The core-shell structure silicon-resin precursor powders were synthesized through coat-mix process and addition of Al2O3-SiO2-Y2O3 composite additives.A series of porous silicon carbide ceramics were produced after mo...The core-shell structure silicon-resin precursor powders were synthesized through coat-mix process and addition of Al2O3-SiO2-Y2O3 composite additives.A series of porous silicon carbide ceramics were produced after molding,carbonization and sintering.The phase,morphology,porosity,thermal conductivity,thermal expansion coefficient,and thermal shock resistance were analyzed.The results show that porous silicon carbide ceramics can be produced at low temperature.The grain size of porous silicon carbide ceramic is small,and the thermal conductivity is enhanced significantly.Composite additives also improve the thermal shock resistance of porous ceramics.The bending strength loss rate after 30 times of thermal shock test of the porous ceramics which were added Al2O3-SiO2-Y2O3 and sintered at 1 650 ℃ is only 6.5%.Moreover,the pore inside of the sample is smooth,and the pore size distribution is uniform.Composite additives make little effect on the thermal expansion coefficient of the porous silicon carbide ceramics.展开更多
In this study, we have investigated how the dielectric loss tangent and permittivity of AlN ceramics are affected by factors such as powder mixing methods, milling time, sintering temperature, and the addition of a se...In this study, we have investigated how the dielectric loss tangent and permittivity of AlN ceramics are affected by factors such as powder mixing methods, milling time, sintering temperature, and the addition of a second conductive phase. All ceramic samples were pre-pared by spark plasma sintering （SPS） under a pressure of 30 MPa. AlN composite ceramics sintered with 30wt%-40wt%SiC at 1600℃ for 5 min exhibited the best dielectric loss tangent, which is greater than 0.3. In addition to AlN and β-SiC, the samples also contained 2H-SiC and Fe5Si3, as detected by X-ray difraction （XRD）. The relative densities of the sintered ceramics were higher than 93%. Experimental results indicate that nano-SiC has a strong capability of absorbing electromagnetic waves. The dielectric constant and dielectric loss of AlN-SiC ce-ramics with the same content of SiC decreased as the frequency of electromagnetic waves increased from 1 kHz to 1 MHz.展开更多
This paper presents the microwave dielectric properties of reaction bonded porous silicon nitride ceramics with variant porosity and pore size, which were prepared by adding pore-forming agent grains into the silicon ...This paper presents the microwave dielectric properties of reaction bonded porous silicon nitride ceramics with variant porosity and pore size, which were prepared by adding pore-forming agent grains into the silicon powders. The experimental results show that the dielectric constant and the dielectric loss of the samples reduce evidently with increasing porosity in the sample. When the porosity is constant, the dielectric constant and the dielectric loss of the ceramics decrease visibly as the pore size increases. Among all the obtained samples, the minimum dielectric constant is about 2.4.展开更多
Si3N4 porous ceramics were fabricated by a combined foam-gelcasting and catalytic nitridation method at 1473-1623 K using silicon powder as the starting material,hexadecyl trimethyl ammonium bromide(CTAB)as the foamin...Si3N4 porous ceramics were fabricated by a combined foam-gelcasting and catalytic nitridation method at 1473-1623 K using silicon powder as the starting material,hexadecyl trimethyl ammonium bromide(CTAB)as the foaming agent,and different amounts of micron Fe powder as the catalyst.The effects of the nitridation temperature and the Fe powder addition on the phase composition,the mechanical properties,and the microstructure of the samples were researched.The results show that when nitriding at 1573 K for 5 h and adding 1 mass%Fe powder(with respect to the Si powder),the sample has a high porosity and suitable mechanical properties:the porosity of 76.5%,the compressive strength of 16.2 MPa,and the specific strength of 22.7 MPa•cm3•g^-1.展开更多
A water-quenching technique has been adopted to evaluate thermal shock fracture and fatigue behaviors of porous Si3N4 ceramics in an air atmosphere. The high-porosity Si3N4 ceramics exhibit a higher strength retention...A water-quenching technique has been adopted to evaluate thermal shock fracture and fatigue behaviors of porous Si3N4 ceramics in an air atmosphere. The high-porosity Si3N4 ceramics exhibit a higher strength retention and a better resistance to thermal shock fatigue because of its role of the pores as crack arresters.A dense and coherent surface oxide layer leads to a significant benefit in residual strength during thermal fatigue, however, an increased fatigue number to 30 th cycle cannot cause a further influence although a thicker oxide layer presents, which is attributed to holes defect and disappearance of part intergranular phase.展开更多
