Advanced Si-Based Ceramics and Composites

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Authors: Sang Jin Lee, Chung Hyo Lee, S.Y. Chun
Abstract: Soft and porous CaO-SiO2 powders (CaSiO3, Ca2SiO4, Ca3SiO5) were fabricated by organic-inorganic solution technique. Calcium nitrate and Ludox SK silica sol were dissolved in D.I. water and then 5 wt% polyvinyl alcohol solution was added as a polymeric carrier. The metal cations were dispersed well in solution and a homogeneous polymeric network was formed. The organic-inorganic precursor gels were turned to porous powder having volume expansion through an explosive oxidation reaction during calcination process. The polyvinyl alcohol content and heating rate were affected on the explosive reaction behavior. It may be speculated that the reaction between oxygen and unstable calcium cations resulted in the vigorous exothermic reaction and simultaneously the reaction made extensive void, which is accompanied with volume expansion in the powder structure, by the thermal decomposition in a moment of organic substance, polyvinyl alcohol. The porous powders were crystallized at relatively lower temperatures, and the crystalline development was dependent on the polymer content.
Authors: Shuhei Tabata, Yoshihiro Hirata, Soichiro Sameshima, Yoshimitsu Uemura
Abstract: The SiC powder of median size 0.8 µm was mixed with polyacrylic acid (PAA, dispersant) in a 0.3 M-R(NO3)3 solution (R=Yb, Y, Gd, Sm, Nd and La) at pH 5 to adsorb uniformly the sintering additive (R3+ ion) on the SiC surface. The addition of PAA to the SiC suspension with R3+ ion increased the amount of R3+ ion fixed to SiC particles. The aqueous 30 vol% SiC suspension with 0.52 mass% PAA and 1.50 mass% R2O3 (as R3+ ion) against the mass of SiC, was consolidated by filtration through a gypsum mold to form green compacts of 50-52 % of theoretical density. The consolidated green compacts were hot–pressed under a pressure of 39 MPa at 1950°C for 2 h in an Ar flow. The green compacts were densified with grain growth to 76 - 99 % relative density. The addition of smaller R3+ ion was effective to enhance the sinterability of SiC and also to achieve smaller grain size of SiC. The sintering mechanisms were discussed based on the analysis of the shrinkage curves of SiC/R2O3 compacts during the hot-pressing.
Authors: J.W. Paek, Beom Seob Kim, Deug Joong Kim
Abstract: Ceramic foams containing MoSi2 were prepared by a self-blowing process of poly-silsesquioxane with MoSi2 as filler. Ceramic foams prepared by polymer pyrolysis were composed of MoSi2 and silicon oxycarbide glass matrix. Densities, pore sizes and mechanical properties of ceramic foams were depended on the filler content and heating rate for curing of polymer. Depending on the foaming condition, ceramic foams with a density of 1.2∼0.4 and a compressive strength of 3∼30 MPa were obtained.
Authors: Beyong Hwan Ryu, Young Min Choi, Jae Do Lee
Abstract: Gelcasting has been expected as a promising approach to prepare near-net-shaped ceramic products. The preparation of highly concentrated gelcasting slip is one of key point in this process. The rheological behavior of concentrated silicon nitride gelcasting slip was investigated with the structure effect of polymeric dispersant and the ratio of dispersant to acrylamide monomer in order to prepare highly concentrated gelcasting slip. The slip was prepared by ball milling of silicon nitride batch composed of acrylamide monomer and polymeric dispersant after premixing them by an attritor. The slip mixed with initiator will be deaired in vacuum and cast into molds, and then polymerized. The consolidated green body will be obtained by drying the gelated slip. The polymeric dispersants with different kinds of polyelectrolyte were poly(methylmetacrylic ammonium salt), poly(acrylic ammonium salt), and poly(acrylic amine salt). Before the polymerization of gelcasting slip, the viscosity vs. shear rate was measured to evaluate the rheological behavior of the slip. In case of using the polyacrylic amine salt (PAAm), the high solid loading of silicon nitride slip was obtained up to 47vol% with a low viscosity.
Authors: Jeong Ho Chang, Kyung Ja Kim, Young Kook Shin
Abstract: Selected MPEG-b-PDLLA block copolymers have been synthesized by ring-opening polymerization with systematic variation of the chain lengths of the resident hydrophilic and hydrophobic blocks. The size and shape of the micelles that spontaneously form in solution are then controlled by the characteristics of the block copolymer template. All the materials prepared in this study showed the tunable pore size of 20-80Å with the increase of hydrophobic chain lengths and up to 660m2/g of specific surface area. The formation mechanism of these nanoporous structures obtained by controlling the micelle size has been confirmed using both liquid and solid state 13C and 29Si NMR techniques. This work verifies the formation mechanism of nanoporous structures in which the pore size and wall thickness are closely dependent on the size of hydrophobic cores and hydrophilic shells of the block copolymer templates.
