The effect of sintering dispersed dispersion and nano-emulsion particles of high molecular weightpolytetrafluoroethylene(PTFE)on a substrate as a function of“melt”time and temperature is described.Folded chain singl...The effect of sintering dispersed dispersion and nano-emulsion particles of high molecular weightpolytetrafluoroethylene(PTFE)on a substrate as a function of“melt”time and temperature is described.Folded chain singlecrystals parallel to the substrate and as ribbons on-edge(with double striations),as well as bands,are produced for longersintering times;particle merger and diffusion of individual molecules,crystallizing as folded chain,single(or few)molecule,single crystals when“trapped”on the substrate by cooling occur for shorter sintering times.It is suggested the observedstructures develop with sintering time,in a mesomorphic melt.The structure of the nascent particles is also discussed.展开更多
The effect of sintering dispersed and bulk,low molecular weight(M_n=50,000 Da),nano-emulsionpolytetrafluoroethylene(PTFE)particles near their melting point is described.With the nascent particles consisting of ca.75 n...The effect of sintering dispersed and bulk,low molecular weight(M_n=50,000 Da),nano-emulsionpolytetrafluoroethylene(PTFE)particles near their melting point is described.With the nascent particles consisting of ca.75 nm diameter,hexagonal,single crystals,sintering at,e.g.,350℃,results,initially,in merger of neighboring particles,followed by individual molecular motion on the substrate and the formation of folded chain,lamellar single crystals andspherulites,and on-edge ribbons.It is suggested these structures develop,with time,in the mesomorphic“melt”.Sintering ofthe bulk resin yields extended chain,band structures,as well as folded chain lamellae;end-surface to end-surface merger,possibly by end-to-end polymerization,occurs with increasing time.展开更多
Atomically dispersed precious metal catalysts maximize atom efficiency and exhibit unique reactivity.However,they are susceptible to sintering.Catalytic reactions occurring in reducing environments tend to result in a...Atomically dispersed precious metal catalysts maximize atom efficiency and exhibit unique reactivity.However,they are susceptible to sintering.Catalytic reactions occurring in reducing environments tend to result in atomically dispersed metals sintering at lower temperatures than in oxidative or inert atmospheres due to the formation of mobile metal-H or metal-CO complexes.Here,we develop a new approach to mitigate sintering of oxide supported atomically dispersed metals in a reducing atmosphere using organophosphonate self-assembled monolayers(SAMs).We demonstrate this for the case of atomically dispersed Rh on Al_(2)O_(3) and TiO_(2) using a combination of CO probe molecule FTIR,temperature programmed desorption,and alkene hydrogenation rate measurements.Evidence suggests that SAM functionalization of the oxide provides physical diffusion barriers for the metal and weakens the interactions between the reducing gas and metal,thereby discouraging the adsorbate-promoted diffusion of metal atoms on oxide supports.Our results show that support functionalization by organic species can provide improved resistance to sintering of atomically dispersed metals with maintained catalytic reactivity.展开更多
采用一次烧成工艺制备了 Mn掺杂的 Sr Ti O3电容 -压敏复合功能陶瓷 ,采用 SEM观察了 Sr Ti O3复合功能陶瓷的微观结构 ,测量了不同氧化热处理温度下 Sr Ti O3陶瓷的电学特性 ,采用 XPS研究了 Sr Ti O3复合功能陶瓷中 Mn的化学状态 ,并...采用一次烧成工艺制备了 Mn掺杂的 Sr Ti O3电容 -压敏复合功能陶瓷 ,采用 SEM观察了 Sr Ti O3复合功能陶瓷的微观结构 ,测量了不同氧化热处理温度下 Sr Ti O3陶瓷的电学特性 ,采用 XPS研究了 Sr Ti O3复合功能陶瓷中 Mn的化学状态 ,并分析了 Mn的掺杂行为。研究结果表明 :一次烧成的 Sr Ti O3陶瓷达到较高的烧成致密度 ,Mn主要以 Mn2 + 的形式存在于晶界。氧化热处理过程中 Mn低价取代晶粒表面的 Ti4+ ,对于 Sr Ti O3陶瓷中产生复合功能特性具有重要影响。展开更多
文摘The effect of sintering dispersed dispersion and nano-emulsion particles of high molecular weightpolytetrafluoroethylene(PTFE)on a substrate as a function of“melt”time and temperature is described.Folded chain singlecrystals parallel to the substrate and as ribbons on-edge(with double striations),as well as bands,are produced for longersintering times;particle merger and diffusion of individual molecules,crystallizing as folded chain,single(or few)molecule,single crystals when“trapped”on the substrate by cooling occur for shorter sintering times.It is suggested the observedstructures develop with sintering time,in a mesomorphic melt.The structure of the nascent particles is also discussed.
文摘The effect of sintering dispersed and bulk,low molecular weight(M_n=50,000 Da),nano-emulsionpolytetrafluoroethylene(PTFE)particles near their melting point is described.With the nascent particles consisting of ca.75 nm diameter,hexagonal,single crystals,sintering at,e.g.,350℃,results,initially,in merger of neighboring particles,followed by individual molecular motion on the substrate and the formation of folded chain,lamellar single crystals andspherulites,and on-edge ribbons.It is suggested these structures develop,with time,in the mesomorphic“melt”.Sintering ofthe bulk resin yields extended chain,band structures,as well as folded chain lamellae;end-surface to end-surface merger,possibly by end-to-end polymerization,occurs with increasing time.
基金support from the Department of Energy,Office of Science,Basic Energy Sciences Program,Chemical Sciences,Geosciences,and Biosciences Division[Grant No.DE-SC0005239]support from National Science Foundation(NSF)CAREER grant number CBET-1554112 for work on organic modification of oxide supports surrounding atomically dispersed metal active sites.
文摘Atomically dispersed precious metal catalysts maximize atom efficiency and exhibit unique reactivity.However,they are susceptible to sintering.Catalytic reactions occurring in reducing environments tend to result in atomically dispersed metals sintering at lower temperatures than in oxidative or inert atmospheres due to the formation of mobile metal-H or metal-CO complexes.Here,we develop a new approach to mitigate sintering of oxide supported atomically dispersed metals in a reducing atmosphere using organophosphonate self-assembled monolayers(SAMs).We demonstrate this for the case of atomically dispersed Rh on Al_(2)O_(3) and TiO_(2) using a combination of CO probe molecule FTIR,temperature programmed desorption,and alkene hydrogenation rate measurements.Evidence suggests that SAM functionalization of the oxide provides physical diffusion barriers for the metal and weakens the interactions between the reducing gas and metal,thereby discouraging the adsorbate-promoted diffusion of metal atoms on oxide supports.Our results show that support functionalization by organic species can provide improved resistance to sintering of atomically dispersed metals with maintained catalytic reactivity.
文摘采用一次烧成工艺制备了 Mn掺杂的 Sr Ti O3电容 -压敏复合功能陶瓷 ,采用 SEM观察了 Sr Ti O3复合功能陶瓷的微观结构 ,测量了不同氧化热处理温度下 Sr Ti O3陶瓷的电学特性 ,采用 XPS研究了 Sr Ti O3复合功能陶瓷中 Mn的化学状态 ,并分析了 Mn的掺杂行为。研究结果表明 :一次烧成的 Sr Ti O3陶瓷达到较高的烧成致密度 ,Mn主要以 Mn2 + 的形式存在于晶界。氧化热处理过程中 Mn低价取代晶粒表面的 Ti4+ ,对于 Sr Ti O3陶瓷中产生复合功能特性具有重要影响。
