Mullite coating on recrystallized silicon carbide was successfully prepared by the sol-gel route. The cycling oxidation of coated recrystallized silicon carbide was performed at 1500℃. For comparison, the oxidation o...Mullite coating on recrystallized silicon carbide was successfully prepared by the sol-gel route. The cycling oxidation of coated recrystallized silicon carbide was performed at 1500℃. For comparison, the oxidation of uncoated recrystallized silicon carbide was also carried out at the same condition. The results in- dicated that a layer of compact, adhesive and crack free mullite coating was found on the recrystallized silicon carbide. After oxidation, the new coatings exhibit adherence and crack resistance under thermal cycling between room temperature and 1500℃, therefore the oxidation resistance capability of silicon carbide was enhanced. With the increase of the dipping frequencies, namely, the increase of the thickness of mullite coating, the oxidation resistance of silicon carbide would be futher improved. The formation mechanism of mullite coating was analyzed and discussed and the oxidation dynamics model of coatedmullite silicon carbide has been also proposed.展开更多
Steam oxidation resistance of Si3N4 and Si2N2O as well as SiAlON bonded SiC refractories at 900℃was tested according to ASTM-C863.Phase composition and microstructure before and after oxidation were analyzed by XRD a...Steam oxidation resistance of Si3N4 and Si2N2O as well as SiAlON bonded SiC refractories at 900℃was tested according to ASTM-C863.Phase composition and microstructure before and after oxidation were analyzed by XRD and SEM.The results show that Si3N4 and Si2N2O bonded SiC refractory presents better steam oxidation resistance than SiAlON bonded SiC.For Si3N4 and Si2N2O bonded SiC,the oxidation speed is higher with more pronounced volume expansion in the early 100 h;afterwards,the volume expansion slows down gradually and starts to level off after 300 h.It is considered that the high silica glass phase formed during the oxidation covers Si3N4 and Si2N2O,and SiC as a protective layer and fills the open pores.But for SiAlON bonded SiC,the volume expands gradually and constantly with the increasing oxidation duration even after 500 h,due to the continuous formation of mullite transformed from oxidation products and Al2O3 in SiAlON.展开更多
In the present paper,a silicon nitride-based composite processed with rare-earth oxidesadditives is presented.Its bend strength can be maintained at a value as high as 1000 MPafrom 1000 to 1370℃ and the fracture toug...In the present paper,a silicon nitride-based composite processed with rare-earth oxidesadditives is presented.Its bend strength can be maintained at a value as high as 1000 MPafrom 1000 to 1370℃ and the fracture toughness measures 9-10 MPa·m<sup>1/2</sup>.The static fa-tigue behavior at 1370℃ of this material is also encouraging.Besides,two another α′/β′sialon composites doped with rare-earth oxides are also described.The effects of processingparameters on the microstructure and the properties of the materials are discussed in somedetails.展开更多
For uniform tube inner coating of non-conductive thin films, the double-ended coaxial magnetron pulsed plasma (DCMPP) method was investigated. In this study, coating of TiN and TiO2 was performed. It was clearly shown...For uniform tube inner coating of non-conductive thin films, the double-ended coaxial magnetron pulsed plasma (DCMPP) method was investigated. In this study, coating of TiN and TiO2 was performed. It was clearly shown that the extended anode effect was strongly influenced by the electric resistance of the coated thin films on the inner surface of an insulator tube. Additionally, high frequency (100 kHz) was better for relatively high plasma density. On the other hand, in the case of titanium oxide deposition, negative ion productions drastically decrease the deposition rate and the shifting velocity of plasma main position for coated TiO2 films.展开更多
The steam oxidation of Si3N4-bonded SiC was determined at 1000℃for 50,100,150,200,250 and 300 h,respectively,according to ASTM C863-2000.The evolution of the phase composition and the microstructure as well as their ...The steam oxidation of Si3N4-bonded SiC was determined at 1000℃for 50,100,150,200,250 and 300 h,respectively,according to ASTM C863-2000.The evolution of the phase composition and the microstructure as well as their relationship was investigated by XRD and SEM.The results show that the oxidation rate of Si3N4-bonded SiC is periodic.The presence of nitrogen element can impede the crystallization of SiO2 glass;the local enrichment of CaO impurities is unfavorable for the existence of fibrous SiO2.SiO2 mainly exists as cristobalite when the CaO/SiO2 ratio reaches a suitable level,but gradually transforms to quartz along with the oxidation time when the SiO2 content increases,or the CaO/SiO2 ratio decreases,due to the insufficient mineralization of CaO.The crystallization of SiO2 glass,especially the formation of quartz is the key factor leading to the volume expansion and structural stress.When the cracks extend and reach the surface,the degradation of the material accelerates.展开更多
