Search

Recently, attempts have been made to use silicon carbide (SiC) ceramics as a fuel compact in nuclear reactors. However, the use of sintering additives in the conventional sintering of SiC leads to a coarse microstructure with high porosity and large grains due to the low sinterability and the large amount of grain boundaries or a second phase, deteriorating the mechanical properties, and hence limiting widespread industrial applications. In this study, polycarbosilane (PCS) and spark plasma sintering (SPS) as the SiC precursor and sintering method, respectively, were applied to overcome the low sinterability and the use of sintering additives. As the raw material, amorphous PCS was prepared by pyrolysis at 1000 °C. To decrease the particle size, pyrolyzed PCS powder was subjected to ball milling by using high-energy ball milling. The ball-milled amorphous PCS powder was sintered by SPS. Sintering was carried out at different temperatures (i.e., 1700, 1800, and 1900 °C) and pressures (i.e., 40 and 80 MPa). After SPS, the core/rim structure of the sintered pellet was formed on account of the oxygen originating from the native oxide layer and free carbon in the PCS powder as the raw materials. In the core/rim structure comprising the core, as well as the intermediate and rim regions, the microstructure and chemistry of each region exhibited marginal difference with respect to the free carbon and oxygen. The phase fraction of α-SiC increased from the rim region to core region due to the discrepancy of actual loaded pressure on account of friction with the wall surface. The existing carbon was verified to be graphitized carbon. Owing to the short reaction time, graphitization did not completely proceed, and deficient graphitization led to graphitized carbon as not graphite. The introduced oxygen changed to amorphous SiO2, which was applied to the sintering-additive-like liquid-phase sintering.

Nacre is a natural organic-inorganic hybrid composite, whose hierarchical structure has a complex influence on its high strength. Many structural features have been discovered, which influence the mechanical properties of nacre, and the authors have a particular interest in the role of the asperities and organic-inorganic interactions. In this study, a composite was prepared which mimics the asperity structure using clay minerals. Organic-inorganic bonding was induced with silane treatment. Both factors increased the yield strength of the composites; however, different deformation behavior was exhibited. It was found that asperities improved the strength of the composite, and that composition influences the stiffness of the composite. The organic-inorganic interaction between the resin and the other components of the composite reduced the deformation of the composite and consequently improved strength.

Foam-type porous silicon carbide (SiC) ceramics without cracks and hollow struts were fabricated using the polymer replica method with polycarbosilane (PCS) and polyurethane (PU) foam as the starting materials. The synthesized porous SiC was analyzed using X-ray diffraction and scanning electron microscopy. The results revealed that a porous SiC ceramic structure was formed with a dense framework at a low temperature of 1200°C. During the heat-treatment process, the PCS experienced an organic–inorganic transformation and then converted to the SiC ceramics. It was determined that the organic–inorganic transformation of PCS, which was the stage of silicon oxycarbide formation, is affected by the curing condition. In this study, the optimum curing condition was determined to be an air atmosphere at 200°C for 7 h.

Mineral filler is used for resin compounds, because it increases the stiffness and thermal stability of a resin compound, and it also cuts down the cost. Calcium carbonate, silica, magnesium oxide, and others are used as filler materials in general, and the type of filler material, the size, and content can affect the physical properties of compounds. Those factors also influence the viscosity of resin mixtures and the workability, and should be adjusted by changing the contents of the filler, which depends on the size. In this study, five kinds of ground calcium carbonate, which were different in size, were used to produce polyester compounds ; the physical properties were compared with the filler size and contents. The mechanical properties were measured by bending strength and tensile strength, and the heat deflection temperature was obtained for thermal stability.

Silicon-based inorganic polymers are promising materials as matrix materials for glass fiber composites because of their good process ability, transparency, and thermal property. In this study, for utilization as a matrix precursor for a glass-fiber-reinforced composite, glassy silicone networks were prepared via hydrosilylation of linear/pendant Si−H polysiloxanes and the C=C bonds of viny-lterminated linear/cyclic polysiloxanes. 13C nuclear magnetic resonance spectroscopy was used to determine the structure of the cross-linked states, and a thermal analysis was performed. To assess the mechanical properties of the glassy silicone networks, we performed nanoindentation and 4-point bending tests. Cross-linked networks derived from siloxane polymers are thermally and optically more stable at high temperatures. Different cross-linking agents led to final networks with different properties due to differences in the molecular weights and structures. After stepped postcuring, the Young’s modulus and the hardness of the glassy silicone networks increased; however, the brittleness also increased. The characteristics of the cross-linking agent played an important role in the functional glassy silicone networks.

