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Becher Paul F. The Korean Ceramic Society 2005 한국세라믹학회지 Vol.42 No.8
Tailoring the microstructure and the composition of silicon nitride ceramics can have profound effects on their properties. Here it is shown that the grain growth behavior, in particular its anisotropy, is a function of the specific additives, which allow one to tune the microstructure from one consisting of more equiaxed grains to one with very elongated grains. Recent studies are discussed that provide an understanding of the atomic level processes by which these additives influence grain shapes. Next the microstructural (and compositional) parameters are discussed that can be used to modify the thermal conductivity, as well as fracture toughness of silicon nitride ceramics. As a result of the open <0001> channels in $\beta-Si_3N_4$, the c-axis conductivity can be exceptionally high. Thus, the formation of elongated c-axis grains, particularly when aligned can result in conductivity values approaching those of AlN ceramics. In addition, the controlled formation of elongated grains can also be used to significantly enhance the fracture toughness. At the same time, both properties are shown to be affected by the composition of the densification additives. Utilizing such understanding, one will be able to tailor the ceramics to achieve the properties desired for specific applications.
Becher, Paul F. The Korean Ceramic Society 2016 한국세라믹학회지 Vol.53 No.6
Microstructural design of ceramics has generally focused on information gathered at the micro- and macro-scales and related this to how specific properties could be improved. Ceramic processing serves as the key to optimizes the final microstructure. However, the advent of nano-scale microstructures and highly advanced characterization tools are forcing us to develop new knowledge of what is occurring not just at the micro-scale but also at the atomic level. Thus we are now beginning to be able to address how microstructure is influenced by events at the atomic scale using atomic scale images and data. Theoreticians have joined us in interpreting the mechanisms involved in the "microstructural" evolution at multiple scales and how this can be used to enhance specific properties of ceramics. The focus here is on delving into the various layers the "microstructure" in order understand how atomic-scale events influence the structure and properties of ceramics.
Paul F. Becher 한국세라믹학회 2005 한국세라믹학회지 Vol.42 No.8
Tailoring the microstructure and the composition of silicon nitride ceramics can have profound effects on their properties. Here it is shown that the grain growth behavior, in particular its anisotropy, is a function of the specific additives, which allow one to tune the microstructure from one consisting of more equiaxed grains to one with very elongated grains. Recent studies are discussed that provide an understanding of the atomic level processes by which these additives influence grain shapes. Next the microstructural (and compositional) parameters are discussed that can be used to modify the thermal conductivity, as well as fracture toughness of silicon nitride ceramics. As a result of the open <0001> channels in Si3N4, the c-axis conductivity can be exceptionally high. Thus, the formation of elongated c-axis grains, particularly when aligned can result in conductivity values approaching those of AlN ceramics. In addition, the controlled formation of elongated grains can also be used to significantly enhance the fracture toughness. At the same time, both properties are shown to be affected by the composition of the densification additives. Utilizing such understanding, one will be able to tailor the ceramics to achieve the properties desired for specific applications.
이세훈,고재웅,박영조,김해두,Hua-Tay Lin,Paul Becher 한국세라믹학회 2012 한국세라믹학회지 Vol.49 No.4
Reaction-bonded silicon nitrides containing rare-earth oxide sintering additives were densified by gas pressure sintering. The sintering behavior, microstructure and mechanical properties of the resultant specimens were analyzed. For that purpose, Lu2O3-SiO2 (US), La2O3-MgO (AM) and Y2O3-Al2O3 (YA) additive systems were selected. Among the tested compositions, densification of silicon nitride occurred at the lowest temperature when using the La2O3-MgO system. Since the Lu2O3-SiO2 system has the highest melting temperature, full densification could not be achieved after sintering at 1950oC. However, the system had a reasonably high bending strength of 527 MPa at 1200oC in air and a high fracture toughness of 9.2 MPa·m1/2. The Y2O3-Al2O3 system had the highest room temperature bending strength of 1.2 GPa.
이새훈,조춘래,박영조,고재웅,김해두,Hua-Tay Lin,Paul Becher 한국세라믹학회 2013 한국세라믹학회지 Vol.50 No.3
The densification behavior and strength of sintered reaction bonded silicon nitrides (SRBSN) that contain Lu2O3-SiO2 additives were improved by the addition of fine Si powder. Dense specimens (relative density: 99.5%) were obtained by gas-pressure sintering (GPS) at 1850oC through the addition of fine Si. In contrast, the densification of conventional specimens did not complete at 1950oC. The fine Si decreased the onset temperature of shrinkage and increased the shrinkage rate because the additive helped the compaction of green bodies and induced the formation of fine Si3N4 particles after nitridation and sintering at and above 1600oC. The amount of residual SiO2 within the specimens was not strongly affected by adding fine Si powder because most of the SiO2layer that had formed on the fine Si particles decomposed during nitridation. The maximum strength and fracture toughness of the specimens were 991 MPa and 8.0 MPa·m1/2, respectively.
Lee, Sea-Hoon,Ko, Jae-Woong,Park, Young-Jo,Kim, Hai-Doo,Lin, Hua-Tay,Becher, Paul The Korean Ceramic Society 2012 한국세라믹학회지 Vol.49 No.4
Reaction-bonded silicon nitrides containing rare-earth oxide sintering additives were densified by gas pressure sintering. The sintering behavior, microstructure and mechanical properties of the resultant specimens were analyzed. For that purpose, $Lu_2O_3-SiO_2$ (US), $La_2O_3$-MgO (AM) and $Y_2O_3-Al_2O_3$ (YA) additive systems were selected. Among the tested compositions, densification of silicon nitride occurred at the lowest temperature when using the $La_2O_3$-MgO system. Since the $Lu_2O_3-SiO_2$ system has the highest melting temperature, full densification could not be achieved after sintering at $1950^{\circ}C$. However, the system had a reasonably high bending strength of 527 MPa at $1200^{\circ}C$ in air and a high fracture toughness of 9.2 $MPa{\cdot}m^{1/2}$. The $Y_2O_3-Al_2O_3$ system had the highest room temperature bending strength of 1.2 GPa.