Pressureless Sintering (pressureless + sintering)

Distribution by Scientific Domains
Distribution within Polymers and Materials Science


Selected Abstracts


Tailoring Texture of ,-Y2Si2O7 by Strong Magnetic Field Alignment and Two-Step Sintering

JOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 8 2008
Z. Q. Sun
In this study, a well-dispersed ,-Y2Si2O7 ethanol-based suspension with 30 vol% solid loading was prepared by adding 1 dwb% polyethylene imine dispersant, which allows feeble magnetic ,-Y2Si2O7 particles with anisotropic magnetic susceptibility to rotate in a 12 T strong magnetic field during slip casting, resulting in the development of a strongtexture in green bodies. Pressureless sintering gives rise to more pronounced grain growth in the textured sample than in the untextured sample prepared without the magnetic field due to the rapid migration of the grain boundaries of the well-oriented grains, which was revealed by constant-heating-rate sintering kinetics. It was found that the use of two-step sintering is very efficient not only for inhibiting the grain growth but also for enhancing thetexture. This implies that controlled grain growth is crucial for enhancing texture development in ,-Y2Si2O7. [source]


Processing of Carbon Nanofiber Reinforced ZrB2 Matrix Composites for Aerospace Applications,

ADVANCED ENGINEERING MATERIALS, Issue 7 2010
Jorge Barcena
Ceramic matrix composites (CMCs) based on zirconium diboride (ZrB2) reinforced by vapor grown carbon nanofibers are a potential constituent of reusable thermal protection systems. A manufacturing procedure was devised that involved the fabrication of thin films by tape casting to obtain a layer that could be integrated into a more complex system. Higher thermal conductivities and improved toughness can be expected for nanofiber additions, as compared to the matrix alone. Consolidation by hot-pressing was more effective than pressureless sintering, in terms of the final relative density and flatness of specimens. Examination of microstructures showed that few carbon nanofibers were present in the matrix after consolidation by sintering, which was attributed to a reaction between the nanofibers and zirconium oxide present on the surface of the ZrB2 powder. As a solution, oxygen impurities from the boride powders were removed by reduction with carbon coatings derived from phenolic resin. The deleterious reaction was avoided, but residual carbon remained at the grain boundaries, likely from decomposition of the binder. The use of an alternative binder (PMMA vs. PVB) will be used in future studies to reduce the residual carbon content. Further, consolidation by Spark Plasma Sintering (SPS) will be explored to further reduce the reaction of surface oxides with the nanofibers. Finally, characterization of the microstructure at the nanometric level and further determination of the mechanical and thermal properties will be conducted as part of future studies. [source]


Microstructure and Properties of Pressureless Sintered HfB2 -Based Composites with Additions of ZrB2 or HfC,

ADVANCED ENGINEERING MATERIALS, Issue 10 2007
L. Silvestroni
Ternary HfB2 -ZrB2 and HfB2 -HfC composites (see figure) with 20,vol.-% MoSi2 were produced from commercial powder and densified through pressureless sintering. Dense materials with fine microstructures were obtained at 1900,1950,C. Flexural strength at 1500,C confirmed the excellent stability of the composites at high temperature. [source]


Nanoparticle Coating for Advanced Optical, Mechanical and Rheological Properties,

ADVANCED FUNCTIONAL MATERIALS, Issue 16 2007
F. Hakim
Abstract Primary titania nanoparticles were coated with ultrathin alumina films using Atomic Layer Deposition (ALD). The deposited films were highly uniform and conformal with an average growth rate of 0.2,nm per coating cycle. The alumina films eliminated the surface photocatalytic activity of titania nanoparticles, while maintained their original extinction efficiency of ultraviolet light. Deposited films provided a physical barrier that effectively prevented the titania surface from oxidizing organic material whereas conserving its bulk optical properties. Parts fabricated from coated powders by pressureless sintering had a 13,% increase in surface hardness over parts similarly fabricated from uncoated particles. Owing to its homogeneous distribution, the secondary alumina phase suppressed excessive grain growth. Alumina films completely reacted during sintering to form aluminum titanate composites, as verified by XRD. Coated particles showed a pseudoplastic behavior at low shear rates due to modified colloidal forces. This behavior became similar to the Newtonian flow of uncoated nanoparticle slurries as the shear rate increased. Suspensions of coated particles also showed a decreased viscosity relative to the viscosity of uncoated particle suspensions. [source]


