Phase transformation and interface segregation behavior in Si3N4 ceramics sintered with La2O3–Lu2O3 mixed additive

Microstructure and mechanical property of silicon nitride (Si₃N₄) ceramic are strongly dependent on the selection of sintering additives. When rare‐earth (RE) oxide is used as the sintering additive, segregation of RE ions at interface between Si₃N₄ grain and intergranular glassy film (IGF) is believed to play a critical role. Although the ionic radius of RE ion is known to be an empirical parameter to modify the mechanical property, the correlation between the segregated ions and their ionic radii is still under controversy. In order to address this issue, (i) rate of α‐β phase transformation and (ii) segregation behavior at the interface were studied for Si₃N₄ ceramics sintered using mixture of La₂O₃ and Lu₂O₃ as additives in this study. Specimens of Lu content 30% and higher exhibited lower activation energies for the α‐β phase transformation as compared with those of Lu content 20% and lower. In terms of the segregation behavior, La was preferably segregated at one site and Lu at the other site along β‐Si₃N₄/IGF interface in the specimens of Lu content 30% and higher. It is understood from these results that Lu segregation site should be more closely related with grain growth.     

The Wulff Shape of Alumina: IV. Ti4+-Doped Alumina

Controlled-geometry cavities, initially ≈20 μm × 20 μm × 0.5 μm, were introduced into     -plane titanium-doped (≈210 ppma;≈500 wt. ppm) sapphire substrates using photolithographic methods, and subsequently internalized by diffusion bonding. The samples were annealed in air for prolonged periods at 1600° and 1800°C to convert the titanium to the 4+ state and to allow the pore shapes to adjust. Pores with an equivalent spherical radius of ≈3.6 μm reached a quasi-equilibrium shape within 160 h at 1600°C and within 48 h at 1800°C. The Wulff shape was determined using optical microscopy, scanning electron microscopy, and atomic force microscopy. The Wulff shape of Ti⁴⁺-doped alumina includes well-defined c(0001),     , and     facets and smoothly curved sections. The     and a     facets, which are components of the Wulff shape of undoped sapphire, are not discernable. In contrast to undoped alumina, for which the r-plane has the lowest energy, the c-plane is the lowest energy plane in Ti⁴⁺-doped alumina. The surface energy sequence of the stable c, r, and p surfaces differs from that in undoped alumina. The Wulff shape varies with temperature. Samples equilibrated at 1800°C were re-annealed at 1600°C. Pore shape changes were reversible, indicating that the observed pore shapes were close to the equilibrium (Wulff) shape.     

Development of a unidirectional cooling technique for synthesizing compound semiconductors in 10 m drop tower

The solidification process and structures of CdTe solidified in microgravity were studied using the unidirectional cooling apparatus in a 10 m drop tower. Since the drop tower provides 1.4 seconds of microgravity, the unidirectional cooling apparatus cools samples rapidly by cooling gas. The system adopts a Pt heater, which accurately heats samples to a maximum of 1300°C. The sample is placed in an ampoule under vacuum conditions. A flat wall in the ampoule divides the inner sealed sample from the outer open side. A nozzle blowing cooling gas is directed on to the outer wall, and cools the sample until solidification. The cooling properties were measured during CdTe solidification in microgravity. The result shows that solidification occurred between 0.9 and 1.3 seconds after release, so solidification is completed in microgravity. Optical microscope (OM) observation of the sample solidified in microgravity revealed that it produces CdTe and Te phases with segregation patterns, and the structures are ordered along the cooling direction, whereas no order is observed in the structures of the terrestrial sample solidified under 1 g.     

The Wulff Shape of Alumina: II, Experimental Measurements of Pore Shape Evolution Rates

The rate at which a facetted tetragonal cavity of nonequilibrium shape approaches a cubic equilibrium (Wulff) shape via surface diffusion was modeled. The shape relaxation rate of a facetted "stretched cylinder" was also modeled. For the first geometry, only an approximate solution based on linearizing the mean potential difference between the source and sink facets was obtained. For the stretched cylinder, both an approximate and an exact solution can be obtained; the approximate solution underestimates the evolution rate by a factor of ∼2. To assess the applicability of the models, nonequilibrium shape pores of identical initial geometry (∼20 μm × 20 μm × 0.5 μm) were introduced into (0001), {10[Onemacr]2}, {1120}, and {100} surfaces of sapphire single crystals using microfabrication techniques, ion-beam etching, and hot pressing. The large (∼20 μm × 20 μm) faces of the pore are low-index surfaces whose nature is dictated by the wafer orientation. A series of anneals was performed at 1900°C, and the approach of the pore shape to an equilibrium shape was monitored. The kinetics of shape evolution are highly sensitive to the crystallographic orientation and stability of the low-index surface that dominates the initial pore shape. The measured variations of the pore aspect ratio were compared to those predicted by the kinetic model. The observations suggest that when the initial bounding surface is unstable, shape relaxation may be controlled by diffusion. However, surface-attachment-limited kinetics (SALK) appears to play a major role in determining the pore shape evolution rate in cases where the initial bounding surfaces have orientations that are part of the Wulff shape.     

