Modeling on ammonothermal growth of GaN semiconductor crystals

Ammonothermal systems are modeled using fluid dynamics and heat and mass transfer models. The nutrient is considered as a porous media bed and the flow is simulated using the Darcy–Brinkman–Forchheimer model. The resulting governing equations are solved using the finite volume method. The effects of baffle design on flow pattern, heat and mass transfer in an autoclave are analyzed. For the research-grade autoclave with an internal diameter of 2.22 cm, the constraint for the GaN growth is found to be the growth kinetics and the total area of seed surfaces in the case of baffle opening of 10% (including the central opening of 5% and ring opening of 5%). The fluid flow across the baffle is a clockwise circulating flow which goes upwards in the central hole and downwards in the ring gap. Transport phenomena have been also studied in large-size ammonothermal growth systems with internal diameters of 4.44 cm and 10 cm. The flow pattern across the baffle changes to an anticlockwise circulating flow which goes upwards in the ring gap and downwards in the central hole in the case of 10% baffle opening. Since ammonothermal growth experiments are expensive and time-consuming, modeling becomes an effective tool for research and optimization of the ammonothermal growth processes.     

Erratum to: Experimental and Theoretical Optimization of Radio Frequency Hollow Cathode Discharge

Plasma Chemistry and Plasma Processing -     

Hydrodechlorination of Dichloromethane by a Metal-Free Triazole-Porphyrin Electrocatalyst: Demonstration of Main-Group Element Electrocatalysis**

In this work, the electrocatalytic reduction of dichloromethane (CH₂Cl₂) into hydrocarbons involving a main group element-based molecular triazole-porphyrin electrocatalyst H2PorT8 is reported. This catalyst converted CH₂Cl₂ in acetonitrile to various hydrocarbons (methane, ethane, and ethylene) with a Faradaic efficiency of 70 % and current density of −13 mA cm⁻² at a potential of −2.2 V vs. Fc/Fc⁺ using water as a proton source. The findings of this study and its mechanistic interpretations demonstrated that H2PorT8 was an efficient and stable catalyst for the hydrodechlorination of CH₂Cl₂ and that main group catalysts could be potentially used for exploring new catalytic reaction mechanisms.     

Operation principle of self-phase compensated optical waveguide isolator

A theory is developed for the self-phase compensated optical waveguide isolator recently reported in the literature. The operation principle of such device is explained in terms of synchronization of phase and power conversion. The effect of balancing phase mismatches of the two convertors on achieving a proper percentage of mode conversion is revealed. The way to make use of the phase mismatches of different sections to accommodate the different requirements in phase relationship for the reciprocal and nonreciprocal mode convertors is discussed. The theory is extended to the case where phase compensator is used. It is demonstrated that the introduction of phase compensator separates the adjustment of phase from the adjustment of power for the mode convertors so that relaxes fabrication tolerances of such devices. An isolator consists of three phase mismatched waveguide sections is designed and simulated. The simulation results confirm the self-phase compensation theory.     

Magnetic properties and large cryogenic low-field magnetocaloric effect of HoCo<Subscript>2</Subscript> nanoparticles without core/shell structure

The HoCo₂ nanoparticles are found to be stable in air without any shell protection. The HoCo₂ nanoparticles display superparamagnetic properties between their blocking temperature of 40 K and Curie temperature of 78 K. The magnetic-entropy change increases with decreasing temperature at a certain magnetic-field change, which is ascribed to the competition between the Zeeman energy and thermal-agitation energy at low temperatures. A large magnetic-entropy change of 19.4 J kg⁻¹ K⁻¹ was found at 7.5 K in an applied-field change from 1 to 7 T, while 6.1 J kg⁻¹ K⁻¹ was achieved in a low field change of 1 T. HoCo₂ nanoparticles are useful for application of magnetic refrigeration at low temperatures.