Key Engineering Materials Vols. 317-318

Abstract: Polycarbosilane was synthesized from polydimethylsilane in the presence of ZSM-5 (Si/Al=30) as a catalyst at 350oC-400oC. Characterization of synthesized polycarbosilane was performed with 29Si Solid NMR, FT-IR, and GPC analysis. Number average of molecular weight (Mn) of the polycarbosilane was ranged from 350 to 2340.

Abstract: By the usage of the hydrolysis of Ti alkoxide, various types of TiO2 were obtained through the addition of catalyst (HCl, NH4OH, and CH3COONH4) and some additives into Ti alkoxide solution at room temperature. In special, the effect of two additives, diethylene glycol (DEG) and hexamethylphospheric triamide (HMPA), on hydrolysis behaviors and microstructure with various catalyst for hydrolysis of Ti-tetraisopropoxide was in detail investigated. In case of TiO2 powder with HMPA as an additive, the crystallinity of TiO2 except with NH4OH as a catalyst decreased in XRD patterns, compared to those of products with no additive and with DEG as an additive. The addition of HMPA prohibited the growth of TiO2 and led to fine TiO2 with the average particle size of approximately 10nm.

Abstract: The photoluminescent properties of BaMgAl10O17: Eu2+ synthesized via a sol-gel process employing citric acid (CA) and ethylene glycol (EG) were investigated in this study. The photoluminescence intensity of Eu2+ emission increased with increase in the doping concentration of Eu2+ up to 15 at %. The photoluminescence intensity of BaMgAl10O17: Eu2+ also varied with the EG: CA ratio. SEM investigation indicates that the sol-gel derived particles exhibited an irregular shape. The EPR studies showed that Eu2+ ions occupied the sites with non-cubic symmetry in the prepared phosphors.

Abstract: Synthesis of bulky Y-zeolite was attempted by a hydrothermal hot-pressing (HHP) method. These bulky products synthesized under hydrothermal conditions were identified as Y-zeolite single phase. Especially, bulky Y-zeolite having translucency, high density and large surface area was obtained by HHP treatment at 423K for 2h with 17wt% of 5M-NaOH solution. This solidified zeolite like single-crystal was considered to be made by dissolution and precipitation mechanism. High-density bulky zeolites can be expected as novel molecular sieves and catalysts with high activity.

Abstract: Tricalcium phosphate [TCP, β-Ca3(PO4)2]/titania composite having high mechanical properties was prepared and characterized. The TiO2 and HAp powders, as starting materials, were synthesized by precipitation and hydrothermal methods, respectively. The sintered β-TCP/TiO2 composite was prepared by hot pressing at 1000°C for 30 min under the pressure of 30 MPa in N2 atmosphere or by pressureless sintering at 1200°C for 2 h in air with moisture protection. The influence of sintering condition on the microstructures and mechanical properties of the composite was investigated. During the sintering process, hydroxyapatite used as a starting material was decomposed to β-TCP and CaTiO3 at elevated temperature because TiO2 could accelerate the decomposition of hydroxyapatite. The hot-pressed composite with 98% of sintered density showed high hardness
value of 1080 Hv compared with the pure hydroxyapatite (600 Hv).

Abstract: Porous mullite with a porosity of about 78% was fabricated using Al(OH)3, SiO2 and AlF3 powders to develop non-asbestos friction materials for brake pads. A mullite composite was fabricated by infiltrating liquid phenolic resins to the porous mullite. The effects of the processing parameters (infiltration frequency, immersion duration and resin temperature) on the properties of the mullite composite obtained by the liquid infiltration technique were studied. SEM analysis reveals that the parameters affected the degree of infiltration and hardness of the mullite composite by changing the infiltration frequency and immersion duration. With increasing immersion duration and infiltration frequency, the resultant porosity had lower values. In the case of hardness, the measured values showed an opposite tendency.

Abstract: SiC was sintered with AlN and Y2O3 as sintering additives by spark plasma sintering (SPS). Using nano-sized β-SiC powder as the starting material, the sintered density reached about 95% of theoretical at 1800-2000oC for 10min. The β to α phase transformation of SiC was not found by XRD. The secondary phase such as Y2O3 decreased as the firing temperature was elevated, and β-SiC monophase was identified at 2000oC. It seems that the residual intergranular glassy phase is present between the SiC grains. Significant SiC grain growth was observed at 1800-1900oC by SEM. The grain size decreased with increasing amount of AlN additive. The maximum value of flexural strength of the sample reached approximately 800MPa. These results are discussed on the basis of the liquid-phase sintering mechanism in AlN-Y2O3 and Al2O3-Y2O3 systems.

Abstract: Intra-type structure of ceramic matrix composites (CMCs) can improve the mechanical properties of ceramic materials. In this work, we used γ-alumina as a matrix including nano-pore for fabricating alumina/silicon carbide composites using pressure-less sintering and pulse electric current sintering (PECS) techniques. We added α-alumina as a seed in order to improve densification of this commercially available γ-alumina. The mixture was sintered from 1250 to 1450
by pressure-less sintering and PECS techniques. Densification of the specimen sintered by pressure-less sintering is very difficult and bulk density of the specimen sintered at 1450
showed lower than 3.0 g/cc. In the case of the PECS technique, the density was improved to 3.8 g/cc in seed added specimen sintered at 1450
. Maximum Vickers hardness and maximum fracture strength were obtained 17 GPa and 500 MPa for the seed added specimens sintered at 1450
, respectively.

Abstract: Two different types of carbon fibre bundles were used for filament winding to obtain C/C preforms. C/C-SiC composites were produced from the C/C preforms by a silicon melt infiltration technique. The effect of the type of carbon fibre bundle on the mechanical and thermal properties of the resultant C/C-SiC composites was compared. The spun fiber preform yields C/C-SiC composites of better mechanical properties than the unidirectional continuous fiber preform. The strength of the composites from the SFP was 1.8 times higher than that from the CFP. The flexural strength and the O-ring strength of the composites from the SFP with a density of 2.35 g/cm3 were about 160 MPa and 170 MPa, respectively.

Abstract: Silicon nitride ceramics were prepared by new nitrided pressureless sintering (NPS) process in this study. The microstructures, strengths and thermal properties of the NPS silicon nitride ceramics containing three types of Al2O3 and Y2O3 sintering additives were investigated. Additionally, we have investigated the effect of silicon metal contents changing with 0, 5, 10, 15 and 20 wt% in each composition. The silicon nitride was successfully densified using NPS process, particularly at the starting composition of 5 wt.% Al2O3, 5 wt.% Y2O3, and 5 wt.% Si addition. The maximum flexural strengths and relative densities of these specimens were 500 MPa and 98%, respectively. The flexural strength of sintered specimens after the thermal shock test between 30oC and 1300oC for 20,000 cycles was maintained with the original laboratory strength of 500MPa by low thermal expansion coefficient, 2.9 × 10-6/oC, and high thermal conductivity, 28 W/m⋅oC.