Porous silicon nitride ceramics（Si3N4） were fabricated by pressureless sintering using different particle size of silicon nitride powder.Lu2O3 was used as sintering additive.According to phase relationships in the t...Porous silicon nitride ceramics（Si3N4） were fabricated by pressureless sintering using different particle size of silicon nitride powder.Lu2O3 was used as sintering additive.According to phase relationships in the ternary system Si3N4-SiO2-Lu2O3,porous Si3N4 ceramics with different phase composition were designed through the change of the content of SiO2 which was formed by the oxidation at 800℃ in air.Porous Si3N4 with different phase compositions was obtained after sintering at 1800℃ in N2atmosphere.A small content of SiO2 favored the formation of secondary phase Lu4Si2O7N2,while large content of SiO2 favored the formation of secondary phase Lu2Si2O7 and Si2N2O.Porous Si3N4 ceramics with secondary phase Lu4Si2O7N2 had a flexural strength of 207 MPa,while that with secondary phase Si2N2O and Lu2Si2O7 had lower flexural strength.展开更多
To improve the corrosion resistance of porous Si3N4 used in the high temperature environments,which contain water vapor and volatile species,porous Si3N4-Lu2Si2O7 composite ceramics were fabricated by a process of oxi...To improve the corrosion resistance of porous Si3N4 used in the high temperature environments,which contain water vapor and volatile species,porous Si3N4-Lu2Si2O7 composite ceramics were fabricated by a process of oxidation bonding and pressureless sintering in flowing N 2 atmosphere at the temperatures lower than 1550 ° C.The pores in ceramics were formed by removing the pore-forming agent（phenolic resin）.SiO2 derived by the oxidation of Si3N4 at 1250 ° C in air reacted with Lu2O3 at various sintering temperatures,leading to the formation of Lu2Si2O7 with excellent oxidation and corrosion resistance.The phase composition,microstructure,and mechanical properties of the porous ceramics depended on the sintering temperatures.Si3N4-Lu2Si2O7 composite ceramics with the porosity of 52% and the pore size of 0.2 μm were obtained by oxidation bonding at 1250 ° C for 1 h and pressureless sintering at 1550 ° C for 2 h using 4 wt pct Lu2O3 additive.展开更多
基金Project(50802052)supported by the National Natural Science Foundation of China
文摘The core-shell structure silicon-resin precursor powders were synthesized through coat-mix process and addition of Al2O3-SiO2-Y2O3 composite additives.A series of porous silicon carbide ceramics were produced after molding,carbonization and sintering.The phase,morphology,porosity,thermal conductivity,thermal expansion coefficient,and thermal shock resistance were analyzed.The results show that porous silicon carbide ceramics can be produced at low temperature.The grain size of porous silicon carbide ceramic is small,and the thermal conductivity is enhanced significantly.Composite additives also improve the thermal shock resistance of porous ceramics.The bending strength loss rate after 30 times of thermal shock test of the porous ceramics which were added Al2O3-SiO2-Y2O3 and sintered at 1 650 ℃ is only 6.5%.Moreover,the pore inside of the sample is smooth,and the pore size distribution is uniform.Composite additives make little effect on the thermal expansion coefficient of the porous silicon carbide ceramics.
基金financially supported by the International S&T Cooperation Program of China(No.2010DFR50360)
文摘In this study, we have investigated how the dielectric loss tangent and permittivity of AlN ceramics are affected by factors such as powder mixing methods, milling time, sintering temperature, and the addition of a second conductive phase. All ceramic samples were pre-pared by spark plasma sintering （SPS） under a pressure of 30 MPa. AlN composite ceramics sintered with 30wt%-40wt%SiC at 1600℃ for 5 min exhibited the best dielectric loss tangent, which is greater than 0.3. In addition to AlN and β-SiC, the samples also contained 2H-SiC and Fe5Si3, as detected by X-ray difraction （XRD）. The relative densities of the sintered ceramics were higher than 93%. Experimental results indicate that nano-SiC has a strong capability of absorbing electromagnetic waves. The dielectric constant and dielectric loss of AlN-SiC ce-ramics with the same content of SiC decreased as the frequency of electromagnetic waves increased from 1 kHz to 1 MHz.
基金supported by the National Natural Science Foundation of China under grant No.90305016.
