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Full Density (full + density)
Selected AbstractsNet Shape Reaction Bonded Ceramic Micro Parts by Mechanical MicrostructuringADVANCED ENGINEERING MATERIALS, Issue 10 2006H.-J. Ritzhaupt-Kleissl Net shape ceramic micro components can be realized by mechanical microstructuring green blanks consisting of precursor materials followed by reaction bonding. The precursor material is composed of mainly zirconium silicide as reactive precursor material, organosilicon polymer (PMSS) as low loss binder and zirconium oxide as an inert phase. Shaping in the green state can easily be performed by micro milling, even if different cutting strategies are applied. Subsequently the components are thermally processed for ceramization and for sintering them to full density. Dependent on the material composition and on the green density the dimensions of the sintered components are retained unchanged compared to the dimensions of the green parts. [source] Microstructural Bases for the Superior Densification of Gels doped with Alumina NanoseedsADVANCED ENGINEERING MATERIALS, Issue 1-2 2006J. Tartaj The densification behavior of alumina gels has been studied. We have found that in addition to the effect of volume transformed, the thermal history of gels plays an important role in their densification process. Particularly, prior to the ,- to , transformation, the nanoseeded gels preserve a close packed and evenly distributed porous structure that helps to achieve full density at lower temperatures. [source] Pressureless Sintering t -zirconia@,-Al2O3 (54 mol%) Core,Shell Nanopowders at 1120°C Provides Dense t -Zirconia-Toughened ,-Al2O3 NanocompositesJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 3 2010Min Kim Zirconia-toughened alumina (ZTA) is of growing importance in a wide variety of fields exemplified by ZTA prosthetic implants. Unfortunately, ZTA composites are generally difficult to process because of the need to preserve the tetragonal zirconia phase in the final dense ceramic, coincident with the need to fully densify the ,-Al2O3 component. We report here that liquid-feed flame spray pyrolysis of mixtures of metalloorganic precursors of alumina and zirconia at varying compositional ratios provide access in one step to core,shell nanoparticles, wherein the shell is ,-Al2O3 and the core is a perfect single crystal of tetragonal (t -) zirconia. Pressureless sintering studies provided parameters whereby these nanopowder compacts could be sintered to full density (>99%) at temperatures just above 1100°C converting the shell component to ,-Al2O3 but preserving the t -ZrO2 without the need for any dopants. The final average grain sizes of these sintered compacts are ,200 nm. The resulting materials exhibit the expected response to mechanical deformation with the subsequent production of monoclinic ZrO2. These materials appear to offer a low-temperature, low-cost route to fine-grained ZTA with varied Al2O3:t -ZrO2 compositions. [source] Spark Plasma Sintered Silicon Nitride Ceramics with High Thermal Conductivity Using MgSiN2 as AdditivesJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 9 2009Gui-hua Peng Silicon nitride ceramics were prepared by spark plasma sintering (SPS) at temperatures of 1450°,1600°C for 3,12 min, using ,-Si3N4 powders as raw materials and MgSiN2 as sintering additives. Almost full density of the sample was achieved after sintering at 1450°C for 6 min, while there was about 80 wt%,-Si3N4 phase left in the sintered material. ,-Si3N4 was completely transformed to ,-Si3N4 after sintering at 1500°C for 12 min. The thermal conductivity of sintered materials increased with increasing sintering temperature or holding time. Thermal conductivity of 100 W·(m·K),1 was achieved after sintering at 1600°C for 12 min. The results imply that SPS is an effective and fast method to fabricate ,-Si3N4 ceramics with high thermal conductivity when appropriate additives are used. [source] Cross-linked Polyvinyl Alcohol as a Binder for Gelcasting and Green MachiningJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 10 2008France Chabert Submicrometer alumina suspensions, dispersed in aqueous acidic solutions of polyvinyl alcohol (PVA) and 2,5-dimethoxy-2,5-dihydrofuran, have been evaluated for suitability as a cross-linkable binder system for casting complex-shaped ceramic components. Suspensions of up to 50 vol% solids have rheological behavior, which is suitable for pouring and filling molds. Complex-shaped green bodies are then formed by heating the suspension in the mold for a period of time (typically 15,60 min) at moderate temperature (60°,80°C) to gel the suspension. High green densities (58%,62% of full density) can be obtained. The dried green bodies have strength in excess of 1 MPa and may be readily machined. No more than 1,3 wt% PVA per weight of alumina is necessary, ensuring burnout that minimizes generation of flaws. The ceramic components can be fired to >96% of full density when fired for 2 h at 1400°,1450°C. Cross-linkable PVA may receive more widespread acceptance in ceramic processing than previous gelcasting formulations because PVA is already a common processing additive. [source] Pressureless Sintering of Zirconium Diboride: Particle Size and Additive EffectsJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 5 2008William G. Fahrenholtz Zirconium diboride (ZrB2) was densified by pressureless sintering using <4-wt% boron carbide and/or carbon as sintering aids. As-received ZrB2 with an average particle size of ,2 ,m could be sintered to ,100% density at 1900°C using a combination of boron carbide and carbon to react with and remove the surface oxide impurities. Even though particle size reduction increased