Sintering is the proof test of ceramic processing.So is flaw-sensitive flexural strength.The calibration and optimization of processing are critical to the further development of ceramic science and technology.Chasing...Sintering is the proof test of ceramic processing.So is flaw-sensitive flexural strength.The calibration and optimization of processing are critical to the further development of ceramic science and technology.Chasing ultra-strength in brittle ceramics such as alumina is thus significant in the sense that it best illustrates how one can eliminate flaws via powders,forming,and sintering.Previous reports in high-strength alumina with three-point-bending strengths above 1 GPa are rare,while some cause confusion and suspicion.Here,we revisited the processing of 1 GPa alumina ceramics via accessible processing route of mild-speed centrifugal casting and pressureless two-step sintering.We demonstrated a high green body density of 63%,a low pressureless sintering temperature of 1175℃,a high sintering density of 99.2%,a fine grain size of 0.52μm,and a high flexural strength of 1036±32 MPa(three-point-bending over a 20 mm span length).We hope our study could set down the questioning on 1 GPa alumina:it is doable and can be done better.展开更多
Porous silicon nitride ceramics have attracted a considerable attention due to their excellent overall performance,but poor porosity homogeneity and structural shrinkage induced by prolonged high temperature sintering...Porous silicon nitride ceramics have attracted a considerable attention due to their excellent overall performance,but poor porosity homogeneity and structural shrinkage induced by prolonged high temperature sintering limit its further application.Herein,as a three-in-one solution for the above issues,for the first time we develop a novel approach that integrates the merits of gelcasting-SHS(self-propagating high-temperature synthesis)to prepare porous Si_(3)N_(4)ceramics to simultaneously achieve high porosity,high strength,high toughness,and low thermal conductivity across a wide temperature range.By regulating the solid content,porous Si_(3)N_(4)ceramics with homogeneous pore structure are obtained,where the pore size falls inbetween 1.61 and 4.41 pm,and the elongated grains are interlaced and interlocked to form micron-sized coherent interconnected pores.At the same time,porous Si_(3)N_(4)ceramics with porosity of 67.83%to 78.03%are obtained,where the compressive strength reaches 11.79 to 47.75 MPa and fracture toughness reaches 1.20 to 6.71 MPa-m1/2.展开更多
The α-Fe_2 O_3@SiO_2 reddish pigments with core-shell structure were successfully prepared by hydrothermal and Stober methods. The structure, morphology, and chromaticity of the synthesized pigments were characterize...The α-Fe_2 O_3@SiO_2 reddish pigments with core-shell structure were successfully prepared by hydrothermal and Stober methods. The structure, morphology, and chromaticity of the synthesized pigments were characterized by XRD, SEM, TEM, FTIR, XPS, and colorimetry. The results indicated that the as-prepared pigments have the characteristics of narrow particle size distribution, high dispersion,and good sphericity. The α-Fe_2 O_3@SiO_2 reddish pigments were uniform and well dispersed in solution. In addition, the pigments with different shell thickness were also prepared, and the effect of shell thickness on the color performance of the pigments was discussed.展开更多
We investigate quantum kinetic theory for a massive fermion system under a rotational field.From theDirac equation in rotating frame we derive the complete set of kinetic equations for the spin components of the 8-and...We investigate quantum kinetic theory for a massive fermion system under a rotational field.From theDirac equation in rotating frame we derive the complete set of kinetic equations for the spin components of the 8-and 7-dimensional Wigner functions.While the particles are no longer on a mass shell in the general case due to therotation-spin coupling,there are always only two independent components,which can be taken as the number andspin densities.With help from the off-shell constraint we obtain the closed transport equations for the two independent components in the classical limit and at the quantum level.The classical rotation-orbital coupling controls thedynamical evolution of the number density,but the quantum rotation-spin coupling explicitly changes the spin density.展开更多
基金supported by the National Natural Science Foundation of China(NSFC,Nos.52472074 and 52372064)Xishan−Tsinghua Special Funding for the Deep Integration of Industry,Academia and Research(No.20242000282).
文摘Sintering is the proof test of ceramic processing.So is flaw-sensitive flexural strength.The calibration and optimization of processing are critical to the further development of ceramic science and technology.Chasing ultra-strength in brittle ceramics such as alumina is thus significant in the sense that it best illustrates how one can eliminate flaws via powders,forming,and sintering.Previous reports in high-strength alumina with three-point-bending strengths above 1 GPa are rare,while some cause confusion and suspicion.Here,we revisited the processing of 1 GPa alumina ceramics via accessible processing route of mild-speed centrifugal casting and pressureless two-step sintering.We demonstrated a high green body density of 63%,a low pressureless sintering temperature of 1175℃,a high sintering density of 99.2%,a fine grain size of 0.52μm,and a high flexural strength of 1036±32 MPa(three-point-bending over a 20 mm span length).We hope our study could set down the questioning on 1 GPa alumina:it is doable and can be done better.
基金supports from the National Natural Science Foundation of China(NSFC,Nos.51872159 and 52072381).
文摘Porous silicon nitride ceramics have attracted a considerable attention due to their excellent overall performance,but poor porosity homogeneity and structural shrinkage induced by prolonged high temperature sintering limit its further application.Herein,as a three-in-one solution for the above issues,for the first time we develop a novel approach that integrates the merits of gelcasting-SHS(self-propagating high-temperature synthesis)to prepare porous Si_(3)N_(4)ceramics to simultaneously achieve high porosity,high strength,high toughness,and low thermal conductivity across a wide temperature range.By regulating the solid content,porous Si_(3)N_(4)ceramics with homogeneous pore structure are obtained,where the pore size falls inbetween 1.61 and 4.41 pm,and the elongated grains are interlaced and interlocked to form micron-sized coherent interconnected pores.At the same time,porous Si_(3)N_(4)ceramics with porosity of 67.83%to 78.03%are obtained,where the compressive strength reaches 11.79 to 47.75 MPa and fracture toughness reaches 1.20 to 6.71 MPa-m1/2.
基金financially supported by the Initiative Scientific Research Program from Jingdezheng Ceramic Institute and SRT Program (No. 1721T0264) from Tsinghua University
文摘The α-Fe_2 O_3@SiO_2 reddish pigments with core-shell structure were successfully prepared by hydrothermal and Stober methods. The structure, morphology, and chromaticity of the synthesized pigments were characterized by XRD, SEM, TEM, FTIR, XPS, and colorimetry. The results indicated that the as-prepared pigments have the characteristics of narrow particle size distribution, high dispersion,and good sphericity. The α-Fe_2 O_3@SiO_2 reddish pigments were uniform and well dispersed in solution. In addition, the pigments with different shell thickness were also prepared, and the effect of shell thickness on the color performance of the pigments was discussed.
基金Supported by Guangdong Major Project of Basic and Applied Basic Research (2020B0301030008)NSFC (11890712, 12005112, 12075129)Supported by the Postdoctoral Innovative Talent Support Program of Tsinghua University
文摘We investigate quantum kinetic theory for a massive fermion system under a rotational field.From theDirac equation in rotating frame we derive the complete set of kinetic equations for the spin components of the 8-and 7-dimensional Wigner functions.While the particles are no longer on a mass shell in the general case due to therotation-spin coupling,there are always only two independent components,which can be taken as the number andspin densities.With help from the off-shell constraint we obtain the closed transport equations for the two independent components in the classical limit and at the quantum level.The classical rotation-orbital coupling controls thedynamical evolution of the number density,but the quantum rotation-spin coupling explicitly changes the spin density.
