Key Engineering Materials Vols. 317-318

Abstract: Microwave sintering is a process in which target materials absorb microwaves and heat themselves from the inside. When microwave energy is effectively absorbed by the material, energy consumption for the sintering can be reduced. Our study is focused on the microwave sintering of BaTiO3 to more rapidly obtain dense ceramics with specific characteristics. For BaTiO3-based electronic components, the sintering temperature is too high for manufacture, so various additives are used to decrease the sintering temperature without undue worsening of the electrical characteristics. In this work, during microwave sintering, BaCO3, H3BO3, BaB2O4 and LiF were added to form a liquid. The effects of the amount of liquid phase on density and dielectric properties were investigated. BaTiO3 sintered with BaCO3 and H3BO3 showed dielectric properties, whereas BaTiO3 sintered with BaB2O4 had semiconducting properties with PTCR characteristics. Also, LiF-added BaTiO3 indicated a dielectric constant in which the peak shifts to lower temperatures with higher LiF concentrations.

Abstract: The aim of this research is, to clarify which route the sol-gel-process is taking in the case of a Al-Mg-spinel slurry, in particular, whether the hydrolysis reaction or the spinel formation is faster and which of the intermediate hydroxide phases Al(OH)3, and Mg(OH)2, or MgO and Al2O3 or MgAl2O4
H2O are formed during the spinel formation. The spinel-alloy was produced using the polymeric route during wet chemical processing. Aluminium-isopropoxide was hydrolyzed in order to form the boehmite-sol and then the same amount of magnesia was added and mixed. This sol precipitated as boehmite (AlOOH) and brucite (Mg(OH)2) after ageing for 12h as confirmed by differential thermal analysis (DTA), and differential thermal gravity (DTG) measurements. After that, the powders were subsequently annealed at 900oC for 3h in air and observed by TEM. Calculations using thermodynamic enthalpy data are in good agreement with the experiments and can be used to predict reaction paths in other system as well.

Abstract: TiO2 coated open-cell mullite ceramics were fabricated with coating TiO2 sol on pores of mullite, and its processing parameters were investigated. Open-cell mullite ceramics were fabricated through a gel-casting process. Two kinds of mullite precursor powders were prepared by dissolution of two kinds of aluminum salts (Al(NO3)3·9H2O; type I and Al2(SO4)3·16H2O; type II) into colloidal silica sols. To produce porous mullite ceramics, both mullite precursor powders and PMMA beads (≈ 5μm) were co-dispersed by electrosteric stabilized mechanism in an aqueous system and then gel-casted. The green bodies were sintered above 1300°C for 3hrs in air. The PMMA was offered as pores in sintered mullite bodies. The prepared open-cell mullite ceramics were dipped in TiO2 precursor solution. The sintered bodies coated with TiO2 sols were re-sintered below 1000°C for 3hrs after drying at room temperature. The TiO2 was successfully coated into the open-cell mullite ceramics. The characteristics of each TiO2 coated porous mullite ceramics were investigated by XRD, SEM, porosimetry, as functions of aluminum salt and pH of sol. It was found that the synthesis behavior and the porosity of the mullite are strongly dependent upon aluminum salt species, resulting in different grain size, morphology, and pore size.

Abstract: Water-based slurries containing barium ferrite particles have been prepared and slip cast in magnetic field. This paper presents the characteristics of the suspensions in terms of Iso-Electric Points (IEP) and zeta potential that were evaluated through pH titration and polymer adsorption. Both enlarging the specific surface area of particles by planetary milling and adjusting the pH to low value apparently increase the zeta potentials. Stable slurry was obtained by adding polyethylene glycol (PEG) into the suspension at pH = 2 ~ 3.5. The steric repulsion plays key role in dispersion and PEG films served as insulative layers and mechanically kept particles from contact each other. The barium ferrite particles formed many stacks of plates during slip casting, which either aligned randomly without magnetic field applied or regularly aligned to form textured structure when magnetic field was applied.

