Key Engineering Materials Vols. 602-603

Abstract: Currently, one of the most popular methods for the production of reticulated porous ceramics (RPCs) is the polymer sponge method, which was invented by Schwartzwalder and Somers. SiC RPCs can be fabricated by using the A1₂O₃-SiO₂ as sintering aids at low temperature as 1200-1300 °C in cases with SiC (20-50 % (mass percent)). However, when the content of SiC powders (relative to total weight of solids) was higher than 50 % (mass percent), the most suitable sintering temperature was higher than 1400 °C. In this work, SiC reticulated porous ceramics (SiC RPCs) was fabricated via the polymer sponge method using Al₂O₃-SiO₂ as sintering aids at 1100-1500 °C. In order to obtain SiC RPCs with optimized mechanical performance, the properties of the slurry and influence of sintering temperature have been investigated. It was found that, rheological properties, solid content of the slurry and sintering temperature all have influence on the physical and mechanical properties of SiC RPCs. In order to achieve optimal properties of SiC RPCs, the optimal pH value ,solid content, sintering temperature and content of silica fume were 11, 73.4 % (mass percent), 1300 °C and 17 % (mass percent), respectively.

Abstract: Carbon nanofiber (CNF) and silicon carbonitride (SiCN) ceramic nanocomposites (SiCN/CNF) are fabricated by in-situ growth of CNFs in SiCN ceramics during ceramic transformation of polymeric precursors of polysilazanes (PSZ). Metal catalyst precursors are mixed into the polysilazane liquid forming metal particles from decomposition under heating during the pyrolysis. At certain temperatures, ethylene was introduced as a carbon source to induce the growth of CNFs over the metal particles in the ceramic body followed by heating to higher temperatures to complete the pyrolysis. In this way, bulk nanocomposites of SiCN/CNF are obtained as crack-free bodies although some pores are left in the sample. Scanning electron microscopy (SEM) analysis performed on the cross-section of nanocomposites revealed the distribution of needle-like nanofibers of diameter ~ 200 nm and exposed length of ~ 2 μm. The CNFs exhibited the unique multiscale nanostructure in micron hollow tubes with branched nanofiber walls. Energy dispersive X-ray spectrometer (EDX) detected carbon as the major element from the nanofibers confirming the formation of carbon nanofibers. Moreover, clusters of nanoparticles are formed on the ceramic surface from carbon depositions. The in-situ growth of CNFs in SiCN ceramics provides a one-step process potentially to be developed for fabrication of structural and functional SiCN/CNF nanocomposites.

Abstract: In this paper, AlON powders were prepared by carbothermal reduction and nitridation method. By adding a mixture of Y2O3, La2O3 and MgO as sintering agent, transparent AlON ceramics with high strength were fabricated by pressureless sintering at 1950°C for 8h under nitrogen atmosphere. The obtained ceramic had a relative density of 99.9% and average grain size of 150μm. The transmittance of the 2mm thick ceramics was 82.4% at 1100nm and flexural strength was 300 MPa.

Abstract: In this project, coal fly ash was transformed into ceramic materials by adding a certain amount of fusing agent. Ash samples were compacted and sintered with the addition of potassium carbonate (K₂CO₃·1/2H₂O) under a suitable sintering temperature range. Mineralogy and microstructure of the obtained products were characterized by means of X-ray diffraction and field emission scanning electronic microscope techniques respectively. The results indicate that K₂CO₃·1/2H₂O facilitates the transformation of mullite and quartz phases above 800°C, and the mineralogy phases of the product is leucite (KAlSi₂O₆) and potassium aluminum silicate (KAlSiO₄). In the process, K⁺ interacts with oxygen atom and destroys the original lattice. The regular morphology of the sintered samples was confirmed by the observation under SEM, which reveals a uniform dense ceramic is formed at 900°C with the 40wt% addition of K₂CO₃·1/2H₂O.

Abstract: The effects of sintering temperatures on phase behavior of products from rutile and quartz by aluminothermic reduction nitridation (ARN) were investigated in this study. The crystalline phase and morphology of the samples were determined by X-ray diffraction (XRD), scanning electron microscopy (SEM), respectively. As the temperature was 1200 oC, the phases were corundum, TiN, Si and a small amount of cristobalite. Mullite were initially observed when it reached 1300 oC. A small amount of Si₃Al₃O₃N₅ was detected when the temperature rose to 1400 oC. More corundum, TiN and Si₃Al₃O₃N₅ were obtained at 1500 oC. The quantities of TiN and corundum increased slightly at 1550 oC while Si₃Al₃O₃N₅ reduced. In addition, the microstructure of well-developed β-Sialon crystals was long columnar and the TiN particles distributed in the composites.

