Microwave synthesis and sintering of Mg4Nb2O9 nanoceramics

Mg₄Nb₂O₉ nanopowders were prepared from MgO and Nb₂O₅ mixtures by using a high energy ball milling method, combined with subsequent annealing at low temperatures by microwave heating. After milling for 20 h, pure phase Mg₄Nb₂O₉ nanopowders with an average grain size of 127 nm were obtained at 850, 130 °C lower than that required by a conventional solid state reaction process. Mg₄Nb₂O₉ ceramics sintered at a low temperature of 1,300 °C using microwave heating showed almost full density and excellent microwave dielectric properties (ε_r = 12.9, Q × f = 174,200 GHz and τ_f = ?68).     

Effect of mechanical activation and microwave heating on synthesis and sintering of nano-structured mullite

Nano-structured mullite was produced from the microwave heating of a mixture of clay and alumina activated mechanically in a planetary ball mill for 30, 50 and 70 h. XRD results showed after 30 h milling time, clay disappeared and alumina and quartz appeared as the only crystalline phases. The maximum specific surface area was 34.92 m²/g for the sample activated mechanically for 30 h. The mullitization was completed for powders milled for 30 and 50 h and heated for 30 min (equal to 1376 °C) in a microwave oven. The maximum density and flexural strength values were measured for samples milled for 30 and 50 h, respectively, and sintered for 30 min. The flexural strength values of these samples were 3 and 4.7 times of the strength value of the sample milled for 2 h and sintered at the same conditions.     

Investigation on rheological behavior of 8 mol% yttria stabilized zirconia (8YSZ) powder using Tiron

In this research, 8 mol% yttria stabilized zirconia (8YSZ) powder were dispersed in de-ionized water by the use of different amounts of Tiron as dispersant. The results of rheological and sedimentation measurements of each suspension were evaluated and the optimum amount of Tiron was selected (0.8% Tiron). Also, various pH were investigated and the best stabilized suspension is achieved at pH 10. Furthermore, the zeta potential of suspension with and without adding dispersant was obtained. The isoelectric point (IEP) of as-received 8YSZ powder was about 8.5 and shifted obviously to acidic region after adding dispersant to the suspension.     

Synthesis of Nanosized Ba(Zn1/3Nb2/3)O3 Using Microwave Heating Process

Ba(Zn_1/3Nb_2/3)O₃ (BZN) powder was synthesized through microwave heating process. The formation of BZN phase started at 500 °C and completed at 1000 °C. BZN was also synthesized through the conventional process for comparison. In the microwave processing, the time needed to heat BZN sample from room temperature to 1000 °C, the temperature at which the synthesis was complete, was 26 min. However, in the conventional heating, to obtain complete formation of BZN phase, the sample was heated at 1000 °C for 60 min. TEM analysis showed that the approximate sizes of the crystallites of the powders formed for both methods were between 100 and 250 nm. TEM observation of BZN powders prepared through both processes also revealed that particles are highly aggregated.     

A new route for the direct synthesis of Al2O3/SiC nanopowder mixtures by the carbothermal reduction of parent oxides

The direct synthesis of Al₂O₃/SiC nanopowders from the parent oxides Al₂O₃ and SiO₂ through mullite carbothermal reduction as intermediate phase has been investigated. The effect of the amount of excess stoichiometric carbon (active charcoal, AC) as sole carbon source on the microstructure evolution has been studied. The effect of type of carbon source (AC, graphite (G), 50 wt % AC 50 wt % G mixture, 57 wt % AC 43 wt % G mixture) on the microstructure evolution was investigated using 30 wt % excess stoichiometric carbon. The effect of reaction temperature, reaction duration, initial green compact thickness, and carbon source on the mullite conversion, morphology, and surface area of the final powders has been thoroughly investigated. The calculated activation energy is in the range of 203–230 kJ mol^?1, depending on the carbon source used. The synergetic effect observed for the AC/G mixtures has been accordingly explained. © 2010 American Institute of Chemical Engineers AIChE J, 2010     

Effect of viscosity of polyvinyl alcohol solution on morphology of the electrospun mullite nanofibres

Mullite nanofibres with diameters of 85–110 nm were obtained by electrospinning of solutions containing different amounts of polyvinyl alcohol (PVA, as the template) ranging from 0 wt% to 10 wt%, which were added to the sol, and sintered at different temperatures. The sol was prepared by the sol–gel method, using aluminium isopropoxide (AIP), hydrated aluminium nitrate (AN) and tetraethylorthosilicate (TEOS) as the precursors. The details of crystal development, microstructure and thermal decomposition behaviour of the electrospun nanofibres were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and simultaneous thermal analysis (STA). The optimal content of 6 wt% of PVA in the electrospinning polymeric solutions was found to be the suitable viscosity for the electrospinning and ultimately resulted in the formation of the most pure and uniform mullite nanofibres.     

Microwave hybrid synthesis of silicon carbide nanopowders

Nanosized silicon carbide powders were synthesised from a mixture of silica gel and carbon through both the conventional and microwave heating methods. Reaction kinetics of SiC formation were found to exhibit notable differences for the samples heated in microwave field and furnace. In the conventional method SiC nanopowders can be synthesised after 105 min heating at 1500 °C in a coke-bed using an electrical tube furnace. Electron microscopy studies of these powders showed the existence of equiaxed SiC nanopowders with an average particle size of 8.2 nm. In the microwave heating process, SiC powders formed after 60 min; the powder consisted of a mixture of SiC nanopowders (with two average particle sizes of 13.6 and 58.2 nm) and particles in the shape of long strands (with an average diameter of 330 nm).