Strain rate sensitivity of hydroxyapatite coatings

The strain-rate sensitivity of strength is a significant factor to evaluate the deformation mode of crystalline materials. The strain rate sensitivity of hardness is experimentally investigated here for hydroxyapatite coatings that are sputter deposited onto titanium-coated silicon wafers. These biocompatible HA coatings can provide a strong, dense interface between metal alloy implants and porous hydroxyapatite that can help in-growth of tissue. The interface to the metal alloy implant is important to transfer stress during loading. So, it is very important to know the behavior of the coating under different conditions of loading. Our dynamic test procedure now takes advantage of nanoscratch testing to measure the change in scratch hardness of the coating over a strain rate range that may well simulate the mechanical loading conditions found at the interface between implants and hydroxyapatite coatings.     

Droplet Combustion Experiments Aboard the International Space Station

This paper summarizes the first results from isolated droplet combustion experiments performed on the International Space Station (ISS). The long durations of microgravity provided in the ISS enable the measurement of droplet and flame histories over an unprecedented range of conditions. The first experiments were with heptane and methanol as fuels, initial droplet droplet diameters between 1.5 and 5.0 m m, ambient oxygen mole fractions between 0.1 and 0.4, ambient pressures between 0.7 and 3.0 a t m and ambient environments containing oxygen and nitrogen diluted with both carbon dioxide and helium. The experiments show both radiative and diffusive extinction. For both fuels, the flames exhibited pre-extinction flame oscillations during radiative extinction with a frequency of approximately 1 H z. The results revealed that as the ambient oxygen mole fraction was reduced, the diffusive-extinction droplet diameter increased and the radiative-extinction droplet diameter decreased. In between these two limiting extinction conditions, quasi-steady combustion was observed. Another important measurement that is related to spacecraft fire safety is the limiting oxygen index (LOI), the oxygen concentration below which quasi-steady combustion cannot be supported. This is also the ambient oxygen mole fraction for which the radiative and diffusive extinction diameters become equal. For oxygen/nitrogen mixtures, the LOI is 0.12 and 0.15 for methanol and heptane, respectively. The LOI increases to approximately 0.14 (0.14 O ₂/0.56 N ₂/0.30 C O ₂) and 0.17 (0.17 O ₂/0.63 N ₂/0.20 C O ₂) for methanol and heptane, respectively, for ambient environments that simulated dispersing an inert-gas suppressant (carbon dioxide) into a nominally air (1.0 a t m) ambient environment. The LOI is approximately 0.14 and 0.15 for methanol and heptane, respectively, when helium is dispersed into air at 1 atm. The experiments also showed unique burning behavior for large heptane droplets. After the visible hot flame radiatively extinguished around a large heptane droplet, the droplet continued to burn with a cool flame. This phenomena was observed repeatably over a wide range of ambient conditions. These cool flames were invisible to the experiment imaging system but their behavior was inferred by the sustained quasi-steady burning after visible flame extinction. Verification of this new burning regime was established by both theoretical and numerical analysis of the experimental results. These innovative experiments have provided a wealth of new data for improving the understanding of droplet combustion and related aspects of fire safety, as well as offering important measurements that can be used to test sophisticated evolving computational models and theories of droplet combustion.     

Variability in expiratory trajectory angles during consonant production by one human subject and from a physical mouth model: Application to respiratory droplet emission

The COVID-19 pandemic has highlighted the need to improve understanding of droplet transport during expiratory emissions. While historical emphasis has been placed on violent events such as coughing and sneezing, the recognition of asymptomatic and presymptomatic spread has identified the need to consider other modalities, such as speaking. Accurate prediction of infection risk produced by speaking requires knowledge of both the droplet size distributions that are produced, as well as the expiratory flow fields that transport the droplets into the surroundings. This work demonstrates that the expiratory flow field produced by consonant productions is highly unsteady, exhibiting extremely broad inter- and intra-consonant variability, with mean ejection angles varying from ≈+30° to −30°. Furthermore, implementation of a physical mouth model to quantify the expiratory flow fields for fricative pronunciation of [f] and [θ] demonstrates that flow velocities at the lips are higher than previously predicted, reaching 20–30 m/s, and that the resultant trajectories are unstable. Because both large and small droplet transport are directly influenced by the magnitude and trajectory of the expirated air stream, these findings indicate that prior investigations of the flow dynamics during speech have largely underestimated the fluid penetration distances that can be achieved for particular consonant utterances.     

An electrochemical and theoretical comparison of ionic transport through a polystyrene-based cobalt arsenate membrane

An organic–inorganic composite material, i.e. polystyrene cobalt arsenate was chemically synthesized by a sol–gel process. The physico-chemical of the membrane was determined using SEM, TGA and XRD studies. The membrane was found to be crystalline in nature with uniform arrangement of particles and no sign of visible cracks. Ionic potentials were measured across polystyrene cobalt arsenate membranes with 1:1 electrolytes at different concentrations. Observed membrane potentials were positive. The membrane potential offered by the electrolytes was in the order of KCl > NaCl > LiCl. The successful application of TMS and Kobatake theory were used for the estimation of the charge density of membranes. The order of charge density for 1:1 electrolytes solution was found to be KCl > NaCl > LiCl. Kobatake's equation was used under two limiting conditions, namely in the high concentration range and in the dilute concentration range. The two limiting forms of Kobatake's equation gave identical values of charge density for the membrane taken in this investigation. The TMS method was used to investigate the transport number of ions, mobility ratio, distribution coefficient, charge effectiveness, perm selectivity and concentration of fixed charge of the membranes.     

Fireside Corrosion of Superheater Materials in Coal/Biomass Co-fired Advanced Power Plants

A series of laboratory-based fireside corrosion exposures were conducted to assess the effect of such conditions on superheater/reheater materials at higher than conventional metal temperatures. Controlled atmosphere furnaces combined with the “deposit recoat” test method were used to generate the exposure conditions; the gaseous environment simulated that anticipated from air-firing 20 wt% cereal co-product mixed with a UK coal. The exposures were carried out at 600, 650 and 700 °C with four candidate materials: T92, HR3C and 347HFG steels; nickel-based alloy 625. After the exposures, the samples were examined by SEM/EDX to characterize the damage. Pre- and post-exposure dimensional metrology were used to quantify the metal damage in terms of metal loss distributions. For the austenitic steels, the combined deposit/gas/temperature exposure conditions enabled quantification of the characteristic ‘bell-shaped’ curves (of damage as a function of temperature) for fireside corrosion.     

A review on environmental barrier coatings: History,current state of the art and future developments

The increasing demand for more efficient and environmental-friendly gas turbines has driven the development of new strategies for material development. SiC/SiC ceramic matrix composites (CMCs) can fulfil the stringent requirements; however, they require protection from the operating environment and debris ingested during operation. Environmental barrier coatings (EBCs) are a protective measure to enable the CMCs to operate under harsh conditions. EBC-coated CMCs will enable an increased efficiency and reduced pollutant and CO₂ emissions. In this review, the fundamentals of SiC/SiC ceramic matrix composites degradation in steam environments and under the presence of corrosive species, namely CaO-MgO-Al₂O₃-SiO₂ (CMAS), are first presented. Then, a summary of EBCs along with a comprehensive summary of the current compositions and their interactions with steam and molten corrosive species is presented. Finally, an overview of the latest research directions for the potential next generation of EBCs are outlined.