Nano hafnium carbide（HfC） powders were synthesized by sol-gel combining hightemperature rapid heat treatment process using citric acid and hafnium tetrachloride as the raw materials. The effects of ball milling trea...Nano hafnium carbide（HfC） powders were synthesized by sol-gel combining hightemperature rapid heat treatment process using citric acid and hafnium tetrachloride as the raw materials. The effects of ball milling treatment on the phase and morphology of pyrolysis products（HfO_2-C） and final HfC product were investigated and the influences of heat treatment temperature and holding time on the structure and properties of the synthesized hafnium carbide powders were also studied. The experimental results showed that the HfO_2-C powders with good uniformity and small particle size were prepared by controlling the milling time. Pure HfC powders with an average particle size of 500 nm were obtained at 1 700 ℃ with a holding time of 3 min, and the oxygen content was about 0.69 wt%, lower than that of the hafnium carbide powders prepared by SPS（0.97%）.展开更多
Electric field-induced point electron source is highly demanded for microscopy,spectroscopy,lithography,X-ray tubes,microwave devices,and data displays.However,the instability in emission current and requirement of ul...Electric field-induced point electron source is highly demanded for microscopy,spectroscopy,lithography,X-ray tubes,microwave devices,and data displays.However,the instability in emission current and requirement of ultrahigh vacuum have often limited its extensive applications.Herewith we report a single-crystalline HfC nanowire with oxycarbide emission surface for stable electron emission at 50 nA with fluctuations less than 1%in a vacuum of 4×10^−7 Pa.The emitter has a low work function of 2.5 eV measured by the field emission Fowler-Nordheim curve and it is in good agreement with density functional theory(DFT)calculations.The energy spread is in a range of 0.21–0.26 eV with a corresponding reduced brightness 1.95×10^11−3.81×10^11 A·m^−2·sr^−1·V^−1.The HfC nanowire with oxycarbide emission surface is a qualified candidate for the next-generation electron source with high brightness,large current,and low energy spread.展开更多
Soluble Hf-containing polymers are significant processable precursors for the fabrication of ultra-high temperature ceramics.In this work,cyclic Hf-Schiff base polymers were synthesized via direct polymerization of ha...Soluble Hf-containing polymers are significant processable precursors for the fabrication of ultra-high temperature ceramics.In this work,cyclic Hf-Schiff base polymers were synthesized via direct polymerization of hafnium alkoxide and bis-salen monomers.The defined structure and molecular weight of the polymers were characterized by NMR spectroscopy,gel permeation chromatography and MALDI-TOF mass spectroscopy.The feed ratio of monomers regulated the molecular weight and solubility of the polymers.This synthetic strategy features simple operation under ambient conditions,efficient reaction with high yield and cyclic polymers as the main products.The Hf-Schiff base polymers were converted to HfC/C materials after pyrolysis under argon at 1600°C,which was identified by XRD measurements,elemental analyses and Raman spectroscopy.This work will inspire more precise and efficient synthesis and applications of metallopolymers.展开更多
基金the National Key R&D Program of China(2017YFB0310400)the National Natural Science Foundation of China(51672197)
文摘Nano hafnium carbide（HfC） powders were synthesized by sol-gel combining hightemperature rapid heat treatment process using citric acid and hafnium tetrachloride as the raw materials. The effects of ball milling treatment on the phase and morphology of pyrolysis products（HfO_2-C） and final HfC product were investigated and the influences of heat treatment temperature and holding time on the structure and properties of the synthesized hafnium carbide powders were also studied. The experimental results showed that the HfO_2-C powders with good uniformity and small particle size were prepared by controlling the milling time. Pure HfC powders with an average particle size of 500 nm were obtained at 1 700 ℃ with a holding time of 3 min, and the oxygen content was about 0.69 wt%, lower than that of the hafnium carbide powders prepared by SPS（0.97%）.
基金This work was supported partially by the NIMS-DENKA Centre of Excellence for Next Generation MaterialsA part of this work was also supported by NIMS Microstructural Characterization Platform as a program of“Nanotechnology Platform”of the Ministry of Education,Culture,Sports,Science and Technology(MEXT),Japan.
文摘Electric field-induced point electron source is highly demanded for microscopy,spectroscopy,lithography,X-ray tubes,microwave devices,and data displays.However,the instability in emission current and requirement of ultrahigh vacuum have often limited its extensive applications.Herewith we report a single-crystalline HfC nanowire with oxycarbide emission surface for stable electron emission at 50 nA with fluctuations less than 1%in a vacuum of 4×10^−7 Pa.The emitter has a low work function of 2.5 eV measured by the field emission Fowler-Nordheim curve and it is in good agreement with density functional theory(DFT)calculations.The energy spread is in a range of 0.21–0.26 eV with a corresponding reduced brightness 1.95×10^11−3.81×10^11 A·m^−2·sr^−1·V^−1.The HfC nanowire with oxycarbide emission surface is a qualified candidate for the next-generation electron source with high brightness,large current,and low energy spread.
基金by the National Natural Science Foundation of China(No.21604090)。
文摘Soluble Hf-containing polymers are significant processable precursors for the fabrication of ultra-high temperature ceramics.In this work,cyclic Hf-Schiff base polymers were synthesized via direct polymerization of hafnium alkoxide and bis-salen monomers.The defined structure and molecular weight of the polymers were characterized by NMR spectroscopy,gel permeation chromatography and MALDI-TOF mass spectroscopy.The feed ratio of monomers regulated the molecular weight and solubility of the polymers.This synthetic strategy features simple operation under ambient conditions,efficient reaction with high yield and cyclic polymers as the main products.The Hf-Schiff base polymers were converted to HfC/C materials after pyrolysis under argon at 1600°C,which was identified by XRD measurements,elemental analyses and Raman spectroscopy.This work will inspire more precise and efficient synthesis and applications of metallopolymers.
