The design of processing procedures for ceramics should be based on an accurate knowledge of the packing structures in the green compacts, as these govern the structures of the ceramics after sintering and thus their properties. We examined packing structures by a variety of methods and we studied processing methods for improving these structures. Granules in dry-pressed compacts were examined by confocal laser scanning microscopy. The humidity during pressing and the type of binder significantly affect the deformation behavior of the granules and thus the structure of the compact. The strength of the sintered body increases with increasing homogeneity of the green body. The structures of the particle orientation were also examined by polarized-light microscopy of green compacts made by slip casting. The degrees of orientation were quantitatively evaluated through the measurement of retardation. The anisotropy thus evaluated was dependent on the particle shape and the draining condition of water during shaping. Particles of near-spherical shape produced homogeneous structures.

View full abstract

High-quality epitaxial film growth for several complex oxides having layered structure by “Reactive Solid-Phase-Epitaxy (R-SPE¹⁾)” is reviewed. Materials focused are InGaO₃(ZnO)_m (m=integer), LaCuOS and Na_xCoO₂ (x~0.8). For the epitaxial film growths of the complex systems with a complicated layered crystal structure, it is necessary to form an epitaxial template layer before the film deposition, followed by the thermal annealing of the bilayer films. The layers subsequently deposited on the template layer may be amorphous, polycrystalline or powder, which allows control of the epitaxially grown films in the R-SPE method.

View full abstract

The doping dependence of high temperature plastic flow behavior in TiO₂-doped, GeO₂-doped and TiO₂ and GeO₂ co-doped tetragonal ZrO₂ polycrystal (TZP) was systematically examined at 1400°C in air under an initial strain rate of 1.3×10^-4 s^-1. The amount of dopant cations was in the range of 0.2-4 mol%. The flow stress in the TZP decreases with the increasing doping amount in TiO₂ and/or GeO₂-doped TZP, but levels off over a 3 mol% addition in the GeO₂-doped and TiO₂-GeO₂ co-doped TZP. The doping dependence of the flow stress in the TiO₂-GeO₂ co-doped TZP is close to that in the GeO₂-doped TZP rather than to that in the TiO₂-doped TZP; the flow stress in the GeO₂-doped and TiO₂-GeO₂ co-doped TZP is significantly lower than that in the TiO₂-doped TZP. The doped cations tend to segregate in the vicinity of the grain boundaries in TZP. The doping dependence of the flow stress is probably related to the amount of the grain boundary segregation of the dopant cations; the grain boundary segregation of the dopant cations is supposed to enhance the grain boundary atomic diffusion in the TZP, and hence the flow stress consequently decreases. On the other hand, elongation to failure in the TZP is highly improved by the TiO₂ and/or GeO₂ doping. The improved high temperature ductility of the TZP is attributed to the flow stress reduction due to the TiO₂ and/or GeO₂ doping.

View full abstract

Yttria-stabilized zirconia (YSZ, Zr_1-xY_xO_2-x/2) and titania (TiO₂) films were prepared by laser chemical vapor deposition (LCVD). The deposition rate increased drastically by laser irradiation, and reached to 660 μm/h for YSZ and 2300 μm/h for TiO₂ films. The increase of the deposition rate was resulted from plasma formation around the deposition zone, and the plasma was formed at high laser power and high substrate pre-heating temperature. Various morphologies of films such as feather-like columnar structure and dense microstructure were obtained depending on deposition conditions. The columnar structure contained a large amount of nano-pores at columnar boundary and in grains.

View full abstract

Hexagonal boron nitride (h-BN) dispersed nanocomposites were fabricated through a chemical process involving sintering of matrix ceramic powders covered partly with turbostratic BN (t-BN), which was synthesized by the reduction of a mixture of boric acid and urea precipitating on the matrix ceramic powder during drying after wet-ball milling. Silicon carbide (SiC), silicon nitride (Si₃N₄), alumina (Al₂O₃) and aluminum nitride (AlN) were employed as matrix for h-BN dispersed nanocomposites. The nano-sized h-BN particles were found to be homogeneously dispersed within the matrix grains as well as at grain boundaries, which is typical of nanocomposite structures. As a result, the present nanocomposites showed unique and excellent properties such as high strength, low Young's modulus, high thermal shock resistance, and good machinability.

View full abstract

In the system of CaO-SiO₂-H₂O, there are three crystalline compounds with calcium/silicon ratio of 2.0, α-dicalcium silicate hydrate (α-C₂SH), hillebrandite, and dellaite. In this paper, these three compounds were synthesized and their stability and phase transformation were investigated under hydrothermal conditions up to 400°C. Hydrothermal treatments of α-C₂SH indicated that Ca-rich jaffeite (C/S=3.0) and reinhardbraunsite (C/S=2.5) were formed as intermediate phases before complete transformation to dellaite at 350°C for 1 d. The phase transformation from α-C₂SH to hillebrandite was not observed. On the other hand, a reversible phase transformation from dellaite to hillebrandite was observed at low temperatures below 300°C with formation of a small amount of the C-S-H compounds with C/S ratio of 1.5. The fact that α-C₂SH was not formed form dellaite suggests that α-C₂SH is a metastable phase.

