Key Engineering Materials Vols. 602-603

Abstract: Lu₂Ti₂O₇ octahedrons were synthesized by molten salt synthesis (MSS) through selecting appropriate source materials and controlling proper external conditions. The phase constituents and microstructure morphologies of as-product were characterized by X-ray powder diffraction, Energy Dispersive Spectrometer, Field Emission Scanning Electron Microscopy and High-resolution Transmission Electron Microscopy. According to the experimental results, the detailed microstructure and growth mechanism of Lu₂Ti₂O₇ octahedron have been analyzed. The presented results in this paper may benefit to a deeper understanding of the fundamental physical and chemical processes of MSS methods.

Abstract: The korshunskite samples were prepared in precipitation by the one-step reaction method at atmospheric pressure. The three-layer structure back-propagation network model based on the non-linear relationship between the amount of the korshunskite whiskers and the technological factors, such as the adding amount of raw materials NaOH, MgCl₂, MgO, and reaction temperature, is established. And the results show that the improved back propagation neural networks model is very efficient for predication of the korshunskite whiskers preparation.

Abstract: Yttria-stabilized zirconia fibers thermal insulation materials may be the main developmental direction of high temperature thermal protection materials, due to their high temperature resistance (above 2000 °C), high strength and toughness, low thermal conductivity ( about 2 W/m·K), oxidation resistance and other excellent properties. However, mechanical property of zirconia fibers thermal insulation materials is decrease after high temperature heat treatment. In this paper, the evolution of microscopic structure of zirconia fibers is studied by using the method of oxyacethlene ablation. The gain growth and high temperature creep are observed after 600 s heat treatment at 1800 °C. Crystal phase of zirconia fibers is not changed and still cubic. Moreover, crystallinity is more higher form XRD patterns. Controlling grain growth can improve high temperature mechanical property of zirconia fibers thermal insulation materials.

Abstract: The industrial watchcases of 5Y-Tetragonal ZrO₂ were prepared by microwave sintering. Samples were sintered in a microwave chamber with TE666 resonant mode at 2.45GHz. The sintering temperature was from 1250°C to 1400°C. XRD and SEM techniques were used to characterize the samples. It is found that microwave sintering improved the phase change from t-ZrO₂ to m-ZrO₂. Dense and homogeneous microstructure was obtained within the microwave-sintered samples and the average grain size was about 500nm. Compared to conventional sintering, microwave-sintered samples show higher density and hardness. And microwave-sintered samples were performed 130°C lower temperature. Optimized sample with the density of 6.1 g/cm³ and Vickers Hardness of 16.2MPa is microwave-sintered at 1350°C for 30min.

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: The total bauxite reserves of China are about 3.7 billion tons, however the principal and the first-class bauxite reserves account for only about 20%. A large number of coal gangue was accumulated on the surface of earth, which not only encroaches land but also pollutes the environment. In this study, low-grade bauxite with alumina content of 53.18 wt.% originating from Shanxi Yangquan and coal gangue with alumina content of 38.07 wt.% originating from Shanxi Datong were used to synthesize mullite materials. Bauxite and gangue were mixed by the ratio of 1 to 1. The ingredients were prepared according to the alumina content of 45.63 wt.% and 70 wt.%, by adding a certain amount of industrial alumina, respectively. The samples were sintered at 1350°C to 1600°C after being milled, dried and formed. The influence of alumina content and sintering temperature on the phase, microscopic morphology and properties of samples were discussed. The results show that, the main phases of the sample with the alumina content of 70 wt.% were mullite (more than 60 %) and corundum after being sintered for 4h at 1600 °C. The phenomenon of columnar mullite and granular corundum staggering had been observed. The bulk density and the bending strength of sample is 3.21 g/cm³ and of 220.73 MPa respectively.

