Nuclear Magnetic Resonance Studies of Silicon Carbide Polytypes

The ²⁹Si MAS NMR spectra of the 2H, 4H, 6H, and 3C polytypes of silicon carbide are presented. An attempt is made to correlate differences in the chemical shifts with local atomic environment. The results of the analysis of the spectra of pure polytypes are used as a basic for the interpretation of the spectra of mixed polytypes and a discussion of the crystallinity and impurity levels of different samples. Carbon-13 chemical shifts obtained from spectra of the same polytypes are also tabulated.     

A Silicon–Silica Nanocomposite Material

The synthesis and characterization of a novel silicon–silica nanocomposite material are reported. A self‐assembly method allows the encapsulation of silicon nanoclusters within the channels of a periodic mesoporous silica thin film. The result is the formation of a silicon–silica nanocomposite film with bright, room‐temperature photoluminescence in the visible range, and a nanosecond luminescence lifetime. The properties of the nanocomposite material have been studied by several analytical techniques, which collectively show the existence within the channels of non‐diamondoid‐structure‐type silicon nanoclusters with various hydrogenated silicon sites. It is estimated that the silicon nanoclusters in the silica mesoporous films occupy up to 39 % of the accessible pore volume. The nanocomposite film shows improved resistance to air oxidation compared to crystalline silicon. The high loading and chemical stability to oxidation under ambient conditions are important advantages in terms of the development of silicon‐based light‐emitting diodes from this class of materials.     

Stepwise-Graded Si3N4–SiC Ceramics with Improved Wear Properties

The processing of stepwise graded Si₃N₄/SiC ceramics by pressureless co-sintering is described. Here, SiC (high elastic modulus, high thermal expansion coefficient) forms the substrate and Si₃N₄ (low elastic modulus, low thermal expansion coefficient) forms the top contact surface, with a stepwise gradient in composition existing between the two over a depth of ∼1.7 mm. The resulting Si₃N₄ contact surface is fine-grained and dense, and it contains only 2 vol% yttrium aluminum garnet (YAG) additive. This graded ceramic shows resistance to cone-crack formation under Hertzian indentation, which is attributed to a combined effect of the elastic-modulus gradient and the compressive thermal-expansion-mismatch residual stress present at the contact surface. The presence of the residual stress is corroborated and quantified using Vickers indentation tests. The graded ceramic also possesses wear properties that are significantly improved compared with dense, monolithic Si₃N₄ containing 2 vol% YAG additive. The improved wear resistance is attributed solely to the large compressive stress present at the contact surface. A modification of the simple wear model by Lawn and co-workers is used to rationalize the wear results. Results from this work clearly show that the introduction of surface compressive residual stresses can significantly improve the wear resistance of polycrystalline ceramics, which may have important implications for the design of contact-damage-resistant ceramics.     

Thermal Shock Behavior of Porous Silicon Carbide Ceramics

Using the water-quenching technique, the thermal shock behavior of porous silicon carbide (SiC) ceramics was evaluated as a function of quenching temperature, quenching cycles, and specimen thickness. It is shown that the residual strength of the quenched specimens decreases gradually with increases in the quenching temperature and specimen thickness. Moreover, it was found that the fracture strength of the quenched specimens was not affected by the increase of quenching cycles. This suggests a potential advantage of porous SiC ceramics for cyclic thermal-shock applications.     

A double-layer current conduction model for high-κ gate dielectric materials with interfacial oxide or silicate layer

The current-voltage (I-V) characteristics of metal-oxide-semiconductor (MOS) structures with hafnium oxide as the gate dielectric film were studied. Sharp shifts from a low-voltage ohmic regime to a tunneling conduction were observed in the high-voltage range. The paper demonstrates that this behavior can be described very well with a double-layer dielectric model. Excellent fittings of the experimental curves were obtained and the related key structural and physical parameters were obtained. The model fitting further suggests the optimal annealing conditions for preparing the hafnium oxide films.     

Phase Composition of Oxide Scales during Reheating in Hot Rolling of Low Carbon Steel

Low carbon steel was oxidized over the temperature range 1000‐1250°C in O₂‐CO₂‐H₂O‐N₂, O₂‐H₂O‐N₂, and O₂‐CO₂‐N₂ gas mixtures. Oxidation times were 12‐120 min. and the scales were 50‐2000 μm thick. The variations of these parameters were chosen to elucidate the phase composition of oxide scales under conditions similar to those of reheating furnaces in hot strip mills, using either thin slab casting or conventional casting and rolling technology. Two types of scales have been observed which are influenced by the furnace atmosphere, oxidation time, and temperature. The first type is a crystalline scale with an irregular outer surface, composed mostly of wustite (FeO), and a negligible amount of magnetite (Fe₃O₄). The second type is the classical three‐layer scale, composed of wustite (FeO), magnetite (Fe₃O₄), and hematite (Fe₂O₃). In general, the experiments showed that an increase in oxidation time decreased the percentage of wustite while the percentages of magnetite and hematite increased. A rise in oxygen concentration in the gas mixture increased the percentages of magnetite and hematite, confirming earlier experimental findings. In water vapour‐free atmospheres O₂‐CO₂‐N₂, the oxide scales had a low percentage of wustite, and high percentage of magnetite and hematite. Carbon dioxide showed a small influence at 1100°C, and a negligible one at 1250°C.     

Effect of temperature on methyl radical generation over Sr/La2O3 catalysts

The role of gas phase CH₃ radicals in the oxidative coupling of CH₄ over 1 wt% Sr/La₂O₃ is supported by the observations that (i) this is a very effective catalyst for the generation of CH₃ · radicals and (ii) the concentration of CH₃ · radicals increases with temperature over the range of 450 °C–800 °C.     

Photodegradation mechanism and stabilization of polyphenylene oxide and rigid‐rod polymers

Poly(2,4‐dimethyl‐1,4‐phenylene oxide) (PPO), poly(benzo[1,2‐d:5,4‐d′]bisoxazole‐2,6‐diyl‐1,4‐phenylene) (PBO) and poly(benzo[1,2‐d:4,5‐d′]bisthiazole‐2,6‐diyl‐1,4‐phenylene) (PBZT), which are polymers with extended conjugated structures, undergo a self‐sensitized photo‐induced electron‐transfer reaction. A second component is not required. This article presents many similar observations on these polymers when they are exposed to light and evidence to support the proposed photo‐induced electron‐transfer mechanism. Methods to stabilize these polymers against photo‐oxidation are also described. Workers investigating other conjugated polymeric systems may find the experimental methods, observations and polymer stabilization approaches discussed in this review useful. Copyright © 2005 Society of Chemical Industry