High-purity mullite ceramics, promising engineering ceramics for high-temperature applications, were fabricated using transient liquid phase sintering to improve their high-temperature mechanical properties. Small amounts of ultrafine alumina or silica powders were uniformly mixed with the mullite precursor depending on the silica-alumina ratio of the resulting ceramics to allow for the formation of a transient liquid phase during sintering, thus, enhancing densification at the early stage of sintering and mullite formation by the reaction between additional alumina and the residual glassy phase (mullitization) at the final stage of sintering. The addition of alumina powder to the silica-rich mullite precursor resulted in a reaction between the glassy silica and alumina phases during sintering, thereby forming a mullite phase without inhibiting densification. The addition of fine silica powder to the mullite single-phase precursor led to densification with an abnormal grain growth of mullite, whereas some of the added silica remained as a glassy phase after sintering. The resulting mullite ceramics prepared using different powder compositions showed different sintering behaviors, depending on the amount of alumina added. Upon selecting an optimum process and the amount of alumina to be added, the pure mullite ceramics obtained via transient liquid phase sintering exhibited high density (approximately 99%) and excellent high-temperature flexural strength (approximately 320 MPa) at 1500 °C in air. These results clearly demonstrate that pure mullite ceramics fabricated via transient liquid phase sintering with compositions close to those of stoichiometric mullite could be a promising process for the fabrication of high-temperature structural ceramics used in an ambient atmosphere. The transient liquid phase sintering process proposed in this study could be a powerful processing tool that allows for the preparation of superior high-temperature structural ceramics used in the ambient processing atmosphere. 相似文献
Behaviormetrika - In this paper, we propose a new clustering method based on the concept of maximum likelihood (ML) estimation. In general, the problem of local minima arises when we try to use the... 相似文献
In this paper, we propose a new hierarchical clustering method, which is useful to find appropriate clusters of attributes from given dichotomous or frequency data. Important features of our method are 1) the similarity between two attributes is defined as a probability of their pattern vectors being observed under the hypothesis of independence, 2) for each generated cluster, one pattern vector is defined in a natural manner, and 3) it can be used freely without distinguishing the frequency data from the dichotomous one.
A typical frequency data is analyzed to illustrate how our method works effectively. The discussion on similarities among objects is also included to propose a new similarity measure based on our clustering method.
Here various approaches to understanding the complex relationship among the local atomic arrangements, the distribution of charge/holes and the superconductivity characteristics are discussed for a variety of multi-layered copper-oxide superconductors. Regarding the structural parameters, i.e. bond lengths and angles, no single parameter is revealed to control the superconductivity characteristics alone. As a matter of fact, superconductivity properties are found to be correlated with the whole delicately balanced fine-structure that is described by a set of multiple parameters: it is shown that Tc and Hirr are enhanced in somewhat opposite ways. Expressing the delicately balanced fine-structure through the charge/hole distribution, it is concluded that the more homogeneously distributed the charge carriers are the more enhanced is the Hirr characteristics, while the opposite is likely to apply to the Tc value. 相似文献
Three grain-shape parameters, f1, f2, and f3, are defined to clarify the morphological effect of grains on the high-temperature creep resistance under the condition that
no grain boundary cavitation occurs. The parameter f1 is characteristic of complicated grain shapes, suggesting that it can be a measure of the interlocking grain structure. The
parameter f2 is an important parameter when torsional stresses are imposed on coiled wires, and f3 is characteristic of the short-range roughness of grain boundaries only when f1 is not greatly changed. The minimum creep rate decreases as the grain aspect ratio, R, increases for R <30, and the creep rate increases as R increases for R >30. The parameter f1, as well as Raj and Ashby’s model, gives a satisfactory explanation for the former behavior. On the other hand, it is proven
that their model must be modified using f3 to explain the latter behavior because of the highly elongated grain configuration that is associated with R >30. 相似文献
Porous and electrically conducting carbon-clay composites were prepared by firing mixtures of carbon powder (0 to 20 wt%) and clay minerals. They showed porosity of 50 to 65% and had high mechanical strength (compressive strength = 130 to 400 kg cm–2) as well as high resistance against thermal oxidation in air. Their electrical conductivity, , increased with increasing carbon content levelling off at about 20 wt% of carbon content to give a value of about 2 S cm–1. Formation of carbon chains is considered to be responsible for the electrical conduction in the composite, and a model to correlate the electrical conductivity with the carbon content has been proposed by modifying a model previously proposed by Scarisbrick. 相似文献
A mist of a hydrosol consisting of silica, alumina/NaAlO2 and sodium hydroxide was produced by a supersonic atomization, and treated successively in three furnaces of different temperatures. The temperatures of the furnaces were adjusted for the evaporation of water, the dehydration and the crystallization of the mixed oxide, respectively. The spherical particles ( 0.5m) of carnegieite were found to be formed in a narrow composition range of the raw materials at temperatures of 650 to 900° C. The factors affecting the properties of the particles were investigated. 相似文献
The effects of support and additive on the oxidation state and catalytic activity of Pt catalyst in the low temperature propane combustion were systematically investigated on Pt/MgO, Pt/Al2O3 and Pt/SiO2–Al2O3. The catalytic activity varied much with both support materials and additives. The catalyst on the more acidic support showed higher activity, and the catalytic activity on every support materials increased as the electronegativity of additives increased, while some additives decreased the activity. The oxidation state of platinum, estimated by white line intensity of Pt LIII-edge XANES spectrum, also varied with the support and additives, and additives with higher electronegativity greatly prevented the platinum from its oxidation under oxidising atmosphere. Among almost all the catalysts with various supports and various additives, a clear relationship was observed between the oxidation state of platinum and the catalytic activity; the more metallic platinum showed higher activity. Thus, it was concluded that the total electrophilic/electrophobic property derived from those of the support and additive controls the oxidation state of platinum, which intensively affects the catalytic activity; i.e. higher electrophilic property provides less oxidised platinum, resulting in high catalytic activity. The mechanism of this effect was also discussed on the basis of thermochemical data, and it was proposed that the electrophobic materials promote the noble metal oxidation since the noble metal oxo-anion such as PtOδ− is more stabilised with electrophobic cation. 相似文献
Titanium dioxide and zinc oxide films were fabricated by spin-on and dip-coating methods. Both types of films exhibited columnar grains when the single coating was thin, ∼10 nm. The columnar TiO2 films were dense, as confirmed by their density values calculated from the refractive index and TEM results. The addition of Al cations into the ZnO suppressed grain growth, because Zn had a lower diffusivity in the doped films. The doped ZnO films had nearly the same electrical resistivity as that of the undoped films. 相似文献