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H. Hyuga,M. I. Jones,K. Yoshida,N. kondo,K. Hirao,H. Kita 한양대학교 세라믹연구소 2009 Journal of Ceramic Processing Research Vol.10 No.3
The tribological behavior of monolithic Si3N4 and solid lubricants dispersed Si3N4-based composites has been assessed under a high load and low speeds in an aqueous environment. The results showed that the friction coefficient of the Si3N4 was not significantly reduced when compared to dry sliding and this was attributed to the failure to maintain a lubricating layer between the solid-solid surfaces. In the case of the composites, the initial high friction coefficient was reduced shortly after the beginning of the wear test and maintained a low value (approximately 0.03) throughout. This was attributed to the solid lubricating effect of the composite resulting in a lower stress at the contact asperities preventing the removal of the lubricating layer. The solid lubricant content did not affect the value of the friction coefficient under hydrodynamic type lubrication, but the running in distance decreased with the solid lubricant dispersion. The type and amount of solid lubricant affected the distance required to reach a low friction regime, but all types of solid lubricant showed decreasing friction with increasing amount. In Stribeck analysis, the addition of the solid lubricant resulted in a highly graphitic transfer layer on the Si3N4 material, and shifted the transition points from hydrodynamic to mixed and from mixed to boundary lubrication regimes to more severe conditions. It also reduced the friction coefficient in the boundary lubrication regime. The tribological behavior of monolithic Si3N4 and solid lubricants dispersed Si3N4-based composites has been assessed under a high load and low speeds in an aqueous environment. The results showed that the friction coefficient of the Si3N4 was not significantly reduced when compared to dry sliding and this was attributed to the failure to maintain a lubricating layer between the solid-solid surfaces. In the case of the composites, the initial high friction coefficient was reduced shortly after the beginning of the wear test and maintained a low value (approximately 0.03) throughout. This was attributed to the solid lubricating effect of the composite resulting in a lower stress at the contact asperities preventing the removal of the lubricating layer. The solid lubricant content did not affect the value of the friction coefficient under hydrodynamic type lubrication, but the running in distance decreased with the solid lubricant dispersion. The type and amount of solid lubricant affected the distance required to reach a low friction regime, but all types of solid lubricant showed decreasing friction with increasing amount. In Stribeck analysis, the addition of the solid lubricant resulted in a highly graphitic transfer layer on the Si3N4 material, and shifted the transition points from hydrodynamic to mixed and from mixed to boundary lubrication regimes to more severe conditions. It also reduced the friction coefficient in the boundary lubrication regime.
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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 $MPa{\cdot}m^{1/2}$.
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Hideo Nakai,Taiju Hyuga,Shina Kawai,Taro Kubo,Shigeru Nakamura 대한비뇨의학회 2017 Investigative and Clinical Urology Vol.58 No.-
Vesicoureteral reflux (VUR) is one of the most common diseases in pediatric urology and classified into primary and secondary VUR. Although posterior urethral valve (PUV) is well known as a cause of the secondary VUR, it is controversial that minor urethral deformity recognized in voiding cystourethrography represents mild end of PUV spectrum and contributes to the secondary VUR. We have been studying for these ten years congenital urethral obstructive lesions with special attention to its urethrographic and endoscopic morphology as well as therapeutic response with transurethral incision. Our conclusion to date is that congenital obstructive lesion in the postero-membranous urethra is exclusively PUV (types 1 and 3) and that severity of obstruction depends on broad spectrum of morphological features recognized in PUV. Endoscopic diagnostic criteria for PUV are being consolidated.
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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.
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