear bond strength at constant compression force and compare with the shear testing methods on cylindrical and prismatic samples, like DIN 50161:1977-10, DIN EN15340-2007. The conducted comparative analysis showed a significant shortcoming of the well-known methods for assessing the shear bond strength on samples for industrial application. The main one is an inability to measure the “actual/real” adhesion shear strength of thermal spray coatings for industrial parts. The shear bond strength of plasma-sprayed coating MgAl₂O₄ with thickness 0.30-has been investigated by applying the prismatic samples (linear dimensions of shear area 1.5-), using Nanovea scratch tester at compression forces ranging from 1 to 70 N. Shear force-displacement curves get essential information about adhesion/cohesion, delamination and cracking properties of thermal spray coating. An increase of shear forces is primarily associated with a rise in friction forces at the coating delamination boundary. Applying for parts of the ITER blanket modules, this research has demonstrated the importance of practical application of the shear testing results at compressive loads for ensuring product life.

Abstract: This study was conducted to investigate the effect of Al2O3 on characteristics of A356 reinforced composites. The addition of Al2O3 as reinforced varied from 2 vf-% to 15 vf-% through Stir Casting methods to obtain the optimum addition Al2O3 as reinforced in mechanical properties of composite A356 as matrix and Al2O3 as reinforcement. Magnesium with of 10 wt-% was added to improve wettability between Al2O3 particles and Al A356. Addition of Magnesium is expected to form the spinel phase MgAl2O4 in interface area between Al2O3 and Al A356.The optimum tensile strength was found at 2 vf- Al2O3 with value 131.55 Mpa. However, the value is still lower than the tensile strength of Al A356 As cast. It is caused due to the porosity and the agglomeration of Al2O3 were found in the microstructure of the composite Al A356/Al2O3. The Hardness increased from material monolithic Al A356 as cast, reached 37.43 HRB.

Abstract: A kind of ceramic composite consisted of Mg-Y co-doped ZrO₂ and MgAl₂O₄ was prepared as candidate material to replace the high destiny graphite crucible which was often used in nuclear industry. When the addition of MgAl₂O₄ was 7 wt%, samples with good thermal shock resistance were obtained. The obtained samples were tested up to 12 h in molten uranium, and no apparent cracks were found on the corrosion surface. Meanwhile, the corrosion rate decreased significantly from 30 mg·cm^-2·h^-1 to 3 mg·cm^-2·h^-1 as the corrosion time prolonged from 2 h to 12 h due to the formation of a protection layer consisted of UO₂, Al₃Zr₂ and ZrC during the uranium corrosion test. These results suggested that the ceramic composite prepared in this study owned both good corrosion resistance to the molten uranium and thermal shock resistance at high temperatures,so it’s a high potential material to replace the high density graphite crucible in nuclear industry.

Abstract: Magnesium aluminate spinel (MgAl₂O₄) is widely used in many engineering applications due to its high melting point (2135°C), high mechanical strength, chemical inertness, and good optical properties. Precipitation method is recognized as a convenient and cost-effective method for the synthesis of nanopowders. In this present work, MgAl₂O₄ nanopowders were prepared by precipitation method using ammonium hydrogen carbonate as a precipitant. The precipitated precursors were a mixture of ammonium dawsonite (NH₄Al (OH)₂CO₃·H₂O) and hydrotalcite (Mg₆Al₂(CO₃)(OH)₁₆·4H₂O). After calcining at 1100°C for 2 hours, The MgAl₂O₄ nanopowders with particle size of 20-170 nm were obtained. The sinterability of the MgAl₂O₄ nanopowders was evaluated by sintering compacts of the MgAl₂O₄ nanopowders at temperature of 1300-1650°C for 2 hours. The relative density of the sintered MgAl₂O₄ ceramics reached about >97% of theoretical density after sintering at 1500°C for 2 hours. The Vicker’s hardness of the sintered ceramics reached a value of 1414 HV (13.9 GPa) after sintering at 1650°C for 2 hours.

Abstract: Magnesium aluminate spinel (MgAl₂O₄) or spinel ceramics have been widely used in engineering fields due to its attractive properties, such as high mechanical strength, good optical properties, and high refractoriness. Precipitation is one of the most common techniques for preparing spinel nanopowders because it offers many advantages including low cost, simple method, and ease of mass production. However, severe agglomeration usually takes place during water removal process, i.e. washing and drying. These hard agglomerates deteriorate sinterability of nanopowders. In this study, spinel nanopowders were prepared by co-precipitation method. The remaining water in the precipitated precursor was removed by washing the precipitated precursor with organic solvents, i.e. ethanol and acetone-toluene-acetone. Conventional drying process was also performed for comparison. The characteristics of the obtained spinel nanopowders were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and dynamic light scattering (DLS) method. The results showed that the water removal process did not have any significant effects on phases of the dried precursors and the calcined powders. However, washing the precipitate precursor with organic solvent is the most effective process to prepare spinel nanopowders with low degree of agglomeration. Whereas, removing water by conventional drying process led to the formation of hard agglomerates. Furthermore, the effects of water removal process on sinterability of the spinel nanopowders were also reported.

