Hexagonal Boron Nitride Passivation Layer for Improving the Performance and Reliability of InGaN/GaN Light-Emitting Diodes

We introduce a low temperature process for coating InGaN/GaN light-emitting diodes (LEDs) with h-BN as a passivation layer. The effect of h-BN on device performance and reliability is investigated. At −5 V, the leakage current of the h-BN passivated LED was -1.15 x 10−9 A, which was one order lower than the reference LED’s leakage current of −1.09 × 10−8 A. The h-BN layer minimizes the leakage current characteristics and operating temperature by acting as a passivation and heat dispersion layer. With a reduced working temperature of 33 from 45 °C, the LED lifetime was extended 2.5 times following h-BN passivation. According to our findings, h-BN passivation significantly improves LED reliability.

Simultaneous solutions for convective heat transfer in dusty-nano- and dusty-hybrid nanoliquids

The present study investigates the heat transfer and flow behaviour of magnetohydrodynamic dusty-nano- and dusty-hybrid nanoliquids caused by the stretched surface. We considered the copper oxide (CuO) and magnesium oxide (MgO) nanoparticle suspension in water (H2O) as the base liquid. Similarity transformations are used to transform the partial differential equations to ordinary differential equations and solved by the Runge-Kutta Fehlberg 45 method with a shooting procedure. Outcomes of the velocity and thermal gradients for diverse physical impacts are depicted via plots and the skin friction factor and heat transfer rate are illustrated via tabulated values. Results reveal that dusty-hybrid nanoliquids and their conductive properties play an important role throughout the study. A growth in the mass concentration of dust particles augments the temperature and the Nusselt number, but the reverse reaction to the friction factor and velocity profile has been seen. The Eckert number has a propensity to magnify the temperature of the fluid phase and dust phase. The interaction of dust and nanoparticles extends to the greater heat transmission in the dust phase associated with the fluid phase. Hybridization showed a positive response in the heat transmission of the nanoliquid. The dusty hybrid-nano liquid shows higher heat dispersion compared to the dusty nanoliquid.

Flexible and high precision thermal metasurface

Thermal metamaterials are artificial materials used to manipulate heat flow in many applications, such as thermal protection, thermal camouflage, and precise temperature control. Most of the existing thermal metamaterials are mainly based on metal, which makes their fabrication complex and time-consuming, and limits their flexibility. Here, we show a strategy to simplify the fabrication process, improve machining accuracy, and realize flexibility in thermal metasurfaces. Our proposed thermal metasurface is fabricated by laser engraving of copper-graphene coating surface, utilizing graphene coating with high thermal conductivity instead of the traditional filling materials of low thermal conductivity. It maintains the integrity of copper substrate, giving the metasurface a good heat dispersion. Controlled temperature gradient patterns are established, and the metasurface can be bent without changing its features, except for a slight variation in its thermal gradient. Finally, its cloaking ability is demonstrated by camouflaging the same heat source in the shape of different objects. Our designed metasurface mitigates the limitations in design and fabrication of existing thermal metamaterials, and can be used in applications requiring large flexibility, thermal illusion, and large thermal gradients on small scales.

Mitigating Intensity of Urban Heat Island by Better Understanding on Urban Morphology and Anthropogenic Heat Dispersion

<p>Anthropogenic heat is one of the key factors that causes intensive urban heat island due to its direct impact on ambient temperature in urban areas. Stagnated airflow due to closely packed tall buildings causes weak dilution and removal of anthropogenic heat. Consequently, research is critically needed to investigate the effect of urban morphology on anthropogenic heat dispersion and provide effective planning strategies to reduce UHI intensity, especially at the extreme scenario, such as with very low wind speed and high heat emission. This study provides scientific understanding and develops a GIS-based modelling tool to support decision-making in urban planning practice. We start from a computational parametric study at the neighbourhood scale to investigate the impact of urban morphology on heat dispersion. Site coverage ratio ( ), and frontal area density ( ) are two urban morphological parameters. Ten parametric cases with two heat emission scenarios are designed to study representative urban areas. Furthermore, based on the energy conservation within the urban canopy layer, we develop a semi-empirical model to estimate spatially-averaged in-canopy air temperature increment, in which the exchange velocity between the street canyon and overlying atmosphere is estimated by the Bentham and Britter model. The performance of the new model is validated by cross-comparing with CFD results from the parametric study. By applying this new model, the impact of anthropogenic heat on air temperature is mapped in residential areas of Singapore for both long-term annually averaged and short-term extreme low wind speed to improve urban climate sustainability and resilience.</p>

Fluid Convection Models for Low-Temperature Grinding and Effect of Fluid Warming

The paper considers fluid convection in low-temperature grinding. Fluid cooling often predominates over all other forms of heat dispersion in the grinding zone particularly in low-temperature grinding. Experimental values of convection heat transfer coefficient (CHTC) up to and in excess of 200,000 W/m2K have been found by various researchers both for water-based emulsions and in one case for mineral oils employed in high wheel-speed grinding. Several convection models have been developed in recent years for the prediction of CHTCs in low-temperature grinding. This paper reviews advances in convection modeling and reconsiders the basic assumptions implied. A proposal is made for improved estimation for highly churned flow assuming a degree of fluid warming. Predicted coefficients are compared with measured values.

Sizing of ballistic arm protection for the VIRTUS body armour and load carriage system

IntroductionSevere haemorrhage from the arm that is unresponsive to direct pressure necessitates the application of a tourniquet. Detachable arm protection, referred to as brassards, are used by the UK Armed Forces to protect the upper arm from fragmentation threats. However, the coverage they originally provided was based on limited medical evidence. Medical consensus has determined that the dimensions of arm protection should in future be related to how far up the arm a tourniquet can be applied.MethodCT scans of 120 male Armed Forces personnel were analysed to ascertain the vertical distances from acromion process to the point at which a tourniquet can applied, equating to the anterior axillary fold. These values were statistically compared with those derived from the 2007 UK Military anthropometric survey using a paired t-test. Additional distances were added to account for tourniquet width and slippage, with the total value compared with VIRTUS brassard length.ResultsNo significant difference (p<0.01) was found in mean acromion to axilla length (114 mm) compared with that found in the anthropometric survey confirming sample validity. The deltoid insertion lay 24 mm below the axillary fold for the 50th percentile value from CT. Essential arm coverage for the 99th percentile male in this study was calculated as 201 mm.ConclusionsBased on this research, a single new brassard for the VIRTUS body armour and load carriage system was recommended and manufactured based on the 99th percentile. This is over 30% shorter than the existing VIRTUS brassard, reducing the overall weight burden for the soldier and improving heat dispersion, integration and interoperability. The new brassard has been issued to Armed Forces personnel since October 2018. The reduced mass of ballistic protective material in conjunction with requiring only a single size of brassard has already saved the Ministry of Defence £20 000 in procurement costs.