Young-Il Lee

Microstructure stability of fine-grained silicon carbide ceramics during annealing

Fine-grained silicon carbide ceramics with an average grain size of ∼140 nm or smaller were prepared by low-temperature hot-pressing of very fine β-SiC powders using Al2O3-Y2O3-CaO (AYC) or Y-Mg-Si-Al-O-N glass (ON) as sintering additives. The microstructure stability of the resulting fine-grained SiC ceramics was investigated by annealing at 1850°C and by evaluating quantitatively the grain growth behavior using image analysis. The β → α phase transformation of SiC in AYC-SiC was responsible for the accelerated abnormal grain growth of platelet-shaped grains. In contrast, the β → α phase transformation in ON-SiC was suppressed, which resulted in a very stable microstructure.

Pressureless sintering of SiC-TiC composites with improved fracture toughness

Composites of SiC-TiC containing up to 45 wt% of dispersed TiC particles were pressureless sintered to ∼97% of theoretical density at temperatures between 1850°C and 1950°C with Al2O3 and Y2O3 additions. An in situ-toughened microstructure, consisted of uniformly distributed elongated α-SiC grains, matrixlike TiC grains, and yttrium aluminum garnet (YAG) as a grain boundary phase, was developed via pressureless sintering route in the composites sintered at ≥1900°C. The fracture toughness of SiC-30 wt% TiC composites sintered at 1900°C for 2 h was as high as 7.8 MPa·m1/2, owing to the bridging and crack deflection by the elongated α-SiC grains.

Surface reaction kinetics in oxygen nonstoichiometry re-equilibration of BaTiO3−δ

Abstract We report the (bare) surface redox-reaction rate constant k that was determined, along with the chemical diffusivity D , by a conductivity relaxation technique on Al-doped single crystal and undoped polycrystal BaTiO 3− δ as a function of oxygen activity in its range of −16≤log a O 2 ≤0 at elevated temperatures of 800–1100 °C. It takes a value in the range of −4 k /cm s −1 )≤−1, which is even larger than that of the oxides that are considered best as oxygen membranes. It has been found that the surface reaction step grows more rate controlling as the electronic transference number gets smaller or the electronic stoichiometric composition ( δ ≈0) is approached. The oxygen potential drop due to the surface reaction was estimated by an oxygen concentration cell technique. The oxygen potential drop grows larger as the stoichiometric composition is approached, that is in accord with the variation of k against oxygen activity.

Effects of additive amount on microstructure and mechanical properties of silicon carbide-silicon nitride composites

Powder mixtures of β-SiC-α-Si3N4 in a weight ratio of 1 : 1 containing 5–20 wt% Y-Mg-Si-Al-O-N oxynitride as a sintering additive were liquid-phase sintered at 1800°C for 3 h by hot-pressing. These materials had a microstructure of “in situ-toughened composites” as a result of the α → β phase transformation of Si3N4 during sintering. The introduction of larger amount of additives accelerated the grain growth of elongated β-Si3N4 grains with higher aspect ratio, resulting in the improved fracture toughness and strength. Typical flexural strength and fracture toughness of SiC-Si3N4 composites containing 15 wt% oxynitride glass were 860 MPa and 5.7 MPa · m1/2, respectively.

Characterization of dislocations in copper electrodeposits

Copper electrodeposits with the [111] orientation were obtained from a copper sulfate bath consisting of 280 g/l CuSO45H2O and 80 g/l H2SO4 at 30°C. The cathode current density was 900 to 1300 A/m2. Dislocations in the electrodeposits were characterized using transmission electron microscopy. Dislocations in each of the (111) grains could be classified into two groups. One group belonged to high density dislocation bundles radiating from one small area, and another group was located between the bundles. The dislocations in the first group were directed along the <112< directions parallel to the (111) plane and of the edge type. About 42 pct of the dislocations in the second group were also parallel to the (111) plane and of the mixed type. In summary, the majority of dislocations in the (111) grains was of edge type lying on the (111) plane.

Microstructure and Mechanical Properties of SiC-BN Composites with Oxynitride Glass

By using an oxynitride glass as a sintering additive, the effects of BN content on microstructure and mechanical properties of the hot-pressed and subsequently annealed SiC-BN composites were investigated. The microstructures developed were analyzed by image analysis. The morphology of SiC grains was strongly dependent on BN content in the starting composition. The aspect ratio of SiC decreases with increasing BN content and the average diameter of SiC shows a maximum at 5 wt% BN and decreases with increasing BN content in the starting powder. The fracture toughness increased with increasing BN content while the strength decreased with increasing BN content. The strength and fracture toughness of SiC or SiC-TiC composites were strongly dependent on the morphology of SiC grains, but the strength and fracture toughness of SiC-BN composites were strongly dependent on BN content rather than morphology of SiC grains. These results suggest that fracture toughness of SiC ceramics can be tailored by manipulating BN content in the starting composition. Typical fracture toughness and strength of SiC-10 wt% BN composites were 8 MPa·m 1/2 and 445 MPa, respectively.

Identification of Three-Parameter Models from Step Response

This paper provides an identification method for three-parameter models i.e. first order with dead time models and second order with dead time models. The proposed identification method is based on step response and can be easily implemented using digital microprocessors. The proposed method first identifies the order of the plant i.e. first order or second order from the behavior of the plant with constant input. After the order of the plant is determined, a test step input is applied to the system and the three parameters of the plant are obtained from the corresponding response of the plant. The output of the plant need not to be zero when the test signal is applied. The efficacy of proposed algorithms is verified through simulation and experiment.

Effect of Additive Composition on Fracture Toughness of In Situ-Toughened SiC-Si₃N₄ Composites

Effect of additive composition on fracture toughness of in situ-toughened SiC-Si₃N₄ composites was investigated for five different additive compositions. The highest toughness (6.4 ㎫ㆍm 1/2 ) in SiC-Si₃N₄ composites investigated herein was obtained when an Y-Mg-Si-Al-O-N oxynitride glass was used as a sintering additive. The improvement in fracture toughness was produced by enhanced bridging and deflection by Si₃N₄ grains.

Identification of three-parameter models in the presence of non-zero initial condition

This paper provides an identification method for three-parameter models i.e. first order with dead time models (FODT) and second order with dead time (SODT) models. The proposed identification method is based on step response and can be easily implemented using digital microprocessors. The proposed method first identifies the order of the plant i.e. first order or second order from the behavior of the plant with constant input. After the order of the plant is determined, a test step input is applied to the system and the three parameters of the plant are obtained from the corresponding response of the plant. The output of the plant need not to be zero when the test signal is applied. The efficacy of proposed algorithms is verified through simulation.