Papers by Keyword: Pressureless Sintering

Abstract: AlN-ZrO₂(Ca) composites were prepared with two kinds of Ca-stabilized zirconia (ZrO₂(Ca), i.e Ca-PSZ, particle size d₅₀=65μm and d₅₀=7.5μm) and three kinds of AlN power (d₅₀=15μm, d₅₀=0.5μm and d₅₀=40nm) as raw material in N2 atmosphere by pressureless sintering at 1850 oC. The phase compositions and microstructures of composites were investigated by XRD, SEM and EDS.The sintering performance of the composites was measured by the checking of the bending strength, and the apparent porosity of samples. The results showed that phase compositions of the composites consisted of major phases C-ZrO₂ and AlN and minor phase CaAl₁₂O₁₉. It was harder that AlN-ZrO₂(Ca) composites were sintered than ZrO₂(Ca) material. It was major cause that phase CaAl₁₂O₁₉ formed and its abnormal growed in the boundary layer between AlN particle and ZrO₂(Ca) matrix. With 2mm AlN particles as raw material, AlN-ZrO₂(Ca) composites had better sintering performance when concent of AlN equaled to 8%.

Abstract: Al₃BC₃-based porous ceramics are prepared by pressureless sintering using Al, B₄C, and graphite powders as the starting materials. The influences of molar ratio of starting materials on the purity, open porosity and pore size of the porous ceramics are investigated. Graphite is the main impurity of the Al₃BC₃-based porous ceramics and can be eliminated by adjusting the molar ratio of starting material. With the molar ratio of Al:B₄C:C = 3.2:0.25:2.30, high pure Al₃BC₃ porous ceramic is successfully fabricated by pressureless sintering in vacuum at 1800 °C for 2 h. The open porosity of the Al₃BC₃-based porous ceramics ranges from 36% to 47%. The pores are uniformly distributed.

Abstract: In this work, porous TiB₂ ceramics were consolidated by pressureless sintering method using metallic Ti and Fe as additives in order to perform sintering at temperatures lower than 1700°C. It was shown that processing parameters including milling time of the starting mixture had a considerable effect on final properties of sintered specimens and their behavior in molten aluminum. Microstructural studies were carried out using optical microscope, SEM and EPMA. It was found that specimens with uniform and crack-free microstructure could be produced using the pre-mixed powders milled for as low as 30 min prior to compaction and sintering. Sessile drop test was performed on the specimens milled for 30 and 240 minutes. Their interaction with molten aluminum was also studied. It was found that 30 min milling time resulted in better electrical conductivity, wettability and stability in liquid aluminum.

Abstract: This paper introduces Si₂ON₂-SiC ceramic fabricated by pressureless sintering method and studies the effect of additives, nitriding temperatures on bulk density, porosity, phase composition and microstructure. It is discovered that additives MgO, CeO₂ can increase the densities of Si₂ON₂-SiC ceramic apparently, and MgO additive has a better effect than CeO₂. Nitriding temperature also is an important factor. The bulk density of the specimen with MgO additive reaches maximum at 1.91 g/cm³ when sintered at 1450 °C, and the bulk density of specimen with CeO₂ additive is 1.86 g/cm³ at the same condition while the bulk density of the specimen without additive is only 1.75 g/c^{Superscript text}m3. The X-ray diffraction and scanning electron microscopy of the specimens show that the amount of Si₂ON₂ increase with the sintering temperature increase. But when the temperature is higher than 1500 °C the Si₂ON₂ grains will decompose into Si₃N₄, and Si₂ON₂ will vanish at 1550 °C

Abstract: Tabular alumina, α-Al₂O₃ powder, TiO₂ powder and metal aluminum powder in purity of 99%(by mass) were uesd to prepare the Al₂O₃-TiN composite ceramics by mechanical mixing and thermal reduction of aluminum in situ reaction sintering in the flow of nitrogen gas under atmospheric pressure;the influences of phase composition and microstructure changes at 1200°C-1500°C nitridation reaction in situ,different amount of TiN formations and sintering temperature on the properties of the samples were analyzed by thermal analysis, XRDand SEM. The results show that: TiO₂ was reduced by Al and then nitrided as TiN in the test temperature, the porosity has been increasesd while the bulk density has been decreased with the increased content of TiN formation because of the process of nucleation growth and the strong exothermic of the reaction.The mechanical properties of the composites are relatively better of TiN generated 10%content,because the structure of the skeleton formed by corundum was filled of part matrix evenly and was integrated closely by the matrix.

