Hideo Toraya

Bio: Hideo Toraya is an academic researcher from Tokyo Institute of Technology. The author has contributed to research in topics: Hydrothermal circulation & Monoclinic crystal system. The author has an hindex of 9, co-authored 20 publications receiving 1017 citations.

Calibration Curve for Quantitative Analysis of the Monoclinic‐Tetragonal ZrO2 System by X‐Ray Diffraction

Abstract: A nonlinear calibration curve for volume fraction vs integrated intensity ratio was obtained for the monoclinic-tetragonal ZrO2 system by using X-ray powder pattern-fitting and pattern-decomposition techniques. The empirical equation agrees well with the theoretical one. By using this equation, the deviation from linearity (6.8% maximum) resulting from the intensity difference of corresponding reflections of the two phases can be estimated quite accurately.

Quantitative Analysis of Monoclinic‐Stabilized Cubic ZrO2 Systems by X‐Ray Diffraction

Abstract: An equation for a nonlinear calibration curve of volume fraction vs integrated intensity ratio is presented for monoclinic-stabilized cubic ZrO2 systems containing YO1.5, CaO, and MgO. A parameter in the equation was evaluated theoretically and is given for a range of cubic solid solution compositions.

Preferred orientation correction in powder pattern-fitting

Abstract: An appropriate form of the preferred orientation correction has been derived for the powder pattern-fitting based on data of the powder diffractometry with a flat specimen holder. The seven powder specimens with different average particle sizes were prepared by grinding the synthetic crystals of taeniolite, KLiMg2Si4O10F2. The intensity data were collected on an X-ray powder diffractometer with CuKα radiation, employing step scan technique. The preferred orientation factor was estimated for each reflection of the respective specimens by comparing the observed intensity with the calculated value after scaling by the least-squares procedure. The factor shows a dependence upon the acute angle φ between the preferred orientation direction and the scattering vector. To obtain an analytical expression of the correction factor, several types of functions were examined by trying to correct the effect of preferred orientation with least-squares calculations. The preferred orientation effect was satisfactorily corrected with a function of the form, p(φ)=P1+(1−P1)·exp(−P2·φ2), where P1 and P2 are parameters determined by least-squares fitting.

Hydrothermal Oxidation of Hf Metal Chips in the Preparation of Monoclinic HfO2 Powders

Abstract: Fine monoclinic HfO2 powders were prepared from Hf metal chips by reaction with high-temperature high-pressure water. Dependence of the reaction on temperature-pressure-time was examined in both closed and open systems. Hydrothermal oxidation of Hf proceeded through three stages: the surface oxidation of Hf metal chips with H2O, the reaction of the Hf with released H2 to form hydrides, and the succeeding oxidation of the hydrides with H2O to form HfO2. Most of the fine HfO2 powders were, however, considered to be formed by the latter two reactions after the cracking and pulverizing of metal chips associated with hydriding. The difference in the reaction rates between the closed and open systems was explained by taking into account the fugacity relations of H2 and H2O in the respective systems.

Hydrothermal Reaction-Sintering of Monoclinic HfO2

Abstract: Sintered monoclinic HfO2 bodies were fabricated below the transformation temperature from Hf metal and water by hydrothermal reaction-sintering. Sintering was observed above 900°C under 100 MPa for 3 h. Generally, both the bulk density and the crystallite size of the sintered bodies increased with increasing temperature. Bodies with the maximum relative density (0.98) were obtained by treatment above 1000°C.

Low-Temperature Degradation of Zirconia and Implications for Biomedical Implants

Abstract: This review describes the mechanisms responsible for low-temperature degradation (LTD) of zirconia ceramics and its detrimental consequences for biomedical devices. Special emphasis is given to the critical issue of zirconia degradation actually observed for hip prostheses. Experimental methods to accurately measure and predict LTD in a given zirconia ceramic are presented. Different solutions to inhibit LTD or at least reduce its kinetics are reviewed, with the objective of highlighting alternative options for the generation of new zirconia-based biomedical ceramic devices.

