Abstract: Based on preparation of Al2O3/ZrO2 (4Y) by combustion synthesis in high-gravity field, the microstructure transformation and properties of the materials are investigated through adjusting the ZrO2 (4Y) content in the composites. As the content of ZrO2 changed from 37% to 40%, the microstructures of the ceramics transformed the sphere-like tetragonal ZrO2 crystals from the rod-shaped colonies with nanocrystalline structures. Al2O3/33%ZrO2 (4Y) had the maximum relative density, hardness and flexural strength due to the low solidification temperature, the highest volume fraction of the colonies, small-size defect and high fracture toughness, whereas Al2O3/44%ZrO2 (4Y) was somewhat weakened in strength in despite of its highest fracture toughness.
Abstract: Based on preparing large bulk Al2O3/ZrO2 (4Y) eutectics, SiO2 additive is used for controlling the microstructures, densification and mechanical properties. XRD pattern showed SiO2 additive in the ceramics was in the form of glass phase. SEM images showed that the microstructure morphologies transformed from the cellular eutectics to the rod-shaped colonies with increasing content of SiO2 additive. With increasing content of SiO2 additive, the relative density of the ceramics increased whereas the hardness of the ceramics decreased. As the content of SiO2 additive reached 4%, fracture toughness of the ceramics had the maximum value due to the coupled toughening mechanisms of crack-pinning, crack- bridging and crack-deflection by rod-shaped colonies. As content of SiO2 additive reached 6%, the highest flexural strength of the ceramics was achieved due to high fracture toughness and small-size critical defect.
Abstract: Using aluminum chloride, aluminite powders and butyl titanate as raw materials, alumina- aluminum titanate fiber was prepared by sol-gel method at 1200°C. The influences of the holding time on the aspect ratio, the bulk density, the thermal conductivity coefficient and the heating linear shrinkage of the fiber were studied systemically. The results show that the holding time has an important effect on the properties of the fiber. With the increase of the holding time from 0 h to 3 h, the aspect ratio of the fiber decreases from 870 to 210 and the bulk density of the fiber adds from 352 Kg•m-3 to 432 Kg•m-3. Furthermore, the thermal conductivity coefficient of the fiber keeps rising from 0.162 W•m-1•K-1 to 0.199 W•m-1•K-1 and the heating linear shrinkage of the fiber decreases from 2% to 1.82%.
Abstract: The Al2O3/YSZ eutectic composite ceramics-lined pipes have been prepared by combustion synthesis. By adding ZrO2 (4Y) to the thermit, the influences of centrifugal force and eutectic composition on the microstructure, mechanical properties, strengthening and toughening of ceramics lining are researched. The growth of melt is based on eutectic transformation under high degree of under-cooling at centrifugal force above 200G. The linings are comprised of rod-shaped Al2O3 matrix eutectic grains. The diameters of tetragonal lattice ZrO2 fiber in rod-shaped grains are on nano-micron scale. Causing the strengthening and toughening of ceramics to be promoted, and the Al2O3/YSZ eutectic composite ceramics-lined pipes have high comprehensive mechanical properties.
Abstract: Nano-Fe particles coating Al2O3 composite powders were prepared by heterogeneous precipitation method with nanometer -Al2O3 and Fe(NO3)3•9H2O as raw materials. The composite powders were analyzed by DSC-TG, XRD,SEM and Zeta potential. Results showed that Fe coating Al2O3 nanometer composite powders were obtained in the condition of being sintered at 500°C for 30min and reduced at 700°C for 1h in H2. The coating Fe nanometer particles are in the shape of sphericity with diameter about 30nm and the dispersion of the powders is uniform. Al2O3/Fe composite ceramics were obtained by hot-pressing (30MPa). The mechanical properties of the composite were investigated after hot press at different temperatures. With the increasing of Fe content in composite ceramics, the hardness of the composite is decreased. Fracture toughness of 10mol%Fe content is 5.62MPa after sintered at 1400°C, which is increasing 57% high than that of monolithic Al2O3 ceramics.
Abstract: Rod-like submicron Al2O3 powder was prepared by the controllable hydrolyzation of aluminium isopropoxide in streams atmosphere. XRD data show the rod-like Al2O3 has a γ- Al2O3 structure. TEM images display the length of the submicron Al2O3 is in the range of 100-300 nm and the slenderness ratio is above 10. The rod-like Al2O3 powder was added into the submicron Al2O3 (D50 = 0.5 μm, purity > 99.99%) to increase the bending strength of the sintered body and to decrease the sintering temperature. SEM images display that the Al2O3 sintered at 1450 °C with rod-like Al2O3 added has a higher density than the virgin Al2O3 and the Al2O3 grains have the trans-crystalline fracture. The bending strength of the sintered Al2O3 with 5 wt% rod-like Al2O3 added is 205 MPa, which increases 241%. A sharp shrinkage in 1300-1400 °C appeared in the sintering shrinkage curves characterized by DIL show the rod-like Al2O3 also acts as the sintering aid.
Abstract: Mullite- corundum ceramic was fabricated using bauxite and silica fume as raw materials by gel-casting process. The fluidity of the slurry, as well as strength, apparent porosity, pore size distribution, phase composition and microstructure of sintered specimen were studied. The results showed that: the slurry had low viscosity, good fluidity for casting if pH=9, dispersant 0.33wt% and solid content 60vol%; the strength of material came up to 165MPa even if the apparent porosity was about 30%; the samples were characterized by mullite 70%, corundum 22%, rutile 3%, glass phase only 5%, the size of majority grains was about 2μm, and most crystals with high melting point bonded directly; the average aperture was 264nm, the pore size distribution were divided into 2~30nm, 45~60nm and 1500~4000nm three regions. The distribution probability of nano- aperture was the largest, the volume of micron -aperture was the maximum, but all pores were less than 4μm in diameter, which suggested that the materials had micro porous characteristics. The microcrystalline, crystals combined directly and micro porous structure leaded to high strength of the refractory materials together.
Abstract: Silicon nitride ceramics were prepared by gas pressure sintering (GPS) with different sintering additives, including La2O3, Sm2O3 and Al2O3. Effect of sintering additives on the phase-transformation, microstructure and mechanical properties of porous silicon nitride ceramics was investigated. The results show that the reaction of sintering additives each other and with SiO2 had key effects on the phase-transformation, grain growing and grain boundaries. With 9MPa N2 atmosphere pressure, holding 1h at 1850°C, adding 10wt% one of the La2O3, Sm2O3, Al2O3, porous silicon nitride was prepared and the relative density was 78%, 72%, 85% respectively. The flexural strength was less than 500MPa, and the fracture toughness was less than 4.8MPam1/2. Dropping compounds sintering additives, such as La2O3+Al2O3, Sm2O3+Al2O3 effectively improves the sintering and mechanical properties. The relative density was 99.2% and 98.7% with 10wt% compounds sintering additives. The grain ratio of length to diameter was up to 1:8. The flexural strength was more than 900MPa, and the fracture toughness was more than 8.9MPam1/2.
Abstract: SiC/Ti3SiC2/TiC composites were fabricated by means of pressureless sintering using the 3TiC/2.2Si/0.2Al as raw materials. At the temperatures from 1000°C to 1200 °C, the main phase of the composite synthesized was Ti3SiC2. At the same time, there was a small amount of TiC and SiC existing. The composite had loose microstructure. At the temperature of 1300 °C, the TiC and Ti3SiC2 were the primary phases. And the SiC was the second phase. The SEM images showed that there were two kinds of microstructures in the composite. One was the Ti3SiC2 and the TiC platelets on its surface, another one was the TiC and the SiC equixed grains in its interior.