The performance of proton exchange membrane fuel cells (PEMFC) is strongly determined by the structure and composition of the electrode layer.The interactions between the ionomer,carbon black particles,and solvent aff...The performance of proton exchange membrane fuel cells (PEMFC) is strongly determined by the structure and composition of the electrode layer.The interactions between the ionomer,carbon black particles,and solvent affect the suspension properties and thus the layer morphology.We analyze the effect of the ionomer-to-carbon (I/C) weight ratio for two different types of carbon black on the suspension and layer characteristics.Highly branched carbon blacks with a high surface area tend to form less cracked layers.As less branched carbons can pack together more closely,a smaller pore size results in a larger capillary pressure during drying and thus more cracks.The added ionomer adsorbs on the carbon particles and improves the colloidal stability of the carbon black particles.The carbon black aggregates are thus smaller,resulting in closer packing and thinner layers.Moreover,the addition of the ionomer increases the critical coating thickness (CCT) of the layers because drying stresses are dissipated by the deformation of the ionomer,preventing crack formation.An optimum I/C weight ratio is identified for optimal layer formation and minimized crack formation.展开更多
For most particle-based applications, formulation in the liquid phase is a decisive step, and thus, particle interactions and stability in liquid media are of major importance. The concept of Hansen solubility paramet...For most particle-based applications, formulation in the liquid phase is a decisive step, and thus, particle interactions and stability in liquid media are of major importance. The concept of Hansen solubility parameters (HSP) was initially invented to describe the interactions of (polymer) molecules and their solubility in different liquids and is increasingly being used in particle technology to describe dispersibility. Because dispersions are not thermodynamically stable, the term Hansen dispersibility parameters (HDP) is used instead of HSP (SiiE, Sobisch, Peukert, Lerche,& Segets, 2018). Herein, we extend a previously developed standardized and non-subjective method for determination of Hansen parameters based on analytical centrifugation to the important class of quantum materials. As a technically relevant model system, zinc oxide quantum dots (QDs) were used to transfer our methodology to nanoparticles (NPs) with sizes below lOnm. The results obtained using the standard procedure starting from a dried powder were compared with those obtained through redispersion from the wet sediment produced during the typical washing procedure of QDs, and drying was observed to play an important role. In conclusion, our study reveals the high potential of HDP for quantifying the interfacial properties of NPs as well as their link to dispersibility.展开更多
Particle distribution in the cross-section of the flighted rotating drum(FRD)is critical to the analysis of heat and mass transfer between gas and solids.In this work,the particle tracking velocimetry(PTV)method is ap...Particle distribution in the cross-section of the flighted rotating drum(FRD)is critical to the analysis of heat and mass transfer between gas and solids.In this work,the particle tracking velocimetry(PTV)method is applied to study the influence of the number of flights on the particle motion in FRDs.The drum,installed with 1,4,8,or 12 rectangular flights,is filled with plastic balls to 15%and operated at various rotating speeds ranging from 10 rpm to 30 rpm.The results show that the number of flights has different effects on the holdup ratio and cascading rate of single flight and active flights.With 8 and 12 flights,the FRD produces a larger and more stable particle ratio of the dilute phase.Moreover,DEM simulations agree with PTV measurements,whereas literature models show significant deviations.展开更多
Fluidized bed opposed jet mills are capable of meeting the continuously growing dema nd for contamination-free fine particles.In this type of jet mill,the solid material is entrained and accelerated by expanding gas j...Fluidized bed opposed jet mills are capable of meeting the continuously growing dema nd for contamination-free fine particles.In this type of jet mill,the solid material is entrained and accelerated by expanding gas jets that are focused onto a focal point in side a fluidized bed.The resulting particle collisions induce breakage.The process is affected by the relative particle velocities and the number of particle-particle collisions.Clearly,both quantities are distributed.However,to date,neither relative particle velocities nor collision frequencies in such units have been determined.The present work introduces an innovative method to assess the stressing conditions in jet mills experimentally.To this end,mixtures of glass and ductile metal microspheres were used,with the latter employed in small amounts.Inter-particle collisions between the aluminum and glass spheres lead to the formation of dents on the microparticles.The size and number of these dents are associated with the individual collision velocities and overall collision frequencies.The correlation between dent size and collision velocity was obtained from finite element calculations based on empirical data.The proposed approach was validated using particle image velocimetry during secondary gas injection into a fluidized bed reactor.In this case the effect of the distance between two opposed nozzles was examined.For a lab-scaled fluidized bed opposed jet mill the effects of gas pressure and hold-up were investigated.Relative particle velocities were found to be sign ificantly lower tha n the gas velocities,while the nu mber of contacts per particle was determined to be extremely high.展开更多
文摘The performance of proton exchange membrane fuel cells (PEMFC) is strongly determined by the structure and composition of the electrode layer.The interactions between the ionomer,carbon black particles,and solvent affect the suspension properties and thus the layer morphology.We analyze the effect of the ionomer-to-carbon (I/C) weight ratio for two different types of carbon black on the suspension and layer characteristics.Highly branched carbon blacks with a high surface area tend to form less cracked layers.As less branched carbons can pack together more closely,a smaller pore size results in a larger capillary pressure during drying and thus more cracks.The added ionomer adsorbs on the carbon particles and improves the colloidal stability of the carbon black particles.The carbon black aggregates are thus smaller,resulting in closer packing and thinner layers.Moreover,the addition of the ionomer increases the critical coating thickness (CCT) of the layers because drying stresses are dissipated by the deformation of the ionomer,preventing crack formation.An optimum I/C weight ratio is identified for optimal layer formation and minimized crack formation.