Si(Al)ON precursor was synthesized by formation of new Si-N bond using organoaluminum imine and liquid type poly(phenyl carbosilane). It was decomposed between 200 - 600℃, and the ceramic yield was 51% after pyrolysis. 150 ? 200 ㎚ in thickness of coating film was obtained by spin coating method. The precursor was easily oxidized during process because it was unstable in air. However the oxygen content was limited to 0.5 - 0.7 to silicon in heat treatment step. Even though the content of nitrogen was decreased by pyrolysis, Al-N and Si-N bonds were formed in ammonia atmosphere, and Si(Al)ON film was formed with 0.2 in content to silicon.

Natural materials often have unique mechanical properties, such as the hierarchical structure of nacre formed through mineral bridges or asperities created between an inorganic particle and a natural-layer surface. As these asperities produce an exceptional shear resistance, in this study, we aimed to emulate the natural structure of nacre by synthesizing inorganic asperities and mineral bridges with different temperatures in the range of 1100 - 1300℃ and clay contents from 10 - 50 wt%. Following the infiltration of methyl methacrylate, we measured the mechanical properties to assess whether they were improved by the asperities. It was confirmed that the asperities improved the bending strength by 10%, and the anchoring effect was observed on the fracture surface.

Foam type porous silicon carbide ceramics were fabricated by a polymer replica method using polyurethane foam, carbon black, phenol resin, and silicon powder as raw materials. The influence of the C/Si mole ratio of the ceramic slurry and heat treatment temperature on the porous silicon carbide microstructure was investigated. To characterize the microstructure of porous silicon carbide ceramics, BET, bulk density, X-ray Powder Diffraction (XRD), and Scanning Electron Microscope (SEM) analyses were employed. The results revealed that the surface area of the porous silicon carbide ceramics decreases with increased heat treatment temperature and carbon content at the 2 nd heat treatment stage. The addition of carbon to the ceramic slurry, which was composed of phenol resin and silicon powder, enhanced the direct carbonization reaction of silicon. This is ascribed to a consequent decrease of the wetting angles of carbon to silicon with increasing heat treatment temperature.

Since mesoporous silica such as MCM-41 and SBA-15 was developed, the study of the properties of high-surface area materials was accelerated. Moreover, the mesoporous silica is used as a template to produce high-surface materials by nanocasting technology. The purpose of this paper is the synthesis of a high surface silicon carbide sphere by the nanocasting technology. In this study, KCC-1 silica sphere was used as a template, and polycarbosilane and poly (phenyl carbosilane) were selected for precursor of silicon carbide. Carbosilane polymer gives advantage of synthesis silicon carbide under low temperature, and hollow spheres were produced. In this study, the polycarbosilane was more effective for the synthesis of SiC hollow spheres by inversion of template structure showing a fibrous morphology on the sphere wall. And it was confirmed that the sphere was composed of nanosized SiC crystals, and has high surface area using TEM, XRD and BET analysis.

β-SiC particles were synthesized using silicon powder and carbon black at 1300°C. The effect of grinding the silicon powder raw material into smaller particles on the synthesis and crystallinity of the SiC particles was investigated. The size of the silicon particles decreased and the surface of the silicon particles increasingly oxidized into silicon oxide with increasing grinding time, thus decreasing the yield of SiC because of the higher SiO2 content in the ground silicon powder, since SiO2 transforms into SiC at a higher temperature than does silicon. Although the sizes of the silicon particles in the raw materials were different, all the synthesized SiC crystallites showed similar size. Transmission electron microscopy-energy dispersive X-ray spectroscopy analysis showed that although most of the synthesized SiC particles were smaller than 100 nm, the primary SiC particles had aggregated, and some unreacted silicon was observed inside the large aggregated SiC particles.

