Porous SiOC beads by freeze-drying polycarbosilane emulsions

Porous silicon oxycarbide (SiOC) beads were prepared by freeze-drying of water-in-oil (w/o) emulsion, containing water and polycarbosilane (PCS) dissolved p-xylene in the presence of sodium xylenesulfonate (SXS) as an emulsifier. The emulsion was frozen by being dropped onto a liquid N₂ bath, which resulted in 1~2 mm sized beads. After curing at 200°C for 1 h in air and subsequent pyrolysis at 800°C for 1 h in an Ar gas flow, porous SiOC beads were obtained. Freeze-dried beads showed lamellae-shaped macro-pore structures at a moderate freezing rate due to a phase separation behavior of PCS during freezing, while no lamellae pores were formed at a very high freezing rate. Lamellae-shaped pores developed along the freezing direction. Water droplets that were formed in w/o emulsion converted to spherical pores after drying. The combined processes of producing PCS emulsion and freeze-drying of emulsion resulted in two types of macro-pores: lamellae-shaped and spherical pores. Meso-pores, of which specific surface area and average pore size were 71.5 m² g⁻¹ and 4.85 nm, respectively, were formed inside the SiOC strut.Keywords: Polycarbosilane, Polymer emulsion, Freeze-drying, Porous SiOC bea

Reduction of Functionally Graded Material Layers for Si 3 N 4 -Al 2 O 3 System Using Three-Dimensional Finite Element Modeling

Numerical analysis method was used to reduce the number of functionally graded material (FGM) layers for joining Si 3 N 4 -Al 2 O 3 using polytypoid interlayer by estimating the position of crack. In the past, hot press sintering of multi-layered FGMs with 20 layers of thickness 500 mm each have been fabricated successfully. In this paper, thermal residual stresses were calculated using finite element method (FEM) to find the optimized number of layers and its thicknesses of FGM joint. The number of layers for FGM was reduced to 15 layers from 20 layers. Thicknesses were varied to minimize residual stresses within the layers while reducing the number of FGM layers. The damage caused by thermal residual stress was estimated using maximum principal stress theory and maximum tensile stress theory. The calculated maximum stress was found to be axial stress of 430 MPa around 90% 12H/10% Al 2 O 3 area. For each case, calculated strength of each FGM layer by linear rule of mixture was compared with computed thermal residual stresses. Thermal analysis results correctly predicted the position of crack, and this position agreed well with fabricated joints. Therefore, this numerical analysis method can be applied to reduced FGM layers of crack free joint. Finally, new composition profile of crack free joint was proposed using FGM method

Improvement of Transparent Conducting Performance on Oxygen-Activated Fluorine-Doped Tin Oxide Electrodes Formed by Horizontal Ultrasonic Spray Pyrolysis Deposition

In this study, highly transparent conducting fluorine-doped tin oxide (FTO) electrodes were fabricated using the horizontal ultrasonic spray pyrolysis deposition. In order to improve their transparent conducting performances, we carried out oxygen activation by adjusting the ratio of O₂/(O₂+N₂) in the carrier gas (0%, 20%, and 50%) used during the deposition process. The oxygen activation on the FTO electrodes accelerated the substitution concentration of F (F_O^•) into the oxygen sites in the FTO electrode while the oxygen vacancy (V_O^•^•) concentration was reduced. In addition, due to growth of pyramid-shaped crystallites with (200) preferred orientations, this oxygen activation caused the formation of a uniform surface structure. As a result, compared to others, the FTO electrode prepared at 50% O₂ showed excellent electrical and optical properties (sheet resistance of ∼4.0 ± 0.14 Ω/□, optical transmittance of ∼85.3%, and figure of merit of ∼5.09 ± 0.19 × 10^–2 Ω^–1). This led to a superb photoconversion efficiency (∼7.03 ± 0.20%) as a result of the improved short-circuit current density. The photovoltaic performance improvement can be defined by the decreased sheet resistance of FTO used as a transparent conducting electrode in dye-sensitized solar cells (DSSCs), which is due to the combined effect of the high carrier concentration by the improved F_O^• concentration on the FTO electrodes and the fasted Hall mobility by the formation of a uniform FTO surface structure and distortion relaxation on the FTO lattices resulting from the reduced V_O^•^•^• concentration