Comparison of hot-corrosion behavior of Al2O3, Lu2O3 and their silicates

Shunkichi Ueno,Tatsuki Ohji,Hua-Tay Lin 한양대학교 세라믹연구소 2006 Journal of Ceramic Processing Research Vol.7 No.3

The hot-corrosion behavior of Al2O3, Lu2O3, mullite and Lu2Si2O7 phases were examined at 1500 oC. In the case of Lu2O3, no phase change was observed and no anisotropical corrosion occurred. In the case of the Lu2Si2O7 phase, anisotropic corrosion was observed. The Lu2Si2O7 crystal grains were well sustained to the hot-corrosion, however, the sodium element diffused though the boundary phase and anomalous grain growth occurred. On the other hand, in the case of Al2O3, the grains at the bulk surface corroded anisotropically. The corroded grain had a characteristic morphology as terrace field. In the case of mullite, the surface of the bulk was decomposed to the Al2O3 phase and the silica component was completely removed from the bulk surface. The sodium ions diffused though the sample during the corrosion test. The hot-corrosion behavior of Al2O3, Lu2O3, mullite and Lu2Si2O7 phases were examined at 1500 oC. In the case of Lu2O3, no phase change was observed and no anisotropical corrosion occurred. In the case of the Lu2Si2O7 phase, anisotropic corrosion was observed. The Lu2Si2O7 crystal grains were well sustained to the hot-corrosion, however, the sodium element diffused though the boundary phase and anomalous grain growth occurred. On the other hand, in the case of Al2O3, the grains at the bulk surface corroded anisotropically. The corroded grain had a characteristic morphology as terrace field. In the case of mullite, the surface of the bulk was decomposed to the Al2O3 phase and the silica component was completely removed from the bulk surface. The sodium ions diffused though the sample during the corrosion test.

Designing lutetium silicate environmental barrier coatings for silicon nitride and its recession behavior in steam jets

Shunkichi Ueno,Tatsuki Ohji,Hua-Tay Lin 한양대학교 세라믹연구소 2006 Journal of Ceramic Processing Research Vol.7 No.1

To protect silicon nitride ceramics from water vapor corrosion and oxidation, a new multi-layered environmental barrier coating (EBC) system was developed. The EBC system consisted of a highly dense Lu2SiO5/Lu2Si2O7 binary phase layer and slightly porous Lu2SiO5/Lu2Si2O7 layer, which were coated by a sputtering and dipping technique, respectively. A steam jet exposure test was conducted at 1300 oC for 500 hours with 35 m/s water vapor velocity. Each grain of the dip-coated layer was peeled off from the surface during the test. Also, the highly dense sputter coated layer converted into a porous layer after the test.

Role of Mn and Co additives on the carbothermal nitridation of fly ash

Shunkichi Ueno,Hideki Kita,Tatsuki Ohji 한양대학교 세라믹연구소 2005 Journal of Ceramic Processing Research Vol.6 No.4

The effects of Mn and Co additives on the carbothermal nitridation of fly ash were examined at 1330, 1425 and 1475 oC in a N2 atmosphere under normal pressure. Above 1425 oC, the nitridation of fly ash occurred and oxy-nitride phases, such as β-type sialon, α-type sialon and X2N2O (X=C or Si) type oxy-nitride phases were formed. The phase ratio among these phases was changed with the amount of additives. In the case of a Mn additive, the a to b phase transformation of sialon was accelerated with the amount of the additive. On the other hand, in the case of a Co additive, no effective role on the transformation was observed. At 1475 oC, the ash completely changed to β-type sialon.

Comparison of water vapor corrosion mechanisms of polycrystalline and eutectic Lu2Si2O7

Shunkichi Ueno,D. Doni Jayaseelan,Tatsuki Ohji,Naoki Kondo,Shuzo Kanzaki 한양대학교 세라믹연구소 2004 Journal of Ceramic Processing Research Vol.5 No.2

To clarify the water vapor corrosion mechanism of the Lu2Si2O7 system, a high temperature water vapor corrosion test was performed on two microstructurally different specimens namely, polycrystalline and eutectic. From the results, we address the problem of the application of the Lu2Si2O7 system for environmental barrier coatings (EBC) with regard to their microstructure. In the case of the polycrystalline sample, the boundaries were easily corroded and the silicon nitride substrate underwent oxidation. In the case of the eutectic sample, no weight change was observed during the corrosion test. However, a trace of corrosion and poly-crystallization was observed on the sample surface. An easy processing technique for forming an eutectic protective layer on silicon nitride ceramics is also proposed.

