Key Engineering Materials Vols. 317-318

Abstract: AlN-SiC ceramics with 0 to 75 mol% of AlN were fabricated through pressureless sintering of very fine AlN and SiC. Powder compacts with different amounts of AlN were fired at 2000°C for 1 h in Argon gas flow using an induction-heating furnace. The microstructure and phases present in the products were evaluated using SEM and XRD. The AlN-SiC ceramics had a porous structure with 30% porosity, and the grain size was increased with the addition of AlN. XRD analysis showed that 2H was a main phase in all samples, though 3C and 6H phases were found in 25 mol%AlN-75 mol%SiC ceramic. The electrical properties of the AlN-SiC ceramics were evaluated at various temperatures ranging from room temperature to 300°C. The electrical conductivity of the AlN-SiC ceramics depended on the amount of AlN and on the temperature. The 75 mol%AlN-SiC ceramic had higher electrical resistance, though the other samples were electrical conductors. The highest electrical conductivity was obtained with the 25 mol% AlN composition, which was 7 S/m at room temperature and 30 S/m at 300°C. The Seebeck coefficient for the AlN-SiC ceramics increased with rising temperatures. The AlN-SiC ceramics with 50 mol%AlN had the highest Seebeck coefficient of 220 2V/K at 300°C.

Abstract: Si3N4 ceramics with V2O5 based glasses as sintering additives were successfully fabricated by a powder mixing process and rapid sintering by the PECS method. The fabricated materials by Pulsed Electric Current Sintering (PECS) exhibited very fine microstructure with α and β grains. Electric conductivity for the Si3N4/(V2O5-B2O3) and the Si3N4/(V2O5-B2O3-Al2O3) at room temperature were four and six orders of magnitude higher at room temperature, and two and three orders of magnitude higher at 1000 oC than the conventional Si3N4, respectively

Abstract: The Si3N4/YSiO2N composite in which crystalline YSiO2N was formed as grain boundary phase was fabricated by hot-pressing the mixture of SiO2, Si3N4 and Y2O3. The fracture toughness of this composite was significantly improved, compared to the Si3N4 composites containing Y5Si3O12N or Y2Si3O3N4 as a grain boundary phases. To clarify the toughening mechanism, the microstructure and the crack propagation profiles were observed.

Abstract: Electrically conductive AlN ceramics were fabricated by the addition of a small amount of B4C and sintering aid, and hot-press sintering in a nitrogen atmosphere. The electrical resistivity of AlN ceramics decreased remarkably from 1014
cm to the range of 100 to 102
cm by a minimum of 2.3 wt% of B4C addition. This resistivity decrease was caused by forming three-dimensional networks composed of boron carbonitride (B-C-N) platelets synthesized during sintering. To produce the networks of B-C-N platelets, two-step sintering with a heat-treatment step at 1600°C before the densification step at 2000°C was needed.

Abstract: In this study, we propose a new process to fabricate electrically conductive alumina by gelcasting and reduction sintering. The process used the conventional gelcasting method except for varying amounts of monomer at 2.8, 5.5, and 8.0 wt.% relative to the weight of the slurry. In the plastic mould, the slurry was under in situ solidification for 3 hrs at 25oC to achieve gelation. The freshly gelled bodies were demolded, carefully dried, and then sintered at 1100oC, 1300oC, and 1550oC in nitrogen atmosphere. The holding times at 1100oC and 1300oC was 2 hours, while at 1550oC were 2, 4, and 6 hrs. The sintered alumina body was characterized by electrical property, X-ray diffraction, and scanning electron microscopy. Results showed that monomer additions and sintering schedule significantly affect in lowering electrical resistance. The low value was 3.6×106 +cm at 8.0wt.% monomer addition and sintering at 1550oC for 2 hrs. The effect of physical properties on electrical conductivity and the corresponding reaction mechanism were discussed in details.

Abstract: Multi Wall Carbon Nanotubes (MWCNTs) with a diameter of 20-30 nm were used as a conductive phase to add electric conductivity to yttria stabilized tetragonal zirconia (3Y-TZP). Almost fully dense 3Y-TZP/MWCNTs nanocomposite was obtained by pressureless sintering under inert atmosphere and Hot Isostatic Pressing (HIP) treatment. The conductivity of the nanocomposites increased with increasing content of MWCNTs. Moreover, the fracture toughness increment of the composite was confirmed at 0.5 wt% addition. Scanning electron microscopy and transmission electron microscopy observation of the microstructures showed that MWCNTs were fairly homogeneously dispersed in the 3Y-TZP matrix.

Abstract: CNTs/Fe/Al2O3 nanocomposites were prepared by thermal CVD and SPS methods. The dispersion of CNTs in the Fe/Al2O3 matrix were controlled by an attrition milling process. FESEM analysis revealed that the CNTs of 5 vol.% were homogeneously dispersed in the Fe/Al2O3. The effects of CNTs dispersion on the mechanical and electrical properties of the specimens were investigated. Fracture strength and electrical conductivity of 5 vol.% CNTs/Fe/Al2O3 specimen were measured at 641 MPa and 2.93
10-11 mS/m, whereas that of a 20 vol.% CNTs/Fe/Al2O3 specimen were 208 MPa and 8.46
10-7 mS/m, respectively. In comparison with an Al2O3 monolith, the specimen with 5 vol.% CNTs showed enhanced fracture strength and increased electrical conductivity.

Abstract: Polymer/layered silicate nanocomposities were prepared by in situ polymerization with microwave process. The influence of the amount of clay on the structure and thermal properties for the synthesized nanocomposites were characterized by means of X-ray diffraction (XRD), transmission electron microscopy (TEM), differential scanning calorimetry (DSC), and thermal gravimetric analysis (TGA). It was found that the structure of nanocomposites, an intercalated/exfoliated structure, depended on the clay content.

Abstract: 3Y-TZP/TiNiCo composites have been successfully fabricated by three step heating (dehydrogenation, hydrogen reduction, hot-pressing) of 3Y-TZP /NiO/TiH2/CoO powder mixtures. XRD analysis revealed that TiNi-base intermetallic compounds such as Ni4Ti3, NiTi, Ni3Ti had formed. The bending strength of 3Y-TZP/TiNiCo composites (~650 MPa) were much higher than those of 3Y-TZP monolith (
350 MPa) sintered at the same condition. The electrical resistivity characteristics indicated that 3Y-TZP/30 vol%TiNiCo composites were good electrical conductors. Cobalt addition to TiNi phase influenced on electrical properties of final composites, while their fracture strength was unchanged by the Co addition.

Abstract: Preparation of monodispersed inorganic-organic hybrid particles has become one of the most attractive research topics in recent years. In this study, phenylsilsesquioxane (PhSiO3/2) particles were synthesized from phenyltriethoxysilane (PhSi(OEt)3), ethanol (EtOH), hydrochloric acid for hydrolysis and ammonia water for polycondensation by the sol-gel method. Spherical PhSiO3/2 particles were obtainable by varying the mole ratios of EtOH, H2O, HCl and NH3, to PhSi(OEt)3. The particle size was changed from 0.1 to 5.0 .m with the mole ratio. Under the optimum conditions, monodispersed particles with 1.50 .m in diameter and 0.05 of normalized standard deviation were successfully prepared.