Leucite (KAlSi2O6) glass-ceramic dental prostheses are fabricated by firing glass-ceramic powder into the shape of a tooth crown on an abutment-tooth frame made from a precious metal alloy. It is known that, if this alloy contains silver, the prosthesis suffers from an unattractive yellow discoloration. Although the addition of antimony trioxide to the glass-ceramic mixture has been shown to reduce such yellowing, this compound is believed to be toxic to humans. In this study, we investigate yellowing with the addition of cerium oxide, which is a promising replacement for antimony trioxide due to its low toxicity to biological organisms. Using scanning electron microscopy, energy dispersive X-ray spectrometry, and ultraviolet and visible spectrophotometry, we analyze the fine-grained structure and composition of the fired glass-ceramic body that comes into contact with silver, as well as the nature and extent of the resulting yellowing. Our findings indicate that, as previously thought, the yellowing is indeed caused by the diffusion of silver ions into the fired glass-ceramic body to form colloids, but they also reveal a contribution from sodium at the interface between the fired body and the silver. To investigate methods suppressing the yellowing, we fabricate several model glasses with different sodium-to-cerium ratios, and study the nature and extent of the yellowing for each glass. Our results reveal that the extent of yellowing is correlated with increased sodium content and that the yellowing may be suppressed by adjusting the sodium-to-cerium ratio.
A new chemical vapor deposition process for coating SiC is proposed in which gaseous SiO and toluene vapor are reacted to generate SiC in the presence of iron oxide as a catalytic component. At a temperature of 1450°C, SiC can be formed substantially from gaseous SiO and toluene vapor only at positions of iron oxide deposition. When iron oxide is deposited as a coating on a substrate, SiC can also be formed as a coating on the substrate by calcinating at 1450°C while supplying gaseous SiO and toluene vapor. In this process, SiC coating of several tens µm in thickness is formed on the substrate, and fibrous material is also formed on the SiC coating. The fibrous material is composed of fibrous SiC as well as an Fe-containing spherical substance and can be easily removed mechanically from the SiC coating. This process for coating SiC is based on a vapor–liquid–solid mechanism.
High-quality forsterite ceramics were successfully prepared using desert drift sands to replace the traditional materials at lower temperatures, which not only saves mineral resources but also greatly reduces the cost of ceramic. Meanwhile, the effects of two different proportions of drift sands on the phase formation, physical and mechanical properties of ceramics were also investigated. The experimental results show that drift sands with impurity cations have the lower melting temperature compared to the pure quartz and easy to form liquid phase. As a result, the formation temperature (800°C) of forsterite phase falls by 300°C compared to the forsterite ceramic prepared using analytical reagent (1100°C). Furthermore, drift sands as raw materials can effectively improve the physical and mechanical properties of forsterite ceramics. Finally, the content of forsterite phase can reach up to 95.70 wt %, and the obtained forsterite ceramics show the best physical and mechanical properties when sintered at 1300°C for 2 h.
Amine-functionalized alumina colloidal particles were prepared using polyethyleneimine (PEI) for enhancing the infiltration process of silica based ceramic core. The surface properties of PEI-grafted alumina particles were adjusted to improve their dispersion behavior in aqueous suspension, and the surface charge was adjusted to enhance their adsorption behavior, in neutral pH atmosphere, via an electrostatic repulsion force with a fused silica matrix containing an implied strong negative charge. The characteristics of the surface-modified particles were optimized as shown by the C–H stretching vibration and N–H bending vibration from FT-IR spectra, and the surface charge alternated from −3 to 45 mV. As a result, the dispersion behavior in aqueous solvent was advanced as indicated by the decrease of the polydispersity index, decrease of the average size from 178 to 58 nm, and sharpness of the particle size distribution parabola. These improvements greatly contributed to the acceleration of the infiltration time and efficiency (up to 30 min). Consequently, the promptly infiltrated specimens improved the flexural strength from 3.2 to 10.2 MPa and reduced the linear shrinkage rate from 1.91 to 1.05% as well.
Magnesium aluminate spinel (MgAl2O4) nanopowders were synthesized via nonhydrolytic sol–gel route using anhydrous ethanol, aluminium chloride and magnesium chloride as raw materials. The samples were characterized using differential thermal and thermogravimetric analyses, X-ray diffraction, Fourier transform infrared spectroscopy, Brunauer–Emmet–Teller analysis, dynamic light scattering, scanning electron microscopy and transmission electron microscopy. The results showed that the powders with a single MgAl2O4 spinel phase were obtained after annealing the gel precursor at 900°C, and their average particle size and BET specific surface area were about 30 nm and 43.57 m2/g. In addition, the synthesized MgAl2O4 spinel nanopowders possessed mesoporous structure with the pore diameter between 3–50 nm. The MgAl2O4 spinel nanopowders also exhibited superior sintering properties.