Hexagonal boron nitride （h-BN） is believed to offer better passivation to metallic surfaces than graphene owing to its insulating nature, which facilitates blocking the flow of electrons, thereby preventing the occu...Hexagonal boron nitride （h-BN） is believed to offer better passivation to metallic surfaces than graphene owing to its insulating nature, which facilitates blocking the flow of electrons, thereby preventing the occurrence of galvanic reactions. Nevertheless, this may not be the case when an h-BN-protected material is exposed to aqueous environments. In this work, we analyzed the stability of mono and multilayer h-BN stacks exposed to H202 and atmospheric conditions. Our experiments revealed that monolayer h-BN is as inefficient as graphene as a protective coating when exposed to H202. Multilayer h-BN offered a good degree of protection. Monolayer h-BN was found to be ineffective in an air atmosphere as well. Even a 10-15 layers-thick h-BN stack could not completely protect the surface of the metal under consideration. By combining Auger electron spectroscopy and secondary ion mass spectrometry techniques, we observed that oxygen could diffuse through the grain boundaries of the h-BN stack to reach the metallic substrate. Fortunately, because of the diffusive nature of the process, the oxidized area did not increase with time once a saturated state was reached. This makes multflayer （not monolayer） h-BN a suitable long-term oxidation barrier. Oxygen infiltration could not be observed by X-ray photoelectron spectroscopy. This technique cannot assess the chemical composition of the deeper layers of a material. Hence, the previous reports, which relied on XPS to analyze the passivating properties of h-BN and graphene, may have ignored some important subsurface phenomena. The results obtained in this study provide new insights into the passivating properties of mono and multilayer h-BN in aqueous media and the degradation kinetics of h-BN-coated metals exposed to an air environment.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
MOS capacitors with hafnium oxynitride（HfON）gate dielectrics were fabricated on Ge and Si substrates using the RF reactive magnetron sputtering method.A large amount of fixed charges and interface traps exist at the...MOS capacitors with hafnium oxynitride（HfON）gate dielectrics were fabricated on Ge and Si substrates using the RF reactive magnetron sputtering method.A large amount of fixed charges and interface traps exist at the Ge/HfON interface.HRTEM and XPS analyses show that Ge oxides were grown and diffused into HfON after post metal annealing.A Si nitride interfacial layer was inserted between Ge and HfON as diffusion barrier.Using this method,well behaved capacitance–voltage and current–voltage characteristics were obtained.Finally hystereses are compared under different process conditions and possible causes are discussed.展开更多
The formation of protective multifunctional coatings on magnesium alloy MA8 using plasma electrolyt- ic oxidation （PEO） in an electrolytic system containing nanosized particles of titanium nitride was investigated. ...The formation of protective multifunctional coatings on magnesium alloy MA8 using plasma electrolyt- ic oxidation （PEO） in an electrolytic system containing nanosized particles of titanium nitride was investigated. Electrochemical and mechanical properties of the obtained layers were examined. It was established that microhardness of the coating with the nanoparticle concentration of 3 gl-1 increased twofold （4.2 ± 0.5 GPa）, while wear resistance decreased （4.97 × 10-6 mm3 N-1 m-1）, as compared to re- spective values for the PEO-coating formed in the electrolyte without nanoparticles （2.1 ± 0.3 GPa, 1.12 × 10.5 mm3 N-1 m-1）.展开更多
The effect of the admixtures of Al and Si metals and B4 C and MgAlON compounds on the oxidation of MgO/ Si3N4 composite refractory has been studied, which is a promising carbon free refractory for steel-making applica...The effect of the admixtures of Al and Si metals and B4 C and MgAlON compounds on the oxidation of MgO/ Si3N4 composite refractory has been studied, which is a promising carbon free refractory for steel-making application . The four kinds of admixtures can be used as anti-oxi-dants for Si3N4, but the mixture of Al and Si achieved the best result. The mixture can not only play the role as anti-oxidant, but also assist the sintering process and help form dense sintering layer, improving the property of the composite.展开更多