Structure and Mineralogy of Clay Minerals

Abstract: Phyllosilicates, and among them clay minerals, are of great interest not only for the scientific community but also for their potential applications in many novel and advanced areas. However, the correct application of these minerals requires a thorough knowledge of their crystal chemical properties. This chapter provides crystal chemical and structural details related to phyllosilicates and describes the fundamental features leading to their different behaviour in different natural or technical processes, as also detailed in other chapters of this book. Phyllosilicates, described in this chapter, are minerals of the (i) kaolin-serpentine group (e.g. kaolinite, dickite, nacrite, halloysite, hisingerite, lizardite, antigorite, chrysotile, amesite, carlosturanite, greenalite); (ii) talc and pyrophyllite group (e.g. pyrophyllite, ferripyrophyllite); (iii) mica group, with particular focus to illite; (iv) smectite group (e.g. montmorillonite, beidellite, nontronite, saponite, hectorite, sauconite); (v) vermiculite group; (vi) chlorite group; (vii) some 2:1 layer silicates involving a discontinuous octahedral sheet and a modulated tetrahedral sheet such as kalifersite, palygorskite and sepiolite; (viii) allophane and imogolite and (ix) mixed layer structures with particular focus on illite-smectite.

Hafnia and hafnia-toughened ceramics

Abstract: Hafnia (HfO2) and hafnium-based materials are traditionally regarded as technologically important materials in the nuclear industry, a consequence of their exceptionally high neutron absorption coefficient. Following the discovery of transformation toughening in the mid 1970s, a considerable research effort has been devoted to zirconia (ZrO2)-toughened ceramics (ZTCs). They are considered to be potentially useful materials for structural applications at low and intermediate temperatures (T 1000 °C) is related to the low temperature of the tetragonal to monoclinic transformation in ZrO2. On the basis that HfO2 exhibits a similar crystal structure and in particular that its tetragonal to monoclinic transformation temperature (∼1700 °C) is approximately 700 °C higher than that for ZrO2, it has been suggested that high-temperature transformation toughening could be possible in HfO2-toughened ceramics (HTCs). Although the concepts behind this suggestion are universally appreciated, only a limited success has been made of the fabrication and the microstructural and mechanical property evaluation of these materials. The fracture toughness values obtained so far in HfO2 toughened ceramics are, in fact, considerably lower than those obtained in their ZrO2 counterparts. A great deal of further research work is therefore required in order to understand fully and to exploit toughened ceramics in the HfO2-based and HfO2-containing systems. This review covers the science and technology of HfO2 and HfO2-toughened ceramics in terms of processing, phase transformation, microstructure, and mechanical properties.

Chapter 2 Structures and Mineralogy of Clay Minerals

Abstract: Publisher Summary This chapter describes structures and mineralogy of clay minerals. Phyllosilicates considered in this chapter ideally contain a continuous tetrahedral sheet. Each tetrahedron consists of a cation, T, coordinated to four oxygen atoms and linked to adjacent tetrahedra by sharing three corners (the basal oxygen atoms, Ob) to form an infinite two-dimensional hexagonal mesh pattern along the a, b crystallographic directions. The free corners (the tetrahedral apical oxygen atoms, Oa) of all tetrahedra point to the same side of the sheet and connect the tetrahedral and octahedral sheets to form a common plane with octahedral anionic position Ooct. Ooct anions lie near to the center of each tetrahedral 6-fold ring, but are not shared with tetrahedra. The 1:1 layer structure consists of the repetition of one tetrahedral and one octahedral sheet, while in the 2:1 layer structure one octahedral sheet is sandwiched between two tetrahedral sheets. In the 1:1 layer structure, the unit cell includes six octahedral sites (i.e., four cis and two trans-oriented octahedral) and four tetrahedral sites. Six octahedral sites and eight tetrahedral sites characterize the 2:1 layer unit cell. Structures with all the six octahedral sites occupied are known as “trioctahedral.” If only four of the six octahedra are occupied, the structure is referred to as “dioctahedral.” The structural formula is often reported based on the half unit-cell content—that is, it is based on three octahedral sites.