The oxide-ion conduction mechanism in apatite-type lanthanum germanate, La10(GeO4)6O3, was theoretically investigated by the nudged elastic band method based on the first-principles calculations and the kinetic Monte Carlo (KMC) method. The dominant conduction process is the cooperative mechanism along the c axis with the lowest potential barrier of 0.64 eV, which is not reported previously in the lanthanum germanate system. The other responsible process was a different type of cooperative mechanism connecting the fast conduction channels with the calculated potential barrier of 0.76 eV. The oxide-ion conductivity and apparent activation energy of 0.71 eV in the crystal was statistically estimated by KMC simulations, which is coincident with the experimentally measured oxide-ion conductivities.
We studied ZnO films grown by rf sputtering using Zn metal targets. During the growth the metal target can be at a metal or at an oxide mode, depending on oxidation of the target surface. At a metal mode the target surface is free of oxide, and the sputtering yield is higher, but deposited ZnO films show poor transistor characteristics. ZnO films deposited at an oxide mode show better transistor characteristics, but the sputtering yield is lower. In order to solve these problems, we supplied oxidizer gas as pulses during the growth. We hoped that the target condition could be controlled by varying parameters of the pulses. Our ZnO was grown at 450°C using CO2 or O2 as an oxidizer. After sputtering growth ZnO films were annealed in mixture of CO2 and H2 at 400°C. With these methods, bottom-gate ZnO thin-film transistor showed 6.5 cm2/Vsec mobility, 5 × 106 on/off ratio, and −5 V threshold voltage.
The effect of the melting temperature on the cerium oxidation state and crystallization of cerium phosphate glasses with a molar composition 30CeO2–70P2O5 was investigated. Ce3+ and Ce4+ ion concentration changes in the glass attributable to the melting temperature were investigated by X-ray photoelectron spectroscopy analysis. The crystallization kinetics of the glasses and the activation energy for crystallization were evaluated under non-isothermal conditions using differential thermal analysis (DTA) performed at different heating rates. Each DTA curve exhibited one exothermic peak associated with the crystallization of the glass. The crystalline phase was identified as CePO4 via X-ray diffractometry analysis. The Kissinger and Marotta methods were used to calculate the local activation energies for the glass samples. The amount of precipitated CePO4 with heating at 890 K increased as the melting temperature (or Ce3+) increased. The catalytic properties were studied by thermogravimetric analysis, which showed that a greater amount of precipitated CePO4 led to poorer catalytic properties of the glass.
Mg-doped ZnO nanoparticles with different Mg contents were synthesized by sol–gel method with subsequently calcination at 600°C for 2 h. The X-ray diffraction results revealed the presence of wurtzite pure ZnO with hexagonal structure in both the undoped and doped samples. The crystallite size of the nanoparticle samples was calculated by Scherrer formula, 92 nm for ZnO and 43 nm for 5 wt % Mg-doped ZnO. Raman spectra of pure and Mg-doped ZnO show peak at 437 cm−1 assigned to ZnO non-polar optical phonon high E2 mode. The size of Mg-doped ZnO characterized by transmission electron microscopy is much smaller than that of pure ZnO. The photocatalytic properties of ZnO and Mg-doped ZnO were investigated by monitoring the methylene blue degradation under UV radiation. For the present study, the 5 wt % Mg-doped ZnO shows the highest photocatalytic activity of 92% within 300 min.
Borosilicate glass crystallized into the borate phases GdBO3:Tm3+ and GdF3:Tm3+ was successfully prepared using the phase separation phenomenon. By optimizing its composition, the glass was ‘binodal’ phase separated into borate droplets inside a silica matrix. The lanthanide oxide and fluoride phosphors were restrictively created in the borate phase and an emission attributable to 1D2→3H4 transition of Tm3+ was observed at 458 nm. Furthermore, the luminescent intensity of lanthanide trifluoride crystallized glass at low vibrational energy was 25% larger than that of oxide glass. Based on these results, glass phosphor appears to be a promising candidate for use as a new Gadolinium Neutron Capture Therapy material in cancerous tissue imaging.
