Papers by Keyword: Alumina

Abstract: Spark plasma sintering enables very rapid fabrication of bulk ceramic materials. Suitability of this technique for preparing of multilayer ceramic discs is up to now relatively seldom reported. Our work is focused on a bi-layered disc consisting of thick Al₂O₃ layer and comparably thick Mg (Ca)TiO₃ (MCT) layer sintered in one run. Al₂O₃ powder was nanometric (less than 40 nm) and composed of the hexagonal α-phase whereas MCT is a solid solution of two orthorombic perovskites. Pre-sintered and crushed MCT powder was coarser than alumina, up to 125 μm. The purpose of this material combination was creating of dielectric sandwich consisting of capacitor MCT material and resistor nanoalumina material. Spark plasma sintering run was done at 1200 °C, using pressure 80 MPa and dwell time only 2 min. Resulting sandwich was subjected to microstructural observations and cross sectional measurement of microhardness. On the border between Al₂O₃ and MCT the microhardness was higher than in MCT but has markedly higher dispersion than in the individual components. The MCT material exhibits birefringence in the polarized light [1]. We attempted to correlate the microhardness of individual MCT grains to their orientation indicated by birefringence.

Abstract: Anisotropy of the compressive strength was investigated in porous alumina based ceramics produced by slip casting method. Aluminium paste was used as pore former similarly as in the aerated concrete technology. Materials were sintered at 1650°C and 1750°C. The apparent porosity of the investigated materials was in the range of 54±1% to 60±2%. The compressive strength of the samples in the directions parallel and perpendicular to the sample expansion direction was in the range from 3.8±0.3 to 13.0±1.1 MPa and from 6.4±1.3 to 27.0±1.1 MPa, respectively. The mechanical anisotropy was related with the flattening of the introduced pores. The level of mechanical anisotropy was affected by the initial composition of the slurry and sintering temperature.

Abstract: The effect of addition nanoAl₂O₃ on sintering behavior and mechanical property of ZrO₂ was studied. It was shown that the addition of nanoAl₂O₃ with the Al₂O₃ content from 0.5mol% to 2.0mol% in ZrO₂ ceramics could accelerate the sintering of ZrO₂ and increase the flexural strength and fracture toughness of ZrO₂. ZrO₂ added with 1.0mol% nanoAl₂O₃ has the maximum in strength of 650MPa and toughness of 15MPa·m^1/2 sintered at 1450°C. Compared with undoped ZrO₂ being sintered at 1500°C, the strength and toughness was improved by 12% and 18%, respectively.

Abstract: Direct coagulation casting (DCC) is a relatively new ceramic near-net-shape forming process which can form homogeneous ceramic green body with complex shape and high density. Direct coagulation casting of aqueous alumina slurries by adding Y³⁺ ions, which have been frequently used as additive for the sintering of alumina ceramics, were studied. Two different kinds of Y³⁺-ion releasing substances, Y₂O₃ powder and Y(NO₃)₃, were used as coagulants and were introduced into stabilized alumina suspensions directly after dispersing. Compared with that of Y₂O₃ powder, both the coagulation time and the added amount required for a reasonable shaping were significantly reduced for using Y(NO₃)₃ as coagulant. The effects of the two coagulants on the properties of the consolidated green bodies and sintered ceramics were studied. The mechanisms of coagulation induced by the addition of Y³⁺-ions were discussed on the basis of interactions between Y³⁺ ions and the added polyelectrolyte dispersant.

Abstract: Alumina lightweight insulation bricks were prepared from alumina powders using foaming method. In this paper the influences of foam addition amount (600, 800, 1000, 1200ml/kg) on properties of Alumina lightweight insulation brick were discussed. The phase composition is analyzed by XRD and the microstructure is characterized by SEM. The results show that the bulk density of alumina lightweight insulation brick is 0.63g/cm³,cold compressive strength is 10.53MPa, linear change rate after firing is -4.5% and reheat linear change rate is -0.27%,which synthesized under 1000ml/kg foam amount after firing at 1550°C for 3h. The results indicate that the density of alumina lightweight insulation brick is reduced with the increase of foam addition amount, when the addition amount is more than 1000ml/kg, cold compressive strength decreased significantly and linear shrinkage increased obviously.

Abstract: A novel method to prepare the copper bonding on alumina substrate was presented. The cuprous Cu₂O on the surface of Cu foil was prepared by covering cupric oxide (CuO) powder on Cu foil and treating at 600°C in N₂ atmosphere. Then eutectic process was executed at 1075°C in N₂ atmosphere to bond the Cu and alumina substrate. The cuprous grew on copper foil and bonding between Cu foil-alumina substrate were identified by XRD spectrum and SEM micrographs.