Interest in energy harvesters has grown rapidly over the last decade. The cantilever shaped piezoelectric energy harvesting beam is one of the most employed designs, due to its simplicity and flexibility for further p...Interest in energy harvesters has grown rapidly over the last decade. The cantilever shaped piezoelectric energy harvesting beam is one of the most employed designs, due to its simplicity and flexibility for further performance enhancement. The research effort in the MEMS Piezoelectric vibration energy harvester designed using three types of cantilever materials, Lithium Niobate (LiNbO3), Aluminum Nitride (AlN) and Zinc Oxide (ZnO) with different substrate materials: aluminum, steel and silicon using COMSOL Multiphysics package were designed and analyzed. Voltage, mechanical power and electrical power versus frequency for different cantilever materials and substrates were modeled and simulated using Finite element method (FEM). The resonant frequencies of the LiNbO3/Al, AlN/Al and ZnO/Al systems were found to be 187.5 Hz, 279.5 Hz and 173.5 Hz, respectively. We found that ZnO/Al system yields optimum voltage and electrical power values of 8.2 V and 2.8 mW, respectively. For ZnO cantilever on aluminum, steel and silicon substrates, we found the resonant frequencies to be 173.5 Hz, 170 Hz and 175 Hz, respectively. Interestingly, ZnO/steel yields optimal voltage and electrical power values of 9.83 V and 4.02 mW, respectively. Furthermore, all systems were studied at different differentiate frequencies. We found that voltage and electrical power have increased as the acceleration has increased.展开更多
文摘Mullite coating on recrystallized silicon carbide was successfully prepared by the sol-gel route. The cycling oxidation of coated recrystallized silicon carbide was performed at 1500℃. For comparison, the oxidation of uncoated recrystallized silicon carbide was also carried out at the same condition. The results in- dicated that a layer of compact, adhesive and crack free mullite coating was found on the recrystallized silicon carbide. After oxidation, the new coatings exhibit adherence and crack resistance under thermal cycling between room temperature and 1500℃, therefore the oxidation resistance capability of silicon carbide was enhanced. With the increase of the dipping frequencies, namely, the increase of the thickness of mullite coating, the oxidation resistance of silicon carbide would be futher improved. The formation mechanism of mullite coating was analyzed and discussed and the oxidation dynamics model of coatedmullite silicon carbide has been also proposed.
文摘Steam oxidation resistance of Si3N4 and Si2N2O as well as SiAlON bonded SiC refractories at 900℃was tested according to ASTM-C863.Phase composition and microstructure before and after oxidation were analyzed by XRD and SEM.The results show that Si3N4 and Si2N2O bonded SiC refractory presents better steam oxidation resistance than SiAlON bonded SiC.For Si3N4 and Si2N2O bonded SiC,the oxidation speed is higher with more pronounced volume expansion in the early 100 h;afterwards,the volume expansion slows down gradually and starts to level off after 300 h.It is considered that the high silica glass phase formed during the oxidation covers Si3N4 and Si2N2O,and SiC as a protective layer and fills the open pores.But for SiAlON bonded SiC,the volume expands gradually and constantly with the increasing oxidation duration even after 500 h,due to the continuous formation of mullite transformed from oxidation products and Al2O3 in SiAlON.
基金the High Technology Research and Development Programme of China.
文摘In the present paper,a silicon nitride-based composite processed with rare-earth oxidesadditives is presented.Its bend strength can be maintained at a value as high as 1000 MPafrom 1000 to 1370℃ and the fracture toughness measures 9-10 MPa·m<sup>1/2</sup>.The static fa-tigue behavior at 1370℃ of this material is also encouraging.Besides,two another α′/β′sialon composites doped with rare-earth oxides are also described.The effects of processingparameters on the microstructure and the properties of the materials are discussed in somedetails.
文摘For uniform tube inner coating of non-conductive thin films, the double-ended coaxial magnetron pulsed plasma (DCMPP) method was investigated. In this study, coating of TiN and TiO2 was performed. It was clearly shown that the extended anode effect was strongly influenced by the electric resistance of the coated thin films on the inner surface of an insulator tube. Additionally, high frequency (100 kHz) was better for relatively high plasma density. On the other hand, in the case of titanium oxide deposition, negative ion productions drastically decrease the deposition rate and the shifting velocity of plasma main position for coated TiO2 films.
基金The project is supported by Municipal Science&Technology Program(No.1901001A),Luoyang,China.
文摘The steam oxidation of Si3N4-bonded SiC was determined at 1000℃for 50,100,150,200,250 and 300 h,respectively,according to ASTM C863-2000.The evolution of the phase composition and the microstructure as well as their relationship was investigated by XRD and SEM.The results show that the oxidation rate of Si3N4-bonded SiC is periodic.The presence of nitrogen element can impede the crystallization of SiO2 glass;the local enrichment of CaO impurities is unfavorable for the existence of fibrous SiO2.SiO2 mainly exists as cristobalite when the CaO/SiO2 ratio reaches a suitable level,but gradually transforms to quartz along with the oxidation time when the SiO2 content increases,or the CaO/SiO2 ratio decreases,due to the insufficient mineralization of CaO.The crystallization of SiO2 glass,especially the formation of quartz is the key factor leading to the volume expansion and structural stress.When the cracks extend and reach the surface,the degradation of the material accelerates.