Using the expanded Tomosawa theory, we simulated the experimental values of the hydration reaction of ordinary Portland cement (OPC) compositions and chose the hydration parameters for each component. Furthermore, we compared the superposition of the simulated data of the components and the measured values of the hydration reaction of the OPC. There was a good agreement between the superposition of the simulation data and experimental data of the OPC.
A series of CoOx–FeOx composite oxides with different Co content was prepared using the hydrothermal method. Low-temperature activity of CoOx–FeOx composite oxides for CO oxidation was significantly improved by the addition of 50 atm % CoOx into FeOx. Further increase in Co content up to 70 atm % caused a decrease in low-temperature activity. Structural characterizations by X-ray diffraction, transmission electron microscope and Fourier transform infrared (FT-IR) revealed that Co species, which were not substituted with Fe sites to form CoFe2O4, are highly dispersed on the surface of CoFe2O4 particles with a perimeter interface for the samples with 50 atm % CoOx. The surface valence state of CoOx–FeOx composite oxides was found to be different by FT-IR spectroscopy following NO adsorption. CoOx–FeOx with 50 atm % CoOx includes a relatively large amount of quasi-tetrahedrally coordinated Co2+ sites on the surface, whereas that with 10 atm % CoOx consists of Fe2+ sites on the surface. Temperature-programmed reduction by H2 measurements suggested that CoOx–FeOx with 50 atm % CoOx possesses the largest amount of active oxygen species, which can be reduced by H2 in the lower temperature region. The surface oxygen species of which the formation is related to the presence of quasi-tetrahedrally coordinated Co2+ sites was concluded to participate in CO oxidation reaction on CoOx–FeOx composite oxides.
Based on a sonochemical route, the effect of trisodium citrate and acetic acid, which served as the structure-directing agents, on the synthesis of Ag3PO4 nano/micro structures was investigated. It is found that the sample prepared at n(AgNO3) = 3 mmol, n(KH2PO4) = 2 mmol, n(Na3Cit) = 1 mmol and n(CH3COOH) = 3 mL is composed of coral-like microspheres. When varying the contents of reagents to other levels or in the absence of acetic acid, it brings about the formation of mesoporous microspheres or polyhedrons. The photocatalytic activity of the as-prepared Ag3PO4 samples was evaluated by the degradation of rhodamine B (RhB) under simulated-sunlight irradiation. Compared to the mesoporous microspheres or polyhedrons, the coral-like microspheres exhibit a superior photocatalytic activity, and the degradation percentage of RhB after photocatalysis reaction for 60 min reaches 96.8%.
Fluorine-doped tin oxide (SnOx:Fy) anti-reflection films coated on the surface of silicon spheres were investigated to clarify the effects of heat treatment on the microstructure and optical properties of the SnOx:Fy films. The lattice constants, crystallite size, and bandgap of the SnOx:Fy films depended on both heat treatment temperature and time. Fluorescence spectra of the SnOx:Fy films showed a broad luminescence peak associated with point defects in the films. These results were attributed to the dependence of the O and F contents in the SnOx:Fy crystals on heat treatment conditions.
In this study, KCe(WO4)2 was synthesized by a solid-state reaction of K2CO3, CeO2, and WO3 at 1273 K for 12 h. The Rietveld refinement of this sample confirmed the scheelite-type (tetragonal) structure for KCe(WO4)2, with a I41/a space group and lattice parameters a = 542.4 pm and c = 1207 pm. The lattice parameters for ACe(WO4)2 (A = Li, Na, or K) were in agreement with the ionic radii of the alkali metal ions, but the W–O distance in KCe(WO4)2 was found to be shorter than those in the other compounds, indicative of the distortion of WO42–, attributed to the larger ionic radius of K+. All of the synthesized ACe(WO4)2 materials exhibited fluorescence, with the relative emission intensity decreasing in the order of KCe(WO4)2 > NaCe(WO4)2 > LiCe(WO4)2.
A novel method of fabricating β-SiAlON powders by salt-assisted nitridation using NaCl and NaF as the molten salt medium is proposed. The salt-assisted nitridation synthesis was carried out with Al, Si, and SiO2 powders, fixed amounts of NaCl and NaF, and varying amounts NH4Cl, which was used as an auxiliary nitrogen source. The phase compositions and particle morphologies of the synthesized powders were analyzed. The results demonstrated that, as the NH4Cl content increased, the amount of β-SiAlON crystals increased, and the morphologies of the β-SiAlON crystals became rod-like with smooth tops and conical tips at 1350°C. As the temperature increased, β-SiAlON crystals became rod-like with smooth tops. In addition, the z-values of β-SiAlON were different when different amounts of NH4Cl were added. These results indicate that NH4Cl serves as a nitrogen source; varying the NH4Cl content can limit the growth of β-SiAlON crystals.
