Key Engineering Materials Vols. 602-603

Abstract: Aluminum silicate/SiO₂ sol composites are prepared by aluminum silicate fiber mats impregnated with different amount of silica sol. The effect of different impregnation process on impregnation amount of composites are compared, Thermal insulation performance and compression strength of composites are tested in this paper. The results show that: Samples prepared by vacuum impregnation process can get its highest impregnant amount, compressive strength of composites could achieve 241KPa.Thermal insulation performance of composites would be a liitle worse than aluminum silicate fiber products

Abstract: Titanium tin carbide (Ti₂SnC) is a member a MAX phases, which are ternary carbides or nitrides with layered structure. Ti powders are normally used as Ti source to synthesize Ti₂SnC. In this paper, TiH₂, a relative cheaper Ti source, was used to synthesize Ti₂SnC. Ti₂SnC was synthesized from TiH₂/Sn/TiC or TiH₂/Sn/graphite powders by a tube furnace at different temperatures under Ar atmosphere. From the analysis of X-ray diffraction results, the lowest temperature to synthesize Ti₂SnC was 1000 °C. Ti₂SnC content increased with temperature, and high purity Ti₂SnC was fabricated at 1200 °C. From scanning electron microscopy, as-synthesized Ti₂SnC from TiH₂/Sn/TiC was with plate-like structure. However, for Ti₂SnC from graphite as carbon source, there was some stripe microstructure. Some large pores existed between the Ti₂SnC particles. The existence of the pores make the mechanical properties of Ti₂SnC block significantly reduced, its compressive strength was only 0.483 MPa or 0.35 MPa respectively for samples synthesized from TiC or graphite as carbon source. The low mechanical properties make the porous sample easy to be broken as powders.

Abstract: In the work reported here, boron source is borax, carbon source is sucrose and the nitrogen source is urea, polymer-network method is applied to prepare the hexagonal boron nitride (h-BN) precursor, and then synthesized the h-BN powders with high purity and high surface area. As the sucrose content is fixed, we study the effect of carbonization temperature on phase composition and adsorption properties of h-BN powders. As the carbonization temperature is fixed, the effect of sucrose content on phase composition and adsorption properties of h-BN powders are studied. The results show that the optimum carbonization temperature is 220°C, and when the sucrose content reach 17.45wt.%, the h-BN powders showed the best adsorption properties.

Abstract: In this paper, porous silicon nitride ceramics were prepared feasibly by combustion synthesis with different glasses as additives. The results reveal that the morphology, strength and porosity are relevant to glass type. When BN is added, the porosity and strength is depended on the glass content. With the increasing glass content, the porosity of the composite decreased and the strength increased.

Abstract: On the basis of magnesiothermy, with Mg powder, TiO₂ powder and B₂O₃ powder as mixed reactants, relatively pure TiB₂ powder was prepared by self-propagating high temperature synthesis (SHS) after leached in diluted hydrochloric acid with the suitable concentration. The product after leached was examined by XRD for qualitative and quantitative analysis, SEM for morphology analysis and infrared spectroscopy for transmittance analysis. It was found that diluted hydrochloric acid could decrease most of the impurities, and the proportion of TiB₂ phase in the product is more than 99%, which was hexagonal shape crystal with the grain size of 200-300 nm. Compare with the infrared spectrogram of the middle infrared spectral region of the product of before and after acid pickling, it was found that the transmittance of TiB₂ is extremely low in the range of 2000-4000 cm^-1, which can be excellent infrared absorption material in the corresponding wave band.

