Advanced environmental barrier coatings (EBCs) with a periodic layered structure consisting of two kinds of oxide materials such as Y2Ti2O7 (YT) and Al2O3, which have a large difference in refractive index, can be expected to effectively reflect thermal energy, in addition to preventing oxidation of an underling SiC/SiC composite. In the present study, a test sample containing layers of YT and Al2O3 was fabricated, and its thermal reflection properties were investigated based on measured and simulated optical reflection spectra. In the wavelength region from 500 to 2500 nm, the measured reflectance was found to be lower than the calculated value, which is thought to be due to the porosity of the YT layers produced by the sol-gel technique. Therefore, it can be expected that if all the layers in the EBC were dense, it would exhibit thermal reflection properties that were as good as those predicted by the calculations. In addition, proper arrangement of dopants in alumina layers, based on the calculated flux distribution for oxygen and Al, was thought to enhance the oxygen shielding capability and structural stability of the EBC.
Porous MgAl2O4-Al2O3 ceramic composites were fabricated by impregnating porous Al2O3 bodies calcined at various temperatures with Mg(NO3)2 aqueous solution under reduced pressure, followed by firing at 1500℃. Porous Al2O3 bodies were with continuously long-ranged pore structure was prepared by a unidirectional freeze casting using aqueous slurries and then calcined at 1300℃ 1400℃. The effects of the calcination temperature of Al2O3 bodies and the number of Mg(NO3)2 impregnation on spinel formation at 1500℃ were investigated. The amount of spinel in the bodies sintered at 1500℃ increased with increasing the number of the impregnation with Mg(NO3)2 solution and decreasing calcination temperature of Al2O3 bodies. When the Al2O3 bodies calcined at 1400℃, Mg(NO3)2 solution was difficult to penetrate into Al2O3 ceramic walls because of the densification of Al2O3 ceramic walls. Thus, spinel tended to form near the Al2O3 bodies calcined at 1300℃, Mg(NO3)2 solution penetrated deeper into Al2O3 ceramic walls. And therefore, Mg widely distributed in the Al2O3 ceramic walls and a large amount of spinel formed ever at less number of the impregnation of Mg(NO3)2 solution.
Previously, the authors reported the possibility for the use of scallop shell powder-gypsum composites as a crystallization-type phosphorus remover. However, these composites broke partially on immersing in a phosphorus solution for a prolonged time due to dissolution of gypsum. In this study, in order to improve durability of the composites in the solution, scallop shell powder-Ca(OH)2-gypsum composites were prepared and carbonated to convert Ca(OH)2 into CaCO3. The surface of the carbonated specimens were covered with dense layers composed of fine CaCO3 particles, and therefore the dissolution rete of Ca from gypsum was reduced. The thickness of CaCO3 layer increased with increasing number of carbonation. Although the carbonated specimen showed low phosphorus removal properties in the early period due to the low Ca dissolution rete, about 80% of phosphorus was removed after 10 days immersion in 50ppm phosphorus solution because Ca2+ concentration gradually increased. Moreover, the carbonation specimen had higher durability compared with the scallop shell powder-gypsum composite in the phosphorus solution.
The effect of HfO2 addition on the crystallization behavior of ZnO-Al2O3-SiO2 glasses, which shows volume crystallization, was studied. The crystallization peak temperature (TC) obtained by DTA for as cast glasses decreased with the addition of HfO2. The crystallization peak temperature also decreased by nucleation heat treatments (NHT), and the difference of TC among samples depending on HfO2 content became less with the increase of NHT time. It was found that HfO2 works as a nucleation agent because the addition of HfO2 suppressed the precipitation of SiO2 by promoting the precipitation of Gahnite(ZnAl2O4). The effect of HfO2 on nucleation is less than TiO2 or ZrO2 which are known well as nucleation agents. The low diffusion rate of Hf due to its heavier atomic weight than Ti and Zr would be a reason for the low nucleation ability of HfO2.
Aluminium titanate (Al2TiO5, AT) has a large uniaxial anisotropy of thermal expansion on the crystallographic orientation, which generates microcracks in the ceramics. Owing to microcraks, which buffer thermal expansion, AT shows especially low thermal expansion. On the other hand, microcracks remarkably decrease a mechanical strength and sometimes induce the destruction of specimens due to crack growth. In this study, AT ceramics containing various amounts of pore forming additives of different particle diameter were prepared by firing at various temperatures. The effects of contents and particle sizes of pore forming additives on microstructures and mechanical properties of AT ceramics were examined. In the case of lower contents of pore forming additives, visible large cracks causing the destruction were observed on the surfaces of the specimens and bending strengths decreased with increasing firing temperature. On the other hand, the bending strength of the specimens containing higher amounts of pore forming additives scarcely depended on firing temperatures. The pore spacings decreased with increasing additives content and decreasing particle size of additives. Therefore, large cracks causing decline of bending strength were difficult to occur in the specimens containing higher amounts of additives because crack growth was effectively prevented around pores, resulting in less decline of bending strength with firing temperature. The inhibition of crack growth was effective in the case of the additive with small particle size. As a result, the bending strength of the specimen containing the additive with small particle size was higher than that of the specimen with the large size additive.
