Transparent MgAl2O4 spinel fabricated by spark plasma sintering from commercial powders (original) (raw)
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Acta Metallurgica Slovaca
Transparent MgAl2O4 ceramics were fabricated by pulsed electric current sintering (PECS) employing two-step sintering mode. First, nanoscale MgAl2O4 powders were produced by solution combustion synthesis from hydrated nitrate compounds and urea. Subsequently, the synthesized powders were sintered by PECS with a heating rate of 100oC/min under an applied pressure of 100 MPa. The sintering process was conducted according to two-step heating profile. At the first step, the temperature increased to 1050, 1100, and 1150oC, followed by a dwell time of 60 min. The second-step sintering was carried out at 1300, 1350, and 1400oC for 20 min. The transparent ceramics sintered at 1050oC/ 60 min – 1400oC/ 20 min exhibited transmittance over 80% in infrared range. In addition, transparent samples presented a Vickers hardness up to 30 GPa for sintering mode of 1150oC/ 60 min – 1400oC/ 20 min.
International Journal of Applied Ceramic Technology, 2014
The study deals with the effect of the SPS parameters and LiF doping on the mechanical and optical properties polycrystalline magnesium aluminate spinel (PMAS) with emphasis on the grain size of the final product. Sintering at 1300°C of undoped powder yielded fully dense submicrometer (0.4-0.6 lm) samples with elevated mechanical properties (1600HV and 300MPa bending strength). Doped samples had a larger, 40 lm grain size, lower, 1450HV, hardness and 150MPa bending strength. The transmittance of the doped samples (80% at 500 nm wavelength) was higher than that of the undoped ones. Thus, the required functionality of the ceramic dictates the choice of parameters for the fabrication of dense transparent PMAS. *rothmana@bgu.ac.il
Transparent MgAl 2 O 4 ceramic produced by spark plasma sintering
A two-step pressure profile was used to prepare transparent MgAl 2 O 4 ceramic without sintering aids by spark plasma sintering (SPS) at 1300°C for 3 min. The influence of the pressure profile was investigated systematically and the discoloration in the present study was attributed to the dislocation formed during the fast densification in SPS. At low pre-load pressure, 5 MPa, with the normal characteristic fast heating rate (100°C min À1 ), high inline transmittance of 51% at 550 nm and 85% at 2000 nm can be achieved.
The Criteria for Optimization of Spark Plasma Sintering of Transparent MgAl2O4 Ceramics
Journal of the Japan Society of Powder and Powder Metallurgy
In this work, criteria for optimization of the Spark Plasma Sintering (SPS) regimes to manufacture transparent MgAl 2 O 4 ceramics have been suggested. These criteria are not derived from the microstructural characteristics of the sintered samples-density or grain/pore size. In our approach, the criteria for optimization are the coefficients in the equation describing the normalized optical density. After the choice of the criteria was justified, they were used for obtaining transparent MgAl 2 O 4 ceramics with improved optical properties by SPS at optimal temperatures, pressures and heating rates. The suggested criteria were determined for representative sets of MgAl 2 O 4 ceramic specimens obtained by SPS using different processing parameters.
A thermodynamic approach to obtaining transparent spinel (MgAl2O4) by hot pressing
Journal of the European Ceramic Society, 2015
Hot pressing has been investigated as a method for producing highly transparent, cost-effective MgAl 2 O 4 for optical applications. The hot pressing of stoichiometric Al 2 O 3 -MgO powder mixtures has been preferred to the direct pressing of spinel powders, given the ready availability of pure powders and the opportunity to exploit the thermodynamic driving force for spinel formation. Ultra-pure LiF has been used as a sintering additive. An accurate thermodynamic study on the reactions involved in spinel formation has shed light on the mechanisms affecting the material's transparency, particularly as concerns the presence of a carbonaceous environment. Transparencies up to 70% in the visible range (maximum 78%) have been obtained. The present study focused on the pressure applied during sintering as one of the main parameters influencing the material's final optical properties.
Spark plasma sintering (SPS) of transparent magnesium-aluminate spinel
Journal of Materials Science, 2007
Transparent ceramics are of great current interest on account of their versatile optoelectronic properties and their potential for transparent armor related applications. Foremost candidates for structural transparent ceramic armor materials are cubic aluminum oxynitride, known under the trade name ALON, cubic magnesium-aluminate spinel and sapphire. The latter has a non-cubic crystal structure and is currently manufactured and used only in the form of single crystals, whereas cubic, isotropic ALON and spinel can be processed as polycrystalline materials. The raw material powders for ALON are proprietary and expensive. Spinel powders can be purchased from commercial suppliers and their synthesis is relatively simple. The critical properties for the preparation of bulk transparent spinel are related to the purity, morphology and size of the starting powder. The consolidation into bulk solids is performed at elevated temperatures conjointly with the application of high pressure either as hot pressing (HP) or as hot isostatic pressing (HIP). Optical requirements dictate the elimination of all second phase particles and of residual porosity. Thus the end product of the consolidation process must be a fully dense solid.
Materials
Magnesium aluminate and other alumina-based spinels attract attention due to their high hardness, high mechanical strength, and low dielectric constant. MgAl2O4 was produced by a solid-state reaction between MgO and α-Al2O3 powders. Mechanical activation for 30 min in a planetary ball mill was used to increase the reactivity of powders. Yttrium oxide and graphene were added to prevent abnormal grain growth during sintering. Samples were sintered by hot pressing under vacuum at 1450 °C. Phase composition and microstructure of sintered specimens were characterized by X-ray powder diffraction and scanning electron microscopy. Rietveld analysis revealed 100% pure spinel phase in all sintered specimens, and a decrease in crystallite size with the addition of yttria or graphene. Density measurements indicated that the mechanically activated specimen reached 99.6% relative density. Furthermore, the highest solar absorbance and highest spectral selectivity as a function of temperature were ...