Phase Transformation and Texture in Hot-Forged or Annealed Liquid-Phase-Sintered Silicon Carbide Ceramics

Starting from three powder mixtures of 80 vol% SiC (100α, 50α/50β, 100β) and 20 vol% YAG, liquid-phase-sintered silicon carbide ceramics were prepared by hot pressing at 1800°C for 1 h under 25 MPa, and then by hot forging or annealing at 1900°C for 4 h under an applied stress of 25 MPa in argon. The phase transformation and texture development in the as-hot-pressed, hot-forged, and annealed SiC ceramics were investigated via X-ray diffraction (XRD) and the pole figure measurements. The 6H → 4H polytypic transformation was observed in samples consisting of both α- and β-SiC phases when subjected to compressive deformation but absent in the case of annealing, suggesting the deformation-enhanced solubility of aluminum in SiC. Deformation was also found to enhance the 3C → 4H transformation in the sample containing entirely β-phase, which is due to the accelerated solution-precipitation process assisted by grain boundary sliding. The current study showed that the β- →α-phase transformation had little effect on texture development in SiC. Hot forging generally produced the strongest texture, with the calculated maximum of 2.2 times random in samples started with pure α-SiC phase. The mechanism for texture development was explained based on the microstructural observations.     

Preparation and Luminescence Spectra of Calcium- and Rare-Earth (R = Eu, Tb, and Pr)-Codoped α-SiAlON Ceramics

Rare-earth-doped oxynitride or nitride compounds have been reported to be luminescent and may then serve as new phosphors with good thermal and chemical stabilities. In this work, we report the photoluminescence (PL) spectra of europium-, terbium-, and praseodymium-doped Ca-α-SiAlON ceramics. The highly dense ceramics were prepared by hot pressing at 1750°C for 1 h under 20 MPa in a nitrogen atmosphere. Europium-doped Ca-α-SiAlON displayed a single broad emission band peaking at λ= 550–590 nm depending on the europium concentration. The emission bands in the spectra of europium-doped Ca-α-SiAlONs were assigned to the allowed transition of Eu²⁺ from the lowest crystal field component of 4 f ⁶5 d to ⁸S_7/2 (4 f ⁷) ground-state level. The emission spectra of terbium- and praseodymium-doped Ca-α-SiAlON ceramics both consisted of several sharp lines, which were attributed to the ⁵D₄→⁷F _j ( j = 3, 4, 5, 6) transitions of Tb³⁺ and ³P₀→³H _j ( j = 3, 4, 5) transitions of Pr³⁺, respectively. In particular, the terbium-doped α-SiAlON ceramics showed a strong green emission among these phosphors.     

Transformation of Lower Alkanes into Aromatic Hydrocarbons over ZSM-5 Zeolites

Much attention has been paid to the transformation of lower alkanes such as propane and butanes into aromatic hydrocarbons from both industrial and academic points of view. The aromatic hydrocarbons can be utilized as a booster for high octane number gasoline and are fundamental raw chemicals in petroleum chemistry. The activation of lower alkanes is an intriguing subject. In early work, Csicsery [1-5] described dehydrocyclodimerization of lower alkanes over bifunctional catalysts such as platinum on alumina and Cr₂O₃ on alumina.     

Nucleation Effects of Nucleobases on the Crystallization Kinetics of Poly(L‐lactide)

The effects of nucleobases, especially uracil, on the nonisothermal and isothermal crystallization, melting behavior, spherulite morphology, and crystalline structure of bio‐based and biodegradable PLLA are studied. The melt‐ and cold‐crystallization rates of PLLA increase with increasing uracil loading. The melting behavior of nonisothermally melt‐ and cold‐crystallized PLLAs depends on the uracil content. The isothermal crystallization kinetics is analyzed based on an Avrami model. The incorporation of uracil changes the t_1/2/T_c profile of PLLA due to the more distinct heterogeneous nucleation effects at small supercooling. The crystalline structure of PLLA is not affected by uracil presence. The nucleation density increases and the spherulite size decreases by uracil incorporation.

     

Microstructure of a Molybdenum Nitride Layer Formed by Nitriding Molybdenum Metal

For pure molybdenum nitrided at 1373 K in NH₃ gas, microstructural observations of a molybdenum nitride layer were conducted through a transmission electron microscope. The molybdenum nitride layer consisted of two molybdenum nitrides: an outer one of γ-Mo₂N and an inner one of β-Mo₂N. A great number of (011)[0     1] type twins were observed in the β-Mo₂N phase, whereas the microstructure of the γ-Mo₂N phase was almost free from lattice defects except for slightly observed {111}〈112〉 type twin.     

Optical and adhesive properties of composite silica-impregnated Ca-α-SiAlON:Eu2+ phosphor films prepared on silica glass substrates

The compaction of a Eu-doped Ca-α-SiAlON phosphor powder was performed by electrophoretic deposition (EPD). The effect on the adhesion and optical properties of the silica precursor as both a binder of the powder and a filler of the air voids were evaluated. The adhesion of the silica impregnated composite film to the silica glass substrate was characterized by the tape test. The improvement of the external quantum efficiency was confirmed from the PL spectra measurement after the silica impregnation. The temperature dependence of the external quantum efficiency was also investigated in order to discuss the advantage of using the silica precursor as a binder for high-brightness LED applications.     