Inhibition of experimental autoimmune uveoretinitis by transforming growth factor-beta 1 in B10. A mice

Experimental autoimmune uveoretinitis (EAU) in mice, an organ specific autoimmune disease, has been investigated as an animal model for human endogenous uveitis. In this study, we report on the immunosuppressive effect of transforming growth factor-beta 1 (TGF-beta 1) on the development of EAU in mice. Inhibition by TGF-beta 1 of proliferation of interphotoreceptor retinoid-binding protein (IRBP)-specific T cell lines in B10.A mice against IRBP antigen was dose-dependent. However, when spleen cells used as the antigen presenting cell were first cultured with TGF-beta 1, this anti-proliferation effect was abolished. When IRBP-immunized mice were injected intraperitoneally with TGF-beta 1, dose-dependent suppression of EAU was obtained. The proliferation response of lymph node cells from TGF-beta 1 injected mice with IRBP-induced EAU was suppressed compared with phosphate buffered saline (PBS)-injected mice. These findings suggest that TGF-beta 1 may be a cytokine that plays a role in suppressing IRBP induced EAU in mice.     

The effect of counter anions on corrosion resistance of steel covered by bi-layered polypyrrole film

The bi-layered polypyrrole (PPy) coatings were investigated for corrosion prevention of a carbon steel. The inner layer was doped with the Keggin structure anions of (PMo₁₂) and anions for stabilization of the passive oxide film at the metal-polymer interface, and the outer layer was doped with four organic anions of dihydroxynaphthalenedisulfonate (DHNDS), naphthalenedisulfonate (NDS), anthraquinonedisulfonate (AnqDS) or dodecylsulfate (DoS) for inhibition of the decomposition and release of PMo₁₂. The corrosion tests were performed in 3.5 wt.% NaCl aqueous solution. The corrosion resistance of the steels covered by the bi-layered PPy films was found in the following order: PPy-PMo₁₂/PPy-DHNDS < PPy-PMo₁₂/PPy-NDS < PPy-PMo₁₂/AnqDS < PPy-PMo₁₂/PPy-DoS. The performance of corrosion protection related to the oxidized state of the polymer was discussed.     

Detection of pathogenic bacteria by using zinc finger protein fused with firefly luciferase

We constructed a novel bacterial genome detection system using zinc finger protein (ZF) fused with firefly luciferase (ZF-luciferase). Taking advantage of the direct recognition of double-stranded DNA (dsDNA) by ZF, we previously constructed bacterial genome detection systems that did not require dehybridization processes. To detect polymerase chain reaction (PCR) products rapidly and with a high sensitivity, we constructed two kinds of ZF-luciferase, Sp1-fused luciferase (Sp1-luciferase), and Zif268-fused luciferase (Zif268-luciferase). ZF-luciferase not only maintains luciferase activity but also shows dsDNA-binding ability and specificity. Furthermore, we succeeded in the detection of 10 copies of the genome of Legionella pneumophila and Escherichia coli O157. ZF-luciferase would be a useful tool for highly sensitive detection of pathogenic bacterial genome.     

Effect of Rare Earth Oxide Additives on the Phase Transformation Rates of Si3N4

Fully dense Si₃N₄ sintered with rare-earth (RE=La, Nd, Gd, and Yb) oxide additives were fabricated by hot pressing. They were mainly composed of the α-phase, which enabled the investigation of the α–β transformation kinetics without the effect of porosity. The phase transformation kinetics was found to be linear. The activation energies for the phase transformation were 677, 546, 487, and 424 kJ/mol for RE=La, Nd, Gd, and Yb, respectively, and showed a good correlation with the ionic radii of RE atoms, which was discussed in conjunction with the segregation behaviors of RE atoms at the interface.     

Formation and Control of Agglomerates in Alumina Powder

High-purity alumina powder with submicrometer particle size was colloidally dispersed and classified in either water or ethanol to remove agglomerates from as-received powder. After the slurries were dried, fine alumina particles cohered to form agglomerates, which were "hard" when formed in a water slurry, and "soft" in an ethanol slurry. Firing of the powder compacts with "hard" agglomerates resulted in inhomogeneous microstructures, and homogeneous microstructures were formed with "soft" agglomerates. The reasons for their formation are discussed and experimentally confirmed. In the case of water slurry, a small amount of transition alumina reacted with water at low pH. After drying, alumina gels were formed and acted as a strong binder between α-alumina particles to form "hard" agglomerates.