基金This work was supported financially by the German Science Foundation （DFG）, Collaborative Research Centre SFB953： ＂Synthetic Carbon Allotropes＇. The authors would like to thank S. Romeis and C. Konnerth for interesting and profound discussions
文摘For most particle-based applications, formulation in the liquid phase is a decisive step, and thus, particle interactions and stability in liquid media are of major importance. The concept of Hansen solubility parameters (HSP) was initially invented to describe the interactions of (polymer) molecules and their solubility in different liquids and is increasingly being used in particle technology to describe dispersibility. Because dispersions are not thermodynamically stable, the term Hansen dispersibility parameters (HDP) is used instead of HSP (SiiE, Sobisch, Peukert, Lerche,& Segets, 2018). Herein, we extend a previously developed standardized and non-subjective method for determination of Hansen parameters based on analytical centrifugation to the important class of quantum materials. As a technically relevant model system, zinc oxide quantum dots (QDs) were used to transfer our methodology to nanoparticles (NPs) with sizes below lOnm. The results obtained using the standard procedure starting from a dried powder were compared with those obtained through redispersion from the wet sediment produced during the typical washing procedure of QDs, and drying was observed to play an important role. In conclusion, our study reveals the high potential of HDP for quantifying the interfacial properties of NPs as well as their link to dispersibility.
文摘Particle distribution in the cross-section of the flighted rotating drum(FRD)is critical to the analysis of heat and mass transfer between gas and solids.In this work,the particle tracking velocimetry(PTV)method is applied to study the influence of the number of flights on the particle motion in FRDs.The drum,installed with 1,4,8,or 12 rectangular flights,is filled with plastic balls to 15%and operated at various rotating speeds ranging from 10 rpm to 30 rpm.The results show that the number of flights has different effects on the holdup ratio and cascading rate of single flight and active flights.With 8 and 12 flights,the FRD produces a larger and more stable particle ratio of the dilute phase.Moreover,DEM simulations agree with PTV measurements,whereas literature models show significant deviations.
基金the German Research Foundation through the DFG priority program 1679"Dynamic Simulation of Interconnected Solids Processes".
文摘Fluidized bed opposed jet mills are capable of meeting the continuously growing dema nd for contamination-free fine particles.In this type of jet mill,the solid material is entrained and accelerated by expanding gas jets that are focused onto a focal point in side a fluidized bed.The resulting particle collisions induce breakage.The process is affected by the relative particle velocities and the number of particle-particle collisions.Clearly,both quantities are distributed.However,to date,neither relative particle velocities nor collision frequencies in such units have been determined.The present work introduces an innovative method to assess the stressing conditions in jet mills experimentally.To this end,mixtures of glass and ductile metal microspheres were used,with the latter employed in small amounts.Inter-particle collisions between the aluminum and glass spheres lead to the formation of dents on the microparticles.The size and number of these dents are associated with the individual collision velocities and overall collision frequencies.The correlation between dent size and collision velocity was obtained from finite element calculations based on empirical data.The proposed approach was validated using particle image velocimetry during secondary gas injection into a fluidized bed reactor.In this case the effect of the distance between two opposed nozzles was examined.For a lab-scaled fluidized bed opposed jet mill the effects of gas pressure and hold-up were investigated.Relative particle velocities were found to be sign ificantly lower tha n the gas velocities,while the nu mber of contacts per particle was determined to be extremely high.
