Papers by Keyword: Reaction Sintering

Abstract: A newly developed high-strength reaction-sintered silicon carbide (SiC), which has two or three times higher strength than conventional sintered SiC, is one of the most promising candidates for lightweight substrates of optical mirrors, because of its fully dense structure, small sintering shrinkage ( < 0.5 %), good shape capability, and low sintering temperature. In this paper, in order to improve the performance of the newly developed reaction-sintered SiC, the effect of the microstructure on the bending strength was investigated by focusing on a physical fracture model using observations from transmission electron microscopy and X-ray stress measurement. As a result, it was confirmed that the bending strength of the newly developed reaction-sintered SiC could be improved by reducing the size of residual silicon. The strengthening mechanism of the newly developed reaction-sintered SiC was assumed to be due to piled-up dislocations at the grain-boundary of residual silicon sites, based on Stroh’s fracture model of polycrystalline solids.

Abstract: A novel technique of Spent FCC equilibrium catalyst utilization is presented and mullite are prepared from it. The chemical composition, structure and thermal stability of the spent FCC equilibrium catalyst from oil refinery are characterized by XRD, FT-IR, DTA-TG, Fully chemical analysis technique, SEM. Mullite specimens are prepared by reaction sintering the spent FCC equilibrium catalyst at different temperatures. The result indicates that at 1300-1350°C, the spent FCC equilibrium catalyst could be used to prepare high purity mullite.

Abstract: Mullite–zirconia composites were synthesized through reaction sintering Algerian kaolin, α-Al2O3, and ZrO2. Phases present and their transformations were characterized using x-ray diffraction. Quantitative phase analysis was performed following the Rietveld method. Hardness and fracture toughness were measured by Vickers indentation. The flexural strength was measured using a Universal Testing Machine. It was found that the microstructure of samples sintered for 2 hours at 1600°C was composed of mullite grains which have whiskers’ shape and ZrO2 particles. In the composite containing 16 wt.% ZrO2, the ratio of tetragonal zirconia transformed to monoclinic zirconia was relatively small and did not exceed 18%. However, in the composite containing 32 wt.% ZrO2 around 75% of the tetragonal structure changed to monoclinic structure. Also, it was found that the increase of ZrO2 content from 0 to 32 wt.% decreased the microhardness of the composites from 14 to 10.8 GPa. However, the increase of ZrO2 content from 0 to 24wt.% increased the flexural strength of the composites from 142 to 390 MPa then decreased it with further increase of ZrO2 content. The fracture toughness increased from 1.8 to 2.9 MPa.m1/2 with the increase of ZrO2 content from 0 to 32 wt.%; and the rate of the increase decreased at higher fractions of ZrO2 content. The average linear coefficient of thermal expansion (within the range 50 to 1450°C) for samples containing 0 and 16 wt.% ZrO2 sintered at 1600°C for 2 hours was 4.7 x10-6 K-1 and 5.2 x 10-6 K-1 respectively.

Abstract: B4C/CNTs and B4C/Ccomposites were prepared by reaction-sintering technique. The density and porosity of the samples were determined and three-points bending strengths of the composites were measured. Wear resistance tests were performed using a HT-500 pin-on-disc tribometer with the sample placed horizontally on a turning table. The morphology of the worn surfaces and fracture of the composites were examined by means of scanning electron microscopy so as to explore the wear mechanisms. The results indicate that B4C/CNTs composites show improved friction and wear properties as compared with B4C/C composites. It is found that CNTs in the specimens play important role in improving the wear properties of the composites. The wear mechanism of the B4C/CNTs composites is characterized by micro-cutting and plowing at ambient temperature, and sub-surface cracking and the resultant localized spalling on the worn surface at the temperatures higher than 500°C.

Abstract: Basic bricks with Cr2O3 from chrome ore, as the spinel forming oxide, are used in the non-ferrous industry because of their corrosion resistance against fayalite-type slags, rich in FeO. Our objective in this study was to replace Cr3+ with Me4+ ions, which along with Fe3+ could maintain the spinel formation capability with MgO and perform similarly against fayalite slags in non-ferrous furnaces. Our preliminary research studies showed that Cr-free spinels in the MgO-Al2O3-FeOx-Me4+O2 systems could perform against fayalite slags similar to the complex (Mg2+, Fe2+)O·(Cr3+, Fe3+, Al3+)2O3 spinel, the main corrosion resistant component in the magnesia-chrome bricks. The incorporation of iron oxide in the MgO-Al2O3-Me4+O2 systems would contribute to reactive sintering and also in decreasing the solubility of both the ferrous and ferric ions present in the fayalite slag. Phase analysis on stoichiometric mixes showed that the use of tetravalent cation oxides like tin dioxide (SnO2) and titanium dioxide (TiO2) can induce high solubility of spinel in magnesia. In order to maintain charge balance, two trivalent cations were replaced by a tetravalent and a bivalent cation causing the additional bivalent cation to occupy the octahedral position thereby creating an inversion in position of the bivalent ions similar to the behaviour exhibited by Fe3+ occupying tetrahedral site in complex spinel phase of magnesia-chrome ceramics. Most of the magnesia-chrome refractories have ~60 wt. % MgO and hence our experimental mixes contained that amount and called “magnesia-rich” compositions, to be distinguished from the stoichiometric MgAl2O4 spinel. Our findings showed that the incorporation of nano TiO2 powders reduces the temperature of spinel formation as the diffusion path is shortened and thus activates both synthesis and sintering. Compositions containing 60 wt. % magnesia with alumina, nano TiO2 and Fe2O3 fired below 1500°C for 3 hours resulted in complete spinel formation and open porosity less than 5%.