Abstract: Alumina and zirconia are important materials for energy and optical applications. In this study, the effect of thermal cycling on grain size and residual stress was reported. Residual stress was measured using X-ray diffraction (XRD) sin2ψ method for the as-received and the samples after thermal cycling up to 900 cycles. For alumina, the measured residual stress is approximately 96 MPa in tensile for the as-received material, and increases to its highest value of 480 MPa after 650 thermal cycles. The residual stress decreases from 480 MPa to 96 MPa in tensile with increased thermal cycling from 650 to 900 cycles. The crystallized grain size calculated from the diffraction pattern shows that the mean crystallized grain size is about 93 nm for the as-received and increases to 232 nm after 650 thermal cycles. This result is consistent with the enlarged grain size observed by scanning electron microscopy for the alumina after 650 thermal cycles reported earlier. With continued thermal cycling up to 900 cycles, the crystallized grain size is greatly reduced to 104 nm. It suggests that evolution of the crystallized grain size is correlated with the residual stress. For yttria-stabilized tetragonal zirconia (Y-TZP), the mechanical properties at room temperature, are consistent with the property values provided by the manufacturer. The Young’s modulus of shows a non-linear inverse relationship with increasing temperature. The degradation of the Young’s modulus mostly occurs prior to 400 °C and to a less extent in the temperature range of 400 °C up to 850 °C. The Vickers hardness number for the as-received Y-TZP material decreases to a very small extent after 560 thermal cycles and increases approximately 2%, after 1200 thermal cycles. This is consistent with the trend of the Young’s modulus for thermal-cycled specimens.

Abstract: By using the orthogonal method to evaluate the optimum experiment parameters for preparing mesoporous Al₂O₃-TiO₂ supports which can be used for resid hydrotreating catalyst by its large pore volume, the two-step nanoscale self-assembly technique was successfully adopted. The experimental conditions were obtained, such as: the amount of surfactant; the ratio of raw materials, the amount of precipitation reagent and the reaction time. TEM was employed to characterize the A1₂O₃-TiO₂ complex supports. The test results revealed that the specific surface area of Al₂O₃-TiO₂ complex supports reached to the largest under the optimized experimental condition. The TEM result also showed that TiO₂ was homogeneously distributed in the complex support. The pore distribution of the complex support obtained was stable, which promises the best candidate for the support to resid hydrotreating catalyst.

Abstract: Template method was used to prepare ordered mesoporous alumina, and with purchased γ-Al₂O₃ contrast. BET results showed that the specific surface area and average pore diameter of the mesoporous Al₂O₃ were 226.95 m².g^-1 and 3.91nm respectively, and with higher degree of order. XRD results showed that the cat-P catalyst with the carrier of prepared ordered mesoporous alumina hold small active component grains, which means better dispersion. H₂-TPR results showed that the SMSI effect of cat-P catalyst was stronger than that of cat-B. Stability tests showed that cat-P catalyst in 50h experiment showed high activity and stability, and the conversion of CH₄ and CO₂ were above 90% and 95% respectively among the reaction time, and the deactivation rate of CO₂ was 0.05%/h only. From TG-DTG curve could be seen that the deposition carbon on cat-P catalyst after 48h reaction was in whisker form, which did not cover the active sites and had little influence on the activity and stability of the catalyst.

Abstract: A series of TiO₂-Al₂O₃ composite supports were prepared by co-precipitation of titanium sulfate and aluminium nitrate , and the nickel phosphide catalysts were prepared by incipient wetness impregnation and in situ H₂ reduction method. The catalysts were characterized by XRD, N₂ adsorption, TPR, TEM and XPS techniques. Their hydrodenitrogenation (HDN) performances were evaluated by using quinoline as model molecules . The results show that TiO₂ was evenly dispersed on massive γ-Al₂O₃ surface. The introduction of TiO₂ weakened the strong interaction between Al₂O₃ and phosphate, and improved the reducibility of the precursors , facilitating to the formation of Ni₂P active phase. TiO₂ acted as an electronic promoter for the Ni-P catalyst and enhanced both the hydrogenation and C-N bond cleavage activities. Different Ti / Al molar ratio had great influence on HDN activity of the catalyst. The Ni₂P /TiO₂-Al₂O₃ with Ti/Al ratio of 1/8, exhibited the highest activity for quinoline HDN.