Microstructure and Mechanical Properties of Lu2O3 -Doped Porous Silicon Nitride Ceramics Using Phenolic Resin as Pore-Forming Agent

INTERNATIONAL JOURNAL OF APPLIED CERAMIC TECHNOLOGY, Issue 3 2010
Xiaowei Yin
The joint process consisting of pressureless sintering and chemical vapor infiltration (CVI) was developed to prepare porous Si3N4 ceramics with controlled microstructure. Lu2O3 and phenolic resin acted as sintering aid and pore-forming agent, respectively. The 5 wt% Lu2O3 -doped ceramics using 12,57 vol% phenolic resin attained a porosity ranging from 46% to 53%. With increasing the resin content, the average pore size increased from 1 to 2 ,m. The porous ceramic infiltrated with CVI Si3N4 had an improved microstructure. The decreased pore size and porosity led to an increase in flexural strength, and the densified surface led to an improved surface hardness. [source]


Fabrication and Luminescent Properties of Nd3+ -Doped Lu2O3 Transparent Ceramics by Pressureless Sintering

JOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 10 2009
Ding Zhou
The fabrication of transparent Nd3+ ion-doped Lu2O3 ceramics is investigated by pressureless sintering under a flowing H2 atmosphere. The starting Nd-doped Lu2O3 nanocrystalline powder is synthesized by a modified coprecipitant processing using a NH4OH+NH4HCO3 mixed solution as the precipitant. The thermal decomposition behavior of the precipitate precursor is studied by thermogravimetric analysis and differential thermal analysis. After calcination at 1000C for 2 h, monodispersed Nd3+:Lu2O3 powder is obtained with a primary particle size of about 40 nm and a specific surface area of 13.7 m2/g. Green compacts, free of additives, are formed from the as-synthesized powder by dry pressing followed by cold isostatic pressing. Highly transparent Nd3+:Lu2O3 ceramics are obtained after being sintered under a dry H2 atmosphere at 1880C for 8 h. The linear optical transmittance of the polished transparent samples with a 1.4 mm thickness reaches 75.5% at the wavelength of 1080 nm. High-resolution transmission electron microscopy observations demonstrate a "clear" grain boundary between adjacent grains. The luminescent spectra showed that the absorption coefficient of the 3 at.% Nd-doped Lu2O3 ceramic at 807 nm reached 14 cm,1, while the emission cross section at 1079 nm was 6.5 10,20 cm2. [source]


Pressureless Sintering of Zirconium Diboride: Particle Size and Additive Effects

JOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 5 2008
William 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 1900C 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 1850C 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]


Pressureless Densification of Zirconium Diboride with Boron Carbide Additions

JOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 5 2006
S. C. Zhang
Zirconium diboride (ZrB2) was densified (>98% relative density) at temperatures as low as 1850C by pressureless sintering. Sintering was activated by removing oxide impurities (B2O3 and ZrO2) from particle surfaces. Boron oxide had a high vapor pressure and was removed during heating under a mild vacuum (,150 mTorr). Zirconia was more persistent and had to be removed by chemical reaction. Both WC and B4C were evaluated as additives to facilitate the removal of ZrO2. Reactions were proposed based on thermodynamic analysis and then confirmed by X-ray diffraction analysis of reacted powder mixtures. After the preliminary powder studies, densification was studied using either as-received ZrB2 (surface area ,1 m2/g) or attrition-milled ZrB2 (surface area ,7.5 m2/g) with WC and/or B4C as a sintering aid. ZrB2 containing only WC could be sintered to ,95% relative density in 4 h at 2050C under vacuum. In contrast, the addition of B4C allowed for sintering to >98% relative density in 1 h at 1850C under vacuum. [source]


Densification of Si3N4 with LiYO2 Additive

JOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 4 2004
Branko Matovic
This paper deals with the densification and phase transformation during pressureless sintering of Si3N4 with LiYO2 as the sintering additive. The dilatometric shrinkage data show that the first Li2O- rich liquid forms as low as 1250C, resulting in a significant reduction of sintering temperature. On sintering at 1500C the bulk density increases to more than 90% of the theoretical density with only minor phase transformation from ,-Si3N4 to ,-Si3N4 taking place. At 1600C the secondary phase has been completely converted into a glassy phase and total conversion of ,-Si3N4 to ,-Si3N4 takes place. The grain growth is anisotropic, leading to a microstructure which has potential for enhanced fracture toughness. Li2O evaporates during sintering. Thus, the liquid phase is transient and the final material might have promising mechanical properties as well as promising high-temperature properties despite the low sintering temperature. The results show that the Li2O,Y2O3 system can provide very effective low-temperature sintering additives for silicon nitride. [source]