Crack healing by high temperature oxidation in fine ceramics is an effective method to overcome their low toughness. The crack healing behavior of conventional Si3N4 was examined. The healing temperature chosen for this study was 1100℃, a temperature at which cracks were imperfectly healed. The effect of stress during crack healing was experimentally investigated based on the strength of the imperfectly healed crack. The primary focus of this study was on the investigation of healing behavior during a tensile stress, under which the oxidization of cracks was accelerated and the Si3N4 oxidized significantly because of opened cracks. The following results were obtained: (1) Healing without stress successfully enhanced both remaining strength and apparent fracture toughness of the cracked specimen at room temperature and temperatures as high as 1000℃. (2) The remaining strength when healing in air was higher than that when healing in a low pressure atmosphere. However, the difference in remaining strength between the two was relatively small. (3) The remaining strength when healing with compressive stress was as well as that without stress. (4) Both remaining strength and apparent fracture toughness of the healed specimen, with tensile healing, hardly increased. It was also concluded that the amount of oxidation did not significantly affect the healing behavior in this study.
This study was conducted to investigate the expansion characteristics of mortar specimens during water immersion test, considering “Ettringite (Ett) -generating origin” in cement paste specimens. Specifically, the expansion characteristics of mortar specimens were estimated, suggesting Ett production for each constituent mineral estimated by XRD-Rietveld and selected dissolution methods as an indicator. Results shows that the expansion characteristics of mortar specimens could require not only simple Ett production change evaluation before and after water immersion, but also the evaluation, considering the generation cite of Ettringite and coexisting materials such as C-S-H around Ettringite.
The relationship between strength development and microstructure was investigated using mortars in which various forms of calcium silicate hydrates were generated under long term autoclave curing. For the purpose of identifying the most stable form of calcium silicate hydrate for the highest strength, the mortar specimens were prepared under different conditions with the silica fume additive rate, autoclave curing time and temperature varied. All specimens cured for 10 to 100 hours exhibited the reduction of compressive strength and formation of a large amount of C-S-H. Those cured over 100 hours exhibited decreased compressive strength and crystallized calcium silicate hydrates. The results showed that the dominant factor for the strength development was the formation of C-S-H which was an amorphous calcium silicate hydrate filling the voids, and that coarsely crystallized tobermorite and tobermorite gel which were formed during the process of C-S-H generation had adverse effect on the strength development. However, no adverse effect was observed when the crystalline tobermorite and tobermorite gel were generated in a small and appropriate amount. The highest strength was achieved in the cases of a mixed presence of a large amount of C-S-H with a certain amount of crystalline tobermorite and tobermorite gel formed during the process. It was also found that the tobermorite was formed under long term autoclave curing even with the addition of silica fume. This suggests that the tobermorite may be formed even when an amorphous silica source is used.
A statistical estimation method of S-N curve for structural carbon steels using their static mechanical properties was proposed. Firstly, S-N data series for pure iron and structural carbon steels were extracted from "Database on Fatigue Strength of Metallic Materials" published by the Society of Materials Science, Japan (JSMS) and S-N curve for each material was obtained by using the JSMS standard; "Standard Evaluation Method of Fatigue Reliability for Metallic Materials -Standard Regression Method of S-N Curve-". Secondly, correlations between regression parameters and static mechanical properties were investigated. As a result, the relationships between regression parameters and static mechanical properties (e.g. fatigue limit E and static tensile strength σB) have showed significant correlations. Based on these correlations, S-N curves for structural carbon steels were successfully predicted from their static mechanical properties. Moreover, using the distribution of fatigue limit E, the percentile points for the predicted S-N curve were also estimated. It was finally confirmed that 81% of S-N data series fall within the range of estimated interval between -3s and +3s, where s means a standard deviation.
This paper presents general solutions for piezoelectric materials and anisotropic materials with elliptical cavity subjected to uniform loads at infinity. General solutions are provided by using complex analytical functions based on Lekhnitskii formalism. In the analysis, similarities of fundamental equation between piezoelectric materials and anisotropic materials are shown. By defining new similarity-based complex functions, expressions of mechanical and electrical quantities are derived by analogous form. And distributions of stress, electric displacement, strain and electric field around elliptical cavity are shown by contour plot. Furthermore, it is shown that theory of anisotropic material is applicable to piezoelectric material by adequately replacing material constants.