the oxygen content of the powders from ,0.9 wt% for the as-received powder to ,2.0 wt%, the reduction in particle size enhanced the sinterability of the powder. Attrition-milled ZrB2 with an average particle size of <0.5 ,m was sintered to nearly full density at 1850°C using either boron carbide or a combination of boride carbide and carbon. Regardless of the starting particle size, densification of ZrB2 was not possible without the removal of oxygen-based impurities on the particle surfaces by a chemical reaction. [source] Effects of Oxidation Curing and Sintering Additives on the Formation of Polymer-Derived Near-Stoichiometric Silicon Carbide FibersJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 2 2008Lifu Chen The effects of oxygen pick-up and sintering additives on the formation of silicon carbide (SiC) fibers from polyaluminocarbosilane are studied. It has been found that the strict control of oxygen pick up during the oxidation curing is essential to produce near-stoichiometric SiC fibers. When the molar ratio of oxygen to excess carbon in the pyrolyzed fibers (SiCxOy) is slightly over 1 (O/CExcess=y/(x,1)>1), the excess carbon is eliminated during the subsequent sintering as CO and CO2 as a result of the decomposition of SiCxOy; the remaining oxygen is removed as SiO and CO vapor, leaving near-stoichiometric SiC as the residue. However, with still increasing oxygen pick up, the final ceramic fibers become more porous and rich in silicon. The evolution of CO, CO2, and SiO generates high porosity in the absence of a sintering additive, leading to low fiber density. The inter-connected and open porosity favors the formation of CO. In contrast, for the fibers containing aluminum (Al) or Al/B sintering additives, the pores are much smaller and essentially closed, favoring the formation of CO2. Therefore, after sintering at 1800°C, the fibers without sintering additives contain excess silicon, while those with sintering additives are near stoichiometric. Al is beneficial to the densification but it alone cannot produce fibers of high density. When B is added in addition to Al, the fibers can be sintered to nearly full density. [source] Spark-Plasma Sintering of Silicon Carbide Whiskers (SiCw) Reinforced Nanocrystalline AluminaJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 12 2004Guo-Dong Zhan The combined effect of rapid sintering by spark-plasma-sintering (SPS) technique and mechanical milling of ,-Al2O3 nanopowder via high-energy ball milling (HEBM) on the microstructural development and mechanical properties of nanocrystalline alumina matrix composites toughened by 20 vol% silicon carbide whiskers was investigated. SiCw/,-Al2O3 nanopowders processed by HEBM can be successfully consolidated to full density by SPS at a temperature as low as 1125°C and still retain a near-nanocrystalline matrix grain size (,118 nm). However, to densify the same nanopowder mixture to full density without the benefit of HEBM procedure, the required temperature for sintering was higher than 1200°C, where one encountered excessive grain growth. X-ray diffraction (XRD) and scanning electron microscopy (SEM) results indicated that HEBM did not lead to the transformation of ,-Al2O3 to ,-Al2O3 of the starting powder but rather induced possible residual stress that enhances the densification at lower temperatures. The SiCw/HEBM,-Al2O3 nanocomposite with grain size of 118 nm has attractive mechanical properties, i.e., Vickers hardness of 26.1 GPa and fracture toughness of 6.2 MPa·m1/2. [source] In Situ Synthesis and Microstructures of Tungsten Carbide-Nanoparticle-Reinforced Silicon Nitride-Matrix CompositesJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 3 2004Tateoki Iizuka A W2C-nanoparticle-reinforced Si3N4 -matrix composite was fabricated by sintering porous Si3N4 that had been infiltrated with a tungsten solution. During the sintering procedure, nanometer-sized W2C particles grew in situ from the reaction between the tungsten and carbon sources considered to originate mainly from residual binder. The W2C particles resided in the grain-boundary junctions of the Si3N4, had an average diameter of ,60 nm, and were polyhedral in shape. Because the residual carbon, which normally would obstruct sintering, reacted with the tungsten to form W2C particles in the composite, the sinterability of the Si3N4 was improved, and a W2C,Si3N4 composite with almost full density was obtained. The flexural strength of the W2C,Si3N4 composite was 1212 MPa, ,34% higher than that of standard sintered Si3N4. [source] Pulsed Electric Current Sintering of Silicon NitrideJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 3 2003Motohiro Suganuma Pulsed electric current sintering (PECS) has been used to densify ,-Si3N4 powder doped with oxide additives of Y2O3 and Al2O3. A full density (>99%) was achieved with virtually no transformation to ,-phase, resulting in a microstructure with fine equiaxed grains. With further holding at the sintering temperature, the ,-to-, phase transformation took place, concurrent with an exaggerated grain growth of a limited number of elongated ,-grains in a fine-grained matrix, leading to a distinct bimodal grain size distribution. The average grain size was found to obey a cubic growth law, indicating that the growth is diffusion-controlled. In contrast, the densification by hot pressing was accompanied by a significant degree of the phase transformation, and the subsequent grain growth gave a broad normal size distribution. The apparent activation energy for the phase transformation was as high as 1000 kJ/mol for PECS, almost twice the value for hot pressing (,500 kJ/mol), thereby causing the retention of ,-phase during the densification by PECS. [source] | ||||||||||