Synthesis and Properties of Porous Single-Phase ,,-SiAlON Ceramics

JOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 7 2002
Jian-Feng Yang
Single-phase ,,-SiAlON (Si6,zAlzOzN8,z, z= 0,4.2) ceramics with porous structure have been prepared by pressureless sintering of powder mixtures of -Si3N4, AlN, and Al2O3 of the SiAlON compositions. A solution of AlN and Al2O3 into Si3N4 resulted in the ,,-SiAlON, and full densification was prohibited because no other sintering additives were used. Relative densities ranging from 50%,90% were adjusted with the z -value and sintering temperature. The results of X-ray diffraction, scanning electron microscopy, and transmission electron microscopy analyses indicated that single-phase ,,-SiAlON free from a grain boundary glassy phase could be obtained. Both grain and pore sizes increased with increasing z -value. Low z -value resulted in a relatively high flexural strength. [source]


Microstructural Control of a 70% Silicon Nitride, 30% Barium Aluminum Silicate Self-Reinforced Composite

JOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 1 2001
Feng Yu
The processing response of a 70% silicon nitride,30% barium aluminum silicate (70%-Si3N4,30%-BAS) ceramic-matrix composite was studied using pressureless sintering, at temperatures ranging from 1740C, which is below the melting point of BAS, to 1950C. The relationship between the processing parameters and the microstructural constituents, such as morphology of the ,-Si3N4 whisker and crystallization of the BAS matrix, was evaluated. The mechanical response of this array of microstructures was characterized for flexural strength, as well as fracture behavior, at test temperatures up to 1300C. The indentation method was used to estimate the fracture resistance, and R -curves were obtained from modified compact-tension samples of selected microstructures at room temperature. [source]


Ion-Exchange Loading of Yttrium Acetate as a Sintering Aid on Aluminum Nitride Powder via Aqueous Processing

JOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 11 2000
Yasuhiro Shimizu
A novel fabrication process of AlN ceramics via aqueous colloidal processing and pressureless sintering has been presented. The chemical stability of AlN powder in water was improved by the surface chemical modification with sebacic acid, while maintaining a hydrophilic surface. The treatment of the sebacic acid-modified powder with yttrium acetate tetrahydrate resulted in strong immobilization of Y3+ ions, as a sintering aid, at a highly dispersive level on the AlN powder surface through ion exchange with the free carboxyl groups of the sebacic acid molecules attached to the AlN surface. By selecting slip compositions for a well-deflocculated condition and firing conditions to burn out organic components in the slip cast compacts, a thermal conductivity of about 250 W/(mK) could be attained by the pressureless sintering at 1900C for 5 h. [source]


Microstructure and electrical properties of (1,x)(K0.5Na0.5)NbO3,x BiFeO3 piezoelectric ceramics

PHYSICA STATUS SOLIDI (A) APPLICATIONS AND MATERIALS SCIENCE, Issue 5 2008
Xiang Li
Abstract Lead,free ceramics (1,x)(K0.5Na0.5)NbO3,x BiFeO3 doped with 1 mol% Fe2O3 (KNNBF/x) have been synthesized by pressureless sintering. With the Fe2O3 doping, the KNNBF/x ceramics can be well sintered at 1085,1100 C and exhibit a pure perovskite structure with x < 0.013. It was found that the crystal structure of the KNNBF/x ceramics changed from orthorhombic to tetragonal and then to pseudocubic phase with the increase of BiFeO3 content. The composition KNNBF/0.013 near the tetragonal symmetry that separates the orthorhombic and pseudocubic phases exhibits improved electrical properties: d33 = 173 pC/N, kp = 0.40, ,r = 905, tan , = 4%, Pr = 26 ,C/cm2, and Ec = 11.2 kV/cm, with a high Curie temperature (TC) of about 388 C. Our results suggest that KNNBF/x are promising lead-free high temperature piezoelectric ceramics. ( 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]