文摘Hexagonal boron nitride （h-BN） is believed to offer better passivation to metallic surfaces than graphene owing to its insulating nature, which facilitates blocking the flow of electrons, thereby preventing the occurrence of galvanic reactions. Nevertheless, this may not be the case when an h-BN-protected material is exposed to aqueous environments. In this work, we analyzed the stability of mono and multilayer h-BN stacks exposed to H202 and atmospheric conditions. Our experiments revealed that monolayer h-BN is as inefficient as graphene as a protective coating when exposed to H202. Multilayer h-BN offered a good degree of protection. Monolayer h-BN was found to be ineffective in an air atmosphere as well. Even a 10-15 layers-thick h-BN stack could not completely protect the surface of the metal under consideration. By combining Auger electron spectroscopy and secondary ion mass spectrometry techniques, we observed that oxygen could diffuse through the grain boundaries of the h-BN stack to reach the metallic substrate. Fortunately, because of the diffusive nature of the process, the oxidized area did not increase with time once a saturated state was reached. This makes multflayer （not monolayer） h-BN a suitable long-term oxidation barrier. Oxygen infiltration could not be observed by X-ray photoelectron spectroscopy. This technique cannot assess the chemical composition of the deeper layers of a material. Hence, the previous reports, which relied on XPS to analyze the passivating properties of h-BN and graphene, may have ignored some important subsurface phenomena. The results obtained in this study provide new insights into the passivating properties of mono and multilayer h-BN in aqueous media and the degradation kinetics of h-BN-coated metals exposed to an air environment.
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supported by the State Key Development Program for Basic Research of China(No.2006CB302704)
文摘MOS capacitors with hafnium oxynitride（HfON）gate dielectrics were fabricated on Ge and Si substrates using the RF reactive magnetron sputtering method.A large amount of fixed charges and interface traps exist at the Ge/HfON interface.HRTEM and XPS analyses show that Ge oxides were grown and diffused into HfON after post metal annealing.A Si nitride interfacial layer was inserted between Ge and HfON as diffusion barrier.Using this method,well behaved capacitance–voltage and current–voltage characteristics were obtained.Finally hystereses are compared under different process conditions and possible causes are discussed.
基金financially supported by the Russian Science Foundation(Project No.14-33-00009)the Russian Federation Government(Federal Agency of Scientific Organizations)
文摘The formation of protective multifunctional coatings on magnesium alloy MA8 using plasma electrolyt- ic oxidation （PEO） in an electrolytic system containing nanosized particles of titanium nitride was investigated. Electrochemical and mechanical properties of the obtained layers were examined. It was established that microhardness of the coating with the nanoparticle concentration of 3 gl-1 increased twofold （4.2 ± 0.5 GPa）, while wear resistance decreased （4.97 × 10-6 mm3 N-1 m-1）, as compared to re- spective values for the PEO-coating formed in the electrolyte without nanoparticles （2.1 ± 0.3 GPa, 1.12 × 10.5 mm3 N-1 m-1）.
文摘The effect of the admixtures of Al and Si metals and B4 C and MgAlON compounds on the oxidation of MgO/ Si3N4 composite refractory has been studied, which is a promising carbon free refractory for steel-making application . The four kinds of admixtures can be used as anti-oxi-dants for Si3N4, but the mixture of Al and Si achieved the best result. The mixture can not only play the role as anti-oxidant, but also assist the sintering process and help form dense sintering layer, improving the property of the composite.
基金Project(kq2208270) supported by the Natural Science Foundation of Changsha City,ChinaProject(kq2208270) supported by the National Key Laboratory of Heavy-duty Fast and High-power Electric Locomotive Open Project Funding,China。
文摘Interest in energy harvesters has grown rapidly over the last decade. The cantilever shaped piezoelectric energy harvesting beam is one of the most employed designs, due to its simplicity and flexibility for further performance enhancement. The research effort in the MEMS Piezoelectric vibration energy harvester designed using three types of cantilever materials, Lithium Niobate (LiNbO3), Aluminum Nitride (AlN) and Zinc Oxide (ZnO) with different substrate materials: aluminum, steel and silicon using COMSOL Multiphysics package were designed and analyzed. Voltage, mechanical power and electrical power versus frequency for different cantilever materials and substrates were modeled and simulated using Finite element method (FEM). The resonant frequencies of the LiNbO3/Al, AlN/Al and ZnO/Al systems were found to be 187.5 Hz, 279.5 Hz and 173.5 Hz, respectively. We found that ZnO/Al system yields optimum voltage and electrical power values of 8.2 V and 2.8 mW, respectively. For ZnO cantilever on aluminum, steel and silicon substrates, we found the resonant frequencies to be 173.5 Hz, 170 Hz and 175 Hz, respectively. Interestingly, ZnO/steel yields optimal voltage and electrical power values of 9.83 V and 4.02 mW, respectively. Furthermore, all systems were studied at different differentiate frequencies. We found that voltage and electrical power have increased as the acceleration has increased.