Abstract: To prepare the vibrated casting slurries of waste tile particles, ultrafine colloids of the waste tile was added as a binder materials and as a thixotropic agent to avoid segregation between large and fine grains. The plasticity of the slurries was controlled by the rheology of colloids of the waste tile. To give thixotropic behavior, the dispersion and coagulation behavior of the colloidal suspension were investigated by the zeta potentiometer. The point of zero charge of the suspension was shown near pH 5 and maximum zeta potential was -55[mV] near pH 11. Maximum agglomerate particles were appeared near to the point of zero charge. The bending strength and water absorption of sintered bodies were more than 8.35kgf/cm2, and less than 5.0% above 1100 oC respectively. In addition, crystalline phase of sintered bodies were quarts and mullite. The micro-structure of the sintered body has a uniform pore size and distribution.

Abstract: Three dimensional functional gradients have been prepared from fine powders by direct ink-jet printing using two methods for ink mixing and for coarser, free-flowing powders by acoustic deposition. Each of these methods can provide compositional accuracy of about 3 wt.%. These are 2nd generation technologies for computer control of both shape and composition. The techniques are described and their potential discussed in terms of resolution and compositional accuracy.

Abstract: The dense Pb(Zr0.52Ti0.48)O3 (PZT) piezoelectric ceramics have been prepared at a low temperature by a spark plasma sintering (SPS) method without excess PbO addition and their structural features including domains were systematically investigated. The fine microstructure consisting of submicrometer-sized grains as well as relative density reaching 99% was achieved by sintering at 950°C which is 400°C lower than that of pressureless sintering (PLS). Transmission electron microscopy (TEM) results confirmed that the sintered specimen contained very dense domain structures inside each grain, showing the nanoscaled single-domains even at the small grains (below 100 nm). The SPS-processed PZT exhibited better piezoelectric properties than those of the PLS-processed one, which is attributed to its fine-microstructural feature.

Abstract: C/C-SiC composites, namely carbon fiber reinforced silicon carbide and pyrocarbon matrices, were fabricated in two steps in this study. Firstly, C/C composites were prepared by a rapid economical densification process of chemical liquid-vaporized infiltration. PAN based felt and 2-Dimensional carbon fibers were chosen as preform, respectively. A liquid hydrocarbon, kerosene, was used as a precursor. The C/C composites were processed in a temperature range of 900-1100°C for 150 minutes. Subsequently, C/C-SiC composites were fabricated from the C/C composites and silicon powder by reactive melt infiltration method. Densities, open porosities of the C/C and the C/C-SiC composites were investigated. Structural properties of the C/C-SiC composites were studied by optical microscopy. X-ray diffraction was used to identify the element and the crystal phase of the composites. It was shown that the density of C/C composite reached to 1.72 g/cm3 based on the 2D carbon fibers by CLVI method. Microstructure observation of the C/C composite revealed that the pyrocarbon is layer concentric around the fibers. It was found that during the RMI processing β-SiC was formed through the reaction only between liquid silicon and pyrocarbon, while carbon fiber was not damaged. Free silicon remains in the C/C-SiC composites because of insufficient reaction with the pyrocarbon.

Abstract: High strength particulate ceramic composites are in general reinforced by strong dispersoids, such as strong ceramic particles (SiC, TiB2, ZrO2, et al) and strong metallic particles (Mo, W, et al). In this work high strength ceramic composites with in-situ synthesized hexagonal boron nitride (h-BN) have been prepared and characterized. As an example, we manufactured mullite-BN composites by reactive hot pressing (RHP) using aluminum borates (9Al2O3·2B2O3 and 2Al2O3·B2O3) and silicon nitride as starting materials. The obtained material RHPed at 1800°C showed a strength of 540 MPa, which was 1.64 times higher than that of the monolithic mullite ceramics. TEM observation revealed that the composite had an isotropic microstructure with a fine mullite matrix grain size of less than 1 μm and a nano-sized h-BN platelets of about 200 nm in length and 60∼80 nm in thickness. The high strength was suggested to be from the reduced matrix grain size and the small toughening effect by the h-BN platelets. In addition, this kind of ceramic composite demonstrates low Young’s modulus that is beneficial to the thermal/mechanical shock resistance, and excellent machinability.

Abstract: Dense SiC/Si ceramics were prepared by a silicon melt infiltration method. Highly porous SiC ceramics with a wood-like microstructure were prepared by silica sol infiltration and carbothermal reduction reaction. Furthermore, laminated SiC/Si composites were also prepared by the carbonization and silicon melt infiltration of stacked papers. The microstructure and characteristics for this series of materials were determined and compared.