Microstructural evolution of a commercial ultrafine alumina powder densified by different methods

The densification and grain growth of bodies made from a commercial ultrafine alumina powder was investigated. The primary powder was initially subjected to dry (uniaxial cold pressing) and wet shaping (slip casting), followed by conventional (CS)-, two step (TSS)-, and microwave (MS) sintering to explore the effect of each series of treatments on the densification and microstructural evolution of the specimens. It was demonstrated that a uniform microstructure with higher density would be obtained using the wet shaping method. In addition, microwave sintering was found to be more effective into the densification of the specimens and in yielding a finer grain structure. It is believed that the high heating rate and effective particle packing are responsible for the improvements in these properties. On this basis, it was also demonstrated that the fracture toughness of the samples increased significantly through the application of microwave sintering.     

Synthesis of mullite nanofibres by electrospinning of solutions containing different proportions of polyvinyl butyral

In this paper, the synthesis of continuous mullite (3Al₂O₃·2SiO₂) nanofibres by combination of the sol–gel and electrospinning technique is reported. To find out the optimum viscosity of the electrospinning solution for obtaining the high quality mullite nanofibers, solutions containing different amounts of polyvinyl butyral (PVB, 0–8 wt%) and the precursor sol were prepared for the electrospinning process. The precursor sol was made by using proper amounts of aluminium isopropoxide (AIP), hydrated aluminium nitrate (AN) and tetraethylorthosilicate (TEOS). Crystal phase, microstructure and thermal decomposition behaviour of the electrospun mullite nanofibres were investigated by conventional methods of analysis. The optimal amount of PVB in the electrospinning polymeric solutions was found to be between 4 and 6 wt% and the mullite nanofibres obtained as such were pure, smooth and uniform with diameter sizes of 85–130 nm after calcination at 1200 °C.     

Microwave-assisted synthesis and sintering of mullite

Mullitization behaviour of a mixture of clay and alumina as the starting materials was examined by microwave heating of (a) mixed powder and (b) compacted powder samples for different soaking times. X-ray diffraction results showed that in compacted samples mullitization process was completed after 20 min heating with a density of about 87%. Densification and microstructure of samples with different green densities heated in a microwave oven and conventional electric furnace were compared. Results showed that the grain growth of mullite was restricted by microwave heating.     

Effect of SiC particles on rehological and sintering behaviour of alumina–zircon composites

The effect of additions of SiC particulates on rheological and sintering behaviour of slip-cast alumina–zircon composites has been investigated. Finely divided alumina, zircon and silicon carbide powders were first processed into slips, using polyacrylite dispersant (0.5 wt.%) to create highly concentrated, stable aqueous suspensions at 40 vol.% loadings, from which test specimens which were then slip cast and dried. They were subsequently sintered in air for 2 h at 1650 °C. Rheological properties of the prepared slips were evaluated and related to the amount of added SiC. After sintering, the resultant porosities, fractional densities, crystallographic phases present, and microstructures were determined.     

Watermarked 3-D Mesh Quality Assessment

This paper addresses the problem of assessing distortions produced by watermarking 3D meshes. In particular, a new methodology for subjective evaluation of the quality of 3D objects is proposed and implemented. Two objective metrics derived from measures of surface roughness are then proposed and their efficiency to predict the perceptual impact of 3D watermarking is assessed and compared with the state of the art. Results obtained show good correlations between the proposed objective metrics and subjective assessments by human observers     

Reaction sintering of nano-sized ZnO–Nb2O5 powder mixture: sintering,microstructure and microwave dielectric properties

Sintering behavior, microstructures and microwave dielectric properties of ZnNb₂O₆ (ZN) ceramics prepared by reaction-sintering method were investigated. The X-ray diffraction patterns of the sintered samples revealed single-phase formation with a columbite structure after 1–5 h sintering at 1,025–1,125 °C. After sintering at 1,125 °C for 5 h, the ZN ceramics with the density of 5.56 g/cm³ (98.9 % of the theoretical density) and good microwave dielectric properties of ε_r = 23.9, Q × f = 51,000 and τ_f = ?62 ppm/°C were obtained. The obtained results demonstrated that the reaction-sintering process is a simple and effective method to prepare the ZN ceramics for applications on microwave dielectric resonators.     

Kinetic investigation of the carbothermal reduction of an Iranian clay

The kinetics of the carbothermal reduction of clay under argon atmosphere has been investigated by the X-ray diffraction method. The clay-carbon (excess) mixture was formed into 2 cm diameter disks of different thicknesses. Experimental data evidences the significant effect of sample thickness on the reaction rate at 1400 °C. Decreasing thickness promotes mullite dissociation and formation of SiC and alumina powders. Mathematical modeling of the reaction system showed the gas diffusion in the Knudsen regime through the pellet to be the rate controlling step. Diffusivity of CO and reacted core tortuosity factor have been calculated.     

Microstructural and microwave dielectric properties of LZT (Li2ZnTi3O8) ceramics sintered in presence of bismuth borate glass for LTCC applications

In the present research, the Li₂ZnTi₃O₈(LZT) ceramics were synthesized throughout solid-state ceramic processing, then mixed with bismuth borate (BiBO) glass prepared based on conventional melt quenching method. Wetting behavior of BiBO glass on the LZT ceramic substrate was monitored by hot stage microscopy. Afterward, dielectric LZT ceramics containing different amounts of BiBO glass (0.25–6 wt%) were sintered at various temperatures. X-ray diffraction and electron back scatter diffraction examinations revealed the presence of two crystalline phases of Li₂ZnTi₃O₈ and Bi₂Ti₂O₇. The maximum value of relative density (above 95%) was obtained in the case of specimens contained more than 5 wt% glass. The microwave dielectric properties of the finally sintered BiBO glass containing LZT ceramics were as follows: dielectric constant (ε_r) = 21.44–25.09, quality factor (Q × f) = 10839–54708 GHz and temperature coefficient of resonant frequency (τ_f) = (? 15.58) ? (? 12.86)ppm/°C.