Polydopamine‐Graphene Oxide Flame Retardant Nanocoatings Applied via an Aqueous Liquid Crystalline Scaffold

A highly effective flame retardant (FR) nanocoating was developed by conducting oxidative polymerization of dopamine monomer within an aqueous liquid crystalline (LC) graphene oxide (GO) scaffold coating. Due to its high water content, the LC scaffold coating approach facilitated fast transport and polymerization of dopamine precursors into polydopamine (PDA) within the water swollen interlayer galleries. Uniform and periodically stacked (14.5 Å d‐spacing) PDA/GO nanocoatings could be universally applied on different surfaces, including macroporous flexible polyurethane (PU) foam and flat substrates such as silicon wafers. Remarkably, PDA/GO coated PU foam exhibited highly efficient flame retardant performance reflected by a 65% reduction in peak heat release rate at 5 wt% PDA/GO loading in an 80 nm thick coating. While many physically mixed flame retardants are usually detrimental to the mechanical properties of the foam, the PDA/GO coating did not affect mechanical properties substantially. In addition, the PDA/GO coatings were stable in water due to the intrinsic adhesion capability of PDA and the transformation of GO to the more hydrophobic reduced GO form. Given that PDA is produced from dopamine, a molecule prevalent in nature, these findings suggest that significant opportunities exist for new polymeric FRs derived from other natural catechols.     

DiFuse: distributed frequency domain user selection for multi-user MIMO networks

Optimal user selection is important in increasing the capacity of Multi-User Multi-Input Multi-Output Wi-Fi networks, yet it faces a significant challenge; the multi-user diversity gain can be overwhelmed by the formidable Channel State Information (CSI) acquisition overhead. To lessen the overhead, existing schemes adopt the greedy user selection which generally takes the projected norm as the user selection metric, since it considers both the channel power gain and the orthogonality. However, the projected norm suffers from occasional poor user selection, since it does not take the optimal sum capacity gain into account. This paper proposes a new distributed user selection protocol called DiFuse. To employ the sum capacity gain as the user selection metric in DiFuse, each user cleverly computes its own estimated capacity gain by overhearing the CSI feedback from others. The users then simultaneously transmit their feedbacks at the frequency domain via the distributed feedback contention, which effectively reduces the feedback overhead. Then the AP collectively utilizes them for user selection that achieves the maximum positive increment to the sum capacity gain. We implemented the prototype of DiFuse on the USRP N210, and evaluated its performance via both testbed experiments and trace-driven emulations. The results showed that DiFuse outperforms the throughput of the existing scheme called OPUS by 1.8\({\times }\) on average, while maintaining better fairness.     

Facile synthesis of ZnBi<Subscript>2</Subscript>O<Subscript>4</Subscript>-graphite composites as highly active visible-light photocatalyst for the mineralization of rhodamine B

Novel highly active visible-light photocatalysts in the form of zinc bismuth oxide (ZnBi₂O₄) and graphite hybrid composites were prepared by coupling via a co-precipitation method followed by calcination at 450 °C. The as-prepared ZnBi₂O₄-graphite hybrid composites were tested for the degradation of rhodamine B (RhB) solutions under visible-light irradiation. The existence of strong electronic coupling between the two components within the ZnBi₂O₄-graphite heterostructure suppressed the photogenerated recombination of electrons and holes to a remarkable extent. The prepared composite exhibited excellent photocatalytic activity, leading to more than 93% of RhB degradation at an initial concentration of 50mg·L^-1 with 1.0 g catalyst per liter in 150min. The excellent visible-light photocatalytic mineralization of ZnBi₂O₄-1.0graphite in comparison with pristine ZnBi₂O₄ could be attributed to synergetic effects, charge transfer between ZnBi₂O₄ and graphite, and the separation efficiency of the photogenerated electrons and holes. The photo-induced h+ and the superoxide anion were the major active species responsible for the photodegradation process. The results demonstrate the feasibility of ZnBi₂O₄-1.0graphite as a potential heterogeneous photocatalyst for environmental remediation.     

Cover Image,Volume 67, Issue 5

The cover image, by Keuk‐Min Jeong et al., is based on the Research Article Effects of crosslinking agents on the physical properties of polyimide/amino‐functionalized graphene oxide hybrid films, DOI: 10.1002/pi.5555 .

