Sintering Additives (sintering + additive)

Distribution by Scientific Domains


Selected Abstracts


Effects of Oxidation Curing and Sintering Additives on the Formation of Polymer-Derived Near-Stoichiometric Silicon Carbide Fibers

JOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 2 2008
Lifu 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 1800C, 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]


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]


Development of a Self-Forming Ytterbium Silicate Skin on Silicon Nitride by Controlled Oxidation

JOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 6 2002
Seung Kun Lee
A dense and uniform polycrystalline ytterbium silicate skin on silicon nitride ceramics was developed by a controlled oxidation process to improve the hot corrosion resistance of silicon nitride. The process consists of purposely oxidizing the silicon nitride by heating it at high temperatures. It was found that the ytterbium silicate phase was formed as an oxidation product on the surface of the silicon nitride when it was exposed to air at temperatures above 1250C. The volume fraction of ytterbium silicate compared with that of SiO2 on the silicon nitride surface increased with increasing oxidation time and temperature. The formation and growth of ytterbium silicate on the surface of silicon nitride is attributed to a nucleation and growth mechanism. Ultimately, a dense and uniform ytterbium silicate skin with 3,4 ,m of skin thickness was obtained by oxidation at 1450C for 24 h. The ytterbium silicate layer, formed by oxidation of the silicon nitride, is associated with the reaction of SiO2 on the surface of silicon nitride with Yb2O3 introduced in the silicon nitride as a sintering additive. Preliminary tests showed that the ytterbium silicate skin appears to protect silicon nitride from hot corrosion. No observable evidence of a reaction between the skin and molten Na2SO4 was found when it was exposed to molten Na2SO4 at 1000C for 30 min. [source]


Microstructure and Mechanical Properties of Silicon Nitride Ceramics with Controlled Porosity

JOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 6 2002
Jian-Feng Yang
Porous silicon nitride ceramic with a porosity from 0,0.3 was fabricated by partial hot-pressing of a powder mixture of ,-Si3N4 and 5 wt% Yb2O3 as sintering additive. Irrespective of the porosity, the samples exhibited almost the same microstructural features including grain size, grain aspect ratio, and pore size. Porosity dependences of Young's modulus, flexural strength, and fracture toughness (KIC) were investigated. All these properties decreased with increasing porosity. However, because of the fibrous microstructure, the decreases of flexural strength and fracture toughness were moderate compared with the much greater decrease of Young's modulus. Thus, the strain tolerance (fracture strength/Young's modulus) increased with increasing porosity. The critical energy release rate also increased slightly with an increasing volume fraction of porosity to 0.166 and remained at the same level with that of the dense sample when the porosity was 0.233. They decreased as porosity increased further. [source]


Sintered Reaction-Bonded Silicon Nitride with High Thermal Conductivity and High Strength

INTERNATIONAL JOURNAL OF APPLIED CERAMIC TECHNOLOGY, Issue 2 2008
You Zhou
Sintered reaction-bonded silicon nitride (SRBSN) materials were prepared from a high-purity Si powder doped with Y2O3 and MgO as sintering additives by nitriding at 1400C for 8 h and subsequently postsintering at 1900C for various times ranging from 3 to 24 h. Microstructures and phase compositions of the nitrided and the sintered compacts were characterized. The SRBSN materials sintered for 3, 6, 12, and 24 h had thermal conductivities of 100, 105, 117, and 133 W/m/K, and four-point bending strengths of 843, 736, 612, and 516 MPa, respectively. Simultaneously attaining thermal conductivity and bending strength at such a high level made the SRBSN materials superior over the high-thermal conductivity silicon nitride ceramics that were prepared by sintering of Si3N4 powder in our previous works. This study indicates that the SRBSN route is a promising way of fabricating silicon nitride materials with both high thermal conductivity and high strength. [source]


Spark Plasma Sintered Silicon Nitride Ceramics with High Thermal Conductivity Using MgSiN2 as Additives

JOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 9 2009
Gui-hua Peng
Silicon nitride ceramics were prepared by spark plasma sintering (SPS) at temperatures of 1450,1600C 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 1450C 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 1500C for 12 min. The thermal conductivity of sintered materials increased with increasing sintering temperature or holding time. Thermal conductivity of 100 W(mK),1 was achieved after sintering at 1600C 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]


Effects of Oxidation Curing and Sintering Additives on the Formation of Polymer-Derived Near-Stoichiometric Silicon Carbide Fibers

JOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 2 2008
Lifu 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 1800C, 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]


Thermodynamic Studies on the AlN Sintering Powders Treated With Phosphate Species

JOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 11 2007
Susana Maria Olhero
The processing of aluminum nitride (AlN) ceramics in aqueous media requires the use of a surface layer to protect the surface of the particles against hydrolysis. This surface layer might influence the densification, affecting the reactions between AlN and sintering additives. The present paper describes a thermodynamic and experimental approach to evaluate the effects of a phosphate-based protecting surface layer on the densification of AlN in the presence of YF3,CaF2 as sintering aids, and to predict the densification behavior during sintering using thermodynamic assessments. Based on thermodynamic calculations and the measured weight loss of the samples during heating to sintering temperature, the chemical reactions occurring during firing were proposed. The proposed reactions were related to the experimental results as well as the final properties of the AlN samples, namely, thermal conductivity, microstructure, secondary phases, and density. [source]


Correlation Between Microstructure and Creep Behavior in Liquid-Phase-Sintered ,-Silicon Carbide

JOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 3 2006
M. Castillo-Rodrguez
The influence of increasing the sintering time from 1 to 7 h on the microstructure evolution and the mechanical properties at high temperature was studied in ,-silicon carbide (,-SiC) sintered in argon atmosphere with Y2O3,Al2O3 (10% weight) as liquid phase (LPS-,-SiC). The density decreased from 98.8% to 94.9% of the theoretical value, the grain size increased from 0.64 to 1.61 ,m, and some of the grains became elongated. The compression tests were performed in argon atmosphere, between 1450C and 1625C and stresses between 25 and 450 MPa, with the strain rate being between 4.2 10,8 and 1.5 10,6 s,1. The stress exponent n and the activation energy Q were determined, finding values of n between 2.40.1 and 4.50.2 and Q=68035 kJ/mol for samples sintered for 1 h, and n between 1.20.1 and 2.40.1 and Q=71090 kJ/mol for samples sintered for 7 h. The correlation between these results and the microstructure indicates that grain-boundary sliding and the glide and climb of dislocations, both accommodated by bulk diffusion, may be two independent deformation mechanisms operating. At the temperatures of the tests, the existence of solid-state reactions between SiC and the sintering additives is responsible of the microstructural changes observed. These effects are not a consequence of the process of deformation, but rather they are because of the thermal treatment of the material during the creep. [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]


Relationship between Microstructure and Fracture Toughness of Toughened Silicon Carbide Ceramics

JOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 6 2001
Sung-Gu Lee
Different microstructures in SiC ceramics containing Al2O3, Y2O3, and CaO as sintering additives were prepared by hot-pressing and subsequent annealing. The microstructures obtained were analyzed by image analysis. Crack deflection was frequently observed as the toughening mechanism in samples having elongated ,-SiC grains with aspect ratio >4, length >2 ,m, and grain thickness (t) <3 ,m (defined as key grains 1). Crack bridging was the dominant toughening mechanism observed in samples having grains with thickness of 1 ,m < t < 3 ,m and length >2 ,m (key grains 2). The values of fracture toughness varied from 5.4 to 8.7 MPam1/2 with respect to microstructural characteristics, characterized by mean grain thickness, mean aspect ratio, and total volume fraction of key grains. The difference in fracture toughness was mainly attributed to the amount of key grains participating in the toughening processes. [source]