Abstract: This paper mainly discusses the influences of heating temperatures and CeO₂ additive contents on the phase transformations of zirconia from zircon ore by carbothermal reduction. The phase transformations of zirconia from zircon ore by carbothermal reduction were monitored by X-ray diffraction. The microstructure of the product was characterized by scanning electron microscopy. The results show that without adding CeO₂, the optimized heating temperature of zircon carbothermal reduction was 1600 °C and the main phases of the product were m-ZrO₂, ZrC and β-SiC, t-ZrO₂; After adding CeO₂, the main phase of the products consists of t-ZrO₂, m-ZrO₂, ZrC and β-SiC when the heating temperature is 1600 °C. CeO₂ additive can be introduced into zirconia lattice and can cause it to form cerium stabilized zirconia. Zirconia in the product would be turned into partially stabilized zirconia with cerium addition from 5 wt% to 20 wt%. However, the form of zirconia in the product is not changed greatly with the amount of CeO₂ additive increase.

Abstract: Yb rare earth doped YAG ultrafine particles were synthesized by the stearate melting method using yttrium stearate, ytterbium stearate and aluminum tristearate as starting materials. The phase formation of Yb:YAG, the properties and the sintering activity of the powders were investigated by means of XRD, SEM, dilatometry and vacuum sintering. The results show that pure Yb:YAG nanopowders can be obtained by calcining the co-melted precursor at a relatively low temperature of 800 °C for 4 h. The powders calcined at 1000°C have better sintering activity than the powders calcined at other temperatures. For the Yb:YAG powders doping with 0.5% TEOS, the compact can be sintered to 99.2% of the theoretical density at 1600 °C and 99.7% at 1700 °C. The transparent Yb:YAG ceramics obtained by vacuum sintering at 1700 °C for 5 h exhibit a pore-free and uniform microstructure.

Abstract: The neutron absorption ball for High Temperature Gascooled Reactor contains 25wt% B4C, dispersed in graphite matrix. Good compressive strength, wear resistance and thermal shock resistance are essential to absorption ball. B4C and graphite were selected as the filler, and phenolic resin was chosen as the binder. A certain amount of phenolic resin and silane coupling agent were used for the surface preparation of inorganic aggregate. Results showed that the residual weights of pretreating powders after carbonizing were higher than that of powders without pretreatment. The compressive strength of the neutron absorption ball was studied. Dipping can effectively improve the strength. Within a certain range, the compressive strength increased with the increase of dipping times. Compared with the thermoplastic resin, higher compressive strength sample was obtained when the thermosetting resin was used as impregnant. The average crushing force of the absorption ball prepared by the optimal process was up to 612 N.

Abstract: By introducing ZrO₂ (4Y) powder into the thermit, the solidified Al₂O₃-ZrO₂ (4Y) ceramic composites with eutectic and hypereutectic microstructures were prepared via combustion synthesis in high gravity field, and the microstructures and mechanical properties of the solidified ceramic composites were discussed. XRD, SEM and EDS showed that the Al₂O₃-33%ZrO₂ (4Y) as the eutectic were composed of random-orientated rod-shaped colonies consisting of a triangular dispersion of orderly submicron-nanometer t-ZrO₂ fibers, surrounded by inter-colony regions consisting of spherically-shaped micrometer t-ZrO₂ grains, whereas Al₂O₃-45%ZrO₂ (4Y) as the hypereutectic were comprised of spherically-shaped micron-meter t-ZrO₂ grains, surround by irregularly-shaped α-Al₂O₃ grains and a few colonies. Compared to the directionally solidified Al₂O₃-ZrO₂ (Y₂O₃), the increase in hardness and flexural strength of the eutectic obtained in current experiment was due to high densification, small-size defect and high fracture toughness induced by residual stress toughening and transformation toughening mechanisms; meanwhile, in despite of the moderate decrease in hardness, high flexural strength of the hypereutectic was considered to be a result of small-size defect and high fracture toughness induced by transformation toughening and microcrack toughening mechanisms.

Abstract: Anti-static materials are widely used in the fields of textile, petrochemical processing, aerospace and electronics. An innovative processing method for fabricating anti-static ceramics has been developed based on Fe-infiltration on the sintered zirconia body. The sintered zirconia ceramics infiltrated by metal Fe in the inner atmosphere showed both perfect anti-resistive and mechanical properties. In this study, infiltration temperature and time was optimized to control the surface resistivity. It was shown that the lower surface resistivity could be obtained at higher infiltration temperature and longer infiltration time. However, the surface hardness was decreased. The surface resistivity reached 5.6×10⁷Ω/ at 1300°C for 12 hours. The phase transformation and microstructure changes were characterized by XRD and SEM. The existence state of Fe was analyzed by EDS.