Materials Science Forum Vols. 534-536

Abstract: Open-celled metal foams were synthesized using a replication technique. Therefore a reticulated polyurethane template was coated by a slurry and removed thermally, followed by a sintering step. Since the process is feasible for a multiplicity of metals the experiments were performed on the example of stainless steel 316L. Highly porous components were obtained showing adjustable densities between 0.3 and 2.0 g/cm³. The cell structure is exceedingly homogeneous and the cell sizes may be chosen in the range of 10 – 80 ppi. In order to characterise the properties, compression tests and acoustical tests were carried out. A significant influence of the density and the cell size on the acoustical and mechanical properties was noticed.

Abstract: Recently, textured microstructure has received particular interest in the processing of advanced Si3N4 ceramics because of significant improvement in the mechanical properties and thermal conductivity. This work will report a highly textured β-Si3N4 ceramic by aqueous slip casting in a magnetic field and subsequent pressureless sintering, using commercial α-Si3N4 raw powder and a mixture of Y2O3 and Al2O3 as sintering aids. To obtain the well-dispersed Si3N4- Y2O3-Al2O3 slurries, polyethylenimine (PEI) was chosen as a dispersant. Effects of the sintering aids, PEI amount, pH and stirring time on the stability of the Si3N4 slurries were studied. It is shown that PEI is an effective dispersant for stabilizing the Si3N4-Y2O3-Al2O3 slurries that does not show a time-dependent behavior at an optimum pH ≈ 10, compared to the case in the absence of PEI. Using the 30 vol% Si3N4-Y2O3-Al2O3 slurries stabilized with 1.5 dwb% PEI at pH 10, the highly textured β-Si3N4 with 97 % relative density could be obtained by slip casting in a magnetic field of 12 T and subsequent sintering at 1800 oC for 1 h. The textured microstructure is featured by the alignment of c-axis of β-Si3N4 crystals perpendicular to the magnetic field, and the Lotgering orientation factor, f, is determined to be 0.8.

Abstract: Biomimetic apatite deposition behaviors on Zr-1Nb and Ti-6Al-4V plate with various surface conditions were examined. Both alloys were polished with abrasive papers to have different roughness and some of them were treated in NaOH before deposition of apatites in the simulated body fluid. After, 10 days immersion in a SBF, NaOH treated Zr-1Nb and Ti-6Al-4V were completely coated with apatite. The deposition rate of apatite was higher on NaOH-treated Ti-6Al- 4V than on NaOH-treated Zr-1Nb initially, but the deposition rate on Zr-1Nb accelerated after 2 days and the total weight gain due to the deposition on Zr-1Nb approached to that of Ti-6Al-4V. NaOH treatment was found to enhance the deposition rate of apatite on Ti-6Al-4V significantly. On the other hand, the deposition rate of Zr-1Nb was not influenced by NaOH treatment. Without NaOH treatment, the polished Zr-1Nb with abrasive paper was found to induce more apatite nucleation than the polished Zr-6Al-4V. The presence of apatite was confirmed by XRD analysis. SEM observation revealed a conglomerated granular structure with elongate plates.

Abstract: Prediction of porosity level for the Si3N4 filter substrate manufactured by reaction bonding of Si compact and subsequent post-sintering was attempted and its validity was verified by comparison between calculation and measurement. Both are well matched in the specimens of asnitrided and low temperature post-sintering, while few percents higher measurement in high temperature post-sintered specimens resulted from weight loss during the process.

Abstract: The microstructure and electrical conductivity of CNTs dispersed Al2O3 nancomposites depending on the powder processing and CNTs content were demonstrated. The composite powders with homogeneous dispersion of CNTs could be synthesized by a catalytic route for direct formation of CNTs on nano-sized Fe dispersed Al2O3 powders. The sintered nanocomposite using the composite powder with directly synthesized CNTs showed homogeneous microstructure and enhanced elelctrical conductivity. The influence of powder processing on the properties of sintered nanocomposites was discussed by the observed microstructural features.

Abstract: When Y2O3 was added to Ti-excess BaTiO3 ((Ba+Y)/Ti =1), the the area occupied by Y3+ ion was confirmed by its microstructure development, electrical conductivity behavior and lattice constant. Grain growth inhibition was observed when the content of donor dopant exceeded a critical value (x≈0.01) in BaTiO3+x(0.5Y2O3+TiO2) system. A donor-doped behavior was observed at various Y contents (0.2~3.0 mol% Y) when Y2O3 was added to TiO2-excess BaTiO3. As Y content was increased, (002) and (200) peaks shifted to higher angles and the lattice constant (a and c axis) decreased gradually. These results confirms that Y3+ ions preferentially occupy Ba sites when Ba/Ti < 1.

Abstract: In an attempt to reduce processing cost and to improve the resolution of PDPs, a micro mold transfer processing route for barrier ribs of plasma display panel was attempted. In this study, the parameters that may cause defects during the process were identified, which include the shrinkage during the UV curing process, stress due to the evaporation of organic components, and sintering shrinkage. Considering such parameters, a UV curable paste was developed and the barrier ribs of PDPs were successfully processed via the process. This work demonstrated the possibility of a build-up route in manufacturing barrier ribs of PDP

Abstract: In this study, hydroxyapatite (HAp) and hydroxyapatite-yttria stabilized zirconia (HAp- 3YSZ) with 20 vol. %– (ZrO2+3 %mol Y2O3) nanopowders were consolidated very rapidly to full density by High-frequency induction heat sintering (HFIHS). Effects of temperature and the addition of 3YSZ on the toughness, hardness and microstructure properties have been studied. 3YSZ second phase toughening HAp composites with higher toughness were successfully developed at relatively low temperatures through this technique. The samples were densified by heating to a sintering temperature in the range of 900 to 1200 °C, and then rapidly cooled to 500 °C within a very short time. A relative density of up to 98.6 % of theoretical density of the composites was achieved after sintering at 1100 °C. Compared with hardness and toughness obtained for pure HAp, the hardness and toughness for HAp-20vol. % 3YSZ were much higher.

Abstract: The Ti3SiC2 materials were synthesized by hot pressing TiCx and Si powder mixtures. The matrix grains were lamellar, having a small amount of TiCx. The high-temperature stability was investigated by subjecting Ti3SiC2 to high-temperature oxidation up to 1200oC in air. Ti3SiC2 began to oxidize appreciably above 850oC. The oxidation resulted in the formation of the oxide layer that consisted of TiO2 and SiO2. The scales formed were adherent.

Abstract: Alumina microcomponents have distinguishing advantages over Si counterparts. However, the shrinkage of alumina, as high as 20%, makes it difficult to produce precision components that require a high tolerance. A new fabrication process is presented to greatly reduce the shrinkage. The process consists of forming an Al powdered component through sintering and transforming the Al powdered component into an alumina part. In this way, the shrinkage occurring in sintering the Al powder component will be compensated by the expansion occurred when Al transforms into alumina. The process involves producing micro-moulds, preparing metallic paste, filling the micro-moulds with the metallic paste, demoulding, sintering the green Al patterns and finally oxidising the sintered Al-based components to achieve alumina components. The process was proven successful. Characterization of the sintered alumina microcomponents has been undertaken, including SEM image analysis, density and scale measurements.