Deock-Soo Cheong

Influence of SiC particle size and drying method on mechanical properties and microstructure of Si3N4SiC nanocomposite

Abstract Si 3 N 4 20 vol% SiC nanocomposites with different SiC particle sizes (mean particle size of 30 nm and 270 nm, which were made by plasma CVD and carbothermal reduction, respectively) were fabricated by hot pressing. The effects of second-phase particle size and drying methods for mixed slurry (microwave drying and rotary evaporator) on microstructure and mechanical properties were analyzed. The nanocomposite with ultra-fine (30 nm) SiC dispersoid had a high average strength of 1680 MPa. This value was 26% higher than that of the nanocomposite with coarser (270 nm) SiC dispersoid and corresponded to the strength of nanocomposite made from amorphous Si-C-N precursor powder. Fracture strength and microstructure of the nanocomposite were significantly influenced by the drying methods as well as the SiC particle size. Refinement of microstructure and deagglomoration of the SiC particles were the main reasons for strengthening of the nanocomposite. Furthermore, consolidation of grain boundary phase caused by free-carbon impurity in ultra-fine SiC powder could also be an important factor in the improvement of strength.

Fabrication, mechanical properties and microstructure analysis of Si3N4/SiC nanocomposite

Ceramic nanocomposites, Si 3 N 4 matrix reinforced with nano-sized SiC particles, were fabricated by hot pressing the mixture of Si 3 N 4 and SiC fine powders with different sintering additives. Distinguishable increase in fracture strength at low and high temperatures was obtained by adding nano-sized SiC particles in Si 3 N 4 with Al 2 O 3 and/or Y 2 O 3 . Si 3 N 4 /SiC nanocomposite added with Al 2 O 3 and Y 2 O 3 demonstrated the maximum strength of 1.9 GPa with average strength of 1.7 GPa. Fracture strength of room temperature was retained up to 1400 as I GPa in the sample with addition of 30 nm SiC and 4 wt% Y 2 O 3 Striking observation in this nanocomposite is that SiC particles at grain boundary are directly bonded to Si 3 N 4 grain without glassy phases. Thus, significant improvement in high temperature strength in this nanocomposite can he attributed to inhibition of grain boundary sliding and cavity formation primarily by intergranular SiC particles, besides crystallization of grai boundary phase.

Structural evolution of alumina membrane prepared on an alumina support using a sol-gel method

We prepared supported and unsupported alumina membranes using a sol–gel method. The supported membrane system consisted of an alumina support, two intermediate α-Al2O3 layers, and a top alumina membrane. The θ- to α-Al2O3 transformation in supported and unsupported alumina membranes was investigated using X-ray diffraction (XRD) and scanning electron microscopy. XRD patterns showed that the supported membrane had a 100°C higher θ- to α-Al2O3 transformation temperature than the unsupported one. A similar effect was observed for microstructures of the membranes. We explained their transformation-temperature difference with a stress generated in the supported top membrane using a theoretical approach.

A Simple Way to Prepare Nanosize NiO Powder by Mixing Acidic Ni Compound with Basic Ni Compound

Nanosize NiO powder was prepared by mixing acidic nickel nitrate with basic nickel carbonate. The particle size and morphology of NiO were mainly governed by the mole ratio of the nitrate to the carbonate. The effects were studied by DSC, XRD, FTIR, and SEM. Heat treatment conditions influence the particle size distribution of produced NiO powder extensively for the case of 3N7C (3 moles of the nitrate and 7 moles of the carbonate) and 4N6C, but only slightly for 1N9C and 2N8C. Uniform pseudospherical NiO particles were obtained in 50~70 ㎚ for 1N9C and 30~60 ㎚ for 2N8C by calcination at 750℃ for 2 h.

A new process for Al2O3/SiC nanocomposites by polycarbosilane infiltration

Abstract Conventional powder processing is commonly used to fabricate Al2O3/SiC nanocomposites. Recently a new processing route using SiC polymeric precursor such as polysilazane, polysilastyrene or polycarbosilane (PCS) has also been attempted, where a polymeric precursor solution is mixed with alumina powder. The present work investigated a new process to make the nanocomposites by modifying such a polymer precursor route. Unlike the existing process, the solution of a SiC polymer precursor was infiltrated into partially sintered (i.e. porous) alumina bodies, sintered at normal pressure. With PCS precursor, it was studied how the precursor solution concentration and sintering condition affected the SiC content, densification, phase and microstructure.

The microscopic features of (Li0.5La0.5)TiO3

Abstract In ionic conducting materials, the crystal structure is closely related to the ionic conductivity. In this research we studied the microscopic features of Li 0.5 La 0.5 TiO 3 which exhibited a lithium ionic conductivity as high as 1×10 −3 Scm −1 at room temperature by XRD, TEM and SIMS. It was found that the superstructure was caused by the ordering of La +3 and vacancy, producing the 2a p ×2a p ×2a p unit cell. This ordering was found to be regular in microscopic region, but became irregular in macroscopic region. Li + showed a random distribution which meet the needs for the fast ionic conduction. The second phase was found to be Li 2 TiO 3 which existed in the grain boundary junctions.

Indium Tin Oxide (ITO) Coatings Fabricated Using Mixed ITO Sols

ITO films were achieved by sintering at 500~550℃. This was possible by inducing a seeding effect on an ITO sol by producing crystalline ITO nanoparticles in situ during heat treatment. Two kinds of ITO sols (named ITO-A and ITO-B) were prepared at 2.0 wt% from indium acetate and tin(IV) chloride in different mixed solvents. The ITO-A sol showed a high degree of crystallinity of ITO without any detectable SnO₂ on XRD at 350℃/1 h, but the ITO-B sol showed a small amount of SnO₂ even after annealing at 600℃/1 h. The 10 wt% ITO-A//ITO-B showed the sheet resistance of 3600Ω/□, while the ITO-B sol alone showed 5200 Ω/□ by sintering at 550℃ for 30 min. Processing parameters were studied by TG/DSC, XRD, SEM, sheet resistance, and visible transmittance.

Effect of Additives on Mechanical Properties of Alumina Bushing Fabricated by Gel-Casting

Alumina bushing used for manufacturing glass fiber was fabricated by gel-casting which can fabricate complex forms. When solid loading is increased, density was increased and shrinkage and absorption were decreased. The sample loading with 57 vol% solid was optimum for alumina suspension, which showed the best physical properties. The cast sample was sintered at 1550℃ for two hours with sintering additive, Y₂O₃. The result showed that the alumina bushing with Y₂O₃ sintering additive has a density of 98%, shrinkage of 11% and bending strength of 196 ㎏/㎠.

Gas Pressure Sintering, Mechanical Properties and Microstructure of Three Kinds of Si₃N₄ Ceramics

Three kinds of Si₃N₄ powders (M-11, SN-ESP, and SN-E10) were gas-pressure sintered at 1700-1900℃ for 2 h under 18 atm N₂. Their densification behavior was investigated and compared as well as the mechanical properties and microstructure of the resulting ceramics. SN-ESP and SN-E10 started to reach nearly full densification at 1750℃ and showed almost no decomposition up to 1900℃. In contrast, M-11 was not fully densified until 1800℃ and showed about 3% weigh loss at 1900℃ indicating poor thermal stability. SN-ESP and SN-E10 showed much higher strength both at room temperature and 1200℃ than M-11 when fully densified. Compared with SN-ESP, SN-E10 was not only a little better in strength (both at room temperature and 1200℃) and fracture toughness but also much higher in the Weibull modulus due to more interlocked microstructure by well elongated grains.