Abstract: A wide application of magnesium aluminate spinel powder has attracted a number of studies concerning the preparation of magnesium aluminate spinel powder. In this study, a precursor for magnesium aluminate spinel was synthesized by a homogeneous precipitation method using urea as a precipitant. The precursor and the calcined powders were characterized by X-ray diffractometry, Fourier transform infrared spectroscopy, scanning electron microscopy, and transmission electron microscopy. After precipitation, the precursor was magnesium aluminium hydrate carbonate compound. By calcining, the precursor decomposed to MgO and an amorphous phase after calcining at 600°C. The formation of magnesium aluminate spinel via a reaction between MgO and the amorphous phase was observed after calcining over 800°C. The equiaxed magnesium aluminate spinel nanoparticles with particle size of 20-40 nm were obtained after calcining at 1100°C for 2 hours. Sinterability of the obtained magnesium aluminate spinel nanoparticles was also investigated by sintering compacts of magnesium aluminate spinel nanoparticles in the temperature interval of 1300-1650°C. Sintering temperature of 1600°C allowed the fabrication of dense magnesium aluminate spinel ceramics with relative density >95%.

Abstract: Low-temperature synthesis of spinel (MgAl₂O₄) was investigated by capsule hot isostatic press (HIP) using hydroxides as starting materials; Mg(OH)₂, MgO, Al(OH)₃, g-AlOOH, g-Al₂O₃. MgAl₂O₄ has been conventionally prepared above 1400 °C, whereas the mixtures containing the hydroxides reacted at 400-500 °C for 1 h under 200 MPa of isostatic pressure to form MgAl₂O₄. The hydroxides showed high reactivity to form MgAl₂O₄ with high crystallinity. However, the products more or less contained Mg(OH)₂. The single phase of MgAl₂O₄ was obtained by adding 20 mass% of water to the starting mixtures of Mg(OH)₂ - Al(OH)₃, Mg(OH)₂ - g-Al₂O₃ and MgO - Al(OH)₃. The average grain size of the MgAl₂O₄ was 70 – 100 nm. It was concluded that the transformation of AlOOH or g-Al₂O₃ was promoted by a small amount of water and that Mg(OH)₂ rapidly reacted with the active intermediate compound between AlOOH or g-Al₂O₃ and a-Al₂O₃.

Abstract: The broad strong red emission in Pr³⁺ doped MgAl₂O₄ phosphor powders have been synthesized by conventional solid state reaction in air. The temperature dependence of particle size and structural phase were investigated. The intense red emission of the phosphors is observed under 250-350 nm excitation, which cover the spectrum of Ultraviolet. The optimal emission intensity was obtained when Pr doping level was 0.002mol. The lifetime decay curves with different concentrations and a possible mechanism were also studied.

Abstract: Three synthetic and stoichiometric 2:3 type spinels (Fd3m symmetry): MgAl2O4, MgFe2O4, NiAl2O4 with a different initial structural order in cation sublattice were investigated. Investigations by means of high temperature XRPD method at temperatures ranging from 25°C to 1100°C were carried out. Diffraction patterns at each temperature in isothermal conditions were measured. For each tested spinel changes in several temperature-dependent parameters were measured: oxygen positional parameter u(T), cell edge a0(T), cation site occupancies Occ(T) in tetrahedral and octahedral positions and a cation-anion distance in tetrahedral TO(T) and octahedral MO(T) positions. Temperatures of initial order-disorder transformation were determined on the basis of the course of these dependences. Basing on changes of the cell edge a0(T), the values of thermal expansion coefficient and for spinel before and after the beginning of order-disorder transformation, respectively, were calculated. The values of measured temperature-dependent parameters were used to calculate the degree of inversion x in the spinel structure defined as the number of 3+ cations in tetrahedral sites. In each case two methods of calculating the degree of inversion from experimental data were applied. The first method involved observing the changes in sites occupancy in the cation sublattice versus temperature, which resulted in a change of diffraction lines intensities. The second method was based on observing the changes in cation-anion distances in tetra- and octahedral coordination versus temperature. The results obtained by both methods were compared, discussed and advantages and disadvantages of each of them were presented. It was shown that when atomic scattering factors of cations 2+ and 3+ in the spinel structure differ significantly, the most precise method is the one based on changes in sites occupancies versus temperature. The method based on calculation of changes in cation-anion distances is recommended when atomic scattering factors of cations differ slightly but oxygen positional parameter and cation-anion distances changes significantly during order-disorder transformation like in normal spinel structure when effective ionic radii of 2+ and 3+ cations differ significantly.

Abstract: MgAl2O4 (MA) spinel nano powder was synthesized using a molten salt technique, by heating stochiometric composition of MgO and AlOOH. NaCl salt was used as reactor media for MA spinel formation. Reactants and molten salt mixtures were fired in an alumina crucible for 3 h at temperature ranges from 850 oC to 1000 oC. The synthesis temperature was decreased from 1300 oC required by the conventional solid–solid reaction process to 850 oC for molten salt approach. The synthesized powders were characterized by X-ray diffraction pattern (XRD), scaning electron microscopy (SEM) and Brunauer-Emmett-Teller technique (BET). XRD revealed complete formation of MA without remaining of any secondary phase for synthesized powder fired at 850 oC for 3 h. Moreover, electron microscopy illustrated that the MA morphology and size was the same as the nano boehmite morphology which confirmed the existence of a ‘template process’ mechanism during the molten salt synthesis. In addition, high surface area for nano MA powders was achieved in this method.