Hydrogen‐bonding interaction between poly(ε‐caprolactone) and low‐molecular‐weight amino compounds

The specific interactions between several low‐molecular‐weight diamino compounds and poly(ε‐caprolactone) (PCL) have been investigated by FT‐IR. It was found that PCL and 3,3′‐diaminodiphenylmethane (3,3′‐DADPM) interact through strong intermolecular hydrogen bonds in the blend. Thermal and mechanical properties of PCL/3,3′‐DADPM blends were investigated by DSC and tensile measurements, respectively. The glass transition temperature of the blend increases while both the melting point and the elongation‐at‐break of the blend decrease with the increase of 3,3′‐DADPM content. Besides 3,3′‐DADPM, several other low‐molecular‐weight compounds containing two amino groups, such as o‐phenylenediamine or 1,6‐diaminohexane, were also added into PCL and the corresponding blend systems were investigated by FT‐IR and DSC. The effect of the chemical structure of the additives on the properties of PCL is discussed. © 2001 Society of Chemical Industry     

Miscibility and phase structure of binary blends of poly(L‐lactide) and poly(vinyl alcohol)

Blend films of poly(L ‐lactide) (PLLA) and poly(vinyl alcohol) (PVA) were obtained by evaporation of hexafluoroisopropanol solutions of both components. The component interaction, crystallization behavior, and miscibility of these blends were studied by solid‐state NMR and other conventional methods, such as Fourier transform infrared (FTIR) spectra, differential scanning calorimetry (DSC), and wide‐angle X‐ray diffraction (WAXD). The existence of two series of isolated and constant glass‐transition temperatures (T_g's) independent of the blend composition indicates that PLLA and PVA are immiscible in the amorphous region. However, the DSC data still demonstrates that some degree of compatibility related to blend composition exists in both PLLA/atactic‐PVA (a‐PVA) and PLLA/syndiotactic‐PVA (s‐PVA) blend systems. Furthermore, the formation of interpolymer hydrogen bonding in the amorphous region, which is regarded as the driving force leading to some degree of component compatibility in these immiscible systems, is confirmed by FTIR and further analyzed by ¹³C solid‐state NMR analyses, especially for the blends with low PLLA contents. Although the crystallization kinetics of one component (especially PVA) were affected by another component, WAXD measurement shows that these blends still possess two isolated crystalline PLLA and PVA phases other than the so‐called cocrystalline phase. ¹³C solid‐state NMR analysis excludes the interpolymer hydrogen bonding in the crystalline region. The mechanical properties (tensile strength and elongation at break) of blend films are consistent with the immiscible but somewhat compatible nature of these blends. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 762–772, 2001     

Research and Development of the Coprecipitation Process for Lanthanum Germanate Oxyapatite

Although lanthanum germanate oxyapatite (La–Ge–O) has shown good potential for use as a solid electrolyte in energy storage applications, its synthesis has been challenging by either solid‐ or solution‐state methods. In this study, a new synthesis of La–Ge–O was developed through a coprecipitation technique, in which a highly concentrated homogeneous aqueous solution of La and Ge was prepared from aqueous ammonium germanate and lanthanum nitrate solutions with the addition of dilute nitric acid. Several precipitates were formed by pH manipulation, including an amorphous material obtained at pH > 3. Compared to the individual precipitation behaviors of the parent compounds, the amorphous precipitate was formed only from the aqueous two‐component mixture, and appeared to contain both metals. This material was transformed into crystalline mixtures upon heating at 1273 K. The crystalline phases were La₂Ge₃O₉ and hexagonal‐type GeO₂ when the precipitate was formed below pH 8, and the La–Ge–O and La₂Ge₂O₇ phases when the precipitate was formed around pH 8. Product formation from the coprecipitate was discussed based on X‐ray diffraction and thermal analyses. The improved availability of La–Ge–O will allow more extensive investigations of its useful properties.     

Effects of Gd Substitution on Sintering and Optical Properties of Highly Transparent (Y0.95−xGdxEu0.05)2O3 Ceramics

Highly transparent (Y_0.95?xGd_xEu_0.05)₂O₃ (x = 0.15–0.55) ceramics have been fabricated by vacuum sintering at the relatively low temperature of 1700°C for 4 h with the in‐line transmittances of 73.6%–79.5% at the Eu³⁺ emission wavelength of 613 nm (~91.9%–99.3% of the theoretical transmittance of Y_1.34Gd_0.6Eu_0.06O₃ single crystal), whereas the x = 0.65 ceramic undergoes a phase transformation at 1650°C and has a transparency of 53.4% at the lower sintering temperature of 1625°C. The effects of Gd³⁺ substitution for Y³⁺ on the particle characteristics, sintering kinetics, and optical performances of the materials were systematically studied. The results show that (1) calcining the layered rare‐earth hydroxide precursors of the ternary Y–Gd–Eu system yielded rounded oxide particles with greatly reduced hard agglomeration and the particle/crystallite size slightly decreases along with increasing Gd³⁺ incorporation; (2) in the temperature range 1100°C–1480°C, the sintering kinetics of (Y_0.95?xGd_xEu_0.05)₂O₃ is mainly controlled by grain‐boundary diffusion with similar activation energies of ~230 kJ/mol; (3) Gd³⁺ addition promotes grain growth and densification in the temperature range 1100°C–1400°C; (4) the bandgap energies of the (Y_0.95?xGd_xEu_0.05)₂O₃ ceramics generally decrease with increasing x; however, they are much lower than those of the oxide powders; (5) both the oxide powders and the transparent ceramics exhibit the typical red emission of Eu³⁺ at ~613 nm (the ⁵D₀→⁷F₂ transition) under charge transfer (CT) excitation. Gd³⁺ incorporation enhances the photoluminescence and shortens the fluorescence lifetime of Eu³⁺.