Comparison of water vapor corrosion behavior of silicon nitride with various EBC layers

Shunkichi Ueno,D. Doni Jayaseelan,Tatsuki Ohji 한양대학교 세라믹연구소 2004 Journal of Ceramic Processing Research Vol.5 No.4

The water vapor corrosion resistance of silicon nitride ceramics with Lu2Si2O7, Yb2Si2O7 and ZrSiO4 environmental barrier coating (EBC) layers prepared by a reaction sintering method were examined at 1500 oC in a static state water vapor environment. The weights of these samples slightly increased during the corrosion test. The weight gain rates for Lu, Yb and zircon EBC samples were 2.218 × 10−6, 4.688 × 10−6 and 2.989 × 10−6 g/cm2·h, respectively. In the corrosion mechanism of Lu and Yb cases, the gas path through channels in the EBC layer were generated by the elimination of the boundary silica phase and then, the silicon nitride substrate was slightly oxidized. On the other hand, in the case of ZrSiO4, many cracks were introduced in the EBC layer during the corrosion test due to the difference in thermal expansion coefficient between the EBC layer and the substrate. However, the adhesion between the substrate and the EBC layer of ZrSiO4 was better than that of Lu and/or Yb EBC samples.

Platinum Nano-Dispersion via In Situ Processing : Preparation and catalytic Property of Porous Nanocomposite

Yoshikazu,Hwang Hae-Jin,Naoki Kondo,Tatsuki Ohji 한국분말야금학회 2001 한국분말야금학회지 Vol.8 No.3

A bulk porous composite with plantinum nano-dispersion was synthesized in air atmosphere through the combination of several in situ reactions, including the pyrolysis of . A mixture of (dolomite), , and LiF (0.5 wt%, as an additive) was cold isostatically pressed at 200 MPa and sintered at for 2 h. The porous composite ( : Pt=99 : 1 in volume) had a uniformly open-porous structure (porosity: 56%) with three-dimensional (3-D) network and a narrow pore-size distribution, similarly to the porous composites reported before. Catalytic Properties (viz., NO direct decomposition and NO reduction by ) of the composite were investigated up to . In the absence of oxygen, the NO conversion rate reached ~52% for the direct decomposition and ~100% for the reduction by , respectively. The results suggest the possibility of the porous composite as a multifunctional filter, i.e., simultaneous hot gas-filtering and in one component.

High Thermal Conductivity Silicon Nitride Ceramics

Hirao, Kiyoshi,Zhou, You,Hyuga, Hideki,Ohji, Tatsuki,Kusano, Dai The Korean Ceramic Society 2012 한국세라믹학회지 Vol.49 No.4

This paper deals with the recent developments of high thermal conductivity silicon nitride ceramics. First, the factors that reduce the thermal conductivity of silicon nitride are clarified and the potential approaches to realize high thermal conductivity are described. Then, the recent achievements on the silicon nitride fabricated through the reaction bonding and post sintering technique are presented. Because of a smaller amount of impurity oxygen, the obtained thermal conductivity is substantially higher, compared to that of the conventional gas-pressure sintered silicon nitride, while the microstructures and bending strengths are similar to each other between these two samples. Moreover, further improvement of the thermal conductivity is possible by increasing ${\beta}/{\alpha}$ phase ratio of the nitrided sample, resulting in a very high thermal conductivity of 177 W/($m{\cdot}K$) as well as a high fracture toughness of 11.2 $MPa{\cdot}m^{1/2}$.

High Thermal Conductivity Silicon Nitride Ceramics

Kiyoshi Hirao,You Zhou,Hideki Hyuga,Tatsuki Ohji,Dai Kusano 한국세라믹학회 2012 한국세라믹학회지 Vol.49 No.4

This paper deals with the recent developments of high thermal conductivity silicon nitride ceramics. First, the factors that reduce the thermal conductivity of silicon nitride are clarified and the potential approaches to realize high thermal conductivity are described. Then, the recent achievements on the silicon nitride fabricated through the reaction bonding and post sintering technique are presented. Because of a smaller amount of impurity oxygen, the obtained thermal conductivity is substantially higher, compared to that of the conventional gas-pressure sintered silicon nitride, while the microstructures and bending strengths are similar to each other between these two samples. Moreover, further improvement of the thermal conductivity is possible by increasing β/α phase ratio of the nitrided sample, resulting in a very high thermal conductivity of 177 W/(m·K) as well as a high fracture toughness of 11.2 MPa· m1/2.

Corrosion and oxidation behavior of ASiO4 (A=Ti, Zr and Hf) and silicon nitridewith an HfSiO4 environmental barrier coating

Shunkichi Ueno,D. Doni Jayaseelan,Tatsuki Ohji,Hua-Tay Lin 한양대학교 세라믹연구소 2005 Journal of Ceramic Processing Research Vol.6 No.1

The oxidation and water vapor corrosion behavior of sintered HfSiO4, ZrSiO4, TiSiO4 compacts and high density HfSiO4 coated silicon nitride ceramics were examined at 1500 oC in a static state water vapor environment. The bulk weights of HfSiO4 and ZrSiO4 decreased after the water vapor corrosion test with a weight loss rate of 7.080 × 10−6 and 1.333 × 10−6 g/cm2·h, respectively. The weight of TiSiO4 remained unchanged within the experimental error after the corrosion test. A trace of corrosion was observed on the surface of grains of corrosion-tested compacts. On the other hand, the weight of high density HfSiO4 layer (Environmental Barrier Coating, EBC layer) coated silicon nitride specimens increased with a weight gain rate 1.495 × 10−6 g/cm2·h, which is attributed by the oxidation of the silicon nitride substrate.