     

Reaction of ozone with polybenzimidazole (PBI)

Ozone reacted with the polybenzimidazole (PBI) film surface and X-ray photoelectron spectroscopy (XPS) detected a rapid increase in O atom concentration up to a saturation level of 27 ± 1 atomic %. Atomic force microscopy measurements showed little change in surface roughness. The water contact angle of the treated film decreased by ca. 60% compared to untreated PBI film indicating an increase in hydrophilicity and hydrogen bonding due the formation of the polar oxygenated functional groups. With treatment, the curve fitting of the XPS chemical states showed a decrease in the C–C sp², C–C sp³, and C=N functional groups, and an increase in C–N, C=O, O–C=O, O–(C=O)–O, and N–C=O moieties which were explained using mechanisms associated with the dissociation of the primary ozonide formed from the addition of ozone to the C=C and C=N bonds and reaction with the amine groups in PBI. Washing the treated surface with water partially washed away the O concentration which indicated the formation of a weak boundary layer due to the breakage of bonds in the decomposition of the primary ozonide.     

Effect of feed concentration and inlet air temperature on the properties of soymilk powder obtained by spray drying

In this study, soymilk powder was produced by spray drying. The inlet air temperature of spray dryer was varied from 200 to 280°C and the feed concentration was varied from 15 to 25% (w/v). Response surface methodology was used to examine the effects of these independent variables on the detailed characteristics in terms of physical, structural, functional properties of powder. Overall, results show that rising the inlet air temperature caused a decrease in tapped and loose bulk density, true density, filling rate, water holding capacity, and water content of powder; and an increase in compressibility, Hausner ratio, porosity, interstitial air volume, and wettability index. An increase in feed concentration led to an increase in true density, compressibility, Hausner ratio, porosity, interstitial air volume, and wettability index; and a decrease in tapped and loose bulk density, filling rate, water holding capacity, and water content; whereas oil holding capacity might be increased or decreased and it depended almost solely on the feed concentration.     

Will smartwatches last? factors contributing to intention to keep using smart wearable technology

This study aims to deepen our understanding of the underlying factors affecting the intention to continue using increasingly popular wearable technology. A new theoretical model is developed and validated to extend traditional technology acceptance theories by identifying several value drivers of the continuous intention and actual usage of wearable devices. Hypotheses were tested using partial least squares path modeling on data collected from 383 actual smartwatch users. The results provide wearable device manufacturers with practical guidance for optimizing competition strategies. They also offer policy-making insights for practitioners to promote better wearable devices on the market, especially during the early stages of adoption.     

Effect of turbulence models on predicting HAWT rotor blade performances

The effect of turbulence models on predicting the aerodynamic performance of horizontal-axis wind turbine (HAWT) rotor blades has been investigated. The flow fields around the 2-D airfoil and the 3-D blade of the NREL phase VI wind turbine rotor have been analyzed, and the results are compared between a correlation-based transition model and two other fully turbulent models. The turbulence models selected are the Spalart-Allmaras fully turbulent one-equation model, the k-ω SST fully turbulent two-equation model, and the transition model. A vertex-centered finite-volume method based on an unstructured mesh technique was used to discretize the governing Navier-Stokes equations. The inviscid fluxes were calculated by using 2^nd order Roe’s FDS, and the viscous fluxes were evaluated in a central-differencing manner. For the time integration, an implicit method based on the Gauss-Seidel iteration was used. The results showed that the transition model well captures the laminar separation bubbles on the surface of the airfoil and the blade, and these separation bubbles trigger the separation-induced transition as the laminar flow separates and re-attaches as turbulent. The separation bubbles change the flow pattern on the surface of the airfoil and on the blade, and the pressure and skin-friction distributions are also changed abruptly across the laminar-turbulent transition. With properly predicted boundary-layer transition, the results of the transition model match well with the experiment. However, the results of the fully turbulent models deviate from the experiment due to the lack of the ability of capturing the boundary-layer transition. The adoption of a proper transition turbulence model is essential for the accurate prediction of the aerodynamic loads and also the rotor performance for horizontal-axis wind turbines.     

A novel approach to control of nonminimum-phase nonlinear systems

We present an approach to control of nonminimum-phase multiple-input-multiple-output (MIMO) nonlinear systems based on a singular perturbation-like technique. First, we show that a MIMO nonlinear system can be converted into a singularly perturbed system through magnitude and time-scaling transformation. In particular, the proposed transformation technique can make the hidden or internal dynamics behave like a fast subsystem. However, the fast subsystem takes a weakly controllable form, which implies that a small positive low-gain parameter multiplies the control input in the fast subsystem. For this reason, the well-known integral manifold approach is then utilized to decompose the resulting singularly perturbed system into two subsystems in separate timescales, each of which is of a lower dimension and, more importantly, controllable in the sense that the control input will explicitly appear in the boundary-layer system. The proposed method is particularly useful to make the input-output dynamic characteristics of a decouplable nonminimum-phase nonlinear system decoupled and linear.