文摘This paper presents the microwave dielectric properties of reaction bonded porous silicon nitride ceramics with variant porosity and pore size, which were prepared by adding pore-forming agent grains into the silicon powders. The experimental results show that the dielectric constant and the dielectric loss of the samples reduce evidently with increasing porosity in the sample. When the porosity is constant, the dielectric constant and the dielectric loss of the ceramics decrease visibly as the pore size increases. Among all the obtained samples, the minimum dielectric constant is about 2.4.
基金National Natural Science Foundation of China(Grant No.51672194 and 51872210)Program for Innovative Teams of Outstanding Young and Middle-aged Researchers in the Higher Education Institutions of Hubei Province(T201602)+1 种基金Key Program of Natural Science Foundation of Hubei Province,China(Contract No.2017CFA004)the China Scholarship Council(CSC)(No.201808420278).
文摘Si3N4 porous ceramics were fabricated by a combined foam-gelcasting and catalytic nitridation method at 1473-1623 K using silicon powder as the starting material,hexadecyl trimethyl ammonium bromide(CTAB)as the foaming agent,and different amounts of micron Fe powder as the catalyst.The effects of the nitridation temperature and the Fe powder addition on the phase composition,the mechanical properties,and the microstructure of the samples were researched.The results show that when nitriding at 1573 K for 5 h and adding 1 mass%Fe powder(with respect to the Si powder),the sample has a high porosity and suitable mechanical properties:the porosity of 76.5%,the compressive strength of 16.2 MPa,and the specific strength of 22.7 MPa•cm3•g^-1.
基金financial support of the project from the National Natural Science Foundation of China (Nos. 51272206 and 51472198)the Program for Changjiang Scholars and Innovative Research Team in University (No. IRT1280)+2 种基金the National Key Laboratory Functional Composite (No. 9140C560109130C56201)the Fundamental Research Funds for the Central University (No. xkjc2014009)the State Key Laboratory for Mechanical Behavior of Materials (No. 20121207)
文摘A water-quenching technique has been adopted to evaluate thermal shock fracture and fatigue behaviors of porous Si3N4 ceramics in an air atmosphere. The high-porosity Si3N4 ceramics exhibit a higher strength retention and a better resistance to thermal shock fatigue because of its role of the pores as crack arresters.A dense and coherent surface oxide layer leads to a significant benefit in residual strength during thermal fatigue, however, an increased fatigue number to 30 th cycle cannot cause a further influence although a thicker oxide layer presents, which is attributed to holes defect and disappearance of part intergranular phase.
基金the State Key Laboratory of Solidification Processing in NWPU(No. KB200920)the Program for New Century Excellent Talents in University
文摘Porous silicon nitride ceramics（Si3N4） were fabricated by pressureless sintering using different particle size of silicon nitride powder.Lu2O3 was used as sintering additive.According to phase relationships in the ternary system Si3N4-SiO2-Lu2O3,porous Si3N4 ceramics with different phase composition were designed through the change of the content of SiO2 which was formed by the oxidation at 800℃ in air.Porous Si3N4 with different phase compositions was obtained after sintering at 1800℃ in N2atmosphere.A small content of SiO2 favored the formation of secondary phase Lu4Si2O7N2,while large content of SiO2 favored the formation of secondary phase Lu2Si2O7 and Si2N2O.Porous Si3N4 ceramics with secondary phase Lu4Si2O7N2 had a flexural strength of 207 MPa,while that with secondary phase Si2N2O and Lu2Si2O7 had lower flexural strength.
基金supported by the "Basic Research Fund" of Northwestern Polytechnical University,China (No. W018105)the Fund of the State Key Laboratory of Solidification Processing in Northwestern Polytechnical University, China (No. KB200920)
文摘To improve the corrosion resistance of porous Si3N4 used in the high temperature environments,which contain water vapor and volatile species,porous Si3N4-Lu2Si2O7 composite ceramics were fabricated by a process of oxidation bonding and pressureless sintering in flowing N 2 atmosphere at the temperatures lower than 1550 ° C.The pores in ceramics were formed by removing the pore-forming agent（phenolic resin）.SiO2 derived by the oxidation of Si3N4 at 1250 ° C in air reacted with Lu2O3 at various sintering temperatures,leading to the formation of Lu2Si2O7 with excellent oxidation and corrosion resistance.The phase composition,microstructure,and mechanical properties of the porous ceramics depended on the sintering temperatures.Si3N4-Lu2Si2O7 composite ceramics with the porosity of 52% and the pore size of 0.2 μm were obtained by oxidation bonding at 1250 ° C for 1 h and pressureless sintering at 1550 ° C for 2 h using 4 wt pct Lu2O3 additive.