Abstract: La₂Zr₂O₇ as the thermal barrier coating material draws more and more attention because of its greater thermal physical properties than YSZ, while its further application is restricted by its poor mechanical properties. In this paper, certain ZrO₂ is added into the La₂Zr₂O₇ ceramics in order to improve its mechanical properties. La₂Zr₂O₇ ceramic superfine powder was synthesized by the co-precipitation method and samples of La₂Zr₂O₇-xZrO₂(x=0, 10%, 20%, 30%)were prepared by pressureless sintering process. The microstructure and properties of the samples were investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD), three-point bend tests, SENB, laser impulse thermal conductivity instrument and high-temperature expansion instrument. Owing to the partial solid solubility in the La₂Zr₂O₇-xZrO₂ ceramics, ZrO₂ exists in the form of m-ZrO₂ and t-ZrO₂. With the addition of ZrO₂, the mechanical properties of La₂Zr₂O₇-xZrO₂ ceramics first increase, then decrease. When the addition amount of ZrO₂ reaches 20%, the fracture toughness comes to the greatest value 2.41 MPa·m^1/2. When the addition amount of ZrO₂ reaches 10%, the bending strength comes to the greatest value 202.4 MPa. The thermal conductivity of the La₂Zr₂O₇-xZrO₂ ceramics is lower than YSZ. The thermal conductivity of the La₂Zr₂O₇-xZrO₂ ceramic decrease at 1400 °C as the addition amount of ZrO₂ increases, resulting from the higher intrinsic thermal conductivity of ZrO₂ than that of La₂Zr₂O₇. The thermal expansion coefficient of the La₂Zr₂O₇-xZrO₂ ceramics decreases slowly after about 350 °C and decrease rapidly at about 1000 °C, and then increase rapidly due to the transformation of m-ZrO₂ to t-ZrO₂.

Abstract: Due to its high melting point, low thermal conductivity, and good phase stability at high-temperature, rare earth zirconates material (A₂B₂O₇) will be one of the candidate materials used for the thermal barrier coating. At high temperature, the chemical stability between rare earth zirconates and TGO layer (mainly Al₂O₃) affects much the performance of coating, so the study of the chemical stability is of great significance. In this paper, (Sm_1-xLa_x)₂Zr₂O₇(x=0, 1) were prepared by the co-precipitation method and the chemical stability between (Sm_1-xLa_x)₂Zr₂O₇(x=0, 1) and Al₂O₃ was investigated at the temperature of 1 200 ° C,1 300 ° C, 1 400 ° C, 1 500 ° C, 1 600 ° C respectively for 8 h¡¢24 h. The microstructures were analyzed by scanning electron microscope (SEM) and the crystallographic phases were determined by X-ray diffraction (XRD). The results showed that the chemical reaction occurred between Sm₂Zr₂O₇ and Al₂O₃ above 1 200 ° C producing SmAlO₃. With the heating temperature increased, the content of SmAlO₃ increased. The chemical reaction between La₂Zr₂O₇ and Al₂O₃ began at 1 200 ° C producing LaAlO₃ and ZrO₂. LaAl₁₁O₁₈ appeared at 1 300 ° C. As the increase of heating temperature, the content of LaAlO₃ and ZrO₂ with tetragonal phase reduced gradually in the heated samples, and the content of LaAl₁₁O₁₈ and ZrO₂ with monoclinic phase increased gradually in the heated samples. With the increase of heating temperature, the doping amount decreases in ZrO₂ with tetragonal phase, the structure of ZrO₂ with tetragonal phase tend to be more complete. So ZrO₂ with tetragonal phase transformed to ZrO₂ with monoclinic phase in cooling process easily, which leads to the increase of ZrO₂ with monoclinic phase in the heated samples. As the time of heating temperature prolonged, the kinds of reaction products didn t change, but the amount of reaction products increased.

Abstract: In this paper, plate-like ReAl₁₁O₁₈ (Re = La, Pr, Nd) toughened ceramics were prepared by pressureless sintering at 1650 °C for 5 h in air. The bulk densities of the sintered samples were between 4.7 to 5.3g/cm³. The mechanical properties of the ReAl₁₁O₁₈ ceramics were studied systematically at room temperature. The flexure strength and fracture toughness of ReAl₁₁O₁₈ ceramics were 97.5 to 102.7 Mpa and 3.8 to 4.2Mpa.m^1/2. The results show that: The optimal temperature to synthesis ReAl₁₁O₁₈ ceramics was 1650 °C; The flexural strength and fracture toughness of the ReAl₁₁O₁₈ ceramics increase with the increasing of its bulk density.

Abstract: To study the impacts of structural change of SiO₂ aerogel on its insulating properties, the samples were treated in long time (0-12h) at 500°C, 600°C, 700°C and 800°C. The results show that with the time increased, the shrink extends in thickness, the weight loss and the density is basically unchanged under the 500°Cand 600°C. As for 700°C and 800°C, the results were treated with 0-12h, the shrink extends in thickness, the weight loss and density increases considerably. Besides, the structure and composition of SiO₂ aerogel after the different temperature treaded with 0.5h were tested. The results show that when the heat treatment temperature is between 200-800°C, the small pores in SiO₂ aerogel collapses and large pores increase, and the density is stil1 able to maintain the 1ow value due to the unbroken skeleton. When the heat temperature is between 800-1000°C, the secondary particle size increases and the rnutual integration of the secondary particles occurs, the density will be significantly increased.