The influence of a liquid medium duringa wet-milling process in the grinding and oxidation of silicon powder was investigated. Distilled water, dehydrated ethanol and diethylene glycol were used as the liquid media. The applied grinding times were 0.5, 3, and 12 h. Ground silicon powder samples were characterized by means of aparticle size analysis, scanning electron microscopy(SEM), xray powder diffraction (XRD), FT-IR spectroscopy and by a chemical composition analysis. From the results of the characterization process, we found that diethylene glycol is the most efficient liquid medium when silicon powder is ground using a wet-milling process. The FT-IR results show that the Si-O band intensity in an unground silicon powder is quite strongbecause oxygen becomes incorporated with silicon to form SiO₂ in air. By applying deionized water as a liquid medium for the grinding of silicon, the SiO₂ content increased from 4.12% to 31.7%. However, in the cases of dehydrated ethanol and diethylene glycol, it was found that the SiO₂ contents after grinding only changed insignificantly, from 4.12% to 5.91% and 5.28%, respectively.

Abalone shell is composed of 95 wt% CaCO 3 platelets and 5 wt% of a protein-rich organic matrix which acts as an adhesive layer, connecting aragonite tablets, thus maintaining the structural integrity of the composite. By mimicking abalone shell, we prepared a silicate plate/polymer nanocomposite by infiltrating PMMA between silicate layers and warm-pressing them at 200 o C for 1 h under 15 tons to make organic-inorganic composite materials. To examine the organic-inorganic composite materials after the warm-pressing procedure, the composite sample was analyzed with FE-SEM and TG. The bending strengths and densities of the composites prepared by a silicate plate and PMMA after the warm-pressing process were ~140 MPa and 1.5, respectively.

SiC powder can be produced generally through the Acheson process and it is required long carbothermic reaction time of SiO2 with carbon powder around 2200 °C ~ 2400 °C. Due to the high reaction temperature and long reaction time of the process, the powders produced have a large particle size and consist of mostly alpha phase SiC. Synthetic temperature of beta phase SiC powder is known to produce at 1700 °C ~ 1900 °C which is lower temperature than that of alpha phase SiC powder. We prepared β-SiC powder by heating precursor derived from the mixture of phenolic resin and tetraethyl orthosilicate. The precursor was heated at 1800 °C for 4 h in an Ar atmosphere. In order to examine the pyrolysis residue after the heat treatment, the SiC powder was analyzed with XRD and SEM. The X-ray diffraction result of the SiC powder shows the diffraction peaks around 35°, 60°, and 73° corresponded to the beta SiC phase. β-SiC powder prepared in this study contains lower metallic impurities compare than that of α-SiC powder prepared from Acheson method and is able to use as a good starting material for SiC single crystal growing.

Antimony doped tin oxide (ATO) is a typical p-type semiconductor and widely used due to its good properties such as electrical conductivity and transparency in visible range, while reflects infrared light. These features allow ATO to be used as transparent electrodes, heat mirrors and energy storage device, display panel. However, the use of tin oxide ceramics is limited by the low densification during sintering due to the dominance of non-densification mechanism such as surface diffusion or evaporation condensation. So, sintering agents such as Cobalt (II) oxide (CoO), zinc oxide (ZnO), manganese dioxide (MnO2) used to improve the densification of SnO2 by forming lattice solid solution or liquid phase. In this study, we studied the effect of the sintering agents. The antimony-doped tin oxide nanoparticles were synthesized by a sol-gel method. After sintering ATO with sintering agents such as MnO2 and CoO, sintered body characteristics were investigated by XRD, SEM, thermal conductivity, resistivity and interesting characteristic.

Recently, variety of organic and inorganic hybrid materials have recently investigated as alternative routes to SiOC, SiO 2 thin film formation at low temperatures for applications in electronic ceramics. Specially, silicon based polymers, such as polycarbosilane, polysilane and polysilazane derivatives have been studied for use in electronic ceramics and have been applied as dielectric or insulating materials. In this study, Polycarbosilane(PCS), which Si-CH 2 -Si bonds build up the backbone of the polymer, has been investigated as low-k materials using a solution process. After heat treatment at 350℃ under N 2 atmosphere, chemical composition and dielectric constant of the thin film were SiO 0.27 C 1.94 and 1.2, respectively. Mechanical property measured using nanoindentor shows 1.37 GPa.

