Key Engineering Materials Vols. 336-338

Abstract: Cordierite honeycomb with hexagonal shaped open channels was formed by extrusion forming technique using specially designed mould for the raw oxide mixture of talc, kaolin, alumina and aluminum hydroxide. The honeycomb showed extremely thin wall thickness of 0.15 mm and low coefficient of thermal expansion (CTE) for the high number of cells per square inch (cpsi) approaching to 600. The extruded honeycomb was sintered at various conditions and characterized by XRF, TGA-DTA, XRD, TMA and SEM techniques. The indialite crystalline showing the coefficient of thermal expansion of 1.9 × 10-6/°C was formed while sintering at 1,450°C for 5 h. The influence of Fe2O3 additive and heating condition on the CTE of cordierite was analyzed in details. This particular cordierite honeycomb with high numbers of hexagonal shaped open cells showed a distinct advantage of saving the amount of precious metal catalyst materials to be coated on the substrate in comparison with the honeycomb with rectangular cells while improving the degree of catalyst efficiency when applying the substrate of automotive catalytic converter.

Abstract: The dispersion and stability of PZT aqueous suspensions were investigated by using ammonium salt of polyacrylic acid (PMAA-NH4) as the dispersant. By observing the sedimentation behavior and measuring the viscosity, the effects of dispersant concentration and pH value on the dispersion and stability of the suspensions were studied. The experiment results showed that the optimum concentration of PMAA-NH4 and the optimum pH were 0.25-0.45 wt.%( by weight of the PZT powders) and 8.5-10, respectively. When the dispersant concentration was about 0.45 wt.%, and pH value was about 9.4, the stable PZT suspensions with high solids loading (up to 52 vol.%) and low viscosity (<1Pa.s) was obtained for gelcasting. The microstructure of ceramic formed by gelcasting was denser and more homogeneous than that by die pressing.

Abstract: Ba-ferrite particles were dispersed into distilled water to make stable slurry, which was then slip cast in transverse magnetic field. The water drainage from the slurry was performed by vacuum evacuation to solidify the slurry into a cake – like sample. To obtain high degree of alignments, the slurries were slip cast in static – / pulsed – magnetic field and by using two separate steps of magnetic orientation and drainage. The particles turned their faces to the magnetic field and formed long chains stacked immediately while the magnetic field was applied, of which high induction density led to high degree of alignments. It is necessary to correspond with the drainage, gravity and magnetism so as to suppress the tendency of distortion or cracking of the sample as the aligned long chains of the particles tend to break into pieces and cave in randomly, which eventually destroys the particle alignments.

Abstract: High purity polycrystalline titanium silicon carbide (Ti3SiC2) was synthesized by spark plasma sintering (SPS) with additive of Aluminum at low temperature of 1100~1300oC. XRD results showed that the content of Ti3SiC2 in the synthesized bulks increased with the addition of Aluminum, however, the lattice parameters of Ti3SiC2 samples with different amounts of additive exhibited scarcely change. With a starting powder of 1Ti/1.2Si/2TiC/0.1Al (molar ratios), dense Ti3SiC2 ceramic (98vol.%Ti3SiC2) was obtained by sintering at 1100oC for 5min under a pressure of 30MPa. Fine Plate-like grains of the samples with sizes of 2~6μm could be identified by scanning electronic microscope (SEM).

Abstract: The γ-TiAl intermetallic compounds were produced at the temperature ranging from 850°C to 1050°C by the Spark Plasma Sintering (SPS) process. The effects of sintering temperature and holding time on the mechanical properties of γ-TiAl intermetallic compounds were investigated. The γ-TiAl intermetallic compounds sintered at 1050°C for 10 min showed a high relative density more than 98%, and had the best three-point bending strength of 643MPa, fracture toughness of 12 MPa·m1/2 and microhardness of 560MPa. The microstructural observations indicated typical characteristics of intergranular fracture, which meant the poor ductility of γ-TiAl intermetallic compounds.

Abstract: Nanocomposite WC-10Co powder produced by spray pyrolysis-continuous reduction & carbonization technology and cubic boron nitride (CBN) plated with titanium by vacuum vapor deposit were used, and this paper adopted spark plasma sintering (SPS) process to prepare CBN enhanced ultrafine WC-10Co cemented carbide cermets composite material. The microstructure and mechanical properties of CBN-WC-10Co composites were investigated. The results show that CBN-WC-10Co composites consolidated by spark plasma sintering can reach 95.0 % relative density, and transverse rupture strength (TRS) is 1050 MPa, the average grain size of cermets matrix is less than 420 nm, and CBN-WC-10Co composites with excellent properties are achieved. The CBN still remains very good crystal shape after 1240°C spark plasma sintering, and there is not obvious clearance between CBN plated with titanium and the cermets matrix, the coated titanium layer can not only improve the thermal stability of CBN, but also increase the properties of CBN-WC-10Co composites.

Abstract: Porous alumina bodies were successfully prepared by spark plasma sintering of alumina powders with different amounts of graphite, and by subsequently burning out the graphite. Highly porous bodies were fabricated by spark plasma sintering at 1000°C for 3 min under a pressure of 30 MPa. The heating rate was 80°C/min, and the pulse pattern (on-off) was 12:2. For example, alumina bodies prepared by the addition of 10 ~ 30 vol% graphite showed high porosity of 50 ~ 57%. Porous alumina bodies prepared by the addition of 10 ~ 30 vol% graphite had a high compressive strength of 200 ± 55 MPa, about 35 times higher than those obtained on samples prepared by pressureless sintering, and about 2.5 times higher than those in samples prepared by hot-pressing. The significant improvement in strength relative to values obtained with conventional sintering was attributed to better sintering resulting from the rapid heating between particles.

Abstract: MgAlON ceramics were prepared by spark plasma sintering (SPS) and hot pressing (HP), respectively. It was found that SPSed MgAlON samples had a single MgAlON phase with almost no open pore while HPed MgAlON had a minor AlN phase. The strength of SPSed samples is higher than 500MP, being much higher than that of the HPed samples. The experimental results revealed that that SPS was a rapid and reliable process for the further development of MgAlON.

Abstract: In this study, spark plasma sintering (SPS) was applied to prepare α-Si3N4 ceramics of different densities with magnesia, silicon dioxide, alumina as the sintering aids. The sintering behavior and liquid phase sintering (LPS) mechanism were discussed and the factors influencing the density of the prepared samples were analyzed. Microstructures of sintered samples were observed and the phase compositions were analyzed. The results showed that α-Si3N4 ceramics can be sintered by SPS based on the reaction among α-Si3N4 and sintering additives which lead to the liquid phase and the density can be well controlled from 2.48 to 3.09 g/cm3 while the content of the sintering aids changes from 10% to 28.5% and sintering temperature from 1400°C to 1500°C.

Abstract: In the present study, we fabricated high purity and electrically machinable Ti3SiC2 ceramics by mechanical alloying and subsequent spark plasma sintering. The effect of a trace amount of Al on these synthesis processes was examined. Our results showed that Ti3SiC2 could be synthesized by high energy milling. Spark plasma sintering of mechanically alloyed powder at the temperatures of 1000-1200°C produced nearly single-phased Ti3SiC2 materials. The purity of the sintered Ti3SiC2 bulk was remarkably increased by addition of a small amount of Al. Ti3SiC2 with a purity of 99.3 wt% and a relative density of 98.9% was obtained by mechanical alloying and subsequent spark plasma sintering from a starting mixture composed of n(Ti) : n(Si) : n(Al) : n(c) = 3 : 1 : 0.2 : 2 at 1100°C.