Abstract: Boron nitride (BN) was prepared by polymer-derived method using precursor poly [(alkylamino) borazin (PABZ). The evolution of composition and structure of precursor PABZ into BN ceramic during curing and pyrolysis process was investigated by FTIR, TG, XRD techniques and chemical analysis. The results showed that PABZ could fulfill curing heated at 80°C for 2hrs in NH₃ atmosphere. During the curing process the both transamination reaction and condensation reaction occurred, resulting in cross-link between monomer molecular. With the proceeding of curing process, the new B-N bonds were formed and continued to expand in three dimensions. After cured treatment the thermal pyrolysis of PABZ in ammonia atmosphere was took place, and inorganic degree and crystallinity of products were better, which was more suitable for preparation of high purity hexagonal BN (h-BN) ceramic.

Abstract: Continuous multi-filament boron nitride fibers have been prepared on a large scale using the melt drawn technique from a low-cost boracic acid. Boracic acid was heated to obtain the molten boric oxide in a melting tank. Molten boric oxide was melt spun in a conventional manner through an 200-tip bushing to produce a continuous multifilament yarn consisting of 200 filaments of boric oxide. Boric oxide fibers were nitrided in an ammonia and were annealed in an inert atmosphere while simultaneously subjecting the fibers to sufficient longitudinal tension as to at least prevent longitudinal shrinkage of the fibers. The resulting fibers, consisting essentially of boron nitride, were less than about 8 μm in diameter and greater than 500 m in length. It indicated that the boron nitride fibers has a sound mechanical function with tensile strength of 1.40 GPa. The continuous boron nitride fibers of high-performance were especially suitable for reinforcing plastic, ceramic or metal matrices in the preparation of fiber reinforced composites.

Abstract: The internal gelation process was used to produce gel microspheres containing Zr. The effect of the broth composition particularly pH value on the stability of the broth and morphology of the dried spheres was investigated. The results indicated that the stability of the broth at room temperature increased with the decrease of the pH of the broth. When the pH of the broth was in the range of 1 to 2 the spheres obtained had good sphericity and uniform size and at pH of 1.35 the spheres were transparent.

Abstract: Gel-casting is promising in fabricating uniform and complex-shaped ceramic body, but it is confined in industrial application for the neurotoxic acrylamid (AM) gel system. Ether-contained oligomer poly(ethylene glycol)(200) dimethacrylate (PEG(200)DMA) with methacrylate end-group on either side of the long chain can act as cross-linking agent in free radical polymerization to form branchy gel network structure, which improves the green body strength. In this study, low-toxic PEG(200)DMA was chosen as the main binder and crosslinker and small dosage AM was applied to adjust the viscosity of the ceramic slurry to gel-cast 3YSZ (PEG(200)DMA: AM = 4:1, mass ratio). Influence of monomer concentration, dispersant concentration and solid loading on the rheological behavior of the slurry and the flexural strength of the green body were investigated using aqueous PEG(200)DMA/AM gel system to gel-cast 3YSZ. The results show that the flexural strength of green bodies notably increases with the increasing of monomer concentration and reaches high up to 14.4 MPa when gel-cast 3YSZ ceramics using PEG(200)DMA/AM gel system with appropriate amount of PAA-NH₄, monomer and solid loading. This gel system obviously reduces the toxicity of the conventional AM system and, at the same time, ensures high flexural strength of green bodies.

Abstract: Direct coagulation casting (DCC) is a relatively new ceramic near-net-shape forming process which can form homogeneous ceramic green body with complex shape and high density. Direct coagulation casting of aqueous alumina slurries by adding Y³⁺ ions, which have been frequently used as additive for the sintering of alumina ceramics, were studied. Two different kinds of Y³⁺-ion releasing substances, Y₂O₃ powder and Y(NO₃)₃, were used as coagulants and were introduced into stabilized alumina suspensions directly after dispersing. Compared with that of Y₂O₃ powder, both the coagulation time and the added amount required for a reasonable shaping were significantly reduced for using Y(NO₃)₃ as coagulant. The effects of the two coagulants on the properties of the consolidated green bodies and sintered ceramics were studied. The mechanisms of coagulation induced by the addition of Y³⁺-ions were discussed on the basis of interactions between Y³⁺ ions and the added polyelectrolyte dispersant.