Abstract: Reaction-sintered silicon carbide of 800 MPa class bending strength had been newly developed. The developed silicon carbide showed good rigidity, high thermal conductivity, and high density, like a conventional sintered silicon carbide. The developed silicon carbide is one of the most attractive materials for large-scale ceramic structures because of its low processing temperature, good shape capability, low-cost processing and high purity. We had fabricated some lightweight space mirrors, such as a high-strength reaction-sintered silicon carbide mirror of 650 mm in diameter. In this study, experiments were conducted to investigate the effect of annealing on the bending strength of high-strength reaction-sintered silicon carbide. The annealing heat treatments were carried out at 1073 K, 1273 K, and 1473 K in an air atmosphere. The maximum bending strength of 1091 MPa at room temperature was achieved by the annealing heat-treatment at 1273 K for 10 h in air. We confirmed that annealing heat treatment was effective to improve the bending strength of reaction-sintered silicon carbide by inducing compressive residual stress at the surface oxide layer.

Abstract: In the present work, the structure and sintering behaviour of mullite-zirconia composites were investigated. The composites were prepared by reaction sintering of Algerian kaolin, α-Al2O3, and stabilized zirconia (3Y-TZP). The raw materials were wet ball milled in a planetary ball mill followed by attrition. The green samples shaped using a uniaxial press were sintered between 1100°C and 1600°C for 2 hours. The density was measured by the water immersion method. Phases present and change of the average crystallite size of the mullite phase as a function of sintering temperature and ZrO2 content were characterized through X-ray diffraction. Mulite grains had whiskers' shape; however, the increase of ZrO2 content changed the morphology of grains to near spherical shape. The microstructure of the samples revealed uniform distribution of ZrO2 particles; also, aluminium was uniformly distributed on all grains exception on zirconia grains. At least a relative density of 95 % was achieved for all samples at a relatively lower sintering temperature of 1500°C. In composites containing up to 16 wt. % ZrO2, the zirconia phase retained its tetragonal structure and the transformed part did not exceed 3 %. However, with the addition of 32 wt. % ZrO2 around 66 % of the tetragonal structure changed to monoclinic structure.

Abstract: Si3N4-Si2N2O-TiN composite ceramics were in-situ fabricated by using following reactions of (1) 3TiO2 + Si3N4 → 3TiN + 3SiO2 + N2 and (2) Si3N4+ SiO2 → 2Si2N2O. The mixed powder of α-Si3N4, Al2O3, Y2O3 and TiO2 was hot-pressed at 24 MPa and 1800°-1900°C for 1-4 h in N2. Sintered composite ceramics were characterized by XRD, SEM, TEM, four-point bending test and Vickers indentation method. XRD results and TEM observation showed that TiN and amorphous SiO2 were formed at 1250°C by the reaction (1), and the Si2N2O phase formed by reaction (2) above 1800°C. Si3N4-Si2N2O-TiN composites consisted of ≥2 m sized Si2N2O grains with TiN and Si3N4 grains. Hardness and fracture strength of the composites were comparable to those of Si2N2O ceramics, with fracture toughness being improved at 5vol% TiN containing composites.

Abstract: A newly developed high-strength reaction-sintered silicon carbide (SiC), which has two or three times higher strength than the normal sintered SiC, is one of the most promising candidates such as the lightweight substrate of optical mirror, because of fully dense, small sintering shrinkage (±1%), good shape capability and low sintering temperature. In this paper, in order to improve the performance of newly developed reaction-sintered SiC, the microstructure was investigated by paying attention to the crystal structures using the observation of transmission electron microscope and X-ray diffraction analysis. As a result, it made clear that the finer-scale microstructure could be observed as consisting of large particles (~1m in diameter) of starting powder -SiC and small particles (~1m in diameter) of -SiC formed during the reaction (Si+CSiC) with the residual silicon filling the remaining porosity. Also, it was identified that the -SiC formed during the reaction referred to the cubic (3C) polytype and the -SiC of starting powder referred to the hexagonal (6H) polytype.