ZnO:Al thin film was deposited on glass substrates at room temperature. We used a ceramic ZnO:Al target. Since the oxygen vacancies were artificially introduced into the target, thin film could be deposited by DC magnetron sputtering. When ceramic targets are used, fabrication of transparent conductive ZnO:Al thin film in the facing-target is impossible, due to direct resputtering by oxygen anions. The substrate was colored yellow, and the conductivity was also lost. In this state, the thin film was considered to be “ZnO1−x:Al”. In order to avoid the damage, we used a shield consisting of an upper plate, side plates, back plate and base plate. The substrate encapsulated in the assembled shield was placed on the anode in the magnetron sputter we used, the target was installed in the upper side of the chamber and the anode was installed in the under side. We also described the method of deciding the size of the shield. “Thermalized” sputter particles entered the channel consisting of upper plate, side plates and base plate, and are deposited on the substrate. The oxygen anions were blocked by the shield plates. The values of the resultant thin films were enumerated. Carrier concentration was 1.25 × 1021 cm−3, a rather high value. Resistivity was 1.13 × 10−3 Ω cm and Hall mobility was 4.43 cm2/Vs. Also in the area of high temperature superconductive oxides, thin films were damaged by resputtering by oxygen anions. A shield was also tried in this area. In such cases, simple metal plates were installed beneath the substrates in an off-axis position. Side shields were not installed beside the substrates. This confirmed the necessity of side shield plates. We removed the side shield plates and conducted sputtering. The carrier concentration was 6.92 × 1019 cm−3, resistivity was 1.15 × 10−2 Ω cm and Hall mobility was 7.86 cm2/Vs. Based on these values, the effectiveness of the side plates was confirmed.
A new bonding process has been developed for producing direct-bonded aluminum (DBA) substrates using aluminum nitride (AlN). A transient eutectic liquid phase forms in aluminum–X (X = silicon, germanium, silver, or copper) systems at the interface between the aluminum foil and the AlN substrate. The aluminum–X liquid phase transiently contacts the AlN substrate prior to isothermal solidification by diffusion of the element X into the aluminum foil. We have prepared DBA substrates using this process and demonstrated that they are highly stable after thermal cycling testing.
Polyethylene-terephthalate (PET) films coated with photocatalytic TiO2 and silicone as a binder were fabricated and attached to a window pane. A white substance was observed on the surface after exposure for several months in an area with air pollution. The white substance was analyzed by scanning electron microscopy and energy dispersive X-ray spectroscopy, ion chromatograph and capillary electrophoresis, and was identified as CaSO4. Although the oxidizing properties of the films were affected by the particle size and the amount of TiO2 in the photocatalytic layer, the hydrophilicizing properties of the films were hardly changed by them. In this study, we found that it was possible to decrease the oxidizing properties of photocatalytic PET films while maintaining the hydrophilicizing properties constant by using small TiO2 particles and controlling the ratio of TiO2/binder.
Silica gel is commonly used as a sampling agent in small glass tube products for work environment measurements of organic solvent vapor, as established by the Industrial Safety and Health Act in Japan. However, to date there has been no national standard for sampling tube products, and the extraction efficiency of adsorbed organic solvent vapor from silica gel significantly affects the accuracy of measurement. Here, we have investigated the material properties of silica gel used in typical Japanese sampling tube products, and the effects on the efficiency as determined by the phase equilibrium method. This study focused particularly on specific surface area, porosity and grain diameter of silica gel specimens.
The mole fraction distributions of CaO and SiO2 under a temperature gradient in a CaSiO3 melt were calculated by molecular dynamics. The temperatures at the cold and hot ends of the gradient were 1800 and 2200 K, respectively, in the simulation. We used two sets of potential parameters proposed by Matsui et al. and Seo et al. The simulation results obtained with the two potentials indicate that the mole fraction of CaO in the cold region was higher than that in the hot region, and the mole fraction of SiO2 in the hot region was higher than that in the cold region. This is qualitatively consistent with previous experimental results obtained by laser local heating inside a CaSiO3 glass[Shimizu et al., Optics letters, 36, 2161–2163(2010)].
Edited and published by : The Ceramic Society of Japan Produced and listed by : Komiyama Printing Co., Ltd.(Vol.115 No.1344-Vol.116 No.1351, Vol.118 No.1376-) Letterpress Co., Ltd.(Vol.116 No.1352-Vol.118 No.1375)