View full abstract

The optical property of Li₂O-Al₂O₃-SiO₂ glass-ceramic, with a near-zero thermal expansion coefficient, crystallized under high pressure has been studied. A hydrostatic pressure of 196 MPa was applied by a hot isostatic pressing (HIP) technique during the nucleation process and the crystal growth process. The temperature coefficient of the optical path length, 1/l•dS/dT, decreased by 0.6×10^-6 K^-1, because the temperature coefficient of the refractive index, dn/dT, was decreased by the HIP treatment. The decrease in dn/dT was ascribed to the suppression of the temperature coefficient of the molar polarizability, φ, of the glass-ceramic crystallized by the HIP treatment. It was concluded that the suppression of φ occurs in the crystalline phase during the HIP treatment, based on the fact that the reduced 1/l•dS/dT was maintained even after the relaxation of the densified structure in the glass phase upon thermal annealing. The decrease in the number of lattice defects in the crystalline phase was hypothesized to be a cause of the reduced φ.

View full abstract

Dense submicron-grained alumina ceramics were fabricated by pulse electric current sintering (PECS) using Mg²⁺-doped transition alumina powders at 1200-1350°C under a uniaxial pressure of 40 or 80 MPa. The Mg²⁺-doped transition alumina powders (0-0.50 mass% MgO base) were prepared through a new sol-gel route using high-purity polyhydroxoaluminum (PHA) and MgCl₂ solutions as starting materials. The composite gels obtained were calcined at 900°C and ground by planetary ball-milling. Upon heating, the composite gels transformed into a single-phase γ-alumina or mixed phase of γ- and χ-aluminas, depending on the MgO content. The resultant transition alumina powders were solid solutions, in which Mg²⁺ cations were substituted into the crystal lattice. The powders were re-calcined to increase the content of α-alumina particles, which act as seeding for low-temperature densification. Densification depended on the MgO content and loading pressure. The critical Mg²⁺-doping for suppressing grain growth was found to be 0.10 mass% MgO. Higher loading pressures led to full densification at lower temperatures, resulting in a more uniform and finer microstructure. Thus, dense alumina ceramics (relative density≥99.6%) with a uniform microstructure composed of fine grains with an average size of 0.47 μm could be obtained by PECS at 1250°C under 80 MPa.

View full abstract

A ceramic coating slurry was designed, which suspended or dissolved ceramic components (silica, titania, alumina and kaolinite) and glass-forming components (alkali silicates and soda ash) in water. SiO₂-Al₂O₃-TiO₂ based composite films (about 100 μm thick) were deposited on a steel plate by coating the slurry with a brush, followed by drying and calcination. The ceramic coatings obtained under proper conditions were found to exhibit excellent stability over a wide range of temperature. As evaluated by copper electrodeposition method, through-holes were present in the coatings calcined at 700°C or below, while they were eliminated completely after calcination at 1100°C in N₂. Vickers hardness of the coatings increased with increasing calcination temperature, while adhesive strength went through a sharp maximum at 800°C.

View full abstract

The effect of a small amount of K₂O on the crystallization behavior of the system of Li₂O-SiO₂ glass was investigated. Lithium disilicate (Li₂O•2SiO₂; L•2S) and lithium metasilicate (Li₂O•SiO₂; L•S) crystals precipitate simultaneously, but only L•2S crystals grow upon further heat treatment in the glass without K₂O (Li glass). In the glass containing K₂O (K3 glass), only L•S crystals precipitate and grow. The stable crystalline phases are L•2S and SiO₂ crystals in both glasses. Phase separation was observed before crystallization; a Li₂O-rich phase formed an isolated droplet phase in Li glass and the continuous phase in K3 glass. The Li⁺-K⁺ ion exchange test confirmed these structures. [SEM-EDX] analysis confirmed that K₂O exists in the Li₂O rich phase after phase separation in K3 glass. Thus, K3 glass separates into two phases, one of which is a continuous phase rich in Li₂O and containing considerable amount of K₂O. The L•S crystal precipitates and grows in this phase upon further heat treatment. Consequently, K₂O suppresses the crystallization of L•2S and promotes the precipitation of L•S crystals.

View full abstract

Cordierite has structural channels and alkaline elements can be incorporated into the channel. In order to study the incorporation effect of the alkaline and alkaline earth elements into the channel on structural and electronic properties, density functional theory calculations were performed to analyze the structural changes associated with the incorporation of alkaline and alkaline earth elements. It was found that alkaline and alkaline earth metals can be stable incorporated in the channel. Good correlation between the calculated ionic radius and the first ionization energy was observed. It was also found that the incorporated elements occupy a specific site in the channel. Incorporated elements cause the expansion of the lattice in c-axis because of the repulsive and attractive forces, interaction of the incorporated elements with the framework cations and oxygen atoms, respectively. The incorporated element at the less symmetrical sites in the channel was found to cause the more remarkable distortion.