The effects of the pair-minerializer (CaSO4,-CaF2) on the burnability of clinker and the formation of mineral are investigated. As experimental results, the addition of F as pair-mineralizer, the burnability of clinker was improved. Especially, the addition of F and SO3 components simultaneously as pair-mineralizer, total amount of free lime in clinker is rapidly decreased compared with the value which mineralizer was added separately. In case of adding K2O only to the raw mixes, the amount of free lime is increased. However, if alkali (K2O) and pair-minerializer (CaSO4,-CaF2) were added simultaneously, the produced amount of clinker mineral is increased due to the formation of stable calcium langbeinite by the combination of alkali (K2O) and sulfate

Bismuth metal, upon activation by zinc fluoride, can effect the crossed aldol reaction between α-bromocarbonyl compounds and aldehydes in aqueous media. The reaction was found to be regiospecific and syn-diastereoselective.Key words: bismuth, zinc fluoride, aldol reaction, regioselectivity, aqueous media.

The introduction of A ring pyrazole modification to the hydrocortisone C-21 heteroaryl thioethers generated compounds with excellent transrepression potency (IL-8 inhibition) compared to their hydrocortisone analogs. However, the transcriptional transactivation activity of these compounds were considerably higher than the corresponding hydrocortisone analogs. Among all the compounds evaluated, a quinoxaline thioether modification demonstrated the best overall in vitro separation.

The prednisolone C-21 heteroaryl thioethers have been synthesized and evaluated in cell based transrepression and transactivation assays. Most of the compounds demonstrated weak transactivational activity in both human and rat tyrosineaminotransferase functional assay while keeping potent anti-inflammatory activity. The benzimidazole thioether 7 exhibited comparable anti-inflammatory activity and improved safety profile compared to the classical oral steroid prednisolone.

Structural features of the substituted 4-piperidinyl urea analogs 1, responsible for the H3 antagonist activity, have been identified. Structure–activity relationship of the H3 receptor affinity, hERG ion channel inhibitory activity and their separation is described. Preliminary pharmacokinetic evaluation of the compounds of the series is addressed.

Bismuth metal, activated by ammonium hydrogen fluoride, was found to reduce various α-halocarbonyl compounds efficiently and chemoselectively in aqueous media to give the dehalogenated reduction products in excellent yields.Key words: bismuth, reduction, α-halocarbonyl compounds, fluoride salts, aqueous media, bismuth enolates, aldol reaction.

The effects of clay, aluminum hydroxide, and carbon powder on the sintering of a Si/SiC mixture from photovoltaic silicon-wafer production were investigated. Sintering temperature was fixed at and the sintered bodies were characterized by SEM and XRD to analyze the microstructure and to measure the apparent porosity, absorptivity, and apparent density. The XRD peak intensity of SiC in the sintered body was increased by adding 5% carbon to the Si/SiC mixture. From this result, it is confirmed that Si in the Si/SiC mixture had reacted with the added carbon. Addition of aluminum hydroxide decreased the cristobalite phase and increased the stable mullite phase. The measurement of the physical properties indicates that adding carbon to the Si/SiC mixture enables us to obtain a dense sintered body that has high apparent density and low absorptivity. The sintered body produced from the Si/SiC mixture with aluminum hydroxide and carbon powder as sintering additives can be applied to diesel particulate filters or to heat storage materials, etc., since it possesses high thermal conductivity, and anticorrosion and antioxidation properties.

In order to meet the requirements of faster speed and higher packing density for devices in the field of semiconductor manufacturing, the development of Cu/Low k device material is explored for use in multi-layer interconnection. SiOC(-H) thin films containing alkylgroup are considered the most promising among all the other low k candidate materials for Cu interconnection, which materials are intended to replace conventional Al wiring. Their promising character is due to their thermal and mechanical properties, which are superior to those of organic materials such as porous , SiOF, polyimides, and poly (arylene ether). SiOC(-H) thin films containing alkylgroup are generally prepared by PECVD method using trimethoxysilane as precursor. Nano voids in the film originating from the sterichindrance of alkylgroup lower the dielectric constant of the film. In this study, methyltriphenylsilane containing bulky substitute was prepared and characterized by using NMR, single-crystal X-ray, GC-MS, GPC, FT-IR and TGA analyses. Solid-state NMR is utilized to investigate the insoluble samples and the chemical shift of . X-ray single crystal results confirm that methyltriphenylsilane is composed of one Si molecule, three phenyl rings and one methyl molecule. When methyltriphenylsilane decomposes, it produces radicals such as phenyl, diphenyl, phenylsilane, diphenylsilane, triphenylsilane, etc. From the analytical data, methyltriphenylsilane was found to be very efficient as a CVD or PECVD precursor.