Abstract: In the present work, 3 mol% Yttria-stabilized tetragonal zirconia (Y-TZP) composite containing 25 wt.% of zirconium diboride (ZrB2) was prepared via pressureless sintering method in an inert atmosphere over the temperature range of 1350-1550°C for one hour. The effect of zirconium diboride content in the zirconia matrix, as well as the sintering temperature on densification, phase stability and electrical properties of sintered samples have been studied. The results revealed that there was a significant increased in electrical conductivity of sintered samples when 25 wt.% of ZrB2 is incorporated into Y-TZP matrix.

Abstract: The transmutation target of nuclear waste material has been fabrication by a powder metallurgy method by using Alumina as the matrix and CNTs as reinforcement. The effect of different nanotube contents on the fracture toughness and the bending strength was investigation. The results showed the fracture toughness and the bending strength of composites increased with increasing CNTs mass fraction when the content of CNTs was less than 1.5%. However, when the contents of CNTs greater than 1.5%, the fracture toughness and the bending strength of composites decreased as the content of CNTs increased. Possible mechanisms are discussed in detail in the paper.

Abstract: The FeAl/Al₂O₃ composites were fabricated by pressureless sintering process. The FeAl intermetallics compounds powders were fabricated by mechanical alloying and heat treatment process. The FeAl intermetallics compounds powders and Al₂O₃ powders were mixed and the FeAl/Al₂O₃ composite powders were prepared. The FeAl/Al₂O₃ composites bulks were fabricated by pressureless sintering process at 1600°C for 2h. The phase composition and microstructure of FeAl intermetallics compounds powders produced by mechanical alloying and heat treatment were investigated. The phase composition and microstructure of the FeAl/Al₂O₃ composites sintered bulks were investigated. The XRD patterns results showed that the Fe-Al intermetallics compounds powders were fabricated by mechanical alloying for 60h. The FeAl intermetallics compounds powders were fabricated by heat treatment at 800°C, 900°C and 1000°C. The microstructure showed that the mean particles size of the FeAl intermetallics compounds powders produced by mechanical alloying and heat treatment process was rather fine and about 4-5μm. The XRD patterns results showed that there existed the FeAl phase and Al₂O₃ phase in sintered composites. The FeAl/Al₂O₃ composites bulks exhibited the homogenous and compact microstructure. The mean particles size of FeAl was about 4-5μm and the mean particles size of Al₂O₃ was about 5-10μm. The density and relative density of the FeAl/Al₂O₃ composites increased gradually with the increase of FeAl content.

Abstract: Composite anodes made of NiO and SDC-(Li/Na)2CO3 were investigated in relation to their structure, morphology, and porosity. As a first step, the anode powder was prepared by mixing the NiO with SDC-(Li/Na)2CO3 via solid state reaction in weight percentage of 60 : 40 wt% and in various compositions of carbonates (20 and 30wt%), namely NiO-SDC8020 and NiO-SDC7030, respectively. The powder mixtures were then calcined at 680oC. The resultant powder was fine with surface area of about 13.10-13.70 m2/g and an average particle size of 0.32-0.37µm. The powders consist of two phases i.e. the cubic NiO and face-centered cubic structure SDC-(Li/Na)2CO3 as confirmed with x-ray diffraction. The microstructures were observed under scanning electron microscopy (SEM). The anode pellets were later compacted at different pressures (27, 32 and 37 MPa) and sintered at 600oC. The optimum porosity (20.99-24.78%) was achieved for samples of NiO-SDC8020 and NiO-SDC7030 sintered at 600oC and cold pressed at 32 and 37 MPa.

Abstract: Composite cathodes made of perovskite La0.6Sr0.4Co0.2Fe0.8O3 (LSCF) and SDC carbonates (SDC-(Li/Na)2CO3) were investigated in relation to their structure, morphology, thermal expansion coefficient and porosity. As a first step, the LSCF powder was prepared by sol-gel technique. This was followed by the preparation of the LSCF-SDC carbonates composite cathode by mixing the LSCF with SDC-(Li/Na)2CO3 electrolyte via solid state reaction in various compositions, i.e. 30, 40 and 50 wt.%, namely 70LSCF-30SDC7030, 60LSCF-40SDC7030 and 50LSCF-50SDC7030, respectively. The powder mixtures were then calcined at 680oC. The resultant powder was fine with surface area of about 3.39-7.42 m2/g and particle size of 0.56-0.66µm. The powder consists of two distinct phases, i.e. LSCF and SDC-(Li/Na)2CO3 as confirmed with x-ray diffraction. The microstructures were observed under scanning electron microscopy (SEM). Increasing the amount of the SDC-(Li/Na)2CO3 electrolyte in the composite cathode was found to bring the thermal expansion of the cathode closer to that of the electrolyte. The cathode pellets were later compacted at different pressures (27, 32 and 37 MPa) and sintered at 600oC. The optimum porosity (20.99-24.98%) was achieved for samples with SDC-(Li/Na)2CO3 content of 30-50% sintered at 600oC and cold pressed at 37 MPa.