View full abstract

Zeolities A and P were successfully prepared by hydrothermal treatments at 60-120°C in the presence of NaOH solutions from waste incineration fly ash as a starting materials, pretreated with acid in addition to water washing and heating. The pretreatment was an important process to reproducibly convert fly ash into zeolites A and P. The relations between product fiels of zeolites A and P and hydrothermal conditions were examined. Consequently, zeolites A and P were successfully synthesized by treatments with 1.0-2.0 M NaOH solutions at 60°C for 20-48 h and with 0.5-2.0 M solutions at 80-100°C for 20-48 h, as major products, respectively. The treatment led the generation of sodalite with increasing NaOH concentration. The hydrothermal products exhibited removal characteristics for NH₄ ⁺ ions, and their uptake amounts were found to depend on zeolites A and P yields.

View full abstract

An electrothermal vaporization (ETV)/inductively coupled plasma mass spectrometry (ICP-MS) technique has been developed to determine the ultratrace metallic impurities of 10 elements (Na, Mg, Al, Cr, Mn, Fe, Ni, Cu, Zn, and Pb) in the natural oxide layer on a Si wafer. SiO₂ reacts to form SiF₆ ^2- with hydrofluoric acid in vapor-phase decomposition. The pyrolysis temperature (150°C) for eliminating SiF₆ ^2- was added to the ETV operating conditions, because the existence of ²⁸Si, ²⁸Si₂, and ²⁸SiO₂ due to SiF₆ ^2- influences the ion intensities of ²⁷Al, ⁵⁶Fe, and ⁶⁰Ni, respectively. Furthermore, adding 25 ng of a palladium modifier improved the linearity of each calibration curve. As a result, it was possible to determine the 10 elements in concentration ranges of 20 (40)-10000 pg ml^-1. The recovery percentages obtained by adding the mixed standard solution to the sample solution were 95-105%, showing that the ETV/ICP-MS measurements are accurate. The detection limit (3σ) was approximately 10⁶ (Pb)-10⁸ (Na) atoms cm^-2 for a Si wafer with a diameter of 130 mm. This method can be applied to the above wafer to determine the 10 metallic elements (10⁹-10¹¹ atoms cm^-2).

View full abstract

The effects of alumina hydrates, namely boehmite gel and bayerite, formed by hydration of hydraulic alumina (HA) on the green strength and microstructure of porous alumina ceramics were studied. A boehmite gel (3-D) network was formed at an early stage of the hydration of HA in HA slurry. The HA slurry was hardened by the formation of this 3-D network. Even without the addition of an organic binder, green bodies containing the boehmite gel network retained their original shapes and demonstrated high compressive strengths. On the other hand, another alumina hydrate, bayerite, was formed in the cured samples (green bodies) following hydration for 24 h. A green body cured for 168 h was composed of bayerite grains and a small amount of the boehmite gel 3-D network. Although the green bodies had no defects and retained their original shapes, the compressive strengths decreased with increasing amount of bayerite. The shrinkage of the bayerite-containing samples upon heating was slightly larger than that of the samples without bayerite. Nevertheless, no significant difference was observed in the microstructures of the sintered bodies. These results suggest that it is preferable to use the green body without bayerite in preparing the porous alumina ceramics.

View full abstract

Rapid microwave drying was examined in ZnO ceramic for slip cast bodies and the characteristics of such bodies were compared with those of cast bodies dried by conventional drying techniques. The extent of warpage of a dried body is smaller in microwave drying than in other conventional drying methods. Clearly, the homogeneous heating by microwave contributed to uniformities in the temperature and water content of a green body during drying. The relative density of sintered bodies reached about 98% and the microstructure was uniform for microwave drying. Furthermore, the number and size of pores are smaller for this drying technique than for other methods. Microwave heating has merits in terms of both the rapid drying and structural uniformity of dried bodies. It not only reduces drying period, but also improves the characteristics of a sintered body.

View full abstract

Consolidation of SiC powder with stacking disordered structure was accelerated by milling with the AlN as the additive. SiC powders without the additive were sintered at 1900°C to a density of 99%. The grain size of the sintered material was approximately 500 nm. On the other hand, with AlN as additive, the sintering temperature to reach a density of 98% was reduced to 1700°C by the addition of 1.0 mol% AlN. The grain size of this material was approximately 50 nm. The effect of AlN addition on SiC sintering with disorder-order transformation was evaluated by shrinkage profile, XRD analysis, and TEM observation.

View full abstract

The group velocity dispersion of Bi₂O₃-based optical fiber is obtained in optical telecommunication wavelength. The material dispersion of Bi₂O₃-based glass and the waveguide dispersion of Bi₂O₃-based optical fiber are estimated from the wavelength dependence of refractive index. It is shown that Bi₂O₃-based optical fiber exhibits a large group velocity dispersion induced by the strong wavelength dependence of refractive index. The experimental result shows a good agreement with the numerical calculation, indicating that this dispersion analysis on Bi₂O₃-based glass can be applied to the dispersion management of Bi₂O₃-based optical fiber.

View full abstract