Authors: Hong Peng, Zhi Jian Shen, Mats Nygren
Abstract: Recently we have introduced a novel processing concept of sialon components implying that an extra liquid phase that is thermodynamically compatible with sialon phases is introduced by increasing the O/N ratio in the general formula (Yb+Y)xSi12-(3x+n)Al3x+nOnN16- n while keeping constant the Yb, Y, Si, and Al proportions. By increasing the oxygen content from its stoichiometric value of 5.16 to 15 eq%, a series of powder mixtures were prepared and their overall compositions are located slightly above the homogeneity region of the a- sialon phase. These compositions were consolidated to full densities by hot pressing (HP) and Spark Plasma Sintering (SPS), respectively. The sintering kinetics in the HP and SPS units is compared. The grain growth kinetics were investigated both by post heat-treatment of SPS pre-consolidated monophasic a-sialon bodies consisting of sub-micron sized equiaxed grains in a conventional graphite furnace using extended holding times (hours) and in the SPS apparatus rapidly heated exceeding the temperature threshold of grain growth and using short holding times (minutes). Post heat treatment in the SPS apparatus yielded in-situ reinforced microstructures no matter if an additional liquid/glass was involved or not while corresponding microstructures could only be obtained for non-stoichiometric compositions by post heat treatment in the graphite furnace. The grain growth kinetics is discussed in terms of static and dynamic ripening mechanisms. We have recently shown that the ductility of covalent bonded silicon nitride based ceramics is dramatically enhanced in presence of a pulsed electric field. Compressive strains rates in the range of 10-2 s-1 can easily be achieved at T ³ 1500oC. The enhanced ductility is explained by that the electric field induces motion of charged species present in the grain boundary glassy/liquid phase that in turn promotes grain sliding along the grain boundaries.
Authors: Toshiyasu Nishimura, Xin Xu, Naoto Hirosaki, K. Kimoto, Yoo Yamamoto, Hiroaki Tanaka
Abstract: A commercial silicon nitride powder with oxide sintering additives was ground with high-energy ball mill to obtain nano-sized powder. Metallic aluminium powder was added as a grinding additive. Effect of high-energy ball milling was evaluated by X-ray diffraction analysis. After milling, height of background increased and peak height of silicon nitride decreased in XRD chart, which suggested that vitrification and/or decrease in grain size of silicon nitride occurred. The milled powders were sintered by spark plasma sintering system. Aluminium nitride was formed during sintering by reaction of aluminium and atmospheric nitrogen. Dense nano-ceramics, which were composed of silicon nitride and sialon, were obtained by sintering at 1550 oC.
Authors: Ji Soon Kim, Young Do Kim, Choong Hyo Lee, Pyuck Pa Choi, Young Soon Kwon
Abstract: The effect of milling on the densification behavior of MoSi2 powder during spark-plasma sintering (SPS) was investigated. MoSi2 starting powder with an average particle size of 10 µm was milled to reduce particle sizes to less than 1 µm. Sintering was performed in a SPS facility, varying the sintering temperature from 1200°C to 1500°C. Changes in relative density and the densification rate were measured as a function of temperature. Additionally, the microstructure of sintered compacts was analyzed by means of SEM and EPMA. The sintered density was lower for ballmilled powder compacts (having 94-95% relative density) than for as-received ones (having 94- 98% relative density) despite a higher densification rate of the former in the early and middle stages of sintering. These apparently contradictory results can be explained by a pick-up of oxygen (from 0.3 to 1.8 wt. % O) during the milling process, leading to the formation of silicon oxide and its decomposition into a gas phase at temperatures above 1200°C.
Authors: Xin Xu, Toshiyuki Nishimura, Naoto Hirosaki, Rong Jun Xie, Yoshinobu Yamamoto
Abstract: The fabrication of dense Si3N4 nano-ceramics via a high-energy mechanical milling process and subsequent spark plasma sintering (SPS) was reported in this paper. A thermodynamically stable β-Si3N4 powder mixed with 5-mol% Y2O3 and 2-mol% Al2O3 was used as starting powders for high-energy mechanical milling. Milling resulted in the significant decrease of crystalline size and the amorphization of the starting powders. Based on XRD measurements, the amount of the amorphous phase and the grain sizes of the remaining crystalline silicon nitride depended on the charge ratio. Milled powders could be SPSed at relatively low temperature. A homogenous dense silicon nitride ceramics with grain diameter of about 70 nm could be fabricated through SPS at a low temperature of 1600oC for 5 min. The significant decrease of grain size greatly increased the hardness of obtained ceramics.
Authors: Monika Willert-Porada, Jens Grosse-Berg, Ilker Sen, Ho Seon Park
Abstract: A new processing route for porous b-Si3N4 –ceramics suitable as substrates for Si3N4 functionally graded materials, FGM`s with an environmental barrier coating, EBC, is described. Such materials could be useful for operation at temperatures up to 1650°C in combustion engines, where oxidation and corrosion under hydrothermal conditions cause severe damage of known dense Si3N4-ceramics. The new process is based on microwave sintering of a-Si3N4–Yb-silicate green parts to obtain up to 60% open porous b-Si3N4 pre-forms equilibrated at high temperature with respect to the RE-silicate content, followed by Si-melt infiltration. A sufficient wetting by molten silicon is achieved by coating the inner surface of the porous pre-form with amorphous carbon prior to infiltration. The influence of different green processing parameters and different heating methods - microwave heating and conventional heating – on the porosity of the pre-form is reported. Infiltration is performed by microwave heating of the carbon coated pre-forms, whereas for nitridation both heating processes are combined.

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